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10.1186_s12879-021-05907-0.pdf
|
Availability of data and materials
All data is publically available through Google Trends and through The
Behavioral Risk Factor Surveillance System (BRFSS).
|
Availability of data and materials All data is publically available through Google Trends and through The Behavioral Risk Factor Surveillance System (BRFSS). Ethics approval and consent to participate Our research was exempted from an ethics review by the University of California at San Diego Human Research Protections Program.
|
Johnson et al. BMC Infectious Diseases (2021) 21:215
https://doi.org/10.1186/s12879-021-05907-0
R E S E A R C H A R T I C L E
Open Access
Monitoring HIV testing and pre-exposure
prophylaxis information seeking by
combining digital and traditional data
Derek C. Johnson1,2*, Alicia L. Nobles1,2, Theodore L. Caputi2,3, Michael Liu4, Eric C. Leas2,5, Steffanie A. Strathdee1,
Davey M. Smith1 and John W. Ayers1,2
Abstract
Background: Public health is increasingly turning to non-traditional digital data to inform HIV prevention and
control strategies. We demonstrate a parsimonious method using both traditional survey and internet search
histories to provide new insights into HIV testing and pre-exposure prophylaxis (PrEP) information seeking that can
be easily extended to other settings.
Method: We modeled how US internet search volumes from 2019 for HIV testing and PrEP compared against expected
search volumes for HIV testing and PrEP using state HIV prevalence and socioeconomic characteristics as predictors. States
with search volumes outside the upper and lower bound confidence interval were labeled as either over or under
performing. State performance was evaluated by (a) Centers for Disease Control and Prevention designation as a hotspot for
new HIV diagnoses (b) expanding Medicaid coverage.
Results: Ten states over-performed in models assessing information seeking for HIV testing, while eleven states under-
performed. Thirteen states over-performed in models assessing internet searches for PrEP information, while thirteen states
under-performed. States that expanded Medicaid coverage were more likely to over perform in PrEP models than states that
did not expand Medicaid coverage. While states that were hotspots for new HIV diagnoses were more likely to over perform
on HIV testing searches.
Conclusion: Our study derived a method of measuring HIV and PrEP information seeking that is comparable across states.
Several states exhibited information seeking for PrEP and HIV testing that deviated from model assessments. Statewide
search volume for PrEP information was affected by a state’s decision to expand Medicaid coverage. Our research provides
health officials with an innovative way to monitor statewide interest in PrEP and HIV testing using a metric for information-
seeking that is comparable across states.
Keywords: Google trends, HIV, PrEP, Internet, HIV testing
* Correspondence: [email protected]
1Department of Medicine, Division of Infectious Diseases and Global Public
Health, University of California San Diego, 9500 Gilman Dr, La Jolla, California
92093, USA
2The Center for Data Driven Health at the Qualcomm Institute, University of
California San Diego, La Jolla, California, USA
Full list of author information is available at the end of the article
© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if
changes were made. The images or other third party material in this article are included in the article's Creative Commons
licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons
licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the
data made available in this article, unless otherwise stated in a credit line to the data.
Johnson et al. BMC Infectious Diseases (2021) 21:215
Page 2 of 7
Background
As people increasingly turn to digital sources of news and
information, online activity has the potential to become a
window into the public’s consciousness [1]. Measuring the
public’s online information seeking has the potential to
predict health behavior, as what people are searching the
internet for can be predictive of what they intend to do in
the future [2]. It is possible that seeking information about
HIV testing and Pre-Exposure Prophylaxis (PrEP) online
could be a new surveillance tool in the fight against HIV.
Previous studies have shown a spike in internet searches
for HIV testing has corresponded with increases in HIV
testing, suggesting that seeking HIV testing information
online could be predictive of testing behavior [3]. Utilizing
internet searches could be a way to enhance the surveil-
lance of the public’s interest in seeking information on
HIV and HIV health seeking behavior. Past efforts to
enhance HIV surveillance relied mostly on upscaling trad-
itional data (e.g., clinical records or surveys) that have in-
trinsic shortcomings, such as a limited ability to provide
current information. For instance, the most recent data
for HIV testing on AIDSvu.org is from 2016 and the most
recent AIDSvu.org data on PrEP usage is from 2018 [4].
These limitations have driven public health to increasingly
turn to digital data, such as news, social media, and inter-
net searches, to learn how people seek HIV information
[5–8]. For example, internet search trends can be used to
investigate public interests as evident by actor Charlie
Sheen’s HIV positive disclosure concurring with record
levels of Google searches for HIV awareness, HIV testing,
and condoms [3]. This finding was valid, as it was later
confirmed by traditional data after a 16 month delay [7].
Internet search histories have potential utility for assessing
both help-seeking behavior regarding public interest in
PrEP for HIV prevention and for HIV testing. For ex-
ample, one study conducted in Hong Kong found that a
direct relation between HIV news trends and online
search behavior for issues regarding HIV/AIDS and men
who have sex with men (MSM) [9]. Other studies have
found that areas with high levels of HIV prevalence have
greater internet search volumes for HIV related terms
then areas of low HIV prevalence [10]. These studies show
that the use of internet search histories combined with
traditional surveillance data has the potential to create
synergies that can yield new insights into HIV related
health behavior.
Our study methods use both internet search histories
and traditional survey data to provide new insights on
information seeking for HIV testing and PrEP informa-
tion that can be easily replicated and extended to other
settings and outcomes. Specifically, we predicted ex-
pected internet search volumes for HIV testing and PrEP
based on statewide HIV prevalence and socioeconomic
(SES) factors and compared them to observed search
volumes in a model that allows us to identify if US states
over or under perform against expectations. Moreover,
we evaluated how state performance varied by (a) states
that are designated as hotspots for new HIV diagnoses
(b) states that received Medicaid expansion funding.
Methods
Our study used data from multiple sources. 1) We ob-
tained the most current state-level prevalence of HIV
from the Center for Disease Control and Prevention
HIV surveillance report [11], which is from 2018. HIV
prevalence was chosen over HIV incidence because we
were interested in look at the association between the
total number of HIV cases in a state and internet
searches for HIV testing and PrEP 2) The following
state-level socioeconomic attributes we obtained from
the 2018 Center for Disease Control and Prevention’s
Behavioral Risk Factor Surveillance System (BRFSS):
proportion of males, white non-Hispanics, people aged
45 years or older, and people with household income
over $50,000 [12]. 3) We obtained 2019 state-level an-
nual internet search volumes for HIV testing and PrEP
from using the Google Trends API. Information avail-
able through this API includes the volume of searches
for each term, the number of searches per unit of time,
and the geographic location of the searches (country, re-
gion, state, city, metropolitan area). Search volume data
was calculated as a query fraction of the proportion of
searches of a specific search term relative to all searches
measured per 10 million searches. Standardizing search
volumes was done in order to account for population
sizes. We defined HIV testing searches as any query that
included the terms “HIV” and “test,” “tests,” or “testing”,
“AIDS test”, or “oraquick”. We defined PrEP searches as
any query that included the terms “PrEP” and “HIV” or
“pre-exposure prophylaxis HIV” or
“Truvada” or
“Descovy”.
Internet search volumes are withheld by Google for
states where searches do not achieve a minimum thresh-
old of searches. As a result, we could not obtain search
data for HIV testing for five states (Alaska, Montana,
South Dakota, Vermont, and Wyoming). PrEP search data
could not be obtained for two states (Vermont and
Wyoming). 4) We obtained data on states that expanded
Medicaid coverage from the Kaiser Family Foundation
[13].
Our analysis followed a four-step process. First, we fit
Poisson regression models with state-level HIV preva-
lence data and state-level socioeconomic attributes to
predict the expected internet search volumes of HIV
testing and PrEP for each state. Second, we fit a centered
least squares regression line of expected search volumes
from our models versus observed search volumes from
Google Trends. Third, we compared the expected search
Johnson et al. BMC Infectious Diseases (2021) 21:215
Page 3 of 7
volumes from our models in step one with the observed
search volumes from Google Trends for each state in
order to assess the level of information seeking for HIV
testing and PrEP by calculating the percent difference
between the observed and expected values of the cen-
tered least squares regression (i.e., (observed-fitted)/fit-
ted * 100%).
States with observed information seeking (measured
by observed internet search volumes for HIV testing and
PrEP) greater than their expected information seeking
(predicted internet search volumes by the Poisson re-
gression model) were considered to be over performing
and exhibit greater information seeking for HIV testing
and PrEP than expected given their prevalence of HIV.
States that were over performing above the 95% confi-
dence interval were highlighted in our results (see ex-
ample of plotting observed vs. expected observations in
Fig. 1). Similarly, states with observed information seek-
ing less than their expected information seeking were
considered to be underperforming and exhibit less infor-
mation seeking for HIV testing and/or PrEP than ex-
pected given their prevalence of HIV. States that were
under performing below the 95% confidence interval
were highlighted in our results To describe statistical
uncertainty between expected and observed search vol-
umes, we used bootstrap sampling to calculate the 95%
confidence interval (CI) for the regression line and la-
beled observations outside the confidence band as states
that over or under performed.
Fourth, to understand which states typically under or
over performed we contrasted the deviations in expected
searches against (a) What states were designated as hot-
spots for new HIV diagnoses by the Centers for Disease
Control and Prevention (CDC) [11], (b) What states re-
ceived Medicaid expansion funding that covered HIV
testing and PrEP [13].
Results
We observed different levels of information seeking for
HIV testing and PrEP across states (Fig. 1). Ten states
over-performed for HIV testing searches. Georgia exhib-
ited the greatest difference with 36.8% more searches
than expected followed closely by Rhode Island (35.2%),
then Indiana (28.8%), Pennsylvania (22.9%), Nevada
(18.6%), Florida (18.0%), Louisiana (17.3%), Washington
(15.3%), Iowa (13.4%), and Virginia (10.0%) (Table 1).
Conversely, eleven states under-performed for HIV test-
ing. New Hampshire exhibited the greatest difference
with − 34.1% less searches than expected, followed by
Maine (− 32.2%), Idaho (− 26.1%), Nebraska (− 21.9%),
Oregon (− 20.5%), New Mexico (− 18.5%), Mississippi (−
16.8%), Arkansas (− 15.7%), Alabama (− 13.6%), Massa-
chusetts (− 12.4%), Arizona (− 10.3%),
Fig. 1 Observed vs. Expected HIV Testing and PrEP Internet Searches
as Compared to a Hypothetical Perfectly Fitting Regression Line
Johnson et al. BMC Infectious Diseases (2021) 21:215
Page 4 of 7
Table 1 Statewide Proportional Differences Between Observed and Expected Internet Search Volumes for Information Seeking
About HIV testing and PrEP
U.S State
Alabama
Alaska
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
District of
Columbia
Florida
Georgia
Hawaii
Idaho
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
HIV prevalence per 100,000
individuals
394
136.6
326.6
278.7
451.9
305
371.9
461.4
2515.5
691.8
745.6
229.4
94.1
384.7
248.6
128.5
154.6
239.2
654.4
158
723.8
Massachusetts
385.2
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada
New
Hampshire
New Jersey
New Mexico
New York
223.1
209.9
456.9
281.9
83.7
161.3
501.4
119
511.3
239.9
822.7
North Carolina 411.4
North Dakota^ 101.0
Ohio
Oklahoma
Oregon
270.1
229.2
227
Pennsylvania
361.8
Rhode Island
318.4
South Carolina 480.8
HIV Search
Differences
−13.6*
NA
−10.3*
−15.7*
−9.3
7
−2.9
−5.6
−10.5
18#
36.9#
−11.3
−26.1*
−5.0
28.8#
13.5#
−0.7
1.0
17.3#
−32.2*
6.7
−12.4*
5.4
−3.5
−16.8*
1.7
NA
−21.9*
18.6#
−34.1*
−2.6
−18.5*
0.5
−5.8
21.6
5.4
2.3
−20.5*
22.9#
35.2#
6.7
PrEP Search
Differences
−6.4
−2.3
14.1#
−1.6
4.8
10.7#
4.1
−7.7*
−1.9
−5.3
0.4
−1.1
− 31*
12.6#
−5.0
−17.2*
−13.3*
10.2#
3.9
11.5
−5.2
23.6#
−9.4*
−5.0
0.1
−6.1*
−28.2*
16.9#
18#
−23.6*
−15.9*
−1.6
12.1#
−8.4*
12.8
0.1
−14.1*
12.4#
17.3#
29.2#
−23.1*
Expanded Medicaid
Coverage@
Designated Hotspot for New
HIV Infections
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
N
Y
N
Y
Y
Y
N
Y
Y
N
Y
Y
Y
Y
N
N
Y
Y
Y
Y
Y
Y
Y
N
Y
Y
N
Y
Y
Y
Y
N
N
N
N
Y
N
N
N
Y
Y
Y
N
N
Y
Y
N
N
Y
Y
N
Y
Y
Y
N
N
N
N
N
N
N
Y
N
Y
Y
N
Y
N
N
Y
N
N
Johnson et al. BMC Infectious Diseases (2021) 21:215
Page 5 of 7
Table 1 Statewide Proportional Differences Between Observed and Expected Internet Search Volumes for Information Seeking
About HIV testing and PrEP (Continued)
U.S State
HIV prevalence per 100,000
individuals
HIV Search
Differences
PrEP Search
Differences
Expanded Medicaid
Coverage@
Designated Hotspot for New
HIV Infections
South Dakota
104.9
Tennessee
Texas
Utah
Vermont
Virginia
352.1
471.3
139.7
149.9
365.9
Washington
245
West Virginia
146.7
NA
−4.3
7
−3.4
NA
10#
15.3#
8.4
−7.9
7.7
2.8
−9.3
−2.9
NA
−6.4
19.9#
30.7#
−27.9*
N
Y
Y
N
Y
N
Y
Y
83.5
148.3
Wisconsin
Wyoming^
#Over performed in search models
*Underperformed in search models
@State decisions on the Affordable Care Act’s Medicaid expansion
^HIV prevalence for 2018 was missing for Wyoming and North Dakota and was substituted with HIV prevalence from 2017
NA
NA
N
N
N
Y
Y
N
N
N
Y
N
N
N
(10.2%). Conversely,
Thirteen states over-performed for PrEP searches
(Table 1). West Virginia exhibited the greatest difference
with 30.7% more searches than expected, followed by
Rhode Island (29.2%), Massachusetts (23.6%), Washing-
ton (19.9%), Nevada (18.0%), Pennsylvania (17.3%), Neb-
raska (16.9%), Arizona (14.1%), Illinois (12.6%), Oregon
(12.4%), New York (12.1%), Colorado (10.7%), and Ken-
tucky
thirteen states under-
performed for PrEP searches. Idaho exhibited the great-
est difference with 30.9.% less searches than expected,
followed by Montana (− 28.2%), Wisconsin (− 27.9%),
New Hampshire (− 23.6%), South Carolina (− 23.1%),
Iowa (− 17.2%), New Jersey (− 15.9%), Oklahoma (−
14.1%), Kansas (− 13.3%), Michigan (− 9.4%), North Car-
olina (− 8.4%), Delaware (− 7.7%), and Missouri (− 6.1%).
States that over or under performed on HIV testing
searches did not necessarily do likewise for PrEP
searches (r = 0.12). For instance, Nebraska ranked 7th
for excess HIV testing searches, but then ranked 43rd
for PrEP searches. Four states (Washington, Nevada,
Pennsylvania, and Rhode Island) over-performed for
both HIV testing and PrEP searches, while only 2 states
(New Hampshire and Idaho) under-performed for both.
States that expanded Medicaid coverage were more
likely to over perform more on PrEP searches compared
to states that did not expand Medicaid coverage (z =
2.04, p < 0.041). States that were hotspots for new HIV
diagnoses were more likely to over perform on HIV test-
ing searches than states that were not hotspots for new
HIV diagnoses (z = 2.08, p < 0.037).
Discussion
Our study derived a method of measuring HIV testing
and PrEP information seeking that is comparable across
states. Several states exhibited information seeking for
PrEP and HIV testing that deviated from what was ex-
pected in our models. A state’s performance in our
models was not affected by its designation as a hotspot
for new HIV infections. However, performance for PrEP
information seeking was associated with a state’s deci-
sion to expand Medicaid coverage. By integrating inter-
net search histories and traditional survey data, our
results provide baseline benchmarks for monitoring
statewide interest in seeking information on HIV testing
and PrEP.
Our research demonstrates a need for increased access
to PrEP information, particularly among states that have
not expanded their Medicaid coverage. Lower interest in
seeking information on PrEP for states that did not ex-
pand Medicaid coverage could be detrimental to increas-
ing PrEP utilization given that insurance coverage affects
PrEP uptake [14, 15]. Approximately 12% of PrEP users
receive PrEP through Medicaid [16] and the refusal to
extend coverage could deny people the ability to access
PrEP. Our results, coupled with the inability to utilize
PrEP due to a lack of health insurance, is a potentially
disastrous combination that could result in an increase
in HIV prevalence in states that underperformed in our
PrEP models.
Underperformance in PrEP models could be due to
the unequal distribution of PrEP across different gen-
ders, ages, and states. Our models control for age, sex,
race, and income at the state level using BRFSS data.
However, our models do not adjust for disparities in the
distribution of PrEP. It is possible that PrEP interven-
tions that do not specifically target key populations with
indications for PrEP use could result in these neglected
populations not searching for PrEP information on line,
Johnson et al. BMC Infectious Diseases (2021) 21:215
Page 6 of 7
which would result in an underperformance in our PrEP
models. For example, five states represented 50% of PrEP
prescriptions and although women represent almost 20%
of new HIV infections, they represented only 7% of PrEP
prescriptions [16, 17]. These types of underlying dispar-
ities in PrEP distribution could possibly be factors influ-
encing how people look for information on PrEP.
Our research suggest the possibility that increased at-
tention to HIV testing, promoted by a state being listed
as a CDC hotspot for new HIV diagnoses, does in fact
result in increased public interest in seeking HIV testing
information [11]. States that are listed as a hotspot for
new HIV infections receive a rapid infusion of additional
resources, expertise, and technology to develop and im-
plement locally tailored HIV interventions [18]. It is pos-
sible that the increased promotion of HIV interventions
results in more public interest in seeking HIV testing in-
formation. This would explain why states that were
listed as hotspots for new HIV diagnoses were more
likely to over perform on HIV testing searches than
states that were not hotspots. Our results support pro-
viding states with more resources to promote HIV test-
ing, given that our models suggest increases in searches
for HIV testing are correlated with more CDC support
for HIV programs.
Our study benefits from several strengths. We use a
nationally representative survey to control for several
SES covariates, ensuring that the US population is accur-
ately represented. Because our methods adjusted for
baseline state level SES characteristics, leaders in each
individual state can use these methods to evaluate their
state-specific progress. Our internet search volume data
is measured in real-time, and while we used annual esti-
mates, it is possible to use the same method to estimate
weekly or monthly search volumes. Most importantly,
our research presents a new method for surveillance and
performance monitoring in HIV prevention.
Our research is not without limitations. Internet search
volume data is aggregated and is susceptible to ecological
confounding. Additionally, it cannot be used to determine
which racial/ethnic, gender, or age groups are or are not
engaged with HIV testing or PrEP. While it is possible that
adding more search terms could affect our results, the ef-
fects of adding additional search terms to our models di-
minishes after the most common search terms are added,
as these terms make up the vast majority of search terms
that the public uses. To insure we were using the most
common search terms for HIV testing and PrEP, we con-
sulted with HIV experts on HIV testing and PrEP nomen-
clature. Using search data may be subject to selection bias,
as not all people access the Internet equally and although
some queries may reflect general curiosity rather than
treatment-seeking, it is well known that internet search
trends mirror many health-related behaviors [1].
Conclusions
Our results are a call-to-action for underperforming
states whose populations are not engaged in searching
for information on HIV testing and PrEP. Our research
provides health officials with an innovative way to moni-
tor statewide interest
in PrEP and HIV testing by
highlighting the states that demonstrate the least online
information seeking, which is critical for the promotion
of HIV testing and PrEP as a way to help end the HIV
epidemic. Further research should examine why certain
states are deficient, and policy makers in deficient states
should make efforts to expand HIV testing and PrEP
promotion, perhaps by replicating the interventions and
policies of better-performing states.
Abbreviations
AIDS: acquired immunodeficiency syndrome; CDC: Centers for Disease
Control and Prevention; CI: confidence interval; HIV: human
immunodeficiency virus; PrEP: pre-exposure prophylaxis; SES: socioeconomic
Acknowledgements
The content of this research is solely the responsibility of the authors and
does not necessarily represent the official views of the California HIV/AIDS
Research Program Office or National Institute of Drug Abuse.
Authors’ contributions (last name of each author is listed under what
they contributed to)
Study Conception and Design
Johnson, Ayers, Nobles, Leas
Acquisition and Preparation of Data
Johnson, Caputi, Liu
Analysis and Interpretation of Data
Johnson, Nobles, Strathdee, Smith
Drafting of Manuscript
Johnson, Nobles, Caputi, Liu, Leas, Strathdee, Smith, Ayers
Critical Revisions/Revising
Johnson, Ayers, Nobles, Leas
All authors read and approved the final manuscript.
Funding
This research was supported by funds from the California HIV/AIDS Research
Program Office of the University of California (OS17-SD-001) and the National
Institute of Drug Abuse (T32 DA023356, R37 DA019829).
Availability of data and materials
All data is publically available through Google Trends and through The
Behavioral Risk Factor Surveillance System (BRFSS).
Ethics approval and consent to participate
Our research was exempted from an ethics review by the University of
California at San Diego Human Research Protections Program.
Competing interests
None of the authors declares any conflicts of interest.
Author details
1Department of Medicine, Division of Infectious Diseases and Global Public
Health, University of California San Diego, 9500 Gilman Dr, La Jolla, California
92093, USA. 2The Center for Data Driven Health at the Qualcomm Institute,
University of California San Diego, La Jolla, California, USA. 3Department of
Health Sciences, University of York, York, UK. 4University of Oxford, Oxford,
UK. 5Department of Family Medicine and Public Health, Division of Health
Policy, University of California San Diego, La Jolla, California, USA.
Johnson et al. BMC Infectious Diseases (2021) 21:215
Page 7 of 7
Received: 8 October 2020 Accepted: 16 February 2021
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org/10.1371/journal.pone.0178737.
15. Doblecki-Lewis S, Liu A, Feaster D, et al. Healthcare access and PrEP
continuation in San Francisco and Miami after the US PrEP demo project. In:
Journal of Acquired Immune Deficiency Syndromes. ; 2017. doi:https://doi.
org/10.1097/QAI.0000000000001236.
16. Huang YLA, Zhu W, Smith DK, Harris N, Hoover KW. Hiv preexposure
prophylaxis, by race and ethnicity — United States, 2014–2016. Morb Mortal
Wkly Rep. 2018. doi:https://doi.org/10.15585/MMWR.MM6741A3
17. AIDSVu (aidsvu.org). Mapping PrEP, First Ever Data on PrEP Users Across the U.
S. Emory University, Rollins School of Public Health. https://aidsvu.org/prep/.
Accessed 10 Mar 2020.
18. U.S. Department of Health and Human Services. 2021. HIV National Strategic
Plaen for the United States: a roadmap to end the epidemic 2021–2025.
Washington, DC.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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10.3390_genes14061211.pdf
|
Data Availability Statement: The whole genome data used in this manuscript are available in the
GenBank database under BioProject accession PRJNA416233, PRJEB10098, PRJEB10854,
PRJNA168142, PRJNA205517, PRJNA230019, PRJNA233529, PRJNA288817 and PRJNA291776.
|
Data Availability Statement: The whole genome data used in this manuscript are available in the GenBank database under BioProject accession PRJNA416233, PRJEB10098, PRJEB10854, PRJNA168142, PRJNA205517, PRJNA230019, PRJNA233529, PRJNA288817 and PRJNA291776.
|
Article
Genome-Wide Assessment of Runs of Homozygosity
by Whole-Genome Sequencing in Diverse Horse
Breeds Worldwide
Chujie Chen 1,†, Bo Zhu 2,†, Xiangwei Tang 1, Bin Chen 1, Mei Liu 1
and Jingjing Gu 1,*
, Ning Gao 1
, Sheng Li 3,*
1 Hunan Provincial Key Laboratory for Genetic Improvement of Domestic Animal,
College of Animal Science and Technology, Hunan Agricultural University, Changsha 410128, China;
[email protected] (C.C.); [email protected] (X.T.); [email protected] (B.C.);
[email protected] (M.L.); [email protected] (N.G.)
2 Novogene Bioinformatics Institute, Beijing 100015, China; [email protected]
3 Maxun Biotechnology Institute, Changsha 410024, China
* Correspondence: [email protected] (S.L.); [email protected] (J.G.)
† These authors contributed equally to this work.
Abstract: In the genomes of diploid organisms, runs of homozygosity (ROH), consecutive segments of
homozygosity, are extended. ROH can be applied to evaluate the inbreeding situation of individuals
without pedigree data and to detect selective signatures via ROH islands. We sequenced and analyzed
data derived from the whole-genome sequencing of 97 horses, investigated the distribution of genome-
wide ROH patterns, and calculated ROH-based inbreeding coefficients for 16 representative horse
varieties from around the world. Our findings indicated that both ancient and recent inbreeding
occurrences had varying degrees of impact on various horse breeds. However, recent inbreeding
events were uncommon, particularly among indigenous horse breeds. Consequently, the ROH-
based genomic inbreeding coefficient could aid in monitoring the level of inbreeding. Using the
Thoroughbred population as a case study, we discovered 24 ROH islands containing 72 candidate
genes associated with artificial selection traits. We found that the candidate genes in Thoroughbreds
were involved in neurotransmission (CHRNA6, PRKN, and GRM1), muscle development (ADAMTS15
and QKI), positive regulation of heart rate and heart contraction (HEY2 and TRDN), regulation of
insulin secretion (CACNA1S, KCNMB2, and KCNMB3), and spermatogenesis (JAM3, PACRG, and
SPATA6L). Our findings provide insight into horse breed characteristics and future breeding strategies.
Keywords: ROH; whole-genome sequencing; inbreeding; horse; Thoroughbred
1. Introduction
Domestication of horses began approximately 5500 years ago in the Eurasian steppe [1–3].
Since then, selective breeding and acclimatization have shaped the horse genome, resulting
in more than 500 horse breeds worldwide [4]. Horses are employed in transportation,
warfare, agriculture, and entertainment and can be categorized according to their usage
(racing, sport, endurance, local, and gait), appearance (body size, coat color, and confor-
mation), and temperament (hot, warm, and cold). Horse genomics has progressed rapidly
since the establishment of the horse reference genome [5,6] and advancements in genomics
technology. The genetic mechanisms of many horse traits have been investigated using
single nucleotide polymorphism (SNP) chips and resequencing of the whole genome [7]. In
contrast to SNP chips, whole-genome sequencing can repeatedly cover the entire genome,
resulting in greater resolution and accuracy.
Inbreeding is inevitable in the horse population, and breeds subjected to intense artifi-
cial selection and/or those with a small population size are more likely to experience the
negative effects of inbreeding (such as inbreeding depression). Calculating the inbreeding
Citation: Chen, C.; Zhu, B.; Tang, X.;
Chen, B.; Liu, M.; Gao, N.; Li, S.; Gu,
J. Genome-Wide Assessment of Runs
of Homozygosity by Whole-Genome
Sequencing in Diverse Horse Breeds
Worldwide. Genes 2023, 14, 1211.
https://doi.org/10.3390/
genes14061211
Academic Editor: Chunjiang Zhao
Received: 25 April 2023
Revised: 29 May 2023
Accepted: 30 May 2023
Published: 1 June 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under
the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Genes 2023, 14, 1211. https://doi.org/10.3390/genes14061211
https://www.mdpi.com/journal/genes
genesG C A TT A C GG C A TGenes 2023, 14, 1211
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coefficient from pedigree-based data [8] is the conventional method for measuring the in-
breeding level. However, pedigree mistakes in farm animals [9] and horse populations [10]
are prevalent. Runs of homozygosity (ROH) are continuous stretches of homozygosity
regions spread across diploid genomes resulting from the transmission of identical haplo-
types from common ancestors [11]. ROHs were first identified in the human genome [12]
and have been used to define the degree of inbreeding [13]. The ROH-based genomic
inbreeding coefficient (FROH) is described by measuring the proportion of the ratio of the
sum of each individual’s ROH lengths to the total genome length [14].
Due to the fact that inbreeding is one of the primary causes of ROH [15], ROH is
able to be applied to evaluate the inbreeding situation of individuals without pedigree
data. In general, long ROHs indicate recent genome-wide inbreeding events, whereas
short ROHs indicate ancient inbreeding. Additionally, population bottlenecks, genetic drift,
and selection may contribute to the emergence of ROHs [16]. ROH are not distributed
indistinguishably across the genome and accumulate in particular regions of the genome
in various populations. The regions of the genome with the highest ROH occurrence in a
population are known as “ROH islands” [17]. Genomic regions with selective signatures
frequently overlap with ROH islands [18]. ROH islands can therefore be used to identify
potentially selected genomic regions and identify the genetic basis of commercially valuable
traits in farm animal populations [19]. In recent years, ROH detections on horses have
become increasingly prevalent. However, most ROH studies on horses have focused on SNP
chip data, and only a few have utilized whole-genome sequencing for ROH analysis [20].
We sequenced and utilized whole-genome sequencing data from 97 horses to identify
and analyze ROH patterns in 16 globally representative horse breeds. Using the Thorough-
bred population as a case study, we further investigated ROH islands containing potential
candidate genes for performance traits. Our findings provide insight into horse breed
characteristics and future breeding strategies.
2. Materials and Methods
2.1. Ethics Statement
The Hunan Agricultural University’s Biomedical Research Ethics Committee approved
this study (No. 202046). No horses were injured during or after the sample collection, and
they remained healthy.
2.2. Sampling and Whole-Genome Sequencing
In our horse panel, 37 horses were whole-genome sequenced at high coverage (~30×).
Using a standard phenol-chloroform method, DNAs were obtained from freshly collected
blood samples. Following instructions provided by the manufacturer, sequencing libraries
were constructed and sequenced using an Illumina HiSeq 4000 sequencer to generate 150 bp
paired-end reads. We also retrieved whole-genome sequencing data for more diverse horse
breeds from NCBI (BioProject accession numbers: PRJEB10098, PRJEB10854, PRJNA168142,
PRJNA205517, PRJNA230019, PRJNA233529, PRJNA288817, and PRJNA291776). We ana-
lyzed a diverse horse panel (breed n = 16; total sample n = 97) with distinct appearances,
breed-defining traits, and geographic origins. The horse breeds included Arabian, An-
dalusian, Akhal-Teke, Criollo, Debao, Friesian, Hanoverian, Jeju, Mongolian, Franches-
Montagnes, Przewalskii, American Quarter Horse, Shetland pony, Standardbred, Thor-
oughbred, and Yakutian.
2.3. Quality Controls and SNP Genotyping
All raw sequencing reads were preprocessed for quality control and filtered using
FastQC. After quality control, the BWA program [21] was employed to map clean reads
to the equine reference genome (EquCab3). Population-scale SNP calling was performed
using the Bayesian approach in the SAMtools package [22]. The EquCab3 genome was used
to conduct SNP annotation using ANNOVAR [23]. According to their genomic location,
SNPs were classified into the following classes: exonic, intronic, splicing sites, upstream,
Genes 2023, 14, 1211
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downstream, and intergenic. Exonic SNPs were further classified as synonymous, non-
synonymous, stop-gain, and stop-loss SNPs.
2.4. Runs of Homozygosity Detection
ROH were calculated utilizing Plink v1.9 [24]. We scanned the entire genome of each
horse using a sliding window strategy to identify the ROH regions. The criteria used
to identify ROH were as follows: (1) the size of the sliding window was set to 500 kb;
(2) the lowest SNP density was one per 50 kb; (3) 1 Mb was the maximum distance between
SNPs; (4) based on the ROH length, 1 heterozygote was allowed in a sliding window;
(5) a maximum of 4 missing genotypes were allowed. The defined ROHs were categorized
according to their length: <1 Mb, 1–5 Mb, 5–10 Mb, and >10 Mb.
2.5. Inbreeding Coefficients
As reported by McQuillan et al. [14], genome-wide inbreeding coefficients were com-
puted. In each individual, to calculate the inbreeding coefficients for each of the five
ROH categories, the total length of each ROH category was divided by the total length of
the autosomes (2280.94 Mb) in the sequenced horse genome. The inbreeding coefficients
were recorded as FROH < 1 Mb (<1 Mb), FROH 1–5 Mb (1 to 5 Mb), FROH 5–10 Mb (5 to 10 Mb),
FROH > 10 Mb (>10 Mb), and FROH all (including ROHs of all lengths).
2.6. Detection of ROH Islands in Thoroughbreds and Candidate Genes
To determine the ROH islands in the Thoroughbred population (n = 22), we calculated
the frequency of each SNP across all ROH regions in the entire Thoroughbred population.
Potential ROH islands were identified as the top 1% of SNPs based on their occurrence
frequency in the empirical distribution [17]. Using information from the Ensembl Genome
Browser (www.ensembl.org, accessed on 20 February 2023), genes contained in the ROH
islands were annotated. Functional analysis of the candidate genes was performed using
Gene Ontology (GO) Biological Process enrichment and Kyoto Encyclopedia of Genes and
Genomes (KEGG) pathway analyses in DAVID 2021 [25], with an adjusted p-value greater
than 0.05 indicating significance.
3. Results
3.1. Whole-Genome Sequencing
Using the whole-genome sequencing method, we sequenced and obtained a total of
56,768.2 million clean reads for 97 horse individuals, and the mean entire genome coverage
for each horse was 25.6× (Table S1). We obtained 22,539,736 informative SNPs that were
evenly dispersed across the equine genome (10 SNPs per kb on average) following a
stringent quality control filtering process. Using the Ensembl horse gene annotation set
(Release 106), these population SNPs were annotated. A total of 8,461,302 (37.5%) SNPs
were mapped within the gene regions, including 7,835,178 SNPs in introns, 208,369 SNPs
in exons, and 417,052 SNPs in untranslated regions.
3.2. ROHs in the 16 Horse Breeds
In this study, ROHs were identified in 16 diverse horse breeds that represented dif-
ferent phenotypes and levels subject to selection (Figure 1). To understand the ROH
characteristics of the studied horse population, we first examined the average total length
of the population ROH and the average number of total ROH for each horse breed. We
found that the three highest average numbers of total ROH per horse breed were discov-
ered in three sport horse breeds: Friesian (637), Arabian (621), and Thoroughbred (568).
The three lowest average numbers of total ROH per horse were observed in Przewalskii
primitive horses (180) and two local horse breeds, Debao (167) and Yakutian (102).
Genes 2023, 14, 1211
4 of 12
Figure 1. Box plots of ROHs detected in 16 different horse breeds. The horse breeds were classified
according to their main usages. The horse breeds included Arabian (AB), Andalusian (AL), Akhal-
Teke (AT), Criollo (CR), Debao (DB), Friesian (FS), Hanoverian (HAN), Jeju (JEJU), Mongolian (MG),
Franches-Montagnes (MON), Przewalskii (PRZ), American Quarter Horse (QT), Shetland pony (ST),
Standardbred (STD), Thoroughbred (TB) and Yakutian (YAK). Hollow dots represent the outliers.
Furthermore, the average total length of ROH maintained the same pattern as the
average number of total ROH for each breed. Friesian horses had the largest average
total length of ROH (635.69 Mb), followed by Arabian (602.63 Mb) and Thoroughbred
(614.86 Mb). The lowest average total length of ROH was still found in the primitive and
local horse breeds (Przewalskii: 159.15 Mb, Debao: 118.61 Mb, and Yakutian: 65.69 Mb).
Of the ROH segments in the four length categories, most are short ROH segments
(<1 Mb), followed by ROH segments of 1–5 Mb, accounting for 69.55% and 29.83% of the
total number of ROHs, respectively. ROH segments (5–10 Mb) were present in 12 horse
breeds, with Thoroughbred having the most abundant (117). ROHs greater than 10 Mb
were also the highest in Thoroughbred (10), followed by Standardbreds and Franches-
Montagnes, each with only one long ROH. No long ROH fragments (>10 Mb) were found
in the other horse breeds. Table 1 provides a summary of the ROH segment statistics for
the 16 horse breeds.
Table 1. Summary statistics of the runs of homozygosity (ROH) based on length classes.
Horse
Population
No. of
Samples
Total
No. a
Mean
No. b
Friesian
Thoroughbred
Arabian
Shetland pony
Andalusian
Akhal-Teke
Standardbred
Hanoverian
American
Quarter Horse
5
22
5
3
4
5
4
4
7
3183
12,490
3104
1435
2102
2112
1549
1282
2173
637
568
621
478
526
422
387
321
310
Total
Length
(Mb) c
3178.47
13,526.86
3013.17
1545.37
2047.94
1997.03
1550.21
1207.67
1997.14
Total Mean
Length
(Mb) d
Max.
Length
(Mb) e
635.69
614.86
602.63
515.12
511.99
399.41
387.55
301.92
285.31
7.82
13.89
7.34
8.18
8.47
8.37
11.38
7.32
8.99
Classification of ROH by Length
<1 Mb
1–5 Mb
5–10 Mb
>10 Mb
2116
8233
2119
918
1428
1508
1094
918
1587
1060
4130
970
504
667
599
437
360
575
7
117
15
13
7
5
17
4
11
0
10
0
0
0
0
1
0
0
Genes 2023, 14, x FOR PEER REVIEW 4 of 12 3.2. ROHs in the 16 Horse Breeds In this study, ROHs were identified in 16 diverse horse breeds that represented dif-ferent phenotypes and levels subject to selection (Figure 1). To understand the ROH char-acteristics of the studied horse population, we first examined the average total length of the population ROH and the average number of total ROH for each horse breed. We found that the three highest average numbers of total ROH per horse breed were discovered in three sport horse breeds: Friesian (637), Arabian (621), and Thoroughbred (568). The three lowest average numbers of total ROH per horse were observed in Przewalskii primitive horses (180) and two local horse breeds, Debao (167) and Yakutian (102). Figure 1. Box plots of ROHs detected in 16 different horse breeds. The horse breeds were classified according to their main usages. The horse breeds included Arabian (AB), Andalusian (AL), Akhal-Teke (AT), Criollo (CR), Debao (DB), Friesian (FS), Hanoverian (HAN), Jeju (JEJU), Mongolian (MG), Franches-Montagnes (MON), Przewalskii (PRZ), American Quarter Horse (QT), Shetland pony (ST), Standardbred (STD), Thoroughbred (TB) and Yakutian (YAK). Hollow dots represent the out-liers. Furthermore, the average total length of ROH maintained the same pattern as the average number of total ROH for each breed. Friesian horses had the largest average total length of ROH (635.69 Mb), followed by Arabian (602.63 Mb) and Thoroughbred (614.86 Mb). The lowest average total length of ROH was still found in the primitive and local horse breeds (Przewalskii: 159.15 Mb, Debao: 118.61 Mb, and Yakutian: 65.69 Mb). Of the ROH segments in the four length categories, most are short ROH segments (<1 Mb), followed by ROH segments of 1–5 Mb, accounting for 69.55% and 29.83% of the total number of ROHs, respectively. ROH segments (5–10 Mb) were present in 12 horse breeds, with Thoroughbred having the most abundant (117). ROHs greater than 10 Mb were also the highest in Thoroughbred (10), followed by Standardbreds and Franches-Montagnes, each with only one long ROH. No long ROH fragments (>10 Mb) were found in the other horse breeds. Table 1 provides a summary of the ROH segment statistics for the 16 horse breeds. Genes 2023, 14, 1211
5 of 12
Table 1. Cont.
Horse
Population
Franches-
Montagnes
Criollo
Jeju
Przewalskii
Mongolian
Debao
Yakutian
No. of
Samples
Total
No. a
Mean
No. b
6
2
2
10
5
5
7
1476
491
501
1802
993
836
711
246
246
251
180
199
167
102
Total
Length
(Mb) c
1649.65
403.68
369.51
1591.60
785.24
593.08
459.81
Total Mean
Length
(Mb) d
Max.
Length
(Mb) e
274.94
201.84
184.76
159.16
157.05
118.62
65.69
10.56
4.12
4.13
8.74
4.70
5.06
2.91
Classification of ROH by Length
<1 Mb
1–5 Mb
5–10 Mb
>10 Mb
939
390
417
1378
809
712
639
520
101
84
423
184
123
72
16
0
0
1
0
1
0
1
0
0
0
0
0
0
a Total No.: The overall amount of ROH found in a horse population. b Mean No.: the average number of total
ROH per horse breed. c Total Length: sum of all ROH lengths obtained within a horse population. d Total Mean
Length: the average of the total length of ROH in each population. e Max. Length: maximum length of ROH
segment detected in a horse population.
3.3. Assessment of Inbreeding Coefficients
According to the different ROH length categories, the inbreeding coefficient was
calculated for each horse, and then the average inbreeding coefficient within the horse
breed was calculated. Friesian had the highest value of FROH all (2.79 × 10−1), followed
by Arabian (2.64 × 10−1) and Thoroughbred (2.58 × 10−1). Primitive and indigenous
breeds, such as Przewalskii (6.98 × 10−2), Mongolian (6.89 × 10−2), Debao (5.20 × 10−2)
and Yakutian (2.88 × 10−2), had relatively low inbreeding coefficient values. In the <1 Mb
and 1–5 Mb ROH range divisions, Friesian had the highest FROH (<1 Mb) (1.16 × 10−1) and
FROH (1–5 Mb) (1.59 × 10−1), whereas Yakutian had the lowest FROH (<1 Mb) (2.26 × 10−2) and
FROH (1–5 Mb) (6.20 × 10−3) among all the horse breeds. In the 5–10 Mb and >10 Mb long
ROH range divisions, Thoroughbreds had the highest inbreeding coefficients FROH (5–10 Mb)
(1.42 × 10−2) and FROH (>10 Mb) (2.27 × 10−3) compared to the rest of the horse breeds. The
mean genomic inbreeding coefficients (FROH) for ROH of different length categories in
horse populations are shown in Table 2.
Table 2. Mean genomic inbreeding coefficients (FROH) for ROH of different length categories in
horse populations.
Horse
Population
Horse
Population
No. of
Samples
FS
AB
TB
AL
ST
AT
STD
HAN
QT
JEJU
CR
MON
MG
DB
PRZ
YAK
Friesian
Arabian
Thoroughbred
Andalusian
Shetland pony
Akhal-Teke
Standardbred
Hanoverian
American
Quarter Horse
Jeju
Criollo
Franches-
Montagnes
Mongolian
Debao
Przewalskii
Yakutian
5
5
23
4
3
5
4
4
7
2
2
5
5
5
10
7
ROH Length Category (Mb)
FROH
(<1 Mb)
1.16 × 10−1
1.15 × 10−1
9.73 × 10−2
9.69 × 10−2
8.26 × 10−2
8.14 × 10−2
7.33 × 10−2
6.00 × 10−2
5.94 × 10−2
5.41 × 10−2
5.19 × 10−2
4.96 × 10−2
4.26 × 10−2
3.54 × 10−2
3.50 × 10−2
2.26 × 10−2
FROH
(1–5 Mb)
1.59 × 10−1
1.41 × 10−1
1.44 × 10−1
1.23 × 10−1
1.31 × 10−1
9.06 × 10−2
8.39 × 10−2
6.95 × 10−2
6.15 × 10−2
2.69 × 10−2
3.66 × 10−2
8.53 × 10−2
2.63 × 10−2
1.62 × 10−2
3.44 × 10−2
6.20 × 10−3
FROH
(5–10 Mb)
3.81 × 10−3
7.50 × 10−3
1.42 × 10−2
4.87 × 10−3
1.19 × 10−2
3.06 × 10−3
1.15 × 10−2
2.79 × 10−3
4.22 × 10−3
0
0
8.80 × 10−3
0
4.44 × 10−4
3.83 × 10−4
0
FROH
(>10 Mb)
0
0
2.27 × 10−3
0
0
0
1.25 × 10−3
0
0
0
0
9.26 × 10−4
0
0
0
0
FROH all
SD
2.79 × 10−1
2.64 × 10−1
2.58 × 10−1
2.24 × 10−1
2.26 × 10−1
1.75 × 10−1
1.70 × 10−1
1.32 × 10−1
1.25 × 10−1
8.10 × 10−2
8.85 × 10−2
1.45 × 10−1
6.89 × 10−2
5.20 × 10−2
6.98 × 10−2
2.88 × 10−2
3.11 × 10−4
3.12 × 10−4
4.25 × 10−4
3.13 × 10−4
3.93 × 10−4
3.17 × 10−4
3.93 × 10−4
3.25 × 10−4
3.26 × 10−4
ND
ND
4.43 × 10−4
2.21 × 10−4
1.90 × 10−4
3.10 × 10−4
1.33 × 10−4
Note: FROH was calculated using this formula: FROH = LROH/LAUTO. The total length of ROH on autosomes is
denoted by LROH. LAUTO is the total autosomal length (2280.94 Mb). ND: not detected. SD: standard deviation
(SD is only calculated for population sample sizes greater than 3).
Genes 2023, 14, 1211
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3.4. The ROH Islands and Candidate Genes in Thoroughbreds
Since Thoroughbreds have been selectively bred for racing performance for more
than 300 years, we further analyzed the ROH genome-wide distribution patterns using
the Thoroughbred population as a case study.
In total, 10,631 ROHs were identified
in 22 Thoroughbred horses (Table S2). We found that ROH segments were not evenly
distributed across chromosomes. Figure 2 displays the number of ROH and percentage of
genomic ROH coverage in the Thoroughbred population on each chromosome. With a high
coverage ratio of 28.2%, chromosome 1 of Equus caballus (ECA1) contains the most ROH
segments (997). In contrast, ECA29 had the fewest ROH segments (102), and its coverage
ratio is the second lowest (11.97%). ECA17 had the highest percentage of coverage (31.63%),
while ECA12 had the lowest (11.15%).
Figure 2. Distribution of ROH in Thoroughbred population. The bars represent the sum of number of
ROH, and the line represents the percentage of genomic ROH coverage on horse chromosomes 1 to 31.
Next, we examined the ROH islands in the Thoroughbred population to identify
genomic regions that might have been subjected to selection pressure. We calculated the
frequency of SNPs occurring in ROHs and selected the top 1% as an indicator of the ROH
islands. The frequency of SNP occurrence within the ROH regions was plotted against the
locations of the SNPs along the chromosome for each individual using the Manhattan plot.
A total of 24 ROH islands containing 72 candidate genes were identified on ECA7, 10, 16,
19, 23, 25, 27, 29, 30, and 31 (Figure 3). The longest ROH island was identified on ECA16
with 3325 contiguous SNPs, whereas the shortest was observed on ECA31. ECA30 had the
largest number of ROH islands (six ROH islands, including five candidate genes).
Most identified ROH islands in Thoroughbreds contained candidate genes. However,
six ROH islands on ECA25, 29, 30, and 31 did not contain any annotated protein-coding
genes. Enrichment analyses for GO and KEGG on all identified candidate genes were
conducted. Nine significant GO biological process terms and three significant KEGG
pathways are listed in Supplementary Table S3. The most significantly enriched GO terms
were neurological signaling, neuronal development, positive regulation of heart rate and
contraction, and metabolic processes. KEGG pathways were significantly enriched in
cholinergic synapses, retrograde endocannabinoid signaling, and insulin secretion. We
found that the candidate genes were involved in neurotransmission (CHRNA6, PRKN, and
GRM1), muscle development (ADAMTS15 and QKI), positive regulation of heart rate and
contraction (HEY2 and TRDN), regulation of insulin secretion (CACNA1S, KCNMB2, and
KCNMB3), and spermatogenesis (JAM3, PACRG, and SPATA6L).
Genes 2023, 14, x FOR PEER REVIEW 6 of 12 QT American Quar-ter Horse 7 5.94 × 10−2 6.15 × 10−2 4.22× 10−3 0 1.25 × 10−1 3.26 × 10−4 JEJU Jeju 2 5.41 × 10−2 2.69 × 10−2 0 0 8.10 × 10−2 ND CR Criollo 2 5.19 × 10−2 3.66 × 10−2 0 0 8.85 × 10−2 ND MON Franches-Monta-gnes 5 4.96 × 10−2 8.53 × 10−2 8.80× 10−3 9.26 × 10−4 1.45 × 10−1 4.43 × 10−4 MG Mongolian 5 4.26 × 10−2 2.63 × 10−2 0 0 6.89 × 10−2 2.21 × 10−4 DB Debao 5 3.54 × 10−2 1.62 × 10−2 4.44 × 10−4 0 5.20 × 10−2 1.90 × 10−4 PRZ Przewalskii 10 3.50 × 10−2 3.44 × 10−2 3.83 × 10−4 0 6.98 × 10−2 3.10 × 10−4 YAK Yakutian 7 2.26 × 10−2 6.20× 10−3 0 0 2.88 × 10−2 1.33 × 10−4 Note: FROH was calculated using this formula: FROH = LROH/LAUTO. The total length of ROH on auto-somes is denoted by LROH. LAUTO is the total autosomal length (2280.94 Mb). ND: not detected. SD: standard deviation (SD is only calculated for population sample sizes greater than 3). 3.4. The ROH Islands and Candidate Genes in Thoroughbreds Since Thoroughbreds have been selectively bred for racing performance for more than 300 years, we further analyzed the ROH genome-wide distribution patterns using the Thoroughbred population as a case study. In total, 10,631 ROHs were identified in 22 Thoroughbred horses (Table S2). We found that ROH segments were not evenly distrib-uted across chromosomes. Figure 2 displays the number of ROH and percentage of ge-nomic ROH coverage in the Thoroughbred population on each chromosome. With a high coverage ratio of 28.2%, chromosome 1 of Equus caballus (ECA1) contains the most ROH segments (997). In contrast, ECA29 had the fewest ROH segments (102), and its coverage ratio is the second lowest (11.97%). ECA17 had the highest percentage of coverage (31.63%), while ECA12 had the lowest (11.15%). Figure 2. Distribution of ROH in Thoroughbred population. The bars represent the sum of number of ROH, and the line represents the percentage of genomic ROH coverage on horse chromosomes 1 to 31. Next, we examined the ROH islands in the Thoroughbred population to identify ge-nomic regions that might have been subjected to selection pressure. We calculated the frequency of SNPs occurring in ROHs and selected the top 1% as an indicator of the ROH islands. The frequency of SNP occurrence within the ROH regions was plotted against the locations of the SNPs along the chromosome for each individual using the Manhattan plot. A total of 24 ROH islands containing 72 candidate genes were identified on ECA7, 10, 16, 19, 23, 25, 27, 29, 30, and 31 (Figure 3). The longest ROH island was identified on ECA16 with 3325 contiguous SNPs, whereas the shortest was observed on ECA31. ECA30 had the largest number of ROH islands (six ROH islands, including five candidate genes). Genes 2023, 14, 1211
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Figure 3. Manhattan plot of the occurrences (%) of each SNP within ROH regions in Thoroughbred
population. Each colorful dot stands for an SNP. The horizontal red dotted line represents the cutoff
level (top 1%).
4. Discussion
4.1. Distribution and Patterns of ROH in 16 Horse Populations
In the diploid genome, ROHs are the contiguous regions in which all SNPs at any
position are homozygous in an individual [13]. In our study, we examined the length
patterns of ROH in 16 diverse horse populations. In general, short ROHs (1 Mb) were the
most prevalent, followed by medium (1–5 Mb) and medium-long ROHs (5–10 Mb), with
only a dozen ultra-long ROHs (>10 Mb) detected. The ROH lengths may approximate the
period during which inbreeding occurs. For instance, short ROHs indicate a history of
ancestral inbreeding, whereas long ROHs usually result from recent inbreeding events. We
found that the average length of the short ROHs was much longer in horse breeds (such as
Friesian, Thoroughbred, and Arabian) that had been subjected to strong artificial selection
than in native horse breeds (such as Mongolian, Debao, and Yakutian).
In conjunction with the number and average length of ROHs based on the length cate-
gories, the results suggested that ancient and recent inbreeding events may have varying
degrees of influence on various horse breeds. However, very recent instances of inbreeding
were uncommon, particularly among indigenous horse breeds. It is worth noting that
inbreeding events are not the only factor affecting ROH length. Owing to dynamic ran-
domness and recombination during gamete formation, the generation and evolution of
ROHs are random events to a certain extent [26]. In addition, reduced population size and
bottlenecks may alter the properties of short ROH (<4 Mb) [27].
4.2. ROH-Based Genomic Inbreeding Coefficients
Traditionally, the inbreeding coefficient has been calculated primarily using data ob-
tained from pedigrees. However, the horse pedigree records often contain errors that may
have occurred long ago and could not be tracked. On the other hand, some native horse
breeds did not even have pedigree records. Recently, calculating inbreeding coefficients
using the genome-wide SNP data of livestock is now achievable thanks to the advent
of high-density SNP genotyping technology (such as SNP chips and whole-genome se-
quencing) [19]. SNP data are more advantageous than pedigree data for evaluating the
impact of inbreeding [28]. Moreover, SNP-based calculations of the inbreeding coefficients
demonstrated authentic relationships between individuals [29].
In our study, we used the whole-genome sequencing method to estimate unbiased
genome-wide inbreeding coefficients. We found that horse breeds that required breed
Genes 2023, 14, x FOR PEER REVIEW 7 of 12 Most identified ROH islands in Thoroughbreds contained candidate genes. How-ever, six ROH islands on ECA25, 29, 30, and 31 did not contain any annotated protein-coding genes. Enrichment analyses for GO and KEGG on all identified candidate genes were conducted. Nine significant GO biological process terms and three significant KEGG pathways are listed in Supplementary Table S3. The most significantly enriched GO terms were neurological signaling, neuronal development, positive regulation of heart rate and contraction, and metabolic processes. KEGG pathways were significantly enriched in cho-linergic synapses, retrograde endocannabinoid signaling, and insulin secretion. We found that the candidate genes were involved in neurotransmission (CHRNA6, PRKN, and GRM1), muscle development (ADAMTS15 and QKI), positive regulation of heart rate and contraction (HEY2 and TRDN), regulation of insulin secretion (CACNA1S, KCNMB2, and KCNMB3), and spermatogenesis (JAM3, PACRG, and SPATA6L). Figure 3. Manhattan plot of the occurrences (%) of each SNP within ROH regions in Thoroughbred population. Each colorful dot stands for an SNP. The horizontal red dotted line represents the cutoff level (top 1%). 4. Discussion 4.1. Distribution and Patterns of ROH in 16 Horse Populations In the diploid genome, ROHs are the contiguous regions in which all SNPs at any position are homozygous in an individual [13]. In our study, we examined the length pat-terns of ROH in 16 diverse horse populations. In general, short ROHs (1 Mb) were the most prevalent, followed by medium (1–5 Mb) and medium-long ROHs (5–10 Mb), with only a dozen ultra-long ROHs (>10 Mb) detected. The ROH lengths may approximate the period during which inbreeding occurs. For instance, short ROHs indicate a history of ancestral inbreeding, whereas long ROHs usually result from recent inbreeding events. We found that the average length of the short ROHs was much longer in horse breeds (such as Friesian, Thoroughbred, and Arabian) that had been subjected to strong artificial selection than in native horse breeds (such as Mongolian, Debao, and Yakutian). In conjunction with the number and average length of ROHs based on the length categories, the results suggested that ancient and recent inbreeding events may have var-ying degrees of influence on various horse breeds. However, very recent instances of in-Genes 2023, 14, 1211
8 of 12
registrations and had studbooks had high overall inbreeding coefficients (high FROH all).
For example, due to the limited number of Thoroughbred founders, their effective popula-
tion size is modest. In contrast, indigenous horse breeds showed relatively low degrees of
inbreeding (low FROH all). We further calculated the FROH using different lengths of ROH as
follows: FROH < 1 Mb, FROH 1–5 Mb, FROH 5–10 Mb, and FROH > 10 Mb, which reflect, respectively,
ancestral inbreeding events that happened 50 generations, 10–50 generations, 5–10 gener-
ations, and 5 generations ago [30]. All 16 horse breeds have historical inbreeding events
dating back to 50 generations. Only three horse breeds (Thoroughbred, Standardbred, and
Franches-Montagnes) had FROH > 10 Mb, indicating that inbreeding events occurred within
five generations. Overall, the ROH-based genomic inbreeding coefficient can be useful for
estimating the inbreeding levels of individual horses lacking pedigree information. It could
also provide useful indicators for monitoring increases in inbreeding, preserving horse
breeds, and minimizing the adverse impacts of inbreeding on horse populations.
4.3. Candidate Genes in ROH Islands in Thoroughbreds Are Associated with Artificial
Selection Traits
ROH can be employed to define genomic regions subject to selection pressure and to
characterize the occurrence of selective sweeps. Using the Thoroughbred population as a
case study, we evaluated the candidate genes within the ROH islands. In contrast to other
domesticated animals, horses are valued for their temperament. Important for the breeding,
selection, and training of horses, temperament is defined as an innate neurological charac-
teristic. Due to the fact that the Thoroughbred horse has traditionally been characterized
as a “hot blood” breed and their temperament has been described as extremely prone to
nervousness [31], several candidate genes discovered by our analysis have been reported to
play crucial roles in neurotransmission. For example, CHRNA6 encodes an α subunit of the
neuronal nicotinic acetylcholine receptor that regulates dopaminergic neurotransmission.
In humans, mutations in this gene most likely result in neuropsychiatric disorders (autism,
depression, bipolar disorder, and schizophrenia), neurodegenerative diseases (Parkinson’s
and Alzheimer’s disease), and lung cancer [32,33]. PRKN encodes Parkin, a component
of the E3 ubiquitin ligase complex, and mutations in this gene have been implicated in
Parkinson’s disease [34] and Autism spectrum disorder [35]. In addition, Prkn-deficient
mice exhibit autistic-like behavior and defective synaptogenesis [36]. The metabotropic
glutamate receptor, which is encoded by the GRM1 gene, is involved in learning, synaptic
activity, and neuroprotection. It is also associated with inherited cerebellar ataxia [37].
Thoroughbreds are considered to have great athletic ability because their maximum
oxygen uptake (VO2max) is nearly double that of elite human athletes [38,39]. Equine
scientists and breeders believe that Thoroughbreds must strengthen their cardiorespiratory
capacity and muscle adaptation to obtain such high athletic ability. Consequently, it
is possible that the cardiovascular and muscular systems of Thoroughbreds have been
subjected to intense artificial selection. Several candidate genes associated with cardiac
development have been identified. For example, HEY2 encodes a member of the basic
Helix-Loop-Helix (bHLH) subfamily. It has been suggested that HEY2 controls heart growth
by limiting cardiomyocyte proliferation [40] and is considered a crucial regulator of human
cardiac development [41]. Triadin, one of the major cardiac sarcoplasmic reticulum proteins
encoded by TRDN, stimulates muscle contraction via calcium-induced calcium release [42].
Humans and mice exhibited aberrant heart rates due to the loss of function of TRDN [43].
In addition, we identified candidate genes associated with muscle development, such as
myoblast fusion (ADAMTS15) [44] and vascular smooth cell differentiation (QKI) [45].
Insulin is secreted by pancreatic β-cells to increase glucose consumption by promoting
glucose uptake, glycogen synthesis, and adipogenesis in muscle and adipose tissue [46].
Insulin is essential for maintaining glucose homeostasis in the body. Studies have demon-
strated that insulin secretion is a complex process in which sodium, potassium, and calcium
channels in the membrane of pancreatic β-cells play crucial roles [47,48]. Thoroughbred
horses are insulin-sensitive [49], and insulin stimulates muscle and protein synthesis [50].
Genes 2023, 14, 1211
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Several candidate genes were significantly associated with insulin secretion regulation
in our study. For instance, KCNMB2 and KCNMB3 are two potassium calcium-activated
channel genes inherited in the linkage region, and CACNA1S encodes the voltage-gated
calcium channel subunit α CaV1.1, which may be jointly involved in regulating insulin
secretion in Thoroughbreds.
Since the vast majority of the sequenced Thoroughbreds we used were males, we also
identified candidate genes involved in spermatogenesis (JAM3, PACRG, and SPATA6L).
The adhesion of germ and Sertoli cells regulates the dynamic process of spermatogenesis.
Junctional adhesion molecule-C (JAM-C, encoded by JAM3) is expressed by germ cells
and localizes to the junctions between germ and Sertoli cells. JAM-C participates in the
formation of acrosomes and germ cell polarity [51]. The development of the flagellum is a
crucial step in spermiogenesis because it enables sperm to reach the egg for fertilization. A
MEIG1/PACRG complex in the manchette transports cargo to the centrioles, which are used
to construct sperm tails [52]. Although SPATA6L (encoding spermatogenesis-associated
6-like protein) is predicted to be located in sperm connecting pieces and to be involved
in spermatogenesis, its molecular function remains unknown. An important paralog of
SPATA6L is SPATA6, which is necessary for the correct assembly of the sperm connecting
component and head-tail junction [53]. In the artificial selection of Thoroughbreds for
breeding, athletic performance and superior pedigree lines take precedence over reproduc-
tive fitness. Therefore, almost no selection pressure was exerted on fertility traits [54,55].
Typically, the conception rate of Thoroughbreds is lower than that of other livestock breeds,
at about 60% per conception cycle [56]. All registered foals in the Thoroughbred horse
industry must be born naturally, and artificial reproduction techniques are prohibited. In
addition, breeding seasons in the Northern and Southern Hemispheres are strictly regulated
by the industry. We hypothesized that the relaxation of reproductive traits could result in
the accumulation of deleterious mutations that could diminish the reproductive ability of
Thoroughbred stallions. These candidate genes associated with spermatogenesis may serve
as targets for the future selection of Thoroughbreds in an effort to improve stallion fertility.
5. Conclusions
The present study examined the distribution of ROH and estimated inbreeding coeffi-
cients based on ROH in 16 diverse horse breeds using whole-genome sequencing data from
97 horses. Our data suggest that ancient and recent inbreeding may affect horse breeds
differently, but recent inbreeding is uncommon, particularly among indigenous horse
breeds. The ROH-based genomic inbreeding coefficient is useful for estimating horse in-
breeding levels in horses without pedigree data and for monitoring inbreeding increments
in the horse population. Moreover, we identified 24 ROH islands containing 72 candidate
genes associated with artificial selection traits in Thoroughbreds. These candidate genes
are associated with neurotransmission, muscle development, positive regulation of heart
rate and contraction, regulation of insulin secretion, and spermatogenesis. These findings
provide insight into the characteristics of horse breeds and future breeding strategies.
Supplementary Materials: The following supporting information can be downloaded at: https://
www.mdpi.com/article/10.3390/genes14061211/s1, Table S1: Mapping results of clean reads against
horse reference genome; Table S2: The statistics of ROH on each chromosome in the Thoroughbred
population; Table S3: The top functional categories enriched for candidate genes located in ROH
islands in Thoroughbreds.
Author Contributions: Conceptualization, S.L. and J.G.; methodology, S.L. and J.G.; software, B.Z.,
S.L. and J.G.; validation, C.C., B.Z., S.L. and J.G.; formal analysis, C.C., B.Z., S.L. and J.G.; investigation,
C.C. and B.Z.; resources, X.T., B.C., M.L. and N.G.; data curation, C.C. and B.Z.; writing—original
draft preparation, C.C. and B.Z.; writing—review and editing, C.C., B.Z., S.L. and J.G.; visualization,
J.G.; supervision, J.G.; project administration, J.G.; funding acquisition, J.G. All authors have read
and agreed to the published version of the manuscript.
Genes 2023, 14, 1211
10 of 12
Funding: This research was funded by a grant from the National Natural Science Foundation of
China (No. 31501000).
Institutional Review Board Statement: This work has been approved by the Biomedical Research
Ethics Committee of Hunan Agricultural University (No. 202046).
Informed Consent Statement: Not applicable.
Data Availability Statement: The whole genome data used in this manuscript are available in the
GenBank database under BioProject accession PRJNA416233, PRJEB10098, PRJEB10854,
PRJNA168142, PRJNA205517, PRJNA230019, PRJNA233529, PRJNA288817 and PRJNA291776.
Conflicts of Interest: The authors declare no conflict of interest.
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10.1038_s41598-021-03242-7.pdf
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OPEN
Pharmacokinetics
and central accumulation
of delta‑9‑tetrahydrocannabinol
(THC) and its bioactive metabolites
are influenced by route
of administration and sex in rats
Samantha L. Baglot1,2*, Catherine Hume1,3, Gavin N. Petrie1,2, Robert J. Aukema1,2,
Savannah H. M. Lightfoot1,2, Laine M. Grace1, Ruokun Zhou4, Linda Parker5,
Jong M. Rho6, Stephanie L. Borgland1,7, Ryan J. McLaughlin8, Laurent Brechenmacher4 &
Matthew N. Hill1,3*
Up to a third of North Americans report using cannabis in the prior month, most commonly through
inhalation. Animal models that reflect human consumption are critical to study the impact of cannabis
on brain and behaviour. Most animal studies to date utilize injection of delta‑9‑tetrahydrocannabinol
(THC; primary psychoactive component of cannabis). THC injections produce markedly different
physiological and behavioural effects than inhalation, likely due to distinctive pharmacokinetics. The
current study directly examined if administration route (injection versus inhalation) alters metabolism
and central accumulation of THC and metabolites over time. Adult male and female Sprague–Dawley
rats received either an intraperitoneal injection or a 15‑min session of inhaled exposure to THC.
Blood and brains were collected at 15, 30, 60, 90 and 240‑min post‑exposure for analysis of THC
and metabolites. Despite achieving comparable peak blood THC concentrations in both groups, our
results indicate higher initial brain THC concentration following inhalation, whereas injection resulted
in dramatically higher 11‑OH‑THC concentration, a potent THC metabolite, in blood and brain that
increased over time. Our results provide evidence of different pharmacokinetic profiles following
inhalation versus injection. Accordingly, administration route should be considered during data
interpretation, and translational animal work should strongly consider using inhalation models.
With recreational use of cannabis recently becoming legal in Canada and some states across the U.S., as well as
medicinal use being legal in many other countries, there is a growing need to better understand the effects of
cannabis on brain and behaviour. Up to a third of North Americans over 16 years of age report using cannabis in
the prior month1, most commonly through pulmonary (i.e. inhalation) administration2. Animal models provide
an extremely valuable approach to studying the effects of cannabis, enabling control over composition, dose,
and timing of exposure. Nevertheless, the majority of animal studies to date examine effects of cannabis through
parenteral (i.e. intraperitoneal [IP]) injections of delta-9-tetrahydrocannabinol (THC; the primary psychoactive
1Hotchkiss Brain Institute | Mathison Centre for Mental Health Research & Education, University of Calgary,
Calgary, AB, Canada. 2Graduate Program in Neurscience, University of Calgary, Calgary, AB, Canada. 3Department
of Cell Biology & Anatomy | Department of Psychiatry, University of Calgary, Calgary, AB, Canada. 4Southern
Alberta Mass Spectrometry (SAMS) Facility, University of Calgary, Calgary, AB, Canada. 5Department of
Psychology and Collaborative Neuroscience Program, University of Guelph, Guelph, Canada. 6Departments
of Neurosciences and Pediatrics, University of California San Diego, and Rady Children’s Hospital San Diego,
San Diego, CA, USA. 7Department of Physiology and Pharmacology, University of Calgary, Calgary, AB,
Canada. 8Department of Integrative Physiology and Neuroscience, Washington State University, Pullman, WA,
USA. *email: [email protected]; [email protected]
Scientific Reports | (2021) 11:23990
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Vol.:(0123456789)www.nature.com/scientificreportscomponent of cannabis) or cannabinoid receptor 1 (CB1R) agonists, which does not reflect a common route of
human cannabis consumption nor reproduce the same physiological or behavioural effects of inhalation.
Inhalation of THC produces a more rapid onset and offset of hypothermia, increases feeding behaviour,
decreases locomotion, and fails to induce cross-sensitization to morphine in rats when compared directly to
injected THC3,4. Further, while THC injections result in conditioned avoidance, inhalation produces a place
preference3 and has reinforcing properties (as demonstrated by robust self-administration) in rats5. The effects
of THC inhalation on temperature, appetite, and locomotion in rodents are also typically shown in humans after
exposure to cannabis smoke6–8. The physiological and behavioural differences between inhalation and injections
are likely due to the distinctive pharmacokinetic differences of each route of administration.
Following IP injection, compounds are absorbed primarily through the portal circulation and pass through
the liver to undergo metabolism before reaching other organs, such as the brain9,10. Alternatively, inhalation
provides rapid delivery of compounds into the blood stream, bypassing initial metabolism by the liver, and
resulting in more immediate uptake by highly perfused tissues, including the brain10–12. The amount and dura-
tion of compound absorption also differs, with injections delivering a single bolus versus inhalation delivering
an ongoing infusion. As such, the pharmacokinetic profile of THC, specifically plasma THC concentrations,
between injections and inhalation differ in timing, magnitude, and duration.
The most common form of cannabis administration in human is inhalation (smoking or ‘vaping’) with smok-
ing historically being predominant but vaporization becoming increasingly popular, especially in youth13–15.
Cannabis vape products are highly variable ranging from dried cannabis (most commonly THC concentrations of
10–25%) to concentrated THC distillate (most commonly THC concentrations of 20–25% but can reach upwards
of 70–90%)14,16. Controlled inhalation (smoking or vaping) of cannabis cigarettes in humans produces peak
plasma THC concentrations 10–15-min after initial administration15,17–22 with relatively rapid clearance of THC
from plasma. In fact, plasma THC concentrations are only 15–20% of peak at 30-min following cannabis use,
8–10% at 60-min, and 2–3% at 180 min20,23. However, because of individual differences in the number, duration,
and spacing of puffs, as well as inhalation volume and hold time, the exact concentration of peak plasma THC in
human studies is extremely variable. Peak plasma concentrations of THC range from 60 to 200 ng/mL following
inhalation of cannabis flower17–21,23,24, making animal models that can control dose and timing extremely valuable.
Rodent studies utilizing THC injections allow for control over both dose and timing; however, peak plasma
THC concentrations are found at a slightly later timepoint following injection than inhalation4,25–27. Further,
clearance of THC from plasma following IP injections is much slower, with concentrations still roughly 65% of
peak at 60-min and 50% at 120 min28. Rodent studies employing IP injections utilize a wide range of dosages
(3–20 mg/kg) producing an extensive span of peak plasma THC concentrations from 40 to 200 ng/mL4,25,26,28,
suggesting that they are comparable to the range seen in humans following cannabis use. Interestingly, brain
THC concentrations following IP administration increase over time, peaking at 60–120 min following initial
administration28.
Animal models utilizing vapor delivery of THC or whole cannabis extract have recently been validated3,4,25
and are able to control for dose and timing of exposure while also employing the most common route of cannabis
consumption in humans. Several rodent studies have found plasma THC concentrations of 100–200 ng/mL fol-
lowing 30-min of exposure to 100–200 mg/mL of THC vapor4,25,26,29. Similar to human inhalation, plasma THC
concentrations peak at around 15-min30 with relatively rapid plasma clearance as suggested by concentrations
of ~ 30% of peak at 60-min and 8–10% at 120 min4,25,26. Finally, opposite to injection, brain THC concentrations
following inhalation peak at 15-min following initial administration and decrease over time30.
In preclinical studies, dosing of THC is typically determined by whether it produces blood THC concentra-
tions in the desired range seen in humans following cannabis consumption. However, whether route of admin-
istration influences how much THC, or its metabolites, accumulates in the brain and activate central CB1R is
not understood. As such, it is not clear if injections of THC that produce similar blood THC levels as those
seen following inhalation are indeed comparable in how much impact they have on activation of the central
cannabinoid system. Consideration of the metabolism of THC is also incredibly important in this context and
is often not measured in most analyses even though the metabolites of THC are highly bioactive, undoubtedly
influenced by route of administration, and known to be significantly impacted by sex25,27,29,31. In the liver, THC is
hydroxylated by cytochrome P450 enzymes into 11-hydroxy-THC (11-OH-THC), which is subsequently oxidized
by the same group of enzymes to create 11-Nor-9-carboxy-THC (THC-COOH) and is excreted in urine11. The
concentrations of THC metabolites are very important factors to consider when examining the impacts of THC
administration, as 11-OH-THC is also psychoactive, is at least equipotent if not more potent than THC, and
diffuses more readily into the brain than THC32–34. Thus, differences in the generation and central accumulation
of 11-OH-THC are not trivial and can have a robust impact on the outcome of studies given its ability to be as,
if not more, efficacious than THC in activating CB1R. THC-COOH is detectable for weeks, lacks any known
psychoactivity, yet may possess anti-inflammatory and analgesic effects32,35.
In our attempts to develop more translationally relevant models of THC and cannabis administration, we
examined if there were pharmacokinetic differences in the metabolism and accumulation of THC between
inhalation and injection administration that could help ascertain if these approaches are interchangeable or if
there are differences that need to be considered when interpreting animal research data through a translational
lens. To this extent, we utilized inhalation and injection paradigms, in both male and female rats, that produced
comparable peak plasma THC concentrations to see if these different routes of administration resulted in dif-
ferential pharmacokinetics or central accumulation of THC and metabolites. To quantify concentrations of THC,
11-OH-THC and THC-COOH, we also developed our own analytical approach using mass spectrometry-liquid
chromatography, which allowed us to quantify these molecules in both plasma and brain tissue.
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Vol:.(1234567890)www.nature.com/scientificreports/Methods
Animals and housing. Adult male (n = 62) and female (n = 66) Sprague–Dawley rats were obtained from
Charles Rivers Laboratories (St. Constant, QC, Canada). Rats were pair-housed in clear polycarbonate cages
with in-cage shelters and aspen-chip bedding, as well as ad libitum access to water and standard laboratory chow,
and were acclimated to a standard colony room (12 h light–dark cycle; constant temperature of 21 ± 1 °C). Fol-
lowing ~ 1 week of acclimation rats were split into two administration groups (injection [parenteral] and inhaled
[pulmonary]) and each group was further sub-divided according to five timepoints (15, 30, 60, 90, and 240-min)
(n = 6 per timepoint per administration group unless otherwise specified). All animal experiments were carried
out in compliance with ARRIVE guidelines, and were performed in accordance with the Canadian Council
on Animal Care (CCAC) guidelines and were approved (protocol #: AC19-0024) by the University of Calgary
Animal Care Committee.
Injections of THC. Dosing for both injection and inhalation studies was based on pilot work establish-
ing doses that produced roughly comparable blood THC levels in the range of 60–100 ng/mL. Age matched
(90–110 days of age) male (438 ± 32 g, n = 26) and female (275 ± 12 g, n = 30) rats received a single injection
of THC intraperitoneally (dose of 2.5 mg/kg in a volume of 2 mL/kg). THC (100 mg/mL in 100% EtOH from
Toronto Research Chemicals) was stored at − 20 °C until dissolved into a 1:1:8 solution of dimethylsulfoxide
(DMSO), Tween 80, and 0.9% saline respectively. DMSO is commonly used in dilution of injectable drugs and
can cause toxicity in concentrations > 10% or at lower concentration with ocular or oral exposure36; importantly,
our study diluted to only 1% DMSO. All injections occurred between 0900 and 1200 h; following injections rats
were euthanized via decapitation at five different timepoints (15, 30, 60, 90 and 240-min, referred to as INJ-15,
INJ-30, INJ-60, INJ-90 and INJ-240 hereafter; n = 6 per group for females and n = 5–6 per group for males);
trunk blood was collected, and brains were extracted for hippocampus dissection. The hippocampus was chosen
as the brain structure for analysis as it is an important site for many of the cognitive and emotional effects of can-
nabinoids, has a high density of cannabinoid receptors and is a brain structure whose isolation and dissection is
consistent and straightforward. Blood samples were collected in EDTA tubes and stored on ice until centrifuged
at 10,000 rpm for 10 min at 4 °C. Plasma was collected and stored at − 80 °C until analysis. Dissected hippocampi
were immediately frozen on dry ice and then stored at − 80 °C until analysis.
Passive inhaled delivery of THC. Male (423 ± 19 g, n = 30) and female (274 ± 16 g, n = 30) rats received a
single (15-min) session of inhaled exposure to a THC-dominant cannabis extract (100 mg/mL; 95% THC from
Aphria Inc., ON, CND) via a validated4,5,25 vapor inhalation system (La Jolla Alcohol Research Inc., CA, USA).
THC-dominant cannabis extract was stored at room temperature until diluted to a concentration of 100 mg/
mL THC in polyethylene glycol (PEG-400). PEG is commonly added to cannabis and nicotine-based vaping
products for human consumption and is generally recognized as safe by the FDA37. PEG-400 can cause toxicity
in rats upon exposure > 8 h38, but importantly our study exposes animals for only 15-min. Both DMSO and PEG-
400 are common solvents used for diluting water-insoluble substances (i.e. cannabinoids), but whether different
vehicles alter the detection of cannabinoids and their metabolites through mass spectrometry remains relatively
unknown. As the goal of our study is to compare the pharmacokinetics and central accumulation of THC and
metabolites across routes of administration the most common vehicle for each route was utilized (DMSO for
injection and PEG-400 for inhalation).
The vapor inhalation system uses machinery similar to electronic cigarettes to deliver distinct “puffs” of
cannabis vapor within airtight chambers. Chamber airflow is controlled by a vacuum compressor (i.e. a “pull”
system) that draws room ambient air through an intake valve at a constant rate of 1 L per minute. At set intervals
(as controlled by MedPC IV software [Med Associates, ST. Albans, VT, USA]) THC-dominant cannabis extract
is vaporized (utilizing a SMOK TFV8 X-baby atomizer [Shenzhen IVPS Technology Co., Shenzhen, China] at
40-watts) and combines with the constant ambient air flow for delivery into the chamber. Air (and vapor) are
evacuated through the back of the chamber via the vacuum compressor, filtered and ventilated out of the building
(Fig. 1 for illustrated depiction of the vapor delivery system). In this study, THC vapor was delivered through
a 10-s “puff ” every 2 min for 15-min. “Puffing” profiles vary greatly in human cannabis consumption with
self-titration to reach desired “high”23, therefore studies examining the effects of inhaled cannabis often control
exposure through both percentage of THC and an allotted inhalation time of ~ 10-min15,20. Our delivery sched-
ule of 15-min was chosen to similarly reflect this previous research, as well as pilot testing showed similar peak
levels to cannabis inhalation in humans17–21,23. In accordance with the injected animals, all inhalation sessions
occurred between 0900 and 1200 h, and following the conclusion of the vapor session rats were euthanized via
decapitation at five different timepoints (15 [immediate], 30, 60, 90 and 240-min, referred to as INH-15, INH-30,
INH-60, INH-90 and INH-240 hereafter; n = 6 per group for females and males). Trunk blood and brains were
collected and stored as previously described.
Body temperature. Body temperature was taken via a rectal thermometer immediately prior to euthanasia
for all rats at the 30, 60, 90, and 240-min timepoints. As peak THC and metabolite levels were imperative to
measure immediately following inhalation, blood and brain collection were prioritized at the 15-min timepoint.
Further, hypothermic onset following THC exposure has been shown to occur at 30-min following inhalation3.
Body temperature following THC administration was compared to control animals. Male (n = 6) and female
(n = 6) control animals were exposed to either vehicle vapor (PEG-400 alone) for 15-min or to vehicle injection
(1:1:8 DMSO, Tween 80, and 0.9% saline) and their body temperature was taken at 30, 60, 90 and 240-min post
administration.
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Vol.:(0123456789)www.nature.com/scientificreports/Figure 1. Vapor delivery system. (A) Illustration of passive vapor delivery system: Schematic of vapor apparatus
components with direction of air-flow (adapted from Freels et al.5). Briefly, the vapor inhalation system uses
machinery similar to electronic cigarettes to deliver distinct “puffs” of vapor within airtight chambers. A vacuum
compressor pulls ambient room air through an intake valve at a constant rate of 1 L per minute. At set intervals
THC-dominant cannabis extract is vaporized combining with the constant ambient air flow for delivery into the
chamber. Air (and vapor) are evacuated through the back of the chamber via the vacuum compressor, filtered
and ventilated out of the building. (B) Image of passive vapor exposure: Picture of a male SD rat within the
vapor apparatus.
Tandem mass spectrometry (LC‑MS/MS). Standard solutions and reagents. Both standard and deu-
terated internal standard (IS) stock solutions were purchased from Cerilliant (Round Rock, TX, USA). The
standard solutions, including Δ9-tetrahydrocannabinol (THC), 11-hydroxy-THC (11-OH-THC) and 11-nor-
9-carboxy-THC (THC-COOH) were dissolved in acetonitrile at a concentration of 1.0 mg/mL. The IS stock
solutions including THC-d3 and THC-COOH-d3 were dissolved in acetonitrile at 0.1 mg/mL. LC/MS grade
acetonitrile, water and formic acid were purchased from Thermo Fisher Scientific (Edmonton, Canada). All
compounds and their serial dilutions were stored at − 80 °C freezer until use.
Calibration curves. The stock solutions of each standard were mixed and diluted in 50% methanol/water to
produce a set of standards ranging from 0.1 to 100 ng/mL (0.1, 0.25, 0.5, 1, 2.5, 5, 10, 25, 50, 100). Internal stand-
ard (IS; d3 analytes) solution contains each compound at 10 ng/mL and was prepared in 50% methanol/water.
Solutions used to establish calibration curves were prepared by mixing 20 µL of each standard solution and 20 µL
of IS solution. The calibrators were analyzed in triplicate and the resulting calibration curves were fit by line of
regression using a weight of 1/x2. R2 of each calibration curve was at least 0.999. Lower limit of quantitation
(LLOQ) of each analyte was determined to be at 0.1 ng/mL.
Sample preparation. Glass tubes containing 2 mL of acetonitrile and 100 µL of IS were prepared to receive
plasma and brain samples. Each plasma sample was thawed at room temperature and 500 µL was directly pipet-
ted into the prepared tubes. Each brain tissue sample was weighed and the frozen piece placed into the prepared
glass tubes for manual homogenization with a glass rod until resembling sand. All samples were then sonicated
in an ice bath for 30-min before being stored overnight at − 20 °C to precipitate proteins. The next day samples
were centrifuged at 1800 rpm at 4 °C for 3–4 min to remove particulates and the supernatant from each sample
was transferred to a new glass tube. Tubes were then placed under nitrogen gas to evaporate. Following evapo-
ration, the tube sidewalls were washed with 250 µL acetonitrile to recollect any adhering lipids and then again
placed under nitrogen gas to evaporate. Following complete evaporation, the samples were re-suspended in
100 µL of 1:1 methanol and deionized water. Resuspended samples went through two rounds of centrifugation
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Vol:.(1234567890)www.nature.com/scientificreports/Compounds/standards
Q1 (Da) Q3 (Da)
Retention time (min) CE (volt)
11-OH-THC-1
11-OH-THC-2
THC-COOH-1
THC-COOH-2
THC-COOH-d3-1
THC-COOH-d3-2
THC-1
THC-2
THC-d3-1
THC-d3-2
331
331
345
345
348
348
315
315
318
318
313
193
327
299
330
302
193
259
196
262
2
2
2
2
2
2
2.7
2.7
2.7
2.7
27
27
24
24
24
24
30
30
30
30
Table 1. Multiple Reaction Monitoring (MRM) Transitions and Collision Energies (CE) of different
compounds/standards.
(15,000 rpm at 4 °C for 20 min) to remove particulates and the supernatant transferred to a glass vial with a glass
insert. Samples were then stored at − 80 °C until analysis by LC-MS/Multiple Reaction Monitoring (MRM).
LC‑MS/MS analysis. LC-MS/MS analysis was performed using an Eksigent Micro LC200 coupled to an AB
Sciex QTRAP 5500 mass spectrometry (AB Sciex, Ontario, Canada) at the Southern Alberta Mass Spectrometry
(SAMS) facility located at the University of Calgary. The LC system consisted of a CTC refrigerated autosampler
(held at 10 °C), a six-port sample injection valve with a 5 µL sample loop as well as a column oven. Chromato-
graphic separation of the analytes was carried out on an Eksigent Halo C18 column (2.7 µm, 0.5 × 50 mm, 90 Å,
AB Sciex) at 40 °C. The mobile phase A was composed of 0.1% formic acid in water and the mobile phase B of
0.1% formic acid in acetonitrile. The analytes (2 µL injection) were eluted at 30 µL/min using a gradient from
25 to 95% B in 2.5 min. The column was then cleaned and regenerated using the following program: 95% B for
2 min, 95 to 25% B in 0.2 min and 25% B for 2.3 min. Before each injection, the column was equilibrated at
initial LC condition for 1 min. Carryover was checked by injection of a blank in between samples. The data were
acquired in positive electrospray ionization (ESI) and MRM mode. MRM transitions and collision energies (CE)
of the different compounds are listed in Table 1. Each compound was acquired with two transitions. The first one
was used to quantify the compound and the second one to discriminate isomers when necessary. Ion spray volt-
age was set at 5500 V. Nebulizer gas (GS 1), auxiliary gas (GS 2), curtain gas (CUR) were set at 30, 30, 35 (arbi-
trary units), respectively. Collision gas was set to Medium. Declustering potential (DP), entrance potential (EP)
and cell exit potential (CXP) were set at 80, 7 and 14 V, respectively. LC-MS/MRM data were processed using
Analyst 1.6 software (AB Sciex). Quantitation of each analyte was calculated using its extracted ion chromato-
gram (XIC; peak area) normalized by the peak area of its corresponding IS. Analyte concentration (in pmol/µL)
were normalized to sample volume/weight and converted to ng/mL or ng/g for statistical analysis and graphing.
Statistical analysis. All data are expressed as mean ± SEM. Data were analyzed using IBM SPSS Statistics
26 and graphed using GraphPad Prism 8. Basal body temperature differed between males and females, so the
two sexes were analyzed separately. Temperature data were analyzed by three-way ANOVA with drug group
(THC and control), administration group (injection and inhalation), and timepoint (30, 60, 90, and 240-min) as
between-subjects’ factors. Analyte data were analyzed by three-way ANOVA with sex (male and female), admin-
istration group (THC-INH or INJ), and timepoint (15, 30, 60, 90 and 240-min) as between-subjects’ factors.
Post-hoc comparisons used Bonferroni post hoc tests and differences were considered significant at p ≤ 0.05.
Results
Body temperature. Body temperature measures were compared to controls and analyzed separately by
sex (female > male, main effect of sex [F(1,136) = 23.219 at p < 0.00001]). THC exposure resulted in hypother-
mia in male rats differentially depending on administration group (interaction effect of group and timepoint:
F(9,52) = 5.831 at p < 0.0001, Fig. 2A). In particular, THC-INH resulted in immediate hypothermia at 30- and
60-min (30-min: THC-INH < CON-INH, CON-INJ, and THC-INJ at p < 0.05; 60-min: THC-INH < CON-INJ
and THC-INJ at p < 0.05), whereas THC-INJ resulted in delayed hypothermia at 90- and 240-min (90-min:
THC-INJ < CON-INJ and THC-INH at p < 0.05; 240-min: THC-INJ < THC-INJ, CON-INH, and THC-INH at
p < 0.01). Along these lines, THC-INH resulted in lower body temperature at 30-min compared to 90- and 240-
min (p < 0.01), as well as remained lower at 60-min compared to 90-min (p < 0.05). THC-INJ resulted in lower
body temperature at 90-min compared to 30- and 60-min (p < 0.05), as well as remained lower at 240-min than
all other timepoints (p < 0.01).
Females had an extremely similar pattern where THC exposure resulted in hypothermia in female rats dif-
ferentially depending on administration group (interaction effect of group and time: F(9,56) = 7.097 at p < 0.00001,
Fig. 2B). In particular, THC-INH resulted in immediate hypothermia at 30- and 60-min (30-min: THC-
INH < CON-INH, CON-INJ, and THC-INJ at p < 0.001; 60-min: THC-INH < CON-INH, CON-INJ and THC-INJ
at p < 0.05), whereas THC-INJ resulted in delayed hypothermia at 90- and 240-min (90-min: THC-INJ < CON-
INJ, CON-INH and THC-INH at p < 0.01; 240-min: THC-INJ < CON-INJ and THC-INH at p < 0.001). Along
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Vol.:(0123456789)www.nature.com/scientificreports/Figure 2. Body temperature. (A) Male temperature: Data are presented as mean ± SEM; n = 5–6 for each group.
Presence of (*) with a solid and dashed green line indicates that THC-INH and THC-INJ respectively differ
from one or more other groups. THC-INH resulted in immediate hypothermia at 30-min (THC-INH < all
groups at *p < 0.05) and at 60-min (THC-INH < CON-INJ and THC-INJ at *p < 0.05), whereas THC-INJ resulted
in delayed hypothermia at 90-min (THC-INJ < CON-INJ and THC-INH at *p < 0.05) and at 240-min (THC-
INJ < all groups at **p < 0.01). (B) Female temperature: data are presented as mean ± SEM; n = 5–6 for each
group. Presence of (*) with a solid and dashed green line indicates that THC-INH and THC-INJ respectively
differ from one or more other groups. THC-INH resulted in immediate hypothermia at 30-min (THC-INH < all
groups at **p < 0.01) and at 60-min (THC-INH < all groups at *p < 0.05), whereas THC-INJ resulted in delayed
hypothermia at 90-min (THC-INJ < all groups at *p < 0.05) and at 240-min (THC-INJ < CON-INJ and THC-
INH at **p < 0.01).
these lines, THC-INH resulted in lower body temperature at 30-min compared to all other timepoints (p < 0.05).
THC-INJ resulted in lower body temperature at 90- and 240-min compared to 30 and 60-min (p < 0.001).
THC. Control values serve as assay controls and were undetectable. Analyte concentrations were compared
across administration groups and sex. THC chromatogram illustrates THC (black line) and THC-d3 (grey line)
with an overlapping peak at 2.7 min (Fig. 3A).
In females, but not males, plasma THC concentrations were higher following INH than INJ regardless of
timepoint (interaction effect of sex and group: F(1,94) = 11.164 at p < 0.01, post hoc female-INH > female-INJ
at p < 0.001, Fig. 3B). Following injection, but not inhalation, males had higher plasma THC concentrations
than females regardless of timepoint (male-INJ > female-INJ at p < 0.001). Further, regardless of group, males
and females differed in plasma THC concentrations across timepoints (interaction effect of sex and timepoint:
F(4,94) = 4.107 at p < 0.01, post hoc male-30 > all timepoints at p < 0.01, male-15 > male-90 at p < 0.05, male
240 < male-15, -30, -60 at p < 0.01, female-15 > all timepoints at p < 0.05, female-30 > female-60, -90, -240 at
p < 0.05 Fig. 3B). Males also had higher plasma THC concentrations than females at 30- and 60-min (post hoc
male-30 > female-30 at p < 0.001 and male-60 > female-60 at p < 0.05). Finally, regardless of sex, plasma THC
concentrations were higher following inhalation than injection at 15-min but not different at any other timepoint
(interaction effect of group and timepoint: F(4,94) = 5.301 at p < 0.01, post hoc INH-15 > INJ-15 at p < 0.0001,
Fig. 3B). Plasma THC concentrations also differed by group across timepoint, such that following inhalation,
plasma THC was higher at 15-min than all other timepoints and higher at 30-min than later timepoints (post
hoc INH-15 > all timepoint at p < 0.01 and INH-30 > INH-60, -90, -240 at p < 0.05, Fig. 3B), whereas following
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Vol:.(1234567890)www.nature.com/scientificreports/Figure 3. THC Chromatogram and levels in blood and brain. (A) LC-MS Chromatogram: THC (black line)
and THC-d3 (grey line) overlapping peaks at 2.7 min. (B) Blood levels: data are presented as mean ± SEM;
n = 5–6 for each group. Presence of (*) indicates an administration difference with INH > INJ at 15-min at
****p < 0.0001. Presence of (#) indicates a timepoint difference with green and purple lines indicating an
INH and INJ difference respectively; specifically, INH-15 > all timepoints and INH-30 > INH-60/90/240 at
#p < 0.05, whereas INJ-30 > all timepoints and INJ-240 < all timepoints at ##p < 0.01. Presence of ($) indicates a
sex difference; specifically, female-INH > female-INJ at $$$p < 0.001 and male-INJ > female-INJ at $$$p < 0.001).
(C) Brain levels: data are presented as mean ± SEM; n = 5–6 for each group. Presence of (*) indicates an
administration difference with INH > INJ at 15, 30, and 60-min at **p < 0.01. Presence of (#) indicates a
timepoint difference with green and purple lines indicating an INH and INJ difference respectively; specifically,
INH-15, 30, and 60 > INH-90 and 240 at ##p < 0.01, whereas INJ-90 > INJ-15 and 240 at ##p < 0.01. Presence of ($)
indicates a sex difference with males > females at $$p < 0.01.
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Vol.:(0123456789)www.nature.com/scientificreports/injection, plasma THC was higher at 30-min compared to all other timepoints and lower at 240-min compared
to all other timepoints (post hoc INJ-30 > all timepoints at p < 0.01 and INJ-240 < all timepoints at p < 0.01).
Brain THC concentrations were higher following INH than INJ at 15-, 30- and 60-min regardless of sex
(interaction effect of group and timepoint: F(4,95) = 7.791 at p < 0.0001; post-hoc INH-15 > INJ-15 at p < 0.00001,
INH-30 > INJ-30 at p < 0.0001, and INH-60 > INJ-60 at p < 0.01, Fig. 3C). Further, brain THC concentrations were
higher at 15-, 30- and 60-min compared to 90- and 240-min following inhalation (post-hoc INH-15 > INH-90
and 240 at p < 0.01; INH-30 > INH-90 and 240 at p < 0.001; INH-60 > INH-90 and 240 at p < 0.01). Alternatively,
brain THC concentrations were higher at 90-min than 15- and 240-min, and trending higher compared to
30-min, following injection (post-hoc INJ-90 > INJ-15 at p < 0.01; INJ-90 > INJ-30 at p = 0.07; INJ-90 > INJ-240 at
p < 0.01). Brain THC concentrations were higher in males than females, regardless of group or timepoint (main
effect of sex: F(1,95) = 9.482 at p < 0.01, Fig. 3C).
11‑OH‑THC. 11-OH-THC chromatogram (Fig. 4A) illustrates 11-OH-THC (black line) and 11-OH-THC-
d3 (grey line) with an overlapping peak at 2.0 min. Plasma 11-OH-THC concentrations were higher following
INJ than INH at all timepoints regardless of sex (interaction effect of group and timepoint: F(4,95) = 4.412 at
p = 0.003; post-hoc INH-15 < INJ-15 at p = 0.03, INH-30 < INJ-30 at p < 0.00001, INH-60 < INJ-60 at p < 0.00001,
INH-90 < INJ-90 at p < 0.00001, INH-240 < INJ-240 at p = 0.037). Further, while plasma 11-OH-THC concen-
trations did not differ across timepoint following inhalation, following injection concentrations were lower at
15-min compared to 30-, 60-, and 90-min (post-hoc INJ-15 < INJ-30 at p = 0.0002, INJ-15 < INJ-60 at p = 0.038,
INJ-15 < INJ-90 at p = 0.032), as well as lower at 240-min compared to 30-, 60- and 90-min (post-hoc INJ-
240 < INJ-30 at p < 0.00001, INJ-240 < INJ-60 at p = 0.0005, INJ-240 < INJ-90 at p = 0.0005). Plasma 11-OH-
THC concentrations were higher in females than males, regardless of group or timepoint (main effect of sex:
F(1,95) = 4.613 at p = 0.034, Fig. 4B).
In both sexes, brain 11-OH-THC concentrations were higher following INJ than INH regardless of time-
point (interaction effect of sex and group: F(1,95) = 5.356 at p = 0.023; post hoc male-INH < INJ at p = 0.0002 and
female-INH < INJ at p < 0.00001, Fig. 4C). Further, following injection, brain 11-OH-THC concentration were
higher in females than males (post hoc male-INJ < female-INJ at p < 0.00001). Regardless of sex, brain 11-OH-
THC concentrations were higher following INJ than INH at 30-, 60- and 90-min (interaction effect of group and
timepoint: F(4,95) = 6.411 at p = 0.0001; post hoc INH-30 < INJ-30 at p = 0.0002, INH-60 < INJ-60 at p < 0.00001,
INH-90 < INJ-90 at p < 0.00001, Fig. 4C). Further, while brain 11-OH-THC concentrations did not differ across
timepoint following inhalation, following injection brain 11-OH-THC concentration were lower at 15-min
compared to 30-, 60-, and 90-min (post-hoc INJ-15 < INJ-30 at p = 0.0009, INJ-15 < INJ-60 at p < 0.00001, INJ-
15 < INJ-90 at p < 0.00001), as well as lower at 30-min compared to 60-min (post hoc INJ-30 < INJ-60 at p = 0.023)
and at 240-min compared to 30-, 60- and 90-min (post-hoc INJ-240 < INJ-30 at p = 0.001, INJ-240 < INJ-60 at
p < 0.00001, INJ-240 < INJ-90 at p < 0.00001).
THC‑COOH. THC-COOH chromatogram (Fig. 5A) illustrates THC-COOH (black line) and THC-COOH-
d3 (grey line) with an overlapping peak at 2.0 min. In both sexes, plasma THC-COOH concentrations were
higher following INJ than INH regardless of timepoint (interaction effect of sex and group: F(1,93) = 12.241
at p = 0.0007; post hoc male-INH < INJ at p = 0.00006 and female-INH < INJ at p < 0.00001, Fig. 5B). Further,
following injection, plasma THC-COOH concentration were higher in females than males (post hoc male-
INJ < female-INJ at p < 0.00001). Regardless of sex, plasma THC-COOH concentrations were higher following
INJ than INH at 30-, 60-, 90- and 240-min (interaction effect of group and timepoint: F(4,93) = 3.0 at p = 0.022;
post hoc INH-30 < INJ-30 at p < 0.00001, INH-60 < INJ-60 at p < 0.00001, INH-90 < INJ-90 at p < 0.00001, INH-
240 < INJ-240 at p = 0.00003, Fig. 5B). Further, while plasma THC-COOH concentrations did not differ across
timepoint following inhalation, following injection plasma THC-COOH concentration were lower at 15-min
compared to all other timepoints (post-hoc INJ-15 < all timepoints at p < 0.003).
Irrespective of group or timepoint, brain THC-COOH concentrations were higher in females than males
(main effect of sex: F(1,95) = 15.442 at p = 0.0002, Fig. 5C). Further, irrespective of sex or timepoint, brain THC-
COOH concentrations were higher following inhalation than injection (main effect of group: F(1,95) = 56.656 at
p < 0.00001, Fig. 5C).
Discussion
Given the increased usage, demand, and potency of cannabis over the last few years1,39,40, establishing an animal
model that closely reflects human consumption is critical to study the impact of cannabis on brain and behav-
iour. Most animal studies to date examine the effects of cannabis through IP injection of THC, which produces
markedly different effects than inhalation3. In efforts to make THC injection studies possess more face validity
for translatability to humans, these previous studies aimed to produce peak plasma THC concentrations that
are comparable to concentrations in human cannabis smokers. Utilizing ‘comparable’ dosages established in
previous literature4,25, as well as through pilot work in our laboratory, we sought to produce similar peak plasma
THC concentrations following injection and inhalation that fell within the range produced in humans from
cannabis17–21,23 in order to determine if route of administration influenced metabolism or central accumulation
of THC. Our data provide clear evidence on the different physiological response and pharmacokinetic profiles of
THC and metabolites following comparable dosing of inhaled versus injected THC, which could have significant
impacts for data interpretation and generalizability. Importantly, we found that inhalation led to immediate
hypothermia and an initial higher plasma and brain THC concentration, while injection led to delayed hypo-
thermia, dramatically higher 11-OH-THC concentrations in both plasma and brain and higher THC-COOH
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Vol:.(1234567890)www.nature.com/scientificreports/Figure 4. 11-OH-THC chromatogram and levels in blood and brain. (A) LC-MS Chromatogram: 11-OH-THC
(black line) and 11-OH-THC-d3 (grey line) peaks at 2.0 min. (B) Blood levels: data are presented as
mean ± SEM; n = 5–6 for each group. Presence of (*) indicates an administration difference with INH < INJ
at all timepoints at *p < 0.05. Presence of (#) indicates a timepoint difference with purple lines indicating an
INJ difference where INJ-15 < INJ-30, 60, and 90 at #p < 0.05 and INJ-240 < INJ-30, 60, and 90 at ###p < 0.001.
Presence of ($) indicates a sex difference; specifically, females > males at $p < 0.05. (C) Brain levels: data are
presented as mean ± SEM; n = 5–6 for each group. Presence of (*) indicates an administration difference with
INH < INJ at 30, 60, and 90-min at ***p < 0.001. Presence of (#) indicates a timepoint difference with purple lines
indicating an INJ difference where INJ-15 and 240 < INJ-30, 60, and 90 at ###p < 0.001. Presence of ($) indicates a
sex difference; specifically, male-INH < male-INJ at $$$p < 0.001, female-INH < female-INJ at $$$$p < 0.0001, and
male-INJ < female-INJ at $$$$p < 0.0001.
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Vol.:(0123456789)www.nature.com/scientificreports/Figure 5. THC-COOH chromatogram and levels in blood and brain. (A) LC-MS Chromatogram: THC-
COOH (black line) and THC-COOH-d3 (grey line) peaks at 2.0 min. (B) Blood levels: Data are presented as
mean ± SEM; n = 5–6 for each group. Presence of (*) indicates an administration difference with INH < INJ at 30,
60, 90 and 240-min at *p < 0.05. Presence of (#) indicates a timepoint difference with purple lines indicating an
INJ difference where INJ-15 < all timepoints at ##p < 0.01. Presence of ($) indicates a sex difference; specifically,
male-INH < male-INJ and female-INH < female-INJ at $$$$p < 0.0001, and male-INJ < female-INJ at $$$$p < 0.0001.
(C) Brain levels: Data are presented as mean ± SEM; n = 5–6 for each group. Presence of (*) indicates an
administration difference with INH > INJ at ****p < 0.0001. Presence of ($) indicates a sex difference with
males < females at $$$p < 0.001.
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Vol:.(1234567890)www.nature.com/scientificreports/concentration in the brain. Males in general had higher THC levels while females had higher metabolite levels,
supporting previous findings of robust sex differences in the pharmacokinetics of THC.
Body temperature.
It is well established that THC exposure induces a hypothermic response in both
humans and animals12,25,29. Hypothermia was found in both males and females following inhalation and injec-
tion of THC. In accordance with previous literature25,29, THC inhalation led to immediate hypothermia with
lower temperatures at 30- and 60-min which normalized by 90- and 240-min, whereas THC injection results
in delayed hypothermia with lower temperatures at 90- and 240-min compared to 30- and 60-min. This hypo-
thermic difference based on route of administration is not surprising given that inhaled THC quickly enters
the bloodstream and is taken up by the brain, whereas injected THC undergoes metabolism by the liver before
reaching systemic circulation10. Along these lines, the onset of the hypothermic response seen in this study
occurs in conjunction with peak brain THC levels following both inhalation and injection, at 30- and 90-min
respectively, indicating that this is a good physiological proxy readout of central accumulation of cannabinoids.
The impact of route of administration on the temporal kinetics of hypothermia was not influenced by sex. Nota-
bly, body temperature was also decreased at 240-min in rats exposed to CON inhalation. The delayed hypother-
mic effect in control animals is likely attributed to removal from their home cage at 30-, 60-, 90, and 240-min to
record body temperature, compared to THC exposed animals in which body temperature was recorded sepa-
rately for each timepoint due to euthanasia for mass spectrometry analysis. Alternately, as stress is known to
produce mild hyperthermic responses41, it is also possible that exposure to the control vapor itself was a mildly
stressful experience that produced a transient hyperthermic response that waned over time.
THC concentrations in plasma and brain. Utilizing ‘comparable’ doses of THC inhalation and injec-
tion, as determined by previous literature and our pilot work4,25, we show altered plasma THC levels across route
of administration and timepoint. Specifically, inhalation led to higher plasma THC concentrations than injection
at 15-min, but the two routes of administration did not differ at any other timepoint. This is not surprising as
inhalation results in much more rapid uptake into the bloodstream than injection. In fact, it is known that inha-
lation produces peak plasma THC levels 10–15-min after initial administration in humans and rodents17,19–21,30,
whereas peak levels are found at a slightly later timepoint following injection in rodents4,25,26,28. Along these lines,
plasma THC concentrations were highest following inhalation at 15- and 30-min compared to 60-, 90-, and 240-
min. Alternatively, plasma THC concentrations were higher following injection at 30-min compared to all other
timepoints. As anticipated, there was no difference between peak plasma THC levels between the two routes of
administration (peak THC following inhalation: 73 ng/mL vs. peak THC following injection: 72 ng/mL), and
importantly these levels fall within the range found in human studies (60–200 ng/mL)17–21,23.
Despite the similar peak plasma THC concentrations, inhalation led to higher brain THC concentrations at
15-, 30-, and 60-min compared to injection. This is not surprising as cannabis inhalation provides rapid delivery
into the blood stream, bypasses initial liver metabolism, and results in more immediate uptake by highly perfused
tissues, such as the brain10–12. Further, in accordance with previous literature showing earlier peak brain THC
concentrations following inhalation than injection30, we found brain THC levels peaked at 15- and 30-min fol-
lowing inhalation, while peak brain THC levels did not occur until 90-min following injection. Interestingly, the
peak subjective “high” in humans following cannabis inhalation is ~ 30-min following onset of administration32,
which corresponds to the higher brain THC concentrations found following inhalation.
11‑OH‑THC levels in plasma and brain. Overall, injection administration yielded significantly higher
plasma and brain concentrations of 11-OH-THC compared to inhalation. Specifically, both plasma and brain
concentrations were relatively low (~ 13 and ~ 22 ng/mL respectively) following inhalation and did not differ
across timepoints. Low concentrations of 11-OH-THC following inhalation are common as concentrations are
recirculated through the enterohepatic pathway (liver to bile to small intestine back to liver) and quickly metab-
olized to THC-COOH32. Alternatively, plasma concentrations of 11-OH-THC were highest at 30-, 60-, and
90-min compared to 15- and 240-min following injection, reaching average peak levels of ~ 88 ng/mL, which is
about 8 times higher than inhalation levels. Further, brain concentrations of 11-OH-THC were also highest at
30-, 60-, and 90-min compared to 15- and 240-min following injection, reaching average peak levels of ~ 98 ng/g,
which is about 4.5 times higher than inhalation levels. This striking difference in the production of 11-OH-THC
is not trivial because 11-OH-THC is an agonist at CB1R, is psychoactive, is believed to pass into the brain more
readily than THC, and is as, or more, potent than THC in its ability to produce behavioural and physiological
effects32–34. Recognizing that it is ultimately the activation of CB1R in the brain, which is readily achieved by
both THC and possibly more so by 11-OH-THC, the overall impact administration of THC will have on central
CB1R activation must take both THC and 11-OH-THC levels into account. Following injection, extremely high
concentrations of 11-OH-THC in the brain will produce different physiological, psychological, and behavioural
effects as compared to inhalation. In fact, given the dramatic accumulation of 11-OH-THC in the brain follow-
ing injection, as well as its significant potency at the CB1R, it seems reasonable to hypothesize that injection
of THC produces a much more robust and sustained activation of brain CB1R than THC administered via
inhalation. As our data indicate that inhalation produces a rapid accumulation of THC in the brain, followed by
relatively rapid clearance, this would suggest that the ability of inhaled THC to activate brain CB1R is likely a
time-limited effect. This is consistent with our hypothermia data and the relatively rapid peak, and diminution,
of psychological effects and intoxication produced by inhaled cannabis or THC. Alternatively, injected THC pro-
duces lower initial brain THC concentrations compared to inhaled with levels accumulating over time to reach
peak THC levels at 90-min that are comparable to inhaled. However, injection administration also includes the
addition of high 11-OH-THC levels, produced through hepatic metabolism, and sequestered into the brain. As
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Vol.:(0123456789)www.nature.com/scientificreports/such, injections of THC will potentially produce profoundly different biological effects since the magnitude of
CB1R activation (through brain levels of both THC and 11-OH-THC) will be much greater than that following
inhaled THC. Given that there is a notable discrepancy between many of the beneficial and adverse impacts of
THC that have been documented in rodent studies using injection-based approaches relative to human studies
examining cannabis users, one must consider that the injection-based approach for THC has limitations for
translational research. As the cellular impacts of CB1R activation will be influenced by the magnitude and dura-
tion of its activation, the impacts of accumulating 11-OH-THC in the brain following injections of THC must
be considered in future rodent studies.
THC‑COOH levels in plasma and brain.
Injection administration yielded higher levels of plasma THC-
COOH but lower levels of brain THC-COOH compared to inhalation. More specifically, plasma concentra-
tions of THC-COOH were relatively low (~ 7 ng/mL) following inhalation and did not differ across timepoints.
Whereas plasma concentrations of THC-COOH were highest at 30-, 60-, 90- and 240-min compared to 15-min,
reaching average peak levels of ~ 60 ng/mL, about 9 times higher than inhalation levels. Along these lines, pre-
vious studies have also shown peak THC-COOH concentrations at later timepoints (60–120 min) following
injection in rats28 and inhalation in humans18. Further, given that plasma 11-OH-THC concentrations were
higher following injection, and 11-OH-THC is the metabolic precursor of THC-COOH, it is not surprising
that THC-COOH follows the same pattern. Low levels of THC-COOH in the brain are anticipated as it is the
primary metabolite for urinary elimination11.
Sex differences. Females exhibited significantly higher levels of 11-OH-THC and THC-COOH in both
brain and blood, indicating that females metabolize THC at a faster rate than males do, which is consistent
with previous work in rats27,31,42 and humans43. Interestingly, many studies in humans report that females are
more sensitive to the effects of THC relative to males, particularly in the context of adverse effects of THC or
subjective ratings of “drug effect”43,44. Further, rodent studies also show sex differences in hypothermia, motor
effects, anti-nociception, anxiety-like behaviour, and feeding behaviour with generally greater effects in females
than males27,29,45. The sex-differences in the subjective experience and behavioural effects of cannabis are likely
attributed to higher levels of 11-OH-THC in females. In line with accelerated metabolism of THC and increased
concentrations of THC metabolites in females in our study, female rats were found to have lower levels of THC
itself relative to males, particularly in the brain. Previous animal studies have shown no differences between
male and female THC blood and brain levels25,27,29,31. This sex difference in the production of 11-OH-THC,
particularly following injections, has relevance for interpreting preclinical studies. Our data suggests that while
females achieve slightly lower brain THC levels than males, they appear to acquire brain 11-OH-THC levels that
are double that of males following injection of THC. Given the bioactivity and potency of 11-OH-THC, this
suggests that injection-based studies of THC may show sex differences in some outcomes, but this effect may be
an artifact of the elevated levels of 11-OH-THC produced by injection; an effect which does not occur following
inhalation where brain 11-OH-THC levels are quite low and comparable between males and females. These sex
differences in THC metabolism may also have implications for human THC consumption, especially when it is
consumed via an oral route as entero-hepatic metabolism will impact THC metabolism.
Limitations and conclusions. Of note, the current studies do not include oral or edible intake of cannabis,
another popular form of consumption. This is an area of future work in our group and has recently been success-
fully executed by others46. Oral consumption would also result in an increase in 11-OH-THC production due to
first pass metabolism; however, given that an intoxicating dose of oral cannabis in humans produces peak blood
levels of THC in the range of 1–5 ng/mL47, which is about 1/20–1/100 of the current level produced by injection
of THC, one can anticipate that the levels of 11-OH-THC produced would be substantially lower than what we
see following injection. Thus, despite the potential similarities in pharmacokinetic trajectories, injections would
not be a suitable comparison for oral routes of administration of cannabis or THC. Along these lines, passive
inhalation approaches, such as those utilized in the current experiments, expose the entire animal to cannabis
vapor resulting in potential exposure through skin and/or through oral exposure by grooming of fur. However,
peak THC levels occur roughly 2–4 h following oral consumption48 and our inhalation exposure groups show
a steady decline in blood and brain THC levels at these timepoints, suggesting any exposure through skin or
grooming is very limited. Finally, our current comparison of injection and inhalation routes utilized different
vehicle solvents. Both solvents were used at levels much lower than concentrations leading to toxicity, however
it remains possible these solvents contributed slightly to the different THC and metabolite levels in blood and
brain. Nevertheless, these solvents are some of the most commonly used diluents for these routes of administra-
tion, which further represents the dissimilarities between injection and inhalation approaches and supports the
importance of modeling human consumption in translational research.
In conclusion, our data demonstrate significant and biologically relevant differences in the pharmacokinetics
and accumulation of THC and metabolites following injection versus inhalation. The current study generally sup-
ports previous findings but provides the first direct comparison of both sex and route of administration of THC to
reveal an accurate picture of how these variables are impacting THC metabolism and central accumulation. These
findings should be considered for translational preclinical studies attempting to model the impacts of cannabis
or THC on the brain and behavioural processes. IP injections are the most frequent route of administration for
animal models examining the effects of cannabis (THC) and many previous studies claim the translatability and
relevance to human consumption by aiming to produce peak plasma THC concentrations that are comparable
to concentrations in human cannabis smokers. Utilizing doses that produced comparable peak plasma THC
concentrations, our study illustrates robust differences in the pharmacokinetics and central accumulation of THC
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Vol:.(1234567890)www.nature.com/scientificreports/and its bioactive metabolites when administered via injection versus inhalation. These differences likely underlie
the inconsistency of reproducible behavioural findings between THC injections and inhalation and support the
importance of appropriately modeling the route of administration in preclinical studies. This is not uncommon
in the drug research field, and in fact studies utilizing injections of ethanol have long been abandoned over the
appropriate use of ethanol vapor or drinking as this produces comparable effects to humans and allows for the
study of volitional administration. Based on the data generated herein, we suggest that researchers conducting
translational work in the realm of THC and cannabis strongly consider utilizing inhalation models, or oral routes
of administration, to ensure that any biological effects they see from THC or cannabis extract administration
are not artifacts produced by the accumulation of 11-OH-THC in the brain and activating CB1R in a temporal
manner that is likely quite distinct from what is occurring with humans during typical cannabis use.
Received: 6 August 2021; Accepted: 29 November 2021
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Acknowledgements
This study was supported by operating funds from the Canadian Institutes of Health Research (CIHR TCP-
431510; MNH) and the Hotchkiss Brain Institute (MNH and SL Borgland). SL Baglot received salary support
from a Vanier Scholarship from CIHR, CH received salary support from Eyes High Postdoctoral Scholarship and
Harley Hotchkiss—Samuel Weiss Postdoctoral Fellowship, GNP received salary support from the Branch Out
Neurological Foundation, RJA received salary support from the Mathison Centre for Mental Health Research &
Education, SHML received salary support from the William H. Davies Medical Research Scholarship. All funding
agencies had no influence on design, execution, or publishing of this work. Thank you to all current and former
members of the Hill laboratory for their assistance and expertise. Thank you to Maury Cole and La Jolla Alcohol
Research Inc. for technical assistance with the vapor chambers and Cece Hillard for input on the manuscript.
Author contributions
The study was conceptualized by S.L.B and M.N.H. with assistance from L.P., S.L.B., and R.J.M.; S.L.B. setup the
vapor chambers, carried out the experimental procedures, and completed data acquisition and analysis with help
from C.H., G.N.P., R.J.A., S.H.M.L., and L.M.G. R.Z. and L.B. carried out mass spectrometry method develop-
ment and sample execution. The original manuscript draft was prepared by S.L.B. and subsequently edited by
R.Z., L.P., J.M.R., S.L.B., R.J.M., L.B., and M.N.H.. All authors read and approved the manuscript.
Competing interests
M.N.H. is a member of the scientific advisory board for Shoppers Drug Mart, Jazz Pharmaceuticals and Lund-
beck; all other authors have no conflicts of interest.
Additional information
Correspondence and requests for materials should be addressed to S.L.B. or M.N.H.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
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© The Author(s) 2021
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Data availability
Raw and processed bulk, scRNA-seq and Visium data from mouse
are available from the Gene Expression Omnibus under super series
accession GSE202159. Human tumor scRNA-seq data are available at
the Human Tumor Atlas Network (HTAN) data coordinating center web
platform (https://humantumoratlas.org/). Source data are provided
with this paper.
|
Data availability Raw and processed bulk, scRNA-seq and Visium data from mouse are available from the Gene Expression Omnibus under super series accession GSE202159 . Human tumor scRNA-seq data are available at the Human Tumor Atlas Network (HTAN) data coordinating center web platform ( https://humantumoratlas.org/ ). Source data are provided with this paper. Code availability No new algorithms were developed for this paper. All analysis code is available at https://github.com/dpeerlab/Treg_ depletion_reproducibility /.
| ERROR: type should be string, got "https://doi.org/10.1038/s41590-023-01504-2\n\nConserved transcriptional connectivity of\nregulatory T cells in the tumor microenvi-\nronment informs new combination cancer\ntherapy strategies\n\nReceived: 25 March 2022\n\nAccepted: 5 April 2023\n\nPublished online: 1 May 2023\n\n Check for updates\n\n 2, Jesse A. Green1, Sham Rampersaud\n\nAriella Glasner1,10, Samuel A. Rose2,10, Roshan Sharma2,10, Herman Gudjonson2,\n 1, Izabella K. Valdez1,\nTinyi Chu\nEmma S. Andretta1, Bahawar S. Dhillon1, Michail Schizas1, Stanislav Dikiy\nAlejandra Mendoza1, Wei Hu\n 1, Ojasvi Chaudhary\nTianhao Xu2, Linas Mazutis3, Gabrielle Rizzuto\nAlvaro Quintanal-Villalonga\nCharles M. Rudin\n\n 4,5,\n 6, Parvathy Manoj6, Elisa de Stanchina7,8,\n & Alexander Y. Rudensky\n\n 1, Zhong-Min Wang\n\n 6, Dana Pe’er\n\n 1,\n 2,\n\n 2,9\n\n 1,9\n\nWhile regulatory T (Treg) cells are traditionally viewed as professional\nsuppressors of antigen presenting cells and effector T cells in both\nautoimmunity and cancer, recent findings of distinct Treg cell functions in\ntissue maintenance suggest that their regulatory purview extends to a wider\nrange of cells and is broader than previously assumed. To elucidate tumoral\nTreg cell ‘connectivity’ to diverse tumor-supporting accessory cell types, we\nexplored immediate early changes in their single-cell transcriptomes upon\npunctual Treg cell depletion in experimental lung cancer and injury-induced\ninflammation. Before any notable T cell activation and inflammation,\nfibroblasts, endothelial and myeloid cells exhibited pronounced changes\nin their gene expression in both cancer and injury settings. Factor analysis\nrevealed shared Treg cell-dependent gene programs, foremost, prominent\nupregulation of VEGF and CCR2 signaling-related genes upon Treg cell\ndeprivation in either setting, as well as in Treg cell-poor versus Treg cell-rich\nhuman lung adenocarcinomas. Accordingly, punctual Treg cell depletion\ncombined with short-term VEGF blockade showed markedly improved\ncontrol of PD-1 blockade-resistant lung adenocarcinoma progression\nin mice compared to the corresponding monotherapies, highlighting a\npromising factor-based querying approach to elucidating new rational\ncombination treatments of solid organ cancers.\n\nDiverse stromal cell types found within the tumor microenvironment\n(TME) can support cancer initiation and progression by acting as acces-\nsory cells, yet their relationships and interdependencies remain poorly\nunderstood. Cells of the innate and adaptive immune system, when\n\nmobilized by immunotherapeutic agents, have been implicated in\nlimiting cancer progression, yet some of the very same cell types can\nsupport tumor growth either directly or indirectly by facilitating\ntumor-promoting functions of other accessory cell types. Treg cells,\n\nA full list of affiliations appears at the end of the paper.\n\n e-mail: [email protected]; [email protected]\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1020\n\nnature immunologyArticle\fa\n\nKrasLSL-G12D/WTTrp53fl/flFoxp3GFP-DTR\n\nAd-Cre\n\nDT/\nPBS\n\nAnalysis\n\n0 wk\n\n18–20 wk\n\nb\n\ns\nl\nl\ne\nc\n+\n4\nD\nC\n\nf\no\ng\ne\nr\nT\n%\n\n40\n\n30\n\n20\n\n10\n\n0\n\n****\n\nCtrl\n\nDT\n\nc\n\ns\nl\nl\ne\nc\n+\n5\n4\nD\nC\n\nf\no\n+\n4\nD\nC\n%\n\n8\n\n6\n\n4\n\n2\n\n0\n\nNS\n\nCtrl\n\nDT\n\ne\nCtrl DAPI Foxp3 (GFP)\n\nDT DAPI Foxp3 (GFP)\n\nNS\n\nNS\n\nNS\n\nd\n\n)\ng\n(\n\nt\nh\ng\ne\nW\n\ni\n\n2.0\n\n1.5\n\n1.0\n\n0.5\n\n0\n\nCtrl\n\nDT\n\nTumor-free PBS\nTumor PBS\nTumor-free DT\nTumor DT\n\n20\n\n15\n\n10\n\n5\n\n0\n\ns\nl\nl\ne\nc\n+\n5\n4\nD\nC\n\nf\no\n+\n8\nD\nC\n%\n\nf\n\n200 µm\n\ng\nLYVE1 Foxp3 (GFP) GP38 DAPI\n\nCD11c Foxp3 (GFP) F4/80 Draq7\n\n200 µm\n\n)\n\n0\n0\n0\n,\n1\n×\n(\n\nr\ne\nb\nm\nu\nn\nG\nE\nD\n\n4\n\n3\n\n2\n\n1\n\n0\n\nh\n\n)\n\nm\nµ\n(\ne\nc\nn\na\nt\ns\ni\nD\n\n200\n\n150\n\n100\n\n50\n\n0\n\nUp\nDown\n\nVECFib\n\nNeu\nMac/DC\nLEC\n\nCD4\n\nCD8\n\n****\n\n****\n\n****\n\n-fib\nTreg\n\n-mac\n-LEC\nTreg\n\n-fib\nTreg\n\n-mac\n-LEC\nTreg\n\nTreg\n\nTreg\n\nTumor-free\nzone\n\nTumor\nnodule\n\nFig. 1 | Early transcriptional responses of principal accessory cell populations\nin the lung adenocarcinoma TME to Treg cell depletion. a, Schematic of the\nexperimental design. b,c, Quantification of Treg (CD4+Foxp3+) one-tailed unpaired\nt-test P = 12.87, d.f. = 7 ****P < 0.0001 and Tcon (TCRβ+CD4+ and TCRβ+CD8+)\ncell populations; left, one-tailed t-test P = 0.3799, d.f. = 7, not significant (NS)\nP = 0.3576; right, one-tailed t-test P= 0.1925, d.f. = 7, NS P = 0.4264, in tumor-\nbearing lungs 48 h after diphtheria toxin (DT) or PBS (Ctrl) administration.\nd, Quantification of lung weight in tumor-free and tumor-bearing mice 48 h after\nDT-induced Treg cell depletion. One-way analysis of variance (ANOVA) followed\nby Sidak’s multiple-comparisons test. Tumor-free PBS versus tumor-free DT,\nP = 0.004037, d.f. = 10 NS P > 0.9999; tumor PBS versus tumor DT, P = 0.7450,\nd.f. = 10, NS P = 0.9787. e, Representative IF staining of Foxp3+ cells in tumor-\nbearing lungs of Ctrl and DT-treated mice. f, Numbers of upregulated (red)\nor downregulated (blue) DEGs (P < 0.05) 48 h after DT or PBS administration\n\nidentified by bulk RNA-seq analysis of the indicated cell subsets. Fib, fibroblasts;\nNeu, neutrophils; Mac, macrophages; CD4 and CD8, effector CD4+ and CD8+\nT cells. g, Representative IF staining of the indicated cell types. h, Quantification\nof distances between Treg cells and the indicated cell types. One-way ANOVA,\nalpha = 0.05, followed by Tukey’s multiple-comparison test Treg-Fib tumor-free\nzone versus tumor nodule, q = 8.041, d.f. = 2544 ****P < 0.0001. Treg-LEC tumor-\nfree zone versus tumor nodule q = 10.08, d.f. = 2544, ****P < 0.0001, Treg-Mac\nversus tumor-free zone versus tumor nodule q = 17.79, d.f. = 2544, ****P < 0.0001.\nAt least 200 cells were counted in each comparison. Three independent sections\nper mouse were analyzed. Three and four mice were used in each group in two\nindependent experiments. Data are presented as the mean ± s.e.m. (b–d)\n(b and c) N = Ctrl-5, DT-4, (d) N = 3 tumor-free PBS, 3 tumor-free DT, 4 tumor PBS,\n4 tumor DT. Data are presented as the mean ± s.e.m.\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1021\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\n\n\fexpressing the transcription factor Foxp3, are highly enriched in\nhuman solid organ cancers and their experimental animal models, and\nat sites of inflammation and injury, where they exert both their essen-\ntial immunosuppressive function and distinct tissue repair-promoting\nmodalities1–4. Depletion of Treg cells results in restraint of tumor growth\nin numerous experimental cancer models5–9. Nevertheless, some\ntumors eventually progress after an initial response to Treg depletion5.\nThe latter can be due to waning functionality of effector T cells due to\nnegative regulation by co-receptors, foremost PD-1, expected to occur\nprimarily in PD-1 blockade-responsive tumors expressing PD-L1. An\nalternative, yet not mutually exclusive, explanation, is that Treg cell\ndepletion induces compensatory modulation of key accessory cell\ntypes in the TME, which may affect predominantly PD-1 nonresponsive\ncancers. Thus, early changes in diverse cellular components of the\nTME upon short-term Treg cell depletion may directly and indirectly\nimpact its overall effect on tumor growth. Thus, we sought to eluci-\ndate the interplay between Treg cells and other cellular components\nof the TME by investigating early changes in their features upon Treg\ndepletion in experimental cancer settings. Specifically, we wished to\nuse a genetically engineered mouse model that is characterized by\nnatural evolution of the TME, pronounced Treg cell presence, resist-\nance to PD-1 blockade and close resemblance to human disease.\nTherefore, we used KrasLSL-G12D/WTTrp53fl/fl mice harboring a Foxp3GFP-DTR\nallele (KP-DTR), in which intratracheal infection with a Cre-expressing\nreplication-deficient adenovirus induces lung adenocarcinoma (LuAd)\nformation. These mice offer a well-established model of non-small cell\nlung cancer (NSCLC) in humans, a disease where only some respond\nto PD-1/PD-L1 blockade-based therapies7,10–12. Our studies revealed\nthat Treg cells profoundly affect the transcriptional programs of key\naccessory cells including endothelial cells, fibroblasts, monocytes\nand macrophages in the TME. Moreover, these Treg cell dependencies\nof the transcriptional states of accessory cells are largely conserved\nin human lung cancer.\n\nResults\nEarly responses of tumor microenvironment cells to Treg cell\ndepletion\nTo enable temporally controlled Treg cell depletion in KP adenocarci-\nnomas, we generated KrasLSL-G12D/WTTrp53fl/flFoxp3GFP-DTR mice, in which\nall Treg cells express the diphtheria toxin receptor (DTR)13. We reasoned\nthat since Treg cells are typically found in the tumor margins, early com-\npensatory responses of key accessory cell types—tumor-associated\nfibroblasts, vascular endothelial cells (VECs) and lymphatic endothe-\nlial cells (LECs), and macrophages (Mac)—to Treg cell depletion likely\nprecede effects on the tumor growth. Because the expansion of acti-\nvated self-reactive T cells, observed 72–96 h after DT-mediated Treg\ncell depletion in Foxp3GFP-DTR mice, induces pronounced inflamma-\ntory responses13, we sought to minimize these confounding factors\nby analyzing early transcriptional responses of KP tumor cells, lung\n\nepithelial cells (ECs), VECs, LECs, macrophages and T cells 48 h follow-\ning DT administration to tumor-bearing KP-DTR mice (Fig. 1a,b and\nExtended Data Fig. 1a,e). As expected, pronounced local and systemic\nT cell activation and inflammation, typically elicited by an extended\nTreg cell depletion regimen, were not observed (Fig. 1c and Supplemen-\ntary Fig. 1c), and tumor volume was unaffected at this early time point\n(Fig. 1d) even though neutrophils were moderately increased (Extended\nData Fig. 1c,k). Highly efficient tumoral Treg cell depletion in situ was\nconfirmed by immunofluorescence (IF) microscopy of DT-treated as\ncompared to control (Ctrl) mice, in which Treg cells were found mainly at\nthe boundaries of tumor foci (Fig. 1e). Bulk RNA-sequencing (RNA-seq)\nanalyses of cell subsets purified by fluorescence-activated cell sort-\ning (FACS) from the lungs of DT-treated KP adenocarcinoma-bearing\nKP-DTR mice showed pronounced changes in gene expression in LECs,\nmacrophages and fibroblasts, while T cells, which are considered the\nmain targets of Treg cell suppression, changed the least (Fig. 1f and\nSupplementary Table 1). Among accessory cells, the most pronounced\ntranscriptional responses were observed in fibroblasts, endothelial\ncells and CD11c+ myeloid cells, highlighting Treg cell ‘connectivity’ to\nthese cell types in tumor-bearing lungs (Extended Data Fig. 1f–h and\nSupplementary Table 1). Importantly, DT-induced Treg cell ablation in\ntumor-free control KP-DTR mice resulted in minor, if any, changes in\ngene expression in all lung cell populations analyzed with the excep-\ntion of VECs (Extended Data Fig. 1j,k). This was consistent with the\npredominantly intravascular localization of Treg cells in the lung of\nunchallenged mice in contrast to their heavy presence in the cancerous\nlung parenchyma (Extended Data Fig. 1f,g)14. These results suggest that\nthe observed transcriptional changes in accessory cells in cancerous\nlungs are not due to a systemic response to Treg cell depletion. Next, we\ninvestigated whether shared groups of genes underwent modulation in\ndifferent accessory cell types and observed correlated gene expression\nchanges in endothelial cells and fibroblasts (Extended Data Fig. 1f, g).\nThese included programs related to endothelial-to-mesenchymal\ntransition (EndMT)-related genes (Id2, Itgav and Cxcl12), which were\npreviously shown to be modulated by Treg cells in the hair follicles15, and\ninflammation-related genes (Il6, Ccl5, Acacb, Ccl22, Arg1 and Tnfrsf18),\nwhose expression is affected by Treg cells in adipose tissue in the con-\ntext of metabolic inflammation and muscle injury16,17 (Extended Data\nFig. 1g). Cell-type-specific gene expression changes confirmed the\nshared gene expression changes were not due to sample impurities\n(Extended Data Fig. 1h). Considering the early transcriptional response\nof several TME cell types to Treg depletion, we assessed whether Treg\ncells were found in the proximity of these ‘first responders’ using IF\nanalysis of tumor-bearing lungs. Indeed, GFP-DTR+ Treg cells were found\nin markedly closer proximity to Lyve-1+ LECs, GP38+ fibroblasts and\nF4/80+ macrophages within and near tumor nodules than in areas\nfurther away from tumor nodules in the same tumor-bearing lung\n(Fig. 1g,h). Collectively, we have shown that Treg cells are highly con-\nnected in the KP TME.\n\nFig. 2 | Single-cell transcriptomic analysis of ‘Treg cell dependencies’\nof accessory cell states in mouse lung adenocarcinoma tumor\nmicroenvironment. a,b, t-distributed stochastic neighbor embedding (t-SNE)\nplots (27,000 cells) representing cell populations from major cell lineages\nisolated from 48 h DT-treated or PBS-treated (Ctrl) tumor-bearing lungs\n(three mice per group) colored by cell type (a) and condition (b). c, A density\nplot showing the distribution of cells between experimental conditions.\nd,e, t-SNE plots (2,815 cells) representing distribution of the VEC populations\ncolored by subtype (d) and condition (e). f, A density plot of endothelial cells\nshowing the distribution of cells between experimental conditions. g, Graph of\nneighborhoods of endothelial cells computed using MiloR and t-SNE embedding.\nEach dot represents a neighborhood and is color coded by the false discovery rate\n(FDR)-corrected P value (alpha = 1) quantifying the significance of enrichment\nof DT cells compared to control in each neighborhood. The size of the dot\nrepresents the number of cells in the neighborhood. h, Swarm plot depicting the\n\nlog fold change of differential cell-type abundance in DT-treated versus\ncontrol samples in each neighborhood across different endothelial cell types.\nEach dot represents a neighborhood and is color coded by the FDR-corrected\nP value (alpha = 1) quantifying the significance of enrichment of DT cells\ncompared to control in each neighborhood. A neighborhood is classified as a cell\ntype if it comprises at least 80% of cells in the neighborhood, otherwise it is called\n‘mixed’. i, Heat map showing average factor cell score in each cluster for each\nexperimental condition in the VEC population. The scores were row normalized\nbetween 0 and 1. Each row represents a factor, and each column represents a\ncluster in a specific experimental condition. The clusters are grouped based on\ntheir phenotype. j, Gene expression heat maps showing the top 200 genes that\ncorrelated the most with the imputed activated VEC factor indicated (Methods).\nEach column represents a cell; cells are ordered based on their factor score (in\nascending order from left to right) indicated by the green bar. Select genes of\ninterest are noted on the right. b, e, h and i; Ctrl, PBS, gray; DT, red.\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1022\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fa\n\nd\n\ng\n\ni\n\nj\n\n4\n\n0\n\n–4\n\nb\n\ne\n\nCtrl\nDT\n\nEpithelial\nLymphatic\nendothelial\nVascular\nendothelial\nMyeloid\nNeutrophil\nFibroblast\nT/NK cell\nB cell\n\nArtery vein\naCap\ngCap\nInflammatory\ncapillary\nLymphatic\nendothelial\n\n50\n100\n150\n\n200\n\nFDR\n\n0.75\n\n0.50\n\n0.25\n\nArtery vein\naCap\ngCap\nInflammatory capillary\nLymphatic endothelial\n\n1.0\n\n0.5\n\n0\n\nc\n\n2\nE\nN\nS\n-\nt\n\nf\n\n2\nE\nN\nS\n-\nt\n\nCtrl\nDT\n\nh\n\ngCap\n\nMixed\n\naCap\n\nLymphatic\nendothelial\n\nArtery vein\n\nInflammatory\ncapillary\n\nCtrl\n\nDT\n\nt-SNE1\n\nCtrl\n\nDT\n\nt-SNE1\n\nHigh\n\nLow\n\nHigh\n\nLow\n\n3.0\n\n2.5\n\n2.0\n\n1.5\n\n1.0\n\n0.5\n\n–\nl\no\ng\n1\n0\n(\nF\nD\nR\n)\n\nCtrl\n\nDT\n\nlogFC\n\n–4.0\n\n–2.0\n\n0\n\n2.0\n\n4.0\n\n6.0\n\n8 - angiogenesis\n15 - inflammation hypoxia\n17 - inflammation EndMT\n3 - activated VEC\n19 – IFN response\n\nActivated VEC (3)\n\nAngiogenesis (8)\n\nInflammation, hypoxia (15)\n\nInflammation, EndMT (17)\n\nCell scores\n\nCtrl\n\nDT\n\n4\n\n0\n\n–4\n\nBcl3\nNoct\nRelb\nTnf\nCcrl2\nCc40\nIrf5\nCsf1\nNfκb2\nIcosl\nEgr2\nDll1\nPim1\nIrf1\nIcam1\nFgf2\nTank\nIl6\nTgif1\nNinj1\nTnip1\n\n4\n\n0\n\n–4\n\nLpl\nCd36\nMiga2\nTap1\nWars1\nCd74\nLy6e\nGbp6\nIdo1\nCiita\nOas2\nVegfa\nThbd\nSlco2a1\nJup\nIcam2\nLima1\nCldn5\nPard6g\nCd47\nFmo1\nAlas1\nBmpr2\nSptbn1\nSmad6\nSema3c\n\n4\n\n0\n\n–4\n\nKlf6\nNfil3\nBhlhe40\nMaff\nSerpine1\nPlaur\nTnfaip3\nIcam1\nNfkbia\nJunb\nHbegf\nRel\nRelb\nFosl2\nHmox1\nTimp3\nIrf8\nBatf3\nNfkbiz\nPvr\nCcr7\nStat3\n\nEmp3\nSerpina3\nPsmg4\nCd63\nIl1r1\nLgmn\nCsrp2\nLcn2\nCfb\nLgals4\nNpm3\nTraf4\nKpnb1\nTimp1\nGda\nCh25\nTgm2\nPrkca\nCsrp2\nNgf\nAmmecr1\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1023\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2Nhood size\fSingle-cell analysis of tumoral Treg cell ‘connectivity’\nTo explore the impact of Treg cells on the diverse cell states in the TME,\nwe performed single-cell RNA sequencing (scRNA-seq) of sorted CD45−\nand CD45+ cell populations using the 10X platform (Extended Data\nFig. 2a). These populations were isolated from tumor-bearing lungs of\nKP-DTR mice treated for 48 h with DT or vehicle control 3 months after\nadenoviral Cre-driven tumor initiation (Fig. 1a). After pre-processing,\nwe clustered cells using PhenoGraph18 and annotated clusters using\nexpression of known markers into major cell types (Extended Data\nFig. 2b–d). To ensure our inferences were robust, we focused on the\nmajor hematopoietic and non-hematopoietic cell types in the TME\nthat had substantial numbers of cells. The final processed datasets\nincluded LECs, VECs, LECs, fibroblasts, lymphoid cells and myeloid\ncells (macrophages, monocytes, dendritic cells (DCs) and neutrophils;\nFig. 2a). Similarly to population-level assessments, scRNA-seq showed\nthat short-term Treg cell depletion had profound effects on transcrip-\ntional features of fibroblasts, myeloid and endothelial cells compared\nto lymphocytes (Extended Data Fig. 2e and Fig. 2a–c). To gain deeper\ninsight into the phenotypic response of accessory cells whose transcrip-\ntomes were most affected by Treg removal—endothelial cells, fibroblasts\nand myeloid cells, we separately clustered and embedded each subtype\nto ascribe finer-grain identities (Fig. 2d and Extended Data Fig. 3; for\nannotation strategy see Methods). Furthermore, we used Milo19 to\nquantify changes in abundance of subpopulations and cell states after\nTreg cell depletion (Methods). We found several cell states affected by\nTreg cell depletion, with the most pronounced phenotypic shifts in capil-\nlary VECs, mesenchymal stem cells (MSCs), Col14a1 matrix fibroblasts,\nmonocytes and macrophages (Fig. 2d–h and Extended Data Fig. 4a–d).\nTherefore, Treg cell depletion markedly affected the distribution and\nabundancies of several cell states and subsets in the TME.\n\nShared and distinct Treg cell-dependent gene programs\nWe then sought to characterize genes that respond to Treg cell depletion\nin these key accessory cell subsets. We used factor analysis to char-\nacterize gene expression programs—sets of genes whose expression\nchanges in a coordinated way in a specific set of cells and assessed\ntheir differential usage in cell populations from control or DT mice to\nelucidate the response to Treg cell depletion. Specifically, factor analysis\nmethods are well suited to decompose data into factors, which rep-\nresent coordinated expression programs across cells and reduce the\nimpact of noise on analysis, which can be dominant at an individual\ngene level20. We used single-cell hierarchical Poisson factorization\n(scHPF), designed specifically for scRNA-seq21,22 and applied it to each\ncell lineage separately to dissect the observed gene expression changes.\nEach cell and gene present in the expression matrix was assigned a\nscore for each factor, enabling biological interpretation of that factor\n(see Supplementary Table 2 for factor gene and cell matrices). Factors\nwere robust to random initializations of the model and robust to slight\nchanges in parameters (Methods and Supplementary Fig. 1).\n\nWe reasoned that gene programs most affected by Treg cell pres-\nence would have differential factor cell scores between the control\nand DT conditions. To evaluate this systematically, we computed the\naverage cell score of every factor in each cluster for each condition\n(Fig. 2i) and identified those that have higher averages in DT compared\nto control. In the endothelial lineage, we identified four major gene\nprograms that were robust to random initializations (Supplementary\nFig. 1), were biologically relevant and had significantly differential cell\nscores (Mann–Whitney test; Methods) following Treg cell depletion\ncompared to control in at least one of the endothelial cell subtypes\n(Fig. 2i). We then visualized expression of the genes with the highest\nfactor loadings in the relevant cell subtype (Fig. 2j). We observed several\nnotable patterns, including the inflammatory or activated capillary\nVEC factor (factor 3), a highly Treg cell-dependent factor character-\nized by cytokine/chemokine-, Notch and nuclear factor-κB (NF-κB)\nsignaling-, and co-stimulation pathway-related gene expression\n(Fig. 2j; see Supplementary Table 3 for endothelial factors of inter-\nest). Other highlighted factors enriched following Treg cell depletion\nin the endothelial cell population included genes related to the NF-κB\nsignaling pathway (Nfkbia, Rel, Hbegf), inflammation/hypoxia (Klf6,\nSerpine1, Plaur; factor 15) and vascularization (Vegfa, Thbd and Slco2a1;\nfactor 8), and genes linked to transforming growth factor-beta-induced\nEndMT (Emp3, Timp1 and Tgm2; factor 17). Besides cancer, the latter\nprocess is induced in aberrant tissue remodeling and fibrosis23,24. These\nobservations indicate that Treg cells impact specific features of certain\nendothelial cell subsets in the TME.\n\nNotably, the observed transcriptomic perturbations were not\nunique to endothelial cells. The Treg cell depletion-induced gene pro-\ngrams related to interferon (IFN) response, inflammatory cytokines\n(ICs) and chemokines, STAT3 and interleukin (IL)-6 signaling appeared\nto be shared across accessory cell populations. The three most differen-\ntially expressed gene (DEG) programs observed in fibroblasts following\nTreg cell depletion included an inflammatory secretory phenotype (Ccl2,\nHif1a, Rel, Cxcl1; factor 22), IFN response (Irf7, Ifit3, Isg15; factor 9) and\nECM-related genes (Fbn1, Fn1, Lamc2, Notch2; factor 14; Extended Data\nFig. 5a,b and Supplementary Table 4). On the other hand, several fac-\ntors in monocytes (factors 2, 5, 7, 13, 17, 21 and 22) and macrophages\n(factors 15, 17 and 23) including IFN and hypoxia response emerged as\ndifferentially abundant (Extended Data Fig. 5c,d and Supplementary\nTable 5; for all significant factors across cell subsets, see Supplementary\nTable 6). These results suggested that Treg cell communication with vari-\nous cells in the TME imparted both shared and distinct transcriptional\nfeatures across and within specific cell populations in either a direct\nor indirect manner.\n\nTreg cell dependency of accessory cell states in lung injury\nTo test whether the Treg cell ‘connectivity’ to key accessory cell types\nobserved in lung cancer represents a generalizable facet of tissue organ-\nization, we examined perturbations of their transcriptional states upon\n\nFig. 3 | Shared early transcriptional responses induced upon Treg cell\ndepletion in mouse lung adenocarcinoma TME and bleomycin-induced\nlung inflammation. a, t-SNE plots (24,592 cells) representing cell populations\nisolated from the lungs of mice administered with diphtheria toxin (DT) or PBS\n(Ctrl) for 48 h. Lung injury-induced inflammation was induced in both groups\nof mice upon bleomycin treatment 21 d before DT/PBS administration. The\ndata represent analysis of three mice per group colored by cell type (left) and\ncondition (middle), and a density of the distribution of cells between conditions\n(right). b, t-SNE embedding of endothelial cells isolated from Ctrl and DT\nafter bleomycin administration color coded by cell type (left) or experimental\ncondition (middle), and density plots of the distribution of endothelial cells\nbetween conditions (right). c, Heat map showing average factor cell score in\neach cell type for each experimental condition for endothelial cell subsets. The\nscores were row normalized between 0 and 1. Each row represents a factor, and\neach column represents an endothelial cell subset in a specific experimental\ncondition. Factors of interest are highlighted by a red box. d, Heat map showing\n\nthe 72 shared genes specific to activated VEC factor in both lung challenge\nmodels (Methods and Supplementary Table 9). Each column represents a\ncell; cells are ordered based on their factor score (in ascending order from\nleft to right), indicated by the green bar. e, Heat map showing factor cell score\nacross experimental conditions averaged over each myeloid cluster in each\nexperimental condition for bleomycin-administered cells. The rows are factors\nand columns are clusters for each experimental condition. The clusters are\ngrouped based on the cell type they are associated with. The heat map shows\nrow-normalized scores from 0 to 1. The left color bar shows the average factor\ncell score. f, Heat maps showing the 54 shared genes between mouse lung tumor\nand injury-induced inflammation in the Arg1+ macrophage factor (tumor factor\n23 corresponding to injury-induced inflammation factor 0; Supplementary\nTable 10). Each column is a cell; cells are ordered based on their factor score\n(in ascending order from left to right) indicated by the green bar. The treatment\ncondition for each cell is indicated by gray for PBS and red for DT bars. Select\ngenes of interest are shown.\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1024\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fa\n\nb\n\nc\n\n1.0\n\n0.5\n\n0\n\ne\n\n1.0\n\n0.5\n\n0\n\nEpithelial\nEndothelial\nMyeloid\nNeutrophil\nFibroblast\nT/NK cell\nB cell\n\nCtrl\nDT\n\naCap\ngCap\nInflammatory capillary\nLymphatic endothelial\n\nCtrl\nDT\n\n2\nE\nN\nS\n-\nt\n\n2\nE\nN\nS\n-\nt\n\nCtrl\n\nDT\n\nHigh\n\nt-SNE1\n\nLow\n\nCtrl\n\nDT\n\nHigh\n\nt-SNE1\n\nLow\n\nCtrl\n\nDT\n\n15 inflammation,\nEndMT, angiogenesis\n\n12 -activated VEC\n\n13 -inflammation,\nhypoxia\n\naCap\ngCap\nInflammatory capillary\nLymphatic endothelial\n\nArg1 mac\nAlveolar mac\nC1qa mac\nCcr7 cDC2\n\nCsf3r mono\nMono\nRetnla mac\n\ncDc1\n\ncDc2\npDc\n\nCtrl\n\nDT\n\n0\n\n15\n\nd\n\n4\n\n0\n\n–4\n\nf\n\n4\n\n0\n\n–4\n\nKP activated VEC 3\n\nInjury activated VEC 15\n\nCell scores\nCtrl DT\n\nKP (23)\n\nInjury (0)\n\nCell scores\nDT\nCtrl\n\nMaff\nBhlhe40\nTnfaip3\nFosl2\nCsf1\nIcoslg\nBcl3\nBirc3\nEgr2\nRelb\nPim1\nNoct\nWsb1\nSoca2\nCasp4\nIrf5\nNrid2\nIcam4\nLat2\nSh2b2\nCtps1\nDll1\n\nVegfa\nSpp1\nVcan\nPlaur\nTgm2\nInhba\nCol4a1\nOlr1\nNdrg1\nSlc2a1\nErrfi1\nSod2\nSphk1\nGsr\nArg1\nUpp1\nAlas1\nSmox\nArg2\nPrdx6\nUck2\nBlcap\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1025\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fa\n\n1 mm\n\nb\n\no\nd\nn\nE\n\nb\nF\n\ni\n\nl\ny\nM\n\nFactor\n\n(17) inflammation, EndMT\n(15) inflammation, hypoxia\n(8) angiogenesis\n(19) IFN\n(3) activated VEC\n(22) inflammatory cytokine\n(21) inflammatory cytokine\n(14) ECM\n(9) IFN\n(23) Arg1+ macrophage\n(2) C1Q+ macrophage\n(15) proliferation\n(21) monocyte, hypoxia\n(17) IFN\n(13) Csf3r monocyte\n(5) monocyte, coagulation\n\nTumor\nRNA %\n0.4\n\n0\n\nTumor\nspot\n\n+\n–\n\np\na\nC\na\n\np\na\nC\ng\n\ne\nt\ny\nc\ni\nr\ne\nP\n\ne\ng\na\nh\np\no\nr\nc\na\nM\n\nt\ns\na\nl\nb\no\nr\nb\nfi\n+\n\n1\na\n4\n1\nl\no\nC\n\nt\ns\na\nl\nb\no\nr\nb\nfi\no\ny\nM\n\nt\ns\na\nl\nb\no\nr\nb\nfi\n+\n\n1\na\n3\n1\nl\no\nC\n\ne\nt\ny\nc\no\nn\no\nM\n\ne\ng\na\nh\np\no\nr\nc\na\nm\n\nr\na\nl\no\ne\nv\nl\nA\n\nl\na\ni\nl\ne\nh\nt\no\nd\nn\ne\nc\ni\nt\na\nh\np\nm\ny\nL\n\nc\n\nInflammatory cytokine\n\nd\n\nIFN response\n\n1 mm\n\n0\n\n0.6\n\n0\n\n0.4\n\ne\n\nTreg depleted - Ctrl\nmean factor score\n\n0.04\n\n0\n\n–0.04\n\n–log10(P.adj)\n\n0\n50\n100\n150\n200\n\nSignaling\nniche\n\nIFN\n\nIC\n\nIFN + IC\n\nother\n\nCtrl\n\nTreg depleted\n\nCtrl\n\nTreg depleted\n\nTreg depleted\n\nA\n\nIA\n\nIA\n\nIA\n\nLV\n\nA\n\nLV\n\nIA\n\nV\n\nV\n\nV\n\nBr\n\nA\n\nIC\n\nIFN\n\nf\n\ne\np\ny\nt\n\nl\nl\ne\nC\n\nTumor\nT cell/ILC2\nNK\nNeutrophil\nMSC\nMonocyte\ncDC\nBasophil/mast\nAT2\nAlveolar macrophage\n\n0\n\n1\n\n2\n\n0\n\n0.2\n\n0.4\n\n0.6\n\nCell-type RNA fraction log2 fold change\n\nAdjusted empirical P\n\n0.1\n\n0\n\nFig. 4 | Spatial transcriptomics identifies distinct inflammatory cytokine and\nIFN signaling niches in lung adenocarcinoma following Treg cell depletion.\na, Tumor region identification in KP LuAd sections using Visium ST. The\nfraction of tumor cell RNA in each Visium spot (top right) was determined by\nBayesPrism deconvolution, binarized (bottom right; Methods), and compared\nto histological H&E images (left). b, Factor scores and Bonferroni-adjusted\ntwo-sided t-test P values differentially expressed factors between control and Treg\ncell-depleted conditions in ST. c,d, Representative tissue sections from control\n(left) or Treg cell-depleted (right) conditions. Tumor regions are outlined, and\nspots are colored by factor score. Scores represent IC (c; 18 genes) or IFN (d;\n\n103 genes) gene programs shared across all lineages (Br, bronchi; A/V, artery/\nvein; LV, lymphatic vessel; IAs, immune cell aggregates). e, ST analysis revealed\ndistinct signaling niches. Spots were assigned to niches based on thresholding\na gamma distribution fitted to IC or IFN signaling module scores across all spots\n(Methods). f, Enrichment of cell-type RNA fractions in signaling niches. Adjusted\nempirical P value corresponds to the probability of obtaining the mean observed\nRNA fraction for that cell type (Methods). Fractions with adjusted P > 0.01 are\nnot shown. In a and c–e, images are representative of, and analysis performed\non (b and f), one of two serial sections for each of four samples (DT and Ctrl, two\nbiological replicates each).\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1026\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\n\n\fLung progenitor-like\n\nEMT\n\nHigh plasticity\n\nTumor\n\nSpot\n\n+\n\n–\n\nTumor state deconvolution\n\nTumor subtype RNA %\n\n200 µm\n\nTumor state\n\nGastric\n\nEndoderm.like\n\nAT2.like\n\nAT1.like\n\nEMT\n\nLung.progenitor.like\n\nHigh.plasticity\n\nc\n\ns\nn\no\ni\ns\ne\nl\n\nf\no\nr\ne\nb\nm\nu\nN\n\n10\n\n5\n\n0\n\n1 mm\n\nF3\n\nAnxa1\n\nPtgs2\n\ne\n\nEMT\nHigh plasticity\n\nLung pro\n\nCondition\n\nImmune response\n\nCtrl\nTreg\ndepletion\n\n10\n\n5\n\n0\n\n–\n\n+\n\nGastric\nHigh plasticity\n\nGastric\nHigh plasticity\n\nPf4\n\nGkn2\n\n90\n\nDlk1\n\nCtsh\n\nId2\n\nApoc1\n\nCol18a1\n\nScd2\n\nMgst1\n\nItgav\nPhlda1\n\nPlin2\n\nEgr1\nFosl1\nPtprn\n\nIl6 Sprr1a\nCxcl3\n\nCxcl1\n\nLgals3\nKrt7\n\nErrfi1\n\nThbs1\n\nJunb\nKrt18\n\nPmvk\n\nIfrd1\n\nNedd4\n\nCxcl16\n\nIl4ra\n\nS100a11\n\nIer5\nDusp1\n\nCxcl2\n\nEreg\n\nMeg3\n\nSox9\n\nLgi3\n\nBpifa1\n\nBex2\n\nCxcl10\n\nSignificant\nFalse\nTrue\n\nImmune\nresponse\n\n–\n+\n\na\n\nb\n\nd\n\nj\n\nP\nd\ne\nt\ns\nu\nd\na\n0\n1\ng\no\nl\n–\n\n60\n\n30\n\n0\n\n−3\n\n−2\n\n−1\n\n0\n\n1\n\n2\n\nlog2 fold change\nTreg depletion response – nonresponse\n\nSox9\n\n0\n\n2\n\nPf4\n\n0\n\n4\n\nFig. 5 | High-plasticity state and heterogeneity revealed by lung\nadenocarcinoma responses to Treg cell depletion. a, ST analysis of tumor states.\nBayesPrism deconvolution using additional labeled tumor cells from Yang et al.28\nwas performed to assign tumor-state-specific RNA fractions. Correspondence\nof regions with highlighted differential tumor states (middle) to H&E section\nis shown (right). Dashed lines denote regions with the indicated dominant\ntumor states (red, high plasticity; yellow, EMT; black, lung progenitor-like).\nb, Spots labeled by tumor-state cluster. In a and b, images are representative\nof, and analysis performed on (c and d), one of two serial sections for each of\n\nfour samples (DT and Ctrl, two biological replicates each). c, Quantification of\ntumor lesion area types across Treg cell depletion and control conditions (left)\nor between tumors with or without detectable immune response in Treg cell-\ndepleted condition (right; N = 85 lesion areas). d, Differential gene expression\n(two-sided Wilcoxon test Benjamini–Hochberg adjusted) of tumor spots\nin lesions with and without immune response to Treg cell depletion. e, Log-\nnormalized expression of Sox9 and Pf4 (Cxcl4) in a representative tumor-bearing\nlung section after Treg cell depletion. Inset at top left indicates immune response\nstatus of tumor lesion areas.\n\nidentical short-term Treg cell depletion in a setting of bleomycin-induced\nfibrotic lung inflammation using scRNA-seq analysis (Fig. 3a,b and\nExtended Data Fig. 6a–d). Not only were all cell populations detected\nin tumor-bearing lungs also present in inflamed lungs, Treg deple-\ntion in this setting also generated similar transcriptional responses\n(Fig. 3a,b and Extended Data Fig. 6c,d). Independent analysis of the\ngene programs in the inflamed lung using scHPF (see Supplementary\nTable 7 for factor matrices) identified Treg cell depletion-associated\nendothelial factors (Fig. 3c and Supplementary Table 8). We correlated\ngene scores associated with each factor from lung tumors to lung injury\nto identify similarities. We found that the activated VEC factor in the\nlung injury (factor 15) correlated strongly (Pearson correlation > 0.70)\n\nwith its counterpart in the tumor setting (factor 3), indicating that\nthe same set of genes responded to the loss of Treg cells in both chal-\nlenges. In fact, 72 of the top 200 genes associated with factor 3 spe-\ncific to the tumor endothelial cell inflammatory capillary subset were\nshared with the top 200 genes associated with factor 15 specific to the\nsame subset of cells in the injury model (Fig. 3d and Supplementary\nTable 9). Other endothelial cell factors, namely inflammation/hypoxia\n(factor 13), NF-κB signaling and EndMT (factor 12), that were observed\nin the inflamed lung upon short-term Treg cell depletion also correlated\npositively, even if weakly, with related tumor factors 15 and 8, respec-\ntively (Extended Data Fig. 6e). Consistently, factor analyses of other\nlineages revealed overlapping differential gene programs between\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1027\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\n\n\fa\n\nb\n\nGastric\n\nIC score\n\n200 µm\n\n0\n\n0.6\n\n0\n\n0.6\n\nHigh plasticity\n\nSox9\n\n0\n\n0.5\n\n0\n\n2\n\nFig. 6 | Local histological and immune response heterogeneity following Treg\ncell depletion. a, H&E staining of representative tumor section characterized by\nhistological and immune response state heterogeneity after Treg cell depletion.\nInsets at bottom represent a zoomed-in view of gastric (left) and high-plasticity\n(right) areas. Black arrows highlight neutrophil infiltration in a high-plasticity\narea. b, Tumor RNA fraction within highlighted high-plasticity and gastric\nepithelial states (left) and gene expression modules (right) of tumor lesion\nshown in a. Images are representative of one of two serial sections for each of four\nsamples (DT and Ctrl, two biological replicates each).\n\ntumor and injury models, including Treg cell depletion-induced gene\nprograms in Arg1+ macrophages (Fig. 3e,f and Supplementary Table 10)\nand IC signatures in Col14a1 matrix fibroblasts. These findings sug-\ngested that Treg cell-dependent transcriptional programs are not limited\nto the TME and can be shared across pathological conditions.\n\nSpatial distribution of Treg cell-dependent tumor\nmicroenvironment gene programs\nTo gain insights into the spatial organization of the identified accessory\ncell populations, gene programs and their relationship to transcrip-\ntional states of tumor cells, we profiled four tissue sections (two control,\ntwo Treg cell depleted) using the 10X Visium platform. We used Bayes-\nPrism25,26, a Bayesian framework that jointly estimates cell-type frac-\ntions and cell-type-specific gene expression using a labeled scRNA-seq\nreference, to deconvolve each spatial transcriptomics (ST) spot into\nconstituent cell populations. Deconvolution was performed using our\nscRNA-seq datasets labeled with 26 distinct cell populations selected\n\nto optimize granularity, robustness and concordance with underly-\ning histological features in paired H&E-stained sections (Methods,\nFig. 4a, Extended Data Fig. 7a–e and Supplementary Table 11). Next, we\nassessed whether the gene factors that changed upon Treg cell deple-\ntion in scRNA-seq were also identified by ST analysis. Consistently,\nwe observed upregulation of endothelial and fibroblast IC and IFN\nsignaling-related gene signatures after Treg cell depletion within spots\nassigned to the corresponding cell type (Fig. 4b). We also observed\nincreased use of genes associated with the activated VEC factor in capil-\nlary aerocyte (aCap) endothelium assigned spots, as well as increased\nIFN and proliferation related gene signatures in myeloid spots. IC and\nIFN factors shared many genes across all three analyzed accessory\nlineages (18 for IC, 103 for IFN), which suggested that similar gene pro-\ngrams were induced across colocalized cell types by common stimuli,\nindicative of a signaling niche. The spatial behavior of shared genes\nin these two programs showed localization to two distinct signaling\nniches in the tissue, with the IC gene program (Cxcl2, Ier3, Fosl1, Il6)\nlocalized to the tumor core and the IFN response gene program (Ifit1,\nStat1, Isg15, Irf7) localized to the periphery of, or distal to tumor lesions.\nInspection of the same H&E-stained sections confirmed dense tumor\ncell presence with potential hypoxia and neutrophil infiltration at IC\nfoci, and immune cell aggregates at sites with strong IFN response\nsignal (Fig. 4c–e and Supplementary Table 12). Further, ST analysis\nrevealed concordant differential distribution of tumor cells and acces-\nsory cell types within these territories with higher frequency of tumor\ncells, basophils/mast cells, neutrophils and MSCs in IC territories and\na high frequency of T cells/type 2 innate lymphoid cells (ILC2s), natu-\nral killer (NK) cells, conventional dendritic cells (cDCs), monocytes\nand alveolar macrophages in IFN territories (Fig. 4f). Taken together,\nthese results point to two primary inflammatory and spatially distinct\nmodes of lung TME response to Treg cell depletion within tumor mass\nand tumor margin.\n\nTumor states associated with response to Treg cell depletion\nKP LuAds adopt a range of recurrent transcriptional states with features\nof differentiated lung ECs, their progenitors or epithelial progenitors\nfrom other tissues including the gastrointestinal tract and liver and\nEMT (epithelial to mesenchymal transition)-associated ones27–30. We\nnext sought to identify potential associations between tumor states\nand the identified TME niches, that is, IC-positive, IFN-positive and cold\n(negative) ones. We first identified tumor cells within our ECs by call-\ning KRAS p.Gly12Asp mutations. Because optimized dissociative TME\nsingle-cell analysis protocols are suboptimal for capturing tumor cells,\nwe identified only 239 tumor cells within our mouse scRNA-seq dataset.\nTo enable robust deconvolution of tumor cell states, we substituted\ntumor cells from our scRNA-seq dataset with those from a published\ndataset that had better capture of KP LuAd cells (KP-tracer dataset;\nN = 18,083)28. With this updated reference, we performed an additional\nspot deconvolution to more accurately capture tumor states in the tis-\nsue. TME fractions for other cell types remained relatively unchanged\nbetween deconvolutions (Extended Data Fig. 7c).\n\nIn spots with tumors, the tumor-state fractions exhibited regional\nvariation in gene expression programs, sometimes within seemingly\nthe same tumor nodule (Fig. 5a). Tumor spots were clustered by their\ntumor-state fractions and typically showed a dominant tumor state\nin each spot (Extended Data Fig. 8a) manifested in the expression of\ncorresponding marker genes (Extended Data Fig. 8b), forming continu-\nous spatial patches of similar phenotypes (Fig. 5b and Extended Data\nFig. 8c). Spots were grouped into tumor lesion areas, or contiguous\npatches of tumor cells belonging to the same cluster and quanti-\nfied across control and Treg cell-depleted conditions. Tumor states\nwere also compared across tumors that had a detectable immune\nresponse (>10% of spots in IC or IFN signaling niches) or not in Treg\ncell-depleted sections. Treg cell depletion resulted in a pronounced\nincrease in tumor lesion areas that corresponded to a high-plasticity\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1028\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fcell state, specifically among tumor nodules associated with an immune\nresponse (Fig. 5c and Extended Data Fig. 8d,e)29. This was consistent\nwith a significant enrichment of high-plasticity cell-state genes upregu-\nlated by tumor cells after Treg cell depletion in scRNA-seq (Extended\nData Fig. 8f,g and Supplementary Tables 13 and 14). Therefore, TME\nresponse to the removal of Treg cells may elicit a gene program in\nLuAds that represents an unstable transitional state, which can give\nrise to other tumor states28,29. While IC and IFN niches were observed\nin the majority of tumor nodules after Treg cell depletion, there were\nsome nodules, and even areas within individual nodules, that did not\n(Fig. 4c). In particular, those with a gastric epithelial lineage gene\nexpression program were selectively devoid of IC or IFN responses\n(Fig. 5c and Extended Data Fig. 8e). We assessed differential gene\nexpression between immune response ‘rich’ and ‘poor’ lesion areas\nand found increased expression of Gkn2 (gastrokine), Pf4 (platelet fac-\ntor 4/Cxcl4) and Sox9 among other genes (Fig. 5d,e and Supplementary\nTable 15). Interestingly, Sox9 expression in lung tumor cells was shown\nto enable their escape from NK cell-mediated killing in certain cases27,\nsuggesting one potential mechanism of immune evasion. Similarly to\na recent analysis of CRISPR-edited tumors31, the observed response\nto Treg cell depletion was spatially restricted, as even ‘nonresponsive’\nareas that were directly adjacent to responsive ones were deprived of\nimmune cell or IC signals (Fig.6a,b). Therefore, regional variation in\ntumor state appears to define the TME response to Treg cell depletion.\n\nConserved Treg cell-dependent features of human and mouse\ntumor microenvironment\nNext, we sought to test whether Treg cell-dependent TME features\nobserved in mice are conserved in human cancer (Fig. 7a) by leveraging\nvariation in Treg cell abundance in 25 primary or local metastatic LuAd\nsamples from 23 individuals, analyzed using scRNA-seq (Supplemen-\ntary Tables 16 and 17). Despite differences in composition and propor-\ntion of accessory cell types in these datasets, we were able to detect all\ncell populations corresponding to those observed in mice (Fig. 7b and\nExtended Data Fig. 9a,b). To determine whether the factors induced\nafter Treg cell depletion in mice are present in human LuAd samples\nwith a low abundance of Treg cells, we first determined the frequency\nof Treg cells among all hematopoietic cells in each sample (Fig. 7c and\nExtended Data Figs. 9c and 10a,b). Next, we performed scHPF analy-\nsis for each of the cell lineages under investigation (Supplementary\nTable 18) and looked for orthologous genes shared between human\nand mouse factors to align gene programs between species (Fig. 7d).\nThen, we assessed the correlation of mean factor usage in single cells to\nTreg cell frequency across human samples. This identified three factors\nnegatively correlated with Treg cell proportion that corresponded to\naspects of the compensatory endothelial response to Treg cell depletion\nin the KP mouse model (Extended Data Fig. 10c). The latter included\nfactors whose most associated genes were related to activated aCap\n(CAR4, CD36, IFNGR1, FAS, CX3CL1, TNFRSF11b, EDN1; factors 3 and\n5; Fig. 7e), inflammation and hypoxia (VEGFB, PLAUR, SERPINE1, IL6,\nCXCL1, BCL3, PVR, IRF4, BATF3, TFP12; factors 4 and 5) and angiogenesis\n\nfactors (factor 3). We used the sum of these three factors as a general\nTreg cell-responsive endothelial gene program to account for potential\nsample-specific, cell-type-specific or condition-specific effects that\nwould separate a shared underlying biological program into separate\nfactors during factorization (Extended Data Fig. 10d). Comparing this\nscore to Treg cell proportion, we observed a clear negative correlation—\nstronger than any factor individually—across tumor samples (Fig. 7f),\nwhich suggested conserved Treg cell influence on this gene expression\nprogram. To further identify specific components of this shared Treg\ncell-responsive endothelial gene expression program, we compared the\nloadings of genes in the factors related to inflammation and hypoxia\nacross species (factors 4 + 5 in human LuAd, factor 15 in KP mouse;\nFig. 7g). This identified genes encoding key inflammatory mediators\n(IL6, CSF3, VCAM1, SELE, PTGS2) and a host of VEGF-induced genes in\nendothelial cells (RND1, ADAMTS1, ADAMTS4, ADAMTS9, AKAP12) as\nconserved members of expression programs induced in endothelial\ncells in Treg cell-poor TMEs across species.\n\nSimilar analyses of fibroblasts and myeloid cells also revealed\ncorresponding Treg cell-dependent mouse and human factors. For\nexample, human fibroblast factors 3, 5 and 22 corresponded to IC\nmouse fibroblast factors 21 and 22, with overlapping genes including\nIL6, IL1RL1, NFKB1, CCL2 and LIF (Extended Data Fig. 10e,f), while factor\n9 (AP1 TF family members, KLF2/4, SOX9, HES1, IRF1) was negatively\nassociated with Treg cell proportion. Additionally, high usage of con-\nserved CSF3R monocyte factor 16 (CSF3R, PROK2, VCAN) in human ‘Treg\ncell-poor’ LuAd samples was consistent with the hypoxia, angiogenesis\nand NF-κB signaling related features (VEGFA, HIF1A, CEACAM1, NOTCH1,\nBCL3, BCL6) of this population in Treg cell-depleted mice (Extended\nData Fig. 10g,h and Extended Data Fig. 5d). Notably, several human\nmyeloid factors and corresponding mouse factors showed positive\ncorrelation with Treg cell presence, such as an SPP1/FOLR2 macrophage\nfactor, a cell cycle factor and a C1Q+ macrophage factor (C1Q, antigen\npresentation-related genes), which included genes encoding known\nnegative regulators of innate and adaptive immunity (CFH, CR1L, LAG3,\nPDCD1LG2, LILRB4, IL18BP; Extended Data Fig. 10i). Interestingly, we\nobserved similarly pronounced downregulation of this gene program\nupon Treg cell depletion in both lung tumors and bleomycin-induced\ninjury, suggesting that Treg cells within both niches sustain certain\nimmunomodulatory myeloid cell states. Further analysis of correla-\ntion between conserved Treg cell-dependent human and mouse factors\nrevealed a set of opposing TME programs (Extended Data Fig. 10j). One\nfactor group in Treg cell-poor or Treg cell-deprived tumors included IL-1β/\nIL-18 signaling-related genes (IL18RAP, IL1RAP) expressed in angiogenic\nmonocytes and tumor necrosis factor (TNF)/IL-1β-induced genes in\nfibroblast and endothelial cells involved in monocyte and neutrophil\nrecruitment (CSF3, CXCL1, CXCL2, CXCL8, CCL2). The other, positively\nassociated with Treg cell presence, featured immunomodulatory genes\nthat inhibit IL-1β/IL-18 signaling (TMEM176B, IL18BP; see Supplemen-\ntary Table 19 for KP/injury/LuAd factors). These findings suggest a\nconserved role of Treg cells in tuning transcriptional states of principal\naccessory cell types in the TME.\n\nFig. 7 | Factor analysis of Treg cell ‘dependencies’ of accessory cell\ntranscriptional states in human and mouse lung adenocarcinomas.\na, Schematic of the experimental design. b, t-SNE plot of all cells (82,991 total\ncells) from 25 primary human LuAd or local metastases labeled by lineage.\nc, t-SNE of T/NK cell lineage colored by unique molecular identifier (UMI) counts\nof Treg cell marker genes (maximum of two). d, Jaccard similarity between genes\nassociated with mouse and human factors in tumor endothelial cells. Factors of\ninterest with high correlation are highlighted by a green box. e, Conservation\nof activated VEC signature genes. Normalized gene loading (fraction of gene\nscore across all factors) of genes within the mouse activated VEC signature\nacross all human endothelial factors. Upper and lower notches of the box plot\ncorrespond to the 75th and 25th quartiles, respectively, and the middle notch\ncorresponds to the median. Whiskers extend to the farthest data point no more\n\nthan 1.5 times the interquartile range from the hinge, with outliers beyond that\ndisplayed as individual points. Select genes with high loadings of factors 3 and 5\nare highlighted (N = 45 genes). f, Mean log2 sum of inflammation/angiogenesis\nassociated human endothelial factor (3, 4 and 5) cell loadings plotted against\nlog2 Treg cell proportion in each human sample. Spearman correlation estimate\n(R) and P value are listed. Trend line represents a linear model fit between\nthe two and shading indicating the 95% confidence interval (N = 19 human\nsamples). g, Normalized gene scores (fraction of gene scores across all factors)\nin orthologous genes between mouse and human inflammation/hypoxia factors.\nGenes significantly attributed to both human factors and mouse factors are\nhighlighted as conserved. VEGF-induced genes in endothelial cells were derived\nfrom the CytoSig database.\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1029\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fa\n\nb\n\ne\n\ns\ne\nn\ne\ng\nC\nE\nV\ng\nn\nd\na\no\n\ni\n\nl\ne\nn\ne\ng\nd\ne\nz\ni\nl\na\nm\nr\no\nN\n\n0\n\n1.00\n\n0.75\n\n0.50\n\n0.25\n\n0\n\ng\n\n5\n+\n4\nn\na\nm\nu\nh\ng\nn\nd\na\no\n\ni\n\nl\ne\nn\ne\ng\nd\ne\nz\ni\nl\na\nm\nr\no\nN\n\nTreg cell depletion\nin mice\n\nCompute associations\nbetween altered genes\n\nIdentify compensatory\nprograms\n\nLevel of Treg cell\npresence in humans\n\nCandidates for\ncombination therapy\n\nMouse KP factors\n\nc\n\nCD3E\n\nCD4\n\nB cell\nBlood endothelial\nEpithelial\nFibroblast\nLymphatic endothelial\nMyeloid\nNeutrophil\nT/NK\n\nFOXP3\n\nIL2RA\n\nd\n\ns\nr\no\nt\nc\na\nf\nd\nA\nu\nL\nn\na\nm\nu\nH\n\n2\n\n1\n\n0\n\n0.15\n\n0.1\n\n0.05\n\n0\n\n16\n7\n5\n10\n19\n21\n22\n3\n20\n14\n18\n13\n9\n6\n15\n17\n8\n1\n2\n4\n12\n111\n\nICAM4\n\nBIRC3\n\nTNFAIP3\n\nTIFA\n\n0.6\n\n0.4\n\nRAB20\nBCOR\n\nELMSAN1\n\n0.2\n\nCDC42EP4\n\nNOCT\n\nBCL3\nPIM1\n\nRELB\n\nSLC25A25\nSHB\nTGIF1\nFOXP4\n\nCSF1\n\n10\n\n1\n\n4\n\n15\n\n0\n\n13 7 12 8 6\n\n2\n\n5 18\n\n14\n\n113\n\n9\n\n19 17 16\n\nf\n\ne\ng\na\ns\nu\n5\n\n,\n\n4\n\n,\n\n3\nr\no\nt\nc\na\nf\n\n2\ng\no\n\nl\n\n2\n\n0\n\n–2\n\n–4\n\n3 = inflammatory capillary\n4, 5 = inflammation\n\nR = –0.41, P = 0.082\n\nCells\n100\n200\n300\n400\n\n1\n\n2\n\n3\n\n4\n\n5\n\n6\n\n7\n\n8\n\n9\n\n10\n\n11\n\n12\n\n13\n\n14\n\n15\n\n16\n\n17\n\n18\n\n19\n\n20 21\n\n22\n\nFactor\n\n–6\n\n–5\n\n–4\n\n-3\n\nlog2 Treg/CD45+\n\nIL6\n\nSELE\n\nTNFAIP3\n\nBIRC3\n\nCSF3\n\nVCAM1\n\nRCAN1\n\nMB21D2\nIER3\n\nKDM6B\n\nRND1\n\nARID5A\n\nADAMTS1\nMAP3K8\n\nSDC4\n\nADAMTS4\n\nADAMTS9\n\nPTGS2\n\nSOX7\n\nTM4SF1\n\nTRIB1\n\nNFKBID\n\nNFKBIA\n\nTGIF2\n\nREL\n\nAKAP12\n\nTNFSF9\n\nSLC25A25\nMTF1\n\nPLAUR\n\nPVR\n\nSAT1\nZBTB10\n\nNOCT\n\nFOSL1\n\nEMP1\n\nARL5B\n\nBMP2\n\nPMAIP1\n\nSLC20A1\n\nPTPRE\n\nPFKFB3\n\nINHBA\n\nConserved\n\nConserved &\nVEGF induced\n\n0\n\n0.25\n\n0.50\n\n0.75\n\nNormalized gene loading mouse 15\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1030\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\n\n\fCombinatorial Treg cell depletion therapy\nThese results highlighted candidate compensatory pathways, whose\ntargeting in combination with current clinical-stage intratumoral Treg\ncell depletion strategies32,33 could improve therapeutic efficacy. In this\nregard, the increased expression of VEGF pathway-related genes upon\nTreg cell deprivation was of particular interest suggesting that height-\nened VEGF signaling may ‘buffer’ the negative impact of Treg cell deple-\ntion on the tumor-supporting TME function and facilitate a rebound in\nthe tumor progression. We tested the above possibility by investigating\nwhether combining short-term Treg cell depletion with VEGF blockade\ncan lead to an improved control of KP tumor progression. We trans-\nplanted KP adenocarcinomas into Foxp3GFP-DTR mice and administered\nthem with DT and mouse VEGF-A neutralizing antibody (aVEGF) after\ntumors became macroscopically detectable (Fig. 8a). While Treg cell\ndepletion and VEGF blockade alone could slow tumor progression,\ntheir combination had a markedly more pronounced therapeutic\neffect (Fig. 8b). Assessment of the overall survival rate, when mice\nwere left untreated after the initial response and killed after rebound\n(tumor volume reached 1 cm3, maximum allowed by the institutional\nguidelines) showed that combination therapy improved survival in\ncomparison to either monotherapy or untreated groups (Fig. 8b). While\nsimilarly increased numbers and activation level of tumoral T cells were\nobserved in ‘DT + aVEGF’ and ‘DT-only’ in comparison to ‘aVEGF-only’\ntumor samples on day 20 of transplantation (Fig. 8c), IFN-γ-producing\nCD4+ T cells and IFN-γ-producing and TNFα-producing CD8+ T cells\nwere markedly increased in the combination treatment group as were\nmonocyte numbers (Fig. 8d). Moreover, we observed further increases\nin tumor hypoxia and apoptosis upon combined Treg cell depletion and\nVEGF blockade in comparison to both monotherapeutic modalities\nand untreated control groups (Fig. 8e,f). Notably, KP tumors failed to\nrespond to PD-1 blockade, which did not offer additional therapeutic\n\nbenefits when combined with VEGF blockade in full agreement with a\nrecent study of antiangiogenic, anti-PD-1 and chemotherapy in a KP lung\ncancer model34. Recent studies revealed high amounts of chemokine\nreceptor CCR8 displayed by Treg cells in human cancers35,36 highlighting\ntheir depletion as a therapeutic strategy33,37,38. Thus, we examined the\ntherapeutic potential of short-term VEGF blockade combined with\nantibody-mediated depletion of CCR8-expressing Treg cells, which\nrepresented only a fraction of intratumoral Treg cells in KP tumors\n(Fig. 8g). While CCR8 antibody treatment alone diminished tumor\ngrowth, a markedly more pronounced effect was observed when it\nwas combined with VEGF blockade (Fig. 8h). Notably, this regimen\nwas associated with a mere 15% decrease in overall population of\ntumor-associated Treg cells in the absence of their noticeable changes\nin the secondary lymphoid organs (Fig. 8i). Besides VEGF-A, whose\nneutralization was conducted as a proof-of-concept approach for the\ndiscovery of orthogonal combination therapy, we noted additional\ncandidate compensatory pathways enriched in the Treg cell-poor or\ncell-depleted TME including the CCR2–CCL2 axis, inhibitors of which\nare currently tested as monotherapies or combination therapies of\nhuman cancers. To further test the utility of assessment of early TME\nresponses to Treg cell depletion for identifying combinatory therapeu-\ntic modalities, we subjected KP tumor transplanted mice to a similar\nshort-term treatment with CCR8 antibody and a selective CCR2 antago-\nnist RS-504393 (CCR2i). The latter combination provided minimal\nadditional therapeutic benefit in comparison to anti-CCR8 and CCR2i\nmonotherapies contrary to aVEGF/CCR8 combination (Fig. 8j,k). These\nresults suggest that CCR2 blockade and Treg cell depletion may converge\non shared or partially overlapping TME states, whereas VEGF blockade\noffers an orthogonal intervention and highlights potential for discovery\nof orthogonal cancer therapies through single-cell and spatial analyses\nof early TME responses to acute perturbation.\n\nFig. 8 | Systemic or intratumoral CCR8+ Treg cell depletion combined with\nVEGF blockade restrains KP adenocarcinoma progression. a, Schematic of\nthe experimental design; s.c., subcutaneous. b, Tumor growth dynamics upon\nthe indicated therapeutic interventions. The data represent mean values of\ntumor volume measurements (left). Adjusted P values for day 20 measurements:\nPBS-IgG versus DT-IgG P < 0.0001; PBS-IgG versus PBS-αVEGF P = 0.0004;\nPBS-IgG versus DT-αVEGF P < 0.0001; DT-IgG versus PBS-αVEGF P = 0.0328;\nDT-IgG versus DT-αVEGF P = 0.0109; PBS-αVEGF versus DT-αVEGF; P = 0.0005.\nRepresentative image of tumor volumes at day 20 (center). Kaplan–Meyer\nsurvival curves followed by log rank (Mantel–Cox) of KP tumor-bearing mice\n(right). The ‘survival’ time reflects the end point of the experiment when tumor\nvolume in individual mice reached 1 cm3; adjusted P values: PBS-IgG versus\nDT-IgG P = 0.0012; PBS-IgG versus PBS-αVEGF P > 0.05 (NS), PBS-IgG versus\nDT-αVEGF P = 0.0078; DT-IgG versus PBS-αVEGF P > 0.05 (NS); DT-IgG versus\nDT-αVEGF P = 0.05; PBS-αVEGF versus DT-αVEGF P = 0.0186. c,d, Quantification\nof the indicated immune cell subsets and frequencies of activated (CD44hi\nCD62lo), proliferating (Ki67+) and IFN-γ-producing TCRβ+ CD4+ and TCRβ+ CD8+\ncells in tumor samples shown in Fig. 8b in the indicated experimental groups\nof mice analyzed on day 20. e, Representative HIF1α and TUNEL staining of KP\ntumor sections. f, Quantification of HIF1α expression and apoptosis (TUNEL\nstaining) in KP tumor sections; staining areas and signal intensity normalized\nby the total area and mean background intensity, respectively. 3–5 tumors from\neach experimental group were analyzed. (PBS-IgG N = 5; DT-IgG N = 4; PBS αVegf\nN = 3; DT αVegf N = 3) with four sections per individual tumor sample. Data\nrepresent the mean ± s.e.m. g, Proportion of intratumoral Treg cells on day 20\nafter KP tumor transplantation. Data represent the mean ± s.e.m. of one of two\nindependent experiments; N = 8. h, Tumor growth dynamics upon the indicated\ntherapeutic interventions. Gray arrows indicate days of neutralizing antibody\nadministration. The data represent mean values of tumor volume measurements\n(left). Adjusted P values for day 20 measurements: IgG versus αCCR8 P < 0.0001;\nIgG versus αVEGF P < 0.0001; IgG versus αCCR8-αVEGF P < 0.0001; αCCR8\nversus αVEGF P = 0.0434; αCCR8 versus αCCR8-αVEGF P = 0.0044; αVEGF\nversus αCCR8-αVEGF P < 0.0001. i, Quantification of proportion and absolute\nnumbers of intratumoral and splenic Treg cells following treatment (left) and the\ncorresponding Treg cell numbers in spleens in the treated animals (right). Data in\n\nh and i represent the mean ± s.e.m. of one of two independent experiments, IgG N\n= 10, CCR8 N = 10, αVegf N = 8, CCR8-αVegf N = 8. j, Tumor growth dynamics upon\nthe indicated therapeutic interventions (left). Gray and black arrows indicate\ntiming of neutralizing antibody and CCR2 inhibitor (CCR2i) administration,\nrespectively. The data represent the mean ± s.e.m. values of tumor volume\nmeasurements. Adjusted P values of day 20 measurements: IgG versus αCCR8\nP = 0.0009; IgG versus αVEGF P < 0.0001; IgG versus CCR2i P < 0.0001; IgG\nversus αCCR8-αVEGF P < 0.0001; IgG versus αCCR8-CCR2i P < 0.0001; αCCR8\nversus αVEGF P = 0.9982; αCCR8 versus CCR2i P = 0.6138; αCCR8 versus αCCR8-\nαVEGF P < 0.0001; αCCR8 versus αCCR8-CCR2i P = 0.0041; αVEGF versus CCR2i\nP = 0.9551; αVEGF versus αCCR8-αVEGF P = 0.0003; αVEGF versus αCCR8-CCR2i\nP = 0.0363; CCR2i versus αCCR8-αVEGF P = 0.0018; CCR2i versus αCCR8-\nCCR2i P = 0.2271; αCCR8-αVEGF versus αCCR8-CCR2i P = 0.4530. Plots include\ndata from two independent experiments combined with nine animals in each\ngroup in experiment 1 (IgG N = 9, αCCR8 N = 9, αVEGF N = 9, CCR2i N = 9, αCCR8\nN = αVEGF-9, αCCR8-CCR2i N = 9) and 4–6 animals per group in experiment 2\n(IgG N = 4; CCR2i N = 6; CCR8-CCR2i N = 6). k, Kaplan–Meyer survival curves\nfollowed by Log-rank (Mantel–Cox) of KP tumor-bearing mice. The ‘survival’ time\nreflects the end point of the experiment when tumor volume in individual mice\nreached 1 cm3. Adjusted P values: IgG versus αCCR8 ***P < 0.0001; IgG versus\nαVEGF ***P < 0.0001; IgG versus CCR2i ***P < 0.0001; IgG versus αCCR8-αVEGF\n***P < 0.0001; IgG versus αCCR8-CCR2i ***P < 0.0001; αCCR8 versus αVEGF\nP = 0.5687 (NS); αCCR8 versus CCR2i P = 0.7411 (NS); αCCR8 versus αCCR8-\nαVEGF ***P = 0.0002; αCCR8 versus αCCR8-CCR2i P = 0.0342; αVEGF versus\nCCR2i P = 0.8054 (NS); αVEGF versus αCCR8-αVEGF ***P = 0.0006; αVEGF versus\nαCCR8-CCR2i P = 0.0666 (NS); CCR2i versus αCCR8-αVEGF ***P = 0.0003;\nCCR2i versus αCCR8-CCR2i P = 0.6749 (NS); αCCR8-αVEGF versus αCCR8-CCR2i\n*P = 0.0489. Plots include data from two independent experiments combined\nwith 5–11 animals in each group in experiment 1 (IgG N = 9, αCCR8 N = 9; αVEGF\nN = 9; CCR2i N = 11; αCCR8-αVEGF N = 9; αCCR8-CCR2i N = 5) and 4–10 animals\nper group in experiment 2 (IgG N = 7; CCR2i N = 10; CCR8-CCR2i N = 4). In b–d,\nh and i, plots are representative of one of two experiments with 8–10 mice per\ngroup each, at day 20 after transplantation. Number of mice per group in b and c:\nPBS-IgG N = 10; DT-IgG N = 10; PBS-αVEGF N = 9; DT-αVEGF N = 9; number of mice\nper group in h and i: IgG N = 10; αCCR8 N = 10; αVEGF N = 8; αCCR8-αVEGF N = 8.\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1031\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fa\n\nGFP-DTR\n\nFoxp3\n\nb\n\n3\n\n)\n\nm\nm\n\ns.c. KP\n\nDT/\nPBS\n\nαVEGF/\nIgG\n\nAnalysis\n\n(\n\nl\n\no\nv\nr\no\nm\nu\nT\n\nPBS-IgG\n\nDT-IgG\n\nPBS αVEGF\n\nDT αVEGF\n\n800\n\n600\n\n400\n\n200\n\n0\n\n*\n*\n*\n\n*\n*\n*\n\n*\n*\n*\n\n*\n\n*\n*\n*\n\n*\n\nDay 0\n\nDay 8,9\n\nDay 9, 12, 15\n\nDay 20\n\n5 7 8 9 1\n\n1\n\n2\n1\n\n3\n1\n\n4\n1\n\n5\n1\n\n6\n1\n\n9\n1\n\n0\n2\n\nDays\n\n***\n\nNS\n\nNS\n\n****\n\n**** ****\n\n****\n\nNS\n\nNS\n\n120\n\n****\n\n**** ****\n\n90\n\n60\n\n30\n\n0\n\n4\nD\nC\n\nf\no\n7\n6\nK\n%\n\nI\n\n100\n\n80\n\n60\n\n40\n\n20\n\n0\n\nPBS-IgG\n\nDT-IgG\n\nPBS-αVEGF\n\nDT-αVEGF\n\nd\n\nr\ne\nb\nm\nu\nn\nγ\n-\nN\nF\nI\n\n4\nD\nC\n\n250,000\n\n200,000\n\n150,000\n\n100,000\n\n50,000\n\n0\n\ni\n\nl\na\nv\nv\nr\nu\nS\n\n100\n\n80\n\n60\n\n40\n\n20\n\n0\n\n0\n\n20\n\n40\n\nDays\n\n****\n\n***\n\nNS\n\n****\n\n*** ****\n\nr\ne\nb\nm\nu\nn\nγ\n-\nN\nF\nI\n\n8\nD\nC\n\n100,000\n\n80,000\n\n60,000\n\n40,000\n\n20,000\n\n0\n\nPBS-IgG\nDT-IgG\nPBS αVEGF\nDT αVEGF\n\n****\n*\n\nNS\n\n**\n\n**\n\n****\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\n****\n\nNS\n\nNS\n\n*\n\nNS\n\nNS\n\n***\n\n**** ****\n\n100\n\n***\n\n**** ***\n\n8\nD\nC\n\nf\no\n7\n6\nK\n%\n\nI\n\n80\n\n60\n\n40\n\n20\n\n0\n\n120\n\n90\n\n60\n\n30\n\n0\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\n**\n\n*\n\nNS\n\nNS\n\nNS *\n\n150,000\n\n100,000\n\n50,000\n\nr\ne\nb\nm\nu\nn\no\nn\no\nM\n\n0\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\n****\n\nNS\n\nNS\n\n1,500,000\n\n****\n\n**** **\n\n1,000,000\n\n500,000\n\n0\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\n**\n\nNS\n\nNS\n\n***\n\n**\n\n**\n\n200,000\n\n150,000\n\n100,000\n\n50,000\n\n0\n\nPBS-IgG\n\nDT-IgG\nDT-αVEGF\nPBS-αVEGF\n\n4\nD\nC\n\nf\no\n\no\n\nl\n\nL\n2\n6\nD\nC\n\ni\n\nh\n\n4\n4\nD\nC\n%\n\n8\nD\nC\n\nf\no\n\no\n\nl\n\n2\n6\nD\nC\n\ni\n\nh\n\n4\n4\nD\nC\n%\n\nPBS -IgG\n\nDT-IgG\n\nPBS-αEGF\n\nDT-αEGF\n\n500 µm\n\nPBS-IgG\n\nDT-IgG\n\nPBS-αEGF\n\nDT-αEGF\n\nc\n\nr\ne\nb\nm\nu\nn\n4\nD\nC\n\nr\ne\nb\nm\nu\nn\n8\nD\nC\n\ne\n\nL\nE\nN\nU\nT\n\na\n1\nF\nI\nH\n\ng\n\n****\n\n*\n\nNS\n\nNS NS **\n\n****\n\n*\n\nNS\n\n**\n\n*\n\n****\n\nf\n\na\ne\nr\na\nl\na\nt\no\nt\n/\nL\nE\nN\nU\nT\n\n100\n\n80\n\n60\n\n40\n\n20\n\n0\n\ny\nt\ni\ns\nn\ne\nt\nn\n\ni\n\nL\nE\nN\nU\nT\n\n120\n\n80\n\n40\n\n0\n\nPBS-IgG\n\nPBS-αVEGF\nDT-IgG\nDT-αVEGF\n\nPBS-IgG\n\nPBS-αVEGF\nDT-IgG\nDT-αVEGF\n\n****\n\nNS\n\nNS\n\n**\n\nNS\n\n**\n\n**\n\n***\n\nNS\n\n100\n\nNS\n\nNS\n\n*\n\na\ne\nr\na\nl\na\nt\no\nt\n/\nα\n1\nF\nI\nH\n\n80\n\n60\n\n40\n\n20\n\n0\n\ny\nt\ni\ns\nn\ne\nt\nn\n\ni\n\nα\n1\nF\nI\nH\n\n120\n\n80\n\n40\n\n0\n\nPBS-IgG\n\nPBS-αVEGF\nDT-IgG\nDT-αVEGF\n\nPBS-IgG\n\nPBS-αVEGF\nDT-IgG\nDT-αVEGF\n\nNS\n\nNS\n\nNS\n\nNS NS NS\n\n***\n\nNS\n\nNS\n\n**\n\n*\n\n**\n\ni\n\n4\nD\nC\nP\nK\nf\no\ng\ne\nr\nT\n%\n\n80\n\n60\n\n40\n\n20\n\n0\n\nn\ne\ne\nl\np\ns\nn\n\ni\n\nr\ne\nb\nm\nu\nn\ng\ne\nr\nT\n\n100,000\n\n80,000\n\n60,000\n\n40,000\n\n20,000\n\n0\n\nIgG\nαCCR8\nαVEGF\nαCCR8-αVEGF\n\nIgG\nαCCR8\nαVEGF\nαCCR8-αVEGF\n\nIgG\n\nαCCR8\n\nαVEGF\n\nCCR2i\n\nαCCR8-αVEGF\n\nαCCR8-CCR2i\n\nIgG /αCCR8/αVEGF\n\nh\n\n)\n\n3\n\nm\nm\n\n(\n\nl\n\no\nv\nr\no\nm\nu\nT\n\n1,000\n\n800\n\n600\n\n400\n\n200\n\n0\n\nIgG\n\nαCCR8\n\nαVEGF\nαCCR8-αVEGF\n\n*\n*\n*\n\n*\n*\n**\n*\n*\n\n*\n\n*\n*\n**\n*\n\nIgG\n\n10864\n\n12 14 16 18 19 20\n\nDays\n\nIgG /αCCR8,/αVEGF,\nCCR2i\n\nNS\nNS\nNS\n\nNS\n\n*\n*\n*\n\n*\n*\n*\n\nNS\n\n*\n\n*\n*\n\n*\n*\n*\n\n*\n*\n*\n\n*\n*\n*\n\n*\n*\n*\n\n*\n*\n\nk\n\ni\n\nl\na\nv\nv\nr\nu\ns\n\nf\no\ny\nt\ni\nl\ni\n\nb\na\nb\no\nr\nP\n\n100\n\n80\n\n60\n\n40\n\n20\n\n0\n\nIgG\n\nαCCR8\n\nαVEGF\n\nCCR2i\n\nαCCR8-αVEGF\n\nαCCR8-CCR2i\n\ng\ne\nr\nT\nP\nK\nf\no\n\n+\n\n8\nR\nC\nC\n%\n\n100\n\n80\n\n60\n\n40\n\n20\n\n0\n\nj\n\n)\n\n3\n\nm\nm\n\n(\n\nl\n\no\nv\nr\no\nm\nu\nT\n\n1,000\n\n800\n\n600\n\n400\n\n200\n\n0\n\n5\n\n7\n\n9\n\n11\n\n13\n\n15\n\n18\n\n20\n\n0\n\n10\n\n20\n\n30\n\n40\n\nTime (days)\n\nDays elapsed\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1032\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\n\n\fDiscussion\nSuccesses in therapeutic targeting of PD-1 and CTLA-4 pathways in T\nlymphocytes are viewed as clinical evidence supporting the notion\nof cancer surveillance by cells of the adaptive immune system akin\nto that of pathogens. On the other hand, a growing realization of\nthe important roles immune cells play in supporting normal tissue\nfunction, maintenance and repair suggests an alternative, even if not\nmutually exclusive view of tumor–immune interactions. Within the\nlatter framework, the TME can be considered as a tissue-supporting\nmulticellular network, which in response to cues emanating from can-\ncerous cells supports their growth. In this regard, cancer represents\na special state of a parenchymal cell, whose support by both immune\nand non-immune cells is guided by common, yet poorly understood\nprinciples of tissue organization. Treg cells suppress immune responses\ndirected against self-antigens and foreign antigens to protect tissues\nfrom inflammation-associated loss of function1. Besides this indirect\ntissue-supporting functionality, Treg cells were also implicated in direct\nresponses to injury and other forms of tissue damage through produc-\ntion of tissue repair factors16,39–42 suggesting that these functions of Treg\ncells are conserved. Furthermore, Treg cells were shown to support skin\nand hematopoietic stem cell niches15,43,44. Therefore, it is reasonable to\nassume that in human solid organ malignancies and in experimental\nmouse cancers Treg cells likely play similar dual roles supporting tumor\ngrowth-promoting accessory cell states.\n\nHere, we showed that Treg cells have a profound impact on states\nof key accessory cells in a genetic autochthonous mouse model of\nNSCLC, in an experimental model of lung injury and in human LuAds.\nUsing robust unsupervised data-driven computational analyses, we\nfound that Treg cells support conserved gene programs—factors—across\nexperimental models of lung cancer and injury, suggesting their role\nin coordinating broad, shared accessory cell functions that extend\nto various conditions and tissue states. The latter included human\nimmunomodulatory C1Q+ (CFH, CR1L, LAG3, PDCD1LG2, LILRB4, IL18BP)\nand SPP1/FOLR2 macrophage factors and their mouse counterparts,\nwhich were positively associated with the Treg cell presence. A similar\nmacrophage gene program is also reported to be enriched in NSCLC\nlesions45 and sustained by Treg cells in mouse models of melanoma and\nbreast cancer46.\n\nOur analysis of the distribution of activated cell types and gene\nexpression programs with respect to their localization within and\naround tumor nodules showed high concordance of characteristic\ngene programs that were identified by scRNA-seq and ST analyses\nin situ. Notably, Treg cell depletion induced the IC response program\nlocalized to tumor nodule cores, while the IFN response program\nwas most notable in the margins of tumor foci. The display of these\nprograms by multiple cell types present within the same local niche\nsuggests that they are elicited by common signals (‘signal niche’), for\nexample, hypoxia response in the tumor nodule cores and a transient\nburst of IFN-γ produced by CD8+ T cells and NK cells concentrated in\nthe tumor margins47. We also observed heterogeneity between Treg cell\ndepletion responsive and nonresponsive tumor foci distinguished by\nthe presence or paucity of the IC gene program. Interestingly, tumor\nnodules that failed to induce the IC gene program in response to Treg\ncell depletion expressed Sox9 in agreement with a recent study where\nupregulation of Sox9 in human LuAd conferred resistance to NK cells27.\nAmong the conserved gene programs negatively associated\nwith Treg cell presence in mouse and human lung cancer, we noted the\nVEGF signaling pathway. This included increased expression of VEGF\nsignaling-related genes in endothelial cells and increased expres-\nsion of VEGF-A in myeloid and other cell types. This most likely rein-\nforces the immunosuppressive TME providing support for tumor\ngrowth consistent with a recent report of tumor ischemia caused by\nthe transient spike in intratumoral IFN-γ following CD25 antibody\nphotoimmunotherapy-induced Treg cell depletion47. In addition,\nlung EC-derived VEGF was shown to specify development of CAR4hi\n\nendothelial cells and promote vascularization and tissue regenera-\ntion following injury48,49. VEGF has also been suggested to exert an\nimmunomodulatory effect on cells of the innate and adaptive immune\nsystem50. Considering VEGF targeting being an approved therapy for\nsome human cancers, combined VEGF-A and Treg cell targeting serves\nas a proof-of-concept for a rational combination therapy instructed\nby the new knowledge of TME transcriptional connectivity. While\nnear complete loss of the Treg cell pool in KP-DTR mice led to systemic\nautoimmunity and inflammation, VEGF blockade coupled with CCR8\nantibody-mediated depletion of intratumoral Treg cells showed impres-\nsive therapeutic efficacy with no adverse effects. The latter owes to the\nfact that CCR8 expression is selectively enriched in highly activated\nintratumoral Treg cell subsets in human and mouse malignancies35,36,38.\nOur observation that a combination of CCR8 antibody-mediated intra-\ntumoral Treg cell depletion with CCR2 blockade did not yield additional\nbenefit in comparison to the corresponding monotherapies suggests\nthat the latter either directly or indirectly converge on a shared regula-\ntory node and highlights the utility of preclinical selection of combina-\ntorial therapeutic strategies informed by scRNA-seq and ST analyses\nof early TME responses.\n\nOur study highlights a generalizable approach where perturba-\ntion of a given cell population in an engineered genetic cancer model\nenabled computational learning of its ‘connectivity’ and influence on\nthe TME and other diseased tissue states, which could then be com-\npared to the human clinical settings. A surfeit of secreted and cell\nsurface molecules has been implicated in Treg cell-mediated immuno-\nsuppressive and tissue-supporting functions. However, none of these\nindividual modalities can predominantly account for the bulk of these\nfunctionalities. Combinatorial targeting of these putative mediators\nwill enable elucidation of the molecular mechanisms of the observed\nTreg cell dependencies in the TME. Our results suggest that Treg cells\nserve as an essential component of a complex network of accessory\ncells of both hematopoietic and non-hematopoietic origin. Shared\nperturbations in their transcriptional states observed across the three\ndifferent settings imply that the identified interdependencies of Treg\ncells and other components of tissue-supporting cellular networks are\nconserved and can be exploited to develop new strategies for rational\ntherapies of cancer and other diseases.\n\nOnline content\nAny methods, additional references, Nature Portfolio reporting sum-\nmaries, source data, extended data, supplementary information,\nacknowledgements, peer review information; details of author contri-\nbutions and competing interests; and statements of data and code avail-\nability are available at https://doi.org/10.1038/s41590-023-01504-2.\n\nReferences\n1.\n\nJosefowicz, S. Z., Lu, L.-F. & Rudensky, A. Y. Regulatory T cells:\nmechanisms of differentiation and function. Annu. Rev. Immunol.\n30, 531–564 (2012).\n\n2. Sakaguchi, S. et al. Regulatory T cells and human disease. Annu.\n\nRev. Immunol. 38, 541–566 (2020).\n\n3. 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Emergence of a high-plasticity cell state\nduring lung cancer evolution. Cancer Cell 38, 229–246 (2020).\n\n30. LaFave, L. M. et al. Epigenomic state transitions characterize\n\ntumor progression in mouse lung adenocarcinoma. Cancer Cell\n38, 212–228 (2020).\n\nOpen Access This article is licensed under a Creative Commons\nAttribution 4.0 International License, which permits use, sharing,\nadaptation, distribution and reproduction in any medium or format,\nas long as you give appropriate credit to the original author(s) and the\nsource, provide a link to the Creative Commons license, and indicate\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1034\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fif changes were made. The images or other third party material in this\narticle are included in the article’s Creative Commons license, unless\nindicated otherwise in a credit line to the material. If material is not\nincluded in the article’s Creative Commons license and your intended\nuse is not permitted by statutory regulation or exceeds the permitted\n\nuse, you will need to obtain permission directly from the copyright\nholder. To view a copy of this license, visit http://creativecommons.\norg/licenses/by/4.0/.\n\n© The Author(s) 2023, corrected publication 2023\n\n1Immunology Program, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, NY, USA. 2Program for Computational and Systems\nBiology, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, NY, USA. 3Institute of Biotechnology, Life Sciences Centre,\nVilnius University, Vilnius, Lithuania. 4Human Oncology & Pathogenesis Program, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center,\nNew York, NY, USA. 5Department of Pathology & Laboratory Medicine, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, NY,\nUSA. 6Department of Medicine, Thoracic Oncology Service, New York, NY, USA. 7Antitumor Assessment Core Facility, New York, NY, USA. 8Molecular\nPharmacology Program, Sloan Kettering Institute, Memorial Sloan Kettering Cancer Center, New York, NY, USA. 9Howard Hughes Medical Institute, Sloan\nKettering Institute, Memorial Sloan Kettering Cancer Center, New York, NY, USA. 10These authors contributed equally: Ariella Glasner, Samuel A. Rose,\nRoshan Sharma.\n\n e-mail: [email protected]; [email protected]\n\nNature Immunology | Volume 24 | June 2023 | 1020–1035\n\n1035\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fMethods\nExperimental model and mouse details\nMice. Animals were housed at the Memorial Sloan Kettering Cancer\nCenter (MSKCC) animal facility under specific pathogen-free condi-\ntions according to institutional guidelines. All studies were performed\nunder protocol 08-10-023 and approved by the MSKCC Institutional\nAnimal Care and Use Committee. Mice used in this study had no previ-\nous history of experimentation or exposure to drugs. Foxp3GFP-DTR and\nKrasLSL-G12D Trp53fl/fl mice were previously described10,13. Adult male and\nfemale mice (6 weeks or older) were used for all experiments.\n\nLung adenocarcinoma and bleomycin-induced fibrotic injury induc-\ntion. Cre recombinase-mediated induction of KP LuAds was previously\ndescribed10. Briefly, mice were anesthetized with a 160–180 μl keta-\nmine–xylazine mixture and infected with Cre recombinase-expressing\nadenovirus (1 × 108 plaque-forming units) via intratracheal administra-\ntion. Tumors developed within approximately 3 months. For the induc-\ntion of fibrotic injury, pharmaceutical-grade bleomycin (Fresenius\nKabi) was administered intranasally to anesthetized mice (0.06 U per\nmouse). For the s.c. KP tumor growth model, KP cells were resuspended\nin sterile PBS and injected to the flank subcutaneous space (1 × 106 KP\ncells in 200 μl per mouse).\n\nDiphtheria toxin, VEGF, PD-1, CCR8 antibody and CCR2i treatments.\nDT (List Biological Laboratories) was administered to mice (1 μg per\nmouse in PBS) via retro-orbital injection twice on two consecutive\ndays. For tumor transplantation experiments, DT was injected on days\n8 and 9 after tumor s.c. transplantation. Mouse polyclonal neutral-\nizing VEGF-A antibody (R&D clone AF-493-M) or control IgG (BioXcell\nclone BE0130) were injected on days 9, 12 and 15 (20 μg per mouse\nper injection) with or without DT, or on days 8, 10, 12, 14 and 17 with\nor without CCR8 antibody (BioLegend clone SA214G2; 240 μg per\nmouse per injection). PD-1 antibody (BioXcell clone BE0146) alone\nor in combination with VEGF-A antibody was administered on days\n8, 10, 12, 14 and 17 (250 μg per mouse per injection). RS-504393 CCR2\ninhibitor (CCR2i; 2517, Tocris) was administered (50 mg per kg body\nweight) daily in combination with CCR8 antibody, VEGF-A antibody or\ntheir combination. In these experiments, CCR8 and VEGF-A antibodies\nwere administered on days 8, 10 and 12, and CCR2i was administered\ndaily starting on day 8 and ending on day 12.\n\nHuman lung adenocarcinoma samples. Individuals with LuAd under-\ngoing a surgical resection or tissue biopsy at MSKCC were identified\nand biospecimens collected prospectively from 2017 to 2020. All par-\nticipants from whom biospecimens were obtained provided informed\nconsent for an MSKCC-wide biospecimen collection and analysis pro-\ntocol. Recruitment was designed to capture a wide, unbiased swath\nof heterogeneous disease, with a slight emphasis on EGFR-mutated\ntumors with a high propensity to transform to more aggressive sub-\ntypes. Biases may be present related to this recruitment design, the\nrace, sex, smoking status and the general population of MSKCC. Use of\nall participant material and data described in this paper was performed\nunder ethical approval obtained from the MSKCC Institutional Review\nBoard (study nos. 06-107 and 12-245). Only continuous trends between\ncell proportion and factor use were assessed across all participants\nand therefore controls based on sample groupings are not relevant.\n\nCell isolation and flow cytometry. For isolation of immune and stro-\nmal cells, lungs were perfused, placed into 5 ml microcentrifuge tubes\ncontaining 400 μl of cold serum-free RPMI and chopped with scis-\nsors (1–2 mm). Lung fragments were placed in 2–3 ml of pre-warmed\ndigestion medium (RPMI 1640, 10 mM HEPES buffer pH 7.2 to 7.6, 1%\npenicillin–streptomycin, 1% l-glutamine, liberase (Sigma-Aldrich,\n05401020001) and 1 U ml−1 DNase I (Sigma-Aldrich, 10104159001;\n2–3 ml)) and incubated for 30 min at 37 °C. After digestion, supernatant\n\nNature Immunology\n\nwas collected and cells were resuspended in ice-cold RPMI 1640 contain-\ning 5% FCS (Thermo Fisher, 35010CV), 1 mM HEPES pH 7.2 to 7.6 (Corn-\ning, MT25060CI), 1% penicillin–streptomycin (Corning, MT30002CI)\nand 200 mM l-glutamine (Corning, MT25005CI). After additional\ndigestion for 1 h of the remaining tissue, both digested cell fractions\npassed through a 100-μm strainer (Corning, 07-201-432), washed and\nFACS sorted. For cell isolation from transplanted KP tumor-bearing\nmice, tumors were placed into 5 ml microcentrifuge tubes containing\n400 μl of cold serum-free RPMI 1640, chopped with scissors and incu-\nbated in digestion medium containing 1 mg ml−1 collagenase (Sigma,\n11088793001) and 1 U ml−1 DNase I (Sigma-Aldrich, 10104159001) and\nbeads on a shaker at 37 °C for 1 h. For cytokine production measure-\nments, cells were incubated at 37 °C, 5% CO2 for 3 h in the presence of\n50 ng ml−1 phorbol-12-myristate-13-acetate (Sigma-Aldrich, P8139),\n500 ng ml−1 ionomycin (Sigma-Aldrich, I0634), 1 μg ml−1 brefeldin A\n(Sigma-Aldrich, B6542) and 2 μM monensin (Sigma-Aldrich, M5273).\nCells were stained with Ghost Dye Red 780 (Tonbo Bioscience, 13-0865)\nor Zombie NIR Flexible Viability Kit (BioLegend, 423106) and a mixture\nof fluorophore-conjugated antibodies for 30 min at 4 °C cells, washed\nand fixed with 1% paraformaldehyde (Electron Microscopy Sciences,\n15710). For intracellular staining, cells were fixed and permeabilized\nwith the BD Cytofix/Cytoperm Kit or with the Thermo Fisher Transcrip-\ntion Factor Fix/Perm Kit according to the manufacturer’s instructions\nand analyzed on a BD LSR II flow cytometer or sorted on a BD Aria II flow\ncytometer. Post-sort cell purity was routinely higher than 95%. Flow\ncytometry data were collected on an LSR II using FACS Diva v8.0 (BD),\nor on Aurora using SpectroFlo v2.2.0.3 (Cytek). Flow cytometry data\nwere analyzed using FlowJo v 10.6.1 (BD).\n\nImmunofluorescence microscopy, histological and spatial tran-\nscriptomic analyses. Perfused lungs were fixed for 1 h at 22 °C in 4%\nparaformaldehyde and dehydrated at 4 °C in 30% sucrose, snap-frozen\nin OCT compound (Sakura Tissue-Tek, 4583). For ST, samples were\nflash frozen without fixation. All samples were sectioned with a Leica\nCM1950 Cryostat at −2 °C, to a thickness of 10 μm. Sections were fixed\nin acetone for 20 min at −20 °C, rehydrated in PBS, blocked with 10%\nnormal donkey serum ( Jackson ImmunoResearch, 017-000-121) in PBS,\n0.3% Triton X-100, and stained overnight with fluorophore-conjugated\nantibodies at 4 °C in a humidified chamber. Thereafter, nuclei were\nstained with DAPI (5 mg ml−1; Abcam, 28718-90-3) or Draq7 (5 μM;\nAbcam, 109202) for 20 min at 22 °C. Sections were imaged in Slow-\nFade mounting medium (Life Technologies, S36938) using a confocal\nLeica SP8 microscope. For histology, tissues were fixed in 10% neu-\ntral buffered formalin, embedded in paraffin, and sectioned. For the\nTUNEL assay, sections were processed under standardized conditions\nusing the DeadEnd Fluorometric Detection System (Promega, G3250),\nand subsequent immunohistochemistry was carried out using BOND\nPolymer Refine Detection Kit (Leica, DS9800), according to the manu-\nfacturer’s instructions. All Images were processed and analyzed using\nImageJ package v2.0.0-rc-69/1.52p. Distances between cells of interest\nwere quantified following the same strategy and using similar code as\ndescribed elsewhere51.\n\nAntibodies. See Supplementary Table 20 for all antibodies used in\nthis study.\n\nRNA-seq library preparation and sequencing. Cell populations\nwere sorted straight into TRIzol (Thermo Fisher, 15596018), RNA was\nprecipitated with isopropanol and linear acrylamide, washed with 75%\nethanol, and resuspended in RNase-free water. After RiboGreen quan-\ntification and quality control by Agilent BioAnalyzer, 0.4–2.0 ng total\nRNA with RNA integrity numbers ranging from 1.0 to 9.9 underwent\namplification using the SMART-Seq v4 Ultra Low Input RNA Kit (Clo-\nnetech, 63488), with 12 cycles of amplification. Subsequently, 1.5–10 ng\nof amplified cDNA was used to prepare libraries with the KAPA Hyper\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fPrep Kit (Kapa Biosystems, KK8504) using 8 cycles of PCR. Samples\nwere barcoded and run on a HiSeq 4000 or HiSeq 2500 in rapid mode\nin a 50 bp/50 bp paired-end run, using the HiSeq 3000/4000 SBS Kit\nor HiSeq Rapid SBS Kit v2 (Illumina). An average of 32 million paired\nreads were generated per sample, and the percentage of mRNA bases\nper sample ranged from 62% to 88%.\n\nRNA-seq analysis. Paired-end RNA-seq reads were mapped to the\ngenome using STAR52 v2.7.3a. Gene annotations were downloaded\nfrom Ensembl release 83, which is based on mouse genome assembly\nGRCm38. R v3.6.0 was used for generating count matrices and DESeq2\n(ref. 53) was used for principal-component analysis, to identify DEGs\nand for Spearman correlations calculations and for hierarchical clus-\ntering and generation of k-means heat maps.\n\nSingle-cell RNA sequencing. Single-cell RNA-seq was performed on\nFACS-sorted mouse lung KP cells or human LuAd samples, on the Chro-\nmium instrument (10X Genomics) following the user guide manual\n(CG00052 Rev E) for 3′ v2 and v3 as previously described54. Briefly,\nsorted cells were washed once with PBS containing 0.04% BSA and\nresuspended in PBS containing 0.04% BSA to a final concentration\nof 700–1,200 cells per μl. Viability of cells was confirmed to be above\n80%, as confirmed with 0.2% (wt/vol) Trypan Blue staining (Countess\nII). Then samples were encapsulated in microfluidic droplets at a dilu-\ntion of ∼70 cells per ml (doublet rate ∼3.9%). Encapsulated cells were\nsubjected to a reverse transcription (RT) reaction at 53 °C for 60 min.\nAfter RT, the emulsion droplets were broken and barcoded cDNA was\npurified with DynaBeads MyOne SILANE, followed by 14 cycles of PCR\namplification (98 °C for 180 s; (98 °C for 15 s, 67 °C for 20 s, 72 °C for\n60 s) × 12 cycles; 72 °C for 60 s). Then, 50 ng of PCR-amplified barcoded\ncDNA was fragmented with the reagents provided in the kit and puri-\nfied with SPRI beads to obtain an average fragment size of 600 bp.\nNext, the DNA library was ligated to the sequencing adaptor followed\nby indexing PCR (98 °C for 45 s; (98 °C for 20 s, 54 °C for 30 s, 72 °C for\n20 s) × 10 cycles; 72 °C for 60 s). An average of 5,000 cells were targeted\nfor each tumor sample. The resulting DNA library was double-size\npurified (0.6–0.8×) with SPRI beads and sequenced on an Illumina\nNovaSeq platform (R1: 26 cycles (KP), 28 cycles (LuAd); i7: 8 cycles; R2:\n96 cycles (KP), 90 cycles (LuAd)) resulting in 184.5–186.1 million reads\nper sample (average reads per single cell, 42,000; average reads per\ntranscript, 4.40–7.14; KP).\n\nVisium spatial gene expression slides were permeabilized at 37 °C\nfor 12–18 min and polyadenylated. mRNA was captured by primers\nbound to the slides. RT, second-strand synthesis, cDNA amplification\nand library preparation proceeded using the Visium Spatial Gene\nExpression Slide & Reagent Kit (10X Genomics PN 1000184) accord-\ning to the manufacturer’s protocol. After evaluation by real-time PCR,\ncDNA amplification included 11–12 cycles; sequencing libraries were\nprepared with 8 cycles of PCR. Indexed libraries were pooled equimolar\nand sequenced on a NovaSeq 6000 in a PE28/120 run using the NovaSeq\n6000 S1 Reagent Kit (200 cycles; Illumina). An average of 219 million\npaired reads were generated per sample.\n\nComputational analysis of scRNA-seq data. For basic pre-processing\nand lineage identification see Supplementary Methods. We performed\ndimensionality reduction using principal-component analysis (specify-\ning 50 principal components (PCs); nPC = 50), then visualized the data\nin two dimensions 2D using t-SNE on the PCs (perplexity parameter set\nto 50 (KP) or 100 (injury)). The cells were grouped into clusters using\nPhenoGraph18 on the PC space, with k = 30 (Extended Data Fig. 2b). We\nestablished that clustering was robust to slight changes in k, by reclus-\ntering the cells under varying k (k ϵ (20, 25, 30, 35, 40, 45)) and measur-\ning consistency using the adjusted Rand index (using the sklearn\npackage in Python), obtaining an average Rand index > 0.85. To anno-\ntate each cluster as a specific lineage, we computed the average\n\nNature Immunology\n\nexpression of known lineage markers (Extended Data Fig. 2c,d). All the\ngenes used for annotation are listed in the heat map48,55–60.\n\nFor human LuAd samples, non-empty droplets were defined using\nCellBender on a per-sample basis61. The expected number of cells was\ndefined by SEQC output after the initial quality filters described above,\nplus 25,000, to ensure an adequate number of empty droplets in each\nhuman sample. A learning rate of 0.0001 (modified to 0.00005 for\nsamples needing a slower learning rate) was used with 300 epochs.\nViable cells were identified with a library size greater than 500 UMIs,\ngene number greater than 250, log10 genes per UMI greater than 0.8\n(complexity), and less than 20% mitochondrial transcripts.\n\nUMI counts from non-empty droplets with doublets removed\nwere normalized by first dividing by the library size (UMI counts per\ndroplet), multiplying by a scale factor of 10,000, and then taking the\nnatural logarithm of 1 + the normalized counts. Before dimensionality\nreduction and clustering, genes were filtered out if they were detected\nin less than 10 cells, had low transcript annotation quality (transcript\nsupport level 4 or 5 in Ensembl 85), or belonged to categories includ-\ning mitochondrial transcripts, highly expressed ncRNAs, ribosomal\nRNAs, immunoglobulin transcripts, hemoglobin genes or T cell antigen\nreceptor variable regions. This resulted in 18,597 retained genes and\n84,909 cells. The median total counts and number of cells per sample\nare listed in Supplementary Table 11.\n\nDoublet detection. For all mouse model samples, we performed\ndoublet detection using Scrublet62 with default parameters (that is,\nexpected_doublet_rate = 0.06, min_counts = 2, min_cells = 3, min_gene_\nvariability_pctl = 85, log_transform = true, n_prin_comps = 30) on each\nsample individually. Since we were more interested in analyzing specific\nlineages, we removed doublets when processing each lineage individu-\nally (as described below).\n\nIn human samples, doublets were identified on non-empty drop-\nlets for each sample individually using DoubleDetection (https://doi.\norg/10.5281/zenodo.2678041) with a P-value threshold of 1 × 10−7 and\na voter threshold of 0.8. This algorithm was used because of its higher\nrelative accuracy among doublet detection methods63, important for\nconsistency across heterogeneous sample mixtures. Doublets were\nremoved before lineage identification.\n\nDensity plots. For analysis of individual lineages (mouse samples), see\nSupplementary Methods. t-SNE plots are valuable to build a hypoth-\nesis but it can be difficult to glean the density of cells from different\nconditions due to cells (dots) overlapping on top of each other. To\ncomplement the t-SNE plots (colored by conditions such as Fig. 2b,e)\nand further highlight the finding that Treg cell depletion has different\neffects in different cell populations, we chose to represent the distribu-\ntion of the cells in the t-SNE plot using a density plot. We used the kde-\nplot implementation in Seaborn package in Python (with non-default\nparameter thres = 0).\n\nscRNA-seq differential expression. Differential expression test-\ning between tumor cells of control or DT conditions was performed\nusing MAST64 on log-normalized values. Only genes in at least 10%\nof cells in either condition and a minimum log fold change of 0.25\n(2,654 genes) were used as input. Significant genes were defined as\nadjusted P value < 0.05 and log fold change > 0.5. Gene-set enrichment\nanalysis was performed using fgsea65 with log fold-change values of\nsignificant genes. Gene sets derived from tumor clustering in work by\nMarjanovic et al.29 were used to assess enrichment (Extended Data Fig.\n8g; scrna_tumor_de_fgsea).\n\nMilo analysis. Milo incorporates information from biological rep-\nlicates to assign a P value for fold changes in neighborhood cellular\nabundance between experimental conditions, where neighborhoods\nare defined as regions of transcriptionally similar cells in a k-nearest\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fneighbor (kNN) graph generated. This method provided us with rigor-\nous statistics to compare the frequency of different transcriptional\nstates between conditions.\n\nFor each lineage, we sought to quantify the changes in density\nof control and DT cells in each neighborhood in the kNN graph using\nMilo19. Conceptually, Milo is analogous to differential gene expression\nanalysis, but instead of identifying genes that are differential between\ntwo groups of cells, Milo tests for differential cell density in (possibly\noverlapping) neighborhoods in the kNN graph, across different condi-\ntions. Milo also considers the originating sample of each cell and treats\nany batch effect as a covariate. However, since we did not observe sig-\nnificant batch effects in our data, the design matrix we supplied only\nincluded the sample identity and experimental condition of each cell.\nTo perform the analysis, we first constructed a kNN graph (k = 30)\non PCs using the buildGraph function in Milo. For each lineage, we used\nthe same number of PCs (nPCs = 50) as for clustering and cell-type\nannotation above. We constructed neighborhoods on top of the kNN\ngraph using the Milo makeNhoods function with default parameters\n(prop = 0.1, refined = true), then counted cells in each neighborhood\nusing the countCells function and assessed statistical significance\nusing testNhoods and calcNhoodDistance for spatial FDR correction.\nWe used default parameters in all these cases. Results were then visu-\nalized using the plotNhoodGraphDA function with alpha set to 1 in all\ncases (implying that neighborhoods with spatial FDR < 1 are colored in\nall visualizations). We further assigned each neighborhood to a cell type\nif more than 80% of the cells in it belonged to that cell type; otherwise,\nthe neighborhood was termed ‘mixed’.\n\nFactor analysis. To identify gene programs and their usage across\ncells, we used the scHPF package21. scHPF is a Bayesian factorization\nmethod that explicitly models sparsity in scRNA-seq count data, using\nhierarchical Poisson factorization to achieve positive-valued loadings\nacross a selected number of factors for individual cells and genes. The\nmethod provides gene scores, which assign each gene a score for gene\nmembership in a factor, and cell scores, which quantify the usage of\neach factor by a cell. Cells with high cell scores for a factor will use the\ngene program represented by that factor at higher levels; the gene pro-\ngram, in turn, consists of genes with high gene scores for that factor. In\nthe context of response to Treg cell perturbation in cells from different\nlineages, scHPF provided an ideal unsupervised and data-driven way\nto extract gene programs (factors) that are systematically altered by\nthe perturbation.\n\nIn the mouse tumor samples, scHPF was run using default hyperpa-\nrameters in the endothelial, fibroblast and myeloid lineages to obtain\n20 endothelial-specific factors, 25 fibroblast-specific factors and 25\nmyeloid-specific factors.\n\nDifferential factor usage between diphtheria toxin and control. We\nexpect that the coordinated gene program response to the impact of\nTreg cell depletion should reflect as factor cell scores being differential\nbetween control and DT conditions. To quantify this, we computed the\naverage cell score of every factor in each cluster of cells (grouped by the\ncell type they belong to) for each condition. This result is presented as a\nheat map in Fig. 2i for endothelial cells, Extended Data Fig. 5a for fibro-\nblasts, Extended Data Fig. 5c for myeloid cells in the tumor model and\nFig. 3g for endothelial cells in the bleomycin injury model. Investigating\naverages at the cluster level ensures that any factors that reflect subtle\nshifts in cell states within a cell type will be identified. We then studied\nthose factors that have higher averages in DT compared to control.\n\nTo ensure that our factors are significantly differential between\ncontrol and DT, we considered cell scores for each factor in each cluster\nand computed P values between the two conditions using a Mann–Whit-\nney U test as implemented in the scipy.stats.mannwhitneyu package\nin Python. The P values are reported in Supplementary Table 6. We\nthen considered factors that were robust to random initialization of\n\nNature Immunology\n\nscHPF (Supplementary Fig. 1 and ‘Robustness analysis of factors’),\nwere biologically relevant and had P values < 0.01 for further analysis.\n\nRobustness analysis of factors. We assessed the robustness of the\nobtained factors in two ways. First, we sought to ensure that the\nobtained factors were robust to random initializations. For this, we\nfixed the number of factors computed and reran the model for 20\niterations. To quantify the similarity across iterations, we computed\nPearson correlation (for both gene and cell scores) between best match-\ning factors between iterations. The best matching factors between\nany two iterations were identified using an implementation of the\nHungarian66 matching algorithm. The algorithm matches each factor\nfrom one iteration to the best matching factor from a second itera-\ntion such that the total cost is minimized, where the cost is defined as\n(1 − pairwise correlation score between two iterations). We used the\nPython (v3.8) implementation of the linear_sum_assignment function\nin the optimize module of SciPy package (v1.7.1). After matching, we\nreported the median correlation score between pairs of iterations\n(Supplementary Fig. 1).\n\nSecond, we sought to ensure that the factors we identified in our\nanalysis (highlighted in red in Figs. 2i and 3g and Extended Data Fig. 5a,\nc) as being different between control and DT conditions (‘Differential\nfactor usage between diphtheria toxin and control’) were robust to\nchanges in parameters, mainly the choice of number of factors. This\ntest ensures that the obtained factors were not identified by chance\nand that they constitute robust signal in the data. For this, we fixed the\nnumber of factors computed above (that is, 20 factors for endothelial,\n25 factors for fibroblasts and 25 factors for myeloid) as the baseline.\nThen, we reran scHPF for a range of number of factors (around the\nchosen value) and computed correlations with the specific factors\nof interest. To compute the correlation to the best matching factor,\nwe used the same strategy of the Hungarian matching algorithm as\ndescribed above. The average correlation over 20 such iterations was\nthen reported (Supplementary Fig. 1).\n\nWe repeated the same computation to assess the robustness of\n\nchosen factors in the bleomycin injury model.\n\nComparison of human and mouse factors. In human samples, scHPF\nwas run with default hyperparameters and ten random initializations in\nthe endothelial, fibroblast and myeloid lineages, using raw UMI counts\nfor genes expressed in at least 1% of cells within the lineage. This left\n12,533 genes in the endothelial lineage, 13,216 genes in the fibroblast\nlineage and 12,253 genes in the myeloid lineage for factor analysis. To\nselect the number of factors for downstream analysis, scHPF was first\nrun with two more factors than the number of PhenoGraph clusters\nwithin the lineage, then subsequently increased nine times by steps\nof one, for a total of nine separate factorizations (that is, k = (17, 18, 19\n… 250). To achieve consistent granularity across lineages, we chose\nthe factorization in which ~90% of the variance in a cells’ expression\n(on average) was explained by the top 7 factors, given by 22 factors\nfor endothelial and fibroblasts lineages and 27 factors for myeloid\nlineage cells.\n\nAfter matrix factorization in human samples, we identified gene\nprograms associated with Treg cell presence in LuAd tumors by calcu-\nlating the Spearman correlation between the log2 average factor cell\nscore and log2 Treg cell proportion of CD45+ cells in each sample. This\ncalculation was also performed using the Treg cell proportion of CD3+\ncells in each sample to ensure consistency; however, the Treg cell propor-\ntion of CD45+ cells are referenced in the primary results (Extended Data\nFig. 9a). We assessed the stability of gene programs using a similar strat-\negy to that used for mouse above, and robustness of factor associations\nto Treg cell presence was assessed by the same correlation calculation\nusing matched factors in a separate run of scHPF factorized using a\ndifferent value of k. The relationship of factors across lineages was\nassessed by the pairwise Spearman correlation of log2 average factor\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fcell scores in each sample from one factor to all other factors. Only\nsamples with enough representative cells were used for correlation\nanalysis in each lineage (>5 cells in endothelial and fibroblast, >20\ncells in myeloid). Sample 16 was removed from all factor correlations\ndue to outlier values driven by high IFN signatures, and sample 17 was\nremoved from endothelial correlations due to outlier values driven\nby low cell numbers.\n\nTo identify conserved gene programs (factors) in endothelial,\nfibroblast and myeloid cells between human and mouse tumors, we\ncompared the gene scores of orthologous genes. First, the genes used\nfor factorization were filtered for orthologs that had a one-to-one\ncorrespondence between species (Ensembl 85 annotations) and were\nexpressed in both species. A gene was assigned to a factor if its gene\nscore was two standard deviations greater than the mean of gene\nscores for all genes in that factor. Then, a Jaccard similarity score was\ncalculated between all mouse and human factors of a given lineage\nby dividing the number of shared assigned genes by the number of\nunique assigned genes in each pair of factors. The z-score of Jaccard\nvalues for all human factors against each mouse factor was used to\nidentify human factors with greater homology to a mouse factor than\nbackground. Typically, a Jaccard similarity score greater than 0.06 in\nthe endothelial lineage and 0.07 in the fibroblast and myeloid lineages\nand would define one (and no more than three) factors in human with\nhomology to a mouse factor.\n\nThe validity of factor mappings across species was assessed by\nexamining the genes shared between conserved factors to ensure they\nbelonged to coherent biological programs (inflammation, angiogen-\nesis, and so on). To find genes with similarly high scores across con-\nserved factors, we normalized the gene score for each gene by the sum\nof its scores across all factors (fraction of total gene score), which also\nenabled comparison across factorizations. This was used to compare\nTreg cell-associated inflammation and hypoxia programs in Fig. 4g; we\ncompared normalized gene scores from the sum of human factors 4 and\n5 to those in mouse factor 15, as these corresponded to the same under-\nlying biological process across species (see below). In Fig. 4g, genes\nwere listed as conserved if they were assigned (as described above) to\nboth the human and mouse factors being compared. VEGF-regulated\ngenes in endothelial cells were identified using gene sets derived by\nDhainaut et al.31 with data from the CytoSig database, which houses\npublic cytokine response datasets for many cell types and treatment\npairs (https://cytosig.ccr.cancer.gov/).\n\nIn certain cases, gene or cell scores for several factors were\nsummed to relate an underlying biological process to similar gene\nexpression programs in mouse (as above) or Treg cell proportion across\nhuman participants. An underlying biological process (for example,\ninflammation) could be split across several factors due to similar but\nnonoverlapping expression programs (for example, cell-type-specific\nsignaling) or very similar expression programs with sample-specific or\ncondition-specific effects. Comparisons including only partial signal in\nthese cases, when only a single factor was compared to another entity,\ncould mask associations to the broader biological program. In Fig. 4f,\nwe summed cell loadings for human endothelial factors 3, 4 and 5 to\nrelate the conserved Treg cell-responsive endothelial expression pro-\ngram to Treg cell proportion across tumor samples. We reasoned that\nthese factors were related to a shared underlying biological process\nbecause they were each individually negatively associated with Treg\ncell proportion across samples to various degrees (Extended Data\nFig. 9c), and their genes aligned with different components of Treg\ncell depletion-induced expression program in mouse tumors: factor\n3, aCap; factor 4/5, inflammation and hypoxia with features of the\nmouse activated VEC (Fig. 4e). Additionally, factors 4 and 5 shared\ninflammation-relevant genes (IL6, CSF3) but with different sample\nspecificities, which indicated that sample-specific effects rather than\nthe underlying biology could have separated this gene program across\ntwo human factors (Extended Data Fig. 9d). Therefore, a summed factor\n\nNature Immunology\n\nscore was found to be more appropriate in capturing certain endothe-\nlial gene program relationships to Treg cell proportion.\n\nExpression heat maps\nOnce we identified the factors of interest in each of the cell types,\nbased on our definition of higher average cell score in DT compared\nto control conditions, we zoomed into the genes that contributed the\nmost to those factors. We were particularly interested in understand-\ning the genes that drive the factor score in a specific subpopulation of\ncells. In our analysis, we sought to focus on specific subtypes with the\nhighest average cell score for the factor. As such, we isolated the cell\ntypes of interest and correlated the factor usage (cell scores) with gene\nexpression. Details of the subsetting are provided in Supplementary\nTable 19. To elaborate, we provide an example: we identified factors 9,\n14 and 22 to be enriched in DT-treated cells compared to control in the\nfibroblast subpopulation in the mouse tumor model. These factors had\nthe highest cell usage scores among the COL14A1 subtype. Therefore,\nto identify genes that are driving these factors and ensure that we focus\non gene programs specific to the COL14A1 subtype, we subset this cell\ntype of interest and correlate factor usage with gene expression. In\ncases where the cell type of interest was small (for example, the inflam-\nmatory capillary subset in endothelial cells in the mouse tumor model),\nwe subsetted the cell type of interest combined with the phenotypically\nmost similar cell type (for example, we grouped the inflammatory cap-\nillary subset with aCap in the mouse tumor model endothelial cells).\nThis ensured we had sufficient cell numbers to compute the correlation\nand allowed us to identify genes specific to the cell type of interest in\ncontrast to its nearest phenotypically similar subtype.\n\nTo this end, we correlated gene expression against the cell scores\nin the isolated set of cells and identified the top 200 most correlated\ngenes as being relevant to that factor for that specific subpopulation.\nTo ensure that the correlation scores were not influenced by any poten-\ntial outliers (cells with deviant cell scores), we compared our results\nagainst correlation computed between the imputed gene expression\nand imputed factor cell scores (using MAGIC57, nPCs = 20, k = 30, ka = 10,\nt = 4). In both scenarios, we obtained highly similar results. The expres-\nsion heat maps (Figs. 2i and 3i and Extended Data Figs. 5a,c, 6b and 7a,d)\ndisplay the result from imputed data.\n\nWe followed the same procedure for the bleomycin injury model\n\ndata.\n\nSpatial transcriptomics\nRead mapping and quantification. We processed Visium ST data with\nthe SpaceRanger pipeline from 10X Genomics (v1.3.1). The mkfastq\nfunction was used to generate FASTQ files from raw base calls and the\ncount function was used in combination with a matched brightfield\nH&E-stained image to align to a modified mm10 genome, perform\ntissue detection and count UMIs for each spot. The modified genome\nconsisted of Ensembl 100 annotations with an added transcript to\ndetect DTR-GFP expressed from the Foxp3 promoter (sDTR-eGFP).\nUMI counts were summed by gene symbol and sDTR-eGFP reads were\nsummed together with Foxp3. All analyses of differential cell-type abun-\ndance or gene expression were performed in the first serial section of\neach biological sample to preserve the independence of observations.\nGene expression counts were log normalized using SCTransform67 with\nSeurat (v4.1.1)68 to compare between spots. Spots with fewer than 1,000\nUMIs were excluded from analysis.\n\nDeconvolution of Visium spots to cell-type RNA fractions. Visium\ncaptures transcripts from sectioned tissue placed over 55-μm-diameter\nspots, such that each spot sums gene expression from multiple cells. We\nused the BayesPrism algorithm25,26 to deconvolve cell types present in\neach spot and thereby improve the effective resolution of the technol-\nogy. As input, BayesPrism accepts a spot-by-gene count matrix and a\nscRNA-seq reference dataset labeled by cell type; it utilizes a Bayesian\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fapproach to jointly model cell-type fractions and cell-type-specific\ngene expression within each Visium spot.\n\nIn our ST analysis, we used two separate scRNA-seq references\nfor deconvolution—one containing the small number of tumor cells\n(N = 239) captured in our study (‘non-merged reference’) and another\ncontaining cells from a separate study that sampled more tumor\ncells (N = 18,083) from specific tumor sub-states (‘merged reference’,\ndetailed below). The merged reference was used to assess the presence\nof granular transcriptional states within tumors, while the non-merged\nreference was used to study accessory cell populations without the\ninfluence of batch effects (data from two separate studies) or con-\nfounding during deconvolution (limited resolution between normal\nepithelial and certain tumor states, that is, AT2 versus AT2-like tumors).\nThe non-merged reference includes scRNA-seq data solely consisting\nof cells from identical experimental conditions to the ST data (KP\ntumor-bearing lungs treated with PBS or DT; data from Fig. 2) and\nwas used for all analyses in Fig. 4 and Extended Data Fig. 7c,e,f. Cell\nfraction estimates from the non-merged reference were used to distin-\nguish tumor spots from normal spots because of the better-matched\nexperimental characteristics, the capture of tumor cells from the Treg\ncell-depleted state and the lower chance of similarity to normal EC\ntypes by using all tumor cells as a single reference population. The\ncell-type fraction estimates of tumor sub-states from the merged\nreference were used for analysis in Fig. 5 and Extended Data Fig. 8 only\nin tumor spots defined using the non-merged reference. Additional\ndetails of scRNA-seq reference construction and applications of the\ncell-type fraction estimates are mentioned below.\n\nSelection of cell types and marker genes. The accuracy and reli-\nability of cell fraction estimates depends on the presence of features\nin Visium data that are specific to labeled populations (highly specific\ncell-type markers give better deconvolution), the transcriptional dis-\ntance between populations (better separated populations give better\ndeconvolution) and how closely matched the scRNA-seq reference is\nwith populations profiled in situ by ST. We thus optimized both gene\nselection and cell-type label granularity in our scRNA-seq reference\nand leveraged the ability of BayesPrism to encode separate cell states\nwithin a population to better match the reference in specific conditions\n(that is, control versus Treg cell depleted).\n\nFeature selection before deconvolution can improve the\nsignal-to-noise ratio by removing genes that are irrelevant to cell type but\nbehave similarly to relevant genes, and it can also mitigate the influence\nof genes that change due to batch effects. We therefore chose to focus on\ncell-type marker genes in our deconvolution, which is a recommended\noption in BayesPrism. Marker genes were computed by conducting\npairwise t-tests across cell types (findMarker function in SCRAN) using\nlog-normalized data. We defined marker genes by a minimum P value of\n0.05 and minimum log fold-change value of 0.25 across all comparisons.\nGenes with fewer than ten counts across all Visium sections, or those\ndetected in fewer than five cells in scRNA-seq data, were removed in\naddition to ribosomal genes, mitochondrial genes and genes associated\nwith the cell cycle (https://github.com/dpeerlab/spectra/).\n\nWe merged highly similar cell types to avoid confounding deconvo-\nlution. To ensure adequate resolution between cell types, we computed\nmarker genes as described above, starting at the most granular level of\nannotation and iteratively merging cell populations with their closest\nneighbor (by transcriptional distance), until each cell population had\nat least 30 marker genes. This included collapsing Artery/Vein with\ngCap cells (labeled as gCap); CD8+ T cells, effector T cells, exhausted\nCD8+ T cells, MAIT, gdT, TH2, naïve T cell, activated T cell, Treg and ILC2\npopulations (T cell/ILC2); B and plasma cells (B cells); monocyte and\nCsf3r+ monocytes (monocyte); cDC1 and cDC2 (cDC); and Csf3r+ neu-\ntrophil, Ccl3+ neutrophil, and Siglecf+ neutrophil (neutrophil). We\nfurther merged the inflammatory capillary population with aCap cells\n(aCap), and Arg1+ with C1q+ macrophage populations (macrophages),\n\nNature Immunology\n\nas these are arguably specialized cell states of the same overarching cell\ntype. Cycling T cells were also removed to prevent misassignment to\ntissue regions with increased expression of cell cycle-related genes. The\nresulting filtered scRNA-seq reference comprised 4,219 marker genes\nand 23,178 cells labeled as 26 cell populations (see Extended Data Fig.\n7a for full list of cell populations included).\n\nWhile our feature selection strategy ensured adequate resolution\nbetween cell types, transcriptional heterogeneity within cell types can\nalso influence deconvolution. BayesPrism initially performs inference\nat the cell-state level, which can account for condition-specific hetero-\ngeneity in transcriptional states within cell types during deconvolution.\nCell states can be captured by the algorithm through cell-type-specific\nexpression estimates but can also be included as labels in the reference\ndata. We observed substantial transcriptional shifts in accessory cell\npopulations between control and Treg cell-depleted conditions by\nscRNA-seq (Fig. 2 and Extended Data Figs. 4 and 5), and thus labeled\ncells from these accessory populations to help capture heterogeneity\nwithin cell types. Control and Treg cell-depleted states were assigned\nfor aCap, gCap, LECs, Col13a1+ and Col14a1+ fibroblasts, pericytes,\nmyofibroblasts, AT1, AT2, cDC, macrophage, alveolar macrophage,\nneutrophil and monocyte populations in the scRNA-seq reference.\nBayesPrism sums cell-state fractions at the cell-type level before the\nfinal update step and downstream analysis.\n\nFollowing cell-state definition, BayesPrism was run jointly on serial\nsections, to allow sharing of information across more spots during the\nfinal update step and filtering out of genes whose expression fraction\n(reads/total reads) was greater than 0.01 in 10% of Visium spots. For the\nrobustness and reproducibility analysis, each section was deconvolved\nindependently.\n\nKP tumor cells are known to adopt a range of recurrent cell states\nas they progress27–30. To assess tumor transcriptional states and their\nrelation to Treg cell depletion, we performed a second deconvolution\nusing BayesPrism across Visium spots with a scRNA-seq reference\ncontaining more granular tumor-state labels. Given the limited number\nof tumor cells in our reference (239 cells), we decided to incorporate\nscRNA-seq data from Yang et al.28, which contains ~50,000 KP tumor\ncells (referred to as KP-Tracer data), to more accurately assign general\nstates within tumor regions. A key advantage of BayesPrism is that it\ncan incorporate single-cell data from multiple sources, which do not\nneed to be matched with our data. The algorithm uses cell types in the\nscRNA-seq reference as a prior for possible cell states in the Visium data,\nwhile disregarding cell-type fractions in the reference.\n\nTo minimize computational burden and sample-specific biases,\nwe processed the KP-Tracer data by removing mutation-specific and\nmesenchymal cell states (these were largely sample-specific), and\ndownsampling the remaining tumor states to a maximum of 2,000 cells\n(for balanced sampling), leaving 18,083 cells. The original tumor-state\nlabels of EMT-1, EMT-2 and pre-EMT were combined (labeled as EMT),\nas were early gastric, late gastric and gastric-like populations (gastric)\nto limit deconvolution to the most representative overarching tumor\nstates. These cells were combined with our accessory cell data in the\nsame count matrix, with tumor cells removed. Marker gene selec-\ntion and gene filtering were performed as above but adding 24 genes\nupregulated in AT2 cells relative to the AT2-like tumor state (t-test on\nlog-normalized scRNA-seq data with adjusted P value < 0.001, log fold\nchange > 1, expressed in 15% more cells relative to AT2-like) to better\ndiscriminate tumor from normal states. The final merged reference\ncontained 40,787 cells and 4,546 genes after cell and feature selection,\nand we ran BayesPrism deconvolution with it using identical settings\nto the non-merged reference.\n\nVisium data are very noisy. To discriminate robust evidence for\ncell type, we only included cell-type fractions above background at\nparticular spots, using the same mixed-model strategy as the compute.\nbackground function from SpaceFold26, with modifications detailed\nbelow. Specifically, for each cell type in each tissue section, a gamma\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fmixture model with two components was fit for cell-type fraction\n(gammamixEM from mixtools69), and a Gaussian mixture model with\ntwo components was fit for the summed deconvolved gene expression\nvalues (Mclust from Mclust70) across all spots. Mixture model distribu-\ntions were checked for agreement with data structure by histogram and\noverlay of the fitted distribution. After determining parameters for the\nmixture components, we identified spots with >70% posterior prob-\nability of being assigned to the mixture component having the higher\nmean and used the minimum value of these as a threshold for calling a\ncell type ‘present’. Cell-type thresholds below 0.001 were reset to 0.001,\nand summed deconvolved gene expression thresholds below 50 were\nreset to 50. To enable comparison across tissue sections and prevent\nerroneous cutoffs due to tissue-specific composition, the median of\ncell fraction and summed deconvolved gene expression cutoffs across\nall eight tissue sections was used for each cell type. These values were\nsubsequently refined with the guidance of H&E staining (see below for\ndetails). Illustrations of spot binarization for the presence of specific\ncell types are shown in Fig. 4a and Extended Data Fig. 7d,e.\n\nAssessing robustness and accuracy. We assessed the robustness\nand accuracy of cell-type RNA fraction estimates before proceeding\nfurther with analysis downstream of our deconvolution. We performed\nbootstrap analysis to determine robustness to the sparse capture of\nVisium. Specifically, we ran BayesPrism on one tissue section with\neach spot randomly downsampled to 90% of its reads, repeated this\n20 times, and calculated the Spearman correlation of cell-type fraction\nestimates between the original and each downsampled deconvolu-\ntion. Cell fraction estimates across spots were highly consistent, with\nSpearman correlations ≥ 0.87 for all trials (Extended Data Fig. 7a). We\nnext compared average cell-type fraction between individually decon-\nvolved serial sections across all samples, validating the expectation\nthat cell-type fractions captured by serial sections are highly similar\n(Spearman R = 0.99; Extended Data Fig. 7b). To ensure consistency\nbetween our two deconvolution approaches, we compared cell-type\nfractions of non-tumor accessory cells with and without additional\ntumor states from the KP-Tracer study in our scRNA-seq reference.\nAverage log(cell-type RNA fraction) values from each tissue section\nwere highly correlated (Spearman R = 0.97), suggesting that decon-\nvolved accessory populations were generally not influenced by tumor\nRNA, and that the inclusion of tumor cells from a separate study did\nnot impact accessory cell deconvolution (Extended Data Fig. 7c). One\nexception was in resolving AT1-like and AT2-like epithelial states, which\nare highly similar to several of the added tumor states; the added states\nlikely improved their resolution in tumor regions, but not in non-tumor\nregions, due to transcriptional similarity with normal epithelial states.\nCell-type assignment was cross-referenced with the underlying\ntissue histology from matched H&E-stained brightfield images to con-\nfirm accurate positioning of cell types where possible (Extended Data\nFig. 7d,e). For example, spots deemed to possess different capillary\ntypes, pericytes and alveolar macrophages were consistent with the\nliterature and anatomical features (Extended Data Fig. 7d). gCap and\nartery/vein cells were localized around blood vessels and alveoli, with\nsome penetration into tumor areas, whereas aCap cells were mainly\ndistributed over alveoli and surrounding tumor areas, consistent with\ntheir propensity to surround areas of injury48. Pericytes were localized\naround blood vessels and bronchi, consistent with published annota-\ntions60, and alveolar macrophages were concentrated in areas sur-\nrounding tumor regions, as previously shown45. LECs, DCs and B cells\nare all expected in areas containing lymphoid aggregates emanating\nfrom a lymphatic vessel and were indeed detected in these regions by\nour ST analysis (Extended Data Fig. 7e). Moreover, regions represent-\ning part of an IC signaling niche were found to have higher neutrophil\ncell-type fraction (Fig. 4f), which was readily apparent in the aligned\ntissue section due to the unique appearance of neutrophils in H&E\nstaining (Fig. 5f).\n\nNature Immunology\n\nUpon assessing marker gene expression and inspecting histology,\nwe noted that several cell types including AT2, gCap, MSCs, monocytes\nand LECs had more modes in the distribution of their cell-type fractions\nand summed deconvolved gene expression values across spots, likely\ndue to regional variation in cell-type composition and read density. To\naccount for this variation, we reset the presence/absence thresholds for\nthese cell types as above but using mixture models with three mixture\ncomponents instead of two. As a result, the minimum value from spots\nassigned to the mixture component with the second highest mean (one\nabove background mixture component) with >70 posterior probability\nwas used as a threshold. The median cell-type fraction and summed\ndeconvolved gene expression threshold values of the three-component\nmixture models across all tissue sections was applied to all spots (as\nfor two-component models above).\n\nAnalysis of gene program usage across conditions. To assess the\ndifferential use of gene programs between control and Treg cell-depleted\nconditions identified by factor analysis in scRNA-seq data, we used\nthe AddModuleScore function in Seurat to compute the relative\nlog-normalized expression of each factor’s genes relative to a ran-\ndom set of background genes with similar average expression in the\ntissue. Specifically, all genes were split into 24 expression bins and 100\ncontrol features were randomly selected for each feature in the input\ngene program from a corresponding bin. The average log-normalized\nexpression of control features was then subtracted from the average\nlog-normalized expression of the features of interest to derive a mod-\nule score. Module scores were computed across spots from all four\nsamples at the same time. A t-test was performed to compare gene\nprogram module scores in control and Treg cell-depleted conditions\nfor gene programs of interest, and P values were adjusted by Benja-\nmini–Hochberg correction. To measure the difference in relevant\ncellular contexts, comparisons were restricted to spots with cell-type\nfractions above background for cell types in which the gene program\nof interest was found to be differential by scRNA-seq, creating a table\ncomparing cell type by gene program of interest across conditions\n(Fig. 4b). The visualization of specific module scores was performed\nin Figs. 4c,d and 5g.\n\nDefinition of signaling niches. Certain gene programs that increased\ntheir abundance in both our scRNA-seq and ST analysis following\nTreg cell depletion shared many genes across endothelial, fibroblast\nand myeloid lineages. This included factors that contained many\nIFN-stimulated genes (IFN factors) and factors that contained genes\nrelated to IC and hypoxia signaling (IC factors). To determine shared\ngenes between IFN and IC factors, genes were assigned to each rel-\nevant factor from the mouse scRNA-seq in the same way as detailed\nin ‘Comparison of human and mouse factors’ and the intersection\nof genes across all three lineages for IFN or IC factors was taken. The\nIFN factors were defined as fibroblast factor 9, endothelial factor 19\nand myeloid factor 17. IC factors were defined as fibroblast factor\n22, endothelial factor 15 and myeloid factor 21. The module score\nof shared genes for IFN (N = 103 genes) or IC (N = 18 genes)-related\ngene programs (See Supplementary Table 12 for gene lists) was then\nused to define ‘signaling niches’ or Visium spots where a common\nsignaling pathway may drive downstream gene expression in several\ncolocalized cell types.\n\nTo assign spots to a signaling niche, we took advantage of the fact\nthat most spots across all tissue sections did not show signal for IFN or\nIC gene programs. Therefore, we modeled the background rate of these\ngene programs by fitting their module scores plus a pseudocount of\none to a gamma distribution using maximum likelihood estimation\n(fitdistr from MASS package71) across all spots on the four biologically\nindependent sections being analyzed. Alignment with the gamma dis-\ntribution was checked by a histogram of the gene scores and density\noverlay of the fit distribution. The module score corresponding to an\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fupper tail probability of 0.01 in the fit distributions was then used as\na threshold above which spots were assigned to that signaling niche.\nAssignment of spots to the IFN or IC signaling niche was not mutually\nexclusive and gave 397 spots assigned to IC niches, 330 spots assigned\nto IFN niches and 21 spots assigned to both. An illustration of spot\nassignment to signaling niches is shown in Fig. 4e.\n\nCell-type enrichment in signaling niches. To assess the presence\nof different cell types within signaling niches relative to background\ncell-type fractions across the tissue (Fig. 4f), we took a random sample\nof 100 spots across all tissue sections and averaged the fraction of each\ncell type, then repeated for 10,000 iterations to form an empirical prob-\nability distribution of mean cell-type fractions of randomly selected\nspots. The empirical P value was calculated as the fraction of iterations\nin our empirical distribution with an average cell-type fraction above\nthe average for all spots in a given signaling niche (IFN or IC). Empirical\nP values were adjusted by Benjamini–Hochberg correction to account\nfor multiple hypothesis testing. To measure the magnitude of enrich-\nment for each cell type, the log2 average cell-type fractions from the\ntotal empirical distribution were subtracted from average cell-type\nfraction values from either signaling niche.\n\nDefinition of tumor-state regions. To classify spots within tumor\nlesions into areas of consistent transcriptional phenotypic state (‘tumor\nlesion areas’), we first selected spots with tumor RNA above back-\nground (detailed above) and used cell-type fractions from the decon-\nvolution with the merged reference. To visualize the co-occurrence\nof tumor states, we z-scored fractions of tumor states in tumor spots\nand hierarchically clustered the spots into seven groups (R cutree with\nk = 7) using Pearson correlation distance and average agglomeration\n(Extended Data Fig. 8a). This analysis revealed that tumor spots were\ntypically dominated by a single tumor state. When plotted in their tissue\ncontext, we found that they often aggregated spatially (Fig. 5a), pro-\nviding further support for the presence of consistent transcriptional\nphenotypes within lesional areas.\n\nEach of the seven clusters was then labeled based on the tumor\nstate with the highest cell-type fraction in the cluster. The validity of\ncluster labels was assessed by the expression of tumor-state marker\ngenes defined by previous studies28. We found clearly higher expres-\nsion of marker genes in their corresponding cluster relative to other\ntumor clusters (Extended Data Fig. 8b). The classification of tumor\nspots in their tissue location is shown in Fig. 5b and Extended Data\nFig. 8c. In H&E staining, the location of different tumor states often cor-\nresponded to a noticeable change in histology, further supporting our\nclassifications. For instance, neighboring gastric and high-plasticity\nregions also exhibited a more differentiated morphology in the gastric\ntumor area and less structure in the high-plasticity area (Fig. 5f,g).\nWhile our strategy increased the resolution of tumor transcriptional\nstates in our deconvolution, there may be additional tumor states\nin situ that are not contained within our merged scRNA-seq refer-\nence due to heterogeneity of the model. In these cases, the cell-type\nfractions from missing tumor states would be assigned to the closest\ntranscriptional neighbor.\n\nTumor lesion areas were defined by separating connected com-\nponents (contiguous spots in tissue) of the same tumor-state cluster.\nAT1-like and AT2-like tumor-state clusters were merged for tumor lesion\narea definition because of the higher degree of mixing between these\ntumor states observed previously29 and in our analysis (Extended Data\nFig. 8a). Only lesion areas greater than six spots were kept for subse-\nquent analysis, to avoid micrometastases or regions dominated by\ntumor edges due to sectioning. This resulted in 47 and 38 tumor lesion\nareas in control and Treg cell-depleted tissue sections, respectively.\nTumor lesion areas were deemed to have an immune response in Treg\ncell-depleted tissue sections if >10% of constituent spots were part of\nan IC or IFN signaling niche (Fig. 5e).\n\nNature Immunology\n\nDifferential expression of tumor areas\nWe were interested in detecting differential gene expression between\ntumor lesion areas. We first collected all spots in tumor lesion areas (1)\nexhibiting an immune response and (2) exhibiting no response after\nTreg cell depletion (defined in the paragraph above), then performed a\nWilcoxon rank-sum test between the two groups of spots. SCTransform\nlog-normalized values were used as input and only genes detected\nin at least 10% of spots in either condition and with an average log\nfold-change value > 0.25 between conditions were tested. We detected\n259 genes that were differentially higher in responding tumors and\n142 genes that were higher in non-responding tumors at Benjamini–\nHochberg-adjusted P value < 0.01 and log fold-change > 0.5 (Fig. 5d and\nSupplementary Table 15). The SCTransform log-normalized expression\nlevels of specific genes associated with non-responsiveness to Treg cell\ndepletion are shown in Fig. 5e.\n\nStatistics\nFor all mouse experiments, statistical analyses were performed using\nGraphPad Prism 9 and are detailed in the figure legends. Mice were allo-\ncated randomly to experimental groups. No statistical methods were\nused to predetermine sample sizes but our sample sizes are similar to\nthose reported in previous publications5,13. Data collection and analysis\nwere not performed blind to the conditions of the experiments.\n\nStatistical tests used for analysis of RNA-seq and ST data are\ndescribed. For scRNA-seq and ST, count data were assumed to be distrib-\nuted according to a negative binomial distribution and log-transformed\ndata according to a normal distribution. In other analyses, data dis-\ntribution was assumed to be normal but this was not formally tested.\nscRNA-seq data analysis was performed using custom code rely-\ning primarily on Python v3.8.11 using Scanpy v1.8.1 package for basic\npre-processing and analysis. Visualization of the data was done using\nMulticoreTSNE v0.1 implementation of t-SNE in Python, and clustering\nwas done using PhenoGraph v1.5.7 package in Python. Factor analysis\nwas done using scHPF v0.5.0 implementation in Python v3.7.11. Dif-\nferential abundance testing between scRNA-seq conditions was per-\nformed using Milo v1.3.4. Identification of factors (Hungarian matching\nalgorithm) was implemented using the linear_sum_assignment module\nin optimize submodule of SciPy (v1.7.1) in Python (v3.8). For human\nfactor analysis, Spearman correlation coefficients and P values were\ncalculated in R using ggpubr (0.4.0) and results were visualized using\nggplot2 v3.3.5.\n\nReporting summary\nFurther information on research design is available in the Nature Port-\nfolio Reporting Summary linked to this article.\n\nData availability\nRaw and processed bulk, scRNA-seq and Visium data from mouse\nare available from the Gene Expression Omnibus under super series\naccession GSE202159. Human tumor scRNA-seq data are available at\nthe Human Tumor Atlas Network (HTAN) data coordinating center web\nplatform (https://humantumoratlas.org/). Source data are provided\nwith this paper.\n\nCode availability\nNo new algorithms were developed for this paper. All analy-\nsis code is available at https://github.com/dpeerlab/Treg_\ndepletion_reproducibility/.\n\nReferences\n51. Levine, A. G. et al. Stability and function of regulatory\n\nT cells expressing the transcription factor T-bet. 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This work was supported by the Irvington Cancer\nResearch Institute Postdoctoral Fellowship (to A.G.), NCI Cancer\nCenter Support grant P30 CA08748, NCI grant U54 CA209975 (to\nA.Y.R., D.P. and C.L.), NIAID grant R01AI034206 (to A.Y.R.), NCI Human\nTumor Atlas Network U2C CA233284 (to D.P.), Immunology T32\ntraining grant 5T32CA009149-44 (to S.A.R), Wrobel Family Foundation\n(to S.A.R) Alan and Sandra Gerry Metastasis and Tumor Ecosystems\nCenter at MSKCC (to R.S. and D.P.), NCI grant R35CA263816 (to\nC.M.R.), the Robert J. and Helen C. Kleberg Foundation (to C.M.R. and\nD.P.) and Ludwig Center for Cancer Immunotherapy at MSKCC. A.Y.R.\nand D.P. are HHMI investigators.\n\nAuthor contributions\nA.G., D.P. and A.Y.R. conceived the study and designed the\nexperiments. A.G., S.A.R., R.S., D.P. and A.Y.R. interpreted the data and\nwrote the manuscript. A.G. and J.A.G. performed the experiments.\nS.R., I.K.V., E.S.A., B.S.D. and Z.-M.W. assisted with cell isolation and\nin vivo tumor experiments. A.G., M.S. and S.D. performed bulk RNA-seq\nanalysis. O.C., T.X. and L.M. prepared scRNA-seq samples. S.A.R., R.S.\nand H.G. performed analysis of the scRNA-seq data. W.H. and A.M.\nassisted with imaging and analysis. S.A.R. and T.C. performed analysis\nof Visium data. G.R. performed pathological analysis. A.Q.-V., P.M.,\nJ.E., E.D.S. and C.M.R. performed scRNA-seq of human LuAd samples.\nD.P. and A.Y.R. supervised the study. Correspondence and requests for\nmaterials should be addressed to the corresponding authors.\n\nCompeting interests\nA.Y.R. is a member of SAB, and has equity in Surface Oncology, RAPT\nTherapeutics, Sonoma Biotherapeutics, Santa Ana Bio and Vedanta\nBiosciences and is an SAB member of BioInvent and Amgen; A.Y.R.\nholds a therapeutic Treg cell depletion IP licensed to Takeda. C.M.R.\nhas consulted regarding oncology drug development with AbbVie,\nAmgen, Astra Zeneca, D2G, Daiichi Sankyo, Epizyme, Genentech/\nRoche, Ipsen, Jazz, Kowa, and Merck, and is a member of the SAB of\nAuron, Bridge Medicines, Earli, and Harpoon Therapeutics. D.P is a\nmember of the SAB and has equity in Insitro. The remaining authors\ndeclare no competing interests.\n\nAdditional information\nExtended data is available for this paper at\nhttps://doi.org/10.1038/s41590-023-01504-2.\n\nan R package for analyzing mixture models. J. Stat. Softw. 32,\n1–29 (2009).\n\n70. Scrucca, L., Fop, M., Brendan, M. T. & Raftery, A. E. mclust 5:\n\nSupplementary information The online version\ncontains supplementary material available at\nhttps://doi.org/10.1038/s41590-023-01504-2.\n\nclustering, classification and density estimation using Gaussian\nfinite mixture models. R J. 8, 289–317 (2016).\n\n71. Venables, W. N. & Ripley, B. D. Modern Applied Statistics with S\n(Springer New York, 2002). https://doi.org/10.1007/978-0-387-\n21706-2\n\nAcknowledgements\nWe thank members of the A.Y.R. and D.P. laboratories for discussions,\nT. Tammela for the KP cell line, MSKCC Single Cell Analytics Innovation\nLab and Integrated Genomics Operation Core facility funded by\nthe NCI Cancer Center Support Grant (CCSG, P30 CA08748),\n\nCorrespondence and requests for materials should be addressed to\nDana Pe’er or Alexander Y. Rudensky.\n\nPeer review information Nature Immunology thanks Shannon Turley\nand the other, anonymous, reviewer(s) for their contribution to the\npeer review of this work. Primary Handling Editor: L. A. Dempsey, in\ncollaboration with the Nature Immunology team.\n\nReprints and permissions information is available at\nwww.nature.com/reprints.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 1 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 1 | Short-term Treg depletion in lung KP adenocarcinoma\nbearing mice. (a, b, d) Representative gating of normal and tumor (EpCAM)\ncells, (D) TCRβ+CD4+ (CD4), TCRβ+CD8+ (CD8) T cells, myeloid cells (MHCII+GR1-\nCD11b+), neutrophils (Neu) (MHCII-GR1+CD11b+), vascular endothelial cells\n(VEC) (CD45-CD31+GP38−), fibroblasts (Fib) (CD45-CD31-GP38+), and lymphatic\nendothelial cells (LEC) (CD45−CD31+GP38+) (A, B) in KP-lungs in diphtheria toxin\n(DT, N = 3) and PBS (Ctrl, N = 4) mice and (D) in tumor-free lungs (DT, N = 4), PBS\n(Ctrl, N = 4). (c, e) Cell frequencies in KP tumors from (A, B, D). Data represent\nmean ± SEM of one of two independent experiments. (c) Two-way ANOVA\nalpha = 0.05, Šídák’s multiple comparisons CD4 t = 2.254, df = 35 ns P = 0.1953,\nCD8 t = 1.235, df = 35 ns P = 0.8320, MHCII+/Gr1- CD11b+ (MAC/DC) t = 0.5098\ndf = 35 ns P = 0.9987, MHCII-/Gr1+ CD11b+ (Neu) t = 2.985, df = 35, * P = 0.0355,\nVEC, t = 0.2030, df = 35 ns P>0.9999, Fib t = 0.09821, df = 35 ns P>0.9999, Lec t =\n0.08549, df = 35 ns P>0.9999. (e) Two-way ANOVA, Alpha = 0.05, Tukey’s multiple\n\ncomparisons EC, t = 1.056. df = 12, ns P = 0.5261 Tumor t = 1.217, df = 12, ns P =\n0.4332. (f) Fold change (FC) deferentially expressed genes (DEG). (g) k-means\nclustering of FC DEG between DT and Ctrl. Columns - log2 FC (DT/Ctrl) for cells,\neach row is a gene. Select genes are labeled. (h) Z-score-normalized counts for\nselected genes in (G). (i) Cell frequencies in tumor-free DT and Ctrl lungs. Two-\nway ANOVA, alpha = 0.05, Šídák’s multiple comparisons. Epcam t = 0.3437, df =\n37, ns P>0.9999, CD4 t = 1.413, df = 32 ns 0.769, CD8 t = 1.434, df = 32 ns P = 0.7550,\nMHCII+/Gr1- CD11b+ (MAC/DC) t = 0.8971, df = 32, ns P = 0.9771, MHCII-/Gr1+\nCD11b+ (Neu) t = 3.664, df = 32 ** P = 0.0071, VEC t = 0.3854, df = 32 ns P>0.9999,\nFib t = 0.008845, df = 32 ns P>0.9999, LEC t = 0.01251, df = 32 ns P>0.9999. Data\nrepresent mean ± SEM of one of two independent experiments N = DT-3, PBS-3.\n(j) DEG Numbers. (k) FC DEG in cells isolated from tumor-free lungs of DT vs Ctrl\nmice. (F, J, K) DEG - differentially expressed genes (p<0.05). Red-upregulated,\nblue-downregulated.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 2 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 2 | scRNA-seq analysis and annotation of major TME cell\ntypes in lung KP adenocarcinomas. (a) Sorting strategy. CD45+ and CD45− cells\nwere sorted from lungs of PBS (Ctrl) and DT treated (48 hr) mice (3 mice per\ngroup) harboring KP lung tumors. (b) t-SNE plots embedding (27,606 cells)\nrepresenting distribution of all the cells isolated in (A), colored by PhenoGraph\nclusters (k = 30) (left), or sample (right) (related to Fig. 2a, which shows major\ncell lineages). (c) Heatmap showing the average expression of cell type specific\n\nmarkers in each cluster. The rows are genes and columns are clusters. Shown\nexpression is row normalized between 0–1 and genes are grouped to indicate\nthe subtype they typically are associated with. All the genes used for annotation\nare shown. (d) t-SNE embedding (same as B) colored by lineages inferred using\nthe average expression of each cluster shown in the heatmap in (C). (e) t-SNE\nembedding reflecting experimental conditions (Ctrl: PBS, gray; DT: diphtheria\ntoxin, red).\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 3 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 3 | Heatmaps of Treg depletion-induced gene\nexpression changes in fibroblasts, endothelial and myeloid cells in lung\nKP adenocarcinomas. (a) Heatmap showing the average expression of known\nendothelial markers in each endothelial cell cluster. Rows indicate cluster and\ncolumns indicate genes. The heatmap is column normalized between 0–1 and\nthe genes are grouped to indicate the subtype they typically are associated\n\nwith (top). t-SNE embeddings (2815 cells) (bottom) representing distribution\nof endothelial cells color coded by their cluster identity inferred using the\ngene expression pattern for each cluster shown in the heatmap (left) and cell\ntype annotation derived from the heatmap above (right). All genes used for\nannotation are shown. (b) Same as (A) for fibroblasts. (c) Same as (A) for myeloid\ncells.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 4 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 4 | Neighborhood analysis of Treg depletion-induced\ngene expression changes in endothelial cells, fibroblast and myeloid cells\nin lung KP adenocarcinomas. (a) t-SNE embedding of fibroblasts (3,791 cells)\n(top) color coded by cell subtype (left) or experimental condition (right). A\ndensity plot of the distribution of fibroblasts between conditions (bottom).\nCtrl – PBS, gray; DT - diphtheria toxin, red. (b) Graph of neighborhoods of\nfibroblast cells computed using MiloR and embedded on t-SNE (top). Each dot\nrepresents a cellular neighborhood and is color coded by the FDR corrected\np-value (alpha = 1) quantifying the significance of enrichment of DT cells\n\ncompared to control in each neighborhood. The size of the dot represents the\nnumber of cells in the neighborhood. (bottom). Swarm plot depicting the log-\nfold change in differential abundance of DT treated cells against control cells in\neach neighborhood across different fibroblast cell types. Each dot represents\na neighborhood and is color coded by the FDR corrected p-value (alpha = 1)\nquantifying the significance of enrichment of DT cells compared to control in\neach neighborhood. A neighborhood is classified as a cell type if it comprises at\nleast 80% of cells in the neighborhood, or called ‘mixed’ otherwise. (c) Same as\n(A) for myeloid cells. (d) Same as (B) for myeloid cells.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 5 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 5 | Factor analysis of Treg-dependent gene expression\nby fibroblasts and myeloid cells in lung KP adenocarcinomas. (a) Heatmap\nshowing factor cell score across experimental conditions averaged over each\nfibroblast cluster in each experimental condition. The rows are factors and\ncolumns are clusters for each experimental condition. The clusters are grouped\nbased on the cell type they are associated with. The heatmap is row normalized\nfrom 0–1. (b) Heatmaps showing the top 200 genes that correlate the most with\n\nimputed cell scores of the indicated factors (see Methods) for fibroblast subsets.\nEach column is a cell; cells are ordered based on their factor score in ascending\norder from left to right indicated by the green bar. The experimental condition\nfor each cell is indicated by the grey for PBS (Ctrl) and red for diphtheria toxin-\ntreated conditions (DT) bar. Select examples of genes of interest are noted. (c)\nHeatmap as in A for myeloid cells. (d) Heatmaps as in B for myeloid cells.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 6 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 6 | Treg-dependent gene expression changes in\nendothelial and myeloid cells in bleomycin-induced lung inflammation vs\nKP adenocarcinomas. (a) Schematic of the experimental design. (b) Numbers\nof Treg and effector T cells in Ctrl (PBS) and DT (Diphtheria Toxin) treated lungs,\nat day 21 after bleomycin administration. (Left) Two-way ANOVA, Alpha = 0.05,\nfollowed by Tukey’s multiple comparisons test was performed. PBS Ctrl vs. PBS\nBL, q = 11.66 df = 8 ***P = 0.0002, PBS Ctrl vs. DT Ctrl q = 0.9285, DF = 8, ns P =\n0.9103. PBS Ctrl vs. DT BL q = 0.1986, df = 8 ns P = 0.9989. DT Ctrl vs. DT BL q =\n0.7299 df = 8, ns P = 0.9529. Center Two-way ANOVA, Alpha = 0.05, followed by\nŠídák’s multiple comparisons test was performed. PBS Ctrl vs DT Ctrl t = 0.3479\ndf = 8 ns P = 0.9997, PBS BL vs DT BL t = 1.575 df = 8 ns P = 0.633. (Right) Two-\nway ANOVA, Alpha = 0.05, followed by Tukey’s multiple comparisons test was\n\nperformed. PBS Ctrl Vs DT Ctrl q = 00.7223 df = 8 ns P = 0.9542 PBS BL vs DT BL t\n= 0.1102 df = 8 ns P = 0.9998. A representative of two independent experiments\nwith 3 mice per group in each is shown. (c, d) t-SNE embedding of endothelial\ncells isolated from lungs of DT treated and Ctrl mice color coded by cell type\n(left) or experimental condition (middle) and density plots of the distribution\nof endothelial cells between conditions (right). (c) fibroblast, (D) myeloid cells.\n(e) Heatmaps showing the top 200 genes that correlate the most with imputed\ncell scores of the indicated factors for endothelial cells. Each column is a cell;\ncells are ordered based on their factor score in ascending order from left to right\nindicated by the green bar. The treatment condition for each cell is indicated by\ngrey (Ctrl) and red (DT) bars. Select genes of interest are shown.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 7 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 7 | Robustness and validation of BayesPrism\ndeconvolution. (a) For each cell type, Spearman’s correlation of cell fraction\nacross all spots was calculated between deconvolution using all available reads\nand 1 of 20 separate deconvolutions using the available reads downsampled to\n90%. Points represent the mean of the 20 Spearman’s correlation calculations\nand error bars are the minimum and maximum correlation values. (b)\nComparison of cell fractions across separately deconvolved serial sections. For\nall four biological samples, the average cell fraction for each cell type is plotted\nin the first serial section relative to the second. Trend line indicates a slope of 1.\nSpearman’s correlation is shown. (c) Comparison of average log cell fractions in\n\neach of 8 tissue sections using the standard scRNA-seq reference or the reference\nwith tumor RNA substituted for KP-Tracer tumor cells. Trend line indicates a\nslope of 1. Spearman’s correlation is shown. (d, e). Examples of positive spots for\ncertain populations of interest are associated with histological features. Images\nare from representative areas of control and Treg depleted tissue sections. Plots\nwith positive spots display the same example areas in the top of each panel\narrangement with the H&E stained image at lower resolution. (Br = bronchi; A/V\n= artery / vein; LV = lymphatic vessel). Analysis performed on (A-C) and images\nare representative of (D-E), one of two serial sections for each of four samples (DT\nand Ctrl two biological replicates each). One experiment was performed.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fA\n\nT\nM\nE\n\nr\ne\nt\ns\nu\nc\n\nl\n\nn\no\ni\nt\nc\na\nr\nf\n\nl\nl\n\ne\nC\n\nCtrl 1\n\nC\n\nCell fraction cluster\n\n1\n2\n3\n4\n5\n6\n7\n\nRNA %\nZ- score\n4\n2\n0\n- 2\n- 4\n\nB\n\ni\n\nn\no\ns\ns\ne\nr\np\nx\ne\n\nd\ne\nz\n\ni\nl\n\na\nm\nr\no\nn\n\ng\no\nL\n\n3\n\n2\n\n1\n\n0\n\n4\n\n2\n\n0\n\n5\n\n4\n\n3\n\n2\n\n1\n\n0\n\n1.5\n\n1.0\n\n0.5\n\n0.0\n\ne\nk\n\ni\nl\n.\n2\nT\nA\n\ne\nk\n\ni\nl\n.\n1\nT\nA\n\nc\ni\nr\nt\ns\na\nG\n\ne\nk\n\ni\nl\n.\n\nm\nr\ne\nd\no\nd\nn\nE\n\ne\nk\n\ni\nl\n.\nr\no\nt\ni\nn\ne\ng\no\nr\np\n.\ng\nn\nu\nL\n\ny\nt\ni\nc\ni\nt\ns\na\np\n.\nh\ng\nH\n\ni\n\nl\n\nHopx\n\nSftpc\n\n8\n\n6\n\n4\n\n3\n\n2\n\n1\n\n0\n\n3\n\n2\n\n1\n\n0\n\nFn1\n\nGkn2\n\nSox2\n\nGc\n\nItga2\n\nAT1.like AT2.like EMTEndoderm.likeGastricHigh.plasticity\n\nT\nM\nE\n\ne\nk\n\ni\nl\n-\n1\nT\nA\n\ne\nk\n\ni\nl\n-\n2\nT\nA\n\nc\ni\nr\nt\ns\na\nG\n\ne\nk\n\ni\nl\n-\n\nm\nr\ne\nd\no\nd\nn\nE\n\ny\nt\ni\nc\ni\nt\ns\na\np\n-\nh\ng\nH\n\ni\n\nl\n\ne\nk\n\ni\nl\n-\nr\no\nt\ni\nn\ne\ng\no\nr\np\n\ng\nn\nu\nL\n\nCtrl 2\n\nTreg depleted 2\n\nGastric\n\nEndoderm.like\n\nAT2.like\n\nAT1.like\n\nEMT\n\nLung.progenitor.like\n\nHigh.plasticity\n\nGastric\n\nEndoderm.like\n\nAT2.like\n\nAT1.like\n\nEMT\n\nLung.progenitor.like\n\nHigh.plasticity\n\n1mm\n\n1mm\n\n1mm\n\nImmune response\n\nTumor state\n\nGastric\n\nEndoderm.like\n\nTumor state\n\nAT2.like\n\nGastric\n\nAT1.like\nEndoderm.like\n\nAT2.like\nEMT\nAT1.like\n\nEMT\n\nLung.progenitor.like\n\nLung.progenitor.like\n\nHigh.plasticity\n\n-\n\n+\nTRUE\n\nCondition\n\nD\n\ns\na\ne\nr\na\nn\no\ns\ne\n\ni\n\nl\n\nf\no\nr\ne\nb\nm\nu\nN\n\n20\n\n10\n\n0\n\nAT1-like AT2-like\n\nEMT Endoderm Gastric\n\nF\n\nGm10076\n\n30\n\nPtma\n\nCtrl\nControl\n\nTreg\nTreg depletion\ndepletion\n\nHigh\nplasticity\n\nLung\nprogenitor\n\nTnfrsf12a\n\nH3f3b\n\nF3\n\nIfrd1\n\nNr1d1\n\nGadd45b\n\nAtf4\n\nCcnl1\n\nClk4\n\nCoq10b\n\nSrsf11\n\nWsb1\n\nCxcl16\n\n20\n\nP\nd\ne\nt\ns\nu\nd\na\n\nj\n\n0\n1\ng\no\nl\n-\n\n10\n\nHbb- bs\n\nHspa8\n\nTle5\n\nGm10073\n\nGm9843\n\nTubb5\n\nSec61b\n\nCcl21a\n\nFtl1- ps1\n\nFkbp1a\n\nTmsb4x\n\nFkbp4\nCrip2\n\nSt3gal4\n\nTrpt1\n\nEif3f\n\nEno1\n\nHsp90ab1\n\nGm10250\n\nStmn1 Gm2000\nGm9493\n\nGm11808\n\nLrrc58\nSh3bgrl3\n\nGstp1\n\nPtms\nRnf5\n\nPrdx2\n\nRtraf\n\nFkbp2\n\nAldoa\n\nTpi1\n\nKrtcap2\n\nMgst3\n\nHsp90b1\n\nTuba1b\nGsta4\nMettl7a1\n\nLdha\n\nAtpif1\n\nDnaja1\n\nSfn\n\nHspb1\n\n0\n-2.5\n\nMapk3\n\nSnrpb\n\nSlc25a25 Ppp1r15a\n\nH4c8\n\n5430416N02Rik\n\nU2af1\n\nGigyf2\n\nZkscan5\n\nEwsr1\nSnhg16\n\nSrsf3\nChtop\n\nRsrc2\n\nUbl3\nNop58\n\nEif5\nSlc6a14\nXpc\n\nKlf5\n\nActn1\n\nNsrp1\n\nTra2b\nMafk\n\nNfkbia\nKlf6\n\nRcan1\nMaff\n\nCcl2\n\nSntb2\n\nFabp3\nPlk2\n\nPtpn14\n\nNrg1\n\nMab21l4\n\nDazl\n\nOser1\n\nPhlda1\n\nEgr1\n\nGc\n\nSprr1a\n\nLog2 fold-change Treg depleted - Control\n\n0\n\n2.5\n\ns\na\ne\nr\na\nn\no\ns\ne\n\ni\n\nl\n\nf\no\nr\ne\nb\nm\nu\nN\n\n10\n\n5\n\n0\n\nE\n\nG\n\nt\ne\ns\n\ne\nn\ne\ng\n\nr\ne\nt\ns\nu\nc\n\nl\n\nr\no\nm\nu\nt\n\nl\n\na\nt\ne\n\ni\n\nc\nv\no\nn\na\nj\nr\na\nM\n\nAT1-like\n\nAT2-like\n\nGastric\n\nHigh\nplasticity\n\nLung\nprogenitor\n\nHPCS\n\n0\n\n5\n\n10\n\n-log10 adjusted P\n\nExtended Data Fig. 8 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\n\n\fExtended Data Fig. 8 | Spatial transcriptomic analysis of tumor cell states\nperturbed in response to Treg cell depletion. (a) Hierarchical clustering of\ntumor spots by tumor state RNA fractions. (b) Log normalized expression of\ntumor state marker genes in assigned spot clusters from A. (c) H&E staining of\n3 independent KP tumor sections (in addition to those shown in Fig. 5b) with\ntumor spots denoted by their assigned cluster in A. (d) Number of tumor lesion\nareas identified across all lung tumor states in control or Treg depleted mice\n(85 tumor lesions total). (e) Number of tumor lesion areas identified in Treg\ndepleted sections across all tumor states colored by immune response status\nin Treg depleted mice (N = 38 tumor lesion areas). (f) Differentially expressed\n\ngenes (Wilcoxon test BH adjusted) between tumor cells between control and Treg\ndepleted conditions. (N = 239 cells total). (g) GSEA of differentially expressed\ngenes in F within gene sets defined by different tumor clusters identified\nin Marjanovic et al.29 which partially align with tumor states identified by\ndeconvolution. Dashed line indicates adjusted p-values <0.05. (NES = normalized\nenrichment score; HPCS = high plasticity cell state). Analysis performed on (A-D,\nB-G) and images are representative of (C), one of two serial sections for each of\nfour samples (DT and Ctrl two biological replicates each). One experiment was\nperformed.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 9 | Clustering and cell linage annotation of scRNA-seq\ndatasets of human lung adenocarcinomas. (a) Heatmap displaying genes used\nto determine lineage assignments for single cell PhenoGraph clusters in human\nLuAd samples. Color bar represents the mean log normalized gene expression\n\nin each PhenoGraph cluster scaled from 0 to 1 for each gene. (b) Global t-SNE\nembedding across of all human lineages as in Fig. 4b colored by sample. ID.\nAll genes used for annotation are shown. (c, d) t-SNE embeddings of the T/NK\nlineage colored by PhenoGraph cluster (C) or sample ID (D).\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 10 | See next page for caption.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\fExtended Data Fig. 10 | Association of Treg abundance with transcriptional\nfeatures of endothelial cells in human LuAd and loadings of human and\nmouse fibroblast and endothelial cell factors. (a) Treg proportion of\nhematopoietic cells (CD45+) calculated from scRNA-seq data across all samples.\n(b) Treg proportion of hematopoietic cells compared to the Treg proportion of\nCD3+ cells across all human samples. (c) Mean log2 cell loading of CAR4+ capillary\n(factor 3) and other inflammation/hypoxia associated human endothelial factors\n(4,5) plotted against log2 Treg proportion in each patient sample. Spearman\ncorrelation estimate (R) and p value are listed. Trend line represents a linear\nmodel fit between the two and shading indicating the 95% confidence interval.\n(d) t-SNE of human endothelial cells colored by factor 3, 4, or 5 cell loading\n\n(max 2.5) or sample ID. (N = 19 patient samples). (e,g) Heatmap showing Jaccard\nsimilarity of genes associated with human and mouse fibroblast (E) or myeloid\n(G) factors. (f,h,i) Mean log2 cell loading of factors negatively associated with\nTreg frequency in fibroblasts (F) and myeloid cells (H), or positively associated\nin myeloid cells (I) plotted against log2 Treg proportion in each patient sample.\nSpearman correlation estimate (R) and p value are listed. Trend line represents\na linear model fit between the two and shading indicating the 95% confidence\ninterval. (fibroblast N = 20; myeloid N = 23). (j) Heatmap showing the Spearman’s\ncorrelation between Treg cell frequency associated human factors with\nconserved trends in mouse Treg-depletion.\n\nNature Immunology\n\nArticlehttps://doi.org/10.1038/s41590-023-01504-2\f\fβ\n\n\fβ\n\n\f μ\n\n"
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10.1103_physrevx.12.021038.pdf
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PHYSICAL REVIEW X 12, 021038 (2022)
Defining Coarse-Grainability in a Model of Structured Microbial Ecosystems
Jacob Moran
Department of Physics, Washington University in St. Louis, St. Louis, Missouri, USA
Mikhail Tikhonov *
Department of Physics and Center for Science and Engineering of Living Systems,
Washington University in St. Louis, St. Louis, Missouri, USA
(Received 5 August 2021; revised 14 March 2022; accepted 13 April 2022; published 16 May 2022)
Despite their complexity, microbial ecosystems appear to be at least partially “coarse-grainable” in that
some properties of interest can be adequately described by effective models of dimension much smaller
than the number of interacting lineages. This is especially puzzling, since recent studies demonstrate that a
surprising amount of functionally relevant diversity is present at all levels of resolution, down to strains
differing by 100 nucleotides or fewer. Rigorously defining coarse-grainability and understanding the
conditions for its emergence is of critical importance for understanding microbial ecosystems. To begin
addressing these questions, we propose a minimal model for investigating hierarchically structured
ecosystems within the framework of resource competition. We use our model to operationally define
coarse-graining quality based on reproducibility of the outcomes of a specified experiment and show that a
coarse-graining can be operationally valid despite grouping together functionally diverse strains.
Furthermore, we demonstrate that a high diversity of strains (while nominally more complex) may, in
fact, facilitate coarse-grainability and that, at least within our model, coarse-grainability is maximized when
a community is assembled in its “native” environment. Our modeling framework offers a path toward
building a theoretical understanding of which ecosystem properties, and in which environmental
conditions, might be predictable by coarse-grained models.
DOI: 10.1103/PhysRevX.12.021038
Subject Areas: Biological Physics, Statistical Physics
I. INTRODUCTION
Microbial communities are complex dynamical systems
composed of a highly diverse collection of interacting
species, and yet they often appear to be at least partially
“coarse-grainable,” meaning that some properties of inter-
est can be predicted by effective models of dimension
much smaller than the number of interacting lineages. For
example, industrial bioreactors consisting of hundreds of
species are well described by models with ≲10 functional
classes [1,2]. What makes this possible? One potential
explanation is that coarse-grainability is a direct conse-
quence of the hierarchically structured trait distribution
across organisms. If 100 interacting phenotypes are all
close variants of only ten species, which can be further
grouped into just two families, it is natural to expect that the
diverse community might be approximately described by a
*[email protected]
Published by the American Physical Society under the terms of
license.
the Creative Commons Attribution 4.0 International
Further distribution of this work must maintain attribution to
the author(s) and the published article’s title, journal citation,
and DOI.
However, recent data reveal
two- or ten-dimensional model. Under this view, effective
models are possible because ecosystems are less diverse
than a naïve counting of microscopic strains might suggest.
this intuition to be too
simplistic: A surprising extent of relevant diversity persists
at all levels of resolution. Numerous studies highlight the
role of strain-level variation in shaping the functional
repertoire of a microbial population [3–8]. A recent work
by Goyal et al. concludes that strains might indeed be “the
relevant unit of interaction and dynamics in microbiomes,
not merely a descriptive detail” [9]. Surprisingly, however,
a greater strain diversity can sometimes enhance predict-
ability instead of undermining it [10]. Equally puzzling, the
notion of a bacterial species is undoubtedly useful, despite
collapsing together strains that famously may collectively
share only 20% of their genes [11]. Moreover, by some
assessments, the species-level characterization of a com-
munity appears to be too detailed and can be coarse-grained
further [12], e.g., to the level of a taxonomic family [13].
Rigorously defining coarse-grainability and understand-
ing the conditions for its emergence is of critical impor-
tance: Harnessing coarse-grainability is our main instrument
for understanding, predicting, or controlling the behavior
of these complex systems. Can an ecosystem be coarse-
grainable for some purposes but not others? Or in some
2160-3308=22=12(2)=021038(14)
021038-1
Published by the American Physical Society
JACOB MORAN and MIKHAIL TIKHONOV
PHYS. REV. X 12, 021038 (2022)
environments but not others? Can we ever expect the coarse-
grained descriptions derived in the simplified environ-
ment of a laboratory to generalize to the complex natural
conditions? Addressing this exciting set of general ques-
tions is an important challenge at the interface of theoretical
microbial ecology and statistical physics.
Here, we introduce a theoretical framework to begin
addressing these questions. The novelty of our approach is
twofold. First, we propose a minimal model for investigat-
ing structured ecosystems. Much recent work studies the
behavior of large microbial ecosystems in the unstructured
regime, where the traits of interacting organisms are drawn
randomly (see, e.g., [14–18]). However, real ecosystems
assemble from pools of taxa whose trait distributions are
highly nonrandom due to functional constraints, common
selection pressures, or common descent. These factors
create structure at all levels, from the distribution of genes
across strains in microbial pangenomes [19–21] to the
distribution of function across taxa [12,22,23], with impor-
tant implications for dynamics, patterns of coexistence, or
responses to perturbations [24–27]. In natural communities,
taxa can often be grouped by identifiable functional roles,
often represented by closely related species or strains. As
we seek to define and characterize ecosystem coarse-
grainability, it seems clear that this structure must play
an important role. Our model implements such structure
within a consumer-resource framework in a simple, prin-
cipled way through trait interactions.
The second novelty of our approach is a framework
for defining and evaluating a hierarchy of coarse-grained
descriptions. The ultimate performance criterion for a
coarse-graining scheme would be its ability to serve as a
basis for a predictive model, capable of predicting ecosys-
tem dynamics or properties. However, finding the “most
predictive model” is a difficult problem. Here, as a simpler
first step, we propose an operational approach which is
inspired by the experiments in Ref. [13] and is based on the
reproducibility of experimental outcomes. Specifically, we
focus on a particular form of coarse-graining in which taxa
are grouped together
into putative functional groups.
Grouping means omitting details, and we say that details
are safe to ignore if they do not change the outcome of
some specified experiment.
Importantly, as we show,
choosing different experiments changes which, or whether,
details can be ignored.
Specifically, we define how ecosystems can be coarse-
grainable in the weak sense, where a desired performance
of a coarse-graining can be achieved in a given environ-
ment, and in the strong sense, where the performance of a
given coarse-graining is maintained even as environment
complexity is increased. We demonstrate that the same
ecosystem can be coarse-grainable under one criterion—
even in the strong sense—and not at all coarse-grainable
under another. This reconciles the apparent paradox men-
tioned above, showing that a coarse-graining can be
operationally valid for some purposes, despite grouping
together functionally diverse strains. We explain how
strong-sense coarse-grainability arises in the model con-
sidered here and show that this property is context specific:
A coarse-graining that works in the organisms’ natural
ecoevolutionary context is easily broken if the community
is assembled in the non-native environment or if the natural
ecological diversity is removed. Finally, we discuss the
extent to which our findings generalize beyond our model.
II. AN ECOEVOLUTIONARY FRAMEWORK FOR
A HIERARCHICAL DESCRIPTION OF THE
INTERACTING PHENOTYPES
In order to study the hierarchy of possible coarse-
graining schemes for ecosystems, we need an eco-
evolutionary framework that would describe players
functionally, by a list of characteristics that can be made
longer (more detailed) or shorter (more coarse-grained). In
addition, for our purposes we also want an ability to tune
the complexity of the environment, for example, to study
the robustness of a coarse-graining between the simplified
conditions of a laboratory and the more complex natural
environment. In this section, we present our model imple-
menting these two requirements.
A. The ecoevolutionary dynamics
A given environment presents various opportunities that
organisms can exploit to gain a competitive advantage.
Imagine a world where all such opportunities or “niches”
are enumerated with index i ∈ f1…L∞g. The notation L∞
highlights that, in general, one expects this to be a very
in
large number, corresponding to a complete (and,
practice, unattainable) microscopic description. A strain
μ is phenotypically described by enumerating which of
these opportunities it exploits, i.e., by a string of numbers
of length L∞ which we denote σμi. For simplicity, we
assume σμi to be binary (σμi ∈ f0; 1g): Strain μ either can
or cannot benefit from opportunity i. This allows us to think
of evolution as acting via bit flips 0 ↦ 1 and 1 ↦ 0,
corresponding to the acquisition or loss of the relevant
machinery (“trait i”) via horizontal gene transfer events or
loss-of-function mutations.
We assume that the fitness benefit from carrying trait i is
largest when the opportunity is unexploited and declines as
the competition increases. For a given set of phenotypes
present in the community, the ecological dynamics are
determined by the feedback between strain abundance and
the strain
opportunity exploitation [Fig. 1(a)]. Briefly,
abundances Nμ determine the total exploitation level Ti ≡
P
μ Nμσμi of opportunity i. The exploitation level deter-
mines the fitness benefit hi ≡ hiðTiÞ from carrying the
respective trait; we choose hiðTiÞ of the form hiðTiÞ ¼
½bi=ð1 þ Ti=KiÞ(cid:2). These hi, in turn, determine the growth
or decline of the strains. Specifically, we postulate the
following ecological dynamics:
021038-2
DEFINING COARSE-GRAINABILITY IN A MODEL OF …
PHYS. REV. X 12, 021038 (2022)
(a)
(b)
(c)
FIG. 1. Our ecoevolutionary framework modifies a standard
model of resource competition. Organisms engage in ecological
competition for limited resources and evolve by gaining or losing
traits. Carrying a trait incurs a cost but enables the organism to
benefit from the corresponding resource. Here, our novelty is to
consider how traits interact with each other. Combinations that
interact unfavorably are costly to maintain; as a result, not all
phenotypes are competitive. (a) A metabolic interpretation of our
model corresponds to an ecosystem in a chemostat. A set of
strains with abundances fNμg compete for a set of substitutable
resources indexed by i, e.g., alternative sources of carbon. In this
interpretation, Ki correspond to resource supply rates, and hi are
the resource concentrations in the effluent. (b) For this work, we
adopt a more general interpretation where the resources i need not
be specifically metabolic. Instead, we think of i as enumerating
any depletable environmental opportunities that the phenotypes
can exploit, which confer a benefit hi that declines with exploi-
tation level Ti. We parametrize this dependence by the maximum
benefit bi and the carrying capacity Ki (the exploitation level
where the benefit is halved); see the text. (c) In our model,
phenotypes are binary vectors described by traits they carry. The
most competitive phenotypes (rows in the cartoon) are not
random but are shaped by pairwise trait interactions Jij. Strongly
synergistic traits (Jij > 0)
tend to cooccur, while strongly
antagonistic traits (Jij < 0) are likely not carried together. Such
structured phenotypes lead to structured ecosystems, as we
investigate.
_Nμ
Nμ
¼
X
σμihi − χμ
i
strain abundance;
ð1aÞ
bi
1 þ Ti=Ki
benefit from carrying i;
ð1bÞ
Ti ≡
Nμσμi
exploitation of i:
ð1cÞ
hi ¼ hðTiÞ ≡
X
μ
In these equations, the parameters bi and Ki describe the
environment, with bi being the fitness benefit of being the
first to discover the opportunity i (at zero exploitation
Ti ¼ 0) and the “carrying capacity” Ki describing how
quickly the benefit declines as the exploitation level Ti
increases [Fig. 1(b)]. The quantities χμ are interpreted as the
“maintenance cost” of being an organism carrying a given
set of traits; more on this below.
The dynamics (1) is basically the MacArthur model of
competition for L∞ substitutable “resources” [28–30]. To
these dynamics, we add the stochastic arrival of new
phenotypes arising through bit flips (“mutations”), as is
standard in studies of adaptive dynamics. The combined
ecoevolutionary process is simulated using a hybrid dis-
crete-continuous method as described in Supplemental
Material [31]. As presented so far, our ecoevolutionary
model is similar to, e.g., Ref. [36]; our key novelty (trait
interactions) is introduced in the next section. We note,
however, that typically the interpretation of resources in
models like (1) is metabolic [16,18,37–40]; for example, i
might label the different forms of carbon available to a
carbon-limited microbial community. Here, we adopt a
more general perspective, where i labels any depletable
environmental opportunity, which need not be specifically
metabolic.
As an example, one way for a strain to survive in
chemostat conditions is to develop an ability to adhere to
the walls of the device [41]. The wall surface is finite and
provides an example of a nonmetabolic limited resource.
Similarly, being physically bigger or carrying a rare toxin
could be a useful survival strategy, but in both cases the
benefit decreases as the trait becomes widespread in the
community. Unlike the forms of carbon, which may be
numerous but are certainly countable and finite, the list
of exploitable opportunities of this kind could be arbitrarily
long (L∞ → ∞), especially when considering the com-
plexity of natural microbial environments. Note that, by
construction, our model allows coexistence of a very large
number of phenotypes. In many studies, explaining such
coexistence is the aim; here, it is our starting point. Rather
than asking how a given environment enables coexistence
of a diverse community, we start from the observation
that natural communities are extremely diverse, interpret
this as evidence for the existence of a very large number of
(potentially unknown) limiting factors, and ask whether
such diversity of types can be usefully coarse-grained.
(1a)]
is
certainly a simplification. It is also worth noting that the
model (1) is special in that it possesses a Lyapunov function
[42]; we return to this point below. Nevertheless, this is a
good starting step for our program, namely, understanding
the circumstances under which coarse-grained descriptions
are adequate. Most crucially, a suitable choice of the cost
model χμ allows us to naturally obtain communities with an
Modeling fitness benefits as additive [Eq.
021038-3
JACOB MORAN and MIKHAIL TIKHONOV
PHYS. REV. X 12, 021038 (2022)
hierarchical structure of trait distributions across organisms
mimicking that of natural biodiversity.
B. A simple cost model leads to hierarchically
structured communities
Several studies investigated dynamics like (1) with costs
assigned randomly (see, e.g., [15–18,40,43]). Here, we
seek to build a model where the phenotypes in the
community are not random but are hierarchically struc-
tured, reproducing phenomena such as divergent
taxa
belonging to identifiable functional groups, the fine-scale
strain diversity found within a species, or the notion of
“core” and “accessory” traits in a bacterial pangenome [44].
For this, consider the following cost structure:
X
X
χμ ¼ c þ
χiσμi −
Jijσμiσμj:
ð2Þ
i
i<j
The parameter c encodes a baseline cost of essential
housekeeping functions (e.g., DNA replication). χi is the
cost of carrying trait i (e.g., synthesizing the relevant
machinery); for most of our discussion, we set c ¼ 0.1
and set all χi ≡ χ0 ¼ 0.5 for simplicity. The key object for
us is the matrix Jij, which encodes interactions between
traits and shapes the pool of viable (low-cost) phenotypes
in our model,
[Fig. 1(c)]. As an example, the enzyme nitrogenase is
inactivated by oxygen, so running nitrogen fixation and
oxygen respiration in the same cell requires expensive
infrastructure for compartmentalizing the two processes
from each other;
this corresponds to a
strongly negative Jij (carrying both traits is costly). An
example for the opposite case of a beneficial interaction
(positive Jij) is a branched catabolic pathway, where
sharing enzymes
to produce common intermediates
reduces the cost relative to running the two branches
independently. Crucially, in our model, the parameters c,
χi, and Jij are the same for all organisms; we refer to
them as encoding the “biochemistry” of our ecoevolu-
tionary world.
We now make our key choice. To set Jij, we generate a
random matrix of progressively smaller elements, as
illustrated in Fig. 2(a). Specifically, we draw the ele-
ment Jij out of a Gaussian distribution with zero mean
and standard deviation J0f½maxði; jÞ(cid:2), with a sigmoid-
shaped fðnÞ ¼ 1=f1 þ exp ½ðn − n(cid:3)Þ=δ(cid:2)g [see Fig. 2(b)].
this work, we set J0 ¼ 0.2, n(cid:3) ¼ 10, and
Throughout
δ ¼ 3. As we will see, this choice for the interaction matrix
J implements a hierarchically structured distribution of
traits. Intuitively, since high-cost phenotypes are poor
the interactions Jij as
competitors, we can think of
FIG. 2. A simple model of trait interactions leads to hierarchically structured ecosystems. (a),(b) In our model, the traits carried by a
given phenotype interact with each other to determine its “maintenance cost” (see the text). The matrix of pairwise trait interactions Jij is
drawn randomly and is the same for all phenotypes, encoding the “biochemical constraints”; (a) shows an example (Jij is triangular with
one element per trait pair i ≠ j). We assume an interaction structure such that a few traits interact strongly while others interact weaker
and weaker (b). (c) An example of ecoevolutionary dynamics generated in our model. Shading corresponds to different phenotypes.
Although new strains continue to emerge and die out throughout the period shown, they can be grouped into several coarse-grained types
of approximately stable abundance (one is highlighted in color). (d) The phenotypes present at the end point of the trajectory shown in
(c). Each of 27 phenotypes is a row of length L∞ ¼ 40 (white pixels are carried traits). The seven highlighted strains are identical in traits
1–24. We say that they belong to the same “L(cid:3)-type,” for level of coarse-graining L(cid:3) ¼ 24. (e) The number of L(cid:3)-types in the community
in (d), shown as a function of L(cid:3). At a coarse-grained level, the community appears to consist of only four types [one of these is
highlighted in (c) using color]; resolving finer substructure requires L(cid:3) > 15. (f),(g) The same as (d) and (e) for a broader set of strains,
pooled over Menv ¼ 50 similar environments. The hierarchical structure is maintained (if the trait matrix were randomized, the number
of L(cid:3)-types would grow exponentially; see the dashed line). Here, we ask: In what sense, if any, could the phenotypic details beyond
L(cid:3) ≈ 20–25 be coarse-grained away in this model?
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determining the “sensible” trait associations. For strongly
interacting traits, only some combinations are competitive,
resulting in traits that are mutually exclusive (Jij < 0)
or that frequently cooccur (Jij > 0) in low-cost (viable)
phenotypes [Fig. 1(c)]. In contrast, a weakly interacting
trait can be gained, be lost, or remain polymorphic, as
dictated by the environment. An example might be a gene
encoding a costly pump that enables the organism to live in
otherwise inaccessible (toxin-laden) regions of the habitat.
Such a trait is “weakly interacting” if the cost of running the
pump does not significantly depend on the genetic back-
ground. As we will see, our model naturally gives rise to
hierarchically structured sets of phenotypes that share some
“core” functions but differ in others to form finer-scale
diversity, resembling the notions of core and accessory
traits of a bacterial pangenome [44].
C. Environment defines a strain pool
To build some intuition about
the model defined
above, consider Fig. 2(c) that shows an example of these
ecoevolutionary dynamics for one random biochemistry,
and an environment where we set bi ≡ b0 ¼ 1 for sim-
plicity, and Ki ¼ K0 ¼ 1010 to set the scale of population
size as appropriate for bacteria. Grayscale shading corre-
sponds to distinct phenotypes; the community is initialized
with a single (randomly drawn) phenotype. The dynamics
of Fig. 2(c) illustrate that our framework allows us to define
a form of ecosystem stability where all
the original
phenotypes may have gone extinct and were replaced by
others, and yet at a coarse-grained level the ecosystem
structure remains recognizably “the same.” Here, starting
from about
the dynamics resemble a stable
coexistence of several coarse-grained “species” (one is
highlighted in color), whose overall abundance remains
roughly stable even as individual strains continue to emerge
and die out. To formalize this observation, we need
the notion of coarse-grained “L(cid:3)-types”, which we now
introduce.
t ≃ 105,
As we continue the simulation, the dynamics converge to
an ecoevolutionary equilibrium (a state where the coexist-
ing types are in ecological equilibrium and no single-bit-
flip mutant can invade). In this example, it consists of 27
coexisting phenotypes and is shown in Fig. 2(d). Note that,
it appears to possess a
confirming our expectations,
hierarchical structure. The seven highlighted strains are
identical over the first 24 components and differ only in the
“tail” (components 25–40). A coarse-grained description
that characterizes organisms only by the first L(cid:3) ¼ 24 traits
would be unable to distinguish these strains; we say that
these strains belong to the same L(cid:3)-type with L(cid:3) ¼ 24.
Figure 2(e) plots the number of L(cid:3)-types resolved at
different levels of coarse-graining L(cid:3) [within the commu-
nity shown in Fig. 2(d)]. For L(cid:3) ¼ 3–15, the number of
types remains stable at
the color in Fig. 2(c)
just 4;
highlights one of them. Beyond L(cid:3) ¼ 15, adding more
details begins to resolve additional types, up until L(cid:3) ¼ L∞
when the number of L(cid:3)-types coincides with the total
number of microscopic strains.
Of course, when discussing the diversity of strains one
expects to find in a given environment, it is important to
remember that no real environment is exactly static, and no
real community is in evolutionary equilibrium. To take this
into account while keeping the model simple, we consider
not a single equilibrium but a collection of communities
assembled in M
env ¼ 50 similar environments where we
randomly perturb the carrying capacity of all opportunities
[Ki ¼ K0ð1 þ ϵηiÞ, with ϵ ¼ 0.1 and ηi are independent
identically distributed from a standard Gaussian]; see
Supplemental Material [31]. Figure 2(f) shows the set of
strains pooled over the 50 ecosystems assembled in this
way. This strain pool is the central object we seek to coarse-
grain. We stress that its construction explicitly depends on
the particular
the environment. (Or, more specifically,
random set of M
env ¼ 50
is large enough that the results we present are robust to their
exact choice.)
env similar environments, but M
As we see in Fig. 2(f), adding more strains to the pool
makes its hierarchical structure even more apparent.
Quantitatively, the number of L(cid:3)-types [Fig. 2(g)] grows
much slower than if the traits of each phenotype were
randomly permuted (the dashed control curve): Micro-
scopically, perturbing the environment favors new strains,
but at a coarse-grained level, these new strains are variations
of the same few types. This is precisely the behavior that we
aim to capture in our model. Beyond L(cid:3) ≈ 20–25, the number
of resolved types begins to grow rapidly. Can this diversity be
coarse-grained away? Is there a precise sense in which these
tail-end traits are “just details”? To answer this question, we
must begin by making it quantitative.
III. COARSE-GRAINING
A. Methodology for defining coarse-grainability
The L∞-dimensional description we define represents the
complete list of niches and opportunities present in a natural
habitat. Any recreation in the laboratory is simplified,
retaining only some of the relevant factors. We model
simplified environments as including resources or oppor-
tunities 1 through L [Fig. 3(a)]. The parameter L represents
environment complexity. The other key parameter is the level
of coarse-graining detail, L(cid:3) [Fig. 3(b)]. For each L(cid:3), the
identity and combined abundance of L(cid:3)-types provides a
candidate coarse-grained description of the ecosystem. We
seek a quantitative metric for assessing its quality.
Ideally,
this assessment would be a comparison of
performance of two models—one highly detailed, the other
coarse-grained—and our test would evaluate the prediction
error for a given property of interest. However, what we
build is not a coarse-grained model but a hierarchy of
coarse-grained variables. These variables could be used to
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(a)
(b)
(c)
(d)
(e)
(f)
FIG. 3. Defining weak and strong coarse-grainability. (a) The complex natural habitat is modeled as including a large number L∞ of
exploitable resources or opportunities. In a laboratory, we can consider a sequence of ever-more-detailed approximations including
resources 1; …; L (with the remaining ones set to zero). (b) For each environment, the model describes the pool of strains we expect to
encounter [the pool of “L-strains”; see Fig. 2(f)]. For a given L, the strains are unlikely to carry traits i for resources not provided
(i > L). As environment complexity L increases, the pool becomes increasingly diverse. (c) The set of L-strains can be coarse-grained to
a varying level of detail L(cid:3) ≤ L. Let QðL; L(cid:3)Þ be any quantitative metric (to be defined later) scoring the quality of the L(cid:3)-coarse-
graining in the environment of complexity L. At L(cid:3) ¼ L, the strain diversity is fully resolved (no coarse-graining). The “coarse-
grainability” of the ecosystem is encoded in the behavior of QðL; L(cid:3)Þ at L(cid:3) < L. Different metrics Q encode different operational
definitions of coarse-grainability. (d) A non-coarse-grainable ecosystem (sensu quality metric Q). The coarse-graining quality remains
poor unless the microscopic strain diversity is fully resolved (at L(cid:3) ¼ L). (e) Weak-sense coarse-grainability: In any given environment
(a fixed L, highlighted), a desired quality can be achieved with a coarser-than-microscopic description (L(cid:3) < L). (f) Strong-sense
coarse-grainability: The same coarse-graining (a fixed L(cid:3), highlighted) provides the desired quality even as the environment complexity
is increased.
build any number of models, and identifying the most
predictive of these is a highly nontrivial task. Here, we
sidestep this problem by proposing an operational approach
that evaluates a coarse-graining based on the reproducibil-
ity of outcomes of a specified experimental protocol.
We will describe and contrast two protocols, each of
which could be seen as verifying the validity of the coarse-
graining and each yielding its own metric of coarse-
graining quality QðL; L(cid:3)Þ; see Fig. 3(c). The “diagonal”
entries of Q (with L(cid:3) ¼ L) correspond to an absence of
coarse-graining: The description of strains resolves all the
traits relevant in a given environment. Coarse-grainability
is encoded in the behavior of QðL; L(cid:3)Þ with L(cid:3) < L
[Figs. 3(d)–3(f)]. Consider first the behavior of QðL; L(cid:3)Þ
as a function of L(cid:3), with L fixed. If we observe that, in a
given environment, sufficient quality can be achieved
already with L(cid:3) < L, we say that
the ecosystem is
coarse-grainable in the weak sense. For strong-sense
coarse-grainability, we ask if the same coarse-grained
description continues to perform well even as the environ-
ment is made more complex (i.e., instead of fixing L and
varying L(cid:3), we fix L(cid:3) and vary L). Strong-sense coarse-
grainability would be a highly desirable property, but
a priori it is unclear if it is even theoretically possible.
Crucially, these definitions depend on the choice of the
operational criterion for assessing coarse-graining validity
(the experiment whose results we require to be reproduc-
ible). Below, we show that the same ecosystem can be
coarse-grainable in the strong sense under one criterion and
yet not coarse-grainable at all under another.
B. Operational definitions of coarse-graining
quality QðL; L(cid:3)Þ
In this section, we describe two “experimental” proto-
cols, each of which could be seen as a sensible test of the
quality of a coarse-graining. They establish two alternative
criteria for a coarse-graining to be operationally valid,
which we then contrast.
1. The reconstitution test
One possible criterion is the reconstitution test. Drawing
a random representative for each of the L(cid:3)-types in the strain
pool, we seed an identical environment with the represent-
atives we choose, allowing them to reach an ecological
equilibrium [Fig. 4(b)]. If the details ignored by the
coarse-graining are indeed irrelevant, we expect such
“reconstituted” replicates to all be alike. If the reconstituted
communities are found to be highly variable depending on
exactly which representative we happen to pick, this signals
that
in fact,
significant.
the distinctions we attempt
to ignore are,
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(a) Candidate coarse-graining
(b)
Reconstitution test
Coarse-graining quality
Good
Poor
One strain
of each
L*-type
(c) Leave-one-out test
Invade with
Invade with
Invade with
Repeat for each L*-type
Time
Time
FIG. 4. Specific criteria for assessing coarse-graining quality
QðL; L(cid:3)Þ. (a) In this cartoon, the community is coarse-grained
into three operational taxonomic units (OTUs), implemented in
our model as L(cid:3)-types. (b) The reconstitution test. Under this
criterion, grouping strains into coarse-grained OTUs is justified if
reconstituting a community from a single representative of each
OTU yields similar communities regardless of which represent-
atives we pick. As a quantitative measure, we compare the OTU
abundances across replicates. (c) The “leave-one-out test.” Under
this criterion, grouping strains into coarse-grained OTUs is
justified if the strains constituting OTU X (green in this cartoon)
all behave similarly when introduced into a community missing
X. As a quantitative measure, we compare the invasion rates of
the left-out strains.
μ
μ
(cid:3)
(cid:3), let us denote nðαÞ
Quantitatively, for each L(cid:3)-type μ
its
final relative abundance (i.e., the fraction of total popula-
tion size) in the reconstituted replicate α. The coefficient of
variation of nðαÞ
(cid:3) over α (denoted CVα½nðαÞ
(cid:3) (cid:2)) provides a
μ
natural measure of variability across replicates. To combine
these into a single number, we compute the average such
variability over all L(cid:3)-types μ
(cid:3), weighted by their mean
relative abundance across replicates (denoted hnðαÞ
X
(cid:3) iα):
μ
Q
rec ¼
μ
(cid:3)
hnðαÞ
(cid:3) iαCVα½nðαÞ
(cid:3) (cid:2):
μ
μ
Since the coefficient of variation is, by definition,
(cid:3) (cid:2)=hnðαÞ
(cid:3) (cid:2) ¼ stdα½nðαÞ
CVα½nðαÞ
(cid:3) iα, our metric simplifies to
μ
μ
μ
P
stdα½nðαÞ
Q
(cid:3) (cid:2). With this definition, a perfect recon-
rec ¼
stitution has Q
rec ¼ 0. Conveniently, this is automatically
the case if L(cid:3) ¼ L (no coarse-graining).
μ
(cid:3)
μ
2. The leave-one-out test
As we will see, the criterion defined above is extremely
stringent and is rarely satisfied. In this section, we introduce
a weaker version. Instead of the composition of the
entire community, we explicitly focus on one particular
property of interest (below, the invasion rate of a strain).
Furthermore,
instead of requiring the grouped-together
strains to be interchangeable in absolute terms, we ask
that they behave similarly in the context of the assembled
community.
Specifically, for a given scheme grouping strains into
coarse-grained types, consider assembling a community
missing a particular coarse-grained type μ
(cid:3) [the ecological
equilibrium reached when combining all the strains in the
pool, except those belonging to type μ
(cid:3); see Fig. 4(c)]. We
judge the coarse-graining as valid if the different strains
constituting the missing type μ
(cid:3) all behave similarly when
introduced into this community. As one example, we can
compare their initial growth rates if introduced into the
community at low abundance, called henceforth “invasion
rate” (other possible choices include the abundance the
strain reaches if established or the level of niche exploi-
tation hi in the resulting community; these are shown in
Supplemental Material [31]). If the invasion rates are
similar, describing the community as missing the coarse-
grained type μ
the
invasion rates vary strongly, we conclude that the features
our coarse-graining is neglecting are, in fact, important.
(cid:3) indeed is consistent. If, however,
Quantitatively, denote the invasion rate of strain μ into a
(cid:3) as rμ;μ
(cid:3). We define
community missing type μ
X
Q
inv ¼
μ
(cid:3)
¯nμ
(cid:3)stdμ∈μ
(cid:3)
rμ;μ
;
(cid:3)
(cid:3)
(cid:3) in the pool and stdμ∈μ
where ¯nμ
is the relative mean abundance of strains
belonging to type μ
(cid:3) denotes the
standard deviation over all strains belonging to μ
(cid:3) weighted
by strain abundance in the pool (i.e., a strain’s combined
abundance observed across the set of M
env environments
used to define the pool). Once again, at L(cid:3) ¼ L we
automatically have Q
inv ¼ 0, as this corresponds to the
fully microscopic description (each type μ
(cid:3) is represen-
ted by exactly one strain). Note that this averaging con-
vention (weighted by abundance in the pool) is slightly
different from that used in the previous section (using
average abundance across the assembled replicates). Using
the same convention for both Q
rec does not
change our results but artificially inflates the latter with
noise from low-abundance (rare) strains. (For details, see
Supplemental Material [31].)
inv and Q
To illustrate the difference between the two criteria,
consider the statement that a community consisting of
Tetrahymena thermophila and Chlamydomonas reinhardtii
cannot be invaded by Escherichia coli [45]. What meaning
should we ascribe to this statement when phrased in terms
of coarse-grained units rather than specific strains? Under
the first criterion, we require that if we combine any single
strain of T. thermophila, any strain of C. reinhardtii, and
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JACOB MORAN and MIKHAIL TIKHONOV
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any strain of E. coli, only the first two survive. Under the
second criterion, we combine a vial labeled T. thermophila,
containing the entire diverse ensemble of its strains, with a
similarly diverse vial of C. reinhardtii and verify that the
resulting community cannot be invaded by any individual
strain of E. coli [46].
Note that, in our model, the existence of a Lyapunov
function [42] means the ecological equilibrium is uniquely
determined by the environment and the identity of the
competing strains; their initial abundance or the order of
their introduction does not matter. While this is a simpli-
fication, this property is very useful for our purposes, since
any lack of reproducibility between reconstituted commun-
ities is then clearly attributable to faulty coarse-graining. In
a model where even identical phenotypes could assemble
into multiple steady states, distinguishing this variability
from the variability due to strain differences adds a layer of
complexity to our analysis.
IV. RESULTS
A. A coarse-graining may be operationally valid despite
grouping functionally diverse strains
Throughout this section, we continue to use an envi-
ronment with Ki ≡ K0 and bi ≡ b0 (all L∞ opportunities
are equally lucrative). In practice, when approximating a
complex environment in the laboratory, we try to capture
the most salient features first. Thus, it would have been
perfectly natural to instead let Ki and/or bi decline with i;
one would expect this to improve coarse-grainability, and
this is indeed the case (see Supplemental Material [31]).
The motivation for our choice is twofold: First, keeping all
Ki and bi the same requires fewer parameters than choosing
a particular functional form of decline with i. Second, the
regime where no niches are obviously negligible only
makes it more striking to find that an ecosystem can be
not only coarse-grainable, but coarse-grainable in the
strong sense.
Figure 5(a) plots Q
invðL; L(cid:3)Þ for the leave-one-out test
comparing the invasion rates of different strains falling into
the same coarse-grained types. We find that any desired
coarse-graining quality can be achieved by a sufficient L(cid:3)
and is almost unaffected by L. As environment complexity
increases and becomes capable of sustaining an ever-
growing number of microscopic strains, each L(cid:3)-type
becomes increasingly diverse. Nevertheless, all the strains
in the same L(cid:3)-type continue to behave similarly by our
invasion-rate-based metric; in other words, under this cri-
terion, the ecosystem is coarse-grainable in the strong sense.
And yet, it would be wrong to conclude that the traits
beyond a given L(cid:3) are “negligible” in any absolute sense.
This is clearly demonstrated by the reconstitution test
[Fig. 5(b)]. If we attempt to reconstruct the community
from its members, every detail matters: No amount of
coarse-graining is acceptable. We now explain this apparent
paradox within our model.
Consider a community at an ecological equilibrium, and
let us focus on a particular phenotype σ carrying one of the
FIG. 5. The same ecosystem can be coarse-grainable under one criterion, but not under another. (a) If coarse-graining quality is
evaluated using the leave-one-out test (assessing reproducibility of strain invasion rates), our ecosystem model is coarse-grainable in the
strong sense: The acceptable level of coarse-graining, determined by the desired quality score (isolines of Q), is robust to environment
complexity [compare to Fig. 3(f)]. (b) In contrast, under the reconstitution test criterion, no amount of coarse-graining is acceptable
[compare to Fig. 3(d)]. This comparison shows that a coarse-graining can be operationally valid for a given purpose (a) even when the
strains it groups together are functionally diverse (b). Both heat maps represent a single random biochemistry, the same in both. Isolines
in (a) are averaged over 20 biochemistries to demonstrate robustness (see Supplemental Material [31]). (c) Explaining the origin of
strong-sense coarse-grainability in our model. The plot shows the scaling with i of jhi − χij [computed for L(cid:3) ¼ 30, L ¼ 40, and
averaged across communities assembled for the leave-one-out test in (a)]. The strong-sense coarse-grainability in (a) is ensured
whenever the decay is faster than 1=i (dashed gray line). Intuitively, this makes the tail-end traits approximately neutral in the assembled
community; see the text. We expect this scaling to be controlled by the sigmoidal decay of trait interaction magnitude jJijj, as confirmed
here [solid gray line; the same as Fig. 2(b) but normalized to a maximum of 1 to show the decay of interaction strength rather than their
absolute magnitude].
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P
j Jji0
σjσi0
weakly interacting (tail-end) traits i0: σi0 ¼ 1. What would
be the fitness effect of losing this trait? Losing the benefit
hi0
from opportunity i0 is offset by the reduction in
maintenance cost; for a weakly interacting trait, the con-
tribution from the term
is negligible, and
the change in cost is simply χi0. We conclude that the fitness
effect of losing the trait is δf ¼ χi0
− hi0. At an evolu-
tionary equilibrium, we therefore have hi0 ¼ χi0
(the
“functional attractor” state [40]). When this condition is
the opportunity or niche i0 is
satisfied, we say that
“equilibrated.” If a weakly interacting niche is equilib-
rated, carrying the respective trait becomes approximately
neutral.
Here, our community is not at the evolutionary equilib-
rium; nevertheless, a sufficiently diverse strain pool sim-
ilarly ensures that the opportunities corresponding to the
weakly interacting (tail-end) traits become approximately
equilibrated: hi ≈ χi. For a simpler model where the
phenotype costs χμ are drawn randomly, the mechanism
for this can be understood analytically (the “shielded
phase” in Ref. [18]; see also Ref. [47]). Here, the costs
are not random, but, as long as trait interactions are weak,
one expects the behavior to be similar (see Supplemental
section S6.2 in Ref. [18]). This expectation is confirmed in
simulations. Figure 5(c) shows the observed niche disequi-
librium hi − χi as a function of 1=i. The plot confirms that
the tail-end niches (1=i → 0) are increasingly well equili-
brated (jhi − χij decays with i). The strong-sense coarse-
grainability in Fig. 5(a) is ensured whenever the decay is
faster than 1=i (dashed gray line). This is because, with this
scaling, the sum of contributions from the omitted tail-end
traits is bounded (see Supplemental Material [31]). The
analytical argument in Ref. [18] leads us to expect the
disequilibrium to be controlled by the decaying typical
magnitude of interactions jJijj (solid gray line). If the tail-
end niches are equilibrated, carrying the respective traits
becomes approximately neutral, and the ability of a strain to
invade is entirely determined by its phenotypic profile over
nonequilibrated niches, explaining the observations in
Fig. 5(a). We conclude that, in our model, the strong-sense
coarse-grainability is a consequence of the faster-than-1=i
decay of interaction strength in Fig. 3(b).
Crucially, however, this approximate neutrality applies
only in the environment created by the assembled
community and does not mean that the distinctions are
functionally negligible. For instance, consider the (Lotka-
Volterra-style) interaction term for a given pair of strains
μ ≠ ν:
Aμν ≡
1
Nμ
∂ _Nμ
∂Nν
¼
X
σμiσνi
i
h2
i
biKi
¼
P
iσμiσνih2
i
b0K0
;
where we substitute bi ≡ b0 and Ki ≡ K0 for our environ-
ment. Even when tail-end niches are equilibrated with
hi ≈ χi ¼ χ0, we find that each of them contributes equally
to the interaction term: No detail is negligible.
This argument directly relates the observed effect to the
distinction between a trait that is truly neutral and one that
is effectively neutral in the assembled community only. A
truly neutral trait, one incurring almost no cost and bringing
almost no benefit, would have hi → 0 and its contribution
to the interaction term Aμν would indeed be small. And,
indeed, if we repeat our analysis for a scenario where both
bi and χi decline with i, we find that neglecting the tail-end
traits becomes an adequate coarse-graining also for the
reconstitution test (see Supplemental Material [31]).
it
this,
The conclusion from contrasting Figs. 5(a) and 5(b) is
worth emphasizing. In the example we construct,
the
coarse-grained description is valid sensu Fig. 5(a). This
means that, for instance, we can meaningfully say that “a
community assembled of OTU#1 and OTU#2 can be
invaded by OTU#4.” We can even measure, e.g.,
the
invasion rate and be assured that
is quantitatively
reproducible, with a bounded error bar, across the many
strains that constitute OTU#4 at the microscopic level.
the interaction between the OTUs as
Despite all
coarse-grained units is not actually definable: Any speci-
fic pair of strains of OTU#1 and OTU#4 may interact
differently with each other, as is indeed observed experi-
mentally [9].
Our focus on reproducibility of L(cid:3)-type abundances
across replicates is inspired by the experiments in Ref. [13].
To complete this parallel, we should mention that, besides
inoculating the same environment with a set of similar
inocula, as we do for our reconstitution test [cf. Fig. 4(b)],
one could also use the same inoculum to seed a set of
similar environments. To implement this in our model, we
use the strain pool constructed as described in Sec. II C to
inoculate a set of environments with slight variations in the
carrying capacities Ki ≈ K0 drawn from a Gaussian dis-
tribution of width ϵ ¼ 0.1. This is meant to represent the
unavoidable variability present in any experimental repli-
cates of the “same” environment Ki ≈ K0, which can affect
fitness even when subtle [48]. After assembling the
replicate communities, we find that community composi-
tion is more reproducible at coarser levels of description
[Figs. 6(b) and 6(c)], consistent with the experimental
observations of Goldford et al. [13] and with the inter-
pretation of
this pattern as resulting from functional
redundancy within coarse-grained types [12,49].
B. Using non-native strain pool
reduces coarse-grainability
The previous section describes a mechanism by which
strain diversity can aid coarse-grainability. As we explain,
in our model ecosystem the diverse set of strains contained
within the coarse-grained units is able to successfully
equilibrate the weakly interacting niches, rendering them
effectively neutral and leading to the behavior shown in
021038-9
JACOB MORAN and MIKHAIL TIKHONOV
PHYS. REV. X 12, 021038 (2022)
(a)
(b)
(c)
FIG. 6. Replicate communities assembled in similar environ-
ments are more reproducible at coarser level of description. (a) A
⃗K þ ⃗ϵ (each carrying capacity
set of similar environments
modified by 10% Gaussian noise) is inoculated with the same
strain pool and brought to ecological equilibrium. (b) Equilibrium
relative abundances of coarse-grained L(cid:3)-types across 20
replicates, shown for two levels of coarse-graining. A coarser
description (L(cid:3) ¼ 5; 7 resolved types) is more reproducible,
consistent with experimental observations [13]. (c) The variabil-
ity of coarse-grained descriptions increases with the level of
detail. Variability is measured as the average coefficient of
variation (CV) in relative abundance of an L(cid:3)-type over 100
replicates, weighted by L(cid:3)-type mean relative abundance across
replicates. Dashed lines mark L(cid:3) ¼ 5, 30 shown in (b). Data
points and shading show mean (cid:4)SD over 20 random choices of
biochemistry fJijg. All simulations performed with L ¼ 40.
Fig. 5(a). However, for this to occur,
the strain pool
diversity needs to be derived from a sufficiently similar
set of environments, as we now show.
To see this, we repeat the leave-one-out analysis in
Fig. 5(a), except now we inoculate the same test environ-
ment of complexity L ¼ 40 (using Ki ¼ K0, bi ¼ b0 as
before) with strain pools derived from other environments
that are increasingly dissimilar to it. Specifically, following
the procedure described in Sec. II C, we generate strain
pools in environments with Ki ¼ K0ð1 þ ϵηiÞ, where ηi are
drawn from the standard normal distribution and ϵ is the
parameter we vary. (The bi are left at bi ¼ b0 for simplic-
ity.) The results are presented in Fig. 7, which shows the
performance of different L(cid:3)-coarse-grainings under the
leave-one-out test.
At ϵ ¼ 0, this is identical to the protocol in Fig. 5(a). We
see that describing phenotypes by 20 traits is sufficient for
the invasion rates of grouped-together strains to be
FIG. 7. A coarse-graining scheme works best when the envi-
ronment is populated by the native strain pool. The same test
environment as in Fig. 5(a) is inoculated with strain pools that
evolve in environments increasingly further away (see the text).
The coarse-graining quality is assessed by leave-one-out experi-
ments and shown as a function of L(cid:3) and environment deviation ϵ
from the test condition. L is fixed at L ¼ 40 for comparison with
the last row in Fig. 5(a). As the environments for generating strain
pools are modified, the traits that were previously negligible can
no longer be coarse-grained. The same random biochemistry as in
Fig. 5(a) is used, and each pixel is averaged over 20 random
environments.
consistent within an error bar of Q < 10−2. However, as
ϵ is increased and the strain pools we use are derived from
the same coarse-
increasingly distant environments,
graining becomes insufficient.
Instead, a substantially
higher level of coarse-graining detail L(cid:3) is required to
maintain the desired quality. In summary, we find that, in
our model, a coarse-graining scheme works best when the
environment is populated by the native strain pool.
V. DISCUSSION
The interface of statistical physics and theoretical ecol-
ogy has a long and highly influential tradition of studying
large, random ecosystems, starting from the work of
May [50]. The key insight of this approach is that pat-
terns that are typical to some ensemble of ecosystems are
more likely to be generalizable and reproducible than the
details specific to any one realization. However, the choice
of the ensemble (and,
in particular, adding constraints
relevant for natural ecosystems) can affect predictions
significantly [51–57]. Which predictions of random-inter-
action models are robust
to introducing more realistic
structures and, conversely, which phenomena cannot be
invoking structural constraints is an
explained without
active area of research [58].
Resource competition models—one of
the simplest
frameworks explicitly linking composition to function—
offer a highly promising context to begin addressing these
questions, with much recent progress. For example, it was
recently shown that cross-feeding interactions structured by
shared “rules of metabolism” (but otherwise random) can
021038-10
DEFINING COARSE-GRAINABILITY IN A MODEL OF …
PHYS. REV. X 12, 021038 (2022)
reproduce a surprising range of experimental observations
[13,43]. This work made it possible to begin disentangling
which experimental observations can be seen as evidence
for nontrivial underlying mechanisms and which can be
reproduced already in the simplest models.
In this work, we presented a simple framework that
allows generating random ecosystems with community
structure as a tunable control parameter. Instead of postu-
lating a fixed architecture, such as a number of discrete
“families” of phenotypes [43], we use a biologically
motivated approach to derive it from functional trade-offs,
parametrized by a matrix of trait-trait interactions J. Simple
(few-parameter) choices for J generate communities with
complex structures,
including hierarchical architectures
which, at least superficially, appear to mimic those of
natural biodiversity. Perhaps the most immediate benefit
from such a framework would be to help develop new ways
to quantify the highly multidimensional concept of com-
munity structure across scales, such as, for example, the
structure of microbial pangenomes.
In this spirit, here we used this framework to quantify
the notion of coarse-grainability. We proposed a way to
operationally define the quality of a coarse-grained descrip-
tion based on the reproducibility of outcomes of a specified
experiment. We demonstrated that an ecosystem can be
coarse-grainable under one criterion while also not at all
coarse-grainable under another.
Specifically, one way to approach the coarse-graining
problem is to group together only the individuals that are to
a sufficient extent interchangeable. This is the criterion we
introduced as a “reconstitution test” and is the criterion
implicitly assumed by virtually all compositional models
of ecosystem dynamics [59]. However, experimental evi-
dence [3–6,9] suggests that, unless we are willing to resolve
types differing by as few as 100 bases, this criterion is
likely violated in most practical circumstances. It is cer-
tainly violated when grouping strains into functional
groups, or taxonomic species or families [11,12,60–63].
One expects, therefore, that explaining the practical suc-
cesses of such descriptions would require a different defini-
tion of what makes a coarse-graining scheme adequate.
We proposed that this can be achieved with only a subtle
change to the criterion: namely, by requiring that
the
grouped strains be approximately interchangeable not in
all conditions but
in the conditions created by the
assembled community itself. As long as the strains we
study remain in a diverse ecological context, and as long as
this diversity is derived from a sufficiently similar envi-
ronment, we find that the coarse-grained description can be
consistent in the sense that the strains grouped together
possess similar properties of interest (e.g., invasion rate and
postinvasion abundance).
In this paper, we focused on a case where the traits were
differentiated only by the strength of their interactions,
which established a unique hierarchy among them (a clear
order in which to include them in the hierarchy of coarse-
grained descriptions). In the more general case, the trait
cost χi or the trait usefulness in a given environment (bi and
Ki) will set up alternative, potentially conflicting hierar-
chies. We expect the model to have a rich phenomenology
in this regime, which we have not considered here. Another
obvious limitation of our analysis is that our model includes
only competitive interactions. A simple way to extend our
framework would be to include cross-feeding interactions;
we leave this extension for future work.
Our analysis introduced a distinction between weak-
sense and strong-sense coarse-grainability based on
whether the performance of a coarse-graining scheme is
robust
to increasing the environment complexity. We
explained how strong-sense coarse-grainability arises in
our model, linking it to a previously described phenome-
non, namely, that a sufficiently diverse community may
“pin” resource concentrations (here, the exploitation of
environmental opportunities) at values that are robust to
compositional details [16,18,37,47]. Tracing its origin
makes it clear that strong-sense coarse-grainability in our
model is only as good as the assumption that the cost of
carrying weakly interacting traits is independent of the
phenotypic background. Whether this assumption is ever a
good approximation in natural ecosystems remains to be
seen. Still, our argument provides an explicit mechanism
for how coarse-grainability can not only coexist, but may,
in fact, be facilitated by diversity.
The fact that strong-sense coarse-grainability is at least
theoretically possible is intriguing also for the following
reason. Throughout this work, we interpreted L as indexing
a sequence of ever-more-complex environments (e.g., a
minimal medium with one carbon source; a mixture of
several carbon sources; resuspended homogenized leaf
matter; or an actual
leaf). An alternative perspective,
however, is to think of a single environment of interest
and take L to be the level of detail at which it is modeled.
Any model we could ever consider, however detailed, is
necessarily incomplete. Consider the example of the human
is the exact geometry of the gut
gut: How important
epithelium, the effect of peristalsis and flow on small-scale
bacterial aggregates, or the exact role of the vast diversity of
uncharacterized secondary metabolites [64,65]? It seems
plausible that the complete list of factors shaping this
ecosystem includes many we will never even know about,
let alone include in our models. Our analysis raises an
intriguing—though, at
this point, purely speculative—
question of whether the tremendous diversity of natural
ecosystems might afford our models some unexpected
degree of robustness to such unknown details.
In conclusion, there are many reasons to believe that
analyzing a species in artificial laboratory environments
might be of limited utility for understanding its function
or interactions in the natural environment [66]. Usually,
however, the concern is that the laboratory conditions are
021038-11
JACOB MORAN and MIKHAIL TIKHONOV
PHYS. REV. X 12, 021038 (2022)
too simple, and, in reality, many more details may matter.
Here, we use our model to propose that, at least in some
conditions, the opposite can be true: Understanding the
interaction of two strains in the foreign conditions of the
Petri dish may require a much more detailed knowledge of
microscopic idiosyncracies. Removing individual strains of
a species from their natural ecoevolutionary context may
eliminate the very reasons that make a species-level
characterization an adequate coarse-graining of the natural
diversity.
All simulations were performed in MATLAB (Mathworks,
Inc.). The associated code, data, and scripts to reproduce all
figures in this work are available at Mendeley Data [67]. A
PYTHON implementation of the model is also available [68].
ACKNOWLEDGMENTS
We thank J. Grilli, C. Holmes, R. S. McGee, and C.
Strandkvist for helpful discussions. This research was
supported in part by National Science Foundation Grant
No. PHY-1748958,
the Gordon and Betty Moore
Foundation Grant No. 2919.02, and the Kavli Foundation.
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10.1155_2019_8132520.pdf
|
Data Availability
The data used to support the findings of this study are
available from the corresponding author upon request.
|
Data Availability The data used to support the findings of this study are available from the corresponding author upon request.
|
Hindawi
BioMed Research International
Volume 2019, Article ID 8132520, 14 pages
https://doi.org/10.1155/2019/8132520
Research Article
Influence of Insertion Torque on
Clinical and Biological Outcomes before and
after Loading of Mandibular Implant-Retained Overdentures
in Atrophic Edentulous Mandibles
,1 Amália Machado Bielemann,2 Alessandra Julie Schuster,2
Fernanda Faot
Raissa Micaella Marcello-Machado ,3 Altair Antoninha Del Bel Cury,3
Gustavo G. Nascimento ,4 and Otac-lio Luiz Chagas-Junior
5
1 Department of Restorative Dentistry, School of Dentistry, Federal University of Pelotas, RS, Brazil
2Graduate Program in Dentistry, School of Dentistry, Federal University of Pelotas, RS, Brazil
3Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, SP, Brazil
4Section of Periodontology, Department of Dentistry and Oral Health, Aarhus University, Aarhus, Denmark
5Department of Oral and Maxillofacial Surgery and Maxillofacial Prosthodontics, School of Dentistry,
Federal University of Pelotas, RS, Brazil
Correspondence should be addressed to Fernanda Faot; [email protected]
Received 10 January 2019; Revised 12 April 2019; Accepted 8 May 2019; Published 2 June 2019
Academic Editor: Konstantinos Michalakis
Copyright © 2019 Fernanda Faot et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Aim. To evaluate the influence of primary insertion torque (IT) values of narrow dental implants on the peri-implant health, implant
stability, immunoinflammatory responses, bone loss, and success and survival rates. Methods. Thirty-one edentulous patients
received two narrow implants (2.9x10mm, Facility NeoPoros) to retain mandibular overdentures. The implants were categorized in
four groups according to their IT: (G1) IT > 10 Ncm; (G2) IT ≥ 10Ncm and ≤ 30 Ncm; (G3) IT >30Ncm and < 45Ncm; (G4) IT ≥
45Ncm, and all implants were loaded after 3 months of healing. The following clinical outcomes were evaluated 1, 3, 6, and 12 months
after implant insertion: (i) peri-implant tissue health (PH), gingival index (GI), plaque index (PI), calculus presence (CP), probing
depth (PD), and bleeding on probing (BOP); (ii) implant stability quotient (ISQ) by resonance frequency analysis; and (iii) IL-1𝛽 and
TNF-𝛼 concentration in the peri-implant crevicular fluid. The marginal bone level (MBL) and changes (MBC) were evaluated. The
2
Chi
test, Kruskal-Wallis test, mixed-effects regression analysis, and the Kendall rank correlation coefficient were used for statistical
analysis (𝛼 = 5%). Results. G1 presented the highest PD at all evaluated periods. G2 presented higher PI at month 6 and 12. G4 showed
increased GI at month 3 and 12 and more CP at month 1 (p=.003). G2 and G4 had higher ISQ values over the study period, while
those from G1 and G3 presented lower ISQ values. The IL-1𝛽 concentration increased until month 12 and was independent of IT
and bone type; G4 had a higher IL-1𝛽 concentration in month 3 than the other groups (p=.015). The TNF-𝛼 release was negatively
correlated with IT, and TNF-𝛼 release was highest in G1 at month 12. The MBL immediately after surgery and the MBC at month
12 were similar between the groups, and G4 presented a positive MBC at month 12. The survival and success rates were 75% for
G1, 81.3% for G2, 64.3% for G3, and 95% for G4. Conclusion. The IT did not influence the clinical outcomes and the peri-implant
immunoinflammatory responses and was weakly correlated with the narrow dental implants primary stability. The observed success
rates suggest that the ideal IT for atrophic fully edentulous patients may deviate from the standardized IT of 32 Ncm.
1. Introduction
Oral rehabilitation with dental implants aims to establish
functional, aesthetic, and phonetic success, with reduced
morbidity and pain, aiming at reduced healing and rehabilita-
tion periods with acceptable cost-effectiveness [1]. One of the
prerequisites for successful osseointegration of the implants
is to achieve adequate primary stability after insertion of
2
BioMed Research International
the implants to prevent early failures [2]. Many of these
failures are biomechanically induced and associated with
risk factors such as low primary stability, low bone density,
short or narrow implants, and occlusal overloading [3]. Bone
density dictates the mechanical properties of the bone bed,
which may suffer changes during healing depending on
the surgical protocol, since the more porous, more elastic,
and well vascularized trabecular bone favors the formation
of a dense cortical bone near the surface of the implant,
guaranteeing the achievement of biological stability and
successful osseointegration [3–5].
Initially, stability of the implant is achieved mechanically
and can be measured by the insertion torque of the implant
(IT) [6], which is characterized by mechanical bonding
between the implant threads and the bone bed and is a
measure of the frictional resistance when the implant is
inserted into the bone bed [7]. The IT is dependent on
bone quality and quantity, surgical technique, and implant
geometry [8] and higher IT indicate greater primary stability
[5, 9, 10]. The literature indicates that the optimal IT to
achieve successful osseointegration is 30 Ncm, which is
sufficient to allow both conventional and immediate occlusal
loading of the implants while avoiding occlusal overload
failures [6, 11].
Although high IT promotes high mechanical primary
stability by compacting the host bone [5], it is still discussed
whether this stability is advantageous to the bone healing
reaction. Implants with high IT in thick cortical bone (Type
1 and Type 2) have been shown to have an increased
chance of osseointegration failure, as high IT values can
generate an inflammatory reaction of exacerbated healing
that contributed to early implant failure [7, 12]. Such defects
may have iatrogenic causes arising from overheating by
surgical drills, excessive osteoplastic forces, microfractures
in the peri-implant bone, debris from the implant surface,
reduced vascularization, or local ischemia, which may result
in necrosis, increased resorption, and/or bone formation [12–
14]. However, a study by Khayat et al. (2013) showed that
excessive IT (≥ 70 Ncm) did not affect osseointegration nor
increase marginal bone resorption around tapered dental
implants, irrespective of the maxillary arch [15]. This con-
trasts with the results from Marcocini et al. (2018), who
investigated 2 types of tapered implants that differed only in
the cutting groove design and found that implants installed
with high IT (50 Ncm) displayed a larger reduction in mean
marginal bone loss values across the follow-up evaluations up
to three years. In addition, this effect was significantly more
pronounced in the mandible and the findings also showed
that the recession of facial soft tissue was significantly higher
in both mandibular and maxillary sites that received high-IT
implants [16].
On the other hand, low IT values are associated with a
lower mechanical primary stability due to reduced osseo-
compression and tension, resulting in a smaller bone-implant
contact area. In these cases, the empty space between the
implant and the host bone is rapidly filled by the blood clot
that will be the precursor of the new bone formation [5, 9].
Thus it is thought that lower stability can promote rapid
formation of new bone in the vicinity of the implant without
reabsorption of the old bone, promoting rapid secondary
stability [17]. The results of a split-mouth, randomized clinical
trial by Verrastro-Neto et al. (2018) for the rehabiliatation
of the mandible in edentulous patients suggested that the
beneficial effect of implants inserted with low IT (19.18±3.56
Ncm) can be observed by the 7th day of healing through
high concentrations of biomarkers like vascular endothelial
growth factor and osteoprotegerin, which favor angiogenesis
and local microcirculation [12, 18].
Norton (2017) [15] performed a study with single implants
inserted in both jaws in healed bone beds or immediately after
extractions, with a minimum IT of < 5 Ncm (spinners) and
maximum IT of 20 Ncm. The results of this study showed
that implants installed with IT between 10 and 20 Ncm can
reach high success rates comparable to implants installed with
high IT, presenting favorable gains in biological stability over
time. Implants with ITs greater than 10 Ncm can produce
more predictable ISQ values due to less variability in the error
bar graph than those with ITs between 5 and 10 Ncm. That
is, implants with IT below 10 Ncm present a greater risk of
instability even if they have high ISQ, and caution is advised
when making decisions regarding occlusal loading of these
implants [13].
A systematic review by Berardini et al. (2016) [19]
evaluated the effect of high and low IT on marginal bone
loss and implant survival in in vivo and clinical studies.
Their meta-analysis indicated that there were no significant
differences in the rate of bone resorption or survival rates
between inserted implants with high or low IT in both
animal and human studies. However, the authors emphasized
that the methodological aspects are described in insufficient
detail to allow comparison and that the reported data are
heterogeneous. In addition, most available clinical studies
describe results from single implant crowns [7, 13–16, 20]
or immediate loading protocols adopted in full arch implant
restorations [18, 21] and implant-retained overdentures [22–
24] or focus on the influence of systemic diseases on IT [25].
There are current no studies investigating the effect of IT in
totally edentulous patients with atrophic mandibles.
Therefore,
this study investigates whether the peri-
implant healing is affected clinically and biologically by IT
values, considering that different ITs can generate distinct
healing responses. To achieve this, this study monitored the
peri-implant health parameters and selected proinflamma-
tory biomarkers (TNF-𝛼 and IL-1𝛽) as secondary variables,
along with marginal bone level (MBL) and changes (MBC)
over a period of 1 year for narrow diameter implants (NDI)
installed as mandibular overdenture (MO)retainers with
extremely low (< 10 Ncm), low to moderate (≥ 10 Ncm and
≤ 30 Ncm), moderate to high (>30 Ncm and < 45 Ncm), and
high IT values (≥ 45 Ncm). The null hypothesis tested is that
the different ITs will not affect the success and survival rates
of NDI retaining MO.
2. Materials and Methods
This prospective longitudinal clinical trial recruited patients
treated at the School of Dentistry of the Federal University of
Pelotas, Brazil. The study was approved by the institutional
BioMed Research International
3
research ethics committee board for human subjects (proto-
col 1.267.086).
2.1. Experimental Design. Between June 2014 and June 2015,
wearers of conventional complete dentures (CD) were invited
to participate in the study performed at the Federal University
of Pelotas - School of Dentistry.
Inclusion criteria:
(i) ≥ 3 months of adaptation to the conventional com-
plete dentures
(ii) Clinical criteria for mandibular atrophy [26]: poor
the
bone availability in the anterior region of
mandible, poor retention and instability of
the
mandibular CD
(iii) Availability for follow-up exams at 3, 6, and 12 months
(iv) Signed informed consent form
Exclusion criteria:
(i) History of radiotherapy in the head or neck region
(ii) Previous history of oral implant treatment
(iii) Treatment with bisphosphonate in the past 12 months
(iv) Heavy smoking (> 11 cigarettes/day)
(v) Severe diabetes
glycemic control)
(hyperglycemia or
inadequate
(vi) Bleeding disorders (hemorrhagic diathesis; drug-
induced anticoagulation)
(vii) Severe systemic diseases (rheumatoid arthritis; osteo-
genesis imperfecta)
(viii) Compromised immune systems (HIV; immunosup-
pressive medications) [27]
Individuals who accepted to participate in the study were
recruited for treatment with implant-retained mandibular
overdentures (IMO). The sample size calculation was based
on data from a previous study by Hof et al. (2014) [24] and
calculated with the statistical program G∗Power (cid:2)3.1. The
calculation was based on the mean baseline ISQ values of the
four IT groups, within an effect size of 1.21, a power of 80%,
a 5% alpha error, and an extra increase by 20% to account for
potential patient losses and refusals, showing that 15 implants
per group were required to complete this study totalizing the
need of at least 30 individuals. A total of 40 patients met the
inclusion criteria. Of those 31 patients agreed to participate
in the study and signed a written informed consent form.
Selected patients were evaluated radiographically for bone
availability and received 2 narrow diameter implants (NDI)
between the mental foramina. After 3 months of healing,
Equator type abutments were installed and the mandibular
overdentures were loaded.
The participants were followed and clinically and radio-
graphically evaluated over a period of 12 months. The follow-
ing peri-implant health parameters were collected over time:
plaque index (PI), calculus presence (CP), gingival index
(GI), probing depth (PD), and bleeding on probing (BOP). In
addition, the implant stability quotient (ISQ) was measured,
the peri-implant crevicular fluid (PICF) was collected, and
the marginal bone level (MBL) and marginal bone level
change (MBC) were measured radiographically. A flow chart
summarizing the clinical study is presented in Figure 1.
2.2. Radiographic Evaluation and Surgical Protocol. Digital
panoramic radiographs were performed to assess the bone
availability before surgical planning as previously described
[28]. A single expert examiner (R.M.M.M.) performed the
linear radiographic measurements to evaluate the mandibu-
lar bone height in the anterior and posterior regions. The
mandibular atrophy level was then determined following the
methodology described by Marcello-Machado et al. (2016)
[29].
A standardized one-stage surgical protocol performed
by an experienced surgeon (O.L.C.J.) was followed to install
two NDI (ø2.9-10mm Facility(cid:2), NeoPoros surface, Neodent
Osseointegrated Implants, Curitiba, Brazil) in the anterior
region of the mandible. The drill was oriented using the
distal face of the upper lateral incisors, respecting a distance
of 5 mm from the mental foramina. The manufacturer's
recommended drill sequence was followed, each implant was
inserted at the bone crest level, and the final stage of insertion
was performed with a manual wrench. After 3 months of
bone healing, the nonsubmerged healing cap was replaced
by Equator abutments and the IMOs were loaded by two
experienced prosthodontists (A.M.B. and R.M.M.M.). The
O-ring attachments that constitute the female part of the
Equator abutment were then connected intraorally using self-
curing denture acrylic resin (VIPIFlash(cid:2), VIPI industry, S˜ao
Paulo, Brazil) to capture the system to the internal surface
of the prosthesis. Denture stability, retention, and occlusion
were checked, and the participants received oral hygiene
instructions. The radiographic evaluation and surgical pro-
tocol used in this study is described in detail by Bielemann et
al. (2018) [28]. The implants were categorized into four groups
according to the insertion torque (IT) registered during the
surgery by a manual wrench: Group 1 (G1), implants with
extremely low IT (< 10 Ncm); Group 2 (G2) with low to
moderate IT (≥ 10 Ncm and ≤ 30 Ncm); Group 3 (G3) with
moderate to high IT (>30 Ncm and < 45 Ncm); and Group
4 (G4) with high IT (≥ 45 Ncm). The implant position was
checked by panoramic radiographs performed immediately
after installation and analyzed by cross-sectional
images
obtained with cone bean computed tomography (CBCT)
after 1 year of loading to determine the type of cortical bone
anchorage (mono- or bicortical anchorage). The anchorage
was classified as follows: (i) implants with apical cortical bone
contact; (ii) implants with bicortical bone contact (apical
and cervical regions); and (iii) implants with cervical cortical
bone contact [30].
2.3. Implant Stability, Clinical Assessment, Crevicular Fluid
Sampling, and Peri-Implant Bone-Level Assessment. A single
experienced prosthodontist (A.M.B.) performed all clinical
evaluations following the methodology described by Biele-
mann et al. (2018) [31]. Resonance frequency analysis (RFA)
(Osstell(cid:2)-Integration Diagnostics AB, G¨oteborg, Sweden),
4
Experimental Design
BioMed Research International
Pre-loading
Post-loading
Installation: 2 NDI
Implant loading
ø2.9x10 mm, Facility
Ⓡ, NeoPoros
Healing cap
Equator Attachment
1
month
PIH
ISQ
PICF
Baseline
“surgery”
ISQ
X-Ray
Insertion Torque
G1: > 10 Ncm
G2: ≥ 10 Ncm ≤ 30 Ncm
G3: > 30 Ncm < 45 Ncm
G4: ≥ 45 Ncm
31 patients
Mandibular atrophy
Outcome variables:
3
months
PIH
ISQ
PICF
6
months
PIH
ISQ
PICF
12
months
PIH
ISQ
PICF
X-Ray
MBL
MBC
Success & Survival
PIH: Peri-implant tissue health
ISQ: Implant stability quotient
PICF: Peri-implant crevicular fluid
Panoramic X-Ray:
- PI; GI; CP; PD; BOP
- Primary and secondary stability
- IL-1 and TNF- concentration
- MBL: Marginal bone level
- MBC: Marginal bone change
Figure 1: Summary of the experimental design.
which provides implant stability quotient (ISQ) measure-
ments (scale 1–100), was performed at implant placement
(baseline) and after 1, 3, 6, and 12 months. Measurements were
made in triplicate in four different directions (mesial, distal,
buccal, and lingual).
At 1, 3, 6, and 12 months after implant insertion, the peri-
implant health parameters [28] were measured: plaque index
(PI) and gingival index (GI) scores, calculus presence (CP),
probing depth (PD), and bleeding on probing index (BOP).
In addition, the PICF was collected.
Panoramic radiographs with standardized settings were
taken immediately after surgery (Baseline) and 12 months
after surgery to measure the peri-implant MBL and MBC. The
images were analyzed using the DBSWin-VistaScan digital
system, and the reference point was the external edge of the
implant head during the evaluation of peri-implant bone level
[31].
2.4. Implant Success and Survival. The success of the implants
was evaluated according to the clinical criteria proposed by
Misch et al. (2008) [32] and Papaspyridakos et al. (2012)
[33]: absence of pain or tenderness upon function, absence
of clinical implant mobility, radiographic marginal bone loss
<1.5 mm from initial surgery, and absence of infections,
dysesthesia, or exudates [32, 33]. Implants that remained in
situ but did not meet the success criteria were included in the
survival group.
was used for comparisons between groups of dichotomous
variables (PI, CP, GI, BOP, and cortical bone anchorage), and
the Kruskal-Wallis test was used for comparisons between
continuous variables (PD, ISQ, IL-1𝛽, TNF-𝛼, MBL, and
MBC).
Mixed-effects linear regression analysis was performed to
test the effect of follow-up time, IT, bone type, atrophy, and
time since edentulism on ISQ, PD, IL-1𝛽, and TNF-𝛼 adjusted
for gender, age, smoking status, bleeding on probing, and
plaque presence, taking into account individual differences
using random intercepts. All variables were standardized to
a mean of 0 and a standard deviation of 1 to allow compar-
ison between the estimates; coefficients and respective 95%
confidence intervals were then estimated. The first assessed
data were used as the reference category for the comparisons.
The Kendall rank correlation coefficient (𝑅𝑘) test was
used to verify the relationships between the variables: ISQ,
PD, MBL, MBC, IL-1𝛽, and TNF-𝛼. The results were stratified
to be interpreted according the intensity of the correlations
as follows: very high, high, moderate,
low, and without
correlation [34].
Kaplan-Meier survival analysis was used to calculate the
survival rate of implants for each group. The level of signif-
icance was set at 5%. All statistical analyses were performed
using the SPSS Version 22 software (IBM SPSS Statistics 22).
3. Results
2.5. Statistical Analysis. The implants were considered as
the statistical unit. The Shapiro-Wilk test indicated that
all data were non-normally distributed. The chi-square test
Table 1 lists the demographic characteristics of the sample
population along with the bone remodeling at 12 months,
according to the proposed groups in which the implants are
BioMed Research International
Table 1: Patients characteristics according to the insertion torque groups.
5
G4
≥45Ncm
(n=20)
11//9
63.65 (6.66)
22.1(15.98)
G1
>10Ncm
(n=12)
9//3
65.83(9.73)
22.17(9.88)
G2
≥10Ncm
≤30Ncm
(n=16)
14//2
70.19(7.46)
28.38(12.45)
G3
>30Ncm
<45Ncm
(n=14)
8//6
63.65(6.66)
22.1(15.98)
7/5
3/13
7/7
7/13
24.43(4.02)
23.64(3.01)
22.93(4.56)
22.92(4.56)
3.60(2.65)
3.66(2.87)
3.38(3.63)
3.38(3.63)
1//11
7//5
2/10
0.00
(-0.82 – 0.71)
0.00
(-0.36 – 0.83)
0.00
(-1.01 – 0.83)
3//13
6//10
6/10
0
(-0.37 – 0.78)
0.00
(-1.08 – 0.55)
0
(-1.08 - 0.91)
1//13
9//5
4/10
0
(-0.78 – 0.0)
0
(-0.97 - 0.66)
0
(-0.51 – 1.44)
3//17
12//8
12/8
-0.23
(-1.09 – 0.77)
0
(-0.75 -1.06)
0.29
(-0.77 – 1.92)
3/2/7
2/7/7
0/8/6
5/11/4
Gender (Female/Male)
Age (years; mean, SD)
Mandibular Time since edentulism
(years; mean, SD)
Mandibular Time since edentulism (<25 /
>25 years)
Mandibular anterior midline (mm; mean,
SD)
Superior bone height from the mental
foramen (mm; mean, SD)
Smokers / Non-Smokers
+
Bone Atrophy (Yes/ No)
Bone Type (Type I / Type II)∗
MBL baseline (median; min – max)
MBL 1 year (median; min – max)
BLC (median; min – max)
Cortical Bone Anchorage (apical/
bicortical / cervical)
the sample unit. A total of 62 implants were installed in the
anterior mandible region of 31 totally edentulous patients, 21
females and 10 males, with a mean age of 66.9 years (59–89)
years. The mean mandibular time since edentulism was 24.74
± 13.12 years, the mean bone height in the anterior midline
of the mandible was 23.36 ± 3.74 mm, and the superior
height from the mental foramen of 4.10 ± 3.40 mm. Group
1 (G1) consisted of 12 implants, Group 2 (G2) consisted of
16 implants, Group 3 (G3) consisted of 14 implants, and
Group 4 (G4) consisted of 20 implants. These 4 groups
presented similar characteristics in terms of mean age, atro-
phy (34 implants were inserted in radiographically atrophic
mandibles), bone type (38 implants were inserted in bone
type II), and proportion of smokers. However, the time since
mandibular edentulism was significantly different between
the groups, with G2 having the highest mean of 28.38 ±12.45
years (P = 0.018). In the anterior midline of the mandible,
the bone height was significantly higher in the G1 (24.43 ±
4.02 mm, P = 0.038). The MBL immediately after insertion
of the implants and after 12 months of osseointegration was
similar between the groups 0.00 (-1.09–0.78) mm and 0.00 (-
1.08–1.06) mm, respectively (P> 0.05). MBC was only positive
for G4, 0.29 (-0.77–1.92) mm, but no significant differences
were found (P> 0.05). The cross-sectional CBCT images
showed that the majority of the implants (n = 28) were
inserted with bicortical bone contact (cervical and apical),
24 implants were inserted with cervical bone contact, and 10
implants were inserted with cortical bone contact. The IT was
not influenced by the type of implant anchoring (p > 0.05).
Tables S1 and S2 show the comparisons between the
medians (min-max) of the implant stability quotient and
proinflammatory markers outcomes, respectively, between
groups at different evaluation periods. Table 2 presents the
results of the peri-implant health clinical outcomes. The PI,
GI, and BOP outcomes were similar between the groups (P
> 0.05),while CP was only significantly more prevalent in
G4 after 1 month (P = 0.003). In addition, all implants were
surrounded by at least 2 mm of keratinized mucosa (data not
shown).
Table 3 displays the results from the mixed-effects regres-
sion analysis. The ISQ increased gradually over time until
6 months; at 12 months, the ISQ values were similar to
those observed at baseline (Figure 2(a)). Implants with low
to moderate IT (G2) and high IT (G4) had higher ISQ values
over the entire follow-up period, whereas those from G1 and
G3 presented lower ISQ values (Figure 3(a) and Table S1).
A gradual but nonsignificant reduction in PD values was
observed over time (Figure 2(b)). Significantly higher PD val-
ues were measured in G3 individuals (Figure 3(b)), implants
installed in bone type 2 and in atrophic bone, and individuals
with time since edentulism ≥ 25 years (Figure 4(a)). Increased
levels of TNF-𝛼 were observed after 12 months of loading
(Figure 2(c)), while reduced values were noted in the medium
to high torque (G3) and high torque groups (G4; Figure 3(c)).
6
BioMed Research International
Table 2: Presence of peri-implant health issues across the follow-up period (in %) and the significant intergroup comparisons (Chi-square
test, p<0.05). ∗ p=0.003.
1M
50
G1
Plaque Index
3M
6M 12M
50
44.4
6/12
5/10
4/9
11.1
1/9
G2
25
57.1
61.5
46.2
4/16
8/14
8/13
6/13
G3
35.7
5/14
58.3
7/12
22.2
0
2/9
0/9
G4
60
55
36.8
31.6
12/20
11/20
7/19
6/19
1M
0
0/12
A
0
0/16
A
0
0/14
A
30
6 /20
B∗
Calculus Presence
Gingival inflammation
Bleeding on Probing
3M
0
0/10
0
11.1
9/9
0
6M 12M 1M
0
0
3M
0
9/9
0/12
0/10
0
0
7.1
6M
0
0/9
0
12M
0
0/9
1M
0
3M 6M 12M
0
0
0
0/12
0/10
0/10
0/9
0
12.5
7.1
0
7.7
0/14
0/13
0/13
0/16
1/14
0/13
0/13
2/16
1/14
0/13
1/13
0
0
0
0
0
0
0/12
0/9
0/9
0/14
0/14
0/9
0
0
0
0
10
0
0/20
0/19
0/19
0/20
2/20
0/19
0
0/9
5.3
1/19
0
0
0/14
0/12
0
5
11.1
1/9
0
0
0/9
5.3
0/20
1/20
0/19
1/19
Table 3: Results from the mixed-effects multilevel analysis of the effects time and insertion torque on the clinical and biological conditions
related to implant healing. The estimates are given as standardized coefficients with their respective 95% confidence intervals. Analyses were
adjusted for gender, age, smoking status, bleeding on probing, and plaque. Statistically significant results are presented in italic.
Time
Baseline
1 Month
3 Months
6 Months
12 Months
Torque type
G1
G2
G3
G4
Bone type
1
2
Atrophy
No
Yes
Time of edentulism
< 25 years
≥ 25 years
ISQ
Coef. (95%CI)
Ref.
-0.7 (-1.0;-0.4)
-0.6 (-0.9;-0.4)
-0.6 (-0.8;-0.3)
-0.2 (-0.5;0.0)
Ref.
0.7 (0.1;1.2)
0.4 (-0.3;1.1)
0.8 (0.2;1.3)
Ref.
0.0 (-0.4;0.4)
Ref.
0.0 (-0.4;0.4)
Ref.
-0.1 (-0.5;0.3)
PD
Coef. (95%CI)
-
Ref.
-0.5 (-0.7;-0.3)
-1.1 (-1.4;-0.8)
-1.4 (-1.7;-1.1)
Ref.
0.3 (-0.1;0.7)
0.4 (0.0;0.8)
0.1 (-0.3;0.4)
Ref.
0.4 (0.1;0.7)
Ref.
0.3 (0.1;0.6)
Ref.
0.3 (0.1;0.6)
TNF-𝛼
Coef. (95%CI)
IL-1𝛽
Coef. (95%CI)
-
Ref.
-0.2 (-0.6;0.2)
0.1 (-0.1;0.4)
0.9 (0.5;1.3)
Ref.
-0.3 (-0.6;0.1)
-0.4 (-0.8;0.0)
-0.3 (-0.6;0.0)
Ref.
0.1 (-0.2;0.3)
Ref.
-0.1 (-0.3;0.2)
Ref.
-0.1 (-0.4;0.1)
-
Ref.
0.4 (0.1;0.6)
0.8 (0.5;1.2)
1.4 (1.1;1.7)
Ref.
-0.1 (-0.5;0.2)
-0.2 (-0.6;0.1)
0.0 (-0.3;0.3)
Ref.
0.0 (-0.3;0.2)
Ref.
-0.1 (-0.4;0.3)
Ref.
0.1 (-0.2;0.3)
Finally, IL-1𝛽 levels tended to increase over the study period,
irrespective of torque and bone type (Figures 2(d) and 3(d)).
Table 4 presents the correlation results between bone
remodeling, clinical parameters,
stability, and
cytokine concentration. The results indicate a weak positive
correlation between IT and primary ISQ (P = 0.01; Rk =
0.252). In G1, a moderate positive correlation was found
between MBL and MBC at 12 months and between MBL
baseline and IL-1𝛽 at 6 months. In G2, there was a strong
positive correlation between MBL and MBC at 12 months
implant
and a moderate positive correlation between the PD in
month 1 with MBL at 12 months and between the PD at
month 1 and month 3 with the MBC. In G3, a weak positive
correlation was found for MBL and MBC at 12 months, along
with a moderate positive correlation between MBL baseline
and ISQ at 12 months and a weak negative correlation
between TNF-𝛼 and ISQ at month 1. G4 showed a weak to
moderate positive correlation between MBL baseline and
IL-1𝛽 at 6 months and a weak negative correlation between
MBL baseline and TNF-𝛼 at month 1.
BioMed Research International
7
70
60
50
40
30
20
10
0
500
450
400
350
300
250
200
150
100
50
0
l
/
g
p
ISQ
A
B
C
D
E
PD
A
A
A
A
m
m
7
6
5
4
3
2
1
0
Baseline
1
3
6
12
1
3
6
12
Months
(a)
TNF-
A
1
A
6
A
3
Months
(c)
B
12
l
/
g
p
900
800
700
600
500
400
300
200
100
0
A
1
Months
(b)
IL-1
B
3
Months
(d)
C
D
6
12
Figure 2: Medians (min-max) of implant stability, probing depth and proinflammatory markers over the evaluation time for al implants
(different letters indicate statistically significant differences; the estimates given are standardized coefficients with respective 95% confidence
intervals).
p = 0.008
ISQ
Baseline
1
3
Months
(a)
TNF-
Q
S
I
70
60
50
40
30
20
10
0
l
/
g
p
600
500
400
300
200
100
0
PD
m
m
7
6
5
4
3
2
1
0
6
12
1
3
6
12
l
/
g
p
900
800
700
600
500
400
300
200
100
0
Months
(b)
IL-1
p = 0.015
1
3
6
12
Months
(d)
1
3
6
12
Months
(c)
Figure 3: Comparisons between the medians (min-max) of the implant stability, probing depth, and proinflammatory markers outcomes
between groups at different evaluation periods (Kruskall-Wallis independent analyses, p<0.05).
8
BioMed Research International
Table 4: Correlation between implant stability quotient, cytokine concentrations (pg/𝜇l), clinical parameters, and marginal bone relation at
each evaluation period. 𝑅𝑝 values are Pearson correlation coefficients, and 𝑅𝑠 values are Spearman correlation coefficients.
BASELINE
1 MONTH
3
MONTHS
6
MONTHS
MBL.0
p= 0.010
Rk =-0.582
p= 0.015
Rk =0.550
IPS
ISQ
TNF-𝛼
IL-1𝛽
G1 MBL.12M
G2 MBL.12M
G3 SUP.HF
G3
ANT.MH
G3 MBL.12M
G2 MBL.12M
G2 MBC
G3 MBL.0
G4 MBC
G3 TNF-
𝛼.1M
G3 MBL.0
G4 MBL.0
G1 MBL.0
G4 MBL.0
IPS
p= 0.026
Rk =0.508
p= 0.003
Rk =0.662
p= 0.045
𝑅𝑘 𝑡=0.332
p= 0.030
Rk =-0.366
ISQ
p= 0.024
Rk= -0.456
TNF-𝛼
p= 0.018
Rk =-0.401
IL-1𝛽
12
MONTHS
MBC
p= 0.023
Rk =0.636
p≤0.001
Rk =0.867
p= 0.020
Rk =-0.612
p= 0.030
Rk =0.580
p= 0.002
Rk =0.853
IPS
IPS
IPS
p= 0.016
Rk =0.539
p= 0.008
Rk =0.476
ISQ
ISQ
ISQ
p= 0.033
Rk = 0.618
TNF-𝛼
IL-1𝛽
TNF-𝛼
IL-1𝛽
TNF-𝛼
IL-1𝛽
p= 0.007
Rk =0.629
p= 0.037
Rk=0.356
G4 MBL.12m
p= 0.026
Rk=0.396
p= 0.041
Rk =-0.358
SUP.HF, superior height of the formanina; ANT.MH, anterior midline height; MBL, manrginal bone level; MBC, marginal bone level change; PD, probing depth.
p= 0.005
Rk =0.461
G4 PD12m
Figure 5 shows the Kaplan-Meier survival curve of the
implants. In the G1 group, 3 implants failed at 2 months
resulting in success and survival rates of 75%. The success
and survival rates in G2 were 81.3%, as 2 failures occurred
before occlusal loading at 3 months and 1 failure occurred
at 4 months. G3 had the worst success and survival rates
of 64.3%, as 3 implants failed before 3 months and 2 more
failures occurred at 6 and 7 months, respectively. G4 had the
highest survival and success rates of 95%, with one failure at
4 months.
4. Discussion
The influence of IT on outcomes related to implant healing
and stability during and after implant osseointegration is
still controversial and dependent on factors such as bone
availability, patient profile, surgical protocol, and implant
macrogeometry [21, 24, 35–37]. Thus, this longitudinal clini-
cal study evaluated the effect of IT values on implant success
and survival during 1 year of function and mapped clinical
and biological endpoints of NDI placed in the anterior region
BioMed Research International
9
of totally edentulous jaws. In the studied population, the null
hypothesis tested was rejected because the different ITs had
different survival and success rates. The high IT group (G4)
had the best survival and success rates of 95% while the
medium to high IT group (G3) had the worst rates of 64.3%.
Multilevel regression analysis enabled evaluating how time,
IT, bone type, atrophy, and time since edentulism influence
ISQ, PD, and the proinflammatory markers IL-1𝛽 and TNF-
𝛼.
This study showed that the PD reduced significantly over
the 12 months, independent of the IT. The ISQ decreased
from the baseline to 1 month and increased significantly by
month 12. In the high torque G4 group, the ISQ showed a
smaller reduction compared to the baseline, reflecting the
relatively small changes in ISQ when the baseline value is
already high. The release of the proinflammatory cytokine
IL-1𝛽 was not influenced by IT, while TNF-𝛼 was negatively
correlated with IT. Factors inherent to the patient such as
bone type, atrophy, and time since mandibular edentulism
were shown to influence only the PD outcome (Figures 4(a),
4(b) and 4(c)). The IT was not influenced by the bone
type nor by atrophic jaw conditions. However, some patient
characteristics were significantly different in 2 groups: (i) time
since edentulism in the mandible was significantly longer in
the low to moderate insertion torque group (G2: 28.38 ± 12.45
years, P = 0.018); and (ii) bone availability in the anterior
mandible was significantly higher in the low insertion torque
group (G1: 24.43 ± 4.02 mm, P = 0.038).
The peri-implant health indexes show similar behavior in
all groups over the follow-up period. The PI was relatively
high in the study population, which has a high average
age and prolonged period of mandibular time since eden-
tulism. Previous studies have shown that patients with such
characteristics may present motor difficulties that inhibit
adequate hygienic maintenance of dental implants [28, 31].
The G4 group showed the highest IT and presented higher
inflammatory indexes (GI and BOP) during pre-loading and
at 6 months after loading. The behavior of these outcomes
could be related to maladaptation to the prostheses or reduc-
tion of peri-implant mucosal height resulting in exposure of
the transmucosal part of the prosthetic abutment, favoring
plaque accumulation. It is also recommended re-evaluate
the transmucosal height of the prosthetic abutments and its
necessity of replacement to promote better adaptation and
stability of the mandibular prostheses and maintenance of
ideal biological sealing [31]. Thus, periodic consultations are
recommended during the first year of implant healing and
adaptation, mainly in patients with prolonged time since
edentulism using IMO [31].
Although IT did not influence the PD during the 12
months of follow-up, at month 12, PD was reduced by 140%
in relation to month 1. This result was expected due to the
peri-implant soft tissue healing that increases the bundles
of collagen fibers parallel to the surface of the implant and
promotes tissue resistance around the prosthetic components
[31, 38]. In addition, the PD decreased with shorter time
since edentulism (Figure 4(a)) and greater bone availability
in the anterior region of the mandible (Figure 4(b)). This
corroborated the study by Ivanovski & Lee (2018), who
affirmed that the peri-implant mucosa width is genetically
predetermined and that the dimensions of this soft tissue
are also preserved through the more apical establishment of
the implant; that is, marginal bone height determines the
biological width of peri-implant soft tissue [38]. The G4 group
with the highest IT had a mean PD that was 30% lower than
the other groups. This is also consistent with the findings
reported by Marconcini et al. (2018), which showed that
higher insertion torque (≥ 50 Ncm) in mandible led to greater
bone resorption and mucosal recession than that registered
for implants placed with a regular IT (< 50 Ncm). Moreover,
sites with a thick buccal bone wall (≥ 1 mm) showed smaller
recession at the facial soft tissue level only after 3 years [16].
Recently, studies have stated that there is no correlation
between IT and the primary stability registered by ISQ [13,
21, 30, 39], indicating that both methods are not comparable
[39]. This study found a weak correlation between IT and
primary ISQ (P = 0.01; Rk = 0.252), which has previously been
reported [39–41]. The present study also showed that there
was a drop in ISQ values after primary stability establishment
up to 6 months that was subsequently counteracted by the
increase in secondary ISQ recording to baseline ISQ values
(Table 3 and Figure 2), in accordance with most previously
published results [13, 21, 24, 37]. The ISQ of G2 and G4 was
approximately 7.5 times higher than G1 and 3 times higher
than that of G3 (Table 3 and Figure 3). These results show that
the G2 and G4 ITs generate more effective results to achieve
an adequate secondary stability, reinforced by the increase in
ISQ in these groups after loading up to the 12th month, since
G1 and G3 had a reduction in ISQ between 3 and 6 months,
followed by an increase between 6 and 12 months. Similar
behavior has been reported by Norton (2017), who observed
increased ISQ after the third month of osseointegration and
occlusal loading of implants inserted with (extremely) low
IT from < 5–20 Ncm [13]. Our results are in agreement with
Marcello-Machado et al. (2018), where NDI as IMO retainers
reached ISQ values similar to those during installation at
12 months of osseointegration, thus demonstrating adequate
secondary stability establishment [31]. In our study it was
also noted that the G4 ISQ reduced 80% less than the G1,
evidencing that minor changes in the ISQ are expected when
the value recorded in the baseline is already high [13], and
the reduction is more pronounced between baseline and
month 1. The proinflammatory markers IL-1𝛽 and TNF-𝛼
may enable osteoclastogenesis and reabsorption of alveolar
bone, especially during the initial bone healing period [12].
Our results showed that the release of these inflammatory
markers had no correlation with each other. However, the 2
cytokines monitored in our study were differently influenced
by the IT (Table 3 and Figure 3). The concentration of
TNF-𝛼 showed a significant increase by 110% at 12 months
compared to the thrid month, as already observed at 12
months in a study with IMO retained by a bar-clip [42].
Thus, it is suggested that the occlusal loading after 3 months
of healing stimulates the release of proinflammatory factors,
i.e., the micromovements generated by the loading may
have a positive effect on bone neoformation [43], favoring
remodeling and bone formation [12, 44]. This effect may be
more noticeable when an inadequately low IT is achieved,
10
BioMed Research International
)
m
m
(
h
t
p
e
D
g
n
i
b
o
r
P
)
m
m
(
h
t
p
e
D
g
n
i
b
o
r
P
7
6
5
4
3
2
1
0
7
6
5
4
3
2
1
0
y = 0.0145x + 2.9177
R² = 0.0344
y = 0.0054x + 2.6414
R² = 0.0078
y = 0.0099x + 1.9645
R² = 0.0394
y = 0.0073x + 1.7425
R² = 0.0368
0
10
20
40
Time since edentulism (years)
30
50
60
PD.1 Month
Linear (PD.1 Month)
PD.3 Months
Linear (PD.3 Months)
PD.6 Months
Linear (PD.6 Months)
PD.12 Months
Linear (PD.12 Months)
(a)
y = -0.0689x + 4.8867
R² = 0.0625
y = -0.0246x + 3.3542
R² = 0.0127
y = -0.0216x + 2.7165
R² = 0.0132
y = -0.0266x + 2.5452
R² = 0.0346
10
15
20
25
30
35
Bone atrophy (anterior region height, mm)
PD.1 Month
PD. 3 Months
PD. 6 Months
PD.12 Months
Linear (PD.1 Month)
Linear (PD. 3 Months)
Linear (PD. 6 Months)
Linear (PD.12 Months)
(b)
)
m
m
(
h
t
p
e
D
g
n
i
b
o
r
P
7
6
5
4
3
2
1
0
B
A
B
A
B
A
Bone type
Bone atrophy
Time since edentulism
Type I
Type II
Non-atrophic
Atrophic
<25 years
>25 years
(c)
Figure 4: (a) Dispersion diagrams showing the correlations between the probing depth (PD) and time since edentulism and (b) between the
PD and bone atrophy according to the anterior mandibular height. (c) Box-plot showing the PD according bone type, bone atrophy, and time
since edentulism. Different letters indicate statistically significant differences; the estimates given are standardized coefficients with respective
95% confidence intervals.
BioMed Research International
11
Survival Function
e
t
a
r
l
a
v
i
v
r
u
S
1,0
0,8
0,6
0,4
0,2
0,0
0
2
4
6
8
10
12
Follow-up (months)
Group
G1: IT <10N
G2: IT >10N<30N
G3: IT >30N<45N
G4: IT ≥45N
G1: IT <10N-censored
G2: IT >10N<30N-censored
G3: IT >30N<45N-censored
G4: IT ≥45N-censored
Figure 5: Kaplan-Meier survival curve for each Insertion Torque group.
as evident in G1, which presented a TNF-𝛼 concentration
that was 40% and 30% higher than G2 and G4, respectively,
suggesting that the instability causes a more exacerbated
TNF-𝛼 proinflammatory response.
After the first month of bone healing, which is charac-
terized by intense cellular and proinflammatory activity, the
TNF-𝛼 release reduced for all groups until the third month,
as expected [12], and this reduction was more pronounced
in the high torque group G4. Similar behavior has been
demonstrated in the same period (1 and 3 months) in MO-
retaining implants with IT > 30 Ncm that received immediate
loading. Conversely, G1 and G4 had a mean TNF-𝛼 concen-
tration that was 80% higher than the G3 and G2 groups at
month 12. This high concentration of G4 corroborates the
results of the study with an immediate loading protocol by
Verrastro-Neto et al. (2018), which suggests that the high
IT favors bone formation and repair, as evidenced by the
greater osteoblastic activity due to the increase of BMP-9,
supraregulation of periostin, involved in the recruitment of
osteoblastic cells, and increased levels of placental growth
factor, which is involved in bone formation and repair [18].
IL-1𝛽 has been associated as a marker of bone resorption,
peri-implant infection, trauma, and iatrogenic conditions
[45–47]. However, in this study, IL-1𝛽 was not able to reflect
the trauma generated during surgery as previous reported by
Hof et al. (2014) [24]. Independently of the IT values achieved,
as the concentration of IL-1𝛽 increased progressively over
time, with a more expressive increase after occlusal loading,
finally it peaked at 12 months with concentrations 140%
higher than at first month. Thus, the progressive release of
this biomarker may have resulted from microtrauma caused
by the functional loading of the implants but imperceptible by
the patient stimulating the interaction between the biological
events and the mechanical forces, which are fundamental
for treatment success. These results are consistent with the
results from Elsyad et al. (2017), who attributed this finding to
the presence of plaque and gingival inflammation suggesting
that IL-1𝛽 is present in PICF irrespective of the presence of
gingival bleeding [48]. In the follow-up study by Hof et al.
(2014) that evaluated the impact of low IT (20 Ncm) and high
IT (>50 Ncm) during preloading (3 months) and postloading
(12 months), it was reported a higher concentration of IL-
1𝛽 in the low torque group; however this finding was not
significant. Their IL-1𝛽 results demonstrated no active stages
of tissue destruction, as the concentrations were comparable
to those reported for peri-implant health sites [24]. A study
that followed implants with a IT of 30 Ncm with immediate
loading found that the concentration of IL-1𝛽 was initially
low and it increased progressively until 12 months [48]. Thus,
it is suggested that the functional loading of the implants
favors IL-1𝛽 release, interacting with the processes of bone
remodeling and osseointegration [12, 37, 44].
12
BioMed Research International
Finally, the groups studied did not present similar success
and survival rates, G3 with intermediate to high torque had
the worst rates (64.3%) as 5 implants failed, and G4 had
the best rates (95%) with only one failure after occlusal
loading. Moreover, G4 also had the highest ISQ across the
different time intervals, with the lowest stability reduction
relative to the other groups, higher IL-1𝛽 concentration,
and lower TNF-𝛼 concentration at month 12, positive bone
remodeling in addition to a positive correlation between MBL
baseline and the ISQ 12 months (P = 0.033; Rk = 0.618).
These results are in agreement with the in vivo study of
Rea et al. (2015), wherein higher IT showed a tendency to
exhibit lower bone crest resorption but also had a reduced
implant bone contact [17]. However, another clinical study
found that single implants installed with an IT of 68.3 (6.0)
Ncm had a 32-fold greater bone loss rate than regular IT
implants installed with 30.4(±6.1) Ncm, within 3 months
of healing. After occlusal loading at 12 months, this rate
decreased drastically, but was still 2 times higher in the high
IT group [20]. Thus, elevated IT can compromise marginal
bone remodeling due to osseocompression in mandibular
cortical bones [16]. Although one recent study found that
bicortical bone contact increases the implant stability [30],
the type of anchorage did not significantly influence the IT
in this study (p > 0.05).
The limitations of this study include the use of NDI
without using other types of implants as a control group; the
IT was determined with a manual wrench and the sample
was split in 4 relatively small IT groups to provide results on
the influence of IT during osseointegration, both pre- and
postoperative occlusal loading. However, in the correlation
analysis, all IT data are grouped together.
5. Conclusion
The null hypothesis was rejected because IT was associated
with different success and survival rates, although IT did not
significantly influence clinical peri-implant health outcomes
nor IL-1𝛽 or TNF-𝛼 biomarker expression. The survival and
success rates suggest that the ideal IT for atrophic fully
edentulous patients may deviate from the standardized IT
implants with IT > 45 Ncm
of 32 Ncm. In this study,
presented more favorable clinical and biological results after
12 months of osseointegration. Weak correlations between
IT and primary stability were observed in this study; the
results also demonstrate that the expected probing depth is a
function of time since edentulism, bone type, and mandibular
atrophy.
Data Availability
The data used to support the findings of this study are
available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest
regarding the publication of this paper.
Acknowledgments
This study was financed in part by Coordenac¸˜ao de Aper-
feic¸oamento de Pessoal de N´ıvel Superior-Brasil (CAPES),
Finance Code 001. This study was funded by the National
Council
for Scientific and Technological Development
(CNPq, Grant 476170/2013-3) and by Neodent’s Research
Support Program.
Supplementary Materials
Tables S1 and S2 list medians (min-max) and present the com-
parisons between groups at different evaluation periods for
the implant stability quotient and proinflammatory markers,
respectively. (Supplementary Materials)
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| null |
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Availability of data and materials Qualified researchers may request access to patient-level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://www.clinicalstudydatarequest.com .
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Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
https://doi.org/10.1186/s12884-019-2192-z
R E S E A R C H A R T I C L E
Open Access
Epidemiology of influenza in pregnant
women hospitalized with respiratory illness
in Moscow, 2012/2013–2015/2016: a
hospital-based active surveillance study
Svetlana Trushakova1*, Lidiya Kisteneva1, Beatriz Guglieri-López2, Evgenia Mukasheva1, Irina Kruzhkova1,
Ainara Mira-Iglesias2, Kirill Krasnoslobodtsev2, Ekaterina Morozova2, Ludmila Kolobukhina1, Joan Puig-Barberà2†
and Elena Burtseva1†
Abstract
Background: To better understand the impact of seasonal influenza in pregnant women we analyzed data collected
during four seasons at a hospital for acute respiratory infection that specializes in treating pregnant women.
Methods: This was a single-center active surveillance study of women 15–44 years of age hospitalized for acute
respiratory diseases between 2012/2013 and 2015/2016 in Moscow, Russian Federation. Women had to have been
hospitalized within 7 days of the onset of symptoms. Swabs were taken within 48 h of admission, and influenza was
detected by reverse transcription-polymerase chain reaction.
Results: During the four seasons, of the 1992 hospitalized women 1748 were pregnant. Laboratory-confirmed influenza
was detected more frequently in pregnant women (825/1748; 47.2%) than non-pregnant women (58/244; 23.8%) (OR for
influenza = 2.87 [95% CI, 2.10–3.92]; p < 0.001). This pattern was homogenous across seasons (p = 0.112 by test of
homogeneity of equal odds). Influenza A(H1N1)pdm09 was the dominant strain in 2012/2013, A(H3N2) in 2013/2014, B/
Yamagata lineage and A(H3N2) in 2014/2015, and A(H1N1)pdm09 in 2015/2016. Influenza-positive pregnant admissions
went to the hospital sooner than influenza-negative pregnant admissions (p < 0.001). The risk of influenza increased by
2% with each year of age and was higher in women with underlying conditions (OR = 1.52 [95% CI, 1.16 to 1.99]).
Pregnant women positive for influenza were homogeneously distributed by trimester (p = 0.37 for homogeneity; p = 0.49
for trend). Frequencies of stillbirth, delivery, preterm delivery, and caesarean delivery did not significantly differ between
influenza-positive and influenza-negative hospitalized pregnant women or between subtypes/lineages.
Conclusions: Pregnant women are at increased risk for hospitalization due to influenza irrespective of season, circulating
viruses, or trimester.
Keywords: Influenza, Pregnancy, Hospitalization, Surveillance
* Correspondence: [email protected]
†Joan Puig-Barberà and Elena Burtseva contributed equally to this work.
1Ministry of Health of the Russian Federation, FSBI “N.F. Gamaleya NRCEM”,
16, Gamaleya str, Moscow, Russia Moscow 123098, Russian Federation
Full list of author information is available at the end of the article
© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 2 of 14
Background
Pregnant women are at increased risk of severe influenza
illness and influenza-related death [1–3] and, during all
trimesters, are at increased risk of hospital admission
due to influenza infection [4]. Influenza illness during
pregnancy also appears to be associated with increased
rates of stillbirth, neonatal death, preterm delivery, and
[5–7]. In 2012,
the World Health
low birth weight
Organization expanded its recommendations for sea-
sonal influenza vaccination to all pregnant women [8].
Maternal influenza vaccination does not pose a risk to
the developing fetus [9] and may reduce stillbirth,
growth restriction, and preterm birth [10–12].
Due to small study populations and designs that limit
interpretation, the real impact of influenza on pregnant
women remains uncertain [4, 13, 14]. Also, many of the
severe cases of influenza analyzed occurred during the
2009 A(H1N1)pdm09 pandemic, when women may have
been
[4].
Additional data are therefore needed to evaluate and
support vaccination policies for pregnant women.
precautionary
hospitalized
reasons
for
The present investigation was conducted as part of the
Global Influenza Hospital Surveillance Network (GIHSN)
that aims to generate data on the impact of influenza virus
infection during hospitalization. The GIHSN is an inter-
national collaboration launched in 2012 to improve un-
derstanding of influenza epidemiology and better inform
public health policy decisions [15]. The GIHSN has run a
multinational, prospective, hospital-based active surveil-
lance study to collect epidemiological data over several
consecutive years. Sites included in the GIHSN use a stan-
dardized protocol and standard operating procedures,
allowing results to be compared and pooled [16].
Since 2012, the Federal Budget Institute of Health “Clin-
ical Hospital for Infectious Diseases No. 1” (CHID#1) in
Moscow, Russian Federation has participated in the
GIHSN. CHID#1 is one of two reference hospitals for
acute respiratory diseases in Moscow and specializes in
treating pregnant women. Here, we analyzed data col-
lected from CHID#1 during the four seasons since its in-
clusion in the GIHSN (2012/2013 to 2015/2016)
to
influenza in hospitalized
describe the epidemiology of
pregnant women, and evaluate the clinical symptoms and
outcomes of influenza-associated acute respiratory illness
in this population.
Methods
Study design
This was a prospective, active surveillance hospital-based
study conducted during the 2012/2013 to 2015/2016 in-
fluenza seasons at CHID#1 (Moscow, Russian Federation).
CHID#1 is a unique, specialized department hospital for
pregnant women with infectious diseases. The study was
performed due to the study site’s participation in the
GIHSN international influenza surveillance project and its
use of the standardized GIHSN protocols [16].
Study conduct
Patients admitted to the participating hospital were in-
cluded, after written consent, if they were residents in the
predefined hospital’s catchment area, presented with an
acute illness possibly related to influenza, were not institu-
tionalized, and were admitted within 7 days of the onset of
symptoms. Patients discharged during the previous 30
days were excluded. Swabs were collected within 48 h
from patients meeting the inclusion criteria and tested by
reverse transcription-polymerase chain reaction (RT-PCR)
for influenza. Influenza-positive samples were sub-typed
by RT-PCR to identify A(H1N1)pdm09, A(H3N2), B/
Yamagata-lineage, and B/Victoria-lineage strains.
The present analysis was limited to women 15–44 years
of age admitted with an acute respiratory infection, in line
with the age range used by others [3, 17, 18]. All other as-
pects of patient selection were in accordance with the
GIHSN study protocol [16]. RT-PCR was conducted at the
World Health Organization National Influenza Centre at
the Ivanovsky Institute of Virology (Moscow, Russian
Federation) using Amplisens® Ribo-sorb and Ribo-prep
(Federal Budget Institute of Science “Central Research In-
stitute for Epidemiology”, Moscow, Russian Federation) or
a PREP-NA DNA/RNA extraction kit (DNA-Technology,
Moscow, Russian Federation) to extract RNA; a Reverta-L
kit (Federal Budget Institute of Science “Central Research
Institute for Epidemiology”) for reverse transcription; and
kits from Federal Budget Institute of Science “Central Re-
search Institute for Epidemiology” and DNA-Technology to
amplify influenza A, A(H1N1), A(H3N2), A(H1N1)pdm09,
and B genes.
Socio-demographic and clinical information were col-
lected by face-to-face interviews with patients or attending
physicians or by reviewing clinical records. Information
collected included socio-demographic characteristics, the
major complaint at admission, smoking habits, underlying
conditions, vaccination status, pregnancy outcome during
the current admission, clinical course, and major diagnosis
at discharge. Registered pregnancy outcomes included
abortion (terminated at < 20 weeks gestational age), still-
birth (≥ 20 weeks gestational age with no heartbeat or re-
spiratory effort), delivery (birth at any gestational age with
heartbeat or respiratory effort), live preterm birth (< 37
weeks of gestation), low birth weight (< 2500 g), perinatal
death (i.e. during the mother’s current admission), and
caesarean delivery. Main discharge diagnoses were re-
corded by the physician using ICD-10 codes.
Statistical analysis
Statistical analyses were restricted to women recruited in
periods with continuous influenza circulation, defined as
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 3 of 14
≥2 admissions positive for influenza in ≥2 consecutive
weeks. Calendar time (admission week) was modeled
using restricted cubic splines with four knots. The num-
ber of knots was set based on the Akaike information
criterion [19]. The odds ratio (OR) of admission with in-
fluenza was calculated by bivariate logistic regression
using the category with the lowest value as the reference.
To estimate the significance of differences among
groups, chi-squared, likelihood ratio, t, and nonparamet-
ric K-sample tests were used. For comparisons of con-
tinuous variables among multiple categories, equality of
medians and one-way analysis of variance were used
with Scheffe correction for multiple comparisons. Likeli-
hood ratio tests were used to check for confounding,
interaction, linearity, and clustering. The adjusted odds
ratio (aOR) of influenza among pregnant women was es-
timated by multivariate logistic regression using minimal
sufficient adjustment by variables identified as con-
founders by causal diagrams (e.g. age, underlying condi-
tions, smoking habits, admissions in the previous year,
time to swab, season, and epidemiological week at ad-
mission). The goodness of fit of the models was assessed
using Akaike and Bayesian information criteria. Condi-
tional plots [20] of the average predicted probability of
influenza positive admission and of pregnancy outcome
during admission were used to assess complex relation-
ships between the pregnant women’s age in years, their
infant’s gestational age, underlying conditions, influenza
infection, and infection by A subtype or B lineage. The
predicted probabilities of either influenza infection or
pregnancy outcomes during admission were adjusted by
age, smoking habits, chronic underlying conditions, ad-
mission in previous 12 months, pregnancy trimester,
time to swab, and calendar time (season-week) as re-
stricted cubic splines. All p-values were two-tailed. A
Table 1 Influenza infection status in pregnant admissions
p-value < 0.05 was considered to indicate statistical
significance. Heterogeneity in the effects of risk factors
were quantified using the I2 test, with heterogeneity defined
as an I2 > 50%. All statistical analyses were performed with
Stata/SE version 14 (College Station, TX, USA).
Results
Included population
The study included 1992 women 15–44 years old admitted
for acute respiratory diseases between 2012/2013 and 2015/
2016. Of these admissions, 1748 were pregnant (Table 1). In-
fluenza was detected in 47.2% (825/1748) of pregnant admis-
sions and 23.8% (58/244) of non-pregnant admissions (OR
for influenza = 2.87 [95% confidence interval (CI), 2.10–3.92];
p < 0.001; data not shown). Proportions of pregnant admis-
sions with influenza were similar during the four influenza
seasons included (48.7% in 2012/2013, 44.5% in 2013/2014,
52.4% in 2014/2015, and 44.9% in 2015/2016; p = 0.112 by
test for homogeneity of equal odds; data not shown). Propor-
tions of non-pregnant women with influenza were not ana-
lyzed further because of insufficient numbers. The main
comparative assessment and conclusions were made by com-
paring hospitalized influenza-positive pregnant women with
hospitalized influenza-negative pregnant women.
Influenza circulation
During the four influenza seasons included in this study, in-
fluenza was detected during similar periods, although the
season varied from as short as 13 weeks in 2014/2015 to as
long as 24 weeks in 2015/2016, and the peak occurred as
early as week 3 in 2015/2016 and as late as week 11 in
2013/2014 (Fig. 1). Influenza A(H1N1)pdm09 was the
dominant strain in 2012/2013, A(H3N2) in 2013/2014, B/
Yamagata lineage and A(H3N2)
in 2014/2015, and
A(H1N1)pdm09 in 2015/2016 (Table 1 and Fig. 1).
RT-PCR result
RT-PCR result
Positivea
Negative
Influenza type
A(H1N1)pdm09
A(H3N2)
B/Yamagata lineage
B/Victoria lineage
A not subtyped
B undetermined lineage
n (%)
2012/2013
N = 520
253 (48.7)
267 (51.3)
155 (61.3)
41 (16.2)
6 (2.4)
10 (4.0)
5 (2.0)
36 (14.2)
Abbreviation: RT-PCR reverse transcription-polymerase chain reaction
aTest of homogeneity (equal odds) between seasons: p = 0.1176
2013/2014
N = 335
149 (44.5)
186 (55.5)
13 (8.7)
100 (67.1)
34 (22.8)
2 (1.3)
0 (0.0)
0 (0.0)
2014/2015
N = 296
155 (52.4)
141 (47.6)
11 (7.1)
62 (40.0)
66 (42.6)
5 (3.2)
1 (0.6)
10 (6.5)
2015/2016
N = 597
268 (44.9)
329 (55.1)
182 (67.9)
21 (7.8)
0 (0.0)
59 (22.0)
2 (0.7)
4 (1.5)
All seasons
N = 1748
825 (47.2)
923 (52.8)
361 (43.8)
224 (27.2)
106 (12.8)
76 (9.2)
8 (1.0)
50 (6.1)
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 4 of 14
Fig. 1 Number of pregnant admissions with acute respiratory infection by influenza subtype/lineage and season
Characteristics of influenza-positive pregnant admissions
Influenza-positive pregnant admissions were slightly older
than influenza-negative pregnant admissions (median = 28.5
vs. 28.0 years; OR = 1.02 [95% CI, 1.00–1.04]; p = 0.021)
(Table 2). The risk of a positive influenza result increased
with each year by 2% (95% CI, 0–4%). When analyzed by
subtype/lineage, the probability of influenza infection in-
creased with each additional year by 3% (95% CI, 1–6%; p =
0.012) for A(H1N1)pdm09 and by 9% (4–13%; p < 0.001)
for B/Yamagata lineage but was unaffected by age for
A(H3N2) and B/Victoria lineage (Table 4). Median ages were
higher for pregnant admissions positive for B/Yamagata-line-
age
for
A(H1N1)pdm09 (29 years; p = 0.01), A(H3N2) (28 years; p =
0.006), or B/Victoria lineage (28 years; p = 0.006) (Table 3).
Minor but significant differences in age were found for preg-
nant admissions by influenza subtype/lineage (p = 0.028).
those positive
than for
(30 years)
viruses
Pregnant admissions who were positive for influenza
more frequently had underlying conditions than those
who were negative for influenza (OR = 1.52 [95% CI,
1.16–1.99], p = 0.003; aOR = 1.56 [95% CI, 1.16–2.08])
(Tables 2 and 4). This was confirmed for influenza
A(H3N2) and A(H1N1) but not for the two B lineages
significant differences between
(Table 4). However,
influenza-positive
pregnant
admissions were not detected for the individual under-
lying conditions (Tables 2 and 3).
influenza-negative
and
age
was
similar
Gestational
in
distribution
influenza-positive and influenza-negative pregnant admis-
sions (p = 0.872) (Table 2). Pregnant admissions positive
for influenza were homogeneously distributed by trimester
(p = 0.37 for homogeneity in the distribution of estimates
and p = 0.49 for trend, data not shown). The OR of admis-
sion with any influenza did not differ between the first
and second trimesters (1.19 [95% CI, 0.92–1.53]; p = 0.16)
or between the first and third trimesters (1.12 [95% CI,
0.86–1.46]; p = 0.39). Most of the pregnant admissions in-
fected with influenza A(H1N1)pdm09 or B/Victoria
lineage were in the first or second trimester, whereas most
of those infected with influenza A(H3N2) or B/Yamagata
lineage were in the second or third trimester, resulting in
a significant difference in strain distribution by trimester
(p = 0.005) (Table 3); however, aORs for each strain did
not differ between trimesters (Table 4). Likewise, distribu-
tions of gestational age at admission differed significantly
by virus subtype/lineage, and median gestational ages were
lower for admissions positive for A(H1N1)pdm09 (21
weeks) than for admissions positive for A(H3N2) (25
weeks; p = 0.032 [data not shown]) or B/Yamagata lineage
(24 weeks; p = 0.027 [data not shown]) (Table 3). Condi-
tional plots did not reveal interactions between trimester
and patient age or presence of underlying conditions for
the risk of admission with any influenza or with each sub-
type/lineage (Additional file 1).
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 5 of 14
Table 2 Characteristics of pregnant admissions by influenza infection status
Characteristic
Age group, n (%)
15–19 years
20–24 years
25–29 years
30–34 years
35–39 years
40–44 years
Median age in years (range)
Underlying conditions, n (%)
One or more
Cardiovascular disease
Chronic obstructive pulmonary disease
Asthma
Diabetes a
Renal impairment
Cirrhosis
Neuromuscular a
Neoplasm
Rheumatic disease a
Antiviral treatment before admission, n (%)
Influenza vaccination, n (%)
Trimester of pregnancy, n (%)
First (0–13 weeks)
Second (14–26 weeks)
Third (27–42 weeks)
Gestational age at admission (weeks), median (range)
Smoking habits, n (%)
Never smoked
Past smoker
Current smoker
Socioeconomic class, n (%)
Managerial
Skilled
Unskilled
Other
Time from onset of symptoms to swabbing, n (%)
0–2 days
3–4 days
5–7 days
Days from onset of symptoms to swabbing, median (range)
aOdds ratio estimated by exact logistic regression
bN = 823
cN = 918
Influenza positive
N = 825
27 (3.3)
153 (18.6)
327 (39.6)
206 (25.0)
89 (10.8)
23 (2.8)
28.5 (15–44)
136 (16.5)
28 (3.4)
12 (1.5)
10 (1.2)
0 (0.0)
68 (8.2)
9 (1.1)
1 (0.1)
2 (0.2)
0 (0.0)
8 (1.0)
3 (0.4)
177 (21.5)
355 (43.0)
291 (35.3)
23 (1–41) b
563 (68.2)
225 (27.3)
37 (4.5)
359 (43.5)
360 (43.6)
34 (4.1)
72 (8.7)
559 (67.8)
228 (27.6)
38 (4.6)
2 (0–7)
Influenza negative
OR (95% CI)
P-value
N = 923
42 (4.6)
214 (23.2)
341 (36.9)
214 (23.2)
94 (10.2)
18 (2.0)
28 (15–44)
106 (11.5)
23 (2.5)
8 (0.9)
6 (0.7)
3 (0.3)
54 (5.9)
8 (0.9)
0 (0.0)
2 (0.2)
1 (0.1)
11 (1.2)
3 (0.3)
221 (23.9)
372 (40.3)
325 (35.2)
23 (3–42) c
602 (65.2)
268 (29.0)
53 (5.7)
383 (41.5)
425 (46.0)
52 (5.6)
63 (6.8)
541 (58.6)
280 (30.3)
102 (11.1)
2 (0–7)
1
1.11 (0.66–1.88)
1.49 (0.90–2.48)
1.50 (0.89–2.52)
1.47 (0.84–2.59)
1.99 (0.91–4.35)
1.02 (1.00–1.04)
1.52 (1.16–1.99)
1.37 (0.78–2.40)
1.68 (0.69–4.14)
1.87 (0.68–5.17)
0.29 (0.00–2.71)
1.44 (0.99–2.09)
1.26 (0.48–3.28)
1.12 (0.03–∞)
1.12 (0.16–7.95)
1.12 (0.00–43.63)
0.81 (0.32–2.02)
0.99 (0.97–1.01)
1
1.19 (0.93–1.52)
1.12 (0.87–1.44)
1.00 (0.99–1.01)
1
0.90 (0.73–1.11)
0.75 (0.48–1.15)
1
0.9 (0.74–1.10)
0.7 (0.44–1.10)
1.22 (0.84–1.76)
1
0.79 (0.64–0.97)
0.36 (0.24–0.53)
0.87 (0.82–0.93)
0.692
0.122
0.128
0.178
0.086
0.021
0.003
0.268
0.256
0.227
0.294
0.053
0.637
0.944
0.912
1.000
0.652
0.392
0.162
0.388
0.872
0.317
0.188
0.324
0.121
0.29
0.027
0.001
0.001
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 6 of 14
Table 3 Characteristics of pregnant admissions by influenza subtype/lineage
Characteristic
Age group, n (%)
15–19 years
20–24 years
25–29 years
30–34 years
35–39 years
40–44 years
Age (years), median (range)
Underlying conditions a, n (%)
None
One or more
Cardiovascular disease
Chronic obstructive pulmonary disease
Asthma
Renal impairment
Cirrhosis
Neoplasm
Trimester of pregnancy, n (%)
First (0–13 weeks)
Second (14–26 weeks)
Third (27–42 weeks)
Gestational age at admission (weeks), median (range)
Time from onset of symptoms to swabbing, n (%)
0–2 days
3–4 days
5–7 days
Days from onset of symptoms to swabbing, median (range)
A(H1N1)pdm09
N = 361
A(H3N2)
N = 224
B/Yamagata
N = 106
B/Victoria
N = 76
11 (3.0)
69 (19.1)
146 (40.4)
91 (25.2)
37 (10.2)
7 (1.9)
29 (17–44)
300 (83.1)
61 (16.9)
16 (4.4)
7 (1.9)
4 (1.1)
31 (8.6)
3 (0.8)
2 (0.6)
87 (24.1)
167 (46.3)
106 (29.4)
21 (2–41)
263 (72.9)
83 (23.0)
15 (4.2)
2 (0–7)
10 (4.5)
40 (17.9)
95 (42.4)
51 (22.8)
21 (9.4)
2 (1.9)
17 (16.0)
31 (29.2)
31 (29.2)
18 (17.0)
2 (2.6)
19 (25.0)
30 (39.5)
23 (30.3)
1 (1.3)
7 (3.1)
28 (15–43)
7 (6.6)
30 (19–42)
1 (1.3)
28 (19–45)
182 (81.3)
42 (18.8)
92 (86.8)
14 (13.2)
7 (3.1)
5 (2.2)
3 (1.3)
20 (8.9)
2 (0.9)
0 (0.0)
46 (20.5)
81 (36.2)
97 (43.3)
25 (4–40)
167 (74.6)
48 (21.4)
9 (4.0)
2 (0–6)
1 (0.9)
0 (0.0)
1 (0.9)
7 (6.6)
2 (1.9)
0 (0.0)
17 (16.0)
44 (41.5)
45 (42.5)
24 (7–41)
57 (53.8)
47 (44.3)
2 (1.9)
2 (0–5)
63 (82.9)
13 (17.1)
3 (3.9)
0 (0.0)
1 (1.3)
8 (10.5)
1 (1.3)
0 (0.0)
11 (14.5)
40 (52.6)
25 (32.9)
27 (7–38)
42 (55.3)
29 (38.2)
5 (6.6)
2 (0–6)
P-value
0.028
0.004
0.666
0.666
0.376
0.276
0.988
0.819
0.803
0.521
0.005
0.017
0.001
0.001
Abbreviation: NC not calculated
aDiabetes, neuromuscular disease, and rheumatic disease are not listed because of low numbers
Table 4 Adjusted estimates of risk factors for influenza in pregnant admissions overall and by subtype/lineage
Characteristic
Age (in years) b
Trimester
First (0–13 weeks)
Second (14–26 weeks)
Third (27–42 weeks)
Underlying conditions
No
Yes
Adjusted odds ratio (95% CI) a
Any influenza
N = 825
1.02 (1.00–1.04)
1
1.19 (0.92–1.53)
1.12 (0.86–1.46)
A(H1N1)pdm09
A(H3N2)
N = 361
1.03 (1.01–1.06)
N = 224
1.00 (0.97–1.03)
1
1.17 (0.82–1.66)
0.82 (0.56–1.19)
1
0.89 (0.58–1.35)
1.23 (0.82–1.87)
B/Yamagata
N = 106
1.09 (1.04–1.13)
1
1.34 (0.75–2.51)
1.57 (0.86–2.88)
B/Victoria
N = 76
0.96 (0.91–1.00)
1
2.28 (1.10–4.74)
1.58 (0.72–3.44)
1
1.56 (1.16–2.08)
1
1.78 (1.19–2.67)
1
1.91 (1.25–2.93)
1
1.24 (0.66–2.34)
1
2.01 (0.39–3.15)
aAdjusted for age, trimester, comorbidity, hospital admission in the previous 12 months, time to swab, and season-week
bComparison group was influenza-negative pregnant women
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 7 of 14
Rates of influenza vaccination and antiviral use before
admission were ≤ 1% and did not differ between
influenza-positive and -negative pregnant admissions
(Table 2). Smoking habits and socioeconomic class also
did not differ between pregnant admissions positive or
negative for influenza.
The probability of laboratory-confirmed influenza de-
creased with the time between symptom onset and swab-
bing (Table 2). The probability was lowest in pregnant
admissions with samples taken 5–7 days after the onset of
symptoms (OR = 0.36 [95% CI, 0.24–0.53]; p = 0.001). The
adjusted probability of a positive result decreased 13%
(95% CI, 7–18%) for each day between onset of symptoms
and swabbing (aOR = 0.87 [95% CI, 0.82–0.93]). Time to
swabbing differed significantly between subtypes/lineages,
although the median was 2 days in all cases (Table 3).
Clinical manifestations of influenza in pregnant
admissions and pregnancy outcomes
Fever (p < 0.001), cough (p < 0.001), and myalgia (p
< 0.001) were reported as presenting complaints more often
in influenza-positive than influenza-negative pregnant admis-
sions (Table 5). Overall, fever, reported by 97.1%, was the
most common presenting complaint in pregnant admissions
positive for influenza. The aOR for influenza-positive vs.
influenza-negative pregnant admissions was 6.34 (95% CI,
4.01–10.03) for fever and 2.76 (95% CI, 2.13–3.43) for cough
(Table 6). Cough was a common presenting complaint in
pregnant admissions infected with B/Victoria lineage (86.8%)
and A(H1N1)pdm09 (82.0%) but less common for those in-
fected with A(H3N2) (69.2%) or B/Yamagata lineage (72.6%)
(p < 0.001) (Table 7). Proportions of pregnant admissions
reporting all other symptoms (headache, malaise, myalgia,
sore throat, and dyspnea) also differed significantly by sub-
type/lineage. For example, dyspnea was a major presenting
complaint in 18.0% of admissions with A(H1N1)pdm09 but
in less than 5% of admissions with A(H3N2) or B/Yamaga-
ta-lineage and in 6.6of admissions with B/Victoria-lineage.
Hospital admission occurred sooner after the onset of
symptoms in influenza-positive than influenza-negative
pregnant admissions (p < 0.001) (Table 5). This was par-
ticularly the case for pregnant admissions positive for in-
fluenza A(H1N1)pdm09 and influenza A(H3N2), where
more than half went to the hospital on the first day
(Table 7). In contrast, more than half of pregnant admis-
sions positive for influenza B went to the hospital by the
second day. As a result, the time to symptom onset dif-
fered significantly by strain (p < 0.001).
The overall length of hospital stay was not significantly
longer for pregnant admissions positive for influenza
than those negative for influenza (Table 5); however,
pregnant women hospitalized for > 4 days had a higher
probability of laboratory-confirmed influenza than those
hospitalized for ≤4 days (OR = 1.62 [95%CI, 1.21–2.16];
p = 0.001; data not shown). The median hospital stay
also differed by subtype/lineage (Table 7) and was longer
for pregnant women infected with B/Yamagata lineage
than for those infected with influenza B/
(6.4 days)
Victoria lineage (4.9 days; p = 0.025 [data not shown]) or
A(H1N1)pdm09 (5.4 days; p = 0.074 [data not shown])
but did not differ for those infected with A(H3N2) (5.9
days; p > 0.05 [data not shown]).
At discharge, the most common diagnosis was other re-
spiratory infections (n = 923; 52.8%) followed by influenza (n
= 725; 41.5%) (Table 5). Pneumonia was the main discharge
diagnosis in 22 cases (0.1%) and did not differ between
influenza-positive and influenza-negative pregnant women.
Only one influenza-positive pregnant admission and
only two influenza-negative pregnant admissions were
hospitalized in an intensive care unit (Table 5). No
deaths were reported.
Pregnancy outcomes
Pregnancy outcomes recorded during the hospital stay
(pregnancy
included 177 live births, 53 abortions
stopped before 20 weeks), and 23 stillbirths (Table 5).
No perinatal deaths were reported (data not shown).
Abortion was more frequent
in influenza-negative
than influenza-positive pregnant admissions (4.2% vs.
1.7%; p = 0.003), although the frequency did not signifi-
cantly differ by subtype/lineage (Table 7). Frequencies of
other pregnancy outcomes did not differ between
strains, but predicted probabilities were higher for still-
birth in women infected with B/Yamagata, for cesarean
delivery in women infected with A(H3N2), and for pre-
term delivery and low birth weight in women infected
with B/Victoria (Table 8 and Fig. 2).
Age had a noticeable impact on pregnancy out-
comes in influenza-positive admissions. The probabil-
ities of abortion and stillbirth increased with the
mother’s age, whereas the probability of live birth de-
creased with the mother’s age (Fig. 2). Probabilities of
preterm delivery, caesarean delivery, and low birth
weight were highest
in the youngest mothers, al-
though a second smaller increase in probability oc-
curred in women 35–40 years of age.
Discussion
In 2012, the World Health Organization identified preg-
nant women and newborns as priority risk groups for
seasonal influenza [8]. However, the full burden of influ-
enza in pregnant women and their infants is poorly
understood due to differences in the design and conduct
of epidemiological studies, and the small numbers of
pregnant women included [21–23]. The present study
describes new epidemiological data of influenza infection
among a large cohort of pregnant women, and the im-
pact of influenza on clinical outcomes in these women
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 8 of 14
Table 5 Clinical manifestations and outcomes in pregnant admissions by influenza infection status
Influenza positive
Influenza negative
N = 825
N = 923
Odds ratio
(95% CI) a
Clinical manifestation/outcome
Signs/symptoms, n (%)
Fever
Headache
Malaise
Myalgia
Cough
Sore throat
Dyspnea
Time from onset of symptoms to admission, n (%)
1 day
2 days
3 days
≥ 4 days
Time from onset of symptoms to admission, median (range)
Length of hospital stay, n (%) b
0–4 days
5 days
6–7 days
Median length of hospital stay (range)
Intensive care unit admission, n (%)
Pregnancy outcome, n (%)
Aborted (< 20 weeks)
Stillborn (≥ 20 weeks)
Live delivery during the current admission, n (%)
Preterm (< 37 weeks) c
Cesarean‡
Low birth weight (< 2500 g) c
Discharge diagnosis, n (%)
Influenza
Pneumonia
Respiratory disease
Other
aEstimated by exact logistic regression
bProportions are relative to deliveries during the current admission
N = 825 for influenza positive, N = 922 for influenza negative
801 (97.1)
445 (53.9)
497 (60.2)
354 (42.9)
630 (76.4)
546 (66.2)
86 (10.4)
392 (47.5)
209 (25.3)
138 (16.7)
86 (10.4)
2 (0–6)
254 (30.8)
155 (18.8)
261 (31.6)
6 (0–35)
1 (0.1)
14 (1.7)
12 (1.5)
78 (9.5)
17 (21.8)
14 (17.9)
3 (3.8)
617 (74.8)
9 (1.1)
187 (22.7)
12 (1.5)
781 (84.6)
489 (53.0)
550 (59.6)
244 (26.4)
511 (55.4)
689 (74.6)
75 (8.1)
384 (41.6)
194 (21.0)
158 (17.1)
187 (20.3)
2 (0–7)
342 (37.1)
128 (13.9)
259 (28.1)
5 (0–31)
2 (0.2)
39 (4.2)
11 (1.2)
99 (10.7)
19 (19.2)
16 (16.2)
12 (12.1)
108 (11.7)
13 (1.4)
736 (79.7)
66 (7.2)
P-value
< 0.001
0.688
0.781
< 0.001
< 0.001
< 0.001
0.098
0.661
0.254
< 0.001
< 0.001
< 0.001
< 0.001
0.627
0.635
0.003
0.631
0.379
0.669
0.753
0.062
6.07 (3.89–9.46)
1.04 (0.86–1.25)
1.03 (0.85–1.24)
2.09 (1.71–2.56)
2.60 (2.12–3.20)
0.66 (0.54–0.82)
1.32 (0.95–1.82)
1
1.06 (0.83–1.34)
0.86 (0.65–1.12)
0.45 (0.34–0.60)
0.85 (0.80–0.91)
1
1.63 (1.23–2.17)
1.36 (1.07–1.72)
1.00 (0.98–1.04)
0.56 (0.05–6.17)
0.39 (0.21–0.73)
1.22 (0.54–2.79)
0.87 (0.64–1.19)
1.17 (0.56–2.45)
1.13 (0.52–2.50)
0.29 (0.08–1.07)
23.34 (18.07–30.13)
0.77 (0.33–1.82)
0.74 (0.06–0.09)
0.19 (0.10–0.36)
< 0.001
0.553
< 0.001
< 0.001
and their infants. Using a prospective, active-surveillance
study design as part of the GIHSN’s hospital-based sur-
veillance [16], we collected data from more than 1700
pregnant women (825 with confirmed influenza) admit-
specializing in acute respiratory
ted to a hospital
infections.
We restricted the analysis to women aged 15–44 years. A
standard age range (often ages 16–49) is usually [24], but
not always [25], chosen to define women of childbearing
age. This age-range includes women who are less likely to
be pregnant [24, 26], as was the case in our investigation –
the number of pregnant women aged 45–50 was exceed-
ingly small (< 1%, data not shown). In addition, the distribu-
tion of pregnant women in the lowest (15–19 year) and
highest (40–44 year) age groups in our study was similar,
and we restricted the childbearing age range (15 to < 44)
according to pregnancy probability in our dataset to ensure
consistency and reasonable estimates.
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 9 of 14
Table 6 Adjusted estimates of risks for clinical manifestations and outcomes in pregnant admissions overall and by subtype/lineage
Adjusted odds ratio (95% CI) a
Any influenza
A(H1N1)pdm09
Characteristic
N = 825
N = 361
Signs and symptoms
A(H3N2)
N = 224
B/Yamagata
N = 106
B/Victoria
N = 76
Fever
Headache
Malaise
Myalgia
Cough
Sore throat
Dyspnea
6.34 (4.01–10.03)
1.03 (0.84–1.25)
1.05 (0.86–1.29)
1.83 (1.48–2.26)
2.76 (2.13–3.43)
0.64 (0.51–0.80)
1.10 (0.78–1.54)
5.81 (3.02–11.17)
1.35 (1.02–1.77)
1.67 (1.26–2.22)
2.62 (1.98–3.47)
4.21 (3.04–5.83)
0.53 (0.39–0.71)
2.31 (1.54–3.49)
6.36 (2.88–10.65)
0.84 (0.62–1.14)
0.72 (0.53–0.98)
1.16 (0.83–1.62)
2.03 (1.47–2.82)
0.71 (0.50–1.00)
0.37 (0.18–0.77)
11.0 (2.65–45.73)
0.72 (0.47–1.09)
0.56 (0.37–0.85)
1.71 (1.11–2.63)
2.14 (1.35–3.38)
0.57 (0.36–0.89)
0.42 (0.16–1.08)
5.56 (1.31–23.51)
1.41 (0.83–2.40)
1.84 (1.01–3.33)
2.12 (1.28–3.52)
4.72 (2.35–9.45)
0.78 (0.44–1.39)
0.98 (0.37–2.63)
Days between symptom onset and admission
Below median
Median or above
Length of stay
Below median
Median or above
Pregnancy outcome
Abortion
Stillbirth
Live delivery
Preterm delivery
Cesarean delivery
Low birth weight
1
1.43 (0.88–2.32)
1
2.06 (1.12–3.79)
1
1.29 (0.56–2.99)
1
1.15 (0.43–3.13)
1
0.96 (0.26–3.51)
1
0.94 (0.77–1.14)
1
1.34 (1.02–176)
1
0.72 (0.53–0.98)
1
0.52 (0.34–0.80)
1
1.08 (0.65–1.77)
0.32 (0.16–0.64)
1.10 (0.47–2.59)
0.74 (0.52–1.06)
0.89 (0.45–1.78)
0.96 (0.45–2.06)
0.34 (0.10–1.10)
0.44 (0.18–1.09)
0.35 (0.07–1.72)
0.78 (0.47–1.29)
1.07 (0.45–2.56)
0.46 (0.12–1.70)
0.28 (0.03–2.13)
0.30 (0.09–1.05)
1.14 (0.30–4.35)
0.64 (0.37–1.10)
0.67 (0.22–2.06)
1.22 (0.44–3.42)
0.42 (0.09–2.06)
0.36 (0.80–1.67)
3 (0.87–10.28)
0.90 (0.45–1.78)
0.42 (0.05–3.31)
0.87 (0.18–4.07)
–
–
1.34 (0.25–7.06)
0.62 (0.20–1.94)
2.42 (0.47–2.38)
–
2.85 (0.29–1.07)
aAdjusted for age, trimester, comorbidity, hospital admission in the previous 12 months, time to swab, and season-week
Influenza infection, confirmed by RT-PCR, accounted for
nearly half of the admissions over the four influenza sea-
sons. The risk of influenza infection was higher in pregnant
than non-pregnant women (OR = 2.87 [95% CI, 2.10–
3.92]), irrespective of subtype/lineage and trimester. This
agrees with a larger multi-country study by the GIHSN
during the 2012/2013 influenza season, which found an
aOR of 3.84 (95% CI, 2.48–5.94) for influenza-related
hospitalization in pregnant vs. non-pregnant women [16].
It also agrees with a meta-analysis of influenza A infection,
which found a combined OR of 2.44 (95% CI 1.22–4.87)
for
vs.
influenza-related hospitalization in pregnant
non-pregnant women, although most of the included stud-
ies were during the 2009 A(H1N1) pandemic season [4].
Underlying conditions, especially anemia, obesity, and
asthma, increase the risk of influenza-related hospitalization
in pregnant women [27, 28]. Although we confirmed this
in the present study, we were unable to detect significant
differences for individual conditions, probably because of
insufficient numbers. The present study also confirmed that
the risk of hospitalization with influenza does not differ by
trimester or influenza subtype, as described by others [29].
Influenza-positive pregnant admissions were hospital-
ized sooner after the onset of symptoms and stayed
slightly longer in the hospital than influenza-negative
pregnant admissions. Pregnant admissions positive for
influenza also more frequently complained of cough,
myalgia, and especially fever than those who were nega-
tive for influenza. This suggests that influenza causes
more severe illness in pregnant women than other kinds
of acute respiratory infection.
Influenza viruses varied substantially between seasons, al-
though all subtypes and lineages resulted in hospitalization.
In addition, demographics, clinical manifestations, and rates
of stillbirth differed slightly between subtypes/lineages. For
example, the risk of influenza infection increased slightly
with the mother’s age. Also, admissions with influenza
A(H1N1)pdm09 or B/Victoria lineage occurred mostly in
the first or second trimester, whereas admissions with influ-
enza A(H3N2) or B/Yamagata lineage occurred mostly in
the second or third trimester. Although influenza B has
been reported to be more frequent in the second trimester
than influenza A [29], these results suggest that the two B
lineages cannot be considered equivalent.
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 10 of 14
Table 7 Clinical manifestations and outcomes in pregnant admissions by influenza subtype/lineage
Manifestation/outcome
Signs/symptoms, n (%)
Fever
Headache
Malaise
Myalgia
Cough
Sore throat
Dyspnea
Time from onset of symptoms to admission, n (%)
1 day
2 days
3 days
≥ 4 days
Time from onset of symptoms to admission (days), median (range)
Length of hospitalization stay, n (%) a
0–4 days
5 days
6–7 days
≥ 8 days
Length of hospitalization stay (days), median (range)
Admission to an intensive care unit, n (%)
Pregnancy outcome, n (%)
Abortion (< 20 weeks)
Stillbirth (≥ 20 weeks)
Live delivery during the current admission
Preterm (< 37 weeks gestational age) b
Cesarean†
Low birth weight (< 2500 g) c
aN = 223 for A(H3N2) and N = 105 for B/Yamagata
bProportions are relative to deliveries
Influenza, especially A(H1N1), is considered a risk
for stillbirth and low birth weight [27, 30–32]. How-
ever, we did not find differences in rates of stillbirth,
preterm delivery, or
caesarean delivery between
influenza-positive and -negative pregnant admissions
In agreement with
or between subtypes/lineages.
this, a recent meta-analysis reported a computed
pooled OR of 1.24 (95% CI, 0.96–1.59) for small for
gestational age, suggesting that
influenza does not
[23]. Unexpectedly, however,
affect birth weight
abortion before 20 weeks was more
in
influenza-negative
than influenza-positive women,
suggesting that other respiratory infections pose a
higher risk of abortion.
frequent
A(H1N1)pdm09
A(H3N2)
B/Yamagata
B/Victoria
N = 361
N = 224
N = 106
N = 76
350 (97.0)
209 (57.9)
240 (66.5)
191 (52.9)
296 (82.0)
215 (59.6)
65 (18.0)
217 (96.9)
104 (98.1)
110 (49.1)
48 (45.3)
114 (50.9)
51 (48.1)
73 (32.6)
42 (39.6)
155 (69.2)
77 (72.6)
74 (97.4)
52 (68.4)
60 (78.9)
35 (46.1)
66 (86.8)
160 (71.4)
71 (67.0)
56 (73.7)
0.009
9 (4.0)
5 (4.7)
5 (6.6)
195 (54.0)
115 (51.3)
29 (27.4)
98 (27.1)
40 (11.1)
28 (7.8)
1 (0–6)
124 (34.3)
85 (23.5)
100 (27.7)
52 (14.4)
5 (0–35)
1 (0.3)
8 (2.2)
2 (0.6)
31 (8.6)
9 (29.0)
3 (9.7)
1 (3.2)
58 (25.9)
31 (13.8)
20 (8.9)
1 (0–6)
61 (27.2)
32 (14.3)
84 (37.5)
46 (20.5)
6 (0–14)
0 (0.0)
32 (30.2)
35 (33.0)
10 (9.4)
2 (0–5)
22 (20.8)
14 (13.2)
39 (36.8)
30 (28.3)
6 (1–16)
0 (0.0)
3 (1.3)
3 (12.5)
2 (1.9)
4 (3.8)
24 (10.7)
14 (13.2)
4 (16.7)
6 (25.0)
1 (4.2)
1 (7.1)
2 (14.3)
0 (0.0)
31 (40.8)
13 (17.1)
16 (21.1)
16 (21.1)
2 (0–5)
26 (34.2)
16 (21.1)
26 (34.2)
8 (10.5)
5 (0–11)
0 (0.0)
0 (0.0)
2 (2.6)
4 (5.3)
2 (50.0)
0 (0.0)
1 (25.0)
P-value
0.924
0.003
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
< 0.001
0.771
0.667
0.050
0.261
0.177
0.391
0.276
These data confirm that pregnant women are at in-
creased risk from seasonal influenza A and B viruses.
With more than 1700 pregnant admissions, this study
provides important and detailed information about the
impact of influenza in pregnant women that can be used
to inform and support vaccination policies in this sus-
ceptible population. Furthermore, our study provides
pregnancy outcome data, which are rarely included in
epidemiological studies of influenza in pregnant women,
and have not been published before.
However, this study had some limitations. The main ana-
lysis was based on comparing hospitalized influenza-positive
with hospitalized influenza-negative pregnant women. We
to
were unable
admissions
pregnant
compare
to
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 11 of 14
Table 8 Predicted probability of pregnancy outcomes during admission according to influenza infection status
Outcome
Abortion (< 20 weeks)
Stillbirth (≥ 20 weeks)
Live delivery during admission
Preterm (< 37 weeks gestational age)
Cesarean delivery
Low birth weight (< 2500 g)
RT-PCR result
No influenza
A(H1N1)pdm09
A(H3N2)
B/Yamagata-lineage
No influenza
A(H1N1)pdm09
A(H3N2)
B/Yamagata-lineage
B/Victoria-lineage
No influenza
A(H1N1)pdm09
A(H3N2)
B/Yamagata-lineage
B/Victoria-lineage
No influenza
A(H1N1)pdm09
A(H3N2)
B/Yamagata-lineage
B/Victoria-lineage
No influenza
A(H1N1)pdm09
A(H3N2)
B/Yamagata-lineage
No influenza
A(H1N1)pdm09
A(H3N2)
B/Victoria-lineage
Adjusted predicted probability a (95% CI)
0.07 (0.05, 0.09)
0.03 (0.01, 0.06)
0.02 (0.00, 0.05)
0.03 (−0.01, 0.06)
0.02 (0.01, 0.03)
0.01 (0.00, 0.02)
0.02 (0.00, 0.05)
0.06 (0.00, 0.12)
0.03 (−0.01, 0.07)
0.11 (0.09, 0.13)
0.09 (0.06, 0.12)
0.09 (0.05, 0.12)
0.11 (0.06, 0.16)
0.08 (0.01, 0.15)
0.13 (0.07, 0.20)
0.27 (0.06, 0.48)
0.19 (0.01, 0.38)
0.06 (−0.05, 0.17)
0.54 (− 0.05, 1.13)
0.18 (0.09, 0.26)
0.09 (−0.02, 0.21)
0.29 (0.09, 0.49)
0.15 (−0.05, 0.36)
0.14 (0.07, 0.21)
0.05 (−0.06, 0.16)
0.06 (−0.06, 0.17)
0.55 (−0.05, 1.15)
n
39
8
3
2
11
2
3
4
2
99
31
24
14
4
19
9
4
1
2
16
3
6
2
12
1
1
1
Abbreviation: RT-PCR reverse transcription-polymerase chain reaction
aAdjusted by age, smoking habits, chronic underlying conditions, admission in last 12 months, trimester, time to swab and season-week
non-pregnant admissions because of insufficient numbers of
non-pregnant women, who are more frequently admitted for
influenza-like illness to other hospitals in Moscow. Nonethe-
less, the unique nature of the CHID#1 study site allowed us
to recruit a substantial number of pregnant women. CHID#1
receives the most pregnant admissions from any of the hos-
pitals in the GIHSN network (over 97% of the total pregnant
admissions based on unpublished GIHSN data from the
2015/2016 season). Another limitation was that we could
not assess the long-term effects of influenza on pregnant
women, or pregnancy outcome beyond the current admis-
sion, because data were collected only from women while
they were hospitalized, and follow-up evaluations until the
end of pregnancy were not within the study protocol. Finally,
the study could have been limited by increasing rates of
hospitalization for pregnant women following the 2009
pandemic due to increased awareness of the risks. However,
this was probably accounted for by recruiting consecutive
admissions without previous knowledge of influenza status.
Furthermore, influenza positivity rates were similar over the
four influenza seasons, suggesting that
this was not a
problem.
Conclusions
Our results confirm that pregnant women are at in-
creased risk from influenza infection irrespective of sea-
son, circulating viruses, or trimester. This supports
recommendations by the World Health Organization [8]
and many countries [33, 34] that pregnant women be
prioritized for seasonal
influenza vaccination. Despite
these recommendations and evidence that influenza vac-
cination is considered safe and effective for pregnant
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 12 of 14
A
C
E
B
D
F
Fig. 2 Predicted probabilities of birth outcomes according to the mother’s age and influenza strain. Predicted probabilities of abortion (a),
stillbirth (b), live delivery (c), preterm delivery (d), cesarean delivery (e), and low birth weight (f) in influenza-positive pregnant admissions.
Probabilities were adjusted by age, smoking habits, chronic underlying conditions, previous admissions, trimester, time to swab, and season-week
women [7, 10, 12, 35, 36], vaccination uptake by preg-
nant women is generally poor [37], as found in the
present study, where < 1% of pregnant women were vac-
cinated. Additional efforts are therefore needed to edu-
cate healthcare workers, public health officials, and
pregnant women about the risks of seasonal influenza
and the importance of vaccination.
Additional file
Additional file 1: Predicted probability of admission with influenza by
(a) trimester, (b-e) subtype/lineage, and (f) overall according to age
group and presence of underlying conditions. Conditional plots
examining interactions between trimester and patient age or presence of
underlying conditions for the risk of admission with any influenza or with
each subtype/lineage. (PDF 35 kb)
Abbreviations
aOR: Adjusted odds ratio; CHID#1: Federal Budget Institute of Health “Clinical
Hospital for Infectious Diseases No. 1”; CI: Confidence interval; GIHSN: Global
Influenza Hospital Surveillance Network; OR: Odds ratio; RT-PCR: Reverse
transcription-polymerase chain reaction
Acknowledgements
Medical writing support was provided by Drs. Phillip Leventhal and Jonathan
M. Pitt (4Clinics, France) and paid for by Sanofi Pasteur.
Funding
The work reported in this manuscript was funded by Sanofi Pasteur.
Trushakova et al. BMC Pregnancy and Childbirth (2019) 19:72
Page 13 of 14
Availability of data and materials
Qualified researchers may request access to patient-level data and related
study documents including the clinical study report, study protocol with any
amendments, blank case report form, statistical analysis plan, and dataset
specifications. Patient level data will be anonymized and study documents
will be redacted to protect the privacy of trial participants. Further details on
Sanofi’s data sharing criteria, eligible studies, and process for requesting
access can be found at: https://www.clinicalstudydatarequest.com.
Authors’ contributions
LK1, IK, and LK2 contributed to data acquisition and enrolling participants; ST,
EM1, KK, and EM2 contributed to laboratory data testing, BG, AM, and ST
contributed to data processing, analysis, or interpretation; JP and EB provided
overall study oversight and JP wrote the first draft of the manuscript. All authors
provided critical review or revisions of the manuscript, approved the final draft,
and agree to be accountable for its accuracy and integrity.
Ethics approval and consent to participate
The study was approved by the “Ethics Committee of Hospital No.1 for
Infectious Diseases”, CHID#1, Moscow Health Department, Russian
Federation. All participants provided written consent. Written consent was
obtained from the parents or legal guardians of participants under the age
of 16 years, in line with the GIHSN protocol and local legislation.
Consent for publication
Not relevant.
Competing interests
ST, LK1, EM1, IK, KK, EM2, LK2, and EB report grants from Fondation Merieux;
grants and non-financial support from Sanofi Pasteur; non-financial support
from The Foundation for the Promotion of Health and Biomedical Research of
Valencia Region (FISABIO) during the conduct of the study (scientific counsel-
ling, discussion on methods, data quality management and analysis, and
provision of the study protocol and questionnaires); and grants from the Influ-
enza Division of the US Centers for Disease Control and Prevention outside of
the submitted work. The remaining authors have no competing interests to
declare.
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
Author details
1Ministry of Health of the Russian Federation, FSBI “N.F. Gamaleya NRCEM”,
16, Gamaleya str, Moscow, Russia Moscow 123098, Russian Federation.
2Fundación para el Fomento de la Investigación Sanitaria y Biomédica
(FISABIO) de la Comunidad Valenciana, Avda Catalunya 21, 46020 Valencia,
Spain.
Received: 5 July 2018 Accepted: 15 January 2019
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| null |
10.1371_journal.pone.0244962.pdf
|
Data Availability Statement: Data cannot be
shared publicly because of this was not permitted
by the consent form signed by participants. Data
are available from the Keller-Lamar Health
Foundation ([email protected]) for researchers
who can provide evidence of IRB approval for
access.
|
Data cannot be shared publicly because of this was not permitted by the consent form signed by participants. Data are available from the Keller-Lamar Health Foundation ( [email protected] ) for researchers who can provide evidence of IRB approval for access.
|
RESEARCH ARTICLE
Feasibility and validation of a web-based
platform for the self-administered patient
collection of demographics, health status,
anxiety, depression, and cognition in
community dwelling elderly
Matthew Calamia1*, Daniel S. Weitzner1, Alyssa N. De Vito1, John P. K. Bernstein1,
Ray AllenID
2, Jeffrey N. Keller2
1 Department of Psychology, Louisiana State University, Baton Rouge, Louisiana, United States of America,
2 Pennington Biomedical Research Center, Baton Rouge, Louisiana, United States of America
* [email protected]
Abstract
The coronavirus disease pandemic has brought a new urgency for the development and
deployment of web-based applications which complement, and offer alternatives to, tradi-
tional one-on-one consultations and pencil-and-paper (PaP) based assessments that cur-
rently dominate clinical research. We have recently developed a web-based application that
can be used for the self-administered collection of patient demographics, self-rated health,
depression and anxiety, and cognition as part of a single platform. In this study we report the
findings from a study with 155 cognitively healthy older adults who received established
PaP versions, as well as our novel computerized measures of self-rated health, depression
and anxiety, and cognition. Moderate to high correlations were observed between PaP and
web- based measures of self-rated health (r = 0.77), depression and anxiety (r = 0.72), and
preclinical Alzheimer’s disease cognitive composite (PACC) (r = .61). Test-retest correla-
tions were variable with high correlations for a measure of processing speed and a measure
of delayed episodic memory. Taken together, these data support the feasibility and validity
of utilization of this novel web-based platform as a new alternative for collecting patient
demographics and the assessment of self-rated health, depression and anxiety, and cogni-
tion in the elderly.
Introduction
The coronavirus disease 19 (Covid-19) pandemic and the resulting direct and indirect impacts
of social distancing dramatically interrupted or stopped clinical research around the world.
These and other realities in the wake of Covid-19 have created a new urgency for the genera-
tion of web-based research platforms which provide alternatives to face-to-face and pencil-
and-paper (PaP) based assessments, and reduce the dependence on the manual transfer of PaP
a1111111111
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a1111111111
a1111111111
a1111111111
OPEN ACCESS
Citation: Calamia M, Weitzner DS, De Vito AN,
Bernstein JPK, Allen R, Keller JN (2021) Feasibility
and validation of a web-based platform for the self-
administered patient collection of demographics,
health status, anxiety, depression, and cognition in
community dwelling elderly. PLoS ONE 16(1):
e0244962. https://doi.org/10.1371/journal.
pone.0244962
Editor: Simone Reppermund, University of New
South Wales, AUSTRALIA
Received: April 17, 2020
Accepted: December 19, 2020
Published: January 19, 2021
Copyright: © 2021 Calamia et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data cannot be
shared publicly because of this was not permitted
by the consent form signed by participants. Data
are available from the Keller-Lamar Health
Foundation ([email protected]) for researchers
who can provide evidence of IRB approval for
access.
Funding: The study was funded by a contract from
the Keller-Lamar Health Foundation (http://www.
PLOS ONE | https://doi.org/10.1371/journal.pone.0244962 January 19, 2021
1 / 15
PLOS ONEkeller-lamar.org/) awarded to MC. The funders had
no role in study design, data collection and
analysis, decision to publish, or preparation of the
manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Web-based platform for self-administered assessments in the elderly
data to an electronic database. Several computerized and web-based applications are currently
available for conducting individualized assessments for a variety of clinical endpoints, includ-
ing cognition [1]. However, these tools typically do not readily interact with a centralized
study database and generally lack the ability to collect the supporting clinical data that accom-
pany clinical research studies (e.g., demographics, secondary endpoint data collection). In
order to address these research gaps, we have created a web-based platform which allows for
the self-administered collection of patient demographics, the delivery and automated scoring
of multiple assessments, and the capability to automatically populate all study data into a single
secure and functional electronic database.
The fastest growing segment of the United States population is those 85 years of age and
older, with age related diseases such as Alzheimer’s disease related dementia (ADRD) expected
to increase from 5 million to 15 million in the next three decades [2, 3]. Recent ADRD research
efforts have focused on developing multicomponent assessments of cognitive function, with
an emphasis on developing composite cognitive assessments that are sensitive enough to mea-
sure the earliest changes relevant to the future development of ADRD. The Alzheimer’s Dis-
ease Cooperative Study Preclinical Alzheimer’s Cognitive Composite (ADCS-PACC) is a PaP
based assessment package that has emerged as the leading clinical research tool for aging, mild
cognitive impairment, and pre-ADRD research. The ADCS-PACC focuses on the assessment
of the three cognitive domains which are the most predictive for the development of ADRD
[4]. The ADCS-PACC is the primary endpoint in one of the largest clinical trials for AD pre-
vention [5], and is a major cognitive endpoint for some of the largest longitudinal and cohort
studies around the world [6]. Computer-based assessments have increasingly been used and
valued for clinical care and research including studies of the elderly [7–9], clinical trials
focused on cognition [10], and longitudinal studies with elderly participants [11, 12]. Cur-
rently there are no computerized/web-based options for the ADCS-PACC even though such
an advance would provide a potential option that decreases the need for face-to-face assess-
ments, manual scoring, manual z-score transformation, and manual data transfer to an elec-
tronic database that currently accompanies all ADCS-PACC efforts.
The current study focused on the validity and feasibility of using the computerized PACC
(cPACC), a novel web-based application which employs a self-administered approach for
elderly participants to provide demographic data as well as undergo assessments of self-rated
health, depression, anxiety, and cognition. Analysis of 155 community dwelling elderly sub-
jects demonstrates the feasibility of collecting data for each of these aspects in a self-adminis-
tered manner that resulted in the automated population of a single, secured, cloud-based
database. We report on the validity for each of the web-based measures with traditional PaP
based assessments and report on their reliability as part of a two-week test-retest design in a
subset of participants.
Methods
The demographic, assessment, and database platform
The platform used in this study was created by developers at Pennington Biomedical Research
Center. The platform consisted of a web application written in Angular v.6 communicating
with an API developed in Microsoft ASP.NET Core v2.1. The participants used the web-based
application to answer a series of questions, and complete the different cognitive tasks, in a self-
guided manner. As each question and assessment was completed the resulting data populated
a central database that contained the demographic profile and assessment scores for each par-
ticipant. All data was stored in a Microsoft SQL Server database. The entire system was oper-
ated as a web application in Microsoft Azure. Data was extracted from Azure using Microsoft
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PLOS ONEWeb-based platform for self-administered assessments in the elderly
SQL Server Management Studio. Administrative rights within the platform were used to con-
trol access to functionality and data.
Participants
Individuals were recruited to the study who were 55–95 years old (inclusive), who did not have
motor or sensory deficits that were sufficient to interfere with the ability of the participant to
complete computerized assessments. Fliers, email blasts to a clinical registry of individuals
aged 50 and over, and word of mouth were used to recruit participants in the Baton Rouge
area. A total of 174 participants met study criteria and provided written informed consent.
Most of these participants (n = 155) had Mini-Mental State Examination (MMSE) scores in a
range suggesting intact global cognition (i.e., greater than or equal to 25) and are the focus of
all analyses other than the one analysis also comparing their scores against a small group of
participants with MMSE scores below 25 (n = 19). Demographic information for the study
sample is provided in Tables 1 and 2. See Table 3 for raw performance data for participants.
Missing data ranged from 0–15 participants across measures.
Table 1. Participant demographics.
MMSE � 25
MMSE < 25
Demographic Variables
Age
Female
Non-Hispanic
Race
Caucasian
African American
Bi-racial
Native American
Highest Degree of Education
GED
Some College
Associate’s Degree
Bachelor’s Degree
Master’s Degree
Doctorate Degree
Marital Status
Married
Widowed
Divorced
Never Married
Common-Law Partner
Living Situation
Living Alone
Residence Type
Single Family Home
Apartment
Assisted Living
Mean (SD)
71.64 (8.13)
-
-
-
-
-
-
-
-
-
-
-
n (%)
-
111 (71.6%)
145 (93.5%)
140 (90.3%)
7 (4.5%)
2 (1.3%)
1 (0.01%)
10 (6.5%)
33 (21.3%)
7 (3.9%)
41 (26.5%)
52 (33.5%)
6 (3.9%)
82 (55.0%)
28 (18.8%)
23 (15.4%)
14 (9.4%)
2 (1.3%)
48 (31.0%)
111 (71.6%)
35 (22.6%)
3 (0.6%)
n (%)
-
6 (31.6%)
18 (94.7%)
17 (89.5%)
1 (5.3%)
-
-
1 (5.3%)
1 (5.3%)
1 (5.3%)
7 (36.8%)
4 (21.1%)
2 (10.5%)
9 (47.4%)
6 (31.6%)
2 (10.5%)
1 (5.3%)
-
5 (26.3%)
11 (57.9%)
4 (21.1%)
3 (15.8%)
Mean (SD)
75.94 (11.10)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Note: Demographic information for some variables was unavailable and therefore not all variables will sum to a total of 155 and 19 individuals. MMSE = Mini-Mental
State Examination
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Table 2. Prevalence of health conditions in entire sample.
Health Condition
Cardiovascular
High Blood Pressure
High Cholesterol
Diabetes
Heart Attack
Atrial Fibrillation
Neurological
Stroke
Parkinson’s Disease
Multiple Sclerosis
Transient Ischemic Attack
Alzheimer’s Disease
Other Dementia
Other Neurological Disease
Concussion/TBI
Psychiatric
Alcohol Abuse
Drug Abuse
Depression
Anxiety
Other
B12 Deficiency
Sleep Apnea
Thyroid Deficiency
Cancer
n (%)
72 (46.5%)
55 (35.5%)
18 (11.6%)
4 (2.6%)
14 (9.0%)
2 (1.3%)
2 (1.3%)
0 (0.0%)
1 (0.6%)
0 (0.0%)
3 (1.9%)
4 (2.6%)
2 (1.3%)
3 (1.9%)
1 (0.6%)
31 (20.0%)
27 (17.4%)
6 (3.9%)
17 (11.0%)
33 (21.3%)
26 (16.8%)
https://doi.org/10.1371/journal.pone.0244962.t002
Procedures
The measures were administered on the same day, with half of the participants completing the
PaP measures first, and the other half completing the web-battery first. Participants completed
the measures in a quiet and private testing room on either a desktop or laptop computer with a
computer mouse. PaP measures were administered by a trained research assistant. Research
assistants remained in the room while participants completed the computerized measures, but
only to address technological issues (e.g., computer froze/internet connection issues) or pro-
vide encouragement to participants.
A subset of the sample with MMSE scores greater than or equal to 25 (n = 55) were ran-
domly selected to complete a second visit approximately two weeks later during which they
repeated the cPACC to assess for test-retest reliability. The first study visit was on June 11,
2018 and the last study visit was on October 9, 2019. All study procedures were approved by
the LSU Institutional Review Board and were conducted according to the principles
expressed in the Declaration of Helsinki. All data collected during the assessment were stored
immediately at the conclusion of each page. Data were written to a Microsoft SQL Server
2014 database and stored as the raw answer provided by the participant. Answers to some
tests such as the participant typing the name and hobby of a person in an image were reported
as the exact text entered by the participant. Other tests using multiple choice answers or clicks
on a grid were scored as number of correct answers and where applicable number of
attempts.
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Table 3. Means and standard deviations for cognitive measures and questionnaires in participants scoring above and below 25 on the MMSE.
MMSE � 25
MMSE < 25
Variables
FNHR-IFR
FNHR-IR
FNHR-DFR
FNHR-DR
GLIR
GLDR
SL
SM
VP
VAS
LM-DR
DSC
FCSRT
GAI
GDS
n
148
149
148
149
155
151
149
140
142
152
152
151
151
152
152
Mean (SD)
6.54 (2.90)
28.33 (2.99)
10.24 (3.52)
14.65 (2.01)
14.05 (3.74)
7.03 (2.67)
10.50 (6.44)
24.54 (7.85)
19.27 (5.58)
83.91 (13.33)
6.19 (3.02)
51.11 (13.49)
47.66 (1.48)
1.61 (3.06)
5.54 (4.84)
Min
0
17
2
0
5
0
0
0
1
18
1
24
31
0
0
Max
14
32
16
16
23
12
24
42
28
100
18
90
49
16
25
n
17
16
15
15
16
19
18
16
16
18
17
17
17
18
17
Mean (SD)
2.53 (2.24)
21.06 (7.04)
4.27 (4.85)
10.00 (3.70)
8.44 (3.98)
3.53 (2.95)
4.00 (3.93)
11.44 (7.25)
9.63 (7.16)
82.33 (12.98)
2.24 (2.93)
28.06 (13.83)
40.24 (10.83)
3.00 (2.68)
8.18 (5.87)
Min
0
10
0
4
4
0
0
0
0
60
0
3
4
0
1
Max
8
31
14
16
18
10
14
24
19
100
8
64
48
10
22
Note: SD = Standard Deviation; MMSE = Mini-Mental State Examination; FNHR-IR = Face Name Hobby Recall Immediate Free Recall; FNHR-IFR = Face Name
Hobby Recall Immediate Recognition; FNHR-DFR = Face Name Hobby Recall Delayed Free Recall; FNHR-DR = Face Name Hobby Recall Delayed Recognition;
GLIR = Grid Locations Immediate Recall; GLDR = Grid Locations Delayed Recall; SL = Symbol Line; VP = Visual Patterns; SM = Speeded Matching; VAS = EQ-5D
Visual Analog Scale; Logical Memory–Delayed Recall; DSC = Digit Symbol Coding; FCSRT = Free and Cued Selective Reminding Test; GAI = Geriatric Anxiety
Inventory; GDS = Geriatric Depression Scale.
https://doi.org/10.1371/journal.pone.0244962.t003
Paper and Pencil (PaP) measures
Questionnaires. Participants completed the EQ-5D Visual Analog Scale (VAS) to assess
self-rated health [13]. For this measure, participants rate their current health on a 0 to 100
scale from the “worst health” to “best health” they can imagine. The EQ-5D VAS is sensitive to
individual differences such as age [14] and physical activity [15]. The Geriatric Anxiety Inven-
tory (GAI) and Geriatric Depression Scale (GDS) were used to assess anxiety and depression,
respectively. The GAI is a 20-item geriatric-focused self-report measure of anxiety-related
symptoms [16]. The GAI demonstrates excellent internal consistency (α = 0.91) and test-retest
reliability (r = 0.91) [16] as well as good convergent validity with worry and anxiety measures
[17]. The Geriatric Depression Scale (GDS) is a 30-item self-report which measures depressive
symptoms in older adults [18]. The GDS demonstrates excellent internal consistency (α =
0.94), good test-retest reliability (r = 0.84) [19], and at least adequate convergent validity with
other depression measures such as the Beck Depression Inventory-II (r = .78) [20]. However,
despite good convergent validity, the discriminant validity of these measures is weak with one
study finding a correlation as high as r = .86 between the GAI and GDS [21]. A 12-item com-
puter proficiency questionnaire [22] was used in order to assess how easily older adults felt
they could perform tasks on a computer (e.g., “Use a keyboard to type”) in a 5-point likert
scale format. The sample had a self-reported mean computer proficiency rating of 3.17 (SD =
.98), indicating that on average, they could somewhat easily perform computer-based tasks.
Alzheimer’s Disease Cooperative Study Preclinical Alzheimer’s Cognitive Composite
(ADCS-PACC). A review by Alzheimer’s disease cooperative study (ADCS) identified episodic
memory, executive function, and orientation as the 3 key cognitive domains linked to the
development of mild cognitive impairment and ADRD [4]. A total of 4 pencil-and- paper
(PaP) cognitive assessments were selected to capture these domains as part of the ADCS
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Preclinical Alzheimer’s Disease Composite (PACC). The ADCS-PACC is comprised of imme-
diate recall on the Free and Cued Selective Reminding Test (FCSRT), delayed recall on one
story from the Logical Memory subtest of the Wechsler Memory Scale-Revised battery, the
Digit-Symbol Test from the Wechsler Adult Intelligence Scale-Revised, and the MMSE.
Studies using the ADCS-PACC have been able to identify study subjects who would go on
to develop clinical biomarkers of ADRDs such as pathological beta amyloid deposition as well
as identify which study subjects exhibit the fastest rate of cognitive decline in longitudinal
studies [23, 24]. Due to these successes, the ADCS-PACC has emerged as one of the most com-
monly utilized cognitive batteries in prominent clinical trials and longitudinal research studies
including the A4 trial and Alzheimer’s Disease Neuroimaging Initiative (ADNI), respectively.
Web-battery measures
Questionnaires. The web battery included survey items regarding participant demo-
graphics (i.e., date, gender, zip code, ethnicity, race, marital status, living situation, and highest
level of education attained), and health history (i.e., a list of conditions presented as a check-
list). Additionally, we collected information on family history of dementia, pain severity and
interference on daily functioning, frequency of exercise, number of medications and medica-
tion adherence, concern about driving and accident history, self-rated health, and subjective
memory complaints that will be a part of future research. Responses to the demographic and
health history questions can be found in Tables 1 and 2.
For the purposes of this study, psychometric validation focused on 1) a self-report measure
of health in which participants make one global rating of their health and 2) a new 17-item
measure of depression and anxiety developed based on widely used measures of depression
and anxiety. Participants were asked to rate how much they felt or experienced certain symp-
toms over the past 2 weeks on a 5-point scale (“not at all” to “extremely”). Given that brief
measures of depression and anxiety show poor discriminant validity [15], these symptoms
were assessed jointly rather than with the aim of developing two separate scales.
Computerized Preclinical Alzheimer’s Cognitive Composite (cPACC). The cognitive measure
in this study was a cognitive composite that was validated against the (ADCS-PACC). Like the
ADCS-PACC, the cPACC was designed to assess the domains of orientation and episodic
memory. The cPACC also includes a measure of processing speed designed to be comparable
to the PaP measure of digit symbol coding which the PACC considers a measure of executive
functioning. Additionally, the cPACC includes measures of working memory given working
memory is related to executive functioning [25], a PACC domain, and a working memory
item is included on the MMSE which is used as part of the PACC.
Orientation. For orientation participants are asked orientation questions on the computer
screen (day, year, time of day, etc.) and select the answers from a list of multiple-choice
response options. Participants receive 1 point for each correct answer.
Face Name Hobby Recall (FNHR). This cPACC component is designed to assess episodic
memory which is one component of the ADCS-PACC. It is based on the short version of the
Face-Name Associative Memory Exam [26–28]. For cPACC Faces and Names the participant
first completes a learning trial in which 8 faces with a name and hobby presented underneath.
The names and hobbies chosen are short in word length (e.g., Amy, Hiker). Faces vary in age,
gender, and race. Stimuli are presented twice and are followed by immediate recall trial each
time in which they have to recall the names and hobbies when presented only with the face by
typing their responses into a text box and then clicking “next” to submit their response. Partic-
ipants receive 1 point each for correctly naming the person’s hobby and their name, for a total
of 2 points per stimulus. An immediate recognition trial then follows in which they must select
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the correct name and hobby from a multiple-choice list by clicking on the correct stimulus.
Participants receive 1 point for correctly selecting each name and hobby for a total of 2 possible
points. After a ~15-minute delay in which they complete other cPACC measures, delayed
recall and recognition trials are completed
Grid locations. The grid location test is a measure of visual episodic memory designed based
on the Visual Spatial Learning Test [29, 30], a measure designed to be a visual equivalent to
verbal list-learning paradigms. Scores on this measure highly correlated with verbal memory
measures [29, 30]. For cPACC grid locations, participants complete two learning and immedi-
ate memory trials in which they see 6 symbols on a 4x4 grid and then have to select the symbols
they saw and put them in the correct location. For each symbol, participants can earn up to 2
points (1 point for selecting the correct symbol and 1 point for placing the symbol in the cor-
rect location), for a possible of 12 points. The same symbols and locations are used for both
learning trials. Participants then complete a delayed memory trial after ~15-minute delay.
Speeded matching. Speeded matching is a measure of processing speed and executive func-
tion that is based on the Wechsler Adult Intelligence Scale—Revised (WAIS-R) Digit Symbol
Coding subtest [31], a measure included in the ADCS-PACC. For the cPACC participants
have 90 seconds to select symbols that correspond to numbers based on a key matching each
unique symbol to a specific number. As participants select symbols, they appear in the blank
boxes above the numbers. As participants complete more matches, additional numbers with
blank boxes above them appear on the screen. Participants receive 1 point for each correctly
selected symbol.
Symbol line. Symbol line is a measure of visual working memory based on Wechsler Mem-
ory Scale—Fourth Edition (WMS-IV) Symbol Span [32]. Participants see a line of symbols and
then have to correctly select which symbols they saw in the correct order (i.e., left to right). Ini-
tially participants are shown only two symbols in a line, but lines of increasing lengths are
added until a participant makes no correct responses or is presented with a trial of 7 symbols.
For each symbol, participants can achieve a total possible of 2 points. If participants recall
incorrect symbols, they receive 0 points. If all of the correct symbols are recalled, but in the
incorrect order, participants receive 1 point. If participants recall the correct symbols in the
correct order, they receive 2 points.
Visual patterns. Visual patterns is a measure of visual working memory based on Wechsler
Memory Scale - 3rd Edition (WMS-III) Spatial Span [33]. Participants see an array of 9 white
boxes and are asked to recall the order in which boxes are turned black. A box that is turned
black returns to white before the next box turns black. Initially participants are shown only
two boxes that are turned black but increasing numbers of boxes are turned black until a par-
ticipant makes no correct responses or is presented with a trial of 7 boxes. Participants receive
1 point for the correct completion for each sequence.
Analyses
Validity. Pearson correlations were used to examine the relationship between scores on
questionnaire measures administered via PaP or the web-battery. For the GAI and GDS, scores
were first converted to z-scores and a composite was created to compare with the web-based
measure of depression and anxiety symptoms. For the web-battery measure of depression and
anxiety, a confirmatory factor analysis (CFA) was first conducted as part of assessing construct
validity to assess whether a one-factor model provided adequate model fit. To compare the
ACDS-PACC and cPACC using a Pearson correlation, individual tests administered were also
first converted to z-scores. Thus, for the PaP, the FCSRT, Logical Memory Delayed Score, and
MMSE total score were individually standardized into Z-scores, and then summed together to
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PLOS ONEWeb-based platform for self-administered assessments in the elderly
create the PaP composite score. To create the cPACC composite score, the number of correct
responses recalled during the Faces and Names immediate and long-delay free recall and multiple
choice, Grid Locations, Symbol Matching, Symbol Line, and Grid Pattern tasks were individually
standardized into Z-scores. These Z-scores were then summed together to create the cPACC com-
posite score. Only two participants in our cognitively intact sample missed an orientation item.
Therefore, orientation was not included when creating a cPACC composite score.
To assess the sensitivity of the cPACC to cognitive impairment, we calculated effect sizes
using Hedges’ g to determine whether the subtests of the cPACC could differentiate between
those with MMSE scores above and below 25. Hedges’ g was used given the large difference in
sample sizes between the cognitively healthy and cognitively impaired group.
Practice effects. Dependent t-tests and Cohen’s d were used to examine practice effects
on the web-battery in the subsample who completed a second visit approximately two weeks
following the initial visit.
Reliability. To assess internal consistency of the measure of depression and anxiety, coef-
ficient alpha was used. To assess the test-retest reliability of the web-battery, Pearson correla-
tions were used to examine the relationship of questionnaire and cognitive test scores
administered within an approximately two-week test-retest interval.
Results
Validity and reliability of the web-based questionnaire
Adequate fit for a one-factor model for the web-battery measure of depression and anxiety
(CFI = 0.91, RMSEA = 0.08) was obtained when allowing for two pairs of correlated residuals
for items with similar content (i.e., “I was easily upset” and “I was easily annoyed”; “I had diffi-
culty stopping myself from worrying” and “I worried a lot.”). Coefficient alpha for this scale
was .91. A high correlation was observed between the web-battery measure of depression and
anxiety and the GDS/GAI composite, r = 0.70. Similarly, a high correlation, r = .77, was
obtained between the web-battery measure of self-rated health and the EQ-5D VAS (see Fig 1).
Fig 1. Relationships between web-based measures and paper and pencil measures. Note: A.) Relationship between
the Computerized Preclinical Alzheimer’s Cognitive Composite (cPACC) and the Alzheimer’s Disease Cooperative
Study Preclinical Alzheimer’s Cognitive Composite (ADCS-PACC). B.) Relationship between the web-battery measure
of depression and anxiety and the GDS/GAI composite score. C.) Relationship between the web-battery measure of
self-rated health and the EQ-5D Visual Analog Scale (VAS).
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PLOS ONEWeb-based platform for self-administered assessments in the elderly
In the 55 older adults in the sample who completed a retest after approximately 2 weeks, the
web-battery measure of depression and anxiety and self-rated health both had high test-retest
correlations, r = 0.85 and r = .0.83, respectively.
Validity and reliability of the Computerized Preclinical Alzheimer’s
Cognitive Composite (cPACC)
The composite scores derived from the cPACC and the ADCS-PACC were found to be mod-
erately related (r = .61) (Fig 1). As an additional exploratory analysis using stepwise regres-
sion showed that this same correlation could be obtained using only a subset of measures:
Speeded Match, the immediate trials of Face Name Hobby Recall, the immediate trials of
Face Name Hobby Recognition, and the delayed trial of Grid Locations (F(4,132) = 22.08,
R2 = .401). See S1 Table for the full results of the regression analysis. In addition, the Speeded
Match task moderately correlated to the Digit Symbol Coding subtest (r = .56), thus demon-
strating convergent validity between a measure of the cPACC and a PaP measure it was
designed to match (see S2 Table for relationships among all measures on the CPACC and the
PaP measures).
All of the measures of working memory, episodic memory, and processing scored as part of
the cPACC battery significantly differed (Hedges’ g ranged from 1.12 to 2.30) between those
above and below an MMSE score of 25, which is a common cutoff for cognitive impairment.
The differences between those above and below an MMSE score of 25 were larger in the com-
posite PaP score compared to the composite cPACC score (Hedges’ g = 2.98 vs 2.31). However,
when the MMSE was removed from the PaP composite score, the differences between those
above and below an MMSE score of 25 were larger in the composite cPACC score compared
to the composite PaP score (Hedges g = 2.31 to 2.17).
High test-retest reliability was obtained on delayed free-recall and multiple-choice subtests
of the Faces and Names test (r = .70 to r = .74) as well as on a measure of processing speed sim-
ilar to digit symbol coding on the PaP (r = .73) (Table 4). However, tasks of visual working
memory demonstrated weak to moderate test-retest reliability (r = .36 to r = .45).
Both episodic memory tasks demonstrated significant practice effects (p’s < .01) on both
immediate and delayed-recall trials. However, measures of processing speed and visual work-
ing memory tasks did not (p’s > .05; see Table 4).
Table 4. Test-retest correlations and practice effects between baseline and follow-up visits.
Test
Face Name Hobby Recall Immediate Free Recall
Face Name Hobby Recall Immediate Recognition
Face Name Hobby Recall Delayed Free Recall
Face Name Hobby Recall Delayed Recognition
Grid Locations Immediate Recall
Grid Locations Delayed Recall
Symbol Line
Visual Patterns
Speeded Matching
R
.56
.59
.70
.74
.57
.48
.36
.45
.73
t
9.52���
6.34���
5.84���
3.93���
6.25���
3.60��
1.08
1.37
.075
d
1.25
.78
.61
.39
.78
.49
.16
.19
.02
Note: All correlations significant at p < .01
�� indicates significant dependent t-test value at the p < .01 level
��� indicates significant dependent t-test value at the p < .001 level
d = Cohen’s d
https://doi.org/10.1371/journal.pone.0244962.t004
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Discussion
The current study demonstrates the feasibility of using a novel web-based application for the
collection of study subject demographics, as well as the results from diverse computer-based
assessments, in the elderly. The current feasibility and validity study was conducted under con-
ditions where data was collected both in traditional research settings (i.e., lab space on a uni-
versity campus) as well as in senior living communities. Although a research assistant was
present in case assistance was needed for participants to navigate the web-battery, for nearly
all participants the interaction was primarily limited to providing encouragement during
testing. Encouragement was needed in large part due to the fact the participants completed
extensive PaP as well as web-based battery in the same day. In a minority of participants
assistance with using the computer and/or providing further clarifications to the questions
and tasks that were being asked. In future studies it will be important to further refine the
delivery of the web-based assessments in order to minimize/eliminate the involvement of
research personnel in the evaluation. Exploratory step wise regression analysis identified that
the use of a greatly abbreviated cPACC battery was sufficient to capture the observed validity
between cPACC and ADCS-PACC (Speeded Match, FNHR, Grid Locations). Together these
observations point to the ability to reduce or eliminate participant frustration by using an
abbreviated cPACC and/or minimizing the amount of PaP assessments in future validation
efforts.
Although further validation is needed, one potential use for this platform is to provide an
option for the self-administered collection of assessments and patient demographics in a clini-
cal setting that involves little to no involvement of clinical staff. Additionally, in the current
study use of this web-based platform occurred in some instances in assisted-livings raising the
potential for conducting evaluations outside of traditional clinic setting, including an individu-
al’s home. Both the limited involvement of clinical staff and ability to administer evaluations
outside of the traditional setting are increasingly important aspect of clinical research given
the impacts of Covid-19.
We observed that multiple assessments within the current platform provided valid mea-
sures for diverse aspects of geriatric health. Specifically, we identified the ability of the platform
to capture self-reported patient demographics as well as valid measurements self-rated health,
depression and anxiety symptoms in a sample of community dwelling elderly. The relationship
that was observed between the web-battery measure of depression and anxiety and the GDS/
GAI composite in the current study was similar to correlations found in other studies report-
ing measures of depression and anxiety (e.g., [34–36]). It is important to point out that the
platform therefore not only contains cognitive assessments but also includes other endpoints
that are routinely required as part of cognition focused studies.
There is a widespread and growing use of the ADCS-PACC in clinical trials and longitudi-
nal studies, and therefore there is a need to produce ADCS-PACC assessment options that
don’t require traditional PaP delivery/capture during periods of significant operational and
safety challenges such as Covid-19. We developed the current web-based battery to provide a
mechanism to capture an ADCS-PACC relevant assessment that could be delivered using a
computer-based application in place of a PaP. While our computer-based assessment taps into
cognitive domains relevant to the ADCS-PACC, and significantly correlates with performance
on a PaP version of the ADCS-PACC (moderate significance), we recognize that there are ver-
bal and mechanical limitations in the current computer-based assessment does not allow for a
complete overlap with the individual assessments comprising the ADCS-PACC. Further, the
tests that comprise the computer-based assessment were designed to address similar constructs
to the ADCS-PACC but the format and demands are different even for tests most similar to
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PLOS ONEWeb-based platform for self-administered assessments in the elderly
one another. For example, orientation was asked using multiple choice questions on the
cPACC while the MMSE asks for a verbal response without cues and the digit symbol coding
requires written copy of symbols while the speeded match task involves using a mouse to click
on a response. In this initial validity study, we identified the cPACC to have a statistically sig-
nificant (moderate correlation) with the PaP version of the ADCS-PACC, and to have compa-
rable discrimination to the ADCS-PACC in terms identifying those with and without
cognitive impairment. Interestingly, when the MMSE was removed from the PaP composite
score, the cPACC was better able to distinguish between those with and without cognitive
impairment. To our knowledge, there has only been one previous validation study of com-
puter-based assessments targeting the ADCS-PACC [10]. That study demonstrated that the
computerized batteries had positive correlations with the ADCS-PACC. Therefore, the results
of the current study add to a limited, but growing literature which represents a potentially
important step in moving from a reliance upon PaP versions of the ADCS-PACC for the mea-
surement of an ADRD relevant cognitive composite. In particular, it will be important to
determine in the near future the ability to extend the findings from this initial validation study
to a larger and more diverse study sample that also includes data as to the feasibility of using
the cPACC for measuring the rates of cognitive change over time.
Inherent cPACC features such as the automated assessment delivery and scoring may facili-
tate cognitive composite measures being conducted in a larger number of clinical and research
settings. The cPACC demonstrated good reliability when assessing delayed memory both
through free recall and when given further cuing through multiple choice on the FHNR test.
To our knowledge this is the first study to describe the use of a recall component in a comput-
erized episodic memory test. The FHNR task is based on the Face-Name Association Memory
Test which has been shown to distinguish between cognitive healthy individuals and those
with MCI and correlates with AD biomarkers such as amyloid deposition [26, 28]. Given the
size of practice effects observed for episodic memory measures, a future goal is to develop
alternate forms to reduce practice effects.
In addition to verbal episodic memory, visual episodic memory has shown to decline in a
similar magnitude in individuals at risk for ADRD [37] and visual episodic memory measures
cognitive impairment beyond verbal episodic memory alone [38]. Measures of visual episodic
memory and visual working memory are extremely feasible and conducive for a computer-
based delivery of cognitive assessments and are components of the cPACC [39, 40]. However,
with the exception of a task assessing processing speed, all other tests demonstrated weak to
moderate test-retest correlations in the current study. One possible solution to improve the
test-retest reliability of the cPACC is to add more trials to the visual episodic memory tests.
Despite this, subtests of the cPACC demonstrated strong effect sizes in distinguishing between
those with and without subtle cognitive impairment. Future studies can explore the ability of
the cPACC to identify subtle cognitive impairments in older adults.
Participants in the current study did not demonstrate variability in responses to the orienta-
tion items (only 3 participants in the entire sample did not get both orientation questions cor-
rect). For the PACC, the MMSE is included given it includes items to measure orientation, a
domain identified in the review as important for assessing preclinical AD. However, the
MMSE, is known to have poor psychometric properties (i.e., ceiling effects and low test-retest-
reliability) in healthy, non-demented, older adults [41]. In some circumstances removing the
MMSE has actually been shown to improve the sensitivity of the ADCS-PACC to measure cog-
nitive decline [42]. Taken together, these data highlight the importance of the need to continue
to optimize the psychometric properties of the ADCS-PACC.
A number of studies have identified important roles of working memory in the develop-
ment of MCI and progression to ADRD. Modifications to the ADCS-PACC which add in a
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PLOS ONEWeb-based platform for self-administered assessments in the elderly
measure of verbal fluency, tasks highly linked to working memory, were found to be better
than the original PACC in capturing longitudinal decline [42, 43]. Verbal fluency measures
can be considered as measures of executive functioning, a domain identified as important in
the assessment of preclinical AD [44]. While verbal fluency measures are difficult to incorpo-
rate into a computerized testing setting, visual working memory measures can be readily
implemented in a computer-based assessment and are sensitive to the identification of cogni-
tive decline associated with ADRD [45]. Further validation efforts and implementation of the
cPACC may identify that it has enhanced sensitivity and utility with which to measure and
monitor cognitive change relevant to the development of ADRD.
The focus of the current study was to conduct an initial validation study of the web-battery,
including the cPACC in a non-demented, community dwelling, sample of older study partici-
pants. Of note, only a subset of the much larger web-battery questionnaire was the focus of
psychometric validation and future studies will need to validate the remaining questions. A
limitation of the current study is observed in the study sample being overwhelmingly Cauca-
sian and well-educated which is not representative of the general population raises caution in
extending the findings from this study to a more ethnically and educationally diverse sample.
Validation of the cPACC was based on cross sectional data and caution should be applied in
determining the ability of the cPACC to measure cognitive change in a longitudinal manner
similar to previous studies reported with the ADCS-PACC. Further, in making comparisons
between those with intact global cognition (i.e., MMSE score of 25 or higher) and reduced
global cognition (i.e., MMSE score less than 25), the current study had a small number of par-
ticipants with reduced global cognition. Future studies can continue to examine the utility of
the cPACC to differentiate between those with intact and reduced cognitive performance.
Supporting information
S1 Table. Stepwise regression of cPACC measures and the PaP composite score.
(DOCX)
S2 Table. Correlations among measures on the cPACC and PaP measures.
(DOCX)
Author Contributions
Conceptualization: Matthew Calamia, Alyssa N. De Vito, John P. K. Bernstein, Jeffrey N.
Keller.
Data curation: Matthew Calamia, Alyssa N. De Vito.
Formal analysis: Matthew Calamia, Daniel S. Weitzner, Alyssa N. De Vito, John P. K.
Bernstein.
Investigation: Matthew Calamia, Daniel S. Weitzner, Alyssa N. De Vito, John P. K. Bernstein,
Ray Allen.
Methodology: Matthew Calamia, Alyssa N. De Vito, John P. K. Bernstein, Ray Allen.
Software: Ray Allen.
Supervision: Matthew Calamia, Alyssa N. De Vito.
Validation: Matthew Calamia, Daniel S. Weitzner.
Writing – original draft: Matthew Calamia, Daniel S. Weitzner, Alyssa N. De Vito, John P. K.
Bernstein.
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PLOS ONEWeb-based platform for self-administered assessments in the elderly
Writing – review & editing: Jeffrey N. Keller.
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| null |
10.1186_s40168-023-01519-9.pdf
|
Availability of data and materials
The raw data in this study is from reference [3]. All analyzed data in this study
is available in Additional file 3. The source code is available at https:// github.
com/ Neina‑ 0830/ WWTP_ commu nity_ pre
|
Availability of data and materials The raw data in this study is from reference [3] . All analyzed data in this study is available in Additional file 3. The source code is available at https:// github. com/ Neina-0830/ WWTP_ commu nity_ predi ction .
|
Liu et al. Microbiome (2023) 11:93
https://doi.org/10.1186/s40168-023-01519-9
RESEARCH
Microbiome
Open Access
Predicting microbial community
compositions in wastewater treatment plants
using artificial neural networks
Xiaonan Liu1, Yong Nie1* and Xiao‑Lei Wu1,2,3*
Abstract
Background Activated sludge (AS) of wastewater treatment plants (WWTPs) is one of the world’s largest artificial
microbial ecosystems and the microbial community of the AS system is closely related to WWTPs’ performance. How‑
ever, how to predict its community structure is still unclear.
1:1 of amplicon sequence variants (ASVs) appearing in at least 10% of samples and core taxa were 35.09%
Results Here, we used artificial neural networks (ANN) to predict the microbial compositions of AS systems collected
from WWTPs located worldwide. The predictive accuracy R2
1:1 of the Shannon–Wiener index reached 60.42%, and the
average R2
and 42.99%, respectively. We also found that the predictability of ASVs was significantly positively correlated with their
relative abundance and occurrence frequency, but significantly negatively correlated with potential migration rate.
The typical functional groups such as nitrifiers, denitrifiers, polyphosphate‑accumulating organisms (PAOs), glycogen‑
accumulating organisms (GAOs), and filamentous organisms in AS systems could also be well recovered using ANN
models, with R2
1:1 ranging from 32.62% to 56.81%. Furthermore, we found that whether industry wastewater source
contained in inflow (IndConInf ) had good predictive abilities, although its correlation with ASVs in the Mantel test
analysis was weak, which suggested important factors that cannot be identified using traditional methods may be
highlighted by the ANN model.
Conclusions We demonstrated that the microbial compositions and major functional groups of AS systems are
predictable using our approach, and IndConInf has a significant impact on the prediction. Our results provide a better
understanding of the factors affecting AS communities through the prediction of the microbial community of AS
systems, which could lead to insights for improved operating parameters and control of community structure.
Keywords Activated sludge, Artificial neural networks, Prediction, Microbial compositions, Functional groups
*Correspondence:
Yong Nie
[email protected]
Xiao‑Lei Wu
[email protected]
1 College of Engineering, Peking University, Beijing 100871, China
2 Institute of Ocean Research, Peking University, Beijing 100871, China
3 Institute of Ecology, Peking University, Beijing 100871, China
Background
With the increasing expansion of urbanization, about 360
billion m3 of wastewater is produced every year globally
[1]. The activated sludge (AS) system in wastewater treat-
ment plants (WWTPs) is at the heart of current sewage
treatment technology [2]. Microorganisms treat almost
60% of this wastewater in AS systems before release
[3]. This process relies on the degradation of organic
compounds, biotransformation of toxic substances,
and removal of pathogens by diverse microorganisms
[4–6]. Thus, the microbial communities present in these
© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or
other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this
licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco
mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
Liu et al. Microbiome (2023) 11:93
Page 2 of 15
systems determine their performance [7]. Predicting the
microbial communities of AS systems and exploring fac-
tors that influence them will provide reasonable sugges-
tions for the design, optimization, and stable operation
of sewage treatment systems [8–11]. However, because
wastewater always contains a multiplicity of resources,
the AS system exhibits an enormous microbial diversity
and varies greatly worldwide. The global activated sludge
community encompasses about 1 billion bacterial phylo-
types, with a small global core bacterial community con-
sisting of only 28 operational taxonomic units (OTUs)
[3]. The overwhelming taxonomic diversity and variabil-
ity of microbial communities in AS systems pose a sig-
nificant challenge for accurate modeling and predicting
their structure and function. It is still unclear how we can
predict the microbial communities in the AS systems of
WWTPs according to the design parameters and envi-
ronmental data.
The AS system contains high biomass and microbial
diversity [3, 12], and predicting the microbial commu-
nity is complicated by diverse factors. For example, AS
systems treating municipal and industrial wastewater
harbor distinct microbial communities [13, 14], suggest-
ing that the type of wastewater impacts microbial com-
position. The influent biodegradability [biological oxygen
demand/chemical oxygen demand (B/C ratio)] also plays
an essential role in shaping the AS microbial community.
A low or high B/C ratio may lead to low microbial diver-
sity and pollutant removal loading [15], indicating that
the impact of the B/C ratio on community structure may
be nonlinear. Recently, the integration of high-through-
put sequencing and multivariate statistical analysis indi-
cated that the microbial communities of AS systems are
significantly correlated with multiple factors, such as
location, geographical distance, dissolved oxygen (DO),
temperature, hydraulic retention time (HRT), sludge
retention time (SRT), inflow and effluent of chemical
oxygen demand (COD), total nitrogen (TN), total phos-
phorus (TP) [16, 17]. The combined influence of multi-
ple environmental factors has made it difficult to predict
the microbial compositions in AS systems and thus has
become an obstacle to guiding the operation of WWTPs.
Mechanism-based kinetic models, such as the Monod
equation, Lotka-Volterra model, and individual-based
dynamic model can predict the structure of microbial
communities based on specific growth and interaction
mechanisms under given conditions [18–21]. However,
these models are limited in their capacity to generalize to
complex natural communities due to simplified growth
or interaction assumptions. Multiple linear regression
models can predict microbial community structure from
multiple environmental factors. A previous study pre-
dicted bacterial and fungal groups in a soil microbial
community from typical soil environmental factors [C
and N concentrations, pH, mean annual temperature
(MAT), mean annual precipitation (MAP) and net pri-
mary productivity (NPP), etc.] using this method [22].
However, since multiple regression models ignore the
interaction effects of environmental factors and non-
linear relationships, the predictability of the microbial
taxa in that study was at most no more than 60%. The
AS system is affected by multiple cross-complex fac-
tors, including geographical factors, design and opera-
tion parameters, and physicochemical parameters, and
multiple regression analysis is not enough to capture this
complex relationship. In addition, no attention has been
paid to the regularity of predictability of microbial taxa
in previous studies, which is essential for a deeper under-
standing and control of microbial community structure.
Artificial neural network (ANN) is a machine learn-
ing method for the automatic and quantitative learning
of a suitable relationship without any specific assump-
tions and guiding system optimization [23]. The ANN
is an ideal alternative to model these complex relation-
ships between microbial communities and environmen-
tal variables as this method is better suited to account
for the non-linear associations between variables and
the interactions among predictors [24]. ANNs have
helped researchers to successfully analyze the relation-
ship between environmental factors and microbial com-
munity structure in many ecosystems [24–26], while the
relevant applications of activated sludge systems are still
lacking. Considering the strong ability of the ANN model
to predict complex systems, we hypothesized that the
ANN model can predict the microbial community struc-
ture of AS system.
Here, we used ANN models and environment data to
predict the microbial community structure of AS sys-
tems from global wastewater treatment plants. We ana-
lyzed the predictability of different taxa and the effects of
environmental factors on the prediction. These analyses
deepened our understanding of the microbial community
of AS systems, provided reasonable suggestions for accu-
rately predicting major functional groups, and provided a
theoretical basis for better design and operating param-
eters, and to control community structure.
Results
Overview of microbial community structure in AS systems
By preprocessing 777 (no data leakage) activated sludge
samples from 269 wastewater treatment plants located
in 23 countries across 6 continents using the QIIME2
pipeline, we obtained the basic information of micro-
bial community structure in AS systems. Specifically,
the Shannon–Wiener index ranged from 2.90 to 6.41
(Fig. 1a), Pielou’s evenness index ranged from 0.50 to
Liu et al. Microbiome (2023) 11:93
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Fig. 1 Overview of microbial community structure in AS system. Distribution of Shannon–Wiener index (a), Pielou’s evenness index (b), species
richness (c), and Faith’s phylogenetic diversity (d). e. Occurrence frequency and average relative abundance distribution of all ASVs in the AS system
0.90 (Fig. 1b), species richness ranged from 217 to 2014
(Fig. 1c), and Faith’s phylogenetic diversity ranged from
22.58 to 148.33 (Fig. 1d). Detailed information about
alpha diversity is provided in Table S1.
In addition, we analyzed the distribution features of the
average relative abundance and occurrence frequency of
the ASVs. The results showed that ASVs in the AS sys-
tems were dominated by low relative abundance (Fig. 1e),
which is in line with the general ecological environment
[27]. In this study, we only predicted 1493 ASVs appeared
in at least 10% of samples, of which 290 belonged to the
core ASVs (Fig. 1e), which was defined as overall abun-
dant, ubiquitous, and frequently abundant ASVs.
Alpha‑diversities of AS systems can be predicted by ANN
models
Predictability of alpha‑diversities
To obtain an overall prediction of AS community
structure, we first constructed predictive models for
different alpha diversity indices, including the Shannon–
Wiener index, Pielou’s evenness index, species richness,
and Faith’s phylogenetic diversity. Here, the predictive
accuracy is measured relative to the 1:1 observed-pre-
dicted line (rather than a best-fit line), named R2
1:1, so
accuracy assessments are both qualitative and quantita-
tive [22]. By comparing the observed and predicted alpha
diversities in test sets, we found that predictive accura-
cies R2
1:1 of the Shannon–Wiener index (Fig. 2b), Pie-
lou’s evenness index (Fig. 2c), species richness (Fig. 2d),
and Faith’s phylogenetic diversity (Fig. 2e) were 60.42%,
54.11%, 49.92%, and 60.37%, respectively.
Comparing the predictability of different alpha diver-
sity indices, we found that the Shannon–Wiener index
and Pielou’s evenness index were more predictable than
species richness, which may be related to the environ-
mental sensitivity of species evenness. Species evenness
has previously been reported to be more sensitive to
human activity and environmental changes than rich-
ness because environmental conditions may significantly
affect ecosystems long before a species is threatened by
extinction [28]. In addition, the predictive accuracy of
phylogenetic diversity was also higher than species rich-
ness, reflecting that species’ evolutionary history may
be influenced by environmental factors surrounding the
microbial community.
Environmental factors important for predicting
alpha‑diversities
During the model training process for predicting alpha-
diversities of AS systems, an importance weight value
was assigned to each environmental factor by Garson’s
connection weight method [29]. The factors with higher
importance weights were more informative when the
model was used to predict alpha diversities.
To assess the importance of different environmental
factors in predicting alpha-diversities of AS microbial
communities, we ranked the average importance weights
of environmental factors in different predictive models
in descending order (Additional file 2: Figure S1). The
results showed that DO was most important for predict-
ing the Shannon–Wiener and Pielou’s evenness indi-
ces, but inflow-relatedIndConInf was most important
Liu et al. Microbiome (2023) 11:93
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Fig. 2 Prediction of alpha diversity. a. The framework of ANN models: input data (blue), output data (red), and a predictive model trained to
compute output data from input data (purple). Correlations between observed and predicted values of Shannon–Wiener index (b), Pielou’e
evenness index (c), species richness (d), and Faith’s phylogenetic diversity (e). The 1:1 relationship is shown as a solid black line, and the best fit
is shown as the dashed light blue line. The blue‑shaded region represents the 95% confidence interval for the best‑fit line. We reported the R2
value of the best‑fit line between predicted and observed and the R2 observations relative to the 1:1 line. f. Heatmap of importance weights of
environmental factors in alpha diversity predictive models
Liu et al. Microbiome (2023) 11:93
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Fig. 3 Prediction of the relative abundance of ASVs>10%. a. Percentage of ASV number and relative abundance of ASVs<10% versus ASVs>10%. b.
Distribution of the predictive accuracy of ASVs>10%. The dark green, dark blue, and dark red text represents the proportion of ASVs with prediction
accuracy exceeding 10%, 30%, and 50%, respectively. c. Principal component analysis (PCA) of environmental factors colored by k‑means clusters. d.
Ranking of environmental factors in descending order of median importance weights
for predicting species richness and Faith’s phylogenetic
diversity. Climatic condition latitude (Lat), design-related
N removal process [nitrification (Nitri) and denitrifi-
cation (Denitri)], COD in the inflow of aeration tank
(AtInfCOD), and the sludge volume index (SVI) were
also environmental factors with high average importance
weights for predicting alpha diversities (Fig. 2f ).
Assessment of the predictivity of community structure
using the ANN model
Predictability of the relative abundances of ASVs
To obtain a deep prediction of AS community structure,
we predicted the relative abundance of ASVs in AS sys-
tems. We constructed predictive models for the 1493
ASVs found in more than 10% of samples (ASVs>10%),
which accounted for 3.2% of the total ASVs and
64.97 ± 0.54% (mean ± SEM) of the relative abundance in
AS samples (Fig. 3a). The results showed that the average
predictive accuracy R2
1:1 of ASVs>10% was 35.09% (Table
S2). Further, we found that 19.83% of ASVs>10% could be
predicted with R2
1:1 over 50%, 60.82% of ASVs>10% could
be predicted with R2
1:1 over 30%, and 91.96% of ASVs>10%
could be predicted with R2
1:1 over 10% (Fig. 3b).
In addition, we also predicted the structures of the
microbial communities of the test samples, by recover-
ing the ASVs>10% subcommunity of each sample in its
entirety [25]. Here, we refer to the observed values of
ASVs>10% subcommunities in different test samples as
“observed communities”, and the corresponding pre-
dicted values as “predicted communities”. By comparing
Liu et al. Microbiome (2023) 11:93
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the intra-group and inter-group differences between the
predicted and observed communities, we found that the
Bray–Curtis similarity between intra-groups was signifi-
cantly higher than that between inter-groups (Additional
file 2: Figure S2a). This result also proved that the ANN
model could predict the microbial community structure
of the AS system from an overall perspective.
Furthermore, we predicted microbial taxa at different
taxonomic levels and found that microbial community
structure had high average predictive accuracy ranging
from 33.32% to 41.6% at all taxonomic levels (Additional
file 2: Figure S2b). The predictive accuracy R2
1:1 of the
three most abundant phyla (Proteobacteria, Bacteroidota,
and Myxococcota) in the AS system were 64.54%, 55.37%,
and 59.04%, respectively. The three most abundant orders
Burkholderiales, Chitinophagales, and Pseudomonadales
could be predicted with R2
1:1 of 63.89%, 56.19%, and
42.81%, respectively (Table S3).
Importance of environmental factors in the prediction
of ASVs
During the model training process for predicting abun-
dances of ASVs, an importance weight value was also
assigned to each environmental factor as above (Table
S4). By displaying the importance weights of environ-
mental factors in different ASVs predictive models, we
found that environmental factors had different weights in
predicting different ASVs (Additional file 2: Figure S3a).
Further, we clustered the environmental factors into three
clusters according to their importance weights using the
k-means clustering algorithm and displayed them using
principal components analysis (PCA) (Fig. 3c). We found
that these three clusters corresponded to three parts
divided by the median of importance weights in descend-
ing order (Fig. 3d). This result showed that environmen-
tal factors of cluster1, which included climatic condition
sampling moment temperature (SMT), design and opera-
tion parameters year of plant build (BY) and Denitri,
inflow conditions IndConInf and total nitrogen in the
inflow of aeration tank (AtInfTN), and physicochemical
properties SVI, etc., contributed the most to the predic-
tion of community structure, cluster2 was second, and
cluster3 was the least important group of factors in pre-
dicting community structure (Table S5).
We then wondered what influences the importance
weights of environmental factors in predicting microbial
taxa. Before constructing the predictive model, we per-
formed a Mantel test analysis on the correlation between
the ASVs>10% subcommunity and ecological environment
factors (Table S5). The correlation analysis showed that
environmental factors significantly associated with the
ASVs>10% subcommunity included climate conditions
Lat, longitude (Lon), MAT, the annual mean of daily
maximum temperature (AMMinT), sampling month pre-
cipitation (SMP), and GDP, design and operation param-
eter Nitri, and physicochemical property mixed liquid
temperature (MIT) (Pearson’s ρ > 0.2, p < 0.01). By com-
paring the importance of environmental factors in pre-
dicting community structure and the correlation between
environmental factors and community structure, we
found that some of the environmental factors that had
high importance weights in many predictive models were
not strongly correlated with the ASVs>10% subcommunity
(Additional file 2: Figure S3). For example, inflow condi-
tions IndConInf and AtInfTN, which were important for
predicting the relative abundance of ASVs, did not signif-
icantly correlate with the ASVs>10 sub-community. How-
ever, despite these differences, there was a significant
positive correlation between the importance weights of
environmental factors and their correlation coefficients
with the ASVs>10% subcommunity (Additional file 2:
Figure S4a, R2 = 0.1271, p < 0.05). These results showed
that in addition to the correlation analysis, importance
weights analysis in ANN predictive models also helped
to expand the range of environmental factors that should
be paid attention to when exploring the performance of
WWTPs.
In addition, we also analyzed the influence of the dis-
tribution of environmental factors on their weights. The
result showed that both the skewness (Additional file 2:
Figure S4b; R2 = 0.6268, p < 0.001) and kurtosis (Addi-
tional file 2: Figure S4c; R2 = 0.7106, p < 0.001) of normal-
ized environmental factors were significantly negatively
correlated with their average importance weights in pre-
dicting ASVs. This suggested that environmental factors
of low skewness and low kurtosis may be more important
in predicting community structure.
Characteristics of ASVs with high predictabilities
ASVs with higher relative abundances and occurrence
frequencies can be better predicted using the ANN model
To investigate the correlation between the predictabil-
ity of ASVs and their distribution features, we compared
the predictability of ASVs with different relative abun-
dances and frequencies. The correlation analysis between
the predictive accuracy R2
1:1 of all 1493 ASVs>10% and
their relative abundances showed that the R2
1:1 of an
ASV was significantly positively correlated with its rela-
tive abundance (Fig. 4a; R2 = 0.05279, P < 0.001), indicat-
ing that the high predictability of an ASV may be related
to its high relative abundance. Furthermore, we found
that the R2
1:1 of an ASV was slightly positively associ-
ated with its occurrence frequency at significant levels
(Fig. 4b; R2 = 0.02602, P < 0.001), indicating that the high
predictability of abundant ASV>10% may also be related
to its high occurrence frequency. Further, we grouped
Liu et al. Microbiome (2023) 11:93
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Fig. 4 Distribution features and predictability of ASV>10%. a. Correlation of test R2
1:1 with the average relative abundance. b. Correlation of test R2
with the occurrence frequency. c. Fit of the neutral community model (NCM) of AS system community assembly. The solid blue lines indicate the
best fit to the NCM as in Sloan et al. [31], and the dashed blue lines represent 95% confidence intervals around the model prediction. R2 indicates
the fit to this model, and m indicates the estimated migration rate. d. The test R2
1:1 of above, neutral, and below partitions. e. Correlation of test R2
with the estimated migration rate. The data was provided by the results of different partitions. f. The test R2
1:1 of core and non‑core taxa. Statistical
analysis was performed using a two‑sample Student’s t‑test: ***, p < 0.001; n.s, p > 0.05, no significance
1:1
1:1
Liu et al. Microbiome (2023) 11:93
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ASVs based on their relative abundances and occurrence
frequencies as in previous studies (see more details in
Additional file 1), and the results showed that ASVs>10%
with a high relative abundance and high occurrence fre-
quency were significantly more predictable than those
with low relative abundance (Additional file 2: Figure
S5a) and low occurrence frequency (Additional file 2:
Figure S5b), which is consistent with the previous result.
It is worth mentioning that the occurrence frequency of
an ASV has a significant positive correlation with its rela-
tive abundance (Additional file 2: Figure S5c, R2 = 0.2978,
P < 0.001), supporting that high relative abundance and
high occurrence frequency can corroborate each other in
their contribution to predictability.
Previous studies had demonstrated that rare taxa were
more dynamic than abundant taxa [30], so we wondered
whether a taxon’s predictability was related to its variabil-
ity across samples. To explore this question, we analyzed
the relationship between R2
1:1 of the ASVs and their coef-
ficients of variation and found that the predictive accu-
racy of an ASV was significantly negatively correlated
with its coefficient of variation (Additional file 2: Figure
S5d; R2 = 0.01946, P < 0.001), implying that taxa with
higher variability were less predictable.
The predictability of an ASV decreases as its potential
migration rate increases
Previous studies have demonstrated that the process of
community assembly is closely related to its predictabil-
ity [22, 32, 33], so we explored the association between
microbial community assembly mechanisms in AS sys-
tems and the predictability of the corresponding taxa.
By neutral community model (NCM) model fitting, we
found that stochastic processes explained 63.3% of the
microbial community variation in AS systems (Fig. 4c).
The ASVs>10% were subsequently separated into three
partitions (above, below, and neutral) depending on their
occurrence frequencies and relative abundance. By com-
paring the distribution features of the three partitions,
we found that the relative abundance (Additional file 2:
Figure S6a) and occurrence frequency (Additional file 2:
Figure S6b) of ASVs>10% in the below partition were sig-
nificantly higher than those of the above partition. Fur-
ther, we found that the predictive accuracy R2
1:1 of the
below partition was also significantly higher than that
of the above partition (Fig. 4d). This result again showed
that ASVs with higher relative abundances and occur-
rence frequencies can be better predicted using ANN
models.
In addition, the estimated migration rates of the dif-
ferent partitions assessed by NCM were also different.
Points above the fitting curve represent taxa found more
frequently than expected, suggesting that they have a
higher migration ability and can disperse to more loca-
tions. Points below the fitting curve represent taxa found
less frequently than expected, suggesting their lower dis-
persal ability in WWTPs on a global scale or that they
have a stronger response to local environmental condi-
tions. The fitting results also confirmed that the taxa in
the above partition had the highest estimated migration
rates, and the taxa in the below partition had the lowest
estimated migration rates (Additional file 2: Figure S7).
Further analysis of the relationship between the migra-
tion rate and predictability of different partitions, we
found that a taxon’s predictability had a high negative
correlation with its estimated migration rate (Fig. 4e,
R2 = 0.996, P = 0.0401), indicating that the predictabil-
ity of an ASV decreased as its potential migration rate
increased.
Core taxa of AS systems can be predicted by ANN models
Due to its highly complex organic environment, activated
sludge selects a unique core community that does not
overlap with the core communities of other habitats [3].
We evaluated the predictabilities of core taxa that were
abundant and ubiquitous using ANN models. As defined
in Methods, we obtained 290 core ASVs and 1203 non-
core ASVs in the ASVs>10% subcommunity (Additional
file 2: Figure S8a). Our analyses found that the relative
abundance (Additional file 2: Figure S8b) and occurrence
frequency (Additional file 2: Figure S8c) of core taxa were
significantly higher than those of non-core taxa, and the
estimated migration rate of core taxa was lower than that
of non-core taxa (Additional file 2: Figure S8d).
By assessing the predictability of the core taxa, we
found more than 37.59% of the core ASVs could be well
predicted with an R2 of over 50%, more than 78.62% could
be well predicted with an R2 of over 30%, and more than
94.48% could be well predicted with an R2 of over 10%,
and the average prediction accuracy was 42.99% (Table
S2), which was significantly higher than the non-core
taxa (Fig. 4f ). Because the core taxa are reported to be
closely related to nitrogen removal, phosphorus removal,
and even flocculation enhancement of activated sludge
[12, 34, 35], the results implied that the ANN model
could be used to assess the performance of WWTPs by
predicting the dynamics of the core taxa.
Prediction of major functional groups in AS systems
To more directly understand and control the performance
of WWTPs, we predicted and analyzed major func-
tional groups of microbial communities in the AS system
using ANN models. Referring to the MiDAS4 database,
the functional groups in AS systems include nitrogen
removal groups (nitrifiers and denitrifiers), phosphorus
removal groups (PAOs), and their competitors (GAOs),
Liu et al. Microbiome (2023) 11:93
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Fig. 5 Prediction of major functional groups. a. The test R2
importance weights of environmental factors in the predictive models of functional groups. Errors bars in these graphs show the 95% credible
intervals of the mean values. Statistical analysis was performed using a two‑sample Student’s t‑test: ***, p < 0.001; n.s, p > 0.05, no significance
1:1 of nitrifiers, denitrifiers, PAOs, GAOs, and filamentous organisms. b. Heatmap of
and filamentous organisms [36] (Table S6). Then, we
calculated the total relative abundance of different func-
tional groups in each sample by summarizing the relative
abundances of ASVs from the same functional groups.
Finally, we predicted the total relative abundance of
each functional group using the ANN model. The results
showed that the predictive accuracy R2
1:1 for nitrifiers,
denitrifiers, PAOs, GAOs, and filamentous organisms
was 32.62%, 62.65%, 53.46%, 53.31%, and 62.86%, respec-
tively (Fig. 5a).
To further understand the prediction results of func-
tional groups, we also analyzed the importance weights
of environmental factors in their predictive models. By
performing Ward clustering analysis on the importance
weights of environmental factors in these predictive
models of different functional groups, we found that the
importance of environmental factors in predicting PAOs
and GAOs was the most similar, followed by nitrifiers and
denitrifiers (Fig. 5b), which implied a consistent contri-
bution of environmental factors when predicting relevant
functions. Overall, the design and operation parameters
BY and Denitri, inflow condition IndConInf, physico-
chemical properties sludge loading (F/M), and nitrate
nitrogen concentration (NO3-N) were important for pre-
dicting nitrogen and phosphorus removal function, while
climatic conditions Lat and SMT, design and operation
parameter Nitri, and physicochemical properties SVI and
MIT significantly affected the prediction of filamentous
organisms. Additionally, SVI also had a crucial impact on
the prediction of denitrifiers, which may be because some
filamentous bacteria also function as denitrifiers [37].
To demonstrate the importance of these environmental
factors, we only used the above 10 high-weight environ-
mental factors to predict functional groups. The results
showed that using only those ten factors allowed us to
predict the abundance of major functional groups in AS
systems with a predictive accuracy of R2
1:1 ranging from
17.25% to 52.00% (Additional file 2: Figure S9).
In summary, the climatic conditions Lat and SMT, the
design and operation parameters BY, Denitri, and Nitri,
the inflow condition IndConInf, as well as some sample
physicochemical properties (F/M, SVI, MIT, and NO3-N)
of AS systems all affect the prediction of functional
groups. Controlling these critical environmental factors
can help us regulate the performance of WWTPs, which
will guide us to design more reasonable operating param-
eters according to environmental changes.
Discussion
In this study, we predicted the diversity and the struc-
tures of the microbial community, as well as the func-
tional groups in AS systems using ANN models. We also
evaluated the importance of environmental factors in the
predictions.
The use of artificial neural network (ANN) models in
this study increased the predictive power of complex
systems of microbial communities. When ANN models
were used to predict ASVs appearing in at least 10% of
the samples, 60.82% of the ASVs>10% had a prediction
accuracy R2
1:1 exceeding 30%. In a previous study, the
multiple regression model could only explain about
15% of the variability in the genus-level taxonomy of a
soil bacterial microbial community [22] and only pre-
dicted the top ten taxa of that community. Compared
Liu et al. Microbiome (2023) 11:93
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with this previous study, our prediction accuracy was
greatly improved, with the prediction range being
increased to all ASVs appearing in at least 10% of sam-
ples, which proves the application potential of ANN
models in predicting the complex systems of microbial
communities. We recommend using the ANN model as
a deep learning method in the prediction of complex
microbial communities.
Using the Neutral Community Model (NCM) pro-
posed by Sloan et al. [31], this study transformed migra-
tion from a vague qualitative concept into a number with
biological meaning, the potential migration rate (m).
Higher values of m indicate that a species is less limited
by dispersal. The low migration rate of high-abundance
taxa and high migration rate of low-abundance taxa in
this study (Additional file 2: Figure S10) indicates that
dispersal limitation has a significant effect on high-
abundance taxa, but not on low-abundance taxa, which
is consistent with findings for some ecosystems [38,
39]. High-abundance taxa with a low migration rate will
appear in some samples due to environmental selection
[40], and their relative abundance can be well predicted
using these environmental factors. However, low-abun-
dance taxa with high migration rates usually appear in
a sample when the migration occurs and the spatial het-
erogeneity of the sample provides them with ecological
niches. Neither the randomness of migration nor the spa-
tial heterogeneity of samples was reflected in our input
environmental variables, as such, these environmen-
tal factors were less predictive of low-abundance taxa.
Nitrosomonas, the main genus of nitrifiers, is a group
with a low relative abundance and high migration in the
AS system (Table S2), so the predictability of nitrifiers is
low (Fig. 5a). In addition, low-abundance taxa has been
reported to have higher abundance variability than high-
abundance taxa [30], and prediction targets with higher
variability are not conducive to the stability of the predic-
tive model, further explaining why the predictability of
the relative abundance of high-abundance taxa was sig-
nificantly higher than that of low-abundance taxa.
The importance of low-abundance rare species in many
ecosystems has been demonstrated [27, 41]. These spe-
cies play important roles in the community by provid-
ing necessary traits or acting as partners in interspecific
interactions [42, 43]. To gain a better understanding of
the importance of rare taxa in the microbial community,
it is essential to develop a prediction model that accu-
rately identifies low-abundance species. As the microbial
community is influenced by both abiotic environmen-
tal factors [16] and species interaction [44], a machine-
learning prediction model that considers the mechanism
of microbial interaction may improve the prediction
accuracy of rare species.
The weight of environmental factors in the predictive
model reflects the influence of environmental factors on
the corresponding prediction targets to a certain extent.
For example, our results showed that the most important
environmental factors affecting the prediction of even-
ness and richness were DO and IndConInf, respectively.
Evenness and richness are two critical indicators to meas-
ure the diversity of ecological communities. The former
describes species differences, and the latter describes the
number of species. Previous studies have demonstrated
that relative abundances of some functional taxa are sen-
sitive to changes in DO [45, 46], and the abundance of
these functional bacteria reflects the differences in spe-
cies abundance of the community. Therefore, DO has a
high weight in predicting the evenness of microbial com-
munities in AS systems. Industrial wastewater contains
many toxic and harmful substances [47, 48], which many
microorganisms cannot survive. Therefore, industrial
wastewater directly affects the population of microor-
ganisms [49], and IndConInf plays an important role in
predicting the richness of microbial communities in AS
systems.
In addition, the impact of environmental factors on
microbial taxa may be related to the specific function.
The environmental factors with top weights in predic-
tive models of nitrogen removal-related taxa ASV6 and
ASV142 were AtInfTN, Nitri, and NO3-N (Table S4). The
SVI is very important for the prediction of filamentous
organisms (Fig. 5b), which is because filamentous bacte-
ria will cause sludge bulking and foaming [50], and thus
affect the SVI. The Denitri has the greatest impact on
PAOs and GAOs, which is consistent with the denitrifi-
cation capacity of the typical PAOs genus Ca_Accumuli-
bacter and GAOs genus Ca_Competibacter [51]. This
correspondence between functions and environmental
factors indicates that environmental factors with high
weights in predicting microbial taxa may play an essential
role in environmental filtering in the deterministic pro-
cess of community assembly.
Important factors that cannot be identified using tra-
ditional methods may be highlighted by ANN modeling.
Conventional studies on AS systems have only focused
on the correlation relationship between environmental
factors and microbial communities [16, 17, 52], which
limited the scope of consideration for key environmen-
tal factors. For example, we found that whether industry
wastewater source contained in inflow (IndConInf ) was
a significant predictor in ASVs>10% predictive models in
this study. This finding is consistent with earlier research
which has demonstrated notable differences in microbial
communities between industrial and municipal sewage
[14, 53], suggesting that the IndConInf may influence
the microbial community structure of AS systems [54].
Liu et al. Microbiome (2023) 11:93
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However, our correlation analysis did not reveal a signifi-
cant association between IndConInf and the microbial
community structure (Table S5). Actually, the correlation
analysis of environmental factors correlation analysis is
limited in its ability to capture more complex relation-
ships and can only reveal simple linear or monotonic
associations [16, 55]. Therefore, its application in explor-
ing the impact of environmental factors is constrained.
By analyzing the importance weights of environmental
factors in predictive models, this study illuminated vari-
ables that require further attention and that can bet-
ter predict and control the microbial community of AS
systems.
Although our work has made some contributions to the
prediction and interpretation of the microbial commu-
nity structure in AS systems, we still cannot explain the
weights of some environmental factors in the predictive
model due to the black-box characteristics of the ANN
model. Our results show that environmental factors with
low skewness and low kurtosis distribution are more
likely to have higher weights in predicting the relative
abundance of microbial taxa, which we cannot explain
using current knowledge. Increasing the interpretability
of the ANN model will help us better use this powerful
predictive tool to analyze our concerns, which is also
the future direction of machine learning-based big data
analysis.
Conclusions
In this work, we used an ANN model to predict the
structure of microbial communities in global AS sys-
tem, including alpha diversity, ASVs appearing in at least
10% of samples, core taxa, and major functional groups.
We found that taxa with high relative abundance, high
occurrence frequency, and low estimated migration rate
were more accurately predicted by the ANN model. Fur-
thermore, the presence of industrial wastewater in the
inflow significantly impacted the prediction of microbial
communities, as demonstrated by the weight analysis
of environmental factors in the ANN models. This find-
ing implies the important role of industrial wastewater
in shaping microbial communities in AS systems. Over-
all, our findings highlighted the importance of the ANN
model in predicting the complex microbial communities.
They also provide new insights into the predictability of
microbial taxa and the influence of environmental factors
on microbial communities.
Methods
Datasets
This study used a previously published dataset of 1186
activated sludge samples taken from 269 WWTPs in 23
countries across 6 continents. In addition to 16S rRNA
sequencing data of these sludge samples, associated
metadata conforming to the Genomic Standards Consor-
tium’s MIxS and Environmental Ontology Standards [56]
were also provided by plant managers and investigators.
Among the 1186 samples collected in the previous
study, some were from different sampling points of the
same WWTP, and some were obtained from the same
sampling point at different times. As such, the environ-
mental factors and community structure between these
samples may have high similarities [3] and when evaluat-
ing a model with all 1186 samples, it may overestimate
the generalization ability of its predictions. Therefore,
we removed the samples with the same environmental
information and minimal weighted-UniFrac distance (no
more than Q1-3*IQR, Q1 is the first quartile, and IQR
is Inter-Quartile Range) of the microbial community in
these 1186 original samples and used the remaining 777
samples (no data leakage) for subsequent construction
and evaluation of the predictive model.
Data preprocessing
To ensure the accuracy, completeness, and consistency of
the data, we preprocessed the original data before build-
ing the machine learning predictive model.
Metadata preprocessing
For the metadata obtained from previous studies (refer-
ence [3]), to reduce the redundancy of environmental
data, we first removed some non-numerical variables
of multiple categories that are difficult to operate and
some variables with no practical meanings, such as site
name, city name, etc. The remaining variables were used
to train the model and their abbreviations and meanings
are shown in Table S7. To have a clearer understanding
of the environmental factors, we classified the differ-
ent types of environmental factors used for prediction
[3], including climate conditions, design and operation
parameters, inflow conditions, effluent conditions, and
physicochemical properties of samples (Table S7). Then,
the LabelEncoder algorithm was used to numeric binary
non-numeric variables, and missing values were com-
pleted according to the two-nearest neighbor principle.
Additionally, all environmental factors were normalized
to 1–100 to eliminate dimensional influence [24].
The final environment data for input in our machine
learning predictive models is shown in Table S8.
Sequencing data preprocessing
The original microbial sequencing data were processed
using Quantitative Insights Into Microbial Ecology
(QIIME2) software (http:// qiime2. org) [57]. All paired
reads were merged, quality filtered, then denoised
through the DADA2 plugin [58] to clustered into 100%
Liu et al. Microbiome (2023) 11:93
Page 12 of 15
amplicon sequence variants (ASVs). Then, ASVs classified
as fungi, ASVs with unassigned taxonomy at the domain
level, and ASVs annotated as mitochondria or chloro-
plasts were removed so that only bacterial and archaeal
sequences were retained. Singletons (ASVs with only
one sequence) were discarded before further analysis to
reduce the impact of sequencing errors. Then we rarefied
each sample to 20,954 sequences, to obtain the maximal
observation of both samples and features, from which
46,628 ASVs were obtained. The final feature sequences
were taxonomically classified using the MiDAS4 refer-
ence database [36], and phylogenetic trees were gener-
ated using phylogeny plugins for further analysis.
Alpha diversity indices such as the Shannon–Wiener
index, ASV count (species richness), and Pielou’s even-
ness were calculated using the vegan package of R 4.0.3
software (http:// www.r- proje ct. org) according to the final
feature table. Faith’s phylogenetic diversity was calculated
using the Picante package of R 4.0.3 software according
to the feature table and phylogenetic tree. The relative
abundance of each ASV was also calculated from this fea-
ture table. Together, these microbial community features
served as target variables for our AS community predic-
tive models.
Artificial neural network model
We employed the PyTorch (v1.7.1, https:// pytor ch. org/)
library in python 3.8 to build fully connected artificial
neural networks (ANNs). After testing, the three-layer
network (including a hidden layer), with relu and sigmoid
as activation functions between layers, had an excellent
prediction effect on the condition that the network topol-
ogy was relatively simple.
The first layer was the input layer, and this study’s input
data was the sewage treatment plants’ environmental
data (Table S8). Therefore, there were 48 nodes in the
first layer. According to previously studied algorithm
optimization results [59], the internal hidden layer had
97 nodes (2n + 1, where n is the number of input nodes).
Meanwhile, the output layer had 1 node, corresponding
to the index of alpha-diversities, the relative abundance
of different ASVs, or the abundance of functional groups
(Fig. 2a). We built predictive models separately for each
target to minimize prediction errors.
There were many random operations in the model
training process, which made the results inconsistent
after repeatedly running the same code. We set a fixed
global seed for the random number generator to obtain
repeatable training results. All models were trained
twenty times by different seeds to avoid the deviation
caused by each randomization, and the averaged results
were used for further analysis.
Alpha diversity and microbial taxa abundance predictive
model
For alpha diversity, we established predictive models for
the Shannon–Wiener index, Pielou’s evenness index, spe-
cies richness, and Faith’s phylogenetic diversity. For taxa,
the relative abundance of taxa with low occurrence fre-
quency was zero in many samples, which made it difficult
for the model to learn useful information on the train-
ing set (underfitting). Therefore, only the relative abun-
dance of ASVs present in at least 10% of samples (named
ASVs>10%, corresponding ASVs<10% represent ASVs that
appear in no more than 10% samples) were modeled to
predict.
There were 777 samples to build the alpha diversity or
ASVs>10% abundance predictive models. To reduce the
risk of overfitting during hyperparameter optimization,
we performed fourfold cross-validation in the train-
ing process. As a result, these models were developed
by applying fourfold cross-validation to 80% of the total
samples. Test sets comprising the remaining 20% of the
whole samples were used to evaluate the performance of
the models. All models were trained 20 times by different
seeds to avoid obtaining model bias. Finally, the model
performance was assessed based on the averaged results.
In the training processes of ANN models, the coeffi-
cient of determination (R2) and mean square error (MSE)
were used to evaluate the accuracies and losses. After
optimization of hyperparameters, we used an Adam opti-
mizer with a batch size of 256, a learning rate of 0.00001,
a drop-out of 0, and a weight decay of 0.01 to train these
models. To obtain the number of iterations when the
model was optimal, we repeatedly tested the variation
of R2 and MSE with the number of iterations (Additional
file 2: Figure S11). The results showed that after reaching
5000 iterations, the R2 and MSE of most models started
to remain stable. The number of iterations for all models
was set to 10,000, considering the trade-off between the
time cost of iteration and the lowest losses.
From neutral community model to neutral and non‑neutral
partitions
To determine the potential importance of stochastic
processes on WWTP community assembly, we used a
neutral community model (NCM) to predict the rela-
tionship between an ASVs’ occurrence frequency and
their relative abundance across the wider metacom-
munity [31]. The model is an adaptation of the neutral
theory adjusted to large microbial populations. In this
model, m is an estimate of dispersal between commu-
nities, being the estimated migration rate. Because the
estimation of migration rate m is affected by the num-
ber of sequences in samples, we flattened the number
Liu et al. Microbiome (2023) 11:93
Page 13 of 15
of sequences in each sample to 2000 before fitting the
neutral community model, allowing us to compare esti-
mated migration rates for different microbial partitions.
In this study, all 46,628 ASVs were separated into
three partitions depending on whether they occurred
more frequently than (above partition), less frequently
than (below partition), or within (neutral partition) the
95% confidence interval of the NCM predictions [60].
To explore the effect of the potential migration rate
of ASVs on their predictability, we analyzed and com-
pared the predictive accuracy of different (above, neu-
tral, and below) partitions belonging to the common
ASVs>10% sub-community.
Definition of core taxa
A global-scale core microbial subcommunity of WWTP
was determined based on multiple reported measures.
In this report, we explored the predictability of micro-
bial taxa at the ASV level (100% similarity), as such
the classification criteria for core ASVs were slightly
different than those for core OTUs [3]. First, ‘over-
all abundant ASVs’ were filtered out according to the
mean relative abundance (MRA) across all samples. We
selected all top 1% ASVs as overall abundant ASVs. Sec-
ond, ‘ubiquitous ASVs’ were defined as ASVs with an
occurrence frequency in more than 20% of all samples.
Finally, ‘frequently abundant ASVs’ were selected based
on their relative abundances within a sample. In each
sample, the ASVs were defined as abundant when they
had a higher relative abundance than other ASVs and
made up the top 80% of the reads in the sample [34].
A frequently abundant ASV was defined as abundant
in at least 10% of the samples. Following the same cri-
teria described above, a core ASV should be one that
was from the top 1% ASVs, a core ASV also had to be
detected in more than 20% of the samples and domi-
nant for more than 10% of the samples. Correspond-
ing to the core taxa, ASVs that did not meet the above
three conditions were called non-core taxa.
Statistical analysis
All alpha diversity measures were conducted using the
vegan and Picante packages of R (v. 4.0.3). Unless indi-
cated otherwise, an unpaired, two-tailed, two-sample
Student’s t-test was performed for comparative statis-
tics using the t.test function in the stats package of R
4.0.3. Linear correlation analyses between different
parameters were implemented using the lm function in
the stats package of R 4.0.3. All analysis and graphing
were done using R4.0.3 or python 3.8.
Supplementary Information
The online version contains supplementary material available at https:// doi.
org/ 10. 1186/ s40168‑ 023‑ 01519‑9.
Additional file 1. Supplementary analysis. Grouping and Comparisonof
ASVs>10%.
Additional file 2: Figure S1. Ranking of importance weights ofenviron‑
mental factors in different alpha‑diversities predictive models. FigureS2.
a. Comparison ofintra‑ and inter‑group Bray‑Curtis similarity between
predicted and observedcommunities. b. Average prediction accuracy
R2
1:1of microbial taxa at different taxonomic levels. Figure S3. Environ‑
mental factor importance weights andPearson’s correlation coefficients.
FigureS4. Correlation ofcorrelation coefficients of environment factors
with ASVs>10%subcommunity, skewness, and kurtosis of normalized
environment variables withtheir Garson’s connection weights. Figure
S5. a. Comparison of predictiveaccuracy R2
1:1 between low,medium,
and high abundance taxa. b. Comparison of predictiveaccuracy R2
1:1
between low,medium, and high‑frequency taxa. c. Correlation of relative
abundance with the occurrencefrequency of ASVs. d. Correlation of
the R2
1:1in test sets with the coefficient of variation of ASVs. Figure S6.
Comparison ofaverage relative abundance and occurrencefrequency
between above, neutral, and below partitions. Figure S7. Fit of theneutral
community model (NCM) of above, neutral, and below partitions. Figure
S8.The taxonomic composition, average relative abundance, occurrence
frequency,and estimated migration rate of core and non‑core taxa. Figure
S9. Predictionof functional groups with 10 high‑weight environmental
factors. Figure S10. Fitof the neutral community model (NCM) of high
abundance, medium abundance, andlow abundance subcommunities.
Figure S11. Changes of mean square errors (MSE) andcoefficients of
determination (R2) on the validation set with epochswhen training the
model.
Additional file 3: Table S1. Alpha‑diversitiesof AS system. Table S2.
Summary of ASVs belonging ASVs>10% sub‑community.Table S3.
Summary of microbial taxa at different taxonomic levels. Table S4.
Averageimportance weights of environmental factors in different ASVs
predictivemodels. Table S5. Summary of different environment variables.
Table S6. Summaryof genera belonging to major functional groups.
TableS7. Abbreviations, meanings, and types of environment variables.
Table S8. Numericaland normalized environmental data.
Acknowledgements
The authors thank the Global Water Microbiome Consortium (GWMC) and all
the people involved for providing samples and plant metadata. We thank the
High‑performance Computing Platform of Peking University for providing the
computing platform.
Authors’ contributions
X.L. conceived the study and performed all analysis and computation. X.L. and
Y.N. co‑wrote the paper, and X.L.W. revised it. All authors discussed the results
and commented on the article. The author(s) read and approved the final
manuscript.
Funding
This study has received funding from the National Key R&D Program of China
(2018YFA0902100 and 2021YFA0910300) and the National Natural Science
Foundation of China (91951204, 32130004, 32161133023, and 32170113).
Availability of data and materials
The raw data in this study is from reference [3]. All analyzed data in this study
is available in Additional file 3. The source code is available at https:// github.
com/ Neina‑ 0830/ WWTP_ commu nity_ predi ction.
Declarations
Ethics approval and consent to participate
Not applicable.
Liu et al. Microbiome (2023) 11:93
Page 14 of 15
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
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10.1371_journal.pwat.0000137.pdf
|
Data Availability Statement: Data have been
uploaded to Zenodo 10.5281/zenodo.7447637.
|
Data have been uploaded to Zenodo 10.5281/zenodo.7447637 .
|
RESEARCH ARTICLE
Disparities in disruptions to public drinking
water services in Texas communities during
Winter Storm Uri 2021
Brianna Tomko1, Christine L. Nittrouer2, Xavier Sanchez-VilaID
3, Audrey H. SawyerID
1*
1 School of Earth Sciences, The Ohio State University, Columbus, OH, United States of America,
2 Department of Management, Rawls College of Business, Texas Tech University, Lubbock, TX, United
States of America, 3 Department of Civil and Environmental Engineering, Universitat Politècnica de
Catalunya, Barcelona, Spain
* [email protected]
Abstract
Winter Storm Uri of February 2021 left millions of United States residents without access to
reliable, clean domestic water during the COVID19 pandemic. In the state of Texas, over 17
million people served by public drinking water systems were placed under boil water adviso-
ries for periods ranging from one day to more than one month. We performed a geospatial
analysis that combined public boil water advisory data for Texas with demographic informa-
tion from the 2010 United States Census to understand the affected public water systems
and the populations they served. We also issued a cross-sectional survey to account for
people’s lived experiences. Geospatial analysis shows that the duration of boil water adviso-
ries depended partly on the size of the public water system. Large, urban public water sys-
tems issued advisories of intermediate length (5–7 days) and served racially diverse
communities of moderate income. Small, mostly rural public water systems issued some of
the longest advisories (20 days or more). Many of these systems served disproportionately
White communities of lower income, but some served predominantly non-White, Hispanic,
and Latino communities. In survey data, “first-generation” participants (whose parents were
not college-educated) were more likely to be placed under boil water advisories, pointing to
disparate impacts by socioeconomic group. The survey also revealed large communication
gaps between public water utilities and individuals: more than half of all respondents were
unsure or confused about whether they were issued a boil water advisory. Our study rein-
forces the need to improve resilience in public water services for large, diverse, urban com-
munities and small, rural communities in the United States and to provide a clear and
efficient channel for emergency communications between public water service utilities and
the communities they serve.
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OPEN ACCESS
Citation: Tomko B, Nittrouer CL, Sanchez-Vila X,
Sawyer AH (2023) Disparities in disruptions to
public drinking water services in Texas
communities during Winter Storm Uri 2021. PLOS
Water 2(6): e0000137. https://doi.org/10.1371/
journal.pwat.0000137
Editor: Majid Shafiee-Jood, University of Virginia,
UNITED STATES
Received: December 16, 2022
Accepted: May 8, 2023
Published: June 21, 2023
Copyright: © 2023 Tomko et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data have been
uploaded to Zenodo 10.5281/zenodo.7447637.
Funding: The authors received no specific funding
for this work.
1. Introduction
Competing interests: The authors have declared
that no competing interests exist.
Water is an essential resource that is unevenly distributed. For at least one month each year,
two-thirds of the world’s population experiences conditions of severe water scarcity [1]. As the
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Drinking water access during winter Storm Uri
climate changes, water scarcity will persist for many communities [2] and expand to new ones.
Extreme weather events such as droughts and floods are also expected to increase in frequency
and severity in the coming decades, creating further disruptions to water supply and accessibil-
ity. Droughts have triggered partial shutoffs of municipal water services in locations around
the world. A prominent example is Cape Town, South Africa’s municipal water crisis in 2018,
when the city narrowly averted a total shutdown of municipal water services due to drought
conditions and water management decisions [3]. Droughts place heavy pressure on surface
water resources and, to a lesser extent, on groundwater resources, which are inherently more
insulated against drought. Floods damage infrastructure and test the limits of water treatment
technologies. Severe weather such as winter storms bring freezing temperatures that damage
pipes and can also disrupt the power supply to water treatment facilities.
In the United States, 97% of the population has access to improved water [4], but supply
was disrupted across Central and Gulf Coast states during Winter Storm Uri (February, 2021).
The state of Texas experienced severe, but not unprecedented, cold temperatures in the teens
and single digits in Fahrenheit (around -10 to -15˚C) [5]. Due to infrastructure damage, the
storm left millions without power and water for days [6, 7]. The storm was estimated to have
caused over 200 deaths and created about $100 billion in financial losses in Texas [8]. Over
two out of three (69%) Texans lost electrical power at some point during the storm. Almost
half (49%) reported losing access to running water, on average, for 52 hours [9]. Those with
uninterrupted access to running water still reported that their water was unpotable for an aver-
age of 40 hours during the week of the storm (for example, they were issued a boil water advi-
sory). More than half (56%) of those who lost potable water considered the loss to be
extremely serious or very serious. Nearly half (45%) also experienced difficulties finding bot-
tled water, rating this impact as very serious or extremely serious. For comparison, loss of cell
phone service, difficulties obtaining food, and illness or injury to immediate family were all
rated less serious in terms of impact [9]. Loss of domestic water disproportionately affects the
health of vulnerable populations such as children, older adults, and low-income individuals
[7]. The combination of winter weather, which made it difficult to use public transportation,
and the ongoing COVID-19 pandemic hindered access to bottled water supplies, particularly
for older adults and low-income individuals [7]. The loss of clean, domestic water also made it
harder for households to follow World Health Organization (WHO) and United Nations Chil-
dren’s Fund (UNICEF) guidelines for handwashing, a key strategy to reduce the spread of the
virus [10].
Amidst these factors, it is important to understand the effects of Winter Storm Uri on the
public supply of drinking water. Winter Storm Uri was the largest known boil water event in
U.S. history [11] and reflects the resilience of public water services in the region. Here, resil-
ience is defined as “the ability to plan for, absorb, recover from, or more successfully adapt to
actual or potential adverse effects” [12]. Existing analyses of this historic boil water event point
to knock-on effects from power outages [6]. In a survey of large, mostly urban public water
utilities, 85% lost power, impacting their ability to produce clean water [13]. Though many
had backup generators, not all were operable due to low fuel supplies or cold temperatures.
Additionally, water losses from burst pipes and leaks caused pressures to drop in many distri-
bution systems below the regulatory minimum, triggering the issuance of boil water advisories
to protect customers from pathogens that tend to infiltrate under low pressure [13]. By under-
standing which public systems were most affected and the contributing factors, new strategies
can be implemented to increase the resilience of public drinking water systems under future
weather extremes.
It is also important to identify communities that may be disproportionately vulnerable to
disruption of services in order to prioritize equitable responses [14]. Across urban areas of the
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PLOS WATERDrinking water access during winter Storm Uri
United States, consistent disparities in piped water access have been linked to unpredictable
housing conditions and racialized wealth gaps [15]. In peri-urban and ex-urban areas, munici-
pal underbounding has excluded low-income neighborhoods and people of color from public
water services altogether [16, 17]. Further, studies demonstrate that Black and Latino individu-
als, and generally those at lower income levels, have less access to clean water [18] and indoor
plumbing [19]. In the specific case of Winter Storm Uri, multiple studies have identified dis-
parities in utility outages across racial, ethnic, and income groups. For example, Nejat et al.
[20] showed that communities with a great proportion of non-Hispanic White residents, single
family homes, and greater income experienced a smaller share of lingering power outages after
the storm. Grineski et al. [21] revealed through a survey that Black participants were more
likely to experience longer water outages. By analyzing 311 calls for the Houston area, Lee et al.
[22] showed that burst pipes were more severe for low-income and racial minority groups.
Glazer et al. [11] examined power and water outages by county and compared them with an
index of social vulnerability. Although there was no clear correlation between the length of
boil water advisories and social vulnerability at the county level, they observed that some of the
most impacted counties had greater percentages of non-English speakers and minority resi-
dents, apartment complexes, and mobile homes. They therefore suggested the need for a more
granular analysis on a census tract level.
Here, we sought to understand the finer-scale impacts to Texas public water systems and
identify groups that were most affected through two related studies: 1) a geospatial analysis of
community public water systems that issued boil water advisories, and 2) a cross-sectional sur-
vey of individuals residing in Texas during February 2021. In the geospatial analysis, we used
principal component analysis to test whether there was any relation between the length of the
advisory (a measure of the recovery time for safe drinking water services after the storm) and
various factors describing the public drinking water system. These factors included size or
location of the public water system, severity of weather, and demographics at the system level
(derived from mapping public census data onto each water system’s service area). We also
asked how the advisories impacted specific demographic groups at the individual level through
the cross-sectional survey using both an analysis of covariance and a multivariate analysis of
covariance on human subjects data. Drawing on the integrated findings from these two stud-
ies, we were also able to examine individual awareness of boil water advisories and explore
communication gaps between public water providers and customers.
2. Method and materials
2.1. Geospatial analysis
To gather data on boil water advisories issued by public water systems, a request for informa-
tion was placed with the Texas Commission of Environmental Quality (TCEQ) under the
Texas Public Information Act. We limited the request to community public water systems,
defined as those that have the potential to serve at least fifteen residential connections or
twenty-five residents on a year-round basis [23], and excluded other public water systems such
as schools, hospitals, and seasonal communities. Most of the Texas population (roughly 27 mil-
lion people, or 93%) is served by community public water systems [24] and therefore is repre-
sented in the geospatial component of this study. Roughly 5% of Texans depend on private
well water [25] and are not included in the geospatial analysis.
TCEQ provided a list of 2,080 community public water systems that reported issuing a boil
water advisory related to Winter Storm Uri. The list includes the name of the public water sys-
tem, a unique identifier code, the county, the issue date of the boil water notice, the rescind
date of the boil water notice, the population served, and the number of connections served.
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Retail service areas for community public water systems were obtained from the Texas Water
Development Board’s (TWDB) water service boundary viewer in November of 2021 [26]. It is
worth noting that as of 2022, Texas is one of only 24 states with geospatial data products for
community water system service area boundaries [27]. The Texas dataset includes 4,572 out of
4,641 community public water systems [28]. Fourteen of the 2,080 public water systems that
issued boil water advisories did not have a service area polygon, so they were excluded from
the analysis. The remaining 2,066 records were screened for completeness and to ensure that
the dates of boil water advisories were consistent with Winter Storm Uri. A small number of
records [28] were removed from the analysis because they were incomplete, or the reported
advisory was not conclusively connected to Winter Storm Uri. The excluded records fit at least
one of these exclusion criteria: 1) the reported advisory was issued and rescinded prior to Win-
ter Storm Uri; 2) the reported advisory was issued before the storm hit, and local minimum
temperatures never fell below freezing; 3) no rescind date was provided. In total, 2,038 public
water systems were retained for analysis.
To relate information on boil water advisories to weather, daily climate summaries were
retrieved from the National Oceanic and Atmospheric Administration (NOAA) from Febru-
ary 1, 2021 to February 28, 2021 for 360 weather stations in the state of Texas [29]. Some of the
longest boil water advisories were not lifted until March, but the month of February fully
encompassed the climatological phenomenon of Winter Storm Uri; thus, we restrict our inves-
tigation of the climatological phenomenon (for example, how long temperatures stayed below
freezing) to February. A point feature class shapefile was created in ArcGIS for the weather sta-
tions with attributes containing the minimum and maximum recorded temperatures for each
day in February. We also calculated the sum of the number of days in February that the maxi-
mum or minimum daily temperature was below freezing. Some stations had missing data for
maximum and minimum temperatures on select days. No attempt was made to interpolate
missing data because it is possible that data gaps are temperature-dependent or biased towards
frozen temperatures. Our estimation of the number of frozen days is therefore conservative,
meaning that the number of days below freezing may be underestimated, and the minimum
daily recorded temperature may be overestimated. Weather data from the nearest station was
attributed to each public water system using a spatial join with the nearest neighbor in ArcGIS.
Information on urban and rural households, population, race, and housing tenure were
obtained for the state of Texas from the 2010 decennial United States Census using the R pack-
age tidycensus [30]. We opted for the 2010 census instead of the more recent 2020 census
because the results of the 2020 decennial census and American Community Survey were
impacted by the COVID-19 pandemic, and income data were only available as experimental
estimates [24]. We acknowledge that Texas has experienced substantial growth and demo-
graphic change since 2010, which introduces additional uncertainty to our analysis. Medium
income and its margin of error (MOE) were taken from the 2010 American Community Sur-
vey. The U.S. Census Bureau organizes data based on spatial hierarchy, ranging from states
down to blocks, the smallest measurement scale. Income data are not available at the block
level, but they are at the next largest block group level. Initial analyses showed that block
group-level calculations compared well with block-level calculations for public water systems
[31]. We, therefore, chose to analyze demographic information for block groups because there
is simplicity and advantage to working at one consistent spatial level for demographic and
income data.
In ArcGIS Pro, areas were calculated for both block groups and public water systems. Over-
lapping areas between the census block groups and public water systems were then used to
compute aerially-weighted average demographics for each public water system. Further infor-
mation is provided in Section 1 in S1 Text.
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To explore relationships between public water system characteristics, we performed infer-
ential statistical analyses, including Pearson correlation coefficients (which provide a measure
of linear correlation between two variables) and principal component analysis (PCA) using
MATLAB. The goal of PCA is to reduce the dimensionality of the data set by finding the com-
bination of variables that best explain the total variance [32]. Variables that displayed strong
positive skewness were logarithmically transformed (specifically, number of advisory days, ser-
vice area, homes served, and median income). All variables were then scaled to have a mean of
0 and a standard deviation of 1. We chose 15 variables to be included in the final matrix of cor-
relation coefficients, detailed in the Results. These variables were selected to represent a range
of conditions (meteorological: extent and duration of freezing temperatures; geographic: lati-
tude and longitude, degree of urbanization; scale of the system: size of service area and number
of homes served; and demographics: race, ethnicity, homeownership, and income), with the
goals of understanding which public water systems took the longest to recover, who was
affected, and for how long. We explored different combinations of variables (e.g., minimum of
daily minimum temperature versus minimum of daily maximum temperature; population
served versus homes served; fraction of White individuals versus fraction of White families)
and found negligible differences; duplicated variables were removed. Last, the dataset was sub-
jected to PCA to test whether there were any underlying patterns in the public water systems
that issued short or long boil water advisories. We removed the length of the boil water advi-
sory as a variable from the analysis, so that public water systems were only described by geo-
graphic, meteorological, and demographic variables. We also chose to eliminate elevation, a
variable strongly correlated with longitude and latitude, which was found in preliminary analy-
sis to have a negligible effect on the amount of variance explained by the first two components
in the PCA analysis, leaving a total of 13 remaining variables for the final statistical analysis.
2.2 Survey
We cross-sectionally surveyed 407 people who lived in Texas during Winter Storm Uri. We
received IRB approval (#IRB2021-882) from Texas Tech University to conduct this study. Par-
ticipants were asked to indicate their consent to participate in the first question on the survey,
and we only collected data from individuals who indicated their consent on this question. Par-
ticipants were permitted to skip any question in the survey they did not feel comfortable
answering without penalty. Participants were recruited from January 2022—April 2022. We
retained only those participants without missing data who could also be located and thus con-
nected to public water system service areas in the geospatial analysis (N = 289). Of these,
N = 23 people in our data set were not on public water systems at all, so we excluded these
individuals from our analyses regarding public water systems. Thus, our final sample consists
of N = 266 participants. Importantly, these individuals self-identify as 56% (149) female, 43%
(114) male, 1% (2) non-binary, and .5% (1) preferred not to say. Regarding race and ethnicity,
these individuals self-identify as 71% (188) White and Non-Hispanic, 21% (57) Latino or His-
panic, 3% (9) Asian or Pacific Islander, 4% (10) Black and Non-Hispanic, and 1% (2) who
chose to write-in their responses (see Section 3 in S1 Text for details regarding how these cate-
gories were combined and how identities differ slightly from categories in the United States
Census). Our sample is on average 22.88 years old (ranging from 18 to 80 years).
We associated participants with their public water system (if they lived in one) by asking
them to provide a zip code and street address where they were living during the storm, or alter-
natively, two nearby cross-streets, if they were uncomfortable listing their street address. This
information was provided by N = 266 participants. Using a Google Sheets plug-in called Geo-
Code, we generated longitude and latitude for each of these participants (Fig B in S1 Text) and
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PLOS WATERDrinking water access during winter Storm Uri
performed a spatial join in ArcGIS to the feature class of public water systems. Because some
participants provided nearby cross-streets rather than exact addresses, we experimented with
including a 500-m buffer, which only affected the spatial join for 4 participants (thus, we did
not use this buffer).
We asked participants a series of thirteen questions related to their access to clean water
during Winter Storm Uri, their experiences with boil water advisories, electricity and Wi-Fi
outages, and burst pipes or water damage due to the storm. Finally, we asked them a series of
demographic questions related to their gender, race and ethnicity, age, income, and familial
education levels (all survey questions are available in Section 3 in S1 Text). We defined “first-
generation” status as any participant whose parents had not completed college (of note, all par-
ticipants had received at least some college training themselves). First-generation status tends
to be positively correlated with families who are also low income [33, 34]. To control for the
differences across these variables’ scales, we z-transformed each variable before analysis. More
information is provided in Section 3 in S1 Text.
To explore how Participant Ethnicity, Income, and First-Generation Status related to their
experiences with Water Access, we conducted first (1) an analysis of covariance (ANCOVA)
on a binary variable indicating whether participants were issued a boil water advisory, derived
from the linked geospatial data; and second (2) a multivariate analysis of covariance (MAN-
COVA) on the thirteen survey items; we also controlled for two variables across both analyses
from the geospatial study: the fraction of rural households (System-Level Rural Housing) as a
measure of urban development in the participant’s area and the total number of households
(System-Level Total Housing) as a measure of the scale of the public water system that served
the participant.
3. Results
3.1. Analysis of public water systems
Most boil water advisories were issued on February 17, approximately one day after freezing
weather descended on the state of Texas (Fig 1A). Advisories were lifted anywhere from 1 to
36 days later, though below-freezing weather only lasted a maximum of 12 days (Fig 1B). The
mean length of an advisory was 7.96 days (d), with an standard deviation of 3.57 d. Meanwhile,
the mean length of below-freezing weather was 3.83 d, and the standard deviation was 3.26 d
(Fig 1B). Some of the coldest weather occurred in the northwestern regions of Texas farther
from the coast (Fig 1D), and consequently at higher latitude and longitude (for example,
r = 0.64, p < 0.01 for latitude and frozen days in Fig 2). In contrast, the duration of advisories
was scattered and showed no clear spatial trends with latitude or longitude (Fig 1C). As further
evidence, global Moran’s I (a measure of spatial autocorrelation calculated here with a binary,
nearest-neighbors weighting system) was 0.977 and 0.988 for minimum recorded temperature
and number of freezing days, respectively, indicating smoothly varying weather conditions.
Moran’s I was only 0.139 for the length of the advisory, indicating a more random
distribution.
Indeed, the length of the boil water advisory was weakly (linearly) correlated with all the
individual weather and sociodemographic factors analyzed, according to the values of bivariate
Pearson correlation coefficients (Fig 2). Public water systems that served a smaller number of
homes had a weak tendency to issue longer advisories (r = -0.25, p < 0.01). Those public water
systems that served a higher proportion of families who owned their homes outright also had a
weak tendency to issue longer advisories (r = 0.19, p < 0.01). It is important to note that at the
scale of public water systems, greater rates of homeownership were consistent with lower
median income (r = -0.44, p < 0.01) and more rural service areas (r = 0.24, p < 0.01), meaning
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PLOS WATERDrinking water access during winter Storm Uri
Fig 1. Patterns in the length of boil water advisories differ from patterns in cold weather. (A) Histograms showing when
advisories were issued and lifted (total number of samples, N = 2,038). (B) Histograms showing length of advisories and length of time
the maximum daily temperature was below freezing in February. (C) Map of boil water advisories duration. (D) Map of freezing
weather duration (number of days in February when the maximum daily temperature was below freezing). Map base layer and
technical documentation available from U.S. Census 2010 TIGER/Line shapefiles for Texas.
https://doi.org/10.1371/journal.pwat.0000137.g001
that homeownership rates are not an indicator of wealth when aggregated by public water sys-
tem and compared across urban and rural areas. In fact, public water systems with greater
median income tended to have a greater fraction of mortgaged homes (r = 0.76, p < .01) and
be more urban (r = 0.24, p < .01).
Within public water systems, the fraction of White families and Black or African American
families was strongly negatively correlated (r = -0.86, p < 0.01), and a weak negative correla-
tion was also evident between White families and families of Hispanic or Latino ethnicity (r =
-0.38, p < 0.01). This outcome is not forced by having compositional variables that sum to
one, as race and ethnicity are independent and overlapping categories in the census data. Also,
families could identify with additional races that include American Indian or Alaska Native,
Asian, and Native Hawaiian or Other Pacific Islander. The public water systems that served
greater proportions of White families tended to be more rural (less urban, r = -0.50, p < 0.01),
whereas the public water systems that served greater proportions of Black or African American
families and Hispanic or Latino families were mostly urban (r = 0.32 and r = 0.33, respectively,
with p < 0.01 in both cases).
The bivariate correlations in Fig 2 do not provide a comprehensive vision of the dataset.
For this reason, we developed a multivariate interpretation by means of PCA. We found that
approximately half (48%) of the geographic, meteorological, and demographic variability
among public water systems was explained by only the first two principal components
(Fig 3B).
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Fig 2. Correlation matrix for public water systems that issued an advisory in response to Winter Storm Uri.
https://doi.org/10.1371/journal.pwat.0000137.g002
PCA resulted in geographic and weather-related variables being projected onto the first and
third quadrants in the plot of components 1 and 2 (Fig 3B). For example, public water systems
at greater latitude that experienced more freezing days in February of 2021 project toward the
first quadrant. Meanwhile, variables that describe the scale and demographics of the service
area projected more or less orthogonally. For example, public water systems that served greater
number of homes and were located in more urban areas are projected toward the second quad-
rant. These public water systems were also associated with a greater proportion of rented
homes and lower proportion of White families. The four public water systems that served the
greatest number of customers (all >1 million, as reported to TCEQ by the providers) clustered
in the second quadrant and all experienced advisories of intermediate length (within mean
plus one standard deviation). These include the City of Houston, San Antonio Water System,
City of Fort Worth, and City of Austin Water & Wastewater (Fig 3B). In contrast, the public
water systems associated with some of the longest advisories tended to cluster in the fourth
quadrant. Specifically, 14 of the 15 systems with the longest advisories (all 20 days or more)
were similar in terms of their small populations and service areas and their tendency to serve
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PLOS WATERDrinking water access during winter Storm Uri
Fig 3. Principal component analysis of public water systems. A) Percent of variance among public water systems explained as a function of the
number of components. B) Projection of public water system data on a graph of principal components 1 and 2. Each point represents a public
water system that issued a boil water advisory, colored according to length of the advisory. Large diamonds indicate the 15 public water systems
with the longest boil water advisories. Large circles show the 4 public water systems that serve the largest populations. Vectors show the
projection of the geographic, meteorological, and demographic variables involved in the analysis.
https://doi.org/10.1371/journal.pwat.0000137.g003
more rural, homeowning families (Table A in S1 Text). The one system that did not cluster
with the 14 others was a small, rural system that served a large proportion (51%) of Black or
African American families (Table A in S1 Text). Interestingly, public water systems with very
short boil water advisories (1–2 days) did not cluster strongly according to the first two princi-
pal components (Fig 3A).
In summary (Table 1), the geospatial analysis suggests that no one factor resoundingly
explains recovery times for public water systems, but large, urban systems consistently issued
advisories of intermediate length affecting large, diverse communities. The longest recovery
times were experienced by small, rural systems of variable community demographics
(Table 1).
3.2. Individual experiences and awareness
Eighteen percent (N = 48) of the 266 participants who were on public water systems stated that
they were under a boil water advisory during the storm. Meanwhile, 37% (N = 98) were not
sure if they were under a boil water advisory, and 45% (N = 120) stated they were not under a
boil water advisory. Interestingly, 53% (N = 29 people) who said they were under a boil water
advisory actually were not, based on their locations at the time of the storm; 10% (N = 11) of
those who were not sure actually were; and 5% (N = 6) of those who said they were not under
a boil water advisory actually were. This finding highlights gaps in the way advisories were
communicated to the public (Fig C and Table B in S1 Text). These gaps did not appear to vary
strongly across racial and ethnic groups (Fig C in S1 Text).
A majority of participants (69%; N = 183) reported that they did not lose access to water
where they were living, while 31% (N = 76) did lose access to some degree. Specifically, 16%
(N = 43) lost access to running water altogether (no water flowed when they turned on their
tap); 12% (N = 33) experienced some change in water pressure. A majority (91%; N = 243)
stated that they did not notice any visible changes in the color, taste, or smell of their water.
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PLOS WATERTable 1. Summary of key findings from geospatial analysis, the survey tool, and their relationships.
Sample
How extensive were boil water
advisories, and what was the impact to
surveyed individuals?
System-Level Geospatial Analysis
2,038 community public drinking water systems
44% of the 4,572 community public water systems with
mapped service areas in Texas issued boil water advisories.
The mean length of an advisory was 7.96 days (d), and the
standard deviation was 3.57 d.
What communities were affected?
There was no one clear demographic variable that explained
the length of boil water advisories, but longer advisories
tended to be issued by smaller public water systems (serving
fewer homes).
Drinking water access during winter Storm Uri
Individual-Level Survey
266 individuals
Boil water advisories were issued to 14% of participants based
on their locations (for comparison, 18% said they were issued
an advisory in the survey); 31% experienced loss of water
pressure, 9% noticed changes in color, taste, or smell of their
water, 18% experienced burst pipes, and 16% experienced
water damage.
ANCOVA results: Although White, non-Hispanic individuals
were under confirmed boil water advisories significantly more
often than Black/Hispanic/Biracial individuals, the difference
was driven by the proportion of first-generation college
participants across ethnic categories who were more likely to
be under boil water advisories.
The 4 public water systems that served the greatest
populations (all >1 million, as reported to TCEQ by the
providers) experienced advisories of intermediate length
(within 1 standard deviation of the mean). These urban
systems served relatively greater proportions of home renters
and were more racially and ethnically diverse.
MANCOVA results: White, non-Hispanic individuals also
identified that they were under confirmed boil water advisories
significantly more often that Black/Hispanic/Biracial
individuals, but these differences (~5%) must be considered in
light of communication gaps between water utilities and the
public.
The 15 pubic water systems with the longest advisories (> 20
days) were similar in terms of their small populations and
service areas. Most served more rural, White, homeowning
families, but 3 of the 15 worst served an above-average
proportion of non-White or Hispanic and Latino families.
How well were boil water advisories
communicated?
There was broad public confusion: 37% of the sample was not sure if they were placed under a boil water advisory; 53% of
people who said they were issued a boil water advisory (18%) actually were not, based on their locations during the storm; 5%
of those who said they were not issued a boil water advisory (45%) actually were, based on their locations.
What are some of the limitations of the
tool?
Not all public water systems may have reported boil water
advisories to the TCEQ; Age of census data; Uncertainties of
calculating system demographics from aerially weighted
census data.
Snowball sampling approach, which favored college students
and specific regions; Small numbers; Uncertainties in
participants’ geographic locations; Delayed survey
dissemination.
https://doi.org/10.1371/journal.pwat.0000137.t001
Further, most participants did not experience burst pipes in their homes (82%; N = 217) or
water damage (84%; N = 224) (Fig D in S1 Text). The detailed omnibus and univariate test sta-
tistics, F-statistics, and effect sizes are reported in Tables C and D of S1 Text, so we summarize
only high-level findings below.
First, using the ANCOVA, we observed main effects of race and ethnicity (p = .01) and
first-generation status (p = .04) on boil water advisories that were issued to participants (as
determined from combining public water system and participant datasets). Although White
individuals (MW = 0.21, SEW = 0.04) were under confirmed boil water advisories significantly
more often than Black/Hispanic/Biracial (MBLB = 0.16, SEBLB = 0.05) participants, these effects
were driven by the experiences of White first-generation participants (MFG = 0.25, SEFG =
0.05). Participants who identified as White first-generation were significantly more likely to be
under boil water advisories than non-first-generation participants (MNFG = 0.14, SENFG = 0.03)
(Table C of S1 Text). Income effects were less clear, but first-generation status may be a better
indicator of socioeconomic status than income in this survey, given that many participants
were college students whose income responses may have been shaped by multidimensional
factors.
Second, using the MANCOVA, we observed that race and ethnicity had a significant main
effect on boil water advisories that were experienced by participants (as determined from par-
ticipant responses alone), such that White, non-Hispanic participants were significantly more
likely (p = .03) to report being under a boil water advisory (MW = 2.12, SEW = 0.11) than
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PLOS WATERDrinking water access during winter Storm Uri
Black/Hispanic/Biracial participants (MBLB = 2.35, SEBLB = 0.14) (Table C of S1 Text). The dif-
ference was approximately 5%. Both the ANCOVA and MANCOVA analyses hold across pub-
lic water system characteristics (the number of households that the participants’ water systems
served, and the fraction of rural households, which we control for in both analyses).
In summary (Table 1), we find that (1) across race and ethnicities, 42% to 49% of partici-
pants were incorrect regarding their actual boil water advisory status, which points to a gaping
opportunity to improve public health communications during extreme weather events and
emergencies. Additionally, (2) when we examined the influence of various socioeconomic fac-
tors on these experiences, we found that although White, non-Hispanic participants were
more likely to report being under boil water advisories than Black, Hispanic and Latino, and
Biracial participants, these results were driven by the experiences of first-generation partici-
pants within the White-identifying group (ANCOVA results). Finally, our (3) MANCOVA
results replicate the race and ethnicity effect we observed, and again point to communication
gaps between drinking water utilities and the public across racial and ethnic groups (Table 1).
4. Discussion
Considering all results in a holistic way (Table 1), we found that large, urban public water sys-
tems and small, rural ones had different recovery response times to Winter Storm Uri. Fur-
thermore, first-generation participants, who may come from more socioeconomically
disadvantaged backgrounds, were more likely to be issued boil water advisories across race,
ethnicity, and rural or urban communities. Advisories were not communicated effectively,
which disadvantaged all racial and ethnic groups. Below, we consider factors behind these
trends and implications for water security and future disaster recovery.
4.1. Response times across big, urban and small, rural systems
Winter Storm Uri impacted water access for large urban and small rural communities differ-
ently, revealing two scales of vulnerability in public water services. A small number of large
water providers serve a majority of the Texas population and issued boil water advisories that
left mostly urban residents without reliable drinking water for 5–7 days. Meanwhile, a very
small number of mostly rural public water systems issued boil water advisories that lasted
weeks and had acute effects on small portions of the Texas population. Of the 15 public water
systems with the longest boil water advisories, 11 served exclusively rural communities (frac-
tion of rural households = 1). All but one served fewer than 1,000 residents (M = 415 and
SD = 398). The cross-sectional survey showed similar effects: the total number of households
served and the fraction of those households being rural in a participant’s public water system
had a significant impact on whether that participant was issued a boil water advisory (Table C
of S1 Text).
The scale or size of public water systems can influence resilience to severe weather in differ-
ent ways. Large (typically urban) providers have more available resources for responding to
power loss and infrastructure damage during extreme weather, but large treatment plants also
require more power to operate and expertise to troubleshoot or maintain under extreme sce-
narios [13, 35]. Large systems also have longer distribution networks or more places where
pipes can burst, requiring more time and resources to identify and repair damage. As a result,
the largest systems are highly vulnerable to extreme weather events. Importantly, this small
number of large public systems impact the greatest share of the population, not only in Texas
but all across the United States. Just 8% of the approximately 52,000 community water systems
in the United States serve 82% of the population [36]. Therefore, investments in upgrades to
large public water systems can yield big returns–particularly for urban communities.
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PLOS WATERDrinking water access during winter Storm Uri
In comparison, smaller systems range widely in their resilience to extreme weather events
because of uncertainties in the human and financial resources available to them [35]. In the
current study, smaller systems (serving less than 1,000 individuals) displayed a wide range in
the lengths of their boil water advisories (range of 1–36 days), consistent with Glazer et al.
[11], who showed that smaller public systems tended to take longer to recover. Of the 15 sys-
tems with the longest recovery times, many were already struggling to meet federal drinking
water standards during typical weather conditions (they had multiple violations to the stan-
dards before and after Winter Storm Uri). Most (12) of these 15 systems used groundwater as
their water source, which often requires little treatment prior to distribution [37], making it
likely that these public water systems had little to no infrastructure to oversee, but also few
staff to address emergencies, leaving their customers water insecure in the face of extreme
weather. In general terms, many rural communities have limited access to resources to make
repairs during a winter storm [13]. Some of the rural systems with the longest advisories in
this study also served residents in unconventional housing such as mobile homes, consistent
with other studies that have noted unreliable water access in mobile home communities
[11, 38].
In total, Winter Storm Uri revealed weaknesses in both water policy and management
across urban and rural areas. The U.S. Federal Energy Regulatory Commission (FERC) and
industry stakeholders had previously identified critical infrastructure to winterize in order to
mitigate the effects of future winter storm events, but the recommendations were generally not
implemented [11]. Policy reform and funding are thus imperative to incentivize weatheriza-
tion and emergency preparedness. Under Texas Senate Bill 3, which passed in response to
Winter Storm Uri, public water utilities were required to have an alternate power source for
emergencies and to establish Emergency Preparedness Plans. In a follow-up survey of large
water utilities conducted one year after the storm, most had either established backup power
systems or were taking steps to do so [13]. However, 90% of these relatively well-resourced
utilities still cited economics as a limit to further action. With the influence of climate change
and urbanization, many large public water systems therefore remain vulnerable to extreme
weather, which creates water insecurities for growing populations [39].
4.2. Water service inequalities
Given the large number of public water systems that issued boil water advisories (Fig 3), it is
perhaps unsurprising that impacts were felt across racial and ethnic groups in both our sys-
tem-level and individual analyses (Table 1), though public water systems are organized around
communities that have been shaped by legacies of discrimination (including redlining and
gentrification). Glazer et al. [11] also observed no clear relationship between duration of boil
water notices and a social vulnerability index at the county level. We note, however, several
limitations in our finer-scale analysis that introduced additional uncertainties (Table 1). For
example, calculating an aerially weighted average set of demographics for a public water sys-
tem assumes that housing density within the service area is uniform. Differences may also be
masked by reducing demographic and income statistics to average values for entire communi-
ties (for example, two public water systems might have very similar average incomes but very
different income distributions).
It is important to note that other studies have revealed clear racial and ethnic disparities in
water supply and outage factors during Winter Storm Uri. Lee et al. [22] showed that more
311 calls related to burst pipes were placed by low-income and minority groups, and a survey
by Grineski et al. [21] showed that Black participants experienced longer water outages. Burst
pipes and loss of water pressure are not necessarily distributed evenly throughout individual
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PLOS WATERDrinking water access during winter Storm Uri
public water systems, unlike boil water advisories, allowing for more disparate impacts to
neighborhoods based on race, ethnicity, and income demographics. Power outages were not
equitably distributed across racial and ethnic identities either [20]. Although power outages
had knock-on effects on water providers, they were only one of many factors that led providers
to issue advisories [6, 40].
The importance of the first-generation status in our cross-sectional survey data may point
to underlying socioeconomic disparities in how boil water advisories were issued. We specifi-
cally found that first-generation, White participants were more often under boil water adviso-
ries than non-first-generation, White participants. Our method of recruiting participants
using snowball sampling via our own networks (e.g., research and conference contacts) and a
human subjects research participant pool of college-aged students within a large, public uni-
versity in Texas likely influenced the types of individuals within each income bracket and
masked income effects. First-generation status may therefore be the best measure of socioeco-
nomic status in our survey. Future research could explore income and education effects across
even a wider community sample.
Lastly, this study underscores other issues with disparity in public water services across
racial and ethnic groups, particularly a tendency for small, rural systems in Texas tend to serve
predominantly White communities (Fig 2), consistent with studies from elsewhere in the U.S.
[19, 41]. For example, a study from North Carolina showed that only 15% of small public
water systems served an above-average proportion of non-White or Hispanic and Latino fami-
lies (>26.96%) [42]. Yet, in our study, 4 out of the 15 (25%) small systems with the longest
advisories served an above-average proportion of non-White families; three served mostly His-
panic and Latino families, and one served mostly Black or African American families. Our
ANCOVA analysis (Table C of S1 Text), similarly shows that the longest recovery times in
rural public water systems in Texas were not consistently concentrated in predominantly
White communities.
To dismantle inequalities in public water system services and improve resilience for all
communities, there should be more investment in vulnerable geographic areas, including low-
income non-White communities [20], and steps should be taken to de-centralize and diversify
water supply systems. Additionally, identifying urban and rural communities that are under-
served by public water systems, and extending those services is important for ensuring equita-
ble water access [42].
4.3. Public awareness gaps
This study revealed wholesale communication gaps between water utilities and surveyed par-
ticipants. Tiedmann et al. [13] Castellanos et al. [43] also highlighted gaps and inconsistencies
in the way utilities communicated with the public. In a post-storm survey of large public water
utilities, roughly 80% cited communication issues with the public as an important complicat-
ing factor during the storm; yet, one year later, few of these utilities had taken steps to improve
communications [13]. A number of strategies have been suggested to improve communica-
tions, particularly to younger ages and minoritized groups. For example, public utilities could
leverage social media more frequently in their communications [13] and translate messages to
languages other than English [43]. Communication gaps can be exacerbated for communities
that rely less on traditional forms of media communication or where there are more non-
Native English speakers [44]. The large communication gap documented in this study has
important public health consequences and reinforces the need for diverse and tailored com-
munication strategies to reach diverse customer populations.
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PLOS WATERDrinking water access during winter Storm Uri
5. Conclusions
The factors that affected public water supplies in Winter Storm Uri were complex, but the
severity of temperatures was not clearly correlated to the length of the advisory. Instead, the
size or scale of the public water system and its urban or rural location were most important.
Smaller systems faced some of the longest boil water advisories lasting for multiple weeks,
while a large portion of the Texas population living in urban areas served by large public water
systems was placed under advisories lasting less than a week. Additionally, cross-sectional sur-
vey data suggest that first-generation, White individuals were more likely to be issued a boil
water advisory than non-first-generation, White individuals. These findings highlight differ-
ences in the resilience of public water services to communities of varying size, urban or rural
location, and socioeconomic status that affect water security for Texas residents. In the wake
of Winter Storm Uri, a clear need exists to help public water service providers prepare for
more extreme weather events in a changing climate. Survey data also revealed massive com-
munication gaps between public water service providers and customers, indicating a need for
new communication strategies.
Supporting information
S1 Text. Additional information on public water system demographics, survey methods,
and results.
(PDF)
Acknowledgments
We thank Michele Risko and Patrick Kading at the Texas Commission on Environmental
Quality for their assistance with data acquisition. We also thank two anonymous reviewers for
their helpful suggestions. This study has IRB (2021–882) approval. All datasets corresponding
to this analysis are freely available at https://doi.org/10.5281/zenodo.7447637.
Author Contributions
Conceptualization: Christine L. Nittrouer, Audrey H. Sawyer.
Data curation: Brianna Tomko, Christine L. Nittrouer, Audrey H. Sawyer.
Formal analysis: Brianna Tomko, Christine L. Nittrouer, Audrey H. Sawyer.
Investigation: Brianna Tomko, Christine L. Nittrouer.
Methodology: Brianna Tomko, Christine L. Nittrouer, Xavier Sanchez-Vila.
Project administration: Audrey H. Sawyer.
Resources: Christine L. Nittrouer, Audrey H. Sawyer.
Supervision: Audrey H. Sawyer.
Validation: Brianna Tomko, Christine L. Nittrouer.
Visualization: Brianna Tomko, Christine L. Nittrouer, Audrey H. Sawyer.
Writing – original draft: Brianna Tomko.
Writing – review & editing: Christine L. Nittrouer, Xavier Sanchez-Vila, Audrey H. Sawyer.
PLOS Water | https://doi.org/10.1371/journal.pwat.0000137 June 21, 2023
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PLOS WATERDrinking water access during winter Storm Uri
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| null |
10.1038_s41467-021-24130-8.pdf
|
Data availability
Single-cell RNA-seq data files are available in “GSE151337”. All other relevant data
supporting the key findings of this study are available within the article and its
Supplementary Information files or from the corresponding author upon reasonable
request. A reporting summary for this article is available as a Supplementary Information
file. Source data are provided with this paper.
Code availability
Codes used in this analysis were deposited onto GitHub:
|
Code availability Codes used in this analysis were deposited onto GitHub: https://doi.org/10.5281/ zenodo.4743036 . Data availability Single-cell RNA-seq data files are available in ' GSE151337' . All other relevant data supporting the key findings of this study are available within the article and its Supplementary Information files or from the corresponding author upon reasonable request. A reporting summary for this article is available as a Supplementary Information file. Source data are provided with this paper.
|
ARTICLE
https://doi.org/10.1038/s41467-021-24130-8
OPEN
TCF21+ mesenchymal cells contribute to testis
somatic cell development, homeostasis, and
regeneration in mice
1,10, Adrienne Niederriter Shami1,10, Lindsay Moritz2,10, Hailey Larose1,10, Gabriel L. Manske2,
Yu-chi Shen
Qianyi Ma1, Xianing Zheng1, Meena Sukhwani3, Michael Czerwinski4, Caleb Sultan1, Haolin Chen5,
Stephen J. Gurczynski4, Jason R. Spence
1,7 &
Saher Sue Hammoud
4, Kyle E. Orwig3, Michelle Tallquist6, Jun Z. Li
1,8,9✉
;
,
:
)
(
0
9
8
7
6
5
4
3
2
1
Testicular development and function rely on interactions between somatic cells and the
germline, but similar to other organs, regenerative capacity declines in aging and disease.
Whether the adult testis maintains a reserve progenitor population remains uncertain. Here,
we characterize a recently identified mouse testis interstitial population expressing the
transcription factor Tcf21. We found that TCF21lin cells are bipotential somatic progenitors
present in fetal testis and ovary, maintain adult testis homeostasis during aging, and act as
potential reserve somatic progenitors following injury. In vitro, TCF21lin cells are multipotent
mesenchymal progenitors which form multiple somatic lineages including Leydig and myoid
cells. Additionally, TCF21+ cells resemble resident fibroblast populations reported in other
organs having roles in tissue homeostasis, fibrosis, and regeneration. Our findings reveal that
the testis, like other organs, maintains multipotent mesenchymal progenitors that can be
future therapies for hypoandrogenism and/or
potentially leveraged in development of
infertility.
1 Department of Human Genetics, University of Michigan, Ann Arbor, MI, USA. 2 Cellular and Molecular Biology Program, University of Michigan, Ann Arbor,
MI, USA. 3 Department of Obstetrics, Gynecology and Reproductive Sciences, Integrative Systems Biology Graduate Program, Magee-Womens Research
Institute, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA. 4 Department of Internal Medicine, University of Michigan, Ann Arbor, MI, USA.
5 Biochemistry and Molecular Biology, Bloomberg School of Public Health, John Hopkins, USA. 6 University of Hawaii, Center for Cardiovascular Research,
Honolulu, HI, USA. 7 Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, MI, USA. 8 Department of Obstetrics
and Gynecology, University of Michigan, Ann Arbor, MI, USA. 9 Department of Urology, University of Michigan, Ann Arbor, MI, USA. 10These authors
contributed equally: Yu-chi Shen, Adrienne Niederriter Shami, Lindsay Moritz, Hailey Larose.
email: [email protected]
✉
NATURE COMMUNICATIONS |
(2021) 12:3876 | https://doi.org/10.1038/s41467-021-24130-8 | www.nature.com/naturecommunications
1
ARTICLE
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-24130-8
Sexual reproduction relies on the generation of distinct sexes
to increase biological diversity. The core of this strategy
rests with a bipotential gonadal primordium that supports
development of sex-specific reproductive organs with functionally
distinct gonadal cell types. The gonadal primordium is comprised
of primordial germ cells and mesenchymal cells that originate
coelomic
from two sources:
and the
mesonephros6. In early development, the coelomic epithelial cells
give rise to multiple cell lineages, including interstitial cells and
Sertoli cells1, but later become restricted to the interstitial com-
partment of the testis. In contrast, mesonephric-derived cells
migrating into the gonad contribute only to fetal/adult Leydig and
interstitial cells, but not Sertoli cells7,8. Hence,
the gonadal
mesenchyme is heterogeneous on the molecular and cellular
scales, and certain somatic lineages (e.g. Leydig and myoid) have
multiple cells of origin9, pointing to a complex developmental
programming of reproductive organs (reviewed in10–12).
epithelium1–5
the
to disruption of
Establishment of a functional somatic microenvironment is
essential for continuous sperm production, germ cell homeostasis,
and regeneration13–15. Disruptions in somatic cell populations
can dramatically alter germ cell development and testis function.
For instance, genetic ablation of macrophages in the adult testis
leads
spermatogonial proliferation and
differentiation16. Others have shown that testicular endothelial
cells14 as well as lymphatic endothelial cells15 support human and
mouse spermatogonial stem cell survival and expansion, and can
and
also modulate
regeneration15. Therefore, these studies underscore the impor-
tance of defining the interstitial cell composition and function.
spermatogonial
homeostasis
cell
The testis interstitial cells are believed to be postmitotic (not
actively proliferating). However, studies in rats have demon-
strated that adult Leydig cells can regenerate after ethane dime-
thane sulfonate (EDS)-induced cell death (reviewed in17).
Multiple interstitial cell (CD90/PDGFRA/COUPTFII/NESTIN)
populations were shown to re-enter the cell cycle upon EDS-
induced Leydig cell death, suggesting that the interstitial com-
partment contains a reserve Leydig or a general somatic cell
progenitor population that can be activated in response to
damage18–26. Without single-cell RNA-seq analysis it remains
difficult to tease apart whether these markers are observed in a
single homogenous population of Leydig stem cells or if these
cells are a heterogenous pool of progenitors. Furthermore, it is
also unclear if a common somatic progenitor could give rise to
additional cell types other than Leydig, and what the role of such
stem/progenitors would be in the normal adult testis.
rare spindle-shaped cells
Using single-cell RNA sequencing (scRNA-seq) we previously
identified a mesenchymal cell population in the adult mouse testis
that expresses the transcription factor Tcf21 and appears in vivo
as
surrounding the seminiferous
tubule13. The Tcf21 gene has known roles in the development of
multiple organs, including the testis27–29. Loss of Tcf21 Tcf21
promotes feminization of external genitalia in karyotypically male
mice30, while overexpression of Tcf21 in primary embryonic
ovary cells leads to in vitro sex-reversal via aberrant anti-
Mullerian hormone expression31. These results provide evidence
for a role of TCF21 in male sex determination and testis somatic
cell differentiation. Recent reports of single-cell sequencing dur-
ing sex determination and cell linage specification also identified
Tcf21 expression among subsets of gonadal somatic cells in both
male and female, although these experiments were limited to
NR5A1-eGFPcells32,33. The similarity between the adult Tcf21+
population and fetal somatic progenitors32,33 suggests that the
Tcf21+ population that persists in adulthood may retain fetal
developmental or functional properties.
In this study, we examine the role of TCF21+ cells in the
developing and adult mouse testis. To answer this question, we
utilize genetic lineage tracing to mark and follow the potential
and fate of the TCF21lin population both in vitro and in vivo.
First, in directed in vitro differentiation paradigms, we find that
flow-sorted TCF21lin cells possess mesenchymal stem cell (MSC)-
like properties and can be directed to differentiate to either myoid
or Leydig cell lineages, thus acting as true multipotent progenitors
in vitro. In vivo, fetal TCF21lin cells are bipotential somatic
progenitors, contributing to all known somatic cell populations in
the fetal and adult testis and ovary. In the adult testis, the
TCF21lin cells replenish somatic populations in response to injury
as well as in normal aging. Furthermore, the adult testis Tcf21+
cells resemble resident fibroblast populations in multiple organs
which have been implicated in tissue homeostasis, fibrosis, and
regeneration. In summary, our work demonstrates the first evi-
dence for a reserve somatic cell population in the adult testis,
representing a potential targetable cell population for develop-
ment of treatments for gonad-related defects and disease.
Results
The Tcf21+ population is
a molecularly heterogenous
mesenchymal cell population that is transcriptomically similar
to myoid and Leydig cells. We recently employed single-cell
RNA-seq (scRNA-seq) to generate a cell atlas of the mouse
testis13,34. Our analysis of ~35,000 cells identified all known
somatic cell types as well as an unexpected Tcf21+ population
(Fig. 1A)13. To better understand its potential function, we first
examined if the Tcf21+ population has molecular similarity to
other known somatic cells in the testis. By using a pairwise dis-
similarity matrix for somatic cell centroids, we find that the Tcf21+
population was distinct from macrophage and Sertoli lineages but
transcriptomically similar to myoid, Leydig, and endothelial cells
(Fig. 1B). In contrast with these cell types, Tcf21+ cells did not
express any of their terminally differentiated markers (Fig. 1C).
Rather, this population was uniquely demarcated by the expression
of Tcf21, Pdgfra, CoupTFII, and mesenchymal progenitor cell (MP)
markers including Sca1 (Fig. 1C, Supplementary Data 1), Arx, and
Vim13. To define molecular properties of Tcf21+ cells more
broadly we identified differentially expressed genes between the
Tcf21+ population and its most similar cell types (using >2-fold
change and FDR < 5%). Gene ontology analysis suggested that the
Tcf21+ population is of mesenchymal origin, and likely involved in
extracellular matrix (ECM) biology, tissue injury, and repair pro-
cesses (Fig. 1D; Supplementary Data 1). Although the ECM was
once considered a passive support scaffold, a wealth of data now
suggest an active role for ECM in many aspects of biology, from
tissue maintenance, regeneration, cell differentiation, to fibrosis
and cancer35,36.
Given the identification of multiple mesenchymal progenitor
markers in the Tcf21+ population, we next sought to validate
expression of these markers in vivo. We took advantage of
previously generated Tcf21mCrem mice,
in which a tamoxifen
inducible Cre recombinase inserted at the Tcf21 locus enables the
long-term tracing of the descendant populations of the Tcf21-
expressing cells, whether in the gonads, heart, kidney, and cranial
muscle37. Specifically, testes were collected from Tcf21mCrem:
R26RtdTom mice after 3 doses of tamoxifen (tdTom+ labeled cells
referred to as TCF21lin), dissociated, stained for a comprehensive
panel of mouse MSC markers (SCA1, CD73, CD29, CD34, and
THY1) while excluding mature Leydig (cKIT+) or immune
(CD45+) cells, and analyzed using flow cytometry (Supplemen-
tary Fig. 1A–F). Since our Tcf21+ cells were initially discovered by
enriching SCA1+ cells in the testis, we examined the hetero-
geneity of the SCA1+ and TCF21lin cells. As expected, the SCA1+
population has a broader representation in the testis than
TCF21lin (~3–5% vs. ~1–2%), with about 45% of SCA1+ cells
2
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Fig. 1 Identification and characterization of a Tcf21-expressing interstitial somatic cell in the adult testis. A Visualization of 6 somatic cell types in PC
space. Data reprocessed from Green et al.13. Note somatic cell populations were enriched using a combination of cell surface markers or transgenic lines,
therefore cell frequencies in PCA plot are not representative of in vivo. B Heatmap of dissimilarity matrix of somatic cell types illustrates high similarity for
the Tcf21+ interstitial population with 3 somatic cell types—myoid, Leydig, and endothelial cells. Gray scale indicates Euclidean distance. C Violin plots of
representative markers for endothelial, myoid, Tcf21+ interstitial population, and Leydig cells. D Heatmap of differentially expressed markers for each of the
4 closely related somatic cell types—endothelial, myoid, Tcf21+ interstitial population, and Leydig cells.
being also positive for TCF21lin (Supplementary Fig. 1G)—a
proportion consistent with our estimates from scRNA-seq data.
Within the SCA1+ population, we identified two TCF21lin
subpopulations (blue and purple in Supplementary Fig. 1B, C)
based on co-expression of MSC markers: a larger TCF21lin
population (13.8%, blue) that strongly expressed CD29 (ITGB1),
CD73, and CD34, and a smaller discrete TCF21lin population
(0.77%, purple) that expressed all MSC markers. In contrast, 80%
of the TCF21lin population expresses SCA1 (Supplementary Fig.
1G), and within the TCF21lin populations we identify multiple
subtypes that are molecularly heterogenous with respect to MSC
marker expression (Supplementary Fig. 1D–F) (see below for the
functional assessment of TCF21lin heterogeneity in vitro).
Given the heterogeneity observed within the TCF21lin popula-
tion, we asked whether TCF21lin subtypes could be further defined
by co-expression of previously described interstitial markers
including PDGFRA, COUPTFII, CD34, and FGF5, respectively.
To answer this question, we co-stained testes from Tcf21mCrem:
R26RtdTom or Tcf21mCrem:R26RtdTom:PdgfraDGFRAGFP mice with
COUPTFII, CD34, and FGF5 antibodies (Supplementary Fig. 1H),
but we did not detect a preferred segregation of TCF21lin cells
within the PDGRA, COUPTFII, CD34 or FGF5 subpopulation
(Supplementary Fig. 1H), suggesting that these populations are
heterogeneous on both the cellular and molecular level. Further-
more, for all markers analyzed we find co-stained cells both in the
interstitium or surrounding tubules—suggesting that the cellular
heterogeneity is not a result of spatial location.
The TCF21lin/SCA1+ population has mesenchymal progenitor
properties in vitro and can be differentiated to Leydig and
myoid cell fates in vitro. Mesenchymal progenitors are typically
characterized by their capacity for forming adherent fibroblast-
like colonies on plastic (measured as CFU-F: fibroblastic colony-
forming units) and for differentiating into adipocytes, osteocytes,
and chondrocytes in vitro (reviewed in38). Such populations have
been isolated from multiple human and mouse organs, including
juvenile or adult testes18,39–41. To examine whether the adult
SCA1+ and TCF21lin cells have mesenchymal progenitor-like
properties in vitro, we sorted four types of cells: SCA1−/cKIT+
interstitial cells (control), SCA1+/cKIT−, SCA1+/TCF21lin, or
TCF21lin cells from Tcf21mCrem:R26RtdTom animals (Supplemen-
tary Fig. 2A) and plated these cells at single-cell density to
measure clonogenic potential in vitro. Although SCA1+/cKIT−,
SCA1+/TCF21lin, and TCF21lin populations all formed colonies
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in vitro, clonogenic potential differed across populations. The
TCF21lin and SCA1+/TCF21lin double-positive cells formed the
highest number of colonies—~100 colonies per 1000 plated cells,
and this was followed by SCA1+/cKIT− cells (regardless of the
TCF21lin status) and cKIT+ cells with ~40 and 10 colonies,
respectively (Supplementary Fig. 2B). Altogether,
these data
demonstrate that SCA1+and SCA1+/TCF21lin populations have
characteristics of mesenchymal progenitors in vitro, but selecting
for the TCF21lin population significantly increases colony for-
mation potential.
chondrocytes, and osteoblasts. To examine
In addition to expanding in culture, mesenchymal progenitors,
under appropriate conditions, can be directed to differentiate into
adipocytes,
if
TCF21lin cells maintain such properties in vitro, we sorted either
SCA1+/cKIT− or cKIT+ cells
from Tcf21mCrem:R26RtdTom
animals and examined whether the TCF21lin within the SCA1+
population contribute to all three lineages (Supplementary Fig.
2A). Importantly, we found that SCA1+ cells robustly differ-
entiated into adipocytes, chondrocytes, and osteocytes as shown
by Oil red, Alizarin red, and Alcian blue staining, respectively
(Supplementary Fig. 2C). Furthermore, co-immunofluorescence
of Osterix (osteocytes), Perilipin (adipocytes), and SOX9
(chondrocytes) with tdTom+ (TCF21lin) cells demonstrates that
the TCF21lin cells within the SCA1+ population can contribute to
all three lineages in vitro (Supplementary Fig. 2D).
Given the ability of the SCA1+/TCF21lin to generate multiple
mesenchymal cell types in vitro and the transcriptomic relationship
of Tcf21+ cells with both Leydig and myoid cells (Fig. 1B), we next
asked whether the TCF21lin/SCA1+ population can be directed to
differentiate to both Leydig and myoid cells in vitro and if the
TCF21lin/SCA1+ population serves as a multipotent progenitor for
Leydig and myoid lineages. To this end, we sorted SCA1+/cKIT−
(regardless of TCF21 status) or TCF21lin/SCA1+/cKIT− cells from
Tcf21mCrem:R26RtdTom adult male testes and directed their
differentiation to either myoid or Leydig cells using a set of
growth factors based on the repertoire of receptors expressed in
our scRNA-seq datasets and earlier in vivo genetic findings of
Leydig or myoid cell specification (Supplementary Fig. 2E–K)42–44.
After treating the bulk SCA1+ or SCA1+/TCF21lin cells with a
myoid differentiation cocktail which includes Smoothened agonist
(SAG, an activator of Desert Hedgehog), PDGFAA, PDGFBB,
ACTIVINA, BMP2, BMP4, and Valproic Acid (outlined in
Supplementary Fig. 2E), we observed a morphological conversion
of spindle-shaped cells to flattened and striated cells resembling
smooth muscle cells (Supplementary Fig. 2F). This conversion was
confirmed by expression of smooth muscle cell markers such as
smooth muscle actin (Supplementary Fig. 2G, H).
that
Previous in vivo and in vitro experiments uncovered that
Desert Hedgehog (DHH), FGF, and PDGF signaling are
stimulatory to Leydig cell differentiation, while Notch signaling
and other factors are inhibitory42–44. By incorporating these
findings from the literature with our scRNA-seq data, we
developed a 14-day differentiation protocol
includes
PDGFAA, PDGFBB, SAG, FGF2, LiCl2, and DAPT (Notch
inhibitor) (detailed in Supplementary Fig. 2I). This protocol
enables successful differentiation of SCA1+/cKIT− or SCA1
+/TCF21lin cells to Leydig cells (Supplementary Fig. 2J–K).
Importantly, the in vitro-derived Leydig cells expressed steroido-
genic factor 1 (SF1) (Supplementary Fig. 2K, L) and secreted
testosterone (Supplementary Fig. 2M, N). Notably, Leydig cell
generation and testosterone secretion was achieved in vitro
independent of LH control
(Supplementary Fig. 2M, N).
Consistent with absence of LH regulation, we detected only low
levels of
luteinizing hormone/choriogonadotropin receptor
(Lhcgr) transcripts in day 14 Leydig cells (Supplementary Data 2).
As the SCA1+/TCF21lin cells were labeled as a population, the
results described above could not distinguish between two scenarios:
(1) each TCF21lin/SCA1+ cell is a multipotent progenitor, capable of
adopting any of multiple fates, or (2) these cells are heterogeneous,
comprised of multiple subtypes of progenitors that each have been
cryptically committed to differentiate into a different somatic
lineage. To distinguish between these two scenarios, we sorted
individual TCF21lin/SCA1+ cells into separate wells on 96-well
plates and allowed each to expand into individual clones. Following
clonal expansion, clones were randomly assigned to either the myoid
or Leydig cell differentiation protocol (Fig. 2A, B). Therefore, if these
cells have already been primed for one or the other lineage, we
would expect that some clones, but not all, that undergo the myoid-
inducing treatment will differentiate to the myoid lineage, and
likewise some clones in the Leydig treatment group will follow the
Leydig lineage. Co-immunostaining with either SMA for myoid cells
or SF1 for Leydig cells reveals that all TCF21lin/SCA1+ clones in
either group differentiate to adopt the induced fate 100% of the time
(number of clones provided in Supplementary Table 1), suggesting
that the TCF21lin/SCA1+ cells are individually multipotent in vitro
(Fig. 2C, D; Supplementary Table 1). However, since the TCF21lin/
SCA1+ population can be further stratified by the expression of
CD105 in vivo, we examined whether TCF21lin/SCA1+ multipotency
may be restricted to a specific subset of TCF21lin/SCA1+ cells. To
address this, we sorted and differentiated TCF21lin/SCA1+/CD105+
or TCF21lin/SCA1+/CD105− cells
to Leydig or myoid cells.
Consistent with our earlier finding using TCF21lin/SCA1+ cells, both
the TCF21lin/SCA1+/CD105+ and the TCF21lin/SCA1+/CD105−
clonal cells differentiated to both lineages with 100% efficiency
(Fig. 2C, D; Supplementary Table 1). These observations suggest that
subpopulations of
multipotency is not due to heterogeneous
TCF21lin/SCA1+ cells identified by flow cytometry (Supplementary
Fig. 1E, F). We caution, however, since these clonal cell experiments
were performed in vitro, the multipotency of these cells in vivo
remains to be confirmed in future studies.
Single-cell time-course analysis reveals timing and diverging
trajectories during Leydig cell differentiation. We next char-
acterized the Leydig cell differentiation process in vitro using
scRNA-seq. To this end, we collected and analyzed ~6500 cells
across four time-points along the in vitro differentiation process
(d0 sorted SCA1+/cKIT−, d4, d7, and d14 in culture). By clus-
tering the merged dataset and using gene expression and marker
gene analysis we identified seven distinct clusters (Fig. 3A, B).
When overlaying time points on the different clusters, we find
that freshly sorted SCA1+/cKIT− cells at d0 contribute to clusters
1 and 2 (Fig. 3C). A small number of cells in cluster 1 express von
Willebrand factor (Vwf) and the receptor tyrosine kinase Tie-1,
indicating that some endothelial cells also express Sca1 (Fig. 1C,
3B; Supplementary Data 2). However, cluster 2, which constitutes
the majority of SCA1+/cKIT− cells, is the interstitial progenitor
population that expresses Tcf21, Pdgfra, and CoupTFII (Fig. 3B;
Supplementary Data 2).
Four days after exposure to expansion media, cells dominate in
clusters 3 and 5 with a smaller number of cells appearing in
clusters 4 and 6 (Fig. 3C). Cells in cluster 3 are actively
proliferating, as reflected by expression of proliferative marker
Mki67, cyclins Cdk1 and Cdc20, and mitotic microtubule
associated proteins (Fig. 3B; Supplementary Data 2). Cells in
cluster 5 are postmitotic myofibroblasts expressing genes involved
in actin cytoskeleton dynamics and remodeling (e.g. S100a4,
Actn1, Lmod1, Nexn, Acta2) (Fig. 3B; Supplementary Data 2).
Three days (d7) after transition to differentiation media, cells
have become directed to an ECM-depositing myofibroblast cell
state (expressing: Clu, Postn, Tnc, Col5A2; cluster 4) or a
4
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Valproic acid
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Clonal
expansion
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individual cells
d4
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DMEM/F12 +
PDGFAA, PDGFBB,
FGF2, SAG
DMEM/F12 + PDGFAA, PDGFBB,
FGF2, SAG, LiCl2, DAPT
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Fig. 2 The adult TCF21lin cells are multipotent and can be directed to differentiate to Leydig and smooth muscle cells in vitro. Schematic representation
of experimental timelines for myoid (A) and Leydig cell (B) directed differentiation. Gating strategy used for FACS is presented in Supplementary Fig. 2G. C
Immunofluorescence staining of smooth muscle actin (SMA) in clonal TCF21lin cells after 7 days of in vitro culture in the presence of differentiation media.
Representative images of n = 13 technical replicates of TCF21lin/cKit−, n = 6 technical replicates of TCF21lin/SCA1+/cKit−, n = 6 technical replicates of
TCF21lin/SCA1+/cKit−/CD105+, and n = 14 technical replicates of TCF21lin/SCA1+/cKit−/CD105. Scale bar: 100 μm for all images in the panel. D
Immunofluorescence staining of steroidogenic factor 1 (SF1) in clonal TCF21lin cells after 14 days of in vitro culture in the presence of differentiation media.
Representative images of n = 19 technical replicates of TCF21lin/cKit−, n = 17 technical replicates of TCF21lin/SCA1+/cKit−, n = 12 technical replicates of
TCF21lin/SCA1+/cKit−/CD105+, and n = 22 technical replicates of TCF21lin/SCA1+/cKit−/CD105−. Scale bar: 100 μm for all images in the panel.
progenitor Leydig cell state (cluster 6) (Fig. 3B, C; Supplementary
Data 2). Although cells in cluster 4 do not ultimately contribute to
Leydig differentiation, they express fibroblast markers (Col5a2,
Postn, Tnc) as well as extracellular-matrix related proteins
involved in tissue remodeling (Fig. 3B, C; Supplementary Data 2).
Interestingly, this population also expresses Pdgfa, raising the
possibility that cluster 4 cells serve as an intermediate supportive
cell population required to promote continued differentiation of
Leydig cells.
By day 14 (10 days of exposure to differentiation media), cells
in clusters 6 and 7 have become more differentiated (Fig. 3C).
Cells in cluster 6 appear to prepare for steroidogenesis by
increasing expression of lysosome/exosome genes, likely employ-
ing autophagy to degrade cellular components into steroid
building blocks like cholesterol and fats (Fig. 3B; Supplementary
Data 2). Previously, autophagy in Leydig cells was shown to be a
rate-limiting step for testosterone synthesis45. Apolipoprotein E
(ApoE) is also expressed at this time, indicating that LDL uptake
is occurring which is critical for steroidogenesis, in line with
genetic evidence in ApoE/Ldlr knockout mice46. Finally, in cluster
7, steroidogenic enzymes Cyp17A1, Hsd3B1, StAR, Cyp11A1 are
expressed, as well as the mature Leydig cell factor Insl3, indicating
a cellular state with functional steroidogenesis (Fig. 3B, F;
Supplementary Data 2). The in vitro developmental progression
sampling also confirmed Monocle3
based on time point
pseudotime analysis (Fig. 3D–F), where sorted progenitors give
rise to a cycle of proliferating and differentiating intermediates.
Cells then branch into two differentiation trajectories, one
aborting in cluster 4, which is an ECM producing myofibroblast,
possibly a support intermediate cell, while the remaining cells
proceed through clusters 6 and 7 which lead to differentiated
Leydig cells (Fig. 3D–F).
(clusters 2–5)
Given our success with generating molecularly functional
(testosterone secreting) Leydig cells in vitro, we next asked
whether the in vitro derived Leydig cells bear resemblance to
in vivo Leydig cells by comparing to previously published adult
and fetal somatic cell states13,33. Notably, the in vitro inter-
in Leydig cell differentiation
mediate states
correlate with early interstitial progenitors in the fetal gonad,
whereas clusters 6 and 7 have a higher correlation to fetal Leydig
cells (Supplementary Fig. 3A). When comparing to the adult
testis, the in vitro intermediate states (Clusters 2–5) correlate
more closely with the Tcf21-expressing interstitial population,
whereas clusters 6 and 7 have the highest correlation to adult
Leydig cells (Supplementary Fig. 3B). Interestingly, overall the
in vitro derived Leydig cells have higher correlation to adult
Leydig cells than fetal Leydig cells (r = 0.84 vs. 0.58, respectively)
(Supplementary Fig. 3A, B).
TCF21lin cells contribute to somatic lineages in the male gonad
in vivo. Given our ability to differentiate TCF21lin cells to Leydig
or myoid cells in vitro, we next asked if the Tcf21 lineage can
serve as a somatic progenitor in vivo. To this end, we performed
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Fig. 3 scRNA-seq differentiation trajectory of in vitro derived Leydig cells. A Single-cell RNA-seq time course analysis of in vitro Leydig differentiation
(days 0, 4, 7, and 14) identifies seven clusters, as visualized in t-SNE space. Note: n = 1 replicate per timepoint, but in vitro differentiation was successfully
completed 8 times in prior experiments. B Heatmap of differentially expressed markers across the seven cluster centroids. C Visualization of the
contribution of each individual time point at day 0, 4, 7, or 14 to the 7 clusters in t-SNE space. D Pseudotime ordering of cells from the 4 time points (days
0, 4, 7, and 14) by Monocle3 in UMAP space, colored by 7 clusters. E Schematic annotation of differentiation trajectory as defined by Monocle. F
Expression profiles of selected markers across the differentiation pseudotime.
time points
in
from early developmental
lineage-tracing
Tcf21mCrem:R26RtdTom mice and analyzed fully formed testes at
E17.5 and adult testes at 10 weeks. Specifically, timed pregnant
females were given a single dose of tamoxifen at either gestational
days E9.5, E10.5, E11.5 or E12.5 (Fig. 4A). We verified that
TCF21lin labeling was absent in embryonic gonads harvested
from vehicle-treated timed pregnant females, confirming tight
regulation of the tamoxifen inducible Cre (Supplementary Fig.
4A). Additionally, lineage traced cells did not overlap with Vasa, a
germ cell marker, confirming specificity of the Tcf21mCrem line
(Supplementary Fig. 4B).
Immunofluorescence and histological analysis of E17.5 male
gonads revealed marked differences in the extent of labeling across
the different
tamoxifen injection time points (Fig. 4B). We
observed relatively fewer TCF21lin cells from E9.5 injected
animals, yet those cells co-localized with multiple somatic lineages,
as evidenced by colocalization with markers for Sertoli cells
(SOX9), interstitial cells (COUPTFII) and, to a lower extent, fetal
Leydig cells (3BHSD), and myoid (SMA) cells (Fig. 4C–F,
Supplementary Fig. 4C–F). In contrast, a greater number of
TCF21lin cells are observed in the E17.5 gonads collected from
animals treated with tamoxifen at E10.5, E11.5, and E12.5,
possibly due to broader expression of Tcf21 in multiple somatic
(Fig. 4C–F,
progenitors at
Supplementary Fig. 4C–F). The TCF21lin cells at these different
timepoints again contributed to Sertoli (SOX9),
fetal Leydig
(3BHSD), interstitial (COUPTFII, PDGFRA, and GLI), and myoid
(SMA) cells (Fig. 4C–F, Supplementary Fig. 4C–H). Curiously, a
significant fraction of E9.5 or E10.5 TCF21lin cells contribute to
Sertoli cells, but these cells account for only a fraction of all SOX9
+ cells in the fetal and postnatal testis (Supplementary Fig. 4C).
This suggests either incomplete labeling of TCF21lin cells or Sertoli
cells arising from multiple somatic progenitor populations.
injection timepoints
To determine the origin of TCF21lin+ cells in the embryonic
gonad and potential overlap with the WT1+ population—a
these later
somatic progenitor previously shown to give rise to Sertoli and
interstitial populations including adult Leydig cells47—we injected
timed pregnant Tcf21mCrem:R26RtdTom;Oct4-eGFP females with a
single dose of tamoxifen at E10.5 and collected and stained whole
mount gonads with WT1 at E11.5. In the E11.5 gonads, the
TCF21lin cells localize both to the coelomic epithelium and the
mesonephros, making it difficult to determine if TCF21lin cells
truly originate from the coelomic epithelium or mesonephros, or
are present in either location. However, we find that TCF21lin cells
partially overlap with the WT1+ cells in the coelomic epithelium,
but many cells are either TCF21lin or WT1+, suggesting these are
possibly two distinct populations (Fig. 4G).
Unlike most somatic cell populations in the testis, the steroid-
producing Leydig cells are unique in that they arise in two distinct
waves. To ascertain whether the fetal-derived TCF21lin cells
persist in the postnatal testis and give rise to adult Leydig cells,
Tcf21mCrem:R26RtdTom time pregnant females received a single
injection of tamoxifen at E10.5 and the pups were fostered and
matured to adulthood (see Methods; Fig. 4A). In 10-week-old
male testes, we found that a fraction of adult Sertoli, peritubular
myoid, endothelial cells, and interstitial cells are tdTom+ (i.e.,
derived from TCF21lin) (Fig. 4C–F). Furthermore, we observe
overlap between TCF21lin and 3BHSD in the adult
testis,
suggesting that the fetal TCF21lin population gives rise to adult
Leydig cells (Fig. 4D). Taken together, we demonstrate that the
fetal TCF21lin contributes to all adult somatic lineages of the
testis. However, since the overlap is incomplete in all somatic
populations this raises two possibilities: incomplete labeling or an
alternative progenitor source population.
Although the fetal TCF21lin cells in vivo contribute to multiple
somatic progenitor populations, the limitations of labeling a
single somatic progenitor in the fetal testis has hampered our
ability to conclusively determine whether
the fetal TCF21
population is a heterogeneous population already committed to
different fates, or if each cell is individually multipotent.
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Fig. 4 The TCF21lin population contributes to multiple somatic lineages in the fetal and adult testis. A Experimental timeline used for Tcf21 lineage
tracing analysis. Tcf21mCrem:R26RtdTom timed pregnant females were injected with a single dose of Tamoxifen at E9.5, 10.5, E11.5 or 12.5 and testes were
analyzed at E17.5 or 10 weeks. B The TCF21lin cells at E10.5 (n = 17) and E11.5 (n = 11) contribute to all major somatic cell populations in the fetal gonad,
whereas the E12.5 (n = 11) TCF21lin cells give rise only to testis interstitial cells. C–F Co-immunostaining of fetal or fostered adult TCF21lin testis cross-
sections with Sertoli cell marker SOX9 (green; C, n = 5 for E9.5 and E12.5, n = 3 for E10.5), Leydig cell marker 3β-HSD (green; D, n = 5 for E9.5, n = 3 for
E10.5 and E12.5), interstitial cell marker COUPTFII (NR2F2; green in E, n = 5 for E9.5, E10.5, and E12.5), and a myoid cell marker alpha smooth muscle actin
(SMA, green in F, n = 5 for E9.5, n = 3 for E10.5, n = 4 for E12.5). G The TCF21lin in the fetal testis of Tcf21mCrem; R26RtdTom; Oct4-eGFP embryos are present
in both the coelomic epithelium and mesonephros. Colocalization of WT1+ (Green) cells in the E11.5 gonad with TCF21lin cells (n = 2). In all panels the
nuclear counterstain is DAPI (white B–G, n = 2). Scale bars for all B–G: 20 μm.
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The TCF21lin gives rise to multiple fetal and adult ovarian
somatic cell types. Prior to sex determination in mammals, the
gonadal primordium is bipotential, meaning that the gonadal
mesenchyme has the ability to give rise to either male or female
support cell and steroidogenic cell lineages32. Once Sry expression
is turned on during a critical window of fetal development, testis
differentiation is initiated48. Despite an early commitment to
either male or female somatic cell types, genetic studies in mouse
have shown that terminally differentiated cell types must be
actively maintained throughout life. For example, loss of either
Dmrt1 in Sertoli cells or Foxl2 in granulosa cells can trigger
reciprocal cell fate conversions (from Sertoli to granulosa cell fate
or granulosa to Sertoli cell fate, respectively)49–51. Taking into
account the known gonadal mesenchyme plasticity and the ability
of TCF21lin cells to give rise to multiple somatic lineages in the
testis, we next asked if TCF21lin cells are present in the fetal ovary
and whether they might give rise to analogous cell types in
females. For our female gonad experiments, we injected timed
pregnant female Tcf21mCrem:R26RtdTom;Oct4-eGFP mice with a
single dose of tamoxifen at E10.5 and collected female embryonic
gonads at E11.5. Our analysis shows that TCF21lin cells are
present in the coelomic epithelium and mesonephros, similarly to
what we had observed in male embryonic gonads (Supplementary
Fig. 4K). We then injected tamoxifen in timed-pregnant females
at E10.5, E11.5, or E12.5 Tcf21mCrem:R26RtdTom mice and found
broad somatic cell labeling in the female gonads at E17.5 (Sup-
plementary Fig. 4L). Co-staining ovarian cross-sections with
terminally differentiated markers shows that TCF21lin cells
overlap with markers for granulosa cells (FOXL2+), interstitial
cells (COUPTFII+), and smooth muscle cells (SMA+), and does
not overlap with the germ cell markers VASA or OCT4 (Sup-
plementary Fig. 4M–P).
In the postnatal mouse ovary, the fetal TCF21lin population
contributes to multiple adult somatic lineages including granulosa
cells (FOXL2+ and WT1+) of primordial and growing follicles,
and endothelial cells (PECAM+), but not germ cells (Supple-
mentary Fig. 4Q). Furthermore, we find that the TCF21lin cells
surround the follicles and co-expresses the theca cell marker
3BHSD+52. Previous studies have shown that theca cells are
derived postnatally from GLI1+ and/or WT1+ populations53,54.
Therefore, we asked if the fetal TCF21lin cells co-express WT1. To
this end, we examined E11.5 embryonic gonads from timed
pregnant Tcf21mCrem:R26RtdTom;Oct4-eGFP mice and co-stained
gonad sections for WT1 (Supplementary Fig. 4K). WT1 and
TCF21lin appear to be largely in distinct populations with the
exception of a few cells in the coelomic epithelium (Supplemen-
tary Fig. 4K). Therefore, the fetal TCF21lin population is largely
distinct from the WT1 population, yet,
it contributes to all
somatic lineages in the adult ovary including: granulosa cells
(FOXL2+ and WT1+) of primordial and growing follicles,
endothelial cells (PECAM+), and theca cells (3BHSD+), suggest-
ing that the female gonad, like the male gonad, may have multiple
somatic progenitors.
Tcf21lin cells regenerate Leydig cells in the adult testis in
response to chemical ablation. Although somatic cells of the
testis are considered to be postmitotic and do not naturally
turnover, our steady state scRNA-seq datasets identified rare
Tcf21+ cells that express low levels of either steroidogenic acute
regulatory protein (StAR) or Sma, suggesting that a rare subset of
Tcf21+ cells transition to either Leydig or myoid cells, respec-
tively. We then examined if the adult TCF21lin is capable of
serving as a somatic progenitor at least for adult Leydig cells in
the testis. Specifically, animals were treated with EDS to reduce
Leydig cell numbers and the testes were collected and assessed to
determine (1) if regeneration does occur and (2) if TCF21lin
contributes to the regeneration.
EDS treatment was previously used in rats to selectively ablate
Leydig cells55–58. Although there is strong species specificity and
animal-to-animal variation in Leydig cell sensitivity to EDS59, two
300 mg/kg EDS injections in C57BL/6 mice spaced 48 h apart
(Supplementary Fig. 5A) resulted in a reduction of mature Leydig
cells in the adult testis as evidenced by cell death and CYP17A1
protein levels (Supplementary Fig. 5B, C). At 12 h post final
injection (hpfi), we detected TUNEL positive Leydig cells, but
given the spacing of 48 h between the first and second injection,
the majority of apoptotic events likely preceded the time of
analysis and could therefore not be quantified (Supplementary
Fig. 5B). However, consistent with Leydig cell loss, we find that
CYP17A1 protein expression decreased as early as 12 hpfi and
had largely recovered by 14 dpfi (Supplementary Fig. 5C). We
confirmed that the recovery of CYP17A1 protein expression was
not simply due to Leydig cell hypertrophy, as the Leydig cell
diameter is similar between the EDS- and vehicle-treated animals
at 14 dpfi (Supplementary Fig. 5D).
To determine if regenerating Leydig cells were derived from
the adult testis TCF21lin we injected 8-week-old Tcf21mCrem:
R26RtdTom mice with three 2 mg tamoxifen injections, and
then treated the mice with two 300 mg/kg EDS injections
spaced 48 h apart (Fig. 5A). Similar to EDS-treated C57BL/6
animals, CYP17A1 protein levels in Tcf21mCrem:R26RtdTom
EDS-treated animals decreased at 3 dpfi and recovered by 24
dpfi (Fig. 5B). To ensure that the reduction in CYP17A1 is due
to Leydig cell loss, we quantified the number of SF1+ cells in
EDS- and vehicle-treated animals at 3 dpfi. In 3 dpfi animals,
~20% of all cells in testis cross-sections are SF1+ in the
vehicle-treated animal, whereas the number of SF1+ cells is
reduced to 5% in the EDS-treated animals, suggesting many
Leydig cells were lost (n = 3 vehicle and n = 3 EDS animals).
To test whether the TCF21lin population contributes to Leydig
cell regeneration, we examined if TCF21lin cells proliferate in
response to injury and contribute to the regenerated Leydig
cells. At 3 dpfi, TCF21lin cells surrounding the tubules re-
entered the cell cycle as detected by colocalization of BrdU
and TCF21lin (Fig. 5C). Importantly, by 24 dpfi in EDS-treated
animals, ~18% of SF1+ Leydig cells are Tcf21lin positive as
compared to ~5% in the vehicle-treated animals (Fig. 5D, E).
Furthermore, we find that the EDS-treated animals have a
higher number of SF1+ cells as compared to controls (Fig. 5F),
which is consistent with an overcompensation in CYP17A1
protein levels observed in EDS-treated animals (Fig. 5B) and
the absence of Leydig cell hypertrophy (Fig. 5G).
To independently validate the role of TCF21lin cells in testis
somatic cell regeneration, we performed allogenic transplants.
Specifically, we sorted adult SCA1+ cells from Tcf21mCrem:
R26RtdTom animals and transplanted them into the testis
interstitium of EDS-treated C57BL/6 animals where no Leydig
cells in the host animal will be TdTom+ (Supplementary Fig. 5E).
the transplanted TCF21lin population
We reasoned that
contributed to Leydig cell regeneration, then we should detect
TCF21lin/SF1 double-positive cells in the C57BL/6 EDS-treated
animal. By 24 hpfi, we
cells homed to
the basement membrane, and by 7 dpfi, we began detecting
TCF21lin/SF1+ cells (Supplementary Fig. 5F), suggesting that the
Tcf21lin cells engrafted in the ablated C57BL/6 mouse testis and
gave rise to SF1+ cells.
found TCF21lin
Therefore, by using the Tcf21mCrem:R26RtdTom EDS model and
the TCF21lin transplant approach in EDS-treated C57BL/6 mice,
we demonstrate that the adult Tcf21lin population in the testis can
at least serve as a reserve Leydig progenitor in response to Leydig
cell loss.
if
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Fig. 5 The TCF21lin population regenerates Leydig cells in vivo after injury. A Schematic representation of the experimental method used to ablate Leydig
cells in the Tcf21mCrem:R26RtdTom background. B Representative CYP17A1 protein immunoblots from EDS- and vehicle-treated animals at 3 dpfi and 24 dpfi.
A total of n = 8 independent experiments were performed. C Representative images of BrdU+ cells in the testis of vehicle- and EDS-treated animals
(representative from n = 2 vehicle, n = 3 EDS). D Colocalization of TCF21lin (tdTom) and Leydig cell marker SF1 in EDS- or vehicle-treated Tcf21mCrem:
R26RtdTom animals at 24 h post final injection (24 hpfi) or 24 days post final injection (24 dpfi). Representative images from n = 6 vehicle, n = 7 EDS
animals. E Quantification of the percentage of Leydig cells expressing both SF1 and TCF21lin (tdTom) per field at 24 dpfi (Note—each circle represents the
percentage per animal. A total of n = 6 vehicle, n = 7 EDS were analyzed). Data are presented as mean ± SEM. P = 0.0018. F Quantification of Leydig cell
number per field of view at 24 dpfi (n = 6 vehicle, n = 7 EDS). Data are presented as mean ± SEM. P = 0.042. G Average Leydig cell diameter
measurements in Tcf21mCrem:R26RtdTom EDS or vehicle-injected animals, after Leydig cell recovery at 24 dpfi (n = 3 per condition). Lines indicate mean and
quartiles. P = 0.092. All statistical tests were performed using Welch’s unpaired, two-sided t-tests. Scale bars: 20 μm for all images in C, D.
Peritubular myoid cells of the testis can regenerate after injury
in adult testis. While we demonstrated that Leydig cells can be
regenerated in response to tissue injury, it is unclear whether
additional somatic cells in the testis can do the same. Previous
studies have shown that Sertoli cells can be replaced by trans-
plantation but cannot be regenerated following targeted diphtheria
toxin (DTX) treatment60–62, but the regenerative ability of peri-
tubular myoid cells has not been assessed. To examine whether (1)
peritubular myoid cells regenerate and (2) if TCF21lin cells con-
tribute to the regeneration, we treated 6–13-week-old Myh11cre-
eGFP; Rosa26iDTR/+ mice (referred to as MYH11-cre:iDTR here-
after) with multiple low doses of DTX to balance animal survival
and myoid cell ablation and collected testes at 12 hpfi and 4 dpfi
(Supplementary Fig. 6A). By 12 hpfi, TUNEL positive cells were
present on the tubule basement membrane in Myh11cre-eGFP;
Rosa26iDTR/+ mice but absent in control mice but were no longer
detectable by 4 dpfi (Supplementary Fig. 6B). However, at 4 dpfi
the testis cross-sections of Myh11cre-eGFP; Rosa26iDTR/+ animals
continue to display vacuoles and disordered tubules, whereas,
these histological features were absent from controls (Supple-
mentary Fig. 6C). By 4 dpfi, we detect BrdU positive smooth
muscle cells surrounding the basement membrane (Supplemen-
tary Fig. 6D, yellow arrow) as well as BrdU positive cells on the
tubule surface which lacked SMA expression (Supplementary Fig.
6D, white arrow), indicating both neighboring peritubular smooth
muscle cells and nearby interstitial cell progenitors re-enter the
cell cycle to regenerate the basement membrane in the DTX-
treated Myh11cre-eGFP; Rosa26iDTR/+mice. However, since our
TCF21 antibody did not yield clear immunofluorescence staining,
it could not be determined whether proliferating cells are TCF21+.
In an attempt to overcome these limitations, we sorted SCA1+/
TCF21lin cells from Tcf21mCrem:R26RtdTom animals and transplanted
these cells into the interstitial space of Myh11cre-eGFP; Rosa26iDTR/+
DTX-treated animals. Within 24 h after transplant, we were able to
detect TCF21lin cells surrounding the damaged tubules (Supplemen-
tary Fig. 6E, F), but failed to observe any TCF21lin become SMA+ at
this time point (Supplementary Fig. 6G). Therefore, unlike with
Leydig cells, for which we relied on EDS to induce cell-specific
ablation, our effort to ablate myoid cells relied on the use of
Myh11cre-egfp; Rosa26iDTR/+ which is expressed in multiple organs,
leading to lower animal viability and preventing the analysis of the
fate of TCF21lin transplanted cells at later time-points.
In summary, we have demonstrated that peritubular myoid
cells can be regenerated. Unlike in the case of Leydig cells
described above, whether myoid cells can be derived from
TCF21lin cells could not be ascertained at this time and will
require additional tools.
Adult TCF21lin cells contribute to somatic turnover in the
testis during natural aging. Once established, the somatic cells of
the testis are maintained throughout a male’s reproductive age.
However, a study in rats using [3H]thymidine labeling to detect
proliferation suggested that Leydig and possibly peritubular
myoid cells may undergo rare events of cellular turnover during
an animal’s natural lifespan63. We then asked to what extent adult
mouse testis somatic cells turnover during natural aging, and
secondly, whether new cells would be derived from TCF21lin
progenitors. To answer this question, we performed a long-term
lineage tracing experiment where 8-week-old (adult) Tcf21mCrem:
R26RtdTom animals were injected with a single dose of 2 mg
tamoxifen. Animals were euthanized either 1-week past final
injection or 1-year past final injection (aged mice) (Fig. 6A). In
animals with 1-week labeling we detected rare TCF21lin cells
surrounding the basement membrane, and these cells do not
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significantly overlap with Leydig cell markers such as SF1
(Fig. 6B). In aged mice, we detected a significant
increase
inTCF21lin and SF1 double-positive cells (Fig. 6C) as well as more
labeling around peritubular cells suggesting that peritubular cells
may also be replenished (Fig. 6B). This extent of labeling varied
by individual animal which could be due to tamoxifen injection
efficiency or true natural biological variability in inbred mice.
Interestingly, while there is no significant difference in Leydig cell
number between short-term labeled and aged individuals, there is
more variability among aged individuals (Fig. 6D). Nevertheless,
these data indicate that somatic cells of the testis naturally
turnover, possibly at low rates, and the TCF21lin population
contributes to the replenishment and maintenance of the adult
Leydig cell population, and likely peritubular myoid cells as well.
Additional intercellular interactions suggested by scRNA-seq
data. To gain a sense of cellular crosstalk between Tcf21+ cells
and germ/somatic cells, we focused on previously documented
ligand–receptor (L–R) pairs that are highly variable among major
cell types in our scRNA-seq data, and calculated Interaction
Scores between germ cells and somatic cells, or among somatic
cells of the testis (Fig. 6E, Supplementary Data 3). We previously
showed that the Tcf21+ population has more potential interac-
tions with spermatogonial populations than other germ cells34
(Fig. 6E, right panel), Similarly, we find that spermatogonia can
potentially signal back to the Tcf21+ population (Fig. 6E, left
panel) via PDGFA, various ADAMs, calmodulins, FGFs, and
guanine nucleotide binding proteins (Supplementary Data 3).
Within the somatic compartment, Tcf21+ cells have the
greatest potential
interactions with endothelial, myoid, and
macrophages (Fig. 6E, middle panel) involving receptor–ligand
signaling systems
such as Lrp1 (Cd91), a multifunctional,
endocytic receptor capable of binding a vast array of ligands64
and known to regulate the levels of signaling molecules by
endocytosis, as well as directly participate in signaling for cell
migration, proliferation, and vascular permeability (reviewed
in65). Like other tissue mesenchymal progenitors, Tcf21+ cells
may also modulate local inflammatory responses, as they express
Thrombomodulin (Tbhd) which interacts with and can proteo-
lytically cleave the pro-inflammatory molecule Hmgb1 (reviewed
in66). Additionally, there are several more cell-specific interac-
tions with myoid cells (Tgfb2-Tgfbr3;Gpc3-Cd81), endothelial cells
(Cxcl12-Itgb1; Pdgfa-Pdgfra; and Vegfa-Itgb1), and macrophages
(Igf1-Igf1r; F13a1-Itgb1) (Supplementary Data 3). Several of these
putative interactions are involved in growth, wound healing,
phagocytosis, and matrix remodeling in various mesenchymal cell
types67. These putative interactions will need to be validated by
spatial analysis and/or functional perturbations of
individual
signaling pathways.
The Tcf21+ population in the testis resembles resident fibro-
blast populations in other tissues. Finally, given the essential
role of Tcf21+ cells in mesenchymal development of many tissues,
lung, and kidney27,29,37,68, we sought to
including the heart,
understand whether the adult testis Tcf21+ population resembles
other Tcf21+ mesenchymal populations found in single-cell
analyses of other tissues. Our comparison of testis somatic cells
with the publicly available scRNA-seq datasets from coronary
lung, and liver69–73 find that the testis Tcf21+
artery, heart,
population most closely resembled the resident fibroblast or
myofibroblast populations (Fig. 7, left), as well as the fibroblast/
myofibroblast populations that appear transiently after tissue
injury (Fig. 7, right). These diverse fibroblast cell types have been
documented across tissues and implicated in fibrotic damage and
tissue regeneration, even when they may differ with respect to
cellular markers or nomenclature (reviewed in74). Our results
indicate that they are transcriptomically similar to the testicular
Tcf21+ population characterized here, suggesting that they col-
lectively represent an emerging class of resident adult progenitor
cells playing similar roles in tissue maintenance and repair across
multiple organ systems.
Discussion
Tcf21 is a basic helix-loop-helix transcription factor, known to
have roles in the development of numerous organs, including the
testis27–29. During testis development, Tcf21 is expressed in the
bipotential gonadal ridge at E10.5 similar to other key transcrip-
tion factors, including WT1 and GATA475, and loss of Tcf21
results in gonadal dysgenesis29,30. Here, our lineage tracing data
show that the fetal TCF21lin population is a bipotential gonadal
progenitor giving rise to most somatic cell types including steroid-
producing cells (fetal and adult), stromal/interstitial cells, and
supporting cells, as well as vasculature, consistent with a common
bipotential progenitor model recently described32,33. Gonadal
organogenesis is a complex process with multiple somatic pro-
genitors have been described in both males and females including:
WT1, COUPTFII, NESTIN, and PDGFRA18,21,23,47,76. To a cer-
tain extent, our data reconcile these findings by showing that
TCF21lin overlaps with many of these markers, but our data
supports a multi-progenitor model for both interstitial and Sertoli
cell populations that are molecularly and cellularly heterogeneous.
Furthermore, we demonstrate that the TCF21lin cells share char-
acteristics with adult mesenchymal progenitors (MPs) for example:
TCF21lin/SCA1+cells can self-renew in vitro and possess numerous
secretion of molecules with anti-
traits
inflammatory and immunoregulatory effects, and have multi-lineage
potential77,78. Furthermore, we show that the SCA1+/TCF21lin cells
can be directed to differentiate to myoid and Leydig cells in vitro.
Importantly, the in vitro derived Leydig cells secrete testosterone and
highly resemble in vivo-derived Leydig cells. The availability of such a
robust differentiation protocol makes it possible to use in vitro-derived
cells to study how environmental toxicants effect steroidogenesis or
Leydig cell function/biology.
like homing ability,
Testosterone is essential for the development and maintenance
of male characteristics and fertility. Reduced serum testosterone
affects millions of men and is associated with numerous pathol-
ogies including infertility, cardiovascular diseases, metabolic
syndrome, and decreased sexual function. Although exogenous
replacement therapies are largely successful in ameliorating these
they carry increased risks of cardiovascular and
symptoms,
prostate disease or infertility79–81. Therefore, identifying a pro-
genitor population and/or natural mechanism to restore testos-
terone levels in vivo and combat hypogonadism or age-related
decline in testosterone levels is critical82,83. Here, we show that
resident TCF21lin cells or TCF21lin allogenic transplants can be
activated to support Leydig cell regeneration and replenish Leydig
cells upon injury or aging. To date there have been several
putative stem Leydig cell populations described in the fetal and
early postnatal testis of multiple species. These likely analogous
populations are demarcated by diverse cell surface markers that
are often species specific, like CD90 for rat and CD51 for mouse
(reviewed in82,84–86) and exhibit species specific properties. For
example, PDGFRA+ cells isolated from rat can be differentiated
to Leydig cells, whereas, although the PDGFRA+ isolated cells
from the human testes exhibit aspects of MSC characteristics
in vitro, they are unable to fully differentiate into Leydig cells, nor
can they produce testosterone41.
Finally, given the critical role of Tcf21 in the development of
other tissues, as well as in aging and disease models87,88, we
examined commonalities between testis Tcf21+ cells and similar
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Fig. 6 The TCF21lin population maintains testis tissue homeostasis during aging. A Schematic representation of TCF21 lineage tracing in the aging testis.
B Colocalization of TCF21lin (tdTom) and Leydig cell marker SF1 (green) at 1 week or 1 year following a single injection of tamoxifen in 8-week-old adults.
Scale bar: 20 μm all panels. DAPI is used as the nuclear counterstain (white). (Oil only control n = 2, demonstrating the tight control of TCF21-Cre, n = 3
for 1 week, n = 5 for 1 year). C Quantification of the percent of Leydig cells expressing both SF1 and TCF21lin (tdTom) per field (Note—each circle/square
represents the percentage per animal; n = 3 for 1 week, n = 5 for 1 year). P = 0.016. Data are presented as mean ± SEM. All statistical tests were performed
using Welch’s unpaired, two-sided t-tests. D Quantification of Leydig cell number per field (Note—each circle/square represents the percentage per
animal; n = 3 for 1 week, n = 5 for 1 year). P = 0.19. Data are presented as mean ± SEM. All statistical tests were performed using Welch’s unpaired, two-
sided t-tests. E Summary of putative ligand–receptor interactions in the mouse testis between the germline and soma (left), within soma (center), and
soma and germline (right). Arrows summarize top 5% of all interactions. ScRNA-seq data from42, processed as described in34. Symbol size indicates the
number of receptor–ligand interactions contributed by a cell type and line width shows the number of interactions between the two cell types.
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Fig. 7 The adult Tcf21+ interstitial population resembles fibroblasts in other tissues. A Rank correlation of scRNA-seq cluster centroids of somatic cells
from wild-type adult mouse testes with other tissues in healthy adult mice (left panels) or after injury (right panels). Gray blocks indicate cell types not
present in healthy datasets.
populations in other organ systems and disease states69–73. The
Tcf21+ population in the adult testis molecularly resembles Tcf21+
fibroblast or fibroblast-like populations that have functional roles in
normal tissue maintenance, injury, or disease in other organs such
as the heart, lung, and liver. While the response to tissue injury is
often context dependent, resulting in fibrosis vs. regeneration, it
remains unknown if a single resident mesenchymal population is
activated to promote either response depending on the levels of
damage or signaling pathways activated or if multiple populations
respond and leading to divergent outcomes. Here in response to
EDS, the TCF21lin restores Leydig cells but in many cases, fibrosis is
often observed in men with impaired spermatogenesis89–93.
Therefore, whether dysregulation of the TCF21+ population may be
involved in the pathogenesis of testis fibrosis in certain contexts of
infertility remains to be examined. A greater understanding of this
population and its regulation may contribute to more informed
strategies to restore testis function, as well as the tissue regeneration
and tissue repair therapies for other organs94.
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Methods
Lead contact and materials availability. Further information and requests for
resources and reagents should be directed to and will be fulfilled by the Lead
Contact, Saher Sue Hammoud ([email protected]).
Mice. All animal experiments were carried out with prior approval of the Uni-
versity of Michigan Institutional Committee on Use and Care of Animals (Animal
Protocols: PRO00006047, PRO00008135), in accordance with the guidelines
established by the National Research Council Guide for the Care and Use of
Laboratory Animals. Mice were housed in the University of Michigan animal
facility, in an environment controlled for light (12 h on/off) and temperature
(21–23 °C) with ad libitum access to water and food (Lab Diet #5008 for breeding
pairs, #5LOD for non-breeding animals).
Colony founders for Rosa26iDTR (Stock #007900), Oct4-eGFP (Stock #004654),
PdgfraDGFRAEGFP (Stock007669), R26RtdTom mice (Stock #007909), and
Myh11cre-egfp (Stock #007742, maintained on a B6 background) were obtained from
Jackson Labs. The Tcf21mCrem and the Gli1EGFP were generously provided by
Michelle Tallquist and Deb Gumuccio, respectively. EDS injection studies were
performed on age-matched C57BL/6 mice obtained from Jackson Labs (Stock
#000664) or Tcf21mCrem:R26RtdTom animals. For detailed mouse strain information,
see below. All primers used for genotyping are provided in Supplementary Data 4.
Interstitial populations single-cell data analysis
Single-cell RNA-sequencing analysis comparing somatic cell types in the adult mouse
testis. Somatic cells (N = 3622) and their cell type classifications were defined by
Green et al.13. To compare somatic cell populations of the testis we obtained the
Euclidean distances for the somatic cell centroids then ordered the cell types using
the optimal leaf ordering (OLO) algorithm in R Package Seriation. Based on this
analysis, we discovered that the Tcf21+ population is highly correlated to endo-
thelial, myoid and Leydig cells, but distinct from immune cells and Sertoli cells. To
get a better understanding of the functional role of the Tcf21+ population, we called
differentially expressed genes in each somatic cell type using a nonparametric
binomial test. The differentially expressed genes have: (1) At least 20% difference in
detection rate; (2) a minimum of 2-fold change in average expression levels, and
(3) p value < 0.01 in the binomial test. Pathway Enrichment analysis for the dif-
ferentially expressed genes was performed with PANTHER tool v.15 (http://www.
pantherdb.org)95. Significance of the over- and under-representation of GO
Complete Biological Process categories was calculated using Fisher’s exact test and
multiple testing correction with the false discovery rate.
Ligand–receptor analysis. We used a previously published curated list of
ligand–receptor (LR) pairs34. We limited the analysis to LR pairs that have either
highly variable ligand genes among the 7 somatic cell centroids, or highly variable
receptor gene among the 26 germ cell centroids (6 SPG and 20 non-SPG clusters).
Thresholds were set for both genes mean and variance across the clusters according
to the density. For each ligand–receptor pair we calculated its apparent signaling
strength as an “Interaction Score”, defined as the product of the mean expression
level of the ligand in one cell type and that of the receptor in another cell type. In
all, we calculated such an Interaction Score matrix of cell type pairs for germ
(ligand)-soma (receptor) interaction, soma (ligand)-soma (receptor) interaction
and soma (ligand)-germ (receptor) interaction (reproduced with permission
from34, respectively. To extract the general signaling pattern for each interaction
matrix, we defined “strong interactions” for each matrix by keeping the highest 5%
Interaction Scores for each matrix. We then calculated the number of such strong
L–R interactions for each pair of cell types as their overall interaction strength and
displayed them as the line width of arrows in the pairwise interaction plots.
Cell type correlations across tissues. We downloaded the single-cell counts data
from GEO for artery, lung, heart and two liver datasets69–73. For the datasets
providing cluster information including our testis datasets, we generated expression
centroid for each cell type. We then calculated the spearman rank correlation for all
cell type pairs between testis and other tissues. For the artery and the liver datasets,
the cell type clusters were not provided, so for these datasets we re-analyzed the
raw data using Seurat—following the analysis descriptions from the original papers.
For parameters that are not specified, we either used default values or set
accordingly. To regenerate the clusters for these raw datasets, we used Louvain
clustering in Seurat and assigned cell types according to markers listed in the two
papers. We then followed the same procedure to calculate cell type expression
centroid and spearman rank correlations with cell types from testis. Summary of all
correlations are illustrated in the heatmap.
In vitro differentiation assays
Flow cytometry. Testes were collected from adult C57BL/6 (JAX®mice, stock
#000664) mice and enzymatically and mechanically dissociated into a single-cell
suspension. Briefly, testes from adult mice were excised and the tunica albuginea
was removed. Seminiferous tubules were transferred to 10 ml of digestion buffer 1
(comprised of Advanced DMEM:F12 media (ThermoFisher Scientific), 200 mg/ml
Collagenase IA (Sigma), and 400 units/ml DNase I (Worthington Biochemical
Corp)). Tubules were dispersed by gently shaking by hand and a 5-min dissociation
at 35 °C/215 rpm. To enrich for interstitial cells, tubules were allowed to settle, and
the supernatant was collected, quenched with the addition of fetal bovine serum
(FBS) (ThermoFisher Scientific), filtered through a 100 μm strainer and pelleted at
600 g for 5 min. The remaining tubules were then transferred to digestion buffer 2
(200 mg/ml trypsin (ThermoFisher Scientific) and 400 units/ml DNase I (Wor-
thington Biochemical Corp) dissolved in Advanced DMEM:F12 media) and dis-
sociated at 35 °C/215 rpm for 5 min each and quenched with the addition of FBS
(ThermoFisher Scientific). The cell pellets from multiple digests were combined
and filtered through a 100 μm strainer, washed in phosphate-buffered saline (PBS),
pelleted at 600 g for 3 min, and re-suspended in MACS buffer containing 0.5% BSA
(Miltenyi Biotec).
The single-cell suspensions were stained with a single antibody or combination
of antibodies depending on the experiment. The antibodies used include anti-Ly6a-
AlexaFluor 488 (1:100; Biolegend, Cat#108115), Biotinylated anti-Ly6a (1:200;
Biolegend, Cat#108103), streptavidin conjugated AlexaFluor 488 (1:1000; Life
Technologies Cat# S11223; RRID: AB_2336881), anti-CD73-APC (1:300;
Biolegend, Cat#127209), anti-CD90.1-Brilliant Violet 650 (1:300; Biolegend,
Cat#202533), anti-CD29-PE/Dazzle 594 (1:300; Biolegend, Cat#102231), anti-
CD105-PerCP/Cy5.5 (1:300; Biolegend, Cat#120415), anti-CD45-Brilliant Violet
510 (1:300; Biolegend, Cat#), anti-CD117-PE/Cy7 (c-KIT) (1:300; Biolegend,
Cat#105813), and anti-CD34-PE (1:300; Biolegend, Cat#128609).
Tri-lineage differentiation assay. Testes were collected from adult C57BL/6 or
Tcf21mCrem:R26RtdTom mice and dissociated into a single-cell suspension and
sorted for SCA1+/cKITit− or SCA1+/TCF21lin/cKITit− cells, respectively. For
adipogenic differentiation, 3 × 104 cells were plated in a monolayer, cultured for
10 days (Stem ProAdipogenic differentiation kit) and stained with either Oil Red O
or Perilipin (1:250, Sigma, Cat#P1873). For chondrogenic differentiation, 5 × 104
cells were plated in micromass, cultured for 21 days (Stem ProChondrogenic dif-
ferentiation kit) and stained with either Alcian blue or SOX9 (1:250, EMD Milli-
pore, Cat#ABE571). For osteogenic differentiation, 1 × 104 cells were plated in
micromass, cultured for 14 days (StemProOsteogenicdifferentiationkit), and
stained with either Alizarin red or Osterix (1:250, Abcam, Cat#ab22552)96.
CFU-assay. Colony-forming unit assays were performed as previously described97.
Briefly, testes were collected from C57BL/6 or adult Tcf21mCrem:R26RtdTom males
following 3 injections of tamoxifen (2 mg) every other day. Following single-cell
dissociations, SCA1+/cKITit−, SCA1+/TCF21lin/cKITit−, TCF21lin/cKITit−, or
cKIT+/SCA1− cells were plated into Corning Primaria 6 well plates at a density of
1000 cells/well. Cells were cultured for 14 days in Mesen Cult MSC medium
(StemCell Technologies) and colonies were stained using Giemsa. Colonies were
defined as clumps having either >20 or >50 cells.
In vitro directed differentiation to Leydig and myoid cells. Testes were collected from
adult C57BL/6 or Tcf21mCrem:R26RtdTom males, dissociated into a single-cell sus-
pension, and sorted for SCA1+/cKit− cells. Cells were plated into a 24 well,
Matrigel-coated plate at a density of 100,000 cells/well in DMEM/
F12 supplemented with 10% FBS and 1X Normocin. For myoid cell differentiation,
after 18 h media was replaced with differentiation media- DMEM/
F12 supplemented with 1X Penicillin/streptomycin, 10 ng/ml PDGFAA, 10 ng/ml
PDGFBB, 0.5 µM SAG, 10 ng/ml BMP2, 10 ng/ml BMP4, 10 ng/ml ActivinA, and
1 mM Valproic acid. After the 7-day differentiation protocol, cells were stained for
SMA (1:200, Sigma, Cat#A5228). For Leydig cell differentiation, the FACs sorted
cells were initially recovered for 18 h in DMEM+10%FBS and then the cells were
expanded for 3 days in DMEM/F12 supplemented with 1X Normocin, 10 ng/ml
PDGFAA, 10 ng/ml PDGFBB, 0.5 µM SAG, and 10 ng/ml FGF2. After 3 days, the
expansion media was replaced with differentiation media- DMEM/
F12 supplemented with 1X Penicillin/streptomycin, 10 ng/ml PDGFAA, 10 ng/ml
PDGFBB, 0.5 µM SAG, 10 ng/ml FGF2, 5 mM LiCl2, and 10 µM DAPT. After
10 days in differentiation media, cells were stained for SF1 (1:100, CosmoBio,
Cat#KAL-KO610). Media was collected every other day for testosterone
measurements.
Clonal expansion and directed differentiation of TCF21lin cells. To assess
multipotency of TCF21lin cells, testes were collected from adult Tcf21mCrem:
R26RtdTom males and dissociated into a single-cell suspension. Individual
TCF21lin/cKit−, SCA1+/TCF21lin/cKit−, SCA1+/TCF21lin/cKit−/Cd105+ or
SCA1+/TCF21lin/cKit−/Cd105− cells were sorted into Corning Primaria 96-well
plates and cultured in Mesen Cult mouse MSC media for ~3 weeks to allow for
colony formation. Individual colonies were then directed to differentiate to either a
myoid or Leydig cell fate following the directed differentiation protocols described
above. Cells were then stained for either SMA (for myoid cells, 1:200, Sigma,
Cat#A5228) or SF1 (Leydig cells, 1:100, CosmoBio, Cat#KAL-KO610).
Drop-seq analysis of the Leydig cell differentiation time-course analysis. Drop-seq
was performed on cells collected from various points of differentiation, where a
single sample per timepoint was diluted to 280 cells/µl and processed as described
previously98. Briefly, cells, barcoded microparticle beads (MACOSKO-2011-10,
Lots 113015B and 090316, ChemGenes Corporation), and lysis buffer were co-
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flown into a microfluidic device and captured in nanoliter-sized droplets. After
droplet collection and breakage, the beads were washed, and cDNA synthesis
occurred on the bead using Maxima H-minus RT (ThermoFisher Scientific) and
the Template Switch Oligo. Excess oligos were removed by exonuclease I digestion.
cDNA amplification was done for 15 cycles from pools of 2000 beads using Hot
Start Ready Mix (Kapa Biosystems) and the SMART PCR primer. Individual PCRs
were purified and pooled for library generation. A total of 600 pg of amplified
cDNA was used for a NexteraXT library preparation (Illumina) with the New-P5-
SMARTPCR hybrid oligo, and a modified P7 Nexteraoligo with 10 bp barcodes.
Sequencing was performed on a NovaSeq (Illumina) for read 2 length of 94 nt with
the Read1 Custom Seq primer. Oligosequences are the same as previously
described13,98.
Single-cell RNA-seq analysis across time points. The paired-end Drop-seq data from
days 4, 7, and 14 of in vitro Leydig cell differentiation were sequenced in the same
batch and were processed using Drop-seq tools v1.13 from the McCarroll laboratory
as previously described98,99. Specifically, the reads were aligned to the mouse
reference genome (GRCm38, version 38) using STAR v2.7.1a100. The pipeline
generated digital gene expression matrices with genes as rows and cells as columns
that served as the starting point for downstream analyses.
The cells from each time point were first filtered by cell size and integrity—cells
with <500 detected genes or with >10% of transcripts corresponding to
mitochondria-encoded genes were removed, resulting in 973–2980 pass-QC cells
for the three time points, for a total of 6124 cells. Among the retained cells, the
average number of detected genes per cell was ~1888, and the average number of
UMIs was ~4838. For each cell, we normalized transcript counts by (1) dividing by
the total number of UMIs per cell and (2) multiplying by 10,000 to obtain a
transcripts-per-10K measure, and then log-transformed it by E = ln(transcripts-
per-10K+1).
For each time point, we standardized the expression level of each gene across
cells by using (E-mean(E))/sd(E) and performed PCA using highly variable genes
(HVG). We obtained 4 clusters using Louvain-Jaccard clustering with top PCs by R
package Seurat (v2.3.4). We calculated cluster centroids and ordered the clusters by
minimizing pairwise Euclidean distance of cluster centroids in R package Seriation.
We evaluated batch effect by comparing the top PC placements and rank
correlation of ordered cluster centroids across the 3 time points. Differentially
expressed markers for each cluster were obtained by comparing it against all other
clusters using a nonparametric binomial test, requiring at least 20% higher
detection rate, a minimum of 1.5-fold higher average expression level, and p value
< 0.01. Clusters were defined based on known markers.
We extracted the somatic cells from the INT4 dataset from13. This dataset was
enriched for the Tcf21+ interstitial population and was used as the starting time
point: day 0. We merged the 3 datasets of in vitro Leydig differentiation with the
somatic cells of INT4, for 6619 good-quality cells and 24,698 detected genes. These
cells on average have 1837 detected genes and 4623 UMIs per cell. We selected
2344 HVG genes in the merged dataset and did PCA using HVG. We performed t-
SNE, UMAP and Louvain-Jaccard clustering using top PCs. We obtained 10
clusters initially, and ordered the clusters as described above. Based on
differentially expressed markers for each cluster and rank correlation across the
cluster centroids, we decided to merge 3 clusters (clusters 4–6) and identified them
as ECM myofibroblast. We merged 2 other clusters (clusters 7–8) as differentiating
myofibroblast. This led to the identification of 7 cell types for in vitro Leydig
differentiation—(1) Endothelial, (2) Interstitial progenitor, (3) Proliferating
progenitor, (4) ECM myofibroblasts, (5) Differentiating myofibroblasts, (6)
Differentiating Leydig, and (7) Leydig. We did pseudotemporal ordering of cells
from the 4 time points (days 0, 4, 7, and 14) by Monocle3 and visualized the single-
cell trajectory in UMAP space.
We compared our in vitro Leydig differentiation data with those of fetal mouse
gonads and adult mouse testis somatic cells. Specifically, we calculated the rank
correlation between our 7 cell type centroids of Leydig differentiation data with the
6 cell type centroids from NR5A1-eGFP+ progenitor cells from E10.5 to E16.5 fetal
male mouse gonads33 using markers present in both data (N = 2692). We also
calculated the rank correlation between our 7 cell type centroids with the 7 cell type
centroids from adult mouse testis (Green et al.13) using the union of markers
present in both datasets (N = 1769).
TCF21 lineage tracing
TCF21 lineage tracing analysis and colocalization with immunofluorescence in fetal
and adult testis and ovary. Tcf21mCrem:R26RtdTom or Tcf21mCrem:R26RtdTom; Oct4-
eGFP timed-pregnant females were administrated with a single dose of 1 mg
tamoxifen via gavage at E9.5, E10.5, E11.5, or E12.5. Embryos were obtained at
E11.5, E17.5, or E19.5 via C-section. Tail clippings from the embryos were used to
identify sex and genotype. Embryonic gonads were fixed in 4% paraformaldehyde
(PFA) at 4 °C for 1 h, transferred to 30% sucrose in 1xPBS at 4 °C overnight and
embedded in OCT (Surgipath cryo-gel, Leica #39475237). To analyze the TCF21lin
contribution in adult testis and ovaries, the E19.5 pups obtained by C-section were
fostered to CD1 females. The foster mice were euthanized at 10 weeks. Adult testes
and ovaries were fixed in 4% PFA at 4 °C overnight, transferred to 30% sucrose in
1xPBS at 4 °C for overnight and embedded in OCT.
For immunofluorescence, 7–10 micron thick OCT sections were cut using a
Leica CM3050S cryostat and sections were refixed with 4% PFA for 10 min and
permeabilized by incubation in 0.1% Triton in PBS for 15 min. Sections were
blocked in 1xPBS supplemented with 3% BSA and 500 mM glycine for 1 h at room
temperature and co-stained with FGF5, SOX9, 3β-HSD, CD34, SMA, VASA,
COUPTFII, CD31, SF1, WT1, FOXL2, and DsRed antibodies. The primary
antibodies and concentrations used are summarized in Supplementary Data 5. All
secondary antibodies (Alexa-488-, Alexa-568-, and Alexa-647-conjugated
secondary antibodies; Life Technologies/MolecularProbes) were all used at a 1:1000
dilution. DAPI was used as a nuclear counterstain at 1:1000. Representative images
were taken with a ZeissAX10 epifluorescence microscope, a Leica SP8 confocal
microscope, or a Nikon A1R-HD25 confocal microscope and processed with
ImageJ.
Quantification of immunofluorescence colocalization. Tissue sections were stained
for immunofluorescence as described above and >20 images per testis were imaged
with a ×40 1.2NA objective on a ZeissAX10 epifluorescence microscope, all at a
single z-section. The percent overlap between the TCF21lin population and several
marker proteins was done using accustom written ImageJ macro (available upon
request). Briefly, nuclear regions of interest (ROIs) were created from DAPI
staining by blurring with a Gaussian filter, making the image binary, separating
overlapping nuclei with a watershed function, then saving the outline of each
binary nucleus to ImageJ’s ROI manager. The signal from TCF21lin and the
immunostaining were made binary using ImageJ’s automatic thresholding function
and the overlap of the binary stain and the nuclear ROI was measured using Image
J’s Measurement function. Cells positive for each stain and double-positive cells
were sorted and identified in Microsoft Excel. The quantification from each testis
was the sum of all quantified images taken from a single testis. The macro was
optimized by contrasting its results to manual quantification from at least five
images per immunostaining.
Lineage-tracing analysis of the Tcf21+ population in the aged testis. Five or eight-
week-old Tcf21mCrem:R26RtdTom males were injected with a single dose of 2 mg
tamoxifen intraperitoneally. The testes were collected 1 week (as control) or 1-year
past injection. The testes were fixed in 4% PFA at 4 °C overnight, transferred to
30% sucrose in 1xPBS at 4 °C overnight and embedded in OCT. The sections were
co-stained for SF1 (1:100, CosmoBio, Cat#KAL-KO610). Overlap of SF1 and
tdTomato were counted manually per field; ~25–100 fields per biological replicate
per condition were collected.
In vivo ablation of Leydig cells
Ethane dimethane sulfonate (EDS) injections. EDS (AABlocks, Cat. No: 4672-49-5)
was dissolved in DMSO at a concentration of 150 mg/mL. Working solutions of
EDS were further diluted in PBS to a final concentration of 50 mg/mL. A total of
300 mg/kg of EDS was injected intraperitoneally every other day for 2 days into
C57BL/6 age-matched or Tcf21mCrem:R26RtdTom mice. Testes and sera were col-
lected 12 h, 24 h, 4 days, 7 days, 14 days, or 21 days past final injections. Negative
controls were given vehicle treatment of DMSO: PBS without EDS.
Diphtheria toxin Injections. DTX (Sigma) was diluted in PBS at a concentration of
2 mg/ml. A final concentration of 150 ng DTX was injected intraperitoneally every
day for 3 or 4 days into Myh11cre-egfp;Rosa26DTR/+ male mice. Testes were collected
12 h, and 4 days past final injections. Littermates that are Myh11-Cre-recombinase
negative were given DTX and served as controls.
TUNEL staining. Testes were collected, fixed in 4% PFA for ~16 h at 4 °C, dehy-
drated in ethanol wash series, and embedded in paraffin. Five-micron FFPE tissue
sections were deparaffinized, rehydrated, and permeabilized in 20 μg/ml Proteinase
K solution for 15 min at room temperature. Samples were further processed fol-
lowing the Promega Dead End Colorimetric TUNEL kit according to the manu-
facturer instructions. All images collected used a Leica Leitz DMRD microscope.
Hormone measurements. Testosterone measurements were performed by the
University of Virginia Center for Research in Reproduction Ligand Assay and
Analysis Core.
FFPE immunofluorescence. Whole testes were fixed in 4% PFA overnight at 4 °C
and processed for formalin fixed paraffin embedding as described in (Fisher et al.
2008). Five-micron FFPE tissue sections were deparaffinized by incubation in
Histoclear 3× for 5 min, followed by incubation in 100% EtOH 2× for 5 min, 95%
EtOH 2× for 5 min, 80% EtOH 1× for 5 min, 70% EtOH 1× for 5 min, 50% EtOH
1× for 5 min, 30% EtOH 1× for 5 min, and deionized water 2× for 3 min each.
Tissue sections were permeabilized by incubation in 0.1% Triton in PBS for 15 min.
For all antibodies, antigen retrieval was performed by boiling in 10 mM sodium
citrate, pH 6.0 for 30 min. Sections were blocked in 1xPBS supplemented with 3%
BSA and 500 mM glycine for 3 h at room temperature. Endogenous peroxidases
and alkaline phosphatases were blocked by a 10-min incubation in BloxAll solution
(VectorLabs, Cat. No: SP-6000). The primary antibodies and concentrations used
are listed below. Alexa-488-, Alexa- 555-, and Alexa-647-conjugated secondary
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antibodies (Life Technologies/MolecularProbes) were all used at 1:1000. DAPI was
used as a nuclear counterstain. For quantification, the overlap of SF1 and tdTomato
were counted manually per field; ~25–50 fields per biological replicate per con-
dition were collected.
Cell diameter measurements. For all cell diameter measurements, FFPE slides were
stained as described above and co-stained with SF1 to mark Leydig cells (1:100,
CosmoBio, Cat#KAL-KO610) and 488-wheat germ agglutinin (WGA, Biotium Cat.
No: 29022-1) to mark cell perimeter. A tubule was centered in the field of view at
×40 magnification and an image was taken. At least 70 well-defined Leydig cells,
marked by both SF1 and clear visible cell perimeter by WGA, were counted per
condition. Cell diameter was measured manually using ImageJ’s Line and Measure
functions.
FFPE immunohistochemistry. Whole testes were fixed in 4% PFA overnight at 4 °C,
processed and deparaffinized as described above. The primary antibodies and
concentrations used are listed in Supplementary Data 5. Horseradish peroxidase
and alkaline phosphatase conjugated secondary antibodies (Abcam) were all used
at a 1:100 concentration and left to incubate for 1-h at room temperature. Slides
were rinsed of developing solution under running DI water for 1 min and mounted
in per mount.
Transplantation of TCF21lin cells into the testis of WT EDS treated or Myh11cre-egfp;
Rosa26DTR/+ animals. 6–18 weeks Tcf21mCrem:R26RtdTom male mice were injected
with 1 mg tamoxifen intraperitoneally every other day for a total of three injections.
Testes were dissociated into a single-cell suspension and the SCA1+/cKITit−
stained cells were collected by FACs, as described above. For each animal
~65,000–150,000 cells were diluted in 10 μl of MEM media plus Trypan Blue and
were injected into the interstitium via the rete testes of either EDS-treated C57BL/6
animals 24 hpfi of EDS or into Myh11cre-egfp;Rosa26DTR/+ mice 24 hpfi of DTX. As
a control, 10 μL of MEM media plus Trypan Blue was injected into the con-
tralateral testis of each experimental animal. Testes were collected at 24 h post
transplant (hpt), 4 days post transplant (dpt) and 7 dpt for FPPE processing as
described above.
Reporting summary. Further information on research design is available in the Nature
Research Reporting Summary linked to this article.
Data availability
Single-cell RNA-seq data files are available in “GSE151337”. All other relevant data
supporting the key findings of this study are available within the article and its
Supplementary Information files or from the corresponding author upon reasonable
request. A reporting summary for this article is available as a Supplementary Information
file. Source data are provided with this paper.
Code availability
Codes used in this analysis were deposited onto GitHub: https://doi.org/10.5281/
zenodo.4743036.
Received: 19 May 2020; Accepted: 4 June 2021;
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Acknowledgements
We thank members of the Hammoud, Li and Yamashita Labs for scientific discussions
and manuscript comments. We thank Drs. Barry Zirkin and Haolin Chen for EDS
compound. This research was supported by National Institute of Health (NIH)
grants 1R21HD090371-01A1 (S.S.H., J.Z.L.), 1DP2HD091949-01 (S.S.H.), R01
HD092084 (K.E.O., S.S.H), F30HD097961 (A.N.S), F31HD100124 (G.L.M.), training
grants 5T32HD079342 (A.N.S.), 5T32GM007863 (A.N.S.), NSF 1256260 DGE (L.M.),
Rackham Predoctoral Fellowship (L.M.), T32GM007315 (L.M), T32HD007505 (G.L.M.),
T32GM007315 (G.L.M.), and Michigan Institute for Data Science (MIDAS) grant for
Health Sciences Challenge Award (J.Z.L., S.S.H.), Open Philanthropy Grant 2019-199327
(5384) (S.S.H.).
Author contributions
S.S.H., H.L., and A.N.S. provided overall project design. Y.S., H.L., A.N.S., L.M., G.L.M.,
M.S., M.C., and C.S. performed experiments. Q.M. J.Z.L, and X.Z. analyzed data. S.J.G.
performed flow cytometry experiments and analysis. A.N.S. and S.S.H. wrote the
manuscript with input from H.C., J.R.S., K.E.O, M.T. and J.Z.L. Q.M. and G.L.M.
contributed equally. Comments from all authors were provided.
Additional information
Supplementary information The online version contains supplementary material
available at https://doi.org/10.1038/s41467-021-24130-8.
Correspondence and requests for materials should be addressed to S.S.H.
Peer review information Nature Communications thanks Miles Wilkinson and the other
anonymous reviewer(s) for their contribution to the peer review of this work.
Reprints and permission information is available at http://www.nature.com/reprints
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in
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Competing interests
The authors declare no competing interests.
© The Author(s) 2021
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17
| null |
10.1371_journal.pone.0254310.pdf
|
Data Availability Statement: All data are public
and available on the Brazilian Institute of
Geography and Statistics website (www.ibge.gov.
br).
|
All data are public and available on the Brazilian Institute of Geography and Statistics website ( www.ibge.gov. br ).
|
RESEARCH ARTICLE
Contextual and individual factors associated
with public dental services utilisation in Brazil:
A multilevel analysis
Maria Helena Rodrigues GalvãoID
Giuseppe Roncalli1
1*, Arthur de Almeida MedeirosID
1,2, Angelo
1 Postgraduate Program in Public Health, Federal University of Rio Grande do Norte, Natal, Rio Grande do
Norte, Brazil, 2 Integrated Health Institute, Federal University of Mato Grosso do Sul, Campo Grande, Mato
Grosso do Sul, Brazil
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
* [email protected]
Abstract
Background
OPEN ACCESS
Citation: Galvão MHR, Medeiros AdA, Roncalli AG
(2021) Contextual and individual factors associated
with public dental services utilisation in Brazil: A
multilevel analysis. PLoS ONE 16(7): e0254310.
https://doi.org/10.1371/journal.pone.0254310
Editor: Ratilal Lalloo, University of Queensland,
AUSTRALIA
Received: March 29, 2021
Accepted: June 23, 2021
Published: July 9, 2021
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0254310
Copyright: © 2021 Galvão et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All data are public
and available on the Brazilian Institute of
Geography and Statistics website (www.ibge.gov.
br).
This study verified the association between contextual and individual factors and public den-
tal services utilisation in Brazil.
Methods
The study was conducted based on a cross-sectional population-based household survey
performed in Brazil (National Health Survey– 2019)). Data was collected between August
2019 and March 2020. Total sample included 43,167 individuals aged �15 years who had
at least one dental appointment in the last 12 months before interview. Study outcome was
‘public dental service utilisation’, and Andersen’s behavioral model was adopted for select-
ing independent variables. A multilevel analysis was performed using individual factors as
first level and federation units as second level.
Results
The highest prevalence of public dental service utilisation on an individual level was
observed among unable to read or write people (PR: 3.31; p<0.001), indigenous (PR:
1.40; p<0.001), black or brown (PR: 1.16; p<0.001), with per capita household income
of up to U$124 (PR: 2.40; p<0.001), living in the rural area (PR: 1.28; p<0.001), and
who self-rated oral health as regular (PR: 1.15; p<0.001) or very bad/bad (PR: 1.26;
p<0.001). On the contextual level, highest PR of public dental service utilisation was
observed among those living in federal units with increased oral health coverage in pri-
mary health care.
Conclusions
Public dental service utilisation is associated with individual and contextual factors. These
results can guide decision-making based on evidence from policymakers, demonstrating
PLOS ONE | https://doi.org/10.1371/journal.pone.0254310 July 9, 2021
1 / 14
PLOS ONEFunding: This study was financed in part by the
Coordenac¸ão de Aperfeic¸oamento de Pessoal de
Nı´vel Superior – Brasil (CAPES) – Finance Code
001. The funding consisted of a postgraduate
studies scholarship to MHRG and payment of
publication fees. Furthermore, it did not interfere
with the study’s design and collection, analysis,
and interpretation of data and writing the
manuscript. There was no additional external
funding received for this study.
Competing interests: The authors have declared
that no competing interests exist.
Factors associated with public dental services utilisation in Brazil
the potential for mitigating oral health inequalities and increasing service coverage in a pub-
lic and universal health system.
Introduction
Brazil is a middle-income country with universal healthcare system covering dental assistance
for all citizens. In 2003, the Brazilian oral health care service was transformed by the National
Oral Health Policy implementation, expanding primary care teams and emphasizing the pri-
mary care-based model [1]. The last Brazilian oral health epidemiological survey, in demon-
strated significant oral health needs, especially in adolescents and adults. Mean values of
decayed, missing, and filled teeth (DMFT) index were 4.2 for adolescents, 16.7 for adults, and
27.5 for older adults. However, “decayed teeth” and “missing teeth” components sharply
reduced compared to previous year. In contrast, “filled teeth” component grew in relative
terms, indicating greater access to dental services for dental restorations [2].
Brazil expanded primary care teams in the public oral health sector, increasing population
coverage from 20.5% (2003) to 43.1% (2019). However, this expansion was not regular over
time. In the first period of policy implementation (2003–2011), a significant expansion
occurred in dental teams, from 6,170 (2003) to 23,076 (2011). A reduction occurred between
2015 and 2018, followed by expansion of 28,991 teams in 2019. Such oscillation was due to
political issues [3]. Furthermore, Brazil has a significant number of dentists (337,137 in Febru-
ary 2021) and has shown a considerable increase in the number of undergraduate courses in
Dentistry in recent years [4].
Although the number of dentists and oral health care teams expanded in the SUS, equity
in dental service access was not reached. For example, 21.6 million people have never had a
dental appointment until 2010 [3]. Furthermore, most dental appointments in Brazil are
paid by either out-of-pocket or private dental insurance plans, despite expansion of public
services, favoring inequalities in dental service utilisation [5]. Last year, dentist appoint-
ments were higher among those with more education, income, and private healthcare cov-
erage and living in the country’s wealthiest regions [5]. Other studies in Brazil revealed that
public dental services are more used by black people from low-income families, living in
small towns, with more than four household residents, and having more dental treatment
need [6,7].
Despite advances in oral health policies, literature lacks studies regarding the profile of den-
tal service users, especially assessing the effectiveness of strategies adopted to expand access to
population with great inequalities. Evaluating multiple determinants of dental service utilisa-
tion based on broader theoretical models and national scope is important to understand the
country’s reality. Therefore, understanding the profile of public dental services helps evaluate
public policy performance regarding equity in oral health. Although other studies [8,9] were
conducted with the same topic, this study presents new and recent contextual elements.
Andersen Behavioral Model comprises a conceptual framework for understanding multiple
dimensions of access to medical and health care outcomes and is valid to evaluate health ser-
vice utilization. The model presents individual and contextual determinants for health service
utilisation, evaluating predisposing, enabling, and need factors at each level [10]. Experts com-
monly use Andersen’s behavioral model to explain access to oral health care [11].
Thus, this study aimed to verify contextual and individual factors associated with public
dental service utilisation by Brazilians aged 15 years or older using concepts of the Andersen
behavior model [10].
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Materials and methods
Participants and database
Data were collected from the National Health Survey—2019 (PNS), a population-based house-
hold survey that assessed Brazilian determinants, conditions, and health needs. PNS provides a
representative database about the country and people living in private households, contribut-
ing to elaborate public health policies in Brazil and allowed territorial coverage using the Mas-
ter Sample of Integrated Household Research System (SIPD) [12,13].
A three-stage cluster sampling method was used: census tracts selection from primary sam-
pling units, household selection in each PSU,) and one resident aged 15 or older from each
household, randomly selected based on the list of residents obtained during the interview. A
total of 108,457 households were selected (100,541 were occupied), resulting in a database of
279,382 responses (94,114 home interviews).PNS 2019 data were collected between August
2019 and March 2020 [12,13].
The questionnaire was divided into three sections and conducted by trained interviewers
using a mobile device. Third section of the questionnaire included oral health with self-
reported information about last dental appointment, number of missing teeth, and oral health
assessment. This study sample included people aged 15 or older who were selected to answer
the survey questionnaire. Answers to the following question were considered: ‘When was the
last time you visited a dentist?’. Information about last dental appointment was obtained only
for the selected resident who had the last dental appointment up to three years before the inter-
view [12]. Thus, sample consisted of 43,167 individuals.
Characterization of variables
Dependent variable. The study outcome was ‘public dental service utilisation’. We con-
sidered only affirmative or negative responses to the question ‘Has dental consultation been
conducted in the Brazilian National Health System (SUS, from the Portuguese acronym)?’.
Independent variables. Andersen’s behavioral model [10] (Fig 1) was adopted to select
independent variables (Box 1).
Individual independent variables. Regarding individual predisposing factors, we consid-
ered sex (male or female), age (stratified into age groups), skin color/race (white, black, indige-
nous, or Asian), educational level (unlettered, incomplete elementary school, complete
elementary school, high school, or higher education), and per capita household income (up to
U$ 124, from U$125 to U$248, or U$249 or more). Individual facilitating factors were
Fig 1. Conceptual framework adapted by Andersen’s behavioral model.
https://doi.org/10.1371/journal.pone.0254310.g001
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PLOS ONEBox 1. Description of individual and contextual variables of the study and adaptation strategies for the analysis model. Brazil, 2019
Factors associated with public dental services utilisation in Brazil
Variable
Age
Source
Reference Year
National Health Survey (PNS)
2019
Description
Age, in years, at the time of the interview
Sex
National Health Survey (PNS)
2019
Sex
Skin color/Race
National Health Survey (PNS)
2019
Self-reported skin color
Educational level
National Health Survey (PNS)
2019
Highest educational level reached
Per capita household
income
National Health Survey (PNS)
2019
Per capita household income, converted into dollars, (considering the
average values of December/2019)
Household area
National Health Survey (PNS)
2019
Place of residence
Enrolled in Primary
Health Care
National Health Survey (PNS)
2019
Information regarding household enrolled in a primary care facility.
Type of dental
attendance
National Health Survey (PNS)
2019
Reason for the last dental appointment
Self-rated oral health
National Health Survey (PNS)
2019
Self-rated oral health
Number of lost teeth
National Health Survey (PNS)
2019
PLOS ONE | https://doi.org/10.1371/journal.pone.0254310 July 9, 2021
Original Categorization
(Adapted Categorization)
Age categorized into groups.
15 to 19 years
20 to 39 years
40 to 59 years
60 years or older
Male
Female
White (White)
Black (Black or Brown)
Asian (Asian)
Brown (Black or Brown)
Indigenous (Indigenous)
Unable to read or write (Unable
to read or write)
Incomplete primary school
(Incomplete primary school)
Primary school (Primary school)
Incomplete High School (Primary
school)
High School (High School)
Undergraduate (High School)
Graduation (Higher education)
Continuous variable categorized
in:
U$ 249 and over
U$ 125 to U$ 248
Up to U$ 124
Urban
Rural
Yes
No
Do not know
Cleaning, prevention, or overhaul
(Preventive dental attendance)
Dental pain (Tooth extraction or
dental pain)
Tooth extraction (Tooth
extraction or dental pain)
Dental treatment (Dental
treatment)
Gum problem (Dental treatment)
Mouth wound treatment (Dental
treatment)
Dental implant (Dental
treatment)
Placement/maintenance of braces
on teeth (dental treatment)
Prosthesis or denture placement/
maintenance (Dental treatment)
Other treatments (Dental
treatment)
Very good (Very good or good)
Good (Very good or good)
Moderate (Moderate)
Bad (Bad or very bad)
Very bad (Bad or very bad)
�Continuous variable
(Continued )
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Box 1. (Continued)
Human Development
Index
(HDI)
Average per capita
income
Gini Index
Brazilian agency of the United
Nations Development Program
(UNDP)
2017
Brazilian agency of the United
Nations Development Program
(UNDP)
2017
Brazilian agency of the United
Nations Development Program
(UNDP)
2017
Oral Health Coverage in
Primary Health Care
Primary Care Management
and Information System
(E-Gestor)
2019
https://doi.org/10.1371/journal.pone.0254310.t001
Human Development Index refers to geometric mean of dimensions:
Income, Education, and Longevity, with equal weights.
�Continuous variable
Sum of income of all household members, divided by the number of
residents.
�Continuous variable
It measures degree of inequality in the distribution of individuals
according to per capita household income. Its value ranges from 0,
when there is no inequality (per capita household income of all
individuals has the same value), to 1, when inequality is maximum
(only one individual holds all income). The universe of individuals is
limited to those living in permanent private households.
Number of oral health teams in primary care services, divided by the
population in the same year.
�Continuous variable
�Continuous variable
household area (urban or rural) and enrolled in primary health care teams (yes, no, or do not
know). Type of dental attendance (preventive care, dental treatment, or tooth extraction/den-
tal pain), self-rated oral health (very good/good, regular, or very bad/bad), and number of lost
teeth were considered individual need factors. Individual independent variables were collected
from PNS questionnaire.
Context-independent variables. Predisposing contextual factors were Human Develop-
ment Index (HDI), Gini index, and average per capita income obtained from the Brazilian
branch of the United Nations Development Program, considering the latest information avail-
able. Enabling contextual factor was oral health coverage in primary health care (December
2019 as reference) obtained from the System of Information and Management of Primary
Care (Brazil, Ministry of Health). Contextual variables were collected considered all Brazilian
Federation Units (FU) (26 states) and the Federal District.
Statistical analysis
Individual and contextual variables were stored in two databases and merged using determin-
istic linkage technique [14], considering FU codification as reference variable.
All variables were analyzed concerning missing data and outliers. Skin color/race and aver-
age per capita household income presented 0.01% (n = 5) and 0.04% (n = 16) of missing data,
respectively. According to Hair et al., missing data of less than 10% can be ignored [15].
Population expansion was performed for descriptive analysis since this study has a complex
sample design. Expansion factors (or sample weights) were defined to analyze PNS data con-
sidering complex sampling design and distinct selection probabilities for selected households
and residents. Final weight applied was a product of the inverse of selection probability expres-
sions of each stage of sampling plan, including correction for non-responses and adjustments
to total populations8. Prevalence was calculated for individual and contextual variables. In
addition, univariate Poisson regression analysis with robust variance was performed to esti-
mate prevalence ratio (PR) and 95% confidence interval (95% CI). Variables presenting
p�0.20 were included in multilevel analysis model.
Multilevel modeling was chosen because contextual characteristics have a significant effect
on people [16]. Therefore, individual factors and FU were considered first and second levels,
respectively.
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Multilevel Poisson regression initiated with null model analysis to identify random effects.
Subsequently, modeling was performed with individual and contextual variables. To analyze
interaction between levels, an interaction term was created from the individual variable
‘Record in primary health care teams?’ and the contextual variable ‘oral health coverage in pri-
mary health care.’
Ethical statement
PNS 2019 met all requirements in research involving humans and was approved by the
National Research Ethics Committee (protocol n. 3,529,376). PNS data are public and available
on the Brazilian Institute of Geography and Statistics website (www.ibge.gov.br). Information
regarding contextual variables was collected from a secondary public database.
Results
Descriptive analysis
Regarding the study’s outcome, it was observed that only 23.1% (CI95% 22.3%; 23.9%) of peo-
ple used public oral health services. Regarding individual predisposing factors, most partici-
pants aged between 20 and 39 years (41.2%, 95%CI 40.3–42.1%), were women (56.6%, 95%CI
55.7–57.5%), had high school degree (38.0%, 95CI% 37.1–38.9%), were black or brown (50.9%,
95%CI 49.8–51.9%), and had household income per capita of up to $124 (16.8%, 95%CI 16.2–
17.5%).Average income met the criterion established by the federal government to register in
the national income transfer program for poor people. For individual facilitating factors,
59.0% (95%CI 57.6%; 60.3%) were enrolled in primary care teams and 88.8% (95%CI 88.8%;
89.8) resided in urban areas. Concerning need factors, 75.7% (95%CI 74.9%; 76.5%) rated oral
health as very good or good and 47.3% (95%CI 46.3%; 48.2%) performed preventive care.
Average number of missing teeth was 2.615 ± 0.045 teeth (Table 1).
Regarding predisposing contextual characteristics, mean HDI of FU was 0.777 ± 0.001,
Gini index was 0.523 ± 0.001, and average per capita income was U$372.219 ± 1.128. Average
oral health coverage in primary health was 51.540 ± 0.168 (Table 1).
Univariate analysis
Univariate analysis (Table 2) showed decreased public dental service utilisation according to
age and low prevalence among males (PR: 0.89, 95%CI 0.86–0.93). Educational level and aver-
age per capita household income showed a dose-response effect. Black or brown and indige-
nous were more likely to use public dental services (58% and 121%, respectively) than white
people. Lack of registration by primary health care teams reduced public dental service utilisa-
tion, whereas people living in rural areas were one-fold more likely to use dental services. Den-
tal service utilisation was associated with worse self-rated oral health and tooth extraction or
dental pain.
Regarding contextual factors, public dental service utilisation was more prevalent in FUs
with low HDI, low average income per capita, and high oral health coverage in primary health
(Table 2).
Multilevel analysis
In multilevel modeling, initial null model indicated a contextual effect on prevalence of public
oral health service utilisation. Variance analysis supports this situation since it was different
from zero (0.19—CI95% 0.11; 0.34) and likelihood ratio was significant (LR: 1631.00—
p�0.001) (Table 3).
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Table 1. Descriptive analysis of individual and contextual characteristics with population expansion. Brazil, 2019.
Variables
n
%
95%CI
Public dental service utilisation
Yes
No
Predisposing
Age
15–19 years
20–39 years
40–59 years
60 years or older
Sex
Female
Male
Educational level
Higher education
High School
Primary school
Incomplete primary school
Unable to read or write
Skin color/Race
White
Black or Brown
Asian
Indigenous
Household income per capita
$249 or more
$125 to $248
$124 or less
Enabling
Registered by primary health care teams
Yes
No
Unknown
Household area
Urban
Rural
Perceived need
Self-rated oral health
Very good or good
Moderate
Bad or very bad
Type of dental attendance
Preventive dental attendance
Dental treatment
Tooth extraction or dental pain
Number of lost teeth
Dependent variable
19,264,898
64,103,920
Individual characteristics
8,805,350
34,369,590
28,248,460
11,945,417
47,188,994
36,179,824
18,056,984
31,684,302
14,927,561
16,684,530
2,015,440
39,762,365
42,396,346
854,911
348,118
46,963,490
22,348,415
14,016,143
49,160,708
24,399,200
9,808,909
74,462,063
8,906,755
63,087,330
17,669,925
2,611,562
39,414,057
30,646,905
13,307,855
83,368,818
23.1
76.9
10.6
41.2
33.9
14.3
56.6
43.4
21.7
38.0
17.9
20.0
2.4
47.7
50.9
1.0
0.4
56.4
26.8
16.8
59.0
29.3
11.8
89.3
10.7
75.7
21.2
3.1
47.3
36.8
16.0
(22.3; 23.9)
(76.1; 77.7)
(9.9; 1.3)
(40.3; 42.1)
(33.1; 34.7)
(13.7; 15.0)
(55.7; 57.5)
(42.5; 44.3)
(20.8; 22.6)
(37.1; 38.9)
(17.2; 18.7)
(19.3; 20.7)
(2.2; 2.7)
(46.6; 48.8)
(49.8; 51.9)
(0.8; 1.3)
(0.3; 0.5)
(55,3; 57,4)
(25,9; 27,7)
(16,2; 17,5)
(57,6; 60,3)
(28,1; 30,5)
(11,1; 12,5)
(88.8; 89.8)
(10.2; 11.2)
(74.9; 76.5)
(20.5; 22.0)
(2.9; 3.4)
(46.3; 48.2)
(35.9; 37.7)
(15.3; 16.6)
2.615 ± 0.045 (2.527; 2.703)
(Continued )
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Table 1. (Continued)
Variables
n
%
95%CI
Contextual characteristics
Predisposing
Human Development Index
Gini Index
Average Per Capita Income
Enabling
Oral Health Coverage in PHC
PHC: Primary Health Care. CI: Confidence interval.
https://doi.org/10.1371/journal.pone.0254310.t002
83,368,818
83,368,818
83,368,818
0.777 ± 0.001 (0.776; 0.778)
0.523 ± 0.001 (0.522; 0.523)
372.219 ± 1.128 (370.006; 374.432)
83,368,818
51.540 ± 0.168 (51.209; 51.871)
In model 1, only individual variables were included, which maintained the significance
level, except for the variable ‘number of lost teeth’. Most significant adjustments observed were
concerning age. Inversion of PR for education level and skin color/race was observed, with
approximately 50% decrease in PR compared to univariate analysis.
In model 2, when contextual variables were included, no changes were observed in the PR
of individual variables. Gini index lost significance, while PR for HDI largely increased com-
pared to univariate analysis (RP: 189.65—CI95% 0.86; 41,383.46).
Final model included variables presenting statistical significance. PR of all variables
included in the model did not change. Although contextual factors influenced public dental
service utilisation (LR: 270.02; p<0.001), they did not mitigate individual effects.
All individual variables—except for ‘number of lost teeth’—and the contextual variable
‘oral health coverage in primary health care’ were included in the final model. Highest preva-
lence of public dental service utilisation was observed among unable to read or write people
(PR: 3.31–95%CI 3.01; 3.78 –p<0.001), indigenous (PR: 1.40–95%CI 1.18; 1.67– p<0.001),
black or brown (PR: 1.16–95%CI 1.10; 1.21– p<0.001), with per capita household income up
to U$124 (PR: 2.40–95%CI 2.27; 2.55– p<0.001), living in rural areas (PR: 1.28–95%CI 1.22;
1.33– p<0.001), who self-rated oral health as regular (PR: 1.15—CI95% 1.10; 1.20– p<0.001)
or very bad/bad (PR: 1.26—CI95% 1.17; 1.37– p<0.001), and living in FU with high oral health
coverage in the primary care.
Variance between initial null and final models decreased 15%, demonstrating the effects of
the Brazilian FU context on public dental service utilisation.
Discussion
This study verified the association between individual and contextual factors and public dental
service utilisation in Brazil, considering the Andersen Behaviour Model. Our results showed
that contextual and individual characteristics influence public dental service utilisation. At an
individual level, after adjustment for age and sex, educational level, skin color or race, and
household income demonstrated an effect on predisposition to public dental services utilisa-
tion. Enabling factors were living in households enrolled in primary care teams or located in
rural areas. Need factors associated with public dental service utilisation were poor self-rated
oral health and absence of restorative treatment in the last dental attendance. Regarding con-
textual factors, public dental service utilisation was associated with percentage of FU popula-
tion covered by oral health teams in primary care.
Public dental service utilisation by vulnerable groups was evident, demonstrating potential
of the national public policy to expand dental health care access. The reduced utilisation of
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Table 2. Univariate associations between outcome and the independent variables according to the individual and contextual levels. Brazil, 2019.
Variables
Public dental service utilisation
p-value
PR (95%CI)
No % (95%CI)
Yes % (95%CI)
Individual characteristics
Predisposing
Age
15–19 years
20–39 years
40–59 years
60 years or older
Sex
Female
Male
Educational level
Higher education
High School
Primary school
Incomplete primary school
Unable to read or write
Skin color/Race
White
Black or Brown
Asian
Indigenous
Household income per capita
$249 or more
$125 to $248
$124 or less
Enabling
Registered by primary health care teams
Yes
No
Unknown
Household area
Urban
Rural
Perceived need
Self-rated oral health
Very good or good
Moderate
Bad or very bad
Type of dental attendance
Preventive dental attendance
Dental treatment
Tooth extraction or dental pain
73.9 (70.7; 76.8)
76.2 (75.1; 77.4)
76.9 (75.6; 78.2)
80.9 (79.4; 82.4)
75.3 (74.3; 76.4)
78.9 (77.8; 88.0)
93.8 (92.6; 94.8)
80.9 (79.7; 82.0)
71.6 (69.9; 73.5)
59.3 (57.5; 61.0)
48.4 (43.7; 53.2)
83.2 (82.1; 8436)
70.9 (69.8; 72.1)
86.1 (76.3; 92.2)
59.6 (49.4; 69.0)
89.2 (88.4; 90.0)
68.7 (66.8; 70.4)
48.8 (46.8; 50.8)
69.7 (68.6; 70.8)
88.3 (87.1; 89.4)
84.7 (82.8; 86.3)
79.9 (79.0; 80.7)
51.9 (49.6; 54.1)
80.0 (79.1; 80.9)
68.8 (67.1; 70.4)
56.4 (52.0; 60.7)
78.3 (77.1; 79.5)
83.1 (82.0; 84.2)
58.2 (56.3; 60.1)
26.1 (23.2; 29.3)
23.8 (22.6; 24.9)
23.1 (21.8; 24.4)
19.1 (17.6; 20.6)
24.7 (23.6; 25.7)
21.1 (20.0; 22.2)
6.2 (5.2; 7.4)
19.1 (18.0; 20.3)
28.4 (26.5; 30.4)
40.7 (39.0; 42.5)
51.6 (46.8; 56.3)
16.8 (15.7; 17.9)
29.1 (27.9; 30.2)
13.9 (7.8; 23.7)
40.4 (31.0; 50.6)
10.8 (10.0; 11.6)
31.3 (29.6; 33.2)
51.2 (49.2; 53.2)
30.3 (29.2; 31.4)
11.7 (10.6; 12.9)
15.3 (13.7; 17.2)
20.1 (19.3; 21.0)
48.1 (45.9; 50.4)
20.0 (19.1; 20.9)
31.2 (29.6; 32.9)
43.6 (39.3; 48.0)
21.7 (20.5; 22.9)
16.9 (15.8; 18.0)
41.8 (39.9; 43.7)
Number of lost teeth
2.470 ± 0.051 (2.371; 2.570)
3.097 ± 0.085 (2.930; 3.264)
Contextual characteristics
0.003
<0.001
<0.001
1
0.89 (0.82–0.96)
0.86 (0.79–0.92)
0.77 (0.71–0.84)
1
<0.001
0.89 (0.86–0.93)
<0.001
<0.001
<0.001
<0.001
<0.001
0.150
<0.001
<0.001
<0.001
<0.001
<0.001
1
2.97 (2.73–3.23)
4.63 (4.24–5.05)
6.06 (5.58–6.58)
6.56 (5.91–7.28)
1
1.58 (1.51–1.65)
0.80 (0.0–1.08)
2.21 (1.86–2.63)
1
2.73 (2.59–2.87)
4.23 (4.03–4.45)
1
0.44 (0.42–0.46)
0.54 (0.50–0.58)
1
<0.001
2.06 (1.98–2.15)
<0.001
<0.001
<0.001
<0.001
<0.001
1
1.43 (1.38–1.50)
1.91 (1.77–2.07)
1
0.75 (0.73–0.80)
1.70 (1.63–1.78)
1.01 (1.01–1.02)
Predisposing
Human Development Index
0.781 ± 0.001 (0.780; 0.782)
0.763 ± 0.001 (0.761; 0.765)
<0.001
0.01 (0.01; 0.01)
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(Continued )
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PLOS ONEFactors associated with public dental services utilisation in Brazil
Table 2. (Continued)
Variables
Public dental service utilisation
p-value
PR (95%CI)
Gini Index
Average Income Per Capita
Enabling
Oral Health Coverage in PHC
No % (95%CI)
Yes % (95%CI)
0.522 ± 0.001 (0.521; 0.522)
383.026 ± 1.208 (380.657; 385.395)
0.527 ± 0.001 (0.525; 0.528)
336.259 ± 2.588 (331.186; 341.333)
0.085
<0.001
24.85 (0.64; 961.75)
0.99 (0.99; 0.99)
49.920 ± 0.178 (49.570; 50.270)
56.930 ± 0.391 (56.164; 57.697)
<0.001
1.01 (1.01; 1.02)
PHC: Primary Health Care. CI: Confidence interval. PR: prevalence ratio.
https://doi.org/10.1371/journal.pone.0254310.t003
dental service care is associated with males [9,17–20], black and brown skin color/race
[9,20,21], indigenous people [9,20], low educational level [12,20,22], low-income [9,17,19,20],
lack of health insurance [9,17,20], poor perception of oral health [9,18,23], and living in rural
areas [1,18].
The Brazilian population also demonstrated a social gradient in public dental service utilisa-
tion. Considering dental appointments within last 12 months, the lower the income and edu-
cational level, the higher the number of dental consultations in the SUS. These results
demonstrate that universal public dental service coverage can be a strategy for tackling
inequalities in dental care utilisation. Despite this, inequalities in dental service utilisation per-
sist in Brazil after the National Oral Health Policy implementation [8,24] and may be related
to greater private dental service utilisation. Although public dental care supply increased, the
private sector performed the highest proportion (77.4%) of dental care appoitments in the last
12 months.
The Brazilian scenario of public oral health differs from other countries. Dental care is part
of a universal healthcare system, free of charge at the moment of use and financed by the fed-
eral government with resources from taxes. Oral health teams are included in primary health
care and offer preventive and restorative treatments [1], explaining the potential individual
factor of enrolling in primary care teams to enable public dental services utilisation. This
enrollment enables families to access (e.g., promotion, prevention) and several aspects of fam-
ily and community care. Moreover, oral health teams proved useful as facilitators for access to
Brazilian public dental services, even after adjusting for other variables. This corroborates with
another study, which observed that individuals registered in the Brazilian Family Health Strat-
egy were more likely to use dental services than those unregistered, reducing private insurance
use [25].
Reorientation of oral health care, emphasizing the care model based in primary care, was
the main goal of the National Oral Health Policy. Primary health care has an essential role in
assuming responsibility for detecting needs, providing necessary referrals, monitoring evolu-
tion of rehabilitation, and maintaining rehabilitation in the post-treatment period. Thus, it is
essential to expand the offer of primary care services in oral health. For this purpose, the gov-
ernment is committed to expand and qualify primary care through the family health strategy
[1]. Our results suggest that dental service coverage in primary care increases public dental ser-
vice access. The World Health Organization recommends incorporating primary dental ser-
vices into primary health care initiatives to use pre-existing medical infrastructure and reduce
oral disease burden [26].
In addition to the influence of individual level, characteristics of FUs might affect public
dental service utilisation, whereas socioeconomic factors did not contribute to predisposing
dental public service utilisation. However, oral health coverage in primary care proved to be a
contextual characteristic enabling public dental service access.
PLOS ONE | https://doi.org/10.1371/journal.pone.0254310 July 9, 2021
10 / 14
PLOS ONEFactors associated with public dental services utilisation in Brazil
Table 3. Poisson multilevel regression analysis for public dental services utilisation according to individual and contextual levels. Brazil, 2019.
Variables
Null Model (n = 41,596) Model 1 (n = 41,575) p-value Model 2 (n = 41,575)
PR (95%CI)
PR (95%CI)
Individual characteristics
p-value Final Model (n = 41,575) p-value
PR (95%CI)
Predisposing
Age
15–19 years
20–39 years
40–59 years
60 years or older
Sex
Female
Male
Educational level
Higher education
High School
Primary school
Incomplete primary school
Unable to read or write
Skin color/Race
White
Black or Brown
Asian
Indigenous
Household income per capita
$249 or more
$125 to $248
$124 or less
Enabling
Are you registered by primary health care teams?
Yes
No
Unknown
Household area
Urban
Rural
Perceived need
Self-rated oral health
Very good or good
Moderate
Bad or very bad
Type of dental attendance
Preventive dental attendance
Dental treatment
Tooth extraction or dental pain
Number of lost teeth
Predisposing
Human Development Index
1
1.08 (1.00; 1.17)
1.06 (0.98; 1.15)
1.04 (0.94; 1.14)
0.032
0.116
0.385
1
1.08 (1.00; 1.17)
1.06 (0.98; 1.15)
1.04 (0.94; 1.14)
0.033
0.118
0.387
1
1.08 (1.00; 1.17)
1.06 (0.95; 1.15)
1.04 (0.94; 1.14)
0.034
0.119
0.388
1
1
1
0.89 (0.86; 0.93)
<0.001
0.89 (0.86; 0.93)
<0.001
0.89 (0.86; 0.93)
<0.001
1
2.06 (2.89; 2.25)
2.67 (2.43; 2.93)
3.19 (2.91; 3.49)
3.38 (3.02; 3.78)
<0.001
<0.001
<0.001
<0.001
1
2.06 (1.89; 2.25)
2.67 (2.43; 2.93)
3.19 (2.92; 3.49)
3.38 (3.02; 3.78)
1
1
1.16 (1.11; 1.21)
<0.001
1.16 (1.10; 1.21)
0.82 (0.61; 1.10)
0.192
0.82 (0.61; 1.10)
1.40 (1.18; 1.67)
<0.001
1.40 (1.18; 1.67)
<0.001
<0.001
<0.001
<0.001
<0.001
0.188
<0.001
1
2.06 (1.89; 2.25)
2.67 (2.43; 2.93)
3.19 (2.91; 3.49)
3.37 (3.01; 3.78)
1
1.16 (1.10; 1.21)
0.82 (0.61; 1.10)
1.40 (1.18; 1.67)
<0.001
<0.001
<0.001
<0.001
<0.001
0.191
<0.001
1
1
1
1.85 (1.75; 1.95)
2.41 (2.27; 2.55)
<0.001
<0.001
1.84 (1.75; 1.95)
2.40 (2.27; 2.55)
<0.001
<0.001
1.84 (1.75; 1.95)
2.40 (2.27; 2.55)
<0.001
<0.001
1
1
1
0.64 (0.61; 0.68)
0.72 (0.67; 0.78)
<0.001
<0.001
0.64 (0.61; 0.68)
0.73 (0.68; 0.78)
<0.001
<0.001
0.64 (0.61 (0.68)
0.73 (0.68; 0.78)
<0.001
<0.001
1
1
1
1.28 (1.22; 1.34)
<0.001
1.28 (1.22; 1.33)
<0.001
1.28 (1.22; 1.33)
<0.001
1
1
1.15 (1.10; 1.20)
1.26 (1.17; 1.37)
<0.001
<0.001
1.15 (1.10; 1.20)
1.26 (1.17; 1.37)
<0.001
<0.001
1
1.15 (1.10; 1.20)
1.26 (1.17; 1.37)
<0.001
<0.001
1
1
1
0.64 (0.61; 0.67)
<0.001
0.64 (0.61; 0.67)
1.04 (0.99; 1.09)
0.99 (0.99; 1.00)
0.062
0.929
1.04 (0.99; 1.09)
-
<0.001
0.066
-
Contextual characteristics
189.65 (0.86; 41,383.46)
0.056
0.64 (0.61; 0.67)
1.04 (0.99; 1.09)
<0.001
0.065
-
-
-
-
(Continued )
11 / 14
PLOS ONE | https://doi.org/10.1371/journal.pone.0254310 July 9, 2021
PLOS ONEFactors associated with public dental services utilisation in Brazil
Gini Index
Average per capita income
Enabling
Oral Health Coverage in PHC
Fixed Effects
Intercept (95%CI)
Random Effects
Variance (95%CI)
LR test (Chi2, p-value)
Table 3. (Continued)
Variables
Null Model (n = 41,596) Model 1 (n = 41,575) p-value Model 2 (n = 41,575)
PR (95%CI)
PR (95%CI)
1.19 (0.17; 8.33)
0.99 (0.99; 1.00)
p-value Final Model (n = 41,575) p-value
PR (95%CI)
0.854
0.044
-
-
-
-
1.00 (1.00; 1.01)
0.004
1.00 (1.00; 1.01)
<0.001
-1.36 (-1.52; -1.19)
0.07 (0.06; 0.08)
0.01 (0.01; 0.07)
0.04 (0.03; 0.05)
0.19 (0.11; 0.34)
1631.00 (<0.001)
0.06 (0.03; 0.12)
369.79 (<0.001)
0.03 (0.02; 0.07)
238.63 (<0.001)
0.04 (0.02; 0.08)
270.02 (<0.001)
Model 1: Individual variables; Model 2: Individual variables, maintaining significance level in model 1 and contextual variables; Final model: Individual and contextual
variables, maintaining significance level. PHC: Primary Health Care; LR: Likelihood Ratio.
https://doi.org/10.1371/journal.pone.0254310.t004
Although oral health policies were one of Brazilian government priorities in 2003, current
national agenda [3] neglected oral health care (i.e., low political priority) and excluded oral
health teams from primary care services since 2017. The limited government budget for oral
health care suggests that dental care is unnecessary and should not be provided by the SUS,
unlike other medical services [26]. As shown in this study, reduced policy expansion in Brazil
may threaten equity in dental service utilisation since public services may mitigate inequalities.
Also, public services, part of a universal system, effectively reduced inequalities in dental ser-
vice utilisation, offering an alternative to adopt private dental insurance.
The present study has some strengths and limitations. We used data from a population-
based survey performed with people living in private households. Interviewers were trained in
two stages, and data was collected using digital mobile devices. Urban and rural areas were
estimated for major national regions, FU, capitals, and metropolitan regions [12]. Nonetheless,
this study presents classic limitations of studies with a cross-sectional design. Data were subject
to information and memory bias since the primary outcome was self-reported. However, bias
is expected to be random and small due to sample size.
Despite limitations, this study provides a valuable analysis regarding the profile of dental
service users in Brazil and demonstrates that individual and contextual factors are associated
with public dental service utilisation. At the individual level, sex, educational level, skin color/
race, and household income are predisposing factors for public dental service utilisation,
whereas enrolling in primary care teams and living in rural areas were enabling factors. At the
contextual level, a high percentage of the population covered by oral health in primary care
was an enabling factor for public dental service utilisation.
According to Andersen and Newman [27], the intervention variable must be mutable to
promote equity of access. Changes in health policies may change health service utilisation.
Demographic and social structure variables associated with dental service utilisation have a
low potential for mutability. Alternatively, enabling variables, such as expanding public service
coverage and enrolling families in primary care, have a high potential for mutability through
government actions. Thus, our study revealed that government action is fundamental for
reducing inequalities, observing a mitigating effect of public policies on inequalities associated
with dental services utilisation. This result may guide evidence-based decision-making for pol-
icymakers. Nevertheless, expansion of government actions are needed because coverage is still
low and inequalities are persistent.
PLOS ONE | https://doi.org/10.1371/journal.pone.0254310 July 9, 2021
12 / 14
PLOS ONEFactors associated with public dental services utilisation in Brazil
Acknowledgments
The authors thank Probatus Academic Services for providing scientific language revision and
editing.
Author Contributions
Conceptualization: Maria Helena Rodrigues Galvão, Arthur de Almeida Medeiros, Angelo
Giuseppe Roncalli.
Formal analysis: Maria Helena Rodrigues Galvão, Arthur de Almeida Medeiros.
Methodology: Maria Helena Rodrigues Galvão, Arthur de Almeida Medeiros.
Supervision: Angelo Giuseppe Roncalli.
Writing – original draft: Maria Helena Rodrigues Galvão, Arthur de Almeida Medeiros.
Writing – review & editing: Maria Helena Rodrigues Galvão, Arthur de Almeida Medeiros,
Angelo Giuseppe Roncalli.
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PLOS ONE
| null |
10.21468_scipostphys.14.3.029.pdf
| null | null |
SciPost Phys. 14, 029 (2023)
Hydrodynamics of higher-rank gauge theories
Marvin Qi⋆, Oliver Hart, Aaron J. Friedman, Rahul Nandkishore and Andrew Lucas†
Department of Physics and Center for Theory of Quantum Matter,
University of Colorado, Boulder CO 80309, USA
⋆ [email protected] , † [email protected]
Abstract
We extend recent work on hydrodynamics with global multipolar symmetries — known
as “fracton hydrodynamics” — to systems in which the multipolar symmetries are gauged.
We refer to the latter as “fracton magnetohydrodynamics”, in analogy to conventional
magnetohydrodynamics (MHD), which governs systems with gauged charge conservation.
We show that fracton MHD arises naturally from higher-rank Maxwell’s equations and
in systems with one-form symmetries obeying certain constraints; while we focus on
“minimal” higher-rank generalizations of MHD that realize diffusion, our methods may
also be used to identify other, more exotic hydrodynamic theories (e.g., with magnetic
subdiffusion). In contrast to semi-microscopic derivations of MHD, our approach eluci-
dates the origin of the hydrodynamic modes by identifying the corresponding higher-form
symmetries. Being rooted in symmetries, the hydrodynamic modes may persist even
when the semi-microscopic equations no longer provide an accurate description of the
system.
Copyright M. Qi et al.
This work is licensed under the Creative Commons
Attribution 4.0 International License.
Published by the SciPost Foundation.
Received 16-05-2022
Accepted 07-10-2022
Published 08-03-2023
doi:10.21468/SciPostPhys.14.3.029
Check for
updates
Contents
1 Introduction
2 Electrodynamics to magnetohydrodynamics
2.1 Rank-one Maxwell’s equations
2.2 Hydrodynamic interpretation
2.2.1 Matter-free limit: The photon
2.2.2 The Ohmic regime: Magnetic diffusion
2.2.3 Conservation of fluxes through surfaces
2.2.4 Absence of diffusion of the conserved electric charge
2.2.5 Magnetic charges and regime of validity
2.3 One-form symmetries
3 Tensor electrodynamics and magnetohydrodynamics
3.1 Rank-two Maxwell’s equations
3.2 Hydrodynamic interpretation
3.2.1 Matter-free limit: The photon
1
2
5
5
6
6
7
9
9
10
10
13
13
14
14
SciPost Phys. 14, 029 (2023)
3.2.2 The Ohmic regime: Magnetic diffusion
3.2.3 Magnetic charge and regime of validity
3.3 One-form symmetries
4 Standard higher-rank generalizations of electromagnetism
4.1 Rank-n Maxwell’s equations
4.2 Microscopic Hamiltonians
4.3 Hydrodynamic interpretation
4.3.1 Matter free limit: The photon
4.3.2 The Ohmic regime: Magnetic diffusion
4.4 One-form symmetries
4.5 Conditions leading to particular higher-rank theories
5 Magnetic subdiffusion
5.1 Maxwell’s equations with vector charge
5.2 Hydrodynamic interpretation
5.3 One-form symmetries
6 Conclusion
A Charge and current belonging to different irreps
A.1 Two-form symmetry: Irrep 1
A.2 Scale- and rotation-invariant hydrodynamics: Irrep 5
A.3 Mixing and matching
B From constraints to the rank-n continuity equation
References
14
15
16
21
22
22
23
24
24
25
26
27
27
28
29
31
32
32
33
34
34
36
1 Introduction
Recent years have seen an explosion of interest in the dynamics of classical and quantum
many-body systems with kinetic constraints [1]. While sufficiently severe constraints and local
dynamics [2, 3] may realize strong Hilbert space fragmentation [2–13], preventing the system
from relaxing, one generally expects that more mild constraints merely delay thermalization
due to anomalously slow dynamics [13]. In certain cases [13, 14], the universal properties
of these theories can be characterized within the framework of hydrodynamics, which is the
coarse-grained effective theory of the long-time and long-wavelength dynamics of systems as
they relax to equilibrium. As an example, consider interacting charged particles on a lattice,
where the Hamiltonian (or quantum circuit, e.g.) that generates the dynamics conserves both
total charge and its dipole moment [15]. The dynamics of thermalization in such a theory
is described by a fourth-order, subdiffusive equation [14, 16]; the resulting hydrodynamic
universality class characterizing the generic features of this and related constrained models
hosts so-called “fracton hydrodynamics”, [14, 17–24], as it describes the thermalization of
fracton systems [25–35] (systems whose elementary excitations can only move in tandem) as
they relax to global equilibrium.
Previous studies of hydrodynamics in fractonic systems explicitly treat the associated
multipolar symmetries as global [14]. However, to characterize actual fracton phases, one
2
SciPost Phys. 14, 029 (2023)
should instead consider gauged multipolar symmetries [31–34], which are relevant to proposed
realizations of fractons, e.g., in the quantum theory of elasticity [36] and in quantum spin
models [37–39]. The latter theories may be regarded as generalized quantum spin liquids
that realize an emergent compact quantum electrodynamics (QED); there, the underlying local
spin model gives rise to emergent electric and magnetic charges, along with gauge fields that
obey compact versions of Maxwell’s equations [40]. Importantly, the emergent gauge fields
in question are typically higher-rank [32], with basic experimental implications that have
recently been considered in the literature [41–44]. Note that in any laboratory realization,
there will inevitably be dissipative effects that spoil the effective higher-rank electromagnetism,
along with nonlinearities in the higher-rank Maxwell’s equations. To make clear predictions
for experiment, it is thus desirable to consider a formalism that does not treat the microscopic
degrees of freedom directly, but instead describes the collective, long-lived degrees of freedom in
the system. In generic interacting systems, these long-lived modes are associated with conserved
densities (or Goldstone bosons), and their dynamics is dubbed “hydrodynamics”1 [45–47]. In
this work, we develop a hydrodynamic theory of systems with exotic conservation laws and
constraints, which give rise to higher-rank variants of electromagnetism.
Somewhat surprisingly, a first-principles derivation of magnetohydrodynamics using one-
form symmetries was not done until the past decade [48–53], and so we begin with a re-
view thereof in Sec. 2. The subtlety lies in the fact that the corresponding hydrodynamic
theory—magnetohydrodynamics (MHD)—is controlled by an unusual type of symmetry, known
as a one-form symmetry. The typical symmetries relevant to conventional hydrodynamics are
associated with the constancy in time of the integral over all space of a finite, local density. In
d spatial dimensions, an n-form symmetry corresponds to the integral of a local density over a
manifold with codimension n: When d = 3, one-form symmetries correspond to integrals of lo-
cal densities over two-dimensional surfaces, while two-form symmetries correspond to integrals
over one-dimensional curves. The one-form conserved charge in Maxwellian electromagnetism
is simply the magnetic flux through arbitrary closed and semi-infinite two-dimensional surfaces.
Still, a precise mathematical framework to interpret the hydrodynamics of such conserved
charges was only recently developed [54]. In the simplest limit—which describes conventional
metals—the only slow (i.e., long-lived) degree of freedom is the magnetic flux density, which
diffuses perpendicular to the field direction [55], as depicted in Fig. 1.
This approach, based on higher-form symmetries, has significant conceptual advantages
over more familiar semi-microscopic derivations of MHD. Specifically, the symmetry-based
approach highlights the underlying symmetries responsible for the observed long-wavelength
modes, while also being less limited in its regime of validity than the semi-microscopic approach.
For example, in conventional (rank-one) MHD, the semi-microscopic derivation invokes ap-
proximate separability of the electromagnetic and matter stress tensors. In the symmetry-based
approach [48], one invokes hydrodynamic principles to recover a coarse-grained theory of the
long-time and long-wavelength dynamics of the fields in the most interesting and physically
relevant regimes, where there may not be a clean separation between the two tensors. This
approach also gives predictions for particular limits of conventional U(1) spin-liquids in which
the relevant symmetries are weakly broken. In the case of emergent electromagnetism in
fractonic spin liquids, the emergent gauge fields are higher rank, leading to additional subtleties
and new universality classes. The hydrodynamic description of these higher-rank theories is
the subject of this work.
We investigate the simplest example of “fracton magnetohydrodynamics”, which arises in
rank-two electromagnetism, in Sec. 3. We show that, in the most interesting regime, where
1Note that our use of the term “hydrodynamics”—the coarse-grained description of systems as they relax
to equilibrium—does not require that momentum be conserved, and need not correspond to the Navier-Stokes
equations, for example.
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Figure 1: Schematic illustration of diffusion of magnetic field lines. The field lines’
dynamics must preserve the flux of the magnetic field B, (cid:82)
B · dS, through any choice
of surface, S. On the left, there exists a high “concentration” of field lines at the
center; on the right, dynamics over time interval δt smooths out the local excess of
magnetic flux while preserving the total magnetic flux through the surface.
S
electric charges proliferate while magnetic charges do not,2 the higher-rank magnetic field
obeys a diffusion equation. This result follows from including electrically charged matter via
Ohm’s law in the higher-rank generalization of Maxwell’s equations in Ref. [32].
More interestingly, we interpret this result independently of the semi-microscopic approach.
We show that higher-rank MHD naturally arises as a consequence of the theory’s one-form
symmetry when the conserved density corresponding to that one-form obeys certain global
constraints. In Sec. 4 we straightforwardly generalize the construction to higher-rank theories
starting either from higher-rank generalizations of Maxwell’s equations, or a one-form conserved
density combined with certain constraints. We show that at every rank, there exists a self-dual
generalization of Maxwell’s equations whose universal behavior is described by diffusion of
magnetic flux lines. For such theories involving traceless symmetric rank-n tensors, every
additional rank introduces two additional diffusing modes, and the diffusion constants at rank
n are given by D m2/n2, with m ∈ {1, . . . , n} and D = τ c2 is the diffusion constant for the
rank-one theory, with the relaxation time, τ, a phenomenological parameter. Additionally,
all of these theories share the same one-form symmetry; we also show how this one-form
conserved density and a set of constraints thereupon uniquely determine the rank and form of
the generalized Maxwell’s equations and the quasinormal mode structure of the corresponding
MHD.
In Sec. 5 we discuss a more exotic scenario, in which the densities of electric and magnetic
matter in Maxwell’s equations themselves carry a vector index. We show how this “vector
charge theory” [32], gives rise to subdiffusive MHD, and elucidate the combination of one-form
symmetries and constraints that lead to subdiffusion, rather than diffusion. Thus, the additional
constraints that lead to the fracton magnetohydrodynamics landscape can realize either diffusion
or subdiffusion, depending on details of the particular model under consideration, in contrast
to systems without multipolar symmetries, which only admit diffusive MHD.
We assume throughout that the systems of interest exist in three spatial dimensions ((cid:82)3) and
enjoy SO(3) rotational invariance. The irreducible representations (irreps) of SO(3) correspond
to integer spins ℓ = 0, 1, 2, . . . , which we denote according to their dimension: 1, 3, 5, . . . . It is
also straightforward to extend our formalism to the reduced point groups relevant to condensed
matter realizations, but we relegate such studies to future work.
2The systems that we consider generally exhibit electromagnetic duality between electric and magnetic fields
and matter. Consequently, analogous results obtain when magnetic charges dominate, instead leading to “electrohy-
drodynamics”, i.e., diffusion of electric field lines.
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2 Electrodynamics to magnetohydrodynamics
Before considering systems with multipolar symmetries, we first review the modern hydrody-
namic interpretation of standard (rank-one) electromagnetism in terms of “magnetohydrody-
namics” — namely, the diffusion of magnetic flux lines in conducting metals (or plasmas) with
mobile, electrically charged particles. Starting from Maxwell’s equations, we discuss several
physical regimes and the corresponding behavior of the electromagnetic fields, and derive
magnetic diffusion generically in the presence of electrically charged matter. We interpret these
results in the language of hydrodynamics, and argue from more abstract perspectives, following
Ref. [48], how a one-form symmetry must arise whenever the charge and current lie in the
same irreducible representation of the SO(3) symmetry. The same approach will be applied to
theories that conserve higher multipole moments of charge density in subsequent sections.
2.1 Rank-one Maxwell’s equations
and corresponding current J (e)
Standard, rank-one electromagnetism is a field theory describing the behavior of the electric
and magnetic fields, E and B, in the presence of electrically charged matter with charge density
ρ(e)
. The dynamics of the E and B fields are governed entirely
by Maxwell’s equations (1). Our discussion applies equally to Maxwell’s equations in the
vacuum as to emergent electromagnetism; while magnetic monopoles do not occur ‘naturally’,
they are to be expected in emergent electrodynamics (and its higher-rank analogues discussed
in later sections). Thus, for generality, we allow for a nonzero magnetic charge density, ρ(m)
,
and corresponding current, J (m)
, in the discussion to follow. In a condensed matter setting, the
equations presented in the following section describe, e.g., U(1) spin liquids with gapless gauge
modes and gapped matter [56] (see also Refs. [57–60]), realized perhaps most prominently in
quantum spin ice [56, 61, 62].
In standard Cartesian coordinates, Maxwell’s equations for the electric and magnetic fields
are given by
∂
i Ei
∂
∂
i Bi
t Bi
ρ(e)
= 1
ϵ
= µ ρ(m)
,
,
= −ε
∂
j Ek
i jk
∂
t Ei
= 1
µ ϵ
ε
∂
i jk
j Bk
− µJ
− 1
ϵ
(m)
i
,
(e)
i
J
,
(1a)
(1b)
(1c)
(1d)
(e)
i
and ρ(m)
(m)
i
where ρ(e)
are the electric and magnetic monopole charge densities, respectively,
the ith components of the corresponding currents, and µ ϵ c2 = 1 defines
with J
the speed of light, c, in terms of the dielectric permittivity, ϵ, and the permeability, µ, which
characterize the system’s linear response to electric and magnetic fields, respectively.
and J
Additionally, because electric (magnetic) charge is locally conserved, charge density and its
associated current are related by a continuity equation,
ρ(e) + ∂
∂
t
(e)
i
i J
= 0 ,
(2)
which follows from taking the divergence of Ampère’s law (1d); the magnetic charge continuity
equation takes the same form, and follows from taking the divergence of Faraday’s law (1c).
Note that magnetic monopoles are not present in standard Maxwellian electrodynamics;
thus, in the context of nonemergent electromagnetic systems, such as conventional metals
(m)
or plasmas in space, one should take ρ(m) = J
= 0. However, in the context of emergent
i
electromagnetism, one generally expects to find both electric and magnetic charges in generic
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temperature and parameter regimes. As we will see shortly, for the purposes of realizing
interesting hydrodynamics in such materials, it is crucial that a separation of scales exists
between the two types of matter so that one of the two species (electric or magnetic) is
sufficiently suppressed in density with respect to the complementary species [63].
2.2 Hydrodynamic interpretation
An important observation is that Maxwell’s equations (1) can be regarded as hydrodynamic
equations of motion for the electromagnetic fields. In the absence of magnetic matter, Faraday’s
law (1c) can be viewed as a hydrodynamic equation of motion for the B field:
∂
t Bi
+ ∂
j
(cid:128)ε
i jk Ek
(cid:138) = 0 ,
(3)
where the second, parenthetical term on the left-hand side plays the role of the hydrodynamic
current conjugate to the vector-valued conserved density, B.
In fact, (3) can be recast in the form of a continuity equation (e.g., (22) in Sec. 2.3) by
identifying Bi as a conserved density, and ε
i jk Ek as the corresponding current. Then (3) takes
the standard hydrodynamic form ∂
i, corresponding
j Ji j
to Bi, with the associated current given by Ji j
i jk Ek. Since the conserved density is a
[pseudo]vector, the current is rank two: Ji j can be interpreted as the current of Bi in the
jth direction. Effectively, Faraday’s law (1c) gives an explicit form for the current, obviating
the need for a standard constitutive relation in which the currents are expressed in terms of
derivative expansions of the conserved densities (in this case, the E and B fields).
= 0 for a vector charge density, ρ
= ε
+ ∂
ρ
t
i
A similar procedure can be applied to the E field: Rearranging Ampère’s law (1d) leads to
∂
t Ei
− c2 ε
∂
j Bk
i jk
= − 1
ϵ
(e)
i
J
,
(4)
which resembles the magnetic analogue (3) but with a [possibly] nonzero source term on the
right-hand side. If the source is removed (J (e) → 0), then the electric field is, like the magnetic
= 0 , with
field, a true conserved density, obeying the standard continuity equation ∂
ρ
i jkBk , mirroring (3) up to the factor −c2 in defining the conjugate
current. When J (e) ̸= 0, the electric field is no longer conserved.3
→ Ei and Ji j
→ −c2ε
j Ji j
+ ∂
ρ
t
i
i
In the presence of magnetic charge, Maxwell’s equations become fully self dual, and the
magnetic field is no longer a conserved density, instead decaying on a time scale set by the
conductivity for magnetic monopoles, in accordance with the magnetic analogue of (14). In
what follows, unless otherwise stated, we will consider Maxwell’s equations (1) without mag-
netic charge, where the equations are no longer self dual under E ↔ B (and, correspondingly,
e ↔ m), but the magnetic flux density is exactly conserved.
2.2.1 Matter-free limit: The photon
We first consider the matter-free sector, where ρ(e) = J
= 0 (and likewise for magnetic
charges). This will serve as a useful point of comparison for the results obtained later on in the
context of higher-rank gauge theories. In the absence of charged matter, both the electric and
magnetic fields obey a continuity equation of the form ∂
= 0 (22), where the current
t
= −c2ε
i jkBk , and the current conjugate to the
corresponding to the conserved density Ei is Ji j
j Ji j
+ ∂
ρ
i
(e)
i
3More precisely, if we apply a Helmholtz decomposition to the electric field, the irrotational (curl-free) component
decays on a time scale τ−1 = σ/ϵ, from the argument presented in Eq. (14). On the other hand, the solenoidal
(divergence-free) component is inextricably tied to the exactly conserved B field (absent magnetic monopoles).
From (1d), at times t ≫ τ, we have a purely solenoidal electric field, E = τc2∇ × B, which is locked to the dynamics
of B, and, hence, diffuses. The “overlap” of E with the diffusing B field vanishes as k → 0, however.
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conserved density Bi is Ji j
Ampère’s law (1d) (and vice versa), e.g., gives the equations of motion
i jk Ek. Taking the curl of Faraday’s law (1c) and inserting into
= ε
∂ 2
t Ei
= c2 ∂ 2Ei ,
∂ 2
t Bi
= c2 ∂ 2Bi ,
(5)
where ∂ 2 is the Laplacian; the expressions above correspond to wave equations for both the
electric and magnetic fields, which propagate ballistically at speed c. Since Faraday’s (1c)
and Ampère’s (1d) laws relate E and B, the above equations are not independent. Taking
(and similarly for the B field), the system’s normal modes are identified
E j
as
(k, ω) ei(kz z−ωt)
∼ E j
ω = c|k| ,
for Ex, y , with ωB = k × E .
(6)
Because the wave vector, k, is taken to be oriented in the ˆz direction, (6) corresponds to
wavelike propagation of the transverse components of the E and B fields along the ˆz direction;
the two transverse normal modes (i.e., those perpendicular to k∥ˆz) correspond to the two
polarizations of the photon.
A longitudinal photon polarization is forbidden by the matter-free Gauss law constraint
= 0, forcing the longitudinal component of
(1a), whose Fourier transform is given by kz Ez
the electric field to vanish. We note that the same holds for the B field, even in the presence
of electrically charged matter. The absence of a propagating longitudinal mode can also be
justified by identifying a “hidden” conserved quantity, as we discuss in Sec. 2.2.3.
2.2.2 The Ohmic regime: Magnetic diffusion
We now consider the hydrodynamic description of the E and B fields in the case most relevant
to experiments in electronic materials: the Ohmic regime. There, electrically charged matter
obeys Ohm’s law, J (e) = σE, where σ is the Drude conductivity; this limit describes the
behavior of mobile charges in conducting materials (including poor conductors), and is the
most analytically tractable scenario in which the electric fields decays while the magnetic field
remains a good hydrodynamic mode (i.e., a conserved density). This limit can arise in actual
electronic materials in the presence of dynamical fields (or in the context of spin liquids, in
which case the charges and fields are emergent) if there is a large separation of scales between
the electric and magnetic conductivities, so that the latter can be ignored [63].
The Ohmic regime is the most generic scenario in which the presence of (electric) charge
breaks the conservation of the electric field, E, leading to its decay, while the magnetic field
remains a good hydrodynamic mode. In Sec. 2.3, we will see that this corresponds to breaking
the E field’s one-form symmetry while preserving that of the B field.
Microscopically, one expects the matter current, J (e)
, to be proportional to the force that
engenders it—in this case the Lorentz force, F ∝ E + v × B. We then introduce the Drude
conductivity, σ (or equivalently, the relaxation time, τ ∼ 1/σ), as the coefficient of propor-
tionality J (e) ∼ σF . Importantly, σ ∼ 1/τ is a phenomenological parameter, which differs
for different materials and must be determined using experiments (likewise, the parameters
σ ∼ 1/τ introduced for higher rank theories of electromagnetism will also differ from the τ
discussed here).
Note that we have already implicitly made some restrictions to this phenomenological
parameter based on symmetry arguments. Generally speaking, σ could be a matrix; however,
since the system is assumed to exhibit SO(3) rotational invariance, we must construct σ out
of SO(3)-invariant objects. Since the only compatible such matrix is the identity, σ reduces
to a scalar. Thus, the current, J, is both proportional and parallel to the force that drives it.
Additionally, because we are interested in linear response (and linearized hydrodynamics), σ
must be independent of both the E and B fields. Finally, because the matter velocity field, v , has
nonzero overlap with other hydrodynamic modes (e.g., the matter current, J (e)
), the magnetic
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SciPost Phys. 14, 029 (2023)
contribution to the Lorentz force, v × B, is nonlinear, and therefore subleading. Hence, we
are left with J (e) = σE with σ a microscopically determined parameter that is independent of
the fields. We do not need to consider k or ω dependence in σ, which amount to subleading
corrections to hydrodynamics: In generic systems, such corrections are suppressed by the
dimensionless combinations of kℓ
mft) corresponds to a microscopic
mean free path (time) for inter-particle scattering. In the context of hydrodynamics, such terms
are generically interpreted as higher-derivative corrections to the constitutive relations.
mft where ℓ
mfp or ωτ
mfp (τ
The effect of including a nonvanishing matter current, J (e) ̸= 0, is to break the conservation
of the electric field, E, as can be seen upon examination of the right-hand side of Ampère’s
law (4). Following the prescription of quasihydrodynamics [52], we further eschew the Drude
conductivity, σ, in favor of the relaxation time, τ, to recover
∂
t Ei
− c2 ε
= − 1
ϵ
where, in an Ohmic metal, τ = ϵ/σ, with σ the Drude conductivity. Note that the relaxation
time for fields, τ, that appears in (7) is not the same as the scattering time that appears in
microscopic expressions for the Drude conductivity.
= − 1
τ
= −
(e)
i
j Bk
Ei ,
Ei
(7)
i jk
∂
J
σ
ϵ
Having recovered an expression governing the dynamics of the electric field in the Ohmic
limit, the hydrodynamic equation of motion for the magnetic flux density is found by taking
the curl of (7), and inserting the resulting expression into Faraday’s law (1c), giving
+ c2 (cid:128)∂
(cid:138) = 0 ,
(8)
∂
∂
j B j
− ∂ 2Bi
i
∂ 2
t Bi
+ 1
τ
t Bi
where we have used the vector calculus identity ε
curl, and we note that ∂
≪ 1, meaning that ∂
τ∂
−∂ 2Bi for the double
mBn
= 0 by the magnetic Gauss’s law (1b). At late times,4 we take
≫ τ∂ 2
t Bi, resulting in the equation of motion
i Bi
t Bi
= ∂
j B j
kmn
i jk
ε
∂
∂
∂
t
i
j
∂
= D ∂ 2Bi ,
corresponding to diffusion of magnetic flux lines, with diffusion constant D ≡ τ c2, depicted
schematically in Fig. 1. In Sec. 2.3, we show how this same result (9) can be derived from the
usual hydrodynamic procedure of constructing the current conjugate to the conserved density,
Bi, via constitutive relations.
t Bi
(9)
Making use of the generalized divergence theorem, the fact that the B field obeys a standard
continuity equation (3) implies that the components, Bi, are conserved quantities over all space.
However, the equations of motion for Bi actually exhibit a much larger set of conservation laws:
The total magnetic flux through any closed or semi-infinite surface is conserved, as we will
see in Sec. 2.3. In fact, this follows already from Faraday’s law (1c) alone [equivalently, the
hydrodynamic continuity equation (22)], without the need to appeal to the magnetic Gauss
law constraint (1b).
From (9), the quasinormal modes for the magnetic field corresponding to a wavevector
(k, ω) ei(kz z−ωt)
, are given by
oriented in the ˆz direction, Bi
(x , t) ∝ Bi
ω = −i τ c2k2
z ,
for Bx, y ,
(10)
and, as in (6), the longitudinal component, Bz, is not a propagating mode since it is constrained
= 0. The transverse
to vanish by the [Fourier-transformed] magnetic Gauss’s law (1b), ki Bi
components of the field are not constrained by Gauss’s law and diffuse, as one would expect
from (9).
4By late times we mean t ≫ τ. At times t ≲ τ, there exist oscillatory solutions for short-wavelength modes
satisfying τck > 1/2, which decay over a time scale set by τ. At late times, the dominant contribution is from long-
wavelength modes with λ ≳ c
1 − 4τ2c2k2 = −iτc2k2 + O(k4).
Analogous arguments are given in Appendix A of Ref. [63].
tτ, whose dispersion is given by ω(k) = − i
2τ + i
2τ
(cid:112)
(cid:112)
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SciPost Phys. 14, 029 (2023)
2.2.3 Conservation of fluxes through surfaces
We now derive the conservation of magnetic flux through arbitrary closed surfaces; this deriva-
tion applies equally to electric flux in the absence of electrically charged matter (ρ(e) = 0). For
simplicity, we restrict our consideration to the magnetic field, where ρ(m) = 0 is guaranteed in
free space, and assumed in the context of spin liquids.
Note that multiplying the magnetic Gauss’s law (1b) by an arbitrary, time-independent test
function, Φ(x ), and integrating over any volume, D, still gives zero:
∂
i Bi
= 0 →
(cid:90)
D
d3 x Φ ∂
i Bi
= 0 ,
and using integration by parts, we then find
(cid:90)
D
d3 x Φ ∂
i Bi
= −
(cid:90)
D
d3 x Bi
(cid:0)∂
i
Φ(cid:1) +
(cid:90)
∂ D
dS Φ Bi ˆni ,
(11)
(12)
where ˆni are the components of the unit vector normal to the surface ∂ D (where ∂ D is the
boundary of the volume, D, and ˆn points outward from D).
Note that applying a total time derivative to this integral also gives zero. Choosing Φ = 1
eliminates the volume integral in (12), and applying the total time derivative leads to
d
dt
(cid:90)
∂ D
dS ˆni Bi
= 0 ,
(13)
for any domain, D, implying that the magnetic flux through the boundary, ∂ D of any volume,
D, is conserved. The same result can be derived alternatively from Faraday’s law (1c) by
considering higher-form symmetries in Sec. 2.3, where we find that the magnetic flux through
semi-infinite surfaces is also conserved.
2.2.4 Absence of diffusion of the conserved electric charge
Here we explore why Fick’s Law of diffusion does not apply to the conserved electric charge,
ρ(e)
. In a conducting medium with conductivity σ, the charge current is given by Ohm’s law,
(e)
= σEi, whenever there are mobile charges. The continuity equation (2) gives rise to
J
i
exponential relaxation of charge in the bulk of the conducting medium,
ρ(e) + σ∂
∂
t
= ∂
i Ei
ρ(e) + 1
τ
t
ρ(e) = 0 ,
(14)
so that the electric charge density in the bulk decays to zero exponentially on the time scale
1
τ
=
σ
ϵ
.
(15)
Essentially, the long-range Coulomb interactions easy pull charges from very far away, and
the resulting interaction rapidly screens any test charge placed in the system. The familiar
diffusion of conserved charges that one expects for locally interacting charges with a global
(rather than gauged) U(1) symmetry is absent here because the charge density is instead driven
by the self-generated electric field.
9
2.2.5 Magnetic charges and regime of validity
SciPost Phys. 14, 029 (2023)
We briefly reinstate magnetic matter in (1b) and (1c) for the purpose of discussing the regime of
validity of the magnetic diffusion recovered in Sec. 2.2.2, which gives rise to a magnetic charge
current J (m) = σ
mB. As discussed in Sec. 2.2.4, the long-ranged nature of the electric (magnetic)
fields implies that electric (magnetic) charge density — i.e., the irrotational component of the
electric (magnetic) field, ρ(e) = ϵ∂
/ϵ
(τ−1
µ). It is, however, the solenoidal components that are responsible for magnetic
m
diffusion. Orienting the wavevector parallel to ˆz, we find that the x and y components of B in
Fourier space satisfy
/µ) — decays on a time scale τ−1
e
i Ei (ρ(m) = ∂
= σ
e
= σ
i Bi
m
(iω)2B⊥ = −c2k2B⊥ + iω(τ−1
m
+ τ−1
e
)B⊥ − τ−1
e
τ−1
m B⊥ ,
(16)
and we assume that there exists a large separation of [time]scales — i.e., τ
m
to wavelengths ckτ
e
to (16) is given by
e. Restricting
≲ 1/2 (so as to preclude oscillatory solutions), the longest-lived solution
≫ τ
i ω = 1
2
(cid:166)τ−1
e
+ τ−1
m
− τ−1
e
τ−1
m
(cid:198)(τ
m
− τ
e
)2 − (2ckτ
e
τ
m
)2(cid:169)
.
(17)
In the long-wavelength limit, ckτ
ec2k2 + O(k4, τ
/τ
); for the
e
e
diffusion pole to dominate over simple exponential decay (implied by a finite τ
m), there must
exist a further restriction on the wavevector, k: Specifically, the wavevector regime relevant to
magnetic diffusion is
≪ 1, we find i ω = τ−1
m
+ τ
m
(cid:118)
(cid:116) τ
τ
e
m
≪ ckτ
e
≪ 1 .
(18)
If there exists a large separation of scales between the two decay rates, τ−1
m , then there
e
exists a nonzero window over which magnetic diffusion prevails. Alternatively, in terms of
energy scales, the relevant regime is simply
≫ τ−1
τ−1
m
≪ i ω ≪ τ−1
e
.
(19)
One scenario that may realize this regime is if the gaps ∆
e(m) to electric and magnetic matter
exhibit a [perhaps O(1)] separation of scales, as is typically the case in simple models of,
e.g., quantum spin ice [62]. Assuming a simple Drude-like expression for the conductivity, the
e(m)/T
conductivity should scale with the density of the corresponding matter, such that τ
at temperature T , leading to (cid:112)τ
e
T ≲ (∆
m
will be predominant.
)/(2T )
. At sufficiently low temperatures,
), we obtain an exponentially large energy window over which magnetic diffusion
e(m) ∼ e
− ∆
e
∼ e
(∆
e
/τ
−∆
∆
m
m
2.3 One-form symmetries
Having presented a very thorough discussion of the hydrodynamic limit of the conventional
Maxwell equations, let us now present a derivation of these properties based on the more
modern language of one-form symmetries [48]. We interpret one-form symmetries in hydrody-
namic theories as being a consequence of demanding that the conserved density, ρ
i , and its
corresponding current, Ji j , both realize vector representations of SO(3), which we denote as
the 3. We then apply these findings to the case of rank-one electromagnetism, and find the
results are equivalent to Sec. 2.2.
The standard hydrodynamic equation of motion for a vector-valued density is given by the
continuity equation,
ρ
∂
t
i
+ ∂
j Ji j
= 0 ,
10
(20)
SciPost Phys. 14, 029 (2023)
and, in general, rank-two objects like Ji j can be decomposed according to 3 ⊗ 3 = 1 ⊕ 3 ⊕ 5 [64],
Ji j
δ
= 1
3
(cid:124)
i j tr [ J ]
(cid:123)(cid:122)
(cid:125)
1
+ ε
i jk Jk
(cid:124) (cid:123)(cid:122) (cid:125)
3
,
+ Si j
(cid:124)(cid:123)(cid:122)(cid:125)
5
(21)
= ε
− 2δ
3Ji j
≡ (cid:128)
+ 3J ji
kmnJmn encodes the antisymmetric components
where the first term is the trace part, Jk
(cid:138) /6 encodes the symmetric, traceless part of
i j Jkk
of Ji j, and the tensor Si j
[64]. In general the current may be in a reducible representation of SO(3), with nonzero
Ji j
overlap with the 1, 3, and 5 irreps. Having a current that overlaps with particular irreps (or
combinations thereof) gives rise to different hydrodynamic theories with different conservation
laws. While one might generally expect the current to have nonzero overlap with all irreps
in (21), we will focus on the case where Ji j is in an irreducible representation of SO(3). This
will generally lead to the most conservation laws and the richest structure. A case where the
current overlaps with multiple irreps is discussed in App. A.3.
In particular, in the case where the current, Ji j is in the 3 of SO(3) (the “spin-one” irrep),
then the current must be expressible entirely in terms of SO(3)-invariant tensors, and a vector-
valued object, Jk. In (21), that vector object can be extracted from the rank-two object by
contracting with the Levi-Civita symbol ε
i jk. Dropping the 1 and 5 pieces from (21), we rewrite
(20) in terms of Jk
∈ 3 as
∂
ρ
+ ε
t
i
∂
j Jk
i jk
= 0 ,
(22)
which is precisely the form of Faraday’s law (1c) (and also Ampère’s law (1d) in the matter-free
limit).
To find the conserved quantities associated with the continuity equation (22), consider the
putatively conserved quantity
Q [ f ] ≡
(cid:90)
(cid:82)3
d3 x fi
ρ
i ,
(23)
where fi is any vector-valued function of x ∈ (cid:82)3, and ρ
vector f is assumed to be time independent, the total time derivative of Q[ f ] is given by
i is a vector-valued density. Since the
dQ
dt
(cid:90)
=
(cid:82)3
d3 x fi
∂
t
ρ
i
= −
(cid:90)
(cid:82)3
d3 x fi
ε
i jk
∂
j Jk
=
(cid:90)
(cid:82)3
d3 x Jk
ε
i jk
∂
j fi
−
(cid:90)
∂ (cid:82)3
ε
dS fi
i jkJk ˆn j ,
(24)
where we have invoked the continuity equation (22) to write ∂
ρ in terms of J and then
integrated by parts. We require that f and J are well-behaved as |x | → ∞, so that the
boundary integral above vanishes, giving
t
dQ
dt
(cid:90)
=
(cid:82)3
d3 x Jk
(cid:128)ε
∂
j fi
i jk
(cid:138)
,
and we then find that Q is conserved when the curl of fi vanishes, i.e.,
ε
∂
j fk
i jk
= 0 ,
(25)
(26)
i
= ∂
ϕ for some scalar function, ϕ(x ), leads to a conserved charge, Q [ f ],
so that the choice fi
of the form (23). One can recover solutions to (26) via Helmholtz decomposition of the vector
field f : Restricting to solutions that are well-behaved as |x | → ∞, the only solution for fi
in Fourier space is one parallel to the wave vector k; (26) precludes a nonzero “transverse”
(or divergence-free, solenoidal) term in the Helmholtz decomposition of f , leaving only the
parallel (or curl-free, irrotational) component, fi
= ∂
ϕ.
i
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Choosing ϕ to be an indicator function for some finite volume, V , i.e.,
ϕ
V
(x ) =
(cid:168)
1 x ∈ V ,
0 x /∈ V ,
(27)
i
V
ϕ
= ∂
(x ) = −δ [x ∈ ∂ V ] ˆni
(x ), where δ [. . .] is a delta function that restricts
implies that fi
x to lie in the boundary, ∂ V , of the volume, V , and ˆni is the unit vector pointing out of V and
normal to ∂ V . Indicator functions can also be chosen for semi-infinite volumes, V , such that
δ [. . .] restricts x to some semi-infinite surface (i.e., a boundaryless surface, such as the x y
plane, that bounds a semi-infinite region of space).
Essentially, the prescription above gives rise to a conserved charge, Q, that is the integral
i over all space,
i jkJk are in the 3 of SO(3) leads to a new, one-form
i through surfaces. That
over a surface, S, of the local density. Thus, in addition to conservation of ρ
the fact that both ρ
conserved charge corresponding to the conservation of the flux of ρ
one-form charge is given by
i and its current, Ji j
= ε
(cid:90)
≡
QS
dS ρ
i ˆni ,
(28)
S
where S is an arbitrary closed or semi-infinite surface (we have ignored an overall sign relating
solely to the definition of “outside” in the indicator function). The flux through any such
surface is exactly conserved by the continuity equation (22). The importance of ρ
i and its
corresponding current, Ji j, being in the same irrep of SO(3) is that this allows Ji j to be expressed
in terms of a lower-rank object, Jk, and the Levi-Civita tensor ε
i jk. The appearance of the
= ε
i jkJk guarantees (26), and thereby a one-form symmetry. This
antisymmetric tensor in Ji j
same argument holds when ρ
i and Ji j each carry additional indices, e.g. in the higher-rank
theories of electromagnetism considered later.
Returning to the particular case of the electromagnetic fields in the Ohmic regime, we note
that Faraday’s law (1c) is already of the form required to realize a one-form symmetry,
ρ
∂
t
i
+ ε
∂
j Jk
i jk
= 0 ,
(22)
i
→ Bi is the magnetic field and Jk
where ρ
∈ 3 is the electric field. This derives from the
→ Ek
ability to write the rank-two current, Ji j , conjugate to the conserved density, Bi, entirely in
terms of the E field.
From the hydrodynamic perspective, the current Ji
∈ 3 can be constructed via derivative
j) and SO(3)-invariant objects
jkℓ). Given that Ji transforms in the vector representation of SO(3), the terms
expansion using the available conserved densities (namely, ρ
(i.e., δ
permitted at lowest order are given by
jk and ε
Ji
= α ρ
+ D ε
ρ
∂
j
k
i jk
i
+ α′∂
ρ
∂
j
j
i
+ O(∂ 3) ,
(29)
where the terms on the right-hand side are the only allowed terms with zero, one, and two
∝ ρ
i is ostensibly allowed, as both objects belong to the 3,
derivatives. While the term Ji
other considerations preclude α ̸= 0. For example, if the density ρ
i, is odd/even under time
reversal, inversion (or parity), or some combination thereof, then the current, Ji, must be
even/odd under the same transformation; since the term proportional to α in Eq. (29) contains
no derivatives, thermodynamics forbid any disagreement under either time reversal or inversion.
Even allowing for the possibility that time-reversal and/or inversion symmetry are broken
microscopically, the effective field theory formalism of Ref. [65] forbids α ̸= 0 in general.5
5If we take α ̸= 0 as the leading contribution, then the equations of motion become ∂
ρ
ρ
i, which is unstable. In the case of MHD, we also have Gauss’s law, ∂
) − α2∂ 2ρ
= α2∂
(∂
ρ
t
i
i
i
= −αε
ρ
∂
k, or
j
= 0 (1b), and
i jk
ρ
i
i
∂ 2
t
so the equation of motion becomes ∂ 2
t
j
j
ρ
i
= −α2∂ 2ρ
i, whose unstable modes are given by ω(k) = ± i α|k|.
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Hence, we take α = 0, so that the leading, symmetry-allowed contribution to the current is
Ji
term in (29) subleading (as it contains an extra derivative),
j
and we are left with
k, with the latter, α′
= ε
i jk
ρ
∂
Ji
= D ε
ρ
∂
j
k ,
i jk
(30)
to leading order, where D is a phenomenological parameter. Using (30) for the current in (22)
gives the equation of motion for the B field (ρ
),
∂
t Bi
= D ∂ 2Bi
− D∂
i
(cid:138) = D ∂ 2Bi ,
(31)
i
→ Bi
(cid:128)∂
j B j
which is simply the diffusion equation, where Maxwell’s equations and Ohm’s law allow us to
make the identification D = τc2. The continuity equation alone (i.e., absent any Gauss law
(k)ei(kz z−ωt)
constraint) gives rise to a nondecaying mode. For a density of the form ρ
,
i
the transverse components diffuse, i.e., ρ
z , while the longitudinal
In the presence of a Gauss law constraint, the nondecaying
component does not decay.
longitudinal mode is removed.
x, y decay with rate τc2k2
∼ ρ
i
3 Tensor electrodynamics and magnetohydrodynamics
Here we consider a rank-two theory of electromagnetism analogous to the standard, rank-one
theory discussed in Sec. 2. Such theories arise in systems hosting charged matter, which
conserve not only electric charge, but also its first moment (i.e., the dipole moment). Regarding
the provenience of higher-rank gauge theories in a condensed matter setting, the emergence
of higher-rank electromagnetism from microscopic spin-liquid Hamiltonians is discussed at
length in Refs. [31, 66–70]. Additionally, certain aspects of these theories are reminiscent of
gravity [71], which is also a rank-two theory.
3.1 Rank-two Maxwell’s equations
The rank-two Maxwell’s equations in which the electric and magnetic monopole (i.e., charge)
densities are scalars, and the E and B fields are [traceless, symmetric] tensors, take the form [32]
∂
∂
∂
∂
i
i
j Ei j
j Bi j
∂
t Bi j
∂
t Ei j
ρ(e)
= 1
ϵ
= µρ(m)
,
,
(cid:128)ε
ikl
(cid:128)ε
= − 1
2
= 1
2 µ ϵ
∂
k El j
+ ε
∂
k El i
jkl
∂
kBl j
+ ε
jkl
∂
kBl i
ikl
(cid:138) − µJ
(cid:138) − 1
ϵ
(m)
i j
,
(e)
i j
J
.
(32a)
(32b)
(32c)
(32d)
As in the rank-one case, we recover continuity equations for the electric and magnetic charges
by taking the divergence on both indices of Faraday’s (32c) and Ampère’s (32d) laws. The
continuity equations are given by
ρ(e/m) + ∂
∂
t
∂
(e/m)
i j
j J
i
= 0 ,
(33)
where the doubled spatial derivative extends the divergence that appears in rank-one theories.
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3.2 Hydrodynamic interpretation
In analogy to the discussion of rank-one electromagnetism in Sec. 2.2, we recover a hydrody-
namic description for the rank-two electric and magnetic fields, Ei j and Bi j, in the absence of
their corresponding matter (in the presence of electric matter, the electric field is no longer a
conserved density, as in the rank-one case, and likewise for the magnetic field). Because we
expect realizations of rank-two quantum electrodynamics (QED) to be emergent, we allow for
magnetic matter, with magnetic charge density, ρ(m)
, in (32). In the context of, e.g., frustrated
magnets, where such higher-rank QED may emerge [31, 33, 34, 66, 67, 69, 70], one generally
expects both electric and magnetic quasiparticles, whose densities will both be nonzero at
nonzero temperature.
3.2.1 Matter-free limit: The photon
ρ
In the absence of both electric and magnetic matter, the components, Ei j and Bi j, of both the
electric and magnetic field tensors are conserved in accordance with the higher-rank continuity
equation ∂
= 0. The dispersion relation for the “photon” can once again be derived,
e.g., by taking the time derivative of the rank-two Ampère’s law (32d), then using Faraday’s
law (32c) to express ∂
t Bi j in terms of the electric field tensor Ei j. This results in the wave
equation
kJi jk
+ ∂
i j
t
∂ 2
t Ei j
= c2
4
(cid:148)(cid:0)δ
in
∂ 2 − ∂
∂
(cid:1) En j
n
i
− ε
ikℓε
∂
k
∂
mEnℓ + i ↔ j
jmn
(cid:151)
,
(34)
where µ ϵ c2 = 1 defines the [maximum] speed of light, c. The equation for Bi j assumes the
same form, by electromagnetic duality. The system’s normal modes can then be found by
orienting the the wave vector k along ˆz for convenience. We find four linearly dispersing
modes, with two doubly degenerate branches
ω = c
2
×
kz
(cid:168)
1 Exz, E yz ,
2 Ex y , Ex x
= −E y y ,
(35)
which correspond to ballistic (wavelike) propagation at speed c/2 (Exz, E yz) and speed c (Ex y ,
= −E y y ). In principle, the symmetric, traceless tensor Ei j has five independent degrees
Ex x
of freedom. However, one of the resulting five modes is dynamically trivialized by the Gauss
= 0.
law constraints (32a) and (32b), i.e., the longitudinal components satisfy k2
z Ezz
= 0, as
Note also that the diagonal elements Ex x and E y y appear in the combination Ex x
required by tracelessness.
= k2
+ E y y
z Bzz
3.2.2 The Ohmic regime: Magnetic diffusion
We now consider the sector in which only one species of charge (electric or magnetic) is present.
This may arise, e.g., due to a separation of scales between the gaps for electric versus magnetic
matter in materials with emergent QED. The self-dual nature of the traceless scalar charge
theory with respect to electric and magnetic fields means that, although we take the limit of
vanishing magnetic charge density, ρ(m) = 0, for concreteness, the results apply equally to the
regime of vanishing electric charge density, ρ(e) = 0, with the roles of the electric and magnetic
fields reversed (up to signs and factors of c). The inclusion of both electric and magnetic matter
and the corresponding regime of validity is considered in Sec. 3.2.3.
In the absence of magnetic charge, Faraday’s law (32c) can be interpreted as a continuity
→ Bi j. The continuity equation takes the form
equation for the rank-two conserved density ρ
i j
∂
t Bi j
(cid:128)ε
+ 1
2
ikℓ∂
k Eℓ j
+ ε
∂
k Eℓi
jkl
(cid:138) = 0 ,
(36)
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that depends on the material. Since J
and, as before, the presence of electric matter in (32d) spoils the conservation of the rank-
two electric field. Following the prescription of quasihydrodynamics [52], we replace the
(e)
τ Ei j, where τ is a phenomenological parameter
electric current in (32d) according to J
i j
(e)
i j and Ei j transform as rank-two tensors, the “electri-
(e)
i j and Ei j is constrained by SO(3) symmetry to be of the form
cal conductivity” relating J
δ
σ
+ γδ
jk. For a traceless symmetric electric field tensor Ei j, the
conductivity is therefore characterized by a single parameter ϵτ−1 = β + γ. Similarly to (15)
in the rank-one case, we obtain
i jkℓ = αδ
+ βδ
= 1
kℓ
δ
δ
iℓ
jℓ
ik
i j
∂
t Ei j
− 1
2
c2(ε
ikℓ∂
kBℓ j
+ ε
jkℓ∂
kBℓi
) = − 1
τ
Ei j ,
(37)
and at late times, when τ∂
with Eq. (36) gives
t
≪ 1, we ignore the time derivative term. Combining this result
∂
t Bi j
+ 1
4
τc2(3∂
∂
n
i Bn j
+ 3∂
∂
n
j Bni
− 4∂ 2Bi j
− 2δ
∂
i j
∂
nBmn
m
) = 0 .
(38)
We then seek quasinormal modes corresponding to a wavevector k oriented in the ˆz direction,
and find four diffusing modes,
ω = − i
4
D k2
z
×
(cid:168)
1 Bxz, B yz ,
4 Bx y , Bx x
= −B y y ,
(39)
where D = τ c2 is the same diffusion constant identified in the rank-one case (9); as with the
quasinormal modes for the matter-free sector (35), the two branches are distinguished by the
propagation speed, c/2 versus c, and Bzz
= 0.
z Bzz
This mode structure is to be expected based on a general counting argument: A conserved
∈ 5 [the traceless, symmetric, rank-two tensor irrep of SO(3)], contains five
density, Bi j
independent elements, one of which is constrained by Gauss’s law (32b)—whose Fourier-
= 0 for propagation in the ˆz direction—along with four propagating modes.
transform is k2
= 0.
Thus, Bzz is trivially zero by Gauss’s law, and tracelessness then requires that Bx x
Interestingly, note that fixing the second index of Bi j to be j = z, gives rise to the same three
modes recovered in the rank-one case (10); additionally, the 5 theory has two additional modes,
distinguished by a fourfold enhancement of the diffusion constant (each higher rank gives rise
= τ c2 m2/n2
to two new propagating modes; the diffusion constants at rank n are given by Dm
for m ∈ {1, . . . , n}).
+ B y y
3.2.3 Magnetic charge and regime of validity
Including magnetic matter, whose leading effect is to give rise to a current J
mBi j, leads
to exponential decay of all rank-two fields at the longest time scales, as was the case for the
rank-one theory discussed in Sec. 2.2.5. Specifically, we find that, for well-separated time scales,
µ, the length scales relevant to magnetic diffusion of the higher-rank
τ
e
/ϵ ≪ τ
= σ
e
= σ
m
(m)
i j
= σ
m
gauge fields are those satisfying
(cid:118)
(cid:116) τ
τ
e
m
≪ ckτ
e
≪ 1 .
(40)
The same condition applies equally to both branches of propagating modes. Above the UV
cutoff, there exist remnants of wavelike propagation, and below the IR cutoff, all fields decay
exponentially at the same rate, irrespective of the characteristic length scales over which they
vary.
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3.3 One-form symmetries
The continuity equation (36) can be recast in the standard form for a tensor conserved quantity,
ρ
∂
t
i j
+ ∂
kJi jk
= 0 ,
(41)
i j, Ji j
i j, and the rank-three current, here Ji jk
),
where both the rank-two density, ρ
transform in the 5 of SO(3), corresponding to traceless, symmetric rank-two tensors (i.e.,
∈ 5). We remind the reader that the vanishing of magnetic charge density can at best
ρ
only be expected to hold approximately in emergent theories; see Sec. 3.2.3 for a discussion of
the length and time scales over which (41) provides an accurate description of the dynamics. We
find the conserved quantities associated with (41) as in the rank-one case (23) by considering
ikℓJℓ j
jkℓJℓi
= 1
2
+ε
(ε
Q[ f ] ≡
(cid:90)
(cid:82)3
d3 x fi j
ρ
i j ,
(42)
where fi j is a traceless, symmetric tensor-valued function of x ∈ (cid:82)3 (note that any components
of f not in the 5 — i.e., the trace and the antisymmetric part — cannot contribute to Q, and
are therefore not physical). Following the same procedure as used in the rank-one case, we
find that Q[ f ] is conserved whenever fi j satisfies
εℓki
∂
k fℓ j
+ εℓk j
∂
k fℓi
− 2
3
δ
εℓkm
∂
k fℓm
i j
= 0 ,
(43)
(ε
= 1
2
) is in the 5 — i.e., it can be written in
which derives from the fact that Ji jk
terms of the traceless, symmetric tensor Jℓ j. The last term in (43) is identically zero when fi j is
3 tr [ f ] δ
symmetric; additionally, the contribution due to a nonzero trace component of fi j
from the first two terms will conspire to cancel, since (ε
ktr [ f ] = 0. Owing to the
antisymmetry of ε
i jk in its indices, (43) is satisfied precisely when fi j is of the form
ikℓJℓ j
jkℓJℓi
⊃ 1
+ ε
+ ε
)∂
ik j
jki
i j
(x ) = ∂
fi j
∂
i
Φ − 1
3
j
δ
i j
∂ 2Φ ,
(44)
where Φ is any scalar function of x , leading to an infinite family of solutions to (43). It is worth
noting that (44) coincides with the structure of time-independent gauge transformations acting
on the vector potential Ai j, canonically conjugate to Ei j. This apparent equivalence derives
from the self-dual nature of the traceless scalar charge theory — i.e., the derivative and tensor
structure of the electric and magnetic Gauss’s laws is identical.
The preclusion of other forms of solutions to (43) can be justified by appealing to the “scalar-
vector-tensor” (SVT) decomposition of fi j, which can be viewed intuitively as a Helmholtz
decomposition on each index of the rank-two tensor:
= f
∥
i j
+ f
⊥
i j
+ f T
i j ,
fi j
(45)
where, in Fourier space, the “scalar” component, f
both indices; the “tensor” component, f T
component, f
index and transverse to k in the other).
, is parallel to the wave vector, k, in
i j , is transverse to k in both indices; and the “vector”
⊥
i j , is mixed (being a symmetric sum of two terms that are parallel to k in one
∥
The decomposition (45) can be realized using the projector,
P⊥
i j
= 1
k2
(k2δ
− ki k j
i j
) = 1
k2
(ε
ikℓkℓ)(ε
jkmkm
) ,
(46)
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which projects onto the subspace orthogonal to k, and its complement, P∥ = 1 − P⊥
P⊥ + P∥ = 1, we resolve the identity on either side of f to recover
. Using
f = P∥
f P∥
(cid:124) (cid:123)(cid:122) (cid:125)
scalar, Φ
+ P∥
(cid:124)
f P⊥ + P⊥
(cid:123)(cid:122)
vector, va
f P∥
(cid:125)
+ P⊥
f P⊥
(cid:124) (cid:123)(cid:122) (cid:125)
tensor, Tab
=
Φ
va
,
va
Ta b
(47)
where, in the last equality, we have written f schematically in a basis in which the wavevector,
k, locally defines the “parallel” vector (1, 0, 0)T , so that Φ lies in the parallel block, Ta b lies in
the transverse (perpendicular) block, and the components va mix between blocks.
The scalar contribution, k4Φ(k) = −ki k j fi j, corresponds to the doubly longitudinal com-
ponent; the vector, v(k), has two independent components,6 and is written in terms of fi j
) f bd contains the two
as k4vi
remaining degrees of freedom (as T is a symmetric 2 × 2 matrix in the subspace orthogonal to
k, whose trace is fixed7).
)kc f bc; and finally, k4Ti j
(k) = −(ε
(k) = −(ε
iabka
iabka
jcd kc
)(ε
Writing out the projectors explicitly — and ensuring that each individual term is traceless
and symmetric — gives
(k)
fi j
(cid:124) (cid:123)(cid:122) (cid:125)
5
ki k j
= − (cid:128)
(cid:124)
− (cid:148)(cid:0)ε
(cid:124)
i j k2(cid:138) Φ
(cid:125)
+ (cid:148)(ε
(cid:124)
δ
− 1
3
(cid:123)(cid:122)
scalar, 1
+ i ↔ j
iabka vb
)k j
(cid:123)(cid:122)
vector, 2
(cid:151)
(cid:125)
iabka
(cid:1) (cid:128)ε
(cid:138)
jcd kc
Tbd
− δ
i j k2Taa
+ δ
i j kakb Ta b
(cid:123)(cid:122)
tensor, 2
,
(cid:151)
(cid:125)
(48)
where each term is labelled below according to its role in the SVT decomposition and the
number of independent degrees of freedom carried.
) f b j
iabka
Equipped with the decomposition (48), we can show that (44) is the only solution that leads
to conserved quantities of the form (42). Inserting (48) for f in the relation (ε
= 0, we
see that the scalar term in (48) is annihilated independently in each term in (43), and therefore
a valid solution to fi j. However, the “vector” and “tensor” parts of the decomposition (48) only
satisfy (43) if they vanish (this is most apparent from the definitions of v and T in terms of fi j).
= 0 implies that f must be “parallel” to k in both indices since fi j
In other words, (ε
and f T in (45) and (47), so
is symmetric, which precludes any contribution from the terms f
that only Φ is nonzero, corresponding to the doubly parallel block in (47). Note that the more
general equation, (ε
(k), does not admit new
solutions in which the first two terms in (43) nontrivially cancel one another (i.e., conspire to
cancel without vanishing individually), and we conclude that there are no additional solutions
beyond those captured by (48).8
= Ai j for some antisymmetric tensor Ai j
iabka
iabka
) f b j
) f b j
⊥
Having identified (44) as the only solutions for f compatible with charges of the form (42),
in analogy to the rank-one case, we take Φ to be an indicator function for the volume, V ⊂ (cid:82)3,
Φ
V
(x ) =
(cid:168)
1 x ∈ V ,
0 x /∈ V ,
(49)
6Note that the decomposition (45) is not unique, since any components v ∝ k will be projected out of v .
7Much like the vector components6, Ta b is not uniquely determined: The trace is only fixed once the components
parallel to k have been projected out.
8Suppose that (ε
ia bka
) f b j
= Ai j, and that the antisymmetric tensor Ai j
j
− (k · λ)k j
the (for now) arbitrary vector field λ(k). Since Ti j must satisfy ki Ti j
k2λ
function fi j
= ki k j
fi j
∝ ki c j, since it belongs to the null space of ε
δ
i j k2, which is already captured by setting Φ ≡ 1 in (48).
= 0, or λ ∝ k. Then ε
iℓmkm is solved by fi j
i jkk j fkℓ
− 1
3
∝ ε
i jk
= ε
λ
= 0, and Ti j
k is parametrized in terms of
)Aid , we find that
i j. However, we can also add any
i jkk j. Demanding symmetry of fi j gives the solution
jcd kc
= (ε
∝ δ
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∂
= ∂
(x ). As in the rank-one case, similar indicator
we have fi j
functions can be chosen for boundaryless, semi-infinite surfaces, S. The conserved quantities
associated with choosing Φ = Φ
δ [x ∈ ∂ V ] ˆni
= −∂
Φ
V
i
j
j
V (49) are
QS
= −
(cid:90)
(cid:82)3
d3 x ρ
∂
δ[x ∈ S]ˆni
j
i j
=
(cid:90)
S
dS ˆni
(cid:128)∂
(cid:138)
,
ρ
j
i j
(50)
where S is either the boundary, ∂ V , of some finite volume, V , or a semi-infinite surface, and ˆn
is the outwardly oriented unit vector normal to S. Essentially, the flux of
ρ
(cid:101)
i
≡ ∂
ρ
i j ,
j
(51)
through any closed or semi-infinite surface is conserved; thus, in systems where the charge and
current transform as the 5 of SO(3), there is an effective one-form symmetry corresponding to
ρ
the one-form charge, (cid:101)
We also note that the rank-two continuity equation (36) (or (41) in terms of the rank-two
ρ
i j (51) realizes
magnetic field) expressed in terms of the one-form conserved quantity, (cid:101)
the rank-one continuity equation (22) for a theory with a one-form symmetry, where both the
density and current are in the 3. Taking the divergence on both sides of (36) and using
i (51).
≡ ∂
ρ
i
j
ρ
(cid:101)
i
≡ ∂
ρ
j
i j
,
(cid:101)Ji
≡ 1
2
∂
j Ji j ,
(52)
we then find that
ikℓ∂
which has the form of a continuity equation associated with one-form symmetries, and is
equivalent to the rank-one Faraday (1c) and/or Ampère (1d) laws.
k (cid:101)Jℓ = 0 ,
ρ
t (cid:101)
(53)
+ ε
∂
i
ρ
Because (cid:101)
ρ
apply—i.e., this describes a theory with a vector-valued conserved density, (cid:101)
ρ
one-form symmetry associated to the flux of (cid:101)
ρ
Unlike the discussion of Sec. 2.3, however, because (cid:101)
the rank-two conserved density, ρ
rank-one case.
i obeys the continuity equation (53), the same arguments invoked in Sec. 2.3
i, along with a
i through arbitrary closed or infinite surfaces.
i arises from taking the divergence of
i in the
i j, it obeys extra constraints that do not apply to ρ
ρ
The first constraint follows from the fact that (cid:101)
i j, which constrains the total “charge” to be zero,
ρ
i is the divergence of a higher-rank object,
(cid:90)
(cid:90)
ρ
d3 x (cid:101)
i
=
d3 x ∂
ρ
j
i j
= 0 ,
(54)
where the latter equality follows from integration by parts and the fact that ρ
as |x | → ∞.
i j is well-behaved
The other constraints relate to the “moments” of charge, and derive from properties of ρ
(specifically, that it’s in the 5 of SO(3)). The fact that ρ
i j is traceless gives the constraint
i j
(cid:90)
ρ
d3 x xi (cid:101)
i
=
(cid:90)
d3 x xi
(cid:128)∂
ρ
j
i j
(cid:138) =
(cid:90)
(cid:90)
d3 x δ
ρ
i j
i j
= −
d3 x tr [ρ] = 0 ,
(55)
ρ
which can be viewed as the “parallel” moment of (cid:101)
i j is symmetric (in i ↔ j) gives rise to
the derivative from ∂
(cid:90)
i j to xi). The fact that ρ
i (again using integration by parts to move
ρ
(cid:90)
(cid:90)
(cid:90)
j
d3 x ε
ρ
i jk x j (cid:101)
k
=
d3 x ε
i jk x j
(cid:0)∂
ℓ
ρ
kℓ
(cid:1) = −
d3 x ε
δ
jℓρ
kℓ = −
i jk
d3 x ε
ρ
i jk
jk
= 0 ,
18
(56)
SciPost Phys. 14, 029 (2023)
ρ
which can be viewed as the “transverse” moment of (cid:101)
ρ
Note that, in the context of constraints on (cid:101)
space; in the context of decomposing fi j, these terms refer to k in Fourier space.
i, and also relies on integration by parts.
i, “parallel” and “transverse” refer to x in real
→ Bi j, and current Ji j
The discussion thus far explains how the rank-two Maxwell’s equations (32) can be viewed
as a continuity equation (36) for the B field in the Ohmic regime, where both the density,
→ Ei j, are in the 5 of SO(3) (41). This leads to a one-form symmetry
ρ
i j
ρ
ρ
i j through boundary surfaces. We then
corresponding to conservation of the flux of (cid:101)
j
ρ
i obeys precisely the continuity equation (53) that gives rise to a one-form symmetry
note that (cid:101)
corresponding to fluxes of ρ
i corresponds
to a rank-two conserved quantity in the 5, it obeys the additional constraints (54), (55), and
(56).
ρ
i in the rank-one case in Sec. 2.3. However, because (cid:101)
= ∂
i
We now argue that it is possible to go in the other direction: Knowing that a vector-valued
ρ
conserved density, (cid:101)
i, obeys the one-form symmetric continuity equation (22) and respects
the above three constraints is sufficient to determine that the underlying theory is second
rank, obeys the rank-two continuity equation (41), and has the quasinormal modes (39)
corresponding to rank-two magnetohydrodynamics in the Ohmic regime.
First, the constraint that the total charge vanishes, (cid:82) d3 x (cid:101)
ρ
i is
the derivative of another object (in this case, that object is higher rank) that need only be
well behaved at infinity. Consider a function, g(x) in one dimension, where the Fundamental
′(x), with G the antiderivative of g. On the circle,
Theorem of Calculus provides that g(x) = G
e.g., the vanishing of total charge is given by (cid:82) 2π
0 dθ g(θ ) = G(2π) − G(0) = 0. The zero charge
constraint therefore implies that the antiderivative G(θ ) is single-valued. On the real line, we
use integration by parts to see
ρ
= 0 (54), implies that (cid:101)
i
(cid:90)
(cid:82)
dx g(x) = G(x)|+∞
−∞ → 0 .
(57)
In this context, the zero-charge constraint implies that the antiderivative, G(x), asymptotically
vanishes such that lim|x|→∞ G(x) = 0 (note that it is unreasonable to require that G be an even
function a priori, since the constraint is nonlocal). Note that the same considerations also hold
for vector-valued g. Essentially, the conclusion is that, while any smooth function, g, can be
written in terms of its antiderivative, G, the zero-charge constraint further implies that G(x) is
well-behaved at infinity (on the real line) or single-valued (on the circle).
The higher-dimensional case is slightly more subtle, as there is no crisp notion of an
antiderivative in (cid:82)d for d > 1. Nonetheless, we posit that any well-behaved vector-valued
j Gi j without
function, gi
loss of generality, where the relation between g and G is determined (nonuniquely) by the
Helmholtz decomposition.9 Using higher-dimensional integration by parts, we find
∈ (cid:82)d , can be written as the derivative of a higher-rank object, gi
= ∂
(cid:90)
(cid:82)d
d3 x gi
=
(cid:90)
(cid:82)d
d3 x ∂
j Gi j
=
(cid:90)
∂ (cid:82)d
dS Gi j ˆn j
→ 0 ,
(58)
(x ) = 0, where the factor of |x |d−1 is hidden
and thus, the constraint implies lim|x |→∞|x |d−1 Gi j
∈ (cid:82)d
in the measure dS . As in the d = 1 case, we see that generic vector-valued functions, gi
j Gi j; however, this becomes especially natural when (cid:82) g = 0 , in which case
can be written as ∂
any choice of G that vanishes sufficiently rapidly at infinity and satisfies ∂
= gi is valid (on
the torus, the constraint is that G is single valued). Effectively, the vanishing of total charge
j Gi j
9One can view the Gi j for a particular i as a vector, where g j
= ∂
j Gi j gives the irrotational component, g = ∇ · G;
the solenoidal component, ∇ × G, is not fixed in this scenario, so the decomposition is not unique.
19
SciPost Phys. 14, 029 (2023)
(54) immediately implies the existence of a well-behaved, higher-rank object:
(cid:90)
(cid:82)3
ρ
d3 x (cid:101)
i
= 0 =⇒
ρ
(cid:101)
i
= ∂
ρ
i j ,
j
(59)
for some ρ
i j that vanishes as |x | → ∞ faster than 1/|x |2.
Next, the constraints (55) and (56) then imply that ρ
i j is traceless and symmetric, respec-
tively. We note that, in principle, it is also possible that the trace and antisymmetric components
of ρ
i j (respectively in the 1 and 3) are themselves divergences of higher-rank objects—however,
as higher-derivative corrections to the definition of ρ
i j, these subleading terms can safely be
ignored.10 Essentially, at leading order, any nonzero trace component of ρ
i j decouples from
the hydrodynamic equations governing the components of ρ
i j in the 5; hence these terms are
unimportant at the level of hydrodynamics.
Taking the time derivative of (56) gives a new constraint on (cid:101)Jk:
0 = d
dt
(cid:90)
=
(cid:90)
(cid:90)
d3 x ε
ρ
i jk x j (cid:101)
i
=
d3 x ε
(cid:0)−ε
∂
ℓ (cid:101)Jm
iℓm
(cid:1)
i jk x j
(cid:90)
d3 x
d3 x
(cid:128)∂
ℓ x j
(cid:138) (cid:128)ε
ε
iℓm
i jk
(cid:138)
=
(cid:101)Jm
(cid:90)
= 2
d3 x (cid:101)Jk
= 0 ,
(cid:128)ε
(cid:138)
ε
i jk
i jm
=
(cid:101)Jm
(cid:90)
d3 x 2δ
km (cid:101)Jm
(60)
ρ
where the second line above relies upon integration by parts; by the same logic used for (cid:101)
i,
(60) implies that (cid:101)Ji
i and (cid:101)Ji in terms of higher-rank
objects into the [ostensibly rank-one] continuity equation (53) gives
ρ
j Ji j . Substituting the expressions for (cid:101)
= 1
2
∂
(∂
∂
t
j
ρ
i j
) + 1
2
ε
ikℓ∂
k
(∂
j Jℓ j
) = 0 ,
(61)
and, extracting an overall ∂
with (53) and the rank-one theory — must be of the form
j, we determine that the equation of motion for ρ
i j — by consistency
ρ
∂
t
i j
+ 1
2
ε
ikℓ∂
kJℓ j
+ 1
2
ε
jkℓ∂
k
Λℓi
= 0 ,
(62)
where the latter term on the left-hand side is the most general term permitted by the constraints
on the index structure and is annihilated by ∂
= Jℓi up to
∂
subleading corrections (i.e., terms of the form ∂
k), while
jkℓ
tracelessness of ρ
i j requires that
j . Symmetry of ρ
i j enforces Λ
ℓi
ℓ . . . , that are annihilated by ε
k
∂
(ε
ikℓJℓi
) = 0 ,
which implies that either Jℓi is symmetric, or that ε
n. As the latter means
that the antisymmetric part of Jℓi appears in the hydrodynamic equation of motion at higher
derivative order, we ignore this possibility and take Jℓi to be symmetric. Furthermore, the
trace component of Jℓi does not contribute to the hydrodynamic equation of motion, since
) = 0. Hence the continuity equation takes the form of (36) with
ε
) + ε
ikℓ
traceless, symmetric charge ρ
i j and traceless, symmetric current Ji j. The normal modes (39)
follow as a consequence of the hydrodynamic equation of motion.
ikℓJℓi
(Jδ
(Jδ
(63)
= ε
kmn
jkℓ
Ω
ℓ j
ℓi
∂
∂
∂
m
k
k
10Precisely, any Green’s functions of interest for ρ
i j would not exhibit any singularities sensitive to the neglected
terms—in fact, the neglected terms would be strictly subleading to those included. For example, again orienting
∼ (Dk2 − iω)−1k2 × (1 + ak2 + · · · ) where the ak2 coefficient comes from total derivative terms we
k = kˆz, GR
ρ
x y
have neglected.
x y
ρ
20
SciPost Phys. 14, 029 (2023)
As a quick aside from the present discussion, note that if the tracelessness condition is
i j transforms in the 1 ⊕ 5 representation of SO(3), but (54) and (56) are
relaxed such that ρ
still imposed, then the resulting equation of motion is
ρ
∂
t
i j
(cid:128)ε
+ 1
2
ikℓ∂
kJℓ j
+ ε
jkℓ∂
kJℓi
(cid:138) = 0 ,
(64)
where, now, Ji j now transforms in the reducible 3 ⊕ 5 representation of SO(3)—i.e., it is
traceless but not symmetric. The resulting rank-two electromagnetic theory then corresponds
to a traceful electric field with scalar charge density [32], and is not self dual: The electric
field tensor, Ei j, is symmetric but not traceless (1 ⊕ 5), while the magnetic field tensor, Bi j, is
traceless but not symmetric (3 ⊕ 5).
Returning to the traceless scalar charge theory (32), we recover a constitutive relation for
i j and SO(3)-invariant objects. To
∈ 5, via derivative expansion of ρ
the rank-two current, Ji j
low order, this takes the form
= αρ
Ji j
+ D
2
i j
(cid:128)ε
ikℓ∂
k
ρℓ j
+ ε
jkℓ∂
k
ρℓi
(cid:138) + O(∂ 2) ,
(65)
i j
ab
i j and ε
ρ
δ
and we have neglected subleading contributions at O(∂ 2). The only SO(3)-invariant objects at
our disposal are δ
i jk: Note that the only other zero-derivative term one could write
i jtr (cid:2)ρ (cid:3) = 0; single-derivative terms require use of ε
= δ
∝ δ
down is Ji j
i jk, but
ab
i jk . . . , and symmetry of ρ
∝ ε
i j forbids contracting two indices
symmetry of Ji j forbids Ji j
of the latter with ε
i jk. In direct analogy to the constitutive relation for the rank-one current
= αρ
(29), the term Ji j
i j is forbidden by arguments based on time-reversal symmetry and
generic results from effective hydrodynamic theories [65]; most convincingly, α ̸= 0 leads to
unstable (and unphysical) solutions with quasinormal dispersions ω = ±iα|k| , ±2 iα|k| . Thus,
we take α = 0, with the leading contribution proportional to D [identified as τc2 in the case of
Maxwell’s equations (32)], giving rise to the quasinormal modes (39).
4 Standard higher-rank generalizations of electromagnetism
Having explained the “higher-form symmetry” formulation of magnetohydrodynamics for both
conventional (rank-one) electromagnetism and its rank-two (fractonic) generalization, we now
turn to generalizing to traceless symmetric rank-n theories. In the interest of simplicity, we
make the “standard” assumption that the electric and magnetic charge densities are scalars,
that the generalized Maxwell equations are self dual, and that the E and B fields are both
in the same irrep of SO(3). While other choices exist, and may lead to exotic hydrodynamic
theories (see, e.g., Sec. 5), the theories that obtain from the aforementioned restrictions are
physically most similar to rank-one MHD, and thus we refer to this class of higher-rank theories
as “standard”. Microscopic Hamiltonians that realize such rank-n theories can be constructed
using ideas analogous to those presented in Refs. [32,41,68]. For completeness, we also discuss
a concrete lattice model realization in the next section.
To generalize the results of the preceding sections to rank-n theories of electromagnetism,
it will first prove convenient to define appropriate generalizations of the divergence and curl
operators that appear in the higher-rank extensions of Maxwell’s equations. The action of these
operators on some totally symmetric tensor Ai1,...,in
is given explicitly by
∂
. . . ∂
∇(n) · A ≡ ∂
i2
i1
≡ 1
(cid:128)ε
i1kℓ∂
n
i1,...,in
Ai1,i2,...,in
,
in
kAℓ,i2,...,in
+ . . .
(cid:138)
,
( (cid:101)∇ × A)
(66a)
(66b)
21
SciPost Phys. 14, 029 (2023)
∂
imkℓ
where the generalized n-fold divergence, ∇(n) · A, amounts to taking the divergence of each
index of A individually, and the symmetrized multi-index curl, (cid:101)∇×A, is given simply by applying
the usual curl, ε
k , to each index, im , of A, and taking the average, so that the resulting
object remains symmetric in all indices. The former has n derivatives, while the latter contains
only one derivative for all n; these generalized derivative operators reduce to the standard
divergence and curl for n = 1. The standard vector calculus identity, div(curl A) = 0, also
applies to the rank-n variants (66). In the discussion to follow, we also make use of the multi-
}, to unencumber notation when working with higher-rank indices.
index In
Using this language, the generalized divergence in (66a), e.g., can alternatively be written
∂
≡ {i1, i2, . . . , in
≡ ∇(n) · A.
AIn
In
4.1 Rank-n Maxwell’s equations
Making use of the generalized derivatives in (66), the natural generalization of the rank-one (1)
and rank-two (32) Maxwell’s equations to rank-n fields and currents with scalar electric and
magnetic charge is
ρ(e)
∇(n) · E = 1
ϵ
∇(n) · B = µρ(m)
,
,
∂
∂
t B = − (cid:101)∇ × E − µJ
µ ϵ (cid:101)∇ × B − 1
t E = 1
ϵ
(m)
,
(e)
J
,
(67a)
(67b)
(67c)
(67d)
, BIn
, are fully symmetric and traceless. As in
where both the rank-n fields EIn
previous sections, we take c to be defined by c2 = (µϵ)−1 despite the presence of multiple photon
branches in the matter-free case, each with its own “speed of light”. Taking the generalized
divergence over all n indices of either Faraday’s (67c) or Ampère’s (67d) law gives rise to the
appropriate continuity equation for the scalar charge density ρ(e)
, and the current JIn
, respectively
or ρ(m)
ρ(e/m) + ∇(n) · J
(e/m) = 0 .
∂
t
(68)
4.2 Microscopic Hamiltonians
For completeness, we provide a sketch of how microscopic lattice models that realize higher-
rank gauge theories can be constructed systematically. The construction follows closely the
approach taken in, e.g., Refs. [32, 41, 68]; the Hilbert space is constructed from rotor degrees
of freedom that live on the sites of a face-centered cubic lattice with an additional lattice site at
the center (as shown in Fig. 2). The Hilbert space of each individual rotor degree of freedom is
spanned by angular momentum eigenstates, with integer angular momentum quantum number
n, and approximates that of a large-S quantum spin under the mapping ˆSz ∼ n − 1/2 and
± ∼ e
ˆS
. While we focus on realizing the symmetric scalar charge theory (whose traceless
variant will be the focus of the following section), other theories can be constructed using the
same Hilbert space in the presence of different Hamiltonians that give rise to different Gauss
law constraints.
±iθ
As is depicted in Fig. 2, the unit cell consists of 10 rotors: Three rotors, nx x x
(r ),
(r ) are placed at the vertices of a simple cubic lattice, whose sites are located at positions
and nzzz
(r ) are
r ; two rotors are then placed at the center of each plaquette, e.g., nx x y
(r ), is placed on the corresponding vertex
situated in the x y plane; finally, a single rotor, nx yz
of the dual simple cubic lattice. Note that rotors are labeled by the unit cell to which they
belong, rather than their actual locations within the unit cell.
(r ) and nx y y
(r ), n y y y
22
SciPost Phys. 14, 029 (2023)
Figure 2: Schematic depiction of the sites that comprise the microscopic lattice
Hamiltonian that realizes a rank-three theory. The red sites, which live on the sites
of a simple cubic lattice, indexed by r , host the three rotors nx x x , n y y y , and nzzz.
The blue sites, which live at the centers of the plaquettes of the cubic lattice, host
two rotors each. For example, nx x y and n y x x in the x y plane. Finally, the green site,
which lives on the sites of the dual cubic lattice, hosts a single rotor: nx yz.
The charge-free electric Gauss’ law, ∂
= 0 , can be reproduced in the rotor language
k Ei jk
by replacing the derivatives that appear in the continuum theory with the corresponding finite
difference operators. One possible discretization uses the one-sided derivative:
∂
∂
i
j
∂
∂
i
j
∂
k Ei jk
↔ (cid:88)
i jk
(cid:2)ni jk
ni jk
ni jk
(r ) + ni jk
(r + ei
(r + ei
+ e j
+ e j
) + ni jk
)(cid:3) .
+ ek
(r + ei
) + ni jk
(r + e j
+ ek
) + ni jk
) + ni jk
(r + ek
(r + ek
)+
+ ei
)+
(r + e j
+r y
+r y
+rz θ
∼ (−1)rx
(r ) = (−1)rx
The lattice variant of the electric field tensor is obtained from the rotor variables ni jk via an alter-
+rz ni jk
(r ) (and, similarly, the vector potential is obtained from
nating sign Ei jk
the operator θ , conjugate to n, via Ai jk
i jk). The corresponding contribution to
the Hamiltonian then takes the form of a soft [quadratic] constraint ∼ λ(∂
)2, which
k Ei jk
ensures that the ground state of the model is free of charge, ∂
= 0, in the limit λ → ∞,
when the constraint is exact. The theory can then be endowed with dynamics by writing down
additional terms that commute with, and hence preserve, the Gauss law constraint (but mix
states within a fixed total charge sector). For further details on how such “E2 + B2” terms can
be constructed, we refer the reader to Refs. [32, 66, 67]. Different theories are obtained by
writing down different Gauss law constraints, which select the configurations of Ei jk that are
energetically penalized.
k Ei jk
∂
∂
∂
∂
i
i
j
j
4.3 Hydrodynamic interpretation
As in previous sections, both the electric and magnetic fields are conserved in the absence
of their corresponding matter. In the following we derive the general mode structure for the
photon in the absence of both species of matter, and then derive diffusion of the rank-n magnetic
field when only electric matter is present. Since the higher rank theories that we discuss are
expected to be emergent, both species of matter are generally expected to be present with
nonzero density at nonzero temperatures. The energy and length scales over which magnetic
23
xyznxxx,...nxxy,...nzzx,...nyyz,...nxyzSciPost Phys. 14, 029 (2023)
diffusion prevails, laid out in Sec. 2.2.5 for the rank-one case, also describe the regime of
validity of the rank-n theories.
4.3.1 Matter free limit: The photon
To discuss the normal modes of the rank-n theory in the absence of both species of matter, we
will find it convenient to introduce a new notation for the components of symmetric tensors
. Since the tensor is fully symmetric by assumption, each component
such as the electric field EIn
≡ E{ni
of the field can be indexed by the number of x’s, y’s and z’s that it contains, i.e., Enx ,n y ,nz
}
→ E2,1,0 in the
with ni
simplified notation. Taking the time derivative of Ampère’s law (67d) and making use of
Faraday’s law to replace ∂
t B, we find the following equation of motion for the rank-n E field,
having oriented the wave vector parallel to ˆz
= n, where d = 3 throughout. For instance, Ex x y
≥ 0 and (cid:80)d
i=1 ni
ω2E{ni
}
=
c2k2
z
n2
(cid:148)(nx
+ n y
+ 2nx n y
)E{ni
}
− n y
(n y
− 1)Enx
+2,n y
−2,nz
− nx
(nx
− 1)Enx
−2,n y
+2,nz
(cid:151)
.
(69)
+2,mz
+2,m y ,mz
+ Emx ,m y
+ Emx ,m y ,mz
It can be verified explicitly that the equation of motion preserves the tracelessness constraint
+ 2 = n), as it must. While it
= 0 (with mx
Emx
may appear from (69) that sectors with fixed nz are not coupled by the dynamics, this is not the
case once the tracelessness constraints are taken into account. For the case n = 3, the theory
has six ballistically propagating modes. The rank-three tensor has d n = 27 components, of
which only (cid:0)d+n−1
(cid:1) = 10 are independent by symmetry, and a further (cid:0)d+n−3
(cid:1) = 3 are removed
n−2
by the tracelessness constraint. This leaves us with the following seven modes
+ m y
+ mz
+2
n
ω = c
3
×
kz
= −4Ex x y , Exzz
= −E y yz ,
1 E yzz
2 Ex yz, Ex xz
3 Ex y y , Ex x y ,
0 Ezzz ,
= −4Ex y y ,
(70)
where the values of the field components Ex x x , E y y y and Ezzz are determined by the traceless-
ness constraints. The longitudinal mode Ezzz is then removed by Gauss’ law, leaving the six
modes that are not three-fold parallel to ˆz. More generally, rank-n traceless symmetric tensors
in d = 3 possess 2n + 1 independent components, giving rise to 2n dynamical modes and one
nondecaying mode E0,0,n. Eliminating this nondynamical mode using Gauss’s law, there are
2n dynamical modes grouped into n two-fold degenerate branches with speeds in the range
[c/n, c] with dispersion relations ω = ckz m/n for m = 1, 2, . . . , n .
4.3.2 The Ohmic regime: Magnetic diffusion
We now permit nonzero electric charge density and current while maintaining a vanishing
density of magnetic charges, ρ(m)
. In this limit, Faraday’s law (67c) may be interpreted as a
continuity equation for the rank-n locally conserved density ρ
. Specifically,
= BIn
In
∂
t B + (cid:101)∇ × E = 0 ,
(71)
t
In
ρ
+ ∂
= 0 with a rank-(n + 1) current. On the other hand, the
may be written ∂
. Following
continuity equation for the electric field is sourced by a nonvanishing current J
the prescription of quasihydrodynamics, the exact conservation of E is broken by introducing a
time scale τ
JIn,in+1
in+1
(e)
E .
(72)
∂
t E + (cid:101)∇ × B = − 1
τ
24
SciPost Phys. 14, 029 (2023)
= σ
(e)
In
eEIn
with a scalar conductivity σ
The time scale, τ, characterizes the decay of the n-fold longitudinal component of the electric
field (i.e., the electric charge density). As explained in detail in Sec. 2.2.2, this procedure can
alternatively be thought of as imposing an Ohm’s law relationship between the current and
the field that drives it; in this case J
e, which describes
the system’s linear response at sufficiently long length and time scales. By analogy with the
discussion above Eq. (37), this is the most general form of the conductivity permitted by SO(3)
symmetry: any rank-2n SO(3)-invariant tensor is expressible in terms of products of δ
i j. All
contributions from δ
vanish by virtue of tracelessness
. A nonvanishing contribution therefore has i associated with J and j contracted with E
of EIn
), for some
(or vice versa), for all terms in the product, giving rise to a contribution J
}; since EIn
permutation π of the indices {i1, . . . , in
∝ EIn
.
≪ 1, we drop the time derivative in (72) and substitute
At sufficiently long times, when τ∂
the resulting relationship between E and B fields into Faraday’s law (67c). This leads to an
equation of motion analogous to (69). Defining D = τc2 in accordance with (15)
is totally symmetric, we obtain J (e)
In
i j with both i and j contracted with EIn
(e) ⊃ Eπ(In
t
iωB{ni
}
=
Dk2
z
n2
(cid:148)(nx
+ n y
+ 2nx n y
)B{ni
}
− n y
(n y
− 1)Bnx
+2,n y
−2,nz
− nx
(nx
− 1)Bnx
−2,n y
+2,nz
(cid:151)
,
(73)
=0
where the B field satisfies the tracelessness constraints Bmx
+ 2 = n). The mode structure mirrors that of the matter free case. For
(with mx
instance, for the n = 3 theory there are six dynamical modes, while the three-fold longitudinal
mode is unable to decay
+Bmx ,m y ,mz
+Bmx ,m y
+2,m y ,mz
+ m y
+ mz
+2,mz
+2
ω = − i
9
Dk2
z
×
= −4Bx x y , Bxzz
= −B y yz ,
1 B yzz
4 Bx yz, Bx xz
9 Bx y y , Bx x y ,
0 Bzzz ,
= −4Bx y y ,
(74)
where the values of the field components Bx x x , B y y y and Bzzz are determined by the traceless-
ness constraints. The longitudinal mode Bzzz is then removed by Gauss’ law, leaving the six
= Dm2/n2 for m = 1, . . . , n, with each branch
transverse modes, with diffusion constants Dm
doubly degenerate. This normal mode structure also generalizes to n > 3.
In the presence of magnetic charge, the regime of validity of (74), i.e., the length and time
scales over which magnetic diffusion occurs, is identical to the rank-one and rank-two theories,
presented in Secs. 2.2.5 and 3.2.3, respectively.
4.4 One-form symmetries
In the absence of magnetic currents, Faraday’s law (67c) can be recast as a continuity equation
for the conserved density ρ
= BIn
In
Following the Secs. 2.3 and 3.3, we consider the putatively conserved quantity
∂
t
ρ + (cid:101)∇ × J = 0 .
Q[ f ] ≡
(cid:90)
(cid:82)3
d3 x fIn
ρ
In
,
(75)
(76)
can be chosen to be traceless and symmetric. In order for Q[ f ] to be conserved, i.e.,
where fIn
˙Q[ f ] = 0, the tensor-valued fIn
satisfies
ε
mk(i1
|
∂
k fm|i2...in
)
= 0 ,
25
(77)
SciPost Phys. 14, 029 (2023)
which follows from integrating (76) by parts and utilizing the symmetry properties of the
JIn
, which is also traceless and symmetric. The parentheses indicate symmetrization over the
), where π is a permutation belonging
surrounded indices, i.e., T(i1,...,in
to the symmetric group Sn. The vertical bars denote indices that are to be excluded from the
symmetrization procedure. We have omitted terms that vanish due to the assumed symmetry of
). Accounting for the tracelessness of f , the appropriate solution to the above is
fi1,...,in
= f(i1,...,in
Tπ(i1,...,in
) = 1
n!
π∈Sn
(cid:80)
fIn
= ∂
i1
· · · ∂
in
Φ −
(cid:1)
(cid:0)n
2
2n − 1
δ
∂
i3
(i1,i2
· · · ∂
)
in
∂ 2Φ +
(cid:1)
3(cid:0)n
4
(2n − 1)(2n − 3)
δ
δ
∂
i5
i3,i4
(i1,i2
· · · ∂
)
in
∂ 4Φ + . . . ,
(78)
where the second and third terms11 on the right-hand side progressively remove the trace part
of f . Taking the curl on any of fIn
i jk. Choosing the
same indicator functions as, e.g., (49), we identify the conserved quantities as
’s indices vanishes by antisymmetry of ε
= −
QS
(cid:90)
(cid:82)3
d3 x ρ
∂
i2
· · · ∂
δ[x ∈ S]ˆni1
in
i1,...,in
= (−1)n
(cid:90)
S
dS ∂
· · · ∂
ρ
in
i2
i1,...,in
ˆni1
,
(79)
ρ
where S = ∂ V for some volume V . That is, the flux of the object (cid:101)
through
any closed or semi-infinite surface is conserved by the rank-n continuity equation (75). Hence,
in systems whose charge and current are both symmetric traceless tensors of the same rank
[in the (2n + 1)-dimensional irrep of SO(3)], there is an effective one-form symmetry whose
· · · ∂
ρ
. This explains the presence of the nondecaying mode
charge is given by (cid:101)
in (70), since for a surface Σ, the rank-n theory conserves
i1,...,in
i1,...,in
· · · ∂
≡ ∂
≡ ∂
ρ
ρ
in
in
i1
i1
i2
i2
QΣ = ρ
i1,i2,...,in
(t)(ki2
· · · kin
)
(cid:90)
dS ni1
Σ
eik·x = ρ
(t)kn−1
z
iz···z
(cid:90)
Σ
dS ni eikz z .
(80)
Taking the surface Σ to be the x y plane, (80) evaluates to Q ∝ ρ
zz···z
is unable to decay in time. On the other hand, taking Σ to be the yz or z x planes places no
yz···z, since (80) evaluates to Q = 0. Furthermore,
constraints on the components ρ
the one-form symmetry does not constrain any components of ρ orthogonal to the projector
ˆki2
, where ˆk is the unit vector in the direction of k.
zz···z, implying that ρ
xz···z and ρ
· · · ˆkin
4.5 Conditions leading to particular higher-rank theories
This origin of the one-form symmetry may alternatively be seen by taking n − 1 derivatives of
· · · ∂
the continuity equation, defining (cid:101)Ji1
Ji1,i2,...,in
≡ 1
n
∂
in
i2
∂
ρ
t (cid:101)
i
+ ε
ikℓ∂
k (cid:101)Jℓ = 0 .
(81)
Hence, common to all systems obeying generalized Maxwell’s equations of the form (67) is
ρ
. In order to proceed in
a one-form symmetry of the conserved density (cid:101)
the reverse direction, i.e., from the equation for a one-form symmetry (81) to the continuity
equation (75) for the object ρ that transforms in the (2n + 1)-dimensional irrep of SO(3), we
ρ
must impose supplementary constraints on (cid:101)
(cid:90)
i. First,
i1,...,in
· · · ∂
≡ ∂
ρ
in
i2
i1
ρ
d3 x f (cid:101)
i
= 0 ,
where
∂
· · · ∂
i1
in−1
f = 0 ,
(82)
i. That is, multipole moments up to and including order n − 2
ρ
for sufficiently well behaved (cid:101)
must strictly vanish (for the rank-two case (54), this reduces to total charge, while for the
11The terms included in Eq. (78) are sufficient to remove the trace part for n < 6. Including the second term only
is sufficient for n < 4.
26
ρ
rank-one case there are no supplementary constraints on (cid:101)
condition on ρ
maps to
i1,i2,...,in
SciPost Phys. 14, 029 (2023)
i). Meanwhile, the tracelessness
(cid:90)
ρ
d3 x (cid:101)
i xi
(xi3
· · · xin
) = 0 .
(83)
The above represents (cid:0)d+n−3
(cid:1) independent constraints, which equals the number of independent
n−2
components in the trace. The constraints that enforce symmetry, on the other hand, are given
by
(cid:90)
d3 x ε
ρ
k jℓ (cid:101)
j xℓ(xi3
· · · xin
) = 0 .
(84)
)
i2
i1
i1
i2
in
in
ρ
ρ
= ∂
= ∂
= ρ
· · · ∂
· · · ∂
i1,...,in
j(k,i3,...,in
, we can assume that ρ
ρ
. The constraints on the various moments of (cid:101)
That the constraints (82) to (84) are sufficient to “canonically” determine the rank-n
continuity equation can be argued as follows. First, note that we can always write (nonuniquely)
ρ
i in (82) can be satisfied by
(cid:101)
i1,...,in
introducing the higher rank object ρ
that is well-behaved at infinity by direct analogy with
the arguments presented for the rank-two case in Sec. 3.3. Next, we make use of the symmetry
ρ
constraints (84). In writing (cid:101)
i1,(i2,...,in
)
i1,...,in
due to the commutativity of derivatives. Integrating (84) by parts n − 1 times gives us that
ρ
), which implies that the tensor is fully symmetric, up to higher derivative
corrections, the possibility of which we ignore. Tracelessness of ρ
then follows from
i1,...,in
= 0. Akin to the manipulations in Eq. (60),
integrating (83) by parts n − 1 times, i.e., ρ
i,i,i3,...,in
the corresponding constraints placed on the current (cid:101)Ji are found by taking the time derivative
of (84), the precise details of which are deferred to Appendix B. There, we discuss carefully
the full reconstruction of the continuity equation (75) in the specific setting of a rank-three
theory. The key steps are as follows: (i) the constraints on (cid:101)Ji motivate the introduction of the
restricts the continuity equation to be of the form (75),
rank-n current JIn
but JIn
to be fully
symmetric and traceless.
needn’t be symmetric or traceless; (iii) tracelessness of ρ
; (ii) symmetry of ρ
then restricts JIn
k( j,i3,...,in
i1,i2,...,in
= ρ
In
In
5 Magnetic subdiffusion
We now consider a higher-rank theory that exhibits subdiffusion of magnetic field lines, the
“traceful vector charge theory” of Ref. [32], in which the electric and magnetic charge (mono-
pole) densities are vector valued.
5.1 Maxwell’s equations with vector charge
The rank-two Maxwell’s equations for symmetric tensor E and B fields and vector densities for
the matter content are given by
∂
∂
∂
∂
j Ei j
j Bi j
t Bi j
t Ei j
,
ρ(e)
i
= 1
ϵ
= µ ρ(m)
,
i
= ε
ikℓε
= − 1
µϵ
∂
∂
k
mEℓn
jmn
− µ J
ε
ikℓε
∂
∂
k
mBℓn
jmn
(m)
i j
,
− 1
ϵ
J
(85a)
(85b)
(85c)
(85d)
(e)
i j
.
Unlike sections 3 and 4, the tensor fields Ei j and Bi j now transform in a reducible representation
of SO(3), 5 ⊕ 1. Continuity equations for the electric and magnetic charges can be recovered
27
SciPost Phys. 14, 029 (2023)
by taking the divergence on one index of Faraday’s (85c) and Ampère’s (85d) laws, respectively.
The continuity equations are given by
∂
t
ρ(e/m)
i
+ ∂
(e/m)
i j
j J
= 0 .
(86)
5.2 Hydrodynamic interpretation
Like the traceless scalar charge theory and rank-one electromagnetism, Maxwell’s equations
can be interpreted as continuity equations for the rank-two electric and magnetic fields, Ei j
and Bi j, in the absence of their corresponding matter.
We begin by considering the matter-free limit. In the absence of electric and magnetic
matter, both Ei j and Bi j are conserved densities. The fields obey wavelike equations, which
derive straightforwardly using the same machinery employed in previous sections, and take
the form
∂ 2
t Ei j
= ˜c2 (cid:128)−∂
∂
∂
∂
j
k
i
mEkm
+ ∂ 2∂
∂
k
j Eki
+ ∂ 2∂
∂
i Ek j
k
− ∂ 4Ei j
,
(87)
(cid:138)
for Ei j, and the equation of motion for Bi j takes the same form due to electromagnetic du-
ality. We have defined µ ϵ ˜c2 = 1, although it should be noted that—in contrast to previous
sections—(µϵ)−1/2 (and hence ˜c) no longer has the dimensions of a speed.
For wavevector k oriented in the ˆz direction, the normal modes are given by
ω = ˜c k2
z
×
(cid:168)
0 Ezi ,
1 Ex x , E y y , Ex y ,
(88)
corresponding to three quadratically dispersing modes. This is to be expected given that the
symmetric tensor, Ei j, has six independent degrees of freedom, with three components removed
= 0 , which
by the Gauss’s law constraints (85a) and (85b). In fact, Gauss’s law constrains kz Ezi
freezes the modes Ezi .
Next we consider how the hydrodynamic description is altered in the presence of elec-
tric charges (with all vector components). The effect of, say, electric matter is to break the
conservation law associated to Ei j while preserving the conservation law associated to Bi j.
Quasihydrodynamics dictates that the conservation law for Ei j is should be broken in the most
general manner permitted by symmetry constraints
∂
t Ei j
+ ˜c2ε
ikℓε
∂
∂
k
mBℓn
jmn
= − 1
3τ
1
δ
i j Ekk
− 1
τ
5
(cid:129)
Ei j
− 1
3
(cid:139)
,
δ
i j Ekk
(89)
1 and τ
where τ
5 are phenomenological parameters characterizing the decay rate of the trace
part and the traceless symmetric part of Ei j, respectively. The right-hand side of Eq. (89)
represents the most general structure permitted by SO(3) rotational invariance; the fact that
Ei j transforms in a reducible representation of SO(3) implies that the “electrical condictivity” is
no longer characterized by a single time scale in general (as was the case in all prior sections).
Specifically, as noted above Eq. (37), SO(3) symmetry forces the electrical conductivity to
be of the form σ
jk, which for Ei j belonging to the reducible
kℓ
5 ⊕ 1 representation gives ϵτ−1
≪ 1 and
1
τ
5
+ βδ
δ
+ γδ
iℓ
jℓ
= 3α and ϵτ−1
= β + γ. In the long time limit, τ
1
5
≪ 1, substituting (89) into (85c) gives
i jkℓ = αδ
δ
δ
∂
∂
ik
i j
t
t
∂
t Bi j
= −τ
5˜c2 (cid:128)−∂
∂
∂
∂
j
k
i
mBkm
+ ∂ 2∂
∂
k
j Bki
+ ∂ 2∂
∂
i Bk j
k
(cid:138)
− ∂ 4Bi j
(cid:138) (cid:0)δ
∂
i
j
∂ 2 − ∂
∂ 2 − ∂
∂
k
m
(cid:1) Bkm ,
km
(90)
+ 1
3
(τ
5
− τ
1
)˜c2 (cid:128)δ
i j
28
and the quasinormal modes for a wavevector, k, oriented in the ˆz direction are
SciPost Phys. 14, 029 (2023)
ω = −i ˜D k4
z
×
0
1
1
3
(cid:128)
1 + 2τ
1
τ
5
Bz j ,
Bx y , Bx x
Bx x
= B y y
= −B y y ,
(91)
(cid:138)
5 ˜c2. In the special case τ
1
with ˜D = τ
5, the normal mode structure mirrors that of the
matter-free case, but with subdiffusing—rather than propagating—modes. When the two time
scales differ, τ
5, the quasinormal mode corresponding to the trace part of Bi j decays with
1
a different rate. In the presence of a Gauss law constraint, the three nondecaying modes, Bz j,
are removed.
= τ
̸= τ
Note that in the presence of magnetic charge, the regime of validity of (91), i.e., the length
and time scales over which magnetic subdiffusion occurs, is determined by analogy to previous
sections, with modifications due to the higher-order nature of the hydrodynamic equations of
motion.
5.3 One-form symmetries
The continuity equation for the rank-two magnetic field takes the general form
ρ
∂
t
i j
+ ∂
∂
k
mJi jkm
= 0 ,
(92)
where both the rank-two charge, ρ
jmnJℓn transform
as the reducible representation 1 ⊕ 5 of SO(3), corresponding to symmetric but not traceless
rank-two tensors. The conserved quantities associated to this continuity equation are of the
form
i j, and rank-four current, Ji jkm
= ε
ikℓ
ε
where the symmetric tensor fi j satisfies
(cid:82)3
(cid:90)
Q[ f ] ≡
d3 x fi j
ρ
i j ,
ε
ikℓ∂
k
ε
∂
m fi j
jmn
= 0 .
(93)
(94)
Note the similarity to (43), which has the curl acting only on a single tensor index of fi j. Here,
we obtain an infinite family of solutions that decay sufficiently quickly as |x | → ∞ of the form
fi j
(x ) = ∂
Ψ
j
i
+ ∂
Ψ
i ,
j
(95)
i
(x ) are arbitrary vector-valued functions. As in Sec. 3.3, the absence of additional solu-
where Ψ
tions that decay sufficiently quickly as |x | → ∞ can be justified using the “scalar-vector-tensor”
⊂ fi j, can be written in terms of
decomposition of fi j. Recall that the tensor contribution, f T
) f bd , up to the usual ambiguity in Helmholtz decompo-
the tensor k4Ti j
sitions, which arises from additional contributions that are projected out when reconstructing
(k) = 0, leaving only the
fi j according to Eq. (48). Equation (94) therefore demands that Ti j
contributions from the “scalar” and “vector” terms, f
space, the general solution therefore assumes the form
∥
i j, respectively. In momentum
⊥
i j and f
(k) = −(ε
iabka
jcd kc
)(ε
i j
(k) = −ki k j
Φ + (ε
fi j
iabka vb
parametrized in terms of the scalar Φ and the vector vi. Note that the scalar term differs
from (48) since fi j is not necessarily traceless. Equation (96) can be rewritten as
ja bka vb
(96)
)k j
+ ki
(ε
) ,
(k) = (cid:2)ε
fi j
iabka vb
− 1
2 ki
Φ(cid:3) k j
+ ki
(cid:148)ε
ja bka vb
− 1
2 k j
Φ(cid:151)
,
(97)
29
SciPost Phys. 14, 029 (2023)
where the expression in the square brackets is identified as the Helmholtz decomposition of a
vector Ψ
Φ. We therefore recover the solution anticipated in Eq. (95),
parametrized by the arbitrary vector field Ψ
i, i.e., Ψ
iabka vb
(x ).
2 ki
= ε
− 1
i
i
To shed light on the conservation laws implied by the solution (95), we make use of the
vector-valued indicator functions
Ψ
(x ) = δ
×
ik
i;k,V
(cid:168)
1 x ∈ V ,
0 x /∈ V ,
which lead to the three-component conserved quantity
(cid:90)
Qi
[S] =
dS ρ
i j ˆn j ,
(98)
(99)
S
for each choice of surface S = ∂ V . Since there are three conserved quantities associated with
each surface S, the continuity equation effectively describes three one-form symmetries.
However, the three one-form symmetries are not completely independent, as the conserved
charges satisfy nontrivial constraints. Consider the conserved quantities
(cid:90)
Qi
(x j
) =
ρ
i j dxkdxℓ ,
for j ̸= k ̸= ℓ .
(100)
These conserved quantities are a particular case of (99) where the surface, S, is taken to be the
xk xℓ plane at a given x j. The constraint that Qi
(cid:90)
) must satisfy is
(x j
(cid:90)
dx j Qi
(x j
) =
dxi Q j
(xi
) ,
(101)
where there is no sum on the repeated indices. The constraint (101) arises from the fact that the
LHS is equal to (cid:82) d3 x ρ
ji. Equality follows from symmetry
of the conserved density ρ
i j while the RHS is equal to (cid:82) d3 x ρ
i j.
We now argue that the converse is true —that is, any theory hosting three independent
one-form symmetries subject to the constraint (101) is necessarily the vector charge theory.
We label our three conserved densities by ρ(a)
, where a ∈ {1, 2, 3} is (for the moment) a flavor
index labeling the conserved densities and i is the usual spatial index. For each a, the charges
satisfy the continuity equation for a one-form symmetry:
i
∂
ρ(a)
i
t
+ ε
ikℓ∂
(a)
kJ
ℓ
= 0 .
(102)
To each plane perpendicular to a coordinate direction xi we associate three conserved charges
(cid:90)
Q(a)(xi
) ≡
ρ(a)
i dx jdxk ,
for i ̸= j ̸= k .
(103)
labeled by the flavor index a. To these conserved charges we impose the constraint
(cid:90)
dxi Q(a)(xi
) =
(cid:90)
dxa Q(i)(xa
) ,
(104)
which is the analogue of (101). Note that the constraint requires that the flavor index a be
identified with a spatial index that can be used to label the coordinates, so we will neglect the
parentheses henceforth. Expanding and rearranging the constraint (104) yields
(cid:90)
d3 x (cid:2)ρa
i
− ρi
a
(cid:3) = 0 .
30
(105)
SciPost Phys. 14, 029 (2023)
We conclude that the antisymmetric part of ρa
i identically vanishes or is the divergence of a
higher-rank object. For simplicity we assume the former; the latter reduces to the former at
long wavelengths. There is hence no reason to distinguish between “raised” and “lowered”
indices, and we rewrite ρa
ai is symmetric leads to a constraint on
i
Jaℓ: It must be of the form ε
mJnℓ so that the second term of (102) is symmetric. We then
amn
recover the continuity equation
ai. The constraint that ρ
∂
→ ρ
ρ
∂
t
ia
+ (ε
ikℓ∂
k
)(ε
∂
)Jℓn
m
amn
= 0 .
(106)
This completes the understanding of the features of the subdiffusive normal modes in (91):
The mode structure arises from the three one-form symmetries, while subdiffusion arises from
the constraint (101), which forces a second derivative in the continuity equation for ρ
i j.
6 Conclusion
We have presented a hydrodynamic formulation capable of dealing with gauged multipolar
symmetries, such as are expected to arise in fracton phases of matter. The formulation we
have presented is a natural generalization of the treatments of ordinary (Maxwell) magne-
tohydrodynamics based on higher-form symmetries. This (somewhat abstract) formulation
has the advantage of not being limited in validity to the weak-coupling regime, unlike more
semi-microscopic approaches where one simply couples a fracton fluid (as developed in, e.g.,
Ref. [14]) to a higher-rank gauge theory. Instead, we have argued that “fracton magneto-
hydrodynamics” is best understood in terms of one-form symmetries, just like conventional
magnetohydrodynamics [48], and the hydrodynamic objects are (linelike) symmetry charges
of this one-form symmetry, which may be viewed as “generalized magnetic flux lines.”
One surprising feature is that the “higher rank” fractonic gauge theories generically exhibit
diffusion of magnetic flux lines, in contrast to the subdiffusion of charge that is seen in theories
with global multipolar symmetry. To gain intuition for this absence of subdiffusion, it is helpful to
recall that whereas charge diffuses in a theory with a global U(1) symmetry, once the symmetry
gets gauged, the charge relaxes exponentially, being driven by long-range interactions carried
by the gauge fields. Similarly, the “subdiffusion of charge” obtained in theories with global
multipolar symmetry generically gives way to exponential relaxation when the symmetry is
gauged. The hydrodynamic modes involve not the charges, but rather the flux lines, and these
generically relax diffusively. Nevertheless, theories with subdiffusion of magnetic field lines
can also be accessed, and we have provided a specific example thereof.
The theories we present describe the generic long-time description of the quantum dynamics
of fractonic phases (at nonzero charge density) exhibiting gauged — as opposed to global
— multipolar symmetries (as relevant to spin liquids and fracton phases). We develop a
symmetry-based approach describing arbitrary higher-rank theories of this type, and showcase
the subdiffusion of magnetic fields as an example of the exotic universal dynamics that may
arise in this context.
One obvious direction for the future is to aim to apply this formalism to emerging experi-
ments on quantum spin liquids, both conventional and fractonic. Any such program would
need to be driven by experiment, so we do not discuss it further in this (theoretically focused)
manuscript. However, there are also important conceptual points of principle that should
be cleaned up in future work. For example, how can the effects of momentum conserva-
tion be incorporated into our hydrodynamic framework? Formally, this necessitates coupling
the higher-rank gauge theory to curved spacetime, which was done for MHD in Ref. [48].
However, this leads to technical challenges associated to the fact that defining moments of
charge on curved spacetime is difficult [72]. Additionally, we have restricted ourselves in
31
SciPost Phys. 14, 029 (2023)
this manuscript to systems in three spatial dimensions. Are there surprises if one changes
the dimension? Furthermore, the theories we have herein considered involve gauged U(1)
symmetries. However, going from such theories to the kinds of lattice models beloved of
the quantum information community requires a sequence of Higgs and partial confinement
transitions [73]. What happens to the hydrodynamic theory as we go through these transitions?
Or again: thus far we have considered gauge theories of Abelian fractons. What if we move
to non-Abelian generalizations? It was argued in [74] that imposing non-Abelian multipolar
global symmetries would totally trivialize the dynamics, but the same need not be true of gauged
non-Abelian symmetries, and the magnetohydrodynamics of non-Abelian fractonic systems
could be a particularly fruitful problem for future work, extending the literature on non-Abelian
versions of magnetohydrodynamics [75–78].
Acknowledgements
MQ was supported by the National Defense Science and Engineering Graduate Fellowship
(NDSEG) program. AL was supported by the Gordon and Betty Moore Foundation’s EPiQS
Initiative via Grant GBMF10279, by the National Science Foundation via CAREER Grant DMR-
2145544, and by a Research Fellowship from the Alfred P. Sloan Foundation under Grant
FG-2020-13795. OH and RN were supported by the U.S. Department of Energy, Office of
Science, Basic Energy Sciences, under Award #DE-SC0021346. AJF was supported in part by a
Simons Investigator Award via Leo Radzihovsky.
A Charge and current belonging to different irreps
A.1 Two-form symmetry: Irrep 1
i belonging to the 3 of SO(3), but whose
Consider a theory that contains a vector charge ρ
associated current Ji j transforms instead in the 1, i.e., the trivial or scalar irrep of SO(3). In
this case the current may be parametrized in terms of the scalar J:
Ji j
= J δ
i j ,
and the continuity equation for vector charge density becomes
ρ
∂
t
i
+ ∂
i J = 0 .
(A.1)
(A.2)
As in the main text, we define the putatively conserved quantity Q[ fi
] ≡ (cid:82) d3 x fi
fields fi
following constraints on the field fi
] parametrized by vector
i . Using the continuity equation (A.2 ), we obtain the
(x ) , i.e., Q[ fi
ρ
(cid:90)
d
dt
Q[ fi
] =
d3 x fi
∂
t
ρ
i
= −
(cid:90)
(cid:90)
d3 x fi
∂
i J =
d3 x J ∂
i fi ,
(A.3)
where in the final equality we have integrated by parts. We then find that, so long as fi is
divergence-free,
∂
i fi
= 0 ,
(A.4)
we are guaranteed that Q[ fi
] is a conserved quantity of the theory.
As is well known, there are infinitely many solutions to (A.4 ), which may be expressed by
k j , in which case (A.4 ) amounts
jk using an antisymmetric tensor Ω
writing fi
= −Ω
= ε
Ω
i jk
jk
32
SciPost Phys. 14, 029 (2023)
to the statement that the two-form, Ω, is “closed,” i.e., dΩ = 0 . Up to topological effects (which
we do not consider here), this implies that Ω = dα is exact, or that
fi
= ε
α
∂
j
k ,
i jk
(A.5)
for any function α
k (i.e., the vector field fi is the curl of some vector-valued function).
In a similar manner to the indicator functions chosen to elucidate the conservation of flux
through surfaces in, e.g., Sec. 2.3 of the main text, here it is instructive to choose a particular
form for the functions α
(x ). Let γ denote a curve (closed, or infinite in extent) in (cid:82)3, and
consider
k
α
k
(x ) ≡
(cid:90)
ε
γ
− x j
)
i jk d yi
( y j
|y − x |3
,
(A.6)
where y denotes the line integral along γ and x denotes an arbitrary point in (cid:82)3. Using the
textbook Biot–Savart law, we see that
(x ) =
fi
(cid:90)
γ
d y δ3 (x − y) ni
(y) ,
(A.7)
where n(y) is the unit vector along γ at point y. We can write this more elegantly: The
conserved charges are generated by the different curves γ and using the notation that ρ is a
one-form,
(cid:90)
Qγ =
ρ ,
(A.8)
is a conserved quantity. For each curve γ there exists a corresponding conserved charge, Qγ.
γ
A.2 Scale- and rotation-invariant hydrodynamics: Irrep 5
Next, suppose Ji j transforms in the 5 of SO(3), corresponding to traceless symmetric tensors,
which may be parametrized by explicitly removing the antisymmetric and trace parts of a
generic rank-two current tensor
(cid:149)
∂
ρ
∂
t
i
+
j Ji j
+ ∂
j J ji
(cid:152)
− 2
3
∂
i J j j
= 0 ,
(A.9)
where the term in the square brackets is manifestly symmetric and traceless. Looking for
conserved quantities Q[ fi
(x ) and repeating the
same logic as before, we find that
] parametrized by vector-valued functions fi
∂
i f j
+ ∂
j fi
− 2
3
δ
∂
k fk
i j
= 0
(A.10)
in order for Q[ fi
to (A.10 ), given explicitly by
] to be conserved. In three spatial dimensions, there is a finite list of solutions
fi
= α xi
+ ε
i jk x j
β
k ,
(A.11)
with α and β
k scalar and vector constants, respectively. Hence, (A.9 ) will generally only
lead to seven conserved quantities (four additional conservation laws from (A.11 ), and the
three original charges corresponding to fi
i is a conserved density). If we were to
interpret ρ
i as a velocity field, then the conserved quantities would correspond to momentum
(ρ
= 1, since ρ
i), angular momentum (ε
k ), and “dilatation” (xi
i jk x j
i ).
ρ
ρ
33
SciPost Phys. 14, 029 (2023)
A.3 Mixing and matching
In general, it’s possible that that the currents may not transform as a single irrep but instead as
a direct sum of different irreps. As an example, hydrodynamics with rotation invariance but
without scale invariance has a current Ji j that transforms in the 1 ⊕ 5 representation. In this
case, the current decomposes as
Ji j
= J δ
+ (cid:101)Ji j ,
i j
(A.12)
where J and (cid:101)Ji j transform in the 1 and 5 irreps, respectively. The continuity equation then
reads
(cid:149)
ρ
∂
t
i
+
∂
j Ji j
+ ∂
j J ji
− 2
3
∂
i J j j
(cid:152)
+ 1
3
∂
i J j j
= 0 .
(A.13)
The quantity Q[ fi
finite list of solutions in (A.11 ) is reduced to
] is conserved when both (A.4 ) and (A.10 ) are satisfied simultaneously. The
fi
= ε
i jk x j
β
k ,
(A.14)
ρ
so that the “dilatation” xi
i is no longer a conserved quantity. This is an example of the general
situation wherein the current decomposes into irreducible representations; if there exists a list
of conserved quantities associated to each irrep, then the set of conserved quantities is reduced
to the intersection of these lists when the current has nonzero overlap with multiple irreps.
B From constraints to the rank-n continuity equation
In this appendix we explicitly work through the steps that one may take to “canonically” derive
ρ
the rank-three continuity equation from the associated constraints on the density (cid:101)
i, which
obeys the equation of motion
ikℓ∂
associated with a one-form symmetry. The generalization to higher rank theories, n > 3, follows
an identical line of reasoning.
k (cid:101)Jℓ = 0
ρ
t (cid:101)
(81)
+ ε
∂
i
In Sec. 4.5, we described how the constraints (82) to (84) lead one to consider a symmetric,
ρ
traceless rank-n tensor ρ
, that is well behaved
i via (cid:101)
at infinity. We now proceed by considering the constraints placed on the effective current (cid:101)Ji
ρ
associated with the density (cid:101)
i. Specializing to rank-three and taking the time derivative of
Eq. (84), we find
ρ
, related to (cid:101)
i1,...,in
i1,...,in
· · · ∂
= ∂
ρ
in
i2
i1
(cid:90)
d
dt
d3 x ε
ρ
k jℓ (cid:101)
j xℓ xi
= −
(cid:90)
d3 x ε
k jℓε
jmn xℓ xi
∂
m (cid:101)Jn
= 0 .
(B.1)
kJi jk
Consequently, the current (cid:101)Ji is also constrained, and it is more natural to write (cid:101)Ji
in terms of a rank-three tensor that need only vanish at infinity. To see this, we write (B.1 ) in
terms of the new tensor Ji jk and integrate by parts:
j
= 1
3
∂
∂
(cid:90)
0 = 3
d3 x ε
k jℓε
jmn xℓ xi
∂
m (cid:101)Jn
=
(cid:90)
d3 x ε
k jℓε
jmn xℓ xi
∂
∂
∂
a
m
bJna b
=
(cid:90)
d3 x ε
k jℓε
∂
mJnℓi ,
jmn
(B.2)
where we have used the fact that Ji jk is symmetric in [at least] its last two indices. Contracting
the two Levi-Cevita symbols, we arrive at
(cid:90)
d3 x (cid:0)∂
ℓJkℓi
− ∂
kJℓℓi
(cid:1) = 0 .
(B.3)
34
SciPost Phys. 14, 029 (2023)
If the second term vanishes, then (B.3 ) implies that Ji jk that decay away sufficiently quickly as
|x | → ∞ will satisfy the constraint in Eq. (B.1 ). That the second term should vanish (since
Ji jk should be symmetric and traceless) is not immediately apparent; we show that this must
be the case below. Writing (81) in terms of the new rank-three degrees of freedom
(cid:129)
∂
ρ
t
i jk
∂
∂
j
k
+ 1
3
ε
∂
mJn jk
imn
(cid:139)
= 0 .
(B.4)
To derive an equation of motion for the higher rank object ρ
leading to
i jk, we integrate up Eq. (B.4 ),
ρ
∂
t
i jk
(cid:128)ε
+ 1
3
∂
mJn jk
+ ε
Λ
∂
m
nki
jmn
imn
+ ε
kmn
∂
m
Λ′
ni j
(cid:138) = 0 ,
(B.5)
∂
where last two terms on the right are the most general terms annihilated by the derivatives
∂
k compatible with the index structure in Eq. (B.4 ). We now require that the equations
j
must respect the symmetry and tracelessness of ρ, which imposes stringent constraints on the
permitted form of Λ and Λ′
. First, requiring that ρ
= ρ
i jk
jik, we find that
ε
∂
(Jn jk
m
imn
− Λ
) + ε
(Λ
∂
m
nki
− Jnik
jmn
) + ε
kmn
∂
m
nk j
(Λ′
ni j
− Λ′
n ji
) != 0 .
(B.6)
= Λ′
The final term on the left-hand side suggests that Λ′
n ji, up to terms that contain a higher
number of derivatives. A similar argument can be made for the other “integration constant”, Λ,
by requiring that ρ
to be symmetric in their last two
indices. Under this assumption, the first two terms in (B.6 ) can be satisfied, to lowest order in
derivatives, by Λ
i jk in its first and
last indices. Requiring that ρ remains traceless under time evolution leads to the requirement
that
k ji. We therefore take both Λ and Λ′
= Ji jk follows from symmetry of ρ
= Ji jk. Similarly, Λ′
= ρ
i jk
i jk
i jk
ni j
−∂
ρ
t
iik
= 1
3
(cid:0)2ε
∂
mJnik
imn
+ ε
kmn
∂
mJnii
(cid:1) != 0 ,
(B.7)
suggesting that we should take Ji jk to be symmetric in its first two indices (making it fully
symmetric when twinned with symmetry in its last two indices), and traceless. While a
fully symmetric and traceless J certainly satisfies (B.7 ), it turns out that this is not the only
choice. There exists one further solution in which the current transforms in the reducible 3 ⊕ 3
representation:
Ji jk
(cid:148)δ
= 2
3
λ
k
i j
+ δ
λ
j
ki
+ δ
λ
i
jk
(cid:151) +
(cid:128)δ
(cid:149) 1
3
λ
k
i j
+ δ
λ
ik
(cid:152)
(cid:138) − 2
3
j
δ
λ
i
jk
= δ
λ
k
i j
+ δ
λ
j ,
ik
(B.8)
(x ). While the existence of such a solution may appear to im-
parametrized by the vector field λ
ply that the rank-three continuity equation cannot be obtained from (81) and the corresponding
constraints alone, we note that
i
ε
∂
(δ
λ
k
n j
m
+ δ
λ
j
nk
imn
) + ε
∂
(δ
λ
k
ni
m
+ δ
λ
i
nk
jmn
) + ε
kmn
∂
(δ
λ
j
ni
m
+ δ
λ
i
n j
) = 0 ,
(B.9)
i.e., the solution (B.8 ) does not contribute to the equation of motion for ρ
[as was the case
for the trace part of Ji j below Eq. (63)], and can therefore be disregarded. This leaves us with
the equation of motion
i jk
ρ
∂
t
i jk
(cid:128)ε
+ 1
3
∂
mJn jk
+ ε
∂
mJnki
jmn
imn
+ ε
kmn
∂
mJni j
(cid:138) = 0 ,
(B.10)
where both ρ
i jk and Ji jk transform in the 7 of SO(3), i.e., they are symmetric traceless rank-
three tensors. This is precisely the form of the continuity equation in Eq. (75) of the main
text.
35
SciPost Phys. 14, 029 (2023)
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| null |
10.1111_eva.13529.pdf
| null |
DATA AVA I L A B I L I T Y S TAT E M E N T Sequence data are hosted at the SRA under BIOPROJECT SUB10359598. Plasmids, ancestor, and mutator clones are available on request. Evolved mutants can be shared subject to Material Transfer Agreements. Experimental data are available at Zenodo with a permanent doi: 10.5281/zenodo.7503995 .
|
Received: 15 July 2022 | Accepted: 27 December 2022
DOI: 10.1111/eva.13529
O R I G I N A L A R T I C L E
Selecting for infectivity across metapopulations can increase
virulence in the social microbe Bacillus thuringiensis
Tatiana Dimitriu1 | Wided Souissi2 | Peter Morwool1 | Alistair Darby3 |
Neil Crickmore2 | Ben Raymond1
1Centre for Ecology and Conservation,
University of Exeter, Penryn, UK
2School of Life Sciences, University of
Sussex, Brighton, UK
3Centre for Genomic Research, Institute
of Integrative Biology, University of
Liverpool, Liverpool, UK
Correspondence
Ben Raymond, Centre for Ecology and
Conservation, University of Exeter,
Penryn Campus, Treliever Road, Penryn
TR10 9FE, UK.
Email: [email protected]
Funding information
Biotechnology and Biological Sciences
Research Council, Grant/Award Number:
BB/S002928/1; Leverhulme Trust, Grant/
Award Number: RPG- 2014- 252
Abstract
Passage experiments that sequentially infect hosts with parasites have long been
used to manipulate virulence. However, for many invertebrate pathogens, passage
has been applied naively without a full theoretical understanding of how best to
select for increased virulence and this has led to very mixed results. Understanding
the evolution of virulence is complex because selection on parasites occurs across
multiple spatial scales with potentially different conflicts operating on parasites with
different life histories. For example, in social microbes, strong selection on replication
rate within hosts can lead to cheating and loss of virulence, because investment in
public goods virulence reduces replication rate. In this study, we tested how varying
mutation supply and selection for infectivity or pathogen yield (population size in
hosts) affected the evolution of virulence against resistant hosts in the specialist insect
pathogen Bacillus thuringiensis, aiming to optimize methods for strain improvement
against a difficult to kill insect target. We show that selection for infectivity using
competition between subpopulations in a metapopulation prevents social cheating,
acts to retain key virulence plasmids, and facilitates increased virulence. Increased
virulence was associated with reduced efficiency of sporulation, and possible loss of
function in putative regulatory genes but not with altered expression of the primary
virulence factors. Selection in a metapopulation provides a broadly applicable tool for
improving the efficacy of biocontrol agents. Moreover, a structured host population
can facilitate artificial selection on infectivity, while selection on life- history traits
such as faster replication or larger population sizes can reduce virulence in social
microbes.
K E Y W O R D S
Bacillus thuringiensis, directed evolution, evolution of virulence, mutators, public goods, social
evolution
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
© 2023 The Authors. Evolutionary Applications published by John Wiley & Sons Ltd.
Evolutionary Applications. 2023;16:705–720.
wileyonlinelibrary.com/journal/eva
| 705
706 |
1 | I NTRO D U C TI O N
have low fitness in clonal infections (Granato et al., 2018; Pollitt
et al., 2014; Rumbaugh et al., 2012). In addition, increased genetic
Passage, the repeated infection and re- isolation of a microbe in a
diversity within infections (low relatedness) will also tend to favor
host, has been used as a tool for the manipulation of parasite vir-
fast- replicating genotypes such as cheaters. These social biology
ulence for decades, as well as a means of testing evolution of vir-
concepts are relevant both for clinically important microbes and for
ulence theory (Ebert, 1998; Raymond & Erdos, 2022). Passage has
biocontrol agents. Experimental evolution with entomopathogenic
been used as a means of producing attenuated live vaccines: se-
nematodes, for instance, shows increasing opportunities for cheat-
quential infection of animal tissue cultures can lead to loss of viru-
ing can lead to attenuation and extinction and may explain historical
lence in human hosts and has been used to produce polio and yellow
problems with unstable virulence in laboratory culture of these par-
fever vaccines among others (Barrett, 2017; Sabin & Boulger, 1973).
asites (Shapiro- Ilan & Raymond, 2016).
Conversely, adaptation of viruses to a particular host via passage can
In this study, we aimed to apply social evolution theory to increase
lead to increased virulence (Ebert, 1998). Although there are com-
the virulence of the Gram- positive invertebrate pathogen, Bacillus
plex relationships between virulence and fitness in natural popula-
thuringiensis (Bt). Bt is a valuable model for testing novel passage
tions (Alizon et al., 2009; Frank, 1996; Gandon et al., 2001), artificial
regimes partly because its virulence factors are extremely well char-
inoculation means that many of the negative consequences of high
acterized (Adang et al., 2014) and because its social biology is well
virulence in terms of reducing opportunities for transmission will
studied (Cornforth et al., 2015; Raymond et al., 2012; van Leeuwen
not operate in laboratory conditions and so it may be possible to
et al., 2015; Zhou et al., 2014). Bt obligately requires pore- forming
increase the virulence of naturally occurring pathogens via artificial
toxins for infection. These are produced in the form of crystalline
selection.
bodies at sporulation in the cadaver but solubilized in the midgut
Increasing pathogen virulence via passage has long been a goal
after ingestion; the toxins disintegrate the midgut epithelium and
in biocontrol research as researchers have sought to manipulate the
allow these microbes access to the hemocoel (Adang et al., 2014). In
efficacy and/or host range of microbes that are potential biocontrol
some insect hosts, the production of quorum- regulated phospholi-
agents (Raymond & Erdos, 2022). Naïve passage designs, in which
pases and cellulases complements the action of the crystal toxins and
the aim is simple infection and re- infection, without any other se-
facilitates host invasion (Salamitou et al., 2000; Zhou et al., 2014). A
lection pressure or modifications, can increase the virulence of
range of other virulence factors, for example, vegetative insecticidal
baculoviruses in terms of reducing doses required to kill 50% of in-
proteins (Vips), are produced by most Bt genotypes, although their
sects (LD50; Berling et al., 2009; Kolodny- Hirsch & Van Beek, 1997;
Maleki- Milani, 1978). Simple passage of baculoviruses can increase
contribution to pathogenicity is less clear (Chakroun et al., 2016).
The entomocidal toxins of Bt have high selective potency against
virulence partly because of the high genetic diversity found in nat-
insect pests and this is the main reason why Bt dominates the mi-
ural populations (Shapiro et al., 1992; Thézé et al., 2014). However,
crobial biocontrol market and supplies the vast majority of insecti-
another reason naïve passage can be effective is if reproductive rate
cidal toxins for genetically modified crops (Bravo et al., 2011). There
within hosts is positively correlated with virulence, as is assumed by
is considerable interest in identifying mutants or proteins that can
many classical models of virulence (Alizon et al., 2009; Frank, 1996;
overcome host resistance in Bt (Badran et al., 2016). From a theo-
Gandon et al., 2001). Even in the simplest passage design, there
retical point of view, it may also make sense to target passage ex-
will be competition between genetic variants within hosts, and we
periments at hosts that are resistant but still vulnerable to some
would expect that this within- host competition would favor higher
level of infection. Based on fitness landscape theory, we expected
replication rate, ignoring issues such as antagonism and evasion or
that rapid, experimentally tractable evolution would be more likely
manipulation of immunity (Massey et al., 2004; Raberg et al., 2006).
in a pathogen of low fitness rather than one already at an adap-
This means that simple isolation and re- infection can select for the
tive peak (Poelwijk et al., 2007). We used a host species commonly
genotypes which grow faster within hosts which can produce the
targeted by Bt products: the diamondback moth Plutella xylostella,
net result of an increase in virulence without the need for any addi-
using a well- characterized genotype with a high level of resistance
tional selection pressure.
to B. thuringiensis kurstaki (Figure 1a). Moreover, for social microbes,
Nevertheless, research on the social biology of microorganisms
the selection pressure acting to maintain virulence does not come
emphasizes that increased replication rate does not necessarily lead
from within- host competition for rapid growth— it has to come from
to increased virulence (Buckling & Brockhurst, 2008; Raymond &
competition between populations in terms of total population size
Erdos, 2022). Many bacterial pathogens, for instance, invest con-
(yield; Griffin et al., 2004) or number of hosts infected (Raymond
siderable resources in producing virulence factors such as toxins
et al., 2012). This is because the fitness benefits (or public goods)
or siderophores that are important for infecting hosts or accessing
provided by cooperation in known social pathogens either increase
host resources such as iron (Diard, Garcia, et al., 2014; Raymond
the efficient use of host resource or, in the case of the crystal toxins,
et al., 2012; West & Buckling, 2003). Investing in these resources
provide access to host tissues.
can slow replication rates, this means that intense within- host com-
In this study, we conducted experimental evolution using cycles
petition can select for cheaters which can outcompete virulent
of Bt infection in resistant hosts alternating with spore production
genotypes in mixed infections, although cheaters are expected to
in vitro (Figure 1b). We tested passage regimes that would maximize
DIMITRIU et al.(a)
1
0.8
0.6
0.4
0.2
y
t
i
l
a
t
r
o
m
0
1
susceptible
resistant
(c)
(d)
Yield selection
Infectivity selection
| 707
100
dose (spores /
10000
l)
100000
Exclude low
mortality groups
(b)
inoculation
spore wash
early infection
ancestor
in vivo selection
in vitro
control
5 passage
rounds
spore wash
sporulation
+ antibiotic
Pool all
cadavers
High mortality
groups inoculate
two groups
F I G U R E 1 Experimental design to increase the virulence of Bacillus thuringiensis against resistant insects. (a) Bioassays using the Bt
ancestor 7.1.o show substantial differences in mortality between our focal Bt- resistant insect VL- FR and its near- isogenic Bt susceptible
counterpart VL- SS (F1,5 = 34.1, p = 0.0021). The LC50 of the susceptible insects was estimated at 7.4 CFU/μl (95% confidence limits 6.9–
7.9 CFU/μl), while the LC50 of the VL- FR resistant was 5 × 105 CFU/μl (Table S1). Selection experiments began with a dose that would kill
20%– 30% of VL- FR insects. (b) Selection cycles performed separately for strains with mutator or wild- type mutation rates. For in vivo
selection treatments, cells selected from killed larvae were inoculated into sporulation medium. In vitro growth, sporulation, and purification
steps were common to the in vivo selection and in vitro control. To prevent the invasion of cheaters not contributing to cooperative
virulence, two in vivo selection treatments were compared. (c) In the yield selection treatment, all cadavers from a replicate lineage were
pooled and inoculated together for spore production. (d) Under infectivity selection, half of the subpopulations, those causing the lowest
mortality, are terminated at the end of each round of selection, while spores from the remaining subpopulations are divided and used to
infect two subpopulations in the next round of infection.
selection based on yield (population size within hosts) or infectivity.
in terms of preserving population structure (Gardner & West, 2006;
Selecting for yield involved combining all cadavers from a replicate
Kümmerli et al., 2009). It should be emphasized that these selection
into a single inoculum pool at each passage, so that genotypes with
treatments are not exclusive or perfect; in other words, we cannot
higher yield are better represented in the next round of infections
prevent some selection for infectivity in the yield treatment (infec-
(Griffin et al., 2004; Shapiro- Ilan & Raymond, 2016; Figure 1c). This
tions have to happen) nor some selection for yield in the infectivity
type of between- host competition has previously produced modest
treatment (bacteria have to grow in cadavers), but we are attempt-
increases in virulence in Bt (Garbutt et al., 2011). Pooling subpop-
ing to maximize different types of selection with our experimental
ulations also mimic a high pathogen dispersal rate within natural
treatments.
populations (Kümmerli et al., 2009), which is plausible for Bacillus
Finally, we tested how increasing the mutation supply would
(Pearce et al., 2009). The second selection regime involved directly
affect virulence evolution, a common approach in directed evolu-
selecting for infectivity. To do this, we imposed competition be-
tion (Selifonova et al., 2001). Bacterial clones with elevated mu-
tween subpopulations within a metapopulation (each metapopu-
tation rates and defective proofreading genes, or mutators, are
lation constituting an independent replicate), so that only the 50%
prevalent in pathogenic species (Taddei et al., 1997), and we in-
most infectious subpopulations are used to initiate the next round
creased mutation supply by approximately 25- fold by carrying out
of infection (Figure 1d). The most infectious subpopulations are
selection in a mutator, made by disruption of the mutS gene, and
divided and initiate two new pathogen subpopulations in the next
in lineages using the wild- type ancestor. At the end of passage
round of infection. The infectivity selection treatment, therefore,
experiments, we bio- assayed for changes in virulence in evolved
uses a form of budding dispersal and so may have additional benefits
lineages (diverse independently evolved populations), as well as
DIMITRIU et al.708 |
clones isolated from those lineages. We also measured changes
in life history (spore production in vitro, competitive fitness) and
was calculated with fluctuation assays. Thirty- two independent
cultures per strain were inoculated into sporulation medium by 107-
tested whether social conflicts might have affected experimental
fold dilution from Luria Broth (LB) overnight cultures. After 24 h,
outcomes. Finally, in order to explore the possible mechanisms for
the observed increase in virulence in evolved clones, we explored
cultures were plated on LB agar (LBA) plates containing 100 μg/ml
rifampicin. Mutation rates and confidence intervals were calculated
if changes in the expression of known virulence factors could ex-
with the Ma- Sandri- Sarkar (MSS)- maximum likelihood method using
plain results and conducted whole genome sequencing on a selec-
FALCOR (Hall et al., 2009).
tion of evolved clones.
2 | M E TH O D S
2.1 | Insects and insect rearing
The Cry1Ac- resistant line “VL- FR” of the diamondback moth
Spores and crystals of Bacillus thuringiensis (Bt) for use in se-
lection experiments or bioassays were grown in sporulation media
(HCO; Lecadet et al., 1980) containing polymyxin (100 IU/ml) (Oxoid)
for 72 h at 30°C with shaking at 150 rpm. Selective antibiotics
(10 μg/ml Erm for wt; 5 μg/ml Chl for mut strain) was used in all cul-
tures except for the production of spores for bioassays which used
antibiotic free culture. When grown from insect cadavers, 30 μg/ml
Erm was used to inhibit growth of Gram- negative bacteria. Standard
P. xylostella used for experiments was previously derived from a
spore production conditions used 1 ml of HCO in 24- well growth
cross of line NOQA- GE with the diet adapted susceptible line Vero
plates. Rows of inoculated wells from the same line of selection were
Beach: This line was reared and selected for resistance to Cry1Ac as
alternated with empty rows to prevent and control for contamina-
described previously (Zhou et al., 2018) and can survive exposure
to ~104- fold higher doses of Bt compared to the susceptible
line (Figure 1a). We established a population that was fixed for
tion between lines. Initial cultures of ancestors at passage 1 used
100 ml cultures in 500 ml flasks. All other routine culture of Bt used
overnight growth in LB or on LBA plates at 30°C with antibiotic se-
resistance using a PCR screen of resistance alleles in the parental
lection unless otherwise stated.
population. An insect strain with similar genetic background (also
derived from a NOQA- GE X Vero Beach cross) was established from
F2 offspring but in this case fixed for susceptibility to Cry1Ac, this
2.3 | Passage experiment
line was denoted “VL- SS” (Zhou et al., 2018). Diamondback moth
larvae used for selection were reared on autoclaved artificial diet
Selection was performed with four replicate lineages per strain for
(Baxter et al., 2011) without supplementary antibiotics until third
each treatment combination, using a factorial design with two strains
instar. All insects were reared in a controlled temperature facility at
with diffferent mutation rates (wt and mut) and three treatments
the University of Exeter's Cornwall campus.
(yield, infectivity, and in vitro control), as detailed below. In total, five
2.2 | Bacteria, plasmids, strain construction, and
in vitro growth conditions
rounds of selection were performed.
For infection, sterile diet was cut into quarters in 55 ml dishes,
and each quarter was inoculated with 90 μl of spore suspension
containing 5 × 104 to 105 spores/μl (chosen to produce 25%– 30%
mortality over 2– 4 days for resistant diamondback moth, Figure 1a)
The ancestor Bt kurstaki strain was obtained by transforming the
and dried. Ten third- instar VL- FR larvae were then added per 55 ml
original Bt 7.1.o field isolate (Raymond et al., 2009) with the pHT315-
dish. Insect death was recorded daily from 2 days after infection.
gfp plasmid containing the ermB gene conferring erythromycin (Erm)
When total mortality reached 25%– 30%, the earliest cadavers
resistance (Zhou et al., 2014). The mutator (mut) strain was obtained
were transferred to microcentrifuge tubes with sterile tooth-
by inactivating the mutS gene in 7.1.o via disruption. The cat gene
picks (Figure 1b). After 2 days at room temperature, 0.85% NaCl
conferring chloramphenicol (Chl) resistance was digested from the
solution (saline) was added to the tubes. Cadavers were homog-
pAB2 plasmid using KpnI (Bravo et al., 1996); this fragment was
enized with pellet pestles, briefly vortexed and the suspension
inserted at position 1439 of the mutS- coding sequence. The sequence
was transferred to sporulation media (Figure 1b). When growth
between positions 601 and 1837 of mutS gene coding sequence
with the cat insert was synthesized in pUC57 from GenScript after
was complete, 200- μl spore aliquots were transferred to 96- well
PCR plates with aluminum sealing film for quantification, infec-
adding BamHI sites on both ends. This synthesized BamHI fragment
tion, and storage. Plates were centrifuged at 3000 g for 15 min,
was cloned into the thermosensitive vector pRN1501 (Lereclus
the supernatant was removed, and spores were resuspended in
et al., 1992). The resulting plasmid was introduced into Bt 7.1.o, and
clones with recombination of the interrupted mutS sequence into
100 μl 0.85% NaCl. Plates containing spores to be used in next
infection step were pasteurized (20 min, 65°C) and kept at 10°C
the chromosome were obtained after growth at 42°C by screening
until use, the others were stored at −20°C. Pasteurization pre-
for Chl resistance and loss of Erm resistance (Lereclus et al., 1992).
vents contamination from gut microbes and also selects against
Insertion was confirmed by PCR and sequencing of mutS locus. The
fast- growing asporogenic mutants. Spore density was estimated
mutation rate of the ancestor with wild- type mutS (wt) and mut strain
by measuring OD600 nm in a 96- well microplate reader of 10- fold
DIMITRIU et al. | 709
dilutions. A subset of the samples was plated at appropriate dilu-
Evolved lineages are genetically diverse populations, which can
tions, in order to visually check for contamination and calibrate
present challenges when repeating experiments or when attempt-
OD600 nm measurements.
ing to link genotypes to phenotypes. Clones were therefore isolated
Selection of cadavers and inoculation into sporulation medium
from lineage samples by streaking single colonies three times on LBA
depended on experimental treatment (Figure 1). The in vitro control
and we conducted bioassays on both clones and lineage after selec-
selection treatment culture and washing steps were performed as
tion was complete. Clone nomenclature in figures and tables indi-
above but without infection of insects (Figure 1b) and was similarly
cates strain (W for wild- type mutation rate, M for mutator), selection
performed on four lineages per strain (wt and mut) with five rounds
of selection. Approximately 5 × 104 spores were used to inoculate
1 ml sporulation medium and cultured as above. After standard cul-
treatment (y for yield, i for infectivity selected, v for in vitro control)
with a letter for independent lineage and a number for clone, for
example, Wia1, wild type, infectivity selected, lineage a, clone 1.
ture, spores were centrifuged and resuspended in 0.85% NaCl, pas-
The viable spore concentration of all cultures used in bioassays
teurized and kept at 10°C until use.
was measured by plating after pasteurization and measuring colony
For the yield selection treatment, each lineage was split into six
insect subpopulations using six dishes, which were pooled together
at each round of selection. Cadavers from all six subpopulations in
the pooled treatment were transferred to the same microfuge tube.
forming units (CFUs). Standard virulence bioassays used at least
three doses with 60 insects per dose (between 104 and 1.2 × 105
spores/μl for Bt kurstaki in resistant P. xylostella) and were repeated
at least twice. The exception to this was the initial screens of clone
Then, 300 μl saline was added and the suspension was distributed
into six wells for inoculation. For replicate lineages with more than
level variation of six clones per lineage which used 15– 30 insects per
dose and two doses. Only strongly melanized cadavers were scored
30% death across insect populations at time of selection, the num-
as Bt- killed insects. The dose response in bioassays of clones was
ber of cadavers taken from subpopulations with most deaths was
analyzed in terms of viable spores but also in terms of dilution factor
reduced in order to select 30% cadavers overall and maintain com-
of inocula relative to purified spore stocks as some clones displayed
parable population sizes across treatments at inoculation (Figure 1c).
variable germination rates. Clones of interest were further charac-
For the infectivity treatment, each lineage was split into 12 in-
terized in additional assays.
sect subpopulations maintained separately in 12 dishes. The six sub-
Viable spore counts of clones grown in sporulation media
populations with highest larval mortality were selected and the six
for bioassays were used in analyses of in vitro spore production.
other subpopulations were discarded (Figure 1d). If the same mor-
Assessment of bacterial development rate used three clones with
tality was observed for several populations, the ones with earliest
elevated virulence, three clones with virulence unchanged, as well
observed deaths were selected. A maximum of three cadavers from
the wild- type ancestor. The proportion of cells that were vegetative,
selected subpopulations were transferred into separate tubes (6
the proportion of cells that had toxin crystals within the exosporium
total), then 50 μl saline was added to each tube, and the suspension
from each tube was inoculated into separate wells. After spore puri-
and proportion of cells at the final stage of lysis of exosporium were
assessed by phase- contrast microscopy after 72 and 144 h of growth
fication, spores from each well were used to inoculate two subpop-
in HCO.
ulations in the next selection round (Figure 1d).
2.4 | Phenotypic and genotypic characterization of
evolved bacteria
2.4.1 | Bioassays
2.5 | Genetic analysis of Cry toxin gene
complements
The ancestor Bt 7.1.o wild type bears genes for several expressed
toxins (Cry1Ac, Cry2Aa, Cry1Aa) on a large plasmid (homologous
to CP009999, 317 kb) and one additional toxin gene (Cry1Ab) on
After five rounds of selection, the evolved lineages were frozen at
a smaller plasmid (homologous to CP010003, 69 kb; Tables S2 and
−80°C (LB 20% glycerol) without pasteurization. These frozen stocks
S3; Méric et al., 2018). Loss of each of those plasmids was detected
were used to inoculate sporulation medium for assays of evolved
in several clones with whole- genome sequencing. We extended
lineages. The virulence of each independently evolved lineage
those results by estimating Cry toxin plasmid loss for all clones from
(Figure 2) was assayed by scoring proportional mortality (Figure 2)
evolved lines by PCR (six clones from each insect- evolved lineage
after 3 days (early mortality) and 5 days (late mortality). This analysis
and two clones from each in vitro lineage). Primers were designed
used data from two independent bioassays using independently
and validated with the sequenced clones. Primers Cry2- F and Cry2- R
grown spore stocks (minimizing effects due to variation in growth/
amplify a 1.1- kb product from the large toxin- encoding plasmid
amplification in vitro). Analyses reported in Figure 2a used average
(CP009999). Primers Cry1- F and Cry1- R amplify a 1.4- kb product
mortality data that were normalized against the ancestor: The
when any Cry1A gene is present; no amplification corresponds to
average mortality (across the three doses) of each evolved lineage
loss of both cry gene- bearing plasmids.
replicate was divided by the average mortality measured in the
PCR used HotStar Taq (Qiagen) with cycling parameters: 5 min at
ancestor within the same assay.
95°C, 30 × (45 s at 94°C, 1 min at 50°C [Cry2 primers] or 53°C [Cry1
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F I G U R E 2 Mutation rate and infectivity selection shape evolution of virulence and retention of public good virulence factors (cry toxins).
Data shown are from two separate bioassays performed with independent spore preparations of evolved lineages after five rounds of
selection. (a) Proportional mortality (relative to the ancestor) at 3 and 5 days after inoculation for different treatments, control refers to
in vitro passaged controls. The center value of the boxplots shows the median, boxes the first and third quartile, and whiskers represent 1.5
times the interquartile range; outliers are shown as dots (N = 8, 4 lineages per treatment in two assays). (b) Barplots show the proportion of
clones containing no toxin genes (light gray), Cry1Ab only (dark gray), or both Cry1A and Cry2A (black) genes for each lineage (N = 6 clones
per lineage). Scatter plots show logit- transformed mortality 3 days after inoculation is shown as a function of Bt dose; lines are fitted models
for each lineage. For reference, the dotted black line shows the dose response for the ancestor, the replicate lineages within each treatment
are color coded. See also Table S1 and Figures S1 and S2 for results of clone- level virulence assays.
primers], 90 s at 72°C), 10 min at 72°C. To provide DNA templates,
were sequenced. DNA was extracted using Qiagen DNeasy Blood
fresh colonies were resuspended into 100 μl dH2O, frozen 20 min at
−80°C, then boiled 10 min. Five microliters of supernatant was used
and Tissue genomic extraction kits, after appropriate lysis for
Gram- positive bacteria. Sequencing was performed on an Illumina
in each 20 μl PCR mix. To exclude false- negative results, each nega-
tive result was repeated three times from fresh colonies.
HiSeq 2500 at a minimum of 30× coverage with 125 bp paired- end
sequencing.
2.6 | Whole- genome sequencing
All Illumina reads were trimmed with Trimmomatic (Bolger
et al., 2014). Unicycler version 0.4.7 was used to perform a hybrid
assembly using PacBio self- corrected reads and Illumina reads of
the mutator ancestor (Wick et al., 2017). Since most evolutionary
The unmarked ancestral Bt kurstaki 7.1.o strain was sequenced
change occurred in the mutator lineages, the mutator ancestor pro-
with PacBio after DNA extraction using the Qiagen high- molecular
vided a better reference for subsequent identification of mutations.
weight kit. Data were produced using SMRTbell® Express Template
This combination of read types resulted in a final assembly that
Prep Kit 2.0 following the manufacturer's recommendations. This
included small plasmids that were otherwise lost from the PacBio
resulted in a 15- to 20- kb insert library which was sequenced on a
data- only assemblies, due to the DNA fragment size section being
PacBio Sequel system using one cell per library and 10- h sequencing
larger than the size of the plasmids. Short read data were used to
movie time. The data were processed to provide CCS and single pass
validate and aid the comparison of plasmid genomes between a ref-
data and assembled using Unicycler version 0.4.7 (Wick et al., 2017).
erence (Bt HD- 1) and Bt 7.1.o (this study) by mapping the short reads
For insect- evolved lineages, the two clones with highest viru-
to the assembly (Li, 2012; Tables S2 and S3).
lence based on a preliminary screening were chosen. At least two
The final assembly was then checked against other Bacillus
clones from each endpoint evolved lineage, in addition to the wt
thuringiensis kurstaki strains using MAUVE (Darling et al., 2011)
ancestor (7.1.o wt pHT315- gfp) and the mutator (7.1.o ΔmutS),
to check for possible genome errors. The assembled genome was
DIMITRIU et al.
| 711
then run though the software PROKKA (Seemann, 2014) providing
acid was then added, the sample was vortexed and incubated
a draft annotation. The PROKKA flag— use genus— genus Bacillus
overnight at 4°C. The precipitate was spun down at 15,000 g for
was used to improve taxon- specific gene annotations. The final
10 min, the supernatant removed, and the pellet was washed twice
assembly was then checked against other Illumina whole- genome
with ice cold 70% acetone then air- dried. Following the addition of
shotgun data from the selection experiments. Sequenced clones
and the final reference assembly were then analyzed with snippy
20 μl water, the mixture was sonicated for 2 min to resuspend the
pellet. For immunodetection of Vip3Aa, samples were boiled with
version 4.4.0 (https://github.com/tseem ann/snippy) to align reads
SDS sample buffer and spotted onto a nitrocellulose membrane.
against the newly assembled reference and used to call single-
A Vip3A antibody (a kind gift from Professor Juan Ferré) was used
nucleotide polymorphisms (SNPs). The Snippy SNP data were used
in association with an anti- rabbit IgG HRP- linked antibody for
to generate a Jukes– Cantor Neighbor- Joining (1000 boot strap
enhanced chemiluminescent detection.
replicates) phylogeny of isolates, implemented with Geneious
Prime 2022.2.1.
2.9 | Statistical analysis
2.7 | Fitness measurements
All statistical analyses were carried out in R (v4.0.4) (https://www.R-
proje ct.org). Bioassay and mortality data were analyzed using one
Measurements of relative fitness used a mutant of the 7.1.o isolate
of two methods. For comparisons between independently evolved
constructed by transformation with a pHT315- rfp plasmid containing
lineages, we used generalized linear modeling (glms) with logit link
the tetR gene conferring tetracycline resistance (Zhou et al., 2014).
functions and quasibinomial errors to correct for overdispersion.
This version of the ancestor, therefore, has a plasmid with a similar
However, to test for treatment effects, or account for random
backbone to the evolved wild- type clones but is distinguishable using
effects associated with different lineages within treatments, we
its distinct antibiotic resistance. Relative fitness was calculated from
used generalized linear mixed models (glmer) in the package lme4
an estimate of relative reproductive rates (Malthusian parameter) of
(Bates et al., 2015). Although we attempted to use glmer models
the two genotypes as described previously (Zhou et al., 2020).
with binomial errors, these commonly failed to converge, especially
A 50/50 mix of spores of the two competitors was used to ini-
tiate competition experiments and was produced under standard
in more complex models with dose × treatment interactions. Instead,
we used the conservative approach of arc- sine transforming
conditions. Fitness was measured in conditions duplicating the se-
proportional mortality and using normal errors. The glm and glmer
lection experiment, that is, infection and transfer of cadaver to spor-
approaches produce qualitatively similar results. Hypothesis testing
ulating media, although insect cadavers were processed individually
and HCO culture did not use antibiotics. After spore washing, mixes
in glmers used likelihood ratio tests after model simplification. LC50s
and their standard errors were calculated using the dose.p function
were plated on antibiotic- free medium as well as tetracycline 10 μg/
ml (for the standard competitor ancestor). The density of evolved
in the MASS package (Venables & Ripley, 2002) after fitting simple
logit models using log10 dose and quasibinomial errors.
clones in cadavers was calculated by subtracting competitor counts
Analyses of viable spore production also used glmers with lin-
from antibiotic- free plates total counts, as the cat gene present in
eage fitted as a random effect, while the analysis of competitive fit-
the mut strain proved unreliable at allowing growth on chloram-
ness used evolved clone as a random effect and genotype (mutator
phenicol on LBA and we wished to use a common method for both
or wild type) and number of toxins as explanatory variables. Model
wt and mut clones.
2.8 | Assessment of toxin production
assumptions (normality, heteroskedasticity) were checked with
graphical analyses and qq plots. Raw experimental data are available
from Zenodo (Dimitriu et al., 2022).
Bt bacteria were cultured in sporulation medium as above. Crystal
3 | R E S U LT S
protein production was assessed by centrifuging 1 ml of culture and
resuspending in 1 ml of water, followed by two rounds of sonication,
3.1 | Characterization of the mut ancestor
centrifugation, and washing to break open un- lysed cells. The final
pellet was resuspended in 1 ml of water. Serial dilutions were plated
out in order to calculate the concentration of viable spores as CFUs.
SDS- PAGE analysis was then performed on equal numbers of CFUs
to compare the production of crystal proteins. For the assessment of
Wild- type (wt) mutation rate toward rifampicin resistance was
7 × 10−10 [95% CI 4.2 × 10−10 to 1.02 × 10−9] while for the ΔmutS strain,
the mutation rate was 1.76 × 10−8 [95% CI 1.48 × 10−8 to 2.08 × 10−8],
showing a 25- fold increase compared to the wt strain. The mutator,
secreted Vip3A production, culture supernatant was passed through
prior to passage, did not show any change in virulence relative to
a 0.22- μm cellulose acetate filter and to 1 ml of the filtrate 50 μl of
2% sodium deoxycholate was added and incubated for 30 min on
ice. One hundred and fifty microliters of 100% Trichloroacetic
the ancestor (log10 LC50 = 4.9, test for difference from ancestor-
effect size 0.21, SE = 1.22, p = 0.87). The competitive fitness of the
mutator strain under the conditions of the passage experiment also
DIMITRIU et al.712 |
did not differ from the wt ancestor (t = 0.49, p = 0.63, means ± SE of
mut and wt are 1.09 ± 0.06 and 1.13 ± 0.06, respectively).
bar plots) that correlated with loss of virulence. In particular, wild-
type lineages lost more plasmids than mutator lineages (Figure 2b;
Fisher's exact test two- tailed p = 0.006). Importantly, the infectivity
selection regime was more effective at retaining these plasmids and
3.2 | Changes in virulence in evolved lineages
preventing invasion of putative cheaters (Figure 2b, Fisher's exact
test two- tailed p = 0.00032).
Duplicated endpoint assays of evolved lineages used total mortality,
normalized to that of the wt ancestor in each experiment, to
assess changes in virulence after five rounds of selection (ca. 60
3.4 | Social cheating and Cry plasmid loss
generations). These assays showed that increased mutation rate and
infectivity selection between subpopulations resulted in increases
Previously, we have seen that Cry toxin production is a public
in normalized mortality relative to the in vitro controls (Figure 2a).
good and loss of investment in Cry toxins can be driven by the
Both strain (wild type or mutator) (F1,44 = 12.3, p = 0.001 at 3 days,
F1,44 = 10.6, p = 0.002 at 5 days) and selection treatment (3 days,
F2,44 = 21.4, p < 0.001; 5 days, F2,44 = 8, p = 0.0011) affected virulence
of evolved lineages. Post hoc tests confirm increased virulence for
selective advantage of increased competitive growth rate within
hosts, or social cheating. In order to test whether mutants that had
lost Cry- toxin plasmids were cheaters, we measured the fitness
of one clone from each lineage in competition experiments that
infectivity versus yield selected treatments and for mutators versus
used a marked RFP mutant derived from the ancestor (Figure 3a).
wild- type lineages (Tukey tests, all p < 0.01). Differences between
treatments were larger when assessing early normalized mortality,
suggesting changes in virulence affected timing and overall levels
of mortality. We also used mixed models of arc- sine transformed
mortality, which fitted independent lineage as a random effect.
These gave qualitatively similar results for day 3 and day 5 mortality,
Mutants carrying fewer Cry toxin genes had higher competi-
tive fitness (mixed model likelihood ratio test (LRT) df = 1, like -
lihood ratio (LR) = 9.05, p < 0.0026, Figure 3a). Mutants which
had lost Cry toxins also had lower virulence, that is, higher LC50
(glm F1, 64 = 67.1, p < 0.0001, Figure 3b). Thus, these data were
consistent with the hypothesis that low- virulence mutants were
except in the mixed model analysis mutators and the infectivity
free- loading on investment in Cry toxins by other variants in their
treatment could be seen to increase virulence by causing more
lineages (Raymond et al., 2012).
mortality at higher doses (dose × selection treatment interactions
and dose × strain interactions, all p < 0.01 day 3, and p < 0.001, day
5 mortality).
Since the evolved lineages were not genetically homogeneous,
we also conducted bioassays of six clones isolated from each lin-
After accounting for toxin production, mutators evolved greater
competitive fitness than wild- type lineages (Figure 3a; mixed model,
df = 1, LR = 7.5, p = 0.006) and had increased virulence (i.e., lower
LC50s; glm F1, 63 = 7.35, p < 0.0157, Figure 3b) suggesting that increas-
ing mutation rate increased the supply of beneficial alleles without
eage. Clonal assays of day 5 mortality show that mutation rate and
reducing overall fitness. Since the original mutator mutant has in-
infectivity selection increased virulence via interactions with dose
distinguishable fitness from the wild- type ancestor (see Section 2,
(mixed effect glm with lineage as random effect: dose × selection
treatment interaction df = 3, LR = 16.95, p < 0.001; dose*strain
interaction df = 1, LR = 12.59, p < 0.001; Figures S1 and S2). We
identified a number of clones with substantial increases in virulence,
Methods), the initial genetic background of these strains does not
explain this pattern.
quantified as a reduction of LC50 of more than an order of magni-
3.5 | Spore production
tude from endpoint assays (Table S1).
The other major life- history trait that we explored was the efficiency
of spore production in the sporulation media used in the selection
3.3 | Patterns of Cry toxin plasmid carriage
experiments. We saw lower spore production in the mutator and
the infectivity- selected lineages, the treatments that produced
We saw variation between independent evolved lineages in terms of
higher virulence (Figure 4a, mixed models, selection treatment
whether or not they increased or decreased virulence with respect
to ancestors (Figure 2b, scatter plots). Low- virulence lineages clearly
df = 3, LR = 11.3, p = 0.01, mutation rate df = 1, LR = 8.36, p < 0.01).
Importantly, between lineages, lower spore production was as-
had low mortality that did not increase with dose (glm lineage × dose
interaction F1,17 = 5.92, p = 0.026). Cry toxins are encoded on two
plasmids in Bt kurstaki— a mega- plasmid containing Cry1Aa, Cry1Ac,
sociated with lower LC50, which corresponds to higher virulence
(Figure 4b, F1,14 = 10.5, p < 0.01, adj. R2 = 0.39). We examined bacte-
rial development and sporulation after 3 days (the typical growth pe-
and Cry2Aa, and a circa 80 kb plasmid- carrying Cry1Ab (Méric
riod in sporulation media) and after 6 days for clones with elevated
et al., 2018). We used PCR primers to screen for loss of plasmids
to test if loss of virulence in evolved lineages could be explained by
changes in plasmid carriage. We observed numerous events of toxin
virulence (n = 3), the wild- type ancestor and clones with unchanged
virulence (n = 3). Elevated virulence was associated with delayed de-
velopment. After 3 days, cultures of high- virulence clones retained
gene loss in clones isolated from in vivo evolved lineages (Figure 2b
5%– 10% vegetative cells, while 85%– 95% cells had completed
DIMITRIU et al. | 713
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F I G U R E 3 Social cheating and fitness in experimentally evolved Bt lines. Evolved clones that had lost virulence plasmids encoding Cry
genes had higher competitive fitness (a) and lower virulence (b) than the ancestor. Toxin complements in A (n = 16) are derived from Illumina
sequencing data; LC50s are shown for all evolved clones with toxin complement data based on PCR (n = 70). Means ± SE of fitness for each
clone are plotted as solid points, raw data are plotted at 50% transparency, the dashed line is a reference line for the ancestor while solid
lines show fitted statistical models (glms fitted with strain and number of Cry toxins).
development and undergone exosporium lysis. For clones with un-
similarity to other Bt kurstaki genomes (Tables S2 and S3, Figure S4).
changed virulence, these figures were 1% vegetative cells and 99%
Resequencing of evolved mutants confirmed that none carried
lysis. After 6 days, all clones showed 98%– 99% lysis indicating that
mutations in their Cry genes or in regions immediately upstream.
clones with elevated virulence could eventually undergo complete
Comparison to the ancestral genome showed both small- scale
sporulation.
mutations and large- scale deletions associated with plasmid loss.
This is biologically significant because toxin production is
Particularly, toxin gene loss patterns detected previously by PCR can
linked to sporulation in Bt, especially the Cry1A toxins that are
be explained by the loss of one or both Cry plasmids.
the dominant virulence factors in our experimental strains (Deng
Phylogenetic analysis of our evolved clones shows that
et al., 2014). In addition, we selected for sporulation by heat-
there was considerably more genome evolution in the mutators
treating preparations at the end of each round of selection to kill
(Figure 5a). However, evolved clones with similar virulence phe-
vegetative cells and possible contaminants. Reduced sporulation
notypes did not group together, nor did clones from the same
would therefore be expected to result in lower Cry toxin produc-
treatments (Figure 5a). Mapping of mutations to the reference also
tion and reduced virulence. Two classes of toxins are produced by
suggests minimal convergent evolution in terms of shared changes
Bt kurstaki earlier in the growth cycle than the Cry1A toxins and are
in DNA (although there is phenotypic convergent evolution in terms
expected to have activity against Cry1A- resistant insects: These
of sporulation). We did identify a small list of genes that acquired
are the Cry2 toxins and the vegetative virulence factor Vip3Aa
mutations in more than one clone (Figure 5b); this list includes three
(Carrière et al., 2015; Deng et al., 2014). However, we found no
transcriptional regulators nprA (in clones Mia3 and Mia4), dagR, and
significant differences in the ratio of Cry1A and Cry2 production
sgrR with nonsynonymous mutations in insect- passaged lineage, but
that correlated with changes in virulence. At the lineage level, nei-
not in the in vitro controls. In general, missense or frameshift muta-
ther mutators nor infectivity- selected replicates had elevated Cry2
tions were prevalent in genes encoding putative transcriptional reg-
production (Figure 4c), nor did we see differences in Cry2 produc-
ulators (Table S4). These included well- described regulators such
tion between the ancestor and evolved clones with high virulence
as NprA but also proteins with helix– turn– helix domains. Mutator
(Figure S3). We did find substantial variation in secretion of Vip3Aa
clones from the infectivity selection treatment accounted for 10
between evolved clones, but no association with increased viru-
of the 16 loss of function mutations in putative transcriptional reg-
lence was seen (Figure 4d).
To seek mechanistic explanations for the reduced sporulation
ulators. If we compared the virulence (LC50) of all the sequenced
mutator clones, there was a significant increase in virulence in the
and increased virulence, the genomes of two clones from each
mutator clones with these loss of function mutations compared to
lineage were sequenced and compared to a long- read assembly of
our ancestor. The ancestral genome consisted of a 5.7- Mb chro-
those with no mutations in regulatory genes (F1,16 = 5.91, p = 0.027,
Figure 6), assuming that the data from these clones are independent
mosome and 14 plasmids that resolved as single contigs with high
observations.
DIMITRIU et al.
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F I G U R E 4 Increased virulence trade- offs against reduced spore production in vitro in evolved Bt lines but is not associated with
increased Vip3 production. (a) Variation in spore production between evolved lineages; asterisks indicate the significance of contrasts
between all evolved lineages and the ancestor in a glm; boxplots summarize the results from two assays of each clone. (b) Mean LC50
of each in vivo passaged lineage (averaged across clones) plotted against spore production, ancestors plotted as hollow circles. (c) SDS
PAGE showing Cry toxin production for all infectivity- selected lineages, see Figure S3 for examples of individual clones. (d) Slot blot
characterization of Vip3 production in the ancestral Bt clone and clones with increased virulence or virulence unchanged relative to the
ancestor (see Table S1). See also Figures S3– S5 and Tables S2 and S3 for additional details on genomic and proteomic characterization of
evolved clones.
4 | D I S C U S S I O N
Virulence factors can behave as public goods both in laboratory
models and infections of insects with biocontrol agents such as
Kin selection theory emphasizes that altered virulence can have
entomopathogenic nematodes and Bt (Deng et al., 2015; Raymond
different impacts on individual- and group- level fitness components
et al., 2012; Shapiro- Ilan & Raymond, 2016; Zhou et al., 2014). In
(Buckling & Brockhurst, 2008). Notably, investment in bacterial
this experiment, it was clear that some clones within lineages had
virulence factors requires resources to be diverted away from
reduced virulence relative to the ancestor and so were putative
growth of individual cells. If virulence factors are diffusible, they
cheaters and we identified a link between the absence of Cry toxins
may behave as “public goods” by conferring fitness benefits that
and increases in competitive fitness.
are shared among members of an infecting group (Diard, Sellin,
This result mirrors previous experiments with Bt strains that
et al., 2014; Harrison et al., 2006; Raymond et al., 2012). If cells
have been cured of Cry toxin- encoding plasmids as well as com-
reduce investment in public goods virulence, these “cheaters”
petition between Bt and naturally Cry null B. cereus (Raymond
can gain a reproductive advantage during growth in the host by
et al., 2007, 2012). The key difference in this study is that loss
freeloading on the products of others (Buckling & Brockhurst, 2008;
of Cry toxin plasmids occurred spontaneously during experimen-
Diggle et al., 2007; West & Buckling, 2003), although this may be
tal evolution in multiple independent lineages and in different
accompanied by a group- level cost such as reduced infectivity.
ways, that is, by the loss of either or both of the large plasmids
F I G U R E 5 Genomic analyses of evolved Bt clones. (a) Consensus neighbor- joining tree of sequenced evolved clones. For ease of
interpretation, bootstrap of values >50 only has been displayed for the nodes. The scale bar indicates the number of substitutions per
nucleotide. The reference refers to the hybrid PacBio/Illumina assembly, all other sequence information is derived from SNPs identified
using Illumina data with reference to this assembly. (b) Histogram of nonsynonymous mutations identified in at least two evolved clones in
our three selection regimes, infectivity selection, yield selection, and the in vitro passage controls.
DIMITRIU et al.
mvd1
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| 715
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DIMITRIU et al.
716 |
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(van Leeuwen et al., 2015), pooling cadavers can reduce relatedness
relative to infectivity selection treatment. In other words, there is
more scope for cheating and greater selection to increase competitive
fitness in the yield treatment. An additional consideration is that the
strain
infectivity selection also imposed selection for gains in virulence at an
ancestor
mutator
wild type
additional spatial scale. In all treatments, there was selection for in-
fectivity at a between- host level (successful infections had to occur in
order for passage to proceed), but by using selection in a metapopula-
tion, we imposed an additional level of competition between subpop-
ulations within a metapopulation. The effects of this level of selection
for infectivity in pathogens require further investigation, but we
would hypothesize that competition between subpopulations is more
important when investment in virulence has costs in terms of growth
rate and population size within hosts (Raymond & Erdos, 2022). The
primary aim of this study was to devise and test different methods of
selection for maintaining and improving virulence, and so we will re-
quire further work to tease out the precise evolutionary mechanisms
behind the success of this particular treatment.
The mutator lines showed clear differences in the rate of mo-
lecular evolution as well as greater increases in virulence relative
to the wild- type background. In contrast to previous work, we did
not see an increase in levels of cheating under high mutation rate
no
yes
regulatory mutations
F I G U R E 6 Virulence of evolved mutators clones with and
without loss of function mutations in putative transcriptional
regulators. Virulence is expressed as log LC50 with evolved wild-
type clones and the 7.1.o ancestor plotted as comparisons, after
excluding data from sequenced clones that are Cry toxin cheaters
(i.e., which carry 0 or 1 Cry toxin genes). Table S4 contains a list of
mutations
that encode these toxins. In addition, the selection pressure we
(Harrison & Buckling, 2005; Racey et al., 2009). Mutagenesis is very
imposed during experimental evolution affected how readily Cry
commonly employed in strain improvement and directed evolu-
toxins were lost: selection pressure to maximize yield (final bacte-
tion with Bacillus sp. and other microbes (Lai et al., 2004; Raju &
rial population size in cadavers) resulted in higher rates of plasmid
Divakar, 2013). The evidence for the ability of mutators to accelerate
loss than in the infectivity selection treatment. The loss of Cry
increases in fitness is complex and has several unresolved questions.
plasmids in response to selection on yield occurred because Cry
Mutators can increase the supply of both beneficial and deleterious
toxins impose costs on growth rate and total production of spores
alleles and it is clear that there is no known direct benefit to having
within hosts (Raymond et al., 2012). Both social evolution and evo-
impaired proofreading: mutator alleles rise in frequency by hitchhik-
lution of virulence theory commonly divide the selective forces
ing on the fitness benefits of linked alleles (Chao & Cox, 1983; de
into individual (within- host)- and group- level (between- hosts)
Visser, 2002; Gentile et al., 2011; Raynes et al., 2013). Mutators are
components; these are often in conflict in microbes (Cressler
often seen at quite high proportions in pathogenic bacteria (LeClerc
et al., 2016; Frank, 1997). Other bacterial virulence factors which
et al., 1996; Matic et al., 1997; Oliver et al., 2000) and mutators can
are public goods, such as siderophores, have a group- level benefit
appear spontaneously in experimental evolution although evolving
in terms of increasing yield within the host (Harrison et al., 2006;
lineages often moderate mutation rates during long- term transfer
West & Buckling, 2003). However, Cry toxins are produced at
experiments (Good et al., 2017; Ho et al., 2021).
sporulation in the cadaver, rather than in the early stages of in-
One reason for the prevalence of mutators among pathogens,
fection, and these Cry toxins require so much protein production
and for their success in this study is that small populations and/or in-
that Bt variants which invest in these toxins produce substantially
fection bottlenecks may limit the supply of beneficial mutations (de
fewer spores per unit resource than their Cry null counterparts
Visser, 2002). While demographics and mutation supply are known
(Deng et al., 2015; Raymond et al., 2012). In essence, Cry toxins
to affect the long- term success of mutators, simulation and exper-
are a different type of public good than siderophores and act to in-
imental studies suggest that bottlenecks or small populations may
crease infectivity rather than the quality of resources available for
not favor high mutation rates (Ho et al., 2021; Raynes et al., 2018).
growth within hosts. It remains to be seen whether other bacterial
Nevertheless, mutators are more successful when large- effect ben-
public goods can also reduce the population size of pathogens in
eficial mutations are available for hitchhiking (Gentile et al., 2011;
hosts but increase infectivity or transmission.
Mao et al., 1997; Thompson et al., 2006) and the strong selection
The infectivity selection was not only more effective at retaining
imposed by new sporulation conditions and a novel host genotype
Cry plasmids but also resulted in greatest gains in virulence relative
in this study may account for the success of mutators in our exper-
to the ancestor. There are several factors that could have contributed
iments. Overall, the use of mutators to overcome host resistance
to these results. First, budding dispersal in a metapopulation involved
has a sensible biological basis. However, this may have had implica-
less pooling of cadavers than the yield selection treatment. Since in-
tions for the stability of virulence (Figure S2). Potentially more sta-
dividual cadavers are likely to be dominated by different genotypes
ble phenotypes can be produced by periodic rounds of chemically
DIMITRIU et al.
| 717
induced mutagenesis or by using initial pathogen populations with
example, cessation of growth can be cooperative and late switch-
high standing genetic variation.
ing to sporulation can provide growth advantages in competition
It was not possible to identify a single simple cause for any gains
(Gardner et al., 2007; Ratcliff et al., 2013). Efficient sporulation is
in virulence in this study, although this was consistently related to
essential for the production of both fungal and bacterial biocontrol
decreased sporulation efficiency. Reduced sporulation means that
agents and can be readily lost during laboratory selection. In gen-
vegetative cells and their secreted proteins will be more prevalent
eral, passage regimes that minimize social conflict and which can
in final inocula. We observed that a proportion of mutants with
maintain valuable cooperative traits could have broad importance
reduced sporulation gave increased virulence per spore but not in
across diverse groups of invertebrate pathogens, and here, we have
terms of dilution of the broth in which they were cultured. This is po-
shown the value of a metapopulation regime that can reduce social
tentially because standardizing doses by spore means that a higher
conflicts. Moreover, experimental evolution offers a mechanism- free
concentration of broth- associated virulence factors are included in
solution to overcoming pest resistance which is potentially applicable
inocula. As yet, none of the classic virulence factors associated with
to many pathogens and pests. In vivo selection experiment may help
broth such as Vip3A appear to be responsible for this increase in
us discover entirely novel virulence factors or virulence modification
virulence. There are many virulence factors secreted into broth by
responses, in contrast to directed evolution methods that require
Bt and its relatives (Chitlaru et al., 2006; Guinebretière et al., 2002;
well- understood protein– protein interactions (Badran et al., 2016).
Stenfors Arnesen et al., 2008). While spores and crystals are by
far the most significant contributors to virulence in Bt (Raymond
AC K N OW L E D G M E N T S
et al., 2010), broth- associated virulence may be more significant
The authors thank Andy Matthews for support and technical advice.
for Cry1A- resistant hosts and are known to be significant for some
This study was supported by the Leverhulme Trust (RPG- 2014- 252)
hosts such as Galleria mellonella (Salamitou et al., 2000).
and BBSRC (BB/S002928/1).
For example, secreted virulence factors are regulated by a quorum-
sensing system (PlcR/PapR) that activates a suite of virulence factors
C O N FL I C T O F I N T E R E S T
involved in host invasion (Salamitou et al., 2000; Zhou et al., 2014).
This work formed part of international patent application number
Although these are typically produced at stationary phase, expression
WO 2019/030529 A1 by BR & NC.
of the PlcR- regulated genes is repressed in low- nutrient sporulation
medium such as HCO (Lereclus et al., 2000). The stationary phase
DATA AVA I L A B I L I T Y S TAT E M E N T
secretome of Bt and B. anthracis in sporulation medium is mainly
Sequence data are hosted at the SRA under BIOPROJECT
composed of metalloproteases such as Inh1A and NprA and is much
SUB10359598. Plasmids, ancestor, and mutator clones are avail-
reduced in the diversity of proteins compared to nutrient- rich media
able on request. Evolved mutants can be shared subject to Material
(Chitlaru et al., 2006; Perchat et al., 2011). These metalloproteases
Transfer Agreements. Experimental data are available at Zenodo
are hypothesized to have a role in the digestion of cadavers in late-
with a permanent doi: 10.5281/zenodo.7503995.
stage infections (Perchat et al., 2016). Nevertheless, they are essen-
tially lytic enzymes and could improve the ability of Bt to invade the
O R C I D
host at high concentration. Other possible virulence factors secreted
Tatiana Dimitriu
https://orcid.org/0000-0002-1604-2622
into sporulating media include chitinase- and chitin- binding proteins
Alistair Darby
https://orcid.org/0000-0002-3786-6209
(Chitlaru et al., 2006). Notably, we did identify mutations in nprA and
Neil Crickmore
https://orcid.org/0000-0002-8448-0763
other transcriptional regulators in several insect- passaged mutators.
Ben Raymond
https://orcid.org/0000-0002-3730-0985
Bt is an important pathogen for both organic horticulture and
modern “biotech” crops, so the question of how to improve strains
R E F E R E N C E S
or discover new toxins is significant, especially given the need to
respond to the evolution of resistance (Adang et al., 2014; Badran
et al., 2016). However, these methods may be applicable to other par-
asites, such as nematodes and fungi, which also produce costly ex-
creted virulence factors (Shapiro- Ilan & Raymond, 2016). Moreover,
in addition to providing a framework for increasing virulence, the
combination of in vivo and in vitro selection used here could be ap-
plied when the aim is adaptation to an alternative growth medium or
the improvement of other phenotypes, while retaining insecticidal ef-
ficacy. For some pathogens, for example, fungi, it is also worth bear-
ing in mind that while virulence traits may not be social (there are no
data yet), other important biocontrol traits such as sporulation are
likely to be subject to social conflicts. The timing of a switch to any
resting state (spore, persister, etc.) can be subject to cheating— for
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S U P P O R T I N G I N FO R M AT I O N
Additional supporting information can be found online in the
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DIMITRIU et al.
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10.15252_embj.2022112118.pdf
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Data availability
The data that support the findings of this study are available from
corresponding author. RNA-sequencing data have been
the
deposited in GEO under accession number (GSE215951; http://
www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE215951).
|
Data availability The data that support the findings of this study are available from the corresponding author. RNA-sequencing data have been deposited in GEO under accession number (GSE215951; http:// www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE215951 ). Expanded View for this article is available online .
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Article
A critical period of prehearing spontaneous Ca2+
spiking is required for hair-bundle maintenance
in inner hair cells
Adam J Carlton1
, Jing-Yi Jeng1
Lara De Tomasi1, Anna Underhill1
Guy P Richardson4
, Fiorella C Grandi2
, Stuart L Johnson1,3, Kevin P Legan4
, Mirna Mustapha1,3 & Walter Marcotti1,3,*
, Francesca De Faveri1
, Corn(cid:1)e J Kros4
,
, Federico Ceriani1
,
Abstract
Sensory-independent Ca2+ spiking regulates the development of
mammalian sensory systems. In the immature cochlea, inner hair
cells (IHCs) fire spontaneous Ca2+ action potentials (APs) that are
generated either intrinsically or by intercellular Ca2+ waves in
the nonsensory cells. The extent to which either or both of these
Ca2+ signalling mechansims are required for IHC maturation is
unknown. We find that intrinsic Ca2+ APs in IHCs, but not those
elicited by Ca2+ waves, regulate the maturation and maintenance
of the stereociliary hair bundles. Using a mouse model in which
the potassium channel Kir2.1 is reversibly overexpressed in IHCs
(Kir2.1-OE), we find that IHC membrane hyperpolarization prevents
IHCs from generating intrinsic Ca2+ APs but not APs induced by
Ca2+ waves. Absence of intrinsic Ca2+ APs leads to the loss of
mechanoelectrical transduction in IHCs prior to hearing onset due
to progressive loss or fusion of stereocilia. RNA-sequencing data
show that pathways involved in morphogenesis, actin filament-
based processes, and Rho-GTPase signaling are upregulated in
Kir2.1-OE mice. By manipulating in vivo expression of Kir2.1 chan-
nels, we identify a “critical time period” during which intrinsic
Ca2+ APs in IHCs regulate hair-bundle function.
Keywords calcium waves; development; hair cell; mechanoelectrical
transduction; spontaneous action potentials
Subject Category Neuroscience
DOI 10.15252/embj.2022112118 | Received 16 July 2022 | Revised 22 November
2022 | Accepted 28 November 2022 | Published online 3 January 2023
The EMBO Journal (2023) 42: e112118
Introduction
Inner hair cells (IHCs) are the primary sensory receptors of the adult
mammalian cochlea and relay acoustic information onto type I
1 School of Biosciences, University of Sheffield, Sheffield, UK
2 Gladstone Institute of Neurological Disease, San Francisco, CA, USA
3 Neuroscience Institute, University of Sheffield, Sheffield, UK
4 School of Life Sciences, University of Sussex, Falmer, Brighton, UK
*Corresponding author. Tel: +44 114 2221098; E-mail: [email protected]
spiral ganglion afferent neurons via the graded release of glutamate
from their specialized ribbon synapses (Fuchs, 2005; Moser et al,
2020). Before hearing onset, however, which in most altricial
rodents occurs at around postnatal day 12 (P12) (Mikaelian &
Ruben, 1964; Ehret, 1983; Romand, 1983), IHCs exhibit patterned
action potential activity that is elicited spontaneously in the absence
of sound-induced stimulation by the activation of CaV1.3 Ca2+
chan-
nels (Marcotti et al, 2003a; Tritsch et al, 2010; Johnson et al, 2011).
This activity has been shown to drive the bursting-like firing pattern
along the neural pathway of the immature auditory system (Lippe,
1994; Jones et al, 2007; Sonntag et al, 2009; Tritsch et al, 2010). As
with other sensory systems (Katz & Shatz, 1996; Stellwagen & Shatz,
2002; Moody & Bosma, 2005; Blankenship & Feller, 2010), patterned
peripheral firing activity was identified as being critical for the
refinement of neural circuits in the brain (Clause et al, 2014, 2017;
M€uller et al, 2019; Maul et al, 2022). Additionally, Ca2+
-dependent
APs in IHCs have been shown to instruct the normal functional dif-
ferentiation of the IHCs themselves (Johnson et al, 2007, 2013),
most likely via regulating gene expression (Dolmetsch et al, 1997).
However, due to the complex extracellular modulation of the intrin-
sic Ca2+
action potentials in developing IHCs, the exact role of this
activity is largely unknown.
Spontaneous intrinsic Ca2+
action potentials first appear in the IHCs
of the mouse cochlea at late embryonic stages (Marcotti et al, 2003a),
and their frequency and pattern are controlled by the transiently
expressed small-conductance Ca2+
current ISK2 (Marcotti
+
et al, 2004) and the inward rectifier K
current IK1 (Marcotti et al, 1999).
The frequency and pattern of the electrical activity in IHCs are also
extrinsically evoked and modulated by the spontaneous release of ATP
from the neighboring nonsensory cells (Tritsch et al, 2010; Wang et al,
2015; Johnson et al, 2017). This complex regulation makes it difficult to
identify and separate the specific functional roles of the intrinsic and
externally driven Ca2+
-dependent AP activity in IHCs.
In this study, we used a mouse model in which the inward recti-
channel Kir2.1 (Yu et al, 2004) was selectively overexpressed
+
-activated K
+
fier K
(cid:1) 2023 The Authors. Published under the terms of the CC BY 4.0 license.
The EMBO Journal 42: e112118 | 2023
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Adam J Carlton et al
in vivo in the IHCs under the control of doxycycline (DOX), lowering
their membrane potential and preventing them from firing the
intrinsic spontaneous Ca2+
action potentials. These “silent” IHCs,
however, retained their ability to respond with AP activity to extrin-
sic modulation by the ATP-induced signaling from the nonsensory
cochlear cells. Our results show that prehearing IHCs require spon-
taneous intrinsic Ca2+
firing to maintain the normal morphological
and biophysical characteristics of the mechanoelectrical transducer
apparatus for a period of time in the second postnatal week, before
the onset of hearing. We also found several key genes that are
upregulated in the absence of the intrinsic Ca2+
action potential
activity in IHCs, several of which are involved in pathways related
to maintaining cytoskeletal homeostasis.
Results
Overexpression of Kir2.1 (Kir2.1-OE) in cochlear IHCs in vivo
prevents spontaneous firing activity
The role of spontaneous Ca2+
action potential activity in IHCs,
which are spikes generated intrinsically as opposed to those induced
by Ca2+
waves originating in the nonsensory cells, was investigated
+
by conditionally overexpressing the inwardly rectifying K
channel
Kir2.1 (Kcnj2) in the IHCs, thereby hyperpolarizing their resting
membrane potential.
; Kir2.1+/(cid:1)
DOX-induced overexpression of Kir2.1 channels in the IHCs was
evident from the presence of Kir2.1 immunofluorescence in the baso-
lateral membrane of P6 (Fig EV1B) and P11 Kir2.1-OE mice
(OtofrtTA+/(cid:1)
: Fig 1B), but not in age-matched littermate con-
trol mice that were also exposed to DOX (OtofrtTA+/(cid:1); Kir2.1+/+
: P6,
Fig EV1A; P9-P11, Fig 1A). OHCs and nonsensory cells surrounding
the hair cells showed no or very little overexpression of Kir2.1
(Appendix Fig S1), indicating specificity of the Otof promoter for tar-
geting the IHCs. Prehearing IHCs overexpressing Kir2.1 showed a sig-
+
nificantly larger inward K
current compared with control cells but
+
currents (Fig 1C–G, P9-P11). The larger inward
normal outward K
+
current in the IHCs from Kir2.1-OE led to a hyperpolarized shift of
K
the resting membrane potential (Vm) of the IHCs of about 10 mV
compared with control cells (Fig 1H). The slope conductance around
the respective resting Vm values was also significantly increased in
IHCs from Kir2.1-OE mice compared with control littermates (Fig 1I).
The overexpression of Kir2.1 in neonatal P4 mice had a similar effect
the IHC basolateral membrane
on the biophysical properties of
(Fig EV1D) as that described in P9-P11 IHCs (Fig 1D). We also found
that
the number of presynaptic ribbons, postsynaptic glutamate
receptors and their co-localization in prehearing IHCs was not
affected by the overexpression of Kir2.1 channels (Fig EV2A–D). In
agreement with the normal morphological profile of the synapses,
exocytosis in IHCs was not significantly different between the two
genotypes (P = 0.4709, 2-way ANOVA, Fig EV2E and F). These data
indicate that the overexpression of Kir2.1 channels is not affecting the
expression of the ion channels that are normally present in develop-
ing IHCs or their ribbon synapses.
We then investigated the ability of IHCs to fire intrinsic and
induced Ca2+
action potentials at near body temperature (34–37°C)
with an in vivo endolymph-like solution surrounding the IHC hair
bundles. IHC Ca2+
action potentials are elicited by the opening of
Ca2+
channels that activate at around (cid:1)60 mV (Marcotti et al,
2003a). During the first postnatal week, the ionic composition of the
endolymph is comparable to that of the perilymph, which contains
1.3 mM Ca2+
(Wangemann & Schacht, 1996). Under these recording
conditions, spontaneous Ca2+
spiking activity was recorded from P4
control IHCs (Fig 2A). The mean spike frequency of IHCs was
2.19 (cid:3) 1.09 Hz (n = 6), and the coefficient of variation (CV) was
1.30 (cid:3) 0.63 (n = 7, duration of the recordings 45–101 s), which
being greater than one, is indicative of a bursting pattern of activity
as previously demonstrated (Johnson et al, 2011). In P4 Kir2.1-OE
mice, due to the more hyperpolarized resting Vm, IHCs do not fire
action potentials spontaneously, although they retain the ability to
do so during large depolarizing current injections (Fig 2B). During
the second postnatal week, spontaneous action potentials in IHCs
disappear when using ex vivo cochlear preparations, which is due to
a progressive hyperpolarization of the IHC resting Vm (Marcotti
et al, 2003b) but could still be elicited by depolarizing current injec-
tions (Fig 2C, top panel). This membrane hyperpolarization is likely
to be compensated in vivo by the resting open probability of the
mechanoelectrical transducer (MET) channel (Johnson et al, 2012).
This is because in vivo the endolymphatic Ca2+
concentration during
the second postnatal week has been estimated to be near 0.3 mM
(Johnson et al, 2012), which will increase the open probability of
the MET channels and thus cause the IHCs to depolarize to around
the action potential threshold (Fig 2C, bottom panel; for spike fre-
quency and CV see Fig 7E). We found that the IHCs from Kir2.1-OE
mice failed to elicit spontaneous action potentials even in the esti-
mated 0.3 mM endolymphatic Ca2+
concentration, causing the IHCs
to remain silent at rest (Fig 2D).
IHCs are surrounded by nonsensory cells in the greater epithelial
ridge (GER, also known as Ko¨lliker’s organ: Fig 2E). The release of
ATP from nonsensory cells of the GER leads to spatially and tempo-
rally coordinated Ca2+
waves that propagate across the epithelium
and cause IHCs to depolarize as much as 28 mV (Tritsch et al,
2010). This depolarization has been shown to produce periodic
bursts of Ca2+
action potentials in IHCs (Tritsch et al, 2007, 2010;
Wang et al, 2015; Johnson et al, 2017). The frequency and duration
of the Ca2+
waves in the nonsensory cells were not affected by the
overexpression of the Kir2.1 channels (Fig EV3A and B). Therefore,
we investigated whether the more hyperpolarized IHCs (by about
10 mV) from Kir2.1-OE mice retained the ability to respond to spon-
taneous Ca2+
waves originating in the GER. We found that in the
presence of the estimated in vivo endolymph-like Ca2+
(0.3 mM),
signals caused by the opening of the Ca2+
the Ca2+
channels in IHCs
followed very closely the time course of the Ca2+
wave originating
in the GER in both control (Fig 2F) and Kir2.1-OE (Fig 2G) P7-P9
mice. Moreover, the correlation between IHC Ca2+
activity and Ca2+
waves in the nonsensory cells was unaffected in Kir2.1 mice
(Fig EV3C). This indicates that the large depolarization caused by
the extracellular input of the nonsensory cells was necessary and
sufficient to depolarize the IHCs in Kir2.1-OE mice and cause the
opening of voltage-gated calcium channels.
Progressive loss of mechanoelectrical transduction in IHCs
lacking intrinsic Ca2+ action potentials
MET currents were recorded from apical-coil IHCs by displacing
their hair bundles using a 50 Hz sinusoidal force stimulus from a
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(cid:1) 2023 The Authors
Adam J Carlton et al
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Figure 1. Basolateral membrane properties of IHCs overexpressing Kir2.1 channels.
A, B Maximum intensity projections of confocal z-stacks taken from the apical cochlear region of control (A) and littermate Kir2.1 overexpressing (B, Kir2.1-OE) mice at
postnatal day 11. Inner hair cells (IHCs) were stained with antibodies against Kir2.1 (green) and the hair cell marker Myo7a (blue). At least 3 mice for each genotype
were used. Scale bars: 10 lm.
C, D Currents from IHCs of control (C, P9) and Kir2.1-OE (D, P10) prehearing mice. Currents were elicited by using depolarizing and hyperpolarizing voltage steps, with a
nominal increment of 10 mV, from a holding potential of (cid:1)84 mV. Test potentials are shown next to some of the traces. Note that the large inward rectifier Kir2.1
current is only present in the IHC of the Kir2.1-OE mouse (D). The outward current is primarily carried by a delayed rectifier current IK. IK1 identifies the small
inwardly rectifying K+ current normally expressed in IHCs.
Steady-state current–voltage curves obtained from IHCs of control (P9-P11) and Kir2.1-OE (P9-P11) mice.
E
F, G Size of the total steady-state outward (F, IK: Control 2.88 (cid:3) 1.07 nA, n = 8; Kir2.1-OE 2.25 (cid:3) 0.97 nA, n = 6) and inward (G, Control, IK1: 0.30 (cid:3) 0.05 nA, n = 8;
Kir2.1-OE, IKir2.1: 3.13 (cid:3) 1.37 nA, n = 6) K+ currents measured at 0 mV and (cid:1) 124 mV, respectively. n.s: P = 0.2836.
Resting membrane potential (Vm) measured in IHCs from Control ((cid:1)62.6 (cid:3) 3.8 mV, n = 7) and Kir2.1-OE ((cid:1)73.5 (cid:3) 3.5 mV, n = 5).
Slope conductance of the current measured at around the respective resting Vm (Control 1.8 (cid:3) 0.4 nS, n = 8; Kir2.1-OE 25.5 (cid:3) 9.6 nA, n = 6).
H
I
Data information: In panels F–I, data are shown as means (cid:3) SD, and the single cell value recordings (open symbols) are plotted with the average data. All statistical tests
were performed using the Student’s t-test. The number of IHCs investigated is shown above the average data points (6 control and 3 Kir2.1-OE mice).
Source data are available online for this figure.
piezo-driven fluid jet (Corns et al, 2018; Carlton et al, 2021). A large
MET current was elicited in all IHCs tested from control (Fig 3A)
and Kir2.1-OE (Fig 3B) mice at P6-P7 when their stereociliary
bundles were moved towards the taller stereocilia (i.e., in the excita-
tory direction) at negative membrane potentials. By stepping the
membrane potential from (cid:1)124 mV to more depolarized values in
20 mV increments, the transducer current decreased in size at first
and then reversed near 0 mV in IHCs from both genotypes (Fig 3A–
C), consistent with the nonselective permeability of MET channels
to cations. The maximal MET current at both (cid:1)124 mV and +96 mV
was not significantly different between control and littermate Kir2.1-
OE mice (P = 0.2269 and P = 0.3620, respectively, t-test, Fig 3D).
The resting open probability of the MET channel, which is derived
from the current flowing through open transducer channels in the
absence of mechanical stimulation (arrows: Fig 3A and B), was also
not significantly different between the IHCs from the two genotypes
((cid:1)124 mV: P = 0.3766; +96 mV: P = 0.2846, Fig 3E). At P8-P9, the
size of the MET currents became more variable in the IHCs overex-
pressing the Kir2.1 channel, with some cells showing a third of the
current recorded from control mice (Fig 4A–C). Overall, the size of
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Figure 2. Kir2.1 overexpression prevents spontaneous, but not induced, Ca2+ action potentials in IHCs.
A, B Whole-cell recordings of Ca2+ action potential activity in apical-coil IHCs from P4 control (A) and Kir2.1-OE (B) mice in the presence of 1.3 mM Ca2+ in the extracel-
lular solution and at body temperature. Note that IHCs from control mice (A) fire spontaneous action potentials (40 s out of 142 s recording time), while those from
overexpressing Kir2.1 IHCs (B) require a substantial current injection to elicit any spikes. For voltage-clamp data see also Fig EV1C–I. Data in Panel A,B, and
Fig EV1C–I were obtained from 8 control IHCs (7 mice) and 11 Kir2.1-OE IHCs (6 mice).
C, D Calcium action potentials in IHCs from control (C) and Kir2.1-OE (D) mice during the second postnatal week. IHC voltage responses were recorded during the appli-
cation of a solution containing 1.3 mM Ca2+ (top panels) or 0.3 mM Ca2+ (bottom panels). The latter Ca2+ concentration (0.3 mM), which was used to mimic the
estimated in vivo Ca2+ concentration in the endolymphatic compartment (Johnson et al, 2012), caused control IHCs, but not those from Kir2.1-OE mice, to elicit
spontaneous action potentials (40 s out of 56 s recording time).
Diagram showing a cross-section of an immature organ of Corti. IHCs: inner hair cells; GER: greater epithelial ridge, which includes nonsensory cells surrounding
the IHCs. Red arrows indicate the propagation of ATP-induced Ca2+ waves from the GER towards the IHCs, which leads to their depolarization (Tritsch et al, 2007;
Wang et al, 2015; Johnson et al, 2017).
E
F, G Representative DF/F0 traces from the IHCs and GER of P7-P9 control (F) and Kir2.1-OE (G) mice in the presence of 0.3 mM Ca2+. Spontaneous ATP-dependent Ca2+
waves from the GER (green traces) were eliciting coordinated Ca2+ signals in the IHCs from both controls and Kir2.1-OE mice. For each genotype, two separate sets
of recordings from 2 mice are shown (top and bottom right), with the top traces being linked to the images on the left: before [1], during [2] and after [3]) the gen-
eration of a large Ca2+ wave from the GER. For details about the frequency and duration of the Ca2+ waves, and the number of mice and recordings see Fig EV3. All
recordings were obtained at body temperature. Traces are computed as pixel averages of regions of interest centred on IHCs.
Source data are available online for this figure.
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Figure 3. Mechanoelectrical transduction in Kir2.1 overexpressing IHCs is normal during the first postnatal week.
A, B Saturating MET currents recorded from apical IHCs of P6 control (A) and Kir2.1-OE (B) mice in response to 50 Hz sinusoidal force stimuli to the hair bundles at
C
D
E
membrane potentials of (cid:1)124 and +96 mV. Driver voltage (DV) stimuli to the fluid jet are shown above the traces, with positive deflections of the DV being excita-
tory. The arrows indicate the closure of the transducer channel in response to inhibitory bundle stimuli at (cid:1)124 and +96 mV.
Peak-to-peak MET current–voltage curves from P6-P7 apical-coil IHCs of 7 control (12 IHCs) and 3 littermate Kir2.1-OE mice (7 IHCs). Recordings were obtained by
mechanically stimulating the hair bundles of IHCs at the same time as stepping their membrane potential from (cid:1)124 mV to +96 mV in 20 mV increments. The
two sets of data are not significantly different: P = 0.6320, 2-way ANOVA.
Maximum size of the MET current recorded at (cid:1)124 mV (left panel) and +96 mV (right panel) in IHCs from both genotypes.
Resting open probability (Popen) of the MET current in IHCs from the two genotypes measured at (cid:1)124 mV (left) and +96 mV (right). The resting open probability
was calculated by dividing the resting MET current (the difference between the current level before the stimulus, indicated by the dashed line, and the current level
at the negative phase of the stimulus when all channels are closed) by the maximum peak-to-peak MET current.
Data information: All comparisons in panels D and E are not significantly different between the two genotypes (D: (cid:1)124 mV: P = 0.2269; +96 mV: P = 0.3620;
E: (cid:1)124 mV: P = 0.3766; +96 mV: P = 0.2846, t-test). In panels C-E, data are shown as means (cid:3) SD, and the single cell value recordings (open symbols) are plotted
behind the average data. The number of IHCs investigated is shown above the averaged data points from 7 control and 3 Kir2.1-OE mice.
Source data are available online for this figure.
the MET current was significantly reduced ((cid:1)124 mV: P < 0.0001;
+96 mV: P = 0.0003, Fig 4D) and the resting open probability
increased ((cid:1)124 mV: P = 0.0080; +96 mV: P = 0.0130, Fig 4E) in
IHCs from Kir2.1-OE mice compared with controls. Since an
increased resting open probability of the MET channel could be
associated with changes, specifically a reduction, in the free Ca2+
inside the stereocilia, we tested this possibility by changing the
intracellular Ca2+
buffering capacity by using different concentra-
tions of the fast Ca2+
chelator BAPTA. Increasing the intracellular
BAPTA from 0.1 to 5 mM significantly augmented the resting open
probability of
in IHCs from both genotypes,
although at both BAPTA concentrations it was significantly higher
in the IHCs of Kir2.1-OE mice (Fig EV4). This indicates that in the
absence of spontaneous intrinsic firing activity in the IHCs of Kir2.1-
OE mice, the MET channels are likely to have a reduced Ca2+
sensi-
tivity during the second postnatal week. By P10-P11, we found that
the MET current in the IHCs of Kir2.1-OE mice was very small or
the MET channel
absent (Fig 4F–I). At this stage, IHCs from P10-P11 Kir2.1-OE mice
also failed to load with the styryl dye FM1-43 (Fig 4J), which is a
permeant blocker of the hair cell MET channel and functions as an
optical readout for the presence of the resting MET current (Gale
et al, 2001).
IHCs from Kir2.1-OE mice undergo progressive loss and fusion of
the stereocilia
We investigated whether the rapid reduction in the MET current
was caused by defects in the growth and/or maintenance of the
stereociliary bundles in IHCs. Using scanning electron microscopy
we found that the hair bundles of the IHCs from Kir2.1-OE mice
were able to develop a staircase structure composed of rows of
stereocilia that were indistinguishable from those present in control
cells (arrows: Fig 5A and B). This is consistent with the presence of
a normal MET current at least up to the end of the first postnatal
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week (Fig 3). However, from about P9 onwards, IHCs from Kir2.1-
OE mice started to lose the shorter third row of stereocilia (Fig 5B).
A few IHCs also started to exhibit stereocilia fusion, which became
more pronounced at older ages. By P26, none of the IHCs in the
Kir2.1-OE mice showed normal-looking bundles, which instead
exhibited profound stereocilia fusion (Fig 5C and D). Kir2.1+/(cid:1)
mice
that were not crossed with OtofrtTA+/(cid:1)
mice showed normal hair-
bundle development when treated with DOX, highlighting the speci-
ficity of the Kir2.1-OE strategy (Fig EV5). These data indicate that
spontaneous Ca2+
actions potential activity during the second
Figure 4.
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◀ Figure 4. Rapid disappearance of the MET current in Kir2.1 overexpressing IHCs during the second postnatal week.
A, B Saturating MET currents recorded from apical IHCs of P8 control (A) and Kir2.1-OE (B) mice. IHC hair bundles were stimulated as described in Fig 3.
C
Peak-to-peak MET current–voltage curves from P8-P9 apical-coil IHCs of 12 control (17 IHCs) and 13 littermates Kir2.1-OE mice (24 IHCs). The two sets of data are
significantly different: P < 0.0001, 2-way ANOVA.
The maximum size of the MET current measured in IHCs at (cid:1)124 mV (left panel) and +96 mV (right panel) from Kir2.1-OE mice was significantly reduced compared
to that of control cells.
The resting open probability (Popen) of the MET current in IHCs was significantly increased in Kir2.1-OE compared with control cells at both (cid:1)124 mV (left) and
+96 mV (right).
D
E
F, G Saturating MET currents recorded from apical IHCs of control (F, P10) and Kir2.1-OE (G, P11) mice. IHC hair bundles were stimulated as described in Fig 3.
H
Peak-to-peak MET current–voltage curves from P10-P11 apical-coil IHCs of 13 control (16 IHCs) and 4 littermate Kir2.1-OE mice (9 IHCs). The two sets of data are
significantly different: P < 0.0001, 2-way ANOVA.
The maximum size of the MET current measured in IHCs at (cid:1)124 mV (left panel) and +96 mV (right panel) from Kir2.1-OE mice is significantly reduced compared
to that of control cells.
Example of FM1-43 uptake by IHCs from P11 control (top) and Kir2.1-OE (bottom) mice, showing the lack of fluorescence labeling in the latter, which is an indica-
tion of the lack of MET channels open at rest at this stage in the IHCs overexpressing the Kir2.1 channels. At least 3 mice for each genotype were used.
I
J
Data information: In panels D, E, I, data are shown as means (cid:3) SD, and the single cell value recordings (open symbols) are plotted with the average data. The number of
IHCs investigated is shown above the average data points from 12 control and 13 littermates Kir2.1-OE mice (panels D and E) and 13 control and 4 littermate Kir2.1-OE
mice (panel I). Statistical tests in panels D, E, I was done using the t-test. The * defines the presence of statistical significance, with the P-value shown above the data.
Source data are available online for this figure.
postnatal week is required for the maintenance of the stereociliary
bundles in the mature IHCs.
Localization of bundle proteins is not affected in Kir2.1-OE mice
To establish whether the progressive loss and fusion of the stere-
ocilia were linked to the mislocalization of some of the key proteins
expressed in the hair bundles, we performed immunostaining exper-
iments on both genotypes. Stereocilia fusion has previously been
documented in hair cells from mice lacking Myo6, the gene encoding
for the (F-actin) minus end-directed unconventional myosin 6 (Self
et al, 1999). We found that MYOSIN VI was expressed in the stere-
ocilia of the IHCs from both control and littermate Kir2.1-OE mice
(Fig 6A and C). The disorganized hair bundle of the IHCs from
Kir2.1-OE mice also showed a normal distribution at the tip of the
taller rows of stereocilia of EPS8, MYOSIN XV-isoform 1 and
WHIRLIN (Fig 6B and D); key proteins required for growth and
maintenance of stereocilia (Belyantseva et al, 2005; Delprat et al,
2005; Manor et al, 2011; Zampini et al, 2011).
IHC action potentials exert their developmental role in
stereocilia maintenance during a critical period
Next, we tested whether Ca2+
action potential activity in IHCs was
regulating hair-bundle maintenance during a specific time window
or “critical period” of prehearing development. This was achieved
by downregulating Kir2.1-OE in vivo by removing DOX from the
drinking water at a specific developmental time point. Considering
that the hair bundles of the IHCs in Kir2.1-OE mice were able to
acquire a staircase structure and have normal MET current at the
end of the first postnatal week, we sought to test whether the role of
the Ca2+
-action potentials was restricted to the second week, just
before hearing onset at ~P12.
As for the above investigation, DOX was continuously supplied
to the females from the time of conception, but for this set of experi-
ments, it was then removed from the drinking water when the pups
were P5. We found that 2–3 days without DOX was sufficient to
strongly downregulate Kir2.1 from the membrane of the IHCs of
Kir2.1-OE mice
the
(Fig 7A and B). This
indicates
that
overexpression of Kir2.1 in IHCs was primarily occurring during the
first postnatal week under these conditions. We then investigated
whether the downregulation of Kir2.1 channels following DOX
removal (Appendix Fig S2) re-established the ability of IHCs to fire
intrinsic spontaneous action potentials.
In 1.3 mM extracellular
Ca2+
, action potentials only occurred during depolarizing current
injections in the IHCs from both control and Kir2.1-OE mice in the
second postnatal week (Fig 7C and D; see also Fig 2C and D). In the
the in vivo endolymph-like Ca2+
presence of
concentration
(0.3 mM), spontaneous intrinsic firing was present not only in the
IHCs of control mice (Fig 7E; see also Fig 2C) but also in Kir2.1-OE
mice (Fig 7F). For long-lasting current clamp recordings, spikes
occurred in a bursting pattern in both controls and Kir2.1-OE mice.
The mean spike frequency (1.17 (cid:3) 0.47 Hz, n = 4) and CV
(1.12 (cid:3) 0.17, n = 4 IHCs, duration of the recordings 62–125 s) in
control mice were not significantly different from those measured in
Kir2.1-OE mice (frequency: 1.29 (cid:3) 0.83 Hz, n = 5 IHCs, P = 0.7766;
CV: 1.15 (cid:3) 0.11, n = 5, duration of
the recordings 32–101 s,
P = 0.7954). The IHC resting membrane potential was not signifi-
cantly different between control and Kir2.1-OE mice in the presence
of both 1.3 mM and 0.3 mM Ca2+
(Fig 7G). These data indicate that
the removal of DOX was effective in downregulating Kir2.1 channels
and restoring the normal physiology of the IHCs. We also found that
when DOX was removed at P5, IHCs were able to maintain their
hair-bundle structure after the onset of hearing (Fig 7H–K). These
findings indicate that Ca2+
regulation via action potentials in IHCs is
required for the final maturation and maintenance of
the hair
bundles after a critical point just before hearing onset.
Identification of genes regulated by the intrinsic Ca2+ action
potentials using RNA-sequencing
To understand the molecular pathways underpinning the changes in
the hair-bundle structure observed in the absence of the intrinsic
Ca2+
action potential activity in IHCs, we performed RNA-seq on P9
controls and littermate Kir2.1-OE mice. At this age, most of the hair
bundles still showed a normal-looking structure, but with some
IHCs having lost the 3rd row of stereocilia and some showing stere-
ocilia fusion (Fig 5B). This was associated with the onset of MET
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IHC bundle morphology progressively deteriorates in Kir2.1 overexpressing mice.
Figure 5.
A, B Scanning electron microscope (SEM) images showing the IHC hair-bundle structure in the apical coil of the cochlea of P11 control (A) and P8-P11 Kir2.1-OE (B)
mice. Control IHCs and the large majority of P8 IHCs from Kir2.1-OE mice show a normal hair-bundle structure composed of three rows of stereocilia: tall, interme-
diate and short (arrows). From about P9 in Kir2.1-OE mice, some IHCs start to lose the third row of stereocilia (arrowheads) and some already exhibit some fusion
of the stereocilia (asterisk). These changes in hair-bundle structure became more prominent at P11. At least 3 mice for each genotype were used. In these panels
and those below, asterisks are used to define some of the abnormal hair bundles.
C, D SEM images of both IHCs and OHCs from the cochlea of P26 control (C, upper panel) and P26 Kir2.1-OE (D, upper panel) mice. Lower panels show a higher-
magnification view of the hair bundle of IHCs from both genotypes, highlighting the profound disruption of the stereocilia in IHCs overexpressing Kir2.1 channels.
At least 3 mice for each genotype were used.
Source data are available online for this figure.
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Figure 6. Hair-bundle proteins involved in stereociliary elongation are not affected in IHCs from Kir2.1-OE mice.
A, B Maximum intensity projections of confocal z-stacks showing images of the hair bundles from apical-coil IHCs of P6 and P11 control (A) and Kir2.1-OE (B) mice
immunostained with antibodies against MYOSIN VI (blue) and EPS8 (magenta). At least 3 mice for each genotype were used.
C, D Confocal images of the hair bundles of P11 IHCs from control (C) and Kir2.1-OE (D) mice immunostained with antibodies against MYOSIN XV-isoform 1 (blue) and
WHIRLIN (magenta). In all panels (A–D), stereocilia are labeled with phalloidin (green). Note that despite the disrupted hair-bundle structure in the IHCs overex-
pressing Kir2.1 channels; the stereocilia retained a normal distribution of these bundle proteins. At least 3 mice for each genotype were used.
Source data are available online for this figure.
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current reduction in at least some of the IHCs (Fig 4A–E). We rea-
soned that by profiling animals at this age we could understand the
early molecular response that leads to abnormal hair-bundle mor-
phology.
RNA-sequencing was performed on three replicates, each with
eight pooled organs of Corti
from four mice. Total RNA was
extracted and sent for library preparation and sequencing. Sequence
data were mapped to the mouse genome (mm10) using the
Figure 7.
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◀ Figure 7. Ca2+ spikes in IHCs regulate bundle morphology over a critical period during the second postnatal week of development.
A, B Maximum intensity projections of confocal z-stacks showing the IHCs of the apical cochlear region from control (A) and Kir2.1-OE (B) pups with the females being
in the continuous presence of DOX in the drinking water from conception up to when the pups were P5 (upper panels). Middle and bottom panels show IHCs at P7
and P14 following the removal of DOX at P5 for both control (A) and Kir2.1-OE mice (B). IHCs were stained with antibodies against the K+ channel Kir2.1 (green)
and Myosin 7a (Myo7a, blue: cell marker). Note that after 2 days following the removal of DOX, Kir2.1 overexpression was already largely downregulated. At least 3
mice for each genotype were used.
E, F
C, D Calcium action potentials in IHCs from control (C) and Kir2.1-OE (D) mice during the second postnatal week (P8–P9). IHC voltage responses were recorded during
the application of 1.3 mM Ca2+ extracellular solution. The voltage-clamp data recorded from the same two IHCs displayed in panels C and D are shown in
Appendix Fig S2; the IHCs from Kir2.1-OE mice show a strongly reduced Kir2.1 current. DOX was removed from the drinking water at P5.
Spontaneous Ca2+ action potentials in IHCs from control (E, 60 s out of 92 s recording time) and Kir2.1-OE (F, 60 s out of 103 s recording time) mice during the sec-
ond postnatal week in the presence of the in vivo endolymph-like 0.3 mM Ca2+. Note that in contrast to when DOX was present throughout development (Fig 2A–
D), the removal of DOX at P5 restored the ability of IHCs from Kir2.1-OE mice to generate spontaneous intrinsic Ca2+ action potentials.
IHC resting membrane potentials from 2 control (4 IHCs) and 3 Kir2.1-OE (7 IHCs) mice in the presence of 1.3 mM Ca2+ (left) or 0.3 mM Ca2+ (right) in the extracel-
lular solution. One-way ANOVA followed by the Bonferroni’s post-test: ns, P > 0.9990 (1.3 mM Ca2+); ns, P = 0.8864 (0.3 mM Ca2+); all other comparisons were
*P < 0.0001.
G
H, I Maximum intensity projections of confocal z-stacks showing images of the hair bundles from apical-coil IHCs of P14 control (H) and Kir2.1-OE (I) mice stained with
J, K
phalloidin. DOX was removed from the mother’s drinking water when the pups were P5. At least 3 mice for each genotype were used.
SEM images showing the normal structure of the hair bundles of the IHCs in the apical coil of the cochlea of P14 control (J) and P14 Kir2.1-OE (K) mice. DOX was
removed from the mother’s drinking water when the pups were P5. Note that the morphological profile of the hair bundles in IHCs is comparable between control
and Kir2.1-OE mice, indicating that the removal of the intrinsic Ca2+ action potentials prior to the second postnatal week has no effect on the mechanoelectrical
transduction apparatus. At least 3 mice for each genotype were used.
Source data are available online for this figure.
NextFlow RNA pipeline and gene counts were performed using Sal-
mon (see Materials and Methods). These raw counts were then used
as the input for differential gene expression analysis using DeSEQ2
(Love et al, 2014). After performing principal component analysis
(PCA) on the top 1,000 expressed genes in the samples, we observed
a clear separation between the different genotypes with PC1, which
separated Kir2.1-OE and control mice, explaining 85% of
the
observed variance. Conversely, PC2, which mostly separated the dif-
ferent biological replicates, explained 8% of variance between sam-
ples (Fig 8A). As expected, we observed a 13-fold increase in Kcnj2
(Fig 8B), validating the overexpression of the Kir2.1.
We next performed differential gene expression analysis (Padj <
0.05 and fold-changes >1.5), yielding 589 upregulated genes and 30
downregulated genes (Dataset EV1; Appendix Fig S3). Pathway
analysis showed an enrichment of GO terms related to cell morpho-
genesis, actin filament-based processes and Rho-GTPase signaling
(Fig 8C). Among the differentially expressed genes were 118 upregu-
lated genes with annotations related to actin filament or microtubule
regulation (Fig 8D). We also noted several genes related to the Golgi
body and the trans-Golgi network (TGN), for example, Golga3, Gol-
ga4, and Trip11, which are all hypothesized to play a role in main-
taining Golgi structure. In line with the stereocilia phenotype, we
observed the upregulation of some components of the stereocilia,
Myo7a and Pcdh15 (2.25 and 2.15-fold, respectively) (Fig 8E).
We also performed network analysis on known protein–protein
interactions on the differentially expressed genes
(Fig 8G,
Dataset EV2). Chromatin remodeling genes were also overrepre-
sented among the upregulated genes, including DNA methylation
(Dnmt1, Dnmt3a) and demethylation (Tet1, Tet2, Tet3) and histone
modifying enzymes (Hdac4, Setd2, Setd5). Several components of
the LINC complex that connects the nuclear lamina to the cytoskele-
tal network, including the subunits of the laminin complex (Lama
1, 2, 4, 5, Lamb1,2, Lamc1 and 2) and the Nesperin family (Syne1,
Syne2, Syne3) that connect the cytoskeletal network to laminin,
were upregulated (Fig 8G, Dataset EV2). Mechanical signals are
directly transduced from extracellular stimulus to the nuclear inte-
rior through the interaction of the nesperin proteins (Khilan et al,
structure and
2021). Moreover,
the maintenance of nuclear
Figure 8. RNA-sequencing reveals upregulation of microtubule and cytoskeletal genes in Kir2.1-OE mice.
A PCA plot of each RNA library. Each point represents one pool of 4 mice (8 cochleae) for both control and littermates Kir2.1-OE mice. Principal components were calcu-
lated using the top 1,000 genes after rlog transformation, using DESeq2. The percent variance for each principal component is noted on the axis.
B Transcripts per million (TPM) counts of Kcnj2 for each RNA-seq library. Counts were performed using Salmon, and length normalized. Each point represents a single
library. Bars represent the mean (cid:3) SD.
C Top GO terms associated with the significantly (Padj. < 0.05) upregulated genes in the Kir2.1-OE mouse.
D Heatmap of the counts for each RNA library for the 118 genes associated with microtubule or cytoskeletal processes. Each row is z-scored. 118 genes were all differ-
entially expressed as per the previous analysis.
E TPM counts of Pcdh15, Myo7a, Macf1, Ank2, Myh9, Map1a, Map1b, Cep290 and Sptbn1 for each RNA-seq library. Counts were performed using Salmon and normalized
to the length of the gene sequence. Each point represents a single library. Bars represent the mean (cid:3) SD. The numbers above the data represent the multiple
hypotheses corrected adjusted P-values derived from DESeq2 using the Wald test.
F Results of the top 15 motifs that were overrepresented in the TSS of the upregulated genes in Kir2.1. Enrichment analysis was performed using HOMER, in the
(cid:3) 2,000 bp region around the transcriptional start site of upregulated genes.
G A network rendering of the top GO processes associated with the upregulated gene set. Networks were seeded with the upregulated genes and their nearest
interaction and visualized using Cytoscape.
Source data are available online for this figure.
▸
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Figure 8.
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organization is regulated by laminins, which also help to transduce
mechanical strain forces into a transcriptional response.
We next sought to determine which transcription factors (TFs)
might be mediating the upregulated genes in Kir2.1 overexpression.
Using the list of 589 upregulated genes, we used HOMER (Heinz
et al, 2010) to scan the region (cid:3) 2,000 bp from the transcriptional
start site (TSS) of each gene for TF binding motifs. Within the top
fifteen enriched motifs were several hair cell-enriched TFs, such as
Isl1, Sox9, and Gfi1 (Fig 8F). Several classic targets of SOX9, includ-
ing Acan, Col2a1, Col4a1, Col5a1, Col11a1/2, Col23a1, and several
ankyrin family proteins (genes: Ank1, Ank2, Ankrd11, Ankrd12)
and Myo9b, were found to be SOX9 target genes in chromatin
immunoprecipitation with sequencing (ChIP-seq) studies conducted
in rib chondrocytes (Ohba et al, 2015). Of the 602 upregulated
genes, 65% overlapped with SOX9 target genes in chondrocytes.
Similarly, SOX9 ChIP-seq in the developing testis found SOX9 bound
on Myo7a (Li et al, 2014). We also observed enrichment for the RFX
family, which plays a conserved role in ciliogenesis in many differ-
ent organisms (Lemeille et al, 2020).
Discussion
Here we show that spontaneous intrinsic Ca2+
action potential activ-
ity present in the developing IHCs, and thus Ca2+
regulation, is cru-
cial for the final stages of maturation and maintenance of the
stereociliary hair bundles. The absence of the intrinsic action poten-
tials during the second postnatal week led to a progressive re-
absorption of stereocilia in the short 3rd row and a fusion of the
tallest rows, generating “giant” stereocilia. The functional conse-
quence of this hair-bundle disruption was a complete loss of mecha-
noelectrical
transduction prior to the onset of hearing at P12.
Furthermore, we show that this intrinsic regulation of IHC develop-
ment occurs during a critical time window that spans the second
postnatal week of development, just before hearing onset. The RNA-
sequencing analysis highlighted that absence of intrinsic APs caused
the upregulation of genes involved in cytoskeleton and Rho-GTPase-
related pathways, several of which have not been previously associ-
ated with cochlear development.
Calcium-dependent activity in the developing cochlea
The initial morphological and functional differentiation of cochlear
sensory hair cells depends on intrinsic genetic programs that are
coordinated by a combination of transcription factors, including Ato-
h1 (Bermingham et al, 1999), Helios (Chessum et al, 2018) and
Tbx2 (Garc(cid:1)ıa-A~noveros et al, 2022), and microRNAs such as miR-96
(Kuhn et al, 2011). However, evidence from other sensory systems,
especially from the visual system (e.g., Grubb & Thompson, 2004;
Blankenship & Feller, 2010), shows that the final maturation of sen-
sory pathways is driven by experience-independent Ca2+
-dependent
activity, which occurs during a critical period of development. This
early electrical activity has been shown to regulate several cellular
responses (Berridge et al, 2000), including the remodeling of synap-
tic connections (Zhang & Poo, 2001) and ion-channel expression
(Moody & Bosma, 2005).
In the mammalian cochlea, spontaneous Ca2+
throughout
recorded
have
been
potentials
-dependent action
postnatal
the
(cid:1)
+
channels and the efflux of K
development of the IHCs (Kros et al, 1998; Glowatzki & Fuchs,
2000; Beutner & Moser, 2001; Marcotti et al, 2003a; Brandt et al,
2007). The firing activity of neighboring IHCs is normally synchro-
nized by spontaneous intercellular Ca2+
signaling originating in the
nonsensory cells via the release of ATP (Tritsch et al, 2007; Johnson
et al, 2011, 2017; Wang et al, 2015; Eckrich et al, 2018). ATP acts
on purinergic autoreceptors expressed in the nonsensory cells sur-
rounding the IHCs, which leads to the opening of TMEM16A Ca2+
-
activated Cl
in the intercellular space,
causing IHC depolarization (Wang et al, 2015). Although Ca2+
action
potential activity in developing IHCs has been linked to the refine-
ment of the tonotopic organization in the brainstem (Clause et al,
2014, 2017; M€uller et al, 2019; Maul et al, 2022) and auditory neu-
ron survival (Zhang-Hooks et al, 2016), its direct role in regulating
and/or maintaining IHC development is still largely unknown. A
previous study has shown that increasing the IHC firing activity pre-
vented linearization of their exocytotic Ca2+
dependence in the adult
cochlea (Johnson et al, 2013), although both the intrinsic and ATP-
dependent mechanisms could have contributed.
The mouse model used in this study (Kir2.1-OE) has allowed us
to specifically silence the intrinsic Ca2+
action potentials in develop-
ing IHCs in vivo while retaining the ability of nonsensory cells to
depolarize the IHCs via ATP-dependent Ca2+
signaling. We found
that the absence of spontaneous Ca2+
action potentials that are
intrinsically generated by the IHCs prevented the full maturation
and maintenance of the hair bundles in IHCs, thus abolishing the
mechanoelectrical transducer current that is required for the conver-
sion of acoustic stimuli into electrical signals. We found that such
crucial control over the hair-bundle structure and function is only
established after a critical time point in the second postnatal week,
just before hearing onset.
Role of Ca2+-dependent action potentials in the maturation of
hair cells
Calcium-dependent electrical activity regulates several cellular
responses (Berridge et al, 2000). Changes in intracellular Ca2+
sig-
nals mediated by L-type Ca2+
channels have been implicated in regu-
lating gene expression in many intracellular pathways including
those associated with remodeling and development (Bading et al,
1993; Dolmetsch et al, 1997; Fields et al, 2005; Hagenston & Bading,
2011). Here we show using RNA-seq analysis that the dysregulation
of Ca2+
in prehearing IHCs, which only retain the extrinsic modula-
tion of the Ca2+
signals from the nonsensory cells, led to 589 upregu-
lated and 30 downregulated genes.
One of the characteristic phenotypes of the Kir2.1-OE mouse
cochlea was the formation of giant or fused stereocilia, which was
previously reported in knockout mice for the protein TRIOBP (Kita-
jiri et al, 2010), which is a component of the stereocilia rootles
(Pacentine et al, 2020), for the unconventional MYO6 (Self et al,
1999) that localizes at the base and all along the length of the stere-
ocilia (Hertzano et al, 2008) and for a protein associated with the
shaft connectors located between stereocilia (PTPRQ, Goodyear
et al, 2003). RNA-seq analysis did not show any significant changes
in the genes encoding the above three proteins in the cochlea of
Kir2.1-OE mice but did identify 118 upregulated genes with annota-
tions related to actin filament or microtubule regulation. This
included cytoskeletal genes Mapt, Sptb, Plec, and Nefh, Kinesin
(cid:1) 2023 The Authors
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The EMBO Journal
Adam J Carlton et al
superfamily proteins, which are microtubule-dependent molecular
motors (Kif1a, Kif5a, Kif5c, Kif21a, Kif21b), and several components
of the Rho-GTPase pathway (Rock1, Iqgap1, Iqgap2, Itpr1, Itpr2,
Itpr3, Arhgap13, Argef11, Argef17, Trio, and Kalrn). Although most
of the identified genes are possible novel candidates involved in hair
cell development, we found some that have previously been associ-
ated with hair-bundle morphology. For example, the actin-binding
protein spectrin isoform SPTBN1 (Sptbn1), which is expressed in
the rootlets actin filaments of the stereocilia (Furness et al, 2008),
and together with TRIOBP contributes to strengthen their insertion
point into the apical membrane of the hair cells (Pacentine et al,
2020), is required for the correct hair-bundle morphology. In the
absence of SPTBN1 mice are deaf (Liu et al, 2019). Furthermore, the
actin crosslinking family protein 7 (ACF7) and the microtubule-
associated protein 1A (MAP-1A), which are encoded by the genes
Macf1 and Macp1a, respectively, are also involved in the organiza-
tion of the cuticular plate of the hair cells (Jaeger et al, 1994;
Antonellis et al, 2014), which is the point of stereocilia insertion of
the hair bundles. Finally, the non-muscle myosin Type IIA (MYH9)
has been linked to both syndromic and nonsyndromic hearing loss
due to the disruption of the hair cell stereociliary bundles (Mhatre
et al, 2006). Among the identified transcription factors, we found
enrichment for regulatory factor X (RFX), which plays a role in
Materials and Methods
Reagents and Tools table
Reagent/Resource
Experimental Models
OtofrtTA (M. musculus)
TetO-Kir2.1-IRES-tauLacZ
Recombinant DNA
Example: pCMV-BE3
Antibodies
Mouse-IgG1 anti-Eps8
Rabbit-IgG anti-Whirlin
Rabbit-IgG anti-Myo6
Rabbit-IgG anti-Myo15 isoform 1
Mouse-IgG1 anti-Kir2.1
Mouse-IgG1 anti-CtBP2
Mouse-IgG2a anti-GluR2
Chemicals, enzymes, and other reagents
Doxycycline
Vitamins
Amino acids
DMEM/F12
Fluo-4 AM
FM1-43
Texas Red-X phalloidin
ciliogenesis in many different organisms (Lemeille et al, 2020). In
mammals, Rfx3 is involved in ciliary assembly and motility, and
Rfx4 is known to modulate Shh signaling and regional control of cili-
ogenesis (Ashique et al, 2009). Moreover, recent work has shown
that the RFX family is essential for hearing in mice, with mice at
3 months of age showing a loss of stereocilia structure (Elkon et al,
2015).
Altogether,
indicate that
these results
the intrinsic Ca2+
-
dependent action potential activity in IHCs during the second post-
natal week is necessary to drive their full morphological and func-
tional maturation into auditory sensory receptors. The absence of
such activity led to the upregulation of
the genetic pathways
involved in the maintenance of cytoskeletal homeostasis, possibly
as an attempt to repair or compensate for the progressive deteriora-
tion of the actin-based hair bundles. Moreover, we found that the
MET channel of the IHCs from Kir2.1-OE mice acquire a reduced
Ca2+
sensitivity, which could be a potential compensatory mecha-
nism for maintaining resting MET current size as the MET current is
rapidly declining. Although genetic compensation responses follow-
ing the mutation of genes have been described in many organisms
including zebrafish and mice (e.g., El-Brolosy & Stainier, 2017;
Buglo et al, 2020), their underlying mechanisms remain largely
unknown.
Reference or Source
Identifier or Catalog Number
Ozgene Pty Ltd
The Jackson Lab
Addgene
BD Biosciences
gift from Dr. Thomas Friedman
Proteus Biosciences
gift from Dr. Thomas Friedman
Alomone Lab
BD Biosciences
Millipore
Karidox 100 mg/ml
Thermo Fisher
Thermo Fisher
Sigma
Thermo Fisher
Molecular Probes
ThermoFisher
n/a
009136
Cat #73021
610143
n/a
25-6791
n/a
APC026
612044
MAB397
11120-037
11130-036
D8062
F14201
T3163
T7471
14 of 20
The EMBO Journal 42: e112118 | 2023
(cid:1) 2023 The Authors
Adam J Carlton et al
The EMBO Journal
Reagents and Tools table (continued)
Reagent/Resource
Reference or Source
Identifier or Catalog Number
Software
pClamp 10
Origin
Python 2.7
ImageJ
DeSeq2
Metascape
Reactome
HOMER
Other
Digidata 1440A
Two-photon laser-scanning microscope (Bergamo II system B232)
Vega3 LMU scanning electron microscope
LSM 880 AiryScan microscope
RNeasy Plus Micro Kit
NovaSeq sequencer
RRID:SCR_011323
RRID:SCR_014212
RRID:SCR_008394
RRID:SCR_003070
RRID:SCR_021038
Molecular Devices
OriginLab
Python Software Foundation
NIH
Love et al, 2014
Zhou et al, 2019
Gillespie et al, 2022
Heinz et al, 2010
Molecular Devices
Thorlabs Inc
Tescan
Zeiss
Qiagen
Illumina
Methods and Protocols
Ethics statement
Animal work was licensed by the Home Office under the Animals
(Scientific Procedures) Act 1986 (PPL_PCC8E5E93) and was
approved by the University of Sheffield Ethical Review Committee
(180626_Mar). For ex vivo experiments mice were killed by cervical
dislocation followed by decapitation. Mice had free access to food
and water and a 12 h light/dark cycle.
Transgenic mice
The transgenic mouse line OtofrtTA expressing rtTA driven by the Otof
promoter was constructed by Ozgene Pty Ltd (Bentley WA, Australia).
In these mice, the expression of a target gene is controlled by a reverse
tetracycline-controlled transactivator rtTA (Tet-On system: Baron &
Bujard, 2000). Otof encodes for the ribbon synaptic Ca2+
sensor otofer-
lin, which in the cochlea is expressed exclusively in hair cells but pri-
marily in IHCs from around birth (Roux et al, 2006). Homozygous
OtofrtTA mice were paired with heterozygous teto-Kir2.1-IRES-tau-lacZ
mice (Kir2.1: Jackson laboratories, 009136, Yu et al, 2004). Both
mouse lines (OtofrtTA and Kir2.1) were maintained on the C57BL/6N
background. The resultant compound heterozygous mice, which we
named Kir2.1-OE (Kir2.1-OverExpression) mice for simplicity, allowed
+
cell-specific overexpression of the inward rectifier K
channel Kir2.1
in the IHCs when mice were treated with doxycycline (DOX). Litter-
mate heterozygous OtofrtTA mice treated with DOX were used as con-
trols. Pregnant, breast-feeding females and weaned pups (controls
and Kir2.1-OE) were given 0.5 mg/ml of DOX daily in their drinking
water, a dose that was previously optimized for the mouse cochlea
(Johnson et al, 2013).
Tissue preparation
Cochleae were dissected out from the inner ear of the mouse using
an extracellular solution composed of (in mM): 135 NaCl, 5.8 KCl,
1.3 CaCl2, 0.9 MgCl2, 0.7 NaH2PO4, 5.6 D-glucose, 10 HEPES.
Sodium pyruvate (2 mM), amino acids, and vitamins were added
from concentrates (Thermo Fisher Scientific, UK). The pH was
adjusted to 7.48 with 1 M NaOH (osmolality ~308 mOsm/kg). The
dissected cochleae were transferred to a microscope chamber and
immobilized via a nylon mesh attached to a stainless-steel ring as
previously described (Marcotti et al, 2003b). The chamber (vol-
ume ~ 2 ml) was perfused from a peristaltic pump and mounted on
the stage of an upright microscope (Olympus BX51, Japan; Leica
DMLFS, Germany) equipped with Nomarski Differential Interference
Contrast (DIC) optics (60× or 64× water immersion objective) and
15× eyepieces. The microscope chamber was continuously perfused
with the extracellular solution by a peristaltic pump (Cole-Palmer,
UK).
Whole-cell electrophysiology
Patch clamp experiments were performed from hair cells positioned
at the 9–12 kHz region of the cochlear apical coil (M€uller et al,
2005). Recordings were performed at room temperature (20–24°C)
using an Optopatch amplifier (Cairn Research Ltd, UK) as previously
described (Jeng et al, 2020; Carlton et al, 2021). Patch pipettes were
pulled from soda glass capillaries, which had a typical resistance in
the extracellular solution of 2–3 MΩ. The intracellular solution used
for the patch pipette contained (in mM): 131 KCl, 3 MgCl2, 1 EGTA-
KOH, 5 Na2ATP, 5 HEPES, 10 Na-phosphocreatine (pH was adjusted
with 1 M KOH to 7.28; 294 mOsm/kg). Data acquisition was con-
trolled by pClamp software using Digidata 1440A (Molecular
Devices, USA). In order to reduce the electrode capacitance, patch
electrodes were coated with surf wax (Mr Zoggs SexWax, USA).
Recordings were low-pass filtered at 2.5 kHz (8-pole Bessel), sam-
pled at 5 kHz, and stored on a computer for offline analysis (Clamp-
fit, Molecular Devices; Origin 2021: OriginLab, USA). Membrane
potentials under voltage-clamp conditions were corrected offline for
the residual series resistance Rs after compensation (usually 80%)
(cid:1) 2023 The Authors
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Adam J Carlton et al
and the liquid junction potential (LJP) of (cid:1)4 mV, which was mea-
sured between electrode and bath solutions. Voltage-clamp proto-
cols are referred to a holding potential of (cid:1)84 mV unless otherwise
stated.
Real-time changes
in membrane capacitance (DCm) were
tracked at body temperature as previously described (Johnson
et al, 2005, 2017). Briefly, a 4 kHz sine wave of 13 mV RMS was
applied to IHCs from the holding potential of (cid:1)81 mV and was
interrupted for the duration of the voltage step. The capacitance
signal from the Optopatch was filtered at 250 Hz and sampled at
5 kHz. DCm was measured by averaging the Cm trace over a
200 ms period following the voltage step and subtracting the pre-
pulse baseline. Data were acquired using pClamp software and a
Digidata 1440A (Molecular Devices). DCm experiments were per-
formed during the local perfusion of the IHCs with 30 mM TEA,
+
15 mM 4-AP (Fluka) to block the outward K
currents (Johnson
+
et al, 2005), and 5 mM CsCl to block the inward rectifier K
cur-
rent (Marcotti et al, 1999).
For mechanoelectrical transducer (MET) current recordings, the
hair bundles of hair cells were displaced using a fluid-jet system
from a pipette driven by a 25 mm diameter piezoelectric disc (Corns
et al, 2014, 2018; Carlton et al, 2021). For these experiments, the
intracellular solution contained (in mM): 131 CsCl, 3 MgCl2, 1
EGTA-KOH, 5 Na2ATP, 5 HEPES, 10 Na-phosphocreatine (pH was
adjusted with 1 M CsOH to 7.28; 290 mOsm/kg). The extracellular
solution was as described above, although for most of the record-
ings we included 5 mM CsCl, which was used to block the inward
+
current (Marcotti et al, 1999). In order to maintain the
rectifier K
osmolality of the extracellular solution constant, NaCl was reduced
to 130 mM in this case.
The fluid-jet pipette tip had a diameter of 8–10 lm and was posi-
tioned near the hair bundles to elicit a maximal MET current (typi-
cally 10 lm). Mechanical stimuli were applied as 50 Hz sinusoids
(filtered at 1 kHz, 8-pole Bessel). Prior to the positioning of the fluid
jet by the hair bundles, any steady-state pressure was removed. The
use of the fluid jet allows for the efficient displacement of the hair
bundles in both the excitatory and inhibitory directions, which is
essential to perform reliable measurements of the resting open prob-
ability of the MET channels.
Two-photon confocal Ca2+ imaging
Acutely dissected cochleae were incubated for 40 min at 37°C in
DMEM/F12, supplemented with fluo-4 AM at a final concentration
of 10 lM (Thermo Fisher Scientific) as recently described (Ceriani
et al, 2019). The incubation medium contained also pluronic F-127
(0.1%, w/v) and sulfinpyrazone (250 lM) to prevent dye sequestra-
tion and secretion. Calcium signals were recorded using a two-
photon laser-scanning microscope (Bergamo II System B232, Thor-
labs Inc., USA) based on a mode-locked laser system operating at
925 nm, 80-MHz pulse repetition rate, < 100-fs pulse width (Mai Tai
HP DeepSee, Spectra-Physics, USA). Images were captured with a
60× objective (LUMFLN60XW, Olympus, Japan) using a GaAsp
PMT (Hamamatsu) coupled with a 525/40 bandpass filter (FF02-
525/40-25, Semrock). Images were analyzed offline using custom-
built software routines written in Python (Python 2.7, Python Soft-
ware Foundation) and ImageJ
(NIH). Calcium signals were
measured as relative changes in fluorescence emission intensity
(DF/F0).
Scanning electron microscopy (SEM)
The isolated inner ear was very gently perfused with fixative for 1–
2 min through the round window. A small hole in the apical portion
of the cochlear bone was made prior to perfusion to allow the fixa-
tive to flow out from the cochlea. The fixative contained 2.5% vol/
vol glutaraldehyde in 0.1 M sodium cacodylate buffer plus 2 mM
CaCl2 (pH 7.4). The inner ears were then immersed in the above fix-
ative and placed on a rotating shaker for 2 h at room temperature.
After fixation, the organ of Corti was exposed by removing the bone
from the apical coil of the cochlea and then immersed in 1%
osmium tetroxide in 0.1 M cacodylate buffer for 1 h. For osmium
impregnation, which avoids gold coating, cochleae were incubated
in solutions of saturated aqueous thiocarbohydrazide (20 min) alter-
nating with 1% osmium tetroxide in buffer (2 h) twice (the OTOTO
technique: Furness & Hackney, 1986). The cochleae were then dehy-
drated through an ethanol series and critical point dried using CO2
as the transitional fluid (Leica EM CPD300) and mounted on speci-
men stubs using conductive silver paint (Agar Scientific, Stansted,
UK). The apical coil of the organ of Corti was examined at 10 kV
using a Tescan Vega3 LMU scanning electron microscope in the
electron microscopy unit at the University of Sheffield.
Immunofluorescence microscopy
As for SEM, the isolated inner ear was initially gently perfused with
4% paraformaldehyde in phosphate-buffered saline (PBS, pH 7.4)
through the round window. Following this initial short fixation, the
inner ear was fixed for 20 min at room temperature and then washed
three times in PBS for 10 min. The apical coil of the organ of Corti
was then washed in PBS, removed by fine dissection, and incubated
in PBS supplemented with 5% normal goat or horse serum and 0.5%
Triton X-100 for 1 h at room temperature. The samples were
immunolabeled with primary antibodies overnight at 37°C, washed
three times with PBS, and incubated with the secondary antibodies
for 1 h at 37°C. Antibodies were prepared in 1% serum and 0.5% Tri-
ton X-100 in PBS. Primary antibodies were mouse-IgG1 anti-Eps8
(1:1,000, BD Biosciences, 610,143), rabbit-IgG anti-WHIRLIN (1:200,
gift from Dr. Thomas Friedman, NIH, USA); rabbit-IgG anti-MYO6
(1:150, Proteus Biosciences, 25–6,791); rabbit-IgG anti-MYO15-
isoform 1 (1:1,000, gift from Dr. Thomas Friedman, NIH, USA)
Israel,
mouse-IgG1 anti-Kir2.1 channel
APC026); mouse IgG1 anti-CtBP2 (1:200, Biosciences, #612044) and
mouse IgG2a anti-GluR2 (1:200, Millipore, MAB397). F-actin was
stained with Texas Red-X phalloidin (1:400, ThermoFisher, T7471)
in the secondary antibody solution. Secondary antibodies were
species-appropriate Alexa Fluor or Northern Lights secondary anti-
bodies. Samples were mounted in VECTASHIELD (H-1000). The
images from the apical cochlear region (8–12 kHz) were captured
with Nikon A1 confocal microscope equipped with a Nikon CFI Plan
Apo 60× Oil objective or a Zeiss LSM 880 AiryScan equipped with
Plan-Apochromat 63× Oil DIC M27 objective for super-resolution
images of hair bundles. Both microscopes are part of the Wolfson
Light Microscope Facility at the University of Sheffield. Image stacks
were processed with Fiji ImageJ software.
(1:100, Alomone Lab,
FM1-43 staining
A 3 mM stock solution of
the dye FM1-43 (T3163, Molecular
Probes) was prepared in water. The dissected organs of Corti (aged
P11–P12) were transferred to the bottom of a chamber filled with
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Adam J Carlton et al
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extracellular solution, and held in position using a nylon mesh, as
described above (see above: Tissue preparation). All experiments
were performed at room temperature (20–24°C), as previously
described (Gale et al, 2001). Briefly,
the solution bathing the
cochleae was very rapidly exchanged with that containing 3 lM
FM1-43 for 10 s and immediately washed several times with normal
extracellular solution. The cochleae were then viewed with an
upright microscope equipped with epifluorescence optics and FITC
filters (excitation 488 nm, emission 520 nm) using a 63× water
immersion objective and a CCD camera.
RNA isolation and library preparation
The sensory epithelium from four control and four littermates
Kir2.1-OE mice under DOX were microdissected in DNase-free ice-
cold PBS 1× and immediately snap frozen in liquid nitrogen. RNA
was extracted using RNeasy Plus Micro Kit (Qiagen) according to
the manufacturer’s instructions. RNA quantity was established
using a Nanodrop spectrophotometer and RNA integrity number
(RIN) was calculated using a BioAnalyzer. All samples had RIN
score greater than 9.1. Preparation of the mRNA library was per-
formed using poly A enrichment and sequenced on the Illumina
NovaSeq sequencer using paired-end 150 bp reads.
RNA-sequencing analysis and differential gene expression
The sequencing libraries were processed using the nf-core RNA
pipeline (Ewels et al, 2020, https://nf-co.re/rnaseq/usage) using the
standard parameters. Reads were mapped to the mouse genome
(mm10). The resulting gene counts were determined using Salmon
(Patro et al, 2017) and used for downstream analysis with DESeq2
(Love et al, 2014). Metascape (Zhou et al, 2019) and Reactome
(Gillespie et al, 2022) were used to query for enriched GO terms and
pathways in the list of differentially expressed genes. HOMER
(Heinz et al, 2010) was used to find known and de novo motifs
among the upregulated genes in a 2000 bp window up and down-
stream of the transcriptional start site (TSS).
Statistical analysis
Statistical comparisons of means were made by the Student’s two-
tailed t-test or, for multiple comparisons, the analysis of variance
(one-way or two-way ANOVA followed by a suitable post-test) and
Mann–Whitney U test (when normal distribution could not be
assumed) were used. P < 0.05 was selected as the criterion for statis-
tical significance. Only mean values with a similar variance between
groups were compared. Average values are quoted in text and figures
as means (cid:3) S.D. Animals of either sex were randomly assigned to
the different experimental groups. No statistical methods were used
to define sample size, which was determined based on previously
published similar work from our laboratory. Animals were taken
from several cages and breeding pairs over a period of several
months. Most of the electrophysiological and morphological (but not
imaging) experiments were performed blind to animal genotyping
and in most cases, experiments were replicated at least 3 times.
Data availability
The data that support the findings of this study are available from
corresponding author. RNA-sequencing data have been
the
deposited in GEO under accession number (GSE215951; http://
www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE215951).
Expanded View for this article is available online.
Acknowledgements
The authors thank Michelle Bird (University of Sheffield) for her assistance with
the mouse husbandry, and Catherine Gennery and Laila Moushtaq-
Kheradmandi for their genotyping work. This work was supported by the
BBSRC (BB/S006257/1 and BB/T004991/1) and Wellcome Trust (224326/Z/21/Z)
to WM, the MRC (MR/S002510/1) to MM. AU was supported by a PhD
studentship from the MRC DiMeN Doctoral Training Partnership to WM. For the
purpose of Open Access, the author has applied a CC BY public copyright license
to any author accepted manuscript version arising from this submission.
Author contributions
Adam J Carlton: Conceptualization; data curation; formal analysis; validation;
investigation; methodology; writing – original draft; writing – review and edit-
ing. Jing-Yi Jeng: Data curation; formal analysis; validation; investigation;
methodology; writing – review and editing. Fiorella C Grandi: Data curation;
formal analysis; validation; investigation; methodology; writing – review and
editing. Francesca De Faveri: Data curation; formal analysis; validation;
investigation; methodology; writing – review and editing. Federico Ceriani:
Data curation; formal analysis; validation; investigation; methodology; writing
– review and editing. Lara De Tomasi: Investigation; methodology. Anna
Underhill: Data curation; formal analysis; investigation; methodology.
Stuart L Johnson: Data curation; formal analysis; validation; investigation;
methodology. Kevin P Legan: Investigation. Corn(cid:1)e J Kros: Methodology;
writing – review and editing. Guy P Richardson: Investigation; methodology;
writing – review and editing. Mirna Mustapha: Resources; funding
acquisition; methodology; writing – review and editing. Walter Marcotti:
Conceptualization; resources; data curation; formal analysis; supervision;
funding acquisition; validation; investigation; methodology; writing – original
draft.
Disclosure and competing interests statement
The authors declare that they have no conflict of interest.
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which permits use, distribution and reproduction in
any medium, provided the original work is properly
cited.
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|
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
|
All relevant data are within the paper and its Supporting Information files.
|
RESEARCH ARTICLE
Yellow fever virus is susceptible to sofosbuvir
both in vitro and in vivo
Caroline S. de Freitas1,2☯, Luiza M. Higa3☯, Carolina Q. Sacramento1,2, Andre´ C. Ferreira1,2,
Patrı´cia A. Reis1, Rodrigo Delvecchio3, Fabio L. Monteiro3, Giselle Barbosa-Lima4,
Harrison James Westgarth3, Yasmine Rangel Vieira2,4, Mayara Mattos1,2,
Natasha Rocha1,2, Lucas Villas Boˆ as Hoelz5, Rennan Papaleo Paes Leme5, Moˆ nica
M. Bastos5, Gisele Olinto L. Rodrigues6,7, Carla Elizabeth M. LopesID
Martins Queiroz-Junior8, Cristiano X. Lima9, Vivian V. Costa6,7, Mauro M. Teixeira6,10,
1, Nubia Boechat5, Amilcar Tanuri3, Thiago
Fernando A. Bozza1,4, Patrı´cia T. BozzaID
1,2,4*
Moreno L. SouzaID
6,7, Celso
1 Laborato´ rio de Imunofarmacologia, Instituto Oswaldo Cruz (IOC), Fundac¸ão Oswaldo Cruz (Fiocruz), Rio
de Janeiro, RJ, Brazil, 2 National Institute for Science and Technology on Innovation on Neglected Diseases
(INCT/IDN), Center for Technological Development in Health (CDTS), Fiocruz, Rio de Janeiro, RJ, Brazil,
3 Laborato´ rio de Virologia Molecular, Instituto de Biologia, Universidade Federal do Rio de Janeiro (UFRJ),
Rio de Janeiro, RJ, Brazil, 4 Instituto Nacional de Infectologia (INI), Fiocruz, Rio de Janeiro, RJ, Brazil,
5 Instituto de Tecnologia de Fa´ rmacos (Farmanguinhos), Fiocruz, Rio de Janeiro, RJ, Brazil, 6 Center for
Research and Development of Pharmaceuticals, Institute of Biological Sciences (ICB), Universidade Federal
de Minas Gerais (UFMG), Minas Gerais, Brazil, 7 Research Group in Arboviral Diseases, Department of
Morphology, Institute of Biological Sciences (ICB), Universidade Federal de Minas Gerais (UFMG), Minas
Gerais, Brazil, 8 Cardiac Lab, Department of Morphology, Institute of Biological Sciences (ICB), Universidade
Federal de Minas Gerais (UFMG), Minas Gerais, Brazil, 9 Departamento de Cirurgia, Faculdade de
Medicina, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil,
10 Immunopharmacology Lab, Department of Biochemistry and Immunology, Institute of Biological Sciences
(ICB), Universidade Federal de Minas Gerais (UFMG), Minas Gerais, Brazil
☯ These authors contributed equally to this work.
* [email protected]
Abstract
Yellow fever virus (YFV) is a member of the Flaviviridae family. In Brazil, yellow fever (YF)
cases have increased dramatically in sylvatic areas neighboring urban zones in the last few
years. Because of the high lethality rates associated with infection and absence of any anti-
viral treatments, it is essential to identify therapeutic options to respond to YFV outbreaks.
Repurposing of clinically approved drugs represents the fastest alternative to discover anti-
virals for public health emergencies. Other Flaviviruses, such as Zika (ZIKV) and dengue
(DENV) viruses, are susceptible to sofosbuvir, a clinically approved drug against hepatitis C
virus (HCV). Our data showed that sofosbuvir docks onto YFV RNA polymerase using con-
served amino acid residues for nucleotide binding. This drug inhibited the replication of both
vaccine and wild-type strains of YFV on human hepatoma cells, with EC50 values around
5 μM. Sofosbuvir protected YFV-infected neonatal Swiss mice and adult type I interferon
receptor knockout mice (A129-/-) from mortality and weight loss. Because of its safety profile
in humans and significant antiviral effects in vitro and in mice, Sofosbuvir may represent a
novel therapeutic option for the treatment of YF. Key-words: Yellow fever virus; Yellow
fever, antiviral; sofosbuvir
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OPEN ACCESS
Citation: de Freitas CS, Higa LM, Sacramento CQ,
Ferreira AC, Reis PA, Delvecchio R, et al. (2019)
Yellow fever virus is susceptible to sofosbuvir both
in vitro and in vivo. PLoS Negl Trop Dis 13(1):
e0007072. https://doi.org/10.1371/journal.
pntd.0007072
Editor: Samuel V. Scarpino, Northeastern
University, UNITED STATES
Received: March 25, 2018
Accepted: December 12, 2018
Published: January 30, 2019
Copyright: © 2019 de Freitas et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
Funding: The financial support was provided by
Fundac¸ão de Amparo à Pesquisa do Estado do Rio
de Janeiro (FAPERJ - http://www.faperj.br/ - Grant
Number E-26/201.573/2014) and Conselho
Nacional de Desenvolvimento Cientı´fico e
Tecnolo´gico (CNPq - http://cnpq.br/ - Grant
Numbers 306389/2014-2 and 425636/2016-0).
TMLS received the funds. This work has received
PLOS Neglected Tropical Diseases | https://doi.org/10.1371/journal.pntd.0007072 January 30, 2019
1 / 22
financial support from the National Institute of
Science and Technology in Dengue (INCT dengue),
a scheme funded by the Brazilian National Science
Council (CNPq, Brazil) and Minas Gerais
Foundation for Science (FAPEMIG, Brazil). Funding
was also provided by National Council for Scientific
and Technological Development (CNPq), Ministry
of Science, Technology, Information and
Communications (no. 465313/2014-0); Ministry of
Education/CAPES (no. 465313/2014-0); Research
Foundation of the State of Rio de Janeiro/FAPERJ
(no. 465313/2014-0) and Oswaldo Cruz
Foundation/FIOCRUZ to National Institute for
Science and Technology on Innovation on
Diseases of Neglected Populations (INCT/IDPN),
Center for Technological Development in Health
(CDTS), Fiocruz, Rio de Janeiro, RJ, Brazil. This
study was financed in part by the Coordenac¸ão de
Aperfeic¸oamento de Pessoal de Nı´vel Superior -
Brasil (CAPES) - Finance Code 001. The funders
had no role in study design, data collection and
analysis, decision to publish, or preparation of the
manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Sofosbuvir inhibits yellow fever virus
Author summary
Yellow fever virus is transmitted by mosquitoes and its infection may be asymptomatic or
lead to a wide clinical spectrum ranging from a mild febrile illness to a potentially lethal
viral hemorrhagic fever characterized by liver damage. Although a yellow fever vaccine is
available, low coverage allows 80,000–200,000 cases and 30,000–60,000 deaths annually
worldwide. There are no specific therapy and treatment relies on supportive care, rein-
forcing an urgent need for antiviral repourposing. Here, we showed that sofosbuvir, clini-
cally approved against hepatitis C, inhibits yellow fever virus replication in liver cell lines
and animal models. In vitro, sofosbuvir inhibits viral RNA replication, decreases the num-
ber of infected cells and the production of infectious virus particles. These data is particu-
larly relevante since the liver is the main target of yellow fever infection. Sofosbuvir also
protected infected animals from mortality, weight loss and liver injury, especially prophy-
latically. Our pre-clinical results supports a second use of sofosbuvir against yellow fever.
Introduction
Yellow fever virus (YFV) is a single-strand positive-sense RNA virus which belongs to the Fla-
viviridae family. Yellow fever (YF) outbreaks were very common throughout the tropical
world until the beginning of the 20th century, when vaccination and vector control limited the
urban virus circulation [1]. Classically, sylvatic and urban cycles of YFV transmission occur.
Non-human primates are sylvatic reservoirs of jungle YFV and non-immunized humans
entering the rain forest and those living in the ecotone (between preserved rain forest and
urban area) are highly susceptible to YFV, which is transmitted by mosquitoes from Haemago-
gus and Sabethes genera [2]. The virus is usually brought to urban settings by viremic humans
infected in the jungle [2]. The urban cycle involves transmission of the virus among humans
by vectors like Aedes spp. mosquitoes [2].
Brazil, an endemic country for YF, failed to vaccinate a large proportion of the susceptible
population. This scenario of low human vaccinal coverage along with increased sylvatic YFV
activity in primates has been occurring in Brazil since 2016, leading to bursts of human cases
of YF. For instance, between the second semester of 2017 and March 2018, 4,847 epizootics
were reported and 920 human cases were confirmed. There were 300 deaths associated with
this outbreak [3, 4]. In fact, cases of YF increased 1.8-times compared to the previous 35 years
[3]. Altogether, these data also show that YFV spread from Brazilian rain forests to the outskirt
of major cities in the Southeastern region of the country. Despite the detection of YFV in some
urban areas in humans and primates during this recent reemergence, the Brazilian Ministry of
Health (MoH) has argued this was a sylvatic cycle with no urban autochthonous transmission.
Indeed, most of the recent activity of YFV was observed in areas adjacent to the Atlantic forest,
where the genotype I was introduced two times in 2005 (95% interval: 2002–2007) and 2016
(95% interval: 2012–2017), spilling over from the Amazon forest [5].
YFV causes massive lethality in new world monkeys and around 30% in humans [6]. Once
displaying signs of severe infection, such as bleeding, shock, liver function decay and jaundice,
infected individuals are likely to progress to poor clinical outcomes. Most often, acute hepatic
failure occurs rapidly. No specific treatment options to YFV exist and patients solely receive
intensive palliative care. Therefore, antivirals with anti-flavivirus activity may represent an
important alternative for drug repurposing in an attempt to improve patient outcome.
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Sofosbuvir inhibits yellow fever virus
Developed in the 1930s, YF 17D live-attenuated vaccine confers long-lasting immunity to
its recipients. Vaccination is recommended for individuals aged � 9 months who are living in
or travelling to areas at risk for YF. Contraindications include hypersensitivity to vaccine com-
ponents, severe immunodeficiency, and age under �6 months [7]. Even though 17D is highly
effective and one of the safest vaccines in history, rare severe adverse events have been
reported. YF vaccine-associated neurological (YEL-AND) and viscerotropic (YEL-AVD) dis-
eases are similar to classic YF caused by wild-type (WT) virus. Reporting rates of YEL-AND
and YEL-AVD are 0.8 and 0.4 cases per 100,000 doses distributed [8]. Specific treatment
would be of utmost importance for individuals with YF vaccine-associated diseases and for
YFV-exposed people for whom vaccination is contraindicated.
Our group and others have recently shown that sofosbuvir, a clinically approved anti-hepa-
titis C virus (HCV) drug, is also endowed with anti-Zika virus (ZIKV) antiviral activity in vitro
and in vivo [9–11]. It has also been shown that sofosbuvir also blocks dengue virus (DENV)
replication [12]. Animal model studies of sofosbuvir on Flaviviruses reveal that this drug is
more effective when used prophylactically or as early as possible. Sofosbuvir was approved by
the Food and Drug Administration (FDA) in 2013. It has been used in therapy regimens there-
after in large worldwide scale to treat HCV-infected individuals, with infrequent registers of
toxicity and adverse effects even for complex patients, such as those co-infected with both
HIV/HCV or with substantial liver damage provoked by HCV [13–15]. Sofosbuvir is very
effective against HCV genotype 1/2/3, and safe doses may range from 400 to 1200 mg daily for
up to 24 weeks [13–15]. When compared to pan-antiviral drugs such as ribavirin, sofosbuvir is
considered safer for pregnant woman [16, 17].
In the intention to have a safe and an active antiviral compound to inhibit YFV replication,
we tested whether the virus was susceptible to sofosbuvir. We found that sofosbuvir binds to
conserved amino acid residues on the YFV RNA polymerase (NS5), inhibiting virus replica-
tion in human hepatoma cells, diminishing YFV-associated mortality and improving the
hepatic condition in animal models.
Materials and methods
Reagents
The antiviral sofosbuvir was kindly donated by Dr. Jaime Rabi (Microbiolo´gica Quı´mica e
Farmacêutica, Brazil; part of the BMK Consortium). Ribavirin and AZT were provided by
Instituto de Tecnologia de Farmacos (Farmanguinhos, Fiocruz). Drugs were dissolved in
100% dimethylsulfoxide (DMSO) and subsequently diluted at least 104-fold in culture medium
before each assay. The final DMSO concentration showed no cytotoxicity. The materials for
cell culture were purchased from Thermo Scientific Life Sciences (Grand Island, NY) unless
otherwise mentioned.
Cells and virus
Human hepatoma cell lines (Huh-7 and HepG2) and African green monkey (Vero) cells were
cultured in DMEM supplemented with 10% fetal bovine serum (FBS; HyClone, Logan, Utah),
100 U/mL penicillin, and 100 μg/mL streptomycin [18, 19]. Cells were incubated at 37˚C in
5% CO2. Aedes albopictus C6/36 cells were cultured at 28˚C in Leibovitz L15 medium supple-
mented with 2 mM L-glutamine, 0.75 g/L sodium bicarbonate, 0.3% tryptose phosphate broth,
non-essential amino acids and 5% FBS.YFV vaccinal and WT strains were passaged at an mul-
tiplicity of infection (MOI) of 0.01 in either Vero (for 24–72 h at 37˚C) or C6/36 (for 6 days at
28˚C) cells. Virus titers were also determined in Vero cell cultures by TCID50/mL [20] or pla-
que forming units (PFU)/mL (described below). The vaccine strain 17D was donated by the
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Reference Laboratory for Flavivirus, Fiocruz, Brazilian Ministry of Health, whereas the WT
strain was isolated in Vero cells from the serum of a symptomatic patient with confirmed
RT-PCR result for YFV (GenBank accession #MH018072) [21].
Cytotoxicity assay
Monolayers of 1.5 x 104 hepatoma cells in 96-well plates were treated for 5 days with various
concentrations of sofosbuvir or ribavirin as a control. Then, 5 mg/ml 2,3-bis-(2-methoxy-
4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) in DMEM was added to the
cells in the presence of 0.01% of N-methyl dibenzopyrazine methyl sulfate (PMS). After incu-
bating for 4 h at 37˚C, the plates were read in a spectrophotometer at 492 nm and 620 nm [22].
The 50% cytotoxic concentration (CC50) was calculated by a non-linear regression analysis of
the dose-response curves.
Yield-reduction assay
Monolayers of 5.5 x 106 Huh-7 cells in 6-well plates were infected with YFV at an MOI of 0.1
for 1 h at 37˚C. The cells were washed with PBS to remove residual viruses, and various con-
centrations of sofosbuvir, or ribavirin, in DMEM with 1% FBS were added. After 24 h, the cells
were lysed, the cellular debris was cleared by centrifugation, and the virus titers in the superna-
tant were determined. A non-linear regression analysis of the dose-response curves was per-
formed to calculate the concentration at which each drug inhibited the plaque-forming
activity of YFV by 50% (EC50).
Immunofluorescence analysis
Huh-7 and HepG2 cells were seeded on black 96-well microplates with clear bottom (Greiner
Bio-One, Kremsmu¨nster, Austria) and infected with YFV at an MOI of 1. After 1 hour, the
viral inoculum was removed and cells were incubated with growth medium containing sofos-
buvir, Ribavirin or AZT for 2 days. Cells were then fixed with 4% paraformaldehyde in PBS for
20 min at room temperature. The fixative was removed and cells monolayers were washed
with PBS. Blocking of unspecific binding of the antibody and permeabilization were per-
formed with 3% bovine serum albumin (BSA, Sigma Aldrich) and 0.1% Triton X-100 in PBS
for 20 min at room temperature. SCICONS J2 antibody (Scicons, Hungary), which recognizes
double-stranded RNA, was diluted 1:1000 in PBS and incubated for 1 h at room temperature.
The primary antibody was removed and cell monolayer was washed twice with PBS. Secondary
antibody Donkey anti-mouse IgG coupled to AlexaFluor488 fluorochrome (Thermo Fisher
Scientific) was diluted 1:1000 in PBS and incubated for 40 min at room temperature. After
washing cells with PBS, nucleus staining with DAPI diluted 1:10,000 in PBS was performed at
room temperature for 10 min and then washed with PBS. Cells were imaged using a Nikon
TE300 (Tokyo, Japan) inverted microscope coupled to a Leica DFC310FX camera (Leica Bio-
systems, Wetzlar, Germany). Images referent to AlexaFluor488 and DAPI signals were merged
using the microscope software.
Flow cytometry
Huh-7 and HepG2 cells were seeded in 6-well plates at density of 6 x104 cells/well and 2.5 x 105
cells/well, respectively. For infection, the growth medium was replaced by serum-free medium
containing the virus at an MOI of 1. Mock-infected cells were incubated with conditioned
medium from uninfected cells prepared exactly as performed for viral propagation. After 1
hour, inoculum was removed and replaced by growth medium containing either the vehicle or
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Sofosbuvir inhibits yellow fever virus
the antivirals at different concentrations and incubated for 48 h (for HepG2) or 72 h (for Huh-
7) cells. After that, cells were harvested by treatment with a 0.25% trypsin solution. Cells were
fixed with 4% paraformaldehyde (Sigma-Aldrich) in phosphate buffered saline (PBS) for 15
min at room temperature and washed with PBS. Cells were permeabilized with 0.1% Triton X-
100 (Sigma Aldrich) in PBS, washed with PBS, and blocked with PBS with 5% FBS. Cells were
incubated with 4G2, a pan-flavivirus antibody raised against the envelope E protein produced
in 4G2-4-15 hybridoma cells (ATCC), diluted 1:10 in PBS with 5% FBS. Cells were labeled
with donkey anti-mouse Alexa Fluor 488 antibody (Thermo Scientific,Waltham, MA, USA)
diluted 1:1000 in PBS with 5% FBS, and were analyzed by flow cytometry in a BD Accuri C6
(Becton, Dickinson and Company, Franklin Lakes, NJ, USA) for YFV infection. The gate strat-
egy to assure accuracy in the analysis is displayed as S1 Fig.
Plaque forming assay
Virus titers were determined as PFU on Vero cells. Supernatants containing virus were serially
diluted and incubated over confluent monolayers. After 1 h, cells were overlaid with semisolid
medium, alpha-MEM (GIBCO) containing 1.4% carboxymethyl cellulose (Sigma-Aldrich) and
1% FBS (GIBCO). Cells were further incubated for 4 to 5 days. Cells were fixed in 4% formal-
dehyde and stained with 1% crystal violet in 20% ethanol for plaque visualization.
Sequence comparisons
The sequences encoding the C-terminal portion of the RNA polymerase from Flaviviruses
were acquired from the complete sequences deposited in GenBank. An alignment was per-
formed using the ClustalW algorithm in the Mega 6.0 software. The sequences were analyzed
by disparity index per site. Compared regions are displayed in the S1 Table.
Comparative modeling
The amino acid sequence encoding YFV RNA polymerase (UniProtKB code: P03314) was
obtained from the EXPASY proteomic portal [23] (http://ca.expasy.org/). The template search
was performed using the Blast server (http://blast.ncbi.nlm.nih.gov/Blast.cgi) with the Protein
Data Bank [24] (PDB; http://www.pdb.org/pdb/home/home.do) as the database and the
default options. The T-COFFEE algorithm was used to generate the alignment between the
amino acid sequences of the template proteins and YFV RNA polymerase. Subsequently, the
construction of the YFV RNA polymerase complex was performed using MODELLER 9.19
software [25], which employs spatial restriction techniques based on the 3D-template struc-
ture. The preliminary model was refined in the same software, using three cycles of the default
optimization protocol. The structural evaluation of the model was then performed using two
independent algorithms in the SAVES server (http://nihserver.mbi.ucla.edu/SAVES_3/): PRO-
CHECK software [26] (stereochemical quality analysis) and VERIFY 3D [27] (compatibility
analysis between the 3D model and its own amino acid sequence by assigning a structural class
based on its location and environment and by comparing the results with those of crystal
structures).
Animals
The procedures described in this study were in accordance with the ethical and animal experi-
ments regulations of the Brazilian Government (Law 11794/2008), guidelines published at the
Brazilian participant Institutions and National Institutes of Health guide for the care and use
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Sofosbuvir inhibits yellow fever virus
of laboratory animals. The study is reported in accordance with the ARRIVE guidelines for
reporting experiments involving animals [28].
Neonate mouse model
Swiss albino mice (Mus musculus) (pathogen-free) from the Oswaldo Cruz Foundation breed-
ing unit (Instituto de Ciência e Tecnologia em Biomodelos; ICTB/Fiocruz) were used for these
studies. The animals were kept at a constant temperature (25˚C) with free access to chow and
water in a 12-h light/dark cycle. The experimental laboratory received pregnant mice (at
approximately the 14th gestational day) from the breeding unit. Pregnant mice were observed
daily until delivery to accurately determine the postnatal day. We established a litter size of 10
animals for all experimental replicates. The Animal Welfare Committee of the Oswaldo Cruz
Foundation (CEUA/FIOCRUZ) approved and covered (license number L-016/2016) the
experiments in this study.
Adult mouse model
In parallel, some experiments were conducted using type I interferon receptor deficient
mice (A129−/−), SV129 background, obtained from Bioterio de Matrizes da Universidade de
Sao Paulo (USP). Adult male and female A129−/− mice were bred and kept at Immunophar-
macology Laboratory of the Universidade Federal de Minas Gerais (UFMG) under specific
pathogen-free conditions. Mice were kept at a constant temperature (25˚C) with free access
to chow and water in a 12-h light/dark cycle. Experimental protocol was approved by the
Committee on Animal Ethics of the UFMG (CEUA/UFMG, Permit Protocol Number 84/
2018).
Experimental infection and treatment. As proof-of-principle, sofosbuvir treatment was
initially performed in three-day-old Swiss mice infected intraperitoneally with different doses
of vaccinal virus. To monitor drug efficiency in a highly aggressive system, adult (7–9 week-
old) A129-/- (type I interferon receptor knockout) mice were infected with different inoculums
of YFV by the intravenous (i.v.) route (tail vein). Both male and female mice were used in the
experiments. Treatments with sofosbuvir were performed at 20 mg/kg/day administered intra-
peritoneally for Swiss newborn and orally (gavage) for A129-/- mice. Regimen was adminis-
tered daily beginning one day prior to infection or one day after infection until control group
(animals infected and untreated) decease. Animals were monitored daily for survival and
weight variation. Of note, both male and female mice were used in the experiments.
If necessary, euthanasia was performed to alleviate animal suffering. The criteria were the
following: i) differences in weight between infected and control groups [> 50% for Swiss new-
born (variation in weight gain) and > 20% for A129-/- (variation in weight loss)], ii) ataxia, iii)
loss of gait reflex, iv) absence of righting reflex within 60 seconds, and v) separation, with no
feeding, of moribund offspring by the female adult mouse (for Swiss newborn mice).
Histopathological liver analysis. Liver samples from adult euthanized mice at day 3 post-
YFV inoculation were obtained. Afterwards, they were immediately fixed in 10% buffered for-
malin for 24 h and embedded in paraffin. Tissue sections (4 mm thicknesses) were stained
with hematoxylin and eosin (H&E) and evaluated under a microscope, Axioskop 40 (Carl
Zeiss, Go¨ttingen, Germany) adapted to a digital camera (PowerShot A620, Canon, Tokyo,
Japan). Histopathology score was performed according to a set of custom designed criteria
modified from evaluating cellular infiltration, hepatocyte swelling and degeneration and then
added to reach a four-points score (0, absent; 1, slight; 2, moderate; 3, marked; and 4, severe)
in each analysis [29]. For easy interpretation, the overall score was taken into account and all
the parameters summed for a maximum possible score of 8 points. A total of two sections for
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Sofosbuvir inhibits yellow fever virus
each animal were examined and results were plotted as the media of damage values in each
mouse.
Statistical analysis. All assays were performed and codified by one professional. Subse-
quently, a different professional analyzed the results before the identification of the experimen-
tal groups. This approach was used to keep the pharmacological assays blind. All experiments
were carried out at least three independent times, including technical replicates in each assay.
The dose-response curves used to calculate the EC50 and CC50 values were generated by Prism
GraphPad software 7.0. The equations to fit the best curve were generated based on R2
values � 0.9. Fisher’s exact and ANOVA tests were also used, with P values <0.05 considered
statistically significant. For flow cytometry and viral titer analysis, data were analyzed by
ANOVA. When ANOVA revealed a significant effect, data were further analyzed with Dunnet
post hoc test to correct for multiple comparisons and to determine specific group differences.
The significance of survival curves was evaluated using the Log-rank (Mantel-Cox) test. P val-
ues of �0.05 were considered statistically significant.
Results
Prediction of the complex between Sofosbuvir triphosphate and YFV RNA
polymerase using comparative modeling
We initially compared the disparities among regions encoding the RNA-dependent RNA poly-
merase (RDRP) region of the contemporary Flaviviruses DENV, ZIKV and YFV (Table 1).
The YFV RNA polymerase shares a conserved domain for catalytic activity with the ortholo-
gous enzymes (S1 Table).
Next, the crystal structures of DENV NS5 complexed with viral RNA (PDB code: 5DTO)
[30],HCV NS5B in complex with Sofosbuvir diphosphate (PDB code: 4WTG) [31], and com-
plexed to UTP (PDB code: 1GX6) [32] were selected and used in a comparative modeling pro-
cedure, covering 100% of the YFV RNA polymerase sequence considered here (residues
Thr252-Ile878). These three template proteins represent orthologous viral RNA polymerases
from the Flaviviridae family. Consequently, the resulting 3D model of YFV RNA polymerase
in complex with Sofosbuvir triphosphate showed good structural quality.
The analysis of the YFV RNA polymerase model suggests that sofosbuvir triphosphate
binds between the palm and the fingers regions, making hydrogen bonds with Gly538,
Trp539, Ser603, and Lys 693 residues and salt bridge interactions with Lys359 and two Mg2+
ions. Interestingly, these interactions are all described as relevant for incorporation of natural
ribonucleotides [31] (Fig 1). Therefore, these results motivate further testing in biologically rel-
evant models to YFV replication.
Table 1. Estimated disparities index per site between amino acid sequences encoding the RDRP domain peptide located at last 680 amino acids of C-terminal from
contemporary flaviviruses.
Order
1
2
3
4
5
6
GenBank #
FJ654700.1
KX197205.1
AB189124.1
DQ672564.1
AY858046.2
GU289913.1
Agent
YFV
ZIKV
DENV1
DENV2
DENV3
DENV4
https://doi.org/10.1371/journal.pntd.0007072.t001
Distance comparisons
1
2
3
4
5
0.000
0.000
0.000
0.000
0.000
0.002
0.000
0.107
0.042
0.056
0.000
0.000
0.062
0.011
0.000
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Sofosbuvir inhibits yellow fever virus
Fig 1. 3D model of YFV RNA polymerase in complex with sofosbuvir triphosphate. (A) Cartoon representation of YFV RNA polymerase 3D
model. The sofosbuvir triphosphate structure is represented by sticks and colored according to atom type (red, oxygen; blue, nitrogen; orange,
carbon; yellow, phosphorus) and the two catalytic Mg2+ ions are shown as orange spheres. (B) Schematic representation of the interaction between
amino acid residues of the enzyme and the sofosbuvir triphosphate structure, where the salt bridges are shown in orange and hydrogen bonds are in
green.
https://doi.org/10.1371/journal.pntd.0007072.g001
Sofosbuvir inhibits YFV replication in different models of human hepatoma cells.
YFV primarily replicates in the liver, where sofosbuvir is majorly converted from prodrug to
its active metabolite [13–15]. Thus, we monitored YFV susceptibility to sofosbuvir using
human hepatocellular carcinoma cells. YFV was yield in Huh-7 cells in the presence of sofos-
buvir for 24 h, and then cell lysates were titered in Vero cells. Indeed, sofosbuvir inhibited
both vaccine and WT strains of YFV replication similarly, in dose-dependent manner (Fig 2).
As a positive control to inhibit virus replication, we used ribavirin, a broad spectrum antiviral
which was previously shown to inhibit YFV replication in vitro and in vivo [33–35] (Fig 2).
Sofosbuvir’s and ribavirin’s potencies over YFV were quite comparable, with EC50 values vary-
ing 12% (Table 2). Of note, AZT, used as a negative control, did not affect virus replication
(Fig 2).
Since sofosbuvir is around 25% less cytotoxic than ribavirin, its selectivity index (SI; ratio
between EC50 and CC50) was also higher (Table 2). Sofosbuvir displayed improved SI values
Fig 2. Pharmacology of sofosbuvir against YFV. Huh-7 were infected with YFV at an MOI of 0.1 and exposed to
various concentrations of the antivirals for 24 h. Supernatants were harvested and titered in Vero cells by TCID50/mL.
The data represent means ± SEM of five independent experiments.
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Sofosbuvir inhibits yellow fever virus
Table 2. Pharmacological parameters related to antiviral activity of sofosbuvir and ribavirin against YFV.
Drugs
Sofosbuvir
Ribavirin
EC50 (μM)
Vaccinal
4.8 ± 0.2
3.9 ± 0.3
WT
4.2 ± 0.2
4.9 ± 0.3
CC50 (μM)
381 ± 25
284 ± 12
SI�
Vaccinal
80
72
WT
90
58
�Selectivity index (SI) = CC50/EC50
https://doi.org/10.1371/journal.pntd.0007072.t002
(when compared to ribavirin) by 10 and 50% with respect to the vaccine and WT strains,
respectively (Table 2).
We further expanded whether YFV susceptibility to sofosbuvir was similar in different line-
ages of hepatoma cells, Huh-7 and HepG2. To do so, titers were determined by PFU, for more
accurate comparisons, with respect to differences in levels of virus replication. Consistently,
sofosbuvir treatment decreased the production of infectious virus particles (Fig 3) in a dose-
Fig 3. Sofosbuvir reduces the production of YFV viral particles. Huh-7 cells were infected with vaccine strain YFV17D (A) or
WT-YFV (B) at an MOI of 1. HepG2 cells were infected with vaccine strain YFV17D (C) or WT-YFV (D) at an MOI of 1. YFV-
infected were exposed to the indicated concentrations of AZT, ribavirin, or sofosbuvir. Supernatants were analyzed by plaque
assay to determine viral titers. Data represent mean ± SEM of at least four independent experiments. Comparison between
untreated and treated YFV-infected cells was performed using ANOVA followed by Dunnet post hoc test. Statistical
significance compared to untreated YFV-infected cells is indicated by asterisks (� P < 0.05, �� P < 0.01 and ��� P <0.001).
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Sofosbuvir inhibits yellow fever virus
dependent manner in Huh-7 (Fig 3A and 3B) and HepG2 cells (Fig 3C and 3D). Ribavirin also
inhibited viral replication, whereas AZT was ineffective (Fig 3). Sofosbuvir at doses of 10 and
50 μM inhibited, respectively, 3- and 5-log10 the production of infectious particles by infected
Huh-7 cells (Fig 3A and 3B). Sofosbuvir was at least 100 times more effective than ribavirin at
inhibiting YFV infectivity (regardless of strain) in infected Huh-7 cells (Fig 3A and 3B). YFV
susceptibility to the tested antiviral drugs in HepG2 cells was lesser when compared to Huh-7
cell (Fig 3A, 3B, 3C and 3D). Sofosbuvir treatment reduced the production of infectious YFV
in HepG2 cells by 1- and 2-log10 at doses of 10 and 50 μM, respectively (Fig 3C and 3D). In
HepG2 cells, ribavirin’s and sofosbuvir’s effects over virus replication were comparable (Fig
3C and 3D).
Knowing the pharmacological activity and putative target, we used a cell-based assay to
demonstrate that sofosbuvir inhibits YFV RNA polymerase. During YFV replication, an anti-
genomic negative-sense RNA strand is synthetized, creating an intermediate double-stranded
(ds) RNA with the positive-sense virus genome. Viral genome replication was thus assessed by
immunodetection of dsRNA. HepG2 and Huh-7 hepatoma cells lines were infected with an
MOI of 1 and treated with sofosbuvir (0.4 to 50 μM) for 48 h. Ribavirin and AZT were used as
positive and negative controls, respectively. Sofosbuvir reduced the genome replication of both
vaccine and WT strains of YFV in Huh-7 and HepG2 cells in a dose dependent manner (Fig
4). Again, YFV replication was more susceptible to sofosbuvir in Huh-7 (Fig 4A and 4B) than
HepG2 (Fig 4C and 4D) cells. Although ribavirin showed activity both against YFV 17D and
WT YFV, full inhibition was only achieved at higher concentrations (when compared to sofos-
buvir) (Fig 4). In Huh-7 cells treated with 10 μM of the drugs, we observe less foci of dsRNA in
sofosbuvir- than in ribavirin-treated cells (Fig 4A and 4B). AZT was inefficient in blocking
either vaccine or WT YFV replication at the tested concentrations (Fig 4).
We next determined antigen production by flow cytometry using the 4G2 antibody, a pan-
flavivirus antibody raised against the envelope protein (Fig 5 and S2 Fig). Huh-7 cells were
infected with vaccine (Fig 5A) or WT (Fig 5B) YFV strain at an MOI of 1 and then treated
with ribavirin or AZT at 10 and 50 μM or with sofosbuvir in concentrations ranging from 0.4
to 50 μM for 72 h. Flow cytometry analysis showed a pronounced reduction in the number of
YFV-infected Huh-7 cells treated with 50 μM ribavirin and 10–50 μM sofosbuvir compared to
untreated YFV-infected cells (Fig 5A and 5B). AZT treatment did not present a significant
anti-YFV effect. Untreated WT YFV-infected cells presented with 89.75% of 4G2+ cells
whereas sofosbuvir treatment at 10 and 50 μM resulted in 0.36 and 0.06% of antigen positive
cells, respectively. Treatment with 10 and 50 μM ribavirin led to 65.43 and 1.79% of YFV-
infected cells, indicating that sofosbuvir was more efficient than ribavirin in reducing the
number of infected cells. HepG2 cells were also infected with vaccine (Fig 5C) or WT (Fig 5D)
YFV and treated with AZT, ribavirin, or sofosbuvir for 48 h and analyzed by flow cytometry.
Similar to the results observed in Huh-7, ribavirin and sofosbuvir treatment led to a reduction
in the number of YFV-infected HepG2 cells. Noteworthy, sofosbuvir and ribavirin-induced
reduction of YFV-infected cells was more pronounced in Huh-7 cells than in HepG2 cells (Fig
5). This result is consistent with our observations from immunodetection of viral dsRNA (Fig
4). Sofosbuvir treatment reduced the number of YFV-infected cells in a dose-dependent man-
ner, both in Huh-7 and HepG2 cells (Fig 5). We did not observe differences in sofosbuvir sus-
ceptibility between vaccine and WT YFV strain, with respect to antigenic production.
Altogether, sofosbuvir demonstrated higher potency in the reduction of viral genome repli-
cation, protein synthesis, and infectious viral particle production than ribavirin in both hepa-
toma cell lines tested. These results indicate that sofosbuvir is a good candidate against YFV
and deserving of further in vivo testing.
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Sofosbuvir inhibits yellow fever virus
Fig 4. Sofosbuvir inhibits viral dsRNA, an intermediate in viral genome replication. Huh-7 cells were infected with
either vaccine strain, YFV-17D (A) or WT-YFV (B); HepG2 cells were infected with YFV-17D (C) or WT-YFV (D).
Infected cells were treated with AZT, ribavirin (RBV), or sofosbuvir (SOF) at the indicated concentrations for 48 h.
Viral replication was analyzed by immunofluorescence using J2 antibody to detect viral replication intermediate
dsRNA (green) and DAPI to stain cell nuclei (blue). White scale bars indicate 50 μm. Images are representative of four
independent experiments.
https://doi.org/10.1371/journal.pntd.0007072.g004
Sofosbuvir enhances survival of YFV-infected mice. To test sofosbuvir in vivo, we used
the dose of 20 mg/kg/day in newborn Swiss outbred mice. This dose is consistent with its pre-
clinical/clinical studies for drug approval [15]. Since in vitro experiments had shown that vac-
cine and WT strains of YFV are equally susceptible to sofosbuvir, we initially used the vaccine
strain because it required easier handling and biosafety containment. Three-days-old Swiss
mice were infected intraperitoneally with 1.0 x104 PFU of vaccine virus. These animals began
to receive treatment 1 day prior to infection (pre-treatment) or 1 day post-infection (late-treat-
ment). Sofosbuvir significantly enhanced the survival of YFV-infected mice who received pre-
treatment, pointing out to prophylactic activity and a possible narrow time frame for antiviral
intervention (Fig 6A). Variations in body weight among groups were marginal (Fig 6B).
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Sofosbuvir inhibits yellow fever virus
Fig 5. Sofosbuvir reduces the number of YFV-infected cells. Huh-7 cells were infected with vaccine strain YFV17D (A) or wild-
type YFV (B) at an MOI of 1. HepG2 cells were infected with vaccine strain YFV17D (C) or wild-type YFV (D) at an MOI of 1.
YFV-infected cells were exposed to the indicated concentrations of AZT, ribavirin, or sofosbuvir. Anti-pan-flavivirus antibody
was used for flow cytometry analysis. The data represent mean ± SEM of at least three independent experiments. Differences
between untreated and treated YFV-infected cells were assessed using ANOVA followed by Dunnet post hoc test. Statistical
significance compared to untreated YFV-infected cells is indicated by asterisks (� P < 0.05, �� P < 0.01 and ��� P <0.001).
https://doi.org/10.1371/journal.pntd.0007072.g005
Despite the fact that mortality starts to occur only 7 days after infection, late-treatment did not
statistically increase animal survival (Fig 6A).
Fig 6. Early/Prophylactic treatment with sofosbuvir increases survival of YFV-infected mice. Three-day-old Swiss mice were infected with YFV-17D (1 x 104 PFU)
and treated with sofosbuvir either 1 day before (pre) or after (late) infection. Survival (A) and weight variation (B) were assessed during the course of treatment. Survival
was statistically assessed by Log-rank (Mentel-Cox) test. Differences in weight are displayed as the means ± SEM. At least three independent experiments were
performed with 10 mice/group. � P < 0.05.
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Sofosbuvir inhibits yellow fever virus
Next, we repeated the same model of three-days-old Swiss mouse infection, but with a
higher dose of vaccine virus (1 x 106 PFU) to achieve higher mortality. Since late-treatment
did not previously enhance survival in this model, the efficacy of sofosbuvir was accessed only
with the pre-treatment condition. Sofosbuvir protected pre-treated mice from YFV-induced
mortality (Fig 7A). Infected mice died within two weeks after infection, whereas 70% of sofos-
buvir pre-treated YFV-infected animals survived to lethal inoculum (Fig 7A). We also evalu-
ated the weight gain during the time course of our experiment in mice. YFV-infected animals
had reduced postnatal development, whereas sofosbuvir-treated YFV-infected mice gained
weight almost as much as the uninfected controls (Fig 7B). We monitored through the course
of infection the levels of alanine aminotransferase (ALT), an important biomarker of liver
integrity. At day 12 post-infection, when untreated animals experience the highest mortality
(Fig 7A), ALT levels became two-times higher in the infected/untreated mice than in those
treated with sofosbuvir (Fig 7C). Consistent with the increase in this biomarker of liver injury,
untreated animals displayed viral replication almost three-times higher in blood plasma and
liver than treated mice (Fig 7D).
Although these initial in vivo results are exciting, documented information of sofosbuvir
protection in a highly virulent in vivo system is noteworthy. Mice with impaired innate
immune response, due to the lack of type I interferon receptor (A129-/-), were infected with
WT YFV and treated with sofosbuvir at 20 mg/kg/day, again beginning one day before (pre-
treatment) or after (post-treatment) infection. Results show that inoculation with 4 x 104 and 4
x 103 PFUs caused 100% mortality within one week along with massive loss in body weight
(Fig 8). At these high virus inputs, pre-treatment with sofosbuvir enhanced the mean time of
survival by 57%, although mortality was the final outcome for all mice (Fig 8A and 8B). At a
virus dose able to cause 50% of mortality, 4 x 102 PFU, pre-treatment with sofosbuvir
enhanced survival significantly (Fig 8C).
Taking this last condition as reference, infectious viral titers were measured in different
organs, along with serum ALT levels, liver histopathology, and white cell counts. WT YFV rep-
licated in different anatomical compartments of the A129-/-, like peripheral blood, liver, spleen
and kidney (Fig 9A–9D). In parallel to viscerotropic replication, animals rapidly progress to
high virus titers in the brain (Fig 9E). Pre-treatment with sofosbuvir reduced the virus levels in
the brain of the infected mice by 2-log (Fig 9E), along with leucocytosis (Fig 9F). Hepatic con-
dition was improved in sofosbuvir-treated mice, regardless of the regimen, indicated by mea-
suring ALT levels (Fig 9G) and liver injury (Fig 9H and 9I).
Although pre-treatment was to be better than late-treatment to enhance survival, hepatic
histology and ALT levels show that benefits from late treatment may exist (Fig 9G–9I). Histo-
pathological analysis revealed mild alterations in the liver of vehicle-treated mice, including
discrete and focal neutrophil inflammatory infiltrate and hepatocyte degeneration. Interest-
ingly, pre-treatment with sofosbuvir abrogated such alterations while post-treatment partially
reverted the observed liver lesions. This last result show that, although limited in efficacy, late
treatment may be better than leaving the animals untreated. Our in vivo data reinforces the
repurposing of sofosbuvir towards YF, with limited timing opportunity for intervention.
Discussion
Brazil has been challenged in recent years by the (re)emergence of arboviruses. Massive cases
of microcephaly associated with ZIKV circulation were registered in 2015/16 [36]. Two differ-
ent genotypes of chikungunya virus (CHIKV) started to co-circulate from 2014 onward [37,
38]. The four DENV serotypes are hyperendemic throughout the country [39]. More recently,
YFV activity increased, affecting both non-human primates and humans without constituted
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Sofosbuvir inhibits yellow fever virus
Fig 7. Pre-treatment with sofosbuvir increases survival and inhibits weight loss of YFV-infected mice. Three-day-old Swiss mice were pre-treated
with sofosbuvir beginning 1 day before infection with YFV 17D (1 x 106 PFU). Survival (A) and weight variation (B) were assessed during the course
of treatment. Survival was statistically assessed by Log-rank (Mentel-Cox) test. Differences in weight are displayed as the means ± SEM. Three-
independent experiments were performed with 10 mice/group. Throughout the course of the experiment, a subset of animals (3 mice/group) were
euthanized to monitor plasma ALT (C) and viral RNA levels (D). Data are presented as means ± SEM. For panels B, C and D, two-way ANOVA for
each day was used to assess the significance � P < 0.05.
https://doi.org/10.1371/journal.pntd.0007072.g007
immunity [3, 6]. These facts clearly demonstrate that strategies of vector control failed and
highlights the necessity of alternative strategies to control or even mitigate the diseases pro-
voked by the arboviruses.
In the context of this article, re-emergence of YFV also points out that poor vaccination
coverage left human population living in or entering the ecotone susceptible to this virus.
Unvaccinated individuals may quickly progress to severe YF, with hepatic and even neurologi-
cal impairment [1]. In addition, due to massive YF vaccine campaigns, a fair amount of vac-
cine-related severe adverse events may be expected. Thus, finding antiviral drugs to treat YFV-
infected individuals is critical for medical intervention over cases provoked by WT or even
vaccine strains.
Several groups demonstrated that the clinically approved anti-HCV drug sofosbuvir is
endowed with antiviral activity towards other flaviviruses, such as ZIKV [10, 40] and DENV
[12]. Considering that these agents belong to the same family, it was plausible to test sofosbuvir
against YFV. Indeed, we observed that sofosbuvir targets the YFV RNA polymerase in silico
and inhibits YFV RNA replication and infectious virus production. The in vitro antiviral
results display that sofosbuvir’s EC50 towards YFV is in micromolar range and comparable to
what has been described for ZIKV and DENV [10, 12, 40].
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Sofosbuvir inhibits yellow fever virus
Fig 8. Treatment with sofosbuvir increases survival and inhibits weight loss of adult YFV-infected A129-/- mice. 7–9
week-old A129-/- mice were inoculated with 4x104, 4x103 or 4x102 PFU of a WT strain of YFV i.v. A, C and E) Body weight
was measured daily until day 14th and expressed as % of body weight loss from day 0 before YFV inoculation. B, D and F)
Survival rates were analysed every 12 hours and expressed as % of survival mice until day 14th post-YFV inoculation. Dashed
lines are representative of MOCK-infected group. Survival was statistically assessed by Log-rank (Mentel-Cox) test.
Differences in weight are displayed as the means ± SEM, and two-way ANOVA for each day was used to assess the
significance. Three independent experiments were performed with at least four mice/group. � P < 0.05.
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Sofosbuvir inhibits yellow fever virus
Fig 9. Treatment with Sofosbuvir ameliorates disease outcomes of adult YFV-infected A129-/- mice. A129-/- mice were infected with 4x102 PFU of low passage
clinical isolate of YFV by intravenous route. A-E) Viral loads from serum (A), liver (B), spleen (C), kidneys (D), and brain (E) assessed by plaque assay. Results are
shown as median f Log10 PFU equivalents per mL or g. F) Total and differential cell counts in blood were represented as number of differential cell counts (leukocytes,
mononuclear cells and neutrophils) normalized to % of total cells counts. G) Hepatic transaminase levels of ALT were measured in serum of MOCK and YFV-infected
mice at days 3 and 6 p.i. Results are shown as ALT (U/L) of serum. H) Histopathological score of liver tissue sections. I) Representative of hepatic damage and
Hematoxylin & Eosin staining of liver sections of control and YFV-infected mice three days after infection (Scale Bar—100 μm). The images presented are representative
animals on the third day of infection. Four animals per group were assayed. � for p<0.05 when compared to MOCK. # for p< 0.05 when compared to YFV.
https://doi.org/10.1371/journal.pntd.0007072.g009
Our data showed that sofosbuvir inhibits YFV in hepatoma cell lines. This is particularly
relevant since YFV targets hepatocytes and the liver is the most affected organ in YF [41]. The
degree of liver damage measured by elevated aminotransferase levels and jaundice is associated
with higher mortality [42]. Our Swiss mouse neonate model of virus infection reproduced the
deleterious association among increased ALT, viral loads, and mortality. By inhibiting virus
replication, sofosbuvir attacked this deleterious loop towards the liver protection and
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Sofosbuvir inhibits yellow fever virus
enhancement of the survival. In humans, massive apoptosis and necrosis of hepatocytes are
reported in fatal cases [43]. Additionally, impaired synthesis of clotting factors caused by YFV-
induced liver injury is key to the pathogenesis of haemorrhagic manifestations in severe YF
[44]. Experimental cases of liver transplant after YFV infection have been conducted during
recent outbreak in Brazil [4, 45, 46] with 50% survival rate. We envision that sofosbuvir may
improve liver function in YF patients, and hopefully enhance the survival rates of transplanted
individuals.
In the highly virulent infection mouse model, A129-/- mice infected with WT YFV, sofosbu-
vir also diminished virus replication in the brain and improved liver condition. Both viscero-
tropic and neurological replication injuries are hallmarks of YFV pathogenesis [8, 47, 48]. Our
data point to a possible benefit of early treatment with sofosbuvir for patients that may prog-
ress to complications at late stages in the natural history of infection. Interestingly, although
sofosbuvir was more efficient when used prophylactically, it was able to improve the hepatic
condition of the infected animals receiving a post-infection treatment.
Sofosbuvir reduced the YFV-induced mortality and lack of weight gain in mice models.
Considering our data and the safety history of sofosbuvir in hepatitis, it is not a hyperbolic
conclusion to consider sofosbuvir in clinical use for YF infection in humans. Primarily, sofos-
buvir could be worthwhile for acutely infected individuals and those displaying neurotropic
and viscerotropic diseases provoked by virus replication. Since vaccine is not recommended
for special groups of individuals at higher risk of severe adverse events, such as elderly and
those with immunodeficiencies, sofosbuvir could be used prophylactically.
Most commonly, we used ribavirin, a pan-antiviral drug with anti-YFV activity demon-
strated both in vitro and in vivo [33–35], as a positive control to inhibit YFV replication and to
compare with sofosbuvir. Our in vitro data showed that sofosbuvir was more efficient than
ribavirin in reducing viral genome replication, number of infected cells, and production of
infectious viral particles in Huh-7 cells. Moreover, we thus understand that sofosbuvir pos-
sesses advantages over ribavirin in terms of safety and efficacy. Ribavirin is more toxic than
sofosbuvir, especially for critically ill hepatic and renal patients [49], such as those with YF.
According to FDA categorization of risk during pregnancy; sofosbuvir is class B (drugs with
the second to lowest risk of causing malformations), whereas ribavirin is class X (forbidden,
even for men having intimate contact with women at gestational age). Thus, ribavirin would
have a more limited scope of use. Indeed, when ribavirin was used in a clinical trial against a
flavivirus, the Japanese encephalitis virus, it failed to be effective [50].
Although our results are translational, it is important to cite that further studies are neces-
sary to precisely determine sofosbuvir’s mechanism(s) of action towards YFV life cycle and the
dose adjustment to treat patients with YF. YFV RNA polymerase is the likely target, based on
docking onto this enzyme and reduction of viral dsRNA levels. As with other RNA polymer-
ases from positive-sense RNA viruses, well-conserved motifs are found the YFV and HCV
RNA polymerase, such as D-x(4,5)-D and GDD, which are spatially juxtaposed, wherein Asp
binds Mg2+ and Asn selects ribonucleotide triphosphates over dNTPs, determining RNA syn-
thesis [9]. It would not be surprising to see sofosbuvir triphosphate bound to these critical resi-
dues for catalysis, because even other positive-sense RNA virus beyond members of the
Flaviviridae family are susceptible to sofosbuvir [51]. Sofosbuvir inhibits HCV RNA polymer-
ase as chain terminator [31]. Nevertheless, this drug is considered a non-obligate chain termi-
nator, due to the presence of 3’-OH moiety. Indeed, sofosbuvir inhibited ZIKV replication by
targeting its RNA polymerase directly and provoking A-to-G mutation in the virus genome
[40]. Whether sofosbuvir acts directly on YFV RNA polymerase, induces an error-prone virus
replication by its incorporation in the virus genome or inhibition inosine monophosphate
dehydrogenase and/or independent mechanisms–it remains to be elucidated. We also
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Sofosbuvir inhibits yellow fever virus
observed that sofosbuvir inhibits 100- to1000-times more potently HCV than YFV produc-
tion/replication [13, 31, 52]. These differences in potencies need to be interpreted in light of
the sofosbuvir’s concentrations found in anatomical compartments and its long-half life [16],
which may already be enough to compensate the limitation in the in vitro pharmacological
potency. Sofosbuvir’s chemical structure allows its vectorization to the liver, where it is found
at 77 μM when patients are treated with the reference dose of 400 mg/day [53]. YFV produc-
tion was reduced up to 3-log10 when hepatoma cells where treated with sofosbuvir at 50 μM.
Since sofosbuvir has been used clinically at doses up to 1200 mg/day [16], it is possible to have
enough sofosbuvir in the liver to inhibit or reduce YFV replication in humans remains. Only
clinical trials will reveal the best regimen and posology for sofosbuvir towards YF. In the our
mouse models, sofosbuvir efficacy was observed at the reference pre-clinical dose previously
studied for HCV, 20 mg/kg/day [15].
The results described here demonstrate for the first time the antiviral activity of sofosbuvir
to YFV, which caused a recent outbreak in Brazil, providing primary scientific evidence for a
new potential use of a clinically approved antiviral drug and reinforcing that its chemical struc-
ture may be used to generate selective anti-YFV specific drugs.
Supporting information
S1 Fig. Gate strategy from flow cytometry analysis. Flow cytometry events were gated on a
dot plot FSC-A x FSC-H to exclude doublets (A-C). Cells were identified on FSC-A x SSC-A
dotplots and gated to eliminate debris from analysis (D-F) and then evaluate percentage of
4G2-positive cells (G-I). Panels are representative of five independent experiments.
(PDF)
S2 Fig. Dot plots from flow cytometry analysis. 4G2 positive cells quantified from Huh-7
(A-E) and HepG2 (F-J) cells infected with YFV and treated with sofosbuvir at indicated con-
centrations. Representative of at least five independent experiments.
(PDF)
S1 Table. Alignment of RNA polymerases from members of the Flaviviridae family. C-ter-
minal region of the RNA polymerase from Zika, Dengue, hepatitis C and yellow fever viruses.
Conserved amino acid residues are highlighted in yellow. Critical amino acid residues are
highlighted in red.
(PDF)
Acknowledgments
Thanks are due to Dr. Karin Bru¨ning and Dr. Jaime Rabi, from the BMK Consortium (Blanver
Farmoquı´mica Ltda; Microbiolo´gica Quı´mica e FarmacêuticaLtda; Karin Bruning & Cia.
Ltda), for donating sofosbuvir, and to Dr. Ana M. B. de Filippis, from Laborato´rio de Flavivı´-
rus, Instituto Oswaldo Cruz, Fiocruz, for kindly providing the YFV strain. We thank Ilma
Marcal and Tania Colina for technical assistance. We also thank the Liver Center at Federal
University of Minas Gerais for technical support.
Author Contributions
Conceptualization: Amilcar Tanuri, Thiago Moreno L. Souza.
Data curation: Thiago Moreno L. Souza.
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Sofosbuvir inhibits yellow fever virus
Formal analysis: Caroline S. de Freitas, Luiza M. Higa, Carolina Q. Sacramento, Andre´ C. Fer-
reira, Patrı´cia A. Reis, Rodrigo Delvecchio, Fabio L. Monteiro, Giselle Barbosa-Lima, Harri-
son James Westgarth, Yasmine Rangel Vieira, Natasha Rocha, Lucas Villas Boˆas Hoelz,
Rennan Papaleo Paes Leme, Moˆnica M. Bastos, Gisele Olinto L. Rodrigues, Carla Elizabeth
M. Lopes, Celso Martins Queiroz-Junior, Cristiano X. Lima, Vivian V. Costa, Mauro M.
Teixeira, Fernando A. Bozza, Patrı´cia T. Bozza, Nubia Boechat, Amilcar Tanuri, Thiago
Moreno L. Souza.
Funding acquisition: Mauro M. Teixeira, Amilcar Tanuri, Thiago Moreno L. Souza.
Investigation: Caroline S. de Freitas, Luiza M. Higa, Carolina Q. Sacramento, Andre´ C. Fer-
reira, Patrı´cia A. Reis, Rodrigo Delvecchio, Fabio L. Monteiro, Giselle Barbosa-Lima, Yas-
mine Rangel Vieira, Mayara Mattos, Lucas Villas Boˆas Hoelz, Rennan Papaleo Paes Leme,
Gisele Olinto L. Rodrigues, Carla Elizabeth M. Lopes, Celso Martins Queiroz-Junior, Cris-
tiano X. Lima, Vivian V. Costa.
Methodology: Caroline S. de Freitas, Luiza M. Higa, Carolina Q. Sacramento, Andre´ C. Fer-
reira, Patrı´cia A. Reis, Rodrigo Delvecchio, Fabio L. Monteiro, Giselle Barbosa-Lima, Harri-
son James Westgarth, Yasmine Rangel Vieira, Mayara Mattos, Natasha Rocha, Lucas Villas
Boˆas Hoelz, Rennan Papaleo Paes Leme, Gisele Olinto L. Rodrigues, Carla Elizabeth M.
Lopes, Celso Martins Queiroz-Junior, Cristiano X. Lima, Vivian V. Costa.
Project administration: Amilcar Tanuri, Thiago Moreno L. Souza.
Resources: Mauro M. Teixeira, Amilcar Tanuri, Thiago Moreno L. Souza.
Supervision: Mauro M. Teixeira, Amilcar Tanuri.
Validation: Caroline S. de Freitas, Luiza M. Higa, Vivian V. Costa, Mauro M. Teixeira, Amil-
car Tanuri, Thiago Moreno L. Souza.
Visualization: Caroline S. de Freitas, Luiza M. Higa, Carolina Q. Sacramento, Andre´ C. Fer-
reira, Patrı´cia A. Reis, Rodrigo Delvecchio, Fabio L. Monteiro, Moˆnica M. Bastos, Vivian V.
Costa, Mauro M. Teixeira, Fernando A. Bozza, Patrı´cia T. Bozza, Nubia Boechat, Amilcar
Tanuri, Thiago Moreno L. Souza.
Writing – original draft: Caroline S. de Freitas, Luiza M. Higa, Carolina Q. Sacramento,
Andre´ C. Ferreira, Patrı´cia A. Reis, Rodrigo Delvecchio, Fabio L. Monteiro, Moˆnica M. Bas-
tos, Vivian V. Costa, Mauro M. Teixeira, Fernando A. Bozza, Patrı´cia T. Bozza, Nubia Boe-
chat, Amilcar Tanuri, Thiago Moreno L. Souza.
Writing – review & editing: Caroline S. de Freitas, Luiza M. Higa, Harrison James Westgarth,
Mauro M. Teixeira, Fernando A. Bozza, Patrı´cia T. Bozza, Nubia Boechat, Thiago Moreno
L. Souza.
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| null |
10.1088_1361-6501_ad095a.pdf
|
Data availability statements
The data cannot be made publicly available upon publication
because they are owned by a third party and the terms of use
prevent public distribution. The data that support the findings
of this study are available upon reasonable request from the
authors.
|
Data availability statements The data cannot be made publicly available upon publication because they are owned by a third party and the terms of use prevent public distribution. The data that support the findings of this study are available upon reasonable request from the authors.
|
Meas. Sci. Technol. 35 (2024) 025117 (13pp)
Measurement Science and Technology
https://doi.org/10.1088/1361-6501/ad095a
Missing data filling in soft sensing using
denoising diffusion probability model
Dongnian Jiang
∗, Renjie Wang, Fuyuan Shen and Wei Li
School of Electrical Engineering and Information Engineering, Lanzhou University of Technology,
Lanzhou 730050, People’s Republic of China
E-mail: [email protected]
Received 8 September 2023, revised 17 October 2023
Accepted for publication 1 November 2023
Published 14 November 2023
Abstract
With the aim of addressing the problem of degradation in soft measurement accuracy due to
missing data in industrial processes, a filling method based on the denoising diffusion
probability model (DDPM) is proposed here to improve the accuracy of soft measurement
modeling. First, missing regions are detected with the help of an improved Isolation Forest
algorithm to obtain information such as the locations and numbers of missing data regions.
Next, a data generation model is constructed based on DDPM and new samples are obtained. By
adjusting the threshold for normal operation of the system and the weight sampler, filler samples
that are similar to the distribution of the original data can be filtered from the new samples to
form a complete dataset. The feasibility of the proposed missing data filling method is explored
through numerical simulations, and its superiority in terms of improving the prediction accuracy
of soft measurements is verified in regard to the nickel flash smelting process.
Keywords: missing data, DDPM, isolation forest, soft measurement
1. Introduction
Due to space constraints, harsh inspection environments, and
the high cost of complex industrial processes, soft meas-
urements are often used to increase the point coverage of
a measurement system [1, 2]. Soft measurement modeling
can be generally classified into mechanism-based and data-
driven approaches [3, 4]. Although data-driven soft measure-
ment modeling has become a research hotspot in recent years,
thanks to its simplicity and efficiency [5], the problem of miss-
ing data has become common during the collection of sensor
data, and is affected by internal sensor failures, the different
sampling rates of the sensors, and communication limitations.
This phenomenon can seriously reduce the prediction accur-
acy of the data-driven soft measurement model, which in turn
can be detrimental to the monitoring of the operational status
of the system and the product quality.
Missing data usually refers to the presence of missing val-
ues in the feature data matrix [6]. Three main categories of
∗
Author to whom any correspondence should be addressed.
missing data can be distinguished: the first relates to poor con-
tacts in the sensor’s internal circuitry, resulting in multiple
missing parts of the data matrix; the second is an imbalance in
the data distribution caused by the different sampling rates of
the sensors (i.e. data on one feature are sparser than for other
features); while the third arises from power supply interrup-
tions or sudden sensor failures in industrial systems that cause
data to be missing during the recovery period. Zhou et al [7]
addressed the problem of missing air quality data due to sensor
failure by using a mask matrix and a generating adversarial
network (GAN) for missing value filling. Zhu et al [8] used
a local outlier factor (LOF) and the K-means++ algorithm to
find sparse regions of the data for small sample datasets and
used a GAN to generate and fill them to obtain a dataset with
balanced distribution. Studies have shown that missing data
affect the prediction accuracy and computational speed of a
soft measurement model, meaning that it is necessary to fill in
the missing areas in the dataset before carrying out soft meas-
urement modeling [9].
The distribution pattern of multivariate time series data
such as finance, medical treatment and weather is clear and
the fluctuation range is large, which is easy to fill the missing
1361-6501/24/025117+13$33.00
1
© 2023 IOP Publishing Ltd
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
value [10–12]. The parameters of industrial systems are relat-
ively stable under normal operating conditions and fluctuate
for a long time within a small range, which means that sensor
data has a unique distribution [13]. Therefore, when dealing
with missing data in industrial systems, it is not only necessary
to consider whether the filling values are reasonable, but also
whether they conform to the distribution of the original data
[14, 15]. K-nearest neighbor (KNN), synthetic minority over-
sampling technique (SMOTE), and random forest (RF) meth-
ods can effectively fill in missing values in ordinary time series
data [16–18]. However, in these methods, it is difficult to meet
the requirements for the numerical rationality of industrial data
and the distribution pattern of normal system operation [19].
Following the development of deep learning techniques,
some researchers have proposed more powerful deep gener-
ative models for filling missing values [20–22]. A stable and
high-precision generation model is the foundation for filling in
missing data. In GANs, generators and discriminators optim-
ize each other during the adversarial process. However, its
mode collapse and unstable training often lead to lower filling
accuracy [23]. Variational auto-encoders adopts an optimiza-
tion method of approximate inference between generated data
and raw data, resulting in significant deviation between the two
types of data. In addition, converting high-dimensional data
into low-dimensional space leads to the loss of some original
information [24]. If the generated data has a significant devi-
ation from the original data, it cannot be used to fill in missing
data.
Based on the continuous developments in generative net-
work architectures, Ho et al [25] proposed denoising diffu-
sion probability model (DDPM) for high-quality image syn-
thesis. Only one neural network was used in the DDPM for
fitting the noise distribution in the forward noise addition pro-
cess, meaning that there was no need to reach Nash equilib-
rium. This approach yielded higher stability and convenience
than the training process of a GAN. Other researchers have
applied DDPMs in the fields of image denoising, image gen-
eration and time series data prediction [26–29]. Wyatt et al
[30] applied a DDPM to image anomaly detection. Rasul et al
[31] introduced the hidden variables in an RNN as a priori
knowledge into the DDPM denoising process to achieve the
task of forecasting time series data. Yan et al [32] proposed
an improved DDPM for the detection task of cyber-physical
system. Although DDPM has achieved good results in areas
such as image denoising, it does not require high accuracy of
the generated data. Due to the closed-loop propagation charac-
teristics of sensor data in industrial processes, if the data gen-
eration process cannot be effectively improved to enhance the
accuracy of generated data, safety problems and accidents may
arise. There is currently a lack of research on the application
of DDPMs to industrial processes.
To solve the problem of degradation in the accuracy of soft
measurement models caused by missing data in industrial pro-
cesses, this paper proposes a missing data filling framework
called IF-DDPM, as shown in figure 1. First, an improved isol-
ated forest algorithm is used to detect missing data regions.
Next, DDPM is used to generate a new batch of samples that
Figure 1. Diagram of the proposed missing data filling framework
based on IF-DDPM.
match the distribution of the original data. Based on the detec-
tion of missing regions, a weight sampler is used to obtain
filler samples that are added into the missing regions to form a
complete dataset. Finally, the complete high-quality dataset is
used to build soft measurement models to improve prediction
accuracy.
In summary, compared with existing missing data filling
methods, the method in this paper makes the following contri-
butions:
2
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
(1) An improved isolated forest algorithm is proposed for the
detection of missing data from continuous time series,
which can provide information on areas of missing data.
(2) A missing data filling framework called IF-DDPM is pro-
posed for industrial scenarios in which the filled data need
to have a high level of similarity to the original data and
to meet the distribution requirements for normal operation
of the system.
2. Missing region detection using an improved
isolated forest algorithm
Traditional outlier detection algorithms typically use data min-
ing methods to find outliers that are inconsistent with the dis-
tribution of the dataset. In this approach, it is necessary to cal-
culate the distance between the points or the density of the
data distribution at a given point [33–35]. The isolated forest
algorithm proposed by Liu et al [36] is based on the idea binary
of a tree. It enables anomaly detection by placing anomalies
independently on the child nodes closest to the root node of
the tree. It has been widely used due to the simple calculations
involved and its high efficiency.
The original isolated forest algorithm can only detect anom-
alies and cannot identify the existence of regions with miss-
ing data. This means that it is not possible to obtain informa-
tion about regions of missing data, including their locations in
the dataset and the amounts of missing data. In this paper, we
therefore propose a missing region detection algorithm based
on an improved version of isolated forest.
We assume that the sensor dataset with missing values is
X = {x1, x2, x3, · · · , xn
}. Slicing is performed based on pairs of
random values p that lie between the maximum and minimum
values in the dataset. Finally, each data point is placed on an
isolated child node of the binary tree or a subspace. At this
point, we have
c (n) = 2H (n − 1) − (2 (n − 1)/ n)
(1)
We calculate the anomaly score for each sample point as
follows:
s (x, n) = 2 − E (h (x))
c (n)
(2)
where h(x) is the average distance to the sample point on a
single subtree, and E(h(x)) is the average of distance of the
sample points in t trees. Anomaly scores are obtained as shown
in algorithm 2 by generating isolated forests.
Algorithm 2. Generation of isolated forests Fforest(X, t, ψ ), where
the anomaly score for a sample point is calculated as s(x).
Input: X is a missing sensor dataset, t is the number of isolated
subtrees, ψ is the random sample size
1: initialize the isolated forest Fforest(X, t, ψ ) and set the tree
maximum ceiling(log2ψ ).
2: for i = 1 to t do
′
X
is a randomly selected sample from the given data set X.
′, 0, l)
Fforest = Fforest∪Ftree(X
3:
4: end for
5: return Fforest
6: for x ∈ X do
7:
8:
9: end for
10: return Fforest,s(x)
calculate s(x) according to equation (2)
return s(x)
In order to detect missing regions, the anomaly score for
each sample point is first obtained using the original isolated
forest algorithm. These anomaly scores are then sorted from
large too small. The sample points at each end of a missing
portion from a period have larger anomaly values than the oth-
ers. Hence, the number of anomaly score values selected is
twice the number of missing regions. This is the anomaly score
corresponding to the data points at both ends of each missing
region. Equation (3) shows the correspondence between the
number of missing regions m and the number of outliers M to
be selected:
where c(n) is the average path length for n sample points.
Algorithm 1 shows the process used to build isolated subtrees.
M = 2 ∗ m.
(3)
Algorithm 1. Generate isolated subtrees Ftree(X, e, l).
Input: X is the sensor dataset with missing parts, l is the height of
the isolated subtree, e is the current isolated subtree height, p is a
random value between the maximum and minimum values
1: if e < l or |X| > l then randomly select a segmentation point
p from the given dataset X.
← X ⩽ p
2:
Xless
Xgreater ← X > p
3:
4: return the leaf node N
{left node Ftree(Xleft, e + 1, l),
right node Ftree(Xright, e + 1, l),
split value p}
return the leaf node N{X}
5: else
6:
7: end if
8: return Ftree
When the data points at both ends of the missing region and
their indices have been found, the location and size of the miss-
ing region in the dataset can be obtained. Algorithm 3 shows
the missing region detection process.
Algorithm 3. Missing area detection.
Input: s(x, n) is the anomaly score for the training data points, m
is the number of missing regions
1: rank all exception scores from large to small according to
s(x, n).
2: screen M anomalies using equation (3).
3: group the M anomalies in pairs, based on the order of
indexing.
4: subtract the index of each set of data points to obtain the
location and size of the missing region.
5: return the location and size of each missing area.
3
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
3. Generation and filling of missing data based on
DDPM
In order to fill the missing sensor data, a new sample bank
first needs to be constructed by obtaining new samples with
a high level of similarity to the original data with the help
of a stable generative model. Next, a sampler is used to fill
the missing areas by sampling data from the new sample pool
based on the distribution of the data. DDPM is currently the
most advanced generative probabilistic model. It uses a neural
network to build a mapping from a simple distribution to a tar-
get distribution. Train the network and make its loss converge
for the purpose of generating data. The model consists of two
stages, adding noise and denoising, and its structure is shown
in figure 2. In the first stage, the original data are input into
DDPM as training data. Uniformly increasing Gaussian noise
is gradually added to the original data distribution q(x0), based
|xt−1), until q(x0) has been corrup-
on the diffusion law q(xt
ted to give a Gaussian distribution. In the second stage, this
Gaussian distribution is reduced to q(x0) using a neural net-
|xt).
work approximation based on the denoising law pθ(xt−1
The addition of noise is essentially a Markov process. It
incorporates a noise distribution based only on the state of the
previous step, and is independent of any state other than the
previous step. Thus, it can be described as a one-step transfer
probability:
(cid:16)
q (Xt
|Xt−1) = N
Xt
p
|
1 − βtXt−1; βtI
(cid:17)
(4)
√
1 − βtXt−1 is the mean, and βtI is the variance. The
where
current value Xt is sampled from this distribution. β ∈ (0, 1)
represents the noise added at the tth instant, and the degree of
noise increases over time. The variables are defined as
q (xt|xt−1, x0) = N
(cid:0)
xt−1
=∝ exp
(cid:1)
|˜u (xt, x0) , ˜βtI
√
(xt −
− 1
2
√
(cid:0)
+
xt−1
(cid:18)
αt−1x0
−
1 − ¯αt−1
(cid:18)(cid:18)
= exp
(cid:18)
−
− 1
2
√
αt
2
βt
αt
βt
+
√
1
1 − ¯αt−1
(cid:19)
¯αt
2
1 − ¯αt
x0
xt +
αtxt−1)2
βt
(cid:1)
2
−
(cid:0)
√
¯αtx0
xt −
1 − ¯αt
(cid:19)
2
xt−1
!!
(cid:1)
2
(cid:19)(cid:19)
xt−1 + C (xt, x0)
.
(8)
The mean ˜ut(xt, x0) and variance ˜βt can be derived from
equation (8) as follows:
√
˜ut (xt, x0) =
˜βt =
¯αt−1βt
1 − ¯αt
1 − ¯αt−1
1 − ¯αt
√
αt (1 − ¯αt−1)
1 − ¯αt
xt
x0 +
βt.
(9)
(10)
At this point, the optimization objective function of the
model can be calculated by the minimum likelihood of pθ(x0):
(cid:21)
(cid:20)
Eq(x0:T)
log
|x0)
q (x1:T
pθ (xθ : T)
⩾ −Eq(x0) log pθ (x0) .
(11)
A one-dimensional convolutional neural network (1D-
CNN) is used in the inverse process to fit the noise distri-
bution added at each step of the forward process. The data
distribution of the previous step is reduced by this network
based on the current step. The parameters in equation (9) are
optimized by this network, which can be rewritten as shown in
equation (12):
"
αt = 1 − βt.
Eq
DKL (q (xT
|x0
||pθ (xT)))
(5)
Hence, each step of the noise addition process can
be represented by the original data distribution q(x0) and
equation (5) as:
+
TX
t=2
DKL (q (xt−1
|xt, x0) ||pθ (xt−1
|xt)) − log pθ (x0
|x1)
.
(12)
(cid:0)
q
Xt
|X0) = N (Xt
|
√
¯αtX0, (1 − ¯αt) I
(cid:1)
.
The parameters εθ to be optimized in equation (12) are
(6)
given in equation (13):
The sampling at each step of the denoising process also
conforms to the characteristics of a Gaussian distribution.
However, it is necessary to introduce the parameter θ for estim-
ation of the previous step, using the sampling formula:
"
Ex0,ε
βt
2
2αt (1 − ¯αt)
2σt
(cid:12)
(cid:12)
(cid:12)
(cid:12)
(cid:12)
(cid:12)ε − εθ
(cid:16)√
¯αtx0 +
p
1 − ¯αtε, t
#
(cid:17)(cid:12)
(cid:12)
(cid:12)
(cid:12)
(cid:12)
(cid:12)
2
(13)
#
(cid:16)
pθ
Xt−1
|Xt) = N (Xt−1
| uθ (Xt, t) ,
X
(cid:17)
(Xt, t)
.
θ
(7)
where:
(cid:18)
uθ (xt, t) = ˜u
(cid:17)(cid:19)
p
1 − ¯αtεt (xt)
(cid:19)
(cid:16)
xt
−
1√
xt,
¯αt
(cid:18)
− βt√
xt
1√
αt
1 − ¯αt
εθ (xt, t)
.
(14)
posterior
The
diffusion
|xt−1, x0) is calculated as shown in equation (8):
conditional
q(xt
probability
=
4
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
Figure 2. Diagram showing the structure of DDPM.
By optimizing the shared convolutional kernel of the 1D-
CNN and other network parameters, εθ is approximated for
fitting to the Gaussian noise ε added in the current step.
Algorithms 4 and 5 show the training and inverse sampling
process of the DDPM model, respectively.
Algorithm 4. DDPM training process.
Input: Initial data x0
∼ q(x0)
1: while not converged do
2:
3:
initialize t ∼ Uniform(1, 2, · · · , T) and ε ∼ N(0, 1)
gradient descent optimization based on
∇θ
(cid:13)
(cid:13)2
1 − ¯αtε, t)
(cid:13)
(cid:13)ε − εθ(
¯αtx0 +
√
√
4: end while
Algorithm 5. Inverse sampling process of DDPM.
Input: Noise distribution xT ∼ N(0, 1)
1: for t = T to 1 do
if t > 1 then
2:
z ∼ N(0, 1)
3:
4:
5:
6:
7:
xt−1 = 1√
αt
z = 0
end if
else
(xt − 1−αt√
1− ¯αt
εθ(xt, t)) + σtz
8: end for
9: return x0
Through the use of 1D-CNN and DDPM optimization, the
model can generate data with a similar distribution to that of
the raw data from the sensor.
In order to meet the requirements for high accuracy of the
sensor filling data, it is necessary to set a reasonable threshold
for normal operation of the system according to the actual
5
production process. Generated data within the threshold range
are selected to ensure the high accuracy and reasonableness of
the filled data. We take the missing area information detected
as a known condition, and use the weight sampler to get a cor-
responding fill sample for each missing area based on the data
distribution law. The distribution of the generated samples is
very similar to that of the original samples. Finally, the filler
samples are sequentially filled into the missing regions to form
a complete dataset.
4. Soft measurement modeling
The proposed soft measurement model uses a fully connected
neural network, consisting of an input layer, a hidden layer,
and an output layer. The predicted value of the target variable
can be calculated in the output layer by adjusting the weights.
The output of each layer is shown in equation (15):
!
Hj = g
nX
i =1
wijxi + aj
(15)
where wi,j is the weight of each node, xi is the output of the
node in the previous layer, aj is the bias of each node, g is the
activation function, and Hj is the final output value of each
node. The calculation of the final output layer is shown in
equation (16):
lX
Ok =
Hjwjk + bk.
j =1
(16)
After backpropagation and weight optimization of multiple
hidden layer gradients, multiple auxiliary variables can be fit-
ted to the target variable.
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
In this study, mean absolute error (MAE) and mean square
error (MSE) were used as metrics to evaluate the prediction
accuracy of the soft sensor model, for both the unfilled data
and the resulting datasets after five different filling methods
had been applied. The calculation formulae are as follows:
MAE =
MSE =
1
m
1
m
mX
i =1
mX
i =1
| fi
− yi
|
( fi
− yi)2
(17)
(18)
where fi are the real data, yi are the predicted data, and m is the
number of training samples.
5. Case study
5.1. Numerical simulation
To validate the proposed method in this paper, a nonlinear two-
dimensional function was set up as shown in equation (19).
The 128 sets of x and y data generated by this relation equation
were used as training data
y = x2 + cos (3π x) ∗ log x0.5 + 0.1 ∗ ε
, ε ∈ N (0, 1)
(19)
where x follows a standard uniform distribution x ∈ U(0, 1),
and ε is the added Gaussian noise. This nonlinear function
is used to account for the complex functional relationships
between variables in industrial processes. It is also used to
validate the applicability of the IF-DDPM method to solve
the problem of missing sensor data in complex industrial
processes.
The 1D-CNN, DDPM models and samplers used for the
training process in this study were implemented using the
PyTorch deep learning environment. The sklearn Python pack-
age was used to implement the isolated forest outlier detec-
tion algorithm and the other machine learning missing data
filling methods. All the procedures in this study were validated
by simulation on an NVIDIA GeForce RTX3090Ti GPU. The
parameters of the isolated forest algorithm were set as shown
in table 1.
The neural network in DDPM used a 1D-CNN, the activa-
tion function was a LeakyReLU, and the network parameters
were optimized using Adam optimizer. The number of noise
addition diffusion steps was 100. The experimental steps car-
ried out to validate the IF-DDPM filling method were as fol-
lows:
Step 1: Each missing region and its missing information in
the training dataset was detected using the improved isolated
forest algorithm.
Step 2: The training data were fed to the DDPM. Table 2
shows the parameters of the model and the structure of
the 1D-CNN network. The model loss curve is shown in
figure 3. In order to verify the superior data generation res-
ults from the proposed DDPM, nine different degrees of
Gaussian noise were added to the nonlinear 2D function given
Table 1. Parameter settings for the improved isolated forest
algorithm.
Parameter
n_estimators
max_samples
contamination
max_features
bootstrap
n_jobs
random_state
verbose
warm_start
outlier threshold
Value
100
811
auto
1
false
none
none
0
false
0.01
Table 2. Parameter settings for the proposed DDPM.
Argument
Value
Number of iterations
Spread steps
Lot size
Optimizer for DDPM
Sequence length
Number of convolution kernels 90
5
Convolution kernel size
3
Convolution step size
200
1000
512
Adam
128
Figure 3. Loss curve for the DDPM model.
in equation (19), to demonstrate that the DDPM could fit
data representing various scenarios. Figure 4 shows graphs for
these nine fitting cases, and it can be seen that the data gener-
ated by DDPM are very similar to the original data, with excel-
lent fitting results. The high similarity of the two types of data
is the basis for high-precision sampling of the generated data
and filling in missing areas.
Step 3: Based on the original data distribution, a weight
sampler was used to obtain new samples from the generated
data that were within the threshold range. These new samples
6
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
Figure 4. Curves for generated data with different degrees of added noise.
Figure 5. Results for the filling of missing areas.
had the same distribution pattern as the original data and con-
formed to the characteristics of normal system operation. The
samples were sequentially filled into the missing regions to
construct the complete dataset. Figure 5 shows the results of
data filling for three different missing states in the dataset with
two, three, and four missing regions. The filled data obtained
using the proposed IF-DDPM method show the same trends
and values as the original data, even if the dataset contains
multiple missing regions.
5.2. Filling missing sensor data for a nickel flash smelting
furnace (NFSF)
Figure 6 shows a NFSF, which is used to
5.2.1. NFSF.
carry out a fire smelting process consisting of three steps: ore
7
◦
◦
preparation, smelting, and refining. Normal operation of the
NFSF is essential to ensure the purity of the nickel. Since
C −
the internal temperature of NFSF is maintained at 1450
1550
C, it is not appropriate to install sensors. Hence, soft
measurements of the temperature variable y inside the fur-
nace need to be modeled in order to monitor the reaction state.
The auxiliary variables affecting the temperature inside the
furnace are shown in table 3. In this study, we used sensor
data from three different time periods during the actual nickel
flash smelting process. Each time period has 1200 samples, of
which 1000 are used to train the model and the rest of the data
is used for testing. Manually set a different number of ran-
dom missing data areas: 2, 3, and 4. The validity of the filling
method is verified by comparing the completed data with the
original complete data set. Information on the missing regions
is shown in table 4.
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
Figure 6. Process flow diagram for the NFSF.
Table 3. Variables of the NFSF.
Number
Data attribute
x1
x2
x3
x4
y
Concentrate humidity (%)
Concentrate particle size (mm)
Sulfur dioxide content in sulfur containing flue gas (mg m3)
Water content of sulfur-containing flue gas (%)
◦
Internal reaction temperature of flash furnace (
C)
Table 4. Information on the missing regions in different states.
Missing area information
Number of missing areas
2
3
4
Missing
area indices
332–402
461–523
322–382
424–494
634–690
250–282
307–354
571–615
721–750
Amount of
missing data
71
63
61
71
57
33
48
45
30
The method pro-
5.2.2. Comparison with other methods.
posed in this paper was compared with other common missing
data filling methods, such as GAN, KNN, RF, and SMOTE, to
verify its effectiveness and superiority.
(a) GANs have mostly been used in areas such as image gen-
eration since they were developed. However, the study in
[8] used this type of deep generative model to generate
missing data and achieved good filling of scarce regions
[37]. In order to prevent the phenomenon of pattern col-
lapse arising during the GAN training process, attribute
features such as the mean, variance and extreme values of
the original data were used as additional information in
the adversarial training of a CGAN. The loss function and
penalty term of WGAN-GP were introduced to ensure that
the model had a stable generation process. We therefore
8
refer to this improved model as C-WGAN-GP, for which
the loss function is shown in equation (20):
L = E
˜x∼P
g
+ λ E
ˆx∼Pˆx
(cid:2)
D (˜x |y)] − E
x∼P
(∥∇ˆxD (ˆx |y )∥
h
r
[D (x| y)
(cid:3)
i
− 1)2
.
2
(20)
(b) KNN has also been used for filling missing values [16]. It
gives an estimated interpolation of missing values by cal-
culating the average of the K nearest neighbors of a given
point to form a complete dataset. This calculation is shown
in equation (21):
q
d =
(x2
− x1)2 + (y2
− y1)2
(21)
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
Figure 7. Loss curves for DDPM.
Figure 8. Original and generated data density distributions from GAN and DDPM.
where d is the result of the distance calculation. (x2, y2) and
(x1, y1) are the coordinates of the two points.
(c) RF treats the problem of missing data filling essentially as
a regression prediction task, in which missing values are
replaced with predicted values [17]. Features with miss-
ing values are used as labels, and the remainder are used
as training samples. The parts of the dataset without miss-
ing values are used as training data, while the missing
data need to be predicted. In this paper, filling is achieved
by replacing the predicted values with the missing values
using the RF algorithm.
(d) SMOTE is an oversampling technique that is used to
expand the number of samples to solve the problem of
sample imbalance [18]. It obtains the K nearest neighbors
of a point in the negative sample and randomly interpol-
ates between each pair of points to give a new sample, as
shown in equation (22):
xnew = x + rand (0, 1) ∗ (˜x − x)
(22)
9
where xnew is the new sample obtained by sampling, x is a point
in the selected minority sample, and ˜x is a K nearest neighbor
point of x.
The data points on
5.2.3. Filling process for missing data.
both sides of the missing regions in the original dataset are on
the fringes of the data cluster compared to the other data points,
meaning that the anomaly scores obtained for them are larger.
First, the top four, six and eight sample points with the max-
imum anomaly scores were selected and the missing region
information was calculated for each case using algorithm 3.
Next, the DDPM was used to generate a batch of data that
matched the distribution of the original data. The normalized
original data with missing values was used as the training data
for the DDPM. Figure 7 shows the loss curves for the DDPM
for these three states. It can be seen from the figure that train-
ing of the model was stable, and the loss value finally stabil-
ized at around zero. Figure 8 shows the density distribution
of the original data and the generated data created using the
two generative models, C-WGAN-GP and DDPM. The black
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
Figure 9. Results from five data filling methods on the NFSF sensor dataset.
line represents the distribution curve of the original data, and
the red line indicates the generated distribution curves. It can
be observed that the data generated by the DDPM meets the
requirements of the original data distribution, while the density
distribution for C-WGAN-GP deviates more. During the train-
ing process, it was found that C-WGAN-GP was more difficult
to train, and there were many occurrences of pattern crashes.
In contrast, DDPM training was relatively stable, and its gen-
erated data density profile was closer to the original data than
C-WGAN-GP. Hence, the performance of DDPM in terms of
filling missing data was superior to that of the GAN.
Finally, a sampler was used to select new samples from the
generated data and fill them into the corresponding missing
regions. Figure 9 shows the data distributions for the five meth-
ods of filling missing data. The blue points represent the ori-
ginal data points, and the red show the filled data points. It can
be seen that the data generated by our IF-DDPM method match
the original data distribution most closely. A comparison with
C-WGAN-GP shows that the generated data is similar to the
original data. In the process of training the model, C-WGAN-
GP undergoes failure of adversarial training caused by the dif-
ficulty of convergence of the training loss, while IF-DDPM
does not experience this situation. The data generated by the
KNN and RF methods are too dense, and do not match the
characteristics of the real industrial data. In contrast, the data
generated using SMOTE and GAN are relatively uniform, and
do not satisfy the condition on the normal runtime data distri-
bution of a Gaussian distribution. Table 5 shows the similarity
results of two data under different methods using KL diver-
gence measurement. The data filled in using the IF-DDPM
method has the highest similarity with the original data, while
the two types of data under the RF method have the lowest sim-
ilarity. Hence, the results of the proposed IF-DDPM method of
Table 5. KL divergence table between filled data and raw data
under different methods.
Number of missing regions Ours GAN KNN
RF
SMOTE
Two missing regions
Three missing regions
Four missing regions
0.5861 0.6059 0.6056 0.6314 0.6009
0.5730 0.6326 0.6041 0.6350 0.6035
0.5749 0.6059 0.6059 0.6265 0.5904
Table 6. Errors for the soft-sensor model when applied to different
training sets.
Error
Unfilled
Ours
GAN
KNN
RF
SMOTE
MSE
44.01
6.68
11.88
10.32
15.98
8.66
missing data filling are more consistent with the original data
distribution than the other methods.
The unfilled dataset and the complete datasets obtained
from the five filling methods were used to build a soft meas-
urement model. The MSE for the soft measurement model was
tested using 200 sets of data. Table 6 shows the MAE results
for the soft measurement model in these six different scen-
arios. It can clearly be observed that the unfilled soft measure-
ment model has the largest error. The soft measurement model
prediction error is relatively small on the training set with com-
plete filling, and the IF-DDPM method gives a smaller error
compared to the other four methods.
Figure 10 shows histograms of the MAE error for the
soft measurement model when applied to the six datasets.
Figure 11 shows box plots of the MAE error in the soft meas-
urement predictions for the unfilled data versus the datasets
obtained with the five filling methods. From figures 10 and 11,
it can be seen that the prediction error is largest for the unfilled
10
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
6. Conclusion
In this paper, an IF-DDPM missing data generation and filling
method has been proposed with the aim of improving the
accuracy of a soft measurement model. First, an improved isol-
ated forest algorithm was used to detect missing regions of data
to obtain information on the missing regions. Next, DDPM
was used to generate data that conformed to the original distri-
bution, and the missing data were filled using a sampler. In the
IF-DDPM framework, the sampled points at both ends of the
missing regions were filtered based on anomaly scores, mean-
ing that the locations of the missing regions in the dataset could
be obtained in addition to the amounts of missing data. By
setting the data screening threshold and weight sampler, new
samples could be generated by DDPM that met the require-
ments and could be filled into the missing regions. Finally, the
high level of accuracy, reasonableness and distribution simil-
arity of the filled data were verified. The effectiveness of the
proposed method was verified using a numerical simulation
and a real industrial sensor dataset, as well as by considering
multiple sets of missing data. The simulation results showed
that the proposed IF-DDPM method could be used to fill miss-
ing sensor data for industrial processes. It was also shown
to effectively improve the prediction performance of the soft
measurement model for industrial processes.
Due to the significant difference between DDPM generat-
ing temporal data and image data, in future work, we will con-
sider capturing the spatiotemporal correlation of feature data
during the generation process to further improve the accur-
acy of filling data. At the same time, having a complete and
high-quality dataset is the foundation for ensuring the accur-
acy of data-driven models. Therefore, the IF-DDPM frame-
work proposed in this article is also suitable for other down-
stream applications, such as fault detection and classification.
Data availability statements
The data cannot be made publicly available upon publication
because they are owned by a third party and the terms of use
prevent public distribution. The data that support the findings
of this study are available upon reasonable request from the
authors.
Acknowledgments
The work was supported by the National Natural Science
Foundation of China (Grant No. 62263020), National Key
R&D Program of China (Grant No. 2020YFB1713600),
Outstanding Youth Fund of Gansu Province (Grant No.
20JR10RA202), Lanzhou Science and Technology Project
(Grant No. 2022-2-69) and Hongliu Outstanding Young
Talents Support Project of Lanzhou University of Technology.
Figure 10. Histograms of prediction error for the soft-sensing
model with various filling methods.
Figure 11. Box plots showing the errors in the soft-sensor model
predictions for unfilled data and the complete datasets obtained with
five filling methods.
dataset, and is relatively small on the filled complete data-
sets. The error for the IF-DDPM method is smaller than for
the other four methods. We can conclude from this validation
process on the flash furnace sensor dataset that the proposed
IF-DDPM data filling method is superior to the other filling
methods.
Conflict of interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
11
Meas. Sci. Technol. 35 (2024) 025117
D Jiang et al
ORCID iDs
Dongnian Jiang https://orcid.org/0009-0003-7891-6297
Fuyuan Shen https://orcid.org/0009-0003-2014-9128
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10.1016_j.enpol.2022.113313.pdf
|
Data availability
The data that has been used is confidential.
|
Data availability The data that has been used is confidential.
|
What causes energy and transport poverty in Ireland? Analysing
demographic, economic, and social dynamics, and policy implications
Lowans, C., Foley, A., Furszyfer Del Rio, D., Caulfield, B., Sovacool, B. K., Griffiths, S., & Rooney, D. (2023).
What causes energy and transport poverty in Ireland? Analysing demographic, economic, and social dynamics,
and policy implications. Energy Policy , 172, Article 113313. https://doi.org/10.1016/j.enpol.2022.113313
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Download date:03. Aug. 2024
Contents lists available at ScienceDirect
Energy Policy
journal homepage: www.elsevier.com/locate/enpol
What causes energy and transport poverty in Ireland? Analysing
demographic, economic, and social dynamics, and policy implications
Christopher Lowans a, *, Aoife Foley a, b, Dylan Furszyfer Del Rio a, c, Brian Caulfield b, Benjamin
K. Sovacool c, g,i, Steven Griffiths e, David Rooney h
a School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Belfast, United Kingdom
b Department of Civil, Structural, and Environmental Engineering, Trinity College Dublin, The University of Dublin, Dublin, 2, Ireland
c Science Policy Research Unit, University of Sussex, Brighton, United Kingdom
e Khalifa University of Science and Technology, Abu Dhabi, United Arab Emirates
g Department of Business Technology and Development, Aarhus University, Denmark
h School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Belfast, United Kingdom
i Earth and Environment, Boston University, United States
A R T I C L E I N F O
A B S T R A C T
Keywords:
Energy poverty
Transport poverty
Covid-19
Ireland
Nationally representative survey
Energy and transport poverty have been postulated as conditions linked by overlapping causal factors such as
structural economic inequality or housing stock and affecting overlapping demographics such as family size or
income. The strength of the overlap of these conditions and their causal mechanisms has not been assessed across
Ireland prior to this study. We apply and analyse existing and novel energy and transport poverty metrics in a
survey of 1564 participants across Ireland and consider results from expenditure and consensual data examining
causal mechanisms and correlations. We find that energy and transport poverty rates are broadly similar across
Ireland at approximately 14% for energy poverty and 18% for transport poverty using the half-median metric,
while participant knowledge of causal factors, such as lack of domestic energy efficiency and perceived desir-
ability of potential poverty solutions, such as increased public transport provision, are low. Furthermore, we find
that self-reported data concerning energy and transport expenditures and preferences do not correspond to ex-
pected outcomes. We thus conclude that ever refined targeting of individuals and households for support
measures is not optimal for either decarbonisation or alleviation of energy and transport poverty conditions and
suggest some salient policy implications.
1. Introduction
Energy and transport poverty can co-occur and reinforce each other
leading to a “double energy vulnerability”. Historically, energy and
transport poverty were treated as different problems with their own
causes and consequences (Simcock et al., 2021). Recently, however, it
has been postulated that these conditions are not distinct and have
overlapping causes and links (Mattioli et al., 2017). One of the key
characteristics of this double energy vulnerability is that it could force
individuals or groups of individuals to choose which service to prioritise,
for example, choosing between heating the home or paying for school
transport (Sovacool and Furszyfer Del Rio, 2022).
This dichotomic issue ought to take more policy relevance. It has
been shown that as many as 6% of neighbourhoods, or 3 million people
in England, are at risk of “double energy vulnerability” clustered in
isolated rural areas, due to a lack of both energy and transport infra-
structure (Robinson and Mattioli, 2020). The current energy crisis is
expected to place enormous pressure on households and public services
during the winter of 2022 (Bolton and Stewart, 2022) (Torjesen, 2022).
We position our research focused on the extent of energy and transport
poverty and their causal mechanisms with a view to uncovering routes
to their alleviation across the Island of Ireland.
The literature has defined fuel (or energy) poverty as the inability to
secure materially and socially-necessitated energy services, such as
heating a home or using appliances (Bouzarovski and Petrova, 2015).
This lack of energy provision results in a range of physical health, mental
* Corresponding author. School of Mechanical and Aerospace Engineering, Queen’s University Belfast, Ashby Building, Stranmillis Road, Belfast, BT9 5AH, United
Kingdom.
E-mail address: [email protected] (C. Lowans).
https://doi.org/10.1016/j.enpol.2022.113313
Received 21 April 2022; Received in revised form 3 October 2022; Accepted 20 October 2022
EnergyPolicy172(2023)113313Availableonline4November20220301-4215/©2022TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).C. Lowans et al.
health and social impacts, including increased risk of circulatory and
respiratory disease, increased social isolation and thousands of excess
winter deaths annually (National Audit Office, 2003) (Rudge and Gil-
christ, 2005) (Marmot Review Team and Friends of the Earth, 2011).
Those most vulnerable to energy poverty are those least able to adapt to
it, i.e., low-income households with children, the elderly, and the
disabled, or those whose pre-existing health vulnerabilities are most
acutely exacerbated (Bednar and Reames, 2020). Assessing energy
poverty typically takes the form of either expenditure measures, where
energy expenses are measured against a certain threshold, or via
consensual measures, which assess the subjective lived experiences of
households to determine poverty. Concerning expenditure measures,
either modelled expenditure or need to spend (to maintain a certain heating
regime) is used, such as with the widespread 10% metric (where
household expenditure on energy exceeds 10% of their income after
deductions), or actual spend, such as in the half-median metric (where
household expenditure on energy is less than half the sample median)
(Thomson et al., 2017), can be used.
Transport poverty, meanwhile, deals with the lack of mobility ser-
vices necessary for participation in society, resulting from the inacces-
sibility, unaffordability or unavailability of transport (Lucas et al., 2016;
Mattioli et al., 2017; Mullen and Marsden, 2016). Depending upon the
definition, up to 90% of households may be affected by transport
poverty (Lucas et al., 2016). The consequences of transport poverty are
no less severe than those of energy poverty, given the increased likeli-
hood of low-income and marginalised groups being exposed to
transportation-related air pollution, violence, sexual harassment and
crime (Furszyfer Del Rio and Sovacool, 2023). Other effects include
restricted access to employment and the increased difficulties posed to
the disabled (Lucas et al., 2016). Transport poverty metrics focus on one
of the aspects of inaccessibility, unaffordability or unavailability of
transport that were outlined by Lucas et al. (2016). However, as there is
no standard definition of transport poverty, the metrics applied are not
(yet) as sophisticated as those in the energy poverty domain.
Assessments of causal mechanisms and options for the alleviation of
energy poverty are scarce but increasing in number. For example, one
recent study has assessed the causal instruments of energy poverty in
eleven countries, while another has examined the global potential for
alleviating energy poverty with renewable energy (Rao et al., 2022)
(Zhao et al., 2022). Such assessments are also increasing in the transport
poverty literature, for example, with recent studies examining the
relationship between income and commute satisfaction in China and the
accessibility of public transport in Oslo (Shi et al., 2022) (Lunke, 2022).
Furthermore, studies are also increasingly paying attention to these is-
sues as a joint subject, for instance, recent work in Iceland concerning
the lived experience of energy and transport poverty (Upham et al.,
2022).
Assessing transport and energy poverty together, however, is not a
simple task. Research in this area has concluded that measuring these
problems poses a key challenge for researchers and impacts real-world
outcomes (Mattioli et al., 2017). Challenges for uniting their measure-
ment begin with the unit of measurement; households are energy poor
while individuals are transport poor. Furthermore, while there are
standards for household energy use, there are no standards for transport
use. To compound this issue, a problem arises as to which comes first,
data collection or metric definition, which creates the common chicken
and egg problem. Additionally, no single metric captures all aspects of
either condition, so using multiple metrics simultaneously is required for
a more complete picture. The introduction of vulnerability lenses, i.e.,
assessing who is more likely to be vulnerable to each condition, is less
technically challenging than measurement. We have argued in previous
work that this ought to be used in conjunction with energy or transport
poverty metrics (Lowans et al., 2021).
Of contextual relevance is the Covid-19 pandemic. The Covid-19
pandemic had a profound impact on energy consumption in 2020,
causing demand to contract by 5% (International Energy Agency, 2020).
Beyond consumption, the Covid-19 pandemic has been noted to have
implications for energy justice, energy poverty, and transport poverty
(Sovacool et al., 2020). However, the pandemic also has been noted to
create opportunities for sustainable responses (Griffiths et al., 2021).
Consequently, a more thorough understanding of the impacts of the
pandemic at the household level is required to assess the impacts of the
pandemic and the opportunities arising from it.
This research examines the Island of Ireland, which is comprised of 2
distinct political and legal jurisdictions, yet shares a common market for
electricity, in addition to many areas of economic interdependence
allowing for the comparison of causal factors. The Island of Ireland has a
large proportion of rural dwellers (a demographic known to be vulner-
able to energy and transport poverty), yet no assessment of energy and
transport poverty as a joint issue exists for either jurisdiction. Further-
more, assessments of energy and transport poverty are not up to date in
either jurisdiction; we, therefore, aim to be more comprehensive with
recent empirical and original data.
To fill data gaps and examine the intersections between energy and
transport poverty and the decarbonisation of the energy and transport
systems, we conducted a nationally representative survey (n = 1564)
with participants from the Island of Ireland. This work is the first cross
border study across the Island of Ireland, which examines the conditions
of energy and transport poverty simultaneously to inform future
research on decarbonising each area in a just manner which is of
particular importance given the ongoing energy price crisis.
This paper has three aims, which are presented here and examined in
Sections 4 and 5.
1. Assess and record self-reported expenditures on energy and transport
services and use these and other collected data to assess contempo-
rary energy and transport poverty on the Island of Ireland.
2. Assess the strength of the causal mechanisms of these conditions and
assess their overlap.
3. Analyse how the Covid-19 pandemic affected energy and transport
usage.
The outcomes and conclusions of this research will be useful to re-
searchers and practitioners seeking to alleviate energy and transport
poverty, individually or as a joint issue.
The article proceeds as follows. First, we begin by outlining the
context of energy and transport trends across the Island of Ireland.
Second, we discuss our research design and subsequently present our
results and discuss them. Last, we derive conclusions from our findings
and suggest some policy implications of these findings.
2. Contextualising energy and mobility trends and energy and
transport poverty in Ireland
As remarked in the introduction, insufficient provision of modern
energy and transport services contributes significantly to deprivation
across the developed world. Considering energy in Northern Ireland (NI)
first, the latest House Condition Survey showed that in 2016, 22% of
households in NI were in fuel poverty1, decreasing to 18% in 2018 due to
a reduction in fuel prices (Northern Ireland Housing Executive, 2016).
According to this official data, energy poverty data in NI is based upon
modelled expenditure, which ignores actual spending patterns, exposing
a data gap. In the Republic of Ireland, energy poverty rates are calcu-
lated using data from the EU SILC database and have also historically
been assessed using the 10% metric. During the development of in-
dicators for EU wide comparison, the Energy Poverty Observatory found
energy poverty rates in Ireland to range from 5% to 18%, depending on
the chosen indicator (Energy Poverty Advisory Hub, 2020).
1 Note that our previous research has already criticised current metrics for
being insufficient, and thus this number may be an underestimate.
EnergyPolicy172(2023)1133132C. Lowans et al.
Work is emerging in interrogating the causal mechanisms of energy
poverty and its effects on household incomes in the Republic of Ireland.
Researchers have found that (using EU SILC data) fuel poverty is
indistinct from general deprivation as defined by the National Measure
of Deprivation for Ireland finding that when aspects of fuel poverty are
included in the National Measure of Deprivation, fuel poverty and
deprivation are subsequently indistinct (Watson and Maitre, 2015).
Transport poverty remains somewhat indirectly quantified and has
not been directly examined for quite some time, but NI’s problem in this
area is considerable (General Consumer Council Northern Ireland,
2001). In NI, transport data forms the Travel Survey for Northern Ireland
(TSNI), while the equivalent in Ireland is the National Travel Survey
(NTS) (Department for Infrastructure, 2020) (Central Statistics Office,
2021). Common survey questions and outcomes include items such as
average journey length and the main mode of transport. However, none
of the data collected are used to explicitly measure transport poverty.
Indicators of energy poverty are used for high level monitoring of
energy poverty rates. However, access to support is often devolved to
sub-national governments and subject to stringent targeting criteria
such as having a very low household income, and often the incentives for
landlords to access such schemes are greatly diminished. In Northern
Ireland for example, access to the main retrofit funding scheme is a
“postcode lottery” where access is only available in areas where fuel
poverty is highest (Northern Ireland Housing Executive, 2022).
Transport poverty indicators and vulnerability lenses are typically
not used explicitly in any context but are implicitly acknowledged in
accessing transport related supports. For example, in Ireland, people
with disabilities are eligible for the Motorised Transport Grant, provided
they require a vehicle to access employment, cannot use public trans-
port, and are subjected to a means test (Citizensinformation.ie, 2022).
However, these support measures can ignore the fact that the causal
mechanism of transport poverty can often be related to the built envi-
ronment. For example the Irish National Travel Survey notes that the
greatest contributor to encouraging more cycling would be safer cycling
routes (Central Statistics Office, 2021).
In the literature concerning the alleviation of energy poverty, access
to support measures for those in energy poverty to undertake building
fabric upgrades is seen as nearly essential. Middlemiss and Gillard find
that social housing providers are the most common source of lasting
built fabric improvements, and that most respondents would not
consider debt mechanisms to improve their dwelling fabric (Middlemiss
and Gillard, 2015). It is noted that in similar research for the Scottish
Government, most participants’ awareness about the availability of
support is low, and few believe that they require help or advice and thus
would not actively seek either (Ipsos MORI Scotland and Alembic
Research Ltd., 2020).
Regarding alleviating transport poverty, many barriers relate to the
insufficient provision of or high cost of public transport or are related to
infrastructural issues. Indeed, research concludes that street connectiv-
ity, bus provision and neighbourhood safety are more significant con-
tributors to spatial variation in transport use than demographic factors
(Lucas et al., 2018). However, some barriers are more related to
perception. In Northern Ireland, for instance, fear of travelling into
unknown areas arises. Thus, not only must more transport options be
available, but they must also be considered safe by users (Crisp et al.,
2017). Overcoming transport poverty, therefore, requires changes to the
provision of public transport and should avoid exacerbating existing
inequalities. Unfortunately, subsidies for more sustainable mobility
options such as EVs, which are noted as inequitable, have been found in
Ireland (Caulfield et al., 2022).
Furthermore, means of alleviating energy poverty and transport are
often linked to climate goals in that they also present effective mecha-
nisms for emissions reduction and are frequently key components of
sectoral targets in climate change laws or plans. Northern Ireland and
the Republic of Ireland have passed net-zero emissions laws with a
deadline of 2050 for net-zero emissions and with various sub-sector
targets (Minister for Agriculture, Environment and Rural Affairs,
2022) (Oireachtas, 2021). Sufficient support for vulnerable groups will
be essential to meet climate goals. Our results also discuss their effec-
tiveness for energy and transport poverty alleviation and climate change
mitigation.
3. Research design
Our survey instrument combines existing energy and transport
poverty measurements as adapted from the EU Energy Poverty Advisory
Hub with new assessments (Energy Poverty Advisory Hub, 2020).
Moreover, it aims to determine how households think about financial
trade-offs between energy and transport and thus points the way for
prioritising current and future solutions. The survey asked questions
according to the research aims of the project as well as to glean infor-
mation beyond the data represented in current statistics (e.g., how
households trade-off between energy/transport services and other es-
sentials). The key objectives of this household survey are the following:
1. Record self-reported expenditure on energy and transport services
and consequently assess energy and transport poverty in the same
data set.
2. Assess the strength of the causal mechanisms and measure the rela-
tionship between energy and transport poverty.
3. Provide an analysis on the effects of the Covid-19 pandemic on re-
spondents’ energy and transport use.
Once the survey data was processed, it was applied to previously
unused energy and transport poverty metrics. These metrics, subjective
experiences and the overlap of these conditions are the key knowledge
gaps we seek to fill. Furthermore, although some data being collected
already exists for Ireland, collecting it again in tandem with data from
Northern Ireland allows for an accurate cross-jurisdiction comparison
across the Island.
3.1. Expenditure metrics of energy and transport poverty
Expenditure metrics can be subcategorised according to the expen-
diture used: either modelled or actual spending. The collection of data
regarding actual energy and transport expenditure is advantageous as
fuel poverty figures in NI are based upon modelled expenditure (i.e.,
what a household needs to spend, according to a household energy
model to maintain a certain heating regime) ignoring actual spending
patterns, which are the means of measurement in Ireland. The main
drawback of actual expenditure is that it makes it difficult to assess
whether a certain level of energy expenditure indicates financial cir-
cumstances or deliberate choice of the household (Lowans et al., 2021).
Hence, we have also collected data for consensual measures. The mea-
sures applied, drawing from the EU Energy Poverty Observatory and
other sources, are as follows.
• 2Mexp: a household (energy) or individual (transport) can be
considered energy/transport poor if expenditure on energy/trans-
port exceeds twice the sample median. For energy, this metric may
capture households that are energy inefficient and spend an exces-
sive amount. However, it may also or instead capture the richest
individuals who have the most to spend and may not therefore be
limited to its ability to measure energy poverty (Energy Poverty
Advisory Hub, 2020). We use this metric for two additional reasons.
First, it comprises half of Mattioli’s “Car related economic stress”
metric in transport (Mattioli et al., 2016). Second, due to data limi-
tations in our survey, we were not able to collect household income
data, only individual respondent income.
• M/2: a household (energy) or individual (transport) is energy/
transport poor if its absolute energy/transport expenditure (in
financial terms) is below half the national median or abnormally low.
EnergyPolicy172(2023)1133133C. Lowans et al.
This could be due to high energy efficiency standards but may also be
indicative of households that are dangerously under-consuming
energy.2
3.2. Consensual measures of energy and transport poverty
In addition to expenditure metrics, we used consensual metrics of
energy and transport poverty as follows.
• Arrears on bills: households that report falling into arrears on their
energy (or transport) bills once or more during the past 12 months.
This metric is useful for uncovering households that self-report
financial difficulties in paying for energy or transport, which may
not be revealed by the M/2 indicator (Energy Poverty Advisory Hub,
2020).
• Inability to keep warm: households that self-report the inability to
keep their home adequately warm when needed are considered en-
ergy poor under this metric. This can uncover either financial
hardship caused by energy bills or the effects of buildings in poor
condition (Energy Poverty Advisory Hub, 2020).
• Essentiality of car ownership: an individual is transport poor if they
consider a car essential to meet their needs. This borrows from
Mattioli’s “Forced Car Ownership” metric, but makes this a consen-
sual measure rather than a financial one (Mattioli, 2017).
• Adequacy of public transport: as a compliment to the essentiality of car
ownership, individuals can be considered transport poor if they do
not believe public transport in their area is sufficient to meet their
needs. This borrows from research by the Social Exclusion Unit
identifying the availability and accessibility of private and public
transport to be a barrier to social inclusion (Social Exclusion Unit,
2003).
3.3. The survey instrument
The main aim of designing the survey was to achieve empirical
novelty rather than aiming for conceptual or methodological novelty,
which is becoming an established practice in the social sciences (Sova-
cool et al., 2021). As with this prior referenced work, the survey
designed and conducted here had no theoretical framework. The aims of
the overall project require quantitative data as a deliverable. We did not
wish to retrofit our hypotheses to fit the collected data (Sovacool et al.,
2021).
The questionnaire was designed to take 15–20 min to complete and
consisted of 39 questions. The first section assessed the demographics of
the respondents. The second section assessed respondents’ attitudes and
behaviours regarding domestic energy use. The third section asked re-
spondents questions regarding their attitudes and behaviours regarding
transport energy use. A mix of answer types were used, ranging from
allowing respondents to input numerical values to ranking categorical
values. Lastly, some questions were open ended (e.g., allowing re-
spondents to describe how they cope with and manage their energy
expenditures.) The survey was implemented online by the market
research company Dynata, which used a representative respondent
panel. Dynata scripted the survey using their software, which the
research team checked before being sent to respondents. These re-
spondents agreed to participate in Dynata’s respondent panels in return
for incentives from Dynata: the researchers had no contact with the
respondents and were not involved in providing incentives. All re-
spondents were at least 18 and resident in one of the study jurisdictions.
A standard data assessment procedure of inspection for incorrect or
inconsistent data, cleaning for removal of anomalies, visual inspection
2 Note that we have not used metrics which include a relative threshold for
income as we have been unable to collect household incomes, as mentioned
above.
and verification, and a recording of the changes made to the stored data
was followed. A total of 328 respondents were removed based on quality
checks. These quality checks included “flat-liners,” i.e., where re-
spondents gave straight-line responses on blocks of questions; those who
gave incomplete, contradictory, or unrealistic responses; and re-
spondents who had unrealistically fast survey completion times. The
final sample comprised 1564 respondents, with 431 in Northern Ireland
and 1133 in Ireland. These provided a representative sample of each
respective jurisdiction and the Island as a whole and are illustrated in
Table 1.
3.4. Statistical testing and analysis
Our analysis has used multiple methods of testing to determine the
strength of relationships between variables. The need for multiple
methods arises from the multiple formats of data collected. The methods
we use are: linear and logistic regression, Pearson correlation coeffi-
cient, chi-square tests, point biserial correlations, Spearman’s rank
correlation, and Cramer’s V tests.
Regression analysis was carried out on the collected survey data to
determine the strength of the drivers of energy and transport poverty.
The Pearson correlation coefficient is used to determine the linear cor-
relation between data sets. The Chi-square test is used to determine
whether there is a statistically significant difference between observed
and expected outcomes in categorical variables. Point biserial correla-
tion coefficients are used when one variable is binary and the other is
continuous. It is equivalent to the Pearson correlation coefficient, which
applies to two continuous variables. Spearman’s rank correlation coef-
ficient is used to test the strength of the association between two ranked
variables, or one ranked variable and one continuous variable. Here we
have used it to measure the correlation between 2 binary variables.
Cramer’s V test is another test of association based upon the chi-squared
test, used to measure the association between nominal variables, and
may be used on variables with multiple categories.
All significance tests are conducted at the 0.05 level. Depending
upon the statistical test used, we calculate significance either as a P-
value or with a two-tailed test.
3.5. Demographics of respondents
The full demographic profile of our respondents is outlined in Table 1
below.
Table 1 shows our respondents’ demographic and socioeconomic
profiles, which were ensured to be representative for the Island of
Ireland in terms of dwelling type, dwelling tenure, personal income, and
location. However, we cannot guarantee representativeness beyond
these categories (e.g., educational attainment). The survey was
completed by respondents in November 2021, making our results very
up to date at the time of publication, albeit preceding 2022 international
energy crisis and the consequences of Russia’s invasion of Ukraine
(European Commission, 2022) (International Energy Agency, 2022).
Note that energy and transport poverty are calculated for each
jurisdiction using the median for each jurisdiction, and when displayed
together as a rate for the whole Island this is the sum of the number of
energy or transport poor for each jurisdiction as a percentage of the
sample size.
3.6. Study limitations
Overall, we identify three key potential limitations related to
surveying as a methodology, namely, the acquiescence bias in responses
(Messick, 1966) (Furr, 2011), perceived social desirability of responses
(Fisher, 1993) (Huang et al., 1998) and respondent knowledge (Mel-
chert, 2011) (van de Mortel, 2008) (Kruger and Dunning, 1999). These
will be discussed in turn.
The acquiescence bias in responses is a phenomenon exhibited by
EnergyPolicy172(2023)1133134C. Lowans et al.
Table 1
Demographic profile of respondents.
Demographics
Jurisdiction of residence
Northern Ireland
Republic of Ireland
Number of Household inhabitants
1
2
3
4
5
6
7
8
9
Age of respondent
18–24
25–39
40–49
50–59
60–74
75+
Gender of respondent
Frequency
Percent
431
1133
27.6%
72.4%
Frequency
Percent
234
444
336
329
131
60
21
7
2
15%
28.4%
21.5%
21%
8.4%
3.8%
1.3%
0.4%
0.1%
Frequency
Percent
127
548
356
242
253
38
8.1%
35%
22.8%
15.5%
16.2%
2.4%
Frequency
Percent
Male
Female
Other
Is the respondent a member of the Black, Asian or other ethnic minority community
637
922
5
40.7%
59%
0.3%
Yes
No
Area of residence
Frequency
Percent
112
1452
7.2%
92.8%
Frequency
Percent
Armagh
Belfast
Derry/Londonderry
Lisburn
Newry
Dublin
Cork
Limerick
Waterford
Galway
Large Town (18,000 inhabitants to 75,000 inhabitants)
Small/Medium Town (4500 inhabitants to 10000
inhabitants)
6
65
8
9
1
136
29
17
13
19
453
342
0.4%
4.2%
0.5%
0.6%
0.1%
8.7%
1.9%
1.1%
0.8%
1.2%
29%
21.9%
Intermediate Settlement/Village (1000 inhabitants to 4500
150
9.6%
inhabitants)
Small Village/Hamlet/Open Country (less than 1000
316
20.2%
inhabitants)
Respondent employment status
Frequency
Percent
Working full-time
Not Working
Retired
Permanently Sick/Disabled or Looking After Family/Home
Working part-time
Respondent home ownership status
845
134
195
135
255
54%
8.6%
12.5%
8.6%
16.3%
Owned by respondent
Rented
Social Housing
Owned by respondent’s family
Respondent dwelling type
Bungalow
Terraced House
Semi-Detached House
Detached House
Frequency
Percent
926
391
79
168
59.2%
25%
5.1%
10.7%
Frequency
Percent
231
263
466
372
14.8%
16.8%
29.8%
23.8%
Table 1 (continued )
Demographics
Flat/Apartment
Caravan
Other
Respondent monthly income
Northern Ireland [GBP]
0–1000
1001–2000
2001–3000
3001–4000
4001+
Republic of Ireland [EUR]
0–1000
1001–2000
2001–3000
3001–4000
4001+
215
2
15
13.7%
0.1%
1%
Frequency
Percent
93
163
107
42
26
Frequency
175
291
354
204
109
21.6%
37.8%
24.8%
9.7%
6%
Percent
15.4%
25.7%
31.2%
18%
9.6%
respondents whereby they have a tendency to give positive answers to
questions, regardless of the content of the question, and do not consider
their “true” response (Messick, 1966). Furthermore, a related known
contributor to this trend occurs when there is an unbalance of positively
or negatively described items in a survey, i.e., a string of positively
described items exacerbates tendencies to respond affirmatively (Furr,
2011).
The perceived social desirability of responses presents an issue such
that respondents may edit their responses in order to be perceived in a
more favourable light (Fisher, 1993). This presents an issue in that the
reported responses do not reflect respondents’ true behaviour. Indeed
the respondents may over or under-report so that their answers could be
seen as more moderate than their true response (Huang et al., 1998).
Respondent knowledge presents issues in several ways. Firstly, re-
spondents unaware of their emotions may not fully understand or be
aware of their behaviours or tendencies and so may give misleading
answers (Melchert, 2011). Secondly, some respondents may prefer to
present a façade rather than be truthful in their responses (note this is
related to but not the same as responding in a way the respondent sus-
pects would be perceived as more socially desirable) (van de Mortel,
2008). Thirdly, a widely recognised issue is that people hold overly
favourable views of their capabilities in many intellectual and social
domains, that do not reflect their true capabilities (Kruger and Dunning,
1999).
Additionally, the length of the survey instrument may have
contributed to some fatigue in responses. This risk was deemed
acceptable when considering the survey aim of understanding energy
and transport poverty among the same respondent panel.
Our survey relies entirely on self-reported data, which can present
limitations e.g., some respondents may not accurately recall informa-
tion. Moreover, we expected collecting household income data to be
highly inaccurate in rented properties, and to have posed ethical ques-
tions for respondents who may be unwilling or unable to disclose in-
formation regarding others in their household who in turn might not
consent. Relatedly, some weakness exists in our methodology in that we
have had to minimise the questions asked, and thus the data collected, to
measure both energy and transport poverty while not exhausting re-
spondents. This has come at the expense of slightly imperfect methods
for each of energy and transport poverty measurement. Ideally, we
would find household incomes in addition to individual incomes so that
we could also run analysis using relative as well as absolute thresholds
for our expenditure metrics of energy and transport poverty. In these
circumstances, we and others argue that the main drawback of actual
expenditure is that it makes it difficult to assess whether a certain level
of energy or transport expenditure indicates financial circumstances or
deliberate choice of the household. However, we believe the merits of
collecting the data to examine the overlap of these conditions outweigh
EnergyPolicy172(2023)1133135C. Lowans et al.
the drawbacks of limiting our data collection in each sub-area.
When calculating expenditure rates of energy and transport poverty,
it was our intention during data collection to account for the support
measures that individuals receive. However, when examining these
collected data, many erroneous entries were noted e.g., where re-
spondents claimed to receive more in supports than is possible. Thus, we
omitted all supports data from energy (and transport) poverty expen-
diture metric calculations. This has unavoidably affected the calculation
of energy and transport poverty rates and possibly also the correlation
analysis, but we are unable to say by how much in either case.
Finally, due to constraints on the length of this paper, we cannot
deeply analyse all 39 questions in this paper or present the entirety of
the results. Thus, the results which are most relevant to the research
aims are presented alongside the most pertinent analysis.
4. Results & analysis
This section presents our results and analysis, beginning with the
energy and transport questions, and lastly, their overlap as outlined by
our paper aims in Section 3. Note that the abbreviations NI and ROI
should be taken to mean Northern Ireland and Republic of Ireland
respectively.
4.1. Patterns of energy use and expenditures
In this section, we discuss the results and analysis pertinent to re-
spondents’ domestic energy use. This pertains to all three of our paper
aims: to uncover the extent of energy poverty, assess the causal mech-
anisms, and examine the effects of the Covid-19 pandemic on energy
usage Ireland wide.
Firstly, Fig. 1 and Table 2 below show respondents’ monthly energy
bills. The distribution of self-reported energy expenses is displayed in
Fig. 1, whilst the median and mean of self-reported energy expenses are
listed in Table 2. As can be seen, the median and mean energy bill rose
across this time by 14% in NI, 10% in ROI and 17% in NI and 16% in
ROI, respectively.
In Tables 3 and 4 we show responses to questions concerning thermal
comfort and dwelling issues. The first questions deal with consensual
Table 2
Median energy expenditures in each jurisdiction.
Median
Monthly
energy Bill
before
pandemic
Median
monthly
energy bill
during the
pandemic
Mean
Monthly
energy Bill
before
pandemic
Mean
monthly
energy bill
during the
pandemic
150
171
181
212
138
151
177
206
Jurisdiction
Northern
Ireland
[GBP]
Republic of
Ireland
[EUR]
Table 3
Respondents’ ability to keep their household comfortably warm when needed.
Q13. Can your household keep your home comfortably warm when needed?
Frequency
Percentage
Island wide
Northern Ireland
Republic of Ireland
Yes
No
Yes
No
Yes
No
1340
224
381
50
959
174
85%
14%
88%
12%
85%
15%
Table 4
Showing the percentage of respondents per jurisdiction reporting issues with
their dwelling.
Do you have any of the following problems with your dwelling/accommodation?
Percentage of respondents
Island
wide
Northern
Ireland
Republic of
Ireland
A leaking roof
Damp walls/floors/foundation
Rot in window frames or floor
Other structural problem(s)
I have none of these problems
with my dwelling
7%
17%
8%
8%
71%
5%
13%
5%
4%
80%
7%
18%
8%
10%
68%
Fig. 1. Respondent’s energy bills in each jurisdiction, prior to and during the Covid-19 pandemic. Panel A) Northern Ireland, Panel B) Republic of Ireland.
EnergyPolicy172(2023)1133136C. Lowans et al.
measures of energy poverty that concern respondents’ self-reported
ability to keep their home comfortably warm and the presence of the
problems with their dwelling that are known to relate to energy poverty.
Under the consensual measure of ability to keep their home comfortably
warm, as shown in Table 3, some 14% of respondents can be considered
energy poor, whilst 17% of respondents report at least one problem with
their dwelling, as shown in Table 4.
In Table 5 we correlate answers displayed in Table 3 with expendi-
ture metrics of energy poverty. Chi-squared tests and Spearman’s ρ tests
show weak, statistically insignificant correlations between these
metrics.
As stated in section 3.2, a key consensual measure of energy poverty
is the presence of arrears on household energy bills (Energy Poverty
Advisory Hub, 2020). As shown in Table 6 below, when asked “In the
past 12 months, have you been unable to pay for such a heating or
electricity bill on time due to financial difficulties?” 10% of respondents
reported falling into arrears at least once, and 11% reported falling into
arrears twice or more; thus, the rate of falling into arrears is 21%.
Table 7 below shows the correlations between the results from
Table 6 and financial metrics of energy poverty. Chi-squared tests and
Cramer’s V tests show weak, statistically insignificant correlations be-
tween the falling into arrears on an energy bill and financial metrics of
energy poverty, despite the design aim of this metric; to capture
households experiencing difficulties paying for energy due to financial
circumstance.
The results displayed in Table 8 below pertain to respondents’
heating fuels. This question provides important context as to which fuels
the energy poor are consuming, and the ease with which the switch to
low carbon options might occur. Regarding heating fuels, when asked
what the primary heating fuel of their dwelling is, 38% of respondents
reported oil, 33% reported gas, 20% reported electricity, with the
remainder made up of coal, peat, other, and “don’t know".
Table 9 shows that chi-squared tests show strong associations be-
tween primary heating fuel and financial metrics of energy poverty;
however, these are statistically insignificant except for the 2Mexp metric
for energy expenditure before Covid-19, which is significant; that is that
over-expenditure on energy is correlated significantly to heating fuel.
Cramer’s V test on the same 2Mexp metric shows a weak but significant
correlation. All other associations found by Cramer’s V tests are weak
and not statistically significant.
Table 10 shows a list of energy technologies and whether re-
spondents have these installed. When asked which energy related
technologies they have installed at home, or plan to install within the
next 12 months, except for low energy lightbulbs, most respondents did
not have, nor planned to install solar PV, smart meters, smart appliances,
EV chargepoints, or “other”. These results suggest that either re-
spondents have limited knowledge that alternative technologies could
lower their energy bills, or that they are aware of these opportunities yet
have no plans to install such technologies regardless of the potential
savings.
As shown in Table 11 below, when asked to identify which option
would make the greatest difference in meeting their domestic energy
needs, 46% of respondents identified lower costs, 26% identified more
income, and 25% identified more efficient home and appliances i.e.,
even though more efficient homes and appliances would decrease
Table 5
Correlations between self-reported ability to keep homes warm and financial
metrics of energy poverty.
Can you keep your household comfortably warm when needed?
Chi Squared
Spearman’s Rho
P value
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
0.066
0.116
0.275
0.036
0.007
0.09
0.013
(cid:0) 0.05
0.797
0.734
0.6
0.849
Table 6
Respondents’ reporting difficulty paying energy bills.
In the past 12 months, have you been unable to pay for such a heating or electricity bill
on time due to financial difficulties?
Percentage of
respondents
Yes, once
Yes, twice or more
No
Island wide
Northern
Ireland
10%
11%
79%
8%
6%
85%
Republic of Ireland
10%
13%
77%
Table 7
Correlations between the inability to pay for an energy bill and financial metrics
of energy poverty.
Correlations between the inability to pay for an energy bill in the past 12 months and
financial metrics of energy poverty
Chi Squared
Cramer’s V
P value
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
1.324
0.175
1.71
2.195
0.029
0.011
0.033
0.037
0.516
0.916
0.425
0.334
Table 8
Respondents’ primary heating fuel.
Primary heating fuel
Percentage of
respondents
Oil
Gas
Electricity
Coal
Peat
Other
Don’t know
Island wide
Northern
Ireland
Republic of Ireland
38%
33%
20%
3%
4%
2%
1%
52%
35%
10%
2%
1%
2%
33%
33%
32%
24%
3%
5%
2%
1%
Table 9
Correlations between primary heating fuel and financial metrics of energy
poverty.
Correlations between primary heating fuel and financial metrics of energy poverty
Chi Squared
Cramer’s V
P value
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
9.217
6.399
12.369
12.082
0.077
0.064
0.09
0.088
0.162
0.38
0.049
0.06
energy bills, lower unit costs are the more popular means of improving
the ability of respondents to meet their needs.
In Fig. 2 below, we depict the rates of energy poverty calculated from
our data, as per our first paper’s aim. Calculated rates of energy poverty
show that under the M/2 metric, both jurisdictions have approximately
the same rates of energy poverty. This suggests that the proportion of the
population that under-consumes energy is roughly the same in each
jurisdiction and has remained approximately constant from before the
Covid-19 pandemic to during the pandemic. However, under the 2Mexp
metric, over-expenditure on energy is roughly 4% higher in ROI than in
NI.
Table 12 below shows the correlations between income and financial
metrics of energy poverty and illustrates that point biserial correlations
uncover no associations between self-reported monthly post-tax income
and financial metrics of energy poverty. Furthermore, a binomial
regression model using all demographic factors together as predictors
could not predict instances of energy poverty. The coefficient results of
this model are not presented here to conserve space.
EnergyPolicy172(2023)1133137C. Lowans et al.
Table 10
Technologies installed, or planned to be installed at respondents’ dwellings.
Technologies installed, or planned to be installed at respondents’ dwellings
Percentage of responses
Solar
panels
This is installed at my home
This will be installed at my home within the next 12
9%
6%
months
I have no plans to install this
85%
Smart
meter
20%
18%
63%
Smart appliances
(networked)
Electric vehicle
chargepoint
Low energy
lightbulbs
16%
15%
69%
4%
8%
88%
71%
11%
18%
Other
8%
6%
73%
Table 11
Respondents’ perception of item which would make greatest difference to
meeting household energy needs.
Respondents’ perception of item which would make greatest difference to meeting
household energy needs
More income
A more efficient home and
appliances
Island
wide
26%
25%
Lower heating and electricity
46%
costs
Other
3%
Northern
Ireland
Republic of
Ireland
25%
23%
48%
4%
27%
26%
45%
3%
4.2. Patterns of transport use and expenditures
In this section we will discuss the results and analysis pertinent to
respondents’ transport energy use. This pertains to all three of our paper
aims: to uncover the extent of transport poverty, assess the causal
mechanisms, and examine the effects of the Covid-19 pandemic on
transport usage Ireland wide.
Firstly, Fig. 3 and Table 13 below show respondents’ monthly
transport bills. The distribution of self-reported transport expenses
(comprising expenses on cars, public transport, and taxis) is displayed in
Fig. 3, whilst the median and mean of self-reported transport expenses
are listed in Table 13. As can be seen, the median and mean transport bill
fell across this time by 22% in NI, 27% in ROI and 17% in both NI and
ROI respectively.
In Tables 14 and 15 we show the results of our questions pertaining
to perceptions of different transport modes. We see in Table 14 that
across the Island, over 90% of responses say that owning a motor vehicle
is essential to fully participate in society. However this contrasts with
the results in Table 15, showing that a combined 48% of respondents
said that public transport in their area is sufficient to meet most or all of
their needs.
Table 16 shows the correlations between the perception based re-
sponses in Table 14 with financial metrics of transport poverty. Chi-
squared tests and Spearman’s ρ tests show no statistically significant
correlations between the belief in the necessity of vehicle ownership and
financial metrics of transport poverty.
Table 17 shows the correlations between the perception based re-
sponses in Table 15 with financial metrics of transport poverty. Chi-
squared tests and Cramer’s V tests show no statistically significant cor-
relations between the belief in the sufficiency of public transport and
financial metrics of transport poverty, except for a very weak correlation
Table 12
Statistical associations between monthly post-tax income and financial metrics
of energy poverty.
Statistical associations between monthly post-tax income and financial metrics of
energy poverty
Point biserial correlation
Significance test
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
0.008
0.031
(cid:0) 0.004
(cid:0) 0.005
0.752
0.225
0.877
0.848
Fig. 2. Calculated rates of energy poverty across the Island of Ireland using the M/2 and 2Mexp metrics. Note NI = Northern Ireland. ROI = Republic of Ireland.
Island = both.
EnergyPolicy172(2023)1133138C. Lowans et al.
Fig. 3. Respondent’s transport bills in each jurisdiction, prior to and during the Covid-19 pandemic. Panel A) Northern Ireland, Panel B) Republic of Ireland.
Table 13
Median and mean transport bills before and during the Covid-19 pandemic in NI
and ROI.
Table 16
Statistical associations between belief in the necessity of owning a vehicle and
financial metrics of transport poverty.
Jurisdiction Median
Monthly
transport Bill
before
pandemic
Median
monthly
transport bill
during the
pandemic
Mean
Monthly
transport Bill
before
pandemic
Mean
monthly
transport bill
during the
pandemic
NI [GBP]
ROI [EUR]
231
237
174
173
298
318
248
265
Statistical associations between belief in the necessity of owning a vehicle and
financial metrics of transport poverty
Chi Squared
Spearman’s Rho
P Value
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
0.587
0.803
0.85
1.669
(cid:0) 0.018
(cid:0) 0.017
(cid:0) 0.020
(cid:0) 0.029
0.498
0.520
0.435
0.268
Table 14
Respondents’ perception of the essentiality of owning their motor vehicle.
Respondents’ perception of the essentiality of owning their motor vehicle
Island wide
Northern Ireland
Republic of Ireland
Essential
Not-essential
N/A
85%
7%
3%
85%
5%
4%
85%
7%
3%
Table 15
Respondents’ perception of public transport sufficiency.
Respondents’ perception of the sufficiency of public transport in their area
Sufficient to meet most
transport needs
Island
wide
37%
Sufficient to meet all transport
11%
needs
Not sufficient
52%
Northern
Ireland
Republic of
Ireland
43%
11%
46%
35%
11%
55%
with the M/2 metric during Covid-19 as shown in Table 17.
The results of asking respondents what would aid in meeting their
transport needs are shown in Table 18. Namely, 28% say they require
more income, 30% say they require lower fuel costs whilst only 16%
would like greater public transport provision. The remaining 25% is
Table 17
Statistical associations between belief in public transport sufficiency and
financial metrics of transport poverty.
Statistical associations between belief in public transport sufficiency and financial
metrics of transport poverty
Chi Squared
Cramer’s V
P Value
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
4.315
6.782
9.019
1.966
0.053
0.066
0.076
0.035
0.116
0.034
0.011
0.374
divided across increased vehicle efficiency, greater access to vehicles, an
increased ability to work from home, increased provision of EV public
charging and “other".
Regarding the inability to pay for personal car use and public
transport, the responses for car and public transport have moved in the
same direction following the Covid-19 pandemic, as shown in Table 19.
Before the pandemic, 13% of respondents report an inability to pay for
their car expenses at least once, but this falls to 10% during the
pandemic, perhaps due to decreasing volumes of travel. As for public
transport, 8% report an inability to pay for public transport at least once
prior to the pandemic, falling to 7% during the pandemic.
Chi-squared tests and Cramer’s V tests, as shown in Tables 20 and 21,
show no statistically significant correlations between respondents’
inability to pay for any of their means of transport before and during the
EnergyPolicy172(2023)1133139C. Lowans et al.
Table 18
Respondents’ perception of item which would make greatest difference to
meeting transport needs.
Table 21
Statistical associations between inability to pay for transport and financial
metrics of transport poverty, during the Covid-19 19 pandemic.
Respondents’ perception of item which would make greatest difference to meeting
transport needs
Statistical associations between inability to pay for transport and financial metrics of
transport poverty, during the Covid-19 pandemic
Northern
Ireland
Republic of
Ireland
Car
Chi Squared
Cramer’s V
P Value
More income
A more efficient vehicle
Lower petrol/diesel/electricity
costs
Island
wide
28%
9%
30%
More public transport
Owning or having access to more
16%
2%
vehicles
Ability to work from home more
More public electric vehicle
10%
2%
charging stations
Other
3%
24%
10%
35%
16%
1%
8%
2%
4%
30%
9%
29%
16%
2%
10%
2%
2%
Table 19
Respondents’ self reported inability to pay for modes of transport, before and
during the pandemic.
Respondents’ self reported inability to pay for modes of transport, before and during
the pandemic
Island
wide
Northern
Ireland
Republic of
Ireland
Personal vehicle – before the pandemic
Yes, once
Yes, twice or more
No
I have adapted my behaviour
5%
5%
84%
6%
instead of not paying
Personal vehicle – during the pandemic
Yes, once
Yes, twice or more
No
I have adapted my behaviour
7%
6%
79%
8%
instead of not paying
Public transport – before the pandemic
Yes, once
Yes, twice or more
No
I have adapted my behaviour
3%
4%
88%
5%
instead of not paying
Public transport – during the pandemic
Yes, once
Yes, twice or more
No
I have adapted my behaviour
5%
3%
86%
6%
instead of not paying
4%
4%
88%
4%
7%
3%
84%
6%
4%
2%
90%
4%
6%
2%
87%
6%
5%
6%
83%
6%
8%
7%
76%
9%
3%
4%
88%
6%
4%
4%
86%
6%
pandemic and financial metrics of transport poverty.
In Fig. 4 we show the rates of transport poverty calculated from our
data, as per our first aim. Expenditure rates of transport poverty show
Table 20
Statistical associations between inability to pay for transport and financial
metrics of transport poverty, prior to the Covid-19 19 pandemic.
Statistical associations between inability to pay for transport and financial metrics of
transport poverty, prior to the Covid-19 pandemic
Car
Chi Squared
Cramer’s V
P Value
M/2
2Mexp
6.305
2.492
Public transport
0.063
0.04
0.098
0.477
Chi Squared
Cramer’s V
P Value
M/2
2Mexp
6.347
4.803
0.064
0.055
0.096
0.187
M/2
2Mexp
1.019
2.48
Public transport
0.026
0.04
0.797
0.479
Chi Squared
Cramer’s V
P Value
M/2
2Mexp
1.776
4.496
0.034
0.054
0.620
0.213
that under the M/2 metric, both jurisdictions have approximately the
same rate of transport poverty during the pandemic, and additionally
the rate was lower prior to the pandemic. This suggests that the pro-
portion of the population that under-consumes transport is roughly the
same in each jurisdiction. As with energy, under the 2Mexp metric, over-
expenditure on transport is higher in ROI than in NI, but the difference is
small.
Table 22 illustrates that, there are no statistically significant corre-
lations between monthly post-tax income and financial metrics of
transport poverty except for the M/2 metric during the pandemic which
is significant, but the association is very weak and negative. Further-
more, as with energy poverty, a binomial regression model using all
demographic factors together as predictors could not predict instances
of transport poverty. The coefficient results are not presented here to
conserve space.
4.3. Intersection of energy and mobility poverty
In this section, we will discuss the results and analysis concerning the
overlap in respondents’ domestic energy and transport use. This pertains
to our second aim concerning the overlap of energy and transport
poverty, and to our third aim of examining the effects of the Covid-19
pandemic on these issues.
Table 23 shows there are statistically significant correlations be-
tween all measures of energy and transport poverty and that these are all
significant. This is likely explained by a strong association between
being not energy poor and not transport poor, i.e., between respondents
measuring as a 0 on each of the binary measures.
Table 24 shows statistically significant but very weak correlations
between monthly energy and monthly transport bills. That is to say that
very little of the change in energy bills is explained by a change in
transport bills, despite the significance of the finding, with the magni-
tude of this influence decreasing by a factor of ten during the Covid-19
pandemic, as would be expected with greatly reduced travel behaviour.
5. Discussion
This paper had three aims. First, we sought to record self-reported
energy and transport services expenditure and assess consensual and
financial metrics of energy and transport poverty. Second, we sought to
determine the strength of the causal mechanisms for energy and trans-
port poverty and measure the relationship between these conditions.
Third and finally, we sought to evaluate the impact of the Covid-19
pandemic on respondents’ energy and transport expenditures.
For aims 1 and 3, our results indicate that mean and median energy
expenses rose from the period preceding the pandemic to the period
during the pandemic, while mean and median transport expenses fell
over this period. We found no statistically significant associations be-
tween self-reported incomes and energy or transport bills. This is
perhaps surprising given that one might expect higher earners to spend
more on these services.
EnergyPolicy172(2023)11331310C. Lowans et al.
Fig. 4. Calculated rates of transport poverty across the Island of Ireland using the M/2 and 2Mexp metrics.
Table 22
Statistical associations between monthly post-tax income and financial metrics
of transport poverty.
Statistical associations between monthly post-tax income and financial metrics of
transport poverty
Point biserial correlation
Significance test
M/2 (pre-Covid-19)
M/2 (during Covid-19)
2Mexp (pre-Covid-19)
2Mexp (during Covid-19)
(cid:0) 0.024
(cid:0) 0.056
(cid:0) 0.009
(cid:0) 0.032
0.349
0.027
0.731
0.213
Regarding financial metrics of energy poverty and aim 1 of this
paper, the rates of energy poverty under the M/2 metric are similar
across the Island of Ireland, while overconsumption is higher in Ireland
than in Northern Ireland. This pattern holds for financial metrics of
transport poverty although the differences are smaller. Possible expla-
nations for these patterns include higher wages or higher fuel prices in
ROI, but we have not been able to determine the causal mechanism from
the data collected.
As for consensual energy poverty measures, up to 21% of re-
spondents can be considered energy poor. However, no statistical as-
sociation was uncovered between financial metrics of energy poverty
and the consensual measures. This is surprising, as we would expect to
find an association between the M/2 metric, designed to uncover under-
consumption of energy, and those who report an inability to keep their
home warm or arrears on bills. As for transport poverty, the lack of as-
sociation between arrears on bills and the M/2 metric persists.
Regarding motor vehicles, 90% of respondents considered owning a
Table 23
Statistical associations between metrics of energy and transport poverty.
Statistical associations between metrics of energy and transport poverty
motor vehicle a necessity, yet 48% of respondents stated that public
transport in their area is sufficient to meet most or all of their needs. This
result suggests that reasons for owning cars or other motor vehicles
extend beyond meeting “needs” and includes wants based on cultural
norms and perhaps negative preconceptions concerning public transport
(Mattioli et al., 2020). As with energy, there are no meaningful corre-
lations between financial and consensual measures of transport poverty.
This would suggest that under-expenditure and over-expenditure on
transport in our results are distinct from the perceptions of the factors
that indicate transport poverty.
A consequence of collecting self-reported individual incomes, which
we cannot verify, and lacking full household income responses is that we
do not have the data needed to determine if energy and transport
poverty are distinct from income poverty to corroborate or refute
research outlined in section 2 regarding energy poverty in the Republic
of Ireland (Watson and Maitre, 2015). However, determining this
distinction may not be very useful to the research or policy communities
given that our results show that with each change of indicator, there is a
change in who is identified as energy or transport poor. This concurs
with our earlier research suggesting that there is no single perfect
Table 24
Regression analysis results showing the relationship between monthly energy
bills and monthly transport bills before and during the Covid-19 19 pandemic.
Monthly transport bills
Before the pandemic
During the pandemic
Monthly energy bills
R2
0.032
0.0037
P value
0.000
0.000
Transport
Energy
M/2
Chi Sq
89.035
93.376
4.433
6.681
M/2
M/2 Covid-19
2Mexp
2Mexp Covid-19
P value
0.000
0.000
0.035
0.01
M/2 Covid-19
Chi Sq
92.862
100.216
3.895
6.488
P value
0.000
0.000
0.048
0.011
2Mexp
Chi Sq
15.036
18.209
20.128
21.007
P value
0.000
0.000
0.000
0.000
2Mexp Covid-19
Chi Sq
20.106
22.163
19.373
24.996
P value
0.000
0.000
0.000
0.000
EnergyPolicy172(2023)11331311C. Lowans et al.
indicator, nor should one be sought. Rather an appropriate set of in-
dicators should be used (Lowans et al., 2021). Given this indicator
imperfection, we suggest that more broad approaches should be adopted
for identification and alleviation of energy and transport poverty. In
policy terms this means identification of the energy poor should
continue to be devolved to local governments who know local situations
best. The Affordable Warmth Scheme in Northern Ireland is one example
of this. The same should apply for transport poverty schemes and criteria
for identifying these people should be widened beyond the current
stringent conditions.
Regarding aim 2 of this paper concerning causal mechanisms, our
analysis has not uncovered any statistically significant correlations be-
tween the demographic data and the energy and transport poverty re-
sults (under expenditure metrics), nor between demographic data and
self-reported energy and transport bills. That is to say that vulnerabil-
ities known to contribute to energy and transport poverty (such as age,
income etc.) do not, according to our results, have a statistical associa-
tion with being energy or transport poor. As for the relationship between
energy and transport poverty, we have observed statistically significant
associations between all financial energy and transport poverty metrics
as outlined in Table 23. However, we suspect that what is being iden-
tified is the link between being not energy poor and not transport poor as
most respondents gave these answers.
Concerning energy poverty alleviation, despite a desire for lower
domestic energy costs, respondents are broadly unwilling to install new
technologies to reduce these costs. Hence, perceived barriers opposing
the uptake of such technologies must be considered. A household might,
for instance, object to taking out debt to finance a solar PV installation
even though the installation would reduce the carbon footprint of the
dwelling and possibly lower per-unit energy costs. Although the net
financial impact on the household might be positive, the perceived
benefits of undertaking the installation may not outweigh perceived
costs (Middlemiss and Gillard, 2015). Therefore, for successful out-
comes, not only must energy cost burdens be lifted, but also any related
barriers must be addressed as well. Lastly, as indicated by low uptake
rates, the reliance on market signals to trigger mass retrofits is insuffi-
cient, which could be overcome by expanding the groups subject to
targeted interventions. The Republic of Ireland has a 2030 climate goal
to roll out 2.7 TWh of district heating in cities and to install 400,000 heat
pumps in existing homes (Department of the Environment Climate and
Communications, 2021). However, at the time of writing, Northern
Ireland lacks housing-specific decarbonisation targets (in the form of
estimated numbers of installations and retrofits per year) (Northern
Ireland Department for the Economy, 2021). Hence, greater efforts are
needed to match policy goals with implementation in the Republic of
Ireland.
As for transport poverty, the most prevalent responses indicated that
more income or lower fuel costs would make the most difference.
Despite widespread policy recognition that technology and modal shifts
in energy and transport, if managed correctly, would benefit consumers,
there is very little recognition of this by consumers themselves. This
finding, in conjunction with the finding in the energy poverty literature
from Middlemiss and Gillard that support schemes should actively seek
participants, suggests that there is much more work to be done yet by
governments to provide and promote sustainable mobility (Middlemiss
and Gillard, 2015). Our finding regarding the perceived need for car
ownership suggests once again that much more work is required to reach
the Irish Government’s 2030 goal of reducing the amount of “fossil
fuelled distance” by 10% (Department of the Environment Climate and
Communications, 2021). If most respondents do not believe public
transport to be sufficient to meet their needs, they are very unlikely to
forego personal vehicles for another transport mode. As with domestic
energy, Northern Ireland lacks quantified targets for transport decar-
bonisation and modal shift.
Lastly, the lack of statistical correlations between our expenditure
metrics, causal factors, and consensual metrics highlights the challenge
associated with defining energy and transport poverty and categorising
those impacted. Our results contradict findings that the drivers of energy
and transport consumption are those that are accounted for in housing
energy models and vulnerability lenses (e.g., house age or dwelling
location). That is, we have found self-reported spending on energy and
transport is distinct from expected behaviour, but we cannot determine
why this is the case. Furthermore, we have found no discernible single or
multiple root causes when examining self-reported energy and transport
poverty, nor can we explain why we cannot find these causes.
6. Conclusion and policy implications
The first policy implication of our work is that in the absence of
revamped national surveying in Northern Ireland to collect actual
expenditure alongside modelled data, the focus on modelled expendi-
ture data that is currently collected will remain necessary going forward
for monitoring energy poverty rates at the national level and should
remain in place for consistent monitoring of “need to spend” and for
assessing energy performance gap purposes in the future. The second
policy implication of our work is that we believe it necessary in both
jurisdictions for official transport poverty indicators to be adopted and
collected alongside energy poverty indicators to monitor overlaps and
characteristics at a national level.
A third policy implication is that the energy or transport poor should
be anyone identifiable by any of a series of energy or transport poverty
indicators or vulnerability lenses, as opposed to stringent targeting
criteria. As we have not been able to correlate our findings with the
outcomes we expected, we believe further refining of targeted support is
a poor policy approach. Indeed, we have noted that support schemes are
most effective when they are comprehensive and when local govern-
ments proactively reach out to the vulnerable, rather than the other way
around.
It is widely recognised that national domestic retrofit programs,
active travel schemes and improvement of public transport services are
among the measures required for meeting decarbonisation targets and at
deployment rates exceeding what is currently observed. With continued
locally devolved selection of support recipients, the more widely defined
and identified energy or transport poor can be the first to access the
support schemes or necessary infrastructure. As we have noted, many
transport poverty barriers are infrastructural which require solutions in
the built environment. Improved and sustainable public transport and
active travel schemes should thus be the focus of transport policy efforts.
Furthermore, and as noted in the discussion, debt mechanisms are the
least attractive means of support for the energy poor (and by analogy,
the same could be said of the transport poor). In many cases, re-
spondents were not inclined to acquire technologies that may ameliorate
their energy and/or transport poverty situation. Therefore, as a final
policy implication of this work, and building on other literature, support
measures should not pose a debt burden to vulnerable households and
should be large enough to enact lasting change rather than merely
lessening the financial burden of consumption of contemporary energy
and transport services.
Regarding furthering the literature, we have two recommendations.
First, we recommend similar studies are carried out in other jurisdictions
(within and beyond Europe) to explore the reported outcomes further
and to see if the reasons for the difference between actual and expected
outcomes can be determined, which is a key weakness of our study.
Second, we recommend that detailed surveys of vulnerable groups, as
identified by vulnerability lenses, are conducted to determine the rea-
sons for the difference between actual and expected outcomes, or to see
if more focused surveys contradict our findings. We are excited to see the
outcomes of such studies regardless of whether they agree or contradict
our findings.
EnergyPolicy172(2023)11331312C. Lowans et al.
CRediT authorship contribution statement
Christopher Lowans: Conceptualization, Methodology, Investiga-
tion, Data curation, Software, Formal analysis, Writing – original draft,
Writing – review & editing, Project administration. Aoife Foley:
Conceptualization, Methodology, Writing – original draft, Writing –
review & editing, Funding acquisition. Dylan Furszyfer Del Rio:
Conceptualization, Methodology, Writing – original draft, Writing –
review & editing. Brian Caulfield: Conceptualization, Methodology,
Writing – original draft, Writing – review & editing. Benjamin K.
Sovacool: Conceptualization, Methodology, Writing – original draft,
Writing – review & editing, Funding acquisition. Steven Griffiths:
Conceptualization, Methodology, Writing – original draft, Writing –
review & editing. David Rooney: Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
The data that has been used is confidential.
Acknowledgements
Mr Christopher Lowans and Dr Aoife Foley’s research is supported by
the Department for the Economy (DfE), Northern Ireland. The views and
opinions expressed in this document do not necessarily reflect those of
DfE. Dr. Dylan Furszyfer del Rio, Dr Steve Griffiths and Professor
Benjamin Sovacool gratefully acknowledge financial support from UK
Research and Innovation through the Centre for Research into Energy
Demand Solutions, grant reference number EP/R035288/1, as well as
Khalifa University of Science and Technology “High Impact Grant."
Nomenclature & Abbreviations
2Mexp A household (energy) or individual (transport) is energy/
transport poor if its absolute energy/transport expenditure (in
financial terms) is below half the national median
European Union
EU
EU SILC European Union Statistics on Income and Living Conditions
EUR
EV
GBP
M/2
Euro
Electric Vehicle
Pound Sterling
A household (energy) or individual (transport) can be
considered energy/transport poor if expenditure on energy/
transport exceeds twice the sample median
Northern Ireland
Republic of Ireland
Terawatt Hours
NI
ROI
TWh
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EnergyPolicy172(2023)11331314
| null |
10.1038_s42255-023-00774-2.pdf
|
Data availability
All data generated or analysed during this study are included in the
article and its Supplementary Information. Results of the ORFeome,
the CRISPR–Cas9 and the PRISM screens are available in Supplementary
Table 1. Data from the Cancer Cell Line Encyclopedia are available at
https://depmap.org/portal/. Source data are provided with this paper.
|
Statistics and reproducibility All data are expressed as the mean ± s.e.m., with the exception of oxygraphic data that are expressed as the mean ± s.d. All reported sample sizes (n) represent biological replicate plates or a different mouse. All attempts at replication were successful. All Student's t-tests were two sided. Statistical tests were performed using Microsoft Excel and GraphPad Prism 9. Data availability All data generated or analysed during this study are included in the article and its Supplementary Information. Results of the ORFeome, the CRISPR-Cas9 and the PRISM screens are available in Supplementary Table 1 . Data from the Cancer Cell Line Encyclopedia are available at https://depmap.org/portal /. Source data are provided with this paper.
|
Salvage of ribose from uridine or RNA
supports glycolysis in nutrient-limited
conditions
https://doi.org/10.1038/s42255-023-00774-2
Received: 3 February 2023
Accepted: 3 March 2023
Published online: 17 May 2023
Check for updates
Owen S. Skinner1,2,3,10, Joan Blanco-Fernández
5, Hongying Shen
Akinori Kawakami
Lena Joesch-Cohen1, Matthew G. Rees
Vamsi K. Mootha
& Alexis A. Jourdain
1,2,3
4
4,10, Russell P. Goodman
1,2,3,7,
1,2,3,8,9, Lajos V. Kemény5,6,
1, Jennifer A. Roth
1, David E. Fisher5,
Glucose is vital for life, serving as both a source of energy and carbon
building block for growth. When glucose is limiting, alternative nutrients
must be harnessed. To identify mechanisms by which cells can tolerate
complete loss of glucose, we performed nutrient-sensitized genome-wide
genetic screens and a PRISM growth assay across 482 cancer cell lines.
We report that catabolism of uridine from the medium enables the growth
of cells in the complete absence of glucose. While previous studies have
shown that uridine can be salvaged to support pyrimidine synthesis in the
setting of mitochondrial oxidative phosphorylation deficiency1, our work
demonstrates that the ribose moiety of uridine or RNA can be salvaged to
fulfil energy requirements via a pathway based on: (1) the phosphorylytic
cleavage of uridine by uridine phosphorylase UPP1/UPP2 into uracil and
ribose-1-phosphate (R1P), (2) the conversion of uridine-derived R1P into
fructose-6-P and glyceraldehyde-3-P by the non-oxidative branch of the
pentose phosphate pathway and (3) their glycolytic utilization to fuel ATP
production, biosynthesis a nd g lu coneogenesis. Capacity for glycolysis
from uridine-derived ribose appears widespread, and we confirm its activity
in cancer lineages, primary macrophages and mice in vivo. An interesting
property of this pathway is that R1P enters downstream of the initial, highly
regulated steps of glucose transport and upper glycolysis. We anticipate
that ‘uridine bypass’ of upper glycolysis could be important in the context of
disease and even exploited for therapeutic purposes.
We sought to identify new genes and pathways that might serve as alter-
native sources of energy when glucose is limiting. We transduced K562
cells with a library comprising 17,255 barcoded open reading frames
(ORFs)2 and compared proliferation in medium containing glucose
and galactose, a poor substrate for glycolysis (Fig. 1a). We used Dul-
becco’s modified Eagle’s medium (DMEM) that contained glutamine,
as well as pyruvate and uridine, for which oxidative phosphorylation
(OXPHOS)-deficient cells are dependent1,3. After 21 d, we harvested cells
and sequenced barcodes using next-generation sequencing (Extended
Data Fig. 1a and Supplementary Table 1). The mitochondrial pyruvate
dehydrogenase kinases 1–4 (encoded by PDK1–PDK4) are repressors of
oxidative metabolism, and all four isoforms were depleted in galactose
(Fig. 1b). Unexpectedly, we found striking enrichment in galactose for
ORFs encoding UPP1 and UPP2, two paralogous uridine phosphorylases
A full list of affiliations appears at the end of the paper.
e-mail: [email protected]; [email protected]
Nature Metabolism | Volume 5 | May 2023 | 765–776
765
nature metabolismLettera
Over-expression screen
K562 cells
ORFeome library
(17,255 ORFs)
Glucose Galactose
(+pyruvate, +uridine)
b
e
u
l
a
v
P
0
1
g
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–
5
4
3
2
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6
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Deoxycytidine
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Thymidine
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Cytidine
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g
7
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Base
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2
<
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–
RNA
Fig. 1 | Uridine phosphorylase activity supports growth on uridine or
RNA. a, Schematic overview of the ORF proliferation screen. b, Volcano plot
representation of the screen hits after 21 d of growth in medium containing
25 mM glucose or 25 mM galactose, 0.2 mM uridine and 1 mM sodium pyruvate
(n = 2). LFC, log2 (fold change). P values were calculated using a two-sided
Student’s t-test. Statistics were not adjusted for multiple comparisons.
c, Reaction catalysed by UPP1 and UPP2 proteins. d–f, Cell growth assays of K562
control cells and K562 cells expressing UPP1-FLAG or UPP2-FLAG in pyruvate-free
media in the presence of: 25 mM glucose or 25 mM galactose or 0.2 mM uridine
(±U; n = 3 replicate wells, P < 1.1 × 10−4 and P < 4.7 × 10−5; d), 10 mM of either
glucose, galactose or uridine (n = 3, P < 2.1 × 10−5; e) or 5 mM of the indicated
nucleosides (n = 3, P < 2.0 × 10−7; f). Data are shown as the mean ± s.e.m. with
two-sided t-test relative to control cells. g, Schematic of RNA highlighting its
ribose groups. h, Intracellular abundance of the four nucleoside precursors of
RNA in control or UPP1-FLAG-expressing K562 cells grown in sugar-free medium
supplemented with 0.5 mg ml−1 purified yeast RNA after 24 h. Data are expressed
as fold changes of sugar-free medium (n = 4, P < 1.2 × 10−6) and shown as the
mean ± s.e.m. with two-sided t-test relative to control. i, Cell growth assays of
control or UPP1-FLAG-expressing K562 cells in sugar-free medium supplemented
with 0.5 mg ml−1 of purified yeast RNA (n = 3, P < 2.6 × 10−5). Data are shown as the
mean ± s.e.m. with two-sided t-test relative to control cells. All growth assays,
metabolomics and screens included 4 mM l-glutamine and 10% dialysed FBS.
catalysing the phosphate-dependent catabolism of uridine into R1P
and uracil (Fig. 1b,c and Extended Data Fig. 1b,c).
To validate the screen, we stably expressed UPP1 and UPP2 ORFs
in K562 cells and observed a significant gain in proliferation in galac-
tose medium (Fig. 1d). This gain was dependent on uridine being pre-
sent in the medium, while expression of UPP1/UPP2, or addition of
uridine, had no effect in glucose-containing medium. Importantly,
we found that UPP1-expressing cells also efficiently proliferated in
medium containing uridine in the complete absence of glucose or
galactose (‘sugar-free’), while control cells were unable to proliferate
(Fig. 1e and Extended Data Fig. 1d). The ability of UPP1 cells to grow
in sugar-free medium strictly depended on uridine, and none of the
other seven nucleoside precursors of nucleic acids could substitute for
uridine (Fig. 1f).
Uridine-derived nucleotides are building blocks for RNA (Fig. 1g),
and RNA is an unstable molecule, sensitive to cellular and secreted
RNases. We tested if RNA-derived uridine could support growth in a
UPP1-dependent manner and supplemented glucose-free medium
with purified yeast RNA. The intracellular abundance of all four ribonu-
cleosides accumulated following addition of RNA to the medium, with
significantly lower uridine levels in UPP1-expressing cells, suggesting
UPP1-mediated catabolism (Fig. 1h). Accordingly, UPP1-expressing
K562 cells proliferated in sugar-free medium supplemented with RNA
(Fig. 1i). We conclude that elevated uridine phosphorylase activity
confers the ability to grow in medium containing uridine or RNA, in
the complete absence of glucose.
We next addressed the mechanism of how uridine supports the
growth of UPP1-expressing cells. Previous studies have noted the
beneficial effect of uridine in the absence of glucose and proposed
mechanisms that include the salvage of uridine for nucleotide synthesis
and its role in glycosylation4–8. Others reported the beneficial role of
uridine phosphorylase in maintaining ATP levels and viability during
glucose restriction in the brain9–11. To further investigate the molecu-
lar mechanism of uridine-supported proliferation, we performed a
secondary genome-wide CRISPR–Cas9 depletion screen using K562
cells expressing UPP1-FLAG grown on glucose or uridine (Fig. 2a,b and
Extended Data Fig. 2a).
We found that, although most essential gene sets were shared
between glucose and uridine conditions, three major classes of
genes were differentially essential in uridine as compared to glucose
(Fig. 2b, Extended Data Fig. 2b and Supplementary Table 1): (1) As
expected from pyrimidine salvage from uridine, all three enzymes
involved in de novo pyrimidine synthesis (encoded by CAD, DHODH and
UMPS) were essential in glucose but dispensable in uridine. (2) Genes
central to the non-oxidative branch of the pentose phosphate pathway
(non-oxPPP; PGM2, TKT, RPE) showed high essentiality in uridine.
Nature Metabolism | Volume 5 | May 2023 | 765–776
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Letterhttps://doi.org/10.1038/s42255-023-00774-2
a
b
Depletion screen
K562 cells
+ UPP1-FLAG
CRISPR library
(77,441 sgRNAs)
)
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GPI
HK2
CAD
UMPS
DHODH
ALDOA
PGM2
RPE
TKT
3
2
1
0
–1
–2
–3
–4
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–6
–6 –5 –4 –3 –2 –1
sgRNA representation in glucose (zglu)
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Ribose-P
Lactate
Citrate
Control
UPP1-FLAG
8 × 109
6 × 109
4 × 109
2 × 109
0
1 × 109
5 × 108
0
M + 0
M + 3
M + 2
M + 1
M + 4
M + 5
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Ribose-P
UPP1
Uracil
Ribulose-5-P
Glucose-6-P
RPE
Pyrimidines
Ribose-1-P
PGM2
Ribose-5-P Xylulose-5-P
TKT
TALDO1
Fructose-6-P
non-oxPPP
Seduheptulose-7-P
+ Erythrose-4-P
Dihydroorotate
Glyceraldehyde-3-P
Gene essentiality
Essential in uridine
Dispensable in uridine
Always essential
GAPDH
ENO1
PKM
Lactate
–
Gln + Asp + HCO3
De novo pyrimidine
PPP
Glycolysis
Fig. 2 | Uridine-derived ribose contributes to the pentose phosphate pathway
and glycolysis. a, Schematic of a genome-wide CRISPR–Cas9 depletion screen
comparing the proliferation of UPP1-FLAG-expressing K562 cells in sugar-free
medium containing 10 mM glucose or uridine after 21 d (n = 2), in the absence
of supplemental pyruvate and uridine. b, Gene-level analysis of a genome-wide
CRISPR–Cas9 screen in glucose versus uridine reported as z-scores relative
to non-cutting controls in glucose (zglu) and uridine (zu; n = 10,442 expressed
genes, n = 2 replicates). c, Differential sensitivity of UPP1-FLAG-expressing K562
cells treated with the indicated sgRNAs targeting enzymes of upper glycolysis
(n = 4, P < 8.7 × 10−11, P < 3.2 × 10−10, P < 6.4 × 10−5), the PPP (n = 4, P < 5.7 × 10−8,
P < 7.6 × 10−8, P < 2.1 × 10−5, P < 6.3 × 10−7) or the salvage of uridine for pyrimidine
synthesis (n = 3) in glucose versus uridine, expressed as the fold change of
glucose and compared to control sgRNAs (n = 11). Data are shown as the
mean ± s.e.m. after 4–5 d. P values were calculated using a two-sided Student’s
t-test relative to control sgRNAs. Statistics were not adjusted for multiple
comparisons. d, Lactate determination in medium containing 10 mM glucose,
galactose or uridine (sugar-free) after 3 h (n = 3 replicate wells, P < 7.8 × 10−7).
Data are shown as the mean ± s.e.m. with two-sided t-test relative to control cells.
e, Labelling with 13C5-uridine ([1′,2′,3′,4′,5′-13C5]uridine; labelled carbon atoms
in the ribose of uridine are indicated in magenta) and 13C5-uridine tracer analysis
of representative intracellular metabolites from the PPP, glycolysis and the TCA
cycle in control or UPP1-FLAG-expressing K562 cells (n = 3). Data are shown as
the mean ± s.e.m. and are corrected for natural isotope abundance. f, 13C5-uridine
tracer analysis of liver metabolites 30 min after intraperitoneal injection in
overnight fasted mice with 0.4 g per kg body weight shown as the percentage
of 13C-labelled intermediates compared to the total pool. Data are shown as the
mean ± s.e.m. and are corrected for natural isotope abundance (n = 4 mice).
g, Schematic of uridine-derived ribose catabolism integrating gene essentiality
results in glucose versus uridine. Gln, glutamine; Asp, aspartate. All growth
assays and metabolomics experiments included 4 mM (DMEM) l-glutamine and
10% dialysed FBS. a.u., arbitrary units.
Among them PGM2, which encodes an enzyme that converts ribose-1-P
to ribose-5-P and connects the UPP1/UPP2 reaction to the PPP, was
highly essential in uridine, but almost fully dispensable in glucose.
Accordingly, uridine-grown cells were particularly sensitive to deple-
tion of PGM2, TKT and RPE, or to TKT inhibition, while they were insensi-
tive to the de novo pyrimidine synthesis inhibitor brequinar(Fig. 2c and
Extended Data Fig. 3a,b). In contrast, genes of the oxidative branch of
the PPP (G6PD, PGLS, PGD) did not score differentially between glucose
and uridine. (3) As expected from their essentiality in glucose-limited
conditions3,12, genes encoding the mitochondrial respiratory chain
were generally more essential in uridine, although to a lesser extent
compared to the non-oxPPP, perhaps due to the low energy supply in
the absence of glucose.
In contrast to the previously proposed mechanisms4–8, ablation of
genes involved in uridine salvage for nucleotide synthesis (UCK1/UCK2,
TYMS) or in glycosylation had no effect on the growth of cells in uridine
when compared to glucose (Fig. 2c, Extended Data Fig. 3b,c and Supple-
mentary Table 1). Central enzymes of glycolysis were essential both in
glucose and in uridine, indicating that a functional glycolytic pathway
is required for survival with uridine alone. However, our comparative
analysis revealed that several upper glycolytic enzymes (encoded by
ALDOA, GPI and HK2) were dispensable in uridine, and only essential
in glucose (Fig. 2b,c and Extended Data Fig. 3b). Not all steps of upper
glycolysis scored in either condition, potentially due to the multiple
genes with overlapping functions encoding glycolytic enzymes, a
common limitation in single gene-targeting screens. Nevertheless,
genes found to be dispensable in uridine included all steps upstream of
fructose-6-P (F6P) and/or glyceraldehyde-3-P (G3P), which connect the
non-oxPPP to glycolysis, pointing to a key role for these two metabolites
in supporting proliferation on uridine.
The essentiality of the non-oxPPP, with the dispensability of upper
glycolysis in uridine (Fig. 2b,c), prompted us to hypothesize that the
ribose moiety of uridine can enter glycolysis and serve as a substrate for
biosynthesis and energy production. Lactate secretion and glycolytic
utilization of uridine, however, were excluded in earlier work4–8. None-
theless, given the importance of PPP enzymes and the dispensability
Nature Metabolism | Volume 5 | May 2023 | 765–776
767
Letterhttps://doi.org/10.1038/s42255-023-00774-2UMPSDHODHCADHK2GPIALDOA
of upper glycolysis, we reinvestigated this possibility and measured
lactate secretion in uridine-grown cells. Strikingly, we found that
UPP1-expressing cells grown in uridine secreted high amounts of lactate
(Fig. 2d). Accordingly, we found using liquid chromatography–mass
spectrometry (LC–MS) that uridine restored steady-state abundance
of most central carbon metabolism detected in the absence of glucose,
strongly suggesting some degree of lower glycolysis activity from
uridine (Extended Data Fig. 4a).
To directly test if uridine-derived ribose could serve as a substrate
for glycolysis, we designed a tracer experiment using isotopically
labelled uridine with five ribose carbons (13C5-uridine) and LC–MS
(Fig. 2e). UPP1-expressing cells avidly incorporated 13C5-uridine, as
seen by the presence of 13C in all the intracellular intermediates of
the PPP and glycolysis analysed, including ribose-phosphate, upper
and lower glycolytic intermediates and lactate, while control cells
showed very little label incorporation. Tricarboxylic acid (TCA) cycle
intermediates, among them citrate, were also partially labelled (mostly
M + 2), indicating potential incorporation of carbon from glycolysis
via pyruvate. To determine whether this labelling pattern extends
in vivo, we next injected overnight fasted mice intraperitoneally with
a 13C5-uridine tracer and measured incorporation in the liver and in
circulating metabolites after 30 min. As in cell lines, we found 13C incor-
poration in ribose-phosphate and glycolysis in 13C5-uridine-treated
animals (Fig. 2f and Extended Data Fig. 4b–e). Incorporation efficiency
was smaller than in cell culture, as expected from low-dose 13C5-uridine
injection, shorter treatment time and competition with other endog-
enous substrates in vivo, including unlabelled uridine. 13C5-uridine
incorporation also occurred in fed animals, albeit to a lesser extent,
and expression of liver Upp1 and Upp2 did not change with feeding
(Extended Data Fig. 4b–f). We also found modest but significant incor-
poration of uridine-derived 13C in glucose, indicating gluconeogenesis
from uridine-derived carbons (Fig. 2f and Extended Data Fig. 4c,d).
Together, our results indicate that in cell lines and in animals in vivo,
uridine catabolism provides ribose for the PPP, and that the non-oxPPP
and the glycolytic pathway communicate via F6P and G3P to replen-
ish glycolysis thus entirely bypassing the requirement for glucose in
supporting lower glycolysis, biosynthesis and energy production in
sugar-free medium (Fig. 2g).
We next sought to determine whether any human cell lines exhibit
a latent ability to use uridine-derived ribose to grow on uridine when
glucose is absent without the need for over-expression. We screened
482 pooled barcoded adherent cancer cell lines spanning 22 solid
tumour lineages from the PRISM collection13 in medium containing
10 mM glucose or uridine, in the absence of any supplemental sugar
(Fig. 3a, Extended Data Fig. 5 and Supplementary Table 1). Cells from
the melanoma and the glioma lineages grew remarkably well in uridine
as compared to the other lineages, whereas Ewing sarcoma cells grew
significantly less well (Fig. 3b). Cell lines from the PRISM collection have
been extensively characterized at a molecular level14, so we searched
for genomic factors that correlate with the ability to grow on uridine
(Supplementary Table 1). Genome wide, the top-scoring transcript, pro-
tein and genomic copy number variant was UPP1 (Fig. 3c–e), in strong
agreement with our ORF screen (Fig. 1b). Expression of UPP1 across the
CCLE collection was the highest in cell lines of skin origin (Extended
Data Fig. 6a,b), where high uridine phosphorylase enzyme activity has
been documented15, and tended to be lowest in the bone lineage. UPP2
was almost never expressed in the CCLE collection (average transcripts
per million (TPM) < 1; Extended Data Fig. 6a). In agreement with these
results, we confirmed significant, UPP1-dependent, proliferation and
uridine catabolism in melanoma cells grown in sugar-free medium
supplemented with uridine or RNA (Fig. 3f–h and Extended Data
Fig. 6c–e). We conclude that the endogenous expression of UPP1 is nec-
essary and sufficient to support the growth of cancer cells on uridine.
We next investigated the factors that promote UPP1 expression and
growth on uridine by integrating our results with CCLE data to prioritize
transcription factors, which highlighted MITF as a strong candidate in
melanoma cells, both at the protein and the transcript level (Fig. 3c,d
and Extended Data Fig. 6a,b). We found that MITF over-expression
promoted UPP1 expression and uridine growth (Extended Data
Fig. 7a,b), while endogenous MITF binding was detected in the tran-
scription start site (TSS) and the promoter (−3.5 kb from the TSS) of
UPP1 in a large-scale chromatin immunoprecipitation (ChIP) study16,
which we experimentally validated (Extended Data Fig. 7c,d). Accord-
ingly, siRNA-mediated depletion of MITF decreased UPP1 expression
in melanoma cells (Extended Data Fig. 7e).
Our solid tumour PRISM cancer cells collection did not include
cells of the immune lineage, where UPP1 is expressed at high levels17,18,
so we asked whether immune cells exhibit the capacity to metabolize
ribose from uridine either at baseline or in a transcriptionally regu-
lated manner. In the human monocytic THP1 cell line, in macrophage
colony-stimulating factor (M-CSF)-matured peripheral blood mono-
nuclear cells (PBMCs), and in primary mouse bone marrow-derived
macrophages (BMDMs), we found that differentiation into mac-
rophages and/or further polarization with immunostimulatory
molecules increased UPP1 expression (Fig. 3i–k and Extended Data
Fig. 8a,b). In contrast, expression of pyrimidine salvage genes
(UCK1/UCK2) and 13C5-uridine incorporation into UMP were not
affected, and even decreased, during this process (Extended Data
Fig. 8c,d). Among the immunostimulatory molecules, RNA enhanced
UPP1 expression, suggesting the existence of a feed-forward loop,
where RNA (and conceivably RNA-containing pathogens and debris)
may trigger UPP1 expression and uridine salvage for building blocks
and energy production. Supporting this idea, stimulation of PBMCs
and BMDMs with a TLR7/TLR8 agonist (R848) lead to a significant,
IκB kinase (IKK)-dependent, increase in UPP1 transcription in BMDMs
(Fig. 3j,k and Extended Data Fig. 8e). Label incorporation from uridine
ribose was also strongly increased in citrate and lactate after differen-
tiation of THP1 and after BMDM stimulation with R848, while it wasn't
further increased in M-CSF-matured PBMCs, possibly due to high base-
line capacity for uridine catabolism in these cells (Fig. 3l and Extended
Data Fig. 8f,g). Together, our results indicate that macrophages have
the capacity to use uridine-derived ribose for glycolysis, and that UPP1
expression and uridine catabolism can sharply increase during cellular
differentiation and in response to immunostimulating molecules, with
cell type and species differences.
We next sought to determine whether glycolysis from uridine is
under acute regulation in the same way as from glucose. Active OXPHOS
tends to keep glucose uptake and glycolysis at lower levels, while acute
inhibition of OXPHOS leads to an immediate and strong increase in
glucose-supported glycolysis, as evidenced by a robust increase in the
extracellular acidification rate (ECAR) following oligomycin treatment
(Fig. 4a,b). Strikingly, we found no ECAR stimulation by OXPHOS inhibi-
tors, no difference in 13C5-uridine incorporation following antimycin
blockage of the electron transport chain, and no increase in uridine
import in OXPHOS-inhibited UPP1-expressing cells grown on uridine
(Fig. 4b,c and Extended Data Fig. 9a,b). Because glycolysis from both
uridine and glucose share a common pathway from G3P (Fig. 2g), dif-
ferential regulation of glycolysis following OXPHOS inhibition must
occur in the upper part of the pathway. Consistent with this notion, we
observed no stimulation of ECAR in mannose-grown cells, a sugar con-
nected to glycolysis by F6P (Extended Data Fig. 9c). We conclude that
substrates such as uridine can enter glycolysis in a constitutive way, in
contrast to glucose, by bypassing regulatory steps of upper glycolysis
such as glucose transport and initial phosphorylation.
In line with this, we next performed a competition experiment to
evaluate if the presence of glucose affects the incorporation of uridine
in cells. Incorporation of uridine in lactate was notably not affected by
competition with glucose in our experimental conditions, despite the
presence of a large molar excess of glucose (Fig. 4c). Therefore, and in
agreement with a bypass of regulatory steps of upper glycolysis, uridine
Nature Metabolism | Volume 5 | May 2023 | 765–776
768
Letterhttps://doi.org/10.1038/s42255-023-00774-2a
f
r
e
b
m
u
n
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PRISM screen
482 cancer cell lines
(adherent)
6 d
Glucose
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Glioma (CNS)
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MITF
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MITF
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MITF
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UPP1 (Chr 7p12.3)
Chr 7p
UPP1
Barcode sequencing
Effect size
Transcript correlation with
growth on uridine (z-score)
Protein correlation with
growth on uridine (z-score)
Copy number correlation with
growth on uridine (z-score)
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2.5 × 108
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–
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BMDMs
Lactate
6
–
0
1
.
×
8
6
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P
3
–
0
1
×
4
.
1
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P
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×
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–
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RNA
LPS
–
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RNA
LPS
M + 0
M + 1
M + 2
M + 3
Fig. 3 | Capacity for glycolysis from uridine is governed by lineage and
transcriptional control of UPP1/UPP2 gene expression. a, Schematic of the
PRISM screen with 482 cancer cells lines grown for 6 d in sugar-free medium
complemented with 10 mM glucose or uridine (n = 2). b, Lineage analysis (n = 22
lineages) highlighting growth on uridine as compared to glucose. False discovery
rates (FDRs) were calculated using a Benjamini–Hochberg algorithm correcting
for multiple comparisons13. c,d, Correlation between uridine growth and
expression of transcripts (n = 8,123; c) and proteins (n = 3,216; d) across cancer
cell lines. e, Correlation between gene copy number (n = 5,950) and growth on
uridine across the cell lines, highlighting chromosome 7p. UPP1 is encoded
on Chr7p12.3. f, Cell growth assay in sugar-free medium complemented with
10 mM glucose or uridine of a panel of melanoma (n = 9) and non-melanoma
(n = 3, 293T, K562 and HeLa) cell lines. Data are shown as the mean ± s.e.m. (n = 4).
MDA, MDA-MB-435S. g, Cell growth assay of melanoma UACC-257 wild-type
(UPP1WT) and knock-out (UPP1KO) clones in sugar-free medium complemented
with 10 mM of glucose or uridine. Data are shown as the mean ± s.e.m. (n = 3,
P < 7.4 × 10−6, P < 2.2 × 10−4, P < 5.3 × 10−6) with two-sided t-test relative to UPP1WT
cells in the same medium. h, 13C5-uridine tracer analysis reporting representative
intracellular metabolites from the PPP, glycolysis and the TCA cycle in UACC-
257 wild-type (UPP1WT) and two knock-out (UPP1KO) clones after 5 h (n = 4,
P < 1.2 × 10−9, P < 2.2 × 10−12, P < 2.6 × 10−8, P < 6.1 × 10−9, P < 8.7 × 10−9, P < 8.2 ×
10−8). i–k, Expression of UPP1 (Upp1) and IL1B (Il1b) in human THP1 cells (n = 4,
P < 1.7 × 10−3, P < 1.9 × 10−6, P < 2.2 × 10−6, P < 1.4 × 10−7; i), human M-CSF-matured
PBMCs (n = 4 donors, P < 1.5 × 10−2, P < 5.5 × 10−5, P < 1.2 × 10−3, P < 8.4 × 10−5; j)
and BMDMs (n = 3 mice, P < 1.6 × 10−2, P < 5.9 × 10−2, P < 2.5 × 10−3, P < 2.0 × 10−2,
P < 1.9 × 10−3, P < 4.4 × 10−4; k) after treatment with 100 nM phorbol myristate
acetate (PMA) for 48 h (THP1), 100 ng ml−1 lipopolysaccharides (LPS; THP1,
BMDMs), 1 mg ml−1 purified yeast RNA (THP1, PBMCs, BMDMs) or 5 µg ml−1 of
TLR7/TLR8 agonist (R848) for 24 h and as determined by quantitative PCR
(qPCR). l, 13C5-uridine tracer analysis reporting incorporation in media lactate
from BMDMs treated for 24 h with 5 µg ml−1 R848 and further grown for 16 h in
glucose-free DMEM containing 5 mM 13C5-uridine and 5 µg ml−1 R848 (n = 3 mice,
P < 1.4 × 10−3, P < 3.6 × 10−5, P < 6.8 × 10−6). Data are shown as the mean ± s.e.m. with
two-sided t-test relative to untreated cells.
can be incorporated into cells even when lactate production from
glucose is saturated, suggesting constitutive import and catabolism.
Cells with severe OXPHOS dysfunction classically have to be grown
on glucose, and uridine must be supplemented1. The traditional expla-
nation has been that glucose is required to support glycolytic ATP
production as OXPHOS is debilitated, and that uridine supplemen-
tation is required for pyrimidine salvage given that de novo pyrimi-
dine synthesis via DHODH requires coupling to a functional electron
transport chain1,3 (Extended Data Fig. 9d). Having observed energy
harvesting from uridine, we finally tested whether uridine-derived
ribose could also benefit OXPHOS-inhibited cells in the absence of
glucose. We found a significant UPP1-dependent rescue of viability in
galactose-grown cells treated with antimycin A (Fig. 4d), now reveal-
ing that supplemental uridine benefits mitochondrial dysfunction in
two ways: (1) pyrimidine salvage when de novo pyrimidine synthesis
is impossible, and (2) energy production in UPP1-expressing cells.
Nature Metabolism | Volume 5 | May 2023 | 765–776
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Letterhttps://doi.org/10.1038/s42255-023-00774-2
a
Feedback inhibition?
Glucose
Uridine
G6P
R1P
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Lactate
Lactate
OXPHOS
OXPHOS
b
i
)
n
m
/
H
p
m
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100
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.
1
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P
Lactate
.
×
9
5
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P
0 mM glucose
0 mM glucose + anti. A
1 mM glucose
5 mM glucose
10 mM glucose
25 mM glucose
P > 0.05
Control
UPP1-FLAG
5
–
0
1
.
×
9
3
<
P
d
)
%
(
h
t
a
e
d
l
l
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C
80
60
40
20
0
25
50
75
Time (min)
M + 0
M + 1
M + 2
M + 3
–U +U –U +U –U +U –U +U
DMSO Anti. A DMSO Anti. A
Glucose
Galactose
Fig. 4 | Glycolysis from uridine bypasses the regulated steps of upper
glycolysis and supports OXPHOS-deficient cells. a, Schematic of glycolysis
inhibition by OXPHOS. G6P, glucose-6-phosphate. b, Representative ECAR
in UPP1-expressing K562 cells grown in sugar-free medium with and without
supplementation of 10 mM of glucose, galactose or uridine, with n = 30 replicate
wells. O, oligomycin; C, CCCP; A, antimycin A. Data are shown as the mean ± s.d.
c, 13C5-uridine tracer analysis reporting intracellular lactate in UACC-257
melanoma cells in glucose-free RPMI medium containing 5 mM 13C5-uridine and in
competition with increasing amount of unlabelled glucose (0, 1, 5, 10 and 25 mM)
or treated with 100 nM antimycin A, all after 5 h (n = 4, P < 1.7 × 10−4, P < 4.9 × 10−3,
P < 1.8 × 10−5, P < 5.9 × 10−5, P > 0.05). Data are shown as the mean ± s.e.m. and
are corrected for natural isotope abundance. P values were calculated using a
two-sided Student’s t-test. Statistics were not adjusted for multiple comparisons.
d, Percentage of dead cells in UPP1-expressing K562 cells grown in 5 mM glucose
or galactose supplemented with 5 mM uridine (U) and antimycin A (anti. A). Data
are shown as the mean ± s.e.m. with two-sided t-test relative to control K562 cells
(n = 4, P < 3.9 × 10−5).
For decades it has been known that cells with mitochondrial defi-
ciencies are dependent on uridine to support pyrimidine synthesis
given the dependence of de novo pyrimidine synthesis on DHODH,
whose activity is coupled to the electron transport chain1. Although it
has been documented, it is less appreciated that uridine supplemen-
tation can support cell growth in the absence of glucose4–10. Here, we
show that, in addition to nucleotide synthesis, uridine can serve as a
substrate for energy production, biosynthesis and gluconeogenesis.
Mechanistically, we show that glycolysis from uridine-derived ribose
is initiated with the phosphorylytic cleavage of uridine by UPP1/UPP2,
followed by shuttling of its ribose moiety through the non-oxPPP
and glycolysis, hence supporting not only nucleotide metabolism
but also energy production or gluconeogenesis in the absence of
glucose (Fig. 2g).
By comparing uridine to other nucleosides and using similar
tracer experiments to ours, Wice et al.7 observed incorporation of
uridine-derived carbons in most cellular fractions in mammalian cell
culture and in chicken embryos. However, they did not detect pyruvate
and lactate in uridine, and concluded that uridine does not participate in
glycolysis, but rather is required for nucleotide synthesis, and proposed
that energy is derived exclusively from glutamine in the absence of
glucose6,7. Loffler et al. and Linker et al. reached the same conclusion4,8.
Our observations based on a genome-wide CRISPR–Cas9 screening and
metabolic tracers (Fig. 2) agree with previous observations that cells
can proliferate in sugar-free medium if uridine is provided, and that
uridine is crucial for nucleotide synthesis—but differ mechanistically
on the role of glycolysis in this condition, as we were able to identify a
significant amount of labelling in glycolytic intermediates and secreted
lactate, as well as a high ECAR, all consistent with glycolytic ATP pro-
duction from uridine. It has previously been reported that uridine
protects cortical neurons and immunostimulated astrocytes from
glucose deprivation-induced cell death, in a way related to ATP, and it
was hypothesized that uridine could serve as an ATP source9. Our genetic
perturbation and tracer studies are consistent with this hypothesis.
Fig. 4c–e). Our gain-of-function and loss-of-function studies suggest
that tissues expressing UPP1/UPP2 will have capacity for glycolysis
from uridine-derived ribose. Based on gene expression atlases18,19, we
predict uridine may be a meaningful source of energy in blood cells,
lung, brain and kidney, as well as in certain cancers. Uridine is the most
abundant and soluble nucleoside in circulation20 and it is possible that
uridine may serve as an alternative energy source in these tissues, or
for immune and cancer metabolism, similar to what has been proposed
for other sugars and nucleosides21–23. It is notable that the strongest
human metabolic quantitative trait loci for circulating uridine cor-
responds to UPP1 (ref. 24), while uridine phosphorylase activity is the
main determinant of circulating uridine in mice25.
A fascinating aspect of glycolysis from uridine is its apparent
absence of regulation, at least at shorter timescales. The ability of uri-
dine to serve as a constitutive input into glycolysis might have clinical
implications for human diseases, as uridine is present at high levels
in foods such as milk and beer26,27, and previous in vivo studies have
shown that a uridine-rich diet leads to glycogen accumulation, glu-
coneogenesis, fatty liver and pre-diabetes in mice28,29. We now report
that glycolysis from uridine lacks at least two checkpoints as (1) it is not
controlled by OXPHOS (Fig. 4a–c), and (2) it occurs even when lactate
production from glucose is evidently saturated (Fig. 4c), or after food
intake in vivo (Extended Data Fig. 4d,e). Although glycolysis from uri-
dine appears to occur at a slower pace than from glucose, we speculate
that constitutive fuelling of glycolysis and gluconeogenesis from a
uridine-rich diet may contribute to human conditions such as fatty liver
disease and diabetes. Such a ‘uridine bypass’ is conceivable because
glycolysis is so strongly controlled in upper glycolysis, for example,
glucose transport30, which we show is bypassed by uridine, because
the non-oxPPP and glycolysis are connected by F6P and G3P (Fig. 2g).
This ability of uridine to bypass upper glycolysis may be beneficial
in certain cases. For example, disorders of upper glycolysis, notably
GLUT1 deficiency syndrome31, may benefit from uridine therapy and
from induction of UPP1/UPP2 expression.
The capacity to harvest energy and building blocks from uridine
appears to be widespread. Here, we report very high capacity for
uridine-derived ribose catabolism in melanoma and glioma cell lines
(Fig. 3a–h), in primary human and mouse macrophages (Fig. 3i–l),
and we also detect labelling patterns from uridine-derived ribose in
the liver and the whole organism in vivo (Fig. 2f and Extended Data
At longer timescales, UPP1 expression and capacity for ribose
catabolism from uridine appear to be determined by cellular differen-
tiation and further activation by extracellular signals. Here we focused
on the monocytic lineage and found that (1) in THP1 cells, UPP1 expres-
sion and activity sharply increased during differentiation and polari-
zation, (2) high baseline rates of glycolysis from uridine are observed
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Letterhttps://doi.org/10.1038/s42255-023-00774-2
in M-CSF-matured PBMCs and (3) treatment with immunostimulat-
ing molecules acutely promote both UPP1 expression and uridine
catabolism in BMDMs (Fig. 3i–l). Whereas we didn’t investigate whether
uridine is required for macrophage activation, we noticed that all the
agonists tested ultimately lead to nuclear factor kappa B (NF-κB) activa-
tion, which binds in the UPP1 promoter17,32. It is thus likely that NF-κB
may serve as a transcription factor for UPP1. Supporting this assertion,
we found that blocking NF-κB signalling with upstream IKK inhibitors
abolished R848-induced Upp1 expression (Extended Data Fig. 8e).
Uridine phosphorylase and ribose salvage by UPP1 appears to
lie downstream of a number of signalling pathways with potential
relevance to disease. We have demonstrated that uridine breakdown
is promoted by MITF, a transcription factor associated with melanoma
progression, which we show binds upstream of UPP1 to promote its
expression (Extended Data Fig. 7). In an accompanying study, Nwosu,
Ward et al., demonstrate that UPP1 expression is under the control of
KRAS–MAPK signalling33. It is notable that both MITF and NF-kB can
act downstream of KRAS–MAPK34–38 and that some pancreatic cell
lines with high uridine phosphorylase activity highlighted by Nwosu,
Ward et al.33 unpublished data, also scored in our PRISM screen (Sup-
plementary Table 1).
Finally, we found that RNA in the medium can replace glucose
to promote cellular proliferation (Fig. 1i and Extended Data Fig. 6e).
RNA is a highly abundant molecule, ranging from 4% of the dry weight
of a mammalian cell to 20% of a bacterium39. Recycling of ribosomes
through ribophagy, for example, plays an important role in supporting
viability during starvation40. Cells of our immune system also ingest
large quantities of RNA during phagocytosis, and we experimentally
showed that the expression of UPP1 increases with macrophage acti-
vation (Fig. 3i–k), including when cells are stimulated with RNA itself,
suggesting the existence of a positive feedback loop. Uridine seems to
be the only constituent of RNA that can be efficiently used for energy
production, at least in K562 cells (Fig. 1h). Whereas the salvage of RNA
to provide building blocks during starvation has long been appreciated
for nucleotide synthesis, to our knowledge, its contribution to energy
metabolism has not been considered in the past, except for some
fungi that can grow on minimum media with RNA as their sole carbon
source41. We speculate that, similar to glycogen and starch, RNA itself
may constitute as large stock of energy in the form of a polymer, and
that it may be used for energy storage and to support cellular function
during starvation, or during processes associated with high energy
costs such as the immune response.
Methods
Cell lines
K562 (CCL-243), 293T (CRL-3216), HeLa (CCL-2), A375 (CRL-1619), A2058
(CRL-11147), SH4 (CRL-7724), MDA-MB-435S (HTB-129), SK-MEL-5 (HTB-
70), SK-MEL-30 (HTB-63) and THP1 (TIB-202) cell lines were obtained
from the American Type Culture Collection (ATCC). UACC-62, UACC-
257 and LOX-IMVI cells were obtained from the Frederick Cancer
Division of Cancer Treatment and Diagnosis (DCTD) Tumor Cell Line
Repository. All cell lines were re-authenticated by STR profiling at
ATCC before submission of the manuscript and compared to ATCC
and Cellosaurus (ExPASy) STR profiles in 2020, with the exception of
THP1 (TIB-202) and U937 (CRL-1593.2), which were purchased from
ATCC for the experiments. Cells lines from the PRISM collection were
obtained from The PRISM Lab (Broad Institute) and were not further
re-authenticated. MDA-MB-435S cells were previously assumed to be
ductal carcinoma cells and recent gene expression analysis assigned
them to the melanoma lineage (ATCC).
Cell culture and cell growth assays
Cell line stocks were routinely maintained in DMEM (HeLa, 293T,
K562, A375, A2058, SK-MEL-5, MDA-MB-435S) containing 1 mM sodium
pyruvate (Thermo Fisher Scientific) with 25 mM glucose, 10% FBS
(Thermo Fisher Scientific), 50 µg ml −1 uridine (Sigma), 4 mM
l-glutamine and 100 U ml−1 penicillin–streptomycin (Thermo Fisher
Scientific); or in RPMI (SH4, UACC-62, UACC-257, SK-MEL-30, LOX-IMVI,
THP1) with 11.1 mM glucose, 10% FBS (Thermo Fisher Scientific), 2 mM
l-glutamine and 100 U ml−1 penicillin–streptomycin (Thermo Fisher
Scientific) under 5% CO2 at 37 °C. All growth assays, metabolomics,
screens and bioenergetics experiments were performed in medium
containing dialysed FBS. For growth experiments, an equal number
of cells was counted, washed in PBS and resuspended in no-glucose
DMEM (Thermo Fisher Scientific), or no-glucose RPMI (Teknova)
complemented with 10% dialysed FBS (Thermo Fisher Scientific),
100 U ml−1 penicillin–streptomycin (Thermo Fisher Scientific) and
5–10 mM of glucose, galactose, uridine or mannose (all from Sigma)
dissolved in water, or with an equal volume of water alone. For RNA
and other nucleoside complementation assays, 0.5 mg ml−1 purified
RNA from Torula yeast (Sigma) or the selected nucleosides (Sigma)
were weighted and directly resuspend in DMEM. In all cases, cells were
counted with a Vi-Cell Counter (Beckman) after 3 to 5 d of growth and
only live cells were considered. Cell viability in glucose and galactose
was determined using the same Vi-Cell Counter assay. Measurements
were taken from distinct samples.
Open reading frame screen
For ORF screening, K562 cells were infected with a lentiviral-carried
ORFeome v8.1 library2 (Genome Perturbation Platform, Broad Institute)
containing 17,255 ORFs mapping to 12,548 genes, in duplicate. Cells
were infected at a multiplicity of infection of 0.3 and at 500 cells per
ORF in the presence of 10 µg ml−1 polybrene (Millipore). After 72 h, cells
were transferred to culture medium containing 2 µg ml−1 puromycin
(Thermo Fisher Scientific) and incubated for an additional 48 h. On the
day of the screen, cells were plated in screening medium containing
no-glucose DMEM supplemented with 10% dialysed FBS, 1 mM sodium
pyruvate (Thermo Fisher Scientific), 50 µg ml−1 uridine (Sigma) and
100 U ml−1 penicillin–streptomycin (Thermo Fisher Scientific) and
25 mM of either glucose or galactose (Sigma) at a concentration of
105 cells per ml and with 500 cells per ORF. Cells were passaged every
3 d and 500 cells per ORF were harvested after 0, 9 and 21 d of growth.
Total genomic DNA was isolated from cells using a NucleoSpin Blood
kit (Clontech) using the manufacturer’s recommendations. Barcode
sequencing, mapping and read count were performed by the Genome
Perturbation Platform (Broad Institute). For screen analysis, log2
(normalized read counts) were used, and P values were calculated
using a two-sided t-test. The presence of lentiviral recombination
within the ORFeome library was not tested and as such genes that
dropped out should be considered with caution, as these may represent
unnatural proteins42.
Stable gene over-expression
cDNAs corresponding to GFP, UPP1-FLAG and UPP2-FLAG were cloned
in pWPI/Neo (Addgene). Lentiviruses were produced according to
Addgene’s protocol. Twenty-four hours after infection, cells were
selected with 0.5 mg ml−1 G418 (Thermo Fisher Scientific) for 48 h.
Polyacrylamide gel electrophoresis and immunoblotting
Cells grown in routine medium were harvested, washed in PBS and
lysed for 5 min on ice in RIPA buffer (25 mM Tris pH 7.5, 150 mM NaCl,
0.1% SDS, 0.1% sodium deoxycholate, 1% NP40 analogue, 1 × protease
(Cell Signaling) and a 1:500 dilution of Universal Nuclease (Thermo
Fisher Scientific)). Protein concentration was determined from total
cell lysates using a DC protein assay (Bio-Rad). Gel electrophoresis
was done on Novex Tris-Glycine gels (Thermo Fisher Scientific) before
transfer using the Trans-Blot Turbo blotting system and nitrocellulose
membranes (Bio-Rad). All immunoblotting was performed in Inter-
cept Protein blocking buffer (Li-cor) or in 5% milk powder in TBST
(TBS + 0.1% Tween-20). Washes were done in TBST. Specific primary
Nature Metabolism | Volume 5 | May 2023 | 765–776
771
Letterhttps://doi.org/10.1038/s42255-023-00774-2antibodies were diluted at a concentration of 1:100–1:5,000 in block-
ing buffer. Fluorescent-coupled secondary antibodies were diluted at
a ratio of 1:10,000 in blocking buffer. Membranes were imaged with an
Odyssey CLx analyzer (Li-cor with Image Studio Lite v4.0) or by chemi-
luminescence. The following antibodies were used: FLAG M2 (Sigma,
F1804), Actin (Abcam, ab8227), TUBB (Thermo, MA5-16308), UPP1
(Sigma, SAB1402388), MITF (Sigma, HPA003259), TYR (Santa Cruz
sc-20035), MLANA (CST, 64718), HK2 (CST, 28675), GPI (CST, 94068),
ALDOA (CST, 8060), TKT (CST, 64414), RPE (Proteintech, 12168-2-AP),
PGM2 (Proteintech, 11022-1-AP), UCK2 (Proteintech, 10511-1-AP), TYMS
(Proteintech, 15047-1-AP), S6 ribosomal protein (Santa Cruz, sc-74459)
and phosphor-S6 (Santa Cruz, sc-293144). Two commercially available
antibodies to UPP2 were tested (Sigma, SAB4301661; Abcam, ab153861),
but no specific band could be detected.
PRISM screen
A six-well plate containing a mixture of 482 barcoded adherent can-
cer cell lines (PR500)13 grown on RPMI (Life Technologies, 11835055)
containing 10% FBS was prepared by The PRISM Lab (Broad Institute)
seeded at a density of 200 cells per cell line. On day 0, the culture
medium was replaced with no-glucose RPMI medium (Life Technolo-
gies, 11879020) containing 10% dialysed FBS and 100 U ml−1 penicil-
lin–streptomycin and supplemented with 10 mM of either glucose
or uridine (n = 3 replicate wells each). The medium was replaced with
fresh medium on days 3 and 5. On day 6, all wells reached confluency
and cells were lysed. Lysates were denatured, (95 °C) and total DNA
from all replicate wells was PCR amplified using KAPA polymerase
and primers containing Illumina flow cell-binding sequences. PCR
products were confirmed to show single-band amplification using gel
electrophoresis, pooled, purified using the Xymo Select-a-Size DNA
Clean & Concentration kit, quantified using a Qubit 3 Fluorometer, and
then sequenced via HiSeq (50 cycles, single read, library concentration
of 10 pM with 25% PhiX spike-in) as previously described43. Barcode
abundance was determined from sequencing, and unexpectedly low
counts (for example, from sequencing noise) were filtered out from
individual replicates so as not to unintentionally depress cell line
counts in the collapsed data. Replicates were then mean-collapsed,
and log fold change and growth rate metrics were calculated accord-
ing to equations (1) and (2):
log2 fold change = log2 (nu/ng)
Growth rate =
log2 (nf/n0)
t
(1)
(2)
where nu and ng are counts from the uridine and glucose supplemented
conditions, respectively, n0 and nf are counts from the initial and final
timepoints, respectively, and t is the assay length in days. Data analysis
and correlation analysis were performed by The PRISM Lab following
a published workflow13.
RNA extraction, reverse transcription and qPCR
qPCR was performed using the TaqMan assays (Thermo Fisher Sci-
entific). RNA was extracted from total cells grown in routine media
with a RNeasy kit (Qiagen) and digested with DNase I before murine
leukaemia virus reverse transcription using random primers (Promega)
and a CFx96 qPCR machine (Bio-Rad) using the following TaqMan
assays: Hs01066247_m1 (human UPP1), Mm00447676_m1 (mouse
Upp1), Mm01331071_m1 (mouse Upp2), Hs01117294_m1 (human MITF),
Hs01075618_m1 (human UCK1), Hs00989900_m1 (human UCK2),
Mm00550050_m1 (mouse Hmgcs2), Hs00427620_m1 (human TBP) and
Mm00782638_s1 (mouse Rplp2), Mm00434228_m1 (mouse Il-1B) and
Mm01277042_m1 (mouse Tbp). An assay for human UPP2 (Hs00542789_
m1) was tested but no amplification could be detected. Human PBMCs
and mouse BMDM data were normalized to TBP, and liver mouse data
were normalized to Rplp2, both using the ΔΔCt method. qPCR primers
for ChIP are described below.
Chromatin immunoprecipitation
MDA-MB-435S cells were washed once with PBS and fixed with 1%
formaldehyde in PBS for 15 min at room temperature. Fixation was
stopped by adding glycine (final concentration of 0.2 M) for 5 min
at room temperature. Cells were harvested by scraping with ice-cold
PBS. Cell pellets were resuspended in SDS lysis buffer (50 mM Tris-HCl,
pH 8.1, 10 mM EDTA, 1% SDS, protease inhibitor (Pierce Protease Inhibi-
tor, EDTA-Free (Thermo Fisher Scientific))), incubated for 10 min at
4 °C, and sonicated to generate DNA fragments (around 500 base pairs)
with a Qsonica Q800R2 system. Samples were centrifuged to remove
debris and diluted tenfold in immunoprecipitation dilution buffer
(16.7 mM Tris-HCl, pH 8.1, 1.2 mM EDTA, 0.01% SDS, 1.1% Triton-X100,
167 mM NaCl, protease inhibitor).
Chromatin (~50 µg) was pre-cleared with normal rabbit IgG (EMD
Millipore) and protein A/G beads (Protein A/G UltraLink Resin (Thermo
Fisher Scientific)) in low-salt buffer (20 mM Tris-HCl, pH 8.1, 2 mM
EDTA, 0.1% SDS, 150 mM NaCl, protease inhibitor) containing 0.25 mg
ml−1salmon sperm DNA and 0.25 mg ml−1 BSA for 2 h at 4 °C. Pre-cleared
chromatin was incubated with 5 µl of anti-MITF (D5G7V (Cell Signal-
ing Technology)) or 5 µg of normal rabbit IgG overnight at 4 °C (~1:10
vol:weight dilution). Samples were incubated with protein A/G beads
for another 2 h at 4 °C. Immune complexes were washed sequentially
twice with low-salt buffer, twice with high-salt buffer (20 mM Tris-HCl,
pH 8.1, 2 mM EDTA, 0.1% SDS, 500 mM NaCl, protease inhibitor), LiCl
buffer (250 mM LiCl, 1% NP40, 1% sodium deoxycholate, 1 mM EDTA,
10 mM Tris-HCl, pH 8.1, protease inhibitor) and twice with Tris-EDTA.
After washes, immune complexes were eluted from beads twice with
elution buffer (1% SDS, 10 mM dithiothreitol, 0.1 M NaHCO3) for 15 min
at room temperature. Samples were de-crosslinked by overnight incu-
bation at 65 °C and treated with proteinase K (Qiagen) for 1 h at 56 °C.
DNA was purified with QIAquick PCR purification kit (Qiagen).
qPCR using KAPA SYBR FAST One-Step RT–qPCR Kit Universal
(KAPA Biosystems) was performed to check MITF enrichments using
the following primers: UPP1-TSS (5′-TGACCTTGGGTTAGTCCTAGA-3′)
and (5′-AGCAGCCAGTTCTGTTACTC-3′); UPP1—3.5 kb (5′-AGCAA
CCTGGGAAAGTGATG-3′) and (5′-CGCCAACTCTCACTCATCATA
TAG-3′); TYR promoter (5′-GTGGGATACGAGCCAATTCGAA
AG-3′) and (5′-TCCCACCTCCAGCATCAAACACTT-3′); ACTB gene
body (5′-CATCCTCACCCTGAAGTACCC-3′) and (5′-TAGAAG
GTGTGGTGCCAGATT-3′)
Gene-specific CRISPR–Cas9 clone knockouts
To generate UPP1KO single-cell clones in MDA-MB-435S and UACC-
257 cells, a sgRNA targeting UPP1 (TTGGATTTAAAAGTCTGACG) was
ordered as complementary oligonucleotides (Integrated DNA Tech-
nologies) and cloned in pLentiCRISPRv2 (Addgene). Purified DNA
was co-transfected with a GFP-expressing plasmid in the cell lines of
interest using Lipofectamine 2000 (Thermo Fisher Scientific). After
48 h, cells were sorted using an MoFlo Astrios EQ Cell sorter and indi-
vidual cells were seeded in a 96-well plate containing routine culture
media for clone isolation. UPP1 depletion in single-cell clones was
assessed by protein immunoblotting using antibodies to UPP1. The
region corresponding to the sgRNA targeting site in the UPP1 gene
was sequenced in MDA-MB-435S using TGGGAGCAACAGGGGTTAAG
and TCAAGCATTTGTGGGTTGGTC primers and showed a homozy-
gous 1-bp deletion in clone 1, heterozygous 4-bp and 9-bp deletions
in clone 2, and heterozygous 1-bp and 2-bp insertions in clone 3. The
9-bp deletion in clone 2 is expected to produce a truncated protein
(hypomorphic allele).
To deplete the expression of ALDOA, GPI, HK2, PGM2, TKT, RPE,
UCK1, UCK2 and TYMS, two sgRNAs were cloned into pLENTICRISPRv2.
UPP1-expressing K562 cells were transduced with lentiviruses carrying
Nature Metabolism | Volume 5 | May 2023 | 765–776
772
Letterhttps://doi.org/10.1038/s42255-023-00774-2these sgRNAs, selected with puromycin and the pooled population was
analysed after 7–10 d. sgRNA sequences were: ALDOA_sg1 AATGGCGA-
GACTACCACCCA; ALDOA_sg2 AGGATGACACCCCCAATGCA; GPI_sg1
TGGGAGGACGCTACTCGCTG; GPI_sg2 TGACCCTCAACACCAAC-
CAT; HK2_sg1 CATCAAGGAGAACAAAGGCG; HK2_sg2 TTACTTTCAC-
CCAAAGCACA; PGM2_sg1
TGATTCTAGGAGCGTGAACA; PGM2_sg2 AATCCCCTGACTGA-
TAAATG; TKT_sg1 GAAACAAGCTTTCACCGACG; TKT_sg2 CCTGCC-
CAGCTACAAAGTTG; RPE_sg1 ATATCTATCTGATTAGCCCA; RPE_sg2
CCCCAGAGTCTAGCATCCGG; UCK1_sg1 TGTGTCACAAAATCATAGGT;
UCK1_sg2 CCGCTCACCCCTATCAGGAA; UCK2_sg1 TCTGCTCCGAG-
GTAAGGACA; UCK2_sg2 TACTGTCTATCCCGCAGACG; TYMS_sg1 TTC-
CAAGGGAGTGAAAATCT; TYMS_sg2 ATGTGCGCTTGGAATCCAAG.
siRNA treatment
UACC-257 and MDA-MB-435S cells were transfected with a non-targeting
siRNA (N-001206-14-05) or an siRNA targeting MITF (M-008674-0005;
Dharmacon) using Lipofectamine RNAiMAX according to the manu-
facturer’s instruction. Cells were analysed 72 h after transfection and
robust MITF knock-down was confirmed by qPCR.
Immune cell isolation and differentiation
Human THP1 cultured cell lines were differentiated in routine
medium containing 100 nM PMA (Sigma). After 2 d, the medium
was changed for medium containing 100 ng ml−1 LPS (O111:B4, Sigma,
L4391) or 1 mg ml−1 Torula yeast RNA (Sigma) and incubated for two
additional days.
Mouse BMDMs were extracted from hips, femurs and tibias of three
13-week-old C57BL/6J male mice and plated in DMEM supplemented
with 50 ng ml−1 M-CSF (ImmunoTools, 12343115), 10% heat-inactivated
FBS, 1% penicillin–streptomycin and 1% HEPES. After 3 d, the medium
was replenished with M-CSF-supplemented DMEM. On day 6, cells were
detached, counted and replated at 2 × 106 ml−1 per well of a six-well plate.
Three hours after plating, cells were further treated with 0.1 µg µl−1 LPS
O111:B4 (Sigma L4391), 1 mg ml−1 RNA (Sigma R6625) or 5 µg ml−1 R848
(Invivogen tlrl-r848) for 24 h. Cells treated with the IKK inhibitor BMS-
345541 (Merck 401480) were pre-treated with 5 µM BMS-345541 for 1.5 h
and then polarized with R848 and BMS-345541 for 24 h.
Human PBMCs were isolated from buffy coats of blood donors
from a local transfusion centre. Buffy coats were centrifuged on a
Lymphoprep (Stemcell, 07851) gradient followed by CD14+ purification
with CD14 microbeads (Miltenyi, 130050201), according to manufac-
turer’s instruction. Isolated CD14+ cells were plated in RPMI medium
supplemented with 50 ng ml−1 M-CSF (ImmunoTools, 11343113), 10%
heat-inactivated FBS, 1% penicillin–streptomycin and 1% HEPES.
After 3 d, the medium was replenished with M-CSF-supplemented
DMEM. On day 6, cells were detached, counted and replated at
1.5–2 × 106 ml−1 per well of a six-well plate. PBMC polarization was per-
formed as with BMDMs.
Genome-wide CRISPR–Cas9 screening
A secondary genome-wide CRISPR–Cas9 screening was performed
using K562 cells expressing UPP1-FLAG and a lentiviral-carried Brunello
library (Genome Perturbation Platform, Broad Institute) contain-
ing 76,441 sgRNAs44, in duplicate. Cells were infected with multiplic-
ity of infection of 0.3 and at 500 cells per sgRNA in the presence of
10 µg ml−1 polybrene (Millipore). After 24 h, cells were transferred
to culture medium containing 2 µg ml−1 puromycin (Thermo Fisher
Scientific) and incubated for an additional 48 h. On day 7, the cells were
plated in no-glucose DMEM containing 10% dialysed FBS and 100 U ml−1
penicillin–streptomycin and supplemented with 10 mM of either glu-
cose or uridine at a concentration of 105 cells per ml and with 1,000 cells
per sgRNA. Cells were passaged every 3 d for 2 weeks and, on day 21,
1,000 cells per sgRNA were harvested. DNA isolation was performed
as for the ORFeome screen.
CRISPR screen analysis was performed using a normalized z-score
approach where raw sgRNA read counts were normalized to reads
per million and then log2 transformed using the following formula:
log2(reads from an individual sgRNA / total reads in the sample 106 + 1)45.
The log2 (fold change) of each sgRNA was determined relative to the
pre-swap control. For each gene in each replicate, the mean log2 (fold
change) in the abundance of all four sgRNAs was calculated. Genes
with low expression (log2 (fragments per kilobase of transcript per
million mapped reads) < 0) according to publicly available K562
RNA-sequencing data (sample GSM854403 in Gene Expression Omni-
bus series GSE34740) were removed. log2 (fold changes) were aver-
aged by taking the mean across replicates. For each treatment, a null
distribution was defined by the 3,726 genes with lowest expression.
To score each gene within each treatment, its mean log2 (fold change)
across replicates was z-score transformed, using the statistics of the
null distribution defined as above.
Metabolite profiling (steady state)
For steady-state metabolomics of glycolytic and PPP intermediates,
an equal number of cells expressing GFP or UPP1-FLAG were washed
in PBS and pre-incubated for 24 h in no-glucose DMEM supplemented
with 10% dialysed FBS (Thermo Fisher Scientific), 100 U ml−1 penicillin–
streptomycin (Thermo Fisher Scientific) and 5 mM of glucose, galac-
tose or uridine (all from Sigma) dissolved in water, or with an equal
volume of water alone. Cells were then re-counted and 2 × 106 cells
were seeded in fresh medium of the same formulation and incubated
for two additional hours before metabolite extraction. Cells were
pelleted and immediately extracted with 80% methanol, lyophilized
and resuspended in 60% acetonitrile for intracellular LC–MS analysis.
13C5-uridine tracer on cultured cells
For tracer analysis on cultured cells, an equal number of cells expressing
GFP or UPP1-FLAG were washed in PBS and pre-incubated in no-glucose
DMEM or RPMI medium supplemented with 10% dialysed FBS (Thermo
Fisher Scientific), 100 U ml−1 penicillin–streptomycin (Thermo Fisher
Scientific) and 5 mM unlabelled uridine (all from Sigma) dissolved
in water. After 24 h, the medium was changed for the same medium
with the exception that 13C-labelled uridine ([1′,2′,3′,4′,5′-13C5] uridine,
NUC-034, Omicron Biochemicals) was used. Cells were incubated for
five additional hours before metabolite extraction. Cells were then
harvested, the medium was removed and saved, and cellular pellets
were resuspended in a 9:1 ratio (75% acetonitrile; 25% methanol:water)
extraction mixture, spun at 20,000g for 10 min, and the supernatant
was transferred to a glass sample vial for LC–MS analysis.
Animal experiments
All animal experiments in this paper were approved by the Massachu-
setts General Hospital, the University of Massachusetts Institutional
Animal Care and Use Committee, or the Swiss Cantonal authorities, and
all relevant ethical regulations were followed. All animals used were
male C57BL/6J mice purchased from The Jackson Laboratory, aged 8–13
weeks. All cages were provided with food and water ad libitum. Food
and water were monitored daily and replenished as needed, and cages
were changed weekly. A standard light–dark cycle of 12-h light exposure
was used. Animals were housed at 2–5 per cage. The temperature was
21° ± 1 °C with 55% ± 10% humidity.
13C5-uridine tracer in mice
For in vivo tracing analysis, 8- to 12-week-old C57BL/6J male mice were
fasted overnight or fed ad libitum and injected intraperitoneally with
0.2 M 13C-labelled uridine diluted in PBS to 0.4 g per kg body weight.
After 30 min, blood and livers were collected from the mice under
isoflurane anaesthesia. Liver was flash frozen in liquid nitrogen before
subsequent analysis, and blood was collected in EDTA plasma tubes,
spun and plasma was stored for further analysis. For plasma metabolite
Nature Metabolism | Volume 5 | May 2023 | 765–776
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Letterhttps://doi.org/10.1038/s42255-023-00774-2analysis, 117 µl of acetonitrile and 20 µl of LC–MS-grade water was
added to 30 µl of plasma, the mixture was vortexed and left on ice for
10 min. The samples were then spun at 21,000g for 20 min, and 100 µl of
the supernatant was transferred to a glass sample vial for downstream
LC–MS analysis.
Intracellular LC–MS analysis
For labelled and unlabelled LC–MS analysis of intracellular metabo-
lites, 5 µl of sample was loaded on a ZIC-pHILIC column (Milipore).
Buffer A was 20 mM ammonium carbonate, pH 9.6 and buffer B was
acetonitrile. For each run, the total flow rate was 0.15 ml min−1 and
the samples were loaded at 80% B. The gradient was held at 80% B for
0.5 min, then ramped to 20% B over the next 20 min, held at 20% B for
0.8 min, ramped to 80% B over 0.2 min, then held at 80% B for 7.5 min
for re-equilibration. Mass spectra were continuously acquired on a
Thermo Q-Exactive Plus run in polarity switching mode with a scan
range of 70–1000 m/z and a resolving power of 70,000 (@200 m/z).
Data were acquired using Xcalibur (v.4.1.31.9, Thermo Fisher). Data
were analysed using TraceFinder (v.4.1, Thermo Fisher) and Progenesis
(v.2.3.6275.47961) software, and labelled data were manually corrected
for natural isotope abundance.
Media/plasma LC–MS analysis
Media and plasma samples were subjected to the following LC–MS
analysis: 10 µl of sample was loaded on a BEH Amide column (Waters).
Buffer A was 20 mM ammonium acetate, 0.25% ammonium hydroxide,
5% acetonitrile, pH 9.0, while buffer B was acetonitrile. Samples were
loaded on the column and the gradient began at 85% B, 0.22 ml min−1,
held for 0.5 min, then ramped to 35% B over 8.5 min, then ramped to
2% B over 2 min, held for 1 min, then ramped to 85% B over 1.5 min and
held for 1.1 min. The flow rate was then increased to 0.42 ml min−1 and
held for 3 min for re-equilibration. Mass spectra were collected on a
Thermo Q-Exactive Plus run in polarity switching mode with a scan
range of 70–1,000 m/z and a resolving power of 70,000 (@200 m/z).
Data were acquired using Xcalibur (v.4.1.31.9, Thermo Fisher). Data
were analysed using TraceFinder (v.4.1, Thermo Fisher) and Progenesis
(v.2.3.6275.47961) software, and labelled data were manually corrected
for natural isotope abundance.
Oxygen consumption and extracellular acidification rates by
Seahorse XF analyzer
Approximately 1.25 × 105 K562 cells were plated on a Seahorse plate in
Seahorse XF DMEM medium (Agilent) supplemented with 10 mM glu-
cose, galactose, mannose or uridine, or with an equal volume of water
alone, and 4 mM glutamine (Thermo Fisher Scientific). FBS was omit-
ted. Oxygen consumption and ECARs were simultaneously recorded
by a Seahorse XFe96 analyzer (Agilent) using the Mito Stress Test pro-
tocol, in which cells were sequentially perturbed by 2 µM oligomycin,
1 µM CCCP and 0.5 µM antimycin (Sigma). Data were analysed using
the Seahorse Wave Desktop Software (v.2.6.3, Agilent). Data were not
corrected for carbonic acid derived from respiratory CO2.
Lactate determination
Lactate secretion in the culture medium was determined using a
glycolysis cell-based assay kit (Cayman Chemical). An equal number
of K562 cells expressing GFP or UPP1-FLAG were washed in PBS and
pre-incubated for 24 h in no-glucose DMEM medium supplemented
with 10% dialysed FBS (Thermo Fisher Scientific), 100 U ml−1 penicillin–
streptomycin (Thermo Fisher Scientific) and 5 mM glucose, galactose
or uridine (all from Sigma) dissolved in water, or with an equal volume
of water alone. Cells were then re-counted and seeded in fresh medium
of the same formulation and incubated for three additional hours. Cells
were then spun down and lactate concentration was determined on the
supernatants (spent media).
Gene Ontology analysis
Gene Ontology (GO) analysis was performed using GOrilla with default
settings and using a ranked gene list as input46. Only GO terms consti-
tuted of < 500 genes and scoring at FDR < 0.001 with a minimum of
two genes were considered significant and are displayed in the figures.
The complete unfiltered data can be found in Supplementary Table 1.
Gene-specific cDNA cloning and expression
cDNAs of interest were custom designed (Genewiz or IDT) and cloned
into pWPI-Neo or pLV-lenti-puro using BamHI and SpeI (New England
Biolabs).
Statistics and reproducibility
All data are expressed as the mean ± s.e.m., with the exception of oxy-
graphic data that are expressed as the mean ± s.d. All reported sample
sizes (n) represent biological replicate plates or a different mouse.
All attempts at replication were successful. All Student’s t-tests were
two sided. Statistical tests were performed using Microsoft Excel and
GraphPad Prism 9.
Reporting summary
Further information on research design is available in the Nature Port-
folio Reporting Summary linked to this article.
Data availability
All data generated or analysed during this study are included in the
article and its Supplementary Information. Results of the ORFeome,
the CRISPR–Cas9 and the PRISM screens are available in Supplementary
Table 1. Data from the Cancer Cell Line Encyclopedia are available at
https://depmap.org/portal/. Source data are provided with this paper.
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Acknowledgements
The authors thank T. Ast (Broad Institute), P. Broz (University of
Lausanne), O. Goldberger (Massachusetts General Hospital),
S. Luther (University of Lausanne), M. Miranda (Massachusetts
General Hospital), M. Rebsamen (University of Lausanne),
M. Ronan (Broad Institute), D. Rosenberg (Broad Institute), R. Sharma
(Massachusetts General Hospital) and T.L To (Broad Institute) for their
help and for sharing reagents. This work was supported by National
Institutes of Health grants R35GM122455 (to V.K.M.), F32GM133047
(to O.S.S.), DK115881 (to R.P.G.), R01AR043369-24 (to D.E.F.),
P01CA163222-07 (to D.E.F.), K99/R00 GM124296 (to H.S.), an SNF
Project Grant 310030_200796 (to A.A.J.), a grant from the Dr. Miriam
and Sheldon Adelson Medical Research Foundation (to D.E.F.) and
a J. Bolyai Research Scholarship of the Hungarian Academy of
Sciences and a grant from the National Research, Development and
Innovation Office OTKA FK138696 (to L.V.K.). V.K.M. is an Investigator
of the Howard Hughes Medical Institute.
Nature Metabolism | Volume 5 | May 2023 | 765–776
775
Letterhttps://doi.org/10.1038/s42255-023-00774-2Author contributions
O.S.S., J.B.-F., L.J.-C., A.K., L.V.K., H.S., R.P.G. and A.A.J. performed the
experiments; M.G.R. and J.A.R. supervised L.J.-C.; D.E.F. supervised
A.K. and L.V.K.; A.A.J. supervised J.B.-F.; V.K.M. supervised O.S.S., and
H.S., R.P.G. and A.A.J. until independence; A.A.J. and V.K.M. designed
the project; A.A.J. and V.K.M. wrote the manuscript with input from
all authors.
Correspondence and requests for materials should be addressed to
Vamsi K. Mootha or Alexis A. Jourdain.
Peer review information Nature Metabolism thanks the anonymous
reviewers for their contribution to the peer review of this work. Primary
Handling Editor: Alfredo Giménez-Cassina, in collaboration with the
Nature Metabolism team.
Funding
Open access funding provided by University of Lausanne.
Reprints and permissions information is available at
www.nature.com/reprints.
Competing interests
V.K.M. is a paid scientific advisor to 5AM Ventures. O.S.S. was
a paid consultant for Proteinaceous Inc. D.E.F. has a financial
interest in Soltego, a company developing salt-inducible kinase
inhibitors for topical skin-darkening treatments that might be
used for a broad set of human applications. The interests of D.E.F.
were reviewed and are managed by Massachusetts General
Hospital and Partners HealthCare in accordance with their
conflict-of-interest policies. The remaining authors declare no
competing interests.
Additional information
Extended data is available for this paper at
https://doi.org/10.1038/s42255-023-00774-2.
Supplementary information The online version
contains supplementary material available at
https://doi.org/10.1038/s42255-023-00774-2.
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org/licenses/by/4.0/.
© The Author(s) 2023
1Broad Institute of MIT and Harvard, Cambridge, MA, USA. 2Department of Molecular Biology and Howard Hughes Medical Institute, Massachusetts
General Hospital, Boston, MA, USA. 3Department of Systems Biology, Harvard Medical School, Boston, MA, USA. 4Department of Immunobiology,
University of Lausanne, Epalinges, Switzerland. 5Cutaneous Biology Research Center, Department of Dermatology, Massachusetts General
Hospital, Harvard Medical School, Charlestown, MA, USA. 6Department of Dermatology, Venereology and Dermatooncology, Faculty of Medicine,
Semmelweis University, Budapest, Hungary. 7Present address: Liver Center, Division of Gastroenterology, Massachusetts General Hospital,
Boston, MA, USA. 8Present address: Cellular and Molecular Physiology, Yale School of Medicine, New Haven, CT, USA. 9Present address: Yale Systems
Biology Institute, Yale West Campus, West Haven, CT, USA. 10These authors contributed equally: Owen S. Skinner, Joan Blanco-Fernández.
e-mail: [email protected]; [email protected]
Nature Metabolism | Volume 5 | May 2023 | 765–776
776
Letterhttps://doi.org/10.1038/s42255-023-00774-2a
)
1
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All ORFs
UPP1 ORFs
UPP2 ORFs
15
10
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0
15
10
5
0
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2
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y
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R = 0.88
R = 0.91
5
0
15
Day 21, glucose, rep1 (log2 TPM+1)
10
0
15
Day 21, galactose, rep1 (log2 TPM+1)
10
5
10
5
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-5
-10
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R = 0.16
-5
-10
0
Day 21, glucose, rep1 (log2 fold of day 0)
10
5
10
5
0
-5
-10
R = 0.18
-5
-10
0
Day 21, galactose, rep1 (log2 fold of day 0)
10
5
UPP1
TRCN0000487722
Glucose
Galactose
b
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l
l
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m
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p
d
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e
R
10000
8000
6000
4000
2000
UPP1
TRCN0000488151
1500
1000
500
0
UPP1
TRCN0000470579
100
90
80
70
60
50
7
14
Time (days)
21
0
7
14
Time (days)
21
0
7
14
Time (days)
21
UPP2
TRCN0000489731
UPP2
TRCN0000488047
UPP2
TRCN0000472567
50
40
30
20
10
0
200
150
100
50
0
7
14
Time (days)
21
0
7
14
Time (days)
21
0
7
14
Time (days)
21
0
0
1000
800
600
400
200
0
0
n
o
i
l
l
i
m
r
e
p
d
a
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R
c
d
kDa
49
49
Glucose
- Uridine
p-S6
S6
ORFs enriched in galactose (relative to glucose)
ORFs depleted in galactose (relative to glucose)
uridine metabolic process
uridine catabolic process
UMP salvage
pyrimidine ribonucleotide salvage
pyrimidine nucleotide salvage
pyrimidine ribonucleoside monophosphate biosynthetic process
UMP biosynthetic process
pyrimidine nucleoside salvage
pyrimidine-containing compound salvage
pyrimidine nucleoside monophosphate biosynthetic process
regulation of acyl-CoA biosynthetic process
regulation of acetyl-CoA biosynthetic process from pyruvate
regulation of sulfur metabolic process
hexose metabolic process
monosaccharide metabolic process
regulation of glucose metabolic process
regulation of cellular carbohydrate metabolic process
protein autophosphorylation
peptidyl-serine phosphorylation
peptidyl-serine modification
5
0
15
log2 fold of enrichment
10
0 2 4 6 8 10
log2 fold of enrichment
Extended Data Fig. 1 | Additional analysis of the ORF screen. (a) Replicate
plot and Pearson correlation (R) of n = 2 replicate ORF screens in glucose and
galactose highlighting UPP1 and UPP2 ORFs shown as log2 TPM + 1, or log2 fold
of day 0. (b) Representation of all six UPP ORFs, expressed as read per million
in the global population of glucose or galactose-grown cells and as a function
of time (n = 2). TRCN0000470579 encodes a splice variant of UPP1 that is
N-terminal truncated and lacks the uridine binding site (NM_001287428.2).
(c) Top 10 ontologies associated with the ORFs enriched and depleted in
galactose relative to glucose. The complete gene ontology analysis is reported in
the Supplementary Data Table 1. (d) Protein immunoblot of K562 cells expressing
UPP1-FLAG grown for 16 h in sugar-free media supplemented with 10 mM of
glucose or uridine and immunolabelled with antibodies to total S6 ribosomal
protein and phosphorylated S6 ribosomal protein (p-S6). Representative of n = 2
experiments. Total S6 loading control was performed on the same gel. All growth
assays and screens included 4mM L-glutamine and 10% dialyzed FBS.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
Extended Data Fig. 2 | Genome-wide CRISPR–Cas9 screen and gene ontology
analysis in glucose and uridine. (a) Replicate plot and Pearson correlation
(R) analysis of n = 2 replicate genome-wide CRISPR–Cas9 screens in glucose
and uridine highlighting genes of de novo pyrimidine synthesis, glycolysis and
the PPP. Data are shown as log2 TPM + 1, or log2 fold of day 7. Top: Data shown
at the individual sgRNA level. Bottom: Data shown at the gene level. (b) Top 10
ontologies associated with the genes enriched and depleted in uridine relative
to glucose. Only 8 terms scored for the analysis of essential genes in uridine. The
complete gene ontology analysis is reported in the Supplementary Data Table 1.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2a
)
O
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Glucose
Uridine
b
3
-
0
1
x
2
.
2
<
P
6
-
0
1
x
9
.
4
<
P
1.2
1.0
0.8
0.6
0.4
0.2
0.0
DMSO OT
Brequinar
Uridine salvage pathway
UCK1/2
Uridine
UMP
TYMS
TMP
Control sgRNAs
HK2 sgRNAs
HK2
Actin
Control sgRNAs
RPE sgRNAs
RPE
TUBB
kDa
64
37
kDa
64
49
Control sgRNAs
GPI sgRNAs
GPI
Actin
Control sgRNAs
PGM2 sgRNAs
PGM2
TUBB
kDa
37
37
kDa
26
37
kDa
115
37
kDa
26
49
ALDOA sgRNAs
Control sgRNAs
kDa
Control sgRNAs
TKT sgRNAs
ALDOA
64
Actin
49
TKT
TUBB
Control sgRNAs
UCK2 sgRNAs
UCK2
Actin
kDa
37
37
Control sgRNAs
TYMS sgRNAs
TYMS
Actin
Extended Data Fig. 3 | CRISPR–Cas9 screen validation. (a) Differential
sensitivity to small molecule inhibitors of the PPP (OT: oxythiamine) or de novo
pyrimidine synthesis (brequinar) in glucose vs uridine in UPP1-FLAG expressing
K562 (n = 3, P < 2.2 × 10−3, P < 4.9 × 10−6) after 4 days, reported as fold of DMSO.
Data are shown as ±SEM with two-sided t-test relative DMSO. (b) Immunoblot
analysis of proteins from upper glycolysis, the PPP and pyrimidine salvage in
UPP1-expressing K562 cells treated with their corresponding sgRNAs. UCK1
is expressed at low levels in K562 cells and its protein could not be detected.
Representative of n = 2 experiments. (c) Simplified representation of the uridine
salvage pathway and thymidine synthesis. TUBB and Actin loading controls
were performed on the same gels. All growth assays and screens included 4mM
L-glutamine and 10% dialyzed FBS.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
a
Pentose phosphate pathway
5
-
0
1
x
2
.
2
<
P
1.5 109
Ribose-P
)
.
U
A
.
(
y
t
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s
n
e
n
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I
1 109
5 108
0
-
Glucose
Galactose
Uridine
2.5 108
Glucose-6-P
4
-
0
1
x
3
.
2
<
P
Uridine
-
Glucose
Galactose
4 107
3 107
2 107
1 107
0
5 107
4 107
3 107
2 107
1 107
0
Sedoheptulose-7-P
6-P-Gluconate
4
-
0
1
x
1
.
8
<
P
5 108
4 108
3 108
2 108
1 108
0
-
Glucose
Galactose
Uridine
Glycolysis
-
Glucose
Galactose
Uridine
Fructose-1,6-BP
4
-
0
1
x
9
.
2
<
P
6 107
4 107
2 107
0
-
Glucose
Galactose
Uridine
Dihydroxyacetone-P
2
-
0
1
x
4
.
1
<
P
Uridine
-
Glucose
Galactose
Liver uridine
Blood uridine
Fed
Fasted
8 108
6 108
4 108
2 108
0
M+0
M+1
M+2
M+3
M+4
M+5
M+0
M+1
M+2
M+3
M+4
M+5
e
)
l
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%
(
n
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1
18
12
6
0
1 107
8 106
6 106
4 106
2 106
0
4 107
3 107
2 107
1 107
0
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UPP1-FLAG
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1
x
7
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P
Erythrose-4-P
4
-
0
1
x
4
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Glucose
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Uridine
Glyceraldehyde-3-P
5
-
0
1
x
3
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9
<
P
-
Glucose
Galactose
Uridine
Fed
Fasted (12h)
Fasted (24h)
Glucose
Lactate
Ribose-P
Upp1
Upp2 Hmgcs2
Liver ribose-P
Blood lactate
Blood glucose
6 106
4 106
2 106
0
M+5
M+4
M+3
M+2
M+1
M+0
M+1
M+2
M+3
M+4
M+5
Liver ribose-P
1 107
8 106
6 106
4 106
2 106
0
)
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U
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(
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2 109
1 109
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8 109
6 109
4 109
2 109
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2 108
1.5 108
1 108
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7 108
6 108
5 108
4 108
3 108
2 108
1 108
0
2.5 107
2 107
1.5 107
1 107
5 106
0
M+6
M+5
M+4
M+3
M+2
M+1
M+0
M+6
M+5
M+4
M+3
M+2
M+1
Blood lactate
1.5 108
Blood glucose
1.5 109
1.5 107
1 108
5 107
0
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M+2
M+3
M+0
M+1
M+2
M+3
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U
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5 108
1 107
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M+2
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M+3
M+4
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Extended Data Fig. 4 | See next page for caption.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
Extended Data Fig. 4 | Additional metabolomics analysis. (a) Steady-state
abundance of representative intracellular metabolites from the pentose
phosphate pathway (PPP) and glycolysis in sugar-free media complemented
with 10 mM glucose, 10 mM galactose or 10 mM uridine, in the presence of 4mM
L-glutamine and 10% dialyzed FBS (n = 3 replicate wells, P < 2.2 × 10−5, P < 8.1 × 10−4,
P < 4.7 × 10−7, P < 1.4 × 10−4). Data are shown as mean ± SEM with two-sided t-test
relative to control cells. (b) 13C5-uridine tracer analysis of liver and blood uridine
30 min after intraperitoneal injection in fed or overnight fasted mice with
0.4 g/kg 13C5-uridine (n = 3 replicate wells, P < 2.3 × 10−4, P < 2.9 × 10−4, P < 1.4 × 10−2,
P < 9.3 × 10−5). Data are shown as mean ± SEM (c) 13C5-uridine tracer analysis of
liver ribose-phosphate (ribose-P) and circulating lactate and glucose 30 min after
intraperitoneal injection in overnight fasted and (d) in fed animals with 0.4 g/kg
13C5-uridine. Data are shown as mean ± SEM and are corrected for natural isotope
abundance (n = 4 mice in each group). (e) 13C5-uridine tracer analysis of liver
ribose-phosphate, blood lactate and blood glucose 30 min after intraperitoneal
injection in fed mice with 0.4 g/kg 13C5-uridine shown as the percentage
of 13C-labeled intermediate compared to the total pool. Data are shown as
mean ± SEM and are corrected for natural isotope abundance (n = 4 mice). See
also (f) qPCR determination in the liver of ad libitum fed mice, or fasted for 12 h
or 24 h, with probes to Upp1, Upp2 and Hmgc2. Hmgc2 transcripts are expected to
increase with fasting. Data are shown as mean ± SEM (n = 3 mice in each group).
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-22
p
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All cell lines
Melanoma
Glioma
R = 0.90
5
0
10
Cell line representation in glucose, rep1
(log2 TPM+1)
20
15
3
p
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s
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R = 0.75
5
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20
10
Cell line representation in uridine, rep1
(log2 TPM+1)
15
20
15
10
5
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15
10
5
0
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+
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R = 0.91
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Cell line representation in glucose, rep1
(log2 TPM+1)
20
15
n
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R = 0.74
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Cell line representation in uridine, rep1
(log2 TPM+1)
15
20
15
10
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R = 0.89
5
0
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Cell line representation in glucose, rep2
(log2 TPM+1)
15
20
R = 0.75
5
0
20
10
Cell line representation in uridine, rep2
(log2 TPM+1)
15
Extended Data Fig. 5 | PRISM screen replicate analysis. Replicate plot and correlation analysis of n = 3 replicate PRISM screens in glucose and uridine highlighting
the melanoma and glioma lineages and showing Pearson correlation between replicates (R).
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
a
)
1
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UPP1
Urinary Tract
Pancreas
Esophagus
Thyroid
Bile Duct
Kidney
Uterus
Engineered
Cervix
Lung
Liver
Gastric
Prostate
Ovary
Eye
Fibroblast
Colorectal
Soft Tissue
Breast
Plasma Cell
Lymphocyte
Embryo
Skin
Central Nervous System
Upper Aerodigestive
Epidermoid Carcinoma
293T
K562
HeLa
A375
LOX-IMVI
SH4
A2058
SK-MEL-30
UACC-257
SK-MEL-5
UACC-62
MDA
WT
kDa UPP1
UPP1
WT
UPP1KO
UPP1KO
UPP1KO
Blood
Bone
Adrenal Cortex
Peripheral Nervous System
4
-
0
1
x
3
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1
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3
-
0
1
x
5
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1
<
P
3
-
0
1
x
5
.
4
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P
c
UACC-257
d
MDA-MB-435S
UPP1
Actin
MDA-MB-435S
WT
UPP1
UPP1
WT
UPP1KO
UPP1hypo
UPP1KO
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-3
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Glucose
Uridine
-
RNA
UPP1
Actin
WT
UPP1
UPP1
WT
UPP1KO
UPP1hypo
UPP1KO
UACC-62
UACC-257
MDA-MB-435S
UPP1
TYR
TUBB
MITF
MLANA
TUBB
25
37
kDa
25
37
b
kDa
25
100
50
50
15
50
Extended Data Fig. 6 | UPP1, UPP2 and MITF expression across the CCLE
collection and in melanoma. (a) UPP1, UPP2 and MITF expression across
the complete CCLE collection (n = 30 lineages). (b) Protein immunoblot of a
panel of melanoma (n = 9) and non-melanoma (n = 3, 293T, K562 and HeLa)
cell lines grown and showing expression of UPP1 as well as MITF, TYR and
MLANA, three melanoma markers. MDA: MDA-MB-435S. (c) Top: Immunoblot
of UACC-257 melanoma cells with wild-type (UPP1WT) and knock-out (UPP1KO).
Bottom: Immunoblot of MDA-MB-435S melanoma cells in wild-type (UPP1WT),
knock-out (UPP1KO) and hypomorphic (UPP1hypo) clones (see methods). (d) Cell
growth assay of MDA-MB-435S clones in sugar-free media containing dialyzed
FBS complemented with 10 mM of either glucose or uridine and dialyzed FBS.
Negative doublings indicate cell death. Data are shown as mean ± SEM with
two-sided t-test relative to UPP1WT cells in the same media (n = 3 replicate wells,
P < 2.9 × 10−6, P < 1.3 × 10−5, P < 3.1 × 10−7). (e) Cell growth assay of three melanoma
cell lines with high UPP1 expression in sugar-free media complemented with
10 mM of glucose or 0.5 mg/mL of RNA. Data are shown as mean ± SEM with two-
sided t-test relative to UPP1WT and were not corrected for multiple comparison
(n = 3 replicate wells, P < 4.5 × 10−3, P < 1.3 × 10−4, P < 1.5 × 10−3. TUBB and Actin
loading controls were performed on the same gels. All growth assays and screens
included 4mM L-glutamine and 10% dialyzed FBS.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2UPP2MITF
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UPP1
TYRO
TUBB
MITF
MLANA
TUBB
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c
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25
100
50
50
15
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Extended Data Fig. 7 | MITF promotes UPP1 expression and growth on
uridine in melanoma cells. (a) Protein immunoblot and (b) cell growth assay of
LOX-IMVI in sugar-free media supplemented with 10 mM of glucose or 10 mM of
uridine. LOX-IMVI is a melanoma cell line with low endogenous MITF expression
and over-expressing MITF or a control gene. Data are shown as mean ± SEM
(n = 3, P < 6.6 × 10−5). P values were calculated using a two-sided student T test.
Statistics were not adjusted for multiple comparison. (c) MITF occupancy in
UPP1 transcription start site (TSS) and promoter (a region 3.5 kb away from the
TSS), as determined by ChIP-Seq in COLO829 melanoma cells16. (d) ChIP-qPCR
validation of MITF binding in UPP1 promoter and TSS in MDA-MB-435S
melanoma cells. TYR is a known transcriptional target of MITF, ACTB is not. Data
are shown as mean ± SEM (n = 3, P < 1.1 × 10−1, P < 2.4 × 10−2, P < 2.9 × 10−2) with
two-sided t-test relative to control IgG. (e) qPCR analysis of five melanoma cells
after treatment with MITF siRNA (n = 3, P < 7.0 × 10−3, P < 9.6 × 10−4, P < 1.3 × 10−2,
P < 3.6 × 10−5). Data are shown as mean ± SEM with two-sided t-test relative to the
indicated control. TUBB loading controls were performed on the same gels. All
growth assays and screens included 2 mM (RPMI) or 4 mM (DMEM) L-glutamine
and 10% dialyzed FBS.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
a
kDa
25
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Extended Data Fig. 8 | UPP1 expression and uridine catabolism for energy
production in the monocytic lineage. (a) Protein immunoblot of human
THP1 monocytic cells treated with 100 nM PMA alone for 48 h followed by the
addition of 100 ng/mL LPS or 1 mg/mL purified yeast RNA for another 48 h and
immunoblotted with antibodies to UPP1 and TUBB. Western blot quantification is
shown as fold of untreated cells (monocytes). (b) Protein immunoblot of human
MCSF-matured PBMC treated with 5 µg/ml R848 for 24 h and immunoblotted
with antibodies to UPP1 and Actin (n = 3 donors). Western blot quantification
is shown as fold of untreated cells and relative to each donor. (c) Expression of
UCK1 and UCK2 in THP1 cells as determined by qPCR after treatment as in (a).
Data are shown as mean ± SEM with n = 4. (d) 13C5-uridine tracer analysis of UMP
in THP1 cells treated as in (a) with the exception that glucose-free RPMI media
containing 5 mM 13C5-uridine was used. Data are shown as mean ± SEM with
n = 4. (e) Expression of Upp1 and Il1b in BMDM as determined by qPCR after co-
treatment for 24 h with 5 µg/ml R848 and 5 µg/ml BMS-345541, an IKK inhibitor.
Data are shown as mean ± SEM with two-sided t-test relative to untreated (n = 3
mice, P < 1.8 × 10−2, P < 4.5 × 10−2). (f) 13C5-uridine tracer analysis of citrate and
lactate in THP1 cells treated as in (a) with the exception that glucose-free RPMI
media containing 5 mM 13C5-uridine was used for the last 6 h. Data are shown as
mean ± SEM with two-sided t-test relative to PMA-treated cells with n = 4, P < 4.7
× 10−4, P < 1.5 × 10−2, P < 1.2 × 10−2, P < 2.2 × 10−2. (g) 13C5-uridine tracer analysis of
media lactate in human MCSF-matured PBMC cells treated as in (b) with the
exception that glucose-free RPMI media containing 5 mM 13C5-uridine was used
was used for the last 6 h. Data are shown as mean ± SEM, with n = 4 donors. TUBB
and Actin loading controls were performed on the same gels. All metabolomics
included 2 mM (RPMI) or 4 mM (DMEM) L-glutamine and 10% dialyzed FBS.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
Control
O
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CoQred
complex III
OXPHOS
Extended Data Fig. 9 | Additional bioenergetics measurement in uridine
or mannose-grown cells. (a) Representative oxygen consumption rate (OCR)
and extracellular acidification rate (ECAR) in control and UPP1-expressing
K562 cells grown in sugar-free media or in media supplemented with 10 mM
of glucose, galactose or uridine, all in the presence of glutamine and dialyzed
FBS (n = 30 replicate wells). O: oligomycin. C: CCCP. A: antimycin A. Data are
shown as mean ± SD with n > 6 replicate wells. (b) Total consumption of uridine
in UACC-257 melanoma cells treated with antimycin A (100 nM) in the same
media. Data are shown as mean ± SEM with n = 4. (c) Representative extracellular
acidification rate (ECAR) in cells in 10 mM of glucose or mannose and treated as
in (a). (d) Schematic representation of de novo pyrimidine synthesis and uridine
auxotrophy during OXPHOS inhibition. CoQ: co-enzyme Q. Ox: oxidized. Red:
reduced. All experiments included 4mM L-glutamine and 10% dialyzed FBS.
Nature Metabolism
Letterhttps://doi.org/10.1038/s42255-023-00774-2
| null |
10.1371_journal.pcbi.1010923.pdf
|
uction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files. RNA-seq of samples of human
left ventricle heart, left atrial appendage, aorta, tibial
arteries, and coronary arteries were downloaded
from the GTEx project [29] via dbGaP (phs000424.
v8.p2). In addition, we downloaded RNA-seq
datasets of dilated cardiomyopathy (DCM),
ischemic cardiomyopathy (ICM), and controls
(SRA database, accession code GSE116250) [40].
The ribosome profiling dataset of DCM patients
was downloaded from the SRA database
(GSE131111) [47]. finally, we downloaded
Adenosine-to-inosine RNA editing is essential to prevent undesired immune activation. This
diverse process alters the genetic content of the
| null |
RESEARCH ARTICLE
Increased A-to-I RNA editing in atherosclerosis
and cardiomyopathies
Tomer D. MannID
1,2☯, Eli Kopel1☯, Eli EisenbergID
3*, Erez Y. Levanon1,4*
1 Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat Gan, Israel, 2 Tel Aviv
Sourasky Medical Center, Tel Aviv, Israel, 3 Raymond and Beverly Sackler School of Physics and Astronomy
and Sagol School of Neuroscience, Tel Aviv, University, Tel Aviv, Israel, 4 The Institute of Nanotechnology
and Advanced Materials, Bar-Ilan University, Ramat Gan, Israel
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
☯ These authors contributed equally to this work.
* [email protected] (EE); [email protected] (EYL)
Abstract
OPEN ACCESS
Citation: Mann TD, Kopel E, Eisenberg E, Levanon
EY (2023) Increased A-to-I RNA editing in
atherosclerosis and cardiomyopathies. PLoS
Comput Biol 19(4): e1010923. https://doi.org/
10.1371/journal.pcbi.1010923
Editor: Ananda Mondal, Florida International
University, UNITED STATES
Received: June 21, 2022
Accepted: February 5, 2023
Published: April 10, 2023
Copyright: © 2023 Mann et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files. RNA-seq of samples of human
left ventricle heart, left atrial appendage, aorta, tibial
arteries, and coronary arteries were downloaded
from the GTEx project [29] via dbGaP (phs000424.
v8.p2). In addition, we downloaded RNA-seq
datasets of dilated cardiomyopathy (DCM),
ischemic cardiomyopathy (ICM), and controls
(SRA database, accession code GSE116250) [40].
The ribosome profiling dataset of DCM patients
was downloaded from the SRA database
(GSE131111) [47]. finally, we downloaded
Adenosine-to-inosine RNA editing is essential to prevent undesired immune activation. This
diverse process alters the genetic content of the RNA and may recode proteins, change
splice sites and miRNA targets, and mimic genomic mutations. Recent studies have associ-
ated or implicated aberrant editing with pathological conditions, including cancer, autoim-
mune diseases, and neurological and psychiatric conditions. RNA editing patterns in
cardiovascular tissues have not been investigated systematically so far, and little is known
about its potential role in cardiac diseases. Some hints suggest robust editing in this system,
including the fact that ADARB1 (ADAR2), the main coding-sequence editor, is most highly
expressed in these tissues. Here we characterized RNA editing in the heart and arteries and
examined a contributory role to the development of atherosclerosis and two structural heart
diseases -Ischemic and Dilated Cardiomyopathies. Analyzing hundreds of RNA-seq sam-
ples taken from the heart and arteries of cardiac patients and controls, we find that global
editing, alongside inflammatory gene expression, is increased in patients with atherosclero-
sis, cardiomyopathies, and heart failure. We describe a single recoding editing site and sug-
gest it as a target for focused research. This recoding editing site in the IGFBP7 gene is one
of the only evolutionary conserved sites between mammals, and we found it exhibits consis-
tently increased levels of editing in these patients. Our findings reveal that RNA editing is
abundant in arteries and is elevated in several key cardiovascular conditions. They thus pro-
vide a roadmap for basic and translational research of RNA as a mediator of atherosclerosis
and non-genetic cardiomyopathies.
Author summary
The human genetic code is highly preserved, yet RNA editing is a process that changes it
in RNA molecules. This may alter the resultant proteins or the properties of the RNA
strands themselves, leading to changes in their special structure and affecting their inter-
action with the immune system. Unedited RNA may be highly immunogenic, and studies
of recent years demonstrated that dysregulated editing causes an inflammatory response.
PLOS Computational Biology | https://doi.org/10.1371/journal.pcbi.1010923 April 10, 2023
1 / 18
PLOS COMPUTATIONAL BIOLOGYadditional RNA-seq datasets of ICM from the left
and right ventricles of left-sided heart failure,
biventricular heart failure, and controls (SRA
database accession code GSE120852) [65]. Tables
with raw data used for figure plotting can be found
in S4 Table.
Funding: E.E and E.Y.L are supported by the Israel
Science Foundation (ISF grants 2673/17 and 1945/
18 to E.E and 231/21 to E.Y.L). https://www.isf.org.
il/#/. The funders had no role in study design, data
collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests: The authors declare that the
research was conducted in the absence of any
commercial or financial relationships that could be
construed as a potential conflict of interest.
Increased A-to-I RNA editing in atherosclerosis and cardiomyopathies
We show that human RNA editing is especially active in the arteries and investigate its
role in cardiovascular diseases such as atherosclerosis and cardiomyopathies. These dis-
eases have an underlying inflammatory component that eventually leads to the loss of nor-
mal structure and function. We show that RNA editing is increased in patients and draw a
line connecting it with a concomitant increase in inflammatory pathways. We also
detected specific coding editing sites, mainly in the IGFBP7 gene, where aberrant editing
is present, resulting in an altered protein product. We delineate these changes and suggest
they may contribute to or are a marker of the underlying pathology. Our findings indicate
that RNA editing is a key player in several cardiovascular diseases.
Introduction
RNA editing is an endogenous post-transcriptional process, catalyzed by the Adenosine deam-
inases acting on RNA (ADAR1 and ADAR 2) enzymes. ADAR1 has 2 isoforms, p110 and p150
which is interferon- inducible. As a result of editing, the RNA sequence is modified from its
genomic blueprint, and genomically-encoded adenosines on the RNA molecule are deami-
nated into inosines [1,2].
Editing occurs in both coding and non-coding sequences of the genome. Editing of a cod-
ing sequence may result in amino-acid substitution in the protein product ("recoding"), similar
to genomic mutations. Some recoding events have an enormous functional impact. For exam-
ple, recoding of the Q/R site within the Glutamate ionotropic receptor AMPA type subunit 2
(GRIA2) gene is essential to prevent an inborn fatal phenotype in mice [3,4], and editing of the
Antizyme inhibitor 1 (AZIN1) site induces an S/G change, which drives the development of
hepatocellular carcinoma [5].
However, virtually all the editing occurs in non-coding repetitive elements, e.g., the human
Alu sequences (Alu editing). The current view is that the primordial role of RNA editing is to
prevent the identification of endogenous double-stranded RNA structures by the innate
immune system. Such structures can be formed when two repetitive Alu elements reside close
to one another in the same RNA transcript but with the opposite orientation. Double-stranded
RNA molecules are potent immune stimulators that the intracellular sensor melanoma differ-
entiation-associated protein 5 (MDA5) and mitochondrial antiviral signaling protein (MAVS)
recognize [6].
Editing introduces mismatches between the two RNA strands and disrupts otherwise per-
fect complementation, the hallmark of viruses [7–9]. This enables escape from immune system
activation (S1 Fig in the Supplementary Material). The association between RNA editing and
disease has been studied in recent years, particularly in cancer, neurology, and autoimmunity
[5,7,10–12]. In cancer, inhibition of ADAR augments the effects of checkpoint inhibitors and
is under development as a drug [13].
Exceptionally high levels of A-to-I editing were demonstrated in arteries [14–16], and this
observation calls for a better understanding of the cardiovascular (CV) editome in homeostasis
and disease. We chose to focus on atherosclerotic cardiovascular disease (ASCVD) and several
key cardiomyopathies (CMPs) to explore the possible pathogenic role of disturbed RNA
editing.
atherosclerotic-related diseases such as ischemic heart disease (IHD) and cardiomyopathy
(ICM), are the most common causes of morbidity and mortality in the western world [17].
They are characterized by narrowing of the heart’s vasculature, structural heart changes and
PLOS Computational Biology | https://doi.org/10.1371/journal.pcbi.1010923 April 10, 2023
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
reduced cardiac function, and may culminate in sudden cardiac death or terminal heart
failure.
It is increasingly appreciated that atherosclerosis is also a result of vascular inflammation
driven by the IFN, IL-1-IL-6 pathway. A number of large-scale clinical trials [18,19], demon-
strated the benefit of anti-inflammatory treatments in atherosclerosis, supporting the inflam-
matory hypothesis. Current evidence suggests that myocardial remodeling occurs in response
to ischemia, and inflammation may therefore contribute to ischemic cardiomyopathy forma-
tion, thereby propagating atherosclerosis. In addition, inflammation also mediates insult-
related repair in ischemic cardiomyopathy, dilated cardiomyopathy, and other cardiomyopa-
thies, since pathological remodeling and scar formation are regulated by inflammatory signals.
Accordingly, in parallel to studying the genetic factors underlying atherosclerosis and coro-
nary artery disease, an understanding of the molecular mechanisms that contribute to vascular
inflammation may provide additional actionable targets for clinical treatments.
A very recent, pivotal work by Li et al [20] has demonstrated that coronary artery disease
(CAD) is heavily influenced by editing and that a genetic tendency to under-edit immuno-
genic Alus is associated with heightened interferon activity and disease prevalence. Common
genetic variants that are associated with RNA editing were recognized to be enriched in
GWAS signals of autoimmune and immune-related diseases, including CAD, and accounted
for a high fraction of disease heritability.
We hypothesized that since RNA editing is associated with inflammation, aberrant editing
might contribute to the development of either atherosclerosis or non-genetic cardiomyopa-
thies. We additionally speculated that a single editing disruption at a critical recoding site may
adversely affect cellular function and lead to a disease, as reported with FLNA editing and car-
diomyopathy [14].
Here we analyzed RNA sequencing data from the GTEx project, encompassing thousands
of human samples, as well as additional datasets of cardiomyopathy patients. We observed an
increase in both intronic and recoding editing in atherosclerosis patients, with a particular
increase in Insulin-like growth factor binding protein 7 (IGFBP7) gene editing. Similarly, we
observed increases in intronic editing in patients with various types of cardiomyopathies. We
have confirmed that these pathologies are accompanied by increased expression of inflamma-
tory genes [21]. Finally, we discuss the possibility that interferon-related inflammation leads to
enhanced ADAR1 p150 expression, resulting in elevated editing in cardiovascular pathologies.
Methods
Construction of the GTEx patient groups
GTEx patients were classified according to their medical records and the medical examiners’
diagnoses. The atherosclerosis group contained all patients who died from or had a history of
myocardial infarction or ischemic heart disease. The cerebrovascular group contained patients
who either died from or had a history of cerebrovascular accident (CVA), intracranial hemor-
rhage (ICH), or cerebral aneurysm. The control group contained all other individuals not
included in the atherosclerosis and cerebrovascular groups, with the exception of patients who
died from an overwhelming infection who were removed from the analysis.
RNA-seq data preprocessing and quality control
Transcriptomic data quality was evaluated using the FastQC quality control tool version 0.10.1
[22]. The reads were mapped uniquely (outFilterMultimapNmax = 1) to the human reference
genome version GRCh38/ hg38 using STAR aligner version 2.5.2b [23].
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Gene expression quantification
We used Salmon tool version 0.11.2 to quantify the transcript levels of the RNA-seq data in
units of transcripts per million (TPM) [24] using default parameters and indexed human
genome version GRCh38/hg38. We then converted the output from transcript to gene-level
using “tximport” R package version 1.12.3.
Global RNA editing levels in Alu repetitive elements
Most editing in humans takes place within Alu repeats [25]. To measure the editing in Alu ele-
ments, we used the Alu Editing Index (AEI) method version 1.0 [15]. Briefly, the global RNA-
editing index was calculated as the ratio of guanosine at all the adenosine positions in Alu over
adenosine and guanosine at the same genomic locus. A higher index indicates a greater per-
centage of editing activity in a sample. To clean the editing signal further, we analyzed an addi-
tional subset of 3,031 genomic Alus that are located within 3’ UTR of genes, have a minimum
length of 250bp, and have an oppositely oriented Alu on the same 3’ UTR.
RNA editing quantification in coding editing sites
We used the REDIToolKnown script to measure A-to-I RNA editing levels in coding editing
sites [26]. The script quantifies the editing levels in a set of 314 previously defined exonic edit-
ing sites [27]. The parameters we used allowed a minimum of one read supporting the varia-
tion (−v 1), minimum 0.001 editing frequency (−n 0.001), exploring one base near splice
junction (−r 1), minimum one read coverage, and trimming of five bases at both ends of the
reads (−T 5–5). For each patient we first calculated the Coding Editing Index (CEI), which
provides a global measure of editing in coding sequences. For this purpose, we calculated the
ratio of the total number of A-to-G mismatches at all the 314 sites together over the total num-
ber of reads aligned to all of these positions. Then we calculated the editing level of each of the
314 sites separately. Only sites with at least ten supporting reads were included in the analyses.
Interferon Signature (ISG)
To evaluate IFN signaling activity modifications, we first calculated the gene expression levels
of the interferon genes using the Salmon tool version 0.11.2 [24] with default parameters. The
output was then used as input for the ISG pipeline, which uses a 38-gene signature to provide
an ISG score [21].
Ribosome profiling preprocess
We used “Trimmomatic” (version 0.33) with the parameters: -phred33 LEADING:3 TRAIL-
ING:3 MINLEN:20 to remove the Illumina trueseq adapters from each ribo-seq fastq format
file and a “bowtie” aligner (version 1.2.3) with default parameters to index the human hg38
genome and align fastq files to SAM format. To avoid contamination, we used “FastqScreen”
(version 0.14.0) to filter out reads that aligned to the rRNA transcriptome. In order to optimize
alignment, we added the -v 2 parameter, which permits a maximum of 2 mismatches in each
read while aligning. We sorted the SAM with “Picard” (version 2.5.0) SortSam with the param-
eter SORT_ORDER = coordinate. We indexed the Bam file using Picard BuildBamIndex and
default parameters.
Statistics and figures
All values in graphs are expressed as the mean ± SEM. Normality was tested with the Shapiro-
Wilk normality test. If the data exhibited a normal distribution, we performed pairwise testing
PLOS Computational Biology | https://doi.org/10.1371/journal.pcbi.1010923 April 10, 2023
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
with two-sided, unpaired, Student’s t-test, and multiple group comparisons with a 2-way
ANOVA followed by Tukey post-test. For non-normally distributed data or N < 30, we per-
formed pairwise testing using the two-sided non-parametric Mann-Whitney U test with multi-
ple group comparisons by the non-parametric Kruskal-Wallis test, followed by Dunn’s post-
test. Corrections for multiple comparisons were made by the Benjamini–Hochberg false dis-
covery rate (FDR) multiple testing procedure.
All tests and analyses were performed using “R” version 3.5.3 (R Core Team 2014) and
Rstudio, an integrated development environment for R, version 1.3.1093. Figures were drawn
with R, Microsoft Excel, and Adobe Illustrator (version 24.0.1).
Results
RNA editing is exceptionally high in arteries
A major resource of human RNA-seq data is the GTEx cohort [28], which includes thousands
of samples from donors with various backgrounds and medical conditions.
A previous analysis of the extensive GTEx dataset [29], which included RNA sequencing of
8848 samples and 47 tissues, showed Alu editing was the highest in the arteries (GTEx tissue
types: Aorta, Coronary arteries, Tibial artery), but not in the heart (Left Ventricle: LV, and Left
Atrial Appendage: LAA(15). However, the scope of editing in coding sequences is unknown.
Here we analyze the same dataset to compare the level of editing within the coding region
across tissues. We extended the previously published Recoding Editing Index [30] to a Coding
Editing Index (see Methods), which also includes synonymous editing sites, and found editing
in coding areas in the arteries to be the highest of all other 47 tissues, including the brain,
which is believed to be the most edited tissue (Fig 1A).
Both the arteries and the brain have highly edited recoding sites. However, unlike the brain,
the sites in the arteries are strongly expressed, leading to a much higher number of edited
transcripts.
To illustrate this point, we compared the total number of edited transcripts, averaged over
donors. Summing over the 314 evaluated coding sites (see Methods), the number of A-to-G
mismatches (each representing an editing event) is an order of magnitude larger in artery tis-
sues than in the cerebellar hemisphere, the brain’s most highly edited tissue (11685, 12014, and
9442 transcripts per tissue in the aorta, tibial, and coronary arteries, respectively, vs. 1,700 in
the cerebellar hemisphere). In the arteries, the number of recoding events in FLNA transcripts
alone (4897, 5582, and 4187 per sample in the aorta, tibial, and coronary arteries, respectively)
is considerably larger than the total number of edited transcripts in the brain [14].
Interestingly, the editing profile across all 314 coding sites reveals tissue-specific patterns
that largely reflect the anatomy and function of cardiovascular tissues. We ran a PCA based
hierarchical clustering analysis and found that arteries cluster separately from the LV and LAA
as two distinct groups. Within the arteries, the aorta and coronaries share a similar pattern
that is distinct from that of the tibial artery (Fig 1B). This suggests that editing patterns indeed
share unique characteristics of each cardiovascular tissue.
An additional difference we discovered between the brain and the cardiovascular tissues is
that cardiovascular editing is highly centralized- i.e., a small number of sites account for the
majority of recoding editing. To illustrate this point, we inspected the distribution of editing in
specific genes. We found that the five most edited targets in cardiovascular tissues are IGFBP7,
FLNA, NEIL1, CCNI, and SRP9. Together, these five sites account for 92%, 91%, 90%, 64%,
and 68% of all coding editing activity in the aorta, coronary arteries, tibial artery, LV, and
LAA, respectively. The markedly different behavior of recoding activity between the tissues is
due to both elevated expression and higher editing levels of the abovementioned few recoding
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Fig 1. RNA editing in coding sequences is exceptionally high in arteries. (a) Coding Editing Index of all GTEx
samples, presenting the editing level per donor as a weighted average over 314 coding editing sites. (b) Clustering tissues
by the profile of editing levels for the 314 sites (calculated for pooled GTEx data). Columns correspond to editing sites
meeting cutoff values of � 5% editing and expression in at least five tissues. Colors represent the editing percent at each
site. The values correspond to the pulled average per site for all samples meeting the specified cutoffs for each site. Arrows
indicate the positions of IGFBP7, FLNA, NEIL1, CCN1 and SRP9, the sites where most cardiovascular editing takes place.
(c) The relative contribution of specific coding sites to the overall recoding activity. Notably, all highly edited sites are
recoded (editing causes an amino-acid change) and evolutionarily conserved across mammals—samples from left
ventricles of unselected GTEx donors.
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
sites. In contrast, the top five edited sites in the cerebellar hemisphere account for only 29% of
the editing activity (Fig 1C).
Recoding in cardiovascular tissues is focused on a few editing sites, in stark contrast to the
complex recoding profile in the brain. This difference may be partly attributed to differences
in the cellular composition of each tissue. Brain tissues exhibit diverse cellular populations
[31], while the majority of heart cells are cardiomyocytes and cardiac fibroblasts [32]. This
leads to a homogenous editing profile across cardiovascular tissues, as opposed to the hetero-
geneous structure of the brain.
Notably, mutations and altered expression of IGFBP7 and FLNA have been observed in
various cardiovascular diseases [14,33–39].
RNA editing in Alu sequences increases in atherosclerosis and
cardiomyopathies
A medical record of each GTEx donor is available upon request. We used this information to
further analyze the GTEx cohort and divided its donors into three groups of interest: patients
with atherosclerosis (n = 104), patients with cerebrovascular disease (CVAs, aneurysms, and
ICH; n = 145), and controls (n = 228, S1 Table in the Supplementary Material; Methods).
In addition to the GTEx cohort, we analyzed other datasets including ventricular samples
from three different cardiac pathologies [40] (S2 Table in the Supplementary Material) with
matching healthy controls. We found that atherosclerosis patients demonstrate a significant
and consistent increase in the Alu editing in all cardiovascular tissues except the tibial artery
(Fig 2A). Consistent with this observation, ischemic cardiomyopathy patients also demon-
strated increased Alu editing (Fig 2B), as did those with non-ischemic types of cardiomyopa-
thies (i.e. dilated cardiomyopathy) (Table 1 and Fig 2B). In contrast, patients with
cerebrovascular disease demonstrated a decrease in Alu editing across all cardiovascular tissues
(Table 2 and Fig 2A).
Fig 2. RNA-editing in Alu sequences in atherosclerosis and cardiomyopathies. AEI demonstrates consistently increased
editing levels of all Alu sequences in (a) atherosclerosis (ASCVD: red; controls: White; Cerebrovascular: yellow) and (b) CMPs
(Patients: red; Controls: white) patients, and hypo-editing in cerebrovascular patients. The DCM and ICM groups are
compared to the same control group. Note that due to differences in read length, the nominal index values cannot be compared
across the two panels. The exact p-values are detailed in Tables 1 and 2.
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Table 1. AEI and CEI in ischemic and dilated cardiomyopathies (ICM, DCM,mean ± SEM). Red upward arrows represent significantly increased editing levels com-
pared with controls (Wilcoxon rank-sum test). Note that ICM and DCM are both compared to the same control group.
AEI ASCVD
Cerebrovascular
disease
Controls
CEI ASCVD
Cerebrovascular
disease
Controls
Artery aorta
Artery tibial
Artery coronary
Heart LV
Heart LAA
3.59 ± 0.14 (n = 47,
p = 1.3e-5) "
2.1 ± 0.09 (n = 86,
p = 6.4e-6) #
2.7 ± 0.1 (n = 113)
42.9 ± 1.0 (n = 46,
p = 1.75e-5) "
36.6 ± 0.7 (n = 85,
p = 0.02) #
2.78 ± 0.05 (n = 76,
p = 0.94)
2.53 ± 0.05 (n = 108,
p = 1e-4) #
2.77 ± 0.04 (n = 173)
42.83 ± 0.62 (n = 74,
p = 0.19)
40.9 ± 0.46 (n = 108,
p = 0.03) #
3.04 ± 0.19 (n = 18,
p = 0.008) "
1.7 ± 0.07 (n = 56,
p = 1.49e-7) #
2.45 ± 0.1 (n = 67)
35.68 ± 2.21 (n = 17,
p = 0.75)
29.43 ± 1.25 (n = 56,
p = 0.001) #
1.51 ± 0.05 (n = 48,
p = 3e-4) "
1.04 ± 0.03 (n = 86,
p = 2.7e-5) #
1.26 ± 0.04 (n = 112)
7.92 ± 0.46 (n = 48,
p = 4.7e-6) "
4.64 ± 0.43 (n = 86,
p = 3e-4) #
2.11 ± 0.06 (n = 53,
p = 1.1e-5) "
1.33 ± 0.08 (n = 67,
p = 5.55e-6) #
1.68 ± 0.05 (n = 97)
10.34 ± 0.49 (n = 53,
p = 0.003) "
7.43 ± 0.39 (n = 67,
p = 0.002) #
38.7 ± 0.6 (n = 113)
41.8 ± 0.4 (n = 173)
34.83 ± 0.8 (n = 67)
5.7 ± 0.34 (n = 112)
8.71 ± 0.33 (n = 94)
https://doi.org/10.1371/journal.pcbi.1010923.t001
We have employed a multivariate regression model, including gender and age as additional
independent variables, to control for gender and age differences between the groups.
A pathological role for Alu editing in atherosclerosis patients related to editing in the 3’
UTR of the CTSS gene has previously been discovered [41]. Inspired by this result, we exam-
ined the editing in all Alus within 3’ UTRs and found that, the editing in these Alus increases
in atherosclerosis, dilated and ischemic cardiomyopathies, while they decrease in cerebrovas-
cular patients (S2 Fig in the Supplementary Material).
Interferon-stimulated genes are increased in cardiomyopathies and
correlate with elevated ADAR1 expression
The ADAR1 p150 isoform is interferon-inducible and has been linked to inflammatory pro-
cesses [42]. We hypothesized that the elevated editing we observed in cardiomyopathies and
atherosclerosis patients results from underlying tissue inflammation that mediates the patho-
logical process.
In order to examine this hypothesis, we measured the ISG score [21], which considers the
expression level of 38 genes in the IFN cascade (including ADAR1). The ISG scores were
higher in cardiomyopathy patients than in controls (Wilcoxon rank-sum test, p-
value = 0.00005, 0.001 for ischemic and dilated cardiomyopathies, respectively, and P = 0.017
for the validation ICM set, Fig 3A and 3B), supporting the association of these pathologies
with inflammation. The other GTEx cohorts did not exhibit any significant change in the ISG
score, presumably because they are associated with milder heart conditions. Furthermore, the
results in Fig 3C and 3D, demonstrate that ADAR1 expression is upregulated in ischemic and
dilated cardiomyopathies compared to controls (Wilcoxon rank-sum test, p-value = 0.00002,
0.000003 in dilated and ischemic cardiomyopathies, respectively), with no change in the other
cohorts. ADAR2 is not known to be affected by interferon. As expected, we did not detect any
change in its expression between patients and controls.
Table 2. AEI and CEI in cardiac and cerebrovascular patients (mean ± SEM). Note that due to differences in read length, the nominal index values cannot be compared
with those in Table 1. The color and direction of the arrows represents the change (upward red, elevated editing levels; downward blue, reduced editing levels) and the sig-
nificance (Wilcoxon rank-sum test).
ICM
DCM
ICM (validation)
AEI
CEI
Patients
Controls
Patients
Controls
0.68 ± 0.02 (n = 13, p = 0.002) "
0.56 ± 0.02 (n = 14)
4.14 ± 0.59 (n = 13, p = 0.002) "
2.36 ± 0.14 (n = 14)
0.67 ± 0.01 (n = 37, p = 0.0001) "
0.56 ± 0.02 (n = 14)
3.51 ± 0.19 (n = 37, p = 9.72e-06) "
2.36 ± 0.14 (n = 14)
0.63 ± 0.01 (n = 20, p = 1.55e-05) "
0.55 ± 0.02 (n = 10)
5.71 ± 0.41 (n = 20, p = 1.29e-05) "
3.22 ± 0.23 (n = 10)
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Fig 3. Interferon stimulated genes and ADAR1 expression in CMPs. Interferon Signature (ISG) Score in (a) CMPs and (b)
ICM patients. ADAR1 expression levels in transcript per million units in patients with (c) CMPs and (d) ICM. The exact p-values
are detailed in Tables 1 and 2.
https://doi.org/10.1371/journal.pcbi.1010923.g003
Moreover, we observed a positive linear relationship between ADAR1 expression, and the
Alu editing and ISG scores in cardiomyopathies samples, which implicates RNA editing in
these conditions, and these three parameters exhibit positive pairwise correlations (Spearman,
rho = 0.48, 0.58 and 0.45; p-value = 1.5e-06, 1.2e-09 and 5.9e-06, for ADAR1 expression vs.
ISG score, ADAR1 vs. Alu editing and Alu editing vs. ISG, respectively). Linear regression
analysis revealed that the Alu editing is significantly correlated to both ADAR1 expression and
the ISG score, with a positive interaction term (p for interaction = 0.004) (S2 Fig).
Editing in recoding sites increases in various cardiovascular diseases
While less than 1% of all RNA editing activity occurs in protein-coding sites, such events may
have a far-reaching pathological impact. Significant differences in editing at specific coding
sites may be of particular interest if they mimic a genetic variant or mutation. We, therefore,
focused our coding site analysis on sites that exhibit a meaningful biological change in the edit-
ing level (defined as a mean difference of at least 5%) in disease. Analyses were performed sep-
arately for each of the cohorts. We found that coding editing was increased in atherosclerotic
cardiac diseases and decreased in cerebrovascular diseases, reminiscent of what we observed in
Alu editing (Fig 4A and 4B). 34 coding sites demonstrated meaningful and significant alter-
ations, with most sites showing increased editing levels in atherosclerosis and conversely- a
decrease in editing in cerebrovascular patients, compared with controls (Fig 4C and S3 Table
in the Supplementary Material). A recent comprehensive analysis of the GTEx cohort [15],
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Fig 4. The landscape of coding editing in cardiovascular patients. Coding Editing Index distribution in (a) Aortae of cerebrovascular
and (b) ASCVD patients. (c) Heatmap summarizing all significant (FDR cutoff of � 0.05) and meaningful (editing index
difference � 5%) coding editing sites in cardiovascular tissues from GTEx data. Columns are editing sites. Rows represent individual
samples of cardiovascular tissue taken from donors with either cardiac or cerebral disease. The heatmap presents 139 patients who have
information for at least 70 editing sites. We calculated the change in editing levels for each patient in each site by subtracting the control
group average at that site. The color represents the direction of change (blue, elevated editing levels; red, reduced editing levels). (d)
Summary of the significant changes in editing in the cardiomyopathies cohorts. Columns are editing sites that differentiate patients from
controls in at least one disease type. Colors represent the mean difference in editing levels compared to controls. Gray cells represent
missing data or differences not meeting the significance cutoff. Significance is defined by the Wilcoxon rank-sum test p-values followed
by the Benjamini–Hochberg procedure with FDR < 0.05.
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Table 3. RNA-editing levels in specific coding sites in CMPs. Statistical analysis was performed using Wilcoxon rank-sum test followed by the Benjamini–Hochberg
procedure for false discovery rate (FDR) for multiple testing (< 0.05).
Site
Gene name
Dataset
Mean control (%)
Mean patient (%)
P. value
chr4 57110068
chr1 160332454
chr4 57110068
chr1 160332454
chr13 45516236
chr4 57110068
IGFBP7
COPA
IGFBP7
COPA
COG3
IGFBP7
ICM (validation)
ICM
ICM
DCM
DCM
DCM
https://doi.org/10.1371/journal.pcbi.1010923.t003
18.95 ± 1.63
12.95 ± 1.79
15.58 ± 1.3
12.95 ± 1.79
12.23 ± 2.05
15.58 ± 1.3
27.41 ± 1.35
21.09 ± 2.78
26.86 ± 2.91
20.84 ± 1.57
21.90 ± 1.98
22.71 ± 1.24
0.00
0.03
0.01
0.01
0.01
0.00
FDR
0.00
0.04
0.02
0.02
0.02
0.01
precluded a major influence of baseline characteristics such as sex and age on the editing levels.
Hence, the main driver of such differences is likely the existence of a disease.
Although the cohort’s sizes of ICM and DCM were relatively small, we could still detect
three differentially edited sites within the IGFBP7, Copper-exporting P-type ATPase (COPA),
and Conserved oligomeric Golgi complex subunit 3 (COG3) transcripts that exhibited a sub-
stantial increase (Table 3 and Fig 4D). These sites are mainly edited by ADAR2 [43–46]. We
did not detect differences in the expression of this enzyme in either disease compared with
controls, and thus the source of the differential editing in these sites is unclear. It is worth men-
tioning that, ideally, the existence of such differences should be discerned in the arteries in
ICM, where ADAR2 expression is also notably high. However, only LV samples were available
for this cohort.
In order to examine whether the recoded RNA transcripts were in fact being translated into
proteins, bearing a potential functional impact, we analyzed ribosome profiling data [47] of
left ventricular samples from DCM donors (n = 30), which are enriched for translated tran-
scripts. Measurements of the editing levels at conserved mammalian sites verified the presence
of increased editing (S3 Fig in the Supplementary Material) [48–51].
IGFBP7 editing as a potential diagnostic marker for underlying ischemic
disease
Cardiac markers of injury greatly facilitate the management of ischemic and heart failure
patients, and novel markers are constantly sought after. IGFBP7 can be sampled from the
peripheral blood and serum levels of this protein increase in atherosclerosis and heart failure
with preserved ejection fraction and correlate with cardiac function [37,52–55].
Our analysis demonstrates that the editing of IGFBP7 is markedly increased with cardiac or
vascular injury (increases in LV editing from 16% to 26% and 26% to 34% in ischemic cardio-
myopathy and atherosclerosis, respectively). Integrating both IGFBP7 serum levels and edited
isoform fraction may thus further increase its diagnostic performance without the need for
sequencing, rendering it an accessible and potentially useful novel clinical marker. This can be
done, for instance, by sampling peripheral blood and utilizing specific antibodies for the edited
and non-edited versions of IGFBP7.
Considering that our findings suggest the functional relevance of RNA editing to a variety
of cardiovascular conditions, we examined how well the tissue editing levels correlate with
those of the peripheral blood. We used the GTEx cohort for this purpose since it contains data
from numerous tissues. Indeed, the Spearman test demonstrated a strong correlation between
the blood Alu editing level and four of the five cardiovascular tissues (rho LV = 0.72; n = 175,
LAA = 0.8; n = 183, aorta = 0.79; n = 184, coronaries = 0.62; n = 113, and tibial artery = 0.38;
n = 268, and p values = < 2.2e-16, < 2.2e-16, < 2.2e-16, < 2.2e-16, and 3.2e-10, respectively).
We next wanted to assess how increased editing translated into a potentially clinically useful
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
test. We tested this by comparing the predictive value of Alu editing and IGFBP7 editing with
expression-based levels of common clinical markers of cardiac injury. We found that, in the
cardiomyopathies, editing of both Alu and IGFBP7 has a predictive value that is comparable to
the expression of top heart failure markers, such as troponin subtypes, atrial natriuretic peptide
(ANP), and B-type natriuretic peptide (BNP, AUC: ANP = 0.73, BNP = 0.9, Troponin I = 0.92,
Troponin T = 0.69, IGFBP7 editing = 0.8, Alu editing = 0.84, S4 Fig in the Supplementary
Material). This suggests that the magnitude of editing changes is on the same scale as state-of-
the-art cardiac markers.
Discussion
Advancements in diagnostics and therapeutics of cardiovascular illnesses rely on a deeper
understanding of the underlying cellular processes. RNA editing is highly abundant in vascular
tissues, is tissue-specific, and participates in pathways that contribute to cardiovascular dis-
eases, such as inflammation and circular RNA regulation. Li et al. recently pointed a spotlight
on RNA editing demonstrating it is a major mediator of the genotypic effects on inflammatory
diseases [20]., including atherosclerosis. Further investigating the role and mechanisms this
process plays in the pathogenesis of common inflammatory-driven diseases and possible ways
to intercept them is therefore timely and warranted.
RNA editing appears to interfere with several relevant mechanisms. Jain et al. [14] recog-
nized that aberrant editing at the recoding FLNA site contributes to dilated cardiomyopathy
and identified a 50% reduction in editing at this site in patients. Mice lacking the ability to edit
the FLNA site develop heart failure and diastolic hypertension. In addition, a strong downre-
gulation of ADAR2 and an increase in ADAR1 expression were observed in patients with con-
genital heart defects [56].
Here we present for the first time a comprehensive analysis of RNA-editing in the cardio-
vascular system. We found that the highest activity of editing in both coding and non-coding
sequences occurs in the cardiovascular system. Patients with coronary or structural heart dis-
ease demonstrated yet higher levels of editing compared with controls. Conversely, editing
decreases in patients with cerebral vascular accidents or hemorrhages but no heart disease.
These opposing trends may represent differences in etiology (i.e., atherosclerosis and remodel-
ing in cardiac patients versus cardio-embolic strokes and bleeding from aneurysms in cerebro-
vascular patients), or else the aberrant editing may serve as an etiology.
In recent years, it has become accepted that inflammation plays a central role in conditions
such as atherosclerosis and the resulting ischemic cardiomyopathy [57–60]. Although cardio-
vascular inflammation has been extensively investigated using various methodologies [61,62],
key drivers of the inflammatory signals remain elusive. RNA editing is known to take part in
the inflammation cascade since ADAR1 p150 expression is strongly influenced by IFN.
Accordingly, we observed a close correlation between inflammation and increased ADAR1
expression and Alu editing in ischemic and dilated cardiomyopathies. Interestingly, Alu edit-
ing is most increased in ICM, where inflammation may mediate both the initial pathology and
subsequent scar formation. It is possible that RNA editing represents a new inflammation-
driving mechanism that is highly active in the arteries. Importantly, attempts are currently
made to both activate and inhibit the editing machinery for therapeutic purposes. This may
render RNA editing an actionable target.
It is important to note that RNA editing is present and maybe upregulated even in the
absence of inflammation. Alu editing was increased in the GTEx data analysis of patients with
atherosclerosis and cerebrovascular disease, despite normal ISG scores. It is possible that the
lack of correlation between the ISG score and Alu editing in this cohort is due to the nature of
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
the GTEx population, which comprises mainly deceased donors, and where tissue ischemia and
other perimortem changes may affect gene expression and blur an analysis that relies on differ-
ential expression. Given this potential caveat, it is noteworthy that the Alu editing in this popu-
lation still differentiates patients from controls, testifying to the robustness of the measurement.
Another possible mechanism by which altered editing promotes structural heart diseases and
atherosclerosis is through effects on the formation of circRNA. An increase in RNA editing in
intronic regions has been shown to reduce circRNA levels [60]. This reduction may in turn con-
tribute to the development of heart failure, dilated cardiomyopathies, and probably also to ath-
erosclerosis [57–59,63,64]. Aberrant editing may therefore be an initiating factor.
Our results identified several specific coding sites with prominent differences between
patients and controls. These recoding changes are enriched for relevant biological processes.
For example, in left ventricle samples, we observed an enrichment of genes controlling actin
and cytoskeleton polarity, a process that plays a crucial role in cardiomyopathy and heart fail-
ure. Similarly, we observed enrichment in atrial cell membrane repolarization regulation in the
left atrial appendage -a process that may be involved in arrhythmias and thrombus formation.
Finally, a primary site of interest for cardiovascular editing is within the IGFBP7 gene. This
gene is highly expressed in cardiovascular tissues and contains two evolutionarily conserved
editing sites. We observed altered editing in the major editing site in atherosclerosis patients.
Editing in this site in cardiovascular tissues is exceptionally high, surpassing 95%, and consti-
tutes a sizable proportion of all recoding editing in the heart and arteries. The mechanism by
which the observed increase in editing of IGFBP7 influences pathogenesis is unclear. It is pos-
sible that editing of this highly expressed cardiovascular gene is a result of the globally
increased ADAR1-mediated editing. However, some indirect evidence suggests that this site is
edited by ADAR2 and is not so much susceptible to ADAR1 overexpression.
IGFBP7 has already been suggested as a biomarker of atherosclerosis and heart failure
[54,55], and mutations in this gene are known to lead to vascular and valvular pathologies
[60]. The notion that IGFBP7 could serve as a clinical marker is supported by our finding that
in addition to the elevated serum levels seen in patients, the editing levels of this protein are
also increased. [65] Editing changes at this site in cardiomyopathy patients are comparable to
the elevations in gene expression of top cardiac biomarkers. Taken together, our findings sug-
gest that RNA editing is involved in atherosclerosis and subsequent ischemic, as well as in
dilated cardiomyopathies. Further research is warranted to assess the role of this emerging
process in the development of these diseases.
Limitations
The main limitation of our study is its in-silico nature, which naturally precludes the ability to
infer causality and mandates further biological experiments.
Another limitation is the use of publicly available data, where complete details on the
donors are sometimes lacking, along with a lack of control over the data production process.
For example, no genotypic information was available in the cardiomyopathies datasets. This
limitation is common in bioinformatical analysis in general, and we addressed this issue by
carefully selecting datasets with high-quality and maximal annotations.
We made every effort to use reproducible, meticulous analysis of the data and to use well-
established and consistent statistical approaches to maximize the robustness of our findings.
Conclusion
RNA editing in both coding and non-coding regions is increased in atherosclerosis, ischemic,
and dilated cardiomyopathies. It is partially correlated with increased inflammatory signals
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
and thus might be involved in maladaptive inflammatory remodeling. RNA editing is not fully
explained by increased inflammation and may represent a discrete additional process.
Supporting information
S1 Fig. RNA editing protects against self-immune attacks. Endogenous dsRNA structures
are formed naturally and may activate the inflammation pathway. To prevent false activation
of the immune system, ADAR1 disrupts endogenous dsRNA structures by editing adenosines
to inosines. Mitochondrial Antiviral Signaling (MAVS); Melanoma Differentiation-Associated
protein 5 (MDA5); Interferon Stimulated Genes (ISG); Adenosine Deaminase Acting on RNA
1 (ADAR1).
(JPG)
S2 Fig. RNA-editing in Alu sequences within 3’ UTR in ASCVD, CMPs. The AEI demon-
strates consistent increased editing levels of all Alu sequences in (a) ASCVD patients, and
hypo-editing in cerebrovascular patients (Two-sided Wilcoxon rank-sum test, p.
value = 0.0004, 0.024, 0.005, 0.0002, 0.78 for ASCVD and 1.7e-6, 4.1e-7, 4.1e-5, 8.6e-5,
0.0001 for cerebrovascular in the aorta, coronary arteries, LAA, LV and tibial artery, respec-
tively). Increased editing levels are also observed in (b) CMP (Two-sided Wilcoxon rank-
sum test, p. value = 4e-06 and 1.1e-05 for DCM and ICM, respectively) and (c) ICM (Addi-
tional validation set) (Two-sided Wilcoxon rank-sum test, p. value = 0.001). Note that due
to differences in read length, the nominal index values cannot be compared between the
two panels.
(JPG)
S3 Fig. Three-way correlation between the Interferon stimulated gene score (ISG), ADAR1
expression, and the Alu editing index (AEI) in CMPs. Spearman correlation rho = 0.48, 0.58
and 0.45; p-value = 1.5e-06, 1.2e-09 and 5.9e-06, for ADAR1 expression vs. ISG score, ADAR1
vs. Alu editing and Alu editing vs. ISG, respectively). Linear regression analysis revealed that
the Alu editing is significantly correlated to both ADAR1 expression and the ISG score, with a
positive interaction term (p for interaction = 0.004)
(JPG)
S4 Fig. Compatible levels of editing between Ribo-seq and RNA-seq data. Levels of A-to-I
editing in left ventricle DCM patients from RNA-seq (n = 37) and Ribo-seq (n = 30) datasets.
Cutoff of � 10 reads for each site.
(JPG)
S5 Fig. Comparison of clinical and potential cardiac injury markers by ROC curves.
(JPG)
S1 Table. Clinical characteristics of GTEx donors.
(XLSX)
S2 Table. Overview of the analyzed datasets.
(XLSX)
S3 Table. Significant alterations in the levels of coding editing sites in disease.
(CSV)
S4 Table. Tables with raw data used for figure plotting.
(XLSX)
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PLOS COMPUTATIONAL BIOLOGYIncreased A-to-I RNA editing in atherosclerosis and cardiomyopathies
Acknowledgments
We thank Orshai Gabay and Roni Cohen for technical assistance on the experiment analysis.
We thank the GTEx consortium for making their RNA sequencing data publicly available.
Author Contributions
Conceptualization: Tomer D. Mann, Eli Eisenberg, Erez Y. Levanon.
Formal analysis: Tomer D. Mann, Eli Kopel.
Investigation: Tomer D. Mann, Eli Kopel.
Software: Eli Kopel.
Supervision: Eli Eisenberg, Erez Y. Levanon.
Visualization: Eli Kopel.
Writing – original draft: Tomer D. Mann, Eli Kopel.
Writing – review & editing: Eli Eisenberg, Erez Y. Levanon.
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10.1007_s43076-022-00253-9.pdf
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Data Availability Data not available due to ethical restrictions.Due to the nature of this research, partici-
pants of this study did not agree for their data (whole transcripts) to be shared publicly. However, some
supporting material concerning the data analysis process will be available for both reviewers and readers.
|
Data Availability Data not available due to ethical restrictions.Due to the nature of this research, participants of this study did not agree for their data (whole transcripts) to be shared publicly. However, some supporting material concerning the data analysis process will be available for both reviewers and readers.
|
Trends in Psychology
https://doi.org/10.1007/s43076-022-00253-9
ORIGINAL ARTICLE
Uncertainty in Child Custody Cases After Parental
Separation: Context and Decision‑Making Process
Josimar Antônio de Alcântara Mendes1
· Thomas Ormerod1
Accepted: 10 November 2022
© The Author(s) 2023
Abstract
Context factors (e.g. a family’s developmental crisis) can affect the child custody
decision-making process and the child’s best interests after parental separation. But
what are these context factors, and how can they vary across different cultures and
legal systems? This paper reports a cross-cultural qualitative study funded by the
Brazilian Ministry of Education and was carried out under a Naturalistic Decision-
making approach. This study addresses context factors that impact the decision-
making of experienced legal actors working in child custody cases. Interviews were
conducted with 73 legal actors (judges, prosecutors, lawyers, psychologists, and
social workers) in Brazil and England. The data gathered were analysed employing a
reflexive thematic analysis that generated seven themes addressing how uncertainty
is structured by context factors in child custody cases after parental separation. The
themes generated encompassed three domains (‘family’, ‘family court’, and ‘legal-
psychosocial’) that draw attention to the sources of uncertainty in child custody
cases, especially to the role of contextual players (family and children) in the child
custody decision-making process.
Keyword Child custody · Decision-making · Divorce · Uncertainty · Thematic
analysis
Cases in which divorced parents cannot reach a settlement and therefore need to go
to trial, are estimated to be about 5% of the total of divorces (Baker, 2012; Kelly,
2007; Wallace & Koerner, 2003). Despite being a small part of the total of divorces,
* Josimar Antônio de Alcântara Mendes
[email protected]
1 University of Sussex, Brighton, UK
Vol.:(0123456789)1 3
Trends in Psychology
these cases pose a challenge to family court professionals as such cases tend to be
very complex and involve different factors that will impact the decision-making pro-
cess and the child’s best interests.1
Extensive scholarship has addressed divorce-related factors that can affect the
decision-making process. For instance, some studies addressed the application
of ‘the best interests of the child’ standard (Eekelaar, 2015; Mendes & Ormerod,
2019), procedures for evaluation (Goldstein, 2016), judges’ attitudes (Stamps et al.,
1996), psychologists’ and lawyers’ views (O’Neill et al., 2018) as well as ‘child and
family features’ that can influence the judges’ decision-making (Wallace & Koerner,
2003). These issues reinforce the assumption that a decision-making process carried
out in natural settings (i.e. in the real world) is affected by uncertainty (Klein et al.,
1993; Lipshitz & Strauss, 1997; Lipshitz et al., 2001; Lipshitz, 1993a, b). However,
there is still a lack of scholarship focused on how context factors can play a role in
the decision-making process in child custody cases – especially factors that are not
related to mental health issues, personality traits, intimate partner violence, child
abuse and neglect.
We understand ‘context factors’ as issues and/or dynamics regarding individual,
organizational and system factors that can influence the decision-making process,
especially by prompting uncertainty into this process. In general, two core domains
constrain most of these factors’ variance: 1) type of legal system (e.g. laws, legal and
technical guidance/practices); and 2) contextual issues (e.g. family’s developmental
struggles) (Mendes & Bucher-Maluschke, 2017; Mendes & Ormerod, 2021).
In this study, our exploration of context factors considers differences across two
nations that differ according to their underlying legal systems: the common law
approach of England, and the civil law approach of Brazil. The English family jus-
tice system bounces between two contrasting practice approaches: (1) behaviour-
focused and (2) outcome-focused (Eekelaar & Maclean, 2013). The former refers
to the emphasis on settlements made by the parties through the modification of their
expectations/behaviours rather than through proceedings and adjudication—in this
scenario, whatever encompasses the settlement is less important than the parties’
agreement and closing the case. The latter approach refers to the idea that family
justice works as an ‘impartial spectator’ that can provide fair outcomes throughout a
fair process.
In Brazil, since the enactment of the New Code of Civil Proceedings in 2015,
the family justice system has specific routes that aim to promote consensual settle-
ments or self-composition.2 However, the Brazilian legal system still is very liti-
gation-driven and, in most cases, these routes are there just pro-forma (Mendes &
Ormerod, 2021). In addition, the Brazilian family justice system has a child custody
1 Despite legal and definitional differences, ‘divorce’ and ‘parental separation’ will be referred to as
the same thing throughout this paper: the relationship breakdown between two people that had a child
together.
2 This is related to processes in which both parties (parents) find a functional way to communicate their
differences, interests and goals regarding the matter under dispute and to thereby reach an agreement by
themselves, without judicial mediation.
1 3
Trends in Psychology
decision-making process that is ‘closed-ended’ as the law points out only two pos-
sible outcomes: (1) joint custody (preferably); and (2) sole physical custody.3
Context factors, tend to define and frame "the space in which decision-making
processes operate" (Jones et al., 2014, p. 203). In this sense, the task of understand-
ing context factors that surround the process of making a decision is crucial (Lip-
shitz, 1993a)—especially because uncertainty is the main impediment to an effec-
tive decision-making process (Lipshitz & Strauss, 1997). This task is challenging
for legal actors because family struggles are more related to psychosocial issues than
legal ones, which leads to more uncertainty in such cases.
Within the legal scholarship, the role of uncertainty is largely addressed as ‘legal
uncertainty’ and it is seen as a consequence of generic legal standards that make it
difficult to say, ex ante, if certain actions are legal and what legal officials might do
(Lang, 2017). However, the legal literature neglects other factors that can lead to
uncertainty within family justice and its decision-making processes.
Some scholars have addressed how legal actors use heuristics to deal with uncer-
tainty in child custody cases (e.g. Enosh & Bayer-Topilsky, 2015), noting that uncer-
tainty is a key player in such cases. However, the literature in this field is lacking
studies that investigate context factors in child custody cases that build and sustain
the levels of uncertainty as well as the consequences of it. This is concerning as
uncertainty “affects real-world decisions by interrupting ongoing action, delaying
intended action, and guiding the development of new alternatives” (Lipshitz, 1993b,
p. 173). Hence, family justice and its professionals should be aware of context fac-
tors because they can lead to errors and biased judgments during the decision-mak-
ing process, which can impair the quality of the decisions made, affecting the child’s
best interests as well as the family’s well-being.
In an attempt to draw attention to context factors (especially those not related to
mental health issues, personality traits, intimate partner violence, child abuse and
neglect and the like) and how they are structured within the child custody decision-
making process, this study presents results from a qualitative inquiry that identified
key context factors responsible for producing and sustaining uncertainty in child
custody cases after parental separation.4
3 For further discussion regarding the Brazilian family justice and child custody after parental separa-
tion, please see Mendes and Ormerod (2021).
4 These results are part of a larger research project that has identified cognitive strategies used by legal
actors to cope with uncertainty prompted by context factors. The project had a naturalistic and cross-cul-
tural design that approached legal and cultural issues in Brazil and England, and aimed to understand: 1)
how the decision-making process is structured in terms of its contextual dynamics and constraints; 2) the
role of legal actors in the decision-making process; 3) how ‘the best interests of the child’ is understood
and applied; and 4) how the type of legal system (civil law in Brazil, common law in England) affects the
decision-making process.
1 3Trends in Psychology
Method
This study’s design incorporated a Naturalistic Decision-Making research meth-
odology, which aims to understand and describe how individuals make their deci-
sions in the real world. This approach highlights “how expert practitioners perform
cognitively complex functions in demanding, real-world situations characterized by
uncertainty, high stakes, and team and organizational constraints” (Patterson et al.,
2016, p. 229).
Instruments, Participants, and Procedures
This study used semi-structured interviews with open-ended and closed questions
– to check the interview questions, please see Online Resource 1.5 The first author
conducted the interviews, which were held for 40 to 70 min, with an average inter-
view time of 55 min. Seventy-three Brazilian and English participants (judges, pros-
ecutors,6 lawyers, psychologists and social workers) took part in this study. The main
inclusion criterion for all participants was to have at least two years of experience in
child custody cases after parental separation. To check participants’ demographics,
please see Online Resource 2.
In both countries, we recruited participants in three ways: a) through the research-
ers’ existing network; b) by sending participation invitations via email and mail; and
c) through snowball recruitment7: each participant was asked if they knew some-
one meeting the inclusion criteria, whom they could recommend to take part in the
study. Access to English participants was difficult because applications to approach
magistrates and social workers (from CAFCASS8) were not granted. Exclusively in
England, we also reached participants via: i) LinkedIn; ii) inviting eligible lawyers
by email invitation based on the list available at http:// www. resol ution. org. uk9; iii)
inviting eligible psychologists by email (we used the list available at the British Psy-
chological Society’s Directory of Expert Witnesses—https:// www. bps. org. uk/ lists/
EWT/ search); iv) emailing authors with papers published on child custody cases
and/or the best interests of the child—they were asked if they would like to take
part in the study or if they would nominate anyone else eligible. Nevertheless, due
5 The questions are based on prior studies that focused on: 1) law, procedures and judicial process
regarding parental separation and child custody and contact/access in Brazil and England—see Mendes
and Ormerod (2021); and 2) a systematic review on ‘the best interests of the child’ in English and Portu-
guese—see Mendes and Ormerod (2019).
6 In Brazil and England, divorce and child custody are a private law matter. However, in Brazil, there
are some cases in which the State is seen as an interested party and non-criminal prosecutors can be
involved. For more clarification, see Mendes and Ormerod (2021).
7 See Sadler et al. (2010) for further information.
8 Stands for Children and Family Court Advisory and Support Service. It is the English “evaluation ser-
vice” and they advise family courts about what is safe for children and what are the child’s best interests
in child custody cases.
9 Resolution’ is an organisation promoting constructive resolution of family disputes and has over 6,500
members among family lawyers and other professionals.
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Trends in Psychology
to the circumstances described, the number of participants in England was smaller
compared to Brazil, but as diverse as the Brazilian group.10 Informed consent was
obtained from all individual participants included in the study and the interviews
were conducted either in person, via Skype or by telephone in both countries, and
recorded with a Sony ICDBX140 Digital Voice Recorder. The study and its materi-
als (e.g. information sheet and consent form) were approved by the University of
Sussex’s Social Sciences & Arts Research Ethics Committee under the Certificate of
Approval number ER/JA454/2. The authors have no competing interests to declare
that are relevant to the content of this article.
Data Analysis
This study adopted thematic analysis as its theoretical framework to understand and
analyse the data gathered. A thematic analysis aims to search for patterns within
qualitative data. Thematic analysis is a process that identifies, organises, and inter-
prets these patterns, leading to analysis and final reporting on those patterns through
the use of ‘themes’ (Boyatzis, 1998; Braun & Clarke, 2006, 2013).
A theme can be seen as a ‘wall’ composed of a lot of ‘bricks’ (codes) connected
by a strong ‘cement’ (meanings). Both ‘bricks’ and ‘cement’ are distinguished and
understood by the researcher’s subjectivity and active role in the data analysis pro-
cess, which is organic and interactive, going beyond the first round of coding, and
extending throughout the whole process of analysis (Braun & Clarke, 2022a; Braun
et al., 2019).11
We propose an Integrative Data-driven Thematic Analysis (IDDTA) that inte-
grates inductive and abductive (theoretical) layers of analysis, revealing manifest
and latent levels of content.12 IDDTA assumes that: (a) neither the data nor the
meanings derived from it are given; both are detected and distinguished as such by
10 In Brazil, three cities were selected: 1) Brasília—it is Brazil’s capital and its court has a large and
solid system for the evaluation of child custody cases; as such it is treated as a reference in Brazil; b) São
Paulo—it is the biggest city in South America, has the biggest court in the world (considering the num-
ber of cases per year) and also has the biggest family court in South America (where participants were
recruited); and 3) Porto Alegre—it has a court known for launching case laws concerning family law that
have spread to other courts, and has also inspired the enactment of acts in this field. Selecting these three
cities enabled this study to economically but effectively achieve a representative sample of the ‘Brazilian
child custody field’. We intended to take the same approach in England by selecting participants from
London, Brighton (southern) and one northern city. However, gathering participants in England was a
herculean task that took over eight months. Hence, we decided to recruit participants from all over Eng-
land.
11 Thematic analysis is a highly flexible methodology, and does not prescribe procedures of data collec-
tion, or limit the theoretical or epistemological perspectives possible within it (Braun & Clarke, 2006,
2013; Braun et al., 2019; Nowell et al., 2017). Boyatzis (1998, p. 1) refers to thematic analysis as a “way
of seeing”, meaning that different people can see different things by looking at and analysing the same
data. Moreover, different people can conceive and use thematic analysis in different ways (Braun et al.,
2019).
12 A similar approach was proposed by Urquhart (2013) for Grounded Theory. She referred to the ‘mid-
dle-range’ coding process in which the coding would emerge from inputs based on the raw data and on
the literature, thus combining induction and abduction processes.
1 3Trends in Psychology
an observer13; (b) qualitative research is inevitably underpinned by the researcher’s
subjectivity, hence no knowledge is neutral14; and (c) qualitative research is a pro-
cess that analytically organises, interprets and reveals patterns of meanings within
the data by means of analytic inputs and outputs that interact in a recursive way.
Braun and Clarke (2022b) reflect on the key role of the researcher’s subjectivity dur-
ing the data analysis process and how the researcher’s work should generate and
report themes that go beyond a ‘topic summary’ by portraying ‘interpretative stories’
concerning consolidated meaning. We agree with this idea but we understand that it
is also important to consider that: a) it is the researcher’s unique views, perspectives,
experience and understanding (therefore, their subjectivity) that will guide them in
the process of organising and describing the data. Hence, the researcher’s subjectiv-
ity is present and is pivotal in the accomplishment of these tasks, even though these
tasks’ outcomes might seem less complex and sophisticated than “interpretative sto-
ries built around [a] uniting meaning” (Braun & Clarke, 2022b, p. 3); and b) qualita-
tive research can be relevant for poly-making (Sale & Thielke, 2018; Tracy, 2010)
and decision-making (Mendes, 2022). In this sense, whenever the outcomes of a
qualitative study are aimed at or relevant for policy-makers and decision-makers,
it is important to ensure this audience’s readership and grasping. Sometimes, this
means providing results that are a little bit more ‘structured’ and descriptive.
Taking these assumptions into account, and based on the assertions of Braun and
Clarke (2022a) and Braun et al. (2019), IDDTA is a reflexive thematic analysis as
it assumes and highlights the researcher’s active role in the process of outlining the
generated themes; it also highlights the meaning rather than quantity of data. This
study’s IDDTA had five phases inspired by and adapted from models in Braun and
Clarke (2006, 2013, 2022a), Braun et al. (2019) and Nowell et al. (2017): Phase
I—Familiarisation (before starting coding, the first author read the interview tran-
scripts, intending to get ‘closer to the data’, its depth and breadth. This familiarisa-
tion was an active process that looked for meanings and patterns by speed-reading
the whole dataset before moving on to Phase II (open coding). During this initial
phase, the first author used the memoing tool15); Phase II—First Level of Analysis:
13 In other words, we assume the assertion, given by Second-order Cybernetic theorists Maturana and
Varela (1991) and Von Foerster (2003), that ‘things’ only become things when observed, distinguished
and pointed out by an observer—i.e. it is the observer and their active perception that give meaning to
things. Thus, reality and its contents (as meaningful constructs) emerge from an observer’s perspective.
In IDDTA, this is set as an essential principle throughout the whole process that leads the observer to
identify, interpret, classify and analyse codes and themes.
14 According to González Rey’s (2011) assertions, in qualitative data analysis, it is the researcher’s
subjectivity, in a dialogic interaction with the data (for extension, with the research participants’ sub-
jectivity too), that drives the process of interpretation (i.e., building up meanings and themes). Hence,
no knowledge is produced outside of historical, social and cultural contexts; neither is it removed from
the researcher’s subjectivity, previous knowledge or experiential framework. Therefore, no knowledge is
totally neutral, pure or inductive.
15 The memoing tool was fundamental for this phase. It is used to take notes regarding any ideas,
insights or interpretations that emerge during the process. This technique was applied throughout the
whole analysis, and it was important to identify links that pointed out patterns and resulting themes. The
notes were also important to embody the latent (interpretative) character of the process.
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Trends in Psychology
open coding16 (process aimed to organise, describe, sort and synthesise the dataset
in a very open way, without restraints—this phase generated 62 codes (see Online
Resource 3)17; Phase III—Second Level of Analysis: generating initial themes
(analysis of initial codes to construct themes—this phase generated 12 candidate
themes and 25 features; see Online Resource 4); Phase IV—Reviewing & Setting the
Themes: definitions and relationships (refinement of candidate themes and features
and trying to set them in a broader context alongside meaningful themes that also
highlighted their connections—this phase generated 7 final themes and 22 features
that will be presented in the next section. During the whole process, some themes
were split or combined with others to compose other more meaningful themes and/
or features); and Phase V—Anchoring18 & Thematic Map (pointing out in which
participants’ data themes and features were based (hence, ‘anchored’) on; a thematic
map to showcase how themes and features are connected and interacting); Phase
VI—Ensuring Trustworthiness: credibility and dependability (peer review/debrief-
ing19 and reflexivity (see Online Resource 5)20).
For the data analysis process of this IDDTA, the unit of coding (the basic seg-
ment of raw data assessed that elicits meanings that help to identify patterns related
to the studied phenomenon) was a sentence.21 Also, the unit of analysis (the entity
considered as the information source upon which interpretation was focused) was
the whole transcript concerning each interview.
Figure 1 summarises the whole process of data analysis.
This study was not preregistered. Also, due to the nature of this study, partici-
pants did not agree for their data (whole transcripts) to be shared publicly. However,
some supplemental material concerning the data analysis process will be available
online.
Results
This study gathered data from 48 Brazilian and 25 English participants. Of these,
64% were female. The proportion of females and males in each country and within
each category of legal actors was similar. The mean years of experience in Brazil
was 14 (SD = 9.7) and 16.5 (SD = 8.9) in England.
16 Inspired by the conceptions of ‘open coding’ by Urquhart (2013) and ‘initial coding’ by Charmaz
(2014).
17 This coding process was helped by the qualitative data analysis software NVivo 10 for Mac OS.
18 This strategy is just a tool used to provide the results’ confirmability. It should not be seen as a quan-
titative measure in which ‘the larger the number of supporters (participants) pointed, the more significant
that theme/feature is’.
19 Four expert practitioners and academics with expertise in child custody cases and/or qualitative
research reviewed this study’s data analysis process and the themes generated.
20 To ensure the final results’ trustworthiness through ‘credibility’, ‘confirmability’ and ‘dependability’
as asserted by Creswell and Poth (2017), Darawsheh (2014), Flick et al. (2004) and Guest et al. (2012).
21 The level of analysis can be ‘line-by-line’, ‘sentence-by-sentence’, ‘paragraph-by-paragraph’ or ‘inci-
dent-by-incident’. The researcher will choose the level of analysis according to their objectives and the
data characteristics.
1 3Trends in Psychology
Fig. 1 Data analysis process
The themes below are presented according to a hierarchy of attributes: a) a theme:
generated according to meaningful content in the dataset; b) feature: signposts charac-
teristics of the theme; and c) highlight: relevant issues arising within a feature. Each
theme is illustrated with participants’ quotations that are linked to their ID, which pre-
sents their country (‘BR’; ‘EN’) and category (‘Jd’ = Judge; ‘Lw’ = Lawyer; ‘Pr’ = Pros-
ecutor; ‘Psy’ = psychologist; SW = Social Worker). In Brazil, participants also have
their city pointed in their ID (BsB = Brasília; POA = Porto Alegre; SP = São Paulo).
Table 1 presents the themes generated and their features (or subthemes). It also
shows how these themes are anchored in the data.
The thematic map presented in Fig. 2 showcases context factors present in child
custody decision-making after parental separation. It shows how the seven themes
are connected and interacting between and within each another. The map also shows
their classification according to specific domains: 1) ‘family’; 2) ‘family court’; and
3) ‘legal-psychosocial’.
Family Domain
Themes that encompass the ‘family’ domain represent issues strictly related to
the family interaction and dynamics after parental separation that can impact the
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Trends in Psychology
Table 1 Themes and features generated by the reflexive thematic analysis and their anchoring on the data
Theme
Data anchoring
Theme CT1: Parental Separation: Crisis and
Family Life Cycle
(CT1.1) Dysfunctionally coping divorce: family
crisis1
(CT 1.2) Misunderstanding and pathologisation
of family interactions and coping strategies in
the context of custody dispute: perspectives on
parental alienation2
(CT 1.2.1) Tricks the decision-making
(CT 1.2.2) Impairs the child’s role
(CT 1.3) Parental separation as part of the family
life cycle3
1 – P2, P8, P9, P11, P17, P20, P21, P24, P31, P35,
P39, P42, P44, P45, P49, P55, P57, P58, P62, P67
2 – P1, P2, P3, P4, P5, P11, P16, P17, P22, P23,
P24, P30, P32, P36, P40, P42, P43, P50, P54,
P60, P62
3 – P1, P2, P12, P14, P18, P19, P24, P26
Theme CT2: Hindering the Best Interests of
4 – P1, P2, P3, P4, P5, P7, P11, P14, P15, P17, P18,
the Child
(CT 2.1) Conjugality Vs. Parenthood4
(CT 2.2) Detaching from the child and attaching
to the litigation5
(CT 2.3) Lack of parenting skills6
(CT 2.4) “No ‘child maintenance’, no contact with
the child”7
(CT 2.5) Misunderstanding joint custody8
(CT 2.6) Involving the child in parental conflict9
P22, P23, P24, P25, P26, P27, P34, P35, P36,
P38, P41, P42, P43, P45, P50, P54, P56, P57,
P58, P62, P63, P66, P67, P68, P70, P72, P73
5 – P1, P2, P3, P4, P5, P6, P7, P8, P12, P13, P15,
P16, P17, P20, P21, P24, P25, P27, P28, P29,
P30, P33, P34, P37, P44, P47, P49, P50, P51,
P52, P53, P56, P58, P59, P62, P63, P64, P65,
P68, P69, P70, P72, P73
6 – P1, P2, P3, P14, P15, P16, P36, P46
7 – P2, P3, P5, P27, P29, P31, P45
8 – P6, P9, P15, P16, P22, P25, P31, P34, P43, P44
9 – P1, P2, P3, P5, P8, P11, P12, P13, P14, P17,
P24, P34, P35, P36, P37, P39, P40, P41, P42,
P43, P44, P47, P50, P54, P56, P57, P60, P62,
P66, P68, P69, P73
Theme CT3: The Judiciary’s Constraints &
10 – P2, P4, P11, P13, P14, P16, P21, P42, P44,
P45, P48, P49
11 – P7, P8, P12, P18, P19, P20, P22, P25, P26,
P29, P31, P34, P35, P42, P54, P57, P59, P71,
P72, P73
12 – P9, P12, P36, P38
13 – P49, P51, P56, P61, P65, P68
Practices
(CT 3.1) “The Law is powerless”: legal and epis-
temological limitations of Law10
(CT 3.1.1) Limits of Law
(CT 3.1.2) Litigious mindset
(CT 3.2) Organisational issues11
(CT 3.2.1) Time & Workflow
(CT 3.2.2) Staff & Workload
(CT 3.2.3) Judges’ career & Courts
(CT 3.2.4) Lack of training and knowledge
(CT 3.3) Between fear and bravery: the psycholo-
gists’ practice in Brazil12
(CT 3.4) An advocate in intractable cases: the
psychologists’ practice in England13
Theme CT4: Applying The Best Interests of the
14 – P3, P5, P8, P9, P10, P20, P37, P41, P45, P46,
Child Principle
(CT 4.1) Indeterminacy14
(CT 4.2) Idiosyncrasy15
P47, P59, P62, P64, P69
15 – P3, P5, P6, P7, P14, P15, P17, P24, P27, P36,
P39, P40, P42, P43, P44, P47, P51, P56, P57,
P59, P63, P64, P71, P72
1 3Table 1 (continued)
Theme
Theme CT5: Making the Decision-Making
(CT 5.1) Misconduct, maltreatment and abuse
Process Harder
allegations16
(CT 5.2) Tied Parents: “I cannot pick one”17
(CT 5.3) Legal actors’ emotional struggles18
Theme CT6: Assessing the Child’s Best
Interests in Child Custody Cases: Evaluation
Services
(CT 6.1) ‘Psychosocial Study’: the Brazilian
model19
Trends in Psychology
Data anchoring
16 – P2, P3, P6, P9, P13, P16, P18, P24, P25, P35,
P36, P37, P38, P44, P45, P54, P56, P57, P59,
P62, P63, P65, P66, P67, P71, P72
17 – P1, P27, P28, P44
18 – P16, P27, P34
19 – P2, P3, P4, P8, P10, P12, P13, P21, P22, P23,
P24, P26, P35, P36, P39, P41, P42
20 – P49, P50, P52, P53, P54, P56, P57, P59, P60,
P69
(CT 6.1.1) Family Firefighters: the role of psycho-
social evaluation
(CT 6.1.2) Interdisciplinarity
(CT 6.1.3) Non-protocol-based practice
(CT 6.2) ‘Children and Family Court Advisory
and Support Service – CAFCASS’: the English
model20
(CT 6.2.1) Protocol-based practice: Children Act’s
Sect. 7 Report
(CT 6.2.2) Non-evidence-based practice
Theme CT7: Making a Child’s Arrangement
21 – P1, P15, P16, P21, P23, P27, P43, P44, P49,
Decision Involving Adolescents
(CT 7.1) “It’s quite impossible to go against their
will”21
P50, P51, P52, P55, P66, P69, P71
22 – P2, P35, P42, P43, P44, P45, P47, P73
(CT 7.2) “They can play the game too”: getting
into the litigating parents’ dynamic22
Fig. 2 Thematic map
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Trends in Psychology
decision-making process. For instance, this domain comprises issues concerning
family life, family development, family member roles, parenting, co-parenting, liti-
gation and coping strategies after the divorce.
Theme CT1: Parental Separation: Crisis and Family Life Cycle
The feature Dysfunctionally coping with divorce: family crisis (CT1.1) captures dys-
functional strategies used by families to cope with times of hardship after parental
separation:
I understand that [parents are] going to court and asking the judge what are
the best interests of the child is a dysfunctionality in the family itself (BR_
SP.Psy.01)
Generally, what tends to happen is that there is a lot of heat when it comes to
[parental] separation and that kind of tends to cloud a lot of the judgements
when it comes to contact [with the child] (EN_Lw.03)
Some legal actors see family dysfunctionality whenever a family goes to court for
the purpose of delegating to a third party (the judge) the power to solve their prob-
lems. This dynamic might be driven by multiple difficulties that the whole family
endures during a separation. The intensification of these difficulties can lead a fam-
ily—especially the parents—to become blind to the child’s interests and the family’s
well-being. This process can be characterised as a family crisis moment:
Everyone is very hurt, and there is no communication. Making a decision
regarding child custody at this moment is very complicated (BR_Pr.01)
[the parents need to] cope and overcome this moment of crisis so they will be
able to see and care for their child again (BR_POA.Psy.01)
The feature Misunderstanding and pathologisation of family interactions and
coping strategies in the context of custody dispute: perspectives on parental aliena-
tion (CT1.2) captures legal actors’ and the judiciary’s conceptions and understand-
ings regarding the family crisis, which see some of the family dysfunctional coping
strategies as examples of ‘parental alienation’. This is considered to be a frequent
issue in judicial custody disputes. For some legal actors, its presence will make deci-
sion-making more difficult and impair the child’s role within it, as it is likely that the
child will be co-opted by one of the parents:
I see as more difficult cases, those in which there is a clear Parental Alienation
Syndrome already installed because we have the practice of alienation already
installed (BR_BsB.Jd.01)
Parental alienation [is a situation] in which the child is in service of the adult’s
desire (BR_POA.Psy.04)
Other legal actors do not rely on parental alienation assumptions or accept its rel-
evance to the decision-making process, due to its broad definition and gratuitous use
within child custody cases:
1 3Trends in Psychology
I don’t like to use the term ‘parental alienation’ because it has a number of
connotations which don’t necessarily help (EN_Jd.02)
I think that parental alienation has become fashionable, when in fact you have
to value how this was built, how the other took part, and not whether or not
there is parental alienation (BR_SP.Psy.02)
The feature Parental separation as part of the family life cycle (CT1.3) cap-
tures conceptions that see parental separation as part of the family’s developmental
cycle, and that non-assertive behaviours might happen in such situations due to the
moment of crisis typical in parental separation:
It is a phase of life transition and that is how I see it. It is a phase of going
through transitions, and sometimes they are very emotional and people,
maybe, do not know how to deal with it in a positive way (BR_POA.SW.03)
Some people sometimes ask me: Does divorce destroy families? It depends on
the family; some get destroyed, others do not, and some [families] understand
that it is something temporary and that time will heal those wounds and the
children need to be protected (BR_BsB.Jd.01)
Theme CT2: Hindering the Best Interests of the Child
The feature Conjugality vs. Parenthood (CT2.1) captures a frequent issue faced by
separated parents involved in high-level litigation: they cannot distinguish parental
issues from conjugal ones:
Well, quite frequently my experience is that when there’s still hostility between
parents about why their marriage is broken down that can influence greatly
influence their attitude towards either visiting contact… to be able to see the
other parent, to be able to facilitate that (EN_Psy.09)
I think that [separating parenting from conjugality issues] it is something that,
many times, [must] pass through strong psychological support. I think the judi-
ciary is not always prepared for that (BR_Pr.02)
These excerpts highlight the risk of unsolved and problematic conjugal issues
overlapping with parental performance, at which point the child’s well-being is jeop-
ardised. Hence, for some interviewees, the acrimony between parents is based not on
the child’s interests but rather on issues stemming from the broken relationship.
The feature Detaching from the child and attaching to the litigation (CT2.2) cap-
tures issues related to situations in which the parents are so involved in their own
matters, and within which they keep up the conflict, that they can neglect and harm
the child’s well-being:
Parents go deep into the dispute and forget the child and the main aim, which
is to protect and ensure a healthy development for the child and promote a
positive familial coexistence (BR_POA.SW.01)
It’s about winning a case and not about what is best for the child at all. You
know, to the extent of completely ignoring what the child wants (EN_Lw.06)
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Trends in Psychology
The feature Lack of parenting skills (CT2.3) captures issues regarding parents
who do not have the necessary parental skills to protect their child:
I am going to call it the emotional immaturity of the parents, you know? This
is when there is no pathology involved (BR_Pr.05)
Sometimes a parent does not have the slightest ability to look after the child,
for various reasons, people who have problems with drugs, with alcohol, so we
have several cases like this (BR_BsB.Jd.01)
The feature “No ‘child maintenance’, no contact with the child” (CT2.4) captures
parents’ perspectives that misunderstand the best interests of the child by making the
contact between the child and the non-custodial parent conditional upon receipt of
maintenance payments:
Those with lower-wage parents misunderstand a lot the issue of alimony and
the issue of coexistence. So, if the father does not want to pay alimony, the
mother says: ok, then I will also not let you see my child. The child becomes a
bargaining chip (BR_BsB.Jd.02)
They [parents] associate alimony with the right to have contact with the child.
It happens especially amongst people who have very little education, this is
rare in the middle class, but it happens there too (BR_BsB.Jd.03)
Conflating child maintenance and the right to keep contact with both parents was
seen only in Brazilian interviews, as in England child maintenance is not a judicial
matter at first. This issue is commonly associated with low-income families in Brazil.
The feature Misunderstanding joint custody (CT2.5) captures misunderstandings
regarding this type of arrangement:
Sometimes the person says: Ah, I want joint custody because I want to see my
son every day. This is not joint custody. The joint custody is joint care, co-
responsibility (BR_BsB.Lw.02)
The parents see the joint custody as a kind of mystery, it is something that
“everybody likes” but they do not have a clear notion about what this kind of
arrangement really is (BR_SP.Lw.04)
This issue was reported only by Brazilian participants, possibly because Brazilian
law contributes to this misunderstanding:
The law does not define well what this joint custody would be, because, you
see, in truth, family power [i.e. parental responsibility] was already enshrined
in the law beforehand (BR_Pr.02)
The feature Involving the child in parental conflict (CT2.6) captures issues related
to high-level litigation situations in which the parents involve the child in their con-
flict, by either co-opting them to one side, forming alliances or neglecting the chil-
dren who are forced to assume roles and functions more suited to adults or parents:
[the parents can harm the child’s best interests when] putting pressure on the
child, or, first of all, by exposing the children to the conflict, by negative talk
about the other parent (EN_SW.01)
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The child feels in the middle of it and is often put in a position of mediating
this dispute between parents. It demands from the child a psychological basis
and structure that are not there. I have seen cases in which the child ends up
somatising these struggles (BR_SP.Psy.03)
Some children become carers for parents who are facing a really difficult mar-
riage breakdown. They take on too much responsibility, emotionally they’re
not really ready for (EN_Psy.09)
These excerpts highlight how reckless parental litigation can prove prejudicial
towards children caught up in such situations, as they can either get triangulated
within their parents’ conflicts (pushed to pick sides and form alliances) or be forced
to assume parental roles and functions that they should not have to.
Theme CT4: Applying the Best Interests of the Child Principle
The feature Idiosyncrasy (CT4.2) captures characteristics that make the assurance of
the best interests of the child principle (BIC) very idiosyncratic:
It [BIC] will depend on the customs, moral and cultural values of each family,
because we know that each family has its principles, its morality, and this will
vary from family to family (BR_BsB.Lw.01)
Therefore, I consider that [BIC] is extremely subjective from case to case
because it varies so much, the way that the guidelines are interpreted (EN_
Psy.04)
These idiosyncratic characteristics indicate that assuring the best interests of the
child depends on moral and cultural variations between families, and consequently
for each child in their respective circumstances. Therefore, this principle cannot be
generalised for all cases.
Theme CT5: Making the Decision‑Making Process Harder
The feature Misconduct, maltreatment and abuse allegations (CT5.1) captures situ-
ations in which there are allegations of abuse, violence or maltreatment against the
child that make the custodial decision-making process even harder:
They [hardest cases] are those in which there are allegations of violence of any
kind (BR_SP.Psy.02)
Cases involving allegations of sexual abuse [are the hardest]. Because they are
almost impossible to prove always. It is very difficult to find pieces of evi-
dence to support them because they sound more like made-up narratives (BR_
SP.Psy.04)
Whether there are domestic violence allegations, true or not, whether there is a
sexual abuse allegation or not… that causes problems, whether it’s true or not
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because the court doesn’t know how to deal with it, only the parties know or
only God knows whether that is true (EN_Lw.02)
Cases with allegations of maltreatment and violence seem to be the most difficult
because they bring into play two essential elements to consider during the decision-
making: 1) jeopardy regarding the child’s physical and psycho-emotional integrity;
and 2) allegations without proof. This can be a dilemma for decision-makers as,
although they value safeguarding the child’s physical and psycho-emotional well-
being, they are committed to making decisions based on concrete and provable facts.
Theme CT7: Making a Custodial Arrangement Involving Adolescents
The feature “They can play the game too”: getting into the litigating parents’
dynamic (CT7.2) captures legal actors’ perceptions that adolescents can consciously
and intentionally involve themselves in the parental conflict:
They tend to make alliances with one or the other according to their own inter-
ests (BR_Pr.06)
The chances of the child finding they can play one off against the other are
massively enhanced and … that’s quite often the case that leads to the kind of
private law proceedings in which I end up getting involved (EN_SW.05)
Apparently, adolescents are not only more capable of expressing their voice and
voting with their feet, they also get involved intentionally in their parents’ conflict to
take advantage or to adjust themselves to the litigation dynamic within their family.
Family Court Domain
The ‘family court’ domain regards themes that comprise factors related to legal
issues that constrain the decision-making process. These issues refer to the applica-
tion of the law and its limits and procedural issues as well as how the court addresses
the child during the decision-making process. Based on the participant’s account-
ings regarding law limitations and legal mindset, we understand that these issues,
alongside the family domain ones, are what most pressurize the decision-making
process in child custody cases.
Theme CT3: the Judiciary’s Constraints and Practices
The feature “The Law is powerless”: legal and epistemological limitations of law
(CT3.1) captures issues that the law cannot affect or control, such as domestic
dynamics, parents’ behaviours outside the court, and daily routines involving the
child. Also, law limitations would refer to the impossibility of preventing the child
from suffering during parental separation:
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I think in every divorce, or almost every divorce to some degree, the child
suffers, that is my perception. But I think the law is powerless to solve this
kind of problem (BR_BsB.Jd.04)
We can make orders about what should happen to a child, but judges have
no power to make sure it will happen (EN_Jd.01)
The [family’s] reality often does not fit into legal guidelines (BR_BsB.
SW.01)
Another factor that constrains the legal work in child custody cases is the
intrinsic adversarial modus operandi of law practice, which tends to lead parents
into acrimonious litigation by encouraging a ‘litigious mindset’:
If people want to fight, they will be able to and they will continue to fight
whether the judgment has closed the case or not, because usually in a case
like this, one parent wins and the other one loses (BR_Pr.03)
Theme CT4: Applying the Best Interests of the Child Principle
The feature Indeterminacy (CT4.1) captures legal and conceptual limitations that
make ‘the best interests’ an unclear and vague construct:
I have no way of giving you a definition [for BIC]. If you are going to look
into the doctrine that underpins it, there is no specific definition for that
principle (BR_BsB.Lw.01)
I think it’s a very fluid concept, the best interests of the child. I think it’s
open to interpretation (EN_Lw.07)
Although the vagueness of ‘the best interests’ can be an issue for some legal
actors, it seems a good thing for others:
So it [BIC] being broad allows us to do this analysis case by case. […] If
it was rigid, we would not be able to interpret it well. I prefer it to be open
(BR_BsB.Jd.03)
In this sense, the ‘best interests’ indeterminacy can highlight the legal actors’
discretionary power by allowing them to freely interpret what are the best inter-
ests of the child according to each case.
Theme CT5: Making the Decision‑Making Process Harder
The feature Tied parents: “I cannot pick one” (CT5.2) captures perceptions
regarding situations in which both parents present similar contexts:
In situations where there is no clarity about who has the best conditions to
protect or at least to take better care of the child [it is hard to make a deci-
sion] (BR_BsB.Jd.01)
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What is more difficult are those cases in which both parents want the cus-
tody and both have similar conditions to be awarded the custody (BR_Pr.03)
Theme CT7: Making a Custodial Arrangement Involving Adolescents
The feature “It’s quite impossible to go against their will” (CT7.1) captures legal
actors’ perceptions that it is impossible to force an adolescent to comply with a
legal custody decision:
The older the children, the judge becomes increasingly powerless (EN_
Jd.01)
They [adolescents] are going to vote with their feet; in other words, the ado-
lescent will go to live with whichever parent he or she wants to live with
(EN_Jd.03)
No judge or legal measure is capable of determining what an adolescent should
do regarding their custody because, at the end of the day, they can do whatever
they want once they leave the court. The older the adolescent, the weaker are
legal custody measures.
Legal‑Psychosocial Domain
The ‘legal-psychosocial’ domain comprises themes that regard the evaluation ser-
vices in Brazil and England. It also refers to some legal actors’ practices and their
emotional struggles during the decision-making process.
Theme CT3: the Judiciary’s Constraints and Practices
The feature Between fear and bravery: the psychologists’ practice in Brazil
(CT3.3) captures Brazilian psychologists’ perceptions on the edges of their work:
It has happened to me that a lawyer questioned my competency and attached
my résumé to the case transcripts in order to question my work. He had his
own retained expert, then he used my résumé to claim that I was not good
enough. […] This aspect, this characteristic of private family law cases
makes us [staff] quite reluctant (BR_SP.Psy.04)
In Brazil, the work of psychologists bounces between the fear of being targeted
by the litigating dynamic (as pointed out by BR_SP.Psy.04) and the bravery to act
as the child’s advocate.
The feature An advocate in intractable cases: the psychologists’ practice in
England (CT3.4) captures English psychologists’ commitment to safeguarding
the child’s welfare in intractable cases:
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[I see myself as] an advocate for the child. So, you are working for... If
you’re working with the child you’re working for the child (EN_Psy.02)
In England, the work of psychologists is required only on complex or intractable
cases. This policy might be justified by the fact that the services of a psychologist in a
child custody case tend to be more expensive than the services of social workers. None-
theless, some psychologists see themselves as an advocate for the child in such cases.
Theme CT6: Assessing the Best Interests of the Child in Child Custody Cases:
Evaluation Services
The feature ‘Psychosocial study’: the Brazilian model (CT6.1) captures the Brazilian
evaluation process carried out by psychosocial staff, called a ‘psychosocial study’.
It is similar to the idea of the ‘case study’ common within psychology and social
work. However, understandings about the goals of such a study can vary amongst
psychosocial staff:
Whenever the case goes to psychosocial study, it is because the parental con-
flict is very serious (BR_BsB.Jd.03)
So not all cases go to a psychosocial evaluation. Only cases in which we notice
a conflict; cases in which the parents agree do not go to psychosocial evalua-
tion (BR_Pr.01)
Judges and prosecutors tend to see psychosocial staff as ‘family firefighters’, the
only solution for intractable cases. In turn, some psychosocial professionals see their
role as a mediator:
I think when I help adults to reflect on what is best for a child, on how the
child will be better, I am doing something the judiciary should do, which is
to protect the child. I think that protection should be present in all instances
(BR_BsB.SW.01)
[The psychosocial staff role] is to promote reflection, and intervention in some
cases, where we perceive cases of vulnerability or risks that are spotted and
referred to the support network (BR_BsB.SW.02)
The lack of guidelines and protocol surrounding the evaluation is another charac-
teristic of the Brazilian system:
We do not have a standard, a rigid methodology (BR_POA.Psy.03)
We do not use any protocol (BR_BsB.Psy.03)
I think professional freedom is important, but I think it is also important to
build a methodology of service, something that is consistent and incorporates
some principles (BR_BsB.Psy.05)
The feature ‘Children and Family Court Advisory and Support Service – CAF-
CASS’: the English model (CT6.2) captures characteristics of the assessment carried
out by English social workers from CAFCASS:
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In most of those cases, there will be a report on section 7 of the Children Act,
prepared either by a CAFCASS office or, if local authorities social services are
involved, by a social worker.” EN_Jd.01
Unlike the Brazilian ‘psychosocial study’, the evaluation process in England is a
more structured assessment with clear guidelines both from the Children Act 1989
(Sect. 7) and CAFCASS. However, there is a lack of evidence-based practice in
England22:
Reading through [the report], it was just absolute nonsense, it was just the
CAFCASS officers’ views, it wasn’t based on facts, or logic or reasonableness
(EN_Lw.02)
I would say that a lot of the guidance we used to follow in CAFCASS was
based on opinion, as opposed to hard research or based on evidence, and I
think that could be a criticism that you might level at the system (EN_SW.01)
Also, there is ‘risk-avoidance’23 related to the CAFCASS officers’ work:
I do think that they are a very risk-averse organization. They certainly have
become that. So, for instance, they will always take the safest route, safest
route even if it means that a child potentially might suffer by not having a rela-
tionship (EN_Lw.04)
Discussion
We understand that context factors displayed throughout the themes resemble what
Wells (1978) called ‘estimator variables’ in eye-witness testimony within criminal
justice. This type of variable affects the legal process but is not under its control.
In the case of eye-witness testimony, they are part of the context in which the per-
son witnessed a crime, and which consequently can influence a person’s testimony.
Similarly, context factors constrain child custody cases and influence the decision-
making process but they are not under the control of the legal system or decision-
makers.24 Therefore, context factors produce uncertainty.
22 The CAFCASS website states that “practitioners use the Child Impact Assessment Framework (CIAF)
when carrying out their analysis. The CIAF is a structured framework that sets out how children may
experience parental separation and how this can be understood and assessed at Cafcass. It builds on
our existing knowledge and guidance and follows a consistent and evidence-informed approach helping
practitioners to find an outcome that is in the best interests of the children involved. The framework is
informed by external research and our experience of supporting 140,000 children per year”. In regards to
‘risk-avoidance practice’, the CAFCASS website also outlines the process by which CAFCASS are asked
to advise the court on what is best for the child, who are ultimately required to make a decision based on
all of the information that is presented to them.
23 Idem 22.
24 Sometimes, the judiciary has the power to exercise control over these issues but it is impeded by
micro or macro issues that limit powers or make them impossible to exercise. Examples are the number
of cases that reach the judiciary, and financial limits on the number of legal civil servants available to
tackle cases.
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These estimator variables can impact the making of a decision in child custody
cases as well as the child’s best interests. On one side, the family uses dysfunctional
strategies to suppress the emotional distress caused by the divorce – these can blur
the way legal actors perceive and understand the context in which the child’s inter-
ests shall be safeguarded. On the other one, the laws and legal actors’ practices,
shape how these interests will be understood and assured in such cases. Hence, the
outcome for what is best for the child will depend on how both families and legal
actors found themselves in each side as well as the quality of the interaction between
them amongst those uncertainty factors.
Every decision-making process that occurs in a natural setting will be surrounded
by uncertainty (Klein et al., 1993). In general, ‘uncertainty’ in real-life decision-
making refers to the doubts generated by the perception of a certain problem and
that struct and shape the search for a solution (Lipshitz & Strauss, 1997; Lipshitz,
1993b). We understand that the assembling of ‘estimator variables’, and interactions
between and within them, is what structures the uncertainty in child custody cases
after parental separation. However, we believe that context factors prompted by the
family are the main source of uncertainty in such cases.
‘Family’: the Foremost Domain of Uncertainty in Child Custody Cases
We believe that context factors in the family domain tend to produce most of the
uncertainty in the decision-making process. The harder it is for the family to deal
with the developmental crisis that parental separation prompts, the more uncertain
the case shall be. That is because individuals and families going through a crisis
are expected to act erratically, in a disorganised way, and usually employ non-asser-
tive coping strategies (Mendes & Bucher-Maluschke, 2017; Sá et al., 2008). In this
sense, it is possible that law professionals might have more difficulties in dealing
with the families’ struggles than dealing with issues regarding the ‘family court’ and
‘legal-psychosocial’ domains because the family’s struggle relates more to psycho-
social issues than legal ones.
Features that encompass the family domain portray some interesting dynam-
ics. For instance: a) family developmental crisis after parental separation (CT1.1;
CT1.3); b) conjugal vs parental issues (CT2.1; CT2.2); c) triangulations and col-
lusion inside the family (CT1.2; CT7); and d) maltreatment and abuse allegations
(CT5.1).
It is known that parental separation is linked to the family’s development, being
part of its life cycle and representing a crisis moment to the family system (McGol-
drick et al., 2014; Mendes & Bucher-Maluschke, 2017). The Family Life Cycle, in
which parental separation occurs, is paced by developmental steps marked by uncer-
tainty, instability and disorganisation, that push family interactions towards a change
of patterns that will lead it to the next step of its development (Mendes & Bucher-
Maluschke, 2017). However, a lot of families struggle with this transitional process
and try to cope by means of dysfunctional and non-assertive strategies. This is a key
point in the child custody decision-making process because this dynamic can shape
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not only the parents’ attitudes and behaviours throughout proceedings but can also
shape the characteristics of the information that shall be evaluated and taken into
account to make decisions.
Those non-assertive coping strategies displayed by the family can misdirect
the decision-making and hinder the child’s role during a child custody dispute
(CT1.2 [CT1.2.1; CT1.2.2]). An example is what some legal actors label as
‘parental alienation’. This is a very fragile concept if one considers its conceptual,
scientific, ethical and technical dimensions (Barbosa et al., 2021; Barnett, 2020;
Bruch, 2001; Mackenzie et al., 2020; Meier, 2020; Mendes & Bucher-Maluschke,
2017; Neilson, 2018; Pepiton et al., 2012; Shaw).
‘Parental alienation’ is a label that derives from the incomplete, imperfect,
ambiguous and/or simplistic information available in child custody cases. Infor-
mation in this scenario is fed and blurred by developmental struggles that the
family display after parental separation. When legal actors are not aware of that,
labelling can be an ‘easy way’ to go through a complex, erratic, multidetermined
and dynamic scenario. This is a problem, as overly simplistic labels like ‘parental
alienation’ can engender a ‘rebound effect’, as they tend to produce more of what
they should tackle: uncertainty and litigation. That is because the type, amount
and shape of uncertainty with which decision-makers must deal with will depend
on the decision-making strategies they are applying (Lipshitz & Strauss, 1997).
Hence, by applying over-simplistic uncertainty-coping strategies, legal actors
might face even more uncertainty. Therefore, these labels can increase the fami-
lies’ struggles (Barbosa et al., 2021; Mendes & Bucher-Maluschke, 2017), which
might enhance the uncertainty and impair the child’s interests. In sum, what
labels such as ‘parental alienation’ do is create a vicious cycle of uncertainty in
child custody cases, as the uncertainty prompted by a family’s developmental
struggles might lead to procedures and decisions that worsen the family’s devel-
opmental struggles and, therefore, add more uncertainty to the decision-making
process.
Adolescents are significant players in the child custody scenario as they might
be consciously involved in parental conflict (CT7.2). This triangulation on the
part of the adolescent in the parents’ conflict shows that adolescents are not only
active players in such cases but that they are also active in similar ways inside
their family. Triangulation and collusion dynamics are common in child custody
cases after parental separation. These dynamics are not necessarily dysfunctional
or even permanent and they can be a way in which the family can go through
and adjust itself to transitional developmental stages, especially very challenging
ones (Emery, 2012; Juras & Costa, 2017). In this sense, some triangulations can
even benefit the family. The problem is when the dynamic of a triangulation loses
its transitional and adaptive character and becomes a long-lasting transactional
structure, highlighting fixed and rigid oppositions that increase tension between
family members. This can lead to coalitions, inflexible loyalties and triangulated
conflicts that impede the family’s progress through its functional development
(Barbosa et al., 2021; Juras & Costa, 2017; Mendes & Bucher-Maluschke, 2017).
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The Main Difference Between Brazil and England
We have observed interesting legal and cultural differences between Brazil and Eng-
land that can impact the decision-making process.25 For instance, there is the way
legal actors perceive divorce/parental separation. Families going through parental
separation and child custody disputes seek judicial aid when they are facing a crisis
moment (Mosten & Traum, 2017). However, only Brazilian participants acknowl-
edged that and the dysfunctional dynamic it brings about. Only 11% of the total
participants (Brazilian) referred to parental separation as part of the family life
cycle. These frequencies yield that Brazilian legal actors might be more aware of the
uncertainty caused by those context factors than English ones. Nevertheless, only a
few of Brazilian legal actors see the separation as a potential phase for the family’s
development.
There are also differences regarding the way professional evaluation is carried out
in each country. It tends to be non-protocol based in Brazil and non-evidence based
in England. In the psychosocial evaluation, the safeguarding of the child’s interests
can be weakened if one considers that the work carried out by psychologists and
social workers in Brazil tends to be non-protocol-based (CT6.1[CT6.1.3]) and non-
evidence based in England (CT6.2[CT6.2.2]). These results are surprising since we
expected the Brazilian evaluation process to be stricter and structured due to its civil
law system, which relies on written law rather than case law and customary practice.
We also expected the English evaluation process to be more loose and marked by
workarounds due to its common law system. However, we saw the opposite. Some
Brazilian participants indicated that “the [family] reality often does not fit into legal
guidelines” (BR_BsB.SW.01), so their practice needs to be more open and worka-
rounds need to be applied so they can properly approach the case and cope with
uncertainty. Even though English participants were working in a more open and cus-
tomary system, they indicated that they rely heavily on protocols: “I tend, certainly,
on a difficult case, to go through each element of the welfare checklist [from Chil-
dren Act 1989] quite slavishly” (EN_Jd.01). Based on this, we understand that the
nature of the legal system itself (civil or common law) is not what makes the tack-
ling of uncertainty easier or harder for legal actors. In fact, this reinforces our belief
that context issues, especially those regarding family developmental struggles are
the greatest source of uncertainty in child custody cases.
What to Make of These Results: a Preliminary Evaluation
We understand that context factors are contingencies that impact legal actors’ per-
formance throughout the decision-making process by influencing the cognitive strat-
egies they choose to cope with uncertainty (Mendes & Ormerod, 2022). However,
25 Brazil is the most catholic country in the world. Hence, religious beliefs are likely to play a role in all
matters concerning society, families and the justice system. However, religious beliefs were not pervasive
or salient within the data. We believe further studies focused on legal actors’ religious issues are needed
to properly investigate the role of these issues in child custody cases after parental separation.
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context factors can also cue strategies that generate errors and biased judgements.
Being aware of these factors, and properly interpreting them, might be the first step
in assertively handling uncertainty in child custody cases as the understanding of
contextual issues is an important part of the decision-making process (Ben-Haim,
2019).
In a scenario of decision-making under uncertainty, any approach to tackling
uncertainty is welcome, especially when ignoring uncertainty is more attractive and
easier than recognising it and properly coping with it (Marchau et al., 2019). There
are three typical strategies used to cope with uncertainty during a decision-making
process (Lipshitz & Strauss, 1997): (1) reduce uncertainty; (2) acknowledge uncer-
tainty; and (3) suppress uncertainty.
The strategies to reduce uncertainty are mainly anchored in collecting additional
information before making a decision. Whenever further information is not avail-
able, the decision-maker can make some extrapolations based on the information
available and then make a decision/take an action. In child custody cases, the strat-
egy to reduce uncertainty would start with the collection of all relevant and avail-
able information that might influence the decision-making process. This includes
the information about the context factors presented in this paper. In principle, the
themes presented in this paper can be used as an informal checklist by legal actors
to ensure that they have considered all possible sources of uncertainty. Even though
some of them might not be very novel for part of the readership, we believe that hav-
ing them structured and organised and published, alongside pertinent discussions, is
an important step for an informed and evidence-based practice within the family jus-
tice system (Danser & Faith‐Slaker, 2019).26 Moreover, providing evidence is also
important to provoke relevant changes and policy-making within organisations like
the judiciary (Sanderson, 2002).
Another strategy to handle uncertainty is to acknowledge and properly manage
the sources of uncertainty. One cannot control or promote ‘harm reduction’ of what
one does not know. Hence, legal actors cannot properly tackle uncertainty if they do
not acknowledge it and how it can affect their decision-making process. In this sense,
we believe this paper can promote awareness regarding the importance of acknowl-
edging the uncertainty in child custody cases and, therefore, be able to select better
courses of action that can avoid or handle risk factors (Lipshitz & Strauss, 1997),
especially for the child’s interests and the family well-being.
The ‘suppression strategy’ regards actions that either deny (e.g. ignoring or dis-
torting information that is unwelcome) or rationalise the uncertainty within the deci-
sion-making process. Our data suggest that this is a strategy invoked by some legal
actors—e.g. CT1.2. We do not believe this is a good strategy to cope with uncer-
tainty in child custody cases as this can lead to increasing uncertainty and, therefore,
can put children and families in jeopardy. Instead, we believe that the best course of
action is to acknowledge the sources of uncertainty (like the ones presented in this
paper), map how they might affect the decision-making process in that specific case
26 Qualitative evidence is important for an evidence-based practice—See Sale and Thielke (2018).
1 3Trends in Psychology
and then, based on evidence-based practice, reduce uncertainty and make decisions
that really are child-centred.27
Limitations and Future Directions
This study’s design and the data gathered do not allow us to determine the optimal
ways with which one can cope with uncertainty in child custody cases.28 They also
do not allow us to properly approach the role of legal actors’ systems of beliefs in
acknowledging and dealing with context factors and the uncertainty they produce.
However, we believe this paper can help legal actors to understand how uncertainty
in child custody cases constrains their performance and, thus, make them more
aware of it—which is an important step in the tackling of uncertainty as mentioned
before.29
Even though the results of this study make progress in understanding how con-
text factors structure uncertainty in child custody cases, there are still processes that
need to be investigated, such as how context factors are measured or weighed by
legal actors when making a decision in a specific case and the role of ‘system of
beliefs’—as mentioned. Also, future work should examine the strategies used to
cope with uncertainty and whether there are optimal ways to cope with uncertainty
in such cases, taking into account the child’s best interests.
Final Considerations
This paper allowed us, for the first time, under a ‘naturalistic decision-making’
approach, to identify and organise issues that shape uncertainty in child custody
cases after parental separation. This is important, both to draw the attention of legal
actors and academia to the role of the context in child custody cases and also to initi-
ate research into ways of coping with uncertainty, aiming to avoid or diminish errors
and biased judgments. We understand that the results presented in this paper not
only further the knowledge in an underresearched field but they can also help legal
27 This is especially needed in Brazil, where family justice tends to adopt non-evidence based as well as
ethically and scientifically questionable practices to mediate and solve conflicts/litigation within family
courts—e.g. ‘systemic constellation work’ or ‘family constellation’: a mediumistic pseudo-psychother-
apy imported from Germany without any sort of transcultural adaptation and/or scientific probe towards
its efficacy within the family justice.
28 In the major study from which these results were extracted, we identified eight cognitive strategies
used by legal actors to cope with uncertainty in child custody cases. Like context factors, we identified
two domains for these strategies: (1) heuristics: strategic knowledge used to search the environment and
set up shortcuts to make a decision; and (2) metacognition: referring to metacognitive knowledge that
serves to monitor the decisions made and to make sure those decisions abide by the goal state. These
domains are further explored by Mendes and Ormerod (2022).
29 The results from this study were also pivotal to helping us develop an experiment that might allow us
further the discussion regarding ways to better cope with uncertainty and arrive at better decisions. It is
a verbal protocol analysis based on a decision-making experiment with legal actors from Brazil and Eng-
land. Currently, we are writing the results to then submit them for publication.
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actors to be more aware of the sources of uncertainty in child custody cases that can
impact their performance during the decision-making process.
Supplementary Information The online version contains supplementary material available at https:// doi.
org/ 10. 1007/ s43076- 022- 00253-9.
Funding This study was funded by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior –
Brasil (CAPES) – Finance Code 001, Ministry of Education, Brazil.
Data Availability Data not available due to ethical restrictions.Due to the nature of this research, partici-
pants of this study did not agree for their data (whole transcripts) to be shared publicly. However, some
supporting material concerning the data analysis process will be available for both reviewers and readers.
Declarations
Ethics Approval This study was approved by University of Sussex’s Sciences & Technology C-REC under
the Certificate of Approval ER/JA454/1.
Informed Consent Informed consent was obtained from all individual participants included in the study.
Consent for Publication All participants gave consent for their data to be used in publication.
Conflict of Interest The authors declare no competing interests.
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10.1112_topo.12275.pdf
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Received: 8 March 2021
Revised: 4 July 2022
Accepted: 20 September 2022
DOI: 10.1112/topo.12275
R E S E A R C H A R T I C L E
Journal of Topology
Toroidal integer homology three-spheres have
irreducible 𝑺𝑼(𝟐)-representations
Tye Lidman1
Juanita Pinzón-Caicedo2
Raphael Zentner3
Abstract
We prove that if an integer homology three-sphere con-
tains an embedded incompressible torus, then its funda-
mental group admits irreducible 𝑆𝑈(2)-representations.
M S C 2 0 2 0
57R58 (primary)
1Department of Mathematics, North
Carolina State University, Raleigh, North
Carolina, USA
2Department of Mathematics, University
of Notre Dame, Notre Dame, Indiana,
USA
3Fakultät für Mathematik, Universität
Regensburg, Regensburg, Germany
Correspondence
Raphael Zentner, Fakultät für
Mathematik, Universität Regensburg,
93040 Regensburg, Germany.
Email: raphael.zentner@mathematik.
uni-regensburg.de
Funding information
Sloan Fellowship; Max Planck Institute
for Mathematics in Bonn; Simons
Foundation, Grant/Award Number:
712377; DFG; NSF, Grant/Award
Numbers: DMS-1709702, DMS-1664567
1
INTRODUCTION
The fundamental group is one of the most powerful invariants to distinguish closed three-
manifolds. In fact, by Perelman’s proof of Thurston’s Geometrization conjecture [28–30],
fundamental groups determine closed, orientable three-manifolds up to orientations of the
prime factors and up to the indeterminacy arising from lens spaces. Prominently, the three-
dimensional Poincaré conjecture, a special case of Geometrization, characterizes 𝑆3 as the
unique closed, simply connected three-manifold. For a three-manifold with non-trivial funda-
mental group, it is then useful to quantify the non-triviality of the fundamental group. Since the
© 2023 The Authors. Journal of Topology is copyright © London Mathematical Society. This is an open access article under the terms of
the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the
original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
344
wileyonlinelibrary.com/journal/jtop
J. Topol. 2023;16:344–367.
17538424, 2023, 1, Downloaded from https://londmathsoc.onlinelibrary.wiley.com/doi/10.1112/topo.12275 by Universitaet Regensburg, Wiley Online Library on [22/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseTOROIDAL HOMOLOGY SPHERES AND 𝑆𝑈(2)-REPRESENTATIONS
345
Geometrization theorem implies that three-manifolds have residually finite fundamental groups
[18], this non-triviality can be measured by representations to finite groups. However, there is
not a finite group 𝐺 such that every three-manifold group has a non-trivial homomorphism
to 𝐺. Therefore, a more uniform measurement of non-triviality can be found in the following
conjecture.
Conjecture 1 (Kirby Problem 3.105(A), [19]). If 𝑌 is a closed, connected, three-manifold other than
𝑆3, then 𝜋1(𝑌) admits a non-trivial 𝑆𝑈(2)-representation.
Note that this conjecture is equivalent to the statement that the fundamental groups of all
integer homology three-spheres other than 𝑆3 admit irreducible 𝑆𝑈(2)-representations. Indeed,
every three-manifold whose first homology group is non-zero admits non-trivial abelian rep-
resentations to 𝑆𝑈(2). Moreover, lens spaces are examples of manifolds that admit non-trivial
𝑆𝑈(2)-representations of their fundamental groups, but no irreducible ones. There are also
three-manifolds with non-abelian fundamental group which do not admit irreducible represen-
tations [26]. However, for representations of perfect groups to 𝑆𝑈(2), non-triviality is equivalent
to irreducibility.
For comparison, the third author showed in [36] that Conjecture 1 is true if one replaces 𝑆𝑈(2)
with 𝑆𝐿2(ℂ). The reader may also relate Conjecture 1 with characterizing the three-manifolds with
simplest instanton or Heegaard Floer homologies. One side of the L-space conjecture predicts
that every prime integer homology three-sphere other than 𝑆3 and the Poincaré homology three-
sphere admits a co-orientable taut foliation. This fact, together with the gauge-theoretic methods
used by Kronheimer–Mrowka in [21], would then imply Conjecture 1.
There are many families of integer homology three-spheres for which Conjecture 1 has been
established, such as those which are Seifert fibred (although the methods go back to Fintushel–
Stern [13], this can be found explicitly in [32, Theorem 2.1]), branched double covers of non-trivial
knots with determinant 1 [8, Theorem 3.1] and [35, Corollary 9.2], 1∕𝑛-surgeries on non-trivial
knots in 𝑆3 [20], those that are filled by a Stein manifold which is not a homology ball [1], or for
splicings of knots in 𝑆3 [36].
It follows again from Geometrization that there are three (non-disjoint) types of prime integer
homology three spheres: Seifert fibred, hyperbolic, and toroidal ones. We remark that although
some toroidal integer homology three spheres are Seifert fibered, they are never hyperbolic. The
third author established that if all hyperbolic integer homology three spheres have irreducible
𝑆𝑈(2)-representations, then Conjecture 1 holds in general. While we are unable to complete the
remaining step in this program, we confirm the existence of 𝑆𝑈(2)-representations for toroidal
integer homology three spheres.
Theorem 1.1. Let 𝑌 be a toroidal integer homology three spheres. Then 𝜋1(𝑌) admits an irreducible
𝑆𝑈(2)-representation.
A proof of Theorem 1.1 could be obtained by showing that toroidal integer homology three
spheres have non-trivial instanton Floer homology. Although we expect the latter to be true (see
[19, Problem 3.106]), we do not prove it in this article. Our proof of Theorem 1.1 instead relies on
holonomy perturbations in a manner similar to the proof of [36, Theorem 8.3]. If 𝑌 is a toroidal
integer homology three-sphere, then 𝑌 can be viewed as a splice of knots 𝐾𝑖 in integer homol-
ogy three spheres 𝑌𝑖 for 𝑖 = 1, 2 (see, for example, [11, Proof of Corollary 6.2]). If some 𝑌𝑖 has an
17538424, 2023, 1, Downloaded from https://londmathsoc.onlinelibrary.wiley.com/doi/10.1112/topo.12275 by Universitaet Regensburg, Wiley Online Library on [22/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License346
LIDMAN et al.
irreducible 𝑆𝑈(2)-representation, then there is a 𝜋1-surjective map from 𝑌 to 𝑌𝑖 and we can pull
back to an irreducible 𝑆𝑈(2)-representation for 𝑌. If not, then we will study the image of the space
of representations of the knot exterior 𝑌𝑖 ⧵ 𝑁(𝐾𝑖)◦ in the character variety for the boundary torus
(that is, in the pillowcase). Here, 𝑁(𝐾𝑖) denotes a closed tubular neighborhood of 𝐾𝑖, and 𝑁(𝐾𝑖)◦
denotes its interior. Similar to the case of non-trivial knots in 𝑆3, if 𝑌𝑖 has no irreducible repre-
sentations, we will show that the image in the pillowcase contains a suitably essential loop. The
loops for the two exteriors will have a non-trivial intersection, and therefore, the spliced manifold
𝑌 will admit an irreducible 𝑆𝑈(2)-representation.
Theorem 1.1 gives a simpler proof of [36, Theorem 9.4] since it avoids the use of a finiteness
result of Boileau–Rubinstein–Wang.
Corollary 1.2 (Theorem 9.4, [36]). Every integer homology three-sphere other than 𝑆3 has an
irreducible 𝑆𝐿2(ℂ)-representation of its fundamental group.
Proof. By the remarks above we have to consider three cases: Seifert fibred, hyperbolic, and
toroidal integer homology three spheres. Let 𝑌 be an integer homology three-sphere other than 𝑆3.
If 𝑌 is hyperbolic, it admits an irreducible 𝑆𝐿2(ℂ)-representation by lifting the holonomy represen-
tation to 𝑃𝑆𝐿2(ℂ) [9]. If 𝑌 is Seifert fibred, then 𝜋1(𝑌) admits an irreducible 𝑆𝑈(2)-representation
□
by [32, Theorem 2.1]. If 𝑌 is toroidal, the result now follows from Theorem 1.1.
In order to generalize the holonomy perturbation machinery developed by the third author
from non-trivial knots in 𝑆3, we will need to establish a non-vanishing result which may be of
independent interest.
Theorem 1.3. Let 𝐽 be a knot in an integer homology three-sphere 𝑌 such that the exterior of 𝐽 is
irreducible and boundary-incompressible. Suppose that 𝐼∗(𝑌) = 0. Then, 𝐼𝑤
∗ (𝑌0(𝐽)) ≠ 0.
Here, and throughout this article, 𝐼∗ denotes Floer’s original version of instanton Floer homol-
∗ denotes instanton Floer homology for an admissible 𝑆𝑂(3)-bundle with second
ogy and 𝐼𝑤
Stiefel–Whitney class 𝑤. (Note that 𝑌0(𝐽) admits only one such bundle.)
The proof of Theorem 1.3 is a combination of (1) Kronheimer–Mrowka’s non-vanishing result
for instanton Floer homology of three-manifolds with a taut-sutured manifold hierarchy [22], (2)
the surgery exact triangle in instanton Floer homology, and (3) Gordon’s description of surgery
on cable knots [17]. The argument is similar to Kronheimer–Mrowka’s proof of Property P [21].
While Theorem 1.3 itself may not be particularly interesting, it does lead to the following
corollary, whose analog in Heegaard Floer homology has been established by Ni [27, p.1144] and
Conway and Tosun [7]. The proof of the corollary appears in Section 2 below.
Corollary 1.4. Let 𝑌 ≠ 𝑆3 be an integer homology three-sphere which bounds a Mazur manifold.
Then, 𝐼∗(𝑌) ≠ 0, and hence 𝜋1(𝑌) admits an irreducible 𝑆𝑈(2)-representation.
Recall that Baldwin–Sivek prove that if an integer homology three-sphere 𝑌 bounds a Stein
domain with non-trivial homology, then 𝜋1(𝑌) admits an irreducible 𝑆𝑈(2)-representation [1,
Theorem 1.1]. In light of Conjecture 1, the following conjecture would be a natural extension of
their work.
17538424, 2023, 1, Downloaded from https://londmathsoc.onlinelibrary.wiley.com/doi/10.1112/topo.12275 by Universitaet Regensburg, Wiley Online Library on [22/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseTOROIDAL HOMOLOGY SPHERES AND 𝑆𝑈(2)-REPRESENTATIONS
347
Conjecture 2. If 𝑌 ≠ 𝑆3 is an integer homology three-sphere which bounds a Stein integer homology
ball, then 𝜋1(𝑌) admits an irreducible 𝑆𝑈(2)-representation.
Since Stein domains admit handlebody decompositions with no three handles [12], Corol-
lary 1.4 proves this conjecture for the boundaries of Stein integer homology balls with the simplest
possible handle decompositions.
Theorem 1.1 also has two simple corollaries. The first one is obtained by considering branched
covers over satellite knots in 𝑆3. Remarkably, its proof requires no use of gauge theory, beyond
our main result. Its proof appears in Section 5 below.
Corollary 1.5. Let 𝐾 be a prime, satellite knot in 𝑆3. Conjecture 1 holds for any non-trivial cyclic
branched cover of 𝐾.
To obtain the second corollary, define a graph manifold integer homology three-sphere to be a
closed, orientable three-manifold whose torus decomposition has no hyperbolic pieces.† As dis-
cussed above, the fundamental groups of Seifert integer homology three spheres other than 𝑆3
admit irreducible 𝑆𝑈(2)-representations, and hence we obtain the following.
Corollary 1.6. Let 𝑌 be a graph manifold integer homology three-sphere other than 𝑆3. Then 𝜋1(𝑌)
admits an irreducible 𝑆𝑈(2)-representation.
A first alternate proof of this corollary can be obtained by noting that every integer homology
three-sphere other than 𝑆3 which is a graph manifold can be realized as the branched double cover
of a non-trivial (arborescent) knot in 𝑆3, see [4]. A second alternate proof can be obtained by not-
ing that every prime graph manifold integer homology three-sphere 𝑌 other than 𝑆3 or Σ(2, 3, 5)
admits a co-orientable taut foliation by [3, Corollary 0.3]. This implies that 𝐼∗(𝑌) ≠ 0, and this in
turn implies that there exists an irreducible 𝑆𝑈(2)-representation. On the other hand, the binary
dodecahedral group is well known to admit two conjugacy classes of irreducible representations,
completing the proof. Note that, unlike for Seifert integer homology three spheres, the Casson
invariant of a non-trivial graph manifold can be zero. For example, the three-manifold 𝑌 obtained
as the splice of two copies of the exterior of the right-handed trefoil has trivial Casson invariant
[5, 16].
Outline
In Section 2 we establish the main technical result Theorem 1.3 whose strategy also leads us to
prove Corollary 1.4 about Mazur manifolds. In Section 3, we review the pillowcase construction
and prove Theorem 1.1 in subsection 3.3, using a technical result about invariance under holon-
omy perturbations in instanton Floer homology reviewed in Section 4. The material in Section 4
is mostly known (or at least folklore knowledge) and can be found elsewhere, but the reader
might appreciate our synthesis of the role of holonomy perturbations and our sketch of invari-
ance in order to follow more easily through the proof of our main results. In Section 5, we prove
Corollary 1.5.
† Some authors impose additional constraints, such as primeness or a non-trivial torus decomposition.
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2
INSTANTON FLOER HOMOLOGY OF 0-SURGERY
LIDMAN et al.
In this section we rely solely on formal properties of instanton Floer homology to prove Theo-
rem 1.3 regarding the instanton Floer homology of 0-surgeries, and Corollary 1.4 regarding the
instanton Floer homology of integer homology three spheres that bound Mazur manifolds. More
concrete aspects of instanton Floer homology groups, in particular those regarding perturbations,
appear in Section 4, but in this section we wish to place the focus on the usefulness of formal
properties for purposes of computations.
We consider instanton Floer homology for admissible bundles, as introduced by Floer [15].
For integer homology three spheres, this is the trivial 𝑆𝑈(2)-bundle over 𝑌. For three-manifolds
with positive first Betti number, this is an 𝑆𝑂(3)-bundle 𝑃 → 𝑌 such that there is a surface Σ ⊆ 𝑌
on which the second Stiefel–Whitney class 𝑤 ∶= 𝑤2(𝑃) evaluates non-trivially, that is, such that
⟨𝑤2(𝑃), [Σ]⟩ ≠ 0. The instanton Floer homology group is defined as a version of Morse homology
of the Chern–Simons function on the space of connections on the admissible bundle [10, 15]. It
is denoted by 𝐼∗(𝑌) for the trivial bundle on integer homology three spheres, and it is denoted by
𝐼𝑤
∗ (𝑌) for 𝑆𝑂(3)-bundles 𝑃 → 𝑌 with 𝑤2(𝑃) = 𝑤. We remark here that for an integer homology
three-sphere, the trivial connection is isolated and is the unique reducible connection (up to gauge
equivalence). In the other cases, the admissibility condition ensures that there are no reducible
flat connections on the bundle.
In the case of a knot 𝐾 in an integer homology three-sphere 𝑌, there is a unique admissible
bundle on the 0-surgery 𝑌0(𝐾), because 𝐻2(𝑌0(𝐾); ℤ∕2) ≅ ℤ∕2. Therefore, the instanton Floer
homology group 𝐼𝑤(𝑌0(𝐾)) is defined without ambiguity.
Proposition 2.1. Instanton Floer homology satisfies the following properties.
(1) For 𝑌 an integer homology three-sphere and any 𝑛 ∈ ℤ, the three-manifolds 𝑌1∕𝑛(𝐾),
𝑌1∕(𝑛+1)(𝐾), and 𝑌0(𝐾) fit into an exact triangle
(2) If 𝑀 is an irreducible three-manifold with 𝑏1(𝑀) = 1, then 𝐼𝑤
(3) For 𝑌 an integer homology three-sphere, if 𝜋1(𝑌) admits no irreducible 𝑆𝑈(2)-representations,
∗ (𝑀) ≠ 0.
then 𝐼∗(𝑌) = 0.
∗ (𝑆2 × 𝑆1) = 0.
(4) 𝐼𝑤
Proof. The surgery exact triangle in (2.1(1)) is originally due to Floer [15, Theorem 2.4] with details
given in [6, Theorem 2.5]. The non-triviality result in (2.1(2)) is precisely [22, Theorem 7.21]. Next,
(2.1(3)) follows from [14, Theorem 1], since if 𝜋1(𝑌) admits no irreducible 𝑆𝑈(2)-representations,
then the generating set for the instanton Floer chain groups is empty. Finally, (2.1(4)) follows
from (2.1(3)) and (2.1(1)), by considering the surgery exact triangle for surgery on the unknot in
∗ (see Section 4), since 𝜋1(𝑆2 × 𝑆1) admits
𝑆3. Alternatively, this follows from the definition of 𝐼𝑤
□
no representations to 𝑆𝑂(3) which do not lift to 𝑆𝑈(2).
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349
We will be particularly interested in integer homology three spheres whose fundamental groups
do not admit irreducible 𝑆𝑈(2)-representations. We therefore establish the following definition.
Definition 2.2. An integer homology three-sphere 𝑌 is 𝑆𝑈(2)-cyclic if every 𝑆𝑈(2)-representation
of 𝜋1(𝑌) is trivial.
Notice that Conjecture 1 states that 𝑆3 is the only 𝑆𝑈(2)-cyclic integer homology three-sphere.
Having stated the above formal properties of instanton Floer homology, the proofs of
Theorem 1.3 and Corollary 1.4 now follow easily.
2.1
Non-vanishing of instanton Floer homology
In this subsection we illustrate the way the formal properties from Proposition 2.1 can be used to
show that the instanton homology groups are non-zero in two cases: (1) three-manifolds obtained
as 0-surgery along knots in 𝑆𝑈(2)-cyclic integer homology three spheres whose exterior is irre-
ducible and boundary incompressible, and (2) three-manifolds other than 𝑆3 obtained as the
boundary of a Mazur manifold.
Proof of Theorem 1.3. We assume that 𝐼𝑤
∗ (𝑌0(𝐾)) is trivial and argue by contradiction. By
Proposition 2.1(1) the three-manifolds 𝑌1∕𝑛(𝐾), 𝑌1∕(𝑛+1)(𝐾), and 𝑌0(𝐾) fit together in an exact
triangle
The assumption 𝐼𝑤
∗ (𝑌0(𝐾)) = 0 implies that there is an isomorphism
𝐼∗(𝑌1∕(𝑛+1)(𝐾)) ≅ 𝐼∗(𝑌1∕𝑛(𝐾)) for each 𝑛 ∈ ℤ.
In particular, if 𝑛 = 0, then 𝐼∗(𝑌1(𝐾)) ≅ 𝐼∗(𝑌) = 0 thus showing that for all 𝑛 ∈ ℤ,
𝐼∗(𝑌1∕𝑛(𝐾)) = 0.
(1)
Now, a result of Gordon [17, Lemma 7.2] shows that 𝑌1∕4(𝐾) is diffeomorphic to 𝑌1(𝐾2,1), where
𝐾2,1 is the (2,1)-cable of the knot 𝐾 (see Figure 5 for an example of 𝐾2,1). This together with
Equation 1 implies 𝐼∗(𝑌1(𝐾2,1)) = 0. An iteration of an exact triangle as in Proposition 2.1(1) for
surgeries along 𝐾2,1 gives 𝐼𝑤
∗ (𝑌0(𝐾2,1)) = 0.
We now consider a decomposition of 𝑌0(𝐾2,1) that includes the knot exterior of 𝐾 in 𝑌. Denote
1∕2 ⊂ 𝑆1 × 𝐷2, and
by 𝐶2,1 a closed curve that lies in the boundary of a ‘small’ solid torus 𝑆1 × 𝜕𝐷2
representing the class 2[𝑆1] + [𝜕𝐷2
1∕2). Notice that the 0-framing of 𝐾2,1 in 𝑌
induces the framing on 𝐶2,1 determined by the curve 𝜆 in 𝜕𝑁(𝐶2,1) that represents the class 2[𝑆1]
in 𝐻1(𝑆1 × 𝜕𝐷2) (see [17, p. 692]). Therefore, the manifold 𝑌0(𝐾2,1) can be expressed as the union
of the knot exterior 𝑌 ⧵ 𝑁(𝐾), and the result of Dehn surgery on 𝑆1 × 𝐷2 along the curve 𝐶2,1
1∕2] in 𝐻1(𝑆1 × 𝜕𝐷2
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LIDMAN et al.
F I G U R E 1 A Mazur manifold with one two-handle attached with framing given by 𝑛 for some 𝑛 ∈ ℕ
with framing given by 𝜆. By hypothesis the knot exterior 𝑌 ⧵ 𝑁(𝐾) is irreducible and boundary-
incompressible, and by [17, Lemma 7.2] the 0-surgery along the curve 𝐶2,1 is a Seifert-fibred space
with incompressible boundary. Hence 𝑌0(𝐾2,1) is an irreducible closed three-manifold with first
Betti number equal to 1 and with trivial instanton Floer homology. However, this contradicts
□
Proposition 2.1(2), which says that 𝐼𝑤
∗ (𝑌0(𝐾2,1)) ≠ 0.
Next, consider integer homology three spheres that bound a Mazur manifold, that is, a four-
manifold that admits a handle decomposition in terms of exactly one 0-handle, one 1-handle, and
one 2-handle which algebraically runs exactly once over the 1-handle, such as in Figure 1. Then
we have the following.
Proof of Corollary 1.4.
If 𝑌 bounds a Mazur manifold, then there exists a knot 𝐽 in 𝑌 such
that 𝑌0(𝐽) = 𝑆2 × 𝑆1. Moreover, if 𝐼∗(𝑌) = 0, a combination of the surgery exact triangle from
Proposition 2.1(1) and the computation 𝐼𝑤
∗ (𝑆2 × 𝑆1) = 0 from Proposition 2.1(4) shows once again
that 𝐼∗(𝑌1∕4(𝐽)) = 0. The same argument used above in the proof of Theorem 1.3 then gives
𝐼∗(𝑌0(𝐽2,1)) = 0.
However, the exterior of a knot in 𝑆2 × 𝑆1 which generates homology is either irreducible and
boundary-incompressible or a solid torus. To see this claim, suppose that the complement of a knot
𝐾 in 𝑆2 × 𝑆1 is reducible. Since 𝐾 is non-trivial in homology, it intersects every non-separating 𝑆2,
and so there are no 𝑆2 × 𝑆1 summands in the exterior of 𝐾. Hence, the exterior of 𝐾 is the con-
nected sum of a closed three-manifold 𝑁 other than 𝑆3 and a three-manifold with torus boundary
𝑀. But we know that there is a filling of the exterior of 𝐾 which is 𝑆2 × 𝑆1, and, in particular, this is
irreducible. Therefore, the separating 2-sphere must bound a ball on the side of 𝑀 after the filling.
But this implies that 𝑁 is 𝑆2 × 𝑆1, and this is a contradiction. On the other hand, if the exterior
of 𝐾 has compressible boundary and is not a solid torus, then that means that the exterior is the
connected sum of a solid torus with a closed three-manifold. The exterior of 𝐾 is then reducible, a
contradiction we have already established. So the only possibility that remains is that the exterior
is a solid torus.
The case where the exterior of a knot in 𝑆2 × 𝑆1 is a solid torus corresponds to 𝑌 = 𝑆3,
so by our assumption the exterior of 𝐽 is irreducible and boundary-incompressible. As in
the proof of Theorem 1.3 we get that 𝑌0(𝐽2,1) is irreducible with 𝑏1 = 1. But this contradicts
□
Proposition 2.1(2).
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351
3
THE PILLOWCASE ALTERNATIVE
In this section we recall the relevant background on 𝑆𝑈(2)-character varieties and generalize work
of the third author [36] to prove Theorem 1.1, our main result.
3.1
The pillowcase
Given a connected manifold 𝑀, we denote by
𝑅(𝑀) = Hom(𝜋1(𝑀), 𝑆𝑈(2))∕𝑆𝑈(2)
the space of 𝑆𝑈(2)-representations of its fundamental group, up to conjugation. We will write
𝑅(𝑀)∗ for the subset of irreducible representations. For example, the space 𝑅(𝑇2) is identified
with the pillowcase, an orbifold homeomorphic to a two-dimensional sphere with four corner
points. To see this, notice that since 𝜋1(𝑇2) ≅ ℤ2 is abelian, the image of any representation
𝜌 ∶ 𝜋1(𝑇2) → 𝑆𝑈(2) is contained in a maximal torus subgroup of 𝑆𝑈(2). Up to conjugation,
]
0
this torus can be identified with the circle group consisting of matrices of the form
𝑒−𝑖𝜃
for 𝜃 ∈ [0, 2𝜋]. Thus, if we denote the generators of 𝜋1(𝑇2) ≅ ℤ2 by 𝑚 and 𝑙, then, again after
conjugation, a representation 𝜌 ∈ 𝑅(𝑇2) is determined by
[
𝑒𝑖𝜃
0
𝜌(𝑚) =
[
𝑒𝑖𝛼
0
]
0
𝑒−𝑖𝛼
and
𝜌(𝑙) =
[
𝑒𝑖𝛽
0
]
,
0
𝑒−𝑖𝛽
and hence we can associate to 𝜌 a pair (𝛼, 𝛽) ∈ [0, 2𝜋] × [0, 2𝜋]. However, conjugation of 𝜌 by
gives rise to the representation associated to the pair (2𝜋 − 𝛼, 2𝜋 − 𝛽). This is
the element
0
−1
[
]
1
0
the only ambiguity, however, as can be seen using the fact that the trace of an element in 𝑆𝑈(2)
determines its conjugacy class. Therefore 𝑅(𝑇2) is isomorphic to the quotient of the fundamental
domain [0, 𝜋] × [0, 2𝜋] by identifications on the boundary as indicated in Figure 2.
If we have a three-manifold 𝑀 with torus boundary, then the inclusion 𝑖 ∶ 𝑇2 ≅ 𝜕𝑀 ↪ 𝑀
induces a map 𝑖∗ ∶ 𝑅(𝑀) → 𝑅(𝑇2) by restricting a representation to the boundary. For instance, if
𝐾 is a knot in a three-manifold 𝑌, then the three-manifold 𝑌(𝐾) ∶= 𝑌 ⧵ 𝑁(𝐾)◦ obtained by remov-
ing the interior of a tubular neighborhood 𝑁(𝐾) of 𝐾 from 𝑌 is a three-manifold with boundary a
two-dimensional torus. Figure 2 shows the image of 𝑅(𝑆3(𝐾)) when 𝐾 is the right-handed trefoil
in 𝑆3, once in the pillowcase, and once in the fundamental domain [0, 𝜋] × [0, 2𝜋]. Here we use
the convention that the first coordinate corresponds to 𝜌(𝑚𝐾), where 𝑚𝐾 is a meridian to the knot
𝐾, and the second coordinate corresponds to 𝜌(𝑙𝐾), where 𝑙𝐾 is a longitude of the knot 𝐾.
For a knot 𝐾 in a three-manifold 𝑌 there is a well-defined notion of meridian 𝑚𝐾, and if the
knot is nullhomologous, there is a well-defined notion of longitude 𝑙𝐾. In particular, this is the
case for any knot 𝐾 in an integer homology three-sphere 𝑌. In what follows, we will use the nota-
tion 𝑅(𝐾) ∶= 𝑅(𝑌(𝐾)) if it is clear which integer homology three-sphere 𝑌 we have in mind, and
we will stick to the above convention of the coordinates in 𝑅(𝑇2) corresponding to the merid-
ian and longitude of 𝐾. With these conventions, all abelian representations in 𝑅(𝐾) map under
𝑖∗ to the thick red line {𝛽 = 0 mod 2𝜋ℤ} ‘at the bottom’ of the pillowcase 𝑅(𝑇2). Indeed, 𝑙𝐾
is a product of commutators in the fundamental group of the knot complement, so an abelian
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LIDMAN et al.
F I G U R E 2
representation variety 𝑅(𝐾) of the trefoil in the pillowcase
The gluing pattern for obtaining the pillowcase from a rectangle, and the image of the
representation necessarily maps 𝑙𝐾 to the identity. Furthermore, for any 𝛼 ∈ [0, 𝜋] we can find an
abelian representation of 𝑅(𝐾) whose restriction to 𝑅(𝑇2) corresponds to (𝛼, 0).
If we cut the pillowcase open along the lines 𝑎0 ∶= {𝛼 = 0 mod 2𝜋ℤ} and 𝑎𝜋 ∶= {𝛼 = 𝜋
mod 2𝜋ℤ}, we obtain a cylinder 𝐶 = [0, 𝜋] × ℝ∕2𝜋ℤ. In the gluing pattern of Figure 2 this means
that we do not perform the identifications along the four indicated vertical boundary lines.
Our main goal is to prove Theorem 3.5 below, which asserts that if 𝐾 is a knot in an 𝑆𝑈(2)-
cyclic integer homology three-sphere whose 0-surgery has non-trivial instanton homology, then
the image of 𝑅(𝐾) in the pillowcase contains a homologically non-trivial embedded closed curve in
the cylinder 𝐶. In order to derive Theorem 1.1 from this, we need a more refined statement, namely,
that there is a homologically non-trivial embedded closed curve in 𝑖∗(𝑅(𝐾)) that is disjoint from a
neighborhood of the two lines 𝑎0 and 𝑎𝜋. Notice that for a knot in 𝑆3, there are no representations
with 𝜌(𝑙𝐾) ≠ id and 𝜌(𝑚𝐾) = ± id. This is because the fundamental group of a knot complement
in 𝑆3 is normally generated by the meridian of the knot. In particular, there are no representa-
tions in 𝑖∗(𝑅(𝐾)) that have coordinates (𝛼, 𝛽) with 𝛽 ≠ 0, and 𝛼 = 0 or 𝛼 = 𝜋. In [36, Proposition
8.1], it is shown that the image of 𝑅(𝐾)∗, the subset of irreducible representations in 𝑅(𝐾), in
fact, stays outside a neighborhood of these two lines. We begin with a generalization of this
fact.
Lemma 3.1. Let 𝐾 be a knot in an 𝑆𝑈(2)-cyclic integer homology three-sphere 𝑌. There is a neigh-
borhood of the lines {𝛼 = 0 mod 2𝜋ℤ} and {𝛼 = 𝜋 mod 2𝜋ℤ} in the pillowcase which is disjoint
from the image of 𝑅(𝐾)∗.
Proof. Suppose by contradiction that the image of 𝑅(𝐾)∗ intersects every neighborhood of the lines
{𝛼 = 0 mod 2𝜋ℤ} and {𝛼 = 𝜋 mod 2𝜋ℤ}. If that was the case, then we could find a sequence of
elements in 𝑅(𝐾)∗ whose image under 𝑖∗ converges to a point on one of the two lines. By the
compactness of 𝑅(𝐾), the limit is the image of a representation 𝜌 ∶ 𝜋1(𝑌(𝐾)) → 𝑆𝑈(2) sending
every meridional curve 𝜇 to ±1. We first claim that 𝜌 must be a central representation (and hence
reducible), and so its image under 𝑖∗ can only be (0,0) or (𝜋, 0).
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353
First, if 𝜌(𝜇) = 1, then 𝜌 ∶ 𝜋1(𝑌(𝐾)) → 𝑆𝑈(2) is really a representation of 𝜋1(𝑌). Since 𝑌 is
assumed to be 𝑆𝑈(2)-cyclic, then the representation is trivial and therefore 𝜌(𝜆) = 1. As a con-
sequence, if the limit of elements in 𝑅(𝐾)∗ is an element of the line {𝛼 = 0 mod 2𝜋ℤ}, then it
is the point (0,0) in the pillowcase. Next, consider the case that 𝜌(𝜇) = −1. If the representation
𝜌 is irreducible, then we obtain an irreducible representation ˜𝜌 ∶ 𝜋1(𝑌) → 𝑆𝑂(3). The obstruc-
tion to lifting an 𝑆𝑂(3) representation into an 𝑆𝑈(2)-representation is an element of 𝐻2(𝑌; ℤ∕2),
and since 𝑌 is an integer homology three-sphere, the obstruction vanishes and ˜𝜌 would lift to
an irreducible representation to 𝑆𝑈(2), contradicting the fact that 𝑌 is 𝑆𝑈(2)-cyclic. Therefore,
a representation 𝜌 ∶ 𝜋1(𝑌(𝐾)) → 𝑆𝑈(2) satisfying 𝜌(𝜇) = −1 is reducible and hence abelian, and
so factors through 𝐻1(𝑌(𝐾)). Because 𝜆 is trivial in 𝐻1(𝑌(𝐾)), we see that 𝜌 is the central repre-
sentation sending 𝜇 to −1 and 𝜆 to 1, and this corresponds to the point (−𝜋, 0) in the pillowcase.
All of this shows that if a sequence of elements in 𝑖∗𝑅(𝐾) converges to a point on the lines {𝛼 = 0
mod 2𝜋ℤ} and {𝛼 = 𝜋 mod 2𝜋ℤ}, then the limit point is a central representation. For notation,
we will call these representations 𝜌± for the sign of the image of 𝜇.
Now, it remains to show that the points (0,0) and (𝜋, 0) cannot be limits of irreducible rep-
resentations. We remark here that this fact does not require that 𝑌 is 𝑆𝑈(2)-cyclic. Let Γ =
𝜋1(𝑌(𝐾)). A result of Weil [33] expanded in [25, Chapter 2] shows that 𝑇𝜌𝑅(𝐾) corresponds
to 𝐻1(Γ; 𝔰𝔲(2)ad◦𝜌). This group is identified with the first cohomology group (with twisted
coefficients) of a 𝐾(Γ, 1)-space, or more generally, with the first (twisted) cohomology of any
CW complex with fundamental group isomorphic to Γ. This shows that 𝐻1(Γ; 𝔰𝔲(2)ad◦𝜌) =
𝐻1(𝑌(𝐾); 𝔰𝔲(2)ad◦𝜌) and so 𝑇𝜌𝑅(𝐾) = 𝐻1(𝑌(𝐾); 𝔰𝔲(2)ad◦𝜌). Next, since each representation 𝜌± is
central, then 𝑎𝑑◦𝜌± is the trivial representation and so
(
(
𝑌(𝐾); ℝ3
𝑌(𝐾); 𝔰𝔲(2)ad◦𝜌±
≅ ℝ3.
= 𝐻1
𝐻1
)
)
This shows that the tangent space to 𝑅(𝐾) at 𝜌± is three-dimensional. Since we obtain three
dimensions of freedom by abelian representations near 𝜌± in 𝑅(𝐾), the entire tangent space to
𝑅(𝐾) consists of tangent vectors to abelian representations and so there cannot be irreducible
□
representations near 𝜌±, completing the proof.
3.2
Essential curves in the pillowcase
In this section, we relate the instanton Floer homology of 0-surgery on a knot to the image of the
character variety of the knot exterior in the pillowcase. This will be the key step in the proof of
Theorem 1.1, found at the end of this subsection.
We next establish some notation, following Kronheimer–Mrowka in [20], which will be useful
in the proof of our next theorem.
Definition 3.2. For a subset 𝐿 ⊆ 𝑅(𝑇2), we denote by 𝑅(𝐾|𝐿) the set of elements [𝜌] ∈ 𝑅(𝐾) such
that [𝑖∗𝜌] ∈ 𝐿.
Theorem 3.3. Let 𝐾 be a knot in an integer homology three-sphere 𝑌, and assume that the instanton
∗ (𝑌0(𝐾)) ≠ 0. Then any topologically embedded
Floer homology of the 0-surgery is non-vanishing, 𝐼𝑤
path from 𝑃 = (0, 𝜋) to 𝑄 = (𝜋, 𝜋) in the associated pillowcase has an intersection point with the
image of 𝑅(𝐾).
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LIDMAN et al.
Before proving the theorem, we point out that this generalizes [36, Theorem 7.1], from knots
in 𝑆3 to knots in general integer homology three spheres. The main difference in the argument
compared to [36, Theorem 7.1] is that here we make use of the non-trivial instanton Floer homol-
ogy of the 0-surgery in an essential way, which is exploited through its connection with holonomy
perturbations of the Chern–Simons functional. The arguments of the third author in [36] instead
use holonomy perturbations of a moduli space which computes the Donaldson invariants of a
closed 4-manifold containing the 0-surgery as a hypersurface. In that case, the non-vanishing
result builds on the existence of a taut foliation on 𝑆3
0(𝐾) for a non-trivial knot 𝐾. In the case
at hand, we do not know whether 𝑌0(𝐾), the 0-surgery on a knot 𝐾 in the integer homology
three-sphere 𝑌, admits a taut foliation.
Proof. Suppose by contradiction that there is a continuous embedded path 𝑐 from 𝑃 to 𝑄 such
that its image is disjoint from 𝑖∗(𝑅(𝐾)) ⊆ 𝑅(𝑇2). (We will not distinguish between paths and their
image for the remainder of this proof.) In other words, 𝑅(𝐾|𝑐) is empty. In particular, we may
suppose that 𝑐 is disjoint from the bottom line {𝛽 = 0} of the pillowcase 𝑅(𝑇2), since any element
of this line lies in the image of 𝑖∗. Since the image 𝑖∗(𝑅(𝐾)) is compact, there is a neighborhood
𝑈 ⊆ 𝑅(𝑇2) of the image of 𝑐 in 𝑅(𝑇2) which is still disjoint from 𝑖∗(𝑅(𝐾)). Since 𝑅(𝐾|𝑐) is empty,
for 𝑐′ sufficiently close to 𝑐, 𝑅(𝐾|𝑐′) is empty as well.
Associated to a three-manifold and admissible bundle, we consider two objects: the Chern–
Simons functional and holonomy perturbations of the Chern–Simons functional. These are
described in detail in Section 4, in particular Sections 4.2 and 4.3, but their definition is not
needed for the proof. Given a three-manifold 𝑍 with admissible bundle represented by 𝑤 and
a holonomy perturbation Ψ, let 𝑅𝑤
Ψ (𝑍) denote the set of critical points of the Chern–Simons func-
tional perturbed by Ψ. By Theorem 4.4 below (which is essentially a synthesis of [36, Theorem
4.2 and Proposition 5.3]), there exists a path 𝑐′ arbitrarily close to 𝑐 and a (holonomy) pertur-
Ψ (𝑌0(𝐾)) is a double cover of 𝑅(𝐾|𝑐′).
bation Ψ of the Chern–Simons functional such that 𝑅𝑤
Therefore, 𝑅𝑤
Ψ (𝑌0(𝐾)) is empty, so computing Morse homology with respect to this perturba-
tion of the Chern–Simons functional produces a trivial group. However, Theorem 4.5 below
asserts that computing Morse homology with respect to the particular perturbation Ψ produces
a group isomorphic to 𝐼𝑤
∗ (𝑌0(𝐾)), which is non-zero by assumption. Therefore, we obtain a
□
contradiction.
Remark 3.4. Although [36, Proposition 5.3] is only stated for knots in 𝑆3, the arguments used in
its proof apply for a knot in an arbitrary 𝑆𝑈(2)-cyclic integer homology three sphere.
If we combine the constraint that 𝑌 is 𝑆𝑈(2)-cyclic with the assumption that 𝐼𝑤
∗ (𝑌0(𝐾)) is non-
trivial, then we obtain the following generalization of [36, Theorem 7.1], which will be the last
step before the proof of our main theorem.
Theorem 3.5 (Pillowcase alternative). Suppose that 𝑌 is an 𝑆𝑈(2)-cyclic integer homology three-
sphere. Suppose that 𝐾 is a knot in 𝑌 such that the 0-surgery 𝑌0(𝐾) has non-trivial instanton Floer
∗ (𝑌0(𝐾)), where 𝑤 is the non-zero class in 𝐻2(𝑌0(𝐾); ℤ∕2) ≅ ℤ∕2. Then the image
homology 𝐼𝑤
𝑖∗(𝑅(𝑌(𝐾))) in the cut-open pillowcase 𝐶 = [0, 𝜋] × (ℝ∕2𝜋ℤ) contains a topologically embedded
curve which is homologically non-trivial in 𝐻1(𝐶; ℤ) ≅ ℤ.
Proof. The hypothesis implies that the lines {(0, 𝛽) ∈ 𝑅(𝑇2) | 𝛽 ≠ 0} and {(𝜋, 𝛽) ∈ 𝑅(𝑇2) | 𝛽 ≠
0} have empty intersection with 𝑖∗(𝑅(𝑌(𝐾))) (Figure 3). The conclusion then follows from
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355
This is a hypothetical image of a representation variety 𝑖∗(𝑅(𝐾)) of a knot 𝐾 in an integer
F I G U R E 3
∗ (𝑌0(𝐾)) ≠ 0 and assumed to not
homology three-sphere 𝑌. The homology three-sphere 𝑌 is assumed to satisfy 𝐼𝑤
be 𝑆𝑈(2)-cyclic. As a consequence, 𝑖∗(𝑅(𝐾)) intersects every path joining 𝑃 and 𝑄 as in Theorem 3.3, but it does
not contain a curve which is homologically non-trivial in the cut-open pillowcase 𝐶 = [0, 𝜋] × (ℝ∕2𝜋ℤ). This
hypothetical example thus illustrates that the 𝑆𝑈(2)-cyclic assumption is necessary in Theorem 3.5.
Theorem 3.3 together with the Alexander duality argument of [36, Lemma 7.3]. For Alexander
duality to work we use the fact that 𝑖∗(𝑅(𝑌(𝐾))) is a semi-algebraic set of dimension 1, and hence
□
a finite graph, see [2].
3.3
Main result
In this subsection we prove that if an integer homology three-sphere contains an embedded
incompressible torus, then the fundamental group of the homology three-sphere admits irre-
ducible 𝑆𝑈(2)-representations. To derive our result we first recall that we can realize a toroidal
integer homology three-sphere as a splice, as in [11, Proof of Corollary 6.2]. We then study the
image of the two knot exteriors in the pillowcase of the incompressible torus. With this in mind,
we include the following definition.
Definition 3.6. Let 𝐾1 ⊂ 𝑌1 and 𝐾2 ⊂ 𝑌2 be oriented knots in oriented integer homology three
spheres. For 𝑖 = 1, 2, denote by 𝜇𝑖, 𝜆𝑖 ⊂ 𝜕𝑁(𝐾𝑖) a meridian and longitude for 𝐾𝑖 in 𝑌𝑖. Form a
three-manifold 𝑌 as
◦
(𝑌1 ⧵ 𝑁(𝐾1)
) ∪
ℎ
◦
(𝑌2 ⧵ 𝑁(𝐾2)
),
where ℎ ∶ 𝜕𝑁(𝐾1) → 𝜕𝑁(𝐾2) identifies 𝜇1 with 𝜆2 and 𝜆1 with 𝜇2. The manifold 𝑌 is called the
splice of 𝑌1 and 𝑌2 along knots 𝐾1 and 𝐾2.
Let 𝑌 be an integer homology three sphere and let 𝑇 be a two-dimensional torus embedded
in 𝑌 in such manner that its normal bundle is trivial. A simple application of the Mayer–Vietoris
sequence shows that 𝑌 ⧵ 𝑁(𝑇)◦ has two connected components 𝑀1, 𝑀2, and that each component
has the same homology groups as 𝑆1. The ‘half lives, half dies’ principle shows that for each 𝑖 = 1, 2
there exists a basis (𝛼𝑖, 𝛽𝑖) for the peripheral subgroup of 𝜕𝑀𝑖 such that 𝛽𝑖 is nullhomologous in
𝑀𝑖. Therefore, if 𝑌𝑖 denotes the union of 𝑀𝑖 and a solid torus 𝑆1 × 𝐷2 in such a way that the curve
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LIDMAN et al.
{1} × 𝜕𝐷2 gets identified with 𝛼𝑖, then 𝑌𝑖 is an integer homology three-sphere. Moreover, since
𝑇 is incompressible in 𝑌, then the core of the solid torus in 𝑌𝑖 is a non-trivial knot 𝐾𝑖. In other
words, every toroidal integer homology three-sphere can be expressed as a splice of non-trivial
knots 𝐾1 and 𝐾2 in integer homology three spheres 𝑌1 and 𝑌2.
With all of this in place, we are ready to prove our main result.
(𝑌2 ⧵ 𝑁(𝐾2)◦), with 𝐾1, 𝐾2 non-trivial
Proof of Theorem 1.1. Realize 𝑌 as a splice (𝑌1 ⧵ 𝑁(𝐾1)◦) ∪
knots. Suppose first that 𝑌𝑖 ⧵ 𝑁(𝐾𝑖)◦ is reducible, in other words, that 𝑌𝑖 ⧵ 𝑁(𝐾𝑖)◦ = 𝑄𝑖#(𝑍𝑖 ⧵
𝑁(𝐽𝑖)◦) where 𝑄𝑖, 𝑍𝑖 are integer homology three spheres and 𝐽𝑖 ⊂ 𝑍𝑖 has irreducible and boundary-
incompressible exterior. As a consequence of van Kampen’s theorem, there exists a surjection
𝜋1(𝑌𝑖 ⧵ 𝑁(𝐾𝑖)◦) → 𝜋1(𝑍𝑖 ⧵ 𝑁(𝐽𝑖)◦), and this surjection induces a 𝜋1-surjection from 𝑌 to the
splice of (𝑍1, 𝐽1) and (𝑍2, 𝐽2). Thus, our proof reduces to the case when 𝑌 is the splice of two
knots with irreducible and boundary-incompressible exteriors, which we assume from now on.
ℎ
Next, by the Seifert–van Kampen theorem, the pieces of the decomposition fit into the following
commutative diagram:
and since each 𝑌𝑖 ⧵ 𝑁(𝐾𝑖)◦ is a homology circle, there exists a 𝜋1-surjection from 𝑌 to each 𝑌𝑖.
Therefore, our proof reduces further to the case when both 𝑌1 and 𝑌2 are 𝑆𝑈(2)-cyclic since an
irreducible representation for 𝑌𝑖 gives rise to one for 𝑌.
To recap, the previous two paragraphs allow us to assume that 𝑌 is the splice of (𝑌1, 𝐾1), (𝑌2, 𝐾2)
with each 𝑌𝑖 an 𝑆𝑈(2)-cyclic homology three sphere, and each 𝐾𝑖 ⊂ 𝑌𝑖 a knot with irreducible
and boundary-incompressible exterior. Then, as a consequence of Proposition 2.1(3) we have that
each 𝑌𝑖 has trivial instanton Floer homology. Moreover, since each 𝑌𝑖 ⧵ 𝑁(𝐾𝑖)◦ is irreducible and
boundary-incompressible, Theorem 1.3 shows that the instanton Floer homology of 0-surgery on
𝑌𝑖 along 𝐾𝑖 is non-zero. Therefore, the hypotheses of both Theorem 3.5 and Lemma 3.1 hold, and
the proof now follows exactly as in [36, Proof of Theorem 8.3(i)] with [36, Theorem 7.1] and [36,
□
Proposition 8.1(ii)] replaced by Theorem 3.5 and Lemma 3.1 respectively (Figure 4).
REVIEW OF INSTANTON FLOER HOMOLOGY AND HOLONOMY
4
PERTURBATIONS
We start this section with a disclaimer: We do not claim to prove any original or new result in
this section. However, we review instanton Floer homology and holonomy perturbations to the
extent which is necessary in order to understand the proof of our main results above. For instance,
Section 4.3 below contains a synthesis of the third author’s results about holonomy perturbations
from [36] which we hope that the reader unfamiliar with this reference will appreciate. Section 4.5
contains a result about invariance under holonomy perturbations in the context of an admissi-
ble bundle with non-trivial second Stiefel–Whitney class, together with a sketch of proof. Again,
this result is already contained in [14] and [10], but by looking up these references it may not be
immediately clear whether these results apply verbatim in our situation.
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357
Let 𝑌 be the three-manifold obtained as the splice of two copies of the exterior of a right-handed
F I G U R E 4
trefoil, and let 𝑇 be the incompressible torus given as the intersection of the two knot exteriors. The figure shows
the image of each copy of 𝑅(𝑇2,3)∗ in the pillowcase. Note that any representation of the splice corresponding to
an intersection of the red and blue curves is irreducible.
The proof of Theorem 3.3 relies on a non-vanishing result of an instanton Floer homology group
𝐼𝑤
∗,Φ(𝑌0(𝐾)), computed with suitable perturbation terms Φ of the Chern–Simons function. We will
review the construction of these perturbation terms below, which are built from the holonomy
along families of circles, parametrized by embedded surfaces. The critical points of the complex
underlying the homology group 𝐼𝑤
∗,Φ(𝑌0(𝐾)) will have a clear interpretation in terms of inter-
sections of the representation variety 𝑅(𝐾) with certain deformations of the path given by the
straight line {𝛽 = 𝜋} in the pillowcase, resulting as the representation variety of the boundary of
the exterior of 𝐾 in 𝑌 as before.
On the other hand, Theorem 1.3 yields a non-vanishing result for 𝐼𝑤
∗ (𝑌0(𝐾)), defined in the
usual way, and in particular without the above class of perturbation terms. We can therefore
complete the proof from the fact that the two instanton Floer homology groups, 𝐼𝑤
∗ (𝑌0(𝐾)) and
𝐼𝑤
∗,Φ(𝑌0(𝐾)), are isomorphic, and we sketch the proof of this below.
∗ (𝑌0(𝐾)) and 𝐼𝑤
Remark 4.1. In the construction of both 𝐼𝑤
∗,Φ(𝑌0(𝐾)) there are typically perturbation
terms involved for the sake of transversality. These can be chosen as small as one likes, in a suitable
sense. We will omit these auxiliary perturbations from our notation. The perturbations labeled by
the terms Φ, however, will have a clear geometric purpose, and the discussion below will focus
on these.
4.1
The Chern–Simons function
For details on the holonomy perturbations we use we refer the reader to Donaldson’s book [10],
Floer’s original article [15], and the third author’s article [36].
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LIDMAN et al.
If we deal with an admissible 𝑆𝑂(3)-bundle 𝐹 → 𝑌 over a three-manifold 𝑌 with second Stiefel–
Whitney class 𝑤, we may suppose that it arises from an 𝑈(2)-bundle 𝐸 → 𝑌 as its adjoint bundle
𝔰𝔲(𝐸), see, for instance, [10, Section 5.6]. Then 𝑤 = 𝑤2(𝐸) ≡ 𝑐1(𝐸) mod 2. The space of 𝑆𝑂(3)-
connections on 𝐹 is then naturally isomorphic to the space of 𝑈(2)-connections on 𝐸 that induce
a fixed connection 𝜃 in the determinant line bundle det(𝐸), which we will suppress from notation.
When dealing with functoriality properties, it is more accurate to consider 𝑤 to be an embedded
1-manifold which is Poincaré dual to 𝑤2(𝐸) = 𝑤2(𝐹), see [23].
We will fix a reference connection 𝐴0 on 𝐸 and consider the Chern–Simons function
CS ∶ A → ℝ
𝐴 ↦ ∫
𝑌
tr(2𝑎 ∧ (𝐹𝐴0
)0 + 𝑎 ∧ 𝑑𝐴0
𝑎 +
1
3
𝑎 ∧ [𝑎 ∧ 𝑎]) ,
defined on the affine space A of connections 𝐴 in 𝐸 which induce 𝜃 in det(𝐸), and where we have
written 𝐴 = 𝐴0 + 𝑎 with 𝑎 ∈ Ω1(𝑌; 𝔰𝔲(𝐸)). The term 𝐹𝐴 denotes the curvature of a connection
𝐴, and (𝐹𝐴)0 denotes its trace-free part, and 𝑑𝐴 denotes the exterior derivative associated to a
connection 𝐴. We denote by G the group of bundle automorphisms of 𝐸 which have determinant
1. The Chern–Simons function induces a circle-valued function CS ∶ B → ℝ∕ℤ on the space B =
A ∕G of connections modulo gauge equivalence, and the instanton Floer homology 𝐼𝑤
∗ (𝑌) is the
Morse homology, in a suitable sense, of the Chern–Simons function CS. To carry this out, one has
to deal with a suitable grading on the critical points, which will only be a relative ℤ∕8-grading,
with suitable compactness arguments (Uhlenbeck compactification and ‘energy running down
the ends’), and with transversality arguments. In particular, one will in general add a convenient
perturbation term to the Chern–Simons function to obtain the required transversality results. This
is usually done by the use of holonomy perturbations that we discuss below. By a Sard–Smale-
type condition, this term can be chosen as small as one wants, in the respective topologies one
is working with. Therefore, we are suppressing these perturbations for the sake of transversality
from our notation. One then needs to prove independence of the various choices involved, and in
particular the Riemannian metric and the perturbation terms required for transversality.
One may also deal with orientations, but we do not need this in our situation, where ℤ∕2-
coefficients in the Floer homology will be sufficient.
4.2
Review of holonomy perturbations
To set up the perturbation of the Chern–Simons function we are using, we need to introduce some
notation. Let 𝜒 ∶ 𝑆𝑈(2) → ℝ be a class function, that is, a smooth conjugation invariant function.
Any element in 𝑆𝑈(2) is conjugate to a diagonal element, and hence there is a 2𝜋-periodic even
function g ∶ ℝ → ℝ such that
])
([
𝜒
𝑒𝑖𝑡
0
0
𝑒−𝑖𝑡
= g(𝑡)
(2)
for all 𝑡 ∈ ℝ. Furthermore, let Σ be a compact surface with boundary, and let 𝜇 be a real-valued
two-form which has compact support in the interior of Σ and with ∫
Σ 𝜇 = 1. Let 𝜄 ∶ Σ × 𝑆1 → 𝑌
be an embedding. Let 𝑁 ⊆ 𝑌 be a codimension-zero submanifold containing the image of 𝜄, and
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359
such that the bundle 𝐸 is trivialized over 𝑁 in such a way that the connection 𝜃 in det(𝐸) induces
the trivial product connection in the determinant line bundle of our trivialization of 𝐸 over 𝑁.
This means that connections in A can be understood as 𝑆𝑈(2)-connections in 𝐸 when restricted
to 𝑁.
Associated to this data, we can define a function
Φ ∶ A → ℝ
which is invariant under the action of the gauge group G . For 𝑧 ∈ Σ, we denote by 𝜄𝑧 ∶ 𝑆1 → 𝑌
the circle 𝑡 ↦ 𝜄(𝑧, 𝑡). A connection 𝐴 ∈ A provides an 𝑆𝑈(2)-connection over the image of 𝜄. The
(𝐴) of 𝐴 around the loop 𝜄𝑧 (with variable starting point) is a section of the bundle
holonomy Hol𝜄𝑧
(𝐴))
of automorphisms of 𝐸 with determinant 1 over the loop. Since 𝜒 is a class function, 𝜒(Hol𝜄𝑧
is well defined. We can therefore define
Φ(𝐴) = ∫
Σ
𝜒(Hol𝜄𝑧
(𝐴)) 𝜇(𝑧) ,
(3)
and this function is invariant under the action of the gauge group G . It depends on the data (𝜄, 𝜒, 𝜇)
and a trivialization of the bundle over a codimension-zero submanifold 𝑁, but we will omit the
latter from notation.
We will have to work with a finite sequence of such embeddings, all supported in a sub-
manifold 𝑁 of codimension zero over which the bundle 𝐸 → 𝑁 is trivial. For some 𝑛 ∈ ℕ, let
𝜄𝑘 ∶ 𝑆1 × Σ𝑘 → 𝑁 ⊆ 𝑌 be a sequence of embeddings for 𝑘 = 0, … , 𝑛 − 1 such that the interior of
the image of 𝜄𝑘 is disjoint from the interior of the image of 𝜄𝑙 for 𝑘 ≠ 𝑙. We also suppose class
functions 𝜒𝑘 ∶ 𝑆𝑈(2) → ℝ corresponding to even, 2𝜋-periodic functions g𝑘 ∶ ℝ → ℝ as above to
be chosen, for 𝑘 = 0, … , 𝑛 − 1, and we assume that 𝜇𝑘 is a two form on Σ𝑘 with support in the
interior of Σ𝑘 and integral 1. Just as in the case of (3), the data determine a finite sequence of
functions
Φ𝑘 ∶ A → ℝ ,
𝑘 = 0, … , 𝑛 − 1 ,
and we are interested in the Morse homology of the function
CS + Ψ ∶ B → ℝ∕ℤ, where Ψ =
𝑛−1∑
𝑘=0
Φ𝑘.
(4)
Definition 4.2. We denote by 𝑅𝑤
Ψ ∶ B → ℝ∕ℤ, where Ψ is specified by the holonomy perturbation data {𝜄𝑘, 𝜒𝑘} as above.
Ψ (𝑌) the space of critical points [𝐴] ∈ A ∕G of the function CS +
If the holonomy perturbation data Ψ are chosen in a way such that 𝑅𝑤
Ψ (𝑌) does not contain
equivalence classes of connections [𝐴] such that 𝐴 is reducible, then the construction for defining
a Floer homology 𝐼𝑤
Ψ (𝑌) with generators given by critical points of the perturbed Chern–Simons
function CS + Ψ, and with differentials defined from negative gradient flow lines, goes through
in the same way as in [10, 15]. This will require additional small perturbations in order to make
the critical points non-degenerate and in order to obtain transversality for the moduli spaces of
flow lines. In fact, we really have not done anything new compared to the constructions in these
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LIDMAN et al.
references since the same perturbations already appear there for the sake of obtaining transversal-
ity of the moduli spaces involved in the construction. The only slight difference is that in Floer’s
work, the surfaces Σ𝑘 appearing in the definition of the embeddings 𝜄𝑘 are always chosen to be
disks, whereas those used in the proof of Theorem 3.3 above, that is, in [36, Theorem 4.2 and
Proposition 5.3], the surfaces Σ𝑘 are all annuli.
More specifically, for completeness, we recall a bit more on the implementation of the holon-
omy perturbations used in the work of the third author as needed in the previous section for
studying 𝑌0(𝐾). Suppose that we are given a smoothly embedded path 𝑐 from 𝑃 = (0, 𝜋) to 𝑄 =
(𝜋, 𝜋) avoiding (0,0) and (𝜋, 0), and which is homotopic to the straight line segment 𝑐0 ∶= {𝛽 = 𝜋}
from 𝑃 to 𝑄 relative to the four corner points of the pillowcase 𝑃, 𝑄, (0, 0) and (𝜋, 0). There is an
isotopy 𝜙𝑡 through area-preserving maps of the pillowcase 𝑅(𝑇2) such that 𝜙1 maps the straight
line 𝑐0 to the path 𝑐, and such that 𝜙𝑡 fixes the four corner points of the pillowcase. [36, Theo-
rem 3.3] states that isotopies through area-preserving maps can be 𝐶0-approximated by isotopies
through finitely many shearing maps. For details on shearing maps we refer the reader to [36, Sec-
tions 2 and 3]. The essential relationship is outlined in the following subsection, which we include
for the sake of clarity and completeness of our exposition.
4.3
Review of holonomy perturbations and shearing maps
We denote by 𝑅(𝑁) the space of flat 𝑆𝑈(2)-connections in the trivial 𝑆𝑈(2)-bundle over 𝑁 = 𝑆1 ×
Σ up to gauge equivalence, where Σ = 𝑆1 × 𝐼 = 𝑆1 × [0, 1] is an annulus. The two inclusion maps
𝑖− ∶ 𝑆1 × (𝑆1 × {0}) → 𝑁 and 𝑖+ ∶ 𝑆1 × (𝑆1 × {1}) → 𝑁 induce restriction maps 𝑟−, 𝑟+ ∶ 𝑅(𝑁) →
𝑅(𝑇2) to the representation varieties of the two boundary tori, which are pillowcases. In this situ-
ation, we have that both 𝑟− and 𝑟+ are homeomorphisms, and under the natural identification of
these tori we have 𝑟− = 𝑟+.
Now if 𝜒 is a class function as in Equation (2) above, then instead of the flatness equation 𝐹𝐴 = 0
for connections 𝐴 on the trivial bundle over 𝑁 one may consider the equation
𝐹𝐴 = 𝜒′(Hol𝑙(𝐴)) 𝜇,
(5)
where the notation means the following: We denote by 𝑙𝑧 = 𝑆1 × {𝑧} the ‘longitude’ in 𝑁 = 𝑆1 × Σ
parametrized by a point 𝑧 ∈ Σ, the holonomy of the connection 𝐴 along such a loop is denoted
(𝐴), the map 𝜒′ ∶ 𝑆𝑈(2) → 𝔰𝔲(2) is the trace dual of the derivative 𝑑𝜒 of 𝜒, and 𝜇 is a 2-
by Hol𝑙𝑧
form in 𝑁 which is the pull-back of a 2-form 𝜇Σ via the projection 𝑆1 × Σ → Σ, and 𝜇Σ has compact
support in the interior of Σ and satisfies ∫
Σ 𝜇Σ = 1. Then the equation we consider instead of the
(𝐴)) 𝜇(𝑤,𝑧), for (𝑤, 𝑧) ∈ 𝑁 = 𝑆1 × Σ.
flatness equation 𝐹𝐴 = 0 is the equation (𝐹𝐴)(𝑤,𝑧) = 𝜒′(Hol𝑙𝑧
It can be proved that solutions 𝐴 of this equation are reducible, and that furthermore the holon-
(𝐴) does not depend on the point 𝑧 ∈ Σ. Both claims can be found in [6, Lemma 4],
omy Hol𝑙𝑧
and are reproved in [36, Proposition 2.1] by the third author. This finally justifies our notation in
Equation (5).
If we denote by 𝑅𝜒(𝑁) the solutions of Equation (5) up to gauge equivalence, then we still
have two restriction maps 𝑟± ∶ 𝑅𝜒(𝑁) → 𝑅(𝑇2). However, in this situation we have the following
relationship.
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361
Proposition 4.3. The two restriction maps 𝑟± are homeomorphisms and fit into a commutative
diagram
(6)
where 𝜙 is a shearing map that relates to 𝜒 as follows.
If we write 𝑚− = {𝑝𝑡} × 𝑆1 × {0} and 𝑚+ = {𝑝𝑡} × 𝑆1 × {1} for ‘meridians’ given by the boundaries
of Σ in {𝑝𝑡} × Σ, and if
Hol𝑚±
(𝐴) =
[
𝑒𝑖𝛽±
0
]
0
𝑒−𝑖𝛽±
, and Hol𝑙(𝐴) =
[
𝑒𝑖𝛼
0
]
,
0
𝑒−𝑖𝛼
which we may suppose up to gauge equivalence, then we have
(
)
𝜙𝜒
𝛼
𝛽−
(
=
)
𝛼
𝛽− + 𝑓(𝛼)
,
(7)
(8)
where 𝑓 ∶ ℝ → ℝ is the derivative of the function g appearing in Equation (2). Here, (𝛼, 𝛽±)
determine points in 𝑅(𝑇2) determined by Hol𝑚±
(𝐴) and Hol𝑙(𝐴) as in Equation (7) above.
Equation (6) is essentially proved in [6, Lemma 4], and a proof also appears in [36, Proposition
2.1].
Of course, one can iterate this construction: One may choose a finite collection of disjoint
embeddings 𝜄𝑘 ∶ 𝑆1 × Σ into a closed three-manifold 𝑌, and class functions 𝜒𝑘. The embed-
dings may chosen to be ‘parallel’ in that the image of 𝜄𝑘 corresponds to 𝑆1 × (𝑆1 × [𝑘, 𝑘 + 1]) ⊆
𝑆1 × (𝑆1 × [0, 𝑛]) ⊆ 𝑌, but the role of ‘meridian’ and ‘longitude’ may be chosen arbitrarily in an
𝑆𝐿2(ℤ) worth of possible choices. In this case the restriction maps to the two boundary compo-
nents of 𝑆1 × (𝑆1 × [0, 𝑛]) in the diagram analogous to Equation (6) will be related by a composition
of shearing maps.
4.4
Holonomy perturbations and the pillowcase
The main application of holonomy perturbations we have in mind is stated as Theorem 4.4
below. To put it into context, note first that for a non-trivial bundle, the critical space of the
Chern–Simons function 𝑅𝑤(𝑌0(𝐾)) is a double cover of 𝑅(𝐾|𝑐0), where 𝑐0 is the straight line
from (0, 𝜋) to (𝜋, 𝜋) in the pillowcase, see [36, Proposition 5.1]. If we choose holonomy per-
turbations associated to some data {𝜄𝑘, 𝜒𝑘}𝑛−1
𝑘=0 as above, where the image of 𝜄𝑘 corresponds to
𝑆1 × (𝑆1 × [𝑘, 𝑘 + 1]) ⊆ 𝑆1 × (𝑆1 × [0, 𝑛]) ⊆ 𝑌 in a collar neighborhood of the Dehn filling torus
in 𝑌0(𝐾), then repeated use of Proposition 4.3 above will imply that for the holonomy perturbation
Ψ (𝑌0(𝐾)) will correspond to 𝑅(𝐾|𝑐′),
Ψ determined by the data {𝜄𝑘, 𝜒𝑘}𝑛−1
𝑘=0, the critical space of 𝑅𝑤
◦ … ◦𝜙0, with ‘directions’
where 𝑐′ is the image of 𝑐0 under a composition of shearing maps 𝜙𝑛−1
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LIDMAN et al.
determined by the embeddings 𝜄𝑘. (In Equation (8) we are dealing with a shearing in direction
)
(
0
1
, but we can pick any direction in ℤ2.)
The main point of [36, Theorem 4.2] is that the area-preserving maps of the pillowcase obtained
by composition of shearing maps is 𝐶0-dense in the space of all area-preserving maps of the
pillowcase, and this yields the following result.
Theorem 4.4 (Theorem 4.2 and Proposition 5.3, [36]). Let 𝐾 be a knot in an 𝑆𝑈(2)-cyclic integer
homology three-sphere 𝑌. Let 𝑐 be an embedded path from (0, 𝜋) to (𝜋, 𝜋) missing the other orbifold
points of the pillowcase, and which is homotopic to the straight line segment 𝑐0 ∶= {𝛽 = 𝜋} from 𝑃
to 𝑄 relative to the four corner points of the pillowcase 𝑃, 𝑄, (0, 0) and (𝜋, 0). Then, there exists an
embedded path 𝑐′ arbitrarily close to 𝑐 and a holonomy perturbation Ψ along disjoint embeddings
of 𝑆1 × (𝑆1 × 𝐼) parallel to the boundary of a neighborhood of 𝐾 such that 𝑅𝑤
Ψ (𝑌0(𝐾)) double-covers
𝑅(𝐾|𝑐′).
(To see that we get a double-cover here we refer the reader to [36, Remark 1.2]).
We only stress the fact that we must assume that there are no reducible connections in 𝑅𝑤
Ψ (𝑌),
since the presence of such solutions will result in a failure of the transversality arguments involved
in the discussion.
4.5
Invariance of instanton Floer homology
The instanton Floer homology groups 𝐼𝑤(𝑌) and 𝐼𝑤
Ψ (𝑌), the latter being defined under the addi-
tional assumption that 𝑅𝑤
Ψ (𝑌) does not contain reducible connections, depend on additional data
that we have already suppressed from notation, notably the choice of a Riemannian metric on
𝑌 and holonomy perturbations just as defined above in order to achieve transversality. More
explicitly, holonomy perturbations have already been implicit in the definition of instanton Floer
homology unless the critical points of CS had been non-degenerate at the start and the moduli
space defining the flow lines had been cut out transversally. In Floer’s original work [15], and
elaborated in more detail in Donaldson’s book [10], invariance under the choice of Riemannian
metric and the choice of holonomy perturbations follows from a more general concept, namely,
the functoriality of instanton Floer homology under cobordisms. See also the discussion in [23,
Section 3.8.]
Theorem 4.5 (Invariance under holonomy perturbations). Suppose that the space of critical points
𝑅𝑤
Ψ (𝑌) of the perturbed Chern–Simons function 4 appearing in Definition 4.2 above does not contain
equivalence classes of reducible connections. Then the associated instanton Floer homology groups
∗ (𝑌) and 𝐼𝑤
𝐼𝑤
∗,Ψ(𝑌) are isomorphic.
Sketch of Proof. The proof of this statement is standard, so we will describe a chain map
determining the isomorphism on homology and outline the ideas along which the result is proved.
Slightly more generally, suppose that we are dealing with a smooth map [0, 1] → 𝐶∞(A , ℝ),
𝑠 ↦ Γ(𝑠). We may suppose that this map is constant near 0 and 1. The Floer differential counts
flow lines of the Chern–Simons function, possibly suitably perturbed. Instead of doing this, we
may also consider the downward gradient flow equation of the time-dependent function CS +Γ(𝑠),
where we extend Γ(𝑠) to a map (−∞, ∞) → 𝐶∞(A , ℝ) which is constant Γ(0) on (−∞, 0] and
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363
constant Γ(1) on [1, ∞). If we are given critical points 𝜌0 of CS +Γ(0) and 𝜌1 of CS +Γ(1) of the same
of connections 𝐀 = {𝐴(𝑡)}𝑡 on
index, then we consider a zero-dimensional moduli space 𝑀𝜌0,𝜌1
𝐸 → ℝ × 𝑌 of finite 𝐿2-norm (inducing 𝜃 on det(𝐸), pulled back to ℝ × 𝑌), such that the equation
𝑑𝐴
𝑑𝑡
= − grad(CS +Γ(𝑡))(𝐴(𝑡))
(9)
holds on ℝ × 𝑌, where grad denotes the 𝐿2-gradient, and 𝐀 limits to 𝜌0 and 𝜌1 in the limit 𝑡 → ±∞,
respectively. Finally, we also require that the moduli space 𝑀𝜌0,𝜌1
is cut out transversally.
We require that the addition of the term − grad(Γ(𝑡))(𝐴(𝑡)) to the gradient flow Equation 9
for the Chern–Simons function does not alter the linearized deformation theory for 𝐀, see, for
instance, [10, Sections 3 and 4]. Furthermore, we have to require that the Uhlenbeck compact-
ification goes through with the perturbation we have in mind. It is shown in [10, Section 5.5]
that both hold for the function Γ built from holonomy perturbations as described in Equation 10.
One essential feature is that the holonomy perturbation term appearing in the flow equation is
uniformly bounded.
A suitable interpolation between the holonomy perturbation data Γ(0) = 0 and Γ(1) = Ψ for Ψ
as in Equation (4) is given, for instance, by the following formula. Suppose that Ψ is determined
by data {𝜄𝑘, 𝜒𝑘}𝑛−1
𝑘=0. Then for 𝑡 ∈ [ 𝑘
] we define
, 𝑘+1
𝑛
𝑛
Γ(𝑡) =
𝑘−1∑
𝑙=0
Φ𝑙 + 𝛽(𝑡 − 𝑘∕𝑛)Φ𝑘
(10)
for any 𝑘 ∈ {0, … , 𝑛 − 1}. Here 𝛽 ∶ [0, 1
𝑛
neighborhood of 0 and 1 in a neighborhood of 1
𝑛
.
] → [0, 1] is a smooth function which is 0 in a
Now the moduli space 𝑀𝜌0,𝜌1
𝜌0 and 𝜌1 in 𝑅𝑤(𝑌) and 𝑅𝑤
the setup for instanton Floer homology for admissible bundles, this does not occur.
does not contain any reducibles, because if it did, then the limits
Ψ (𝑌), respectively, would also be reducible, and by our assumption and
One defines a linear map 𝜁 ∶ 𝐶𝑤(𝑌) → 𝐶𝑤
Ψ (𝑌) of the underlying chain complexes such that the
‘matrix entry’ corresponding to the elements 𝜌0 ∈ 𝐶𝑤(𝑌) and 𝜌1 ∈ 𝐶𝑤
Ψ (𝑌) is given by the signed
, where the sign is determined in the usual way by the choice
count of the moduli space 𝑀𝜌0,𝜌1
of a homology orientation. That 𝜁 is a chain map follows from analyzing the compactification of
suitable 1-dimensional moduli spaces, making use of Uhlenbeck compactification — no bubbling
can occur here due to the dimension of the moduli space — and the chain convergence discussed
in [10, Section 5.1], together with suitable gluing results.
That different interpolations yield chain homotopic chain maps follows from studying the com-
pactification of (−1)-dimensional moduli spaces over a 1-dimensional family, defining a chain
homotopy equivalence between the two different interpolations.
That 𝜁 defines a chain homotopy equivalence follows from the functoriality property: One
may consider a further path Γ′ ∶ [1, 2] → 𝐶∞(A , ℝ) such that Γ′(1) = Γ(1), similar as above. This
defines a corresponding chain map 𝜁′ ∶ 𝐶𝑤
Ψ → 𝐶𝑤
. On the other hand, one may concatenate
the path Γ(𝑡) and the path Γ′(𝑡) and build a corresponding interpolation Γ′′ ∶ [0, 2] → 𝐶∞(A , ℝ),
resulting in a chain map 𝜁′′ as above. A neck stretching argument then shows that 𝜁′′ and 𝜁′◦𝜁 are
chain homotopy equivalent, and hence induce the same maps on homology. In our situation we
take Γ′(2) to be 0, meaning that this defines again the ‘unperturbed’ chain complex 𝐶𝑤(𝑌) (which,
again, may contain some perturbations for the sake of regularity omitted in our notation). One
Γ′(2)
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LIDMAN et al.
Left: The pattern representing the (2,1)-cable satellite operation. Right: The (2,1)-cable for the
F I G U R E 5
right-handed trefoil. The extra twisting appears as a consequence of the requirement that a longitude in 𝑆1 × 𝐷2
maps to the canonical longitude of the trefoil.
may finally interpolate between Γ′′ and the 0-term along a 1-dimensional family. Analyzing again
the compactification of suitable (−1)-dimensional moduli spaces over a 1-dimensional family, one
□
obtains a chain homotopy equivalence between 𝜁′′ and the identity.
Remark 4.6. There is some confusion about invariance under ‘small’ and ‘large’ holonomy pertur-
bations in the field. If one is given holonomy perturbation data for which the underlying space of
critical points and moduli spaces defining the differentials are already cut out transversally, then
for small enough perturbations the same will still hold, and the resulting chain complexes will
be isomorphic. This is due to the fact that the condition of being cut out transversally is an open
condition, expressed as the surjectivity of the deformation operators involved in the linearized
equation together with the Coulomb gauge fixing.
If, on the other hand, one is given a situation where the critical points and the unperturbed
moduli spaces are not cut out transversally, then one needs to perturb, and even if these perturba-
tions are chosen ‘small’, the resulting chain complexes will in general not be isomorphic but only
chain homotopy equivalent. In this situation, the proof of invariance is really the same as proving
the invariance under ‘large’ perturbations, and already present in [15] and [10].
5
BRANCHED COVERS OF PRIME SATELLITE KNOTS
In this section, we prove Corollary 1.5, establishing the existence of a non-trivial 𝑆𝑈(2) represen-
tation for cyclic branched covers of prime satellite knots. We begin with a definition of satellite
knots.
Definition 5.1. Let 𝑃 ⊂ 𝑆1 × 𝐷2 be an oriented knot in the solid torus. Consider an orientation-
preserving embedding ℎ ∶ 𝑆1 × 𝐷2 → 𝑆3 whose image is a tubular neighborhood of a knot 𝐾 so
that 𝑆1 × {∗∈ 𝜕𝐷2} is mapped to the canonical longitude of 𝐾. The knot ℎ(𝑃) is called the satellite
knot with pattern 𝑃 and companion 𝐾, and is denoted as 𝑃(𝐾). The winding number of the satellite
is defined to be the algebraic intersection number of 𝑃 with {∗} × 𝐷2. See Figure 5 for an example.
Corollary 1.5. Let 𝐾 be a prime, satellite knot in 𝑆3 and let Σ(𝐾) be any non-trivial cyclic cover of
𝑆3 branched over 𝐾. Then 𝜋1(Σ(𝐾)) admits a non-trivial 𝑆𝑈(2) representation.
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365
Proof. Let 𝐾 be a prime satellite knot in 𝑆3. If Σ(𝐾) is not an integer homology sphere, then there
is a non-trivial abelian representation. In the case when Σ(𝐾) is an integer homology three sphere,
then by Theorem 1.1 it suffices to show that Σ(𝐾) is toroidal. Write 𝐾 = 𝑃(𝐽) and observe that if
Σ(𝐾) is the 𝑑-fold cover of 𝑆3 branched over 𝑃(𝐽), then there is a decomposition of Σ(𝑃(𝐽)) as the
union of Σ(𝑆1 × 𝐷2, 𝑃), the 𝑑-fold cover of 𝑆1 × 𝐷2 branched over 𝑃, and a 𝑑-fold covering space
of the knot complement 𝑆3 ⧵ 𝑁(𝐽). The isomorphism type of this latter covering space depends
only on the greatest common divisor between 𝑑 and the winding number of 𝑆1 × 𝐷2, see, for
example, [31] or [24, p. 220]. Since the exterior of 𝐽 has incompressible boundary, the same is true
of any cover. Therefore, we just need to show that Σ(𝐷2 × 𝑆1, 𝑃) has incompressible boundary.
We claim the following. Let 𝑃 be a non-trivial pattern knot in 𝐷2 × 𝑆1 which does not correspond
to a connect-sum and which is not contained in an embedded 𝐵3. Then for any cyclic branched
cover over 𝑃, Σ(𝐷2 × 𝑆1, 𝑃) has incompressible boundary. This claim is standard and proved in
□
the lemma below for completeness.
Lemma 5.2. Let 𝑃 be a non-trivial pattern knot in 𝐷2 × 𝑆1 which does not correspond to a connect-
sum and which is not contained in an embedded 𝐵3. Then for any cyclic branched cover over 𝑃, the
manifold 𝑀 = Σ(𝐷2 × 𝑆1, 𝑃) has incompressible boundary.
Proof. Suppose that 𝛾 is an essential loop on 𝜕𝑀, which is nullhomotopic in 𝑀. Let 𝐺 denote
the group of covering transformations of 𝑀 and consider the action of 𝐺 on the boundary. We
first claim that 𝛾 can be isotoped on the boundary such that for each g ∈ 𝐺, either g(𝛾) ∩ 𝛾 = ∅
or g(𝛾) = 𝛾. Of course, we only need to consider the elements of 𝐺 that fix setwise the boundary
component containing 𝛾. Since the action of 𝐺 on 𝑇2 is of finite order and free, it must be the
standard covering transformation of a covering map from 𝑇2 to itself, which fixes all homology
classes. In particular, the homology class of 𝛾 in 𝜕𝑀 is fixed by this action, and the claim easily
follows. Now, because of this claim, and because the curve 𝛾 is disjoint from the lift of 𝑃, the
equivariant Dehn’s lemma [34] implies that there exists a disk 𝐷 in 𝑀 bounding 𝛾 such that for all
g, either g(𝐷) ∩ 𝐷 = ∅ or g(𝐷) = 𝐷, and furthermore, 𝐷 is transverse to the lift of the branch set.
g∈𝐺 g(𝐷). Then, Σ∕𝐺 is a collection of disks
Consider the (possibly disconnected) surface Σ =
in 𝐷2 × 𝑆1 and Σ → Σ∕𝐺 is a branched cover (although some components of Σ may have trivial
branch locus). Furthermore, each component of the boundary of Σ∕𝐺 is an essential curve on the
boundary of the solid torus. For homology reasons, it is necessarily a meridional curve on the solid
torus and each component of Σ∕𝐺 is a meridional disk. (The components cannot have any other
topology, since a disk can only cover/branch cover another disk.) Now, if any component of Σ∕𝐺
does not intersect 𝑃, then we can cut 𝐷2 × 𝑆1 along one of these disks, and see that 𝑃 is contained
in 𝐵3 and we have a contradiction. If some component of Σ∕𝐺 does intersect 𝑃, it must intersect
in exactly one point, since a disk cannot be a cyclic cover of a disk branched along more than one
point. (Here we are using the fact that the branch points all correspond to intersections of 𝑃 with
the disk.) In other words, 𝑃 is the pattern for a connect-sum, and again we have a contradiction.
□
This proves the claim and completes the proof of the lemma.
⋃
A C K N O W L E D G E M E N T S
Tye Lidman was partially supported by NSF grant DMS-1709702 and a Sloan Fellowship. Juanita
Pinzón-Caicedo is grateful to the Max Planck Institute for Mathematics in Bonn for its hospitality
and financial support while a portion of this work was prepared for publication. She was partially
supported by NSF grant DMS-1664567, and by Simons Foundation Collaboration grant 712377.
17538424, 2023, 1, Downloaded from https://londmathsoc.onlinelibrary.wiley.com/doi/10.1112/topo.12275 by Universitaet Regensburg, Wiley Online Library on [22/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License366
LIDMAN et al.
Raphael Zentner is grateful to the DFG for support through the Heisenberg program. We would
also like to thank John Baldwin, Paul Kirk, and Tom Mrowka for helpful discussions.
Open access funding enabled and organized by Projekt DEAL.
J O U R N A L I N F O R M A T I O N
The Journal of Topology is wholly owned and managed by the London Mathematical Society,
a not-for-profit Charity registered with the UK Charity Commission. All surplus income from its
publishing programme is used to support mathematicians and mathematics research in the form
of research grants, conference grants, prizes, initiatives for early career researchers and the
promotion of mathematics.
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17538424, 2023, 1, Downloaded from https://londmathsoc.onlinelibrary.wiley.com/doi/10.1112/topo.12275 by Universitaet Regensburg, Wiley Online Library on [22/02/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License
| null |
10.7554_elife.88204.pdf
| null |
Data availability The 3D cryo-EM density maps have been deposited in the Electron Microscopy Data Bank under the accession number EMD-29861. Atomic coordinates for the atomic model have been deposited in the Protein Data Bank under the accession number 8G94. All other data needed to evaluate the conclusions in the paper are present in the paper and/or the supplementary materials.
|
RESEARCH ARTICLE
Transmembrane protein CD69 acts as an
S1PR1 agonist
Hongwen Chen1, Yu Qin1, Marissa Chou2, Jason G Cyster2,3*, Xiaochun Li1,4*
1Department of Molecular Genetics, The University of Texas Southwestern Medical
Center, Dallas, United States; 2Department of Microbiology and Immunology,
University of California, San Francisco, San Francisco, United States; 3Howard Hughes
Medical Institute, University of California, San Francisco, San Francisco, United
States; 4Department of Biophysics, The University of Texas Southwestern Medical
Center, Dallas, United States
Abstract The activation of Sphingosine- 1- phosphate receptor 1 (S1PR1) by S1P promotes
lymphocyte egress from lymphoid organs, a process critical for immune surveillance and T cell
effector activity. Multiple drugs that inhibit S1PR1 function are in use clinically for the treatment of
autoimmune diseases. Cluster of Differentiation 69 (CD69) is an endogenous negative regulator
of lymphocyte egress that interacts with S1PR1 in cis to facilitate internalization and degradation
of the receptor. The mechanism by which CD69 causes S1PR1 internalization has been unclear.
Moreover, although there are numerous class A GPCR structures determined with different small
molecule agonists bound, it remains unknown whether a transmembrane protein per se can act as
a class A GPCR agonist. Here, we present the cryo- EM structure of CD69- bound S1PR1 coupled to
the heterotrimeric Gi complex. The transmembrane helix (TM) of one protomer of CD69 homodimer
contacts the S1PR1- TM4. This interaction allosterically induces the movement of S1PR1- TMs 5–6,
directly activating the receptor to engage the heterotrimeric Gi. Mutations in key residues at the
interface affect the interactions between CD69 and S1PR1, as well as reduce the receptor internal-
ization. Thus, our structural findings along with functional analyses demonstrate that CD69 acts in cis
as a protein agonist of S1PR1, thereby promoting Gi- dependent S1PR1 internalization, loss of S1P
gradient sensing, and inhibition of lymphocyte egress.
Editor's evaluation
This important study provides unprecedented molecular insight into the activation and internal-
ization of an important cell surface receptor induced by another membrane protein. The data
supporting the conclusions are compelling, which include rigorous electron microscopy analysis, and
biochemical and cell- based functional assays. The findings here not only reveal important mecha-
nisms of S1P GPCR regulation, but also have implications for other fields such as receptor pharma-
cology and immunity.
Introduction
Sphingosine- 1- phosphate (S1P) plays an essential role in the immune system by promoting the egress
of lymphocytes from lymphoid organs into blood and lymph via a direct interaction with one of its five
cognate G protein–coupled receptors, S1PR1 (Baeyens and Schwab, 2020; Cartier and Hla, 2019;
Cyster and Schwab, 2012; Pappu et al., 2007; Rosen et al., 2013; Spiegel and Milstien, 2003).
After egressing from spleen, lymph nodes, or mucosal lymphoid tissues, T and B lymphocytes travel
to other lymphoid organs in a cycle of continual pathogen surveillance. When an infection occurs,
*For correspondence:
[email protected] (JGC);
xiaochun.li@utsouthwestern.
edu (XL)
Competing interest: The authors
declare that no competing
interests exist.
Funding: See page 12
Preprinted: 15 February 2023
Received: 04 April 2023
Accepted: 09 April 2023
Published: 11 April 2023
Reviewing Editor: Jungsan
Sohn, Johns Hopkins University
School of Medicine, United
States
Copyright Chen et al. This
article is distributed under the
terms of the Creative Commons
Attribution License, which
permits unrestricted use and
redistribution provided that the
original author and source are
credited.
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
1 of 16
Research article
there is a temporary shutdown of lymphocyte egress from the responding lymphoid organ(s) and this
enables increased accumulation of lymphocytes and enhances the immune response (Cyster and
Schwab, 2012). Egress shutdown is mediated by type I interferon (IFN) inducing lymphocyte CD69
expression. CD69, a type II transmembrane C- type lectin protein, intrinsically inhibits the function
of S1PR1 in T and B cells (Shiow et al., 2006). CD69 also regulates T cell egress from the thymus
(Nakayama et al., 2002; Zachariah and Cyster, 2010). A disulfide- bond in the extracellular domain
links CD69 as a homodimer (Ziegler et al., 1994). Biochemical studies demonstrated that CD69 may
associate with S1PR1 through interactions between their transmembrane domains (TMs) to facilitate
S1PR1 internalization and degradation (Bankovich et al., 2010). Unlike S1P, CD69 has been shown to
bind S1PR1 but not the other S1PRs (Bankovich et al., 2010; Jenne et al., 2009; Shiow et al., 2006).
However, the mechanism of CD69- induced S1PR1 internalization and thus functional inactivation has
been unclear.
Importantly, several S1PR1 modulators (e.g. Fingolimod, also known as FTY720, Siponimod, Ozan-
imod, and Etrasimod), have been approved for treating the autoimmune diseases multiple sclerosis
and ulcerative colitis (Brinkmann et al., 2010; Chun et al., 2021; Dal Buono et al., 2022; Kappos
et al., 2010). These immunosuppressants are believed to act by inhibiting S1PR1 function and thereby
preventing autoimmune effector lymphocytes exiting lymphoid organs, blocking the autoimmune
attack. Either sphingosine or FTY720 is metabolically catalyzed by two intracellular sphingosine kinases
into the phosphorylated form (S1P or FTY720- P) and then exported to the extracellular space via S1P
transporters (Baeyens and Schwab, 2020; Spiegel et al., 2019). There, S1P binds to its receptors for
initiation of the signal while FTY720- P activates the S1PR1 but causes a persistent internalization and
degradation of S1PR1 to attenuate the signal (Brinkmann et al., 2010).
Recently, cryogenic electron microscopy (cryo- EM) structures of S1PR1 complexed with different
small molecule ligands have been determined (Liu et al., 2022; Xu et al., 2022; Yu et al., 2022; Yuan
et al., 2021). These findings reveal a mechanism of how S1PR1 engages its endogenous ligand S1P
and its modulators to adopt the active conformation for recruiting the heterotrimeric Gi protein. The
previously determined crystal structure of antagonist ML056- bound S1PR1 reveals its inactive state
(Hanson et al., 2012). However, the molecular mechanism remains unknown of how CD69 binds
to S1PR1 to trigger its internalization. Therefore, structural study on the S1PR1- CD69 complex will
provide molecular insights into the CD69- mediated functional inhibition of S1PR1 and reveal how a
class A GPCR can be regulated by a transmembrane protein modulator. In this manuscript, we deter-
mined the structure of CD69- bound S1PR1 coupled to Gαiβ1γ2 heterotrimer by cryo- EM at 3.15 Å reso-
lution. Our findings reveal that TM of CD69 contacts TM4 of S1PR1 to activate the receptor allowing
it to engage the α5 helix of Gαi in the absence of S1P ligand, thereby disrupting the receptor’s egress-
promoting function.
Results
Since serum contains an abundance of lipids including S1P, we expressed human S1PR1 or CD69 in
HEK293 cells cultured in a medium with lipid- deficient serum. Then, we purified human S1PR1 protein
alone to validate its activation in the presence of S1P using the GTPase- Glo assay (Figure 1A). We
then tested the effect on S1PR1 of adding the CD69 homodimer in the absence of S1P. Remarkably,
addition of CD69 alone caused a similar amount of Gi activation as addition of S1P indicating that
CD69 functions as a protein agonist of S1PR1 (Figure 1A).
To perform structural studies, we mixed lysates from HEK293 cells that independently expressed
human CD69 and human S1PR1. The CD69- S1PR1 complex was then incubated with Gαiβ1γ2 hetero-
trimer and scFv16 (Maeda et al., 2018) at 1:1.2:1.4 molar ratio. After gel- filtration purification, the
resulting complex was concentrated for cryo- EM analysis (Figure 1—figure supplement 1A). We
obtained over 1 million particles from ~4000 cryo- EM images. The overall structure of the CD69-
bound S1PR1 coupled to heterotrimeric Gi was determined at 3.15 Å resolution by 293,516 particles
(Figure 1—figure supplement 1B–F; Table 1). The structure shows that one S1PR1 binds one CD69
homodimer and one Gi heterotrimer. It also revealed well- defined features for the canonical seven
transmembrane helices (7- TMs) of S1PR1, the Gαi Ras- like domain, the Gβ and Gγ subunits and scFv16
(Figure 1B, Figure 1—figure supplement 2A). The intracellular loop 3 (ICL3) and the C- terminus of
S1PR1 and the intracellular and extracellular domains of CD69 were not found in the cryo- EM map
indicating their flexibility in the complex. In contrast, the TMs of the CD69 homodimer were clearly
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
2 of 16
Structural Biology and Molecular Biophysics
Research article
A
C
)
%
(
r
e
v
o
n
r
u
t
P
T
G
d
e
z
i
l
a
m
r
o
N
80
70
60
50
40
B
****
****
CD69-a
CD69-b
****
****
****
****
S1PR1
no ligand
1 nM S1P
10 nM S1P
100 nM S1P
0.5 µM CD69
2.5 µM CD69
10 µM CD69
scFv16
Gβ
Gγ
Giα
Extracellular
Space
N
CD69-a
S1PR1
90°
6
Cytosol
ICL3
C
ICL2
Giα
Gβ
CD69-b
scFv16
Gγ
CD69-a
CD69-b
5
7
1
4
3
2
ligand-binding
pocket
Figure 1. Overall structure of human CD69- S1PR1- Gi complex. (A) S1PR1- induced GTP turnover for Gi1 in the
presence of purified CD69 or S1P. Luminescence signals were normalized relative to the condition with Gi1 only.
Data are mean ± s.e.m. of three independent experiments. One- way ANOVA with Tukey’s test; ****p<0.0001.
Experiments were repeated at least three times with similar results. (B) Cryo- EM map of human CD69 bound
S1PR1- Gi complex. (C) Cartoon presentation of the complex in the same view and color scheme as shown in (B).
Slab view of S1PR1 from the extracellular side showing that the orthosteric binding pocket is vacant.
The online version of this article includes the following source data and figure supplement(s) for figure 1:
Figure supplement 1. Cryo- EM reconstruction of CD69 bound S1PR1- Gi complex.
Figure supplement 1—source data 1. Original uncropped SDS- PAGE gels for data in Figure 1—figure
supplement 1.
Figure supplement 1—source data 2. Uncropped SDS- PAGE gels for data in Figure 1—figure supplement 1
with the relevant bands labeled.
Figure supplement 2. The cryo- EM density map of CD69- bound S1PR1- Gi complex.
Figure supplement 3. Structural comparison between CD69- bound S1PR1 and S1P- bound S1PR1.
Figure supplement 4. Structures of homodimeric and heterodimeric GPCRs.
defined in the map owing to their interactions with S1PR1 (Figure 1B, Figure 1—figure supplement
2A; the interacting TM helix is referred to as CD69- a). Because no lipid was supplemented into the
protein during the expression and purification, there is no notable lipid ligand in the 7- TM bundle of
S1PR1, which is different from the previous structural discoveries on S1PRs (Chen et al., 2022; Liu
et al., 2022; Xu et al., 2022; Yu et al., 2022; Yuan et al., 2021; Zhao et al., 2022).
Structural comparison shows that the entire complex and the S1P bound S1PR1- Gi complex share
a similar conformation with a root- mean- square deviation (RMSD) of 0.82 Å (Figure 1—figure supple-
ment 3A). The receptors in both complexes can be aligned well; however, the F1614.43 in TM4 presents
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
3 of 16
Structural Biology and Molecular Biophysics
Research article
Table 1. Cryo- EM data collection, processing,
and refinement statistics.
Structure
CD69- S1PR1- Gi- scFv16
PDB
EMDB
Data collection/
processing
Magnification
Voltage (kV)
Pixel size (Å)
Defocus range (μm)
Electron exposure (e-/Å2)
Symmetry imposed
8G94
EMD- 29861
105,000
300
0.83
1.0–2.0
60
C1
Initial particles (No.)
~1.1 million
Final particles (No.)
Map resolution (Å)
FSC threshold
Map resolution range (Å)
Refinement
Model Resolution (Å)
FSC threshold
Map sharpening B- factor
(Å2)
Model composition
Non- hydrogen atoms
Protein residues
Ligand
B- factors (Å2)
Protein
R.m.s. deviations
Bond lengths (Å)
Bond angles (°)
Validation
MolProbity score
Clashscore
Rotamers outliers (%)
Ramachandran plot (%)
Favored
Allowed
Outliers
293,516
3.14
0.143
25–3.0
3.3
0.5
–60
9223
1187
0
98.23
0.006
0.702
1.64
6.49
0.00
95.87
4.13
0.00
a notable shift due to CD69 binding (Figure 1—
figure supplement 3B). We docked the S1PR1
bound to the other TM of the CD69 homodimer
which showed that the modeled receptor would
sterically clash with the Giα subunit (Figure 1—
figure supplement 3C). This may explain why
only one receptor binds one CD69 homodimer in
the presence of the heterotrimeric G- protein.
Receptor activity- modifying protein 1 (RAMP1),
a type I transmembrane domain protein, binds
the calcitonin receptor- like receptor (CLR) class
B GPCR to form the Calcitonin gene- related
peptide (CGRP) receptor which is involved in the
pathology of migraine (Russell et al., 2014). The
structure of Gs- protein coupled CGRP receptor
uncovers that TM of RAMP1 interacts with TMs
3–5 of CLR and the extracellular domains of
RAMP1 and CLR have extensive interactions
(Liang et al., 2018; Figure 1—figure supple-
ment 4A). Both CLR and RAMP1 contribute to the
engagement of their agonist CGRP. However, in
our structure, CD69 acts as an agonist to activate
S1PR1 through a direct binding to TM4 of S1PR1
in the absence of a canonical agonist (e.g. S1P or
FTY720- P). The extracellular domain of CD69 is
completely invisible in the complex and may not
interact with the extracellular loops of S1PR1.
Another type of intramembrane interaction
observed for GPCRs is the formation of either
homodimers or heterodimers. The metabotropic
glutamate receptor 2 (mGlu2), a Class- C GPCR,
employs TM4 to maintain its inactive dimeric state
or TM6 to assemble as a homodimer in the pres-
ence of its agonist (Du et al., 2021; Figure 1—
figure supplement 4B). The structure of inactive
mGlu2–mGlu7 heterodimer shows that TM5 plays
a key role in the complex assembly (Du et al.,
2021; Figure 1—figure supplement 4C). More-
over, TM1 of the class D GPCR Ste2 is responsible
for engaging the TM1 of another Ste2 to form a
homodimer (Velazhahan et al., 2021; Figure 1—
figure supplement 4D). These findings elucidate
that GPCRs could employ distinct TMs to recruit
their transmembrane binding partners.
The TM of one protomer of CD69 homodimer
interacts with the TM4 of S1PR1 (Figure 1C). The
interface area between TMs is about 600 Å2. Struc-
tural analysis shows that residues V41, V45, V48,
V49, T52, I56, I59, A60 of CD69 mediate its exten-
sive interactions with the receptor (Figure 2A,
Figure 1—figure supplement 2B). Residues
L1604.42, F1614.43,
I1644.46, W1684.50, V1694.51,
L1724.54, I1734.55, G1764.58, I1794.61 and M1804.62
of S1PR1- TM4 contribute to the interaction with
CD69 (Figure 2B, Figure 1—figure supplement
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
4 of 16
Structural Biology and Molecular Biophysics
Research article
A
C
E
A60
I179
M180
I59
I173
I56
L172
T52
V169
W168
I164
V49
V48
V45
F161
TM4
L160
CD69-a
V41
90°
B
I173
V169
M180
T52
I59
I56
A60
L172
V49
I179
–50
S1PR1
–37
–50
CD69
–37
–50
CD69
–37
–50
Tubulin
F
S1PR1
–
WT
V169Y
M180Y
CD69
WT
WT
WT
WT
Lane #
1
2
3
4
WT
V48F,
V49F
5
WT
I56F,
I59F
6
IP: Flag
WB: Flag
IP: Flag
WB: Strep
Lysates
WB: Strep
Lysates
WB: Tubulin
D
%
e
s
a
e
e
r
l
F
G
T
-
P
A
14
12
10
8
6
4
2
0
-2
S1PR1WT
S1PR1V169Y
S1PR1M180Y
-12
-11
-10
-9
-7
-8
Log [S1P (M)]
-6
-5
-4
)
%
(
r
e
v
o
n
r
u
t
P
T
G
d
e
z
i
l
a
m
r
o
N
70
65
60
55
50
Control
CD69(V48F/V49F)
CD69(WT)
CD69(I56F/I59F)
S1PR1
S1PR1 + CD69
S1PR1 + CD69(V48F/V49F)
S1PR1 + CD69(I56F/I59F)
Figure 2. The binding interface between CD69 and S1PR1. (A) and (B) Detailed interactions between CD69- a and
TM4 of S1PR1. Residues that contribute to complex formation are labeled. CD69 is shown in green and S1PR1
in slate. (C) S1PR1- Flag and CD69- StrepII co- immunoprecipitation assay in transfected HEK293 GnTI- cells from
one experiment that is representative of three. (D) Dose- response curves of S1PR1WT, S1PR1V169Y and S1PR1M180Y
for the TGFα shedding assay using S1P. Data are mean ± s.d. (n=3). (E) S1PR1- induced GTP turnover for Gi1
in the presence of purified wild- type and mutant CD69. Luminescence signals were normalized relative to the
condition with Gi1 only. Data are mean ± s.e.m. of three independent experiments. One- way ANOVA with Tukey’s
test; ***p<0.001, ****p<0.0001. Experiments in (C)-(E) were repeated at least twice with similar results. (F) Flow
cytometric analysis of S1PR1 surface expression on WEHI231 lymphoma cells transduced with S1PR1 and CD69
wild- type and mutant constructs as indicated. From one experiment that is representative of three.
The online version of this article includes the following source data and figure supplement(s) for figure 2:
Source data 1. Original uncropped western blots for data in Figure 2.
Source data 2. Uncropped western blots for data in Figure 2 with the relevant bands labeled.
Figure supplement 1. Size exclusion column profiles of CD69 wild type and mutants.
Figure supplement 1—source data 1. Original uncropped SDS- PAGE gels for data in Figure 2—figure
supplement 1.
Figure supplement 1—source data 2. Uncropped SDS- PAGE gels for data in Figure 2—figure supplement 1
with the relevant bands labeled.
2B). However, the TM of another CD69 does not have any interactions with the receptor and hetero-
trimeric Gi protein (Figure 1C). Further structural comparison with the S1P- bound S1PR1- Gi complex
indicates that the heterotrimeric Gi proteins in both complexes exhibit a similar state with a RMSD
of 0.45 Å. Also, the intracellular regions of the heptahelical domain adopt a similar conformation
to accommodate the Gi proteins. This finding implies that S1P and CD69 stimulate the receptor to
engage the heterotrimeric Gi proteins in an analogous fashion.
To validate our structural observations, we performed the co- immunoprecipitation (co- IP) assay
using S1PR1 and CD69 variants. Compared to the wild- type S1PR1, two mutants (V1694.51Y and
M1804.62Y) present reduced binding to CD69 (Figure 2C). The TGFα shedding assay showed that
these two mutants retained normal activity in response to S1P (Figure 2D). We also tested two CD69
double mutations (V48F/V49F and I56F/I59F) for their association with S1PR1. The co- IP results
show that the interaction between S1PR1 and either mutant is considerably attenuated, thus directly
supporting the role of CD69- TM in the complex assembly (Figure 2C). Moreover, we have purified
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A
C
ML056
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(3V2Y)
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V49
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V41
L174
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E
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W269
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S1PR1
(7TD3)
L174
I170
Figure 3. Comparison between CD69- bound S1PR1 and ML056- or S1P- bound S1PR1. (A) Overall structures of
S1PR1 binding with CD69 and ML056. The CD69- bound S1PR1 structure was aligned to ML056- bound inactive
S1PR1 (PDB code: 3V2Y). ML056- bound receptor is shown in brown, CD69- bound receptor in blue, and the TM
of CD69 in green. The same color scheme is used (C) and (D). (B) The movements of TM4 and TM6 of CD69-
bound S1PR1 compared with ML056- bound inactive S1PR1. (C) Residues involved in the TM movements. (D) TM6
movement around W2696.48 and F2736.52. (E) Comparison between CD69- bound S1PR1 and S1P- bound S1PR1 (PDB
code: 7TD3). Residues in the ligand binding pocket are shown. CD69- bound receptor in blue and S1P- bound
S1PR1 in cyan. S1P is shown as balls and sticks in yellow.
The online version of this article includes the following source data and figure supplement(s) for figure 3:
Figure supplement 1. S1PR1 specificity for CD69 binding.
Figure supplement 1—source data 1. Original uncropped western blots for data in Figure 3—figure
supplement 1.
Figure supplement 1—source data 2. Uncropped western blots for data in Figure 3—figure supplement 1 with
the relevant bands labeled.
Figure supplement 2. Structures of GPCRs with their positive allosteric modulators.
CD69(V48F/V49F) and CD69(I56F/I59F) individually and mixed with S1PR1 and Gαiβ1γ2 to conduct a
GTPase- Glo assay (Figure 2—figure supplement 1). Consistent with the results of our co- IP assays,
the activation of Gi proteins in the presence of either variant was decreased (Figure 2E). To further
validate the physiological role of the CD69- S1PR1- Gi complex, we tested the two CD69 variants, for
their influence on CD69- mediated S1PR1 internalization in WEHI231 B lymphoma cells. In accord with
the biochemical data, CD69(V48F/V49F), and CD69(I56F/I59F) were both reduced in their ability to
downregulate S1PR1 (Figure 2F).
The structures of the S1PR1 complex with its small molecule modulators (including S1P,
FTY720- P, BAF312, and ML056) uncover that the TMs 3, 5, 6, and 7 contribute to accommodate
the modulators in the orthosteric site (Hanson et al., 2012; Liu et al., 2022; Xu et al., 2022;
Yu et al., 2022; Yuan et al., 2021). In contrast, S1PR1 employs its TM4 to associate with CD69
which functions as a protein agonist for triggering receptor activation. Structural comparison with
the inactive state of ML056 bound S1PR1 reveals a unique mechanism of CD69- mediated S1PR1
activation (Figure 3A). The binding of CD69 induces a 4 Å shift at the intracellular end of TM4
causing the residues C1674.49, I1704.52, and L1744.56 in TM4 to face TM3 (Figure 3B). C1674.49 and
I1704.52 have hydrophobic contacts with the F1333.41 in TM3, and L1744.56 pushes the F2105.47 in TM5
towards the edge of the receptor to form the hydrophobic interactions with W2696.48 and F2736.52
in TM6 (Figure 3C, Figure 1—figure supplement 2C). These interactions trigger the notable
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movement of TM5 and TM6 allowing the opening of intracellular regions to engage the hetero-
trimeric Gi proteins (Figure 3A and D). Residues A1373.45, I1704.52, L1744.56, F2105.47, W2696.48, and
F2736.52 are conserved among S1PR1, S1PR2 and S1PR3, but not F1333.41 and C1674.49. Remarkably,
further comparison shows that the key residues, which are crucial for the S1P binding and receptor
activation, present similar conformations in the structures of S1P- bound S1PR1 and CD69- bound
S1PR1, although S1P and CD69 have different structural natures and completely distinct binding
sites in the receptor (Figure 3E).
To date, five S1PRs have been identified. These receptors have different tissue distributions, and
they also function via distinct kinds of G proteins (including Gi, Gq, and G12/13) (Cartier and Hla, 2019).
Previous work showed that CD69 specifically binds to S1PR1, and it does not associate with S1PR2,
S1PR3 or S1PR5 (Bankovich et al., 2010; Jenne et al., 2009; Shiow et al., 2006). To dissect the
binding specificity of CD69, we carried out the co- IP assays to show a very weak interaction between
S1PR2 and CD69 (Figure 3—figure supplement 1A). Although the sequence homology among five
S1PRs is high, residues L1574.51 and L1684.62 in S1PR2- TM4 are not conserved with those in S1PR1
and are determinants for specific recognition of CD69 (Figure 3—figure supplement 1B). We spec-
ulated that converting these two residues to those in S1PR1 may prompt the interaction between
S1PR2 variant and CD69. Our co- IP result clearly shows that S1PR2(L1574.51V/L1684.62M) could interact
with CD69 albeit the interactions are weaker than that between S1PR1 and CD69 (Figure 3—figure
supplement 1A). This finding further demonstrates the essential role of S1PR1- TM4 in the CD69-
mediated S1PR1 signaling.
All the known small molecule S1PR1 agonists or antagonists bind to the orthosteric site in the
heptahelical domain (Hanson et al., 2012; Liu et al., 2022; Xu et al., 2022; Yu et al., 2022; Yuan
et al., 2021). Interestingly, the CD69 binding site is akin to that of the allosteric agents which attach
to receptors (Draper- Joyce et al., 2021; Mao et al., 2020; Yang et al., 2022), although the nature
of these agents and CD69 is quite different. The diversity of the allosteric modulator binding sites in
GPCRs has been revealed by numerous structures (Figure 3—figure supplement 2). When the ortho-
steric site is occupied, the positive allosteric modulator attaches to the receptor and then increases
agonist affinity and/or efficacy. CD69 binds to the edge of S1PR1, but it acts as a protein agonist to
directly activate the receptor in the absence of any agonists in the orthosteric site. Thus, our finding
suggests CD69 is different from other S1PR1 agonists in that it functions via a direct binding to the
edge of the receptor.
It remains unknown whether the antagonist of S1PR1 bound to the 7- TMs will affect the CD69-
mediated regulation of S1PR1. We co- transfected S1PR1- GFP and CD69- mCherry into HEK293 cells
in a lipid depleted medium. After 24 hr, the fluorescence images show that substantial receptors
(~80%) have been internalized with CD69. However, when we added the Ex26, a potent S1PR1 antag-
onist (Cahalan et al., 2013), into the cells 6 hr after transfection, the images show that just ~50%
receptors have been internalized (Figure 4A and B). This finding indicates that the CD69- mediated
S1PR1 activation could be reversed when the 7- TMs pocket is preoccupied by an antagonist.
It has been known that S1P, FTY720- P and CD69, could promote the internalization of S1PR1.
However, the mechanisms of S1P- and FTY720- P- mediated internalization appear to be different.
While both S1P and FTY720- P activate Gi- signaling, FTY720- P is considered as a β-arrestin- biased
agonist, and FTY720- P- induced S1PR1 internalization is β-arrestin- dependent (Oo et al., 2007;
Xu et al., 2022). The pathway of S1P- mediated internalization can be β-arrestin- dependent or
independent (Galvani et al., 2015; Reeves et al., 2016). To test the mechanism of how CD69
induces the receptor internalization, we performed a fluorescence imaging assay to check the
internalization of S1PR1 in the presence of either Gi inhibitor Pertussis toxin (PTX) or β-arrestin
inhibitor Barbadin. The plasmids encoding S1PR1- GFP and CD69- mCherry were co- transfected
into HEK293 cells in a lipid depleted medium. After 6 hr, we added PTX or Barbadin. On day 2, we
calculated the fraction of internalized S1PR1 in each group by fluorescence imaging. The results
show that Barbadin does not interfere with CD69- induced receptor internalization (Figure 4C and
D), but PTX could prevent half of the receptors from internalization (Figure 4E and F). Our finding
also supports that Barbadin was effective in reducing FTY70- P- induced S1PR1 internalization
(Figure 4—figure supplement 1). Thus, CD69 agonism of S1PR1 induces Gi- dependent internal-
ization of the complex.
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A
Vehicle
Ex26
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Figure 4. CD69 induced S1PR1 internalization. (A) HEK293 cells were treated with 2 μM Ex26 or vehicle for 12 hr
and imaged using confocal microscopy. Scale bar, 10 μm. (B) Quantification of intracellular S1PR1 of the cells in
(A). (C) HEK293 cells were treated with 20 μM Barbadin for 12 hr and imaged for analysis. Scale bar, 10 μm. (D)
Quantification of intracellular S1PR1 of the cells in (C). (E) HEK293 cells were treated with 200 ng/ml pertussis toxin
(PTX) for 12 hr and imaged for analysis. Scale bar, 10 μm. (F) Quantification of intracellular S1PR1 of the cells in
(E). Data are mean ± s.e.m. Two- sided Welch’s t- test; ns, not significant, **p<0.01, ****p<0.0001. All experiments
were repeated at least three times with similar results.
The online version of this article includes the following figure supplement(s) for figure 4:
Figure supplement 1. Barbadin alters the FTY720- P mediated S1PR1 internalization.
Figure supplement 2. S1PR1- induced GTP turnover for Gi1 in the presence of purified CD69 and S1P.
Discussion
Our studies provide a model for understanding how the lymphocyte activation marker CD69 controls
lymphocyte egress and thus augments adaptive immunity. As an immediate early gene, CD69 is
strongly transcriptionally induced in lymphocytes within an hour of exposure to type I IFN, toll- like
receptor (TLR) ligands, or antigen receptor engagement (Grigorova et al., 2010; Shiow et al., 2006;
Ziegler et al., 1994). Following induction, CD69 protein engages S1PR1 as an agonist, causing S1PR1
internalization and loss of the ability to sense S1P gradients. We speculate that even prior to internal-
ization, CD69 disrupts S1PR1’s egress promoting function by acting as a high concentration agonist
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
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Structural Biology and Molecular Biophysics
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and thus making the receptor ‘blind’ to S1P distribution. Consistently, our functional analysis reveals
that CD69 could not synergize with S1P to trigger S1PR1 activation (Figure 4—figure supplement 2).
Previous work has shown the critical importance of correctly distributed S1P and thus correctly
localized S1PR1 activation for effective lymphocyte egress (Schwab et al., 2005). As well as promoting
egress, S1PR1, transmits signals needed for maintaining T cell survival (Mendoza et al., 2017) and
CD69 has been implicated in transmitting signals that influence T cell differentiation (Cibrián and
Sánchez- Madrid, 2017; Kimura et al., 2017). Whether the CD69- S1PR1 complex contributes to these
signals before undergoing degradation merits further study. GRK2 (Arnon et al., 2011; Oo et al.,
2007; Watterson et al., 2002) and dynamin (Willinger et al., 2014) participate in S1PR1 internaliza-
tion in response to S1P. In accord with these factors possibly having a role in CD69- mediated S1PR1
internalization, they have been shown to promote internalization of some receptors independently of
β-arrestins (Moo et al., 2021). The selectivity of CD69 for S1PR1 is important for allowing activated
CD69+ lymphocytes and natural killer cells to employ other S1PRs, such as S1PR2 and S1PR5, to carry
out functions without interruption by CD69 (Jenne et al., 2009; Laidlaw et al., 2019; Moriyama
et al., 2014). The lack of conservation of key residues that mediate the S1PR1- CD69 interaction in
TM4 of S1PR2, S1PR5 and the other S1PRs provides an explanation for this selectivity (Figure 3—
figure supplement 1B). In summary, we provide the first example of GPCR activation by interaction in
cis with a transmembrane ligand and thereby explain the mechanism of lymphocyte egress shutdown.
The structure also offers insights that may enable introduction of transcriptionally inducible GPCR
switches into CAR- T cells and other engineered cell types.
Methods
Constructs
For expression and purification, the wild- type human S1PR1 (a.a.1–347, UniProt: P21453) and CD69
(full- length, UniProt: Q07108) were separately cloned into pEZT- BM vector (Morales- Perez et al.,
2016) with a C- terminal Flag tag and StrepII tag, respectively. Plasmids of Gαi1, Gβ1/Gγ2 and scFv16
are kind gifts from Brian Kobilka (Stanford University). For co- immunoprecipitation assay, the full-
length wild- type human S1PR1 fused with a C- terminal Flag tag and CD69 fused with a C- terminal
StrepII tag, were separately cloned into pCAGGS vector (Niwa et al., 1991) with modified multiple
cloning sites. For fluorescence microscopy, the plasmids pCAGGS- S1PR1- Flag- GFP and pCAGGS-
CD69- StrepII- mCherry were constructed.
Protein expression and purification
S1PR1- Flag and CD69- StrepII were separately expressed using baculovirus- mediated transduction
of mammalian HEK293S GnTI− cells (ATCC CRL- 3022) in a medium containing FreeStyle 293 (Gibco
Cat# 12338018) supplemented with 2% charcoal- dextran stripped fetal bovine serum (Gibco Cat#
12676029), penicillin (100 U/mL), and streptomycin (100 μg/mL) (Corning Cat# 30–002 CI). Baculo-
viruses were generated in Sf9 cells, and P2 virus was used to infect HEK293S GnTI− cells at 37 °C.
At 8 hr after infection, sodium butyrate at a final concentration of 10 mM was added to the culture.
After further incubation for 64 hr at 30 °C, cells expressing S1PR1- Flag and CD69- StrepII were mixed
together and resuspended in buffer A (20 mM HEPES, pH 7.5, 150 mM NaCl) supplemented with
protease inhibitors and then homogenized by sonication. The protein was solubilized with 1% LMNG
(lauryl maltose neopentyl glycol) /0.1% CHS (cholesteryl hemisuccinate) for 1 hr at 4 °C. Insoluble
material was removed by centrifugation at 40,000 g, 4 °C for 30 min, and the supernatant was incu-
bated with Strep- Tactin XT resin (IBA Cat# 2- 5030- 025) for batch binding. The resin was washed with
20 column volumes (CV) of buffer A containing 0.01% LMNG/0.001% CHS. The protein complex was
eluted with 6 CVs of buffer A containing 0.01% LMNG/0.001% CHS and 50 mM biotin, followed by
a second affinity purification by anti- Flag M2 resin (Sigma- Aldrich). The excessive CD69- StrepII was
washed off with 20 CVs of buffer A containing 0.01% LMNG/0.001% CHS, and the complex was
eluted with 5 CVs of 3×Flag peptide (0.1 mg/ml; ApexBio). The eluted protein was further purified
by gel filtration using a Superose 6 Increase 10/300 GL column (Cytiva) with 20 mM HEPES, pH 7.5,
150 mM NaCl, 0.001% L- MNG/0.0001% CHS, and 0.0025% glyco- diosgenin (GDN). The peak frac-
tions were collected for complex assembly.
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To assemble the CD69- S1PR1- Gi- scFv16 complex, purified CD69- S1PR1 was mixed with the Gi
heterotrimer at a 1:1.2 molar ratio. This mixture was incubated on ice for 1 hr, followed by the addi-
tion of apyrase to catalyze the hydrolysis of unbound GDP on ice for 1 hr. Then, scFv16 was added
at a 1.4:1 molar ratio (scFv16: CD69- S1PR1) followed by 30 min incubation on ice. The mixture was
diluted 10- fold by gel filtration column buffer. To remove excess Gi and scFv16 proteins, the mixture
was purified by anti- Flag M2 affinity chromatography. The complex was eluted and concentrated
using an Amicon Ultra Centrifugal Filter (molecular weight cutoff 100 kDa). The complex was further
purified by gel filtration (Superose 6 Increase 10/300 GL) with buffer 20 mM HEPES, pH 7.5, 150 mM
NaCl, 0.001% L- MNG/0.0001% CHS, and 0.0025% GDN. Peak fractions consisting of CD69- S1PR1- Gi
complex were concentrated to ~10–12 mg/ml for cryo- EM studies.
Cryo-EM sample preparation and data acquisition
The freshly purified CD69- S1PR1- Gi- scFv16 complex was added to Quantifoil R1.2/1.3 400- mesh Au
holey carbon grids (Quantifoil), blotted using a Vitrobot Mark IV (FEI), and vitrified in liquid ethane.
The grids were imaged in a 300- kV Titan Krios (FEI) with a Gatan K3 Summit direct electron detector.
Data were collected in super- resolution mode at a pixel size of 0.415 Å with a dose rate of 23 electrons
per physical pixel per second. Images were recorded for 5 s exposures in 50 subframes with a total
dose of 60 electrons per Å2.
Imaging processing and 3D reconstruction
A total of 4,239 dose- fractionated image stacks of CD69- S1PR1- Gi complex were collected and
subjected to single particle analysis using RELION- 3.1 (Zivanov et al., 2018) and cryoSPARC v3.3
(Punjani et al., 2017). MotionCor2 (Zheng et al., 2017) was used for motion correction and dose
weighting, CTFFIND- 4.1 Rohou and Grigorieff, 2015 for contrast transfer function (CTF) estimation,
and crYOLO Wagner et al., 2019 for particle picking with a general model. A total of 1,113,446 parti-
cles were extracted with a pixel size of 1.66 Å in RELION and imported to cryoSPARC. The imported
particles were subjected to ab initio model reconstruction and several rounds of alternating 2D classi-
fication and heterogeneous refinement. Then 336,669 particles from the best class were re- extracted
at full pixel size (0.83 Å) in RELION and imported to cryoSPARC again. Two heterogeneous refine-
ments were performed in parallel and the resulting particles from the two best classes were combined
with duplicates removed. These 293,516 particles were subjected to CTF refinement and Bayesian
polishing followed by masked 3D auto refinement. RELION postprocessing was used for sharpening
of the final map.
Model construction and refinement
The cryo- EM structure of the S1PR1- Gi bound to S1P (PDB: 7TD3) (Liu et al., 2022) was used as
initial models and manually docked into cryo- EM density map with UCSF Chimera- 1.15 (Pettersen
et al., 2004). The transmembrane helix of CD69 was manually built using Coot- 0.9.6 (Emsley and
Cowtan, 2004). Due to the limited local resolution, the TM of CD69- b was built as polyalanine. The
resulting model was subjected to iterative rounds of manual adjustment and rebuilding in Coot and
real- space refinement in Phenix- 1.16 (Adams et al., 2010). MolProbity (Williams et al., 2018) was
used to validate the geometries of the model. Structural figures were generated using UCSF Chime-
ra- 1.15, ChimeraX- 1.5 (Pettersen et al., 2021), and PyMOL- 2.3 (https://pymol.org/2/).
GTP turnover assay
GTP turnover was analyzed using GTPase- Glo Assay kit (Promega Cat# V7681). Briefly, the purified
S1PR1 was first incubated with purified CD69 and/or S1P followed by mixing with isolated Gi protein
in an assay buffer containing 20 mM HEPES, pH7.5, 150 mM NaCl, 0.01% LMNG/0.001% CHS, 10 mM
MgCl2, 100 μM TCEP, 10 μM GDP and 5 μM GTP. After incubation for 60 min, the reconstituted
GTPase- Glo reagent was added to the sample and incubated for 30 min at room temperature. The
amount of remaining GTP was assessed by measuring luminescence after adding and incubation with
the detection reagent for 10 min at room temperature. The luminescence signal was normalized in
each case to that of G- protein alone. Data were analyzed using GraphPad Prism 9.
Co-immunoprecipitation and immunoblotting assay
HEK293 GnTI- cells were transfected with plasmids encoding CD69- StrepII and S1PR1- Flag using
FuGene 6 transfection reagent in 60 mm dishes. Forty- eight hr post transfection, cells were harvested
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and whole cell lysates were prepared using IP lysis buffer (Thermo Scientific) supplemented with
protease inhibitor cocktail (Roche). Lysates were cleared by centrifuging at 20,000 g for 15 min at 4
°C. Supernatants were incubated with anti- Flag M2 affinity beads (MilliporeSigma) with end- over- end
rotation for 2 h at 4 °C. Beads were washed three times with lysis buffer for 5 min per wash with
end- over- end rotation at 4 °C. Proteins were eluted from beads with lysis buffer supplemented with
0.4 mg/ml 3×Flag peptide. Protein samples were loaded Bolt 4–12% Bis- Tris plus gels (Invitrogen)
and transferred to TransBlot Turbo nitrocellulose membranes (Bio- Rad). Membranes were blocked
for 1 hr at room temperature with 5% milk in PBS with 0.05% Tween 20 (PBST) followed by primary
antibody incubation, three- times wash, secondary antibody incubation, and three- times wash again.
Membranes were developed for 2 min at room temperature using SuperSignal West Pico PLUS Chemi-
luminescent Substrate (Thermo Scientific) and then imaged using the LI- COR Odyssey Fc imaging
system. The following primary antibodies were used: Tubulin (D3U1W), Cell Signaling Cat# 86298
(1:3000 dilution); Flag tag (FLA- 1), MBL International Cat# M185- 3L (1:3000 dilution); StrepII tag,
IBA GmbH Cat# 2- 1507- 001 (1:2000 dilution). Anti- mouse IgG HRP- linked secondary antibody (Cell
Signaling Cat# 7076) was used for chemiluminescent detection (1:3000 dilution).
TGFα shedding assay
The agonist activity of S1P for the mutant S1PR1s was determined by the TGFα shedding assays
(Inoue et al., 2012). Briefly, three pCAGGS plasmids encoding the human full- length S1PR1 variant
(empty vector as negative control), the chimeric Gαq/i1 subunit and alkaline phosphatase- fused TGFα
(AP- TGFα) were co- transfected into HEK293 cells using FuGene 6 transfection reagent in a 12- well
plate. After 24 hr, the transfected cells were collected by trypsinization, washed with phosphate-
buffered saline (PBS), and resuspended in Hanks’ balanced salt solution (HBSS) with 5 mM HEPES (pH
7.4). Then, the cells were seeded into a 96- well culture plate and treated with S1P, which was serially
diluted in HEPES- containing HBSS with 0.01% fatty acid–free bovine serum albumin. After incubation
with S1P, the cell plate was spun, and conditioned media was transferred to an empty 96- well plate.
AP reaction solution (120 mM Tris- HCl, pH 9.5, 40 mM NaCl, 10 mM MgCl2, and 10 mM p- nitrophenyl
phosphate) was added into the cell plates and the conditioned media plates. The absorbance at
405 nm was measured using a microplate reader (Synergy Neo2, BioTek) before and after 2 hr incuba-
tion at 37 °C. Ligand- induced AP- TGFα release was calculated as described previously (Inoue et al.,
2012). AP- TGFα release signal of empty vector- transfected cells were subtracted from that of S1PR1
cells at the corresponding S1P concentration points. Then, the vehicle- treated AP- TGFα release signal
was set as a baseline and ligand- induced AP- TGFα release signals were fitted to a four- parameter
sigmoidal concentration–response curve using GraphPad Prism 9 software.
Fluorescence microscopy
HEK293 cells were plated in 35 mm glass bottom dishes (Cellvis Cat# D35141.5N) followed by trans-
fection with S1PR1- GFP and/or CD69- mCherry using FuGene 6 reagent on the next day. Twenty-
four hr post transfection, the cells were stained with Hoechst 33342 reagent (Thermo Fisher Cat#
R37605) and fluorescence images were acquired using a Zeiss LSM 800 microscope system with ZEN
imaging software (Zeiss).
For fluorescence quantification of intracellular S1PR1 and CD69, outside and inside of plasma
membrane were circled manually in Fiji software (Schindelin et al., 2012). The fluorescence intensi-
ties in each circle were measured and regarded as whole- cell and intracellular fluorescence intensity,
respectively. The intracellular fluorescence intensity was normalized to its corresponding whole- cell
fluorescence intensity. For each data point, ~30 cells were randomly selected for quantification. The
data shown in the figures are representative of two or more independent experiments.
WEHI231 cell retroviral transduction
WEHI231 B lymphoma cells were co- transduced with retroviral constructs encoding OX56 N- ter-
minal tagged human S1PR1 containing an IRES- hCD4 reporter and either empty vector or constructs
encoding wildtype, V48F/V49F or I56F/I59F human CD69 and an IRES- GFP reporter using methods
previously described (Lu et al., 2019). After 3–5 days, the cells were harvested and rested for 20 min
at 37 °C in PBS without serum, then stained to detect OX56, CD69, and hCD4. OX56 (S1PR1) staining
on hCD4 +GFP + CD69 + cells were plotted.
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
11 of 16
Structural Biology and Molecular Biophysics
Research article
Acknowledgements
The data were collected at the UT Southwestern Medical Center Cryo- EM Facility (funded in part
by the CPRIT Core Facility Support Award RP170644). We thank L Beatty, L Esparza, and Y Xu for
technical support. This work was supported by NIH P01 HL160487, R01 GM135343, and Welch
Foundation (I- 1957) (to XL) and R01 AI040098 (to JGC). JGC is an investigator of Howard Hughes
Medical Institute. This article is subject to HHMI’s Open Access to Publications policy. HHMI lab
heads have previously granted a nonexclusive CC BY 4.0 license to the public and a sublicensable
license to HHMI in their research articles. Pursuant to those licenses, the author- accepted manu-
script of this article can be made freely available under a CC BY 4.0 license immediately upon
publication.
Additional information
Funding
Funder
National Institutes of
Health
National Institutes of
Health
Grant reference number Author
P01 HL160487
Xiaochun Li
R01 GM135343
Xiaochun Li
Welch Foundation
I-1957
Howard Hughes Medical
Institute
National Institutes of
Health
Xiaochun Li
Jason G Cyster
R01 AI040098
Jason G Cyster
The funders had no role in study design, data collection and interpretation, or the
decision to submit the work for publication.
Author contributions
Hongwen Chen, Conceptualization, Investigation, Writing – review and editing; Yu Qin, Marissa Chou,
Investigation, Writing – review and editing; Jason G Cyster, Xiaochun Li, Conceptualization, Supervi-
sion, Funding acquisition, Writing – original draft, Writing – review and editing
Author ORCIDs
Hongwen Chen
Jason G Cyster
Xiaochun Li
http://orcid.org/0000-0002-1065-9808
http://orcid.org/0000-0002-4735-9745
http://orcid.org/0000-0002-0177-0803
Decision letter and Author response
Decision letter https://doi.org/10.7554/eLife.88204.sa1
Author response https://doi.org/10.7554/eLife.88204.sa2
Additional files
Supplementary files
• MDAR checklist
Data availability
The 3D cryo- EM density maps have been deposited in the Electron Microscopy Data Bank under the
accession number EMD- 29861. Atomic coordinates for the atomic model have been deposited in the
Protein Data Bank under the accession number 8G94. All other data needed to evaluate the conclu-
sions in the paper are present in the paper and/or the supplementary materials.
Chen et al. eLife 2023;12:e88204. DOI: https://doi.org/10.7554/eLife.88204
12 of 16
Structural Biology and Molecular Biophysics
Research article
The following datasets were generated:
Author(s)
Chen H, Li X
Year
2023
Chen H, Li X
2023
Dataset title
Dataset URL
Database and Identifier
Structure of CD69-
bound S1PR1 coupled to
heterotrimeric Gi
Structure of CD69-
bound S1PR1 coupled to
heterotrimeric Gi
https://www. rcsb. org/
structure/ 8G94
RCSB Protein Data Bank,
8G94
https://www. ebi. ac.
uk/ emdb/ EMD- 29861
EMBD, EMD- 29861
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Information Systems Frontiers
https://doi.org/10.1007/s10796-023-10369-7
Snakes and Ladders: Unpacking the Personalisation‑Privacy Paradox
in the Context of AI‑Enabled Personalisation in the Physical Retail
Environment
Ana Isabel Canhoto1
· Brendan James Keegan2 · Maria Ryzhikh3
Accepted: 4 January 2023
© The Author(s) 2023
Abstract
Artificial intelligence (AI) is expected to bring to the physical retail environment the kind of mass personalisation that is
already common in online commerce, delivering offers that are targeted to each customer, and that adapt to changes in the
customer’s context. However, factors related to the in-store environment, the small screen where the offer is delivered, and
privacy concerns, create uncertainty regarding how customers might react to highly personalised offers that are delivered to
their smartphones while they are in a store. To investigate how customers exposed to this type of AI-enabled, personalised
offer, perceive it and respond to it, we use the personalisation-privacy paradox lens. Case study data focused on UK based,
female, fashion retail shoppers exposed to such offers reveal that they seek discounts on desired items and improvement
of the in-store experience; they resent interruptions and generic offers; express a strong desire for autonomy; and attempt
to control access to private information and to improve the recommendations that they receive. Our analysis also exposes
contradictions in customers’ expectations of personalisation that requires location tracking. We conclude by drawing an anal-
ogy to the popular Snakes and Ladders game, to illustrate the delicate balance between drivers and barriers to acceptance of
AI-enabled, highly personalised offers delivered to customers’ smartphones while they are in-store.
Keywords Artificial intelligence · Personalisation · Privacy · Personalisation-privacy paradox · Retail · Geo-location
1 Introduction
Artificial intelligence (AI) is expected to transform business
practice in in-store retailing (Davenport et al., 2020), by
bringing to the physical retail environment the kind of mass
personalisation that is already common in online commerce
(Kumar et al., 2017). Personalisation benefits retailers
because targeted messages get noticed amid the noise of other
communications (Balan & Mathew, 2020), increase sales,
and support customer intimacy, involvement with the brand
* Ana Isabel Canhoto
[email protected]
Brendan James Keegan
[email protected]
Maria Ryzhikh
[email protected]
1 University of Sussex, Sussex, UK
2 Maynooth University, Maynooth, Ireland
3 Weber-Stephen Products EMEA GmbH, Berlin, Germany
(Gardino et al., 2021) and customer loyalty (Pappas et al.,
2018). Moreover, campaign response can be monitored directly
and corrective action can be taken promptly, thus improving
conversion rate (Chou & Shao, 2021). In the physical retail
environment, personalisation is typically provided by the
salesperson, which has several limitations. On the supply side,
sales staff have access to limited customer data in-store which
constrains their ability to adapt their recommendations (van
de Sanden et al., 2019). On the demand side, increasingly,
customers do not want to interact with a salesperson,
particularly in the wake of the Covid-19 pandemic (Mondada
et al., 2020; Yoganathan et al., 2021). Where technology is used
for in-store recommendations, but not drawing on AI, these
are based on customer segmentation rather than individual
behaviours. Moreover, such recommendations tend not to
reflect real time changes in the context, such as the customer’s
location, the store’s inventory levels or the level of crowding
in specific area. AI can overcome these limitations of in-store
personalisation, due to its ability to integrate multiple sources
of information, and create data-driven offers (Kietzmann et al.,
2018). Moreover, given that many retail customers use their
Vol.:(0123456789)1 3
mobile phones while shopping (Rippé et al., 2017), retailers can
deliver the AI-created, targeted messages to customers’ mobile
devices while they are in—or near – their store. We refer to this
type of targeted offer, which has been personalised by artificial
intelligence technology and is delivered to individual shoppers’
phones, in the physical retail environment as “artificial
intelligence enabled personalisation” (hereafter referred to as
AI-EP).
While there is a rich body of work examining consumer
experiences of personalisation in the online environment
(see Boerman et al., 2017 for a review), this has not been
replicated for physical retail (van de Sanden et al., 2019).
However, attitudes towards personalisation vary signifi-
cantly with the context in which it takes place (Aguirre et al.,
2016). First, as consumers’ motivations vary for online vs in-
store retail (Haridasan & Fernando, 2018), their perceptions
and evaluation of personalisation in the physical environ-
ment may differ from those identified in the extant literature
on personalisation. Second, the interface through which the
message is delivered influences the perception of the extent
to which the message has been personalised, with high qual-
ity interfaces increasing the perception of personalisation
(Ameen et al., 2022). The small screen of mobile phones
may impact negatively on consumers’ involvement with the
message (Grewal et al., 2016), offsetting their suitability as
targeting devices. Third, privacy concerns negatively impact
consumers’ evaluation of personalisation in online shopping
environments (Li et al., 2017). However, paradoxically, this
effect was not detected in Ameen et al (2022)’s study of
consumer interactions with smart technologies in shop-
ping malls. In summary, while, from a technical perspec-
tive, AI-EP may be similar to online personalisation, factors
related to the context of message delivery (in-store), the for-
mat of message delivery (small screen) and the salience of
privacy concerns in different media suggest that consumer
acceptance of personalisation may vary significantly across
the two environments. This uncertainty represents a limita-
tion in the current conceptual understanding of consumer
acceptance of personalisation and is also a key barrier to
adoption AI by businesses (Bughin et al., 2017). That is
why Ameen et al (2022), Riegger et al (2021) and van de
Sanden et al (2019), among others, have called for empiri-
cal research examining consumers’ attitudes towards AI-EP.
This paper aims to advance the conceptual understanding
of AI-EP by investigating the following research question:
“How do consumers experience and respond to AI-EP?”.
To frame this investigation, we draw on the personali-
sation-privacy paradox, particularly Sutanto et al’s (2013)
research on smartphone users. This lens allows us to go
beyond understanding whether consumers accept or reject
AI-EP, to identify the reasons for their behaviour, as well as
how they manage any tensions that may arise while inter-
acting with AI-EP, as urged by Riegger et al (2021). We
Information Systems Frontiers
investigate these dynamics empirically by focusing on a UK
fashion retail personalisation app. We focused on one spe-
cific app in order to develop an holistic understanding of the
usage climate of this technology, as recommended by Wang
et al (2015). We chose fashion retail because this is a highly
dynamic industry, which benefits from targeted, location-
based communication with customers (Kumar et al., 2017);
and because this is one of the most promising sectors for
AI applications (Davenport et al., 2020). Finally, we chose
the UK because it is at the forefront of the digital retailing
revolution (Ameen et al., 2022).
Given that AI-EP is a relatively unexplored phenomenon
(Riegger et al., 2021), and the paradoxical findings that are
beginning to emerge (e.g., Ameen et al., 2022), we opted
for an exploratory approach. Specifically, a qualitative case
study which included in-depth interviews with 18 female,
millennial fashion retail shoppers, who had been exposed to
a personalised advert.
The paper makes three contributions. First, we show that
customers welcome this innovative way of interacting with
them in the retail environment. However, their experiences
with online personalisation create very high expectations
of the extent of AI-EP, as well as additional services such
as creation of wish lists or the ability to edit their prefer-
ences. These findings can guide practitioners’ investment
in AI-EP. Second, we provide empirical evidence of how
the impact of the context of message delivery, the format
of message delivery and the salience of privacy concerns
differs for AI-EP vs online personalisation. This can guide
the application of findings from extant research, and guide
future research efforts. Third, we identify the content
and process gratifications derived from AI-EP, extending
Sutanto et al (2013)’s work on the manifestation of the
personalisation-privacy paradox among smartphone users.
The paper is organized as follows. Section 2 considers the
emerging literature on the opportunities and challenges for
AI use in physical retail. Section 3 presents the theoretical
background. Section 4 articulates the approach to data col-
lection and analysis. Section 5 communicates the empirical
findings. Section 6 discusses the findings, and uses the motif
of the Snakes and Ladders game to capture the factors that
support or prevent acceptance of AI-EP, Finally, Sect. 7 cap-
tures the contributions of this empirical investigation to the
advancement of theory and practice of AI deployment for
personalisation in physical retail environments.
2 Research Background
2.1 Prior Studies in AI in Retail
AI studies have seen a significant amount of attention in
recent years from many different disciplines, and applied
1 3
Information Systems Frontiers
to many different settings, including retail (Dwivedi et al.,
2021).
Several studies propose that AI can help retailers develop
new and innovative applications from the various datasets
available to them (e.g., Davenport et al., 2020), and in doing
so, achieve competitive advantage. However, they tend to lack
empirical evidence, and to overlook the customer perspective.
There is also a growing a body of work focusing on the obsta-
cles to effective use of AI (e.g., Boratto et al., 2018). Authors
mention the risk of consumer backlash and of negative impact
for firms. Though, the lack of customer focused research results
in insufficient understanding of consumers’ perceptions of AI
use in retail.
In turn, the literature on digital personalisation (e.g.,
Boerman et al, 2017) suggests that AI-EP could enhance but
also frustrate customers. Yet, except for Ameen et al (2022),
these studies examine personalisation in controlled experiments
rather than actual in-store experience. Finally, the effectiveness
of personalisation efforts tends to be limited by customers’
privacy concerns (e.g., Aguirre et al, 2016). While some of
these studies focus on smartphones (e.g., Sutanto et al., 2013),
they provided limited insight into how customers manage the
tensions arising.
Table 1 summarises the notable themes identified in the
stream of literature related to AI and its use for personalisa-
tion. The right-hand column emphasises the research gaps.
2.2 Personalisation‑Privacy Paradox
The review of the literature revealed a lack of customer
focused, evidenced based understanding of how AI-EP benefits
retail customers, and which factors may create resistance to
acceptance of AI-EP or destroy value for customers. While
personalisation can bring benefits to consumers, they may
resist personalisation if they deem that the collection and use
of personal data that underpin personalisation is too invasive
(Moore et al., 2015). This tension has been termed the
Personalisation-Privacy paradox.1 To unpack the conditions
under which the personalisation-privacy paradox manifests
in each context, it is necessary to identify the gratifications
that users derive from interacting with the medium through
which personalisation is delivered, as well as their desires and
concerns about information privacy (Sutanto et al., 2013).
2.2.1 Gratifications from Personalisation
Sutanto et al (2013) put forward two types of gratification
arising from personalisation: content gratification, referring
1 The term “personalisation-privacy paradox” is also, sometimes,
used to refer to the disparity between users’ privacy protection inten-
tions and their privacy protection behaviours (e.g., Norberg, Horne &
Horne 2007).
to the enjoyment derived from the personalised message
itself; and process gratification, referring to the enjoyment
derived from the medium in which the personalised offer is
delivered.
The personalisation literature identifies various content
related gratifications such as receiving offers that reflect cus-
tomers’ preferences (Krishnaraju et al., 2016; Pappas et al,
2017) and context (Xu et al., 2011), reducing the effort or
time required to complete the purchase (Tam & Ho, 2006),
and enabling cost savings and other financial gains (Schmidt
et al., 2020). However, personalised messages can also stir
negative emotions such as irritation (Haghirian et al., 2005)
or anger (Pappas et al., 2018), thus rendering personalisation
efforts ineffective (Demoulin & Willems, 2019). Customers
are likely to resist offers that are seen as a threat to their
freedom of choice (Brehm & Brehm, 2013). AI-EP may be
perceived as restricting the options available to customers,
which may result in customers rejecting the AI offer, in order
to reaffirm their autonomy (André et al., 2018).
In turn, process gratification arises from the ability to
control how messages are received (Brusilovsky & Tasso,
2004), such as being able to filter out certain messages, or
to control when and how they are displayed (Sutanto et al.,
2013). Research has also shown that being able to control
which information is collected and how it is used increases
message effectiveness (Tucker, 2014), while lack of trans-
parency from firms has the opposite effect (Aguirre et al.,
2015). AI algorithms are, typically, opaque (Burrell, 2016),
preventing customers to see – and influence – how they
produced a specific recommendation, which may result in
resistance to AI-EP.
While Sutanto et al (2013) found, in the context of smart-
phones, that personalisation gives users process gratification
but not content gratification, by and large, the personalisa-
tion literature focuses on the latter (Boerman et al., 2017).
2.2.2 Privacy Concerns
The effectiveness of personalisation efforts may be offset
by users’ concerns over the privacy of their personal infor-
mation (Awad & Krishnan, 2006). For instance, online ads
that closely match customers’ browsing history reduce pur-
chase intentions, because they raise concerns over firms’
surveillance practices (Aguirre et al., 2016). Customers set
boundaries – psychological or physical – around their per-
sonal data (Stanton & Stam, 2003), and attempts to cross
those boundaries raise concerns, and are met with resistance
(Xu et al., 2008). Customers manage information boundaries
by selectively sharing or withholding information (Sutanto
et al., 2013). In addition, they may purposefully provide
false information, such as using a false name or birth date
(Miltgen & Smith, 2019), when firms attempt to collect per-
sonal data that they deem private.
1 3Information Systems Frontiers
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1 3
Information Systems Frontiers
The literature indicates that customers may be comfort-
able disclosing information deemed to be relevant for the
intended outcome (Xu et al., 2011), when access to the ser-
vice is time critical (Hubert et al., 2017), and where the
information is routinely requested in that context (Stanton &
Stam, 2003). However, customers resist sharing information
that is deemed sensitive, such as their health status (Sutanto
et al., 2013); or which could be used for discrimination
(Stanton & Stam, 2003). They also resist sharing informa-
tion when they feel that they lack control over what data are
collected, how data are used, and with whom they are shared
(Liu et al., 2019; Schmidt et al., 2020). However, informa-
tion boundaries vary across individuals and are dynamic.
Namely, those customers that value information transpar-
ency are also most likely to resist the data collection that
underpins personalisation (Awad & Krishnan, 2006). Cus-
tomers also change whether they share information depend-
ing on the perceived gains or losses of each situation (Kar,
2020). The perception of being under surveillance is particu-
larly prevalent in online interactions and in smart services
(Bues et al., 2017).
Therefore, in addition to providing privacy features
(Awad & Krishnan, 2006), firms also need to identify which
information customers are comfortable to share, and what
trade-offs they are prepared to make in order not to break
their personal information boundaries (Pentina et al., 2016).
This is particularly relevant for AI-EP, given the need for
large volumes of data to support the development of targeted
offers (Davenport et al., 2020).
3 Research Design
The aim of our study was to advance the conceptual under-
standing of AI-EP by investigating the following research
question: “How do consumers experience and respond
to AI-EP?”. Hence, a qualitative, exploratory case study
methodology (Sarker et al., 2018) was adopted. The unit
of analysis was shoppers’ interactions with an AI-enabled
smartphone application, in the context of fashion retail. This
methodology offered an opportunity to collect primary data
from customers in situ experiencing the AI-enabled per-
sonalisation offer, guided by key studies in the field (e.g.,
Ameen et al., 2022; Riegger et al., 2021). It also offered the
unique opportunity to collect rich and diverse perspectives
from participants, as they reflected upon the hybrid digital-
physical experience of AI-EP, extending previous works
in the area, particularly Sutanto et al. (2013). In doing so,
the method adopted allowed us to understand and analyse a
broad range of participant views, and to theorise and concep-
tualise (Eisenhardt, 1989), in line with other case studies that
have examined the impact of technology upon personalisa-
tion (e.g., Griva et al., 2021).
3.1 The Selected App
The mobile app selected as the focus for this case study was
the Regent Street App. The app was first launched in 2012 to
enhance the shopping experience of visitors to this famous
shopping district, in London (UK). As shown in Fig. 1, the
app included the option to receive personalised offers while
shopping in the area. To create and deliver these offers, the
app combined “two technologies: geofencing beacons that
use location aware to offer content to users within a specified
proximity to the store and cloud-based artificial intelligence
(AI) to ensure personal relevancy of offers” (Lemmon, 2017).
Circa 80% of the stores in this shopping district joined
the scheme, implementing the associated technology in their
premises, such as beacons around the store and microchips
in the items on sale (Scott, 2014), in addition to artificial
intelligence programme to personalise the offers. Moreover,
98.6% of app users created a personal profile and signed up
to receive personalised content (Lemmon, 2017).
The AI-EP messages are delivered when app users are in
the vicinity of the stores that signed-up to the app (Demp-
sey, 2015), resulting in a 7.4% increase in response rate for
AI-EP vs. untargeted offers (Lemmon, 2017).
3.2 Data Collection
To gather customer experiences, we used in-depth, semi-
structured interviews, to allow participants to articulate their
actions and intentions towards the AI-EP, as well as implica-
tions for their personal data.
In order to recruit participants, one of the authors (who
conducted all the interviews) positioned themselves outside
a specific fashion store in Regent Street, which was known
to use the Regent Street App for the delivery of AI-enabled
personalised offers. As shoppers walked past the store, the
interviewer approached them, showed them the advert in
Fig. 2, and invited them to participate in an interview. This
approach is in line with Kar (2020)’s recommendation that
research on customer perceptions of digital technology
should take place immediately after encounter with that
technology.
Some interviews took place outside the store, others at
a nearby café. No financial incentives were offered to the
interview participants. The interview protocol (Table 2)
reflected the key themes identified in the extant literature.
The questions focused on perceptions of the message rather
than the technology underpinning it, as customers don’t
always understand the technology behind personalisation.
This approach allowed us to move beyond a simplistic view
of positive vs. negative attitudes, and to understand the black
box of the customers’ response (Belk, 2017).
As resistance to AI-EP may depend on customer char-
acteristics (Yoganathan et al., 2021), we recruited an
1 3Information Systems Frontiers
Fig. 1 Case study App. Image source: http:// okosv aros. lechn erkoz pont. hu/ en/ node/ 558
homogeneous sample via purposive sampling (Bryman &
Bell, 2015), to give direction to the data collected in support
of the case study (Yin, 2012). We focused on female shop-
pers aged 18 to 30 years old, because, in the UK, this demo-
graphic group cares the most about looking trendy (YouGov,
2020). Women in this age group are twice as likely than
men to agree that they spend a lot on clothes and to value
immediate access to fashion items; and they are also more
likely than men and then older women to shop at multiple
retailers (YouGov, 2020). Consequently, this demographic
group are a key target for high street fashion retailers’ pro-
motional efforts. This demographic group are also more
open than others to sharing their personal data with firms,
given that they grew up in a digital world (Liu et al., 2019).
However, women may resist AI, especially when outcomes
are consequential (Castelo et al., 2019). We conducted 18
audio-recorded interviews, each lasting between 30 min and
one hour. Each recording was transcribed with an average of
9,000 words, equating to just over 160,000 words in the final
dataset. The data was checked for accuracy and prepared for
analysis.
3.3 Data Analysis
The interview data was analysed using NVIVO and fol-
lowing Krippendorff's (2004) systematic approach to
thematic analysis. As is customary of exploratory case
studies in the information systems discipline (see Sarker
et al., 2018), the theory was used to guide the design of
the study and to set the general direction of data analysis.
In practice, this meant that a preliminary coding book
was developed based on the themes identified in the lit-
erature, and this was used in stage 1 of data analysis
to deductively code the transcripts into a) gratifications
from personalisation, b) privacy concerns and c) reaction
to AI-EP. Subsequently, in stage 2, for each of the themes
1 3
Information Systems Frontiers
Fig. 2 Interview prompt
Table 2 Interview protocol
Section
Stimulus
AI-EP – Gratifications
AI-EP – Outcomes
AI-EP – Privacy concerns
Question
Participant receives targeted prompt. Upon opening the screen, the participant learns that the offer is exclusive to
users of the Regent Street mobile app walking past that store, and who have bought in that store, previously
1. What do you think of this offer?
2. This offer has been personalised based on your location and shopping preferences. Is this offer useful?
3. How does it enhance your shopping experience?
4. When the brand sends real-time, relevant offers to your mobile phone, are they mostly trying to sell more, or try-
ing to build relationships with customers like you, by serving your specific needs?
5. Do you think that the company will always make the best offer specifically for you?
6. Why do you suppose that?
7. Do personalised offers help you develop bonds with this brand?
8. Would receiving this type of offer discourage you from switching to another brand? Why?
9. Which personal information would enhance your experience with this retailer?
[Probe for location and behavioural data]
10. Are you willing to share that information with the company, so that they can develop offers specifically for you?
11. Where should the limit be?
12. What are the benefits of letting the company access your personal data?
13. What are the risks of letting the company access your personal data?
14. Through the app, the company can track your movements not only in-store but also in the proximity of stores on
Regent Street? How does that make you feel?
in the code book, the analysis of the data proceeded in an
inductive fashion, with subsequent codes emerging from
the data. The final set of codes is depicted in Table 3.
The findings emerging from this analytical process
are presented in the next section, following a polyphonic
account. This approach presents the range of perspectives
offered by the research participants in order to develop a
layered account of the phenomenon being investigated
(Travers, 2001), as is customary of interpretive research.
This is in contrast with identifying the dominant narrative or
single shared reality typical of positivist approaches to data
analysis (Sarker et al., 2018).
1 3Table 3 Coding structure
Aggregation
1st order
2nd order
Illustrative quotes
Information Systems Frontiers
Gratifications
Content
Relevancy of the offer that is better than
humans
Time saving attributes to the customer experi-
ence
Financial benefits that are attractive to modern
customer base through appetite for discounts
Other benefits
Process
Message Delivery
Information collection process
Information use processes enhancing value to
the in-store experience
Privacy concerns
Information boundaries Boundary management practices
Information—Willingness to share
Information—Desire to protect
Acceptance of AI-EP Perceptions
Positive
Negative
Behaviour
Acceptance of AI-EP and Customers are
willing to share information to receive
personalisation offer
Rejection due to irritation from notifications,
interruptions, lack of control
I mean if you can choose what you like and then
they will remember it that would be so much
easier to go and shop there and maybe you
would buy a bit even more
It’s really useful because you get to know what
is there
I would value it a lot. There is nothing to lose
for customers and it is not like we are com-
mitting to a sale of any sort or a purchase of
any sort
But also the things like pretty macarons or
lemonade
If I would receive an offer from a store I really
like and I already have a 10% offer, I would
definitely go inside and check out the stuff
they have
I would rather have a setting in application—
right now I am shopping for my dad. Rather
than registering it under me. Or buying gifts
for him or for her rather but still that the
information being given
It would definitely help because I can make a
profile of things I like. It is an amazing tool
definitely
I only share information about fashion. Only
information where I know it can create value
for me
if somebody wants to track me down they can
do it, they have (the data), anyway… but on
the other hand, it does not really matter what
they are going to do because they can have it
anyway
I want to know if it’s going to be used for more
than just trying to fulfil my needs within the
shop
Sometimes I just want to have something which
I already have, which is different from what I
already have. So, personalizing is useful for
me in terms of fashion
If the company has bad intentions, there may be
some downside in sharing the information
Telling them about your style, so they would
know what specific things to target to you,
and maybe saying your age group and gender,
because that might help them to target you
towards particular things as well
If I am not shopping I would not want that sort
of notifications or if I am doing something
else I do not know. If you end up passing there
every day it could be quite annoying
1 3
Information Systems Frontiers
4 Findings
4.1 Gratifications from Personalisation
Our participants were very positive about using the app
while shopping in Regent Street and receiving personalised
recommendations on their phones: “You are going to (Regent
Street) in your free time, and want to have a nice day, and,
through, the app it might be even nicer.” (Interviewee 3).
Contrary to the participants in Sutanto et al (2013)’s
experiment, for whom personalisation via smartphone apps
delivered process but not content gratification, our partici-
pants identified both types of personalisation. The analysis
of the data (Table 4) showed that the participants experienced
relevance, time savings and financial gratifications, in line
with the literature on online personalisation. However, for
most of our interviewees, discounts seemed to be the main
benefit expected from AI-EP, undermining the promise that
this form of personalisation can increase basket variety and
improve retailer profitability (Kumar et al., 2017). For those
interviewees, discounts might be complemented by other
benefits, such as time savings, but not replaced by them.
As for process gratifications (Table 5), some deemed
receiving personalised notifications on their phones as supe-
rior to relying on shop window information to gain informa-
tion about new products or about deals (e.g., participant 11);
Table 4 Content Gratifications
or receiving offers via e-mail (e.g., participant 16). However,
many more commented that, at times, the volume of notifica-
tions became a nuisance. This is particularly relevant for the
Regent Street app, as this is a central London location, next
to theatres, cafés and other leisure venues, as well as a com-
muting route, as mentioned by participant 4. A high volume
of notifications could result in information overload (e.g.,
participant 13), intrude in relaxation time (e.g., participant
6), as well as drain the phone’s battery (e.g., participant 18).
While most participants mentioned the option of switching
off their Bluetooth to stop notifications, this was an unsat-
isfactory solution for many. Instead, many expressed the
desire to control effortlessly when to receive notifications
and what type, which is in line with literature on the role of
customer autonomy in technology interactions (e.g., André
et al., 2018).
Opinions varied as to whether the app was an effective
way of collecting and using information for AI-EP. Some,
like participant 14, were happy with the data collection
process. However, others felt that the app should integrate
with other data sources (e.g., participant 7). Still, others,
like interviewee 5, lamented the lack of ability to edit pur-
chase histories, or to select when not to collect data (e.g.,
for gifts and other one-off purchases). Because AI doesn’t
understand the reasons behind a purchase (Woo Kim &
Duhachek, 2020), the research participants predicted that
1 3Table 5 Process Gratifications
Information Systems Frontiers
one-off purchases would be added to their purchase his-
tory, undermining the quality of future recommendations.
Two interviewees (4 and 17) indicated an explicit desire to
understand why they had received specific recommenda-
tions. In Sutanto et al (2013)’s examination of app users’
willingness to share personal information, the process
benefits referred to the experience with the medium itself
(namely, navigation of the app). However, interviewees 3
and 17 also seemed to value process benefits at the level
of the in-store experience broadly, emphasising the hybrid
nature of AI-EP.
4.2 Privacy Concerns
In terms of boundary management behaviours, as detailed in
Table 6, we found various instances of selective information
disclosure to tap into benefits. For instance, the interview-
ees were willing to provide information directly into the app
or via surveys (e.g., participant 9) to improve the accuracy
and relevance of the resulting recommendations. They also
engaged in redemptive behaviours (Stanton & Stam, 2003),
whereby they shared information to reduce the losses gen-
erated by irrelevant recommendations, as illustrated by
1 3
Information Systems Frontiers
Table 6 Privacy Concerns
Interviewee 5. The interviewees were also keen to engage
in information withdrawal. In particular, they wanted to
remove records of one-off purchases, as well as historical
information that was no longer relevant (e.g., participant 6),
corresponding to Stanton and Stam (2003)’s political and
protective behaviours. However, those options were seen to
be unavailable or too difficult to access. Finally, we did not
find evidence of interviewees disclosing fake information to
1 3Information Systems Frontiers
manage the benefits and risks of AI-EP, contrary to what was
reported in Miltgen and Smith (2019).
The interviewees were aware that, by using the app, a range
of companies could access their personal data, including the
mobile phone operator, the app developer, and the fashion
brand. This situation was seen as the default in the digital era,
as illustrated by interviewee 6’s quote. As shown in Table 6, an
in line with extant literature (e.g., Miltgen & Smith, 2019), the
interviewees were willing to share information such as clothes’
size, specific body measures, preferred styles, or favourite
colours, to obtain relevant recommendations. As Interviewee
13 said: “The use of these (types of) data does not bother me
because I think it is a win–win situation”. In contrast, and in
line with Sutanto et al. (2013), most were unwilling to share
personal information which they did not deem essential for the
task at hand, or which could leave them vulnerable to manipu-
lation, nuisance, or fraud (e.g., Interviewee 4).
The topics of location and social media data divided opin-
ions, however. Regarding the former, interviewees 3, 12 and
15 expressed the view that sharing geo-tracking was a natu-
ral extension of what already happens on other media and
was useful to develop targeted offers. However, the others
expressed reservation towards various aspects of the track-
ing of their location. They described this activity as “creepy”
(e.g., interviewee 11) and, in line with Schmidt et al. (2020),
they expressed a strong desire to limit the app’s ability to
track their movements (e.g., interviewee 2). Regarding social
media data, interviewees 1, 5 and 15 were in favour. But the
remaining felt that these data should be off limits to the app.
Some, like interviewee 6, rejected this because they felt that
the data would be too revealing; others, like interviewee 7,
because social media data were deemed irrelevant.
Two key nuances emerged regarding privacy concerns
associated with AI-EP. The first nuance relates to control
over access to personal information. Specifically, inter-
viewees would be willing to share more information if they
could be in control of what data was collected and when
(e.g., interviewee 1), or if they were reassured that the
app provider would not take advantage of the situation to
access other areas of their phones (e.g., Interviewee 3). The
second nuance refers to trusted parties. The app provider
was, implicitly, a trusted party, but this sentiment did not
necessarily extend to specific stores on the app, particularly
smaller ones (see Interviewee 17) due to concerns of the lat-
ter’s ability to fend off security attacks. On the other hand,
there were other parties that the interviewees trusted more
than the app provider – namely, Apple (as mentioned by
interviewee 10).
Table 7 Perceptions of AI-EP
1 3
Information Systems Frontiers
4.3 Acceptance of AI‑EP
The literature’s enthusiasm for AI-EP (e.g., Bues et al., 2017)
was mirrored in our research participants’ reactions. The
analysis of the findings (Table 7) reveals that some participants
found this type of offers interesting (e.g., participant 8), exciting
(e.g., participant 1) and useful (e.g., participant 17). Many felt
valued by the company behind the offers (e.g., participant
10) and, as a result, developed a positive attitude towards the
company (e.g., participant 3), which indicates the potential of
AI-EP for relational benefits (Liu et al., 2019). Having said
that, 10 out of the 18 participants could not see any relational
benefits. They expressed scepticism about the intentions
behind AI-EP offers, seeing them as mostly an attempt to get
customers to increase their expenditure (e.g., participant 12).
Participant 4 also expressed scepticism about AI-EP’s ability
to meet her needs, due to limitations of the technology, as
well as the associated cost. Other negative emotions reported
were annoyance (e.g., participant 16), and creepiness or the
feeling of being stalked (e.g., participant 9). Some participants
also reported a feeling of intrusion in what is meant to be a
leisurely, relaxing activity, with interviewee 8 describing it
as thus: “It’s like a sales assistant running out into the street
and grabbing me.” In addition, interviewee 15 reported a fear
of over-spending as a result of AI-EP, while participant 18’s
comment that “You would be less aware of what is going on.
Table 8 Behavioural Outcomes
You would be in a loop” echoes the perceived threat to freedom
of choice identified by Brehm and Brehm (2013).
The positive sentiments translated in willingness to act on
the offers delivered via AI-EP, particularly if they came in the
form of exclusive, time-limited discounts, for their favourite
stores, as exemplified by participant 3’s quote (Table 8). While
participants 13 and 17 said that AI-EP might lead them to try
new stores, most ignored offers from stores that they did not
usually shop at, or which they were not familiar with. That is,
it seems that AI-EP works better for customer retention than
for customer acquisition, and for the pre-approach stage of the
sales process, which contradicts claims that AI can add value
at any stage of the sales process (e.g., Syam & Sharma, 2018).
However, participants have very high expectations of
AI-EP. While some are willing to accept some trial and
error (participant 4), in general, they expect extremely tar-
geted and unique offers (e.g., participant 8). This expectation
might reflect the participants’ experience with personalisa-
tion in the online environment, where users typically receive
very unique recommendations (Griva et al., 2021). Failing
to meet such expectations seems to result in disappointment
with the app (e.g., participant 5, Table 8), rather than with
the brand (e.g., participant 4, Table 7). This reaction is in
contrast with extant literature on online personalisation (e.g.,
Baek & Morimoto, 2012), but aligned with literature on
mobile shopping apps (e.g., Shankar et al., 2016).
1 35 Discussion
The extant literature argues that fashion retailers may
enhance the customer experience through the use of AI-EP
by harnessing company-owned as well as external datasets
to create highly individualised offers (van de Sanden et al.,
2019). Though, the broader personalisation literature implies
that the effectiveness of AI-EP may be compromised by pri-
vacy concerns (Aguirre et al, 2016), and that AI-EP may
even result in customer dissatisfaction, due to inflated expec-
tations or negative experiences (e.g., Karumur et al., 2018).
Our focus on the customer perspective, and the exploration
of an actual in-store experience, provides empirical evidence
of the tensions in place, and how customers navigate them,
as discussed next.
5.1 The Personalisation‑Privacy Paradox
in the AI‑EP Context
Our findings are aligned with those from research on per-
sonalisation in the online environment, which established
that personalised offers may deliver content gratification in
the form of relevance (Krishnaraju et al., 2016), plus time
(Tam & Ho, 2006), and cost savings (Schmidt et al., 2020).
Though, in AI-EP, the opportunity for cost savings seems
to dominate over the other forms of content gratification
mentioned in the online personalisation literature.
The limited importance of relevance in AI-EP might
reflect the nature of shopping in the physical environment
where, typically, there are fewer options on display than in
online shopping (Kumar et al., 2017). Therefore, custom-
ers may feel less overwhelmed by choice in the physical
environment. Moreover, some of our interviewees seemed to
associate fashion shopping in the physical environment with
an hedonic experience (Gardino et al., 2021), rather than a
functional one. The pleasant nature of in-store shopping may
explain the reduced importance of time savings in AI-EP vs.
online personalisation. The resistance to suggestions by the
AI could also indicate that customers do not trust that AI
has the skill to make such recommendations (Woo Kim &
Duhachek, 2020), given that fashion shopping is a task rich
in intuition and subjectivity (Castelo et al., 2019).
This familiarity with personalisation in online fashion retail
suggests a compelling path for future adoption by retailers.
However, the emphasis on discounts contradicts the predic-
tion that AI-EP will generate additional sales opportunities
and improve retailer profitability (e.g., Kumar et al., 2017) by
prompting customers to consider complementary items, or
generating impulse purchases (e.g., Griva et al., 2021).
Our findings also show the need for a careful approach
to the process of delivering the personalised offer. In line
with previous research on personalisation online (e.g.,
Information Systems Frontiers
Brusilovsky & Tasso, 2004) and on smartphones (Sutanto
et al., 2013), many participants expressed a strong desire
for controlling notifications and other aspects of message
delivery. Moreover, we observed intricate interactions
between the receipt of notifications and various contextual
factors such as phone battery depletion, the purpose of visit
(e.g., shopping vs meeting friends) or additional information
provided.
Customers also expressed a strong desire to be in control
of the information held in the system and used to create
personalised recommendations, which is line with findings
from online personalisation research (e.g., Aguirre et al.,
2015; Tucker, 2014). Moreover, customers wanted the
ability to edit information held by the retailers and which
they perceived to be undermining the quality of the AI-EP.
However, it is not clear that enabling customers to engage
in such boundary management behaviours (Stanton & Stam,
2003) would deliver the results sought by retailers. As shown
in the context of online personalisation, messages need to
be persuasive in order to be successful (Pappas et al, 2017);
and fashion retailers need access to large and stable datasets
about customers and their context (Ameen et al., 2022) in
order for the AI to create high quality, persuasive messages.
Some customers also expressed a desired to understand
why they received specific recommendations. It will be
difficult for retailers to meet this particular customer
expectation because algorithms are opaque, and it is difficult
to trace exactly which data inputs are generating which outputs
(Burrell, 2016). As a result, some customers may reject the
AI-EP offer to reaffirm their autonomy (André et al., 2018).
Exposure to widespread collection of personal data in the
online environment may have influenced our respondents’
willingness to share data for AI-EP (Stanton & Stam, 2003).
Many also showed willingness to participate in ad-hoc data
collection initiatives, as they saw these as an opportunity
to improve their shopping experience. However, there were
noticeable nuances in terms of comfort with disclosing
certain types of personal data, which require a very careful
approach from retailers in order not to violate customers’
personal information boundaries (Pentina et al., 2016).
Mobile apps are useful tools to collect data such as unique
customer identifier and transaction history, due to the high
penetration of mobile phones, and because they can be
linked to individual users (Shankar et al., 2016). However,
customers need to perceive a link between the information
requested and the resulting offer (Xu et al., 2011). Moreover,
firms need to avoid collecting information which customers
deem likely to be misused, or to leave them in a vulnerable
position. Some participants also opposed the collection of
social media activity.
Another data input that is essential for instore AI-EP is
location (Schmidt et al., 2020). This can either be individual
1 3
Information Systems Frontiers
data such as the customer’s whereabouts, or contextual data
such as the weather or crowd levels (Verhoef et al., 2017).
However, the emotionally charged descriptors used by some
of our participants, indicate that customers intensely dislike
extensive tracking in the physical environment. This presents
a challenge for fashion retailers: one the one hand, location
data enables them to take full advantage of AI’s capabilities
for personalisation; on the other hand, customers may see
this as an invasion of privacy (Xu et al., 2008), which may
result in negative attitudes towards AI-EP and, ultimately,
its rejection (Shankar et al., 2016).
5.2 Effectiveness of AI‑EP
Based on our findings, attempts to use AI-EP for customer
acquisition may be ineffective (Demoulin & Willems,
2019), or even detrimental (Baek & Morimoto, 2012) for
the brand. This finding was somehow surprising given that
the app considered in this case study was provided by a
trusted party which offered discounts to a variety of stores
in a given shopping district. Trust has been shown to impact
the perception of a personalised offer (Aguirre et al., 2016)
and, as such, familiarity with the Regent Street app might
lead customers to be receptive to AI-EP attempts from new
brands (Chen & Dibb, 2010).
Furthermore, we found that customers expressed a strong
desire for autonomy and freedom of choice, as reported in
the context of online personalisation (Balan & Mathew,
2020). Though, while previous research focused on choice
and agency in relation to the content of the message, we
witnessed a willingness to control message delivery, too.
Granting this flexibility might return a sense of control to
customers (Brehm & Brehm, 2013), but may increase the
complexity of the app (e.g., in terms of navigation), which
will negatively impact the user experience (Shankar et al.,
2016). Moreover, it reduces the retailers’ ability to collect
data and deliver targeted messages (Chou & Shao, 2021).
While AI can integrate multiple sources of customer,
contextual and transactional data, our study exposes
limitations to the extent of in-store personalisation (Ameen
et al., 2022; Boratto et al., 2018). Namely, in contrast with
the online environment, where personalisation may influence
the search and evaluation stages (Davenport et al., 2020),
AI-EP was revealed to be most valued at point of purchase
stage, albeit not for payment purposes. Furthermore, whilst
algorithms underpinning AI-EP need to be rigorously tested
(Sutanto et al., 2013), our findings indicate that fashion
shoppers have low tolerance for such trial and error. As in the
online environment, consumer trust and positive emotions
are essential for successful personalisation (e.g., Pappas,
2018). As with personalisation in the online environment
(e.g., Pappas et al, 2017), customers have high expectations
of AI-EP. The inflated expectations and the low tolerance
for mistakes, are likely to result in disappointment and app
abandonment (Riegger et al., 2021; Shankar et al., 2016),
represents a waste of resources, and inability to continue
collecting data about customers.
Figure 3 presents an overarching view of how in-store
AI-EP can enhance customer experiences, capturing both the
enabling factors from content and process gratifications, and
the detracting factors related to unmet process gratification
expectations and from privacy concerns. We represent the
AI- EP journey consisting of opportunities and threats for
retailers, as encapsulated in the well-known game of Snakes
and Ladders. This model highlights the potential as well as
the risk for brands about to embark upon such an endeavour.
Moreover, from our review of personalisation in both
retail and digital spheres, this is the first such conceptual
framework of its kind representing the user-end perspective
of such innovations in technology.
The game begins from the moment a user/player is within
proximity of the store. The player is then faced with two
options, either an enabling force (indicated by a ladder)
moving them higher up the personalisation journey, or
a detractor (indicated by a snake) preventing progress on
the board. Each factor is described with key attributes as
generated from the findings of the study. We envisage that
AI-EP is not a one shoe fits all experience for users, and
that it may take a circuitous route. As retailers continue
to innovate, the blank squares represent the stages of the
journey not relevant to AI-EP. The final goal is where the
AI-EP has delivered a positive in-store experience and
created value for customers and retailers.
6 Conclusion
The deployment of AI technology for personalisation
promises to address some of the business challenges
faced by high-street retailers (Kumar et al., 2017), such as
increased competition, heightened price sensitivity or the
emergence of the show-rooming phenomenon. AI-EP apps,
such as the one analysed in this paper, enable the creation
of offers that draw on individual behaviours and contextual
information, as opposed to aggregate segment information
(as in the case of non-AI, automated personalisation) or
intuition (as in the case of sales staff personalisation). As
a result, AI-EP offers can be more relevant, granular and
timely than either of those alternatives. However, factors
related to the context of message delivery, the format of
message delivery, and the salience of privacy concerns
may impact the relevance of extant research on technology-
enabled personalisation—mostly performed in the online
environment—to help us understand consumers’ acceptance
of AI-EP. Therefore, we responded to calls by Ameen et al
(2022), Riegger et al (2021) and van de Sanden et al (2019),
1 3Information Systems Frontiers
Fig. 3 The Snakes and Ladders of AI-Enabled Personalisation
among others, for empirical research on how consumers
experience and respond to AI-EP.
The qualitative investigation of consumers’ interac-
tion with AI-DP in a shopping district with London, UK,
through the lens of the personalisation-privacy paradox
enabled us to identify the perceived content and process
benefits derived from AI-EP, as well as how privacy con-
cerns undermine these benefits and inform the custom-
ers’ boundary management tactics. Together, these factors
result in a carefully orchestrated process whereby cus-
tomers either accept or reject the artificial intelligence-
derived personalisation offer, but with a high degree of
control over their interaction with the offer, and in par-
ticular the use of their personal information.
6.1 Theoretical Contributions
Our study makes the following three contributions. First,
we showed that customers welcome this innovative way of
interacting with them in the retail environment as posited by
Davenport et al. (2020) and others, which should give con-
fidence to practitioners considering adoption of AI (Bughin
et al., 2017). However, we found that customers’ experiences
with online personalisation create very high expectations of
the extent of personalisation possible via AI-EP, address-
ing the gaps identified in Table 1. Those high expectations
may be difficult to meet, given not only the technological
restrictions of AI-EP but also consumers’ discomfort with
location tracking as well as the safeguarding of data which
is essential for the efficacy of the offer, which can create
customer backlashes and reputation damage (Castillo et al.,
2020). Customers’ online experiences also shape their desire
for additional services and functionalities, such as the crea-
tion of wish lists or the ability to edit their preferences. This
desire presents unique challenges from the point of view of
interface design which have not been reported, yet. We rep-
resented the range of factors impacting positively vs nega-
tively on customers’ experiences with – and assessment of
– AI-EP via the motif of Snakes and Ladders boardgame.
Second, we provided empirical evidence of how the
impact of the context of message delivery, the format of
message delivery and the salience of privacy concerns dif-
fers for AI-EP vs online personalisation. Specifically, regard-
ing the impact of the different motivations for online vs.
in-store retail on customers’ perception and evaluation of
personalisation efforts (Haridasan & Fernando, 2018), we
found that customers may be in a particular physical location
for reasons other than shopping, and that this may result in
1 3
Information Systems Frontiers
heightened irritation from app notifications. Moreover, cus-
tomers seem more sensitive to evidence of tracking of past
purchase behaviour in the physical environment than online,
and more likely to resist the tracking of location and shelf-
browsing behaviour than online browsing. In terms of the
impact of message delivery interface, our findings confirm
that the small screen of mobile phones impact negatively on
consumers’ involvement with the message (Grewal et al.,
2016), and that there is a need for attention-grabbing subject
lines to make shoppers want to check the message, imme-
diately. Future research could test the effectiveness of the
same message delivered online vs via AI-EP, to quantify the
effect of delivery interface on the effectiveness of personali-
sation campaigns. Another factor that could limit the impact
of AI-EP was the high number of notifications that mobile
phone users typically receive on their devices, not just from
direct messages from other users, but also from social media
apps, calendar apps and others. Having said that, AI-EP
could be more effective than e-mail offers, possibly because
of the relative novelty of this form of personalisation, but
also because of the volume of traffic that e-mail may attract
(including spam content). Finally, regarding the impact of
privacy concerns on consumers’ evaluation of AI-EP, our
findings – like Ameen et al (2022)’s study of consumer inter-
actions with smart technologies in shopping malls – seem to
contradict Li et al. (2017). Unlike studies of personalisation
in the online environment (e.g., Pappas, 2018), customers
do not seem too concerned with the firm’s access to their
personal information, in principle. This could be because
the collection of such information is now seen as a condition
for accessing services in the digital era. However, it could
also be because of the particular type of app used in our
case study. Like Ameen et al (2022)’s app, ours was valid
for a shopping area, rather than a specific retailer. This fact
may decrease the customers’ perception of surveillance, and
increase their trust in the firm behind the AI-EP. Further
research is needed to separate the effect of type of app (i.e.,
retailer vs location specific) from the overall privacy con-
cerns with AI-EP. However, customers did express concerns
over access to information which they did not deem essential
for the task at hand, and access by unfamiliar retailers. Our
findings thus assist in contextualising extant literature on
AI-enabled personalisation online vs in-store.
Third, we identified the specific content and process
gratifications derived from AI-EP, and how they enhance or
detract from the value of AI-EP for retail customers. Con-
tent gratifications included discounts, time savings and rel-
evance of offers, with the first one seemingly dominating the
others. Receiving notifications on the phone was a process
gratification for some but detracted from the overall benefit
for others. Likewise, opinions were divided on the process
gratification derived from how this app collected and used
information for AI-EP. Our findings, thus, extend Sutanto
et al (2013)’s work on the manifestation of the personali-
sation-privacy paradox among smartphone users, in hybrid
(physical-digital) environments.
6.2 Practical Contributions
Collectively, these findings mean that the use of AI tech-
nology for personalisation in the physical environment can
address some of the business challenges faced by high-street
retailers as suggested in Davenport et al. (2020), but with
significant differences vis a vis personalisation in the online
environment. Specifically, our findings have the following
managerial implications.
First, AI-EP is more suitable for customer retention
efforts, than for customer acquisition. This is both because of
the type of dataset required to deliver on customer expecta-
tions of AI-EP and avoid the risk of customer backlash, and
because of customers’ intense negative reaction to receiving
personalised offers from brands that they usually do not buy
from. A better way to acquire customers in this demographic
group might be through the use of dynamic, entertaining
adverts on social media; or by including their items in cloth-
ing subscription services (YouGov, 2020).
Second, to attract customers, retailers should offer entic-
ing discounts on desired items. This is because, contrary to
the online environment and to what is suggested in the litera-
ture (e.g., Kietzmann et al., 2018), we found that customers
weren’t driven by hedonic offers, and that there was limited
scope for shopping basket expansion.
Third, retailers should focus on providing information
about items’ features, availability and other attributes that
are important in the pre-purchase stage. This is because,
while shoppers may interact with their smartphones across
all stages of the purchase process (e.g., Syam & Sharma,
2018), they seemed most receptive to AI-EP offers in the
lead-up to the purchase, rather than during the purchase
(e.g., payment options) or afterwards (e.g., asking for
feedback).
Fourth, retailers need to test various aspects of offer deliv-
ery, in order to minimise the concerns and irritants detected
in our study. These include the number of notifications, to
address shoppers' concerns with battery depletion and the
fact that customers may be in the store’s neighbourhood for
different reasons; and the wording of the message, to assuage
customers’ desire to understand why they got a specific offer.
It is also important for retailers to unpack which personal-
ised offers are rejected because customers want to reaffirm
their autonomy vs the AI (André et al., 2018), rather than
because the offer itself was not persuasive.
Fifth, retailers need to approach data collection and use,
carefully. Our study revealed that the use of location and
social media data, which is accepted in the online context,
caused intense negative reactions among some customers.
1 3Conversely, the relative novelty of in-store AI-EP means
that customers may be willing to participate in ad-hoc data
collection initiatives, if they perceive a link between the
information requested and improvements in their shopping
experience.
6.3 Research Limitations and Further Research
It is important to recognise the limitations resulting from
the focus and characteristics of our approach. The focus
on fashion retail, on a multi-store app, and on the UK may
limit the transferability of our findings to other research
contexts. Research into other empirical settings is needed
before claims can be made about consumer perceptions and
experiences of AI-EP, generally. Likewise, young female
consumers exhibit distinct attitudes to fashion shopping,
sharing digital data and interacting with AI, meaning that
our findings may not be directly applicable to older female
shoppers, or to male shoppers of similar age. Findings
from personalisation in the online environment indicate
that perception of personalisation benefits is a key a factor
in acceptance of personalisation (Pappas et al., 2017).
Therefore, it is important to identify which messages most
clearly communicate the desired content gratification valued
by different types of customers and/or different contexts.
Moreover, by adopting a qualitative approach, we were
able to identify a range of issues relevant for fashion
retail customers. However, we are not able to quantify
their absolute or relative importance. Further research
employing quantitative approaches, namely natural
experiments (e.g., Tag et al, 2021), is needed before claims
can be made about the salience of specific gratifications
and privacy concerns, or about the magnitude of their
impact on consumer acceptance of AI-EP. Likewise,
the use of methodologies such as fuzzy-set qualitative
comparative analysis (see Pappas, 2018) would enable the
identification of how the different factors identified in this
study combine to amplify – or not – purchase intention
when exposed to AI-EP.
Furthermore, our focus on consumers overlooks the
retailers’ perspective of AI-EP, which is a worthy area of
further study. In particular, an avenue of further study that
would advance our findings, as well as the work of Yoga-
nathan et al. (2021), is to examine the relationship between
AI-EP and access to onsite retail staff, homing in on the
digital-physical customer experience dynamic. Given the
practical nature of such an investigation, and the need for
close collaboration with the organisation deploying the
AI-EP solution, it would be beneficial to adopt the clinical
inquiry approach method (see Schein, 2008). In this meth-
odological approach, academic researchers and practitioners
work together to shape the project, with the explicit goal of
improving practice. Clinical inquiry is particularly useful for
Information Systems Frontiers
instigating digital innovation from within the organisation,
as demonstrated in Vassilakopoulou et al (2022)’s analysis
of the potential for creating hybrid human/AI service teams.
Declarations
Conflicts of Interest The authors have no relevant financial or non-
financial interests to disclose.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article's Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article's Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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Ana Isabel Canhoto is Professor of Digital Business at the University
of Sussex, UK. Her research focuses on the role of digital technology
(including Artificial Intelligence Big Data and the Metaverse) in
interactions between firms and their customers. She examines drivers
of adoption, user experiences, consequences of adoption, and the role
of context. She is also committed to the pedagogical use of digital
technology, including using machine learning to support pupil
performance, creating quasi-simulations for experiential learning, and
training early career researchers to use social media to develop and
disseminate their work.
Brendan James Keegan is an Assistant Professor in Marketing at
Maynooth University, Ireland. His current research activities are within
the areas of business to business relationships, artificial intelligence
and machine learning applications in digital marketing, and digital
placemaking. His work is published in Information Systems Frontiers,
Industrial Marketing Management, European Journal of Marketing,
European Management Review. He is the Principal Investigator for
the ongoing digital placemaking research within the H2020 funded
GoGreenRoutes Project.
Maria Ryzhikh is the E-Commerce Manager for the EMEA region
at Weber-Stephen Products EMEA GmbH. She completed the MSc
Marketing program at Oxford Brookes University in 2016 and
then continued to pursue her career in digital marketing in various
technologically-driven companies in the UK and Germany. Her
particular areas of interests lie in performance marketing, digital
marketing analytics, online consumer behaviour and psychology, and
conversion rate optimization.
1 3
| null |
10.1103_physrevd.107.035006.pdf
| null | null |
PHYSICAL REVIEW D 107, 035006 (2023)
Constraining feeble neutrino interactions with ultralight dark matter
Abhish Dev ,1,* Gordan Krnjaic,1,2,3,†
Pedro Machado ,1,‡
and Harikrishnan Ramani
4,§
1Theoretical Physics Department, Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA
2Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637, USA
3Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA
4Stanford Institute for Theoretical Physics, Stanford University, Stanford, California 94305, USA
(Received 8 July 2022; accepted 17 January 2023; published 7 February 2023)
If ultralight (≪ eV), bosonic dark matter couples to right-handed neutrinos, active neutrino masses and
mixing angles depend on the ambient dark matter density. When the neutrino Majorana mass, induced by the
dark matter background, is small compared to the Dirac mass, neutrinos are “pseudo-Dirac” fermions that
undergo oscillations between nearly degenerate active and sterile states. We present a complete cosmological
history for such a scenario and find severe limits from a variety of terrestrial and cosmological observables.
For scalar masses in the “fuzzy” dark matter regime (∼10−20 eV), these limits exclude couplings of order
10−30, corresponding to Yukawa interactions comparable to the gravitational force between neutrinos and
surpassing equivalent limits on time variation in scalar-induced electron and proton couplings.
DOI: 10.1103/PhysRevD.107.035006
I. INTRODUCTION
The identity of dark matter and the origin of neutrino
masses are two fundamental mysteries that unambiguously
require new physics beyond the Standard Model (SM).
Unlike other deficiencies of the SM (e.g., the lack of an
inflaton candidate), these problems may be resolved with
interactions at terrestrially and astrophysically accessible
energy scales. Thus, exploring all viable connections
between these problems is generically interesting, espe-
cially when they economically combine known tools that
address each problem separately.
Ultralight
(≪ eV) bosonic dark matter
characterized by a macroscopic de Broglie wavelength
(DM) ϕ is
λϕ ¼
1
mϕvϕ
≈ 200 km
(cid:1)
(cid:3)(cid:1)
neV
mϕ
10−3
vϕ
(cid:3)
;
ð1Þ
which exceeds the interparticle separation, where vϕ is the
field velocity. If ϕ is misaligned from the minimum of
quadratic potential, it oscillates as a classical field about
this minimum according to
*[email protected]
†
[email protected]
[email protected]
§[email protected]
‡
ϕð⃗r; tÞ ¼
p
ffiffiffiffiffiffiffiffiffiffiffiffiffi
2ρϕðtÞ
mϕ
cos½mϕðt þ ⃗vϕ · ⃗rÞ þ φð⃗rÞ(cid:2);
ð2Þ
and the corresponding energy density redshifts like non-
relativistic matter ρϕ ∝ a−3, where a is the cosmic scale
factor and φ is a possible phase. This phase may encode
additional information about spatial variation—e.g., differ-
ent ϕ domains arising from cosmological initial conditions1
or the incoherent virialization in the Galaxy leading to
variation on the scale of λϕ.
If ϕ couples to SM particles, their masses, spins, and
time dependence from
coupling constants may inherit
Eq. (2). In the context of charged SM particles, there are
many searches for such phenomena, which typically place
very strong limits on the ϕ-SM interaction strength (see
Ref. [1] for a review). By contrast, there are relatively few
bounds on DM-induced time dependence in the neutrino
sector [2–15] and the corresponding limits constrain com-
paratively large interaction strengths primarily via flavor
oscillations.
In this paper, we introduce the possibility that an
ultralight DM candidate ϕ induces a time-dependent
Majorana mass for right-handed neutrinos,
mM ¼
yϕ
2
ϕðtÞ;
ð3Þ
Published by the American Physical Society under the terms of
license.
the Creative Commons Attribution 4.0 International
Further distribution of this work must maintain attribution to
the author(s) and the published article’s title, journal citation,
and DOI. Funded by SCOAP3.
1This can happen, e.g., due to oscillation starting at slightly
times in different Hubble patches when the field
different
becomes dynamical at H ∼ mϕ.
2470-0010=2023=107(3)=035006(9)
035006-1
Published by the American Physical Society
DEV, KRNJAIC, MACHADO, and RAMANI
PHYS. REV. D 107, 035006 (2023)
where yϕ is a coupling constant and the time dependence
arises from Eq. (2). Since misaligned scalar fields must be
thermally decoupled from SM particles, they can only
induce a tiny Majorana mass without spoiling their cold
cosmic abundance through interactions with neutrinos.
This requirement eliminates many possible connections
between neutrinos and ultralight dark matter, but is com-
patible with the pseudo-Dirac regime, which is the focus of
our paper.
When the neutrino Majorana mass is small compared to
the Dirac mass mD, the mass eigenstates form a pair of
pseudo-Dirac fermions; one “active” νa and one “sterile” νs
(per generation). These states oscillate into each other with
a characteristic probability governed by their squared mass
difference δm2 [16],
Pðνa → νsÞ ¼ sin2ð2θÞ sin2
(cid:1)
(cid:3)
δm2L
4Eν
To understand the impact of ϕ on neutrino oscillations, it
is instructive to describe the “1 þ 1” scenario, in which
there is only one generation of l and N. For simplicity,
assume that the active state here is an electron flavor
neutrino. In the broken electroweak phase, the first term in
Eq. (5) generates a Dirac mass of neutrinos. When the ϕ
field is misaligned according to Eq. (2), the second term in
Eq. (5) generates a Majorana mass for N, so we have
mD ¼ yνvffiffiffi
p ;
2
mM ¼
yϕ
2
ϕðtÞ;
ð6Þ
for the Dirac and Majorana contributions, respectively,
where v ¼ 246 GeV is the Higgs vacuum expectation
value. When mM ≪ mD, we obtain two nearly degenerate
neutrino mass-squared eigenstates
;
ð4Þ
m2
h;l ¼ m2
D
(cid:3) mDmM ≡ m2
ν (cid:3)
1
2
δm2;
ð7Þ
where L is the baseline, Eν is the energy of the propagating
neutrino, and θ ≈ π=4 is the mixing angle, which is near
maximal in the pseudo-Dirac limit where mM ≪ mD.
The Majorana mass governing δm2 in Eq. (4) is time
dependent, so the oscillation rate becomes sensitive to the
dark matter density and to its cosmic evolution. This
dependence can impact various terrestrial and cosmological
observables. In this work, we extract resultant bounds and
impose extremely strong limits on the induced Majorana
mass; depending on the value of mϕ, we find some limits on
the coupling yϕ corresponding to a ϕ mediated Yukawa
force comparable to that of gravity.
This paper is organized as follows: in Sec. II we present
our theoretical framework, in Sec. III we delineate the
qualitatively different neutrino oscillation regimes that ϕ
can induce, in Sec. IV we compute the terrestrial bounds,
in Sec. V we determine the cosmological bounds on this
scenario, and in Sec. VI we make some concluding
remarks.
II. ULTRALIGHT DARK MATTER AND
PSEUDO-DIRAC NEUTRINOS
We consider a scalar DM candidate ϕ with lepton
number 2 and a cosmic abundance due to misalignment.
In Weyl fermion notation, the Lagrangian in this scenario
contains
L ⊃ yνHlN þ
yϕ
2
ϕNN þ H:c:;
ð5Þ
where yν is the neutrino Yukawa coupling, H is the SM
Higgs doublet, l is the SM lepton doublet, and N is a SM
neutral fermion, i.e., a right-handed neutrino. As we will
see next, the presence of a feeble interaction between the
scalar DM and the right-handed neutrino can have dramatic
effects in neutrino oscillation phenomenology.
and we define δm2 ≡ yϕmD
p
ffiffiffiffiffiffiffiffi
2ρϕ
=mϕ, where
δm2 ≈ 2 × 10−15 eV2
(cid:1)
yϕ
10−10
(cid:3)(cid:1)
10−15 eV
mϕ
(cid:3)(cid:1)
mD
0.1 eV
(cid:3)
;
ð8Þ
for the splitting between Weyl fermions, as opposed to the
usual Δm2
ij measured in oscillation experiments; here we
have taken the local density to be ρ⊙
ϕ ¼ 0.4 GeV=cm3 [17].
The active-sterile mixing angle in this case is
tan ð2θÞ ¼
2mD
mM
≫ 1;
ð9Þ
which is nearly maximal, θ ≈ π=4 in our full parameter
space of interest.
The diagonalization of the mass terms in Eq. (6) is
obtained by defining the flavor fields in terms of the mass
eigenstates approximately as
jνei ¼
1
p ðjνhi þ jνliÞ;
ffiffiffi
2
jνsi ¼
1
p ðjνhi − jνliÞ:
ffiffiffi
2
The time evolution of a νe state is given by
(cid:3)
(cid:1)
− i
2Eν
Z
UðtÞjνei ¼
1ðt0Þ
(cid:3)
1
p
ffiffiffi
2
dt0m2
Z
(cid:5)
0
t
exp
(cid:1)
− i
2Eν
þ exp
t
0
dt0m2
2ðt0Þ
jν1i
(cid:6)
jν2i
;
which yields a νe → νe survival probability
ð10Þ
ð11Þ
ð12Þ
035006-2
CONSTRAINING FEEBLE NEUTRINO INTERACTIONS WITH …
PHYS. REV. D 107, 035006 (2023)
PeeðtÞ ¼ jhνðtÞjνeij2 ¼ cos2
(cid:1)
1
4Eν
Z
t
0
(cid:3)
dt0δm2ðt0Þ
:
ð13Þ
Using Eqs. (2) and (6) we obtain
Z
t
0
1
2
dt0δm2ðt0Þ ¼
p
ffiffiffiffiffiffiffiffi
2ρϕ
yϕmD
mϕ
Z
t
0
dt0 cos ðmϕt0 þ φÞ;
where we have absorbed the vϕ dependence in Eq. (2)
into the definition of φ for brevity. Thus, for a neutrino
emitted at t ¼ 0 and observed at some later time t, the
resulting electron-neutrino disappearance probability can
be written as
1 − Pee ¼ sin2
(cid:7)
mD
2Eν
p
ffiffiffiffiffiffiffiffi
2ρϕ
yϕ
m2
ϕ
ðsin ½mϕt þ φ(cid:2) − sin φÞ
;
(cid:8)
ð14Þ
where we have treated the phase φ as a constant over the
propagation time.
Generalization for more neutrino flavors is straightfor-
ward and can be derived following similar steps as those
taken in Ref. [18]. Moreover, to simplify the discussion on
the constraints and because the electron-neutrino admixture
in ν3 is small (jUe3j ≪ 1), when ϕ couples to ν1 or ν2 we
will only consider nonstandard νe disappearance, while
when ϕ couples to ν3 we will only consider nonstandard
νμ;τ disappearance; in both regimes, we treat the active-
sterile oscillation in a two-flavor (active-sterile) framework.
As written in Eq. (2), the phase φ need not be constant
over the full neutrino trajectory. Indeed, in the Galaxy,
virialization will disrupt any constant phase value down to
coherence patches of order the de Broglie wavelength in
Eq. (1). Thus, the full oscillation probability will depend
crucially on the relative size of the oscillation baseline and
this coherence scale.
Finally, we note that our scalar mass is not protected by
any symmetry, so it will be sensitive to irreducible one-loop
corrections of order
δmϕ ∼
yϕmD
4π
∼ 10−18 eV
(cid:1)
yϕ
10−15
(cid:3)(cid:1)
mD
10 meV
(cid:3)
;
ð15Þ
from the interactions in Eq. (5). Thus, for small yϕ in the
pseudo-Dirac limit, this contribution does not destabilize
the ultralight scalar mass, assuming no ϕ couplings to
heavier states.2
2The operator kH†Hjϕj2 is also allowed by all symmetries
and can induce a large correction to mϕ if the coefficient is not
suppressed. Exponential k ≪ 1 suppression can be achieved in
UV models where H and ϕ are localized on different branes in a
higher-dimensional spacetime.
III. NEUTRINO OSCILLATION REGIMES
In what follows, we will consider three distinct regimes
for neutrino oscillations in the presence of the ultralight
scalar
fields. These regimes arrive from the relation
between the neutrino oscillation length and the modulation
frequency of ϕ or the coherence length that defines the
overall phase φ. Instead of performing a detailed fit of
experimental data, we will recast existing constraints on
pseudo-Dirac neutrinos from Ref. [19] on our parameters of
interest, yϕ and mϕ. As neutrinos are ultrarelativistic, we
identify t ¼ L in Eq. (14).
A. Constant ϕ: mϕL ≲ 1
In the low-frequency mϕL ≲ 1 regime,
the neutrino
encounters a constant phase φ domain over the course
of its propagation. Expanding Eq. (14) around mϕL → 0
yields an oscillation probability
1 − Pee ≈ sin2
(cid:1)
L
4Eν
2yϕmD
mϕ
p
ffiffiffiffiffiffiffiffi
2ρϕ
(cid:3)
cos φ
:
ð16Þ
We can interpret this oscillation probability as follows.
Since the period of the field ϕ is too long compared to the
neutrino time of flight, the pseudo-Dirac mass splitting
induced by the field is constant
for each neutrino.
Nevertheless, as an experiment collects data, the mass
splitting will evolve as the field ϕ displays time modula-
tion. In practice, several neutrino experiments have a high
enough rate of events to observe time modulation of
oscillation probabilities with periods as short as 10 min,
which would correspond to mϕ ∼ 10−18 eV [3–5,12]. Since
any small pseudo-Dirac mass splitting leads to maximal
mixing, time modulation of neutrino oscillation probabil-
ities due to ϕ modulation would lead to large, observable
effects on oscillation data.
Both constant and time-dependent pseudo-Dirac mass
splittings would be ruled out by neutrino data if observed
and can be used to set limits on the coupling strength yϕ
for a given mϕ. Since sterile neutrino oscillation constraints
are typically reported as bounds on δm2, we can define an
effective mass-squared δm2
eff by equating the arguments of
Eqs. (4) and (16) to obtain
δm2
eff
≡
2yϕmD
mϕ
p
ffiffiffiffiffiffiffiffi
2ρϕ
;
ð17Þ
assuming cos φ ∼ 1. Recasting pseudo-Dirac neutrino lim-
its on δm2 in Eq. (17) allows us to constrain
yϕ <
mϕ
2mD
δm2
p ;
limffiffiffiffiffiffiffiffi
2ρϕ
ð18Þ
where we have identified δm2
δm2
eff with the constrained value
lim. Note that, depending on context, ρϕ can either be the
035006-3
DEV, KRNJAIC, MACHADO, and RAMANI
PHYS. REV. D 107, 035006 (2023)
cosmological DM density at a given cosmic era or the
present day local density.
in any of the three regimes outlined in Sec. III, so the
relationship between yϕ and mϕ will differ in each case.
B. Modulating ϕ: ðmϕvϕL < 1 ≪ mϕLÞ
When the ϕ modulating frequency is high, mϕL ≫ 1, the
to
accumulated phase due to propagation is sufficient
induce many modulation cycles on ϕ over the neutrino
trajectory. However, as long as mϕvϕL ≲ 1, the neutrino
time of flight is shorter than separation time of ϕ wave
packets. A neutrino propagating in this regime will
encounter the same value of φ across its trajectory; that
is, the modulation of ϕ throughout the neutrino trajectory is
coherent. Without loss of generality, we can set the initial
condition φ ¼ 0. The effective oscillation probability in
this regime is given by a time average of Eq. (14) over the
duration of propagation,
h1 − Peei ≈ sin2
(cid:1)
yϕmD
2Eνm2
ϕ
(cid:3)
p
ffiffiffiffiffiffiffiffi
2ρϕ
;
ð19Þ
where we have assumed that ρϕ does not change appreci-
ably across the baseline. In this intermediate regime, we
repeat the argument leading up to Eq. (18) and constrain
ϕ ¼
ylim
δm2
2mD
limm2
ϕL
p :
ffiffiffiffiffiffiffiffi
2ρϕ
ð20Þ
C. Random walk: 1 ≪ mϕvϕL
Finally, in the mϕvϕL ≫ 1 regime, the neutrino time of
flight is longer than the wave packet separation of ϕ, so the
neutrino traverses a random sample of ϕ field patches, each
with a different phase φ. Along this trajectory, there are
approximately mϕvϕL patches whose contributions add
incoherently, so the effective phase can be approximated by
φ
, assuming random distribution of phases φ
ffiffiffiffiffiffiffiffiffiffiffiffiffiffi
mϕvϕL
p
∼
eff
and the phase-averaged probability can be written
h1 − Peei ≈ sin2
p
(cid:1)
yϕmD
(cid:3)
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2ρϕvϕL
3=2
ϕ
2Eνm
:
ð21Þ
A. Solar neutrinos
12 þ s4
For electron neutrinos, the pseudo-Dirac splitting can be
constrained by measurements of the solar neutrino flux.
In the standard three neutrino oscillations paradigm, 8B
neutrinos undergo an adiabatic evolution due to large
matter effects in the Sun [20]. This leads to a survival
13s2
probability Pðνe → νeÞ ≃ c4
13 ≃ 0.3, where sij and
cij are the sines and cosines of mixing angle θij. Low-
energy solar neutrinos, on the other hand, are not affected
by matter effects, and thus Pðνe → νeÞ ≃ c4
12Þ þ
s4
13 ≃ 0.55. These probabilities describe well the experi-
mental data [21–29]. This can be used to extract an order
of magnitude bound on the splitting in our scenario
this prediction is not affected by
by demanding that
an order 1 amount. Here we use ρ⊙
ϕ and vϕ ≈ 10−3 with
L ¼ 1.5 × 108 km, which requires
12 þ s4
13ðc4
δm2
lim < 10−12 eV2
ð23Þ
and can be translated into a bound on our model parameters
using the relations in Sec. III, where the appropriate regime
is determined by mϕ. Since solar neutrinos are essentially
almost pure ν2 or incoherent νe, and νe has but a small
admixture ν3 mass eigenstate, the corresponding solar limit
on the yϕ applies only to the right-handed partners N1;2.
Applying the solar limit from Eq. (23) to the three regimes
from Sec. III A, and assuming that the Dirac mass of ν1
satisfies m2
21 ¼ 7.4 × 10−5 eV2 [30], we find the
D
following constraints.
¼ Δm2
For mϕ ≲ 10−18 eV, solar neutrinos are in the constant ϕ
regime, so from Eq. (18), we find a limit
ϕ ≈ 3 × 10−26
ylim
(cid:1)
(cid:3)
mϕ
10−18 eV
;
for mϕ < 10−18 eV: ð24Þ
Note that, if mϕ ≲ 10−24 eV, the period of ϕ is larger than
20 yr, and the observation of pseudo-Dirac mass splittings
become dependent on the initial condition φ. For
10−18 ≲ mϕ ≲ 10−15 eV, we are in the modulating ϕ
regime, where Eq. (20) yields a limit of order
The corresponding limit on the coupling reads
s
ffiffiffiffiffiffiffiffiffiffiffiffi
m3
ϕL
2ρϕvϕ
:
δm2
lim
mD
ϕ ¼
ylim
IV. TERRESTRIAL OBSERVABLES
ϕ ≈ 3 × 10−26
ylim
ð22Þ
(cid:3)
1=2
(cid:1)
mϕ
10−18 eV
;
for mϕ < 10−15 eV:
ð25Þ
Finally, for mϕ ≳ 10−15 eV, solar neutrinos will traverse a
random sample of phases φ, corresponding to the random
walk regime, so the bound from Eq. (22) applies to give
We now consider various terrestrial bounds on pseudo-
Dirac neutrinos in the context of our scenario. Depending
on the values of yϕ and mϕ, a particular constraint can apply
ϕ ≈ 2 × 10−20
ylim
(cid:3)
3=2
(cid:1)
mϕ
10−15 eV
; for mϕ > 10−15 eV:
ð26Þ
035006-4
CONSTRAINING FEEBLE NEUTRINO INTERACTIONS WITH …
PHYS. REV. D 107, 035006 (2023)
These results are plotted in the left panel of Fig. 1, which
shows constraints on ϕ coupled only to ν1 or ν2, corre-
sponding to νe oscillations measurements.
B. Atmospheric neutrinos
Measurements of the atmospheric neutrinos can place
limits on the ϕ coupling to ν3 since muon neutrinos have a
large admixture of the ν3 eigenstate. If ν3 is split in a
pseudo-Dirac pair, a substantial deficit of atmospheric νμ
flux would be observed, contradicting experimental data
[33–36]. The characteristic atmospheric baseline is Earth’s
radius L ≈ 6000 km, and the Super-Kamiokande constraint
on constant pseudo-Dirac mass splittings is [19]
δm2
lim < 10−4 eV2;
ð27Þ
which translates into a bound on the ϕ coupling to ν3. For
ultralight ϕ masses, atmospheric oscillations are in the
constant ϕ regime of Sec. III A, so translating the constraint
from Eq. (27) with Dirac mass satisfying m2
32 ¼
D
2.4 × 10−3 eV2 [30] yields
(cid:1)
¼ Δm2
(cid:3)
ϕ ≈ 10−14
ylim
mϕ
3 × 10−14 eV
;
ð28Þ
which is valid for mϕ ≲ 3 × 10−14 eV. For larger ϕ masses
in the modulating ϕ regime of Sec. III B, we impose
the limit
3 ≈ 10−14
ylim
(cid:1)
mϕ
3 × 10−14 eV
(cid:3)
2
:
ð29Þ
These bounds are presented in the orange shaded region
of Fig. 1 (right panel). Note that for mϕ ≳ 10−10 eV,
atmospheric oscillations are in the long baseline regime
the bound in this mass range is
of Sec. III C, but
subdominant to other constraints in Fig. 1 and is not
shown. In principle, atmospheric neutrinos also bound the
ϕ coupling to ν1;2, but solar constraints are stronger.
V. COSMOLOGY
A. Scalar evolution
Throughout our analysis, we assume that the ϕ potential
can be written as
VðϕÞ ¼ m2
ϕjϕj2 þ
λϕ
4
jϕj4 þ Oðjϕj6Þ;
ð30Þ
where, in principle, the size of the quartic is unconstrained
by symmetry arguments and can take on any value.
However, there is an irreducible contribution to the quartic
interaction generated through a Coleman-Weinberg inter-
action with the neutrinos
λmin
ϕ ≈
y4
ϕ
16π2 ;
ð31Þ
FIG. 1. Left: parameter space for ϕ coupled only to ν1 or ν2 mass eigenstates, which is predominantly constrained νe oscillation
bounds. Here we show bounds from cosmic microwave background (CMB) and big bang nucleosynthesis (BBN) from Sec. V,
Milky Way (MW) satellites from Sec. V B, scalar thermalization with neutrinos from Sec. V D, solar neutrino oscillations from
Sec. IVA, and model-independent limits on light DM from ultrafaint dwarf (UFD) heating [31]. For points below the gray dotted line,
the ϕ mediated force between right-handed neutrinos is weaker than gravity, which is theoretically disfavored by the weak gravity
conjecture [32] Right: same as the left, only ϕ now couples only to ν3, so the limits are driven by νμ;τ oscillations for which the solar
bound is subdominant to the atmospheric bound described in Sec. IV B.
035006-5
DEV, KRNJAIC, MACHADO, and RAMANI
PHYS. REV. D 107, 035006 (2023)
which is always present in the absence of fine-tuning. In an
expanding Friedmann-Robertson-Walker universe, ϕ sat-
isfies the equation of motion
of the total dark matter abundance, in which case it need not
redshift
like nonrelativistic matter at early (or even
later) times.
̈ϕ þ 3H _ϕ þ V0 ¼ 0;
ð32Þ
where the prime denotes a derivative with respect to ϕ.
If ϕ is initially displaced from its minimum, it is frozen
by Hubble friction until H _ϕ ∼ V0, so if the mass term
dominates the potential, V0 ∼ m2
the field becomes
dynamical when mϕ ∼ H and oscillates about ϕ ¼ 0 while
redshifting like nonrelativistic matter ρϕ ∝ a−3.
ϕϕ,
In this scenario,
the initial misalignment amplitude
ϕi during inflation sets
the DM abundance. Since
mϕ=Hi ≪ 1, where Hi is the Hubble scale during inflation,
ϕ generates isocurvature perturbations, which are con-
strained by CMB measurements [37]. However, as long
as ϕi=Hi ≫ 1, isocurvature perturbations can be parametri-
cally suppressed; so for a given Hi, a suitable choice of ϕi
can account for the DM abundance while being safe from
this constraint. Furthermore, since mϕ=Hi ≪ 1, ϕ evolu-
tion is predominantly classical during inflation, so the
initial amplitude ϕi remains a free parameter throughout
our analysis and can be chosen to yield the observed DM
density [38].
B. Milky Way satellites
In order for ϕ to account for the full DM abundance, it
must redshift like nonrelativistic matter ðρϕ ∝ a−3Þ in the
early Universe, starting at least at matter-radiation equality
at a critical redshift z⋆ ∼ 106, corresponding to a temper-
ature T⋆ ∼ keV [39]. Since the ϕNN interaction in Eq. (5)
yields an irreducible quartic scalar self-interaction term, we
need to ensure that the ρϕ is not dominated by the quartic
contribution at Teq; otherwise,
like
radiation ρϕ ∝ a−4 (or faster if even higher polynomial
terms dominate instead) [40]. Avoiding this fate requires
it would redshift
C. Black hole superradiance
the
The phenomenon of black hole superradiance,
growth of ultralight boson clouds around spinning black
holes, has been used to set limits in certain regions of
ultralight boson parameter space irrespective of the boson
couplings to the Standard Model [41]. However, these
limits apply only when the self-interactions are negligible.
We find that the upper limit on the quartic coupling derived
in [42] is orders of magnitude weaker than the one allowed
by the Milky Way satellites in Eq. (34). As a result, the
quartic can always be chosen between these bounds so as to
not suffer from either constraints, and in the spirit of being
conservative, we do not display the superradiance limits.
D. Neutrino free streaming
The Yukawa interaction in Eq. (5) enables ϕν → ϕν
scattering, which can modify the neutrino free streaming
length and affect CMB observables. Since active neutrinos
do not couple directly to ϕ, the cross section for this process
two Dirac mass
requires
insertions and scales as
D=T4. Ensuring that the typical neutrino not scatter
σ ∼ y4
ϕm2
∼ 0.1 eV requires
in a Hubble time at recombination Trec
yϕ ≲
(cid:3)
1=4
0mϕ
(cid:1)
p
ffiffiffiffiffi
g⋆
recT3
T3
1.66
DM;0mPlm2
ρ
D
(cid:3)
(cid:1)
≈10−11
10 meV
mD
(cid:1)
1=2
(cid:3)
1=4
mϕ
1 eV
ð35Þ
;
ð36Þ
where g⋆ ¼ 3.36 is the number of effective relativistic
species at recombination and T0 ¼ 2.72 K is the present
day CMB temperature. This bound is shown in Fig. 1 as the
green shaded region.
m2
ϕjϕ⋆j2 >
y4
ϕ
16π2
jϕ⋆j4;
ð33Þ
E. CMB/BBN
where ϕ⋆ ≡ ϕðT⋆Þ. Using the scaling in Eq. (2), we find
(cid:3)
3=2
(cid:6)1=4
≈ 5 × 10−9
(cid:1)
(cid:3)
;
mϕ
neV
yϕ ≲
(cid:5)
8π2m4
ϕ
ρ
Ω
dm
c
(cid:1)
T0
T⋆
(cid:1)
λϕ ≲ 4 × 10−36
(cid:3)
4
mϕ
neV
;
ð34Þ
where ρc is the present day critical density. The inequality
in Eq. (34) defines the gray shaded regions in Fig. 1 where
this effect would erase the Milky Way satellites already
observed. However, note that
this bound is model-
dependent, as it can be evaded if ϕ is only a small fraction
In this section, we investigate the effects of the scalar
field in the early Universe, specifically, active to sterile
oscillations, which increase the effective number of neu-
trino species ΔNeff.
1. Cosmological field density
If the relic density was set by the misalignment mecha-
nism, then the DM density grows as T3 and remains as
the temperature TH when
nonrelativistic DM until
mϕ ¼ 3HðTHÞ, where H is
the Hubble parameter.
Above this temperature,
the field is constant due to
Hubble friction and only contributes to the vacuum energy,
so we have
035006-6
CONSTRAINING FEEBLE NEUTRINO INTERACTIONS WITH …
PHYS. REV. D 107, 035006 (2023)
ρϕðTÞ ¼ ρϕðT0Þ
(cid:1)
(cid:3)(cid:1)
g⋆;SðT0Þ
g⋆;SðTÞ
min ðT; THÞ
T0
(cid:3)
3
;
ð37Þ
δm2L
T
∼ mDmM
FT6
G2
∼
(cid:1)
yϕ
10−29
(cid:3)(cid:1)
(cid:3)(cid:1)
(cid:3)
mD
10 meV
10−12 eV
mϕ
;
ð43Þ
where T0 ¼ 2.72 K is the present day CMB temperature
and ρϕðT0Þ ¼ Ω
ρc is the cosmological DM density,
dm
via
the
which
related
is
ρϕðT0Þ ≈ 3 × 10−6ρ⊙
insert
In what
ϕ .
Eq. (37) into mMðTÞ ¼ yϕϕðTÞ=2 using Eq. (2) to model
the Majorana mass as function of cosmic temperature.
follows, we
overdensity
local
to
2. Cosmological sterile neutrino production
To compute the early Universe sterile neutrino yield, it is
to define rβ as the ratio of active/sterile
convenient
momentum moments
rβ ≡
hpβi
s
hpβi
a
;
ð38Þ
the sterile and active distributions,
where angular brackets h(cid:4) (cid:4) (cid:4)i
s;a denote a thermal average
over
respectively.
Generalizing the formalism of Ref. [43], rβ satisfies the
Boltzmann equation
Z
drβ
dT
¼ −
1
2HThpβi
a
d3p
ð2πÞ3
pβΓ sin2ð2θMÞ
ep=T þ 1
;
ð39Þ
where Γ ¼ 7π
24 G2
FpT4, and the mixing angle is
sin2ð2θMÞ ¼
sin2ð2θ0Þ
½cosð2θ0Þ − 2pVeff=Δm2(cid:2)2 þ sin2ð2θ0Þ
;
ð40Þ
where the effective matter potential for each flavor a ¼ e,
μ, τ can be written as
Va
eff
¼ (cid:3)C1ηGFT3 − Ca
2
α G2
FT4p;
ð41Þ
Þ=nγ ¼ 6 × 10−10 is the lepton asym-
where η ¼ ðnL − n ¯L
μ;τ
2 ≈ 0.17, and the (cid:3) refer
2 ≈ 0.61, C
metry, C1 ¼ 0.95, Ce
to neutrinos and antineutrinos [44]. Here the vacuum
mixing angle θ0 in Eq. (40) is ϕ dependent,
θ0 ¼ tan−1
(cid:1)
p
ffiffiffiffiffiffiffiffi
2ρϕ
yϕ
mDmϕ
(cid:3)
;
ð42Þ
where we have used Eqs. (6) and (9). Note that the first two
moments of the active neutrino distribution yield the
number and energy densities [hp0i
a
¼ na; hp1i
a
In the following subsections, we derive detailed ΔNeff
limits from BBN and CMB based on νa → νs oscillations
around T ≈ MeV;
light
element yields or the Hubble rate. The oscillation proba-
bility is maximized when the argument of Eq. (4) is order 1,
implying
later oscillations do not affect
¼ ρa].
p
where we have used δm2 ¼ mDmM and mM ∼ yϕϕ from
ffiffiffiffiffi
=mϕ ∝ T3=2 from Eq. (2), and approxi-
ρϕ
Eq. (6), ϕ ∼
mated L ∼ ðG2
FT5Þ−1 as the neutrino mean free path, setting
T ¼ MeV throughout. Thus, the blueshifted DM density at
BBN greatly enhances the neutrino Majorana mass and
yields on order 1 oscillation probability for extremely
feeble couplings yϕ ∼ Oð10−29Þ. Note that, in our numeri-
cal study below, we use the full temperature dependence
from Eq. (37), which also accounts for the Hubble damped
regime when T > TH and is relevant for the smallest values
of mϕ we consider.
However, from Eq. (37), for sufficiently large values of
yϕ and ρϕ, mM > mD, so neutrinos are no longer pseudo-
Dirac fermions at high temperatures. In this regime, νa →
νs oscillations are sharply suppressed as θ0 → π=2 in
Eq. (42), so there is a ceiling to the couplings that can
be probed in the early Universe; this effect yields concave
regions for the BBN/CMB regions in Fig. 1.
3. Extracting the CMB ΔNeff limit
For temperatures before active neutrino decoupling,
sterile neutrinos produced via νa → νs oscillations contrib-
ute to ΔNeff, which can be constrained using cosmic
microwave background anisotropy data. Oscillations that
take place after neutrino decoupling interchange active
and sterile states, but do not contribute to ΔNeff. In terms of
r in Eq. (39), sterile production via a flavor oscillations
predicts
ΔNCMB
eff
¼ r1ðT
νa
dec
Þ;
ð44Þ
νμ;ντ
≈ 3.2 and T
dec
νa ¯νa → eþe−
νe
≈ 5.34 MeV are the temper-
where T
dec
atures of
chemical decoupling [44].
Assuming the Λ CDM cosmological model, the Planck
Collaboration constraints ΔNeff
≲ 0.28 [37] and we show
this constraint in Fig. 1 as the blue shaded region alongside
projections from future measurements with CMB-S4 [45].
4. BBN ΔNeff limit
A nonzero ΔNeff from sterile production also yields a
larger initial neutron/proton fraction at the onset of BBN,
which increases the primordial helium fraction. As in
Eq. (44), for ϕ coupled to νμ;τ, the effect on BBN arises
purely from the expansion rate via
ΔNBBN
eff
¼ r1ðT
νμ;τ
dec
Þ;
ð45Þ
where r1 is the solution to Eq. (39) with β ¼ 1 evaluated at
decoupling, assuming no initial population of steriles. The
blue contour of Fig. 1 (right panel) shows parameter space
035006-7
DEV, KRNJAIC, MACHADO, and RAMANI
PHYS. REV. D 107, 035006 (2023)
where ΔNBBN
eff > 0.5 [46,47] for ϕ coupled to the ν3 mass
eigenstate, implying oscillations from νμ and ντ flavor
states.
However, for νe → νs oscillations, there are two distinct
effects that impact the n=p ratio: oscillations before νe
≈ 3.2 MeV change the expan-
chemical decoupling at T
sion rate as above, and oscillations after decoupling deplete
the νe density. Both effects can be captured with a shift3 in
the effective Fermi constant via
νe
dec
G2
F
→
1
2 G2
F
½2 þ r2ðT
νe
dec
Þ − r2ðTnuc
Þ(cid:2)
ð46Þ
and a simultaneous shift in g⋆ via
g⋆ → g⋆;SM
þ
7
4 r1ðT
νe
dec
Þ;
ð47Þ
≈ 0.8 MeV is
¼ 10.75 during BBN, and Tnuc
where g⋆;SM
the temperature at which nucleon interconversion freezes
out in the SM. We can economically capture both effects
with an equivalent ΔNBBN
[48] to obtain
eff
ΔNBBN
eff
≈ r1ðT
νe
dec
Þ þ
4
7 g⋆;SM
½r2ðTnuc
Þ − r2ðT
νe
dec
Þ(cid:2):
ð48Þ
In Fig. 1 (left panel), the blue shaded region shows the BBN
exclusion for which ΔNBBN
eff > 0.5.
VI. CONCLUSIONS
In this paper, we have presented the first cosmologically
in which neutrino masses acquire time
viable model
3Note that hp2i
a
FT5, so r2 ¼ hp2i
Γ ∼ G2
from this rate.
∝ T5, which sets the weak scattering rate
s yields the fractional departure
s=hp2i
dependence through their coupling to ultralight dark matter.
In our scenario, the DM interaction sets the right-handed
neutrino Majorana mass and neutrinos are pseudo-Dirac
fermions with small mass splittings between active and
sterile states. Since in the pseudo-Dirac regime the mixing
angle between active and sterile is maximal, we extract
limits on ultrafeeble Yukawa couplings between DM and
right-handed neutrinos, constraining values of order yϕ ∼
10−30 for mϕ ∼ 10−19 eV in the fuzzy DM regime [49]; for
the ϕ mediated Yukawa force
such small couplings,
between right-handed neutrinos is comparable to that of
gravity.
Throughout our analysis, we have emphasized bounds
from solar and atmospheric neutrino oscillations,
large
scale structure, and the CMB/BBN eras. However, addi-
tional limits on this scenario may also be extracted by
studying cosmic ray propagation [18] or diffuse supernova
background neutrinos
[50,51], which we leave for
future work.
ACKNOWLEDGMENTS
This work is supported by the Fermi Research Alliance,
LLC under Contract No. DE-AC02-07CH11359 with the
U.S. Department of Energy, Office of Science, Office of
High Energy Physics. H. R. acknowledges the support from
the Simons Investigator Grant No. 824870, DOE Award
No. DE-SC0012012, NSF Grant No. PHY2014215, DOE
HEP QuantISED Award No. 100495, and the Gordon and
Betty Moore Foundation Grant No. GBMF7946. This work
was performed in part at the Aspen Center for Physics,
which is supported by NSF Grant No. PHY-1607611. This
project has received support from the European Union’s
Horizon 2020 research and innovation program under the
Marie Skłodowska-Curie Grant Agreement No. 860881-
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035006-9
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10.1016_j.jbc.2023.105073.pdf
|
Data availability
All relevant data are contained within the main article or
supplemental information. Please email [email protected] with
requests for raw data or reagents.
| null |
RESEARCH ARTICLE
Mitochondrial double-stranded RNA triggers induction
of the antiviral DNA deaminase APOBEC3A and nuclear
DNA damage
, Rémi Buisson4,5
Received for publication, May 8, 2023, and in revised form, June 27, 2023 Published, Papers in Press, July 19, 2023,
https://doi.org/10.1016/j.jbc.2023.105073
Chloe Wick1, Seyed Arad Moghadasi1, Jordan T. Becker1
Elodie Bournique4,5
From the 1Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, Minnesota, USA;
2Department of Biochemistry and Structural Biology, University of Texas Health San Antonio, San Antonio, Texas, USA;
3Department of Life and Environmental Sciences, University of Cagliari, Cittadella Universitaria di Monserrato, Monserrato,
Cagliari, Italy; 4Department of Biological Chemistry, School of Medicine, and 5Center for Epigenetics and Metabolism, Chao
Family Comprehensive Cancer Center, University of California Irvine, Irvine, California, USA; 6Howard Hughes Medical Institute,
University of Texas Health San Antonio, San Antonio, Texas, USA
, and Reuben S. Harris1,2,6,*
, Elisa Fanunza2,3
, Sunwoo Oh4,5
,
Reviewed by members of the JBC Editorial Board. Edited by Karin Musier-Forsyth
APOBEC3A is an antiviral DNA deaminase often induced by
virus infection. APOBEC3A is also a source of cancer mutation
in viral and nonviral tumor types. It is therefore critical to
identify factors responsible for APOBEC3A upregulation. Here,
we test the hypothesis that leaked mitochondrial (mt) double-
stranded (ds)RNA is recognized as foreign nucleic acid,
which triggers innate immune signaling, APOBEC3A up-
regulation, and DNA damage. Knockdown of an enzyme
responsible for degrading mtdsRNA,
the exoribonuclease
polynucleotide phosphorylase, results in mtdsRNA leakage into
the cytosol and induction of APOBEC3A expression. APO-
BEC3A upregulation by cytoplasmic mtdsRNA requires RIG-I,
MAVS, and STAT2 and is likely part of a broader type I
interferon response. Importantly, although mtdsRNA-induced
APOBEC3A appears cytoplasmic by subcellular fractionation
its induction triggers an overt DNA damage
experiments,
response characterized by elevated nuclear γ-H2AX staining.
Thus, mtdsRNA dysregulation may induce APOBEC3A and
contribute to observed genomic instability and mutation sig-
natures in cancer.
The apolipoprotein B mRNA editing catalytic polypeptide-
like 3 (APOBEC3 or A3) family of proteins comprises seven
members in humans (1). As single-stranded (ss)DNA cytosine
deaminases, these enzymes normally function as antiviral
factors capable of
inhibiting virus replication, suppressing
infectivity, and blocking pathogenesis (2). However, this potent
DNA editing activity can also be directed at the human
genome in cancer and cause mutations in chromosomal DNA
(3–5). A3-catalyzed genomic C-to-U deamination events
become immortalized as C-to-T transition and C-to-G trans-
version mutations, most frequently in TCA and TCT trinu-
cleotide motifs. Collectively, these single base substitution
(SBS) mutation patterns in cancer are known as SBS2 and
* For correspondence: Reuben S. Harris, [email protected].
SBS13 or, more simply, as the “APOBEC mutation signature.”
The APOBEC mutation signature is found in over 70% of
cancers and can be the largest fraction of somatic variation in
many individual tumors and tumor types (6, 7).
APOBEC3A (A3A) and APOBEC3B (A3B) are the most
likely sources of APOBEC signature mutations in cancer (most
recently addressed by (8, 9)). Both enzymes are potent ssDNA
cytosine deaminases that intrinsically prefer TC motifs due to
identical
loop regions that engage the thymine nucleobase
immediately upstream of a target cytosine (10, 11). Ectopic
expression of both enzymes inflicts APOBEC signature mu-
tations in model bacteria and yeast systems, the chicken cell
line HAP1 (3, 9, 12–16).
line DT40, and the human cell
Recently, CRISPR knockout studies have shown that both
enzymes contribute to ongoing mutagenesis in human cancer
cell lines, with A3A accounting for a larger fraction of the
overall APOBEC signature (8). Importantly, each of these
human enzymes is capable of catalyzing mutagenesis and
promoting tumor formation in mice, which demonstrates that
this mutational process is capable of uniquely driving carci-
nogenesis (and is not simply a passenger phenomenon despite
the fact that most APOBEC signature mutations are likely to
be aphenotypic) (17–21).
A3A expression is suppressed in most normal human tissues
(22–24). However, consistent with its function as an antiviral
innate immune factor, its transcription can be induced by viral
infection (25–27). For instance, human papillomavirus infec-
tion of normal immortalized keratinocytes or human tonsillar
epithelial cells, human polyomavirus infection of human uro-
thelium, and human cytomegalovirus infection of decidual
tissues are all reported to trigger increased expression of A3A
(26, 28–30). Furthermore, consistent with antiviral function,
A3A is induced by type I interferons (IFNs) in multiple cell
types including monocytes, macrophages, and dendritic cells
(22, 31–34). This pathway is initiated by IFN binding to its cell
surface receptor, JAK/STAT signal transduction, and STAT2
binding the A3A promoter and transcriptional activation (25).
J. Biol. Chem. (2023) 299(9) 105073 1
© 2023 THE AUTHORS. Published by Elsevier Inc on behalf of American Society for Biochemistry and Molecular Biology. This is an open access article under the CC
BY license (http://creativecommons.org/licenses/by/4.0/).
APOBEC3A upregulation by mitochondrial dsRNA
However, it is important to note that infection by other viruses
such as the lentivirus HIV-1 and the herpesvirus Epstein–Barr
virus fails to induce A3A expression (35, 36). Moreover, most
cancer types with an APOBEC signature SBS2 and SBS13 and
A3A expression lack viral etiologies (7, 23, 24, 37–39). It is
to understand nonviral
therefore of considerable interest
mechanisms of A3A upregulation.
Extrinsic nucleic acids from dead cells and intrinsic nucleic
acids from chromosome missegregation (micronuclei) and
aberrant endogenous virus and transposon activity can, like viral
nucleic acids, activate nucleic acid sensors and trigger strong
IFN responses including A3A upregulation (31, 40–42). Mito-
chondria are another potential source of endogenous immu-
nostimulatory nucleic acids (43–45). For instance, bidirectional
transcription of mitochondrial genes can result in double-
stranded (ds)RNA, which is normally recycled by a degrado-
some comprising the exoribonuclease polynucleotide phos-
phorylase (PNPase) and the ATP-dependent RNA helicase
SUPV3L1 (46–50). Knockdown of either component of this
complex results in accumulation of mitochondrial dsRNA
(mtdsRNA) (45, 51). Moreover, PNPase depletion additionally
allows mtdsRNA to escape into the cytosol (45, 52). Cytosolic
mtdsRNA is then free to engage the RNA sensors RIG-I and
MDA5 and potentiate an IFN response (45). Therefore, a
combination of genetic, biochemistry, and cell biology ap-
proaches is used here to test the hypothesis that mtdsRNA can
be mistaken as foreign and trigger a virus-like innate immune
response that leads to A3A induction and nuclear DNA damage.
Results
Mitochondrial and nuclear dsRNA trigger A3A upregulation
To test the hypothesis that mtdsRNA leads to an induction
of A3A expression, the breast epithelial cell line MCF10A was
transfected with siRNAs to deplete the mitochondrial exori-
bonuclease PNPase and the RNA helicase SUPV3L1 and
immunofluorescent microscopy was used to quantify dsRNA.
Strong cytoplasmic staining with the dsRNA-specific mono-
clonal antibody J2 was observed in PNPase- and SUPV3L1-
depleted cells after membrane permeabilization with 0.2%
triton-X100 (45) (Fig. 1A). A stringent 0.2% digitonin per-
meabilization protocol yielded similar results (Fig. S1A). The
majority of the dsRNA signal in these conditions appeared
coincident with mitochondria as indicated by overlapping
staining with MitoTracker (Red CMXRos). Interestingly, a
milder 0.02% digitonin protocol, which permeabilizes only the
plasma membrane (and not mitochondrial or nuclear mem-
branes (53)), indicated that only PNPase depletion selectively
triggers cytosolic dsRNA accumulation (Fig. 1B; additional
images in Fig. S1B). In comparison, when using the same
0.02% digitonin treatment to preferentially permeabilize the
cytoplasmic membrane, SUPV3L1 depletion did not lead to
significant mtdsRNA leakage into the cytosol (Fig. 1B; addi-
images in Fig. S1B). As a negative control, non-
tional
digitonin-permeabilized cells
staining
(images in Fig. S1C). Quantification of imaging results from
confirmed
the 0.2% and 0.02% digitonin experiments
showed little
J2
2 J. Biol. Chem. (2023) 299(9) 105073
significant overlap between dsRNA and MitoTracker staining
(Fig. S1D) and significant numbers of dsRNA foci accumu-
lating in PNPase-depleted cells (Fig. S1E).
To assess if knockdown of PNPase and subsequent release
of dsRNA into the cytosol causes an IFN response in
MCF10A cells, the expression of the interferon-stimulated
gene ISG15 was measured as an indicator of a type I IFN
production. PNPase knockdown, but not SUPV3L1 knock-
down, resulted in strong upregulation of both ISG15 and A3A
(Fig. 1, C and D). The two isoforms of A3A beginning at Met1
and Met13 are both evident, consistent with a transcriptional
induction mechanism. Indeed, A3A mRNA levels increased
15- to 20-fold through PNPase knockdown in comparison with
a nontargeting siRNA (Fig. 1D). A3B mRNA levels were also
induced significantly, but other A3 mRNAs appeared un-
changed (Fig. 1D; quantification of all A3 mRNAs in Fig. S2A).
Similar results for A3A and A3B were obtained in the lung
carcinoma epithelial cell
line A549 but not in HeLa cells,
which are defective in interferon synthesis (Fig. S2, B and C).
Taken together, these data indicated that leakage of mito-
chondrial dsRNA into the cytosol leads to a strong upregula-
tion of A3A and a weaker but still significant induction of A3B.
To determine if dsRNA of a nonmitochondrial origin might
also lead to A3A induction, the RNA regulatory protein TAR
DNA-binding protein 43 (TDP-43) was knocked down, which
is known to result in cytoplasmic RNA polymerase III tran-
script accumulation (54, 55). Thus, TDP-43 was depleted from
MCF10A cells and, as anticipated from this prior literature,
this knockdown caused an accumulation of dsRNA puncta in
the cytoplasm (Fig. 1, A and B). Importantly, this dsRNA signal
showed little overlap with mitochondrial staining by Mito-
Tracker Red CMXRos. However, similar to depletion of
PNPase above, immunoblot and reverse transcription-quanti-
tative PCR (RT-qPCR) experiments showed a >10-fold in-
crease in A3A levels following TDP-43 depletion (Fig. 1, C and
D). It is not clear why TDP-43 depletion results in higher A3A
protein levels in comparison with PNPase depletion, despite
similar fold-induction at the mRNA level and similarly high
IFN responses as assessed by ISG15 levels. Nevertheless,
despite this additional protein-level curiosity, these results
combined to demonstrate that an accumulation of cytosolic
dsRNA from mitochondrial or nuclear origins leads to a robust
induction of A3A expression.
Cytosolic sensing of mitochondrial dsRNA requires the RNA
sensor RIG-I
To determine the RNA sensor responsible for A3A upre-
gulation in response to mtdsRNA accumulation in the cyto-
plasm, MCF10A cells were codepleted of PNPase and
candidate RNA sensors and then A3A levels were quantified as
above. In comparison with the induction of A3A observed in
cells depleted for PNPase, codepletion of PNPase and the
cytosolic RNA sensor RIG-I prevented A3A upregulation. In
contrast, treatment with siRNAs against MDA5 had no sig-
nificant effect (Fig. 2, A and B). To further substantiate these
knockdown results, MCF10A cells engineered by CRISPR-
A
Triton
B
0.02% Digitonin
APOBEC3A upregulation by mitochondrial dsRNA
C
D
Figure 1. Leaked mitochondrial dsRNA triggers A3A upregulation. A and B, immunofluorescence microscopy images of MCF10A cells treated with siCtrl,
siPNPase, siSUPV3L1, or siTDP-43 for 72 h and permeabilized with (A) 0.2% Triton X-100 or (B) 0.02% digitonin after which they were stained with the dsRNA-
binding antibody J2. Mitochondria were stained with MitoTracker, and nuclei were stained with Hoechst (the scale bar represents 10 μm). C, immunoblot
analysis of the indicated proteins expressed in MCF10A cells treated with siCtrl, siPNPase, siSUPV3L1, or siTDP-43 for 72 h. Tubulin was used as a loading
control. All subpanels are from the same representative blot. D, Reverse transcription-quantitative PCR analysis of A3 mRNA levels in MCF10A cells after
treatment with siCtrl, siPNPase, siSUPV3L1, or siTDP-43 for 72 h. Expression refers to A3 mRNA fold change relative to the negative control (set to 1)
normalized to TBP. Mean values ± SEM of three independent experiments (*p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001 by Student’s t test and not shown if
insignificant).
Cas9 to lack RIG-I also demonstrated that this sensor is
required for A3A induction by cytoplasmic dsRNA (Fig. 2, C
and D). As anticipated from prior work (55), RIG-I null
MCF10A cells also failed to induce A3A following TDP-43
depletion (Fig. 2E).
MAVS is an adaptor in RNA sensing that typically functions
downstream of RIG-I (56, 57). To further test whether A3A
induction is dependent on the sensing of dsRNA, RNAi ex-
periments were done to investigate the involvement of MAVS.
As above for RIG-I experiments, codepletion of PNPase and
MAVS reduced A3A expression to uninduced levels and
MAVS-null clones showed a complete abrogation of A3A
induction following knockdown of PNPase (Fig. 2, A, B, F and
G). These results combined to further demonstrate that A3A is
J. Biol. Chem. (2023) 299(9) 105073 3
APOBEC3A upregulation by mitochondrial dsRNA
A
C
F
B
D
E
G
Figure 2. The RIG-I/MAVS axis is required for upregulating A3A in response to endogenous mitochondrial dsRNA. A, RT-qPCR analysis of A3A after
treatment with siCtrl, siPNPase, and codepletions of siPNPase with siCtrl, siRIG-I, siMDA5, and siMAVS for 72 h in MCF10A cells. Expression refers to mRNA
fold change relative to the negative control (which was set to 1) and was normalized to TBP. Mean values ± SEM of three independent experiments (*p ≤
0.05 by Student’s t test and not shown if insignificant). B, immunoblot analysis of A3A in MCF10A cells treated with siCtrl, siPNPase, and codepletions of
siPNPase with siCtrl, siRIG-I, siMDA5, and siMAVS. Tubulin was used as a loading control. C and D, RT-qPCR and immunoblot analysis of A3A mRNA and
protein levels, respectively, in control or RIG-I KO MCF10A cells following siCtrl or siPNPase treatment. Expression refers to mRNA fold change relative to the
negative control (which was set to 1) and was normalized to TBP. Mean values ± SEM of three independent experiments (***p ≤ 0.001 by Student’s t test and
not shown if insignificant). E, RT-qPCR analysis of A3A mRNA levels, respectively, in control or RIG-I KO MCF10A cells following siCtrl or siTDP-43 treatment.
Expression refers to mRNA fold change relative to the negative control (which was set to 1) and was normalized to TBP. Mean values ± SEM of three
4 J. Biol. Chem. (2023) 299(9) 105073
upregulated through the sensing of cytosolic mtdsRNA by the
RIG-I/MAVS pathway.
A3A upregulation by cytosolic mtdsRNA requires STAT2
Accumulation of immunostimulatory dsRNA can trigger a
wide array of cellular responses, and therefore a set of IFN-
responsive genes was analyzed to determine whether the ca-
nonical IFN pathway is involved. We found that five canonical
IFN-stimulated genes (ISG15, IFI44, DDX60, MX1, and OAS1
(58)) were all significantly upregulated following siPNPase
treatment (Fig. 3A). Genes encoding the inflammatory cyto-
kines tumor necrosis factor alpha and interleukin 6 were also
induced by PNPase knockdown (Fig. S4). The expression of
specific IFN genes was not examined, but several are also bona
fide ISGs and induction is anticipated based on prior reports
(e.g., IFN-β in ref. (45)). To confirm that A3A is upregulated
through a type I IFN response, knockdown of the IFN-α/β
receptor in siPNPase-treated cells effectively reduced A3A
expression levels to those of the control (Fig. 3, B and C).
IFNAR1 depletion was confirmed by RT-qPCR in these ex-
periments because available commercial antibodies did not
work in our hands (Fig. 3D). These results indicated that,
following activation of RIG-I and MAVS, A3A induction oc-
curs through a type-I IFN-dependent signaling pathway.
The most
IFN-dependent
likely mediators of a type-I
response based on the prior literature (25, 59, 60) are the IFN-
inducible transcription factors STAT1 and STAT2. These
factors were therefore depleted from MCF10A cells with
siRNA, and the effect of PNPase knockdown was examined as
described above. Interestingly, only STAT2 (and not STAT1)
depletion was able to block A3A induction by PNPase
knockdown (Fig. 3, B and C; independent STAT1 knockdown
results in Fig. S3). This important result was confirmed using
STAT2-knockout MCF10A clones, where A3A is no longer
inducible by PNPase knockdown (Fig. 3, E and F).
We also extended these results to another cell line using a
completely orthologous approach. A549 cells were transfected
with vectors expressing the Zika virus proteins NS2A and
NS4B as tools to block the JAK-STAT signaling cascade that
occurs following IFN induction. NS2A mediates the degrada-
tion of STAT1 and STAT2, and NS4B suppresses the phos-
phorylation of STAT1 (61, 62). A549 cells were transfected
concurrently with siPNPase and plasmids encoding FLAG-
tagged NS2A and NS4B (Fig. 3G). A3A upregulation was
eliminated by the addition of NS2A. In contrast, transfection
of NS4B into the cells had little effect with A3A levels still
rising 15- to 20-fold after PNPase knockdown. As NS2A (but
not NS4B) interferes with STAT2 activation, these data sup-
port the knockdown and knockout results above showing that
A3A induction requires STAT2. Thus, activation of RIG-I/
MAVS by endogenous dsRNA causes a type I IFN response
that induces A3A via STAT2.
APOBEC3A upregulation by mitochondrial dsRNA
A3A induction by mtdsRNA triggers a DNA damage response
To investigate the kinetics of A3A induction by mtdsRNA
leakage, A3A, A3B, and PNPase mRNA expression levels were
analyzed every 24 h over a 4-day period following PNPase
depletion (Fig. 4A). This analysis revealed that A3A expression
peaks, approximately 15-fold, at around 72 h after siRNA
transfection and quickly recovers to 2-fold induction by 96 h.
A3B mRNA levels peak with similar kinetics, although only
around 3-fold, roughly plateauing between 48 and 72 h post
transfection, and A3B mRNA levels may also persist slightly
longer. In the same time course, PNPase mRNA levels are
depleted maximally by 72 h post transfection and begin to
recover by 96 h (Fig. 4A). These results indicate that A3A (and
A3B) mRNA levels correlate inversely with PNPase levels (and
thereby also with cytosolic mtdsRNA levels) and are likely to
be transient in nature.
To determine where mtdsRNA-induced A3A protein ac-
cumulates within cells, PNPase was depleted from MCF10A,
subcellular fractionation was used to separate nuclear and
cytoplasmic components, and immunoblots were done to
detect relevant proteins. Phorbol 12-myristate 13-acetate was
used as a positive control to induce A3A and A3B, as shown
previously (63–65). This biochemical approach showed that
the majority of mtdsRNA-inducible A3A is localized to the
cytoplasm, with tubulin as a positive control (Fig. 4B). In
comparison, the majority of A3B localizes to nuclear fractions,
with histone H3 as a positive control (Fig. 4B). Cytosolic
localization of IFNα-induced endogenous A3A has been re-
ported for another cell line (THP1), and nuclear localization of
endogenous A3B has been reported for MCF10A and a
multitude of cell lines by many groups (63, 66–68).
as
above
Last, we asked whether the A3A protein induced under
these conditions of PNPase depletion/cytosolic mtdsRNA
accumulation is capable of inflicting nuclear DNA damage.
from
This was done by depleting PNPase
MCF10A cells and then using immunofluorescence micro-
scopy to visualize and quantify the DNA damage marker γ-
H2AX. Interestingly, PNPase depletion causes strong increases
in both pan-nuclear and focused γ-H2AX staining including a
doubling of the number of γ-H2AX foci (Fig. 4, C and D;
quantification in Fig. S5). An independent experiment with
doxorubicin as a positive control confirmed this result and
suggested that the overall level of DNA damage inflicted by
A3A is less than that caused by this chemotherapeutic
(Fig. S5). Importantly, MCF10A cells engineered by CRISPR to
lack endogenous A3A demonstrated that the majority of these
nuclear γ-H2AX foci are dependent upon this enzyme (Fig. 4,
C and D), despite the majority of protein localizing to the
cytosol as described above. A3A knockout was confirmed by
immunoblot and by sequencing the gRNA-binding site where
each allele has multiple mutations including a frameshift
mutation (Fig. 4, E and F). These experiments combined to
independent experiments (***p ≤ 0.001 by Student’s t test and not shown if insignificant). F and G, RT-qPCR and immunoblot analysis of A3A mRNA and
protein levels, respectively, in control or MAVS KO MCF10A cells following siCtrl or siPNPase treatment. Expression refers to mRNA fold change relative to the
negative control (which was set to 1) and was normalized to TBP. Mean values ± SEM of three independent experiments (**p ≤ 0.01 by Student’s t test and
not shown if insignificant). RT-qPCR, reverse transcription-quantitative PCR.
J. Biol. Chem. (2023) 299(9) 105073 5
APOBEC3A upregulation by mitochondrial dsRNA
A
C
F
B
D
E
G
Figure 3. A3A is upregulated via a STAT2-dependent interferon response. A, RT-qPCR analysis of a panel of interferon-responsive genes (ISG15, IFI44,
DDX60, MX1, OAS1) after siPNPase treatment of MCF10A cells. Expression refers to mRNA log2 fold change relative to the negative control (which was set to
0) and was normalized to TBP. Mean values ± SEM of three independent experiments (*p ≤ 0.05, **p ≤ 0.01 by Student’s t test and not shown if insignificant).
B, RT-qPCR analysis of A3A in MCF10A cells treated with siCtrl, siPNPase, and siPNPase in combination with siCtrl, siIFNAR1, siSTAT1, and siSTAT2 (*p ≤ 0.05
by Student’s t test and not shown if insignificant). C, immunoblot analysis of MCF10A cells treated with siCtrl, siPNPase, and codepletions of siPNPase in
combination with siIFNAR1, siSTAT1, and siSTAT2. D, RT-qPCR analysis of IFNAR1 in MCF10A cells treated with siCtrl or siIFNAR1 and siPNPase. Mean values ±
SEM of three independent experiments (**p ≤ 0.01 by Student’s t test). E and F, RT-qPCR and immunoblot analysis of A3A mRNA and protein levels,
respectively, in control or STAT2 KO MCF10A cells following siCtrl or siPNPase treatment. Expression refers to mRNA fold change relative to the negative
6 J. Biol. Chem. (2023) 299(9) 105073
indicate that cytosolic mtdsRNA accumulation leads to a
strong A3A-dependent DNA damage response.
Discussion
that
Here, we report
the cytoplasmic accumulation of
endogenous dsRNA of mitochondrial origin triggers a strong
increase in the expression of A3A and a slight increase in the
expression of A3B. While it has been previously reported that
foreign and synthetic nucleic acids are able to trigger the in-
duction of A3A through a type-I IFN response (22, 25, 31, 33,
69, 70), our results are the first to examine how dysregulation
of endogenous dsRNA may act as a natural source of immu-
nostimulatory nucleic acids and lead to strong upregulation of
A3A. We show that the upregulation of A3A by endogenous
dsRNA is dependent on the RIG-I/MAVS signaling axis and
proceeds through a type I
IFN response in a STAT2-
dependent manner. Moreover, upregulated A3A, although
almost entirely cytoplasmic, is also able to cause chromosomal
DNA damage as evidenced by elevated γ-H2AX staining.
Taken together, these results support a model
in which a
breach in mitochondrial integrity can leak dsRNA into the
triggers RIG-I/MAVS/STAT2-dependent
cytosol, which
upregulation of the IFN response including A3A expression
and, importantly, DNA damage (Fig. 5). This pathway could be
directly relevant to cells with mitochondrial dsRNA leakage as
well as to bystander cells due to the auto/paracrine nature of
the IFN response. These observations may help explain the
periodic (also called episodic) occurrence of APOBEC3
signature mutations in cancer cell lines, which were shown
recently to involve A3A (8, 71).
The
chromosomal DNA damage observed following
knockdown of PNPase and accumulation of mtdsRNA is
surprising given that the bulk of induced A3A protein is
cytoplasmic. In fact, our subcellular fractionation experiments
indicate no detectable A3A in the nucleus of PNPase-depleted
cells. This observation is consistent with a prior report of
endogenous A3A localization to the cytoplasm following IFN-
α treatment of the cell line THP1 (67). However, given the
strong genetic dependence of γ-H2AX accumulation here on
A3A following PNPase knockdown and cytoplasmic mtdsRNA
accumulation, we hypothesize that a low level of induced A3A
is able to diffuse through nuclear pores (due to its small size),
deaminate single-stranded regions of chromosomal DNA, and
trigger DNA breaks as evidenced by elevated levels of nuclear
γ-H2AX foci.
In addition to the robust A3A upregulation observed upon
knockdown of PNPase, A3B was also significantly induced,
although to a much lower extent. A3B has also been found to
deaminate genomic DNA, and it is also a major source of
mutations in cancer and is found at much higher levels in the
nuclear compartment of a wide range of tumors and cancer
lines (22–24, 63, 66). Although the majority of DNA
cell
APOBEC3A upregulation by mitochondrial dsRNA
damage observed here following PNPase depletion is depen-
dent on A3A, A3B is predominantly nuclear with direct access
to chromosomal DNA and, thus, also able to contribute to the
overall landscape of APOBEC signature mutations observed in
cancer.
Here, we propose that sporadic induction of A3A caused by
mitochondrial stress and cytoplasmic dsRNA accumulation
over the course of human lifetime may contribute to the
overall burden of DNA damage and mutation accumulation in
cancer. To investigate the volatility of A3A upregulation due to
mitochondrial dysfunction, the kinetics of A3A induction
following the depletion of PNPase were investigated. The in-
duction of A3A is transient in nature with A3A peaking at 72 h
after knockdown of PNPase and returning from a 15-fold to a
2-fold induction after an additional 24 h. Thus, the transient
induction of A3A by the dysregulation of endogenous nucleic
acids could be responsible for some of the proposed episodic
bursts of A3A mutagenesis observed in cancer (8, 71).
In the context of both A3A and A3B, episodic mutagenesis
by A3A may cause “mutational flares” and A3B may contribute
to a continuous “mutational smolder,” which together account
for the overall landscape of APOBEC signature mutations in
cancer. Because of its potent deaminase activity and capacity to
damage the genome, A3A is tightly regulated and only induced
in response to infection, inflammation, and other stresses to
the cell including mitochondrial dysfunction as shown here.
Thus, the transient upregulation of A3A during these condi-
tions could lead to nuclear DNA damage and mutation
accumulation. However, A3B, which is nuclear and often
expressed at much higher levels in tumors, may result in a
continuous but slower accumulation of APOBEC3 signature
mutations to the nuclear genome over time. Thus, A3A and
A3B can together explain the bulk of the overall APOBEC
mutation signature
and the mechanism
described here through endogenous dsRNA may be particu-
larly relevant to tumor types with nonviral, nonchronic, or
otherwise unclear etiologies.
cancer
across
Experimental procedures
Cell culture
MCF10A cells were cultured in Dulbecco’s modified Eagle’s
medium/F12 (Thermo Fisher Scientific #11320033) supple-
mented with 5% horse serum (Sigma-Aldrich #H1270), 20 ng/
ml EGF (Peprotech #AF-100-15), 0.5 μg/ml hydrocortisone
(Sigma #H0888), 100 ng/ml cholera toxin (Sigma #C8052),
10 μg/ml insulin (Sigma #91077C), and 1% penicillin/strep-
tomycin (Thermo Fisher Scientific #15140122). HeLa and
A549 cells were cultured in Dulbecco’s modified Eagle’s
medium (Thermo Fisher Scientific #SH30022FS) supple-
mented with 10% fetal bovine serum (Life Technologies
#1043702) and 1% penicillin/streptomycin (Thermo Fisher
Scientific #15140122). Cells were maintained at 37 (cid:1)C and 5%
control (which was set to 1) and was normalized to TBP. Mean values ± SEM of three independent experiments (**p ≤ 0.01 by Student’s t test and not shown
if insignificant). G, RT-qPCR analysis of A3A in A549 cells treated with siCtrl/siPNPase and transfected with plasmids encoding NS2A, NS4B, or GFP. Expression
refers to mRNA fold change relative to the negative control (which was set to 1) and was normalized to TBP. Mean values ± SEM of two independent
experiments (*p ≤ 0.05, **p ≤ 0.01 by Student’s t test and not shown if insignificant). RT-qPCR, reverse transcription-quantitative PCR.
J. Biol. Chem. (2023) 299(9) 105073 7
APOBEC3A upregulation by mitochondrial dsRNA
A
B
C
E
D
F
Figure 4. DNA damage induced by the upregulation of PNPase is A3A dependent. A, Reverse transcription-quantitative PCR analysis of A3A, A3B, and
PNPase in MCF10A cells treated with siCtrl/siPNPase after 24, 48, 72, and 96 h (mean ± SEM of three independent experiments). B, immunoblot analysis of
whole cell, cytoplasmic, and nuclear fractions of MCF10A cells treated with 10 nM siCtrl/siPNPase for 72 h or DMSO/25 ng/ml PMA for 24 h (C and D)
representative immunofluorescence microscopy images of γ-H2AX in control or A3A knockout cells treated with siCtrl/siPNPase with quantification of the
number of γ-H2AX foci per nucleus (the scale bar represents 10 μm; mean ± SEM of n > 50 cells per condition; *p ≤ 0.05 by Student’s t test and not shown if
insignificant). E, immunoblot of A3A in two control (lacZ) clones and in an A3A knockout clone following 24 h of stimulation with 25 ng/ml PMA. F, DNA
sequence of the CRISPR-disrupted A3A alleles in MCF10A cells (5/10 sequenced plasmids had allele 1 and 5/10 allele 2). Frameshift-induced premature stop
codons are highlighted in yellow, and insertions, deletions, and substitutions are shown in red. DMSO, dimethyl sulfoxide; PMA, phorbol 12-myristate 13-
acetate.
8 J. Biol. Chem. (2023) 299(9) 105073
APOBEC3A upregulation by mitochondrial dsRNA
Figure 5. Working model for A3A upregulation by endogenous dsRNA. Mitochondrial dsRNAs that accumulate inappropriately in the cytosol are sensed
by RIG-I, which signals through the adaptor protein MAVS and leads to a type I interferon response, induction of A3A by STAT2, and chromosomal DNA
damage. Three distinct panels are shown to illustrate the fact that interferon signaling can act in both cis and trans (autocrine and paracrine).
CO2. MCF10A cells were purchased from Horizon, and
A549 cells, HeLa cells, and 293T cell lines were obtained from
the American Type Culture Collection (ATCC).
RNA interference
See Table S1 for all oligonucleotide sequences including
siRNA sequences. Duplex siRNAs (IDTDNA) were resus-
pended at 20 μM in nuclease-free duplex buffer (IDTDNA
#11-01-03-01), and cells were treated at a final concentration
of 10 nM. siRNAs were reverse transfected using Lipofect-
amine RNAiMAX (Thermo Fischer Scientific #13778150) in
OptiMEM (Thermo Fischer Scientific #31985062). RNAiMAX
was used at a ratio of 5 μl to 1 μl of 20 μM siRNA. To transfect
plasmid DNA and siRNAs concurrently, TransIT-X2 (Mirus
#MIR6000) was used to transfect the plasmid DNA and
RNAiMAX was used to transfect the siRNA 24 h later.
Transfections of siRNAs were completed in antibiotic-free
medium for 72 h before harvesting. siRNA transfection effi-
ciency was assessed using TYE 563 (IDTDNA #51-01-20-19),
and knockdown of desired proteins was evaluated via immu-
noblot and RT-qPCR analysis.
CRISPR knockout cells
MCF10A cells engineered by CRISPR to lack MAVS and
STAT2 were described recently (25, 72). See Table S1 for all
oligonucleotide sequences including gRNAs. The construct
encoding the gRNA was created by cloning the gRNA into the
LentiCRISPR1000 (73) plasmid via Golden Gate cloning using
the Esp3I sites. Virus was created using HEK-293T cells
(ATCC) transfected with LentiCRISPR1000 plasmids encoding
the gRNA, gag, and VSVG. The gRNA for the RIG-I knockouts
was 50- GCGCCTGGACAATGGCACCT-30, and the gRNA
the A3A knockouts was 50- GAAAAACAACAAGG
for
GCCCAA-3’. MCF10As were then transduced and selected
with puromycin (1 μg/ml) (Gold Biotechnology #P-600-500)
after 24 h. Surviving cells were single cell cloned in a 96-well
plate and grown until 80% confluent. Cells were maintained
in 1 μg/ml puromycin for all subsequent passages. Knockout of
the target gene was verified by immunoblot and pJet
sequencing (Thermo Fisher Scientific #K1231) of the target
region. After harvesting genomic DNA from the cells, primers
were used to amplify 200 bp surrounding the target sequence
on each end (50-GATGCTCGGTGTGGTAGGAG-30 and
50-CCCTGAGTCCTCAGATCCCA-30
for A3A), which was
then cloned into a pJet vector using the CloneJet PCR Kit
(Thermo Fisher Scientific #K1231). Ten different plasmids
were confirmed using Sanger DNA sequencing (GeneWiz) for
each gene.
Quantitative reverse-transcription PCR
(Roche Life
See Table S1 for all oligonucleotide sequences including
PCR primers. Total RNA was extracted from cells using the
High Pure RNA Isolation Kit
Science
the manufacturer’s
#11828665001) per
instructions. The
total RNA was transcribed into cDNA in a 20-μl reaction
using 50 μM random hexamer primers (50-NNNNNN-30)
(IDTDNA), 1 mM dNTPs (Millipore Sigma #DNTP-RO), 20 U
transcriptor
Science
transcriptase
#3531317001), and 20 U protector RNase inhibitor (Roche Life
Science #3335399001). Quantitative PCR was carried out on a
LightCycler 480 II (Roche Life Science) in technical triplicate
using SsoFast Eva Green Supermix (Bio-Rad #1725200).
(Roche Life
reverse
Immunofluorescence microscopy
See Table S2 for information on all primary and secondary
antibodies. Cells were fixed in 4% formaldehyde (Thermo
Fisher Scientific #28906) for 15 min and permeabilized using
PBS containing 0.2% Triton X-100 (Sigma-Aldrich #T8787).
J. Biol. Chem. (2023) 299(9) 105073 9
APOBEC3A upregulation by mitochondrial dsRNA
Cells were blocked in immunofluorescence microscopy
blocking solution (2.8 μM KH2PO4, 7.2 μM K2HPO4, 5% goat
serum, 5% glycerol, 1% gelatin from cold water fish, 0.04%
sodium azide, pH 7.2) with 0.1% Triton X-100 for 1 h at room
temperature. Cells were incubated overnight at 4 (cid:1)C in primary
antibody, which was diluted in immunofluorescence blocking
buffer. Incubation of cells with fluorophore-conjugated sec-
ondary antibody diluted in immunofluorescence blocking
buffer was completed for 2 h at room temperature, and nuclei
were stained with Hoechst 33342 (1 μg/ml) (Thermo Fisher
Scientific #PI62249). Images were collected at 20× magnifica-
tion (or 10× magnification for Fig. 1B) using a Cytation 5 Cell
Imaging Multi-Mode Reader (BioTek) or an EVOS FL Cell
Imaging System (Thermo Fisher Scientific). For the γ-H2AX
images, a Cytation 5 Cell Imaging Multi-Mode Reader was
used to image five slices that were 2 μM apart, which were then
combined using a maximum intensity projection to create the
final image.
Immunofluorescence microscopy with differential
permeabilization
Immunofluorescence microscopy with differential per-
meabilization was conducted in a manner similar to the
immunofluorescence microscopy protocol listed above. How-
ever, instead of permeabilizing the cells with PBS containing
0.2% Triton X-100, cells were permeabilized with 0.2% digi-
tonin (Sigma-Aldrich #D141) to ensure permeabilization of all
membranes, or 0.02% digitonin to permeabilize only the
plasma membranes, or 0% digitonin to permeabilize none of
the membranes. Blocking was completed in the same immu-
nofluorescence microscopy blocking solution but without the
added 0.1% Triton X-100. The rest of the immunofluorescence
microscopy is the same as the immunofluorescence micro-
scopy protocol for the permeabilization of all membranes with
Triton X-100.
Immunoblotting
See Table S2 for information on all primary and secondary
antibodies. Cells were lysed in 2.5× RSB (125 mM Tris HCl,
20% glycerol, 7.5% SDS, 5% β-mercaptoethanol, 250 mM DTT,
0.05% Orange G, pH 6.8) and boiled for 10 min. Lysates were
run on a 4 to 20% gradient SDS-PAGE gel (Bio-Rad #3450033)
and then transferred to a PVDF membrane (Millipore
#IPFL00010). Membranes were blocked in 5% milk in PBS for
1 h at room temperature. Primary antibody was diluted in 5%
milk in PBS and applied overnight at 4 (cid:1)C. Blots were then
incubated in secondary antibody in 5% milk in PBS
supplemented with 0.1% Tween 20 (Thermo Fisher Scientific
#BP337-500) and 0.02% SDS (Thermo Fisher Scientific
#419530010) for 1 h at room temperature. Blots using the
antibody 5210-87-13 (66) for A3A and A3B, as well as anti-
bodies against MAVS, STAT2, ISG15, and SUPV3L1, utilized
an HRP-labeled anti-rabbit
secondary antibody (Jackson
ImmunoResearch #111-035-144), which was visualized using
SuperSignal West Femto Maximum Sensitivity Substrate
10 J. Biol. Chem. (2023) 299(9) 105073
(Thermo Fisher Scientific #PI34095). Blots were imaged on the
LI-COR Odyssey Fc imaging system (LI-COR Biosciences).
Subcellular fractionation
Approximately 106 cells were pelleted for 2 min at 3000
RPM and then washed in 150 μl cold PBS. Cells were pelleted
again at 3000 RPM for 2 min. The supernatant was then
decanted, and the pellet was resuspended in 90 μl ice-cold
0.1% IGEPAL CA-630 (Sigma-Aldrich #I8896). This resus-
pension was the whole-cell fraction, and a portion (30 μl) of
the resuspension was removed and treated with RSB. The
remaining resuspension was pelleted at 3000 RPM for 5 min at
4 (cid:1)C. The supernatant was the cytosolic fraction, and a portion
(30 μl) of the resuspension was removed and treated with RSB.
The pellet was then washed in 80 μl ice-cold 0.1% IGEPAL
CA-630 (Sigma-Aldrich #I8896) and spun again at 3000 RPM
for 5 min. The supernatant was discarded, and the pellet was
resuspended in 10 μl HED buffer (25 mM Hepes, 15 mM
EDTA, 1 mM DTT, 10% glycerol, pH 7.4). This resuspension
was the nuclear fraction and was subsequently treated with
RSB.
Chemicals and inhibitors
Cells were treated with MitoTracker CMXRos (Thermo
Fisher Scientific #M7512) for 30 min at a concentration of
500 nM in order to stain the mitochondria prior to fixation.
3p-hpRNA/LyoVec (Invivogen #tlrl-hprnalv) was used at 1 μg/
ml for 16 h to stimulate and screen for RIG-I in the control
and RIG-I knockout cells. PMA (Sigma-Aldrich #P1585), a
known inducer of A3A and A3B (63–65), was used at 25 ng/ml
over 24 h. Doxorubicin (Sigma-Aldrich #D1515) was used as a
positive control for DNA damage response by treating cells for
24 h at a concentration of 1 μM.
Data availability
All relevant data are contained within the main article or
supplemental information. Please email [email protected] with
requests for raw data or reagents.
Supporting
information.
information—This
article
contains
supporting
Acknowledgments—We thank Harris
helpful feedback during these studies.
laboratory members
for
Author contributions—C. W., S. A. M., J. T. B., R. S. H. conceptu-
alization; S. A. M., J. T. B., E. F., S. O., E. B., R. B. methodology; C.
W., S. A. M., J. T. B. formal analysis; C. W. investigation; E. F., S. O.,
E. B., R. B. resources; C. W., R. S. H. writing – original draft; C. W.,
S. A. M., J. T. B., E. F., S. O., E. B., R. B., R. S. H. writing – review &
editing; S. A. M., J. T. B., R. S. H. supervision; R. B., R. S. H. funding
acquisition.
Funding and additional information—These studies were supported
by NCI, National Institutes of Health P01 CA234228 (to R. S. H.),
NIAID, National Institutes of Health R37 AI064046 (to R. S. H.),
NCI, National Institutes of Health R37 CA252081 (to R. B.), and
a Recruitment of Established Investigators Award from the Cancer
Prevention and Research Institute of Texas (CPRIT RR220053 to R.
S. H.). J. T. B. received partial salary support from the National
Institute for Allergy and Infectious Diseases (F32-AI147813). C. W.
received part-time support
from the University of Minnesota
Undergraduate Research Opportunities Program (UROP). C. W. is
the Marvin and Christine Ballard Scholar, the Leon Snyder Scholar,
and the Harold Paul Morris Memorial Scholarship holder. S. O. is a
Dr Lorna Calin Scholar and was supported by the Faculty Mentor
Program from the University of California, Irvine. R. S. H. is the
Ewing Halsell President’s Council Distinguished Chair at University
of Texas San Antonio and an Investigator of the Howard Hughes
Medical Institute. The content is solely the responsibility of the
authors and does not necessarily represent the official views of the
National Institutes of Health.
Conflict of interest—The authors declare that they have no conflicts
of interest with the contents of this article.
Abbreviations—The abbreviations used are: APOBEC3, apolipo-
protein B mRNA editing catalytic polypeptide-like 3; IFN, inter-
feron;
polynucleotide
IFN-stimulated
phosphorylase; SBS, single base substitution.
PNPase,
gene;
ISG,
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| null |
10.1038_s41598-019-40420-0.pdf
|
Data Availability
ESBL or carbapenemase gene sequence data that support the findings of this study have been deposited into
GenBank with the accession numbers listed in Table 1. The data that support the descriptive statistical analyses of
sample characteristics and resistance gene presence are available from the corresponding author upon reasonable
request.
|
Data Availability ESBL or carbapenemase gene sequence data that support the findings of this study have been deposited into GenBank with the accession numbers listed in Table 1 . The data that support the descriptive statistical analyses of sample characteristics and resistance gene presence are available from the corresponding author upon reasonable request.
|
opeN
Received: 8 November 2018
Accepted: 11 February 2019
Published: xx xx xxxx
Multi-state study of
Enterobacteriaceae harboring
extended-spectrum beta-lactamase
and carbapenemase genes in U.s.
drinking water
Windy D. tanner1, James A. VanDerslice1, Ramesh K. Goel1, Molly K. Leecaster1,2,
Mark A. Fisher1, Jeremy olstadt3, Catherine M. Gurley4, Anderson G. Morris4,
Kathryn A. seely4, Leslie Chapman5, Michelle Korando5, Kalifa-Amira shabazz6,
Andrea stadsholt7, Janice VanDeVelde7, ellen Braun-Howland8, Christine Minihane8,
pamela J. Higgins9, Michelle Deras10, othman Jaber11, Dee Jette12 & Adi V. Gundlapalli1,2
Community-associated acquisition of extended-spectrum beta-lactamase- (esBL) and carbapenemase-
producing Enterobacteriaceae has significantly increased in recent years, necessitating greater
inquiry into potential exposure routes, including food and water sources. In high-income countries,
drinking water is often neglected as a possible source of community exposure to antibiotic-resistant
organisms. We screened coliform-positive tap water samples (n = 483) from public and private water
systems in six states of the United states for blaCtX-M, blasHV, blateM, blaKpC, blaNDM, and blaOXA-48-type
genes by multiplex pCR. positive samples were subcultured to isolate organisms harboring esBL or
carbapenemase genes. Thirty-one samples (6.4%) were positive for blaCtX-M, esBL-type blasHV or blateM,
or blaOXA-48-type carbapenemase genes, including at least one positive sample from each state. esBL
and blaOXA-48-type Enterobacteriaceae isolates included E. coli, Kluyvera, Providencia, Klebsiella, and
Citrobacter species. the blaOXA-48-type genes were also found in non-fermenting Gram-negative species,
including Shewanella, Pseudomonas and Acinetobacter. Multiple isolates were phenotypically non-
susceptible to third-generation cephalosporin or carbapenem antibiotics. These findings suggest that
tap water in high income countries could serve as an important source of community exposure to esBL
and carbapenemase genes, and that these genes may be disseminated by non-Enterobacteriaceae that
are not detected as part of standard microbiological water quality testing.
Antibiotic-resistant infections are responsible for an estimated 2 million illnesses and 23,000 deaths in the United
States each year1. The rising prevalence of multidrug-resistant bacteria is especially alarming, as infections with
these organisms have led to increasing use of broad spectrum antibiotics such as third and fourth generation
cephalosporin and carbapenem antibiotics1,2. Enzymes such as extended-spectrum beta-lactamases (ESBLs)
and carbapenemases can render these antibiotics ineffective. Enterobacteriaceae harboring these enzymes are
ranked among the most urgent antibiotic resistance threats according to the U.S. Centers for Disease Control and
1University of Utah, Salt Lake city, Ut, USA. 2VA Salt Lake city Healthcare System, Salt Lake city, Ut, USA.
3Wisconsin State Laboratory of Hygiene, Madison, Wi, USA. 4Arkansas Department of Health Public Health
Laboratory, Little Rock AR, USA. 5illinois Department of Public Health, carbondale, iL, USA. 6illinois Department
of Public Health, chicago, iL, USA. 7Illinois Department of Public Health, Springfield, IL, USA. 8Wadsworth center,
Albany, nY, USA. 9Pennsylvania Department of environmental Protection, Harrisburg, PA, USA. 10Weber Basin
Water conservancy District, Layton, Ut, USA. 11Utah Public Health Laboratory, taylorsville, Ut, USA. 12Davis county
Health Department, Clearfield, UT, USA. Correspondence and requests for materials should be addressed to W.D.T.
(email: [email protected])
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www.nature.com/scientificreportsFigure 1. Water sample coliform screening process.
Prevention and the World Health Organization1,3. Additionally, genes encoding these enzymes are often found on
mobile genetic elements that can be transferred horizontally to other bacterial species4.
ESBL- and carbapenemase-producing bacteria are commonly associated with healthcare contact1; however,
community-associated infections have significantly increased in recent years5. Between 2009 and 2011, the occur-
rence of ESBL-producing bacteria in community-associated infections increased from 3.1% to 12.6%, while the
occurrence in hospital-associated infections remained the same5. It is estimated that as many as two-thirds of all
ESBL-producing Enterobacteriaceae infections are community-associated5,6. In 2013, the U.S. Centers for Disease
Control warned that spread of carbapenem-resistant Enterobacteriaceae (CRE) into the community could reason-
ably be expected, as experienced with ESBLs1. A 2017 review on CRE in the community noted that the prevalence
of CRE among U.S. community-associated study samples ranged from 5.6 to 10.8%7.
ESBL and carbapenemase genes are frequently found in Enterobacteriaceae such as Klebsiella pneumo-
niae, Escherichia coli, Enterobacter cloacae, and Citrobacter species4, but can also be found in non-fermenting
Gram-negative species8. These bacteria are most commonly spread via fecal-oral transmission routes, including
direct transmission (e.g. via hands) and indirect transmission (e.g. via the environment). Enterobacteriaceae,
including ESBL- and carbapenemase-producing strains, have been reported in a number of environmental com-
partments including food, animals, surface waters, and drinking water9–11. Studies reporting carbapenemase or
ESBL genes in drinking water have largely been performed in low-income countries9,12. In high income countries,
reports of ESBL- and carbapenemase-producing bacteria in drinking water have been limited to single-cases
or intrinsic genes in nonpathogenic environmental bacterial species10,13, but comprehensive studies are lacking.
In clinical infections, ESBL and carbapenemase genes are most frequently found in Enterobacteriaceae spe-
cies, but these organisms are typically uncommon in chlorinated public drinking water supplies in high income
countries. To potentially increase detection of ESBL and carbapenemase genes in U.S. drinking water, we targeted
water samples testing positive for Enterobacteriaceae (coliform bacteria). The objectives of our study were (1) to
determine whether ESBL- and carbapenemase-producing genes are present in U.S. drinking water samples that
have tested positive for E. coli or total coliform bacteria, (2) to describe the sample and water system characteris-
tics associated with samples testing positive for ESBL or carbapenemase genes, and (3) to determine if the ESBL
and carbapenemase genes are present in viable bacteria isolated from the coliform-positive water samples.
Methods
Water sample collection and coliform testing. Between July 2015 and November 2016, regulatory and
investigational drinking water samples testing positive for E. coli or total coliform bacteria (Enterobacteriaceae)
were acquired from multiple local and state public health laboratories that perform regulatory water quality test-
ing for water utilities in their state or county. Participating laboratories included the state public health and envi-
ronmental laboratories in Wisconsin (99 samples), New York (22 samples), Pennsylvania (69 samples), Illinois
(125 total samples from three laboratories), Arkansas (64 samples), and Utah. Additionally, one water utility and
one county health laboratory in Utah supplied coliform-positive samples (total of 104 Utah samples). Sample
acquisition was opportunistic, and may not have consistently included all coliform-positive samples from each
site. Private well samples were only included in the study if positive for E. coli, to avoid testing a high proportion
of private well samples, which frequently test positive for coliform bacteria. Drinking water samples collected for
regulatory water quality testing are required to be 100 milliliters in volume and must be preserved with sodium
thiosulfate and tested at the laboratory within 30 hours of collection, preferably held at a temperature between 0
and 10 degrees Celsius during transport14.
Upon arrival to each public health laboratory location, drinking water samples were tested using a conven-
tional enzyme-substrate method that indicates the presence of E. coli and total coliform bacteria (Fig. 1)15. The
substrate reagent includes a nutrient medium that creates culture conditions in the water sample and promotes
bacterial growth and has chromogenic and fluorogenic indicators to detect total coliforms and E. coli, respectively.
Samples were tested in either a presence/absence format using the original 100-mL collection vessel, or in a quan-
titative format by dispensing the 100 mL sample into a multi-well tray (Quanti-Tray, IDEXX, Westbrook, ME)
for quantification by the most-probable-number technique. When E. coli and/or coliform bacteria were detected
in cultured presence/absence samples, 1 mL of the positive enriched sample was placed in a sterile cryovial with
Scientific RepoRts | (2019) 9:3938 | https://doi.org/10.1038/s41598-019-40420-0
2
www.nature.com/scientificreportswww.nature.com/scientificreports/1 mL 40% glycerol (final concentration 20% glycerol). If a sealed Quanti-Tray sample was positive, the back of the
tray was disinfected and a sterile syringe was used to extract the culture liquid from positive wells. When multiple
wells indicated coliforms, a composite 1 mL of enriched sample was produced by extracting liquid from several
wells, and the composite was mixed with glycerol as described above. All samples were cryogenically frozen, and
samples from locations other than Utah were shipped on dry ice overnight to the research laboratory in Salt Lake
City, Utah.
Resistance gene detection and pCR amplicon sequencing. Upon arrival to the research laboratory,
preserved samples were screened for the three ESBL genes most common in the U.S. (blaSHV, blaTEM, and blaCTX)
by multiplex PCR, as previously described16. Samples were also screened for the three most common carbapen-
emase genes (blaOXA-48-type, blaNDM, and blaKPC) by a separate multiplex PCR17. Primer sequences are available
in Supplemental Table 1. Four microliters of the preserved sample culture was directly used as a template for the
PCR in a total reaction volume of 25 µL. Positive controls included American Type Culture Collection isolate
BAA-2146 for blaSHV, blaTEM, blaCTX-M, and blaNDM detection, a clinical KPC-producing K. pneumoniae for blaKPC
detection, and an OXA-48-producing Shewanella for blaOXA-48 detection. PCR products were run on a 1% agarose
gel, and amplicon from samples putatively positive for any of the genes of interest were sequenced by the Sanger
method using the forward and reverse primers used for PCR screening. If samples were positive for multiple
genes, PCR was repeated with individual primer pairs in separate reactions, followed by amplicon sequencing.
Sequences were searched against the Comprehensive Antibiotic Resistance Database (CARD) and the GenBank
(BLASTn) database to confirm the presence of one of the six target ESBL or carbapenemase genes. SHV and TEM
variants identified by these databases were compared against the Lahey Clinic designation of ESBL-type blaSHV
and blaTEM genes18. Lack of specificity of the multiplex CTX-M primer can result in amplification of several other
16; as a result, chromatograms from all CTX-M –positive amplicon sequences
beta-lactamase genes, such as blaOXY
were closely inspected to see if multiple genes may have been present in the sample. In the case of mixed chroma-
tograms, samples were tested as described above using a CTX-M group-specific multiplex PCR19 (Supplemental
Table 1). Samples confirmed as positive for any of the target genes by Sanger sequencing were subsequently
tested for the specific gene using primers that amplified larger segments of the genes to better identify the specific
alleles20–23.
Bacterial isolation and identification. Bacteria carrying the target resistance genes were isolated through
selective and non-selective culture of the preserved samples as previously described24,25. Samples confirmed for
the presence of blaTEM, blaSHV, or blaCTX-M genes were plated on CHROMagar OrientationTM agar plates (DRG
International, Springfield, NJ) with and without a proprietary ESBL supplement. Approximately 50 µL of the
preserved sample was spread onto each plate, and cefotaxime (30 µg), ceftazidime (30 µg), and aztreonam (30 µg)
disks were placed on the non-selective CHROMagar plates. After overnight incubation, colonies on the ESBL
CHROMagar plate and colonies falling within a distinct zone forming around the disks on the non-selective plate
were tested by PCR for blaTEM, blaSHV, or blaCTX-M genes using the primer set from the ESBL multiplex PCR.
For samples confirmed as having a blaOXA-48-type gene, 50 µL of preserved sample was spread onto an mSuper-
CARBATM plate (DRG International, Springfield, NJ) and a non-selective CHROMagar OrientationTM plate (DRG
International, Springfield, NJ) with a temocillin disk (Rosco, Taastrup, Denmark). In cases where the blaOXA-
48-type producer could not be isolated by direct plating, 20 µL of the original culture sample was added to a Luria
broth with 0.5, 1, 2, and 4 µg/mL imipenem. Broths testing positive by PCR were plated as described above and to
sheep’s blood agar with 0.125 µg/mL ertapenem. Colonies growing on the mSuperCARBATM plate and colonies
growing within the distinct zone that formed around the temocillin disk were tested for blaOXA-48-type genes using
the OXA-48 primer set from the carbapenemase gene multiplex PCR and confirmed by Sanger sequencing of the
amplicon. Bacterial species identification was performed by MALDI-TOF (Biotyper, Bruker, Bellerica, MA) using
the full spectral library which is composed of spectra representing roughly 2750 species of microorganisms from
approximately 470 genera26.
susceptibility testing. Minimum inhibitory concentrations of isolates were determined using Thermo
ScientificTM SensititreTM Extended Spectrum Beta-lactamase plates (Trek Diagnostic Systems, Inc., Independence,
OH) according to manufacturer’s directions. For isolates in which the genes were lost upon subculture, fresh
colonies from the original sample were used in preparation of the antimicrobial susceptibility testing inoculum,
if sufficient growth was present.
supplementary sample data. Laboratories supplying coliform-positive samples also provided limited
data on sample characteristics such as water source (e.g. groundwater, [treated] surface water, or blended),
sample type (regulatory or investigational), and system type (e.g. community public water system [CPWS],
non-community public water system [NCPWS], or private well, as defined by the U.S. Environmental Protection
Agency)27. Sample results were described with respect to these characteristics. Statistical analyses to test specific
hypotheses or make statements of inference would have been inappropriate because the sample collection was
opportunistic.
Results
sample characteristics. A total of 483 non-duplicate samples, representing 361 public and private water
systems, were collected. Of the 483 samples, 27% were from private water systems, 31% were from CPWSs, and
42% were from NCPWSs. Spring water sources comprised 5% of the samples, while surface and ground water
sources made up 9% and 78%, respectively. Approximately 6% of the samples were blended, meaning that they
were from systems where both groundwater and treated surface water were supplied to consumers during periods
of high demand.
Scientific RepoRts | (2019) 9:3938 | https://doi.org/10.1038/s41598-019-40420-0
3
www.nature.com/scientificreportswww.nature.com/scientificreports/pCR detection of esBL and carbapenemase genes and amplicon sequencing. The 483 sam-
ples were screened for blaTEM, blaSHV, blaCTX-M, blaKPC, blaNDM, and blaOXA-48-type genes. Sixty-four samples
appeared to be positive for blaCTX-M by initial PCR and agarose gel screening; however, comparison of PCR
amplicon sequences to the GenBank database revealed that the majority of the putative blaCTX-M positives were
attributed to extended-spectrum beta-lactamase genes that are typically chromosomally-encoded in various
Enterobacteriaceae species, including blaRAHN (Rahnella aquatilis), blaFONA (Serratia fonticola), blaOXY (Klebsiella
oxytoca), blaSMO (strain RUS)28, and Citrobacter amalonaticus class-A beta lactamase. Because these were not
target genes in the study, we did not pursue any further analysis of these samples.
Thirty-one samples (6.4%) from twenty-six (7.2%) of the water systems were positive for the target blaCTX-M,
blaOXA-48-type, or ESBL-type blaTEM or blaSHV genes. Thirteen of the samples (2.7%) from twelve different water
systems (3.3%) were confirmed to have blaCTX-M genes based on amplicon sequences. Thirteen (2.7%) and ten
(2.1%) samples were positive for blaSHV and blaTEM genes, respectively. One SHV-positive amplicon sequence most
closely matched an ESBL-type blaSHV (blaSHV-38) and one TEM-positive amplicon sequence closely matched both
ESBL- and non-ESBL-type blaTEM genes. The carbapenemase multiplex PCR screen did not reveal any NDM- or
KPC-positive samples; however, blaOXA-48-type genes were detected in 19 samples (3.9%) from 15 (4.2%) of the
water systems. The samples with blaOXA-48-type genes were from four states and represented between 2–7% of
samples from those states. Three of the samples that tested positive for blaOXA-48-type genes were also positive
for blaCTX-M genes and were each from different states. At least one ESBL or carbapenemase gene was detected
in all six states comprising between 1% and 15% of the coliform-positive samples tested from each state. Of the
185 water systems with repeat samples, six had samples that tested positive for at least one ESBL or carbapene-
mase gene; half of these water systems had only one positive sample and one water system had all three samples
positive.
Isolation of esBL- or carbapenemase-producing bacteria and antimicrobial susceptibility testing.
Bacteria carrying target ESBL or carbapenemase genes were isolated from 21 of the 31 positive samples (Table 1).
The blaCTX-M gene was detected in Klebsiella oxytoca, Citrobacter freundii complex, Kluyvera ascorbata and
Kluyvera georgiana, the latter two being the progenitor of blaCTX-M genes. The ESBL-type blaSHV-38 gene was pres-
ent in Klebsiella pneumoniae and a blaTEM variant gene exhibiting an ESBL phenotype was present in E. coli.
Species carrying blaOXA-48-type genes included E. coli, Providencia rettgeri, Acinetobacter baumannii complex,
Pseudomonas putida, Pseudomonas koreensis, Pandoraea sputorum, Shewanella putrefaciens, and other Shewanella
(n = 3) and Pseudomonas (n = 1) that could not be identified at the species level. The stability of ESBL and blaOXA-
48-type genes were noted based on the number of subcultures where the resistance gene could still be detected in
culture. The ESBL genes were very stable and were still detected after two or more subcultures. The blaOXA-48-type
genes were typically lost after one subculture in non-Shewanella non-fermenting Gram-negative rods, and after
two or more subcultures in Enterobacteriaceae, with or without selective pressure from imipenem concentrations
ranging from 0.125 ug/mL to 4 ug/mL. Nucleic acid sequences of the PCR amplicon from isolates and from pos-
itive samples where an isolate could not be recovered are available in GenBank (Table 1). Antimicrobial suscepti-
bility testing results for selected isolates are presented in Table 2.
Resistance gene presence and tap water source characteristics. The blaCTX-M, blaOXA-48-type, or
ESBL-type blaTEM or blaSHV genes were found in 5.6% of samples from ground water sources and 7.0% of samples
from treated surface water sources, and from 10.0% of private and 5.1% of public water systems: 8.7% of CPWS
and 2.5% of NCPWS samples. Overall, 6.8% of the coliform-positive samples from public water systems also
tested positive for E. coli; however, when considering those samples that tested positive for an ESBL or blaOXA-
48-type gene, 22.2% were positive for E. coli.
Discussion
Community-associated antibiotic-resistant infections caused by ESBL- and carbapenemase-producing bacteria
have increased significantly in the U.S. over the past decade. Many possible transmission routes have been stud-
ied; but in high-income countries, drinking water has not been adequately assessed as a potential source. We
detected ESBL genes (blaCTX-M, blaTEM, or blaSHV) or blaOXA-48-type carbapenemase genes in more than 6% of the
coliform-positive U.S. drinking water samples screened. Coliform-positive drinking water samples were targeted
for testing because ESBL and carbapenemase genes are most commonly found in Enterobacteriaceae in clinical
settings.
Non-E. coli coliforms, such as Klebsiella, Citrobacter, Enterobacter, and Serratia species are considered to be
non-pathogenic by regulatory agencies and are primarily used as indicators of potential fecal contamination or
water distribution system breaches. However, it is important to consider that these species are also some of the
most common carriers of ESBL and carbapenemase genes, regardless of pathogenicity24. In 2015, the presence of
coliform bacteria was reported in 1909 U.S. water systems serving over 10 million in total people, although audits
and other compliance reports have found that underreporting of drinking water contaminants in public water
systems is likely a widespread issue29. Coliform bacteria are even more common in private wells, which supply
approximately 15% of the U.S. population with drinking water30.
This study utilized a convenience sample of coliform-positive water samples from state and local public
health and environmental laboratories to increase the potential for finding ESBL- or carbapenemase-producing
Enterobacteriaceae, not to estimate the overall frequency of occurrence of these genes in U.S. water supplies. The
study design limits the conclusions that can be drawn regarding the overall prevalence of these genes in U.S.
water systems and limits access to some information associated with individual samples, such as the type of water
treatment used by the water systems. There is also a slight possibility that some coliform-positive samples resulted
Scientific RepoRts | (2019) 9:3938 | https://doi.org/10.1038/s41598-019-40420-0
4
www.nature.com/scientificreportswww.nature.com/scientificreports/Gene variant or group
with highest DNA
sequence similaritya
Sample ID
CTX-M-205
CTX-M-205
CTX-M-40 or
CTX-M-63
Gene
homology
with
reference
sequence
Protein
homology
with
reference
sequence
79%
80%
84%
86%
88%
94%
GenBank Nucleotide
Reference (GenBank
Protein reference)
MG028655 (ATJ25945.1)
MG028655
Organism
Citrobacter freundii-
complex
Citrobacter freundii-
complex
E. coli
present/
absent in
sample
System typeb Water source
GenBank
Accession
Number
Absent
CPWS
Ground
MG560059
Present
CPWS
Ground
MG560060
NG_048991 NG_049014
Kluyvera georgiana
Present
Private
Ground
MG560061
OXA-181
100%
100%
CP023897
Acinetobacter baumannii
complex
Present
Private
Ground
MG560077
CTX-M-40 or
CTX-M-63
CTX-M-40 or
CTX-M-63
CTX-M-9 family
CTX-M-10 or
CTX-M-34
CTX-M-40 or
CTX-M-63
CTX-M-12
OXA-48b
OXA-48b
OXA-181
OXA-48b
CTX-M-3
OXA-181
OXA-48b
OXA-252
OXA-48b
OXA-48b
TEM-1 variante
98%
84%
86%d
100%
96%
100%
100%
100%
85%d
100%
100%
85%
100%
100%
99%
99%
99%
OXA-48b or OXA-252
100%
CTX-M-3
CTX-M-2-like
OXA-48b
OXA-252
OXA-199
CTX-M-36
OXA-48b
OXA-48b
OXA-252
OXA-48b
OXA-48b
OXA-252
SHV-38
99%
89%
100%
100%
99%
99%
100%
100%
100%
100%
100%
100%
100%
95%
100%
100%
100%
93%
100%
100%
94%
100%
100%
99%
99%
99%
100%
99%
96%
100%
100%
99%
100%
100%
100%
100%
100%
100%
100%
100%
99%
94%
95%
NG_048991 NG_049014
Kluyvera georgiana
Absent
CPWS
Ground
MG560062
NG_048991 NG_049014
Kluyvera georgiana
Absent
CPWS
Ground
MG560063
NG_049028.1
Not Isolated
Absent
CPWS
Surface
MG560055
100%
NG_048898 NG_048984
Kluyvera species
Present
Private
Ground
MG560064
NG_048991 NG_049014
Not Isolated
Present
Private
Unknown
MG560056
DQ821704
KU820807
KU820807
KJ620504
KU820804
KU200455
KJ620504
KU820804
CP022089
KC902850
KC902850
Not Isolated
Not Isolated
E. coli
E. coli
Shewanella species
Klebsiella oxytoca
Not Isolated
Shewanella species
Not Isolated
Present
Present
Present
Present
Present
Present
Present
Present
Present
Private
Private
Private
Private
Private
Private
Private
Private
Private
Shewanella putrefaciens;
Absent
NCPWS
Pandoraea sputorum
Absent
NCPWS
KU664682.1
E. coli
KU820804 NG_050608.1
Shewanella species
Y10278
KX377894
KU820807
NG_050608
NG_049495
NG_048986
KU820807
KU820807
NG_050608
KU820802
KU820807
KU820800
NG_050077
Not Isolated
Kluyvera ascorbata
Providencia rettgeri
Not Isolated
Not Isolated
Klebsiella oxytoca
Not Isolated
Not Isolated
Pseudomonas species
Not Isolated
Pseudomonas koreensis
Pseudomonas putida
Present
Present
Absent
Absent
Absent
Private
Private
CPWS
CPWS
NCPWS
Present
NCPWS
Present
NCPWS
Absent
Absent
Absent
Absent
Absent
Absent
Absent
CPWS
CPWS
CPWS
CPWS
CPWS
CPWS
CPWS
Klebsiella pneumoniae
Present
NCPWS
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Surface
Ground
Ground
Ground
Ground
Blendf
Blendf
Blendf
Blendf
Blendf
Blendf
Blendf
Ground
MG560057
MG560068
MG560078
MG560079
MG560080
MG560065
MG56006
MG560081
MG560070
MG560082
MG560083
MG560088
MG560084
MG560058
MG560066
MG560085
MG560071
MG560072
MG560067
MG560073
MG560074
MG560075
MG560076
MG560086
MG560087
MG560089
A1c
A2c
A3a
A3b
A4
A5
A6
B1
B2
B3a
B3b
B4
B5
B6
B7
B8
B9
B10
B11a
B11b
B12
B13
C1
C2
D1c
D2c
D3c
E1a
E1b
E2c
E3
E4
E5c
E6c
F1
Table 1. Characteristics of sources of samples positive for blaCTX-M, blaOXA-48-type, and putative blaSHV or blaTEM
genes; aGene variant or cluster with greatest homology in GenBank; bCPWS: Community public water system,
NCPWS: Non-community public water system; cSamples collected from the same water systems: A1 and A2;
D1, D2, and D3; E2, E5, and E6; d100% nucleotide homology to uncultured bacterium clones; eGene sequence
is similarly homologous with both ESBL and non-ESBL blaTEM genes, but exhibits an ESBL phenotype fBlended
water is a composite of ground water and treated surface water.
from accidental contamination during original sample collection; however, regulatory samples are typically col-
lected by public water system personnel trained in proper sample collection technique.
The true prevalence of the target ESBL and carbapenemase genes in these samples may be underestimated
due to testing limitations. A very small proportion of the preserved water sample culture was tested, potentially
missing ESBL and carbapenemase genes present in low copy numbers in the samples. Additionally, DNA was not
extracted from the preserved samples prior to PCR testing, and PCR inhibitors may have been present, affecting
gene detection. We were also only able to determine presence or absence of the target resistance genes and could
not assess the abundance in the original water sample due to the culture step in the coliform screening process.
Scientific RepoRts | (2019) 9:3938 | https://doi.org/10.1038/s41598-019-40420-0
5
www.nature.com/scientificreportswww.nature.com/scientificreports/Sample
ID
A1
A2
A3a
A3b
A4
A5
B1
B4
B5
B6
B7
B11
B12
B13
C2
D1
E1a
E3
F1
Organism (gene type)
C. freundii-complex (CTX)
C. freundii-complex (CTX)
K. georgianab (CTX)
No
No
No
A. baumannii complex (OXA-48) Yes
K. georgianab (CTX)
K. georgianab (CTX)
Kluyvera speciesb (CTX)
Yes
No
No
E. coli (OXA-48)
E. coli (OXA-48)
Shewanella speciesb (OXA-48)
K. oxytoca (CTX)
S. putrefaciensb (OXA-48)
E. coli (TEM)
Shewanella speciesb (OXA-48)
K. ascorbatab (CTX)
P. rettgeri (OXA-48)
K. oxytoca (CTX)
Pseudomonas speciesb (OXA)
K. pneumoniae (SHV)
Yes
Yes
No
No
Yes
Yes
No
No
No
No
No
Yes
ESBL
pheno-typea TAZ
FOT
FAZ
FEP FOX CEP
POD
AXO IMI MEM GEN AMP CIP P/T4
≤1
16*
>16* ≤1
≤1
≤0.25 ≤0.25
≤0.25
0.5
≤0.25 ≤0.25 >16
8
>16
8
≤0.25 ≤0.25 >16
0.5
32
8
8
≤0.25
1
0.5
0.5
2
8
4
2
>16
>16
16
≤8
>16
≤0.25 ≤8
>16
2
8
≤8
8
≤0.25 ≤0.25 >16
≤0.25 ≤0.25 >16
≤0.25 >16*
1
0.5
0.5
1
64
2
4
≤8
>16
≤8
8*
8*
≤4
64
≤4
≤4
8
64
>16* 1
>16* 1
>16
2
>16
>16
>16
>16
>16
8
1
2
16
0.5
2
≤1
≤1
8
2
>16 ≤4
≤4
≤1
1
>16
>16 ≤0.25
≤1
≤1
>16
≤1
≤1
2
≤1
≤1
≤1
≤4
8
8
≤4
≤8
≤8
≤0.25
≤0.25
>16
16
>16 ≤1
64
≤4
>16
1
>16* ≤0.25
≤4
>16
4
>64 >16
2
≤4
>16
4
≤1
≤1
≤1
8
≤1
2
4
≤1
≤1
≤1
≤1
64
4
≤1
≤1
≤1
16
8
≤1
1
≤1
≤0.5 ≤1
≤0.5 ≤1
≤0.5 ≤1
≤1
4
≤0.5 ≤1
≤0.5 ≤1
≤0.5 ≤1
≤1
1
≤0.5 >8
≤0.5 ≤1
≤0.5
4
≤0.5 ≤1
≤0.5
2
≤0.5 ≤1
≤0.5 ≤1
≤0.5 ≤1
≤0.5 ≤1
≤0.5 ≤1
≤4
≤4
≤4
≤4
≤4
≤4
≤4
8
≤4
≤4
≤4
≤4
≤4
16
≤4
≤4
≤4
≤4
16
32/4
≤8* ≤1
≤1 ≤4/4
16*
>16 ≤1 ≤4/4
≤1
16*
>16 ≤1
64/4
64/4
>16 ≤1 ≤4/4
>16 ≤1 ≤4/4
≤8
≤1 >64/4
≤8
≤1 >64/4
≤1 ≤4/4
16
>16* ≤1 ≤4/4
≤1 ≤4/4
16
>16 ≤1 >64/4
≤1 ≤4/4
≤8
≤8
≤1 ≤4/4
≤8* ≤1 ≤4/4
≤8* ≤1 ≤4/4
>16 ≤1 ≤4/4
>16* ≤1 >64/4
Table 2. Minimum inhibitory concentrations (MIC) (µg/mL) of isolates to selected antibiotics by broth
microdilution. TAZ: ceftazidime, FOT: cefotaxime, FAZ: cefazolin, FEP: cefepime, FOX: cefoxitin, CEP:
cephalothin, POD: cefpodoxime, AXO: ceftriaxone, IMI: imipenem, MEM: meropenem, GEN: gentamicin,
AMP: ampicillin, CIP: ciprofloxacin, P/T4: piperacillin/tazobactam. Cells marked with a * indicate antibiotics
to which the organism is considered intrinsically resistant according to the 2019 Clinical and Laboratory
Standards Institute (CLSI) M100 ED29:2019 document34; aAn ESBL phenotype is defined as an bacterial
phenotype that exhibits a ≥3 two-fold concentration decrease in a MIC for ceftazidime or cefotaxime tested in
combination with clavulanate vs the MIC of ceftazidime or cefotaxime when tested alone. Although the result is
listed for all organisms in Table 2, this test is intended for Klebsiella pneumoniae, Klebsiella oxytoca, E. coli, and
Proteus mirabilis per CLSI m10034; bIndicates organisms for which an intrinsic resistance profile is not available
in the CLSI m100 document.
ESBL and carbapenemase genes other than the target genes were also clearly present in these water samples.
Some of these genes may have been carried by the bacterial isolates in addition to the target ESBL or carbap-
enemase genes that were detected, potentially contributing to any observed phenotype. The majority of these
non-target genes are known to be chromosomally-located and carried by nonpathogenic bacterial species; how-
ever, some, such as blaOXY, can be plasmid-mediated, and may still be a cause for clinical concern31. The blaCTX-M
and blaOXA-48-type genes also arise from relatively nonpathogenic bacteria that are commonly found in water
sources (Kluyvera and Shewanella species, respectively), but have been widely disseminated to other bacterial
species via mobile genetic elements. In this study, blaOXA-48-type genes in non-Shewanella species were lost in
subsequent subcultures, a phenomenon also observed with non- fermenting Gram-negative rods carrying the
blaNDM-1 carbapenemase gene isolated from New Delhi drinking water9.
Bacterial isolates were identified using the full spectral Bruker MALDI-TOF library used by diverse labo-
ratories. Despite the broad organism coverage, it is possible that some environmental species may have limited
representation in this system. In our study, all of the blaCTX-M carriers were identified as Enterobacteriaceae, with
approximately half being classified as Kluyvera species. Enterobacteriaceae carrying blaOXA-48-type genes were also
isolated from some water samples. The blaOXA-48-like genes were also found in non-Enterobacteriaceae species,
such as Shewanella, Acinetobacter, and Pseudomonas. The coliform species harboring resistance genes would
trigger a positive coliform water test, which would theoretically be followed by attempts to remediate the contam-
ination issue; however non-Enterobacteriaceae species harboring ESBL and carbapenemase genes would evade
detection by the most commonly used coliform screening methods resulting in “silent” dissemination of these
genes via tap water that meets all current regulatory requirements.
Studies reporting ESBL- and carbapenemase-producing bacteria in drinking water in high-income coun-
tries are extremely rare. CTX-M-producing E. coli was discovered in drinking water in France in a single water
sample10, and carbapenemase-producing Serratia fonticola has previously been reported in drinking water from
Portugal32, as have non-fermenting intrinsic carbapenemase-producers13. To our knowledge, this is the first
extensive study of drinking water from multiple regions in a high income country that has revealed the geograph-
ically widespread distribution of ESBL- and carbapenemase-producing isolates of serious clinical concern. Our
findings suggest that community exposure to these organisms may be more common than currently realized, and
consequently, their prevalence in the general population may be underestimated.
Scientific RepoRts | (2019) 9:3938 | https://doi.org/10.1038/s41598-019-40420-0
6
www.nature.com/scientificreportswww.nature.com/scientificreports/In this era of increasing antimicrobial resistance, it is critical to determine the public health significance of
antibiotic resistance genes in community drinking water regardless of country income-level classification. Public
water systems provide an effective means by which pathogens and antibiotic-resistant organisms can be transmit-
ted to large segments of the population. In high-income countries, approaches such as increased consumption of
bottled water are not adequate solutions, as bottled water is often derived from the same sources as tap water, and
may also contain trace levels of total coliform bacteria33. More research is needed to better characterize the prob-
lem, understand the associated risk, and devise solutions to combat antimicrobial-resistant bacteria in tap water2.
Data Availability
ESBL or carbapenemase gene sequence data that support the findings of this study have been deposited into
GenBank with the accession numbers listed in Table 1. The data that support the descriptive statistical analyses of
sample characteristics and resistance gene presence are available from the corresponding author upon reasonable
request.
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Acknowledgements
A portion of this work was performed in the laboratory of Catherine Loc-Carrillo, PhD at the Veterans Affairs
Medical Center campus in Salt Lake City, Utah and through resources provided by the Salt Lake City VA IDEAS
center (VA Center of Innovation Award #I50HX001240 from the Health Services Research and Development
of the Office of Research and Development of the US Department of Veterans Affairs). Funding for this project
was provided through the Health Studies Fund of the University of Utah Department of Family and Preventive
Medicine and through internal University Seed Funding. We are grateful for the outreach efforts of Sarah
Wright and the Association of Public Health Laboratories (APHL) in finding participating APHL member
laboratories. We also appreciate the assistance of the Arkansas Department of Health Public Health Laboratory,
Illinois Department of Health laboratories in Chicago, Springfield, and Carbondale, Pennsylvania Department
of Environmental Protection Laboratories, Biggs Laboratory at the New York State Department of Health
Wadsworth Center, Wisconsin State Laboratory of Hygiene, Utah Public Health Laboratory, Davis, Utah County
Health Department Laboratory, and the Weber Basin Water Conservancy District laboratory (Utah) in providing
and preserving samples for this project. We are also grateful to the Derek Warner and the University of Utah Core
DNA Sequencing Laboratory for consulting on and assistance in preparations for the PCR amplicon sequencing.
Author Contributions
W.T. performed all PCR testing, bacterial isolation, and susceptibility testing. J.V., R.G. and A.G. participated
in study design and manuscript preparation. M.L. performed all descriptive statistical analysis. M.F. performed
MALDI-TOF testing. Authors from the Arkansas, Illinois, New York, Pennsylvania, Utah, Wisconsin, and
Davis County public health laboratories and the Weber Basin Water Conservancy District assisted in preserving
coliform-positive samples and compiling associated sample data. All authors were involved in compiling and
reviewing data for the report and approved the final version. Authors from the Arkansas, Illinois, New York,
Pennsylvania, Utah, Wisconsin, and Davis County public health laboratories and the Weber Basin Water
Conservancy District all contributed equally.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-40420-0.
Competing Interests: The authors declare no competing interests.
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© The Author(s) 2019
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| null |
10.1088_1748-605x_acf90a.pdf
|
Data availability statement
All data that support the findings of this study are
included within the article (and any supplementary
files).
|
Data availability statement All data that support the findings of this study are included within the article (and any supplementary files).
|
OPEN ACCESS
RECEIVED
10 April 2023
REVISED
29 July 2023
ACCEPTED FOR PUBLICATION
12 September 2023
PUBLISHED
26 September 2023
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Biomed. Mater. 18 (2023) 065009
https://doi.org/10.1088/1748-605X/acf90a
Biomedical Materials
PAPER
Development and optimisation of hydroxyapatite-polyethylene
glycol diacrylate hydrogel inks for 3D printing of bone tissue
engineered scaffolds
Mina Rajabi1, Jaydee D Cabral2, Sarah Saunderson2, Maree Gould1 and M Azam Ali1,∗
1 Centre for Bioengineering & Nanomedicine, Faculty of Dentistry, Division of Health Sciences, University of Otago, PO Box 56,
Dunedin 9054, New Zealand
2 Department of Microbiology and Immunology, University of Otago, PO Box 56, Dunedin 9054, New Zealand
∗
Author to whom any correspondence should be addressed.
E-mail: [email protected]
Keywords: extrusion 3D printing, hydroxyapatite, polyethylene glycol diacrylate, pluronic F127, mesenchymal stem cells,
bone regeneration
Supplementary material for this article is available online
Abstract
In the event of excessive damage to bone tissue, the self-healing process alone is not sufficient to
restore bone integrity. Three-dimensional (3D) printing, as an advanced additive manufacturing
technology, can create implantable bone scaffolds with accurate geometry and internal
architecture, facilitating bone regeneration. This study aims to develop and optimise
hydroxyapatite-polyethylene glycol diacrylate (HA-PEGDA) hydrogel inks for extrusion 3D
printing of bone tissue scaffolds. Different concentrations of HA were mixed with PEGDA, and
further incorporated with pluronic F127 (PF127) as a sacrificial carrier. PF127 provided good
distribution of HA nanoparticle within the scaffolds and improved the rheological requirements of
HA-PEGDA inks for extrusion 3D printing without significant reduction in the HA content after
its removal. Higher printing pressures and printing rates were needed to generate the same strand
diameter when using a higher HA content compared to a lower HA content. Scaffolds with
excellent shape fidelity up to 75-layers and high resolution (∼200 µm) with uniform strands were
fabricated. Increasing the HA content enhanced the compression strength and decreased the
swelling degree and degradation rate of 3D printed HA-PEGDA scaffolds. In addition, the
incorporation of HA improved the adhesion and proliferation of human bone mesenchymal stem
cells (hBMSCs) onto the scaffolds. 3D printed scaffolds with 2 wt% HA promoted osteogenic
differentiation of hBMSCs as confirmed by the expression of alkaline phosphatase activity and
calcium deposition. Altogether, the developed HA-PEGDA hydrogel ink has promising potential as
a scaffold material for bone tissue regeneration, with excellent shape fidelity and the ability to
promote osteogenic differentiation of hBMSCs.
1. Introduction
The limitations of existing bone grafting procedures
and the rising incidences of bone and joint disorders
have necessitated the development of scaffold-based
tissue engineering strategies as an alternative treat-
ment for bone regeneration. For bone tissue engineer-
ing, the characteristics of the three-dimensional (3D)
design along with the material properties of a scaffold
are critical. In this regard, additive manufacturing
(AM), and particularly 3D printing technologies,
have shown promising advantages over conventional
methods (e.g. solvent casting and electrospinning)
for fabricating complex 3D tissue engineered scaf-
folds. The advantages of this technology are the pre-
cise control over the architectural features of the
scaffolds, fabricating customised and patient-specific
constructs with interconnected pores to allow cell
migration and waste and nutrient transportation,
as well as direct printing of various materials such
© 2023 The Author(s). Published by IOP Publishing Ltd
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
as metals, ceramics, cells and biomolecules [1, 2].
3D printing technologies use computer aided design
(CAD) to generate a 3D model, convert it to a
Standard Triangulate Language format, and eventu-
ally print the component through the layer-by-layer
automated deposition of inks onto a substrate [1, 3].
Various biomaterial inks based on polymers, ceram-
ics and their composites have been developed for 3D
printing of bone tissue scaffolds [4–6]. However, low
shape-fidelity of natural-based inks and comprom-
ised biological properties of synthetic-based inks
remarkably limit the number of potential inks for 3D
printing of bone tissue scaffolds [1, 7, 8]. Therefore,
the development of suitable 3D printing inks, which
fulfils both biological requirements and print fidel-
ity satisfying the biofabrication window, is still very
challenging [9].
Hydroxyapatite (HA, Ca10(PO4)6(OH)2) has the
closest composition to natural bone of mammali-
ans among the various calcium phosphate ceramics
(CPCs). HA is thermodynamically the most stable
and least soluble CPC in physiological conditions
after implantation, making it beneficial when long
term healing is expected [3, 10, 11]. In addition, HA
can provide nucleating sites on its surface to pre-
cipitate apatite crystals [12]. HA has been shown to
promote adhesion and proliferation of osteoblasts, as
well as inducing osteogenesis differentiation of MSCs
when studied in vitro and in vivo [13, 14]. However,
like other CPCs, HA is brittle and has low tough-
ness, which restricts its use mainly to filling teeth
and bone, or coating orthopaedic and dental implants
[15]. Therefore, the incorporation of HA in a poly-
meric system is of high interest to produce com-
posite materials with good bioactivity and compress-
ive strength resulting from HA, and good toughness
and biodegradability granted by the polymer matrix
[3, 16]. Polyethylene glycol (PEG), a water soluble
and biocompatible polymer, is one of the most used
biomaterials in various biomedical applications [17].
Polyethylene glycol diacrylate (PEGDA) is a non-
ionic hydrophilic derivative of PEG with two acrylate
groups showing low toxicity, non-immunogenicity,
and antifouling properties [18]. The acrylates allow
free radical photopolymerisation of PEGDA in the
presence of a photoinitiator [19, 20], converting from
a low-viscosity monomer (liquid) to a stable polymer
(solid), thereby forming high-precision and com-
plex geometries. These advantages make PEGDA a
promising candidate in 3D printing for biomedical
applications.
Although a variety of nanocomposites have been
fabricated using HA as the filler and PEGDA as
the polymer matrix, most were prepared by con-
ventional
fabrication methods, exhibiting disad-
vantages such as poor control over geometrical
design and long processing time. A few studies have
focused on using 3D printing techniques to fabric-
ate hydroxyapatite-PEGDA (HA-PEGDA) nanocom-
posites for bone regeneration. For instance, Zhou
et al [21] incorporated HA into PEGDA ink for
fabricating nanocomposite bone scaffolds using a
stereolithography (SLA)-based 3D bioprinter. They
demonstrated that HA containing 3D printed scaf-
folds under low intensity pulsed ultrasound (LIPUS)
treatment promoted mesenchymal stem cells (MSCs)
proliferation, alkaline phosphatase (ALP) activity,
and calcium deposition. Mondal et al [22] used
PEGDA to reduce the overall ink viscosity of acrylated
epoxidized soybean oil (AESO)/HA ink required for
masked stereolithography-based 3D printing. At 10
vol% HA, they reported improved tensile strength,
apatite formation on the nanocomposite surfaces
within 7 d of incubation in simulated body fluid
(SBF), as well as good viability and proliferation of
differentiated mouse pre-osteoblastic cells (MC3T3-
E1) on the nanocomposites. Deng et al [23] applied
continuous liquid interface production (CLIP) for 3D
printing of HA/PEGDA nanocomposites. The incor-
poration of HA resulted in significant improvement
of both compression strength and biocompatibility
of the final 3D prints. However, in these studies often
PEGDA is mixed with HA powder to form a slurry,
and there is a risk of the sedimentation of ceramic
particles, or there is a need for additional support
during printing.
Herein, we explored a new processing method,
i.e., extrusion-based 3D printing, to prepare HA-
PEGDA scaffolds with high resolution. To the best
of our knowledge, there is no study in the literat-
ure that utilises extrusion-based 3D printing for fab-
ricating bone tissue scaffolds based on HA-PEGDA
inks. In order to provide adequate rheological beha-
viour and gelation mechanism for HA-PEGDA inks
required for extrusion-based 3D printing, pluronic
F127 (PF127) was added to the inks as a sacrificial
carrier. PF127 is an amphiphilic and water-soluble
triblock copolymer with hydrophilic block polyethyl-
ene oxide (PEO) and hydrophobic block polypropyl-
ene oxide (PPO) [24]. Above the critical micelle con-
centration, PF127 can go through thermo-reversible
gelation when the temperature increases. We hypo-
thesised that PF127 can be a suitable carrier for HA-
PEGDA inks providing proper distribution of HA
within the PEGDA network before photopolymerisa-
tion, and sufficient shear thinning properties during
processing to 3D print high resolution complex scaf-
folds. For this purpose, a comprehensive investigation
was carried out on the effect of HA content on rhe-
ological properties and printability of HA-PEGDA
hydrogel inks, and the optimal printing parameters
were evaluated. The swelling behaviour, compress-
ive strength, and in vitro degradation of these 3D
printed scaffolds were investigated. Furthermore, the
2
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
cytocompatibility, cell attachment, osteogenic differ-
entiation of human bone mesenchymal stem cells
(hBMSCs), and bone-bioactivity in the presence of
3D printed HA-PEGDA scaffolds were evaluated.
2. Materials
3-(4,5-dimethyl-2-yl)-2,5-diphenyltetrazolium bro-
mide (MTT), foetal calf serum (FCS), and Dulbecco’s
Modified Eagle’s Medium (DMEM)-low glucose were
supplied by Sigma-Aldrich. Penicillin/Streptomycin
(Pen/Strep), TrypLE™ Express Enzyme, and sodium
dodecyl sulphate (SDS) were obtained from Gibco™,
ThermoFisher Scientifics. Trypan blue, propidium
iodide (PI; P-3566), Calcein AM (C3099), and pro-
longed gold antifade mountant were purchased
from Molecular Probes, Invitrogen, ThermoFisher
Scientifics. Dimethyl sulfoxide (DMSO) and para-
formaldehyde were obtained from Merck. PEGDA
(Mn 700 Da), PF127 (Mn 12 600 Da, 70% w/w PEO),
HA (<200 nm nanoparticle size), 2-hydroxy-4′-
(Irga-
(2-hydroxyethoxy)-2-methylpropiophenone
cure 2959), Alizarin red stain were purchased
from Sigma-Aldrich. Magnesium chloride hexahy-
drate (MgCl2.6H2O, ⩾98.5%, CAS 7791-18-6),
sodium hydrogen carbonate (NaHCO3, ⩾99.7%,
CAS 31437), and calcium chloride (CaCl2, 97%,
CAS 10043-52-4) were obtained from ROMIL-
Pure Chemistry, Riedel-deHa¨en, and AppliChem,
respectively.
3. Methods
3.1. Preparation of hydrogel inks
After
(Supplementary
some preliminary studies
data), the hydrogel inks were prepared by dispers-
ing three different concentrations of HA (1, 2 and
5 wt%) and 20 w% PEGDA (700 kDa) in distilled
water. The maximum concentration of HA was set
at 5 wt% because it did not block the dispensing
nozzle. Then, 0.1 wt% Irgacure 2959 was added, and
stirred to make homogenous solutions. Afterwards,
25 wt% PF127 was added, and the hydrogel inks were
stored in the fridge for two days for complete dissol-
ution of PF127. Finally, all hydrogel inks were loaded
into 10 ml cartridges (Polypropylene, UV/light block
amber barrels, Nordson EFD, USA), and allowed to
equilibrate to room temperature for one hour before
printing to initiate the physical gelation of PF127.
3.2. Rheological characterisation of the hydrogel
inks
The effect of adding HA on the rheological properties
of hydrogel inks was investigated using a HR-3 rheo-
meter (Discovery Hybrid rheometer, TA Instrument)
with a 40 mm diameter parallel plate geometry, a
plate-to-plate gap of 150 µm, and a solvent trap to
3
prevent hydrogel drying. To characterise their shear-
thinning properties, G′ and G′′ were recorded as a
function of oscillatory strain sweeps (0.01%–1000%)
at a constant frequency of 10 rad s−1. An oscillat-
ory temperature sweep was used to study the gelling
behaviour of the inks from −5 ◦C to 10 ◦C at a con-
stant temperature step of 1 ◦C. Samples were equilib-
rated for 5 min before testing and for 1 min at each
subsequent temperature to minimise thermal gradi-
ents within the sample. The temperature at which the
elastic modulus made a drastic jump towards a higher
value was recorded as the gel point of the hydrogel. All
measurements were performed within the linear vis-
coelastic region at 25 ◦C and at least by duplicate.
3.3. Optimising and evaluating the shape fidelity of
3D printed scaffolds
In order to optimise the printing conditions, print-
ability of each hydrogel ink was tested by extruding
the ink using a pneumatic bioplotter (BioScaffolder
3.1, GeSiM, Germany) in a 2-layered construct with
grid-like patterns (0/90◦) with adjacent filaments of
210 µm in diameter at inter-spacing of 300 µm using
a 27 gauge dispensing nozzle (Polyethylene Smooth-
flow tapered tips, Nordson EFD, USA). The strand
diameter was measured at the second layer. The z
value of the printing point was specified to be the
printer’s substrate with z value of 0, and nozzle off-
set value of 210 µm (strand diameter) + 160 µm
(height of the petri dish) + 30 µm (error) = 0.4 mm
was specified in the 3D printing process. Printing was
performed at a room temperature of 25 ◦C, pressure
60–100 kPa, printing speed 1–15 mm s−1, and UV
curing for 8 s after printing each layer (OmniCure
Series1500, 365 nm, 25 mW cm−2). The Pr val-
ues of hydrogel inks under different conditions were
determined based on equation (1), which compares
printed area versus those in digital design [25]
Pr =
Ae
At
(1)
where Ae represents the area of printed mesh in exper-
iments determined by images, while At stands for the
area of mesh according to the design, which is the
area of a square hole of 300 mm on each side. An
ideal pore exhibited a square (or rectangular) shape
resulting in Pr = 1, while Pr < 1 and Pr > 1 corres-
pond to a more round or irregular shaped, respect-
ively. The Pr values in the range of 0.9–1.1 were con-
sidered acceptable for the 3D printed hydrogel con-
struct based on the literature [25]. Immediately after
fabrication, 3D printed structures were imaged using
an inverted optical microscope (Olympus IX71 inver-
ted microscope, Japan) using a 4× objective. To meas-
ure the Pr value of each set of printing parameters,
optical images of the scaffolds were analysed using
ImageJ software (ImageJ v.1.51r, National Institutes
of Health, USA). At least 20 squares were measured to
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
determine A mean values. After optimising the con-
ditions, a cylindrical geometry (8 × 1 mm2) embed-
ded with strands in a 0/90◦ pattern in five layers was
designed using BioCAD software and fabricated using
a 27 gauge nozzle.
scaffolds (8 × 5 mm2 in 25-layers) were placed on a
compression flat surface fixture, and a force with a
frequency of 1 Hz and a displacement of 10 mm was
applied by an oscillating plate at the isothermal tem-
perature of 37 ◦C and humidity of 80%.
3.4. Post-printing rinsing process
The post-printing rinsing process was optimised and
performed to remove PF127 from the 3D printed scaf-
folds, thereby improving the cell viability on scaf-
folds (supplementary data). Briefly, the scaffolds were
washed twice with cold phosphate buffered saline
(PBS; pH = 7.4), and left in PBS for 48 h at 4 ◦C with
daily changes of PBS to equilibrate. In addition, prior
to cell studies, scaffolds were immersed in cell culture
media overnight to exchange the PBS with media.
3.5. Thermogravimetric analysis (TGA)
The effect of the post-printing rinsing process on the
HA content of the scaffolds was investigated using a
Q50 TGA analyser (TA Instruments, New Castle, DE,
USA). Approximately 5 mg of air-dried scaffolds (as-
prepared or washed) were loaded in a platinum pan
and heated from 25 ◦C to 900 ◦C at a constant heating
rate of 10 ◦C min−1 under N2 supply with a flow rate
of 10 ml min−1.
3.6. Fourier-transform infrared spectroscopy
(FTIR)
Fourier transform infrared spectroscopy (Varian 610-
IR FTIR, Bruker, USA) was used for the identification
of the chemical structure of 3D printed scaffolds, HA,
and PEGDA monomer. Scans (n = 24) for each FTIR
spectrum were obtained in ATR mode over the region
400–4000 cm−1 with a 4 cm−1 spectral resolution.
3.7. Swelling behaviour
After post-printing rinsing and drying the scaffolds in
the oven at 37 ◦C for 48 h, the initial dry mass, mdry,
was obtained. Then, the scaffolds (N = 5) were incub-
ated in PBS (pH = 7.4) for 48 h at 37 ◦C. The swollen
scaffolds were removed from the PBS and weighed
after blotting off the excess PBS from the sample sur-
face with kimwipes (KIMTECH Science) to obtain
mswollen. The swelling ratio was calculated based on
equation (2):
Swelling ratio =
(
mswollen − mdry
mdry
)
.
(2)
3.8. Dynamic mechanical properties
Dynamic mechanical analysis (DMA) was performed
using a DMA 242 E Artemis (Netzsch, Germany) in
compressive mode for the 3D printed scaffolds with
the aim of evaluating their mechanical properties.
After post-printing rinsing, the scaffolds (N = 3)
remained submerged in PBS to stay hydrated until
the time of testing, which occurred within 1–2 d. The
4
3.9. In vitro degradation
Scaffolds (N = 5) were dried in oven at 37 ◦C for 48 h
to obtain the initial dry mass, minitial. Then, the scaf-
folds were immersed in PBS (pH = 7.4) for 1, 3, 7, 14,
and 21 d. Finally, the scaffolds were removed, oven-
dried at 37 ◦C for 48 h, and weighed to obtain the
final weight after degradation, mdegraded. Degradation
was assessed using equation (3):
Mass percent remaining =
)
(
minitial − mdegraded
minitial
× 100.
(3)
3.10. Biocompatibility assessment
The scaffolds were placed into 96-well plates and
20 µl of human immortalised bone-marrow derived
mesenchymal stem cells (T0523-hTERT, resolving
IMAGES, Australia) were seeded on the scaffolds at
a density of 1.5 × 104 cells/well. Same density of cells
was directly seeded onto the wells for both the pos-
itive control group, where cells were cultured only
with media, and the negative control group, where
cells were treated with 10% SDS. Media with no cells
served as blank. The cells were incubated for 2 h at
37 ◦C to allow the cells to adhere to the scaffold before
the addition of 80 µl of culture media (DMEM sup-
plemented with 10% FCS, 1% penicillin/streptomy-
cin). Scaffolds were not washed after 2 h of cell adhe-
sion to maintain the absolute number of cells across
all the samples so direct comparisons with controls
could be made. After 48 h, the media was discarded
and subsequently, 100 µl of MTT (0.5 mg ml−1)
was added to each well. After 4 h of incubation,
the supernatant of the culture was gently discarded,
and 100 µl of DMSO was added to end the reaction
by dissolving the formazan crystals formed in living
cells. Three replicates were used in three independ-
ent experiments, and absorption was measured at
570 nm using a plate reader (Varioskan Lux, Thermo
Fisher Scientific). The relative cell viability (%) com-
pared to positive control was calculated according to
equation (4):
%Cell viability =
ODsample
ODpositive control
× 100.
(4)
3.11. Live/dead assay
The scaffolds were placed into 96-well plates and
hBMSCs were seeded on the scaffolds at a dens-
ity of 1.5 × 104 cells/well. After 48 h of cell
seeding, the live/dead reagent containing calcein
AM (1.5 µM) and PI (3 µM) was added to the
scaffolds. Data was then acquired using a fluorescence
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
microscopy (Olympus IX71 inverted microscope,
Olympus Corporation, Japan).
3.12. Osteogenic differentiation of hBMSCs
The scaffolds were placed in 48-well plates and 50 µl
of the hBMSCs were seeded on the scaffolds at a
density of 4.5 × 104 cells/well. The cells were incub-
ated at 37 ◦C for 2 h to allow the cells to adhere to
the scaffold before the addition of 250 µl of culture
medium. The normal culture media was changed to
osteogenic media after cells reached 90% confluence
(after 2 d). The osteogenic medium was prepared by
adding 100 nM dexamethasone, 50 µg ml−1 ascorbic
acid, and 10 mM glycerophosphate disodium salt into
the normal culture medium, and it was changed every
two days. After 7, 14, and 21 d, the osteogenic differ-
entiation of the cells was analysed by ALP activity and
calcium deposition.
3.12.1. ALP activity
ALP activity of the cells was evaluated as an early
marker of osteoblast differentiation. An ALP sub-
strate kit (SensoLyte pNPP ALP assay kit) was used
according to the manufacturer’s protocol. Firstly, cells
were gently washed twice with 1× assay buffer, and
incubated in lysis buffer (0.2% Triton X-100 in 1×
assay buffer) for 10 min. Then, the cells were sonic-
ated on iced water for 1 min in cycles of 1.5 s (on)
followed by 1 s (off) at 50% amplitude (SFX150,
Branson Ultrasonic Corporation, USA). Afterward,
the cell lysates were transferred into a 96-well plate
(50 µl/well). Subsequently, 50 µl of pNPP substrate
solution was added, gently mixed for 30 s on a
shaker, and incubated in the dark at room temper-
ature for 60 min. The reaction was then terminated
by adding 50 µl of the stop solution. The produc-
tion of P-nitrophenol was determined by measuring
the absorbance at 405 nm using a plate reader. ALP
activity was expressed as the amount of p-nitrophenol
released, and calculated using equation (5):
ALP activity = (B × D) / (∆T × V)
(5)
where, B is the amount of pNP in the sample well
calculated from the standard curve (µmol), D is the
sample dilution factor, ∆T is the reaction time (min),
and V is the original sample volume added into the
reaction well (ml). Finally, all values were normal-
ised to the corresponding total protein concentration
measured by using a bicinchoninic acid (BCA) assay
kit (Thermo fisher).
3.12.2. Alizarin red S (ARS) staining
ARS staining was used to detect calcium deposition
on the scaffolds after 7, 14 and 21 d of cell cul-
ture. Briefly, the scaffolds were gently washed three
times with PBS, and then the attached cells were fixed
with 4% paraformaldehyde and incubated for 20 min
5
at room temperature. Afterwards, cells were washed
twice with deionised water, and stained with 40 mM
Alizarin Red S solution (pH 4.1–4.3) for 30 min at
room temperature with gentle shaking in the dark.
Finally, the scaffolds were washed several times with
distilled water, air-dried and observed under a light
microscope (Olympus CKX41 inverted microscope,
Olympus, Japan).
3.13. Evaluation of the mineralisation capacity of
the 3D printed scaffolds using SBF
The in vitro mineralisation capacity of the 3D printed
scaffolds was investigated after immersion in SBF at
37 ◦C. The SBF was prepared according to Kokubo’s
protocol [26]. Briefly, the scaffolds were immersed
in plastic vials containing SBF for 21 and 35 d.
Afterward, the 3D printed scaffolds were taken out
from SBF and carefully rinsed with deionised water
to remove the soluble inorganic ions from the SBF.
The results of the mineralisation test were assessed by
scanning electron microscopy (SEM) and x-ray dif-
fraction (XRD) analysis.
3.13.1. Evaluation of the mineralisation capacity of
SBF conditioned scaffolds using SEM
Morphological characterisation of the surface was
carried out using SEM (JEOL 6700F FE-SEM, JEOL
Ltd, Japan). All the samples were coated with palladi-
um/platinum using an Emitech K575X Peltier-cooled
high-resolution sputter coater (EM Technologies Ltd,
Kent, England) prior to scanning. The elemental ana-
lysis of the mineral deposits was evaluated by using
energy dispersive x-ray spectroscopy (EDS) in con-
jugation with SEM. Four spots on each sample were
used for the EDS analysis. Ca/P ratio of each sample
was calculated as an average of these four spots.
3.13.2. Evaluation of the mineralisation capacity of
SBF conditioned scaffolds using XRD
To investigate the components of the apatite layer,
the SBF conditioned 3D printed scaffolds were ana-
lysed using an XRD (Agilent Technologies Supernova
system) with monochromatic Cu Kα radiation.
The spectra for all
the scaffolds were recorded
from 10◦ to 80◦ 2θ and treated using CrysAlisPro
software.
3.14. Statistical analysis
Measurements are indicated as mean ± standard
deviation (SD). Statistical comparisons were made
using either one-way or two-way ANOVA followed
by a Tukey’s multiple comparison test performed by
GraphPad Prism 9.0. A value of p < 0.05 was con-
sidered statistically significant, and ∗, ∗∗, ∗∗∗, ∗∗∗∗
represent p < 0.05, p < 0.002, p < 0.0002, p < 0.0001,
respectively.
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
4. Results and discussion
4.1. Effect of HA on shear thinning behaviour of
the hydrogel ink
A suitable ink for extrusion-based 3D printing must
have high extrudability and good shape-fidelity,
which in turn defines its printing accuracy. Both char-
acteristics are directly related to the rheological prop-
erties of the ink. It is known that shear thinning beha-
viour enables decreasing proportional stress along-
side increasing flow, facilitating uniform extrusion of
the ink from the nozzle [27]. The effect of HA con-
tent on the viscoelastic behaviour of the hydrogel inks
is shown in figure 1. At the beginning, all the hydro-
gel inks formed a consistent 3D network and were in
gel-state (G′ > G′′), critical for extrusion-based 3D
printing. Afterwards, the breakdown of the structure
started with some micro cracks, although the elastic
portion of the viscoelastic behaviour still prevailed.
As the shear strain increased above the critical yield
strain of ∼1%, all the hydrogel inks underwent struc-
tural breakdown. The crossover points of the G′ and
G′′ curves in all four hydrogels were found within a
comparable shear strain range of 4%–6%. By addi-
tional increase of the strain and passing the cros-
sover point (G′ = G′′), the individual micro cracks
grew further and formed a macro crack that eventu-
ally ruptured the entire sample, resulting in the flow
of the ink (G′′ > G′). It is worth mentioning that the
viscoelastic plateau did not change remarkably with
increasing HA content. This means that the addition
of HA, within the applied range, increased the stiff-
ness, while not significantly affecting the physical sta-
bility and viscoelastic behaviour of the inks. As expec-
ted, by increasing the HA concentration from 0 to
1, 2 and 5 wt%, the G′ value of the inks increased
from 15.1 kPa to 17.3 kPa, 19.3 kPa and 27.5 kPa,
respectively. The HA nanoparticles acted as physical
crosslinks or reinforcements of the amorphous phase,
thereby the free movement of polymer chains was
restricted, and the relaxation of the polymer chains
became difficult, increasing the moduli [28].
segments became energetically unfavourable. This
phenomenon resulted in more PPO molecules accu-
mulating on HA through hydrogen bonding, exerting
the bridging effect of HA and increasing the mod-
ulus significantly. Therefore, the elastic response of
the hydrogel inks dominated the viscous response
(G′ > G′′). By increasing the HA content, the gela-
tion temperature decreased from 8 ◦C for the ink
without HA to 7 ◦C, 5 ◦C, and 1 ◦C for the inks
with 1 wt%, 2 wt%, and 5 wt% HA, respectively.
This result might be attributed to the structure-
making properties of the ions at the surface of HA
nanoparticles. Structure-making effect is referred to
the orientation of water molecules around charged
ions due to the electrostatic interaction [30]. Based
on Jones–Dole viscosity B-coefficients, the degree of
water structuring depends on the ion-solvent inter-
actions. Generally, positive values of the B-coefficient
indicate kosmotropes (small ions of high charge dens-
ity, which are strongly hydrated), while negative val-
ues represent chaotropes (large ions of low charge
density, which are weakly hydrated) [31]. The ions
3−
at the surface of HA nanoparticles (Ca2+, PO4
and OH−) possess B-coefficients of 0.284, 0.590,
and 0.122, respectively [31]. Therefore, HA prefer-
entially bound the water molecules, taking up water
before it is energetically favourable to be released.
As a result, the degree of hydrogen bonding between
water and the OH groups of the PPO units decreased
and the hydrophobic interactions between the PPO
residues increased, which consequently decreased the
gelation temperature [32]. As the HA concentration
increased, the gelation process was accelerated due to
the higher ion content in the ink that binds adjacent
water molecules tightly, thus immobilising them, and
allowing the PPO groups to hydrophobically associ-
ate at lower temperatures [29]. It is worth mentioning
that although the thermos-sensitivity was not directly
exploited in the ambient printing process, it facilit-
ated HA dispersion and homogenisation with PF127,
as well as loading cartridges with PF127 solutions at
low temperature (0 ◦C).
4.2. Effect of HA on gelation temperature of the
HA-based inks
To investigate if the physical gelation of PF127 was
hampered by adding HA, the temperature sweeps
were conducted, and the results are shown in figure 2.
At temperatures lower than the gelation temperat-
ure, all hydrogel inks displayed a viscoelastic beha-
viour (G′ > G′′) because the interaction of water with
HA was weak, and the formation of more hydrogen
bonds with the relatively hydrophilic PEG in PEGDA
and PEO block in PF127 was more favourable [29].
However, above the gelation temperature, the inter-
action of PPO block in PF127 and HA increased
because the overall hydrophobicity of PPO block in
PF127 increased, and the adsorption of water on these
4.3. Evaluation of printability and shape fidelity of
HA-based hydrogel inks
inks was
The printability of HA-based hydrogel
evaluated using a combination of different print-
ing pressures (60–100 kPa), and printing rates (1–
15 mm s−1). Generally, a lower printing pressure and
higher printing rate resulted in a thinner strand dia-
meter. The extent of decrease in the printed strand
diameter was more significant on the less viscous
hydrogel (0 wt% HA) compared to the most viscous
hydrogel (5 wt% HA). By increasing the printing pres-
sure, higher printing rates were required to fabric-
ate parallel strands using the inks with 0 wt% and
1 wt% HA, whereas lower printing rates were needed
to extrude uniform strands using the inks with 2 wt%
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Figure 1. Strain-sweep graph for HA-based hydrogel inks. The closed and open symbols represent the storage modulus (G′) and
loss modulus (G′′), respectively.
Figure 2. Oscillatory temperature sweeps of the HA-based hydrogel inks.
and 5 wt% HA. For instance, at a constant pressure
of 90 kPa, the minimum printing rate required to
fabricate consistent parallel strands was 10, 8, 3, and
2 mm s−1 for 0, 1, 2, and 5 wt%, respectively. It
is likely attributed to the same fact that the higher
the printing rate the lower the ink viscosity during
extrusion.
Additionally, higher pressures were required to
extrude more viscous hydrogels. It was observed that
the strand diameter of ink with 0 wt% HA increased
exponentially with increasing printing pressures. For
example, at a constant printing ratio of 10 mm s−1,
by increasing the printing pressure from 70 to 80
and 90 kPa, the strand diameter increased from 176
to 266 and 531 µm, respectively (figure 3(a)). This
is probably due to the intrinsic lower viscosity of
0 wt% HA ink, which caused a higher extent of
strand spreading when a larger printing pressure was
used. By increasing the viscosity, a linear relationship
between printing pressures and strand diameter was
observed in 1 wt% HA ink; where at a constant print-
ing ratio of 9 mm s−1, by increasing the printing pres-
sure from 70 to 80 and 90 kPa, the strand diameter
increased from 188 to 336 and 442 µm, respectively
(figure 3(b)). For 5 wt% ink, at a constant printing
ratio of 2 mm s−1, the strand diameter was 128, 143,
305, 412, and 498 µm, for the printing pressure of
60, 70, 80, 90, and 100 kPa, respectively (figure 3(d)).
The high viscosity of these hydrogels likely reduced
the extent of filament spreading at higher printing
pressures. It was also observed that the SD of prin-
ted strand diameter decreased with hydrogel inks
of higher viscosity. Hence, a more viscous hydrogel
offered higher printing consistency and better control
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Figure 3. Effect of printing pressure (kPa) and printing rate (mm s−1) on the strand diameter (mm) measured by ImageJ. (a)
0 wt% HA, (b) 1 wt% HA, (c) 2 wt% HA, and (d) 5 wt% HA. Shaded area indicates desired strand diameter (210 ± 10.5 µm).
over the printed strand diameter at increasing print-
ing pressure. The shaded orange areas in figure 3
show the optimum combination of printing pres-
sure and printing rate to obtain a strand diameter of
210 ± 10.5 µm for different hydrogel inks. A five per-
cent error was selected as the tolerance threshold for
the strand diameter based on the literature [33]. As it
can be seen, lower printing pressures of 60 and 70 kPa
were suitable for inks with 0 wt% and 1 wt% HA,
whereas a printing pressure of >80 kPa was required
for inks with 2 wt% and 5 wt% HA.
Figure 4 shows the Pr value of HA-based inks with
different concentrations of HA under different print-
ing pressures and printing rates. A very low printing
rate resulted in excessive ink deposition and fusion
of the printed strands on the cross site, where mesh
areas were nearly zero, making it unfeasible to man-
ufacture a 3D scaffold (Pr < 1). On the other hand,
a very high printing rate showed irregular or frac-
tured morphology, resulting in incomplete pattern-
ing (Pr > 1). Under optimum conditions, smooth and
uniform strands were extruded continuously, result-
ing in a grid-like pattern close to a square with reg-
ular edges. Therefore, the optimal combination of
both printing pressure and printing rate that enable
the fabrication of complete grid-like patterns at the
highest printing resolution were selected for further
experiments.
4.4. Evaluating the HA content of 3D printed
scaffolds by using TGA
TGA analysis was performed to determine the
amount of HA after the scaffolds were subjected to
the washing process, and the mass loss versus tem-
perature curves are shown in figure 5. Three stages
of mass loss were observed for all the as-prepared
scaffolds (before washing), two minor and one major
mass loss. The initial stage corresponded to the evap-
oration of residual water in the samples, at a temper-
ature between 50 ◦C–200 ◦C (mass loss 1%–2%). The
great and rapid mass loss occurred within the narrow
range of temperature (350 ◦C–450 ◦C) during the
thermal decomposition of PEGDA and PF127. Based
on the published literature, PF127 shows a single step
degradation between 300 ◦C–452 ◦C via a random
scission mechanism, and the pyrolysis of PEGDA
occurs around 410 ◦C [23, 34–36]. The mass loss of
2.5%–3.5% in the third region corresponded to the
dehydration of HA. It is known that HA is thermally
stable up to 700 ◦C, and then by increasing the tem-
perature between 850 ◦C and 1100 ◦C, gradual weight
loss occurred due to the partial desorption of water
[37, 38]. The TGA curves of washed scaffolds were
also divided into three regions. In the first region,
the initial weight loss of less than 2% occurred below
200 ◦C. In the second region, between 250 ◦C and
450 ◦C a loss of 88%–97% of the initial weight was
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Figure 4. Ink printability assessment under different printing pressures and printing rates. (a) 0 wt% HA, (b) 1 wt% HA, (c)
2 wt% HA, and (d) 5 wt% HA; scale bar = 200 µm.
Figure 5. Mass loss of the 3D printed HA-PEGDA scaffolds. The solid lines represent as-prepared and the dashed lines represent
washed scaffolds.
observed. The third region showed a gradual mass
loss until the temperature reached 890 ◦C.
By increasing the HA content, in both as-prepared
and washed scaffolds, the weight loss decreased, and
the thermal stability of the scaffolds increased. This
thermal stability enhancement can be ascribed to
(I) the larger amount of physical crosslinking that
HA made within the network through the formation
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Table 1. TGA critical points of 3D printed HA-based scaffolds with different mass fraction of HA nanoparticles. T5 and T50 represent
the decomposition temperature of the scaffolds when the mass loss was 5% and 50%, respectively.
T5 (◦C)
T50 (◦C)
Residual mass at 890 ◦C (%)
Scaffold
As prepared
Washed
As prepared
Washed
As prepared
Washed
0 wt%
1 wt%
2 wt%
5 wt%
319.39
374.05
354.60
340.34
161.23
247.60
280.24
299.68
407.61
412.75
407.75
410.78
325.81
374.04
384.57
390.03
2.20
5.02
7.43
11.37
4.35
6.70
9.26
13.44
Figure 6. FTIR spectra of 3D printed scaffolds with different HA content, HA, and PEGDA monomer.
of strong hydrogen interactions and van der Waals
forces, which restricted the segmental mobility of
the macromolecular chains, thereby decreasing the
weight loss [39, 40]; and (II) the barrier effect of
the HA nanoparticles that effectively obstructed the
diffusion of volatile products from the bulk of the
polymer to the gas phase, thereby slowing down the
decomposition process [41].
The onset temperature corresponding to 5%
weight loss (T5), the temperature corresponding to
the 50% weight loss (T50), as well as the total weight
loss measured at 890 ◦C, for all the scaffolds are
shown in table 1 The remarkable difference between
the as-prepared and washed scaffolds was that washed
scaffolds had lower rapid thermal degradation onset
temperatures compared with as-prepared scaffolds,
which can be attributed to the less compact network
and increased porosity after removal of PF127. It can
be observed that the residual mass at 890 ◦C increased
gradually with increasing the HA content due to the
high thermal stability of the mineral nanoparticles
[40]. The residual mineral masses for the washed
scaffolds were measured to be 9.09%, 4.91%, and
2.35% for scaffolds with 5 wt% HA, 2 wt% HA, and
1 wt% HA, respectively. These values were close to the
residual mineral masses for the as-prepared scaffolds,
which are 9.17%, 5.23%, and 2.82% for scaffolds with
5 wt% HA, 2 wt% HA, and 1 wt% HA, respectively.
Therefore, HA was incorporated efficiently through-
out the scaffolds without significant loss of HA during
the washing process.
4.5. Analysis of chemical structure by using FTIR
The chemical structures of the 3D printed scaffolds
with varying HA concentrations as well as the PEGDA
monomer and HA were identified using FTIR ana-
lysis. As shown in figure 6, for PEGDA monomer,
the peak at 2867 cm−1 and 1721 cm−1 are assigned
to the stretching vibrations of C–H in the alkyl
and stretching vibrations of CO in the carbonyl
group, respectively. Also, the peaks at 1452 cm−1,
1349 cm−1, and 1093 cm−1 correspond with the
bending of C–H and the stretching vibrations of
C–O and C–O–C, respectively. As for pure HA,
2−, and the oth-
the peak at 882 cm−1 is for HPO4
ers at 1000–1100 cm−1 (1087 cm−1, 1033 cm−1),
600 cm−1, and 561 cm−1 are for PO4
3− [42–44]. Also,
peaks between 1420 and 1455 cm−1 correspond to
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Figure 7. Equilibrium degree of swelling of the 3D printed HA-PEGDA scaffolds; bar = mean ± SD. Statistical analysis was
carried out using a one-way ANOVA followed by Tukey’s multiple comparisons test, where p < 0.002 (∗∗).
2+) [45]. For the 3D printed scaf-
carbonates (CO3
folds, FTIR spectra demonstrated successful curing
as there were no peaks associated with unreacted
acrylate double bond (810 cm−1, 1191 cm−1 and
1410 cm−1) remaining after curing. In addition, the
carbonyl (C=O) peak of acrylate group slightly shif-
ted towards a higher wavenumber, from 1721 cm−1
to 1730 cm−1, implying an increased carbonyl bond
strength due to the loss of conjugation between the
C=C and carbonyl groups during the photo poly-
merisation reaction [46]. With increasing the HA
content in the scaffolds, the characteristic absorption
peak at 600 cm−1 and 561 cm−1 gradually increased.
The peaks at approx. 1455 cm−1, 1093 cm−1, and
1033 cm−1 correspond and overlap with the bands of
carbonate and phosphate groups of HA and charac-
teristic bands of PEGDA [47]. In addition, the broad
band at 3500 cm−1 is attributed to the O–H stretching
vibration due to the absorption of water molecules on
PEGDA and HA. Nevertheless, there was no remark-
able difference between the spectra of pure PEGDA
and HA-PEGDA scaffolds likely because the peaks for
the functional groups of HA overlapped by the peaks
of PEGDA due to its relatively low concentration of
HA [23]. FTIR analysis confirmed the presence of
both the polymer phase and the mineral phase in the
fabricated scaffolds.
4.6. Analysis of swelling behaviour influenced by
HA concentration
Swelling is based on occupying the free volume within
a sample until it reaches a state of equilibrium and is
important in the fabrication of bone scaffolds because
a decrease in swelling may increase the mechanical
properties [48]. The effect of HA addition on the
swelling of the 3D printed HA-PEGDA scaffolds is
shown in figure 7. For scaffolds without HA (0 wt%),
the equilibrium degree of swelling was found to be
244.8 ± 12, while this value decreased to 237.0 ± 10
and 226.3 ± 7.0 for scaffolds with 1 wt% and 2 wt%,
respectively. A significant decrease in swelling to
215.4 ± 6 was also observed with the incorporation
of 5 wt% HA compared to 0 wt% (p = 0.0095). This
reduction may be attributed to the physical interac-
tion of HA nanoparticles with PEGDA, which acted as
a filler that occupy free space within the polymer net-
work, thereby impeding the penetration of liquid into
the interior network of the hydrogel. Furthermore,
interaction with HA restricted the PEGDA polymeric
chains motion, which promoted the densification
of the hydrogel [49]. Similar studies showed that
increasing the amount of HA in PEGDA composites
decreased the swelling ability of the tested materials,
confirming the contribution of HA in the formation
of a more crosslinked network [50, 51].
4.7. Analysis of compressive modulus influenced
by HA concentration
The addition of HA nanoparticles to the PEGDA
matrix slightly increased the compressive modulus of
the 3D printed scaffolds as shown in figure 8. The
scaffolds without HA (0 wt% HA) had a compress-
ive modulus of 176.77 ± 22 kPa. Upon the addi-
tion of 1 wt% HA, the compressive modulus slightly
increased to 179 ± 35 kPa. As the HA concentra-
tion further increased to 2 wt% and 5 wt%, the
compressive modulus reached 189 ± 38 kPa and
219 ± 17 kPa, respectively, with no significant differ-
ence between them. The increase in the compressive
modulus of 3D printed scaffolds can be attributed to
three factors: (I) the effective dispersion of HA nan-
oparticles and the physical crosslinking between HA
and the PEGDA matrix, (II) a reduction in PEGDA
mobility near the HA nanoparticles and a decrease in
the swelling percentage, and (III) the ability of HA
nanoparticles to absorb and distribute compressive
loads [48, 52]. Consequently, the incorporation of
HA nanoparticles within the polymer matrix, in an
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Figure 8. Compressive modulus of the 3D printed HA-PEGDA scaffolds in hydrated state at 37 ◦C.
appropriate loading range, enhances the compressive
modulus of the composite and facilitates load trans-
fer between the components. Our results fall within
the range reported in the literature for HA-PEGDA
composites [53, 54]. However, it should be noted that
studies utilizing continuous 3D printing techniques
such as SLA and CLIP have reported higher com-
pressive moduli for printed scaffolds. For example,
Huang et al [55] reported a compressive modulus of
459.1 kPa for SLA-printed PEGDA scaffolds. Deng
et al [23] fabricated 3D printed PEGDA-HA scaf-
folds with 1 wt% HA using CLIP technology res-
ulting in a compression modulus of approximately
40 MPa, while pure PEGDA scaffolds exhibited a
value of around 20 MPa. However, an increase in HA
loading to 2 wt% decreased the compressive modu-
lus due to the agglomeration of HA nanoparticles.
In another study, Kumar et al [56] demonstrated
that SLA-printed PEGDA-HAP scaffolds with 1 wt%
HA reached a compressive modulus of approximately
50 MPa, which was not significantly different from
unfilled PEGDA.
Therefore, it can be inferred that not only the
molecular weights and concentrations of the polymer
and filler, as well as the crosslinking method, affect
the mechanical properties of the scaffolds, but also
the specific 3D printing technique employed signific-
antly impacts the final mechanical characteristics of
the printed scaffolds. Continuous printing processes
like SLA and CLIP ensure solidification of the inner-
most sections of the print, contributing to the over-
all robustness of the structure. Furthermore, the con-
ditions under which mechanical testing is conduc-
ted also play a role. In our study, mechanical tests
were performed in a hydrated state at 37 ◦C, which
is closer to the physiological environment, whereas
other studies conducted testing in a dry state or did
not provide information on the testing conditions. It
is well known that hydration influences the mechan-
ical properties of scaffolds. For instance, a study by
Such´y et al [57] fabricated scaffolds with various com-
positions and demonstrated that the elastic modulus
12
and compressive strength decreased by approximately
95% in the hydrated state compared to the dry state.
This highlights the importance of analysing scaffolds
in the hydrated state, as it more accurately simulates
the real in vivo environment for which these scaffolds
are designed. Overall, although the compressive mod-
ulus of the 3D printed HA-PEGDA scaffolds in our
study remains lower than that of native human bone
tissue, it is postulated that in a biologically functional
implant, the surrounding native bone would provide
the initial mechanical support while scaffold miner-
alisation and the healing process occur [58].
4.8. Analysis of in vitro degradation influenced by
HA concentration
The in vitro degradation behaviour of 3D printed
HA-PEGDA scaffolds in PBS at 37 ◦C was evalu-
ated, as shown in figure 9. During the experimental
period, all scaffolds maintained their structural integ-
rity. However, an increase in the HA content of the
scaffolds led to a decrease in mass loss, indicating a
slower degradation rate. The mass loss percentages
for scaffolds with 0 wt%, 1 wt%, 2 wt%, and 5 wt%
HA on day 21 were 18.74% ± 1.16, 16.09% ± 0.86,
14.00% ± 0.80, and 11.18% ± 2.19, respectively.
Significantly lower mass loss was observed in scaffolds
with 5 wt% HA compared to those with 0 wt% HA
(p = 0.0002).
Several factors may contribute to the observed
differences in degradation behaviour. HA possesses
higher stability and resistance to degradation com-
pared to PEGDA. Therefore, the incorporation of
HA within the scaffolds enhanced their stability and
hindered their degradation to some extent. Moreover,
this reduction in degradation rate can be attributed to
the fact that HA nanoparticles acted as physical cross-
linking centres within the polymer network, resulting
in decreased hydrophilicity. Consequently, the water
penetration became more difficult within the more
organised polymer chain network, leading to slower
hydrolytic cleavage of the ester bonds in PEGDA
and subsequently reducing the degradation rate [59].
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
Figure 9. Degradation rate of the 3D printed HA-PEGDA scaffolds in PBS for different times, bar = mean ± SD.
Figure 10. Cell viability results of hBMSCs on 3D printed HA-PEGDA scaffolds for 48 h measured by MTT assay. Results are
represented as the mean ± SD of three independent experiments with three technical replicates. One-way ANOVA followed by
Tukey’s comparisons test was used for statistical analysis, p < 0.05 (∗).
Therefore, the ratio of PEGDA and HA in the scaf-
folds can be tailored to control the degradation rate to
match the rate of bone healing, which varies depend-
ing on bone type, size, location, as well as individual
factors such as age, comorbidities, lifestyle.
It is important to note that all scaffolds exhib-
ited an increase in mass loss over the 21 d period.
For scaffolds with 2 wt% HA, the degradation rate
appeared to reach a plateau from day 7, while scaffolds
with 5 wt% HA showed a slowing down or reaching
a steady state of degradation by day 14 after an ini-
tial period of mass loss. These time points correspond
to the equilibrium point of degradation rate, where
minimal or no changes in mass loss occur until the
end of the assay [60]. This behaviour may be attrib-
uted to the release of HA into the media, where the
higher HA content and more compact structure of
scaffolds with 5 wt% HA prolonged the time needed
to reach the equilibrium point compared to scaffolds
with 2 wt% HA. Nevertheless, further investigation
and characterization are necessary to gain a compre-
hensive understanding of the underlying mechanisms
responsible for these observations.
4.9. Biocompatibility assessment of 3D printed
scaffolds by using MTT assay
To investigate whether cell viability is compromised
by the presence of HA in the fabricated 3D printed
scaffolds, the viability of hBMSCs after 48 h culture
was analysed. Figure 10 demonstrates that the addi-
tion of HA (1 and 2 wt%) into the scaffolds did not
significantly decrease cell viability (72.5% for 1 wt%
and 72.7% for 2 wt%). However, by increasing HA to
5 wt%, a significant reduction in cell viability (67.8%)
was observed compared to scaffolds with 0 wt% HA
(84.3%) (p = 0.0444). This result could be correl-
ated to the presence of a higher amount of HA on
the surface of the scaffold with 5 wt% HA, and hence
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M Rajabi et al
Figure 11. Cell viability and attachment on 3D printed HA-PEGDA scaffolds with live/dead assay (green, calcein AM; red, PI) (a)
after 48 h, 200 µm (b) after 14 d, 100 µm.
the release of higher amounts of HA into the static
culture medium within the first 48 h of cell culture.
There are three different scenarios on the cytotox-
icity of nanoparticles [61]. (I) Nanoparticles can react
with the surrounding fluids, causing the release or
depletion of ions and proteins crucial for cell func-
tion; (II) nanoparticles can interact with cell mem-
brane receptors, potentially inducing apoptotic sig-
nalling cascades; and (III) nanoparticles can be inter-
nalised by cells, exerting their effects inside the cell.
The third assumption is the most accepted hypo-
thesis, which relies on the degradability of the nan-
oparticles upon internalisation. Based on this hypo-
thesis, after HA nanoparticles uptake by endocytosis,
they degrade under the acidic conditions in the lyso-
some, yielding an increase of calcium and phosphorus
ions. A slow and sustained dissolution of HA in the
lysosomes would benefit transfection, while a faster
internalisation of nanoparticles would release a high
concentration of calcium ions that would lead to cell
death. For example, Huang et al [62] reported that
HA crystals were endocytosed by A7R5 cells, where
a high degree of crystal endocytosis corresponded
to a high intracellular calcium concentration, lead-
ing to cell membrane rupture. Nonetheless, the size,
shape, concentration, and exposure duration of HA
nanoparticles as well as the cell type may influence
the cellular responses and degree of potential dam-
age. In our study, it was observed that by increas-
ing the culture time and changing the cell culture
media, the cell viability increased as evidenced by live-
dead assay. The cellular response to HA nanoparticles
is an active area of research, and further studies are
needed to understand the detailed mechanisms and
concentration-dependent patterns of cellular damage
associated with HA nanoparticles.
4.10. Cell attachment and growth on the 3D
printed scaffolds
Due to the complete inertness of scaffolds with 0 wt%
HA, hBMSCs showed a preference for cell–cell and
cell–plate contact rather than cell–scaffold interac-
tion, thereby no cell attachment was observed on the
scaffolds with 0 wt% HA. The attachment and viab-
ility of hBMSCs at different layers of the 3D printed
HA-PEGDA scaffolds after 48 h of culture are shown
in figure 11. High cell viability (>95%) of attached
hBMSCs was observed on all three scaffolds with
no significant difference (figure 11(a)). However, cell
attachment on scaffolds with 5 wt% HA was signific-
antly higher compared to 1 wt% (p = 0.0372), which
can be attributed to the rougher surface of scaffolds
with 5 wt% HA. Moreover, HA has the ability to
adsorb protein from the culture media [63], and the
absorption of protein likely increased with increasing
the HA content, resulting in higher adhesion of hBM-
SCs to the scaffolds with 5 wt% HA. It can also be seen
the cells on scaffolds with 5 wt% HA already started
to spread out along the pores after 2 d. This indicated
that the addition of HA nanoparticles into the inert
3D printed PEGDA scaffolds significantly improved
the adhesion of hBMSCs. After 14 d, the cells filled up
the pores and covered the entire scaffolds, exhibiting
a vigorous proliferation of hBMSCs, and the biocom-
patibility of all scaffolds for further use (figure 11(b)).
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Figure 12. Comparison between alkaline phosphatase (ALP) activity of hBMSCs in various 3D printed scaffolds cultured in either
osteogenic or normal media at day 7, 14, and 21. ‘OS’ and ‘N’ stand for scaffolds cultured in osteogenic and normal media,
respectively. Results are expressed as mean ± SD from three independent experiments with three technical replicates. Statistical
analysis was carried out using two-way ANOVA followed by Tukey’s multiple comparisons test, p < 0.0001 (∗∗∗∗) and p < 0.002
(∗∗).
Finally, these images suggested that lower cell viabil-
ity of scaffolds with 5 wt% HA obtained by MTT assay
(figure 10), was a result of a higher rate of HA leach
out within the first 48 h, which significantly improved
by changing the cell culture media. Therefore, the 3D
printed scaffolds with 2 wt% and 5 wt% HA were
selected for further experiments as they showed the
lowest swelling ratio, highest compression strength,
and improved cell attachment.
4.11. Measuring osteogenic differentiation of
hBMSCs by using ALP activity
The ALP activity of the hBMSCs was measured for
the comparison of their osteogenic activities at day
7, 14 and 21, and results are shown in figure 12. All
data were normalised with regard to the total pro-
tein content to confirm that the higher ALP activ-
ity was not due to the increased cell number. In the
stimulated group, ALP activity of hBMSCs in all three
scaffolds showed a similar pattern with a peak dur-
ing osteogenic differentiation. In detail, ALP activ-
ity significantly increased with increasing culture time
from day 7 to day 14 in scaffolds with 0 wt% HA
(p < 0.0001), 2 wt% HA (p < 0.0001), and 5 wt%
HA (p < 0.0001). This was followed by a significant
decrease in ALP activity at day 21 in scaffolds with
0 wt% HA (p < 0.0001), 2 wt% HA (p < 0.0001),
and 5 wt% HA (p < 0.0001). The lower ALP activ-
ities of cells at day 21 in all groups indicated that
the cells on these scaffolds switched to the next dif-
ferentiation state with the down regulation of the
ALP gene [64]. Furthermore, 3D printed HA-PEGDA
scaffolds showed an increase in ALP activity in a
dose-dependent pattern, and scaffolds with 5 wt%
HA showed a lower ALP activity compared to 0 wt%
HA at all time points. Similarly, Wang, Hu [65] repor-
ted that HA affects ALP activity of MC3T3E1 cells in
a dose-dependent manner. In their study, the ALP
activity at day 7 increased when the concentration
of HA was ⩽100 mg ml−1, while it decreased when
the concentration reached 200 mg ml−1. When com-
paring the influence of HA in normal media, the
ALP expression on all the scaffolds was dependent on
the culture time. The ALP activity on scaffolds with
2 wt% HA and 5 wt% HA continued to increase up
to day 21, with a significant increase in ALP activ-
ity of scaffolds with 2 wt% HA from day 7 to day
14 (p = 0.0194). This indicated that the cells cul-
tured in normal media on scaffolds with 2 wt% HA
and 5 wt% HA were still at an earlier differentiation
stage. Moreover, all the scaffolds showed significantly
higher ALP activity of cells cultured in osteogenic
media compared to that of normal media at day 14
(p < 0.0001). Significantly higher ALP activity was
also observed at day 21 in cells cultured in osteogenic
media compared to that of normal media on scaf-
folds with 0 wt% HA (p = 0.0100). This indicated
the synergic effect of the scaffold composition and
the osteogenic media on cell response. Overall, these
results demonstrated that hBMSCs undergo similar
changes of ALP activity during osteogenic differenti-
ation on 3D printed HA-PEGDA scaffolds cultured in
either normal or osteogenic media, and an appropri-
ate HA incorporation can promote hBMSCs differen-
tiation into mature osteoblasts phenotypes in normal
media.
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M Rajabi et al
Figure 13. Calcium deposition of hBMSCs on the 3D printed HA-PEGDA scaffolds visualised by Alizarin red staining.
4.12. Measuring calcium deposition by using ARS
staining
Matrix mineralisation is a controlled biological pro-
cess whereby differentiating osteoblasts accumulate
environmental calcium and phosphate to form cal-
cium apatite. In bone tissue engineering, extracel-
lular mineralisation serves as a strong indicator of
osteogenic differentiation [66, 67]. Figure 13 presents
optical microscopic images of 3D printed scaffolds
stained with ARS after 7, 14, and 21 d of culture
in either normal or osteogenic media to qualitat-
ively assess the mineralisation of hBMSCs. Scaffolds
without HA (0 wt% HA) exhibited no calcium depos-
ition on days 7 and 14, regardless of the culture
medium used. However, on day 21, hBMSCs cultured
on scaffolds with 0 wt% HA in osteogenic media dis-
played orange-red stains indicative of mineralisation,
while hBMSCs in normal media did not show miner-
alisation. On the other hand, mineralisation continu-
ously improved on 3D printed HA-PEGDA scaffolds
during osteogenic differentiation. hBMSCs on scaf-
folds with 2 wt% HA and 5 wt% HA showed greater
amounts of mineral deposits and bone nodules com-
pared to cells cultured on the scaffolds with 0 wt% HA
at day 7, 14, and 21.
Recent evidence has demonstrated that HA nan-
oparticles possess remarkable osteoinductive proper-
ties on hMSCs by up-regulating osteogenic genes such
as ALP, COL1, and RUNX2 when hMSCs are grown
on HA-based biomaterials [68]. Moreover, multiple
studies have suggested that the presence of extracel-
lular HA can influence the commitment and func-
tion of osteoblasts by releasing Ca2+ continuously.
For example, Sattary et al [69] showed that the addi-
tion of 15 wt% HA nanoparticles in PCL/Gel scaf-
folds promoted the rate of mineral deposition by MG-
63 cells compared to scaffolds without HA at day 7
and 21. In another study, Gleeson et al [70] demon-
strated that the addition of different concentrations of
HA (50, 100, and 200 wt%) to a collagen-based scaf-
fold enhanced the mineralisation of MC3T3-E1 pre-
osteoblast cells in a concentration-dependent man-
ner as early as day 7. Similarly, Kim et al [71] showed
enhanced mineralisation of BMSCs on poly (lactic-
co-glycolic acid) scaffold by increasing the concentra-
tion of HA from 0% to 10%, 20%, 40% and 60%, in
a timely manner from day 1 to day 7, 14, 21 and 28.
Therefore, our findings aligned with previous stud-
ies that have shown enhanced mineralisation on HA-
based scaffolds, with HA acting as a chelating agent
for mineral deposition on MSCs as early as day 7 [72].
Moreover, it is plausible that the partial dissolution
of HA led to elevated levels of calcium and phos-
phate ions in the immediate vicinity, thereby promot-
ing the activity and mineralisation of osteoblast-like
cells [73].
Even in the absence of osteogenic media, the addi-
tion of HA can induce osteogenic differentiation. For
example, in a study conducted by Jamshidi et al [74],
the addition of HA in gellan gum beads induced nod-
ule formation in MC3T3-E1 cells. It also signific-
antly enhanced the osteogenic differentiation of bone
marrow stromal cells, even in the absence of osteo-
genic media, when compared to tissue culture plastic
under similar conditions. These effects were observed
within a timeframe of 5 d. Similar study by Calabrese
16
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
Figure 14. SEM images, EDS results, and XRD patterns of mineral deposits on the 3D printed scaffolds after (a), (b) 21 d, and (c),
(d) 35 d of incubation in SBF.
et al [75] investigated the effects of collagen–HA scaf-
folds loaded with human adipose-derived stem cells
(hADSCs) in both normal and osteogenic media.
The results revealed that in the presence of normal
media, calcium deposits began to appear after 2 weeks
of culture. However, when the samples were grown
in the presence of osteogenic factors, mineralisation
of the extracellular matrix initiated as early as the
first week, with a statistically significant increase over
time. Furthermore, the presence of osteogenic factors
resulted in a greater amount of calcium deposition at
each assessed time point compared to samples cul-
tured in normal media. These results indicated that
the scaffold itself induced osteogenic differentiation
of hADSCs in vitro, while the presence of osteo-
genic factors accelerated this process. Another study
examined the osteogenic properties of collagen–HA
scaffolds compared to collagen scaffolds. Enhanced
ARS staining was observed in MSCs cultured on
collagen–HA scaffolds, both in normal and osteo-
genic media, suggesting that the addition of HA in the
scaffold improved its osteoinductive and osteocon-
ductive properties. Furthermore, the presence of HA
in the collagen-based scaffold significantly increased
calcium deposition, reducing the need for high levels
of BMP2 to induce the osteogenic ability of MSCs
[76]. In addition, the amount of calcium deposition
in hAD-MSCs cultured in cell culture media contain-
ing HA was observed to increase as early as day 7,
compared to hAD-MSCs cultured in normal media
alone [77]. Similarly, Fu et al [78] reported an increas-
ing number of stained bone nodules recovered from
HA scaffolds as the culture time progressed from
day 3 to day 12. Therefore, consistent with previ-
ous reports, our findings indicated that the presence
of HA in the 3D printed HA-PEGDA scaffolds can
enhance mineralisation, even in the absence of osteo-
genic supplements.
4.13. Evaluation of bone bioactivity of the 3D
printed scaffolds by using SEM and XRD
The morphology and composition of mineral depos-
its on the 3D printed scaffolds after 21 d and 35 d
incubation in SBF are shown in figures 14(a) and (c).
There was an increased apatite deposition on the sur-
face of the scaffolds over time. After soaking in SBF
for 21 d, a few small granules were visible on the sur-
face of scaffold with 0 wt% HA. After 35 d, the surface
of these scaffolds formed more heterogeneous min-
eral deposits with small aggregations. A higher mag-
nification examination showed that the granules were
composed of crystals with the cauliflower morpho-
logy of HA, the same morphology observed by Ni
et al [79]. Despite the absence of HA in the original
17
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
composition of the scaffold with 0 wt% HA, a few
mineral deposits were observed on the scaffolds due
to ions sedimentation. Similarly, Zhang and Ma [80]
reported the formation of an apatite layer on the poly-
l-lactic acid (PLLA) surface in SBF, where PLLA was
slowly hydrolysed in SBF, leading to the formation of
new –OH hydroxyl and –COOH carboxyl groups on
its surface. The presence of these groups facilitated
3− through electro-
the accumulation of Ca2+ and PO4
static forces and hydrogen bonding on the surface.
The surface of scaffolds with 2 wt% HA and 5 wt%
HA formed many more apatite clusters and mineral
deposits compared to the pristine scaffolds. The sur-
face of scaffolds with 2 wt% HA exhibited rough min-
eral deposits on day 21. The surface of scaffolds with
5 wt% HA showed a deposited layer with typical glob-
ular cauliflower morphology of HA. After 35 d, the
surface of scaffold with 2 wt% HA formed uniform
spherical petal-like particles (∼5 µm), the same mor-
phology seen by Gao, Wei [81]. The surface of scaf-
folds with 5 wt% HA exhibited larger mineral depos-
its after 35 d. Previous studies have shown that miner-
alisation was higher on scaffolds containing HA than
on pristine scaffolds [72]. The major reason for the
enhancement of apatite formation on the 3D printed
HA-PEGDA scaffolds might be that the HA particles
acted as nucleation initiation sites [82]. Nucleation
is affected by the surface charge of the HA scaffolds
3− from the metastable SBF
that absorb Ca2+ and PO4
solution and form an apatite layer [83]. The more
HA content in the composite scaffolds, the more nuc-
leation initiation sites existed, as a result the apatite
formation was faster. Once the apatite nuclei were
formed, they grew spontaneously by consuming the
3− present in the surrounding fluid [82].
Ca2+ and PO4
Moreover, the rougher and bumpy surface of scaf-
folds with 2 wt% HA and 5 wt% HA compared to
the pristine scaffolds provided a higher surface area
and induced higher deposition of minerals. The main
elements on the surface of the scaffolds detected by
EDS were calcium, phosphorus, oxygen, and carbon,
indicating that calcium (Ca) and phosphate (P) were
present in the minerals and was not contaminated
by other ions in the SBF. The Ca/P ratio was not
significantly different between various scaffolds and
remained constant at ∼1.9–2. This is slightly higher
than the Ca/P ratio of natural HA (1.67). However,
studies have shown that the Ca/P ratio between 1.3
and 2.0 can be classified as HA [84, 85], which con-
firms the formation of HA on the surface of the 3D
printed HA-PEGDA scaffolds.
The compositions of these newly formed miner-
als were also determined by using XRD analysis. Based
on 2013 international centre for diffraction data, the
standard pattern of HA (PDF#09-0432) consists of
primary peaks located at 25.87◦, 31.77◦, 32.2◦, 32.9◦,
34.05◦, 39.82◦, 46.71◦, 49.47◦, 50.5◦, 53.15◦ assigned
to the Miller indices of (0 0 2), (2 1 1), (1 1 2),
18
(3 0 0), (2 0 2), (3 1 0), (2 2 2), (2 1 3), (3 2 1), and
(0 0 4)planes of HA, respectively. The XRD patterns
of 3D printed scaffolds after 21 d of immersion in
SBF are shown in figure 14(b). The XRD data sug-
gested that scaffolds with 0 wt% HA at day 21 con-
tained a mixture of sodium chloride (NaCl) arising
from the SBF solution and HA. The peak at 45.5◦ was
from NaCl (JCPDS 01-077-2064) [86], while diffrac-
tion peaks at 32◦ corresponded to HA. After 35 d of
immersion in SBF (figure 14(d)), the pristine scaf-
fold clearly exhibited the peaks of HA such as (0 0
2), (2 1 1), (3 0 0), (2 0 2), (3 1 0) and (2 2 2) at
32◦, 40.5◦ and 48.5◦, respectively. The XRD analysis
also demonstrated that the mineralisation on scaf-
folds with 2 wt% HA and 5 wt% HA showed their
most intense diffraction peaks at 25.9, 31.74, 32.12,
and 32.9, which coincided with the main reflection
planes of natural HA. The presence of separated peaks
with high intensity indicated a high degree of crys-
tallinity on the surface of these scaffolds [87]. No sig-
nificant difference was observed between the intens-
ity of the peaks on different scaffolds, indicating that
the crystallinity degrees of these deposits were likely
equal.
5. Conclusion
In conclusion, printing pressures and printing rates
of a pneumatic extrusion-based 3D bioplotter were
optimised to fabricate HA-PEGDA scaffolds with
high shape fidelity and resolution (200 µm). HA
incorporation had no negative impact on the rheolo-
gical properties of the developed HA-based hydro-
gel inks, showing excellent printability for extrusion-
based 3D printing. PF127 was an excellent sacrificial
carrier, which provided good distribution of HA nan-
oparticles within the scaffolds. The HA content in
the 3D printed scaffolds did not significantly decrease
after the post-printing rinsing process was applied
to remove PF127. Among all the 3D printed scaf-
folds, scaffolds with 5 wt% HA showed the least swell-
ing ratio and degradation rate, and the highest com-
pression strength and hBMSCs attachment onto the
scaffolds. On the other hand, scaffolds with 2 wt%
HA showed slightly higher cell viability and higher
ALP activity, while also improving the calcium depos-
ition, compared to scaffolds containing 5 wt% HA.
Collectively, these results show that the fabricated 3D
printed HA-PEGDA scaffolds have promising applic-
ations for bone regeneration by exhibiting excellent
shape fidelity and promotion of hBMSCs adhesion
and osteogenic differentiation.
Data availability statement
All data that support the findings of this study are
included within the article (and any supplementary
files).
Biomed. Mater. 18 (2023) 065009
M Rajabi et al
Acknowledgments
The authors are particularly grateful for the import-
ant contribution of Professor Lyall R Hanton for
providing the XRD facility. We also gratefully
acknowledge Dr C John McAdam for his support
with DMA. The authors also appreciate the fund-
ing provided by the University of Otago Doctoral
Scholarship supporting this research.
ORCID iDs
Mina Rajabi https://orcid.org/0000-0002-1536-
1584
Jaydee D Cabral https://orcid.org/0000-0002-
5620-0330
Sarah Saunderson https://orcid.org/0000-0002-
6003-0653
Maree Gould https://orcid.org/0000-0003-3733-
1359
M Azam Ali https://orcid.org/0000-0002-0136-
6800
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10.1038_s41398-023-02450-1.pdf
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The source data can be available from the corresponding authors on reasonable
request.
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DATA AVAILABILITY The source data can be available from the corresponding authors on reasonable request.
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Translational Psychiatry
www.nature.com/tp
OPEN
ARTICLE
The association between gut microbiota and postoperative
delirium in patients
Yiying Zhang 1 ✉
Timothy T. Houle3, Edward R. Marcantonio4 and Zhongcong Xie
, Kathryn Baldyga1, Yuanlin Dong 1, Wenyu Song2, Mirella Villanueva1, Hao Deng3, Ariel Mueller3,
1 ✉
© The Author(s) 2023
Postoperative delirium is a common postoperative complication in older patients, and its pathogenesis and biomarkers remain
largely undetermined. The gut microbiota has been shown to regulate brain function, and therefore, it is vital to explore the
association between gut microbiota and postoperative delirium. Of 220 patients (65 years old or older) who had a knee
replacement, hip replacement, or laminectomy under general or spinal anesthesia, 86 participants were included in the data
analysis. The incidence (primary outcome) and severity of postoperative delirium were assessed for two days. Fecal swabs were
collected from participants immediately after surgery. The 16S rRNA gene sequencing was used to assess gut microbiota. Principal
component analyses along with a literature review were used to identify plausible gut microbiota, and three gut bacteria were
further studied for their associations with postoperative delirium. Of the 86 participants [age 71.0 (69.0–76.0, 25–75% percentile of
quartile), 53% female], 10 (12%) developed postoperative delirium. Postoperative gut bacteria Parabacteroides distasonis was
associated with postoperative delirium after adjusting for age and sex (Odds Ratio [OR] 2.13, 95% Confidence Interval (CI):
1.09–4.17, P = 0.026). The association between delirium and both Prevotella (OR: 0.59, 95% CI: 0.33–1.04, P = 0.067) and Collinsella
(OR: 0.57, 95% CI: 0.27–1.24, P = 0.158) did not meet statistical significance. These findings suggest that there may be an association
between postoperative gut microbiota, specifically Parabacteroides distasonis, and postoperative delirium. However, further
research is needed to confirm these findings and better understand the gut-brain axis’s role in postoperative outcomes.
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Translational Psychiatry
(2023) 13:156 ; https://doi.org/10.1038/s41398-023-02450-1
INTRODUCTION
Postoperative delirium one of the most common postoperative
complications in older patients [1], and is associated with
nosocomial complications [2]; extended hospital stays [3], a
higher chance of institutional discharge [4, 5], and increased
morbidity [5–8] and mortality [9, 10]. One study has estimated the
annual healthcare costs in the United States attributable to
postoperative delirium to be $32.9 billion [11]. Despite its clinical
importance, there are currently no targeted interventions for
postoperative delirium due to an incomplete understanding of its
pathogenesis.
Inflammation, neuroinflammation, and Alzheimer’s disease
neuropathogenesis (e.g., phosphorylated tau) have been reported
as biomarkers and pathogenesis of postoperative delirium
[1, 12, 13]. However, identifying the causes, contributing factors,
pathogenesis, and biomarkers of postoperative delirium remains
to be fully elucidated, hindering progress in the field.
The gut microbiota constitutes up to 95% of the total human
microbiota [14], and it is widely recognized that the gut-brain axis
role in regulating brain function [15–18].
plays an essential
Dysregulation of the gut microbiota has been associated with
alterations in immune function and increased risk of certain
diseases [19]. Specifically, gut microbiota dysbiosis, an imbalance
of gut microbiota associated with adverse outcomes, has been
linked to cognitive impairment and Alzheimer’s disease neuro-
pathogenesis [14, 20–23].
In a previous study of 18-month-old mice, anesthesia and
surgery were associated with changes in gut microbiota and
cognitive impairment 48 h and 5–8 days after the anesthesia/
surgery,
respectively [24]. A recent study showed that gut
microbiota alteration contributes to developing postoperative
cognitive impairment in mice [25]. Our recent animal study
showed that anesthesia/surgery reduced the abundance of gut
lactobacillus and induced delirium-like behavior in the 18-month-
old mice [26]. Treatment with lactobacillus or probiotics mitigated
the anesthesia/surgery-induced delirium-like behavior in the mice
[26]. These pre-clinical data suggest that gut microbiota may
contribute to the pathogenesis and serve as a biomarker of
postoperative delirium. However, to our knowledge, no clinical
studies have shown the association between gut microbiota and
postoperative delirium in patients.
This prospective observational cohort study aimed to investi-
gate the association between changes in postoperative gut
microbiota and postoperative delirium in patients. Specifically,
1Geriatric Anesthesia Research Unit, Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Harvard Medical School, Charlestown, MA 02129,
USA. 2Department of Medicine, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA 02115, USA. 3Department of Anesthesia, Critical Care and Pain Medicine,
Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA. 4Divisions of General Medicine and Gerontology, Department of Medicine, Beth Israel
Deaconess Medical Center, Harvard Medical School, Boston, MA 02215, USA.
email: [email protected]; [email protected]
✉
Received: 17 January 2023 Revised: 21 April 2023 Accepted: 25 April 2023
Y. Zhang et al.
2
we aimed to test the hypothesis that alterations in gut microbiota
may be associated with an increased risk of postoperative
delirium. The findings of this study could advance our under-
the risk factors, biomarkers, pathogenesis, and
standing of
potential
interventions for postoperative delirium in patients.
They may encourage further research in this area.
METHODS
Study enrollment
Upon approval by the Mass General Brigham Institutional Review Board,
this prospective observational cohort study was performed at Massachu-
setts General Hospital, Boston, MA, between 2016 and 2020. Patients 65
years or older, proficient in English, and scheduled for elective knee
replacement, hip replacement, or laminectomy under general or spinal
anesthesia at the study hospital were included.
(2) severe visual or hearing impairments;
Patients were excluded from participation if they had any of the
followings:
(1) past medical history of neurological and psychiatric
diseases, including Alzheimer’s disease (AD), other forms of dementia,
stroke, or psychosis;
(3)
current smokers; or (4) taking antibiotics within one week of surgery.
Trained clinical research coordinators approached eligible patients for
participation during preoperative clinic visits. Written informed consent
was obtained at the time of enrollment, prior to the initiation of the
study procedures. This manuscript is being reported following the
STrengthening the Reporting of OBservational studies in Epidemiology
(STROBE) criteria.
Anesthesia, surgery, and fecal sample collection and
measurement
Enrolled participants had a knee replacement, hip replacement, or repair of
spinal stenosis under general or spinal anesthesia. All participants received
standardized perioperative care,
including standard postoperative pain
management (e.g., patient-controlled analgesia with hydromorphone).
Depth of sedation was at the discretion of the treating provider but was
not captured in the current study. There have been no significant changes
in the surgery or anesthesia practice since the start of the study. Fecal
swabs were collected from participants immediately after surgery. The 16S
rRNA gene sequencing was performed by BGI America (Cambridge, MA) as
previously described [26]. The relative abundance in 16S rRNA sequencing
refers to the proportion of a specific bacterial species or group of bacteria
relative to the total number of bacterial sequences in each sample. In this
study, we employed relative abundance values to compare the bacterial
composition between different samples or groups.
Determination of postoperative delirium
Trained clinical research coordinators interviewed participants to deter-
mine the presence or absence of postoperative delirium on postoperative
days one and two when applicable. The Confusion Assessment Measure-
ment (CAM) was used in this study as a diagnostic algorithm to determine
the presence or absence of delirium, which has high reliability [27, 28]. The
incidence of postoperative delirium, the primary outcome, was assessed
using the CAM assessment once per day between 8:00 a.m. and 12:00
noon. Sixty-four of the 86 participants were evaluated on both days. One of
the 10 participants with postoperative delirium and 21 of
the 76
participants without postoperative delirium had the CAM only on one
day. Delirium was defined as present if it occurred on either postoperative
day one or day two. The preoperative CAM was not performed in the
present study because previous studies [29, 30] have shown that
participants who underwent elective surgeries had a very low incidence
of preoperative delirium.
The secondary outcome was the severity of postoperative delirium,
represented by the Memorial Delirium Assessment Scale (MDAS) [28, 31],
quantifying delirium-related symptoms based on 10 features. Each feature
is scored from 0 (best) to 3 (worst symptom) with a maximal score of 30.
We determined the MDAS scores for all patients, regardless of whether
they had the presence of delirium, based on the results from CAM, on that
two
day. The average MDAS score (averaged from both days if
postoperative tests were performed or
from one day if only one
postoperative test was performed) or peak MDAS score was used to
assess delirium severity independent of the results obtained from CAM. We
performed the postoperative MMSE as part of CAM [27, 32] and also for
MDAS calculation on postoperative day one and day two [32].
losing important
input variables without
Dimension-reduction algorithm (DASH)
We developed a specialized feature engineering process to reduce the
information.
complexity of
Specifically, principal component analysis (PCA) was performed among
740 gut bacteria to reduce these variables’ dimensions (one variable per
gut bacteria). The eigenvalue above the 3.0 PC component was chosen,
representing 72.8% of the total variance in the dataset. Second, bacteria
from the selected PC component were ranked according to the
contribution index from highest score (e.g., ±0.12) to lowest score (e.g.,
± 0.003), resulting in the top 35 gut bacteria. Third, gut bacteria without
information at the species level were excluded, resulting in 15 gut bacteria.
Fourth, eight gut bacteria were selected from the 15 gut bacteria based on
domain expertise, showcasing that these eight bacteria were associated
with human diseases from previous studies. Finally, three gut bacteria
(Parabacteroides distasonis, Prevotella, and Collinsella), excluding the other
five gut bacteria, were investigated in the present study based on their
relevance with inflammation, which contributes to cognitive dysfunction
[33–38].
Statistical analysis
Descriptive statistics were conducted using methods appropriate for the
variables under this study. Means and standard deviations were used for
continuously scaled variables that were normally distributed. Medians and
25th and 75th percentiles were used for skewed or ordinal data. Frequency
counts and percentages or proportions were used for categorical variables.
Differences in baseline characteristics between those who did and did not
develop delirium were assessed with a t-test, Mann–Whitney U test (for
non-normal continuous data), chi-square, or Fisher’s exact test (in the case
of small cell counts), as appropriate.
A principal component analysis (PCA) was conducted to reveal hidden
patterns across high-dimensional bacteria abundance measurements at
the species level. Robust logistic regression was then used to assess the
relationship between the three selected gut bacteria and postoperative
delirium as the binary outcome. Results were presented as odds ratio (OR)
per one unit of relative abundance of gut bacteria change in the
biomarker and their associated 95% confidence intervals (CI). Linear
regression was used to evaluate the association between bacteria and
delirium severity (using both average and peak MDAS scores) as the
continuous outcome, with results presented as a mean difference (beta
coefficient [β]) and its associated 95% confidence interval. Models were
created to adjust for the associations between the biomarkers and
outcomes for age and sex for both the primary and secondary outcomes.
Variables for adjustment were based on previous studies as deemed
clinically relevant.
The present study did not consider multiple comparisons because of the
exploratory nature in which the final association model was based,
reduced inputs with a small number (e.g., three bacteria) of independent
variables according to the statistical principles described before [39]. All
analyses were conducted using R version 4.0.5 statistical software (Vienna,
Austria). All analyses used two-tailed hypothesis testing where appropriate,
with statistical significance interpreted at p < 0.05.
and
Archive
Reporting summary
The paper’s raw sequence data has been deposited in the Genome
Sequence
https://
www.ncbi.nlm.nih.gov/sra/PRJNA967718. The accession number for the
data has been deposited in accordance with the guidelines set out by
Genomics, Proteomics & Bioinformatics in 2021 and Nucleic Acids Res in
2022. This information is essential for anyone wishing to access or use the
data for further research or analysis. Simplesubmission.com performed the
submission of the raw sequence data.
accessible
publicly
at
is
RESULTS
A total of 491 patients were screened, of which 220 participants
were enrolled. A total of 117 participants were excluded owing to
becoming ineligible after enrollment (N = 3), no longer expressing
interest in participating (N = 22), cancellation/rescheduling sur-
gery (N = 24), not having enough DNA during extraction (N = 63),
or not completing the postoperative CAM testing (N = 5). Thus,
103 participants were included in the gut microbiota cohort. Of
these, 17 participants were further excluded due to DNA sample
contamination (N = 5) and poor quality of 16S rRNA gene
Translational Psychiatry
(2023) 13:156
sequencing (N = 12). Thus, 86 participants were included in the
data analysis (Fig. 1). There were no significant differences in the
demographic characteristics between the participants included
Fig. 1 Flow diagram. The flow diagram shows that 491 participants
were screened for the studies, and 220 were initially enrolled. One
hundred seventeen participants were excluded after enrollment,
and 103 were included in the gut microbiota cohort. During the
analysis, 17 additional participants were excluded, resulting in 86
participants for the final data analysis.
Y. Zhang et al.
3
(N = 86) and those excluded (N = 134; Supplemental Table 1).
There were no significant complications among participants
during the immediate postoperative period.
Ten of the 86 (12%) participants developed postoperative
delirium (Table 1). The baseline demographic and clinical
characteristics of the 86 participants were presented in Table 1.
There were no significant differences in age, gender, ethnicity,
surgery type, anesthesia type, or preoperative Mini-Mental State
Examination (MMSE) score between the participants with post-
operative delirium (N = 10) and those without postoperative
delirium (N = 76). The participants who developed postoperative
delirium had lower postoperative MMSE scores and higher MDAS
scores than the participants who did not develop postoperative
delirium (Table 1).
Postoperative gut microbiota abundances were associated
with the incidence of postoperative delirium
A total of 740 postoperative gut bacteria were identified and used
in the principal component analysis (PCA). The participants with
and without postoperative delirium showed a significant differ-
ence in the index of principal component 8 (Supplementary
Fig. 1). Using the methods described above, three gut bacteria
were identified, with the association between these bacteria and
the incidence and severity of delirium presented in Table 2. In
unadjusted analyses, the abundance of postoperative gut bacteria
Parabacteroides distasonis was associated with postoperative
delirium (OR 1.97, 95% CI: 1.04–3.74, P = 0.038). There was no
statistically significant association between the abundances of
Prevotella (OR 0.62, 95% CI: 0.36–1.07, P = 0.085) or Collinsella (OR
0.60, 95% CI: 0.28–1.30, P = 0.197) and postoperative delirium
(Table 2).
After the adjustment for age and sex, similar results were
observed. The abundance of postoperative Parabacteroides
distasonis was significantly associated with postoperative delirium
(OR 2.13, 95% CI: 1.09–4.17, P = 0.026). Prevotella (OR: 0.59, 95% CI:
0.33–1.04, P = 0.067) and Collinsella (OR 0.57, 95% CI: 0.27–1.24,
P = 0.158) were not significantly associated with the incidence of
postoperative delirium. Further, the abundance of postoperative
Parabacteroides distasonis, Prevotella and Collinsella were not
Table 1. Demographic characteristics of the participants.
Age, median (25–75% percentile of quartile)
Female, n (%)
Non-white or Hispanic, n (%)
Education years, median (25–75% percentile of quartile)
Surgery type, n (%)
Knee replacement
Hip replacement
Spinal stenosis
Anesthesia type, n (%)
General
Spinal
MMSE, median (25–75% percentile of quartile)
Pre-surgery score
Post-surgery score
MDAS score (average)
mean ± SD
MDAS score (peak)
mean ± SD
Delirium (N = 10)
72.0 (70.5–75.3)
4 (40)
1 (10)
16 (16.0–16.0)
No Delirium (N = 76)
71.0 (69.0–76.8)
42 (46)
3 (3.9)
16.4 (16.0–18.0)
5 (50)
3 (30)
2 (20)
6 (60)
4 (40)
48 (63)
23 (30)
5 (7)
40 (53)
36 (47)
29.0 (29.0–30.0)
27.5 (26.5–28.6)
5.15 ± 1.53
29.0 (28.0–30.0)
29.0 (28.5–30.0)
1.98 ± 1.39
6.60 ± 1.78
2.45 ± 1.64
P value
0.499
0.521
0.289
0.866
0.332
0.661
0.734
0.004
<0.01
<0.01
MMSE mini-mental status examination, MDAS Memorial Delirium Assessment Scale, SD standard deviation.
Translational Psychiatry
(2023) 13:156
4
Y. Zhang et al.
Table 2. Association between gut bacteria and postoperative deliriuma.
Presence of postoperative deliriumb
Parabacteroides distasonis
Prevotella
Collinsella
The severity of postoperative deliriumc
Unadjusted
Odds ratio (95%CI)
1.97 (1.04–3.74)
0.62 (0.36–1.07)
0.60 (0.28–1.30)
Unadjusted
Parabacteroides distasonis
Prevotella
β coefficient (95% CI)
0.23 (−0.06 to 0.52)
−0.12 (−0.32 to 0.08)
−0.14 (−0.45 to 0.17)
P value
0.038
0.085
0.197
P value
0.123
0.242
Adjusted for age and sex
Odds ratio (95% CI)
2.13 (1.09–4.17)
0.59 (0.33–1.04)
0.57 (0.27–1.24)
Adjusted for age and sex
β coefficient (95% CI)
0.21 (−0.13 to 0.55)
−0.11 (−0.32 to 0.09)
−0.10 (−0.45 to 0.24)
P value
0.026
0.067
0.158
P value
0.226
0.288
Collinsella
aModels were created to adjust the associations between the bacteria and outcomes for age and sex based on previous studies as deemed clinically relevant.
bResults are presented as odds ratio (OR) per one unit change in gut bacteria value and its associated 95% confidence intervals (CI).
cResults are presented as the beta coefficient (β) per one unit change in gut bacteria value and its associated 95% CI.
0.564
0.383
Fig. 2 Different postoperative gut bacteria between participants with and without postoperative delirium. Participants who developed
postoperative delirium (N = 10) had a higher postoperative relative abundance of gut bacteria of Parabacteroides distasonis (A) a lower
abundance of postoperative gut bacteria of Prevotella (B) but not Collinsella (C) than the participants who did not develop postoperative
delirium (N = 76). The box indicates the median (50th percentile), the first quartile (25th percentile), and the third quartile (75th percentile) of
the abundance of bacteria. Mann–Whitney U test was used to determine the differences in bacteria abundance between the participants with
postoperative delirium and those without postoperative delirium.
associated with the severity of postoperative delirium, both before
and after adjusting for age and sex (Table 2).
Postoperative gut microbiota abundances were different
between the participants with and without postoperative
delirium
Our analysis revealed that participants who developed post-
operative delirium had higher Parabacteroides distasonis in their
postoperative gut microbiota (average abundance 1.659 ± 0.984)
compared to those who did not develop delirium (average
abundance 0.850 ± 1.080) although, this difference was only
marginally significant (Mann–Whitney U test, P = 0.064, Fig. 2A).
Additionally, we observed lower levels of postoperative gut
Prevotella in participants with postoperative delirium (average
abundance 0.803 ± 1.321) compared to those without post-
operative delirium (average abundance 1.687 ± 1.459), again
significant
making
(Mann–Whitney U test, P = 0.085, Fig. 2B). However, we did not
observe any significant difference in postoperative gut Collinsella
abundance between the two groups (average abundance
0.442 ± 0.932 vs. 0.903 ± 1.040, Mann–Whitney U test, P = 0.212,
Fig. 2C).
only marginally
difference
this
DISCUSSION
In this prospective observational cohort study, we observed an
association between the gut bacteria Parabacteroides distasonis and
the incidence of postoperative delirium in patients. These findings
suggest that gut microbiota dysbiosis,
linked to several brain
dysfunctions [15–18], may play a role in the development of
postoperative delirium. Further studies are needed to confirm these
results and explore the potential of gut bacteria as biomarkers,
pathogenesis, and intervention targets for postoperative delirium.
In the present study, the incidence of postoperative delirium
was 12%, consistent with the incidence rates reported in other
clinical investigations of postoperative delirium [40–43] that have
reported rates ranging from 5.1% to 19.4% in patients. The
characteristics of the participants,
including their average and
peak MDAS scores, are presented in Table 1. Future studies will
include a table that
lists the characteristics of participants
according to various abundances of the gut microbiota.
Increases in the abundance of postoperative gut bacteria
Parabacteroides distasonis were associated with an increased
incidence of postoperative delirium in the patients after adjusting
age and sex. Although generally lower in delirious patients,
decreases in the abundance of Prevotella and Collinsella were not
Translational Psychiatry
(2023) 13:156
significantly associated with delirium. Moreover, patients who
developed postoperative delirium had a higher abundance of
postoperative gut Parabacteroides distasonis and a lower abun-
dance of postoperative gut Prevotella and Collinsella. These data
suggest
the gut-brain axis to
postoperative delirium, and certain postoperative gut bacteria
that are associated with postoperative delirium.
the potential contribution of
The objective of the present study was to investigate the
potential association between postoperative gut microbiota and
postoperative delirium in patients. We used postoperative
samples in this proof-of-concept study to establish a system for
determining whether postoperative gut microbiota may contri-
bute to the development of postoperative delirium in patients.
Parabacteroides distasonis is a bacteria implicated in Crohn’s
Disease, ulcerative colitis [36] and Prevotella is associated with
chronic inflammatory disease [37]. Therefore, future studies should
investigate the potential association between postoperative
delirium and Crohn’s Disease, ulcerative colitis, and chronic
inflammatory diseases.
In addition to inflammation, Parabacter-
oides distasonis may influence postoperative delirium by generat-
ing metabolites that could directly impact brain function. For
instance, some gut bacteria, such as those producing short-chain
fatty acids (SCFAs), have been shown to affect brain function and
behavior, and Parabacteroides distasonis may produce SCFAs or
other metabolites with similar effects. Moreover, changes in the
gut microbiome’s composition, such as changes in the abundance
of Parabacteroides distasonis, may have downstream effects on
other gut bacteria, which could affect brain function. Future
studies to test this hypothesis are warranted.
Previous studies have shown that patients who developed
postoperative delirium (N = 20) and who did not develop post-
operative delirium (N = 20) had a different abundance of preopera-
tive gut bacteria [44]. Specifically, gut bacteria Proteobacteria,
Enterobacteriaceae, Escherichia shigella, Klebsiella, Ruminococcus,
Roseburia, Blautia, Holdemanella, Anaerostipes, Burkholderiaceae,
Peptococcus, Lactobacillus, and Dorea were abundant in the patients
with postoperative delirium, and Streptococcus equinus and Blautia
in the patients without postoperative
hominis were abundant
delirium [44]. However, this previous study is different from the
current study, as it determined preoperative, not postoperative, gut
microbiota, did not establish an association with the incidence of
postoperative delirium, and did not assess the severity of delirium
with the MDAS. Thus, the previous study did not demonstrate the
association between gut microbiota and postoperative delirium in
patients. Future studies should include the systematical determina-
tion of the association between postoperative delirium and both
pre-and postoperative gut microbiota in a larger-scale study.
Interestingly, the present study did not find associations between
the three gut bacteria and the severity of postoperative delirium in
patients, as represented by average MDAS scores (Table 2) or peak
MDAS scores (data not shown). However, a previous study also
indicates that some biomarkers are only associated with the
incidence, not severity, of postoperative delirium in patients [45].
Notably, the average and peak MDAS scores of the present
study participants were 5.15 and 6.60, respectively (Table 1).
Although Breitbart et al. stated that MDAS Scores ≥13 indicate the
the study’s participants included
presence of delirium [31],
psychiatry consult patients [31]. Marcantonio et al. showed that
the best MDAS cutoff for postoperative delirium was 5 in the
participants with surgery for hip fracture repair [28]. Therefore, it is
reasonable that the average MDAS score was 5.15 (average) and
6.60 (peak) in the present study.
One strength of our study was the use of a Dimension-reduction
Algorithm in Small Human-datasets
that combined
statistical dimensionality reduction algorithms and domain
expertise to efficiently extract accurate signals from the noise
background in high-throughput data screening. Given the small
sample size and complex data structure, data-driven methodology
(DASH)
Translational Psychiatry
(2023) 13:156
Y. Zhang et al.
5
alone was insufficient in finding the relationship between gut
microbiota and postoperative delirium in patients. By incorporat-
ing our current knowledge into the data-driven methodology, we
could filter through several hundred variables in a small dataset,
making this method particularly powerful for analyzing small but
high-dimensional patient-level datasets. However,
it should be
noted that the selection strategy used with principal component
analysis (PCA), such as excluding bacteria without species, could
result in selection bias in the present study. Nevertheless, in the
present study, we identified Parabacteroides distasonis because it
is associated with inflammation-related disorders
[36], and
inflammation is associated with postoperative delirium [12].
Prevotella was selected because of its association with chronic
inflammatory disease [37], and Collinsella was chosen because of
its known association with cumulative inflammatory response [38].
This study had limitations, including a small sample size from a
single center and a low number
(10) of participants who
developed postoperative delirium. However, similar studies with
smaller sample sizes (N = 11 [46] and N = 14 [47]) have drawn
solid conclusions. Out of 220 enrolled participants, 134 were
excluded mainly due to insufficient DNA amounts in the swapped
samples. There were no significant differences in the character-
istics between the 86 participants in the final data analysis and the
134 excluded patients, except for anesthesia type (Supplemental
Table 1). However, previous studies have shown that anesthesia
type does not affect the incidence of postoperative delirium
[40, 48]. In addition, we did not perform preoperative CAM in the
participants since these participants had elective cases and the
rate of preoperative delirium would be very low based on the
findings from previous studies by Mei et al. (0 of 606 participants)
[29] and Shi et al. (3 of 192 participants) [30]. The presence of
postoperative delirium in the present study may not be called
incident delirium but rather postoperative delirium.
In conclusion, this proof-of-concept study established a potential
link between postoperative gut bacteria Parabacteroides distasonis
and postoperative delirium in patients. Patients who developed
postoperative delirium had a higher abundance of this bacteria than
those who did not. However, the association between postoperative
gut microbiota and postoperative delirium was relatively weak in
this study, and therefore, the clinical relevance of these findings
needs further investigation in future research. Nevertheless, these
findings suggest
that gut microbiota dysbiosis may influence
postoperative delirium, but more research is necessary to under-
stand the role of gut microbiota in this condition.
DATA AVAILABILITY
The source data can be available from the corresponding authors on reasonable
request.
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ACKNOWLEDGEMENTS
This research was supported by R21 AG065606 to YZ, R21 HD098754, R01 AG062509,
and RF1 AG070761 from the National Institutes of Health, Bethesda, MD, and Henry L.
Beecher Professorship from Harvard University to ZX. The authors appreciate Xin Ma
from Shanghai Tenth People’s Hospital and Tongji University School of Medicine for
parts of the data calculation. This study received assistance from the Anesthesia
Research Center (ARC), housed within the Department of Anesthesia, Critical Care
and Pain Medicine at Massachusetts General Hospital.
AUTHOR CONTRIBUTIONS
Study concept and design: ZX, YZ, and ERM. Acquisition of data: KB, YD, MV, and YZ.
Analysis and interpretation of data: YZ, WS, HD, AM, TTH, and ZX. Drafting of the
manuscript: YZ and ZX. Critical revision of the manuscript for important intellectual
content: ZX, AM, and ERM. Obtained funding: ZX and YZ. Administrative, technical,
and material support: ZX, YD, KB, and YZ. Study supervision: ZX. All authors approved
the manuscript.
COMPETING INTERESTS
The authors declare no competing or conflicting interests to disclose for the present
study. ZX provided consulting services to Shanghai’s 9th and 10th hospitals, Baxter
(invited speaker), NanoMosaic, and Journal of Anesthesiology and Perioperative Science
in the last 36 months.
ADDITIONAL INFORMATION
Supplementary information The online version contains supplementary material
available at https://doi.org/10.1038/s41398-023-02450-1.
Correspondence and requests for materials should be addressed to Yiying Zhang or
Zhongcong Xie.
Reprints and permission information is available at http://www.nature.com/
reprints
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(2023) 13:156
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Translational Psychiatry
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| null |
10.1126_sciadv.adg1671.pdf
|
Data and
materials availability: All data are available in the main text and/or the Supplementary
Materials. The raw FASTQ files of the scRNA-seq and the processed files (output from
CellRanger) are accessible through GEO (accession number: GSE224031). The code for data
analyses is available on figshare (doi: 10.6084/m9.figshare.22490956) and via the link: https://
figshare.com/s/ca568d8f44d020cb6389.
| null |
S C I E N C E A D VA N C E S | R E S E A R C H A R T I C L E
D E V E LO P M E N TA L B I O LO G Y
Atoh1 drives the heterogeneity of the pontine nuclei
neurons and promotes their differentiation
Sih-Rong Wu1,2, Jessica C. Butts2,3,4, Matthew S. Caudill1,2, Jean-Pierre Revelli2,3,
Ryan S. Dhindsa2,3, Mark A. Durham2,5,6, Huda Y. Zoghbi1,2,3,4,7*
Pontine nuclei (PN) neurons mediate the communication between the cerebral cortex andthe cerebellum to
refine skilled motor functions. Prior studies showed that PN neurons fall into two subtypes based on their an-
atomic location and region-specific connectivity, but the extent of their heterogeneity and its molecular drivers
remain unknown. Atoh1 encodes a transcription factor that is expressed in the PN precursors. We previously
showed that partial loss of Atoh1 function in mice results in delayed PN development and impaired motor learn-
ing. In this study, we performed single-cell RNA sequencing to elucidate the cell state–specific functions of Atoh1
during PN development and found that Atoh1 regulates cell cycle exit, differentiation, migration, and survival of
PN neurons. Our data revealed six previously not known PN subtypes that are molecularly and spatially distinct.
We found that the PN subtypes exhibit differential vulnerability to partial loss of Atoh1 function, providing in-
sights into the prominence of PN phenotypes in patients with ATOH1 missense mutations.
Copyright © 2023
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
License 4.0 (CC BY).
INTRODUCTION
Motor skills such as picking up a cup of coffee or hitting a speedy
baseball with a bat require communication between the cerebral
cortex and the cerebellum (1). These connections are mediated
through the pontine nuclei (PN) that are located in the ventral
pons and composed of mostly glutamatergic neurons (2–7).
Given their pivotal role in motor functions, several efforts have
been made to understand how the PN develop in mammals. PN
neurons originate from a group of proliferating neuroepithelial
cells residing in the rhombic lip (RL) located in the developing
hindbrain. Specifically, Wnt1- and Atoh1-expressing cells within
the caudal RL (cRL) give rise to glutamatergic PN neurons (8–
10). PN neurons are born during mouse embryonic day 12.5
(E12.5) to E18.5 and migrate tangentially along the anterior extra-
mural stream (AES) until they reach the ventral pons (11, 12).
A previous study of PN in rabbit and cat has suggested that PN
can be categorized into subpopulations on the basis of their ana-
tomical location and connectivity (13). Anatomically, the PN are
divided into the basal pontine nucleus (BPN) and the reticuloteg-
mental nucleus (RtTg) at the anterior-ventral and posterior-dorsal
part of the PN, respectively. Subpopulations of PN neurons have
been proposed on the basis of their positions along the rostro-
caudal axis, which inherit the expression pattern of Hox2-Hox5
from their progenitors at the cRL (14). Moreover, tracing experi-
ments demonstrated that the corticopontine connectivity was estab-
lished in a partially region-specific manner (15, 16), suggesting that
PN neurons are functionally diverse on the basis of their regional
cortical inputs. However, the extent of PN heterogeneity remains
1Department of Neuroscience, Baylor College of Medicine, Houston, TX, USA. 2Jan
and Dan Duncan Neurological Research Institute at Texas Children’s Hospital,
Houston, TX, USA. 3Department of Molecular and Human Genetics, Baylor
College of Medicine, Houston, TX, USA. 4Howard Hughes Medical Institute,
Baylor College of Medicine, Houston, TX, USA. 5Program in Developmental
Biology, Baylor College of Medicine, Houston, TX, USA. 6Medical Student Scientist
Training Program, Baylor College of Medicine, Houston, TX, USA. 7Department of
Pediatrics, Baylor College of Medicine, Houston, TX, USA.
*Corresponding author. Email: [email protected]
unclear, and the molecular determinants of PN heterogeneity are
not known.
Another outstanding question is how a pool of seemingly ho-
mogenous Atoh1 + progenitors give rise to diverse progenies in
the PN. Atoh1, or Atonal homolog 1, encodes a basic helix-loop-
helix transcription factor ATOH1 that is required for the develop-
ment of a variety of neurons in the hindbrain (9) and the dorsal
spinal cord (17), hair cells in the inner ear (18), Merkel cells in
the skin (19), and secretory cells in the gut (20). In the hindbrain,
Atoh1-lineage neurons contribute to many key components of the
proprioceptive pathway, including the PN (9, 21). A recent report
implicated a homozygous missense variant in ATOH1 in two
human patients with global developmental delay, motor function
deficits, pontocerebellar hypoplasia, and hearing loss (22). It is
thus of great interest and clinical relevance to understand how
Atoh1 shapes proper PN development.
Loss of both copies of Atoh1 in mice results in complete absence
of the PN, whereas loss of one copy of Atoh1 in mice has no observ-
able change in the PN (23), making it difficult to study the function
of Atoh1 during PN development using either the Atoh1 knockout
or the heterozygous mouse model. We previously found that substi-
tuting the serine at position 193 with an alanine (S193A) resulted in
an Atoh1 hypomorphic allele (24). Mice carrying Atoh1S193A over an
Atoh1 null allele (Atoh1S193A/−) showed delayed development in the
PN neurons at postnatal day 0 (P0) and impaired motor learning as
adults. Given the heterogeneity of the PN neurons, it is unclear
whether this hypomorphic mutation affects the development of
all PN subtypes equally. In other contexts, we learn that different
cell types have differential vulnerability to perturbation in genes im-
plicated in neurodevelopmental disorders (25, 26). For example,
despite being expressed in both neurogenic niches, increased level
of the transcription factor Foxg1 compromises the neurogenesis of
excitatory neurons but not inhibitory neurons (26). We therefore
hypothesized that PN subtypes are molecularly distinct and have
differential vulnerability to partial loss of function of Atoh1.
In this study, we profiled the single-cell transcriptomes of the PN
neurons and characterized the molecular and cellular phenotypes of
Wu et al., Sci. Adv. 9, eadg1671 (2023) 30 June 2023
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S C I E N C E A D VA N C E S | R E S E A R C H A R T I C L E
the PN in Atoh1S193A/− mice at the single-cell resolution. We found
cell state–specific roles of Atoh1 during PN development including
regulating cell cycle exit, differentiation, migration, survival, and
cellular heterogeneity of the PN neurons. In addition, our single-
cell RNA sequencing (scRNA-seq) data revealed six unreported
subtypes of the PN neurons at P5 and identified subtype-specific
markers. We found that the PN subtypes have differential vulnera-
bilities to partial loss of function of Atoh1. Our study provides com-
prehensive evidence of how a transcription factor plays multiple
roles during neuronal development and demonstrates that neuronal
subtypes could have differential sensitivity to perturbation of a gene
that is expressed in the precursors of all subtypes.
RESULTS
Phospho-mutation of Atoh1 at serine-193 leads to PN
hypoplasia in mice
To test whether partial loss of function of Atoh1 results in PN ab-
normalities postnatally, we compared the morphology of the PN in
mice with hypomorphic mutation to those in control mice at P0 and
P21. We crossed Atoh1S193A/+ mice to Atoh1lacZ/+, whereby the lacZ
allele replaced the coding region of Atoh1, creating a null allele. Fol-
lowing the tangential migration along the AES (Fig. 1A), most of the
PN neurons finished migrating at P0 in control mice (Fig. 1B, left,
arrow), with little lacZ signal in the AES (arrowhead). We observed
a decreased intensity of the lacZ staining in the PN in Atoh1S193A/lacZ
mice (Fig. 1B, right, arrow), indicating potential neuronal loss at P0
in Atoh1S193A/lacZ mice. In addition, consistent with our previous
study (24), we found that there were more lacZ+ neurons retained
at the AES in Atoh1S193A/lacZ mice (Fig. 1B, right, arrowhead), sug-
gesting a developmental delay or a migration deficit in some PN
neurons upon partial loss of Atoh1 function.
To test whether those neurons retained at the AES ever reach
their destination, we examined the PN morphology at a juvenile
age, P21. Given that Atoh1 is turned off postnatally in the PN, we
permanently labeled the Atoh1-lineage neurons using a Cre-depen-
dent lacZ reporter (27) in combination with an Atoh1Cre/+ knock-in
mouse in which one copy of Atoh1 is functionally a null allele
because the Cre replaced the coding region of Atoh1 (28). This
allows us to visualize the PN postnatally (Fig. 1C). We found that
the size of the PN was reduced in Atoh1Cre/S193A; Rosalsl-lacZ/+
animals compared to Atoh1Cre/+; Rosalsl-lacZ/+ animals (Fig. 1D).
We further confirmed this phenotype quantitatively by performing
immunofluorescence staining with osteopontin antibody, a pan-PN
neuronal marker (fig. S1). These data suggested that partial loss of
function of Atoh1 not only leads to delayed development of PN but
also leads to PN hypoplasia in mice reminiscent of the malforma-
tion of the PN in patients with ATOH1 missense variant (22). To-
gether, these data reinforced the importance of Atoh1 in PN
development and demonstrated that Atoh1 hypomorphic allele
could provide a useful mouse model to dissect the functions of
Atoh1 during PN development.
fluorescent
Progression of the PN development is impaired in
Atoh1S193A/− animals
To investigate the mechanisms by which Atoh1 regulates normal
PN development, we performed scRNA-seq in developing
hindbrains from control mice and mice with Atoh1 hypomorphic
reporter
mutation. We used a Cre-dependent
(Rosalsl-TdTomato) to permanently label Atoh1-lineage neurons with
Atoh1Cre/+ mice (Fig. 2, A and B). During development, PN
progenitors and migrating PN neurons are spatially dispersed
between the cRL to ventral pons. Therefore, to ensure that we
capture all PN progenitors and migrating neurons, we collected
whole hindbrains from Atoh1Cre/+; Rosalsl-tdTomato/+ (hereafter re-
ferred to as control) and Atoh1Cre/S193A; Rosalsl-tdTomato/+ (hereafter
referred to as Atoh1S193A/−) embryos. After single-cell dissociation,
Atoh1-lineage (TdTom+) neurons were isolated by fluorescence-
activated cell sorting (FACS), followed by scRNA-seq with 10X
Genomics Chromium platform (Fig. 2C). We collected samples at
two time points, E14.5 and E18.5, to examine the phenotypes of
Atoh1S193A/− mice in the middle and at the end of neurogenesis
of the PN neurons, respectively.
We first focused on the E14.5 time point to explore the role of
Atoh1 during PN development. After filtering out low-quality cells
(see Materials and Methods), a total of 22,191 cells were retained
from control hindbrains, and 22,206 cells were retained from
Atoh1S193A/− hindbrains (n = 3 animals per genotype). Among
the cells in the hindbrain, we identified the PN cells based on the
clusters that expressed the established markers of the AES and PN
(fig. S2, A to D). This resulted in 2451 control PN cells and 3124
Atoh1S193A/− PN cells for downstream analyses. After unbiased clus-
tering of the PN cells, we annotated each cluster by the expression of
the known markers (Fig. 2D, fig. S2E, and data S1). Cells at different
Fig. 1. Phospho-mutation of Atoh1 at serine-193 leads to PN hypoplasia in mice. (A) Schematic representation of the PN development in mice. PN neurons migrate
from cRL to ventral pons through AES during E12.5 to E18.5. Mb, midbrain; Cb, cerebellum. (B) Whole-mount X-galactosidase (X-gal) staining on mouse hindbrain at P0
(ventral view). The arrows and arrowheads denote the neurons at the PN and in the AES, respectively. Scale bars, 1 mm. (C) Strategy to label the Atoh1-lineage neurons.
Atoh1Cre/+ knock-in mouse was crossed with mouse carrying either wild-type or hypomorphic Atoh1 (Atoh1S193A) and a Cre-dependent lacZ allele. (D) Whole-mount X-gal
staining on mouse hindbrain at P21 (ventral view). The dashed lines outline the area of the PN. Scale bars, 1 mm.
Wu et al., Sci. Adv. 9, eadg1671 (2023) 30 June 2023
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S C I E N C E A D VA N C E S | R E S E A R C H A R T I C L E
Fig. 2. Atoh1S193A/− animals exhibited an impaired progression of PN development. (A) Strategy to label the Atoh1-lineage neurons. Atoh1Cre/+ knock-in mouse was
crossed with mouse carrying either wild-type or hypomorphic Atoh1 (Atoh1S193A) and a Cre-dependent TdTomato (TdTom) reporter. (B) An image of three-dimensional
rendering of E14.5 mouse head using lightsheet microscopy (back view). The TdTom represents the Atoh1-lineage neurons in developing hindbrain. EGL, external granule
layer; SC, spinal cord. (C) Workflow for scRNA-seq of Atoh1-lineage neurons from E14.5 and E18.5 mouse hindbrain. Hindbrains were collected from E14.5 and E18.5
control and Atoh1S193A/− embryos (n = 3 per genotype for each time point). After enrichment by sorting, single-cell transcription profiles were captured by 10X Genomics
Chromium platform. (D) Expression levels of the selective markers visualized on uniform manifold approximation and projection (UMAP). Ccnd1, Atoh1, Nhlh1, and Mapt
are known markers for progenitors, intermediate progenitors, migrating neurons, and differentiated neurons, respectively. (E) UMAP of the five cell states identified in
developing PN at E14.5 by scRNA-seq. (F) Proportion of the cells in each cell state. The arrows indicate the direction of the change in Atoh1S193A/− animals. *False discovery
rate (FDR) < 0.01 and #FDR < 0.05.
progenitors
cell states during PN development were fully captured in our
dataset, including proliferating progenitors (Ccnd1+), postmitotic
(Atoh1 high), migrating neurons
intermediate
(Nhlh1+), and differentiated neurons (Mapt+) in both control and
Atoh1S193A/− samples (Fig. 2E). The migrating neurons constituted
the largest population and were divided into two clusters. We there-
fore annotated the two migrating populations as migrating neurons-
1 and migrating neurons-2 based on their maturity. The migrating
neurons-1 are neurons starting to differentiate and migrate away
from the cRL (Atoh1 lowNhlh1high), whereas
the migrating
neurons-2 are neurons expressing not only high level of the
marker for migration but also low level of the differentiated neuro-
nal marker (Nhlh1highMapt low) (Fig. 2, D and E).
Next, we calculated the percentage of the cells in each cell state in
control and Atoh1S193A/− animals and performed differential abun-
dance test using propeller with speckle R package (29). We found
that the proportion of the progenitors increased markedly from
9.3 to 31% with a significant decrease in the percentage of the mi-
grating neurons-2 from 29.3 to 19.7% in Atoh1S193A/− animals
(Fig. 2F and fig. S3A). The shift in the proportion of the cells in
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each state indicates that the progression of the PN development was
impaired in Atoh1S193A/− mice. Trajectory analysis with Slingshot
(30, 31) showed that in control embryos, more cells progressed
further in pseudo-time than in Atoh1S193A/− embryos (fig. S3, B
and C). Collectively, these data demonstrate that partial loss of
Atoh1 function leads to accumulation of the PN progenitors at
the expense of the differentiating PN neurons.
Partial loss of function of Atoh1 leads to decreased cell
cycle exit in PN progenitors and deficits in differentiation
and migration
We sought to investigate the mechanisms underlying the altered
proportions of the cells in each state in Atoh1S193A/− animals.
While we observed an increased proportion of the proliferating pro-
genitors in Atoh1S193A/− mice, the proportion of its downstream cell
state (i.e., the intermediate progenitors) was not altered (Fig. 2F).
Instead, the fraction of the migrating neurons-2 was significantly
decreased. Therefore, we proposed two mechanisms underlying
the observed phenotypes: Robust function of Atoh1 is required to
(i) drive cell cycle exit and (ii) promote differentiation and migra-
tion (fig. S3D). To validate our scRNA-seq data, we performed im-
munofluorescence staining and histological analyses on the
embryonic tissues. First, we performed immunofluorescence stain-
ing for MKI67, a marker for proliferating cells, on E14.5 hindbrains
of control and Atoh1S193A/− animals. On the basis of the staining of
ATOH1 and MKI67 at the cRL in control animals (fig. S4), the
MKI67+ proliferating progenitors and ATOH1+ intermediate pro-
genitors reside at the ventromedial and dorsolateral cRL, respective-
ly (Fig. 3A). In control animals, there were only few TdTom+ cells at
the cRL (Fig. 3B, asterisks), suggesting that the proliferating PN pro-
genitors become postmitotic and leave the cRL upon the onset of
Atoh1 expression. In contrast, there was an increased percentage
of the MKI67+ cells that overlapped with TdTom at the cRL in
Atoh1S193A/− mice (Fig. 3, B and C), indicating an accumulation
of proliferating progenitors at the cRL. The data suggest that
robust Atoh1 function is important for the cycling progenitors to
become postmitotic.
Next, to test whether the differentiating and migrating neurons
are reduced in Atoh1S193A/− mice, we used the thymidine analog, 5-
chloro-20-deoxyuridine (CldU) to label and quantify the number of
PN neurons born within the 24-hour window between E13.5 and
E14.5. We injected the pregnant dams with CldU at E13.5 and har-
vested the brains from E14.5 embryos. It has been shown that it
takes 1 to 2 days for the PN neurons to migrate from cRL to
ventral pons (12). Consistent with the literature, we found that
most of the CldU-labeled PN neurons (CldU+TdTom+) were
located at the AES 24 hours after injection in control animals
(Fig. 3, D and E). The number of the CldU+TdTom+ cells at the
AES was significantly reduced in Atoh1S193A/− mice (Fig. 3F), sug-
gesting that fewer neurons differentiated and migrated away from
the cRL in Atoh1S193A/− animals. These data validate our scRNA-
seq data in which we found that migrating population was decreased
in Atoh1S193A/− mice. Together, our data demonstrate that Atoh1
governs multiple biological processes in addition to specifying PN
neurons. Atoh1 is important for both cell cycle exit of the PN pro-
genitors and the differentiation and migration of the intermediate
progenitors.
The cell state–specific dysregulated pathways in
Atoh1S193A/− mice
To understand how Atoh1 mediates different biological processes
during PN development, we sought to identify the genes that
were dysregulated in each cell state in Atoh1S193A/− mice compared
to control mice. Using differential gene expression analysis, we
identified 362 differentially expressed genes (DEGs) [log2 fold
change (log2FC) > 0.25 and false discovery rate (FDR) < 0.05]
(data S2). We found that intermediate progenitors had the highest
number of DEGs, followed by migrating neurons-1 and progenitors
(Fig. 4A). We verified that the different number of DEGs per cell
state was not simply a result of differences in total cell number in
each cell state (fig. S5A). The cell states with the greatest transcrip-
tional dysregulation (i.e., progenitors, intermediate progenitors,
and migrating neurons-1) also expressed high levels of Atoh1
(Figs. 4A and 2D), which led us to test whether these DEGs were
directly regulated by Atoh1. We calculated the percentage of the
DEGs that had ATOH1 binding peak(s) identified by ATOH1 chro-
matin immunoprecipitation sequencing (32) and performed en-
richment analysis for each cell state. We found a significant
enrichment of DEGs with ATOH1-binding in progenitors (47%),
intermediate progenitors (48%), and migrating neurons-1 (42%)
(Fig. 4B), highlighting the direct impact of Atoh1 in these cell
states. We found several genes that were direct targets of Atoh1
and have been reported to play a role in PN development
(Fig. 4C, shaded box). For instance, Atoh1, which is known to reg-
ulate itself (33), was down-regulated in the progenitors. Barhl1, im-
portant for migration and survival of the PN neurons (34), was
down-regulated in both progenitors and intermediate progenitors
upon partial loss of Atoh1 function. Last, Nhlh1, essential for migra-
tion of the PN neurons (35), was decreased in the intermediate pro-
genitors and the migrating neurons. Moreover, we also identified
other DEGs such as Pcp4 and Rab15, whose roles have not been
characterized in PN development but have robust changes in ex-
pression levels in multiple cell states (Fig. 4C). Pcp4 has been iden-
tified as an Atoh1 target in cochlear hair cells (36). Rab15 was also
reported as an Atoh1 target in different Atoh1-lineage cells includ-
ing developing cerebellar granule neurons (32), Merkel cells (37),
cochlear hair cells (36), and neurons in dorsal neural tube (38).
Next, we focused on the three most affected cell states and per-
formed gene ontology (GO) enrichment analysis to identify the
pathways that were dysregulated in Atoh1S193A/− mice beyond the
known Atoh1 targets (Fig. 4D and data S3). In line with the
finding of increased proliferating cells in Atoh1S193A/− mice, cell
proliferation and cell cycle regulation including Notch signaling
were among the top enriched GO terms for the up-regulated
genes in the progenitor state (Fig. 4D, left). Consistent with the tra-
jectory analysis that showed an impaired differentiation process in
Atoh1S193A/− mice, down-regulated genes were enriched for neuron
differentiation and cytoskeleton organization ontologies across all
three cell states (Fig. 4D). These data supported our earlier findings
that Atoh1 plays roles in both cell cycle regulation, neuronal differ-
entiation, and migration. We found that up-regulated genes in both
intermediate progenitors and migrating neurons-1 were enriched
for regulators of apoptosis (Fig. 4D, middle and right). For instance,
the average level of Hrk, a member of proapoptotic Bcl-2 family, was
significantly increased in both intermediate progenitors and mi-
grating neurons-1 in Atoh1S193A/− mice (fig. S5B). In addition,
other genes involved in programmed cell death such as Pak3 and
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Fig. 3. Partial loss of function of Atoh1 leads to decreased cell cycle exit in PN progenitors and deficits in differentiation and migration. (A) Schematic illustration
of mouse coronal section at E14.5. The area of cRL was enlarged on the bottom with locations of three cell states being labeled. Me, medulla. (B) Immunofluorescence
staining of MKI67 on E14.5 mouse brain. The cropped view of cRL in control (top) and Atoh1S193A/− (bottom) mice. Atoh1-lineage neurons were labeled with TdTom, and
the nuclei were labeled with 40,6-diamidino-2-phenylindole (DAPI). The asterisk denotes the progenitors at cRL. Scale bars, 50 μm. (C) Quantification of the percentage of
MKI67 fluorescence overlapping with TdTom in control and Atoh1S193A/− mice. The gray symbols represent the five regions of interest quantified from each animal (n = 3
per genotype). The average percentage for each animal was shown in colored shape. The crossbar denotes the mean per genotype. ***Χ2(1) = 53.95, P = 2.06 × 10−13 by
mixed-model analysis of variance (ANOVA). (D) Schematic illustration of coronal section at E14.5. The dashed gray box indicates the region of interest shown in (E). V,
ventricle; PB, parabrachial nuclei; CN, cerebellar nuclei; VC, ventral cochlear nucleus. (E) Immunofluorescence staining of CldU on E14.5 mouse brain. The areas of AES for
control (left) and Atoh1S193A/− (right) animals were shown. Atoh1-lineage neurons were labeled with TdTom, and the nuclei were labeled with DAPI. Scale bars, 50 μm. (F)
The average number of the CldU+TdTom+ cells per region of interest (ROI) in control and Atoh1S193A/− mice. The gray symbols represent the 10 regions of interest quan-
tified from each animal (n = 3 per genotype). The average number for each animal was shown in colored shape. The crossbar denotes the mean per genotype. ***Χ2(1) =
39.81, P = 2.80 × 10−10 by mixed-model ANOVA.
Dap were also up-regulated in the intermediate progenitor state (fig.
S5, C and D). These data indicate that Atoh1 hypomorphic muta-
tion might lead to increased cell death. To test this hypothesis, we
performed terminal deoxynucleotidyl transferase–mediated deoxy-
uridine triphosphate nick end labeling (TUNEL) assay and found
that the number of TUNEL-positive cells was significantly increased
in Atoh1S193A/− mice at the lateral cRL (Fig. 4, E and F), where the
intermediate progenitors (Atoh1high) are located (Fig. 3A and fig.
S4). The increased cell apoptosis in Atoh1S193A/− animals may
count for the reduced size of PN at older age and suggest that
Atoh1 is important for the survival of the intermediate progenitors.
In summary, these data highlight the important roles of Atoh1 in
progenitors, intermediate progenitors, and migrating neurons by
regulating cell cycle, differentiation, migration, and survival of the
developing PN neurons.
PN neurons are molecularly heterogeneous
To determine whether the cellular identities of the differentiated PN
neurons are altered in Atoh1S193A/− mice, we performed scRNA-seq
in control and Atoh1S193A/− hindbrains at E18.5, when most of the
PN neurons have been born and migrated to the ventral pons. We
analyzed 1196 control and 1176 Atoh1S193A/− PN cells (n = 3
animals per genotype) (fig. S6, A to D). After unbiased clustering,
we annotated the clusters based on the known marker genes (Fig. 5,
A and B). As expected, most of the cells at E18.5 are differentiated
neurons marked by Mapt expression (Fig. 5A). The differentiated
neurons were classified into four subtypes (Fig. 5B), suggesting
that they are molecularly heterogeneous. We characterized the
marker genes for each subtype based on differential gene expression
analysis and named the differentiated PN neuron subtypes as em-
bryonic PN1(ePN1) to ePN4 (Fig. 5C, fig. S6E, and data S1). Similar
to the E14.5 data, we found a slightly increased proportion of the
progenitors in Atoh1S193A/− mice at E18.5 (Fig. 5D). In addition,
the percentage of ePN1 cells was significantly reduced from 14.9
to 5.5% (Fig. 5D and fig. S6F), while there was no significant
change in other ePN subtypes. These data indicate that ePN sub-
types have differential vulnerability to the Atoh1 hypomorphic
mutation.
To test whether the differential vulnerability of the PN subtypes
in Atoh1S193A/− embryos proceeds to the postnatal stage, we first de-
termined whether the PN neurons maintained their molecular het-
erogeneity at P5. We enriched the PN neurons by dissecting and
pooling the PN from control mice at P5 (n = 11 to 13 per replicate)
and performed scRNA-seq for TdTom+ cells. Six PN subtypes (PN1
to PN6) were uncovered using unbiased clustering (Fig. 6A), sug-
gesting that PN neurons maintained molecularly distinct at P5. In
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Fig. 4. Partial loss of Atoh1 function leads to multiple dysregulated pathways. (A) The number of the up-regulated (left) and down-regulated (right) genes at each
cell state in Atoh1S193A/− compared to control. The DEGs were filtered by log2FC > 0.25 and FDR < 0.05. (B) The enrichment of the DEGs with ATOH1-binding in each cell
state. The percentage of the DEGs with ATOH1-binding peak was shown in number. The odds ratios were presented with 95% confidence interval performed by Fisher’s
exact test. P value was adjusted by Bonferroni. (C) Volcano plot of the DEGs with ATOH1-binding peak grouped by cell state. The shaded box indicates the known Atoh1
targets that have been reported in PN development. (D) Gene ontology (GO) enrichment analysis of the DEGs. The representative biological processes are shown with
−log10(FDR). (E) TUNEL staining on cRL in control (top) and Atoh1S193A/− (bottom) at E14.5. The Atoh1-lineage cells were labeled with TdTom, and the nuclei were stained
with DAPI. (F) The average number of the TUNEL+ cells per region of interest in control and Atoh1S193A/− mice. The gray symbols represent the five ROIs quantified from
each animal (n = 3 per genotype). The average number for each animal was shown in colored shape. The crossbar denotes the mean per genotype. ***Χ2(1) = 82.41, P =
2.20 × 10−16 by mixed-model ANOVA.
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Fig. 5. PN neurons at E18.5 are heterogeneous and exhibit differential vulnerability to Atoh1 hypomorphic mutation. (A) Expression levels of the selective markers
visualized on UMAP. Ccnd1, Atoh1, Nhlh1, and Mapt are known markers for progenitors, intermediate progenitors, migrating neurons, and differentiated neurons, re-
spectively. (B) UMAP of the major cell states and ePN subtypes at E18.5. The dashed line denotes the subtype that was significantly reduced in Atoh1S193A/− mice. (C) Violin
plot showing the expression levels of the marker genes. Prox1, Ephb1, Hoxb5, and Ntng1 are the marker genes for ePN1, ePN2, ePN3, and ePN4, respectively. (D) Proportion
of the cells in each cell state and PN subtype. The arrow indicates the direction of the change in Atoh1S193A/− animals. *FDR < 0.01.
Fig. 6. scRNA-seq reveals six PN subtypes in mice at P5. (A) UMAP of the major subtypes of the PN neurons at P5. (B) Violin plot showing the expression levels of the
marker genes for each PN subtype. (C) Matching the cell type between E18.5 ePN subtypes and P5 PN subtypes by FR test.
addition, we identified the marker genes for each subtype by differ-
ential gene expression analysis (Fig. 6B, fig. S7A, and data S1).
Notably, several PN subtypes at P5 share similar markers with
those at E18.5 (figs. S6E and S7A), indicating that PN subtypes
might be conserved between these two time points. Thus, we per-
formed Friedman-Rafsky (FR) test to match the cell type in E18.5
and P5 datasets (39). We found concordant signatures between the
two time points (Fig. 6C), suggesting that the PN neurons have
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partially acquired their molecular signatures by E18.5. Notably, we
did not find a matched cell type for PN5 and PN6 in the E18.5 data
(Fig. 6C). Given that PN5 and PN6 are two of the smallest popula-
tions among all subtypes, those two subtypes could be underrepre-
sented in the E18.5 data due to the low total number of the PN
neurons being sampled. Together, these data confirm that differen-
tiated PN neurons are molecularly heterogeneous at both E18.5 and
P5. Moreover, the preferential loss of ePN1 subtype at E18.5 in
Atoh1S193A/− embryos spurred our interest in testing whether the
six PN subtypes at P5 are affected by Atoh1 hypomorphic mutation
equally.
The six molecularly defined PN subtypes are spatially
segregated
We ultimately wanted to test whether the six PN subtypes were dif-
ferentially compromised in the Atoh1S193A/− mice. However, exam-
ining the cellular phenotypes in PN subtypes is challenging without
knowing where those subtypes are located. Unlike the well-charac-
terized laminated structure in the cerebral cortex, cerebellum, and
retina, the cellular organization of the PN has not been delineated.
Thus, we first characterized the anatomic location of the six PN sub-
types in control animals and then used the spatial information to
identify the cellular phenotypes in Atoh1S193A/− animals. We per-
formed fluorescence RNA in situ hybridization (ISH) with RNA-
Scope HiPlex assay on serial coronal sections from control mouse
brain at P5 (Fig. 7A). We used TdTom probes to label Atoh1-lineage
neurons and define the region of interest (i.e., PN). By binning the
images, we calculated the average expression of the marker genes
within each bin across the PN (Fig. 7B). We found that all the
markers exhibit spatial specificity except Cdh8. For example,
Hoxb5 is expressed in the caudal part of the PN, which is consistent
with the previous study (14). Another marker, Etv1, is highly ex-
pressed in a restricted part of the medioventral PN. In contrast,
Cdh8 is globally expressed across PN, which is expected based on
our P5 scRNA-seq data (Fig. 6B). Notably, we found that Somatos-
tatin (Sst), a neuropeptide that is typically expressed in inhibitory
neurons, is expressed in a subset of the PN neurons that exhibit a
shell-like pattern at the rostral PN (Fig. 7B). To date, there are only
few studies showing that Sst is coexpressed in subsets of excitatory
neurons in pre-Bötzinger complex, lateral hypothalamus, and dor-
solateral periaqueductal gray matter (40–42). Here, we demonstrat-
ed that Sst is coexpressed in a subset of Slc17a6 + [Vesicular
Glutamate Transporter 2 (VGLUT2+)] PN neurons (fig. S7, B
and C).
To find the corresponding PN subtypes between ISH and
scRNA-seq data, we implemented K-nearest neighbor classification
method to annotate the six PN subtypes on the representative
regions of interest (Fig. 7C). PN neurons have been categorized
into two nuclei, RtTg and BPN, based on their anatomic location.
However, there has been no evidence showing that they are molec-
ularly different. We found that PN6 is largely restricted to BPN,
while PN3 and PN5 mostly reside at RtTg. In contrast, PN1, PN2,
and PN4 are distributed across both nuclei. These findings suggest
that RtTg and BPN have both shared and distinct neuronal sub-
types. Together, our ISH data validate the expression of the
marker genes identified by scRNA-seq and establish the spatial
map of the PN subtypes that provides a higher resolution of the cy-
toarchitecture in the PN.
PN subtypes have differential vulnerability to Atoh1
hypomorphic mutation
With the spatial map of the PN subtypes being established, we next
performed fluorescence ISH to characterize PN subtypes in control
and Atoh1S193A/− animals at P5. First, we used TdTom probes to
identify PN across serial coronal sections. On the basis of the ana-
tomic landmarks other than PN, we aligned the sections from dif-
ferent animals to proximity and quantified the size of PN in control
and Atoh1S193A/− animals. Consistent with the whole-mount lacZ
staining at P21 (Fig. 1D), we observed a reduced size of PN in
Atoh1S193A/− animals at P5 (Fig. 8A). Moreover, we found that
the caudal PN were most affected with 80% reduction at the most
caudal section, while there is no significant difference at the rostral
PN (Fig. 8, A and B).
Given that different PN subtypes exhibit spatial specificity along
the rostral-caudal axis (Fig. 7C), we hypothesized that the PN sub-
types are differentially affected in Atoh1S193A/− mice. We thus exam-
ined three PN subtypes (PN3, PN4, and PN6) in control and
Atoh1S193A/− at P5 by performing dual ISH using TdTom probes
and marker genes Cdkn1c, Hoxb5, and Etv1, respectively. Cdkn1c
is the marker gene for PN3 subtype, which matches ePN1 subtype
at E18.5 (Fig. 6C) and is predicted to be reduced in Atoh1S193A/−
animals according to our E18.5 scRNA-seq data (Fig. 5D). More-
over, on the basis of the spatial map of the PN (Fig. 7C) and the
selective loss of the caudal PN in Atoh1S193A/− mice (Fig. 8, A and
B), we predicted that Hoxb5+ (PN4) was compromised by partial
loss of function of Atoh1. Last, to test whether there is one
subtype that is not sensitive to Atoh1 hypomorphic mutation, we
chose Etv1+ subtype (PN6). Given its specific location within the
PN (Fig. 7C), changes in PN, if any, should be easily detected. We
examined serial sections across the whole PN to exclude the possi-
bility that certain subtype might be mislocated. We found fewer
Cdkn1c+TdTom+ PN3 neurons in Atoh1S193A/− mice, accompanied
by reduced size in the area where the Cdkn1c+ PN3 neurons are
normally located in controls (Fig. 8, C and D, top). Hoxb5+TdTom+
PN4 neurons were also reduced in Atoh1S193A/− animals at the
caudal sections (Fig. 8, C and D, middle). In contrast, Etv1+TdTom+
PN6 neurons were not affected in Atoh1S193A/− animals (Fig. 8, C
and D, bottom). Together, these data demonstrated that although
all PN subtypes were derived from Atoh1 + progenitors, certain
PN subtypes such as PN3 and PN4 neurons were more vulnerable
to partial loss of function of Atoh1, suggesting that the robustness of
Atoh1 function is critical for PN subtype cell fate decisions.
DISCUSSION
In this study, we used a mouse model carrying an Atoh1 hypomor-
phic mutation and implemented scRNA-seq technology to eluci-
date the roles of Atoh1 in different cell states during PN
development. We demonstrate that Atoh1 is involved in multiple
biological processes including regulating the cell cycle, differentia-
tion, cell survival, migration, and the heterogeneity of the PN
neurons. Moreover, we show that Atoh1-lineage PN neurons are
classified as six subtypes based on their molecular signatures and
provide a list of marker genes for future studies. PN subtypes
display differential vulnerability to Atoh1 hypomorphic mutation,
which opens up questions such as how cell fate decisions are
made and how perturbation of a transcription factor results in
subtype-specific phenotypes.
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Fig. 7. The spatial map of the PN subtypes in mice at P5. (A) Workflow of the RNAScope HiPlex Assay on P5 control mouse. (B) Expression patterns of the selective
markers for each PN subtype across the rostral to caudal axis of the PN. (C) Schematic summary of the distribution of the PN subtypes across the PN. The dashed line
denotes the border between RtTg and BPN.
The interplay between Atoh1 and Notch signaling
One interesting phenotype that we observed in Atoh1S193A/− mice
was the marked increase in proliferating progenitors (Figs. 2F and
3B), suggesting that Atoh1 might promote cell cycle exit. In addi-
tion, differential gene expression analysis revealed several dysregu-
lated pathways including up-regulated Notch signaling in the
proliferating progenitors (Fig. 4D). The interaction between
Atoh1 and Notch signaling has been demonstrated in the mamma-
lian intestine (43, 44). In the epithelium lining the crypts in the
small intestine, the stem cells differentiate into secretory cells
when they escape Notch activation by up-regulation of Atoh1. In
contrast, the stem cells in which Notch is activated remain as
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Fig. 8. PN subtypes have different vulnerabilities to Atoh1 hypomorphic mutation. (A) The size of the PN was determined by the areas across seven coronal sections
(n = 3 per genotype). Data are presented as means ± SD. *P < 0.05; **P < 0.01; ***P < 0.001 by two-tailed unpaired t test. (B) Representative RNA ISH images for the most
rostral (top) and the most caudal (bottom) sections from control (left) and Atoh1S193A/− (right) mice at P5. The PN neurons were labeled with TdTom probes. The nuclei
were stained with DAPI. The dashed line encloses the area of the PN. Scale bars, 500 μm. (C) RNA ISH on control (left) and Atoh1S193A/− (right) mice at P5 using Cdkn1c (top),
Hoxb5 (middle), and Etv1 (bottom) probes. PN neurons were labeled with TdTom probes. The nuclei were stained with DAPI. The box denotes the area shown in (D). Scale
bars, 200 μm. (D) The zoom-in view of PN from the boxes in (C). The expression of Cdkn1c (top), Hoxb5 (middle), and Etv1 (bottom) were shown in green. Scale bars, 20 μm.
progenitors at the crypts where Wnt is high. Here, we hypothesize
that in cRL, partial loss of function of Atoh1 leads to failure to
escape Notch activation, which maintains the cells at the proliferat-
ing state. In line with this hypothesis, we observed up-regulation of
the Notch receptor Notch1 and its ligand Dll1 as well as genes down-
stream from active Notch signaling including Hes1 and Hes5 in the
proliferating progenitors of Atoh1S193A/− mice (data S2). Moreover,
a recent study in zebrafish showed that inhibition of Notch activity
at lower RL led to increased atoh1b+ postmitotic precursors at the
expense of atoh1a+ proliferating progenitors (45). This study also
supports our hypothesis that Atoh1 and Notch antagonize each
other at the cRL in mammals.
The molecular heterogeneity of the PN neurons
One of the most puzzling questions in neurobiology is how to define
a neuronal subtype. In the case of the PN, they have been considered
as a group of heterogeneous neurons based on their anatomical lo-
cation, origins at the cRL, and functions based on the cortico-
ponto-cerebellar connectivity (13–16, 46). However, whether the
PN neurons can be further defined by their molecular signature
has not been shown. scRNA-seq provides a powerful approach to
classify the cell type unbiasedly based on transcription profiles.
Here, we uncovered six PN subtypes in P5 mice (Fig. 6, A and B).
These six PN subtypes were spatially segregated (Fig. 7), which
raises an interesting question regarding whether this spatial map re-
flects the topographic connectivity between the cerebral cortex and
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the PN. For example, Sst+ PN subtype reside at the rostral dorsal
part of the PN (Fig. 7B). This location coincides with where PN
receive inputs from brain regions involved in visual pathway includ-
ing visual cortex, superior colliculus, inferior colliculus, and pretec-
tum (47). Whether there is a subtype-specific connectivity that also
reflects the functionality needs further investigation. However,
those studies cannot be done without genetic tools to target differ-
ent PN subtypes. Our study identified and validated the marker
genes for the six PN subtypes. In future studies, one can test
whether different PN subtypes preferentially connect with specific
groups of neurons in the cerebral cortex and/or cerebellum by in-
tersectional labeling and viral tracing approaches with the molecu-
lar markers. In addition to the connectivity, the molecular markers
that we identified can be used to manipulate PN neurons in a
subtype-specific manner for functional characterization.
From a pool of Atoh1+ progenitors to diverse PN subtypes
How does a pool of seemingly homogeneous progenitors give rise to
a diverse population of neurons? In the case of Atoh1-lineage
neurons in the hindbrain, Atoh1 + progenitors contribute to
neurons in the cerebellum and dozens of brainstem nuclei depend-
ing on the rostrocaudal origins at RL and the timing of leaving from
RL (9, 10, 48). PN, for example, were derived from rhombomere 6 to
8 where Hox2 to Hox5 are expressed (10). It has not been addressed,
however, whether Atoh1 contributes to the diversity of the subpo-
pulation within one lineage (i.e., PN in this study). To this end, we
tested whether PN subtypes display differential vulnerability to
Atoh1 hypomorphic mutation. On the basis of our scRNA-seq
data at E18.5 and the histological analysis at P5, partial loss of func-
tion of Atoh1 not only reduced the size of the PN but also reduced
the diversity of the PN neurons by preferentially affecting PN3 and
PN4 subtypes (Fig. 8, C and D). These data suggest that Atoh1 may
contribute to the acquisition or maintenance of the heterogeneity of
the PN neurons.
In summary, this study dissected the functions of Atoh1 during
PN differentiation by characterizing the phenotypes in Atoh1 hypo-
morphic mutant at single-cell resolution. Our data demonstrate that
Atoh1 regulates cell cycle exit, differentiation, migration, and sur-
vival during PN development and contributes to the diversity of
the PN subtypes. In addition, this study also uncovers the molecular
heterogeneity of the PN, which opens new doors for understanding
the neural fate decisions, connectivity, and functionality of the
PN neurons.
MATERIALS AND METHODS
Mice
The following mouse lines were used in this study: Atoh1lacZ/+ (23),
Atoh1S193A/+ (24), Atoh1Cre/+ (28), Rosalsl-lacZ/+ (JAX:02429), and
Rosalsl-TdTomato/+ (JAX:007914). All mice were housed in a level 3,
American Association for Laboratory Animal Science (AALAS)–
certificated facility on a 14-hour light cycle. Husbandry, housing,
euthanasia, and experimental guidelines were approved by the In-
stitutional Animal Care & Use Committee (IACUC) at Baylor
College of Medicine.
Whole-mount X-galactosidase staining
The brains of P0 pups and P21 mice were dissected out in ice-cold
phosphate-buffered saline (PBS). The samples were fixed in 4%
paraformaldehyde (PFA) at 4°C for 1 hour (P0) or 2 hours (P21).
After brief PBS wash at room temperature (RT), the samples were
incubated in equilibration buffer [2 mM MgCl2, 0.05% sodium de-
oxycholate, 0.02% NP-40, and 0.1 M sodium phosphate (pH 7.3)] at
4°C for 15 min, followed by 2-hour incubation at 37°C in X-galac-
tosidase (X-gal) reaction solution [X-gal (1 mg/ml), 5 mM potassi-
um ferrocyanide, and 5 mM potassium ferricyanide in equilibration
buffer]. After staining, the samples were washed with PBS three
times at RT and fixed again in 4% PFA at 4°C for 1 hour (P0) or
4 hours (P21) before imaging.
Sample preparation for scRNA-seq
The brains of embryos and P5 pups were dissected out in ice-cold
HEBG medium (0.8× B27 and 0.25× GlutaMAX in hibernate E
medium). For embryonic studies, hindbrains were collected from
Atoh1S193A/− and its littermate control (one embryo per genotype,
three independent replicates). For P5 study, PN were microdis-
sected out and pooled from 11 to 13 pups (two independent
replicates). Tissues were cut into small pieces and transferred to a
1.5-ml microcentrifuge tube using wide-bore pipette tips. The
single-cell dissociation protocol was modified from the previous
study (49). Briefly, the tissues were incubated with Worthington
Papain solution at 37°C for 30 min at 800 rpm. At the end of incu-
bation, the samples were transferred to a 15-ml falcon tube, followed
by gentle trituration with serologic pipette. The cell pellets were col-
lected by centrifuge at 200 rcf for 3 min at 4°C, washed, and
resuspended in ice-cold sorting buffer (PBS with 0.05% fetal
bovine serum). The single-cell resuspension was loaded to 30-μm
cell drainer (CellTrics, SYSMEX 04-004-2326) to remove debris,
followed by 40,6-diamidino-2-phenylindole (DAPI) staining at RT
for 5 min. TdTom+DAPI− cells (120,000 to 150,000 cells per
sample) were sorted into bovine serum albumin–coated 15-ml
falcon tube by Sony SH800S cell sorter. The cells were pelleted by
centrifuge at 200 rcf for 5 min at 4°C and resuspended in sorting
buffer to make the final concentration of 1000 cells/μl.
Library construction and sequencing
The cDNA libraries were constructed by 10X Genomics 30 v3.1 kit
following the user guide. Briefly, ~16,500 cells from one sample
were mixed with reversed transcription master mix before loaded
into Chromium Chip G. Droplets containing cells, reversed tran-
scription reagents, and barcoded gel beads were generated by
Chromium Controller. The first strand cDNA was amplified,
fragmentated, and ligated with sequencing adaptors and sample
indices. The cDNA libraries were sequenced by Illumina
NovaSeq 6000.
RNAScope HiPlex assay
The brain of the P5 pup was flash-frozen and embedded in optimal
cutting temperature (OCT) compound. Coronal sections were
made by cryostat (Leica) at 20 μm and store in −80°C until use.
The RNAScope HiPlex assay (ACD, catalog no. 324419) was per-
formed according to the manufacturer’s instructions. Briefly, sec-
tions were fixed in 4% PFA for 1 hour at RT, followed by
dehydration with 50, 70, and 100% ethanol. The protease treatment
was done by incubating the sections with protease III at RT for 30
min. Sections were hybridized with the 12 probes (catalog nos.
487941-T1, 845141-T2, 480301-T3, 557891-T4, 319171-T5,
458331-T6, 485461-T7, 503461-T8, 404631-T9, 503481-T10,
Wu et al., Sci. Adv. 9, eadg1671 (2023) 30 June 2023
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S C I E N C E A D VA N C E S | R E S E A R C H A R T I C L E
317041-T11, and 433411-T12) at 40°C for 2 hours, followed by three
rounds of amplification. The TdTom reporter was helpful during
sectioning to ensure that we covered the whole PN. However, it
also introduced hazy background in HiPlex assay. Thus, we
quenched the TdTom signal by incubating the sections with 5% for-
malin-fixed paraffin-embedded (FFPE) reagent (included in the kit)
at RT for 30 min before developing the first round of the targets (T1
to T3). The nuclei were stained with DAPI. The 12 targets were de-
tected in four rounds of imaging with three targets per round.
Between each round, the fluorophores were cleaved and washed
away before the next round of signal development. After image ac-
quisition of all targets, the fluorophores were cleaved, and a blank
image of each section was taken to serve as background images.
Immunofluorescence staining
The heads of the E14.5 embryos were dissected in ice-cold PBS.
After brief wash, the samples were fixed in 4% PFA at 4°C for 4
hours, followed by PBS wash and incubation in 30% sucrose in
PBS at 4°C for 14 to 16 hours. The samples were cryopreserved
and stored at −80°C until use. The coronal sections were collected
on slides by cryostat (Leica) with 20 or 25 μm in thickness. The
slides were rinsed with PBS to remove OCT, followed by permeabi-
lization with 0.3% Triton X-100 in PBS for 15 min at RT. Antigen
retrieval was performed by heating in antigen retrieval buffer [10
mM sodium citrate and 0.05% Tween-20 (pH 6.0)] at 85°C for 10
min [MKI67/red fluorescent protein (RFP) staining] or 30 min
(CldU labeling). After blocking with blocking buffer (5% normal
goat serum with 0.3% Triton X-100 in PBS) for 2 hours at RT, the
sections were incubated with primary antibodies in blocking buffer
at 4°C for 24 hours, followed by PBS wash three times. The sections
were incubated with secondary antibodies in blocking buffer at RT
for 2 hours. The counterstain was performed by DAPI staining at
RT for 10 min. The slides were mounted with ProLong Gold
mounting media (Invitrogen). The following antibodies were used
in this study with the indicated dilutions: rat anti–5-bromo-20-de-
oxyuridine (BrdU)/CldU (1:250; Abcam, ab6326), mouse anti-Ki67
(1:100; R&D Systems, AF7649), rabbit anti-RFP (1:2000; Rockland,
600-401-379), goat anti-rat Alexa Fluor 488 (1:500; Invitrogen, A-
11006), goat anti-rabbit Alexa Fluor 555 (1:500; Invitrogen, A-
21428), goat anti-mouse Alexa Fluor 647 (1:500; Invitrogen,
A-21236).
CldU labeling
CldU was prepared in sterile saline at 4.82 mg/ml and injected into
pregnant dams intraperitoneally at E13.5 (85 mg/kg). Twenty-four
hours later, the embryos were collected at E14.5, followed by immu-
nofluorescence staining using rat anti-BrdU/CldU antibody (1:250;
Abcam, ab2326).
TUNEL assay
The sample preparation follows the same protocol that we used for
immunofluorescence staining. Coronal sections were made by cryo-
stat (Leica) at 25 μm. TUNEL assay was performed using DeadEnd
Fluorometric TUNEL system (Promega) according to the user
guide. Briefly, sections were fixed in 4% PFA at RT for 15 min, fol-
lowed by proteinase K (20 μg/ml) treatment at RT for 12 min and a
second fixation with 4% PFA for 5 min. The sections were equili-
brated and incubated with fluorescein-labeled nucleotides and ter-
minal deoxynucleotidyl transferase reaction mix at 37°C for 1 hour.
After stopping the reaction by 2× saline-sodium citrate (SSC), the
nuclei were stained with DAPI.
Dual-color fluorescence RNA ISH
The brains from P5 pups were embedded in OCT, frozen on dry ice,
and stored in −80°C until use. The sections were cut by cryostat
(Leica) at 20 μm. We generated a digoxigenin (DIG)–labeled
mRNA antisense probes against Slc17a6 and TdTom and fluoresce-
in isothiocyanate (FITC)–labeled mRNA against Sst, Etv1, and
Hoxb5 using reverse-transcribed mouse cDNA as template and
DIG or FITC RNA labeling kits from Roche (Sigma-Aldrich).
Primer sequences for Sst, Slc17a6, and TdTom probes are available
in Allen Brain Atlas (www.brain-map.org). The following primers
were used: 50-ttcagaactcgggtctgctt-30 and 50-gaatcatgcaaaaggtggct-30
for Etv1 probe and 50-gatggatctcagcgtcaacc-30 and 50-tatgagtctggcta-
cagccg-30 for Hoxb5 probe. ISH was performed by the RNA In Situ
Hybridization Core at Baylor College of Medicine using an auto-
mated robotic platform as previously described (50) with modifica-
tions of the protocol for double ISH. Briefly, two probes were
hybridized to the tissue simultaneously. After the wash and block-
ing steps, the DIG-labeled probes were visualized by incubating
with tyramide-Cy3 Plus (1:75; PerkinElmer) for 15 min. After
washing in TNT (0.05% Tween-20 in 150mM NaCl and 100mM
Tris-HCl, pH7.5), the remaining horseradish peroxidase (HRP) ac-
tivity was quenched by a 10-min incubation in 0.2 M HCl. The sec-
tions were then washed in TNT and blocked in TNB for 15 min,
followed by incubation with HRP-labeled sheep anti-FITC antibody
(1:500 in TNB; Roche) at RT for 30 min. After washes in TNT, the
FITC-labeled probes were visualized by incubating with tyramide-
FITC Plus (1:75; PerkinElmer) for 15 min. Following washes in
TNT, the slides were stained with DAPI (Invitrogen), washed
again, removed from the machine, and mounted with ProLong
Diamond (Invitrogen).
Image acquisition
Images of the whole-mount tissues were obtained by Zeiss Axio
Zoom.V16. All fluorescence images were obtained by Nikon Ti2E
Inverted Motorized Microscope equipped with CSU-W1 Dual
Camera-Dual Disk System with 405/488/561/640-nm lasers with
10×, 20×, or 40× objectives.
Data preprocessing
The reads were aligned to customized genome that composed of
mouse mm10 reference genome and woodchuck hepatitis virus
post-transcriptional regulatory element (WPRE) sequence to
ensure capturing the TdTom transcripts. The alignment and quan-
tification of unique molecular identifier (UMI) were performed on
10× cloud analysis platform by CellRanger pipeline v5.0.1 with
default parameters. Individual expression matrix of each sample
was filtered by Seurat v4 package (51), followed by doublet
removal using DoubletFinder v2 package (52). Briefly, cells with
at least 1000 genes expressed and less than 1% of total UMIs that
were mitochondrial genes were retained (see the code for detailed
criteria depending on the age of the samples.) Doublets with high
confidence score identified by DoubletFinder with default parame-
ters were removed.
Wu et al., Sci. Adv. 9, eadg1671 (2023) 30 June 2023
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Integration and clustering
The data integration and clustering were performed by Seurat
package. The filtered datasets of the samples from the same age
were combined by integration pipeline. Briefly, the filtered matrices
were normalized and scaled by regressing out the percentage of mi-
tochondrial genes with SCTransform (53). For E14.5 and E18.5
samples in which different genotypes were compared, a reference-
based integration method was used by setting the first replicate as
reference to cut down the computational power and time needed.
For P5 study, the default integration method was used. Following
the integration, principal components analysis (PCA) was per-
formed. We selected the top 30 PCAs to generate the K-nearest
neighbor graph, which was used to perform clustering with variated
resolutions depending on the complexity of the dataset. To annotate
the clusters, the top 10 genes that were highly expressed in each
cluster were identified by FindAllMarkers function with default
parameters.
Trajectory analysis
The RNA assay of the Seurat object was extracted and transformed
into SingleCellExperiment object (sce) by as.SingleCellExperiment
function. The sce was used as input to infer the trajectory by sling-
shot (30). Kolmogorov-Smirnov test was used to assess whether the
distribution of pseudo-time is identical between the genotypes.
Differential expression analysis
We performed differential expression analysis between control and
Atoh1S193A/− samples for each cell state by FindMarkers function
from Seurat package. Only genes that were expressed in at least
10% of the cells were included. MAST (54) was used as test
method. The P value was corrected by Benjamini-Hochberg
FDR method.
GO analysis
GO analysis was performed using a web-based tool called g:Profiler
(55) with custom statistic domain scope. The up- and down-regu-
lated DEGs (log2FC > 0.25 and FDR < 0.05) within progenitors, in-
termediate progenitors, and migrating neurons-1 were used as
inputs independently. GO biological process was selected for the
analysis.
Image processing for RNAScope HiPlex assay
Five confocal z-stacks of images were collected at 100- and 200-μm
intervals along the rostral-caudal axis of the pontine nucleus for
each of the nine transcripts. In addition, DAPI images were also col-
lected to serve as reference images. Individual z-stacks consisted of
10 images separated by 0.9 μm. The z-stacks were first processed by
taking the max intensity projection and cropping the resulting tran-
script image such that the midline of the pontine aligned with the
right-hand border of each image.
Visualizing transcript expression
Overlaying the transcript images onto the DAPI reference images
(56) revealed that some transcripts were not consistently colocalized
to the nucleus. To address this ambiguity in our analysis, we did not
associate transcript expression with individual cells. Rather, we
binned each image into a set of tiles measuring 34 μm × 31 μm
and measured the transcript expression in each tile (57). Since tran-
scripts may be expressed at very different levels, we normalized the
expression for each transcript by the max expression recorded
across the five image locations in the pontine. Thus, in Fig. 7B,
the expression for each transcript varies from 0 to 1 across the
five rostral to caudal sections. To identify the boundaries of the
pontine, the max intensity projection across all the transcripts at a
given imaging location was smoothed with a Gaussian filter with an
SD of 35 μm. This filtered image was converted to a binary image by
mean thresholding, and any remaining holes were morphologically
closed using a 5-μm × 5-μm structuring element. Last, the pontine
boundary was taken to be the edge of the largest single region in the
closed binary image.
Quantification of the immunostaining and statistics
The cell number of the CldU+TdTom+ (Fig. 3F) and TUNEL+
(Fig. 4F) was determined by manual counting on imageJ. The per-
centage of MKI67 signal overlapping with TdTom (Fig. 3C) was cal-
culated based on Manders’ coefficients using JACoP (58). For the
statistical
test, we used mixed-model analysis of variance
(ANOVA) to compare genotype using lme4 package on R (59) to
count for variations among technical and biological replicates.
The person who performed the quantification was blinded to the
genotypes of the samples.
Classification of RNA ISH transcripts
The transcript expression levels for each tile in the ISH images can
be represented as a nine-dimensional vector, one component for
each transcript. To classify these transcript expression vectors as
one of the six classes determined from the single-cell RNA-seq anal-
ysis, we built a K-nearest neighbor classifier (60). Specifically, for
each cell (n = 7029) from the RNA-seq clustering results, we extract-
ed the same transcripts as measured in the ISH and the cluster iden-
tity. Each of these nine-component vectors was normalized (60)
and, along with their respective cluster IDs, was used to train and
validate our classifier. The K-nearest neighbor classifier measures
the distance between each nine-component vector and a preset
number of neighbor vectors to predict class membership. To deter-
mine the number of neighbors, we performed sixfold cross-valida-
tion on the training dataset and found that 25 neighbors provided
an average test accuracy of 80%. We then used this 25 nearest neigh-
bor model to predict the cluster identity of each tile in the ISH
images. The border of the RtTg and BPN in Fig. 7C was defined
using the reference atlas of P6 mouse brain (61).
Supplementary Materials
This PDF file includes:
Figs. S1 to S7
Table S1
Legends for data S1 to S3
References
Other Supplementary Material for this
manuscript includes the following:
Data S1 to S3
View/request a protocol for this paper from Bio-protocol.
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Acknowledgments: The content is solely the responsibility of the authors and does not
necessarily represent the official views of the Eunice Kennedy Shriver National Institute Of Child
Health and Human Development or the National Institutes of Health. We thank the Baylor
College of Medicine Center for Comparative Medicine for mouse colony management, D. Yu for
assistance with microscopy, C. Ljungberg for assistance with RNA ISH, and C.-W. Logan Hsu for
assistance with the tissue clearing and lightsheet microscopy. We thank the members of the
Zoghbi lab and M. E. Van Der Heijden for discussions and comments on the manuscript.
Funding: This project was supported by Howard Hughes Medical Institute and funding from a
Shared Instrumentation grant from the NIH (S10 OD016167) and the NIH IDDRC Grant P50
HD103555 from the Eunice Kennedy Shriver National Institute Of Child Health and Human
Development for use of the RNA In Situ Hybridization Core and Neurovisualization Core. J.C.B.
was supported by NIH F32 (NS117723). R.S.D. was supported by NIH NINDS F32 (NS127854).
This work was supported by Howard Hughes Medical Institute (to H.Y.Z.), National Institutes of
Health grant S10 OD016167, National Institutes of Health IDDRC P50 HD103555, National
Institutes of Health grant NS117723 (to J.C.B.), and National Institutes of Health grant NS127854
(to R.S.D.). Author contributions: Conceptualization: H.Y.Z., S.-R.W., J.C.B., and M.A.D.
Methodology: S.-R.W., J.C.B., and M.A.D. Investigation: S.-R.W., J.C.B., and J.-P.R. Software: S.-R.W.,
M.S.C., R.S.D., and M.A.D. Visualization: S.-R.W. and M.S.C. Writing—original draft: S.-R.W. Writing
—review and editing: H.Y.Z., S.-R.W., J.C.B., M.S.C., and R.S.D. Funding acquisition: H.Y.Z.
Competing interests: The authors declare that they have no competing interests. Data and
materials availability: All data are available in the main text and/or the Supplementary
Materials. The raw FASTQ files of the scRNA-seq and the processed files (output from
CellRanger) are accessible through GEO (accession number: GSE224031). The code for data
analyses is available on figshare (doi: 10.6084/m9.figshare.22490956) and via the link: https://
figshare.com/s/ca568d8f44d020cb6389.
Submitted 6 December 2022
Accepted 26 May 2023
Published 30 June 2023
10.1126/sciadv.adg1671
Wu et al., Sci. Adv. 9, eadg1671 (2023) 30 June 2023
15 of 15
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10.1371_journal.ppat.1012032.pdf
|
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
|
All relevant data are within the manuscript and its Supporting Information files.
|
RESEARCH ARTICLE
A tick saliva serpin, IxsS17 inhibits host innate
immune system proteases and enhances host
colonization by Lyme disease agent
Thu-Thuy NguyenID
Samuel Kiarie Gaithuma1, Moiz Ashraf Ansari1, Tae Kwon Kim2, Lucas Tirloni3,
Zeljko Radulovic4, James J. Moresco5, John R. Yates, III6, Albert MulengaID
1, Tae Heung Kim1, Emily Bencosme-Cuevas1, Jacquie Berry1, Alex
1*
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
1 Department of Veterinary Pathobiology, School of Veterinary Medicine and Biomedical Sciences, Texas
A&M University, College Station, Texas, United States of America, 2 Department of Diagnostic Medicine/
Pathobiology, College of Veterinary Medicine, Kansas State University, Manhattan, Kansas, United States of
America, 3 Tick-Pathogen Transmission Unit, Laboratory of Bacteriology, NIAID, Hamilton, Montana, United
States of America, 4 Department of Biology, Stephen F. Austin State University, Nacogdoches, Texas,
United States of America, 5 Center for Genetics of Host Defense, UT Southwestern Medical Center, Dallas,
Texas, United States of America, 6 Department of Molecular Medicine, The Scripps Research Institute, La
Jolla, California, United States of America
* [email protected]
Abstract
Lyme disease (LD) caused by Borrelia burgdorferi is among the most important human vec-
tor borne diseases for which there is no effective prevention method. Identification of tick
saliva transmission factors of the LD agent is needed before the highly advocated tick anti-
gen-based vaccine could be developed. We previously reported the highly conserved
Ixodes scapularis (Ixs) tick saliva serpin (S) 17 (IxsS17) was highly secreted by B. burgdor-
feri infected nymphs. Here, we show that IxsS17 promote tick feeding and enhances B.
burgdorferi colonization of the host. We show that IxsS17 is not part of a redundant system,
and its functional domain reactive center loop (RCL) is 100% conserved in all tick species.
Yeast expressed recombinant (r) IxsS17 inhibits effector proteases of inflammation, blood
clotting, and complement innate immune systems. Interestingly, differential precipitation
analysis revealed novel functional insights that IxsS17 interacts with both effector proteases
and regulatory protease inhibitors. For instance, rIxsS17 interacted with blood clotting prote-
ases, fXII, fX, fXII, plasmin, and plasma kallikrein alongside blood clotting regulatory serpins
(antithrombin III and heparin cofactor II). Similarly, rIxsS17 interacted with both complement
system serine proteases, C1s, C2, and factor I and the regulatory serpin, plasma protease
C1 inhibitor. Consistently, we validated that rIxsS17 dose dependently blocked deposition of
the complement membrane attack complex via the lectin complement pathway and pro-
tected complement sensitive B. burgdorferi from complement-mediated killing. Likewise, co-
inoculating C3H/HeN mice with rIxsS17 and B. burgdorferi significantly enhanced coloniza-
tion of mouse heart and skin organs in a reverse dose dependent manner. Taken together,
our data suggests an important role for IxsS17 in tick feeding and B. burgdorferi colonization
of the host.
OPEN ACCESS
Citation: Nguyen T-T, Kim TH, Bencosme-Cuevas
E, Berry J, Gaithuma ASK, Ansari MA, et al. (2024)
A tick saliva serpin, IxsS17 inhibits host innate
immune system proteases and enhances host
colonization by Lyme disease agent. PLoS Pathog
20(2): e1012032. https://doi.org/10.1371/journal.
ppat.1012032
Editor: Catherine A. Brissette, University of North
Dakota School of Medicine and Health Sciences,
UNITED STATES
Received: June 9, 2023
Accepted: February 6, 2024
Published: February 23, 2024
Copyright: This is an open access article, free of all
copyright, and may be freely reproduced,
distributed, transmitted, modified, built upon, or
otherwise used by anyone for any lawful purpose.
The work is made available under the Creative
Commons CC0 public domain dedication.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
Funding: This research was supported by National
Institutes of Health grants (AI093858, AI074789,
AI138129, and AI119873) to AM, National Center
for Research Resources (5P41RR011823) and
National Institute of General Medical Sciences
(8P41GM103533) to JRY, and Intramural
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
1 / 31
PLOS PATHOGENSResearch Program of the National Institute of
Allergy and Infectious Diseases (Z01 AI001337-01)
to LT. The funders had no role in study design, data
collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Tick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Author summary
Ticks feed on animals and humans for their survival. During blood meal feeding, ticks
inject saliva along with disease causative agents into the hosts. Here, we demonstrate that
I. scapularis tick saliva protein, IxsS17 inhibits host innate immune system proteases and
enhances B. burgdorferi colonization of the host. Recombinant IxsS17 (rIxsS17) inhibits
blood clotting and inflammation systems serine proteases including pancreatic trypsin
and trypsin IV (~100%), blood clotting factor Xa and XIa (~60–80%), plasmin and cathep-
sin G (~50%). Similarly, rIxsS17 interacts with complement system factors, C1s, C2 and
factor I and blocks complement membrane attack complex via the lectin complement
pathway by up to 97%. We found that, in the mouse model for Lyme disease, rIxsS17 sig-
nificantly increases B. burgdorferi colonization of mouse heart and ear tissues by 5.7 and
2.3 times. Taken together, we conclude that IxsS17 is a key protein in tick feeding and B.
burgdorferi colonization of the host, and thus, a potential target antigen for developing
tick antigen-based vaccines against Lyme disease agent transmission.
Introduction
Ticks and tick-borne diseases (TBD) impact public and veterinary health globally. Among
those, Lyme disease (LD) caused by Borrelia species is one of the most important human TBD
that has the most world-wide public health impact. The spirochete, Borrelia burgdorferi that is
transmitted by Ixodes spp. ticks is responsible for LD in United States (US) and Europe, while
B. afzelii and B. garinii are responsible for LD in Eurasia [1–3]. Recently, a second LD patho-
gen B. mayonii was described in the US [4]. Like other TBD agents, except a vaccine against
tick-borne encephalitis approved by FDA in United States in 2021, there is currently no effec-
tive human vaccine against the LD agent.
In the absence of effective vaccines against the LD agent, avoidance of infectious tick bites
is the only prevention method against LD currently. Despite a plethora of methods aimed at
reducing infectious tick bites [5–7], LD cases have continued to increase. Confirmed and prob-
able LD cases reported to the US Centers for Disease Control and Prevention have steadily
risen from just under 20,000 in 1996 to more than 40,000 annual cases since 2008 (www.cdc.
gov). According to insurance database, between 2010–2018, 476,000 LD cases were diagnosed
and treated each year, with economic losses estimated at ~$786 million annually [8,9].
Given the ongoing rise in LD cases and search for better preventative measures, tick-anti-
gen based vaccines have emerged among the most promising LD prevention approaches. This
is based on evidence that repeatedly infested model animals that acquire immunity against tick
feeding are protected against transmission of TBD agents including B. burgdorferi [10–13].
Similarly, in a recent study, repeatedly infested primates were also protected against B. burg-
dorferi transmission [14]. Likewise, active immunization of mice with tick saliva proteins con-
ferred immunity that reduced transmission of LD agents [15–17]. Similarly, tick saliva and
tick salivary gland extracts promoted LD agent replication [18,19] and innate immunity eva-
sion ex vivo [20], and enhanced organ colonization in needle inoculated mice [21,22]. From
these perspectives, tick saliva factors that promote feeding and transmission of TBD agents
have been highly sought after [23–27].
To date, there is no evidence of transovarial transmission (or passed from female ticks to
larval ticks) of LD agents. Larval ticks acquire the spirochetes during feeding on infected reser-
voir hosts and then transtadially transmit to nymphs, which in turn transtadially transmit to
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
2 / 31
PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
adult ticks [28]. Major transmission events of B. burgdorferi occur after the tick has fed for
more than 48 h [29–31]. The small size of the nymph tick and pain suppressants in its saliva
that mask its presence on human skin allows the tick to go unnoticed and feed long enough for
more than 36–48 h to transmit the LD agent [32]. For that reason, although both nymph and
adult ticks are capable of transmitting LD agents to the human host, most reported LD cases
were associated with infectious nymph bites [3,33]. On this basis, we recently identified tick
saliva proteins of B. burgdorferi infected I. scapularis nymphs that were secreted every 12 h
throughout feeding [25].
This study was initiated to understand functional roles of I. scapularis tick saliva serine pro-
tease inhibitor (Serpin; GenBank accession# EEC18973.1 or XP_002415308.5) in tick feeding
and B. burgdorferi colonization of the host. We later found that IxsS17 was among homologs
(orthologs) to Amblyomma americanum serpin (AAS) 19 that were characterized by the func-
tional domain reactive center loop (RCL) being 100% conserved in all tick species according to
currently available data [34,35]. We also reported that IxsS17 and its homologs are among the
proteins being injected into animals by adult I. scapularis [23], A. americanum [35], and Rhipi-
cephalus microplus [27] ticks. In our recent study, we found that B. burgdorferi infected I. sca-
pularis nymphs predominantly secreted IxsS17 at 48h feeding time point when major B.
burgdorferi transmission events are expected [25]. Additionally, we showed that RNAi silenc-
ing of IxsS17 [36] and its A. americanum homolog, AAS19 [23] caused mortality and reduced
tick feeding efficiency. This evidence suggested that functions of IxsS17 and its homologs are
related to tick feeding and transmission of tick-borne pathogens including B. burgdorferi. Con-
sistent with functional analyses of IxsS17 homologs in A. americanum (AAS19; [35]), R. micro-
plus (RmS-15; [37,38]), and recently in R. haemaphysaloides (RHS8; [39]) and I. ricinus
(Iripin-8;), we provide new information that IxsS17 is an anticoagulant that is potentiated by
binding heparin. Significantly, we further show that IxsS17 promotes B. burgdorferi coloniza-
tion of the host by inhibiting host inflammation, blood clotting, and complement system effec-
tor proteases.
Results
I. scapularis serpin (IxsS) 17 is not redundant and is conserved across
Ixodidae tick species
BLASTP search of IxsS17 (EEC18973.1 or XP_002415308.5) amino acid sequence against
entries in GenBank retrieved one sequence match of more than 77% amino acid identity per
tick species except for Dermacentor silvarum, which has two matches (S1 Fig). The next highest
matches to IxsS17 in I. scapularis and other tick species showed amino acid identity levels of
less than 50%. This indicates that IxsS17 and its homologs, in other tick species, are not redun-
dant except for D. silvarum, which has two matches that differ by an 11 amino acid deletion to
IxsS17: KAH7955208.1 and XP_049521536.1 (S1 Fig). With Homo sapiens antithrombin III
(CAA48690.1) set as an outlier, neighbor joining phylogeny tree segregated IxsS17 in group A
with other Ixodes spp. tick serpins: I. ricinus (ABI94058.1) and I. persulcatus (KAG0414503.1)
that show 99% amino acid identity to IxsS17 (Fig 1A, group A). IxsS17 is 80% identical to its
homologs in metastriata ticks including D. andersoni (XP_050039672.1) and D. silvarum,
(KAH7955208.1 and XP_049521536.1) in group B. Likewise, IxsS17 is 77–79% identical to
Hyalomma asiaticum (KAH6936909.1), Rhipicephalus microplus serpin 15 (RmS15;
AHC98666.1), R. sanguineous (XP_037506920.1), and R. haemaphysoloides (QHU78941.) in
cluster C (Fig 1A). Finally, in group D, IxsS17 is 78–80% identical to Amblyomma americanum
(AAS19; GAYW01000076.1), A. maculatum (AEO34218.1), A. triste (A0A023GPF9), A. cajee-
nense (A0A023FM57), and Haemaphysalis longicornis (KAH9373177.1) (Fig 1A, group D).
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 1. Amino acid sequence analysis of IxsS17. (A) Phylogenetic tree was constructed using the MEGA-X software, Maximum Likelihood method and
Le_Gascuel_2008 model with Bootstrap set to 1,000 replications. Group A, B, C and D represent amino acid identity levels of IxsS17 to its homologs in
percentage. (B) Multiple sequence alignment of IxsS17 reactive center loop (EEGSEAAAVTGFVIQLRTAAF) and its homologs as well as the antithrombin III
outlier was done in MacVector using T-Coffee specifications. Amino acids in the grey box are identical.
https://doi.org/10.1371/journal.ppat.1012032.g001
Although overall amino acid identity is below 100% (S1 Fig), the 21 amino acid sequence of
IxsS17 functional reactive center loop (RCL: EEGSEAAAVTGFVIQLRTAAF) is 100% con-
served in all tick serpins analyzed in this study (Fig 1B). IxsS17 was initially described among
45 I. scapularis serpin sequences that were extracted genome contigs [40]. In this manuscript,
we show that the I. scapularis genome (RefSeq GCF-016920785.2) encode for 62 serpins
including IxsS17 as revealed by unique serpin RCLs (S1 Table). Pairwise and global alignment
of the 62 RCLs with coverage set to between 80–100% confirmed that IxsS17 RCL was not
redundant as all other RCLs are 29–52% identical to IxsS17.
When compared to its homologs in other tick species both EEC18973.1 [41, 42] and
XP_002415308.5 [43] have a 53 amino acid sequence extension at the amino terminus end.
SignalP 6.0 software did not identify the signal peptide in EEC18973.1 or XP_002415308.5
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
4 / 31
PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
unless the first 53 amino acids were removed (S2A Fig). However, the subcellular localization
prediction software DeepLoc-2.0 indicated that EEC18973.1 or XP_002415308.5 were an extra
cellular protein with signal peptides at the position 50 to 70 (S2B Fig). In this study we charac-
terized mature EEC18973.1 sequence with the first 70 amino acid sequences removed.
Yeast expressed rIxsS17 inhibits trypsin-like proteases and its inhibitory
function is affected by hexa-histidine tag location
Canonical mode of serpin inhibitory activity is mechanical disruption of the target protease
which starts with the C-terminal reaction center loop (RCL) irreversibly trapping the target
protease [44]. Determined to investigate the effect of the hexa-histidine (His) fusion tag on
inhibitory functions of rIxsS17, we successfully expressed, and affinity purified three rIxsS17
constructs: (1) the hexa-histidine tag located at the N- terminal or (2) C-terminal ends or (3)
cleaved off using the inhouse produced Tobacco etch virus (TEV) protease (Fig 2). The
rIxsS17 are glycosylated like other tick (IxsS-1E1 [AID54718.1], AAS19 [JAI08902.1], AAS27
[GAGD01011247.1], and AAS41 [JAI08957.1]) and human serpins (antithrombin III and vas-
pin) [24,35,45–48]. After deglycosylation treatment, protein sizes reduced ~ 2.5–5.0 kDa
(S3 Fig). Glycosylation is the most common post-translational modification of proteins when
the carbohydrate units are attached to the protein backbone either by N- or O-glycosidic
bonds or both [49,50]. In serpins, glycosylation is important for proper protein secretion, sta-
bility, and their half-life extension [46,51].
We initially used the C-terminal hexa-His-tagged rIxsS17 in substrate hydrolysis assays of
17 serine proteases related to host responses against tick feeding (Fig 3A and S2 Table). This
screen showed that rIxsS17 (1 μM) inhibited pancreatic trypsin (1.5 nM) by 96–100%, rat skin
trypsin IV (2.0 nM; in house expressed) by 90–99%, blood clotting factor (f) Xa (2.3 nM) by
79–80% followed by inhibition of blood clotting fXIa (3.7 nM), plasmin (33.7 nM), and
Fig 2. Expression and affinity purification of recombinant (r) IxsS17. (A) Graphical illustration of three different
rIxsS17 expression constructs that were custom synthesized: (1) C-terminal hexa-histidine tag, (2) N-terminal hexa-
histidine tag and Tobacco Etch Virus (TEV) cutting site (ENLYFQG) included, and (3) hexa-histidine tag is cleaved off
at the TEV cutting site in the non-tagged rIxsS17. Please note all three recombinant constructs contain full-length
sequence of rIxsS17. (B) Western blotting analysis of daily expression of rIxsS17 in Pichia pastoris culture. Culture (1
mL) were precipitated by ammonium sulfate saturation and resolved on 10% SDS-PAGE. rIxsS17 were detected in
western blot using the HRP-conjugated monoclonal antibody to the hexa-histidine tag. (C) Silver staining and (D)
Western blotting analysis of affinity purified rIxsS17. The hexa-histidine tag was detected in the C-terminal (Lane 1)
and N-terminal-His-rIxsS17 (Lane 2) but not in the non-tagged rIxsS17 (Lane 3).
https://doi.org/10.1371/journal.ppat.1012032.g002
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 3. Inhibition profiling of rIxsS17 against 17 serine proteases related to host responses during tick feeding. (A) Inhibition rates of C-terminal-Histidine
tagged rIxsS17 (1 μM) against 17 serine proteases (with indicated concentrations) in the substrate hydrolysis assays. Substrate hydrolysis was monitored at
A405nm every 11s for 30 min at 30˚C. Inhibition rate was calculated using the formula: 100-Vi/V0 x 100, where Vi = activity in the present of, and V0 in the
absence of rIxsS17. (B) Inhibition activity of C-terminal, N-terminal and non-Histidine tagged rIxsS17 against trypsin, factor Xa and human thrombin was
determined using the substrate hydrolysis assay. Data represents mean ± SEM calculated from 3 biological replicates. The difference was analyzed using
ANOVA in GraphPad Prism 9 and is statistically significant when P value � 0.05.
https://doi.org/10.1371/journal.ppat.1012032.g003
cathepsin G (281 nM) by 52–65%, 55–61%, 56–61% respectively. Next, rIxsS17 also inhibited
human chymase (21 nM) by 26–31%, as well as native purified rat and mouse chymase by 26
and 10–25% respectively. Finally, rIxsS17 also inhibited blood clotting fXIIa (15 nM) by 18–
33%, neutrophil elastase (22 nM) by 18–25%, pancreatic chymotrypsin (1.4 nM) by 10–27%,
human thrombin (19 nM) by 28–34%, fIXa (311.4 nM) and pancreatic kallikrein (20 nM) by
~10%. Lastly, rIxsS17 had no inhibitory activity against bovine thrombin (undefined) and pan-
creatic elastase (19 nM). Heat-inactivated rIxsS17 did not inhibit serine proteases (inhibition
rate = 0%) suggesting that its inhibitory activity is heat sensitive.
Next, we tested if proximity of the hexa-His-fusion tag to the RCL or its absence affected
inhibitory activity of rIxsS17 against selected proteases (Fig 3B). As shown, rIxsS17 with N-ter-
minal hexa-His-fusion tag had an 8.6% decrease in the inhibitory activity against trypsin. This
suggests that beside the C-terminus domain that contains RCL, extension of the N-terminus
region of the serpin might affect its inhibitory activity against trypsin. For both factor Xa and
thrombin, inhibitory activity of the three constructs were similar. Since C-terminal histidine
and non-tagged rIxsS17 have equal inhibitory activity, either of them was used in our down-
stream assays.
To determine the efficiency and rate at which rIxsS17 inhibits pancreatic trypsin, trypsin
IV, and factor Xa, stoichiometry of inhibition (SI) and association rate of constant (ka) were
calculated (Fig 4). As shown, the SI (amount of rIxsS17 needed to inhibit one molecule of pro-
tease) for C-terminal His-tagged rIsS17 against trypsin, trypsin IV, and factor Xa was esti-
mated at 12.9, 10.5, and 68 respectively (Fig 4A–4C). The rate of rIxsS17 (ka) inhibition of
trypsin, trypsin IV, and factor Xa was 2.7 ± 0.003 x103 M-1 s-1, 3.9± 0.0001 x103 M-1 s-1, and
5.4 ± 1.1 x 102 M-1 s-1, respectively (Fig 4D–4F).
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 4. rIxsS17 is a moderate inhibitor of trypsin, rat trypsin IV and factor Xa. Stoichiometry inhibition (SI) analysis calculated the amount of rIxsS17
needed to inhibit one molecule of bovine trypsin (A), rat trypsin IV (B), and factor Xa (C). Various molar ratios of rIxsS17 to proteases (0, 2.5, 5, 10, 20, 25, 50)
were incubated for 15 min at 37˚C with constant concentration of bovine trypsin (1.5 nM) or rat trypsin IV (2.0 nM) or factor Xa (13.9 nM). In the presence of
appropriate substrates, residual enzymatic activity was measured and plotted against rIxsS17: protease molar ratio. The SI was determined by extrapolating to
the rIxsS17: protease ratio where protease activity is zero (Y axis = 0). The inhibition rate (ka) of rIxsS17 was determined against bovine trypsin (D), rat trypsin
IV (E) and factor Xa (F). Different concentrations of rIxsS17 (50, 100, 200, 400, 600 and 1000 nM) were incubated with constant amounts of bovine trypsin
(14.6 nM), trypsin IV (12.5 nM) or factor Xa (13.9 nM) for different periods of time (0, 1, 2, 4, 6, 8, 10 and 15 min) at 37˚C. The residual protease activity was
measured and plotted against time to determine the pseudo-first order constant, kobs. Consequently, the second-rate constant (ka) was determined by the best
fit line slope of the kobs values that were plotted against rIxsS17 concentration.
https://doi.org/10.1371/journal.ppat.1012032.g004
The concentration of rIxsS17 (1 μM) used in the inhibitory assays may not reflect the physi-
ological levels of this protein in tick saliva, however, is at optimal concentration of a single tick
salivary recombinant protein to be biologically active in-vitro or ex-vivo (1–6 μM) according to
Chmelař et al., 2016 [52]. The reason is in the complex salivary mixture, this high concentra-
tion could be achieved by combination with numerous redundant proteins.
rIxsS17 interacts with both innate immune system effector proteases and
regulatory protease inhibitors as revealed by protein-to-protein interaction
analysis
Fig 5A–5C, and Table 1 and S1 File summarize the differential precipitation protein-protein
interaction analysis of rIxsS17 and human plasma proteins. Since low amounts of rIxsS17 were
detected in fractions 1–6, we pooled fractions 1–3, and 4–6 while fractions 7–10 where individ-
ually analyzed in LC-MS/MS analysis (S1 File). Next, we used PSOPIA (prediction server of
protein-to-protein interactions; https://mizuguchilab.org/PSOPIA/) to analyze the interaction
if NSAF (normalized spectral abundance factor) value was higher in rIxsS17 and human
plasma mixture compared to plasma only controls. The interactions that were predicted with
more than 75% likelihood by PSOPIA were considered true (Table 1). This analysis confirmed
substrate hydrolysis results and revealed novel insights that rIxsS17 likely interacts with both
effector proteases and regulatory protease inhibitors of the innate immune system (Table 1).
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 5. Protein-to-protein interaction using differential precipitation of proteins (DiffPOP) analysis reveals novel
IxsS17 functional insights. 25 μg of affinity purified rIxsS17 (A-C) or rIxsS4 (D-F) was incubated with human plasma
in reaction buffer (20mM Tris-HCL and 150mM NaCl pH 7.4) overnight at 37˚C. The reaction was stabilized using
Phosphoprotein Kit- Buffer A and subjected to repeated precipitation (X10) using methanol and acetic solution (90%
methanol to 1% acetic acid). Appropriately washed precipitates of each fraction were resolved on 10% SDS-PAGE and
transferred onto PVDF membrane for western blot analysis using monospecific antibodies to rIxsS17. A and
D = human plasma only, B and E = human plasma mixed with rIxsS17 or rIxsS4, and C and F = rIxsS17 or rIxsS4
alone. Ladder (L), Number (1–9) represents each fraction from differential precipitation. Please note that, fraction 10
for rIxsS17 is not shown in this figure; however, its LC-MS/MS data analysis is available in S1 File.
https://doi.org/10.1371/journal.ppat.1012032.g005
Differential precipitation and PSOPIA analysis revealed that rIxsS17 interacted with blood
clotting system factors (f) II (prothrombin), fX, fXII, plasma kallikrein, and plasminogen
alongside blood clotting regulatory protease inhibitors; antithrombin III (serpin), heparin
Table 1. Fast fractionation and LC-MS/MS analyses identification of human plasma proteins that interacted with rIxsS17 and validation using in silico protein to
protein interaction prediction PSOPIA software.
Accession
Description
P00742
P00748
P01042
H0YAC1
A8K9A9
H0VJK2
P00734
P06681
P09871
P05156
P01023
P01008
P05546
P05155
P01019
P36955
P00739
Coagulation factor X
Coagulation factor XII
Kininogen-1
Plasma kallikrein
Plasma kallikrein B
Plasminogen
Prothrombin
Complement C2
Complement C1s subcomponent
Complement factor I
Alpha-2-macroglobulin
Antithrombin-III
Heparin cofactor 2
Functional Classification
Protease—Blood Coagulation
Protease—Blood Coagulation
Protease—Blood Coagulation
Protease—Blood Coagulation
Protease—Blood Coagulation
Protease—Blood Coagulation
Protease—Blood Coagulation
Protease–Complement proteins
Protease—Complement proteins
Protease—Complement proteins
Protease Inhibitor—Blood Coagulation
Protease Inhibitor—Blood Coagulation
Protease Inhibitor—Blood Coagulation
Plasma protease C1 inhibitor
Protease Inhibitor—Complement
Angiotensinogen
Protease inhibitor—Non inhibitory Serpin
Pigment epithelium-derived factor
Protease inhibitor—Non inhibitory Serpin
Haptoglobin-related protein
Metal Binding Proteins
Q5VY43
Platelet endothelial aggregation receptor 1
Receptor
PSOPIA score of 0.75 to 1.0 represent likely protein to protein interactions (Murakami and Mizuguchi, 2014)
https://doi.org/10.1371/journal.ppat.1012032.t001
PSOPIA P2P prediction score
0.8573
0.7918
0.9505
0.8573
0.8573
0.891
0.996
0.8204
0.8054
0.8054
0.7279
0.9794
0.9794
0.9794
0.9695
0.9794
0.8442
0.7513
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
cofactor II (serpin), alpha-2 macroglobulin (non-serpin inhibitor), and Kininogen-1 (non-ser-
pin inhibitor) (Table 1). Similarly, rIxsS17 interacted with complement system serine prote-
ases, C1s, C2, and factor I alongside the complement system regulatory serpin, plasma
protease C1 inhibitor (Table 1). Complement component C3, C4 and C5 were detected in the
differential precipitation of proteins analysis (S1 File); however, PSOPIA predicted weak likeli-
hood for interaction. We also found that rIxsS17 interacted with non-protease blood clotting
system proteins (fibronectin and fibrinogen), non-inhibitory serpins (angiotensinogen, and
pigment endothelium derived factor), and non-proteases (haptoglobin and platelet endothelial
aggregation receptor 1) (Table 1 and S1 File). Notably rIxsS4 (XP_040066711.2 or
XP_040066712.2), an inhibitor of trypsin that similar to IxsS17 has basic amino residue (R) at
its P1 site did not interact with human plasma in the differential precipitation protein-protein
interaction (Fig 5D–5F). This finding confirmed that the rIxsS17 and plasma protein-to-pro-
tein interactions were specific.
rIxsS17 binds glycosaminoglycans (GAGs)
Homology modeling predicted that IxsS17 secondary structure, which was scored at Coulom-
bic electrostatic values of -10 to 10 on ChimeraX server has a single basic positive patch located
near the RCL (Fig 6A and 6B). Basic positive patch could potentially bind negatively charged
ligands such as GAGs [53,54]. Consistently, docking analysis conducted by AutoDock Vina
and ADT v1.5.4 demonstrated that the IxsS17 secondary structure is likely to bind with hepa-
rin (Fig 6B). For the docking, nine poses were predicted and the result with the binding affinity
of -12.6 Kcal/mol and the lower bound and upper bound RMSD as 0 were selected to be the
best docked conformation. Further, the result generated was visualized by PyMOL, which
Fig 6. IxsS17 binds heparin and the putative binding sites are located on the positive basic patch. (A) Comparative
modeling of IxsS17 secondary structure was predicted on Chimera X server and heparin binding predicted using
Autodock Vina and Auto Dock Tools. Heparin ligand (in blue) was arranged accordingly to be flexible to rotate and to
explore the most probable binding positions (in red) while the receptor was kept rigid. RCL = reactive center loop. (B)
Two heparin binding sites at Lysine 188 and 210 are located on the positive basic patch (Electrostatic potential is color
coded: positive is blue; neutral is white and negative is red). (C) Binding affinity of rIxsS17 to 4 different GAGs:
heparin (black circle), heparan sulphate (red square), dermatan sulphate (green triangle) and chondroitin sulphate
(yellow upside-down triangle). The rIxsS17 was added into 96-well microplates previously coated with different GAG
at the concentration of 0, 1, 2, 5, 10 and 20 μg/mL. Binding was detected using HRP-conjugated antibody to the
histidine tag and documented as A450nm. The data represent mean ± SEM from 3 biological replicates. (D) Silver
staining of rIxsS17, rAAS19 (positive control), and r1E1 (negative control) eluted from heparin column. Recombinant
proteins (~300 μg) were applied to the heparin column. After washing, the proteins were eluted using a gradient
concentration of NaCl (0.25–0.5–1.0–2.0–3.0 M). Serpins = the proteins before applying to the column, FT = Flow
through.
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
shows that 5 amino acids of IxsS17 (Asp185, Lys188, Lys210, Ser211, Thr212) were interacting
with heparin; and out of 5 binding sites, 2 heparin binding sites (Lys 188 and 210) lies within
the positively charged or basic patch which was predicted by Chimera X. Finally, we confirmed
that rIxsS17 has high binding affinity for heparin followed by chondroitin sulphate and derma-
tan sulphate (Fig 6C). However, it was interesting to observe that rIxsS17 bound to heparin,
but not to heparan sulphate (Fig 6C). It might be because of structural variations between hep-
arin and heparan sulphate, such as the chain of heparan sulphate is generally longer, with
higher molecular weight (30kDa) than heparin (15kDa). Furthermore, l-iduronic acid pre-
dominates in heparin while d-glucuronic acid represents most of the uronic acid found in
heparan sulfate. This changes the structure configuration resulting into alteration in binding
affinity. Most importantly, heparin is the complete modified version of heparan sulphate and
contains highest negative charge density of any known biological macromolecule which will
increase its binding affinity to the positive patch of rIxsS17 [55]. The relative binding affinity
to heparin was further determined showing that rIxsS17 bound on the heparin column and
was eluted at 0.25-1M of NaCl (Fig 6D). For positive control, IxsS17 A. americanum homolog,
rAAS19 which is known for high binding affinity to heparin and having 4 basic patches [35]
was eluted at higher concentrations of NaCl (1-3M). The negative control r1E1 (KF990169)
does not bind heparin and does not have a basic patch [45], therefore, came out in the flow-
through.
Binding of heparin significantly enhances anti-blood clotting effects of
rIxsS17
In preliminary studies, we empirically determined that 2 μM of rIxsS17 delayed plasma clot-
ting by more than 60 seconds compared to buffer control. Next, we tested if the combination
of rIxsS17 and 17 kDa heparin had synergistic anti-plasma clotting effect. As heparin is an
approved blood clotting disorder therapeutic [56–58], it is interesting to note that pre-incubat-
ing plasma with the rIxsS17 and heparin mixture significantly delayed plasma clotting up to
532.9 seconds compared to clotting time for buffer control (64.5 seconds), rIxsS17 only (184
seconds), and heparin only (407 seconds) (Fig 7). It is also notable that plasma clotting was
also delayed to 474 seconds when a reaction was assembled from plasma that was incubated
separately with rIxsS17 and heparin.
rIxsS17 inhibits complement activation via the mannose-binding lectin
pathway and rescues B. burgdorferi from complement-mediated killing
Consistent with protein-to-protein interaction (in silico and ex vivo) showing that rIxsS17
interacted with complement system serine proteases (C2, C1s and factor I), our data shows
that IxsS17 is an inhibitor of the complement system (Table 1 and Fig 8). We successfully used
the WIESLAB complement system kit to independently assess three complement activation
pathways. The results demonstrated that rIxsS17 significantly inhibited deposition of the com-
plement membrane attack complex (MAC) via the mannose-binding lectin (MBL) comple-
ment activation pathway and moderately via the classical and alternative complement
activation pathways (S4 Table). In the initial screen, rIxsS17 molar excess (4 μM) reduced
MAC deposition by ~40, 62, and 99% via the classical, alternative and MBL pathway, respec-
tively (S4 Table). Moreover, we found that rIxsS17 dose dependently reduced by more than
55% MAC deposition through 31 nM of rIxsS17 (Fig 8). In this study, dose response analysis
was not done for the classical and alternative complement activation pathway.
B. burgdorferi can activate three complement pathways resulting in several host defense
mechanisms that include: opsonization, phagocyte recruitment, priming of the adaptive
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 7. Heparin binding enhances rIxsS17 anti-coagulant activity. Anti-blood clotting effects of heparin binding on
rIxsS17 was determined in the recalcification time assay. Universal coagulation reference human plasma was pre-
incubated with (1) reaction buffer control, or (2) heparin only, or (3) rIxsS17 only, or (4) rIxsS17 and heparin pre-
incubated together, or (5) rIxsS17 and heparin pre-incubated separately. CaCl2 was added to trigger blood clotting and
the reaction was monitored at A650nm every 20s for 20 min. The A650nm data were then fitted in the Sigmoidal dose-
response lines: blue (buffer control), red (heparin only), green (rIxsS17 only), black (rIxsS17 and heparin pre-
incubated together), and brown (rIxsS17 and heparin pre-incubated separately). Clotting time was interpolated from
the sigmoid line when A650nm increases by 10% with 95% confident interval. Drop vertical lines A, B, C, D, and
E = clotting time for buffer only (circle), rIxsS17 only (triangle), heparin only (square), rIxsS17 and heparin pre-
incubated with plasma separately (upside-down triangle), rIxsS17 and heparin pre-incubated with plasma together
(diamond).
https://doi.org/10.1371/journal.ppat.1012032.g007
immune system, and bacteriolysis [59,60]. Serum-mediated bacteriolysis has been used to test
the sensitivity of LD spirochetes to normal human serum [61,62]. Next, we tested if rIxsS17
was able to rescue B. burgdorferi from complement-mediated killing in vitro (Fig 9A–9D).
Consistent with its inhibitory effect against complement activation (Fig 8), rIxsS17 dose
dependently rescued the complement sensitive B. burgdorferi strain B314/pBBE22luc from
complement killing. At 1 h post incubation, B. burgdorferi survival rates ranged from 73–100%
and were not different among the tested groups (Fig 9A). At 2 and 2.5 h, only the positive con-
trol (complement resistant B. burgdorferi strain B314/pCD100) and the 1μM rIxsS17 groups
had higher survival rates than negative control, heat-inactivated rIxsS17 and PBS (protein
buffer control) (Fig 9B and 9C). At 3 h of incubation, 0–14% of the negative control survived
while survival increased to 19–21% (22 ± 2.7%), 25–31% (28 ± 1.8%), 35–47% (42 ± 3.7%) and
55–70% (64 ± 7.9%) in the presence of 0.25, 0.5, 0.75 and 1 μM rIxsS17, respectively (Fig 9D).
The positive control had survival rates of 43–67% (56 ± 12.1%). In heat-inactivated normal
human serum, survival rates of all the tested groups ranged from 97–100%.
Co-injecting rIxsS17 enhances B. burgdorferi colonization of C3H/HeN
heart and skin (ear)
Next, we tested if rIxsS17 supported B. burgdorferi colonization of the C3H/HeN Lyme disease
mouse model. We initially inoculated six groups of mice (4 mice per group) with BSK-II
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 8. rIxsS17 dose dependently inhibited membrane attack complex deposition of the Mannose-Binding Lectin
pathway. A Blank, Negative control, Positive control or Positive control incubated with 2-fold-serial dilution of
rIxsS17starting from 4 μM were added to the Mannan binding lectin (MBL) pathway Kit and incubated at 37˚C for 60
min. After the washing step, conjugate and substrate were subsequently added following the instructions of the
manufacturer. Mac deposition rate was calculated using the formular: (Sample-NC)/(PC-NC) x100%, where NC is
negative control and PC (or 0.0 μM of rIxsS17) is positive control. Data is presented as percent inhibition of MAC
deposition mean ± SEM calculated from 3 biological replicates.
https://doi.org/10.1371/journal.ppat.1012032.g008
medium (negative control), B. burgdorferi in BSK-II (positive control) and B. burgdorferi
mixed with various amounts of rIxsS17 (1, 2, 5 and 10 μM). B. burgdorferi infection was con-
firmed in all treated groups by in-vitro cultivation and conventional PCR (S5 and S6 Tables).
Spirochete burden in the ear, heart, joint and bladder tissues did not show significant differ-
ences between the groups (S4 Fig). However, we observed a pattern of B. burgdorferi load
decreasing with increasing concentration of rIxsS17 in the heart and bladder (S4 Fig). More-
over, IgG antibody titer against B. burgdorferi was statistically lower in the higher rIxsS17 con-
centration groups (S5 Fig).
We decided to repeat the experiment with reduced concentrations of rIxsS17: 0.06, 0.125,
0.25 and 0.5 μM (Fig 10). We show that the spirochete load in the heart tissue of rIxsS17
injected mice with 0.06 and 0.125 μM of rIxsS17 was 5.7 and 4.3 folds significantly higher than
B. burgdorferi only group (Fig 10A). Likewise, there is an apparent trend (P < 0.1) that the spi-
rochete load in 0.06 and 0.125 μM rIxsS17 injected mice was 1.8 and 2.3 folds higher than the
B. burgdorferi only group in ear tissues (Fig 10C). Similarly, there is an apparent high B. burg-
dorferi load in joints of mice that were co-injected with 0.125 and 0.25 μM rIxsS17 than control
(Fig 10B) and there is no apparent difference in the bladder (Fig 10D). To determine whether
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 9. rIxsS17 impaired complement mediated killing of Borrelia burgdorferi. Normal human serum (NHS) was
pre-incubated with serial dilutions of rIxsS17 (0.25, 0.5, 0.75, and 1.0 μM) or heat-inactivated rIxsS17 (1.0 μM) or
Phosphate buffered saline (PBS) at 37˚C for 30 min prior to addition of 85 μl of 106 cells/mL of B. burgdorferi B314/
pBBE22luc (complement sensitive strain) and incubated in a bio-shaker at 32˚C, 100 rpm. NHS incubated with B.
burgdorferi B314/pPCD100 (complement resistant strain) were used as positive control. Survival rates of B. burgdorferi
were assessed at 1.5 h (A), 2 h (B), 2.5 h (C) and 3 h (D) post incubation. Data represents mean ± SEM of 3 biological
replicates. Statistical significance was evaluated using t test in GraphPad Prism 9 (ns: no significance, *:P value � 0.05,
**: P value � 0.01, ***: P value � 0.001). Negative control: black circle, PBS: red diamond, HI-rIxsS17: green cross,
0.25 μM rIxsS17: maroon square, 0.5 μM rIxsS17: green triangle, 0.75μM rIxsS17: purple upside-down triangle, 1μM
rIxsS17: blue hexagon, Positive control: orange star.
https://doi.org/10.1371/journal.ppat.1012032.g009
high concentrations of rIxsS17 affected the survival of B. burgdorferi in the inoculum, we incu-
bated B. burgdorferi with 0, 0.06, 0.125, 0.25, 0.5, 1, 5, and 10 μM of rIxsS17 in vitro. This analy-
sis revealed rIxsS17 did not have a negative effect on B. burgdorferi in culture as spirochete
survival ranged from 95–98% up to 24h of observation (S7 Table).
It is also notable that IgG titers to B. burgdorferi lysate antigen detected in ELISA of the B.
burgdorferi control and rIxsS17-treated groups were not statistically different (S6 Fig). How-
ever, IgM titers of the 0.06 and 0.125 μM rIxsS17 co-injected groups were significantly higher
than 0.25 and 0.50 μM of rIxsS17 co-injected groups. Interestingly the IgM antibody of mice
that were co-injected with 0.25 and 0.50 μM of rIxsS17 did not show any significant difference
with B. burgdorferi control mice (Fig 11A and 11B). Furthermore, immune sera of 0.06 and
0.125 μM rIxsS17 co-inoculated mice bound multiple bands on western blots of lab cultured B.
burgdorferi lysate (Fig 11C).
Discussion
This study provides data showing that I. scapularis serpin (IxsS) 17 regulates key functions that
are important to tick feeding and B. burgdorferi colonization of the host. It builds on previous
studies done by our lab that characterized the A. americanum tick serpin 19 as the only tick
serpin that has its functional domain RCL perfectly conserved (100%) in all tick species as per
available data [34,35]. Like most parasites, ticks have the propensity for encoding redundant
molecular systems. For tick serpins, it is common for multiple paralogs or isoforms showing
more than 70% amino acid identity being transcribed by the same tick species [34,40,63] sug-
gesting redundancy. Thus, the finding that the IxsS17 amino acid sequence does not show any
matches to other I. scapularis serpins with more than 50% amino acid identity suggests that
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 10. Mouse groups co-inoculated with low dose of rIxsS17 have higher B. burgdorferi load in organs than high dose injected mice. Four mice/group
were inoculated with B. burgdorferi only (104 cells) mixed with or without various amounts of rIxsS17 (0.060, 0.125, 0.250, 0.500 μM). At 21 days post
inoculation, B. burgdorferi burden in mouse heart (A), joint (B), ear (C) and bladder tissues (D) was quantified by real-time qPCR method. The data were
presented as fold change of the rIxsS17 treated groups in comparison with B. burgdorferi group (2 -ΔΔCt = [(Ct Flab—Ct β-Actin) B. burgdorferi-rIxsS17 co-
injected group—(Ct Flab—Ct β-Actin) Bb only group]). Blue: B. burgdorferi only group, Red: B. burgdorferi-rIxsS17 co-injected group.
https://doi.org/10.1371/journal.ppat.1012032.g010
this protein represents a non-redundant tick serpin. It is notable that except for D. silvarum
which encodes for two homologs to IxsS17 with more than 77% amino acid identity, 12 other
tick species encoded single serpin homologs to IxsS17, with the RCL being 100% conserved. It
is important to note that the two IxsS17 homologs in D. silvarum have the same RCL and the
difference is limited an 11 amino acid deletion. We would like to note while amino acid
sequence of IxsS17 suggested no redundancy, we are unable to know in this study if IxsS17 is
also not functionally redundant.
The RCL plays an important role in the inhibitory functions of serpins [44], and thus it is
not surprising that the functional roles of IxsS17 is similar to its homologs in A. americanum,
AAS19 [35], R. microplus, RmS15 [37,38], and recently I. ricinus, Iripin-8 [64,65]. Collectively,
substrate hydrolysis and protein-to-protein interaction data in this study indicate that IxsS17
is an inhibitor of innate immunity effector proteases associated with inflammation, nocicep-
tion (pain sensing), hemostasis, and complement innate immune defenses all of which must
be blocked by ticks to feed and transmit tick-borne pathogens. In addition to food digestion
[66], pancreatic trypsin which was highly inhibited by rIxsS17 is also found in blood circula-
tion, accelerates blood clotting in the presence of calcium ions, pro-thromboplastic lipid, factor
V, VII, and X [67], and it is also the major activator of protease-activated receptor 2 (PAR2)
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Fig 11. Mouse groups co-inoculated with low dose of rIxsS17 have higher IgM titers compared to high dose injected mice. ELISA
plates previously coated with B. burgdorferi crude antigen (200 ng/well) were tested with mouse sera that diluted at 1:250 (A) and
1:500 (B). IgM titers were determined using anti-mouse IgM monospecific antibody conjugated with HRP and absorbance were read
at 450 nm. The data were presented as mean ± SEM; each dot is individual mouse. NC = negative control; Bb = B. burgdorferi.
Statistical significance was evaluated using t test in GraphPad Prism 9 (***:P value � 0.001, ****: P value � 0.0001). (C). Western blot
analysis of Bb lysate (2 μg) incubated with antisera from mice (diluted to 1:200) and anti-mouse IgM antibody-HRP conjugate (diluted
to 1:5000). Images were taken at 18 seconds of exposure. Asterisks indicate extra and intense bands detected in mice challenged with
Bb plus 0.06 and 0.125 μM rIxsS17.
https://doi.org/10.1371/journal.ppat.1012032.g011
that initiates inflammation signaling [68]. Likewise, trypsin IV, also highly inhibited by rIxS17
is associated with signaling of cutaneous local inflammation and nociception by activation of
PAR2 on cutaneous neurons [69]. Cathepsin G regulates the inflammatory responses by stim-
ulating production and maturation of cytokines and chemokines and controls the functional
state of immune cells [70]. It is interesting to note that like I. ricinus Iripin-8 and A. ameri-
canum AAS19 [35,65], rIxsS17 inhibited plasmin. At a glance, IxsS17 inhibition of plasmin is
counterintuitive because plasmin is known for degradation of fibrin clots and preventing
platelet aggregation by cleaving PAR1 [68,71], which will benefit tick feeding. However, plas-
min has additional functions on inflammation and wound healing by directly interacting with
various cell types including leukocytes (monocytes, macrophages, and dendritic cells) and cells
of the vasculature (endothelial cells, smooth muscle cells) as well as soluble factors of the
immune system and components of the extracellular matrix [72,73]. In these interactions, plas-
min contributes to inflammation and thus, its inhibition by IxsS17 will help tick feeding
through prevention of inflammatory processes. While substrate hydrolysis reveals that rIxsS17
weakly inhibited human thrombin and did not inhibit bovine thrombin, our protein-to-pro-
tein interaction data show that this protein interacted with thrombin (or blood clotting factor
II) similar to its homologs; AAS19, Iripin-8, and Rm-15 [38,65,74].
Our protein-to-protein interaction data also revealed functional insights of rIxsS17. The
finding that rIxsS17 interacted with both effector proteases and regulatory protease inhibitors
of the innate immune system is intriguing because serpins are known for their role in inhibit-
ing functions of effector serine and cysteine proteases [44]. Thus, it is surprising to note that
rIxsS17 interacted with both effector proteases and serpin regulators of the blood clotting path-
way, antithrombin III (AT), heparin cofactor II (HCII), kininogen-1 and protease C1 inhibitor
(C1 inhibitor) of the complement system. Glycosaminoglycans (GAGs) such as heparin [75]
serve as cofactors to enhance the activity of some mammalian serpins such as AT [76,77] and
HCII [78–80]. Comparative secondary structure modeling predicted basic patches in IxsS17,
which were confirmed as functional heparin binding sites using GAG plate binding and
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
heparin column assays. As IxsS17 and host serpins (AT and HCII) bind heparin, it is poten-
tially possible that the observed interaction between IxsS17 and host serpins was because of
heparin serving as a bridge between IxsS17 and host serpins. Likewise, as C1 inhibitor is
heavily glycosylated [81,82], we hypothesize that IxsS17 may interact with the C1 inhibitor by
binding onto GAGs linked to this protein. It is important to also note that in silico analysis pre-
dicted direct interactions between rIxsS17 and protease inhibitors with more than 95% chance.
These observations warrant further investigations.
The function of serpins is effected by mechanical disruption of the target protease that starts
with the target protease being trapped at P1 residue in the RCL [44]. Our results demonstrated
that positioning the histidine fusion tag at the amino-terminus end and not the C-terminus
end reduced the mechanical efficiency of rIxsS17 which was restored by cleaving off the Histi-
dine tag. The observed stoichiometry inhibition of rIxsS17 against trypsin, rat trypsin IV, and
factor Xa were high and not close to the ideal 1:1 serpin-to-target protease ratio. These findings
could be explained by evidence some of the serpins such as blood clotting regulatory serpins
antithrombin III and heparin II that require binding of glycosaminoglycans (GAGs) to
enhance their inhibitory potency [58]. Similarly, we have shown that heparin binding potenti-
ated functions of AAS19, the IxsS17 homolog in A. americanum [74]. Likewise in this study,
we determined that the putative basic patch in IxsS17 comparative tertiary structure was func-
tional and bound GAGs including heparin, an approved therapeutic against blood clotting dis-
orders [56,71]. Consistent with AAS19 [74], heparin binding by rIxsS17 significantly increased
its anti-coagulation activity. The finding that heparin binding potentiated the function of
rIxsS17 suggests that when tick injects this protein into the host, native IxsS17 binds GAGs to
enhance its anticoagulant functions.
Our protein-to-protein interaction findings showing that rIxsS17 interacted with comple-
ment system proteases. rIxsS17 likely blocks complement activation by potentially interfering
with complement component C2 (associated with classical and MBL pathway), C1s (classical
pathway), and factors I (alternative pathway). The evidence led us to investigate the effect of
this protein on complement activation. Complement system activation can be initiated via
binding of specific antibodies (Classical pathway), mannose binding lectins (MBL pathway) or
small-scale activation of complement component C3 (Alternative pathway) [59,60]. B. burg-
dorferi can activate all 3 complement pathways and results in direct complement-mediated
killing of the spirochete [83]. Moreover, Schuijt et al., [84] showed that the MBL pathway is of
paramount importance in the eradication of B. burgdorferi. Here, we showed that rIxsS17 facil-
itated B. burgdorferi survival and promoted localization via inhibition of MBL pathway. By
inhibiting the deposition of MAC in MBL pathway, rIxsS17 rescued B. burgdorferi from com-
plement-mediated killing in vitro.
Prompted by evidence that IxsS17 is highly secreted by B. burgdorferi infected nymphs [25]
within 24-48h of tick feeding, an open-window for transmission of LD agent, suggested that
this protein is important to the transmission of B. burgdorferi. Kota´l et al., [65] reported that
Iripin-8 significantly influenced nymphal I. ricinus feeding but did not promote B. afzelii
transmission as revealed by RNAi silencing. Here, we took a different approach and assessed
the effect of co-injecting C3H with rIxsS17 and B. burgdorferi and show that this protein pro-
moted colonization of C3H mice. rIxsS17 co-inoculation increased B. burgdorferi loads in
heart and ear tissue, but not distal tissues like joint and bladder of C3H/HeN mice at 21 days
post inoculation. This finding is relatively in agreement with in vivo experiment of Coumo
et al., [85] that MBL deficiency mice had higher antibody titers and harbored significantly
higher B. burgdorferi in skin tissue than deeper tissue (heart, joint and bladder) at 14 days post
inoculation. Interestingly, our results showed that rIxsS17 effects on B. burgdorferi localization
of mouse tissue is inverse dose dependent as revealed by B. burgdorferi load and IgM titers
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
being higher in mice that were injected with low dose (0.06 and 0.125 μM) than high dose
(0.25, 0.5, 1, 2, 5 and 10 μM) of rIxsS17. According to a study of Fikrig et al., [86], IgM to B.
burgdorferi whole cell in infected mice peaks at day 14 post inoculation while IgG increases
continuously even at day 180 after infection. Since IgG response to B. burgdorferi is much later
during infection, it would explain why IgG titer in our experiment which we detected at day 21
(3 weeks) post inoculation did not show significant difference.
Although actual amount of IxsS17 that the tick injects into the mammalian host during
feeding is unknown, it is estimated to be in picomolar range. Thus, it makes logical sense that
low concentrations of rIxsS17 supported B. burgdorferi colonization and vice versa for the high
dose. We hypothesize that the effects of low dose rIxsS17 used in this study better resemble the
dose of the native protein in tick saliva. It will also be interesting to further investigate if the
low spirochete load in mice that received the high dose of rIxsS17 was due to direct toxicity of
this protein to B. burgdorferi or that the high dose over stimulated the innate immune system
leading to clearance of the spirochetes.
LD control and prevention is challenging. The rise and spread of LD, and the fact that indi-
viduals can get LD more than once when bitten by an infected tick requires the development
of novel effective vaccine against this vector-borne disease [87]. The search for vaccine target
antigens is shifting from the pathogen toward tick molecules, with the purpose of reducing
tick density and B. burgdorferi infection among tick population and blocking the transmission
of LD agents [17,87,88]. In line with this, our data show that IxsS17 is an important protein in
both tick feeding and B. burgdorferi colonization of the host, and it represents a possible target
antigen in vaccines to prevent transmission of tick-borne disease agents. The mechanism by
which IxsS17 protected B. burgdorferi from host innate immune response warrants future
investigations.
Materials and methods
Ethics statement
The use of animals strictly followed the animal use protocol approved by Texas A&M Univer-
sity Animal Care and Use Committee under the number IACUC 2020–0089.
Phylogeny and comparative sequence analysis
Amino acid sequence of IxsS17 was obtained from NCBI protein database (Accession #
EEC18973.1) and blasted using the protein-protein BLAST (blastP) tool (non-redundant pro-
tein sequences (nr) and transcriptome shotgun assembly proteins (tsa_nr) database), resulting
in 12 IxsS17 homolog sequences from different tick species. A homo sapiens antithrombin
(Accession # CAA48690.1) was used as the out group in phylogenetic analysis of the sequences
using MEGA X software [89]. Pairwise sequence alignment analysis between IxsS17 and its
homologs was used to determine protein identities. Based on the multiple sequence alignment
analysis, the best protein model prediction and overall mean distance calculation, a phyloge-
netic tree was generated using maximum likelihood statistical method, bootstrap 1000 repli-
cates, Le_Gascuel_2008 model and Gamma distributed with invariant sites (G+I) [90]. In
previous studies, we showed that IxsS17 sequence was likely not redundant because its func-
tional domain RCL did not show amino acid identity of more than 55% to other I. scapularis
serpins [34,35]. To confirm these analyses, we compared the amino acid sequence of IxsS17
RCL to other I. scapularis serpin RCLs that were annotated in the recently updated I. scapularis
genome [43]. Briefly, the extracted RCL regions from previously published serpins [40] were
used as a query database to search each of the 34,235 protein sequences from the current I. sca-
pularis reference genome (ASM1692078v2) using DIAMOND blastp v2.0.15 [91]. Each of the
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
proteins with a positive hit to the RCL query was manually curated to confirm that it is a serpin
with a complete RCL region. Finally, RCL regions from each of the confirmed I. scapularis ser-
pins was compared to IxsS17 RCL using Needleman-Wunsch Global Alignment tool from
NCBI.
The protein signal peptides and subcellular localization of IxsS17 were predicted using the
software SignalP 6.0 (https://services.healthtech.dtu.dk/service.php?SignalP-6.0) and deepLoc
2.0 (https://services.healthtech.dtu.dk/service.php?DeepLoc).
Expression of recombinant (r) IxsS17
Expression of mature rIxsS17 (without signal peptide) was done in Pichia pastoris using the
pPICZαA yeast expression plasmid as described [35, 92]. The coding domain for mature
IxsS17 nucleic acid sequence was retrieved from GenBank (Accession # EEC18973.1) and opti-
mized for expression in P. pastoris. Two rIxsS17 expression constructs with His-fusion tag
placed at amino- or carboxyl-terminus were custom synthesized and cloned into pPICZαA
(Biomatik, Wilmington, Delaware). To facilitate cleaving off the His-fusion-tag, the Tobacco
etch virus (TEV) protease cutting site (ENLYFQG) was inserted between His-fusion tag and
the IxsS17 coding sequence. The TEV protease was produced in house (see below).
Routinely, rIxsS17 expression plasmid was transformed into P. pastoris X33 (Thermo Fisher
Scientific, Hanover Park, IL, USA), cultured at 28˚C in a bio-shaker (MaxQ 400, Thermo
Fisher Scientific), and expression of rIxsS17 induced by feeding cultures daily with 5% metha-
nol as a carbon source. For large scale expression (1 L batches), cultures were incubated for 5
days. The pPICZαA yeast expression plasmid secretes the recombinant protein into culture
media. Thus, spent media was collected and rIxsS17 was precipitated out by ammonium sul-
fate saturation (525 g/L) at 4˚C overnight with stirring. Subsequently, precipitated rIxsS17 was
dialyzed against His-tag affinity column binding buffer (100 mM Tris, 500 mM NaCl, 5mM
imidazole, pH 7.4) and processed for affinity purification using the Hi-Trap Chelating HP col-
umn (Cytiva, Marlborough, MA, USA) under native conditions. Affinity purification was then
confirmed by standard sodium dodecyl sulfate-polyacrylamide gel electrophoresis
(SDS-PAGE), silver staining using the Pierce Silver Stain Kit staining kit (ThermoScientific,
USA), and western blotting analysis using the mouse monoclonal anti-Histidine antibody
(GenScript, Piscataway, NJ) to determine purity. The concentration of rIxsS17 was determined
using a bicinchoninic acid (BCA) kit (ThermoScientific, USA). The protein was stored at
-80˚C until use.
Cleaving off the histidine tag from affinity purified rIxsS17
The TEV protease was produced in house using expression vector MBP-TEVcs (ENLYFQ/G)-
His6-TEVΔ (220–242)-R5, a gift from Alice Ting (Addgene plasmid # 135456; http://n2t.net/
addgene:135456; RRID: Addgene_135456) and Escherichia coli BL21 (DE3) (ThermoScientific,
USA). In brief, the expression vectors were transformed into the E. coli BL21 strain. The trans-
formed E. coli cells were grown in SOB medium (RPI Research Product International, Mount
Prospect, IL) at 37˚C until the OD600 reached 0.4–0.6. The TEV expression was then induced
by cultivation with 1 mM isopropyl-β-d-thiogalactopyranoside (IPTG) at 30˚C for 5 h. The
cells were collected following by sonication in binding buffer (100 mM Tris, 500 mM NaCl,
5mM Imidazole, 10% glycerol, pH 7.4) and centrifugation at 10,000 × g, 4˚C for 30 min. After
that, TEV was purified from the supernatants using the Hi-Trap Chelating HP column (Cytiva,
Marlborough, MA, USA) under native conditions as described previously. The purity of the
protein was assessed by Coomassie blue staining following SDS-PAGE gel electrophoresis. The
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
concentration of TEV was determined using a NanoDrop 8000 spectrophotometer (Thermo-
Scientific, USA). Finally, TEV was stored at -80˚C until use.
To cleave off the His fusion tag, affinity purified rIxsS17 (3 μg) and TEV (1 μg) were mixed
and incubated at 4˚C overnight in cleaving buffer (50 mM Tris-HCl, 0.5 mM EDTA, 1 mM
DTT, pH 8.0). The rIxsS17 and TEV ratio was empirically determined in preliminary studies.
Subsequently, the reaction mix was dialyzed into Hi-Trap chelating column binding buffer
and then purified as described above. Fractions of non-tagged-rIxsS17 were expected to elute
into flow through and column wash fractions while both His-tagged TEV and non-cleaved
rIxsS17 bound onto the affinity column matrix and were recovered into elution fractions.
SDS-PAGE with silver staining and western blotting analysis using the His-tag antibody were
used to confirm purification of His-tag free rIxsS17. Quantification was done as described
above.
rIxsS17 deglycosylation.
In a 50 μl reaction, 10 μg of rIxsS17 was deglycosylated using
2 μl of protein deglycosylation mix II (NEB, MA, USA) under denaturing condition following
the instructions from the manufacturer. The reaction was incubated at 37˚C for 16 h. Deglyco-
sylated rIxsS17 was analyzed by SDS-PAGE and silver staining.
Profiling inhibitor functions against innate immune system proteases
Inhibitory activity of rIxsS17 against 17 serine proteases related to host responses against tick
feeding was determined using substrate hydrolysis assays as described [35,48,93]. The serine
proteases and their substrates used in this study are listed in S2 Table. 1μM of rIxsS17 (His-
tagged at N- or C-terminal or His-tag removed) was incubated with empirically verified serine
protease amount at 37˚C for 15 min in reaction buffer (20 mM Tris–HCl, 150 mM NaCl, 0.1%
BSA, pH 7.4). Subsequently, 200 nM of the appropriate peptide substrate was added to the
final reaction volume of 100 μL and hydrolysis was monitored at 405 nm wavelength every 11s
for 30 min at 30˚C using microplate reader (Biotek Synergy H1, Winooski, VT, USA). The
assay was performed in duplicates of 3 biological replications. Data were subjected to one-
phase decay analysis PRISM 9 to determine plateau values as proxies for initial velocity of sub-
strate hydrolysis or residual enzyme activity.
Stoichiometry of inhibition (SI). Preliminary substrate analysis revealed that molar
excess of rIxsS17 inhibited pancreatic trypsin, trypsin IV, and blood clotting factor Xa by more
than 70%–nearly 100%. To estimate the molar ratio of rIxsS17 to the target protease (pancre-
atic trypsin, trypsin IV, and factor Xa) required for 100% inhibition of enzyme activity of the
target protease, stoichiometry of inhibition (SI) analysis was done as described [35,48,74]. Var-
ious molar ratios of His-tagged rIxsS17 to proteases (0, 2.5, 5, 10, 20, 25, 50) were incubated
for 15 min at 37˚C with constant concentration of bovine trypsin (1.5 nM) or rat trypsin IV
(2.0 nM) or factor Xa (13.9 nM). The colorimetric substrate was added; and residual protease
activity was determined as described above. SI or the molar ratio of rIxsS17 to protease was
determined by plotting the percentage residual protease activity against serpin to protease
ratios, fitting data onto the linear regression in PRISM 9, and extrapolation to the ratio which
resulted in total loss of protease activity [94].
Affinity constant (ka) calculation. The rate of rIxsS17 inhibiting bovine trypsin, trypsin
IV and fXa was determined using the discontinuous method [93, 94]. Different concentrations
of rIxsS17 (50, 100, 200, 400, 600 and 1000 nM) were incubated with constant amounts of
bovine trypsin (14.6 nM), trypsin IV (12.5 nM) or fXa (13.9 nM) for different periods of time
(0, 1, 2, 4, 6, 8, 10 and 15 min) at 37˚C. The colorimetric substrate was added; and residual pro-
tease activity was assayed as described above. The pseudo-first order constant, kobs, was deter-
mined from the slope of a semi-log plot of the residual protease activity against time. The
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
second-rate constant (ka) was determined by the best fit line slope of the kobs values that were
plotted against rIxsS17 concentration [94].
Homology modeling and prediction of basic patches and docking. Secondary structure
modeling of IxsS17 was done on ChimeraX molecular modeling server [95]. The mature
IxsS17 protein amino acid sequence was pasted into Alphafold comparative modeling software
on ChimeraX server, and the best IxsS17 comparative secondary structure model was reported.
Putative basic patches were predicted using the electrostatic potential calculator in ChimeraX.
Molecular docking study using the ligand molecule heparin (PubChem ID 22833565) with
IxsS17 protein was conducted using Autodock Vina and Auto Dock Tools (ADT) v 1.5.4 from
the Scripps Research Institute [96, 97]. The ligand was oriented suppositionally to allow flexi-
ble rotation and thus explore the most probable binding positions, while the receptor was kept
rigid. The grid maps were calculated by Autogrid which represents the center of active site
pocket for the ligand. The generated results were visualized by using PyMOL viewer (https://
pymol.org/2/).
Glycosaminoglycan (GAG) binding and effect on rIxsS17 function. Secondary structure
modeling predicted at least one basic patch in IxsS17 comparative modeling structure. To test
if the rIxsS17 basic patch is functional, rIxsS17 binding affinity of glycosaminoglycans (GAGs):
heparin (Sigma-Aldrich, MO, USA), chondroitin sulphate A (Sigma-Aldrich), heparan sul-
phate (Galen Laboratory Supplies, North Haven, CT) and dermatan sulphate (Galen Labora-
tory Supplies) was done as previously described [48]. A GAG-binding microplate (Galen
Laboratory Supplies) was coated with 200 μL of GAG at the concentration of 25 μg/mL in
binding buffer (100 mM NaCl, 50 mM Na-acetate, 0.2% Tween, pH 7.2) and incubated over-
night at room temperature. After washing with binding buffer, the plate was blocked with
250 μL of 1% bovine serum albumin in PBS for 1 h at 37˚C. Thereafter, different concentra-
tions of rIxsS17 (0, 1, 2, 5, 10 and 20 μg/mL) in 200μL of blocking buffer was added and incu-
bated for 2 h at 37˚C. After the wash, 200 μL of HRP-conjugated anti-histidine antibody
(1:5,000 dilution) was added. Following by addition of 200 μL of 1-step Ultra TMB ELISA sub-
strate (Thermo Scientific), 100 μL of hydrochloric acid (1N) was used to stop the reaction; and
the OD450 nm was determined using a microplate reader (Biotek Synergy H1).
Heparin binding assay. Approximately 300 μg of rIxsS17 or rAAS19 (positive control) or
r1E1 (negative control) was applied to the Hi-Trap heparin column (Cytiva). After washing
with 10 mM phosphate buffer pH 7.4, the proteins were eluted using a gradient concentration
(0.25–0.5–1.0-.2.0–3.0 M) of NaCl. Samples included the protein before binding, flow-
through, wash, and elution were collected, resolved on 10% gel of SDS-PAGE, and subjected
to silver staining for analysis.
Recalcification time assay. Prompted by preliminary findings that rIxsS17 was an inhibi-
tor of blood clotting factors and it bound GAGs including heparin, we assayed the effect of
heparin and rIxsS17 mixture on plasma clotting in a recalcification time assay, which evaluates
the blood clotting system holistically [98]. Five groups: (1) Buffer control (20 mM Tris-HCl,
150 mM NaCl pH 7.4), (2) 17 kDa heparin sodium salt (0.5 μg/mL) (Sigma-Aldrich, USA), (3)
rIxsS17 (2 μM: empirically determined to delay plasma clotting by more than 60 seconds), (4)
rIxsS17 and heparin mixture incubated separately, and (5) rIxsS17 and heparin incubated
together were incubated in 40 μL of 20 mM Tris-HCl, 150 mM NaCl, pH 7.4 buffer for 5 min
at 37˚C. Subsequently, 50 μL of pre-warmed (37˚C) universal coagulation reference human
plasma (UCRP) (ThermoScientific, USA) was added to each group and incubated at 37˚C for
an additional 5 min. After adding 10 μL CaCl2 (final concentration of 150 mM) to trigger
plasma clotting, optical density (A) was monitored at 650 nm wavelength every 20s for 20 min
using the microplate reader (Biotek Synergy H1). Data from the recalcification time assay were
plotted onto sigmoid line in PRISM 9. Initiation of plasma clotting (or clotting time) was
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
interpolated from the sigmoid line when A650nm increased by 10%, with 95% confident interval
as published [74].
Differential Precipitation of Proteins (DiffPOP) and in silico protein to protein interac-
tions. Differential precipitation of protein-to-protein interaction between rIxsS17 and
human plasma was done as described [99]. In a 1.5 mL vial, a reaction mix of 150 μL reaction
containing 25 μg rIxsS17 and 20 μL of human plasma in reaction buffer (20 mM Tris–HCl, 150
mM NaCl, pH 7.4) were incubated at 37˚C overnight. Human plasma only and rIxsS17 only
were also incubated in reaction buffer as negative controls. After the incubation, 100 μL of
Phosphoprotein Kit- Buffer A (Clontech Laboratories, New York, NY) was added to stabilize
the reaction prior to fractionation. To fractionate, precipitation solution (90% methanol/ 1%
acetic acid) was added to the stabilized reaction mix, vortexed and incubated at room tempera-
ture for 5 min. Precipitates were collected by centrifugation at 14,000 rpm (or max speed) at
4˚C. The supernatant was transferred into a new 1.5 mL tube and process repeated until
desired fractions are obtained (10 fractions in total). The pellet was washed in 400 μL of ice-
cold acetone, air-dried, re-suspended in 100 μL reaction buffer and stored in -80˚C until use.
The expectation for this approach is that rIxsS17 will co-precipitate with its interactors. To
determine fractions that co-precipitated with rIxsS17, each fraction was resolved on 10%
SDS-PAGE gels and subjected to standard western blotting analysis using the monospecific
antibody to rIxsS17. The monospecific antibody to rIxsS17 was purified from immune serum
of rabbits that were repeatedly fed on by I. scapularis as previously described [24,48]. The posi-
tive signal was developed using chemiluminescent substrates (ThermoScientific, USA).
Fractions that contain potential complexes with rIxsS17 were processed for LC-MS/MS
analysis using the method published previously [25]. To identify proteins, extracted tandem
mass spectra was searched against the database of non-redundant human proteins from Gen-
Bank using the Prolucid program in the Integrated Proteomics Pipeline (IP2) as published
[100]. The parameters used to identify potentially true rIxsS17 interactors included detecting
at least two peptides in two of three independent LC-MS/MS runs and normalized spectral
abundance factors (index for relative protein abundance in exceeded plasma only control val-
ues). Subsequently, these interactions were validated using in silico methods on the PSOPIA
(prediction server of protein-to-protein interactions; https://mizuguchilab.org/PSOPIA/)
server [101] and readouts with more than 75% likelihood to interact were considered as true.
Complement activity assay. Following up on protein-to-protein interaction results that
rIxsS17 also interacted with complement system factors, we investigated its effect on comple-
ment pathway activations using the WIESLAB Complement Classical, Alternative and Man-
nose-binding Lectin (MBL) pathway Kits (Malmo¨, Sweden). The kits allowed for independent
assessment of rIxsS17 on the three complement activation pathways as measured by C5b-C9
or Membrane Attack Complex (MAC) deposition. For initial screening of rIxsS17 possible
effect on the complement pathways, we started with high dose of rIxsS17, at 4 μM (20 μg) in
100 μL reactions. Subsequently, inhibition activity assessment of a serially dilute rIxsS17
(0.0078–4 μM) was done. First, rIxsS17 was pre-incubated with positive control (human serum
provided with the kit) at 37˚C for 30 min. Then, the samples were added to the wells (provided
in the kits) along with a blank (diluent only), negative control and positive control, and incu-
bated at 37˚C for 60 min. The assay was performed in duplicates. After the washing step, con-
jugate and substrate were subsequently added following the instructions of the manufacturer.
Finally, absorbance was read at 405 nm on a microplate reader (Biotek Synergy H1, Winooski,
VT, USA). The effect of rIxsS17 on MAC deposition was calculated as follow: (Sample-NC)/
(PC-NC) x100 where NC is negative control and PC is positive control.
Borrelia burgdorferi complement sensitivity assay. Prompted by preliminary findings
that rIxsS17 dose dependently reduced deposition of the MAC, we assessed its effect on
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
rescuing complement sensitive spirochete as previously described [61,62,102]. The comple-
ment sensitive B. burgdorferi (B314/pBBE22luc) and complement resistant (B314/pCD100)
strains were kindly gifted by the Skare lab (TAMU Health Science Center). Both strains were
propagated in BSK-II media at 32˚C, 1% CO2. For the assay, 15 μL of normal human serum
(NHS) (Complement technology, TX, USA) was pre-incubated with serial dilutions of either
C-terminal-His/non-tagged rIxsS17 (0.25, 0.5, 0.75, and 1μM), heat-inactivated rIxsS17 (1μM)
or protein buffer (PBS; Phosphate buffered saline) at 37˚C for 30 min prior to addition of 85 μl
of B. burgdorferi B314/pBBE22luc at the concentration of 106 cells/mL and inoculated in a bio-
shaker at 32˚C, 100 rpm. NHS with B. burgdorferi B314/pBBE22luc or B314/pPCD100 were
included as negative and positive controls. Survival of spirochetes was assessed at 1.5, 2, 2.5
and 3 h post incubation. Spirochetes were counted from randomly chosen fields (10–15 fields)
under dark-field microscope. Spirochete viability was judged based on cell mobility, mem-
brane integrity, and cell lysis as described [102]. Spirochete survival rates were calculated from
3 biological replicates. Heat-inactivated NHS (hiNHS) was used as the no complement-activity
control and for normalization.
Effect of IxsS17 on B. burgdorferi colonization of C3H/HeN Lyme disease mouse
model. Routinely, B. burgdorferi strain B31 (MSK5; kindly gifted by the Skare lab) were cul-
tured in BSK-II medium and virulence plasmid Ip25 and Ip28-1 were verified using PCR
primers (S3 Table) as described [103]. Groups of C3H/HeN mice (Charles River Laboratories,
Wilmington, MA) (4 mice/group) were intradermally inoculated with 104 B. burgdorferi spiro-
chetes or 104 B. burgdorferi spirochetes with various amounts of rIxsS17 (0.06, 0.125, 0.25, 0.5,
1, 2, 5 and 10 μM). Another group of 4 mice were inoculated with BSK-II + PBS as the negative
control. At 21 days post inoculation, blood and tissue samples were collected from all mice.
Serum was extracted from blood; and genomic DNA was extracted from tissues using DNeasy
Blood and Tissue kit (Qiagen, MD, USA). B. burgdorferi infection was assessed by ELISA,
western blot, in-vitro cultivation, PCR, and real-time qPCR methods.
ELISA. ELISA was used to determine IgM and IgG antibody titer to B. burgdorferi in
mouse sera. Ninety-six-well-microplates (Nunc MaxiSorp, ThermoScientific) were coated
with 200 ng/well of B. burgdorferi lysate antigen, blocked with 5% skim milk at 4˚C overnight
and incubated with serially diluted mouse sera (at 1:250-500-1,000–2,000) at room tempera-
ture for 2 h. Signal was detected using either goat-anti mouse IgM antibody-HRP conjugated
(ThermoScientific) or Clean-Blot IP detection reagent (ThermoScientific) at a 1:5,000 dilution,
following by addition of the TMB substrate (3,3’,5,5’-tetramethylbenzidine) (ThermoScienti-
fic). The reaction was stopped using 2N sulfuric acid and absorbance was read at 450 nm using
the microplate reader (Biotek Synergy H1).
Western blotting. Two μg of Bb lysate was resolved by SDS-PAGE and transferred to
PDVF membrane. The membrane was blocked in 5% skim milk and then incubated with
mouse antisera (diluted to 1:200) overnight at 4˚C. The anti-mouse IgM-HRP conjugates
(ThermoScientific) at 5,000-fold dilution was used to detect primary antibodies. Finally, signal
was detected using SuperSignal West Femto Maximum Sensitivity Substrate (ThermoScienti-
fic) under Biorad ChemiDoc MP Imaging system.
In-vitro cultivation of B. burgdorferi.
In-vitro cultivation of B. burgdorferi was used to
confirm colonization in mice organs. Within 1 h of completing necropsy, mice organs were
submerged in 3–5 mL of BSK-II with appropriate antibiotics and antifungals and incubated at
32˚C, 1% CO2. The culture was examined bi-weekly for the presence of B. burgdorferi under
dark-field microscope.
Real-time quantitative PCR. Real-time qPCR was used to quantify B. burgdorferi (Bb) in
mouse organs targeting Bb flab; and Murine β-Actin was used as an internal control and for
normalization (Primer sequences are listed in S3 Table) [103,104]. The qPCR assays were
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
performed in 10 μL reactions with 5 μL iTaq Universal SYBR Green Supermix (Bio-rad, Her-
cules, CA), 300 nM each primer and 10–50 ng mouse organ gDNA on a Bio-rad CFX96 real
time system (Bio-rad). Data was analyzed by the comparative (Ct) method with the equation:
Fold change = 2-ΔΔCt = [(Ct Flab—Ct β-Actin) Bb-rIxsS17 co-injected group—(Ct Flab—Ct β-
Actin) Bb only group] [105]. The spirochete burden in mouse organs was expressed as the fold
change of B. burgdorferi load in mice that were co-injected with rIxsS17 compared with mice
injected with B. burgdorferi only.
Statistical analysis
Data was analyzed using GraphPad Prism 9 software and represented as mean ± SEM with sta-
tistical significance (P < 0.05) detected using the Student’s t-test and two-tailed ANOVA.
Supporting information
S1 Fig. Multiple sequence analysis of IxsS17 and its homologs. Amino acid sequences of
IxsS17 and its homologs as well as antithrombin III were aligned in MacVector using T-Coffee
specifications. The broken line red box denotes the functional domain reactive center loop.
Please note that accession numbers are indicated.
(TIF)
S2 Fig. Prediction of signal peptides and subcellular localization for EEC18973.1. Subcellu-
lar localization software DeepLoc-2.0 predicted extracellular location for EEC18973.1 (A). Sig-
nalP 6.0 software predicted the signal peptide for EEC18973.1 after first 53 amino acid were
removed (B). The predicted signal peptides were marked with broken green line.
(TIF)
S3 Fig. rIxsS17 under native and deglycosylated forms. (A) Native C-terminal-his and non-
tagged-IxsS17 were resolved in clear native PAGE following by silver staining analysis. (B) Sil-
ver staining image of C-terminal-his and non-tagged-IxsS17 before and after treatment with
deglycosylation enzyme under denaturing condition.
(TIF)
S4 Fig. Quantitative real-time PCR analysis of B. burgdorferi load in mice co-inoculated
with 1–10 μM of rIxsS17. Four mice/group were inoculated with B. burgdorferi only (104 spi-
rochetes) with or without different amounts of rIxsS17 (1-2-5-10 μM). At 21 days post inocula-
tion, B. burgdorferi burden in mouse heat, ear, joint and bladder tissues was quantified by real-
time qPCR method. The data were presented as fold change of the rIxsS17 treated groups in
comparison with Bb group (2 -ΔΔCt = [(Ct Flab—Ct β-Actin) Bb-rIxsS17 co-injected group—
(Ct Flab—Ct β-Actin) Bb only group]). Red arrows indicate decrease on B. burgdorferi load.
Bb: B. burgdorferi, Bb + rIxsS17: B. burgdorferi co-inoculated with rIxsS17 groups.
(TIF)
S5 Fig. IgG titers against B. burgdorferi in mice co-inoculated with 1–10 μM of rIxsS17.
IgG titers against B. burgdorferi lysate antigen were detected using ELISA. Mouse sera was
tested at 1:500 dilution. The data were presented as mean ± SEM; each dot is individual
mouse. NC = negative control; Bb = B. burgdorferi group. Bb = B. burgdorferi, Bb + rIxsS17 =
B. burgdorferi co-inoculated with rIxsS17. Statistical significance was evaluated using Student’s
t-test in GraphPad Prism 9 (*:P value � 0.05, ns: no significance). Red arrows indicate decrease
on IgG titers.
(TIF)
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
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PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
S6 Fig. IgG titers against B. burgdorferi in mice co-inoculated with 0.06–0.50 μM of
rIxsS17. IgG titers against B. burgdorferi lysate antigen were detected using ELISA. Mouse sera
was tested at 1:250 and 500 dilutions. The data were presented as mean ± SEM; each dot is
individual mouse. NC = negative control; Bb = B. burgdorferi group. Bb = B. burgdorferi, Bb
+ rIxsS17 = B. burgdorferi co-inoculated with rIxsS17.
(TIF)
S1 Table. Amino acid residue identity between IxsS17 RCL and other I. scapularis serpins
with coverage above 80%.
(DOCX)
S2 Table. List of proteases and substrates used in the substrate hydrolysis assay.
(DOCX)
S3 Table. List of oligonucleotide primers used in the study.
(DOCX)
S4 Table. Percentage complement activity of the human serum treated with rIxsS17.
(DOCX)
S5 Table. B. burgdorferi positive rate of mouse tissues by in-vitro cultivation.
(DOCX)
S6 Table. B. burgdorferi positive rate of mouse tissues by PCR.
(DOCX)
S7 Table. Survival rates B. burgdorferi incubated with different concentrations of rIxsS17
in-vitro.
(DOCX)
S1 File. LC-MS/MS analysis of 10 fractions resulted from differential precipitation of pro-
tein assay of IxsS17 with human plasma.
(XLSX)
Acknowledgments
The author would like to thank Drs. Jon T Skare and Alexandra D Powell-Pierce for providing
the B. burgdorferi strains and sharing their complement sensitivity assay protocols. We are
grateful to Texas A&M core facility on Molecular genomics workspace and Dr. Andrew Hill-
house for their technical assistance with real-time qPCR experiment.
Author Contributions
Conceptualization: Thu-Thuy Nguyen, Tae Kwon Kim, Lucas Tirloni, Zeljko Radulovic,
Albert Mulenga.
Data curation: Thu-Thuy Nguyen.
Formal analysis: Alex Samuel Kiarie Gaithuma, Moiz Ashraf Ansari, Tae Kwon Kim, Lucas
Tirloni, Zeljko Radulovic.
Funding acquisition: James J. Moresco, John R. Yates, III, Albert Mulenga.
Investigation: Thu-Thuy Nguyen, Tae Heung Kim, Emily Bencosme-Cuevas, Jacquie Berry,
Tae Kwon Kim, Lucas Tirloni, Zeljko Radulovic.
PLOS Pathogens | https://doi.org/10.1371/journal.ppat.1012032 February 23, 2024
24 / 31
PLOS PATHOGENSTick saliva serpin inhibits innate immune responses and enhances colonization of Lyme disease agent
Methodology: Thu-Thuy Nguyen, Tae Kwon Kim, Lucas Tirloni, Zeljko Radulovic, Albert
Mulenga.
Supervision: Albert Mulenga.
Validation: Thu-Thuy Nguyen, Tae Kwon Kim, Lucas Tirloni, Zeljko Radulovic, James J.
Moresco, Albert Mulenga.
Visualization: Thu-Thuy Nguyen, Moiz Ashraf Ansari.
Writing – original draft: Thu-Thuy Nguyen, Albert Mulenga.
Writing – review & editing: Thu-Thuy Nguyen, Tae Heung Kim, Emily Bencosme-Cuevas,
Jacquie Berry, Alex Samuel Kiarie Gaithuma, Moiz Ashraf Ansari, Tae Kwon Kim, Lucas
Tirloni, Zeljko Radulovic, James J. Moresco, John R. Yates, III, Albert Mulenga.
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PLOS PATHOGENS
| null |
10.1111_jcmm.15814.pdf
| null |
DATA AVA I L A B I L I T Y S TAT E M E N T The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.
|
Received: 23 February 2020 | Revised: 5 August 2020 | Accepted: 8 August 2020
DOI: 10.1111/jcmm.15814
O R I G I N A L A R T I C L E
Cardiac fibroblast miR-27a may function as an endogenous
anti-fibrotic by negatively regulating Early Growth Response
Protein 3 (EGR3)
Lifeng Teng1 | Yubing Huang1 | Jun Guo2 | Bin Li1 | Jin Lin1 | Lining Ma1 |
Yudai Wang1 | Cong Ye1 | Qianqian Chen3
1Department of Cardiology, Hainan General
Hospital, Haikou, China
Abstract
2Department of Cardiology, The First
Affiliated Hospital of Jinan University,
GuangZhou, China
3Nursing Department, Hainan Maternal and
Child Health Hospital, Haikou, China
Correspondence
Jun Guo, Department of Cardiology, The
First Affiliated Hospital of Jinan University,
No. 613, Huangpu Street, Tianhe District,
Guangzhou 510630, China.
Email: [email protected]
Funding information
Central South University Innovation Fund
for Independent Graduate Exploration,
Grant/Award Number: 72150050587;
National Nature Science Foundation for
the Youth of China, Grant/Award Number:
81202005; Technology Plan Fund of Hunan
Science, Grant/Award Number: 2013FJ4109
Pathological myocardial fibrosis and hypertrophy occur due to chronic cardiac stress.
The microRNA-27a (miR-27a) regulates collagen production across diverse cell types
and organs to inhibit fibrosis and could constitute an important therapeutic avenue.
However, its impact on hypertrophy and cardiac remodelling is less well-known. We
employed a transverse aortic constriction (TAC) murine model of left ventricular
pressure overload to investigate the in vivo effects of genetic miR-27a knockout,
antisense inhibition of miR-27a-5p and fibroblast-specific miR-27a knockdown or
overexpression. In silico Venn analysis and reporter assays were used to identify miR-
27a-5p's targeting of Early Growth Response Protein 3 (Egr3). We evaluated the ef-
fects of miR-27a-5p and Egr3 upon transforming growth factor-beta (Tgf-β) signalling
and secretome of cardiac fibroblasts in vitro. miR-27a-5p attenuated TAC-induced
cardiac fibrosis and myofibroblast activation in vivo, without a discernible effect on
cardiac myocytes. Molecularly, miR-27a-5p inhibited transforming growth factor-
beta (Tgf-β) signalling and pro-fibrotic protein secretion in cardiac fibroblasts in vitro
through suppressing the pro-fibrotic transcription factor Early Growth Response
Protein 3 (Egr3). This body of work suggests that cardiac fibroblast miR-27a may
function as an endogenous anti-fibrotic by negatively regulating Egr3 expression.
K E Y W O R D S
cardiac fibrosis, cardiac remodelling, EGR3, miR-27a, TGF-β
1 | I NTRO D U C TI O N
microRNAs (miRNAs) are small non-coding RNAs (ncRNAs) ap-
proximately 22 nucleotides in length that control expression of
(mRNAs).1 It is becoming increasingly evident that a significant frac-
tion of genes and biochemical pathways are regulated by miRNA
or ncRNAs.2-4 Consequently, and unsurprisingly, miRNAs serve
numerous functions under healthy and pathological conditions.5
genes at the level of transcription, by binding to messenger RNAs
Involvement of miRNAs in regulation of the cardiovascular system
Lifeng Teng and Yubing Huang contribute equally to this work.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
© 2020 The Authors. Journal of Cellular and Molecular Medicine published by Foundation for Cellular and Molecular Medicine and John Wiley & Sons Ltd.
J Cell Mol Med. 2021;25:73–83.
wileyonlinelibrary.com/journal/jcmm
| 73
74 |
is well-established and includes processes such as chronic stress-in-
miR-27a-5p knockdown across the four major cardiac cell types (CFs,
duced cardiac remodelling due to aortic stenosis, which is char-
acterized by fibrosis and hypertrophy.6 Among miRNAs known to
CMs, cardiac endothelial cells [CECs] and cardiac vascular smooth
muscle cells [CVSMCs]20) in miR-27a−/− mice (Figure 1A). This de-
regulate cardiac fibrosis are miR-21, miR-29, miR-30 and miR-133,
crease in miR-27a-5p did not affect baseline cardiac characteristics
while cardiac hypertrophy is regulated by are miR-212/132, miR-133
and miR-208.7-14 Fibrosis and hypertrophy are interrelated and can
elicit one another,15 and this is borne out by overlapping miRNAs,
(Figure 1B-E) or overall body weight (Figure S2A).
Next, we assessed whether diminished miR-27a-5p levels would
affect cardiac characteristics under pressure overload conditions.
such as miR-133, that regulates both these processes.
Transverse aortic constriction (TAC) procedures were performed to
One notable miRNA—miR-27a—is dysregulated in both animal mod-
els of fibrosis and human fibrotic disease.16-19 Interestingly, miR-27a has
generate a mouse model of pressure overload in the left ventricle.
miR-27a−/− animals that underwent TAC displayed no discernable dif-
been shown to suppress fibrosis in kidney, bladder, liver and lung pa-
thologies.16-19 These findings suggest that targeting miR-27a could con-
stitute a possible method to prevent cardiac fibrosis. However, little is
ferences in fibrosis levels in lung, liver and kidney tissues from WT
TAC mice (Figure S2B). Notably, miR-27a−/− TAC mice had inferior left
ventricular function than WT TAC mice (Figure 1B). miR-27a−/− TAC
known about miR-27a's role (if any) in cardiac fibrosis. Therefore, better
mice exhibited greater levels of heart mass (Figure 1C), left ventri-
elucidating the net effect of miR-27a on cardiac remodelling will have
cle fibrosis (Figure 1E) and fibrosis markers (Tgfb2, Col1a1, Col1a2,
important ramifications for its potential as a drug target.
To address this gap in our knowledge, we sought to delineate
the involvement of miR-27a on cardiac remodelling by using both in
Col3a1, and Lox mRNA levels as well as Tgfβ2, Smad2 phosphory-
lation, Smad3 phosphorylation, Col I, Col III and Lox protein levels)
(Figure 1F,G) compared to WT TAC animals. Moreover, miR-27a−/−
vivo and in vitro murine models. We demonstrate that genetically
TAC mice exhibited greater levels of myofibroblast activation mark-
or pharmacologically blocking miR-27a-5p enhanced myocardial fi-
brosis in vivo, without a discernible effect on cardiomyocytes (CMs).
ers (α-SMA (Acta2), Fn-EDA, total Fn and Postn mRNA and protein
levels) (Figure 1H,I) compared to WT TAC animals. However, there
We also performed in vitro mechanistic studies in cardiac fibroblasts
were no significant effects on CM size, CM counts (Figure 1D) or
(CFs) to characterize miR-27a-5p function more fully and discovered
hypertrophy biomarker expression (Nppa transcript expression and
it inhibited the pro-fibrotic transforming growth factor-beta (Tgf-β)
signalling pathway through suppressing the pro-fibrotic transcription
factor Early Growth Response Protein 3 (Egr3). This effect resulted
in lower CF release of pro-fibrotic proteins. This body of work high-
lights the beneficial role of CF miR-27a-5p in cardiac remodelling.
Myh7/Myh6 transcript ratio) (Figure S3A).
3.2 | Administration of miR-27a-5p inhibitor
promotes transverse aortic constriction-induced
heart fibrosis in vivo
2 | M ATE R I A L S A N D M E TH O DS
It is possible that compensating mechanisms in miR-27a−/− animals
might conceal other effects. Therefore, we evaluated the effect of
All experiments received approval from the Ethics Review
acute locked nucleic acid (LNA)-based miR-27a-5p inhibition in adult
Committee at Hainan General Hospital (Haikou, China). All mice
mice by injecting three doses of an anti-miR-27a-5p LNA that targets
used in this study were male and were housed and cared for ac-
mmu-miR-27a-5p (Figure 2A). Anti-miR-27a-5p injections induced a
cording to the guidelines outlined in the National Institutes of
profound decrease in miR-27a-5p levels across the four major cardiac
Health's (NIH) ‘Guide for the Care and Use of Laboratory Animals’
cell types (CFs, CMs, CECs and CVSMCs) in comparison to anti-miR-Ctrl
(8th edition). The methods are fully detailed in the Supplementary
injections at both baseline and TAC-induced conditions (Figure 2B). As
Information.
3 | R E S U LT S
3.1 | Genetic knockout of miR-27a promotes
transverse aortic constriction-induced heart fibrosis
in vivo
Since we found that miR-27a mimics did not affect CM cell size in
vitro (Figure S1A,B), we next sought to determine whether manipu-
lation of miR-27a levels in vivo may affect heart function in models
of cardiac disease. For this purpose, we employed mice with ge-
netic knockout (KO) of miR-27a (miR-27a−/−). As expected, analysing
we had observed in mice with genetic KO of miR-27a, anti-miR-27a-5p
administration reduced left ventricular function in TAC mice (Figure 2C).
Anti-miR-27a-5p also promoted heart mass (Figure 2D), left ventricle
fibrosis (Figure 2F), fibrosis marker expression (Figure 2G,H) and my-
ofibroblast activation marker expression (Figure 2I,J). Moreover, there
were no significant effects on CM size, CM counts (Figure 2E) or hyper-
trophy biomarker expression (Figure S3B).
3.3 | Cardiac fibroblast miR-27a-5p levels decline
with age and transverse aortic constriction-
induced stress
As modulating miR-27a-5p expression had an effect on cardiac fi-
miR-27a-5p expression in the four major cardiac cell types revealed
brosis without impacting CM size or counts, we undertook a more
TENG ET al. | 75
F I G U R E 1 Genetic knockout of miR-27a promotes transverse aortic constriction-induced heart fibrosis in vivo. Pulse-wave Doppler
echocardiography and tissue harvesting performed 21 d after transverse aortic constriction (TAC) or sham procedure. n = 9 animals per
cohort. (A) miR-27a levels in cardiac fibroblasts (CFs), cardiomyocytes (CMs), cardiac endothelial cells (CECs) and cardiac vascular smooth
muscle cells (CVSMCs) isolated from left ventricular tissue assessed by quantitative real-time PCR (qPCR) in miR-27a−/− and wild-type
(WT) animals. (B) Fractional shortening and ejection fraction as measured by echocardiography in TAC- or sham-treated miR-27a−/− and
WT animals. (C) Typical haematoxylin & eosin (H&E) and Picrosirius Red/Fast Green FCF-stained myocardial sections (scale bar = 2 mm)
(left panel); heart hypertrophy evaluated by heart weight-to-tibia length ratio (HW/TL) from the cohorts outlined in (B) (right panel). (D)
Cardiomyocyte (CM) hypertrophy evaluated in wheat germ agglutinin (WGA)-stained mid-ventricular tissue sections from the cohorts
outlined in (B); scale bar = 50 µm. CM hypertrophy quantified by CM area and CM count. (E) Extent of fibrosis quantified from Picrosirius
Red/Fast Green FCF-stained cardiac tissue sections from the cohorts outlined in (B). (F, G) Expression of fibrosis markers in left ventricle
tissue from TAC-treated miR-27a−/− and WT animals quantified by (F) qPCR and (G) Western blotting. (H, I) Expression of myofibroblast
activation markers in left ventricle tissue from TAC-treated miR-27a−/− and WT animals quantified by (H) qPCR and (I) Western blotting.
Full immunoblotting images are displayed in Figure S5. Data expressed as means ± standard errors of the mean (SEMs). For panels (A, F-I):
*P < .05, **P < .01 vs WT or WT TAC [Student's t test]. For panels (B-E): *P < .05, **P < .01 vs matching Sham group; †P < .05, ††P < .01 vs WT
TAC group [two-way ANOVA with post hoc Bonferroni]
TENG ET al.76 |
TENG ET al. | 77
F I G U R E 2 Administration of miR-27a-5p inhibitor promotes transverse aortic constriction-induced heart fibrosis in vivo. Pulse-wave
Doppler echocardiography and tissue harvesting performed 21 d after transverse aortic constriction (TAC) or sham procedure. n = 9 animals
per cohort. (A) Design of the locked nucleic acid (LNA) inhibitor against miR-27a-5p (left panel) and experimental overview (right panel). (B)
miR-27a-5p levels in cardiac fibroblasts (CFs), cardiomyocytes (CMs), cardiac endothelial cells (CECs), and cardiac vascular smooth muscle
cells (CVSMCs) isolated from left ventricular tissue assessed by quantitative real-time PCR (qPCR) following anti-miR-27a-5p LNA (Anti-miR-
27a-5p), control LNA (Anti-miR-Ctrl), or vehicle (PBS) in sham mice (top panel) and TAC mice (bottom panel). (C) Fractional shortening and
ejection fraction as measured by echocardiography from the cohorts outlined in (B). (D) Heart hypertrophy evaluated by heart weight-to-
tibia length ratio (HW/TL) ratio from the cohorts outlined in (B). (E) Cardiomyocyte (CM) hypertrophy evaluated in wheat germ agglutinin
(WGA)-stained mid-ventricular tissue sections from the cohorts outlined in (B); scale bar = 50 µm. CM hypertrophy quantified by CM area
and CM count. (F) Extent of fibrosis quantified from typical Picrosirius Red/Fast Green FCF-stained left ventricle tissue sections from the
cohorts outlined in (B). (G, H) Expression of fibrosis markers in left ventricle tissue from TAC-treated Anti-miR-27a-5p, Anti-miR-Ctrl, and
PBS mice quantified by (G) qPCR and (H) Western blotting. (I, J) Expression of myofibroblast activation markers in left ventricle tissue from
TAC-treated Anti-miR-27a-5p, Anti-miR-Ctrl and PBS mice quantified by (I) qPCR and (J) Western blotting. Full immunoblotting images are
displayed in Figure S5. Data expressed as means ± standard errors of the mean (SEMs). For panels (B, G-J): *P < .05, **P < .01 vs PBS or TAC
PBS [one-way ANOVA with post hoc Bonferroni]. For panels (C-F): *P < .05, **P < .01 vs matching Sham group; †P < .05, ††P < .01 vs TAC PBS
group [two-way ANOVA with post hoc Bonferroni]
in-depth examination of CF miR-27a-5p expression in normal and
miR-27a KO from CFs produced the same phenotypic effect as sys-
diseased cardiac tissue. Left ventricle-derived CF miR-27a-5p levels
temic miR-27a KO and LNA inhibition of miR-27a-5p.
declined with time as mice grew older (Figure S4A), in accordance
with its putative role in the regulation of body growth.21 Notably,
CF miR-27a-5p levels dynamically changed following the TAC proce-
dure, with an initial decline 2 days post-surgery and increases there-
after (Figure S4B). Moreover, miR-27a-5p levels in neonatal rat CFs
(NRCFs) and adult mouse CFs were much lower after 2 weeks under
3.5 | Selective overexpression of cardiac fibroblast
miR-27a-5p suppresses transverse aortic constriction-
induced heart fibrosis in vivo
continuous culture (Figure S4C,D). Cumulatively, these results show
A reverse approach was also performed in which miR-27a-5p was se-
that cardiac fibroblast miR-27a-5p levels decline with age and TAC-
induced stress, motivating a more in-depth study on the function of
miR-27a-5p in CFs.
3.4 | Selective knockout of cardiac fibroblast
miR-27a-5p promotes transverse aortic constriction-
induced heart fibrosis in vivo
We employed a tyrosine-mutant adeno-associated virus serotype
2 vector (AAV2Tyr-mut) under the control of the murine Postn pro-
moter (AAV2Tyr-mut-Postn) that selectively drives gene overexpres-
sion in murine CFs.22 Selective miR-27a inactivation in CFs was
achieved using AAV2Tyr-mut-Postn carrying a copy for an enhanced
Cre recombinase (AAV2Tyr-mut-Postn-iCre) in mice with floxed in-
sertion within the miR-27a allele (miR-27afl/fl) (Figure 3A). Due to
more pronounced Postn promoter activity in TAC-induced activated
murine CFs,22 miR-27afl/fl TAC mice treated with the AAV2Tyr-mut-
Postn-iCre exhibited lower CF miR-27a-5p levels vs WT TAC mice
treated with the AAV2Tyr-mut-Postn-iCre or mice treated with the
AAV2-Ctrl (Figure 3B); this miR-27a-5p knockdown was specific
to CFs and did not impact the other cardiac cell types (Figure 3C).
miR-27afl/fl mice treated with the AAV2Tyr-mut-Postn-iCre exhib-
ited reduced left ventricular function (Figure 3D). miR-27afl/fl mice
treated with the AAV2Tyr-mut-Postn-iCre displayed increased cardiac
lectively overexpressed (OE) in CFs in vivo using the same tyrosine-
mutant AAV2Tyr-mut vector under the control of the Postn promoter
(AAV2Tyr-mut-Postn-miR-27a) (Figure 4A). Due to more pronounced
Postn promoter activity in TAC-induced activated murine CFs,22 WT
TAC mice receiving AAV2Tyr-mut-Postn-miR-27a expressed higher
CF miR-27a-5p levels vs WT TAC mice treated with AAV2Tyr-mut-
Postn-Ctrl or mice treated with the AAV2Tyr-mut-Ctrl (Figure 4B);
this miR-27a-5p knockdown was specific to CFs and did not impact
other cardiac cell types (Figure 4C). Upon undergoing a TAC pro-
cedure, AAV2Tyr-mut-Postn-miR-27a mice exhibited reduced left ven-
tricular function (Figure 4D). Moreover, AAV2Tyr-mut-Postn-miR-27a
TAC mice exhibited lower heart mass with no effect on CM size or
counts (Figure 4E,F). AAV2Tyr-mut-Postn-miR-27a TAC mice exhibited
reduced left ventricular fibrosis (Figure 4G), lower fibrosis marker
expression (Figure 4H,I), and lower myofibroblast activation marker
expression (Figure 4J,K). Moreover, there were no significant effects
on hypertrophy biomarker expression (Figure S3D). The results from
selective miR-27a-5p OE in CFs in vivo support the beneficial func-
tion of CF miR-27a-5p in cardiac remodelling.
3.6 | miR-27a-5p suppresses cardiac fibroblast's
pro-fibrotic activity via Early Growth Response
Protein 3 (Egr3) in vitro
mass (Figure 3E), left ventricle fibrosis (Figure 3F), fibrosis marker
Our in vivo experiments showed that miR-27a-5p displays an anti-
expression (Figure 3G,H) and myofibroblast activation marker ex-
fibrotic effect in TAC mice. We have been suggested that miR-27a-
pression (Figure 3I,J). Moreover, there were no significant effects on
hypertrophy biomarker expression (Figure S3C). Overall, selective
5p may modulate Tgf-β signalling activity in CFs. Indeed, miR-27a-5p
mimic in NRCFs decreased bioluminescence in a SBE reporter assay
TENG ET al.78 |
F I G U R E 3 Selective knockout of cardiac fibroblast miR-27a promotes transverse aortic constriction-induced heart fibrosis in vivo. (A)
miR-27afl/fl mice or wild-type (WT) mice (aged 5 d) were administered tyrosine-mutant adeno-associated virus serotype 2 (AAV2Tyr-mut)
carrying a copy of enhanced Cre recombinase, or an inactive C elegans miR-39 control sequence, under the control of the murine Postn
promoter (AAV2Tyr-mut-Postn-iCre or AAV2-Ctrl, 5 × 1011 viral particles per µL) by intrapericardial injection. Transverse aortic constriction
(TAC) or sham procedure was performed 7 wk post- AAV2Tyr-mut injection. Pulse-wave Doppler echocardiography and tissue harvesting
performed 21 d after TAC or sham procedure. n = 9 animals per cohort. (B) miR-27a levels in cardiac fibroblasts isolated from left ventricular
tissue assessed by quantitative real-time PCR (qPCR) in TAC- or sham-treated AAV2Tyr-mut-Postn-iCre and AAV2Tyr-mut-Ctrl animals. (C) miR-
27a levels across all major cardiac cell types in left ventricle tissue from TAC-treated AAV2Tyr-mut-Postn-iCre and AAV2Tyr-mut-Ctrl animals.
(D) Fractional shortening and ejection fraction as measured by echocardiography from the cohorts outlined in (B). (E) Typical haematoxylin
& eosin (H&E)-stained heart tissue sections from the cohorts outlined in (B); scale bar = 2 mm. Heart hypertrophy evaluated by heart
weight-to-tibia length ratio (HW/TL) ratio. (F) Extent of fibrosis quantified from typical Picrosirius Red/Fast Green FCF-stained cardiac
tissue sections from the cohorts outlined in (B); scale bar = 2 mm. (G, H) Expression of fibrosis markers in left ventricle tissue from TAC-
treated AAV2Tyr-mut-Postn-iCre and AAV2Tyr-mut-Ctrl mice quantified by (G) qPCR and (H) Western blotting. (I, J) Expression of myofibroblast
activation markers in left ventricle tissue from TAC-treated AAV2Tyr-mut-Postn-iCre and AAV2Tyr-mut-Ctrl mice quantified by (I) qPCR and (J)
Western blotting. Full immunoblotting images are displayed in Figure S5. Data expressed as means ± standard errors of the mean (SEMs).
For panels (C, G-J): *P < .05, **P < .01 vs AAV2Tyr-mut-Ctrl TAC [Student's t test]. For panels (B, D-F): *P < .05, **P < .01 vs matching Sham
group; †P < .05, ††P < .01 vs AAV2Tyr-mut-Ctrl TAC group [two-way ANOVA with post hoc Bonferroni]
TENG ET al. | 79
TENG ET al.80 |
F I G U R E 4 Selective overexpression of cardiac fibroblast miR-27a suppresses transverse aortic constriction-induced heart fibrosis in
vivo. (A) Wild-type (WT) animals (aged 5 wk) were administered tyrosine-mutant adeno-associated virus serotype 2 (AAV2Tyr-mut) carrying
a copy of miR-27a, or an inactive C elegans miR-39 control sequence, under the control of the murine Postn promoter (AAV2Tyr-mut-Postn-
iCre or AAV2Tyr-mut-Ctrl, 5 × 1011 viral particles per µL) by intrapericardial injection. Transverse aortic constriction (TAC) or sham procedure
was performed 7 wk post- AAV2Tyr-mut injection. Pulse-wave Doppler echocardiography and tissue harvesting performed 21 d after TAC
or sham procedure. n = 9 animals per cohort. (B) Cardiac fibroblast miR-27a levels in left ventricle tissue assessed by quantitative real-time
PCR (qPCR) in TAC- or sham-treated AAV2Tyr-mut-Postn-miR-27a and AAV2Tyr-mut-Ctrl animals. (C) miR-27a levels across all major cardiac
cell types in left ventricle tissue from TAC-treated AAV2Tyr-mut-Postn-miR-27a and AAV2Tyr-mut-Ctrl animals. (D) Fractional shortening and
ejection fraction as measured by echocardiography from the cohorts outlined in (B). (E) Typical haematoxylin & eosin (H&E)-stained heart
tissue sections from the cohorts outlined in (B); scale bar = 2 mm. Heart hypertrophy evaluated by heart weight-to-tibia length ratio (HW/
TL) ratio. (F) Cardiomyocyte (CM) hypertrophy evaluated in wheat germ agglutinin (WGA)-stained mid-ventricular tissue sections from
the cohorts outlined in (B); scale bar = 50 µm. CM hypertrophy quantified by CM area and CM count. (G) Assessment of fibrosis in left
ventricle cardiac tissue sections from the cohorts outlined in (B). (H, I) Expression of fibrosis markers in left ventricle tissue from TAC-treated
AAV2Tyr-mut-Postn-miR-27a and AAV2Tyr-mut-Ctrl mice quantified by (H) qPCR and (I) Western blotting. (J, K) Expression of myofibroblast
activation markers in left ventricle tissue from TAC-treated AAV2Tyr-mut-Postn-miR-27a and AAV2Tyr-mut-Ctrl mice quantified by (J) qPCR
and (K) Western blotting. Full immunoblotting images are displayed in Figure S5. Data expressed as means ± standard errors of the mean
(SEMs). For panels (C, H-K): *P < .05, **P < .01 vs AAV2Tyr-mut-Ctrl TAC [Student's t test]. For panels (B, D-G): *P < .05, **P < .01 vs matching
Sham group; †P < .05, ††P < .01 vs AAV2Tyr-mut-Ctrl TAC group [two-way ANOVA with post hoc Bonferroni]
of Tgf-β activity (Figure 5A), while anti-miR-27a-5p LNA produced
the opposite effect (Figure 5B). We also evaluated the secretome of
miR-27a-5p's anti-fibrotic action in CFs. We analysed the overlap-
ping putative TargetScan-derived target genes for human, mouse
NRCFs to determine whether miR-27a-5p was associated with secre-
and rat miR-27a-5p that were also up-regulated in the left ventricu-
tion of fibrosis-related proteins from NRCFs. Following transfection
of NRCFs with anti-miR-27a-5p LNA, the conditioned media were har-
lar transcriptome of WT TAC mice relative to WT sham mice (GEO
accession: GSE1822425). This Venn analysis uncovered gasdermin
vested and subjected to proteomic analysis by tryptic digest followed
(Gsdma) and Egr3 as potential targets of miR-27a-5p that are up-reg-
by mass spectrometry (Figure 5C). Anti-miR-27a-5p LNA in NRCFs
increased the secretion of multiple mediators of fibrosis compared
to NRCFs transfected with anti-miR-Ctrl LNA (Figure 5D). Many of
the genes encoding for the secreted factors have been associated
with the Tgf-β signalling cascade, such as lysyl oxidase-like 3 (Loxl3)23
and the latent Tgf-β binding proteins (Ltbp1, Ltbp2).24 Using immu-
nofluorescence, we confirmed that anti-miR-27a-5p LNA enhanced
ulated by TAC (Figure 5F). Considering that Gsdma is an apoptosis
mediator primarily expressed in epithelial cells and T-lymphocytes26
while Egr3 is a transcription factor involved in the Tgf-β-induced
fibrotic response in fibroblasts,27 we selected Egr3 for further
analysis. Follow-up TargetScan analysis revealed two putative
miR-27a-5p binding sites in the 3′-untranslated region (3′-UTR) of
Egr3 that are conserved across human, mice and rats (Figure 5G).
Ltbp1 secretion from NRCFs (Figure 5E). These combined observa-
Through mutating these two conserved miR-27a-5p binding sites
tions imply that miR-27a-5p negatively regulates pro-fibrotic activity
in the EGR3 3′-UTR, our 3′-UTR reporter assay in HEK293 cells
in cardiac fibroblasts.
revealed that the WT EGR3 3′-UTR is a direct regulatory target
We then conducted an in silico Venn analysis to determine
of miR-27a-5p (Figure 5H). We confirmed that miR-27a-5p mimic
the target gene(s) of miR-27a-5p that may be responsible for
in NRCFs decreased Egr3 protein expression (Figure 5I). We also
F I G U R E 5 miR-27a-5p suppresses pro-fibrotic activity in cardiac fibroblasts via Egr3 in vitro. (A, B) Transforming growth factor-beta
(Tgf-β) activity in NRCFs assessed by SBE luciferase reporter assays 48 h after treatment with (A) miR-27a-5p mimic or miR-Ctrl or (B) locked
nucleic acid (LNA) against miR-27a-5p (Anti-miR-27a-5p) or control LNA (Anti-miR-Ctrl). (C) Experimental overview for neonatal rat cardiac
fibroblast (NRCF) secretome analysis. (D) Differential secretion of pro-fibrotic proteins by NRCFs treated with Anti-miR-27a-5p expressed by
fold-change relative to NRCFs treated with Anti-miR-Ctrl. (E) Immunofluorescent imaging showing extracellular secretion of Ltbp1 by NRCFs
treated with Anti-miR-27a or Anti-miR-Ctrl. Nuclei stained blue with DAPI. (F) In silico Venn analysis of putative hsa-miR-27a-5p (human)
targets (blue), mmu-miR-27a-5p (mouse) targets (red), rno-miR-27a-5p (rat) targets (green), and up-regulated genes in the TAC murine left
ventricular transcriptome (yellow). (G) Targetscan analysis identifies two conserved miR-27a-5p binding sites in Egr3's 3′ untranslated region
(3′-UTR) across humans, mice, and rats. (H) Dual-fluorophore reporter assay of miR-27a-5p binding to wild-type (WT) or mutated (MUT)
EGR3 3′-UTR in HEK293 cultures that underwent co-transfection with miR-27a-5p mimic or miR-Ctrl; ratiometric calculations performed
with first fluorophore eGFP (miR-27a-5p binding standard) and second fluorophore tdTomato (transfection standard). (L) Differential
secretion of pro-fibrotic proteins by NRCFs treated with Anti-miR-27a-5p + siEgr3 and Anti-miR-27a-5p + siCtrl expressed by fold-change
relative to NRCFs treated with Anti-miR-Ctrl. (M) Immunofluorescence showing extracellular secretion of Ltbp1 by NRCFs treated with Anti-
miR-27a-5p + siEgr3 or Anti-miR-27a-5p + siCtrl. Nuclei stained blue with DAPI. Full immunoblotting images are displayed in Figure S5. All
experiments: n = 3 biological replicates × 3 technical replicates. Data expressed as means ± standard errors of the mean (SEMs). For panels
(A, B, D, I): *P < .05, **P < .01 vs Anti-miR-Ctrl or miR-Ctrl group [Student's t test]. For panel (H): *P < .05, **P < .01 vs matching miR-Ctrl
group [two-way ANOVA with post hoc Bonferroni]. For panel (J, K): *P < .05, **P < .01 vs Anti-miR-Ctrl group; †P < .05, ††P < .01 vs Anti-
miR-27a-5p group [one-way ANOVA with post hoc Bonferroni]. For panel (L): *P < .05, **P < .01 vs Anti-miR-27a-5p + siCtrl [Student's t test]
TENG ET al. | 81
showed that anti-miR-27a-5p LNA in NRCFs increased Egr3 protein
LNA-treated NRCFs (Figure 5M). Overall, our findings advocate that
expression, an effect abrogated by addition of a small-interfering
miR-27a negatively regulates pro-fibrotic Tgf-β activity in CFs via Egr3.
RNAs against Egr3 (siEgr3) (Figure 5J). Moreover, anti-miR-27a-5p
LNA in NRCFs increased SBE reporter bioluminescence, an effect
abrogated by addition of siEgr3 (Figure 5K).
4 | D I S CU S S I O N
Next, we determined whether Egr3 silencing could block anti-miR-
27a-5p LNA-elicited secretion of pro-fibrotic factors by NRCFs. siEgr3
Herein, we performed a series of experiments that cumulatively
blocked anti-miR-27a-5p LNA-triggered secretion of pro-fibrotic me-
support miR-27a-5p's suppression of pathological cardiac fibro-
diators (Figure 5L). By immunofluorescence, we confirmed that the
sis during heart remodelling. Selective genetic KO of miR-27a or
addition of siEgr3 reduced Ltbp1 deposition from anti-miR-27a-5p
miR-27a-5p LNA-based
inhibition enhanced cardiac stress-in-
duced fibrosis in mice that had undergone TAC procedure without
TENG ET al.82 |
impacting CMs, whereas selective OE of CF miR-27a-5p produced
in fibrosis. However, miR-27a-5p's role in fibrotic disease models
the reverse outcome. Secretome analysis in CFs identified several
of these organs remains to be elucidated.
pro-fibrotic factors that were differentially expressed upon miR-27a
Cumulatively, our results underscore the beneficial role of miR-
KD. Luminescence reporter assays of Egr3 3′-UTR binding pointed
27a-5p in cardiac remodelling. Other animal models, such as the
to the pro-fibrotic transcription factor Egr3 as a major mediator of
left-anterior descending coronary artery myocardial infarction (LAD
miR-27a-5p's suppressive effect on fibrosis.
MI) model and Ang infusion model, are needed to confirm the thera-
Several pathological illnesses stimulate fibrosis and the release
peutic potential of miR-27a-5p agomiR therapy in vivo.
of extracellular matrix (ECM) proteins. miR-27a has been shown to
suppress fibrosis under pathological conditions in the kidney, blad-
der, and liver in vivo16-18 and CF collagen gene expression in vitro.12
AC K N OW L E D G E M E N T S
This work was supported by the National Nature Science Foundation
Therefore, here we have been suggested that miR-27a-5p may have
for the Youth of China (grant no. 81202005), the Technology Plan
an anti-fibrotic effect in the heart. The prominent anti-fibrotic effect
Fund of Hunan Science (grant no. 2013FJ4109), and the Central
of miR-27a-5p in CFs is indicated by several lines of evidence: (a) CF
South University Innovation Fund for Independent Graduate
miR-27a-5p levels decline with age and TAC-induced stress; (b) selec-
tive KO of miR-27a from CFs using CF-targeting AAV2Tyr-mut-Postn-
iCre in miR-27afl/fl mice worsens TAC-elicited cardiac fibrosis, while
Exploration (grant no. 72150050587).
C O N FL I C T O F I N T E R E S T
selective miR-27a-5p OE in WT mice CFs improves TAC-triggered
The authors confirm that there are no conflicts of interest.
pathology; and (c) TAC-triggered increases in fibrosis marker and
myofibroblast activation marker expression are heightened in mice
AU T H O R C O N T R I B U T I O N S
with selective CF miR-27a KO and lowered in mice with selective CF
Lifeng Teng: Conceptualization (equal); data curation (equal). Yubing
miR-27a OE.
Huang: formal analysis (equal); investigation (equal). Jun Guo: Data
Our 3′-UTR reporter and immunoblotting assays identified Egr3
curation (equal); formal analysis (equal); software (equal); writing
as a direct regulatory target of miR-27a-5p. We also found that miR-
– original draft (equal). Bin Li: Data curation (equal); formal analy-
27a-5p KD in NRCFs in vitro promotes pro-fibrotic factor secretion
sis (equal); software (equal). Jin Lin: Data curation (equal); formal
in an Egr3-dependent manner. Egr3 is a member of the Early Growth
analysis (equal); software (equal). Lining Ma: Data curation (equal);
Response (Egr) family of transcription factors (Egr-1, Egr-2 and Egr-
resources (equal); validation (equal). Yudai Wang: Data curation
4) that all share a conserved zinc-finger domain targeting the Egr
response element present in several gene promoters.28 Specifically,
(equal); resources (equal); validation (equal). Cong Ye: Formal analy-
sis (equal); validation (equal); visualization (equal). Qianqian Chen:
Egr3 has been shown to be a TGF-ß-induced transcription factor
that bolsters pro-fibrotic gene expression in human fibroblasts.27
Data curation (equal); resources (equal); validation (equal).
Moreover, murine fibroblasts with Egr3 knockout show down-reg-
DATA AVA I L A B I L I T Y S TAT E M E N T
ulated levels of several key fibrotic genes (ie Col1a1, Acta2, Tgfß1,
The data that support the findings of this study are available on re-
Ctgf and Pai1) in response to Tgf-ß2 stimulus, revealing that Egr3 is
necessary and sufficient for Tgf-β-induced fibrotic responses.27 This
is notable considering the Tgf-β2 up-regulation observed in our TAC
mouse left ventricular tissue samples. Our proposition is that miR-
27a-5p suppresses Tgf-β-induced cardiac fibrosis by inhibiting Egr3
expression in CFs.
quest from the corresponding author. The data are not publicly avail-
able due to privacy or ethical restrictions.
O R C I D
Jun Guo
https://orcid.org/0000-0002-4765-6571
Additional research could shed light on several aspects not
R E F E R E N C E S
yet investigated. First, mice can be followed for longer durations
after TAC surgery to see if, and to what extent, miR-27a-5p OE
decreases cardiac fibrosis over time. Second, researchers can fur-
ther elucidate the molecular details of miR-27a-5p's mechanism
of action to determine how the miR-27a-5p/Egr3 axis affects CF-
mediated myocardial fibrosis and the significance of the Tgf-β cas-
cade in this biological process. Third, it remains unknown whether
miR-27a-5p adopts different distributions in other organs, and
whether it shares any overlap in anti-fibrotic function in these or-
gans. We partially addressed this question here, as the impact of
miR-27a-5p KO on organ fibrosis was examined in lung, liver and
kidney tissues in miR-27a−/− animals following TAC. Histological
1. Jonas S, Izaurralde E. Towards a molecular understanding of mi-
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10.1085/jgp.202213131
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ARTICLE
The archaeal glutamate transporter homologue GltPh
shows heterogeneous substrate binding
Krishna D. Reddy1, Didar Ciftci1,2, Amanda J. Scopelliti1, and Olga Boudker1,3
Integral membrane glutamate transporters couple the concentrative substrate transport to ion gradients. There is a wealth of
structural and mechanistic information about this protein family. Recent studies of an archaeal homologue, GltPh, revealed
transport rate heterogeneity, which is inconsistent with simple kinetic models; however, its structural and mechanistic
determinants remain undefined. Here, we demonstrate that in a mutant GltPh, which exclusively populates the outward-facing
state, at least two substates coexist in slow equilibrium, binding the substrate with different apparent affinities. Wild type
GltPh shows similar binding properties, and modulation of the substate equilibrium correlates with transport rates. The low-
affinity substate of the mutant is transient following substrate binding. Consistently, cryo-EM on samples frozen within
seconds after substrate addition reveals the presence of structural classes with perturbed helical packing of the extracellular
half of the transport domain in regions adjacent to the binding site. By contrast, an equilibrated structure does not show such
classes. The structure at 2.2-˚A resolution details a pattern of waters in the intracellular half of the domain and resolves classes
with subtle differences in the substrate-binding site. We hypothesize that the rigid cytoplasmic half of the domain mediates
substrate and ion recognition and coupling, whereas the extracellular labile half sets the affinity and dynamic properties.
Introduction
Membrane glutamate transporters pump substrates against
their concentration gradients, serving critical biological func-
tions across all kingdoms of life. In mammals, excitatory amino
acid transporters recycle glutamate from the synaptic cleft into
the glia (Freidman et al., 2020). In prokaryotes, orthologs take
up nutrients, including glutamate, aspartate, neutral amino ac-
ids, or dicarboxylic acids (Kim et al., 2002; Burguiere et al.,
2004; Youn et al., 2009). Transporters utilize energy from
downhill ionic electrochemical gradients to carry concentrative
substrate uptake. Excitatory amino acid transporters rely on
inward Na+ and proton gradients and an outward K+ gradient
(Zerangue and Kavanaugh, 1996). Prokaryotes couple transport
to either proton or Na+ gradients (Tolner et al., 1995; Ryan et al.,
2009).
These transporters are homotrimers with each protomer
composed of a rigid scaffold trimerization domain and a mobile
transport domain containing the ligand-binding sites. Protomers
functions independently (Erkens et al., 2013; Ruan et al., 2017;
Riederer et al., 2018; Georgieva et al., 2013; Koch et al., 2007;
Grewer et al., 2005; Koch and Larsson, 2005). Transport do-
mains translocate ligands across membranes by moving ∼15 ˚A
from the outward-facing state (OFS) to the inward-facing state
(IFS), in an elevator motion (Reyes et al, 2009; Garaeva et al,
2019; Arkhipova et al, 2020; Qiu et al, 2021). Studies in archaeal
Na+-coupled transporters GltPh and GltTk led to a simple kinetic
model of transport (Boudker et al., 2007; Reyes et al., 2013; Verdon
et al., 2014; Oh and Boudker, 2018; Guskov et al., 2016; Riederer
and Valiyaveetil, 2019; Wang and Boudker, 2020; Arkhipova et al.,
2020; Alleva et al., 2020). Briefly, in the OFS, ion binding to Na1
and Na3 sites reveals the substrate and an additional sodium
(Na2)-binding site through an opening of helical hairpin 2 (HP2),
also preventing the translocation of Na+-only bound transport
domain. Subsequent binding of the substrate and Na2 closes HP2,
allowing translocation to the IFS and ligand release into the
cytoplasm.
Recently, high-speed atomic force microscopy, single-
molecule FRET (smFRET) TIRF microscopy, and 19F-NMR re-
vealed a more complex picture of GltPh transport and dynamics
(Huang et al., 2020; Ciftci et al., 2020; Akyuz et al., 2015; Matin
et al., 2020; Erkens et al., 2013; Akyuz et al., 2013; Huysmans
et al., 2021; Ciftci et al., 2021). These studies established the
existence of additional conformational substates in OFS and
IFS, of which some translocate and transport at different rates.
Although it is expected that cryo-EM studies would resolve
these proposed conformational substates from heterogeneous
datasets, this so far does not appear to be the case (Wang and
.............................................................................................................................................................................
1Department of Physiology and Biophysics, Weill Cornell Medicine, New York, NY;
Hughes Medical Institute, Weill Cornell Medicine, New York, NY.
2Tri-Institutional Training Program in Chemical Biology, New York, NY;
3Howard
Correspondence to Krishna D. Reddy: [email protected]; Olga Boudker: [email protected].
© 2022 Reddy et al. This article is available under a Creative Commons License (Attribution 4.0 International, as described at https://creativecommons.org/licenses/by/4.0/).
Rockefeller University Press
J. Gen. Physiol. 2022 Vol. 154 No. 5
e202213131
https://doi.org/10.1085/jgp.202213131
1 of 13
Boudker, 2020; Arkhipova et al., 2020). Therefore, the struc-
tural and mechanistic determinants of kinetic heterogeneity
remain unclear.
Using a GltPh mutant that exclusively occupies the OFS, we
show that substrate binding in the OFS is heterogeneous, con-
sistent with at least two substates with different affinities. Salt
composition and temperature modulate the substate pop-
ulations, suggesting that they are in equilibrium. However, the
conformational exchange must be slow, in the order of at least
tens of seconds, for the substates to manifest in binding iso-
therms. Notably, a similar conformational equilibrium also
exists in WT protein and potentially contributes to heteroge-
neous transport kinetics. The substrate-bound high-affinity
state of the mutant transporter is expected to predominate
after equilibration. Thus, to gain insights into the structure of
the transient low-affinity substate, we conducted an extensive
analysis of the cryo-EM imaging data collected on the trans-
porter frozen within seconds after substrate addition. The
identified structural classes reveal subtle differences. Specifi-
cally, we observed a subset of structural classes with differently
packed helices in the extracellular half of the transport domain
adjacent to the binding site, suggesting that the region is labile.
We hypothesize that the ensemble of transient, more dynamic,
less uniquely packed conformations comprises the low-affinity
substate.
Images of an equilibrated protein produced re-
constructions at a uniquely high 2.2 ˚A resolution, revealing a
complement of structured waters in the cytoplasmic side of the
transport domain that may contribute to its conformational
rigidity. We did not observe a conformational heterogeneity of
the extracellular half of the transport domain in these data,
which we attribute to the relaxation of the protein to the higher
affinity state. Classifications instead revealed subtle differences
in the substrate-binding site and the global orientations of the
transport domains, which could also contribute to kinetic het-
erogeneity. Our results provide a framework in which modal
kinetic behavior demonstrated by GltPh may be a result of
subtle but long-lived structural differences.
Materials and methods
DNA manipulations and protein preparation
Mutations were introduced to the previously described GltPh
CAT7 construct (Yernool et al., 2004) using PfuUltra II, and
sequences were verified using Sanger sequencing (Macrogen).
Proteins were expressed as C-terminal (His)8 fusions, separated
by a thrombin cleavage site. Proteins were purified as previously
described (Yernool et al., 2004). Briefly, crude membranes of
DH10B Escherichia coli cells overexpressing GltPh were solubilized
for 2 h in 20 mM HEPES/NaOH, pH 7.4, 200 mM NaCl, 5 mM
monopotassium L-aspartate (L-Asp), and 40 mM n-dodecyl-β-
D-maltopyranoside (DDM; Anatrace). After solubilization, the
DDM was diluted to ∼8–10 mM, and after high-speed ultra-
centrifugation (40,000 rpm, Ti45 rotor), the supernatant was
applied to pre-equilibrated Ni-NTA affinity resin (Qiagen) for
1 h. The resin was washed with seven column volumes of
20 mM HEPES/NaOH, pH 7.4, 200 mM NaCl, 5 mM L-Asp, and
40 mM imidazole. Subsequently, the resin was eluted with the
same buffer with increased imidazole (250 mM). The (His)8
tag was cleaved by thrombin digestion overnight at 4°C, and
the proteins were further purified by size-exclusion chro-
matography (SEC) in the appropriate buffer for subsequent ex-
periments. The concentration of GltPh protomers was determined
in a UV cuvette with a 10-mm pathlength (Starna Cells), using
protein diluted 1:40, and an experimentally determined extinction
coefficient of 57,400 M−1 cm−1 (Reyes et al., 2013).
Reconstitution and uptake assays
Liposomes able to maintain proton gradients were prepared
using a 3:1 mixture of POPE and POPG (1-palmitoyl-2-oleoyl-sn-
glycero-3-phosphoethanolamine and 1-palmitoyl-2-oleoyl-sn-
glycero-3-phospho-[19-rac-glycerol]). Lipids were dried on the
rotary evaporator for 2 h and under vacuum overnight. The
resulting lipid film was hydrated by 10 freeze–thaw cycles at a
concentration of 5 mg/ml in 50 mM potassium phosphate buffer
and 100 mM potassium acetate, pH 7. The suspensions were
extruded using a Mini-Extruder (Avanti) through 400-nm
membranes (Whatman) 10 times to form unilamellar lipo-
somes, and Triton X-100 was added to liposomes at a ratio of 1:2
(wt/wt).
P-GltPh for reconstitution was affinity purified, thrombin
cleaved, and purified by SEC in 20 mM HEPES/Tris, pH 7.4,
200 mM NaCl, 1 mM L-Asp, and 7 mM DDM. Purified protein
was added to destabilized liposomes at a ratio of 1:1,000 (wt/wt)
and incubated for 30 min at 23°C. Detergent was removed with
four rounds of incubation with SM-2 beads (Bio-Rad) at 80 mg
beads per 1 ml of liposome suspension (2 h at 23°C twice,
overnight at 23°C once, and 2 h at 23°C once). Before use, SM-2
beads were washed in methanol, rinsed thoroughly with dis-
tilled water, and equilibrated in the liposome internal buffer.
After detergent removal, proteoliposomes were concentrated to
50 mg/ml by ultracentrifugation at 86,000 g for 40 min at 4°C,
freeze–thawed three times, and extruded through 400-nm
membranes 10 times.
Uptakes were initiated by diluting reconstituted proteolipo-
somes 1:100 in the appropriate reaction buffer preincubated at
30°C. At the indicated time points, 200-μl reaction aliquots were
removed and diluted in 2 ml of ice-cold quenching buffer
(20 mM HEPES/Tris, pH 7, and 200 mM LiCl). The quenched
reaction was immediately filtered through a 0.22-µm filter
membrane (Millipore Sigma) and washed three times with 2 ml
quenching buffer. Washed membranes were inserted into
scintillation vials, and the membranes were soaked overnight
in 5 ml Econo-Safe Counting Cocktail. Radioactivity in lip-
osomes was measured using an LS6500 scintillation counter
(Beckman Coulter).
Isothermal titration calorimetry (ITC)
Substrate-free P-GltPh and GltPh proteins were affinity purified,
thrombin cleaved, and purified by SEC in 20 mM HEPES/KOH,
pH 7.4, 99 mM potassium gluconate, 1 mM sodium gluconate,
and 1 mM DDM. Proteins were immediately concentrated to
>5 mg/ml and diluted 2.5-fold to a final concentration of 30–50
µM. When diluted, the sample was supplemented with a final
concentration of 1 mM DDM, 58 mM HEPES/KOH, pH 7.4, and an
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
2 of 13
appropriate amount of sodium salt. 350 μl of protein samples
was degassed and equilibrated to the temperature of the exper-
iment, and ∼300 μl was loaded into the reaction cell of a small-
volume NanoITC (TA Instruments), or in the case of TFB-TBOA
experiments, an Affinity Auto ITC (TA Instruments). Titrant was
prepared in a buffer matching the reaction cell, except that it
contained the appropriate amount of L-Asp (A52100; RPI) and no
protein or DDM. Dilution of DDM in the reaction cell over the
course of the experiment had negligible effects on the injection
heats, as previously noted (Boudker and Oh, 2015). 2 μl of titrant
was injected every 5–6 min, at a constant temperature and a
stirring rate of 250 rpm (125 rpm for TFB-TBOA experiments).
Injection heats measured after protein was saturated with the
ligand were used to determine the dilution heats subtracted from
the ligand binding heats. Data were analyzed using NanoAnalyze
software (TA Instruments) applying either the “independent” or
“multiple sites” (referred to as “single-state” and “two-state,”
respectively, throughout the text) binding models. For two-state
binding, where each state is independent and nonidentical, the
binding polynomial can be expressed as
Σ (cid:2) 1 + K1[S] + K2[S] + K1K2[S]2,
where Ki-s are the binding constants and [S] is the concentration
of free L-Asp. The fraction of total protein bound is given by the
following:
[PS]
[P]
(cid:2) n1K1[S]
1 + K1[S]
+ n2K2[S]
1 + K2[S]
,
where ni is the apparent number of sites per protein molecule.
For the TFB-TBOA competition experiments, protein was
first titrated with TBOA to saturation. Then, the concentration of
TFB-TBOA was increased to a final concentration of 150 μM of
TFB-TBOA, so that the overflow protein was also saturated. The
appropriate amount of L-Asp was subsequently titrated into the
saturated protein to yield a binding curve.
Cryo-EM imaging data collection
In both data sets, 3.5 μl protein at ∼4.5 mg/ml was applied to a
glow-discharged QuantiFoil R 1.2/1.3, 300-mesh, gold grid
(Electron Microscopy Sciences). Grids were blotted at room
temperature and 100% humidity for 3 s at 0 blot force and
plunge frozen in liquid ethane using a VitroBot Mark IV (FEI).
Data S1 was collected on P-GltPh, which was affinity-purified,
concentrated, and buffer-exchanged into 20 mM HEPES/Tris,
pH 7.4, 99 mM K-gluconate, 1 mM Na-gluconate, and 0.8 mM
DDM to remove the substrate. The protein was then SEC puri-
fied in 20 mM HEPES/Tris pH 7.4, 250 mM NaNO3, and 0.8 mM
DDM. 2.5 μl of substrate-free protein was applied to the grid,
then 1 μl of L-Asp was added and mixed just prior to freezing so
that the final substrate concentration was 1 mM, the final pro-
tein concentration was ∼4.5 mg/ml, and protein was exposed to
the substrate for ∼5 s prior to freezing (including blot time).
Data S2 was collected on P-GltPh SEC purified in 20 mM HEPES/
Tris, pH 7.4, 250 mM NaNO3, 1 mM L-Asp, and 0.8 mM DDM.
The final buffer conditions of Data S2 were identical to Data S1.
All imaging data were collected on Titan Krios microscopes
(FEI) operated at 300 kV. Data S1 was collected on a K2 Summit
direct electron detector (Gatan). Automated data collection was
performed in counting mode using Leginon software (Suloway
et al., 2005), with a magnification of 22,500×, electron exposure
of 70.23 e−/ ˚A2, 50 frames/s, a defocus range of −1.0 to −2.0 μm,
and pixel size of 1.073 ˚A. Data S2 was collected with a K3 Summit
direct electron detector (Gatan). Automated data collection was
performed in superresolution counting mode using SerialEM
software (Mastronarde, 2005) with a magnification of 81,000×,
electron exposure of 47.91 e−/ ˚A2, 30 frames/s, defocus range of
−0.5 to −2.5 μm, and pixel size of 0.53 ˚A.
Image processing
The frame stacks were motion-corrected using MotionCor2
(Zheng et al., 2017), with 2× binning in the case of Data S1, and
contrast transfer function (CTF) estimation was performed us-
ing CTFFIND 4.1 (Rohou and Grigorieff, 2015). All datasets were
processed using cryoSPARC 3.0 and Relion 3.0.8 simultaneously
with default parameters unless otherwise stated (Su et al., 2020;
Punjani et al., 2017; Zivanov et al., 2018). Specific information on
the processing of each dataset is in Figs. S5 and S7. In brief,
particles were nonspecifically picked from micrographs using
the Laplacian of Gaussian (LoG) picker, aiming for ∼2,000 picks
per micrograph. These particles were extracted at a box size of
240 pixels with 4× binning and then imported to cryoSPARC.
The particles underwent one round of 2-D classification to re-
move artifacts, and then multiple rounds of heterogeneous re-
finement (C1) using eight total classes, seven of which were
noisy volumes (created by one iteration of ab initio) and one was
an unmasked 3-D model obtained from a previous processing
pipeline. Once >95% of particles converged on a single class,
the particles were converted back to Relion format via PyEM
(Asarnow et al, 2019) and re-extracted at full box size. These
particles were reimported to cryoSPARC, underwent three more
rounds of heterogeneous refinement, then nonuniform (NU)
refinement using C3 symmetry (dynamic mask threshold, dy-
namic mask near, and dynamic mask far were always set to 0.8,
12, and 28, respectively; Punjani et al., 2020). These particles
were converted back to Relion format and underwent Bayesian
polishing, using parameters obtained using 5,000 random par-
ticles within the set. After three more rounds of heterogeneous
refinement in cryoSPARC and one round of NU-refinement, we
performed local CTF refinement (minimum fit res 7 ˚A). After
three more rounds of heterogeneous refinement and one round
of NU-refinement, we performed global CTF refinement (three
iterations, minimum fit res 7 ˚A; fitting trefoil, spherical aber-
ration, and tetrafoil), three more rounds of heterogeneous re-
finement, and one round of NU-refinement.
Data S2 was also further processed to obtain the high-
resolution structure by three additional rounds of polishing,
local CTF refinement, and global CTF refinement as described
above. During the polishing rounds, the box size and pixel size
were rescaled as indicated in the supplement. After the second
round of polishing, we classified single protomers by employing
the relion_particle_symmetry_expand function in Relion to ar-
tificially expand the particle set three times (C3) so that each
protomer rotated to the same position (Scheres, 2016). The ex-
panded particle set was subjected to 3-D classification without
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
3 of 13
alignment with T = 400 and 10 classes, using the refined C3
structure as a reference map. The exceptionally high T value was
chosen to separate out subtle structural differences, and lower T
values were also tested during processing. The local mask was
created using a 20 ˚A map of the transport domain of Chain A of
PDB accession no. 2NWX, with an initial binarization threshold
of 0.01, extended by 3 pixels, and a soft-edge of 10 pixels. Of
these, particle stacks from subsets of interest were separately
used in cryoSPARC’s local refinement (C1), using the mask and
map obtained from the most recent NU refinement. Single
protomers in Data S1 were classified similarly, except we per-
formed symmetry expansion after the first round of polishing
due to the limited resolution of the dataset. After processing, the
resulting half-maps were used as inputs for density modification
implemented in PHENIX 1.19.1–4122 (Terwilliger et al., 2020;
Adams et al., 2010), using a mask created from the NU-
refinement job (threshold 0.1, dilation radius 15, soft padding
width 5). All density maps were displayed using ChimeraX
(Pettersen et al., 2021).
Model building and refinement
For atomic model building, the crystal structure of WT GltPh in
the OFS (PDB accession no. 2NWX) was docked into the densities
using UCSF Chimera (Pettersen et al., 2004). The model was first
real-space refined in PHENIX (Adams et al., 2010). Then, chain A
was adjusted manually, and ions were added in COOT (Emsley
and Cowtan, 2004). Waters were initially added using phe-
nix.douse, and subsequently manually inspected and adjusted.
The resulting model underwent additional rounds of real-space
refinement and validated using MolProbity (Chen et al., 2010).
All structural models were displayed using ChimeraX (Pettersen
et al., 2021). Per-residue Cα RMSDs were generated with Chi-
mera (Meng et al., 2006). To cross-validate models, refined
models (FSCsum) were randomly displaced an average of 0.3 ˚A
using phenix.pdbtools. The displaced model was real-space re-
fined against half-map 1 obtained through density modification
to obtain FSCwork. The resulting model was validated against
half-map 2 to obtain FSCfree.
Single-molecule dynamics assay
Thrombin-cleaved P-GltPh, containing C321A and N378C muta-
tions, was labeled and reconstituted as described previously
(Akyuz et al., 2013). Protein was SEC purified in 20 mM HEPES/
Tris, 200 mM NaCl, 1 mM L-Asp, and 1 mM DDM. Purified
protein was labeled using maleimide-activated LD555P-MAL
and LD655-MAL dyes and biotin-PEG11 at a molar ratio of 4:5:10:
2.5. Excess dyes were removed on a PDMiniTrap Sephadex G-25
desalting column (GE Healthcare).
All experiments were performed on a previously described
home-built prism-based TIRF microscope constructed around a
Nikon Eclipse Ti inverted microscope body (Juette et al., 2016).
Microfluidic imaging chambers were passivated with biotin-
PEG, as previously described (Akyuz et al., 2015). After passiv-
ation, the microfluidic channel was incubated with 0.8 μM
streptavidin (Invitrogen) in T50 buffer (50 mM NaCl and 10 mM
Tris, pH 7.5) for 7 min, then thoroughly rinsed with T50 buffer.
Detergent-solubilized protein was immobilized by slowly
flowing over the channel, and excess protein was removed by
washing with 1 ml of 25 mM HEPES/Tris, pH 7.4, 200 mM KCl,
and 1 mM DDM.
Buffers were supplemented with an oxygen-scavenging
system composed of 2 mM protocatechuic acid and 50 nM
protocatechuate-3,4-dioxygenase, as described previously
(Aitken et al., 2008). The smFRET movies were recorded with
100-ms integration time using 80–100 mW laser power. All
conditions tested were in the presence of 25 mM HEPES/Tris,
500 mM sodium salt, and 1 mM DDM, in the presence or absence
of 1 mM L-Asp. Slides were washed with 25 mM HEPES/Tris,
200 mM KCl, and 1 mM DDM between experiments. Traces were
analyzed using Spartan software (Juette et al., 2016). Trajectories
were corrected for spectral cross talk and preprocessed auto-
matically to exclude trajectories that lasted ≤15 frames and had a
signal-to-noise ratio of ≤8. Traces with multiple photobleaching
events (indicative of multiple sensors in a protein) or inconsis-
tent total fluorescence intensity were also discarded.
Single-molecule transport assay and analysis
Thrombin-cleaved GltPh (C321A/N378C) was SEC purified in
20 mM HEPES/Tris, pH 7.4, 200 mM NaCl, and 0.1 mM L-Asp.
Protein was labeled with maleimide-activated biotin-PEG11 (EZ-Link;
Thermo Fisher Scientific) in the presence of N-ethylmaleimide at a
molar ratio of 1:2:4 as previously described (Ciftci et al., 2020).
Liposomes were prepared from a 3:1 (wt/wt) mixture of E. coli polar
lipid extract (Avanti Polar Lipids) and egg phosphatidylcholine in
SM-KCl buffer (50 mM HEPES/Tris, pH 7.4, and 200 mM KCl).
Liposomes were extruded through 400-nm filters (Whatman Nu-
cleopore) using a syringe extruder (Avanti), and destabilized by the
addition of Triton X-100 at 1:2 (wt/wt) detergent-to-lipid ratio. La-
beled GltPh was added to the liposome suspension at 1:1,000 (wt/wt)
protein-to-lipid ratio at room temperature for 30 min. Detergent was
removed by six rounds incubation of Bio-Beads (two rounds at 23°C
for 2 h each, one round at 4°C overnight, and three rounds at 4°C for
2 h each). The excess substrate was removed by three rounds of
centrifugation for 1 h at 49,192 g at 4°C, removal of the supernatant,
addition of 1 ml fresh SM buffer, and three freeze/thaw cycles.
Liposomes were concentrated to 50 mg/ml, and ccPEB1a-Y198F la-
beled with activated LD555P-MAL and LD655-MAL dyes as de-
scribed (Ciftci et al., 2020) was added at a final concentration of
0.6 μM and encapsulated by two freeze–thaw cycles. To remove
unencapsulated ccPEB1a-Y198F, 1 ml of SM-KCl buffer was added,
and liposomes were centrifuged for 1 h at 49.192 g at 4°C. Super-
natants were discarded, and liposomes were suspended at 50 mg/ml
and extruded 12 times through 100-nm filters.
Single-transporter smFRET assays were performed on the
same microscope described above, and the microfluidic imaging
chambers were prepared in the same way. After coating with
streptavidin, the channel was rinsed thoroughly with SM-K(X)
buffer (50 mM HEPES/Tris, pH 7.4, containing 200 mM potas-
sium salt buffer, where the anion (X) was changed based on the
condition tested). Extruded liposomes were immobilized by
slowly flowing over the channel, and excess liposomes were
removed by washing with 1 ml SM-K(X) buffer.
Buffers were supplemented with an oxygen-scavenging
system as above, and the smFRET movies were recorded with
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400-ms integration time using 20–40-mW laser power. To
confirm that liposomes were not leaky, a video was taken in SM-
K(X) buffer containing 1 μM L-Asp and 1 μM valinomycin. No
L-Asp uptake was observed under these conditions lacking Na+
gradient. After completion of this video, another video was
initiated to record transport events. At ∼3 s into this video, SM-
Na(X) buffer containing 1 μM L-Asp and 1 μM valinomycin was
perfused into the channel.
Traces were analyzed using Spartan software (Juette et al.,
2016). Trajectories were corrected for spectral cross talk and
preprocessed automatically to exclude trajectories that lasted
≤15 frames, had a signal-to-noise ratio of ≤8, and had an initial
FRET efficiency <0.4 or >0.7. Traces with multiple photo-
bleaching events (indicative of multiple sensors in a liposome)
or inconsistent total fluorescence intensity were also discarded.
Remaining traces were sorted by either the presence or absence
of observable transport events (responding and nonresponding
traces, respectively), determined by an increase in FRET effi-
ciency from ∼0.55–0.6 to ∼0.75–0.8.
Responding traces from each video were plotted as time-
dependent mean FRET efficiency. Buffer replacement time was
determined as described (Ciftci et al., 2020). Within each data-
set, the data were normalized so that 0% was the first time point
and 100% was the first time point +0.2 (the change in FRET
efficiency upon saturation of ccPEB1a-Y198F). The resulting
normalized data were multiplied by the fraction of the responding
traces relative to the total traces. Data from three independent
reconstitutions were merged and fitted to a triexponential func-
tion in GraphPad Prism 8.4.2, where Y0 = 0, plateau = 1, per-
centages were set between 0 and 100, and all rate constants were
set to be shared between all data sets and >0.
Online supplemental material
Fig. S1 shows the location of P-GltPh mutations and their effect
on function. Fig. S2 illustrates how complex ITC isotherms
change depending on parameters. Fig. S3 shows binding of TFB-
TBOA to WT GltPh. Fig. S4 shows the fitted parameters of WT
GltPh uptake in the presence of different anions. Figs. S5 and S6
are processing flowcharts and model validations of structures
from Data S1. Figs. S7, S9, and S10 are processing flowcharts and
map validations of structures from Data S2. Fig. S8 shows the
effect of P-GltPh mutations on structure. Fig. S11 shows the an-
gles of protomer tilts between OFSout, OFSmid, and OFSin. Fig. S12
shows the changes in adjacent protomers in OFSout, OFSmid, and
OFSin. Fig. S13 shows the conformation of extracellular helices in
OFSout, OFSmid, and OFSin. Video 1 shows different viewing an-
gles of Fig. 3 a. Videos 2 and 3 show 3-D variability (3DVA)
components of OFSout/OFSmid and OFSmid/OFSin, respectively.
Video 4 shows structural transitions between OFSout, OFSmid,
and OFSin. Tables S1 and S2 show fitted parameters of L-Asp
binding to P-GltPh at 10°C and 15°C, respectively. Tables S3 and
S4 show model validations of Data S1 and S2, respectively. Table
S5 shows structural differences between structures with pack-
ing heterogeneity and OFSout, OFSmid, and OFSin. Table S6 show
the tilt states arising from 3-D classification of Data S2. Data S1
provides maps and models from P-GltPh where substrate was
added just before freezing. Data S2 provides maps and models
from P-GltPh where substrate was present throughout the
purification.
Results
P-GltPh (S279E/D405N) reveals two outward-facing substrate-
binding conformations modulated by temperature and salts
We generated a mutant, P-GltPh, that eliminates Na+ binding to
Na1 (D405N) and introduces a protonatable residue at the tip of
HP1 (S279E), mimicking amino acid sequence features of proton-
coupled orthologues (Fig. S1, a and b). Our original intention was
to test the pH dependence of this mutant. While proton gra-
dients stimulated P-GltPh activity in the presence of Na+ (Fig.
S1 c), this is not the focus of this study. We observed that P-GltPh
had greatly diminished transport compared to the WT trans-
porter (Ryan et al., 2009), prompting us to test its ability to
translocate substrate using smFRET. We introduced a single
cysteine mutation into a cysteine-free background (P-GltPh
C321A/N378C), purified the protein in DDM, labeled with donor
and acceptor fluorophores, and analyzed by smFRET as in earlier
studies (Akyuz et al., 2013; Akyuz et al., 2015; Huysmans et al.,
2021). Conformations of protomer pairs within individual GltPh
trimers can be distinguished by FRET efficiency (EFRET) as either
both in OFS (∼0.4), a mixture of OFS and IFS (∼0.6), or both in
IFS (∼0.8). Most P-GltPh molecules occupy a low EFRET state in
the presence of 500 mM sodium salts, regardless of anion or
substrate presence (Fig. 1, a and b). Thus, this mutant is mainly
in OFS or the intermediate iOFS (Huang et al., 2020; Verdon and
Boudker, 2012), which our smFRET setup cannot distinguish.
Further characterization of substrate binding to the mutant
yielded results incompatible with our current understanding of
the glutamate transporter mechanism. We expected to observe
simple 1:1 binding of aspartate to a single P-GltPh binding site in
ITC experiments. Instead, we observed bimodal binding iso-
therms in 500 mM NaCl at 15°C (Fig. 1 d). Because this mutant
exclusively occupies OFS, this result suggests heterogeneity in
substrate binding to this state. A two-state model assuming two
independent, nonidentical binding states (OFS1 and OFS2; Freire
et al., 2009) is the simplest model that fits this data reliably.
Several other binding models for complex equilibria, including
cooperative and sequential binding, cannot fit our data. Notably,
lack of coupling between protomers has been well established
(Georgieva et al., 2013; Riederer et al., 2018; Erkens et al., 2013;
Ruan et al., 2017). Taken together, the data suggest there are two
dominant binding states, although there could be additional
underlying complexity. Furthermore, it is likely that these two
states represent two different conformations of the same site in
the transporter population rather than two distinct binding
sites within a protomer, since the sums of the apparent stoi-
chiometries (n1 and n2 values) averaged 0.97 ± 0.26 (range of
0.74–1.53, n = 6; Tables S1 and S2). The bimodal isotherms reflect
the presence of a conformation with a higher affinity and lower
exothermic binding enthalpy (OFS1) and a conformation with
a lower affinity and higher exothermic enthalpy (OFS2;
Brautigam, 2015; Le et al., 2013). The two conformations must
interconvert only slowly, or not at all, during the ITC experi-
ment to manifest two distinct binding states. We do not observe
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Figure 1. Two outward-facing substrate-binding states in P-GltPh (S279E/D405N). (a and b) FRET efficiency population histograms of P-GltPh in the
presence of 500 mM sodium salts, in the absence (a) or presence (b) of 1 mM L-Asp. N is the number of molecules analyzed. Data shown are an aggregate of
two independent experiments. Population contour plots are color-coded from tan (lowest population) to red (highest). Expected conformations according to
EFRET values are indicated by arrows. (c–g) ITC experiments on P-GltPh at 15°C were performed at least twice on independently prepared protein samples with
similar results. Insets show the thermal power with the corresponding scales. (c–e) Aspartate binding isotherms derived from the ITC experiments in the
presence of different amounts of NaCl (green squares): 50 mM (c); 500 mM (d); and 1 M (e). The 50-mM data were fitted to the single-state model, with fitted
Kd = 917 nM, ΔH = −2.3 kcal mol−1, and an apparent number of binding sites, n = 1.18. 500 mM NaCl and 1 M NaCl data were fitted to the two-state binding
model. The 500-mM NaCl data gave the following fitted parameters for the two states: Kd, 1.3 and 60.8 nM; ΔH, −3.3 and −7.1 kcal mol−1; n, 0.64 and 0.22. The
1-M NaCl data: Kd, 0.5 and 27.4 nM; ΔH, −3.7 and −8.7 kcal mol−1; n, 0.77 and 0.38. (f and g) Aspartate binding isotherms were obtained in 500 mM Na-
gluconate (f, red circles), or NaNO3 (g, blue triangles). All data were fitted to the two-state model. The 500-mM Na-gluconate data: Kd, 4.4 and 138.3 nM; ΔH,
−2.3 and −5.5 kcal mol−1; n, 1.00 and 0.28. The 500-mM NaNO3 data: Kd, 0.9 and 34.3 nM; ΔH, −2.1 and −6.5 kcal mol−1; n, 0.62 and 0.37. (h) Comparison of the
n2 fraction in 500 mM NaCl or NaNO3. Each point is an independent experiment. (i) Schematic representations of the conformational and binding equilibria
obtained experimentally at 10°C and 15°C in 500 mM NaCl (solid lines) or inferred (dashed lines). The thermodynamic parameters were estimated under the
assumptions that there are two nonexchanging binding states. Directions of the arrows indicate directions of the free energy changes, ΔG-s, shown. All values
are in kilocalories per mole. Binding ΔG-s are from Tables S1 and S2. The free energy differences between sodium-bound OFSs were calculated from equi-
librium constants Keq = n
2. Thin lines represent steps that are slow on the time scale of ITC experiments.
1/n
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bimodal isotherms in 50 mM NaCl (Fig. 1 c), likely because the
lower substrate affinity at lower Na+ concentrations (Reyes
et al., 2013; Boudker et al., 2007) blurs the distinctions be-
tween the states.
It is possible, in principle, that heterogeneous binding re-
flects incomplete occupancy of
the sodium-binding sites.
P-GltPh does not have Na1 due to D405N mutation, and Na2
requires aspartate binding and closure of HP2. Thus, Na3 is
the only site that could have incomplete occupancy before
substrate binding. However, the measured Na+ KD-s for WT
GltPh, 99–170 mM (Riederer and Valiyaveetil, 2019; Reyes et al.,
2013; Hanelt et al., 2015), suggests that the sites are saturated at
500 mM NaCl. Furthermore, if the lower-affinity OFS2 were due
to incomplete occupancy of the sodium sites, increasing NaCl
concentration would eliminate it. Instead, in 1 M NaCl, we ob-
served qualitative differences in the isotherms, consistent with
an increased fraction of OFS2 (Fig. 1 e). Thus, the relative pop-
ulations are unlikely to depend on the occupancy of the Na3
binding site, and NaCl-dependent changes might reflect general
salting effects.
To test this, we determined OFS2 fraction in the presence of
500 mM sodium salts containing anions on different ends of the
Hofmeister lyotropic series (gluconate− < Cl− < NO3
−; Zhang and
Cremer, 2006). Gluconate and nitrate have the opposite effects
on protein structure; respectively, they decrease and increase
the solubility of nonpolar moieties: “salting out” and “salting
in” effects. The biphasic shape of the binding isotherms is less
pronounced in gluconate than chloride and nitrate (Fig. 1, d, f,
and g). The fitted OFS2 fraction increases from ∼28% in NaCl
to ∼38% in NaNO3 (Fig. 1 h). It decreases in Na+-gluconate,
although the binding parameters were difficult to model re-
producibly. Decreasing temperature to 10°C resulted in the
OFS2 fraction falling to ∼20% in NaCl (Tables S1 and S2).
Increasing temperatures above 15°C resulted in protein
aggregation.
The observation that temperature and chaotropic salts in-
crease the OFS2 fraction suggests that OFS1 and OFS2 are in a
slow equilibrium. Because later ions in the Hofmeister series
favor OFS2, the state might feature greater solvent exposure of
hydrophobic regions than OFS1. It is possible that looser helical
packing leads to more extensive water accessibility, with energy
costs offset by increased conformational entropy. We estimated
the free energy differences between OFS1 and OFS2 based on the
measured populations and binding free energies at 10°C and 15°C
under the assumption that the states do not exchange signifi-
cantly during the ITC experiment (Fig. 1 i). OFS1 and OFS2 are
nearly isoenergetic before substrate binding, but OFS1 pre-
dominates when bound to L-Asp, reflecting higher affinity.
Transiency of OFS2 may explain why alternate substrate-
binding conformations have not been visible in structures.
Nevertheless, it must be kinetically stable over the course of
ITC titrations. Notably, nanomolar apparent binding affinities
measured for P-GltPh suggest that substrate dissociation con-
tributes little to the binding process observed in ITC. Thus, the
two states may differ primarily in the binding on-rates, with the
high-affinity state binding substrate faster than the low-affinity
state. Regardless, these results strongly suggest that there is
conformational heterogeneity in the transporter, manifesting in
different binding mechanisms.
Heterogeneous substrate binding in WT GltPh
We also performed ITC experiments on the WT protein. When
we used high protein concentrations to increase experimental
sensitivity, we observed unusual features in binding isotherms
(Fig. 2, a–c). Specifically, the initial L-Asp injections do not have
constant heats as expected for a high-affinity single-site binding
process. Instead, injection heats steadily decrease until an
abrupt drop occurs when the ligand saturates the protein. The
two-state model, where the two affinities are close but not
identical and the higher-affinity state has a higher exothermic
binding enthalpy, fits data well, though the binding parameters
are not uniquely determined (Fig. S2 a). As in P-GltPh, we ob-
served qualitative differences in the isotherms in different salts
and temperatures. The isotherm in 500 mM Na-gluconate at
15°C has a particularly unusual shape (Fig. 2 a) but becomes
more reminiscent of a single-site binding in more chaotropic
salts (Fig. 2, b and c) or at higher temperatures (Fig. 2, d and e).
As in P-GltPh, changes in the state populations can account for
the changing isotherm shapes (Fig. S2 b), though faster ex-
change between states or altered enthalpies could also con-
tribute. Collectively, our data suggest that WT GltPh has
multiple binding states in temperature- and salt-modulated
equilibrium.
WT transporter in saturating Na+ concentrations is pre-
dominantly in the OFS (Akyuz et al., 2015; Akyuz et al., 2013),
but we cannot exclude that inward-facing protomers contrib-
ute to heterogeneous L-Asp binding. However, binding of the
transport blocker TFB-TBOA, which has a 100-fold preference
for OFS (Mcilwain et al., 2016; Boudker et al., 2007; Wang and
Boudker, 2020), also produced bimodal ITC isotherms remi-
niscent of P-GltPh (Fig. S3, a–c). Due to the lower TFB-TBOA
affinity, we could not determine precise binding parameters
and quantify the salt effects. When GltPh saturated with TFB-
TBOA was competed with L-Asp, we again observed bimodal
isotherms (Fig. S3, d–f). Thus, the inhibitor binding is hetero-
geneous, and some level of conformational heterogeneity per-
sists after binding.
We used a recently developed smFRET-based single-
transporter assay to test if salt-modulated state populations
correlated with transport rates (Ciftci et al., 2020). P-GltPh
C321A/N378C mutant was labeled with PEG11-biotin and
N-ethylmaleimide and reconstituted into liposomes; low pro-
tein-to-lipid ratios enriched vesicles containing at most one GltPh
trimer. The proteoliposomes were then loaded with periplasmic
glutamate/aspartate binding protein (PEB1a) Y198F/N73C/K149C
mutant with reduced aspartate affinity labeled with maleimide-
activated donor (LD555P) and acceptor (LD655) fluorophores (re-
ferred to altogether as ccPEB1a-Y198F). The proteoliposomes were
immobilized in microscope chambers via biotinylated transporter
and assayed for transport after perfusion with saturating Na+ and
L-Asp concentrations. An increase in mean EFRET from ∼0.6 to ∼0.8
reflects saturation of the ccPEB1a-Y198F sensor by L-Asp molecules
transported into vesicles. This assay previously established kinetic
heterogeneity in WT GltPh transport (Ciftci et al., 2020), where at
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Figure 2. Heterogeneous substrate binding in WT GltPh. (a–c) Aspartate binding isotherms derived from the ITC experiments performed at 15°C in the
presence of 500 mM Na-gluconate (red circles; a); NaCl (green squares; b); or NaNO3 (blue triangles; c). (d and e) Aspartate binding isotherms in 500 mM Na-
gluconate at 25°C (d) or 35°C (e). Experiments in a–e were performed at least twice on independently prepared protein samples, producing similar results. All
data were fitted to the two-state model (black lines); however, the binding parameters were not uniquely determined (see Fig. S2 for further information).
Insets show the thermal power with the corresponding scales. (f) Aspartate transport of GltPh (C321A/N378C) was measured using the single-transporter
FRET-based assay in NaNO3 (blue) or NaCl (green). Transport was initiated by perfusing surface-immobilized proteoliposomes with buffer containing 200 mM
sodium salt, 1 μM valinomycin, and 1 μM L-Asp. Data are shown as fractions of total observable transport over time, fitted to triexponential functions (black
lines). The fitted parameters are in Fig. S4 b. Data are means and SE from three independent experiments.
least three observable populations (fast, intermediate, and slow)
transport at vastly different rates, and all contribute to mean uptake
measured in bulk. Most WT transporters are slow, with turnover
times of tens to hundreds of seconds.
Because GltPh mediates an uncoupled anion conductance,
which dissipates the buildup of membrane potential due to
electrogenic transport, it shows faster uptake in the presence of
more permeant anions (gluconate− < Cl− < NO3
−; Ryan and
Mindell, 2007). Thus, we measured transport in K+-loaded
proteoliposomes in the presence of ionophore valinomycin
clamping the potential. Triexponential fits of the uptake ki-
netics suggest that most molecules are in the slow transporting
population regardless of the salt, as expected. The fractions of
the slow transporters were similar in Na-gluconate (81.1 ± 3.1)
and NaCl (79.5 ± 3.4%) conditions but increased in NaNO3
conditions (87.3 ± 2.2%; Fig. 2 f and Fig. S4, a and b). Therefore
NaNO3-favored OFS2 conformation might correlate with a slower
transporter population.
Transient transport domain structures following
substrate binding
smFRET has shown that P-GltPh is exclusively outward facing,
making this mutant an excellent model to dissect differences
between OFS1 and OFS2 using cryo-EM, where structural het-
erogeneity should reflect the binding heterogeneity. Because
OFS2 is transient and OFS1 predominates at equilibrium, we
optimized conditions to increase the probability of imaging
OFS2. ITC analysis showed that elevated temperatures and
chaotropic salts increased the OFS2 fraction (Fig. 1). Thus, we
pre-equilibrated P-GltPh in 250 mM NaNO3 at 25°C and froze
grids within ∼5 s after adding 1 mM L-Asp (Data S1).
When we refined particles with imposed C3 symmetry, we
˚
obtained density maps with an overall resolution of 3.0
A (Fig.
S5). To maximally retain heterogeneity, we used a data pro-
cessing approach designed to pick the highest-quality particles
regardless of conformation (Su et al., 2020). We then used
symmetry expansion and focused classification of single proto-
mers into 10 classes followed by local refinement, a processing
approach that previously revealed OFS and iOFS in the WT GltPh
ensemble (Huang et al., 2020). We did not find any iOFS classes
and observed only OFS classes with similar overall structures.
We refined models for four classes with the highest resolution,
from 3.15 to 3.85 ˚A (Figs. S5 and S6, and Table S3). When we
superimposed their isolated transport domains on the intracel-
lular regions (HP1, TM8b, and TM7a), below the substrate-
binding site, they aligned well (Fig. 3 a). In contrast, we
observed displacements of helices in the extracellular halves
above the substrate-binding site, most noticeable in HP2, TM8a,
and TM7b (Fig. 3 a and Video 1). These observations suggest
heterogeneity in packing of the extracellular half of the transport
domain immediately after substrate binding. Notably, we did
not observe any differences between the Na3 sites of these
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Figure 3. Mobility of transport domains helices. (a) Superimposition of transport domains from Data S1. A1 is salmon, A3 is teal, A6 is yellow, and A7 is
magenta. (b) Superimposition of crystal structures of substrate-bound (PDB accession no. 2NWX; light yellow), TBOA-bound (PDB accession no. 2NWW; dark
blue), and Na+-bound (PDB accession no. 7AHK; maroon) GltPh. The domains were superimposed on HP1 and TM7a (residues 258–309). The views are from the
intracellular (left) and extracellular (right) sides of the transport domain. (c) Cartoon representation of the transport domain resolved in the equilibrated Data
S2 after refinement in C3. Resolved waters are shown as blue spheres. Sodium ions are purple spheres, the substrate is green, HP1 is yellow, and HP2 is red.
structures, further suggesting that heterogeneous substrate
binding does not result from incomplete Na3 occupancy. Similar
superpositions of transport domains from the previously re-
ported structures of substrate-, inhibitor-, and Na+-only bound
GltPh (Alleva et al., 2020; Boudker et al., 2007; Yernool et al.,
2004) also picture differences in positions of TM7b and TM8a in
addition to the expected differences in gating HP2 (Fig. 3 b).
These observations further support conformational lability of
these helices and suggest that ligand binding entails the re-
structuring of the entire region.
High-resolution equilibrated structures of P-GltPh
ITC experiments suggest that the low-affinity OFS2 should be
transient in the presence of substrate, and the high-affinity OFS1
should predominate at equilibrium (Fig. 1 i). To visualize OFS1,
we imaged P-GltPh in equilibrium conditions, purified in the
presence of 250 mM NaNO3 and 1 mM L-Asp (Data S2). We
˚
refined the maps to 2.2
A after imposing C3 symmetry (Fig. S7).
The increased resolution compared with Data S1 may reflect
reduced structural heterogeneity but can also be due to different
microscopes, imaging parameters, or variations in grid prepa-
rations. The D405N mutation abolishes Na+ binding at Na1
(Boudker et al., 2007; Riederer et al., 2021 Preprint); in its place,
we observed an excess density, suggesting that a water molecule
replaces the ion (Fig S8 a). The S279E side chain points into the
extracellular milieu, away from the transport domain (Fig. S8 b).
We found that the equilibrated transport domain structure is
nearly identical to class A3 (with overall RMSD of 0.30 ˚A). Thus,
we propose that this conformation corresponds to the higher-
affinity OFS1 substate. The ensemble of other conformations
observed in Data S1 (A1, A6, and A7), showing different helical
packing of the transport domain, together make up the low-
affinity OFS2 substate. Previously described V366A and A345V
mutations in HP2 are on the interface between the hairpin and
TM8a and TM7b helices, where they can disrupt the helical
packing. Indeed, HDX-MS measurements suggested increased
local dynamics in this region in Y204L/A345V/V366A GltPh
mutant. Consistent with our hypothesis, these mutations also
decreased substrate affinity, even though the crystal structure of
the mutant pictured preserved substrate coordination and
binding site details (Huysmans et al., 2021; Ciftci et al., 2021).
Interestingly, we observed several water densities within the
transport domain of the equilibrated structure contributing
to the hydrogen bond network between substrate- and ion-
coordinating residues. All six resolved buried water molecules
are below the substrate-binding site. In contrast, the extracel-
lular half of the domain, corresponding to the labile helices in
Data S1, appears “dry” (Fig. 3 c). We speculate that the extensive
hydrogen bond network in the cytoplasmic half of the transport
domain ensures its rigidity. In contrast, the extracellular half,
less constrained by polar interactions, can sample multiple
conformations with altered packing. Notably, chaotropic salts
and elevated temperature favor OFS2 consistent with less well-
packed, more dynamic, water-accessible structures.
We also looked for structural heterogeneity in Data S2. 3DVA
analysis on a single protomer using the symmetry-expanded
particle stack (Punjani and Fleet, 2021) revealed small move-
ments of the transport domain (Videos 2 and 3). Focused 3-D
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classification of ∼1.6 million symmetry-expanded particles
showed only protomers in OFS and yielded structural classes
corresponding to the density variations seen in 3DVA analysis.
We refined these classes to 2.36–2.65 ˚A resolutions (Figs. S9 and
S10; and Table S4). Superimpositions of the refined trimers on
trimerization regions (residues 150–195) showed three subtly
different tilts of the classified protomers, consisting of move-
ments of the transport domain and the peripheral parts of the
scaffold (OFSout, OFSmid, and OFSin; Video 4). The largest tilt
difference of 2.1° is between OFSout and OFSin transport do-
mains. The tilt differences for OFSout/OFSmid and OFSmid/OFSin
were ∼1.1° each (Fig. S11). The adjacent protomers are unaf-
fected, suggesting that the movements occur independently in
individual protomers (Video 4 and Fig. S12). Notably, we ob-
served no rearrangements of the extracellular helices regardless
of the tilts, and all tilt states most closely resembled class A3 in
Data S1, consistent with the high-affinity OFS1 predominating at
equilibrium (Fig. S13 and Table S5). The mechanistic basis of the
tilts and their role in transport remain unclear. Similar transport
domain tilts might also be present in Data S1; however, the
moderate resolution of the maps prevents their visualization.
Analysis of the substrate-binding sites of Data S2 classes re-
vealed that Asp-390 sampled multiple rotameric states. The
highly conserved Asp-390 in TM8 does not coordinate the
substrate but is critical for high-affinity binding—D390A mu-
tant has a 1,000-fold lower affinity (Riederer and Valiyaveetil,
2019). Arg-397 in TM8 is the principal substrate-coordinating
residue. Its guanidinium group forms hydrogen bonds with the
L-Asp sidechain carboxylate and cation-π interactions with Tyr-
317 in TM7. We found that Asp-390 can be in down or up ro-
tamers, hydrogen-bonding to Arg-397 or Tyr-317, respectively
(Fig. 4, a and b). Classifications also revealed a middle rotamer,
perhaps representing an average of the two rotamers or a
unique state (Fig. 4 c). Different Asp-390 rotamers do not result
in an observable change of Arg-397 conformation but might alter
the local electrostatics. Furthermore, tyrosine hydrogen bonding
through the OH group potentiates cation-π interactions com-
pared with phenylalanine (Gallivan and Dougherty, 1999), and
Y317F mutation leads to a 10-fold loss of L-Asp affinity in GltPh
(Riederer and Valiyaveetil, 2019). Thus, the up and down ro-
tamers might alter substrate affinity. After additional rounds of
sorting, we found that only subpopulations of OFSout, compris-
ing ∼10% of all particles, featured up or middle rotamers
(Fig. 4 c, Table S6, and Fig. S9). Thus, the preference of Asp-390
to hydrogen-bond with Arg-397 or Tyr-317 might be allosteri-
cally coupled to the position of the transport domain. Whether
these structural heterogeneities contribute to the elevator dy-
namic or substrate binding heterogeneities is yet unclear.
Discussion
Serendipitously, we found that P-GltPh mutant has two slowly
exchanging outward-facing conformational substates, OFS1 and
2, binding aspartate with different affinities and enthalpies
(Fig. 1). smFRET and cryo-EM showed that P-GltPh is predomi-
nantly outward facing (Fig. 1, a and b). Thus, P-GltPh is an ex-
cellent model to consider the mechanism of heterogeneous
binding in OFS. WT GltPh is less suitable because its conforma-
tional ensemble includes iOFS and IFS (Wang and Boudker,
2020; Reyes et al., 2013; Akyuz et al., 2013). Nevertheless, WT
binding isotherms also suggest multiple binding conformations
modulated by salts and temperature (Fig. 2). Interestingly, a
recent saturation transfer difference NMR study reported an
unusually low Hill coefficient of 0.69 for aspartate binding to
liposome-reconstituted GltPh (Hall et al., 2020). A Hill coefficient
below one may reflect negative cooperativity or result from
multiple binding states with distinct affinities (Cattoni et al.,
2015; Sevlever et al., 2020; Wang and Pan, 1996). Distinguish-
ing these possibilities requires a kinetic approach (Cattoni et al.,
2009).
Recent studies showed that GltPh, originating from a hyper-
thermophilic archaeon, exhibits activity modes at ambient
temperature, where transporter subpopulations function with
rates differing by orders of magnitude (Ciftci et al., 2020).
Switching between the modes is rare, occurring on a timescale
of hundreds of seconds. These modes were attributed to sub-
populations with different elevator dynamics and intracellular
substrate-release rates (Matin et al., 2020; Huysmans et al.,
2021; Ciftci et al., 2021). Here, we observed that NaNO3 mod-
ulated the populations of the binding substates and increased
the fraction of the slow transporters (Fig. 2 f). Therefore, the
substrate-binding heterogeneity might contribute to the het-
erogeneous uptake rates or reflect different substrate-binding
properties of fast and slow subpopulations. Some gain-
of-function mutations, including R276S/M395R, A345V, and
V366A, both increase GltPh elevator dynamics and reduce
substrate affinity (Huysmans et al., 2021; Ciftci et al., 2021).
Thus, structural perturbations can affect binding and translo-
cation in concert, suggesting that the two processes share some
of the determinants. Therefore, insights into the structural bases
of heterogeneous binding might also illuminate the origins of
heterogeneous transport kinetics.
Our cryo-EM structures of P-GltPh, imaged in conditions
nearly identical to the ITC experiments, demonstrate that the
extracellular half of the transport domain has a continuum of
packing states upon binding (Fig. 3, a and b). Yet, after equili-
bration, we observed no such heterogeneity. We found no other
pronounced structural heterogeneities that exist immediately
after binding but are gone after equilibration. Furthermore, our
structural classes do not display any evidence of altered sub-
strate coordination that could explain affinity differences. Thus,
heterogeneous binding observed in ITC correlates with different
helical packing observed in cryo-EM structures. We propose that
structural flexibility in the extracellular regions, including the
gating HP2 and adjacent helices, alter the energetics of substrate
binding. We further suggest that the structural class with the
optimal packing corresponds to the high-affinity substate, and
the classes with the continuum of helical packing arrangements
together correspond to the low-affinity substate. In this model,
following substrate binding, the transporter relaxes to the op-
timally packed state on a very slow time scale. The equilibration
process involves helical repacking and might be slow because it
requires rearrangement of a large transport domain or substrate
release and rebinding. Consistently with our hypothesis,
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
10 of 13
Figure 4. Multiple rotameric states of D390. (a and b) Cartoon representations of OFSout classes with down (a) and up (b) D390 rotamers. The mesh
objects are density maps contoured at 4 σ; only densities within 1.5 ˚A of labeled residues are displayed for clarity. HP1 is in yellow, and HP2 was removed for
clarity. (c) Percentage of particles (total = 1,623,456) that classified into certain D390 rotamers. Numbers in parentheses represent the number of 3-D classes,
further detailed in Fig. S9 and Table S6.
packing mutations A345V and V366A in the extracellular hel-
ices lead to decreased affinity and increased binding and dis-
sociation kinetics (Huysmans et al., 2021; Ciftci et al., 2021).
Notably, the closure of the HP2 tip over the substrate-binding
site is unlikely to contribute to binding heterogeneity because it
is a rapid process (Riederer and Valiyaveetil, 2019), and
achieving tight binding requires a kinetically slow step (Ewers
et al., 2013; Hanelt et al., 2015). Thus, we see helical repacking
as the most, if not the only, feasible explanation of the exper-
imentally observed heterogeneous binding.
Our structural analysis suggests that there is a structural
specialization within the transport domain of glutamate trans-
porters. A network of buried waters and polar residues within
the cytoplasmic half of the domain (TM8b, TM7a, and HP1)
might ensure exquisitely specific, evolutionarily conserved
structure responsible for sodium selectivity and allosteric cou-
pling between ion and substrate binding (Fig. 3 c). In contrast,
the extracellular half (TN7b, HP2, and TM8a) is more hydro-
phobic and contains no resolved waters. Fewer constraining
polar interactions may permit variable helical packing in this
region, setting the dynamic properties—substrate affinities and
elevator dynamics—of GltPh substates and, perhaps, different
glutamate transporter homologues. Consistently, extensive
mutagenesis in HP1 and adjacent helices did not identify gain-
of-function mutants analogous to those in HP2 (Huysmans
et al., 2021). These observations of structural specializations
are reminiscent of “protein sectors,” where proteins can have
discrete, independently evolving functional units in tertiary
structure (Halabi et al., 2009).
The structural differences between the high- and low-affinity
states are small and require analysis of high-resolution cryo-EM
datasets. Similarly, modal gating in KcsA was attributed to
subtle sidechain rearrangements (Chakrapani et al., 2011).
Functional heterogeneity has been observed in ion channels,
transporters, and enzymes. For example, nicotinic acetylcholine
receptors feature opening bursts interspersed with short or long
closed periods (Colquhoun and Sakmann, 1985), and ionotropic
glutamate receptors show complex kinetics with multiple gating
modes (Popescu, 2012). P-type ATPase also displayed periods of
rapid transport interspersed with prolonged pauses (Veshaguri
et al., 2016). As in GltPh, these distinct modes may be due to
concerted subtle restructuring of protein regions occurring on
long timescales.
Acknowledgments
Joseph A. Mindell served as editor.
We thank Drs. Eva Fortea, Maria Falzone, Philipp Schmid-
peter, Biao Qiu, and Xiaoyu Wang for helpful discussions on
cryo-EM data processing. We especially thank Navid Paknejad
for in-depth assistance with optimizing cryo-EM processing
workflows. We thank the Scientific Computing Unit at Weill
Cornell Medical College for maintenance and support of com-
putational resources. Also, we thank Bryce Delgado for prelim-
inary ITC experiments, Will Eng for protein expression, and
Vishnu Ghani for preparation of the PEB1a protein. We thank Dr.
Scott Blanchard and members of the Blanchard lab for support
and resources for smFRET experiments. Finally, we thank Drs.
Erika Riederer, Francis Valiyaveetil, and Yun Huang for helpful
discussions and exploratory experiments.
The work was supported by National Institutes of Health
(NIH) grant F32 NS102325 (to K.D. Reddy), American Heart
Association grant 19PRE34380215 (to D. Ciftci), and NIH grants
R01NS064357 and R37NS085318 (to O. Boudker). Data S1 was
collected with assistance from Carolina Hernandez at the Si-
mons Electron Microscopy Center and National Resource for
Automated Molecular Microscopy located at the New York
Structural Biology Center, supported by grants from the Simons
Foundation (349247), NYSTAR, and the NIH National Institute of
General Medical Sciences (GM103310) with additional support
from Agouron Institute (F00316) and NIH S10 OD019994-01.
Data S2 was collected at the UMass cryo-EM facility with help
from Dr. Kangkang Song and Dr. Chen Xu.
The authors declare no competing financial interests.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
11 of 13
Author contributions: K.D. Reddy and O. Boudker designed
the experiments, analyzed the data, refined the molecular
models, and wrote the manuscript with input from all authors.
K.D. Reddy and A.J. Scopelliti performed the cloning and radi-
oactive transport assays. K.D. Reddy and D. Ciftci performed the
smFRET dynamics and transport assays. K.D. Reddy performed
the ITC and cryo-EM sample preparation and processing.
Submitted: 14 February 2022
Accepted: 10 March 2022
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Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
13 of 13
Supplemental material
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S1
Figure S1. P-GltPh (S279E/D405N) has partial proton dependence. (a) Structural model of substrate-bound GltPh (PDB accession no. 2NWX). (b) Sequence
alignment of Na+-coupled (GltPh and GltTk) and H+-coupled (GltPEc, GltTBc, and DctABs) transporters; structural elements are indicated below the alignment.
Residues mutated in P-GltPh are in bold. (c) pH-dependent aspartate uptake of P-GltPh. Proteoliposomes were loaded with 50 mM potassium phosphate buffer,
pH 7, and 100 mM potassium acetate and diluted into the following 50 mM buffers containing 1 μM [3H]L-Asp: MES/NMDG, pH 6 (triangles); HEPES/Tris, pH 7
(diamonds); or HEPES/Tris, pH 8 (squares). Buffers contained either 100 mM KCl (filled symbols) or 100 mM NaCl (empty symbols). Solid lines are shown to
guide the eye, and error bars (SD) not displayed represent errors smaller than the size of the symbol.
Figure S2. Simulated fits of WT binding isotherms. (a) WT GltPh, 500 mM Na-gluconate, 15°C; kD1 and kD2 of Fit 1 (solid) and Fit 2 (dashed) are constrained
to 0.3 and 0.4 nM, respectively. Fit 1 and Fit 2 have parameters of n1, 0.82 and 0.37; ΔH1, −22.7 and −50.0 kcal mol−1; n2, 0.36 and 0.89; ΔH2, 39.4 and 16.2 kcal
mol−1. (b) WT GltPh, 500 mM Na-gluconate, 35°C; kD1 and kD2 are constrained to 1.6 and 3.6 nM, respectively. Fit has parameters of n1, 0.16; ΔH1, −24.6 kcal
mol−1; n2, 1.30; ΔH2, −4.7 kcal mol−1.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S2
Figure S3. TFB-TBOA binds to two states in WT GltPh. All ITC experiments were performed at 15°C in buffers containing 500 mM Na-gluconate (red circles),
NaCl (green squares), or NaNO3 (blue triangles). Insets show the thermal power with the corresponding scales. All data were fitted to the two-state model;
however, exact binding parameters cannot be reliably determined. (a–c) TFB-TBOA binding isotherms. (d–f) Aspartate competition isotherms in the presence
of saturating TFB-TBOA concentrations (see Materials and methods).
Figure S4. Fitted parameters of L-Asp uptake by WT GltPh in the presence of various anions. (a) Aspartate transport of GltPh (C321A/N378C) was
measured using the single-transporter FRET-based assay in NaNO3 (blue) or Na-gluconate (red). Data are means and SE from three independent experiments
performed as in Fig. 2 f. (b) Data in a and Fig. 2 f were fitted to triexponential functions (a three-phase association model; GraphPad Prism). Initial and final
fractions of total possible uptake were set to 0 and 1, respectively. The three turnover rates (fast, intermediate, and slow), corresponding to the heterogeneous
transporter populations, were constrained to be the same for all datasets (see Materials and methods for a detailed description of data processing). Three
independent experiments per condition were analyzed, and values shown are means and SEM of the fitted parameters.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S3
Figure S5. Processing flowchart for Data S1. Protomer maps from masked classification are unsharpened, with the two other protomers removed for
clarity. All maps are contoured at 8 σ.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S4
Figure S6. Validation statistics for models from Data S1. From left to right, map FSC from NU-refinement in cryoSPARC, model-to-data validation in
Phenix of the single protomer, and local resolution estimation of the unsharpened map. All maps are contoured at 8 σ. The top protomer is the subject of
focused classification and model refinement.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S5
Figure S7. Processing flowchart for Data S2. Unsharpened maps are contoured at σ of 8.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S6
Figure S8. Effects of mutations on P-GltPh. (a) Close-up view of S279 mutation. The model and density map are from Data S2 refined in C3 (PDB accession
no. 7RCP). A water molecule replacing Na1 and coordinated by D405N is emphasized as a red ball. Hydrogen bonds are displayed using hbond in ChimeraX.
(b) Cartoon representation of a protomer viewed in the membrane plane (left) and from the extracellular space (right), showing S279E points away from the
transport domain.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S7
Figure S9. Focused classification of Data S2. Protomer maps from masked classification are unsharpened, with the two other protomers removed for
clarity. Colored classes were used for further model refinement (blue, OFSin; green, OFSmid; purple, OFSout, D390 down; wheat, OFSout, D390 up). All maps are
contoured at σ of 8.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S8
Figure S10. Validation statistics for models from Data S2. (a–e) OFS-C3 (a); OFSout, D390 down (b); OFSout, D390 up (c); OFSmid (d); OFSin (e). From left to
right, map FSC from NU-refinement in cryoSPARC, model-to-data validation in Phenix of the trimer, and local resolution estimation. All maps are contoured at 8
σ except a, which is contoured at 10 σ. The top protomer in b–e is the subject of focused classification and model refinement.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S9
Figure S11. Protomer tilts in the OFS. Trimers were superimposed on the trimerization regions (residues 150–195) of all three protomers. (a–c) Comparison
of the tilts for OFSout/OFSmid (a); OFSmid/OFSin (b); and OFSout/OFSin (c). OFSout, OFSmid, and OFSin are purple, green, and blue, respectively. Although parts of
the scaffold domain also move (see Videos 2, 3, and 4), only transport domains of the classified protomers are shown for clarity. Black bars represent tilt axes
and angles calculated using the align command in ChimeraX. The corresponding per-residue Cα RMSDs are on the right. Transmembrane domains are labeled
and alternatively shaded. L, the flexible loop between TM3 and TM4.
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S10
Figure S12. Protomers adjacent to chain A do not display concerted movements. Trimers were aligned along the trimerization regions of all three
protomers (residues 150–195). (a and b) Per-residue Cα RMSDs of chain B (a) and chain C (b). Lines are OFSout/OFSmid (green); OFSmid/OFSin (red); and OFSout/
OFSin (blue). Transmembrane domains are labeled and alternatively shaded. L, the disordered loop between domains 3 and 4.
Figure S13. P-GltPh at equilibrium does not have mobility in extracellular helices. Transport domains were superimposed on HP1 and TM7a (residues
258–309). Superimposition of transport domains from Data S2. OFSout (D390 down) is purple, OFSout (D390 up) wheat, OFSmid green, and OFSin blue. The
views are from the intracellular (left) and extracellular (right) sides of the transport domain.
Video 1. Different viewing angles of structures in Fig. 3 a.
Video 2. 3DVA component approximating transitions from OFSout to OFSmid.
Video 3. 3DVA component approximating transitions from OFSmid to OFSin.
Video 4. Structural transitions between OFSout (pink), OFSmid (green), and OFSin (blue). Models were superimposed on immobile regions of all three
protomers (residues 150–195).
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S11
Provided online are six tables and two datasets. Table S1 shows L-Asp binding to P-GltPh (S279E/D405N) at 10°C in 500 mM NaCl.
Table S2 shows L-Asp binding to P-GltPh (S279E/D405N) at 15°C in 500 mM NaCl. Table S3 shows model refinement and validation
statistics for Data S1. Table S4 shows model refinement and validation statistics for Data S2. Table S5 shows comparison between
structures from Data S1 and tilt states from Data S2. Table S6 shows correlation of tilt states and D390 rotamers from Data S2
processing. Data S1 shows maps and models generated from P-GltPh purified in substrate-free (Na+ only) conditions, and substrate
(L-Asp) was added ~5 s prior to freezing. Data S2 provides maps and models generated from P-GltPh purified in the presence of
substrate and ions (Na+ and L-Asp).
Reddy et al.
Functional heterogeneity of glutamate transporters
Journal of General Physiology
https://doi.org/10.1085/jgp.202213131
S12
| null |
10.1103_physrevresearch.4.l032021.pdf
| null | null |
PHYSICAL REVIEW RESEARCH 4, L032021 (2022)
Letter
Monitoring-induced entanglement entropy and sampling complexity
Mathias Van Regemortel,1,* Oles Shtanko,2 Luis Pedro García-Pintos,1 Abhinav Deshpande,3 Hossein Dehghani
Alexey V. Gorshkov ,1 and Mohammad Hafezi
1
,1
1Joint Quantum Institute and Joint Center for Quantum Information and Computer Science, NIST/University of Maryland,
College Park, Maryland 20742, USA
2IBM Quantum, IBM Research – Almaden, San Jose, California 95120, USA
3Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
(Received 1 February 2022; accepted 12 July 2022; published 9 August 2022)
The dynamics of open quantum systems is generally described by a master equation, which describes the
loss of information into the environment. By using a simple model of uncoupled emitters, we illustrate how
the recovery of this information depends on the monitoring scheme applied to register the decay clicks. The
dissipative dynamics, in this case, is described by pure-state stochastic trajectories, and we examine different
unravelings of the same master equation. More precisely, we demonstrate how registering the sequence of
clicks from spontaneously emitted photons through a linear optical interferometer induces entanglement in the
trajectory states. Since this model consists of an array of single-photon emitters, we show a direct equivalence
with Fock-state boson sampling and link the hardness of sampling the outcomes of the quantum jumps with the
scaling of trajectory entanglement.
DOI: 10.1103/PhysRevResearch.4.L032021
The coupling of a quantum system to an environment
generally leads to decoherence and, under certain conditions,
can be modeled by a Markovian master equation that could
generically result in a mixed (nonpure) density matrix [1].
An alternative but equivalent approach describes the “un-
raveling” of the same density matrix in terms of pure-state
stochastic wave-function trajectories [2–5]. Interestingly, for
a given master equation, the unraveling in terms of stochastic
trajectories is not unique. For example, note that a Lindblad
master equation,
∂t ρ = γ
(cid:2)
(cid:3)
c jρc†
j
j
− 1
2
(cid:4)
{c†
j c j, ρ}
,
(1)
(cid:5)
is invariant under any transformation ci →
j Ui jc j, where
U is a unitary matrix and γ is the decoherence rate. Here, c j
are the jump operators that describe dissipative coupling to the
environment (see Supplemental Material [6]). In particular,
this implies that any observable (cid:3)O(cid:4) = Tr(ρO) preserves its
expectation value, independent of the choice of U . In the
unraveling picture, on the other hand, the unitary U is of
direct importance for the stochastic quantum states, as can be
understood by evaluating the effect of a quantum jump ci|ψ(cid:4).
Nevertheless, averaging expectation values over different tra-
jectory states will converge back to the U -independent result
*[email protected]
Published by the American Physical Society under the terms of the
Creative Commons Attribution 4.0 International license. Further
distribution of this work must maintain attribution to the author(s)
and the published article’s title, journal citation, and DOI.
from the master equation, Eψ (cid:3)ψ|O|ψ(cid:4) = Tr(ρO), where Eψ
is the expectation over all individual trajectories |ψ(cid:4). This is
in contrast with the case of nonlinear quantities, such as bi-
partite entanglement entropy, which may show an unraveling
dependence.
Physically, the specific choice of unraveling of a master
equation is determined by the physical observable that is mon-
itored in a dissipative process [7–12], e.g., detecting the decay
of a two-level system by observing the emitted single pho-
ton. Remarkably, such stochastic quantum trajectories were
observed in several pioneering experiments in trapped-ion
systems [13–16] and circuit quantum electrodynamics (circuit
QED) [17]. Moreover, it has been shown that monitoring such
trajectories can be used to manipulate stochastic quantum
systems [18–22], with potential applications in quantum error
correction [23,24].
Furthermore, from a theoretical perspective, monitoring
may have a profound impact on the stochastic trajectory
states when it competes with coherent processes. Specifi-
cally, it was shown in Refs. [25–28] that a scaling transition
for averaged trajectory entanglement entropy can occur. In
these works, dissipation was studied in the context of a
measurement-induced phase transition [29,30], and the master
equation associated with the dissipative dynamics was chang-
ing across the phase transition. This implies that the effect of
the monitoring protocol itself and the corresponding choice
of unraveling remain largely unexplored for the scaling of
entanglement entropy in the stochastic trajectory states.
In this Research Letter, we consider different monitoring
schemes that correspond to different unravelings of the same
master equation and analyze the associated impact on stochas-
tic quantum dynamics. We consider an array of uncoupled
single-photon emitters whose decay can be monitored by
2643-1564/2022/4(3)/L032021(6)
L032021-1
Published by the American Physical Society
MATHIAS VAN REGEMORTEL et al.
PHYSICAL REVIEW RESEARCH 4, L032021 (2022)
1: --
2: --
3: --
4: --
5: X
6: --
7: --
8: --
9: --
Jump outcomes
Jump outcomes
Boson sampling
FIG. 1. (a) A schematic illustration of the setup, consisting of a
chain of N two-level emitters, M of which are initially in the |↑(cid:4)
state, with the remaining N − M in the |↓(cid:4) state. The quantum jumps
from the spontaneous emissions in the chain are monitored through
the output ports of a linear optical network represented by an N × N
unitary U , giving new jump operators ci. (b) The case N = M = 22
and U sampled from the N × N Haar measure: half-chain entropy
for some stochastic trajectories (red) and the averaged value (blue).
The inset shows the volume-law scaling of the maximal averaged
entanglement entropy Smax. (c) After registering M clicks, the jump
outcome probabilities are given by Fock-state boson sampling from
Eq. (5). A comparison is given for N = 7, M = 4, giving 210 possi-
ble outcomes, and a Haar-random U , sampled with 10 000 quantum
trajectories from the associated unraveling.
detected photons. A linear optical network (LON) is posi-
tioned between the emitters and the detectors, as shown in
Fig. 1(a), so that the new jump operators correspond to a
LON-determined linear combination of the decay jump oper-
ators. As the sequence of jump clicks is recorded, a buildup
and a decay of entanglement entropy are generated in the
state of the emitters; see Fig. 1(b). When the LON unitary is
Haar random (see, e.g., Ref. [31]), the averaged entanglement
entropy reaches a maximum over time that has volume-law
scaling, as shown in the inset of Fig. 1(b). Moreover, since a
series of single-photon emissions is recorded, we analytically
verify a direct equivalence between sampling the outcomes
of the decay jumps and the Fock-state boson sampling prob-
lem [32], as we also numerically demonstrate in Fig. 1(c).
Finally, we illustrate in Fig. 2 that the depth of the LON
determines the scaling of maximal trajectory entanglement
entropy over time, ranging from area law for constant depth
to volume law when the depth is proportional to the number
of emitters. Given the connection of our system to Fock-state
boson sampling, we relate the scaling of maximal trajectory
entanglement entropy to the hardness of classically sampling
the jump-outcome probabilities: polynomial vs superpolyno-
mial time, respectively [33,34]. Utilizing the setup described
above, we therefore establish clear connections between the
invariance properties of the master equation, the scaling of the
associated trajectory entanglement entropy, and the sampling
complexity of jump outcomes.
The model. Our setup consists of a chain of N two-level
systems that emit photons via deexcitation and are monitored
through the output arms of a LON, represented by an N × N
unitary U . We start from a state with M two-level systems
in the excited state |↑(cid:4) and N − M in the ground state |↓(cid:4),
i.e., |ψ0(M, N )(cid:4) ≡ | ↑1 · · · ↑M↓M+1 · · · ↓M−N (cid:4), and assume a
FIG. 2. The entanglement generated in the chain of emitters is
studied by monitoring the decays through a LON. (a) Schematic of
the setup, where N emitters are excited and monitored through a
D-layered LON consisting of staggered layers of 2 × 2 Haar random
unitaries from Eq. (3). (b) and (c) A network of constant depth D
(D)
shows area-law scaling: In (b), increasing N, S
max remains stable, and
N (l, kmax(D)) for N = 100, selected
in (c), entanglement profiles S
after kmax, when the maximal entropy is reached, saturate in the bulk
(we find that kmax is independent of l). (d) and (e) Taking D to
scale with system size as D = pN gives a volume law for entropy,
converging to the result of an N × N Haar-random unitary for large
(D)
p. (d) Scaling of S
max with system size shows linear growth, and (e)
N (l, kmax(p)) for N = 22 show a strong dependence on
the profiles S
subsystem size l.
(D)
(D)
uniform rate γ for the excited emitters to spontaneously emit
a photon and relax to the ground state, as depicted in Fig. 1(a).
It is assumed that τd (cid:8) 1/(Mγ ), with τd comprising the
time for a photon to traverse the LON and the detector dead
time. A jump click recorded in output arm i of the LON U
now corresponds to applying the jump operator
ci ≡
N(cid:2)
j=1
Ui jσ −
j
,
(2)
j
(cid:5)
− iσ y
= (σ x
j
with σ −
j
j and σ x,y,z
j )/2 being the decay operator of emitter
being the Pauli (x, y, z) operator acting on site j.
As was emphasized earlier and shown in more detail in
the Supplemental Material [6], the Lindblad master equation,
ρσ +
given by ∂t ρ = γ
, ρ}), is invariant
i
under unitary mixing of the jump operators (2). On the level
of the master equation, the dynamics of the (uncoupled)
emitters is a simple classically mixed state, for which the
single-emitter density matrix entries evolve for each emitter
independently as ρ↑↑ = 1 − ρ↓↓ = e−γ t , ρ↓↑ = ρ↑↓ = 0 with
ρi j = |i(cid:4)(cid:3) j|.
i (σ −
{σ +
i
− 1
2
σ −
i
i
Stochastic quantum trajectories. A crucial element in this
Research Letter is the explicit monitoring and recording of the
jumps ci (2). The stochastic dynamics resulting from register-
ing the photon clicks in the output arms of U can be simulated
with pure-state trajectories [2–4]. Given a state |ψ (t )(cid:4), we
evaluate the probability for jump ci to occur in a short time
interval (cid:8)t as pi(t ) = γ (cid:8)t(cid:3)ψ (t )|c†
i ci|ψ (t )(cid:4). The probability
pjump(t ) =
i pi(t ) determines whether a jump happens at
time t or not. If a jump happens, then ci is selected with
probability ∝pi(t ), and we evaluate |ψ (t + (cid:8)t )(cid:4) = ci|ψ (t )(cid:4).
If there is no jump, the system evolves for time (cid:8)t under the
(cid:5)
L032021-2
MONITORING-INDUCED ENTANGLEMENT ENTROPY AND …
PHYSICAL REVIEW RESEARCH 4, L032021 (2022)
effective non-Hermitian Hamiltonian Heff = − iγ
j c j. In
both scenarios, the state is renormalized after each time step.
In the limit (cid:8)t → 0, averaging (cid:3)O(cid:4) over sampled trajectory
states is equivalent to computing (cid:3)O(cid:4) via the master equa-
tion (1).
j c†
2
(cid:5)
(cid:5)
i
Note that Heff only depends on the number of excited
(cid:5)
j c j, and that |ψ (t )(cid:4) is an
emitters Nexc =
eigenstate of Nexc between jumps if we start from |ψ0(N, M )(cid:4).
This means that, after renormalization, the evolution between
jumps does not change the stochastic state |ψ (t )(cid:4).
σ +
i
σ −
i
j c†
=
For the rest of this work, we will therefore discard the
explicit time dimension and express the evolution in terms of
the jump sequence (m1, . . . , mM ), with mk representing the
kth click in output arm 1 (cid:2) mk (cid:2) N and 1 (cid:2) k (cid:2) M. This
sequence can be obtained reliably when τd (cid:8) 1/(Mγ ), since
the photon clicks are now registered with an accuracy sig-
nificantly higher than the duration of emission (the temporal
extent of the photonic wave packet).
Connection to remote entanglement of two emitters. To
intuitively explain the idea and illustrate the underlying cor-
respondence with bosonic statistics, we start with the simple
case of two excited emitters and a 2 × 2 LON (N = M = 2)
parametrized as
(cid:6)
U =
a
−eiφb∗
b
eiφa∗
(cid:7)
,
(3)
2
√
(σ −
1
+ σ −
2 ) and ca = 1√
with |a|2 + |b|2 = 1, quantifying the mixing between the
modes, and φ being the relative phase shift. Setting a = b =
1/
2 and φ = π , corresponding to a 50 : 50 beam splitter,
(σ −
gives two new jumps cs = 1√
−
1
2
σ −
2 ), the symmetric and antisymmetric jump, respectively.
In case a symmetric click is observed, the symmetric jump
cs is applied to the initial state |↑↑(cid:4), giving the symmetric
Bell state |ψs(cid:4) = 1√
(|↑↓(cid:4) + |↓↑(cid:4)). This state can only decay
2
another time with the same symmetric jump cs, as seen im-
mediately by evaluating the probabilities Pi ∝ (cid:3)ψs|c†
i ci|ψs(cid:4),
with i = (a, s). The same story holds for the antisymmetric
jump ca, and therefore, upon monitoring the output arms of the
beam splitter, either the jump sequence (ms, ms) or (ma, ma)
is detected, each with probability 1
2 , and never the sequence
(ms, ma) or (ma, ms). This is equivalent to the celebrated
Hong-Ou-Mandel effect for two indistinguishable photons,
incident on the two input arms of a 50 : 50 beam splitter
[35]. In our case, however, the indistinguishable photonic
wave packets are detected after a time much shorter than the
duration of emission. As a result, an intermediate maximally
entangled (anti)symmetric Bell state between the two emitters
is established to convey the interference between the emit-
ted photons. A similar procedure was considered to generate
entanglement between cold atoms in a lattice configuration
[36] and experimentally implemented to entangle two distant
trapped ions [37]. The effect can also be viewed as super-
radiant emission [38].
Correspondence with boson sampling. We now general-
ize the system to N emitters, of which M are excited, and
an N × N unitary U , representing the LON with monitored
output arms; see Fig. 1(a). After having registered all M
clicks, an observer knows that all emitters have reached the
ground state |ψ(cid:4) = |↓↓ · · ·(cid:4). The probability of detecting the
M clicks in the Markovian sequence (cid:12)m ≡ (m1, m2, . . . , mM )
can be evaluated as (see Supplemental Material [6])
(cid:3)ψ0(M, N )|c†
m1
(cid:2)
P( (cid:12)m) = 1
M!
= 1
M!
(cid:12)k,(cid:12)l
· · · c†
mM
cmM
· · · cm1
|ψ0(M, N )(cid:4)
U ∗
m1,k1
· · · U ∗
mM ,kM
UmM ,lM
· · · Um1,l1
×(cid:3)ψ0(M, N )|σ +
k1
|Per(UT )|2
M!
.
=
· · · σ +
kM
σ −
lM
· · · σ −
l1
|ψ0(M, N )(cid:4)
(4)
(cid:5)
(cid:8)
σ ∈SM
M
Here, Per(A) =
i=1 Ai,σ (i) is the permanent of an
M × M matrix A, with SM being the symmetric group, i.e., the
summation is performed over the M! possible permutations of
the numbers 1, . . . , M. UT is the M × M matrix constructed
from U by taking the first M columns and repeating the ith row
ni times, where ni is the number of times detector i appears
in the sequence (cid:12)m. |Per(UT )|2 arises from gathering all terms
that give unit (nonzero) expectation value in the second line of
Eq. (4). Expression (4) can also be obtained with multiboson
correlation sampling, i.e., by evaluating the Mth-order tempo-
ral correlation function of the photonic quantum state at the
output ports of the LON [39].
We see that P( (cid:12)m) is the same for all (cid:12)m that give rise to a
given (cid:12)n = (n1, . . . , nN ). Therefore the probability of register-
i ni = M is obtained simply by multiplying
ing clicks (cid:12)n with
the expression (4) by the number of sequences (cid:12)m that give rise
to this (cid:12)n, so that
(cid:5)
P((cid:12)n) =
(cid:8)
|Per(UT )|2
i ni!
.
(5)
The jump outcome probabilities P((cid:12)n) in Eq. (5) are exactly
the ones found for Fock-state (conventional) boson sampling
when M indistinguishable photons are sampled after pass-
ing through an N × N interferometer [32,40], as verified in
Fig. 1(c). When U is drawn from the Haar measure and
N = O(M2), it has been proven that sampling from the output
distribution is classically hard (takes superpolynomial time)
unless the polynomial hierarchy collapses to the third level.
This follows from the #P hardness of classically computing
the output probabilities in Eq. (5).
Experimentally, Fock-state boson sampling has been im-
plemented for small numbers of photons, well within the
classically simulable regime [41–43]. Gaussian boson sam-
pling [44], using squeezed states instead of single photons
as input, can be scaled up further, leading to one of the
first claims of experimental quantum advantage [45]. Inter-
estingly, by engineering long-range interactions, Fock-state
boson sampling was also proven to be equivalent to sampling
spin measurement outcomes after a short Hamiltonian time
evolution [46,47].
Trajectory entanglement entropy. Our primary interest lies
in evaluating nonlinear properties of the stochastic trajectory
states of the emitters. For this, we focus on the averaged
trajectory entanglement entropy of a subsystem of size l < N,
after having registered 0 (cid:2) k (cid:2) M clicks in the output arms
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PHYSICAL REVIEW RESEARCH 4, L032021 (2022)
of a network U , evaluated as
M (l, k) = 1
Ns
(U )
S
Ns(cid:2)
i=1
(cid:9)(cid:10)
(cid:10)ψ (U )
M (k)
(cid:12)
,
(cid:11)
i
S(l )
(6)
· · · cm1
|ψ0(M, N )(cid:4),
with Ns being the number of samples taken and |ψ (U )
M (k)(cid:4)i ∝
cmk
the state after some sequence
i.e.,
(cid:12)m of k detected jumps cm j (2). Furthermore, S(l )[|ψ(cid:4)] =
−Tr[ρA ln ρA] is the von Neumann entanglement entropy of
state |ψ(cid:4), with ρA = TrB|ψ(cid:4)(cid:3)ψ| being the reduced density
matrix of subsystem A, containing l adjacent sites starting
from the boundary, and B, containing the remaining N − l
sites.
From a photonic perspective, an equivalent state |ψ (U )
M (k)(cid:4)i
can be obtained by subtracting k single photons from the M-
photon wave function at the output ports (m1, . . . , mk ) from
U and sending the remaining M − k photons back through U .
By sampling stochastic trajectories using matrix-product
states (MPSs) [48], we show in Fig. 1(b) that when U is drawn
from the Haar measure, a volume-law scaling for entangle-
ment entropy is observed, as seen in the inset. In this case,
each new jump ci (2) generally has a nonzero overlap with
any σ −
j and will induce long-range entanglement between all
emitters in the chain. Yet, the initial growth of entanglement is
(U )
M (N/2, k = 1) (cid:2) ln 2, independent of N,
upper bounded by S
which is obtained from the concavity of entanglement entropy
[49] (see Supplemental Material [6] for details).
LON and the sampling procedure. In what follows, we
restrict ourselves to the case N = M, i.e., all M emitters are
initialized in the excited state |ψN (k = 0)(cid:4) = |↑↑ · · ·(cid:4). The
N × N unitary U (N, D) that encodes the quantum jumps is
implemented through a LON that consists of D staggered
layers of Haar random 2 × 2 unitaries, each of which can be
written as Eq. (3) [see Fig. 2(a)]. For a sufficiently deep LON,
one can show that sampling instances from the LON converge
to drawing the N × N unitaries from the Haar measure [50].
Each instance in the sample set is obtained by (i) sam-
pling a U (N, D) and (ii) sampling a quantum trajectory, thus
yielding a jump sequence mk and the corresponding stochastic
series of (pure) states |ψN (k)(cid:4), with 0 (cid:2) k (cid:2) N being the
number of registered jump clicks. After repeating this proce-
dure Ns times, we obtain a set of sampled trajectories, and
(D)
N (l, k) for subsystem
the averaged entanglement entropies S
size l can be evaluated, yielding the entanglement of the
trajectories averaged over unitaries U (N, D).
Previously, a number of works have investigated the entan-
glement entropy of the M-photon wave function for Fock-state
boson sampling in an N-mode LON. In the Haar regime, the
photonic wave function shows volume-law scaling of entan-
glement entropy when exiting the LON [51,52]. In this chain
of two-level emitters, on the other hand, the spontaneously
emitted photons themselves are short-lived (stemming from
the Born-Markov approximation of the quantum trajectory
approach), and we study the buildup and decay of entan-
glement entropy between the emitters induced by registering
and applying the jumps c j (2). Additionally, this also marks
a significant difference with the measurement-induced phase
transition studied in circuit models [29,30] since no projective
measurements are performed on the emitters.
Numerical results and scaling of complexity. The stochastic
simulations were run with MPSs [48,53], using the C++
package ITENSOR [54].
(D)
N,max
In Figs. 2(b) and 2(c), we first study the scaling of entan-
glement entropy by monitoring outputs of a LON with fixed
depth D. The largest achieved averaged entanglement entropy
(D)
N (k, l )] shows an area-law behavior. In
≡ maxk,l [S
S
(D)
Fig. 2(b), it is seen that S
N,max does not scale with system
size for fixed D. This is further confirmed in Fig. 2(c) for
subsystem scaling for the case N = 100, where it is seen that
(D)
N (k, l ) converges to a finite value in the bulk. Note that, for
S
(D)
N (k, l ) is always reached for l = N/2.
any k, the maximal S
Intuitively, after detecting a click from a jump c j when D =
const, an observer can pinpoint a subset of adjacent emitters
of size 2D from which the decay could have originated, inde-
pendent of N. Therefore registering a click can only generate
local entanglement in the chain. LONs of fixed depth D (cid:8) N
are represented by a unitary U that is formulated as a banded
matrix of width 2D. Interestingly, there exist polynomial-time
algorithms to efficiently evaluate Per(UT ) of banded matrices,
which encode output probabilities of outcomes with few or no
collisions via Eq. (5) [33,34,55]. The efficient evaluation of
the output probabilities is in line with our result: The area law
of entanglement entropy ensures that the output configurations
P((cid:12)n) can be efficiently sampled using MPSs of fixed maximal
bond dimension to represent the quantum state of the emitters
after k clicks [48].
As shown in Figs. 2(d) and 2(e), the situation drastically
changes when the network depth D scales linearly with system
size: D = pN. In Fig. 2(d), we show the maximal averaged
(pN )
entanglement entropy S
N,max, which now has a clear linear
dependence on system size N, thus establishing a volume
law. The simulation quickly gets out of reach for efficient
simulation with MPSs of a given maximal bond dimension
χmax (set to χmax = 700). Also, the entanglement profiles of
subsystem size l, shown in Fig. 2(e), acquire a strong depen-
dence on subsystem size l when p is increased, which we
identify as volume law for the scaling for subsystem entan-
glement entropy. As p increases, the entanglement entropy
approaches the value obtained by sampling U from the N × N
Haar measure [black dashed line in Figs. 2(d) and 2(e)].
In order to secure the classical sampling hardness, the
original proof for Fock-state boson sampling requires that
N = O(M2) to ensure collision-free samples [32]. While we
are not in that regime, to our knowledge no efficient classical
algorithm is known to sample the jump outcomes if N = M
and D ∝ N. In our unraveling picture, we face a correlation in
complexity: The entanglement entropy between the emitters
in the trajectory states has volume-law scaling and quickly
surpasses the limit of efficient simulation with MPSs.
In contrast, when the trajectory-averaged entanglement en-
tropy scales as an area law, the sample complexity (the number
of trajectory states required in order to accurately sample the
density matrix) may be expected to increase exponentially.
This is captured by the scaling of the (classical) Shannon en-
tropy of the distribution over quantum trajectory states. Hence
there is a trade-off between sample complexity of trajectories
and the complexity of simulating each trajectory. It might
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PHYSICAL REVIEW RESEARCH 4, L032021 (2022)
be possible to practically exploit this trade-off in a classical
algorithm; see Supplemental Material [6] for a more detailed
explanation.
Conclusions and outlook. It was illustrated that chang-
ing the unraveling of a straightforward, uncoupled master
equation of emitters may cause drastic changes in both the
entanglement of stochastic trajectory states and the sampling
hardness of jump outcomes. Moreover, changing the unravel-
ing is immediately related to an observer monitoring the decay
clicks in the output arms of a LON, resulting in the unitary
mixing of the decay jumps. Sampling the jump outcomes
in the established monitoring scheme is equivalent to the
problem of Fock-state boson sampling. Finally, a connection
was established between the scaling of entanglement entropy
between emitters and the classical hardness of sampling the
jump outcomes.
While we have reported different scaling behavior for
the trajectory entanglement entropy, we have not yet seen a
conclusive signature of a scaling transition for the trajectory
entanglement entropy across a critical point, such as pre-
sented in, e.g., Refs. [26,28]. For example, one can investigate
fermionic or Gaussian models to access larger systems for the
scaling analysis.
Note added. Recently, we became aware of a recent work,
where an entanglement scaling transition was reported in a
homodyne monitoring scheme [56].
Acknowledgments. We acknowledge stimulating discus-
sions with Alireza Seif and Dominik Hangleiter. M.V.R.,
H.D., and M.H. were sponsored by ARO W911NF2010232,
AFOSR FA9550-19-1-0399, NSF OMA-2120757, QSA-
DOE, and the Simons Foundation. L.P.G.-P. and A.V.G.
acknowledge funding by the DOE ASCR Accelerated
Research in Quantum Computing program (Award No.
DE-SC0020312), DARPA SAVaNT ADVENT, NSF QLCI
(Award No. OMA-2120757), DOE QSA, ARO MURI, DOE
ASCR Quantum Testbed Pathfinder program (Award No.
DE-SC0019040), NSF PFCQC program, AFOSR, AFOSR
MURI, and U.S. Department of Energy Award No. DE-
SC0019449. A.D. acknowledges support from the National
Science Foundation (RAISE-TAQS 1839204). The Institute
for Quantum Information and Matter is an NSF Physics Fron-
tiers Center (PHY-1733907). This work used the Extreme
Science and Engineering Discovery Environment (XSEDE),
supported by National Science Foundation Grants No. ACI-
1548562 and ACI-1928147, at the Pittsburgh Supercomputing
Center (PSC) [57].
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erating scientific discovery, Comput. Sci. Eng. 16, 62 (2014).
L032021-6
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10.1371_journal.pone.0285473.pdf
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Data Availability Statement: All relevant data are
available at: https://www.ebi.ac.uk/biostudies/
studies/S-BSST1078.
|
All relevant data are available at: https://www.ebi.ac.uk/biostudies/ studies/S-BSST1078 .
|
RESEARCH ARTICLE
Cellular apoptosis and cell cycle arrest as
potential therapeutic targets for eugenol
derivatives in Candida auris
Hammad Alam1, Vartika Srivastava1, Windy Sekgele2, Mohmmad Younus WaniID
Abdullah Saad Al-Bogami3, Julitha Molepo2*, Aijaz Ahmad1,4
3*,
1 Faculty of Health Sciences, Department of Clinical Microbiology and Infectious Diseases, School of
Pathology, University of the Witwatersrand, Johannesburg, South Africa, 2 Faculty of Health Sciences,
Department of Oral Biological Sciences, School of Oral Health Sciences, University of the Witwatersrand,
Johannesburg, South Africa, 3 Department of Chemistry, College of Science, University of Jeddah, Jeddah,
Saudi Arabia, 4 Infection Control, Charlotte Maxeke Johannesburg Academic Hospital, National Health
Laboratory Service, Johannesburg, South Africa
* [email protected] (MYW); [email protected] (JM)
Abstract
Candida auris, the youngest Candida species, is known to cause candidiasis and candide-
mia in humans and has been related to several hospital outbreaks. Moreover, Candida auris
infections are largely resistant to the antifungal drugs currently in clinical use, necessitating
the development of novel medications and approaches to treat such infections. Following up
on our previous studies that demonstrated eugenol tosylate congeners (ETCs) to have anti-
fungal activity, several ETCs (C1-C6) were synthesized to find a lead molecule with the req-
uisite antifungal activity against C. auris. Preliminary tests, including broth microdilution and
the MUSE cell viability assay, identified C5 as the most active derivative, with a MIC value of
0.98 g/mL against all strains tested. Cell count and viability assays further validated the fun-
gicidal activity of C5. Apoptotic indicators, such as phosphatidylserine externalization, DNA
fragmentation, mitochondrial depolarization, decreased cytochrome c and oxidase activity
and cell death confirmed that C5 caused apoptosis in C. auris isolates. The low cytotoxicity
of C5 further confirmed the safety of using this derivative in future studies. To support the
conclusions drawn in this investigation, additional in vivo experiments demonstrating the
antifungal activity of this lead compound in animal models will be needed.
Introduction
Candida auris was identified as a novel human pathogen in 2009, and it has subsequently
caused considerable healthcare problems by producing systemic infections in individuals with
underlying diseases [1–3]. Even though C. auris shares a strong evolutionary relationship with
other pathogenic Candida species, it differs from them in terms of biology, genetics, epidemi-
ology, antifungal resistance, virulence, host adaptation, and transmission [4,5]. It was recently
identified as a critically important fungal pathogen due to its inherent resistance to the major-
ity of presently used antifungal medications, as well as pan-resistance in certain isolates. [6].
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OPEN ACCESS
Citation: Alam H, Srivastava V, Sekgele W, Wani
MY, Al-Bogami AS, Molepo J, et al. (2023) Cellular
apoptosis and cell cycle arrest as potential
therapeutic targets for eugenol derivatives in
Candida auris. PLoS ONE 18(6): e0285473. https://
doi.org/10.1371/journal.pone.0285473
Editor: Muhammad Yasir, University of New South
Wales, AUSTRALIA
Received: January 7, 2023
Accepted: April 24, 2023
Published: June 21, 2023
Copyright: © 2023 Alam et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
available at: https://www.ebi.ac.uk/biostudies/
studies/S-BSST1078.
Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
PLOS ONE | https://doi.org/10.1371/journal.pone.0285473 June 21, 2023
1 / 15
PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Echinocandins are often used to prevent C. auris infections, particularly in patients in critical
care or who have had invasive surgical procedures [7,8]. The high mortality rates associated
with this pathogen have been attributed to multidrug resistance, accompanying hospital out-
breaks, and invasive infections [9,10]. Another clinically important problem is the misdiagno-
sis of C. auris, as laboratory yeast identification methods frequently mistake it with other
yeasts [11]. Hence, to combat infections caused by C. auris and other developing fungal dis-
eases, it is imperative to develop novel, secure, and effective antifungal drugs, and treatment
strategies with a range of pharmacological targets.
Natural product-based antifungal medications are typically regarded as being efficient,
affordable, and safer with less toxicity [12]. Due to the antibacterial, antiviral, antifungal, anti-
cancer, anti-inflammatory, and antioxidant properties associated with eugenol and its deriva-
tives, they have long been used in cosmetology, medicine, and pharmacology [13,14]. Eugenol
and other monoterpene phenols have been demonstrated to inhibit the production of ergos-
terol and, also inhibit efflux pump inhibitors in C. albicans and other non-albicans Candida
species, resulting in the reversal of drug resistance among these pathogens [15]. In our previ-
ous studies, derivatization of eugenol led to the development of eugenol tosylates (ETC’s) with
improved antifungal efficacy and safety profile than the parent compound eugenol [14,16,17].
In this study different ETCs (C1–C6) were synthesized in a quest to find a lead molecule with
desired antifungal activity against C. auris.
Materials and methods
The chemical reagents and solvents were procured from Sigma Aldrich and Merck Germany.
TLC plates used were precoated aluminium sheets (silica gel 60 F254, Merck Germany) and
visualization was done by UV light in a UV cabinet. Heraeus Vario EL III analyser was used
for elemental analysis. Bruker ALPHA FT-IR spectrometer (Eco-ATR) was used for FTIR
analysis. Bruker AVANCE 400 spectrometer (400 MHz) was used for 1H and 13C NMR spectra
using DMSO-d6/CDCl3 as solvent with TMS (Tetramethylsilane) as standard. ESI-MS positive
ion mode was recorded on Micromass Quattro II triple quadrapole mass spectrometer.
Synthesis
Eugenol, also known as 2-methoxy-4-(prop-2-en-1-yl) phenol, was used as the starting mate-
rial for all the compounds C1–C6. For their synthesis, eugenol was treated with various phenyl
and substituted phenylsulfonyl chlorides in refluxing pyridine for 18–24 hours. (Completion
of reaction was monitored by TLC). After the completion of the reaction, the reaction mass
was quenched with distilled water and extracted with dichloromethane. Finally, the combined
organic layers were washed with distilled water again and dried over anhydrous Na2SO4. The
compounds were further purified using column chromatography using dichloromethane and
methanol in the ratio of 7:3 as eluant. The detailed synthesis route has been discussed previ-
ously [18].
2-methoxy-4-(prop-2-en-1-yl)phenyl-4-methylbenzenesulfonate (C1). Yield: 85%;
Anal. Calc. for C17H18O4S; C, 64.13; H, 5.70%, found; C, 64.28; H, 5.56%; IRmaxcm-1: 3035
(C-H stretch), 1585 (C = C, Ar), 1385, 1155 (S = O); 1H NMR (DMSO-d6) (ppm): 7.87–7.59
(4H, m, Ar-H), 6.88–6.78 (3H, m, Ar-H), 6.25 (1H, m), 4.86 (1H, dd, J = 15.2 Hz, 6.8 Hz), 4.48
(1H, dd, J = 15.2 Hz, 6.8 Hz), 3.90 (2H, d, CH2), 3.64 (3H, s, OCH3), 2.25 (3H, s, CH3);
13CNMR (DMSO-d6) (ppm): 148.9, 144.6, 138.5, 137.0, 135.4, 133.5, 130.6, 129.3, 128.0, 122.5,
118.5, 117.0, 115.7, 56.4, 47.4, 24.8; ESI-MS m/z [M+H]+ 319.10; [M+Na]+ 341.08.
2-methoxy-4-(prop-2-en-1-yl)phenyl-4-nitrobenzenesulfonate (C2). Yield: 89%; Anal.
Calc. for C16H15NO6S; C, 55.01; H, 4.33%; N, 4.01; found; C, 55.28; H, 4.30%, N, 4.20;
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
IRmaxcm-1: 3035 (C-H stretch), 1582 (C = C, Ar), 1385, 1152 (S = O); 1H NMR (DMSO-d6)
(ppm): 8.48–8.36 (4H, m, Ar-H), 6.88–6.78 (3H, m, Ar-H), 6.28 (1H, m), 4.97 (1H, dd, J = 15.2
Hz, 6.8 Hz), 4.46 (1H, dd, J = 15.2 Hz, 6.8 Hz), 3.94 (2H, d, CH2), 3.40 (3H, s, OCH3);
13CNMR (DMSO-d6) (ppm): 152.8, 150.5, 141.7, 139.0, 138.0, 135.3, 128.4, 124.7, 122.1, 120.9,
116.4, 112.6, 56.5, 43.0; ESI-MS m/z [M+H]+ 350.42; [M+Na]+ 372.40.
2-methoxy-4-(prop-2-en-1-yl)phenyl-4-iodobenzenesulfonate (C3). Yield: 82%; Anal.
Calc. for C16H15IO4S; C, 44.67; H, 3.51%, found; C, 44.76; H, 3.62%; IRmaxcm-1: 3038 (C-H
stretch), 1584 (C = C, Ar), 1384, 1155 (S = O); 1H NMR (DMSO-d6) (ppm): 7.64–7.39 (4H, m,
Ar-H), 6.99–6.94 (3H, m, Ar-H), 6.25 (1H, m), 4.79 (1H, dd, J = 15.2 Hz, 6.8 Hz), 4.49 (1H, dd,
J = 15.2 Hz, 6.8 Hz), 3.90 (2H, d, CH2), 3.61 (3H, s, OCH3); 13CNMR (DMSO-d6) (ppm):
150.9, 142.5, 138.5, 138.0, 135.1, 134.2, 130.7, 123.8, 121.5, 117.5, 112.1, 101.9, 55.8, 40.3;
ESI-MS m/z [M+H]+ 331.30; [M+Na]+ 345.26.
2-methoxy-4-(prop-2-en-1-yl)phenyl-4-bromobenzenesulfonate (C4). Yield: 80%;
Anal. Calc. for C16H15BrO4S; C, 50.14; H, 3.95%, found; C, 55.25; H, 4.05%; IRmaxcm-1: 3034
(C-H stretch), 1585 (C = C, Ar), 1386, 1158 (S = O); 1H NMR (DMSO-d6) (ppm): 7.84–7.68
(4H, m, Ar-H), 7.00–6.80 (3H, m, Ar-H), 6.21 (1H, m), 4.83 (1H, dd, J = 15.2 Hz, 6.8 Hz), 4.45
(1H, dd, J = 15.2 Hz, 6.8 Hz), 3.87 (2H, d, CH2), 3.64 (3H, s, OCH3); 13CNMR (DMSO-d6)
(ppm): 150.4, 139.6, 137.8, 136.9, 134.8, 133.7, 129.5, 122.8, 119.9, 116.4, 112.6, 56.5, 44.9;
ESI-MS m/z [M+H]+ 384.26; [M+Na]+ 406.30.
2-methoxy-4-(prop-2-en-1-yl)phenyl-4-(bromomethyl)benzenesulfonate (C5). Yield:
70%; Anal. Calc. for C17H17O4S; C, 64.13; H, 5.70%, found; C, 64.28; H, 5.56%; IRmaxcm-1:
3035 (C-H stretch), 1585 (C = C, Ar), 1387, 1155 (S = O); 1H NMR (DMSO-d6) (ppm): 7.84–
7.68 (4H, m, Ar-H), 7.00–6.80 (3H, m, Ar-H), 6.21 (1H, m), 4.83 (1H, dd, J = 15.2 Hz, 6.8 Hz),
4.46 (1H, dd, J = 15.2 Hz, 6.8 Hz), 4.20 (2H, s, CH2), 3.87 (2H, d, CH2), 3.64 (3H, s, OCH3);
13CNMR (DMSO-d6) (ppm): 150.6, 143.2, 138.8, 138.0, 136.7, 133.8, 130.6, 127.8, 123.5, 123.0,
117.2, 113.7, 56.4, 42.5, 34.0; ESI-MS m/z [M+H]+ 398.30; [M+Na]+ 420.40.
2-methoxy-4-(prop-2-en-1-yl)phenyl-4-propylbenzenesulfonate (C6). Yield: 75%;
Anal. Calc. for C19H22O4S; C, 65.87; H, 6.40%, found; C, 65.95; H, 6.48%; IRmaxcm-1: 3036
(C-H stretch), 1587 (C = C, Ar), 1385, 1156 (S = O); 1H NMR (DMSO-d6) (ppm): 7.80–7.54
(4H, m, Ar-H), 6.68–6.57 (3H, m, Ar-H), 6.25 (1H, m), 4.83 (1H, dd, J = 15.2 Hz, 6.8 Hz), 4.57
(1H, dd, J = 15.2 Hz, 6.8 Hz), 3.91 (2H, d, CH2), 3.61 (3H, s, OCH3), 2.65 (2H, t, CH2), 1.63
(2H, m, CH2), 1.11 (3H, t, CH3); 13CNMR (DMSO-d6) (ppm): 151.0, 146.1, 139.5, 138.0, 135.4,
132.6, 128.8, 126.6, 121.4, 120.1, 115.0, 112.8, 56.5, 42.7, 36.7, 24.3, 13.8; ESI-MS m/z [M+H]+
347.45; [M+Na]+ 369.46.
Ethics statement
All the Candida auris isolates (n = 5) were obtained from the Division of Mycology, National
Institute of Communicable Diseases (NICD), Johannesburg, South Africa. To use these isolates
in this study, an ethics waiver was obtained from the Human Research Ethics Committee of
University of the Witwatersrand (M140159) and performed according to guidelines outlined
in the Helsinki Declaration. All the isolates were revived on Sabouraud Dextrose Agar (SDA)
before the experiments.
Antifungal activity of eugenol derivatives (C1–C6)
Antifungal activity of the newly synthesized compounds was done against C. auris isolates fol-
lowing Clinical and Laboratory Standards Institute (CLSI) recommendations M27-A3 guide-
lines [19]. The stock concentrations of all the test congeners were prepared to 5000 μg/ml
using 1% DMSO, leading to the final test concentrations ranged from 1250–11.34 μg/ml after
PLOS ONE | https://doi.org/10.1371/journal.pone.0285473 June 21, 2023
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
serial dilution. After incubation at 37˚C for 24 hours MIC values were visually observed as the
lowermost concentrations of the test compounds (C1–C6) at which no fungal growth was
seen. Amphotericin B and 1% DMSO were used as positive and negative controls in the sus-
ceptibility assays.
To further determine MFC, all wells that showed no growth were sub-cultured on SDA
plates. After incubation at 37˚C for 24 hours, MFC were recorded as the least concentrations
of test congeners that resulted in total fungal mortality (99.9%).
Cell viability assay
In presence or absence of the active compound (C5) the cell viability of C. auris 6057 was
checked with the Guava1 Muse1 Cell Analyzer following manufacturer’s instructions.
Briefly, for cell viability C. auris cells were grown till log-phase and then centrifuged for 5 min-
utes at 4000 g. The cells were then washed three times with phosphate buffer saline (PBS), fol-
lowed by exposing with the different concentrations (½ MIC, MIC, 2MIC) of C-5 at 37˚C for 4
hours. After incubation cells were washed and resuspended in PBS. Cell viability was quanti-
fied by adding 20 μL of cell suspension and 380 μL of count & viability reagents at room tem-
perature for 5 minutes in dark. The results were calculated by reading cell count and cell
viability using the Guava1 Muse1 Cell Analyzer.
Apoptotic studies
Protoplast preparation. The apoptotic studies were done by making the protoplast of C.
auris healthy or C5 treated cells. Protoplasts were made from C. auris MRL6057 cells using the
previously published protocol [18].
Effect of C5 on membrane potential of mitochondria (Δψm). The effect of the most active
compound C5 on C. auris mitochondrial membrane potential (MMP Δψm) was quantified by
using the JC-10 kit (Abcam, UK), according to the directions of manufacture. Briefly, 90 μL of the
prepared protoplasts were added with 50 μL JC-10 dye into vibrant bottom and blackened walled
96-well plates and left in dark at room temperature for 1 hour. After incubation period, 50 μL vol-
ume of kit’s buffer-B was added, and then microtiter plate was rotated at 800g for 2 minutes. The
excitation-emission maxima ratio (Ex/Em = 490/530nm and 540/590nm) was calculated using a
microplate reader (Spectra-Max iD-3 multi-mode, USA). The fluorescence intensity (green fluo-
rescence) referred to as X was calculated by using Ex/Em = 490/530nm, while the red fluorescence
referred to as Y was calculated by using Ex/Em = 540/590nm. The reduction in mitochondrial
membrane potential (MMP) in exposed cells was measured using the aggregate/monomeric (Y-
mean/X-mean) ratio of JC10 dye. The decrease in ratio was regarded as depolarization of the
mitochondrial membrane. During the tests, negative (untreated cells) and positive (treated with
10 mM H2O2) controls were also included.
Effect on C. auris cytochrome c oxidase discharge. Using a previously described proto-
col, the effect of a C5 on cytochrome c oxidase release in C. auris was studied [20]. Briefly
1×106 CFU/mL C. auris cells were exposed with different concentrations (½ MIC, MIC and 2
MIC) of compound C5 for 4 hours at 37˚C with shaking. The positive control (10 mM H2O2
treated cells) and the negative control (untreated C. cells) were also included. After exposure
to test compounds cells were harvested, rinsed with PBS, and then homogenized in buffer-A
(EDTA 1mM, phenylmethylsulfonyl fluoride (PMSF) 1mM, tris base 50mM, 7.5pH). After
homogenization cells were subjected to another cycle of centrifugation at 4000g for 10 min-
utes. The supernatant was collected separately in microcentrifuge tubes and centrifuged at
15,000 g for 45 minutes. The cell free suspension was pipetted out in microcentrifuge tubes
and used to calculate the cytochrome c concentration in the cytoplasm. Meanwhile, the pellet
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
was dispersed in buffer-B (EDTA 2mM, tris base 50mM; 5.0pH) and used to calculate the
quantity of cytochrome c in mitochondria. Before taking absorbance ascorbic acid (500 mg/
mL) was added in 1:1 ratio, to calculate the amount of cytochrome c in mitochondria and cyto-
sol by taking absorbance at 550 nm with a UV-1800 SHIMADZU spectrophotometer.
DNA disintegration analysis by TUNEL-assay. The TUNEL assay, also referred to as ter-
minal deoxy-nucleotidyl transferase dUTP nick end labelling staining, is used to identify DNA
breaks or fragmentations produced during the last stages of apoptosis. For the TUNEL assay,
overnight grown C. auris cells were collected and resuspended in PBS followed by treatment
with ½MIC, MIC, and MFC values of the compound C5. H2O2 treated C. auris cells acts as the
positive control and healthy cells serve as the negative control. The cells were rinsed three
times with PBS and fixed in fixative solution (4% paraformaldehyde) for 30 minutes. The cells
were put in 0.25% triton X-100 for 2 minutes for permeabilization. After permeabilization the
C. auris cells were washed with PBS and incubated with Click-iT Plus TUNEL-assay kit
(Thermo Fisher Scientific’s) for 30 minutes in the dark. Additionally, Candida cells were
stained with 50μl of Hoechst 33342 (1X) dye (Thermo-Fisher Scientific USA) and left-over to
sit for 15 minutes in the dark at room temperature, and cell were washed two time with PBS,
before going to observe under the Confocal Microscopy (Zeiss Laser-Scanning Confocal-
Microscope (LSM) 780 Jena, Germany). The excitation-wavelength of 495nm and an emis-
sion-wavelength of 519nm for the Click-iT Plus TUNEL assay dye and for Hoechst-33342 dye
had an excitation wavelength of 350nm and an emission wavelength of 461nm was used.
Cell cycle arrest. The cell cycle arrest is a point in the cell cycle at which cells no longer par-
ticipate in the processes of cell duplication and division. The effect of C5 on the cell cycle was
determined by allowing yeast cells to grow for 8 hours, harvested at 4000g followed by suspension
of 0.5 McFarland cells in SD broth. Different concentrations of C5 (½ MIC, MIC, and 2MIC) was
added to different tubes followed by incubation for 4 hours with shaking. After, incubation the
protoplasts were prepared according to protocol as above describe. The fixed cells were then incu-
bated with the MuseTM Cell Cycle reagents, which contains ribonuclease and propidium iodide
for 30 minutes and cell cycle analysis was determined using Muse1 Cell Analyzer. The percent-
age of dead and live cells in each phase of the cell cycle was then visualised.
Cytotoxicity of C5 compound. The cytotoxic potential of C5 was assessed using horse
red blood cells (purchased from National Health Laboratory Service, Johannesburg, South
Africa) following a previously described method. [18]. Briefly, 50ml of sterile falcon tubes con-
taining 10 mL of horse blood were spun for 10 minutes at 2000rpm. The resultant red blood
cells (RBSs) pellet was thoroughly washed three times with a cooled PBS solution before the
cells were suspended once more in a cold PBS solution to produce a 10% RBC suspension.
This RBC sample was once more diluted with a PBS solution ten times. As a result, a positive
control and three different C5 compound concentrations (½MIC, MIC, and MFC) were
added to the resulting 1mL RBC suspension. The treated RBC suspension was then maintained
at room temperature for 1 hour and then centrifuged for 10 minutes at 2000rpm. 200μL super-
natant was aliquoted in flatbottom 96-well plate (Thermo Fisher Scientific, Germany), and
absorbance was measured at 450nm with SpectraMax iD3 multi-mode microplate reader. In
this experiment Triton X 100 (1%) was kept as positive control and whereas, the fresh PBS as
negative control. According to Lone et al., the percent hemolysis was calculated [18].
Results and discussion
Chemistry
Eugenol (EUG) or 4-allyl-2-methoxyphenol is a naturally occurring compound that is known
to show antifungal properties [21]. Eugenol has been known to scavenge free radicals, inhibit
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Fig 1. Scheme 1: Structures of derivatives C1-C6.
https://doi.org/10.1371/journal.pone.0285473.g001
the generation of reactive oxygen species and increase cyto-antioxidant potential. Besides it is
also known to increase H2O2 and Ca2+ concentrations in the cytoplasm, which links the anti-
fungal activity of this compound to plasma membrane damaging and destabilizing properties
[22]. Despite having potent antifungal properties, eugenol has also been found to show hepato-
toxicity, contact stomatitis, and allergic cheilitis besides other complications. It also displays
low chemical stability and is sensitive to oxidation and various chemical interactions [13]. To
mitigate these side effects and to improve the biological profile of this molecules, various mod-
ifications of the structure of this molecule have been carried out [13,23]. In a previous study, it
was reported that eugenol tosylate and its congeners, prepared by the functionalization of the
hydroxyl group of eugenol with different sulfonyl chlorides under basic conditions, showed
promising antifungal activities compared to eugenol. In this study, more derivatives were syn-
thesized and screened to find a suitable molecule with desired antifungal activity as illustrated
in Scheme 1 (Fig 1). Elemental analysis, FTIR, 1HNMR, 13CNMR, and MS-ESI+ spectrum
studies were used to determine the structure of the compounds. The synthesis of eugenol-tosy-
late and its congeners C1-C6 was demonstrated using FTIR spectra. The presence of bands
around 1156–1158 cm-1 and 1385–1387 cm-1 corresponding to the sulfonyl group of the
respective derivatives and the absence of any free or H-bonded band at/or about 3200–3700
cm-1 corresponding to -OH provides strong proof for the formation of the desired com-
pounds. The absence of any signal for the OH proton in 1H and 13CNMR, as well as the emer-
gence of distinctive peaks at predicted chemical shifts and integral values for phenyl ring and
allyl protons, give strong evidence for the synthesis of the C1-C6 derivatives. All of the deriva-
tives (C1-C6) had a [M+H]+ peak in their mass spectra, which corresponded to the chemical
formula of the compounds, further verifying their synthesis. The molecular ion peaks in some
derivatives were found as [M+Na+]+ (Metal adduct ions). The physical and spectral data are
presented in the experimental section.
Antifungal activity
C. auris is a public health concern due to its resistance and proclivity to create nosocomial epi-
demics. Fungal infections caused by this pathogen have high morbidity and mortality rates. C.
auris, despite being the youngest Candida species, is resistant to all major antifungal drug clas-
ses [24]. Therefore, there is an inevitable need of developing new antifungal drugs with novel
approaches to curb the infections caused by multidrug resistant pathogens like C. auris. Euge-
nol, a natural compound from clove essential oil, has been thoroughly studied for its antimi-
crobial activities [25]. Derivatization of eugenol has also been reported to enhance its
antimicrobial properties and safety usage [14]. Modifying natural compounds to improve their
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
efficacy, solubility, and safe use has been appreciated in the discovery of innovative medica-
tions against a variety of infectious disorders, including candidiasis [26,27].
Susceptibility testing
The in-vitro susceptibility of six newly synthesized eugenol derivatives (C1–C6) was assessed
against 5 different C. auris strains by measuring minimum inhibitory concentrations (MIC)
and minimum fungicidal concentrations (MFC) (Table 1). All the test compounds showed anti-
fungal activity with varying MIC and MFC values against C. auris isolates with compound C5
being the most potent with MIC and MFC values of 0.98 and 1.95 μg/mL respectively. Regard-
ing the selected C. auris strains, all the strains except C. auris 6057 were susceptible to ampho-
tericin B (AmB) which served as a positive control in this study. Interestingly C5 showed
equally potent antifungal activity against the amphotericin B susceptible and resistant isolates.
Based on MIC and MFC results, C5 showed considerably high anticandidal activity out of the
six compounds examined and resistant isolate C. auris 6057 carried out for further study.
Cell viability
Cell count and viability assay was performed to further demonstrate the susceptibility of C.
auris 6057 cells against the most active compound C5. Fig 2 depicts the cell viability profile
and population profile of C. auris before and after treatment with ½MIC, MIC, and 2MIC con-
centrations of C5 compound. The percentage (%) of cell viability at ½MIC, MIC, and 2MIC of
C5 compound was 46.5%, 34.5%, and 19.9%, respectively (Fig 2). These findings established
that the test compound C5 suppresses the growth and viability of C. auris in a concentration
dependent manner and thereby validating the susceptibility results. As expected, negative con-
trol contained 86.8% live cells, whereas the positive control contained only 23.0% live cells.
Cell cycle arrest
Following the susceptibility assays, effect of the C5 compound on the cell cycle of C. auris 6057
cells was determined using MUSE cell analyzer. The percentage of cells distributed among the
Table 1. Minimum inhibitory concentrations (MIC) and minimum fungicidal concentrations (MFC) of eugenol tosylate congeners (C1-C6) against different C.
auris isolates.
C1
C2
C3
C4
C5
C6
AmB
Compounds
MIC (μg/mL)
MFC (μg/mL)
MIC (μg/mL)
MFC (μg/mL)
MIC (μg/mL)
MFC (μg/mL)
MIC (μg/mL)
MFC (μg/mL)
MIC (μg/mL)
MFC (μg/mL)
MIC (μg/mL)
MFC (μg/mL)
MIC (μg/mL)
MFC (μg/mL)
C. auris 6005
125
C. auris 6015
125
C. auris 6057
>125
C. auris 6059
125
C. auris 6065
125
>125
31.25
125
3.91
7.81
31.25
62.5
0.98
1.8
125
>125
1
2
>125
15.62
62.5
3.91
7.81
15.62
62.5
0.98
1.8
62.5
>125
0.25
0.5
>125
31.25
62.5
3.91
7.81
31.25
125
0.98
1.8
62.5
>125
4
8
>125
15.62
31.5
3.91
7.81
31.25
31.5
0.98
1.8
125
>125
0.5
1
>125
31.25
62.5
3.91
7.81
31.25
62.5
0.98
1.8
62.5
>125
1
2
amphotericin B (AmB)*.
https://doi.org/10.1371/journal.pone.0285473.t001
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Fig 2. Cell viability of C. auris 6057. Cell viability of C. auris 6057 cells was recorded when cells were exposed to
½MIC, MIC and 2MIC of C5 compound. Cells without any exposure and with 10 mM H2O2 exposed serve as negative
and positive controls.
https://doi.org/10.1371/journal.pone.0285473.g002
various stages of the cell cycle in the untreated cells represent the normally developing cells,
while as in comparison cells stuck in one phase indicate cell cycle arrest. The cell cycle results
of C. auris revealed that healthy cells were uniformly distributed with 56% in G0/G1 phase,
32% in S phase and 11% in G2/M phase. In contrast cells treated with C5 halted in G0/G1, and
the cell growth was predominantly limited to G0/G1 phase 76.8%, 91.5%, and 94.0% after the
exposer with ½MIC, MIC and 2MIC respectively, while as only 15.6%, 6.5% and 4.6% of cells
were arrested in S phase when exposed with ½MIC, MIC and 2MIC respectively (Fig 3). In the
positive control where cells were treated with H2O2, then most of the cells (92.3%) were seen
arrested in G0/G1 phase and only 4.6% and 3% are arrested in S and G2/M phases
respectively.
Apoptotic studies
Apoptosis is an evolutionarily conserved mechanism used in the regulation of growth and dif-
ferentiation by all multicellular organisms [28]. Apoptosis and cell cycle are linked to each
other and are important in cell survival, proliferation, and reproduction. Arrest of C. auris
cells in G1 phase of cell cycle as seen above could be related to the apoptosis in these cells and
thereby their inability to enter S phase. Therefore, the present study determined the effect of
C-5 compound against different apoptotic markers in C. auris 6057.
Mitochondrial membrane potential (MMP) (Δψm)
The membrane potential in mitochondria (Δψm) is an indicator of mitochondrial energetic
status. MMP (Δψm) is used to assess the activity of proton-pumps in mitochondria, electron
transport systems (ETS) and the activation of mitochondrial permeability due to a variety of
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Fig 3. C. auris cell cycle analysis. The figure shows the effect of compound C5 at ½MIC, MIC and 2MIC on the cycle
in C. auris 6057. Positive controls were cells exposed to H2O2, and negative controls were healthy untreated C. auris
cells.
https://doi.org/10.1371/journal.pone.0285473.g003
causes. Loss of MMP is regarded as very crucial for the survival and death of cells and is neces-
sary stage for the apoptotic cascade. Therefore, C-5 compound was studied for its effect on
MMP in C. auris cells to determine its ability to cause apoptosis. The constant Δψm in living
yeast cells allowed the JC-10 to dye to clump together which can be detected by red fluoresce.
In contrast apoptotic cells show decreasing Δψm, which restricts the JC-10 dye to monomeric
form and thereby resulting in green fluorescence. Δψm was measured by calculating the ratio
of JC-10 aggregates to JC-10 monomers; a decrease in values when compared to the untreated
control indicated Δψm deprotonation. The results showed that in comparison of untreated
cells a significant rise in JC-10 monomer was observed, means fluorescence levels was
detected, indicating mitochondrial membrane depolarization (Fig 4). The ratio was 2.32 in the
untreated cells or the cells with intact mitochondrial membrane and 1.91 in the positive con-
trol cells. In terms of C-5 compound exposure, on increasing the concentration of the com-
pound (½MIC, MIC, and 2MIC) membrane potential decreases from 1.92, 1.49, and 1.42
respectively. These results suggested that the C-5 compound causes membrane disintegration
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Fig 4. Mitochondrial depolarization in C. auris. Fluorescence ratio (Y mean/X mean) representing Δψm and thereby
apoptosis. In comparison to the negative untreated cells, C. auris exposed cells with ½MIC, MIC and 2MIC of C-5
compound showed depolarization of the mitochondrial membrane. Positive control represents C. auris 6057 cells
treated with H2O2.
https://doi.org/10.1371/journal.pone.0285473.g004
in mitochondria by lowering the mitochondrial membrane potential in C. auris cells. Mito-
chondrial membrane depolarization is caused by uncontrolled mitochondrial membrane
pores, which induces movement and the activation of various pro-apoptotic proteins [29].
Reactive oxygen species (ROS) that are produced by mitochondria were also recognized to
play role in causing apoptosis [30].
Cytochrome c oxidase activity
Determination of cytochrome c oxidase activity is an established method for studying apopto-
tic cell death. This quantitative method involves the measurement of the amount of cyto-
chrome c oozing out from mitochondria to cytosol. This study examined the changes in
cytochrome c levels in mitochondria and cytosol in untreated and treated cells with various
concentrations of compound C-5. According to the observations, there was a considerable
increase of cytochrome c level in cytosol and decrease in mitochondrial cytosol when cells
were treated with C-5 in comparison to untreated cells (Fig 5). Untreated negative control cells
had relative fluorescence values which were consider 1.0 for cytochrome c in both mitochon-
dria and cytosol. In comparison, congener C-5 treatment caused significant ooze out of cyto-
chrome c in C. auris cells at ½MIC, MIC, and 2MIC values, with relative fluorescence values of
0.8, 0.78 and 0.47 for mitochondrial and 1.15, 1.24 and 1.35 for cytosolic cytochrome c, corre-
spondingly (Fig 5). The cytochrome c in positive control was 0.49 for mitochondrial cyto-
chrome c and 1.43 for cytosolic cytochrome c. Cytochrome c is a component of the electron
transport chain (ETC) that is loosely connected to the inner mitochondrial membrane. Cas-
pases are activated by the release of cytochrome c from the mitochondria to the cytoplasm,
which is a critical step in the induction of apoptotic cell death [18].
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Fig 5. Cytochrome c movement in C. auris. Movement of apoptotic factor cytochrome c from mitochondria to
cytosol in C. auris 6057 in absence (negative control), H2O2 treated (positive control) and C-5 treated at ½MIC, MIC
and 2MIC. At 550nm, cytochrome c level in the mitochondria (orange line) and the cytosol (blue line) were calculated.
https://doi.org/10.1371/journal.pone.0285473.g005
DNA damage by TUNEL assay
The phosphatidylserine externalization is another important parameter for apoptosis studies.
The terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) test
was used to investigate this hallmark of apoptosis in response to the active drugs. The TUNEL-
assay is based on the incorporation of modified dUTP at the 30-OH ends of fragmented DNA.
C. auris cells were additionally stained with a Hoechst 33342 dye for differentiation, as it pro-
duces blue color for live cells. From the results, C. auris cells exposed to various concentrations
(½MIC, MIC, and 2MIC) of the compound C-5, showed considerable increase in the amount
of TUNEL positive nuclei (green color fluorescent spots) when compared to the unexposed
cell population (Fig 6).
The results are suggesting the dose dependent effect, with higher concentrations of the test
compound C-5 by increasing the population and intensity of green fluorescence cells in
TUNEL positive nuclei. Positive control (10 mM H2O2 treated) also showed enhanced green
fluorescence, showing a higher population of late apoptotic cells, in comparison of negative
control with more blue fluorescence live cell population.
Cytotoxicity
The cytotoxicity of C-5 at various doses ½MIC, MIC, and 2MIC was tested against horse
blood cells (RBCs). We corelate the Triton X-100 as a positive control, which induces 100%
RBSs lysis, test compound C-5 caused hemolysis 0.6%, 3.92%, and 7.3% at ½MIC, MIC, and
2MIC values (Fig 7). PBS was utilized as a negative control, and no RBCs were lysed. The
results confirmed that the newly synthesized C-5 compound has lower cytotoxicity, implying
that it is potentially safe to use for in-vivo studies and thus provide a potential candidate for
antifungal drug development.
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Fig 6. TUNNEL assay representing apoptosis in C. auris. The confocal microscopy of C. auris 6057 cells treated with
different concentrations (½MIC, MIC, and 2MIC) of C-5 compound. Hoechst 33342 dye (blue fluorescence) showing
live cells, and Alexa Fluor 488 dye (green fluorescence) represent apoptotic cells.
https://doi.org/10.1371/journal.pone.0285473.g006
Fig 7. Hemolytic activity of C-5. Hemolysis of horse red blood cells was done in presence of Triton-X (control) and
different concentrations of C-5.
https://doi.org/10.1371/journal.pone.0285473.g007
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PLOS ONEEugenol derivatives as antifungal agents against Candida auris
Conclusion
Significant antifungal activity caused by apoptosis and cell cycle arrest in C. auris by one of the
eugenol derivatives (C5) was observed in this study. This compound showed potent antifungal
activity against the tested C. auris isolates, and therefore shows promise as a lead molecule that
could be studied further. Furthermore, unlike eugenol this lead compound also displayed low
toxicity against red blood cells urging its safe use for in vivo assays. To put together, these
results are quite encouraging and therefore further detailed studies of the active compound
against different fungal pathogens would reveal more about the potential of this compound as
a lead molecule.
Statistical analysis
All experiments were performed independently in triplicates (n = 3), and data were presented
as mean ± standard deviation (SD). The statistical analysis was performed using two-way
ANOVA test by GraphPad Prism software, version 8.0.1.
Supporting information
S1 File. Supporting information contains 1H and 13CNMR data of the compounds C1-C6,
and Scheme (S1 Scheme) for the synthesis of the compounds.
(DOCX)
Author Contributions
Conceptualization: Mohmmad Younus Wani, Julitha Molepo, Aijaz Ahmad.
Formal analysis: Vartika Srivastava, Mohmmad Younus Wani, Abdullah Saad Al-Bogami,
Julitha Molepo, Aijaz Ahmad.
Investigation: Mohmmad Younus Wani, Abdullah Saad Al-Bogami, Aijaz Ahmad.
Methodology: Hammad Alam, Vartika Srivastava, Windy Sekgele, Mohmmad Younus Wani.
Project administration: Vartika Srivastava.
Resources: Aijaz Ahmad.
Software: Hammad Alam, Mohmmad Younus Wani.
Supervision: Abdullah Saad Al-Bogami, Julitha Molepo, Aijaz Ahmad.
Validation: Windy Sekgele, Mohmmad Younus Wani, Aijaz Ahmad.
Writing – original draft: Hammad Alam, Windy Sekgele, Mohmmad Younus Wani.
Writing – review & editing: Vartika Srivastava, Mohmmad Younus Wani, Abdullah Saad Al-
Bogami, Aijaz Ahmad.
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10.1088_2752-5295_acc08a.pdf
|
Data availability statement
The data that support the findings of this study are available upon request from the authors.
|
Data availability statement The data that support the findings of this study are available upon request from the authors.
|
Environ. Res.: Climate 2 (2023) 025002
https://doi.org/10.1088/2752-5295/acc08a
PAPER
Projected expansion of hottest climate zones over Africa during
the mid and late 21st century
Alima Dajuma1,2,∗, Mouhamadou Bamba Sylla1, Moustapha Tall1, Mansour Almazroui3,4,
Nourredine Yassa5,6, Arona Diedhiou7 and Filippo Giorgi8
1 African Institute Mathematical Sciences (AIMS), AIMS Rwanda Center, KN 3, P.O. Box 7150, Kigali, Rwanda
2 Université Peleforo Gon Coulibaly, BP 1328, Korhogo, Cˆote d’Ivoire
3 Center of Excellence for Climate Change Research/Department of Meteorology, King Abdulaziz University, Jeddah, Saudi Arabia
4 Climatic Research Unit, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom
5 Commissariat aux Energies Renouvelables et `a l’Efficacité Energétique, 23 avenue Slimane Asselah, Telemly, Algiers, Algeria
6 Faculty of Chemistry, University of Sciences and Technology Houari Boumediene, BP 32 EL-Alia, Bab-Ezzouar, 16111 Algiers, Algeria
7 University Grenoble Alpes, IRD, CNRS, Grenoble-INP, IGE, 38000 Grenoble, France
8 Abdus Salam International Centre for Theoretical Physics, Earth System Physics section, Trieste, Italy
∗
Author to whom any correspondence should be addressed.
E-mail: [email protected] and [email protected]
Keywords: thermal type, climate classification, global climate models, PET
Supplementary material for this article is available online
OPEN ACCESS
RECEIVED
29 August 2022
REVISED
8 February 2023
ACCEPTED FOR PUBLICATION
2 March 2023
PUBLISHED
27 March 2023
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under the terms of the
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Attribution 4.0 licence.
Any further distribution
of this work must
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citation and DOI.
Abstract
Projected shifts in thermal climate zones over Africa during the mid and late 21st century are
assessed by employing the Thornthwaite thermal classification applied to 40 CMIP6 global climate
models under the SSP1-2.6, SSP2-4.5 and SSP5-8.5 forcing scenarios. The CMIP6 multimodel
ensemble mean reproduces the observed pattern of thermal zones during the reference period,
albeit with some discrepancies. The projections reveal a gradual expansion of the hottest thermal
type consisting of a northward and southward displacement of torrid climate zones, with this effect
intensifying as greenhouse gas (GHG) forcing increases and the time horizon moves from the mid
to the end of the century. In particular, the Mediterranean region, almost all southern African
countries, part of East Africa and most Madagascar predominantly warm in present-day
conditions, are projected to face mostly hot climates in the mid—21st century and torrid by the
end of the 21st century in the high-end forcing scenario. Generally, in the mid—21st century,
torrid climates expand by up to ∼15%, 20% and 27% of total Africa’s land areas for the SSP1-2.6,
SSP2-4.5 and SSP5-8.5, respectively, with these fractions increasing to ∼16%, 28% and 42% in the
late 21st century. Therefore, at the end of the 21st century for the high-end GHG concentration
scenario, the African continent will be covered by 81%–87% of torrid climate type, which will have
enormous impacts on the sustainable development of African countries.
1. Introduction
Human induced climate change by increased anthropogenic greenhouse gas (GHG) forcing affects the globe
ubiquitously (IPCC 2021), in particular causing changes in local and regional climates (Sylla et al 2016a,
Dosio et al 2021, Ranasinghe et al 2021, Seneviratne et al 2021). These changes have lead to disastrous
impacts including heatwaves, droughts, floods, landslides and bush fires (e.g. Russo et al 2015, Cloutier et al
2016, Mukherjee et al 2018, Tabari 2020).
Of great concern within the global change debate is the occurrence of possible shifts in thermal climate
zones. These are areas predominantly identified by temperature patterns and impact for example
biodiversity, agricultural systems, public health and the energy sector (Mahlstein et al 2013, Porter et al
2014). In fact, thermal climate zones distribution determines the ecosystem of living species and the energy
consumption from residential buildings (Bai et al 2019, Kim and Bae 2021). As temperatures will continue to
rise due to anthropogenic warming (IPCC 2021), changes in thermal climate zones will accelerate ecosystem
© 2023 The Author(s). Published by IOP Publishing Ltd
Environ. Res.: Climate 2 (2023) 025002
A Dajuma et al
disruption and increase stress on the energy and other socioeconomic sectors (Mascarelli 2013, Trisos et al
2022). This problem will be particularly important for poorer countries, e.g. in the African continent, due to
their greater vulnerability. In fact, Africa is highly vulnerable to human-induced climate change (Trisos et al
2022) and is already facing a lot of damage in key sectors including water resources, agriculture, biodiversity
and the entire ecosystems. As temperature is projected to increase, this implies change in magnitude and
frequency of events which affects the ecosystem, including shift in thermal climate zones. The agricultural
sector is sensitive to the change in thermal climate zones and these changes might affect the food security
over the continent. Moreover, the change in thermal zones will threaten the population health to heat stress
as they will be exposed to hot conditions, thus affect their work productivity.
At the global scale, previous studies have suggested that climate zones have already shifted in recent
decades (Fraedrich et al 2001, Chen and Chen 2013, Belda et al 2014, Yoo and Rohli 2016) and these shifts
are projected to expand further and rapidly under foreseeable future climate conditions (Rubel and Kottek
2010, Elguindi et al 2014, Zhang et al 2017, Navarro et al 2022).
At the regional scale, numerous studies have been undertaken to investigate changes in regional climate
zones using different climate classification methods. For example, de Castro et al (2007) showed that
European climate types would shift toward warmer or drier types by the end of the 21st century. As another
example, Rahimi et al (2019) projected that torrid and arid climate types covering small land areas of
Southeast Asia under present climate conditions would expand and cover larger areas under the RCP4.5 and
RCP8.5 scenarios, by the end of the 21st century. Elguindi and Grundstein (2013) found a recession of cold
climate zones across the eastern part of the United States and projected a growth of torrid climate across the
southern part of the country.
In Africa, a study by Sylla et al (2016b) projected an extension of torrid climate throughout West Africa
by the end of the 21st century using CORDEX data (Coordinated Regional climate Downscaling Experiment;
Giorgi et al 2009). However, other regions of Africa such as areas of North Africa, East Africa, Central Africa,
Southern Africa lack this information. Additionally, with the sixth phase of the Coupled Model
Intercomparison Project (CMIP6; Eyring et al 2016), the availability of a large number of models
participating to ScenarioMIP (O’Neill et al 2016) provide valuable resources to assess and/or update future
thermal conditions over these regions under different shared socioeconomic
pathways (SSPs).
Based on these considerations, we here apply the revised Thornthwaite thermal climate classification,
which is a temperature-based potential evapotranspiration (PET) estimate methodology, to investigate
changes in thermal climate zones during the mid and late 21st century deriving from the CMIP6 projections.
In particular, we examine the spatial extension of high-end thermal types (i.e. hot and torrid types). For
reliability and robustness of our results, we analyze the multimodel ensemble (MME) mean of 40 CMIP6
global climate models (GCMs) (Eyring et al 2016) and employ two formulations of PET estimates:
Thornthwaite (1948) and Hamon (1963).
This study provides useful information for stakeholders to identify hottest thermal zones (i.e. hot and
torrid) over Africa and ultimately reduce the potential risk related to the impacts of their intensification and
shifts.
2. Data and methods
2.1. Data description
The ensemble mean of 40 GCMs from CMIP6 for the reference period (1985–2014) and two future periods,
i.e. mid (2041–2070) and late (2071–2100) 21st century, are used to examine changes in thermal climate
zones over Africa (see figure 1). To consider a range of possible future climates, we analyze different future
SSPs (O’Neill et al 2016): SSP1-2.6 (i.e. low forcing: sustainability pathway), SSP2-4.5 (i.e. medium forcing:
middle-of-the-road pathway) and SSP5-8.5 (high-end forcing: fossil fueled development pathway). The list
of GCMs in the ensemble is presented in table 1. The analysis also includes regionalization over nine (9)
sub-regions over Africa (see figure 1) identified by the Intergovernmental Panel on Climate Change (IPCC)
Assessment Report 6 (AR6) (IPCC AR6 WGI 2021). Monthly near-surface air temperature from the GCMs is
used to compute monthly PET to be used into the thermal climate classification methods as previously done
by (Feddema 2005, Elguindi and Grundstein 2013, Sylla et al 2016b, Lang et al 2017).
For model validation purpose we use the near-surface monthly mean temperature data from the Climatic
Research Unit (i.e. CRU TS4.05; 0.5◦ × 0.5◦ of resolution; www.cru.uea.acuk/data) (Harris et al 2020) and
from the fifth generation of European Re-analysis (ERA5; 31 km of resolution) (Hersbach and Dee 2016,
Hersbach et al 2020). Use of different observation datasets allows us to account for uncertainties in historical
temperature data over Africa (Diallo et al 2012).
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Figure 1. Africa domain, Topography (in m) including nine (9) IPCC AR6 sub-regions in Africa (MED-AF: Mediterranean; SAH:
Sahara, WAF: West-Africa, CAF: Central Africa, NEAF: North-East-Africa, CEAF: Central-East-Africa, WSAF: West-South-Africa,
ESAF: East-South-Africa, MDG: Madagascar).
Table 1. Summary of CMIP6 GCMs considered, their modeling centers, resolution and references.
Model
number Model name
1
2
3
4
5
6
7
8 9
10
11
ACCESS-CM2
ACCESS-ESM1-5
AWI-CM-1-1-MR
BCC-CSM2-MR
CanESM5
CAMS-CSM1-0
CAS-ESM2-0
CESM2
CESM2-WACCM
CIESM
CMCC-CM2-SR5
Resolution
(◦lon × ◦lat)
1.875 × 1.25
References
Bi et al (2020)
1.875 × 1.25
Ziehn et al (2020)
0.937 × 0.935
Semmler et al (2020)
1.125 × 1.125
Wu et al (2019)
2.8 × 2.8
Swart et al (2019)
1.125 × 1.121
Rong et al (2018)
1.406 × 1.417
Jin et al (2021)
1.25 × 0.942
1.25 × 0.942
1.25 × 0.942
1.25 × 0.942
Danabasoglu et al
(2020)
Lin et al (2020)
Cherchi et al (2019)
(Continued.)
Modeling center (country)
Commonwealth Scientific and
Industrial Research Organization
(Australia)
Commonwealth Scientific and
Industrial Research Organization
(Australia)
Alfred Wegener Institute,
Helmholtz Centre for Polar and
Marine Research (Germany)
Beijing Climate Centre China
Meteorological Administration
(China)
Canadian Center for Climate
Modelling and Analysis (Canada)
Chinese Academic of
Meteorological Sciences (China)
Chinese Academic of Sciences
(China)
National Center for Atmospheric
Research (NCAR) (USA)
National Center for Atmospheric
Research (NCAR) (USA)
Euro-Mediterranean Centre on
Climate Change Foundation (Italy)
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Model
number Model name
12
CMCC-ESM2
13 14 15
CNRM-CM6-1-HR
CNRM-CM6-1
CNRM-ESM2-1
16 17 18
19
20
21
22
23
24
EC-EARTH3
EC-EARTH3-Veg
EC-EARTH3-
Veg_LR
FGOALS-f3
FGOALS-g3
FIO-ESM-2-0
GFDL-ESM4
GISS-E2
IITM-ESM
25 26
INM-CM4-8
INM-CM5-0
27
28
29
30
31
32
33
34
35
36
37 38
39
40
IPSL-CM6A-LR
KACE-1-0-G
KIOST-ESM
MCM-UA-1-0
MIROC6
MIROC-ES2L
MPI-ESM1-2-HR
MPI-ESM1-2-LR
MRI-ESM2-0
NESM3
NorESM2-LM
NorESM2-MM
TaiESM1
UKESM1-0-LL
Table 1. (Continued.)
Modeling center (country)
Euro-Mediterranean Centre on
Climate Change Foundation (Italy)
Centre National de Recherches
Météorologiques- Centre Européen
de Recherches et de Formation
Avancée en Calcul Scientifique
(France)
EC-EARTH consortium, Rossby
Center, Swedish Meteorological
and Hydrological Institute/SMHI
(Sweden)
Chinese Academic of Sciences
(China)
Chinese Academic of Sciences
(China)
First Institute of Oceanography,
Ministry of Natural Resources
(China)
NOAA Geophysical Fluid Dynamic
Laboratory (USA)
Goddard Institute for Space Studies
(USA)
Center for Climate Change
Research, Indian Institute of
Tropical Meteorology Pune (India)
Institute for Numerical
Mathematics. Russian Academy of
Science (Russia)
Institut Pierre-Simon Laplace
(France)
National Institute of
Meteorological Sciences, Korea
Meteorological Administration
(Republic of Korea)
Korea Institute of Ocean Science
and Technology (Republic of
Korea)
Department of Geosciences,
University of Arizona (USA)
Japan Agency for Marine-Earth
Science and Technology (Japan)
Japan Agency for Marine-Earth
Science and Technology (Japan)
Max Planck Institute for
Meteorology (Germany)
Max Planck Institute for
Meteorology (Germany)
Meteorological Research Institute
(Japan)
Nanjing University of Information
Science and Technology (China)
Norwegian Climate Center
(Norway)
Research Center for Environmental
Changes, Academia Sinica
(Taiwan)
Met Office Hadley Center (UK)
4
Resolution
(◦lon × ◦lat)
1.25 × 0.942
0.5 × 0.5
1.4 × 1.4
1.4 × 1.4
0.7 × 0.7
0.7 × 0.7
1.125 × 1.121
1.25 × 1
2 × 2
References
Lovato et al (2022)
Voldoire et al (2019)
Döscher et al (2022)
HE et al (2020)
Pu et al (2020)
1.25 × 0.942
Bao et al (2020)
1.25 × 1
2.5 × 2
Dunne et al (2020)
Schmidt et al (2014)
1.875 × 1.9
Raghavan et al (2021)
2 × 1.5 2 × 1.5
Volodin et al (2013)
Volodin et al (2017)
2.5 × 1.267
Boucher et al (2020)
1.875 × 1.25
Lee et al (2020)
1.875 × 1.894
Pak et al (2021)
3.75 × 2.235
Delworth et al (2002)
1.4 × 1.4
Tatebe et al (2019)
2.8125 × 2.79
Hajima et al (2020)
0.9375 × 0.935
(Müller et al 2018)
1.875 × 1.865
Mauritsen et al (2019)
1.125 × 1.121
Yukimoto et al (2019)
1.875 × 1.865
Cao et al (2018)
2.5 × 1.895
1.25 × 0.942
1.25 × 0.9.42
Seland et al (2020)
Lee et al (2020)
1.875 × 1.25
Sellar et al (2019)
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2.2. Methods
The revised Thornthwaite thermal climate classification method (Feddema 2005) is applied to monthly
near-surface mean temperature of CRU, ERA5 and each of the CMIP6 GCMs (see table 1). This thermal
climate classification uses PET and we compute annual PET from the Thornthwaite formulation. This
method is widely used to estimate PET for climate classification (Elguindi and Grundstein 2013, Elguindi
et al 2014, Sylla et al 2016b, Rahimi et al 2019). In order to enhance robustness of our results we also use the
Hamon PET method (Lu et al 2005, McCabe et al 2015, Zhao et al 2021, Abeysiriwardana et al 2022), which
also employs near-surface mean temperature. All datasets (i.e. CRU, ERA5, CMIP6 GCMs) are reggrided
onto a 1◦ × 1◦ common grid. The following section describes the different formulations of the Thornthwaite
and Hamon PET formulations.
2.2.1. Thornthwaite method
The Thornthwaite PET formulation was developed by (Thornthwaite 1948) and calculated monthly PET as
follows:
PET = 16 ∗ N ∗
)
a
(
10T
I
12∑
I =
Ii
i =1
)
1.514
(
Ii =
Tmean,i
5
a = 6.75 × 10−7 ∗ I3 − 7.71 × 10−5 ∗ I2 + 1.79 × 10−2 ∗ I + 0.492
where,
PET: monthly potential evapotranspiration [mm.month−1]
Tmean,i: monthly mean near-surface air temperature [◦C] for month i
N: mean length of daylight
I: annual heat index
Ii: monthly heat index for month i
a: function of the annual heat index.
(1)
(2)
(3)
(4)
2.2.2. Hamon method
The Hamon method was developed by Hamon (1963) to estimate PET from mean monthly near-surface air
temperature and day length. The formulation is as follows:
PET = k ∗ 0.165 × 7 ∗ N ∗
(
)
es
T + 273.3
where,
PET: monthly potential evapotranspiration [mm.day−1]
k: proportionality coefficient =11 [unitless]
N: daytime length [x/12 h]
es: saturation vapor pressure [mb]
T: average monthly temperature [◦C]
es—saturation vapor pressure is given by the following equation:
es = 6.108e
17.27T
T+237.3 .
(5)
(6)
Each of the estimated monthly PET is computed annually before performing the climatological mean for
the reference period (1985–2014) and the two future periods (2041–2070; 2071–2100) under each of the SSP
forcing scenario. The thermal climate classification (i.e. thermal types; Feddema 2005) derived from total
annual PET is given in table 2. Six categories of thermal index are identified: frigid (PET between 0 and
300 mm yr−1), cold (PET between 300 and 600 mm yr−1), cool (PET between 600 and 900 mm yr−1), warm
(PET between 900 and 1200 mm yr−1), hot (PET between 1200 and 1500 mm yr−1) and torrid (PET above
1500 mm yr−1), depending on the range of values of the total annual PET. These thermal types are expressed
in mm.yr−1 and the range of values of each type is assigned in table 2.
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Table 2. Description of thermal types according to the range of values of annual PET (Feddema 2005, adapted by permission of the
publisher (Taylor & Francis Ltd, http://www.tandfonline.com.)).
Thermal type
Annual PET (mm.yr−1)
Torrid
Hot
Warm
Cool
Cold
Frigid
>1500
1200–1500
900–1200
600–900
300–600
0–300
Future shift of thermal zones with respect to the reference period is assessed under SSP1-2.6, SSP2-4.5
and SSP5-8.5 forcing scenarios for the CMIP6 MME. A shift is defined here as an extension or recession of a
thermal climate type or a change in the thermal category occupying a region. We consider the shift robust if
the information is similar when using both methodologies to calculate PET (i.e. Thornthwaite and Hamon).
For a quantitative assessment of the shift in each thermal type, historical and projected changes (i.e.
SSP1-2.6/SSP2-4.5/SSP5-8.5 minus reference period) of land-only area extent are computed over the Africa
continent and for each of the 9 IPCC AR6 sub-regions (as defined in figure 1). The area cover of a climate
type is expressed as percentage of the total Africa area.
3. Results
3.1. Validation
The thermal classification applied to CRU, ERA5 and CMIP6 MME using the Thornthwaite and Hamon PET
methods during the reference period (1985–2014) are intercompared in figure 2. For all methods, robust
features of torrid climate type are observed (in both CRU and ERA5 and for both methodologies) over the
WAF region including the Sahel (i.e. Senegal, Mali, Niger, Chad, Sudan, Burkina Faso) and the northern part
of Gulf of Guinea (i.e. Cˆote d’Ivoire, Ghana, Benin, Togo, Nigeria), the western and central Saharan Desert
(SAH), parts of northern and central East Africa (i.e. NEAF and CEAF) including South Sudan, northeast
Kenya and Horn of Africa. The hot climate type mainly prevails over part of the northeastern SAH (i.e. Libya
and Egypt), Central Africa (i.e. Congo Basin, Congo, Central Africa Republic), in some coastal countries of
ESAF (i.e. Mozambique) and in western Madagascar. The warm climate type is mainly found in the WSAF
region including Angola, Namibia, western Zambia, parts of ESAF (i.e. eastern Zambia, Zimbabwe, southern
Botswana), and eastern Madagascar. The cool climate type occurs in South Africa and the coastal
Mediterranean regions of Africa (MED-AF).
A few discrepancies, however, exist across the results with different datasets and methodologies. The most
notable ones are in the coastal areas of MED-AF and in South Africa, along the coastlines of the Gulf of
Guinea in WAF and over the CAF. In fact, the cool thermal type is less extended in the coastal region of
MED-AF (i.e. in Morocco and northern Algeria) and in South Africa when using the Hamon PET compared
to the Thornthwaite method. In addition, along the coastlines of the Gulf of Guinea (i.e. southern Cˆote
d’Ivoire and southern Ghana), the torrid thermal type prevails based on CRU data while the hot thermal type
is found based on ERA5, indicating that the latter has lower temperatures than CRU. Finally, over Cameroon
and Gabon, using CRU with both methods and ERA5 with Hamon PET yields a hot dominant climate type,
whereas ERA5 along with the Thornthwaite PET produces a prevailing warm thermal type. Note, however,
that these discrepancies are very localized and do not alter the estimated global pattern of thermal zones in
Africa.
The CMIP6 MME yields an overall agreement with CRU and ERA5 in terms of patterns of thermal types
over most of the regions. In particular, it captures the torrid climate type over WAF, western and central SAH
and part of northern and central East Africa (i.e. NEAF and CEAF), including the Horn of Africa. It also
captures the dominant hot thermal type over the CAF, northeastern SAH, the coastal countries of CEAF and
ESAF and the western region of Madagascar. In addition, it reproduces areas of warm climate conditions, and
specifically along the coastal regions of MED-AF, some localized areas in CEAF, most of WSAF and eastern
Madagascar. The cool thermal type observed over South Africa country is also well captured.
The CMIP6 MME also exhibits some biases, such as the extension of the hot type in the northern
portions of the Southern Africa region (i.e. northern WSAF and ESAF), and the lack of cool thermal type in
the coastal MED-AF region (i.e. Morocco and northern Algeria).
These biases in thermal types are evidently related to corresponding biases in surface air temperature.
Figures SM1 and SM2 show the distribution of mean monthly temperature for the CRU observations (figure
SM1(a)), ERA5 reanalysis (figure SM1(b)) and CMIP6 MME (figure SM1(c)) along with the mean monthly
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Figure 2. Distribution of thermal types in the reference period (1985–2014) using Thornthwaite (left panel) and Hamon (right
panel) for CRU observations (a), (b), for ERA5 reanalysis (c), (d) and CMIP6 multimodel ensemble mean (MME) (e), (f).
temperature bias in the CMIP6 MME with respect to the CRU observations (figure SM2(a)) and the ERA5
reanalysis (figure SM2(b)) during the reference period (1985–2014). The CMIP6 MME shows a warm bias in
the northern region of WSAF of up to 1.2 ◦C compared to both the CRU dataset and ERA5 reanalysis. Over
northern ESAF, CMIP6 shows a warm bias compared to both CRU and ERA5 but more pronounced with
respect to the latter. In addition, a warm bias is also present in MED-AF of about 0.5 ◦C with respect to both
the CRU and ERA5 datasets.
For a more quantitative assessment of the CMIP6 MME thermal type simulation during present climate
conditions, the percentage of land-only areas occupied by each thermal type for the reference period in the 9
IPCC AR6 sub-regions and in the entire African continent are intercompared in figure 3 for the CRU
observations, the ERA5 reanalysis and the CMIP6 MME using both the Thornthwaite and Hamon PET
formulations. Figures SM3 and SM4 provide similar information but separately for the Thornthwaite and
Hamon methods.
The CRU observations (ERA5 reanalysis) indicate that the SAH, WAF, CAF, and NEAF (figures 3(a)–(c)
and (e)) are affected by torrid climate over a fractional area of about 18%–23% (16%–20.6%), 7%–7.65%
(5.77%–6.83%), 2.7%–3.65% (3%–3.85%) and 4.35%–5% (4.43%–5%) of total Africa area, respectively. The
corresponding observed hot thermal type fractional cover based on CRU (ERA5) in the same regions exhibit
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Figure 3. Thermal types area extent (in area percent of land-only Africa) in 9 IPCC AR6 sub-regions (a)–(i) and Africa (j) during
reference period (1985–2014) for CRU observation, ERA5 reanalysis and CMIP6 Multi-Model Ensemble (MME) using
Thornthwaite and Hamon potential evapotranspiration (PET) methods. Thor-CRU: Thornthwaite applied to CRU; Thor-ERA5:
Thornthwaite applied to ERA5; Thor-CMIP6: Thornthwaite applied to CMIP6; Ham-CRU: Hamon applied to CRU; Ham-ERA5:
Hamon applied to ERA5; Ham-CMIP6: Hamon applied to CMIP6.
ranges of 5.5%–11% (6.7%–12.3%), 0.65%–1.3% (1.34%–2.52%), 8.9%–10.88% (7.35%–10.42%) and
1.3%–2.33% (1.18%–2%). The warm climate type covers very small fractions of these regions while the cool
climate does not occur except in NEAF (less than 1% total Africa fractional area).
In CEAF, ESAF and WSAF, the warm thermal type fractional cover is 2.7%–3.85% (2.9%–3.25%),
3.85%–4.5% (1.1%–4.33%) and 5.6%–6.57% (6%–7.37%) in CRU observations (ERA5 reanalysis),
respectively. In these three regions, the hot thermal type with respect to total Africa fractional extents are
estimated to be 0.45%–2.3% (0.53%–1.65%), 2.36%–3.85% (1.65%–4.15%) and 1.3%–3.3% (0.4%–2.75%)
in the CRU observations (ERA5 reanalysis), respectively. Over Madagascar and MED-AF, hot and warm
thermal types cover only few areas that do not add up to 3% of total Africa area.
As a result, for the whole Africa (land-only areas) based on the CRU observation dataset (ERA5
reanalysis), torrid, hot and warm climate conditions dominate with a range of 37.77%–47.36%
(35.95%–43.98%), 24.3%–41.35% (22.29%–40.48%) and 18.13%–22.18% (20.03%–25.52%) area extent,
respectively. The proportion of observed cool climate varies between 1.7%–6.13% (3.09%–7.76%). It should
be emphasized that for the whole Africa the climate classification using the Thornthwaite PET shows larger
areas with torrid and warm thermal types compared to the Hamon PET, while this exhibits greater areal
extents occupied by hot climate type.
Compared to the observed thermal-based estimates, the CMIP6 MME captures very well the fraction of
land occupied by each thermal type, with only a few exceptions. For example, over the CAF, the CMIP6
MME slightly overestimates the areas covered by torrid climate types, whereas it somewhat underestimates
the areas under hot climate conditions in both PET formulations. Furthermore, in the SAH, the models
slightly underestimate the torrid thermal type area extent while overestimating the warm thermal type
fraction. Overall, for the whole Africa, the CMIP6 MME captures very well the observed fractional area
extent of each thermal category considering both PET formulation.
In summary, torrid, hot and warm climate conditions are predominant in Africa. These categories are
primarily found in the SAH, WAF, CAF and NEAF regions irrespective of the PET formulation considered.
SAH experiences the largest proportion of areas under torrid climate, followed by WAF, CAF and NEAF, with
greater fractions of hot conditions in CAF. In CEAF, ESAF and WSAF hot and warm conditions prevail with
similar proportions. The CMIP6 MME is able to capture this regional distribution, with small differences
compared to observations over a few regions (i.e. SAH and CAF).
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Figure 4. Distribution of thermal types for reference period (1985–2014; (a), (b)), future SSP1-2.6 (2041–2070; (c), (d)), SSP2-4.5
(2041–2070; (e), (f)), and SSP5-8.5 (2041–2070; (g), (h)) using Thornthwaite potential evapotranspiration (PET) method ((a),
(c), (e), and (g): left panels) and Hamon PET method ((b), (d), (f) and (h): right panels) for CMIP6 Multi-Model Ensemble
(MME).
3.2. Future changes of thermal climate zones
3.2.1. Mid—21st century (2041-2070)
The reference period and projections of thermal climate types under the SSP1-2.6, SSP2-4.5 and SSP5-8.5
forcing scenarios using the Thornthwaite and Hamon PET methods for the mid—century period (i.e.
2041–2070) are shown in figure 4 while figure 5 displays their future shifts with respect to their reference
period. Robust increases in torrid and hot climates spatial extent are observed in all SSP scenarios. Under the
SSP1-2.6, the torrid climate type, initially confined within the Sahara Desert (i.e. SAH), WAF and part of
East Africa (i.e. NEAF and CEAF), extends north into the Mediterranean region and south to cover part of
Central Africa (i.e. northern Congo basin). The Southern Africa region (i.e. WSAF and ESAF), dominated in
present day climate by the warm type, shifts to a hot thermal type in the mid—century, mainly in countries
such as Namibia, part of Angola, southern Botswana and western Zambia.
Under the SSP2-4.5 scenario, the areas covered by torrid thermal type increase towards the south
reaching the central Congo basin and the coastlines of CEAF and ESAF (i.e. Kenya, Tanzania and
Mozambique) at the expenses of hot and warm climate conditions.
Under the most extreme SSP5-8.5 scenario, torrid climate is projected to extend north and south
covering most areas of the continent, with only a few exceptions in localized areas. For example, a large part
of WSAF and ESAF is still dominated by the hot thermal type, the southern part of ESAF still experiences
cool thermal conditions, and the coastal areas of Morocco and Algeria, central Angola, Ethiopian highlands
and East African highlands are still subject to the warm thermal type.
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Figure 5. Spatial shifts of thermal climate zones for future SSP1-2.6 (2041–2070), SSP2-4.5 (2041–2070) and SSP5-8.5
(2041–2070) relative to reference period (1985–2014) using Thornthwaite (a), (c), (e) and Hamon (b), (d), (f) potential
evapotranspiration (PET) method. no chg stands for no change, Ht -> Td: change from hot to torrid, Wm -> Td: change from
warm to torrid, Wm -> Ht: change from warm to hot, Cl -> Wm: change from cool to warm.
There is thus a robust recession of hot and warm climates and an extension of the torrid climate type
with increasing GHG concentrations already at mid—century. In fact, the Mediterranean region, part of
CEAF and most countries in WSAF, ESAF and Madagascar progressively shift from warm to hot while the
remaining areas of the Sahara Desert and most part of central Africa (SAH and CAF) gradually shift from hot
to torrid (i.e. figure 5). Note that there are some differences between results with the Thornthwaite and
Hamon methods. For instance, for the low forcing scenario (i.e. SSP1-2.6), the northward and southward
displacement of torrid climate boundaries is more extended when using Thornthwaite compared to Hamon,
while for the higher forcing scenario (i.e. SSP5-8.5) the spatial extension of climate type change is similar
with both methods.
3.2.2. Late 21st century (2071-2100)
Figure 6 presents the reference period and projected climate types under the SSP1-2.6, SSP2-4.5 and SSP5-8.5
scenarios using the Thornthwaite and Hamon PET methods for the late 21st century period (i.e. 2071–2100).
Figure 7 shows the corresponding shifts from one climate type to another. Similar patterns of changes are
projected as for the mid—century period, with larger spatial expansion of areas under torrid conditions. For
example, under the SSP1-2.6 scenario in the Congo basin (in CAF) the torrid climate has wider spatial extent
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Figure 6. Distribution of thermal types for reference period (1985–2014; (a), (b)), future SSP1-2.6 (2071–2100; (c), (d)), SSP2-4.5
(2071–2100; (e), (f)), and SSP5-8.5 (2071–2100; (g), (h)) using Thornthwaite potential evapotranspiration (PET) method ((a),
(c), (e), and (g): left panels) and Hamon PET method ((b), (d), (f) and (h): right panels) for CMIP6 Multi-Model Ensemble
(MME).
compared to mid—century. For the SSP2-4.5 scenario, more areas in southern Africa (i.e. WSAF and ESAF)
experience torrid climate, including northern Botswana, compared to the mid—century.
In the SSP5-8.5 scenario, torrid climate is projected to cover almost the entire Africa continent, stretching
to the coastal Mediterranean region in the north and to WSAF and ESAF in the south. The coastal areas of
South Africa previously dominated by cool thermal type during the reference period and the mid—century,
shift to warm climate type during the late 21st century.
These changes in thermal types are closely related to corresponding changes in near-surface temperatures
pattern. Figure SM5 presents the distribution of the late 21st century change in annual mean, maximum and
minimum temperature climatologies for SSP1-2.6, SSP2-4.5 and SSP5-8.5 scenarios. The annual mean,
maximum and minimum temperatures display a spatial pattern of a uniform increase of about 0.5 ◦C under
SSP1-2.6. Under SSP2-4.5, maximum increase of 3 ◦C–3.5 ◦C is found over the Sahara, the Mediterranean
and Sahelian regions of West Africa and a minimum increase of 1.5 ◦C–2 ◦C is found over southern part of
West Africa, Central Africa and part of East Africa (WAF, CAF, CEAF and NEAF). We also note a
considerable warming of 3 ◦C prevailing for maximum temperature over the southern part of the continent
(i.e. WSAF and ESAF). Under SSP5-8.5, the larger increase of up to 5 ◦C is projected over most of the regions
except in West and Central Africa regions that will face an increase in minimum temperature of about 4 ◦C.
As expected, areas of pronounced temperature changes coincident with areas of shift toward hottest thermal
type.
Overall, there is a clear shift of thermal climate zones toward the torrid thermal type across Africa
throughout the 21st century, with this expansion covering more regions during the late century compared to
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Figure 7. Spatial shifts of thermal climate zones for future SSP1-2.6 (2071–2100), SSP2-4.5 (2071–2100) and SSP5-8.5
(2071–2100) relative to reference period (1985–2014) using Thornthwaite (a), (c), (e) and Hamon (b), (d), (f) potential
evapotranspiration (PET) method. no chg stands for no change, Ht -> Td: change from hot to torrid, Wm -> Td: change from
warm to torrid, Wm -> Ht: change from warm to hot, Cl -> Wm: change from cool to warm.
the mid—century. At the end of the century and under the high-end fossil fueled development pathway (i.e.
SSP5-8.5), torrid climate occupies almost the entire Africa continent, except for localized areas, e.g. in the
southern part of South Africa. In fact, the Mediterranean region, a large part of CEAF and most countries in
southern Africa (i.e. WSAF and ESAF) and Madagascar steadily shift from warm to torrid. In addition, the
shift from hot to torrid in many areas of the central, eastern and northern Sahara Desert and most part of
central Africa (SAH and CAF) during the mid—century increasingly expands spatially in the late 21st
century.
These findings are partly consistent with previous findings by Elguindi et al (2014) and Sylla et al
(2016b), who projected an extension of torrid climate into central and parts of southern Africa using CMIP5
and in West Africa using CORDEX, respectively.
3.3. Projected areal extent
To quantify the shift in thermal zones, the change in area extent of each thermal type is assessed over the 9
Africa sub-regions and the whole African continent (land only) in figures 8 and 9, which present the changes
in the fractional cover of different thermal types using the Thornthwaite method under the SSP1-2.6,
SSP2-4.5 and SSP5-8.5 scenarios in the mid (2041–2070; figure 8) and late century (2071–2100; figure 9).
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Figure 8. Mid—21st century change (SSP1-2.6/SSP2-4.5/SSP5-8.5: 2041–2070 minus reference period: 1985–2014) of area extent
(in area percent of land-only Africa) occupied by each thermal type for 9 IPCC AR6 sub-regions (a)–(i) and Africa continent
(j) using Thornthwaite potential evapotranspiration (PET) method. The red crosses indicate the outliers.
Figure 9. Late 21st century change (SSP1-2.6/SSP2-4.5/SSP5-8.5: 2071–2100 minus reference period: 1985–2014) of areas extent
(in area percent) occupied by each thermal type for 9 IPCC AR6 sub-regions (a)–(i) and Africa continent (j) using Thornthwaite
potential evapotranspiration (PET) method. The red crosses indicate the outliers.
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The unit for the changes is percentage of total land area of the whole Africa continent. Results using the
Hamon method are not shown as they show similar changes compared to Thornthwaite (see supplementary
materials figures SM6 and SM7). Our results first indicate that the sign of change is the same across all SSP
scenarios and the changes are greater under the SSP5-8.5 scenario than the SSP2-4.5 and SSP1-2.6.
Figure 8 shows a general increase in area extent of torrid thermal type across all regions and scenarios
with the largest increases projected in SAH (8.25%) and CAF (5.45%) under SSP5-8.5. We also find a
decrease in fractional area cover of hot and warm types of about −5.15%% and −3.08% in SAH and
−3.25% and −2.2% in CAF, respectively, for the same scenario and period. Another notable feature is the
slight increase in area extent of both torrid and hot thermal types and a decrease of warm climate type area
coverage over the ESAF and WSAF regions. In the MDG and MED-AF regions, the changes are small and not
significant. Considering the whole Africa continent, there is an increase of torrid thermal type area extent in
the range of 15.2%–27%, whilst hot and warm climate area coverages decrease by 6.7%–1.35% and
16.56%–11.12%, respectively. As expected, the increase and decrease in area extent are larger under the high
end SSP5-8.5 scenario compared to the other two.
Figure 9 shows similar changes as in figure 8, but with larger magnitudes. For example, the change in the
proportion of torrid climate type during the late 21st century reaches 9.45%% in SAH and 7.85% in CAF.
The corresponding decreases in hot and warm conditions area coverage are 6.2% and 3.2% in SAH and
5.31% and 2.55% in CAF under the SSP5-8.5. In addition, over WSAF and ESAF a moderate spatial
expansion of torrid and hot climates and a recession of warm areas prevail as in the mid—century but again
with larger values. Consequently, considering the whole Africa continent, the torrid climate type increases in
spatial extent by up to ∼16% for SSP1-2.6, 28% for SSP2-4.5 and 42% under SSP5-8.5 scenario at the
expenses of warm and hot zones.
Overall over Africa, warm and hot thermal types are projected to be progressively converted into the
torrid thermal type across almost all land areas as GHG concentrations increase into the 21st century. By
mid—century, the torrid climate type using both Thornthwaite and Hamon PET estimates covers a total
fractional area of the African continent in the range of 49%–60%, 53%–65% and 61%–72% under SSP1-2.6,
SSP2-4.5 and SSP5-8.5. At the end of the century, Africa will be covered by the torrid climate type at
50%–61%, 62%–73% and 81%–87% under SSP1-2.6, SSP2-4.5 and SSP5-8.5, respectively.
4. Conclusion and discussion
In this paper, we investigated the shift in thermal climate zones over Africa during the mid (2041–2070) and
late 21st century (2071–2100) with respect to the reference period (1985–2014) under three SSP forcing
scenarios: a low forcing scenario (SSP1-2.6), a medium forcing scenario (SSP2-4.5) and a high-end forcing
scenario (SSP5-8.5). This was achieved using multiple PET temperature-based estimation methods, the
Thornthwaite and Hamon methods, applied to CRU observations, the ERA5 reanalysis and the CMIP6 MME
mean (MME).
The MME is able to reproduce the general thermal conditions of the African continent, although it
overestimates the spatial extent of hot climate zones in the northern part of Southern Africa region
compared to the observation and reanalysis. In particular, among the well simulated thermal types by the
MME we identified the torrid climate type of West Africa (i.e. Sahel and Gulf of Guinea), western and central
Sahara Desert (SAH), and part of north and central Eastern Africa (i.e. NEAF and CEAF), the hot climate
type in central Africa, northeastern SAH and western Madagascar, the warm and cool types in some portions
of the coastal Mediterranean of Africa (MED-AF) and CEAF, most part of WSAF and eastern Madagascar,
and the cool type characterizing the South Africa country.
Overall in Africa, torrid, hot and warm climates are the dominant thermal conditions, with the torrid
type exhibiting the largest fractional area cover and in general, the CMIP6 MME is able to capture this area
distribution.
Projected changes in CMIP6 MME exhibit future shifts of thermal types consistent across methods but
with different magnitudes. Overall in Africa, the greatest expansion occurs in the torrid thermal type using
the Thornthwaite (Hamon) PET estimate with an increase, expressed in % of total Africa land area, ranging
from 16% (14.3%) under the SSP1-2.6%–28% (26.33%) under the SSP2-4.5 and 42% (45.6%) under the
SSP5-8.5 by the end of the 21st century. Such expansion occurs at the expenses of hot and warm thermal
types. Therefore, by the end of the century the fraction of Africa covered by torrid climates will be, according
to the Thornthwaite (Hamon) PET estimate, 61% (50%), 73% (62%) and 87% (81.3%), the fraction covered
by hot climate 21.3% (38.7%), 16% (31.3%) and 9% (16.2%) and by warm climate 15% (10.5%), 9.7%
(6.25%) and 4% (2.4%), under the SSP1-2.6, SSP2-4.5 and SSP5-8.5, respectively. Corresponding values for
mid—century are 60% (49.1%), 65% (53.7%) and 72% (61.3%) for torrid climate, 21.65% (39.27%),
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A Dajuma et al
19.75% (36.9%) and 16.3% (31.7%) for hot climate, and 15.5% (10.75%), 12.9% (8.7%) and 10% (6.48%)
for warm climate.
The most important climate shifts occur in the Mediterranean region, CEAF, southern Africa and
Madagascar. These areas initially warm will become mostly hot by mid—21st century and overall torrid by
late 21st century. In addition, areas in central Africa, particularly the Congo basin previously hot will become
torrid by the end of the 21st century.
It is important to note that although the multimodel ensemble of the 40 CMIP6 GCMs shows a good
performance in simulating temperature and PET, these results are still subject to some uncertainties. They
can be originated from future emissions, internal climate variability and inter-model difference (RÄISÄNEN
2007, Knutti and Sedlácˇek 2013, Northrop and Chandler 2014, Zhang and Chen 2021). Nevertheless, the use
of a large ensemble (i.e. 40 CMIP6 GCMs here) provides more robust results (Semenov and Stratonovitch
2010, Gao et al 2019, Doi & Kim 2022).
From our study, it is clearly evident that anthropogenic climate change will cause a shift of thermal zones
and further displace the boundaries of the hottest thermal types over all African regions during the mid and
late 21st century. In particular, most of the continent will be subject to torrid climate conditions by the end
of the 21st century in the SSP5-8.5 scenario.
The projected shifts (i.e. extention of torrid thermal zones and recession of hot, warm and cool thermal
types) will result in a disruption of existing ecosystem structure and a loss of biodiversity (Mahlstein et al
2013, Román-Palacios and Wiens 2020). In fact, climate controls the distribution of species ranges and rate
of primary productivity (Grimm et al 2013, Elguindi et al 2014).
Another implication of our results is an increase in the level of heat stress for human, crops and animals.
Shifting towards the hottest thermal climate type can substantially affect the most vulnerable population to
heat stress, i.e. outdoor intensive workers in sectors such as agriculture, forestry, fishing, construction, and
utility suppliers (Xiang et al 2014, Acharya et al 2018, Moda et al 2019). In addition, changes towards hottest
thermal climate conditions in regions previously favorable to farming, can cause heat stress on crops
resulting in loss of crop production (Teixeira et al 2013). Finally, as livestock cattles substantially contributing
to food production, grow in specific climate conditions, the expansion of the hottest thermal type can trigger
heat stress on livestock in previously favorable regions, resulting in an alteration of the livestock production
systems (Salem et al 2011, Renaudeau et al 2012).
A final concern is the energy demand and consumption and more generally the design for
energy-efficient buildings. Because of the displacement of boundaries of the torrid thermal type, some
regions will experience novel climate types and unprecedented warming, leading to an increase in energy
demand for African countries that already struggle to meed to the present-day demand. Our results thus call
for an update in the design of buildings for more efficient energy savings.
An assessment of the impacts of these thermal zones shifts on heat stress for human, crops and animals as
well as energy demand and availability will be presented in subsequent papers.
Data availability statement
The data that support the findings of this study are available upon request from the authors.
Acknowledgments
This work was funded by a grant from the African Institute for Mathematical Sciences, www.nexteinstein.
org, with financial support from the Government of Canada, provided through Global Affairs Canada, www.
international.gc.ca, and the International Development Research Centre, www.idrc.ca.
Mansour Almazroui was funded by Institutional Fund Projects under grant no. (IFPIP: 1195-155-1443)
and gratefully acknowledge technical and financial support provided by the Ministry of Education and King
Abdulaziz University, DSR, Jeddah, Saudi Arabia.
The CMIP6 data used in this study are available from https://esgf-node.llnl.gov, CRU data from www.
cru.uea.ac.uk/data and ERA5 data from https://cds.climate.copernicus.eu. The authors are thankful to the
modeling centers that contributed to the CMIP6 and developers of CRU and ERA5 for providing datasets to
the research community.
Funding
This work was funded by the African Institute for Mathematical Sciences (AIMS) and the International
Development Research Centre (IDRC).
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A Dajuma et al
Conflict of interest
The authors declare no competing interests.
ORCID iDs
Alima Dajuma https://orcid.org/0000-0002-4422-8256
Arona Diedhiou https://orcid.org/0000-0003-3841-1027
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18
| null |
10.1038_s41586-023-06157-7.pdf
|
Data availability
The authors declare that the data supporting the findings of this study
are available within the paper and its supplementary information files.
Sequencing data are available from the Sequencing Read Archive under
BioProject identifiers PRJNA602546 and PRJNA867730. The raw data
and all other datasets generated in this study are available from the
corresponding authors upon reasonable request. Source data are pro-
vided with this paper.
Code availability
Scripts and pipelines used for all sequencing data analysis and for image
analysis are available at the GitHub online repository (https://github.
com/chengzhongzhangDFCI/nature2023)47.
|
Data availability The authors declare that the data supporting the findings of this study are available within the paper and its supplementary information files. Sequencing data are available from the Sequencing Read Archive under BioProject identifiers PRJNA602546 and PRJNA867730 . The raw data and all other datasets generated in this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper. Code availability Scripts and pipelines used for all sequencing data analysis and for image analysis are available at the GitHub online repository ( https://github. com/chengzhongzhangDFCI/nature2023 ) 47 .
|
Heritable transcriptional defects from
aberrations of nuclear architecture
https://doi.org/10.1038/s41586-023-06157-7
Received: 22 December 2021
Accepted: 2 May 2023
Published online: 7 June 2023
Open access
Check for updates
Stamatis Papathanasiou1,2,11 ✉, Nikos A. Mynhier1,2,14, Shiwei Liu2,12,14, Gregory Brunette1,2,
Ema Stokasimov1,2, Etai Jacob3,4,13, Lanting Li3,4,5, Caroline Comenho6,7,8, Bas van Steensel9,
Jason D. Buenrostro6,7,8, Cheng-Zhong Zhang3,4,5,6 ✉ & David Pellman1,2,3,6,10 ✉
Transcriptional heterogeneity due to plasticity of the epigenetic state of chromatin
contributes to tumour evolution, metastasis and drug resistance1–3. However, the
mechanisms that cause this epigenetic variation are incompletely understood. Here
we identify micronuclei and chromosome bridges, aberrations in the nucleus common
in cancer4,5, as sources of heritable transcriptional suppression. Using a combination
of approaches, including long-term live-cell imaging and same-cell single-cell RNA
sequencing (Look-Seq2), we identified reductions in gene expression in chromosomes
from micronuclei. With heterogeneous penetrance, these changes in gene expression
can be heritable even after the chromosome from the micronucleus has been
re-incorporated into a normal daughter cell nucleus. Concomitantly, micronuclear
chromosomes acquire aberrant epigenetic chromatin marks. These defects may
persist as variably reduced chromatin accessibility and reduced gene expression after
clonal expansion from single cells. Persistent transcriptional repression is strongly
associated with, and may be explained by, markedly long-lived DNA damage. Epigenetic
alterations in transcription may therefore be inherently coupled to chromosomal
instability and aberrations in nuclear architecture.
Nuclear atypia, which encompasses aberrations in nuclear size and
morphology, is a hallmark feature of many tumours that is commonly
used to assign tumour grade and predict patient prognosis4,6,7. Recently,
our group and others demonstrated that structural abnormalities of
the nucleus—micronuclei or chromosome bridges—can lead to various
simple and complex chromosomal rearrangements, including chromo-
thripsis8–11. This process is an extensive form of chromosome fragmen-
tation and rearrangement that is common in cancer12–14. Although the
role of nuclear abnormalities in the generation of genetic instability
is now appreciated, other consequences of nuclear atypia have been
little studied. For example, although micronuclei can have transcription
defects and altered chromatin marks15–17, the functional consequences
of these alterations remain unclear.
Transcriptome analysis by Look-Seq2
Micronuclei form from mis-segregation of intact chromosomes or
acentric chromosome fragments. In the first cell cycle after the for-
mation of the micronucleus (hereafter termed generation 1), >50% of
micronuclei undergo nuclear envelope (NE) rupture and acquire DNA
damage15,18,19, which is partly explained by a pathological form of DNA
base excision repair20. There is a second wave of DNA damage that can
occur on any of these chromosomes when the cell enters mitosis, even
if the NE of the micronucleus remains intact until mitotic entry11. After
cell division, the micronuclear chromosome (MN chromosome) can
remain in the cytoplasm and reform a micronucleus, be re-integrated en
bloc into one daughter cell nucleus or have fragments re-incorporated
into both daughter nuclei8,18. End joining of chromosome fragments
in daughter nuclei generates chromothripsis12,21.
A direct assessment of the transcriptional consequences of micro-
nucleation requires single-cell transcriptome analysis, which we
performed with a modified method for live-imaging and single-cell
whole-genome sequencing8,11 (Methods). We induced chromosome
mis-segregation and generated micronucleated RPE-1 cells using a
nocodazole-induced mitotic block and release procedure8. We assessed
the loss of micronuclear NE integrity by live-cell imaging (Supplemen-
tary Videos 1 and 2). Micronucleated cells or their daughter cells were
then isolated for transcriptome analysis22 (Extended Data Fig. 1a).
Initially, we isolated cells using the approach we used for single-cell
whole-genome sequencing8,11. However, for most of the experiments
in this study, we developed an improved contact-free laser capture
microdissection23 method (Extended Data Fig. 1b and Methods). The
updated capture method is optimized for isolating cells with minimal
perturbation and, because of an in-house fabricated culture chamber,
1Department of Cell Biology, Blavatnik Institute, Harvard Medical School, Boston, MA, USA. 2Department of Pediatric Oncology, Dana-Farber Cancer Institute, Boston, MA, USA. 3Single-Cell
Sequencing Program, Dana-Farber Cancer Institute, Boston, MA, USA. 4Department of Data Sciences, Dana-Farber Cancer Institute, Boston, MA, USA. 5Department of Biomedical Informatics,
Blavatnik Institute, Harvard Medical School, Boston, MA, USA. 6Broad Institute of MIT and Harvard, Cambridge, MA, USA. 7Department of Stem Cell and Regenerative Biology, Harvard
University, Cambridge, MA, USA. 8Gene Regulation Observatory, Broad Institute of MIT and Harvard, Cambridge, MA, USA. 9Division of Gene Regulation and Oncode Institute, The Netherlands
Cancer Institute, Amsterdam, The Netherlands. 10Howard Hughes Medical Institute, Chevy Chase, MD, USA. 11Present address: Institute of Molecular Biology, Mainz, Germany. 12Present address:
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA. 13Present address: AstraZeneca, Waltham, MA, USA. 14These authors contributed equally: Nikos A.
Mynhier, Shiwei Liu. ✉e-mail: [email protected]; [email protected]; [email protected]
184 | Nature | Vol 619 | 6 July 2023
Articleit is also optimized for the isolation of daughter cells, sister cells or
niece cells of the micronucleated cell. We refer to this method, using
either type of cell capture technique, as Look-Seq2.
(Extended Data Fig. 4i). Together, these data demonstrate that almost
all newly generated micronuclei—ruptured or intact—exhibit defective
transcription.
To assess micronucleation-induced transcriptional changes, we
needed to identify the chromosome that was in the micronucleus,
determine the copy number of this chromosome and then compare the
transcriptional output of this chromosome to the expectation based on
the DNA copy number (Extended Data Fig. 1c). These goals were accom-
plished using haplotype-resolved transcriptome analysis of Look-Seq2
data of the cell of interest combined with transcriptome analysis of its
family members (Methods, Supplementary Table 1 and Extended Data
Figs. 1 and 2). Haplotype-resolved transcriptome analysis correctly
identified clonal 10q trisomy (based on a 2:1 allelic imbalance) and the
low transcription output from the inactive X chromosome in female
RPE-1 cells (Extended Data Fig. 1d).
We next needed to identify the MN chromosome and determine its
copy number, which sets the expectation for the normal transcription
output of that chromosome. The identity of the MN chromosome was
inferred from the pattern of mis-segregation, which we determined
from the transcriptomes of the family members of the micronucle-
ated cell. Because the family member cells have normal nuclei, their
transcription output is proportional to DNA copy number24. Once the
chromosome content of the family members is known, the pattern
of mis-segregation that generates the micronucleated cell can be
deduced, which then enables the determination of the copy number
of the chromosome in the micronucleus (Extended Data Fig. 2a and
Methods). As an example, monosomic transcription in the sister of a
micronucleated cell indicated that the micronucleated cell has to be
trisomic for that chromosome (a 1:3 segregation; Fig. 1a). This solves the
problem of assigning DNA copy number without making assumptions
about whether the chromosome from the micronucleus is normally
transcribed or not (Methods).
Transcription defects in micronuclei
As an initial validation of Look-Seq2, we analysed micronuclei con-
taining acentric 5q chromosomal arms generated by CRISPR–Cas9
cleavage25. We focused on micronuclei that had undergone NE rup-
ture because NE rupture causes abrupt transcriptional silencing15. We
identified the transcriptional defects expected from partitioning of
Cas9-generated acentric fragments into micronuclei25 (Extended Data
Fig. 2b). As further validation, we generated micronuclei by random
whole chromosome mis-segregation and confirmed near-complete
transcriptional silencing of chromosomes from micronuclei after NE
rupture (Fig. 1b, Extended Data Fig. 3a,b and Supplementary Table 2).
We further used Look-Seq2 to assess transcription before NE rup-
ture (generation 1) and found that most intact micronuclei exhibited
significant transcriptional suppression (Fig. 1c and Extended Data
Fig. 3c). Among 11 intact micronuclei, 2 were inferred to have normal
transcription, 1 showed a partial defect and the rest showed signifi-
cantly reduced transcription or near-complete transcriptional silencing
(Fig. 1d, Extended Data Fig. 3a and Supplementary Table 2). Defective
transcription from both intact and ruptured micronuclei was confirmed
by fluorescence intensity (FI) measurements for a marker of active,
phosphorylated RNA polymerase II (RNAP2-Ser5ph; Extended Data
Fig. 4). The transcription defect of intact micronuclei was evident from
the beginning of interphase (Fig. 1e and Extended Data Fig. 4b–d),
and the degree of RNAP2-Ser5ph loss was positively correlated to the
extent of the defect in nuclear pore complex assembly (Fig. 1f and
Extended Data Fig. 4h). Our previous studies demonstrated that the
defect in assembly of the nuclear pore complex is itself correlated
with micronuclear defects in nuclear import26,27. Consistent with the
idea that micronuclei lack the normal complement of transcription
machinery proteins, CDK9 and CDK12, which are both required for tran-
scription elongation, exhibited reduced recruitment to micronuclei
The transcriptional defects in MN chromosomes correlated with
alterations in epigenetic chromatin marks. There was a modest increase
in the repressive marks histone 3 lysine 9 dimethylation (H3K9me2)
and histone 3 lysine 27 trimethylation (H3K27me3) that accumulated
on a subset of micronuclei with NE disruption late during interphase
(Extended Data Fig. 5a,b), a result consistent with a previous report16.
Moreover, micronuclei exhibited loss of the active chromatin marks
histone 3 lysine 27 acetylation (H3K27ac) and histone 3 lysine 9 acetyla-
tion (H3K9ac)15,16 from the beginning of interphase, which correlated
with reductions in the level of active RNAP2 (Fig. 1g and Extended Data
Fig. 5c). This highly penetrant loss of H3K27ac is notable because recent
studies have indicated that recovery of H3K27ac is essential for the
normal reestablishment of transcription after mitosis28–30. Multiple
factors probably contribute to the transcription defects of micronu-
clei because inhibition of HDACs partially restored H3K27ac, but it
was not sufficient to rescue the levels of RNAP2-Ser5ph (Extended
Data Fig. 5d).
In summary, both intact and ruptured micronuclei exhibit transcrip-
tional defects and chromatin alterations. The correlated acquisition of
altered chromatin states raised the possibility that the transcription
defects could be inherited.
Heritable transcription defects
After cell division, around 40% of MN chromosomes are incorporated
into newly formed daughter cell nuclei (generation 2; Fig. 2a). To deter-
mine whether the transcription defects in MN chromosomes can persist
even in a normal daughter cell nuclear environment, we performed
Look-Seq2 analysis on 37 pairs of daughter cells with re-incorporated
MN chromosomes (generation 2, termed MN daughters). The average
time interval from chromosome re-incorporation until cell isolation
was 16 h, which substantially exceeded the time required for normal
chromosomes to recover transcription after mitosis (about 90 min)28–30.
We also sequenced one (7 out 37) or both daughters (22 out 37) of the
MN sister cell (MN nieces). The MN nieces provide the same informa-
tion about the segregation of the MN chromatid as the generation 1
sister cell and, when it was possible to isolate both nieces, provide the
information in biological replicate (Extended Data Fig. 2a). Eight of the
37 MN daughter pairs were processed using our old capture method
and lacked contemporaneous isolation of the nieces. We were never-
theless able to infer the transcription status of the re-incorporated
MN chromosome based on patterns observed in the samples
with MN nieces (Methods, Supplementary Table 2 and Extended
Data Fig. 6).
There was heterogeneous transcriptional recovery of the MN chro-
mosomes that were re-incorporated into daughter cell nuclei. Among
44 re-incorporated MN chromosomes, 12 (27%) exhibited a significant
reduction or near-complete loss of transcription (Fig. 2b,c, Extended
Data Fig. 6 and Supplementary Table 2). Reduced transcription cannot
be explained by interspersed DNA losses associated with chromoth-
ripsis because transcriptional reduction seemed to be uniform across
the chromosome (Extended Data Fig. 7). Moreover, we inferred recipro-
cal distributions in fragments of the MN chromosome into both MN
daughters in four cases and calculated the transcriptional output of
the re-incorporated MN chromosome as the combined transcription
from both daughters. In 3 out of 4 cases, the combined transcription
of re-incorporated fragments still showed a significant reduction (nor-
malized transcriptional output of about 0.18–0.38).
These single-cell transcriptome data indicated that a subset of MN
chromosomes acquire heritable transcription defects. The frequency
of this defect was probably underestimated because we assessed
transcription averaged across 10 Mb bins and were not able to detect
Nature | Vol 619 | 6 July 2023 | 185
a
Mitosis
1:3
segregation
MN sister
G1
G2
MN cell
2:2
segregation
PN
MN
PN
PN
MN
PN
PN
MN
PN
PN
MN
PN
Normalized transcription of each homologue
A
B
NE disruption
10q duplication
Inactive X
b
3
2
1
0
3
2
1
0
Chr. 1
Chr. 2
Chr. 3
Chr. 4
Chr. 5
Chr. 6
Chr. 7
Chr. 8
Chr. 10b
Chr. 17
Chr. 16
Chr. 18
Chr. 15
Chr. 14
Chr. 13
Chr. 10a
Chr. 9
Chr. 11
Chr. 12
Chr. 20
Chr. 19
Chr. 21
Chr. 22
Chr. X
NE intact
c
3
2
1
0
3
2
1
0
Chr. 1
Chr. 2
Chr. 3
Chr. 4
Chr. 5
Chr. 6
Chr. 7
Chr. 8
Chr. 10a
Chr. 10b
Chr. 11
Chr. 9
Chr. 14
Chr. 13
Chr. 12
Chr. 16
Chr. 15
Chr. 20
Chr. 19
Chr. 18
Chr. 17
Chr. 22
Chr. 21
Chr. X
d
)
%
(
i
l
e
c
u
n
o
r
c
M
i
n =11
n =10
Normal
Defective
Silencing
P < 0.0001
f
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:
N
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2
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:
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p
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e
S
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1.5
1.0
0.5
0
Intact
NE
disruption
2 h
6 h
23 h
100
80
60
40
20
0
Spearman’s correlation = 0.83
1.5
1.0
0.5
0
g
)
:
N
P
N
M
(
c
a
7
2
K
3
H
3.0
2.5
2.0
1.5
1.0
0.5
0
0
0.5
RNAP2-Ser5ph (MN:PN)
1.0
P < 0.0001
Spearman’s correlation = 0.81
3
2
1
0
)
:
N
P
N
M
(
c
a
7
2
K
3
H
2 h
23 h
0
1
RNAP2-Ser5ph (MN:PN)
Fig. 1 | Transcription defects in newly generated micronuclei. a, Two patterns
of chromosome segregation that generate a micronucleated cell (MN cell)
and its sister (MN sister). Filled magenta shapes indicate the mis-segregated
chromatid in the micronucleus (MN) and its sister chromatids in the primary
nucleus (PN); open magenta shapes indicate sister chromatids of the other
homologue. Top, a 1:3 mis-segregation generates monosomy in a MN sister cell
and trisomy in a MN cell. Bottom, a 2:2 segregation generates disomy in both
cells. In G2 (when cells were isolated), chromosomes in the primary nucleus are
replicated but the MN chromatid is poorly replicated. Lollipops represent
transcripts (open circles for transcripts from the normal homologue; filled
circles for transcripts from the MN homologue; dashed lines for transcripts from
the MN chromatid). b, Normalized transcription yield of each chromosome in a
MN cell and sister cell pair after 1:3 mis-segregation. Filled and open bars indicate
transcription from different homologues assessed by the parental haplotypes;
filled magenta bars for the MN homologue (Chr.2B); open magenta bars for the
normally segregated homologue (Chr.2A). Monosomic transcription of Chr.2B
in the MN cell (bottom) is from the normally segregated chromatid in the primary
nucleus and indicates near-complete silencing of the MN chromatid.
c, Chromosome-wide silencing of an intact micronucleus generated by a
2:2 segregation, similar to b. The MN homologue is Chr.1B. d, Summary of
transcription output in 21 MN cell–MN sister pairs, grouped by the status of MN
nuclear envelope (NE) integrity. See also Supplementary Table 2 and Extended
Data Fig. 3 for more information related to b–d e, RNAP2-Ser5ph signal (MN:PN
ratios of background normalized fluorescent intensities) at the indicated
time points after MN formation (left to right, n = 644, 212 and 605 from 2 or 3
experiments). Boxes indicate median ratio with a 95% confidence interval
(CI), P values from two-tailed Mann–Whitney test. f, Correlation between
micronuclei transcription (RNAP2-Ser5ph intensity) and nuclear pore complex
density (POM121, 2 h after mitotic shake-off), (n = 334 from 3 experiments).
Two-tailed Spearman’s correlation. g, Left, MN:PN ratios for H3K27ac (left to
right, n = 187 and 118 from 2 experiments), analysed as in e. Right, correlation
between H3K27ac and RNAP2-Ser5ph signals (2 h after shake-off; n = 187 from
2 experiments). Two-tailed Spearman’s correlation.
transcriptional aberrations at the level of individual genes. We devel-
oped single-cell imaging approaches to both verify and further study
these heritable defects in transcription.
Visualization of nascent transcripts
We adapted the U2OS 2-6-3 nascent transcription reporter system31
to assess the transcriptional activity of re-incorporated MN chromo-
somes. The 2-6-3 transcription reporter construct contains lac operator
arrays, which enabled visualization of the reporter locus on chromo-
some 1. The reporter also contains an inducible mRNA containing
MS2 aptamers, which enabled visualization of inducible nascent
transcripts. We induced random micronucleation in this cell line, and
cells with micronuclei containing the chromosome 1 reporter were
identified by imaging (under these conditions, the most frequently
mis-segregated chromosome is chromosome 1 (refs. 18,32)). After
the division of these micronucleated cells, we identified examples of
chromosome 1 re-incorporation into daughter cells. Transcriptional
activity of the reporter locus was assessed qualitatively by measuring
the presence or absence of the MS2-containing transcript (Fig. 2d and
Supplementary Video 3). We also quantitatively assessed transcrip-
tional activity by measuring the FI of marked nascent transcripts using
186 | Nature | Vol 619 | 6 July 2023
Article
MN nieces
b
Normalized transcription per chromosome
A
B
a
PN
MN
PN
MN sister
1:3
segregation
MN cell
N1
N2
D1
D2
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1
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at –13 h
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n = 10 n = 34
Defective
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n = 31
n = 39
d
e
t
a
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o
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o
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a
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60
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disruption
e
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d
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(
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o
s
o
m
o
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c
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100
80
60
40
20
0
g
–6
–4
–2
0
Time (h)
2
4
6
8
10
Late G2
rupture
at –1 h
Mitosis
Recovered
–6
–4
–2
2
0
Time (h)
4
6
8
10
t = 26.8 h
33.5 h
36.5 h
4
0 h
40.5 h
e
g
r
e
M
P
A
N
S
-
I
c
a
L
l
o
a
H
-
P
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M
Fig. 2 | Variably penetrant memory of MN chromosome transcription
compromise after re-incorporation into a normal nucleus. a, Example of
copy number and transcriptional yield after two generations following a 1:3
mis-segregation in generation 1. The MN sister cell generates two monosomic
MN nieces (N1 and N2), whereas the MN cell generates MN daughters (D1 and D2).
Only one MN daughter is trisomic because the MN chromatid is poorly replicated.
See Extended Data Fig. 2a for the outcome of 2:2 mis-segregations. b, Near-
complete loss of transcription of a re-incorporated MN chromosome 5 (magenta)
after a 1:3 mis-segregation in generation 1. Shown are the normalized transcription
yields as in Fig. 1b. Chromosome 13 (green) underwent a 2:2 mis-segregation in
generation 1 and displays transcription recovery after re-incorporation. See
also Extended Data Fig. 7. c, Transcription output of 44 re-incorporated MN
chromosomes from 37 families using Look-Seq2. Defective indicates a significant
reduction in the transcriptional yield (Methods, Supplementary Table 2 and
Extended Data Fig. 6). d, Transcription status of the U2OS 2-6-3 transcription
reporter (n = 70 from 13 experiments). Defective indicates little or no visible
MCP–Halo signal. e, Example of defective MN chromosome transcription after
re-incorporation. Grey line indicates the mean and s.e.m. of the FI in controls
(normal nuclei; n = 23 LacI reporters; Extended Data Fig. 8a). Red horizontal
line indicates minimum detectable value in the controls. Black line indicates
reporter transcription in a ruptured MN (no detectable generation 1 signal)
that does not reach a normal level after re-incorporation (generation 2).
f, Example of full transcription recovery, analysed as in e. Red vertical line
indicates the time point of MN nuclear envelope rupture. g, Best single focal
plane confocal images from a time-lapse series showing defective transcription
after re-incorporation. Green, GFP–H2B; blue triangles, reporter locus; magenta
triangles, MS2 reporter expression; open arrowheads, cell that enters the field,
providing an adventitious MCP–Halo bleaching control. Scale bar, 5 µm.
an automated time-lapse image analysis pipeline (Fig. 2e,f, Extended
Data Fig. 8 and Methods).
Consistent with our Look-Seq2 data, imaging of nascent transcripts
confirmed that a subset of re-incorporated MN chromosomes (24 out
of 70 live-imaging movies following 2 generations) exhibited persistent
defects in transcription (Fig. 2d,e,g and Extended Data Fig. 8). Fur-
thermore, 83% (20 out of 24) of examples exhibiting a generation 2
transcriptional defect had undergone rupture of the micronucleus
NE in generation 1, during the interphase of the previous cell cycle
(Fig. 2d,e,g, Extended Data Fig. 8d and Supplementary Video 3).
Nature | Vol 619 | 6 July 2023 | 187
Transcription defects and DNA damage
Previous studies have shown that DNA damage responses can trigger
transcriptional silencing33–35. We therefore considered the possibility
that heritable defects in the transcription of MN chromosomes might
be linked to DNA damage.
As an initial test of this hypothesis, we used a correlated live-cell
same-cell fixed imaging protocol11,26 to follow MN chromosomes
through cell division, observed their re-incorporation into a normal
nucleus and detected γH2AX-marked DNA damage by immunoflu-
orescence imaging (Methods). Using live-cell imaging of GFP–H2B
signals, we followed the division of 13 micronucleated cells that had
re-incorporation of the MN chromosome because neither daughter
cell had detectable micronuclei. In 8 out of 13 of these cell divisions,
we observed large γH2AX-labelled subnuclear territories that were
typically restricted to one of the two daughter nuclei (Extended Data
Fig. 9a). We term these structures MN bodies.
To determine whether these γH2AX-labelled MN bodies are derived
from re-incorporated MN chromosomes, we used live-cell imaging of
the γH2AX-binding protein mediator of DNA damage checkpoint 1
(MDC1) fused to a tag that can be visualized with a fluorescent dye
(SNAP-tag). The SNAP–MDC1 fusion protein was not visible on cyto-
plasmic MN chromosomes during interphase, presumably because it
is sequestered in the main nucleus. However, after mitotic NE break-
down, some MN chromosomes were brightly labelled, which enabled
us to track them from mitosis into the next interphase (Extended Data
Fig. 9b,c and Supplementary Videos 4–6). After division, 31 out of 69 of
these chromosomes were incorporated into normal daughter nuclei to
become nuclear MN bodies (Extended Data Fig. 9c). We independently
confirmed that damaged MN bodies originated from re-incorporated
micronuclei using the U2OS 2-6-3 reporter system (Extended Data
Fig. 8e). Notably, the DNA damage detected in MN bodies persisted for
an extended period (average of >21 h; Extended Data Fig. 9d), longer
than the normal time course of DNA double-strand break repair36.
Same-cell live-fixed imaging showed that damaged MN bodies exhib-
ited reduced levels of both RNAP2-Ser5ph and H3K27ac (Fig. 3a–d
and Extended Data Fig. 9e–g). MN bodies accumulated γH2AX and
endogenous MDC1 as well as the DNA damage response protein 53BP1
(94% of MN bodies were positive for γH2AX and 82% were positive for
53BP1; Fig. 3e,f and Extended Data Fig. 9e). The formation of dam-
aged MN bodies correlated with micronucleus rupture in the previous
interphase. However, there were examples of MN bodies derived from
micronuclei that remained intact until mitotic NE breakdown (25%, 7
out of 28 cases). In these latter examples, DNA damage was probably
acquired during mitosis11.
We observed a small, but significant, increase in the repressive his-
tone marks H3K9me2 and H3K27me3 in MN bodies (P < 0.0001 and
P = 0.0028, respectively, two-tailed Mann–Whitney test, Extended
Data Fig. 9h). The deficiency in active chromatin marks did not cor-
relate with persistence of the mitotic chromatin marks H3S10ph or
H3T3ph (Extended Data Fig. 9i). Therefore, the loss of H3K27ac and
RNAP2-Ser5ph seem to be the primary features associated with herit-
able transcriptional defects of MN chromosomes.
The above data show that damaged chromosomes acquire transcrip-
tional defects. However, they do not address whether it is primarily
damaged chromosomes that acquire this defect, which would sug-
gest that DNA damage and altered transcription could be mechanisti-
cally linked. Testing this association necessitated an imaging system
that could track all MN chromosomes, irrespective of whether they
are damaged or not. We therefore developed a chromatin tagging
system that we refer to as DamMN. DamMN is based on the ability of
DNA-adenine methyltransferase (Dam) to methylate adenine residues
in DNA, which results in N6-methyladenine (m6A)37. An inducible Dam
methyltransferase was fused to three tandem copies of mCherry, which
restricted it to the cytoplasm because it lacks a nuclear localization
188 | Nature | Vol 619 | 6 July 2023
a
TP53
siRNA
Nocodazole
Mitotic
shake-off
Division
Live imaging
0 h
18 h
24 h
Fixation
for IF
45 h
GFP–H2B
SNAP–MDC1
RNAP2-Ser5ph
H3K27ac
N
M
d
e
r
u
t
p
u
R
b
e
v
i
t
a
e
r
l
y
d
o
b
N
M
l
o
r
t
n
o
c
N
P
o
t
1.5
1.0
0.5
0
Ruptured
Intact
P = 0.411
P = 0.0013
P = 0.0447
P < 0.0001
e
v
i
t
a
e
r
l
y
d
o
b
N
M
l
o
r
t
n
o
c
N
P
o
t
1.5
1.0
0.5
0
H3K27ac
RNAP2-Ser5ph
P < 0.0001
2.5
2.0
1.5
1.0
0.5
0
d
o
i
t
a
r
I
F
RNAP2-Ser5ph
H3K27ac
P < 0.0001
e
o
i
t
a
r
I
F
30
25
20
15
10
5
0
P < 0.0001
Control MN body
P < 0.0001
c
o
i
t
a
r
I
F
f
o
i
t
a
r
I
F
2.5
2.0
1.5
1.0
0.5
0
40
35
30
25
20
15
10
5
0
Control MN body
Control MN body
Control MN body
Fig. 3 | MN bodies exhibit transcription defects and extensive DNA damage.
a, Defective transcription and H3K27ac in MN bodies. Top, scheme of the
experiment (time points are approximate). IF, immunofluorescence imaging.
Bottom, representative images of a daughter cell with a MN body from a
ruptured micronucleus. Magenta dashed lines indicate a MN body with low
RNAP2-Ser5ph and low H3K27ac levels. Scale bars, 5 µm. b, Aggregate data of
relative MN body fluorescence intensities (FI) for RNAP2-Ser5ph and H3K27ac
as in a (left to right, n = 43 and 41 from 8 experiments). Boxes are median with
95% CI; P values from two-tailed Mann–Whitney test comparing the FI ratio
between MN and control PN region in the same cell. c, Decrease in RNAP2-
Ser5ph in MN bodies verified by fixed imaging. Cells were fixed approximately
45 h after mitotic shake-off. MN bodies were identified on the basis of the
endogenous MDC1 signal. Data points represent relative FI of RNAP2-Ser5ph
in MN bodies against control regions (n = 1,447 from 12 experiments). Boxes
are median with 95% CI; two-tailed Mann–Whitney test. d, Decrease in H3K27ac
in MN bodies (n = 341 from 2 experiments). e, DNA damage in MN bodies.
FI measurements of γH2AX intensity (94% of MN bodies were positive, >3 s.d.
above the mean of the corresponding nuclear background; n = 195 from 2
experiments). f, 53BP1 accumulation within MN bodies as in e (82% of MN bodies
were positive for 53BP1; n = 211, from two experiments). Analyses in d–f are
similar to c.
signal and is larger than the size-exclusion limit for passive diffusion
across nuclear pores (mega-Dam; Fig. 4a and Extended Data Fig. 10a–c).
The fusion protein contained two tandem degrons that were used to
induce mega-Dam degradation to restrict its expression to the inter-
phase when micronuclei formed. In many G2/M synchronized cells, we
could eliminate mega-Dam, which prevented adventitious labelling
of all other chromosomes following mitotic NE breakdown (approxi-
mately 50% efficiency of specific labelling; Fig. 4a,b and Extended Data
Fig. 10a–d).
Article
a
b
megaDam
TET-ON
DAM
3×Cherry
AID-Smash
TP53 siRNA
+ RO 3306
Release
into MPS1
inhibitor
Induce
megaDam
megaDam
degradation
Division
Fixation
for IF
0 h
2 h
22 h
45 h
Hoechst
m6A-Tracer
γH2AX
RNAP2-Ser5ph
i
h
g
h
-
X
A
2
H
γ
w
o
l
-
X
A
2
H
γ
d
Induce
LacI–SNAP Induce MCP Halo
Nocodazole Mitotic
add fluorescent ligands
Track MS2 expression
(MN re incorporation)
TP53 siRNA
shake off
Division
Fixation
for IF
0 h
18 h
24 h
Live imaging
45 h
LacI–SNAP
MCP–Halo
c
o
i
t
a
r
I
F
P < 0.0001 P < 0.0001
P > 0.999
1.5
1.0
0.5
0
High γH2AX
Low γH2AX
Intermediate γH2AX
Control
e
h
t
i
w
N
M
d
e
t
a
r
o
p
r
o
c
n
i
-
e
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)
%
(
e
g
a
m
a
d
A
N
D
100
80
60
40
20
0
No γH2AX focus
γH2AX focus
n = 32
n = 17
Recovery Defect
2
S
M
d
e
z
i
l
a
m
r
o
N
)
.
u
.
a
(
e
s
n
e
c
s
e
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fl
12
10
8
6
4
2
0
2
S
M
d
e
z
i
l
a
m
r
o
N
)
.
u
.
a
(
e
s
n
e
c
s
e
r
o
u
fl
12
10
8
6
4
2
0
Mitosis
Defective transcription recovery
Hoechst
γH2AX
RNAP2-Ser5ph
Normal transcription recovery
0
2
4
6
8
10
Time (h)
12
14
16
18
Fig. 4 | Damaged MN bodies are more likely to have persistent transcription
defects. a, Transgenerational tracking of MN chromosome fate. Top, cartoon
of the gene expressing megaDam. Bottom, scheme of manipulations that restrict
DamMN expression to the first interphase when MN form. b, Representative
images of re-incorporated MN with high (top) and low (bottom) DNA damage
in MN bodies. Top, MN body (m6A-Tracer, dashed magenta outline) with high
γH2AX signal (top quartile, see c on the right) but low RNAP2-Ser5ph labelling.
Bottom, MN body with low γH2AX signal (bottom quartile) and normal RNAP2-
Ser5ph labelling. c, Aggregate data of relative RNAP2-Ser5ph intensities in MN
bodies with DNA damage levels (Extended Data Fig. 10e; left to right, n = 220,
111, 220 and 112 from 4 experiments). Boxes are median with 95% CI; Kruskal–
Wallis with Dunn’s multiple comparisons. d, Correspondence between high
damage level and low transcriptional activity for re-incorporated MN
chromosome 1 using the U2OS 2-6-3 nascent transcription reporter. Top,
scheme of the experiment. During live imaging, cells with the reporter in a
micronucleus were identified by LacI–SNAP and followed through cell division
to identify MN body formation. Transcription activity of the reporter was
tracked in live cells (MCP–Halo foci) and, after re-incorporation, was scored for
γH2AX-marked DNA damage. Bottom left, MS2 signal used to detect reporter
transcription activity as in Fig. 2e (grey line indicates the mean and s.e.m. of FI
in controls as shown in Extended Data Fig. 8a). Bottom right, after live imaging,
cells were fixed to detect γH2AX and RNAP2-Ser5ph (which correlated with the
MS2 signal). Shown are representative images. Yellow arrowheads indicate MN
bodies. e, Summary of the transcriptional output and presence of damage for
49 re-incorporated chromosome 1 with the reporter. The correlation between
transcription defect (Fig. 2d) and DNA damage is significant (11 experiments,
P < 0.0001, two-sided Fisher’s exact test). Scale bars, 5 µm (b,d).
Using DamMN, we identified MN chromosomes that formed MN
bodies. The MN bodies from the top quartile of γH2AX labelling
more frequently acquired heritable transcription defects than the
MN bodies from the bottom quartile (Fig. 4b,c and Extended Data
Fig. 10e). We confirmed this result using the U2OS 2-6-3 reporter sys-
tem (Fig. 4d,e). Same-cell live-fixed imaging showed that 28 out of
32 cells that recovered transcription lacked detectable DNA damage
after MN chromosome re-incorporation. By contrast, 16 out of 17 of the
chromosomes that exhibited persistent transcriptional suppression
exhibited extensive DNA damage. Therefore, DNA damage and heritable
transcriptome defects of MN chromosomes may be mechanistically
linked.
Long-term effects of aberrant nuclei
To assess potential long-term epigenetic consequences of nuclear
aberrations, we analysed samples from a previously described clonal
evolution experiment11. In this experiment, we had generated chro-
mosome bridges through CRISPR–Cas9-engineered chromosome 4
sister-chromatid fusion11 (Fig. 5a). After live-cell imaging, we isolated
12 clones from cells that formed and then broke chromosome 4 bridges
(hereafter termed bridge clones). Beneficial for our design, the broken
chromosome 4 was preserved after clonal expansion, despite under-
going extensive downstream genetic evolution. We acquired detailed
information about the copy number alterations, rearrangements and
Nature | Vol 619 | 6 July 2023 | 189
a
Bridge
clones
(12 total)
RPE-1 bulk
ATAC-seq
RNA-seq
DNA-seq
Control
clones
(10 total)
b
1.3
1.2
1.1
1.0
0.9
0.8
Control
Bridge
d
Bridge
Control
c
ATAC peaks
26–38 Mb
l
Parental
i
ii
iii
iv
v
vi
vii
viii
ix
x
o
r
t
n
o
C
CRISPR–Cas9
l
i
a
n
g
s
C
A
T
A
7
H
D
C
P
1.00
0.75
0.50
0.25
I
0.10
IV
II
III
e
g
d
i
r
B
0.25
0.50
1.00
*
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
log(TPM ratio of
PCDH7 expression)
PCDH7
Fig. 5 | Long-term epigenetic and transcriptional consequences of exposure
of a chromosome to the cytoplasm. a, Generation of RPE-1 clones with broken
chromosome 4 and control RPE-1 clones. Chromosome 4 bridges were generated
by sister chromatid fusion after CRISPR–Cas9 breakage at the subtelomeres
of either 4p or 4q (left, bottom to top). ATAC-seq, RNA-seq and DNA-seq were
performed on the bridge clones, control clones, and bulk (parental population)
controls. b, Average ATAC signal variation in 10 Mb intervals in control (n = 10)
and bridge clones (n = 12). White dots indicate the median; black boxes indicate
first and third quantiles; red dots indicate a 10 Mb region on chromosome 4
(27–37 Mb) with significantly reduced chromatin accessibility in bridge clones.
Regions with significantly reduced ATAC intensities in the control clones (fold
change < 0.65) are from pericentric regions of chromosome 1, chromosome
9 and chromosome 15 with few ATAC peaks. c, Copy number normalized ATAC-
seq peak intensities in chromosome 4 (26–38 Mb) in the parental clone, control
clones and bridge clones. The region with an asterisk in bridge clone III has DNA
copy number zero from homozygous deletion and is masked. See Extended
Data Fig. 11 for haplotype-specific DNA copy number alterations and
rearrangements in bridge clones. There is a significant reduction in the ATAC
signal in 6 out of 12 clones (I–V and XII) (P < 0.0001, one-sided permutation
test; Methods). Bridge clones are ordered by the level of PCDH7 expression
(ascending), the only gene with significant expression within this region. Arrow
indicates the PCDH7 promoter. d, Correlation between PCDH7 expression
(log-transformed transcripts per million ratio) and chromatin accessibility in
the promoter and gene body of PCDH7 (normalized ATAC peak intensity) in
control (green dots) and bridge clones (purple dots). Four bridge clones
displaying a reduction in both chromatin accessibility and gene expression are
on the lower left labelled with sample identifiers.
subclonal architecture of these populations, which was necessary to
distinguish epigenetic or transcriptional changes from genetic changes
associated with chromothripsis. The DNA copy number was confirmed
by re-sequencing of these clones (Extended Data Fig. 11).
Chromosome bridges are functionally similar to micronuclei11. That
is, micronuclei and chromosome bridges share the same defect in NE
and nuclear pore complex assembly. Moreover, both can undergo NE
membrane collapse and expose chromatin to the cytoplasm, and both
cause chromothripsis through similar mechanisms9,11. We found that
broken bridge chromosomes form MN-body-like structures with DNA
damage and reduced RNAP2-Ser5ph levels (Extended Data Fig. 12a). In
addition to shared functional defects, during clonal evolution, broken
bridge chromosomes from one generation often form micronuclei in
the next generation and vice versa11,25. This means that during down-
stream evolution, the broken bridge chromosome may be frequently
trapped in a secondarily formed micronucleus.
We performed bulk assay for transposase-accessible chromatin with
sequencing (ATAC-seq) and RNA sequencing (RNA-seq) analyses on
12 chromosome 4 bridge clones, the parental clone and 10 parental
subclones (Fig. 5a and Methods). In general, the ATAC-seq profiles of
both control and bridge clones exhibited little variation over 5–10 Mb
of genomic intervals (Fig. 5b and Extended Data Fig. 12b) after normali-
zation to the DNA copy number derived from the re-sequencing data
(Extended Data Fig. 11). In the bridge clones, however, we identified
a variably penetrant but significant reduction in the ATAC-seq sig-
nal within a 10 Mb region of chromosome 4p (P < 0.0001, one-sided
permutation test; Fig. 5b,c and Extended Data Fig. 12c). RNA-seq
analysis of the one, non-essential gene in this region, PCDH7, verified
that the reduction in the ATAC-seq signal across PCDH7 was associ-
ated with a corresponding reduction in its expression (Fig. 5c,d). In
bridge clone I, which had the lowest PCDH7 expression, this region
exhibited the most significant and largest fold reduction in ATAC
signal (Extended Data Fig. 12d and Methods). In addition to the chro-
mosome 4p region, we identified several regions on other chromo-
somes with significant reductions in accessibility (Extended Data
Fig. 12c).
Because the ATAC peak densities were normalized to the DNA copy
number, the reduced ATAC signal on chromosome 4p is independent
of DNA loss and therefore reflects reduced chromatin accessibility. The
reduction in chromatin accessibility over the chromosome 4p region
(27–37 Mb) also cannot be attributed to rearrangements. Three bridge
clones with the most significant levels in ATAC signal reduction (clones
I, II and IV) had no rearrangement breakpoints on chromosome 4p.
Moreover, rearrangements in this region (27–37 Mb) in bridge clone III
were restricted to a 30-kb interval (32.19–32.22 Mb) that was far away
from the region of the most significant reduction in ATAC signal (Fig. 5c
and Extended Data Fig. 11). In addition, bridge clones VIII and XI had
the most breakpoints within or flanking the region of 27–37 Mb on
chromosome 4p but did not display a significant reduction in ATAC
signal or PCDH7 expression.
The chromosome 4p region may have either been in a bridge or in a
subsequently formed micronucleus. Consistent with this notion, the
clones with reduced 4p chromatin accessibility either had a 4q-terminal
deletion (clones I, II and IV) or had rearrangement breakpoints on both
190 | Nature | Vol 619 | 6 July 2023
Article
telomeric and centromeric sides of this region (clone III) (Extended Data
Fig. 11). Furthermore, two clones (I and III) showed near-complete loss
of the B homologue. This result indicated that the reduced accessibility
and expression were both on the remaining, rearranged A homologue.
Together, these data suggest that chromatin state alterations
acquired in chromosome bridges or micronuclei can, with variable
penetrance, be propagated long-term. This effect can occur even in cell
culture conditions that lack selection for specific epigenetic changes.
Discussion
We established that micronuclei, which are common features of cancer
nuclear atypia, can generate heritable defects in transcription. These
findings should have relevance for tumour evolution1–3 and for contexts
during normal development in which micronucleation occurs38. We
propose the following model for the acquisition of these heritable
defects (Extended Data Fig. 13). When micronuclei form, even before
NE rupture, they exhibit defects in post-mitotic transcriptional recovery
along with variably reduced H3K27ac that probably results from defec-
tive nuclear import into micronuclei and the corresponding abnormal
composition of the nucleoplasm26,27. The reduced levels of H3K27ac
persist after micronuclear rupture. However, it can be reversed after
the MN chromosome is re-incorporated into a daughter cell primary
nucleus, unless the re-incorporated chromosome acquires extensive
DNA damage. Persistent DNA damage may have a direct role in repress-
ing transcription because previous work has established that DNA
damage or abnormal DNA replication generates transcriptional silenc-
ing and/or epigenetic plasticity33,39.
There are notable similarities between MN bodies and previously
described 53BP1 bodies40–42. 53BP1 bodies form during interphase when
DNA damage or under-replicated DNA is carried over from the previous
cell cycle. Similar to MN bodies, 53BP1 bodies show persistent DNA
damage and accumulate a subset of damage response factors. Through
incompletely understood mechanisms, 53BP1 bodies are thought to
shield DNA lesions until they can be repaired later in the cell cycle40.
Notably, 53BP1 bodies also exhibit transcriptional suppression, again
for unclear reasons and through unknown mechanisms41.
There are several ways in which transcription and epigenetic variation
from MN chromosomes could be translated into phenotypic variability
and long-term epigenetic alterations. One possibility would be that the
initial transcriptional alterations are stably and permanently propa-
gated. However, this idea can be excluded because a substantial fraction
of MN chromosomes restore transcription after re-incorporation into a
normal nucleus. Therefore, the epigenetic alterations from cytoplasmic
chromatin are dynamic, although lasting suppression may become
fixed in a subset of cases. Indeed, our analysis of cell populations that
evolved long-term after breakage of a chromosome 4 bridge identi-
fied a large, gene-poor region of chromosome 4p with heterogene-
ous suppression of chromatin accessibility and transcription. The
preservation of altered chromatin only in a gene-poor region makes
sense because the clonal expansion was done without selection for
any epigenetic or transcriptional change. Without selection, random
changes to the normal transcription programme of the cell from ran-
dom epigenetic alteration of gene expression would compromise fit-
ness, and cells with such alterations should be lost from the population
during clonal growth24. Non-essential, gene-poor genomic regions are
therefore most likely to preserve the footprint of epigenetic changes
acquired from initial bridge formation and/or resulting micronuclea-
tion. In addition to direct effects on transcription, chromosome-wide
transcription silencing may promote evolutionary adaptation indi-
rectly through genetic mechanisms. For example, transcriptional
suppression of a trisomic chromosome might allow cells undergo-
ing chromothripsis or other genetic alterations to this chromosome
to persist longer in the population, thereby increasing their chance
of fixation.
In summary, our results suggest that chromosomal instability is
inherently coupled to variation in chromatin state and gene expres-
sion through aberrations in the nucleus that are common in cancer.
Online content
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ries, source data, extended data, supplementary information, acknowl-
edgements, peer review information; details of author contributions
and competing interests; and statements of data and code availability
are available at https://doi.org/10.1038/s41586-023-06157-7.
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ArticleMethods
Cell culture and cell line construction
Cells were cultured at 37 °C in 5% CO2 atmosphere with 100% humid-
ity. Telomerase-immortalized RPE-1 retinal pigment epithelium cells
(CRL-4000, American Type Culture Collection), U2OS osteosarcoma
cells (HTB-96, American Type Culture Collection) and derivative cell
lines were grown in DMEM/F12 (1:1) medium without phenol red (Gibco)
supplemented with 10% FBS, 100 IU ml−1 penicillin and 100 µg ml−1
streptomycin. For cell lines with doxycycline-inducible constructs,
tetracycline-free FBS (X&Y Cell Culture) was used.
Stable cells lines H2B–eGFP and TDRFP–NLS RPE-1, mRFP–H2B and
eGFP–BAF RPE-1, mRFP–H2B RPE-1 and TDRFP–NLS U2OS were gener-
ated by transduction of RPE-1 or U2OS cells using lentivirus or retro-
virus vectors carrying the genes of interest as previously described26.
RPE-1 cells with transient expression of a dominant-negative variant
of telomeric repeat-binding factor 2 (TRF2-DN)43 were treated as
previously described11. RPE-1 clones derived from single cells with
CRISPR–Cas9-mediated telomere loss on chromosome 4 (chromo-
some 4 bridge) and their derived clones were generated in a previ-
ous study11. Control parental RPE-1 subclones were generated by
FACS and expansion in 96-well plates. The HDAC inhibitor vorinostat
(SAHA, Sigma-Aldrich, SML0061) was used at 0.5 µM concentration,
as described in the Extended Data Fig. 5d.
Generation of cells expressing SNAP-MDC1. The RPE-1 GFP-H2B
RFP–NLS SNAP–MDC1 cell line (Fig. 3 and Extended Data Fig. 9) was
generated by lentiviral transduction of the SNAP–MDC1-bearing lenti-
viral vector. This vector was generated by cloning a synthesized SNAPf
fragment (sequence from pBS-TRE-SNAPf-WPRE; plasmid 104106,
Addgene) with AgeI and BstBI restriction sites into the pLenti CMV/
TO GFP-MDC1 (779-2) (plasmid 26285, Addgene, gift from E. Campeau;
Genewiz) backbone, substituting SNAPf with eGFP at the N terminus of
MDC1. Stably transduced cells were selected by FACS around 10 days
after transduction for SNAP–MDC1 expression.
Generation of the modified U2OS 2-6-3 transcription system. Our
modified U2OS 2-6-3 cells contain GFP–H2B, Cuo-LacI–SNAP and MS2–
Halo (Figs. 2 and 4 and Extended Data Fig. 8). These cells were generated
from the original U2OS 2-6-3 cells31 (gift from D. Spector). In brief, the
2-6-3 transgene consists of 256 tandem copies of the lac operator, which
enables visualization of the transgene genomic locus, 96 tetracycline
response elements (TREs) to control the reporter transgene and 24
MS2 translational operators (MS2 repeats) for the visualization of the
reporter nascent transcript31. The 2-6-3 transgene was introduced into
a single euchromatic locus on chromosome 1p36 (ref. 31). We modified
the system as follows. We introduced a lentivirus with the coding se-
quence of LacI fused to SNAP, under the control of a cumate-inducible
promoter. Independent control of LacI–SNAP and the MS2 reporter
enabled the identification of the reporter in micronuclei in generation 1,
followed by assessment of MS2-marked transcription in generation 2.
We also stably introduced genes expressing LacI–SNAP, rtTA and MS2
coat protein (MCP) used for visualizing the MS2 aptamers.
Specifically, U2OS 2-6-3 cells were transduced with pLenti CMV rtTA3
Blast (w756-1, plasmid 26429, Addgene; gift from E. Campeau) for the
expression of rtTA, a lentiviral vector, phage ubc nls ha 2×mcp HALO,
for the expression of MCP–Halo (plasmid 64540 Addgene; gift from
J. Chao) and lenti Cuo-LacI-SNAP, for the expression of LacI–SNAP. Our
LacI–SNAP expression vector, CuO-LacI-NLS–SNAPf, contains the cod-
ing sequence for the SNAPf-Tag (sequence from pBS-TRE-SNAPf-WPRE;
plasmid 104106, Addgene) followed in-frame with the coding sequence
for LacI-NLS (sequence taken from Cherry-LacRep; plasmid 18985,
Addgene). This sequence was subcloned into pCDH-EF1-CymR-T2A-Puro
(QM200VA-1, System Biosciences SBI) using NheI and BstBI restriction
sites. The final modified U2OS 2-6-3 cell line was obtained by selection
for hygromycin resistance conferred by the 2-6-3 transgene and for
blasticidin resistance conferred by the rtTA expression construct,
followed by FACS to identify MCP–Halo and LacI–SNAP expression.
Note that binding of LacI–SNAP to LacO was transiently inhibited by
adding 1 mM isopropyl β-d-1-thiogalactopyranoside before FACS.
Transient inhibition was done to avoid genetic instability from LacI
binding to the Lac operators, which are a barrier to replication fork
progression. Full maps of the constructs used are available upon
request. All cell lines used in this study were monitored for mycoplasma
contamination.
Generation of RPE-1 megaDam cells. The RPE-1 3×Cherry Dam AID
Smash cell line (RPE-1 megaDam) (Fig. 4 and Extended Data Fig. 10)
was generated by lentiviral transduction of the megaDam vector
into a RPE-1 cell line that has a doxycycline-inducible transgene ex-
pressing the E3 ligase, OsTIR1, integrated at the ROSA26 locus44. The
megaDam vector (Fig. 4a) was generated by synthesizing (Genewiz) a
sequence containing three copies of mCherry (based on the sequence
from pHAGE-EFS-N22p-3XRFPnls; plasmid 75387, Addgene) and the
sequences encoding the mAID and SMASh degrons (from ref. 44). The
Dam coding sequence was taken from TS52_pT_damonly (van Steensel
lab). The sequence encoding the Dam–mCherry double-degron fusion
was cloned and introduced into the lentiviral vector pCW57.1 (plasmid
41393, Addgene; gift from D. Root, Genewiz). A stably expressed RPE-1
megaDam cell line was obtained by puromycin selection.
Cell cycle synchronization and methods to generate
micronuclei or bridges
To synchronize cells and to generate micronuclei, most experiments
in this study used a previously described nocodazole block and release
protocol8,11,26 unless otherwise stated. In brief, approximately 15 h
after TP53 siRNA (Horizon Discovery) treatment, cells were treated
with 100 ng ml–1 nocodazole for 6 h followed by a mitotic shake-off
procedure. Alternatively (for Figs. 1f and 4a–c and Extended Data
Figs. 4f and 10), cells were synchronized at the G2/M border with a
treatment of 9 µM RO-3306 (MilliporeSigma), a CDK1 inhibitor, for
18 h. G2/M-arrested cells were next released into mitosis by washing five
to seven times with medium, followed by addition of 1 µM NMS-P715
(MilliporeSigma) to impair chromosome segregation through inhibi-
tion of the MPS1 kinase45.
For the analysis of cells after bridge formation (Extended Data
Fig. 12a), RPE-1 TRF2-DN cells were treated as previously described11.
In brief, the cells were incubated in 0.1 µg ml–1 doxycycline (Millipore
Sigma) for about 20 h to generate chromosome bridges. Bridges begin
to form during cell divisions that occur at least 8 h after the washout of
doxycycline. At 20 h after doxycycline washout, cells were synchronized
in G2 using 9 µM RO-3306 (Millipore Sigma) for another 18 h. Finally,
the cells were fixed and analysed 6 h after release from RO-3306, at
the next interphase after bridge resolution and cell division. Data in
Fig. 5 and Extended Data Figs. 11 and 12b–d were generated using RPE-1
clones derived in a previous study11.
Detection of nascent transcripts marked with 5-ethynyl uridine
To detect nascent transcripts, cells were incubated for 30 min with 1 mM
5-ethynyl uridine, which was added approximately 23 h after mitotic
shake-off from nocodazole release. Incorporation of 5-ethynyl uridine
was detected using a Click-iT RNA Alexa Fluor 488 imaging kit according
to the manufacturer’s instructions (ThermoFisher Scientific).
Cas9 RNP transfection
The method for targeting a specific chromosome arm to a micronucleus
in RPE-1 cells, complete characterization of the editing efficiency of the
sgRNA used in this study and the frequency of generation of micronuclei
harbouring the targeted chromosome have been previously described
in detail25. In brief, a Trueguide Synthetic gRNA system (ThermoFisher
Scientific) was used to generate the sgRNA for chromosome 5q with
the sequence 5'-G*U*U*GGCCUCCCAAACCACUA-3' (asterisks indi-
cate modified 2′-O-methyl bases with phosphorothioate linkages).
RPE-1 GFP–H2B RFP–NLS cells were synchronized in G0 by serum star-
vation for 23 h and were then transfected with the Cas9–gRNA RNP
complexes 22 h after release. Cell synchronization and transfection
were performed on cells seeded onto MembraneRing 35 rings (415190-
9142-000, Carl Zeiss), which enabled cell isolation by laser capture
(see below). Live-cell imaging started 3–5 h after transfection, and cells
with micronuclei and their siblings were followed until late G2 phase
before cell capture for single-cell RNA-seq (scRNA-seq).
Live-cell imaging
For the majority of the live-cell imaging experiments, images were
collected on Nikon (Ti-E) or (Ti2) wide-field inverted microscopes
equipped with Perfect Focus, an environmental enclosure to main-
tain cell culture conditions (37 °C and humidified 5% CO2), a ×20/0.75
NA Plan Apochromat Lambda objective (Nikon) or a ×40/0.95 NA Plan
Apochromat Lambda objective, and a Zyla 4.2 sCMOS camera (Andor).
At each time point, three 2-µm-spaced Z-focal plane image stacks were
acquired. Live imaging by confocal microscopy (for the experiments
with the adapted U2OS 2-6-3 and MDC1-expressing cells, see below) was
performed at 15 min time intervals on a Ti2 inverted microscope fitted
with a CSU-W1 spinning disk confocal head (Nikon). At each time point,
three 2-µm-spaced Z-focal plane image stacks were acquired using a
×40/0.95 NA Plan Apochromat Lambda objective. The microscopes
were controlled using Metamorph (v.7.10.2.240; Molecular Devices)
or NIS Elements (v.4.30 AR or newer versions; Nikon Instruments).
Live-cell imaging for Look-Seq2 experiments. Imaging for Look-Seq2
experiments (Figs. 1 and 2 and Extended Data Figs. 1–3, 6 and 7) was
performed using a wide-field inverted microscope and a ×20 objective
(see above). Note that at this imaging resolution, we can confidently
detect the presence of micronuclei and micronuclear rupture. RFP–NLS
or GFP–BAF were used for the assessment of NE integrity of the genera-
tion 1 samples as previously described26.
We acknowledge, however, that with ×20 wide-field imaging, some
events such as micronuclei of small size and borderline cases of NE
rupture, fine bridges and rare cases of micronuclei-like structures
connected to the PN may not be resolved.
Live-cell imaging of cells containing the U2OS 2-6-3 transcription
reporter. For live-cell imaging of our modified U2OS 2-6-3 nascent
transcript reporter cells (Figs. 2 and 4 and Extended Data Fig. 8), cells
were seeded on 35-mm ibiTreat Grid-500 dishes (Ibidi) with a gridded
imaging surface after mitotic shake-off. SNAP-tagged and Halo-tagged
proteins were labelled using 250 nM JF549-cpSNAP-tag and 50 nM
JF646-HaloTag ligands ( Janelia Materials) for 15 min before the start of
imaging. To induce LacI–SNAP expression, cumate (30 µg ml–1; System
Biosciences) was added in the medium immediately after the mitotic
shake-off step and washed out before imaging. Doxycycline (1 µg ml–1;
MilliporeSigma) was used to induce expression of the MS2 transcription
reporter approximately 2 h before the start of imaging and was main-
tained in the medium for the remainder of the experiment (Fig. 4d).
Confocal imaging started 16–19 h after mitotic shake-off and was per-
formed as described above for about 24 h or until most of the cells of
interest had divided and could be imaged in the generation 2 cycle.
Live-cell imaging of MDC1-expressing cells. For the live-cell im-
aging experiments tracking damaged MN chromosomes marked by
MDC1 (RPE-1 GFP–H2B RFP–NLS SNAP–MDC1 cells; Fig. 3 and Extended
Data Fig. 9), cells were seeded in 35-mm ibiTreat dishes (ibidi) after mi-
totic shake-off and SNAP-tagged proteins were labelled using 250 nM
JF646-SNAP-tag ligand ( Janelia Materials) for 15 min before the start
of imaging. Imaging was started 16–19 h after mitotic shake-off and
images were collected at 15 min intervals with a ×40 objective and 2 × 2
image stitching. An exception was the experiments with high tem-
poral resolution, for which images were collected at 6 min intervals
(Extended Data Fig. 9c and Supplementary Video 4) and at 3 min intervals
(Supplementary Videos 5 and 6) without image stitching.
Capture of single-cells for Look-Seq2
The isolation of live cells for scRNA-seq was performed in two ways.
Our initial experiments were performed in an analogous manner to our
early Look-Seq procedure8 for single-cell whole-genome sequencing
(which was subsequently refined in ref. 11). For these experiments,
cell isolation was accomplished by trypsinization followed by FACS of
single cells into 384-well µClear imaging plates (Greiner). Micronucle-
ated cells were identified by imaging, and then after the division of
these cells, daughter cells were isolated for scRNA-seq by trypsinization
and replating into 384-well µClear plates after serial dilution. This pro-
cedure was used for a subset of the generation 2 experiments to assess
the effect of MN chromosome re-incorporation into normal daughter
nuclei (10 out of 127 of the total generation 2 samples; Supplementary
Table 1). We subsequently developed the laser capture microdissection
(LCM) procedure (Extended Data Fig. 1b, described below) and used
this method to collect MN cells and MN sisters (generation 1) and MN
daughters and MN nieces (the majority of generation 2 samples) for
data presented in Figs. 1 and 2, Extended Data Figs. 1–3, 6 and 7 and
Supplementary Tables 1 and 2. The main advantages of our modified
system over previous LCM methods23 are as follows: (1) minimized
cell stress (because the cells are kept in medium in the microcham-
ber setup that prevents the culture to dry out) throughout capturing;
(2) higher throughput; and (3) an enhanced ability to capture cell fami-
lies (again because the cells are maintained in medium throughout
capture of multiple cells).
Live-cell imaging for Look-Seq2 experiments. Cells were treated
as described in ‘Cell cycle synchronization and methods to generate
micronuclei or bridges’ for the induction of micronuclei. After mitotic
shake-off, cells were handled as described below.
For experiments using the older cell capture method8,11, cells were
plated into 384-well µClear imaging plates and imaged using wide-field
fluorescence microscopy at time intervals of 15 min for up to 48 h or
until the majority of cells had progressed through mitosis. For LCM
capture experiments, cells were instead plated on MembraneRing 35
rings (hereafter membrane rings; 415190-9142-000, Carl Zeiss) (see
details in ‘Development of the modified LCM capture method’ below)
and imaging was performed at 15 min intervals as described above,
with the difference that image stitching (2 × 2) was used to track mobile
cells across different fields of view.
Development of the modified LCM capture method. We adapted a
previously developed LCM system (Palm Microbeam, Carl Zeiss) and
re-designed the capturing and imaging setup as described below and
in Extended Data Fig. 1b. A custom-designed aluminium adapter was
constructed (SeqTech) to enable imaging of cells plated on the mem-
brane rings. We also custom-designed and 3D printed an adapter using
VeroWhite material (opaque white Polyjet resin, SeqTech) to allow
placement of the membrane rings in a flipped orientation on the Palm
Microbeam LCM microscope with a DishHolder 50 CC (415101-2000-
841, Carl Zeiss; Extended Data Fig. 1b). This enabled capturing of cells
in a multi-well capture plate that could be placed close to the cells,
which helped increase the capturing speed, efficiency and therefore
the throughput of the method. The designs of the adapters are available
upon request. The Look-Seq2 method is described in detail in a provi-
sional patent46. A hydrophobic barrier was applied at the periphery of
the surface of the membrane rings using an ImmEdge PaP Pen (Vector
Laboratories) to prevent evaporation of the medium. Next, the cells
were plated on the membrane rings.
ArticleAt the time of cell capturing, the cells were supplemented with
medium containing HEPES buffer. Next, the membrane rings were
flipped upside down and positioned on the custom-made adapter after
the application of a glass 20 mm glass coverslip (Neuvitro), which we
refer to as the microchamber (Extended Data Fig. 1b). The cells in the
microchamber were transferred to a Palm Zeiss LCM microscope on the
custom-made adapter. Cells of interest were identified by extrapolation
of the coordinates on the imaging microscope to the LCM microscope
using a custom MatLab script and reference marks that were applied
to the membrane rings. Snapshots of all imaging channels of the cells
of interest were taken immediately before LCM to ensure accurate
assessment of the micronuclear NE integrity and cell viability (from the
maintenance of nuclear RFP–NLS). Cells of interest on small membrane
surfaces were then catapulted into single wells in 5.5 µl of lysis buffer
(see ‘Generation of scRNA-seq data’ below) in a 96-well capture plate
(CapturePlate 96 (D), 415190-9151-000, Carl Zeiss). The cell lysates were
quickly transferred to 96-well PCR plates (Eppendorf) by centrifuga-
tion and stored in −20 °C for cDNA library generation and scRNA-seq.
Generation of scRNA-seq data
cDNA synthesis and amplification were performed using a modified
protocol of the SMART-Seq v4 Ultra Low Input RNA kit for sequencing
(Takara Bio). In brief, the manufacturer’s instructions were used with
the following modifications: (1) 3 µl of RNAse inhibitor was added per
20 µl for the 10x reaction buffer solution; and (2) all the reaction vol-
umes were decreased by half to maintain reactant stoichiometry. cDNA
amplification of single cells was performed by PCR for 21 cycles and
the amplified products were purified using AMPure XP paramagnetic
beads (Beckman Coulter).
The quality and quantity of the amplified cDNA libraries were
assessed using a dsDNA HS Assay kit on a Qubit fluorometer (Ther-
moFisher Scientific) and an Agilent High Sensitivity DNA kit on a 2100
Bioanalyzer system (Agilent Technologies). cDNA libraries with con-
centrations below 0.2 ng µl–1 and/or fragment size distributions not
showing a peak at 2 kb as expected for the size distribution of full-length
mRNAs were excluded from subsequent analyses. Sequencing librar-
ies were generated by tagmentation using a Nextera XT DNA Library
Preparation kit (Illumina) with minor modifications of the manufac-
turer’s instructions. In brief, 0.1–0.2 ng µl–1 of cDNA samples were used
in one-quarter of the suggested volumes for all subsequent reactions.
Barcodes from the Nextera XT Index kit v2 Sets AD (Illumina) were used
for multiplexing, and the quality of RNA-seq libraries was assessed using
a Qubit fluorometer (ThermoFisher Scientific) and a 2100 Bioanalyzer
(Agilent Technologies). Sequencing was performed on MiSeq and HiSeq
2500 sequencing instruments (2× 100 bp) after quantity normalization
and additional quality assessment of the individual libraries by low-pass
sequencing on a MiSeq Nano flow cell.
scRNA-seq data processing
The complete workflow of scRNA-seq data processing and downstream
analysis was implemented as a snakemake pipeline (publicly available
at https://github.com/chengzhongzhangDFCI/nature2023)47. Details
of individual steps are described below.
Alignment and post-alignment processing of sequencing data.
Sequencing reads were aligned using STAR (v.2.7.6a) (https://github.
com/alexdobin/STAR) to the Gencode v.25 reference (–twoPassMode
basic; –quantMode: TranscriptomeSAM and GeneCounts) and sorted
by genomic coordinate. For post-alignment processing, we followed
the best practice of GATK (https://gatk.broadinstitute.org/hc/en-us/ar
ticles/360035531192-RNAseq-short-variant-discovery-SNPs-Indels-),
which included adding read group information and executing Split-
NCigarReads (both using GATK v.4.1.9.0). We skipped duplicate removal
as the estimated fractions of duplicated reads was below 5% for all
libraries.
Quality assessment of scRNA-seq data. The STAR program outputs
various alignment metrics of the RNA-seq data. We report the following
information in Supplementary Table 1: the percentages of unmapped
reads, multi-mapped reads, reads mapped to no features according to
gene annotations, and reads mapped to multiple features according to
gene annotations; the number of genes (transcripts) represented by
at least 1, 5 or 10 reads; and the average number of reads covering each
gene. The primary metric for removing low-quality scRNA-seq libraries
was the number of genes covered by ≥5 reads. For control (untreated)
RPE-1 cells isolated by FACS, Look-Seq or Look-Seq2 procedures, we
excluded cells with <6,000 genes covered by ≥5 reads; for cells with or
related to micronucleation, including MN cells, MN sisters, MN daugh-
ters and MN nieces, isolated by either Look-Seq and Look-Seq2, we
excluded cells with <4,000 genes with 5 or more reads. A total of 464
cells were included in the final analysis, 434 of which were sequenced
on HiSeq (Illumina) and another 30 by MiSeq (Illumina). All of these
samples are listed in Supplementary Table 1 with annotations of the
experimental setup.
Single-cell gene expression analysis
Quantification of total gene expression. We calculated the TPM
for each gene using RSEM (https://deweylab.github.io/RSEM/) with
rsem-calculate-expression. We excluded genes with low expression
(mean TPM in control cells ≤ 25) that displayed more cell-to-cell vari-
ability due to both transcriptional noise and technical variation.
Quantification of allele-specific expression. We assessed allelic gene
expression based on allelic depths of RNA-seq reads at heterozygous
sites (only single-nucleotide variants) calculated using the ASERead-
Counter module of GATK (v.4.1.9.0). The list of heterozygous variants
and the haplotype phase of variant genotypes on parental chromo-
somes were both taken from a previous study48. To calculate the aver-
age allelic fraction of transcripts of each gene, we first summed the
total number of haplotype-specific (A or B) reads at all variant sites
in coding (exonic) and untranslated regions and then calculated the
fraction of haplotype-specific read coverage (fA and fB; fA + fB = 1). The
averaging of allelic coverage at the gene level helps to improve the ac-
curacy of allelic fraction calculation when there are multiple variants
in the transcribed sequence. To eliminate allelic gene expression bias
in parental RPE-1 cells (for example, from imprinting), we only assessed
allelic expression of genes with roughly equal allelic contributions from
both parental homologues in control RPE-1 cells (average allele frac-
tion in control cells is within the range of 0.3–0.7). Two chromosomes
required special treatment. For chromosome X transcripts originating
from one normally transcribed active X and one epigenetically silenced
inactive X, we included all X-linked genes. To account for the presence of
a duplicated copy of chromosome 10q (60.78 Mb-qter on GRCh38) that
is translocated to the q-terminus of active X, we adjusted the normal
range of allele fractions of the single-copy homologue in the trisomic
10q region to be 0.2−0.4. As we had previously determined the allelic
identities of both the active X and the extra copy of chromosome 10q,
we used the transcription of the active X and the normal (single-copy)
chromosome 10 homologue as reference to calibrate the transcription
of the inactive X and the trisomic 10q segment.
Quantification of transcriptional changes relative to normal disomic
transcription. We assessed transcriptional changes using both the
total transcriptional yield (measured by TPM) and the allelic fraction
of transcripts from each gene (Extended Data Fig. 1c). For the total tran-
scription yield, we calculated the transcription ratio by normalizing the
TPM in each single cell by the mean TPM in control RPE-1 cells (listed in
Supplementary Table 1). To mitigate variations in the TPM ratios derived
from individual genes due to transcriptional noise, we calculated the
average TPM ratio in 10 Mb genomic intervals for regional transcription
analysis and across entire chromosomes or chromosome arms for
chromosome (or arm)-level transcription analysis. As different genes
show varying degrees of transcriptional variation, we performed a
weighted average to attenuate the contributions of genes with more
variability that is due to either transcriptional noise49,50 or technical
variability51,52. This averaging strategy is described in the Supplemen-
tary Information. A similar strategy was used to estimate the average
allelic fraction. We further introduced a scaling factor for TPM ratios in
each cell to eliminate global changes to TPM values due to significant
upregulation or downregulation of one or a few highly transcribed
genes (Supplementary Information).
The normalized haplotype-specific transcription value was calcu-
lated by multiplying the average TPM ratio by the average allele frac-
tion. For normal disomic transcription, the average TPM ratio is 1 and
the average allelic fraction is 0.5, thus the average haplotype-specific
transcription is 0.5. For monoallelic transcription that is due to DNA
loss or complete epigenetic silencing, we expect the TPM ratio to be
around 0.5 (assuming a linear relationship between gene transcription
output and copy number24) and the silenced or lost chromosome to
have allelic fraction 0; for trisomies with a 2:1 allelic ratio, we expect
the TPM ratio to be 1.5. In both cases, the unaltered homologue will
have close to normal allelic transcription (0.5) and serve as an intrinsic
control for the altered homologue.
We note that experimental perturbations (for example, nocodazole
treatment) can cause gene expression changes that are independent of
chromosome-specific transcriptional changes on the MN chromosome.
We found that the majority of these differentially expressed genes
had both low transcription level and low transcriptional variability in
control RPE-1 cells. These genes were excluded from the TPM and allelic
fraction calculation by the total TPM cutoff (TPM > 25).
Quantification of normal transcriptional variation. We derived a
reference distribution of normal transcription of each homologous
chromosome from the haplotype-specific transcription in control
RPE-1 cells (Extended Data Fig. 1d). These reference distributions were
used to assess whether the observed average transcription of a chro-
mosome in a RPE-1 cell is significantly different from normal transcrip-
tion. Three chromosomes required special treatment. (1) RPE-1 cells
frequently acquire alterations to chromosome 12, including trisomic
12, tetrasomic 12p or 12p uniparental disomy. We manually reviewed
chromosome 12 transcriptional levels (of both homologues) in the
control cells and removed those with chromosome 12 alterations.
(2) RPE-1 cells share an extra copy of a 10q segment that is attached
to Xa. To match the expression of the single-copy homologue to the
mean expression of other autosomes, we multiplied the expression
of both homologues by a factor of 1.5. (3) For chromosome X, we simi-
larly multiplied the expression of both Xa and Xi by a constant factor
(about 0.6) to match the mean expression of Xa to the mean expression
of an autosome. The normalization of chromosome 10q expression
and chromosome X expression only affects the visualization of tran-
scriptional changes of individual chromosomes. It does not affect the
assessment of whether the observed transcriptional change is within
the normal range of variation, which was done separately for each
chromosome.
Estimation of transcriptional changes due to chromosomal gain
and loss. In addition to normal disomic transcription, we estimated the
range of normal transcription of monosomies or trisomies to assess the
normality of gene transcription after chromosome mis-segregation,
micronucleation or re-incorporation of MN chromosomes. For monoso-
mies, we estimated the residual fraction of transcripts from the deleted
homologue based on the RNA-seq data of bona fide monosomies—when
a pair of daughter cells showed approximately 0:2 allelic ratio. For
trisomies, we estimated the range of normal trisomic transcription
using three strategies.
First, assuming the average transcriptional yield from each parental
homologue to be similar in both trisomies and disomies, we expected
the total allelic transcription yield from two copies of the duplicated
homologue in a trisomic cell to be similar to the total transcription yield
from both homologues in disomic cells. Therefore, we compared the
observed allelic transcription yield of the duplicated homologue to the
distribution of total transcription (from both homologues) in control
RPE-1 cells to assess the normality of transcription of the duplicated
chromosome.
Second, assuming the transcription yield of the single-copy homo-
logue is similar to the transcription yield from either copy of the dupli-
cated homologue, we used the allelic ratio between the duplicated
homologue and the single-copy homologue in trisomic cells to assess
the normality of transcription of the duplicated chromosome. For this
comparison, we used the allelic ratios of the trisomic 10q segment in
control RPE-1 cells to assess the normality of transcription of sponta-
neous trisomies.
Finally, we used the transcription data of RPE-1 cells with de novo
trisomies either induced by nocodazole treatment or generated spon-
taneously during cell culture. To identify bona fide trisomies, we used
the following three criteria: (1) we required that there was approxi-
mately proportional changes in the chromosome-wide average TPM
ratio (1.5); (2) we required that the transcriptional allele fractions were
consistent with the DNA allelic fractions (1/3 or 2/3); (3) importantly, we
required that each trisomy is either shared by a pair of sibling cells or
accompanied by a monosomy in another sibling cell, thereby indicating
a de novo mis-segregation event. The last requirement is equivalent to
a biological replicate and should exclude random transcriptional varia-
tion that affects individual cells. Reference monosomies and trisomies
are annotated in Supplementary Table 2 (from both generation 1 and
generation 2 samples).
One advantage of using de novo trisomies as a reference is that the
observed transcriptional changes are not affected by long-term adap-
tive changes that may occur in clonal trisomies (for example, 10q). We
note that even in de novo trisomies, there is a slight decrease in the
expression of each DNA copy, which resulted in a transcription ratio
slightly lower than 1.5.
Classification of chromosomal transcription in single RPE-1 cells.
We used haplotype-specific transcription to determine whether the
observed transcription yield of each chromosome in a single cell is
consistent with a normal RPE-1 genome or indicates gain or loss of
transcription due to chromosome mis-segregation (including micro-
nucleation). To classify the transcriptional copy-number state based
on haplotype-specific transcription yield, we first compared the aver-
age haplotype-specific transcription of every chromosome in a RPE-1
cell to the normal range of transcription (‘reference’) derived from
control RPE-1 cells (Extended Data Fig. 2d). The normal transcription
distribution (Extended Data Fig. 1d) reflects transcriptional variation of
a single chromosome and was calculated separately for each parental
chromosome. For chromosomes in which transcription levels were
outside the normal range (red dots in Extended Data Fig. 2d), we then
compared the haplotype-specific transcription yield to normal disomic
transcription or complete DNA loss (‘nullisomic’) to determine whether
the transcriptional changes were consistent with whole-chromosome
gain or loss. If the transcriptional level of a chromosome did not fall
within normal ranges of monosomic (1), disomic (2) or nullisomic (0)
transcription states, it was classified as intermediate (1+ or 1–). For
the duplicated 10q segment or any chromosome inferred to be dupli-
cated, we only assessed whether the transcriptional level was within
the normal range of disomic transcription or displayed significantly
reduced transcription.
To assess whether the observed transcription yield of a chromo-
some is within the range of normal monosomic or disomic tran-
scription, we used two-tailed z-tests and considered deviations with
ArticleBonferroni-corrected P values of ≥0.05 to be non-significant. For the
comparison against nullisomic transcription, we did not calculate the
P value as the transcription yield should be strictly zero (that is, no
variation); any deviation from zero reflects technical errors (phasing
errors, amplification errors, sequencing errors, among others), for
which we did not have sufficient data to estimate the null distribution.
We classified a chromosome as being nullisomic if the normalized
transcription yield was below 0.1 based on the observations of nul-
lisomic chromosomes in bona fide monosomies. The classification
of the transcriptional states of all chromosomes with non-disomic
transcription in micronucleation-related cells is summarized in Sup-
plementary Table 2.
Identification of mis-segregated chromosomes and chromosomes
in micronuclei. We identified mis-segregated chromosomes based
on changes in the total and haplotype-specific transcription in all sib-
ling cells from each experiment (family). We first used allele-specific
transcription to identify homologous chromosomes with transcrip-
tion levels significantly deviating from normal (monosomic for that
haplotype) transcription (summarized in Supplementary Table 2).
We then considered both allelic and total transcription levels across
all cells in each family (MN cell, MN sister or their daughters) to de-
termine the integer DNA copy number states of chromosomes with
non-monosomic transcription and the chromosome segregation
pattern in the family. We also assessed whether the observed tran-
scriptional variation is consistent with the expected outcome of mi-
cronucleation, micronucleation-independent mis-segregation that
generates reciprocal loss and gain between sibling cells or random
transcriptional noise.
The identification of chromosomes that were partitioned into micro-
nuclei is based on matching the allelic imbalance and DNA copy number
states of a chromosome in all sibling cells inferred from the transcrip-
tome data to the expected outcomes of different mis-segregation or
segregation patterns of the MN chromosome (Extended Data Fig. 2a).
This inference automatically determines the parental haplotype of the
MN chromatid. Notably, the pattern of micronucleus-related transcrip-
tional changes can be identified independent of the transcription level
of the MN chromatid either in the MN cell or in the MN daughter cell
that has re-incorporated the MN chromatid. Therefore, the inference
of the MN chromatid based on the predicted patterns of transcriptional
changes does not affect the assessment of transcriptional normality of
the MN chromatid either in a micronucleus or after re-incorporation.
We note that the most definitive features of micronucleus-related
transcriptional changes are the loss of transcription of the MN chroma-
tid, which occurs in two scenarios: (1) in the MN sister cell or its progeny
(MN nieces) owing to mis-segregation of the MN chromatid; (2) in one
of two MN daughter cells that is missing the incompletely replicated
MN chromatid in the MN mother cell. In both scenarios, the inference
of the MN chromosome relies on the detection of near-complete tran-
scriptional loss in a non-MN cell (MN sister or MN nieces) or in one MN
daughter cell that did not re-incorporate the MN chromatid. Therefore,
the inference of the MN chromosome is insensitive to both spontaneous
transcriptional variability in non-MN cells (as it relies on the detection
of complete transcriptional loss) and potential transcriptional changes
due to the presence (MN cell) or re-incorporation (MN daughter cell)
of the MN chromosome.
We further note a few special cases. First, we identified two MN
cell–MN sister pairs (F84 and F206) with no chromosome displaying
significant deviations from the normal range of transcriptional varia-
tion. We inferred that the MN cell in these two families contained MN
chromosomes that had undergone 2:2 segregation and had normal
transcription output. Under these circumstances, the MN chromo-
some is transcribed like a normal chromosome and therefore ‘invisible’
based on the transcriptome data. We nonetheless cannot rule out other
possibilities, for example, when the micronucleus contains an acentric
chromosome arm (13p, 14p, 15p, 21p or 22p), the transcription output
of which cannot be assessed by RNA-seq. The inference of normal MN
transcription in these two families reflects a conservative estimate of
transcriptional deficiency in micronuclei. Second, in family F71, we
identified chromosome 18 to have a 3:1 transcriptional ratio between
the MN cell and the MN sister cell. This ratio indicated that the MN cell
contained an extra chromosome 18 (due to 3:1 mis-segregation) that
is being transcribed to normal levels. The extra chromosome 18 copy
could be either contained in the PN or partitioned in the micronucleus;
we inferred the extra chromosome 18 copy to be in the PN because
we identified chromosome 1p that displays the transcriptional pat-
tern expected for a MN chromosome with defective transcription.
Third, there were nine families of MN daughters for which we did not
obtain MN niece cells (most of these were collected using the original
Look-Seq method). For these cases, we inferred the identity of the MN
chromosome by comparing the total and allelic transcriptional imbal-
ance between the MN daughters to the transcriptional profiles of MN
daughters for which both the MN chromosome and its segregation
pattern can be directly inferred from the data of MN nieces. Specifi-
cally, when the re-incorporated MN chromosome displayed normal
transcription, the MN daughters showed either about 3:2 or 2:1 tran-
scriptional ratio, which reflected the presence of an extra, normally
transcribing chromosome in one MN daughter; we used this informa-
tion to infer normal transcription of re-incorporated MN when the
MN daughters showed the same transcriptional ratios even when no
MN niece is available. When the re-incorporated MN chromosome
displayed deficient transcription with transcriptional yield a, the MN
daughters would show transcriptional ratios of either 2 + a:2 or 1 + a:1.
When a ≈ 0, the MN daughters showed identical transcription patterns;
we can nonetheless conclude that the extra MN chromatid being pre-
sent in either daughter cell produced no transcriptional output and
therefore must be epigenetically silenced. We inferred family F254 to
correspond to this scenario.
Finally, we noted that chromosome 18 and acrocentric chromo-
somes (chromosomes 13, 14, 15, 21 and 22) displayed more transcrip-
tional variability than other chromosomes. The more pronounced
variability of these chromosomes is obvious from the reference distri-
butions. Such variation was generally not shared by sibling cells and/or
is inconsistent with the patterns of transcriptional changes predicted
by micronucleus-related or micronucleus-independent chromosome
mis-segregation events. Therefore, the variable transcription of these
chromosomes does not pose a problem for the identification of MN
chromosomes.
Quantification of the transcriptional yield of MN chromosomes.
After identifying the MN chromatid (both the chromosome identity and
the parental haplotype), we estimated the transcriptional yield of the
MN chromatid based on the haplotype-specific transcription yield. For
MN cells (generation 1) of 2:2 segregation, they contained a single copy
of the MN chromatid in the micronucleus, we therefore directly derived
the transcriptional yield of the MN chromatid from the transcriptional
yield of the MN haplotype. For MN cells having undergone 3:1 (MN cell:
MN sister) mis-segregations, the haplotype-specific transcriptional
yield of the MN haplotype represented the combined transcription
output from both the MN chromatid and its intact sister chromatid
in the PN. In this scenario, we compared the transcriptional yield of
the MN haplotype to the transcriptional yield of reference disomic
transcription levels to assess whether the MN chromatid displayed
normal or deficient transcription.
For re-incorporated MNs, if the MN chromosome was inferred to
have undergone a 2:2 segregation in generation 1, then the single-copy
MN chromatid is distributed to one or both MN daughter cells. In this
scenario, we estimated the transcription yield of the re-incorporated
MN chromatid using the combined transcriptional yield of the MN hap-
lotype in both MN daughters (this accounts for possible fragmentation
and reciprocal distribution of fragments of the MN chromatid into
both daughters). We then compared the transcription yield of the MN
haplotype to the range of normal transcription of a single homologue to
assess transcriptional normality or deficiency. If the MN chromosome
was inferred to have undergone a 3:1 segregation in generation 1, then
each MN daughter contained an extra, intact copy of the MN chromo-
some in addition to the re-incorporated MN chromatid. In this scenario,
we compared the transcriptional yield of the MN haplotype in each
MN daughter cell to the ranges of both monosomic transcription and
disomic transcription to assess the normality or deficiency of transcrip-
tion of the re-incorporated MN chromatid.
The data of normalized transcription ratios, inferred DNA copy
number states and the transcriptional yields of MN chromatids and
haplotypes are summarized in Supplementary Table 2.
Same-cell correlative live-fixed imaging
For the same-cell correlative live-fixed imaging experiments using
MDC1-expressing cells (Fig. 3a,b), cells were seeded on 35-mm ibiTreat
Grid-500 dishes (Ibidi) with a gridded imaging surface. Live-cell imaging
was performed using wide-field fluorescence microscope as described
in the ‘Live-cell imaging’ section. At the end of live-cell imaging, cells
were immediately fixed by incubation with methanol for 10 min at
−20 °C. A snapshot of the last imaging frame including a differential
interference contrast image was taken to visualize the grids of the
coverslip dish. The grid coordinate information and the last snapshot
of the time-lapse images were used to locate the cells of interest after
fixation and indirect immunofluorescence imaging.
For experiments using the RPE-1 RFP–NLS GFP–H2B cells (Extended
Data Fig. 9a) and the modified U2OS 263 cells (Fig. 4d,e), live-cell imag-
ing was performed as described above. At the end of the live-cell imag-
ing, cells were fixed by incubation with methanol for 10 min at −20 °C
for RPE-1 RFP–NLS GFP–H2B cells or 4% paraformaldehyde for 20 min
at room temperature (modified U2OS 263 cells). Cells of interest were
located according to the grid coordinates for subsequent indirect
immunofluorescence analysis.
Indirect immunofluorescence and confocal microscopy of fixed
cells
Cells were fixed and prepared for indirect immunofluorescence and
confocal microscopy as previously described11,26.
Images were acquired on a Nikon Ti-E inverted microscope (Nikon)
with a Yokogawa CSU-22 spinning disk confocal head with the Borealis
modification or a Ti2 inverted microscope fitted with a CSU-W1 spinning
disk. Z-stacks of 0.4–0.7 µm spacing were collected using a CoolSnap
HQ2 CCD camera (Photometrics) or a Zyla 4.2 sCMOS camera (Andor)
with a ×60/1.40 NA or a ×100/1.45 NA Plan Apochromat oil-immersion
objective (Nikon).
The following antibodies were used for indirect immunofluores-
cence imaging: phospho γH2AX (Ser139) (Millipore, 05-636-I; 1:400);
H3K27ac (Active Motif, 39133; 1:200); MDC1 (Abcam, ab11171; 1:1,000);
MDC1 (Sigma-Aldrich, M2444; 1:1,000); phospho RNA PolII S5 (Milli-
pore, MABE954, clone 1H4B6; 1:400); Cdk9 (Cell Signaling, 2316; 1:10);
CDK12 (Abcam, ab246887; 1:400); 53BP1 (Santa Cruz, 22760S; 1:100);
H3K27me3 (ThermoFisher Scientific, MA511198; 1:1,000); H3K9ac
(Cell Signaling, 9649S; 1:400); H3K9me2 (Cell Signaling, 9753S; 1:400);
POM121 (Proteintech, 15645-1-AP; 1:200); phospho H3T3 (Millipore,
07-424, 1:12,000); phospho H3S10 (Abcam, ab47297; 1:200); and
fibrillarin (Abcam, ab4566; 1:500). Staining of Dam-methylated DNA
in fixed cells was done using purified GFP-tagged m6A-Tracer protein as
previously described53.
Image analysis of fixed-cell experiments
Two image analysis pipelines were used in this study. To characterize
the transcriptional state and chromatin alterations in micronuclei
(Fig. 1e–g and Extended Data Figs. 4 and 5), we used customized ImageJ/
Fiji macros as previously described26. To characterize MN bodies or
MN-body-like structures (Figs. 3 and 4 and Extended Data Figs. 9, 10
and 12), we used a Python-based analysis pipeline47 with additional
preprocessing procedures performed using ImageJ/Fiji software. Both
pipelines overall consisted of the following steps: (1) cells of interest
were identified and their primary nuclei were segmented; (2) micronu-
clei or re-incorporated MN chromosomes (or chromosome bridges)
were identified and segmented; (3) mean FI values for labelled proteins
or DNA were quantified over the segmented regions of interest (ROIs).
Analysis of the transcription and chromatin alterations in micro-
nuclei. Image analysis of micronuclei in immunofluorescence experi-
ments were performed as previously described26.
Image segmentation and ROI identification. First, the three-
dimension (xyz) images of primary nuclei and micronuclei were seg-
mented using the Li or Otsu thresholding method in ImageJ/Fiji with
the DNA (Hoechst) signal as input. Second, the nuclear segmentations
were further refined using the ImageJ/Fiji functions Watershed and
Erode to remove connecting pixels bordering abutting nuclei. Third,
nuclear segmentations containing primary nuclei and micronuclei
were manually selected as ROIs using the ImageJ/Fiji functions Wand
Tool. ROIs from one single focal plane where primary nuclear and
micronuclear DNA signal were in focus were manually selected and
used for the following quantification.
FI quantification. The mean FI of labelled proteins or DNA was quanti-
fied over the selected ROIs from their corresponding microscope fluo-
rescence channels. For quantification of nuclear proteins (for example,
RNAP2-Ser5ph, RFP–NLS; Fig. 1e–g and Extended Data Fig. 4) or labelled
DNA (Hoechst), the mean FI values were calculated for micronuclear
ROIs and primary nuclear ROIs, respectively. These mean FI values were
subtracted by the mean FI value of the non-nuclear background to ob-
tain the background-subtracted mean FI. The background-subtracted
mean FI of micronuclei were divided by the background-subtracted
mean FI of the corresponding PN to obtain the MN/PN mean FI ra-
tios. For quantification of histone modifications, including H3K27ac,
H3K9ac, H3K9me2, H3K27me3 and γH2AX, the MN/PN mean FI ratios
of these marks were further divided by the MN/PN mean FI ratio of
DNA (Hoechst) to obtain the DNA-normalized FI ratios. To analyse mi-
cronuclei with intact or ruptured NE, micronuclei with MN/PN mean FI
ratios of NLS below 0.1 relative to the PN were considered ruptured and
above 0.3 were considered intact. Micronuclei with MN/PN FI ratios in
between were excluded for analysis, as the assessment of NE integrity
is not definitive.
In addition, the background-subtracted mean FI of RNAP2-Ser5ph
are shown as exact FI values without normalizing to the
background-subtracted mean FI of the corresponding PN (Extended
Data Fig. 4b).
Analysis of generation 2 re-incorporated MN chromosomes.
Analysis of incorporated MN chromosomes were performed primar-
ily using an automated script written in Python47. Further details are
available upon request.
Image segmentation and ROI identification. Step 1, all candidate
primary nuclei within the three-dimensional images were identified
and segmented either using the Li thresholding method with DNA
(Hoechst) signal as the input or the Otsu or Li thresholding method
with RNA Pol2S5 signal as the input. The nuclear segmentations were
further refined using binary mask operations similar to the procedures
described above for the micronuclei analysis pipeline.
Step 2, a smaller cropped three-dimensional image (z-stack) was gen-
erated for each segmented PN object to minimize variability in the fluo-
rescence signal across the entire image. Only PN objects located within
Articlethe middle 50% of our images were analysed to minimize the uneven
illumination due to the large field of view of the camera (2,048 × 2,048
pixels). From these cropped three-dimension images, a single focal
plane in which the MDC1 or m6A-Tracer (hereafter m6T) signal was in
focus was selected. This single focal plane was determined as the focal
plane with the largest standard deviation (s.d.) in the FI distribution of
all pixels (which is used as an estimator of the strongest overall signal)
from the MDC1 or m6T channel. The cropped xy images and segmenta-
tions for each candidate PN objects were then analysed.
Step 3, primary nuclei that contained potential re-incorporated MN
chromosomes were located using the presence of large foci of MDC1
or m6T. To identify MDC1 or m6T large foci for each candidate PN, the
FI of all pixels within the corresponding nuclear segmentation were
quantified to generate a nuclear FI distribution. Positive pixels were
selected if their FI > 2 s.d. above the mean for the nuclear FI distribution.
These positive pixels were subject to an area size filter (300 pixels) to
remove small noise pixels so that only connected-positive pixels larger
than the size filter were kept to generate the final ROIs for MDC1 and
m6T. For nuclei with multiple valid MDC1 and m6T foci (for example,
from two or more MN chromosomes), all foci were analysed together
per each nucleus. Additionally, to increase detection accuracy of m6T
foci from cells with a variable m6T expression, candidate nuclei of
interest were manually screened using ImageJ/Fiji. The xy coordinates
of these candidate nuclei were supplemented as additional inputs
for the analysis pipeline and used for locating valid nuclei containing
m6T foci according to the above criteria (FI > 2 s.d. and > 300 pixels)
using Python.
Step 4, segmentations for other objects (nuclear or subnuclear struc-
tures) that were used for the analysis were generated. Specifically, the
ROIs for the primary nuclei were defined by excluding the MDC1 and
m6T segmentations as well as the nucleoli segmentations from the
original nuclear segmentation (see step 1). The nucleoli segmentations
were generated using the lower 10% of the primary nuclear FI distribu-
tion of RNAP2-Ser5ph. This 10% (percentile) cutoff was determined by
comparing with the nucleoli segmentations using fibrillarin-positive
signals (which are pixels for which FI > 3 s.d. above the mean for its total
nuclear FI distribution; Extended Data Fig. 9f): the highest overlap with
the nucleoli segmentations using the fibrillarin-positive signal was
achieved using the lower 10% of the nuclear RNAP2-Ser5ph FI as the
cutoff for nucleoli segmentations. ROIs or the γH2AX-positive areas
were defined by γH2AX-positive pixels for which FI > 3 s.d. above the
mean for the nuclear FI distribution. The ROI area occupancy ratio
of the γH2AX-positive pixels within the m6T foci was used to define
different levels of γH2AX in re-incorporated m6T micronuclei (Fig. 4c
and Extended Data Fig. 10e).
Step 5, a randomized control ROI was segmented by randomly pick-
ing a smaller area (at a size similar to the MDC1 or m6T ROI) within the
primary nuclear ROI generated above for each cell containing a MDC1
or m6T foci. The random picking process was performed using our
Python-based analysis pipeline.
To validate the accuracy of MDC1 and m6T foci identification, the ROI
segmentations of a random subset of cells were manually examined. The
mis-identification rate of our automated pipeline using random subsets
of cells was typically lower than 10%. Additionally, for the m6T dataset
after quantification (see below), outliers were also manually exam-
ined. The mis-identification rate for the outliers of m6T dataset was
25%. These mis-identified m6T foci (n = 27) were mostly m6T-positive
micronuclei immediately next to the primary nuclei and were distrib-
uted near-symmetrically at the top and bottom of the measurement
distribution. These images were excluded during the analysis.
FI quantification. The mean FI of labelled proteins or DNA was quanti-
fied over the segmented ROIs above from their corresponding micro-
scope channels. All mean FI values were then background subtracted
by the corresponding mean FI of the non-nuclear background. The
background-subtracted mean FI of MDC1 or m6T ROIs (see step 3 above)
and the background-subtracted mean FI of the randomized control
ROI (see step 5 above) were normalized to the background-subtracted
mean FI of the corresponding primary nuclear ROI (see step 4
above) to obtain the normalized mean FI ratios of labelled proteins
or DNA.
For quantification of histone modifications, including H3K27ac,
H3K9ac, H3K9me2, H3K27me3, γH2AX, H3S10ph and H3T3ph, the nor-
malized mean FI ratios of these marks were further divided by the nor-
malized mean FI ratio of DNA (Hoechst) to obtain the DNA-normalized
FI ratios. This controlled for signal enrichment due to chromosome
compaction.
Analysis of re-incorporated fragments from chromosome bridge
resolution. Quantification of RNAP2-Ser5ph of incorporated bridge
segments after bridge resolution and cell division (Extended Data
Fig. 12a) was performed in a similar manner to the re-incorporated
micronuclei as described above, with ROIs for MN-body-like structures
from bridge segments identified using MDC1-positive signal (FI > 2 s.d.
and >300 pixels).
Analysis of incorporated MN chromosomes for the correlative
live-fixed imaging. For quantification of RNAP2-Ser5ph and H3K27ac
in incorporated MN chromosomes marked by MDC1 foci (Fig. 3a,b),
the analysis was performed in a similar manner as described above
except for the following differences: (1) The MN-body segmentations
were manually drawn along the MDC1-enriched pixels; and (2) the
control (or PN) segmentations were manually defined as a large PN
region excluding nucleoli. ROIs for MN bodies and controls were manu-
ally selected over these segmentations for a single Z-plane where MN
bodies were in focus. The mean FI MN body-to-control ratios were
obtained by dividing the background-subtracted mean FI of the MN
body ROIs to the background-subtracted mean FI of the control ROIs.
Note that the time-lapse images were analysed manually to assign the
re-incorporated daughters and rupture events, and the low sample size
allowed for the manual quantification analysis. In addition, note that
some daughter cells with MN bodies that were included in the analysis
had new micronuclei in the generation 2 samples, independent of the
detected re-incorporated MN chromosome.
Graphical data from the imaging analyses were plotted and statisti-
cal analyses were performed using GraphPad Prism (v.9.4.0; GraphPad
Software).
Analysis of live-cell imaging data for MS2-marked nascent
transcription
To quantify the MS2-marked transcription level (Figs. 2e,f and 4d and
Extended Data Fig. 8), an automated script written in Python was used
with assists using ImageJ/Fiji.
Image segmentation and LacO and MS2 foci tracking. MN cells with
the LacO and MS2 (LacO/MAS) reporter or control cells without MN
were manually identified from the image series, and image series of
interest were divided into three parts: generation 1 interphase, mitosis,
and generation 2 interphase.
For time frames covering the generation 1 interphase (for both con-
trol cells and for MN cells), all primary nuclei were segmented using
the cellpose package54, and all LacO/MS2 foci (in both MN and PN)
were segmented using the Yen segmentation method55 for each time
frame. The primary nuclei and LacI foci of interest in the first time point
were identified by finding the object segmentation with the shortest
distance to a user-provided xy centroid coordinate of the nucleus and
the LacO/MS2 focus, respectively. For the following time points, the
same nuclei and LacO/MS2 foci were automatically identified by find-
ing the object segmentation for which the distance was the shortest to
the identified nuclei and the LacO/MS2 foci segmentations from the
previous time point (or time points). The identification of the LacO/
MS2 foci and their corresponding primary nuclei was manually evalu-
ated to assist the tracking of the correct LacO/MS2 foci. The xy centroid
coordinates of the segmentations were used for estimating the object
moving distance above.
For time points around mitosis, the nuclei and LacO/MS2 foci were
manually tracked in ImageJ/Fiji to accurately identify the partitioning
of LacO/MS2 foci into daughter cells during mitotic exit.
For time frames after mitosis (generation 2), daughter primary nuclei
were segmented and tracked as described for the first interphase. For
LacO/MS2 foci segmentation tracking, we used a combination of sev-
eral criteria for technical reasons. Because long-term binding of LacI
to LacO can impair DNA replication, we terminated LacI gene expres-
sion at around 18 h after mitotic shake-off. This led to a loss of LacI
signal in a subset of daughter cells during the generation 2 interphase,
particularly evident at later time points. For these cells that had lost
the LacI signal, we quantified the FI distribution for the nuclear MS2
signal and identified positive MS2 pixels for which FI > 3 s.d. above
the mean for the nuclear MS2 FI distribution. The enrichment of such
nuclear MS2-positive foci (if present) was then used for the LacO/MS2
foci segmentation tracking for the subsequent time points. For time
points in which LacI foci persisted, we tracked the LacO/MS2 foci as
described for generation 1 interphase. If no LacO/MS2 foci could be
identified during generation 2 interphase, time points were annotated
as having no MS2 expression, and therefore no segmentation was
performed.
Additionally, we manually examined the nuclei and LacO/MS2 foci
tracking because the estimation of object movement using minimal
centroid moving distance could lead to incorrect tracking when objects
swap positions between time points. For these time points, additional
xy pixel centroid coordinates for the nuclei and LacO or MS2 foci were
obtained using ImageJ/Fiji and supplemented the automated object
tracking.
Fl quantification for ROIs. ROIs for the LacO or MS2 foci and the cor-
responding PN were selected from their segmentation as described
above. ROIs from one single focal plane where the LacI signal was in
focus were used for FI quantification. Based on these ROIs, the mean FI
of the MS2 signal for LacO/MS2 foci and matching PN pairs and of the
non-nuclear background was quantified. The background-subtracted
mean FI of LacO/MS2 foci was divided by the background-subtracted
mean FI of the primary nuclear areas (excluding the LacO/MS2 foci) to
obtain the normalized MS2 level.
For time points that were annotated as having no MS2 expression
in generation 2, a value of 1.7 was assigned because this value is the
minimal detectable normalized MS2 signal for the positive MS2 foci in
the controls (see details below) for the purpose of plotting the graphs.
To obtain this value, we analysed 23 control cells over two cell-cycles
for which the LacO/MS2 focus was located within the PN for both
generations 1 and 2. We quantified the normalized MS2 level during
the generation 2 interphase for time points at which the cells had lost
the LacI signal after we stopped LacI expression. These control cells
maintained MS2 reporter transcription, and their LacO/MS2 foci were
detected and segmented from MS2-positive pixels (FI > 3 s.d. above
the mean for the nuclear MS2 FI distribution, as described above).
The lowest of the normalized mean FI for all detected MS2-positive
foci (n = 477) from all imaged time points above was 1.7, defining 1.7 as
the minimum detectable mean MS2 signal in the control experiments.
Therefore, 1.7 was used as the normalized MS2 signal when no positive
MS2 foci could be detected. Note that this is a conservative estimate
because the actual MS2 level can be lower as measured for some MN
bodies in which the LacI signal was present and can be used for seg-
mentation. In other words, this should underestimate the degree of
MS2 signal loss for MN chromosomes that are in a normal generation
2 daughter cells.
SDS–PAGE and western blotting
Lysis of RPE-1 Dam and control RPE-1 cells (Extended Data Fig. 10c)
was performed after trypsinization and washes with PBS by adding
an equal volume of a 2× lysis buffer (100 mM Tris-HCl pH 6.8, 4% SDS
and 12% β-mercaptoethanol). Whole-cell lysates were denatured at
100 °C for 10 min, Laemmli–SDS sample buffer (Boston BioProd-
ucts) was added, and the samples were subjected to SDS–PAGE on
NuPAGE 4–12% Bis-Tris gradient gels (Novex Life Technologies). The
proteins were then transferred onto a nitrocellulose membrane
(Millipore). The membranes were blocked using Odyssey blocking
buffer (LI-COR) and were incubated with primary antibodies for
1 h at room temperature or overnight at 4 °C. The primary antibod-
ies and dilutions used were anti-mCherry rabbit 1:1,000 (ab167453,
Abcam) and anti-GAPDH mouse 1:5,000 (ab9485, Abcam). After
washes with PBS-T, we incubated the membranes with the fluores-
cent secondary antibodies IRDye 680RD donkey anti-rabbit 1:5,000
(926-68073, LI-COR) and IRDye 800CW donkey anti-mouse 1:5,000
(926-32212, LI-COR) for 1 h at room temperature. Membranes were
visualized using a ChemiDoc MP imaging system (Bio-Rad). Note that
the images shown in Extended Data Fig. 10c were cropped to show the
bands at the protein size. The full scan (uncropped) blots are shown in
Supplementary Fig. 1.
FACS
RPE-1 megaDam cells (see the section ‘Cell culture and cell line con-
struction’) were analysed by FACS for mCherry expression using a
LSR Fortessa flow cytometer (BD) (Extended Data Fig. 10b). Cells were
stained with DAPI for dead-cell exclusion and live cells were analysed for
their percentage of mCherry-positive cells (excluding autofluorescent
cells by gating PE relative to FITC). Data were recorded using FACSDiva
(v.8.0; BD) software, and FlowJo (v.10.7.1; BD) was used for data analysis.
Examples of the gating strategy are shown in Supplementary Fig. 2.
Genomic analysis of bridge clones
DNA sequencing. Genomic DNA was purified using a DNeasy Blood
and Tissue kit (Qiagen) and was then fragmented on a Covaris M220
instrument according to the manufacturer’s protocol. Libraries were
prepared using Swift S2 Acel reagents on a Beckman Coulter Biomek
i7 liquid handling platform from approximately 200 ng of DNA with 14
cycles of PCR amplification. DNA libraries were quantified on a Qubit
2.0 Fluorometer (Life Technologies) and fragment size distributions
were evaluated on a Agilent TapeStation 2200 (Agilent Technologies).
Pooled libraries were further evaluated with low-pass sequencing on an
Illumina MiSeq and then sequenced to approximately 5× mean genome
coverage on an NovaSeq 6000 instrument (Illumina) with 2× 150 bp
paired-end configuration in the Molecular Biology Core Facilities at
Dana-Farber Cancer Institute. Haplotype-specific DNA copy number
was calculated using the same workflow as previously described11,48.
DNA rearrangements shown in Extended Data Fig. 11 were taken from
previous analyses11.
RNA-seq. RNA extraction, library preparation and sequencing were
conducted at Azenta Life Sciences. In brief, total RNA was extracted
from fresh-frozen cell pellet samples using a RNeasy Plus Universal
mini kit (Qiagen). RNA samples were quantified using a Qubit 2.0 Fluo-
rometer (Life Technologies), and RNA integrity was evaluated using a
TapeStation 4200 (Agilent Technologies). An ERCC RNA Spike-In Mix
kit (4456740, ThermoFisher Scientific) was added (but not used) and
sequencing libraries were prepared using a NEBNext Ultra RNA Library
Prep kit for Illumina (NEB). The quality of the sequencing libraries
were validated on a Agilent TapeStation (Agilent Technologies), and
the concentration of the libraries were quantified using a Qubit Fluo-
rometer and by quantitative PCR (KAPA Biosystems). The samples
were sequenced on an Illumina instrument (4000 or equivalent) with
Article2× 150 bp paired-end configuration with an average of around 60 mil-
lion reads per sample.
Bulk RNA-seq data were aligned using STAR (v.2.7.10a) with the same
parameters as single-cell RNA-seq data processing. As the estimated
fraction of duplicate reads in bulk RNA-seq data was above 5%, we
followed all steps of post-alignment processing (including duplicate
removal) as described in the best practice of GATK. All post-alignment
processing was carried out using GATK (v.4.2.6.1). The remaining
steps of RNA-seq data processing were identical to the processing of
scRNA-seq data.
We first generated feature counts from analysis-ready RNA-seq bam
files using featureCounts from Subread 2.0.1 (https://subread.source-
forge.net) and then calculated total TPM47. We performed a similar
global TPM normalization step for each sample by scaling the TPM val-
ues by a constant factor to match the median expression of genes that
are transcribed bi-allelically and have mean TPM between 1 and 1,000
(6,683 total). After global normalization, we calculated allelic tran-
scription of each gene using the same procedure as for the single-cell
transcriptome analysis. To assess the transcriptional yield of each gene
copy, we further divided both the total and allelic transcriptional levels
by the DNA copy number in 250 kb local intervals; the DNA copy number
was determined from whole-genome DNA sequencing data generated
on the same culture. To quantify transcriptional changes relative to
normal transcription, we normalized the transcription yield (both total
and allelic) in bridge and control clones by the transcriptional level in
the parental RPE-1 sample. The final TPM ratio (gene level) was then used
to assess transcriptional changes (both total and haplotype-specific)
independent of copy-number variation.
ATAC-seq. The preparation of nuclei, transposition and amplification
by PCR were performed as previously described56. In brief, cells were
trypsinized and washed twice with PBS. Then, 10,000 cells in 5 µl PBS
were transposed in 42.5 µl of transposition buffer (33 mM Tris acetate
buffer, 66 mM potassium acetate, 10 mM magnesium acetate, 0.1% NP-
40, 16% DMF, 0.004× protease inhibitor cocktail and ddH2O to 42.5 µl)
and 2.5 µl of TDE1 Illumina Tn5 transposase. The transposition reac-
tion was conducted for 30 min at 37 °C, followed immediately by DNA
purification using a ZYMO DNA Clean and Concentrator 5 kit (Zymo
Research). Cycle-determining quantitative PCR was conducted to am-
plify libraries and stop amplification before saturation. The amplified
libraries were purified using a ZYMO DNA Clean and Concentrator 5
kit and quantified by using a Qubit 2.0 Fluorometer. The libraries were
normalized and pooled based on quantitative PCR analysis and were
subsequently sequenced on a NovaSeq S1 instrument (Illumina) with 2×
50 bp paired-end configuration or a NextSeq instrument (Illumina) with
2× 38 bp configuration at the Bauer Core Facility of Harvard University.
Reads were trimmed to remove adapter sequences and then aligned
to hg38 using Bowtie2 (ref. 57) with the following parameters: -X2000–
rg-id. Chromatin accessibility peak calling was conducted as previ-
ously described58. In brief, we first performed peak-calling on each
sample using MACS2 (ref. 59) with the following parameters/options:
–nomodel,–nolambda,–keep-dup all,–call-summits.
We then combined and merged overlapping peaks (within 400 bp)
called from all samples to create a unique list of peaks (259,036 total).
The fragment count within each peak was calculated using the get-
Counts function from chromvar60, and then normalized using the
preprocessCore normalize.quantiles function61.
To assess changes in chromatin accessibility in the bridge clones,
we first divided the quantile-normalized fragment count60 for every
peak by the local DNA copy number (250 kb bins) to account for DNA
gain or loss, which was almost exclusively restricted to chromosome 4.
To account for technical variation during library preparation, we
applied a permutation approach to generate a reference ATAC profile for
each individual clone based on the ATAC profiles in control clones. First,
for each ATAC-seq peak, we generated a replicate set of 50 peaks with
similar GC content and average accessibility in the control samples (ten
control RPE-1 subclones) using the getBackgroundPeaks(<normalized.
counts>, bias = <gc.bias>) command from chromvar60.
Here <gc.bias> was calculated for each peak region (300 bp) and
<normalized.counts> denotes the ATAC fragment counts in the ten
control subclone samples. Assuming the replicate peaks are subject to
similar technical variation, we then used the ATAC-seq densities of rep-
licate peaks as the null distribution for the peak of interest to perform
intra-sample background normalization by random permutations.
During each permutation, we randomly selected 1 out of 50 replicate
peaks for each peak in a given genomic interval to create a random
reference ATAC profile. By generating a sufficient number of reference
ATAC profiles through permutations, we could assess the statistical
deviation of the observed ATAC profile in each genomic interval from
the null distribution generated by permutations. To enable a sufficient
number of permutations, we only considered intervals with at least 10
peaks per Mb (with a maximum of 5010 ≈ 9.8 × 1016 permutations). We
performed around 106 permutations for each interval (lower than the
number of all possible permutations) to identify outliers with P values
on the order of 10−6.
The above-described permutation sampling was performed on the
fragment counts in each sample in 1, 5 or 10 Mb intervals. Based on
the null distributions derived from random permutations, we then
calculated the fold change of the observed ATAC density relative to
the mean of the null distribution. The re-centered fold change of ATAC
signal is shown in Extended Data Fig. 12b. We further estimated the
likelihood of the observed average ATAC density of each interval in each
sample based on the null distributions generated by permutations (an
example is shown in Extended Data Fig. 12d). Shown in Figs. 5b and 12c
are the average fold change of ATAC signals across all 12 bridge clones.
Our permutation sampling directly accounts for GC bias. It also
accounts for non-uniform peak density across the genome. Addition-
ally, in our analysis, we primarily focused on clonal or near-clonal
changes that are more likely generated by the initial formation and
resolution of bridges than subclonal changes that are more likely to
have arisen downstream. We therefore focused on intervals with a
significant reduction in the average ATAC fold change (<0.70).
Reporting summary
Further information on research design is available in the Nature Port-
folio Reporting Summary linked to this article.
Data availability
The authors declare that the data supporting the findings of this study
are available within the paper and its supplementary information files.
Sequencing data are available from the Sequencing Read Archive under
BioProject identifiers PRJNA602546 and PRJNA867730. The raw data
and all other datasets generated in this study are available from the
corresponding authors upon reasonable request. Source data are pro-
vided with this paper.
Code availability
Scripts and pipelines used for all sequencing data analysis and for image
analysis are available at the GitHub online repository (https://github.
com/chengzhongzhangDFCI/nature2023)47.
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Acknowledgements We are grateful to staff at the Janelia Research Campus, D. Spector, T. de
Lange and T. van Schaik for reagents; R. Nicol, H. Zhang and Y. Brody for assistance and advice
with LCM microscopy; J. Wang, R. Parasuram, M. Leibowitz and H. Ndiaye for help with
preliminary experiments; N. Sebryn and R. Davidowitz for help with scheme illustrations and/or
video editing; S. Armstrong for use of a LSR Fortessa flow cytometer; R. Jaenisch, M. Meyerson,
A. Spektor and members of the Pellman laboratory for discussions; and staff at the Center for
Cancer Genomics of Dana-Farber Cancer Institute for sequencing services. G.B. is supported
by the training grant T15LM007092 from the National Library of Medicine. C.-Z. Z. was
supported by a NCI K22 award (K22CA216319) and by the Claudia Adams Barr Program for
Innovative Cancer Research from the Dana-Farber Cancer Institute. D.P. is a HHMI Investigator,
a member of the HMS/Boston Ludwig Center and is supported by NIH R01 CA213404-24 and
an award from the G. Harold and Leila Y. Mathers Foundation.
Author contributions S.P., C.-Z.Z. and D.P. conceived the project. S.P. designed all experiments
with the supervision of D.P. S.P. performed most of the experiments and invented the modified
LCM system. E.S. helped with the cell culture experiments. S.P., N.A.M., G.B., E.J. and C.-Z.Z.
designed the bioinformatics analysis. N.A.M. performed the scRNA-seq data analysis. G.B.
performed the ATAC-seq and DNA sequencing data analyses. L.L. performed the bulk RNA-seq
data analysis and C.-Z.Z. supervised the computational genomic analysis. S.L. developed the
automated image analysis pipelines. S.P., N.A.M., S.L., E.J., E.S., C.-Z.Z. and D.P. analysed data.
C.C. and J.D.B. helped with the ATAC-seq analysis. B.v.S. contributed reagents and advice for
the DamMN experiments. D.P., S.P. and C.-Z.Z. wrote the manuscript with edits from all the
authors. D.P. supervised the study.
Competing interests J.D.B. holds patents related to ATAC-seq and is a scientific advisory board
member of Camp4 and seqWell. C.-Z.Z. is a scientific adviser for Pillar BioSciences. D.P. is a
member of the Volastra Therapeutics scientific advisory board. Dana-Farber Cancer Institute is
in the process of applying for a patent application (PCT/US20 19/023696, September 26, 2019)
covering “Systems and methods for capturing cells” that lists S.P. as inventor. All other authors
declare no competing interests.
Additional information
Supplementary information The online version contains supplementary material available at
https://doi.org/10.1038/s41586-023-06157-7.
Correspondence and requests for materials should be addressed to Stamatis Papathanasiou,
Cheng-Zhong Zhang or David Pellman.
Peer review information Nature thanks Lilian Kabeche and the other, anonymous, reviewer(s)
for their contribution to the peer review of this work.
Reprints and permissions information is available at http://www.nature.com/reprints.
Articlea Look-Seq2 overview
Look-Seq2
MN nieces
MN sister
generation 1
mitotic
shakeoff
G1
G2
M
generation 2
MN cell
b Design of the single-cell capture apparatus
microchamber
hydrophobic
barrier
cell
media
glass
coverslip
cut cells of interest
Look-Seq
MN daughters
capture
Smart-seq2
membrane
ring
membrane
invert
imaging
microscope
LCM
microscope
c
Total and allele-specific expression assessed by single-cell RNA-Seq
disomic
monosomic
trisomic
A
B
total mRNA
allelle-specific
mRNA
A
B
d
3
2
1
0
Normal transcriptional variation determined from single-cell RNA-Seq of control RPE-1 cells
Normalized haplotype-specific chromosomal transcription
Chr
1
total
A B
2
3
4
5
6
7
8
9
10a 10b
11
12
13
14
15
16
17
18
19
20
21
22
X
active X
61Mb-qter
duplication
inactive X
Extended Data Fig. 1 | See next page for caption.
Extended Data Fig. 1 | Overview of experimental and analytical workflows.
(a) Scheme of Look-Seq2. There are two key improvements compared to the
original Look-Seq. First, live-cell imaging starts before the first cell division
that leads to micronuclei; this enables tracking, isolation, and transcriptome
analysis of both the MN cell and its sister cell (“generation 1”). Moreover, we
can image cells over two cell divisions (“generation 2”) and analyze both the
daughters of the MN cell (“MN daughters”) and the daughters of the MN sister
cell (“MN nieces”). The second improvement is that single cells are isolated
using a new capture strategy with minimal mechanical perturbation that is
illustrated in (b). (b) Second generation experimental strategy for single-cell
capture and sequencing. We adapted a previously developed LCM system
(Palm Microbeam, Carl Zeiss) and re-designed the imaging and capture setup.
The modifications enable the inversion of the membrane rings relative to
the microscope objective. This allows medium to be present continuously
throughout capture, which provides more time for the capture of family
member cells. The setup is also compatible with laser catapulting into 96 well
plates, which further increases throughput. See Methods for details. (c) Two
measures of transcription yield from single-cell RNA-Seq data: (1) The total
transcriptional yield is assessed by the transcripts per million (TPM) calculated
from all RNA-Seq fragments overlapping with annotated coding regions.
(2) The fraction of transcripts derived from each parental homologue is
estimated from the counts of haplotype-specific sequencing reads. The
haplotype-specific transcription yield is estimated by multiplying the total
transcriptional yield by the haplotype fraction of transcripts. The transcription
level of each gene in a single cell is further normalized by its mean in normal
RPE-1 cells to obtain the normalized transcription of each gene. Details of
the computational analysis are provided in Methods. (d) Normal range of
transcriptional variation of each parental homologue derived from single-cell
RNA-Seq data of control RPE-1 cells (n = 198; for Chr.12 n = 190 after excluding
trisomies). Shown are the range of mean transcription of each chromosome
(mean TPM ratio across all genes on a chromosome in each cell; shaded boxes)
and the range of haplotype-specific transcription (mean haplotype-specific
TPM ratio across all genes on a chromosome in each cell, open boxes) calculated
from the total transcription and the haplotype fractions. Box plots indicate
the 1st (bottom edge) and 3rd (top edge) quartiles and the median (horizontal
line), with whiskers indicating 1.5x the interquartile range. The range of total
transcriptional variation is used to estimate the range of normal disomic
transcription (i.e., transcription of two copies of a chromosome, either from
one copy of both parental homologues or from two copies of one homologue);
the range of haplotype-specific transcriptional variation is used to estimate
the range of normal transcription from each parental homologue. For the
trisomic Chr.10q segment (61Mb-qter), the two haplotype-specific TPM ratios
reflect the transcriptional output of the single-copy homologue (A) and the
duplicated homologue (B); for Chr.X, the haplotype-specific TPM ratios reflect
the transcriptional output of the active X (Xa) and the inactive X (Xi). For the
10q segment and Chr.X, the haplotype-specific TPM ratios are calculated by
normalizing the TPM ratio of the intact 10q (A homologue) and Xa to 1. The
duplicated 10q segment is appended to the q-terminus of the active X.
Articlea
MN daughters
MN nieces
2:2 segregation
1:3 segregation
MN
PN
MN cell
MN sister
PN
D2
D1
N1
N2
MN sister
MN cell
MN
PN
PN
MN nieces
N1
N2
D1
D2
MN daughters
b
MN generated by CRISPR/Cas9
c
normalized transcription (10 Mb/bin)
MN sister
MN haplotype
intact haplotype
CRISPR-Cas9
MN cell
PN
MN
PN
2
1
0
2
1
0
Chr5
~64 Mb
cumulative TPM
from low to high expression
40000
trisomy
disomy
monosomy
0
0
40000
40000
0
0
40000
d
Haplotype-specific transcriptional yield (TPM ratio) of all chromosomes in MN experiments (nocodazole)
reference (mostly monosomic except 10q and Xi)
non-reference
0
20
40
60
80
100 120 140 160 180 (Mb)
2
1
0
Chr
1
2
3
4
5
6
7
8
9
10a
10b
11
12
13
14
15
16
17
18
19
20
21
22
X
Extended Data Fig. 2 | See next page for caption.
Extended Data Fig. 2 | General strategy for the inference of haplotype-
specific DNA copy number and chromosome mis-segregation events from
single-cell RNA-Seq data. (a) Two segregation patterns of MN chromosomes
(left: 2:2 segregation; right 1:3 segregation) generated by nocodazole-block-
and-release and the predicted copy-number outcomes over two generations.
MN chromatids (filled magenta) and chromatids of the other haplotype (open
magenta) are represented in the same fashion as in Figs. 1 and 2. Under 2:2
segregation, the MN sister cell or the MN nieces (dashed boxes) should display
bi-allelic disomic transcription but one of the two MN daughter cells (shaded
boxes) should display mono-allelic transcription of the intact haplotype
(open magenta) due to deficient replication of the MN chromatid; under 1:3
segregation, the MN sister cell or the MN nieces should display mono-allelic
transcription from the intact haplotype (open magenta). The predicted
monosomic transcription outcomes are used to identify micronuclear
chromosomes and their segregation pattern in each experimental family.
(b) and (c) Validation of the transcriptional outcomes of MN (“generation 1”)
using an experimental strategy (b) of inducing MN with acentric Chr.5q
fragments generated by CRISPR-Cas9 as reported in our recent study25. The
data shown in (c) demonstrate the predicted transcriptional outcome when the
micronucleus contains only one copy of Chr.5q fragment arm that most closely
resembles the segregation patterns generated by nocodazole block-and-
release. Two measures of gene transcription are shown: in the left plot, filled
and open magenta circles are the normalized allelic expression of the
broken and the intact haplotype in 10 Mb bins; on the right are the cumulative
TPM (from low to high expression). The MN sister cell shows normal disomic
transcription. In the MN cell, monoallelic transcription of the MN haplotype
(filled circles) extending from near 64 Mb to the q-terminus indicates
silencing of an acentric Chr.5 fragment partitioned into the micronucleus
after Cas9-breaks generated at ~64 Mb. Reduced transcription of Chr.5 in the
MN cell is also evident from the cumulative TPM plot on the right that shows
a reduction in total transcription relative to normal disomic Chr.5. As the
cumulative TPM plot is generated for all genes on Chr.5, it does not distinguish
chromosome-wide transcriptional reduction from regional loss of transcription.
(d) Identification of chromosomes with non-reference transcriptional states
(red dots) in MN families (n = 173 cells) based on reference transcription
distributions determined from control RPE-1 cells (Extended Data Fig. 1d).
Based on the inferred DNA copy-number states of these chromosomes (assuming
proportional transcriptional yield and DNA copy number), we further identify
chromosomes with mis-segregation patterns consistent with the predicted
outcomes in (a). Error bars represent normal range of transcription estimated
based on the 5 % and 95 % values in control cells. Red dots represent chromosomes
with significant deviations (Bonferroni corrected P <0.05, two-tailed Z-test; for
48 chromosomes including both homologues).
ArticleExtended Data Fig. 3 | See next page for caption.
Extended Data Fig. 3 | Additional data on the loss of transcription in newly
generated (generation 1) MN. (a) Summary of the transcriptional yield of MN
chromosomes in all generation 1 families. Each bar plot represents the average
transcriptional yield of both homologues of the MN chromosome (filled for the
micronuclear homologue that is annotated below each plot; open for the intact
homologue) in the MN cell (left) and the MN sister (right) in each family. Two
families with near identical transcription from all chromosomes (indicating
normal transcription yield of the MN chromosome) are not shown. In family
F98, the MN haplotype (Chr.4B, green) displays reduced transcription in the
MN cell; in all the other families, the MN haplotype in the MN cell displayed near
complete silencing as indicated by either near complete loss of transcription
(2:2 segregation, left) or close to monosomic transcription (1:3 segregation,
right) from the intact sister chromatid of the MN haplotype in the primary
nucleus. In family F71, the transcriptional imbalance is restricted to the 1p arm
with near complete silencing (see Supplementary Table 2). In family F230 and
F203, the transcriptional pattern indicates an extra copy of the MN homologue
that is shared between the MN cell and the MN sister reflecting a pre-existing
duplication of the MN homologue (i.e., a pre-existing trisomy). The examples
shown in panels b (F220) and c (F216) are highlighted. (b) Chromosome-wide
transcriptional data of both Chr.2 haplotypes (left) in the MN family shown in
Fig. 1b (F220) and plots of cumulative TPM from low to highly expressed genes
(right) plots that validate the inference of monosomic and disomic Chr.2
transcription. (c) Chromosome-wide transcriptional data of both Chr.1
haplotypes in the MN family shown in Fig. 1c (F216) and cumulative TPM plots
that validate the inference of monosomic and disomic Chr.1 transcription.
ArticleExtended Data Fig. 4 | See next page for caption.
Extended Data Fig. 4 | Transcription defects and chromatin modifications
in newly formed (generation 1) micronuclei. (a) Transcription is present at a
reduced level in intact MN and is nearly absent in ruptured MN. Data points are
background normalized MN:PN fluorescence intensity (FI) ratios of RNAP2-
Ser5ph at 23 h post mitotic shake-off. RFP-NLS levels were used to assign the
micronuclei in the two groups (n = 83 and 82, left to right, from three
experiments). Micronuclei with NLS ratios below 0.1 relative to the PN were
considered ruptured and above 0.3 were considered intact. Median with 95%
confidence interval (CI); Two-tailed Mann–Whitney test. (b) Data from Fig. 1e,
but instead of the MN:PN FI ratios what is shown here are the background
normalized intensity values at 2, 6 and 23 h post release from nocodazole and
mitotic shake-off (n = 644 for 2 h, 212 for 6 h and 605 for 23 h, from two or
three experiments). Median with 95% CI; Kruskal-Wallis with Dunn’s multiple
comparisons test. (c) In U2OS cells, active transcription (RNAP2-Ser5ph) is also
reduced in newly formed MN. Performed and analyzed as in Fig. 1e (n = 88 and
104, left to right, from two experiments). (d) Representative images from the
data shown in Fig. 1e. Yellow arrows indicate micronuclei. Scale bars 5 µm.
(e) Independent confirmation of the MN transcription defect by 30 min EU
pulse labeling. Left, representative images of S/G2 cells with MN (generation 1).
Right, correlation between RNAP2-Ser5ph and EU levels. Cells with varying
levels of RNAP2-Ser5ph intensity were selected and then EU intensity levels
were measured (n = 37, from one experiment). Note the strong EU signal in the
nucleoli which lack RNAP2-Ser5ph, because rDNA is transcribed primarily by
RNA polymerase I and RNA polymerase III. Two-tailed Spearman’s correlation.
Scale bar 5 µm. (f) MN transcription defects verified in MN generated by G2
arrest with CDK1 inhibition, followed by release into an MPS1 inhibitor. This
synchronization and MN induction method differs from the nocodazole block
and release protocol primarily used in this study because it shortens rather
than lengthens mitosis (excluding hypothetical artifacts from prolonged
mitotic arrest). Left: scheme of the experiment. RPE-1 cells were analyzed 2 h
after release from the G2 block (n = 334, from three experiments). Right:
quantification and analysis of the results as in Fig. 1e. (g) Transcription and
chromatin defects in spontaneously generated micronuclei. Decreased levels
of RNAP2-Ser5ph and H3K27ac in spontaneous micronuclei of untreated RPE-1
(left) and U2OS cells (right). Performed and analyzed as in Fig. 1e (n = 134 and
295, left to right, from two experiments). (h) Representative images from data
shown in Fig. 1f. Yellow arrows indicate micronuclei with nucleoporin signal
(POM121) and transcription (RNAP2-Ser5ph). In contrast, red arrows indicate a
micronucleus with decreased nucleoporin signal and much lower transcription
signal. Note that we evaluated the specificity of POM121 staining by confocal
microscopy, showing the typical nuclear pore complex dot-like pattern at the
nuclear surface and the increased rim signal at the nuclear periphery by imaging
a focal plane in the middle of the cell. Scale bar 5 µm. (i) Reduced accumulation
of CDK9 and CDK12 in the micronuclei. The levels of CDK9, CDK12 and RNAP2-
Ser5ph were analyzed in micronuclei 23 h post release from a nocodazole block
followed by a mitotic shake-off. The experiment was performed and analyzed
as in Fig. 1e (n = 285 and 291, left to right, from two experiments). An analysis
of intact micronuclei also showed the defective accumulation of both CDK9
and CDK12 (MN:PN ratios: 0.35 for CDK9 and 0.09 for RNAP2-Ser5ph; n = 111,
P < 0.0001; 0.28 and 0.08 MN:PN for CDK12 and RNAP2-Ser5ph groups,
respectively, n = 102, P < 0.0001; from two experiments).
Articlea
)
:
N
P
N
M
(
o
i
t
a
r
I
F
9
3
2
1
0
P < 0.0001
P = 0.126
2 h 23 h
2 h 23 h
H3K9me2
H3K27me3
b
s
r
h
3
2
t
a
N
M
d
e
r
u
t
p
u
r
Hoechst
RFP-NLS
H3K9me2
)
Hoechst
RFP-NLS
H3K27me3
:
N
P
N
M
(
2
e
m
9
K
3
H
8
6
4
2
0
P = 0.0095
)
:
N
P
N
M
(
3
e
m
7
2
K
3
H
intact
ruptured
8
6
4
2
0
P = 0.225
intact
ruptured
c
s
r
h
2
t
a
N
M
Hoechst
RFP-NLS
H3K9ac
)
Hoechst
RFP-NLS
H3K27ac
:
N
P
N
M
(
c
a
9
K
3
H
d
P < 0.0001
2.5
2.0
1.5
1.0
0.5
0.0
P < 0.0001
10
8
6
4
3
2
1
0
P = 0.061
)
:
N
P
N
M
(
h
p
5
r
e
S
-
2
P
A
N
R
P < 0.0001
4
2
1.0
0.5
0.0
)
:
N
P
N
M
(
c
a
7
2
K
3
H
2 h
23 h
DMSO HDACi
DMSO HDACi
Extended Data Fig. 5 | Epigenetic alterations in micronuclei. (a) Modest
increase of repressive chromatin marks in a subset of late S/G2 MN. Performed
and analyzed as in Extended Data Fig. 4a (n = 129, 179, 114 and 105, left to
right, from two experiments for H3K9me2; from one or two experiments for
H3K27me3). (b) Left, selected example images from (a) of cells with ruptured
MN that show apparent enrichment for H3K9me2 and HK27me3 in the MN.
Arrowheads: micronuclei lacking normal RFP-NLS accumulation. Right: related
to (a) but comparing intact and ruptured MN for H3K9me2 (n = 48 and 40, left
to right, from two experiments) and H3K27me3 (n = 35 and 26, left to right,
from two experiments) at 23 h post mitotic shake-off. Performed and analyzed
as in Extended Data Fig. 4a. Scale bars 5 µm. (c) Loss of H3K9ac and H3K27ac in
MN at the indicated timepoint during interphase. Left: representative images
of the indicated histone modifications at 2 h post mitotic shake-off. Right:
quantification and analysis of data for H3K9ac as in Fig. 1e (n = 148 and 124, left
to right, from two experiments). Scale bars 5 µm. (d) HDAC inhibition is not
sufficient to rescue the transcription defect of chromosomes in micronuclei.
Cells were analyzed after incubation with a pan-HDAC inhibitor (SAHA) for 23 h
post release from a nocodazole block followed by a mitotic shake-off (n = 302
and 335, left to right, from three experiments for both H3K27ac and RNAP2-
Ser5ph). For the intact micronuclei, a significant rescue of H3K27ac levels was
also observed after HDACi treatment (0.36 and 0.87 MN:PN for H3K27ac in
DMSO and HDACi groups, respectively, P < 0.0001) and this was also not
detectably accompanied by rescue of the transcription defect (0.14 and 0.07
MN:PN for RNAP2-Ser5ph in DMSO and HDACi groups, respectively) (n = 165
and 131 for both H3K27ac and RNAP2-Ser5ph). Performed and analyzed as in
Fig. 1e. All pairwise comparisons between DMSO and HDACi have P < 0.0001.
For the intact MN, all pairwise comparisons between MN and PN have P < 0.0001,
except for the HDACi of H3K27ac group that has P = 0.502.
Normalized MN transcription of MN daughters (left) & MN nieces (right)
NE disruption
NE intact
F108
F146
F219
F225
F229
F235
F251
F252
F145
F227
F240
2B
1A
1A
1B
9B
1A
4B
1A
1B
1A
7A
F253
F274
F236
F258
F12
F24
F34
F231
F233
F25
7B
2A
2qA
13B
10qB
Xa
8B^
8B
12qA
12A
10qB
F148
F205
F259
F275
F261
F37
near normal
(monosomic)
defective
(sub-monosomic)
8B
1B^
1A^
2A
12A
1A^
NE disruption
NE intact
F189
F238
F184
F154
F155
F281
F34
F233
F25
1pA
1pA
8A
8A
8A
1pB
1qB
9B
11A
6A
19A
2:2
3
2
1
0
Chr
3
2
1
0
Chr
3
2
1
0
Chr
3
2
1
0
Chr
1:3
with significantly reduced transcription
F147
F262
F273
F283
F258
F261
near normal
(disomic)
defective
(sub-disomic)
2B
1A
5B
11B
5B
13B
3
2
1
0
Chr
Extended Data Fig. 6 | See next page for caption.
ArticleExtended Data Fig. 6 | Summary of the transcriptional yield of
reincorporated MN chromosomes in all generation 2 families. Each
bar plot shows the transcriptional yield of both homologues of the MN
chromosome (filled for the MN homologue that is annotated below each plot;
open for the intact homologue) in the MN daughters (two on the left) and one
or both MN nieces (on the right) in a family. Families are grouped based on the
status of MN nuclear envelope integrity (left: NE disruption during generation 1
interphase; right: intact NE) and the segregation pattern of MN chromosomes
(top, 2:2; bottom 1:3). Nine families (NE disruption: F12, F24, F34, F37, F154,
F155, F281, F34; intact NE: F25) without MN nieces are shown separately from
the remaining families with MN nieces. MN chromosomes with near normal
transcriptional yield are shown in green: Under a 2:2 segregation, the MN
haplotype displays a transcriptional ratio of 1:0 between the MN daughters and
normal (monosomic) transcription in the MN nieces; under a 1:3 segregation,
the MN haplotype displays a transcriptional ratio of 2:1 between the MN
daughters and complete transcriptional loss in the MN nieces. (See Extended
Data Fig. 2a for the segregation patterns.) MN chromosomes with significantly
reduced transcription are shown in magenta. For these chromosomes, the MN
haplotype shows a statistically significant lower transcription than normal
transcription (monosomic transcription under 2:2 segregation and disomic
transcription under 1:3 segregation, two-sided z-test). In families F24, F205,
F259, and F37, we identified transcription of the MN haplotype in both daughter
cells that is consistent with chromosome fragmentation; we combined the
transcriptional yield in both MN daughters in these samples to assess the
normality of transcription of reincorporated MN chromosomes. All MN
chromosomes with deficient transcription are associated with NE disruption.
The summary bar charts of normal and deficient transcription in 2:2 segregation
samples and 1:3 segregation samples only include MNs with the predicted
patterns of transcriptional imbalance. Deviations from the predicted patterns
are explained below. In family F233 and F261, the presence of an extra copy of
the Chr.12A homologue in all family members indicates a pre-existing duplication
of Chr.12A that is a frequent alteration in RPE-1 cells. In family F12, we inferred
the MN chromosome to be the active X (the transcription yield of the inactive X
is not shown) that also contains a duplicated 10q segment. In seven families
(F236, F231, F25, F189, F238, F281, F34), we inferred that the MNs contain only a
chromosome arm; in family F238, we inferred the 1p arm was reincorporated
into one MN daughter and the 1q arm was persistent in a MN niece cell based on
live-cell imaging. We note that for MN chromosomes that underwent 1:3
segregations, the normality of transcription is assessed by comparing the level
of MN haplotype-specific transcription to the level of total transcription of
normal disomies (2). The proportional gain of transcription of duplicated
homologue (2) is verified using observations from 18 spontaneous trisomies.
NE disruption
normalized TPM per chromosome
A
B
MN
daughters
MN
nieces
chr1
chr2 chr3
chr4
chr5 chr6 chr7
chr8 chr9 chr10a chr10b chr11 chr12 chr13 chr14 chr15 chr16 chr17
chr18
chr19
chr20 chr21 chr22 chrX
1:3
segregation
MN nieces
Binned TPM ratio by coordinate (10Mb)
cumulative TPM
from low to high expression
MN haplotype
intact haplotype
trisomy
a
3
2
1
0
b
c
PN
MN sister
MN cell
N1
N2
D1
D2
2:2
segregation
MN nieces
MN sister
MN cell
N1
N2
D1
D2
MN
PN
PN
MN
PN
Extended Data Fig. 7 | See next page for caption.
MN daughters
MN daughters
Chr5
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
2
1
0
3
2
1
0
3
2
1
0
40000
0
0
40000
disomy
monosomy
40000
0
0
40000
40000
0
0
40000
40000
0
0
40000
0 20 40 60 80 100 120 140 160 180 (Mb)
Binned TPM ratio by coordinate (10Mb)
MN haplotype
intact haplotype
3
2
1
0
3
2
1
0
3
2
1
0
3
2
1
0
Chr13
0 20 40 60 80 100 (Mb)
10000
0
10000
0 500010000
0
0
10000
10000
0
0
10000
10000
0
0
10000
ArticleExtended Data Fig. 7 | Complete data of the F258 family. Data are for the
family shown in Fig. 2b. (a) Haplotype-specific chromosomal transcriptional
ratios showing non-disomic transcription of Chr.5 (magenta) and Chr.13 (green).
The first two cells are the MN daughters; the second two are MN nieces. Regional
transcription data of Chr.5 and Chr.13 are shown in (b) and (c). (b) The segregation
pattern, expected transcriptional yield, and observed transcriptional levels of
Chr.5 in all four cells. The presence of monosomic expression in both nieces
and disomic/biallelic expression in both MN daughters indicate a 1:3 segregation
of Chr.5. As the two MN daughters both display close to disomic transcription
but one or both of them have reincorporated the Chr.5 copy from the
micronucleus, we conclude that the reincorporated Chr.5 is not actively
transcribed. (c) The segregation pattern, expected transcriptional yield, and
observed transcriptional data of Chr.13 in all four cells. In contrast to the pattern
of Chr.5, the two nieces both display disomic/biallelic expression, and one MN
daughter displays monosomic expression; this pattern establishes a 2:2
segregation of Chr.13. The presence of transcripts phased to the MN haplotype
(filled green circles in the bottom cell) indicates transcription of the
reincorporated Chr.13 in the bottom cell. We note that there is more regional
transcription variation in Chr.13 than in Chr.5 that is partially due to the lower
gene density on Chr.13.
t = 20.8 h
t = 21 h
t = 23.3 h
t = 23.8 h
t = 25 h
t = 28 h
t = 30.5 h
t = 39.8 h
d
-
B
2
H
P
F
G
P
A
N
S
-
I
c
a
L
l
-
o
a
H
P
C
M
e
P
A
N
S
-
I
c
a
L
+
B
2
H
P
F
G
-
P
A
N
S
-
I
c
a
L
l
-
o
a
H
P
C
M
t = 20.5 h
t = 38.8 h
t = 39.5 h
t = 42.5 h
f
)
n
m
i
(
e
m
i
t
e
r
u
t
p
u
r
o
a
H
P
C
M
-
l
spearman r = 0.999
2000
1000
0
0
1000
2000
NLS-RFP rupture time (min)
Extended Data Fig. 8 | See next page for caption.
Article
Extended Data Fig. 8 | Analysis of nascent transcription from reincorporated
micronuclei. (a) Control U2OS 2-6-3 reporters to assess nascent transcription
of normally expressing reporters in the main nucleus. Normalized FI of MS2
signal (MCP-Halo) were measured from reporters that were in the main nucleus
during both generation 1 and 2 (n = 23 LacI reporters). Grey bar: mitosis. Error
bars: mean +/− SEM). Red line: minimum detectable normalized MS2 value of
the controls (see Methods). (b) Example of a MN with late G2 rupture in
generation 1 that recovered transcription after reincorporation into a daughter
nucleus in generation 2. Performed and analyzed as in (a), above. Grey line:
mean intensity of the control reporters in main nucleus. (c) Aggregated data
of nascent transcription from reincorporated MN assessed by the U2OS 2-6-3
reporter, similar to (a) and Fig. 2e and f. Normalized FI of the MS2 signal
(MCP-Halo) were measured from reporters that were in a MN in generation 1
and then incorporated into a daughter nucleus in generation 2. Top, a subset of
cells with MN that ruptured during generation 1 interphase and then recovered
transcription after reincorporation into a daughter nucleus in generation 2
(n = 7 analyzed out of 19 similar cases). Note that prior to mitosis there is
variable MS2 signal because of variable MCP-Halo accumulation in intact
micronuclei and because of variability in the timing of MN NE rupture. Bottom,
aggregate data for a similar subset of samples where the MN ruptured during
generation 1 interphase and then displayed a generation 2 transcription defect
after reincorporation (black line, n = 9 analyzed out of 20 similar cases). Note:
(1) for ease of visualization, error bars (mean +/− SEM) are shown only for the
experimental samples, but not the controls; (2) when there was no detectable
MS2 signal in an experimental sample, we assigned the minimal detectable
normalized value in control cells (1.7, see Methods) to this sample (This explains
the complete overlap between the black and red lines after the 10-hour
timepoint). (d) Images from a timelapse series for the experiment in (b), above.
Green arrowhead: MN rupture. Red arrowheads: MS2 expression from the
reporter after reincorporation into a daughter nucleus in generation 2. Time:
hours post release from the G2 block. Scale bars 5 µm. (e) Validation that MN-
bodies originate from MN chromosomes, using same-cell live/fixed imaging.
Left, images from a time-lapse series (U2OS 2-6-3 system, see Figs. 2e–g, 4d).
A cell with a MN harboring Chr.1 (with the reporter integrated in Chr.1p, yellow
arrowheads) was identified. The MN ruptured in the interphase that it was
formed. After mitosis, the MN chromosome was reincorporated into a daughter
cell PN (blue arrowheads, LacI-SNAP) but was not expressed (magenta
arrowheads, MCP-Halo), even though it was in a normal nuclear environment.
Right, at the end of the time-lapse imaging (t = 42.5 h) cells were fixed and the
same cells were analyzed by immunofluorescence microscopy, revealing a
large γH2AX-positive MN-body is at the location of the reincorporated MN
chromosome identified by LacI-SNAP (open magenta arrowheads). Scale bars
5 µm. (f) Validation of the method to assess MN rupture with the U2OS 2-6-3
reporter system. For all experiments with this reporter, loss of the general
nuclear MCP-Halo signal (MCP-Halo contains an NLS) was used to determine
the time of MN NE rupture. We verified that MCP-Halo signal loss from MN
corresponds to RFP-NLS by two-color live-cell imaging in U2OS 2-6-3 cells
expressing both MCP-Halo and RFP-NLS (n = 40 from four experiments;
two-tailed Spearman’s correlation).
a
nocodazole mitotic
TP53si
shake-off
division
live-imaging
0 h
~18 h
~24 h
Hoechst
γH2AX
N = 4
N = 9
100
s
u
c
o
f
e
g
a
m
a
d
A
N
D
)
%
(
s
r
e
t
h
g
u
a
d
n
i
80
60
40
20
0
fixation
for IF
~45 h
b
H2AX focus
no H2AX focus
Hoechst
MDC1
Intact NE disruption
t = 24.7 h
24.8 h
27 h
27.1 h
27.2 h
27.5 h
27.8 h
29.1 h
32.6 h
c
A
r
e
t
h
g
u
a
D
B
r
e
t
h
g
u
a
D
-
B
2
H
P
F
G
-
S
L
N
P
F
R
1
C
D
M
P
A
N
S
-
d
MN-body duration (MDC1)
imaging duration
MN-body duration
e
y
t
i
s
n
e
n
t
i
1
C
D
M
e
v
i
t
l
a
e
r
f
g
P < 0.0001
P < 0.0001
P < 0.0001
25
20
15
10
5
0
2.5
2.0
1.5
1.0
0.5
)
l
o
r
t
n
o
c
:
y
d
o
b
N
M
(
h
p
5
r
e
S
-
2
P
A
N
R
0.0
fibrillarin-pos
random
fibrillarin-neg
RNAP2-Ser5ph
P < 0.0001
2.5
2.0
1.5
U2OS
2.5
2.0
1.5
o
i
t
a
r
H3K27ac
P < 0.0001
o
i
t
a
r
I
F
1.0
0.5
0.0
.
I
.
F
1.0
0.5
0.0
control MN-body
control MN-body
0
10
20
duration (hours)
30
40
control MN-body
H3K9ac
P < 0.0001
h
o
i
t
a
r
I
F
2.5
2.0
1.5
1.0
0.5
0.0
H3K9me2
P < 0.0001
o
i
t
a
r
I
F
4.0
3.0
2.0
1.0
0.0
o
i
t
a
r
I
F
4.0
3.0
2.0
1.0
0.0
H3K27me3
P = 0.0028
i
H3S10ph
3.0
P = 0.0537
o
i
t
a
r
I
F
2.0
1.0
0.0
H3T3ph
P = 0.001
o
i
t
a
r
I
F
4
2.0
1.0
0.0
control MN-body
control MN-body
control MN-body
control MN-body
control MN-body
Extended Data Fig. 9 | See next page for caption.
Article
Extended Data Fig. 9 | Characterization of MN-bodies. (a) Same-cell live-
fixed experiment supporting the fixed imaging shown in Fig. 3a, b. Top, scheme
of the experiment. MN were induced in RPE-1 cells and the MN fate was tracked
with GFP-H2B and RFP-NLS (to visualize MN NE rupture in generation 1). After
most cells progressed into generation 2, they were fixed and labeled to detect
γH2AX. Bottom left: representative images of a daughter cell pair, one with and
one without an MN-body. Bottom right: summary of 13 cell pairs tracked and
analyzed by the same-cell live-fixed experiments (from two experiments).
Scale bars 5 µm. (b) MDC1 accumulation on a mitotic chromosome in an RPE-1
cell that had a micronucleus in the prior interphase (generated by nocodazole
block and release), shown by immunofluorescence staining of endogenous
MDC1 (representative images from two experiments). Note that the micronuclear
chromosome can be identified because it is decondensed, a known feature of
mitotic micronuclear chromosomes. Scale bar 5 µm. (c) Images from a timelapse
series tracking damaged MN chromosomes through cell division and MN-body
formation. GFP-H2B: chromosomes; green arrowheads: MN chromosome; RFP-
NLS: NE integrity; blue arrowheads: MN NE rupture; red arrowheads: SNAP-
MDC1-marked MN DNA damage. Time: hours post release from the nocodazole
block for MN induction. Scale bars 5 µm. (d) Durations of MN-bodies assessed
by live-cell imaging of SNAP-MDC1 indicate MN-bodies persist throughout
most of the generation 2 interphase. Each row shows the lifetime of a MN-body
(black bar) and the duration of imaging (light grey bar). In all but four cases, the
MN-bodies persisted until the end of the imaging (see Extended Data Fig. 9c for
an example of a time lapse series). Note that analysis of the live-cell imaging
experiments showed that 68% of cells with MN-bodies were derived from
mother cells with a micronucleus that ruptured, 22% were derived from mother
cells with intact micronuclei and 10% from non-micronucleated mother cells.
(e) Distribution of signal intensities for MN-body by immunofluorescence
staining for the endogenous MDC1. Performed and analyzed as in Fig. 3c
(n = 341, from two experiments). Median with 95% CI. Two-tailed Mann–
Whitney. (f) Determination of the background nuclear RNAP2-Ser5ph signal
in nucleoli. We measured the background RNAP2-Ser5ph signal in nucleoli
(fibrillarin positive), which should lack active RNA polymerase II, and in nuclear
regions lacking nucleoli. These values were then normalized to the density of
fluorescence intensity from a nuclear mask excluding the nucleoli. The detection
of measurable RNAP2-Ser5ph signal in the nucleoli means that we likely
underestimate the extent of RNAP2-Ser5ph signal loss in MN-bodies (see
Methods; n = 650, from two experiments). Median with 95% CI. Kruskal-Wallis
with Dunn’s multiple comparisons test. (g) Verification of low transcription
and H3K27ac loss in MN-bodies in U2OS cells. Performed and analyzed as in
Fig. 3c (n = 138, from two experiments). (h) Reduced H3K9ac (left) but not
H3K9me2 (middle) or H3K27me3 (right) in MN-bodies. Performed and analyzed
as in Fig. 3c (n = 222, 234 and 244, left to right, from two experiments). (i) H3S10ph
and H3T3ph levels show no increase but a minor decrease in MN-bodies
compared to the control. Performed and analyzed as in Fig. 3c (n = 130 left;
n = 124 right, from two experiments).
Extended Data Fig. 10 | See next page for caption.
ArticleExtended Data Fig. 10 | DamMN system characterization. (a) Validation of
the DamMN system. Shown are representative single-focal plane confocal images
of RPE-1 megaDam cells ~45 h post release from the CDK1-induced G2 block at
the start of the experiment (see Fig. 4a and Methods). There is no m6A DNA
methylation if megaDam transcript is not induced (left, no Dox); if megaDam is
not degraded prior to mitotic entry, all primary nuclei show m6A DNA methylation
because of labeling during mitosis (middle, Dox, no ASV no IAA); if megaDam is
degraded prior to mitosis because of size exclusion through the NE by passive
import, primary nuclei are mostly not m6A methylated (right, Dox, +ASV +IAA).
m6A methylation is visualized with the m6A-Tracer (see Fig. 4a and Methods; four
experiments). Scale bars 20 µm. Note that even in the condition of degrading
megaDam before mitosis (Dox, +ASV +IAA), many cells still show whole nucleus
labeling with the m6A-Tracer. This could either result from cells that were in
mitosis at the time of megaDam induction or from cells where nuclear exclusion
of megaDam was not complete. (b) Efficient induction and degradation of
megaDam. FACS analysis to detect mCherry-tagged megaDam. All samples are
unsynchronized RPE-1cells with or without megaDam, with or without megaDam
transcriptional induction or megaDam degradation for the indicated periods
of time. The controls are RPE-1 cells lacking the megaDam construct showing
no background autofluorescence without or with Dox treatment. Shown is the
percentage of cells expressing mCherry (PE channel, from two experiments).
(c) Western blot to detect megaDam for the indicated samples corresponding
to the experiment shown in (b), above. Shown is a cropped image of a gel from
the region at the megaDam molecular weight (~130 kDa). Note: the a-mCherry
Ab detects non-specific background bands, but megaDam is readily
distinguished from these background bands (two experiments). * indicates a
background band. For gel source data, see Supplementary Fig. 1. (d) Specific
labeling of MN chromosomes in mitotic cells (two examples) using the DamMN
system. Top shows an MN chromosome in a prometaphase cell lacking DNA
damage. Bottom shows an MN chromosome in a metaphase cell with DNA
damage. Note that the MN chromosome is less condensed during mitosis, as
has been previously described62. Performed as described in Fig. 4a. Yellow
dashed line: an MN chromosome positive for γH2AX and m6Tracer (n = 4
experiments). Scale bars 5 µm. (e) Control for Fig. 4c showing the distribution
of MN-body γH2AX FI units relative to the general nuclear background (lacking
nucleoli). The MN-body region of interest corresponds to the m6A-Tracer signal
(see Methods). The γH2AX low MN-bodies were designated if the total area of
γH2AX positive pixels (>3SD above background, see Methods) occupy less than
21% of the MN-body area (corresponding to the bottom quartile of γH2AX
positive MN-bodies). The designation of γH2AX intermediate MN-bodies was
between 21% and 65.7% of the MN-body area (the middle two quartiles), and
γH2AX high was >65.7% of the MN-body area (the top quartile of MN-bodies)
(left to right: n = 220, 111, 220 and 112, four experiments). Error bars: median
with 95% CI.
Haplotype-specific DNA copy number and rearrangements
haplotype A
haplotype B
Haplotype-specific transcriptional ratio (avg. Bridge Clone : parental RPE-1)
I
II
III
IV
V
VI
VII
VIII
IX
X
XI
XII
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Extended Data Fig. 11 | See next page for caption.
ArticleExtended Data Fig. 11 | Haplotype-specific DNA copy number,
rearrangements, and transcriptional levels of chromosome 4 in 12 bridge
clones. Top: Haplotype-specific Chr.4 DNA copy number (250 kb bins)
determined from newly generated DNA-Seq data (~5X mean sequencing depth)
of the bridge clones. Black arcs represent intra-chromosomal rearrangements
determined from previous whole-genome sequencing of the same bridge
clones and subclones (10-60X mean depth)11. Rearrangements are phased to
each homologue based on haplotype-specific copy-number changes at
rearrangement breakpoints. The shaded box denotes the region of 27-37 Mb on
Chr.4p with reduced ATAC-Seq signal as shown in Fig. 5c. We detected no clonal
or subclonal breakpoints in this region (based on both DNA copy-number
changepoints and rearrangement junctions) in bridge clones I, II, IV, VI, VII, IX,
X. Bridge clone VIII and XI contain the most breakpoints within this region but
display no significant change of ATAC density after normalization for copy-
number variation. In bridge clone III, ATAC reduction is most prominent
between 27 and 30.5 Mb and this region is far away from rearrangements that
affect two segments between 32.19-32.23 Mb. Bridge clones V and XII both
contain multiple copy-number and rearrangement breakpoints in this region
and display modest but significant ATAC reduction (based on the permutation
test). Bottom: Haplotype-specific gene transcription (TPM) ratio on Chr.4.
Each dot represents the average TPM ratio of a single gene calculated from all
12 bridge clones, excluding samples with complete DNA deletion. Arrows point
to the PCDH7 gene residing in the region with reduced ATAC signal.
Extended Data Fig. 12 | See next page for caption.
ArticleExtended Data Fig. 12 | Long-term effects on chromatin and expression
after chromosome bridge formation. (a) MN-body-like structures in the
daughter cells after chromosome bridge formation. Top, schematic presentation
of the experiment. Bottom left, representative images of immunofluorescence
analysis for MDC1 and RNAP2-Ser5ph, showing MDC1-positive nuclear bodies
(dashed magenta line) after chromosome bridge formation and cell division of
RPE-1 cells expressing TRF2-DN (see Methods). We observed a high frequency
of cells with MDC1-positive nuclear structures of varying size (~10%) that likely
represent reincorporated chromosome bridges. This number is expected,
since the frequency of chromosome bridge formation is ~30% per cell division
under the conditions described in Methods11. Scale bar, 5 µm. Right,
quantification of the RNAP2-Ser5ph levels in the MDC1-positive nuclear
structures after bridge chromosome reincorporation compared to the PN
control. Performed and analyzed as in Fig. 3c (n = 309, from two experiments).
(b) Genome-wide ATAC signal variation in control (i-x, left) and bridge (I-XII,
right) clones in 1 Mb (top), 5 Mb (middle), and 10 Mb (bottom) intervals. The
ATAC change in each interval (1 Mb increment) is assessed by normalizing the
observed total ATAC signal (only from peaks) in each interval by the mean ATAC
density of the null distribution generated by random permutations of
individual peaks (see Methods, n = 2637 of 1 Mb genomic intervals). Bins with
less than 10 ATAC peaks/Mb are excluded. Box plots indicate the first (bottom
edge) and third (top edge) quartiles and the median (horizontal line), with
whiskers indicating 1.5x the interquartile range. In each plot, red dots represent
bins overlapping with the region of 27-38 Mb of Chr.4 that displays the most
significant ATAC reduction across all bridge clones (see below). (c) Average
ATAC signal variation in 10 Mb intervals across all 12 bridge clones. We only
consider 10 Mb regions with 100 or more peaks. As the calculation is performed
on all 10 Mb intervals with 1 Mb increment, a single region with a significant
reduction in ATAC signal may result in multiple 10 Mb intervals with significant
ATAC reduction; these consecutive 10 Mb bins are merged. Bins with the most
significant ATAC reduction (fold change < 0.8) mostly come from two regions:
Red dots are from the 4p region (26-38 Mb) shown in Fig. 5; purple dots are from
a region from Chr.13q (54-76 Mb). Among 10 Mb regions with ATAC signal < 0.85,
two are from Chr.4 and Chr.13: Chr.4:129-139 Mb (red circles) and Chr.13:78-94
Mb (purple circles). The other regions with ATAC signal < 0.85 are likely to have
a non-epigenetic origin: Two regions (Chr.3:88-99 Mb, green dots; Chr.6:58-70
Mb, blue dots) span centromeres and have low confidence; another region on
Chr.12p (12-30 Mb, light green dots) shows a similar reduction in the control
clones and the variation is likely related to 12p gain or uniparental disomy that
are frequent subclonal alterations in RPE-1 cells. The significant reduction in
ATAC signal in Chr.4 and Chr.13 is unlikely to reflect random technical variation
as they are specific to the bridge clones. For the Chr.13 region, we do not
exclude a biological source for this variation, for example, an unidentified trans
signaling effect that is related to bridge formation, breakage, or downstream
evolution. It is known that certain genomic regions display more intrinsic
variability of ATAC-Seq signals63 and such regions may be more prone to effects
from chromosome bridge formation or breakage. Box plots indicate the first
(bottom edge) and third (top edge) quartiles and the median (horizontal line),
with whiskers indicating 1.5x the interquartile range. (d) Scatter plot of the fold
change of ATAC signal (log2 transformed) and the P-value of ATAC signal variation
estimated from permutations in bridge Clone I (two-sided permutation test, up
to 5 million permutations without additional adjustment; see Methods). The
two red dots are both from Chr.4:27-38 Mb. The cap of p-value at 2 x 10−7 reflects
5 million permutations performed for each interval.
cell cycle
progression
Mitosis I
(NE Breakdown)
Generation I
Mitosis II
(NE Breakdown)
Generation II
many generations
normal
transcription
no transcription
normal
defective
no transcription
recovered
defective
Import defect
& low H3K27ac
No rupture
or damage
Mitosis II
Mitosis I
MN rupture
& damage
damage
at mitosis
MN-body
(Persistent damage)
genetic &
epigenetic
variation
aneuploid,
no variation
aneuploid,
genetic &
epigenetic
variation
Extended Data Fig. 13 | Transcriptional and epigenetic consequences of
micronucleation (Model summarizing the results). Top, transcriptional
outcomes of chromosomes transiently in micronuclei or chromosome bridges.
Green line shows the transcriptional yield of chromosomes in MN without
persistent DNA damage (generation 2). Red line shows the transcription yield
of MN chromosomes with DNA damage that persists into generation 2. Bottom,
cellular events leading to heritable transcription defects. Mitosis I: A cell with a
lagging chromosome divides, generating the MN cell and its sister (shaded).
Deficient nuclear import of MN prevents the establishment of H3K27 acetylation
and causes significantly reduced or complete loss of transcription. If the MN
nuclear envelope remains intact and the MN chromosome does not acquire
DNA damage during mitosis (top), the MN chromosome can recover
transcription after Mitosis II. If the MN chromosome acquires DNA damage
either due to MN nuclear envelope rupture during interphase (bottom, red) or
subsequently during mitosis (dashed arrow), the damaged chromosome may
form MN-bodies with varying degrees of transcriptional silencing (bottom,
with red MN-body in the primary nucleus) after Mitosis II. With partial
penetrance, transcriptional silencing may persist for multiple generations,
generating transcriptional heterogeneity that can be subject to selection.
ArticleCorresponding author(s): David Pellman
Last updated by author(s): Apr 3, 2023
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Data exclusions
For image analysis of MDC1-labeled MN-bodies (Fig 3 and Extended Data Fig. 8), only cells within the middle 50% of the field of views were
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Antibodies
Antibodies used
Validation
The following antibodies were used for indirect immunofluorescence in this study: phospho gammaH2AX (Ser139) (Millipore
#05-636-I, 1:400), H3K27ac (Active Motif #39133, 1:200), MDC1 (Abcam #ab11171, 1:1000), MDC1 (Sigma-Aldrich # M2444,
1:1000), phospho RNA PolII S5 (Millipore #MABE954, clone 1H4B6, 1:400), Cdk9 (Cell Signaling #2316, 1:10), Cdk12 (Abcam
#ab246887, 1:400), 53BP1 (Santacruz #22760S, 1:100), H3K27me3 (Thermo Fisher #MA511198, 1:1000), H3K9ac (Cell Signaling
#9649S, 1:400), H3K9me2 (Cell Signaling #9753S, 1:400), POM121 (Proteintech 15645-1-AP, 1:200), phospho H3T3 (Millipore
#07-424, 1:12000), phospho H3S10 (Abcam #ab47297, 1:200) and Fibrillarin (Abcam # ab4566, 1:500). For western blots, the
following primary antibodies were used: The primary antibodies and dilutions used were anti-mCherry rabbit 1:1000 (ab167453,
Abcam) and anti-GAPDH mouse 1:5000 (ab9485, Abcam). The fluorescent secondary antibodies are IRDye 680RD Donkey anti-rabbit
1:5000 (926-68073, LICOR Biosciences) and IRDye 800CW Donkey anti-mouse 1:5000 (926-32212, LICOR Biosciences).
gammaH2AX (Ser139) (Millipore #05-636-I) antibody was previously used and validated by irradiation in Crasta et al., Nature 2012.
RNA PolII S5 (Millipore #MABE954) was used in Lin et al., EMBO J 2018
H3K27ac (Active Motif #39133) was used in Alekseyenko et al., Genes & Development 2015,
MDC1 (Abcam #ab11171) and MDC1 (Sigma-Aldrich # M2444) were used in Lukas et al., Nature 2011
53BP1 (Santacruz #22760S) was used in Passerini et al., Nature Communications 2016,
H3K9ac (Cell Signaling #9649S) was used in Weinert et al., Cell 2018,
H3K27me3 (Thermo Fisher #MA511198) was used in Beuzelin et al., Front Physiol 2020,
Fibrillarin (Abcam # ab4566) was used in Wang et al., Cell 2018,
mCherry (Abcam #ab167453-100ul) was used in Lattao et al., Dev Cell 2021,
Cdk9 (Cell Signaling #2316) was recommended by the R. Young lab (MIT) and was used in Verma et al., Mol Cell Biol 2019,
Cdk12 (Abcam #ab24688) was recommended by the R. Young lab (MIT) and was used in Liu et al., Cancer Gene Ther 2022,
phospho H3T3 (Millipore #07-727, 1:12000) was validated by signal enrichment on mitotic chromosomes and was used in Hadders et
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U2OS and hTERT RPE-1 were purchased from ATCC or obtained from other laboratories as described in the Methods section.
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| null |
10.1038_s41467-022-29759-7.pdf
|
Data availability
The data generated in this study have been deposited in the figshare database under the
accession code: https://doi.org/10.6084/m9.figshare.17026607.v1.
Code availability
Code used to analyse data in this manuscript are available from the corresponding author
upon reasonable request.
|
Data availability The data generated in this study have been deposited in the figshare database under the accession code: https://doi.org/10.6084/m9.figshare.17026607.v1 . Code availability Code used to analyse data in this manuscript are available from the corresponding author upon reasonable request.
|
ARTICLE
https://doi.org/10.1038/s41467-022-29759-7
OPEN
Singlet and triplet to doublet energy transfer:
improving organic light-emitting diodes with
radicals
1,2,6, Alexander J. Gillett
Feng Li
William K. Myers
4, Richard H. Friend
2,6, Qinying Gu2, Junshuai Ding1, Zhangwu Chen1, Timothy J. H. Hele
2,5✉
2✉
3,
& Emrys W. Evans
;
,
:
)
(
0
9
8
7
6
5
4
3
2
1
Organic light-emitting diodes (OLEDs) must be engineered to circumvent the efficiency limit
imposed by the 3:1 ratio of triplet to singlet exciton formation following electron-hole capture.
Here we show the spin nature of luminescent radicals such as TTM-3PCz allows direct
energy harvesting from both singlet and triplet excitons through energy transfer, with sub-
sequent rapid and efficient light emission from the doublet excitons. This is demonstrated
with a model Thermally-Activated Delayed Fluorescence (TADF) organic semiconductor,
4CzIPN, where reverse intersystem crossing from triplets is characteristically slow (50%
emission by 1 µs). The radical:TADF combination shows much faster emission via the doublet
channel (80% emission by 100 ns) than the comparable TADF-only system, and sustains
higher electroluminescent efficiency with increasing current density than a radical-only
device. By unlocking energy transfer channels between singlet, triplet and doublet excitons,
further technology opportunities are enabled for optoelectronics using organic radicals.
1 State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Qianjin Avenue 2699, Changchun 130012,
P. R. China. 2 Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK. 3 Department of Chemistry, University College
London, Christopher Ingold Building, London WC1H 0AJ, UK. 4 Centre for Advanced Electron Spin Resonance (CAESR), Department of Chemistry, University
of Oxford, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK. 5 Department of Chemistry, Swansea University, Singleton Park, Swansea
SA2 8PP, UK. 6These authors contributed equally: Feng Li, Alexander J. Gillett.
email: [email protected]; [email protected]
✉
NATURE COMMUNICATIONS |
(2022) 13:2744 | https://doi.org/10.1038/s41467-022-29759-7 | www.nature.com/naturecommunications
1
ARTICLE
NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-29759-7
Spin management is an important consideration for organic
light-emitting diode (OLED) efficiency in display and
lighting technologies. For closed-shell molecules with
singlet-spin-0 ground state, spin statistics with electrical exci-
tation leads to the formation of 25% singlet (spin-0, S1) and
75% triplet (spin-1, T1) excitons1,2. In first-generation OLEDs,
this results in maximum electroluminescence (EL) internal
quantum efficiency (EQE) of 25% as singlet emission (fluores-
þ hν)
triplet emission
cence, S1
þ hν) is spin-forbidden. In com-
(phosphorescence, T1
mercial applications, triplet–triplet annihilation- and enhanced
phosphorescence-based schemes have been used to obtain
efficient luminescence from triplet states3–7. Other technologies
under development include thermally activated delayed fluor-
electron donor–acceptor
escence
molecular designs promote reduced exchange interaction
and minimised S1-T1 energy gap for reverse intersystem
is allowed whereas
↛ S0
(i.e., TADF)8–12, where
! S0
crossing (rISC, T1
electroluminescence mechanism is shown in Fig. 1a.
! S1) and delayed S1 emission. The TADF
Another possibility to extract emission from the otherwise dark
T1 state is to transfer its energy to another energy acceptor
molecule, which then emits light. However, if the acceptor is a
ground-state singlet, converting the donor triplet to an acceptor
excited-state emissive singlet is spin-forbidden:
(cid:3)
(cid:1)
D T1
(cid:1) (cid:3)
þ A S0
(cid:1) (cid:3)
↛ D S0
(cid:1) (cid:3)
þ A S1
ð1Þ
where DðXÞ stands for the energy donor molecule in state X and
AðXÞ for the energy acceptor, and !=↛ denotes spin-allowed/
forbidden. It is possible to convert the donor triplet to an acceptor
triplet, but emission from this state is spin-forbidden
(cid:1) (cid:3)
! D S0
(cid:1) (cid:3)
↛ D S0
(cid:1) (cid:3)
þ A S0
(cid:1) (cid:3)
þ A S0
(cid:1)
þ A T1
(cid:1)
D T1
þ hν
(cid:3)
(cid:3)
ð2Þ
TADF-only OLED
Radical-only OLED
(b)
y
g
r
e
n
E
(e)
(a)
(d)
y
g
r
e
n
E
rISC
ISC
y
g
r
e
n
E
Electrical excitation
−
+
hv
S1
T1
S0
TADF:radical energy transfer OLED
Electrical excitation
−
+
rISCC
ISC
2{D(S1)A(D0)}
4{D(T1)A(D0)}
FRET
2{D(T1)A(D0)}
Dexter
2{D(S0)A(D1)}
hv
D = Energy donor
(4CzIPN, non-radical)
A = Energy acceptor
(TTM-3PCz, radical)
2{D(S0)A(D0)}
TADF:non-radical energy transfer OLED
'hyperfluorescence'
(c)
Electrical excitation
−
+
Electrical excitation
−
+
Q1
hv
y
g
r
e
n
E
D1
D0
rISC
ISC
1{D(S1)A(S0)}
3{D(T1)A(S0)}
FRET
Dexter
hv
1{D(S0)A(S1)}
3{D(S0)A(T1)}
non-radiative
D = Energy donor
(non-radical)
A = Energy acceptor
(non-radical)
1{D(S0)A(S0)}
(f)
TTM-3PCz
Cl
Cl Cl
4CzIPN
NC
N
N
N
CN
N
Cl
Cl
Cl
Cl
N
Cl
)
1
–
m
c
1
–
M
4
0
1
(
i
t
n
e
c
i
f
f
e
o
c
n
o
i
t
c
n
i
t
x
E
4
3
2
1
0
400
4CzIPN
TTM-3PCz
1.0
0.8
0.6
0.4
0.2
N
o
r
m
a
l
i
s
e
d
P
L
(
a
r
b
.
u
n
i
t
s
)
800
0.0
900
500
600
700
Wavelength (nm)
Fig. 1 Light emission mechanisms and the radical energy transfer system. Electroluminescence mechanisms for TADF-only, radical-only and energy
transfer OLEDs. Spin-allowed radiative transitions from excited to ground states are indicated by blue arrows labelled ‘hv.’ a Scheme for TADF OLED
mechanism with emission from singlet S1 exciton, and singlet–triplet intersystem crossing (ISC) and reverse intersystem crossing (rISC) processes with
non-emissive triplet T1 exciton. b Scheme for radical OLED mechanism with emission from doublet D1 exciton, formed by direct electrical excitation. Higher
energy and non-emissive quartet Q1 exciton state are shown. c Scheme for TADF:non-radical energy transfer OLED mechanism. Electrical excitation
generates singlet D(S1) and triplet D(T1) excitons, with FRET singlet-singlet energy transfer to non-radical energy acceptor (A) to form emissive singlet
excitons, A(S1). Dexter triplet–triplet energy transfer forms non-emissive triplet excitons, A(T1); non-radiative decay to the ground state is shown by a wavy
arrow. ISC and rISC steps between D(T1) and D(S1) are indicated. Spin multiplicity of D and A pairs are denoted by 2 S+1 in 2S+1{D A}. d Scheme for
TADF:radical energy transfer OLED mechanism. Electrical excitation generates singlet D(S1) and triplet D(T1) excitons, with singlet–doublet FRET and
triplet–doublet Dexter energy transfer to radical energy acceptor (A) to form emissive doublet excitons, A(D1). e Chemical structures for 4CzIPN and
TTM-3PCz used to test the mechanism in (d). f Absorption (black) and normalised PL (red) profiles for 4CzIPN (dotted lines) and TTM-3PCz (solid lines).
2
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ARTICLE
such that the process converts one dark state to another dark
state. A donor singlet can transfer its energy to the acceptor
singlet by Förster transfer:
(cid:1) (cid:3)
(cid:1) (cid:3)
! D S0
þ A S0
(cid:1) (cid:3)
! D S0
(cid:1) (cid:3)
þ A S0
(cid:1) (cid:3)
þ A S1
(cid:1) (cid:3)
D S1
þ hν
ð3Þ
but since this converts one bright state to another bright state, it
does not improve the device efficiency, though could improve
other device characteristics such as colour purity. Singlet to
singlet energy transfer has been achieved, in previous work13–17,
where TADF materials have been used as sensitisers in Förster-
type energy transfer of TADF S1 to non-radical fluorescent
molecules in a ‘hyperfluorescence’ scheme as depicted in Fig. 1c.
In these systems, energy transfer of TADF triplet excitons is
indirect and proceeds following reverse intersystem crossing to
the TADF S1. However, the undesirable triplet–triplet energy
transfer to lower energy triplets on the ‘hyperfluorescent’ mole-
cule, as mentioned above, as well as undesirable triplet-
annihilation interactions, must therefore be suppressed.
! D0
In contrast to OLED technologies employing electronic excita-
tions with paired electrons, efficient radical-based OLEDs offer an
alternative route to overcoming the spin-statistics limit using doublet
þ hν
excitons with spin-allowed doublet emission (D1
fluorescence), since the dark quartet state Q1 lies above the D1 state
in energy18–24 (note that Dx denotes doublet electronic states, and D
denotes energy donor). The radical OLED photophysical mechan-
ism is shown in Fig. 1b. However, despite demonstrating an excel-
lent peak EQE at low injection current densities, the ‘roll-off’—
decreasing efficiency with increasing current density—is severe in
reported radical devices using single-dopant emissive layers where
charge trapping directly forms doublet excitons20,25. The role of
exciton-exciton and exciton-charge annihilation effects were ruled
out by transient PL measurements on electrically-driven OLEDs,
leading to the conclusion that the charge-trapping mechanism for
EL must be improved to advance the performance of radical-based
devices25.
Here we consider if the desirable properties of radical emitters
could be used to ‘brighten’ otherwise dark (or slowly emissive)
triplet states where emission efficiency cannot easily be improved
by using a ground-state singlet acceptor. In the SI section 1, we
show how, using a ground-state radical acceptor, triplet energy
transfer leading to an emissive excited-state doublet can be
quantum mechanically spin-allowed by Dexter transfer:
(cid:1)
þ A D0
(cid:1) (cid:3)
! D S0
(cid:1) (cid:3)
! D S0
(cid:1)
þ A D1
(cid:1)
þ A D0
(cid:1)
D T1
þ hν
(cid:3)
(cid:3)
(cid:3)
(cid:3)
ð4Þ
unlike the case of a ground-state singlet acceptor considered
earlier. Energy transfer from an excited-state singlet to a doublet
is also allowed via a Förster-type mechanism
(cid:1) (cid:3)
(cid:1) (cid:3)
! D S0
! D S0
(cid:1)
þ A D1
(cid:1)
þ A D0
(cid:1)
þ A D0
(cid:1) (cid:3)
D S1
þ hν
(cid:3)
(cid:3)
(cid:3)
ð5Þ
meaning that the radicals’ doublet-spin nature enables energy
harvesting of all electronic excitations in standard organic semi-
conductors. In addition, rapid EL emission can be enabled in
radical energy transfer-based devices, which is desirable:
to
enhance EL efficiency in OLEDs by outcompeting non-radiative
channels, and to avoid building up of high excitation densities at
high drive currents that can cause efficiency roll-off. Previously,
triplet
to doublet energy transfer has been demonstrated in
experiments using transient radical acceptors26, but to the best of
our knowledge has not been demonstrated using a stable, emissive
radical nor in an optoelectronic device.
We have combined non-radical organic semiconductors as
energy donors with radical emitters as energy acceptors to form
light-emitting layers. In principle, the strategy we propose can
work with a wide range of standard OLED semiconductors so
long as their singlet and triplet states are higher in energy than
the doublet exciton in the radical material. It is desirable to
choose systems for which the spin-exchange energy is kept low,
so that the singlet energy is kept low, and (as in the case of
‘hyperfluorescence’ mentioned earlier) we use here TADF
materials that are engineered to reduce the exchange energy to
thermally accessible values. A further advantage here is that
TADF systems undergo intersystem crossing following photo-
excitation, allowing us to follow singlet and triplet dynamics in
transient all-optical measurements. Thus our energy donors and
acceptors in double-dopant emissive layers were chosen to be the
benchmark TADF material, 1,2,3,5-tetrakis(carbazole-9-yl)-4,6-
tris(2,4,6-trichlorophenyl)
dicyanobenzene
methyl-3-substituted-9-phenyl-9H-carbazole (TTM-3PCz) radi-
cal from our previous work20. Transient PL (trPL) and absorp-
tion (TA) measurements were used to probe the singlet–doublet
and triplet–doublet
showing
rapid energy transfer on picosecond and microsecond timescales
from singlet
respectively. Magneto-
electroluminescence studies support the role of triplet–doublet
energy transfer in radical-based OLEDs. The TADF:radical
devices show improved device characteristics, with reduced turn-
on voltage and roll-off in the EQE, as well as better device sta-
bility than single-dopant radical structures. TADF:radical sys-
tems extend the spin space of organic optoelectronics, where
advantageous ‘hyperfluorescence’ can be retained, dark triplet
states removed, and more direct triplet–doublet energy transfer
used for efficient radical-based optoelectronics.
energy transfer mechanisms,
(4CzIPN)8,
and triplet
excitons,
and
Results and discussion
Radical energy harvesting for doublet emission. Figure 1d
shows an energy level diagram for radical-based OLEDs using
double-dopant emissive layers containing non-radical organic
components (D, energy donor) and radical emitters (A, energy
acceptor). General design rules are formulated: singlet (S1) and
triplet (T1) excitons of D can transfer energy to the doublet (D1)
of A for efficient doublet emission where
As
energy
donors
acceptors,
Þ > EðA; D1
Þ and EðD; T1
Þ where EðD; S1
1. The singlet and triplet energy levels of the donor are higher
Þ > EðA; D1
Þ
Þ are
Þ is the
than the D1 state of the acceptor, i.e., EðD; S1
and EðD; T1
the S1 and T1 exciton energies of D, and EðA; D1
radical A D1 exciton energy;
2. The donor-cation/acceptor-anion, D•+ A•− or donor-anion/
acceptor-cation, D•− A•+ states must be higher energy
than the radical D1-exciton, i.e., E(D•+ A•−) > E(A, D1)
and E(D•−A•+) > E(A, D1).
and
4CzIPN (EðD;
HOMOÞ = −5.8 eV; EðD; LUMOÞ = −3.4 eV)27 and TTM-3PCz
(EðA; HOMOÞ = −5.8–6 eV; EðA; SOMO reductionÞ = −3.7 eV)20
were chosen, and their molecular structures are given in Fig. 1e.
Singlet–doublet
transfer (Fig. 1d, dotted arrow) by a dipolar
fluorescence resonance energy transfer, FRET, mechanism results
in conservation of doublet-spin multiplicity from 2S1 to 2S0. This
was promoted by spectral overlap of TTM-3PCz A-absorption and
D-fluorescence of 4CzIPN (Fig. 1f), a well-studied TADF emitter
with a singlet–triplet exchange energy gap of <50 meV28,29. The
small singlet–triplet energy gap also allows substantial spectral
overlap of D-phosphorescence and A-absorption, which also leads
to a resonant energy condition. This sets up conditions for
triplet–doublet
energy transfer by electron-exchange Dexter
mechanism (Fig. 1d, dotted arrow) from long-lived (>microsecond)
4CzIPN triplet excitons, which can be harvested for light emission.
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The reverse process—doublet
to triplet energy transfer—was
previously demonstrated by us and others with TTM-carbazole
and anthracene derivatives30. Triplet–doublet energy transfer to
form 2S0 is spin-allowed by the 2T1 state, which is mixed with the
4T1 state because of the negligible doublet–quartet 2;4T1 energy
difference (estimated to be ~10 µeV from the intermolecular
approach with no bond formation where antiferromagnetic
coupled doublet is the lowest energy state31, meaning they are
effectively degenerate) and spin mixing terms such as the triplet
zero-field splitting interaction32. The mixed 2;4T1 states allow
unlocked triplet–doublet channels for direct energy transfer with
organic radicals. The theoretical considerations for singlet–doublet
and triplet–doublet energy transfer by FRET and Dexter mechan-
isms are discussed further in Supplementary Information 1.
the
energy
radical
transfer
studying
Energy transfer photophysics with radical emitters. In order to
understand the photophysics of combined TADF:radical materi-
als we firstly studied films that were radical-only, TADF-only and
TADF:radical blends. We used time-resolved optical spectroscopy
measurements to probe energy transfer from 4CzIPN to TTM-
3PCz on pico- to microsecond timescales. The film composition
for
concept was
4CzIPN:TTM-3PCz:CBP (ratio = 0.25:0.03:0.72). Reference films
were studied for TTM-3PCz radical only (TTM-3PCz:CBP,
0.03:0.97) and 4CzIPN TADF only (4CzIPN:CBP, 0.25:0.75). The
composition is based on the starting point of our previous work
on TTM-3PCz OLEDs20, which here allows us to test energy
transfer mechanisms in proof-of-principle studies. 4CzIPN and
TTM-3PCz were blended in CBP (4,4’bis(N-carbazolyl)-1,1’-
biphenyl) to reduce the effects of exciton self-quenching33, and
with higher doping of 4CzIPN than the radical to promote charge
trapping at the TADF sites and subsequent energy transfer to
TTM-3PCz for light emission.
TrPL profiles for nano-to-microsecond time ranges (with
355 nm excitation, all fluences = 5 μJ/cm2) of 4CzIPN:TTM-
3PCz:CBP films are found to be superpositions of TTM-3PCz
(~700 nm) and 4CzIPN (~530 nm) emission. PL timeslices
(2.5 ns) are given in Fig. 2a for 4CzIPN:TTM-3PCz:CBP (red),
4CzIPN:CBP (black) and TTM-3PCz:CBP (blue). In Fig. 2b,
normalised PL spectra with respect to radical emission (timeslices
from 2.5 to 50 ns) show substantial quenching of 4CzIPN on
nanosecond timescales. For OLED applications it is desirable to
reduce the overall emission time to minimise exciton quenching
mechanisms34, leading us to consider plots of the integrated PL
fraction for total emission (Fig. 2c). From this, we observe in
4CzIPN:TTM-3PCz:CBP that 95% of all photons are emitted by
1 μs, and over 80% of emission occurring by 100 ns. This
compares favourably to 4CzIPN:CBP where only ~50% of
emission happens by 1 μs, such that the donor–acceptor blend
shows faster emission than the 4CzIPN-only blend.
We have performed TA studies of 4CzIPN:TTM-3PCz:CBP,
TTM-3PCz:CBP and 4CzIPN:CBP films in order to elucidate the
energy transfer processes from excited-state absorption kinetics.
In Fig. 3a, ΔT/T spectral timeslices are presented for short-time
TA of 4CzIPN:TTM-3PCz:CBP from 0.2–0.3 ps to 1000–1700 ps.
Excitation at 400 nm allowed for the preferential formation of
excitons on 4CzIPN, owing to its strong absorption in this region
and significantly higher loading fraction. The initial TA spectrum
of 4CzIPN:TTM-3PCz:CBP (0.2–0.3 ps) closely resembles that of
4CzIPN:CBP, where we have assigned the 4CzIPN ground-state
bleach between 360–460 nm, the 4CzIPN stimulated emission
overlaid on a photoinduced absorption (PIA) between 480 and
700 nm, and the primary 4CzIPN S1 PIA at 830 nm (see
Supplementary Figs. 1 and 2 for TA of 4CzIPN:CBP films). By
10 ps, we observe new PIA bands that grow in for 4CzIPN:TTM-
3PCz:CBP at 620, 950 and 1650 nm. These features match with
the TTM-3PCz D1 spectral profile obtained from studies of TTM-
3PCz:CBP films (Supplementary Figs. 3 and 4), showing energy
transfer from TADF singlet to radical doublet. In Fig. 3b, the
normalised ΔT/T kinetic profiles for 4CzIPN:TTM-3PCz:CBP in
and 4CzIPN S1
TTM-3PCz D1
(800–830 nm, orange line) PIA regions are shown. We highlight
an additional quenching of 4CzIPN in 4CzIPN:TTM-3PCz:CBP
compared to 4CzIPN:CBP films (black line, Fig. 3b). The
quenching of 4CzIPN S1 PIA and the growth of TTM-3PCz D1
PIA on picosecond timescales prior to nanosecond 4CzIPN
intersystem crossing is attributed to Förster-type singlet–doublet
energy transfer35. As the 4CzIPN S1 PIA lies in a region where
there is reduced absorption by the TTM-3PCz D1, we can use the
ΔT/T with and without the presence of TTM-3PCz to estimate a
lower bound for the fraction of singlet–doublet energy transfer.
By 1.7 ns, the 4CzIPN S1 PIA falls to approximately 45% and 60%
of the initial signal with (orange) and without (black) TTM-3PCz
present, respectively, suggesting that ≥15% of S1 from 4CzIPN
have already undergone fluorescence resonance energy transfer
(FRET) to TTM-3PCz in 4CzIPN:TTM-3PCz:CBP. With selective
excitation of TTM-3PCz at 600 nm (below the 4CzIPN bandgap)
(610–630 nm,
red line)
(a)
)
s
t
i
n
u
.
b
r
a
(
L
P
d
e
s
i
l
a
m
r
o
N
1.0
0.8
0.6
0.4
0.2
0.0
4CzIPN:TTM-3PCz:CBP
TTM-3PCz:CBP
4CzIPN:CBP
(b)
4CzIPN:TTM-3PCz:CBP
2.5 ns
2.5 ns
5 ns
10 ns
20 ns
50 ns
)
s
t
i
n
u
.
b
r
a
(
L
P
d
e
s
i
l
a
m
r
o
N
1.0
0.8
0.6
0.4
0.2
0.0
(c)
n
o
i
t
c
a
r
F
L
P
d
e
t
a
r
g
e
t
n
I
1.0
0.8
0.6
0.4
0.2
0.0
450
500
550
600
650
700
750
800
850
450
500
550
600
650
700
750
800
850
101
Wavelength (nm)
Wavelength (nm)
4CzIPN (450 - 800 nm)
TTM-3PCz (575 - 840 nm)
4CzIPN:TTM-3PCz (650 - 840 nm)
102
Time (ns)
103
104
Fig. 2 Transient photoluminescence studies of 4CzIPN and TTM-3PCz with 355 nm excitation. a PL timeslices at 2.5 ns for 4CzIPN:TTM-3PCz:CBP
(ratio = 0.25:0.03:0.72, red line); 4CzIPN:CBP (0.25:0.75, black line); TTM-3PCz:CBP (0.03:0.97, blue line), showing emission from both TADF and
radical in the combined film. b PL timeslices for 4CzIPN:TTM-3PCz:CBP at various times from 2.5 to 50 ns, showing the 4CzIPN emission decaying relative
to the radical emission at longer times. c Integrated PL fraction time profiles from 2.5 ns to 25 µs for 4CzIPN:TTM-3PCz:CBP in 650–840 nm range (red
line); 4CzIPN:CBP in 450–800 nm range (black line); and TTM-3PCz:CBP in 575–840 nm range (blue line), showing faster luminescence for the combined
TADF:radical film than the TADF-only film.
4
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(a)
1.0×10-3
5.0×10-4
/
T
T
∆
0.0
-5.0×10-4
-1.0×10-3
(d)
/
T
T
∆
d
e
s
i
l
a
m
r
o
N
1.0
0.8
0.6
0.4
0.2
4CzIPN:TTM-3PCz:CBP
0.2 - 0.3 ps
1 - 2 ps
10 - 20 ps
100 - 200 ps
1000 - 1700 ps
400 500 600 700 800 900
1400 1500 1600
Wavelength (nm)
Probe range: 610-630 nm
4CzIPN:TTM-3PCz:CBP
TTM-3PCz:CBP
Bi-exponential Fit
τ
1 = 18.8 ns
τ
2 = 1.6 μs
Mono-exponential Fit
τ = 16.8 ns
(b)
/
T
T
∆
d
e
s
i
l
a
m
r
o
N
1.0
0.8
0.6
0.4
0.2
0.0
0.1
(e)
/
T
T
∆
d
e
s
i
l
a
m
r
o
N
1.0
0.8
0.6
0.4
0.2
4CzIPN:TTM-3PCz:CBP
(c)
0.0
-1.0×10-4
-2.0×10-4
/
T
T
∆
-3.0×10-4
-4.0×10-4
-5.0×10-4
-6.0×10-4
610 - 630 nm (TTM-3PCz D1 PIA)
800 - 830 nm (4CzIPN S1 PIA)
1500 - 1600 nm (TTM-3PCz D1 PIA)
4CzIPN S1 PIA without TTM-3PCz
ARTICLE
4CzIPN:TTM-3PCz:CBP
1 - 2 ns
5 - 6 ns
10 - 15 ns
20 - 30 ns
50 - 60 ns
100 - 200 ns
1000 - 2000 ns
1
10
100
1000
500
600
700
800
900
1000
Time (ps)
Wavelength (nm)
Probe range: 800-830 nm
4CzIPN:TTM-3PCz:CBP
4CzIP:CBP
Bi-exponential Fit
τ
prompt = 7.8 ns
τ
delayed = 1.0 μs
Bi-exponential Fit
τ
prompt = 12.1 ns
τ
delayed = 2.5 μs
(f)
)
K
3
9
2
=
T
(
L
P
d
e
y
a
e
D
l
/
)
T
(
L
P
d
e
y
a
e
D
l
1
0.8
0.6
0.4
0.2
50
4CzIPN:CBP
4CzIPN:TTM-3PCz:CBP
100
150
200
250
300
Temperature (K)
0.0
100
101
103
102
Time (ns)
104
105
0.0
100
101
103
102
Time (ns)
104
105
Fig. 3 Transient absorption and temperature dependence studies of 4CzIPN and TTM-3PCz. Picosecond to nanosecond (a) timeslices and (b) kinetic
profiles from transient absorption studies of 4CzIPN:TTM-3PCz:CBP (ratio = 0.25:0.03:0.72). 400 nm excitation, fluence = 89.1 μJ/cm2. This shows the
decay of the singlet PIA around 830 nm and the growth of the radical PIAs around 620 and 1650 nm. c Nanosecond to microsecond timeslices of the
4CzIPN:TTM-3PCz:CBP blend (0.25:0.03:0.72). 355 nm excitation, fluence = 17.0 μJ/cm2. Discontinuities in timeslice spectral profiles for (a) and (c) arise
because multiple experiments are used to cover the studied wavelength probe regions. Transient absorption kinetic profiles for photoinduced absorption
features of (d) TTM-3PCz (610–630 nm) and (e) 4CzIPN (800–830 nm). d TTM-3PCz excited-state kinetics are shown for 4CzIPN:TTM-3PCz:CBP
(0.25:0.03:0.72, red squares); and TTM-3PCz:CBP (0.03:0.97, black circles). This shows delayed radical emission is active in 4CzIPN:TTM-3PCz:CBP
(TADF:radical) from triplet–doublet energy transfer. e 4CzIPN excited-state kinetics are shown for 4CzIPN:TTM-3PCz:CBP (red squares); and 4CzIPN:CBP
(0.25:0.75, black circles). This shows delayed radical emission in 4CzIPN:TTM-3PCz:CBP (TADF:radical) is more rapid than delayed emission in
4CzIPN:CBP (TADF only). Mono- and bi-exponential fits are indicated by solid lines in (d and e). f Ratio of integrated delayed PL contribution for
4CzIPN:CBP (black circles) and 4CzIPN:TTM-3PCz:CBP (red circles) at different temperatures. Three-point moving average and trends for these profiles
are indicated by square and line plots, and show different temperature dependencies.
in 4CzIPN:TTM-3PCz:CBP, the resulting TA profiles resemble
TTM-3PCz:CBP, showing that the D1 exciton—once formed—
does not interact with 4CzIPN by further energy or charge
transfer processes (Supplementary Figs. 5 and 6).
We have studied energy transfer for timescales beyond 1 ns
with long-time TA measurements of 4CzIPN:TTM-3PCz:CBP
films (excited at 355 nm). ΔT/T spectral timeslices (1–2 ns to
1000–2000 ns) in Fig. 3c display features at 620, 830 and 1600
nm, which can be attributed to the TTM-3PCz D1 PIA and
4CzIPN S1 PIA from radical-only (Supplementary Figs. 3 and 4)
and TADF-only films (Supplementary Figs. 1 and 2). The kinetic
decay profile of
the TTM-3PCz PIA (600–630 nm) has an
extended lifetime in 4CzIPN:TTM-3PCz:CBP films (red squares,
Fig. 3d) compared to TTM-3PCz:CBP (black circles). The
4CzIPN:TTM-3PCz:CBP kinetic profile can be fitted to a bi-
= 1.6 μs.
exponential with time constants of τ
The presence of a long-lived D1 state in 4CzIPN:TTM-3PCz:CBP,
beyond the D1 excited-state lifetime measured from TTM-
3PCz:CBP (τ = 16.8 ns, Supplementary Fig. 4), suggests energy
transfer from 4CzIPN triplet (T1) states. By comparing the kinetic
traces of the PIA associated with 4CzIPN from 800 to 830 nm in
4CzIPN:CBP (black circles, Fig. 3e) and 4CzIPN:TTM-3PCz:CBP
(red squares), we observed reductions in both the prompt and
from 12.1 to 7.8 ns and 2.5 μs to 1.0 μs,
delayed lifetimes,
respectively, from the presence of TTM-3PCz. This provides
further evidence for energy transfer from 4CzIPN T1 (delayed
= 18.8 ns and τ
2
1
(cid:3)
(cid:3)
(cid:3)
(cid:3)
or
transfer,
(cid:1)
D T1
(cid:1)
þ A D0
(cid:1)
þ A D1
kinetic), and additionally from 4CzIPN S1 (prompt kinetic), to
form TTM-3PCz D1.
(cid:1) (cid:3)
! D S0
Triplet–doublet energy transfer
from 4CzIPN, a TADF
molecule, can be attributed to a hyperfluorescent-type mechan-
ism by breakout from S1-T1 ISC and rISC cycles36,
(cid:1) (cid:3)
! D S1
(cid:1)
ð6Þ
þ A D0
i.e., 4CzIPN reverse intersystem crossing, then singlet–doublet
triplet–doublet
Förster
direct Dexter-type
mechanism37,38 as given in Eq. (4). Both mechanisms lead to
reduced T1 lifetime. In order to distinguish the energy transfer
mechanisms, we have performed temperature dependence studies
(50–293 K) on trPL of 4CzIPN:CBP (Supplementary Fig. 10) and
4CzIPN:TTM-3PCz:CBP (Supplementary Fig. 11). In both films
there is negligible temperature dependence on trPL up to 100 ns,
which we define as the prompt emission; we classify light
emission from 100 ns onwards as delayed-type. The ratio of
integrated delayed emission at different temperatures (T) with
respect to the integrated value at 293 K is shown in Fig. 3f (i.e.,
delayed PL(T)/delayed PL(T = 293 K)). The delayed PL ratio is
reduced in 4CzIPN:CBP films compared to 4CzIPN:TTM-
3PCz:CBP,
falling to 0.2 and 0.8 at 50 K, respectively. This
supports a Dexter-type triplet–doublet energy transfer channel in
4CzIPN:TTM-3PCz:CBP, with lower activation energy than
reverse intersystem in 4CzIPN:CBP for thermally activated
delayed fluorescence. However, the signal:noise for delayed PL
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5
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(a)
)
V
e
(
y
g
r
e
n
E
–2
–3
–4
–5
–6
–7
LiF/
Al
C
P
A
T
P
B
C
ITO/
MoO3
40 nm
30 nm
M
P
M
Y
P
3
B
60 nm
4CzIPN
TTM-3PCz
4CzIPN:TTM-3PCz:CBP
TTM-3PCz:CBP
4CzIPN:CBP
(d)
25
)
%
(
E
Q
E
20
15
10
5
(b)
103
)
2
–
102
m
c
A
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(
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t
i
s
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e
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d
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o
N
1
0.8
0.6
0.4
0.2
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4CzIPN:TTM-3PCz:CBP
TTM-3PCz:CBP
4CzIPN:CBP
5
6
Voltage (V)
7
0
1
2
3
4
3V
3.5V
4V
4.5V
5V
5.5V
6V
7V
8V
9V
10V
(c)
)
2
–
m
1
–
r
s
W
(
e
c
n
a
d
a
R
i
103
102
101
100
10–1
10–2
10–3
10–4
10–5
(f)
)
%
(
L
E
M
6
5
4
3
2
1
8
9 10 11 12
4CzIPN:TTM-3PCz:CBP
TTM-3PCz:CBP
4CzIPN:CBP
0
1
2
3
4
5
6
Voltage (V)
7
8
9 10 11 12
4CzIPN:(TTM-3PCz):CBP
(4CzIPN):TTM-3PCz:CBP
(4CzIPN):CBP
0
10–3
10–2
10–1
100
101
102
Current density (mA cm–2)
0
400
500
600
700
800
900
Wavelength (nm)
0
–300
–200
0
–100
Magnetic field (mT)
100
200
300
Fig. 4 4CzIPN and TTM-3PCz organic light-emitting diodes. a Device architecture for OLEDs with varying emissive layer: 4CzIPN:TTM-3PCz:CBP,
4CzIPN:CBP; TTM-3PCz:CBP. b–d Current density–voltage (J–V), radiance–voltage, EQE–current density (from 10−3 mA/cm2) curves for OLEDs.
e Normalised EL profiles for 4CzIPN:TTM-3PCz:CBP OLEDs with varying voltage, and 4CzIPN and TTM-3PCz emission contributions. f Magneto-
electroluminescence (MEL) studies of TTM-3PCz (red squares) and 4CzIPN (red diamonds) emission in 4CzIPN:TTM-3PCz:CBP OLEDs; 4CzIPN emission
in 4CzIPN:CBP (black triangles). OLED devices were biased at 8 V. MEL studies show different magnetic field dependencies for 4CzIPN and TTM-3PCz
emission from 4CzIPN:TTM-3PCz:CBP devices, which supports Dexter triplet–doublet energy transfer and not the hyperfluorescence mechanism of
4CzIPN triplet exciton energy harvesting.
ratio varies in 4CzIPN:TTM-3PCz:CBP with changing tempera-
ture, restricting further quantitative analysis.
From the film photophysical studies, we have demonstrated
efficient singlet–doublet and triplet–doublet energy transfer in
4CzIPN:TTM-3PCz:CBP from picosecond to microsecond time-
scales, which we have attributed to Förster and Dexter
mechanisms that enable luminescent TADF:radical films with
emission from radical D1.
Radical OLEDs and magneto-electroluminescence studies.
Following our demonstration of singlet–triplet–doublet energy
transfer photophysics, we aimed to exploit these processes in more
efficient radical-based OLED designs. We fabricated TADF:radical
OLEDs using the device structure in Fig. 4a. B3PYMPM (4,6-
bis(3,5-di(pyridine-3-yl)phenyl)-2-methylpyrimidine) and TAPC
(1,1-bis[(di-4-tolylamino)phenyl]cyclohexane) were used as elec-
tron transport and hole transport layers, respectively. The emissive
layer (EML) was 4CzIPN:TTM-3PCz:CBP (0.25:0.03:0.72)—the
same composition as the photophysics studies. Single-dopant
OLEDs were also fabricated where EML was 4CzIPN:CBP
(0.25:0.75) for TADF reference devices; and EML was TTM-
3PCz:CBP (0.03:0.97) for radical reference OLEDs.
The current density–voltage (J–V), radiance–voltage and EQE
plots for the 4CzIPN:TTM-3PCz:CBP (red squares), 4CzIPN:CBP
(black triangles) and TTM-3PCz:CBP (blue circles) OLEDs are
shown in Fig. 4b–d. We found that the turn-on voltages decrease
from 2.9 V (TTM-3PCz:CBP device) to 2.3 V (4CzIPN:TTM-
3PCz:CBP) to 2.2 V (4CzIPN:CBP). Here, we define the turn-on
voltage to be that corresponding to current density >0.1 µA/cm2,
above the electrical noise level of the devices. The trend in turn-
on voltage suggests that the inclusion of the TADF sensitiser
leverages more energy-efficient doublet exciton formation in
electroluminescence. However, the higher turn-on voltage for
TADF:radical OLEDs compared to TADF, and different J–V
profiles in Fig. 3b, imply that both CBP and 4CzIPN mediate
some electrical excitation of TTM-3PCz in TADF:radical devices.
If all doublet electroluminescence originated by energy transfer
from TADF sensitisation as in Fig. 1d, the J–V curves and turn-on
voltages would be identical for 4CzIPN:CBP and 4CzIPN:TTM-
3PCz:CBP OLEDs.
are
achievable
We note there is a plateau in maximum radiance of
~1 W sr−1 m−2 from 5 V for TTM-3PCz:CBP devices in Fig. 4c;
radiance values up to 10 W sr−1 m−2
in
4CzIPN:TTM-3PCz:CBP. At voltages higher than 5 V, there is
an increasing component of 4CzIPN emission in the total EL of
4CzIPN:TTM-3PCz:CBP OLEDs. At 10 V the EL from the device
contains 89% TTM-3PCz and 11% 4CzIPN contributions. The
higher radiance at 10 V for 4CzIPN:TTM-3PCz:CBP (5.0 W
sr−1 cm−2) compared to TTM-3PCz:CBP (1.1 W sr−1 cm−2) in
Fig. 4c is therefore consistent with increasing energy transfer
contribution from electrically excited 4CzIPN. The EL profile at
the steady-state PL profile for
10 V in Fig. 4e resembles
4CzIPN:TTM-3PCz blends (Supplementary Fig. 8).
Figure 4d shows that there is substantial increase in maximum
EQE on going from 4CzIPN:CBP (7.8%) and TTM-3PCz:CBP
(10.7%) devices to 4CzIPN:TTM-3PCz:CBP (16.4%) OLEDs. The
EQE is evaluated for the total EL output. We note that the 25% wt.
4CzIPN:CBP reference device shown here has lower EQE than
previous reports with 3% wt. 4CzIPN concentration due to exciton
self-quenching effects8,33. The high 4CzIPN concentration is
necessary to promote charge trapping at the TADF component
in 4CzIPN:TTM-3PCz:CBP blends. Here the higher EQE on going
from 4CzIPN:CBP to 4CzIPN:TTM-3PCz:CBP OLEDs suggests
efficient energy transfer from 4CzIPN to TTM-3PCz,
leading
6
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is not
J0,
limited by the EL efficiency of
to performance that
the 4CzIPN:CBP device.
the critical current density that
corresponds to the device current at half the maximum EQE,
increases from 2.1 mA cm−2 for TTM-3PCz:CBP to 9.5 mA cm−2
for 4CzIPN:TTM-3PCz:CBP. The better roll-off and sustained EL
efficiency in 4CzIPN:TTM-3PCz:CBP OLEDs is also attributed to
an increasing contribution of 4CzIPN energy transfer to the EL at
higher current densities. At lower voltages (<5 V) and current
densities (<0.1 mA cm−2), the EL shows TTM-3PCz emission only
(Fig. 4e). We performed studies to obtain the device’s half-lifetime,
T50 (time for luminance to fall to half of the initial value under a
constant current density). The T50 of energy transfer-type
4CzIPN:TTM-3PCz:CBP OLEDs was found to be 42 min at
0.4 mA/cm2 (see Supplementary Fig. 7), indicating some improve-
ment over charge-trapping-type devices that we have previously
reported for
radical OLEDs with TTM-derivative:host EML
(10 min at 0.1 mA/cm2)25.
Magneto-electroluminescence (MEL) and magnetoconduc-
tance (MC) studies have been performed on the 4CzIPN:CBP
and 4CzIPN:TTM-3PCz:CBP devices. The devices were biased at
8 V and the data for magneto-EL and magnetoconductance were
collected simultaneously. In 4CzIPN:CBP devices, MEL and MC
profiles show enhanced EL and current density upon application
of magnetic field (Fig. 4f and Supplementary Fig. 9). The profiles
are fitted to double Lorentzian functions that capture low
(<10 mT) and high (>10 mT) magnetic field effects (MFEs).
The low field dependence is characteristic of magnetic field effects
on hyperfine-mediate spin mixing of singlet and triplet polaron
pair39, the precursors of excitons, which affect the ratio of singlet
and triplet exciton formation. High field effects can arise from
triplet exciton–polaron quenching and singlet–triplet dephasing
effects40,41. MFEs of 4CzIPN:CBP devices are positive and show
typical behaviour for MEL and MC from non-radical dopant
systems, as previously reported42.
that
In TADF:radical OLEDs (4CzIPN:TTM-3PCz:CBP) we have
studied magnetic field effects on EL from TTM-3PCz
(680–800 nm) and 4CzIPN (500–550 nm) emission contributions.
We observe positive magnetic field effects for both TTM-3PCz
and 4CzIPN contributions, which indicates
the main
magnetic field sensitivity originates from hyperfine-mediated
spin mixing of singlet–triplet polaron pairs, as found in the
TADF-only devices. However the size of MEL for 4CzIPN (+4%
at 250 mT) and TTM-3PCz
emission
components are different in TADF:radical OLEDs. We consider
that non-identical MEL profiles for 4CzIPN and TTM-3PCz
emission in 4CzIPN:TTM-3PCz:CBP devices supports a Dexter
triplet–doublet energy transfer mechanism because an identical
field sensitivity would be expected for the 4CzIPN and TTM-
3PCz MEL in TADF:radical hyperfluorescent-type devices.
(+1% at 250 mT)
We have demonstrated efficient energy transfer of 4CzIPN
singlet and triplet excitons to obtain emissive doublet excitons of
TTM-3PCz.
In trPL studies we observed more rapid light
emission in 4CzIPN:TTM-3PCz:CBP blends than 4CzIPN:CBP,
as up to 95% and 50% of photons are emitted by 1 µs,
respectively. TA measurements revealed singlet–doublet and
triplet–doublet energy transfer on 10–100 ns and 100 ns–1 µs
timescales, though the observed timescale of triplet transfer is
limited by the time taken for intersystem crossing to take place on
4CzIPN and, as a spin-allowed process, may be faster than this.
layer were
OLEDs with 4CzIPN:TTM-3PCz:CBP emissive
= 9.5 mA/cm2,
demonstrated with max EQE = 16.4% and J0
EQE = 10.7%,
which
(max
outperforms TTM-3PCz:CBP
= 2.1 mA/cm2) for the same charge transport layer architec-
J0
ture. With also an order of magnitude improvement in device
stability, the energy transfer-type radical OLEDs therefore show a
substantial improvement in device characteristics compared to
previous reports of charge-trapping radical OLEDs. The MEL
results allow us to rule out a fully hyperfluorescence-type (Eq. (6))
mechanism for EL, and support Dexter-type T1-D1 energy
transfer pathways enabled by organic radicals, here TTM-3PCz.
We highlight that Dexter triplet–triplet transfer from energy
donor to acceptor is a loss route for light emission with non-
radicals, and must be suppressed in energy transfer devices using
non-radical fluorescent emitters, for example, hyperfluorescence-
type devices15. However fluorescent radical (doublet) emitters can
exploit the triplet–doublet energy transfer pathway for radical
OLEDs as we have demonstrated here, without a lower-lying
radical ‘triplet state’ that must be avoided for emission losses. In
future work, our device concepts can be used in improved
material combinations for more efficient energy transfer with
reduced exciton quenching, and with increased radical lumines-
cence for advancing the performance beyond this starting point.
By unlocking new energy transfer channels, an optoelectronic
design for improved radical-based light-emitting devices
is
enabled by their unpaired electron spin properties.
Methods
Materials. TTM-3PCz precursor was synthesised by Suzuki coupling of tris(2,4,6-
trichlorophenyl)methane (HTTM) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl-
3PCz20. In this procedure, TTM-3PCz radicals were generated from the precursor
by treatment with potassium t-butoxide in tetrahydrofuran, followed by oxidation
with p-chloranil. 4CzIPN, TAPC, B3PYMPM, CBP of sublimed grade and other
OLED materials were obtained from Ossila, Xi’an Polymer Light and Lumtec.
Photophysics. TrPL and TA studies were performed on home-built setups pow-
ered by a Ti:sapphire amplifier (Spectra Physics Solstice Ace, 100 fs pulses at
800 nm, 7 W output at 1 kHz). TrPL profiles were recorded using an Andor
spectrometer setup with electrically gated intensified CCD camera (Andor SR303i;
Andor iStar). Sample excitation with 400 nm pump pulse was provided by
frequency-doubled 800 nm pulse from Ti:sapphire amplifier in trPL and short-time
(ps–ns) TA studies. Short-time TA studies with 600 nm excitation were achieved
from the wavelength tuneable output of TOPAS optical parametric amplifier (Light
Conversion), which was pumped by the 800 nm laser pulses from the Ti:sapphire
amplifier. Long-time (ns–µs) TA studies were performed with 355 nm pump pulses
from an Innolas Picolo 25. Probe pulses for TA were obtained from non-collinear
optical parametric amplifier (NOPA) systems for the visible (500–780 nm), near-
infrared (830–1000 nm) and infrared (1250–1650 nm) wavelength ranges. The
NOPA probe pulses were divided into two identical beams by a 50/50 beamsplitter;
this allowed for the use of a second reference beam for improved signal:noise. The
probe pulse for the UV (350–500 nm) region was provided by a white light
supercontinuum generated in a CaF2 crystal. The probe pulses were detected by Si
(Hamamatsu S8381-1024Q) and InGaAs (Hamamatsu G11608-512DA) dual-line
array with a custom-built board from Stresing Entwicklungsbüro.
Device fabrication and characterisation. Organic semiconductor films and
devices were fabricated by vacuum-deposition processing (<6 × 10‒7 torr) using an
Angstrom Engineering EvoVac 700 system. Current density, voltage and electro-
luminescence characteristics were measured using a Keithley 2400 sourcemeter,
Keithley 2000 multimeter and calibrated silicon photodiode. The EL spectra were
recorded by an Ocean Optics Flame spectrometer. Magneto-EL measurements
were performed with Andor spectrometer (Shamrock 303i and iDus camera) for
modulation of EL in presence of magnetic field applied by GMW 3470
electromagnet.
Reporting summary. Further information on research design is available in the Nature
Research Reporting Summary linked to this article.
Data availability
The data generated in this study have been deposited in the figshare database under the
accession code: https://doi.org/10.6084/m9.figshare.17026607.v1.
Code availability
Code used to analyse data in this manuscript are available from the corresponding author
upon reasonable request.
Received: 30 August 2021; Accepted: 2 March 2022;
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Acknowledgements
J.D., Z.C. and F.L. are grateful for financial support from the National Natural Science
Foundation of China (grant no. 51925303). E.W.E. is grateful to the Leverhulme Trust for
an Early Career Fellowship; and the Royal Society for a University Research Fellowship
(grant no. URF\R1\201300). TJHH thanks the Royal Society for a University Research
Fellowship (grant no. URF\R1\201502). WKM and the Centre for Advanced Electron
Spin Resonance is supported by EPSRC (EP/L011972/1). F.L. is an academic visitor at the
Cavendish Laboratory, Cambridge, and is supported by the Talents Cultivation Pro-
gramme (Jilin University, China). A.J.G. and RHF acknowledge support from the Simons
Foundation (grant no. 601946) and the EPSRC (EP/M01083X/1 and EP/M005143/1).
This project has received funding from the ERC under the European Union’s Horizon
2020 research and innovation programme (grant agreement no. 670405 and 101020167).
Author contributions
E.W.E. and F.L. fabricated thin films and OLED devices, which were characterised by
photoluminescence, J–V-radiance measurements and magnetic field studies. AJG per-
formed transient absorption measurements. A.J.G. and E.W.E. carried out the transient
PL measurements. Q.G. conducted OLED time dependence studies. J.D. and Z.C. syn-
thesised the radical materials. TJHH formulated theory on the photophysical mechan-
isms. W.K.M. conducted spin physics studies. EWE, RHF and FL conceived the project
and supervised the work. The results were analysed and the manuscript was written with
input from all authors.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains supplementary material
available at https://doi.org/10.1038/s41467-022-29759-7.
Correspondence and requests for materials should be addressed to Richard H. Friend or
Emrys W. Evans.
Peer review information Nature Communications thanks Nadzeya Kukhta and the
other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer
reviewer reports are available.
Reprints and permission information is available at http://www.nature.com/reprints
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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Availability of data and materials
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Wangler and Jansky BMC Family Practice (2021) 22:252
https://doi.org/10.1186/s12875-021-01601-x
RESEARCH
Open Access
Prerequisites for providing effective support
to family caregivers within the primary care
setting – results of a study series in Germany
Julian Wangler* and Michael Jansky
Abstract
Background: General Practitioners are considered to be well placed to monitor home-care settings and to respond
specifically to family caregivers. To do this, they must be sensitive to the needs and expectations of caregivers. In order
to determine the current status of GP care in terms of the support given to family caregivers, a series of studies were
conducted to gather the perspectives of both caregivers and GPs. The results are used to derive starting points as to
which measures would be sensible and useful to strengthen support offered to family caregivers in the primary care
setting.
Methods: Between 2020 and 2021, three sub-studies were conducted: a) an online survey of 612 family caregivers;
b) qualitative interviews with 37 family caregivers; c) an online survey of 3556 GPs.
Results: Family caregivers see GPs as a highly skilled and trustworthy central point of contact; there are many differ-
ent reasons for consulting them on the subject of care. In the perception of caregivers, particular weaknesses in GP
support are the absence of signposting to advisory and assistance services and, in many cases, the failure to involve
family caregivers in good time. At the same time, GPs do not always have sufficient attention to the physical and
psychological needs of family caregivers. The doctors interviewed consider the GP practice to be well suited to being
a primary point of contact for caregivers, but recognise that various challenges exist. These relate, among other things,
to the timely organisation of appropriate respite services, targeted referral to support services or the early identifica-
tion of informal caregivers.
Conclusions: GP practices can play a central role in supporting family caregivers. Caregivers should be approached
by the practice team at an early stage and consistently signposted to help and support services. In order to support
care settings successfully, it is important to consider the triadic constellation of needs, wishes and stresses of both the
caregiver and the care recipient. More training and greater involvement of practice staff in the support and identifica-
tion of caregivers seems advisable.
Keywords: Caregivers, General practitioner, Ientification, Strain, Needs, Care, Support
*Correspondence: [email protected]
Center for General Medicine and Geriatrics, University Medical Center
of the Johannes Gutenberg University Mainz, Am Pulverturm 13,
55131 Mainz, Germany
Background
In the EU-27, over 20% of the population is already
over 65 years old [1, 2]. This results in a growing
need for care and support. In Germany, this need is
documented on the basis of approx. 4.1 million peo-
ple formally classed as needing care [3]. If informal
unremunerated care and support activities are also
© The Author(s) 2021. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or
other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this
licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco
mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 2 of 12
considered, this number increases to approximately
5.5 million who receive care or support [4, 5].
Informal care is predominantly provided in the home
environment by private citizens, who bear a considera-
ble share of the caregiving burden in caring for people
close to them who are in need of care [6–8]. Accord-
ing to representative data, more than 17% of 40- to
85-year-olds regularly support at least one person in
coping with everyday life; of these, a good third pro-
vide care in the stricter sense [9, 10].
Although research has shown that a caring role can
provide a subjective sense of purpose [11, 12], it is
associated with a greater health risk due to the physi-
cal and mental strain involved [8, 10, 13–19]. If the
consequences of the illness have not been considered
in advance and precautionary measures have not been
taken, it is not uncommon for caregivers to become
burnt-out and exhausted [15, 20–22]. In order to avoid
such crises and to promote the resilience of caregiv-
ers, various support services have been established in
Germany, including care support centres, outpatient
psychiatric services and dementia networks [23]. How-
ever, studies show that such services are only used by a
proportion of caregivers [24–26].
Since they have provided ongoing care to the patient
over many years and know them well, GPs are consid-
ered to be well suited to provide support for home care
settings and to respond to the particular concerns of
family caregivers [6, 27–29]. Apart from diagnosing
and treating health problems, GPs can provide infor-
mation and advice to caregivers, offer psychosocial
support and gain an overall picture of the care con-
ditions so that needs can be addressed promptly. By
referring patients to support and counselling services,
GPs can set the course for successful long-term care
and show caregivers ways to offset and relieve the bur-
den of caregiving [24, 30].
In 2018, the National Association of Statutory Health
Insurance Physicians (KBV) carried out a telephone
survey of 6043 randomly selected citizens representa-
tive of the German resident population. The study
concluded that about 60% of family caregivers talk to
their GP about their caring role [29]. Of these, around
two-thirds had been made aware of concrete offers of
help by their GP. Up to now, there has been a lack of
reliable studies, especially for the German-speaking
countries, which shed light on the status of GP support
for the target group of family caregivers, but also on
the practical challenges experienced, both broadly and
from multiple perspectives, i.e. from the point of view
of doctors and caregivers.
Methods
Overall study and research interest
This paper wants to help determine the current status of
German GP care in terms of the support given to fam-
ily caregivers. By doing so, it summarises the results of
a series of explorative studies conducted to gather the
perspectives of both family caregivers and GPs, and com-
pares the results with existing research.
The study, which consists of three sub-studies, stands
as an independent supplementary study in the broader
context of an Innovation Fund model project on outpa-
tient medical and nursing dementia care (DemStepCare)
[31]. All three sub-studies have already been published or
accepted for publication. The specific purpose of the pre-
sent work was to bundle commonalities of the individual
studies from an overarching perspective and to draw con-
clusions in terms of an overall view of the study series.
We are convinced that interconnecting the three studies
in this way opens up a concentrated view and increases
the informative value of all studies.
The aim was to explore the attitudes, experiences and
wishes of caregivers and GPs with regard to the support
of caregivers provided by the GP setting. The focus was
on the importance of GP support for caregivers and how
GPs perceive their own remit as contact partners. One
focus was to compare the needs of caregivers with the
support they actually experience. Another aim was to
identify the challenges for GP care.
Against the backdrop of the joint consideration of all
central results, the article aims to derive starting points
as to which measures would be sensible and useful to
strengthen support offered to family caregivers in the pri-
mary care setting. In view of this focus, special attention
is paid to weaknesses in the GP setting.
Sub‑studies
Initially, an online survey of 612 family caregivers [32]
was conducted in spring 2020 to identify care needs
and experiences in relation to GP care. The anony-
mous survey was posted on 17 German-language Inter-
net forums aimed at family care and family caregivers.
The selected forums were usually embedded in general
information portals on the subject of care. These web-
sites are intended to support family caregivers across
the board on a wide variety of questions relating to care
in a domestic setting (no specific clinical pictures) and
enable an exchange. Based on the registered number
of members, the authors assume that the forums theo-
retically reach up to 11,000 family caregivers in total.
In order to obtain the broadest possible picture of the
reality of care in Germany, the inclusion criterion was
deliberately kept general; accordingly, the survey target
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 3 of 12
group included all kinds of family caregivers. The mean
age of the respondents was 54 years, with 93% of the
respondents being women.
In a next step, we wanted to explore these results
in more detail, a total of 37 family caregivers were
recruited from the same online care forums and inter-
viewed between autumn 2020 and spring 2021 [33]. In
the respective Internet forums, a call was made in the
form of a thread in which information was given on the
general topic. People who were willing to be available
for an interview could contact the given email address.
As in the online survey, the interviews with family car-
egivers essentially included all types of caregivers and
care constellations. The inclusion criterion was that the
caregivers had regularly cared for at least one relative,
friend or neighbour in the last 12 months.
In a next step, the attitudes and experiences of GPs
with regard to the care of caregivers were gathered
by means of an online survey [34]. In spring 2021,
all 13,170 GPs in Baden-Württemberg, Hesse and
Rhineland-Palatinate were invited to participate in
the anonymised survey by post. In the one-time let-
ter of invitation, the doctors were given, among other
things, password-protected access to the online survey.
Of the 3595 questionnaires processed, 3556 fully com-
pleted questionnaires were included in the evaluation
(response rate: 27%). This survey determined, inter alia,
the priorities set by GPs when supporting caregivers
and to what extent they use the available resources to
make care more effective. The mean age of the GPs sur-
veyed was 55 years, with about half of the doctors hav-
ing their practice in rural regions.
Incentives were not used in any of the three studies.
Development of survey instruments
Since the studies built on each other, there was a continu-
ous learning process with regard to the design of the sub-
sequent sub-study. In addition, the survey instruments
developed were supported by other elements:
• Preparations for the multi-part study series (includ-
ing interviews with family caregivers in the context of
DemStepCare, focus group with 8 GPs)
• Further preliminary studies by the authors on
dementia care by GPs (e.g., [35])
• General literature search (papers used here focused
on caregivers and their support in the GP-based set-
ting [12, 17, 24, 28, 30, 36, 37], including those by
Geschke et al. [24], Greenwood et al. [28, 36] and Jol-
ing et al. [17].
• Carrying out pre-tests in the run-up to data collec-
tion
The aim was to keep the instruments used to interview
family caregivers and GPs mutually compatible. For this
purpose, certain question models were adapted to facili-
tate comparison of the results.
Data analysis
Data from the quantitative studies were evaluated using
SPSS 23.0. Apart from the descriptive analysis, a T-test
was applied to independent random samples in order to
identify significant differences between two groups. In
the case of the survey of family caregivers, binary logis-
tic regression was used to identify possible influencing
variables. Evaluation of the interview study as well as the
open questions in the questionnaires is based on a quali-
tative content analysis [38].
Results
Figure 1 shows the starting points condensed from the
analysis of the sub-studies with a view to more effective
GP support for family caregivers. In the following, each
of the dimensions presented will be discussed with ref-
erence to the respective central findings and correlated
with the existing research.
Support, approach, communication
As shown in the survey of family caregivers [32], car-
egivers experience GPs as highly skilled and trustworthy
central points of contact. Three out of four respondents
(72%) talk to their GP about their caring role, with 54%
doing so frequently. The way in which support is pro-
vided is judged positively in important contexts, espe-
cially the GP’s knowledge of the personal care situation,
approachability on a wide range of problems and the
attention given to the person in need of care.
At the same time, the same survey shows that the
wishes of caregivers with regard to an early approach
by the GP practice are frequently not matched by their
own experience. Fewer than one in two caregivers (42%)
report having been promptly identified as such by their
GP. One in five (18%) reports that the responsible GP
did not wait for them to voice questions or problems
regarding care but (pro-)actively approached the car-
egiver. In the qualitative interviews [33], some of the car-
egivers stated that they had initially felt uncertain about
the extent to which their needs and problems should
be a matter for GP support; there was hesitance, which
sometimes deferred problematic situations. Accordingly,
counselling sessions in the initial or preparatory phase of
care are comparatively less frequent.
Such findings from the interviews with family caregiv-
ers are in line with the survey of GPs [34], which showed
that the latter perceive it as a great challenge to system-
atically identify informal caregivers in their daily practice
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 4 of 12
Fig. 1 Derived starting points for effective GP support for caregivers (own diagram)
(59%). As other papers have pointed out, transitions to
becoming informal caregivers are often fluid, so it can
be difficult for the GP team to identify them [7, 27, 28,
36, 39]. Difficulties arise especially if the person receiv-
ing care is not registered with the same practice as the
caregiver [10]. Krug et al. [40] note that identification of
relatives and their problems is often more likely to be in
response to stress behaviours identified by the practice
team. Because of this, caregivers are often not registered
until stress or even decompensation processes are well
advanced. In this respect, it is extremely important that
GP responsibility for caregivers is explicitly signalled [37,
41, 42].
As the results of the sub-studies have shown, there is
a comparatively large variation in the regularity and thus
the interval between GP support consultations. In sur-
veys and interviews [32, 33], caregivers take issue with a
not infrequently rushed, irregular and sometimes rather
casual treatment of their caring situation, which it is
often taken up by other reasons for consultation (e.g.,
health check-ups, vaccinations).
GPs often cite time constraints as a significant chal-
lenge to providing adequate advice to caregivers in
everyday practice (68%); in addition, many GPs find it
challenging to ensure a regular exchange with caregiv-
ers (43%), e.g. because the caregiver has a different GP.
Linked with such barriers to support is the fact that GPs
are often unable to adequately meet the need expressed
by caregivers for a stabilising, psychosocial discussion
[32, 33].
Care triad and needs of caregivers
As already mentioned, the study results from both per-
spectives reflect that GPs see themselves as contacts who
are well acquainted with the situation of family caregiv-
ers and have a generally accurate picture of their personal
situation. Family caregivers are remarkably positive about
the way in which GPs create insight on the part of the
care recipient by offering explanations (85%) and involve
them in decisions (82%). In contrast, slightly more than
half of the caregivers interviewed report feeling that their
views, needs and stresses were adequately considered by
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 5 of 12
the GP; 23% feel encouraged to address their own health
situation [32].
advisory role when it comes to organising the framework
conditions for care.
The latter finding should be regarded with caution,
since informal caregivers often trivialise their own com-
plaints compared to the extent of the problems of the
person cared for and consider potential complaints
to be relative [39, 43]. Nevertheless, the results of the
other studies also confirm that caregivers and their con-
cerns generally receive far less consideration, given the
time and resource constraints for GPs. In the interviews
with caregivers [33], it was expressed that GPs are often
mainly focused on the person being cared for, without
considering the needs and stresses of the caregiver. On
the other hand, 44% of GPs report that they find it chal-
lenging to consider the needs and wishes of both the car-
egiver and care recipient in their daily practice [34].
The research literature confirms such findings. Due to
the often tardy and inconsistent identification of caregiv-
ers and the somewhat sporadic contacts with them, GPs
find it difficult to involve caregivers from the outset [32,
37, 38]. On the other hand, in the triadic constellation
there is a tendency for GPs to perceive caregivers primar-
ily in terms of their function relative to the person being
cared for, so that psychosocial effects are marginalised
[27, 36, 39, 43].
Against this background, the GP team should empathi-
cally encourage caregivers to voice their own health con-
cerns and offer support (consultations independently of
the care recipient, where appropriate), as well as refer
them to specific support services [37, 41, 42]. It is also
important to involve caregivers in decision-making pro-
cesses about adaptation of the care (organisation) [6, 15,
17]. Home visits can also help to better assess care and
stress situations. The survey of family caregivers [32] has
shown that it is not yet possible to adequately fulfil car-
egivers’ wish to be visited by the GP team in their private
premises.
Information, advice, mediation
In general, caregivers positively rate the information and
advisory activities of GPs with regard to specific clini-
cal pictures and courses of disease, as well as diagnostic
and treatment options. One weakness identified in all the
sub-studies is that GPs do not always provide referrals
to counselling or support services. In the survey of fam-
ily caregivers [32], for example, 60% report having been
referred to support and care services by their GP at least
once. The results of a regression analysis show that refer-
rals to such services are an important factor influencing
the feeling of being able to cope with the care situation.
The interviews [33] also showed that a considerable pro-
portion of caregivers would like the GP to play a greater
Among the GPs surveyed [34], more than three quar-
ters (79%) found it very challenging to point caregivers to
the appropriate support and respite services in the area.
48% of doctors interviewed believe that they have made
at least half of the family caregivers they have supported
aware of concrete offers of help in recent years, day-
care facilities or short-term care and care services being
mentioned in particular. In response to open questions,
doctors with rural practices in particular cite the lack of
interprofessional structures (e.g. bridging care services,
inpatient palliative care facilities) and bureaucratic hur-
dles as the cause for limited mediation.
In general, these results correspond with the finding in
the research literature that GPs often do not have an ade-
quate overview of external forms of support for caregiv-
ers [24, 30] and are mostly not integrated into community
health networks or (in)formal collaborative networks
[43–46]. Another frequently encountered problem is the
lack of availability of certain forms of assistance at short
notice. For example, the GP survey [34] showed that 89%
of all doctors experience the rapid availability of care or
psychosocial respite services as a challenge.
Use of resources
In the course of the overall study, we were able to identify
several practical resources which, if their use was manda-
tory, can contribute to more effective support for family
caregivers. One of these is the involvement of practice
staff. Particularly when it comes to the identification of
informal caregivers, it is important that this should not
be seen as the exclusive task of GPs but, if it is to be effec-
tive, as a task for the entire practice team [37, 42, 47].
Accordingly, it is important to make non-clinical practice
staff (e.g. non-clinical practice assistants, primary care
assistants) aware of the need to identify caregivers.
Assuming that they genuinely receive appropriate
training and are involved in the support of caregivers,
non-clinical practice staff can also offer potential syner-
gies when it comes to carrying out home visits. Even the
assumption of advisory and coordinating roles (e.g. refer-
ral to local support services) can relieve the burden on
GPs and at the same time strengthen the mediator role of
the GP practice [40]. Last but not least, practice staff can
offer a lot of added value when it comes to providing (ini-
tial) psychosocial support and, if necessary, linking this to
referrals to stabilisation services.
Practice management is particularly important when
it comes to involving practice staff. Firstly, this is about
creating the conditions that allow caregivers to be readily
observed (e.g. rotation of staff between different duties)
[37]. Secondly, obligatory and systematic arrangements
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 6 of 12
are required for documenting specific problems (e.g. ref-
erences in the patient notes about caregiving role or signs
of stress) [40, 47].
One problem is that, so far, GPs have only partially
involved their practice staff in the identification and sup-
port of caregivers. For example, 47% of the GPs surveyed
[34] reported having members of their non-clinical prac-
tice team who regularly support their own work in terms
of identifying and supporting family caregivers. Similarly,
only some GP practice staff are trained to undertake
specific tasks associated with this. Similar findings have
already been identified in several studies on the diagnosis
of dementia in the primary care setting, where the gen-
eral practice team has so far only been involved to a lim-
ited extent in observing elderly patients and looking out
for and/or documenting warning signs [35, 47].
Beyond the practice staff, another significant resource
is the application of and compliance with evidence-
based guidelines. The S3 guideline “Family caregivers”
was published in Germany for GPs as early as 2005 and
has since been updated and expanded [41]. With regard
to the above-mentioned DEGAM guideline, 40% of the
GPs surveyed report that they are aware of it. Of these,
55% reported using the guideline frequently or occasion-
ally (44% rarely). Such results are consistent with other
reports of the critical distancing of some GPs from guide-
lines published by medical societies in particular [48–50].
Status quo and starting points for optimisation
Overall, 68% of the caregivers surveyed who talk to their
GP about care say they feel (very) well supported by the
GP. 70% feel that their GP is usually good at helping them
when they approach them with a care-related question.
47% of the GPs surveyed stated that there was a (very)
good possibility of meeting the needs of family caregiv-
ers in their everyday practice (53% less good or not good
at all). The possibilities and structures that exist for GPs
within the healthcare system to provide good support
for caregivers are assessed positively by 44%, and rather
negatively by 52%.
In terms of an overall assessment, it appears that the
vast majority of doctors (77%) consider the GP setting
as the primary contact point for the needs of caregiv-
ers. However, many respondents (56%) say that, when
it comes to playing a more proactive role for this tar-
get group, they are limited by the current framework
conditions.
In response to an open question, some of the doctors
said that, in order to be able to better support caregivers
in the future, they wanted to see better integration of GPs
within local health and care structures or a closer col-
laboration in the interprofessional network, so as to give
them a better overview of existing services and the ability
to make targeted referrals. In addition, they express the
wish for the health insurance funds to systematically sup-
port family caregivers, thereby assisting the work of GPs.
Another suggestion is the creation of a low-threshold
support programme, in which caregivers can be enrolled
by GPs and which, on the basis of an individual risk
stratification, guarantees them ongoing information and
advice, as well as intervention measures when needed.
Discussion
Principal findings and comparison with prior work
The study series was able to generate a broad picture
of the current status of GP care with regard to support
for family caregivers. Due to their position in the Ger-
man health care system, GPs perform extensive primary
care tasks. GPs are the first point of contact for patients
and therefore often familiar with their patients and the
patients’ family members for many years; there is a trust-
ing doctor-patient relationship [6, 27–29].
The results obtained in the course of the sub-studies
show that the GP setting has great potential to act as a
central support for this group. Discussions with fam-
ily caregivers about care (organisation) and care cir-
cumstances are widespread in everyday practice and are
based on a high level of trust on the part of caregivers.
Especially the low-threshold accessibility for various
problems, the familiarity with the personal circumstances
as well as the attention to the person in need of care are
experienced positively.
This confirms previous studies which underline the
major importance of GP support for the target group
under consideration and see GPs as being in a position
to make key contributions to the longer-term stabilisa-
tion of home-care settings [6, 7, 14, 28–30, 51, 52]. Both
caregivers and GPs believe that the primary care setting
has great potential to address and deal with the problems
of caregivers [7, 14, 29, 30, 52]. For example, a study con-
ducted in Ireland highlights the priority role of the GP
in developing longer-term coping and resilience strate-
gies in home-care settings [53]. For their part, Green-
wood and colleagues [30] were able to work out that the
primary care setting can play a central role in support-
ing and relieving the burden on caregivers and effectively
coordinate further care.
Nevertheless, the results of the present study also
reveal weaknesses which mean that, despite being very
aware of the need to support family caregivers, GPs are
not always able to meet the needs of home-care situa-
tions as part of their everyday practice [6, 51, 54]. This is
true, for example, with regard to the role of GPs in identi-
fying and anticipating care difficulties. Caregivers would
also like the GP to play a greater advisory role when it
comes to organising the framework conditions for care
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 7 of 12
and signposting them to help and support services. Addi-
tionally, the sub-studies confirmed the findings from
previous studies that GPs do not always consider the
physical and emotional needs of family caregivers to the
same extent as those of the person requiring care [30, 36,
37, 39, 42, 52].
In particular, the comparatively low level of GP referral
activities and collaboration with support services in the
provision of care results in restrictions and delays in the
effective support and (preventive) stabilisation of caregiv-
ers. As noted in various studies, GPs in Germany - espe-
cially in rural regions - are often solitary providers and
cannot access interprofessional networks and collabora-
tions [24, 26, 30, 40, 43–46, 54]. The results of the sur-
vey of family caregivers are confirmed, for example, by a
Canadian study conducted by Parmar et al., who find that
GPs fail to consistently address the need of caregivers and
care recipients for early and regular signposting to respite
services [45, 55]. When family caregivers are referred to
such support services, they benefit from timely access to
information on organising care [8, 52], which allows the
caregiver to stay at home longer without care crises (e.g.,
hospitalisations) arising [24, 56].
Another issue is that the GP team does not always
identify family caregivers in a timely and systematic way,
making it harder to identify specific needs and antici-
pate pressures. Overall, the results demonstrate the value
of active communication by the GP team in relation to
the family caregiver group. In the qualitative studies by
Burridge et al. conducted in Australia, it is notable that
caregivers do not always feel confident to voice their
problems, if GPs do not signal to them that they see
themselves as a point of contact [39, 57]. Against this
backdrop, it makes sense to strengthen GPs’ conversa-
tion skills in dealing with caring relatives through further
training. If communication can be more open between
both parties, family caregivers will be less reluctant to
report feelings of burden, depression, and stress [51]. A
systematic assessment of the caregivers’ general well-
being, performed by the GP, is essential for the prompt
adjustment of home care [58].
A fundamental problem not only of the German, but
also of other health systems is fragmentation, meaning
that the sectors are separated. As a result, primary care is
often not integrated into multi-professional care, which
also affects the care of family carergivers [59]. In Ger-
many in particular, there is often a lack of staff who can
relieve and supplement the GP, offer support to caregiv-
ers and competently assign them to support services [30].
In this context, it is worth mentioning that only a
proportion of GPs train non-clinical practice staff and
involve them so that they can take on specific tasks such
as identifying and supporting family caregivers [24, 30,
40, 47]. Studies like those by Krug et al. [40] show that
the detection of exhaustion in caregivers is not system-
atic among staff members, but rather a reaction to warn-
ing signals that the caregivers show to the practice team.
This problem is often related to a lack of knowledge and
awareness [32, 35]. At the same time, various studies
show that there is a great need for delegation in primary
care since GPs are often overworked already in most
countries [47]. Therefore, practice staff should be more
systematically involved in the detection and support of
family caregivers [35]. Staff members who have under-
gone appropriate training can also take on referring and
mediating activities to advisory and support networks. If
the practice team is networked with other service provid-
ers, this not only relieves caregivers, but also the prac-
tices themselves; the mediator role of the GP’s practice
can be strengthened. Requests made to the practice team
could then be passed on to competent actors in the net-
work. For example, closer cooperation with long-term
care insurance funds, which GPs sometimes use in the
context of care advice [40], and the local care support
points could help relieve caregivers. Where such collabo-
rative solutions exist in everyday practice, GPs also find it
much easier to meet the needs of caregivers [51]. Practice
management is of particular importance with regard to
the involvement of the practice staff. On the one hand,
prerequisites should be created under which it is possible
to identify and observe caregivers (e.g. regularly changing
work areas). On the other hand, it depends on system-
atic arrangements with regard to the documentation of
abnormalities (e.g. entering signs of stress in the patient
file) [37, 42].
In order to stabilize home care settings, there is also
the need for structured interdisciplinary forms of care
that combine medical, nursing and further care offers in
order to offer person-centered and evidence-based sup-
port [60–62]. The lack of effective outpatient crisis inter-
vention structures often leads to hospital admissions in
crisis situations, which may result in serious complica-
tions for patients [63]. There is some discussion on the
introduction of case and support managers to assist
GPs in supporting family care situations [52, 59, 64, 65].
Case managers offer the advantage that they are cross-
sectorally networked and can act as a link between GPs
and other care providers (e.g. care services, support net-
works, emergency clinics), so that risk stratifications for
those in need of care and carergivers can be carried out
at an early stage [59].
Also important in care planning is the issue of ade-
quate referral to care-supporting systems, networks
and services. In this context, however, it has been
found that GP teams often complain about inad-
equate integration into professional care and advisory
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 8 of 12
networks [40]. A central lever for making GP support
for family caregivers more effective is undoubtedly the
closer integration of GPs into counselling and sup-
port services [66]. To this end, it will be important to
strengthen interdisciplinary communication, to estab-
lish collaborative municipal networks in the area of
health promotion [44, 58] and to provide GPs with a
reliable knowledge of advisory services in their area in
order to facilitate the straightforward referral of car-
egivers. A systematic review by Plöthner et al. points
out the importance of strengthening outpatient care
structures [51]. The researchers draw the conclusion
that establishing an outpatient care system, which sup-
ports families and friends in providing (elderly) care
while meeting the needs and wishes of informal car-
egivers, is of high relevance. An important prerequi-
site for this is to take into account family doctors with
their own contractual elements, ensuring that they are
appropriately remunerated when they take on advisory,
mediating and caring activities for a caregiver network
[21, 51, 56]. Scientifically supported model projects are
already trying to strengthen the anchoring of GP-based
care in regional advisory and support networks [30, 31,
59, 60].
The increased focus on evidence-based guidelines is
also an important tool for better addressing the needs of
caregivers. For example, manageable care plans derived
from guidelines could help GPs tailor care management
to the care needs of the caregiver and the patient [46,
49]. In doing so, the assessment of the care situation and
its impact on the general well-being of the caregiver can
approached in a structured way [66]. Clear and efficient
guidelines from early diagnosis to adequate referrals can
certainly improve the GP’s ability to support time- and
energy-consuming home-care situations. Consequently,
intervention trials focusing on the skills of GPs could be
helpful in improving home-care outcomes regarding the
family caregiver [32, 37].
Not only in Germany, but also internationally, there
is a lack of longitudinal studies that include doctors,
nurses (e.g. palliative care patients) and family caregiv-
ers in order to support the development and effec-
tiveness of family GP-related interventions [67] that
maintain or increase the quality of life of patients and
their relatives [68]. An exception is the implementa-
tion of the Gold Standards Framework in Great Britain,
in which family caregivers are explicitly included [69].
The caregivers‘perspectives and experiences were taken
into account, e.g. the need for a professional coordinator
[70] and the support of district nurses [71]. The extent to
which such approaches can be adopted in the more frag-
mented German health system is part of future research
projects.
Starting points
The following starting points for effective GP support
for family caregivers can be stated against the back-
ground of the findings as well as the results from previ-
ous studies:
• Early identification, approach and involvement of
(informal) caregivers is essential for providing good
support [72]. For example, possible care activities can
be consistently queried using anamnesis question-
naire when new patients have initial contact with the
practice. In addition, it seems worthwhile to design
postings in the reception area of the practice and give
advice from the practice team that family caregivers
should identify themselves (if necessary, design in
several languages). People in need of care should be
asked who their informal caregivers are. In the case
of new diagnoses that are known to be associated
with a need for care, the practice team should ask
about possible caregivers. Information with regard
to care constellations can also be requested dur-
ing home visits as well as informal caregivers can be
identified. Patients with a presumed role as caregivers
should be addressed about the issue [37, 42]. Moreo-
ver, it would be beneficial if caregiving relatives also
voluntarily point out their care activities and speak to
GPs about this issue. This also requires health policy
activities that emphasise how important it is for fam-
ily caregivers to approach GPs on their own initiative
and build a stable relationship [41, 42].
• In the context of early identification and crisis pre-
vention, non-clinical practice staff could be more
closely involved, and tasks could be delegated by
the GP. To this end, GPs should invest more in spe-
cial further training and in optimised practice man-
agement. So far, practice teams do not systemati-
cally record signs of stress and exhaustion in caring
relatives. An entry in the patient file stating whether
someone is a caring relative or which family mem-
ber is mainly responsible for the care could pro-
vide a remedy here and provide an initial indication
of whom to pay particular attention to in terms of
excessive demands from a care situation. The same
applies to the observation and documentation of
warning signals. Caregiving relatives could be pro-
actively identified by the practice team during initial
contact and during house calls, but also by actively
asking the person in need of care [37, 51, 55, 65].
For internal communication and observation in the
practice, a catalog of questions can be developed on
perceptions with regard to contact with family car-
egivers [41]. The systematic recording of burdens and
resources of caring relatives and the networking with
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 9 of 12
other service providers as well as the knowledge of
their offers facilitate appropriate intervention.
• Family caregivers should be made aware that their
support falls within the remit of the GP practice,
so that any health concerns are articulated without
delay. Similarly, it seems advisable not to wait for
caregivers to raise problems but rather to proac-
tively take the initiative (e.g. via opportunities such
as health checks or vaccinations). Caregivers should
be empathically encouraged to raise their own health
concerns [37].
• GP practice teams should be made more aware that,
within the triadic constellation, the needs, wishes
and pressures of caregivers are key to the success
of longer-term care [27, 32, 36, 39, 43]. If necessary,
consultations should be arranged independently of
the person being cared for and sufficient time should
be made available, e.g. during a home visit [27, 46].
• The potential of practice staff can be used through
targeted training, not only in identifying caregivers
but also in advising caregivers and making home vis-
its to better address care problems. In this context,
psychosocial skills could be also expanded through
further training. Such additional tasks taken over by
members of the practice staff should be given greater
consideration in the remuneration of the practice
team [47, 65].
• It seems advisable to raise GPs awareness of the exist-
ence and benefit of evidence-based guidelines, espe-
cially with regard to supporting family caregivers [42,
72]. The Burden Scale for Family Caregivers (BSFC-s)
should be used for the standardized recording of bur-
dens [41].
• Family caregivers should be consistently involved
in decisions with respect to the organisation of
care right from the start. Caregivers too often feel
bypassed when it comes to support of home care.
In addition, studies reveal that interventions that
are not previously discussed with the caregiver and
which occur in an acute situation fail to achieve the
expected result [66].
• Consistent and early mediation to concrete help and
support services gives family caregivers timely access
to information about the organisation of care; the
risk of caregiver ‘burnout’ is significantly minimised
[8, 10, 21, 24, 30, 45]. If family caregivers are suitably
monitored, outpatient care can be arranged so that
caregivers can stay at home longer [56, 64].
• The structural support for primary care as well as the
intersectoral connection of GPs’ practices should be
strengthened. In the role of multi-professional actors,
case managers can mediate between GPs, patients
and caregivers as well as other offers of help and,
thereby, overcome the limits of a fragmented health
system [52, 59, 61, 64]. A central lever for mak-
ing GP support for family caregivers more effective
is undoubtedly the closer integration of GPs into
counselling and support services. To this end, it will
be important to strengthen interdisciplinary com-
munication, to establish collaborative networks in
the area of health promotion [44, 58] and to provide
GPs with a reliable knowledge of advisory services in
their proximity in order to facilitate the straightfor-
ward referral of specific caregivers (e.g. Parkinson-
ism, Stroke, Dementia). A good knowledge of local
conditions and effective networking of the practice
team with other professional providers contribute
to improved care for caregivers while strengthening
their information and education as well as the pre-
vention of care crises [21, 51, 56]. To this end, help
and support services need to be systematised so
that GPs have an overview and consultations can
be structured and still be tailored to the individual
needs of those affected. For some family caregivers,
advice and written information will be sufficient; oth-
ers will need more support and guidance. It could
be worthwhile for GPs to take initiatives to improve
their formal and informal cooperation with coun-
seling and support actors in the field of community
care. In that regard, e.g. doctor or practice networks
offer great opportunities. However, this is primarily a
task for structured municipal health promotion [44].
The establishment of health and prevention networks
is associated with considerable advantages.
Strengths and limitations
This paper has helped in a comprehensive and multi-
methodical way to identify information on the status
quo of caregiver support in primary care. Because of the
broad, heterogeneous and widely dispersed samples the
results have national significance.
However, the sub-studies fail to provide a representa-
tive picture of opinion, due to the limited number of
cases and the self-selection of respondents, since the
surveys were conducted online. One has to allow for the
fact that older people are less au fait with technology,
so that older caregivers and GPs might be under-rep-
resented in the sample. Accordingly, it can be assumed
that the recruitment of caregivers in other settings (e.g.
waiting room surveys in GP offices) would lead to poten-
tially more generalisable statements about the population
under consideration. Such studies should be conducted
with a view to optimising primary care with regard to the
needs of family caregivers.
Wangler and Jansky BMC Family Practice (2021) 22:252
Page 10 of 12
It should also be borne in mind that caregivers were
deliberately considered very broadly and that the spe-
cific needs of different subgroups (e.g., those caring for
dementia patients) could therefore not be considered
separately.
Conclusions
GPs are very important to family caregivers for providing
information on planning and organising care, as well as
psychosocial support and reassurance. By responding to
the needs of caregivers, GPs are able to stabilise home-
care settings in the longer term and avert care crises.
The results show that family caregivers see GPs as a
highly skilled and trustworthy central point of contact.
In the perception of caregivers, particular weaknesses
in GP support are the absence of signposting to advisory
and assistance services and, in many cases, the failure to
involve family caregivers in good time. At the same time,
GPs do not always have sufficient regard for the physical
and psychological needs of caregivers. The doctors inter-
viewed consider the GP practice to be well suited to being
a primary point of contact for caregivers, but recognise
that various challenges exist. These relate, among other
things, to the timely organisation of appropriate respite
services, mediation to appropriate assistance or the early
identification of informal caregivers.
Ideally, family caregivers – provided that the GP team
is aware of their care activities – should be approached
by the practice team at an early stage and consistently
signposted to help and support services. To this end, it
will be important to strengthen interdisciplinary commu-
nication, to establish collaborative (municipal) networks
in the area of health promotion and to provide GPs with
a reliable knowledge of advisory services in their prox-
imity. In order to support care successfully, it is impor-
tant to consider the triadic constellation of needs, wishes
and stresses of both the caregiver and the care recipient.
More training and greater involvement of practice staff
in the support and identification of caregivers seems
advisable.
Abbreviation
GP(s): General Practitioner(s).
Acknowledgements
Not applicable.
Authors’ contributions
The authors alone are responsible for the content and the writing of the
paper. JW prepared, coordinated and implemented the project. Both JW and
MJ contributed to the project design, analysis of transcripts and drafting of the
manuscript. Both authors read and approved the final manuscript.
Funding
Open Access funding enabled and organized by Projekt DEAL.
Availability of data and materials
All major data generated or analysed during this study are included in this
published article. Additional information can be provided on request.
Declarations
Ethics approval and consent to participate
All methods were carried out in accordance with relevant guidelines and
regulations.
Since this is an overview article on the status of the topic under discus-
sion, the Ethics Commission of the State of Rhineland-Palatinate, Germany,
informed us that approval by an ethics committee was not necessary.
Written informed consent for participation was obtained from all participants
before the start of the sub-studies [32–34]. The respondents received informa-
tion about the aim and purpose of the respective study and were informed
that it was an anonymous survey/interview study in accordance with the
existing data protection standards. Furthermore, it was made clear that the
data will only be used for scientific purposes.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Received: 14 September 2021 Accepted: 3 December 2021
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| null |
10.1371_journal.pgen.1010804.pdf
|
Data Availability Statement: All the relevant data
are within the manuscript and its Supporting
Information files.
|
All the relevant data are within the manuscript and its Supporting Information files.
|
RESEARCH ARTICLE
The CERV protein of Cer1, a C. elegans LTR
retrotransposon, is required for nuclear
export of viral genomic RNA and can form
giant nuclear rods
Bing Sun1,2,3☯, Haram KimID
4☯, Craig C. Mello1,2,3, James R. PriessID
4*
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
1 RNA Therapeutics Institute, University of Massachusetts Medical School, Worcester,United States of
America, 2 Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, United
States of America, 3 Howard Hughes Medical Institute, Chevy Chase, Maryland, United States of America,
4 Fred Hutchinson Cancer Center, Seattle, Washington, United States of America
☯ These authors contributed equally to this work.
* [email protected]
Abstract
OPEN ACCESS
Citation: Sun B, Kim H, Mello CC, Priess JR (2023)
The CERV protein of Cer1, a C. elegans LTR
retrotransposon, is required for nuclear export of
viral genomic RNA and can form giant nuclear
rods. PLoS Genet 19(6): e1010804. https://doi.org/
10.1371/journal.pgen.1010804
Editor: Andrew D. Chisholm, University of
California San Diego, UNITED STATES
Received: April 10, 2023
Accepted: May 31, 2023
Published: June 29, 2023
Copyright: © 2023 Sun et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All the relevant data
are within the manuscript and its Supporting
Information files.
Funding: This work was supported by NIH
RO1GM098583 grant to J.R.P with salary support
for HK and J.R.P; NIH RO1GM58800 and a Howard
Hughes Medical Institute award to C.C.M with
salary support for B.S. and C.C.M. The funders had
no role in study design, data collection and
analysis, decision to publish, or preparation of the
manuscript.
Retroviruses and closely related LTR retrotransposons export full-length, unspliced geno-
mic RNA (gRNA) for packaging into virions and to serve as the mRNA encoding GAG and
POL polyproteins. Because gRNA often includes splice acceptor and donor sequences
used to splice viral mRNAs, retroelements must overcome host mechanisms that retain
intron-containing RNAs in the nucleus. Here we examine gRNA expression in Cer1, an LTR
retrotransposon in C. elegans which somehow avoids silencing and is highly expressed in
germ cells. Newly exported Cer1 gRNA associates rapidly with the Cer1 GAG protein,
which has structural similarity with retroviral GAG proteins. gRNA export requires CERV
(C. elegans regulator of viral expression), a novel protein encoded by a spliced Cer1 mRNA.
CERV phosphorylation at S214 is essential for gRNA export, and phosphorylated CERV
colocalizes with nuclear gRNA at presumptive sites of transcription. By electron microscopy,
tagged CERV proteins surround clusters of distinct, linear fibrils that likely represent gRNA
molecules. Single fibrils, or groups of aligned fibrils, also localize near nuclear pores. During
the C. elegans self-fertile period, when hermaphrodites fertilize oocytes with their own
sperm, CERV concentrates in two nuclear foci that are coincident with gRNA. However, as
hermaphrodites cease self-fertilization, and can only produce cross-progeny, CERV under-
goes a remarkable transition to form giant nuclear rods or cylinders that can be up to 5
microns in length. We propose a novel mechanism of rod formation, in which stage-specific
changes in the nucleolus induce CERV to localize to the nucleolar periphery in flattened
streaks of protein and gRNA; these streaks then roll up into cylinders. The rods are a wide-
spread feature of Cer1 in wild strains of C. elegans, but their function is not known and might
be limited to cross-progeny. We speculate that the adaptive strategy Cer1 uses for the iden-
tical self-progeny of a host hermaphrodite might differ for heterozygous cross-progeny sired
by males. For example, mating introduces male chromosomes which can have different, or
no, Cer1 elements.
PLOS Genetics | https://doi.org/10.1371/journal.pgen.1010804 June 29, 2023
1 / 44
PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Competing interests: The authors have declared
that no competing interests exist.
Author summary
LTR retrotransposons are closely related to retroviruses and are enormously abundant in
animals and plants. Cer1 is the most prevalent LTR retrotransposon in the nematode C.
elegans, where it is expressed at high levels in adult germ cells. Cer1 produces typical retro-
viral proteins except for a novel protein called CERV. CERV appears to allow unspliced
Cer1 genomic RNA to be exported from the nucleus, escaping host mechanisms that nor-
mally retain unspliced RNA. The nuclear export of gRNA is regulated by multiple ON/
OFF switches controlled by sex, developmental stage, and environment. CERV is a diffuse
nucleoplasmic protein in the OFF states, but in the ON states CERV colocalizes with
nuclear gRNA. By transmission electron microscopy, CERV is associated with unusual
and distinct linear fibrils that appear to represent gRNA molecules. In older germ cells,
CERV undergoes a remarkable transition to form giant cylindrical rods of unknown func-
tion that can equal the nuclear diameter in length. Rod formation occurs when C. elegans
hermaphrodites can no longer fertilize their own oocytes, and instead require mating with
males. Thus, the rods might be part of adaptive strategies Cer1 uses to distinguish poten-
tially heterozygous cross-progeny from homozygous self-progeny.
Introduction
Long terminal repeat (LTR) retrotransposons are ubiquitous in animals and plants; they com-
prise about 8% of the human genome and over 70% of plant genomes such as maize and wheat
[1,2]. LTR retrotransposons appear to contribute to human aging and to chronic diseases such
as autoimmune disorders [3–7], and are pathogens in several plants [8,9]. Conversely, LTR ret-
rotransposons can have positive roles; some elements have been co-opted for host functions
such as viral defense, and LTR retrotransposons have had major impacts on the evolution of
gene regulatory networks and genome variation [10–13].
LTR retrotransposons are closely related to retroviruses but lack Envelope (Env) proteins
involved in horizontal transmission. Retroviruses such as Rous Sarcoma Virus (RSV) and
Murine Leukemia Virus (MLV) have a simple genomic organization in the form 5’ LTR-gag-
pol-env-3’ LTR. GAG and POL are the products of a large, unspliced mRNA that can double as
the viral genomic RNA (gRNA), while ENV is the product of a separate, spliced mRNA [14].
GAG is the major structural component of the virion and forms the protein shell or capsid for
the viral genome. The GAG polyprotein has three major domains: The nucleocapsid (NC)
domain contains one or more Cys-Cys-His-Cys (CCHC) zinc fingers that bind viral gRNA,
the capsid (CA) domain multimerizes to form the immature viral particles, and matrix (MA)
typically targets the developing particles to the plasma membrane [14]. The POL (polymerase)
polyprotein is proteolytically cleaved into smaller, conserved enzymes such as Protease,
Reverse Transcriptase, RNase H, and Integrase. Retroviral ENV proteins generally are class I
membrane fusion proteins, although the ENV protein of the hookworm Atlas retrovirus is a
structurally unrelated class II membrane protein which resembles the C. elegans cell-cell fuso-
gen EFF-1/AFF-1 [15]. Endogenous retroviruses often lose env genes as they adapt to their
hosts, and some LTR retrotransposons appear to acquire env genes de novo [16,17]. Many
LTR retrotransposons, particularly in plants, contain an open reading frame (ORF) in the
expected position for an env gene, but in most cases this ORF has no clear homology and its
function is unknown [18].
PLOS Genetics | https://doi.org/10.1371/journal.pgen.1010804 June 29, 2023
2 / 44
PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
The genomes of Caenorhabditis elegans wild strains contain several families of LTR retro-
transposons and a few retroviruses, collectively called Cer elements (C. elegans retrotranspo-
son) [19–22]. The most prevalent element is Cer1, a member of the Gypsy/Ty3 family of
retroviruses/retrotransposons [19,21]. Cer1 has inserted at multiple sites over all chromosomes
in wild populations of C. elegans; for example, Cer1 LTRs occur at 182 unique sites in 208
sequenced strains [20]. The laboratory N2 strain of C. elegans contains multiple remnants of
extinct Cer1 elements plus a single, potentially intact element, Cer1[N2, LGIII:-0.04], which
inactivates the mucin-like gene plg-1; this insertion appears to be recent and is the basis for
copulatory plug dimorphism in wild strains [21,23,24]. Viral-like particles of Cer1 GAG are
highly abundant in adult hermaphrodite gonads, where they are visible by transmission elec-
tron microscopy (TEM) and by immunostaining [24]. Cer1 GAG particles initially escaped
experimental detection because most laboratory culture of C. elegans is at 20–22˚C, while
GAG is expressed preferentially at 15˚C and not detected at 25˚C [24]. The GAG particles
appear to bind and bundle gonad microtubules, and contribute to cytoskeletal defects and
temperature-dependent infertility in aging adults [24]. These observations raise the question
of how or why Cer1 expression is tolerated, given that C. elegans has robust small RNA-medi-
ated silencing pathways that repress DNA transposons and other types of Cer retroelements
[25–29]. Recent studies suggest that Cer1 has been co-opted by C. elegans to transfer learned
memories of pathogen avoidance, both horizontally and transgenerationally; although the
mechanism is unclear, the transfer appears to involve Cer1 GAG particles and information
that is somehow relayed to neurons [30]. Interestingly, the mammalian neuronal protein Arc
is involved in long-term memory and appears to be derived from a GAG protein [31–33]; Arc
forms viral-like particles that can transfer horizontally as extracellular vesicles [34–37].
Our present understanding of Cer1 protein functions is largely inferred by sequence com-
parisons with retroviruses and other LTR retrotransposons. The predicted Cer1 POL polypro-
tein has clear similarity to the enzymatic domains of retroviral POL proteins, such as reverse
transcriptase and integrase, and these domains occur in the same 5’ to 3’ order characteristic
of the Gypsy/Ty3 family of retroviruses [19,21]. The predicted Cer1 GAG polyprotein contains
an NC domain with three CCHC fingers, but lacks sequence similarity to the MA or CA
domains of retroviral GAG polyproteins [21]. Cer1 has a large open reading frame (ORF)
between pol and the 3’LTR in the expected position for a retroviral ENV protein [19]. How-
ever, the Cer1 ORF does not resemble retroviral ENV proteins or the fusogen EFF-1/AFF-1,
and has no obvious homology with proteins outside of Caenorhabditis [24]. The ORF forms
the C-terminal half of a larger protein which is encoded by a spliced mRNA [24]; based on the
findings in the present study, we designate this protein as CERV (C. elegans regulator of viral
RNA).
Here we focus on characterizing the GAG and CERV proteins, which are the most species-
specific components of Cer1. Using structural modeling we identify a domain in Cer1 GAG
which closely resembles known structures for retroviral CA domains and is predicted to have a
similar ability to multimerize. We demonstrate that a large fraction of GAG particles in germ
cells are associated with gRNA, and that these particles appear to protect Cer1 gRNA from nat-
ural, age-dependent degradation and from experimentally imposed RNAi-mediated degrada-
tion. We show that CERV is a nuclear protein which is not required for gRNA transcription
but is essential for gRNA export from the nucleus to the cytoplasm. gRNA is exported in sex,
stage, and region-specific conditions where CERV concentrates at nuclear foci of gRNA, the
presumptive sites of Cer1 transcription. Conversely, gRNA is not exported under different
conditions where CERV does not concentrate on nuclear gRNA. This ON/OFF switch in
export is coincident with, and dependent on, phosphorylation of CERV at residue S214. Trans-
mission electron microscopy of germ nuclei shows that CERV is concentrated around clusters
PLOS Genetics | https://doi.org/10.1371/journal.pgen.1010804 June 29, 2023
3 / 44
PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
of small, linear fibrils which likely correspond to gRNA molecules; other CERV-associated
fibrils are aligned in linear arrays suggestive of export intermediates. Most remarkably, we
show that CERV can form giant, cylindrical nuclear rods, at least some of which contain
gRNA. The rods occur primarily after hermaphrodites have finished producing self-progeny,
but retain the ability to produce cross-progeny when mated with males. The CERV rods occur
in various wild strains of C. elegans with different Cer1 insertion sites, suggesting they are a
general but mysterious addition to our understanding of Cer1 biology.
Results
Background
Cer1 viral particles, called GAG particles, are most abundant in adult hermaphrodites cultured
at 15˚C [24], the temperature used for all experiments in this study unless indicated otherwise;
a timeline of development at 15˚C is shown for reference (Fig 1A). Hermaphrodites produce
and store "self-sperm" during the fourth and final larval stage (L4), then switch to producing
oocytes as adults (A). The self-sperm are largely depleted by adult day 4 (A4); thereafter,
oocytes can only be fertilized by sperm from males. Each of the two arms of the hermaphrodite
gonad resembles an elongated, U-shaped cylinder lined with about 1000 germ cells (Fig 1A;
for general reviews of gonad biology see [38,39]). Germ cells develop in a linear sequence
beginning with a mitotic or proliferation zone of germline stem cells at the distal tip of the
gonad; germ cells progress through the successive stages of meiosis after leaving the zone.
GAG particles first appear in early pachytene, and accumulate in enormous numbers through-
out the pachytene region [24]. The gonad is syncytial; each germ cell is connected to a large,
shared region of cytoplasm called the gonad core (Fig 1A and 1B). Cytoplasmic materials flow
longitudinally through the gonad core toward and into expanding oogonia, which cellularize
to become oocytes (Fig 1A) [40]. Pachytene germ nuclei have large nucleoli that occupy most
of the nuclear volume; the duplicated and paired homologous chromosomes (tetrads) are dis-
tributed in a thin shell of nucleoplasm between the nucleolus and the nuclear envelope (Fig 1B
and 1C). In general models for nuclear architecture, the nucleus is thought to resemble a
sponge-like body of inactive/silenced chromatin perforated by channels of transcriptionally-
active chromatin [41,42]. C. elegans pachytene nuclei appear to conform with this model, with
the compacted, paired chromosomes separated by channels which contain nascent mRNA
(Fig 1B and 1C) [43]. By transmission electron microscopy (TEM), the channels often contain
small and irregular, electron-dense bodies that likely represent ribonucleoprotein (RNP) gran-
ules (Fig 1D, brackets). Most of the mRNA in germ nuclei is exported through clusters of
nuclear pores and perinuclear assemblages of proteins, called nuage or P granules, which over-
lie the nuclear channels (Fig 1B and 1C; reviewed in [44]).
Cer1 GAG contains a Capsid-like domain
The predicted Cer1 GAG protein is 690 amino acids, which is larger than typical retroviral
GAG proteins; for example, GAG proteins from Ty3, HIV-1, and RSV are about 280, 500, and
577 amino acids, respectively [45–47]. Cer1 GAG has a predicted zinc finger nucleocapsid
(NC) domain, but no obvious sequence similarity to the matrix (MA) or capsid (CA) domains
of retroviral GAG proteins [21]. Cer1 elements are abundant in wild strains of Caenorhabditis
elegans [20], but are too closely related to suggest functionally important GAG domains. For
example, the entire 2272 amino acid polyprotein encoded by Cer1 [QX1794, LGI:3.91] and
Cer1 [ECA36, LGI:13.94] differ from the laboratory strain Cer1[N2, LGIII:-0.04] by only 2 and
7 amino acids, respectively. Thus, we used blast searches to identify and align Cer1 elements
from five diverse, male-female species of Caenorhabditis (S1, S2, and S3 Figs). A summary plot
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 1. C. elegans gonad and pachytene germ cells. A. Diagrams representing longitudinal (top) and cross-sectional (bottom left) views of one of the two arms
of the adult hermaphrodite gonad. Germ cells are covered by peripheral somatic cells called sheath cells (green). The distal end of the gonad contains
proliferating germ cells, and cells exiting the proliferation zone enter meiotic prophase. Hermaphrodite germ cells initially differentiate as sperm which are
stored, but all later germ cells differentiate as oocytes. The hermaphrodite sperm are used for self-fertilization until those sperm are depleted, defining the self-
fertile period, but hermaphrodites continue to produce oocytes that can be fertilized by male sperm. For most experiments here, worms were grown at 15˚C
and adults (A1-A5) were synchronized from fourth stage (L4) larvae (arrow in timeline). B. Diagram of pachytene germ cells in cross-sectional (left) and
surface (right) views. Each germ cell is connected with the gonad core through an opening called a ring channel. Nuclei have large nucleoli (no) that occupy
most of the nuclear volume, and the paired homologous chromosomes (Chr, blue) occupy the space between the nucleolus and the envelope. Perinuclear P
granules (Pg, white) are associated with clustered nuclear pores (red) and overlie channels between each set of chromosomes. Cer1 GAG particles concentrate
on stable microtubules which surround the germ nuclei and extend for long distances into the gonad core. C. The image shows a single germ nucleus in surface
(left) and cross-sectional (right) optical planes; DNA (blue), P granules (PGL-1; green), and nucleoli (FIB-1; red). Note that the perinuclear P granules are
aligned above the channels, corresponding to the positions of clustered nuclear pores. In this and selected images below, the DAPI channel is shown in cyan
rather than RGB blue for resolution. D.TEM images of three pachytene nuclei; the surrounding cytoplasm is false-colored yellow. Brackets indicate
presumptive RNPs in the channels between the compacted chromosomes (Chr). Examples of P granules (Pg) are outlined by dashed lines. Scale bars in
microns = (C) 1.0; (D) 0.5.
https://doi.org/10.1371/journal.pgen.1010804.g001
of this alignment (Fig 2A) shows that Cer1 GAG is overall divergent relative to the POL pro-
tein, but shows conservation of the NC domain and of a second, previously uncharacterized
domain in the position expected for a retroviral CA protein (Fig 2A, green block). We used
AlphaFold and ColabFold [48,49] to generate structural predictions for the second domain
and used the DALI server [50] to search for related structures in the Protein Database (PDB)
[51]. This analysis showed that the predicted structure for the second domain, now designated
CA-like, is highly similar to CA structures from diverse retroviruses and endogenous LTR ret-
rotransposons (S4 Fig). Retroviral capsids are assembled from hexamers and pentamers of CA
proteins; for example, RSV and HIV capsids contain about 250–300 hexamers of CA and small
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 2. Cer1 GAG and gRNA. A. The protein products of Cer1 are diagrammed at top; the GAG polyprotein contains a nucleocapsid
domain (NC) and a capsid-like (CA-like) domain defined in this study. The POL protein contains protease (PR), reverse transcriptase
(RT), ribonuclease H (RNH) and integrase (INT) domains; boundaries of each domain are described in S1 Fig, legend. The N-terminal
half of CERV shares three peptide sequences with GAG, and one unique peptide; splicing joins the four exons encoding these peptides to
a fifth exon encoding the C-terminal half of CERV. The plot represents conservation of aligned Cer1 sequences from C. elegans, C. nigon,
C. remanei, C. zanzibari, C. latens, and C. inopinata; the alignment for the GAG and CERV regions is shown in S1, S2, and S3 Figs. The
plot was generated using EMBOSS Plotcon [130], which computes local similarity from a sliding window, here 4 amino acids; higher
values on the y axis indicate higher conservation. The position of the phosphorylated residue S214 described in the text is indicated
(asterisk). The red chevrons at bottom indicate the positions of gRNA-specific oligo probes used for smFISH; oligo sequences are
provided in S2 Table. B. The ribbons/slab model at left shows the AlphaFold prediction for a hexamer of the CA-like domain of Cer1
GAG (amino acids 372–541) color coded as per the AlphaFold pLDDT table, a per-residue estimate of confidence on a scale of 0–100
[131]. The middle and right images show the space-filling model of the same hexamer with arbitrary coloring for each subunit. The
model has a dome shape; the middle image shows a view inside the dome, and the image at right shows a side view with arrows indicating
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
the relative positions of the N-terminal and NC regions of GAG. C. The left panel shows a near surface plane of a pachytene germ
nucleus after smFISH for Cer1 gRNA (red). Four foci of Cer1 gRNA are visible, two on each side of the presumptive LGIII homologs
(dotted line). Panel 2 shows a cross-sectional plane through a second germ nucleus; note that the four gRNA foci are coincident with
CERV (see below). Arrowheads indicate the relatively low signals from cytoplasmic gRNA. D. Image of a single pachytene germ nucleus
in an animal raised at 25˚C but shifted to the permissive temperature of 15˚C for 45 minutes before fixation; the image shows gRNA,
GAG, and CERV as indicated. The gRNA exposure was increased relative to Fig 2C to visualize the cytoplasmic foci of gRNA
(arrowheads). The increased exposure typically obscures the four foci of nuclear gRNA, such that there appears to be only two, larger foci
(asterisks). Most of the perinuclear gRNA and GAG foci overlie nuclear channels, where perinuclear P granules are localized. Note that
CERV is localized to the nuclear foci of gRNA, but not the perinuclear, cytoplasmic foci of gRNA. E. Low magnification of the pachytene
region of an A1 gonad, showing GAG and gRNA dispersed throughout the gonad core; the image is a 3-micron Z-projection. The insets
at bottom show that many of the brighter foci of gRNA (arrowheads) colocalize with GAG particles while the dimmer foci (arrows) do
not. F. Panels 1 and 2 shows linear arrays of cytoplasmic gRNA foci (arrowheads) near pachytene germ nuclei; most of the cytoplasmic
but not nuclear (asterisks) foci of gRNA colocalize with GAG particles, which have been shown to form linear arrays through association
with microtubules (Fig 1B and [24]). Small arrays are evident beginning at the A2 stage (panels 1 and 2), but additional large, linear
aggregates occur in A3 and older animals (panel 3). Panel 4 shows gRNA in the post-pachytene region of the gonad where oogonia
increase in size and cellularize as oocytes. gRNA and coincident GAG particles in this region become heavily concentrated around nuclei
(see also S1 Video). The oogonia and oocytes also contain dispersed, cytoplasmic gRNA without coincident GAG particles; cytoplasm
signals in the boxed regions are shown with increased exposures in the insets. G. Arrested oocytes in the proximal arm of an A6 fog-2
(q72) gonad, shown as a 4-micron Z-projection; the inset shows a higher magnification of the boxed region in three oocytes. Cytoplasmic
gRNA and coincident GAG particles are concentrated at the cortical region of each oocyte; note that there is relatively little cytoplasmic
gRNA that is not associated with GAG. H. Wild-type gonad exposed to gRNA(RNAi) for 48 hrs beginning on A1. Panel 1 shows a surface
view of pachytene germ nuclei, and panel 2 shows a longitudinal view of the gonad at lower magnification. The cytoplasmic foci of gRNA
(arrowheads) are generally coincident with GAG particles (see also S5 Fig). Scale bars in microns = (C, D) 1.0; (E-H) 5.0.
https://doi.org/10.1371/journal.pgen.1010804.g002
numbers of pentamers [52,53]. We used AlphaFold to test whether the CA-like domain could
multimerize, and found that it was predicted to form hexamers with high confidence scores
(Fig 2B). The N-terminal region of GAG preceding the CA-like domain is predicted to form a
long alpha-helical coiled-coil (S4 Fig) and does not resemble known structures for retroviral
MA proteins. We conclude that Cer1 GAG resembles other retroviral GAG polyproteins in
having both CA and NC domains. However, Cer1 GAG lacks the MA domain which can target
retroviral particles to the plasma membrane for horizontal transmission [14].
Most Cer1 GAG particles are associated with microtubules, and show complex region- and
stage- specific patterns of localization that are proposed to result from a load/release/transfer
mechanism [24]: First, newly formed GAG particles in the pachytene region are proposed to
load onto a stabilized subset of gonad microtubules; this binding anchors the particles against
cytoplasmic flow (Fig 1A) and allows particle accumulation in the core. Second, the accumu-
lated GAG particles are released abruptly as germ cells exit pachytene and microtubules
become destabilized. Finally, the released GAG particles transfer to dynamic microtubules and
move toward and around oogonia nuclei. We used Green Fluorescent Protein (GFP) to exam-
ine the dynamics of particle localization in live animals (S1 Video). GFP particles were present
in the expected spatial and temporal patterns in adult hermaphrodite gonads, but were not
detected in somatic cells (neurons, muscles, pharyngeal cells, or intestinal cells; n>100 adult
hermaphrodites). As predicted by the load/release/transfer model, large aggregates of GAG
particles persisted with little movement in the pachytene region, but individual particles
moved toward and around nuclei once germ cells exited pachytene (S1 Video).
Newly exported Cer1 gRNA associates rapidly with GAG
Retroviral genomic RNA (gRNA) can serve as an mRNA template which is translated into
GAG and POL proteins, and/or function as the viral genome which is packaged into GAG par-
ticles [14]. Previous in situ hybridization experiments on pachytene germ cells showed that
Cer1 gRNA is concentrated in two nuclear foci which appear to be at or near sites of Cer1 tran-
scription: The nuclear foci are near the middle of the chromosome, consistent with the Cer1
insertion site in the laboratory N2 strain; there are no nuclear foci in the strain CB4856 which
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
lacks Cer1; and there is only one nuclear focus in heterozygous N2/CB4856 animals [24]. That
study showed Cer1 gRNA is highly abundant in gonad cytoplasm, but did not determine
whether any of the cytoplasmic gRNA was associated with GAG particles [24]. Because retrovi-
ral capsids package only two molecules of single stranded gRNA [54], we re-examined gRNA
localization using smFISH (single molecule fluorescence in situ hybridization) in combination
with immunostaining for GAG. Mitotic nuclei typically showed 1–2 foci of gRNA, but staining
in pachytene nuclei could usually be resolved into four, closely spaced foci which presumably
represent each of the four LGIII chromatids (Fig 2C, panels 1 and 2). Numerous, relatively
faint foci of gRNA were detected in the cytoplasm, but visualizing the cytoplasmic foci
required longer exposures that typically saturated signals from the nuclear foci (Fig 2D, aster-
isks). There were two classes of cytoplasmic gRNA foci that differed in staining intensity: the
brighter foci usually colocalized with GAG (Fig 2D and 2E, arrowheads), but the fainter foci
often did not (Fig 2E, arrows). In comparable regions of two A2 gonads, for example, 53%
(n = 812) and 73% (n = 1143) of the brighter cytoplasmic foci colocalized with GAG, as did
77% (n = 2103) of the brighter foci in an A4 gonad. To address where newly exported gRNA
first associates with GAG, we took advantage of the fact that gRNA export is dependent on
temperature: Animals cultured at 25˚C have nuclear gRNA, but lack cytoplasmic gRNA and
GAG expression [24]. We found that cytoplasmic gRNA and GAG could both be detected
within 45 minutes after 25˚C adults were downshifted to the permissive temperature of 15˚C
(Fig 2D). gRNA and coincident GAG were often localized to perinuclear foci (Fig 2D, arrow-
heads), most of which were in the expected positions for P granules (Fig 1B and 1C). This
result suggests that GAG can associate with newly exported gRNA trafficking within or emerg-
ing from P granules.
There was a relatively uniform distribution of cytoplasmic gRNA in the gonad core of A1
animals (Fig 2E). By contrast, older adults showed a progressive accumulation of gRNA in
lines or giant linear aggregates in the pachytene region, and in perinuclear arrays in the post-
pachytene region (Fig 2F). These aggregates colocalized with GAG (Fig 2F), consistent with
the expected concentration of Cer1 capsids on microtubules [24]. In addition to perinuclear
localization of gRNA and GAG particles, oogonia and oocytes contained substantial amounts
of cytoplasmic gRNA which did not appear to localize with GAG (Fig 2F, insets in panel 4).
Retroviral capsids protect gRNA from antiviral factors in the host cytoplasm, in addition to
their roles in viral assembly and transmission [55]. Thus, we wondered whether cytoplasmic
gRNA was protected by an association with GAG. For this analysis we examined gRNA in the
oocytes of fog-2(q72) mutant gonads: Oocytes are transcriptionally quiescent and cannot syn-
thesize additional gRNA, and because oocytes are cellularized they cannot take up additional
gRNA from the gonad core (Fig 1A) [56]. Wild-type oocytes persist for only a few hours before
they are fertilized and laid, but fog-2 mutants can hold large numbers of unfertilized, arrested
oocytes for several days [57]. We found that fog-2 oocytes contained numerous foci of cyto-
plasmic gRNA through at least A6; nearly all of the gRNA colocalized with GAG particles near
the plasma membranes (Fig 2G) where microtubules become concentrated [58]. Because
many of the fog-2 oocytes in A6 adults are expected to have formed on A1, this suggests that
GAG-associated gRNA can persist for at least six days. We next asked whether GAG-associ-
ated gRNA was resistant to RNAi-mediated degradation. A1 hermaphrodites, which have
abundant cytoplasmic gRNA and coincident GAG particles (Fig 2E), were transferred to
gRNA-specific RNAi feeding plates for an additional 15, 24, or 48 hours before processing by
smFISH and immunostaining. Variable but often large amounts of cytoplasmic gRNA per-
sisted at each timepoint, most of which was coincident with GAG particles (Fig 2H; see S5 Fig
for additional data). By contrast, A1 hermaphrodites exposed to RNAi targeting the spliced
cerv mRNA lost most of the cytoplasmic signal within 6 hours (S5 Fig). Together, these results
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
suggest that Cer1 GAG has structural and functional similarity to retroviral GAG proteins,
and appears to package and protect a large fraction of cytoplasmic gRNA.
CERV is a nuclear protein required for gRNA export and GAG expression
The Cer1 CERV protein is 517 amino acids and is the product of a spliced, 5 exon mRNA (Fig
2A) [24]. cerv exons 1–3 have the same reading frame as gag, but cerv exon 4 has a unique read-
ing frame (Figs 2A and S1). CERV has no obvious homology to proteins outside of nematodes
and, like GAG, is highly divergent between Caenorhabditis species (Figs 2A, S1, S2 and S3).
CERV contains a potential leucine zipper (Leu-X6-Leu-X6-Leu-X6-Leu) that it shares with
GAG, a candidate nuclear localization sequence (KRKK), and a candidate nuclear export
sequence MLILADGLRL [59]. CERV does not have an RNA-binding motif recognized by the
RPB2GO database [60], but contains a cysteine-rich region that might form a C4-type zinc fin-
ger [61]. The AlphaFold model for CERV (P34431; https://alphafold.ebi.ac.uk) predicts three
structured domains which are separated by flexible linkers; we term the structured domains H,
M, and G (Figs 3A and S4). The H domain is a helix-hairpin-helix which contains the potential
leucine zipper, and M and G are both globular domains.
We used AlphaFold and ColabFold [48,49] to generate de novo structural predictions for
CERV proteins from each of the five different species of Caenorhabditis aligned in S1 Fig and
represented by the plot in Fig 2A. Despite considerable sequence variation, the predicted struc-
tures were remarkably similar for each of the respective H, M, and G domains (S4 and S6
Figs). We used the DALI server [50] to compare separately each predicted CERV domain with
structures in the Protein Data Bank (PDB), and found that the H and M domains did not have
a close resemblance to known proteins. By contrast, the predicted structure for the G domain
was highly similar to diverse G-proteins and structural mimics of GTPases, but lacked critical
residues required for GTP hydrolysis (S4 Fig). We next used AlphaFold and ColabFold to ask
whether any of the three CERV domains were predicted to multimerize. M domains from
each of the Caenorhabditis CERV proteins were predicted to multimerize with high confidence
scores: M domains could form closed rings with a minimum of 5 subunits, but larger rings
were possible (Figs 3A and S6). Notably, the multimer predictions oriented the potential cyste-
ine finger from each M subunit toward the central axis of the ring (Figs 3A and S6). Multimer
models for the nearly complete CERV protein showed the G domains extending from flexible
spokes at the periphery of the ring; the H domains from adjacent CERV proteins were linked
together at the predicted leucine zippers, and extended at several possible angles from one face
of the ring (Fig 3A).
To localize CERV, we began by using Green Fluorescent Protein (GFP) to tag the N-termi-
nus of CERV, which is shared with GAG, and separately tagged the C-terminus of CERV.
Unfortunately, subsequent experiments showed that the tagged proteins were only partially
functional and often mislocalized in homozygous animals, although the tagged proteins
appeared to localize normally in heterozygotes with wild type (S7 Fig and see below). A previ-
ous study raised monoclonal antibodies against a partial GAG fusion protein that included
one of the peptides sequences shared with CERV (encoded by cerv exon 3; Fig 2A) [24]. That
study characterized a GAG-specific antibody, mAbP3E9, which recognizes a single band at the
expected size for GAG on Western blots of wild-type worm extracts, and recognizes a single,
larger band in extracts from a strain expressing GAG::GFP (Fig 3B). We screened the addi-
tional monoclonal antibodies on extracts of worm proteins, and found that mAbP3C6 stained
GAG plus a second band at 62kD, the approximate size of CERV (Fig 3B). We used mAbP3C6
to immunostain pachytene-stage germ cells, and found that staining was concentrated in two
nuclear foci which coincided with gRNA (Figs 2C, 2D and 3C). Longer exposures showed
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 3. CERV structure and function in gRNA export. A. The left image is the AlphaFold-predicted structural model for the complete
CERV protein (aa1-517), with the pLDDT color-coded confidence scores as in Fig 2A. The H, M, and G domains and the cysteine-rich
loop described in the present study are indicated. Both images at right show different views of a space-filling model of the CERV
hexamer; only the indicated amino acids are depicted in the model, and the subunit coloring is arbitrary. The central ring (top right,
base view) is built from multimers of the M domain with the conserved Cys loops near the central axis (see also S6 Fig). The G domains
extend radially from the M domains by flexible spokes (see also S4 Fig). H domains from adjacent subunits are predicted to bind
together in coiled coils that project at variable angles from the ring (bottom right). B. Western blot of protein extracts from the indicated
strains: cerv(stop) = WM746, gfp:cerv/gag = WM638, and gag:gfp = WM743 (see S1 Table for details). The blot was probed sequentially
with α-GAG, mAbP3C6, and α-ACTIN. Note that the single band recognized by mAbP3C6 in the gfp:cerv/gag extract is GFP:CERV; this
strain does not express GAG:GFP (see analysis in S7 Fig). C. Panel 1 (top row) shows A2 pachytene germ nuclei stained for CERV
(mAbP3C6); the inset at right shows CERV, gRNA, and DNA (blue and cyan) at higher magnification. Panel 2 shows A4 pachytene
germ nuclei stained for CERV. The intense, nuclear foci of CERV have disappeared; CERV is dispersed in the nucleoplasm and present
in irregular aggregates at the center of nucleoli (arrows). Note that the level of gRNA in the nuclear foci (double arrows) has decreased
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
and is comparable to the signal from cytoplasmic foci of gRNA (see also S8 Fig). D. Germ nuclei from A2 adults with a CERV-specific
stop mutation. Panel 1 is an 8 micron Z-projection, showing there is no gRNA detectable in the cytoplasm (compare with the
cytoplasmic gRNA visible in Fig 3G, which is a 3 micron Z-projection of similar germ nuclei). The lack of CERV expression is shown in
the right half of panel 1. Panel 2 is a 3 micron projection showing the absence of GAG particles in the gonad core. E. Pachytene germ
nuclei in a wild-type hermaphrodite cultured at the non-permissive temperature of 25˚C. The gonad has prominent nuclear, but not
cytoplasmic, foci of gRNA, and GAG is not expressed. Note that CERV is present in the nucleoplasm but not concentrated at the nuclear
gRNA foci (arrows). F. A2 adult male gonad at 15˚C. The gonad has nuclear, but not cytoplasmic, foci of gRNA, and GAG is not
expressed. Note that CERV is present in the nucleoplasm but not concentrated on the nuclear gRNA (arrows). G. A2 adult
hermaphrodite gonad at 15˚C, showing the boundary (arrowhead) between the proliferation zone (inset 1) where gRNA is not exported,
and the pachytene region (inset 2) where gRNA is exported and GAG is expressed. Note that the appearance of gRNA and GAG in the
cytoplasm corresponds to where CERV first concentrates on the nuclear foci of gRNA. These images are 3-micron Z-projections, so
signals from a few cytoplasmic foci of gRNA are artificially superimposed on the nuclei. Scale bars in microns = (A-G) 1.0.
https://doi.org/10.1371/journal.pgen.1010804.g003
staining throughout the nucleoplasm but, fortuitously, there was little or no detectable staining
of GAG particles in the cytoplasm (Fig 2D, arrowheads). This suggests that mAbP3C6 recog-
nizes SDS-denatured GAG and CERV, but cannot recognize GAG in formaldehyde-fixed,
non-denatured gonadal tissues. Because cerv exon 4 uses a different reading frame than gag
(Figs 2A and S1) we used CRISPR gene editing to create a cerv-specific stop codon in exon 4,
cer1(ne4881stop), which would not affect the amino acid sequence of the GAG protein. We
found that mAbP3C6 did not stain protein extracts or gonads from the cerv(stop) mutant (Fig
3B and 3D, respectively). Thus, mAbP3C6 can be used as a CERV-specific stain on fixed tis-
sues. The cerv(stop) gonads had prominent foci of nuclear gRNA, indicating the CERV is not
required for gRNA transcription, but gRNA was not detected in the cytoplasm (compare
gonad core region in Figs 3D with 2E). Cytoplasmic gRNA in retroviruses is expected to serve
as the mRNA template for GAG synthesis; as expected, the cerv(stop) mutant lacked GAG
expression by Western blot analysis and by immunostaining (Fig 3B and 3D, respectively).
Because the smFISH protocol can recognize single RNA molecules, and does in fact recog-
nize what are likely two gRNA molecules in GAG particles, the complete absence of cyto-
plasmic gRNA in the cerv(stop) mutant suggests that the gRNA is not exported, or exported
and degraded instantly. By comparison, germline mRNAs targeted for RNAi-mediated degra-
dation can be detected in perinuclear P granules and in cytoplasm by conventional FISH, and
mRNAs targeted for degradation by the nonsense-mediated decay (NMD) pathway can be
detected in oocyte cytoplasm [43]. Moreover, multiple unspliced germline mRNAs exit the
nucleus and accumulate in the cytoplasm in mutants defective in splicing [62]. These several
observations and the finding that CERV is a nuclear protein argue that CERV is required for
gRNA export, rather than preventing degradation of cytoplasmic gRNA.
CERV phosphorylation and localization to nuclear gRNA foci is required
for gRNA export
Our experiments and previous studies [24] found multiple conditions where wild-type germ
cells have prominent nuclear foci of gRNA, but do not appear to export gRNA to the cyto-
plasm and do not express GAG: these non-permissive conditions include hermaphrodites cul-
tured at 25˚C (Fig 3E), males cultured at any temperature (Fig 3F), and germ cells in the
proliferation zone (Fig 3G). We examined these non-permissive conditions and found that in
each case CERV was present in the nucleoplasm, but was not concentrated at the nuclear foci
of gRNA (Fig 3E–3G). Hermaphrodite gonads have a sharp boundary between the prolifera-
tion and meiotic regions where cytoplasmic gRNA and GAG first appear, and this boundary
(arrowhead in Fig 3G) coincided with where nucleoplasmic CERV first concentrates on the
foci of nuclear gRNA. Thus, the ON/OFF switches in gRNA export and GAG expression are
correlated with changes in the association of CERV with nuclear gRNA.
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
We wondered whether post-translational modifications such as phosphorylation caused
nucleoplasmic CERV to concentrate on nuclear gRNA. CERV contains 48 Ser, Thr, or Tyr res-
idues at possible phosphorylation sites (MusiteDeep and NetPhos-3.1). However, only two of
these residues were conserved in all five Caenorhabditis Cer1 elements (S3 Fig), including
Ser214 in the predicted multimerization domain and near the Cys loop (Fig 4A). We used
CRISPR gene editing to create a S214A substitution in CERV and found that Western blots of
the mutant extracts had the expected CERV band, but lacked GAG expression (Fig 4A). By
immunostaining, the S214A mutant germ cells had abundant nucleoplasmic CERV; indeed,
the levels of both nucleoplasmic and cytoplasmic CERV appeared higher than in wildtype (Fig
4B and see Discussion). However, CERV was not concentrated on the nuclear foci of gRNA,
and neither gRNA nor GAG were detected in the cytoplasm (Fig 4B). By contrast, control het-
erozygotes showed the normal concentration of CERV on the nuclear foci of gRNA, and
gRNA was abundant in the cytoplasm (Fig 4B, panel 2). We used CRISPR gene editing to cre-
ate additional alanine substitutions for other Ser and Thr residues flanking S214 (Fig 4A), but
each of the double and triple mutants appeared essentially identical to the single S214A
mutant. We conclude that S214 is essential for CERV to concentrate on nuclear gRNA, and is
required for gRNA export and GAG expression.
We next wanted to determine if CERV was phosphorylated at S214. S214 is followed by an
invariant proline residue, P215 (Fig 4A), suggesting that S214 might be phosphorylated by a
proline-directed serine/threonine kinase [63]. Multiple proline-directed serine/threonine
kinases function in the C. elegans gonad, including CDK-1/cyclin-dependent kinase, GSK-3/
glycogen synthase, and mitogen-activated kinases such as MPK-1 [64,65]. These kinases are
expected to have numerous substrates in the gonad, some of which are known for MPK-1 [66].
We generated a worm strain where the N-terminus of CERV was tagged with the FLAG pep-
tide [67], then probed immunoprecipitated extracts from this strain with a commercial anti-
body that recognizes phosphorylated Ser/Thr-Pro peptides (α-pS/T-P; abcam ab9344). α-pS/
T-P stained a prominent band at the expected molecular weight for CERV (Fig 4C; arrow-
head). Control experiments (Fig 4C) showed that (1) the band was not detected in immuno-
precipitated extracts from wild-type worms which lacked the FLAG tag, (2) the band was not
detected in extracts from worms with a FLAG tag on the C-terminus of GAG, and (3) α-pS/
T-P did not detect the band when the extracts were pre-treated with phosphatase.
We found that α-pS/T-P showed intense staining of nuclear foci that coincided with the
nuclear foci of CERV and gRNA in pachytene germ cells (Fig 4D, panel 1), but showed rela-
tively little staining in the gonad cytoplasm. Because α-pS/T-P is expected to stain several
germline-expressed phosphoproteins (see above) and stained multiple bands with comparable
intensity in worm extracts (Fig 4C), the prominence of the nuclear foci might result from
CERV concentration rather than abundance. α-pS/T-P showed only diffuse, nucleoplasmic
staining in S214 mutant hermaphrodites (Fig 4D, panel 2), indicating that the staining of
nuclear foci in wildtype is specific for CERV. α-pS/T-P showed only diffuse, nucleoplasmic
staining in wild-type males and 25˚C hermaphrodites, where CERV does not concentrate on
the nuclear foci of gRNA and gRNA is not exported (Fig 4E). By contrast, α-pS/T-P stained
the nuclear foci of CERV and gRNA within 1 hour after hermaphrodites raised at 25˚C were
downshifted to the export-permissive temperature of 15˚C (Fig 4E). Similarly, CERV phos-
phorylation at S214 correlated with the gonad boundary where CERV first concentrates on
nuclear gRNA (Fig 4E, panel 2; compare Fig 3G). After germ cells exit pachytene they decrease
and eventually cease transcription [56]. The level of phosphorylated CERV diminished
abruptly as germ nuclei exited pachytene, coincident with the disappearance of the CERV foci
(Fig 4E, panel 3). In the same region, non-phosphorylated CERV accumulated in irregular
bodies in the interior of the nucleolus (arrows) which stained positively for ubiquitin (Fig 4E,
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 4. CERV phosphorylation and the cysteine-rich loop are required for gRNA export. A.The peptide sequence at top left shows the
beginning of the CERV M; residues in bold are invariant in diverse species of Caenorhabditis (see also S3 Fig). Most of this region is
contained in a flexible cysteine-rich loop (AlphaFold model at right) which faces the central axis of the predicted ring multimers (Fig 3A;
see S6 Fig for additional analysis of the M domain). The Cys loop brings multiple Cys residues into close proximity: predicted distances
between the cysteine sulfur atoms are C195-C193 (3.3 Å), C193-C200 (3.8 Å), C200-C269 (3.9 Å), and C269-C195 (3.3 Å) (ChimeraX
[132]). The blot (inset) shows protein extracts from the following strains: WT, WM746 [cerv(stop)], and JJ2706 [(cerv(S214A)]; see S1
Table for strain details. The blot was probed with mAbP3C6, which stains a prominent CERV band and a weaker GAG band in the wild-
type extract. The cerv-specific STOP mutation eliminates both the CERV and GAG band, while the S214A substitution eliminates only
the GAG band. B. Immunostained gonads from a homozygous mutant with a CERV S214A substitution (panel 1), and from a
heterozygous strain with the same mutation plus a wild-type copy (panel 2). C. Immunoprecipitation assays of FLAG-tagged CERV and
FLAG-tagged GAG followed by Western Blot analysis. Extracts are from WT worms, a strain with a FLAG tag on the N-terminus of
CERV (WM894), and a strain with a FLAG tag on the C-terminus of GAG (WM895); see S1 Table for strain details. The extracts were
immunoprecipitated with an anti-FLAG antibody and blotted; a duplicate blot is shown at right after treating the same extracts with
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
phosphatase. Both blots were stained with the α-pS/T-P antiserum; the arrowhead points to a band at the predicted size of CERV that is
absent after phosphatase treatment. D. Germ nuclei in an A2 wild-type gonad (panel 1) and a CERV S214A mutant (panel 2) stained for
CERV and phospho-S/T-P. Note that the S214A mutant nuclei fail to stain with α-pS/T-P and CERV is present in the nucleoplasm but
not concentrated into foci. E. Panel 1 shows different types of germ nuclei as listed after immunostaining for CERV and α-pS/T-P. Note
that CERV only concentrates into foci after hermaphrodites are shifted to the export-permissive temperature of 15˚C, where CERV
becomes phosphorylated. Panel 2 shows the gonad boundary (arrowhead) where CERV first concentrates at the nuclear foci of gRNA,
and panel 3 shows the subsequent, post-pachytene boundary where the CERV foci disappear. Some CERV in the post-pachytene region
localizes to nucleolar inclusions (arrows) that stain positively for ubiquitin (panel 4, red). F. Mutant gonad with serine substitutions at
each of the cysteines C193, C195, and C200 in the M domain of CERV (cer1(zu527) and strain JJ2700, Fig 4A). Panel 1 shows that
nuclear, but not cytoplasmic foci of gRNA are present, and that CERV does not concentrate on the nuclear foci. Panel 2 shows that
CERV appears to be phosphorylated at S214. Arrows indicate perinuclear foci of CERV that occur frequently in this strain. G. Mutant
gonad with an R194A substitution in the M domain of CERV (cer1(zu531) and strain JJ2704). Panel 1 shows that gRNA is present in
nuclear but not cytoplasmic foci, and that GAG is not expressed. Panel 2 shows that the nuclei contain bright foci of phosphorylated
CERV. Panel 3 is a 4-micron Z-projection to visualize entire nuclear volumes, and shows that nuclei have more than the two expected
CERV foci and that none of the CERV foci are coincident with the nuclear gRNA foci. Scale bars in microns (B,D-G) 1.0 micron.
https://doi.org/10.1371/journal.pgen.1010804.g004
panel 4). We conclude that S214 phosphorylation is essential for CERV to concentrate on
nuclear gRNA, and that this concentration is strongly correlated with gRNA export and GAG
expression.
S214 is near the candidate Cys finger in the M domain, which includes invariant cysteine
and arginine residues (Fig 4A; see also alignment in S3 Fig). We generated a triple mutant,
cer1(zu527), with the cysteines C193, C195, and C200 mutated simultaneously to serine (Fig
4A). CERV was phosphorylated and expressed at abnormally high levels in the triple mutant
nuclei, but CERV did not concentrate on the gRNA foci and GAG was not expressed (Fig 4F).
The triple mutant also had bright cytoplasmic foci of CERV (Fig 4F, arrows) as observed infre-
quently in the S214 mutant but not in wildtype. We next generated a mutant, cer1(zu531),
with an R194A substitution in CERV (Fig 4A). Like the S214A and triple cysteine mutants, the
R194A mutant had nuclear foci of gRNA, but little or no cytoplasmic gRNA or GAG expres-
sion (Fig 4G, panel 1). Based on the above results, we anticipated that CERV would not be con-
centrated into nuclear foci, but instead found that CERV was concentrated in extremely bright
foci that stained intensely with α-pS/T-P (Fig 4G, panel 2). However, the mutant nuclei typi-
cally had more than the two foci seen in wildtype, and none of the CERV foci colocalized with
nuclear gRNA (Fig 4G, panel 3). Thus, these foci possibly represent non-functional aggregates
of CERV. Together, our results suggest that gRNA export and GAG expression require S214
phosphorylation and invariant residues in the candidate Cys finger of the M domain.
CERV transitions from small nuclear foci to giant rods
By the A4 stage the nuclear foci of CERV diminish or disappear in many pachytene germ
nuclei, and some CERV accumulates in nucleolar bodies (Fig 3C, panel 2). The nuclear foci of
gRNA persist in A4 germ cells, but often with lower signal intensities that are comparable to
cytoplasmic foci of gRNA (Fig 3C, panel 2; see S8 Fig for additional data). In both respects,
this class of A4 pachytene nuclei closely resembles younger nuclei exiting pachytene (Fig 4E;
panel 3, arrows). Remarkably, however, a second class of A4 pachytene nuclei shows a very dif-
ferent change in CERV localization: The overall level of CERV appears to increase rather than
decrease, and CERV forms giant, rod-shaped nuclear structures (Fig 5A, panel 1). The CERV
rods have a cylindrical geometry with apparently closed ends; they resemble circles in cross
section, and appear as two parallel lines in longitudinal section (Fig 5A, panels 2,3 and S2
Video). CERV is concentrated along the walls of the cylinder, but additional and variable
amounts of CERV can occur in the interior. The rods make broad contacts with the periphery
of the nucleolus, but never appear to penetrate the nucleolus (Fig 5A, panel 3 and S2 Video).
The rods are much larger than CERV foci or cytoplasmic GAG particles (Fig 5A, panel 2): rods
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 5. CERV rods in germ nuclei. A. Panel 1 is a 3-micron Z-projection of an A4 gonad, showing nuclei with CERV rods in addition to
nuclei with CERV foci. Note that the apoptotic cell (x) does not have a CERV rod. Panel 2 compares the sizes of nuclear rods, seen in
both longitudinal (left) and cross-sectional (right) profiles, with the sizes of cytoplasmic GAG particles (red). The arrowhead indicates
one end of a rod that appears to protrude slightly from the nucleus; additional examples of protruding rods are shown in panel 4. Panel 3
illustrates that rods are associated with the periphery of the nucleolus (FIB-1, red). Panel 5 shows a rod-containing nucleus with
perinuclear P granules, which are lost in early apoptosis. Panel 6 shows that the rods are highly phosphorylated, similar to CERV foci.
Panel 7 shows CERV rods in an apoptosis-defective ced-3(n717) mutant, and panel 8 shows CERV rods in the wild strain MY16 with a
Cer1 insertion on LGX [20]. B. Panel 1 shows serial optical sections of a single nucleus, demonstrating that the rings of CERV represent
cross sections of CERV rods (see also S2 Video). Note that both ends of the CERV rod appear to fill the nuclear channel, but the middle
of the rod (at z = -0.6 microns) shifts asymmetrically toward the nucleolus. Panels 2 and 3 indicate the paired LGIII homologs (dotted
line) between the two nuclear foci of gRNA (asterisks), and show that the CERV rod localizes to only one of the two flanking nuclear
channels (arrows). Increased exposures (+ exp) show gRNA in some rods (panels 3 and 4, arrowheads). C. The plot shows the total
number of rod-containing germ nuclei per gonad (left vertical axis) and the percentage of gonads with at least one rod-containing
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
nucleus (right vertical axis, grey bars). Mated animals in the wild-type and fog-2 mutant series were marked and mixed with wild-type
males 24 hours before processing; mating was confirmed for each gonad by sperm in the spermatheca, but this experiment did not
determine when mating occurred. P-values were calculated using nonparametric Mann-Whitney U test and graphing was performed
using GraphPad Prism software version 9.5. **** P< 0.0001, ***P � 0.01, ns = not significant. D. Gonad of a mated wild-type animal at
A5 showing large numbers of CERV rods, all of which stained positively with α-pS/T-P. Scale bars in microns = (A,B) 1.0; (D) 5.0.
https://doi.org/10.1371/journal.pgen.1010804.g005
are generally 0.4–0.6 microns in diameter (S1 data) and have variable lengths, but usually
extend the entire nuclear diameter (4.0 to 5.5 microns). Some rods slightly exceed the nuclear
diameter and are either curved (Fig 5A, panel 1) or are associated with small protrusions of the
nuclear surface (Fig 5A, arrowheads in panels 2 and 4). The CERV rods stain intensely with α-
pS/T-P, similar to the nuclear foci of CERV (Fig 5A, panel 6). Germ cells with CERV rods
could be adjacent to, or surrounded by, germ cells which only had CERV foci or diffuse, nucle-
oplasmic CERV (Fig 5A, panel 1). This heterogeneity was surprising because pachytene germ
cells are syncytial and interconnected by ring channels (Fig 1A); they only cellularize during
apoptosis, which happens to many germ cells for reasons that are largely unknown [68,69].
However, CERV rods did not appear to result from apoptosis: First, most apoptotic germ
nuclei did not contain CERV rods (Fig 5A, "x" in panel 1). Second, nearly all rod-containing
nuclei had perinuclear P granules (Fig 5A, panel 5), which normally are lost during the earliest
stages of apoptosis [69]. Finally, ced-3(n717) mutants which lack germ cell apoptosis had
numerous rod-containing germ nuclei (Fig 5A, panel 7). CERV rods were not present in the
cerv(stop) mutant, or in the mutants with S214A or R194A substitutions in CERV, or in the
CB4856 Hawaiian strain of C. elegans which lacks Cer1 [24]. We examined 7 wild strains of C.
elegans with novel insertions of Cer1 [20] and found that each of these contained variable
numbers of germ nuclei with CERV rods (Fig 5A, panel 8; C. elegans strains MY16, EG4946,
JU406, MY23, CB4507, CB4932, and CX11307). Thus, CERV rods appear to be a general fea-
ture of the Cer1 retroelement.
Most rod-containing germ nuclei in A4 gonads had only one CERV rod (Fig 5A, panel 1):
In a set of 518 rod-containing nuclei, 93.3% had one rod, 5.6% had two rods, and 1.1% had
three rods. By contrast, nuclei in older, mated animals frequently contained multiple rods (S8
Fig). The vast majority of rods in A4 gonads aligned with nuclear channels, either filling the
channel or displaced toward the nucleolus (Fig 5B, panel 1). Rods were usually found in only
one of the two nuclear channels flanking LGIII, where Cer1 is inserted (Fig 5B, asterisks in
panels 2,3). However, a small percentage of rods were in nuclear channels that did not align
with LGIII, or that were even perpendicular to LGIII (S8 Fig). Several rods appeared to contain
gRNA when imaged at increased exposures sufficient to visualize cytoplasmic gRNA foci (Fig
5B, arrowheads in panels 3,4), but the number of rods with gRNA was highly variable between
different gonads in the same preparation.
The number of germ nuclei with CERV rods varied considerably between different animals
at the same stage, but showed a general increase between A4 and A6 (Fig 5C and S1 Data).
This increase raised the possibility that rods were induced by non-specific, age-related cellular
stress. C. elegans lifespan can be doubled by mutations in the insulin receptor DAF-2 (insulin/
IGF-1 receptor), and old daf-2 mutant hermaphrodites retain many features associated with
younger, healthier adults [70–72]. However, the numbers of rod-containing nuclei in A6 daf-2
(e1370) gonads appeared similar to A6 wild-type gonads (Fig 5C and S3 Video). Rods first
appear as hermaphrodites approach the end of their self-fertile period and effectively become
females which require mating to produce additional eggs. Indeed, we found that A5 adults
which were mated on A4 had significantly more CERV rods than unmated A5 adults (Fig 5C
and 5D). Ancestral C. elegans appears to have been a gonochoric species (separate males and
females) which evolved into self-fertilizing hermaphrodites by acquiring the sperm-
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
determining gene fog-2; C. elegans mutants homozygous for fog-2(null) mutations are healthy
females [73,74]. We found that fog-2(q71null) females produced rods much earlier than wild-
type hermaphrodites: all of the A1 and A2 fog-2 gonads had at least some rod-containing
nuclei, compared with the absence of rods in A1 and A2 wild-type gonads (Fig 5C). The num-
bers of rod-containing nuclei in the fog-2 mutants remained relatively constant from A2-A8
(Fig 5C), and the nuclear foci of gRNA and CERV gradually diminished with age (S8 Fig).
However, mating restored high levels of both nuclear gRNA and CERV (S8 Fig), and signifi-
cantly increased the number of rods relative to unmated controls (Fig 5C).
CERV rods and the nucleolus
To understand how CERV rods form, we searched for potential intermediate structures. Rings
of CERV were frequently visible in germ nuclei, and we considered whether these might tem-
plate rod formation. However, optical Z-stacks showed that all rings scored were cross-sections
of long rods (n = 76; Fig 5B, panel 1; see also S2 Video). The nuclear foci of gRNA did not have
a consistent position relative to rod geometry, and could be either at the end or near the mid-
dle of a rod (Fig 5B; asterisks in panels 2,3). We noticed that some of the A4 germ nuclei
appeared to have slightly elongated foci of CERV and nuclear gRNA (Fig 6A), or highly elon-
gated, flattened streaks of CERV and gRNA (Fig 6B, panels 1 and 2). Similar to CERV rods,
the streaks were usually confined to one of the two nuclear channels flanking LGIII, and the
streaks could extend either unidirectionally or bidirectionally away from the nuclear foci (Fig
6B; asterisks in panels 1 and 2). The Cer1 insertion in the N2 laboratory strain is near the mid-
dle of LGIII, and in some nuclei LGIII made a sharp, U-shaped bend near the nuclear gRNA
foci (Fig 6A and 6B, panel 3). In these nuclei, the CERV streak could make a similar, U-shaped
bend in one of the curved, flanking channels. In a few cases, however, the CERV streak
extended into an adjacent but nearly orthogonal channel (Fig 6B, panel 3); we consider these
latter streaks to be possible precursors of CERV rods which do not align with LGIII.
Most of the streaks had widths comparable to the dimensions of nuclear channels. How-
ever, the streaks did not fill the channels, and instead were closely associated with the surface
of the nucleolus (Fig 6B, panel 4–7). A few streaks were much wider than nuclear channels,
and projected views of optical Z-stacks showed that these structures were large and flattened,
oval-shaped bodies we term caps (Fig 6C, arrowheads in panel 1). The CERV caps were associ-
ated with the surface of the nucleolus, similar to the streaks, but showed no relationship to the
boundaries of nuclear channels (Fig 6C; arrowheads in panel 2). This observation suggests that
CERV is associating primarily with the nucleolus, rather than channel-specific structures.
Interestingly, we found that the structure of the nucleolus changed during the stages when
streaks and rods appear: Consistent with results from previous studies [75,76] nucleoli in
A1-A2 germ cells had a finely reticulated or net-like appearance, as seen with the nucleolar
markers FIB-1/fibrillarin and NST-1/nucleostemin (Fig 6D, panel 1) and as visible by TEM of
A1 nuclei (Fig 6E). However, by A5 both FIB-1 and NST-1 appeared to segregate into large,
nearly exclusive domains (Fig 6D, panel 2), and TEM images showed a major and highly vari-
able segregation of nucleolar components (Fig 6E). Because CERV rods form earlier in fog-2
germ cells than in wildtype (Fig 5C), we next examined A2 fog-2 germ cells and found that
they had segregated nucleoli resembling those in older, wild-type germ cells (Fig 6F). Studies
in many types of cells have shown that inhibiting ribosomal RNA synthesis causes a segrega-
tion or separation of nucleolar components [77], and can cause a relocalization of some
nuclear proteins to the periphery of the nucleolus [78]. Thus, streak and rod formation might
be initiated by a decrease in rRNA synthesis as adults near the end of their self-fertile period
(See Discussion).
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 6. Candidate intermediate stages in the formation of CERV rods. A. A4 germ nucleus showing the major nuclear foci of gRNA and
CERV (asterisks) with additional signals (arrowheads) in a nuclear channel flanking LGIII (dotted line). B. Panels 1 and 2 show near-surface
views of germ nuclei with extended, flattened streaks (arrowheads) of gRNA and CERV extending unidirectionally (panel 1) and
bidirectionally (panel 2) from the main foci (arrowheads). Panel 3 shows an example of a CERV streak that extends in a channel which is
nearly orthogonal to LGIII; a summary reconstruction of the streak and chromosomes is shown at right. Panels 4–7 show multiple examples of
flattened streaks of CERV and gRNA at the periphery of the nucleolus. C. Panel 1 shows a single focal plane (left) and a 3-micron Z-projection
of a group of germ nuclei. The CERV streak at top left remains rectangular in the projected view, but the bottom two streaks (arrowheads) are
seen to be much larger oval-shaped "caps" of CERV. Note the main nuclear foci of gRNA and CERV (asterisks) at the perimeters of the caps.
Panel 2 shows a germ nucleus with a CERV cap (indicated by the 3-micron Z-projection at right) where the nucleolus is stained for FIB-1/
fibrillarin (red). Note that the CERV cap extends below and between multiple nuclear channels (arrowheads), suggesting that the cap is
associated primarily with the surface of the nucleolus. D. Panel 1 shows dispersed nucleolar components in A1 nuclei, and panel 2 shows
segregation of the same components in A5 nucleoli. E. TEM images of nucleoli in A1 or A5 wild-type germ nuclei, as listed. Note that the
nucleoli in the A5 nuclei have variable patterns of segregation. F. Images comparing nucleolar components in A2 wild-type nuclei with the
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
same components in A2 fog-2 "female" nuclei. Note that the A2 fog-2 nucleoli resemble older, A5 wild-type nucleoli (see Fig 6D). Scale bars in
microns = A-C (1.0), D, F(2.0), E(1.0).
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TEM analysis of CERV foci and rods
We wanted to compare the ultrastructure of CERV foci, streaks, and rods using the genetically
encoded tag APEX2 (enhanced ascorbate peroxidase 2) [79,80]. With this technique, glutaral-
dehyde-fixed tissues are treated with 3,3-diaminobenzidine, which is converted to an insoluble
polymer near the localized enzyme and visualized by staining with electron-dense osmium.
We created a hybrid strain for the analysis which contained three Cer1 elements: Cer1, Cer1
(APEX2-CERV), and Cer1(GFP-CERV) (see Methods for rationale and strain construction).
The APEX2-CERV hybrid strain was compared to a control strain containing Cer1 plus Cer1
(GFP-CERV). For both strains, GFP-CERV expression was used to pre-screen live animals for
ones with the largest numbers of rods, and these were pooled for TEM analysis. After process-
ing and embedding, 70 nm thin sections were collected at intermittent intervals throughout
the gonads, with additional serial sections from selected regions.
By comparison with our conventional TEM preparations of C. elegans germ cells [24,43],
the APEX2 protocol markedly reduced the background staining of RNA-rich structures such
as the nucleolus (compare Figs 7A with 6E). However, many germ nuclei in the APEX2-CERV
sample had one or two large and prominent electron-dense foci (Fig 7A, white arrowheads)
which were not present in the control sample (Fig 7B). We consider these foci to represent
CERV localization: The foci had the expected sizes for CERV foci, they were in nuclear chan-
nels, and they were abundant in the A2 sample but largely absent in the A5 sample. At high
magnification, the electron-dense foci surrounded a cluster of relatively electron-lucent fibrils
(Fig 7A, black arrows). Germ nuclei in the control sample without the APEX2 tag often con-
tained one or two foci with a similar size and location as the APEX2-CERV foci, but which
were nearly reciprocal in appearance: these foci consisted of a cluster of electron-dense fibrils
surrounded by an electron-lucent zone (Fig 7B, red circles). The clustered fibrils appeared to
be randomly oriented and intermingled with small, electron-dense "dots" of the same thickness
that likely represent cross sections of fibrils (Fig 7B, panel 2). Because CERV foci are associated
with gRNA, we hypothesize that the fibrils represent gRNA molecules. The apparent lengths of
individual fibrils varied considerably from an average of about 200 nm up to a maximum of
500 nm (S9 Fig and S1 Data), but these measurements likely underestimate the true average as
they were taken from 70 nm thin sections. The fibrils were easily distinguished from presump-
tive RNP granules which are common in germ nuclei (compare Fig 1D). In addition to the
clusters of fibrils, both the APEX2-CERV and control samples contained individual fibrils
which were approximately orthogonal to the nuclear envelope and adjacent to nuclear pores
and P granules (Fig 7C and 7D, panel 1). Remarkably, some fibrils appeared to be coaligned in
narrow, linear tracks near the envelope (Fig 7D, panel 2). At high magnification, individual
fibrils typically had a series of fine striations at right angles to the long axis of the fibril, creating
a zig-zag appearance that might represent a helical structure (Fig 7D, panel 3; see also S9 Fig).
The APEX2-CERV sample at A5, but not A2, had several examples of electron-dense
streaks or caps at the nucleolar periphery (Fig 7E, white arrowheads). At high magnification,
the electron-dense streaks contained electron-lucent fibrils which were aligned in the plane of
the nucleolar surface (Fig 7E, black arrows). The control sample had streaks or caps of material
at the nucleolar periphery with a reciprocal appearance; electron-dense fibrils surrounded by
an electron-lucent matrix (Fig 7F, black arrows and white arrowheads, respectively). The A5
APEX2-CERV sample contained several nuclei with longitudinal or cross-sectional profiles of
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 7. TEM of CERV-containing nuclear structures. A. A2 germ nuclei expressing APEX2-CERV. The low magnification images at
top show prominent electron-dense regions that appear to be in nuclear channels next to P granules (dotted outlines). Panel 2 shows
one such region at high magnification; note the electron-lucent fibrils (black arrows) within the electron-dense region. Panel 3 shows
additional examples of the electron-lucent fibrils within APEX2-CERV foci. B. A2 control nuclei. Panel 1 shows a nucleus with two
clusters of electron-dense fibrils (outlined in red), and panel 2 shows similar regions in other nuclei. White arrowheads indicate
electron-lucent regions around the fibrils. C. Examples of aligned fibrils in A2 germ nuclei expressing APEX2-CERV; arrows indicate
individual, electron-lucent fibrils. D. Panel 1 shows three examples of individual fibrils oriented perpendicular to the nuclear envelope;
nuclear pores are indicated by small white arrowheads. Panel 2 shows groups of aligned fibrils near the envelope. Panel 3 is a high
magnification of a single fibril; note zig-zag appearance (black arrowheads), with striations or flanges that are perpendicular to the long
axis of the fibril (see also S9 Fig). E. Examples of flattened streaks or caps of APEX2-CERV at the perimeter of nucleoli (no) in A5 germ
cells. Note that the streaks contain numerous electron-lucent fibrils (black arrows) which are aligned in a plane parallel to the nucleolar
surface. F. Control A5 gonads showing electron-dense fibrils (black arrows) surrounded by an electron-lucent matrix (white
arrowheads) at the surfaces of nucleoli.
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
giant, electron-dense cylinders (Fig 8A, panels 1,2 and 1,3, respectively; see S9 Fig for serial
section data). As expected for CERV rods, the cylinders were closely associated with the nucle-
olar periphery, and the ends of the cylinders could be adjacent to slight protrusions of the
nuclear envelope (Fig 8A, arrowheads in panel 2; see also Fig 5A, arrowheads in panel 4). The
A5 control nuclei had longitudinal and cross-sectional profiles of giant cylinders with similar
dimensions, but with a reciprocal appearance; electron-dense fibrils surrounded by an elec-
tron-lucent matrix (Fig 8B; black arrows and white arrowheads, respectively). Some fibrils at
the periphery, or inside of, a cylinder were oriented parallel to the long axis of the cylinder
(Fig 8A, panels 1 and 2 and 8B, panel 1). However, many of the fibrils visible in cross sections
of the cylinders had a curvature matching the circumference of the cylinder (black arrows in
Fig 8A and 8B). This curvature suggests that rods might form from flattened streaks of CERV
and gRNA which rolled lengthwise into cylinders (Fig 8C). This model predicts that interme-
diate hemicylinders of CERV might be present in some germ cells. Our immunostaining
experiments did not detect convincing examples of hemicylinders with the dimensions
expected for precursors of typical rods. However, 3/318 A5 germ cells had larger than expected
hemicylinders of CERV, and the same population contained a few CERV rods with unusually
large diameters (S8 Fig). Thus, we speculate that the large hemicylinders and rods might both
be derived from large, ovoidal caps of CERV (Fig 6C, panel 1), while most rods are derived
from narrow streaks.
Discussion
Retroviral gRNA is contained and protected by a protein shell, the capsid, which is assembled
from hexamers and pentamers of CA, a subdomain of the GAG polyprotein. Cer1 GAG lacks
sequence similarity to CA, but we showed that it contains a CA-like domain with predicted
structural similarity to CA and the potential to form hexamers. Thus, Cer1 GAG resembles ret-
roviral GAG proteins in having both CA and NC domains, but lacks sequence or structural
similarity with the N-terminal MA (matrix) domain of retroviruses [81]. Most LTR retrotran-
sposons in the Ty3/Gypsy family, which includes Cer1, lack MA proteins and their GAG pro-
teins begin with the CA domain [81]. By contrast, Cer1 GAG has an N-terminal domain
which is larger than most retroviral MA proteins (S4 Fig). MA proteins direct the site of capsid
assembly; retroviruses such as HIV use MA proteins to assemble capsids at the plasma mem-
brane, but others such as mouse mammary tumor virus (MMTV) assemble capsids in the cyto-
plasm [82,83]. The N-terminal domain of Cer1 GAG might have a role in targeting to the
perinuclear P granule that overlie clustered nuclear pores, where GAG would be positioned to
intercept newly exported gRNA. Consistent with this hypothesis, our previous TEM studies
showed that Cer1 capsids are often found by P granules in C. elegans, and presumptive Cer1
capsids in C. japonica form grape-like clusters on P granules [24]. We showed here that newly
exported gRNA appears to associate with GAG at or near P granules in C. elegans (Fig 2D), but
we do not know whether those GAG particles represent entire capsids, or smaller intermedi-
ates in capsid assembly. For example, small numbers of retroviral GAG proteins or hexamers
are thought to bind gRNA before the GAG-gRNA complex moves to capsid assembly sites
[84]. The N-terminal GAG domain of Cer1 might also function in targeting assembled capsids
to microtubules, where they accumulate in enormous numbers (Fig 2F). TEM studies have
shown that Cer1 capsids are coated with fine, radially-oriented spikes which extend about 12–
19 nm [24]; interestingly, the N- terminal GAG domain of Cer1 contains a long, 100 amino
acid alpha helix which would be predicted to measure about 15 nm (S4 Fig).
Cer1 GAG particles are highly abundant in adult hermaphrodite gonads, but genetic experi-
ments which detected spontaneous transposition of DNA transposons did not observe novel
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Fig 8. TEM of CERV rods. A. Panels 1–3 show longitudinal or cross-sections of APEX2-CERV rods in A5 germ nuclei. Note the
electron-lucent fibrils in the rods (black arrows). The top row in panel 2 shows two examples of rods associated with small protrusions
(arrowheads) of the nuclear envelope (compare with arrowheads in Fig 5A, panels 2 and 4). In the cross-sectional images (panel 3),
note the curvature of the fibrils (black arrows) within the electron-dense matrix (white arrowhead). B. Longitudinal or cross-sections
of rods in A5 control nuclei; an example of serial sections through a rod is provided in S9 Fig. The rods consist of electron-dense fibrils
(black arrows) surrounded by an electron-lucent matrix (white arrowheads). The cross-sectional images of rods in panel 2 are shown
as insets at higher magnification; note the curvature of fibrils at the perimeter of the rods. One of the nuclei in panel 2 has a rare double
rod which is also seen in immunostained preparations (S8 Fig). Panel 3 shows additional examples of cross-sections through rods. C.
Speculative model for CERV rod formation, shown in cross-sectional view. CERV and gRNA are proposed to form flattened streaks or
caps on the surfaces of nucleoli. Unknown events cause the streaks to roll lengthwise into cylinders, such that gRNA molecules which
were previously parallel to the nucleolar surface now become curved. Scale bars in microns as listed.
https://doi.org/10.1371/journal.pgen.1010804.g008
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
insertions of Cer1 [85,86]. Endogenous retroelements are expected to accumulate disruptive
mutations over time, or become truncated by unequal homologous recombination between
LTRs [87]. Thus, the discovery that Cer1 GAG particles have a role in memory transfer
strengthens the possibility that Cer1[N2] is a disabled relic, co-opted or repurposed solely for
host functions [30]. Our findings that Cer1 GAG particles are not empty, and can instead con-
tain gRNA, supports a view that Cer1[N2] remains intact. Further support comes from recent
genomic sequencing of wild strains of C. elegans which have nearly identical Cer1 elements at
different insertion sites ([20] and this study). The failure to detect Cer1 transposition could be
because the culture temperature was not optimal for Cer1 expression [24], or that host factors
normally restrict transposition [88]. An additional possibility is that Cer1 transposition is dis-
favored in unmated hermaphrodites: All of the self-progeny of a hermaphrodite will be homo-
zygous for the maternal copy of Cer1 by default, and transposition risks disrupting essential
host genes or creating novel anti-sense insertions that silence expression [89]. In natural popu-
lations, Cer1 transposition might be stimulated by environmental stresses that challenge host
survival [90,91] or by mating with males. Because Cer1 capsids accumulate on stable microtu-
bules, the vast majority of capsids never enter self-progeny [24]. However, our results suggest
that the capsids provide a potential reservoir of age- and RNAi-resistant Cer1 genomes that
might impact cross-progeny sired by males [20].
CERV and nuclear export of unspliced gRNA
Intron-containing pre-mRNAs normally are retained in nuclei or degraded in the cytoplasm,
but the retroviral life cycle requires some mechanism to export and maintain unspliced gRNA.
Moreover, exported but unspliced RNA can trigger gene silencing through poorly understood
mechanisms that retroviruses likely overcome [92]. Intron-containing mRNAs often contain
premature stop codons, and are degraded in the cytoplasm by the nonsense-mediated decay
(NMD) pathway [93]. Most retroviruses have a stop codon between gag and pol, and require
mechanisms to prevent NMD. For example, Rous sarcoma virus gRNA has an RNA stability
element which appears to protect against NMD [94]. Cer1 gRNA does not contain a stop
codon between gag and pol, and would not be expected to be targeted by NMD after nuclear
export [24]. However, Cer1 gRNA retains conventional C. elegans splice sequences which are
recognized and used to create the spliced cerv mRNA [24].
The initial steps in nuclear export begin as pre-mRNAs are loaded co-transcriptionally with
proteins in the TREX (Transcription and Export) complex, and are further marked after splic-
ing by exon junction complex (EJC) proteins [95–97]. These complexes have unique core com-
ponents, and share some subunits such as UAP56 and Aly/REF. Depletion of the C. elegans
homologs of UAP56 or AlyREF allows many unspliced germline RNAs to be exported to the
cytoplasm through the CRM1/Exportin pathway, suggesting that these proteins normally have
important roles in retaining unspliced RNA in germ nuclei [62]. In principle, therefore, viral
gRNAs might become competent for export simply by blocking their association with proteins
like UAP56 and Aly/REF. However, retroviruses such as Mason-Pfizer monkey virus achieve
export by a secondary structure in the gRNA which directly binds the host nuclear export pro-
tein NXF1/NXT1 [98,99]. Other retroviruses such as HIV-1 use an intermediate protein, Rev,
to couple the gRNA with the host nuclear export protein CRM1/Exportin; Rev binds and mul-
timerizes on a structured region of the gRNA, and contains a nuclear export sequence recog-
nized by CRM1 [100].
We showed that the Cer1 CERV protein is required for gRNA export: First, a cerv-specific
STOP mutation or single S214A or R194A substitutions in CERV resulted in an absence of
cytoplasmic gRNA and prevented GAG expression. Second, CERV is a nuclear protein which
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
colocalizes with nuclear gRNA (Fig 2C and 2D). Third, our TEM experiments showed that
CERV and likely gRNA molecules can localize next to the nuclear envelope by nuclear pores
(Fig 7C and 7D), but APEX2-CERV was not detected outside the envelope. Cer1 gRNA export
is regulated by ON/OFF switches that depend on sex, developmental stage, cell cycle, and cul-
ture temperature; at each switch, export occurs only when CERV colocalizes with the nuclear
gRNA. For example, gRNA is not exported at 25˚C, but appears in the cytoplasm after a brief
shift to 15˚C in parallel with CERV concentration on nuclear gRNA (Fig 2D). Similarly, gRNA
is exported in meiotic germ cells at the same spatial boundary where CERV first concentrates
on nuclear gRNA (Fig 3G). CERV localization to gRNA requires phosphorylation at S214, and
we showed that an antibody which recognizes phosphorylated S214 stains CERV at each of the
ON switches (Fig 4E). We have not identified the presumptive proline-directed Ser/Thr kinase
that phosphorylates CERV, but the finding that phosphorylation does not occur at 25˚C raises
the possibility that kinase activity is temperature-sensitive; GAG expression has been shown to
have a similar temperature-sensitivity in each of several wild strains that express Cer1 [24].
CERV localization to gRNA might have several possible functions that direct or permit gRNA
export. For example, CERV has a candidate nuclear export signal, but we have not determined
whether CERV functions as a shuttling protein like the HIV-1 Rev protein. Wild-type gonads
have very little detectable CERV in the cytoplasm, but CERV might only need to shuttle
between the nucleus and perinuclear P granules to achieve export.
Blocking nuclear export can increase the nuclear abundance of some mRNAs. For example,
treating C. elegans with leptomycin B, a specific inhibitor of CRM1/Exportin-mediated nuclear
export, increases the level of tra-2 mRNA in intestinal nuclei [101]. However, none of the sev-
eral cerv mutations described here caused an obvious increase in the level of gRNA in germ
nuclei. Conversely, each of the mutations appeared to increase the level of CERV relative to
wildtype (Fig 4B and 4F). Thus, one likely possibility is that gRNA splices by default into cerv
mRNA in these mutants, and that cerv mRNA is exported by conventional pathways. CERV is
a large protein which lacks sequence or structural similarity to any known retroviral protein,
but is predicted to have three structured domains which are conserved in Cer1 elements found
in different species of Caenorhabditis. The G domain has an interesting resemblance to G-pro-
tein structures but is not expected to have similar enzymatic activity (S4 Fig). Thus, the G
domain might have originated from a G-protein but been maintained solely to recruit acces-
sory proteins [102]. The M domain has two opposing surfaces with high confidence potentials
for binding the complementary surfaces of adjacent M domains (S6 Fig); these interactions
could allow CERV to form closed rings with 5 or more subunits, or might create the potential
for open, helical stacking (see S9 Fig). Each M domain has a cysteine-rich loop which faces the
central axis of the ring and might represent an RNA-binding, C4-type zinc finger (Fig 4A).
Structural and biochemical studies will be required to test possible interactions between CERV
and gRNA, but we showed here that the invariant cysteines and an arginine in the Cys loop are
each essential for CERV concentration at the nuclear foci of gRNA and for gRNA export.
CERV and nuclear fibrils
Our TEM analysis showed that CERV surrounds electron-dense fibrils in germ nuclei, and we
propose that these fibrils represent Cer1 gRNA molecules. Animal nuclei typically contain
RNP or mRNP granules such as Cajal bodies, nuclear speckles, paraspeckles, PML bodies, and
ribosome intermediates [103–105]. These nuclear granules have diverse roles that include pre-
mRNA and ribosomal RNA processing, and presumptive RNP granules are common in C. ele-
gans germ nuclei (Fig 1D). By contrast, there appear to be very few reports in any system of lin-
ear fibrils in nuclei, although viruses often form rods or linear filaments in the cytoplasm
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
[106]. Human cells infected with Rift valley fever phlebovirus develop nuclear fibrils that con-
tain the viral encoded NSs protein, and some amoeba and ciliate nuclei contain electron-
dense, helical fibrils of unknown identity that can be up to 700 nm in length [107–110]. The
helical fibrils appear to contain protein and RNA, and can be oriented perpendicular to the
envelope and adjacent to nuclear pores [107].
The Cer1 gRNA is about 8.4 kb, which is much larger than most mRNAs in C. elegans; only
two germline-expressed mRNAs appear to be larger (encoding HIM-4/hemicentin and HMR-
1/cadherin) [111]. The fibrils have some resemblance to electron-dense fibrils which are visible
in TEM images of Cer1 capsids [24], and our present study showed that many of the capsids
contain gRNA. The nuclear fibrils were often found in prominent clusters in A2 gonads (Fig
7A and 7B); these clusters likely correspond to the nuclear foci of gRNA seen by smFISH (Fig
2C), and thus are at or near the sites of Cer1 transcription. However, individual fibrils could be
perpendicular to the nuclear envelope and directly adjacent to nuclear pores, suggesting inter-
mediate stages of nuclear export (Fig 7C and 7D). Fibrils were up to 500 nm in length (S9 Fig),
raising the question of what length is expected for an 8.4 kb RNA? Previous studies used elec-
tron microscopy to measure contour lengths of single, denatured RNA molecules of various
known sizes that were spread on protein films [112]. Those studies showed an average metric
of 1 micron per 4.3 kb, implying that a fully unstructured Cer1 gRNA could extend nearly 2
microns. However, most RNAs are thought to be folded; purified RNAs in solution fold into
highly compacted structures, and RNAs as large as 5 kb can have 5’ to 3’ end separations of less
than 10 nm [113,114]. We found that programs that predict RNA secondary structures, such
as RNAfold [115], generate highly folded models for Cer1 gRNA, suggesting that gRNA would
not form linear fibrils unless it was associated with factors that restrict folding. For example,
the 6.4 kb genomic ssRNA of tobacco mosaic virus is twisted into a helix by multimers of coat
protein to create 300 nm, rod-shaped viral particles [116].
A surprising feature of the C. elegans fibrils is their ability to form linear arrays (Fig 7D,
panel 2). FISH experiments in other systems described curvilinear tracks of viral RNA that
appeared to extend from sites of transcription toward the nuclear envelope [117,118]. How-
ever, the prevailing view is that mRNPs do not travel in linear tracks to nuclear pores, but
instead move by random diffusion within channels bordered by compacted chromatin (chan-
neled diffusion) [119,120]. Restricted movements are most apparent with large RNAs such as
the 14kb Duchenne muscular dystrophy mRNA [121], and where there are dense chromatin
barriers such as Drosophila polytene chromosomes [122]. Typical channel widths in C. elegans
pachytene nuclei appear to be about 0.5–0.75 microns, as measured between the DAPI-stained
pairs of chromosomes or between the electron-dense chromatin visualized by TEM (Fig 1C
and 1D). By contrast, the linear arrays of fibrils have a combined width of less than 0.2
microns, and do not appear to be directly adjacent to compacted chromatin (Fig 7D). These
observations raise the possibility that the fibril arrays form by association with unidentified lin-
ear structures, such as nuclear actin filaments. We suggest that the large size of the fibrils in C.
elegans, the ability to rapidly induce gRNA export by shifting temperature, and the density of
germ nuclei make the gonad an attractive system for analyzing gRNA export in vivo.
CERV rods and the nucleolus
Several types of retroviral GAG proteins are capable of self-assembly in vitro to form spherical
or rod-shaped capsids [93]; retroviral capsids average 100 to 200 nm in size, and the largest
known viruses are about 1.5 microns [123,124]. By contrast, CERV rods can be nearly 5
microns in length, and our results suggest that rod morphogenesis involves host structures in
addition to any contribution from self-assembly. By TEM, the fibrils in the streaks or caps of
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
CERV are uniformly aligned parallel to the plane of the nucleolar surface (Fig 7E and 7F), but
the fibrils visible in cross sections of rods have a curvature matching the circumference of the
cylinder (Fig 8A and 8B). As a working model, we propose that rod formation begins as CERV
and gRNA localize to the surface of the nucleolus, and that the flattened streaks roll up longitu-
dinally into cylinders (Fig 8C). Additional growth at both ends of the cylinder could extend
rod length to match or exceed the nuclear diameter. The CERV streaks and rods that form at
A4 have relatively uniform widths, and usually track a nuclear channel flanking the Cer1 inser-
tion on LGIII. However, the caps and many of the streaks of CERV that form in later germ
nuclei are not delimited by nuclear channels (Fig 6C, panel 2); at those stages some nuclei have
multiple rods, and some of the rods can have atypically large diameters (S8 Fig). A rolling
mechanism could accommodate this variation, and explain the rare examples of giant hemicy-
linders of CERV (S8 Fig).
What causes CERV and gRNA to relocalize from nuclear foci into nucleolar-associated
streaks? Previous studies on induced gene expression in C. elegans germ cells [43] and analo-
gous observations in this report (Fig 2D) suggest that germ cell RNAs in early adults move
freely within the network of interconnected nuclear channels. Localized streaks of gRNA and
CERV appear on the surfaces of nucleoli at a transitional period in the reproductive cycle as
older hermaphrodites deplete their stores of self-sperm and effectively become female. fog-2
females can produce rods much earlier than wild-type hermaphrodites, suggesting that the
female state is relevant to understanding how rods form. We showed that the structure of the
nucleolus changes markedly near the time that hermaphrodites become depleted of sperm,
and that similar changes occur in much younger fog-2 females. In particular, the nucleolus
loses its reticulated appearance, and the various nucleolar components segregate into largely
separate domains. Nucleolar morphology is closely linked with activity, and nucleolar segrega-
tion occurs frequently in plant and animal cells that repress rRNA transcription during normal
development, or after Pol I transcription is inhibited experimentally by low concentrations of
actinomycin D or oxaliplatin [77,125]. Moreover, conditions that induce nucleolar segregation
cause a subset of nucleoplasmic proteins to translocate to the nucleolus, often in structures
called nucleolar caps [78]. For example, p80 coilin, a protein normally found in Cajal bodies,
and the splicing factor PSF both translocate to the periphery of the nucleolus [78].
Sperm-depleted hermaphrodites must wait indeterminant periods before mating, and
unfertilized oocytes arrest development until they are activated by signals from male-provided
sperm [126]. Relative to young wild-type adults, gonads in sperm-depleted hermaphrodites
and unmated fog-2 females have a reduced mitotic index [127,128] but grow appreciably in
size (S8 Fig). This indicates that gonad development is not fully arrested in the absence of self-
fertilization. We are not aware of studies that directly examined rRNA transcription in those
types of gonads. However, transcriptome studies showed that sperm-depleted hermaphrodites
and fog-2 females have similar gene expression profiles, and that these profiles differ from self-
fertile hermaphrodites primarily by the downregulation of ribosomal components [129]. Thus,
a decrease in rRNA synthesis could be a molecular signature Cer1 uses to distinguish the self-
fertile phase of reproduction from the cross-fertile phase, and serve to trigger an association of
CERV and gRNA with the nucleolar periphery (Fig 6B).
CERV rods; a mysterious structure with unknown function
CERV rods appear to be a widespread, if not ubiquitous feature of Cer1 in wild strains, but we
do not understand what function, if any, they serve. The large sizes of the rods raise the possi-
bility that they contain significant quantities of unidentified host proteins or RNAs. Our FISH
experiments suggest that only some of the rods contain Cer1 gRNA, but our TEM analysis
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
showed fibrils in essentially all rods. This apparent difference needs to be resolved, but an asso-
ciation with rod components might restrict the penetration of FISH probes, and normally
serve to protect the gRNA under harsh environmental or stress conditions. With standard cul-
ture conditions, CERV rods only rarely occur in oogonia that will be fertilized by hermaphro-
dite self-sperm, but rods can be present in many oogonia that have the potential to be fertilized
by male sperm (cross-progeny). In natural populations, the optimal adaptive strategy a resi-
dent Cer1 uses for the identical self-progeny of a hermaphrodite might be very different than
for cross-progeny sired by males. For example, mating could introduce male-derived chromo-
somes that lack Cer1, and present opportunities for colonization, or that have anti-sense inser-
tions in germline-expressed genes which could silence the host element; the available genome
sequences of wild strains of C. elegans indicate that both types of males are expected in nema-
tode populations [20]. C. elegans genetics and site-directed gene editing provide powerful tools
to dissect rod formation and function, and the expanding database of diverse Cer1 elements
provides an important resource for analyzing potential roles in Cer1 ecology. Finally, many
LTR retrotransposons have 3’ ORFs of unknown function in the same position as the Cer1
CERV protein, and it will be interesting to learn whether those ORFS have analogous roles in
gRNA export and/or contribute to novel viral structures [18].
Methods
Worm culture
Nematode were maintained as described [113] with 15˚C as the standard culture temperature
unless stated otherwise. Plate media was made using Bactopeptone (Difco) and Bacto-agar
(Difco) as described [113]. Culture plates were allowed to dry at room temperature for 2 days
in the dark, then seeded with a fresh, overnight culture of E. coli strain OP50. Seeded plates
were stored in the dark at room temperature for no more than 5 days. All worm cultures were
grown for a minimum of two generations without starving before analysis. All strains used for
this study are listed in S1 Table. A minimum of 30–40 animals were analyzed for each experi-
ment reported.
Antibodies
The following antibodies/antisera were used in this study. Actin (Cell Signaling Technology);
Cer1GAG (mAbP3E9) [24]; fibrillarin (abcam ab4566), FLAG (Sigma Aldrich); GFP (Wako);
PGL-1 (gift from Susan Strome), GFP (Wako); phospho Ser/Thr-Pro (abcam ab9344); ubiqui-
tin (FK2, Enzo Life Sciences). The secondary HRP-conjugated anti-mouse antibodies were
from Life Technologies.
Analysis of Cer1 elements from wild strains and hybrid strain construction
Previous genomic sequence studies on wild strains of C. elegans identified a candidate Cer1
LTR at nucleotide 10632248 on LGX in strain ED3046 [20], and we showed this sequence was
part of an intact Cer1 element. Cer1[ED3046, LGX] has 491bp 5’ and 3’ LTRs that are identical
and differ from the 492 bp LTRs of Cer1[N2, LGIII] only by the absence of an adenosine at
position 1, and by a g347t substitution. The remaining differences from Cer1[N2, LGIII] are
a957g (5’UTR), t1175a (5’UTR), g5480t (Ribonuclease H domain, Trp to Leu), and t8262ttt
(3’UTR). During this analysis, we discovered that ED3046 has a second, unannotated Cer1 ele-
ment on LGIII, Cer1[ED3046, LGIII]. The second element is nearly identical to Cer1[ED3046,
LGX] except for an a7914c substitution that does not change the protein sequence. ED3046
was outcrossed into the Hawaiian wild strain CB4856 which lacks Cer1; expression was
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
monitored by immunostaining for GAG. CRISPR gene editing was used to tag the N-terminus
of CERV, and Cer1[ED3046, LGIII; GFP-CERV] was separated from Cer1[ED3046, LGX] by
outcrossing 10X against N2 to create strain JJ2669. A strain containing both tagged and wild-
type copies of CERV on LGIII was created by crossing JJ2669 into glo-2(zu455); Cer1[N2,
LGIII]unc-32(e189) hermaphrodites and scoring the homozygous Unc F2 progeny for GFP
expression. This strain was backcrossed an additional ten times using N2 males to create the
strain JJ2698 used in this study.
Immunostaining and smFISH
Standard procedures for worm dissection, fixation, and mounting were as described [69,152].
Most images were acquired with a DeltaVision microscope and processed using deconvolution
software (GE Healthcare). Images were exported to Adobe Photoshop for contrast/brightness
adjustments and cropping. Other images were collected with a spinning disk confocal micro-
scope [Hamamatsu C9100 camera, Nikon TE2000 inverted microscope, Yokogawa CSU-10
spinning disk] using image acquisition software (Volocity 5.3.3, Improvision). Orthogonal
projections of optical Z-stacks were generated and analyzed either with Volocity software or
with ImageJ.
Cy5- and Cy3-labeled oligo probes for Cer1 gRNA and gfp are listed in S2 Table (Integrated
DNA Technologies). For smFISH plus immunostaining, worms were collected in 30 ul of M9
buffer on taped slides [152] then rinsed in M9 buffer as needed to remove residual bacteria.
The M9 buffer was removed and replaced with 30 μl of gonad buffer [48% Leibovitz L-15
(GIBCO), 9.7% Fetal Calf Serum (GIBCO), 1% sucrose, 2 mM MgCl2; adjustments to the
osmolality were necessary with different stocks of Fetal Calf Serum, and determined by exam-
ining live, dissected gonads under a compound microscope for abnormal shrinkage or swell-
ing. Worms in general were dissected with a single cut below the pharynx to release the gonad.
In experiments comparing staining intensities in two groups of worms, one group was dis-
sected near pharynx and mixed with a second group which was dissected near the tail. After
dissection, an equal volume was added to the drop of 2X fix [5% formaldehyde (Sigma), 25
mM HEPES (7.4), 40 mM NaCl, 5 mM KCL, 2 mM MgCl2]. The tissues were fixed for 15
mins, then rinsed with RNase-free 0.2M Tris (7.5) for two changes of 1 min then 10 min. The
rinse buffer was exchanged with 0.3% Triton X100 in RNase-free PBS (Fisher) and incubated
for 15 mins; during this period the worms were further dissected to isolate the gonads and dis-
card other body parts. The detergent solution was removed and replaced with RNase-free PBS
for three change, 2 mins each. About 75% of the PBS rinse was removed and replaced with
hybridization wash buffer [10% deionized, nuclease-free formamide (Fisher), 2X RNAse-free
SSC (Saline-Sodium Citrate, Fisher)] for 2 mins; this solution was removed and replaced with
hybridization wash buffer for 10 mins at room temperature. The hybridization wash buffer
was removed and replaced with probe mix containing 10% formamide and 90% Stellaris RNA
FISH Hybridization Buffer (Stellaris, SMF-HB1) for 4 hours at 37˚C in a sealed, humidified
chamber. At the completion of hybridization, the gonads were rinsed with several changes of
RNAase-free PBS over a total of 10 minutes at room temperature. The PBS was removed and
replaced with 50 mM DTT (Dithiothreitol, Sigma) in RNAase-free PBS, then incubated for 30
mins at 37˚C in a sealed, humidified chamber. The DTT solution was removed, and the gonads
were washed with three brief changes of PBS and incubated in PBS for 10 mins at room tem-
perature. All subsequent steps were as for standard immunofluorescence (above), expect that
RNAse-free PBS was used for all washes and to dilute antibodies, and all incubation solutions
included 20 units of RNasin Plus (Promega) per 50ul of total solution. After immunostaining,
the sample was rinsed in PBS and stained with 1μg/mL DAPI in ddH2O for 10 min, then
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
mounted in Vectashield mounting media (Vector laboratories) and covered with a No 1.5 cov-
erslip (VWR).
TEM and APEX2 staining
N-terminal tags on CERV impair localization and function in homozygous animals, but
appear to be localized normally in heterozygous strains with untagged CERV (see S7 Fig).
Hence, for the APEX2 analysis we tagged the N-terminus of CERV in Cer1[N2, LGIII], and
crossed this strain with JJ2698 described above; GFP-CERV was included in the hybrid for
pre-selecting animals with the largest numbers of CERV rods. The APEX2-CERV strain was
compared with a control heterozygous strain, Cer1[N2, LGIII]/Cer1[N2, LGIII; GFP-CERV].
Worms were dissected as above in gonad buffer [48% Leibovitz L- 15 (GIBCO), 9.7% Fetal
Calf Serum (GIBCO), 1% sucrose, 2 mM MgCl2]. After dissection an equal volume was added
of freshly prepared fixative [2.0% glutaraldehyde (Electron Microscopy Sciences, EMS), 50mM
cacodylate (pH 7.4), 2mM CaCL2) for 2 min, then replaced with ice-cold fixative for 1 hour.
Processing for APEX2 staining was essentially as described [80] with minor modifications.
The fixative was removed, and the sample rinsed with three quick changes of ice-cold 50 mM
Cacodylate (7.4), 2mm CaCl2, then incubated in ice-cold 50 mM cacodylate (7.4), 2 mM
CaCl2, 20 mM Glycine for 5 mins on ice. The sample was rinsed four times for 2, 2, 2, and 10
mins in ice-cold 50 mM Cacodylate (7.4), 2 mM CaCl2. The rinse solution was removed and
replaced with an ice-cold fresh solution of DAB/H2O2 [500 ul of a frozen/thawed 10X DAB
stock (prepared in advance by dissolving 50 mg diaminobenzidine in 10 ml 0.1 N HCL); 1.7
ml 150 mM cacodylate (7.4), 6mM CaCl2; 2.8 ml H20; and 5 ul of 30% (wt/wt) hydrogen per-
oxide]. After a 6–9 min incubation (sufficient to visualize faint, nuclear spots by light micros-
copy) the DAB/H2O2 solution was removed and the sample was rinsed three times for 1, 1,
and 10 mins with ice-cold 50 mM cacodylate (7.4). The sample was then postfixed/stained
with ice-cold 1% osmium (Electron Microscopy Sciences) in 50mM cacodylate (pH 7.4) for 30
mins, then rinsed several times in ddH2O at room temperature. The gonads were then col-
lected, encased in agar and embedded as described [152]. The same procedures but without
DAB/H2O2 were used for the control sample without the APEX2 tag.
CRISPR/Cas9 genome editing and transgene construction
The Cas9 ribonucleoprotein (RNP) CRISPR strategy [153] was used for genome editing; guide
and donor RNAs are listed in S3 Table. Plasmid pRF4 containing rol-6 (su1006) was used as
co-injection marker. For short insertions like FLAG and deletion mutations, synthesized single
strand DNAs were used as the donor; for long insertions like GFP and APEX2 the PCR prod-
ucts were used instead.
Immunoprecipitation
About 500,000 synchronized adult worms of indicated strains were homogenized by FastPrep-
24 (MP Biomedicals) in lysis buffer [50mM Tris-HCl(7.5), 150 mM NaCl, 1% TRITON X-100,
1 mM EDTA, supplemented with 1 mM PMSF and 1 tablet of cOmplete Protease inhibitor
(Roche) per 50 ml]. About 500 μg of cleared worm lysates were incubated with FLAG magnetic
beads (Pierce) at 15˚C for 1 hour. Beads were washed three times with TBST and treated with-
out or with Lambda Protein Phosphatase (New England Biolabs) according to the manufactur-
er’s instruction.
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
Western blot
Worms were lysed with NuPAGE LDS Sample Buffer (Thermo Fisher Scientific), and each
lysate sample was loaded into a polyacrylamide gel and separated by electrophoresis followed
by transfer onto PVDF membranes (Millipore). Membranes were blocked for 20 min at room
temperature with StartingBlock Blocking Buffer (Thermo Fisher Scientific) then incubated
with primary antibodies overnight at 4˚C. Blots were developed using a Luminata Crescendo
Western HRP substrate (Millipore).
Supporting information
S1 Table. List of strains used in this paper.
(DOCX)
S2 Table. Oligo probes used for smFISH hybridization.
(DOCX)
S3 Table. Oligo sequences used for CRISPR gene constructions.
(DOCX)
S1 Data. Individual Tables show measurements for CERV rod diameters (Fig 5), CERV
rods per gonad (Fig 5C), and fibril lengths measured by TEM (S9 Fig).
(XLSX)
S1 Video. Movie of live, dissected gonad showing Cer1 Protease:GFP (strain JJ2506; GFP
inserted after Q1031), which is incorporated into GAG particles. The movie represents an
elapsed time of 10 mins and shows the transition as germ cells move between the between the
mid-pachytene and post-pachytene regions. Most GAG particles in the mid-pachytene region
are in large aggregates that show relatively little movement when imaged for as long as 40 min-
utes. By contrast, GAG particles in the post-pachytene region are highly mobile, with streams
of particles moving toward and around nuclei (inset).
(MOV)
S2 Video. The video shows successive 0.5-micron optical Z-planes through the pachytene
region of an A5 wild-type gonad. The gonad is stained for CERV (green) and FIB-1/fibrillarin
(red). Note that FIB-1 is localized near the periphery of most nucleoli, by contrast with the
appearance of nucleoli in younger gonads (compare Fig 6D).
(MOV)
S3 Video. The video shows successive 1.0-micron optical Z-planes through the pachytene
region of a A6 daf-2(e1370) gonad; CERV is green and DAPI-stained DNA is white. The
first image is a Z-projection of the entire stack.
(MOV)
S1 Fig. Alignment of Cer1 polyproteins in Caenorhabditis species; amino acids 1–445. The
diagram at top summarizes protein domains in Cer1 as described previously [21,24] and
extended here (see below), plus an additional capsid-like (CA-like) domain in GAG. M and G
subdomains of CERV are indicated and described in the text. The sizes shown for the various
POL domains were updated as per InterPro protein family classifications (http://www.ebi.ac.
uk/interpro/) [133]: protease (PR, aa735-859, IPR021109), reverse transcriptase (RT, aa1052-
1231, IPR000477 plus 1241–1326, IPR043128), and ribonuclease H (RH, aa1327-1429,
IPR041373). A previously described integrase (IN) domain in Cer1 (approximately aa1546-
1603, IPR041588 plus aa1616-1774, IPR001584) was extended here to aa1873 based on struc-
tural prediction: AlphaFold and ColabFold [48,49] were used to generate a structural model
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
for the extended region, which was compared with structures in the Protein Database (PDB)
using the Dali server [51]. This analysis showed that the extension had structural similarity to
integrase proteins from retroviruses such as Foamy Virus (PDB ID: 5frn-A; Dali Z score 6.8)
and Rous Sarcoma Virus (PDB ID:1c0m-B; Dali Z score 5.4). The sequence alignment com-
pares GAG and CERV regions of Cer1 in C.e (elegans, N2 strain) with the corresponding
regions in Cer1 elements present in five diverse, male-female species of Caenorhabditis: C.n
(nigoni, JU1422 [134]); C.r (remanei, PX506 [135]); C.z (zanzibari, JU2190 [136]); C.l (latens,
PX534 [137]); C.i (inopinata, TK-2017 [138]). Cer1 elements were identified from public data-
bases as sequences that (1) had significant homology to the POL domain of C.e Cer1, (2) were
flanked by direct repeats >100 base pairs, and (3) had a predicted tRNA-Pro primer binding
sequence (TGGGGGCCG) adjacent to the 5’LTR, as is characteristic of the Cer1 family [24].
Cer1 chromosomal insertion sites, or Genbank identifiers for unassembled contigs, were as
follows: C.n (insertion at LGX:20,441,949), C.r (Genbank: WUAV01000020.1), C.z (Genbank:
UNPC02004551), C.l (Genbank: NIPN02000054.1), C.i (insertion at LGI:17980975). The com-
plete alignment including the POL regions was used to generate the summary plot in Fig 2A.
C. elegans CERV is the product of a spliced, five exon mRNA; exons 1–3 and exon 5 have the
same reading frame as the CER1 polyprotein, and exon 4 has a different reading frame as
described [24]. Splicing of CERV has not been examined in other Caenorhabditis species, but
nucleotide alignments for each of the listed species showed consensus splice acceptor
sequences similar to those in C. elegans preceding the predicted initiator ATG, and near each
of the known exon boundaries. For example, the experimentally verified splice acceptor for
exon 5 in C. elegans is TTTCAG [24], and the aligned sequences had either TTTCAG (C.n, C.r,
C.z and C.i) or TTGCAG (C.l). The N-terminal half of CERV shares some peptide sequences
with GAG, and includes a predicted Leucine-zipper dimerization motif (LKEKSEELMQKS-
QILVETTLKL; see also S4 Fig) and a monopartite nuclear localization sequence (KRKK; con-
sensus K-(K/R)-X-(K/R). GAG contains a predicted Leucine-rich nuclear export signal
440LNALANRLRL449 in the form L-x(2,3)-[LIVFM]-x(2,3)-L-x-[LI] that is removed in the
spliced CERV product [139].
(TIF)
S2 Fig. Alignment of Cer1 polyproteins in Caenorhabditis species; amino acids 446–766.
(TIF)
S3 Fig. Alignment of Cer1 polyproteins in Caenorhabditis species; amino acids 1871–2272.
A previous report described limited amino acid similarities between the ORF encoded by
CERVCTD and the Env protein of Drosophila Gypsy (Genbank AAK52059.1), and suggested
that the ORF might be an envelope protein [19]. Using Clustal Omega [140], we identified 79/
517 amino acids in the complete CERV protein that were shared with Gypsy Env, but only
nine of these were present in three or more of the Caenorhabditis Cer1 elements. Those resi-
dues, numbered relative to the Cer1 polyprotein/CERV, are R1949/194, F1957/202, I1979/224,
E2033/278, W2036/281, L2048/293, I2055/300, I2174/419, and Y2199/444.
(TIF)
S4 Fig. Structural models for Cer1 GAG and CERV. A. Diagram comparing sizes and
domains of GAG proteins from Ty3 [45]; HIV-1 [46]; and RSV [47] with the predicted GAG
region of Cer1. For reference below, the CERVNTD is aligned with Cer1 GAG to show the posi-
tion of in-frame peptides common to both (exons 1–3) and the unique peptide (exon 4). Some
retroviral GAG proteins contain short, disordered regions between MA, CA, and NC that
might function in particle morphogenesis or GAG-RNA interactions, but which are removed
from mature virions [141]. Cer1 GAG is predicted to have three intrinsically disordered
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
regions acids (grey boxes: aa1-31, 263–358, 564–630) as scored by IUPRED server (https://
iupred3.elte.hu/) using "long disorder" parameters [142], but it has not been determined
whether these represent processing sites. B. Structural prediction for Cer1 GAG residues 387–
546 generated with AlphaFold and colored as per the AlphaFold pLDDT table, a per-residue
estimate of confidence on a scale of 0–100 [131]. The DALI server was used to compare this
model against experimentally determined structures in the PDB as described for S1 Fig. The
model showed the highest similarity to CA proteins from diverse retroviruses and endogenous
LTR retrotransposons, and particularly the C-terminal half of CA which mediates subunit
multimerization [143]. CA domains from RSV (PDB:3g29) and PEG10 (PDB:7lga) are shown
in the middle panel for comparison, along with a superimposition of those structures with the
Cer1 model (ChimeraX Matchmaker [144]). The table shows quantitative data for representa-
tive structural alignments with DALI Z-scores and root-mean-square deviations (RMSD)
from backbone in Angstroms; DALI Z-scores above 2 usually correspond to similar folds
[145]. C. AlphaFold structural prediction for the large region of Cer1 GAG preceding the CA-
like domain, colored as per the pLDDT table [131]. The model shows three long, anti-parallel
alpha-helices, the longest of which contains a predicted leucine zipper (LZ). The red brackets
mark the boundaries of the three peptides (e1-e3) that are spliced in-frame to make the
CERVNTD. The model was searched against the PDB as above, but did not appear to have sig-
nificant similarity to known proteins, including retroviral MA proteins. D. AlphaFold struc-
tural prediction for CERV. CERV residues here and below are numbered with respect to the
CERV protein, rather than the entire Cer1 polyprotein. The complete CERV protein (aa1-517)
was used for the structural prediction, but for clarity the terminal unstructured regions (1–20
and 470–517) are hidden. Red bars correspond to exon boundaries. The H domain consists of
long, anti-parallel alpha-helices shared in part with GAG, and does not appear to have signifi-
cant similarity to known proteins. The G and M domains are described below and in S6 Fig. E.
The AlphaFold structural prediction for the G domain of C.e CERV is shown at top left, col-
ored arbitrarily to indicate alpha-helices (salmon) and beta-strands (green). Highly similar
structural models for the G domain were obtained in independent modeling for each of the
Caenorhabditis species aligned in S1 Fig, shown here at smaller scale for C. zanzibari and C.
inopinata. The G domain consists of five alpha helices (α1-α5) surrounding a five-stranded
parallel β sheet; α1 and α5 are on one side of the sheet, and α2–4 are on the opposite side. This
basic structure is termed a Rossmann-type fold, and is a common structural motif in nucleo-
tide-binding proteins [146]. Most of those proteins have conserved motifs associated with
ATP or GTP hydrolysis, such as the Walker A motif or P-loop (phosphate binding loop) [147],
but some structurally similar proteins of unknown function have been identified, such as pseu-
doGTPases [148]. The G domain of CERV lacks critical residues in the P-loop and other motifs
(bottom right), indicating that the G domain is not predicted to function in GTP hydrolysis.
The DALI server was used as above to search for structures in the PDB with similarity to the G
domain, and matches were found to numerous and diverse nucleotide-binding proteins, here
shown for human Rag GTPase (PDB: 6wj2-F; DALI Z-score 9.2). The G domain also showed
structural similarity to a few proteins that are not predicted to be NTP-binding proteins. One
of these, TSR1 (PDB: 5wwn-A; DALI Z-score 6.2), is a pre-rRNA-processing protein that
appears to be an inactive, structural mimic of a GTPase [149]. A second example is the TIR
domain of the plant Arabidopsis immune receptor RRS1 (PDB: 4c6t-A; DALI Z-score 9.7).
The TIR (Toll-interleukin-1 receptor) domain is thought to mediate protein heterodimeriza-
tion in response to pathogen infection [150].
(TIF)
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
S5 Fig. Cer1 gRNA persistence after RNAi. For each experiment in this figure, worms were
grown on empty-vector (control) bacteria until the stage indicated in brackets (see Fig 1A for
staging). The worms were either processed immediately, transferred to a second plate of con-
trol bacteria, or transferred to a culture plate of bacteria with a dsRNA insert specific for the
target gene. For example, "[A1] plus 6 hrs gfp(RNAi)" means that staged A1 animals on control
bacteria were transferred to gfp(RNAi) feeding plates for 6 hrs before processing. Sets of 30–40
gonads were analyzed for each experiment; the panels show either single, representative
images for a time point, or show multiple images at the same timepoint where the results var-
ied appreciably. A. Comparison of gfp-cerv mRNA expression (strain WM638) in a control
worm without RNAi and in a worm treated with gfp(RNAi) for 6 hrs; the gonads were hybrid-
ized with oligos specific for gfp (S3 Table). gfp(RNAi) markedly reduced the gfp-cerv mRNA
signal by 4 hrs, and most of the signal was gone by 6 hrs as shown; additional time points ana-
lyzed (8 hrs, 10 hrs) appeared similar to 6 hrs. In all RNAi experiments, many of the few
remaining cytoplasmic foci (arrows) were much larger than typical mRNA foci in control
gonads but were not analyzed further. WM638 has gfp inserted at the 5’ end of cerv; this inser-
tion disrupts CERV function such that gRNA is not exported (see S7 Fig). Thus, the gfp probe
is expected to recognize nuclear signals from both gfp:cerv and gfp-containing gRNA, but can
only recognize gfp:cerv in the cytoplasm. We conclude that RNAi is highly effective in remov-
ing cytoplasmic, spliced gfp:cerv mRNA, which is not associated with GAG. B. This experiment
addresses whether RNAi can block newly synthesized gRNA from accumulating in the cyto-
plasm. L4 wild-type larvae, which have little detectable cytoplasmic gRNA, were placed for the
indicated times on control bacteria, or on bacteria with a dsRNA insert specific for Cer1
reverse transcriptase [hereafter gRNA(RNAi)]. gRNA(RNAi) for 24 hrs caused a moderate
decrease in cytoplasmic gRNA compared to control gonads, but gRNA(RNAi) for 48 hrs
caused a much larger relative decrease. The projected images from the 48 hour control and
RNAi gonads were indexed and scored blindly as one of four classes [++++ / +++ / ++ / +]
based on the abundance of immunostained GAG particles. Control gonads were scored as
[37.8% / 47.2% / 15% / 0%, n = 36], and gRNA(RNAi) gonads were scored as [6.7% / 37.8% /
40.0% / and 15.5%, n = 45]. The images shown for the 48 hr control and RNAi experiment
(panel 2) represent the most common class for each (+++ and ++, respectively). Panel 3 is a
single optical plane of the post-pachytene region of a 48 hr gRNA(RNAi) gonad, and shows
that the few remaining gRNA foci colocalize with GAG; asterisks indicate nuclear foci of
gRNA. We conclude (1) that gRNA(RNAi) does not block newly synthesized gRNA from accu-
mulating, but substantially reduces the accumulation compared to controls without RNAi, and
(2) most of the cytoplasmic gRNA remaining after RNAi is associated with GAG. C. This
experiment addresses the effectiveness of gRNA(RNAi) in removing previously accumulated
gRNA. RNAi-sensitive wild-type animals were compared with control, RNAi-resistant rde-1
(ne300) mutant animals [151]. A1 adults, which have accumulated substantial amounts of
cytoplasmic gRNA (see Fig 2E) were placed on gRNA(RNAi) bacteria for 48 hrs before process-
ing. Gonads were analyzed by smFISH for gRNA, then photographed at identical exposures
(shown in black/white for better resolution). Single images representing 10-micron Z-projec-
tions were generated for each gonad, and the images of the wildtype and rde-1 gonads were
scored blindly for the abundance of gRNA [++++ / +++ / ++ / +] as above. The images depict
each of the four classes of gRNA abundance, and were taken from the RNAi-resistant rde-1
animals exposed to gRNA(RNAi). Classification of the rde-1 and WT gonads is quantified at
the bottom of the panel. The data suggests that gRNA(RNAi) lowers the level of gRNA in WT
gonads relative to RNAi-resistant rde-1 controls, but that a considerable amount gRNA
remains. D. This experiment compares wild-type animals which were selected at the A1 stage,
then divided into two populations which were fed for the indicated times with either empty-
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
vector control bacteria or gRNA(RNAi) bacteria. Note the persistence of gRNA after gRNA
(RNAi), and the colocalization of gRNA with GAG.
(TIF)
S6 Fig. Structural models of the CERV M domain. A. The model at left shows the AlphaFold
structural prediction for the M domain of the CERV protein in C. elegans Cer1, with pLDDT
coloring for confidence scores as above. The models at right show independent predictions
using AlphaFold for M domains from Cer1 elements in each of the five Caenorhabditis species
aligned in S3 Fig. Despite the low degree of sequence conservation in the M domain (S3 Fig),
the models are closely similar with the principal difference being short beta-strands present in
C.e, C.r, and C.l, but not in C.n, C.z, or C.i; models for each of the two groups are shown in
superimposition and colored arbitrarily. B. The two top images show octamer models for the
M domains of CERV from C.e and C.l, colored according to pLDDT scores. Space-filling mod-
els for each octamer (below) show the similar electrostatic potentials (red negative, blue posi-
tive; ChimeraX [132]). The M domains from each of the Caenorhabditis species are predicted
to form similar ring-shaped multimers; rings form with a minimum of 5 subunits, and rings
consisting of 5–8 subunits have high confidence scores. The inset at right is a high magnifica-
tion ribbon view of the indicated 3 subunits, shown with arbitrary colors. The lines between
the subunits represent residue contacts which are 3 Å or less and colored according to the
AlphaFold pLDDT confidence scale (ChimeraX [132]. The asterisk indicates the S214 phos-
phorylation site described in the text.
(TIF)
S7 Fig. N-terminal tags disrupt CERV function. A. Characterization of WM638, which has
gfp inserted at the shared 5’ terminus of cerv and gag. The strain was expected to make GFP:
CERV and GFP:GAG, but the live animals had very few GFP:GAG particles and did not make
GFP:CERV rods. Panel 1 shows that WM638 expresses abundant gfp-containing mRNA in the
core cytoplasm, as detected by smFISH with probes specific for gfp. By contrast, panel 2 shows
that a gRNA-specific probe does not detect gRNA in the WM638 core, and that GAG is not
expressed. Thus, the core contains spliced gfp:cerv mRNA, but does not contain gRNA (with
unspliced gfp). Additional experiments showed that the WM638 animals do not make CERV
rods. Panel 3 shows that immunostained CERV foci (red, mAbP3C6) are present in the
WM638 germ nuclei, and that nearly all of these foci are coincident with GFP:CERV (green,
anti-GFP). However, the WM638 nuclei usually do not have closely paired CERV foci as
found in wild-type nuclei, but instead have single foci or dispersed, multiple foci. Panel 4
shows a 6-micron Z-projection of a field of WM638 germ nuclei stained for gRNA (red) and
GFP (green). The inset at right shows that most of the GFP:CERV foci do not colocalize with
nuclear gRNA. These combined results suggest that the N-terminal GFP tag disrupts CERV
function such that gRNA is not exported and GFP:GAG is not expressed. B. Characterization
of WM638/WT heterozygotes. Images are from heterozygous animals generated by crossing
wild-type males into WM638 hermaphrodites. By contrast with the WM638 homozygotes,
both fixed and immunostained heterozygotes showed GFP:CERV localization in foci, streaks,
and rods (panel 1), and cytoplasmic gRNA was abundant in the core (panel 2). The heterozy-
gotes made abundant GFP:GAG particles which were visible in live animals and by staining
for GFP (panel 3), many of which colocalized with gRNA (panel 3). Thus, the heterozygotes
appear to incorporate the GFP-tagged CERV and GAG proteins into the correct structures,
presumably by association with the respective, untagged proteins. C. Characterization of
JJ2698. Because GFP:CERV must be combined with an untagged copy of CERV to localize
properly, this strain was built to be homozygous for both tagged and untagged copies of CERV
(see Methods for details) The strain has the N2 copy of Cer1 plus a linked copy of Cer1 derived
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
from the wild strain ED3046 in which CERV was tagged with GFP. Similar to WM638 hetero-
zygotes, the homozygous JJ2698 animals had nuclear foci of GFP which colocalized with
CERV and nuclear gRNA (panel 1, asterisks), they made GFP-containing GAG particles that
colocalized with cytoplasmic gRNA (panel 2, arrowheads), and they made GFP-containing
CERV rods (panel 3; 6-micron Z-projection).
(TIF)
S8 Fig. Stage-dependent changes in nuclear gRNA in wild-type and fog-2 mutant germ
cells. A. Age-dependent decrease in nuclear gRNA in wild-type hermaphrodites. The images
show pachytene germ nuclei in wild-type hermaphrodites at A1 or A4, stained for gRNA,
GAG, and CERV as indicated. Fixed gonads from each population were dissected to create a
population-specific identifying mark, then mixed together for immunostaining and photogra-
phy; each channel was imaged at the same exposure used for the wild-type A1 animals. Aster-
isks mark nuclear foci of gRNA, and arrowheads mark representative foci of cytoplasmic
gRNA; arrows indicate unusually bright foci of cytoplasmic gRNA in both samples that coloca-
lize with GAG and likely represent aggregates of capsids. The boxed regions are shown at
higher magnification in the insets at right, except that CERV is shown instead of GAG. Note
that the level of nuclear gRNA (asterisks) decreases appreciably by A4. In the low magnifica-
tion image of the A4 gonad, most germ nuclei appear to lack nuclear gRNA foci: This is an
artifact of the sectioning plane; gRNA signals are present but reduced, and are typically visible
in only a single optical plane. By contrast, the bright nuclear gRNA signals in the A1 gonad are
visible in several optical planes. B. Age-dependent changes and effect of mating on nuclear
gRNA in fog-2 females. gRNA, GAG, and CERV as indicated are compared for wild-type A1
animals, unmated fog-2 females from A1-A3, and an A4 fog-2 female that was mated at the A3
stage. Calibration of signal intensities was as described for S8A. The level of nuclear gRNA and
CERV (asterisks) in A1 fog-2 animals appears comparable to wildtype, but decreases markedly
by A3. However, mating for 24 hours restores the levels of nuclear gRNA and CERV. C.
Mated-induced increase in CERV rods in fog-2(q71) mutant. The images compare rods in
unmated A1 and A5 fog-2 mutant gonads with rods in an A5 fog-2 animal that was mated at
A4; see Fig 5C for quantification). Note the large increase in the size of the gonad between A1
and A5, indicating that the gonad does not arrest development in unmated fog-2 animals. D.
Rod variations in mated wild-type animals. Most rods in older mated animals resemble those
in younger animals, but with the following variations. The short arrow in panel 1 indicates a
rod with a diameter of 0.5 microns, which is similar in size to rods in younger, unmated ani-
mals (see Fig 5D). However, a small percentage of rods (3/112) have abnormally large diame-
ters; the long arrow indicates a rod with a diameter of 1.0 microns. Panel 2 shows an example
of a large, hemicylinder of CERV with gRNA shown in red; similar hemicylinders were found
in 3/318 germ nuclei. Panel 3 shows a group of germ nuclei with multiple rods. Panel 4 shows
a 3D optical projection of a germ nucleus with adjacent, parallel rods (arrowheads); "X’s" visi-
ble in the background are 3D reference marks. Similar double rods occur in less than 1% of
nuclei (1/112); see Fig 8B, panel 2 for TEM of a double rod. Note that the rods do not track the
S-shaped LGIII (dotted line; identified by the nuclear foci of gRNA). Bars = 2.0 microns.
(TIF)
S9 Fig. TEM of nuclear fibrils. A. Measurements of individual fibril lengths (n = 150) taken
from TEM micrographs; graphing with GraphPad Prism software version 9.5. B. High magni-
fication of three long fibrils (344, 477, and 497 nm); sample as in Fig 7D. Note the numerous
striations (arrows) which are approximately orthogonal to the long axis of the fibril. C. Images
of sequential, 70 nm thick Z-sections through a curved CERV rod. The perimeter of the rod
has an electron-lucent margin (white arrowheads), and electron-dense fibrils are near the
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PLOS GENETICSThe novel CERV protein of the C. elegans LTR retrotransposon Cer1 functions in the export of viral gRNA
perimeter and inside the rod. The arrows at 140nm and 210nm indicate a few electron-dense
fibrils which are associated with the nucleolus, but do not appear to have become incorporated
into the rod.
(TIF)
Acknowledgments
We thank Ujwal Sheth, who first noticed rods of unknown identify in some germ nuclei; Shan-
non Dennis for advice on Cer1 gene constructions and gift of the JJ2506 strain; Daniel Durn-
ing for advice on APEX2 staining protocols; Steve MacFarlane and Bobbie Schneider for
assistance with TEM; and Anne Villeneuve for advice on germ cell biology. We thank Paul
Davis and Stavros Diamantakis for providing raw data on CoDing Sequence (CDS) lengths in
C. elegans. Some strains were provided by the CGC, which is funded by NIH Office of
Research Infrastructure Programs (P40 OD010440).
Author Contributions
Conceptualization: Craig C. Mello, James R. Priess.
Funding acquisition: Craig C. Mello, James R. Priess.
Investigation: Bing Sun, Haram Kim, Craig C. Mello, James R. Priess.
Methodology: Bing Sun, Haram Kim, Craig C. Mello, James R. Priess.
Supervision: Craig C. Mello, James R. Priess.
Writing – original draft: Bing Sun, Haram Kim, Craig C. Mello, James R. Priess.
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| null |
10.1126_science.adg7883.pdf
|
Data and materials availability:
The cryo-EM map has been deposited in the Electron Microscopy Data Bank with accession
code EMD-40033. The coordinates for the atomic model have been deposited in the Protein
Data Bank with accession code 8GH6. The raw cryo-EM data have been deposited in
EMPIAR with accession code EMPIAR-11458.
|
Data and materials availability: The cryo-EM map has been deposited in the Electron Microscopy Data Bank with accession code EMD-40033. The coordinates for the atomic model have been deposited in the Protein Data Bank with accession code 8GH6. The raw cryo-EM data have been deposited in EMPIAR with accession code EMPIAR-11458.
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HHS Public Access
Author manuscript
Science. Author manuscript; available in PMC 2023 September 13.
Published in final edited form as:
Science. 2023 April 21; 380(6642): 301–308. doi:10.1126/science.adg7883.
Structure of the R2 non-LTR retrotransposon initiating target-
primed reverse transcription
Max E. Wilkinson1,2,3,4,5, Chris J. Frangieh1,2,3,4,5,6, Rhiannon K. Macrae1,2,3,4,5, Feng
Zhang1,2,3,4,5,*
1Howard Hughes Medical Institute; Cambridge, MA 02139, USA.
2Broad Institute of MIT and Harvard; Cambridge, MA 02142, USA.
3McGovern Institute for Brain Research, Massachusetts Institute of Technology; Cambridge, MA
02139, USA.
4Department of Brain and Cognitive Science, Massachusetts Institute of Technology; Cambridge,
MA 02139, USA.
5Department of Biological Engineering, Massachusetts Institute of Technology; Cambridge, MA
02139, USA.
6Department of Electrical Engineering and Computer Science, Massachusetts Institute of
Technology; Cambridge, MA 02139, USA.
Abstract
Non-LTR retrotransposons, or Long Interspersed Nuclear Elements (LINEs), are an abundant
class of eukaryotic transposons that insert into genomes by target-primed reverse transcription
(TPRT). During TPRT, a target DNA sequence is nicked and primes reverse transcription of the
retrotransposon RNA. Here, we report the cryo-electron microscopy structure of the Bombyx
mori R2 non-LTR retrotransposon initiating TPRT at its ribosomal DNA target. The target DNA
sequence is unwound at the insertion site and recognized by an upstream motif. An extension of
the reverse transcriptase (RT) domain recognizes the retrotransposon RNA and guides the 3′ end
into the RT active site to template reverse transcription. We used Cas9 to retarget R2 in vitro to
non-native sequences, suggesting future use as a reprogrammable RNA-based gene-insertion tool.
One-Sentence Summary:
A retrotransposon structure shows the principles of target DNA selection and self RNA
recognition.
This work is licensed under a Creative Commons Attribution 4.0 International License, which allows reusers to distribute, remix,
adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for
commercial use.
*Corresponding author. [email protected].
Author contributions: M.E.W. and F.Z. conceived the project. M.E.W. designed and performed experiments and solved the cryo-EM
structure. C.J.F. generated and analyzed sequencing data. F.Z. supervised the research and experimental design with support from
R.K.M. M.E.W. wrote the manuscript with input from all authors.
Competing interests: F.Z. is a scientific advisor and cofounder of Editas Medicine, Beam Therapeutics, Pairwise Plants, Arbor
Biotechnologies, and Aera Therapeutics. F.Z. is a scientific advisor for Octant.
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Non-long terminal repeat (non-LTR) retrotransposons are the most abundant class of mobile
genetic element (MGE) in the human genome, mostly represented by the LINE-1 and SINE
(or Alu) elements (1). Despite their prevalence and contribution to genetic diversity and
dysregulation through mutagenicity and recombination (1–3) and their prospective use as
gene insertion tools, there is much left to understand about their mobility mechanisms (4).
Pioneering research on the Bombyx mori (silk moth) R2 element (R2Bm), which selectively
inserts into the 28S rRNA gene, has contributed significantly to our understanding of
this type of MGE (5). R2, like all non-LTR retrotransposons, encodes an open reading
frame (ORF) with DNA-binding, endonuclease, and reverse transcriptase activities (Fig.
1A). The endonuclease domain (restriction-like endonuclease, RLE) nicks the target DNA,
and the reverse transcriptase domain uses the exposed 3’ end from the nick to prime
reverse transcription of the R2 RNA, resulting in a new genomic copy of the R2 element
(Fig. 1B) (6, 7). This process is called target-primed reverse transcription (TPRT), and
is characteristic of non-LTR retrotransposons and their group II intron ancestors (8, 9).
The nicked strand that primes reverse transcription is referred to as the bottom strand.
Complementarity between the bottom strand and the 3′ end of the R2 RNA (3′ homology)
is not required to initiate reverse transcription (10) Non-LTR retrotransposons are specific
for reverse transcribing their own RNA; for R2, this specificity requires an element in
the 3′UTR but the precise motif has not been located (11). It is also unclear how R2
specifically recognizes the 28S rRNA target gene, or how DNA nicking is coupled to reverse
transcription within the same protein. To address these questions, we solved a cryo-EM
structure of the Bombyx mori R2 protein (R2Bm) initiating TPRT at the 28S rRNA gene
using its own 3′UTR. The structure reveals an extensive interface with the target DNA, a
small core region of the 3′UTR required for TPRT, and shows that R2Bm can be engineered
to reprogram its insertion site.
Reconstitution and cryo-EM structure of an R2 TPRT complex
We overexpressed R2Bm in Escherichia coli and purified it to apparent homogeneity (fig.
S1). The purified protein was active in vitro, reproducing previously found biochemical
activities, including RNA-stimulated nicking of the target DNA bottom strand, site-specific
TPRT when supplied with in vitro transcribed 3′UTR RNA, and low levels of template
jumping (Fig. 1C) (6, 12). It is unclear if 3′ homology is required for TPRT in vivo;
however, consistent with previous findings, we found that downstream sequences up to 10
nt do not inhibit activity in vitro (Fig. 1C) (10). Sequencing of TPRT junctions confirmed
that homology-mediated TPRT is more likely to initiate reverse transcription at the 3′ end of
the 3′ UTR rather than skipping bases or inserting untemplated nucleotides (fig. S2). (10).
To assemble a complex stalled during initiation of TPRT, we incubated R2Bm with target
DNA, 3′UTR RNA, and the chain-terminator nucleotide 2′,3′-dideoxythymidine (ddT),
which mimics the first nucleotide incorporated in the TPRT reaction (dT) but does not allow
further elongation. Purified TPRT complexes contained stoichiometric amounts of R2Bm,
3′UTR RNA, and target DNA with > 99% of the bottom strand nicked (fig. S1). Initial
attempts at cryo-EM imaging failed due to the preferred orientation and flexibility of the
complex. To overcome these issues, we used a carbon support on the cryo-EM grid and
added 5 nt of downstream 28S RNA sequence to the 3′ end of the 3′UTR RNA to stabilize
the complex by forming a primer-template duplex with the target DNA bottom strand. With
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these modifications, we obtained a cryo-EM reconstruction of the R2 TPRT complex at 3.1
Å resolution (Fig. 1D, fig. S3–4, table S1).
The core of the R2Bm protein is a reverse-transcriptase domain (RT) similar to group
II intron RTs (13), followed by a C-terminal ɑ-helical thumb domain and preceded by a
characteristic N-terminal extension domain (NTE0) implicated in template switching (14),
but the R2Bm RT includes a further N-terminal extension (NTE-1) that binds the 3′UTR
RNA (Fig. 1E, F) (15). Preceding the NTE-1 element are two DNA binding domains: the
N-terminal C2H2 zinc finger domain (N-ZnF) and a Myb domain. C-terminal to the thumb
domain lies an ɑ-helical linker domain that packs against the thumb, followed by a CCHC
zinc-finger domain (ZnF) conserved in many LINE ORFs (4). The ZnF then links to the
C-terminal RLE domain, which cleaves the target DNA. This domain arrangement closely
resembles Prp8 (13, 16, 17), the core protein of the spliceosome, underscoring the close
relationship between pre-mRNA splicing and retrotransposons.
There are several key interactions between the R2Bm protein, 3′UTR RNA, and target DNA
(Fig. 1E, F). The two strands of the target DNA separate around the ZnF domain, with
the bottom strand feeding into the RLE active site where the scissile phosphate remains
bound, while the top strand snakes along the opposing surface of the RLE. The RT active
site contains a heteroduplex formed by the nicked bottom strand of the target DNA (5′ to
the cleavage site) and the 5 nt of 28S RNA homology extension beyond the 3′UTR RNA
(Fig. 1G). This target heteroduplex is surrounded by residues important for RT activity (18),
and the cryo-EM density shows incorporation of the ddT chain terminator nucleotide into
the bottom strand (Fig. 1H). The 5′ end of the bottom strand remains base-paired to the top
strand as it leaves the RLE, and this downstream DNA region has weak cryo-EM density,
suggesting it is not tightly bound by R2Bm. The 248-nt 3′UTR RNA is mostly not resolved
in the cryo-EM density except for a core 40-nt region, which wraps around the NTE-1 ɑ
helix of R2Bm and the 3′ end of which is guided into the RT active site via the NTE0
domain.
R2Bm recognizes a sequence motif upstream of the cleavage site
The target 28S DNA sequence has extensive interactions with R2Bm (summarized in Fig.
2A). Upstream bases from –38 to –7 and downstream bases from +6 to +21 are respectively
paired, whereas the 11 base pairs from –6 to +5 are melted around the RLE domain (bases
are numbered relative to the bottom strand cleavage site). The upstream DNA has a 40° bend
and binds along the surface of the RT, linker, and thumb domains in a manner similar to
the DNA in a recent group IIC intron maturase structure (Fig. 2B, fig. S5, S6) (19). Many
of the contacts between R2Bm and the DNA are via the phosphate backbone, suggesting
that they are not sequence-specific. Based on the structure, however, we predicted that
two regions are key for sequence-specific DNA recognition by R2Bm: a 13-bp upstream
motif from –34 to –22, which is bound by the N-terminal N-ZnF and Myb domains, and
the 7 bp from –6 to +1, which are bound by the RLE (Fig. 2A). We term these regions
the Retrotransposon Upstream Motif (RUM) and Retrotransposon-Associated INsertion site
(RASIN), respectively.
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Consistent with the importance of the RUM region for R2 activity, mutating the entire
upstream sequence between –38 to –7 eliminated bottom strand cleavage, whereas mutating
the downstream sequences between +6 and + 37 preserved wild-type levels of bottom strand
cleavage and TPRT (Fig. 2C) (20). Adding just the 13-bp RUM region to the upstream
mutant at positions –34 to –22 restored near-wild-type activity, whereas a point mutant
RUM (G–27 to C) did not rescue activity (Fig. 2C). This region of the target was strongly
protected in a previous DNase footprinting assay (21). To systematically determine the
importance of each base within the RUM, we performed an R2 cleavage assay on a DNA
target with the upstream region (–38 to –7) mutated and the RUM (–34 to –22) replaced with
a 13N library (Fig. 2D). Sequencing of cleaved targets revealed a consensus RUM sequence
A–31WWWGCNNNA–22, where W is A/T and N is any nucleotide, with minor preferences
in other positions (Fig. 2E). This consensus is a close match to the wild-type 28S sequence
A–31ACGGCGGGA–22 , with the differences underlined.
The RUM is recognized by three domains: N-ZnF, Myb, and an R2-specific insertion ‘6a’
in the RT domain between motifs 6 and 7 (Fig. 2B, fig. S7). The N-ZnF has the classical
C2H2 fold with a zinc ion coordinated between an ɑ-helix and a β-hairpin, but unusually
the ɑ-helix binds in the widened minor groove of the DNA from bases –18 to –23 instead
of the typical major groove (Fig. 2F, fig. S6) (22). The preference for A at base –22 in the
RUM is likely due to N-ZnF Arg125, which hydrogen bonds with the minor-groove–facing
side of the A–T base pair (Fig. 2F). The Myb domain forms a typical three-helix bundle,
with the third helix bound in the major groove from bases –31 to –34 (22) while its linker to
N-ZnF engages with base –30 (Fig. 2G). This is reminiscent of other Myb–DNA structures,
including telomere-interacting protein Rap1 (23). The Myb domain recognizes the A at base
–31 via hydrogen bonds with Lys149 (Fig. 2G). Although Arg198 contacts bases at positions
–33 and –34, these contacts appear not to be sequence specific, as the RUM screen showed
only weak sequence preferences in this region (Fig. 2E, G). Deletion of the N-ZnF and
Myb domains together (ΔN mutant) completely inhibits target DNA nicking and subsequent
TPRT (Fig. 2C) (20). The central GC of the RUM is recognized by His673 and Lys675
of the loop 6a of the RT domain (Fig. 2H). Structural predictions suggest that this loop
is unique among non-LTR RT domains to R2 proteins (fig. S7). We found that deletion
of the 6a loop inhibits target DNA nicking (Fig. 2C). Finally, we found that the distance
between the RUM and the bottom strand cleavage site (the RASIN) is important: increasing
the distance by one base was tolerated, but further increase or any decrease to the distance
inhibited target cleavage (Fig. 2I).
Target DNA interactions at the cleavage and integration site
The second key region for DNA target recognition by R2Bm is the target site for nicking
by the RLE domain and R2 insertion, which we term the RASIN. In our structure, the 11
base pairs of the RASIN from –6 to +5 are melted around the RLE domain. The ZnF appears
to act as the “zip,” stacking on the last upstream pair C–G(–7) with Arg922 and Arg924
and holding unzipped strands apart (Fig. 3A). Strand melting may be enhanced by the 40°
bend in target DNA around the RUM (Fig. 1F). Bases –6 to –1 on the bottom strand then
follow a cleft between the ZnF and the RLE, which adopts a canonical PD-(D/E)xK-family
nuclease fold, but with the characteristic Lys1026 on an ɑ helix instead of the usual β strand
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(Fig. 3B) (24). This lysine, along with catalytic residues Asp996 and Asp1009, are 4 – 6 Å
from the scissile phosphate of C(–1), suggesting C(–1) may be close to its position during
catalysis of bottom strand cleavage. On the top strand, bases –6 to +2 all make extensive
contacts along a cleft between the RLE and linker domains, except for A(–4), which flips
out and contacts C126 of the 3′UTR (Fig. 3C). To determine the relative importance of
the bases in the RASIN, we mutated each of the 11 bp individually and tested the effect
on bottom strand cleavage. Mutating T(+1) to A abolished cleavage entirely, and mutating
T(–6), T(–5), and A(–3) severely decreased activity, whereas other changes were tolerated
(Fig. 3D). This suggests the following RASIN motif for cleavage, given in top strand sense:
T–6TNANNT+1.
Because only the bottom strand of the RASIN enters the RLE active site, we tested the
activity of R2Bm on a single-stranded DNA with the bottom strand sequence and found
that it was cut, albeit weakly (Fig. 3E). Endonuclease activity was strongly stimulated by
providing a 60-nt top strand spanning the RASIN and upstream and downstream sequences,
but was similarly stimulated by a 32-nt top strand complementary only to the upstream
region containing the RUM. A 17-nt top strand complementary to the downstream sequence
did not stimulate activity (Fig. 3E). This suggests that the RUM in a double-stranded
state is important for recruiting the R2Bm RLE to the RASIN bottom strand, and that
the top strand of the RASIN, despite its extensive interaction with R2Bm, is dispensable
for specific bottom strand cleavage. However, when we added deoxynucleotides to these
reactions, TPRT activity was eliminated in the absence of the top strand from the RASIN
downstream but was partially rescued if the 3′UTR RNA contained 3′ homology to the
target site (Fig. 3E). The top strand RASIN bases A(–4), A(–3), and G(–2) are grasped
by Arg901 and Asp902 of the R2Bm linker (Fig. 3C). We mutated these two residues to
alanine and tested TPRT activity on a fully double-stranded substrate, and found that TPRT
activity was reduced and partially rescued by 3′ homology (Fig. 3E). These results suggest
two important factors for initiating TPRT when the 3′UTR RNA lacks 3′ homology. One:
presence of a top strand downstream of the RASIN, which may help retain the nicked
bottom strand, and two: contacts between R2Bm and the top strand RASIN, which help the
nicked bottom strand “pivot” into the RT active site.
R2Bm binds a small core region of the 3′UTR
R2Bm can only initiate TPRT on RNAs containing the R2 3′UTR (self-specificity), but
the molecular basis for this is not known (25). Multiple models have been proposed for
the secondary structure of the R2 3′UTR, and the divergent sequences of R2 RNAs have
hindered identification of key bases (26, 27). A model for the R2 3′UTR secondary structure
based on chemical probing is shown in Fig. 4A and has at least 11 stems (26). In our
cryo-EM map, we resolved density for two stems and their flanking single-stranded regions
(Fig. 4B). Based on nomenclature commonly used for structured RNAs, we name these
stems P1 (nucleotides 33 – 38 and 120 – 135) and P2 (nucleotides 131 – 137 and 236 – 242),
and term the single-stranded junction between P1 and P2 as J1/2 and the single-stranded
region preceding P1 as J0/1. The rest of the 3′UTR may occupy a diffuse cloud of cryo-EM
density next to these core regions (Fig. 4C).
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P1 and J1/2 are mainly recognized by an ɑ helix from the R2Bm NTE-1 domain, which
packs into the major groove of P1 and is wrapped by J1/2 (Fig. 4B). Arg307 recognizes
the Hoogsteen edge of P1 G33, and the interaction is secured by Arg310 and Arg311.
Consistently, these residues were previously shown to be essential for RNA binding (15),
and the first 45 bases of the 3′UTR are essential for TPRT activity (11). J1/2 makes
numerous sequence-specific contacts (Fig. 4D): A127 forms a sugar-edge pair with the
Watson-Crick face of J0/1 A32 , A128 hydrogen bonds to Leu732 and Lys733 of the R2Bm
thumb domain and stacks on NTE-1 Tyr314, U129 hydrogen bonds to Glu319 and Lys322
of NTE-1, and C126 stacks on and hydrogen bonds with A(−4) from the top strand of the
DNA target (Fig. 4B, D).
To test if regions of the R2 3′UTR not clearly visible in the cryo-EM density are required
for TPRT activity, we designed a 43-nt minimal 3′UTR – “R2 tag” – that contains only
the sequences visible in the cryo-EM density, linked by tetraloops (Fig. 4E). The R2 tag
was reverse transcribed as efficiently as the full 248-nt 3′UTR in a TPRT reaction. We
tested the importance of the J1/2 linker by making single base transversions and found that
A127U reduced activity and A128U almost completely abolished TPRT activity (Fig. 4F).
Mutating G33 to C to disrupt base pairing at the bottom of stem P1 also reduced activity
but could be rescued by the compensatory C125G mutation (Fig. 4F). Mutation of J0/1 A32
to G reduced activity, but mutations to C or U were tolerated. Equivalents to P1, P2, J0/1,
and J1/2 can be identified in the secondary structures of diverse R2 elements (26) (fig. S7).
The P1 and P2 stems have different sizes and base compositions, but positions 2 and 3 of
J1/2, corresponding to A127 and A128, are conserved as adenosines, consistent with their
importance for TPRT.
Because the R2 tag alone is efficiently integrated in a TPRT reaction, we tested if adding
the R2 tag to the 3′ end of a “cargo” RNA would allow its integration at the 28S target site.
We added the R2 tag to the 3′ end of a 239-nt CMV promoter RNA. This tagged RNA was
used as efficiently as wild-type R2 3′UTR in a TPRT reaction, whereas an untagged RNA
was not used, nor was an RNA tagged with R2-tag A128U mutant (Fig. 4G). A larger RNA
containing the 720-nt coding sequence for GFP and a 3′ R2 tag was also reverse transcribed
in a TPRT reaction (Fig. 4G).
R2Bm can be retargeted with CRISPR-Cas9
Our structural and biochemical observations suggest a multi-step model for initiation of
TPRT: the R2Bm N-terminal domains first detect a RUM sequence, followed by cleavage of
the bottom strand at the RASIN site, possible pivoting of the nick around the top strand into
the RT active site, annealing of any 3′ homology to the nicked bottom strand, and finally
initiation of reverse transcription (Fig. 5A). This model implies that R2Bm could prime
reverse transcription off an exogenously nicked bottom strand close to the R2Bm binding
site (Fig. 5B). To test this, we replaced the RASIN and downstream sequences of the 28S
DNA target with an unrelated sequence containing an efficient SpCas9 target sequence, but
kept the RUM sequence to anchor R2Bm (Fig. 5B). This substrate could not be cleaved
by R2Bm, but was nicked efficiently by a SpCas9 H840A nickase mutant (Fig. 5C). When
SpCas9 and R2Bm were added together with a single-guide RNA (sgRNA) and an R2
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3′UTR RNA with 5 nt of 3′ homology to the sgRNA nick site, we detected low amounts of
TPRT activity. This activity was enhanced when the R2Bm and SpCas9 proteins were fused
with a 33XTEN flexible linker (Fig. 5C). The RUM was not required for Cas9-directed
TPRT, as mutating the RUM did not reduce activity (Fig. 5C). This suggests Cas9 might
be able to direct R2Bm to perform TPRT at loci other than the 28S target. We mixed the
R2Bm-Cas9(H840A) fusion protein with a 192-bp target sequence from Drosophila virilis,
various sgRNAs, and R2 3′UTRs with 10 nt of 3′ homology to the nick site dictated by
the sgRNA (Fig. 5D). We found TPRT activity at all Cas9 nick sites, with one sgRNA
(guide 2) giving efficient activity (Fig. 5E). Adding R2Bm and SpCas9(H840A) as separate
polypeptides also yielded efficient TPRT with guide 2, but was less robust with other guides
(fig. S9). The 239-nt CMV promoter RNA with a 3′ R2 tag and 10 nt of homology to the
guide 2 nick site was also reverse transcribed efficiently; this activity required the R2 tag and
was reduced in the absence of 3′ homology or with the R2 tag A128U mutation (Fig. 5E).
Larger RNAs like GFP could also be reverse transcribed at the guide 2 nick site (fig. S9). In
summary, R2Bm can be retargeted by Cas9 to perform TPRT at unrelated loci, and the R2
tag can direct incorporation of cargo RNAs at these sites.
Discussion
Here we show the structure of a non-LTR retrotransposon during transposition, and we
dissect the principles of target DNA and self-RNA recognition. Our structure suggests that
R2Bm uses its N-ZnF and Myb domains to locate the endonuclease target sequence, a model
that contrasts with the model for other non-LTR retrotransposons where the endonuclease
domain is the only determinant of target site selection (28, 29). We identified two essential
target site motifs - the RUM and RASIN - that are recognized by R2Bm, but we note that
searching the B. mori genome with a RUM-RASIN consensus motif yields many potential
off-target sites outside of the ribosomal DNA arrays (fig. S10). We examined the sequence
of a previously identified B. mori non-28S insertion in (30) and found the target site had
limited similarity with 28S but had a TTAAcG|T RASIN motif (‘|’ indicates insertion site,
lower-case is deviation from 28S) and a GCTACTTGCGCAT RUM the correct distance
upstream of the RASIN (fig. S10). Non-28S insertions however are rare, and so it is
likely other factors are important in regulating R2Bm transposition, including chromatin
accessibility, other sequence motifs, or the ability of the target DNA to bend and melt.
Non-LTR retrotransposons form a diverse family, and even within the R2 superclade there
are notable differences between elements. R2Bm is a representative of the R2-D clade of
elements, which have a single C2H2 N-terminal ZnF domain, but R2-A clade elements
have three tandem N-terminal ZnF domains (31) that may create a more extensive DNA-
binding interface with greater stringency in target site selection. More broadly, non-LTR
retrotransposons can be divided into two types based on their endonuclease domains:
those that like R2Bm use a C-terminal restriction enzyme-like (RLE) domain, and those
that, like human LINE-1, use an unrelated N-terminal apurinic/apyrimidinic endonuclease
(APE) domain (32, 33). Structure prediction using AlphaFold (34) suggests that, in these
retrotransposons, the APE domain has a distinct position to the RLE domain in R2Bm,
suggesting there may be mechanistic differences in how target cleavage is coupled to reverse
transcription (fig. S5) (35). Nonetheless, the similarity between the DNA interface on the
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R2Bm thumb domain and the corresponding interface in the group IIC intron (fig. S5)
suggests this interface might be conserved amongst most non-LTR retrotransposons (19).
Indeed, the upstream DNA from R2Bm was easily modeled into an AlphaFold model of
human LINE-1 ORF2, including the thumb interactions but also strand separation by the
CCHC ZnF domain, which in LINE-1 ORF2 corresponds to the C-terminal cysteine-rich
domain (fig. S5).
Overall, the results of this work advance our understanding of transposition by non-LTR
retrotransposons and suggest avenues for engineering these transposons for targeted gene
insertions.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments:
We thank S. Zhu and M. Walsh for valuable discussions; E. Brignole and C. Borsa for the smooth running of
the MIT.nano cryo-EM facility, established in part with financial support from the Arnold and Mabel Beckman
Foundation; S. Lövestam for a critical reading of the manuscript; and the entire Zhang lab for support and advice.
We thank T. H. Eickbush and colleagues for their inspiring and pioneering work on R2 elements.
Funding:
Helen Hay Whitney Foundation Postdoctoral Fellowship (MEW)
Howard Hughes Medical Institute (MEW, FZ)
National Institutes of Health grant 2R01HG009761-05 (FZ)
Hock E. Tan and K. Lisa Yang Center for Autism Research at MIT (FZ)
Yang-Tan Molecular Therapeutics Center at McGovern (FZ)
BT Charitable Foundation (FZ)
The Phillips family (FZ)
J. and P. Poitras (FZ)
Data and materials availability:
The cryo-EM map has been deposited in the Electron Microscopy Data Bank with accession
code EMD-40033. The coordinates for the atomic model have been deposited in the Protein
Data Bank with accession code 8GH6. The raw cryo-EM data have been deposited in
EMPIAR with accession code EMPIAR-11458.
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Fig. 1. Cryo-EM structure of the R2Bm retrotransposon.
(A) Domains of the R2Bm retrotransposon. ZnF, zinc finger; NTE, N-terminal extension;
RT, reverse transcriptase; RLE, restriction-like endonuclease. (B) Schematic of target-
primed reverse transcription (TPRT). (C) Denaturing gel of in vitro TPRT reactions on a
labeled 211-bp 28S DNA target. The same gel was visualized by Cy5 fluorescence and
toluidine blue staining. (D) Cryo-EM density of the R2Bm TPRT complex. (E) Cartoon
of the cryo-EM structure. Stars represent active sites. (F) Atomic model for the R2Bm
Science. Author manuscript; available in PMC 2023 September 13.
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TPRT complex. (G) Reverse transcriptase domain and template/primer duplex. (H) Reverse
transcriptase active site. Cryo-EM density is shown as a gray transparent surface.
Science. Author manuscript; available in PMC 2023 September 13.
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Fig. 2. Target DNA recognition upstream of the R2 cleavage site.
(A) Schematic of interactions with the target DNA. Bases are numbered relative to the
bottom strand cleavage site. Positions of protein domains are shown by shaded rectangles.
(B) Structure of R2Bm around the upstream DNA sequences. (C) Effect of upstream DNA
mutations on target cleavage. The schematic shows the sequences of five DNA sequences
tested in top-strand sense; dots represent bases identical to wildtype. Red triangle, bottom
strand cleavage site. Denaturing gels show in vitro TPRT reactions on labeled 211-bp 28S
DNA targets. ΔN, deletion of N-terminal N-ZnF and Myb domains. ΔRT6a, deletion of
Science. Author manuscript; available in PMC 2023 September 13.
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residues 672 – 677 (DGHRKK) of the RT6a loop. (D) Screen for identifying active RUM
sequences. Nicking sites of R2Bm and the restriction endonuclease Nt.BbvCI are shown by
triangles. (E) Sequence logo for sequences enriched in the RUM screen. (F, G, H) Details
of interactions between the target DNA and the N-ZnF, Myb, and RT6a loop. (I) Effect of
altering the distance between the RUM and RASIN motifs. Denaturing gel shows in vitro
TPRT reactions on labeled 211-bp 28S DNA targets.
Science. Author manuscript; available in PMC 2023 September 13.
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Fig. 3. Target DNA recognition at the R2 cleavage site.
(A) Interactions of the top and bottom strands of the target DNA with the ZnF domain
of R2Bm. Star, RLE active site. (B) Interactions of the DNA bottom strand with the RLE
domain. (C) Interactions of the DNA top strand with the RLE domain. Residues mutated
in the RD>AA mutant are highlighted. (D) RASIN sequence requirements for bottom
strand cleavage. The labeled 211-bp 28S DNA targets were incubated with R2Bm and 3′
UTR RNA in the absence of dNTPs. The reactions were analyzed with a denaturing gel.
Mutations are notated in top-strand sense, but both strands were mutated. (E) Denaturing
gel showing R2Bm cleavage and TPRT activity on partially-stranded substrates. Reactions
contained a fluorescein-labeled 76-nt bottom strand. Reactions as indicated also contained
17 nt of downstream top strand sequence (17d), 32 nt of upstream top strand sequence (32u),
or 60 nt of top strand sequence fully complementary to the bottom strand spanning the
upstream and downstream regions. RD>AA; R2Bm R901A D902A.
Science. Author manuscript; available in PMC 2023 September 13.
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Fig. 4. Interactions of R2Bm with the 3′ UTR RNA.
(A) Secondary structure diagram of the 3′ UTR RNA, based on (26). Thicker strokes
represent nucleotides visible in the cryo-EM density. Nucleotides are numbered from the
first base of the 3′ UTR (the base following the stop codon). (B) Structure of the 3′ UTR
RNA core and the R2Bm NTE-1 domain. Dotted lines, hydrogen bonds. (C) Low-pass
filtered cryo-EM map. (D) Interactions between 3′ UTR bases. Dotted lines, hydrogen
bonds. (E) Secondary structure of the R2 tag RNA. Unshaded bases are not in the full-length
3′ UTR. (F) Denaturing gel of in vitro TPRT reactions on a labeled 211-bp 28S DNA target
using various R2 RNAs. Highlighted mutants are in the J1/2 region. The same gel was
visualized by Cy5 fluorescence and toluidine blue staining. (G) The R2-tag allows TPRT of
cargo RNAs. Denaturing gel shows TPRT reactions with equimolar amounts of the indicated
RNAs and a labeled 211-bp 28S DNA target. R2 tag (43 nt) was added to the 3′ end of a
239-nt RNA encoding the CMV promoter or a 764-nt RNA encoding GFP.
Science. Author manuscript; available in PMC 2023 September 13.
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Fig. 5. Mechanism and retargeting of first strand synthesis by R2Bm.
(A) Model for the initial stages of target site cleavage and first strand synthesis. (B) Design
of R2Bm + Cas9 experiments. (C) Complementation of DNA target site mutants by Cas9
cleavage in trans and cis. The denaturing gel shows in vitro TPRT reactions on a labeled
211-bp target corresponding to the wild-type 28S target, or two 235-bp targets: one where
the RASIN TAAGGTA is replaced by 31 bp of unrelated sequence, and another where the
13-bp RUM is additionally scrambled. R2Bm and SpCas9(H840A) were added in trans, or
in cis connected by a 33XTEN linker ( fusion indicated by a shaded box). The sgRNA is
complementary to the inserted sequence and nicks 40 nt from the last RUM base. The R2
RNA is the 3′ UTR with 5 nt of 3′ homology to the nick site. (D) Sequences used for
retargeting R2Bm to an unrelated locus from the Drosophila virilis genome. (E) Denaturing
Science. Author manuscript; available in PMC 2023 September 13.
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gel of in vitro TPRT reactions on the labeled 192-bp Drosophila virilis target. sgRNAs are
numbered as in (D); all R2 RNAs or R2-tagged RNAs have 10 nt of 3′ homology to the nick
site of the sgRNA.
Science. Author manuscript; available in PMC 2023 September 13.
| null |
10.2196_39479.pdf
|
Data Availability
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
|
Data Availability The data supporting the findings of this study are available from the corresponding author upon reasonable request.
|
JMIR HUMAN FACTORS
Original Paper
White et al
Understanding the Subjective Experience of Long-term Remote
Measurement Technology Use for Symptom Tracking in People
With Depression: Multisite Longitudinal Qualitative Analysis
Katie M White1, BSc; Erin Dawe-Lane2, MSc; Sara Siddi3, PhD; Femke Lamers4, PhD; Sara Simblett2, PhD; Gemma
Riquelme Alacid3, MSc; Alina Ivan1, MSc; Inez Myin-Germeys5, PhD; Josep Maria Haro3, PhD; Carolin Oetzmann1,
MSc; Priya Popat1, BSc; Aki Rintala5, PhD; Elena Rubio-Abadal3, PhD; Til Wykes2, PhD; Claire Henderson6, PhD;
Matthew Hotopf1, PhD; Faith Matcham1,7, PhD
1Department of Psychological Medicine, King's College London, London, United Kingdom
2Department of Psychology, King's College London, London, United Kingdom
3Parc Sanitari Sant Joan de Déu, Fundació Sant Joan de Déu, CIBERSAM, Universitat de Barcelona, Barcelona, Spain
4Department of Psychiatry, Amsterdam University Medical Center, Vrije Universiteit, Amsterdam, Netherlands
5Center for Contextual Psychiatry, Department of Neurosciences, UK Leuven, Leuven, Belgium
6Health Service & Population Research Department, King's College London, London, United Kingdom
7School of Psychology, University of Sussex, Falmer, Sussex, United Kingdom
Corresponding Author:
Katie M White, BSc
Department of Psychological Medicine
King's College London
Institute of Psychiatry, Psychology and Neuroscience
16 de Crespigny Park
London, SE5 8AB
United Kingdom
Phone: 44 7850684847
Email: [email protected]
Abstract
Background: Remote measurement technologies (RMTs) have the potential to revolutionize major depressive disorder (MDD)
disease management by offering the ability to assess, monitor, and predict symptom changes. However, the promise of RMT data
depends heavily on sustained user engagement over extended periods. In this paper, we report a longitudinal qualitative study of
the subjective experience of people with MDD engaging with RMTs to provide insight into system usability and user experience
and to provide the basis for future promotion of RMT use in research and clinical practice.
Objective: We aimed to understand the subjective experience of long-term engagement with RMTs using qualitative data
collected in a longitudinal study of RMTs for monitoring MDD. The objectives were to explore the key themes associated with
long-term RMT use and to identify recommendations for future system engagement.
In this multisite, longitudinal qualitative research study, 124 semistructured interviews were conducted with 99
Methods:
participants across the United Kingdom, Spain, and the Netherlands at 3-month, 12-month, and 24-month time points during a
study exploring RMT use (the Remote Assessment of Disease and Relapse-Major Depressive Disorder study). Data were analyzed
using thematic analysis, and interviews were audio recorded, transcribed, and coded in the native language, with the resulting
quotes translated into English.
Results: There were 5 main themes regarding the subjective experience of long-term RMT use: research-related factors, the
utility of RMTs for self-management, technology-related factors, clinical factors, and system amendments and additions.
Conclusions: The subjective experience of long-term RMT use can be considered from 2 main perspectives: experiential factors
(how participants construct their experience of engaging with RMTs) and system-related factors (direct engagement with the
technologies). A set of recommendations based on these strands are proposed for both future research and the real-world
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implementation of RMTs into clinical practice. Future exploration of experiential engagement with RMTs will be key to the
successful use of RMTs in clinical care.
(JMIR Hum Factors 2023;10:e39479) doi: 10.2196/39479
KEYWORDS
remote measurement; technology; qualitative; engagement; telehealth; depression; mental health; mobile phone
Introduction
(MDD)
Background
Depressive disorders, characterized by periods of persistent low
mood and anhedonia, are the third leading cause of disability
worldwide
is
[1]. Major depressive disorder
characterized by a longitudinal trajectory of relapse and
remission [2]. The economic burden of MDD is currently
estimated at US $326 billion [3], with high recurrence associated
with increased comorbidity burden and health care resource use
[4]. Traditional assessment of MDDs is limited in its ability to
detect moment-by-moment symptom changes because it relies
on retrospective questionnaires completed at sporadic time
points, is prone to recall bias, and is often only undertaken at
the point of relapse [5]. Working toward the timely diagnosis
and treatment of MDD remains an urgent priority [5].
Novel remote measurement technologies (RMTs) have the
potential to become an asset for chronic disease management.
Multiparametric RMT systems can provide
real-time,
longitudinal symptom tracking by combining active symptom
reporting via smartphone apps (active RMT) with physiological
and behavioral wearable sensor data (passive RMT) [6].
Continuous data can be collected on mood variability [7],
sociability [8], physical activity [9], cognition [10], speech
acoustics [11], and sleep [12]. Integration of RMT data into
MDD care may help to more accurately assess, monitor, and
predict depressive symptom trajectories, ultimately enabling
personalized interventions [13].
and
The promise of remote tracking in MDD depends almost entirely
on user engagement. Engagement with mobile health (mHealth)
technologies comprises the initial and sustained active use of a
device [14]. High engagement with RMTs is imperative given
the high-frequency data needed to identify symptom patterns
and changes over time. Several systematic reviews have
highlighted the heterogeneity of engagement metrics reported
in remote tracking studies [15-17]. The Remote Assessment of
Disease
Relapse-Major Depressive Disorder
(RADAR-MDD) study is currently the largest multisite
longitudinal study of a multiparametric RMT system for tracking
depression [6]. The RADAR-MDD study has recently reported
promising engagement, both in terms of initial recruitment rates
[18] and sustained retention and data availability [19] over a
2-year follow-up of 623 participants across 3 European sites
(United Kingdom, Spain, and the Netherlands). A large
proportion of participants (79.8%) completed follow-up, and
approximately 50% of the participants had >76% data
completion for passive data streams [19].
When evaluating engagement, an understanding of the subjective
experience of using RMTs should complement objective data
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completion statistics [17]. Subjective engagement with mHealth
technologies can be understood as an experiential construct of
what it feels like [20]. Exploring subjective engagement with
RMTs provides a richer insight into system usability and
perceived utility of, and satisfaction with, the technology [17].
The drivers for sustained user engagement with RMT systems,
which, in contrast to typical mHealth technologies, require long
periods of use for little direct rewards or intervention [21], are
currently unknown.
Several studies have qualitatively explored subjective
engagement with RMTs for depression. A multisite exploration
of the perceived barriers and facilitators to RMT use by Simblett
et al [22] informed the design of the RADAR-MDD study.
Functional (technological convenience, accessibility, and
intrusiveness) and nonfunctional (user cognition, perceived
rewards) factors influenced patients when considering remote
symptom tracking [22]. These findings have been replicated
across patient and physician perspectives [23-25]. Two
systematic reviews [26,27] on broader mHealth technologies
for depression explored the experiences of participants’ actual
use for up to 1 year. Factors such as lower symptom severity,
perceived usefulness of the technology, lower privacy concerns,
lack of technical issues, and access to responsive personal
support were associated with enhanced motivation to engage
with technologies [26,27]. A handful of studies have also
suggested the beneficial effects of symptom monitoring,
including
[28], adaptation of
self-management strategies [29], and access to a “safety net”
of support [30]. However,
typically use
hypothetical scenarios or evaluate short-term system use. As a
result, little is known about the subjective experience of
long-term, real-world use of RMTs.
increased self-awareness
these studies
Objective
This study aims to understand the subjective experience of
long-term engagement with RMTs for monitoring depression
symptoms. It uses qualitative data from the RADAR-MDD
study as an example of sustained RMT use across a 2-year
follow-up period. This study builds on previous qualitative work
by Simblett et al [22] on perceived barriers to and facilitators
of intended RMT use in depression, providing a comparison
with user experiences over 2 years of sustained engagement.
Our objectives were (1) to explore key themes associated with
long-term RMT use and (2) to identify recommendations for
future system engagement. The findings will complement the
objective engagement data and provide a basis for further
promotion of engagement with RMTs for symptom tracking in
research and clinical practice.
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Methods
Design
This study used a multisite longitudinal qualitative research
[31] approach with thematic analysis. Semistructured interviews
were conducted with participants at 3-, 12-, and 24-month time
points at 3 RADAR-MDD sites: King’s College London
(London, United Kingdom), Centro de Investigación Biomédica
en Red (Barcelona, Spain), and Amsterdam University Medical
Centre (Amsterdam, the Netherlands). The design of the
interview topic guide was informed by recent work on the
barriers to and facilitators of RMT use in those living with
depression [16,22].
Procedure
The RADAR-MDD study used the RADAR-base system [32]
for data collection. The study active RMT smartphone app
fortnightly validated mood and self-esteem
delivered
questionnaires and 6-weekly, high-frequency experience
sampling methodology (ESM) questionnaires on current state,
cognitive games, and a speech task. The study passive RMT
smartphone app collected passive data on ambient noise and
light, Bluetooth connection, and GPS location. Participants were
provided with a wearable device, the Fitbit Charge (Fitbit Inc),
measuring their step count, sleep, and physical activity. Further
information on the RADAR-MDD procedure is available in the
protocol paper by Matcham et al [6].
Eligibility criteria for inclusion in this study were (1) current
participation in RADAR-MDD (full eligibility criteria provided
in the study by Matcham et al [6]) and (2) willingness to
participate in a 1:1 interview with a researcher discussing their
experiences of the study. Participants provided written informed
consent for the interviews as part of their RADAR-MDD study
participation.
The interviews were managed by the research team lead at each
site. Participants were recruited using convenience sampling at
each time point to maximize data collection. Interviews were
face-to-face (at the respective research site) or via telephone or
video call (United Kingdom and the Netherlands only). All
interviewers were female and part of the participant-facing
research team. Face-to-face interviews were not conducted
during the COVID-19 pandemic lockdown. Participants were
reimbursed for relevant travel costs and paid per interview (£10
or €10 [US $1.2]).
The interviews were semistructured using open-ended questions,
designed to elicit discussions around using the study technology
in daily life (Multimedia Appendix 1). The content of each topic
guide reflected the expected differences between time points.
For example, the 3-month guide focused on immediate
problem-solving and troubleshooting, where later interviews
included data sharing.
White et al
The topic guides were translated from English into Spanish and
Dutch, and interviews were conducted by native speakers at
each site. The interviews lasted between 30 and 60 minutes and
were conducted between February 2018 and April 2021.
Ethics Approval
The semistructured interviews were approved by the ethics
committee of RADAR-MDD [6]. Ethical approvals for
conducting the study were obtained from Camberwell St Giles
Research Ethics Committee (reference: 17/LO/1154) in London,
from Clinical Research Ethics Committee Fundacio Sant Joan
de Déu (CI: PIC-128-17) in Barcelona, and from Medische
Ethische Toetsingscommissie VUms (2018.012–NL63557.
029.17) in the Netherlands.
Data Analysis Strategy
The interviews were audio recorded and transcribed verbatim.
A preliminary coding framework was developed in English
based on previous findings of barriers to and facilitators of RMT
use in hypothetical scenarios [22]. All sites first coded example
interviews for a cross-site consistency check and a discussion
on revisions to the coding framework, accounting for novel
codes. Each site then proceeded to recode all interviews in the
native language using NVivo software (version 12; QSR
International [33]) according to the final coding framework
(Multimedia Appendix 2 provides a comparison of the
preliminary and final coding framework). The coding was
performed by independent researchers at each site. Each site
sent coded NVivo data sets to the London site, with all quotes
translated into English by a third-party translator briefed on the
study topic [34]. The data were stored on a secure server at the
London site.
Multisite data were merged into one data set and thematic maps
for 3-month, 12-month, and 24-month time points were
developed by 3 researchers (KW, EDL, and PP), identifying
key themes and subthemes. To align with previous longitudinal
qualitative research work [31], data are presented not as a
longitudinal narrative but as contributing to each theme.
Results
Participant Characteristics
A total of 124 interviews with 99 participants were conducted
across 3 sites. Of these 124 interviews, 40 (32.2%) interviews
were conducted at the 3-month time point (15/40, 38% in United
Kingdom; 15/40, 38% in Spain; and 10/40, 25% in the
Netherlands), 42 (33.9%) at the 12-month time point (16/42,
38% at United Kingdom; 16/42, 38% at Spain; 10/42, 24% at
the Netherlands), and 42 (33.9%) at the 24-month time point
(15/42, 36% at United Kingdom; 16/42, 38% at Spain; 11/42,
26% at the Netherlands). A total of 17 participants took part in
an interview at 2 time points; 4 participants were interviewed
across all 3 time points. Participant characteristics according to
time points are shown in Table 1.
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Table 1. Participant characteristics by interview time point.
Characteristics
Site, n
United Kingdom
Spain
the Netherlands
Age (years), mean (SD)
Female, n (%)
Depression severity categorya, n (%)
None
Mild
Moderate
Severe
Very severe
Not reported
Anxiety severity categoryb, n (%)
None
Mild
Moderate
Severe
Not reported
Time point
3-month (n=40)
15
15
10
44.6 (12.1)
30 (75)
4 (10)
7 (18)
10 (25)
7 (18)
11 (28)
1 (3)
7 (18)
7 (18)
12 (30)
13 (33)
1 (3)
12-month (n=42)
24-month (n=42)
16
16
10
49.4 (13.5)
32 (76)
3 (7)
5 (12)
13 (31)
10 (24)
9 (21)
2 (5)
5 (12)
10 (24)
13 (31)
12 (329)
2 (4)
15
16
11
51.9 (15.0)
29 (69)
5 (12)
5 (12)
7 (17)
6 (14)
5 (12)
14 (33)
7 (17)
8 (19)
7 (17)
5 (12)
15 (36)
aMeasured as the Inventory of Depressive Symptomatology-Self Report total score nearest to the interview time for each participant. None=0-13,
mild=14-25, moderate=26-38, severe=39-48, and very severe=49-84.
bMeasured as the Generalized Anxiety Disorder-7 item total score nearest to the interview time for each participant. None=0-5, mild=6-10, moderate=11-15,
and severe=16-21.
Themes
This study aimed to explore the subjective experience of
long-term engagement with RMTs over a 2-year follow-up
period. We present our results under five themes: (1)
research-related factors, (2)
the utility of RMTs for
self-management, (3) technology-related factors, (4) clinical
factors, and (5) system amendments and additions.
Research-Related Factors
When considering initial motivations for engaging with an RMT
study, contributing toward novel research findings was the most
prevalent reason for taking part. Across all time points, research
team support was also a key facilitator of sustained engagement
in the study.
Altruism and Academia
Taking part in the study was an opportunity to use personal
experiences of depression to help others, to advance scientific
understanding, and to “give back” to the system:
I’ve suffered with depression the whole of my adult
life, I’ve obviously had a lot out of the system. If I can
do anything to put back, do you see what I mean—I
will. [P30, 24 months, United Kingdom]
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Taking part for “the future, for the people who come after me”
(P8, 3 months, Spain) was a strong theme that arose in all sites
when discussing reasons for enrolling in the research study.
Altruistic motivations continued across later time points
regardless of whether participants felt they had experienced any
direct benefits:
I am actually quite proud to say that I am doing this
as part of research. Some people will ask me what it
is [the wearable], and I say well it is good if more
people get to know about it. And for the long-term
benefits, might not be for me but for other people,
because it might show. [P18, 12 months, United
Kingdom]
With regard to the RMT aspect of the study, some mentioned
that it “piqued my interest” (P37, 24 months, United Kingdom)
and “I was very intrigued by a study that kind of has consistent
monitoring” (P39, 24 months, United Kingdom). However,
many participants signed up with limited knowledge of the study
procedure, or of the use of RMTs for health care monitoring.
Thus, a lack of prior understanding of RMTs is not a barrier to
initial engagement.
Privacy was not a barrier to participants upon entering the study
or throughout their participation. A key reason for this was that
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the research was conducted in a clinical and academic setting.
In the Spanish cohort, one participant viewed the study as
parallel to their clinical care:
It’s not data about, about privacy, things about you,
no, it’s related to a medical condition, isn’t it? A case
of depression, that’s what it’s about. So if they ask
you for medical data, well, it’s normal. [P25, 24
months, Spain]
Any initial privacy or data security concerns were largely
alleviated by the 3-month point through conversations with the
research team. At later time points, privacy was not discussed
frequently.
Research Team Support
Support from the research team was a facilitator to continued
engagement with the RMTs. This was primarily practical; at 3
months, the research team provided support on how to use the
devices and study apps, which was often imperative to
successful enrollment into the study:
I tried it once [the wearable] and wasn’t able
to...to...put it on the phone. If it hadn’t been for
[researcher name]’s help I wouldn’t have made it.
[P1, 3 months, Spain]
The need for practical support remained a key theme at 12
months, this time concerning technological malfunctions. Ability
to contact the research team through various methods and
receiving a timely reply was important. Some felt comfortable
with initiating support themselves: “I didn’t need that much
contact personally, I could get in contact easily, if it were
necessary” (P21, 24 months, the Netherlands). Others wanted
more contact, for example, more points of researcher-initiated
contact, or specific contact from specialists. At-hand support
was essential for continued participation:
I think it is really important to have the practical
support ‘cause you don’t want to be offline or not
working for long than is necessary. Otherwise it goes
against the purpose of the study really. [P18, 12
months, United Kingdom]
There was a consensus at all time points that the research team
was approachable, patient, and reassuring, helping to alleviate
technological concerns.
The research team also provided emotional support to the
participants. Some participants sought comfort in the knowledge
that they were being monitored as part of a study: “I liked it a
lot because, jeez knowing, I felt safe, you know? Because
knowing that you were there...” (P25, 24 months, Spain). Others
had specific examples of receiving mental health support from
the research team. One participant in the British cohort received
direct signposting, which was noted in both their 12-month and
24-month interview as a crucial part of their study experience:
because of the letter from [researcher] to the GP
clinic I was able to get an immediate referral, and
the problem is if you’re the system it’s great, if you’re
not in the system it’s difficult to get in. I couldn’t have
done it on my own. [P27, 12 months, United
Kingdom]
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Benefits of RMTs for Self-management
Despite primarily engaging with the study for altruistic reasons,
many participants experienced unexpected benefits of using
RMTs for symptom monitoring during their time in the study.
These comprised symptom awareness and communication, both
of which were integrated into self-management of depression.
Symptom Monitoring and Awareness
Across all 3 time points, the most frequently reported benefit
was an increase in symptom awareness. Monitoring various
factors related to depression, for example, mood, sleep, and
exercise, increased self-reflection, and the ability to identify
patterns. For example, having access to objective sleep data
provided clarification and reassurance:
I loved that [the wearable data], I found that so
reassuring to just relax, of course you’ve slept and
then you go ok, the next time you’re lying in bed you
go I’m not ever gonna sleep again but actually you
have, you’ve seen that you do I think that’s brilliant,
really reassuring. [P14, 3 months, United Kingdom]
Although the app did not provide feedback on symptom scores,
many felt that the act of answering the questionnaires prompted
them to analyze how they had been feeling:
I’m more aware of it, the questions on the
questionnaire, especially those that ask how I’m
feeling right now raise my awareness, I feel quite
average or, I’m feeling not great, sometimes you
ignore these things. And if you can take more time to
think about these things...maybe I need to meditate
more, I really feel self-conscious... [P10, 3 months,
the Netherlands]
For some, answering the questionnaires and viewing the Fitbit
data simply provided an understanding of their experience of
depression: “I have noticed that my answers have gotten more
positive throughout the year” (P22, 24 months, the Netherlands).
For others, these data directly motivated behavior changes. At
3 months, the discussion focused on the motivational effects of
the Fitbit data; participants felt encouraged to complete their
daily step count or achieve target physical activity “badges.”
Toward the later time points, these data came to act as prompts
for self-care, for example, increased exercise or relaxation:
Wearing a watch and knowing that my activity
matters, you know? I mean, like the steps I take have
a direct effect on my health, both physical and mental,
all my activity makes me more aware of it, more
conscious of it and it has also been like a driving force
for me to put my batteries in sport or stress
management...a habit forever, so I do not want to do
without it. [P26, 24 months, Spain]
This became especially apparent during
the 24-month
interviews, when the Fitbit data were used to monitor sleep and
mood symptom changes during the COVID-19 pandemic.
Disruption to usual routines during this time allowed some to
reflect more than ever on the benefit of monitoring exercise:
I knew in theory, exercising and getting out and so
on was good for your mental health, but over Covid,
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the monitor helped, and the benefit would have been
even better. I think I might have been worse during
Covid without it. [P36, 24 months, United Kingdom]
Communication
At each time point, the RMT data were also used for
communicating personal experiences to others. Participants
used their increased understanding of their depression to inform
others: “For the first time it kind of occurred to me to let me
partner know when I could feel it was starting...so if you see
my behaviour change or I’m unresponsive this is why” (P39,
24 months, United Kingdom).
Access to the Fitbit data also facilitated joint decision-making,
both for immediate symptom management and long-term
strategies:
There are also days that I don’t reach 5000 steps,
which will make me think oh I haven’t done that many
today...my spouse will say that too, go for another
walk. [P2, 3 months, the Netherlands]
Overall Value and Utility
There was a consensus throughout that the benefits of
participating in the study outweighed the costs, of which there
were relatively few. Many had not envisioned any personal
benefits when enrolling as they were aware that they would not
receive personalized outcomes; however, had been pleasantly
surprised by the integration of RMT data into their depression
self-management, as early as the 3-month time point:
I think its empowering to know more about myself to
understand more so I think once I can see more what
the data is from collecting from data when the other
apps are working and being able to see what the data
is and notice any correlations then I think that will
be really valuable. [P12, 3 months, United Kingdom]
Technology-Related Factors
Experience of the technology used in the study (smartphone
apps and Fitbit) was the most widely cited theme across all sites.
This covered the convenience of integrating the RMTs into
daily life, the usability of the technology, technological
malfunctions that occurred, and the extent to which participants
found the technologies intrusive.
Convenience
Using a mobile phone and wearing a watch were already an
integral part of many participants’ daily routine. The Fitbit
device, “it’s basically wearing a watch” (P7, 3 months, United
Kingdom), collected data passively without the need to input
information, and continual wear, syncing, and charging were
integrated into the routine as early as the 3-month time point.
Reminder messages across the system were useful in the process
of long-term integration.
One aspect that participants found more difficult to integrate
into their routine was the app questionnaires. Timing of the
questionnaires was often inconvenient, for example when at
work, driving, or in social situations: “Obviously I’m less likely
to stop my conversation to be like oh this questionnaire, because
that’s a bit rude” (P4, 3 months, United Kingdom). Frequency
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of the ESM questionnaires was also too high from some: “it’s
impossible to have a routine with that. If you have a full-time
job, it’s always a bother” (P17, 24 months, the Netherlands).
The participants were rarely able to change their routine to
accommodate answering the questionnaires, which sometimes
caused guilt. One participant in the Spanish cohort reflected on
how work affected their ability to respond to app notifications
during their 2-year participation:
At the beginning it was a bit difficult because I was
working, then as I was on sick leave for two years,
the truth is that I’ve been able to adapt quite well.
And in the end, when I went back to work again, it
was a bit difficult... [P1, 24 months, Spain]
Usability
For those who received a smartphone upon enrollment, a large
technological barrier was the process of “relearning” a new
operating system. This was described by some as “more difficult
than anticipated” (P3, 3 months, United Kingdom), particularly
during the 3-month interviews, owing to adapting to a new user
interface and decreased connectivity with other devices. At 24
months, some participants had adjusted to using the new device,
whereas others planned to swap back upon study completion:
No, my only peeve was that I’m an Apple user and
having this bloody awful Android phone, the first
thing I shall do on April 1st is take my SIM card out
of the Motorola thingy. [P35, 24 months, United
Kingdom]
Technological Malfunctions
The participants reported a range of technological malfunctions
that affected their participation in the study. Issues with the
study apps were particularly prevalent during the 3-month
interviews owing to ongoing technological challenges during
the early phases of the study. These included not receiving
notifications, apps crashing, apps logging out, and difficulties
with rescanning QR codes. Participants sometimes had limited
time or motivation to report issues to the team:
I tried opening a questionnaire I wouldn’t be able to
see it, I wouldn’t be able to do it and there was no
way of saying this is happening or why this is
happening so maybe I should have contacted you
about it but I just kind of ignored it. [P4, 3 months,
United Kingdom]
Issues with missing data persisted throughout the 3 time points.
Participants were aware of the times when the active app had
been unable to submit the completed data, or the passive app
had ceased monitoring. Such malfunctions often led to anxiety
or guilt that they were not “correctly” participating: “Well, yes,
when it didn’t work, I became a bit nervous...” (P15, 3 months,
Spain).
Participants also reported frequent missing data with the Fitbit,
caused either by a syncing error or inaccurate recording. These
issues caused some to question the integrity of the study: “It
just didn’t work and that’s not what you expect from a research
study” (P18, 24 months, the Netherlands).
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A participant in the Spanish cohort reflected on how these
technological malfunctions affected not only their ability to
participate in the study but also their experience of being able
to use the resulting data:
There is data that I have missed here, and of course
I was analyzing it with me in important situations of
how I was, and that I have missed them, for more than
a month. [P32, 24 months, Spain]
Intrusiveness
Generally, the concept of remote monitoring, or the use of the
technologies, was not regarded as intrusive. Rather, passive
data collection was noted as a preferable method because “at
some point you don’t notice it. You don’t notice that you’re
wearing it anymore” (P18, 24 months, the Netherlands).
However, one area that caused disruption was the wearability
of the Fitbit device. Several issues associated with the Fitbit
strap were reported, including skin irritation, increased sweating,
and allergic reactions. Some had briefly chosen to remove the
device while experiencing discomfort, whereas others had
purchased straps with alternative materials. At 12 months, many
reported that their strap had broken, and by 24 months, some
had to apply for a full device replacement. One participant felt
guilty when asking the research team for their device to be
repaired:
I know that the money allocated to research programs
or projects is minimal, and of course, when the strap
broke or the Fitbit wouldn’t charge me and then I felt
really bad because I thought “oh my God, now they
have to change my Fitbit.” [P26, 24 months, Spain]
Waiting for a replacement strap or device meant that participants
were unable to continue to use the Fitbit for self-management:
if I was going to continue and for the others who will
be continuing, it will probably begin to happen more
and more depending on how much people are actually
exercising with them on. It only grows, that’s the
problem, in my experience with the other Fitbit, that
definitely happens. [P3, 12 months, United Kingdom]
Clinical Factors
The participants were asked to reflect on whether and how they
could see the RMT data being used in a clinical setting.
Discussions included the extent to which participants felt
comfortable sharing the data, how they envisioned clinicians
using the data, and how feasible this was in the current climate.
Views on Data Sharing
At the 12- and 24-month time points, the participants were
specifically asked to comment on data sharing with medical
professionals. In general, allowing trusted clinicians to view
RMT data alongside medical records was acceptable, or even
essential: “let’s say my whole history, my doctor already has
it, if she has it more extensive, then all the better for me.” (P30,
24 months, Spain). There was some discrepancy over whether
these data should automatically be available to clinicians or
mediated by the patient. Some thought that medical professionals
“would be in a better position to evaluate what they needed
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from it than me to decide that” (P32, 24 months, United
Kingdom). Others worried about interpretation of the data
without context:
I suppose, [I would like to] understand what it is that
is proposed to be shared, and if there’s something
there that would not be appropriate at that time,
because I don’t know what it is until I see it, then yes,
I would like to have a choice...I would want to make
sure that my health record reflects actuality rather
than something that can be interpreted by people
incorrectly. [P31, 24 months, United Kingdom]
Clinical Uses of RMT Data
The participants suggested several ways in which they might
expect RMT data to be beneficial in clinical care. These included
(1) allowing the clinician to view the “whole picture” of
individual experience, (2) allowing the clinician insight into
new symptoms, (3) as a way for patients to report specific areas
of concern, and finally (4) as a basis for making decisions about
suitable treatment or care. Importantly, treatment decisions
should be reached as a joint decision involving the clinician,
the patient, and the data:
I think they could actually look at the data that’s being
produced, and that could assist them in helping me
to come to another decision. Like, if I was deciding
that I would like to move my medication down, but
they’ve got the data that says, no you’re not...but if
it backs it up as well, so it can work both ways, so I
think it does have those benefits. [P33, 24 months,
United Kingdom]
Sleep data were repeatedly cited as a data stream that would
cause change in treatment. Participants from all sites provided
examples of conversations with their mental health clinicians.
One participant in the British cohort also discussed their
experience of integrating the sleep data into their sleep clinic
appointments:
It’s too expensive for the NHS to keep on doing [sleep
tests]...I said, well, actually, I can show you any time
in the last six months or so...an indication of when
I’m sleeping...It helped them choose what exercises
I needed to do and what therapy was required, so,
yes, it was extremely helpful. [P22, 12 months, United
Kingdom]
Presentation of objective sleep data was seen as helpful “proof”
of the participant’s recent experiences:
You can tell your GP that you sleep terribly, but of
course your GP can also think that you’re just
worried, but with the data it’s a fact that you can
prove, so that’s nice, that you have concrete
info...whether you worry or complain about it or not
doesn’t matter, the facts are there. [P10, 12 months,
the Netherlands]
Current Clinical Utility of RMTs
Although the potential for RMTs in clinical care was recognized,
2 key barriers to their implementation were envisioned. First,
the level of technological acceptance of medical professionals
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influenced participant views on the long-term utility of the data.
Participants in the Spanish cohort, who were recruited through
their clinical care, generally reported acceptance of the study
from their clinicians: “even my psychiatrist here and in
Barcelona had the same way of thinking and saw that this was
very useful for me and encouraged me” (P9, 24 months, Spain).
Others described more negative experiences, often causing them
to question the use of the data:
I thought it would be more relevant for my
neurologist, but my neurologist wasn’t particularly
interested when I told him about what I was doing in
the study. [P17, 12 months, United Kingdom]
Second, lack of funding, resources, and time was perceived as
a major roadblock to using RMT data in appointments. This
was particularly apparent in the British cohort with regard to
the National Health Service. For the data to be monitored and
reflected on, new procedures would need to be put in place:
I would be amazed if there was sufficient funding for
that...I don’t believe that the NHS have got the
resources to have people monitoring this sort of stuff.
[P22, 12 months, United Kingdom]
Given the perceived lack of resources to effectively use RMT
data in the National Health Service, some have considered how
best to come to a compromise:
I think realistically, if they had that [data] and I went
to them with a problem, then I would like them to be
able to use it at that point. But I don’t see it as
something that they would be—so, for example, if I
went to them with something and if somehow, it was
a part of my NHS records, if they could access that,
that might be helpful to them. But I don’t see them
using it other than that really. [P32, 24 months,
United Kingdom]
System Amendments and Additions
Participants discussed various changes or additions to the RMT
system used in this study to further encourage long-term
engagement. These included suggestions for questionnaire data
collection and feedback.
Data Collection
Across all sites and time points, the most prevalent suggestions
for changes to the study design were the content of active RMT
questionnaires. Participants felt that they were frequently being
asked to complete the same questions, particularly within the
ESM schedule, which often prompted them to provide the same
answers, for example, with regard to mood changes. This
affected motivation:
At first, I was more excited about it, but as time has
passed, sometimes I don’t feel much like answering
since the same questions get repeated. [P19, 12
months, Spain]
Some also suggested the ability to postpone questionnaires if
feeling too low to complete them and the ability to provide
contextual information. As early as the 3-month time point,
some noted that external factors affecting their mood were not
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being monitored within the validated mood and self-esteem
questionnaires: “I notice that when my home situation isn’t
great, I also fill in the questionnaires less positively” (P5, 3
months, the Netherlands). On reflection, some would have liked
to have given more information at certain points:
The answers are very closed, so you can’t really
answer what you feel. You know? It’s very...it’s very
up in the air. [P1, 24 months, Spain]
Data Feedback
When asked how they might wish to view their symptom data
in future use, the majority felt that this was best displayed
visually through in-app graphs. Many also expressed that this
would need to be accompanied by a “human explanation for
what those things mean” (P3, 12 months, United Kingdom).
There was a discrepancy between when these data would be
best received; some only expected to receive it at the end of the
study, some felt that it would be more useful in real time,
whereas others were cautious that receiving data during periods
of low mood would be detrimental:
If I’m well I want to see it, if I’m unwell, no. If I was
reporting that I was feeling suicidal I don’t think I’d
want to revisit it. [P27, 24 months, United Kingdom]
Furthermore, some participants considered the potential for
RMT data to provide feedback on symptom patterns and changes
over time, correlations with other factors, and depressive relapse
prediction. Specific examples included relationships between
exercise and mood, sleep and mood, and mood and
concentration: “At some point I had a burn out. I’m very curious
as to how my ability to concentrate changed, and if that maybe
shows on the THINC-it app” (P3, 24 months, the Netherlands).
It was generally accepted that having access to data of this nature
would be useful for both self-management and integration into
clinical care. Looking forward at the 24-month time point, one
participant at the British site explained their hopes for the future
of this field:
I think trends are really quite important for me in
managing what is going on...I think one of the things
I am thinking would be good to come out of this is an
ability to see patterns over time and then maybe being
able to use that as a predictor or, I need to do some
intervention here so that I don’t end up there again
if that makes sense.
[P30, 24 months, United
Kingdom]
Discussion
Principal Findings
An exploration of the subjective experience of long-term
engagement with RMTs for depression symptom management
could prove a necessary complement to objective engagement
statistics, providing insights into technology usability, user
experience, and facilitators of sustained use. This study aimed
to (1) explore the key themes associated with long-term RMT
use and (2) identify recommendations for future engagement
through longitudinal qualitative analysis at 3-month, 12-month,
and 24-month time points of the RADAR-MDD study.
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The themes uncovered suggest that long-term engagement with
RMTs can be understood from two main perspectives: (1)
experiential factors and (2) system-related factors (Figure 1).
Experiential factors relate to the ways in which participants
construct their experiences of engaging with RMTs for symptom
monitoring. Experiential factors comprise research altruism,
support from a professional team, and the benefits of using
RMTs for depression management. System-related factors refer
to direct engagement with the RMT systems. The factors include
the usability, convenience, and intrusiveness of the technologies
and the recommended system improvements for successful
clinical implementation.
On the basis of these perspectives, we present a set of
considerations for the promotion of engagement with RMTs
for depression. Given the breadth of use cases proposed for
RMTs in MDD, we focused on two areas: (1) engagement with
research and (2) engagement with real-world implementation.
Recommendations for engagement with future RMT research
are outlined in Multimedia Appendix 3.
Although our data were derived from research participants, we
believe that our findings can also be useful when considering
implementation into clinical practice. Participants identified the
following opportunities for RMTs in clinical care: (1) provision
of feedback-informed care, (2) strengthening the therapeutic
relationship, and (3) the specific clinical value of sleep
monitoring. However, this potential was acknowledged with
the caveat of a perceived lack of time and resources in clinical
care across all 3 countries. Our findings indicate that a large
difference between engagement with RMTs for research and
long-term clinical engagement could be research altruism. In
this study, an important facilitator of both initial and sustained
engagement was the experiential factor of taking part in a novel,
academic study to advance understanding and help others. To
this end, participants forewent privacy concerns and initial
receipt of personal benefit. They were also willing to engage
despite the implementation concerns. In the absence of research
altruism, Figure 1 can be used to identify further experiential
facilitators that could instead be harnessed to promote
engagement when RMTs become integrated into evidence-based
practice. For example, clinical onboarding sessions could include
a clear summary of the proposed uses and benefits of RMT data
and symptom monitoring for an individual’s care. Multimedia
Appendix 4 provides a set of considerations for
the
implementation of RMTs into clinical care based on the
experiential and system-related factors identified.
Figure 1. Experiential and system-related factors in the subjective experience of longitudinal remote measurement technology (RMT) use.
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Figure 2. Recommendations for future remote measurement technology (RMT) use in observational research.
Figure 3. Considerations for remote measurement technology (RMT) implementation in real-world clinical settings.
Comparison With Previous Work
This study builds on previous qualitative analyses of the barriers
to and facilitators of intended RMT use for depression
management. The functional and nonfunctional requirements
set out by Simblett et al [22] roughly align with the system and
experiential factors found here. However, a comparison of
coding frameworks (Multimedia Appendix 2) revealed several
differences in this study. First, nonfunctional, user-related
factors such as cognition, symptom severity, and emotional
resources were not acknowledged as barriers to long-term RMT
engagement. Second, the overall utility of RMTs was discussed
mainly in terms of benefits and rewards, and less so in terms of
costs such as privacy and security. Third, studying long-term
RMT use has revealed an additional layer of understanding
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surrounding nonfunctional requirements; experiential factors
include the impact of professional support and the effects of
symptom monitoring on self-awareness and communication.
When comparing our findings with those from the wider
mHealth literature, technological and system-related factors
remained a common theme. Borghouts et al [26] and Patel et
al [27] found that lack of technical issues, flexible usability of
the platform, personalization, and access to training were
associated with increased long-term engagement with digital
health intervention platforms. One clear difference with digital
health intervention work is the focus on “a desire to actively
improve one’s health” [27] as a main facilitator of initial and
sustained engagement. Our work has shown that in the absence
of a direct or tangible benefit, users remain willing to interact
with RMTs for long periods within a research context.
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Experiential factors such as advancing scientific understanding
and, at later periods, experiencing indirect benefits of mood
tracking, seem to operate as a supplement to the user-related
factors currently reported in the field.
Strengths and Limitations
To the best of our knowledge, this is the largest study to
qualitatively explore long-term RMT use for depression across
multiple countries. Data collection and analyses were conducted
in the native language of each country and only quotes were
translated into English, aiding the transfer of meaning process
[34]. However, this study has some limitations. First, where we
did not anticipate any major intercountry differences in terms
of attitudes toward remote mental health tracking, participants
in the Spanish cohort were invited to participate by the clinicians
involved in their care. This might have overinflated some themes
in our analyses; for example, perceived benefits of the
interviews were conducted via
technologies. Second,
convenience sampling of the participants who remained enrolled
at each time point. This increased the risk of selection bias;
those who enjoyed using the RMTs were more likely to continue
to engage and as a result more likely to agree to an interview.
This could explain the absence of themes relating to symptom
severity or cognitive barriers present in the current work,
although recent analyses have suggested that these factors did
not contribute to sustained engagement in the study [35].
in 21 participants
Convenience sampling also resulted
completing the interviews at ≥2 time points. Preliminary
sensitivity checks on a subset of this sample showed no clear
signs of changes in themes over time. The data were not deemed
rich enough to undertake a full, longitudinal analysis on this
sample. Third, because of resource constraints, no sites
undertook double coding. Fourth, data-driven themes were not
explored in relation to demographic or clinical factors, as this
was deemed beyond the scope of this study. Although previous
work suggests that perceived usability, and actual use, of the
RADAR-base system remains robust across severity of clinical
characteristics [35], understanding demographic differences in
subjective engagement is an important avenue for future
research. Finally, the COVID-19 pandemic occurred during the
study follow-up period. Given the transition to remote working
and health care across all 3 countries during this time, the
subjective experience of using RMTs might have been positively
skewed; for example, with regard to the positive impact of the
research team during social isolation. It should also be noted
that the topic guide primarily asked participants to review their
experience of using RMTs for this specific research project,
and specific use cases for clinical implementation were not
outlined by interviewers. Thus, the themes that arose from this
work relate primarily to long-term engagement with RMT
research, and the transferability of the findings to engagement
in clinical care should be taken with caution.
for
clear
Applications for Future Research
Future work should continue to explore subjective engagement
with RMTs, conceptualized in terms of both experiential and
system-related factors. Where system-related factors often
represent
technological
recommendations
improvements, understanding the experiential effects of
engaging with RMTs is a novel finding that could prove
fundamental in promoting future engagement. A recent
systematic review [17] found that 5 studies have begun to
explore the correlational relationship between objective and
subjective engagement with RMTs. Higher daily assessment
counts from an active RMT app were correlated with increased
app satisfaction ratings at 3-month and 6-month time points
[36,37]. Understanding the link between experiential factors,
such as increased self-awareness, and objective engagement
could bolster this field further.
Our findings explore the initial and sustained engagement with
RMTs for depression symptom monitoring in a research setting.
The next step would be to replicate this work in a clinical setting.
Recent qualitative analyses have reported positive views from
patients and clinicians on the potential for implementation of
RMT into psychological services [38]. This paper provides
considerations for adapting RMT systems for use in clinical
settings and a framework for continuing to analyze the subjective
experience of long-term clinical engagement to allow for further
iterations.
Conclusions
This study aimed to understand the subjective experience of
long-term engagement with RMTs for depression symptom
monitoring as a complement to the high rates of objective
engagement observed in the RADAR-MDD study. Key
experiential and system-related
themes associated with
long-term RMT use were identified along with a set of
recommendations and considerations for promoting future
system use in both research and clinical settings. Further
understanding of the construction of the “experience” of using
RMTs will be key to promoting long-term engagement in
clinical care and depression management in comparison with
general mHealth interventions that offer immediate or tangible
rewards. In the wake of the rapid expansion of this field, we
urge professionals to continue monitoring the subjective
experience of RMT engagement to maximize the potential of
remote monitoring as both a method for data collection and a
tool for symptom management.
Acknowledgments
This paper represents an independent research part funded by the National Institute for Health Research (NIHR) Maudsley
Biomedical Research Centre at South London and Maudsley National Health Service (NHS) Foundation Trust and King’s College
London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR, or the Department of
Health and Social Care.
The authors would like to thank all the members of the Remote Assessment of Disease and Relapse-Central Nervous System
(RADAR-CNS) patient advisory board for their contribution to the device selection procedures and their invaluable advice
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throughout the study protocol design. The authors would also like to acknowledge the work of Felice Fernhout in conducting
coding on the data.
Participant recruitment in Amsterdam was partially accomplished through Hersenonderzoek.nl, a Dutch web-based registry that
facilitates participant recruitment for neuroscience studies [39]. Hersenonderzoek.nl is funded by ZonMw-Memorabel (project
number 73305095003), a project in the context of the Dutch Deltaplan Dementie, Gieskes-Strijbis Foundation, the Alzheimer’s
Society in the Netherlands, and Brain Foundation Netherlands. Participants in Spain were recruited through the following
institutions: Parc Sanitari Sant Joan de Déu network of mental health services (Barcelona), Institut Català de la Salut primary
care services (Barcelona), Institut Pere Mata-Mental Health Care (Terrassa), and Hospital Clínico San Carlos (Madrid).
The authors would like to thank all Genetic Links to Anxiety and Depression study volunteers for their participation and gratefully
acknowledge the NIHR BioResource, NIHR BioResource centers, NHS Trusts, and staff for their contributions. The authors
would also like to acknowledge NIHR Biomedical Research Centre (BRC), King’s College London, South London and Maudsley
NHS Trust and King’s Health Partners. The authors would like to thank the NIHR, NHS Blood and Transplant, and Health Data
Research United Kingdom, as part of the Digital Innovation Hub Program.
This research was reviewed by a team with experience of mental health problems and their caregivers, who were specially trained
to advise on research proposals and documentation through the Feasibility and Acceptability Support Team for Researchers
(FAST-R): a free, confidential service in England provided by the NIHR Maudsley BRC via King’s College London and South
London and Maudsley NHS Foundation Trust.
The RADAR-CNS project received funding from the Innovative Medicines Initiative (IMI) 2 Joint Undertaking under grant
115902. This joint undertaking received support from the European Union’s Horizon 2020 research and innovation program and
European Federation of Pharmaceutical Industries and Associations. This communication reflects the views of the RADAR-CNS
consortium and neither IMI nor the European Union and European Federation of Pharmaceutical Industries and Associations are
liable for any use that may be made of the information contained herein. The funding body was not involved in study design,
data collection or analysis, or data interpretation.
Data Availability
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
Authors' Contributions
KMW contributed to the design and coordination of the study in London as well as to the data processing, coding, analysis, and
writing of this manuscript. EDL contributed to the data coding and analysis. S Siddi contributed to the design and coordination
of the study in Barcelona as well as the data coding. FL contributed to the design and coordination of the study in Amsterdam,
as well as the data coding. S Simblett contributed to the development and design of the study and advised on the analyses. GRA
has contributed to data coding. AI contributed to the study conducted in London. IM-G contributed to the development and design
of the study. JMH contributed to the development and design of the study. CO contributed to the study conducted in London. PP
contributed to the data coding and analysis. AR contributed to the development and design of the study. ER contributed to
participant recruitment for the study. TW contributed to the development and design of the study. CH contributed to data
interpretation and supervision of the first author. MH secured funding and is the principal investigator of the study, and contributed
to the overall study design and conduct. FM contributed to the design and coordination of the study. Patient advisory board
members contributed to the design and development of the study.
Conflicts of Interest
MH is the principal investigator of the RADAR-CNS program, a precompetitive public-private partnership funded by the Innovative
Medicines Initiative and the European Federation of Pharmaceutical Industries and Associations. The program received support
from Janssen, Biogen, Merck & Co, Union Chimique Belge, and Lundbeck. JMH has received economic compensation for
participating in advisory boards or giving educational lectures from Eli Lilly & Co, Sanofi, Lundbeck, and Otsuka. CO is supported
by the UK Medical Research Council (MR/N013700/1) and King’s College London member of the MRC Doctoral Training
Partnership in Biomedical Sciences.
Multimedia Appendix 1
Main interview questions at 3-month, 12-month, and 24-month follow-up time points.
[DOCX File , 21 KB-Multimedia Appendix 1]
Multimedia Appendix 2
Preliminary and final codes in the coding framework.
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[DOCX File , 20 KB-Multimedia Appendix 2]
Multimedia Appendix 3
Recommendations for future remote measurement technology (RMT) use in observational research.
[DOCX File , 319 KB-Multimedia Appendix 3]
Multimedia Appendix 4
Considerations for remote measurement technology (RMT) implementation in real-world clinical settings.
[DOCX File , 366 KB-Multimedia Appendix 4]
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https://humanfactors.jmir.org/2023/1/e39479
XSL•FO
RenderX
JMIR Hum Factors 2023 | vol. 10 | e39479 | p. 14
(page number not for citation purposes)
JMIR HUMAN FACTORS
White et al
37.
38.
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compliance and satisfaction. J Telemed Telecare 2016 Nov 10;24(2):93-100. [doi: 10.1177/1357633x16679049]
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39. Hersenziekten de wereld uit helpen kan alleen met onderzoek. hersenonderzoek nl. URL: https://hersenonderzoek.nl/
[accessed 2023-01-13]
Abbreviations
ESM: experience sampling methodology
MDD: major depressive disorder
mHealth: mobile health
RADAR-MDD: Remote Assessment of Disease and Relapse-Major Depressive Disorder
RMT: remote measurement technology
Edited by A Kushniruk; submitted 13.05.22; peer-reviewed by A AL-Asadi, H Hsin; comments to author 06.10.22; revised version
received 07.10.22; accepted 07.11.22; published 26.01.23
Please cite as:
White KM, Dawe-Lane E, Siddi S, Lamers F, Simblett S, Riquelme Alacid G, Ivan A, Myin-Germeys I, Haro JM, Oetzmann C, Popat
P, Rintala A, Rubio-Abadal E, Wykes T, Henderson C, Hotopf M, Matcham F
Understanding the Subjective Experience of Long-term Remote Measurement Technology Use for Symptom Tracking in People With
Depression: Multisite Longitudinal Qualitative Analysis
JMIR Hum Factors 2023;10:e39479
URL: https://humanfactors.jmir.org/2023/1/e39479
doi: 10.2196/39479
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an open-access
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Myin-Germeys, Josep Maria Haro, Carolin Oetzmann, Priya Popat, Aki Rintala, Elena Rubio-Abadal, Til Wykes, Claire Henderson,
Matthew Hotopf, Faith Matcham. Originally published in JMIR Human Factors (https://humanfactors.jmir.org), 26.01.2023. This
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| null |
10.1088_2050-6120_acf97b.pdf
|
Data availability statement
All data that support the findings of this study are
included within the article (and any supplemen-
tary files).
|
Data availability statement All data that support the findings of this study are included within the article (and any supplementary files).
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Methods Appl. Fluoresc. 12 (2024) 015002
https://doi.org/10.1088/2050-6120/acf97b
PAPER
RECEIVED
6 February 2023
REVISED
27 April 2023
ACCEPTED FOR PUBLICATION
13 September 2023
PUBLISHED
12 October 2023
Emission color tuning and dual-mode luminescence thermometry
design in Dy3+/Eu3+ co-doped SrMoO4 phosphors
Vaibhav Chauhan1
Praveen C Pandey1,∗
, Prashant Dixit1,2, Prashant Kumar Pandey1, Satyam Chaturvedi1 and
1 Department of Physics, Indian Institute of Technology (Banaras Hindu University), Varanasi—221005, U.P., India
2 Department of Basic Science and Humanities, Maharana Pratap Engineering College (Affiliated to Abdul Kalam Technical University,
Lucknow), Kanpur, U.P., India
∗ Author to whom any correspondence should be addressed.
E-mail: [email protected]
Keywords: energy transfer, color tuning, dual-mode luminescence thermometry, temperature sensitivity, SrMoO4
co-doped SrMoO4 phosphors were developed as tunable color-
Abstract
The challenge of building a highly reliable contactless temperature probe with high sensitivity, good
temperature-induced color discriminability, and economical synthesis has prompted the research
community to work in the field of rare-earth-based luminescence thermometry. Moreover, the fast-
growing market for optoelectronic devices has increased the demand for tunable color-emitting
phosphors. In this study, Dy3+/Eu3+
emitting source and dual-mode luminescence thermometer. A facile and cost-effective auto-
combustion method was used to synthesize the phosphors. Our work demonstrates a viable scheme
for tailoring the emission of single-phase phosphors by precisely controlling the dopant concentra-
tions and by modulating excitation wavelength. The overall emission is tuned from greenish-yellow to
white and greenish-yellow to reddish-orange. A detailed energy transfer process from the host to the
Ln3+
Dy3+
studied by analyzing the fluorescence intensity ratio of Dy3+
The maximum relative sensitivity value for 4% Eu3+
−1 at 300 K. Furthermore, the configurational coordinate diagram is presented to elucidate the
K
nature of temperature-dependent emission. Therefore, our research opens up new avenues for the
development of color-tunable luminescent materials for various optoelectronic and temperature-
sensing applications.
ions and between the Ln3+
ions is discussed. Further, anti-thermal quenching in the emission of
ion is observed when excited with 297 nm. The dual-mode luminescence thermometry has been
ions upon excitation at 297 nm.
and Eu3+
co-doped SrMoO4:4%Dy3+
phosphor is 1.46%
1. Introduction
Scientists have endeavored in the exploration and
modification of new phosphors for the acquisition of
tunable color emission owing to their demand in a
variety of applications such as multicolor display
devices, encrypted information storage, white light-
emitting diode (wLED), biological applications, fluor-
escent sensors, etc [1–4]. In general, color tunability is
achieved by modifications in the crystal structure,
doping multiple rare-earth (RE) ions, altering the
excitation wavelength, crystal site engineering, or
changing the temperature [5–7]. Recently, much
attention has been paid to developing single host
material phosphors for the production of tunable light
© 2023 IOP Publishing Ltd
sources for different applications such as field emis-
sion displays, multicolor three-dimensional displays,
full-color flat panel displays, etc [8, 9]. These phos-
phors with tunable emission possess certain advan-
tages
correlated color
temperature (CCT) and adjustable color rendering
index (CRI) values, which are desirable for developing
tunable light sources.
specifically
suitable
for
The intriguing 4f–4f, 5d-4f, and charge transfer
(CT) transitions (ligand to metal or metal to ligand) of
RE ions have made these ions key components in var-
ious emerging applications such as luminescence ther-
mometry, lighting devices, optical fibres, solar cells,
display devices, lasers, and optical probes in biological
applications [10–16]. The 5d-4f and 4f-4f transitions
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
of the different lanthanide ions result in broad emis-
lines, respectively,
sion bands and sharp spectral
which range from UV to infrared region, and these
transitions are influenced by the polarizability and the
ligand field of the parent crystal. Over the past few
years, RE-doped phosphors have sparked a lot of inter-
est in developing adjustable luminescence via co-acti-
vation. Among the different lanthanide ions, the Dy3+
ion with its yellow (4F9/2 → 6H13/2) and blue (4F9/2 →
6H15/2) emissions has been utilized for the emission of
neutral white light [17, 18]. However, such white light
is devoid of a red component which results in low CRI
and elevated CCT value. Nevertheless, doping of red-
emitting RE ions such as Eu3+
can shift the overall
white light emission of the phosphor towards the red
region. Therefore, cool to warm white light color tun-
ability could be accomplished by precisely controlling
the doping concentration of Dy3+
ions in the
host and also by altering the excitation energy.
and Eu3+
One of the essential considerations for RE-based
phosphors is the host lattice. Different host lattices
such as vanadates [19], sulfides [20], phosphates [21],
silicates [22], garnets [23], aluminates [24], molyb-
dates [25, 26], tungstates [27], nitrides [28], or oxyni-
trides [29] has been explored as a host lattice for RE
ions for specific applications. Among all these, RE-
doped molybdate compounds have near-ultraviolet
(UV) excitation, blue-green broadband emission, and
good chemical and thermal stability, drawing atten-
tion to serving as excellent luminescent host phosphor
[30–32]. In this work, we have chosen SrMoO4 as our
model host lattice. Particularly, the SrMoO4 phosphor
is known as a self-activated luminous phosphor that
shows a CT absorption band of [MoO4]2−
groups
owing to charge transfer from O2−
to Mo6+
, situated
near the UV region and has greenish-blue emission
[33, 34]. The SrMoO4 phosphor shows the successful
transfer of the energy from the Mo6+
to the excited
energy levels of the RE ions and this property can be
utilized for exhibiting spectral color tunability in RE-
doped SrMoO4 phosphor.
Apart from tunable color light sources, phosphor
materials have also been studied to develop a lumines-
cent thermometer [35]. Luminescence thermometry
alleviates the problems inherent with traditional
temperature sensor systems as it provides a non-inva-
sive mode of operation, electromagnetic passivity, fast
response, remote readouts, and high-temperature
sensitivity. Various temperature-dependent spectral
parameters such as spectral position, polarization,
fluorescence intensity ratio (FIR), fluorescence life-
time, and bandwidth can be adopted to determine
temperature [36–38]. Among these, FIR is one of the
reliable thermometric parameters for luminescence
thermometry because of its relatively high resistance
to environmental interference. Many RE ions have
thermally coupled energy levels (TCELs) such as Eu3+
(5D0 and 5D1) [35], Dy3+ (4I15/2 and 4F9/2) [39], Er3+
(2H11/2 and 4S3/2) [40], and Nd3+ (4F5/2 and 4F3/2)
2
to Eu3+
[41] which is utilized for FIR based luminescence ther-
and Dy3+
mometry. Among these RE ions, the Eu3+
ions exhibit red, blue, and yellow emissions as a result
of the 5D0 → 7F2 (Eu3+), 4F9/2 → 6H15/2 (Dy3+), and
4F9/2 → 6H13/2 (Dy3+) transitions under UV excita-
tion. In this work, we have used a mixed lanthanide
, Eu3+) approach, which is based on the simulta-
(Dy3+
neous emission of two Ln3+
ions from the same host
material to design luminescence ratiometric thermo-
meters. However, a small overlap between Dy3+
exci-
tation spectra and Eu3+
emission spectra impose
difficulty in realizing the effective transfer of energy
from Dy3+
ions, therefore, it is crucial to find
an appropriate host phosphor having a wide absorp-
tion band and the ability to sensitize Dy3+
and Eu3+
ions simultaneously. To overcome this limitation we
have chosen SrMoO4 as a host phosphor for Dy3+
and
Eu3+
ions and observed an efficient transfer of energy
from the excited band of the host to the excited energy
levels of Dy3+
ions. We have observed that
the rise in temperature leads to the anti-thermal
quenching in Dy3+
emission and thermal quenching
in Eu3+
emission, causing contrasting variations in the
emission peak intensity of the two RE ions. We have
exploited this contrasting nature of Dy3+
and Eu3+
emission with rising temperature to investigate the
FIR-based temperature sensing.
and Eu3+
as
it offers
synthesis methods
In this paper, we have adopted a urea-based auto-
combustion process for the synthesis. The auto-com-
bustion process is an attractive alternative to conven-
tional
various
advantages such as simplicity, narrow particle size dis-
tribution, and cost-efficiency [42, 43]. We have stu-
died the luminescent properties and decay dynamics
thoroughly and explored the involved transfer of
energy from host to dopant ions and from Dy3+
to
Eu3+
ions. We have also accomplished spectral tun-
ability by adjusting the concentration of RE ions in
SrMoO4 and by altering the excitation energy. The
phosphors were also studied for the luminescence
thermometry application. Thus, our studied phos-
phors can potentially be used in crucial applications
such as w-LEDs, spectrally tunable devices, and lumi-
nescence thermometers.
2. Experimental
2.1. Chemicals
The raw materials for the synthesis were strontium
oxide (SrO), dysprosium nitrate (Dy(NO3)3 · xH2O),
europium nitrate (Eu(NO3)3 · 5H2O), ammonium
molybdate tetrahydrate ((NH4)6Mo7O24 · 4H2O), and
Urea. All precursors were purchased from Sigma-
Aldrich. The Dy3+ (0%, 2%, 3%, 4%, 5%) doped
SrMoO4 and Eu3+ (1%, 2%, 3%, and 4%) co-doped in
SrMoO4:4% Dy3+
phosphor are classified as D0, D2,
D3, D4, D5, E1, E2, E3, and E4, respectively.
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
2.2. Synthesis process
A simple urea-assisted auto-combustion procedure was
used to synthesize the phosphors. The stoichiometric
quantity of strontium oxide and RE nitrates were taken
in a clean beaker then HNO3 was gradually poured over
it. After adding around 10 ml of nitric acid, the solution
was constantly stirred for 1 h. To eliminate the surplus
acid, the mixture was placed under constant heating
(80 °C) and enriched with double-distilled (DD) water.
The stoichiometric quantity of molybdate precursor
was mixed with DD water in another beaker and placed
with constant stirring for 2 h. The Urea was mixed with
the molybdate solution. Metal nitrates and urea had a
mole ratio of 2:1. After that both the solutions were
mixed and then the resulting solution was maintained
at 100 °C until the surplus water was evaporated. After
drying, the mixture was placed in a hot air oven preset
to 250 °C for 12 h. The material was then powdered and
put in a closed furnace for 4 h of calcination at 1000 °C.
The same process was used to synthesize all of the
phosphors.
2.3. Characterisation
The XRD data was recorded using Cu Kα radiation
(l = 1.54 Å) in the powder x-ray diffractometer
(Rigaku-Mini FlexII). The structural refinement was
done with the help of FULLPROF software [44]. The
spectrometer (JASCO FT/IR 4600 having an attenu-
ated total reflection setup with a diamond disc as an
internal reflection element) was used to record the
FTIR scans of the phosphors. The absorption analysis
was done using the UV–vis-NIR spectrophotometer
(JASCO V770) equipped with an integrating sphere
setup. The absorption spectra of the powdered sam-
ples were obtained in an absorption mode after the
baseline correction using barium sulfate as a reference.
The Photoluminescence excitation (PLE), emission
(PL), and temperature-dependent PL (TDPL) spectra
analysis were done using a spectrophotometer (Horiba
Fluorolog-3). The slit width of the PL spectrophot-
ometer was fixed at 1 nm for all the measurements.
The PL lifetime was measured with a FLS920 Fluores-
Instruments)
cence
equipped with a 60 W xenon flash lamp. The x-ray
photoelectron spectrometer (Thermo Fischer Scienti-
fic ESCALAB Xi) was utilized for XPS measurements.
(Edinburgh
spectrometer
3. Results and discussion
3.1. XRD analysis
The Rietveld refinement of the XRD pattern was
performed to identify the crystal structure of the
prepared samples. Figure 1(a) illustrates the Rietveld
refined XRD patterns of D0, D4, and E4 phosphors
scanned over the 20° 2θ 80° range. The tetragonal
crystal structure with the I41/a space group of all the
phosphors is confirmed. Some prominent peaks are
labeled in the XRD graph and found in accord with
3
JCPDS file No. 85-0586 (a = b = 5.394 Å and
c = 12.020 Å). The Dy3+ (1.02 Å) and Eu3+ (1.06 Å)
ions are likely to replace Sr2+ (1.26 Å) ions because all
three have a comparable ionic radius for the system
with coordination number = 8 [45]. Figure 1(b)
depicts the tetragonal crystal structure of SrMoO4.
The Sr2+
ions are forming SrO8 dodecahedral with
eight oxygen ions, whereas, Mo6+
ions are forming
MoO4 tetrahedron which is coordinated by four
oxygen ions [34]. The lattice parameters, atomic
positions, and volume of the unit cell of D0, D4, and
E4 phosphors are summarized in table 1. The decre-
ment in the value of lattice parameters and consequent
contraction of unit cell volume results from the
variation between the ionic radius of the lanthanide
dopants (Dy3+
and Eu3+) and Sr2+
ion [33, 34].
3.2. FTIR analysis
The FTIR technique is employed to investigate the IR
active modes present in the phosphors. The FTIR
spectra of D0, D4, and E3 phosphors in the transmit-
tance mode are shown in figure 2. The antisymmetric
stretched vibrations corresponding to O–Mo stretching
in SrMoO4 are represented by the vibrational band from
−1 [46, 47]. The vibrational band
545 cm
−1 reflects the O–Mo–O bending vibra-
near 400 cm
tional mode present in the SrMoO4 [46, 47]. The band
−1 is because of the presence
from 2315 cm
of the C–O asymmetric stretching [43]. No significant
shift in the vibration bands is detected as a result of
doping. The FTIR spectra further justify the develop-
ment of the crystalline phase of D0, D4, and E3 samples.
−1 to 2375 cm
−1 to 908 cm
3.3. XPS analysis
The confirmation of the oxidation state of the elements
that existed on the surface of the D4 and E4 phosphors
is done using the XPS investigation. The C 1 s line at
284.6 eV, which arises owing to the existence of
unintended carbon on the surface of the phosphors
during ambient exposure, is used for charge correction
of XPS spectra of all the elements. Survey scans of both
the phosphors, shown in figure 3(a), validate the
existence of all elements such as Mo, Sr, Dy, Eu, and O
in D4 and E4 samples. All the peaks in the survey scan
are verified by the national institute of a standard
technology XPS database [48]. The survey scans also
confirm that there are no other impurities in the sample
than carbon. Figures 3(b) to (f) shows the XPS spectra of
Sr 3d, Mo 3d, O 1 s, Dy 3d, and Eu 3d for the D4 and E4
phosphors, respectively. Table 2 lists the corresponding
binding energy of the elements present in D4 and E4
samples based on their XPS spectra. The strontium XPS
spectrum as presented in figure 3(b), exhibits two main
peaks corresponding to 3d3/2 and 3d5/2 positioned at
134.0 eV and 132.4 eV, respectively [49]. The strontium
XPS graph verifies the divalent oxidation state of
strontium. Figure 3(c) presents the Mo 3d core-level
XPS spectra with peaks at ∼234.8 eV and ∼231.6 eV
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 1. (a) XRD patterns of (i) E4, (ii) D4, and (iii) D0 phosphors along with their Rietveld refinement profile. (b) SrMoO4 crystal
structure with SrO8 dodecahedron and MoO4 tetrahedron.
Table 1. Atomic positions, unit cell volume, and lattice parameters of D0, D4, and E4 phosphors.
Parameters
D0
D4
E4
) in degree
,
Atomic positions (x,y,z):
Eu/Dy/Sr
Mo
O
Angles (a b g
,
Lattice parameters (Å)
a
c
Unit cell volume (Å3)
Bond lengths (Å)
Sr/DyEu/-O1
Sr/Dy/Eu-O2
Mo-O
RFactors
Rp
Rwp
χ 2
(0,0.250,0.625)
(0,0.250,0.125)
(0.2384,0.1132,0.0449)
(90, 90, 90)
(0,0.250,0.625)
(0,0.250,0.125)
(0.2396,0.1128,0.0444)
(90, 90, 90)
(0,0.250,0.625)
(0,0.250,0.125)
(0.2383,0.1096,0.0442)
(90, 90, 90)
5.398
12.033
350.658
2.597 (3)
2.588 (3)
1.766 (3)
5.98
6.90
4.09
5.392
12.016
349.377
2.595 (3)
2.581 (3)
1.775 (3)
8.76
9.24
3.98
5.384
12.000
347.530
2.569 (3)
2.616 (3)
1.752 (5)
18.4
16.5
8.25
that are accredited to 3d3/2 and 3d5/2 levels of Mo6+
,
respectively. The Mo 3d XPS spectra confirm the +6
oxidation state of Mo [49]. Figure 3(d) depicts the O 1 s
peak at ∼529.5 eV. Figure 3(e) presents the XPS spectra
of Dy 3d for both phosphors. The two strong peaks at
∼1334.4 eV and ∼1303.2 eV are attributed to the 3d3/2
and 3d5/2 core level spin–orbit splitting components of
Dy, respectively [50]. The XPS spectra substantiate the
presence of dysprosium ions in the trivalent oxidation
state. The XPS spectra in figure 3(f) depict the peaks
and Eu3+
corresponding to Eu 3d for E4 phosphor. The spectra
contain four peaks ascribed to Eu2+
levels.
The peaks at 1163.8 eV and 1133.5 eV are accredited to
3d3/2 and 3d5/2 levels of Eu3+
. The peak-to-peak value
for these two peaks is 29.9 eV, which again validates the
trivalent oxidation state of Eu [24]. Two additional
peaks at 1153.4 eV and 1133.5 eV are accredited to the
3d3/2 and 3d5/2 levels of Eu2+
. The Eu 3d XPS spectrum
validates the existence of Eu ions in the +2 and +3
oxidation states in the E4 phosphor [43].
4
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 2. FTIR spectra of D0, D4, and E3 phosphor.
Figure 3. XPS Survey scan (a) and XPS graph of (b) Sr 3d, (c) Mo 3d, (d) O 1 s, (e) Dy 3d spectra for E4 and D4. (f) XPS spectrum of Eu
3d for E4 sample.
3.4. Absorption and bandgap study
The absorption spectra of the D and E phosphor series
are shown in figure 4(a). At 264 nm, the CT broadband
for D0 is detected, which is because of the charge
transfer from O2− → Mo6+
ions [51]. The absorption
band of Dy3+
doped phosphors is slightly moved to
the lower energy (higher wavelength) and peaks at
272 nm. The absorption peak further shifts to 317 nm
for Eu3+
co-doped phosphors. The red-shift in the
absorption peak is because of the presence of disorder
and defect states due to Dy3+
and Eu3+
ions [50]. The
Dy3+
4 f electrons introduce new energy
and Eu3+
5
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 4. (a) UV-visible absorption spectrum of the phosphors. (b) Tauc plot of all the phosphors. Inset shows the variation of
bandgap.
Table 2. The B.E of the elements present in D4 and E4
samples based on their XPS spectra.
Binding
energy (eV)
Elements
Oxidation state
D4
E4
accord with our previous work [33]. The value of Eg is
lowered as the Dy3+
and Eu3+
ions are doped in the
SrMoO4 matrix. The formation of intermediate defect
energy levels of Dy3+
and Eu3+
ions allows the flow of
electrons from O2−
to these levels, which results in the
lowering of bandgap energy.
Sr
Mo
O
Dy
Eu
Sr2+
Sr2+
Mo6+
Mo6+
3d3/2
3d5/2
3d3/2
3d5/2
O 1 s
Dy3+
Dy3+
Eu3+
Eu3+
Eu2+
Eu2+
3d3/2
3d5/2
3d3/2
3d5/2
3d3/2
3d5/2
134.0
132.4
234.8
231.6
529.5
1334.4
1303.2
—
—
—
—
134.0
132.3
234.7
231.6
529.5
1334.1
1304.1
1163.8
1133.4
1153.4
1124.0
levels close to the conduction band of SrMoO4. Below
the conduction band, new defect band forms, and the
transfer of electrons for the O2−
to this defect band
causes a red shift in the absorption band. A small peak
ascribed to the 7F0 → 5D2 electronic transition is
observed around 464 nm for Eu3+
co-doped samples.
The tauc equation is used to compute the bandgap
of phosphors [52].
a n
h
=
(
A h
n
-
E
g
)
n
,
( )
1
Where a signifies the absorption coefficient,
nh
signifies the energy of photons, and n represents the
nature of electronic transitions. The n is taken to be ½
because SrMoO4 exhibits direct allowed electronic
transitions [34, 53]. Figure 4(b) depicts the tauc plot of
the prepared phosphors. The Eg is estimated by
extending the linear section of the tauc figure and the
x-intercept for (a nh )2 = 0 yields the value of Eg in eV.
The calculated value of Eg for D0 is 4.12 eV, and is in
6
3.5. PLE and PL analysis
Figure 5 presents the PLE spectrum of D0 upon
monitoring the emission at 500 nm. The broadband
around 297 nm is the ligand to metal charge transfer
(LMCT) band, which is corresponding to the charge
transfer from O2− → Mo6+
groups of
SrMoO4 phosphor.
in [MoO4]2−
Figure 6(a) represents the PLE spectrum of Dy3+
doped SrMoO4 phosphors
examined at 486 nm
(4F9/2 → 6H15/2). The broadband is observed because of
the overlap between the O2− → Mo6+
LMCT and O2− →
Dy3+
charge transfer bands (CTBs). The broadband is
blue-shifted as the concentration of Dy3+
increases.
Some characteristic 4f-4f transition peaks corresp-
onding to Dy3+
ions are also observed. These peaks are
at 327 nm (6H15/2 → 6P3/2), 352 nm (6H15/2 → 6P7/2),
366 nm (6H15/2 → 4I11/2), 388 nm (6H15/2 → 4I13/2),
427 nm (6H15/2 → 4G11/2), and 453 nm (6H15/2 →
4I15/2). The PLE intensity of all the peaks increases with
increasing Dy3+
concentration upto 4% concentration
and decreases with further doping. The broadband,
resulting from the LMCT and CTB, has the maximum
intensity and is therefore used as an excitation wave-
length while recording the PL spectrum. The 352 nm
peak, which is solely because of the Dy3+
ion is also
chosen as the excitation wavelength for recording the
PL spectrum, as its intensity is highest among the other
characteristic peaks of Dy3+
ions. Figure 6(b) presents
the PLE spectrum of Eu3+
co-doped phosphors exam-
ined at 615 nm (5D0 → 7F2) emission transition of Eu3+
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 5. PLE (in violet) and PL (in green) spectrum of D0 phosphor examined at 500 nm emission wavelength and 297 nm excitation
wavelength, respectively.
Figure 6. PLE spectrum of (a) D series and E series phosphors monitored at 486 nm emission wavelength, and (b) Eu3+
phosphors.
co-doped
ion. Similar broadband as in the case of the D series is
observed for the E series as well which is accredited to
LMCT and O2− →
the overlap of O2− → Mo6+
Dy3+/Eu3+
CTBs. In addition to the broadband, some
characteristic 4 f–4 f transition peaks of Eu3+
ions are
also observed at 362 nm (7F0 → 5D4), 376 nm (7F0 →
5G3), 384 nm (7F0 → 5G4), 394 nm (7F0 → 3L6), 416 nm
(7F0 → 5D3), 464 nm (7F0 → 5D2), and 534 nm (7F0 →
5D1). All the observed excitation peaks in the spectrum
are in good agreement with the reported literature [43].
doped SrMoO4 presented
in figures 7(a) and (b), consist of bands centered at
The PL spectra of Dy3+
486 nm (blue region) and 572 nm (yellow region),
corresponding to 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2
electronic transitions, respectively [50]. Additionally,
two bands of low intensity, located at 454 nm and
660 nm, corresponding to 4I15/2 → 6H15/2 and
4F9/2 → 6H11/2 transitions were also observed. All the
observed emission peaks corresponding to Dy3+
ions
are in accord with the previous reports [50]. The
4F9/2 → 6H13/2 transition is a hypersensitive electric
dipole (ED) transition and its domination in the spec-
trum indicates that the Dy3+
ions occupy sites of non-
inversion symmetry. The 4F9/2 → 6H15/2 transition is
7
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 7. PL spectrum of Dy3+
intensity as a function of Dy3+
doped phosphors upon excitation at (a) 297 nm and (b) 352 nm. Inset in 7 (a) showing the normalized
concentration.
the magnetic dipole (MD) transition. The inset in
figure 7(a) shows that the intensity of 572 nm peak
increases with the increasing Dy3+
concentration. The
peak intensity increases with the increasing Dy3+
con-
centration in the host matrix and attains maximum for
the D4 sample. The emission intensity decreases for
D5 phosphor because of the concentration quenching.
Figure 8(a) presents the comparative PL spectra of
D4 and E series phosphors examined at 297 nm excita-
tion wavelength. In addition to the emission peaks
corresponding to Dy3+
ions, three peaks at 536 nm
(5D1 → 7F1), 615 nm (5D0 → 7F2), and 700 nm (5D0 →
7F4), originating from Eu3+
ions are also observed. The
normalized intensity variation of 573 nm (4F9/2 →
6H13/2) peak and 615 nm (5D0 → 7F2) peak with differ-
ent Eu3+
co-doping concentrations is shown in
figure 8(b). It is noticed that with increasing doping
concentration of Eu3+
ion, the intensity of 573 nm
peak decreases while the intensity of 615 nm peak
increases, which suggests that the energy transfer from
[MoO4]2−
LMCT band to Eu3+
ions is more probable
than energy transfer to Dy3+
ions. To examine the
energy transfer from Dy3+
ions, the Eu3+
co-doped SrMoO4:4%Dy3+
phosphors were excited
with 352 nm, depicted in figure 8(c). The intensity of
the emission peaks corresponding to the electronic
transitions in the Eu3+
ions is very weak as compared
to the Dy3+
ion emission peaks, which clarifies that the
energy transfer from Dy3+
to Eu3+
ions is poor.
Figure 8(d) depicts the PL spectrum of the E series
phosphors examined at 464 nm excitation wave-
length. Some of the characteristic emission peaks of
Eu3+
ions are observed at 509 nm (5D2 → 7F3), 535 nm
(5D1 → 7F1), 555 nm (5D1 → 7F2), 591 nm (5D0 → 7F1),
615 nm (5D0 → 7F2), 654 nm (5D0 → 7F3), and 700 nm
(5D0 → 7F4) [54, 55]. The 5D0 → 7F1 transition of Eu3+
at 591 nm is allowed by MD transition and is
ions to Eu3+
8
at the Sr2+
unaffected by Eu3+
ion surrounding, whereas the
5D0 → 7F2 transition of Eu3+
at 615 nm is allowed
by ED transition and is hypersensitive to the local
symmetry of the Eu3+
ion [55]. The fact that the ED
transition is much more intense than the MD trans-
ition indicates that Eu3+
site deviates from
its inversion symmetry and the local symmetry around
ion is low [55]. Moreover, when Eu3+
the Eu3+
is co-
doped in SrMoO4:4%Dy3+
, there is a charge and size
mismatch which causes cation vacancies and lattice
strain, respectively. These cause a distorted local
environment around Eu3+
ions and a lowering of sym-
metry, which causes an intense ED transition peak. The
splitting in the emission peaks is called stark splitting
and it is due to the ligand field effect when rare-earth
ions are inserted into a ligand environment.
3.6. Energy transfer dynamics
The non-radiative energy transfer among the RE ions
at higher doping concentrations results in the concen-
tration quenching of emission peaks. A non-radiative
energy transfer can occur by one of three mechanisms:
exchange interaction, radiative reabsorption, or elec-
tric multipolar interaction. The radiative reabsorption
requires a substantial overlap between the emission
and the excitation spectra. As a consequence of the
limited spectrum overlap between the Dy3+
emission
and Eu3+
radiation reabsorption is
neglected in this scenario. The donor and acceptor
wave functions must have a considerable direct or
indirect overlap for exchange interactions to take
place. According to the Van Uitert theory, the critical
separation distance (Rc) between the emitting ions for
wavefunction overlapping must be less than 5 Å [56].
The Rc in the case of Dy3+/Eu3+
co-doped SrMoO4 is
evaluated using the formula derived by Blasse [57],
excitation,
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 8. (a) Comparative PL spectra of D4 and E series upon excitation at 297 nm. (b) The normalized intensity of 4F9/2 → 6H13/2 and
5D0 → 7F2 transitions as a function of Eu3+
excitation wavelength. (d) Comparative PL spectra of E series monitored at 464 nm excitation wavelength.
concentration. (c) Comparative PL spectra of D4 and E series monitored at 352 nm
=R
c
2
1
3
3
V
p
4
x N
c
⎜
⎛
⎝
⎟
⎞
⎠
( )
2
, Eu3+
Where the volume of SrMoO4 is designated by V and
from XRD analysis, it is calculated to be 347.53 Å3. The
value of xc represents the total critical concentration of
Dy3+
and Eu3+
ions which is 0.08. The number of
Dy3+
occupied sites is represented by N, which is 4 for
the SrMoO4 lattice. The evaluated critical distance of
SrMoO4:Dy3+
phosphor is 12.75 Å, which is
substantially higher than the critical distance con-
straint. This rules out the possibility of an exchange
interaction process for non-radiative energy transfer.
Therefore, the non-radiative energy transfer could be
only due to the electric multipolar interaction. Electric
multipolar interactions are classified as dipole–dipole,
dipole-quadrupole, and quadrupole-quadrupole. The
type of multipolar interaction can be asserted by
Dexter’s energy transfer mechanism for multipolar
interaction and Reisfeld’s approximation [58],
I
0
I
3
⎛
⎝
/µ
n
C
⎞
⎠
Where I and I0 denote the PL intensity of Dy3+
presence and absence of Eu3+
in the
, respectively. The total
( )
3
C ,
to Eu3+
interactions,
and Eu3+
doping concentration of the Dy3+
ions is
represented by C. I0/
n 3 with n = 6, 8, and 10
/µI
related to dipole–dipole, dipole–quadrupole, and
quadrupole–quadrupole
respectively.
The plots of I0/I as a function of
/Cn 3 are presented in
figure 9(a). It is observed that the linear dependency of
the dipole–dipole interaction is the best-fitting solu-
tion. This suggests that the transfer of energy from
Dy3+
is primarily a dipole–dipole interaction.
Figure 9(b) depicts the schematic of the energy
transfer process utilized to inspect the mechanism of
energy transfer in SrMoO4:Dy3+
phosphors.
The emission broadband of the SrMoO4 host peaks
around 500 nm (in figure 5) and it overlaps with the
excitation spectra of Dy3+
ions (in figure 6),
implying the possibility of the energy transfer from the
[MoO4]2−
ions. When
the electrons in the 2p valance band of the O2−
absorb
297 nm UV photons, they are excited to the Mo6+
4d
conduction band. After that, electrons are transferred
to the Dy3+
energy levels, resulting in yellow
and red emissions, respectively. It should be noted that
on increasing the Eu3+
concentration, the intensity of
the Eu3+
peaks increases while the intensity of the
LMCT band to Dy3+
and Eu3+
and Eu3+
and Eu3+
, Eu3+
9
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 9. (a) I0/ I as a function of (1) C6/3, (2) C8/3, and (3) C10/3 (b) Schematic of the energy transfer phenomenon in SrMoO4:Dy3+
Eu3+
.
,
LMCT band to Eu3+
Dy3+
peaks decreases (in figure 8(a)). For the E4 sam-
ple, the intensity of the peak at 615 nm (5D0 → 7F2) is
more than the peak at 573 nm (4F9/2 → 6H13/2). The
reason behind this could be that the intense excitation
peak of the Eu3+
ion at 464 nm lies much closer to the
broadband emission peak of the [MoO4]2−
LMCT
band (500 nm), which prompts effective energy trans-
fer from [MoO4]2−
levels. To
to Eu3+
study the energy transfer process from Dy3+
ion, we have recorded the PL spectra of co-doped sam-
ples excited by 352 nm. The intensity of the Eu3+
emission peaks is very less in comparison to the Dy3+
emission peaks, indicating the weak energy transfer
from Dy3+
. The energy transfer from
[MoO4]2−
energy
levels are marked with ET1 and ET2, respectively, and
the energy transfer from Dy3+
ions is marked
with ET3 in the energy diagram schematic presented
in figure 9(b).
LMCT band to Dy3+
and Eu3+
to Eu3+
to Eu3+
3.7. PL Decay study
Figures 10(a) and (b) present the luminescence decay
curves of D4 and E4 phosphors with emission
monitored at 572 nm and excited at 352 nm. The bi-
exponential function in equation (4) is used to fit the
decay curves [59],
( )
I t
= +
I
0
B
1
exp
-
⎜
⎛
⎝
t
t
1
⎟
⎞
⎠
+
B
2
exp
-
⎜
⎛
⎝
t
t
2
⎟
⎞
⎠
( )
4
Where t1 signifies fast decay time and t2 signifies the
slow decay time of the 4F9/2 level of the Dy3+
ion; I0 is
the background intensity after prolonged excitation;
B1 and B2 are constants. The average lifetime is
evaluated using the following equation [33]:
t
o
=
(
A
2
t
1 1
+
A
2
t
2 2
/
) (
A
t
1 1
+
A
t
2 2
)
( )
5
According to the preceding relation, the average
lifetime of the 4F9/2 level of the Dy3+
for D4 and E4 is
10
0.936 ms and 0.894 ms, respectively. After Eu3+
doping, the lifetime of Dy3+
the energy transfer from Dy3+
co-
ions is reduced, validating
ions.
to Eu3+
The following relation is utilized for the computa-
tion of energy transfer efficiency (hET
),
h
ET
= -1
t
x
t
o
( )
6
Where to and tx are the average lifetime of the 4F9/2
level. The calculated value of hET is 4.69%, indicating
weak energy transfer from Dy3+
ions in
SrMoO4.
to Eu3+
co-doped
concentration,
3.8. Tunable color study of Dy3+/Eu3+
SrMoO4
Figure 11 presents the Commission Intentional de
I’eclairage (CIE) diagram of the prepared phosphors as
determined by their respective PL spectra. The CIE
coordinates along with color-correlated temperature
(CCT) and color purity are tabulated in table 3.
Figure 11(a) depicts the CIE diagram of the Dy3+
doped and Dy3+/Eu3+
co-doped phosphors when
excited by 297 nm wavelength. With an increase in
Dy3+
the overall emission shifts
towards the yellow region but after Eu3+
co-doping,
red region.
the
Figure 11(b) depicts the CIE diagram of Dy3+
doped
phosphors when excited by 352 nm wavelength. It is
observed that the overall emission of D2 phosphor lies
in the neutral white region with 4.1% color purity and
the emission of D4 phosphor is shifted more towards
the yellow region with 61.1% color purity. Therefore,
the variation in the overall color of Dy3+
doped
phosphors
is more when excited by 352 nm.
Figure 11(c) depicts the CIE diagram for the E4
sample, excited with different wavelengths. It
is
observed that the overall emission of the E4 phosphor
can be adjusted from warm white to red color by
emission shifts
towards
the
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 10. PL decay curve of (a) D4 and (b) E4 phosphors.
Figure 11. CIE chromaticity diagram of (a) Dy3+/Eu3+
SrMoO4 phosphors examined at 352 nm excitation, and (c) E4 phosphor under different excitation wavelengths.
co-doped SrMoO4 phosphors under 297 nm excitation, (b) Dy3+
doped
altering the excitation wavelength. As a result, by
altering concentration and excitation wavelength, the
color of the Dy3+/Eu3+
doped SrMoO4 phosphors
can be tuned.
The CCT of the prepared phosphors is calculated
using McCAMY’s approximated equation [60].
CCT
=
5520.33
-
6823n
+
2
3525n
-
3
449n
( )
7
Where x
e are the chromaticity epicenters
(0.338, 0.186), x and y represent the CIE coordinates,
and y
e
11
line
slope
inverse
.
represented by
is
and the
= -
)
(
x
x
e
n
The calculated CCT values for the E4
-
)
(
y
y
e
sample at 297 nm, 352 nm, and 454 nm are 1590 K,
3612 K, and 1174 K. The lower value of CCT at
454 nm signifies that the overall emission lies in the
red region. Whereas, the higher value of CCT at
352 nm signifies that the overall emission is warm
white. The CCT values verify the shift in the overall
emission of
the E4 phosphor with changing
temperature.
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 12. Temperature-dependent PL of D4 phosphor excited at (a) 297 nm, (b) 352 nm. Temperature-dependent PL of E4
phosphor examined at (c) 464 nm, (d) 297 nm. Inset shows the thermal dependence of the emission intensity of the bands at 572 nm
and 615 nm for E4 phosphor.
Table 3. CIE coordinates, CCT value, and CP of prepared
phosphors.
Sample
code
Excitation
wavelength
(x, y)
CCT (K)
CP (%)
D2
D3
D4
E1
E2
E3
E4
297
352
297
352
297
352
297
297
297
297
352
464
(0.3874, 0.3983)
(0.3408, 0.3395)
(0.3812, 0.4201)
(0.3695, 0.4043)
(0.3939, 0.4409)
(0.4055, 0.4642)
(0.4612, 0.4168)
(0.3121, 0.3945)
(0.3205, 0.3952)
(0.5589, 0.3750)
(0.4054, 0.4053)
(0.6302, 0.3663)
4072
5135
4363
4461
4195
3975
2826
2072
2002
1590
3612
1174
35.8
4.1
40.5
32.3
50.6
61.1
63.6
13.3
15.3
80.3
43.3
99.1
The following expression is used to evaluate the
color purity (CP) which is the measure of mono-
chromaticity,
color purity
(
%
)
=
(
x
s
-
2
x
)
i
+
(
y
s
-
2
)
y
i
(
x
d
-
2
x
)
i
+
(
y
d
-
2
)
y
i
( )
8
Where (x y,s
) are the sample emission color coordi-
s
nates relative to the standard CIE1931 coordinates
( =
)
0.3333
x
the
i
and (x
0.3333,
are
=
)
y,d
d
y
i
12
) and (x y,s
s
dominant wavelength coordinates. The dominant
wavelength coordinates are derived by drawing a
) points to cut
straight line from the (x y,i
i
the point on the perimeter of the CIE color space. The
higher value of color purity refers to the high mono-
chromaticity of particular color. It is observed that for
the E4 sample, the CP value varies significantly from
43.3% to 99.1% by varying the excitation wavelength
from 352 nm to 464 nm. The CP values for the E4
sample establish that the emission of the E4 sample
can be tuned by changing the excitation wavelength.
and Eu3+
3.9. Luminescence thermometry
The temperature dependence of emission intensity of
Dy3+
ions can be considered for applications
in luminescence thermometry. Figures 12(a) and (b)
present the temperature-dependent PL spectra of D4
phosphor examined at 297 nm and 352 nm excitation
wavelength, respectively, for the 300 K–520 K temper-
ature range. It is observed that when excited with
297 nm wavelength, all emission peaks of Dy3+
ions
show an increasing trend with temperature. This
increase in PL intensity of all the peaks is because of the
anti-thermal quenching phenomenon. However,
when excited with 352 nm, the peaks originating from
the 4F9/2 energy level of Dy3+
ion decrease while the
intensity of the peak corresponding to 4I15/2 → 6H15/2
(at 454 nm) increases. This contrasting PL intensity
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 13. (a) The FIR variation with temperature, (b) plot of ln (FIR) versus 1/T, (c) absolute sensitivity (Sa), and (d) relative
sensitivity (Sr) as a function of temperature for E4 phosphor.
Table 4. Comparison of relative sensitivity for reported phosphors
and synthesized E4 phosphor.
Host
Activated ions
Sr
(% K
−1)
T (K)
Reference
KBaGd(WO4)3
MgAl2O4
BaLa4Si3O13
GdVO4
LaOBr
Mg3Y2Ge3O12
SrMoO4
Dy3+/Eu3+
Dy3+/Eu3+
Dy3+/Eu3+
Dy3+/Eu3+
Ce3+/Tb3+
Eu3+/Mn4+
Dy3+/Eu3+
0.64
0.24
1.46
0.016
0.42
0.019
1.46
0.74
0.48
458
420
497
298
433
300
300
420
520
[24]
[62]
[64]
[65]
[66]
This work
variation in D4 phosphor with temperature is because
of the thermally coupled nature of 4I15/2 and 4F9/2
energy levels of Dy3+
ion [61]. As the temperature
increases, the relative population of the thermally
coupled levels is governed by the Boltzmann distribu-
tion law and electrons are transferred from 4F9/2 to
4I15/2 energy level because of which the PL intensity of
454 nm peak increases while that of others decreases
[61]. Figure 12(c) and (d) depict the temperature-
dependent PL spectra of E4 phosphor examined at
464 nm and 297 nm, respectively, for the 300 K–520 K
temperature range. The PL intensity corresponding to
the Eu3+
ion gradually decreases in both cases. The
13
inset in figure 12(d) depicts the normalized intensity
variation of 572 nm (4F9/2 → 6H13/2) and 615 nm
(5D0 → 7F2) peaks of Dy3+
ions with
temperature, respectively.
and Eu3+
The contrasting response of Dy3+
emis-
sion to temperature can be utilized for FIR-based
luminescence thermometry. The FIR (IDy/IEu) is given
by the following equation [62],
and Eu3+
FIR
=
I
572
I
615
= +
A
B
exp
-
( ∆
/
E kT
)
( )
9
Where ΔE is the necessary activation energy for the
non-radiative process, k represents the Boltzmann
constant, A represents the proportional parameter,
and B represents an offset parameter. The FIR
variation with temperature is presented in figure 13(a).
Figure 13(b) depicts the ln (FIR) versus 1/T plot,
where the slope of the linearly fitted line reveals the
−1. The
value of activation energy is 1315.632 cm
thermal sensitivity can further be investigated by
evaluating the absolute and relative sensitivity by the
(10) and (11) equations [62],
S
a
=
¶
FIR
¶
T
=
C
exp
-
( ∆
/
E kT
)
´
∆
E
2
kT
(
)
10
r
S
=
1
FIR
¶
FIR
¶
T
Where the relative sensitivity (Sr) represents the
relative change of the FIR per degree of temperature
100%
)
11
´
(
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
Figure 14. (a) Configurational coordinate diagram for explaining the temperature-induced luminescence processes in E4 phosphor.
(b) CIE color coordinates of E4 sample examined at 297 nm excitation wavelength and different temperatures presented in CIE
diagram.
−2 K
−1). The dependence of Sa and Sr on
change (in % K
temperature is depicted in figures 13(c) and (d),
respectively. The enhancement in Sa is seen with
−1)
temperature with the maximum Sa (1.06 ´ 10
at 520 K. The Sr value is considered the critical
parameter for practical application. The maximum Sr
−1) is obtained at 300 K and decreases
value (1.46% K
with increasing temperature. The Sr value at various
temperatures and its comparison with some of the
data reported in other literature is tabulated in table 4.
Moreover, commercial optical thermometers based
on the green luminescence of Er3+
ion, ascribed to the
2H11/2/4S3/2 → 4I15/2 transitions, has been a useful
tool for optical ratiometric thermometry for more
than thirty years. However, its relative sensitivity is
−1 at room temperature
relatively low, being 1.1% K
[63]. The high Sr value indicates the good signal-
sensing capability of the phosphor. The CIE coordi-
nates of the E4 sample at 300 K, 420 K, and 520 K are
presented in the CIE diagram depicted in figure 14(b).
The well-separated peaks of Dy3+ (572 nm) and Eu3+
(615 nm) result in the good color discriminability of
the E4 phosphor with increasing temperature, which
is vital for temperature detection.
and Eu3+
The configurational coordinate diagram (CCD),
shown in figure 14(a), helps explain the process under-
lying the contrasted variance in PL intensity related to
the Dy3+
ions. The CCD depicts Born–
Oppenheimer potential energy parabolic Ug and Ue
curves representing the ground and excited states of the
system, respectively, as a function of generalized config-
urational coordinate ‘Q’. The Ue state is formed as a
14
and Eu3+
and Eu3+
LMCT and O2− → Dy3+
result of O2− → Mo6+
CTB.
The yellow and red parabola represents the electronic
states of Dy3+
ions, respectively. At room
temperature (300 K), the electrons after absorbing pho-
tons will be excited from the Ug to Ue state (path 1). After
absorption, the electrons reach the bottom of the Ue
state by vibrational relaxation (path 2). Then the elec-
trons can transfer to the 4F9/2 level of Dy3+ (path 3) or
5D0 level of Eu3+
ion (path 4). The electrons then revert
to the corresponding ground states emitting character-
istic emission lines of Dy3+
ions. With temp-
erature rise, the lattice vibrations become stronger and
the thermally active phonons increase. The electrons
near the bottom of the Ue band pair with the thermally
active phonons, and their energies approach the cross-
over point A. These electrons move to the 4F9/2 energy
level of the Dy3+
ion (path 5). Therefore, as the temper-
ature rises, the number of electrons in the 4F9/2 energy
level of the Dy3+
ion increases (by paths 3 and 5), which
increases the PL intensity of the Dy3+
ion. The electron–
phonon interaction becomes stronger as temperature
rises and more and more electrons follow path 5, there-
fore, there is a shortage of electrons following path 4
which results in a decrease in the PL intensity of Eu3+
ions. This explains the nature of temperature-depen-
dent PL of Dy3+/Eu3+
co-doped SrMoO4.
4. Conclusion
In conclusion, a facile urea-assisted auto-combustion
method was used to prepare Dy3+/Eu3+
co-doped
Methods Appl. Fluoresc. 12 (2024) 015002
V Chauhan et al
and Eu3+
SrMoO4 phosphors. The structural analysis validates
the formation of the tetragonal crystal structure of the
phosphors. The absorption spectra ascertain the shift
in the absorption peak after Dy3+
doping,
which is attributed to the energy levels generated
within the bandgap of SrMoO4. The PL emission of
Dy3+/Eu3+
co-doped samples was monitored at
different excitation wavelengths. For the co-doped
samples, the PL intensity of Eu3+
ions is greater for
297 nm excitation than for 352 nm excitation. This is
the effective transfer of energy from
because of
[MoO4]2−
LMCT band to Eu3+
energy levels and the
inefficient transfer of energy from Dy3+
to Eu3+
ions.
The reduction in the lifetime of the 4F9/2 level of Dy3+
with Eu3+
co-doping confirms the weak energy
transfer from Dy3+
to Eu3+
ions. Dexter’s energy
transfer theory indicates that the energy transfer from
Dy3+
follows a non-radiative dipole–dipole
interaction. The emitting color of
the co-doped
samples can be adjusted from white to greenish-yellow
and greenish-yellow to reddish-orange by precisely
controlling the contents of Dy3+
ions and by
modulation of excitation wavelength. The synthesized
Dy3+
doped SrMoO4 phosphors exhibit an anti-
thermal phenomenon owing to the energy transfer
to the Dy3+
from the conduction band of [MoO4]2−
energy levels. The contrasting nature of Dy3+
and
Eu3+
PL emission with temperature is used to probe
the temperature sensing property of the Dy3+/Eu3+
co-doped SrMoO4 phosphor. Based on FIR mode, the
co-doped SrMoO4:4%Dy3+
Sr value for 4% Eu3+
−1 at 300 K. The configurational
phosphor is 1.46% K
coordinate diagram is used to demonstrate the nature
temperature-dependent PL of Dy3+/Eu3+
co-
of
doped SrMoO4 phosphor.
and Eu3+
to Eu3+
Acknowledgments
Vaibhav Chauhan is thankful to CSIR, India for the
financial support provided in the form of a senior
research fellowship grant. Prashant Dixit is thankful to
UGC, India for the financial support provided in the
form of a senior research fellowship grant. Prashant
Kumar Pandey acknowledges the Ministry of educa-
tion, India for the teaching assistantship. The authors
are thankful to Prof. S. B. Rai for PL lifetime measure-
ments. The authors are thankful to DST-FIST for
providing a UV–vis spectrophotometer and FTIR
spectrometer. The authors have declared that no
conflicting interests exist.
Data availability statement
All data that support the findings of this study are
included within the article (and any supplemen-
tary files).
15
ORCID iDs
Vaibhav Chauhan
4853-0225
Praveen C Pandey
6051-1994
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https://orcid.org/0000-0002-
https://orcid.org/0000-0001-
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17
| null |
10.1371_journal.pone.0238646.pdf
|
Data Availability Statement: All relevant data are
within the paper.
|
All relevant data are within the paper.
|
RESEARCH ARTICLE
Endoscopic soft palate augmentation using
injectable materials in dogs to ameliorate
velopharyngeal insufficiency
Emiko Tanaka IsomuraID*, Makoto Matsukawa☯, Kiyoko Nakagawa☯, Ryo Mitsui☯,
Mikihiko Kogo☯
First Department of Oral and Maxillofacial Surgery, Osaka University, Graduate School of Dentistry, Suita
City, Osaka, Japan
☯ These authors contributed equally to this work.
* [email protected]
Abstract
Background
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
OPEN ACCESS
Citation: Isomura ET, Matsukawa M, Nakagawa K,
Mitsui R, Kogo M (2020) Endoscopic soft palate
augmentation using injectable materials in dogs to
ameliorate velopharyngeal insufficiency. PLoS ONE
15(9): e0238646. https://doi.org/10.1371/journal.
pone.0238646
Velopharyngeal structure augmentation methods are used as alternatives to pharyngeal
flap operations. Recently, we investigated the sites of velopharyngeal structure augmenta-
tion in dogs and reported that the most effective injection location is the soft palate. How-
ever, there have been no reports regarding the optimal materials for implantation or
injection. In this study, we aimed to investigate the injectable materials used in soft palate
augmentation in dogs to ameliorate velopharyngeal insufficiency (VPI).
Editor: Mrinmoy Sanyal, Stanford University
School of Medicine, UNITED STATES
Methods
Received: February 25, 2020
Accepted: August 20, 2020
Published: September 4, 2020
Copyright: © 2020 Isomura et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper.
Funding: This study was supported by Grants-in-
Aid for Scientific Research from the Japan Society
for the Promotion of Science (Grant Number
19K19229). Neither of the authors has a financial
interest in any of the products, devices, or drugs
mentioned in this manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Endoscopic soft palate augmentation (ESPA) was performed in dogs using purified sodium
hyaluronate, atelocollagen, or autogenic fat tissue. ESPA is an original technique devel-
oped by our group, and this is the first report of its performance. Moreover, we assessed
the amount of nasal air leakage during inspiration at rest and during expiration under the
rebreathing system at 1, 2, 3, 4, 5, and 6 months after injection of these materials.
Results
The amount of nasal air leakage during expiration under the rebreathing system was signifi-
cantly decreased in all dogs injected with the ESPA materials, but neither apnea nor hypop-
nea was observed.
Conclusions
We investigated the optimal materials for use in ESPA, such as purified sodium hyaluronate,
atelocollagen, or autogenic fat tissue. We found that all of them reduced nasal air leakage
and only autogenic fat tissue showed significant histologic differences in dogs at 6 months.
This technique may also be useful for the treatment of patients with VPI.
PLOS ONE | https://doi.org/10.1371/journal.pone.0238646 September 4, 2020
1 / 11
PLOS ONEEndoscopic soft palate augmentation in dogs
Introduction
When treating patients with cleft palate, velopharyngeal insufficiency (VPI) can sometimes
occur after palatoplasty. VPI is the failure of the nose and mouth to separate during speech
because of an anatomical dysfunction of the soft palate. Many cases of VPI are due to shortfall,
poor movement of the soft palate caused by scarring, or poor reconstruction of the muscles of
the soft palate. Furthermore, patients with 22q11.2 deletion syndrome have VPI due to inade-
quate soft palate muscle formation.
In our hospital, speech therapy is the first step in the treatment of VPI. If VPI cannot be
managed with speech therapy, a speech aid is used for closure of the nasopharynx by lifting the
soft palate or filling the gap. Then, after VPI is shown to be controlled with the speech aid, pha-
ryngeal flap surgery is performed to wean the patient off the speech aid [1–3]. However, it is
difficult to apply this treatment in children because it causes fundamental changes to the velo-
pharyngeal form, which may result in sleep apnea or inability to perform nasal intubation dur-
ing future orthodontic surgeries [4–7].
Several reports have described another method for treating VPI [8–22]. Velopharyngeal
structure augmentation is an alternative to pharyngeal flap surgery that utilizes an injectable
material implanted into the tissue around the velopharynx. However, it is not yet a standard
treatment because it has not been extensively studied. Anatomic sites and injection materials
vary widely, owing to the lack of standardized criteria, and their effects also differ among
institutions.
Recently, we investigated the sites of velopharyngeal structure augmentation in dogs and
reported that the most effective injection location is the soft palate, rather than the posterior
pharyngeal wall or bilateral pharyngeal walls [23]. Dogs’ velopharynx exhibit inherently like
VPI; thus, the rhinopharynx is not completely closed, even when the soft palate is lifted [24].
We injected saline intraorally, in 1-mL increments, into the nasal side of the soft palate, poste-
rior pharyngeal wall, or bilateral pharyngeal walls of each dog. The soft palate that was injected
with saline achieved steady augmentation, and nasal air leakage disappeared following the
5-mL saline injection. Conversely, nasal air leakage persisted in the dogs with saline injected in
the posterior pharyngeal wall or bilateral pharyngeal walls.
There have been no reports about the optimal materials for implantation or injection,
however. There are various artificial and biological materials that may be used in the velophar-
yngeal structure, including silicone, Teflon, porous polyethylene, Gore-Tex1, calcium
hydroxyapatite, auricular or costal cartilage, and autologous fat [8–22]. However, when we
augment the soft palate, the material needs to be injectable, because it cannot be implanted
into the nasal side of the soft palate without damaging the levator veli palatini.
In this study, we aimed to investigate injectable materials for soft palate augmentation in
dogs for treatment of VPI. Furthermore, we sought to introduce the endoscopic soft palate
augmentation (ESPA) technique, because it causes less damage to the levator veli palatini than
other techniques. ESPA is an original technique developed by us, and this is the first report of
its performance. If the materials can keep the volume after injection, this technique may also
be useful for the treatment of patients with VPI.
Materials and methods
ESPA was performed at the Large Animal Laboratory of the Graduate School of Dentistry at
Osaka University using 11 beagles (TOYO beagle; Oriental Yeast Co., Tokyo, Japan), aged 20–
24 months and weighing 9–12 kg. All dogs were housed in separate cages and were provided
solid food (Oriental Yeast Co., Tokyo, Japan) and water ad libitum. All experimental protocols
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were reviewed and approved by the Intramural Animal Care and Use Committee of Osaka
University Graduate School of Dentistry (approval number: 29-004-0).
All procedures were performed under general anesthesia administered via an intramuscular
injection of medetomidine (0.02 mg/kg) and midazolam (0.3 mg/kg), followed by an intraperi-
toneal injection of sodium pentobarbital (25 mg/kg) 15 minutes later. Animals were fixed in
the supine position after the ventilation tube was passed through the mouth, and all efforts
were made to minimize suffering.
Using an electric knife, an approximately 8-mm hole was made in the most anterior part of
the soft palate in each dog. This hole was needed because the endoscope cannot be inserted
nasally in dogs, due to narrowness of the canine nasal cavity. The endoscope (i-Vets 8.0;
SCETI K., Tokyo, Japan) was then inserted into the nasal side of the soft palate, and purified
sodium hyaluronate (Hyaluronate Na1, Sawai Pharmaceutical Co., Ltd. Osaka, Japan, n = 3),
atelocollagen (Koken Atelocollagen implant1, Koken Co., Ltd. Tokyo, Japan, n = 3), or auto-
genic fat tissue (n = 4) was injected into the nasal mucosal side of the anterior two-thirds of the
soft palate using a 23-G needle (Interject™, Boston Scientific, Natick, USA) under endoscopic
guidance to directly confirm entrance into the nasal mucosa (Fig 1).
Autogenic fat tissue was taken from the greater omentum and refined using the Coleman
method [25]. Vascular tissue was removed visually from the extracted greater omentum and
centrifuged (3000 rpm, 3 minutes) to separate the three layers (upper layer: oil from crushed
fat cell, middle layer: fat cells, bottom layer: blood, water, and lidocaine used as local anesthe-
sia). Only the middle layer was used as an injection material. Approximately 2 ml of each
material was injected into each dog until the soft palates slightly touched the post-pharyngeal
walls (Fig 2).
We then assessed the amount of nasal air leakage during inspiration at rest and expiration
under a rebreathing system prior to ESPA (= non-treated) and 1, 2, 3, 4, 5, and 6 months after
injection of the materials, as described previously [23]. The tip of the ventilation tube was with-
drawn from the trachea to the oral cavity to allow expiration through the nasal cavity. After
removing the electrode and ventilation tube to prevent oral air leakage during measurement,
the oral cavity was filled with an alginate impression material. While the dogs were under the
rebreathing system, the amount of air leakage from the nasal cavity was measured by a flow
meter (TSD117; BIOPAC Systems Inc., Japan) using the rubber tubes connected to the flow
meter’s sensor in front of both nasal apertures. The external portion of the rubber tubes was
packed with quick, self-curing acrylic resin (UNIFAST II; GC Co., Tokyo, Japan) to prevent
air leakage. Data from the flow meter was recorded on a personal computer (U24a-px3210r
Windows7; ASUSTek Computer Inc., Japan) using data acquisition and analysis software
(Labchart7; AD Instruments, Japan) through a DC Amplifier (DA100C; BIOPAC Systems
Inc., Japan), an analog output module (HLT100-C; BIOPAC Systems Inc., Japan), and an AD
converter (Power lab; AD Instruments Co., Tokyo, Japan).
Data from the nasal air leakage of one breath was separated into the inspiration phase and
expiration phase, and their integral value was measured. We assessed the amount of nasal air
leakage during inspiration at rest to determine the presence of apnea or hypopnea and nasal
air leakage during expiration under the rebreathing system to evaluate the effect of ESPA.
After euthanasia at 6 months post-injection, histological examinations were performed in all
dogs. The soft palates were dissected and stained with hematoxylin and eosin.
Normality of the data was evaluated and, owing to their nonparametric nature, analyzed
using the Kruskal–Wallis test post-hoc Mann–Whitney’s U test (p-value < 0.05). All statistical
analyses were conducted using R version 2.8.1 (CRAN: https://cran-archive.r-project.org/bin/
windows/base/old/2.8.1/).
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Fig 1. The schema of endoscopic soft palate augmentation (ESPA). An approximately 8-mm hole was created in the most
anterior part of the soft palate using an electric knife to insert the endoscope. This hole needed because the endoscope could not
insert by nasal approach in dog, due to narrowness of the canine nasal cavity. Then, the endoscope was inserted to the nasal side
of the soft palate.
https://doi.org/10.1371/journal.pone.0238646.g001
Results
The changes of nasal air leakage, presented in medians and inter-quartile ranges that occur
over time are shown in Figs 3 and 4. Fig 3 shows nasal air leakage during inspiration at rest
(soft palate was not lifted), and Fig 4 shows nasal air leakage during expiration under
Fig 2. Endoscopic image during ESPA. Approximately 2-ml of the materials was injected to each of the dogs until the soft palate
slightly touched the post-pharyngeal wall.
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Fig 3. The amount of nasal air leakage during inspiration at rest. The changes of nasal air leakage during inspiration at rest
(soft palate was not lifted). The amount of nasal air leakage during inspiration in each dog decreased slightly, compared to the
pre-ESPA value, but the decrease was not enough to cause apnea or hypopnea. (0 = Immediately after ESPA).
https://doi.org/10.1371/journal.pone.0238646.g003
rebreathing (soft palate was lifted due to levator veli palatini action). The amount of nasal air
leakage during inspiration in each dog decreased slightly, compared to the pre-ESPA value,
but the decrease was not enough to cause apnea or hypopnea (Fig 3). Comparison of data at 6
months post-injection among the three materials is shown in Fig 5, and no significant differ-
ence was found.
Conversely, the amount of nasal air leakage during expiration under the rebreathing system
was significantly decreased in all dogs injected with materials used for ESPA, compared with
pre-ESPA (p<0.05) (Fig 4). The median amount of nasal air leakage during expiration in the
non-treated dogs (pre-ESPA: n = 10) was 0.16 L/sec, whereas at 6 months after ESPA, the
median amount of nasal air leakage during expiration was 0.055, 0.089, and 0.049 L/sec in
dogs injected with purified sodium hyaluronate, atelocollagen, and autogenic fat tissue, respec-
tively (Fig 6).
Histologically, the maximum soft palate thickness between the dogs injected with sodium
hyaluronate or atelocollagen and non-treated dogs was the same (Fig 7; Table 1). Fat tissues
were observed around the soft palate injection site of the dogs injected with purified sodium
hyaluronate, whereas fibrous tissues were observed in those injected with atelocollagen. In
contrast, fibrous tissues and vasculatures, appearing as lymphatic or blood vessels with mini-
mal muscle tissues, were observed around the injection site in the dogs injected with autoge-
nous fat.
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Fig 4. The amount of nasal air leakage during expiration under rebreathing. The changes of nasal air leakage
during expiration under rebreathing (soft palate was lifted due to levator veli palatini action). The amount of nasal air
leakage during expiration under the rebreathing system was significantly decreased in all dogs injected with materials
used for ESPA, compared with pre-ESPA (p<0.05). (0 = Immediately after ESPA).
https://doi.org/10.1371/journal.pone.0238646.g004
Fig 5. The amount of nasal air leakage during inspiration at rest. The amount of nasal air leakage during inspiration at rest
decreased in all dogs compared to the pre-ESPA value, but apnea or hypopnea was not observed. Moreover, no significant
difference in the outcomes was observed among the materials.
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Fig 6. The amount of nasal air leakage during expiration under the rebreathing system. The amount of nasal air leakage
during expiration under the rebreathing system decreased significantly in all the dogs injected with any of the materials during
ESPA. The median amount of nasal air leakage of the non-treated dogs (pre-ESPA: n = 10) was 0.16 L/sec, whereas at 6 months
after ESPA, the median amount of nasal air leakage during expiration was 0.055, 0.089, and 0.049 L/sec in dogs injected with
purified sodium hyaluronate, atelocollagen, and autogenic fat tissue, respectively.
https://doi.org/10.1371/journal.pone.0238646.g006
Fig 7. Histological results. The maximum value of the soft palate thickness between the dogs injected with sodium hyaluronate,
atelocollagen and non-treated dogs was the same. Fat tissues were observed around the injection site of the soft palate of the dogs
injected with purified sodium hyaluronate, whereas fibrous tissues were observed in those injected with atelocollagen. In contrast,
fibrous tissues and vasculatures, appearing as lymphatic or blood vessels, with minimal muscle tissues (arrow) were observed
around the injection site in the dogs injected with autogenous fat. (Asterisks are levator veli palatini).
https://doi.org/10.1371/journal.pone.0238646.g007
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Table 1. The thickness of the soft palate at 6 months after the endoscopic soft palate augmentation (ESPA) (mm).
Non-treatment
7.03 (6.95–7.63)
Sodium hyaluronate
6.84 (6.55–6.85)
atelocollagen
7.56 (7.45–8.19)
Fat tissue
10.0 (8.58–11.1)
https://doi.org/10.1371/journal.pone.0238646.t001
Discussion
In this study, we investigated various materials that can be used in ESPA. ESPA is especially
useful because it does not cause injury to the musculus levator veli palatine, and injectable
materials can be implanted while simultaneously monitoring the volume of augmentation
through an endoscope. Injection of materials intraorally has been demonstrated previously,
but this method is challenging when trying to inject into the mucosal layer, as the needle may
penetrate the nasal mucosa [22]. Using our technique, we had to create a hole to insert the
endoscope because the device cannot pass through the nasal cavity in dogs. However, in
humans it can pass through the nasal cavity; thus, creating a hole is not necessary.
We selected purified sodium hyaluronate, atelocollagen, and autogenic fat tissue as the
materials to investigate because they can be injected into the soft palate through a needle.
Sodium hyaluronate is a mucopolysaccharide, and its molar weight is 1,000,000 g/mol, while
atelocollagen is a protein and its molar weight is 300,000 g/mol. One gram of sodium hyaluro-
nate can hold 6,000 ml of water; hence, even if sodium hyaluronate itself is absorbed gradually,
the surrounding water remains, and that makes it easy to retain the entire volume for an
extended period. And all the materials reduced the amount of nasal air leakage during expira-
tion under rebreathing. Soft palate movement was not hindered by any of the materials, and
apnea or hypopnea did not occur in any of the dogs.
Yasuda investigated histological changes in the skin after injection with sodium hyaluronate
or atelocollagen [26]. He reported that the bulging area where the sodium hyaluronate was
injected disappeared after 60 days, and only a small amount of sodium hyaluronate remained
in the site. He also reported minimal fibrous tissue was observed histologically at 180 days
after injection. Conversely, the bulging area where atelocollagen was injected remained even at
180 days after injection, and the material converted to collagen. In our study, sodium hyaluro-
nate was not observed histologically at 6 months after injection, whereas atelocollagen still
remained at the 6-month follow-up, and new collagen was observed around the musculus leva-
tor veli palatini after injection, consistent with Yasuda’s findings.
Multiple studies have reported that 30%–70% of the autologous transplant of fat graft was
resorbed within a year [27–30]. Guerrerosantos et al. reported that fat grafts injected intramus-
cularly were successful, owing to the excellent circulation of muscular tissue, compared to
those injected subcutaneously [30]. In our study, we could not confirm whether the autogenic
fat tissue was injected intramuscularly or subcutaneously, but histologically, no damage to the
levator veli palatini by the injection was observed. Only a small amount of fat tissue remained
around the muscle tissue during follow-up, and vascularization was observed histologically in
the mucosa layer. Soft palate thickness was biggest in the dogs injected with autogenic fat tis-
sue. It was unclear, however, if this observation was due to increased muscle tissue in the soft
palate. By the final observation, no fat tissue remained in any of the animals, instead it was
replaced by fibrous tissue. This result may indicate that the thickness of the soft palate may
remain for a long period of time.
Although ESPA using purified sodium hyaluronate, atelocollagen, or autogenic fat tissue
showed good results after 6 months, its long-term effects should still be considered. Several
reports have described that human lipoaspirate contains multipotent cells and may represent an
alternative stem cell source for bone marrow-derived mesenchymal stem cells [31, 32]. Li et al.
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reported that fat grafts consisting of 105/ml adipose-derived stem cells constitute an ideal trans-
plant strategy, which may result in decreased absorption and accelerated fat regeneration [33].
Adipose-derived stem cells are one of the most widely used stem cell types for the treatment of
bone and cartilage disease, Crohn’s disease, heart diseases, kidney disease, neurological disease
and respiratory disease, as well as for cosmetic and plastic surgery [34]. In addition, some reports
describe the effectiveness of injectable adipose tissue-derived stem cells in treating stress urinary
incontinence or vocal fold paralysis [35–37]. In the future, we should also investigate whether
transplantation of adipose tissue- derived stem cells can help in treating VPI. If we can make the
musculus levator veli palatini stronger with this method, it may be the best treatment for VPI.
This study has several limitations. The structure of the velopharynx in dogs and that in
humans are quite different, and the assessment of speech cannot be performed in dogs. In
dogs, the larynx is located directly behind the base of the tongue and soft palate and lies
between the pharynx and trachea. The larynx covers the trachea during swallowing so that
food does not enter the windpipe. However, the soft palate of dogs may lift like that of humans
during respiration. VPI cannot strictly be assessed in dogs; only analogous comparisons can be
made. Therefore, we eventually must conduct further investigations in humans. We plan to
perform ESPA on humans in a clinical setting soon.
In a systematic review, Nigh E et al. described that autologous fat injection has been advo-
cated for correction of mild to moderate VPI, but it was difficult to adapt in severe VPI [22].
At present, no ideal treatment is available for severe VPI. However, we have previously
reported that the optimal site for injection is the nasal side of the soft palate and improvement
of VPI was dependent on the amount of injected autologous fat [23]. ESPA may control the
amount of fat easier than traditional methods; hence, we believe it could be adapted to treat
severe VPI as well.
In conclusion, we investigated the optimal materials for use in ESPA, such as purified
sodium hyaluronate, atelocollagen, or autogenic fat tissue. We found that all of them reduced
nasal air leakage and only autogenic fat tissue showed significant histologic differences at 6
months in dogs. These results suggest that this technique may also be useful for the treatment
of patients with VPI.
Acknowledgments
We are grateful to Y. Sato for taking care of the animals.
Author Contributions
Conceptualization: Emiko Tanaka Isomura, Mikihiko Kogo.
Data curation: Emiko Tanaka Isomura, Makoto Matsukawa, Kiyoko Nakagawa, Ryo Mitsui.
Formal analysis: Emiko Tanaka Isomura, Makoto Matsukawa, Kiyoko Nakagawa, Ryo Mitsui.
Funding acquisition: Kiyoko Nakagawa.
Methodology: Emiko Tanaka Isomura, Makoto Matsukawa, Kiyoko Nakagawa, Ryo Mitsui.
Project administration: Emiko Tanaka Isomura, Makoto Matsukawa, Kiyoko Nakagawa, Ryo
Mitsui, Mikihiko Kogo.
Resources: Emiko Tanaka Isomura, Makoto Matsukawa, Kiyoko Nakagawa, Ryo Mitsui.
Writing – original draft: Emiko Tanaka Isomura.
Writing – review & editing: Emiko Tanaka Isomura.
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10.3389_fphy.2022.1005333.pdf
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Data availability statement
Publicly available datasets were analyzed in this study. This
data can be found here: https://inspirehep.net.
|
Data availability statement Publicly available datasets were analyzed in this study. This data can be found here: https://inspirehep.net .
|
OPEN ACCESS
EDITED BY
Antonino Marciano,
Fudan University, China
REVIEWED BY
Vladimir Dzhunushaliev,
Al-Farabi Kazakh National University,
Kazakhstan
Seyed Meraj M. Rasouli,
Universidade da Beira Interior, Portugal
*CORRESPONDENCE
Sergei V. Ketov,
[email protected]
SPECIALTY SECTION
This article was submitted to
Cosmology,
a section of the journal
Frontiers in Physics
RECEIVED 28 July 2022
ACCEPTED 31 August 2022
PUBLISHED 04 October 2022
CITATION
Frolovsky D, Ketov SV and Saburov S
(2022), E-models of inflation and
primordial black holes.
Front. Phys. 10:1005333.
doi: 10.3389/fphy.2022.1005333
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TYPE Original Research
PUBLISHED 04 October 2022
DOI 10.3389/fphy.2022.1005333
E-models of inflation and
primordial black holes
Daniel Frolovsky 1, Sergei V. Ketov 1,2,3* and Sultan Saburov 1
1Interdisciplinary Research Laboratory, Tomsk State University, Tomsk, Russia, 2Department of Physics,
Tokyo Metropolitan University, Hachioji, Japan, 3Kavli Institute for the Physics and Mathematics of the
Universe, The University of Tokyo Institutes for Advanced Study, Kashiwa, Japan
We propose and study the new (generalized) E-type α-attractor models of
inflation, in order to include formation of primordial black holes (PBHs). The
inflaton potential has a near-inflection point where slow-roll conditions are
violated, thus leading to large scalar perturbations collapsing to PBHs later. An
ultra-slow roll (short) phase exists between two (longer) phases of slow-roll
inflation. We numerically investigate the phases of inflation, derive the power
spectrum of scalar perturbations and calculate the PBHs masses. For certain
values of the parameters, the asteroid-size PBHs can be formed with the masses
of 1017 ÷ 1019 g, beyond the Hawking evaporation limit and in agreement with
current Cosmic Microwave Background observations. Those PBHs are a
candidate for (part of) dark matter in the present Universe, while the
gravitational waves induced by the PBHs formation may be detectable by
the future space-based gravitational interferometers.
KEYWORDS
cosmological inflation, black holes, cosmic microwave “background (CMB), dark
matter, cosmology
1 Introduction
Measurements of the Cosmic Microwave Background (CMB) radiation by the Planck
mission provide tight observational constraints on cosmological inflation in the early
Universe [1–3]. Nevertheless, the simple Starobinsky model of inflation [4], proposed the
long time ago, is still consistent with the current precision measurements of the CMB
spectral tilt ns of scalar perturbations [1–3],
ns (cid:1) 0.9649 ± 0.0042
68% C.L.
(
)
(1)
The Starobinsky model also gives a prediction for the value of the CMB tensor-to-
scalar ratio r up to an uncertainty in the duration of inflation measured by the number of
e-folds Ne as
rS ≈ 12
N2
e
t
end
, where Ne (cid:1) (cid:1)
H t( )dt ,
t
initial
and H(t) is the Hubble function. The current observational bound [1–3].
r < 0.036
95% C.L.
(
)
(2)
(3)
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Frolovsky et al.
10.3389/fphy.2022.1005333
is already fulfilled for Ne
> 20, whereas the duration of
inflation is expected at Ne = 55 ± 10. This estimate for Ne
comes from the predicted value of ns
in the Starobinsky
model via the Mukhanov-Chibisov formula [5].
ns ≈ 1 − 2
Ne
.
(4)
Equations 2–4 for the tilts r and ns show only the leading
terms with respect to the inverse e-folds number Ne. Given higher
precision of the ns-measurements, the subleading terms may also
be important. For example, in the case of the Starobinsky model,
one finds [6].
ns (cid:1) 1 − 2
Ne
+ 3 ln Ne
2N2
e
− 4
N2
e
+ O ln2Ne
(cid:3)
N3
e
(cid:4) .
(5)
The scalar potential of the canonical inflaton field ϕ in the
Starobinsky model reads1
VS ϕ(cid:5) (cid:6) (cid:1) 3
4
M2
Pl
M2 1 − yS
(cid:5)
(cid:6)2 ,
where we have introduced the dimensionless field
yS (cid:1) exp −
(cid:3)
(cid:7)
(cid:8)
2
3
ϕ
MPl
(cid:4)
(6)
(7)
and the inflaton mass M ~ 10−5MPl, whose value is
determined by the known CMB amplitude. The scale of
inflation can be estimated by the Hubble function H during
slow-roll, which is related to the (unknown) tensor-to-scalar
ratio r. As regards the Starobinsky inflation, the scale of inflation
HS ~ M corresponds to super-high energy physics far beyond the
electro-weak scale and not far from the GUT scale.
The flatness of the inflaton potential during slow roll is
guaranteed by the smallness of yS during inflation. Therefore,
the inflationary observables for CMB will be essentially the same
(in the leading approximation with respect to N−1
e ) after a
generalization of the scalar potential (6) to
e or N−2
Vζ ϕ(cid:5) (cid:6) (cid:1) 3
4
M2
Pl
M2 1 − yS + y2
(cid:9)
Sζ yS(cid:5) (cid:6)
(cid:10)2 ,
(8)
where ζ(yS) is a function regular at yS = 0. Some generalizations of
the Starobinsky model, like Eq. 8, were studied in Ref. [7]. In this
paper, we take the inflaton potential to be a real function squared
because it can always be minimally embedded into supergravity
as a single-field inflationary model [8].
Another simple way of generalizing the Starobinsky model of
inflation is given by the cosmological α − attractors [9, 10] that
come in two families called E-models and T-models. The
E-models have the same scalar potential V(y) as in Eq. 6 but
in terms of the new variable
y (cid:1) exp −
(cid:3)
(cid:8)
(cid:7)(cid:7)(cid:7)
2
3α
ϕ
MPl
(cid:4)
(9)
that depends upon the parameter α > 0. The Starobinsky model
corresponds to α = 1. The E-models lead to the same Eq. 4 for the
tilt ns but significantly change the tilt r as
rα ≈ 12α
N2
e
,
(10)
thus making this theoretical prediction more flexible against
future measurements.
An opportunity of changing the inflaton potential by
arbitrary function ζ(y) can be exploited in order to generate
primordial black holes (PBHs) [11, 12] at smaller values of ϕ or,
equivalently, at lower energy scales. Those energy scales (below
the scale of inflation) are not tightly constrained by observations
yet. Technically, the PBHs production can be engineered by
demanding a near-inflection point in the potential within the
double inflation scenario with an ultra-slow-roll phase between
two slow-roll regimes of inflation, leading to an enhancement of
the power spectrum of scalar perturbations [13–15].2 The PBHs
born in the very early Universe are considered as a candidate for
cold dark matter in the present Universe [16–19].
A generalization of the Starobinsky model for PBHs formation
was proposed and studied in Ref. [20] by using a model very
from the α-attractors. As regards the generalized
different
T-models of α-attractors, the PBHs production was studied in
Refs. [21, 22] for single-field inflation with the scalar potentials
VT ϕ(cid:5) (cid:6) (cid:1) f2
(cid:3)
tanh
ϕ(cid:11)MPl(cid:7)(cid:7)(cid:7)
√
6α
(cid:4) ,
(11)
where f is a regular function. In this paper, we propose and
investigate the generalized E-models of inflation with a near-
inflection point along similar lines.
Our paper is organized as follows. In Section 2 we introduce
our model and investigate its scalar potential. Section 3 is devoted
to the slow-roll approximation during the first stage of inflation
relevant to CMB. In Section 4 we give our results for the power
spectrum of scalar perturbations and its enhancement leading to
PBHs formation. Our conclusion is Section 5.
2 The model
Let us consider the following potential of the canonical
inflaton ϕ:
1 See e.g., Refs. [7, 26, 37] for details about the Starobinsky model, various
extensions and applications. We do not reproduce here the standard
equations describing background dynamics, perturbations and their
power spectrum in single-field inflation, because they are well known
and easily can be found in the literature.
2 See Ref. [27] for a current review of PBHs formation in single-field
inflationary models.
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10.3389/fphy.2022.1005333
V ϕ(cid:5) (cid:6) (cid:1) 3
4
MPlM(
)2 1 − y + y2 β − γy
(cid:6)
(cid:5)
(cid:9)
(cid:10)2 ,
(12)
with the dimensionless parameters (α, β, γ), where the
(17) has the small bump, associated with the local maximum,
and the small dip, associated with the local minimum, with
both being close to the inflection point, similarly to the
models of Ref. [23].
function y(ϕ) is given by
y (cid:1) exp −
(cid:12)
(cid:8)
(cid:7)(cid:7)(cid:7)
2
3α
(cid:6)
0
ϕ + ϕ
(cid:5)
MPl
(cid:13) .
(13)
3 Slow-roll inflation
Compared to Eqs 8, 9, we have Taylor-expanded the function
ζ(y) up to a linear term, ζ(y) = β − γy, and have shifted the field ϕ
by a constant ϕ
0 in order to have a Minkowski minimum at ϕ = 0
with V (0) = 0. Hence, the ϕ
0 is fixed by other parameters. We do
not give here an explicit formula for ϕ
0 because it is not very
illuminating.
Demanding the existence of a near-inflection point in the
potential with a coordinate ϕ
i allows us to replace the
parameters (β, γ) by the new dimensionless parameters (ϕ
i,
ξ) as follows:
(cid:8)
(cid:7)(cid:7)(cid:7)
2
3α
(cid:12)
exp 2
β (cid:1)
γ (cid:1)
1
(cid:12)
1 − ξ2 exp
1
3 1 − ξ2
(cid:14)
(cid:15)
(cid:8)
(cid:6)
(cid:13) ,
ϕi + ϕ
(cid:5)
0
MPl
ϕi + ϕ
(cid:5)
0
MPl
(cid:7)(cid:7)(cid:7)
2
3α
(14)
(cid:6)
(cid:13) .
Since the flatness of the scalar potential during inflation,
the standard slow-roll approximation well describes both the
inflaton dynamics and the power spectrum of perturbations
away from the inflection point and the end of inflation. We
use the slow-roll approximation in order to calculate the
to CMB and estimate the power
observables
relevant
spectrum of scalar perturbations.
the
slow-roll approximation generically fails in the ultra-slow-
roll (non-attractor) regime near an inflection point [21, 22].
Therefore, after having fixed our parameters in the slow-roll
the power
approximation, we numerically
inflection
the
using
spectrum near
Mukhanov-Sasaki
leading to a
correct answer.
the
(MS) equation [24, 25]
recalculate
by
point
is known that
It
The parameters (ϕ
potential has the inflection point at ϕ = ϕ
the potential has a local minimum y−
the inflection point ϕ
side of the inflection point ϕ
separated from the inflection point,
i, ξ) have the clear meaning: when ξ = 0, the
i only; when 0 < ξ ≪ 1,
ext on the right hand side of
ext on the left hand
i, while both extrema are equally
i and a local maximum y+
y±
ext
(cid:1) yi 1 ± ξ
(
) .
(15)
The
(running) number of
e-folds
in the
slow-roll
approximation is given by
tend
Ne (cid:1) (cid:1)
t
H t( )dt ≈ 1
M2
Pl
ϕ
(cid:1)
ϕ
end
V ϕ(cid:5) (cid:6)
V′ ϕ(cid:5) (cid:6)
dϕ ,
(18)
where the prime denotes differentiation with respect to the given
argument. The integral can be taken analytically in the case of our
potential (17). We find
Equations 14, 15 are easily derivable from considering
extrema of the cubic polynomial inside the square brackets in
(12), which leads to a quadratic equation (cf. Ref. [22]). The
inverse relations are given by
(cid:7)(cid:7)(cid:7)
2
3α
ξ2 (cid:1) 1 − 3γ
β2 .
ϕi + ϕ
(cid:5)
MPl
3γ
β
(cid:1) ln
(16)
(cid:8)
(cid:6)
,
0
In terms of the new parameters our scalar potential takes the
form
V ϕ(cid:5) (cid:6) (cid:1) 3
4
MMPl
(
(cid:16)
)2 1 − exp −
(cid:12)
(cid:8)
(cid:7)(cid:7)(cid:7)
2
3α
(cid:8)
+ 1
(cid:12)
1 − ξ2 exp
1
3 1 − ξ2
(cid:14)
−
(cid:15)
(cid:12)
exp
(cid:6)
(cid:13)
(cid:8)
ϕ + ϕ
(cid:5)
0
MPl
− 2ϕ
(cid:6)
(cid:7)(cid:7)(cid:7)
2
3α
ϕi − 2ϕ
(cid:5)
0
MPl
2ϕi − 3ϕ
(cid:5)
0
MPl
(cid:7)(cid:7)(cid:7)
2
3α
An example of
leading to viable
the scalar potential
inflation and PBHs formation is given in Figure 1. The
potentials in the original E-models of α-attractors, arising
in the case of β = γ = ϕ
0 = 0, do not have a near-inflection
point and thus do not lead to PBHs formation. Our potential
Ne ϕ(cid:5) (cid:6) + N0 ≈ 3α
4
√
(cid:7)(cid:7)
α
(cid:3)
exp
(cid:8)
1 − 2 exp
(cid:12)
(cid:3)
− 3
4
(cid:8)
(cid:7)(cid:7)(cid:7)
2
3α
(cid:7)(cid:7)(cid:7)
2
3α
(cid:4)
0
(cid:6)
ϕ + ϕ
(cid:5)
MPl
(cid:8)
(cid:7)
2
3
(cid:4)
(cid:13)
ϕi
MPl
(cid:6)
0
ϕ + ϕ
(cid:5)
MPl
,
(19)
where N0 is an integration constant close to one. We ignore this
constant for simplicity in what follows because it merely shifts
counting of Ne. The standard slow-roll parameters are given by
ϵ (cid:1)
M2
Pl
2
(cid:3)
V′ ϕ(cid:5) (cid:6)
V ϕ(cid:5) (cid:6)
2
(cid:4)
(cid:1) 3α
4N2
e
+ O ln2Ne
(cid:3)
N3
e
(cid:4)
(20)
+ 3α 1 − 2e
(cid:20)
(cid:1) − 1
Ne
+ O ln2Ne
(cid:3)
N3
e
√
(cid:7)(cid:7)
2
3α
ϕi
M
Pl
(cid:21) ln Ne
4N2
e
+
3α 1 − 2e
(cid:20)
√
(cid:7)(cid:7)
2
3α
ϕi
M
Pl
(cid:21)ln 4
3α
4N2
e
(cid:4) .
(21)
(cid:13)−
(17)
and
− 3ϕ
(cid:6)
2
⎫⎬
⎭
(cid:13)
.
η (cid:1) M2
Pl
V″ ϕ(cid:5) (cid:6)
V ϕ(cid:5) (cid:6)
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10.3389/fphy.2022.1005333
FIGURE 1
A profile of the scalar potential (A) and the inflaton dynamics (B) for the parameters α = 0.739, ϕi + ϕ0 = 0.664MPl and ξ = 0.012 with the vanishing
initial velocity. The location of the inflection point is specified by the value of (ϕi + ϕ0)/MPl.
It yields
ns (cid:1) 1 + 2η − 6ϵ (cid:1) 1 − 2
Ne
+ a 3 ln Ne
2N2
e
+
b
N2
e
+ O ln2Ne
(cid:3)
N3
e
(cid:4) ,
whose coefficients are given by
√
(cid:7)(cid:7)
2
3α
√
ϕi
M
Pl
(cid:7)(cid:7)
2
3α
a (cid:1) α 1 − 2e
(cid:20)
b (cid:1) 3α
2
(cid:20)
(cid:22)
1 − 2e
(cid:21) and
ϕi
M
Pl
(cid:21)ln
4
3α
− 3
(cid:23) .
(22)
(23)
Equation 20 reproduces Eq. 10 because r = 16ϵ. When
→ −∞, Eq. 5 is also recovered up to a
choosing α = 1 and ϕ
i
small correction (= 0.05) in the value of the coefficient b due to
our approximation.
4 Power spectrum and PBH masses
We numerically solve the inflaton equation of motion by
using initial conditions with the vanishing initial velocities and
then substitute the background solutions into the equations for
perturbations. All our inflationary solutions are attractors
inflaton field
(during slow roll) by construction. The initial
value is fixed by a desired number of e-folds, see e.g., Refs.
[26, 27] for details.
A typical numerical solution to the Hubble function
during double inflation is given on the left-hand-side of
Figure 2. Demanding a peak in the power spectrum of
scalar perturbations, required for PBHs production, we
find the parameter α has to be restricted to the interval
between 0.5 and 0.9, whereas the parameter ϕ
i also has to
be fixed, as is shown on the right-hand-side of Figure 2. There
is a short phase of ultra-slow-roll between the two stages of
slow-roll inflation (corresponding to two plateaus), which
leads to large perturbations in the power spectrum and PBHs
production.
The standard formula for the power spectrum of scalar
perturbations in the slow-roll approximation [13].
PR (cid:1)
H2
8M2
Pl
π2 ϵ
(24)
is useful for analytic studies of the power spectrum and its
dependence upon the parameters. However, it cannot be used in
the ultra-slow-roll phase where the slow-roll conditions are
violated. Instead, one should use the MS equation [24, 25].
We used both in our calculations in order to see a difference
between the two methods.
The scalar tilt ns is related to the power spectrum by a relation
ns (cid:1) d ln PR
d ln k , where k = aH = da/dt and a(t) is the cosmic factor in
the Friedman-Lemaitre-Robertson-Walker metric. Our results
for the power spectrum are given in Figure 3 for a particular
choice of the parameters. Our results are qualitatively similar for
other values of
the parameters, see the right-hand-side of
Figure 2.
As is clear from Figure 3, the exact results based on the MS
equation vs. the slow-roll approximation increase the hight of the
peak by one or
the
amplification of the peak vs. the CMB spectrum (on the very
left-hand-side of the power spectrum) is given by the seven
orders of magnitude.
two orders of magnitude, whereas
The PBHs masses can be estimated from the peaks as
follows [28]:
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FIGURE 2
The Hubble function H (A) and the relation between the parameters ϕi and α for the power spectrum enhancement and PBHs production (B).
ns
r
α
ξ
ϕi
0.95452
0.00307
0.95491
0.00360
0.5
0.6
0.0102
0.0106
0.95658
0.00409
0.739
0.0122
0.95672
0.00439
0.95650
0.00496
0.8
0.9
0.0115
0.0111
−0.334
−0.455
−0.611
−0.671
−0.765
ϕi
+ ϕ0
0.606
0.633
0.664
0.677
0.696
ΔN MPBH
15.08
15.35
13.28
13.96
13.74
1.06 · 1019 g
1.04 · 1019 g
1.89 · 1017 g
7.75 · 1017 g
8.84 · 1017 g
The ns values below 0.9545 are certainly excluded by CMB
observations, so we do not include our results for the lower values
of ns, see Eq. 1. The values of ns above 0.9565 are in good
agreement with CMB observations at the 95% C.L. The values of
the tensor-to scalar ratio r in the Table are well inside the current
observational bound (3). We also found that lowering the value of
the parameter α leads to narrowing the peaks in the scalar
perturbations spectrum. The PBHs masses are very sensitive
to the value of ΔN.
PBHs may be part of the present dark matter when the
PBH masses are beyond the Hawking evaporation limit of
1015 g, which is required for survival of those PBHs in the
present Universe. However, consistency with the measured
CMB value of ns restricts ΔN from above, as is clear from the
Table.
5 Conclusion
Our approach is this paper is phenomenological and classical.
However,
is not excluded that our deformations of the
E-models of inflation proposed in this paper could appear as
it
FIGURE 3
The power spectrum PR(k) of scalar perturbations from a
numerical solution to the MS equation (in red) vs. an analytic
derivation from Eq. 24 in the slow-roll approximation (in black),
with the same parameters as in Figure 1.
MPBH ≃
M2
Pl
H tpeak
(cid:5)
⎡⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎣
(cid:6) exp 2 Ntotal − Npeak
(cid:5)
t
total
(cid:6) + (cid:1)
ϵ t( )H t( )dt
t
peak
⎤⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎦ .
(25)
The right-hand-side of this equation is mainly sensitive to the
− Npeak, whereas the integral gives a sub-
value of ΔN = Ntotal
leading correction.
Our findings are summarized in the Table below where we
give the values of the CMB tilts ns and r associated with the
values of
together with the
corresponding values of ΔN and PBHs masses MPBH in our
model.
the parameters α, ϕ
i and ξ,
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quantum corrections from a more fundamental
quantum gravity.
theory of
scales, while keeping success
We modified the scalar potential of the single-field E-models
of α-attractors in order to allow PBHs formation in those models
at
in the theoretical
lower
large-single-field inflation in agreement with
description of
CMB measurements. We
PBHs
production consistent with CMB measurements restricts the α
parameter to approximately 0.7 ± 0.2 and leads to the asteroid-
size PBHs with masses of the order 1017 ÷ 1019 g. The masses of
the PBHs formed in the very early Universe may grow further
with time via accretion and mergers.
efficient
found
that
A similar approach was realized in the T-models of α-
attractors [21, 22]. In terms of pole inflation [10] with a non-
canonical inflaton field having just a mass term, the kinetic terms
in the E-models have a pole of order two and exhibit the SL (2,R)
symmetry, whereas the kinetic terms in the T-models also have a
pole of order two but with the SU(1, 1) symmetry. Since those
symmetries are equivalent, the main predictions of the standard
E- and T-models for inflation are essentially the same. The
inflation proposed in this paper
generalized E-models of
simultaneously describe viable inflation and PBHs formation.
The next generation of CMB measurements will probe
deeper regions of parameter space, leading to a discrimination
among currently viable models of inflation, which may falsify the
in particular. The α-attractors add more
Starobinsky model
flexibility on the theoretical side, as regards the tensor-to-
scalar ratio. We demonstrated that certain deformations of the
scalars potentials in the E-models can also lead to efficient PBHs
production capable to describe a whole (or part of) dark matter in
the present Universe.
We tuned the parameters of our model
in order to
overcome the Hawking radiation bound 1015 g for
the
PBHs masses, so that those PBHs may contribute to the
current dark matter. Remarkably,
the PBHs with the
masses between 1017 g and 1019 g belong to the current
those PBHs may
observational mass window where
constitute the whole dark matter [18, 19]. With lower
PBHs masses we found no strong constraints on the
parameters, but those PBHs should all evaporate until now.
Still, those PBHs may have dominated the early Universe,
while their remnants could form dark matter at present.
The PBHs formation in the very early Universe should lead to
a stochastic background of gravitational waves (GW) at present
[29].3 The frequency of those GW can be estimated as
fGW ≈
(cid:3)
−1/2
MPBH
1016 g
(cid:4)
Hz .
(26)
3 See e.g., Ref. [38] for a current review.
It was argued in the literature [30–32] that those GW may be
detectable
gravitational
interferometers such as LISA [33], TAIJI [34], TianQin [35]
and DECIGO [36].
space-based
future
the
by
Data availability statement
Publicly available datasets were analyzed in this study. This
data can be found here: https://inspirehep.net.
Author contributions
All authors listed have made a substantial, direct, and
for
intellectual contribution to the work and approved it
publication.
Funding
This work was supported by Tomsk State University under
the development program Priority-2030. SK was also supported
by Tokyo Metropolitan University, the Japanese Society for
Promotion of Science under the grant No. 22K03624, and the
World Premier International Research Center Initiative (MEXT,
Japan).by the University of Tokyo.
Acknowledgments
SK thanks G. Domenech, S. Mishra and C. Unal
for
correspondence.
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the
authors and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed or
endorsed by the publisher.
Frontiers in Physics
06
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Frolovsky et al.
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10.1007_s00213-019-05336-7.pdf
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Psychopharmacology (2019) 236:3641–3653
https://doi.org/10.1007/s00213-019-05336-7
ORIGINAL INVESTIGATION
Atomoxetine modulates the relationship between perceptual
abilities and response bias
Carole Guedj 1,2
Martine Meunier 1,2 & Fadila Hadj-Bouziane 1,2
& Amélie Reynaud 1,2 & Elisabetta Monfardini 1,2 & Romeo Salemme 1,2 & Alessandro Farnè 1,2 &
Received: 18 February 2019 / Accepted: 16 July 2019
# The Author(s) 2019
/ Published online: 5 August 2019
Abstract
Elucidation of how neuromodulators influence motivated behaviors is a major challenge of neuroscience research. It has been
proposed that the locus-cœruleus-norepinephrine system promotes behavioral flexibility and provides resources required to face
challenges in a wide range of cognitive processes. Both theoretical models and computational models suggest that the locus-
cœruleus-norepinephrine system tunes neural gain in brain circuits to optimize behavior. However, to the best of our knowledge,
empirical proof demonstrating the role of norepinephrine in performance optimization is scarce. Here, we modulated norepi-
nephrine transmission in monkeys performing a Go/No-Go discrimination task using atomoxetine, a norepinephrine-reuptake
inhibitor. We tested the optimization hypothesis by assessing perceptual sensitivity, response bias, and their functional relation-
ship within the framework of the signal detection theory. We also manipulated the contingencies of the task (level of stimulus
discriminability, target stimulus frequency, and decision outcome values) to modulate the relationship between sensitivity and
response bias. We found that atomoxetine increased the subject’s perceptual sensitivity to discriminate target stimuli regardless of
the task contingency. Atomoxetine also improved the functional relationship between sensitivity and response bias, leading to a
closer fit with the optimal strategy in different contexts. In addition, atomoxetine tended to reduce reaction time variability. Taken
together, these findings support a role of norepinephrine transmission in optimizing response strategy.
Keywords Monkey . Atomoxetine . Discrimination . Signal detection theory . Line of optimal response
Introduction
The locus cœruleus-norepinephrine (LC-NE) system is cur-
rently viewed as a key component of behavioral flexibility
(Aston-Jones et al. 1999; Bouret and Sara 2004), energizing
behavior during cognitive and/or physical effort (Robbins
1997; Raizada and Poldrack 2007; Bouret and Richmond
2009; Malecek and Poldrack 2013; Kalwani et al. 2014;
Varazzani et al. 2015). For instance, the LC activity is posi-
tively modulated by the level of difficulty in a context
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s00213-019-05336-7) contains supplementary
material, which is available to authorized users.
* Carole Guedj
[email protected]; [email protected]
* Fadila Hadj-Bouziane
[email protected]
Amélie Reynaud
[email protected]
Elisabetta Monfardini
[email protected]
Romeo Salemme
[email protected]
Alessandro Farnè
[email protected]
Martine Meunier
[email protected]
1
Present address: INSERM, U1028, CNRS UMR5292, Lyon
Neuroscience Research Center, ImpAct Team, 16 Avenue Doyen
Lépine, 69500 Bron, France
2 University UCBL Lyon 1, F-69000 Villeurbanne, France
3642
Psychopharmacology (2019) 236:3641–3653
involving a reward/effort trade-off (Varazzani et al. 2015),
suggesting an involvement of the NE system in mobilizing
resources to face challenges (Raizada and Poldrack 2007;
Bouret and Richmond 2015). Accumulating evidence in be-
havioral studies manipulating NE transmission has demon-
strated its impact on a variety of cognitive processes (Coull
et al. 1995; Doucette et al. 2007; Robinson et al. 2008;
Decamp et al. 2011; Baarendse et al. 2013). For example,
atomoxetine, a NE-reuptake inhibitor that increases NE avail-
ability in the synaptic cleft, was found to improve executive
control in healthy subjects performing a response inhibition
task (Chamberlain et al. 2009). Both theoretical and compu-
tational models suggest that the LC-NE system tunes neural
gain in brain areas to optimize cognitive processes (Servan-
Schreiber et al. 1990; Aston-Jones and Cohen 2005; Eldar
et al. 2013; Devilbiss 2018). We recently reported NE-
dependent brain network reorganization with a reduction in
functional connectivity within and between several networks
at rest (Guedj et al. 2016). Similar tuning of brain activity
could be dependent upon the LC-NE system to optimize cog-
nitive processes (Harris and Thiele 2011; Rodenkirch et al.
2019). Optimizing refers to the refinement of response selec-
tion to maximize reward rate, depending on the context (Gold
and Shadlen 2007; Bogacz 2007); (Summerfield and Tsetsos
2012). To date, direct empirical evidence demonstrating
whether the NE-mediated effects reflect optimization of the
performance, as suggested by the theoretical and computation-
al models (Brown et al. 2005; Shea-Brown et al. 2008;
Eckhoff et al. 2009), is scarce. Providing such evidence will
help clarify the role of NE in cognitive functions.
Here, we tested the optimization hypothesis by assessing
perceptual sensitivity and response bias—within the frame-
work of signal detection theory (SDT) (Green and Swets
1966; Wickens 2001)—in monkeys performing a Go/No-Go
discrimination task. Typically, sensitivity refers to the aptitude
at discriminating a target stimulus in a noisy background,
while bias reflects the extent to which one response (e.g.,
“target present”) is favored compared with another (e.g., “tar-
get absent”). In addition, we implemented Lynn and Barrett’s
(2014) framework describing a functional relationship be-
tween sensitivity and bias that can be mathematically de-
scribed as the line of optimal response (LOR) (Lynn and
Barrett 2014; Lynn et al. 2015). This functional relationship
considers the level of signal/noise interference (i.e., how hard
or easy it is to discriminate the target stimulus) and is influ-
enced by the outcome value of the task (i.e., the cost/benefit
balance of each response type) and the target frequency (i.e.,
rate of signal occurrence). The LOR thus defines the amount
of bias that maximizes utility (i.e., the net benefit earned over a
series of responses) at any given sensitivity level for a specific
environmental context. Here, we examined monkeys’ perfor-
mance in a Go/No-Go discrimination task after injection of
atomoxetine (ATX). In addition, we manipulated the task
contingencies (i.e., level of signal/noise interference, target
frequency, and outcome values) to modulate the relationship
between sensitivity and bias measures. We also examined
whether ATX modulated response time. Based on the optimi-
zation hypothesis, we predicted that enhancing NE transmis-
sion would modulate the functional relationship between sen-
sitivity and response bias to bring the animals’ performance
closer to the LOR.
Methods
Subjects
Four female rhesus monkeys (Macaca mulatta, 7 to 15 years
of age, 6 to 10 kg) participated in this study. Animals had free
access to water and were maintained on a food regulation
schedule, individually tailored (70–90 kcal/kg/day) to main-
tain a stable level of performance for each monkey. Work
complied with European Union Directive 2010/63/EU and
was approved by French Animal Experimentation Ethics
Committee #42 (CELYNE).
Experimental setup
Monkeys were seated in a primate chair approximately 10 cm
in front of a 19 in. high-resolution touchscreen. Stimuli were 13
Latin letters, white on a black background (size 10 × 10 cm),
appearing one-by-one at the center of the touchscreen. The
whole experiment, i.e., the presentation of the stimuli, delivery
of reward, and behavioral data acquisition was controlled by
Presentation® software (https://www.neurobs.com/).
Behavioral task
The task was a Go/No-Go continuous performance task
designed to assess the ability to discriminate a target with-
in a series of distractors (Decamp et al. 2011). The mon-
keys were trained to place their right hand on a starting
point lever affixed to the chair to initiate the task and keep
it running. The task consisted of a series of 200 letters
(Fig. 1a). For each series, one letter, the one appearing
first, was the target, while the other 12 possible letters
served as distractors. The monkey was required to touch
the target (Go response) and to refrain from touching the
distractors (i.e., to keep the hand on the starting point
lever; No-Go response). Several 200-letter series were
presented per testing session, each with a different target.
Within each series, target and distractors were pseudo-
randomized in order to enforce a target frequency of ei-
ther 30% or 70% target letter presentations per block of
50 letters (Fig. 1b). A letter was presented for a maximum
of 1 s. Correct responses led to a reward consisting of 1–
Psychopharmacology (2019) 236:3641–3653
3643
15 drops of the animal’s favorite among a choice of slur-
ries (applesauce, banana smoothie, vanilla milkshake,
etc.). Incorrect responses were followed by a 3-s time out.
The Go/No-Go continuous performance task named here the
“reference task” used as follows: (1) the presence of letter
distractors, (2) a low target frequency (30%), and (3) unbal-
anced outcome value—50-ms valve opening time for each cor-
rect response to the rare target (HIT) and 35-ms valve opening
time for each correct response to the frequent distractors (cor-
rect rejection (CR)). A longer valve opening time leads to a
larger amount of reward compared with shorter valve opening
time. To manipulate the task contingencies, hence the relation-
ship between sensitivity and bias measures, we designed three
“contrast” conditions of the task as follows (Fig. 1a): (1) an
interference contrast replaced letter distractors by a black
screen, (2) a target frequency contrast used a high target fre-
quency (70%) with a reversal of the amount of reward attributed
to correct responses compared with the reference task, and (3)
an outcome value contrast where CRs were unrewarded and
HITs were rewarded by a large amount that corresponded to a
valve opening time of 150 ms.
Monkeys CE and CA were tested on one version of the task
(the outcome value contrast). Monkeys LI and CI were tested
on the other three versions of the task (the reference task, the
interference contrast, and the target frequency contrast) pre-
sented in pseudo-random order within and between sessions.
Each testing session was composed of a variable number of
200-letter runs (according to the monkey’s willingness to per-
form the task). As detailed in Table 1, monkeys completed 4 to
8 sessions. Each session lasted on average between 30 and
60 min. Each daily session ended when the monkey stopped
responding during 10 consecutive min.
Drug administration
After stable baseline performance was established,
atomoxetine, a NE-reuptake inhibitor (ATX, Tocris
Bioscience, Ellisville, MO) and saline (control) adminis-
tration sessions began. The experimenter administered in-
tramuscular injections of ATX or saline 30 min prior to
testing (Gamo et al. 2010; Seu et al. 2009). For monkeys
CI and LI, we tested four doses of ATX: 0.1, 0.5, 0.75,
and 1.0 mg/kg. Each dose was administrated during
1 week, each separated by at least 7 days of washout.
The smallest efficient dose in these two animals
(0.5 mg/kg; see “Statistical Analysis” below and Fig. 1c)
was then administered to the other two monkeys (CA and
CE) with the following protocol per week: 1 day of ATX
administration followed by 2-day washout and 1 day of
saline control condition. Each monkey completed 4 to 5
ATX sessions with the 0.5 mg/kg dose and 4 to 8 saline
sessions. The drug administration schedule for each ani-
mal is detailed in Table 1.
Data analysis
We first computed, for each monkey and each task contrast
condition, the HIT (% Go correct) and CR (% No-Go correct)
rates per 50-trial blocks. We then computed a perceptual sen-
sitivity index (d-prime—Eq. (1)) (Stanislaw and Todorov
1999), reflecting the subject’s ability to discriminate targets
from distractors, and a response bias index (c—Eq. (2))
reflecting the subject’s tendency to respond by a “Go” or a
“No-Go” (Stanislaw and Todorov 1999), two parameters tak-
en from signal detection theory.
0 ¼ Φ−1 HIT proportion
ð
Þ−Φ−1 False alarm proportion
ð
Þ ð1Þ
d
c ¼ −
Φ−1 HIT proportion
ð
Þ þ Φ−1 False alarm proportion
Þ
ð
2
ð2Þ
The Φ−1 function is the inverse of the normal cumulative
distribution function.
A c value significantly superior to 0 reflected a “No-Go”
bias whereas a c value significantly inferior to 0 reflected a
“Go” bias.
Finally, we examined the median and standard deviation of
the reaction times (RTs) for each 50-trial blocks. The standard
deviation of RTs allowed assessing block-by-block variability
in reaction times.
Relationship between sensitivity and response bias:
distance to the line of optimal response
We then investigated the relationship between sensitivity and
bias. We estimated the line of optimal response (LOR) for
each task contrast, i.e., the amount of bias that will maximize
utility (maximize benefits and minimize costs) over d-prime
values (i.e., coptimal) (Eq. (3)) (Lynn and Barrett 2014). Note
that any given set of environmental target frequency and out-
come values lead to a specific LOR. The optimal bias was
defined as follows:
coptimal ¼
(cid:3)
(cid:1)
log β
optimal
d0
ð3Þ
where βoptimal value (Eq. (4)) could be calculated from the
target frequency and outcome values (Tanner Jr. and Swets
1954):
β
optimal
Þ
¼ 1−αð
α
Þ
(cid:2) j−að
Þ
h−mð
ð4Þ
where α is the target frequency and j, a, h, and m are the
outcome values for correct rejections (CR), false alarms
3644
Psychopharmacology (2019) 236:3641–3653
(FA), correct detections (HIT), and missed detections (MISS),
respectively. Importantly, the outcome values’ array (j, a, h,
m) was defined similarly across monkeys as objective values
(Lynn et al. 2012; Lynn and Barrett 2014). Within this context,
it is reasonable to assume that the benefits and costs associated
with the different task contingencies could be ranked based on
the objective outcomes from the lowest value to the highest
value as 3-s wait (3-s time out) and no juice, 1-s wait and no
juice, small amount of juice (valve opening time of
35 ms), middle amount of juice (valve opening time of
50 ms), large amount of juice (valve opening time of
150 ms). As such, the overall goal of the present experi-
ment focused on the ability of ATX to change the per-
ceiver’s distance to our estimate of the objective LOR
rather than computing subjective utilities or individual,
subjective LORs. For Eq. (4), we chose outcome values
depending on the outcome values contrast conditions. We
assigned the actual valve opening times in milliseconds to
correct responses leading to a liquid reward, 0 for correct
responses leading to no reward (1 s wait and no juice) and
− 10 to incorrect responses leading to a penalty time and
no reward (3-s time out). Thus, for the elements (j, a, h,
m) of Eq. (4), we used (35, − 10, 50, − 10) for the refer-
ence task and interference contrast task, (50, − 10, 35, −
10) for the target frequency task where in addition to
changing the frequency of target occurrence, we also
rewarded CRs more than HITs responses, and (0, − 10,
150, − 10) for the outcome value contrast task, where
reward was only delivered for correct “Go” responses.
Then, we evaluated the Euclidean distance to the LOR
for each pair of d-prime and c values to characterize
how the monkeys adjusted their bias to their level of sen-
sitivity (Lynn and Barrett 2014).
Statistical analysis
Selection of the smallest efficient dose of ATX The smallest
efficient dose of ATX was determined in the reference task
using the sensitivity index as an indicator of the subjects’
performance, as in previous literature (e.g., Coull et al.
1995). We computed d-prime values in the reference task for
monkey CI and LI (Fig. 1c). Then, for each dose, d-prime
values were normalized as the percent change from saline
control condition:
Individual Δ scores
d0
ð
¼
Þ−d0
ð
ATX dose condition
jd0
Þ
mean of saline condition
Þj
(cid:2) 100
ð
mean of saline condition
One sample t tests were performed to determine whether
these individual Δ scores significantly differed from 0.
Generalized linear mixed models We examined the effect of
the smallest efficient dose of ATX on the different variables
computed above (i.e., HIT and CR responses, sensitivity, re-
sponse bias, LOR, median and standard deviation of the reac-
tion times) for each monkey, using generalized linear mixed
models (“lmer” R-package). The predictor tested was the
pharmacological condition, and for monkeys LI and CI, we
also used the task contrast as an additional predictor. The term,
sessions, was also included in the model as a random inter-
cept. Post hoc comparisons were carried out using pairwise
comparisons through the “emmeans” package for R (p-adjust-
ed with the false discovery rate method (Lenth 2016). The
behavioral data and the scripts are available as supplementary
materials.
Results
Baseline performances
As shown in Table 2, in the control (saline) condition, the
performance of the animals ranged from 57 to 100% cor-
rect for the HIT responses and from 69 to 92% correct for
the CR responses. Two animals (LI and CI) were tested on
three different versions of the task as follows: (1) the
reference task with a low target frequency (30%) and
distractors, (2) the interference contrast with a low target
frequency (30%) and distractors, and (3) the target fre-
quency contrast with a high target frequency (70%) and
with distractors. In the saline condition, we found that,
compared with the reference task, removing distractors
or increasing the target frequency significantly enhanced
performance, improving CR responses (F(2,29.31) =
17.35, p < 0.001 and F(2,56.93) = 6.53, p < 0.01, respec-
tively for monkeys CI and LI). It also significantly im-
proved the sensitivity index for monkeys LI (F(2,56.99) =
Fig. 1 Behavioral task—a The reference task (i.e., 30% of target stimuli
with distractors and larger reward for HITs (correct Go responses to
targets) compared with correct rejection (correct No-Go withholding of
response to distractors) and the other 3 task contrasts. Compared with the
reference task, the other three variants of the task differed as follows: the
interference contrast (black screens in place of letter distractors), the target
frequency contrast (70% of target stimuli and smaller reward for HIT
responses compared with correct rejection responses), and the outcome
value contrast (increased reward for HIT responses and no reward for
correct rejection responses). b Timeline of the task. A session was divided
into runs that consisted of 200-letter series presented at a pace of 1 Hz.
Each 200-letter series were pseudo-randomized in blocks of 50 letters,
resulting in four blocks per run. c ATX dose-response curves (mg/kg) for
sensitivity, for monkeys LI and CI. Results are plotted as mean ± SEM
(one sample t test on Δ scores, i.e., percentage change from saline control
condition (dotted blue boxes)—***p value < 0.0001; **p value < 0.001;
*p value < 0.05). The smallest efficient dose was based on the perfor-
mance in the reference task (orange boxes)
Psychopharmacology (2019) 236:3641–3653
3645
7.63, p < 0.01) and significantly modulated the response
bias in both animals (F(2,28.89) = 19.45, p < 0.001 and
F(2,56.98) = 13.64, p < 0.001, respectively for monkeys
CI and LI). These results indicate that reducing interfer-
ence and response inhibition improved performance and
modulated the response strategy.
a
b
c
Reference task
F
A
E
1Hz
200-letter-serie
Distractor
B
Target
30%
A
Interference contrast
Target frequency contrast
Outcome value contrast
A
A
A
E
F
A
E
Black screen
Target
30%
A
Distractor
B
Target
70%
A
Distractor
B
Target
30%
A
Session
Run 1
Run 2
Block 1
Block 2
Block 3
Block 4
Block 1
Block 2
Block 3
Block 4
... 1 run
= 200-letter-serie
1 block
= 50 pseudo-randomized
letters
monkey CI
***
***
**
***
***
**
***
*
*
y
t
i
v
i
t
i
s
n
e
S
l
)
s
e
u
a
v
e
m
i
r
p
-
d
(
3.5
3.0
2.5
2.0
1.5
monkey LI
***
***
***
**
**
***
***
***
***
*
Reference task
Interference contrast
Target frequency contrast
SALINE
ATX 0.1
ATX 0.5
ATX 0.75
ATX 1
SALINE
ATX 0.1
ATX 0.5
ATX 0.75
ATX 1
3646
Table 1 Drug administration schedule
Psychopharmacology (2019) 236:3641–3653
Week
number
Monkey CI
Monkey LI
Day
number
Monkeys
CA & CE
1
2
3
4
5
6
7
8
9
Saline
(4 sessions,
36 blocks)
Saline
(5 sessions,
60 blocks)
Washout
Washout
ATX 0.1mg/kg
(4 sessions,
72 blocks)
ATX 0.5mg/kg
(5 sessions,
112 blocks)
Washout
Washout
ATX 0.5mg/kg
(4 sessions,
104 blocks)
ATX 0.1mg/kg
(4 sessions,
68 blocks)
Washout
Washout
ATX 0.75mg/kg
(4 sessions,
68 blocks)
ATX 0.75mg/kg
(4 sessions,
124 blocks)
Washout
Washout
ATX 1.0mg/kg
(5 sessions,
92 blocks)
ATX 1.0mg/kg
(5 sessions,
156 blocks)
1
2
3
4
5
6
7
Saline
Saline
ATX 0.5mg/kg
Monkeys CI and LI were tested with either saline or ATX throughout a week, that included 4 to 8 sessions. The numbers in parenthesis indicates the
numbers of blocks completed by each monkey for a given condition. Gray boxes represent washout periods, not included in the data analysis
Monkeys CE and CA were tested with either saline or ATX on different days across the week. Gray boxes represent days not included in the data analysis
(washout periods). [Monkey CA: 8 saline sessions, 132 blocks - 4 ATX sessions, 109 blocks; Monkey CE: 8 saline sessions, 201 blocks - 4 ATX sessions,
116 blocks]
Effect of ATX on response type, bias, and sensitivity
Smallest efficient dose of ATX We then tested the effect of four
ATX doses on the performance of monkeys CI and LI. The
results on the animals’ response types (HIT and CR responses)
for all the task contrasts and ATX doses are provided in
Table 2. They show that both HIT and CR responses were
differently modulated depending on the dose of ATX and
the task contrast. The CR responses were significantly modu-
lated by the pharmacological condition in both animals
(F(4,356.47) = 4.12, p < 0.01 and F(4,488.81) = 16.68, p <
0.001, respectively for CI and LI) and the HIT responses were
significantly modulated by the pharmacological condition in
monkey CI (F(4,356.43) = 13.07, p < 0.001). To determine the
smallest efficient dose of ATX, we computed a normalized
sensitivity index (see “Statistical Analysis”). As shown in
Fig. 1c, for monkey CI, the sensitivity to discriminate target
from distractors was significantly impaired under ATX
0.1 mg/kg (t(23) = − 2.1, p < 0.05), whereas it was improved
for the three other doses compared with the saline (control)
condition (all p values < 0.005; t(31) = 5.4, t(15) = 6.4, and
t(27) = 3.6, respectively for the doses 0.5, 0.75, and 1.0 mg/kg).
For monkey LI, all ATX doses significantly improved the
sensitivity compared with saline condition (all p values <
0.01; t(23) = 3.1, t(39) = 3.0, t(43) = 5.8, and t(55) = 10.3, respec-
tively for the doses 0.1, 0.5, 0.75, and 1.0 mg/kg). Based on
these results, we selected the ATX dose of 0.5 mg/kg as the
smallest efficient dose for both animals. The other two mon-
keys, CA and CE, were only tested under 0.5 mg/kg ATX and
the saline condition.
Sensitivity The boxplots in Fig. 2 (left panels) illustrate the
sensitivity of each monkey and task contrast in both saline
(blue) and ATX 0.5 mg/kg (orange) conditions. After ATX
administration, the sensitivity to discriminate target stimuli
was significantly improved in three out of four monkeys, re-
gardless of the task contrast (monkey CI, F(1,134) = 6.03, p <
0.05; monkey LI, F(1,165.84) = 15.99, p < 0.001; and mon-
key CA, F(1,238.68) = 34.23, p < 0.001). In monkey CE,
which reached ~ 90% correct on both HIT and CR responses
in the saline condition, ATX did not improve sensitivity
(F(1,280.55) = 2.14, p = 0.14).
Bias and response type Figure 2 (right panels) illustrates the
response bias of each monkey and task contrast in both saline
(blue) and ATX 0.5 mg/kg (orange) conditions. ATX signifi-
cantly affected the response bias in two of the four monkeys
(i.e., monkeys LI and CE), regardless of the task demand. In
both of these animals, boosting NE transmission tended to
reduce or suppress the bias toward Go responses and/or in-
crease the bias toward “No-Go” responses (F(1,165.49) =
44.51, p < 0.001 and F(1,305.92) = 12.92, p < 0.001, respec-
tively for LI and CE). Accordingly, for these animals, mon-
keys LI and CE, only the CR responses were improved under
ATX (post hoc comparisons are provided in Table 2 showing
significant differences between saline and ATX 0.5 mg/kg for
CR responses—p < 0.05—and no differences for HIT re-
sponses). For the other two monkeys, CI and CA for which
ATX did not significantly change the response bias, we ob-
served a significant improvement for the HIT responses (post
hoc comparisons are provided in Table 2 showing significant
differences between saline and ATX 0.5 mg/kg for HIT re-
sponses—p < 0.01). Monkey CA also improves its CR re-
sponses (F(1,152.14) = 56.63, p < 0.001).
Taken together, our results show that increasing NE avail-
ability improves sensitivity when the level of interference,
response inhibition, or the outcome values are manipulated
and could in addition influence the animals’ response bias
by either reducing their bias toward Go responses and/or in-
creasing their bias toward “No-Go” responses. We did not find
Psychopharmacology (2019) 236:3641–3653
3647
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any interaction between task and pharmacological conditions
for response type, bias, or sensitivity.
Relationship between sensitivity and response bias:
Distance to the line of optimal response
To examine the relationship between the sensitivity scores and
the response bias and integrate them into the economic frame-
work of decision-making, we modeled the LOR depending on
the four task contrasts (see “Methods” for details). As shown
in Fig. 3, the task contrast modifies the relationship between
sensitivity and response bias resulting in different shapes of
the LOR. Regardless of the task contrast, the LOR follows a
general trend such that lower sensitivity scores are related to
more pronounced bias. By definition, the LOR is tightly
linked to the amount of expected utility (Lynn and Barrett
2014)—animals whose performance puts them closer to the
LOR should obtain a more optimal balance of rewards and
punishments.
We overlaid the animal performance (i.e., d-prime and c
values) on the modeled LOR to estimate the Euclidean dis-
tance to this LOR. The boxplots in Fig. 3 display the
Euclidean distances to a given LOR as a function of the task
contrast for each monkey in both saline (blue) and ATX
0.5 mg/kg (orange) conditions. First, we found that, in the
saline condition, the distance to the LOR varied across ani-
mals and was significantly impacted by task contingencies for
monkey LI (F(2,56.94) = 7.88, p < 0.001). Second, after ATX
administration, we found that the distance to the LOR was less
variable and decreased in three out of four monkeys (monkey
CI (F(1,134) = 5.53, p < 0.05), monkey LI (F(1,165.63) =
27.37, p < 0.001), and monkey CA (F(1,239) = 16.52,
p < 0.001)). An interaction was found between task contrast
and pharmacological condition in monkey LI (F(2,165.35) =
3.51, p < 0.05), revealing that ATX only affected the relation-
ship between the sensitivity scores and the response bias on
the reference task and the interference task contrast but not on
the target frequency contrast. Overall, our results show that
boosting NE transmission altered both sensitivity and re-
sponse bias and their functional relationship bringing the an-
imals’ performance closer to the line of optimal response.
Effect of ATX on reaction times
RTs tended to increase and/or their variability tended to de-
crease after ATX (0.5 mg/kg) administration in all four mon-
keys. Specifically, RTs significantly increased in three out of
four animals (monkey CI (F(1,131.38) = 29.50, p < 0.001),
monkey LI (F(1,165.16) = 7.07, p < 0.01), and monkey CE
(F(1,311.65) = 5.01, p < 0.05). RT variability decreased in
three out of four monkeys, as shown by the significantly
smaller sta ndard devia t io n (Fig . 4) (m on key C I
Psychopharmacology (2019) 236:3641–3653
3648
Fig. 2 ATX effect on sensitivity
and response bias—Sensitivity
index (left panels) and response
bias (right panels). For the
boxplots illustrating response
biases (right panels), the gray
dashed line divides the c values
according to “Go” (negative
values) and “No-Go” (positive
values) biases. Orange boxplots
correspond to ATX 0.5 mg/kg
conditions and blue boxplots cor-
respond to saline (control) condi-
tions. Black stars with arrow
flankers indicate the main effect
of statistical differences between
saline and ATX 0.5 mg/kg condi-
tions. (***p value < 0.001; **p
value < 0.01; *p value < 0.05)
(F(1,132.88) = 15.47, p < 0.001), monkey LI (F(1,165.89) =
10.85, p < 0.01), and monkey CA (F(1,205.09) = 11.11, p <
0.01)). The only exceptions to the above finding about ATX
effects were (1) in monkey CE, an increased RT variability
(F(1,311.55) = 5.20, p = 0.02) and (2) in monkey CI, a RT
increase restricted to the interference and target frequency task
contrast (interaction (F(2,131.30) = 4.42, p = 0.01), and RT
variability decrease restricted to the reference task (interaction
(F(2,132.41) = 18.48, p < 0.001)).
Discussion
We tested whether the modulatory effects following ATX in-
jection in monkeys translate into an adjustment of the behav-
ior toward the line of optimal response, reflected in the func-
tional relationship between sensitivity index and response bias
(Lynn and Barrett 2014). The animals’ performances were
assessed in a Go/No-Go task under different task contingen-
cies where we varied the level of signal/noise interference, the
target frequency, and the outcome values. We found that
boosting NE transmission tuned the functional relationship
between sensitivity and response bias leading to a closer fit
with the optimal strategy in the different task manipulations
tested. Furthermore, under ATX, the subjects’ response time
tended to increase and show less variability. Altogether, these
findings support the hypothesis that enhancing NE availability
optimizes response strategies.
Boosting NE transmission fine tunes
the functional relationship
between sensitivity and response bias
In agreement with previous reports, we confirm that boosting
NE transmission improved performance in a Go/No-Go task.
This effect has been documented in humans and monkeys, on
both correct detection (e.g. Coull et al. 1995; Decamp et al.
2011) and correct rejection (e.g. Usher et al. 1999). To tease
apart some of the main components of the decision process
that could be selectively affected by NE, we further manipu-
lated the task contingencies (level of interference, target fre-
quency, and outcome values) and analyzed perceptual sensi-
tivity and response bias, in addition to simple accuracies
(which confound the discriminability and bias elements of
performance; Lynn and Barrett 2014). In the control condition
(saline), increasing the level of interference and decreasing the
target frequency altered the animals’ performance (monkeys
LI and CI). After injection of ATX (a NE-reuptake inhibitor),
the animals’ sensitivity index improved in all task contingen-
cies. In other words, boosting NE transmission improves the
sensitivity to discriminate a target stimulus whether or not the
discrimination process involves interfering distractors, a rare
or frequent event, or different outcome values. Future studies
further manipulating the context might reveal NE-dependent
contextual specificities. Two not mutually exclusive mecha-
nisms might explain this pattern of results. The improvement
in the different task variants might reflect a general arousal
effect following ATX injection (Robbins 1997; Coull et al.
Psychopharmacology (2019) 236:3641–3653
3649
Fig. 3 ATX effect on the distance
to the line of optimal response
(LOR)—Plots illustrating the
relationship between sensitivity
and response bias are depicted on
the left. The red line represents the
LOR for each task contrast, which
depends on both the target
frequency and the outcome values
of the task. Each dot represents
the average d-prime and c values
for each block of a given task
contrast and pharmacological
condition (blue = saline and
orange = ATX 0.5 mg/kg) for
monkeys CI, CA (circles) and
monkeys LI, CE (triangles). The
ellipses surrounding the dots were
drawn using a confidence level of
0.5. Adjacent boxplots on the
right display the Euclidean dis-
tance to the LOR in each monkey
and task contrast, in blue and or-
ange, respectively for the saline
(control) and ATX 0.5 mg/kg
conditions
Fig. 4 Standard deviation of reaction times—Box plots illustrate the
standard deviation of reaction times in each monkey and task contrast
under saline condition (blue) and ATX 0.5 mg/kg condition (orange). At
the center of the plots are represented the median of the standard deviation
of reaction times across blocks and dots represent outliers. Black stars
with arrow flankers indicate the main effect of statistical differences be-
tween saline and ATX 0.5 mg/kg conditions. (***p value < 0.001; **p
value < 0.01; *p value < 0.05)
3650
Psychopharmacology (2019) 236:3641–3653
2004; Berridge et al. 2012) and/or the mobilization of energy
or resources to face challenges (Raizada and Poldrack 2007;
Malecek and Poldrack 2013; Kalwani et al. 2014; Bouret and
Richmond 2015; Varazzani et al. 2015).
Does this improvement in terms of sensitivity scores
following ATX injection reflect optimization of the ani-
mals’ response strategy? To address this question, we
modeled the line of optimal response (LOR) for each task
contrast, which describes the amount of bias needed de-
pending on the subjects’ sensitivity. This relationship
varies with the task contingencies (perceptual aspects of
the decision and the outcome value associated with a given
choice). The four animals did not perform the tasks with
the same strategy in the control condition. Two animals
(monkeys CE and LI) reached a high response rate in both
HIT and CR responses while performances of the remain-
ing two animals were lower (between 60 and 80% correct
responses). As a result, ATX significantly modified the
bias in the two animals that performed more poorly on
CR compared with HIT responses (i.e., monkeys LI and
CE). As suggested by Lynn and Barrett (2014), a given
perceiver is able to adjust his bias to optimally accommo-
date his level of sensitivity. We found that ATX injection
helps promote this adjustment, as previously inferred from
the pupil size (Gee et al. 2014). In line with Lynn and
Barrett’s (2014) the proposal, we found that this adjust-
ment led to a closer fit of the performance with the LOR
defined by the contingency of the task. The Euclidean dis-
tance between the performance and the LOR was reduced
in the majority of the animals under ATX. One animal
exhibited a significant interaction between task contingen-
cy and pharmacological condition for the distance to the
LOR, suggesting that specificity based on the task at hand
might emerge following a boost in NE transmission and
future studies further manipulating the context might re-
veal NE-dependent specificities. Note that our experiment
focused on manipulating the task contingency to change
the perceiver’s distance to an estimate of the objective
LOR using ranked values (Lynn et al. 2012; Lynn and
Barrett 2014). While examining the relationship between
individual NE receptor polymorphisms and the perceiver’s
subjective distance to the LOR was beyond the scope of
the current study, our results demonstrate that ATX re-
duced the perceiver’s distance to an estimate of the objec-
tive LOR. In Lynn and Barrett’s (2014) terminology, ATX
might be changing the perceiver’s “subjective estimate” of
the objective payoffs such that the perceiver values the
payoffs differently in the two pharmacological conditions.
It is equally possible that ATX affects the perceiver’s LOR
by altering subjective values rather than its bias or sensi-
tivity, per se. Optimization of behavior requires finding the
best adjustment based on the evaluation of the different
outcomes of given choices and the sensitivity and bias of
the perceptual system and it is conceivable that the wide-
spread projections of the LC-NE system, especially those
directed toward the prefrontal cortex, influence or facilitate
such computations as discussed in the next paragraph
(Rich and Wallis 2016). Here, we suggest that the closer
fit with the LOR following the NE challenge provides ex-
perimental support in favor of the role of NE in optimizing
behavioral performance, in a constant environment. It
would be interesting in future studies to assess the effect
of ATX on individual’s subjective utility of gains and
losses by systematically varying the levels of gains and
losses and incorporating, for instance prospect theory, to
translate objective into subjective gain and loss differences
(Kahneman and Tversky 2012).
How does enhanced NE availability adjust
the performance in a perceptual
discrimination task?
The optimization of the response strategy found in the
ATX condition was accompanied in the majority of the
cases by increased RTs and/or decreased RT variability.
Increased RT measures could reflect a prolonged period
during which information about the stimuli is accumulated
before an option is selected (Bogacz et al. 2010). Trial-by-
trial variability is considered a hallmark of how we select
an option over multiple choices (Bellgrove et al. 2004;
Johnson et al. 2007). A recent study by Murphy et al.
(2014) found a correlation between variation in pupil di-
ameter (considered as a proxy of the LC activity) and re-
sponse time variability in perceptual decision-making in
humans. In humans and rats, a series of experiments have
demonstrated that NE agents (alpha-2-agonists or NE-
reuptake inhibitors) affect response inhibition and response
time variability (e.g. Bari and Robbins 2013). In the pres-
ent study, we also altered NE transmission with a NE-
reuptake inhibitor (ATX). In most of the task variants, the
target appeared in 30% of the trials such that in most of the
trials, the subjects were required to withhold their re-
sponse. The decision to go or not to go had to be taken
rapidly due to the frequency of stimulus presentation (≈
1 Hz). On the one hand, the improvement in correct rejec-
tion (No-Go response) suggests that response inhibition
was improved under ATX (e.g. Robinson et al. 2008;
Chamberlain et al. 2009). On the other hand, the improve-
ment in the HIT and the narrowing of the RT variability
suggest an influence of ATX on attentional and/or deci-
sional processes (Gee et al. 2014; Murphy et al. 2016;
van den Brink et al. 2016). Within the framework of signal
detection theory, our results highlight an improvement of
the sensitivity to the target and the tendency of the animals
to either shift their response bias toward a No-Go response
Psychopharmacology (2019) 236:3641–3653
3651
or reduce their Go bias under ATX leading to a closer fit
with the LOR. ATX acts by preventing the reuptake of NE
in cortical and subcortical regions, leading to an increase of
the post-synaptic effect of LC activation in each target
area. The resulting increase in NE availability is also ex-
pected to act on LC inhibitory autoreceptors (Aghajanian
et al. 1977) and reduce LC activity (Bari and Aston-Jones
2013). Our results suggest that together with a shift of the
performance toward the LOR, ATX could also alter trial-
by-trial variability in RTs. It is possible that this narrowing
in RT distribution reflects an adjustment of the LC activity
and neural gain to optimize performance (Servan-Schreiber
et al. 1990). This adjustment could result in changes in
functional connectivity at the whole-brain level, similar
to those that we recently reported at rest (Guedj et al.
2016). In line with our recent proposal (Guedj et al.
2017), the effect reported here on the performance and
response strategy optimization could be supported by
NE-dependent local-to-global modulations of brain dy-
namics that depends on the context (de Gee et al. 2017).
Conclusion
In the present study, implementing the utility-based approach to
the signal detection theory (Lynn and Barrett 2014) that inte-
grates both perceptual aspects of the decision and the outcome
value associated with a given choice, we provide empirical evi-
dence for a role of NE transmission in optimizing response strat-
egy in a constant environment. Boosting NE transmission mod-
ified the functional relationship between sensitivity index and
response bias leading to a closer fit with the optimal strategy in
different contexts. It also tended to reduce the variability in reac-
tion times. This neuromodulator, with widespread projections
onto virtually the whole brain, facilitates behavioral adaptation
in a variety of contexts. Here, we show that this facilitation results
in fine tuning of the functional relationship between perceptual
and decisional processes.
Acknowledgments We thank Gislène Gardechaux and Frédéric Volland
for their technical assistance. We thank Spencer Lynn for discussions
about signal detection theory and for comments on the manuscript.
Funding information This work was funded by the French National
Research Agency (ANR) ANR-14-CE13-0005-1 grant. It was also support-
ed by the NEURODIS Foundation and the James S. McDonnell Scholar
award. It was performed within the framework of the LABEX CORTEX
(ANR-11-LABX-0042) of the Lyon University within the program
“Investissements d’Avenir” (ANR-11-IDEX-0007) operated by the ANR.
Compliance with ethical standards
Work complied with European Union Directive 2010/63/EU and was
approved by French Animal Experimentation Ethics Committee #42
(CELYNE).
Conflict of interest The authors declare that they have no conflict of
interest.
Disclaimer Funders had no role in study design, data collection and
analysis, decision to publish, or preparation of the manuscript.
Open Access This article is distributed under the terms of the Creative
C o m m o n s A t t r i b u t i o n 4 . 0 I n t e r n a t i o n a l L i c e n s e ( h t t p : / /
creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a link
to the Creative Commons license, and indicate if changes were made.
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| null |
10.1371_journal.pcbi.1011928.pdf
|
Data Availability Statement: Code can be
downloaded from https://git.exeter.ac.uk/mv286/
hormonebayes.
|
Code can be downloaded from https://git.exeter.ac.uk/mv286/ hormonebayes .
|
RESEARCH ARTICLE
HormoneBayes: A novel Bayesian framework
for the analysis of pulsatile hormone
dynamics
Margaritis VoliotisID
1
S. Dhillo2, Krasimira Tsaneva-AtanasovaID
1*, Ali Abbara2, Julia K. Prague2,3,4, Johannes D. Veldhuis5, Waljit
1 Department of Mathematics and Living Systems Institute, College of Engineering, Mathematics and
Physical Sciences, University of Exeter, Exeter, United Kingdom, 2 Department of Metabolism, Digestion and
Reproduction, Imperial College London, Hammersmith Hospital, London, United Kingdom, 3 Department of
Diabetes and Endocrinology, MacLeod Diabetes and Endocrine Centre, Royal Devon and Exeter Hospital,
Exeter, United Kingdom, 4 College of Medicine and Health, University of Exeter, Exeter, United Kingdom,
5 Emeritus Mayo Clinic, Rochester, Michigan, United States of America
* [email protected]
Abstract
The hypothalamus is the central regulator of reproductive hormone secretion. Pulsatile
secretion of gonadotropin releasing hormone (GnRH) is fundamental to physiological stimu-
lation of the pituitary gland to release luteinizing hormone (LH) and follicle stimulating hor-
mone (FSH). Furthermore, GnRH pulsatility is altered in common reproductive disorders
such as polycystic ovary syndrome (PCOS) and hypothalamic amenorrhea (HA). LH is mea-
sured routinely in clinical practice using an automated chemiluminescent immunoassay
method and is the gold standard surrogate marker of GnRH. LH can be measured at fre-
quent intervals (e.g., 10 minutely) to assess GnRH/LH pulsatility. However, this is rarely
done in clinical practice because it is resource intensive, and there is no open-access,
graphical interface software for computational analysis of the LH data available to clinicians.
Here we present hormoneBayes, a novel open-access Bayesian framework that can be
easily applied to reliably analyze serial LH measurements to assess LH pulsatility. The
framework utilizes parsimonious models to simulate hypothalamic signals that drive LH
dynamics, together with state-of-the-art (sequential) Monte-Carlo methods to infer key
parameters and latent hypothalamic dynamics. We show that this method provides esti-
mates for key pulse parameters including inter-pulse interval, secretion and clearance rates
and identifies LH pulses in line with the widely used deconvolution method. We show that
these parameters can distinguish LH pulsatility in different clinical contexts including in
reproductive health and disease in men and women (e.g., healthy men, healthy women
before and after menopause, women with HA or PCOS). A further advantage of hormone-
Bayes is that our mathematical approach provides a quantified estimation of uncertainty.
Our framework will complement methods enabling real-time in-vivo hormone monitoring
and therefore has the potential to assist translation of personalized, data-driven, clinical
care of patients presenting with conditions of reproductive hormone dysfunction.
a1111111111
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OPEN ACCESS
Citation: Voliotis M, Abbara A, Prague JK, Veldhuis
JD, Dhillo WS, Tsaneva-Atanasova K (2024)
HormoneBayes: A novel Bayesian framework for
the analysis of pulsatile hormone dynamics. PLoS
Comput Biol 20(2): e1011928. https://doi.org/
10.1371/journal.pcbi.1011928
Editor: Marc R. Birtwistle, Clemson University,
UNITED STATES
Received: June 16, 2023
Accepted: February 19, 2024
Published: February 29, 2024
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pcbi.1011928
Copyright: © 2024 Voliotis et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Code can be
downloaded from https://git.exeter.ac.uk/mv286/
hormonebayes.
PLOS Computational Biology | https://doi.org/10.1371/journal.pcbi.1011928 February 29, 2024
1 / 11
PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
Funding: MV and KTA acknowledge the financial
support of the EPSRC via grants EP/T017856/1
and EP/N014391/1, and BBSRC via grants BB/
S000550/1 and BB/S001255/1. JKP is supported
by a NIHR academic fellowship, MRC (MR/
M024954/1), and Expanding Excellence in England
(E3) - Exeter Diabetes Research Unit. AA is
supported by an NIHR Clinician Scientist Award
(CS-2018-18-ST2-002). WSD is supported by an
NIHR Senior Investigator Award. The funders had
no role in study design, data collection and
analysis, decision to publish, or preparation of the
manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Author summary
Pulsatile hormone secretion is a widespread phenomenon underlying normal physiology
and is also disrupted in many common endocrine disorders. To aid assessment and quan-
tification of hormonal pulsatility, we developed hormoneBayes, a novel open-access Bayes-
ian framework for analyzing hormonal measurements. The framework uses mathematical
models to describe pulsatile dynamics, together with Bayesian methods to infer model
parameter from data. We demonstrate HormoneBayes utility by analysing pulsatility of
luteinising hormone (LH) data in different clinical contexts including in reproductive
health and disease. Our framework in combination with real-time in-vivo hormone moni-
toring has the potential to assist translation of personalized, data-driven, clinical care of
patients presenting endocrine disorders.
Introduction
Pulsatile hormone dynamics are ubiquitous and play a crucial role in the regulation of many
bodily functions related to metabolism, stress, and fertility [1,2]. Hormones are typically
secreted in both a basal manner to maintain steady state levels, as well as with superimposed
interspersed transient bursts (pulses) [3]. It is now established that the pulsatile nature of hor-
monal secretion affects their interaction with receptors and downstream effector action [4–6].
With regards to fertility, the hypothalamus is the central regulator of the reproductive endo-
crine axis. Notably, gonadotropin releasing hormone (GnRH) is secreted in a pulsatile man-
ner, and seminal studies have demonstrated that this pulsatility is fundamental for its action to
stimulate GnRH receptors on pituitary gonadotropes [5]. Moreover, disturbances in GnRH
pulsatility are observed in common reproductive disorders including polycystic ovary syn-
drome (PCOS) in which GnRH pulsatility is increased [7], and hypothalamic amenorrhea
(HA) in which GnRH pulsatility is reduced [8]. However, despite this disparate alteration in
GnRH pulsatility, differentiation of these two common reproductive disorders, which may
both present similarly with menstrual disturbance, can be challenging [9]. LH is measured rou-
tinely in clinical practice using an automated chemiluminescent immunoassay method and is
the gold standard surrogate marker of GnRH. Furthermore, LH can be measured at frequent
intervals (eg 10minutely) to assess GnRH/LH pulsatility, and accurate assessment of LH pulsa-
tility could help facilitate diagnosis and treatment of patients presenting with reproductive
endocrine disorders [9]. However, this is rarely done in clinical practice because it is resource-
intensive, inconvenient for patients, and there is a lack of software for computational analysis
of the LH data readily available to clinicians.
Analysis of hormone pulsatility is a challenging computational problem, primarily due the
stochastic nature of hormone dynamics and the consequent pulse-to-pulse variability, but also
due to extrinsic factors (such as measurement error) obscuring the observed hormone dynam-
ics [3]. Several computational methods for the analysis of endocrine data have been proposed
in the literature [3,10–15], and deconvolution analysis, is the current gold-standard method
for analyzing LH pulsatility in humans [3]. However, all methods lack open-access software
implementation, with a user-friendly graphical interface that can be readily used by clinicians.
To meet these challenges, we have developed HormoneBayes, a novel, open-access Bayesian
framework for the analysis of hormone pulsatility data. HormoneBayes uses a stochastic
model, describing hormone levels in the circulation incorporating measurement error, and
leverages Bayesian statistics [16] to infer model parameters and latent variable dynamics. We
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
note that this approach is distinct to the deconvolution-based approach [3,14,15], which
employs a single-pulse model to represent the data as a sequence of independent pulses. In this
deconvolution-based method, the number of pulses becomes a model parameter that needs to
be inferred from the data. In a Bayesian context, this leads to a posterior with unknown dimen-
sions, hence posing significant challenges to inference [17,18]. We show that HormoneBayes
can be used to accurately identify LH pulses and estimates clinically relevant measures such as
inter-pulse intervals and secretion rates. The framework also provides a handle on estimation
uncertainty via Bayesian posterior distributions. We showcase how this feature can be used to
enable the understanding of alterations in LH pulsatility by analyzing the effect of direct hypo-
thalamic stimulation using the neuropeptide kisspeptin on a subject-by-subject basis. We also
demonstrate that HormoneBayes can be used to analyze LH pulsatility in different clinical con-
texts/reproductive states (including healthy men, women before and after menopause, and
women with reproductive disorders such as HA or PCOS). Importantly, the framework comes
with an open-access graphical interface that make the core functionality of the framework eas-
ily accessible to clinicians and clinical researchers.
Results
Analyzing pulsatile hormone dynamics using the hormoneBayes
framework
The hormoneBayes framework allows inference of key physiological parameters describing
pulsatile hormone dynamics. The framework utilizes stochastic mathematical models describ-
ing circulating hormone levels and state-of-the-art Bayesian machinery to calibrate these mod-
els to data of hormone profiles and infer model parameters. Fig 1 presents a parsimonious
model (a simple model with great explanatory power) describing circulating LH levels. The
model assumes that there are two modes of LH secretion, namely pulsatile and basal. The
Fig 1. HormoneBayes: a Bayesian framework for analyzing pulsatile LH dynamics. The framework uses a
parsimonious mathematical model to describe LH levels in circulation as the net effect of secretion and clearance. In
the model secretion is driven by a basal hypothalamic signal and a pulsatile signal (mimicking the dynamics of the
GnRH pulse generator which can be turned ‘on’ or ‘off’). An efficient Markov-Chain Monte-Carlo (MCMC) method
performs the Bayesian inference and extracts model parameters and latent hypothalamic dynamics, which are
compatible with the observed data.
https://doi.org/10.1371/journal.pcbi.1011928.g001
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
former corresponds to an on/off signal that randomly switches between two states (corre-
sponding to a high and a low activity) while the latter corresponds to a continuous noisy signal.
Furthermore, the model incorporates LH clearance as a linear first order process leading to an
exponential decay of LH levels following a pulse [3]. We note that more detailed clearance
models, such as the bi-exponential model [3], could be easily integrated. By considering the
processes of LH release and clearance, the model predicts LH circulating dynamics in terms of
five key parameters that can be recovered from data: 1. LH clearance rate; 2. maximum LH
release rate; 3. strength of the pulsatile signal relative to the basal signal; 4. mean time in the on
state; 5. mean time in the off state. Moreover, the model incorporates measurement error as an
additional parameter that is determined based on the assay coefficient of variation (CV).
HormoneBayes relies on the Bayesian paradigm to extract information from the observed
data. Using the Bayes theorem, the method revises our prior beliefs regarding model parame-
ters by transforming the parameters’ prior probability density distributions into posterior dis-
tributions. Parameter prior distributions enable the user to input context-specific information
into the analysis, hence enhancing the flexibility of the method to handle different datasets.
For example, when dealing with data from post-menopausal women the user could choose to
adjust the parameter priors to acknowledge a higher LH secretion rate and/or more sustained
basal secretion. Fig 1 illustrates how the Bayesian machinery allows us to calibrate the model
to the data and extract information regarding model parameter and latent hypothalamic sig-
nals with an estimate of certainty. As we explain in the sections to follow, this information can
be used to identify pulses; summarise the data (e.g., providing mean and standard deviation
estimates); and perform statistical tests.
Identifying LH pulses
Using data to infer the latent hypothalamic signal provides a transparent way to identify pulses
based on their likelihood under the model. As explained above, the model assumes LH pulses
are partly driven by an on/off hypothalamic signal. This latent variable (i.e., inferred variable)
takes two values indicating the ‘on’ (1) and ‘off’ (0) state of the hypothalamic pulse generator,
and therefore the expected posterior estimate (inferred from LH profiling data) can be inter-
preted as the probability that at any given time the hypothalamic pulse generator is ‘on’. This
quantitative measure for accessing the likelihood of a pulse can significantly ease the job a cli-
nician trying to decide whether an upstroke in the LH profile represents a pulse or not. Fig 2
Fig 2. Pulse identification using HormoneBayes. (A) Pulses can be identified using the expected value of the pulsatile
hypothalamic signal, which can be interpreted as the probability of a pulse at a given timepoint. Here, we mark the
onset of a pulse when the pulsatile hypothalamic signal crosses the 0.5 threshold, indicating that at this point a pulse is
the most likely event. (B) The majority of the identified pulses (89%, 77/87) are in line with those obtained using the
deconvolution method. For the analysis we used LH data obtained from healthy pre-menopausal women in early
follicular phase (n = 16).
https://doi.org/10.1371/journal.pcbi.1011928.g002
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
illustrates a representative example of an LH trace with two obvious pulses occurring at times
100min and 300min. These are indeed identified by inspecting the hypothalamic signal profile
which peaks at around those times. At time 200min a less pronounced bump in LH could be
indicative of an LH pulse, however the inferred pulsatile hypothalamic signal remains well
below 0.5, indicating that under the current model there is higher probability that the bump is
a measurement artefact rather than a pulse.
To validate our pulse identification method, we used a database of LH profiles obtained
from healthy pre-menopausal women and compared the identified pulses with those previ-
ously obtained by the deconvolution method [19], which uses a maximum likelihood approach
to fit a series of pulses to the data [3]. Here, we identify a pulse when the posterior probability
that the hypothalamic signal is ‘on’ exceeds a threshold value. We use 0.5 as the threshold
value, which signifies there is higher probability that the hypothalamic pulse generator is on
(rather than off). This value yields the highest agreement between hormoneBayes and the
deconvolution method (see Fig D in S1 Text). We find that hormoneBayes agrees with the
deconvolution method in 77 out of 87 identified pulses (89%). Moreover, 13 pulses identified
by hormoneBayes were not identified by the deconvolution method. Overall, this suggests a
good agreement between the two methods, with hormoneBayes having the added advantage of
providing a measure for the likelihood of each pulse that clinicians and researchers can use to
inform their clinical decision making.
Variation of model parameter within and across groups
To test the applicability of hormoneBayes in different contexts we compile a database of LH
profiles from four groups with diverse LH dynamics (men, healthy pre-menopausal women
with regular menstrual cycles, post-menopausal women, women with HA, and women with
PCOS). As illustrated in Fig 3, the model successfully captures the differences in LH dynamics
in all four groups. Moreover, the model allows us to summarize LH data through model
parameters and assess the variability across and within groups. We find that two model param-
eters explain most of the variability between groups, namely the maximum secretion rate and
pulsatility strength (Fig 3C). The first parameter describes how much LH can be secreted over
time, whilst the second parameter quantifies the strength of the pulsatile signal relative to the
basal signal. Based on these two parameters there is a strong distinction between women with
PCOS and women with HA, who have lower LH secretion rates and/or diminished LH pulsati-
lity strength (i.e., pulsatile signal is weak relative to the basal signal). Furthermore, post-meno-
pausal women display higher secretion rates compared to pre-menopausal women but also
reduced pulsatility strength (i.e., pulses are less pronounced when higher level of LH are estab-
lished post menopause). Interestingly, healthy men and women illustrate a much lower param-
eter variability as compared to HA and post-menopausal women, which could be indicative of
various degrees of severity of HA/PCOS and tighter LH regulation in healthy individuals of
reproductive age.
Discussion
We have presented HormoneBayes, a novel computational framework for analyzing hormone
pulsatility. The framework combines (i) mathematical (mechanistic) models describing hor-
mone dynamics with (ii) computational Bayesian machinery for inferring model parameters
from data. HormoneBayes, comes with an open-access graphical user interface that make the
core functionality of the framework easily accessible, a feature lacking from currently available
analysis methods.
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
Fig 3. HormoneBayes handles LH pulsatility analysis in different contexts. (A) Inferred pulsatility strength and maximum
secretion rate parameters for different individuals: healthy men (n = 10); healthy post-menopause women (n = 13); healthy
pre-menopausal women (n = 4). (B) Inferred parameters for healthy pre-menopausal women (n = 4); women with PCOS
(n = 6) and women with HA (n = 5) illustrating how the assessment of LH pulsatility could help facilitate diagnosis of patients
presenting with reproductive endocrine disorders. (C) Representative fits of the model are given for one subject in each
dataset.
https://doi.org/10.1371/journal.pcbi.1011928.g003
Using a parsimonious mathematical (generative) model of LH secretion, we have demon-
strated the clinical utility of HormoneBayes in accurately describing LH profiles in various
contexts (healthy men, healthy pre-menopausal women, post-menopausal women, women
with PCOS and women with HA), and for identifying pulses. A novel feature of hormoneBayes
is that it summarizes hormone profiles in terms of model parameters that can be used to pre-
dict the underlying clinical conditions or reproductive state. Therefore, in the clinic hormone-
Bayes could assist diagnosis based on hormonal profiles by evaluating how well the profile is
described by different model/prior configurations, representing distinct physiological states
corresponding to different clinical conditions (e.g., PCOS, HA).
Ultimately, data analysis using HormoneBayes is as credible as the underlying generative
model used to describe hormonal dynamics. Unlike deconvolution methods, where the num-
ber of pulses is one of the model parameters to be inferred from the data, our approach relies
on a generative model that assumes two modes of LH secretion: pulsatile and basal. This
assumption is in par with current physiological understanding of the system and the hypotha-
lamic pulse generator hypothesis [20,21]. Furthermore, to model LH circulation levels the
model assumes a linear clearance rate. At least one other model used for the analysis of LH pul-
satility has used more complex (multiple timescale) clearance dynamics, however we expect
this assumption should have a minimal impact for the purpose of assessing LH pulsatility.
Nevertheless, the modular design of HormoneBayes allows future extensions of the model
with the scope of comparing how well different models capture LH dynamics as well as
enabling the analysis of hormone dynamics beyond LH [22,23].
HormoneBayes utilizes the Bayesian paradigm to infer model parameters from the data.
Within this paradigm, for each profile the method will output a (posterior) density distribution
of model parameters, quantifying how probable parameter values are given the observed pro-
file. This is fundamentally different from non-Bayesian methods, which provide point-
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
estimates of model parameters, as it allows for statistical testing. For instance, inferred poste-
rior distributions can be used to evaluate the impact of hormonal interventions on LH secre-
tion parameters, and crucially, this statistical evaluation can be conducted not only at the
population level but also on an individual basis (see Fig E in S1 Text for an example of this
type of personalised analysis). We expect these features of our method regarding personalized
analysis will be crucial as measurement technologies mature enabling cheap sampling of hor-
mone levels in real time [22,24].
Methods
Ethics statement
Data included in this manuscript were obtained from five different clinical research studies,
involving healthy men [25], healthy pre-menopausal women [19], post-menopausal women
[26], women with PCOS and women with HA [8]. Ethical approval for these studies was
granted by: the Hammersmith and Queen Charlotte’s and Chelsea Hospitals Research Ethics
Committee (registration number 05/Q0406/142) [8,19]; the UK National Research Ethics
Committee-Central London (Research Ethics Committee number 14/LO/1098) [25]; and the
West London Regional Ethics Committee (15/LO/1481) [26]. Written informed consent was
obtained from all subjects. All studies were conducted according to Good Clinical Practice
Guidelines.
Data collection
Participants attended a clinical research facility for 8 hour study visits hat included baseline
(vehicle treatment) LH measurements according to the relevant trial protocol as previously
described [8,19,25,26]. A cannula was inserted into a peripheral vein under aseptic conditions
(time at least -30 minutes), through which all subsequent blood samples were taken every 10
minutes from time 0 until 480 minutes. All participants were ambulatory and could eat and
drink freely during the study visit. All blood samples were left to clot for at least 30 minutes
prior to centrifugation at 503 rcf for 10 minutes, after which the serum supernatant was
extracted and immediately frozen at -20˚C prior to subsequent analysis using an automated
chemiluminescent immunoassay method (Abbott Diagnostics, Maidenhead, UK) in batches
after study completion. Reference ranges were as follows: LH 4–14 IU/L; respective intra-assay
and inter-assay coefficients of variation were 4.1% and 2.7%; analytical sensitivity was 0.5 IU/L.
Stochastic model of LH
We used a discrete-time, stochastic model to describe pulsatile LH dynamics. The model com-
prises of three dynamical variables, Pt, Bt, and LHt, that describe the pulsatile and basal hypo-
thalamic signals and the LH concertation in circulation, respectively.
The pulsatile hypothalamic signal Pt can take two values: Ht = 0 corresponding to the ON
state; and Ht = 1 corresponding to the OFF state. The stochastic dynamics of Ht are governed
by the following probability matrix
Hs
0
1
https://doi.org/10.1371/journal.pcbi.1011928.t001
Hs+δt
0
1 (cid:0)
1
tOFF
� dt
1
tON
� dt
1
1
tON
1 (cid:0)
� dt
1
tOFF
� dt
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
i.e., parameters τON and τOFF govern the probabilities that the value of H will either flip or
remain the same over the time interval (s, s+δt).
The evolution of the basal hypothalamic signal, Bt, is described using a discrete time autore-
gressive model obeying the following equation
Xtþdt ¼ Xt (cid:0)
p
ffiffiffiffiffi
dt
� εt;
Xt þ
dt
2
Bt ¼
1
1 þ e(cid:0) Xt
;
where εt is a normally distributed random variable with zero mean and unit variance. Note
that both Bt and Ht are bounded in the interval [0,1].
The two hypothalamic signals drive LH secretion, and along with LH clearance dictate the
circulating LH levels, LHt. The equation describing the time evolution of LHt, is
LHtþdt ¼ LHt þ ½kðPt � f þ Bt � ð1 (cid:0)
f ÞÞ (cid:0) d � LHt� � dt
where d denotes the clearance rate, k denotes the maximum secretion rate, and parameter f
(termed pulsatility strength) describes the relative strength of the two hypothalamic signals.
Finally, the model assumes measurement error in the form:
LHobs
t ¼ LHtð1 þ ZtÞ
where ηt is a normally distributed random variable with zero mean and std. deviation equal to
the CV of the assay. Throughout our analysis we have used δt = 1 min, hence, assuming the
system dynamics do not change significantly over shorter times.
Bayesian inference
The hormoneBayes framework uses Bayesian inference to obtain model parameters Θ = (τON,
τOFF, k, d, f) and latent variable (Ht, Bt) dynamics from LH profiling data D. In particular, hor-
moneBayes solves the inference problem by sampling from the target posterior distribution:
ð
P Y; Ht; BtjD
Þ ¼
PðD; Ht; BtjYÞ � PðYÞ
PðDÞ
;
where P(Θ) is the prior parameter distribution; PðD; Ht; BtjYÞ ¼ PðDjY; Ht; BtÞ � PðHt; BtÞ is
the likelihood of the data given the parameters; and PðDÞ ¼
marginal likelihood or model evidence.
PðD; Ht; BtjYÞ � PðYÞ is the
R
Sampling from the full posterior distribution is performed using a Gibbs sampler, which is
an iterative Monte Carlo Markov Chain (MCMC) scheme. The algorithm is initialised with
parameter values drawn from the prior distribution, i.e., Θ0~P(Θ) and each subsequent itera-
tion, i = 1,. . .,M involves two steps: (1) sampling latent variables (Ht, Bt)i given the data, D, and
the current parameter values Θi−1 and (2) sampling new parameter values Θi given D and the
latent variables (Ht, Bt)i. The first step is performed using Sequential Monte Carlo (SMC) with
ancestral sampling [27]. The second step is further broken down into two parts, first parame-
ters (τON, τOFF) are sampled using an adaptive Metropolis-Hastings sampler and then parame-
ters (k, d, f) are sampled using the simplified version of the manifold Metropolis adjusted
Langevin algorithm (sMMALA) presented in [28].
For the analysis of all datasets in this study we considered the following independent prior
distributions: log10tON � Uðlog10ð5Þ; log10ð240ÞÞ and log10tOFF � Uðlog10ð5Þ; log10ð240ÞÞ,
based on the sampling rate (10min) and duration (480min) used in the LH profiling studies;
log10ðkÞ � N ð0; 5Þ, set as a broad uninformative prior; logð2Þ
Þ, based on LH half-
ð
� N 80; 9:3
d
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PLOS COMPUTATIONAL BIOLOGYHormoneBayes: a novel Bayesian framework for the analysis of pulsatile hormone dynamics
life data; and log(f)~U(0,1), to accommodate analysis of different LH profiles with high or low
pulsatility. Evaluation of the algorithm on synthetic dataset can be found in Fig A-C in
S1 Text.
HormoneBayes also allows the user to access the effect of pharmacological interventions on
LH pulsatility, by fitting in tandem two LH profiling datasets: corresponding to periods before
and after the intervention. In this case a composite model is used to allow inference of parame-
ters Θc = (τON, τOFF, k, d, f), corresponding to the baseline period (before the intervention),
and parameters Θp = (τON,i, τOFF,i, ki, fi) corresponding to the period after the intervention.
Here we assume the intervention does not affect the clearance rate d, hence this parameter
does not appear in Θp. In mathematical terms the target posterior is now given by
�
P Yc; YpjDc; Dp
�
¼
PðDc; DpjYc; YpÞ � PðYc; YpÞ
PðDc; DpÞ
;
and sampling from the posterior is performed as described above. An example of this analysis
is Fig E of the S1 Text.
An open access C++ implementation of HormoneBayes accompanied with a graphical
interface implemented in Python and a user manual can be found at https://git.exeter.ac.uk/
mv286/hormonebayes.
Supporting information
S1 Text. Supplementary figures. Fig A: Testing HormoneBayes on synthetic data. Fig B:
Assessing the effect of the prior for the LH clearance rate. Fig C: Tuning HormoneBayes when
pulses are not clear by using a more informative prior on parameter f. Fig D: Pulse identifica-
tion using HormoneBayes. Fig E: Using HormoneBayes to identify the effect of interventions
on LH pulsatility.
(PDF)
Author Contributions
Conceptualization: Margaritis Voliotis.
Data curation: Ali Abbara, Julia K. Prague.
Formal analysis: Margaritis Voliotis.
Investigation: Margaritis Voliotis, Ali Abbara, Julia K. Prague, Waljit S. Dhillo, Krasimira Tsa-
neva-Atanasova.
Methodology: Margaritis Voliotis, Ali Abbara, Krasimira Tsaneva-Atanasova.
Resources: Ali Abbara, Julia K. Prague, Johannes D. Veldhuis, Waljit S. Dhillo.
Software: Margaritis Voliotis.
Writing – original draft: Margaritis Voliotis.
Writing – review & editing: Ali Abbara, Julia K. Prague, Johannes D. Veldhuis, Waljit S.
Dhillo, Krasimira Tsaneva-Atanasova.
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PLOS COMPUTATIONAL BIOLOGY
| null |
10.1080_14756366.2019.1626375.pdf
| null | null |
JOURNAL OF ENZYME INHIBITION AND MEDICINAL CHEMISTRY
2019, VOL. 34, NO. 1, 1164–1171
https://doi.org/10.1080/14756366.2019.1626375
RESEARCH PAPER
Appraisal of anti-protozoan activity of nitroaromatic benzenesulfonamides
inhibiting carbonic anhydrases from Trypanosoma cruzi and Leishmania donovani
Alessio Nocentinia, Sameh M. Osmanb, Igor A. Rodriguesc, Veronica S. Cardosod, Fatmah Ali S. Alasmaryb,
Zeid AlOthmanb, Alane B. Vermelhod
, Paola Gratteria and Claudiu T. Supurana
aDepartment of Neuroscience, Psychology, Drug Research and Child’s Health, Section of Pharmaceutical and Nutraceutical Sciences, University
of Florence, Sesto Fiorentino, Italy; bDepartment of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia; cDepartment of
Natural Products and Food, School of Pharmacy, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil; dBIOINOVAR – Biotechnology
Laboratories: Biocatalysis, Bioproducts and Bioenergy, Institute of Microbiology Paulo de G(cid:1)oes, Federal University of Rio de Janeiro, Rio de
Janeiro, Brazil
ABSTRACT
Chagas disease and leishmaniasis are neglected tropical disorders caused by the protozoans Trypanosoma
cruzi and Leishmania spp. Carbonic anhydrases (CAs, EC 4.2.1.1) from these protozoans (a-TcCA and
b-LdcCA) have been validated as promising targets for chemotherapic interventions. Many anti-protozoan
agents, such as nitroimidazoles, nifurtimox, and benznidazole possess a nitro aromatic group in their struc-
ture which is crucial for their activity. As a continuation of our previous work on N-nitrosulfonamides as
anti-protozoan agents, we investigated benzenesulfonamides bearing a nitro aromatic moiety against
TcCA and LdcCA, observing selective inhibitions over human off-target CAs. Selected derivatives were
assessed in vitro in different developmental stages of T. cruzi and Leishmania spp. A lack of significant
growth inhibition has been found, which has been connected to the low permeability of this class of
derivatives through cell membranes. Further strategies necessarily need to be designed for targeting
Chagas disease and leishmaniasis with nitro-containing CA inhibitors.
ARTICLE HISTORY
Received 28 April 2019
Revised 26 May 2019
Accepted 28 May 2019
KEYWORDS
Carbonic anhydrase; Chagas
disease; Leishmania;
Trypanosoma;
nitroaromatics
1. Introduction
Chagas disease (American trypanosomiasis) and leishmaniasis are
potentially life-threatening illnesses that have been included in
the list of neglected tropical diseases (NTDs) by the World Health
Organization (WHO). These infections belong to the vector-borne
diseases affecting 20 million people and killing more than 50,000
every year and are caused by parasites of the kinetoplastida family
(Trypanosoma cruzi and Leishmania sp.)1. Kissing bugs of
the
Triatoma and Rhodnius genera naturally transmit T. cruzi that is
primarily diffused in Latin America. Chagas disease progresses by
damaging organs in the cardiac, digestive, or nervous systems1.
The bite of infected phlebotomines instead is the main cause of
Leishmania transmission and potentially generates skin or visceral
fatal damages. Leishmaniasis is the first-in-class NTD in terms of
mortality and morbidity1.
To date, a limited arsenal of anti-protozoan agents is available
for the treatment of these NTDs. These drugs are marked by high
toxicity and limited efficacy, and resistance phenomena are con-
stantly increasing worldwide2–4. The poor interest shown by the
pharmaceutical industry in searching new effective drugs for NTDs
treatment is related to high costs and expected low financial
return. On the contrary, it should be considered a priority to find
new approaches in the treatment of these parasitosis2,5. Large-
scale analysis on the completely known genome sequence of
both protozoans have recently provided the identification of new
enzymatic targets6,7.
The enzymes carbonic anhydrases (CAs, EC 4.2.1.1) identified in
these protozoans, TcCA in T. cruzi and LdcCA in L. donovani (a
parasite from the Leishmania complex, causing visceral leishmania-
sis) have recently been recognised as suitable targets to fight
these infections6,8,9. CAs are natural catalysts that speed up the
rate of CO2 conversion to bicarbonate and proton. This reaction
was shown to be basic in the growth and virulence of pathogenic
microorganisms9. TcCA and LdcCA were both cloned and charac-
terised in 201310–12. Many inhibitors of these isoforms have been
identified, which represent potential anti-protozoan agents acting
by a new mechanism of action which is probably devoid of cross-
resistance to the existing drugs.
TcCA is an a-class enzyme that contains the three highly con-
served histidines (His94, His96, and His119) coordinating to zinc
ion in the enzyme active site, and glutamic acid (Glu 106) as the
gate-keeping residue10. Measurement of the catalytic activity of
TcCA in CO2 hydration showed a kcat of 1.21 (cid:1) 106 s–1, Km of
8.1 (cid:1) 10(cid:3)3 M and kcat/Km of 1.49 (cid:1) 108 M–1 s–1 10. TcCA is inhibited
in the nanomolar range by many CA inhibitory chemotypes such
as aromatic/heterocyclic sulfonamides10,13,14, sulfamates10, thiols10,
anions15, dithiocarbamates15, hydroxamates16, benzoxaboroles17,
[email protected]
CONTACT Paola Gratteri
Nutraceutical Sciences, University of Florence, via Ugo Schiff 6, Sesto Fiorentino 50019,
Neurofarba, University of Florence, via Ugo Schiff 6, Sesto Fiorentino 50019, Italy
This article has been republished with minor changes. These changes do not impact the academic content of the article.
(cid:1) 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided the original work is properly cited.
Department of Neuroscience, Psychology, Drug Research and Child's Health, Section of Pharmaceutical and
Department of
Italy; Claudiu T. Supuran
[email protected]
and N-nitrosulfonamides18. Thiols, hydroxamates, and N-nitrosulfo-
namides show in vitro anti-trypanosomal activity, deterring mul-
tiple phases in the life cycle of the pathogen10,16,18.
LdcCA is a b-class CA whose catalytic activity evaluation reported
a kcat of 9.35 (cid:1) 105 s–1, Km of 15.8 (cid:1) 10(cid:3)3 M, and kcat/Km of
5.9 (cid:1) 107 M–1 s–1 12. LdcCA was shown to be efficiently inhibited by
sulfonamides, heterocyclic thiols, and N-nitrosulfonamides with nano-
molar inhibition constants12,18. Some compounds of the two latter
classes showed in vitro anti-leishmanial activity in preliminary assays,
causing the reduction of the parasites growth and their death12,18.
N-Nitrosulfonamides have been designed by us based on the
presence of the nitro group in the structure of many anti-protozoan
agents, such as the nitroimidazoles, this moiety being pivotal for
the drug mechanism of action18,19. For instance, nifurtimox and
benznidazole (Bzn) have been the first effective drugs for treating
acute-phase human Chagas infection, with the first being no longer
available on the market because of undesirable side effects6.
Considering that sulfonamides are the most effective CAIs known
to date20, we first attached the nitro group on the sulfonamide
itself, providing the N-nitro derivatives as a new chemotype exhibit-
ing a selective inhibition of protozoan CAs over human ubiquitous
isoforms18. As second design strategy, we report herein, consists in
the incorporation of the nitro group on the benzene scaffold bear-
ing the sulfonamide, driven by the aromatic character shown by
the nitro moieties present in many anti-protozoan agents, men-
tioned above. A set of 3-nitrobenzenesulfonamide bearing a variety
of substituents on the main scaffold has thus been reported. This
set has been recently evaluated also for the inhibition of the
human tumour-associated CA IX and XII over the ubiquitous CA I
and II and for hypoxia-enhanced anti-proliferative activity on
tumour cell lines21. In fact, nitroaromatic groups are subjected to
bioreduction processes in hypoxic tissues, which can be exploited
to selectively generate cytotoxins against tumour cells21. Here, the
set of nitro-benzenesulfonamides has been screened for the inhib-
ition of TcCA and LdcCA and the most effective derivatives were
studied in vitro against different species of Leishmania and T. cruzi.
2. Materials and methods
2.1. Chemistry
synthesis of
The
reported earlier by our group21.
3-nitro-4-hydroxy-sulfonamides 4–24 was
2.2. Carbonic anhydrase inhibition
An Applied Photophysics stopped-flow instrument has been used
for assaying the CA-catalysed CO2 hydration activity22. Phenol red
(at a concentration of 0.2 mM) has been used as indicator, working
at the absorbance maximum of 557 nm, with 20 mM Hepes (pH
7.5) as buffer, and 20 mM Na2SO4 (for maintaining constant the
ionic strength), following the initial rates of the CA-catalysed CO2
hydration reaction for a period of 10–100 s. The CO2 concentra-
tions ranged from 1.7 to 17 mM for the determination of the kin-
etic parameters and inhibition constants. For each inhibitor, at
least six traces of the initial 5–10% of the reaction have been
used for determining the initial velocity. The uncatalysed rates
were determined in the same manner and subtracted from the
total observed rates. Stock solutions of inhibitor (0.1 mM) were
prepared in distilled-deionised water and dilutions up to 0.01 nM
were done thereafter with the assay buffer. Inhibitor and enzyme
solutions were preincubated together for 15 min at room tempera-
ture prior to assay, in order to allow for the formation of the E–I
JOURNAL OF ENZYME INHIBITION AND MEDICINAL CHEMISTRY
1165
complex. The inhibition constants were obtained by nonlinear
least-squares methods using PRISM 3 and the Cheng–Prusoff
equation, as reported earlier, and represent the mean from at
least three different determinations23–26. All CA isoforms were
recombinant ones obtained in-house as reported earlier27,28.
2.3. Biological assays
obtained
2.3.1. Cell cultures
2.3.1.1. Trypansoma cruzi and Leishmania parasites cultures.
Epimastigote forms of the T. cruzi clone Dm28c29 and T. cruzi Y30
Cellular
from the
strain were
Ultrastructure, FIOCRUZ. L.
infantum MHOM/BR/1974/PP75 and
L. amazonensis IFLA/BR/1967/PH8 were donated by the Leishmania
Type Culture Collection (LTCC) of Oswaldo Cruz Institute/Fiocruz
(Rio de Janeiro, Brazil). The parasites were maintained by weekly
subcultures in PBHIL medium supplemented with 10% foetal
bovine serum (FBS) at 28 (cid:4)C16.
Laboratory
of
and Technology
2.3.1.2. RAW 264.7 macrophage cell line cultures. RAW 264.7 mur-
Institute of
ine macrophages were obtained from the National
(Instituto Nacional de
Metrology, Quality
Metrologia, Qualidade e Tecnologia,
INMETRO, Rio de Janeiro,
Brazil) and maintained in DMEM medium supplemented with 10%
FBS at 37 (cid:4)C in a 5% controlled CO2 atmosphere. Cell maintenance
was performed every 48–72 h, time necessary for cells to achieve
confluent monolayers.
2.3.2. Inhibitory activity on epimastigotes of Trypanosoma cruzi
and promastigotes of Leishmania
The evaluation of anti-parasites activity was performed in 96 well
plates where the synthetic compounds were serially diluted in the
PHBIL medium supplemented with 10% FBS in concentrations rang-
ing from 2 to 400 mM. Then, parasites (1.8 (cid:1) 106) were added to
each well and the plates incubated for 48 h at 28 (cid:4)C. The experi-
ment controls were: negative control
(culture medium without
parasite) and positive culture (culture medium with parasite).
Benznidazole and amphotericin B (Amp) were used as reference
drugs of T. cruzi and Leishmania, respectively. The minimum inhibi-
tory concentration (MIC) for epimastigotes (T. cruzi DM28c and Y)
and promastigotes (L. amazonensis and L. infantum) was performed
using resazurin (125 mM) as an indicator of cellular metabolic func-
tion. MIC was determined as the lowest concentration of the inhibi-
tor capable of
the parasites by
inhibiting in vitro growth of
spectrophotometric analysis at 490 and 59531. The concentration of
drug which reduces parasites number by 50% (IC50) was deter-
mined by regression analysis using Microsoft Excel 2013.
2.3.3. Cytotoxicity assay in macrophages
Cytotoxicity was performed using tetrazolium dye (MTT) colorimet-
ric assay. RAW 264.7 macrophages cells were harvested after con-
fluent monolayer achievement32. The cells were washed twice
with PBS and a cellular suspension of 106 cells/ml was prepared in
fresh DMEM culture medium. Aliquots of 100 ml of the cellular sus-
pension were placed into polystyrene 96-well plates, and then
incubated at 37 (cid:4)C in a 5% CO2 atmosphere for 6 h in order to
allow macrophage adherence. After this period, the adherent cells
were subjected to treatment with several concentrations of the
drugs (2–256 mM), and then incubated for additional 48 h. Finally,
20 ml of MTT solution (5 mg/ml) were added to each well and the
plates incubated for 4 h. Macrophage viability was determined
1166
A. NOCENTINI ET AL.
after formazan crystals solubilisation with DMSO followed by the
absorbance measurement at 570 nm using a SpectraMax M5 spec-
trophotometer (Molecular Devices, Sunnyvale, CA).
2.3.4. Determination of selectivity index
The selectivity index (SI) of tested drugs was calculated as a ratio of
RAW 264.7 macrophages CC50 to parasites IC50. Benznidazole
(Sigma-Aldrich, Milan, Italy) and Amp were used as reference drugs.
3. Results and discussion
3.1. Chemistry
A set of variably substituted 3-nitrobenzenesulfonamides was
prepared starting from 4-hydrobenzenesulfonamide 121. The
to avoid
sulfonamide moiety was protected (compound 2)
decomposition to sulfonic acid that occurs in the nitrating condi-
tions. Both mono- and di-nitro derivatives were obtained in differ-
ent yields and deprotected in acidic media (compounds 4 and 6).
3,5-Dinitro compound 6 was benzoylated to afford 7. A key inter-
mediate, 3-amino-4-hydroxy-5-nitro-benzenesulfonamide 8 was
achieved by reduction of a unique nitro group of 5 with Na2S2O4
and sequential
sulfonamide deprotection in acidic media.
functionalisation reac-
Intermediate 8 was subjected to several
tions. Acylation reactions produced the di-benzoyl compounds 9
and 10. The pyridinium salt 11 was prepared by the reaction of 8
with the proper pyrylium salt. The light-sensitive derivative 12
was obtained by diazonium salt formation and N2 release in aque-
21. A set of ureas (13–24) was prepared by the reaction
ous NaNO2
of 8 with commercially available isocyanates,
in addition to the
freshly prepared one obtained from 1,3,4,6-tetra-O-acetyl-glucosa-
mine21. Compound 24 was de-acetylated with sodium methoxide
to give the glycoside 25 (Schemes 1 and 2).
Scheme 1. Synthetic routes to 3-nitrobenzenesulfonamides 3–2421.
3.2. Carbonic anhydrase inhibition
The TcCA and LdcCA inhibitory profiles of compounds 4–25 were
evaluated by applying a stopped flow carbon dioxide hydrase
assay22 in comparison to AAZ as standard CAI and compared to
those against the human off-target CA I and II. The following SAR
can be built from the inhibition data shown in Table 1.
TcCA was effectively inhibited by most 3-nitrobenzenesulfona-
mides investigated here. Inhibition constants (KIs) span in medium
nanomolar to low micromolar range between 0.08 and 10.7 mM.
The derivative showing the lowest steric hindrance, namely 4, acts
as the most potent TcCA inhibitor with a KI of 80 nM. The incorp-
oration of a nitro, amino or hydroxy moiety in position 5 of com-
pound 4 as in 6, 8, and 11 lowered the inhibition efficacy to 160,
240, and 110 nM, respectively. Lowering of inhibition potency was
observed by benzoylation of 6 as in 7 (KI of 2.5 mM) or dybenzoy-
lation of 8 as in 9 and 10 (KIs of 3.5 and 4.8 mM). Hence, enhance-
ment of steric hindrance at position 4 has a deleterious effect on
the compounds binding to TcCA.
Incorporation of a positively
charged moiety, such as pyridinium at position 5 in compound
11, caused an evident drop of inhibitory efficacy (KI of 10.7 mM).
Within the set of ureas, the unsubstituted phenyl derivative 13
and the pentafluorinated one 17 showed the most effective inhib-
ition, with KIs of 0.32 and 0.28 mM. All other substitutions on the
ureido-aromatic
to
0.35–1.35 mM. Insertion of aliphatic linkers between the urea and
the outer aromatic portion also has a negative outcome on the
KIs of 22 and 23, which increase to 0.74 and 0.4 mM. The glyco-
sidic derivatives 24 and 25 are micromolar TcCA inhibitors with
KIs of 2.47 and 2.14 mM.
negatively
inhibition
affect
ring
the
JOURNAL OF ENZYME INHIBITION AND MEDICINAL CHEMISTRY
1167
LdcCA inhibition profiles show analogies with those against
TcCA. Again, the simplest derivatives 4, 6, 8, and 12 act as the
best LdcCA inhibitors with KIs of 0.21, 0.34, 0.46, and 0.39 mM,
respectively. Benzoylation of the hydroxy moiety at position 4
markedly reduced the LdcCA inhibitory properties of 7, 9, and 10
(KIs of 4.68, 3.87, and 8.49 mM) as well as incorporation of the
charged pyridinium portion as in 11 (KI of 6.57 mM). Ureido deriva-
tives 13–25 inhibited LdcCA in a rather flat range spanning from
0.86 to 3.65 mM.
As a general trend, most compounds were more effective
against TcCA than CA I, with an SI from 2 to >150, with the
exception of 9 and 10 (Table 2). On the other hand, only few
derivatives (8, 12, 13, 18, 19, 22, and 23) inhibited TcCA more
efficiently than CA II, with SI spanning between 2.5 and 6. LdcCA
was found to be better inhibited than hCA I by most compound,
though the SIs were lower than those TcCA/CA I, and spanned in
the range of 2–50. Most compounds inhibited CA II better than
LdcCA. All compounds inhibited the screened isoforms worse than
the standard AAZ, but the latter did not show selectivity for the
target TcCA and LdcCA compared to the ubiquitous hCAs
(Table 2).
3.3. Anti-protozoan activity
3.3.1. Trypanosoma cruzi strain DM28c and Y
Ten selected derivatives bearing different substituents at the 3-
nitrobenzenesulfonamide scaffold (4, 6, 8, 10, 11, 17, 18, 19, 21,
and 24) were screened for their inhibition activity different species
of Leishmania and Trypanosoma cruzi.
Scheme 2. Synthesis of derivative 25.
Table 1.
mide (AAZ) by a stopped flow CO2 hydrase assay.
Inhibition data of TcCA, LdCA and human CA I and II with sulfonamides reported here and the standard inhibitor acetazola-
KI (lM)a
R
TcCA
LdCA
hCA I
C6H5
4-F-C6H4
Compound
4
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
AAZ
aMean from three different assays, by a stopped flow technique (errors were in the range of ±5–10% of the reported values).
C6H5
4-F-C6H4
4-CF3-C6H4
4-F-3-CH3-C6H3
C6F5
3-CH3O-C6H4
3,4-(OCH2O)-C6H3
3,5-CH3-C6H3
3,5-CF3-C6H3
CH2CH2C6H5
CH2-(2-furyl)
2,4,5-triacetoxy-6-acetoxymethyl-tetrahydro-pyran-3-yl
2,4,5-trihydroxy-6-hydroxymethyl-tetrahydro-pyran-3-yl
–
0.08
0.16
2.52
0.24
3.54
4.79
10.7
0.11
0.32
0.46
0.51
0.38
0.28
0.91
1.02
0.69
1.35
0.74
0.41
2.47
2.14
0.06
0.91
4.35
4.79
6.18
1.38
2.92
>50
6.21
>50
5.39
5.20
7.58
0.69
8.21
>50
8.33
5.99
9.29
>50
5.67
4.92
0.25
0.21
0.34
4.68
0.46
3.87
8.49
6.57
0.39
1.06
0.98
2.34
2.96
1.36
0.95
2.03
1.86
3.65
0.86
1.02
3.64
2.97
0.09
hCA II
0.24
0.18
0.84
0.61
0.39
0.46
1.81
0.64
2.78
0.53
0.24
0.21
0.27
5.15
4.33
0.45
1.72
3.08
2.53
1.89
0.86
0.012
1168
A. NOCENTINI ET AL.
The MIC and IC50 values against T. cruzi epimastigote forms of
these compounds are shown in Table 3. The experiments showed
that no compounds significantly affect the growth of the patho-
gen below 256 mM. The reference drug Bzn showed IC50 values
against Dm28c
and Y strains of 16.56 ± 1.51 and
6.54 ± 1.82 mM, respectively. The assessment of the toxicity of the
selected 3-nitrobenzensulfonamides for Raw 267.4 macrophages
cells showed that most derivatives were less toxic than Bzn (CC50
172.65 ± 10.44 mM.
of
CC50
Compounds 4 and 6 showed instead comparable toxicity with Bnz
with CC50 values of 97.65 ± 11.13 and 100.21 ± 17.27 mM.
115.14 ± 9.48 mM) with
above
clone
3.3.2. L. amazonensis and L. infantum
The MIC and IC50 values of the selected compounds against two
Leishmania species are shown in Table 4. The experiments on
infantum strains did not show MIC values
L. amazonensis and L.
below 400 mM. Despite the compounds showed micromolar inhib-
ition of LdcCA, their efficacy turned out to be insignificant when
translated in vitro against the pathogen cell cultures. The refer-
ence drug Amp exhibited IC50 values against the two strains of
1.65 ± 0.28 and 1.77 ± 0.35 mM, respectively. The tested sulfona-
mides showed anyhow remarkably minor toxicity for Raw 267.4
Table 2. Selectivity index (SI) for target protozoan CAs over hCA I and II.
SI (KI1/KI2)
Compound
4
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
CA I/TcCA
11.4
27.2
1.9
25.8
0.4
0.6
>4.7
56.5
>156
11.7
10.2
19.9
2.5
9.0
>49.0
12.1
4.4
12.6
>125
2.3
2.3
CA II/TcCA
3.0
1.1
0.3
2.5
0.1
0.1
0.2
5.8
8.7
1.2
0.4
0.6
1.0
5.7
4.2
0.7
1.3
4.2
6.3
0.8
0.4
CA I/LdcCA
4.3
12.8
1.0
13.4
0.4
0.3
>7.6
15.9
>47.2
5.5
2.2
2.6
0.5
8.6
>24.6
4.5
1.6
10.8
>49.0
1.6
1.7
CA II/LdcCA
1.1
0.5
0.2
1.3
0.1
0.1
0.3
1.6
2.6
0.5
0.1
0.1
0.2
5.4
2.1
0.2
0.5
3.6
2.5
0.5
0.3
macrophages cells compared to Amp, that has a CC50 of 1 mM
against both strains.
Unfortunately, the tested 3-nitrobenzenesulfonamides turned
out
to be ineffective in vitro against strains of T. cruzi and
Leishmania. The lack of activity is not a totally new issue in the
field of sulfonamide CAIs against pathogens. For instance, some
sulfonamide derivatives demonstrated remarkable in vitro efficacy
in inhibiting the b-CA from the yeast Malassezia globosa, arousing
anyhow complications in vivo because of permeability problems
through biological membranes33.
In the context of T. cruzi and Leishmania, some previously
tested sulfonamides showed an absence of anti-protozoan effi-
cacy, which has been related to the lack of permeability through
the biological membranes of the pathogen34–36. Hence, a formula-
tion of such sulfonamides in nano-emulsions (NEs) of clove oil
was attempted to enhance their bioavailability and penetrability
through membranes34,35. The drugs–NEs formulations potently
inhibited the growth of T. cruzi and Leishmania in vitro, with a
huge increase of efficacy over the sulfonamide CAI alone. NEs
turned out as a novel vehicle for the delivery of such hydro-
philic drugs.
Indeed, it should be noted that 3-nitro-4-hydroxybenzenesulfo-
namides reported here are even more hydrophilic, which can
cause difficulties for the compounds to cross the protozoa cell
membrane and inhibit the cytoplasmatic CAs or exert further
actions due to the nitro group. Hence, formulation to enhance the
compounds bioavailability, such as NEs,
is being prepared to
evaluate the real anti-protozoan efficacy of these set of nitroaro-
matic CAIs.
4. Conclusions
We proposed here nitroaromatic sulfonamides for the treatment
of Chagas disease and leishmaniasis based on CA inhibition. As a
continuation of a previous work of us on N-nitrosulfonamides as
anti-protozoan agents, we studied here benzenesulfonamides
(4–24) bearing a nitro moiety on the aromatic scaffold against
TcCA from T. cruzi, responsible of Chagas disease, and LdcCA from
Leishmania spp. The compounds reported valuable micromolar
inhibition of these two enzymes, in some cases even selective for
the target CAs over
the human ubiquitous CA I and II.
Unfortunately, a selected set of such derivatives tested in vitro
against multiple strains of T. cruzi and Leishmania did not produce
growth inhibition of the parasites. The lack of anti-protozoan effi-
cacy of sulfonamide type derivatives had been already reported
Table 3. Minimum inhibitory concentration (MIC), IC50 values derived from growth inhibition assays of T. cruzi Dm 28c and Y, determination of cytotoxicity (CC50),
selectivity index (SI50) of compounds 4, 6, 8, 10, 11, 17, 18, 19, 21, and 24.
T. cruzi Dm 28c
MIC (mM)a
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
32
Compound
4
6
8
10
11
17
18
19
21
24
Bzn
aMinimum inhibitory concentration.
bmM – concentration which reduced the proliferation of epimastigotes by 50%.
cSelectivity index of 50% ¼ CC50/IC50.
dmM – concentration cytotoxic which reduced 50% of RAW 267.4 cells.
IC50 (mM)b
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
16.56 ± 1.51
SIc
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6.54 ± 1.82
MIC (mM)a
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
32
T. cruzi Y
IC50 (mM)b
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
18.45 ± 0.32
SIc
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
6.168 ± 0.81
CC50 (mM)d
97.65 ± 11.13
100.21 ± 17.27
>256
179.93 ± 21.31
>256
>256
>256
172.65 ± 10.44
>256
>256
115.14 ± 9.48
Table 4. Minimum inhibitory concentration (MIC), IC50 values derived from growth inhibition assays of L. amazonensis and L. infantum, determination of cytotoxicity
(CC50) and selectivity index (SI50) of compounds 4, 6, 8, 10, 11, 17, 18, 19, 21, and 24.
L. amazonensis
L. infantum
JOURNAL OF ENZYME INHIBITION AND MEDICINAL CHEMISTRY
1169
MIC (mM)a
>400
>400
>400
>400
>400
>400
>400
>400
>400
>400
8
Compound
4
6
8
10
11
17
18
19
21
24
Amp
aMIC – minimum inhibitory concentration.
bIC50 mM – concentration which reduced the number of promastigotes by 50%.
cSI50–selectivity index of 50% ¼ CC50/IC50.
dCC50 mM – cytotoxic concentration which reduced 50% of RAW 267.4 viability.
IC50 (mM)b
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
1.65 ± 0.28
SIc
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.60
by us and justified by low permeability of this class of derivatives
through the cell membranes. The use of carriers such as nanoe-
mulsions allowed to overcome this issue. The application of this
approach has been being carried out
for 3-nitrosulfonamides
4–24 to elucidate whether the combination of CA inhibition and
further anti-protozoan actions related to the nitro group could be
a winning anti-infective strategy.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was financed in part by a DSFP Project to Profs. Z.
AlOthman and C.T. Supuran from King Saud University, Riyadh,
Saudi Arabia, by the Coordenac¸~ao de Aperfeic¸oamento Pessoal de
N(cid:1)ıvel Superior-Brasil
(CAPES), Grant Code 001, by grants from
Fundac¸~ao de Amparo (cid:3)a Pesquisa do Estado do Rio de Janeiro
(FAPERJ, Ed. 124/2013; Ed. 15/2015), Brazil, and by Conselho
Nacional de Desenvolvimento Cient(cid:1)ıfico e Tecnol(cid:1)ogico (MCTI-
CNPq, produtividade em pesquisa 303265/2015-9), Brazil.
ORCID
Alane B. Vermelho
Claudiu T. Supuran
http://orcid.org/0000-0001-5926-4172
http://orcid.org/0000-0003-4262-0323
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| null |
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All data that supports the findings of this study are included within the article.
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New J. Phys. 25 (2023) 123044
https://doi.org/10.1088/1367-2630/ad121c
PAPER
X-ray absorption near-edge, terahertz and Raman spectroscopies
evidence growth-orientation dependent cation order, phase
transitions and spin–phonon coupling in half-metallic Ca2FeMoO6
thin films
Ekta Yadav1, G L Prajapati3, Parasmani Rajput4,5 and Krushna R Mavani1,2,∗
1 Department of Physics, Indian Institute of Technology (IIT) Indore, Simrol, Khandwa Road, Indore 453 552, M.P., India
2 Centre for Advanced Electronics, Indian Institute of Technology (IIT) Indore, Simrol, Khandwa Road, Indore 453 552, M.P., India
3 Deparment of Physics, Indian Institute of Science Education and Research (IISER) Bhopal, Bhopal, M.P. 462 066, India
4 Beamline Development and Application Section, Bhabha Atomic Research Centre (BARC), Trombay, Mumbai 400 085, India
5 Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India
∗
Author to whom any correspondence should be addressed.
E-mail: [email protected]
Keywords: double perovskites, spin-phonon coupling, anti-site disorder, Raman spectroscopy, terahertz spectroscopy, half-metallic,
thin films
Abstract
The disorder due to anti-site cation distribution is intrinsic to the double perovskites wherein the
crystal orientations of the substrate template are predicted to offer different degrees of cation order
in thin film form. To demonstrate this effect, epitaxial thin films of half-metallic double perovskite
Ca2FeMoO6 (CFMO) were prepared on (100) and (111) oriented LaAlO3 substrates in vacuum
and nitrogen atmospheres. The findings using X-ray absorption near-edge structure, Terahertz
(THz) and Raman spectroscopies, in combination with magnetization show that (111) epitaxial
template effectively restricts the Fe–Mo anti-site cation disorder. A resultantly enhanced cation
order in (111) films induces dramatic transformations in its properties as follows: (i) significantly
enhanced ferromagnetic exchange interactions and saturation magnetization, (ii) a significant
increase in the Curie temperatures, (iii) a metallic behavior down to much lower temperature
(∼75 K) compared to that down to 200 K for (100) film, (iv) an enhanced spin–phonon coupling.
The complex THz optical conductivity spectra evaluated in the framework of Drude and
Drude–Smith phenomenological models and the temperature-dependent Raman data fitted to the
Balkanski model corroborate well to indicate an enhanced cation order in (111) films. While this
study establishes a dominant role of crystallographic orientation in the much-desired control of
cation order in double perovskites, a demonstration of the same in room temperature half-metallic
CFMO system could reinforce its technological utility both as active and passive components in
emergent spintronic functionalities.
1. Introduction
Half-metallic oxides, having an unbalanced spin population at the Fermi level, facilitate the spin-polarized
charge transport, making them promising materials for advanced spintronics. In this regard, half-metallic
A2BB′O6 (A = Ca, Sr, and Ba, B = Fe, B′ = Mo) double perovskites have gained tremendous attention owing
to the relevance of high Curie temperature (TC), 100% spin polarization at room temperature (RT) and the
spin control of transport properties which help in realizing spintronic applications at RT or above [1, 2]. The
impetus in research on these compounds was imparted by the discovery of low-field magnetoresistance at RT
in Sr2FeMoO6 (SFMO) [3]. In A2FeMoO6 (AFMO) compounds, a high Curie temperature of 360 K, 415 K,
and 380 K for A = Ca, Sr, and Ba, respectively, makes them more promising vis-`a-vis mixed-valent
manganites with both lower TC and smaller spin polarization [4, 5]. Despite such promising attributes of all
© 2023 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft
New J. Phys. 25 (2023) 123044
E Yadav et al
the A2FeMoO6 compounds, most recent research has focused on SFMO, while developments in its Ca and
Ba-based analogs are yet to be realized [5–7].
Ca2FeMoO6 (CFMO) crystallizes in monoclinic structure with a P21/n space group [7]. This lower
symmetry of the structure, as compared to that of orthorhombic SFMO and cubic Ba2FeMoO6 (BFMO)
systems, can be understood in terms of higher orthorhombic distortion due to the smaller ionic radius of
Ca2+ ion. In an ideal structure of CFMO, octahedra of FeO6 and MoO6 are arranged alternatively in a
three-dimensional checkerboard arrangement. The magnetic ground state is described by an ordered
arrangement where ferrimagnetic superexchange interactions across Fe–O–Mo happen between Fe2+/3+ and
Mo6+/5+ ions, giving rise to a theoretically calculated saturation magnetization (MS) of 4 µB/f.u [4, 5]. Here,
the contribution of Fe ions to the net magnetization is predominant with a spin-up state. The majority of Fe
ions acquire trivalent state in these double perovskites. If Fe3+ ions have Mo5+ as the next nearest neighbor,
the Mo sublattice in ordered arrangement contributes negatively to the spin-down state [5]. However, the
experimentally observed value of MS is always found to be lower than the theoretically predicted value [8]
due to the presence of anti-site disorder. The term ‘anti-site’ means the exchange of ions between Fe and Mo
sites. The anti-site disorder occurs due to a similarity in ionic radii and ionic valence of B and B′ cations,
which provides an ease to readily exchange their positions and thereby modify the physical properties of the
system [8, 9]. In the AFMO double perovskite system with an anti-site disorder, when the alternate
arrangement is disturbed, the Fe–O–Fe and Mo–O–Mo interactions increase in number, causing
antiferromagnetic and paramagnetic contributions, respectively. These contributions finally reduce the net
magnetization of the overall ferromagnetic system [9, 10]. Thus, the Fe–O–Mo alternate arrangement is
often possible if the anti-site disorder is reduced, contributing to the higher net magnetization. With a high
degree of anti-site disorder, the half-metallicity of these materials is also lost [5]. Therefore, controlling
anti-site disorder in these double perovskites is a much-needed attribute to realize the functionality.
Many efforts have been made, both theoretical and experimental, in the last years to improve the cation
ordering in AFMO double perovskites. Improving cation ordering is relatively easier in thin films due to the
availability of a range of controlling parameters such as substrate-induced strain, oxygen stoichiometry,
substrate temperature, and thickness variation [10–12]. It is seen that cation ordering can be enhanced by
synthesizing thin films using the pulsed laser deposition (PLD) method at high substrate temperature
(>1000 ◦C) or by deposition in a gas atmosphere of Ar and 5% H2, or low oxygen pressure (10−4 mbar)
[10–12]. Till now, no other means have been reported to improve B-site ordering in AFMO. However, such
growth parameters are difficult to achieve in every growth chamber. These parameters are not so suitable for
all the double perovskites including CFMO. In 2017, Kleibeuker et al proposed and explained a new
hypothesis to achieve B-site ordering in La2MnCoO6 thin films [13]. In their model, they suggested that
(100) oriented substrate, providing compressive strain, creates four reduced in-plane B–O bonds (dB–O) and
two elongated out-of-plane B–O bonds. However, all the six B–O bonds elongate themselves to preserve the
unit cell volume in the case of (111) oriented substrate [13]. With octahedral rotations, strain causes a
uniform effect on all the octahedra in the (100) oriented system. However, in the presence of octahedral
rotations, as shown in the schematic (figure 1), one set of BO6 octahedra tilts towards the substrate plane,
and the other set tilts along the out-of-plane direction. This way, an ordered arrangement of two different
B-site cages can be formed in double perovskites using (111) oriented substrate with compressive strain [13].
In order to validate the hypothesis mentioned above, we chose Ca2FeMoO6 as a model system,
considering that the FeO6 and MoO6 octahedra are noticeably tilted due to the relatively smaller size of the
Ca2+ cation. Also, the anti-site disorder can be affected by a variation in the background gas pressure in the
PLD chamber. Considering these points, we prepared two sets of CFMO thin films on LaAlO3 (LAO)
single-crystal substrates with varied anti-site disorder. One set of films was deposited on LAO (100) while the
other was deposited on LAO (111) substrate to verify the effect of substrate orientation on B-site ordering.
Moreover, for each set of two films, one was deposited in vacuum and the other in nitrogen atmosphere. The
orientation-dependent changes in the structural, electrical, magnetic, vibrational, and optical properties of
these thin films are reported here.
2. Experimental
We synthesized two sets of CFMO thin films (∼65 nm) on LaAlO3 (LAO) single crystal substrate with two
different orientations, i.e. (100) and (111), using the PLD technique. For each set of films, one film was
deposited in vacuum and the other in nitrogen atmosphere. For the sake of clarity, the indexing is done as
CFMO-Vac (100), CFMO-N2 (100) for the CFMO films deposited on LAO (100), and CFMO-Vac (111),
CFMO-N2 (111) for the other set deposited on LAO (111). The bulk pellet of CFMO, used as the target
material in the PLD chamber, was synthesized via conventional solid-state reaction method. For that
purpose, high-purity (>99.99%) powders of CaCO3, Fe2O3, and MoO3 (Sigma Aldrich), in proper
2
New J. Phys. 25 (2023) 123044
E Yadav et al
Figure 1. Schematic showing the FeO6 and MoO6 octahedra containing anti-phase rotations in double perovskite CFMO films
for; (a) (100) orientation and (b) (111) orientation. The zig-zag pattern of Fe–O–Mo chains is shown by black dashed line.
stoichiometric amounts, were ground and calcined at 900 ◦C for 12 h. The resulting mixture was reground
and pressed into pellets (15 mm) using a hydraulic press, and then sintering was carried out at 1150 ◦C for
12 h in Ar + 5% H2 atmosphere. This pellet was used for the deposition in the PLD chamber. A KrF excimer
laser (λ = 248 nm, Coherent Compex Pro) with laser energy 350 mJ operating at a frequency of 5 Hz was
used to ablate the target material in the PLD chamber (Excel Instruments). The substrate temperature was
maintained at 770 ◦C, and the target to substrate distance was 4 cm. For one set, vacuum (∼10−4 Pa) was
maintained during deposition, while 0.5 Pa nitrogen partial pressure was maintained for the other set. After
deposition, the samples were cooled to RT in the same atmosphere at 10 ◦C min−1. The dimension of
prepared CFMO films is 10 mm∗6 mm with 65 nm thickness.
The phase-purity and growth orientation of the synthesized thin films have been investigated by X-ray
diffraction (XRD) measurements in Bruker D8 Diffractometer in Bragg–Brantano geometry using Cu kα
radiations. The Fe K-edge x-ray absorption near edge structure (XANES) spectra were recorded at BL-9,
Scanning EXAFS Beamline of Indus-2 at RRCAT, Indore. The measurements were carried out in fluorescence
mode using an energy-dispersive detector. The beamline consists of an Rh/Pt coated meridional cylindrical
mirror for collimation and a Si (111) double crystal monochromator (DCM) to select the excitation energy
of Fe (7112 eV) K-edge. The second crystal of the DCM is a sagittal cylinder that provides a beam focused in
the horizontal direction. The thickness of the films has been decided by x-ray reflectivity (XRR)
measurements. The electrical properties of the thin films have been studied by performing dc electrical
resistivity measurements in a temperature range of 300–10 K using the four-point probe method. For this
purpose, Keithley (2612 A) made source and measurement meters were used. To understand the variation in
charge carrier dynamics with anti-site disorder, Terahertz time-domain spectroscopic (THz-TDS)
measurement has been performed in a temperature range from 300 to 5 K and frequency range from 0.2 to
1.2 THz. For this purpose, an LT-GaAs photoconductive antenna-based THz spectrometer has been used. A
careful substrate signal background subtraction has been employed to accurately determine the optical
constants. Vibrational characteristics of the thin films have been understood by Raman spectroscopy
measurements using a Horiba LabRAM-made HR-Evolution Raman microscope consisting of a charge
coupled device detector. Micro-Raman signals were recorded in back-scattering mode using a HeNe laser
with excitation light 632.8 nm and a laser power of 1 mW. Temperature-dependent Raman measurements
were carried out by placing the sample on a commercial Linkam stage and varying the temperature using
temperature controllers and the LN2 module. The Magnetization measurements for all the samples were
performed using SQUID-VSM (Quantum Design) magnetometer in the temperature range of 1.8–300 K and
in a magnetic field ranging from −5 T to 5 T.
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New J. Phys. 25 (2023) 123044
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Figure 2. (a) XRD patterns of CFMO films deposited on LAO (100) substrates in vacuum and nitrogen atmospheres. The label ‘s’
represents peaks from substrate having reflections other than (100). (b) Magnified view of the (400) Bragg’s reflection of these
films.
3. Results and discussion
3.1. Structural properties
Figures 2(a) and 3(a) show the full-scale XRD patterns of the CFMO thin films grown on LAO (100) and
LAO (111) substrates, respectively, suggesting that the films are grown single-phase, impurity-free, and
highly oriented toward the crystallographic orientation of the substrates. Figures 2(b) and 3(b) show the
magnified view of the second-order Bragg’s reflection of the CFMO films oriented along (100) and (111)
directions, respectively. The c-axis lattice parameter of (100) oriented CFMO films derived from XRD data is
∼3.92 Å, while it is ∼3.81 Å for (111) oriented films. The CFMO films oriented along the (111) axis have
narrower peaks, indicating more crystalline structure as compared to (100) films. LAO (100) [3.79 Å] and
LAO (111) [3.82 Å] impart lattice mismatch of −3.6% and −0.49% to CFMO [3.86 Å] films, respectively.
Further, this lattice mismatch and the FWHM of the XRD peaks increase for nitrogen-grown films. The
thickness of the films has been estimated by the number of laser shots required for the deposition. Earlier,
XRR measurements were carried out for an SFMO film grown in exactly the same deposition conditions and
laser parameters, in the same deposition chamber. The exact growth rate has been obtained from XRR and
hence thickness of CFMO films is estimated to be ∼65 nm.
Depending upon the preparation conditions, the Fe and Mo ions in the A2FeMoO6 family can arrange in
a random or ordered fashion at their respective sites [4–7]. Previous x-ray absorption spectroscopy data of
polycrystalline SFMO at the Fe L-edge conclude that Fe is either in +3 oxidation state or intermediate
valence Fe2+/Fe3+ [14]. Furthermore, Mössbauer data were also interpreted for either Fe3+ or Fe2+/Fe3+
valence states in these compounds [15]. Many efforts are made to resolve the oxidation state of Fe ions in
these compounds, but this is not understood clearly. Therefore, to investigate the valence state and cation
ordering in the present films, we carried out XANES spectroscopy on the set of CFMO films grown in
vacuum. Figure 4(a) shows the Fe K-edge XANES spectra of the CFMO films and standard references of Fe2+
and Fe3+. To quantify the amount of Fe2+ and Fe3+, Fe XANES data were calculated using linear
combination fitting (LCF) with Athena software [16] within an energy range of −20 eV below to +20 eV
above the edge (figure 4). The LCF method is used to quantify the relative percentage of mixed oxidation
state present in a material. LCF of CFMO-Vac (100) (figure 4(b)) and CFMO-Vac (111) (figure 4(c) were
done using a combination of Fe2+ and Fe3+ standard spectra and goodness of fit parameters (reduced χ2)
along with the percent that contributes to each fit. The accuracy of this method depends on how well the
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New J. Phys. 25 (2023) 123044
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Figure 3. (a) XRD patterns of CFMO films deposited on LAO (111) substrates in vacuum and nitrogen atmospheres. The label ‘s’
represents peaks from substrate having reflections other than (111). (b) Magnified view of the (444) Bragg’s reflection of these
films.
Figure 4. (a) X-ray absorption near edge spectra (XANES) of CFMO-Vac (100) and CFMO-Vac (111) films at Fe-K edge. (b), (c)
linear combination fitting (LCF) done on the XANES spectra.
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Table 1. Relative percentage of the mixed oxidation state of Fe ions obtained from the linear combination fitting (LCF) done on XANES
spectra of CFMO films.
Thin films
Fe2+ (%)
Fe3+ (%)
CFMO-Vac (100)
CFMO-Vac (111)
28
36
72
64
Figure 5. Magnetization (ZFC and FC) versus temperature plots for (a) CFMO-Vac (100) and (b) CFMO-Vac (111) films.
spectra of the chosen reference compounds represent the components in the samples [16]. The relative
percentage of Fe2+/Fe3+ extracted from LCF fitting is presented in table 1. The obtained reduced χ2 for
best-fit χ2 = 0.011 for all the samples. It is clear from the table that for both the CFMO films, Fe ions are
present in divalent and trivalent states with the majority being in the trivalent state. However, it should be
noted that the concentration of Fe2+ ions is higher in (111) oriented films than in (100) oriented films.
3.2. Magnetic properties
The effect of substrate orientation and background gas atmosphere on the magnetic properties of CFMO
thin films has been investigated by magnetic field and temperature-dependent magnetization measurements.
The diamagnetic contribution from the LAO substrate has been eliminated from the data to present the
actual sample contribution. The temperature-dependent magnetizations of the vacuum-grown CFMO thin
films of different orientations are shown in figures 5(a) and (b). Both films exhibit qualitatively similar
magnetization curves, and the Curie temperature (TC) is assigned as the onset temperature of the rapid
increase in magnetization. The TC for (100) and (111) oriented CFMO films is found to be 320 K and 340 K,
respectively. The magnetic hystereses have been recorded at 300 K, 150 K, 10 K, and 2 K for all the CFMO
films. For a comparison, figure 6(a) shows the magnetic hysteresis loops for all the films at 150 K. It can be
noted here that CFMO-Vac (111) film possesses the highest saturation magnetization of 3.2 µB/f.u., while the
CFMO-N2 (100) film shows the least saturation magnetization of 2.4 µB/f.u. For the brevity of the
presentation, the hysteresis plots recorded at different constant temperatures only for CFMO-Vac (111) film
are shown in figure 6(b). It can be seen from the figure that the maximum saturation magnetization of
3.5 µB/f.u. is achieved at 2 K, which is quite close to the theoretically predicted value of 4 µB/f.u. for a perfectly
ordered system. As expected, the saturation magnetization decreases with an increase in temperature.
It is imperative from the data that the saturation magnetization of CFMO films is quite lower than the
theoretically predicted value of 4 µB/f.u., which could be attributed to the finite amount of anti-site disorder
in CFMO thin films. As described earlier in a perfectly ordered CFMO system, the alternate arrangement of
Fe and Mo gives rise to ferrimagnetic interactions with a predominant contribution from Fe ions with a
spin-up state. In the present case, a higher degree of anti-site disorder in (100) oriented CFMO films affects
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Figure 6. (a) Magnetization versus magnetic field curves for all CFMO films recorded at 150 K. (b) Magnetization versus
magnetic field curves of CFMO-Vac (111) thin film recorded at 300 K, 150 K, 10 K and 2 K.
the Fe–O–Mo alternate arrangement and causes paramagnetic and antiferromagnetic contributions, which
finally decreases the magnetization and Curie temperature compared to those of (111) oriented films. The
strength of anti-site disorder present in these A2FeMoO6 type samples can be quantified as [17]:
MS = (4 − 8χ ) µB
(1)
where χ is the concentration of anti-site disorder and MS is the saturation magnetization. The anti-site
disorder is found to be 17.2% and 22.8% for the CFMO films grown on LAO (111) and LAO (100)
substrates, respectively, confirming higher cation ordering in CFMO (111) films.
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New J. Phys. 25 (2023) 123044
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Figure 7. Temperature-dependent resistivity plots of (a) CFMO-N2 (111), (b) CFMO-Vac (111), (c) CFMO-N2 (100), and (d)
CFMO-Vac (100) thin films. Red lines show the fitting of data to the parallel spin channel model.
3.3. Electrical properties
The electrical properties of the Fe–Mo based double perovskites, whether in bulk or thin film forms, are
highly dependent on the synthesis conditions [5, 12]. In AFMO films, a variation in deposition temperature
or background gas atmosphere brings only a moderate change in the electronic transport [2, 18]. CFMO may
show metallic, semiconducting, or insulating behavior based on the degree of anti-site disorder [12].
However, the B-site ordered bulk CFMO exhibits metallicity below the Curie temperature of 350 K.
Figure 7 shows the dc resistivity of all the CFMO films. The (100) oriented CFMO films grown in
vacuum and nitrogen atmospheres display semiconducting behavior below 210 K and 225 K, respectively. In
contrast, the CFMO (111) films grown in vacuum and nitrogen exhibit a semiconducting to metallic
transition at 45 K and 65 K, respectively. The appearance of the majority of the metallic state in (111)
oriented CFMO films is attributed to a large enhancement in the B-site cation ordering of the system by
changing the underneath substrate orientation to (111) direction. This proves to be an alternate method to
improve the cation ordering in CFMO films by the PLD method at optimum deposition temperature and
pressure, without the requirement of the growth parameters which are difficult to achieve.
In the present half-metallic CFMO system, two spin channels act parallel to each other [19, 20]. Here, the
spin-up channel with a band-gap at the Fermi level is semiconducting; while the spin-down channel without
any gap has a metallic nature. The half-metallicity is preserved in the ordered CFMO; however, the
introduction of anti-site disorder reduces the half-metallic character and a large amount of anti-site disorder
gives rise to a semiconducting state. The band structure calculations show that the majority spin band mainly
separates the Fe eg states from Mo t2g states finally creating a gap, while the minority band consists of
strongly hybridized Fe t2g and Mo t2g states [3, 21–23], giving rise to metallicity, in an overall half-metallic
system. In general, the Fe3+–O–Mo5+ arrangement is predominantly present in CFMO facilitating the
spin-up charge conduction with a semiconducting nature as well as the spin-down charge conduction with a
metallic nature as schematically presented in figure 8. However, the conduction channel through
Fe2+–O–Mo6+ arrangement can support only spin-down metallic type conduction.
In the metallic state, the resistivity as a function of temperature can be described as [24, 25]:
ρm (T) = ρ0 + ρnT n
8
(2)
New J. Phys. 25 (2023) 123044
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Figure 8. Schematic showing the superexchange interactions in Fe3+–O–Mo5+ (type-I) and Fe2+–O–Mo6+ (type-II)
configurations in CFMO.
where ρ0 is residual resistivity which is a temperature-independent term, existing due to lattice imperfections,
impurities, grain boundary contributions, etc, ρn governs the strength of electron–electron interactions, and
n is an adjustable parameter. According to the classical Fermi liquid model, the value of exponent n remains
2, which explains the quadratic dependence of resistivity over temperature [21]. However, in case of strong
electronic correlation, two other values of n are often reported i.e., 1.6 and 1.3 which define the non-Fermi
liquid (NFL) state. This model describes the electrical conduction mechanism in the spin-down metallic
state only. On the other hand, the resistivity of the spin-up semiconducting band can be described as [19]:
ρSC (T) = ρ0 + ρSC
d e
Eg
KT
(3)
where ρ0 is temperature-independent term, ρSC
d is a constant governing electrical charge density and Eg is the
band-gap of the material in spin-up channel. In this context, the total resistivity for CFMO films, which takes
into account the resistivity from spin-up semiconducting band (ρSC), spin-down metallic band (ρm) and the
temperature-independent term (ρ0), can be described by parallel spin channel given as [19, 26]:
1
ρ
=
1
ρm
+
1
ρSC
.
(4)
For the present data analysis, this parallel spin-channel model is applied to CFMO thin films where the
anti-site disorder, oxygen vacancies and strain play a dominating role in deciding the transport properties
and temperature dependent band-gap. The equation (4) was fitted to the resistivity data of all the CFMO
films as shown in figure 7 in red line. The one-to-one correspondence of the experimental data with the
model fitting shows that the resistivity is absolutely defined by the parallel spin channel model. A small
upturn in the resistivity at low temperatures indicates that the semiconducting channel is rather more
actively contributing for charge transport. The contributions of spin-down and spin-up channels towards the
total resistivity of the samples have been separately estimated from the extracted parameters as presented in
table 2. It is worth mentioning here that the residual resistivity in spin-down channel (ρ0) is quite low for all
the CFMO films as compared to that in spin-up channel (ρSC
d ) due to the ease of conduction in the metallic
channel. The values of residual resistivities in both channels increase with rise in anti-site disorder, showing
the minimum value for CFMO-Vac (111) film and the maximum value for CFMO-N2 (100) film. From
table 2, it can be seen that the spin-down channel is rather more active in (111) oriented films. As shown
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New J. Phys. 25 (2023) 123044
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Table 2. Parameters extracted from the parallel spin model fitting to the temperature-dependent resistivity plots of all the CFMO films.
Thin films
CFMO-Vac (111)
CFMO-N2 (111)
CFMO-Vac (100)
CFMO-N2 (100)
Spin-down channel
residual resistivity
(mΩ·cm)
0.38
0.53
1.7
4.5
n
1.3
1.3
1.6
1.6
Electron–electron scattering
strength (×10−7Ω·cm)
Spin-up channel
residual resistivity
(mΩ·cm)
Band gap (meV)
0.008
0.03
2.3
3.5
7.5
8.9
58
98
0.13
0.19
3.7
4.3
Figure 9. The complex conductivity i.e. (a) real and (b) imaginary THz conductivity spectra of CFMO-Vac (111) film. (c), (d):
Drude model fitted to the complex conductivity data at 300 K, and 10 K.
earlier by XANES results too, a higher concentration of Fe2+–O–Mo6+ channels are available for spin-down
conduction. Hence, the resistivity and XANES data analyses, in a combined way, show that Fe2+–O–Mo6+
channels in (111) oriented films facilitate a dominant metallic conductivity.
The NFL exponent as described in equation (2) is n = 1.3 for (111) oriented films and n = 1.6 for (100)
oriented CFMO films, suggesting a NFL behavior for metallic state in all the films. The parameter governing
electron–electron scattering strength (ρn), increases abruptly with change in the substrate orientation, which
indicates an increase in the defect concentration or the anti-site disorder. The CFMO (111) films have a
smaller band-gap as compared to the CFMO (100) films. In this context, CFMO-Vac (111) film exhibits the
lowest band gap of 0.13 meV and CFMO-N2 (111) film has the highest band gap of 4.3 meV.
3.4. Terahertz spectroscopy
Terahertz (THz) spectroscopy is a potential tool to investigate various intriguing phenomena such as charge
or spin density waves, orbital ordering, topological phases, phase transitions etc [27, 28]. For the present
investigations, the set of CFMO films grown in vacuum show highest cation ordering and hence the effects
have also been studied by temperature-dependent THz spectroscopy. Figures 9 and 10(a), (b) show the real
(σ1) and imaginary (σ2) parts of the complex optical conductivity (σ∗) in the investigated frequency
(0.2–1.2 THz) and temperature (5–300 K) ranges for both the vacuum grown CFMO films. It is worth
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Figure 10. The complex conductivity i.e. (a) real and (b) imaginary conductivity spectra of CFMO-Vac (100) film. (c), (d):
Drude–Smith model fitted to the THz complex conductivity data at 300 K, and 10 K.
noting here that for (111) oriented CFMO film, both (σ1 and σ2) exhibit positive values. Further, σ2
increases with increasing THz frequency, while σ1 decreases. These are characteristic features of Drude type
of optical conductivity which describes the free carrier dynamics of nearly disorder-free systems. According
to the Drude model, the complex conductivity as a function of frequency (ω) can be written as follows [27]:
σ∗ (ω) =
ε0ω2
p
Γ − iω
− iε0ω (ε∞ − 1)
(5)
where ε0 is the permittivity of vacuum, ωp is the plasma frequency, ε∞ is the permittivity of the medium at
higher frequencies, and Γ is the scattering rate of charge carriers. The optical conductivity of CFMO-Vac
(111) film has been fitted to the Drude model (equation (5)). Here, the real and imaginary parts are fitted
simultaneously at all temperatures. The fittings for maximum (300 K) and minimum (10 K) temperatures of
the measured range are presented in figures 9(c) and (d).
The optical conductivity of CFMO-Vac (100) film (figure 10) shows that the values of imaginary
conductivity remain negative in the investigated ranges of frequency and temperature. These features defy
the Drude type of carrier dynamics. Rather, such features are often observed in disordered systems and can
be described well using the Drude–Smith model. This model has been successfully applied to explain
non-Drude like conductivity of many nanostructured metals, semiconductors, oxides, disordered materials
so far [29]. It is an extension of Drude model which takes account of the backscattering of charge carries due
to the disorder present in the system. As discussed in magnetization results, (100) orientated films have larger
anti-site disorder in the system. Therefore, it creates irregularity at B and B′ sites causing a larger scattering of
the charge carriers. In other words, the anti-site disorder disrupts the charge transport network connected
through B and B′ site ions and increases back-scattering of carriers. The Drude–Smith model is expressed as
[29]:
)
(
1 +
cT
Γ − iω
− iε0ω (ε∞ − 1)
σ∗ (ω) =
ε0ω2
p
Γ − iω
11
(6)
New J. Phys. 25 (2023) 123044
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Figure 11. (a), (b) Temperature-dependent optical parameters derived from the Drude–Smith model fitted to THz conductivity
of CFMO-Vac (100) film, (c), (d) temperature-dependent optical parameters derived from the Drude model fitted to THz
conductivity of CFMO-Vac (111) film.
where, c is the disorder parameter accounting charge carrier backscattering. In general, the more negative
value of c suggests larger backscattering of charge carriers which, in turn, highlights more amount of disorder
present in CFMO-Vac (100) film.
As shown in figure 10, the THz conductivity data of (100) oriented CFMO film fits well with the
Drude–Smith model. Again, the data is fitted simultaneously to both σ1 and σ2 of the film. Here, the value of
disorder parameter c varies weakly with temperature, i.e. it varies from −0.99 at RT to −0.95 ± 0.05 at 10 K.
This feature indicates a static disorder in the film. Any static disorder is inherent to the film and depends on
the sample fabrication process. Unlike this, the disorder in RNiO3 systems is dynamic in nature which brings
strong dependency of parameter c on the temperature [28]. The derived values of ωp and Γ for both the
CFMO-Vac films are presented in figure 11. For CFMO-Vac (111) film, the ωp increases with rising
temperature suggesting strong electron–electron correlation in the metallic state. Also, Γ increases with
increasing temperature. Both ωp and Γ show variations presenting metallic conductivity in CFMO-Vac (111)
film throughout the temperature range. These results of CFMO-Vac (111) agree well with the resistivity data.
In contrast to this, for CFMO-Vac (100) film, ωp first decreases from low temperature to TMI and then it
continues increasing up to RT (figure 11(c)). Γ also exhibits similar temperature-dependence as ωp
(figure 11(d)). These features also corroborate well with the dc resistivity data as described earlier.
3.5. Vibrational properties
Earlier, Raman spectroscopy has been used to study the impact of cation ordering and spin–phonon
coupling in La2CoMnO6 thin films [30, 31]. In the present case, Raman spectroscopy measurements have
been performed on the CFMO films to explore the B-site ordering and spin–phonon interactions in the
films. Figure 12 displays the room-temperature Raman spectra of all the CFMO thin films. The Raman
modes are observed at 258, 294, 315, 422 and 450 cm−1. The curves are de-convoluted, and the
corresponding peak position and FWHM of the Raman modes are presented in table 3. The Raman spectra
show that (111) oriented CFMO films show sharp and well-defined Raman modes. These intense and
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New J. Phys. 25 (2023) 123044
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Figure 12. Room-temperature Raman spectra of (a) CFMO-Vac (100), (b) CFMO-Vac (111), (c) CFMO-N2 (100), and (d)
CFMO-N2 (111) thin films.
Table 3. Raman peak position and FWHM extracted from the deconvolution of the room-temperature data of CFMO thin films.
Thin films
Peak position
(cm−1)
FWHM (cm−1)
Peak position
(cm−1)
FWHM (cm−1)
Peak position
(cm−1)
FWHM (cm−1)
CFMO-Vac (100)
CFMO-N2 (100)
CFMO-Vac (111)
CFMO-N2 (111)
259.47
258.11
257.45
255.82
112.21
165.21
57.64
117.81
299.54
298.38
297.24
295.77
24.56
27.84
16.25
19.59
312.20
314.89
319.17
318.77
61.71
114.29
38.81
50.67
well-defined Raman modes indicate B-site ordering due to the Brillouin zone folding [31]. In a disordered
double perovskite, the B and B′ ions are randomly distributed in the lattice. However, the B and B′ are
alternatively arranged in a long-range cation-ordered double perovskite system, which gives rise to doubling
of the pseudocubic unit cell lattice parameter with respect to the primitive cell. It eventually causes
Brillouin-zone folding and changing of symmetry. The present results suggest that CFMO-Vac (111) film has
the highest degree of B-site ordering, while CFMO-N2 (100) film exhibits the lowest B-site ordering. Here,
we emphasize the point that although all the CFMO films are phase-pure, as observed by XRD, a significant
difference in the cation ordering has been probed by Raman spectroscopy.
It is imperative to mention that any thermal perturbation in the lattice, spin or orbital degrees of freedom
of a system can be sensed by temperature-dependent Raman spectroscopy [30–32]. Bulk polycrystalline
CFMO exhibits Curie transition at ∼350 K [12]. However, in thin films, variation in lattice strain and
anti-site order significantly modifies the transition temperature of the system. The thermal evolution of
vibrational properties of the CFMO system has been understood by temperature-dependent Raman
spectroscopy from 90 K to 400 K. Figures 13(a) and (b) shows the temperature-dependent Raman spectra of
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New J. Phys. 25 (2023) 123044
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Figure 13. Raman spectra of (a) CFMO-Vac (111) and (b) CFMO-Vac (100) thin films at different temperatures from 90 K to
400 K.
CFMO-Vac (100) and CFMO-Vac (111) films, respectively. The temperature-induced thermal expansion of
the lattice produces red-shift of the Raman modes as expected. For a magnetic material, the red-shift of
Raman modes can be attributed to various factors such as [24, 32, 33]:
ω (T) = ω0 + ∆ωph−ph + ∆ωsp−ph + ∆ωanharmonic
(7)
where ω0 is the Raman shift corresponding to 0 K, ∆ωph–ph is the cell volume contribution, ∆ωsp–ph is the
spin–phonon coupling contribution and ∆ωanharmonic signifies the contribution of anharmonic terms in the
system. Here, ∆ωsp–ph arises because of modulation of spin exchange integral by a change in the lattice
vibrational frequencies and hence signifies contribution from spin–phonon interactions. In the present case,
the term ∆ωph–ph represents the isotropic variation in volume which is found negligible here. Thus, the
observed red-shifts of the Raman modes are mainly contributed by higher-order anharmonicity and
spin–phonon coupling in the system.
Balkanski model [34] is valid in the absence of any structural phase change and can quantify the
∆ωanharmonic contribution for the phonon behavior with temperature variation. To estimate the contribution
due to anharmonic terms in the Raman shift, the temperature-dependent variations in the Raman shift and
FWHM are fitted by Balkanski model (equations (8) and (9)) as shown in figures 14(a) and (b):
ω (T) = ω0 − A
1 +
∑
Υ (T) = Υ (0) + A
1 +
1
)
(
exp
¯hωi
kβ T
− 1
∑
1
)
(
exp
¯hωi
kβ T
− 1
(8)
(9)
where, ω0 is the Raman shift at 0 K, Υ(0) is the FWHM at 0 K, parameter A is the anharmonic constant
which describes the contribution from higher order terms for three phonon processes, and [e
corresponds to the thermal population factor of Raman modes. In a three-phonon process, three phonons
interact such that both energy and momentum are collectively transferred between lattice vibrations.
The Balkanski model fits to the data in both the temperature regions i.e. above and below Curie
temperature, as shown in figure 14. The solid lines represent the Balkanski fit for the Raman shift plot
(equation (8)), and the dashed lines show the fits to FWHM (equation (9)) plots. As this model purely
defines the contributions from anharmonic phonon vibrations, the deviation from the fits at Curie
2kβ T ]−1
¯hω
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New J. Phys. 25 (2023) 123044
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Figure 14. (a), (b) Raman shift and FWHM plots as a function of temperature for CFMO-Vac (111) and CFMO-Vac (100) thin
films. Red and pink lines show the Balkanski fit to the Raman shift and FWHM plots.
Table 4. Parameters derived from the Balkanski model fitted to temperature-dependent Raman spectra of CFMO thin films.
Thin films
ω before TC (cm−1)
ω after TC (cm−1)
∆ω (cm−1)
CFMO-Vac (100)
CFMO-Vac (111)
282.42
269.59
326.54
348.87
44.12
79.28
temperature strongly suggests the presence of spin–phonon coupling. At the vicinity of magnetic phase
transition, the phonon renormalization takes place which influences the spin-lattice coupling and gives rise
to an anomaly in the plot of anharmonicity.
Here, the difference in anharmonic coefficients (∆ω0) is the difference between ω0 in both the fitted
curves (red and pink fitted lines in figure 14). The value of ∆ω0 is 79 cm−1 and 44 cm−1 for (111) and (100)
oriented CFMO films, respectively (table 4). Literature reports suggest that the higher value of ∆ω0 implies
higher spin–phonon coupling strength in the system [33]. Thus, the temperature-dependent Raman data
suggest that the CFMO (111) films with higher cation ordering exhibit stronger spin–phonon coupling with
higher ∆ω0. Similar studies of enhanced spin–phonon coupling by chemical doping at A-site in ordered
manganese-based double perovskites such as A2BMnO6 (A = La, Pr, Nd, Sm, Gd; B = Co, Ni) has been
reported earlier [35, 36].
The spin–phonon coupling arises from the phonon modulation of the superexchange integral which
depends on the net amplitude of spin-spin correlation <Si.Sj> functions where Si and Sj are the localized
spins at the ith and jth sites, respectively. Under the mean-field approximation, the phonon renormalization
function δω (T) is related to the magnetization as follows [35]:
δω (T) α
M2 (T)
M2
0
where M(T) and M0 are the magnetization of the sample at temperature T and 0 K, respectively. Hence, the
amplitude of the spin–spin correlation function and the strength of the spin–phonon coupling depends on
the level of cation ordering in double perovskites. This discussion clearly indicates that higher cation
ordering in CFMO (111) films gives rise to stronger spin–phonon coupling in the films. Additionally,
enhanced ferrimagnetic interactions and hence higher magnetization in cation-ordered Fe–Mo system give
rise to higher spin–phonon coupling.
Additionally, the deviation from normal anharmonic behavior takes place is exactly at the transition, TC.
The deviation appears at 325 K and 350 K for the CFMO-Vac (100) and CFMO-Vac (111) thin films,
respectively. The estimated value of TC from Raman data matches well with that observed from the
temperature-dependent magnetization measurements shown earlier. For example in [37], a detailed study of
temperature-dependent Raman spectroscopy is reported on SFMO and CFMO bulk samples by other
researchers, where they have discarded the possibility of spin–phonon coupling in their bulk samples. The
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New J. Phys. 25 (2023) 123044
E Yadav et al
absence of spin–phonon coupling in bulk samples suggests that the spin–phonon coupling in the present
case of 65 nm CFMO thin films appears because of the substrate-induced strain. Consecutively, the coupled
spin and lattice degrees of freedom in these thin films manifest in the temperature-dependent Raman results.
4. Conclusions
In summary, a successful effort has been made to achieve a state of significantly improved cation order in
half-metallic Ca2FeMoO6 (CFMO) thin films by precisely choosing the substrate-orientation and deposition
condition. XANES spectroscopy suggests that the CFMO films contain both the divalent and the trivalent Fe
ions with a dominating presence of Fe3+. In spite of overall dominant present of Fe3+ ions in all films,
CFMO (111) films show slightly higher concentration of Fe2+ ions as compared to that in CFMO (100)
films. As anti-site disorder disturbs the alternate Fe–O–Mo arrangement, it results to weakened ferrimagnetic
interactions in (100) films. Thus, a reduced saturation magnetization and lower TC observed for CFMO
(100) films indirectly show a higher anti-site disorder in these films. This indirect observation of higher
anti-site disorder in CFMO(100) films have also been supported by other measurements, namely, (i) a
drastic change from insulating to metallic state takes place by changing the growth orientation from (100) to
(111), respectively; (ii) terahertz spectroscopy suggests that (111) oriented CFMO film follows Drude
conductivity, however, Drude–Smith model is followed by CFMO-Vac (100) film due to higher degree of
cation disorder; (iii) well-defined and intense Raman modes are observed for CFMO films grown on LAO
(111). The spin-up and spin-down channel contributions to the resistivity have been distinguished by data
analysis which is further supported by XANES data too. The Curie temperatures and the parameter
indicating spin–phonon coupling strength have been derived for CFMO films using temperature-dependent
Raman spectroscopy. The TC derived using Raman data agrees very well with the magnetization. The
spin–phonon coupling occurs in CFMO films in spite of its absence in the bulk counterpart. Spin–phonon
coupling is stronger in CFMO (111) films than in CFMO (100) films. As shown in the present study, the
substrate orientation plays a key role in modifying the structural, electronic, magnetic, vibrational, and
optical properties of this half-metallic double perovskite system.
Data availability statement
All data that supports the findings of this study are included within the article.
Acknowledgments
The work is partially supported by SERB, India (Project No: EMR/2017/001821) of KRM. EY acknowledges
CSIR, New Delhi, for providing fellowship through Grant No. 1061751693. The facility of Raman
Spectrometer under DST-FIST Project No. SR/FST/PSI-225/2016 of Discipline of Physics, IIT Indore, is
acknowledged. RRCAT, Indore is acknowledged for extending XANES facility from BL-09.
ORCID iDs
Ekta Yadav https://orcid.org/0009-0004-4102-2318
Krushna R Mavani https://orcid.org/0000-0001-7962-6097
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10.1080/19420889.2020.1729601
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COMMUNICATIVE & INTEGRATIVE BIOLOGY
2020, VOL. 13, NO. 1, 27–38
https://doi.org/10.1080/19420889.2020.1729601
RESEARCH PAPER
Does regeneration recapitulate phylogeny? Planaria as a model of body-axis
specification in ancestral eumetazoa
Chris Fields
a and Michael Levin b
aCaunes Minervois, France; bAllen Discovery Center, Tufts University, Medford, MA, USA
ABSTRACT
Metazoan body plans combine well-defined primary, secondary, and in many bilaterians, tertiary
body axes with structural asymmetries at multiple scales. Despite decades of study, how axis-
defining symmetries and system-defining asymmetries co-emerge during both evolution and
development remain open questions. Regeneration studies in asexual planaria have demonstrated
an array of viable forms with symmetrized and, in some cases, duplicated body axes. We suggest
that such forms may point toward an ancestral eumetazoan form with characteristics of both
cnidarians and placazoa.
ARTICLE HISTORY
Received 19 December 2019
Revised 7 February 2020
Accepted 9 February 2020
KEYWORDS
Bioelectricity; BMP pathway;
eumetazoa; nervous system;
symmetry breaking;
whole-body regeneration;
Wnt pathway
Introduction
What is the connection between spatial symmetry break-
ing and multicellularity? To what extent can an ur-
metazoan ancestor by envisaged as an initially adventi-
tious, spherically symmetric aggregation of ancestral uni-
suggested by the
cells, e.g. of choanoflagellates as
“choanoblastaea” model [1], see also [2,3]? Spatial asym-
metries clearly predate multicellularity: the Bacilli and the
spiral bacteria are classified by their non-spherically sym-
metric shapes. Even E. coli exhibits substrate-dependent
chirality at the colony scale [4]. Unicellular eukaryotes
exhibit a vast array of internal and external spatial asym-
metries. How are such spatial asymmetries translated to
scale of a multicellular organism, particularly
the
a metazoan with well-defined cell
layers and multiple
distinct organ systems arranged in a specific, population-
invariant pattern? The ability to systematically manipulate
body-axis asymmetries during whole-body regeneration
(WBR) may provide a route toward answering these
questions. Organisms capable of WBR are found in all
five primary metazoan clades, including the placozoa [5],
sponges [6], and ctenophores [7] as well as bilaterians
and cnidarians [8]; hence WBR is widely regarded
as an ancestral metazoan trait [8–10]. Here we will
focus on WBR outcomes in asexual freshwater planaria
(Platyhelminthes, Turbellaria, Tricladida), by far the most
extensively manipulated WBR model system [11,12],
from acoel worms
mentioning
(Hydrazoa) where
(Xenacoelomorpha)
available.
and Hydra
supporting
results
Distinct body axes, along which differentiated structures
can be asymmetrically arranged, provide the basis for
Eumetazoan morphologies. With the advent of whole-
genome sequencing and transcriptomics, it has become
evident that the eumetazoan sister clades of cnidarians
and bilaterians employ homologous “developmental toolk-
its” for body-axis specification [13–16]. Considerable
molecular as well as embryological evidence supports
homology between the primary cnidarian aboral – oral
(A-O) and bilaterian anterior – posterior (A-P) axes
[3,17–19]. While a second, dorsal – ventral (D-V) axis
breaking the otherwise cylindrical symmetry around the
A-P is a defining bilaterian trait, both molecular and ana-
tomical evidence support a secondary (“directive”) axis in
at least some cnidarians [20–23]. A third, left – right (L-R)
asymmetry appears in some arthropods (e.g. in lobsters)
and is ubiquitous in vertebrates [24,25]. We focus here on
the early-appearing A-P and D-V axes and their morpho-
logical correlates, particularly the gut and central nervous
system (CNS) axes.
While many treatments are known that specifically
disrupt axis specification in multicellular systems (e.g.
Wnt, BMP, or bioelectrical pathways for the AP, DV,
and LR axes; see below), these processes remain difficult
to manipulate arbitrarily and with full control with mole-
cular or embryological methods in either cnidarians and
bilaterians. It is not, for example, completely clear at the
molecular or cellular level how the morphological asym-
metries of the CNS or the gut, or the behavioral asymme-
try of forward locomotion, are aligned along the A-P axis
CONTACT Chris Fields
© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted
use, distribution, and reproduction in any medium, provided the original work is properly cited.
23 Rue des Lavandières, Caunes Minervois 11160, France
[email protected]
28
C. FIELDS AND M. LEVIN
systems without
in bilaterals. Nor is it known, outside of planaria and
acoels (see below), whether such morphological or beha-
vioral asymmetries can be selectively reversed, e.g. to
produce A-P symmetric nervous systems or guts. Some
putatively basal, acoel bilaterians have rudimentary, net-
like nervous
evident ganglia or
nerve cords, while others have more elaborated structures
[26–28], suggesting that the correlation between CNS axis
and A-P axis is not universal in bilaterians. The vermi-
form myxozoa, e.g. Buddenbrockia [29,30] exhibit for-
ward locomotion driven by coordinated, A-P aligned
muscle groups, but are radially symmetric cnidarians
that altogether lack nervous systems. While such organ-
isms are morphological outliers and may exhibit substan-
tial derived loss of function, their existence renders
reconstruction of ancestral axis-specification mechanisms
and, in particular, the morphology and expected beha-
vioral repertoire of the common eumetazoan ancestor less
than straightforward even given extensive comparative
genomics.
Here we suggest that WBR [8–10] provides a tractable
alternative to embryonic development for asking funda-
mental questions about body-axis specification and deep
ancestral morphology. We use the term “WBR” to indi-
cate regeneration of the whole body from non-germ cells
following either natural or laboratory-induced injuries. In
the asexual freshwater planaria of primary interest here,
specific experimental manipulations of WBR can symme-
trize the A-P axis, including the nervous system and gut
[11], add ectopic A-P axes [31], or remove the A-P axis
altogether to produce outcomes radially symmetric
around the remaining D-V axis [32]; see also [12] for
review and specific details below. These manipulations
support a suggested homology between the ventral nerve
cord (VNC) of bilaterians and the circumoral nerve ring
of cnidarians. We reconstruct a hypothesized ancestral
eumetazoan characterized by a D-V axis, a blind gut,
a nerve ring with a surrounding nerve net, and asexual
reproduction. We suggest that the primary function of the
nervous system is this animal was not locomotion or
feeding but the regulation of body size and morphology.
Planaria exemplify basal bilaterian morphology
and WBR capability
While the early phylogeny of the Metazoa remains con-
troversial, there is broad agreement across models that the
Cnidarians and the Bilaterians are sister clades [15,33].
The early phylogeny of Bilaterians is similarly controver-
sial, with numerous models now recognizing the
Acoelamorpha as basal bilaterians [34–38]; see [39] for
conflicts between molecular and
a discussion of
phylogenetic
developmental-morphological
analyses.
These animals are characterized by an unsegmented
body plan, blind gut, and in some species, an
L-R symmetric, multiple-VNC nervous system [18,40],
although as noted above, nervous-system morphology is
highly variable [26,28]. The development of robust acoel
model systems including Isodiametra pulchra [41,42] and
Hofstenia miamia [43–45] has allowed the biology of
these organisms, including their regenerative capabilities,
to be characterized. Srivastava et al. [45] showed that
Hofstenia miamia is capable of WBR mediated by the
Wnt and BMP pathways as it is in both planaria and
Hydra [see also 43]. Regeneration is enabled by somatic
stem cells (neoblasts) expressing piwi homologs, as it is in
WBR-capable planaria [39]. In contrast to Hofstenia mia-
mia, Isodiametra pulchra is capable of posterior regenera-
tion, but not WBR [42]. Such variability in WBR
capability is also observed in planaria [12].
Despite recent progress with acoels, the asexual planar-
ian model systems Dugesia japonica and Schmidtea med-
iterranea remain the best-characterized and most
extensively manipulated organisms with which to study
WBR. While the rhabditophoran Platyhelminths, which
include the planarians, are no longer regarded as a basal
taxon, they share many of the morphological character-
istics of the acoels, including unsegmented body plan,
blind gut, and L-R symmetric, two-VNC nervous system
[18]. Whether these morphological commonalities are
ancestral or derived in either extant acoels or extant
planaria remains unknown. Both acoels and planaria
exhibit atypical embryonic development [46,47]; whether
these characteristics are ancestral or derived also remains
unknown.
As with morphology, basal bilaterian reproductive
strategy remains controversial [48,49]. While sexual
reproduction far pre-dates multicellularity, obligate sexu-
ality appears to be a multicellular innovation in both
animal and plant lineages, consistent with Red Queen
type arguments [50]. Demosponges and cnidarians such
as Hydra exhibit opportunistic sexuality with budding
and WBR [51,52], suggesting that obligate sexuality is
derived from this more
[9,53].
Characterized acoels include male-female and cross-
fertilizing hermaphroditic species as well as asexuals that
reproduce by budding or fission [40]. Characterized pla-
naria include obligate sexual, opportunistic sexual, or
asexual species, with some species alternating between
sexual reproduction and parthenogenesis or between sex-
ual and vegetative (fission followed by WBR) reproduc-
tion [54]. Asexual planaria can be sexualized by feeding
them closely related sexual planaria, suggesting that inter-
cellular morphogen-based signaling promotes or enforces
strategy
flexible
sexuality [55,56], inducing stem-cell lineages that would
otherwise reproduce to replicate themselves instead to
undergo a stem – germ – stem lineage cycle [53,57].
Manipulating WBR in planaria
Asexual planaria reproduce by fission transverse to the
A-P axis followed by WBR of missing anterior or pos-
terior structures [58]; fission is a size and environmen-
tal conditions dependent biomechanical process [59]
regulated in part by Wnt and BMP pathways [60].
Experimental transverse amputation of both head and
tail produce trunk fragments that regenerate both ante-
rior and posterior structures. While amputation of both
head and tail does not occur during reproductive fis-
sion in the wild, both it and the other manipulations
described below are possible outcomes of predation in
the wild and engage the same molecular and bioelectric
pathways active in reproductive transverse fission.
A large number of molecular, pharmaceutical, and bio-
electric manipulations have been shown to disrupt
trunk, and smaller fragments
WBR in head,
[11,61]. It is now well-established that the Wnt pathway
implements A-P axis specification [62–65], with either
bioelectric asymmetry [66] or morphogen transport to
the wound site [65] as initiating events. Elements of the
Hedgehog (Hh) pathway regulate Wnt pathway activity
in both anterior and posterior compartments [67].
Regeneration of specific anterior structures including
brain and eyes also depends on the ERK and FGF
pathways [65,68,69]. Molecular manipulations impli-
cate the BMP pathway as specifying the D-V axis [11]
as in other bilaterians [14].
tail,
COMMUNICATIVE & INTEGRATIVE BIOLOGY
29
penetrance
dose-dependent
Here we are primarily interested in manipulations
that symmetrize the A-P axis, i.e. replace the asym-
metric A-P axial morphology with a symmetric
A-P-A morphology, or introduce one or more ectopic
A-P axes, with radially symmetric forms in which the
A-P axis appears to have been altogether eliminated as
the limiting case. If a morphologically normal worm is
cut at 60% and 80% of its length to make a “pre-tail”
(PT) fragment and the fragment is allowed to regener-
ate, a morphologically normal worm will result. If,
however, the PT fragment is treated immediately post-
amputation with β-catenin RNAi
[70,71], octonol
(8OH), a gap-junction blocker [31], or a depolarizing
ionophore [66], a two-headed (2H) phenotype results
[complementary
with
manipulations produce two-tailed phenotypes;
see
31,61]. Examination of these 2 H worms reveals that
the pharynx has also been duplicated, and the ventral
cilia are oriented toward the point of duplication, i.e. in
the “posterior” direction from each head [31] as shown
in Figure 1a. The nervous system is also duplicated,
with both copies functional in directing behavior [72].
Crucially, the VNCs are not only duplicated but are
continuous across
the duplication point, yielding
a nervous system with two complete brains connected
by two uninterrupted and apparently fully functional
VNCs [31,65,72]. Hence, not only has a head grown
from the posterior wound, but the entire anatomy
anterior to the anterior-facing wound has been dupli-
cated from the posterior wound. The A-P axis has, in
other words, been symmetrized to an A-P-A axis
around a point at roughly 70% of the worm’s length,
as shown in Figure 1b.
Figure 1. (a) Cutting a PT fragment from a WT worm and treating with 8OH, a depolarizing ionophore, or β-catenin RNAi yields
a dose-dependent 2 H phenotype in which all structures anterior to the anterior-facing wound are duplicated. (b) This transforma-
tion resymmetrizes the A-P axis around a point at roughly 70% of the worm’s length, equivalent to acting with abstract operators
“Copy70(π)((cid:129))” and “Rotate70((cid:129))” in sequence.
30
C. FIELDS AND M. LEVIN
The symmetrization of the A-P axis can be repre-
sented geometrically as an abstract rotation by π
radians (180°) of a copy of the anterior 70% of the
anatomy of the animal (Figure 1b). The axis of rotation
is the preserved D-V axis. This “rotation” of
the
A-P axis is implemented by regenerative growth from
regenerative
the posterior-facing blastema, while
growth from the anterior-facing blastema reproduces
the original A-P axis [31,65,72]. Symmetrization of the
A-P axis does not erase the distinction between anterior
and posterior;
to produce
a bidirectional A-P-A axis with its midpoint at 70% of
the original length. The symmetrized animal has dupli-
cated anterior and no posterior anatomy.
rather duplicates
it
it
Symmetrized 2 H planaria regenerate to produce 2 H
progeny for as many generations as have been
observed, indicating a stable alteration of morphology.
Intriguingly, the morphologically normal outcomes of
8OH treatment under the above conditions are not
wild-type, but are rather “cryptic” worms that continue
to regenerate 2 H progeny, at the same percentage as in
the original experiment, for multiple rounds of regen-
eration in plain water with no further perturbations
[73]. The production of 2 H progeny can be reversed
by ionophore treatment, indicating that the “memory”
for the 2 H morphology is bioelectric.
Prima facie similar axis duplication results have been
obtained in acoels [45,74] and Hydra [75]; however,
the axis-
neither multi-generation inheritance of
rounds of
duplicated phenotype
across multiple
regeneration or any analog of the “cryptic” phenotype
has been demonstrated in these systems.
Experiments in which the two VNCs are nicked
midway through a PT fragment produce symmetric
4 H worms as shown in Figure 2a [31]. Here again,
the entire anterior anatomy is regenerated from the two
side nicks, producing two symmetrized A-P axes at
right angles. As in 2 H animals, the continuity of the
VNCs is preserved, with each of the four brains con-
nected by VNCs to the two neighboring brains [31].
This outcome can be represented geometrically as
a repeated copy-and-rotate operation as shown in
Figure 2b.
The effective duplication of a symmetrized A-P axis
in the cruciform 4 H animals produced by Oviedo et al.
[31] suggests that radially symmetric, hypercephalized
outcomes such as sketched in Figure 3a could be pro-
duced by making multiple “copies” of the A-P axis and
“rotating” them around a central D-V axis. From
a geometric point of view, making a large number of
copies of the anterior morphology and rotating them in
such a way that the heads are evenly spaced is equiva-
lent
to simply deleting the A-P axis to produce
a radially symmetric, completely anterior morphology.
Every radial direction from the central D-V axis is, in
this case, “anterior”; hence, any regenerative mechan-
ism that “anteriorized” the animal in a radially sym-
metric way could be expected to yield this outcome.
Such radially symmetric, hypercephalized outcomes
were observed by Iglesias et al. [32] up to 4 weeks
Figure 2. (a) Nicking the two VNCs produces symmetric outcomes with two A-P axes. (b) The outcome can be represented by
a repeated copy-and-rotate operation.
COMMUNICATIVE & INTEGRATIVE BIOLOGY
31
Figure 3. (a) Radially symmetric, hypercephalized outcome of multiple A-P axis duplication and symmetrization as the number n of
duplicates becomes large. Such outcomes have been observed following β-catenin RNAi [32]. (b, c) Radially symmetric, hyperce-
phalized outcomes, visualized with synapsin staining, obtained by allowing PT fragments from cryptic worms [73] to regenerate in
plain water. d) detail of apparently duplicated circumferential VNC in (c), showing nearly continuous clustering of neurons into
apparently proto-cephalic structures [cf. 32, Figure 3].
following β-catenin RNAi
treatment of amputation
fragments. Consistently with the 2 H and cruciform
4 H regenerates discussed above, these outcomes have
a continuous, circumferential “VNC” nerve cord [32].
As predicted, the D-V axis remains unaffected, indicat-
ing a lack of
significant cross-talk between the
A-P (Wnt) and D-V (BMP) axis specification systems.
Pharyngeal anatomy is, however, disorganized or lost
altogether in these radially symmetric animals, in con-
trast to its preservation and apparent function in 2 H
and 4 H animals, suggesting that radially symmetric
anteriorization disorganizes tissue specification near
the “origin” of the radial axis. Eyes with optic nerves
are present in association with some, but not all, of the
apparently proto-cephalic clusters of neurons distribu-
ted roughly uniformly along the circumferential “VNC”
[32], suggesting some loss of tissue specification distally
along the radial axis.
Radially symmetric, hypercephalized outcomes with
continuous, circumferential nerve cords (Figure 3b–d)
have also been observed following regeneration,
in
plain water without further perturbation, of PT frag-
ments of cryptic worms [73]. The VNCs appear to be
duplicated in some of these regenerates (Figure 3c,d).
Neither pharyngeal structures nor eyes are observed in
these preparations. These observations suggest that the
bioelectric changes that define the cryptic phenotype
can have far-reaching and variable consequences for
regenerative morphology that await further, detailed
investigation.
In summary, the planarian A-P axis appears to be not
just symmetrizable, but highly manipulable, in regenera-
tion-based assays. Both molecular (e.g. β-catenin RNAi)
and bioelectric (8OH,
ionophore) manipulations can
lead to axis symmetrization and duplication. The cryptic
phenotype identified with 8OH treatment is the first
known example of reversible, bioelectric, epigenetic
inheritance [73]; a similar reversible bioelectric manip-
ulation has now also been demonstrated in Hydra [76].
In the absence of rescue manipulations, the altered phe-
notypes are stable across multiple generations in viable
individual planaria, and may be permanent. These
results suggest that while the A-P axis is “primary” in
planaria as in other bilaterians, it is in a highly plastic
state that may reflect loss of evolved constraints on the
standard bilaterian body plan.
The planarian A-P axis as a transitional state
In cnidarians, Wnt pathway components including the
Disheveled (Dsh) receptor and β-catenin effector are
expressed in a decreasing gradient from the Oral to the
Aboral pole [18,19]. Let us call this the “Wnt – anti-
Wnt” axis, where here “anti-Wnt” refers to either
a Wnt inhibitor or opposite-pole determinant. The
Wnt – anti-Wnt axis is aligned with the gut in cnidar-
ians, orthogonal to the circumoral nerve ring, and
aligned with the long axis of the nerve net driving
whole-body contractions and motility [77].
32
C. FIELDS AND M. LEVIN
In bilaterians, Wnt pathway components are
expressed in a decreasing gradient from posterior
to anterior. In bilaterians possessing a through-gut,
the Wnt – anti-Wnt axis is aligned with the gut and
the nerve cord(s) extending from the anterior brain.
The anterior, anti-Wnt direction is the direction of
both motion and the mouth.
The structure of the pharynx and hence the loca-
tion and orientation of the combined mouth/anus is
variable in both acoels and flatworms. In the pla-
naria of interest here, the mouth opens ventrally,
aligned with the D-V axis [78], along which the
pharynx can also be extended as sketched in Figure
4a. The planarian blind gut has the orientation with
respect to the D-V axis that the cnidarian blind gut
has with respect to the O-A axis. Symmetrizing the
A-P axis in planaria duplicates the mouth and phar-
ynx while maintaining their D-V orientation (Figure
4b).
shown in
Figure 3, the D-V axis has become the “primary”
body axis about which anatomical structures are
symmetry in this
the radial
radially symmetric;
case is analogous to the radial symmetry of cnidar-
ians around the O-A axis.
In the radially symmetric forms
From this perspective, the idea of a “primary axis”
appears somewhat ambiguous in planaria. Known manip-
ulations of the D-V axis, moreover, are neither as thor-
ough-going or as extensive as the A-P manipulations
reviewed here [11]; no D-V analogs of the multiple
A-P duplications shown in Figure 2 or 3 are known.
Topologically, the planarian A-P axis is analogous to the
cnidarian directive (secondary) axis; each specifies two
opposing “sides” of a central, invaginated body cavity.
Could the plasticity of the planarian A-P axis reflect an
ancestral state in which this axis was secondary, as the
directive axis is in cnidarians?
Reconstructing an ancestral eumetazoan
Deep metazoan phylogeny remains highly controver-
sial, with active disagreement about whether porifera or
ctenophores are more basal and considerable uncer-
tainty about the placement of placazoa [79–81]. All
empirical phylogeny, however, equally suffers from
the problem that only extant (or well-preserved fossil)
species are accessible for analysis. An empirically
informed theoretical phylogeny may therefore have
value in considering questions of eumetazoan ancestry.
Standard models of the emergence of animal multicel-
lularity are based on the aggregation of closely related cells
[82], typically choanoflagellates [1–3]. We have recently
proposed an alternative, non-aggregative model in which
ancestral, free-living stem cells produce a protective “body”
comprising their own reproductively disabled progeny as
a means of self-defense in a challenging environment [83].
The principal regulator in this scenario is a “do not pro-
liferate” (DNP) signal that the parent stem cell employs to
shut down proliferative capability in its progeny, rendering
them fully “somatic” cells with no independent genetic
interests or fitness. As Wnt-pathway components are
already used for proliferation suppression of prestalk cells
in Dictyostelium [84,85], it is plausible on phylogenetic
grounds that this DNP signal may be a Wnt or Wnt analog.
The DNP signal is assumed to be secreted only by prolif-
erative stem cells and to be short-range; hence its distribu-
tion within a multicellular system will depend on whether
the system’s proliferative cells are dispersed or clustered, as
sketched in Figure 5a,b. A primitive organism comprising
Figure 4. (a) The planarian mouth opening is aligned along the D-V axis, with respect to which the blind gut has the radial
symmetry of the blind gut in cnidarians. (b) Symmetrizing the A-P axis duplicates the mouth-opening axis while preserving its
D-V orientation.
COMMUNICATIVE & INTEGRATIVE BIOLOGY
33
Figure 5. (a) An ancestral proliferative cell produces progeny for protection, employing a short-range “do not proliferate” (DNP)
signal to suppress their proliferation. (b) A somatic cell layer enclosing dispersed proliferate cells has uniform [DNP]; if proliferative
cells cluster, [DNP] is non-uniform. (c) A primitive organism comprising a cell layer enclosing dispersed proliferative cells is stable;
one enclosing clustered proliferative cell has insufficient [DNP] to prevent rogue proliferation at its margins, so is not stable.
a cellular envelope enclosing a uniform distribution of
dispersed proliferative cells may be expected, assuming
cell-cycle synchronization or some other mechanism to
coordinate stem-cell proliferation, to have relatively uni-
form internal [DNP] and to be stable. However, such an
organism with clustered proliferative cells is expected to
have non-uniform internal [DNP] and to be unstable due
to uncontrolled reproduction by “somatic” cells in which
independent proliferation has not been fully suppressed
(Figure 5c).
Reproductive stability is possible with clustered prolif-
erative cells if longer-range communication of a DNP-like
signal that suppresses rogue cell division by somatic cells
is possible. Neurons provide an ideal solution to this
problem, as they allow long-range, error-correcting com-
munication between source cells and specific target cells
[86]. Neurons likewise provide a means of coordinating
the proliferation of dispersed populations of stem cells
that are too far apart or too distant within a cell lineage to
be reproductively coordinated by other mechanisms. An
34
C. FIELDS AND M. LEVIN
Figure 6. (a, b) A reproductively unstable system can achieve stability by employing neurons to transmit a DNP-like signal (green
curves) to distant somatic cells in order to suppress rogue cell division. (c) Neurons enable the development of complex anatomies,
e.g. invaginated body cavities.
organism with neurons can adopt a more complex body
plan, e.g. by elongating its periphery into an invagination
as sketched in Figure 6. The three extant animal lineages
with complex body plans – the ctenophores, cnidarians,
and bilaterians – all have neurons. Structural, biochem-
ical, and molecular differences between ctenophore neu-
rons and those of cnidarians and bilaterians suggest
convergent evolution to a common function [87]. We
have suggested that the primary ancestral function of
neurons in all three lineages is the long-distance control
of cell proliferation that enables a stable multicellular
morphology even with clustered stem cells [86]. While
this hypothesis remains to be tested, manipulations in
Xenopus embryos provide initial evidence for CNS regu-
lation of distal morphogenesis [88,89].
These theoretical considerations suggest an interpreta-
tion of the radially symmetric, hypercephalized regenera-
tion outcomes shown in Figure 3 as regressions toward an
ancestral state, one that may pre-date not only planaria
but eumetazoa in general. Only one extant animal lineage
comprises flat, approximately radially symmetric organ-
isms: the placozoa [5,90]. Placozoa have no differentiated
neurons but have neurotransmitters and behavior. The
dispersed, interior “fiber” cells of placozoa are extended
and may serve a communicative function, e.g. by para-
crine signaling as in sponges [91]. Characterized placozoa
do not have differentiated mouths or guts, but appear to
digest food externally and absorb nutrients through dis-
tributed ventral-surface cells [5]. Placozoa are predomi-
nately asexual, reproducing by budding or fission with
WBR, but also exhibit opportunistic sexuality. They are
often regarded as amorphous but have a primary
D-V axis, the axis normal to the substrate, around
which they are approximately radially symmetric. Their
anatomy resembles the left side of Figure 5c.
Do the radially symmetric, gutless, hypercephalized
regeneration outcomes in Figure 3b, c resemble placozoa
with neurons added? While gross morphology suggests
that placozoa may be their closest affinity, further investi-
gation of both parties is clearly required to answer this
question. If these planarian regenerates indeed resemble
placozoa at more than a superficial level, they may be
pointing toward an ancestral eumetazoan with radial sym-
metry around a primary D-V axis, a blind or undifferen-
tiated gut, a rudimentary circumferential nerve cord with
radial branching, and asexual reproduction with WBR.
Conclusion
We have suggested here that WBR provides an alternative
to embryology for studying the mechanisms of body-axis
specification and their contributions to the evolution of
complex morphologies. Asexual planaria appear to be
COMMUNICATIVE & INTEGRATIVE BIOLOGY
35
particularly attractive model systems in this regard. The
A-P axis of planaria, in particular, is highly malleable using
molecular, pharmacological, and bioelectric manipulations.
This axis can not only be symmetrized but also duplicated
to such an extent that it effectively disappears, leaving
a radially symmetric, fully anteriorized form. Whether the
A-P axis of acoels, or of other bilaterians, is similarly
manipulable remains to be seen; commonalities in axis-
specification mechanisms across the bilaterians as well as
specific results in acoels [45,74] suggest that they may be.
The evolutionary emergence of complex body plans
appears intimately connected to the emergence of neu-
rons as specialized long-range signaling systems [86]. We
suggest that the emergence of neurons in a radially sym-
metric, placozoan-like animal may have set the stage for
the differentiation of the eumetazoan lineages.
Further work is clearly required to elaborate and test
these hypotheses. Life-history studies of the symmetrized
forms shown in Figure 3b, c have already been initiated; if
such forms can be reliably produced and maintained,
functional investigation of their nervous and digestive
systems will be possible. The results of Müller [75] and
Braun and Ori [76] suggest that Hydra may be an attrac-
tive Cnidarian model system with which to pursue similar
axis-symmetrization studies. Thorough investigation of
cell-cell signaling mechanisms in Placozoa would comple-
ment these investigations. More broadly, the study of the
regulation of morphogenesis outside of the nervous sys-
tem per se by neural activity remains in its infancy. Recent
as well as classical evidence of regulation of regeneration
[92] and of transformation and tumor growth by the
nervous system [93–95] suggests that such studies may
also have significant clinical relevance.
Author Contributions
CF and Ml jointly conceived of the ideas and wrote the paper.
Acknowledgments
We gratefully acknowledge support by an Allen Discovery
Center award from the Paul G. Allen Frontiers Group (No.
12171), and the Templeton World Charity Foundation (No.
TWCF0089/AB55).
Disclosure statement
No potential conflicts of interest were disclosed.
Funding
This work was supported by the Paul G. Allen Frontiers
Group [12171];Templeton World Charity Foundation
[TWCF0089/AB55].
36
C. FIELDS AND M. LEVIN
ORCID
Chris Fields
Michael Levin
http://orcid.org/0000-0002-4812-0744
http://orcid.org/0000-0001-7292-8084
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10.1088_1361-665x_acf970.pdf
|
Data availability statement
The data that support the findings of this study are available
upon reasonable request from the authors.
|
Data availability statement The data that support the findings of this study are available upon reasonable request from the authors.
|
Smart Mater. Struct. 32 (2023) 115009 (16pp)
Smart Materials and Structures
https://doi.org/10.1088/1361-665X/acf970
Identification and reconstruction of
anomalous data in dam monitoring
considering temporal correlation
Yongjiang Chen1, Kui Wang1,∗
and JianFeng Liu1
, Mingjie Zhao1,2, Yong Xiong1, Chuanzhou Li1,3
1 Engineering Research Centre of Diagnosis Technology of Hydro-Construction, Chongqing Jiaotong
University, Chongqing, People’s Republic of China
2 Chongqing University of Science and Technology, Chongqing, People’s Republic of China
3 State Grid XINYUAN Company LTD., Chongqing PANLONG Pumped Storage Power CO., LTD,
Chongqing, People’s Republic of China
E-mail: [email protected]
Received 19 May 2023, revised 29 August 2023
Accepted for publication 13 September 2023
Published 3 October 2023
Abstract
In dam monitoring, anomalous data is often removed directly by researchers. However, some
anomalous data may be due to sudden changes in the state of the dam itself and should not be
removed. In this study, anomalous data in dam monitoring is divided into two categories:
anomalous error data caused by anomalies in the monitoring equipment, and anomalous
warning data caused by sudden changes in the state of the dam itself. Then we propose a method
for identifying and reconstructing anomalous data in dam monitoring that takes into account
temporal correlation. This method is able to identify and retain anomalous warning data, while
removing and reconstructing anomalous error data. To determine the temporal correlation
between dam monitoring parameters (e.g. water level, horizontal displacement, etc), we use
association rules, and to reconstruct the removed dam monitoring data in the case of an
incomplete dataset, we propose a dam monitoring data reconstruction network (DMDRN) based
on generative adversarial network. On this basis and in combination with the density-based
spatial clustering of applications with noise algorithm, the types of anomalous data in dam
monitoring are identified, and the anomalous error data is reconstructed based on DMDRN. Our
approach has been successfully validated in two experiments to identify and reconstruct
anomalous data at a particular dam in China.
Supplementary material for this article is available online
Keywords: dam safety monitoring, association rules, anomalous data, deep learning
(Some figures may appear in colour only in the online journal)
1. Introduction
The identification and reconstruction of anomalous data in
dam monitoring is an important aspect of analyzing inform-
ation for dam safety monitoring and forms the basis of
dam safety evaluation. Traditional methods for processing
∗
Author to whom any correspondence should be addressed.
anomalous data in dam monitoring include hydrography [1],
space-time discriminant [2–4], mathematical statistics [5] and
mathematical modeling [6, 7]. As the field of artificial intel-
ligence grows, methods such as machine learning and deep
learning are increasingly used in structural health monitor-
ing (SHM). SHM for lightweight complex composite struc-
tures was being investigated in the literature [8] with a data-
driven deep learning approach to facilitate automated learning
1361-665X/23/115009+16$33.00 Printed in the UK
1
© 2023 IOP Publishing Ltd
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
of the map of transformed signal features to damage classes.
In the literature [9], a machine learning framework was
presented using bag of visual words for SHM of a composite
sandwich structure using ultrasonic wave signals. In literature
[10], a vibration data-based machine learning architecture was
designed for SHM of a steel plane frame structure. The intro-
duction of such methods also makes dam monitoring smarter
and more accurate in terms of processing anomalous data in
dam monitoring. The literature [11] had developed a novel
data-driven workflow specific to the water and hydropower
sector, which successfully separated normal from anomal-
ous data observations. The literature [12] described the use
of machine learning techniques to detect anomalies in dam
inclinometers, simplifying the process of identifying areas
requiring attention. The literature [13] proposed a data-driven
approach for detecting anomalous conditions in dams based
on simultaneous processing.
Currently, however, most researchers only remove and
reconstruct anomalous data from a specific single monitor-
ing parameter (e.g. water level, horizontal displacement, etc)
without considering the relationship between different monit-
oring parameters. Additionally, anomalous data in dam mon-
itoring is directly removed by most researchers, but some of it
is caused by sudden changes in the condition of the dam itself.
Such anomalous data is an important indicator of feedback
with the condition of the dam itself and should not be removed.
There is some correlation between the different monitoring
parameters of the dam [14]. If two monitoring parameters that
correlate with each other both show anomalies, this type of
error is not due to human measurement error or inaccurate
monitoring instruments [15], but to anomalous data that actu-
ally reflects the sudden change in system condition. In this
case, the data should be retained.
In this study, anomalous data in dam monitoring is cat-
egorized into anomalous error data and anomalous warning
data. We present a method for identifying and reconstruct-
ing anomalous data in dam monitoring that considers tem-
poral correlation. Our approach utilizes association rule min-
ing to investigate the temporal correlation of dam monitoring
parameters [16] and combines it with the density-based spa-
tial clustering of applications with noise (DBSCAN) algorithm
to detect anomalous data. If two dam monitoring parameters
with strong temporal correlation both exhibit abnormal data,
then the anomalous data is classified as anomalous warning
data. On the other hand, if only one dam monitoring para-
meter displays abnormal data, then the anomalous data is clas-
sified as anomalous error data. Additionally, we draw on the
enhanced concept of generative adversarial network (GAIN)
[17] and propose a dam monitoring data reconstruction net-
work (DMDRN) base on GAN [18, 19]. DMDRN consists of
a dam data generator (DG), a dam-data discriminator (DD), a
dam-data random matrix, and a dam-data mask matrix. The
DG has been enhanced with the recurrent neural network
(RNN) capable of capturing temporal features. It uses condi-
tional probability to generate reconstructed data and trains the
DD to differentiate between the reconstructed values gener-
ated by DG and the original values. Through iterative training,
the reconstruction capability of the DG and the recognition
capability of the DD reach the optimal Nash equilibrium state
[18, 19]. Finally, the DG in DMDRN can learn the distribu-
tion model in the dam monitoring data, capture the inherent
uncertainty of the dam monitoring data, and achieve the recon-
struction of the dam monitoring data even when the database
is incomplete.
2. Methodology
2.1. Analysis of temporal correlation between dam monitoring
parameters
2.1.1. Association rules. Association rules refer to a data
mining technique that identifies relationships between vari-
ables in a data set. It is used to discover interesting patterns
or associations between different items or variables in a data
set. The main concepts of association rules [20–23] are as
follows:
‹ Transaction database—The transaction database D is the
database consisting of all subset transactions, |D| is the total
number of subset transactions T. f(α) denotes the frequency
of occurrence of an itemset α.
› Association rules—If the itemset α ⊂ D, an itemset β ⊂ D
and α ∩ β = ϕ , then α → β is said to be an association rule,
α and β are called the antecedent and consequent parts of
this association rule, respectively.
fi Support—The Support PS (α → β) is the probability of
α ∪ β appearing in transaction database D. The closer
PS (α → β) is to 1, the higher association between the ante-
cedent α and the consequent β [20],
Ps (α → β) = P (α ∪ β) =
n (α ∪ β)
|D|
(1)
where: n (α ∪ β) is the number of occurrences of α ∪ β in
D
fl Frequent itemset—When the Support of an itemset is
greater than the set minimum Support, it is said to be a fre-
quent itemset.
(cid:176) Confidence—The probability that the D contains both the
α and β is called the Confidence. Confidence is a measure
of the trustworthiness of the association rules for each set
[21],
Pc (α → β) = P (α|β) =
P (α ∪ β)
Pα
.
(2)
2.1.2. Apriori-based temporal correlation analysis between
Based on the original asso-
dam monitoring parameters.
ciation rules, formulas for temporal correlation have been
defined between dam monitoring parameters (e.g. water level,
horizontal displacement, etc). If there are n association rules
→ βi satisfying the minimum Confidence in the monit-
αi
oring data of dam monitoring parameter A and monitor-
ing parameter B, then the formulas of temporal correlation’s
Support Pts and temporal correlation’s Confidence Ptc for dam
monitoring parameters A and B are as follows [20, 21, 23, 24]:
2
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Table 1. Symbolic representation of dam monitoring data.
Interval
[−1.0, −0.6]
[−0.6, −0.2]
[−0.2,0.2]
[0.2,0.6]
[0.6,1.0]
αi
a1
a2
a3
a4
a5
βi
b1
b2
b3
b4
b5
Pts (A → B) =
nX
Ps (αi
→ βi)
Ptc (A → B) =
i
P
n
i Pc (αi
n
→ βi)
.
(3)
(4)
Pts (A → B) reflects the level of temporal correlation
between dam monitoring parameters A and B. The greater
the value of Pts (A → B), the higher the degree of temporal
correlation between A and B. Ptc (A → B) reflects the level
of Confidence in the temporal correlation between A and B.
The higher Ptc (A → B) is, the more credible the degree of
temporal correlation between monitoring quantities A and B
is. Following the guidelines for determining the minimum
Support and Confidence in general association rules, the min-
imum threshold for Support of temporal correlation between
dam monitoring parameters is set at 0.5 and the minimum
threshold for Confidence is set at 0.5. If both the Support and
Confidence values of the two dam monitoring parameters meet
the minimum threshold, they are considered to have a strong
temporal correlation; otherwise, they are considered to have a
weak temporal correlation.
The procedure for analyzing the temporal correlation
between dam monitoring parameters is as follows:
(1) The dam monitoring parameter data are symbolized and
divided into subseries. As the data is all numerical, it
needs to be converted to the Boolean type supported by
the Apriori algorithm [25]. The input length of the original
data is truncated into a number of subseries using a sliding
window. The subseries are linearly fitted and normalized
so that the final slope values all fall between the interval
[1, −1]. The interval is equally spaced into five segments,
as shown in table 1.
(2) The Apriori algorithm is used to find out the frequent item-
set with Confidence greater than the minimum threshold
value in the itemset. Then, equations (3) and (4) are used
to calculate the Support and Confidence of temporal cor-
relation between the dam monitoring parameters. Finally,
it is determined whether there is a strong temporal correl-
ation between the dam monitoring parameters.
2.1.3. Example of analysis of temporal correlation between
dam monitoring parameters. The experimental data utilized
in this paper has been obtained from a specific dam located in
Chongqing, China. The schematic diagram of the dam monit-
oring system can be observed in figure 1. The primary monitor-
ing devices are enlisted in table 2. The water level of the dam is
assessed through a float-type water level gauge, the horizontal
displacement of the dam surface is evaluated with the help of
an SWT-50 telemetry tension wire transducer, and the seepage
pressure of the dam is measured with a vibrating wire piezo-
meter. The specific details regarding these devices are depicted
in table 3. The amount of dam monitoring data selected in this
paper is 1648, with a total of four groups of data: water level,
EX4 (horizontal displacement monitoring point), P2 (seepage
monitoring point) and P3 (seepage monitoring point), and the
amount of data in each group is 412. The sampling frequency
is basically once a day, and if the data fluctuates a lot, it is
sampled and recorded several times on the same day.
Water level monitoring data and EX4 (horizontal displace-
ment monitoring point) monitoring data from 1 January 2020
to 25 January 2021 were selected for temporal correlation ana-
lysis. A sliding window was set to L = 10, the corresponding
subseries were extracted, and the results of linear fit of water
level and EX4 were obtained as shown in figure 2.
According to the Apriori algorithm, the frequent itemset in
database with confidence greater than the minimum threshold
was found, and equations (3) and (4) were used to calculate the
Support and Confidence of the temporal correlation between
water level and EX4, and the results were shown in table 4.
From table 4, it can be inferred that the temporal correla-
tion’s Support is 0.8293, with the Confidence of 0.8846, which
satisfies the minimum threshold requirement. Therefore, it
can be concluded that there is a strong temporal correlation
between water level and EX4.
2.2. DBSCAN algorithm
The DBSCAN algorithm [26–28] is used to detect anomalous
data points and provide a reference for subsequent classifica-
tion of the anomalous data. The specific process is as follows:
(1) Select an unprocessed data point from the dam monitoring
data.
(2) If the point is a core point, the objects that are accessible
in density from that point are detected and clustered.
(3) Otherwise, the point is considered an edge point, i.e. a
non-core point, the next point is searched, and step (1) is
repeated.
To accurately detect anomalous points in dam monitoring
data, the appropriate parameters must be determined through
several trials [29]. After several trials, the relevant parameters
for the monitoring data of the certain dam in Chongqing were
determined as shown in table 5.
2.3. DMDRN—dam monitoring data reconstruction network
The GAN [18, 19] in deep learning enables data reconstruc-
tion and consists of two models: a generative model (G) and a
discriminative model (D). The objective function of the GAN
model [18] is as follows:
min
G
max
D
V (D, G) = Ex∼pdate(x) [log D (x)]
+ Ez∼pz(z) [log (1 − D (G (z)))] . (5)
3
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 1. Schematic diagram of a dam monitoring system in China.
Table 2. Primary monitoring devices of a dam in China.
NO.
item
code name
quantity
type
1
2
3
4
5
6
7
8
9
10
11
12
13
Wire transducer
Pendulum telecoordinometer
Piezometer
Measurement and control device
Sewing gauge
Interferometric synthetic aperture radar
Weir
Water level gauge
Sinking punctuation
Baseline
Water level gauge
Rain gauge
Temperature monitoring device
EX
IP
UP/P
MCU
CF
M
WE
LD
LS
SW
YL
QW
8
2
13
5
3
2
2
2
10
2
1
1
1
SWT
STC
Vibrating wire piezometer
Vibrating wire piezometer
SFL-300
Stainless steel
Stainless steel
Float-type
Tipping bucket
Resistance-type
Table 3. Detailed list of some of the monitoring devices at a dam in China.
Seepage pressure
Dam monitoring parameter
Water level
Horizontal displacement (EX4)
P2
P3
Monitoring instrument
Instrument type
Float-type water level gauge
SFL-300
Telemetry tension wire transducer
STC-5
Factory No.
Range
Accuracy
Fundamental frequency
Date of burial
3013002
0–300 mm
0.01 mm
Fo = 1.006
2014/4/13
13050931
0–50 mm
0.01 mm
Fo = 30.3
2014/4/10
Piezometer
Vibrating wire
piezometer
P00259
0–350 KPa
0.5%FS
Fo = 5254
2012/9/6
P00590
Fo = 5168
The D distinguishes the labels of the training samples
with maximum probability by maximizing log D(x) and
log(1 − D(G(z)), while the G minimizes log(1 − D(G(z))),
i.e. maximizes the loss of D. G and D thus form a dynamic
‘adversarial training’. The training process fixes one side and
updates the parameters of the other network, alternating iter-
ations, so that G can eventually estimate the distribution of
the sample data and generate the data that the discriminative
model D considers to be true.
After removing anomalous data from dam monitoring data-
set, data reconstruction is required. However, the resulting
dataset is incomplete and cannot be reconstructed using GAN.
Therefore, we draw on the enhanced concept of generative
adversarial network (GAIN) [17] and propose a GAN-based
DMDRN.
2.3.1. DMDRN struction. A network structure of DMDRN
is shown in figure 3. The dam-data generator, DG, gen-
erates reconstructed values based on dam-data conditional
probability DP = (DX| f
DX) through an incomplete dam-
data matrix, a dam-data random matrix, a dam-data mask
matrix, and the RNN, and trains DMDRN’s DD to dis-
tinguish the reconstructed values from the true values in
the dam-data imputed matrix, and through iterative train-
ing of adversarial reconstruction, the reconstruction ability
of DG and the recognition ability of DD reach the optimal
state of Nash equilibrium. Ultimately, the DG in DMDRN
can learn the distribution model
in the dam monitoring
data, grasp the uncertainty inherent in the dam monitoring
data, and achieve the reconstruction of the dam monitoring
data.
4
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 2. The results of linear fit of water level and EX4.
Table 4. Support and confidence calculation results for water level
and EX4.
Frequent itemset
Support
Confidence
a1→b5
a2→b3
a2→b4
a3→b3
a4→b3
Temporal correlation
0.0244
0.0244
0.0244
0.7317
0.0244
0.8293
1
0.5
0.5
0.8108
1
0.8846
Table 5. Parameters setting of the certain dam in DBCSAN
algorithm.
Parameters
Water level
data
Horizontal
displacement data
Piezometer data
Eps
MinPts
1.005
5
1.050
25
1.005
5
The components of the DMDRN are described as follows:
×
Consider a d-dimensional dam-data space Dχ = Dχ 1
× . . . × Dχ d. Suppose that DX = (DX1, . . . , DXd) is a
Dχ 2
dam-data random variable (either continuous or binary) taking
values in Dχ , whose distribution we will denote DP(DX).
Suppose that DM = (DM1, DM2, . . . , DMd) is a dam-data ran-
dom variable taking values in {0, 1}d. We will call DX the dam-
data vector, and DM the dam-data mask vector.
For each i ∈ {1, . . . , d}, we define a new dam-data space
∪ {#} where # is simply a point not in any
×
Dχ d. We define a new dam-data ran-
Dχ in the following
g
Dχ i = Dχ i
Dχ i, representing an unobserved value. Let
× g
g
Dχ 2
Dχ 3
dom variable
way,
× . . . × g
DX = ](DX1, . . . , g
f
Dχ = g
g
Dχ 1
DXd) ∈ g
(cid:26)
g
DXi =
DX1
# if DM1
if DM1 = 1
̸= 1
.
(6)
5
(7)
(8)
‹ The dam-data generator, DG, takes (realizations of)
f
DX,
DM and a dam-data noise variable, DZ, as input and
outputs DX, a dam-data vector of imputations. Let DG,
g
Dχ = {0, 1}d × [0, 1]d → Dχ be a function, and DZ =
(DZ1, . . . , DZd) be d-dimensional dam-data noise (inde-
pendent of all other variables).
Then we define the dam-data random variables DX,
c
DX ∈ Dχ by
DX = DG
(cid:16)
(cid:17)
f
DX, DM, (1 − DM) ⊙ DZ
DX = DM ⊙ f
c
DX + (1 − DM) DX
where ⊙ denotes element-wise multiplication. DX corres-
ponds to the dam-data vector of imputed values (note that
DG outputs a value for every component, even if its value
c
was observed) and
DX corresponds to the completed dam-
data vector, that is, the vector obtained by taking the partial
f
DX and replacing each # with the correspond-
observation
ing value of DX.
In order to capture the temporal correlation of dam
monitoring data and generate a more accurate reconstruc-
tion matrix of dam monitoring data, the encoder–decoder
framework is used to access the RNN into it, and the gen-
erated feature vectors are inputted into the decoder to gen-
erate a reconstruction network of dam monitoring data in
chronological order.
› The dam-data discriminator, DD, is trained as an adversary
to DG, where D in GAN is either true or false to G, while
DD in GAIN tries to dam-data determine the truth or fals-
ity of each part of the vector by predicting the value of Dm
in DM, not the truth or falsity of the whole vector.
fi The dam-data hint mechanism is a random variable, DH,
taking values in a dam-data space DH, both of which
we define. We allow DH to depend on DM and for each
c
(imputed) dam-data sample (
Dx, Dm), we draw Dh accord-
ing to the distribution DH|DM = Dm. We pass Dh as a
dam-data additional input to the DD and so it becomes
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 3. A network structure of DMDRN.
a function DD: Dχ × DH → [0, 1]d, where now the ith
component of DD(c
Dx, Dh) corresponds to the probability
c
that the ith component of
DX was observed conditional on
DX = c
c
Dx and DH = Dh.
dam-data
Let
random variable DB =
(DB1, . . . , DBd) ∈ {0, 1}d be defined by first sampling k
from {1, . . . , d} uniformly at random and then setting:
the
(cid:26)
DBi =
1 if j = k
0 if j ̸= k
.
(9)
Let DH = {0, 0.5, 1}d and, given DM, define
DH = DB ⊙ DM + 0.5 (1 − DB) .
(10)
Observe first that DH is such that DHi = t ⇒ DMi = t for
t ∈ {0, 1} but that DHi = 0.5 implies nothing about DMi.
In other words, DH reveals all but one of the components
of DM to DD. Note, however, that DH does contain some
information about DMi since DMi is not assumed to be
independent of the other components of DM.
By defining DH in different ways, we control the
amount of information contained in DH about DM, From
there, it is possible to train DG with multiple distributions
and choose the optimal one based on the results of DD.
fl The dam-data objective function, DD is trained by maxim-
izing the probability of correctly predicting DM, and DG
is trained by minimizing the probability that DD can cor-
rectly predict DM. Define the dam monitoring data evalu-
ation function DV(DD, DG) [18, 19],
DV(DD, DG) = Ec
[DMTlogDD( c
DX, DH)
DX,DM,DH
+ (1 − DM)T log(1 − DD( c
DX, DH))]
(11)
6
At
this point
the objective function is
min
DG
max
DD
DV(DD, DG).
For DD is a simple binary classification problem
that uses cross-entropy to define the loss function
L (a, b) [30]
L (a, b) =
dX
i =1
[ai log (bi) + (1 − ai) log (1 − bi)]
(12)
where ai denotes the elements in the DD prediction out-
come matrix and bi denotes the elements in the DM corres-
ponding to ai. Let
, then the object-
ive function can be transformed into a function of whether
DD can correctly predict DM [18, 19],
d
DM = DD
c
DX, DH
(cid:17)
(cid:16)
min max
DG DD
h
(cid:16)
L
E
(cid:17)i
DM, d
DM
.
(13)
2.3.2. DMDRN training. DMDRN constructs the potential
distribution of the missing data by generating adversarial
learning multiple times, and therefore solves the network’s
very large and very small optimization problem by iterative
means.
Firstly, The DG parameters are fixed and the DD parameters
are optimized. A mini-batch training approach was used for
parameter search, with kDD samples including ( f
DX(j), Dm(j))
from the training set and kDD samples Dz(j) and Db(j) from
DZ and DB. When DG is fixed, the displacement output of DD
corresponds to the bi = 0 part of each sample, so only DD is
trained to give the output of the bi = 0 part. Define the loss
function of DD [31],
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
L
DD(Dm, c
Dm, Db) =
X
[Dmilog(d
Dmi)
i:bi=0
+ (1 − Dmi)log(1 − d
Dmi)]
(14)
DD is then trained according to
P
kDD
j =1
L
DD(Dm(j),
min
DD
c
Dm(j), Db(j)). Then, the parameters of the generator DG are
optimized by fixing the parameters of the updated discrimin-
ator DD, which outputs a vector of all data, and during the
training of the DG, it is important not only to make the filled
values in the missing places successfully ‘fool’ DD (Dmj = 0),
but also to ensure that the output of the real observed data
is also close to the real value (Dmj = 1). Define two loss
functions [17, 32],
(cid:16)
L
DG
(cid:17)
Dm, c
Dm, Db
= −
X
(cid:16)
i:bi=0
1 − Dmilog
(cid:17)(cid:17)
(cid:16)
d
Dmi
(15)
It is used to assess the quality of the fill, and a smaller value
means that the probability of Dmi = 0 being discriminated by
DD as Dmi = 1 is closer to 1,
L
DM (DX, DX
′) =
dX
i =1
(DmiLDM (Dxi, Dxi
′))
(16)
where
(cid:26)
LDM(Dx, Dx
′
i ) =
′
i
−Dxi)2
(Dx
−Dxi log(Dx
if Dxi is continuous
′
i ) otherwise
It indicates the reconstruction error of the dam monitoring
data and is used to assess the difference between the output
value of the true observation after DG and the true value. The
smaller its value is, the closer the reconstructed value is to
the true value. Therefore, the DG training process is evaluated
using a complete loss function [17, 31],
min
DG
(cid:16)
L
DG
kDGX
j =1
Dm (j ) , c
Dm (j ) , b (j ) + αLDM
(cid:16)
(cid:17)(cid:17)
DX (j ) , c
f
DX (j )
where α is a hyper-parameter.
The DMDRN training pseudo-code [17] is shown in table 6.
(17)
Table 6. DMDRN training pseudo-code.
Input: Training dataset
n(cid:16)
(cid:17)o
f
DX( j), Dm( j)
Train
j =1
Output: Optimized DMDRN parameters (DG,DD).
1: Fixed parameters: DG’s structure and DD’s structure,
hyper-parameter α of equation (17).
2: Optimization of DD parameters and fix DG parameters.
3: Sample kDD samples from the training data
n(cid:16)
kDD
(cid:17)o
f
DX( j), Dm( j)
.
j =1
(cid:16)
(cid:17)
f
DX( j), Dm( j), Dz( j)
DX( j) ← Dm( j) ⊙ f
c
4: Sample kDD samples Dz( j) and Db( j) from DZ and DB
5: for j = 1, . . . , kDD do
6: DX( j) ← DG
7:
8: Dh( j) = Db( j) ⊙ Dm( j) + 0.5(1 − Db( j))
9: end for
10: Calculate:
LDD
∇DD −
(cid:1)
1 − Dm( j) ⊙ DX( j)
c
DX( j), Dh( j), Db( j)
Dm( j), DD
DX( j) +
(cid:17)(cid:17)
kDDP
(cid:16)
(cid:16)
(cid:0)
j =1
11: Update parameters of DD based on stochastic gradient descent
method
12: Optimization of DG parameters and fixing DD parameters
13: Sample kDG samples from the training data
n(cid:16)
(cid:17)o
f
DX( j), Dm( j)
kDG
j =1
(cid:17)
(cid:16)
f
DX(i), Dm(i), Dz(i)
DX(i) ← Dm(i) ⊙ f
c
14: Sample kDG samples Dz(i) and Db(i) from DZ and DB
15: for i = 1, . . . , kDG do
16: DX(i) ← DG
17:
18: Dh(i) = Db(i) ⊙ Dm(i) + 0.5(1 − Db(i))
19: end for
20: Calculate:
(cid:16)
kDGP
Dm(i), c
LDG
∇DG
(cid:1)
1 − Dm(i) ⊙ DX(i)
Dm(i), Db(i) + αLDM
(cid:16)
DX(i), f
DX(i) +
(cid:0)
DX(i)
(cid:17)(cid:17)
i =1
21: Update the parameters of DG based on stochastic gradient
descent method
Table 7. Comparison of original and reconstructed data of P2.
Date
Original data (m)
Reconstructed data (m)
Error
2020/4/26
2020/4/27
2020/4/28
2020/4/29
2020/4/30
419.7082
419.7122
419.7042
419.7109
419.7242
419.7924
419.7889
419.7856
419.7827
419.7823
0.0841
0.0767
0.0813
0.0719
0.0581
2.3.3. DMDRN experiment. To demonstrate the accuracy of
the data reconstructed by DMDRN, The P2 (seepage monitor-
ing point) monitoring data from 1 January 2020 to 25 January
2021 were selected and randomly deleted. Specifically, we
removed the P2 monitoring data from 26 April to 30 April
2020, and input the incomplete P2 monitoring dataset into
DMDRN to reconstruct the removed data. The reconstruc-
ted data were then compared to the original data as shown in
table 7.
Based on table 7, it can be seen that the error between the
reconstructed values by DMDRN and the original values does
not exceed 0.1 m, which meets the accuracy requirements for
dam seepage monitoring, indicating that DMDRN can be used
to reconstruct monitoring data for dams.
Compared to traditional mathematical reconstruction meth-
ods such as the mean or linear interpolation, DMDRN
has more advantages. Dam monitoring data often contains
numerous nonlinear relationships, and traditional mathemat-
ical reconstruction methods necessitate the construction of
suitable mathematical models to explore these relationships
in order to reconstruct abnormal data accurately. DMDRN,
on the other hand, is an enhanced network based on GAN
and serves as a deep learning framework that can better
explore the nonlinear relationships [33–35] in dam monitoring
data and generate precise reconstruction values. Additionally,
7
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
when it comes to processing large-scale dam monitoring data,
traditional statistical methods are time-consuming and inef-
ficient, whereas DMDRN (as a deep learning framework) is
more advantageous [33].
In addition, compared to other deep learning networks,
DMDRN also has unique advantages. In the field of dam mon-
itoring data reconstruction, LSTM and GRU or other deep
learning networks are generally used to predict the monitor-
ing data and replace the original values with the predicted val-
ues for data reconstruction. However, this requires the data
set to be complete, meaning that when LSTM or other net-
works make predictions, abnormal data as the original data
will also become part of the learning process, which is clearly
not scientific. If the abnormal data is removed, the data set is
no longer complete, and LSTM or other networks cannot be
used. DMDRN effectively solves this problem and achieves
the reconstruction of dam monitoring data even when the data
set is incomplete.
In summary, compared to traditional reconstruction meth-
ods, DMDRN, as a deep learning framework, can better
explore the nonlinear relationships among dam monitoring
data, and it is more advantageous for large-scale data pro-
cessing. Compared to other deep learning models, DMDRN
can reconstruct dam monitoring data even when the data set
is incomplete. Therefore, DMDRN is more suitable for the
reconstruction of dam monitoring data.
(2) For two dam monitoring parameters with strong tem-
poral correlation, the DBSCAN algorithm is used to detect
anomalous data points. If anomalous data points occur in
both dam monitoring parameters, they are considered as
anomalous warning data and should be retained and aler-
ted to the system. If anomalous data points occur in only
one of the dam monitoring parameters, they shall be con-
sidered as anomalous error data and should be removed
and reconstructed using DMDRN.
(3) For dam monitoring parameters with weak temporal cor-
relation, the DBSCAN algorithm is used to detect anom-
alous data points one by one. Remove the abnormal data
and proceed to step (4) to reconstruct the dam monitoring
data and determine the type of anomalous data.
(4) DMDRN is used to reconstruct the removed dam monitor-
ing data and then the reconstructed values are compared to
the original values. When the deviation is large, it is con-
sidered as anomalous error data and should be replaced
with the reconstructed data generated by DMDRN. When
the deviation is small, it is considered as anomalous warn-
ing data and should be retained and alerted to the system.
The size of the deviation is based on the actual monitor-
ing data fluctuations, such as the data fluctuation of dam
horizontal displacement monitoring, which is generally
0.1 mm. When the deviation does not exceed 0.1 mm, it
is considered as a small deviation.
3. A method for identifying and reconstructing
anomalous data in dam monitoring considering
temporal correlation
We have utilized some ideas from data cleaning for trans-
former monitoring data [15] and combined the advantages of
DMDRN to propose a method for identifying and reconstruct-
ing anomalous data in dam monitoring considering temporal
correlation. In this study, anomalous data in dam monitoring
is categorized into anomalous error data and anomalous warn-
ing data. Anomalous error data refers to data that is anomalous
due to monitoring equipment malfunctions or human measure-
ment errors and does not truly reflect the state of the dam. For
anomalous error data, it needs to be removed and reconstruc-
ted. Anomalous warning data refers to abnormalities caused
by sudden changes in the structure of the dam itself. At this
time, the data reflects the status of the dam itself and should
be retained and alerted to the system, to provide data support
for subsequent dam safety monitoring and to identify potential
safety hazards.
3.1. Specific implementation steps of the method
The method comprises the following steps:
A flowchart of the implementation of the method is shown
in figure 4.
3.2. Applicability of the method
the method lies in identifying and deal-
The core of
ing with anomalous data by leveraging the temporal cor-
relation between monitoring parameters (e.g. water level,
horizontal displacement, etc), combined with the DMDRN
and DBSCAN algorithms. Therefore, a dam/structure/system
using this method needs to satisfy the following conditions:
• It should have multiple monitoring parameters (e.g. water
level, horizontal displacement, etc). This method relies on
mining the temporal correlation between monitoring para-
meters, so it is not the most suitable approach for systems
with only a single monitoring parameter.
• Sufficient training data should be available for DMDRN.
Since this method requires the use of DMDRN for both
anomalous data identification and data (anomalous error
data) reconstruction, a monitoring system needs to have a
substantial dataset for DMDRN to learn from. For monit-
oring systems that are newly established or have less than
six months of data, DMDRN may not be able to accurately
reconstruct the data.
(1) The Apriori algorithm in association rule is used to analyze
the temporal correlation between dam monitoring para-
meters (e.g. water levels, horizontal displacements, etc),
and thus to determine whether there is a strong temporal
correlation between dam monitoring parameters.
In summary, this method is applicable to dams/structures/sys-
tems that have multiple monitoring parameters and have accu-
mulated a certain amount of monitoring data. Therefore, it is
suitable for most operational dam monitoring systems.
8
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 4. A flowchart of the implementation of the method.
3.3. Advantage of the method
In order to highlight the advantage of the proposed method,
the proposed method is compared with the method of the ori-
ginal monitoring system. The steps of the method used by the
original monitoring system are as follows:
(1) Dam monitoring data is collected and anomalous data is
detected based on the DBSCAN algorithm.
(2) The detected anomalous data is directly removed
and reconstructed based on traditional mathematical
reconstruction methods such as the mean interpola-
replacing the removed anomalous values with
tion,
reconstructed values, thus constituting a complete dam
monitoring dataset.
(3) The complete dataset is monitored, and the system issues
an alarm when a value of the monitored data is out of the
specified range. In this way, the safety monitoring of the
dam is achieved.
From the above steps, it can be concluded that the ori-
ginal monitoring system directly removes and reconstructs
the detected anomalous data and does not further classify the
detected anomalous data, and is unable to identify whether the
anomalous data is anomalous error data or anomalous warning
data, and thus is unable to retain the anomalous warning data.
The proposed method, on the other hand, can further classify
the anomalous data, remove and reconstruct only anomalous
error data, while retaining anomalous warning data. Moreover,
the reconstruction method used by the original monitoring
system is the traditional mathematical reconstruction method,
while the proposed method uses the more efficient DMDRN
to reconstruct the data.
A comparison of the proposed method with the method of
the original monitoring system is shown in figure 5.
4. Experiments and results
The method has different ways of identifying abnormal data
types for dam monitoring parameters with different levels of
temporal correlation (strong or weak). In this method, for
measurements with strong temporal correlation, the type of
abnormal data is determined based on whether the abnormal-
ity occurs simultaneously. For parameters with weak temporal
correlation, the type of abnormal data is determined based on
the deviation from the reconstructed values (refer to section 3.1
and figure 4 for detailed steps). Therefore, to validate the feas-
ibility of this method, it is necessary to separately verify the
applicability of this method for parameters with strong tem-
poral correlation and parameters with weak temporal correla-
tion in dam monitoring.
In order to demonstrate the scientific validity and feasib-
ility of the method proposed in this paper, two experiments
were designed. The first experiment focuses on identifying and
reconstructing abnormal data in dam monitoring parameters
with strong temporal correlation, while the second experiment
focuses on the same task but with weak temporal correlation.
4.1. Identification and reconstruction of anomalous data in
dam monitoring with strong temporal correlation
The method was applied to the monitoring data of EX4
(horizontal displacement monitoring point) from 1 January
2020 to 25 January 2021. From section 2.1.3 of this paper it
can be concluded that there is a strong temporal correlation
9
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 5. A comparison of the proposed method with the method of the original monitoring system.
Figure 6. Water level data anomaly detection chart.
between water level and EX4, Following the step (2) of the
method, which states ‘For two dam monitoring parameters
with strong temporal correlation, the DBSCAN algorithm is
used to detect anomalous data points’, the DBSCAN algorithm
was used to detect anomalous points in the monitoring data
of water level and EX4, and the results were shown in
figures 6 and 7.
Based on the DBSCAN algorithm and figures 6 and 7,
the anomalous time points for EX4 were identified as fol-
lows: 2020/07/17 08:00:00, 2020/07/18 08:00:00, 2020/07/19
08:00:00, 2020/11/3 11:00:00, 2020/11/3 12:00:00, and
2020/11/3 13:00:00. Among these, only the monitoring data
for EX4 were found to be anomalous at 2020/07/17 08:00:00,
2020/07/18 08:00:00, and 2020/07/19 08:00:00, while the
water level monitoring data had no anomalies. According to
step (2) of the method, which states ‘If anomalous data points
occur in only one of the dam monitoring parameters, they shall
be considered as anomalous error data and should be removed
and reconstructed using DMDRN’, it is concluded that the
monitoring data of EX4 at these time points are considered as
anomalous error data and should be removed and reconstruc-
ted using DMDRN.
At 2020/11/3 11:00:00, 2020/11/3 12:00:00, and 2020/11/3
13:00:00, both the monitoring data for EX4 and the water level
data were found to be anomalous. According to step (2) of the
method, which states ‘If anomalous data points occur in both
dam monitoring parameters, they are considered as anomalous
warning data and should be retained and alerted to the system,’
it is concluded that the monitoring data for EX4 at these time
points are considered as anomalous warning data and should
be retained and alerted to the system. Table 8 shows the identi-
fied anomalous data for EX4. And the anomalous error data of
10
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 7. EX4 data anomaly detection chart.
Table 8. Identification of EX4 abnormal data.
No.
‹
›
fi
fl
(cid:176)
–
Time
Water level data
2020/07/17 08:00:00
2020/07/18 08:00:00
2020/07/19 08:00:00
2020/11/3 11:00:00
2020/11/3 12:00:00
2020/11/3 13:00:00
Normal
Normal
Normal
Abnormal
Abnormal
Abnormal
EX4 data
Abnormal
Abnormal
Abnormal
Abnormal
Abnormal
Abnormal
EX4 data anomaly type
Anomalous error data
Anomalous error data
Anomalous error data
Anomalous warning data
Anomalous warning data
Anomalous warning data
No.
‹
›
fi
Table 9. Reconstruction results of EX4 anomalous error data.
Time
Original data (m)
Reconstructed data (m)
2020/07/17 08:00:00
2020/07/18 08:00:00
2020/07/19 08:00:00
−2.21
−2.01
−2.12
−1.01
−1.12
−1.21
EX4 was removed and DMDRN was used for reconstruction,
as shown in table 9.
The anomalous warning data of EX4 was retained. This
reservoir is mainly used for agricultural irrigation and urban–
rural water supply, which caused the abnormal data on 17 July
2020 due to a large amount of water usage on that day. The
comparison between the data before and after the reconstruc-
tion of EX4 is shown in figure 8. It can be observed from the
figure that the reconstructed data is more reasonable than the
original data.
The method detected six anomalous data points for EX4,
which perfectly matched the anomalous data points detected
by the original monitoring system of the dam. This indicates
that the method is effective. However, the original monitor-
ing system did not further classify these six anomalous data
points, but merely deleted and reconstructed them based on
mathematical statistics, which is not sufficiently scientific. The
EX4 anomalous data points observed at 2020/11/3 11:00:00,
2020/11/3 12:00:00, and 2020/11/3 13:00:00 were caused by
a large amount of water usage on that day and represent the
feedback of the dam’s own state changes. Therefore, they
should not be eliminated. The proposed method effectively
identifies the anomalous error data and the anomalous warning
data of EX4, and only removes and reconstructs the anomalous
error data. For the EX4 anomalous data points at 2020/11/3
11:00:00, 2020/11/3 12:00:00, and 2020/11/3 13:00:00, the
method recognizes them as anomalous warning data, retains
these data, and generates warnings. Therefore, this method
provides a more scientifically valid approach to processing
monitoring data for the dam.
4.2. Identification and reconstruction of anomalous data in
dam monitoring with weak temporal correlation
Water level monitoring data and P3 (seepage monitoring point)
monitoring data from 1 January 2020 to 25 January 2021 were
selected for temporal correlation analysis. A sliding window
was set to L = 10, the corresponding subseries were extracted,
and the results of linear fit of water level and P3 were obtained
as shown in figure 9.
11
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 8. EX4 data reconstruction before and after comparison.
Figure 9. Water level and P3 data linear fit results.
Table 10. Support and confidence calculation results for water level
and P3.
Frequent itemset
Support
Confidence
a1→b5
a2→b2
a2→b4
a4→b3
Temporal correlation
0.0244
0.0244
0.0244
0.0244
0.0976
1
0.5
0.5
1
0.75
According to the Apriori algorithm, the frequent itemset in
database with confidence greater than the minimum threshold
was found, and equations (3) and (4) were used to calculate the
support and confidence of the temporal correlation between
water level and P3, and the results were shown in table 10.
From the results in the table, the temporal correlation’s
Support is 0.0976, with the Confidence of 0.75, which does
not meet the minimum threshold requirement, so the temporal
correlation between the water level and P3 is considered weak.
Following the step (3) of the method, which states ‘For dam
monitoring parameters with weak temporal correlation, the
DBSCAN algorithm is used to detect anomalous data points
one by one’, the DBSCAN algorithm was used to detect anom-
alous points in the monitoring data of P3, and the result was
shown in figure 10.
Based on the DBSCAN algorithm and figure 10, it can
be concluded that the abnormal data points for P3 appeared
at 2021/1/19 8:00, 2021/1/20 8:00,2 and 2021/1/21 8:00.
Following steps (3) and (4) of this method, these abnormal data
points of P3 were removed and reconstructed with DMDRN,
as shown in table 11.
12
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 10. P3 data anomaly detection chart.
Table 11. Reconstruction results of P3 anomalous data points.
Time
Original data (m)
Reconstructed data (m)
2021/1/19 8:00
2021/1/20 8:00
2021/1/21 8:00
421.2792
421.1428
421.0627
420.9371
420.9242
420.9156
Table 12. P3 abnormal data identification.
Time
P3 original anomalous data
P3 reconstruction data Deviation Anomaly type
2021/1/19 8:00
2021/1/20 8:00
2021/1/21 8:00
421.2792
421.1428
421.0627
420.9371
420.9242
420.9156
Big
Big
Big
Anomalous error data
Anomalous error data
Anomalous error data
According to step (4) of the method, which states ‘DMDRN
is used to reconstruct the removed dam monitoring data and
then the reconstructed values are compared to the original val-
ues. When the deviation is large, it is considered as anomalous
error data and should be replaced with the reconstructed data
generated by DMDRN. When the deviation is small, it is con-
sidered as anomalous warning data and should be retained and
alerted to the system.’, the original values and reconstructed
values of the anomalous data points for P3 were compared.
The comparison showed that the difference between the ori-
ginal values and reconstructed values was significant, with
both being greater than 0.1 m. As a result, these three abnor-
mal data points were deemed to be anomalous error data, as
indicated in table 12.
According to the table 12, the abnormal data are anomalous
error data, which should be replaced with the reconstruction
values generated by DMDRN. And the comparison between
the data before and after the reconstruction of P3 is shown
in figure 11. It can be observed from the figure that the
reconstructed data is more reasonable than the original data.
The method detected three abnormal data points for P3,
which were completely consistent with the abnormal data
points detected by the original monitoring system of the
dam. This once again proves the effectiveness of the method.
After removing the abnormal data, the original monitoring
system reconstructs the missing values using mathematical
statistics. In contrast, this method reconstructs the monit-
oring data using DMDRN. Compared to traditional recon-
struction methods, DMDRN, as a framework of deep learn-
ing, can better explore the nonlinear relationship among dam
monitoring data, and it is more advantageous for large-scale
data processing. Compared to other deep learning models,
DMDRN can reconstruct the monitoring data of the dam even
in the presence of incomplete datasets. Therefore, this method
provides a more scientific and effective approach for pro-
cessing dam monitoring data.
5. Discussion
The method for identifying and reconstructing anomalous
data in dam monitoring considering temporal correlation
effectively distinguishes anomalous error data and anomalous
warning data, and preserves the abnormal warning data that
13
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
Figure 11. P3 data reconstruction before and after comparison.
can feedback the occurrence of sudden changes in the dam’s
state, which was not considered by previous researchers. The
method preserves the abnormal data that can feedback the
occurrence of sudden changes in the dam’s state and issues
warnings, which can timely investigate potential safety risks
and prevent disasters such as dam failures. This is very
significant.
In addition, compared to traditional reconstruction meth-
ods, DMDRN, as a framework of deep learning, can bet-
ter explore the nonlinear relationship among dam monitoring
data, and it is more advantageous for large-scale data pro-
cessing. Compared to other deep learning models, DMDRN
can reconstruct the monitoring data of the dam even in the
presence of incomplete datasets. Therefore, DMDRN is obvi-
ously more suitable for reconstructing abnormal data in dam
monitoring.
6. Conclusion
the DBSCAN algorithm and
Based on association rules,
DMDRN, this paper develops the method for identifying
and reconstructing anomalous data in dam monitoring con-
sidering temporal correlation and applies it to specific pro-
jects, which proved the feasibility and scientificity of the
method.
• In this study, abnormal data in dam monitoring is divided
into anomalous error data caused by sensor failures or
human errors, and anomalous warning data caused by sud-
den changes in the condition of the dam itself. Unlike previ-
ous methods that remove and reconstruct all abnormal data,
this study proposes to remove and reconstruct only anomal-
ous error data, while retaining anomalous warning data that
can provide feedback on the dam’s own state. This makes
the handling of abnormal data in dam monitoring more
scientific.
• The DMDRN based on GAN is proposed, which can quickly
and efficiently reconstruct dam monitoring data. Compared
to traditional reconstruction methods, DMDRN, as a deep
learning framework, can better explore the nonlinear rela-
tionship among dam monitoring data, and it is more advant-
ageous for large-scale data processing. Additionally, com-
pared to other deep learning models, DMDRN can recon-
struct the monitoring data of the dam even in the presence
of incomplete datasets.
• The method for identifying and reconstructing anomalous
data in dam monitoring considering temporal correlation is
proposed. This method can effectively identify and retain
anomalous warning data, remove and reconstruct anom-
alous error data, and efficiently handle different types of
anomalous data, making the processing of dam monitoring
data more scientific and efficient.
Data availability statement
The data that support the findings of this study are available
upon reasonable request from the authors.
Acknowledgments
The authors acknowledge the financial support provided
by Scientific and Technological Research Program of
Chongqing Municipal Education Commission
(Grant
No. KJZD-K202100705), Chongqing Water Conservancy
Science and Technology Project
(Grant No. CQSLK-
2022002)and Research and Innovation Program for Graduate
Students in Chongqing Jiaotong University (Grant No.
14
Smart Mater. Struct. 32 (2023) 115009
Y Chen et al
2023B0005). The touch-ups to this paper by Editors are greatly
acknowledged.
ORCID iD
Yongjiang Chen https://orcid.org/0009-0001-6210-6682
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| null |
10.1186_s40478-022-01399-4.pdf
|
Availability of data and materials
The datasets that were used and analyzed during the current study are avail‑
able from the corresponding author on reasonable request.
|
Availability of data and materials The datasets that were used and analyzed during the current study are available from the corresponding author on reasonable request.
|
Valentino et al.
Acta Neuropathologica Communications (2022) 10:103
https://doi.org/10.1186/s40478-022-01399-4
RESEARCH
Open Access
Mitochondrial genomic variation
in dementia with Lewy bodies: association
with disease risk and neuropathological
measures
Rebecca R. Valentino1, Chloe Ramnarine1, Michael G. Heckman2, Patrick W. Johnson2,
Alexandra I. Soto‑Beasley1, Ronald L. Walton1, Shunsuke Koga1, Koji Kasanuki1,3, Melissa E. Murray1,
Ryan J. Uitti4, Julie A. Fields5, Hugo Botha6, Vijay K. Ramanan6, Kejal Kantarci7, Val J. Lowe7, Clifford R. Jack7,
Nilufer Ertekin‑Taner1,4, Rodolfo Savica6, Jonathan Graff‑Radford6, Ronald C. Petersen6, Joseph E. Parisi8,
R. Ross Reichard8, Neill R. Graff‑Radford4, Tanis J. Ferman9, Bradley F. Boeve6, Zbigniew K. Wszolek4,
Dennis W. Dickson1 and Owen A. Ross1,10*
Abstract
Dementia with Lewy bodies (DLB) is clinically diagnosed when patients develop dementia less than a year after par‑
kinsonism onset. Age is the primary risk factor for DLB and mitochondrial health influences ageing through effective
oxidative phosphorylation (OXPHOS). Patterns of stable polymorphisms in the mitochondrial genome (mtDNA) alter
OXPHOS efficiency and define individuals to specific mtDNA haplogroups. This study investigates if mtDNA haplo‑
group background affects clinical DLB risk and neuropathological disease severity. 360 clinical DLB cases, 446 neuro‑
pathologically confirmed Lewy body disease (LBD) cases with a high likelihood of having DLB (LBD‑hDLB), and 910
neurologically normal controls had European mtDNA haplogroups defined using Agena Biosciences MassARRAY iPlex
technology. 39 unique mtDNA variants were genotyped and mtDNA haplogroups were assigned to mitochondrial
phylogeny. Striatal dopaminergic degeneration, neuronal loss, and Lewy body counts were also assessed in different
brain regions in LBD‑hDLB cases. Logistic regression models adjusted for age and sex were used to assess associa‑
tions between mtDNA haplogroups and risk of DLB or LBD‑hDLB versus controls in a case‑control analysis. Additional
appropriate regression models, adjusted for age at death and sex, assessed associations of haplogroups with each dif‑
ferent neuropathological outcome measure. No mtDNA haplogroups were significantly associated with DLB or LBD‑
hDLB risk after Bonferroni correction.Haplogroup H suggests a nominally significant reduced risk of DLB (OR
P
after additionally adjusting for the number of APOE ε4 alleles (OR
suggestive association with reduced ventrolateral substantia nigra neuronal loss (OR
haplogroup H may be protective against DLB risk and neuronal loss in substantia nigra regions in LBD‑hDLB cases but
further validation is warranted.
0.61,
0.34). The haplogroup H observation in DLB was consistent
0.006) but no association of LBD‑hDLB (OR
0.004). Haplogroup H also showed a
0.033). Mitochondrial
0.59, P
0.87, P
0.44, P
=
=
=
=
=
=
=
=
*Correspondence: [email protected]
1 Department of Neuroscience, Mayo Clinic, 4500 San Pablo Road,
Jacksonville, FL 32224, USA
Full list of author information is available at the end of the article
© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or
other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this
licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco
mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 2 of 15
Keywords: Dementia with Lewy‑bodies, Lewy body disease, Mitochondrial DNA, Mitochondrial haplogroups,
Neuropathology
Introduction
Lewy body dementia are comprised of two distinct, but
clinically related, disorders—Dementia with Lewy bod-
ies (DLB) and Parkinson’s disease dementia (PDD) [22,
30]. The timing of dementia onset determines the exact
clinical diagnosis, whereby dementia onset before or less
than a year after parkinsonism is classified as DLB, and
dementia onset more than one year after parkinsonism
is classified as PDD [30]. DLB is one of the most com-
mon forms of dementia after Alzheimer’s disease (AD),
accounting for approximately 23% of all dementia cases
[49]. Currently there is no treatment to prevent or cure
DLB and disease course is progressive and eventually
fatal.
Neuropathologically, DLB and PDD are very similar
and fall under the pathological term of Lewy body dis-
ease (LBD). Lewy body disorders are characterized by
the presence of Lewy bodies (LB) and Lewy neurites in
the brain, causing neurodegeneration. LB are complex
masses of aggregated phosphorylated alpha-synuclein
(aSyn), p62, and ubiquitin proteins, as well as lipids and
membranous organelles [29]. The location and distri-
bution of LBs in the brain and the associated neuronal
dysfunction determines clinical phenotypes observed.
For example, LB accumulation in the brainstem and
midbrain regions, and the associated neurodegenera-
tion, typically induces Parkinson’s disease (PD) symp-
toms of tremor, rigidity, and slowness of movement [38],
whereas LB accumulation in the neocortical and limbic
regions is associated with cognitive and neuropsychiatric
symptoms, such as cognitive impairment, fluctuations,
visual hallucinations, and behavioral changes—which are
reflective of PDD or DLB [12, 30, 37]. Classical brainstem
and nigral LB consist of dense, spherical cores with irra-
diating filaments, and a surrounding halo (when stained
with hematoxylin/eosin), whereas LB in neocortical
regions typically have pale, fibrillary structures without a
halo or central core [29, 42]. Paler, fibrillary LB have been
described as premature and are thought to develop into
classical LB structures with disease progression [17]. In
addition to LB, pathological aggregates of extracellular
amyloid-beta (Abeta) plaques and intracellular neurofi-
brillary tangles of hyperphosphorylated tau proteins are
often present in LBD making the disease spectrum very
heterogenous [10]. Neuropathologists use defined cri-
teria to assess aSyn and tau Braak stage, as well as beta-
amyloid Thal phase, to neuropathologically determine
accurate LBD diagnosis and characterize disease severity,
and use available medical records to determine the likeli-
hood of clinical phenotypes [6].
Within the past decade, ongoing efforts have continued
to work towards understanding genetic markers influenc-
ing LB disorders, particularly PD, whereby current case-
control studies consist of tens of thousands of cases [18,
34, 35]. Recent smaller case-control studies of DLB have
identified overlapping genetic markers between PD and
AD [3, 18, 27], further demonstrating the overlapping
pathologies of these diseases, but despite such efforts, the
genetic etiology of DLB is yet to be defined. Thus, provid-
ing additional scope to characterize other genetic factors
which may be driving dementia onset in DLB.
Age consistently remains the major risk factor for neu-
rodegeneration and both healthy ageing and aSyn accu-
mulation is influenced by mitochondrial health, whereby
increased reactive oxygen species (ROS) production
accelerates ageing and aSyn aggregation over time [20,
28]. ROS are a byproduct from oxidative phosphoryla-
tion (OXPHOS) which occurs on the inner mitochon-
drial membrane [26]. Mitochondria contain their own
genomic information (mtDNA), independent to the
nuclear genome, which codes for 13 essential subunits in
OXPHOS complexes. Patterns of stable polymorphisms
across the mtDNA molecule define individuals to specific
mtDNA haplogroups, and each mtDNA haplogroup has
a unique metabolic profile which influences ROS pro-
duction over time [15, 16]. As a result of their distinct
metabolic backgrounds, mtDNA haplogroups have been
associated with age-related and multiple neurodegen-
erative diseases, including PD and AD [4, 21], but have
not been examined in relation to dementia onset in large
cohorts of patients. Therefore, the aims of this study were
to evaluate the association between mtDNA haplogroups
and risk of clinical DLB and pathologically confirmed
LBD cases with a high likelihood of having clinical DLB
(LBD-hDLB) in a case-control analysis. In analysis of
the LBD-hDLB group, we also examined associations of
mtDNA haplogroups with severity of neuropathological
measures, such as LB counts and distribution, neuronal
loss, and dopaminergic degeneration across several brain
regions.
Material and methods
Study subjects and data collection
A total of 806 DLB subjects (N=360 clinically diag-
nosed DLB cases and N=446 autopsy-confirmed Lewy
body disease cases that were assessed as having a high
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 3 of 15
likelihood of DLB (LBD-hDLB) – of which N=48 were
present in both series) and 910 controls were included in
this study. Clinical DLB patients were diagnosed by neu-
rologists at Mayo Clinic in Jacksonville, FL or Rochester,
MN and were recruited as part of the Alzheimer’s Disease
Research Center and the Mayo Clinic Study of Aging.
Pathologically confirmed LBD cases were obtained from
the brain bank for neurodegenerative disorders at Mayo
Clinic in Jacksonville, FL and were evaluated by a single
neuropathologist (Dr. Dennis Dickson). LBD cases were
all assessed as having a high likelihood of DLB according
to the criteria of the fourth report of the DLB consortium
[30]. Controls were recruited by Dr. Zbigniew Wszolek
and his colleagues from Mayo Clinic in Jacksonville, FL
and were absent of neurological disease. All subjects
provided written consent prior to study commencement
and were Caucasian, non-Hispanic, and unrelated. Age
at DLB diagnosis in clinically diagnosed DLB cases, age
at death in pathologically confirmed LBD-hDLB cases,
and age at blood draw in controls, and sex was collected
for all subjects (Table 1). Additionally, neuropathologi-
cal measures for Lewy body counts and substantia nigra
(SN) neuronal loss were available for 242 (54.3%) LBD-
hDLB cases (Table 1).
Neuropathological assessment in LBD‑hDLB
Assessment of neurofibrillary tangles, senile plaques,
and Lewy bodies
Neuropathological methodologies used to assess neurofi-
brillary tangles (NFTs), senile plaques (SPs), and Lewy
bodies (LBs) have been described previously [33]. Briefly,
neuroanatomical sampling and thioflavin-S fluorescence
microscopy was performed, where counts of NFTs and
SPs were measured manually in six cortical regions, four
sections of the hippocampus, and two regions of the
amygdala [43]. Formalin-fixed, paraffin-embedded tissue
samples from limbic and cortical regions were sectioned
and mounted on glass slides. Assessment of LB pathology
was performed using an aSyn antibody (NACP, 1:3000
rabbit polyclonal, Mayo Clinic antibody) with formic acid
pretreatment for 30 minutes and was processed using the
DAKO Autostainer (DAKO Auto Machine Corporation,
Carpinteria, CA) with DAKO Envision+ HRP System.
LB counts were measured in five cortical regions—mid-
dle frontal, superior temporal, inferior parietal, cingulate,
and parahippocampal. The distribution of LB pathol-
ogy was assessed using the staging scheme defined by
Kosaka et al. to categorize samples as either brainstem,
transitional, or diffuse [25]. Braak NFT stage [1] and Thal
amyloid phase [44] were assigned according to the distri-
butions of NFTs and SPs respectively. These neuropatho-
logic measures are summarized in Table 1.
Quantification of striatal dopaminergic degeneration
Quantitative assessment of striatal dopaminergic degen-
eration by measurement of tyrosine hydroxylase immu-
noreactivity (TH-ir) has been described previously [24].
To summarize, the putamen was assessed at the level of
the anterior commissure from a section made from the
hemi-brain in a standardized dissection plane defined
by three points in the fundibulum, uncus, and posterior
margin of the anterior commissure in the third ventricle.
Digital images of the putamen were parcelled into ven-
tromedial and dorsolateral areas [19], and dopaminergic
degeneration was quantitatively assessed.
The basal ganglia section was processed for immuno-
histochemistry with a commercially available antibody
to TH (rabbit polyclonal, 1:600; Affinity Bioreagants,
Golden, Colorado) with Proteinase K pretreatment for 5
minutes. The immunostained sections were captured by
ScanScope XT (Aperio Technologies, Vista, California),
and images were annotated with ImageScope (version
12.1). Regions of interest were manually edited to exclude
artifacts, large blood vessels and their perivascular
spaces, and large fiber bundles. The putamen was divided
into ventromedial and dorsolateral regions. Quantifica-
tion of TH-ir used an algorithm that detected positive
pixels based on optical density. TH-ir was expressed as
a percentage, calculated as the number of positive pix-
els divided by the sum of inverse pixels and background
pixels. A lower TH-ir value represents a greater degree
of putaminal dopaminergic degeneration. Table 1 sum-
marizes dorsolateral and ventromedial putaminal TH-ir
in DLB cases.
Assessment of substantia nigra pigmented neuronal loss
The midbrain was a transverse section at the level of
the third nerve, similar to what has been recommended
for diagnostic evaluation of PD [7]. A semi-quantitative
assessment of SN cell groups was ascertained on hema-
toxylin and eosin-stained sections at 100x magnification.
Our assessment was restricted to pigmented neurons of
SN pars compacta and divided into medial and ventro-
lateral sections—similar to previous studies [14, 39]. We
used a human atlas of SN cell groups to identify medial
and ventrolateral regions of the SN [36]. The density of
nonpigmented neurons was not taken into consideration
for the assessment of the semi-quantitative scores, which
were based on a 4-point scale (0=none, 1=mild, 2=mod-
erate, and 3=severe) (Table 1).
Genetic analysis
Peripheral blood was collected from clinical DLB patients
and control subjects, and frozen cerebellum brain tissue
was provided from pathologically confirmed LBD-hDLB
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 4 of 15
Table 1 Patient characteristics
Variable
Age (years)a
Sex
Male
Female
Braak NFT stage
0
I
II
III
IV
V
VI
Thal amyloid phase
0
1
2
3
4
5
LBD subtype
Transitional
Diffuse
Lewy body counts
Middle frontal gyrus
Superior temporal gyrus
Inferior parietal gyrus
Cingulate gyrus
Parahippocampal gyrus
Putaminal TH-ir
Dorsolateral
Ventromedial
Substantia nigra neuronal loss score
Ventrolateral
0.0
none
0.5
none/mild
1.0
mild
1.5
mild/moderate
=
2.0
2.5
moderate
=
moderate/severe
3.0
severe
Medial
0.0
none
0.5
none/mild
1.0
mild
1.5
mild/moderate
2.0
moderate
2.5
moderate/severe
3.0
severe
=
=
=
=
=
=
=
=
=
=
=
=
Controls (N
=
910)
Clinical DLB (N
=
360)
79 (41, 102)
388 (42.6%)
522 (57.4%)
73 (50, 100)
270 (75.0%)
90 (25.0%)
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
LBD with a high
likelihood of DLB
(N
446)
=
78 (48, 103)
292 (65.5%)
154 (34.5%)
13 (2.9%)
23 (5.2%)
138 (30.9%)
143 (32.1%)
129 (28.9%)
0 (0.0%)
0 (0.0%)
40 (10.7%)
37 (9.9%)
23 (6.1%)
129 (34.4%)
56 (14.9%)
90 (24.0%)
89 (20.0%)
357 (80.0%)
5 (0, 35)
10 (0, 50)
4 (0, 30)
12 (2, 32)
16 (1,45)
2.93 (0.26, 21.61)
8.99 (0.26, 27.42)
0 (0.0%)
2 (1.0%)
10 (5.1%)
7 (3.6%)
21 (10.8%)
23 (11.8%)
132 (67.7%)
2 (1.1%)
1 (0.6%)
25 (14.0%)
16 (8.9%)
26 (14.5%)
25 (14.0%)
84 (46.9%)
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 5 of 15
Table 1 (continued)
Sample median (minimum, maximum) is given for continuous variables
For LBD cases with a high likelihood of DLB, information was unavailable for Thal amyloid phase (N
temporal gyrus Lewy body count (N
Lewy body count (N
(N
a Age represents age at blood draw for controls, age at DLB onset for clinical DLB cases, and age at death for LBD with a high likelihood of DLB cases
=
251), and medial substantia nigra neuronal loss score (N
213), inferior parietal gyrus Lewy body count (N
71), middle frontal gyrus Lewy body count (N
211), cingulate gyrus Lewy body count (N
250), ventromedial putaminal TH-ir (N
232), dorsolateral putaminal TH-ir (N
267).
=
=
=
=
=
=
=
=
250), ventrolateral substantia nigra neuronal loss score
211), superior
=
213), parahippocampal gyrus
cases. Genomic DNA was extracted from peripheral
blood lymphocytes and cerebellum tissue using Autogen
Flex Star and Autogen 245T (Holliston, MA) methods
respectively. DNA was diluted to 15 ng/µl and 39 unique
mitochondrial DNA variants were genotyped by a single-
user (RRV) using two custom-designed Agena Bioscience
iPLEX arrays on Sequenom MassARRAY technology
[11]. More detailed methods for genetic assessments have
been previously published [46, 47]. Individual mitochon-
drial DNA haplogroups were defined to mitochondrial
phylogeny for each subject [48] (Table 2). Phylogeneti-
cally related haplogroups were also grouped into family
haplogroups (e.g. sub-haplogroups H, H1, H2, H3, and
H4 are all part of the family H haplogroup) and four dif-
ferent super-haplogroups (e.g. family J and family T hap-
logroups are super-haplogroup JT) for secondary analysis
assessments. Haplogroups that occurred in fewer than 10
subjects in a given association analysis were not analyzed
in that specific analysis. All cases were examined for pop-
ulation stratification prior to conducting this study [3].
Statistical analysis
Associations of mitochondrial haplogroups with risk
of clinical DLB and LBD-hDLB, each separately versus
controls, were examined using logistic regression mod-
els that were adjusted for age and sex. Odds ratios (ORs)
and 95% confidence intervals (CIs) were estimated. Addi-
tionally, clinical DLB and LBD-hDLB series were com-
bined into one overall DLB series, and associations of
haplogroups with risk of DLB in comparison to controls
were assessed. The 48 cases that were present in both the
clinical DLB series and the LBD-hDLB series, were only
included once in the overall DLB series.
In the LBD-hDLB series, associations of haplogroups
with each different neuropathological outcome meas-
ure were assessed using age at death and sex-adjusted
regression models that are appropriate for the nature
of the given outcome measure. Specifically, associa-
tions of haplogroups with dorsolateral and ventromedial
putaminal TH-ir were examined using linear regression
models, where due to their skewed distributions, lat-
eral putaminal TH-ir was considered on the logarithm
(base-10) scale and medial putaminal TH-ir was con-
sidered on the square root scale. Regression coefficients
and 95% CIs were estimated and are interpreted as the
additive increase on the mean outcome measure (on the
logarithm or square root scale) for the given haplogroup.
Associations of haplogroups with ventrolateral and
medial SN neuronal loss scores were assessed using pro-
portional odds logistic regression models. ORs and 95%
CIs were estimated and are interpreted at the multiplica-
tive increase on the odds or a more severe neuronal loss
score for the given haplogroup. Neuronal loss scores ≤1
(ventrolateral) and ≤1.5 (medial) were combined into one
category in proportional odds logistic regression analysis
due to their low frequencies. Associations between hap-
logroups and cortical LB counts were evaluated using
negative binomial regression models. Multiplicative
effects and 95% CIs were estimated and are interpreted as
the multiplicative increase on the mean LB count for the
given haplogroup. Finally, binary logistic regression mod-
els were used to assess associations between haplogroups
and LBD subtype. ORs and 95% CIs for presence of dif-
fuse LBD were estimated.
We utilized a Bonferroni correction for multiple test-
ing separately for each outcome measure in the primary
analysis that did not involve super-haplogroups (P-values
<0.05 were considered statistically significant in second-
ary super-haplogroup analysis). As haplogroups that
occurred in less than 10 subjects in a given association
analysis were not analyzed in that specific analysis, and
the degree of missing data differed between outcomes,
the Bonferroni-corrected statistical significance level
correspondingly varied between outcomes (see table
footnotes for details). All statistical tests were two-sided.
Statistical analyses were performed using R Statisti-
cal Software (version 3.6.2; R Foundation for Statistical
Computing, Vienna, Austria).
Results
Associations of haplogroups with risk of clinical DLB and
LBD-hDLB are detailed in Table 2. After adjusting for
age and sex, no statistically significant associations were
observed after Bonferroni correction (P <0.0024 con-
sidered significant). However, a nominally significant (P
<0.05) association was reported between sub-haplogroup
H and lower risk of clinical DLB (OR=0.61, P=0.006).
This association was consistent when additionally adjust-
ing for the number of apolipoprotein E4 (APOE4) alleles
(OR=0.59, P = 0.004). No other associations approached
statistical significance in any other series (all P ≥ 0.057,
Table 2). Interestingly though, despite mitochondrial
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 6 of 15
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Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 7 of 15
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Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 8 of 15
sub-haplogroup H not being strongly associated with
LBD-hDLB (OR=0.87, P = 0.34), the protective associa-
tion observed in the clinical DLB series was almost nomi-
nally significant when examining the combined DLB
series (OR=0.78, P = 0.057) (Table 2).
In an exploratory analysis, we also evaluated asso-
ciations of haplogroups with disease risks separately for
males and females (Additional File 1: Tables S1 and S2).
The aforementioned protective association between sub-
haplogroup H and clinical DLB was observed relatively
consistently in males (OR=0.64, P = 0.042) and females
(OR=0.56, P = 0.066). Also, haplogroup HV/HV0a was
suggestively associated with an increased risk of LBD-
hDLB in males (OR=3.33, P = 0.044) and haplogroup V
suggested an association with increased risk of both clini-
cal DLB (OR = 4.29, P = 0.009) and overall DLB (OR =
3.56, P = 0.006) in females.
Associations of individual mitochondrial haplogroups
with putaminal TH-ir and SN neuronal loss (Table 3),
cortical Lewy body counts (Table 4), and diffuse LBD
subtype (Additional File1: Table S3) were also assessed.
No statistically significant associations were observed
after correcting for multiple testing. A nominally sig-
nificant association was noted between sub-haplogroup
H and a less severe ventrolateral SN neuronal loss score
(OR = 0.44, P = 0.033, Table 3) and also between sub-
haplogroup T2 and a lower superior temporal LB count
(multiplicative effect: 0.76, P = 0.044, Table 4). Both
nominally significant associations remained consistent
when additionally adjusting for the number of APOE ε4
alleles (P = 0.033 and P = 0.036 respectively). No asso-
ciations between super-haplogroups and either risk of
DLB or neuropathological outcomes were observed (all P
≥ 0.21).
Discussion
Efforts to understand the genetic etiology of DLB have
identified shared genetic markers between PD and AD
[3, 23, 27, 34]. Physiologically, mitochondrial dysfunction
is consistently reported in synucleinopathies [32] and
mitochondrial phenotypes are predisposed by variation
in mtDNA [15, 16]. Interestingly, LB pathology is more
prevalent in older individuals with mitochondrial dis-
ease compared to controls [9], further emphasizing the
importance of mtDNA in disease pathology.
Acknowledging this, our examinations of mtDNA vari-
ation, in the form of mitochondrial haplogroups, with
DLB risk and neuropathological measures in this study
reported no statistically significant associations after
applying Bonferroni correction. However, mtDNA sub-
haplogroup H reported a suggestive protective effect
with clinical DLB risk (OR=0.61, P = 0.006) which was
not observed in LBD-hDLB cases (OR=0.87, P = 0.34).
Interestingly though, sub-haplogroup H also indicated
an association with less severe ventrolateral SN neuronal
loss (OR=0.44, P = 0.033) in LBD-hDLB cases.
Mitochondrial haplogroup H is the most common hap-
logroup in European populations, accounting for more
than 40% of individuals [45]. Mitochondrial haplogroup
H has more than 80 sub-haplogroups which are predomi-
nantly defined by variation in mtDNA coding regions
[48], with sub-haplogroups H1 and H3 being the most
common. Mitochondrial haplogroup H has previously
been associated with increased risk of PD [21] and DLB
[5]. Notably, the results reported in Hudson and col-
leagues’ study was not an exact association of haplogroup
H with PD risk, as they grouped haplogroup H and hap-
logroup V cases together. Although this was statistically
more reliable, genetically the results do not clarify what
mtDNA variants are driving PD risk because haplogroups
H and V are phylogenetically related but are genetically
different. Moreover, Chinnery et al. reported increased
risk of haplogroup H with DLB in only 84 DLB cases and
did not evaluate H sub-haplogroups. In this study, we
assessed associations between mtDNA haplogroups and
sub-haplogroups with DLB risk more comprehensively
and in a much larger DLB cohort. It is possible that the
elevated risk of DLB with mtDNA haplogroup H back-
ground reported by Chinnery et al. may be an artefact of
common H sub-haplogroups, such as H1 and H3, induc-
ing more detrimental risk outcomes with DLB—which is
also observed in our data. Overall, these studies demon-
strate the functional heterogeneity even within a given
haplogroup and reinforce the need to stratify haplo-
groups into sub-haplogroups in genetic studies [13]. Rep-
lication will be important to validate our findings.
Disappointingly, we did not replicate the sub-haplo-
group H association with reduced DLB risk in pathologi-
cally confirmed LBD-hDLB cases, nor in the combined
cases group. This may be because one cohort was clini-
cally defined whereas the other cohort was neuropatho-
logically defined. Neuropathologically confirmed cases
were included in this study if they were deemed as hav-
ing a high likelihood of clinical dementia—which was
determined from pathology propensity in cortical regions
and available medical records. It is possible that the neu-
ropathologically defined LBD-hDLB cohort may also
contain clinical PDD cases. PDD has a different disease
course to DLB and is diagnosed when dementia devel-
ops more than a year after parkinsonism onset and can
be considered a much slower progression of dementia
than DLB [30]. Interestingly, this data suggests that mito-
chondrial sub-haplogroup H may be protective against
DLB but not PDD, which may suggest that mitochondrial
background influences rate of dementia progression in
PD.
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 9 of 15
Table 3 Associations of haplogroups with putaminal TH‑ir (dorsolateral and ventromedial) and substantia nigra neuronal loss
(ventrolateral and medial)
Mitochondrial
DNA Haplogroup
Haplogroup
frequency,
No. (%),
242
N
=
Association with dorsolateral
putaminal TH
ir
−
Association with
ventromedial putaminal
TH
ir
−
Association with
ventrolateral substantia
nigra
neuronal loss score
Association with
medial substantia nigra
neuronal
loss score
Regression
coefficient (95%
CI)
P
−
value Regression
coefficient (95%
CI)
P
−
value OR (95% CI)
P‑value OR (95% CI)
P‑value
0 (0.0%)
0 (0.0%)
8 (3.3%)
2 (0.8%)
4 (1.7%)
1 (0.4%)
8 (3.3%)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
91 (37.6%)
0.05 (
−
−
0.15, 0.05)
0.32
0.11 (
−
−
0.40, 0.18)
0.45
0.80 (0.43, 1.46)
0.46
1.12 (0.63, 1.97)
0.70
0.92, 0.34)
0.36
0.90 (0.25, 3.31)
0.88
0.55
0.57
–
–
–
–
0.81
–
0.78
–
–
–
0.05 (
0.09, 0.18)
0.11 (
0.27, 0.49)
0.23, 0.09)
0.58, 0.32)
0.51
0.42
–
−
0.13 (
−
−
–
−
0.07 (
−
−
–
0.04 (
−
−
–
–
–
0.26, 0.19)
0.75
–
–
–
0.29 (
−
−
–
–
–
0.06 (
−
–
0.07 (
−
0.09, 0.21)
0.42
0.05 (
0.37, 0.47)
−
–
0.09, 0.22)
0.39
–
0.06 (
0.38, 0.50)
−
–
–
–
−
–
–
−
–
–
–
–
–
–
0.19, 0.13)
0.70
–
–
0.82
0.47
–
–
–
–
–
–
−
–
–
−
–
–
–
0.02 (
0.15, 0.19)
−
0.05 (
−
0.18, 0.08)
0.14 (
0.35, 0.63)
−
0.15 (
−
0.53, 0.23)
0.44 (0.21, 0.93)
0.033
0.54 (0.26, 1.10)
0.090
1.33 (0.48, 3.68)
0.58
1.28 (0.52, 3.13)
0.59
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.87 (0.36, 2.09)
0.75
0.62 (0.28, 1.39)
0.25
–
–
–
–
0.93 (0.37, 2.33)
0.87
0.72 (0.31, 1.69)
0.46
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0.58
0.44
–
–
–
–
–
–
–
–
–
–
–
1.35 (0.42, 4.30)
1.75 (0.72, 4.24)
0.62
0.21
2.52 (0.72, 8.80)
1.21 (0.57, 2.55)
0.15
0.62
–
–
–
–
–
–
–
–
–
–
–
–
0.03 (
−
0.07 (
−
0.52, 0.38)
0.74
1.42 (0.49, 4.11)
0.52
2.60 (0.87, 7.81)
0.088
0.04 (
−
−
–
–
0.20, 0.12)
0.66
–
–
0.17 (
−
−
–
–
0.00 (
−
0.18, 0.17)
0.98
0.04 (
0.45, 0.54)
−
0.62, 0.28)
0.45
1.59 (0.56, 4.51)
0.39
1.41 (0.56, 3.53)
0.47
–
–
–
–
–
–
–
–
–
–
0.87
0.60 (0.23, 1.59)
0.31
1.29 (0.49, 3.39)
0.61
For associations with putaminal TH-ir, regression coefficients, 95% CIs, and p-values result from linear regression models that were adjusted for age at death and sex,
where due to their skewed distributions, lateral putaminal TH-ir was considered on the logarithm (base-10) scale and medial putaminal TH-ir was considered on the
square root scale. Regression coefficients are interpreted as the additive increase on the mean outcome measure (on the logarithm or square root scale) for the given
haplogroup. For associations with ventrolateral and medial substantia nigra neuronal loss scores, ORs, 95% CIs, and p-values result from proportional odds logistic
regression models; ORs are interpreted at the multiplicative increase on the odds or a more severe neuronal loss score for the given haplogroup. After applying a
Bonferroni correction for multiple testing separately for each outcome measure, P-values < 0.0045 (associations with lateral putaminal TH-ir, medial putaminal TH-ir,
and ventrolateral substantia nigra neuronal loss score) and < 0.0050 (association with medial substantia nigra neuronal loss score) were considered statistically
significant.
a Haplogroups that occurred in < 10 subjects in a given association analysis not examined in that analysis. TH-ir
confidence interval; OR
tyrosine hydroxylase immunoreactivity; CI
odds ratio.
=
=
=
Na
N1a
Ia
Wa
Xa
R and R0a
HV and HV0aa
H, H1, H2, H3 and
H4
H
H1
H2a
H3a
H4a
Va
JTa
J, J1, J1d, J2a and
J2b
Ja
J1
J1ad
J2aa
J2ba
T, T1 and T2
T1
T1a
T2
U, U1, U3, U5, U6
and U8b’c
U1
U1a
U3a
U5
U6a
U8b’ca
K
40 (16.5%)
27 (11.2%)
8 (3.3%)
12 (5.0%)
4 (1.7%)
8 (3.3%)
0 (0.0%)
26 (10.7%)
0 (0.0%)
23 (9.5%)
0 (0.0%)
2 (0.8%)
1 (0.4%)
30 (12.4%)
0 (0.0%)
4 (1.7%)
26 (10.7%)
41 (16.9%)
12 (5.0%)
2 (0.8%)
1 (0.4%)
25 (10.3%)
0 (0.0%)
1 (0.4%)
23 (9.5%)
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 10 of 15
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Page 11 of 15
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Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 12 of 15
Interestingly, we did observe a suggestive associa-
tion between mtDNA sub-haplogroup H background
and less severe ventrolateral SN neuronal loss in LBD-
hDLB cases. Albeit not statistically significant, this data
is important because SN degeneration is a classical
hallmark of PD and may behave as an important medi-
ator in LB spread in LBD and may be a defining media-
tor between DLB and PDD. This concept also supports
the rationale that sub-haplogroup H may be protective
against DLB risk, as reported in this study. Functionally
this could be explained by SN cells being more sensitive
to physiological pressures than other neuronal types.
More specifically, dopaminergic SN cells are highly
metabolically active, long and thin, and have little to no
myelination [2], and they heavily rely on healthy mito-
chondria for efficient OXPHOS to ensure sufficient ATP
is produced to maintain their metabolic capacity. Mito-
chondria carrying mtDNA haplogroup H are reported
to have the most efficient OXPHOS coupling capac-
ity in all European haplogroups and produce more ATP
and ROS than other groups [50]. This may be advanta-
geous in protecting SN cells from accumulating LB with
age. On the contrary though, SN cells with a haplogroup
H background may be more susceptible to physiological
pressures as functional studies in cybrid cell lines have
demonstrated these cells have an increased susceptibility
to oxidative stress compared to non-haplogroup H cells
[31]. This suggests that mitochondrial background may
provide cell or regional-specific biological benefits, but
under additional physiological pressures may enhance
disease progression.
As LBD pathology is very heterogenous and presents
with pathological aggregates of tau, beta-amyloid, and
TDP-43 proteins, it is important to also consider the
role nuclear genetic risk factors play in driving disease
risk relative to mtDNA background. More specifically,
APOE4 is consistently an increased genetic risk factor for
clinical DLB [18, 40] and AD [23, 41], and APOE4 influ-
ences LB pathology independently to AD pathology [8].
The mtDNA haplogroup associations reported in this
study were all adjusted for APOE4 allele status which did
not change any observations after adjustments. Reas-
suringly, mtDNA haplogroup associations in DLB and
AD have been reported independent of APOE4 status
in prior studies [5], suggesting both mitochondrial and
nuclear genomic background influence disease pheno-
types. Future studies should consider evaluating major
nuclear genetic risk factors relative to mitochondrial
genetic background to avoid any possible bias.
Several limitations of our study are important to note.
The main limitation being that even though the sample
size of DLB and LBD-hDLB cases are relatively large given
the prevalence of DLB, sample numbers are small for a
genetic association study and therefore the possibility of
a type II error is important to consider. This is especially
true when considering adjustment for multiple testing
and for rare haplogroups. In addition, although all cases
in this study were examined for population stratification
prior to conducting this work [3]; population stratifica-
tion in the control cohort may influence false positive
findings (noting all subjects carried European mtDNA
haplogroups). Global access to well described cohorts of
DLB cases is required and validation of this work in larger
cohorts of DLB and LBD-hDLB cases will be important to
further investigate the role mitochondrial haplogroup H
has in DLB risk and neuropathological development.
Conclusions
We have conducted a comprehensive study of the role of
mitochondrial genomic variation, in the form of mito-
chondrial haplogroups and sub-haplogroups, in clinical
DLB and pathologically confirmed LBD-hDLB. Moreo-
ver, this is one of the first studies to explore the associa-
tion of mtDNA background with neuropathological LB
counts and neuronal loss measures in LBD-hDLB brains.
Our data suggests that mitochondrial sub-haplogroup H
may be protective against clinical DLB risk, independent
of APOE4 background, and this may be indirectly influ-
enced by the suggestive association that sub-haplogroup
H is protective against neuronal loss in substantia nigra
tissue. Additional assessments and replication studies are
warranted to further validate and expand on this data.
Abbreviations
AD: Alzheimer’s disease; APOE4: Apolipoprotein E4 allele; aSyn: Alpha‑synu‑
clein; ATP: Adenosine triphosphate; CIs: Confidence intervals; DLB: Dementia
with Lewy bodies; DNA: Deoxyribonucleic acid; LB: Lewy bodies; LBD: Lewy
body disease; LBD‑hDLB: Lewy body disease with a high likelihood of having
clinical DLB; mtDNA: Mitochondrial DNA; NFTs: Neurofibrillary tangles; ORs:
Odds ratios; OXPHOS: Oxidative phosphorylation; PDD: Parkinson’s disease
dementia; ROS: Reactive oxygen species; SN: Substantia nigra; SP: Senile
plaques; TH‑ir: Tyrosine hydroxylase immunoreactivity.
Supplementary Information
The online version contains supplementary material available at https:// doi.
org/ 10. 1186/ s40478‑ 022‑ 01399‑4.
Additional file 1. Supplementary Tables.
Acknowledgements
We would like to thank all those who contributed towards our research,
particularly the patients and families who donated brain, blood, and DNA
samples—without their donation this study would not have been possible.
We also thank Audrey Strongosky for processing research subjects’ consents,
drawing bloods, and handling collection procedures, as well as Linda G. Rous‑
seau, Virginia R. Phillips, and Monica Castanedes‐Casey for their continuous
commitment, technical support, and teamwork. Mayo Clinic is an American
Parkinson Disease Association (APDA) Mayo Clinic Information and Referral
Center, an APDA Center for Advanced Research, and the Mayo Clinic Lewy
Body Dementia Association (LBDA) Research Center of Excellence.
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 13 of 15
Author contributions
Rebecca R. Valentino designed mitochondrial DNA genotyping assays,
performed all genotyping and quality control assessments, and drafted the
manuscript. Chloe Ramnarine assisted with genotyping samples. Michael G.
Heckman and Patrick W. Johnson performed all statistical analysis and Michael
G. Heckman provided manuscript improvements. Alexandra I. Soto‑Beasley
provided training for genotyping design and methods. Ronald L. Walton
prepared all genomic DNA extracts for all samples from donated human
tissues. Shunsuke Koga and Dennis W. Dickson provided brain tissue samples
for all cases and Dennis W. Dickson performed neuropathological assessments
of LBD cases. Koji Kasanuki conducted all TH‑ir stainings and evaluations in
LBD cases. Ryan J. Uitti, Julie A. Fields, P, Hugo Botha, Vijay K. Ramanan, Kejal
Kantarci, Val J. Lowe, Clifford R. Jack, Nilufer Ertekin‑Taner, Rodolfo Savica,
Jonathan Graff‑Radford, Ronald C. Petersen, Joseph E. Parisi, R. Ross Reichard,
Neill R. Graff‑Radford, Tanis J. Ferman, Bradley F. Boeve, Zbigniew K. Wszolek,
recruited clinical patients, characterized clinical, radiological, and pathologi‑
cal phenotypes and organized blood collections. Zbigniew K. Wszolek also
obtained funding for the collection of control subjects, recruited all control
subjects, and maintained IRB protocols. Owen A. Ross led the study and
oversaw all method developments and analysis. He takes responsibility for the
integrity of the data and the accuracy of the data analysis. All authors read and
approved the final manuscript.
Funding
Shunsuke Koga receives funding from CurePSP and the Rainwater Charita‑
ble Foundation.. Koji Kasanuki is supported by JSPS KAKENHI grant number
19K17119 (Japan). Melissa E. Murray receives funding from the State of
Florida (20A22), LEADS Neuropathology Core (U01AG057195), and the Chan
Zuckerberg Initiative Collaborative Pairs Grant, which are paid directly to the
institute. Julie A. Fields is supported by NIA (U54AG 44170, RF1AG 57547,
U19AG 63911, R01AG 68128, U01AG 45390, R43AG 65088) and NINDS grants
(UH3NS 95495, U01NS 100620), as well as Boston Scientific and PCORI (CER‑
1306‑01897). Funding is paid directly to the institute. Hugo Botha is supported
by NIH/NIDCD (R01 DC12519‑06), NIA (U19 AG63911‑02 and P30 AG62677‑
02), and NINDS (RO3 NS114365‑01) grants, of which are paid directly to the
institute. Vijay K. Ramanan receives research support from the NIH (NIA, NCI).
Kejal Kantarci is supported by NIH grants (P30 AG62677, U01 NS100620, RF1
AG57547); Alzheimer’s Drug Discovery Foundation; Robert H. and Clarice
Smith and Abigail Van Buren Alzheimer s Disease Research Program of the
Mayo Foundation; Schuller Foundation; and the Katrine B. Andersen Professor‑
ship. Clifford R. Jack is supported by NIH, GHR Foundation, and the Alexan‑
der Family Alzheimer’s Disease Research Professorship of the Mayo Clinic.
Jonathan Graff‑Radford is supported by NIH. Nilufer Ertekin‑Taner receives
funding from NIH‑NIA (U01 AG046139, RF1 AG051504, and R01 AG051504).
Jonathan Graff‑Radford and Ronald C. Petersen receive funding from NIH.
R. Ross Reichard is supported by funding from Mayo Clinic, NIA, NHLBI, and
ADRC, which are all paid to the institution. Bradley F. Boeve is supported by
NIH grants (P30 AG62677, U01 NS100620, R34 AG056639); the Robert H. and
Clarice Smith and Abigail Van Buren Alzheimer s Disease Research Program of
the Mayo Foundation; the Lewy Body Dementia Association; the Mayo Clinic
Dorothy and Harry T. Mangurian Jr. Lewy Body Dementia Program; the Little
Family Foundation; the Turner Foundation. Zbigniew K. Wszolek is partially
supported by the Mayo Clinic Center for Regenerative Medicine, Mayo Clinic
in Florida Focused Research Team Program, the gifts from The Sol Goldman
Charitable Trust, and the Donald G. and Jodi P. Heeringa Family, the Haworth
Family Professorship in Neurodegenerative Diseases fund, and The Albertson
Parkinson’s Research Foundation. Owen A. Ross and Dennis W. Dickson are
both supported by NINDS Tau Center without Walls Program (U54‑NS100693)
and NIH (UG3‑NS104095). DWD receives research support from the NIH (P30
AG062677; U54‑NS100693; P01‑AG003949), CurePSP, the Tau Consortium, and
the Robert E. Jacoby Professorship. OAR is supported by NIH (P50‑NS072187;
R01‑ NS078086; U54‑NS100693; U54‑ NS110435), DOD (W81XWH‑17‑1‑0249)
The Michael J. Fox Foundation, The Little Family Foundation, the Mangurian
Foundation Lewy Body Dementia Program at Mayo Clinic, the Turner Founda‑
tion, Mayo Clinic Foundation, and the Center for Individualized Medicine.
Mayo Clinic is also an LBD Center without Walls (U54‑NS110435). Samples
included in this study were clinical controls from Mayo Clinic Rochester and
Mayo Clinic Jacksonville as part of the Alzheimer’s Disease Research Center
(P30 AG062677) and the Mayo Clinic Study of Aging (U01 AG006786) or tissue
donations to the Mayo Clinic Brain Bank in Jacksonville which is supported by
CurePSP and Mayo Clinic funding.
Availability of data and materials
The datasets that were used and analyzed during the current study are avail‑
able from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
This research was approved by the Mayo Clinic Institutional Review Board and
all human participants involved in this work provided written consent prior to
study commencement.
Consent for publication
Not applicable.
Competing interests
Melissa E. Murray personally receives consulting fees from VID Radiopharma‑
ceuticals and has received reimbursements for Mild Cognitive Impairment and
NIH study section. MEM is also Co‑chair of Digital Pathology working group,
Chair/Immediate Past Chair of Atypical Alzheimer’s disease Professional Inter‑
est Area group and is on the organizing committee of the Southeastern Neu‑
rodegeneration Conference. Julie A. Fields is on the Oversight and Monitoring
Board for the SWAN‑Aging study that is funded by the NIA but receives no
compensation. Hugo Botha received free AAN registration for the Geschwind
Award. Vijay K. Ramanan consults for Bayer Schering Pharma, Piramal Life
Sciences, Life Molecular Imaging, Eisai Inc., AVID Radiopharmaceuticals, and
Merck Research, and receives research support from GE Healthcare, Siemens
Molecular Imaging, and AVID Radiopharmaceuticals. Kejal Kantarci consults
Biogen Inc, and receives research support from Avid Radiopharmaceuticals
and Eli Lilly, and funding from NIH and Alzheimer’s Drug Discovery Founda‑
tion. Val J. Lowe receives research support from AVID Radiopharmaceuticals
and Siemens Healthcare and is a consultant for AVID Radiopharmaceuticals,
Eisai, Inc., Bayer Schering Pharma, and Merck Research. Clifford R. Jack is on
the Roche advisory board but receives no payments. CRJ is also on the writing
committee for the NIA AA research framework. Jonathan Graff‑Radford has
received payment from American Academy of Neurology for lecturing. Nilufer
Ertekin‑Taner received payments as a conference speaker for 11th ISABS
and the Department of Electrical and Computer Engineering and Computer
Science, University of Urbana‑Champaigne, and additionally for being a
visiting professor at the Department of Neurology, Indiana University School
of Medicine, Bloomington, Indiana, as well as at National Center for Geriatrics
and Gerontology Nagoya, Japan. NET is a Framingham Heat Study Executive
Board member and NIH/NIA TREAT‑AD external advisory Board member. Jona‑
than Graff‑Radford received a payment from American Academy of Neurology
for presenting a lecture. Ronald C. Petersen receives royalties from Oxford
University Press in UpToDate and consulting fees from Roche, Merck, Biogen,
Genentech, and Eisai. RCP is also on the Genentech DSMB advisory board. R.
Ross Reichard is the President of American Association of Neuropathologists.
Neill R. Graff‑Radford receives research support from multicenter studies with
Biogen, Abbvie, Lilly, and Novartis, and receives personal royalties for a chapter
on NPH in UpToDate. Bradley F. Boeve has served as an investigator for clinical
trials sponsored by Biogen, Alector, and EIP Pharma, and serves on the Scien‑
tific Advisory Board of the Tau Consortium. Zbigniew K. Wszolek serves as PI or
Co‑PI on Biohaven Pharmaceuticals, Inc. (BHV4157‑206 and BHV3241‑301) and
Neuraly, Inc. (NLY01‑PD‑1), and serves as Co‑PI of the Mayo Clinic APDA Center
for Advanced Research and as an external advisory board member for the
Vigil Neuroscience, Inc. All other authors declare that they have no competing
interests to report.
Author details
1 Department of Neuroscience, Mayo Clinic, 4500 San Pablo Road, Jacksonville,
FL 32224, USA. 2 Division of Biomedical Statistics and Informatics, Mayo Clinic,
Jacksonville, FL 32224, USA. 3 Department of Neuropsychiatry, St. Marianna
University School of Medicine, Kanagawa, Japan. 4 Department of Neurology,
Mayo Clinic, Jacksonville, FL 32224, USA. 5 Department of Psychiatry and Psy‑
chology, Mayo Clinic, Rochester, MN 55905, USA. 6 Department of Neurology,
Mayo Clinic, Rochester, MN 55905, USA. 7 Department of Radiology, Mayo
Clinic, Rochester, MN 55905, USA. 8 Laboratory Medicine and Pathology, Mayo
Clinic, Rochester, MN 55905, USA. 9 Department of Psychiatry and Psychology,
Mayo Clinic, Jacksonville, FL 32224, USA. 10 Department of Clinical Genomics,
Mayo Clinic, Jacksonville, FL 32224, USA.
Valentino et al. Acta Neuropathologica Communications (2022) 10:103
Page 14 of 15
Received: 26 April 2022 Accepted: 18 June 2022
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| null |
10.1371_journal.pone.0267316.pdf
|
Data Availability Statement: Data were submitted
to European Bioinformatic Institute, EBI (https://
www.ebi.ac.uk/). The accession was assigned as
ArrayExpress accession E-MTAB-11136.
Additionally, this information is available as a
footnote in Tables 2 and 3 of the manuscript.
|
Data were submitted to European Bioinformatic Institute, EBI ( https:// www.ebi.ac.uk/ ). The accession was assigned as ArrayExpress accession E-MTAB-11136. Additionally, this information is available as a footnote in Tables 2 and 3 of the manuscript.
|
RESEARCH ARTICLE
Transcriptomic analysis of chloride tolerance
in Leptospirillum ferriphilum DSM 14647
adapted to NaCl
Javier Rivera-Araya1, Thomas Heine2, Renato Cha´ vez1, Michael Schlo¨ mann2,
Gloria Levica´ nID
1*
1 Biology Department, Faculty of Chemistry and Biology, University of Santiago of Chile (USACH), Santiago,
Chile, 2 Environmental Microbiology, Institute of Biosciences, TU Bergakademie Freiberg, Freiberg,
Germany
* [email protected]
Abstract
Chloride ions are toxic for most acidophilic microorganisms. In this study, the chloride toler-
ance mechanisms in the acidophilic iron-oxidizing bacterium Leptospirillum ferriphilum DSM
14647 adapted to 180 mM NaCl were investigated by a transcriptomic approach. Results
showed that 99 genes were differentially expressed in the adapted versus the non-adapted
cultures, of which 69 and 30 were significantly up-regulated or down-regulated, respectively.
Genes that were up-regulated include carbonic anhydrase, cytochrome c oxidase (ccoN)
and sulfide:quinone reductase (sqr), likely involved in intracellular pH regulation. Towards
the same end, the cation/proton antiporter CzcA (czcA) was down-regulated. Adapted cells
showed a higher oxygen consumption rate (2.2 x 10−9 ppm O2 s-1cell-1) than non-adapted
cells (1.2 x 10−9 ppm O2 s-1cell-1). Genes coding for the antioxidants flavohemoprotein and
cytochrome c peroxidase were also up-regulated. Measurements of the intracellular reactive
oxygen species (ROS) level revealed that adapted cells had a lower level than non-adapted
cells, suggesting that detoxification of ROS could be an important strategy to withstand
NaCl. In addition, data analysis revealed the up-regulation of genes for Fe-S cluster biosyn-
thesis (iscR), metal reduction (merA) and activation of a cellular response mediated by dif-
fusible signal factors (DSFs) and the second messenger c-di-GMP. Several genes related
to the synthesis of lipopolysaccharide and peptidoglycan were consistently down-regulated.
Unexpectedly, the genes ectB, ectC and ectD involved in the biosynthesis of the compatible
solutes (hydroxy)ectoine were also down-regulated. In line with these findings, although
hydroxyectoine reached 20 nmol mg-1 of wet biomass in non-adapted cells, it was not
detected in L. ferriphilum adapted to NaCl, suggesting that this canonical osmotic stress
response was dispensable for salt adaptation. Differentially expressed transcripts and
experimental validations suggest that adaptation to chloride in acidophilic microorganisms
involves a multifactorial response that is different from the response in other bacteria
studied.
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OPEN ACCESS
Citation: Rivera-Araya J, Heine T, Cha´vez R,
Schlo¨mann M, Levica´n G (2022) Transcriptomic
analysis of chloride tolerance in Leptospirillum
ferriphilum DSM 14647 adapted to NaCl. PLoS
ONE 17(4): e0267316. https://doi.org/10.1371/
journal.pone.0267316
Editor: Benjamin J. Koestler, Western Michigan
University, UNITED STATES
Received: May 21, 2021
Accepted: April 6, 2022
Published: April 29, 2022
Copyright: © 2022 Rivera-Araya et al. This is an
open access article distributed under the terms of
the Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data were submitted
to European Bioinformatic Institute, EBI (https://
www.ebi.ac.uk/). The accession was assigned as
ArrayExpress accession E-MTAB-11136.
Additionally, this information is available as a
footnote in Tables 2 and 3 of the manuscript.
Funding: This work was funded by Fondo Nacional
de Desarrollo Cientı´fico y Tecnolo´gico, FONDECYT,
from the government of Chile (Grant N˚ 1170799/
1211386). JR-A and RC received support from
DICYT_USACH (Proyecto POSTDOC_DICYT, COD
PLOS ONE | https://doi.org/10.1371/journal.pone.0267316 April 29, 2022
1 / 18
PLOS ONE022043CR_POSTDOC). MS thanks Dr. Erich
Kru¨ger foundation for generous support as part of
the Biohydrometallurgical Center Freiberg. The
funders had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Transcriptomic of Na-Cl adapted Leptospirillum ferriphilum
Introduction
Leptospirillum ferriphilum is a Gram-negative, and obligately aerobic iron-oxidizing chemoau-
totroph able to thrive in a pH range from 1.3 to 2.0 [1]. This bacterium belongs to the bioleach-
ing microbial communities involved in solubilization of metals from sulfide ores [2,3].
L. ferriphilum, like most acidophilic microorganisms, shows extreme sensitivity to chloride and
other anions (with the notable exception of sulfate) [4]. Acidophiles possess positive membrane
potentials which facilitates the influx of permeable anions into cells [5]. The mass entry of chloride
and other anions favor the influx of protons causing the collapse of positive internal potential, and
therefore the disruption of the proton motive force, as well as acidification of the cytoplasm and a
general detrimental effect in the cell [4,6,7].Nevertheless, the molecular basis of the response to
chloride in L. ferriphilum and other acidophilic microorganisms are poorly understood.
The mechanisms operating in acidophiles in response to chloride have been investigated just
during recent years. In L. ferriphilum, the osmotic stress induced by sodium chloride leads to the
up-regulation of genes encoding the potassium transporter Kdp, and for the biosynthesis or uptake
of compatible solutes such as (hydroxy)ectoine and trehalose [8–11]. In members of the genus
Acidithiobacillus, like At. ferrooxidans and At. caldus, the use of proline and betaine as osmoprotec-
tants has been reported [12,13], whilst the moderately halotolerant Acidihalobacter prosperus has a
response based on the synthesis and uptake of ectoine [14]. In addition, A. prosperus also seems to
have developed a more specific adaptive response that involves changes in the amino acid compo-
sition of rusticyanin to protect the copper ion present in the active site of this protein [14].
To respond to cytoplasm acidification induced by chloride exposure, acidophiles synthesize
a greater number and diversity of cation/H+ antiporters, proteins that modify the cell mem-
brane, and proteins of the electron-transport chain. These changes result in the presumed
export of protons, at the expense of increasing the respiratory rate [1,4,14,15]. Recently,
Rivera-Araya et al. [4] described that exposure of L. ferriphilum to chloride led to a significant
increase in intracellular reactive oxygen species (ROS). It is believed that ROS enhancement is
produced by the increment in respiratory activity and by disruption of metallic centers of pro-
teins due to osmotic imbalance. In addition, Fe2+ and other cations can trigger Fenton chemis-
try and induce the generation of hydroxyl radicals [16]. In agreement with these observations,
the activation of antioxidant mechanisms seems to play an important complementary role in
the response to chloride. The exposure of L. ferriphilum to 50–150 mM NaCl has been shown
to up-regulate the activity of thioredoxin and cytochrome c peroxidase [4]. Similarly, in other
microorganisms, like At. caldus and Acidimicrobium ferrooxidans, the up-regulation of antiox-
idative proteins in response to NaCl has also been reported [7,13].
Therefore, based on the evidence from the individual studies described above, it is possible to
state that in L. ferriphilum and other acidophilic microorganisms, the exposure to chloride trig-
gers a response that involves the participation of different mechanisms to withstand osmotic, acid
and oxidative stress. However, it is envisioned that a chloride challenge activates a global and
complex physiological response that has yet to be well deciphered. In the present study, we report
on transcriptomic analyses conducted in L. ferriphilum DSM 14647 adapted and exposed to 180
mM NaCl. This study also included the measurements of specific parameters such as oxygen con-
sumption rate, intracellular pH, and ROS and (hydroxy)ectoine content.
Materials and methods
Bacterial strains and growth conditions
L. ferriphilum DSM 14647 [17] used in this study was provided by Leibniz Institute DSMZ.
The bacterial cells were cultured in DSMZ 882 medium (pH 1.8) supplemented with 72 mM
PLOS ONE | https://doi.org/10.1371/journal.pone.0267316 April 29, 2022
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
ferrous sulfate (FeSO4�7H2O). Bacterial growth was carried out in Erlenmeyer flasks at 180
rpm and 37˚C.
Adaptation of L. ferriphilum DSM 14647 to 180 mM NaCl
The adaptation of L. ferriphilum DSM 14647 was performed in growth medium (see above)
with increasing NaCl concentrations (50-100-120-150-180 mM) and supplementation with 1
mM ectoine as a compatible solute (Sigma-Aldrich). The adaptation was performed sequen-
tially and with 3 passages per salt concentration. Cultures were maintained until the late expo-
nential phase and used to inoculate fresh NaCl and ectoine-containing medium (10% v/v) and
generate a new culture. After the 180 mM NaCl-adapted culture had been obtained, the com-
patible solute was gradually (1–0.5–0 mM) removed from the medium. Adapted cells were
constantly grown in the presence of 180 mM NaCl.
Growth curve determination
The experiment was carried out in 250 mL Erlenmeyer flasks. Each flask contained 100 mL
DSMZ 882 medium with 0 or 180 mM NaCl for non-adapted and adapted cells, respectively.
Samples were taken periodically for determination of cell growth, which was measured by
direct microscopic counting using a modified Neubauer chamber. The initial cell density was
1 x 106 cells mL-1.
Measurement of minimum inhibitory concentrations (MIC) of NaCl
This assay was carried out on planktonic cells according to Rivera-Araya et al. [11] with some
modifications. Briefly, to determine the MIC of NaCl, non-adapted and adapted cells of L. fer-
riphilum were cultured in DSMZ 882 medium at pH 1.4, 1.8, 2.4 or 3.0 in the presence of dif-
ferent NaCl concentrations, ranging from 0 to 600 mM. The experiments were performed in
triplicate in 6-well plates, each well containing 5 mL of the medium. Bacteria were inoculated
to a concentration of 1 x 106 cells mL-1 and later incubated at 37˚C for 72–86 h, until the con-
trol sample (without salt) reached the stationary phase. The MIC value corresponds to the
minimal NaCl concentration where no bacterial growth was observed.
mRNA isolation and transcriptomic analysis
mRNA isolation. Cells from control (non-adapted, non-exposed to NaCl), and adapted
in 180 mM NaCl conditions were grown until the late exponential phase. Cells were harvested
by centrifugation at 8,000 x g for 15 min (at 4˚C) and washed once with cold 10 mM H2SO4
and twice with 10 mM sodium citrate pH 7.0. Total RNA was isolated using the RNeasy Mini
Kit (Qiagen). DNA was removed by DNase I treatment (New England, Biolabs) according to
the manufacturer’s instructions.
cDNA library preparation and Illumina sequencing. The quality and integrity of the
total RNA were evaluated using an Agilent Bioanalyzer 2100 and an RNA 600 Nano Kit (Agi-
lent Technologies). Three RNA preparations of high quality (RNA integrity number above 7)
were pooled together and submitted for transcriptome analysis as previously described
[18,19]. Before library preparation, ribosomal RNA was depleted using the MICROBExpress
kit (Thermo Fisher). Then, a TruSeq stranded mRNA library prep kit (Illumina) was used to
generate cDNA libraries for whole transcriptome analysis. The resulting libraries were
sequenced on an Illumina MiSeq system with v3 chemistry and 2 x 75-nucleotide read lengths
(paired end).
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
Differential expression analysis. Raw reads from RNA sequencing of non-adapted and
adapted cells were processed to remove adaptors, and filtered to obtain reads with a quality
higher than Q20, by using the CLC Genomics Workbench software. Then, the filtered reads
were aligned onto the reference genome of L. ferriphilum DSM 14647 [NCBI accession num-
ber: PGK00000000] by CLC Genomics Workbench software. Transcriptomic data was submit-
ted to European Bioinformatic Institute database (ArrayExpress Accession: E-MTAB-11136)
Raw counts for each ORFs features were subjected to differential analysis with statistical R
software, using the DESeq2 package [20]. A gene was considered differentially expressed with
a p-value < 0.05. The assignment of genes to a functional category was carried using the Go
Feat Tool and the public Gene Ontology (GO) database [21].
Oxygen consumption
The oxygen consumption rate was determined by means of optodes (Fibox 3, PreSens-Preci-
sion Sensing GmbH, Regensburg, Germany) [22]. In short, fresh iron-grown 100 mL-cultures
of L. ferriphilum DSM 14647 were harvested by centrifugation at 8,000 x g for 15 min, the
supernatant was removed, and the pelleted cells were resuspended in 0.1 mL the remaining
growth medium, before being added to a 3-mL cuvette containing 2.6 mL of DSM 882 culture
medium pH 1.8 with 0 or 180 mM NaCl for non-adapted and adapted cell cultures, respec-
tively. Afterwards, 0.15 mL of ferrous iron solution were added to the cuvettes (final concen-
tration 72 mM), and the suspension mixed cautiously. The cuvette was then carefully closed
with a glass lid. An oxygen-sensing optode spot had previously been embedded inside the mea-
suring cuvette. Fibre-optics located outside the cuvette on the opposite side of the oxygen sen-
sor spot were connected with a 4-channel fiber–optics oxygen meter (Firesting O2), also
equipped with a receptacle for a temperature sensor. The optode signal was evaluated using
the software Pyro Oxygen Logger. Due to the strong temperature dependence of fluorescence,
measurements were performed in a thermostatic cabinet (UVP Hybridizer HB-1000) at 37˚C.
Optode measurements were performed in triplicate using biological replicates.
Analysis of intracellular (hydroxy)ectoine content
The compatible solutes ectoine and hydroxyectoine were quantified by HPLC analysis, using
an Ultimate 3000–2015 HPLC (Thermo Scientific) system with a 250 mm × 4.6 mm Hypurity
Aquastar C-18 column with particle size of 5 μm (Thermo Scientific), as described previously
[4]. Chromatography was performed with a gradient of two solutions as mobile phase—eluent
A (0.8 mM KH2PO4, 6.0 mM Na2HPO4, pH 7.6) and eluent B (acetonitrile)—at a flow rate of
1.0 mL min-1 at 25˚C. The presence of compatible solutes was monitored at 215 nm by a UV/
VIS detector. The retention times of ectoine and hydroxyectoine were determined using com-
mercially available compounds (purity � 95%, Sigma-Aldrich) as standards. Intracellular
ectoine and hydroxyectoine content was calculated as ng mg-1 of wet biomass, using a calibra-
tion curve.
Determination of ROS levels
The intracellular level of total ROS was measured in non-adapted and adapted cultures using
the fluorescent probe 2’,7’-dichlorodihydrofluorescein diacetate (H2DCFDA) according to
Ferrer et al. [23]. Fluorescent emission values were normalized to the total protein concentra-
tion. Protein quantification was performed by the colorimetric Bradford assay [24]. Since ROS
determination included a last incubation step with the fluorescent probe under neutral pH
conditions, the viability of the cell cultures was tested. For this purpose, a control was
PLOS ONE | https://doi.org/10.1371/journal.pone.0267316 April 29, 2022
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
performed by incubating cells in 100 mM potassium phosphate buffer pH 7.4 without the
probe and then re-inoculating them into fresh medium as described [4].
Statistical analysis
Statistical analysis was performed using the one-way ANOVA test followed by Tukey’s test in
GraphPad Prism 5. The differences were considered to be significant at p < 0.05.
Results and discussion
Characterization of growth and NaCl-tolerance of L. ferriphilum DSM
14647 adapted to 180 mM NaCl
The adaptation of L. ferriphilum to 180 mM NaCl led eventually to a culture with the same cell
density (8 x 107 cells mL-1) as the non-adapted cell culture (Fig 1). However, salt approxi-
mately tripled the time of cellular duplication (td) from 6 to 17 h. A retarding effect on growth
rate and iron oxidation has been observed in different studies of NaCl-susceptible acidophilic
microorganisms, including L. ferriphilum and other species (At. ferrooxidans and S. thermosul-
fidooxidans) [14,25].
It is important to highlight that although the addition of ectoine favored the sequential
acclimation of L. ferriphilum to 180 mM NaCl (data not shown), the adapted cell culture could
grow steadily without ectoine supplementation, indicating that cells were physiologically
adapted to face this stress condition.
It has been widely reported that decreasing the external pH contributes significantly to the
toxicity of chloride in this species and in other acidophilic bacteria [4,15]. In agreement with
this, Fe2+ oxidation in the presence of NaCl is highly influenced by the pH of the growth
medium [5]. Thus, in order to evaluate the tolerance of NaCl-adapted cells, we determined the
MIC of adapted and non-adapted cell cultures exposed to a range of pH values. As shown in
Table 1, the MIC of the adapted culture was higher than that of the non-adapted culture. In
addition, the MIC significantly increased as the pH of the medium increased within the range
of 1.4–3.0. However, it was also observed that at a higher pH of the medium, the difference of
Fig 1. Growth of L. ferriphilum DSM 14647. Curves represent the growth of adapted and exposed to 180 mM NaCl
(grey circles), and non-adapted and non-exposed (white circles) cell cultures. Data represent the average of 3
independent experiments. Error bars represent standard deviation. Initial cell density, 1x106 cells mL-1.
https://doi.org/10.1371/journal.pone.0267316.g001
PLOS ONE | https://doi.org/10.1371/journal.pone.0267316 April 29, 2022
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
Table 1. Minimum inhibitory concentration of NaCl in L. ferriphilum DSM 14647 adapted to 180 mM NaCl at dif-
ferent external pH (pHex).
pHex
1.4
1.8
2.4
3.0
NaCl MIC [mM]
L. f. non-adapted
175
L. f. adapted to 180 mM NaCl
350
225
350
400
375
425
500
https://doi.org/10.1371/journal.pone.0267316.t001
the MIC between adapted and non-adapted cultures was lower. For example, at pH 1.4 the
adapted culture had a MIC value twice (350 mM) that of the non-adapted culture (175 mM),
while at pH 3.0, the MIC of the adapted culture was just 25% higher (500 mM) than that of the
non-adapted culture (400 mM). The results clearly show that the prior adaptation of L. ferri-
philum conferred a higher tolerance against NaCl, but that this tolerance was more noticeable
at a low external pH.
Transcriptomic profile of L. ferriphilum DSM 14647 adapted to 180 mM
NaCl
Screening of differentially expressed genes (DEG). The differential expression analysis
was performed comparing cultures adapted to 180 mM NaCl versus non-adapted non-exposed
cell cultures as described in Materials and Methods. In this analysis, 99 out of 2736 genes
showed significant differential expression (p<0.05) of which 69 (2.5%) and 30 (1.1%) were up-
regulated and down-regulated, respectively. Table 2 lists the genes that were up-regulated
(excluding 43 ORFs predicted as hypothetical proteins, S1 Table) in a range of 4.1- to 91.7–
fold change. Table 3 shows the down-regulated genes (excluding 8 ORFs predicted as hypo-
thetical proteins, S2 Table) in a range of -4.3 to -9.3–fold change. Classification of genes by
their functionality revealed a number of genes involved in metabolism and energy conserva-
tion, the cell envelope, transport and osmoregulation, and stress response and signal transduc-
tion, among others.
-) and protons in the reaction: CO2 + H2O ,
Metabolism and energy conservation. The adaptation of L. ferriphilum to 180 mM NaCl
resulted in the identification of a number of DEGs related to metabolism and energy conserva-
tion. A significant increase in the expression of a carbonic anhydrase (CA, 8.1-fold) was
observed in the adapted culture. This metalloenzyme catalyzes the reversible hydration of car-
bon dioxide to form bicarbonate ions (HCO3
HCO3− + H+ [26]. In autotrophic bacteria that fix CO2 through the Calvin-Benson-Bassham
cycle, CA is involved in the transport and supply of CO2 to Rubisco (D-ribulose 1,5-bispho-
sphate carboxylase/oxygenase) in the carboxysome [27]. However, since this enzyme produces
and uses protons and bicarbonate ions, it also plays a key role in the regulation of pH [28]. In
acidophiles, genes encoding CA and the carboxysomal shell proteins have been described in
At. ferrooxidans and At. thiooxidans [29–31]. Moreover, in At. ferrooxidans, the expression of
the cbb5 operon that encodes the inorganic carbon transporter SulP and CA is dependent on
the CO2 concentration regimen [31]. In L. ferriphilum and other leptospirilli, carbon fixation
is performed by the reductive tricarboxylic acid cycle (RTCA) [29] in which, as far as it is
known from the literature, CA does not seem to play a role. Thus, the predicted CA of L. ferri-
philum could play a major role by contributing towards neutralizing the acidification of the
cytoplasm that is expected to occur in the presence of chloride. In this way, the up-regulation
of the CA-encoding gene could represent a direct strategy of cellular pH homeostasis. The con-
tribution of CA to this purpose deserves to be experimentally addressed.
PLOS ONE | https://doi.org/10.1371/journal.pone.0267316 April 29, 2022
6 / 18
PLOS ONETable 2. Up-regulated genes in L. ferriphilum DSM 14647 adapted to 180 mM NaCl in relation to non-adapted non-exposed control cells.
Accession number
Gene product
Metabolism and energy conservation
Fold change
Transcriptomic of Na-Cl adapted Leptospirillum ferriphilum
KGA94808.1
WP_036082816.1
KGA94222.1
Cell envelope
WP_014961534.1
WP_081938081.1
Transport and osmoregulation
WP_036082891.1
Stress response
WP_036083168.1
WP_052157908.1
WP_023524701.1
KGA93200.1
WP_036079670.1
KGA93006.1
KGA94361.1
Signal transduction
WP_049713715.1
WP_036081469.1
WP_161781719.1
WP_036081415.1
Others
KGA93962.1
WP_036081724.1
KGA94115.1
WP_036083266.1
WP_161781749.1
WP_036081132.1
WP_036080943.1
WP_020859441.1
WP_036082283.1
Carbonic anhydrase (CA)
Cytochrome c oxidase subunit CcoN
Sulfide:quinone reductase (Sqr)
Regulator of protease activity HflC
Fatty acid desaturase
Outer membrane efflux protein TolC
Flavohemoprotein
Cytochrome c peroxidase
Heat-shock protein Hsp20
Transcriptional Regulator IscR
FAD-dependent pyridine nucleotide-disulfide oxidoreductase
Radical SAM domain protein
Mercuric reductase, MerA
Diguanylate cyclase/phosphodiesterase
DSF synthase (RpfF)
Diguanylate cyclase/phosphodiesterase
Diguanylate cyclase/phosphodiesterase
Transposase
Phage related integrase
Methyl-accepting chemotaxis protein
Methyl-accepting chemotaxis protein
Periplasmic serine protease DO (HtrA)
Flagellin protein FlaB
DNA-binding protein HU
Prokaryotic ubiquitin-like protein Pup
Shufflon-specific DNA recombinase
a: Transcriptomic data can be found in EBI database (https://www.ebi.ac.uk/) as indicated in Materials and Methods.
b: Values correspond to the average fold change of 3 biological replicates.
https://doi.org/10.1371/journal.pone.0267316.t002
8.1
4.9
4.6
6.6
4.9
7.4
14.3
10.9
8.1
7.2
5.6
5.2
4.3
11.2
10.7
6.6
5.2
7.7
5.4
5.4
5.1
4.9
4.4
4.3
4.2
4.2
Genes coding for proteins from electron-transport chains such as cytochrome c oxidase
CcoN subunit (4.9-fold) and sulfide:quinone reductase Sqr (4.6-fold) were also significantly
up-regulated. CcoN is the component of the cbb3-type cytochrome oxidase, a complex enzyme
of the respiratory chain which has previously been reported in Leptospirillum spp. [32]. CcoN
is the catalytic subunit of the enzyme in charge of the four-electron reduction of molecular
oxygen to water, a process which is coupled to translocation of protons across the membrane
[33]. The Sqr enzyme could play a role in the detoxification of endogenously generated H2S, a
common product of cysteine metabolism that negatively impacts the redox status of bacterial
cells [34,35]. The enzyme obtains electrons from H2S oxidation and transfers them to the qui-
none pool, thus increasing the activity of the electron-transfer chain. The increase of both cyto-
chrome c oxidase and Sqr activities should increase the respiratory rate of this bacterium to
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PLOS ONETable 3. Down-regulated genes in L. ferriphilum DSM 14647 adapted to 180 mM NaCl in relation to non-adapted non-exposed control cells.
Accession numbera
Metabolism and cell envelope
Gene product
Fold change
Transcriptomic of Na-Cl adapted Leptospirillum ferriphilum
WP_036081541.1
WP_036081614.1
WP_036081511.1
WP_036081618.1
WP_036081521.1
WP_036081553.1
WP_036081550.1
WP_036081600.1
WP_052157773.1
WP_036081557.1
WP_036081546.1
WP_036081519.1
Transport and osmoregulation
WP_036080892.1
WP_036080895.1
WP_036080909.1
WP_036081492.1
WP_020859429.1
WP_020859430.1
WP_020859431.1
Stress response
WP_036080906.1
WP_036080898.1
WP_052157774.1
Glycosyl transferase, group 1 family protein
Glutamine-fructose-6-phosphate aminotransferase
Glycosyl transferase family 2 protein
UDP-glucose dehydrogenase
UTP-glucose-1-phosphate uridylyltransferase
Undecaprenyl-phosphate galactose phosphotransferase
Polysaccharide export protein
dTDP-glucose 4,6-dehydratase
Glycosyltransferase involved in cell wall biosynthesis
Tyrosine-protein kinase EpsD
Polysaccharide deacetylase
Eight transmembrane protein EpsH
Outer membrane efflux protein TolC
RND efflux transporter
RND family efflux transporter MFP subunit
ABC transporter ATP-binding protein MdlB
Diaminobutyrate-2-oxoglutarate transaminase (EctB)
L-ectoine synthase (EctC)
Ectoine hydroxylase (EctD)
Cobalt-zinc-cadmium resistance protein CzcA
Two component sigma54 specific transcriptional regulator
Sigma-54 dependent transcriptional regulator
a: Transcriptomic data can be found in EBI database (https://www.ebi.ac.uk/) as indicated in Materials and Methods.
b: Values correspond to the average fold change average of 3 biological replicates.
https://doi.org/10.1371/journal.pone.0267316.t003
-5.3
-5.7
-5.3
-6.4
-9.0
-4.8
-4.9
-5.0
-6.0
-6.9
-8.4
-9.3
-5.1
-5.4
-6.2
-5.9
-6.5
-5.6
-8.0
-6.8
-4.3
-10.2
provide energy (ATP), reducing power (NAD(P)H), and mainly the possibility of extruding
protons from the cytoplasm to avoid acidification induced by chloride exposure [4,14]. In
order to evaluate whether adapted cells showed a higher respiratory rate, the oxygen consump-
tion of non-adapted and adapted cells of L. ferriphilum exposed to 180 mM NaCl was mea-
sured. As shown in Fig 2, non-adapted L. ferriphilum exposed to 180 mM NaCl was not able to
respire. Interestingly, the O2 consumption rate in adapted cell cultures treated with 180 mM
NaCl was significantly greater than that of non-adapted untreated cells (1.2 x 10−9 versus 2.2 x
10−9 ppm O2 s-1cell-1; p<0.01). This result supports the idea that up-regulation of electron-
transport chain genes contributes towards the increase in the oxygen respiratory activity in
adapted cells exposed to NaCl. A similar effect was observed in a proteomic study of Ac. pros-
perus in which cytochrome c1, rusticyanin and ATP synthase subunit b were over-expressed in
the presence of 500 mM NaCl [14], indicating that proton extrusion by respiration may be a
widely distributed chloride response mechanism in acidophiles.
Cell envelope. One of the up-regulated genes codes for the regulator HflC (6.6-fold)
which modulates the FtsH protease and may serve to maintain quality control of some mem-
brane proteins [36,37]. Additionally, a gene coding for a fatty acid desaturase, which belongs
to a group of enzymes in charge of double-bond insertion at specified positions of fatty acyl
chains, necessary for membrane-lipid fluidity [38], was up-regulated (4.9-fold). In
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
Fig 2. Effect of NaCl adaptation on oxygen consumption rate in L. ferriphilum DSM 14647. The measurements
were carried out in adapted (A) cells exposed to 180 mM NaCl and non-adapted (NA) cells exposed to 0 or 180 mM
NaCl. The data represent the average of 3 independent experiments. Error bars represent standard deviation. Statistical
analysis was carried out by an ANOVA test.
https://doi.org/10.1371/journal.pone.0267316.g002
Synechocystis, strains overexpressing a desaturase gene were found to be more robust under
salt stress conditions [39]. In addition, a correlation between the unsaturation of fatty acids in
membrane lipids and tolerance to salt stress in this genus and other bacteria has been reported
[39,40]. For L. ferriphilum, the up-regulation of the fatty acid desaturase gene suggests an
increase in the unsaturated/saturated fatty acid ratio. Whether this confers higher fluidity to
the membrane in salt stress compared to normal conditions, or it is to compensate a salt-
induced decrease in the fluidity and thus ensure a fluid membrane at high salt, remains to be
determined.
Several genes involved in carbohydrate metabolism had lower expression in adapted versus
non-adapted cells. Among them were genes encoding two glycosyl transferases (-5.3 fold), a
UDP-glucose dehydrogenase (-6.4 fold) and a UTP-glucose-1-phosphate uridylyltransferase
(-9.0 fold) which are directly related to the synthesis of glycosaminoglycans, critical precursors
of peptidoglycans and other cell-surface polymers, such as lipopolysaccharides [41–43].
Another significantly repressed gene under high-salt conditions was glutamine-fructose-
6-phosphate aminotransferase/glucosamine-6-phosphate synthase (-5.7 fold), a dimeric
enzyme that catalyzes the first step in hexosamin metabolism, converting D-fructose-6-phos-
phate (Fru6P) and glutamine (Gln) into D-glucosamine-6-phosphate (GlcN6P) and glutamate
[44]. The end product of the hexosamine pathway, uridine diphosphate N-acetylglucosamine
(UDP-GlcNAc), plays an important role as a precursor of peptidoglycan and glycolipids [45].
Other genes related with the biosynthesis of the cell envelope that were down-regulated in
L. ferriphilum grown at 180 mM NaCl encode undecaprenyl-phosphate galactose phospho-
transferase (-4.8 fold) and dTDP-glucose 4,6-dehydratase (-5.0 fold), two enzymes involved in
the generation of intermediate nucleotide sugars for O-antigen polysaccharide biosynthesis in
the biogenesis of the outer membrane [46,47]. Altogether, these findings suggest that synthesis
of cell surface polymers such as peptidoglycan and lipopolysaccharide were diminished as a
result of the physiological salt adaptation in L. ferriphilum. Abiotic stressors jeopardize the
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
integrity of peptidoglycans and other components of the cell envelope by introducing lesions,
which must be rapidly repaired to prevent cell lysis [48]. As a consequence, upon osmotic
stress induction, cells respond by upregulating the activity of enzymes or genes essential for
cell wall synthesis [49]. Thus, based on these antecedents, we envisioned that adapted cells of
L. ferriphilum exposed to 180 mM NaCl did not generate the corresponding response to
osmotic stress.
Transport and osmoregulation. The adaptation of L. ferriphilum to NaCl also resulted in
the up-regulation of several genes encoding proteins related to cellular transport. These
included the gene encoding TolC protein (7.4-fold), a key component of multidrug efflux sys-
tems such as AcrAB-TolC, AcrEF-TolC, EmrAB-TolC and MacAB-TolC of the outer mem-
brane, which are important for bacterial survival and oxidative stress responses in acidic
environments [50,51].
Conversely, genes encoding several transporters were repressed. In agreement with decreas-
ing the biosynthesis of surface polymers, the expression of a gene encoding a polysaccharide-
transport protein implicated in the export of polysaccharides across the outer membrane [52]
was significantly lower in salt-adapted cells (-4.9-fold). Genes encoding an outer-membrane
efflux protein TolC (different from the one referred to above; -5.1-fold), two genes coding for
RND (Resistance-Nodulation-Division) efflux transporters (-5.4 and -6.2-fold, respectively)
that form complexes with AcrAB-TolC, and play a role in the active efflux of antimicrobial
agents [53], and ABC transporter ATP-binding protein MdlB (-5.9 fold), which is an integral
membrane protein named Mdl (Multidrug resistance-like) that actively transports molecules
across the lipid membrane against a concentration gradient, were also reduced in expression
[54,55].
Regarding osmoregulation, it was unexpected that 3 genes involved in the biosynthesis of
(hydroxy)ectoine-diaminobutyrate-2-oxoglutarate transaminase (ectB, -6.5-fold), L-ectoine
synthase (ectC, -5.6-fold) and ectoine hydroxylase (ectD, -8.0-fold)—were all significantly down-
regulated. Since hydroxyectoine plays an important role in protecting the cells of L. ferriphilum
against saline stress [4], we were interested in evaluating the intracellular content of ectoine and
hydroxyectoine in adapted cells exposed to 180 mM NaCl. As shown in Fig 3, ectoine was not
detected in either adapted or non-adapted cells. However, hydroxyectoine reached 20 nmol mg-1
of wet biomass (p<0.01) in non-adapted cells cultured without NaCl while it was not detected in
extracts of L. ferriphilum adapted to 180 mM NaCl. Taken together, these results reinforce the
idea that the 180 mM NaCl-adapted culture of L. ferriphilum does not develop an active response
to osmotic stress based on the synthesis of compatible solutes. Interestingly, in non-adapted cells,
the compatible solute-mediated response appears to be functionating, and in this way these cells
could be actively responding to the osmotic challenge.
Stress response. Presumed stress response genes that exhibited a significant increase in
their transcript levels encoded the following proteins: a flavohemoprotein (14.3-fold), an
enzyme able to reduce nitric oxide (NO) from reactive nitrogen species (RNS) [56]; a cyto-
chrome c peroxidase (10.9-fold) able to reduce periplasmic hydrogen peroxide [57]; an FAD-
dependent pyridine nucleotide-disulfide oxidoreductase (5.6-fold) which catalyzes disulfide
bond formation and reduction [58,59]; and a radical S-adenosyl-methionine (SAM, 5.2-fold)
precursor for the biosynthesis of the antioxidant cobalamin [23]. These data strongly suggest
that in L. ferriphilum, antioxidant proteins form part of the mechanisms that are activated to
enable this species to face the stress induced by NaCl, and thereby manage redox homeostasis
under these conditions.
In agreement with the induction of antioxidative proteins, in a previous study carried out
by our research group, it was established that exposure to NaCl induced a severe condition of
oxidative stress in L. ferriphilum, leading to an increase in intracellular ROS levels and
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
Fig 3. Effect of NaCl adaptation on the content of compatible solutes in L. ferriphilum DSM 14647. Ectoine and
hydroxyectoine content was measured in adapted cells exposed to 180 mM NaCl (A) and non-adapted (NA) cells
exposed to 0 or 180 mM NaCl. The data represent the average of 3 independent experiments. Error bars represent
standard deviation. Statistical analysis was carried out by ANOVA and a T Test. N.D.: not detected.
https://doi.org/10.1371/journal.pone.0267316.g003
activation of the antioxidant response [4]. In order to establish whether the adapted cells are
able to maintain the redox balance, the intracellular ROS level was measured using a fluores-
cent probe as described in Material and Methods. The measurements were performed in non-
adapted and adapted cultures grown in DSMZ 882 medium supplemented (or not) with 180
mM NaCl. As shown in Fig 4, non-adapted cells exposed to 180 mM NaCl had significantly
Fig 4. Effect of NaCl on ROS generation in L. ferriphilum. ROS were measured in adapted cells exposed to 180 mM
NaCl (A) and non-adapted (NA) cells exposed to 0 or 180 mM NaCl. Cytoplasmic ROS content is expressed as relative
fluorescence units (RFU) of the activated fluorescent probe H2DCFDA per mg of protein. The data represent the average
of 3 independent experiments. Error bars represent standard deviation. Statistical analysis was carried out by ANOVA
and a T Test.
https://doi.org/10.1371/journal.pone.0267316.g004
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
higher intracellular ROS levels (p<0.01) compared with the control condition (without NaCl).
Interestingly, salt-adapted L. ferriphilum treated with 180 mM NaCl showed similar, and even
slightly lower, intracellular ROS levels compared to the control without NaCl, suggesting that
these cells maintain correct redox homeostasis. This condition is most likely managed through
the up-regulation of the antioxidant mechanisms described above.
Another gene that showed up-regulation (7.2-fold) encodes the IscR regulator, potentially
involved in regulating the biosynthesis of [Fe-S] clusters of proteins [60]. The [Fe-S] clusters
are susceptible to being oxidized by superoxide anions, thus releasing Fe2+, and thereby trig-
gering Fenton chemistry and the generation of highly harmful hydroxyl radicals [16,23].
Therefore, these results imply that under high salt conditions, [Fe-S] clusters of proteins suffer
oxidative damage and, in consequence, the cells respond through the activation of the biosyn-
thesis pathway for [Fe-S] clusters.
Interestingly, the merA gene that encodes a mercuric reductase was over-expressed
(4.3-fold) under the high-NaCl regimen. In bacteria, the mercury-resistance (mer) genes are
activated and repressed by the metalloregulatory MerR protein, which has a high degree of
selectivity for mercury (Hg2+) but can additionally be partially stimulated by a variety of transi-
tion metals such as Cd2+, Zn2+, Ag+, Au+, and Au3+ [61]. For example, in the metal-tolerant
bacterium Cupriavidus metallidurans, the genes merA, merT, and merP were up-regulated
when this bacterium was exposed to cadmium [62,63]. A similar phenomenon has been
described in Nitrosomonas europaea, since the mer operon was also induced by cadmium [64].
Although merR was not up-regulated in our study, this gene was detected in the genome of L.
ferriphilum and could contribute to regulate the transcriptional activity of the merA gene in
response to mercury or other metals. We speculate that in L. ferriphilum, chloride stress could
cause oxidation of metalloproteins releasing oxidized metals to the intracellular space that may
activate merA transcription. In L. ferriphilum this response could be relevant, since it has a
high content of cytochromes and [Fe-S] proteins [32,65] that could contribute to increasing
the intracellular free iron and copper contents under stress conditions. Whether the mer
operon has a role in protection and /or avoiding toxicity toward these metals should be
elucidated.
Interestingly, some genes related to stress responses were repressed. Such is the case for the
gene coding for the cobalt-zinc-cadmium resistance protein CzcA (-6.8-fold), one of the three
components of the CzcABC efflux pump [66]. This pump functions as a cation-proton anti-
porter mediating resistance against divalent metals such as cadmium (Cd2+), zinc (Zn2+), and
cobalt (Co2+), among others [67]. As chloride exposure is known to induce cytoplasmic acidifi-
cation by favoring entry of protons into the cell, the response to this anion should involve
strategies that contribute to keeping the intracellular pH closer to neutrality. Thus, repression
of the czcA gene and eventual down-regulation of CzcABC pump activity in adapted L. ferri-
philum could participate towards avoiding the entry of protons into the cytoplasm.
Signal transduction. Among the genes that were up-regulated in the culture adapted to
NaCl, we detected one gene encoding a diffusible signal factor (DSF) synthase (10.7-fold) and
3 genes coding for diguanylate cyclase phosphodiesterase (5.2, 6.6 and 11.2-fold). The protein
DSF synthase (RpfF) synthesizes diffusible signal factors, widely conserved quorum sensing
signals in many Gram-negative bacterial species that play important roles in regulating various
biological functions such as biofilm formation, virulence, and antibiotic and stress resistance
[68,69]. RpfF synthesizes DSF by dehydration of a 3-hydroxyacyl-acyl carrier protein (ACP)
fatty acid intermediate and also cleaves the thioester bond linking DSF to ACP [70]. When
DSFs reach a threshold concentration outside the cell, bacteria activate their cognate receptor
RpfC, a hybrid membrane sensor kinase that phosphorylates the intracellular response regula-
tor RpfG [70]. The activated RpfG possesses c-di-GMP phosphodiesterase activity, which
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
hydrolyzes c-di-GMP to produce GMP. The change in c-di-GMP level affects the transcrip-
tional expression of target genes, thus configuring a physiological response or modulating a
biological process [70]. Therefore, based on the up-regulation of genes encoding DSF synthase
and diguanylate cyclase phosphodiesterase, it is possible to infer that adaptation of L. ferriphi-
lum to NaCl involves the activation of a cellular response mediated by DSF signals and the sec-
ond messenger c-di-GMP. However, the target genes that are modulated by this mechanism
remain to be elucidated.
Others. Other genes up-regulated by NaCl adaptation were two methyl-accepting chemo-
taxis proteins (5.4-fold and 5.1-fold) and a flagellin protein FlaB (4.4-fold) which are related
with movement of microorganisms in response to chemical gradients, and biosynthesis of fla-
gella, respectively [71]. An effect of osmolarity challenges on flagellar function has previously
been reported in bacteria. Specifically, in Desulfovibrio vulgaris, cells were observed to be
highly motile when subjected to salt stress and several key chemotaxis genes were very highly
and reproducibly up-regulated [72]. More recently, Escherichia albertii showed swimming
motility when cultured at low osmotic pressure. Under this condition, the biosynthesis of fla-
gella was also induced [73]. It has been predicted that flagellar induction increases E. albertii
survival in intestinal epithelial cell cultures. Whether motility and flagellum assembly are acti-
vated by NaCl exposure, and the corresponding impact of their activation on adaptation and
fitness of leptospirilli should be addressed.
Another group of genes overexpressed in the NaCl-adapted culture encodes a transposase
(7.7-fold), a phage-related integrase (5.4-fold), a DNA-binding protein HU (4.3-fold) and a
shufflon-specific DNA recombinase (4.2-fold). All are involved in bacterial DNA transaction
systems including transposition and recombination, among others [74]. Therefore, genetic/
genomic modifications could underlie physiological stress responses and/or may pre-adapt a
small subset of the population to face this environmental stress.
Conclusions
Despite its high chloride sensitivity, L. ferriphilum could be stably adapted to 180 mM NaCl. In
adapted cells, the MIC and thus tolerance to NaCl increased considerably compared to non-
adapted non exposed cells. The MIC of adapted and non-adapted cells was shown to be
directly dependent on the pH of the medium, and so the comparison of tolerance to chloride
or other anions in acidophilic microorganisms should be carried out whilst strictly monitoring
the pH of the growth medium.
Transcriptomic data and experimental validations showed that the most significant
responses of L. ferriphilum to chloride adaptation included neutralization and/or expulsion of
protons through activation of carbonic anhydrase, respiratory cytochrome c oxidase and sul-
fide:quinone reductase. Thus, the regulation of pH homeostasis seems to play a key role in the
adaptive response. Towards the same goal, a cation/proton antiporter system CzcA that
extrudes cations through the entry of protons was down-regulated. In addition, the increase in
respiratory activity and oxygen consumption correlated with activation of antioxidant
responses in which genes encoding for ROS scavenging properties and biomolecule protection
seem to play a relevant role in controlling the intracellular ROS level and the redox status of
adapted cells. The response detected shows that oxidative stress is an important element of the
toxicity induced by chloride, and this could largely explain the reason why iron-oxidizing
microorganisms have been reported to be more sensitive to the presence of anions than sulfur-
oxidizers or other acidophiles [75]. Under cultivation conditions, iron-oxidizing microorgan-
isms are exposed to high concentrations of iron as an energy substrate, while sulfur oxidizers
are exposed to trace concentrations of this element that is used only as a micronutrient. Since
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PLOS ONETranscriptomic of Na-Cl adapted Leptospirillum ferriphilum
ferrous iron can trigger Fenton chemistry, its presence in high concentrations leads to a higher
risk of redox stress, making the microorganisms more sensitive to other oxidative stress elici-
tors. Chloride adaptation also correlated with a predicted increase in chemotaxis and biosyn-
thesis of flagella, and predicted cellular communication and signaling via DSFs and c-di-GMP.
Finally, an induction of genetic/genomic modifications by transposition and/or recombination
also seemed to form part of the adaptive response to NaCl exposure. Although there was an
increase in the activity of the electron-transport chain that likely led to an increase in ATP and
NAD(P)H synthesis, carbohydrate metabolism and synthesis of polysaccharide polymers of
the cell surface seemed to suffer significant decreases. Surprisingly, the canonical osmotic
stress response did not appear to be necessary in salt-adapted cells, since genes for biosynthesis
of the compatible solutes ectoine and hydroxyectoine were down-regulated, and only hydro-
xyectoine could be detected and only in non-adapted cells without NaCl. Our results suggest
that L. ferriphilum might have a response to long-term NaCl exposure that is different from
other bacteria since it does not involve the upregulation of canonical mechanisms for facing
osmotic stress. This study thus provides an important reference for future studies on NaCl
adaptation in acidophilic bacteria.
Supporting information
S1 Table. Up-regulated hypothetical genes in L. ferriphilum NaCl-adapted cells.
(XLSX)
S2 Table. Down-regulated hypothetical genes in L. ferriphilum NaCl-adapted cells.
(XLSX)
Acknowledgments
We thank Dr. Luis Valenzuela, Instituto Nacional de Tecnologı´a de los Alimentos (INTA),
Universidad de Chile, for his help with data analyses.
Author Contributions
Conceptualization: Michael Schlo¨mann, Gloria Levica´n.
Formal analysis: Javier Rivera-Araya, Gloria Levica´n.
Funding acquisition: Renato Cha´vez, Michael Schlo¨mann, Gloria Levica´n.
Investigation: Javier Rivera-Araya, Thomas Heine.
Methodology: Javier Rivera-Araya, Thomas Heine.
Project administration: Michael Schlo¨mann, Gloria Levica´n.
Visualization: Javier Rivera-Araya.
Writing – original draft: Javier Rivera-Araya, Gloria Levica´n.
Writing – review & editing: Michael Schlo¨mann, Gloria Levica´n.
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PLOS ONE
| null |
10.1371_journal.pone.0227835.pdf
|
Data Availability Statement: All serum biomarker
values, imaging analysis results, and appropriate
demographic data files are available via Open
Science Framework: DOI 10.17605/OSF.IO/92ERQ.
|
All serum biomarker values, imaging analysis results, and appropriate demographic data files are available via Open Science Framework: DOI 10.17605/OSF.IO/92ERQ .
|
RESEARCH ARTICLE
An IL-18-centered inflammatory network as a
biomarker for cerebral white matter injury
Marie Altendahl1, Pauline Maillard2, Danielle Harvey3, Devyn Cotter1, Samantha Walters1,
Amy Wolf1, Baljeet Singh2, Visesha Kakarla4, Ida Azizkhanian5, Sunil A. Sheth6,
Guanxi Xiao4, Emily Fox1, Michelle You1, Mei Leng7, David Elashoff7, Joel H. Kramer1,8,
Charlie Decarli2, Fanny Elahi1, Jason D. HinmanID
4*
1 Memory & Aging Center, Department of Neurology, University of California San Francisco, San Francisco,
CA, United States of America, 2 Department of Neurology and Center for Neurosciences, University of
California, Davis, CA, United States of America, 3 Department of Public Health Sciences, University of
California, Davis, CA, United States of America, 4 Department of Neurology, David Geffen School of
Medicine, University of California Los Angeles, Los Angeles, CA, United States of America, 5 School
of Medicine, New York Medical College, Vahalla, NY, United States of America, 6 University of Texas
Health McGovern School of Medicine, Department of Neurology, Houston, TX, United States of America,
7 Department of Medicine Statistics Core, Department of Medicine, University of California Los Angeles, Los
Angeles, CA, United States of America, 8 Department of Psychiatry, University of California San Francisco,
San Francisco, CA, United States of America
* [email protected]
Abstract
Chronic systemic sterile inflammation is implicated in the pathogenesis of cerebrovascular
disease and white matter injury. Non-invasive blood markers for risk stratification and dis-
section of inflammatory molecular substrates in vivo are lacking. We sought to identify
whether an interconnected network of inflammatory biomarkers centered on IL-18 and all
previously associated with white matter lesions could detect overt and antecedent white
matter changes in two populations at risk for cerebral small vessel disease. In a cohort of
167 older adults (mean age: 76, SD 7.1, 83 females) that completed a cognitive battery,
physical examination, and blood draw in parallel with MR imaging including DTI, we mea-
sured cerebral white matter hyperintensities (WMH) and free water (FW). Concurrently,
serum levels of a biologic network of inflammation molecules including MPO, GDF-15,
RAGE, ST2, IL-18, and MCP-1 were measured. The ability of a log-transformed population
mean-adjusted inflammatory composite score (ICS) to associate with MR variables was
demonstrated in an age and total intracranial volume adjusted model. In this cohort, ICS
was significantly associated with WMH (β = 0.222, p = 0.013), FW (β = 0.3, p = 0.01), and
with the number of vascular risk factor diagnoses (r = 0.36, p<0.001). In a second cohort of
131 subjects presenting for the evaluation of acute neurologic deficits concerning for
stroke, we used serum levels of 11 inflammatory biomarkers in an unbiased principal com-
ponent analysis which identified a single factor significantly associated with WMH. This
single factor was strongly correlated with the six component ICS identified in the first
cohort and was associated with WMH in a generalized linear regression model adjusted
for age and gender (p = 0.027) but not acute stroke. A network of inflammatory molecules
driven by IL-18 is associated with overt and antecedent white matter injury resulting from
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OPEN ACCESS
Citation: Altendahl M, Maillard P, Harvey D, Cotter
D, Walters S, Wolf A, et al. (2020) An IL-18-
centered inflammatory network as a biomarker for
cerebral white matter injury. PLoS ONE 15(1):
e0227835. https://doi.org/10.1371/journal.
pone.0227835
Editor: Niels Bergsland, University at Buffalo,
UNITED STATES
Received: September 27, 2019
Accepted: December 30, 2019
Published: January 24, 2020
Copyright: This is an open access article, free of all
copyright, and may be freely reproduced,
distributed, transmitted, modified, built upon, or
otherwise used by anyone for any lawful purpose.
The work is made available under the Creative
Commons CC0 public domain dedication.
Data Availability Statement: All serum biomarker
values, imaging analysis results, and appropriate
demographic data files are available via Open
Science Framework: DOI 10.17605/OSF.IO/92ERQ.
Funding: This study was supported by the
following funding agencies: NIH AG062422
(UCSF), NIH AG010129 (UCD), AHA Grant
#15CRP22900006 (UCLA), AHA Grant-in-Aid
#16GRNT31080021 (UCLA), and the MarkVCID
Consortium Project UH2/UH3 NS100608 (UCSF/
UCLA/UCD). Additional support provided by the
PLOS ONE | https://doi.org/10.1371/journal.pone.0227835 January 24, 2020
1 / 20
Medical Student Training in Aging Research
Program (NIH T35AG026736) and the Lillian R.
Gleitsman Foundation. The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing interests: Dr. DeCarli serves as a
consultant to Novartis Pharmaceuticals on a trial
studying the safety of heart failure medication. The
University of California has filed U.S. patent (16/
487,332) application for: “Serologic assay for silent
brain ischemia” for which Drs. Hinman and Xiao
are co-inventors. This does not alter our adherence
to PLOS ONE policies on sharing data and
materials.
IL-18-mediated inflammation and white matter injury
cerebrovascular disease and may be a promising peripheral biomarker for vascular white
matter injury.
Introduction
Cerebral small vessel disease (cSVD) is a leading contributor to vascular cognitive impairment
and is estimated to cause 1/5th of strokes in older adults [1]. cSVD has been associated with
global cognitive decline, decreased executive function and reduced processing speeds [2–4].
Individuals with poorer cardiovascular health have higher cross-sectional cSVD burden and
accelerated disease progression [5, 6]. Early detection of those at risk for cSVD would allow
patients to improve their vascular health and possibly slow the progression of cSVD and cogni-
tive decline associated with poor vascular health [7, 8].
Currently, diagnosis and risk stratification of cSVD relies on imaging techniques such as
quantification of high signal intensities, or white matter hyperintensities (WMH) on
T2-FLAIR imaging. In the context of cSVD, WMH are thought to be evidence of irreversible
white matter injury with axonal and myelin damage [9, 10]. Recent studies using diffusion ten-
sor imaging (DTI), found that DTI-derived measures, including fractional anisotropy (FA)
and extracellular free water (FW), constitute sensitive biomarkers of early-stage white matter
injury resulting from cSVD that occurs in advance of the lasting tissue injury measured by
WMH [11–13]. However, MRI scans are costly and not indicated in the absence of neurologic
symptoms, therefore limiting the ability to prevent or intervene in early cSVD to prevent cog-
nitive decline late in life. Therefore, efforts are needed to identify tools that are both easily
accessible and reproducible to facilitate earlier diagnosis and identification of patients at risk
for cSVD.
At the cellular level, cSVD is hypothesized to result from endothelial dysfunction leading to
subtle dysfunction of the blood brain barrier (BBB), resultant tissue damage, and progressive
inflammatory responses within the brain [14–16]. High levels of chronic inflammation result-
ing from systemic vascular risk factors such as hypertension and diabetes are proposed to exac-
erbate cSVD by damaging cerebral endothelia. A number of systemic inflammatory indicators
have been implicated in cSVD [17, 18], yet do not coalesce around a specific molecular path-
way. In this study, we aimed to investigate the association of a biologically interconnected net-
work of systemic inflammatory markers centered on the pleiotropic pro-inflammatory
cytokine IL-18 with sCVD burden. IL-18 is associated with cardiovascular risk factors and dis-
ease [19–21], increases the expression of cell adhesion molecules on endothelial cells [22, 23],
and may serve as a central coordinator for pathogenic inflammatory signaling [23]. Therefore,
our investigation centered on IL-18 and associated proteins and their cross-sectional correla-
tion with traditional and advanced neuroimaging measures of white matter integrity in a
cross-sectional cohort design involving two populations at risk for cSVD. In a community-
based aging population referred for cognitive evaluation, we used concurrent blood samples
and MRI to develop a composite measure of inflammatory markers (IL-18, MPO, GDF-15,
RAGE, ST2, and MCP-1) [18, 24–27] and correlated this inflammatory composite score (ICS)
with MRI indicators of white matter injury. In a second cohort of acutely ill neurologic
patients presenting for evaluation of stroke, we used an unbiased principal components analy-
sis on a larger set of serum markers to derive inflammatory factors correlated with ICS to and
test their association with cSVD as measured by Fazekas scoring of WMH as further validation
of the ICS as a biomarker. Our findings suggest that an IL-18-centered network of systemic
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IL-18-mediated inflammation and white matter injury
inflammation is associated with overt and antecedent white matter injury resulting from cere-
brovascular disease and may be a promising biomarker of cSVD.
Subjects/materials and methods
MarkVCID study cohort
Research involving human subjects was approved by the Institutional Review Boards of the
University of California, San Francisco (IRB #17–22314) and University of California, Davis
(IRB #215830–47) and was conducted in compliance with the Health Information Portability
and Accountability Act. One hundred and sixty-seven (167) community-dwelling older adults
with normal cognition or mild cognitive impairment (MCI) were recruited from the Univer-
sity of California, San Francisco Memory and Aging Center or the Alzheimer’s Disease Center
at University of California, Davis. Formal written consent including an estimation of capacity
judged by study investigators was obtained and participants completed a baseline neuropsy-
chological testing, neurological evaluation with a trained neurologist, Clinical Dementia Rate
(CDR) completed with a study partner, and a blood draw. Blood samples were collected by
peripheral vein venipuncture into serum-separating tubes, centrifuged immediately, processed
for serum, aliquoted and stored at -80˚C. One hundred and ten (110) study participants com-
pleted an MRI scan within six months of their baseline assessment, and a subset of 49 partici-
pants completed DTI. Participants were included in this study if they were considered non-
demented by a formal consensus panel with a CDR total score of 0.0 or 0.5.
ASPIRE study cohort
Research involving human subjects was approved by the Institutional Review Board of the
University of California, Los Angeles (IRB # 14–001798) and was conducted in compliance
with the Health Information Portability and Accountability Act. Formal written consent was
obtained for all participants prior to the collection of blood samples. Capacity to provide con-
sent was judged by study co-investigators based on the subject’s ability to articulate risks and
benefits of participating after reviewing the consent form. Surrogate consent was approved by
the IRB. Consecutive participants were patients presenting to the UCLA Emergency Depart-
ment with symptoms concerning for stroke between December 2014 and June 2016 and
offered to participate in the study. Study inclusion criteria were: onset of stroke symptoms
within 8 hours of presentation (or within 2 hours of presentation if symptoms were present
upon awakening); greater than 18 years of age and able to consent or had a suitable surrogate
individual to consent on their behalf. Final clinical diagnosis was determined by a board-certi-
fied vascular neurologist. Blood samples were collected by peripheral vein venipuncture into
heparin-containing tubes. Samples were kept on ice and then centrifuged immediately at
13,000 x g for five minutes at 4˚C. The serum was collected and aliquoted into freezer vials for
storage at -80˚C. Subjects with evidence of CNS infection, known CNS malignancy, or recent
head trauma as a potential cause of neurologic symptoms were excluded.
Protein interactions
Tests for protein interactions among biomarkers was performed using the STRING database
v11.0 (string-db.org) [28]. Multiple protein search tool was used to input GDF-15, MPO, ST2,
IL-18, MCP-1, and RAGE. Settings for tests of interactions were: meaning of network
edges = confidence; active interaction sources = all; minimum interaction score = medium
confidence. A second shell of interactors limited to 5 was added for visual representation.
Resulting analysis data were exported and are available via permanent web link (S1 File).
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IL-18-mediated inflammation and white matter injury
Luminex assay and composite score generation
Serum levels of six markers of inflammation: myeloperoxidase (MPO), growth differentiation
factor 15 (GDF-15), receptor for advanced glycation end products (RAGE), ST2, interleukin-
18 (IL-18), and monocyte chemoattractant protein-1 (MCP-1) were measured in duplicate
using a custom assay run across two plates on the Luminex platform (R&D Systems) measur-
ing a total of 15 analytes: TNF-α, IL-6, ST2, MCP-1, RAGE, GDF-15, IL-18, CXCL5, CXCL6,
IGFBP-2, MPO, ITGB3, BDNF, FGF-23, IL-17. The manufacturer protocol was followed and
antigen binding within the assay was measured on a Luminex 200 System and analyzed using
Milliplex Analyst 5.1. Four markers (TNF-α, BDNF, FGF-23, IL-17) were removed prior to
analysis due to missing data and/or high proportions of values less than the limits of detection.
Data points with coefficient of variance greater than 0.15 were excluded. To create a variable
that measures inflammation of the whole network, we calculated an inflammation composite
score (ICS) by normalizing raw inflammatory marker concentrations (pg/mL) using a log
transformation, then standardizing data into z-scores, and finally taking the average of the z-
scores across all six inflammatory markers. z-score generation was performed independently
for each cohort.
MarkVCID cohort MRI acquisition
Participants at the University of California, San Francisco completed MRI on a Siemens Trio
3T machine or Siemens Prisma 3T machine. T1, diffusion, and FLAIR sequences were col-
lected. T1 acquisition: Volumetric MPRAGE sequences were used to acquire T1-weighted
images of the entire brain (Sagittal slice orientation; slice thickness = 1.0 mm; slices per slab =
160; in-plane resolution = 1.0x1.0 mm; matrix = 240x256; TR = 2,300 ms; Trio: TE = 2.98 ms
(Prisma: TE = 2.9); TI = 900 ms; flip angle = 9˚). Diffusion (Trio) parameters: TR/TE 8200/86
ms; B = 0 image and 64 directions at B = 2000 s/mm2; FOV 220×220 mm2 and 2.2 mm thick
slices; matrix 100×100 with 60 slices yielding 2.2 mm3 isotropic voxels / (TR/TE 8000/109 ms;
B = 0 image and 64 directions at B = 2000 s/mm2; FOV 220×220 mm2 and 2.2 mm thick
slices; matrix 100×100 with 55 slices yielding 2.2 mm3 isotropic voxels). Diffusion (Prisma)
parameters: FOV 220×220 mm2 and 2.0 mm slice thickness; matrix 110×110 with 69 slices
yielding 2.0 mm3 isotropic voxels; B = 0 images with TR/TE 7080/72.20 ms; 96 directions at
B = 2500 s/mm2, 48 directions at B = 1000 s/mm2, and 30 directions at B = 500 s/mm all
with TR/TE 2420/72.20 ms. FLAIR (Trio) parameters: slice thickness = 1.00mm; slices per
slab = 160; in-plane resolution = 0.98x0.98mm; matrix = 256x256; TR = 6000ms; TE = 388ms;
TI = 2100ms; flip angle = 120˚. FLAIR (Prisma) parameters: slice thickness = 1.00mm; slices
per slab = 176; in-plane resolution = 1.0x1.0mm; matrix = 256x256; TR = 5000ms; TE =
397ms; TI = 1800ms; flip angle = 120˚.
All brain imaging at the University of California, Davis Imaging Research Center was per-
formed on a 3T Siemens TIM Trio MRI System. Three sequences were used: an axial-oblique
3D T1 acquisition (FSPGR, TE: 2.9ms (min), TR: 2500ms (min), TI: 1100ms, flip angle: 7
degrees, slice thickness: 1mm, number of slices: 192, FOV: 256 x 256 mm, matrix size: 256 x
256, phase encoding direction: A/P), an axial-oblique 2D FLAIR Fast Spin Echo (TE: 90ms,
TR: 9000ms, TI: 2500ms, flip Angle: 150 degrees, slice thickness: 1 mm interleaved, FOV: 256
x 256 mm, matrix size: 256 x 256, phase encoding direction: A/P, Options: Superior/Inferior
saturation pulse On, 80 mm thick) and an axial-oblique 2D DTI sequence (Base sequence: Sin-
gle-shot spin-echo echo planar imaging, TE: 101ms, TR: 9000ms, flip angle: 90 degrees, slice
thickness: 2mm, FOV: 256 x 256 mm, matrix size: 128 x 128, phase encoding direction: P/A,
Options: bandwidth: 1628Hz/Px, echo spacing: 0.7ms, EPI factor: 128). Diffusion weighted
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IL-18-mediated inflammation and white matter injury
images were generated using gradients applied in 60 directions, with total gradient diffusion
sensitivity measured at b = 1000 s/mm2, and 5 volumes with b = 0 s/mm2.
ASPIRE cohort MRI acquisition
MRI was performed on either Siemens Avanto 1.5T or Siemens Trio 3T machines. Axial T2-
weighted images were obtained continuously in 5-mm-thick sections with repetition time of
3800 milliseconds and time to echo of 116 milliseconds. The field of view was 220 mm, and the
matrix was 384x384. Axial FLAIR images were obtained continuously in 5-mm-thick sections
with repetition time of 9000 milliseconds and time to echo of 89 milliseconds. The field of
view was 220 mm, and the matrix was 320x216. Axial diffusion-weighted images were
obtained continuously in 5-mm-thick sections with repetition time of 5600 milliseconds and
time to echo of 106 milliseconds. The field of view was 255 mm, and the matrix was 192x192.
MarkVCID MRI processing
We used DTI measures of free water content (FW), FW-corrected fractional anisotropy
(FACOR) and FW-corrected mean diffusivity (MDCOR). Briefly, DTI images were first prepro-
cessed using FSL software tools [29], including correction for eddy current-induced distor-
tions and participant head movements. Individual uncorrected FA maps were co-registered to
the FSL FA DTI template using linear and nonlinear transformations. Resulting transforma-
tion parameters were inversed and applied to the FSL white-matter labels atlas to provide a
mask of WM region in the native DTI space of the individual. For each individual, overall
measures of mean FW, FACOR and MDCOR were computed by superimposing individual WM
masks onto the respective individual DTI-derived maps and averaging values within these
WM voxels. Segmentation of WMH, hippocampus and total cranial volume (TCV) were per-
formed from FLAIR designed to enhance WMH segmentation [30] and T1-weighted images
by automated procedures previously described and which demonstrates high inter-rater reli-
ability [31–33]. For each individual, overall WMH burden and hippocampus volume were
computed and normalized by TCV to account for differences in head volume. Resulting
WMH burden was also log-transformed to normalize population variance.
ASPIRE cohort Fazekas scoring
Two blinded authors (I.A. and V.K.) evaluated WMH on axial T2-weighted FLAIR images
using the modified Fazekas rating scale to measure hyperintensity burden in periventricular
and deep white matter regions [34, 35]. The total Fazekas score (FS) was obtained by summing
the scores from periventricular and deep white matter regions and the average score used in
subsequent analysis.
Statistical analyses
All statistical analyses were conducted using SPSS (IBM Corp. Released 2013. IBM SPSS Statis-
tics for Windows, Version 22.0. Armonk, NY: IBM Corp.) and SAS 9.4 (SAS Institute Inc.).
Heat maps of ICS scores were generated in Prism (GraphPad). Means, standard deviations
and frequencies are reported for the discovery and validation cohorts. Cohorts, including
those with and without imaging, were compared using t-tests for continuous measures or Chi-
square tests for categorical variables. Linear regression, controlling for age and total intracra-
nial volume, was used to investigate the association of the inflammatory composite score (ICS)
with measures of white matter integrity: WMH, FW, FACOR, and MDCOR. Volumetric WMH
were log-transformed prior to analysis to better meet the assumptions of the regression model.
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IL-18-mediated inflammation and white matter injury
Standardized betas are reported. Correlation coefficients were estimated to assess the associa-
tion between ICS and the total number of vascular risk factors. Principal component analysis
was performed on the serum markers in the ASPIRE cohort to generate two main factors.
Pearson correlations were calculated to assess the association of the principal components and
the ICS score. Linear regressions models, controlling for age and gender, were used to evaluate
the association of the serum principal components as well as the ICS itself on the outcome of
Fazekas score.
Results
MarkVCID and ASPIRE cohort demographics
Fig 1 describes the subject identification, sample collection, and imaging workflows for each
cohort. MarkVCID participants had a mean age of 76.4 ± 7.1 years, mean education of
15.3 ± 3.8 years, and 83 (49.7%) participants identified as female. All participants were func-
tionally intact with 111 participants having a CDR total score of 0.0 and 56 participants having
a CDR total score of 0.5. Participants had an average WMH volume (ml) of 6.94 ± 9.8, FW of
0.23 ± 0.03, FACOR of 0.49 ± 0.08, and MDCOR of 0.54 ± 0.05. Overall, participants with brain
imaging had better vascular and cognitive health (Table 1). ASPIRE participants had a mean
age of 70.8 ± 1.2 years, 60 (45.8%) participants identified as female, and 10 (7.6%) participants
had dementia. MarkVCID participants had an average ICS of 0.004 ± 0.56 and ASPIRE partici-
pants had an average ICS of 0.000 ± 0.60. Individual marker data is shown in Table 2. In the
MarkVCID cohort, inflammatory markers measured in participants with T2-FLAIR imaging
(n = 110) did not significantly differ from subjects without imaging. Participants with DTI
(n = 49) did significantly differ from those without imaging in measures of GDF-15 (t = 2.6,
p = 0.01) and IL-18 (t = 2.7, p = 0.007); participants with DTI had significantly lower levels of
GDF-15 and IL-18. Additional raw imaging and serum inflammatory data are available upon
request.
Although evaluated in different clinical settings, the MarkVCID and ASPIRE Cohorts had
similar levels of vascular factors that increase the risk for cSVD. There were no statistical differ-
ences between the cohorts in gender (p = 0.50), history of myocardial infarction (p = 0.15),
Fig 1. Imaging and fluid analysis workflows in the MarkVCID and ASPIRE cohorts. Workflow diagram of the MarkVCID cohort of 167 subjects that underwent
detailed cognitive evaluations, MRI including DTI, and serum collections (left). Workflow diagram of the ASPIRE cohort of 202 subjects presenting with acute
neurologic symptoms who underwent MRI and concurrent serum collection (right).
https://doi.org/10.1371/journal.pone.0227835.g001
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Table 1. MarkVCID and ASPIRE cohort demographics and vascular health history.
IL-18-mediated inflammation and white matter injury
MarkVCID Cohort
Total
FLAIR
DTI
ASPIRE Cohort
Total
FLAIR vs. None
χ2 (p)
DTI vs. None
χ2 (p)
Total
Gender (F)
CDR = 0
Dementia
Stroke
MI
AFib
HTN
HCL
Diabetes
Age
Education
BMI
Systolic BP
Diastolic BP
n (%)
167 (100)
83 (49.7)
111 (66.5)
0 (0)
30 (18.2)
13 (7.8)
18 (10.8)
107 (64.1)
105 (64.0)
50 (30.3)
Mean (SD)
76.4 (7.1)
15.3 (3.8)
27.6 (5.6)
139 (15.6)
72.3 (7.71)
110 (65.8)
58 (52.7)
80 (72.7)
0 (0)
12 (11.1)
6 (5.5)
11 (10.0)
63 (57.3)
59 (54.6)
23 (21.1)
76.0 (6.9)
15.6 (3.6)
26.6 (5.3)
138 (15.9)
72.9 (7.8)
49 (29.3)
27 (55.1)
40 (81.6)
0 (0)
6 (12.2)
4 (8.2)
5 (10.2)
23 (46.9)
21 (43.7)
5 (10.4)
76.6 (7.0)
16.3 (3.0)
25.9 (4.9)
135 (17.7)
73.4 (8.7)
1.2 (0.33)
5.7 (0.024)�
10.5 (0.001)��
2.4 (0.1)
0.2 (0.6)
6.5 (0.01)�
12.1 (0.001)��
12.9 (0.001)��
t (p)
1.4 (0.2)
-1.3 (0.1)
2.9 (0.004)��
1.6 (0.10)
-1.2 (0.2)
0.8 (0.40)
7.2 (0.007)��
1.6 (0.20)
0.01 (0.9)
0.02 (0.9)
8.84 (0.003)��
12.1 (0.001)��
12.7 (0.001)��
t (p)
-0.24 (0.8)
2.5 (0.01)�
2.6 (0.01)�
2.3 (0.02)�
-1.1 (0.3)
Total
Gender (F)
CDR = 0
Dementia
Stroke
MI
AFib
HTN
HCL
Diabetes
Age
Education
BMI
Systolic BP
Diastolic BP
n (%)
131 (100)
60 (45.8)
N/A
10 (7.63)
51 (38.9)
5 (3.8)
17 (13.0)
69 (52.7)
45 (34.4)
30 (22.9)
Mean (SD)
70.8 (1.2)
N/A
N/A
159.0 (2.9)
86.5 (1.5)
Demographic and vascular health information for 167 MarkVCID Cohort participants and 131 ASPIRE Cohort participants. Chi squared and T-tests evaluated the
group differences between MarkVCID Cohort participants with FLAIR imaging or DTI, and those without. Overall, participants with brain imaging had better vascular
and cognitive health. Missing data for Mark VCID: Stroke (n = 2), HCL (n = 3), Diabetes (n = 2). F = Female, CDR = Clinical Dementia Rating, MI = Myocardial
Infarction, AFib = Atrial Fibrillation, HTN = Hypertension, HCL = Hypercholesterolemia, BMI = Body Mass Index, BP = Blood Pressure
https://doi.org/10.1371/journal.pone.0227835.t001
history of atrial fibrillation (p = 0.56), or history of diabetes (p = 0.17). As expected by partici-
pant enrollment procedures, the ASPIRE Cohort participants had more strokes (p<0.0001),
and higher rates of dementia (p = 0.0003). The MarkVCID cohort was older in age (p<0.0001)
and had a higher proportion of participants with hypertension (p = 0.047) and hypercholester-
olemia (p<0.0001).
Protein interactions
With independent evidence supporting a role for MPO, GDF-15, RAGE, ST2, IL-18, and
MCP-1 in the development of white matter hyperintensities, we asked whether this group of
validated biomarkers might be interconnected biologically. We performed STRING database
Table 2. MarkVCID and ASPIRE cohort inflammation levels.
MarkVCID Cohort (n = 167)
Marker (pg/mL)
IL-18
ST2
MPO
MCP-1
GDF-15
RAGE
Mean (SD)
363.4 (134.3)
13146.1 (6995.1)
111567.7 (103133.0)
300.8 (161.9)
1887.8 (1764.4)
1949.3 (958.9)
ASPIRE Cohort (n = 131)
Marker (pg/mL)
IL-18
ST2
MPO
MCP-1
GDF-15
RAGE
Mean (SD)
397.6 (211.2)
22113.2 (33073.9)
254458.16 (394569.63)
457.5 (222.1)
2758.4 (3575.5)
2543.5 (1358.6)
Inflammatory marker data measured on 167 participants in the MarkVCID Cohort and 131 participants in the
ASPIRE Cohort.
https://doi.org/10.1371/journal.pone.0227835.t002
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IL-18-mediated inflammation and white matter injury
Fig 2. ICS components form an inflammatory network. STRING database query of the six ICS component analytes
reveals a biologically interconnected network centered on IL-18 and highly related to inflammation (p-value for protein
interactions = 0.00022). ICS component analytes are shown as colored nodes (bold) while first level interacting proteins
are shown as white nodes. Line width reflects the strength of data support.
https://doi.org/10.1371/journal.pone.0227835.g002
analysis of these six components to determine if they function in a biologic network. Using the
6 validated protein biomarkers with a first shell of interactors, we identified a biologic network
centered on IL-18 that was enriched for protein-protein interactions (p = 2.14x10-8) (Fig 2).
This network was enriched for 75 gene ontology terms including positive regulation of leuko-
cyte activation (GO.0002696, FDR = 0.00064); positive regulation of inflammatory response
(GO.0050729, FDR = 0.0012); cytokine receptor binding (GO.0005126, FDR = 0.0015); cyto-
kine activity (GO.0005125, FDR = 0.0015), and extracellular region (GO.0005576, FDR =
0.00029). The major signaling pathways center on interleukin signaling and IL-18 is the most
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IL-18-mediated inflammation and white matter injury
connected node at the center of the network. Step-wise expansion of the network by known
protein-protein interactions reveals a complex and tightly interconnected network that is
strongly enriched for cytokine and immune system regulatory elements.
Association of an IL-18 inflammatory network and white matter injury
To determine how these interconnected biomarkers relate to vascular risk factors, white matter
hyperintensities, and DTI measures of white matter injury, we used the means and standard
deviations across the MarkVCID sample (n = 167) to create z-scores for each analyte for each
participant. Z-scores for each analyte were averaged to generate an inflammatory composite
score (ICS) for each subject. This approach reduces the impact of the relatively high population
standard deviations common in biomarker studies. The z-scores for each ICS component ana-
lyte for each individual subject are shown in Fig 3A demonstrating that cumulative ICS was not
driven by one outperforming analyte but rather reflect a true composite of the interacting
inflammatory network. ICS was significantly associated with white matter hyperintensities
(logWMH) (Fig 3B; β = 0.222, p = 0.013) as well as DTI FW (Fig 3C; β = 0.3, p = 0.01) but not
with DTI FACOR (β = 0.004, p = 0.98) or MDCOR (β = -0.2, p = 0.2). Spatial maps of average FW
and WMH distributions on MRI in those subjects with low ICS (below median; upper panel)
and high ICS (above median; middle panel) as well as the difference (lower panel) demonstrates
the effect of high levels of IL-18 driven inflammation on subcortical white matter injury (Fig 4).
The inflammatory composite score and vascular risk
Vascular risk factors such as hypertension, hyperlipidemia, and diabetes increase the risk of
developing white matter hyperintensities. Therefore, we reasoned that if ICS positively associ-
ates with white matter injury by MRI, then ICS should also scale with the burden of cardiovas-
cular risk factors. The number of vascular risk factors significantly correlates with ICS (0.36,
p<0.001). Categorization of the MarkVCID cohort by number of vascular risk factor diagnoses
compared to those with fewer vascular risk factor diagnoses reveals a step-wise increase in
mean ICS (Fig 5). Notably, the magnitude of the difference in mean ICS values increases as the
number of vascular factors increases. Table 3 shows the group differences between MarkVCID
cohort subjects with specific vascular risk factor diagnoses and those without.
Association of ICS with white matter injury in those at risk for stroke
To confirm the ability of ICS to detect WMH, we utilized serum samples from the ASPIRE
biomarker study of acute stroke (n = 202), a single center study designed to identify acute bio-
markers for ischemic stroke. Acutely obtained MRI scans (n = 168) were independently evalu-
ated using the modified Fazekas scoring method. In those subjects with acutely obtained
serum samples (n = 131), the average mean modified Fazekas score was 2.50 ± 1.53. Serum
samples were assayed for biomarker levels and a principal component analysis was performed
on 11 serum markers described in the methods section. This PCA identified two factors with
eigenvalues >1 that account for 53% of the variance (Fig 6A). In this independent cohort, Fac-
tor 1 significantly correlated with ICS (r = 0.94, p<0.0001) (Fig 6B). The most significant con-
tributors to Factor 1 were the log-normalized values of ST2, RAGE, GDF15, and IL-18, all core
markers included in the ICS.
In an age- and gender-adjusted generalized linear regression model, the addition of Factor
1 significantly improved the detection of WMH as measured by the average modified Fazekas
score in this cohort (p = 0.0267). Due to the strong correlation between Factor 1 and ICS in
this cohort, we also assessed associations between ICS and WMH. In bi-variate analysis, ICS
significantly correlated with average modified Fazekas score (p<0.0001) (Fig 6C) highly
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IL-18-mediated inflammation and white matter injury
Fig 3. ICS correlates with MRI measures of cerebrovascular injury. Heatmap of z-scores for each of the individual analytes composing the ICS for each included
subject in the MarkVCID cohort ordered left to right by ICS (average z-score of each analyte) (A). Scatter plot and regression line of logWMH vs. ICS (n = 110) (B).
Scatter plot and regression line of free water vs. ICS (n = 49) (C). Red dashed lines indicate 95% confidence intervals.
https://doi.org/10.1371/journal.pone.0227835.g003
similar to the relationship of ICS with volumetric WMH in the MarkVCID cohort. In an age-
and gender-adjusted model, the association between ICS and Fazekas score was p = 0.083.
Representative FLAIR images from ASPIRE subjects with low and high ICS demonstrate the
association with WMH (Fig 6D). Other demographic factors available in the ASPIRE cohort
including stroke, hypertension, diabetes, and obesity were excluded from the model as they
did not demonstrate significant effects on Fazekas score.
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IL-18-mediated inflammation and white matter injury
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IL-18-mediated inflammation and white matter injury
Fig 4. Association of ICS with overt and antecedent white matter injury. Average intensity maps of free water (FW) and
frequency maps of white matter hyperintensities (WMH) of groups with low (upper) and high (middle) ICS groups dichotomized
around median ICS. Lower panels illustrate voxel differences in FW and WMH between low and high ICS groups.
https://doi.org/10.1371/journal.pone.0227835.g004
Discussion
Cerebral small vessel disease contributes to dementia [36] and increases both the risk of stroke
[37] and poor outcomes after stroke [38]. The progressive, silent development of cerebral
small vessel disease necessitates the development of alternative methods for identifying those
at risk. In population studies, mid-life vascular risk factors increase the risk of white matter
injury resulting from cSVD [17]. However, the identification of biomarkers to assist in separat-
ing those with concurrent vascular risk factors yet no brain injury from those with vascular
risk factors who already have evidence of pathology from cerebral small vessel disease is critical
to developing therapeutic strategies to stem this growing public health challenge. Advances in
imaging techniques such as DTI free water are one approach to detecting a higher risk popula-
tion [12]. Reliable fluid-based biomarkers for early detection are another approach with cer-
tain advantages over imaging including accessibility, applicability, and the ability to test
frequently enabling repeated measurements. Various proteomic and single molecule
approaches for fluid biomarkers that associate with WMH have shown associations but lack an
integrated conceptual framework that drives at disease pathogenesis. Here, we show that a bio-
logically interconnected network of molecules reflecting a composite measure of inflammation
is associated with T2/FLAIR white matter hyperintensities in both an aging community-based
population and a population presenting for evaluation of acute neurologic symptoms. We also
show that this composite measure of inflammation is associated with increases in DTI free
water, further implicating an IL-18-centered inflammatory network in the disease process
underlying cerebral small vessel disease. These data demonstrate a new, reproducible tool to
identify those with and at risk for cSVD.
Fig 5. ICS increases with vascular risk factors. Mean ICS in groups with one or more vascular risk factor diagnoses
(red) vs. those with less vascular risk factor diagnoses (black). All group comparisons were statistically significant at
adjusted p<0.008 except between those with 6 vascular risk factor diagnoses (n = 2).
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IL-18-mediated inflammation and white matter injury
Table 3. ICS associates with vascular risk factor diagnoses.
Vascular Risk Factor Diagnosis
Diff in Mean ICS
Atrial fibrillation (n = 18/167)
Stroke (n = 30/165)
Hypertension (n = 107/167)
Hyperlipidemia (n = 105/164)
Diabetes (n = 50/165)
Myocardial infarction (n = 13/167)
0.415
0.427
0.253
0.209
0.399
0.146
p-value
0.003
0.000
0.004
0.021
<0.001
0.364
Group differences between MarkVCID cohort subjects with specific vascular risk factor diagnoses. Missing data for
Mark VCID: Hx Stroke (n = 2), Hx HCL (n = 3), Hx Diabetes (n = 2).
https://doi.org/10.1371/journal.pone.0227835.t003
Existing data on purported serum and plasma biomarkers for cSVD strongly implicate
inflammation in the cSVD disease process. In a study on 163 lacunar stroke patients and 183
hypertensive patients, patients with evidence of cSVD on brain MRI had significantly elevated
levels of inflammatory markers: neopterin, sICAM-1 and sVCAM-1 [39]. Elevated levels of
inflammation are associated with increased risk of major vascular events, infarct size, and
death [14, 40, 41]. In the Framingham Study, men and women in the highest quartile of CRP
levels at baseline had two to three times the risk of ischemic strokes compared to those in the
lowest CRP quartile [42]. Similar increases in lacunar stroke risk were seen in subjects with ele-
vated CRP in the SPS3 trial [43]. In this study, we selected MPO, GDF-15, RAGE, ST2, IL-18,
and MCP-1 because research using each marker independently showed that the markers may
be related to cSVD. Unlike previous studies, our team investigated the mechanistic relation-
ship between MPO, GDF-15, RAGE, ST2, IL-18, and MCP-1 and discovered that the markers
were related in a biological pathway. Via STRING database analysis, we identified a biologic
network centered on IL-18.
IL-18 is a pleotropic pro-inflammatory cytokine implicated in multiple autoimmune disor-
ders [44], vascular disease [19–21], acute stroke [45], and can be both generated and have action
within the CNS [46]. Within the brain, IL-18 is largely produced by neurons [47, 48] but can
also be found in infiltrating immune cells after ischemia [49, 50]. Here, we propose that the
action of this IL-18 inflammatory network is to damage cerebral small vessels at the blood-brain
barrier interface. The role this pathway plays in regulating downstream white matter injury
resulting from IL-18-mediated cerebral vessel injury is unknown. The action of IL-18 is tightly
regulated by IL-18 binding protein (IL-18BP) [51] and in autoimmune diseases, the serum IL-
18/IL-18BP ratio is associated with disease severity [52–54]. Indeed, blocking the action of IL-
18 using recombinant human IL-18BP (Tadekinig Alfa) is in late stage clinical trials for a num-
ber of autoimmune disorders [55, 56]. Future studies may consider measuring IL-18BP levels
and/or targeting IL-18 as a novel therapeutic strategy for cerebral white matter injury.
By identifying the biologic connectivity of previously reported inflammatory cytokines and
molecules, we begin to apply a more rigorous systems biology approach to the identification of
reliable fluid biomarkers for cerebral small vessel disease. Harnessing this biologic connectivity
provided a clear advantage in this study as evidenced by a strong correlation of ICS with the
results of an unbiased principal components analysis (PCA) in a distinct cohort at increased
risk for cSVD. By using PCA to identify an independent association of a collection of biomark-
ers (F1) that strongly correlates with our previously generated ICS in a different cohort, we
functionally validate the use of the population mean-adjusted ICS to detect cSVD.
Systemic vascular risk factors have long been associated with increased inflammation that
can be indirectly or directly measured [57]. High-sensitivity CRP (hsCRP) is the best example
PLOS ONE | https://doi.org/10.1371/journal.pone.0227835 January 24, 2020
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IL-18-mediated inflammation and white matter injury
Fig 6. ICS is associated with white matter injury in those at risk for stroke. Scree plot of principal components analysis of data from ASPIRE cohort serum biomarker
panel with two main factors (Factor 1 and Factor 2) driving variance (A). Scatter plot of Factor 1 values versus ICS in this cohort demonstrating a significant correlation
(r = 0.94) (B). Scatter plot and regression line of modified Fazekas score and ICS for individual subjects (C). Red dashed lines indicate 95% confidence intervals.
Representative T2/FLAIR MR images of ASPIRE subjects with low (left) or high (right) ICS scores (D).
https://doi.org/10.1371/journal.pone.0227835.g006
of an indirect inflammatory marker associated with vascular risk, white matter hyperintensi-
ties, and recurrent lacunar stroke. Vascular risk factors drive hsCRP levels upwards but pro-
vide no pathogenic clues to the underlying disease process and therefore require a large study
population to verify their association with a disease outcome [58]. A number of inflammatory
pathways with more direct signaling cascades have been associated with vascular risk factors
including IL-18. Here we show evidence that a composite inflammatory measure (ICS) steadily
increases as the number of vascular risk factors increases and that this associates with measures
of silent cerebrovascular injury. Therefore, ICS could add to a clinical evaluation of stroke and
dementia risk by providing a numerical severity to an individual subject’s cerebral microvascu-
lar injury and ongoing risk assessment [59]. Our cohorts lack sufficient data to determine the
effect of vascular risk factor control on ICS. Presumably, sustained uncontrolled risk factors
such as hypertension and diabetes would promote higher inflammatory composite scores
PLOS ONE | https://doi.org/10.1371/journal.pone.0227835 January 24, 2020
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IL-18-mediated inflammation and white matter injury
however this remains to be determined. Precisely how these molecules damage the cerebral
endothelia and promote the development of white matter injury is unknown. The present data
implicate a signaling pathway centered on IL-18 as a potential driver of cerebral endothelia
damage. Future studies can systemically test how persistent elevations of inflammatory cyto-
kines directly damage cerebral endothelia and lead to white matter damage. Advances in iso-
lating endothelial exosomes [2] will likely prove helpful to elucidate these mechanisms.
Establishing the extent to which these silent changes may be reversible seems particularly criti-
cal to establish.
DTI free water is an emerging MR metric that indirectly measures leakage of extracellular
fluid into white matter and precedes the development of T2/FLAIR white matter hyperintensi-
ties. DTI free water is associated with vascular risk factors including systolic blood pressure
and arterial stiffness [11] and more recently has been shown to be associated with cognitive
decline [13]. Exactly what DTI free water is measuring in tissue is unknown, however one
hypothesis is that excess extracellular fluid results from blood-brain barrier leakage with leak-
ing serum proteins damaging myelin and axons. Leakage of the blood-brain barrier is pro-
posed to play a central role in the pathogenesis of cSVD with increased contrast-enhancement
within T2/FLAIR white matter hyperintensities compared to normal appearing white matter
using dynamic contrast enhanced MRI (DCE-MRI) techniques [60, 61]. In a population of
recent lacunar stroke patients, blood-brain barrier leakage within white matter was also associ-
ated with impaired cognition at 1 year. The present study provides a mechanistic link between
a cocktail of markers of peripheral inflammation and blood-brain barrier leakage in relation-
ship to cSVD by demonstrating clear cross-sectional relationships between ICS and DTI free
water. Further studies will be needed to establish causality between ICS and BBB leakage using
longitudinal measures of fluid biomarkers and imaging with DCE-MRI.
In the presented imaging data from the MarkVCID cohort, we did not observe any clear
regional pattern to the differences in either WMH or free water measures between those indi-
viduals with high or low ICS. This finding suggests that the observed association between
WMH and free water with ICS is independent of differences in blood pressure and flow
between anterior and posterior circulations. This result is not surprising given that the whole
brain vasculature is exposed to elevated systemic inflammatory signals and whatever changes
are induced are likely distributed globally throughout the brain. Notably, we also did not see
any association between ICS and clinical stroke in the ASPIRE cohort, indicating that the ele-
vated levels of inflammation measured by ICS are specifically linked to cSVD rather than an
overall increased risk of cerebrovascular disease.
This study establishes a cross-sectional relationship between interconnected inflammatory
molecules and MRI indicators of cerebral small vessel disease in two distinct populations. Lim-
ited to cross-sectional relationships, it does not firmly establish that IL-18-mediated inflamma-
tion is associated with cognitive decline nor with other conditions associated cSVD indicators,
namely the risk of stroke and/or dementia. However, because ICS scales additively with
increased vascular risk factors, which in turn are known to increase the risk of white matter
hyperintensities and impaired cognition, we expect that longitudinal studies will demonstrate
that ICS is predictive of longitudinal declines in cognition and/or the risk of future stroke.
Additionally, we did not observe an association between ICS and other DTI metrics such as
FA or MD. DTI free water and WMH are more directly linked on a continuum of white matter
injury related to inflammation and blood-brain barrier leakage while FA and MD more
directly reflect the integrity of axons within a functional tract. Moreover, the MarkVCID
cohort is largely cognitive normal and with relatively healthy brains, and therefore lacks a wide
range of FA/MD values. A further limitation of this study is the contrast in imaging methodol-
ogies used in the varying cohorts. The lack of precise volumetric assessment of white matter
PLOS ONE | https://doi.org/10.1371/journal.pone.0227835 January 24, 2020
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IL-18-mediated inflammation and white matter injury
lesion volume in the ASPIRE cohort may underestimate the true burden of cSVD in this popu-
lation. However, the ability of this set of inflammatory markers to retain its relationship with
subjectively graded WMH in this population suggests that our approach in generating a popu-
lation mean-adjusted composite inflammatory score may have broad generalizability as a bio-
marker for cSVD in at-risk with different risk factor profiles and demographics.
Conclusion
Cerebral small vessel disease is provoked by cardiovascular risk factors through increased sys-
temic sterile inflammation. This increase in systemic inflammation may be associated with a
specific inflammatory pathway involving IL-18 signaling that can be targeted for therapeutic
engagement. Fluid-based biomarkers to reliably identify those at risk for and with early indica-
tors of cerebral small vessel disease resulting from inflammation can provide a widely accessi-
ble method for risk assessment, monitoring, and therapeutic development.
Supporting information
S1 File. Permanent weblink to STRING database results for ICS components.
(DOCX)
Acknowledgments
The authors are grateful to the support staff of the Memory and Aging Center and the UCLA
Stroke Force for help in subject enrollment.
Author Contributions
Conceptualization: Marie Altendahl, Pauline Maillard, Sunil A. Sheth, Guanxi Xiao, Fanny
Elahi, Jason D. Hinman.
Data curation: Marie Altendahl, Devyn Cotter, Samantha Walters, Amy Wolf, Baljeet Singh,
Visesha Kakarla, Ida Azizkhanian, Emily Fox, Mei Leng, Jason D. Hinman.
Formal analysis: Marie Altendahl, Pauline Maillard, Danielle Harvey, Baljeet Singh, Visesha
Kakarla, Ida Azizkhanian, Sunil A. Sheth, Mei Leng, David Elashoff, Jason D. Hinman.
Funding acquisition: Sunil A. Sheth, Joel H. Kramer, Charlie Decarli, Fanny Elahi, Jason D.
Hinman.
Investigation: Marie Altendahl, Sunil A. Sheth, Emily Fox, Michelle You, Joel H. Kramer,
Charlie Decarli, Fanny Elahi, Jason D. Hinman.
Methodology: Pauline Maillard, Danielle Harvey, Guanxi Xiao, Mei Leng, David Elashoff, Joel
H. Kramer, Fanny Elahi.
Project administration: Devyn Cotter, Samantha Walters, Amy Wolf, Sunil A. Sheth, Emily
Fox, Michelle You.
Resources: Joel H. Kramer, Charlie Decarli.
Software: Pauline Maillard, Charlie Decarli.
Supervision: Sunil A. Sheth, Joel H. Kramer, Charlie Decarli, Jason D. Hinman.
Validation: Danielle Harvey, Mei Leng, Jason D. Hinman.
Visualization: Pauline Maillard, Baljeet Singh, Visesha Kakarla, Ida Azizkhanian, Charlie Dec-
arli, Jason D. Hinman.
PLOS ONE | https://doi.org/10.1371/journal.pone.0227835 January 24, 2020
16 / 20
IL-18-mediated inflammation and white matter injury
Writing – original draft: Marie Altendahl, Pauline Maillard, Danielle Harvey, Mei Leng, Joel
H. Kramer, Charlie Decarli, Fanny Elahi, Jason D. Hinman.
Writing – review & editing: Danielle Harvey, Ida Azizkhanian, Charlie Decarli, Jason D.
Hinman.
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10.1371_journal.pone.0283603.pdf
|
Data Availability Statement: All relevant data are
within the paper and its Supporting information
files.
|
All relevant data are within the paper and its Supporting information files.
|
RESEARCH ARTICLE
When crisis hits: Bike-Sharing platforms amid
the Covid-19 pandemic
Ecem BasakID
1*, Ramah Al Balawi1, Sorouralsadat Fatemi2, Ali Tafti2
1 Zicklin School of Business, Baruch College, City University of New York, New York, New York, United
States of America, 2 College of Business Administration, University of Illinois at Chicago, Chicago, Illinois,
United States of America
* [email protected]
Abstract
In this work, we examine the changes in demand for bike-sharing platforms with the onset of
the Covid-19 pandemic. Using the fixed-effects regression formulation of difference-in-differ-
ences, we evaluate how the demand for bike-sharing platforms changed after the first cases
of Covid were discovered and after the first executive orders were implemented. Accounting
for weather conditions, socio-economic characteristics, time trends, and fixed effects across
cities, our findings indicate that there is an increase in daily bike-sharing trips by 22% on
average after the first Covid-19 case diagnosis, and a decrease of 30% after the first execu-
tive order implementation in each municipality, using the data up to August 2020. Moreover,
we observe a 22% increase in weekday-specific trip frequency after the first Covid-19 case
diagnosis and a 28% decrease in weekend-specific trip frequency after the first executive
order implementation. Finally, we find that there is an increase in the frequency of trips on
bike-sharing platforms in more bike-friendly, transit-friendly, and pedestrian-friendly cities
upon both the first Covid-19 case diagnosis and the first executive order implementation.
Introduction
The Covid-19 pandemic has significantly affected our societal and economic structures. Man-
dated lockdowns and voluntary precautions, which are taken to reduce the spread of the virus,
have affected the demand for all modes of transportation, including public transport in cities.
For example, Aloi et al. [1] indicate a fall of 76% in overall human mobility and a 93% decrease
in public transport usage in Santander, Spain. Using aggregated mobility data from mobile
phones in numerous urban areas in the U.S., Kishore et al. [2] show a surge in travel out of the
cities immediately preceding the stay-at-home advisory. Another study points out a significant
reduction in traffic volumes of 30% to 50% for select highways in California compared to prior
shelter-in-place orders [3].
Individuals have changed their transportation patterns as personal travel decisions affect
the spread of Covid-19 [4]. In response to the social distancing order, people have been less
inclined to board packed buses and trains where social distancing is impossible. Accordingly,
individuals reevaluate their transportation options in the face of the Covid-19 pandemic and
shift to more isolated modes such as biking or walking.
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OPEN ACCESS
Citation: Basak E, Al Balawi R, Fatemi S, Tafti A
(2023) When crisis hits: Bike-Sharing platforms
amid the Covid-19 pandemic. PLoS ONE 18(4):
e0283603. https://doi.org/10.1371/journal.
pone.0283603
Editor: Charitha Dias, Qatar University, QATAR
Received: February 6, 2022
Accepted: March 13, 2023
Published: April 7, 2023
Copyright: © 2023 Basak et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting information
files.
Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
PLOS ONE | https://doi.org/10.1371/journal.pone.0283603 April 7, 2023
1 / 20
PLOS ONEBike-Sharing platforms and Covid-19
With the Covid-19 pandemic, we witness an increasing awareness of bicycles as an alterna-
tive means of transport, as many people either avoid using mass transit or encounter reduced
mass transit services. In the U.S., sales of bicycles and related equipment also almost doubled
in March 2020 compared with the same period in 2019 [5]. In times of crisis, bicycles can pro-
vide resilience in transport systems, satisfying our mobility needs when mass transit systems
are inaccessible. For instance, during the national public transit strike in France in December
2019, Parisians adapted and learned that bikes are dependable and credible modes of transport.
The bike-sharing system in Paris, Ve´lib, gained popularity during the strike [6]. Other exam-
ples are the 2005 New York City transit worker strike and Hurricane Sandy in 2012, which
severely disrupted New York’s subway system. These events led to an increase in bicycle rider-
ship in the city of New York by about 20% [7]. Bicycle sales surged in Japan after earthquakes
struck that country in 2011 [8].
With the surge in demand for bikes, the popularity of bike-sharing platforms has also
increased in March compared to the same period in the year before. They have become a viable
transport alternative that reduces the risk of contracting or spreading the virus and relieves the
fear of overcrowding [9, 10]. Compared to other means of transportation systems such as
buses or trains, bicycling is an open-air activity and helps to avoid close contact with other
travelers. Therefore, people have a more positive attitude toward bike-sharing for traveling
amidst the pandemic [11]. For instance, Citi Bike in New York City announced a 67% increase
in demand between March 1, 2020, and March 11, 2020, compared with the same period in
2019. Divvy in Chicago has also reported that the number of trips doubled in the same period
[12]. A report from Foursquare and Apptopia shows that bike-share mobile application instal-
lations in May and June of 2020 were up 15.6% and 23.3%, respectively, compared to the prior
year [13]. A recent study by Li et al. [14] analyzed the demand for the bike-sharing platform in
London over the period from January 2019 to June 2020. They found that the number of bike-
sharing trips in London decreased after the lockdown; however, it was followed by an increase
in demand over time. Heydari et al. [15] investigated the impact of the Covid-19 pandemic on
the London bike-sharing platform over the period from March 2020 to December 2020. They
initially observed a reduction in bike trips between March and April 2020; however, demand
increased in May and June 2020.
Moreover, Bouhouras et al. [16] found that the demand for bike-sharing platforms in
Greek cities such as Igoumenitsa, Chania, and Rhodes increased in a short period of time
before the lockdown period and peaked during the lockdown. Wang and Noland [17] exam-
ined the impact of Covid-19 on both bike-sharing and subway usage. They found that both
subway ridership and bike-sharing usage plummeted at the beginning; however, bike-sharing
usage has almost returned to normal, whereas subway ridership has remained substantially
below pre-pandemic levels. Other recent studies [18, 19] also revealed that bike-sharing plat-
form usage in many cities has reached or surpassed pre-pandemic levels [20].
It is argued that bike-sharing demand has plummeted because of lockdown waves, even
though it shows higher resiliency and lower drop than subway systems [10]. Following the
stay-at-home advisories, many companies began to allow their employees to work from home,
resulting in a significant reduction in travel within cities. Studies suggest that the demand for
bike-sharing platforms has also decreased due to increased levels of remote working and stay-
at-home advisories, but not as much as other means of transportation. A study conducted in
Budapest, Hungary, shows that there has been an 80% decrease in public transport demand
and only a 2% reduction in the use of bike-sharing platforms during the pandemic [21].
Another study that used ridership data from New York in 2020 showed that bike-sharing trips
have decreased by less than 71%, whereas subway trips have decreased by 90% compared to
February and March of 2019 [10].
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PLOS ONEBike-Sharing platforms and Covid-19
Another study by Li et al. [22] examined the changes in demand for micro-mobility services
such as bike-sharing platforms in Zurich, Switzerland, before and during the lockdown period.
Their spatial and temporal analysis results showed a decrease in the number of trips during the
lockdown period. Their study also revealed that leisure- and shopping-related micro-mobility
trips decreased while grocery-related trips increased. Apple mobility data shows that, by the
end of May 2020, there was a decrease in all modes of transportation including driving, walk-
ing, public transit, and bike-sharing; however, the reduction in public transit ridership was
down much more than bike-share usage [23]. According to a press release by the Bureau of
Transportation Statistics, ridership on eight of the largest docked bike-share systems in the U.
S. declined by 44% from March through May 2020, compared to the same period in 2019 [24].
This could be explained by the fact that people have been traveling less due to stay-at-home
advisories and limited business operations, and this might be affecting the demand for bike-
sharing platforms like other transportation modes.
In this paper, we examine how demand for bike-sharing platform usage changed immedi-
ately following the first Covid-19 case and the first executive order in the U.S., using the data
from January 2019 to July 2020. We use a fixed-effects econometric formulation of the differ-
ence-in-differences (DID) estimation framework, which exploits a natural experiment to
examine how bike-sharing trips have changed with the introduction of the first Covid-19 case
and the first executive order. The DID estimation method is a suitable technique in our con-
text, where randomization on the city level is not possible. DID requires panel data, which is
part of the fixed-effects strategy, to capture the differences in post-treatment periods across the
treatment and control groups [25]. One benefit of the DID model is that it allows us to avoid
“the endogeneity problems that typically arise when making comparisons between heteroge-
neous individuals” [26].
We also consider how the frequency of bike-sharing platform use can be different on week-
days compared to weekends due to the changing travel patterns during the pandemic. In gen-
eral, weekday travel is primarily made up of commuting to and from work, whereas weekend
travel behavior is motivated by recreational activities. Differences in activity types can lead to
different travel patterns that can be hypothesized on weekends and holidays, compared to
weekdays [22, 27, 28]. For instance, Agarwal [27] suggests that there is a decrease in vehicle
trips on weekends compared to weekdays at the household level. Kim et al. [28] find different
weekend and weekday bike-sharing patterns. Their results point out that there is an increase
in bike-sharing traffic volume on the weekends at the stations near parks and schools, which
can be due to the rise in leisure and school activities on the weekends. In contrast, residential
areas and subway stations are found to have less bike-sharing traffic volume on the weekends
than on weekdays. Li et al. [22] find that there was a decrease in the number of micro-mobility
service trips on weekdays during the lockdown period in Zurich. In contrast, there are only
slight changes on weekends compared to before the lockdown period.
In addition, we investigate what factors strengthen or weaken the impact of the pandemic on
the frequency of bike-sharing use. Transportation infrastructure, land use, and neighborhood
attributes contribute to individuals’ preference for bike-sharing [29]. Several studies examine
the effects of the built environment, cycling facilities, transit proximity, and transportation facil-
ity features on bike-sharing frequency [30–32]. The findings are consistent: More bicycle facili-
ties and more excellent transit proximity lead to greater use of bike-sharing. Recent studies [33,
34] also find that better biking infrastructure is linked to higher bike-sharing demand during
the Covid-19 pandemic. For instance, according to Bergantino et al. [33], safer cycling condi-
tions and the creation of dedicated infrastructures encourage individuals to use bike-sharing
platforms during the pandemic. Therefore, we also test the heterogeneous effects depending on
such factors as the city’s bike-friendliness, transit-friendliness, and pedestrian-friendliness.
PLOS ONE | https://doi.org/10.1371/journal.pone.0283603 April 7, 2023
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PLOS ONEBike-Sharing platforms and Covid-19
The remainder of the paper is organized as follows. The next section introduces the data
collection, variable definitions, and research method. Section 3 presents the main results. Sec-
tion 4 shows the heterogeneous effects of bike-friendliness, transit-friendliness, and pedes-
trian-friendliness. Finally, Section 5 concludes with a discussion of the findings.
Materials and methods
Data and variables
We collected data from multiple sources. First, we collected the historical daily trip data avail-
able to the public from bike-sharing programs in Austin, Boston, Chicago, Columbus, Minne-
apolis, New York, Philadelphia, Pittsburgh, Portland, San Francisco, and Washington, D.C.
The daily trip data includes trip duration, start time, end time, starting station, ending station,
and subscription type (i.e., member, single rider). Based on this data, we compute our depen-
dent variable, the daily trip frequency of bike-sharing platform trips (TripFrequencyij). It is cal-
culated as the total daily trips of bike-sharing platform i at time j. During the construction of
this variable, we excluded the bike-sharing platform trips with a duration of two minutes or
less as there might be an issue while renting the bike (i.e., the bike is in a bad condition). We
also excluded the bike-sharing platform trips of forty-five minutes or more, as these trips are
more likely to represent leisure and recreational trips. A single ride for subscribers of these ser-
vices includes forty-five minutes of ride time.
Second, we collected data from various online sources to construct our treatment measures,
FirstCaseij and FirstExecutiveOrderij. Our first treatment variable is the first Covid-19 case
diagnosis (FirstCaseij), which is coded as 1, indicating that the first Covid-19 case is identified
in city i as of day j’ such that j > = j’. The data on Covid-19 cases comes from The New York
Times [35], which is based on reports from state and local health agencies. Our second treat-
ment variable is FirstExecutiveOrderij, which is coded as 1, indicating that an executive order is
issued in city i as of day j’ such that j > = j’. Specifically, we refer to the first executive action
taken by the state governments against the Covid-19 pandemic, which is the stay-at-home
advisories announced by the state governments. While stay-at-home advisories were lifted
before August 2020, restrictions continued in most cities in various forms. For instance, Min-
nesota’s stay-at-home advisory expired on May 18, 2020; however, it was replaced with a "stay
safe Minnesota" order. Moreover, the state extended the state of emergency by another 30
days. Therefore, as the restrictions were still in effect, cities did not fully reopen before August
2020. Consequently, we used the entire period after the first executive order implementation.
Table 1 lists the start date for each of our treatment variables in each city in our study.
Table 1. Treatment start dates.
City
Austin, TX
Boston, MA
Chicago, IL
Columbus, OH
Minneapolis, MN
New York, NY
Philadelphia, PA
Pittsburgh, PA
Portland, OR
San Francisco, CA
Washington D.C.
First Covid-19 case diagnosis
First executive order
03/13/2020
02/01/2020
01/24/2020
03/14/2020
03/12/2020
03/02/2020
03/09/2020
03/14/2020
03/10/2020
03/05/2020
03/08/2020
03/24/2020
03/24/2020
03/21/2020
03/23/2020
03/27/2020
03/22/2020
03/23/2020
03/23/2020
03/23/2020
03/17/2020
04/01/2020
https://doi.org/10.1371/journal.pone.0283603.t001
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PLOS ONEBike-Sharing platforms and Covid-19
To construct the control variables, we collected weather data from the National Oceanic
and Atmospheric Administration (NOAA). Control variables are included in the model as
pre-treatment variables, as weather conditions can have different impacts on the demand for
bike-sharing trips. Evidence from empirical studies [36–39] indicates that favorable weather
conditions, such as higher temperatures, increase bike-sharing platform usage. In contrast,
unfavorable conditions, such as precipitation and strong winds, will decrease such use. For
example, Gebhart and Noland [36] suggest that cold temperatures, rain, and high humidity
levels are likely to reduce the demand for bike-sharing platform trips in Washington, DC. In
contrast, high temperatures are linked to an increased number of such trips. Similarly, the
findings of Morton [37] point out that higher temperatures are associated with higher demand
rates, whereas heavy precipitation, high wind speed, and relative humidity are negatively asso-
ciated with the demand for the London bike-sharing system. Consistent with previous studies,
An et al. [39] find out that there is a higher demand for the CitiBike bike-sharing platform in
NYC in good weather, which is characterized by favorable temperature conditions, lack of
winds, humidity, and rain. On the other hand, El-Assi et al. [31] show that weather conditions
such as precipitation and high humidity decrease the demand for the Toronto bike-sharing
system. Based on the background evidence, first, we control for the following weather-related
variables: 1) Temperatureij, a measure of the average temperature for day j in city i in Fahren-
heit (˚F); 2) Windij, a measure of the average wind speed for day j in city i in knots; 3) Snowij, a
measure of snow depth for day j in city i in inches; 4) Rainij, a measure of total precipitation
for day j in city i in inches; and 5) Humidityij, a measure of the average dew point for day j in
city i in Fahrenheit (˚F).
Furthermore, we collected data on the socio-economic characteristics of the cities from the
U.S. Census Bureau. We include the population (Populationij), median income (Incomeij), the
number of the elderly population (Elderlyij), the number of houses with two cars (Vehicleij),
and the number of people commuting to work with bikes (Commuteij).
When combined, we end up with a panel data set that comprises eleven cities spanning from
January 2019 through July 2020. To make the interpretation of the socio-economic characteris-
tics easier, we include the log-transformed values in our analyses. Following prior literature, we
keep the weather-related variables in their original form [36, 37, 39]. Tables 2 and 3 present the
summary statistics and the correlation of the critical variables, respectively. We use the log-
transformed values of the socio-economic characteristic to interpret linear regression results.
Table 2. Descriptive statistics.
Variable
FirstCase
FirstExecutiveOrder
ln(TripFrequency)
ln(Population)
ln(Elderly)
ln(Income)
ln(Vehicle)
ln(Commute)
Temperature
Rain
Snow
Wind
Humidity
https://doi.org/10.1371/journal.pone.0283603.t002
Obs.
6,052
6,052
6,052
6,052
6,052
6,052
6,052
6,052
6,052
6,052
6,052
6,052
6,052
Mean
0.26
0.23
7.27
15.15
13.22
11.28
13.14
9.75
56.89
0.12
0.08
7.23
44.19
Std. Dev.
0.44
0.42
2.06
0.75
0.81
0.17
0.60
0.96
16.49
0.30
0.51
3.45
16.89
Min
0.00
0.00
0.00
14.25
12.05
11.01
12.39
7.81
-13.50
0.00
0.00
0.50
-24.60
Max
1.00
1.00
11.45
16.75
14.92
11.62
14.32
11.07
91.00
4.42
9.10
24.00
77.90
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PLOS ONEBike-Sharing platforms and Covid-19
Table 3. Correlation.
Variable
FirstCase
FirstExecutiveOrder
ln(TripFrequency)
ln(Population)
ln(Elderly)
ln(Income)
ln(Vehicle)
ln(Commute)
Temperature
Rain
Snow
Wind
Humidity
1
1.00
0.90
-0.07
0.03
0.05
0.12
0.03
0.02
0.16
0.00
-0.05
0.05
0.12
1
2
3
4
5
6
7
8
9
10
11
12
13
2
3
4
5
6
7
8
9
10
11
12
13
1.00
-0.08
0.00
0.02
0.10
0.00
-0.01
0.24
0.00
-0.07
0.03
0.20
1.00
0.87
0.85
0.51
0.85
0.86
0.13
-0.02
-0.05
0.08
0.08
1.00
0.95
0.27
0.96
0.78
1.00
0.21
0.95
0.75
-0.09
-0.12
0.02
0.07
0.11
0.02
0.07
0.13
-0.12
-0.14
1.00
0.21
0.66
0.08
-0.05
-0.07
0.18
0.05
1.00
0.74
-0.09
0.03
0.09
0.12
-0.13
1.00
-0.04
-0.03
0.00
0.14
-0.06
1.00
0.02
-0.27
-0.23
0.93
1.00
0.01
0.11
0.12
1.00
0.06
-0.22
1.00
-0.24
1.00
https://doi.org/10.1371/journal.pone.0283603.t003
Model-free evidence
Before we introduce our model specification, we present visual model-free evidence of the role
of the first Covid-19 case diagnosis and the first executive order implementation on the use of
the bike-sharing platforms in Figs 1 and 2. It is worth noting that Figs 1 and 2 do not account
for time-fixed effects. In Fig 1, we plot the daily trip frequency 60 days before and after the first
reported Covid-19 case in each of the eleven cities (excluding Minneapolis, MN, as its bike-
sharing systems do not report daily trip data between December and March). The solid vertical
line represents the first Covid-19 diagnosis. The dashed horizontal lines represent the average
daily bike-sharing trip frequency before and after the first Covid-19 diagnosis. In Fig 1, we
observe that the daily bike trip frequency decreases in most cities following their first reported
Covid-19 case, except for Boston and Chicago, which had cold winters and were beginning to
warm up in the ensuing weeks. In Fig 2, we plot the daily bike trip frequency 60 days before
and after the first executive order implementation across the cities. We observe a similar trend
in Fig 2. Across all cities, the daily trip frequency declined in the days immediately after the
first executive orders were implemented.
Similar to the plots in Figs 1 and 2, we plot the difference in the bike-sharing trip frequency
before and after the first Covid-19 case diagnosis and the first executive order implementation
by weekday and weekend (see Fig A1 and A2 in S1 Appendix for more details). In Fig A1 in S1
Appendix, we show the weekday bike trip frequencies 60 days before and after the first Covid-
19 case reported in each city. We notice a decrease in the trip frequency on weekdays following
the first reported Covid-19 case in most cities, with a few exceptions in which we see a close
average daily trip frequency after the first Covid-19 case was reported, such as in Boston. In
Fig A2 in S1 Appendix, we show the weekend bike trip frequencies 60 days before and after the
first Covid-19 case that was reported. When we look at the changes in the weekend trip fre-
quency, we see opposing results suggesting an increase in the daily trip frequency in some cit-
ies, such as Philadelphia and Pittsburgh.
Moreover, we also notice a similar trend in the daily weekday and weekend trip frequency
after the first executive order implementation across the cities in this study (see Figs A3 and A4
in S1 Appendix). Finally, Fig A5 in S1 Appendix shows the bike-sharing seasonal trend of Feb-
ruary-June 2019 (pre-covid) compared to February-June 2020 (post-covid) for each city in this
study. Relative to the patterns observed in 2019, we see a short-term decrease in bike-sharing
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PLOS ONEBike-Sharing platforms and Covid-19
Fig 1. Frequency of bike-sharing platform trips over time, before and after the first Covid-19 case.
https://doi.org/10.1371/journal.pone.0283603.g001
trip frequency following the pandemic’s start (towards the end of the first quarter of 2020).
These plots provide further model-free evidence of the changes in the use of the bike-sharing
system due to Covid-19.
Figs 3 and 4 show dumbbell charts that compare the average daily bike-sharing trip fre-
quency before and after each of our two treatments (the first Covid-19 case diagnosis and the
first executive order implementation) that occurred over the entire period of study. We use the
log transformation of trip frequencies for better visualization. Generally, we see short-term
decline in bike-sharing frequency after the first reported infections and the first executive
order implementation within the same U.S. cities. The blue point represents the log average
daily trip frequency for the period before the treatment occurred, and the red point represents
the log average daily trip frequency in the period after the treatment occurred. Fig 3 shows
that the frequency of average daily bike-sharing platforms decreases after the first Covid-19
case diagnosis, except for a few cities such as New York, Philadelphia, and Chicago, in which
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PLOS ONEBike-Sharing platforms and Covid-19
Fig 2. Frequency of bike-sharing platform trips over time, before and after the first executive order implementation.
https://doi.org/10.1371/journal.pone.0283603.g002
the average daily bike-sharing trips did not change significantly; and also except for Columbus,
in which we see an apparent increase. Fig 4 also shows that the frequency of average daily bike-
sharing trips decreases upon the first executive order implementation, again with the afore-
mentioned exceptions. However, it is essential to note that these plots do not consider the cit-
ies’ weather conditions and socio-economic characteristics. Therefore, in the following
subsection, we propose a statistical model to evaluate how bike-sharing frequency changed fol-
lowing the Covid-19 pandemic, accounting for weather conditions and socio-economic
characteristics.
Model specification
The model-free evidence shows that the frequency of trips on bike-sharing platforms generally
decreased in U.S. cities following the first Covid-19 cases and the first executive order
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PLOS ONEBike-Sharing platforms and Covid-19
Fig 3. Dumbbell chart comparing the average daily bike-sharing trip frequency before and after the first Covid-19 case.
https://doi.org/10.1371/journal.pone.0283603.g003
implementation. However, as mentioned earlier, model-free evidence does not account for
many factors that could influence the trip frequency for bike-sharing and the spread of the
virus. Therefore, before incorporating such factors into our statistical analysis, we need to
know the pandemic’s actual effect on bike-sharing trip frequency. However, we may observe a
decrease in the trip frequency in some cities and an increase in others.
Thus, we use a fixed-effects econometric formulation of the DID estimation framework to
examine how the Covid-19 pandemic affected the frequency of bike-sharing trips. The primary
benefit of this estimation model is that we can mimic an experimental design using observa-
tional data. This method compares the differences in bike-sharing trip frequency in treated cit-
ies before and after the treatment event—the onset of the pandemic—to the differences in the
untreated cities (i.e., those cities yet to report a coronavirus case or to implement a first execu-
tive order). The longitudinal nature of the data allows us to use the yet untreated observations
in the data as controls for the treated observations; that is, those cities that have yet to have a
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PLOS ONEBike-Sharing platforms and Covid-19
Fig 4. Dumbbell chart comparing the average daily bike-sharing trip frequency before and after the first executive order
implementation.
https://doi.org/10.1371/journal.pone.0283603.g004
first Covid-19 case or a Covid-related executive order. To facilitate estimation, we use the
fixed-effects regression formulation of the DID model, a formulation described in [25], as fol-
lows:
ln TripFrequency
ð
Þij ¼ b0 þ b1Treatmentij þ gWij þ yj þ mi þ εij;
ð1Þ
where ln(TripFrequency)ij is the log-transformed value of our dependent variable in city i dur-
ing day j. Treatmentij refers to the treatment variables in city i during day j: FirstCaseij or First-
ExecutiveOrderij. They are applied in different cities at different times. To control for existing
time-invariant differences among the heterogeneous geographical locations, i.e., cities, we
included city-fixed effects, μi, in our model. In addition, we included time-fixed effects, θj, to
control for common temporal shocks. This allows for non-linear time-varying effects in the
DID model. Wij is the set of control variables, which includes ln(Population), ln(Income), ln
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PLOS ONEBike-Sharing platforms and Covid-19
(Elderly), ln(Vehicle), ln(Commute), Temperature, Rain, Snow, Wind, and Humidity. Finally,
εij is the error term.
Results
Table 4 reports the coefficient estimates of Eq (1) for the dependent variable ln(TripFre-
quency). As shown in Column (1), we estimate an increase in the log of bike-sharing platform
trip frequency of 0.196 on average across eleven cities after the first Covid-19 case, adjusted for
covariates. An economic interpretation of this result suggests an average adjusted increase in
the number of daily bike-sharing trips by 22% (rounded from the following: [exp(0.196)-1]
*100 = 21.65%). On the other hand, from Column (2), we observe a decrease in the log of bike-
sharing platform trip frequency by 0.353 after the stay-at-home order implementation.
Table 4. Bike-sharing trip frequency: Fixed-effects regression results.
Dependent Variable
Treatment variable
ln(Population)
ln(Elderly)
ln(Income)
ln(Vehicle)
ln(Commute)
Temperature
Rain
Snow
Wind
Humidity
Observations
R2
R2(Adjusted)
F-statistic
Daily fixed effects
City fixed effects
*** p<0.01
** p<0.05
* p<0.10
Robust standard errors are given in parentheses.
https://doi.org/10.1371/journal.pone.0283603.t004
(1)
ln(TripFrequency)
First Case
0.196**
(0.098)
-1.748
(14.555)
7.876
(9.519)
6.751*
(3.567)
7.591***
(2.505)
0.332
(0.429)
0.043***
(0.003)
0.012
(0.022)
-0.172***
(0.038)
-0.026***
(0.005)
-0.017***
(0.003)
6,052
0.284
0.206
197.145***
Yes
Yes
(2)
ln(TripFrequency)
First Executive Order
-0.353*
(0.205)
-4.171
(14.779)
8.446
(9.667)
7.408**
(3.652)
7.355***
(2.502)
0.357
(0.437)
0.044***
(0.003)
0.012
(0.021)
-0.170***
(0.038)
-0.026***
(0.005)
-0.017***
(0.002)
6,052
0.283
0.205
196.389***
Yes
Yes
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PLOS ONEBike-Sharing platforms and Covid-19
Economically, this result suggests a reduction of 30% (rounded from the following: [exp
(-0.353)-1]*100 = -29.74%) in the number of daily bike-sharing trips. We also further examine
the robustness of our model to temporal trends using a relative time model (see Fig A6 and Fig
A7 in S1 Appendix for more details).
Furthermore, we divide our dataset into three panels to compare weekday, weekend, and
bank holiday travel behavior. We include the bank holidays (i.e., New Year’s Day, Martin
Luther King Jr. Day, or Independence Day) observed by the Federal Reserve System. The results
are given in Tables 5 and 6. We estimate a 22% (rounded from the following: [exp(0.197)-1]
*100 = 21.77%) increase in the trip frequency on average across cities during the weekdays
upon the first Covid-19 case (see Column 1 in Table 5), whereas our results do not suggest a
Table 5. Bike-sharing trip frequency: Weekday-, weekend-, and bank holiday-specific fixed-effects regression results where the treatment is the first Covid-19 case.
(1)
(2)
ln(TripFrequency)
ln(TripFrequency)
Weekday
First Case
0.197**
(0.098)
-3.378
(13.776)
10.125
(9.625)
6.780**
(2.961)
7.441***
(2.145)
0.440
(0.392)
0.037***
(0.002)
0.006
(0.016)
-0.158***
(0.032)
-0.024***
(0.005)
-0.014***
(0.002)
4,161
0.271
0.189
126.577***
Yes
Yes
Weekend
First Case
0.174
(0.113)
0.892
(18.152)
4.440
(10.328)
7.152
(5.179)
8.527**
(3.777)
0.007
(0.674)
0.059***
(0.008)
0.019
(0.039)
-0.175***
(0.036)
-0.028***
(0.006)
-0.025***
(0.003)
1,736
0.337
0.257
71.746***
Yes
Yes
Dependent Variable
Travel Behavior
Treatment variable
ln(Population)
ln(Elderly)
ln(Income)
ln(Vehicle)
ln(Commute)
Temperature
Rain
Snow
Wind
Humidity
Observations
R2
R2(Adjusted)
F-statistic
Daily fixed effects
City fixed effects
*** p<0.01
** p<0.05
* p<0.10
Robust standard errors are given in parentheses.
https://doi.org/10.1371/journal.pone.0283603.t005
(3)
ln(TripFrequency)
Bank Holiday
First Case
0.159
(0.420)
0.824
(29.735)
-13.653
(17.933)
2.036
(7.694)
-0.633
(5.811)
1.003
(1.025)
0.061***
(0.010)
0.141
(0.234)
-0.199***
(0.068)
-0.039**
(0.016)
-0.028***
(0.009)
155
0.438
0.273
8.443***
Yes
Yes
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PLOS ONETable 6. Bike-sharing trip frequency: Weekday- and weekend-specific fixed-effects regression results where the treatment is the first executive order.
Bike-Sharing platforms and Covid-19
(1)
ln(TripFrequency)
Weekday
First Executive Order
-0.358
(0.257)
-5.982
(2)
ln(TripFrequency)
Weekend
First Executive Order
-0.323**
(0.156)
-1.063
(13.890)
10.773
(9.797)
7.410**
(3.070)
7.273***
(2.147)
0.466
(0.396)
0.037***
(0.002)
0.007
(0.016)
-0.155***
(0.032)
-0.025***
(0.005)
-0.014***
(0.002)
4,161
0.270
0.188
125.921***
Yes
Yes
(18.444)
4.819
(10.404)
7.819
(5.169)
8.181**
(3.700)
0.025
(0.684)
0.059***
(0.008)
0.021
(0.038)
-0.174***
(0.037)
-0.029***
(0.006)
-0.025***
(0.003)
1,736
0.337
0.257
71.657***
Yes
Yes
Dependent Variable
Travel Behavior
Treatment variable
ln(Population)
ln(Elderly)
ln(Income)
ln(Vehicle)
ln(Commute)
Temperature
Rain
Snow
Wind
Humidity
Observations
R2
R2(Adjusted)
F-statistic
Daily fixed effects
City fixed effects
*** p<0.01
** p<0.05
* p<0.10
Robust standard errors are given in parentheses.
https://doi.org/10.1371/journal.pone.0283603.t006
statistical significance of the same effect on weekends (see Column 2 in Table 5) and the bank
holidays (see Column 3 in Table 5). Table 6 shows the results when the treatment variable is
FirstExecutiveOrder in which we observe opposing results. We estimate a statistically signifi-
cant decrease in the trip frequency of 28% (rounded from the following: [exp(-0.323)-1]*100 =
-27.60%) on the weekends (see Column 2 in Table 6), whereas we find no evidence of the same
effect during the weekdays (see Column 1 in Table 6). However, we do not observe any effect
for the bank holidays data panel as the variable in question is being omitted by the regression
(see Column 3 in Table 6). The reason behind this is that any variables that are constant within
every unit are redundant in a fixed-effects model and will be omitted from the model. Due to
the launch dates of the first executive order, Memorial Day 2020 and Independence Day 2022
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PLOS ONEBike-Sharing platforms and Covid-19
Table 7. Description: BikeScore1, WalkScore1, and TransitScore1.
90–100
70–89
50–69
0–49
BikeScore1
Biker’s Paradise
Very Bikeable
Bikeable
Somewhat Bikeable
WalkScore1
Walker’s Paradise
Very Walkable
Walkable
Car-Dependent
TransitScore1
Rider’s Paradise
Excellent Transit
Good Transit
Some/Minimal Transit
https://doi.org/10.1371/journal.pone.0283603.t007
are treated the same for each unit. Therefore, our treatment variable becomes constant for
each city and does not create any variation.
These results generally show the differences in residents’ travel behavior between weekdays,
weekends, and bank holidays. After the first Covid-19 diagnosis, individuals might have
started using bike-sharing platforms as an alternative to other modes of transportation on
weekdays, especially, for journeys to and from work. However, with the first executive order
implementation, on average, individuals might tend to stay inside more rather than go out.
We also ran the analysis on daily trip frequencies with fewer than thirty minutes, as a single
ride for non-subscribers includes thirty minutes of ride time. The results are consistent.
Heterogeneity analyses
While our empirical estimations thus far suggest a significant impact of the Covid-19 pan-
demic on the frequency of bike-sharing platform trips, it is worth examining the factors that
might amplify the strength of the effect. Prior literature [29–31] suggests that transportation
infrastructure, land use, built environment, and neighborhood attributes contribute to individ-
uals’ preference for bike-sharing systems.
One crucial factor that can moderate the impact of Covid-19 on bike-sharing platforms’ trip
frequency is the pre-existing biking infrastructure. First, in cities with more bike lanes, longer
bike route lengths, fewer hills, higher road connectivity, and bicycle-aware traffic, bike-sharing
platforms should more likely be adopted by individuals and used as an alternative transportation
mode. Second, in walkable cities with better access to amenities, residents might be embracing
these platforms more due to easy and comfortable access to bike stations. Lastly, in cities with
access to public transit, bike-sharing platforms might be used more by the residents due to better
connectivity of the transit network. Therefore, we test the heterogeneous effects depending on
such factors as the city’s bike-friendliness, transit-friendliness, and pedestrian-friendliness.
We collected data from Walk Score [40] to measure 1) bike-friendliness (BikeScore1) [41],
which measures the built environment’s ability to support biking for a given location, 2) pedes-
trian-friendliness (WalkScore1) [41], which measures the walkability of any address by analyz-
ing the walking routes to nearby amenities within a 5-minute walk, and 3) transit-friendliness
(TransitScore1), which measures how well a location is served by public transit [41]. These
measures range from 0 to 100 and divide cities into different groups [1]. Based on the classifi-
cation of BikeScore1, WalkScore1, and TransitScore1, the cities in our dataset scored less
than 90 in all measures. Detailed information on the groups and descriptive statistics of the
scores are given in Tables 7 and 8, respectively.
Table 8. Descriptive statistics: BikeScore1, WalkScore1, and TransitScore1.
Variable
BikeScore1
WalkScore1
TransitScore1
Obs.
6,052
6,052
6,052
Mean
67.71
70.13
61.43
Std. Dev.
9.84
15.54
16.33
Min
49.90
40.50
32.80
Max
83.50
88.30
84.30
https://doi.org/10.1371/journal.pone.0283603.t008
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PLOS ONEBike-Sharing platforms and Covid-19
Then, we re-estimate Eq (1) incorporating interaction terms for these classifications with
the treatment. The new equation including the interaction terms is given below. Note that as
the moderators are static, fixed-effects panel regressions do not yield estimates for β2. The
results are given in Table 9.
ln TripFrequency
ð
Þij
¼ b0 þ b1Treatmentij þ b2Moderatorj þ b3Treatmentij ∗ Moderatorj þ gWij þ yj þ mi
þ εij;
ð2Þ
Surprisingly, these findings suggest interesting differences. First, we see that the impact of
the first Covid-19 case and the first executive order implementation on bike-sharing platforms’
trip frequency is more substantial in bikeable cities. We estimate that the effect of the first
Covid-19 case on Trip Frequency is approximately 90% (rounded from the following: [exp
(0.640)-1]*100 = 89.64%) higher in “very bikeable” cities than in “bikeable” cities (see Table 9,
Column 1). We observe that the impact of the first executive order implementation on Trip
Frequency is approximately 103% (rounded from the following: [exp(0.708)-1]
*100 = 102.99%) higher in “very bikeable” cities than in “bikeable” cities (see Table 9, Column
4). Followed by the first Covid-19 case and the first executive order implementation, we esti-
mate a decrease that is respectively greater by approximately 33% (rounded from the following:
[exp(-0.405)-1]*100 = -33.30%) (see Table 9, Column 1) and 30% (rounded from the following:
[exp(-0.356)-1]*100 = -29.95%) (see Table 9, Column 4) in Trip Frequency in “somewhat bike-
able” as compared to “bikeable” cities. These results might suggest that the residents in bike-
friendly cities embrace these platforms more due to better pre-existing biking infrastructure.
With safe and comfortable biking afforded by good biking infrastructure, residents are more
likely to use bike-sharing platforms for commuting and recreational purposes.
Moreover, we observe a more substantial impact on the Trip Frequency in very walkable
cities upon both the first Covid-19 case (see Table 9, Column 2) and stay-at-home advisory
implementation (see Table 9, Column 5). In pedestrian-friendly cities, residents might be
embracing these platforms more as a result of easy and comfortable access to bike stations by
walking. Moreover, similar to bike-friendliness and pedestrian-friendliness, we observe a
more substantial impact on the Trip Frequency in cities with excellent transit upon both the
first Covid-19 case (see Table 9, Column 3) and the first executive order implementation (see
Table 9, Column 6). Lastly, unlike the “somewhat bikeable” classification of cities, we do not
observe that car dependence or having some transit (compared to good transit) has any moder-
ating influence on the effect of the first Covid-19 case (see Table 9, Column 3) and the first
executive order implementation (see Table 9, Column 6) on the use of bike-sharing platforms.
Discussion and conclusion
We used a DID framework formulated as a fixed-effects regression model to examine how
bike-sharing trip frequency in the United States changed with the onset of the Covid-19 pan-
demic. We modeled the first reported Covid-19 cases and the implementation of the first exec-
utive order in each municipality as two treatment events. We also accounted for socio-
economic factors, weather, and fixed effects for each day and city. First, our results indicate
that, on average, the first reported Covid-19 cases had a positive and statistically significant
effect on the frequency of bike trips in U.S. cities. This could be explained by the fact that the
existence of the first reported Covid-19 case in U.S. cities has heightened individuals’ sensitiv-
ity to cleanliness and social distance. Therefore, individuals were compelled to change their
travel behavior and look for alternative systems of mobility that may offer more resilient urban
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PLOS ONETable 9. Bike-sharing trip frequency: Fixed-effects regression results by heterogeneous effects. The reference indicator for the bike-friendliness scale is Bikeable; the
reference indicator for the pedestrian-friendliness scale is Walkable; and the reference indicator for the transit-friendliness is Good Transit.
Dependent Variable
ln(TripFrequency)
ln(TripFrequency)
ln(TripFrequency)
ln(TripFrequency)
ln(TripFrequency)
ln(TripFrequency)
(1)
(2)
(3)
(4)
(5)
(6)
Bike-Sharing platforms and Covid-19
First Case
0.049
(0.194)
First Case
0.078
(0.113)
0.824***
(0.312)
-0.020
(0.186)
-23.398
(20.104)
15.485
(10.788)
9.591**
(4.682)
2.517
(2.677)
1.647***
(0.624)
0.041***
(0.003)
0.010
(0.021)
-0.174***
(0.038)
-0.025***
(0.006)
-0.016***
(0.002)
6,052
0.323
0.248
200.193***
Yes
Yes
0.797***
(0.263)
-0.106
(0.133)
-13.360
(16.429)
-5.067
(11.572)
1.747
(4.748)
2.087
(3.753)
1.225**
(0.587)
0.042***
(0.003)
0.006
(0.024)
-0.173***
(0.039)
-0.025***
(0.005)
-0.017***
(0.002)
6,052
0.325
0.250
201.941***
Yes
Yes
Treatment variable
Interaction: VeryBikeable
Interaction: SomewhatBikeable
Interaction: VeryWalkable
Interaction: CarDependent
Interaction: ExcellentTransit
Interaction: SomeTransit
ln(Population)
ln(Elderly)
ln(Income)
ln(Vehicle)
ln(Commute)
Temperature
Rain
Snow
Wind
Humidity
Observations
R2
R2(Adjusted)
F-statistic
Daily fixed effects
City fixed effects
First Case
0.314**
(0.152)
0.640***
(0.228)
-0.405*
(0.215)
-28.236
(21.704)
17.118
(11.362)
10.441**
(5.129)
2.032
(2.710)
1.701***
(0.639)
0.041***
(0.003)
0.004
(0.024)
-0.173***
(0.038)
-0.025***
(0.005)
-0.017***
(0.002)
6,052
0.326
0.252
203.65***
Yes
Yes
*** p<0.01
** p<0.05
* p<0.10
Robust standard errors are given in parentheses.
https://doi.org/10.1371/journal.pone.0283603.t009
First Executive Order First Executive Order First Executive Order
-0.429*
(0.025)
-0.387*
(0.213)
-0.189*
(0.112)
0.708***
(0.225)
-0.356*
(0.194)
-14.129
(16.143)
-3.335
(11.246)
2.514
(4.512)
2.545
(3.484)
1.182**
(0.565)
0.041***
(0.003)
0.008
(0.022)
-0.173***
(0.038)
-0.025***
(0.005)
-0.016***
(0.002)
6,052
0.330
0.257
207.319***
Yes
Yes
0.833**
(0.324)
-0.023
(0.198)
-14.108
(14.231)
-4.122
(10.632)
1.668
(3.898)
2.322
(3.314)
1.228**
(0.539)
0.042***
(0.003)
0.008
(0.023)
-0.173***
(0.038)
-0.025***
(0.005)
-0.017***
(0.002)
6,052
0.326
0.252
203.57***
Yes
Yes
0.806***
(0.272)
-0.104
(0.139)
-14.436
(14.436)
-2.551
(10.408)
2.534
(3.727)
2.659
(3.139)
1.219**
(0.524)
0.042***
(0.003)
0.011
(0.020)
-0.173***
(0.038)
-0.025***
(0.004)
-0.017***
(0.002)
6,052
0.328
0.254
205.077***
Yes
Yes
PLOS ONE | https://doi.org/10.1371/journal.pone.0283603 April 7, 2023
16 / 20
PLOS ONEBike-Sharing platforms and Covid-19
transportation. Bike-sharing platforms offer alternative transportation to avoid crowds in the
cities. Second, we observe that the first executive order advisories had a negative and statisti-
cally significant effect on the frequency of bike trips in U.S. cities. This could be explained by
the fact that lockdown restrictions and working from home have led to a decline in commuting
bike trips and other modes of transportation such as public transit.
We also examined sources of heterogeneity in the effect of the pandemic on bike-sharing
use. We compared how bike-sharing use changed between weekends and weekdays with the
onset of the pandemic. We observed an increase in weekday-specific trip frequency as a result
of the first Covid-19 case diagnosis and a decrease in weekend-specific trip frequency due to
the first executive order implementation. We also tested for heterogeneous impacts across a set
of city-level characteristics. We found that there is a greater increase in the frequency of bike-
sharing trips in more bike-friendly, transit-friendly, and pedestrian-friendly cities upon both
the first Covid-19 case diagnosis and the first executive order implementation. We might con-
clude that bike-sharing platforms have an essential role in individuals’ travel behavior, espe-
cially in cities with better bike and transit infrastructure. These platforms are perceived as a
sustainable and resilient transportation option by individuals due to the unprecedented conse-
quences of the Covid-19 pandemic.
Bike-sharing platforms offer a sustainable and active mode of transportation, and hence it
is important to better understand the factors that affect their adoption by the populace. The
Covid-19 pandemic represents an opportunity for cities to embrace new paradigms for urban
mobility. Bike-sharing platforms represent one way in which cities provide a resilient and
adaptive transport network to face the potential challenges of disruptive events like the Covid-
19 pandemic. The pandemic has already highlighted the importance of rethinking the design
of urban transit for greater resilience to such disruptive events. Cities may consider how to
encourage greater use of bike-sharing platforms. Decisions by city authorities such as offering
free or reduced membership could break down barriers to the adoption of bike-sharing. With
the support of local authorities in creating more bike lanes and accessibility to public spaces,
bike-sharing platforms can attract more individuals. Proper incentives followed by infrastruc-
ture adjustments could ensure that individuals will become accustomed to bike-sharing plat-
forms and continue to use them even after the pandemic. For instance, in New York, city
officials are already planning to expand Citi Bike and add more docking stations in its busiest
areas [12]. Investing in bike-sharing platforms and cycling infrastructure could lead to an
increase in memberships because individuals’ willingness to bike is closely linked to how safe
they feel [42].
It is important to note that our research is subject to several limitations. First, the adjusted
R2 values of the models are low, ranging from 0.18 to 0.26. Low values might be an indicator of
the models would not be suitable for use in the predictive modeling of the outcome variables.
Hence, the aims of our model interpretations are limited to assessing the direction and signifi-
cance of coefficient estimates for causal inference.
Second, the main challenge with DID estimation is to ensure that no pre-treatment trends
in the absence of treatment [25]. The violation of this assumption can lead to biased causal esti-
mates. Although there is no statistical test for this assumption, we examine the robustness of
our model to temporal trends using a relative time model. Our findings suggest that there are
no pre-existing trends in bike-sharing demand across the cities that experience the first Covid-
19 diagnosis (see Fig 6A in S1 Appendix). As seen in Fig 7A in S1 Appendix, we observe that
pre-treatment trends exist in bike-sharing demand across the cities experiencing the first exec-
utive order implementation. Future research could explore different estimators that could
overcome this challenge. There are a few recent papers that focus on different ways to relax
this assumption [43–45]. They propose alternative estimators when the parallel trends
PLOS ONE | https://doi.org/10.1371/journal.pone.0283603 April 7, 2023
17 / 20
PLOS ONEBike-Sharing platforms and Covid-19
assumption is violated and examine the robustness of the results to the potential violations of
parallel trends.
Third, our model only examines the short-term effect of the Covid-19 pandemic on bike-
sharing demand due to the data collection period. In the future, depending on data availability
by the bike-sharing providers, the effects can be examined for longer periods.
Fourth, we use the first executive order implementation and the first Covid-19 case as prox-
ies of the Covid-19 pandemic; however, we should keep in mind that the first executive order
implementation might be correlated with the first Covid-19 case. This might be also one of the
reasons that might explain the pre-treatment trend in Fig 7A in S1 Appendix.
Lastly, our reported results are based upon the actual usage of bike-sharing platforms, along
with public transit data for eleven cities in the U.S. Despite the lack of data available for other
U.S. cities, we believe that the exogenous nature of the Covid-19 pandemic provides robust
insights into the relationship between the Covid-19 pandemic and travel behaviors. Given that
we are still in the midst of the pandemic, we expect that forthcoming data could reveal more
about the pandemic’s long-term effects on travel behavior. It is very plausible that the effects
observed thus far may serve as a signal for more lasting changes to come in urban travel
behaviors.
Supporting information
S1 Dataset.
(CSV)
S1 Appendix.
(DOCX)
Author Contributions
Conceptualization: Ecem Basak, Ali Tafti.
Data curation: Ecem Basak, Ramah Al Balawi, Sorouralsadat Fatemi.
Formal analysis: Ecem Basak, Ramah Al Balawi.
Methodology: Ecem Basak, Ramah Al Balawi, Sorouralsadat Fatemi, Ali Tafti.
Project administration: Ecem Basak.
Supervision: Ali Tafti.
Visualization: Ramah Al Balawi.
Writing – original draft: Ecem Basak, Ramah Al Balawi, Sorouralsadat Fatemi, Ali Tafti.
Writing – review & editing: Ecem Basak, Ramah Al Balawi, Sorouralsadat Fatemi, Ali Tafti.
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PLOS ONE
| null |
10.7554_elife.80854.pdf
|
Data availability
Source files of all original gels and Western Blots were provided for the following figures: Figure 1—
figure supplement 2B; Figure 4—figure supplement 1A, C, D, E; Figure 5—figure supplement 2B, F,
G. RNA sequencing and ChIP sequencing data files that support the findings of this study have been
deposited in the Gene Expression Omnibus under the accession code GSE85055, as well as in the
Dryad digital repository (doi:10.5061/dryad.7pvmcvdwm; doi:10.5061/dryad.f1vhhmh0h). Sequences
of sgRNAs, shRNAs, and primers used in this manuscript are included in the Supplementary File
|
Data availability Source files of original gels and Western Blots were provided for the figures: Figure 1 The following datasets were generated: Author(s) Year Dataset title Dataset URL Database and Identifier Soto-Feliciano MY, Zhu C, Morris JP, Huang C-H, Koche RP, Y-J Ho, Banito A, Chen C-W, Shroff A, Tian S, Livshits G, Chen C-C, Fennell M, Armstrong SA, Allis CD, Tschaharganeh DF, Lowe SW 2022 Mll3 suppresses tumorigenesis by activating the Ink4a/Arf locus https:// doi. org/ 10. 5061/ dryad. 7pvmcvdwm Dryad Digital Repository, 10.5061/dryad.7pvmcvdwm Soto-Feliciano MY, Zhu C, Morris JP, Huang C-H, Roche RP, Y-J Ho, Banito A, Chen C-W, Shroff A, Tian S, Livshits G, Chen C-C, Fennell M, Armstrong SA, Allis CD, Tschaharganeh DF, Lowe SW 2022 MLL3 ChIP sequencing in murine and human HCC cells https:// doi. org/ 10. 5061/ dryad. f1vhhmh0h Dryad Digital Repository, 10.5061/dryad.f1vhhmh0h Lowe SW 2017 Mll3 suppresses tumorigenesis by activating the Ink4a/Arf locus https://www. ncbi. nlm. nih. gov/ geo/ query/ acc. cgi? acc= GSE85055 NCBI Gene Expression Omnibus, GSE85055
|
RESEARCH ARTICLE
MLL3 regulates the CDKN2A tumor
suppressor locus in liver cancer
Changyu Zhu1†, Yadira M Soto- Feliciano2,3*†, John P Morris1,4†, Chun- Hao Huang1,
Richard P Koche5, Yu- jui Ho1, Ana Banito1, Chun- Wei Chen1, Aditya Shroff1,
Sha Tian1, Geulah Livshits1, Chi- Chao Chen1, Myles Fennell1, Scott A Armstrong6,
C David Allis2, Darjus F Tschaharganeh7*, Scott W Lowe1,8*
1Department of Cancer Biology and Genetics, Memorial Sloan Kettering Cancer
Center, New York, United States; 2Laboratory of Chromatin Biology and Epigenetics,
The Rockefeller University, New York, United States; 3Koch Institute for Integrative
Cancer Research, Massachusetts Institute of Technology, Cambridge, United
States; 4Department of Pharmacology, The University of North Carolina at Chapel
Hill, Chapel Hill, United States; 5Center for Epigenetics Research, Memorial Sloan
Kettering Cancer Center, New York, United States; 6Dana- Farber Cancer Institute,
Boston, United States; 7Helmholtz- University Group "Cell Plasticity and Epigenetic
Remodeling", German Cancer Research Center, Heidelberg, Germany; 8Howard
Hughes Medical Institute, New York, United States
*For correspondence:
[email protected] (YMS- F);
[email protected] (DFT);
[email protected] (SWL)
†These authors contributed
equally to this work
Competing interest: See page
18
Funding: See page 19
Received: 07 June 2022
Preprinted: 09 June 2022
Accepted: 31 May 2023
Published: 01 June 2023
Reviewing Editor: Hao Zhu,
University of Texas Southwestern
Medical Center, United States
Copyright Zhu, Soto- Feliciano,
Morris et al. This article is
distributed under the terms
of the Creative Commons
Attribution License, which
permits unrestricted use and
redistribution provided that the
original author and source are
credited.
Abstract Mutations in genes encoding components of chromatin modifying and remod-
eling complexes are among the most frequently observed somatic events in human cancers.
For example, missense and nonsense mutations targeting the mixed lineage leukemia family
member 3 (MLL3, encoded by KMT2C) histone methyltransferase occur in a range of solid tumors,
and heterozygous deletions encompassing KMT2C occur in a subset of aggressive leukemias.
Although MLL3 loss can promote tumorigenesis in mice, the molecular targets and biological
processes by which MLL3 suppresses tumorigenesis remain poorly characterized. Here, we
combined genetic, epigenomic, and animal modeling approaches to demonstrate that one of the
mechanisms by which MLL3 links chromatin remodeling to tumor suppression is by co- activating
the Cdkn2a tumor suppressor locus. Disruption of Kmt2c cooperates with Myc overexpression
in the development of murine hepatocellular carcinoma (HCC), in which MLL3 binding to the
Cdkn2a locus is blunted, resulting in reduced H3K4 methylation and low expression levels of the
locus- encoded tumor suppressors p16/Ink4a and p19/Arf. Conversely, elevated KMT2C expres-
sion increases its binding to the CDKN2A locus and co- activates gene transcription. Endogenous
Kmt2c restoration reverses these chromatin and transcriptional effects and triggers Ink4a/Arf-
dependent apoptosis. Underscoring the human relevance of this epistasis, we found that genomic
alterations in KMT2C and CDKN2A were associated with similar transcriptional profiles in human
HCC samples. These results collectively point to a new mechanism for disrupting CDKN2A activity
during cancer development and, in doing so, link MLL3 to an established tumor suppressor
network.
Editor's evaluation
This paper convincingly shows that MLL3 regulates the CDKN2A tumor suppressor in MYC- driven
liver cancers. The use of in vivo models and epigenomic analysis made the findings particularly
robust. This work significantly advances our understanding of the function of MLL3 in cancer.
Zhu, Soto- Feliciano, Morris et al. eLife 2023;12:e80854. DOI: https://doi.org/10.7554/eLife.80854
1 of 25
Research article
Introduction
Hepatocellular carcinoma (HCC) is a deadly primary liver cancer with a 5 year survival rate of only
18% (Jemal et al., 2017). HCC is currently the fourth most frequent cause of cancer- related mortality
worldwide, and its incidence continues to grow (Llovet et al., 2021). Genomic alterations found in
HCC are highly diverse and are characterized by promoter mutations in TERT (telomerase reverse
transcriptase), amplifications, or chromosomal gains encompassing the MYC oncogene, activating
hotspot mutations in CTNNB1 (β-catenin), and inactivating mutations and deletions in the TP53 and
CDKN2A tumor suppressor genes (2017; Schulze et al., 2015).
Among these alterations, genetic gain of MYC and inactivation of tumor suppressor p53 are
known to cooperate to drive tumorigenesis in HCC (Molina- Sánchez et al., 2020). Mechanistically,
oncogenic MYC activation triggers increased expression of the tumor suppressor ARF, one of two
proteins encoded in CDKN2A in alternative reading frames. ARF binds to the E3 ubiquitin ligase
MDM2 to prevent p53 degradation, leading to apoptosis to restrain MYC- driven tumorigenesis (Lowe
and Sherr, 2003). However, it is unclear how the CDKN2A locus is regulated in response to MYC
overexpression.
Beyond these well- studied drivers, HCC frequently harbors mutations in one or more chromatin
modifying enzymes, including MLL3 (encoded by KMT2C; Fujimoto et al., 2012; Kan et al., 2013).
MLL3 is a component of the COMPASS- like complex that has structural and functional similarities
to the developmentally essential Drosophila Trithorax- related complex (Schuettengruber et al.,
2017). This multiprotein complex controls gene expression through its histone H3 lysine 4 (H3K4)
methyltransferase activity, which establishes chromatin modifications most often associated with tran-
scriptional activation (Shilatifard, 2012). Most studies have shown that MLL3 and its paralog MLL4
(encoded by KMT2D) typically catalyze H3K4 monomethylation (H3K4me1) at enhancers (Herz et al.,
2012; Hu et al., 2013), while the MLL1/2 complex is responsible for H3K4 trimethylation (H3K4me3)
at promoters and enhancers in a locus- specific manner (Denissov et al., 2014; Rickels et al., 2016;
Wang et al., 2009).
While less characterized, MLL3/4 regulation of promoter activity is emerging as an additional mech-
anism connecting the COMPASS- like complex to gene expression. Some publications report that
H3K4me1 enrichment at promoters has been associated with gene repression (Cheng et al., 2014),
and MLL3 inactivation decreases H3K4me3 levels at the promoters of metabolism- related genes
in normal murine livers (Valekunja et al., 2013) and human liver cancer cells (Ananthanarayanan
et al., 2011). Furthermore, a recent study in leukemia cells demonstrated that MLL3 and MLL4, in the
absence of MLL 1/2 complex, are capable of binding to promoters to activate tumor suppressor genes
(Soto- Feliciano et al., 2023). These divergent results suggest that the genomic binding pattern and
functions of MLL3 are highly context dependent.
Notably, HCC also harbors mutations in KMT2D (Cleary et al., 2013), while KDM6A/UTX, an
H3K27 demethylase within the COMPASS- like complex, has been functionally established as a potent
tumor suppressor in pancreatic and liver cancers (Revia et al., 2022). These observations suggest
that epigenetic- based mechanisms of gene regulation controlled by the MLL3 complex may constrain
HCC development. However, because chromatin regulators such as ARID1A often exhibit context-
specific tumor suppressive and oncogenic roles in liver cancer development (Sun et al., 2017), it is
unclear whether MLL3 is a bona fide tumor suppressor in HCC. We therefore employed mouse models
of HCC to investigate the molecular targets of MLL3 and the biological processes it affects.
Results
MLL3 is a tumor suppressor in Myc-driven liver cancer
To better understand the functional significance of genes commonly inactivated in HCC, including
a number of chromatin regulators, we selected 12 genes with recurrent inactivating mutations in
human HCC (Cancer Genome Atlas Research Network, 2017; Ahn et al., 2014; Fujimoto et al.,
2012; Figure 1—figure supplement 1A) and performed a CRISPR- based in vivo screen to determine
whether they behave as tumor suppressors in HCC. Specifically, the screen tested whether loss of
each of these 12 genes would drive hepatic tumorigenesis in cooperation with Myc—one of the most
frequently gained and/or amplified oncogenes in HCC (Huang et al., 2014). We applied hydrody-
namic tail vein injection (HTVI) in wild- type mice to directly introduce genetic manipulations into adult
Zhu, Soto- Feliciano, Morris et al. eLife 2023;12:e80854. DOI: https://doi.org/10.7554/eLife.80854
2 of 25
Cancer Biology
Research article
hepatocytes in vivo (Bell et al., 2007). We introduced both a transposon vector for stable genomic
integration of oncogenic Myc cDNA and plasmids designed for transient expression of Cas9 and
single guide RNAs (sgRNAs; a mix of two for each gene; Figure 1B; Largaespada, 2009; Moon et al.,
2019; Tschaharganeh et al., 2014; Xue et al., 2014). 3 months after HTVI, only sgKmt2c resulted in
liver tumor formation with high penetrance (Figure 1—figure supplement 1B), suggesting that MLL3
likely acts as a tumor suppressor to constrain Myc- driven liver cancer. Supporting this idea, KMT2C
mutations co- occur with MYC genomic gains and amplifications in human HCC tumors (Figure 1A).
To validate and extend the results from the screen, we applied the same approach to test whether
the screen phenotype could be recapitulated with oncogenic Myc and single Kmt2c- targeted sgRNAs
(Figure 1B). Mice injected with an Myc cDNA transposon combined with either of two independent
Cas9/Kmt2c sgRNAs (Myc; sgKmt2c.1 or Myc; sgKmt2c.2) developed liver tumors, with a slightly
later onset and slightly longer survival than mice receiving the Myc transposon combined with an
sgRNA targeting Trp53 (Myc; sgTrp53; Figure 1C and D). In contrast, mice injected with Myc and
a control sgRNA (sgChrom8) did not succumb to disease over the observation period (Figure 1C).
These findings were confirmed in an independent cohort of mice (Figure 1—figure supplement
2b). Analyses of tumor- derived genomic DNA revealed insertions and deletions (indels) in either
Kmt2c or Trp53 depending on the genotype of tumor- derived cells (Figure 1—figure supplement
2B). DNA sequencing of the CRISPR- targeted region from two independent Myc; sgKmt2c tumors
revealed indels predicted to generate premature stop codons (Figure 1—figure supplement 2C). In
one case, the indel was heterozygous, implying that even partial suppression of Kmt2c can promote
tumorigenesis. In support of this, GFP- linked Kmt2c shRNAs efficiently cooperated with Myc overex-
pression to drive liver cancer, producing tumors with 50–80% reduction in Kmt2c mRNA expression
(Figure 1—figure supplement 2D–G). shKmt2c.2 resulted in less potent knockdown than shKmt2c.1
yet produced faster tumor formation, suggesting that, as in acute myeloid leukemia (Chen et al.,
2014), MLL3 can likely act as a haploinsufficient tumor suppressor in liver cancer (Figure 1—figure
supplement 2E, G).
Apart from MYC, CTNNB1 (β-catenin) is a frequently mutated oncogene in human HCC (Rebou-
issou et al., 2016), although the co- occurrence between CTNNB1 and KMT2C mutations was not
statistically significant (Figure 1A). To test whether MLL3 loss can cooperate with oncogenic CTNNB1
to promote liver tumorigenesis, we performed analogous HTVI of a transposon vector expressing
constitutively active β-catenin (Ctnnb1- N90; Tward et al., 2007) in combination with Kmt2c- or Trp53-
targeted sgRNAs (Figure 1—figure supplement 3A). However, no tumor formation was observed in
mice that received the Kmt2c- targeted sgRNAs (Figure 1—figure supplement 3B), indicating that
unlike p53, the tumor- suppressive role of MLL3 is specific to the oncogene and contexts.
MLL3 loss alters the chromatin landscape of liver cancer cells
MLL3 and MLL4 are histone methyltransferases that can deposit the H3K4 monomethylation mark at
genomic enhancers and intergenic regions during organ development (Hu et al., 2013). However,
more studies indicate that MLL3 and MLL4 are also capable of binding to promoter regions (Chen
et al., 2014; Dhar et al., 2016; Wang et al., 2010), especially in the context of cancer (Soto- Feliciano
et al., 2023). To determine the genomic binding patterns of MLL3 in HCC, we performed MLL3 chro-
matin immunoprecipitation (ChIP)- sequencing (ChIP- Seq) analysis in Myc; sgKmt2c (sgKmt2c.1 which
generates heterozygous or homozygous indels) and Myc; sgTrp53 liver cancer cell lines. Compared
to the sgTrp53 cells, sgKmt2c cells had a marked reduction in MLL3 chromatin binding at a subset
of genomic loci (Figure 2A). Approximately 40% of the peaks that were selectively lost in Kmt2c-
deficient cells occurred at promoter regions, whereas unchanged MLL3 peaks between the two geno-
types were more likely to be within intergenic regions (Figure 2B, Figure 2—figure supplement 1).
Therefore, our data suggest that, beyond the canonical action of MLL3 at gene enhancers, MLL3
can also occupy promoter regions in Myc- induced liver cancer. Of note, the residual ChIP- seq signal
observed in the sgKmt2c cells most likely reflects the binding of MLL4 and/or remnant MLL3 since
the antibody used in these experiments can recognize both MLL3 and MLL4 proteins (Dorighi et al.,
2017). Nonetheless, the downregulated peak signals in Myc; sgKmt2c cells were specifically due to
MLL3 disruption.
Similar to the Drosophila Trithorax- related complex (Schuettengruber et al., 2017), the mamma-
lian MLL3 and MLL4 complexes facilitate gene transcription by establishing permissive modifications
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Figure 1. MLL3 constrains Myc- driven liver tumorigenesis. (A) Oncoprints displaying genomic mutations and deletions of KMT2C and TP53, gains and
amplifications of MYC, and activating CTNNB1 mutations in merged publicly available datasets (TCGA, MSK, INSERM, RIKEN, AMC, and MERCi) of
1280 sequenced hepatocellular carcinomas, and the table showing their relationships. p- Values were calculated by Fisher exact tests. (B) Schematic for
hydrodynamic tail vein injection (HTVI) of gene delivery into murine livers. Vectors permitting stable expression of Myc transposon (top) and transient
expression of Cas9 and single guide RNAs (sgRNAs) targeting putative tumor suppressors (bottom) via sleeping beauty transposase were introduced
into hepatocytes by HTVI. (C) Survival curves of mice injected with Myc transposon and pX330 expressing two independent sgRNAs targeting Kmt2c
after HTVI (Myc; sgKmt2c.1, n=5; Myc; sgKmt2c.2, n=5). Myc; sgTrp53 (n=5), and Myc; sgChrom8 (n=5) serve as controls. Survival curves were compared
using log- rank tests. (D) Representative images (left, liver macro- dissection, scale bar: 0.5 cm; right, H&E staining, scale bar: 100 μm) of mouse liver
tumors generated by HTVI delivery of Myc transposon and in vivo gene editing. The dashed lines indicate the boundaries between liver tumors and
non- tumor liver tissues.
The online version of this article includes the following source data and figure supplement(s) for figure 1:
Figure supplement 1. In vivo screen identifies MLL3 as a tumor suppressor in Myc- driven liver cancer.
Figure supplement 2. Suppression of Kmt2c by CRISPR or RNAi promotes Myc- driven liver cancer.
Figure supplement 2—source data 1. Original gel for surveyor assays in Figure 1—figure supplement 2B.
Figure supplement 3. MLL3 loss does not cooperate with CTNNB1 oncogene to drive liver cancer.
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Figure 2. MLL3 disruption alters the chromatin and transcriptional landscape of liver cancer cells. (A) Tornado plots showing MLL3 chromatin
immunoprecipitation- sequencing (ChIP- Seq) signal (peaks) that were down or remained unchanged in Myc; sgKmt2c cells relative to Myc; sgTrp53 cells.
Center: transcriptional start site (TSS). (B) Alluvial plot showing the percentages of MLL3 ChIP- Seq peaks in different genomic elements in Myc; sgKmt2c
vs Myc; sgTrp53 cells, including 16,999 peaks down, 48,815 peaks unchanged, and 265 peaks up in sgKmt2c cells. Promoter regions were defined as
Figure 2 continued on next page
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Figure 2 continued
TSS±2 kb. (C) Heatmaps of histone modification ChIP- Seq signals (H3K4me3, H3K4me1, H3K27ac, left panel) and MLL3 ChIP- Seq signal (right panel)
at promoter or intergenic regions in three independent Myc; sgKmt2c and Myc; sgTrp53 liver tumor- derived cell lines. Cluster 1: loss of promoter and
enhancer activity (loss of H3K4me3, H3K4me1, and H3K27ac); cluster 2: gain of enhancer activity (gain of H3K4me1 and H3K27ac); and cluster 3: gain of
promoter activity (increase of H3K4me3). Representative top five loci for each cluster were listed on the right. (D) Volcano plot of differentially expressed
genes revealed by RNA- sequencing of three independent Myc; sgKmt2c and Myc; sgTrp53 hepatocellular carcinoma (HCC) cell lines. Genes in sgKmt2c
cells with more than twofold expression change and exceeding adjusted p- value<10–5 are color- labeled (orange: upregulated; green: downregulated).
Some differentially expressed genes are labeled with gene symbols, and p53 targets are bolded.
The online version of this article includes the following figure supplement(s) for figure 2:
Figure supplement 1. MLL3 deficiency disrupts its binding at promoters in liver cancer cells.
Figure supplement 2. MLL3 disruption impacts transcriptional and histone modification profiles in liver tumors.
on histone H3K4 via the MLL3 and MLL4 methyltransferase (Shilatifard, 2012). To determine whether
MLL3 disruption impacts the local or global chromatin landscape of HCC cells, we performed
ChIP- Seq analyses for H3K4 methylation and H3K27 acetylation in six independently derived tumor
cell lines: three each for Myc; sgKmt2c and Myc; sgTrp53 (Figure 2C). Cluster analysis on genomic
areas revealed three clusters of genomic loci that showed enrichment or depletion between Myc;
sgKmt2c and Myc; sgTrp53 tumor cells for each tested histone modification (Figure 2C, Figure 2—
figure supplement 2A–C). Loci in cluster 1 (reduced H3K4me3, H3K4me1, and H3K27ac in sgKmt2c
cells) showed the most pronounced differences in chromatin modifications between the two liver
tumor genotypes. In contrast, the loci in cluster 2 showed increased H3K4me1 and H3K27ac marks,
most of which mapped to intergenic regions. The loci in cluster 3 showed increased H3K4me3 and
included some p53 target genes such as Cdkn1a and Eda2r.
To determine whether these drastic changes in the chromatin landscape were associated with
changes in MLL3 binding, we integrated the chromatin modifications results with our MLL3 ChIP- Seq
results (Figure 2C). Interestingly, the loci in cluster 1, which displayed the most substantial changes
in histone modifications, involved genes that showed enriched MLL3 binding in Myc; sgTrp53 cells
compared to the Myc; sgKmt2c genotype. These data support a model whereby MLL3 binding to these
loci facilitates the acquisition of a chromatin environment conducive for active gene transcription.
MLL3 regulates specific tumor suppression programs in liver cancer
cells
Transcriptional profiling helped hone in on potentially critical targets of MLL3. Specifically, we deter-
mined the output of these chromatin landscape changes by transcriptional profiling of the same set of
Myc; sgTrp53 and Myc; sgKmt2c liver cancer cell lines described above. Despite the broad binding of
MLL3 across the genome, we found only 248 differentially expressed genes (DEGs): 132 significantly
upregulated (p<0.05, log2 fold- change >2) and 116 significantly downregulated (p<0.05, log2 fold-
change <−2) in Myc; sgKmt2c liver tumor cells compared to Myc; sgTrp53 controls.
As predicted, transcripts encoding p53 and p53 target genes such as Ccng1, Cdkn1a, and Zmat3
(Bieging- Rolett et al., 2020) were upregulated in Myc; sgKmt2c cells, consistent with nonsense-
mediated decay of truncated p53 transcripts and a concomitant reduction in p53 effector genes.
Strikingly, some of the downregulated genes in Myc; sgKmt2c lines mapped to loci enriched in cluster
1, including Cdkn2a, Bmp6, and Lrp2 (Figure 2C–D, Figure 2—figure supplement 2D, E). Of note,
sgKmt2c did not lead to compensatory changes in the transcript levels of other major components of
the COMPASS- like complexes, including Mll4 (Kmt2d), Utx (Kdm6a), Mll1 (Kmt2a), and Mll2 (Kmt2b;
Figure 3—figure supplement 1A), suggesting that the alterations in MLL3 binding, histone modifica-
tion, and transcription were specifically attributed to MLL3 disruption.
We reason that the mediators of MLL3 actions in tumor suppression should be within cluster 1 with
reduced transcription and MLL3 binding in Kmt2c- deficient cells. To further characterize the gene
repertoire directly regulated by MLL3 genomic binding, we integrated the results of MLL3 ChIP- seq
and RNA- seq. Specifically, we selected downregulated DEGs that show concordant decreased
binding of MLL3 in Myc; sgKmt2c lines and subjected them to gene ontology analysis. Apart from
the cluster 1 genes noted above, the integrative analysis revealed multiple MLL3- regulated tumor
suppressive programs (Figure 3A, Figure 3—figure supplement 1B), including both cell- autonomous
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Figure 3. MLL3 regulates specific transcription programs including tumor suppressor CDKN2A. (A) Network plot showing the major biological
processes and related genes directly regulated by MLL3 binding. p- Values and cluster sizes were calculated by the integrative analyses of RNA- seq and
MLL3 chromatin immunoprecipitation- sequencing (ChIP- Seq), as detailed in the Materials and methods. (B) Gene set enrichment analysis (GSEA) plots
of transcriptional signatures derived from mouse hepatocellular carcinomas (HCCs; Myc; sgKmt2c vs Myc; sgTrp53) against transcriptomics of HCCs
Figure 3 continued on next page
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Figure 3 continued
with CDKN2A vs TP53 mutations. (C) GSEA plots of transcriptional signatures derived from KMT2C mutated/deleted human HCCs against the ones
with CDKN2A mutations or homozygous deletions. HCCs with TP53 mutations were used as the controls for both comparisons. Normalized enrichment
scores (NES) and false discovery rate (FDR) q- values were calculated by GSEA.
The online version of this article includes the following figure supplement(s) for figure 3:
Figure supplement 1. MLL3 loss impacts specific transcription programs.
Figure supplement 2. CDKN2A and KMT2C mutations cause similar transcriptional changes in human hepatocellular carcinoma (HCC).
mechanisms (cellular metabolism) and non- autonomous mechanisms (interaction with extracellular
matrix and immune system).
KMT2C and CDKN2A mutations result in similar transcriptomes in
human HCC
One genomic locus that stood out in our integrative analysis was Cdkn2a, which encompasses both
the p16/Ink4a and p19/Arf (p14 in human) tumor suppressors (Gil and Peters, 2006). CDKN2A is
located on the human chromosome 9p and is deleted or epigenetically silenced in many cancer types
(Sherr, 2012), including HCC (2017). While MLL3 likely regulates a plethora of genes that contribute to
its tumor- suppressive potential, the well- defined and potent antitumor functions of Cdkn2a- encoded
proteins make them attractive candidates as functionally relevant MLL3 effectors.
Furthermore, in our analysis publicly available genomic data on 1280 HCC samples, we found
that CDKN2A alterations, like KMT2C alterations, showed significant co- occurrence with MYC gains
and amplifications. However, we were unable to conduct a meaningful test of mutual exclusivity
between CDKN2A and KMT2C alterations (Figure 3—figure supplement 2A), given the constraints
of sample size and the modest frequencies of alteration in each gene. Further dissection of transcrip-
tional profiling datasets from human and mouse HCCs harboring known gene alterations using gene
set enrichment analysis (GSEA) revealed that human tumors with CDKN2A deletions transcriptionally
resembled both mouse and human HCC harboring KMT2C alterations (Figure 3B and C) but not
those harboring RB1 loss (Figure 3—figure supplement 2B), even though the tumor suppressor RB1
is regulated by CDKN2A/p16INK4A and their genomic alterations exhibit mutual exclusivity in multiple
cancer types (Knudsen et al., 2020). While we cannot rule out the possibility that other factors drive
these associations, our results support a biologically meaningful relationship between MLL3 and
CDKN2A.
Cdkn2a locus is a genomic and transcriptional target of MLL3 in liver
cancer
To explore the relationship between MLL3 and Cdkn2a locus in more detail, we tested whether
genes encoded by Cdkn2a were direct targets of MLL3- regulated transcription. Indeed, Cdkn2a is a
cluster 1 locus that, in Myc; sgKmt2c cancer cells, displays significant reduction in (1) expression, (2)
H3K4me1/3 and H3K27ac levels, and (3) MLL3 binding at the Cdkn2a promoter compared with Myc;
sgTrp53 cells (Figure 2C–D, Figure 4A). Of note, MLL3 binding peaks were also observed within the
gene body of Cdkn2a. The differential expression of Ink4a and Arf was confirmed by qPCR, immu-
noblotting, and ChIP- qPCR analyses on multiple Myc; sgKmt2c and Myc; sgTrp53 liver cancer lines
(Figure 4B, Figure 4—figure supplement 1A, B). These results imply that Cdkn2a locus is a genomic
and transcriptional target of MLL3 in liver cancer cells.
Since the Myc; sgTrp53 and Myc; sgKmt2c cells we studied above are not isogenic, we performed a
series of additional experiments to demonstrate a direct transcriptional effect of MLL3 on the Cdkn2a
locus. Because p53 inactivation can lead to compensatory increases in Ink4a and Arf expression (Stott
et al., 1998), representing an alternative possibility accounting for the observed difference of Cdkn2a
expression in sgTrp53 vs sgKmt2c cells. However, p53 suppression in sgKmt2c cells produced only
a subtle and inconsistent effect on the expression of Ink4a and Arf, whereas Kmt2c suppression in
sgTrp53 cells consistently attenuated p16Ink4a and p19Arf protein levels (Figure 4—figure supple-
ment 1C, D). As another means of ruling out the p53 pathway as an explanation for altered Cdkn2a
expression in Myc; sgKmt2c cells, we tested the ability of MLL3 to regulate Cdkn2a transcripts in
an orthogonal liver cancer model driven by Myc and inactivation of Axin1, which is a well- defined
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Figure 4. CDKN2A locus is a genomic and transcriptional target of MLL3 in liver cancer. (A) Genome browser tracks for MLL3 and H3K4me1 chromatin
immunoprecipitation- sequencing (ChIP- Seq) in Myc; sgTrp53 (red) and Myc; sgKmt2c (blue) hepatocellular carcinoma (HCC) cell lines at the Cdkn2a
locus. (B) qPCR analysis for mRNA expression of Ink4a and Arf from three independent Myc; sgKmt2c and Myc; sgTrp53 HCC lines (n=3 cell lines each
genotype). Values are shown as mean ± SD. ***=p<0.001 (unpaired two- tailed t- test). (C) Schematic for CRISPR activation (CRISPRa) system of nuclease-
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Figure 4 continued
dead Cas9 (dCas9) and VP64- p65- Rta (VPR) guided by sgKMT2C to activate KMT2C expression in human HLE HCC cell line. (D) qPCR analysis for mRNA
expression of KMT2C in HLE cells with sgGFP (control) or two different CRISPRa single guide RNAs (sgRNAs) targeting KMT2C (n=4 cell lines each
genotype). Each data point represents the average of technical duplicates. Data are shown as mean ± SEM. ***=p<0.001 (one- way ANOVA followed
by post- hoc t- tests). (E) Genome browser tracks for MLL3 ChIP- Seq at the CDKN2A locus in HLE cells with sgGFP (control, black) or sgKmt2c.1 (blue). (F
and G) qPCR analysis for mRNA expression of (F) INK4A and (G) ARF in HLE cells with sgGFP (control) or sgKMT2C (n=4). Each data point represents the
average of technical duplicates. Data are shown as mean ± SEM. ***=p<0.001 (one- way ANOVA followed by post- hoc t- tests).
The online version of this article includes the following source data and figure supplement(s) for figure 4:
Figure supplement 1. MLL3 directly regulates Cdkn2a expression in liver cancer cells.
Figure supplement 1—source data 1. Original western blots for Figure 4—figure supplement 1A,C,D,E.
tumor suppressor that negatively regulates β-catenin activity in HCC (Satoh et al., 2000). Liver cancer
cells produced by hydrodynamic delivery of the Myc transposon vector and Axin1 sgRNAs displayed
reduced Ink4a and Arf expression upon Kmt2c knockdown without targeting p53 (Figure 4—figure
supplement 1E,F). Importantly, MLL3 binding peaks at the Cdkn2a locus were also detected in Myc;
sgAxin1 liver cancer cells (Figure 4—figure supplement 1G), suggesting that Cdkn2a transcription is
directly regulated by MLL3 rather than an indirect outcome of p53 loss. These data imply that MLL3
supports a chromatin environment at the Cdkn2a locus that facilitates the transcription of both Ink4a
and Arf and raises the possibility that these factors contribute to the tumor suppressor activity of
MLL3 in liver cancer.
We next set out to determine whether MLL3 binding is sufficient to induce transcriptional activa-
tion of the CDKN2A locus and, in doing so, extend our analysis to human liver cancer cells. As the
KMT2C transcript is too large (14,733 bp) for cDNA transduction, we turned to the CRISPR activation
(CRISPRa) system (Chavez et al., 2015) in a human hepatocellular carcinoma cell line (HLE). Following
stable integration of the nuclease dead Cas9 fused to the VP64- p65- Rta (VPR) transcriptional acti-
vator, cells were transduced with two orthogonal sgRNAs targeting the human KMT2C promoter (or,
as control, transduced with sgRNA against GFP; Figure 4C). Cells expressing the KMT2C sgRNAs
showed a marked and specific increase in the expression of endogenous KMT2C, but not of KMT2D
or TP53 (Figure 4D, Figure 5—figure supplement 1A, B), which was accompanied by an increase in
MLL3 binding to the CDKN2A locus (Figure 4E) and transcriptional upregulation of both CDKN2A
transcripts (Figure 4F and G). Therefore, MLL3 directly binds and co- activates transcription of the
CDKN2A locus in human liver cancer cells.
MLL3 mediates oncogene-induced apoptosis in a Cdkn2a-dependent
manner
The above results raise the possibility that the Cdkn2a products, INK4A and ARF, may contribute to
the tumor suppressive activity of MLL3. In this regard, Myc overexpression in primary cells (mouse
embryonic fibroblasts; MEFs) often triggers apoptosis (Evan et al., 1992), and this in turn limits
tumorigenesis in a manner that is dependent on Cdkn2a (Zindy et al., 1998). This pathway also
suppresses liver tumorigenesis since concomitant disruption of Ink4a and Arf using CRISPR, or germ-
line deletion of Arf alone, cooperated with Myc overexpression to rapidly promote tumor develop-
ment (Figure 5—figure supplement 1C). Similarly, Kmt2c suppression also attenuated MYC- induced
apoptosis, as shown by tumor histology and apoptosis by TUNEL assay (Negoescu et al., 1997),
5 days after hydrodynamic delivery of transposon vectors encoding Myc together with GFP- linked
shRNAs targeting Kmt2c (or Renilla luciferase as a control; Figure 5A and B). This difference in apop-
tosis correlated with an increase in retention of GFP- shKmt2c expressing cells 10 days after injection
(Figure 5—figure supplement 1D, E). Altogether, these results show that Kmt2c suppression impairs
Myc- induced apoptosis in vivo in a manner that is reminiscent of the anti- apoptotic effects of Cdkn2a
loss in the context of aberrant Myc activation (Eischen et al., 1999; Jacobs et al., 1999; Schmitt
et al., 1999).
To model the interaction between Myc overexpression, MLL3 function, and Cdkn2a regulation,
we transduced liver progenitor cells (LPCs) with retroviral vectors encoding Myc linked to a reverse
tetracycline transactivator (rtTA3), together with doxycycline (dox)- inducible Kmt2c shRNAs to enable
reversible Kmt2c silencing (Figure 5—figure supplement 2A). Infection of LPCs with Myc in the
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Figure 5. MLL3 mediates oncogene- induced apoptosis in a Cdkn2a- dependent manner. (A) Representative images of TUNEL- positive nuclei (red
staining) in murine livers 5 days after hydrodynamic injection of the indicated vector combinations. DAPI(4′,6- diamidino- 2- phenylindole) was used to
visualize nuclei. (B) Quantification of TUNEL- positive nuclei in mouse livers 5 days after hydrodynamic tail vein injection (HTVI) of the indicated vector
combinations. Data points represent the number of TUNEL- positive cells in five different high- power fields in three independent murine livers per
group. ***=p<0.001 (one- way ANOVA followed by post hoc t- tests). (C) Chromatin immunoprecipitation (ChIP)- qPCR analysis for H3K4me3 signals at Arf
Figure 5 continued on next page
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Figure 5 continued
and Ink4a promoters 4 days after doxycycline (dox) withdrawal in Myc- rtTA3; TRE- shKmt2c cells. Values are mean ± SD from technical replicates (n=3),
and the experiments were conducted in two independent liver progenitor cell (LPC) lines with different shKmt2c. (D) qPCR analysis for mRNA expression
of Arf and Ink4a 4 days after dox withdrawal in two independent LPC lines with different shKmt2c. Values are mean ± SD from technical replicates (n=3).
***=p<0.001 and **=p<0.01 (unpaired two- tailed t- test). (E) Representative images of colony formation assay of the indicated cell lines 5 days after
dox withdrawal. (F) Quantification of colony formation assay. Values are mean ± SD of three independent experiments with two independent LPC lines.
*=p<0.05 (unpaired two- tailed t- test). (G) Time course analysis of Draq7- positive (dead or permeabilized) cells as a fraction of Venus- positive, Myc- rtTA3;
TRE- shKmt2c cells expressing constitutive shRNAs targeting both Ink4a and Arf (shCdkn2a) or Renilla luciferase (shRen) off and on dox. Values represent
mean ± SEM of triplicate wells of each genotype at each timepoint of two independently derived LPC lines, infected with either shRen or shCdkn2a.
*=p<0.05 (unpaired two- tailed t- test of final average percentage Draq7+/GFP+). NS, not significant (p>0.05).
The online version of this article includes the following source data and figure supplement(s) for figure 5:
Figure supplement 1. Kmt2c suppression reduces cell clearance upon enforced Myc expression in vivo.
Figure supplement 2. Endogenous Kmt2c restoration triggers apoptosis and is accompanied by increased Cdkn2a expression.
Figure supplement 2—source data 1. Original western blots for Figure 5—figure supplement 2B,F,G.
presence of MLL3 (i.e. cells infected with Myc- rtTA3 and a dox- inducible shRNA targeting Renilla
luciferase) acutely activated INK4A and ARF expression (Figure 5—figure supplement 2B), and these
cells could not be maintained in culture. Phenocopying the ability of Myc and Kmt2c suppression to
transform liver cells in vivo, combined Myc and shKmt2c expression facilitated the persistent growth
of cells maintained on Dox (Figure 5—figure supplement 2C,D). By contrast, dox withdrawal induced
Kmt2c mRNA expression and H3K4me3 deposition at the Cdkn2a promoters, ultimately leading
to elevations in Arf and Ink4a mRNA and protein (Figure 5C and D), reduced colony formation,
and increased apoptosis (Figure 5—figure supplement 2D–F). Furthermore, constitutive shRNA-
mediated knockdown of Arf and Ink4a through targeting of the shared exon 2 (shCdkn2a) significantly
rescued colony- forming capacity and prevented cell death following Kmt2c restoration as determined
by time- lapse microscopy of cells cultured with a fluorescent dye that stains dead cells (Figure 5E–G,
Figure 5—figure supplement 2G). These data support a model whereby a prominent tumor suppres-
sive output of MLL3 in liver cancer involves direct upregulation of Cdkn2a that, when impaired, atten-
uates the MYC- induced apoptotic program and permits tumor progression.
Figure 6. Model of MLL3 as a tumor suppressor in liver cancer. MLL3 restricts MYC- induced liver tumorigenesis by directly activating the Cdkn2a locus
to mediate tumor cell apoptosis.
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Discussion
Our study combined genetic, epigenomic, and animal modeling approaches to identify Cdkn2a as
an important regulatory target of MLL3 in both mouse and human liver cancers. Our results support
a model whereby oncogenic stress, herein produced by MYC, leads to an increase in the binding of
MLL3 to the CDKN2A locus, an event that is associated with the accumulation of histone marks linked
to the biochemical activity of MLL3- containing complexes and conducive to gene activation (Figure 6).
Accordingly, these events are accompanied by transcriptional upregulation of two key Cdkn2a gene
products, Ink4a and Arf. Moreover, suppression of Kmt2c phenocopies the effects of Cdkn2a inac-
tivation in abrogating MYC- induced apoptosis. Conversely, suppression of Cdkn2a diminishes the
anti- proliferative effects of Kmt2c restoration. As such, our results establish a conserved epistatic rela-
tionship between the chromatin modifier MLL3 and a well- characterized tumor suppressor network.
The epistatic relationship described above might be expected to lead to mutual exclusivity of
KMT2C and CDKN2A alterations; however, we did not observe significant mutual exclusivity in liver
cancer, which is likely due to insufficient samples sizes needed to obtain statistical power. Alterna-
tively, other functionally important components linked to the CDKN2A locus could produce CDKN2A-
independent forces that drive selection for chromosome 9p deletions, including type I interferon
genes, CDKN2B, and MTAP (Barriga et al., 2022). Alternatively, mutual exclusivity between KMT2C
and CDKN2A alterations would be expected only under circumstances where CDKN2A action is the
most dominant MLL3 effector. Indeed, it seems likely that multiple downstream genes, including
factors involved in interactions with stromal and immune populations, contribute to MLL3- driven
tumor suppression, and their relative importance may vary between cell and tissue types. Such a vari-
able output in cancer- relevant gene regulation has been noted for other chromatin regulators that,
at the extreme, serve as pro- oncogenic factors in some contexts and tumor suppressors in others
(Fountain et al., 1992; Schmid et al., 2000; Sun et al., 2017; Xia et al., 2021). Furthermore, our
observation that Kmt2c deficiency cooperated with MYC but not CTNNB1 to drive HCC highlights
such context specificity and is in line with recent findings that chromatin context could favor particular
oncogenic alterations over others (Weiss et al., 2022).
UTX (KDM6A), MLL3 (KMT2C), and MLL4 (KMT2D), the core catalytic components of the COMPASS-
like complex, are all considered tumor suppressors, with frequent loss- of- function genomic alterations
found in a broad spectrum of human cancers (Revia et al., 2022; Sze and Shilatifard, 2016). While
each of these components regulates redundant sets of genes (Hu et al., 2013; Lee et al., 2009),
they may exert their tumor suppressive functions through different mechanisms. In liver and pancreas
cancer models, UTX can control the expression of negative regulators of mTOR such as DEPTOR, and
its disruption prevents their transcription and facilitates tumorigenesis through increased mTORC1
activity (Revia et al., 2022). Additionally, while the mechanisms of MLL4 activity have not been exam-
ined in liver cancer, studies suggest that MLL4 suppresses skin carcinogenesis by promoting lineage
stability and ferroptosis independently of MLL3 (Egolf et al., 2021). Our study demonstrates that
MLL3 is both necessary and sufficient for efficient transcriptional activation of the CDKN2A locus that
drives oncogene- induced apoptosis. The molecular basis for this heterogeneity in effector output
remains to be determined, but it seems likely that different subsets of target genes are preferen-
tially disabled by haploinsufficiency of individual components and/or subject to compensation by the
remaining COMPASS complex activities. Systematic studies comparing the binding, histone modifi-
cations, and transcriptional output of cells across a spectrum of allelic configurations of COMPASS
complex factors will be needed to achieve a more holistic understanding of their functions and inter-
actions in different contexts.
The most well- established role for MLL3/4- UTX- containing complexes is the control of H3K4
monomethylation at enhancers during development (Herz et al., 2010; Hu et al., 2013). While our
ChIP- Seq studies also revealed binding of MLL3/4 to enhancers in liver tumor cells, an even larger
fraction of genes—including Cdkn2a—showed MLL3/4 chromatin enrichment at gene promoters, and
indeed, transcription of this class of genes was most affected by Kmt2c disruption. Interestingly, Kmt2c
suppression preferentially limited the MLL3/4 enrichment at promoters and shifted residual complex
binding toward intergenic regions. Such dynamic regulation of distinct cis- acting elements by the
MLL3/4 complex has also been observed in other contexts (Cheng et al., 2014; Soto- Feliciano et al.,
2023), where the non- canonical binding of MLL3/4 at promoters is a recurrent tumor suppressive
mechanism in cancer cells. MLL3/4 has also been observed to bind within the exons and introns, which
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may enable chromatin looping of enhancers to activate gene expression (Panigrahi and O’Malley,
2021). Further studies into the action and regulation of MLL3/4 complexes at promoters and gene
bodies will be informative and may yield new insights into the actions of the COMPASS- like complex
in cancer.
While CDKN2A showed a surprisingly dominant role in mediating the tumor- suppressive effects of
the broadly acting MLL3 enzyme, there are precedents for a predominant contribution of a single gene
to the functional output of chromatin- complex disruption. Indeed, polycomb repressive complexes
(PRCs) broadly repress gene expression in different cell types through the coordinated action of PRC1
and PRC2 complexes that deposit and maintain repressive H3K27me3 marks on the enhancers of
target genes, including CDKN2A (Bracken et al., 2007; Kotake et al., 2007). Despite these similarly
broad effects, CDKN2A is often the most functionally relevant target of PRC- mediated repression,
as genetic deletion of either the PRC1 component Bmi1 or the PRC2 component Ezh2, or treatment
with small molecule inhibitors of EZH2, can facilitate Cdkn2a induction in normal and tumor cells. This,
in turn, triggers anti- proliferative responses that can be rescued by Cdkn2a deletion (Jacobs et al.,
1999; Richly et al., 2011). Notably, the COMPASS- like complexes are biochemically and functionally
similar to Trithorax complexes in Drosophila, which have an evolutionarily conserved antagonistic
relationship with PRC1 and PRC2 that controls epigenetic memory and cell fate during development
(Mills, 2010; Piunti and Shilatifard, 2016). Our findings suggest such antagonism extends to tumor
suppression in mammalian cells, likely via regulation of Cdkn2a and other tumor suppressor genes
(Soto- Feliciano et al., 2023).
Materials availability statement
Source files of all original gels and western blots were provided for the following figures:
Figure 1—figure supplement 2B;
Figure 4—figure supplement 1A, C, D, E;
Figure 5—figure supplement 2B, F, G.
RNA sequencing and ChIP- Seq data files that support the findings of this study have been depos-
ited in the Gene Expression Omnibus under the accession code GSE85055, as well as in the Dryad
digital repository (doi:10.5061/dryad.7pvmcvdwm; doi:10.5061/dryad.f1vhhmh0h). Sequences of
sgRNAs, shRNAs, and primers used in this manuscript are included in the Supplementary file 1.
Materials and methods
Animal experiments
8- to 10- week- old female C57BL/6 animals were purchased from Envigo (formerly Harlan). Each exper-
iment was performed in mice from the same order. Arf- null animals (C57BL/6 background), originally
provided by Dr. Charles Sherr, St. Jude Children’s Research Hospital, were maintained in our breeding
colony. For HTVI, a sterile 0.9% NaCl solution/plasmid mix was prepared containing oncogene trans-
posons (5 µg DNA of pT3- Myc or 10 µg pT3-Ctnnb1 N90) with either 20 µg of pX330 expressing the
indicated sgRNAs or 20 µg of pT3- EF1a- GFP- miRE plasmid together with CMV- SB13 Transposase (1:5
ratio). Mice were randomly assigned to experimental groups and injected with the 0.9% NaCl solu-
tion/plasmid mix into the lateral tail vein with a total volume corresponding to 10% of body weight
in 5–7 s as described before (Largaespada, 2009; Moon et al., 2019; Tschaharganeh et al., 2014;
Xue et al., 2014). Injected mice were monitored for tumor formation by abdominal palpation. All
animal experiments were approved by the Memorial Sloan Kettering Cancer Center (MSK) Institu-
tional Animal Care and Use Committee (protocol 11- 06- 011). Animals were monitored for signs of ill
health by veterinary staff at the Research Animal Resource Center at MSK, and efforts were made to
minimize suffering.
Vector constructs
The pT3- Myc vector Addgene (#92046) and pT3- EF1a- GFP- miRE plasmid were described before
(Huang et al., 2014). The pT3-Ctnnb1 N90 vector (Tward et al., 2007) was obtained from Addgene
(#31785). For CRISPR/Cas9- mediated genome editing, sgRNAs were subcloned into pX330 (Addgene,
#42230; Hsu et al., 2013). All shRNA and sgRNA sequences are listed in Supplementary file 1.
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Derivation of primary liver tumor cell lines
Liver tumors were resected with sterile instruments, and 10–50 mg of tumor tissue was minced and
washed in sterile PBS, incubated in a mix of 1 mg/mL collagenase IV and 3 mg/mL dispase (dissolved
in sterile, serum- free DMEM(Dulbecco's Modified Eagle Medium)) with gentle shaking, washed with
PBS, incubated for 5 min in 0.05% (w/v) trypsin, and washed and plated in complete DMEM (10%
FBS(fetal bovine serum), 1× penicillin/streptomycin) on collagen- coated plates (PurCol, Advanced
Biomatrix). Primary cultures were passaged until visibly free from fibroblasts. Cell lines were authen-
ticated on a routine basis using short tandem repeat profiling, as well as tested for mycoplasma
contamination and immediately discarded upon a positive test.
Analysis of CRISPR-directed mutations
CRISPR- mediated insertions and deletions were detected by surveyor assay as directed by the manu-
facturer (Transgenomic/IDT). Briefly, after overnight lysis of primary tumors and cell lines at 37°C in
buffer containing 0.4 mg/mL proteinase K, 10 mM Tris, 100 mM NaCl, 10 mM EDTA, and 0.5% SDS,
pH 8.0, genomic DNA was extracted by isopropanol precipitation. ~250–500 bp regions flanking
predicted CRISPR cleavage sites were PCR amplified with Herculase II taq polymerase, column puri-
fied (Qiagen), heated to 95°C, and slowly cooled to promote annealing of heteroduplexes. Following
treatment with Surveyor nuclease, products were analyzed by electrophoresis on a 2% polyacrylamide
gel. Primers used for surveyor assay are listed in Supplementary file 1. Amplified PCR products were
separately gel purified and ligated into blunt- end digested pBlueScript (Stratagene). DNA from 48
transformed colonies was analyzed by Sanger sequencing using a T7 primer.
CRISPR activation
Human HCC cell line HLE, purchased from JCRB Cell Bank (JCRB0404), was transduced by the lenti-
virus expressing nuclease- dead Cas9 (dCas9) fused with VPR (Chavez et al., 2015) and sgRNAs
against KMT2C (sequence in Supplementary file 1) to generate stable MLL3 CRISPRa HLE line by
puromycin selection.
Generation and modification of primary cells
LPCs from E13.5–15.5 C57BL/6 embryos were isolated and grown in hepatocyte growth media
(HGM) as previously described (Zender et al., 2005). To simultaneously overexpress Myc and condi-
tionally suppress Kmt2c, LPCs were co- infected with a retroviral construct constitutively expressing
both Myc and a reverse tet- transactivator (rtTA) (MSCV- Myc- IRES- rtTA) along with retroviral TRMPV
vectors (MSCV- TRE- dsRed- miR30/shRNA- PGK- Venus- IRES- NeoR) (Zuber et al., 2011) expressing
ds- Red linked, teint- responsive shRNAs targeting Kmt2c cloned into an optimized mir- 30 context
(‘mir- E,’ TRPMVe; Zuber et al., 2011). For selection of infected cells and sustained shKmt2c expres-
sion, cells were maintained in HGM with neomycin (1 mg/mL) and dox (1 µg/mL) starting 2 days after
infection. To introduce constitutively expressed shRNAs in the setting of inducible shKmt2c, retro-
viral MLPe vectors (MSCV- LTR- mir- E- PGK- Puro- IRES- GFP; Dickins et al., 2005). GFP- linked shRNAs
targeting either Cdkn2a or Renilla luciferase (as control) were co- infected with MSCV- Myc- IRES- rtTa
and TRMPVe- shKmt2c. Triple- infected cells were maintained in media with neomycin, puromycin
(2 µg/mL), and dox 2 days post infection. Infected continuously proliferating cells were transitioned to
growth in complete DMEM and maintained on collagen- coated plates.
Colony assays
For measurement of cell proliferation, 5000 transduced and selected LPCs or MEFs were plated in
triplicate in 6- well plates. Tetracycline- inducible shKmt2c–expressing LPCs were grown in the pres-
ence or absence of dox, and after 5 days, cells were fixed with formalin and methanol and stained
with 0.05% crystal violet. MEFs were fixed after 6 days with formalin and methanol and stained with
0.05% crystal violet.
Apoptosis assays
Apoptosis was measured in LPCs via Annexin V staining according to the manufacturer’s instructions
(eBiosciences, Annexin- V APC). 25,000 cells were grown with and without dox for 3 days, trypsinized,
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washed with Annexin- V binding buffer, and ~100,000 cells were incubated with Annexin- V APC and
analyzed on an LSRII flow cytometer (BD).
Live imaging
Imaging was performed on LPCs immortalized by linked overexpression of Myc and two independent,
inducible Kmt2c shRNAs constitutively expressing shRNAs targeting Renilla luciferase or Cdkn2a
(generated as detailed above). 1000 cells were plated on collagen- coated, 96 well, clear bottom
imaging plates in media supplemented with 300 nM Draq7 (Invitrogen) with and without dox, in trip-
licate by genotype. 18 hr after plating cells, Venus (marking all plated cells) and Draq7 fluorescence
was collected in two, 10× fields of each well every 15 min for 41 hr using an automated, high content
microscope (InCell 6000, General Electric).
Chromatin immunoprecipitation
Histone ChIP was performed as previously described (Lee et al., 2006). Briefly, cell samples were
cross- linked in 1% formaldehyde for 10 min, and the reaction was stopped by addition of glycine
to 125 mM final concentration. Fixed cells were lysed in SDS lysis buffer, and the chromatin was
fragmented by sonication (Covaris). Sheared chromatin was incubated with antibodies (final concen-
tration 10 µg/mL) against H3K4me3 (Abcam, ab8580, Lot:GR164706- 1), H3K27ac (Abcam, ab4729,
Lot:GR200563- 1), or H3K4me1 (Abcam; ab8895, Lot:GR114265- 2) or with normal rabbit IgG (Abcam,
ab46540) at 4°C for overnight. Antibodies were recovered by binding to protein A/G agarose (Milli-
pore), and the eluted DNA fragments were used directly for qPCR or subjected to high- throughput
sequencing (ChIP- Seq) using a HiSeq 2000 platform (Illumina). High- throughput reads were aligned to
mouse genome assembly NCBI37/mm9 as previously described (Barradas et al., 2009). Reads that
aligned to multiple loci in the mouse genome were discarded. The ChIP- Seq signal for each gene was
quantified as total number of reads per million in the region 2 kb upstream to 2 kb downstream of the
transcription start site (TSS). Primers used for ChIP- qPCR of mouse Cdkn2a promoter (Barradas et al.,
2009) are listed in Table S1.
The complete dataset is available at NCBI Gene Expression Omnibus (GSE85055), as well as the
Dryad digital repository (doi:10.5061/dryad.7pvmcvdwm).
For the MLL3 ChIP- Seq, the following protocol was used. Cross- linking ChIP in mouse and human
HCC cells was performed with 10–20×107 cells per immunoprecipitation. Cells were collected, washed
once with ice- cold PBS, and flash- frozen. Cells were resuspended in ice- cold PBS and cross- linked
using 1% paraformaldehyde (PFA; Electron Microscopy Sciences) for 5 min at room temperature with
gentle rotation. Unreacted PFA was quenched with glycine (final concentration 125 mM) for 5 min
at room temperature with gentle rotation. Cells were washed once with ice- cold PBS and pelleted
by centrifugation (800 g for 5 min). To obtain a soluble chromatin extract, cells were resuspended in
1 mL of LB1 (50 mM HEPES pH 7.5, 140 mM NaCl, 1 mM EDTA, 10% glycerol, 0.5% NP- 40, 0.25%
Triton X- 100, and 1× complete protease inhibitor cocktail) and incubated at 4°C for 10 min while
rotating. Samples were centrifuged (1400 g for 5 min), resuspended in 1 mL of LB2 (10 mM Tris- HCl
pH 8.0, 200 mM NaCl, 1 mM EDTA, 0.5 mM EGTA, and 1× complete protease inhibitor cocktail),
and incubated at 4°C for 10 min while rotating. Finally, samples were centrifuged (1400 g for 5 min)
and resuspended in 1 mL of LB3 (10 mM Tris- HCl pH 8.0, 100 mM NaCl, 1 mM EDTA, 0.5 mM EGTA,
0.1% sodium deoxycholate, 0.5% N- lauroylsarcosine, and 1× complete protease inhibitor cocktail).
Samples were homogenized by passing seven to eight times through a 28- gauge needle, then Triton
X- 100 was added to a final concentration of 1%. Chromatin extracts were sonicated for 14 min using
a Covaris E220 focused ultrasonicator. Lysates were centrifuged at 20,000 g for 10 min at 4°C, and 5%
of the supernatant was saved as input DNA. Beads were prepared by incubating them in 0.5% BSA
in PBS and antibodies overnight (100 μL of Dynabeads Protein A or Protein G [Invitrogen] plus 20 μL
of antibody). The antibody was anti- MLL3/4, which was kindly provided by the Wysocka laboratory
(Dorighi et al., 2017). Antibody- beads mixes were washed with 0.5% BSA in PBS and then added to
the lysates overnight while rotating at 4°C. Beads were then washed six times with RIPA buffer (50 mM
HEPES pH 7.5, 500 mM LiCl, 1 mM EDTA, 0.7% sodium- deoxycholate, and 1% NP- 40) and once with
TE- NaCl Buffer (10 mM Tris- HCl pH 8.0, 50 mM NaCl, and 1 mM EDTA). Chromatin was eluted from
beads in Elution buffer (50 mM Tris- HCl pH 8.0, 10 mM EDTA, and 1% SDS) by incubating at 65°C for
30 min while shaking, supernatant was removed by centrifugation, and crosslinking was reversed by
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further incubating chromatin overnight at 65°C. The eluted chromatin was then treated with RNase A
(10 mg/mL) for 1 hr at 37°C and with proteinase K (Roche) for 2 hr at 55°C. DNA was purified by using
phenol- chloroform extraction followed with ethanol precipitation. The NEBNext Ultra II DNA Library
Prep kit was used to prepare samples for sequencing on an Illumina NextSeq500 (75 bp read length,
single- end, or 37 bp read length, and paired- end).
The complete dataset for MLL3 ChIP- Seq is available at the Dryad digital repository (doi:10.5061/
dryad.f1vhhmh0h).
Immunoblotting
Cell pellets were lysed in Laemmli buffer (100 mM Tris- HCl pH 6.8, 5% glycerol, 2% SDS, and 5%
2- mercaptoethanol). Equal amounts of protein were separated on 12% SDS–polyacrylamide gels and
transferred to PVDF(polyvinylidene difluoride) membranes (90 V, 75 min). β-actin was used as a control
to ensure equal loading, and images were analyzed using the AlphaView software (ProteinSimple).
Immunoblotting was performed using antibodies for MYC (1:1000, Abcam, ab32072), p53 (1:500,
Leica Biosystems, NCL- p53- 505), p19 (1:250, Santa Cruz Biotechnology, sc- 32748), p16 (1:250, Santa
Cruz Biotechnology, sc- 1207), Axin1 (1:1000, Cell Signaling, #2074), and β-actin (1:10000, Sigma-
Aldrich, clone AC- 15). Source files of all western blots were provided for Figure 1—figure supple-
ment 2, Figure 4—figure supplement 1, Figure 5—figure supplement 2.
Quantitative RT-PCR
Total RNA was isolated using RNeasy Mini Kit, QIAshredder Columns, and RNase- Free DNase Set
(Qiagen). cDNA synthesis was performed using TaqMan Reverse Transcription Reagents (Thermo
Fisher Scientific). Real- time PCR was carried out using Power SYBR Green Master Mix (Thermo Fisher
Scientific) and the Life Technologies ViiA 7 machine. Transcript levels were normalized to the levels of
mouse or human Actb mRNA expression and calculated using the ΔΔCt method. Each qRT- PCR was
performed in triplicate using gene- specific primers (sequences listed in Table S1).
RNA sequencing and differential expression analysis
For RNA sequencing, total RNA from three independent tumor- derived cell lines (Myc; sgTrp53 and
Myc; sgKmt2c) was isolated using RNeasy Mini Kit, QIAshredder Columns and RNase- Free DNase
Set (Qiagen). RNA- Seq library construction and sequencing were performed according to proto-
cols used by the integrated genomics operation Core at MSK. 5–10 million reads were acquired
per replicate sample. After removing adaptor sequences with Trimmomatic, RNA- seq reads were
aligned to GRCm38.91(mm10) with STAR (Dobin et al., 2013). Genome- wide transcript counting was
performed by HTSeq to generate an FPKM(Fragments Per Kilobase per Million mapped fragments)
matrix (Anders et al., 2015). DEGs were identified by DESeq2 (v.1.8.2, package in R) and plotted in
the volcano plot. The complete dataset is available at NCBI Gene Expression Omnibus (GSE85055).
Integrative analyses of RNA-seq and MLL3 ChIP-seq
Differential peaks from ChIP- Seq data were annotated by assigning all intragenic peaks to that gene
while intergenic peaks were assigned using linear genomic distance to the TSS. Genes that were coor-
dinately regulated (fold change >1.5 and adjusted p- value<0.1) in MLL3 ChIP- seq and RNA- seq data
were selected for the integrated analysis. Enriched pathways were scored using the enrichGO function
with ‘biological process’ in the clusterProfiler R package. Redundant pathways were collapsed using
the ‘simplify’ function with a cutoff of 0.7 with the p.adjust metric. Network analysis was performed
using differential peaks and genes by running enrichplot::cnetplot in R with default parameters.
Human cancer analyses
RNA sequencing data of selected samples with somatic mutations or homozygous deletions of
KMT2C, CDKN2A, TP53, or RB1 in the TCGA HCC dataset were downloaded from Broad Institute
TCGA Genome Data Analysis Center. To obtain transcriptional signatures of HCC with genomic muta-
tions and deletions of either KMT2C, CDKN2A, and RB1, differential gene expression analyses were
performed by DESeq2 (with TP53- mutated HCCs as controls). The oncoprints of homozygous dele-
tions and somatic mutations of KMT2C, CDKN2A, and TP53, as well as MYC gains and amplifications
from human HCC datasets (Cancer Genome Atlas Research Network, 2017, MSK [Harding et al.,
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2019; Zheng et al., 2018], INSERM [Schulze et al., 2015], RIKEN [Fujimoto et al., 2012], AMC [Ahn
et al., 2014], and MERCi [Ng et al., 2022]) were generated by cBioPortal (https://www.cbioportal.org
; Cerami et al., 2012; Gao et al., 2013).
Gene set enrichment analysis
GSEA was performed using the GSEAPreranked tool for conducting GSEA of data derived from RNA-
seq experiments (version 2.07) against other signatures. The metric scores (normalized enrichment
scores and false discovery rate q- values) were calculated using the sign of the fold change multiplied
by the inverse of the p- value (Subramanian et al., 2005). Specifically, transcriptional signatures were
derived based on significantly changed genes (p- adjusted<0.05, absolute fold change >2) from RNA-
seq of mouse HCC cell lines (Myc; sgKmt2c vs Myc; sgTrp53, n=3 each genotype), and in human
HCCs with mutations in KMT2C vs TP53 (p- adjusted<0.05, absolute fold change >2). These signatures
were compared to the transcriptional comparison of TCGA human HCCs with genomic inactivation of
CDKN2A vs TP53.
Statistical analyses
Data are presented as mean ± D or SEM as specified. The statistical comparison between two groups
was accomplished with the two- tailed student’s t- test or one- way ANOVA followed by post hoc t- tests
among three or more groups. The analyses for co- occurrence or mutual exclusivity of mutations were
performed using Fisher Exact test. Comparisons of survival curves were performed by log- rank tests.
All statistical tests were performed using the Prism 8 software. All data presented in the manuscript
have been replicated in independent cohorts of mice or in at least three biological replicates for
in vitro experiments. On the basis of predicted effects of oncogene- tumor suppressor interaction
introduced by HTVI in mice, with a power of 0.8 and p<0.05, we calculated a minimum sample size
of 5 mice per group. Animals within the same cage were randomly allocated into control and exper-
imental groups, with the group assignment recorded in a master spreadsheet and unmasked only
when all samples of the respective experiments were analyzed. Data collection of each experiment
was detailed in the respective figures, figure legends, and methods. No data were excluded from
studies in this manuscript.
Acknowledgements
We thank Charles Sherr and Janet Novak for constructive guidance and advice on all aspects of this
study. We thank Ali Shilatifard, Lu Wang, and all members of the Lowe lab for helpful and stimu-
lating discussions. We gratefully thank A Chramiec for excellent technical assistance. We thank Joanna
Wysocka (Stanford University) for kindly sharing the anti- MLL3/4 antibody used in our ChIP- Seq
experiments. This work was supported by grants to SWL (P01 CA013106 and R01 CA233944) from
the NIH/NCI, as well as by the National Center for Tumor Disease, Heidelberg, and grants of the
German Research Foundation to DFG (SFB/TRR77). This work was also supported by the NIH/NCI
Cancer Center Support Grant to Memorial Sloan Kettering Cancer Center (P30 CA008748). YMSF is
supported by a MOSAIC K99/R00 Award from the NIH/NIGMS (1K99GM140265- 01). CZ is supported
by an F32 Postdoctoral Fellowship (1F32CA257103) from the NIH/NCI. JPM was a recipient of a
Postdoctoral Fellowship (PF- 14- 066- 01- TBE) from the American Cancer Society. DFT is supported by
a Young Investigator Group (VH- NG- 1114) by the Helmholtz foundation. SWL is the Geoffrey Beene
Chair for Cancer Biology and an investigator of the Howard Hughes Medical Institute.
Additional information
Competing interests
C David Allis: is a co founder of Chroma Therapeutics and Constellation Pharmaceuticals and a Scien-
tific Advisory Board member of EpiCypher. Scott W Lowe: is an advisor for and has equity in the
following biotechnology companies: ORIC Pharmaceuticals, Faeth Therapeutics, Blueprint Medicines,
Geras Bio, Mirimus Inc, Senescea, and PMV Pharmaceuticals. S.W.L. also acknowledges receiving
funding and research support from Agilent Technologies and Calico, for the purposes of massively
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parallel oligo synthesis and single- cell analytics, respectively. The other authors declare that no
competing interests exist.
Funding
Funder
Grant reference number Author
National Cancer Institute
P01 CA013106
Scott W Lowe
National Cancer Institute
R01 CA233944
Scott W Lowe
National Institute of
General Medical Sciences
1K99GM140265-01
Yadira M Soto-Feliciano
National Cancer Institute
1F32CA257103
Changyu Zhu
American Cancer Society
PF-14-066-01-TBE
John P Morris
Helmholtz foundation
VH-NG-1114
Darjus F Tschaharganeh
National Cancer Institute
P30 CA008748
Scott W Lowe
The funders had no role in study design, data collection and interpretation, or the
decision to submit the work for publication.
Author contributions
Changyu Zhu, Yadira M Soto- Feliciano, Conceptualization, Data curation, Formal analysis, Validation,
Investigation, Visualization, Methodology, Writing – original draft, Writing – review and editing; John
P Morris, Conceptualization, Data curation, Formal analysis, Validation, Investigation, Methodology,
Writing – original draft, Writing – review and editing; Chun- Hao Huang, Conceptualization, Data
curation, Formal analysis, Validation, Investigation, Methodology, Writing – original draft; Richard P
Koche, Yu- jui Ho, Software, Formal analysis, Visualization, Methodology; Ana Banito, Data curation,
Methodology; Chun- Wei Chen, Formal analysis; Aditya Shroff, Sha Tian, Geulah Livshits, Chi- Chao
Chen, Myles Fennell, Scott A Armstrong, Methodology; C David Allis, Supervision, Writing – review
and editing; Darjus F Tschaharganeh, Conceptualization, Data curation, Formal analysis, Supervision,
Validation, Investigation, Methodology, Writing – original draft, Writing – review and editing; Scott W
Lowe, Conceptualization, Resources, Supervision, Funding acquisition, Investigation, Writing – orig-
inal draft, Project administration, Writing – review and editing
Author ORCIDs
Changyu Zhu
Yadira M Soto- Feliciano
Richard P Koche
Ana Banito
Chun- Wei Chen
Scott A Armstrong
Scott W Lowe
http://orcid.org/0000-0003-3583-3638
http://orcid.org/0000-0002-8523-7917
http://orcid.org/0000-0002-6820-5083
http://orcid.org/0000-0003-2188-0003
http://orcid.org/0000-0002-8737-6830
http://orcid.org/0000-0002-9099-4728
http://orcid.org/0000-0002-5284-9650
Ethics
All animal experiments were approved by the MSKCC Institutional Animal Care and Use Committee
(protocol 11- 06- 011). Animals were monitored for signs of ill- health by veterinary staff at the Research
Animal Resource Center (RARC) at MSKCC and efforts were made to minimize suffering.
Decision letter and Author response
Decision letter https://doi.org/10.7554/eLife.80854.sa1
Author response https://doi.org/10.7554/eLife.80854.sa2
Additional files
Supplementary files
• Supplementary file 1. Tables displaying the sequences of single guide RNA (sgRNA), shRNA,
qPCR primers, and chromatin immunoprecipitation (ChIP)- qPCR primers used in the studies of this
manuscript.
Zhu, Soto- Feliciano, Morris et al. eLife 2023;12:e80854. DOI: https://doi.org/10.7554/eLife.80854
19 of 25
Cancer Biology
Research article
• MDAR checklist
Data availability
Source files of all original gels and Western Blots were provided for the following figures: Figure 1—
figure supplement 2B; Figure 4—figure supplement 1A, C, D, E; Figure 5—figure supplement 2B, F,
G. RNA sequencing and ChIP sequencing data files that support the findings of this study have been
deposited in the Gene Expression Omnibus under the accession code GSE85055, as well as in the
Dryad digital repository (doi:10.5061/dryad.7pvmcvdwm; doi:10.5061/dryad.f1vhhmh0h). Sequences
of sgRNAs, shRNAs, and primers used in this manuscript are included in the Supplementary File 1.
The following datasets were generated:
Year
2022
Dataset title
Dataset URL
Database and Identifier
Mll3 suppresses
tumorigenesis by activating
the Ink4a/Arf locus
https:// doi. org/
10. 5061/ dryad.
7pvmcvdwm
Dryad Digital Repository,
10.5061/dryad.7pvmcvdwm
2022
MLL3 ChIP sequencing in
murine and human HCC
cells
https:// doi. org/
10. 5061/ dryad.
f1vhhmh0h
Dryad Digital Repository,
10.5061/dryad.f1vhhmh0h
Author(s)
Soto- Feliciano MY,
Zhu C, Morris JP,
Huang C- H, Koche
RP, Y- J Ho, Banito
A, Chen C- W, Shroff
A, Tian S, Livshits G,
Chen C- C, Fennell M,
Armstrong SA, Allis
CD, Tschaharganeh
DF, Lowe SW
Soto- Feliciano MY,
Zhu C, Morris JP,
Huang C- H, Roche
RP, Y- J Ho, Banito
A, Chen C- W, Shroff
A, Tian S, Livshits G,
Chen C- C, Fennell M,
Armstrong SA, Allis
CD, Tschaharganeh
DF, Lowe SW
Lowe SW
2017
Mll3 suppresses
tumorigenesis by activating
the Ink4a/Arf locus
https://www. ncbi.
nlm. nih. gov/ geo/
query/ acc. cgi? acc=
GSE85055
NCBI Gene Expression
Omnibus, GSE85055
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Appendix 1
Appendix 1—key resources table
Reagent type (species)
or resource
Designation
Source or reference Identifiers
Additional information
Strain and strain
background (M. musculus) Wild- type C57BL/6 J
The Jackson
Laboratory
Stock #000664
Cell line (Homo- sapiens)
HLE HCC cell line
JCRB Cell Bank
JCRB0404
Cell line (M. musculus)
Myc; sgTrp53 HCC cell
lines
This paper
Cell line (M. musculus)
Myc; sgKmt2c HCC cell
lines
This paper
Cell line (M. musculus)
Myc; sgAxin1 HCC cell
lines
This paper
Cell line (M. musculus)
Cell line (M. musculus)
TRE- shKmt2c.1 liver
progenitor line
TRE- shKmt2c.2 liver
progenitor line
This paper
This paper
NA
NA
NA
NA
NA
Three independent cell lines derived from different mice
were used as biological replicates
Three independent cell lines derived from different mice
were used as biological replicates
Three independent cell lines derived from different mice
were used as biological replicates
Antibody
Recombinant DNA
reagent
Recombinant DNA
reagent
Recombinant DNA
reagent
Recombinant DNA
reagent
Recombinant DNA
reagent
Anti- MLL3/4 (Rabbit
polyclonal)
Dorighi et al., 2017;
PMID:28483418
NA
ChIP- seq (1:500)
pT3- Myc
Addgene
#92046
pT3-Ctnnb1 N90
Addgene
#31785
PX330- Cas9- U6- sgRNA
Addgene
#42230
CMV- SB13
pT3- EF1a- GFP- miRE
Huang et al., 2014;
PMID:25128497
Huang et al., 2014;
PMID:25128497
NA
NA
Zhu, Soto- Feliciano, Morris et al. eLife 2023;12:e80854. DOI: https://doi.org/10.7554/eLife.80854
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10.1371_journal.pstr.0000097.pdf
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Data Availability Statement: We have no data to
report.
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We have no data to report.
|
RESEARCH ARTICLE
Epistemic outsiders: Unpacking and utilising
the epistemic dimension of disruptive agency
in sustainability transformations
Sergiu SpatanID
Franziska EhnertID
1*, Daniel PeterID
2,3, Gundula ThieleID
4, Marc WolframID
2,3,
2, Stefan ScherbaumID
4
4, Moritz Schulz1, Caroline SurreyID
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
OPEN ACCESS
Citation: Spatan S, Peter D, Thiele G, Wolfram M,
Ehnert F, Scherbaum S, et al. (2024) Epistemic
outsiders: Unpacking and utilising the epistemic
dimension of disruptive agency in sustainability
transformations. PLOS Sustain Transform 3(2):
e0000097. https://doi.org/10.1371/journal.
pstr.0000097
Editor: Ana Delicado, Universidade de Lisboa
Instituto de Ciencias Sociais, PORTUGAL
Received: May 16, 2023
Accepted: January 10, 2024
Published: February 14, 2024
Copyright: © 2024 Spatan et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: We have no data to
report.
Funding: This paper is an outcome of the project
"The Disruptivity of the Others in Urban
Transformations" (DOUbT), which is part of the
Excellence Measure "Disruption and Societal
Change" at TU Dresden (TUDiSC) and is funded by
the Federal Ministry of Education and Research
(BMBF) and the Free State of Saxony under the
Excellence Strategy of the Federal Government and
1 Department of Philosophy, Dresden University of Technology, Dresden, Germany, 2 Leibniz Institute of
Ecological Urban and Regional Development, Dresden, Germany, 3 Faculty of Environmental Sciences,
Dresden University of Technology, Dresden, Germany, 4 Department of Psychology, Dresden University of
Technology, Dresden, Germany
* [email protected]
Abstract
Disruptions (systemic disturbances) are crucial to initiate and accelerate sustainability trans-
formations of large-scale social systems (be they socio-ecological, socio-technical, or socio-
institutional). Their emergence, characteristics and effects strongly relate to the role of
agents who aim to disrupt and transform the status quo, and which thus possess what we
call disruptive agency. In this paper, we highlight the epistemic dimension of disruptive
agency in social transformations, first by conceptualizing disruptive agents as epistemic out-
siders with respect to the social system that they intend to disrupt and transform, and sec-
ond by connecting this conceptualization to notions of belief, social practices, social
networks, discourses, or institutions. We identify five advantages of this approach. Firstly, it
informs and conceptually enables various promising interdisciplinary avenues to explore
and potentially influence transformative change towards sustainability. Secondly, an episte-
mic conception of disruptive agency offers a key for an integrated analysis of the individual
and collective levels of agency involved in sustainability transformations. Thirdly, the notion
of epistemic outsiders conceptually connects agent positions across system boundaries
that are understood to be of crucial importance for sustainability transformations respec-
tively (e.g., “niche innovators” or “regime intermediaries”) but which lack an integrated
understanding. Fourthly, an epistemic perspective additionally highlights the changing
requirements and challenges resulting in two principal stages of transformations unfolding
over time, namely before/after a new epistemic layout is shared by a majority of agents.
Finally, the above features allow to derive and conceive of new intervention formats and
strategies.
Author summary
What can I do to change society for the better? How can I contribute to a more sustainable
world? It often seems like there is only so much that individual humans can do to change
PLOS Sustainability and Transformation | https://doi.org/10.1371/journal.pstr.0000097 February 14, 2024
1 / 23
PLOS SUSTAINABILITY AND TRANSFORMATIONthe La¨nder. The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Epistemic outsiders
entire societies. The question of how human agency relates to ample social transforma-
tions lies at the heart of our paper. Our hope is that, by looking at the epistemic dimension
of agency in societal transformations, we can learn more about how sustainability trans-
formations can be developed and supported, both at an individual level and at a collective
level. By ‘epistemic’ we mean the way people think about themselves and about the norms,
rules, and standards that they are ready to follow. Our contention is that every agent-
driven societal transformation is enabled by people who have a diverging set of beliefs
from the system that they try to dislocate. They are epistemic outsiders to that system, as
we call them. By looking closer at epistemic outsiders and at the epistemic dimension of
social change, we hope to better understand how agents can drive sustainability transfor-
mations. Thus, we note that one of the tacit aims of epistemic outsiders aiming at sustain-
ability transformations is to change the minds of the people composing the status quo–the
epistemic layout of the reference system, as we call it. We aim to better understand how
this can happen by combining our epistemic reading of societal transformations with
existing research on the topics of belief change, networks, discourses, institutions, and
social practices. This allows us to cut across different levels (from individuals to collect-
ives) at which transformative processes occur and connect different strands of research
that otherwise are approached separately.
1 Introduction
In an urgent call for sustainability transformations, societies across the globe face the challenge
of shaping path-deviant and rapid systemic change. Ecological boundaries and tipping points
force us to acknowledge that planetary justice and well-being require accelerating deep trans-
formations in our current energy, transportation, housing, food, or health systems [1,2]. Cor-
respondingly, we also witness the emergence of more and more examples of individuals and
collectives actively striving to disrupt and change the way our societies operate. This can be
schoolchildren skipping classes and demanding climate action (e.g., Fridays for Future move-
ment [3,4]), scientists pushing for new modes of knowledge co-production [5], or activists
blocking busy highways (e.g., the Last Generation movement [6]). While these are highly visi-
ble forms of human agency aimed at sustainability transformations, they are however neither
the only ones nor necessarily the most effective and efficient.
Transformative change occurs through an intricate interplay of external pressures and
opportunities, structural shifts and disruptions, as well as emerging novelties–all of which facil-
itated by particular forms of agency. Understanding how human agents contribute to such
complex transformation dynamics has thus formed an important focus of a large and diverse
body of literature. This has provided conceptual and empirical insights regarding the distinc-
tive role of various types of “change agents” and their agency in system transformations
[7,8,9,10,11]. On the one hand, high importance has been attributed to actors outside the
mainstream that create innovation niches in which alternative system configurations of limited
scale and scope are trialled. In such contexts, the protagonists follow values, goals, rules and
practices that differ substantially from those of the prevailing regime [12,13,14,15]. On the
other hand, also agency exercised by certain incumbent actors within the mainstream has been
recognized to be crucial if providing, e.g., opportunity spaces for niche actors or direct sup-
port, as well as contributing to regime destabilization. Similarly, such action outside the rules of
the established regime is seen to be essentially motivated by not sharing the prevailing world-
views [16,17]. In all of these cases, change agents are therefore seemingly acting from a posi-
tion of an “outsider” to the system they would like to see transformed.
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PLOS SUSTAINABILITY AND TRANSFORMATIONEpistemic outsiders
Against this backdrop, the goal of our paper is to offer an integrated perspective on disrup-
tive agency in transformations that can provide new insights into their particular dynamics,
but also suggest novel intervention strategies aimed at steering transformations towards sus-
tainability. In order to bridge between various actor typologies that have been developed to
understand and illustrate the relevance of particular agency forms for sustainability transfor-
mations [7,18,19,20], which are usually analysed and interpreted separately, we propose the
concept of “epistemic outsiders” as an overarching ontological category that characterizes all
forms of disruptive agency directed towards transformations. By so doing, we aim to enable
more agile analytical approaches and interventions that connect between the role of individu-
als and collectives, thereby also addressing relations across agency levels, system boundaries
and transformation phases, and the problem of scaling innovations [21,22].
In what follows, we will first lay down the ontology assumed throughout the paper (section
2). We will then present our perspective on the epistemic dimension of transformation and dis-
ruptive agency (section 3). Finally, we will discuss ways in which this epistemic reading can
enrich understandings of and intervention strategies for sustainability transformations by
invoking four prevalent schools of thought in related scientific debates (social practice theory,
network theory, discourse theory, and institutional theory) as well as the psychology of mental
constructs (section 4).
2 Ontological assumptions
In this section we set out the ontological assumptions we adopt regarding the nature of social
systems, the transformation of social systems, and disruptive agency as a basic condition for
social system transformations. We also reflect on the fundamental importance of normativity
in such processes. These conceptual prerequisites will allow us to subsequently identify and
elaborate on the role of the epistemic dimension in transformation dynamics.
2.1 Social systems
For understanding social systems, we refer to Anthony Giddens’ structuration theory ([23];
see also [24] with a view to transformations). According to Giddens, a social system is a pat-
terned spatiotemporal set of interrelationships existing between agents (individuals, groups, or
organizations) acting in institutional, technical, and ecological contexts. Their interrelation-
ships are governed by rules, norms and standards that constitute the structural properties of
the social system. The structures inform individual and collective agency, stipulating, e.g., how
to relate to each other and to the environment, what technology to use and how, or what social
behaviours to accept and in which circumstances, etc.
Following Giddens, structures imply a duality in the sense that while structural properties
do enable and constrain agency, they simultaneously also depend on their continuous repro-
duction through agents. For a social system to maintain a particular configuration most agents
must therefore follow the rules, norms, and standards specific to that system. In turn, this
entails that when the share of deviating agents rises above a critical threshold, social systems
can start to destabilize and potentially become reconfigured or even transformed entirely.
Deviant thinking and acting of individuals or collectives thus needs to be understood as a fun-
damental precondition for any deeper change in social systems.
2.2 Transformations and disruptions
While incremental changes happen all the time in all social systems, transformations refer to
nonlinear change processes that fundamentally alter the structures and practices that charac-
terize a given system [25,26]. This particular type of systemic change dynamic depends on
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PLOS SUSTAINABILITY AND TRANSFORMATIONEpistemic outsiders
numerous coevolving factors (social, economic, ecological, cultural, institutional, technologi-
cal, etc.) that together create disruptions of the system. According to a well-known definition
from the socio-technical system literature [27, p.119], disruption is “[. . .] a high-intensity
effect in the structure of the sociotechnical system(s), demonstrated as long-term change in
more than one dimension or element, unlocking the stability and operation of: incumbent
technology and infrastructure, markets and business models, regulations and policy, actors,
networks and ownership structures, and/or practices, behaviour and cultural models”. To this
understanding we need to add two important twists: Firstly, we acknowledge that disruptions
do not always lead to a change in the structural properties of the system, even if they interfere
with them. A system may just as well return to its baseline configuration after the disruption
ends (depending on its resilience). Secondly, we add an epistemic dimension by recognizing
that the interferences with the structural properties of the system cannot be generated by the
disrupted system itself. Therefore, we consider an event (or a chain of events) E a disruption of
the reference system R if and only if (i) E is a high-intensity interference with the structural
properties of R and (ii) E is unanticipated and unplanned by R. Disruptions thus represent
major windows of opportunity for leveraging transformations, but they require epistemic posi-
tions from “outside” of the system, i.e., not derived from its rules, norms and standards—an
important point that we will expand on in subsection 3.3.
With a view to the temporality of change we have to note that transformations imply accel-
eration and unfold rapidly compared to the established pathway. Social systems are dynami-
cally stable, i.e., changes happen continuously either as a result of adaptation or simply because
incremental shifts are stipulated by the rules of the system. Even structural properties can be
changed over a very long period of time through incremental steps without this constituting a
transformation: Those changes have been anticipated and planned, but not elicited by a dis-
ruption. Consequently, in order to deal with the grand challenges of the Anthropocene, dis-
ruptions can offer an important lens to explore options for purposively accelerating and
scaling up transformations.
2.3 Disruptive agency
We assume a basic conception of agency as “the ability to act with intention–as opposed to just
reacting” [19, p.279]; cf. [28,29,30,31]. As noted above, we assume that social structures are
continuously recreated by individual and collective action, while the intentions behind indi-
vidual action are in turn influenced by social structures. Therefore, it is always uncertain how
independent and deviant the agency of actors within a social system can be, given how much
they are influenced by path dependencies, socialization, social pressures etc.
With a view to transformations, this demands to specify and distinguish forms of agency
that are explicitly driven by the motive to transform the reference social system, i.e., to create
purposive disruptions. Admittedly, one may also imagine agents who aim to disrupt without
pursuing any aspirations in terms of transformation, but although such cases could exist and
also contribute to transformation dynamics, they lack plausibility and provide little justifica-
tion for further theorising. Therefore, we define disruptive agency as the ability to act with the
intention of disrupting a social system in order to transform it. Arguably, actors pursuing trans-
formation strive to produce destabilization and deeper change in the system instead of tacitly
following its rules as the majority of conformists does. Correspondingly, the literature on sus-
tainability transformations has identified diverse types of actors who exercise such disruptive
agency, labelled, e.g., “forerunners”, “niche innovators”, “institutional entrepreneurs” or
“knowledge brokers”, and who significantly influence the transformation dynamics observed
[19,32,10]. While differing in their respective role, these actors share an underlying motive of
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PLOS SUSTAINABILITY AND TRANSFORMATIONEpistemic outsiders
system transformation informed by epistemic and normative orientations and their intertwin-
ing. Hence, before expanding on the epistemic dimension in section 3, we need to also account
for normativity in disruptive agency and social system transformations.
2.4 Normativity
Transformations of social systems can happen in any direction. Hence, the pursuit of transfor-
mations by design apparently raises fundamental ethical questions that require societal delib-
eration. Also, a normative concept like “sustainability” that may seem to be supported by a
broad (inter-) societal consensus in fact remains (and must remain) subject to contestation
regarding its particular normative postulates when it comes to the grand challenges of the
Anthropocene [33,34,35]. Nevertheless, the processes and dynamics we aim to unpack here in
principle apply to any transformations of socio-technical, socio-ecological and/or socio-insti-
tutional systems, independent from the value propositions they embrace. In this, we do
acknowledge that actors’ compliance with or deviance from established rule systems for the
sake of transformations (i.e., disruptive agency—not delinquents escaping the rule of law) is
also driven by particular normative orientations. Therefore, we subsequently address the cru-
cial role of normativity in two ways: First, we situate values and normative claims in the con-
text of broader belief sets that underpin the structuration of social systems (section 3). Second,
in the light of the grand challenges we adopt the normative stance of sustainability asking for
new insights and strategies for intervention that our approach can offer (e.g., regarding the
need to overcome the reluctance of incumbents to change, the need for building networks of
change, etc.) to help accelerate deep and path-deviant change (section 4).
3 The epistemic dimension of transformations and disruptive
agency
As outlined above, we are interested in human agents who aim to disrupt and transform an
unsustainable social system. Having recognised the important role such agents play in sustain-
ability transformations, our aim is to further illuminate how their agency, responsibility and
ethical concerns can be instrumental in fostering disruptions. In this paper, we do not aspire
to present an exhaustive framework to capture how transformations occur, or of all the mecha-
nisms through which agents contribute to transformations. More modestly, we want to high-
light the existence of an epistemic dimension in this which is largely overlooked or only
implicit in the common approaches used to study sustainability transformations. Acknowledg-
ing for and analysing this dimension, however, can benefit new understandings of sustainabil-
ity transformations, as well as different forms of intervention (see section 4). There are three
claims that circumscribe our epistemic reading of transformations and disruptive agency:
a. The transformation of a social system involves a modification of the epistemic layout of that
system.
b. The agents who attempt to disrupt and transform a social system are epistemic outsiders to
that social system. In turn, agents reproducing and stabilizing the system can be considered
epistemic insiders.
c. Drawing on their perspective as epistemic outsiders, disruptive agents aiming for social sys-
tem transformation always strive to alter the epistemic layout of that system.
In what follows we explore each of these points. We will first introduce the concept of an
epistemic layout of a social system (section 3.1), which will then help us define epistemic out-
siders more sharply (section 3.2). We can then show that various types of disruptive agents can
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PLOS SUSTAINABILITY AND TRANSFORMATIONEpistemic outsiders
be conceptualized as epistemic outsiders (section 3.3). This will allow us to revisit the basic
mechanisms of social system transformations, this time having an epistemic reading in mind
(section 3.4).
3.1 Epistemic layouts of social systems
So far, we assumed a fairly traditional ontology of social systems that contains two categories
(in the same vein as structuration theory; [23]): (i) agents and their agency and (ii) the struc-
tural properties of social systems. In what follows, we want to highlight a third ontological cate-
gory, which contributes to the structuration of social systems and thus helps explain how a
social system is created and perpetuated: (iii) the epistemic layout of a social system.
Consider any socio-technical system such as the energy or transport system as an example.
Call this system X. The current configuration of X is shaped by a set of beliefs about the values,
the aims, the hierarchy, the expectations, etc.–in other words, the rules, the norms, and the
standards (or “grammar”; [36, p.340])–that underlie the working of the system. X functions
the way it does because, presumably, its stakeholders accept these beliefs (expressed, e.g., in
regulations, policies, markets, contracts, signs) and have confidence that other agents compos-
ing the system also accept these beliefs and act based on them. On the one hand, these are
structural beliefs about the rules, norms, values, and standards of the system. On the other
hand, these are also relational beliefs regarding the behaviour of others (i.e., if I don’t do this,
another agent will react in that way, etc.).
All these beliefs together constitute the epistemic layout of X. Were the agents composing
the system holding alternative beliefs, the social system would be very differently configured.
How we interact with each other is based on our beliefs about rules of interaction and on our
beliefs about what others believe about those rules of interaction. In this sense, our social sys-
tems are “republics of beliefs” [37]–a notion also inspired by game-theoretical considerations
about social conventions and law-abiding behaviour [38,39].
Of course, people don’t simply decide ex nihilo about an epistemic layout they want to sup-
port. People are born and socialized in particular social systems (be it a family, a community, a
religion, a nation or capitalism), such that their structural and relational beliefs regarding that
system are passed on to them in the process of socialization. This reflects the relation between
social structure and agency: The epistemic layout of a social system is part of the deep struc-
turation process of that system (see Giddens [23]). This enables a social system to perpetuate
itself. That being said, it is also possible for people to change their beliefs based on new experi-
ences or evidence. If sufficiently many people do so, the epistemic layout of the system changes
as well, making it possible for the system to become transformed. This leads us to further
explore the possibility of particular agents deviating from a given epistemic layout.
3.2 Epistemic outsiders
In this paper we introduce the conception of epistemic outsiders understood as those agents
who disagree with some or all of the rules, norms and standards constituting the epistemic lay-
out of a social system. In other words, epistemic outsiders “fail” to hold the structural beliefs
corresponding to the reference system. Formally, we define an epistemic outsider in the fol-
lowing way:
Supposing that N = {p, q, r . . .} is the set of all structural propositions corresponding to
the reference system R (describing the rules, norms and standards of R), an agent A is an
epistemic outsider to the system R if A disagrees with at least one of the propositions from
the set N.
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A few observations are in place. Firstly, for A to disagree with one of the propositions from
N, say p, is for A to have doubts about p or to believe that not-p. This applies both to when A is
merely an individual or when A is a group, as groups can presumably also hold beliefs [40]. It
goes without saying that A can be an epistemic outsider to R while at the same time being an
insider to another reference system, say S. Agents normally belong to several social systems
simultaneously (A might be part of a family, of a company, of a political party, etc.).
Secondly, epistemic outsiders can be differentiated by degrees, depending on how many
normative propositions underlying R the agent disagrees with and how essential they are. Sup-
posing that A1 disagrees merely with p, while A2 disagrees with all the propositions from set
N, A2 is more of an epistemic outsider to R than A1. Also, if p represents a core value the
degree of being an epistemic outsider is higher than if it refers to a behavioural rule, for
instance.
Thirdly, focusing on the concept of epistemic outsiders allows to draw parallels between
agents occupying very different positions regarding the reference system. In particular, those
who play an active role within the system, engaging in its institutions and practices and repro-
ducing them, and those who are not part of this process but relate to it from the system’s envi-
ronments. We therefore suggest acknowledging for endogenous and exogenous outsiders. Both
are agents who disagree with at least some of the key tenets of the reference system, but whose
distinct position regarding the system implies different options for taking action in order to
address tensions between the epistemic layout and their own deviant belief sets. For instance,
in the sustainability transformations literature endogenous outsiders are sometimes framed as
“forerunning” incumbent actors or certain types of intermediaries ([10,16], see also section
3.3). Looking back at the socio-technical system example from above, an exogenous outsider
to X can be environmental NGOs or civic initiatives who criticise the current energy/transport
system without having any concrete influence on its development.
Finally, we are of course aware that the term ‘outsider’ as such has also been used and defined
in many different ways. One could speak about institutional outsiders as those individuals who do
not formally belong to a reference institution. Or about marginalized outsiders as those who are
discriminated against or refused access to resources and privileges. Nevertheless, we are focusing
here on defining what it means to be an outsider strictly from an epistemic point of view, acknowl-
edging that these different understandings often overlap and constitute one another.
3.3 Disruptive agents as epistemic outsiders
Considering the notions introduced above it seems plausible to assume that all agents usually
associated with disruptions and transformative action are also epistemic outsiders to the social
system they intend to transform. There are two arguments we employ in order to substantiate
this claim: (i) conceptually, based on the definitions of disruption, disruptive agency and epi-
stemic outsiders presented above; (ii) interpretatively, drawing on sustainability transforma-
tions literature and its findings regarding disruptive agents.
The first argument recognises that, for an agent or a cluster of agents to non-arbitrarily
(that is, on purpose, and not by accident) attempt to disrupt a social system R, it must be the
case that these disrupting agents are epistemic outsiders to R. This can be reconstructed as
follows:
Premise 1. Agent-driven disruptions to a reference system R are planned by the agents who
initiate them.
Premise 2. In principle, such agents could be either epistemic insiders to R or epistemic outsid-
ers to R.
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Premise 3. However, agent-driven disruptions cannot be planned by epistemic insiders to R.
Therefore, agent-driven disruptions to a system R are planned by epistemic outsiders to R.
As defined above, disruptions are unanticipated and unplanned interferences with the struc-
tural properties of R. This means that planning such interference cannot be part of the shared
set of beliefs of the epistemic insiders of R.
The second argument is based on a scoping review of the literature on sustainability trans-
formations, which suggests many different conceptions of transformative agents and their
agency. Despite the fact that these different types of change agents are not informed by the
concept of epistemic outsiders as we present it here, such an understanding is somewhat inher-
ent to them. A prominent example forms the concept of niche, as proposed in the Multi-Level
Perspective (MLP) [12,13,36], or the Strategic Niche Management (SNM) framework [36,41].
Niches are conceived as “protected spaces” that are kept free from the institutional constraints
and path dependencies of the dominant socio-technical regime. They allow agents to develop
their own rationalities, institutional structures and technologies with the goal and the potential
to change or disrupt the “paradigmatic core” [42, p.1] of the status quo specific to a given sec-
tor. In these frameworks the agency required for change has essentially been located inside
niches and in their interactions with the regime. Furthermore, the niche concept has also been
broadened to capture social innovation contexts in which civil society agents (such as civic ini-
tiatives, activist groups, non-governmental organizations) try to initiate transformative change
from below [14,43,44]. In this sense, niche actors can well be interpreted as epistemic outsiders
to the systems they strive to transform.
Also various conceptions of entrepreneurship have become quite influential in this litera-
ture. For instance, “institutional entrepreneurs” try to transform institutions through identify-
ing opportunities, mobilising stakeholders, and leveraging resources [45,46]. “Social
entrepreneurs” focus on business undertakings with the goal of creating social and ecological
value and innovations instead of mere economic profit in a strict sense [47,48]. Following this
strand, more specific concepts such as “sustainable entrepreneurship” [49,50] or “ecopreneur-
ship” [51,52] were developed. In the context of the MLP, Antadze & McGowan [53] propose to
call agents who aim at disrupting the existing system through “questioning normative rules at
the landscape level that support the regime in question” [53, p.2] “moral entrepreneurs”. All of
these entrepreneurial agents try to initiate and facilitate change on the basis of their own per-
spectives and practices which deviate from the norm. This characterises them as epistemic out-
siders trying to shape a more desirable mainstream according to their views. Yet, the
entrepreneurial metaphor focuses more on the specific activities agents do or the skills they
require, less on their deviating belief systems or their general relation to the prevalent reference
system (as an epistemic perspective would do).
Similarly, the crucial role of intermediaries in transformation processes, acting at the vari-
ous interfaces of reference systems and niches, has received increasing attention [54]. They are
facilitators, networkers, mediators or brokers who establish links and translate between stake-
holders, thus playing an interesting role regarding the epistemic insider/outsider dichotomy.
While incumbent actors are usually depicted as inhibiting institutional change and thereby
slowing down transformative processes [55,56,57], certain individual incumbents may also act
as regime intermediaries who effectively foster emerging transformations by influencing
niche-regime interactions. Niche intermediaries in turn support niche formation and amplifi-
cation, connecting between niche agents but also towards the regime [58,59]. Intermediaries
thus share an epistemic outsider position but can form part of both the reference system
(endogenous) or niches (exogenous) in some way, which appears to be crucial for creating
disruptions.
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It appears that all of the specific agent types so far identified as necessary driving forces
behind system transformations can be understood as epistemic outsiders, questioning or
rejecting the essential structural propositions of a given social system. From our perspective, it
is this rejection that genuinely enables the disruptive momentum these agents can bring for-
ward. Although not framed in such terms, this is widely recognised in the pertinent literature
on the subject. The epistemic reading introduced above thus enables conceptual connections
between diverse research strands and a more integrated approach for analysis and interpreta-
tion–a perspective expanded on in section 4.
3.4 Changing roles of disruptive agents
Finally, we want to highlight some overarching implications regarding the role(s) of disruptive
agents in social system transformations. As noted in section 3.1, such transformations neces-
sarily involve the alteration of the epistemic layout underpinning that system. This requires
that sufficiently many agents composing that system abandon their current belief set and
adopt an alternative one within a shorter period of time (acceleration).
This is precisely where an understanding of disruptive agents as epistemic outsiders mat-
ters. Regardless of their diversity, disruptive agents are characterised by a shared intention to
transform a reference system because they hold alternative beliefs, including specific norma-
tive orientations such as sustainability. Their actions thus implicitly or explicitly pursue the
take-up, diffusion and institutionalisation of an alternative belief set in accordance with that
normativity, e.g., by creating new imaginaries and narratives, confronting and/or coordinating
with other individual or collective actors (both from inside and from outside the system), or
enabling joint learnings in experimental settings. In other words, disruptive agents intervene
and strive to change social systems in such a way that their epistemic position moves from out-
sider to insider while maintaining their own belief sets.
Therefore, we can distinguish two basic stages in the process of a social system transforma-
tion that imply rather different roles and (epistemic) strategies for disruptive agents: (i) The
outsider stage, when disruptive agents attempt to destabilize the reference system; (ii) The
insider stage, when disruptive agents act upon successful disruptions and attempt to secure the
prevalence of their own belief sets, thus transforming the epistemic layout of the system. Each
stage suggests different strategies, tactics and approaches, operating at individual and collective
levels–and pointing towards available insights and research frameworks from a variety of sci-
entific disciplines.
4 Epistemic outsiders and sustainability transformations:
Interdisciplinary avenues for future research and action
While in section 3 we have referred to social system transformations in general, we will now
discuss how an epistemic reading can potentially benefit the understanding and also shaping
of transformations with a particular normative orientation, namely towards sustainability.
Apparently, we make this choice with a view to the urgency of the grand challenges that
demand purposive socio-ecological transformations. To this end we will first briefly outline
the utility of an epistemic perspective for interdisciplinary analyses that cut across system
boundaries, connect individual and collective agency and account for critical stages in such
transformations (section 4.1). In the following five sections we will then sketch how the con-
cept of epistemic outsiders can open up promising future research avenues for tackling the
complex dynamics of sustainability transformations by providing a fundamental boundary
object—not only between the specific research strands working on this topic so far (cf. section
3.3) but additionally inviting theoretical approaches to social change that resonate with
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sustainability transformation research respectively, but are usually applied separately: psychol-
ogy of mental constructs (section 4.2), practice theory (section 4.3), network theory (section
4.4), discourse theory (section 4.5), and institutional theory (section 4.6). Focusing on the
emergence, roles and impacts of epistemic outsiders across these complementary epistemolo-
gies of social change can thus build an interdisciplinary bridge to integrate methods, data and
insights. Additionally, each section also sketches resulting options for novel intervention
forms and strategies, even if space for a more in-depth elaboration is obviously limited here.
4.1 Mapping disruptive agency across system boundaries, levels, and time
A basic advantage of an epistemic perspective resides in its ability to equally address change
processes at individual, inter-personal and collective levels, including at various scales (e.g.,
household, organization, sector, society). As noted in section 3, the transformation of a social
system involves the modification of the epistemic layout of that system, which is shared and
reproduced by all individual and collective agents. Correspondingly, disruptive agents may
develop actions tailored towards triggering a re-assessment of and change in individual and/or
collective belief sets, thus ranging, e.g., from personal conversations, discussion groups or pub-
lic happenings to media campaigns, large-scale demonstrators or policy pilots. For each of
these levels (and partly also their relations), however, there are frameworks and concepts avail-
able that can help to explain the particular dynamics of stability/change at play (e.g., in individ-
ual behaviour, everyday practices, social networks, discursive or institutional settings), as well
as corresponding success factors regarding a normative orientation at sustainability. Tracing
the re-/configuration of an epistemic layout across these levels thus sheds light on the dis-/
alignment between very different change dynamics responsible for often neglected conflicts
and synergies in system transformations. Additionally, the changing role of disruptive agents
in the course of a transformation process must be taken into account, focusing especially on
the critical transition between the two stages identified above (outsider and insider stage). For
instance, moving from awareness-raising activism to co-developing regulation proposals does
not happen automatically, but relies on social change processes occurring at different levels.
Therefore, we conceive of sustainability transformation dynamics in epistemic terms by
mapping out disruptive agency across system boundaries (endogenous/exogenous), levels
(individual to society) and time (Fig 1). In this, the range of theoretical perspectives
Fig 1. The epistemic dimension of sustainability transformations: Mapping disruptive agency across system
boundaries, levels and time.
https://doi.org/10.1371/journal.pstr.0000097.g001
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represented here remains only indicative and may well be further expanded, even if it does in
fact reflect current debates in sustainability transformations research. As a side note we
acknowledge that the processes summarized in Fig 1 also apply to cases in which the disruptive
agents do not themselves initiate the disruptions but harness external disruptions (e.g. pan-
demics, earthquakes, etc.). In such cases one may equally identify the different positions, levels
and stages at which attempts to modify the epistemic layout are made so as to bring about sus-
tainability transformations.
4.2 Beliefs
The psychology of belief change provides important insights and explanations regarding at
least two key aspects of transformations and disruptive agency: (i) the reluctance of incum-
bents to change [60,61]; and (ii) the importance of sense making and bridge building
[62,63,64,65]. Especially combining Kelly’s [66] theory of personal constructs with a complex
systems view on belief structures [67] can be very instructive here.
The notion of personal constructs proposes that every person builds a unique representation of
the world by extracting regularities, striving to anticipate events and actively exploring their envi-
ronment to make sense of their experiences. Characteristically, this works through making dis-
tinctions between so-called elements (mental representations of real-world objects or people)
based on idiosyncratic dimensions, so-called constructs, which result in a personal map of the
world [66]. These elements and constructs can be added and modified according to the experi-
ences of an individual. Apart from its cognitive component, every element is also assumed to have
an emotional component, according to the theory of emotional coherence [68,69]. In general,
individuals strive to keep thematically self-contained parts of the construct system coherent to
avoid suffering from cognitive or emotional dissonance [70,68,69].Cognitive and emotional
coherence in a system of constructs could be conceptualized as holding as few contradictory
beliefs as possible at a certain moment, with less contradictory beliefs indicating higher coherence.
This leads the system to stabilize in a state of the best satisfaction of all constraints provided by the
different constructs (see [68,69,71]for a view of parallel constraint satisfaction networks).
These theoretical considerations straightforwardly suggest how to examine and interpret
the two aspects mentioned above. i) Looking at the need for cognitive and emotional coher-
ence is crucial for understanding reluctance to change. Avoiding cognitive and emotional dis-
sonance can hinder an individual to integrate new beliefs (belonging to an alien, outsider
position) into their belief system, even if those would be a more accurate representation of the
world. The fact that “[i]ncumbent regime actors initially tend to downplay the need for trans-
formation” [72, p.244], or even oppose it altogether [73,74] is partly explained by their reluc-
tance to change their own structural beliefs. Doing so is mentally costly, while the epistemic
layout of an existing social system offers both comfort and stability. ii) Disrupting structural
beliefs by only doubting or deconstructing them is not enough as beliefs are embedded in a
cognitive-emotional network striving for coherence. Presenting alternative views and narra-
tives is important to fill the “gap” in the belief system. This explains why the creation of “new
social imaginaries” [75, p.1], and the generation of a diversity of new ideas, alternative view-
points and novel solutions is so crucial. In other words, the outsider perspective needs to be
made palpable as a narrative that offers high positive returns and will soon become an insider
one. It is also important to note that an overlap in beliefs can facilitate communication
between individuals to allow for belief change [67]. Such overlaps can therefore be an entry
point or mutual understanding that serves as the foundation of communication. This gives
certain agents (endogenous outsiders) an important role in changing beliefs [16,17], as they
already share some beliefs with insiders.
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Furthermore, the psychology of belief change can also help to operationalize the concept of
epistemic outsiders at the individual and the social level. On the individual level, internal
dynamics striving for cognitive and emotional coherence drive action and communication.
On a social level, the forming of group beliefs can be described as a loose coupling of the
agents’ individual networks of beliefs. This yields collectively coordinated (but individually
implemented) belief networks and can cause individual beliefs to partly align with the groups
view through shared constructs and valences [67]. It equally offers methods for observing
belief systems and their dynamics, not only in Kelly’s [66] repertory grid method, but also in
tools like cognitive-affective maps [76].
Having these concepts in mind also allows to think about new ideas for intervention. To
overcome the mental cost of belief change, incumbents of unsustainable social systems must
be persuaded of the benefits of changing their structural beliefs towards more sustainable ones.
This refers not only to rational persuasion in order to maintain cognitive coherence, but also
to ways of providing emotional coherence, given the perceived threats of potential transforma-
tions [67]. For instance, socio-spatial intervention formats such as cooperatives or innovation
districts that provide for novel social or human-nature experiences and can therefore create
emotional responses are plausible options here that address this need. As we will see, other per-
spectives on social change (like social practices, networks, discourses and institutions) are also
instrumental in identifying relevant mechanisms to provide the comfort and stability needed
for emotional and cognitive coherence.
4.3 Social practices
Practice theory acknowledges how reality is continuously performed through multiple routin-
ized actions people undertake in their daily lives [77,78,79]. It accounts for an important cor-
nerstone of sustainability transformation processes that has become addressed increasingly in
the literature, i.e., it was not recognized from the outset [80,81]. Notably, it also incorporates
an epistemic dimension and therefore adds a complementary focus to an analysis of disruptive
agency regarding the role of epistemic outsiders in both discontinuing unsustainable social
practices and adopting novel and sustainable ones.
Practice theory expands from the premise that social practices are the building blocks of
society, and are deeply embedded in cultural, institutional and physical contexts. Shove et al.’s
[78] well-known conceptualization of practices as routinized behaviours emerging from inter-
dependent relations between meanings, materials and competences underlines their distinctive
role in the formation, perpetuation but also alteration of an epistemic layout. Especially mean-
ings largely correspond to the concept of belief sets as they refer to ideas or symbols reflecting
social and cultural norms. But also competences, i.e., the knowledge and skills enabling partic-
ular practices share an epistemic dimension.
Routinized actions provide comfort and stability and therefore also cognitive and emotional
coherence. However, changing them turns out to form a specific challenge regarding the
implicit entanglement of meanings and competences with material settings, objects and tools,
which strengthens their obduracy and resistance to change. All practice components are also
closely linked to social networks, discourses and institutions since they can contribute signifi-
cantly to enable or constrain their performance and proliferation.
In order to escape an unjust or unsustainable status quo of a social system, the transforma-
tion of practices thus forms another crucial lever. Epistemic outsiders are individuals who can
potentially trigger such processes as they develop different meanings and competences com-
pared to those shared in the practice performance of insiders and may also link these mean-
ings/competences to existing materialities and their re-interpretation and re-use, or the design
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of entirely new ones. Given their reliance on the prevailing epistemic layout, this task is
unlikely to be realized by insiders who will rather prioritize change in the material dimension
instead, thereby supporting the widely observed bias towards technological fixes [82].
Therefore, practice-oriented intervention approaches that attend to the epistemic dimen-
sion of disruptive agency would seek to dis-/connect between the practice components
involved in order to foreground and modify especially the underpinning belief sets. This may
entail actions designed to question or disrupt established routines, creating space for a reinter-
pretation of existing material realities (as in “pop-up” lanes or parks). But this needs to go
hand-in-hand with the creation of novel physical-material settings or tools that enable the per-
formance of sustainable practices, linked to corresponding meanings and competences. In
this, the epistemic reading allows to also consider the direct influence of related social change
dynamics involving social networks and discourses.
4.4 Social networks
The driving role of social networks in sustainability transformations has been pointed out fre-
quently in the literature [83,84,85]. Recent advancements have been made both conceptually
[86,87,88] and empirically [89,90,91] that deepen the understanding of their particular role
and relevance. Here we want to connect these contributions to the perspective on belief change
outlined above to show how the structure of social networks can inform the analysis of an epi-
stemic layout of collectives and social systems as well as efforts to change it towards a more sus-
tainable configuration.
Since humans strive for cognitive and emotional coherence (cf. section 4.2), changing a per-
son’s core belief set while maintaining cognitive and especially emotional coherent appears to
be a challenging task. In this regard, social networks can prove extremely important. Accord-
ing to insights from the network modelling of social contagion [92,93] the network structure
plays a crucial role for the quality and success of spreading beliefs, knowledge, behaviours or
even practices. The underlying mechanism requires distinguishing between weak ties (loose
relationships between people such as acquaintances) and strong ties (strong relationships
between people such as close friends or family) in a network [94]. While weak ties are charac-
terized by great reach, strong ties are characterized by redundancy. These features–reach or
redundancy–are extremely relevant for what type of contagion these networks facilitate best.
Weak-tie networks facilitate simple contagions. These are spreading processes that do not
encounter resistance (like infection during the Covid-19 pandemic). Strong-tie networks, on
the other hand, facilitate complex contagions. A complex contagion is a spreading process that
needs to overcome substantial resistance (like the adoption of a new social behaviour).
Considering these different configurations of social networks, the spreading of structural
beliefs apparently requires strong-tie networks. Redundancy and social approval of values,
norms, rules, etc. from an agents’ strong ties is key for them to accept the appropriateness of
those beliefs. Since strong ties exist especially with people one depends on the most, changing
one’s own structural beliefs is greatly facilitated if new beliefs, e.g., about how to live sustain-
ably are socially approved by these people. This can be expected to most effectively support the
persistence of a high level of emotional coherence. Indeed, numerous studies have shown that
the influence of redundant strong ties is much more effective in spreading behaviour-deter-
mining beliefs than merely weak ties [93,95].
Consequently, from the perspective of disruptive agents striving for sustainability transfor-
mations, a suitable strategy for changing belief sets and epistemic layouts would thus be to
focus attention and resources not on broad awareness-raising and a wide distribution of infor-
mation (e.g., by influencers), but on rather tightly knit social networks. Research has also
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shown that practices are directly transmitted through social networks [96]. Moreover, chang-
ing belief sets in a well-connected trans-/local community or neighbourhood can provoke
snowball effects that lead to systemic changes (see, e.g., [97,98,99] for the role of strong ties in
the adoption of rooftop photovoltaic technologies).
4.5 Discourses
The study of discourses forms another prominent strand of social change research that is of
crucial importance for an epistemic perspective on transformations. Discourses are under-
stood as sets of ideas, concepts, arguments or narratives that are continuously produced and
reproduced by agents and through which meaning is given to reality [100]. Obviously, they
incorporate beliefs about the values, norms, rules and standards that structure (inter)actions in
this social system. Therefore, analysing discourses is very relevant for understanding (i) how
the epistemic layout of the status quo is perpetuated in practice, (ii) how disruptions occur in
processes of belief change, and (iii) how moving to a novel epistemic layout may work out. We
will address these topics in turn:
First, the epistemic layout of a social system is to a large extent produced and reproduced dis-
cursively by the insiders of this social system. For instance, regulations as well as practices and
routines are framed and argued for on the basis of the belief sets of the agents who form the
system. Thereby discourses continuously shape agents’ behaviour in practice but also inform
their expectations of what is generally believed in or perceived as normal, as well as how other
agents should behave. Taking up Basu’s [37] notion of ‘republics of beliefs’ again, discourses
thus form a linguistic and semiotic backbone for their constitution and stability.
Second, in the literature on sustainability transformations the possible impacts of certain
‘outsider’ agents on a given discourse have been prominently highlighted by Pesch [18]. While
using a different notion of outsiders (more as agents outside design and decision-making pro-
cedures), he describes their unique discursive agency as their capability to bring new views in,
stipulating “out-of-the-box patterns of thinking, thereby creating space for discursive change”
(p. 386). In our term this means that they may be able to disrupt discourses perpetuating the
status quo by questioning the established framework and bringing in new views. Similarly, also
other scholars in this field have explored conceptually and empirically how particular agents
are sometimes able to interfere in discourses and modify them [53,101]. In our reading, this
refers to epistemic outsiders because they are not bound by the epistemic layout of the refer-
ence social system and can therefore view things differently and articulate their perspectives
correspondingly.
Third, the importance of shifting discourses for sustainability transformations has already
been shown for various empirical contexts such as, e.g., energy and water transitions
[102,103,104,105], financial services [106] or urban mobility planning [107]. These studies
illustrate how discourses are highly instrumental for the process of providing cognitive and
emotional coherence, connecting rational choices and evidence-based orientations with narra-
tives and imaginaries. In the process of transformative change, they can help to reframe the
position of epistemic outsiders, placing them at the core of a desirable future system configura-
tion. A transformation is thus complete when alternative discourses supporting a new episte-
mic layout have become mainstream (i.e., “hegemonic” in discourse theoretical terms).
From the perspective of disruptive agents pursuing sustainability transformations, discur-
sive approaches are therefore highly instructive [75], e.g., for conceiving of frames, tropes and
concepts, as well as communication and media strategies. Here, an epistemic perspective adds
a crucial success criterion in that such discursive interventions need to consistently focus on
the underlying belief sets and their coherence, striving, e.g., to exhibit and critique the
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unsustainability of the current system (as in “extinction rebellion”) while simultaneously pre-
senting liveable and attractive alternative futures (as in “nature-based solutions”).
4.6 Institutions
Institutional analysis is concerned with the influence of social rule structures on processes of
societal change and stagnation [108,109]. Studies of sustainability transformations have there-
fore often recurred to an institutional perspective, analyzing the juridical, administrative, terri-
torial and political fabrics of certain systems to illuminate how these affect system change
dynamics, including the role of agency [110,111,112]. The focus often lies on institutional log-
ics which presuppose and purport certain beliefs and behaviours [113,114,74]. In this regard,
institutional approaches help to identify Gidden’s “duality of structure” in societal realities.
For our perspective here, institutions represent perhaps the most change-resistant sedimen-
tation of belief sets into social structures, compared to practices, networks and discourses.
Their establishment requires large amounts of resources and societal coordination. The belief
sets institutions incorporate also shape those of the agents acting within them, simultaneously
enabling certain actions (in conformity) and constraining others (in deviation). Some authors
in sustainability transformation studies also point towards the specific sets of beliefs required
to follow and comply with institutions [115,116,42]. Due to this, it is very difficult for most
agents to even consider a change in their beliefs or social practices since they are bound by the
rewards and sanctions imposed by institutions, as has been shown extensively with a view to
institutional lock-ins, i.e., complete stagnation [117,118,119].
With a view to sustainability transformations, it appears that disruptive agency may thus
have to rely on two known mechanisms of institutional change: i) Instances of agency typically
emerge in the context of institutional tensions since these offer opportunities for agents to
intervene, thereby inducing change [42,112]. ii) Certain influential key agents who act as
endogenous outsiders can use their resources to drive institutional change from within
[120,121]. Hence, a belief change of these few agents can have a disproportionate effect on sys-
tem disruptions. The literature around institutional entrepreneurship addresses such issues on
a strategic level [122,123,57], but so far largely neglects their epistemic dimension.
A disruptive agency perspective would thus entail to focus on the availability and targeted
take up of sustainable belief sets in such (rare) instances of institutional change, considering
the role and contribution of epistemic outsiders (including e.g. as advisers, intermediaries or
via social networks). It would equally ask for and pursue the (disruptive) appropriation or con-
ception of institutions to embrace and support deviating beliefs while in turn constraining the
pursuit of established ones. With a view to the insider stage of transformations, such processes
of transformative institutionalization have been discussed extensively in the literature
[124,87,125], although without recognising their fundamental epistemic dimension. However,
as Haslanger has pointed out, institutional changes most often follow the changes that occur in
the “cultural techne¯” [126], referring to what we characterised as an epistemic layout.
5 Conclusion
In this paper, we have proposed an epistemic reading of disruptive agency in social system
transformations. It suggests that alterations in the belief sets that structure social systems
require a particular type of agents that hold deviant beliefs, at least in part affirming values,
rules, norms or practices that differ from the epistemic layout of the system. These epistemic
outsiders play a fundamental role in enabling but also initiating disruptions, which form a pre-
requisite for systemic change. By concentrating on this role, we have then unpacked what it
implies for research on and interventions for sustainability transformations, identifying
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PLOS SUSTAINABILITY AND TRANSFORMATIONEpistemic outsiders
notions of epistemic outsiderness across a selected range of highly pertinent theoretical
approaches to social change dynamics. We justified our normative focus on sustainability and
purposive disruption with a view to urgently required socio-ecological transformations while
recognising the general applicability of the conceptual approach.
In result, it appears that the notion of epistemic outsiders holds considerable potential both
as a genuine conceptual framework enabling new understandings and interpretations of social
system change, and as a boundary object for productively integrating existing approaches.
More specifically, we recognise the following five advantages of this perspective:
First of all, it generally informs and conceptually enables various promising interdisciplinary
avenues to explore and potentially influence transformative change towards sustainability. An
epistemic reading connects not only between strands within sustainability transformation
studies dealing with different forms and conceptions of disruptive agency already. Most
importantly, it also points to essential contributions from psychology regarding the under-
standing of processes of belief change, and in turn relates these to key theoretical approaches
to social change (social practices, networks, discourses, institutions). Tracing epistemic outsid-
ers in sustainability transformations across these complementary perspectives thus enables to
devise novel interdisciplinary lenses that can further illuminate the complex dynamics of
whole system change.
Second and more specifically, an epistemic conception of disruptive agency offers a key for
an integrated analysis of the individual and collective levels of agency involved in sustainability
transformations. From personal mental constructs to social networks or complex multi-level
governance settings it allows to scrutinize social change dynamics more seamlessly and across
scales by “zooming in/out”, recognising the role and relevance of specific epistemic relations
between individuals and society at large.
Third and similarly, the notion of epistemic outsiders conceptually connects agent positions
across system boundaries that are understood to be of crucial importance for sustainability
transformations respectively (e.g., “niche innovators” or “regime intermediaries”) but lack an
integrated understanding. Conceiving of endogenous and exogenous outsiders focuses on
their common epistemic grounds rather than on obvious distinctions, suggesting a potentially
important role of their mutual awareness, direct interactions and coordinated actions. In par-
ticular, this also applies with a view to multi-system transformations, i.e., the boundaries or
interrelations between different social systems (e.g., energy, mobility, housing)–a crucial aspect
considering the inter-sectoral character of sustainability challenges.
Fourth, an epistemic perspective additionally highlights the changing requirements and
challenges resulting in two principal stages of transformations unfolding over time, namely
before/after a new epistemic layout is shared by a majority of agents. This adds to a deeper
understanding of the different perspectives, needs and strategies of epistemic outsiders and
insiders in the course of a transformation, as well as a focus on the critical momentum and
movements when roles become inverted (regarding existing phase models, e.g., pre-develop-
ment, take-off, acceleration, stabilisation [127]).
Last but not least, the above features allow to derive and conceive of new intervention for-
mats and strategies as discussed in section 4, tailored to their respective epistemic contribu-
tions. It thereby acknowledges a fundamental condition for successful transformations and
suggests ways of addressing it in sustainability oriented policy and practice. In particular,
thinking of disruptive agency in epistemic terms may be helpful to explore the conflicts and
synergies of novel policy mixes (e.g., linking behavioural, organisational and institutional
change in the public, private and civic domains) for effective sustainability transformations.
Some caveat seems in place though. We are of course aware that an epistemic perspective
can and should not replace other valuable and necessary approaches for analysing and
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PLOS SUSTAINABILITY AND TRANSFORMATIONEpistemic outsiders
navigating sustainability transformations, notably those including conceptions of power and
capital. Rather, it provides a complementary conceptual canvas that enables novel cross-overs
towards and between such approaches, considering for instance that each body of literature on
social change invoked here already includes strands that explicitly account for both.
Future research approaches in this direction will require suitable research policy frame-
works and funding instruments that enable the kind of broader inter- and also transdisciplin-
ary (given the need for stakeholder participation) research on complex sustainability
challenges sketched here. While disciplinary piecemeal studies can certainly contribute, this
would likely not yield the added value targeted. Correspondingly, both conceptual and empiri-
cal studies should focus on gaining novel insights through the latitude of an epistemic
approach (across boundaries, levels, stages) by priority.
Author Contributions
Conceptualization: Sergiu Spatan, Daniel Peter, Gundula Thiele, Marc Wolfram, Franziska
Ehnert, Stefan Scherbaum, Moritz Schulz, Caroline Surrey.
Funding acquisition: Marc Wolfram, Franziska Ehnert, Stefan Scherbaum, Moritz Schulz,
Caroline Surrey.
Methodology: Sergiu Spatan, Daniel Peter, Gundula Thiele, Marc Wolfram, Stefan Scher-
baum, Moritz Schulz, Caroline Surrey.
Supervision: Marc Wolfram, Franziska Ehnert, Stefan Scherbaum, Moritz Schulz, Caroline
Surrey.
Visualization: Sergiu Spatan.
Writing – original draft: Sergiu Spatan, Daniel Peter, Gundula Thiele.
Writing – review & editing: Sergiu Spatan, Daniel Peter, Gundula Thiele, Marc Wolfram,
Franziska Ehnert, Stefan Scherbaum, Moritz Schulz, Caroline Surrey.
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PLOS SUSTAINABILITY AND TRANSFORMATION
| null |
10.1371_journal.pone.0287011.pdf
|
Data Availability Statement: All relevant data are
within the paper and its Supporting information
files.
|
All relevant data are within the paper and its Supporting information files.
|
RESEARCH ARTICLE
Time series and power law analysis of crop
yield in some east African countries
Idika E. Okorie1, Emmanuel Afuecheta2,3, Saralees NadarajahID
4*
1 Department of Mathematics, Khalifa University, Abu Dhabi, UAE, 2 Department of Mathematics and
Statistics, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia, 3 Interdisciplinary Research
Center for Finance and Digital Economy, KFUPM, Dhahran, Saudi Arabia, 4 Department of Mathematics,
University of Manchester, Manchester, United Kingdom
* [email protected]
Abstract
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OPEN ACCESS
Citation: Okorie IE, Afuecheta E, Nadarajah S
(2023) Time series and power law analysis of crop
yield in some east African countries. PLoS ONE
18(6): e0287011. https://doi.org/10.1371/journal.
pone.0287011
Editor: Steven Arthur Loiselle, University of Siena,
ITALY
Received: July 7, 2022
Accepted: May 27, 2023
Published: June 13, 2023
Copyright: © 2023 Okorie et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting information
files.
Funding: The authors received no specific funding
for this work.
Competing interests: The authors have declared
that no competing interests exist.
We carry out a time series analysis on the yearly crop yield data in six east African countries
(Burundi, Kenya, Somalia, Tanzania, Uganda and Rwanda) using the autoregressive inte-
grated moving average (ARIMA) model. We describe the upper tail of the yearly crop yield
data in those countries using the power law, lognormal, Fre´ chet and stretched exponential
distributions. The forecast of the fitted ARIMA models suggests that the majority of the
crops in different countries will experience neither an increase nor a decrease in yield from
2019 to 2028. A few exceptional cases correspond to significant increase in the yield of sor-
ghum and coffee in Burundi and Rwanda, respectively, and significant decrease in the yield
of beans in Burundi, Kenya and Rwanda. Based on Vuong’s similarity test p–value, we find
that the power law distribution captured the upper tails of yield distribution better than other
distributions with just one exceptional case in Uganda, suggesting that these crops have the
tendency for producing high yield. We find that only sugar cane in Somalia and sweet potato
in Tanzania have the potential of producing extremely high yield. We describe the yield
behaviour of these two crops as black swan, where the “rich getting richer” or the “preferen-
tial attachment” could be the underlying generating process. Other crops in Burundi, Kenya,
Somalia, Tanzania, Uganda and Rwanda can only produce high but not extremely high
yields. Various climate adaptation/smart strategies (use of short-duration pigeon pea varie-
ties, use of cassava mosaic disease resistant cassava varieties, use of improved maize vari-
eties, intensive manuring with a combination of green and poultry manure, early planting,
etc) that could be adapted to increase yields in east Africa are suggested. The paper could
be useful for future agricultural planning and rates calibration in crop risk insurance.
1 Introduction
Africa is the poorest continent. It is struggling to feed its people. Hence, enhancement of crop
production is important.
Furthermore, farmers are more interested in investing in crops that are capable of produc-
ing high yields not crops that can produce extremely low yield. They want to maximize the
profit on their investment. Crops that have the potential for high yield are likely to attract low
premium in crop yield insurance.
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
1 / 36
PLOS ONECrop yield in some east African countries
There have been several papers on high crop yield in African countries. While discussing
nutrients in the west African Sudano-Sahelian zone, [1] noted that “shrubs and trees with their
alternating periods of nutrient storing and recycling in leaves and wood, micro-depressions,
termite mounts and ant nests become localised points of nutrient concentration and high crop
productivity”. While investigating the importance of liming acid soils, [2] demonstrated that
“severely acidified soils of the western highlands of Cameroon should be limed at moderate
rates to sustain crop productivity”. While examining the seed supply system for maize produc-
tion in southwestern Nigeria, [3] observed that “about 39% of farmers used improved varieties
for high crop yields, 24% for disease resistance and 22% for market preferences, whereas local
varieties were cultivated by 37% of farmers because of market preferences and availability, 16%
because of low cost and 12% because of disease resistance”. [4] demonstrated that continuous-
flow drip irrigation in Bauchi state of Nigeria delivers “high crop yields especially if the crops
are grown under appropriate agronomic practices that enable protraction of the growth sea-
son”. [5] demonstrated that high maize yields on sandy soils in Zimbabwe can be achieved by
using mineral fertilizers. According to [6], among many oilseed crops (for example, sunflower,
soybeans, rapeseed/mustard, sesame, groundnuts, etc) grown in Kenya, oilseed rape is pre-
ferred because of its high yields (1.5 tons—4.0 tons / hectare) with high oil content of 42–46%.
While comparing three fertigation strategies of grapes in the Berg River Valley region of South
Africa, [7] found that “less berry crack contributed to a higher yield and higher export percent-
age of grapes”. While analysing the benefits of soil conservation in the Kondoa eroded area of
Tanzania by conducting a household survey of 240 households, [8] observed that 56% of the
respondents gained high crop yields. [9] investigated limited nitrogen content, a major chal-
lenge to sustainable and high crop production, for agricultural soils of lower eastern Kenya.
While evaluating small holder farmers’ preferences for climate smart agricultural practices in
Tehuledere district, northeastern Ethiopia, [10] found that “high and moderate climate resil-
ience and high crop yield agricultural practices had a positive utility”. [11] demonstrated that
phosphorus treatment for rice fields in lowlands in the central highlands of Madagascar signif-
icantly and consistently accelerated initial production with high crop growth rate and short-
ened days to heading. According to [12], “rain fed agriculture has a high crop yield potential if
rainfall and soil nutrient input resources are utilized effectively”.
But none of these papers discuss the distribution of crop yield or forecasts. The distribu-
tions of crop yields is very useful in agribusiness. These distributions can help to tackle food
shortages and insecurity by understanding how natural resources and farmers attitude towards
crops selection and cultivation can control agricultural productivity, in agricultural policy
assessment and to calibrate rates and premiums in crop insurance. Similarly, understanding
the trend of crop yield and the insights gained from crop yield predictions can go a long way
in helping to address the current global issue of increase in food prices and demand as well as
to understand the associated risk of food production by helping farmers to make informed
decisions especially on what and where to grow.
We are also not aware of any previous research that has focused on predicting crop yield in
east Africa let alone doing so in such an almost holistic manner as we have done in this paper;
so, to bridge this research gap, we follow [13] to provide some crop yield forecast in some east
African countries. We believe that the results herein will be of extreme importance to east Afri-
can regional farmers.
The aim of this paper is two folded. First, to forecast the crop yield and secondly to identify
cash crops that are capable of producing extremely high yield in some east African countries
by modelling the tail region of crop yield data. The remainder of this paper contains data in
Section 2, methods in Section 3, results and discussion in Section 4 and conclusions in Section
5.
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
2 / 36
PLOS ONECrop yield in some east African countries
We use two methods for analyzing the data: time series analysis and fit of heavy tailed distri-
butions. Time series analysis and forecasting is a branch of statistics. Time series forecasting
uses models to predict future outcomes based on past observations. With time series visualiza-
tions, trends and seasonal patterns could be identified. We could then seek to gain deeper
insight as regards to the reason behind these trends. Several time series models have been devel-
oped, studied and widely applied in many fields. Box-Jenkins’ auto-regressive integrated mov-
ing average (ARIMA) model [14] arguably stands out among others as the most widely used
perhaps due to its simplistic application appeal and high precision in modelling. For instance,
[15] used the ARIMA model to forecast rice production, consumption, importation, exporta-
tion and self-sufficiency in the Benin Republic. [16] used the ARIMA model to forecast the
consumption of some livestock products such as eggs, milk, chicken and cow meat to see if the
forecast of consumption was on the increase. [17] highlighted that the past century has wit-
nessed significant rise and fall of cocoa production in Nigeria due to diverse institutional and
climate changes. They used the ARIMA model to predict cocoa production in Nigeria between
2018 and 2025. Their forecast showed a decreasing trend where cocoa production is expected
to fall by more than 20% in 2025 against the 2017 value. [18] used the ARIMA model to forecast
maize production in India from 2018 to 2022. The model predicted about 13.76% increase in
maize production in India. [19] used the ARIMA model to forecast soybean yield in Zambia.
The forecast suggested 23430.3 hectogram / hectare yield increase in 2020 compared to the
2016 figure of 19624 hectogram / hectare. [20] used the ARIMA model to forecast Kharif rice
production in West Bengal, India which contributes about 15% of the total paddy in India. [21]
used the ARIMA model to forecast sorghum production in South Africa from 2017 to 2020.
Their forecast depicted an increasing trend. [22] used the ARIMA model to forecast sugar cane
production in Pakistan from 2019 to 2030. Their forecast indicated a significant increase.
Quantifying the tail of the crop yield distribution is vital for managing agricultural produc-
tion risk and rating crop insurance [23]. The simplest and the most widely used distribution
for modelling rare outcomes occurring in the tail region is the power law distribution. Many
processes follow the power law over large magnitude of values. Recent examples are the distri-
bution of stock returns [24], income [25, 26], wealth of world billionaires [27], persisters-anti-
biotic-tolerant cells [28], duration size of unhealthy air pollution events [29], tourism
recommendations [30], cumulative coal production [31], agricultural land size [32], rates of
wetland loss [18], union size [33], strike size [34] and growth rate of CO2 [35]. Popular alterna-
tives to the power law distribution are the lognormal, stretched exponential, and Fre´chet
distributions.
2 Data
Yearly data from 1961 to 2018 on the yield of cash crops like banana, plantain, beans, cassava,
coffee, sorghum, potato, sweet potato, maize, rice, sugar cane, wheat, millet and cotton seed
from six countries in east Africa (namely, Burundi, Kenya, Somalia, Tanzania, Uganda and
Rwanda) were obtained from Food and Agriculture Organization of United Nations-FAO, see
http://www.fao.org/faostat/en/#home. The data obtained were yields aggregated at national
levels.
The time plots of the crops in different countries are shown in Figs 1 and 2. Some sudden
changes, particularly big drops and falls could be seen at different times indicating periods of
high and low yields. These changes could be as a result of the global economic outlook, envi-
ronmental/climate changes or even changes in farming practices.
Some descriptive statistics of the data for crops are presented in Table 1. The statistics
include the mean, median, standard deviation, minimum, maximum, skewness and kurtosis.
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
3 / 36
PLOS ONECrop yield in some east African countries
Fig 1. Time series plots for crop yield in different countries.
https://doi.org/10.1371/journal.pone.0287011.g001
The discrepancy between the mean and the median values appears not to be large for almost
all the crops across the countries. The mean is larger than the standard deviation for all the
crops across the countries. This suggests that the data are underdispersed. Note that underdis-
persion could be as a result of serial correlation which is typical of time series data. We can
remove serial correlation by random variable transformation. But, this may lead to (a) loss of
data information and (b) limits us to specific class of models to use. The data exhibit varying
degrees of skewness and kurtosis across crops and countries. The lowest (highest) positive
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
4 / 36
PLOS ONECrop yield in some east African countries
Fig 2. Time series plots for crop yield in different countries.
https://doi.org/10.1371/journal.pone.0287011.g002
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
5 / 36
PLOS ONECrop yield in some east African countries
Table 1. Descriptive measures for the crop yield data sets.
Country
Crop
Min.
1st qu.
Median
Mean
3rd qu.
Max.
Std. dev.
Skewness
Kurtosis
Burundi
Kenya
Somalia
Tanzania
Uganda
Rwanda
Banana
Beans
Cassava
Coffee
Sorghum
Sweet potato
Beans
Coffee
Maize
Rice
46915
6044
41867
2687
5890
59048
3127
9212
10713
13076
Sugar Cane
297552
Wheat
Banana
Maize
Sorghum
9212
88430
4149
2040
54433
9067
85880
6961
9837
63302
4783
14918
12957
34802
689809
14918
169641
8173
3320
56947
10036
89890
8251
10000
63837
5556
16839
15813
39621
808548
16839
170374
9758
3522
60700
9599
85078
8100
10426
68215
5413
18293
15507
39988
774305
18293
185025
10065
3992
62984
10380
90894
9379
11881
65848
6122
21078
17266
46510
885805
21078
198616
11765
4280
127352
13184
112378
11598
14042
133015
8382
31991
20712
61813
1211845
31991
317500
17901
9824
12204.970
1375.755
12870.850
1769.862
1886.384
13063.090
1034.896
5003.894
2605.723
10166.130
217545.200
5003.894
46006.370
3029.413
1306.699
Sugar Cane
272727
350000
407143
604201
887500
1455975
346839.200
Maize
Millet
Rice
Cotton Seed
Sorghum
Sweet potato
Banana
Cassava
Coffee
Millet
Plantain
Sweet potato
Beans
Cassava
Coffee
Potato
Sorghum
Sweet potato
4808
4522
7143
2328
4423
10448
23298
32973
3839
8092
42971
24009
5606
11778
2678
22821
6850
34388
9170
7010
12826
4356
6554
18029
39412
44771
5443
11486
52141
35504
6980
55212
5267
64313
10014
53682
12722
8308
16286
5136
9151
29252
42070
66988
6131
14017
56585
41558
8020
91644
5994
68656
11000
62550
12734
8697
16396
5117
8832
34621
40891
71711
6402
13341
59014
40660
7858
82820
6051
71994
11109
63978
14414
9982
19172
5783
10100
49412
44927
89971
7150
15986
60867
44017
8522
116873
6776
82871
12086
75039
31359
19507
27382
7936
17963
72759
48333
144083
10283
16751
84235
62075
10258
164000
11019
130600
15084
96163
4893.517
2401.514
4933.184
1245.132
2717.268
19284.670
5636.359
32354.340
1408.237
2637.066
11223.650
6398.938
1037.677
37331.23
1473.642
20435.600
1754.313
13269.530
3.0704
-0.8491
-2.0220
-0.5861
-0.3327
3.1150
0.0585
0.7453
0.0261
-0.0934
-0.4537
0.7453
0.7969
0.5619
2.2982
1.0554
1.3077
1.5969
0.2870
0.2257
0.4293
0.5541
-1.3614
0.7664
0.6347
-0.6120
0.8573
0.3221
-0.0566
-0.2683
0.6369
0.2417
0.2655
0.0977
https://doi.org/10.1371/journal.pone.0287011.t001
13.0916
0.9814
4.2142
0.1126
-0.1633
10.0345
0.3111
0.2387
-1.0478
-0.0593
-0.0124
0.2387
0.7965
-0.1600
6.9370
-0.2674
2.9604
5.1457
-0.7743
-0.4752
0.5903
-1.0485
1.613
-0.5467
-0.0043
-0.945
-0.0373
1.5048
-0.8005
-1.0465
1.6200
0.7926
0.0410
-0.4860
skewness of 0.0261 (3.1150) corresponds to maize (sweet potato) in Kenya (Burundi). The low-
est (highest) negative skewness of -0.0934 (-2.0220) corresponds to rice (cassava) in Kenya
(Burundi). The lowest (highest) positive kurtosis of 0.0410 (13.0916) corresponds to sorghum
(banana) in Rwanda (Burundi). The lowest (highest) negative kurtosis of -0.0043 (-1.0478)
corresponds to coffee (maize) in Uganda (Kenya). Crop yield skewness has been used to char-
acterize crop yield tendencies. [36] reported that crop yield is positively skewed in the presence
of independent, identical and uniform resource availability distribution. Crop yield is nega-
tively skewed whenever the distributions are Gaussian, i.e. skewness depends on asymmetries
in resource availabilities, meaning that a negatively skewed yield occurs whenever production
is tightly controlled so that the left tails of some resources availabilities distributions are thin
[36]. However, in addition to the observable similarities between the mean and the median
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PLOS ONECrop yield in some east African countries
crop yield values, we notice that for majority of the cases, the skewness and kurtosis values are
close to zero, suggesting possible symmetry and mesokurtosis.
Figs 3 and 4 show boxplots to support the descriptive statistics in Table 1 and to compare
the yield performance of some of the crops that are produced in more than one east African
country. We see that Somalia recorded the highest banana and sugar cane yields. Burundi
recorded the highest beans, coffee and sweet potato yields. Rwanda recorded the highest cas-
sava yield. Kenya recorded the highest rice yield. Tanzania recorded the highest sorghum,
maize and millet yields. Also, evident enough in Figs 3 and 4 are the presence of extreme (high
Fig 3. Box plots for crop yield in different countries.
https://doi.org/10.1371/journal.pone.0287011.g003
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PLOS ONECrop yield in some east African countries
Fig 4. Box plots for crop yield in different countries.
https://doi.org/10.1371/journal.pone.0287011.g004
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PLOS ONECrop yield in some east African countries
and low) yields for some of the crops which are indicated by observations lying outside of the
whiskers in the box plots. The power law distribution discussed later is especially useful for
modelling unusually high yields.
We tested heavytailedness of the each data set using [37]’s test based on Kolmogorov-Smir-
nov statistic corrected for correlation [38]. The p–values of this test for banana, beans, cassava,
coffee, sorghum and sweet potato in Burundi were 0.182, 0.0664, 0.151, 0.102, 0.156 and 0.115,
respectively. The p–values for the crops in Kenya were 0.162, 0.171, 0.059, 0.120, 0.166 and
0.145. The p–values for the crops in Somalia were 0.167, 0.157, 0.098 and 0.115. The p–values
for the crops in Tanzania were 0.168, 0.112, 0.095, 0.068, 0.096 and 0.125. The p–values for the
crops in Uganda were 0.177, 0.114, 0.087, 0.171, 0.105 and 0.077. The p–values for the crops in
Rwanda were 0.075, 0.169, 0.068, 0.098, 0.061 and 0.158. The p–values reported show that
there is no significant evidence against the fact that each data has a heavy tail. Hence, unusually
high yields can be modeled by heavy tailed distributions as done in Section 4.
3 Methods
3.1 Time series analysis of crop yields
One possible technique for time series analysis is to assume that the overall mean is either con-
stantly increasing or constantly decreasing with respect to time. In this case, the fit of a sloping
line might be appropriate for the time series. This type of line is typically referred to as a linear
trend model or a trend-line model and it is a special case of a simple linear regression model
with time index t as the only predictor variable, i.e. t = 1, 2, 3, . . .. The estimated trend line is
the line that minimizes the sum of the squared vertical deviations from the data. Trend lines
serve as important visual aids. However, they often perform poorly in forecasting beyond the
historical data. In practice, majority of the time series data that arise in different areas cannot
be described by some straight lines because their trends often undergo evolution. Given the
past observations, the trend-line model attempts to find the intercept and slope that give the
best average fit to the data. Unfortunately, the deviation of the linear trend model from the
data is usually greatest at the end of the time series where the forecasting starts. Therefore, in
time series analysis and forecasting, the important question ‘what is the appropriate model?’
can first be addressed by visually inspecting the time series data for any constantly changing
trend or randomly changing trend. Based on Figs 1 and 2, we see that assuming a steady
upward or downward linear trend for any of the crop yield data is apparently illogical and out
of place because a randomly changing trend is overwhelmingly evident for all the time series
data. To model the nonlinear trend in all the time series, we may need to regress the time series
on second or higher order terms of t and this may require some trial and errors which may
possibly lead to some overestimated or underestimated models. To circumvent the issue of
model selection, we consider the most reliable models for nonlinear trends in time series and
they are referred to as stochastic time-series models. Examples of such models are the one pro-
posed by [14] which involve straightforward laid down iterative procedures for model fitting
unlike the nonlinear regression method mentioned earlier.
In this section, we carry out a time series analysis to study the yield pattern of crops over a
specified period of time. We need to isolate first the impact of trends (the overall pattern in the
series) and second the impact of random disturbances (the vigorous wiggles in the series). The
impact of trends could be due to planting strategies and techniques, advanced mechanized
farming, farm management, irrigation, the use of fertilizers and genetically improved seed-
lings/crops. The impact of random disturbances could be due to pandemics, crop disease out-
breaks, wars, recessions, environmental degradations (for example, erosion) and extreme
weather conditions such as droughts and floods.
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PLOS ONECrop yield in some east African countries
Let xt denote the observed yield of a crop at time t. Suppose we denote all the observed
information up to time t by I t. We are interested in forecasting xt. We can specify the forecast
as xtjI t or more specifically as ^xtþhjt. The forecast of xt+h given all previous observations up to
time t (x1, x2, . . ., xt) is known as the h–step forecast. The h–step forecasting method can be
easily implemented through the famous Box-Jenkins autoregressive integrated moving average
(ARIMA) modelling framework. ARIMA models are used for trend analysis and forecasting.
The ARIMA (p, d, q) model is defined by
�
1 (cid:0)
�
Xp
�iBi
i¼1
�
ð1 (cid:0) BÞdxt ¼ c þ
1 þ
Xq
yjBj
�
xt;
j¼1
where ϕ’s are the autoregressive (AR) parts of the model, θ’s are the moving average (MA)
parts of the model, d is the order of difference, B is known as the backshift operator, c is a con-
stant which is equal to μ(1 − ϕ1 − � � � − ϕp), μ is the mean of the dth differenced series (1 −
B)dxt and ξt is white noise. ξt are generally assumed to be independent, identically distributed
variables sampled from a normal distribution with zero mean. In ARIMA modelling, we make
the following assumptions about the time series: there are no seasonality or cyclical trends,
there are no outliers, and that the variation about the mean is consistent. After fitting the
ARIMA model, we can check the model adequacy viz-a-viz a popular portmanteau test called
Ljung-Box test by simply testing whether the residuals from the fitted model are white noise.
For Ljung–Box test, we test the hypothesis H0: ρk = 0 versus H1: ρk 6¼ 0. The test statistic of
Ljung-Box test is
Q? ¼ nðn þ 2Þ
Xh
k¼1
^r2
k
ðn (cid:0) kÞ
;
where n is the sample size, ^rk is the sample autocorrelation at lag k, and h is the number of lags
being tested. Under H0, the statistic Q? is asymptotically chi-square distributed with h degrees
of freedom. At α significance level, the critical region for rejecting the hypothesis of random-
ness is Q? > w2
1(cid:0) a;h, where w2
1(cid:0) a;h denotes the (1 − α)th quantile of the chi-squared distribution
with h degrees of freedom.
A detailed discussion of Box-Jenkins ARIMA (p, d, q) model could be read from [39] and
[40]. In Figs 1 and 2, we find some evidence of changing variance in some of the series. Each
series appears clearly non-stationary as the series wanders up and down. Before proceeding
with the data analysis, we ensured that the variance for each series is stabilized by the Box-Cox
transformation [41].
The Box Cox transformation involves an exponent, λ 2 [−5, 5]. In this paper, all values of λ
are considered but the optimal value for each data is applied. The optimal value of λ is the one
that gives the best approximation of the Gaussian distribution. The transformation of xt has
the form:
8
>>>>>>>><
>>>>>>>>:
xtðlÞ ¼
xl
t (cid:0) 1
l
;
if l 6¼ 0;
ln ðxtÞ;
if l ¼ 0:
ð1Þ
The formula in (1) is not as simple as it appears because testing for all possible values one by
one is unnecessarily time consuming. However, most software packages include an option for
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PLOS ONECrop yield in some east African countries
a Box-Cox transformation. In this paper, we used the 0auto:arima0 function in the 0forecast0
package in the R (R Core Team, 2022) software to fit the ARIMA (p, d, q) models. Setting the
0lambda0 argument to 0auto0 allows a transformation to be automatically selected and imple-
mented using the Box-Cox method. The routinely transformed data are then coerced into sta-
tionarity by implementing first or second order differences whenever there is any need to do
so before estimating the appropriate model.
Each coerced series was tested for stationarity using [42]’s test. The null hypothesis was that
the series is stationary. The p–values for banana, beans, cassava, coffee, sorghum and sweet
potato in Burundi were 0.085, 0.085, 0.089, 0.095, 0.075 add 0.083, respectively. The p–values
for the crops in Kenya were 0.057, 0.081, 0.051, 0.069, 0.086 and 0.056. The p–values for the
crops in Somalia were 0.098, 0.078, 0.089 and 0.083. The p–values for the crops in Tanzania
were 0.067, 0.078, 0.082, 0.083, 0.081 and 0.052. The p–values for the crops in Uganda were
0.094, 0.086, 0.095, 0.051, 0.092 and 0.090. The p–values for the crops in Rwanda were 0.098,
0.099, 0.053, 0.073, 0.090 and 0.080.
3.2 Analysis of the maximum crop yields
Suppose we denote the crop yield random variable by X with realizations xi, i = 1, 2, . . ., n,
where n represents the number of observations. For the convenience of fitting distributions
to the available data, we assume that the xi are random. The assumption of independence is
not technically correct as the data are actually serially correlated. But ignoring dependence
in a data set and treating the data as being independent has no effect on parameter estimates,
it only affects standard errors (see, for example, [43]). Hence, the results presented later on
the fit of heavy tailed distributions are correct as accuracy of estimation is not taken into
account.
The probability density functions (PDFs) of the fitted heavy tailed distributions are
1. The power law distribution also known as Pareto distribution of type I [44] specified by the
PDF
f ðxÞ ¼
�(cid:0) a
a (cid:0) 1
xmin
�
x
xmin
for x � xmin > 0, where xmin is the lower bound and α > 0 is the exponent. At or above
xmin, the distribution exhibits properties of a power law distribution.
2. The lognormal distribution specified by the PDF
f ðxÞ ¼
�
1
p exp (cid:0)
ffiffiffiffiffiffi
2p
bx
�
ðln x (cid:0) aÞ2
2b2
for x > 0, where −1 < a < 1 and b > 0 are the location and scale parameters,
respectively.
3. The stretched exponential distribution specified by the PDF
f ðxÞ ¼
� �b(cid:0) 1
x
a
b
a
�
exp (cid:0)
�
� �b
x
a
for x > 0, where a > 0 is the scale parameter and b > 0 is the shape parameter.
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PLOS ONECrop yield in some east African countries
4. Fre´chet distribution [45] specified by the PDF
�
f ðxÞ ¼ babx(cid:0) 1(cid:0) b exp (cid:0)
�
� �(cid:0) b
x
a
for x > 0, where a > 0 is the scale parameter and b > 0 is the shape parameter.
We estimated the parameters of all the distributions by the method of maximum likelihood
through the optim routine in R [46]. We estimated xmin in the power law distribution by fol-
lowing the method in [47]. That is, we chose xmin that minimized
KS ¼ max
x�xmin
jFnðxÞ (cid:0) ^FðxÞj;
where Fn(x) and ^FðxÞ denote, respectively, the empirical and fitted power law distribution
functions for x � xmin.
We have used the method of maximum likelihood because of its popularity. There are other
methods for estimation; in particular, for estimating α of the power law distribution. Some of
these estimators include the rank estimator due to [48], [49]’s estimator and the median esti-
mator due to [50].
Note that each of the four distributions has two free parameters. So, no one distribution is
more flexible than the others in terms of the number of parameters. Unlike the power law dis-
tribution, the lognormal, Fre´chet and stretched exponential distributions model the entire
data. We can compare their fits by the following goodness-of-fit measures:
1. Bayes information criterion (BIC) due to [51] defined by
BIC ¼ (cid:0) 2 ^L þ k ln ðnÞ;
2. Akaike information criterion with a correction (AICc) due to [52] defined by
AICc ¼ AIC þ
2kðk þ 1Þ
n (cid:0) k (cid:0) 1
;
where ^L and k denote, respectively, the maximized log likelihood value and the number of
unknown parameters.
We can also compare all of the fitted distributions through the Kolmogorov–Smirnov test.
Its statistic is given by
KS ¼ max
x2Data
jFnðxÞ (cid:0) ^FðxÞj;
which was corrected as in [38] to account for correlation in the data. The larger the value of
the corresponding KS p–value the better the fitted distribution. We require the p–value of the
Kolmogorov–Smirnov test to be greater than 0.05 to conclude that the distribution is a reason-
able model for the data. A p–value less than 0.05 suggests an absolute rejection of the distribu-
tion as a candidate for the data. However, one major drawback of the Kolmogorov–Smirnov
p–value is that it depends on fixed parameters, hence it does not reflect sampling variability.
We can calculate more conservative p–values by a bootstrapping method in [47]. We imple-
mented this method by using 5000 bootstrap replications to obtain the final p–value for the
Kolmogorov–Smirnov test. In this paper, we shall use the non-bootstrapped KS p–value to
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PLOS ONECrop yield in some east African countries
verify the plausibility of each distribution as a candidate model for data. We use the boot-
strapped KS p–value to discriminate among competing distributions and to generalize our
findings.
Vuong test [53] can be used to discriminate between two non-nested models by testing the
ln Pðxj ^Y2Þ. The test statistic for Voung’s test is L ¼
null hypothesis that the models provide indistinguishable fits for the same data. Suppose we
denote the probabilities for models 1 and 2 by Pðxj ^Y1 Þ and Pðxj ^Y2 Þ, respectively, where ^Y1
and ^Y2 denote the parameter estimates for models 1 and 2, respectively. Let
d ¼ ln Pðxj ^Y1Þ (cid:0)
denote the mean and standard deviation of d, respectively. A large, positive test statistic value
provides evidence that model 1 is superior to model 2. A large, negative test statistic value
gives evidence that model 2 is superior to model 1. Under the null hypothesis that the models
are inseparable, the test statistic Λ is asymptotically standard normal distributed. Two finite
sample corrections of Vuong’s test are sometimes considered based on the AIC and BIC pen-
alty terms, depending on the complexity of the two models. However, these corrections some-
times generate conflicting conclusions.
d�
, where d� and sd
ffiffi
n
sd
p
4 Results and discussion
Ljung–Box p–values in Table 2 are > 0.05 suggesting that the residuals of the fitted ARIMA
models are not statistically significant from white noise at 0.05 significance level for all the
crops except for plantain in Uganda which is not statistically significant from white noise at
0.01 significance level. All of the fitted models are suitable for prediction based on the residual
analysis. From the 10 years (2019–2028) point forecast (solid blue lines) of the fitted ARIMA
models in Figs 5 to 10, we observe the following for Burundi: an initial sharp drop in 2019 fol-
lowed by an upward swing of yield for banana; a sharp increase in 2019 followed by increasing
oscillations of yield for sweet potato; the yield for sorghum shows a quick increase from 2019
to 2028; the yield for beans shows an immediate decline from 2019 to 2028; neither cassava
nor coffee indicate any increasing or decreasing pattern from 2019 to 2028. In Kenya, we
observe the following: the yield for beans shows a continuous decline from 2019 to 2028; nei-
ther upward nor downward yield trend is evident for coffee, rice, wheat and sugar cane from
2019 to 2028; the yield of maize shows a sharp drop in 2019 followed by an increase and then a
stable trend. In Somalia, we observe the following: the yield for maize or sugar cane does not
indicate any pattern; the yield for banana shows an initial moderate increase in 2019 followed
by a period of no trend up to 2028; the yield for sorghum first experienced a sharp drop in
2019 followed by a stable period of no trend up to 2028. In Tanzania, we observe the following:
no significant trend could be identified for maize, rice, sweet potato and cotton seed for the
entire forecast period; millet is characterized by a slight yield decrease in 2019 followed by a
period of no significant trend up to 2028. In Uganda, we observe the following: the forecast for
banana, cassava, millet, plantain and sweet potato did not show any significant trend from
2019 to 2028; the yield for coffee shows a slight increase in 2019 followed by a period of neither
increase nor decrease. In Rwanda, we observe the following: the yield for beans shows a persis-
tent decline from 2019 to 2028; the yield for sweet potato shows initial jump followed by a slow
decline; coffee indicated an upward trend tendency from 2019 to 2028; cassava, potato and sor-
ghum did not indicate any significant trend.
The changes observed in Figs 5 to 10 are consistent with findings in the literature. [54]
established that both intra- and interseasonal changes in temperature and precipitation influ-
ence cereal yields in Tanzania. [55] reported that climate change will reduce mean yields in
Africa by 17% for wheat, 5% for maize, 15% for sorghum and 10% for millet. No mean change
in yield for rice was detected. Using data from the northern Tanzanian highlands, [56]
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PLOS ONETable 2. Ljung–Box test statistic (Q?), its degree of freedom and its p–value for the fitted ARIMA models at lag 10 (i.e. h = 10).
Crop yield in some east African countries
Country
Burundi
Kenya
Somalia
Tanzania
Uganda
Rwanda
Crop
Banana
Beans
Cassava
Coffee
Sorghum
Sweet potato
Beans
Coffee
Maize
Rice
Sugar Cane
Wheat
Banana
Maize
Sorghum
Sugar Cane
Maize
Millet
Rice
Cotton Seed
Sorghum
Sweet potato
Banana
Cassava
Coffee
Millet
Plantain
Sweet potato
Beans
Cassava
Coffee
Potato
Sorghum
Sweet potato
Q?
Fitted ARIMA (p, d, q) a
ARIMA(2,1,2)
ARIMA(0,1,1) with drift
ARIMA(0,1,0)
ARIMA(0,1,1)
ARIMA(1,0,0) with non-zero mean
ARIMA(2,1,1)
ARIMA(1,0,0)
ARIMA(0,1,1)
ARIMA(0,1,3)
ARIMA(0,1,0)
ARIMA(0,1,0)
ARIMA(0,1,1)
ARIMA(0,0,4) with non-zero mean
ARIMA(0,1,0)
ARIMA(1,1,1)
ARIMA(0,1,0)
ARIMA(1,1,1)
ARIMA(1,0,0) with non-zero mean
ARIMA(0,1,1)
ARIMA(0,1,1)
ARIMA(2,1,0)
ARIMA(0,1,1)
ARIMA(0,1,2)
ARIMA(0,1,0)
ARIMA(1,0,0) with non-zero mean
ARIMA(0,1,1)
ARIMA(0,1,0)
ARIMA(0,1,0)
ARIMA(1,0,0) with non-zero mean
ARIMA(1,1,0)
ARIMA(1,0,0) with non-zero mean
ARIMA(0,1,2)
ARIMA(1,0,0) with non-zero mean
ARIMA(2,0,2) with non-zero mean
1.5746
5.9108
12.0820
8.3159
8.9597
7.8161
12.0780
13.2300
3.4473
10.4230
17.9800
13.9030
4.1658
15.1300
3.8376
13.8200
11.8720
6.2974
7.7560
18.8250
4.2521
12.8150
3.8909
12.1600
15.3660
6.1006
19.0490
17.7340
7.9098
2.3680
3.5770
4.4913
4.4304
2.4531
df
6
8
10
9
8
7
8
9
7
10
10
9
5
10
8
10
8
8
9
9
8
9
8
10
8
9
10
10
8
9
8
8
8
5
p–value
0.9544
0.6572
0.2796
0.5027
0.3457
0.3491
0.1478
0.1525
0.8408
0.4042
0.0553
0.1258
0.5258
0.1274
0.8715
0.1814
0.1570
0.6140
0.5589
0.0267
0.8337
0.1711
0.8668
0.2745
0.0524
0.7298
0.0397
0.0596
0.4423
0.9842
0.8931
0.8103
0.8164
0.7835
aDue to space constraints, we omit the coefficients of the fitted ARIMA models; interested readers can obtain them from the authors upon reasonable request.
https://doi.org/10.1371/journal.pone.0287011.t002
demonstrated that increasing night time temperature is the most significant climatic variable
responsible for diminishing coffea arabica yields between 1961 and 2012. According to [57],
annual food crops in the Kilimanjaro region of Tanzania were particularly sensitive to the
drought and maize and beans yields were lower than perennial crops during the years of
drought. Through a simulation study, [58] predicted climate change in east Africa and found
its negative impact on crop production in that region. They projected that the crop output
decrease will lie between 1.2% and 4.5%. [59] identified soil erosion by water as one of the
major causes of land degradation and dwindling agricultural produce in Africa resulting in an
estimated yearly crop yield loss of about 280 million tons. [60] provided evidence to suggest
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PLOS ONECrop yield in some east African countries
Fig 5. Time series plots and 10 years yield forecast with fitted ARIMA models showing 80% and 95% confidence
bands for crops in Burundi.
https://doi.org/10.1371/journal.pone.0287011.g005
that climate change severely impacted rice production in Rwanda. [61] produced evidence to
suggest that temperature increases lead to decline in maize and cassava crops for Tanzania,
Malawi, Zambia and South Africa. [62] observed that the yields for maize, sorghum or millet
fluctuated at a decreasing trend in the Kongwa district of Tanzania. According to [63],
increased temperatures in Kenya due to climate change have a general tendency to reduce rice
yields. [64] showed that the impacts of projected changes in climate on maize production areas
are the reduction in the suitability of the crop, especially around central and western Tanzania,
mid-northern and western Uganda, and parts of western Kenya by 20–40%, and patches of
east Africa will experience a reduction as high as 40–60%, especially in northern Uganda, and
western Kenya. According to [65], maize production in southern highlands of Tanzania has
decreased during the past two decades, since the year 2000. According to [66], climate change
has induced a devastating effect on agricultural production in Somalia leading to crop yield to
decline including sorghum.
Tables 3 and 4 give the BIC, AICc and the KS p–values of the fitted distributions. The BIC
and AICc values for the power law distribution are smaller than those for the remaining distri-
butions. The KS p–value > 0.05 in all the cases except for Millet in Uganda indicating that the
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PLOS ONECrop yield in some east African countries
Fig 6. Time series plots along with 10 years yield forecast for the fitted ARIMA models showing 80% and 95%
prediction confidence bands for crops in Kenya.
https://doi.org/10.1371/journal.pone.0287011.g006
power law is not a plausible distribution in this case. We cannot compare the values of the
goodness-of-fit measures of the power law distribution with those of the other distributions
because the power law distribution fits only the tails whereas the lognormal, Fre´chet, and the
stretched exponential distributions fit the entire data. Thus, we can only compare the BIC and
AICc values of the lognormal, Fre´chet, and the stretched exponential distributions. Based on
the KS p–value, we can observe that the lognormal distribution could be a plausible distribu-
tion for banana and coffee in Burundi, all the crops in Kenya except for sugar cane, maize and
sorghum in Somalia, all the crops in Tanzania, all but banana in Uganda and all but cassava in
Rwanda. Fre´chet distribution appears to be a plausible distribution for banana in Burundi,
maize and wheat in Kenya, maize and sorghum in Somalia, all except for maize and sorghum
in Tanzania and cassava, coffee and plantain in Uganda and all except for cassava and potato
in Rwanda. The stretched exponential distribution appears to be a plausible distribution for
beans and coffee in Burundi, all the crops in Kenya, maize in Somalia, all the crops in Tanza-
nia, all except for plantain in Uganda and all the crops in Rwanda.
Based on the AICc and BIC values in Tables 3 and 4, we can see that none of the three dis-
tributions that model the entire data (i.e. the lognormal, Fre´chet and the stretched exponential
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PLOS ONECrop yield in some east African countries
Fig 7. Time series plots along with 10 years yield forecast for the fitted ARIMA models showing 80% and 95%
prediction confidence bands for crops in Somalia.
https://doi.org/10.1371/journal.pone.0287011.g007
distributions) consistently provide the best fit. None of them consistently gave the smallest
AICc or smallest BIC values across the countries. The bootstrapped KS p–values in Table 5
indicate that the power law distribution is a plausible model for all the crop yield data. In gen-
eral, the distribution with the smallest AICc and smallest BIC values corresponds to the distri-
bution with the largest bootstrapped KS p–values. Fitting of such distributions to the tail of the
data can be compared with that of the power law distribution by using Vuong’s test. The
results of this comparison are presented in Table 6. We can observe that the stretched expo-
nential distribution emerges as the best model for millet in Uganda and the power law distri-
bution emerges as the best model for the rest of the crops except for a few cases where the
winner is undecided. For instance, for sorghum in Burundi (power law and lognormal), sweet
potato in Burundi (power law and Fre´chet) and for banana in Somalia (power law and lognor-
mal). The log-log plots of the fitted distributions superimposed with the empirical distribu-
tions are displayed in Figs 11 to 16. We can see that the power law distribution fits all the crop
yield data well across the countries.
Since the power law model appears to be a plausible distribution for virtually all the crops
across countries, we present the estimate for the parameters of the distribution in Table 7. We
see that the power law mechanism may occur at varying degrees depending on the type of crop
and country. See the ntail values for crops in Table 7, where ntail denotes the total number of
observations equal to or above the threshold value xmin, i.e. the total number of data points fol-
lowing the power law distribution. The occurrence of such extremely high crop yield definitely
has positive impact on farmers and food security. In this case, farmers can make huge profits.
Crop yield risk insurance policies for such crops can attract relatively lower premium rates
compared to crops with lower yields. The α value of the fitted power law model describes the
heaviness of the tail distribution corresponding to extremely high crop yield events with yield
> xmin. According to Table 7, the estimates of α are all > 2 indicating that the data in the right
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
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PLOS ONECrop yield in some east African countries
Fig 8. Time series plots along with 10 years yield forecast for the fitted ARIMA models showing 80% and 95%
prediction confidence bands for crops in Tanzania.
https://doi.org/10.1371/journal.pone.0287011.g008
tail of the distribution show significant high inequality (i.e. large crop yield). However, there
are two special cases satisfying 2 < α � 3 specifically in Somalia for sugar cane and Tanzania
for sweet potato. In these cases, the variance and higher-order moments for the crop yields are
infinite regardless of whether their mean yield exists or not. Hence, the classical central limit
theorem does not hold for these yield data. The consequence of the infinite variance and
higher order moments is that empirical estimates of the means converge very slowly due to the
regular occurrence of extremely large crop yield values. These characteristics suggest that crop
harvest with extremely large yield could sometimes occur for sugar cane in Somalia and sweet
potato in Tanzania. Such events could often be of great importance to the farmers and other
investors in agribusiness. This behavior is referred to as the black swan mechanism (see [67]).
The black swan mechanism describes events coming as a surprise. It has a major effect (posi-
tive or negative) and is often inappropriately rationalized. Farmers can have the tendency to
break even and even enjoy lower crop yield risk insurance policies in Somalia and Tanzania if
they invest in sugar cane and sweet potato, respectively, due to their potential for extremely
high yield.
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
18 / 36
PLOS ONECrop yield in some east African countries
Fig 9. Time series plots along with 10 years yield forecast for the fitted ARIMA models showing 80% and 95%
prediction confidence bands for crops in Uganda.
https://doi.org/10.1371/journal.pone.0287011.g009
All the estimated α values for the power law distribution in Table 7 are > 3 except for sugar
cane in Somalia and sweet potato in Tanzania. This indicates that the sample means for these
crops are Gaussian distributed and that their variances are finite. Hence, the standard central
limit theorem applies for these crop yield data. The finite mean and variance and the observed
evidence of underdispersion in Table 1 suggest that east African regional food security does
not seem to be extremely volatile as regular crop yields for these crops tend to cluster around
the mean crop yield.
Ignoring the impacts of climate and environment, soil structures and compositions/nutri-
ents, crop species, mechanization and technology, etc on crop yields, the observed black swan
behaviour for the yields of sugar cane in Somalia and sweet potato in Tanzania could be
explained by the so called “rich getting richer” principle or the “preferential attachment” princi-
ple. Based on these principles, these two crops have potentials for extremely high yield perhaps
because of either high demand (so every farmer tends to make them their choice crops for cul-
tivation) or common practice such as irrigation adopted by all the farmers being capable of
increasing crop yield [36]. So, speaking of crop harvest, yield could follow the pattern of the
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
19 / 36
PLOS ONECrop yield in some east African countries
Fig 10. Time series plots along with 10 years yield forecast for the fitted ARIMA models showing 80% and 95%
prediction confidence bands for crops in Rwanda.
https://doi.org/10.1371/journal.pone.0287011.g010
rich getting richer or the preferential attachment principle. The extremely high yields for sugar
cane in Somalia and sweet potato in Tanzania are not just a little bit higher than the normal
yield for the same or different crops in the same or other countries. Instead they are so much
higher that they cause their distributions to skew significantly.
5 Conclusions
We have analyzed the trend and tail of some yearly crop yield data such as banana, plantain,
beans, cassava, coffee, sorghum, potato, sweet potato, maize, rice, sugar cane, wheat, millet and
cotton seed from 1961 to 2018 in six east African countries: Burundi, Kenya, Somalia, Tanza-
nia, Uganda and Rwanda. An exploratory analysis of the crop yield data reveals three structural
patterns in each of the series. They are: increasing, decreasing and stagnant trends. Ten years
(2019–2028) time series point forecast based on the fitted ARIMA models shows that majority
of the crops will experience stagnant yield in different countries with only sorghum and coffee
showing the tendency for significant and persistent upward trend in Burundi and Rwanda,
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PLOS ONECrop yield in some east African countries
Table 3. AIC, BIC, AICc and KS p–values (as defined in Section 3) for the fitted distributions.
Country—Crop Power law
BIC
AICc
Burundi—Banana
723.717
Beans
519.823
Cassava
585.320
Coffee
307.680
Sorghum
78.290
Sweet potato
134.892
Kenya—Beans
493.325
Coffee
308.382
Maize
352.530
Rice
785.164
Sugar Cane
771.239
Wheat
418.665
Somalia—Banana
306.400
Maize
577.876
Sorghum 299.456
719.814
515.920
581.417
303.777
74.387
130.989
489.422
304.479
348.627
781.262
767.336
414.762
302.498
573.973
295.554
Sugar Cane
1561.743
1557.841
Tanzania—Maize
602.620
598.718
Millet
357.070
Rice
645.884
Cotton Seed
480.611
Sorghum 264.040
Sweet potato
823.410
Uganda—Banana
269.292
Cassava
688.846
Coffee
614.601
Millet
700.174
Plantain
884.580
Sweet potato
618.508
353.168
641.981
476.708
260.138
819.507
265.389
684.944
610.698
696.271
880.678
614.605
p–value
0.954
0.532
0.132
0.931
0.930
0.901
0.504
0.721
0.756
0.303
0.944
0.835
0.889
0.811
0.954
0.252
0.632
0.922
0.532
0.332
0.834
0.200
0.909
0.518
0.833
0.003
0.667
0.654
https://doi.org/10.1371/journal.pone.0287011.t003
Lognormal
BIC
AICc
1239.032
1235.129
1019.932
1016.029
1293.669
1289.766
1054.181
1050.278
1053.391
1049.489
1243.862
1239.959
980.215
976.313
1043.274
1039.371
1084.795
1080.893
1251.779
1247.876
1613.395
1609.492
1153.186
1149.283
1412.801
1408.899
1097.639
1093.737
978.390
974.487
1621.627
1617.724
1146.010
1142.107
1061.506
1057.604
1157.416
1153.513
999.445
995.543
1088.711
1084.809
1303.455
1299.553
1187.916
1184.013
1361.801
1357.898
1007.740
1003.837
1094.174
1090.271
1245.936
1242.033
1189.226
1185.324
p–value
0.051
0.004
0.000
0.112
0.001
0.000
0.223
0.109
0.332
0.121
0.007
0.821
0.000
0.943
0.131
0.009
0.100
0.966
0.802
0.854
0.211
0.661
0.005
0.535
0.907
0.074
0.138
0.087
Fre´chet
BIC
AICc
1214.761
1210.859
1048.091
1044.188
1337.395
1333.492
1091.141
1087.238
1076.397
1072.495
1184.906
1181.003
999.376
995.474
1390.577
1386.674
1093.394
1089.491
1282.416
1278.513
1639.809
1635.906
1160.712
1156.810
1430.688
1426.786
1109.941
1106.038
976.162
972.259
1608.998
1605.095
1157.454
1153.551
1066.259
1062.356
1169.649
1165.746
1016.260
1012.358
1100.409
1096.506
1307.969
1304.066
1224.341
1220.438
1361.721
1357.818
1013.23
1009.328
1113.196
1109.294
1242.476
1238.573
1209.564
1205.662
p–value
0.629
0.001
0.000
0.001
0.000
0.000
0.010
0.000
0.112
0.012
0.000
0.712
0.000
0.423
0.203
0.019
0.005
0.533
0.242
0.166
0.023
0.183
0.000
0.515
0.516
0.008
0.384
0.012
Stretched exponential
BIC
AICc
1282.202
1278.299
1005.110
1001.208
1250.241
1246.338
1036.031
1032.129
1045.201
1041.298
1290.785
1286.882
979.803
975.900
1038.009
1034.106
1085.083
1081.180
1242.221
1238.318
1595.751
1591.848
1162.411
1158.508
1421.742
1417.839
1102.289
1098.386
1006.621
1002.718
1636.254
1632.351
1155.097
1151.194
1080.704
1076.801
1157.795
1153.893
999.811
995.908
1089.489
1085.586
1305.698
1301.795
1158.594
1154.691
1369.707
1365.804
1017.461
1013.558
1079.048
1075.145
1261.018
1257.115
1196.161
1192.258
p–value
0.000
0.092
0.000
0.645
0.000
0.000
0.432
0.323
0.532
0.231
0.321
0.402
0.000
0.434
0.015
0.003
0.103
0.121
0.499
0.664
0.512
0.402
0.443
0.323
0.420
0.187
0.004
0.065
respectively, while beans indicates significant and persistent yield decrease in Burundi, Kenya
and Rwanda.
We used the power law, lognormal, Fre´chet and stretched exponential distributions to
describe high yields in all the crops across the countries. Based on Vuong’s test, we observed
that the stretched exponential distribution gave the best fit for millet in Uganda while the
Table 4. Continuation of Table 3.
Country—Crop Power law
BIC
AICc
Rwanda—Beans
171.782
167.879
Cassava
152.243
148.340
Coffee
578.925
575.022
Potato
945.602
941.699
Sorghum 617.552
613.650
Sweet potato
278.219
274.317
Lognormal
BIC
AICc
978.580
974.677
1410.743
1406.840
1017.965
1014.062
1332.154
1328.251
1038.192
1034.289
1274.881
1270.979
p–value
0.524
0.019
0.332
0.051
0.543
0.675
Fre´chet
BIC
AICc
989.914
986.011
1433.857
1429.954
1040.145
1036.243
1363.027
1359.124
1054.371
1050.468
1292.205
1288.303
p–value
0.165
0.004
0.054
0.001
0.223
0.263
Stretched exponential
BIC
AICc
978.688
974.785
1392.647
1388.744
1022.827
1018.925
1324.835
1320.932
1043.405
1039.502
1274.463
1270.560
p–value
0.642
0.176
0.105
0.142
0.091
0.243
p–value
0.935
0.909
0.903
0.810
0.732
0.983
https://doi.org/10.1371/journal.pone.0287011.t004
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PLOS ONETable 5. Bootstrap KS-test p–values (as defined in Section 3) for the fitted distributions.
Crop yield in some east African countries
Country
Burundi
Kenya
Somalia
Tanzania
Uganda
Rwanda
Crop
Banana
Beans
Cassava
Coffee
Sorghum
Sweet potato
Beans
Coffee
Maize
Rice
Sugar Cane
Wheat
Banana
Maize
Sorghum
Sugar Cane
Maize
Millet
Rice
Cotton Seed
Sorghum
Sweet potato
Banana
Cassava
Coffee
Millet
Plantain
Sweet potato
Beans
Cassava
Coffee
Potato
Sorghum
Sweet potato
https://doi.org/10.1371/journal.pone.0287011.t005
Power law
Lognormal
Fre´chet
Stretched exponential
0.966
0.902
0.832
0.943
0.952
0.903
0.913
0.928
0.929
0.805
0.945
0.905
0.923
0.933
0.955
0.903
0.956
0.931
0.902
0.933
0.939
0.804
0.965
0.901
0.955
0.201
0.932
0.909
0.958
0.987
0.943
0.924
0.932
0.949
0.732
0.424
0
0.831
0.305
0
0.902
0.821
0.931
0.821
0.611
0.962
0.016
0.942
0.813
0.523
0.787
0.962
0.951
0.971
0.827
0.965
0.514
0.961
0.980
0.612
0.831
0.810
0.951
0.612
0.902
0.732
0.933
0.910
0.925
0.112
0
0.322
0.101
0.221
0.591
0
0.820
0.461
0.132
0.901
0.012
0.912
0.828
0.631
0.511
0.936
0.921
0.822
0.609
0.919
0.055
0.919
0.911
0.423
0.925
0.424
0.901
0.301
0.613
0.321
0.841
0.906
0.094
0.732
0.021
0.941
0.324
0
0.944
0.922
0.954
0.844
0.940
0.922
0
0
0
0
0.723
0.902
0.952
0.949
0.906
0.915
0.931
0.924
0.923
0.919
0.506
0.734
0.911
0.812
0.832
0.815
0.822
0.931
power law distribution gave the best fit for the other crops except for a few undecided cases.
The log-log plots were used to visually inspect the performance of the fitted distributions. The
power law distribution appeared to fit the upper tail of all the crop yield data better than the
other distributions in all the countries. Based on the estimated α value of the fitted power law
model, we found potential for extremely high yield in sugar cane in Somalia and sweet potato
in Tanzania indicating the inappropriateness of the Gaussian distribution for describing these
crop yields. Other crops in Burundi, Kenya, Somalia, Tanzania, Uganda and Rwanda can pro-
duce only high but not extremely high yields. Though the time series point forecasts for major-
ity of the crops show yield stagnancy with a few exceptions, the evidence from the power law
analysis indicates the potential for high yield for all the crops and provides specific calibrations
for the yield of all the crops in terms of what quantity of yield is considered high.
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PLOS ONETable 6. Vuong test statistic (Λ) and its p–value for comparing the upper tail (i.e. x > xmin) of the fitted power law distribution and the best among the rest of the
competing distributions.
Crop yield in some east African countries
Country
Burundi
Kenya
Somalia
Tanzania
Uganda
Rwanda
Crop
Banana
Beans
Cassava
Coffee
Sorghum
Sweet potato
Beans
Coffee
Maize
Rice
Sugar Cane
Wheat
Banana
Maize
Sorghum
Sugar Cane
Maize
Millet
Rice
Cotton Seed
Sorghum
Sweet potato
Banana
Cassava
Coffee
Millet
Plantain
Sweet potato
Beans
Cassava
Coffee
Potato
Sorghum
Sweet potato
Contest
Statistic (Λ)
Power law vs Fre´chet
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Lognormal
Power law vs Fre´chet
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Lognormal
Power law vs Lognormal
Power law vs Lognormal
Power law vs Fre´chet
Power law vs Fre´chet
Power law vs Lognormal
Power law vs Lognormal
Power law vs Stretched exponential
Power law vs Lognormal
Power law vs Stretched exponential
Power law vs Lognormal
Power law vs stretched exponential
Power law vs Lognormal
Power law vs Lognormal
Power law vs Stretched exponential
Power law vs Fre´chet
Power law vs Lognormal
Power law vs Stretched exponential
Power law vs Stretched exponential
Power law vs Lognormal
Power law vs Stretched exponential
Power law vs Lognormal
Power law vs Lognormal
2.6671
6.7366
10.4399
4.1021
1.2454
0.7442
3.9727
3.5192
2.8513
4.4303
4.3048
2.5905
1.7339
2.6642
1.9648
4.5875
5.4006
3.8995
2.8193
2.8774
2.8893
2.3193
2.9194
2.4766
2.7274
−2.8155
3.1373
3.9022
3.5970
2.9573
4.3420
5.4570
4.1122
2.6648
p–value
0.0077
0
0
0
0.2130
0.4568
1.0×10−4
4.0×10−4
0.0044
0
0
0.0096
0.0829
0.0077
0.0494
0
0
1.0×10−4
0.0048
0.0040
0.0039
0.0204
0.0035
0.0133
0.0064
0.0049
0.0017
1.0×10−4
3.0×10−4
0.0031
0
0
0
0.0077
Winner
Power law
Power law
Power law
Power law
Undecided
Undecided
Power law
Power law
Power law
Power law
Power law
Power law
Undecided
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Stretched exponential
Power law
Power law
Power law
Power law
Power law
Power law
Power law
Power law
https://doi.org/10.1371/journal.pone.0287011.t006
We characterize the evidence for extremely high yield for sugar cane and sweet potato in
Somalia and Tanzania, respectively, as black swan where the “rich getting richer” or the “prefer-
ential attachment” could be the underlying generating process, meaning that either the two
crops are increasingly at lower risk of climate change and environmental challenges such as
being drought resistant or farmers are constantly doing many things right (such as adopting
favorable planting strategies, large crop areas, etc) as far as the cultivation of the two crops are
concerned in the two countries.
ARIMA(0,1,1) was used to model and predict coffee in Burundi and Kenya; beans in
Burundi; wheat in Kenya; rice, cotton seed, and sweet potato in Tanzania; millet in Uganda.
ARIMA(2,1,0) was used to model and predict sorghum in Tanzania. ARIMA(2,1,2) was used
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
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PLOS ONECrop yield in some east African countries
Fig 11. Log-log plots for crops yield in Burundi where the red line corresponds to the value of xmin in Table 7.
https://doi.org/10.1371/journal.pone.0287011.g011
to model and predict banana in Burundi. ARIMA(0,1,2) was used to model and predict banana
in Uganda and potato in Rwanda. ARIMA(0,1,0) was used to model and predict cassava in
Burundi and Uganda; sugarcane in Kenya and Somalia; rice in Kenya; maize in Somalia; plan-
tain and sweet potato in Uganda. ARIMA(1,0,0) was used to model and predict Sorghum in
Burundi and Rwanda; beans in Kenya and Rwanda; millet in Tanzania; coffee in Uganda.
ARIMA(2,1,1) was used to model and predict sweet potato in Burundi. ARIMA(0,1,3) was
used to model and predict maize in Kenya. ARIMA(0,0,4) was used to model and predict
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PLOS ONECrop yield in some east African countries
Fig 12. Log-log plots for crops yield in Kenya where the red line corresponds to the value of xmin in Table 7.
https://doi.org/10.1371/journal.pone.0287011.g012
banana in Somalia. ARIMA(1,1,1) was used to model and predict sorghum in Somalia; maize
in Tanzania. ARIMA(2,0,2) was used to model and predict sweet potato in Rwanda.
The yield forecast in Burundi shows an initial quick decline in 2019 followed by an increase
for banana; a sharp increase in 2019 followed by an increase for sweet potato; sorghum shows
a quick increase from 2019 to 2028; beans shows a sharp decrease from 2019 to 2028; neither
cassava nor coffee show any tendency to increase or decrease from 2019 to 2028. The forecast
of the crop yield in Kenya indicates continuous decline of beans yield from 2019 to 2028; no
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PLOS ONECrop yield in some east African countries
Fig 13. Log-log plots for crops yield in Somalia where the red line corresponds to the value of xmin in Table 7.
https://doi.org/10.1371/journal.pone.0287011.g013
decrease or increase pattern in yield is evident for coffee, rice, wheat and sugar cane from 2019
to 2028; maize shows a sharp decline in 2019 with an immediate increase followed by a stable
trend. In Somalia, the yield forecast for maize and sugar cane does not indicate any pattern;
banana shows an initial moderate increase in 2019 followed by the lack of pattern until 2028;
sorghum experienced a sharp drop in 2019 followed by a period of no trend up to 2028. The
yield forecast in Tanzania indicates no significant trend for maize, rice, sweet potato and cot-
ton seed for the whole forecast period; millet is slightly decreased in 2019 and remained stag-
nant until 2028. The yield forecast in Uganda indicates that banana, cassava, millet, plantain
and sweet potato did not show any significant pattern from 2019 to 2028; coffee shows a slight
increase in 2019 followed by a period of no change in yield. The yield forecast in Rwanda indi-
cates that beans persistently decreased from 2019 to 2028; sweet potato shows initial increase
followed by a slow decrease; coffee indicated an upward trend from 2019 to 2028; cassava,
potato and sorghum did not show any significant pattern.
In our discussion in Section 4, we saw how the literature points in the direction of climate
change as the major cause of the observed yield stagnancy and decline. On this backdrop, we
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PLOS ONECrop yield in some east African countries
Fig 14. Log-log plots for crops yield in Tanzania where the red line corresponds to the value of xmin in Table 7.
https://doi.org/10.1371/journal.pone.0287011.g014
suggest that a promising future in favour of high crop yield could await east Africa if urgent
changes or improvements on the cropping systems and infrastructures that currently exist in
east Africa could be made in order to meet up with the inevitable future demand of agricultural
produce due to the increasing population and the challenge of negative impacts of climate
change. Science and technology could be useful in showing how agricultural production can
be significantly improved in east Africa. For instance, the construction of irrigation systems
and rainwater harvesting structures could help cushion the impact of climate change.
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PLOS ONECrop yield in some east African countries
Fig 15. Log-log plots for crops yield in Uganda where the red line corresponds to the value of xmin in Table 7.
https://doi.org/10.1371/journal.pone.0287011.g015
Further, various climate adaptation/smart strategies could be adapted to increase yields in
east Africa. According to [68], short-duration pigeon pea varieties developed by the Interna-
tional Crops Research Institute for Semi-Arid Tropics and the Kenya Agricultural Research
Institute can give high yields and escape drought, but require non-traditional management
practices (for example, sole-cropping, spraying against insect pests). According to [69], NER-
ICA, a new rice for Africa, has shown high potential to revolutionize rice farming, producing
high yield with minimum inputs in stress-afflicted ecologies. [70] observed that cassava mosaic
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
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PLOS ONECrop yield in some east African countries
Fig 16. Log-log plots for crops yield in Rwanda where the red line corresponds to the value of xmin in Table 7.
https://doi.org/10.1371/journal.pone.0287011.g016
disease (CMD) resistant cassava varieties released in western Kenya and Uganda yielded up to
three times more than local varieties. [71] demonstrated that high yields of maize were
recorded from certain varieties (Pwani Hybrid 4-PH4, Coast Composite Maize-CCM and the
local check-Mdzihana) but they usually required relatively high rainfall amounts in order for
them to produce better yields. [72] showed that increased knowledge of varieties, environment
and management factors can double total yield of maize, sorghum, millet and groundnut from
1.67 to 3.29 tons per hectare from the average 5.1 hectares that farmers usually crop in south
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
29 / 36
PLOS ONETable 7. Parameter estimates for the power law distribution for all the crop yield data sets (xmin and α are parameters of the power distribution; αse is the standard
error corresponding to α; ntail is the number of data exceeding xmin).
Crop yield in some east African countries
Country
Burundi
Kenya
Somalia
Tanzania
Uganda
Rwanda
Crop
Banana
Beans
Cassava
Coffee
Sorghum
Sweet potato
Beans
Coffee
Maize
Rice
Sugar Cane
Wheat
Banana
Maize
Sorghum
Sugar Cane
Maize
Millet
Rice
Cotton Seed
Sorghum
Sweet potato
Banana
Cassava
Coffee
Millet
Plantain
Sweet potato
Beans
Cassava
Coffee
Potato
Sorghum
Sweet potato
n
ntail
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
58
35
35
35
35
5
6
32
20
21
39
30
22
13
32
18
57
33
21
34
30
15
37
16
30
37
39
43
33
12
7
36
44
37
14
xmin
55455
9888
89591
9180
12964
89127
5429
6669
16922
37568
806694
19359
222222
9544
4143
300000
12427
9530
15583
5132
10133
25323
44748
66988
5873
12496
53474
40960
8914
120265
5817
64088
10688
75333
α
αse
7.4026
19.9503
52.1287
15.9219
33.5816
8.1898
9.4680
11.9168
14.5952
6.7330
8.5033
6.5818
8.2251
5.4249
5.3367
2.7445
6.0445
8.3193
5.4751
7.2276
7.3606
2.9081
36.7026
4.0035
6.3153
6.6790
7.3752
12.6778
28.4564
13.0159
7.6296
6.0225
9.6451
15.2064
1.0822
3.2032
8.7685
3.3366
14.5710
2.9352
1.4969
2.4411
2.9667
0.9180
1.3699
1.1901
2.0039
0.7822
1.0222
0.2311
0.8781
1.5972
0.7675
1.1370
1.6423
0.3137
8.9257
0.5484
0.8738
0.9094
0.9722
2.0328
7.9260
4.5416
1.1049
0.7572
1.4212
3.7968
https://doi.org/10.1371/journal.pone.0287011.t007
east Zimbabwe. [73] showed that improved maize varieties outyielded the traditional control
variety by 26–46% across sites and season in central Mozambique. [74] showed that the use of
organic soil management practices such as reduced tillage, mulching and leguminous crops in
the northern part of Tanzania increased the production of food crops from an average of 0.5
ton per hectare to 1.5 ton per hectare; subsequently, maize yields increased from 12,000 kilo-
gram to 20,000 kilogram per 4.8 hectares. [75] suggested that relaxing liquidity constraints
could help to encourage farmers’ adaptation through the implementation of soil, water and
land management strategies; thereby, positioning east Africa for food sufficiency in the face of
the current global food crisis. [76] noted that intensive manuring with a combination of green
and poultry manure produced high yields of maize in central Uganda that were comparable to
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
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PLOS ONECrop yield in some east African countries
those with mineral fertilizers. [77] demonstrated that households in Kenya adapting to climate
change and climate variability through uptake of technologies such as early planting, use of
improved crop varieties, and crop diversification produced 4877 kilograms of maize yield
equivalent / hectare per year against 3238 kilograms of maize yield equivalent / hectare per
year for households that did not adapt (a 33.6% difference between the two groups). [78]
found that fertilizer application in the intercropping system is eastern and southern Africa
improved cereal yields by 71–282% and pigeon pea yields by 32–449%, increased benefit-cost
ratios by 10–40%, and reduced variability in cereal yields by 40–56% and pigeon pea yields by
5–52% compared with unfertilized intercrops. [79] showed that drought resistant climate-
smart maize hybrids in Kenya increased yields 33 to 54% relative to conventional hybrids.
According to [80], climate adaptation strategies in the central highlands of Kenya included the
use of fertilizer and manure in combination (71%), terracing (66%), and crop rotation (60%).
[81] showed that climate-smart adaptation practices significantly enhanced wheat yield by
34.35% in southern Ethiopia. [82] showed that use of mulching and permanent planting basin
dimensions on maize in western Uganda relatively increased yield by 11–66% and water use
efficiency by 33–94% compared to conventional practices.
The findings in this paper underscore the importance of using climate-smart agricultural
alternatives to improve resilience farming system and the livelihood of subsistence farmers
due to the impact of climate change in east Africa. Currently, crop yield for majority of the
crops in different countries has been confirmed to neither increase nor decrease with only
few crops experiencing all time increase or decrease in yield. Urgent attention should be
paid to beans production in the affected countries in order to reverse the persistent down-
ward trend of its yield. This paper brings good news of hope for crop yield increase in east
Africa if adaptive farming methods and strategies are adequately harnessed in the region in
the face of climate and environmental challenges and rising global demand for agricultural
produce.
The data from 1961 to 2018 consist of only 58 observations. Hence, the results and forecasts
in this paper should be treated conservatively. A future work is to see if more frequent and
more up-to-date data are available. Another is to consider multivariate modelling of yield by
considering country and crop. The disadvantage of the length of the observed series can be
interpolated by explaining the common factor for each country and crop.
Supporting information
S1 Data.
(CSV)
S2 Data.
(CSV)
S3 Data.
(CSV)
S4 Data.
(CSV)
S5 Data.
(CSV)
S6 Data.
(CSV)
PLOS ONE | https://doi.org/10.1371/journal.pone.0287011 June 13, 2023
31 / 36
PLOS ONECrop yield in some east African countries
S7 Data.
(CSV)
S8 Data.
(CSV)
S1 File.
(TXT)
Acknowledgments
The authors would like to thank the Editor and the two referees for careful reading and com-
ments which greatly improved the paper.
Author Contributions
Formal analysis: Idika E. Okorie, Emmanuel Afuecheta, Saralees Nadarajah.
Methodology: Idika E. Okorie, Emmanuel Afuecheta, Saralees Nadarajah.
Resources: Emmanuel Afuecheta.
Software: Idika E. Okorie, Saralees Nadarajah.
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10.1371_journal.pone.0286598.pdf
|
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
|
All relevant data are within the
|
RESEARCH ARTICLE
Enteral nutrition management in critically ill
adult patients and its relationship with
intensive care unit-acquired muscle
weakness: A national cohort study
Ignacio Zaragoza-Garcı´aID
Daniel Martı´5☯, Elisabet Gallart6☯, Alicia San Jose´ -Arribas7☯, Tamara Raquel Velasco-
Sanz1,8☯, Eva Blazquez-Martı´nez9☯, Marta Raurell-Torredà10☯
1,2☯*, Susana Arias-Rivera3☯, Marı´a Jesu´ s Frade-Mera1,4☯, Joan
1 Department of Nursing, Faculty of Nursing, Physiotherapy and Podology, University Complutense of
Madrid, Madrid, Spain, 2 Invecuid, Instituto de Investigacio´n Sanitaria Hospital 12 de Octubre (imas12),
Madrid, Spain, 3 University Hospital of Getafe, CIBER Enfermedades Respiratorias, Instituto de Salud Carlos
III, Getafe, Spain, 4 Department of Critical Care, 12 Octubre University Hospital, Madrid, Spain, 5 Clinic
University Hospital, Barcelona, Spain, 6 Department of Critical Care, Vall Hebron University Hospital,
Barcelona, Spain, 7 Escola Universitaria d’Infermeria Sant Pau, Hospital de la Santa Creu i Sant Pau,
Barcelona, Spain, 8 Department of Critical Care, San Carlos University Hospital, Madrid, Spain, 9 Bellvitge
University Hospital, Hospitalet de Llobregat, Llobregat, Spain, 10 Department d’Infermeria Fonamental i
medicoquiru´ rgica, Facultat d’Infermeria, Universitat de Barcelona, Barcelona, Spain
☯ These authors contributed equally to this work.
* [email protected]
Abstract
Objective
To assess the incidence and determinants of ICU-acquired muscle weakness (ICUAW) in
adult patients with enteral nutrition (EN) during the first 7 days in the ICU and mechanical
ventilation for at least 48 hours.
Methods
A prospective, nationwide, multicentre cohort study in a national ICU network of 80 ICUs.
ICU patients receiving invasive mechanical ventilation for at least 48 hours and EN the first
7 days of their ICU stay were included. The primary outcome was incidence of ICUAW. The
secondary outcome was analysed, during days 3–7 of ICU stay, the relationship between
demographic and clinical data to contribute to the onset of ICUAW, identify whether energy
and protein intake can contribute independently to the onset of ICUAW and degree of com-
pliance guidelines for EN.
Results
319 patients were studied from 69 ICUs in our country. The incidence of ICUAW was 153/
222 (68.9%; 95% CI [62.5%-74.7%]). Patients without ICUAW showed higher levels of
active mobility (p = 0.018). The logistic regression analysis showed no effect on energy or
protein intake on the onset of ICUAW. Overfeeding was observed on a significant proportion
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OPEN ACCESS
Citation: Zaragoza-Garcı´a I, Arias-Rivera S, Frade-
Mera MJ, Martı´ JD, Gallart E, San Jose´-Arribas A,
et al. (2023) Enteral nutrition management in
critically ill adult patients and its relationship with
intensive care unit-acquired muscle weakness: A
national cohort study. PLoS ONE 18(6): e0286598.
https://doi.org/10.1371/journal.pone.0286598
Editor: Sebastien Kenmoe, University of Buea,
CAMEROON
Received: February 11, 2023
Accepted: May 19, 2023
Published: June 7, 2023
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0286598
Copyright: © 2023 Zaragoza-Garcı´a et al. This is an
open access article distributed under the terms of
the Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
PLOS ONE | https://doi.org/10.1371/journal.pone.0286598 June 7, 2023
1 / 14
PLOS ONEFunding: This work was supported by 2018
european federation of critical care nursing
associations (EfCCNa) Research Awards. The
funders had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Enteral nutrition management and ICUAW
of patient-days, while more overfeeding (as per US guidelines) was found among patients
with obesity than those without (42.9% vs 12.5%; p<0.001). Protein intake was deficient (as
per US/European guidelines) during ICU days 3–7.
Conclusions
The incidence of ICUAW was high in this patient cohort. Early mobility was associated with
a lower incidence of ICUAW. Significant overfeeding and deficient protein intake were
observed. However, energy and protein intake alone were insufficient to explain ICUAW
onset.
Relevance to clinical practice
Low mobility, high incidence of ICUAW and low protein intake suggest the need to train,
update and involve ICU professionals in nutritional care and the need for early mobilization
of ICU patients.
Introduction
Patients admitted to Intensive Care Units (ICUs) are subject to increased metabolic stress. Ele-
vated catabolism requires nutritional resources for the body to perform anabolism adequately
[1]. If oral intake is not possible, enteral nutrition (EN) is recommended over parenteral nutri-
tion, because it has fewer complications [2]. Inappropriate management of enteral nutrition
support in these patients can lead to malnutrition, a common finding in ICU patients [3], for
which the incidence ranges from 39% to 50% of patients, depending on the country and ICU
type [4].
Various authors have described possible causes of malnutrition in critically ill patients.
Delayed initiation of nutrition support has been found in 60% of cases. In addition, an incor-
rect EN regimen can lead to under- or overfeeding, which, together with the inflammatory
response typical for this metabolic state, can contribute to hyperglycaemia, loss of muscle mass
and strength, prolonged rehabilitation, as well as an increase in comorbidities resulting in
deteriorated quality of life in the long term [5].
Loss of muscle mass together with other factors, such as physical immobility, can lead to
the onset of bilateral and symmetric neuromuscular complications, referred to as ICU-
acquired muscle weakness (ICUAW), which contributes to significant functional impairment.
Specifically, the muscles of the limbs and the diaphragm may become weak and atrophic,
impairing patients’ autonomy, prolonging mechanical ventilation, and increasing weaning
time and length of hospital stays [6, 7].
The most studied predictors in ICUAW are related to gender, time on mechanical ventila-
tion, length of ICU stay, age, more days on renal replacement therapy. On the other hand, the
presence of delirium and being actively mobilised during the first 5 days in the ICU are consid-
ered protective factors [8, 9]. Some international bodies specialised in EN suggest the need for
research on the relationship between EN and ICUAW, but due to lack of evidence, they do not
yet make any recommendations in this regard [2, 10].
As a result, various international nutrition-related societies publish specific recommenda-
tions for critically ill patients. Recent studies suggest that diet-only interventions are insuffi-
cient to improve patients’ nutritional status and reduce comorbidities, and this is now
reflected in current recommendations [2]. To mitigate this deterioration, early mobilization in
PLOS ONE | https://doi.org/10.1371/journal.pone.0286598 June 7, 2023
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PLOS ONEEnteral nutrition management and ICUAW
the ICU is recommended [5]. The combination of nutrition plus exercise may modify the cata-
bolic effects of critical illness, muscle wasting, and the development of ICUAW, which has
been identified as a research priority [11].
Currently, no national multicentre studies have evaluated the management of EN in criti-
cally ill patients or the degree of mobility of these patients related to the incidence of ICUAW.
The aim of this study was to assess the incidence and determinants of ICUAW in adult
patients with EN during the first 7 days in the ICU and receiving mechanical ventilation for at
least 48 hours.
Materials and methods
Design
A prospective multicentre observational cohort study was conducted during four months
(2019–2020) in a Spanish national ICU network of 80 ICUs.
Data collection
Patients were recruited consecutively. The data were collected starting from day 3 of ICU
admission. Inclusion criteria were adult patients receiving invasive mechanical ventilation
(IMV) for at least 48 hours in an ICU and EN for at least the first 7 days of their ICU stay.
Exclusion criteria were pregnant women, patients <18 years, those referred to the ICU from
other hospitals, patients with primary neurologic or neuromuscular pathology, those unable to
walk, recent limb amputees, users of orthopaedic devices and patients with body mass index
(BMI) >35.
Sample/Participants
The minimum sample size was 316, calculated according to the 46% incidence of ICUAW
found in a sample of 1421 patients by Stevens et al. [12], a confidence level of 95%, an esti-
mated standard error of 5 and an expected loss of 5%.
Ethical considerations
The study was approved by the Ethics and Clinical Research Committees of the participating
sites under reference protocol PI16/00771. Written informed consent was obtained. The rele-
vant STROBE checklist was followed for reporting the study.
Research variables and measures
Primary outcome. The primary outcome was incidence of ICUAW, assessed by the Med-
ical Research Council Scale (MRC-Sum score) following the assessment protocol described by
Hermans [13]. ICUAW was diagnosed for values lower than 48 out of 60 (the maximum
score) in the first measure of MRC (baseline MRC) [14].
The measure of MRC was conducted after the first awakening of the patient, with the
patient fully awake. See S1 File. Measurement tools.
Secondary outcomes. The secondary outcome were, on the one hand, analysis of the rela-
tionship between demographic and clinical data contributing to the onset of ICUAW during
days 3–7 of ICU stay. On the other hand, we proceeded to identify whether energy or protein
intake during 3–7 days of the ICU stay, taking into account the US and European recommen-
dation, can contribute independently to the onset of ICUAW. Finally, the degree of compli-
ance with current US and European guidelines for target dietary intake in EN during the acute
phase (days 3–7) of ICU admission was analysed.
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PLOS ONEEnteral nutrition management and ICUAW
Specifically, the following recommendations were used for reference in the study [2, 15]:
Target energy and protein intake: According to ASPEN (American Society for Parenteral
and Enteral Nutrition) guidelines [15]: target energy and protein intake should be 25–30 kcal/
kg/day and 1.2–2 g/kg/day, respectively. During the first week, trophic EN is permitted. For
patients with BMI �30 kg/m2 the energy target is 11–14 kcal/kg/day actual body weight/day
and the protein target is 2 g/kg ideal body weight/day.
According to ESPEN (European Society for Clinical Nutrition and Metabolism) guidelines
[2]: target energy and protein intake is 20–25 kcal/kg/day and 1.3 g/kg/day delivered progres-
sively, respectively. During the first week, trophic EN is permitted. Actual body weight is used
for patients with BMI �25 kg/m2 and adjusted body weight for BMI >25 kg/m2.
Other recommendations discussed were interruptions to EN should be avoided. It is rec-
ommended that stopping feeding to evaluate oral tolerance should be limited to once daily at
the most. In addition, gastric residual volume < 500 mL indicates EN tolerance. Finally, insu-
lin therapy should be used to control blood glucose levels and blood glucose levels should be
maintained at <180 mg/dL.
Variables.
Independent variables related to the patient’s baseline condition as well as hos-
pital admission variables were collected. Specifically, age, gender, and BMI, diagnosis on
admission, Barthel and Charlson index, and APACHE II scores were collected. All parameters
were collected from the medical records by a collaborating research nurse. See supplementary
material for definitions and classifications.
The principal dependent variable that was collected was presence of ICUAW according to
MRC sum-score, conducted by a physiotherapist. Secondary dependent variables were energy
and protein intake via EN, level of mobility, continuous renal replacement therapy (CRRT),
airway management, ICUAW-related drugs, vasopressors, moderate and severe
hyperglycaemia.
All these variables were collected during days 3–7 of ICU stay: ICU Mobility Scale (IMS)
score. IMS is a 10-point scale ranges from 0 (patient immobile lying in bed) to 10 (independent
ambulation). The IMS was categorized using a binary system (where <4 represents, in-bed
activities, and �4, active out-of-bed mobilization); Days on which the patient requires CRRT;
Type of airway management (invasive mechanical ventilation (IMV) or no IMV); ICUAW-
related drugs, understood as cumulative doses of drugs such as neuromuscular blocking
agents, steroids [methylprednisolone, dexamethasone, and hydrocortisone in mg equivalent
dose] and aminoglycosides; Administered doses of Vasopressors (epinephrine, adrenaline,
noradrenaline, dopamine and dobutamine). In both cases above intravenous administration is
considered (continuous infusion, stat dose, and bolus injection on demand); Moderate (gly-
caemia >181 and �215 mg/dl) or severe hyperglycaemia (�216 mg/dl) of the total blood glu-
cose results on day 3–7 of the ICU stay multiplied by 100.
A team of trained professionals recorded the variables. Detailed of the measurement tools
are provided in the S1 File.
Data analysis. Categorical variables were expressed as frequency and percentage, using
Fisher or Chi-squared test for comparison between groups. Quantitative variables were
expressed as mean and standard deviation (SD) or median and interquartile range (IQR), and
groups were compared using Student-t or Mann–Whitney U test. To study the correlation
between quantitative variables (actual body weight, energy and protein intake), Pearson or
Spearman was used. A multivariate analysis was used to investigate the association between
EN during 3–7 days of the ICU stay (energy and protein administration, days with overfeeding
and days with protein >0.8 g/kg/day) and ICUAW, also controlling other explanatory vari-
ables: baseline variables (age, gender, BMI, Barthel and Charlson scores) and those related to
ICUAW onset during days 3–7 of the ICU stay (days with CCRT, doses of ICUAW-related
PLOS ONE | https://doi.org/10.1371/journal.pone.0286598 June 7, 2023
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PLOS ONEEnteral nutrition management and ICUAW
drugs and vasopressors, days with IMS �4, and days with moderate and severe hyperglycae-
mia). Data were analysed using SPSS 25.0.
Results
We analysed 319 patients, corresponding to 1595 EN days and 69 ICUs in our country (Fig 1).
The incidence of ICUAW was 68.9% (153/222 patients; 95% CI [62.5%-74.7%]). In 30.4%
(97/319 patients; 95% CI [25.6%-35.7%]), the MRC assessment was unfeasible. Among the
patients with ICUAW, females were at higher risk than males and the most prevalent diagnosis
was sepsis. Patients with ICUAW had higher rates of comorbidity (Charlson), were more
dependent (Barthel) and had greater disease severity (APACHE), but these results were not
statistically significant (Table 1). More overweight and, conversely, fewer obese patients devel-
oped ICUAW (p<0.05 in both cases) (Table 1).
On ICU days 3–7, although the general cohort had low active mobility out of bed (IMS�4),
ICUAW patients had significantly lower values during this period (p<0.001) (Table 2). In
addition, ICUAW patients received significantly more vasopressors (p = 0.029) and had more
days of IMV (p = 0.032). No significant differences were found in median days of CRRT, of
severe or moderate hyperglycaemia, or of administration of ICUAW-related drugs (Table 2)
in the same period. Of the patients who developed ICUAW, 67.4% (273/405 patient/days) had
deep sedation (RASS-3-5) vs 47.7% (83/174 patient/days) of those who did not have ICUAW.
Patients in whom ICUAW could not be assessed had significantly more days of deep sedation
than those in whom ICUAW could be assessed (87.3% (261/299) vs 61.5% (356/579);
p<0.001).
Energy intake during days 3–7 was similar among patients who did and did not develop
ICUAW, independently of which guidelines were followed (ASPEN or ESPEN). Likewise, no
differences were observed in the percentage of patients with overfeeding or number of days of
overfeeding when comparing patients with and without ICUAW (Table 3). Patients receiving
propofol had a median energy intake of 188.0 kcal/day [62.7–380.6 kcal/day] over a total of
727 patient/days.
With regard to protein intake during days 3–7 in all groups, independently of which guide-
lines were followed (ASPEN or ESPEN), no differences were observed between number of
days with >0.8 g/kg/day and ICUAW onset (Table 3).
Median protein intake was below 0.8 g/kg/day, and the median in obese patients with
ICUAW (as per ASPEN guidelines) was closer to the recommended value, at 0.77 g/kg/day
[0.48–0.99] (Table 3).
The logistic regression analysis for ICU days 3–7 showed no effect of energy or protein
intake on the onset of ICUAW. Neither could ICUAW be explained by the increase in days
with overfeeding (S2 File). The days with overfeeding on ICU days 3–7 showed an OR of 1.085
[0.934–1.261]; p = 0.286. This remained after adjusting for baseline variables (OR: 1.109
[0.948–1.296]; p = 0.197). The results were similar after adjusting for ICU stay variables (OR:
1.106 [0.945–1.294]; 0.209) and after adjusting for all variables (baseline and ICU stay vari-
ables) (OR:1.128 [0.956–1.332]; p = 0.154).
Median daily energy intake was close to recommended levels during the first week of the
ICU stay, except for obese patients, who were found to receive slightly above the recom-
mended energy intake levels according to the US guidelines (S1 Fig).
The degree of compliance with energy intake depends on which recommendations are con-
sidered. Overfeeding was observed according to US and European guidelines. Using the US
guidelines and considering patients with BMI <30kg/m2, overfeeding was found on 12.5%
patients/day; 95% CI [10.8%-14.5%] whereas for patients with BMI �30kg/m2 the rate was
PLOS ONE | https://doi.org/10.1371/journal.pone.0286598 June 7, 2023
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PLOS ONEEnteral nutrition management and ICUAW
Fig 1. Flow diagram showing patients’ movement through the study.
https://doi.org/10.1371/journal.pone.0286598.g001
42.9% patients/day; 95% CI [38.0%-48.0%] and according to the European guidelines the rate
was 43.1% patients/day; 95% CI [40.7%-45.5%] (S1 Table).
Median protein administration was low throughout days 3–7 of the ICU stay according to
US and European guidelines (S1 Fig and S1 Table). A third of patients received less than 0.5 g/
kg/day of protein during the first week (S1 Table).
A high percentage of patient-days showed glycaemia <180mg/dl. Patients without ICUAW
had a significantly higher proportion of patient-days with glycaemia <180 mg/dl (p = 0.006)
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PLOS ONEEnteral nutrition management and ICUAW
Table 1. General characteristics of the study population.
Patients with EN who developed ICUAW n = 153
(48.0%)
Patients with EN who did NOT develop ICUAW n = 69
(21.6%)
Female
Sepsis
Trauma
Neurosurgery
Cardiovascular
surgery
Other surgeries
Overdose
Other medical
patients
Underweight
Normal
Overweight
Obese
Gender
Age, years
Dx. on
Admission
BMI (kg/m2)
BMI
Barthel
Charlson index
APACHE IIa
52 (34.0%)
68.0 [55.0–76.0]
32 (20.9%)
10 (6.5%)
4 (2.6%)
15 (9.8%)
18 (11.8%)
3 (2.0%)
71 (46.4%)
27.1 [24.3–30.3]
2 (1.3%)
46 (30.1%)
66 (43.1%)
39 (25.5%)
100 [95–100]
5.0 [2.0–7.0]
23 [18–28]
14 (20.3%)
63.0 [47.5–74.5]
9 (13.0%)
7 (10.1%)
0 (0%)
4 (5.8%)
8 (11.6%)
2 (2.9%)
39 (56.5%)
26.7 [24.0–30.8]
0 (0.0%)
23 (33.3%)
24 (34.8%)
22 (31.9%)
100 [95–100]
4.0 [1.0–6.0]
21 [16–27]
p value
0.041
0.079
<0.001
0.467
-
0.012
0.050
0.655
0.002
0.752
-
0.006
<0.001
0.030
0.933
0.247
0.537
EN: enteral nutrition; ICUAW: intensive care unit-acquired muscle weakness; n: sample; %: percentage; BMI: body mass index; MRC: Medical Research Council scale.
aAPACHE II (assessed in 45 patients without ICUAW, 67 with ICUAW and 59 with missing ICUAW data). Categorical variables are expressed as frequency and
percentage (n (%)) and quantitative variables with non-normal distribution as median [25th -75th percentile]
https://doi.org/10.1371/journal.pone.0286598.t001
(S2 Table). On most patient/days there was one or zero interruptions or pauses in EN. Gastric
residual volume (GRV) was <500 ml on most patient-days. No differences were found
between patients with or without ICUAW for glycaemia or GRV (S2 Table).
A weak correlation was found between patients’ actual body weight and energy (kcal/kg/
day) (r = -0.121; p<0.031) and proteins (g/kg/day) (r = -0.112; p<0.045) delivery.
Table 2. ICU variables, by ICUAW onset on days 3–7 of ICU stay.
Days with IMS�4
Days with CRRT
Days according to Airway
management
• IMV
• no IMV
ICUAW-related drugs (mg)
Vasopressors (mg)
Moderate hyperglycaemia (rate)
Severe hyperglycaemia (rate)
Patients with EN who developed ICUAW n = 153
(48.0%)
Patients with EN who did NOT develop ICUAW n = 69
(21.6%)
0.0 [0.0–0.0] / 0.02±0.14
0.0 [0–0]
0.0 [0.0–0.0] / 0.12±0.44
0.0 [0–0]
5.0 [5.0–5.0]
0.0 [0.0–0.0]
60.0 [0.0–307.4]
24.2 [0.0–123.5]
5.6 [0.0–20.9]
0.0 [0.0–18.0]
5.0 [4.0–5.0]
0.0 [0.0–1.0]
0.0 [0.0–240.0]
7.7 [0.0–39.2]
5.9 [0.0–17.7]
0.0 [0.0–11.5]
p value
<0.001
0.327
0.032
0.028
0.111
0.029
0.386
0.223
EN: enteral nutrition; ICUAW: intensive care unit-acquired muscle weakness; n: sample; %: percentage; IMV: invasive mechanical ventilation; IMS: ICU mobility scale;
CRRT: continuous renal replacement therapy; SD: standard deviation. Quantitative variables are expressed as median [25th -75th percentile] or mean and SD.
https://doi.org/10.1371/journal.pone.0286598.t002
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PLOS ONETable 3. Energy and protein intake via EN and association with ICUAW on ICU days 3–7.
Patients with EN who developed ICUAW n = 153
(48.0%)
Patients with EN who did NOT develop ICUAW n = 69
(21.6%)
p value
Enteral nutrition management and ICUAW
Energy. 2016 ASPEN guidelines
Energy (BMI <30 kg/m2)
Overfeeding (BMI <30 kg/m2)n (%)
Days with overfeeding (BMI <30 kg/m2)
Energy (BMI >30 kg/m2)
Overfeeding (BMI >30 kg/m2)n (%)
Days with overfeeding (BMI >30 kg/m2)
Energy. 2019 ESPEN guidelines
Energy
Overfeeding n (%)
Days with overfeeding
Protein. 2016 ASPEN guidelines
Prot (BMI <30 kg/m2)
Days with protein >0.8 g/kg/day (BMI <30
kg/m2)
Protein (BMI >30 kg/m2)
Days with protein >0.8 g/kg/day (BMI >30
kg/m2)
Protein. 2019 ESPEN guidelines
Protein
Days with protein >0.8 g/kg/day
16.8 [10.6–22.4]
10 (8.8%)
0.0 / [0.0–0.0]
12.6 [9.1–18.3]
12 (30.8%)
2.0 [0.0–4.0]
16.7 [10.7–23.0]
62 (40.5%)
2 [0–4]
0.65 [0.46–0.89]
1.0 [0.0–3.3]
0.77 [0.48–0.99]
3.0 [0.0–4.0]
0.69 [0.46–0.92]
2 [0–4]
16.2 [11.1–22.5]
5 (10.6%)
0.0 / [0.0–0.0]
12.0 [9.7–17.2]
8 (36.4%)
1.5 [0.0–3.0]
15.9 [12.0–22.4]
26 (37.7%)
1 [0–3]
0.69 [0.44–0.91]
2.0 [0.0–4.0]
0.69 [0.52–0.87]
1.0 [0.0–3.0]
0.68 [0.46–0.91]
2 [0–4]
0.899
0.555
0.998
0.988
0.778
0.787
0.916
0.767
0.330
0.873
0.598
0.409
0.238
0.793
0.654
EN: enteral nutrition; ICUAW: intensive care unit-acquired muscle weakness; n: sample; %: percentage. Energy is calculated as Kcal/Kg/day; protein as g/Kg/day.
Categorical variables are expressed as frequency and percentage (n (%)) and quantitative variables as median [25th -75th percentile].
https://doi.org/10.1371/journal.pone.0286598.t003
Discussion
The 68.9% incidence of ICUAW found in this study was higher than the 40% incidence (95%
CI [38–42]) reported in a systematic review [16]. However, the percentage of patients without
MRC assessment during the ICU stay was similar (26% IC 95% [16, 24–28]). Missing ICUAW
data is explained by the patients in whom it was impossible to perform MRC due to insuffi-
cient awakening and comprehension (97/97 [100%] patients), which in itself is considered a
factor that hinders early mobilization [9, 16, 17].
As in other studies, ICUAW was found predominantly in females and patients with sepsis
[18]. We found an association between overweight and ICUAW onset, although the opposite
occurred in the case of obesity. A study conducted in obese and non-obese septic mice [19]
found that sepsis reduced body weight similarly in both groups, but there was attenuated mus-
cle wasting and weakness in the obese mice. This is known as the ‘obesity paradox’.
Mechanical ventilation can lead to a daily muscle loss of 1–2% [12]. In addition, the side
effects of inappropriate nutrition support include hyperglycaemia, muscle loss, prolonged
weaning from MV, and delayed rehabilitation [1, 5].
A review suggests that EN support alone is insufficient to reduce early muscle catabolism,
proposing a combination of early mobilization and optimal rehabilitation [20]. A current
study, investigated the association between these variables, finding that high protein intake
and early mobilization preserves muscle mass [21]. Similarly, other study conducted a clinical
trial with three groups (early mobilization, early mobilization and enteral nutrition protocol
based on ESPEN guidelines, and control group), and found an improvement in ICUAW in
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PLOS ONEEnteral nutrition management and ICUAW
both intervention groups versus the control group [10]. Despite this, there was little difference
between the intervention groups, except for the improvement in muscle strength found in the
enteral nutrition and early mobilization group versus only early mobilization. The patients in
our study had deficient protein intake and few achieved early active mobility. According to
Hermans [22], higher levels of patient mobilization are achieved when physiotherapists lead
mobilization decisions. Our study found that despite the low active mobility on days 3–7, low
mobility is associated with the onset of ICUAW (p = 0.018).
We found no differences in drug administration (neuromuscular blocking agents, steroids
and aminoglycosides) with regard to ICUAW, as corroborated by other recent study [9] except
for the administration of vasopressors, which was also described by other authors [23]. Our
study population was found a high percentage of overfeeding, but insufficient protein intake.
Despite there being some controversy, some authors suggest that protein deficiency may lead
to muscle deterioration and risk for ICUAW [21, 24], yet we found no differences in protein
intake between patients who developed ICUAW or not. This finding may, however, be due to
generalized low protein delivery [25]. In our case, a third of patient/days were below 0.5 g pro-
tein/kg/day, which is defined in the European guidelines as a low protein diet. Therefore,
according to our results, the onset of ICUAW appears to be unrelated to protein intake,
because although protein intake was low in most patients, some of those patients did not
develop ICUAW.
ICUAW-related guideline recommendations: Energy and protein
administration
A high percentage of patients received trophic EN or were below 80% of the US recommenda-
tions [15] for target energy during the first week. However, current evidence and the European
recommendations [2], along with the most recent US guideline update [26], show a tendency
towards lower energy intake during this period. Arabi et al. [27], noted that anorexia is a com-
mon characteristic of critically ill patients. However, during the acute phase, full nutrient pro-
vision can be detrimental because it inhibits autophagy, giving cause for concern considering
that in our study, overfeeding, defined as “energy administration of 110% above the defined
target” [2], was found on almost half of patient/days (43.1%), applying the European recom-
mendations, and in 42.9% of obese patients, applying the 2016 US recommendations. Despite
this, in our study we have not been able to establish overfeeding as defined in the European
guidelines as an explanation for ICUAW.
Although some authors question the optimal amount of protein to deliver, most agree that
early initiation is more important than energy provision [28].
According to both guidelines, protein intake was insufficient in most patients in our sam-
ple. Cahill et al. [29] audited 20 countries to evaluate protein support and concluded that only
2.5% of hospitals achieved >80% of the protein target. Similarly, more recent studies have
found below-target protein intake, specifically 52% (±30%) of the prescribed goal [30] and 10–
12% of the total calorie intake, instead of 24–32% [31]. Furthermore, although our study had
few patients with CRRT patient/days, ongoing use of these therapies may reduce the protein
available for muscle formation [15, 32], and this would worsen protein intake deficiency.
Several studies have described various barriers to delivering the nutritional target in criti-
cally ill patients. A study identified three factors involved in compliance, which are related to
patient, clinical, and site-specific considerations [30]. A Canadian study reported on a nutri-
tional improvement programme in the ICU whereby patients attained over 80% of recom-
mended target energy and protein intakes [33]. This success was attributed to 1) Presence of
registered dietitians in the ICU; 2) Education of the clinical team regarding the need for good
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PLOS ONEEnteral nutrition management and ICUAW
nutritional practice; 3) Encouragement of a culture of interest in bedside nutritional care
among all ICU staff. Furthermore, in our results, a weak inverse correlation between weight
and kcal/kg/day and proteins/kg/day may suggest that EN was administered through a stan-
dard regimen, regardless of weight or patient state. Similar results were reported in a study
conducted in 46 countries over 7 years, observing that patients were undernourished because
EN was not guided by weight or disease status [34].
Furthermore, Peterson et al. [35] found that no enteral product is able to provide adequate
protein intake without excess calorie intake. This observation is important because the current
trend is for permissive underfeeding [36]. McCall et al. [33] reported that they increased pro-
tein intake by delivering additional protein in powder boluses. Other authors have proposed
the use of parenteral amino acids [37], although this route of administration appears to result
in lower protein availability (83% vs complete protein) [38].
Other recommendations related to ICUAW prevention
Patients with ICUAW had fewer patient-days with glycaemia <180 mg/dl. Although hypergly-
caemia was not found to be a risk factor for developing ICUAW in this study, other authors
have found an independent relationship between ICUAW and more than 3 days of hypergly-
caemia [22, 39].
EN cessation was observed on a third of patient/days, which contrasts with few patient/days
with GRV >500 ml and a high energy vs poor protein intake. Unlike other authors’ findings
[40], this study appears to show that EN cessations are not the main cause of inappropriate
nutrition support.
In view of various authors’ findings and ours, it seems reasonable to combine various
actions, including the use of up-to-date EN protocols in all ICUs and the presence in ICUs of
professionals trained in critical care nutrition, and with ICU care team members trained and
motivated to provide early mobilization, who can monitor patients throughout their stay and
be involved in discharge plans [6, 41–43].
Limitations
The lack of a cohort of patients attaining protein goals limits the results on a potential associa-
tion with ICUAW. Measuring ICUAW by means of the MRC scale requires patients’ coopera-
tion, which may have caused a delay or absence in diagnosing ICUAW. The patient’s actual
weight was only recorded on admission and at no other time during the patient’s stay. Like
other authors, we found a lack of reliable instruments available in the ICUs to measure body
weight. In addition, weight estimation is hard because of fluid loss and gain, and changes in
lean tissue mass [44]. Patients’ actual energy requirements could not be measured due to a gen-
eralised absence of indirect calorimetry techniques in the ICUs. Instead, energy requirements
were estimated following the general recommendations in international guidelines. Finally,
use of parenteral nutrition alone or in combination with EN was not investigated, and some
authors have found that parenteral nutrition is detrimental in ICUAW prevention [14, 18].
Implications and recommendations for practice
This study highlights the need for better adherence to international enteral feeding guidelines
among patients admitted to the ICU. The existence of low mobility, high incidence of ICUAW
and low protein intake suggest a need to continue future research to further inform the nutri-
tion–early mobilization binomial, which has recently been observed for the first time. Such an
approach–considering nutrition as a priority but never alone–will enable us to overcome the
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PLOS ONEEnteral nutrition management and ICUAW
undesirable effects of ICUAW. We believe that it is necessary to train, update and involve ICU
professionals in nutritional care and the need for early mobilization of ICU patients.
Conclusions
The incidence of ICUAW was high in patients receiving EN for at least one week.
Early mobilization is associated with lower incidence of ICUAW. Energy and protein intake
alone was insufficient to explain the onset of ICUAW. The influence of protein intake on
ICUAW was unclear, because significant protein deficiency was found in almost all patients
throughout days 3–7 of the ICU stay. Although overfeeding was a common finding in this
patient population, we were unable to confirm an association between overfeeding and
ICUAW onset. Despite adequate compliance with some recommendations, a high percentage
of patients were malnourished according to the guidelines.
Future studies are needed in which early mobilization is more widely implemented and
nutritional requirements are calculated according to individual patients’ baseline situation and
clinical condition, thereby permitting further investigation of the onset of ICUAW in critically
ill patients.
Supporting information
S1 File. Measurement tools.
(PDF)
S2 File. Logistic regression.
(PDF)
S1 Fig. Daily energy and protein intake via enteral nutrition, measured by kg of body
weight and day.
(TIF)
S1 Table. Energy and protein intake by target recommendation.
(PDF)
S2 Table. Other recommendations for enteral nutrition on ICU days 3 to 7.
(PDF)
Acknowledgments
To Sociedad Española de Enfermerı´a Intensiva y Unidades Coronarias (SEEIUC) who pro-
moted the study.
To the MoviPre Group who collaborated in the data collection.
Author Contributions
Conceptualization: Ignacio Zaragoza-Garcı´a, Susana Arias-Rivera, Marı´a Jesu´s Frade-Mera,
Joan Daniel Martı´, Elisabet Gallart, Alicia San Jose´-Arribas, Tamara Raquel Velasco-Sanz,
Eva Blazquez-Martı´nez, Marta Raurell-Torredà.
Data curation: Ignacio Zaragoza-Garcı´a, Susana Arias-Rivera, Marı´a Jesu´s Frade-Mera, Joan
Daniel Martı´, Elisabet Gallart, Alicia San Jose´-Arribas, Tamara Raquel Velasco-Sanz, Eva
Blazquez-Martı´nez, Marta Raurell-Torredà.
PLOS ONE | https://doi.org/10.1371/journal.pone.0286598 June 7, 2023
11 / 14
PLOS ONEEnteral nutrition management and ICUAW
Formal analysis: Ignacio Zaragoza-Garcı´a, Susana Arias-Rivera, Marı´a Jesu´s Frade-Mera, Ali-
cia San Jose´-Arribas, Tamara Raquel Velasco-Sanz, Eva Blazquez-Martı´nez, Marta Raurell-
Torredà.
Funding acquisition: Alicia San Jose´-Arribas, Marta Raurell-Torredà.
Investigation: Ignacio Zaragoza-Garcı´a, Susana Arias-Rivera, Marı´a Jesu´s Frade-Mera, Alicia
San Jose´-Arribas.
Methodology: Ignacio Zaragoza-Garcı´a, Marı´a Jesu´s Frade-Mera, Joan Daniel Martı´, Elisabet
Gallart, Eva Blazquez-Martı´nez, Marta Raurell-Torredà.
Supervision: Susana Arias-Rivera, Marı´a Jesu´s Frade-Mera, Elisabet Gallart, Alicia San Jose´-
Arribas, Marta Raurell-Torredà.
Writing – original draft: Ignacio Zaragoza-Garcı´a, Marta Raurell-Torredà.
Writing – review & editing: Ignacio Zaragoza-Garcı´a, Susana Arias-Rivera, Marı´a Jesu´s
Frade-Mera, Joan Daniel Martı´, Elisabet Gallart, Alicia San Jose´-Arribas, Tamara Raquel
Velasco-Sanz, Eva Blazquez-Martı´nez, Marta Raurell-Torredà.
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10.1093_cvr_cvab085.pdf
|
Data availability
The data underlying this article will be shared on reasonable request to the
corresponding author.
|
Data availability The data underlying this article will be shared on reasonable request to the corresponding author.
|
Cardiovascular Research (2022) 118, 883–896
doi:10.1093/cvr/cvab085
Targeting angiotensin type-2 receptors located on
pressor neurons in the nucleus of the solitary tract
to relieve hypertension in mice
1, Dominique N. Johnson
Mazher Mohammed
Wanhui Sheng
U. Muscha Steckelings7, Karen A. Scott1, Charles J. Frazier
Eric G. Krause1,8,9, and Annette D. de Kloet2,8,9*
1, Eliot A. Spector
1, Khalid Elsaafien
2, Lei A. Wang1, Scott W. Harden
1,
1, Michael Bader3,4,5,6,
1,8,9, Colin Sumners
2,8,9,
1Department of Pharmacodynamics, University of Florida College of Pharmacy, 1345 Center Dr. JHMHC Room P1-20, Gainesville, FL 32610, USA; 2Department of Physiology and
Functional Genomics, University of Florida College of Medicine, 1345 Center Drive, Room M552, Gainesville, FL 32610-0274, USA; 3Max-Delbru¨ck Center for Molecular Medicine
(MDC), Robert-Ro¨ ssle-Str. 10, 13125 Berlin-Buch, Germany; 4University of Lu¨beck, Institute for Biology, Ratzeburger Allee 160, 23562 Lu¨beck, Germany; 5Charite´ University Medicine,
Charite´platz 1, 10117 Berlin, Germany; 6German Center for Cardiovascular Research DZHK-Gescha¨ftsstelle, Potsdamer Str. 58, 10785 Berlin, Germany; 7Department of Cardiovascular
& Renal Research, Institute of Molecular Medicine, University of Southern Denmark, J.B. Winsløws Vej 21-25, Odense C - DK-5000, Denmark; 8Center for Integrative Cardiovascular and
Metabolic Diseases, University of Florida, 1149 Newell Dr. Gainesville, FL 32610, USA; and 9Evelyn F. and William L. McKnight Brain Institute, University of Florida, 1149 Newell Dr.
Gainesville, FL 32610L, USA
Received 17 July 2020; editorial decision 7 March 2021; accepted 10 March 2021; online publish-ahead-of-print 16 March 2021
Time for primary review: 27 days
Aims
These studies evaluate whether angiotensin type-2 receptors (AT2Rs) that are expressed on c-aminobutyric acid (GABA)
neurons in the nucleus of the solitary tract (NTS) represent a novel endogenous blood pressure-lowering mechanism.
....................................................................................................................................................................................................
Methods and
Experiments combined advanced genetic and neuroanatomical techniques, pharmacology, electrophysiology, and
results
optogenetics in mice to define the structure and cardiovascular-related function of NTS neurons that contain
AT2R. Using mice with Cre-recombinase directed to the AT2R gene, we discovered that optogenetic stimulation
of AT2R-expressing neurons in the NTS increases GABA release and blood pressure. To evaluate the role of the
receptor, per se, in cardiovascular regulation, we chronically delivered C21, a selective AT2R agonist, into the brains
of normotensive mice and found that central AT2R activation reduces GABA-related gene expression and blunts
the pressor responses induced by optogenetic excitation of NTS AT2R neurons. Next, using in situ hybridization,
we found that the levels of Agtr2 mRNAs in GABAergic NTS neurons rise during experimentally induced hyperten-
sion, and we hypothesized that this increased expression may be exploited to ameliorate the disease. Consistent
with this, final experiments revealed that central administration of C21 attenuates hypertension, an effect that is
abolished in mice lacking AT2R in GABAergic NTS neurons.
....................................................................................................................................................................................................
Conclusion
These studies unveil novel hindbrain circuits that maintain arterial blood pressure, and reveal a specific population
of AT2R that can be engaged to alleviate hypertension. The implication is that these discrete receptors may serve
as an access point for activating an endogenous depressor circuit.
....................................................................................................................................................................................................
* Corresponding author. Tel: þ352 294 8490, fax: þ352-846-0270; E-mail: adekloet@ufl.edu
Published on behalf of the European Society of Cardiology. All rights reserved. VC The Author(s) 2021. For permissions, please email: [email protected].
884
Graphical Abstract
M. Mohammed et al.
Keywords
...................................................................................................................................................................
Baroreflex (cid:129) GABA (cid:129) Blood pressure (cid:129) Hindbrain (cid:129) RAS
1. Introduction
Hypertension is the leading risk factor for the development of cardiovas-
cular disease and stroke, which are the first and fifth leading causes of
death in the USA.1 Despite lifestyle changes and multi-drug based thera-
pies, nearly 20% of all hypertensive patients in the USA remain with high
blood pressure.2 The brain controls fluid consumption, neuroendocrine
secretion, and autonomic responses that maintain blood pressure and
osmolality at
these
responses is requisite for the blood volume expansion and/or vasocon-
striction that promotes hypertension, and as such, it has been proposed
that the brain is complicit in the aetiology of the disease.3 Accordingly,
understanding the neuronal plasticity that underlies the onset of hyper-
tension may provide insight towards new and effective treatments.
for survival. Dysregulation of
levels optimal
The brain senses increased arterial pressure via baroreceptors that trans-
duce stretch exerted on the vasculature into neural signals that excite sec-
ond order neurons in the nucleus of the solitary tract (NTS). These second
order neurons integrate baroreceptor inputs with other neural, humoral,
and chemosensory signals, and subsequently, reduce cardiac output and vas-
cular resistance to lower blood pressure.4,5 Collectively, this is known as
the baroreflex. Chronically elevated blood pressure, as with hypertension,
requires its resetting such that higher pressures are maintained.6 The activity
of second order neurons is modulated by c-aminobutyric acid (GABA) neu-
rons also localized to the NTS and experimentally induced hypertension is
associated with enhanced GABAergic signalling that causes baroreflex re-
setting and increased blood pressure.7–9 The inference is that GABA actions
...............................................................................
within the NTS contribute to the onset of hypertension; however, GABA
and its receptors are poor therapeutic targets because they are ubiquitous
in the CNS. Consequently, characterization of discrete NTS neurons that
influence arterial blood pressure may allow selective manipulation of
GABAergic neurons to relieve hypertension.
The intriguing presence of the angiotensin type-2 receptor (AT2R) on
a subset of GABAergic neurons in the NTS,10 in conjunction with
reports that central AT2R stimulation attenuates experimentally induced
hypertension in rodents,11,12 led us to hypothesize that the AT2R is a
phenotypic marker for GABAergic neurons in the NTS whose function
is coupled to the development of hypertension. Here, we implemented
an integrated circuit mapping approach to characterize the structure and
cardiovascular-related function of neurons in the NTS that express
AT2Rs. First, we generated a novel mouse line with the expression of
Cre-recombinase directed to the AT2R gene (Agtr2), which we used in
experiments designed to determine the impact of optogenetic manipula-
tion of neurons in the NTS that synthesize AT2R on GABA release and
arterial blood pressure. To evaluate the role of the receptor, per se, in
these processes, we chronically delivered Compound 21 (C21), a selec-
tive AT2R agonist,13 into the brains of normotensive and hypertensive
mice and assessed indices of GABA synthesis in the NTS, as well as car-
diovascular responses to optogenetic stimulation. Final experiments ex-
amined the necessity of AT2Rs on GABAergic neurons in the NTS for
the antihypertensive effects of C21. Taken together, our results suggest
that neurons within the NTS that express AT2Rs can be targeted to re-
verse the development of hypertension.
Angiotensin type-2 receptors on pressor neurons
885
2. Methods
Full details of Section 2 are described in the Supplementary material
online.
2.1 Animals
Studies were conducted in adult male wild-type C57BL/6J-, AT2R-eGFP
reporter- (Mutant Mouse Resource and Research Centers), Agtr2Cre/y
(AT2R-Cre)- or Agtr2loxp/y
(AT2R-flox; Welcome Trust Sanger
Institute) x ROSA-stop-flox-tdTomato (Ai9; Jackson Laboratory Stock #
007909) mice that were 10–12 weeks old at the initiation of the experi-
ments and were approved by the University of Florida Animal Care and
Use committee. The principles governing the care and treatment of ani-
mals, as stated in the Guide for the Care and Use of Laboratory Animals
published by the National Academy of Sciences (eighth ed., 2011), were
followed at all times during this study.
2.2 Anaesthesia and analgesia
For all surgical procedures, anaesthesia was induced using 100% oxygen/
4% isoflurane, USP (Patterson Veterinary; 1.5 L/min), and was maintained
throughout the surgeries by the administration of 100% oxygen/1.5–2%
isoflurane (1.5 L/min). During these surgeries/procedures, the level of
anaesthesia was monitored by checking the eye blink reflex and a reac-
tion to paw pinch, and was adjusted if necessary. For recovery surgery,
buprenorphine (0.1 mg/kg, sc) was administered immediately prior to
the surgical procedure and also during the post-surgical recovery period
(every 12 h for 48 h).
2.3 Euthanasia
For euthanasia, mice were administered a lethal dose of ketamine (ip,
10 mg/0.1 mL, KetaVed), followed either by decapitation or by transcar-
dial perfusion with isotonic saline and 4% paraformaldehyde.
2.4 Administration of viral vectors
Transfer of light-sensitive channel-2 rhodopsin (ChR2) or the control
eYFP into AT2R-containing NTS neurons was achieved through stereo-
taxic microinjection of Cre-dependent AAVs into AT2R-Cre mice.
Deletion of AT2R from neurons within the dorsal vagal complex (DVC)
was achieved by stereotaxic microinjection of AAV8-VGATp-iCre into
AT2R-flox x Ai9 mice; controls expressed only the Ai9 gene.
2.5 In vitro patch-clamp electrophysiology
Horizontal brain slices (300 mm) were prepared using a vibratome and
maintained in a recording chamber. Experiments were performed using
a MultiClamp 700B amplifier paired with a Digidata 1440 A digitizer
(Axon Instruments) and pCLAMP 10 software. Real-time and offline
analysis of electrophysiological data were performed using custom soft-
ware. Optogenetic activation evoked synaptic currents were recorded
from ChR2 negative NTS and dorsal motor nucleus of the vagus
(DMNX) neurons in the presence of bath applied ionotropic glutmate
receptor antagonists [6,7-dinitroquinoxaline-2,3-dion (DNQX; 20 mM)
and (2 R)-amino-5-phosphonovaleric acid; (2 R)-amino-5-phosphono-
pentanoate (AP5; 40 mM)], and using a high chloride K-gluconate internal
solution, were considered likely to be mediated by synaptic activation of
GABAergic receptors. A subset of such responses (n=7) was challenged
with bath application of the GABA receptor antagonists picrotoxin
(PTX; 100 mM) and CGP55845 (10 mM).
...................................................................................................................................................................................
2.6 Millar catheter cardiovascular
recording
In anaesthetized mice, blood pressure and heart rate were measured us-
ing a Millar catheter (Model SPR1000) implanted into the aortic arch and
connected to a PowerLab signal transduction unit (AD Instruments).
Data were analysed using Labchart8 software (AD Instruments).
2.7 Telemetric cardiovascular recording
Catheters of the radiotelemetry transmitters (PAC-10; Data Sciences
International) were implanted into the distal
left carotid artery.
Subsequently, the device itself was positioned subcutaneously in the left
flank region. Upon completion of the surgical procedure, mice recov-
ered for 2 weeks before initiating cardiovascular recordings that were
performed using DataQuest ART or Ponemah software (Data Sciences
International).
2.8 DOCA-Salt hypertension
Mice were rendered hypertensive via subcutaneous implantation of
100 mg DOCA pellets (Innovative Research of America) in the inter-
scapular region and subsequent ad libitum access to isotonic saline
drink.14
2.9 Chronic icv administration of C21
Some mice underwent stereotaxic surgery to implant osmotic mini-
pumps and brain infusion kits (Alzet, DURECT Corporation) to chroni-
cally deliver the selective AT2R agonist, C21 (7.5 ng/kg/h) or aCSF
vehicle into the lateral cerebral ventricle (icv). The dose of C21 used for
these studies is based on experience and on published studies that have
revealed selectivity of this non-peptide agonist for AT2R at comparable
doses.15,16 As seen in Supplementary Figure S2, this dose of C21 is indeed
AT2R-selective.
2.10 In vivo optogenetics
Mice injected with AAV-ChR2 and AAV-eYFP into NTS and/or implanted
with osmotic minipumps and brain infusion kits for the icv delivery of
C21 or saline were implanted with a Millar catheter and then fixed to the
stereotaxic frame. Fibre optic posts were positioned immediately dorsal
to the NTS. After a period of stable baseline recording, blue laser light
was turned on for 1 min at 1, 5, 10, and 15 Hz (20 ms pulses; 10 mW).
2.11 Tissue collection
For real-time (RT)–PCR studies, mice were euthanized and whole brains
were removed and flash frozen in dry ice-cooled isopentane for gene ex-
pression analysis. For in situ hybridization (ISH) and immunohistochemis-
try studies, mice were anaesthetized and perfused transcardially with
isotonic saline followed by 4% paraformaldehyde. Brains were then
post-fixed for 3–4 h, after which they were stored in 30% sucrose until
sectioning.
2.12 RNA isolation, cDNA synthesis, and
semi-quantitative RT–PCR
RNA extraction and DNase treatment were performed using RNeasy
columns
instructions.
Subsequently,
iScript (Bio-Rad) was used to synthesize cDNA from
200 ng of total RNA. Finally, gene expression was assessed by RT–PCR
using a StepOne RT–PCR system, TaqMan Gene Expression Master Mix,
and validated TaqMan probes (Applied Biosystems).
(Qiagen) according to the manufacturer’s
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M. Mohammed et al.
2.13 ISH (RNAscope)
Fluorescent RNAscope ISH studies were performed on brains collected
from reporter or AAV-injected mice as per the manufacturer’s instruc-
tions, using the same modifications that allow for visualization of mRNA
transcripts in the presence of preserved eGFP, eYFP, or tdTomato pro-
tein.10 Immunohistochemistry for reporter genes was conducted after
completion of ISH.
2.14 Immunohistochemistry
Immunohistochemistry was performed using standard procedures10
with Chicken IgY Anti-GFP (1:500) or anti-HuC/D (1:1000) as primary
antibodies.
2.15 Image capture and processing
Images were captured and processed using Axiovision 4.8.2 software
and a Zeiss AxioImager fluorescent Apotome microscope. For dual im-
munohistochemistry/RNAscope ISH, z-stacks of the proteins and tran-
scripts of
interest were captured at 20(cid:2) and 40(cid:2) magnifications
throughout the NTS identified using neuroanatomical landmarks found
in a mouse brain atlas.17
For immunohistochemistry studies, that did not incorporate ISH,
images were captured at lower magnification (5–10(cid:2)). Projection images
of the z-stacks were generated and all final figures were then prepared
using Adobe Photoshop, where brightness and contrast were adjusted
to provide optimal visualization.
2.16 Image analysis
The 20(cid:2) magnification z-stacks of ROIs were used for the manual deter-
mination of the percentage of eGFP or eYFP neurons that contain
...........................................................................................
mRNAs of interest and also to quantify the total Agtr2 mRNA within the
intermediate NTS using ImageJ. Importantly, reporter gene-expressing
neurons were considered to contain the mRNA if at least three visible
transcripts, defined as an individual punctate dot, were observed within
the volume of the eGFP, eYFP, or tdTomato fluorescence. Data are
reported as the percentage of eGFP, eYFP, or tdTomato cells that con-
tain the mRNA within the NTS. ImageJ was used to ascertain the average
integrated density of Agtr2 mRNA within the entire intermediate NTS.
Zen Lite image analysis software (Zeiss) was used to determine the aver-
age quantity of Agtr2-mRNA transcripts per AT2R-eGFP þ Gad1
mRNA-containing cell.
2.17 Statistics
Statistical analyses were performed using GraphPad Prism Software. In
all cases, statistical significance was set at P < 0.05. Specific details of the
tests performed are included in the Supplementary tables.
3. Results
3.1 Development and validation of an
AT2R-Cre knock-in mouse line
Prior anatomical studies from our group and others determined that the
NTS contains a discrete population of neurons that express AT2Rs.10,18
To ascertain the function of these neurons, we engineered mice with the
expression of Cre-recombinase directed to the Agtr2 gene (AT2R-Cre)
(Figure 1A). Initial studies used a Cre-inducible AAV to direct eYFP to the
cells in the NTS that express AT2R (Figure 1B). After allowing 3 weeks
for stable transfection of the virus, we conducted RNAscope ISH for
Figure 1 Development and validation of AT2R-Cre knock-in mice. (A) AT2R-Cre mouse line schematic. (B) Procedure used to generate experimental
subjects that express eYFP in AT2R cells of the NTS. (C) Representative image of AT2R-containing cells expressing eYFP (green); the neuronal marker,
HuC/D is depicted in red. AP, area postrema; 4v, fourth cerebral ventricle; cc, central canal. (D–F) Representative images of (D) eYFP, (E) Agtr2 mRNAs,
and (F) their co-localization in the NTS of an AT2R-Cre mouse injected with the Cre-dependent AAV-eYFP. (G) Quantification of co-localization of eYFP
and Agtr2-mRNA (n=4). Bars=SEM.
Angiotensin type-2 receptors on pressor neurons
887
Agtr2 mRNA and immunohistochemistry for eYFP protein to assess the
fidelity of Cre direction to AT2R-expressing cells. Figure 1C depicts the
location of virus transfected neurons in a representative coronal section
through the intermediate NTS. We determined that Agtr2 mRNA was
present in (cid:3)80% of eYFP-labelled NTS neurons that were surveyed, in-
dicating that Cre-recombinase activity faithfully follows Agtr2 mRNA ex-
pression within the NTS (536 of 670 neurons sampled from 4 mice)
(Figure 1D–G).
3.2 AT2R-expressing neurons in the NTS
increase arterial blood pressure and
release GABA onto post-synaptic neurons
To evaluate whether the activity of AT2R-containing neurons in the
NTS affects cardiovascular function, we used a Cre-inducible AAV to di-
rect the expression of the light-sensitive excitatory opsin, ChR2, and the
fluorophore, eYFP, to these cells (Figure 2A). Live brain slices through the
NTS were used for in vitro whole-cell patch-clamp recordings (Figure 2B
and C). All eYFP-expressing cells tested (n=9) demonstrated strong in-
ward currents in response to continuous blue light exposure in voltage-
clamp configuration (611.80±78.55 pA) and fired action potentials when
exposed to pulses of blue light
in current-clamp configuration
(Figure 2C). These results validate functional opsin expression within
AT2R-eYFP-expressing neurons.19
To test the hypothesis that the activity of neurons in the NTS that ex-
press the AT2R is coupled to changes in vascular resistance and cardiac
output, we injected AAV-eYFP or AAV-ChR2-eYFP into the NTS of
AT2R-Cre mice. Three weeks later, mice were anaesthetized and a
Millar catheter was inserted into the carotid artery. Mice were then
placed into a stereotaxic frame, a micro-craniotomy was performed and
a fibre optic connected to a laser-light source was positioned dorsal to
the NTS, thereby allowing for optogenetic stimulation during cardiovas-
cular recordings. In this way, we were able to assess the effects of intrin-
sic (i.e. non-AT2R-mediated) activation of the AT2R-expressing neurons
on blood pressure. Relative to control mice given AAV-eYFP, mice with
ChR2 directed to AT2R-expressing neurons exhibited significant eleva-
tions in blood pressure, heart rate, and heart rate variability (HRV) upon
exposure to blue light (10 Hz, 1 min) (Figure 2D) that persisted after the
cessation of optical stimulation. That is, there was significant effect of
AAV condition, time, and a time by condition interaction for the cardio-
vascular parameters assessed (see Supplementary Tables S1 and S2 for
ANOVA results and absolute values for cardiovascular parameters, re-
spectively). Interestingly, these elevations in blood pressure were fre-
quency-dependent (Figure 2E and Supplementary Table S3). These
results suggest that intrinsic excitation of neurons in the NTS that ex-
press AT2Rs is sufficient to elevate blood pressure, and based on the
HRV data, the mechanism includes sympathoexcitation.
We next conducted additional neuroanatomical and electrophysio-
logical experiments to better understand how excitation of AT2R-
expressing neurons in the NTS affects blood pressure and heart rate.
Initial experiments investigated the phenotype of neurons in the NTS
that express AT2R. To accomplish this, AT2R-Cre mice that had re-
ceived the AAV-ChR2-eYFP were perfused, and then their brains were
extracted and processed for eYFP immunohistochemistry with
RNAscope ISH for vesicular GABA transporter (VGAT). Figure 2F–H
depicts the co-localization of VGAT mRNAs to AT2R-eYFP cells within
images obtained from four mice
the NTS. Quantitative analysis of
revealed that (cid:3)83% of the AT2R-eYFP neurons in the NTS express
VGAT mRNAs (556 of 665 cells from 4 mice) (Figure 2I). These results
...................................................................................................................................................................................
are consistent with our published data, which demonstrate that AT2R-
expressing neurons in the NTS are by and large GABAergic.10 Further
examination of these images (Figure 2F–H; Supplementary Figure S1 for
additional representative images) reveals that although the majority of
AT2R-eYFP neurons express VGAT mRNA, not all neurons expressing
VGAT mRNAs also express AT2R-eYFP. The implication is that AT2Rs
are expressed on a subset of GABAergic neurons in the NTS.
Follow-up experiments evaluated the connectivity of neurons in the
NTS that express AT2R. AT2R-Cre mice were injected with AAV-ChR2-
eYFP into the NTS and, 3 weeks later, horizontal brain slices through the
NTS were used for in vitro whole-cell patch-clamp recordings. A combina-
tion of epifluorescence and differential interference contrast microscopy
was used to record from neurons that were in close proximity to eYFP-la-
belled fibres but that were themselves devoid of eYFP-labelling (Figure 2J–L).
These NTS neurons were voltage-clamped (-70 mV) in the presence of glu-
tamate receptor antagonists (DNQX and AP5) and subjected to a high
chloride internal solution. Notably, 18 of 19 non-eYFP-expressing NTS neu-
rons tested under these conditions demonstrated clear light-evoked inward
currents (Figure 2K), suggesting synaptic activation of GABAA receptors.
Consistent with this interpretation, light-evoked inward currents were ef-
fectively eliminated in all NTS neurons (3/3) directly challenged by bath ap-
plication of the GABA receptor antagonists (PTX and CGP55845,
P=0.001) (Figure 2L). Overall, these data indicate that AT2R-expressing
NTS neurons predominantly release GABA, and that they make synaptic
contacts with other NTS neurons.
A similar set of experiments was performed on neurons within the ad-
jacent DMNX that also did not express AT2R-eYFP and that were iden-
tified by their large soma and location proximal to the caudal end of the
fourth ventricle. Remarkably, 19 out of 19 DMNX motoneurons were
also responsive to optogenetic stimulation identical to that used to test
NTS neurons (Figure 2M–O). Also similar to results in the NTS, we found
that light-evoked responses in putative DMNX neurons were effectively
eliminated by bath application of GABAergic receptor antagonists in
four of five cells tested (P=0.0001, n=4, Figure 2O). Collectively, these
results establish that within the NTS, AT2Rs are expressed by
GABAergic neurons that form functional inhibitory synapses onto neigh-
bouring neurons residing in the NTS and DMNX.
3.3 Central delivery of C21 lowers basal
and light-evoked elevations in blood
pressure, effects that are accompanied by
decreased GABA-related gene expression
While the previous experiments revealed that intrinsic activation of
AT2R-expressing neurons in the NTS is coupled to increases in blood
pressure and heart rate, they did not address whether stimulation of the
AT2R itself using an AT2R agonist affects neuronal and cardiovascular func-
tion. Thus, our next experiments examined the effects of chronic central
administration of the selective AT2R agonist, C21 (7.5 ng/kg/h, icv), on
GABA-related gene expression in the NTS and on cardiovascular responses
to optical stimulation. Normotensive mice were outfitted with brain infu-
sion kits and osmotic minipumps that chronically infused aCSF control
(CON) or C21 (icv) (Figure 3A). Two weeks after the initiation of the exper-
iment, mice were euthanized, brains were extracted and the DVC, encom-
passing the NTS, area postrema, and DMNX, was micro-dissected for gene
expression analyses. Relative to CON, chronic delivery of C21 significantly
decreased mRNAs for GABA synthetic enzymes, glutamate decarboxylase
1 (Gad1) and glutamate decarboxylase 2 (Gad2), and GABAB receptor 2
(Gabbr2) in the DVC (Figure 3B). In contrast, the levels of mRNAs for VGAT,
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M. Mohammed et al.
Figure 2 AT2R-expressing neurons increase blood pressure and release GABA onto post-synaptic neurons residing in the NTS. (A) Schematic depict-
ing the experimental design. (B) ChR2-eYFP-containing AT2R neurons (arrow) were targeted for whole-cell recording using a combination of epifluores-
cence and differential interference contrast microscopy. (C) Blue light pulses elicited action potentials in all cells tested (n=9). (D) Time course of the
impact of optogenetic stimulation (10 Hz; 10 mW; 20 ms pulses; 60 s) in the NTS of AT2R-Cre mice that received AAV-ChR2-eYFP or AAV-eYFP on
changes in systolic blood pressure (DSBP), mean arterial pressure (DMAP), heart rate (DHR), and HRV; n=5/group. Bars=SEM. Two-way repeated meas-
ures ANOVA revealed an effect of AAV condition, time, and a time by condition interaction for the cardiovascular parameters assessed (see
Supplementary Table S1). (E) Frequency-dependent impact of optogenetic stimulation of AT2R neurons on DSBP, DMAP, DHR, and HRV (1, 5, 10, and
15 Hz; 10 mW; 60 s); n=5/group; *=slope different than AAV-eYFP, P<0.05, linear regression analysis. Bars=SEM. (F–H) Representative images of coronal
sections through the NTS of AAV-ChR2-eYFP mice that were processed for visualization of (F) eYFP immunofluorescence (green) and (G) VGAT mRNAs
(magenta); (H) the merged image. (I) Number of eYFP neurons containing VGAT mRNA. Bar=SEM. (J) NTS neurons were targeted for whole-cell record-
ings (arrow). Scale bar=20 mm. (K) Light-evoked IPSC in a representative NTS neuron. (L) Light-evoked IPSCs in NTS were attenuated by exposure to
PTX and CGP (n=3). (M) DMNX neurons were targeted for whole-cell recording (arrow). Scale bar=20 mm. (N) Light-evoked IPSC in a representative
DMNX neuron. (O) Light-evoked IPSCs in DMNX were greatly attenuated by exposure to PTX and CGP (n=4). Solid and shaded lines in (L) and (O) de-
pict mean and SEM. Effectiveness of bath applied antagonists tested against light-evoked synaptic responses observed in vitro was evaluated using a 1-sam-
ple Student’s t-test (null hypothesis: mean =1, for data normalized to the baseline mean). See Supplementary Tables S1–S3 for results of statistical
analyses.
for subunits 1, 3, and 5 of the GABAA receptor (Gabar1, Gabar3, and
Gabar5), for the GABAB receptor 1 (Gabbr1), for GABAA receptor gamma2
subunit (Gabrg2), and for the vesicular glutamate transporter 2 (VGlut2)
........
within the DVC (Figure 3B) were not affected by C21. Central infusion of
C21 affected neither the levels of mRNAs for Gad1-, Gad2-, and GABABR
nor those of arginine vasopressin (AVP) and corticotrophin releasing
Angiotensin type-2 receptors on pressor neurons
889
Figure 3 Stimulation of AT2R within the brain reduces blood pressure and indices of GABA release in the DVC. (A) Schematic depicting the experi-
mental design used to evaluate the impact of chronic C21 administration (icv; 7.5 ng/kg/h; 2 weeks) on GABA-related gene expression. (B and C) Gene ex-
pression within the (B) DVC or (C) hypothalamus (HYP) as assessed by qRT–PCR in mice given chronic icv C21 or controls (CON) given icv aCSF; VGAT,
Gad1, Gad2, Gabbr1, Gabbr2, GABAA receptor subunits 1, 3, and 5 (Gabar1, Gabar3, and Gabar5), GABAA receptor gamma2 subunit (Gabrg2), VGlut2, AVP,
and CRH; n=8/group; *=P<0.05; t-test. (D) Schematic depicting the experimental design used to evaluate the impact of chronic icv C21 on optical stimula-
tion of AT2R neurons within the NTS. (E) Time course of the impact of optogenetic stimulation of AT2R neurons in the NTS (10 Hz; 10 mW; 20 ms
pulses; 60 s) on DSBP, DMAP, DHR, and HRV; n=7–8/group. Two-way repeated measures ANOVA revealed an effect of C21, time and an interaction be-
tween C21 and time for the cardiovascular parameters assessed. (F) Frequency-dependent impact of optogenetic stimulation of AT2R neurons on DSBP,
DMAP, DHR, and HRV (1, 5, 10, and 15 Hz; 10 mW; 20 ms pulses; 60 s; n=5/group). *=slope is different than 0; and #=slope is different than AAV-eYFP,
P < 0.05, linear regression analysis. See Supplementary Tables S4–S6 for results of statistical analyses.
hormone (CRH) within a hypothalamic micro-dissection containing the para-
ventricular nucleus of the hypothalamus (Figure 3C). To verify that the effects
of C21 on gene expression were mediated by AT2R(s) expressed on
GABAergic neurons, we bred mice with a knock-in mutation of LoxP-sites
flanking the Agtr2 gene (AT2R-flox) with mice that have Cre-recombinase di-
rected to the VGAT (VGAT-ires-cre; 016962 JAX). These AT2R-GABA-KO
mice were chronically delivered aCSF or C21 as described but subsequent
RT–PCR analysis found that patterns of gene expression were similar be-
tween groups (Supplementary Figure S2). One the one hand, this result indi-
cates that C21 exerted effects on GABA-related gene expression via
selective activation of AT2R; on the other,
it indicates that AT2Rs on
GABAergic cells are necessary for the decreased mRNA expression that was
observed.
We next hypothesized that the down-regulation of GABAergic signal-
ling that accompanied chronic C21 administration would alter
.............................................
cardiovascular function during optogenetic excitation of AT2R neurons
in the NTS. To test this hypothesis, AT2R-Cre mice were injected with
AAV-ChR2-eYFP into the NTS and, 3 weeks later, were implanted with
osmotic minipumps and brain infusion kits to chronically deliver aCSF or
C21 as above. The goal was to determine whether altered gene expres-
sion observed after chronic AT2R stimulation is predictive of blunted
cardiovascular responses to light-evoked excitation of AT2R-expressing
neurons in the NTS (Figure 3D). As above, optogenetic stimulation of
AT2R neurons within the NTS elicited frequency-dependent elevations
in blood pressure, heart rate, and HRV in control mice that were deliv-
ered aCSF vehicle into the brain (Figure 3E). Intriguingly, under these
anaesthetized conditions, baseline blood pressure was unaltered by
chronic central activation of AT2R with C21 (see Supplementary Tables
S4 and S5 for ANOVA results and absolute values for cardiovascular
parameters, respectively). However, C21 significantly blunted the
890
M. Mohammed et al.
Figure 4 Experimentally induced hypertension augments the co-expression of Gad1 and Agtr2 mRNAs within the NTS. (A) Daily SBP during the estab-
lishment of DOCA-Salt hypertension. (B) A 24 h SBP trace of DOCA-Salt hypertensive vs. normotensive control (CON) mice. (C) Relative gene expres-
sion of Agtr2, Agtr1a, and Gad1 within the DVC of DOCA-Salt mice vs. CON as determined via RT–PCR (n=8/group). Representative projection images
through the NTS of (D–F) CON or (G–I) DOCA-Salt AT2R-eGFP mice depicting (D, G) eGFP, (E, H) Agtr2, and Gad1 mRNAs in magenta and cyan, re-
spectively, and (F, I) the merged images. Distribution of AT2R-eGFP cells (J) across various distances caudal to bregma and (K) throughout the entire
NTS. (L) Total Agtr2 mRNA levels, (M) percentage of AT2R-eGFP neurons containing Gad1 mRNA, and (N) level of Agtr2 mRNAs per Gad1þ AT2R-
eGFPþ cell throughout the NTS of DOCA-Salt mice vs. CON. n=4/group. *=P<0.05, t-test. Bars=SEM.
increased blood pressure and heart rate responses to blue light stimula-
tion and these effects persisted across all patterns of stimulation
(Figure 3E and F; see Supplementary Table S4 for ANOVA results reveal-
ing significant effects of time, treatment, and time–treatment interac-
tions). Furthermore, linear regression analyses revealed that the impact
of optical stimulation on SBP and MAP was frequency-dependent for
...................
both groups; however, this effect was significantly blunted in the mice
that received C21. Notably, the increase in HRV produced by blue light
stimulation was abrogated by C21, at all stimulation frequencies
(Figure 3E and F). Collectively, these results suggest that chronic icv ad-
ministration of C21, disrupts indices of GABAergic signalling within
the NTS to uncouple blood pressure and heart rate from the firing of
Angiotensin type-2 receptors on pressor neurons
891
Figure 5 DOCA-Salt induced elevations in blood pressure are reversed by chronic icv C21. (A) Schematic of experimental design used to evaluate the
ability of C21 (7.5 ng/kg/h; 8 days) to reduce blood pressure in mice rendered DOCA-Salt hypertensive. (B) Daily SBP (top), MAP (middle), and HR (bot-
tom) throughout the study for the CON and C21 groups. Two-way repeated measures ANOVA revealed significant main effects of time for both MAP
and SBP, and of treatment for MAP, P<0.05. (C) Average SBP (top), MAP (middle), and HR (bottom) of normotensive controls (n=12) and of the DOCA-
Salt mice given C21 (n=7) or vehicle (n=5) during the period post icv minipump implantation, *P<0.05, one-way ANOVA. Bars=SEM. See Supplementary
Table S7 for results of statistical analyses.
AT2R-expressing neurons. Based on the HRV data, these effects of C21
appear to involve suppression of sympathetic activity.
3.4 Experimentally induced hypertension
augments the co-expression of Gad1 and
Agtr2 mRNAs within the NTS
Experimentally induced hypertension is associated with enhanced
GABAergic signalling within the NTS.20 Given that AT2R-expressing
...........................
neurons in the NTS are largely GABAergic and that their excitation is
coupled to elevated blood pressure, we hypothesized that hypertension
would modulate the expression of AT2R within the NTS. The prediction
is that this plasticity may then be exploited to treat the disease. To probe
for such alterations in AT2R expression, we first used wild-type mice
that were implanted with telemetry devices. After surgical recovery and
baseline recordings, mice were implanted with pellets containing DOCA
(100 mg, sc; DOCA-Salt) or sham (CON) and given ad libitum access to
isotonic saline. We selected the DOCA-Salt model of experimentally
892
M. Mohammed et al.
Figure 6 Deletion of AT2R from GABAergic neurons in the NTS prevents the ability of C21 to reduce blood pressure in mice previously rendered hy-
pertensive. (A) Schematic of experimental design. (B–D) Validation of the specific deletion of Agtr2 from tdTomato neurons in the AT2R-cKO mice rela-
tive to Ai9 controls. Images through the NTS of (B) a control mouse and (C) an AT2R-cKO mouse depicting mRNAs for Agtr2 (Cyan) and thereby
highlighting the lack of Agtr2 mRNA within these neurons. (D) Quantification of the number of tdTomato neurons containing Agtr2 mRNA in CON (n=3
mice; 350 of 374 tdTomatoþ cells contain Agtr2 mRNAs) vs. AT2R-cKO mice (n=3 mice; 0 of 489 tdTomatoþ cells contain Agtr2 mRNAs). *P<0.05,
t-test. (E) Change in SBP (top), MAP (middle), and HR (bottom) subsequent to the initiation of icv C21 administration to AT2R-cKO mice and CON (rel-
ative to the baseline DOCA-Salt hypertensive levels). Two-way repeated measures ANOVA revealed significant main effects of time on all cardiovascular
parameters assessed, main effects of genotype on MAP and SBP, and a significant time by genotype interaction for SBP, P < 0.05. n=7/group. Bars=SEM.
See Supplementary Tables S8 and S9 for results of statistical analyses.
induced hypertension since it results in consistent increases in blood
pressure in mice and has a strong neurogenic component.21
Cardiovascular parameters were recorded for 3 weeks, after which
microdissections of the DVC were used to assess Agtr2, Agtr1a, and
Gad1 mRNAs via RT–PCR. As expected, DOCA-Salt significantly in-
creased daily SBP (Figure 4A) and this effect persisted across the light-
dark cycle as determined by mean hourly SBP (Figure 4B). Within the
DVC, DOCA-Salt hypertension significantly increased expression of
Agtr2, Agtr1a, and Gad1 mRNAs relative to normotensive controls
(Figure 4C). We have previously determined that DOCA-Salt does not
lead to a redistribution of AT2R from neurons to glia, indicating that the
rise is Agtr2 mRNAs are not likely due to increased expression on
microglia or astrocytes.14 Here,
follow-up experiments evaluated
whether (i) DOCA-Salt hypertension altered the number or distribution
of neurons in the NTS that express Agtr2 and (ii) if augmented expres-
sion of Agtr2 occurred in NTS neurons that also express Gad1.
Mice genetically engineered to have the expression of eGFP driven by an
Agtr2 bacterial artificial clone gene (AT2R-eGFP)10 underwent DOCA-pel-
let implantation or sham surgeries and were provided access to isotonic sa-
line as described. Afterwards, sections through the NTS were processed
for immunohistochemistry for eGFP and dual RNAscope ISH for Agtr2 and
Gad1 mRNAs (Figure 4D–I). Cell counts throughout the NTS found no ef-
fect of DOCA-Salt on the overall number or distribution of eGFP-labelled
neurons (Figure 4J and K). Rather, ISH revealed that DOCA-Salt resulted in
...........................................................................
elevated Agtr2 mRNA expression in the NTS (Figure 4L). Further quantifica-
tion revealed that DOCA-Salt hypertension is not associated with a shift in
the percentage of AT2R neurons that are GABAergic (Figure 4M), indicating
that DOCA-Salt does not shift the phenotype of AT2R neurons in the
NTS. Instead, DOCA-Salt increased the number of Agtr2 mRNA transcripts
specifically within NTS neurons that contain Gad1 mRNA and report Agtr2
gene transcription with the expression of eGFP (Figure 4N), an effect that
we hypothesize may represent an endogenous depressor mechanism that
can be engaged to reverse augmented GABA synthesis and the develop-
ment of hypertension. Taken together, these results suggest that chronic
elevations in blood pressure increase the synthesis of AT2R in NTS neurons
that express GABA.
3.5 Central delivery of C21 reduces
experimental hypertension and this effect
is abrogated by the selective deletion of
AT2R from GABAergic neurons in the NTS
Based on the localization and plasticity of AT2R in hypertension, we hy-
pothesized that activation of AT2R in the NTS would suppress increased
GABA production and thereby represent a depressor mechanism that can
be engaged to reverse hypertension. Consistent with this notion, elevations
in blood pressure that followed DOCA-Salt treatment in wild-type mice
were abolished with chronic central delivery of C21 using the same infusion
protocol as above, an effect that was absent in controls given the aCSF
Angiotensin type-2 receptors on pressor neurons
893
vehicle (Figure 5A–C). Thus, a final series of experiments tested the hypothe-
sis that AT2Rs expressed on GABAergic neurons in the NTS are necessary
for the antihypertensive effects of centrally delivered C21.
An experimental timeline is depicted in Figure 6A. In order to visualize
and quantify cells undergoing Cre-recombination, we bred female mice
heterozygous for AT2R-flox with homozygous Ai9 reporter male mice.
This produced male offspring carrying: (i) the AT2R-flox gene and ROSA
driven expression of the tdTomato gene preceded by a stop codon
flanked by loxP-sites (AT2R-flox mice) or (ii) littermate Ai9 control mice
carrying only the tdTomato stop-flox manipulation. Mice were implanted
with telemetry devices to measure cardiovascular parameters. To target
Cre-recombination specifically to GABA-synthesizing cells in the NTS,
mice were injected with an AAV that expresses Cre-recombinase under
the control of the VGAT promoter (AAV8-VGAT-Cre). Mice were then
rendered hypertensive using the DOCA-Salt paradigm, afterwhich, they
were chronically delivered C21 icv using osmotic minipumps. Figure 6B
and C highlight the co-localization of tdTomato and Agtr2 mRNA in con-
trol and AT2R-flox mice administered AAV-VGAT-Cre into the NTS. In
control mice (n=3) (cid:4)93% of tdTomato expressing cells (n=374) were
also found to express Agtr2 mRNA. In contrast, AT2R-flox mice (n=3)
given AAV-VGAT-Cre had no co-localization of tdTomato and Agtr2
mRNA in the cells examined (n=489). Importantly, Agtr2 mRNA was
detected in cells devoid of tdTomato that were presumably non-
GABAergic. Collectively, these results confirm that delivery of AAV8-
VGAT-Cre into the NTS has no effect on Agtr2 mRNA expression in con-
trol mice but deletes AT2R from GABAergic neurons within the NTS of
AT2R-flox mice. As expected, due to the overwhelming actions of en-
dogenous Ang-II at AT1aR during hypertension, the expression of which
we found also to be elevated in the DVC during DOCA-Salt (Figure 4C),
the deletion of AT2Rs specifically from GABAergic neurons in the NTS
had no effect on blood pressure during establishment of hypertension
(Supplementary Table S7). Rather, this deletion abolished the antihyper-
tensive effects of C21 (Figure 6E). That is, two-way ANOVA analyses
revealed a significant genotype–time interaction on SBP (Supplementary
Table S8). Main effects of genotype and time were also revealed for MAP
and SBP. Furthermore, analyses of mean SBP, MAP, and HR similarly
highlighted a blunted antihypertensive effect of C21, without an impact
on HR. Taken together, these results suggest that AT2Rs expressed on
GABAergic neurons residing within the NTS are required for the reduc-
tions in blood pressure that follow central administration of C21.
4. Discussion
These experiments reveal an unique population of GABAergic neurons
residing within the NTS whose firing is coupled to changes in blood pres-
sure. These neurons express AT2Rs and form dense inhibitory synapses
onto other neurons residing in the NTS and DMNX. Experimentally in-
duced hypertension up-regulates Agtr2 transcription within these
GABAergic NTS neurons; and chronic central activation of these ele-
vated AT2R with the selective agonist, C21, reverses hypertension.
Moreover, AT2Rs on GABAergic neurons in the NTS appear to be re-
quired for these actions because their selective deletion prevents the
lowered blood pressure that follows central delivery of C21. The collec-
tive implication is that neurons in the NTS that express AT2R may serve
as an access point for reversing hypertension. Whether or not activation
of these NTS AT2R also reverses the hypertension-induced damage to
target organs, such as the kidney and heart, remains to be investigated.
...................................................................................................................................................................................
The expression of AT2R on discrete neurons in the NTS that are inti-
mately linked to blood pressure regulation is a discovery that is both
novel and intriguing. Prior neuroanatomical studies determined that
AT2R are localized to the NTS,18 and over a decade later, the advent of
genetic reporting revealed that
they are mostly distributed on
GABAergic neurons.10 The present results functionally link the activity
of these neurons to changes in blood pressure. Specifically, optogenetic
excitation of neurons in the NTS that express AT2Rs increases blood
pressure in a frequency-dependent manner and these elevations persist
for minutes after blue light illumination ceases. The mechanisms by which
these neurons regulate blood pressure are unknown; however, insight
can be gained from their connectivity and responsiveness. The brain
monitors arterial pressure, in part, via baroreceptor afferents that make
excitatory synapses onto second order neurons in the NTS. Canonically,
second order neurons are depicted as glutamatergic neurons whose ex-
citatory projections to the caudal ventrolateral medulla form a portion
of the efferent limb of the baroreflex arc. However, electrophysiological
recordings from brain slices obtained from GABA reporter mice indicate
that (cid:4)70% of GABAergic neurons in the NTS receive visceral sensory
afferents,22 suggesting that some portion of second order neurons are
GABAergic. Therefore, it is possible that AT2R-expressing neurons in
the NTS, which are also GABAergic, sense arterial pressure via direct
connections from primary baroreceptor afferents. While our studies do
not probe for conditions that acutely activate or inhibit these neurons,
they do reveal that chronic elevations in blood pressure, lead to plasticity
in the expression of the AT2R, implying that elevations in blood pressure
may change their functionality. Furthermore, our findings clearly indicate
that once excited, these neurons prompt a robust rise in arterial pres-
sure, suggestive of a prominent role in the neural circuits that regulate
blood pressure.
When considering the ubiquitous actions of GABA within the NTS,
there is clear evidence for a pressor action of the inhibitory neurotrans-
mitter. For example, microinjections of GABA receptor agonists into
the NTS increase blood pressure,7 an effect mediated by augmented
sympathetic outflow.23 The current thinking is that application of GABA
to the NTS inhibits second order neurons, which elevates blood pres-
sure by liberating sympathetic outflow from tonic inhibition, thus damp-
ening the impact of baroreflex stimulation. Despite the general
acceptance that GABA exerts pressor actions within the area, the spe-
cific circuitry underlying these pressor actions has not been defined. The
NTS contains an abundance of GABAergic interneurons that may medi-
ate these effects; however, the transmitter also arises from projections
from forebrain areas like the central amygdala.24 Our in vitro optogenetic
experiments determined that AT2R-expressing neurons within the NTS
form dense inhibitory synapses onto neighbouring neurons in the NTS
and DMNX. Thus, it is likely that excitation of AT2R-expressing neurons
increases blood pressure by inhibiting second order neurons to augment
sympathetic outflow. Indeed, this notion is supported by our HRV data
in Figures 2 and 3, which suggest that optogenetic activation of AT2R-
expressing neurons increases sympathetic activity. Alternatively, or in ad-
dition, AT2R-expressing neurons in the NTS may increase blood pres-
sure via connections to the forebrain,25 that regulate release of
hormones like AVP into the systemic vasculature.7 Ultimately, cardiovas-
cular disorders, like hypertension, result from autonomic and humoral
dysregulation that chronically increases blood pressure.26 Given that
AT2R-expressing neurons in the NTS appear to play a role in the aetiol-
ogy of hypertension, it is likely that such neurons also engage autonomic
and humoral nodes to couple their activity to hemodynamic status.
894
M. Mohammed et al.
Collectively, our optogenetic studies reveal a specific population of
GABAergic neurons that are involved in the central control of blood
pressure and suggest that AT2R serve as a phenotypic marker for target-
ing this specific neural circuit. To determine the role of AT2R, per se, in
the central regulation of blood pressure, we combined the use of the
AT2R agonist, C21, with optogenetics and with the Cre-loxP-mediated
deletion of AT2Rs. Our initial experiments indicated that central infusion
of C21 attenuated the increase in blood pressure elicited by optogenetic
stimulation of AT2R-expressing neurons in the NTS (Figure 3), and HRV
analyses of the data revealed that this C21 effect involved abrogation of
the sympathetic component of the optical stimulation. While AT2R has
been considered a potential therapeutic for cardiovascular disease for
decades,27 the central site of action and mechanism(s) underlying its pro-
tective effects have remained an enigma. Whole body deletion of AT2Rs
in mice results in heightened pressor responses to systemically delivered
angiotensin II.28,29 Follow-up studies implicated the CNS as a potential
site of action by demonstrating that central administration of angiotensin
II to AT2R KO mice also produces augmented pressor responses30 and
virally mediated overexpression of AT2Rs within the baroreflex circuit
reduces blood pressure and improves baroreflex sensitivity.31 The devel-
opment of C21 as a selective agonist13 provides a pharmacological tool
to probe the effects of AT2R activation. Indeed, consistent with our
results, central administration of C21 lowers blood pressure during ex-
perimentally induced hypertension.11,12 However, recent studies using
microdialysis revealed that acute application of C21 to the NTS has no
effect on levels of GABA sampled from the same nucleus, which casts
uncertainty on AT2R and GABA interactions that affect blood pres-
sure.32 This contrasts with our results showing that chronic administra-
tion of C21 elicits robust and consistent decreases in mRNAs for GABA
synthetic enzymes and blood pressure that counteract the development
of hypertension; these effects of chronically administered C21 require
the presence of the AT2R on GABAergic neurons in the NTS. An expla-
nation for the differences between our current findings and those of
Legat et al.32 may reside in the time course of C21 administration. Our
results indicate that the blood pressure-lowering action of chronic
AT2R activation is associated with reduced levels of GABA synthetic en-
zyme mRNAs. If the antihypertensive action of C21 requires a decrease
in GABA synthesis in AT2R neurons, reductions in GABA levels would
not be achieved by an acute perfusion protocol as employed by Legat
et al.32 Regardless of the mechanism, our collective results reveal novel
pathophysiology that can be reversed with chronic stimulation of brain
AT2Rs and this discovery may be exploited to develop and improve
therapeutics aimed at relieving resistant hypertension. While our data
strongly suggest that the locus of this AT2R-mediated antihypertensive
effect is the NTS (Figure 6), at this point, we are unable to completely
rule out an involvement of other AT2R-containing brain regions. In addi-
tion, we cannot rule out that C21 influences neurohormonal processes,
which may decrease blood pressure. For example, even though our data
indicate that icv infusion of C21 does not alter hypothalamic AVP
mRNA levels (Figure 3C), our previous studies demonstrated that icv in-
fusion of C21 lowers circulating AVP.33
An interesting facet of our data is the differing effects of C21 on blood
pressure and heart rate. For example, central C21 infusion attenuated
the increase in blood pressure produced by optogenetic stimulation of
AT2R-expressing neurons in the NTS, but at the same time completely
blocked the tachycardia induced by optical stimulation, and even low-
ered heart rate. An explanation may reside in the phenotype(s) of cells
in the NTS that contain AT2R. As discussed above, the majority of the
AT2R-containing neurons in the NTS are GABAergic, and we believe
...................................................................................................................................................................................
that C21 lowers blood pressure through reducing GABA synthesis. A re-
duction in GABA synthesis may lead to reduce optogenetic-stimulation
induced GABA release onto DMNX neurons that provide parasympa-
thetic input to the heart, thereby contributing to reduced/abrogated
blood pressure/heart rate responses. Another layer of complexity is that
in addition to being expressed on GABA neurons, there is also a subset
of AT2R neurons in the DMNX itself that are cholinergic,10 and optoge-
netic stimulation of DMNX cholinergic neurons, in general, is known to
reduce heart rate.34 While the particular cholinergic neurons that ex-
press AT2R were not directly adjacent to the fibre optic post in our sub-
jects, the close proximity of the area to the NTS, combined with the
known contribution of the area to the parasympathetic innervation of
the heart, as well as the observed bradycardic response to C21 adminis-
tration (Figure 3), bring up the possibility that
the removal the
GABAergic influence of AT2R neurons in the NTS by way of chronic
C21 administration, effectively ‘unmasks’ a direct effect of the DMNX
neurons to reduce heart rate.
Another thing that is apparent from our data is that deletion of AT2R
from NTS GABAergic neurons does not alter baseline blood pressure
or the development of DOCA-Salt hypertension. These findings were
not unexpected, for a number of reasons. Hypertension is a multi-facto-
rial disease and it is doubtful that removing one protective factor, such as
the AT2R would provide a significant influence over baseline blood pres-
sure. This is especially true as the levels of AT2R in the NTS are low to
begin with,18 and even though their levels increase in hypertension
(Figure 4), their endogenous activity is likely overridden by the pro-hy-
pertensive AT1R, which are expressed at higher levels in the NTS and
also increase in hypertension (Figure 4). A lack of changes in baseline
blood pressure after deletion of brain angiotensin receptors has also
been demonstrated in other studies. Namely, while deletion of AT1R
from catecholaminergic neurons in the rostral ventrolateral medulla at-
tenuated angiotensin II-induced hypertension, it did not alter baseline
blood pressure.35 The key points are that the endogenous AT2R-de-
pressor mechanism that we have discovered to exist in the NTS is likely
to be a failed compensatory mechanism during high blood pressure;
however, it can be taken advantage of by using a selective AT2R-agonist
to decrease BP in hypertension, something that is clearly demonstrated
by our data. Furthermore, combining an AT2R agonist with an AT1R
blocker may result in an enhanced antihypertensive strategy.
Increasing evidence implicates the nervous system in the development
or reversal of resistant hypertension. Accordingly, emerging therapies
are using medications,36 devices,37 or surgical approaches38 that alter
neural and humoral control of circulation to lower blood pressure. For
example, firibastat, a new class of drug that targets the brain renin-angio-
tensin system by suppressing production of angiotensin III and thus de-
creasing its interaction with AT1Rs, was recently found to be efficacious
at lowering blood pressure in patient populations that have historically
suffered from resistant hypertension.36,39 Here, we propose that the an-
tihypertensive effects of centrally acting therapies, especially those tar-
geting the brain renin-angiotensin-system,
likely engage subsets of
GABAergic neurons in NTS, possibly via activation of AT2Rs, to reverse
the neuronal plasticity underlying the autonomic and neuroendocrine
dysfunction that promotes the development of resistant hypertension.
This might be taken advantage of in the clinical setting by developing
means (including drugs) that would produce selective- and long-lasting
activation of the AT2R on GABAergic neurons in the NTS. This is a rea-
sonable suggestion, as it has been demonstrated that certain blood ves-
sels in the dorsomedial NTS lack a blood-brain barrier (BBB),40 and that
during hypertension the BBB in the NTS becomes leaky.41 Alternatively,
Angiotensin type-2 receptors on pressor neurons
895
as these NTS GABAergic neurons appear to be an important compo-
nent of the mechanisms that contribute to resistant hypertension, strate-
gies that dampen their activity, with or without AT2R agonists, might
prove useful in this disease.
Supplementary material
Supplementary material is available at Cardiovascular Research online.
Authors’ contributions
A.D.d.K., E.G.K., and C.S. conceived, designed, supervised, and coordi-
nated the study; M.M., D.N.J., L.A.W., S.W.H., W.S., E.A.S., K.A.S., and
A.D.d.K. conducted experiments and acquired data; M.M., D.N.J., L.A.W.,
S.W.H., K.E., C.J.F., and A.D.d.K. analysed data; U.M.S., M.B., and A.D.d.K.
generated mice; A.D.d.K., E.G.K., C.S., M.M., K.E., C.J.F., M.B., and U.M.S.
wrote, edited, and revised the article.
Conflict of interest: none declared.
Funding
This work was
supported by American Heart Association grant
17GRNT33660969 and National Institute of Health (National Heart Lung
and Blood Institute) grants HL-125805 (ADdK), HL-145028 (ADdK), HL-
093186 (CS), HL-136595 (EGK/CS), HL-096830 (EGK), and HL-122494
(EGK).
Data availability
The data underlying this article will be shared on reasonable request to the
corresponding author.
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Translational perspective
Hypertension is a widespread health problem and risk factor for cardiovascular disease and stroke. Although treatment options exist, many patients
suffer from resistant hypertension, which is associated with enhanced sympathetic drive. Thus, many available therapeutics focus on dampening pres-
sor mechanisms. The present studies take the alternative approach of treating hypertension by exploiting an endogenous depressor mechanism.
| null |
10.1103_physrevresearch.5.013169.pdf
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PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
Monopole Josephson effects in a Dirac spin liquid
Gautam Nambiar
,1,* Daniel Bulmash,1,2 and Victor Galitski1
1Joint Quantum Institute, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
2Condensed Matter Theory Center, Department of Physics, University of Maryland, College Park, Maryland 20742, USA
(Received 18 July 2022; accepted 4 January 2023; published 13 March 2023)
Dirac spin liquids (DSLs) are gapless featureless states, yet interesting by virtue of the effective field theory
describing them—(2 + 1)-dimensional quantum electrodynamics (QED3). Further, a DSL is known to be a
“parent state” of various seemingly unrelated ordered states, such as antiferromagnets and valence bond solids in
the sense that one can obtain ordered states by condensing magnetic monopoles of the emergent gauge field. Can
operators in the effective field theory, such as the emergent electric field, be externally induced and measured? In
this work, we exploit the parent state picture to argue that the answer is yes. We propose a range of “monopole
Josephson effects” that arise when two ordered states are separated by a region of the parent DSL. In particular,
we show that one can induce an AC monopole Josephson effect, which manifests itself as an AC emergent electric
field in the spin liquid, accompanied by a measurable spin current. Further, we show that this AC emergent
electric field can be measured as a sharp tunable peak in Raman scattering. This work provides a theoretical
proof of principle that emergent gauge fields in spin liquids can be externally induced, manipulated, and probed
using more conventional states, which offers a generic platform for studying the exotic spin phases.
DOI: 10.1103/PhysRevResearch.5.013169
I. INTRODUCTION
Consider a spin-1/2 system in its ground state. Flipping
a single spin creates a spin-1 excitation. If the ground state
is conventional, such an excitation would disperse creating
a superposition of spin-wave modes with spin 1. However,
there is strong theoretical reason [1,2] to expect exotic systems
where, in addition to creating spin-1 modes, the spin-flip can
fractionalize into two spin-1/2 excitations, which can then
move away from each other. One interesting class of such
systems in 2 + 1D are Dirac spin liquids (DSLs). The effective
field theory describing DSLs is usually written in terms of
Dirac fermions strongly coupled to an emergent U (1) gauge
field. This strongly coupled theory is believed to flow at low
energies to a conformally invariant fixed point QED3 [3–5].
To detect such an exotic state in a given physical system,
say a material with spins, we would need to probe the low
energy degrees of freedom of the effective field theory de-
scribing the state in question. For example, the low energy
excitations of gapped spin liquids in 2 + 1D are anyonic quasi-
particles. There have been proposals in the past for accessing
emergent degrees of freedom in such gapped spin liquids with
the assistance of more conventional ordered phases, which is
helpful because one typically has better control over ordered
phases. Examples of these include Refs. [6–8] for Z2 spin
liquids, and Ref. [9] for Kitaev spin liquids.
*Corresponding author: [email protected]
Published by the American Physical Society under the terms of the
Creative Commons Attribution 4.0 International license. Further
distribution of this work must maintain attribution to the author(s)
and the published article’s title, journal citation, and DOI.
For a strongly coupled field theory in 2 + 1D, such as
QED3 on the other hand, the low energy degrees of freedom
are not well-defined quasiparticles, but instead the primary
operators of the conformal field theory (CFT). Previous works
have proposed ways to measure correlation functions of such
operators in the ground state of a DSL [3,10,11]. However
there appears to be a lack of proposals to directly control
such operators externally and measure them. In this paper,
we explore this direction and propose a way to induce and
measure an emergent electric field in a DSL. Our proposal
relies crucially on coupling to monopole operators.
Monopole operators insert an integer multiple of 2π flux of
the emergent U (1) gauge field. Polyakov showed [12] that in
2 + 1 D, monopoles are always relevant in the renormalization
group (RG) sense and proliferate in a pure U (1) gauge theory,
leading to confinement of test charges. Such a theory would
not describe a spin liquid phase. However, including gapless
fermions in the theory increases the scaling dimension of
the monopoles, and in the limit of large number of fermion
flavors N f , monopoles can become irrelevant [4,13]. Indeed,
using a symmetry analysis followed by a large N f analy-
sis, Ref. [14] found that on the triangular lattice, monopoles
are either disallowed by symmetry or irrelevant, suggesting
that a DSL could be a stable phase. Such a phase has an
emergent U (1)top symmetry corresponding to the conserva-
tion of total emergent flux (the subscript “top” is used to
differentiate U (1)top from U (1) gauge redundancy). In fact,
monopoles are charged under an enlarged emergent internal
symmetry GIR = SO(6) × U (1)top/Z2 (see Sec. II for a re-
view). Because spatial symmetries have a nontrivial action
in GIR, the monopoles transform under the microscopic sym-
metries like order parameters for magnetic orders including
the 120◦ antiferromagnet and the
12 valence bond
12 ×
√
√
2643-1564/2023/5(1)/013169(19)
013169-1
Published by the American Physical Society
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
can be induced via the monopole Josephson effect. In Sec. IV,
we propose a way to detect this emergent electric field using
Raman scattering. In Sec. V, we discuss other phenomena
related to the monopole Josephson effect. Finally, we offer
some general conclusions and discussion in Sec. VI.
II. REVIEW OF DIRAC SPIN LIQUIDS
DSLs are described by an effective field theory with N f =
4 flavors of gapless Dirac fermions at zero equilibrium den-
sity strongly coupled to a compact U (1) gauge field. The
fermions carry spin-1/2 under the microscopic SO(3) spin
rotation symmetry. One way to get to this theory is the parton
construction, which we will briefly review below.
Before proceeding, we explain some notation. We use (cid:4)V
for 2-component vectors in real-space, (cid:4)V for 3-component
vectors in internal spin or valley space, and V for vectors
in other internal spaces such as SO(6) (see Appendix B 4 for
other remarks on notation).
Consider a spin system whose microscopic Hilbert space
consists of spin-1/2 at each lattice site. We assume that the
Hamiltonian realizes a DSL ground state and respects some
set of symmetries GUV which include lattice symmetries,
time-reversal, and spin-rotation symmetry. We will work on
the triangular lattice for concreteness, but the results are gen-
eral except where otherwise noted. The objective of the parton
construction is to come up with a mean-field theory even in
the case when the spin operators (cid:5) ˆ(cid:4)Si(cid:6) = 0 (likewise for other
local spin operators) for all sites i. In this approach, the Hilbert
space at each site i is doubled by writing spin operators in
terms of fictitious spin-1/2 fermionic “spinon” operators ˆfi,α:
(cid:2)
ˆ(cid:4)Si = 1
2
α,β∈{↑,↓}
iα (cid:4)σαβ ˆfiβ .
ˆf †
(1)
Here (cid:4)σ is a vector of Pauli matrices. This description has a
U (1) gauge redundancy
ˆfiα → eiλi ˆfiα,
(2)
(cid:3)
α=↑,↓ ˆf †
in the sense that the physical spin operators are invariant
under such a transformation. The spin Hilbert space is re-
covered by imposing the constraint that there is exactly one
fermion at each site. One now rewrites the spin Lagrangian
in terms of these fermions. A quadratic term in spins be-
comes a quartic term in fermions, which is then decoupled so
iα ˆf jα(cid:6) acquire
that the fermion hopping coefficients (cid:5)
mean-field expectation value χi j. For a DSL on the triangu-
lar lattice, the mean-field configuration for {χi j} consists of
alternating π -flux and 0-flux on upward and downward tri-
angles, respectively. Diagonalizing this quadratic mean-field
Hamiltonian gives a spectrum with two Dirac cones (valleys).
To zeroth order, the single-occupancy-per-site constraint is
relaxed to demanding single-occupancy on average, i.e., that
the fermions are at half filling. This forces the chemical po-
tential to lie exactly at the Dirac points. So, to zeroth order,
the low-energy theory has 4 flavors of Dirac fermions—2
valleys for each spin. U (1) gauge fluctuations χi j → χi jeiai j
are now reintroduced. The single particle per site constraint
is reintroduced only weakly as a Gauss’s law by assuming a
FIG. 1. Monopole Josephson effect, general idea: We consider a
junction of two ordered phases (OL and OR) separated by a DSL. OL
and OR are viewed as monopole condensates such that their expec-
tation values are related by a generalized phase (unitary matrix) ei(cid:3)
(possibly time-dependent). This leads to a monopole current across
the junction, which is equivalent to an electric field in the emergent
U (1) gauge field in the perpendicular direction inside the DSL.
solid. Therefore, if the 2π monopoles somehow do proliferate
(say as a result of spontaneous symmetry breaking, or due to
a symmetry-breaking perturbation), then the system exits the
DSL phase. The resulting phase is an ordered phase deter-
mined by which combination of monopoles proliferates. In
this sense, it was suggested that the DSL is a parent state
for several seemingly unrelated magnetic and VBS orders
[15,16].
Can this parent state picture guide us towards finding
experimental probes for the low energy theory (QED3) that
describes the DSL? Can ordered states in proximity to a
spin liquid have an interesting effect on the spin liquid, and
vice-versa? In this paper, we argue that the answer to both
these questions is yes, by proposing a Josephson junction-like
setup shown in Fig. 1 with two ordered phases separated by a
middle region in the DSL phase. The main idea is that since
the ordered states can be viewed as monopole condensates,
monopoles can tunnel between the ordered states through the
DSL.
We show that in certain circumstances, applying a Zeeman
field gradient across the junction has the same effect as a volt-
age difference across a regular Josephson junction (between
superconductors) and thus gives rise to an AC monopole cur-
rent flowing across the DSL. In 2 + 1 dimensions, a monopole
current is equivalent to an electric field but in the perpendic-
ular direction. Therefore, this “monopole Josephson effect”
provides a way to externally induce an emergent electric field
through the DSL. We suggest a way to measure the AC
emergent electric field optically as a peak in Raman scattering
intensity by identifying microscopic operators corresponding
to the emergent electric field. In addition to this signature
within the DSL region, we show that when the ordered phases
are 120◦ antiferromagnets, the same monopole Josephson
effect leads to a spin current across the junction. This spin
current can in principle be measured on both the ordered side
and the DSL using techniques proposed in Refs. [17,18]. We
also discuss three other conceptually related effects which all
fall under the umbrella of the monopole Josephson effect.
The rest of the paper is organized as follows. In Sec. II,
we provide a brief review of DSLs, emphasizing the relevant
features of monopole operators and the parent state picture. In
Sec. III, we show that an emergent electric field in the DSL
013169-2
MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
finite coupling constant g:
(cid:2)
α=↑,↓
iα ˆfiα − 1 = 1
ˆf †
g2
(cid:2)
j
ˆei j,
(3)
where ei j is the emergent electric field, i.e., the electric flux of
ai j. This leads us to a field theory Lagrangian density, which
schematically is
L =
N f =4(cid:2)
i=1
¯ψiγ μ(∂μ − iaμ)ψi + 1
8π g2
((cid:12)μνλ∂μaν )2,
(4)
where each ψi is a two-component spinor and a0 has been
introduced as a Lagrange multiplier enforcing Eq. (3). The
theory of the DSL is then described by the low-energy fixed
point, called QED3, of Eq. (4). By itself, Eq. (4) is not useful
to understand the fixed point because the coupling constant
g2 has dimensions of [Length−1]. So, at low energies g2
flows to ∞ making gauge fluctuations uncontrolled. However,
progress can be made by treating 1/N f as a small parameter.
Then, because of screening of gauge fluctuations by the many
gapless fermions, g2 approaches a fixed point value which
scales as (cid:14)/N f where (cid:14) is an inverse length scale of order
of the lattice spacing [4]. Most of the current understanding
of the QED3 fixed point comes from this limit—thinking of
the spinons ψi essentially as almost free fermions with gauge
fluctuations controlled by the large N f expansion. At the same
time, one should keep in mind that the most important low
energy operators to study are primary operators of the CFT
with the lowest scaling dimensions.
The essential features of the nontrivial fixed point theory
QED3 are:
(1) Monopole operators: Among the primary operators in
i . These operators insert 2π
QED3 are magnetic monopoles ˆ(cid:15)†
flux of the emergent gauge field a, that is,
[ ˆb(x), ˆ(cid:15)†
i (x(cid:12))] = 2π δ(x − x(cid:12)) ˆ(cid:15)†
where ˆb(x) = (∂1 ˆa2 − ∂2 ˆa1)(x). In a path integral, the inser-
tion of these operators corresponds to instanton events whose
role is to restore the compactness of the gauge field in the low
energy theory.
i (x(cid:12)),
(5)
Z2
(cid:4)
(2) Enlarged emergent symmetry group [14,16]: While the
microscopic Hamiltonian has the symmetries listed above,
the DSL theory (QED3) has an enlarged internal symme-
try GIR = SO(6)×U (1)top
. The U (1)top symmetry corresponds to
the conservation of total emergent magnetic flux through the
plane: ˆbtot ≡ 1
d 2x ˆb(x). Clearly monopole operators are
2π
charged under U (1)top. The total flux is conserved because the
monopole operators (i.e., flux creation/annihilation operators)
have zero expectation value in the wave function described
by DSL theory. The SO(6) symmetry corresponds to the in-
ternal rotation between the spin and valley indices, and the
monopole operators transform as a vector under SO(6).
More concretely, inserting 2π flux leads to one Landau
zero-mode per fermion flavor, and to maintain half filling,
two of four zero-modes need to be filled. The resulting six
choices lead to six independent monopole creation operators
ˆ(cid:15)†
i (i ∈ {1, . . . 6}) which together transform as a vector under
SO(6). A complementary way to understand this is to observe
that the fermionic partons enjoy an SU (4) symmetry near the
Dirac points. Upon carefully keeping track of redundant fac-
tors of Z2,1 one arrives at GIR above. While the microscopic
SO(3) ⊂ GUV spin-rotation symmetry is directly SO(3)spin ⊂
GIR, elements of the space group generally embed nontrivially
into SO(3)valley × U (1)top ⊂ GIR (in addition to the spatial
transformation).
(3) Parent state of competing orders [14–16]:
If a
monopole operator condenses, i.e., acquires a nonzero expec-
tation value, then the spinons confine, and the low-energy
excitations are unfractionalized spin-1 modes. The resultant
phase is simply a conventional magnetically ordered phase.
Many seemingly unrelated ordered phases can appear depend-
ing on which monopole operator condenses and what the
microscopic symmetries are. For example, on the triangular
lattice, a 120◦ coplanar order can be obtained by condens-
ing spin triplet monopoles, and valence bond solids with a
unit-cell area of 12 times the elementary unit cell can be
obtained by condensing spin singlet monopoles. The DSL
thus serves as a “parent state” for many ordered states, in
the sense that one mechanism (monopole condensation) in the
DSL is responsible for driving transitions to many different
ordered states.
Such a transition could happen for multiple reasons. A
monopole operator may be relevant in the renormalization
group (RG) sense and symmetry allowed; in this case, the
DSL represents a critical point separating ordered phases. On
the other hand, if there are no relevant monopole operators
that are symmetry allowed, then the DSL is a stable phase of
matter—a gapless spin liquid. However, if there is explicit or
spontaneous breaking of a symmetry which was previously
forbidding some monopole operator from condensing, then
this would also lead to a transition to an ordered state.
A. Monopole condensation and unbroken symmetries
The goal of this section is to highlight one fact that will
play a crucial role in our work—in a 120◦ AFM, applying
spin rotation about a certain axis on the condensed monopoles
is equivalent to applying a U (1)top phase rotation.2 To do so,
we will review a particular mechanism for driving monopole
condensation. We first summarize the key facts:
(1) Under this mechanism, a phase transition occurs when
a certain linear combination of 2π monopole operators that is
an eigenvector of a specific SO(6) generator condenses.
(2) From the GIR transformation properties of the con-
densed monopole operator, one can determine which ordered
phase arises.
(3) The GIR symmetry is not fully broken in the ordered
state.
In Appendix A, we review previous works on the stabil-
ity of DSLs, which suggest that a DSL could be a stable
phase on the triangular lattice. In this case, 2π monopole
1The Z2 subgroup of SU (4) generated by fermion parity is actually
a U (1) gauge transformation rather than a symmetry, reducing the
SU (4) symmetry to SO(6) ∼= SU (4)/Z2. The element −1 ∈ SO(6)
is identical to a π rotation in U (1)top.
2The reader can skip to Sec. III, and return to this section when
required.
013169-3
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
operators are symmetry-disallowed in the Langrangian. How-
ever, they are still the operators with the lowest scaling
dimensions in the low-energy theory. The following mech-
anism was proposed [16,19,20] for destroying the DSL by
proliferating 2π monopoles. First, due to interactions, a
fermion bilinear, or mass term, spontaneously acquires a
nonzero expectation value. Although other possibilities can
occur, we will generally assume that a single bilinear
(cid:5) ¯ψσ ατ βψ(cid:6) (cid:16)= 0,
(6)
where σ α and τ β are Pauli matrices acting in the spin and
valley spaces, respectively (here α, β ∈ {0, 1, 2, 3} but α and
β are not both 0). This fermion bilinear serves as a mass
term that splits the degeneracy of the fermionic Landau zero
modes associated with adding a 2π magnetic flux, lowering
the energy (and hence scaling dimension) of one specific lin-
ear combination of monopole operators. In particular, given a
choice of generator σ ατ β of SU (4), we can find a correspond-
ing generator T [σ ατ β] of SO(6). This linear combination
of monopoles then becomes the most relevant operator and
condenses, i.e., acquires a nonzero expectation value. The
resulting state is an ordered state in which the fermions are
confined [12] and the condensed monopole operator serves as
an order parameter.
The linear combination of monopoles which condenses
corresponds to the eigenvector of T [σ ατ β] with the largest
eigenvalue. As an example, suppose that the bilinear ¯ψσ 3ψ
has a nonzero expectation value. One can check that three
ˆ(cid:15)1,2,3, transform
monopole operators, which we shall call
as SO(3)spin singlets, and the other three transform as an
SO(3)spin triplet ˆ(cid:4)(cid:3). In this basis, the SO(6) generator corre-
sponding to σ 3 is
⎛
⎜
⎜
⎝
03×3
...
T [σ 3] =
. . .
0 −i
i
0
0
0
⎞
⎟
⎟
⎠.
0
0
0
(7)
The eigenvector of T [σ 3] with maximal eigenvalue (equal to
1) is
(cid:5) ˆ(cid:3)(cid:6) = ((cid:5) ˆ(cid:15)1(cid:6)
= |(cid:15)|(0
(cid:5) ˆ(cid:15)2(cid:6)
(cid:5) ˆ(cid:15)3(cid:6)
0
0
1
i
(cid:5) ˆ(cid:15)4(cid:6)
0)T .
(cid:5) ˆ(cid:15)5(cid:6)
(cid:5) ˆ(cid:15)6(cid:6))T
(8)
The interpretation of this fact is that the fermion mass term
makes the energy of the monopole ( ˆ(cid:15)†
5)|GS(cid:6) negative,
4
where |GS(cid:6) is the ground state of the DSL in the absence of the
fermion mass term (whose energy we set to 0). Accordingly,
the DSL ground state becomes unstable and a transition occurs
to a state with (cid:5) ˆ(cid:15)†
4
+ i ˆ(cid:15)†
5
(cid:6) (cid:16)= 0.
+ i ˆ(cid:15)†
Although the fermion mass terms pick up nonzero ex-
pectation values, the fact that the monopole operators have
a lower scaling dimension means that we should treat the
condensed monopole operator as the order parameter. Dif-
ferent approaches can be used to determine a microscopic
order parameter corresponding to each monopole operator.
References [14,16] used a symmetry analysis combined with
a Wanner center study of mean-field free fermion bands. In
Appendix B 2, we combine symmetry analysis with opera-
tor algebra constraints to independently motivate the same
results. The key results are as follows. First suppose that a
spin-triplet monopole operator condenses. If (cid:5) ˆ(cid:4)(cid:3)(cid:6) (the vector
notation refers to a vector under SO(3)spin) is given by the
eigenvector with positive eigenvalue of (cid:4)dspin · (cid:4)Tspin[σ ] for a
unit 3-vector in spin-space, (cid:4)dspin, then the ordered state is a
120◦ coplanar AFM order in the plane (in spin-space) normal
√
to (cid:4)dspin. Similarly, various
12 VBS phases can be
obtained if the condensed spin-singlet monopole is an eigen-
vector of (cid:4)dvalley · (cid:4)Tvalley[τ ] for some unit 3-vector (cid:4)dvalley in
valley-space. In this VBS phase, the area of the unit cell is
12 times the area of the unit cell of a triangular lattice.3
12 ×
√
Having identified the monopole order parameters, we now
notice that the GIR symmetry is not completely broken. Sup-
pose that the condensed monopole is an eigenvector [in the
sense of Eq. (8)] of a generator ˆQ of SO(6). We focus for later
use on the case where ˆQ ∈ SO(3)spin. Then
(cid:5)eiθ ˆQ ˆ(cid:3) e−iθ ˆQ(cid:6) = e−iθQ(cid:5) ˆ(cid:3)(cid:6) = e−iθ (cid:5) ˆ(cid:3)(cid:6).
(9)
Note also that under a U (1)top phase rotation,
(cid:5)e−iθ ˆbtot ˆ(cid:3) eiθ ˆbtot (cid:6) = eiθ (cid:5) ˆ(cid:3)(cid:6).
(10)
Hence, (cid:5) ˆ(cid:3)(cid:6) is invariant under e−iθ ˆbtot eiθ ˆQ. Such a transfor-
mation generates a SO(2) diagonal subgroup of SO(2)spin ×
U (1)top that is an unbroken symmetry. This “redundancy”
between spin rotations and U (1)top phase rotation in the 120◦
AFM state will play a crucial role in the AC Josephson setup
proposed in Sec. III.
For completeness, we mention the concrete connection
between the 120◦ AFM order parameter and (cid:4)(cid:15):
(cid:2)
ˆ(cid:4)(cid:3) =
e−i (cid:4)Q.(cid:4)n( ˆ(cid:4)S(cid:4)n + · · ·),
(11)
(cid:4)n
√
3
3 ((cid:4)b1 − (cid:4)b2). Here (cid:4)b1 ≡
2 ˆy and (cid:4)b2 ≡ ˆy are
where (cid:4)Q = 2π
reciprocal lattice vectors satisfying (cid:4)ai · (cid:4)b2 = δi j, where (cid:4)a1 ≡
√
ˆx, (cid:4)a2 ≡ 1
3
2 ˆy are the basis vectors for the triangular lat-
tice. In Eq. (11), the “. . .” refers to operators supported on
three or more sites.
2 ˆx − 1
2 ˆx +
From Eq. (11), we can see that the ordering pattern
(cid:5) ˆ(cid:4)S(cid:4)n(cid:6) = [cos( (cid:4)Q.(cid:4)n), − sin( (cid:4)Q.(cid:4)n),
0],
(12)
corresponds to (cid:5) ˆ(cid:4)(cid:3)(cid:6) = |(cid:15)|(1
ple we considered in Eq. (8).
i
0)T , which was the exam-
III. MONOPOLE JOSEPHSON EFFECTS
In the previous sections we reviewed how various ordered
states can be obtained from a DSL by condensing combi-
nations of six monopole operators related to each other by
the enlarged [SO(6) × U (1)top]/Z2 symmetry. Now we will
argue how this can have physical consequences in the form of
“monopole Josephson effects,” by which we mean a flow of
3One could also consider condensation channels that are eigenvec-
tors of the mixed generators T [σ iτ j]. These “unconventional orders”
were considered in Supplemental Note 5 of Ref. [16]. We will not
consider these in this paper.
013169-4
MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
monopole current between two symmetry-broken regions of a
system. Consider the setup shown in Fig. 1 where a lattice is
split into three regions—two ordered phases (OL and OR) sep-
arated by a DSL region in the middle. Instead of considering
three different materials kept next to each other, we assume
that within the same sample, perturbations localized to regions
L and R drive those regions to ordered phases. This allows us
to view the ordered states OL and OR as being obtained via
monopole condensation from the DSL. Monopoles can now
tunnel from OL to OR through the middle DSL resulting in a
monopole current.
We note that the net monopole current (i.e., current of
U (1)top charge) is just the emergent electric field rotated by
90◦. This is because Faraday’s law takes the form of a conser-
vation law in 2 + 1 D:
∂t
ˆb
2π
+ ∂i
(cid:12)i j ˆe j
2π
= 0.
(13)
2π and ˆJ i[U (1)top](x) = (cid:12)i j ˆe j (x)
So, ˆQ[U (1)top](x) = ˆb(x)
. There-
fore, this monopole Josephson setup provides a way to induce
an emergent electric field inside the DSL (see Fig. 1). We
will show in Sec. IV that if this induced emergent electric
field is time-dependent, it can be optically detected via Raman
scattering.
2π
For a given configuration of OL and OR, our goal is to
make predictions for the resulting monopole currents. Since
the monopoles are charged under GIR = SO(6) × U (1)top/Z2,
there are 16 different conserved currents in principle, cor-
responding to each generator of GIR. These are ˆ(cid:4)J[U (1)top]
(analogous to electric Josephson current across superconduc-
tors), and 15 currents for each generator of SO(6), of which
3 are spin currents. Deep inside the DSL, since GIR is a sym-
metry, all 16 currents are conserved at low energies. However,
outside the DSL and at the boundaries, generically only the
3 spin currents will be conserved (assuming that SO(3)spin is
respected throughout the system). So we will make statements
about two kinds of quantities—(1) spin currents that can be
measured in either the ordered phases or the DSL, for example
using techniques proposed in Refs. [17,18], and (2) currents
corresponding to the emergent symmetries of the DSL, which
can be probed only within the DSL. The most interesting
result of this work is a time-dependent (AC) U (1)top current
in the DSL arising due to either a gradient in Zeeman field or
due to a gradient in staggered spin chirality applied across the
junction.
In this work, we focus on qualitatively determining, for a
given configuration of OL and OR and external fields, which
monopole currents are nonzero and their dependence on the
external fields. In principle, one might also want to calculate
the way that the magnitude of the currents |(cid:5) ˆ(cid:4)JJosephson((cid:4)x)(cid:6)|
scale with the width of the DSL region and thickness of the
boundaries. Qualitatively, we expect the currents to decay as
a power law in the width w of the DSL region since the DSL
is a critical phase. Since the scaling dimension of a conserved
current is d in a d + 1-dimensional CFT, we expect
ORDERED
PHASE
DIRAC SPIN
LIQUID
ORDERED
PHASE
ORDERED
PHASE
ORDERED
PHASE
~
(a)
(b)
FIG. 2. Schematic of
the (a) effective monopole tunneling
Hamiltonian Eq. (15) and (b) our proxy Hamiltonian Eq. (18) used
to capture qualitative features of the Josephson currents obtained by
schematically “integrating out” the DSL.
where (cid:19)b,L and (cid:19)b,R are the boundary scaling dimensions
of the monopole operators on the left and right boundaries,
respectively. [Here, we have also defined the right-hand side
(RHS) of Eq. (14) as E for later convenience.] If the details of
the interface provide an additional length scale, then this could
modify the above scaling. Calculating (cid:19)b,L, (cid:19)b,R and any
additional interface effects is a complicated boundary CFT
problem beyond the scope of this work, so we will not address
this issue in any more quantitative detail.
A. Effective Hamiltonian
Our first task is to write a low energy Hamiltonian coupling
the two ordered regions L and R to the DSL. In the DSL,
monopole operators are the most relevant in the RG sense, and
hence coupling terms involving monopole tunneling should
be the most important at low energy (we expect this from
the large N f scaling dimensions of monopole operators when
one sets N f = 4; see Table I). This motivates the following
coupling Hamiltonian [see Fig. 2(a)]:
(cid:11)
(cid:12)
6(cid:2)
Hc = −
(cid:20)i j,L
dy ˆ(cid:15)†
iL(xL, y) ˆ(cid:15) jD(xL, y)
i, j=1
(cid:12)
(cid:13)
+(cid:20)i j,R
dy ˆ(cid:15)†
iD(xR, y) ˆ(cid:15) jR(xR, y)
+ H.c.,
(15)
where the left (right) interface is at x = xL (x = xR) and y runs
parallel to the boundary (in both terms above). A remark on
notation—to emphasize a monopole-tunneling interpretation,
we have used the same symbol ˆ(cid:15)i for both the monopole
operator in the DSL side ( ˆ(cid:15)iD) and on the ordered sides
ˆ(cid:15)i,L/R. But we note that in general, they would have different
scaling dimensions. For example, as one crosses the interface,
the system goes through a phase transition and the monopole
scaling dimension at the transition is known to be smaller at
the phase transition than deep in the DSL (from a large N f
calculation [19,21]).
Since we assumed that the coupling matrix (cid:20)i j,L/R pre-
serves spin rotation symmetry,
(cid:20)i j,L/R ≡ (cid:20)Sδi j,
for 4 (cid:2) i, j (cid:2) 6.
(16)
|(cid:5) ˆ(cid:4)JJosephson((cid:4)x)(cid:6)| ∝
|(cid:5) ˆ(cid:3)L(cid:6)||(cid:5) ˆ(cid:3)R(cid:6)|
w2−(cid:19)b,L−(cid:19)b,R
≡ E,
Now, since the boundary breaks spatial symmetries, but pre-
serves spin-rotation symmetries, we are also allowed to add
single monopole terms for the spin-singlet monopoles, but not
(14)
013169-5
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
spin triplet monopoles:
ˆHsource =
(cid:12)
3(cid:2)
i=1
dy[Vi,L ˆ(cid:15)i(xL, y) + Vi,R ˆ(cid:15)i(xR, y)] + H.c.
(17)
ˆHsource, i.e.,
ˆH = ˆHDSL + ˆHOL
We argue in Appendix C that ˆHsource does not contribute sig-
nificantly to the currents we are interested in. Then the full
+
Hamiltonian without
ˆHc has a global spin rotation SO(3) symmetry, and formally,
also a global U (1)top symmetry. We will now use the fol-
lowing strategy—we first write a schematic Hamiltonian for
monopole tunneling between OL and OR, where the DSL
region is assumed to have been “integrated out” (we ignore
any potential retardation effects coming from integrating out
gapless modes in the DSL):
+ ˆHOR
ˆHeff = ˆHOL
+ ˆHOR
− (cid:20)eff
S
6(cid:2)
i=4
( ˆ(cid:15)†
iL
ˆ(cid:15)iR + H.c.)
3(cid:2)
(cid:14)
−
i, j=1
(cid:20)eff
i j
ˆ(cid:15)†
iL
ˆ(cid:15) jR + H.c.
(cid:15)
,
(18)
where we have neglected the spatial dependence of ˆ(cid:15)iL [see
Fig. 2(b)]. The parameter
to U (1)top, T [U (1)top] = 16×6. For explicit formulas for T r
when G = SO(6), see Eq. (B13).
The set of operators ˆ(cid:15)i will serve as the order parameter.
They will acquire expectation value when the symmetry is
broken. Assuming that ˆ(cid:15)i is a sum of local operators, Eq. (20)
holds approximately even when the operators are restricted
to small regions. Suppose the expectation value (cid:5) ˆ(cid:15)iR(cid:6) on the
right differs from that on the left (cid:5) ˆ(cid:15)iL(cid:6). We now compute the
current for each generator ˆQr from left to right.
To do this, let us write an effective Hamiltonian. It is
identical to Eq. (18), except that the following Hamiltonian
assumes that the coupling respects the full symmetry G:
ˆH = ˆHL + ˆHR − (cid:20)
N(cid:2)
( ˆ(cid:15)†
iL
ˆ(cid:15)iR + ˆ(cid:15)†
iR
ˆ(cid:15)iL ),
(21)
i=1
where (cid:20) is an effective coupling constant depending on the
details of the intermediate region between L and R. The cur-
rent of generator r from left to right ˆI r
L→R can be calculated
from the Heisenberg equation of motion
ˆI r
L→R
≡ − d ˆQr
dt
L
(cid:16)
ˆQr
L
(cid:17)
.
, ˆH
= i
(22)
Qr
L commutes with HL because it is conserved, and with HR
because HR has support only on side R. The only nonzero
contribution comes from the coupling term,
(cid:2)
i, j
(cid:20)eff
S
∼ EL,
(19)
ˆI r
L→R
= −i(cid:20)
ˆ(cid:15)†
iLT r
i j
ˆ(cid:15) jR + H.c.
(23)
where the factor of L, the system length along y, comes from
the integration along the y direction and E was estimated in
Eq. (14). We compute the conserved spin current and U (1)top
current flowing from OL to OR using the above Hamilto-
nian, and assume that by current conservation (justified in
Appendix C), the same current also flows through the spin
liquid.
B. Brief review of the generalized Josephson effects
Equation (18) is of the form which is usually used to derive
generalized Josephson currents between ordered phases which
break symmetries belonging to a continuous group G [22–24].
Here, we provide a brief review of this formalism which com-
putes the DC and AC Josephson currents of a given symmetry
generator, following Ref. [22]. Let ˆQr for r ∈ {1, . . . M} be
quantum operators corresponding to the M generators of G.
ˆQr are the 15 SO(6) charges ˆQtot[σ ατ β]
For our problem,
(where α and β are not both 0) and the emergent flux ˆbtot.
Now suppose the system is divided into left and right parts L
and R [Fig. (2)]. We assume that each ˆQr can be written as
a sum of local operators (see Appendix B for a discussion).
This allows us to define ˆQr
L/R as the restriction of ˆQr to the
respective region L/R. Let ˆ(cid:15)i be N operators charged under
G, i.e., they transform under the group action. The group
action is
[ ˆQr, ˆ(cid:15)i] =
N(cid:2)
−T r
i j
ˆ(cid:15) j,
(20)
j=1
where T r is an N × N Hermitian matrix of c numbers and
is a representation of ˆQr on CN . When r above corresponds
The expectation value of the RHS above has a disconnected
component and a connected component. Since the two sides
of the system are symmetry breaking, (cid:5) ˆ(cid:15) jL/R(cid:6) is macroscopic.
So, to lowest order, we will ignore the connected piece. Thus,
(cid:18)
(cid:19)
ˆI r
L→R
≈ −i(cid:20)
(cid:2)
(cid:5) ˆ(cid:15)†
iL
(cid:6)T r
i j
(cid:5) ˆ(cid:15) jR(cid:6) + H.c.
(24)
i, j
This is the DC Josephson effect. The same formula can also
be used for the AC Josephson effect as follows. A term is
added to the Hamiltonian that couples to the difference in
− ˆQs
a conserved charge across the two sides:
L )
(for example, the electric potential difference between the
two superconductors). As we will see below, this results in
an oscillatory time dependence for (cid:5) ˆ(cid:15)i(L/R)(cid:6), and therefore
according to Eq. (24), the current ˆI r
L→R also acquires an os-
cillatory time dependence,
ˆHμ = μ
2 ( ˆQs
R
d ˆ(cid:15)iR
dt
d ˆ(cid:15)iL
dt
= −i
= +i
μ
2
μ
2
(cid:17)
(cid:16)
ˆ(cid:15)iR, ˆQs
R
= +i
(cid:17)
(cid:16)
ˆ(cid:15)iL, ˆQs
L
= −i
μ
2
μ
2
(cid:2)
j
(cid:2)
j
(T s)i j ˆ(cid:15) jR,
(25)
(T s)i j ˆ(cid:15) jL.
(26)
The solution is (suppressing the indices of ˆ(cid:3)L/R and T r)
2 tT s ˆ(cid:3)R(0) and ˆ(cid:3)L(t ) = e−i
ˆ(cid:3)R(t ) = ei
2 tT s ˆ(cid:3)(0).
μ
μ
(27)
Substituting in Eq. (24), we get
≈ −i(cid:20)(cid:5) ˆ(cid:3)†
(cid:19)
(cid:18)
μ
ˆI r
L→R(t )
2 tT s (cid:5) ˆ(cid:3)R(0)(cid:6) + H.c. (28)
If T r commutes with T s, then from Eq. (28), the current
oscillates at frequency μ—the familiar AC Josephson effect.
L(0)(cid:6)ei
T rei
2 tT s
μ
013169-6
MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
other:
(cid:5) ˆI[U (1)top](cid:6) = (cid:5) ˆI[σ 3](cid:6) = 2(cid:20)eff
S
|(cid:15)L||(cid:15)R| sin(ϕ).
(30)
Physically, the reason the two currents are the same is that
the carriers of conserved spin and the carriers for conserved
U (1)top charge are the same—the spin triplet monopoles. The
total current (cid:5) ˆI r(cid:6) is related to the current density |(cid:5) ˆ(cid:4)J r(cid:6)| as
|(cid:5) ˆ(cid:4)J r(cid:6)| = (cid:5) ˆI r(cid:6)/L where L is the length of the boundary. These
currents are perpendicular to both the OL-DSL and OR-DSL
boundaries. Therefore, the emergent electric field is parallel
to the boundaries. Since (cid:20)eff
S
∼ EL, we have
(cid:5)ˆe(cid:6) and (cid:5) ˆJ[σ 3](cid:6) ∼ E sin(ϕ),
(31)
where E has been estimated in Eq. (14). As we remarked pre-
viously, Eq. (31) should not be taken quantitatively, hence the
∼ symbol. The important takeaway is the sin ϕ dependence
on the angle mismatch and the observation that the Josephson
currents are those of the U (1)top and ˆSz generators, and are in
fact equal to each other.
2. OL = 120◦ AFM, OR = 120◦ AFM: AC Josephson effect
We again consider a junction with two 120◦ AFMs sepa-
rated by a DSL. For this configuration, the expectation values
of spin triplet monopole operators on either side of the junc-
tion are
(cid:5) ˆ(cid:4)(cid:3)L(cid:6) = |(cid:15)L|(1
i
0)T and (cid:5) ˆ(cid:4)(cid:3)R(cid:6) = |(cid:15)R|(1
i
0)T .
(32)
We now propose two scenarios that lead to an AC Josephson
effect. The first scenario is analogous to the AC Josephson
effect in superconductors obtained by applying a potential
difference, a term that couples to the difference in number
of particles on the right and left. Analogously, here we can
apply the following term to the Hamiltonian that couples to the
difference in emergent magnetic flux across the two sides (the
conserved U (1)top charge). On a triangular lattice, we show in
Appendix B 1 that such a term takes the form of the sum of
staggered spin chiralities
(cid:2)
ˆHμ =
≡
(cid:4)n
(cid:2)
(cid:4)n
(cid:14)(cid:14)
ˆ(cid:4)S(cid:4)n × ˆ(cid:4)S(cid:4)n+(cid:4)a1
(cid:15)
μ(x)
· ˆ(cid:4)S(cid:4)n+(cid:4)a1− (cid:4)a2
−
(cid:14)
ˆ(cid:4)S(cid:4)n × ˆ(cid:4)S(cid:4)n+(cid:4)a1
(cid:15)
· ˆ(cid:4)S(cid:4)n+(cid:4)a2
(cid:15)
μ(x)( ˆχ(cid:2),(cid:4)n − ˆχ(cid:19),(cid:4)n),
(33)
ˆQs is applied and the current in generator
where μ(x) has a gradient from L to R such that μR − μL ≡
μ. Now, we will use the formula in Eq. (28) to determine
the Josephson currents. In this formula, a perturbation in
ˆQr is
generator
calculated. For the perturbation considered in Eq. (33), s cor-
responds to U (1)top. Using Eq. (32) in Eq. (28), and noting
that T s = T [ ˆbtot] = 1 we see that the channels r in which
we get nonzero currents are U (1)top and ˆSz
tot (i.e., emergent
electric field and spin current). Like before, the two are equal:
(cid:5) ˆI[U (1)top](t )(cid:6) = (cid:5) ˆI[σ 3](t )(cid:6) = 2(cid:20)eff
S
|(cid:15)L||(cid:15)R| sin(μt ).
(34)
Here, we have made use of the observation in Eqs. (9) and
(10) that (cid:5) ˆ(cid:4)(cid:3)L(cid:6) and (cid:5) ˆ(cid:4)(cid:3)R(cid:6) are both eigenvectors of T [σ 3] with
eigenvalue 1. The above equation says that a difference in spin
FIG. 3. DC Josephson effect: a (120◦ AFM—DSL—120◦ AFM)
arrangement induces a DC electric field inside the DSL. The spins
of the 120◦ AFM on the left obey Eq. (12), while those on the right
are rotated with respect to Eq. (12) by angle ϕ. This results in a spin
current, whose carriers inside the DSL are monopoles. The resulting
monopole current is equivalent to an emergent electric field.
C. Monopole Josephson currents in a DSL
We will now use the above framework to qualitatively
determine the Josephson currents in the setup shown in Fig. 1.
The symmetry generators ˆQr are the total magnetic flux ˆbtot
and the 15 generators of SO(6), namely, ˆQtot[σ ατ β] (where α
and β are not both 0. Note that for β = 0 and α ∈ {1, 2, 3},
ˆQtot[σ α] is just the total conserved spin ˆSα
tot). The operators
charged under ˆQr are the six monopoles ˆ(cid:15)i which transform
as an SO(6) vector. In Sec. II A, we saw that the monopoles
serve as order parameters for 120◦ AFMs and
12 va-
lence bond solids. Now consider the scenario shown in Fig. 1.
Deep inside the DSL, we assume that Gspace of the triangular
lattice is obeyed. Therefore, (cid:5) ˆ(cid:3)(cid:6) = 0 here. At the same time,
deep inside OL and OR, monopoles are condensed and acquire
macroscopic expectation value (cid:5) ˆ(cid:3)L(cid:6) and (cid:5) ˆ(cid:3)R(cid:6), respectively.
These ordered phases act as a source of monopoles which can
tunnel through the DSL. We show below how one can get DC
and AC Josephson effects for the setup where both OL and OR
are in the 120◦ AFM phase.
12 ×
√
√
1. OL = 120◦ AFM, OR = 120◦ AFM with angle mismatch:
DC Josephson effect
Assume both ordered phases are in a 120◦ AFM state,
in which the spin triplet monopoles have acquired nonzero
expectation value. Suppose the plane of ordering in spin-space
is the same for OL and OR, which we take to be the xy plane.
Now consider the situation where the ordering pattern on OR
is misaligned with respect to OL by an angle ϕ (see Fig. 3). By
this, we mean that if the spins in OL form the ordering pattern
given in Eq. (12), all the spins in OR are rotated by angle ϕ
about the z − axis with respect to the configuration dictated
by Eq. (12). (Here, we have assumed that the lattice does not
contain any defects.) For this situation, the expectation values
of the spin triplet monopoles on either side take the form
(cid:5) ˆ(cid:4)(cid:3)L(cid:6) = |(cid:15)L|(1
i
0)T , (cid:5) ˆ(cid:4)(cid:3)R(cid:6) = eiϕ|(cid:15)R|(1
i
0)T .
(29)
Now, we can apply the formula for Josephson current Eq. (24).
Due to the redundancy between ˆSz spin rotation and U (1)top
phase rotation that we observed in Eqs. (9) and (10), we get
both a U (1)top current and a spin current that are equal to each
013169-7
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
a time-dependent monopole current, or an emergent electric
field in the DSL.
Describing and probing this electric field is what we will
now focus on. What does an emergent electric field mean in
the language of microscopic spins? Using the transformation
of electric field under microscopic symmetries (see first row
of Table III in Appendix B), we can write the following ex-
pression for the zero momentum electric field (i.e., integrated
over space), keeping only nearest-neighbor terms:
(cid:12)
FIG. 4. The proposed setup (120◦ AFM—DSL—120◦ AFM) to
induce and probe the AC Josephson effect. An out of plane (w.r.t.
magnetic ordering) Zeeman field gradient of magnitude h is applied
across the junction, causing the spins on the left to precess at a
different rate than the spins on the right. This precession results in
a spin current, whose carriers inside the DSL are monopoles. The
resulting emergent electric field within the DSL can be probed via
Raman scattering.
chirality terms applied across the two ordered phases leads to
a time-dependent spin current and an equal emergent electric
field. This is a nontrivial prediction of the theory.
However, applying an external term Eq. (33) is not simple
experimentally (although there has been a proposal to get a
spin chirality term in the effective Floquet Hamiltonian of
a spin system driven with a laser [25]). Therefore, we now
propose a simpler way to get the same time dependent electric
field and spin current as before, but this time exploiting our
observation in Eqs. (9) and (10).
In this second scenario, we apply a Zeeman field gradient
across the junction instead of ˆHμ above, as shown in Fig. 4,
(cid:2)
ˆHh =
(cid:15)
,
(cid:14)
ˆSz
(cid:4)n
h(x)
(cid:4)n
(35)
where h(x) has a gradient from L to R such that hR − hL ≡
h. The only difference now as far as the formula Eq. (28)
is concerned, is that T s = T [σ 3] instead of 1. But since
T [σ 3](cid:5) ˆ(cid:4)(cid:3)L(R)(cid:6) = (cid:5) ˆ(cid:4)(cid:3)L(R)(cid:6), this difference does not change the
currents. We therefore again obtain an AC spin current and an
equal AC emergent electric field inside the DSL given by
(cid:5) ˆI[U (1)top](t )(cid:6) = (cid:5) ˆI[σ 3](t )(cid:6) = 2(cid:20)eff
S
|(cid:15)L||(cid:15)R| sin(ht ).
(36)
We can understand this physically as follows. The presence
of the Zeeman field gradient leads to a precession of the
macroscopic 120◦ order parameter with a different rate on the
two sides of the junction, resulting in a spin current. Similar
phenomena have been studied theoretically in several works
previously, for conventional magnetically ordered systems
[26–31], and 3He and spinor BECs [32–34].
What is different for our setup is that the proximity to
the DSL, and the assumption that the 120◦ AFMs are close
to a phase transition to a DSL imply that the carriers of the
spin current in the DSLs are monopole operators. Therefore,
any time-dependent spin current should be accompanied by
(ˆex )tot ≡
d 2x ˆex
(cid:2)
(cid:14)
ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2
(cid:2)
(cid:4)n
d 2x ˆey = v
= v
(cid:12)
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2−(cid:4)a1
(cid:15)
+ · · · ,
(37)
(cid:14)
2
(cid:4)n
(ˆey)tot ≡
ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a1
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2
1√
3
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2−(cid:4)a1
where v is a constant of the order of the Dirac velocity, which
in turn is of the order of Ja where J is the exchange coupling
strength and a is the lattice spacing. As a consequence of the
AC Josephson effect discussed above, we expect
+ · · · ,
(38)
(cid:15)
((cid:5)ˆex(t )(cid:6), (cid:5)ˆey(t )(cid:6)) = E sin(ht ) (cos θ , sin θ ),
(39)
where θ is the angle made by the electric field with x axis (the
direction of the electric field is tangential to the DSL-AFM
boundaries). E is given by the right-hand side of Eq. (14),
calculating which is beyond the scope of this work. The key
point is that since the DSL is described by a CFT, E decays
only as a power law in the width of the DSL.
We see that the operators ˆex and ˆey have a nontrivial spa-
tial structure. To detect this “electric field” consistently, we
would need a probe that is sensitive to rotational form-factors.
Optical probes are well-suited for this purpose because of the
control one gets from the direction of polarization of light
[35,36]. We now propose a way to measure the emergent AC
electric field inside the DSL using Raman scattering.
IV. RAMAN SCATTERING PROBE OF EMERGENT
ELECTRIC FIELD
Suppose the DSL region is irradiated with a laser of fre-
quency ωi. A Raman signal corresponds to inelastic scattering
of light, i.e., the outgoing photon’s frequency ω f is different
from ωi. We will now argue that the presence of an emergent
electric field in the DSL of frequency h, and in particular
the one produced in the setup considered in Sec. III C 2 (see
Fig. 4), will lead to peaks at Raman frequency shifts ω(cid:19) ≡
ω f − ωi = ±h.
It was shown in Refs. [37,38] that the Raman scattering
rate R for a spin system (not necessarily a spin liquid) is given
by the following correlation function calculated in an energy
eigenstate of the spin system |i(cid:6):
(cid:12) ∞
R =
−∞
dteiω(cid:19)t (cid:5)i| ˆM†
(cid:4)q (0) ˆM (cid:4)q(t )|i(cid:6),
(40)
where (cid:4)q = (cid:4)q f − (cid:4)qi is the momentum transferred to the photon.
(For simplicity, we will ignore this small momentum transfer
ˆM acts on the Hilbert space of
from now on.) The operator
013169-8
MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
the spin system, and depends on the underlying lattice for the
spin system as well as the polarizations and momenta of the
ˆM was calculated
incident and scattered light. The operator
for some relevant cases in Refs. [10,37], and we will present
the leading-order results later in Eq. (42).
However, if the expectation value of some operator (in our
case, the emergent electric field) (cid:5)ψ|ˆ(cid:4)e(t )|ψ(cid:6) in a state |ψ(cid:6)
were to depend sinusoidally on time, then |ψ(cid:6) is clearly not
an energy eigenstate but rather a nonequilibrium state. In such
a state, we show in Appendix D that Eq. (40) gets modified
and the Raman scattering rate now measures the following
time-averaged correlation function of the same operator ˆM:
R = lim
T →∞
(cid:12) T
2
− T
2
1
T
(cid:12) T
2
− T
2
dt0
dt(cid:5)ψ| ˆM†(t0) ˆM(t + t0)|ψ(cid:6)eiω(cid:19)t .
(41)
We will use the following (Fleury-Loudon [39]) form for ˆM,
(cid:13)
,
· ((cid:4)n(cid:12) − (cid:4)n)}{(cid:4)(cid:12)i · ((cid:4)n(cid:12) − (cid:4)n)}
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n(cid:12)
ˆM =
(cid:2)
(cid:11)
{(cid:4)(cid:12)∗
f
2t2
(cid:4)n,(cid:4)n(cid:12) ˜e2
U − ωi
(cid:4)n,(cid:4)n(cid:12)
1
4
(42)
which requires a bit of explanation. Here (cid:4)(cid:12)i and (cid:4)(cid:12) f are the
polarizations of the incoming and outgoing photons, respec-
tively. Equation (42) assumes that the spins arise from a single
band Hubbard model at half filling in the large U limit, of the
following form:
⎛
⎞
(cid:2)
ˆHel = −
⎝
t(cid:4)r,(cid:4)r(cid:12) ˆc†
(cid:4)rσ ˆc(cid:4)r(cid:12)σ eie ˆ(cid:4)A( (cid:4)r+(cid:4)r
2
(cid:12)
+ U
(cid:4)r,(cid:4)r(cid:12),σ
(cid:2)
(cid:4)r
ˆn(cid:4)r,↑ ˆn(cid:4)r,↓.
)·((cid:4)r−(cid:4)r(cid:12) ) + H.c.
⎠
(43)
Here ˆ(cid:4)A((cid:4)r) is the electromagnetic field and has the following
expansion in photon creation and annihilation operators
ˆ(cid:4)A((cid:4)r) =
(cid:2)
(cid:4)k
1(cid:20)
2εV ω(cid:4)k
((cid:4)(cid:12)(cid:4)k ˆa(cid:4)k
+ (cid:4)(cid:12)∗
−(cid:4)k
ˆa†
−(cid:4)k
)ei(cid:4)k.(cid:4)r,
(44)
where (cid:4)(cid:12)(cid:4)k is the polarization of mode (cid:4)k, and ε and V are the
dielectric constant and laser mode volume, respectively. We
√
have defined the coupling constant ˜e2 ≡ e2a2
where e is
√
2εV
the electron charge and Ni is the initial number of photons in
the mode of frequency ωi. The driving is assumed to be near
resonance, but at the same time satisfying trr(cid:12) (cid:21) |U − ωi| (cid:21)
U . Under this assumption, one can calculate the scattering rate
perturbatively in both t/(ωi − U ) and ˜e, the light-matter cou-
pling constant; one obtains Eq. (42) at order ˜e2t2/(ωi − U ).
Ni
ωiω f
f )∗(cid:12)k
It is convenient to decompose the tensor ((cid:12) j
into two
one-dimensional (A1g, A2g) and one two-dimensional (Eg) ir-
reducible representations of the triangular lattice point group
(cid:15)∗(cid:12)x
(cid:15)∗(cid:12)y
(cid:14)
−
(cid:14)
(cid:12)x
f
(cid:15)∗(cid:12)y
(cid:15)∗(cid:12)x
,
(cid:15)∗(cid:12)y
(cid:14)
(cid:12)y
f
(cid:15)∗(cid:12)x
−
(cid:15)∗(cid:12)x
(cid:12)x
f
(cid:15)∗(cid:12)y
+
(cid:14)
(cid:12)x
f
(cid:14)
(cid:12)x
f
(cid:21)
+
(cid:14)
(cid:12)y
f
(cid:14)
(cid:12)y
f
(cid:14)
(cid:12)y
f
A2 ≡
(cid:13)
A1 ≡
E1
E2
(45)
+
≡
(cid:22)
(cid:11)
,
.
i
i
i
i
i
i
i
i
i
On the triangular lattice, using this basis reduces Eq. (42) to
+ E2 ˆOE2
− E1 ˆOE1
+ ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2
+ ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2−(cid:4)a1
(cid:15)
, where
(cid:15)
,
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2−(cid:4)a1
(cid:15)
,
(cid:14)
A1 ˆOA1
ˆM = 4t2 ˜e2
U − ωi
(cid:2)
(cid:14)
ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a1
=
ˆOA1
(cid:4)n
√
ˆOE2
=
ˆOE1
= 1
4
(cid:4)n
(cid:2)
(cid:14)
ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2
3
4
(cid:4)n
(cid:2)
(cid:14)
2 ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a1
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2
− ˆ(cid:4)S(cid:4)n · ˆ(cid:4)S(cid:4)n+(cid:4)a2−(cid:4)a1
(cid:15)
. (46)
t2 ˜e2
U −ωi
, there is no term in the A2g channel. For the
Up to order
particular case of a Dirac spin liquid, we can use Eq. (37) to
relate the emergent electric fields to microscopic quantities.
Up to corrections involving longer range terms, we see that
ˆOE2 and ˆOE1 are indeed proportional to the emergent electric
fields (ˆex )tot and (ˆey)tot, respectively (because the symmetry
transformation of the emergent electric fields on the triangular
lattice is identical to that of the E2g channel.) On the other
ˆOA1 is proportional to the Hamiltonian of the system.
hand,
This lets us write the above expression as
ˆM = 4t2 ˜e2
U − ωi
(cid:23)
1
J
A1 ˆH +
√
3A
4v
(E2 ˆex − E1 ˆey) + · · ·
(cid:24)
,
(47)
where A is the area of the DSL region. We can now relate
the Raman scattering rate in a DSL to correlation functions of
the electric field and the Hamiltonian by inserting the above
expression into Eq. (41).
Before we proceed, we highlight two main differences
from previous theoretical literature, arising due to the pres-
ence of the AC Josephson effect in the setup in Sec. III C 2,
on Raman scattering. First, the Raman scattering rate is usu-
ally derived when the spin system is in an equilibrium state,
where one-point functions (cid:5) ˆO(t )(cid:6) for interesting operators
ˆO typically equal zero, in which case a correlation function
(cid:5) ˆO1(t1) ˆO2(t2)(cid:6) would be given entirely by its connected com-
ponent. However, in our case, the DSL is in a nonequilibrium
steady state where (cid:5)ˆ(cid:4)e(t )(cid:6) ∝ sin(ht ) [see Eq. (39)]. Hence, the
correlation function also has a disconnected component. In
what follows, we will assume that the contribution to the au-
tocorrelation function coming from the monopole Josephson
effect is dominated by the disconnected piece
(cid:5)ˆei(t1)ˆe j (t2)(cid:6) ≈ (cid:5)ˆei(t1)(cid:6)(cid:5)ˆe j (t2)(cid:6).
(48)
Since
lim
T →∞
1
T
(cid:12)
T /2
−T /2
lim
T →∞
dt0 sin(ht0) sin(h(t + t0)) = 1
2
(cid:12)
cos(ht ) and
dt0 sin(h(t + t0)) = 0,
(49)
1
T
T /2
−T /2
013169-9
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
dt0(cid:5)ˆex(t0) ˆH (t0 + t )(cid:6) ≈ 0.
(50)
B. Mixed current: OL = 120◦ AFM, OR =VBS
we find that the autocorrelation functions are sharply peaked
in frequency as follows:
(cid:12) T
dt0(cid:5)ˆex(t0)ˆex(t0 + t )(cid:6) ≈
E 2 cos2 θ
2
cos(ht ),
dt0(cid:5)ˆex(t0)ˆey(t0 + t )(cid:6) ≈
dt0(cid:5)ˆex(t0)ˆey(t0 + t )(cid:6) ≈
E 2 sin2 θ
2
cos(ht ),
E 2 sin(2θ )
4
cos(ht ),
lim
T →∞
lim
T →∞
lim
T →∞
lim
T →∞
1
T
1
T
1
T
1
T
2
− T
2
(cid:12) T
2
− T
2
(cid:12) T
2
− T
2
(cid:12) T
2
− T
2
This brings us to the second difference—an equilibrium cor-
relation function in a symmetry preserving state is diagonal
in the A1, A2, E2, E1 basis. But due the monopole Josephson
effect, the steady state no longer has rotational symmetry,
leading to mixing within the Eg channel [see Eq. (50)]. Now,
we are ready to write down the final result for the Raman
scattering rate
R = K|E1 sin θ − E2 cos θ |2{δ(ω(cid:19) − h) + δ(ω(cid:19) + h)},
(51)
t2 ˜e2
U −ωi
E 2( A
v
where K ≡ 3π
)2 is a constant. By tuning the polar-
2
izations of the incoming and detected photons, one can tune
the values of A1, A2, E1, E2. By measuring the scattering rate
R for each such choice, one can separately measure the corre-
lation function in each channel, and thus verify the prediction
in Eq. (51).
We have shown that for a junction with two 120◦ AFMs
separated by a DSL, if we apply a Zeeman field gradient h
across the junction, the AC emergent electric field resulting
from the monopole Josephson effect produces sharp peaks at
Raman frequency shifts ±h in the Eg channels. The strength
of the peak decays as a power law in the width of the junction.
The above setup provides a way to induce and directly probe
the emergent electric field in a Dirac spin liquid.
V. OTHER MONOPOLE JOSEPHSON EFFECTS
In this section, we present other effects which fall under
the general umbrella of monopole Josephson effects.
A. Josephson energy—Long-range phase rigidity
For the DC Josephson current, we assumed that the two
120◦ orders had an angle misalignment. This misalignment
could have arisen due to an external pinning potential, with
a strength smaller than the coupling (cid:20)S
eff, so that our as-
sumption that the spin SO(3) is conserved continues to hold.
Here, on the other hand, we suppose that there is no external
pinning and we let the relative angle between ˆ(cid:4)(cid:3)L and ˆ(cid:4)(cid:3)R
to fluctuate. In other words, if (cid:5) (cid:4)(cid:3)L(cid:6) = |(cid:15)L|(1
0) and
(cid:5) (cid:4)(cid:3)R(cid:6) = eiϕ|(cid:15)R|(1
0), we let ϕ be a dynamical degree
of freedom.
i
i
As a first approximation, one can calculate the energy of
such a configuration from the Hamiltonian Eq. (15) from
just the disconnected piece of the two-monopole correlation
function
E [ϕ] ≈ −EL((cid:5) ˆ(cid:4)(cid:3)†
(cid:6) · (cid:5) ˆ(cid:4)(cid:3)R(cid:6) + H.c.)
L
∼ −2EL|(cid:15)L||(cid:15)R| cos(ϕ).
(52)
This Josephson energy implies that there is a restoring force
that tries to align the angles of two 120◦ AFM puddles sep-
arated by a DSL. This restoring force is proportional to E,
which we expect to decay only as a power law in the width
of the DSL [see Eq. (14)] since the DSL is a critical phase.
Therefore, puddles of ordered phases separated by regions
of DSL will display a tendency for their order parameters to
align, a behavior we call long-range phase rigidity.
pletely different orders: OL = 120◦ AFM and OR =
√
Consider a junction with a DSL region separating two com-
12 ×
12 VBS. Using the parent state picture, we view OL as the
condensate of spin triplet monopoles and OR as the condensate
of spin singlet monopoles, as follows:
√
= |(cid:15)L|(0
(cid:5)((cid:15)1 (cid:15)2 (cid:15)3 (cid:15)4 (cid:15)5 (cid:15)6)(cid:6)T
L
0)T ,
(cid:5)((cid:15)1 (cid:15)2 (cid:15)3 (cid:15)4 (cid:15)5 (cid:15)6)(cid:6)T
R
0)T .
= |(cid:15)R|(1
1
0
0
0
i
i
0
0
(53)
We now argue that in this configuration, the DSL will have
currents of the mixed spin-valley generators of SO(6). As a
toy model, we start with the coupling Hamiltonian in Eq. (21).
This assumes an SO(6) symmetric term for monopole tunnel-
ing. In this case, one can directly use Eq. (24) to calculate
the Josephson currents. We then see that the currents with
nonzero expectation value are those for the the mixed SO(6)
generators: ˆ(cid:4)J[σ iτ j]. For the example we picked in Eq. (53),
ˆ(cid:4)J 14 ≡ ˆ(cid:4)J[σ 1τ 1] and ˆ(cid:4)J 25 ≡ ˆ(cid:4)J[σ 2τ 2] are nonzero and equal, and
the remaining independent currents are 0.
The presence of these mixed currents can be identified
by their symmetry breaking patterns in the bulk of the DSL
region. In Appendix B, we summarize the symmetry proper-
ties of all such currents (Table III) on the triangular lattice.
We observe that when a general combination of the mixed
currents (last three rows of Table III) has a nonzero expec-
tation value, time-reversal symmetry is broken and discrete
translation symmetry is reduced to translations by two lattice
spacings along both (cid:4)a1 and (cid:4)a2, i.e., a four-site unit cell forms
in the DSL region.
We point out, however, that the microscopic model does
not have an SO(6) symmetry. Therefore, our assumption
above of an SO(6)-symmetric coupling at the interface is
not strictly justified. Nevertheless, we can qualitatively argue
that the physical consequence of having mixed currents in
the DSL—unit-cell expansion and time-reversal symmetry
breaking (within the DSL bulk) continues to hold. Consider
the effective Hamiltonian Eq. (18) with a coupling that breaks
SO(6) to SO(3)spin. Then we have
(cid:5)− ˙ˆQL[σ i](cid:6) = (cid:5) ˙ˆQR[σ i](cid:6) = 0 and
(cid:5)− ˙ˆQL[τ i](cid:6) = (cid:5) ˙ˆQR[τ i](cid:6) = 0, while
(cid:5)− ˙ˆQL[σ iτ j](cid:6) (cid:16)= (cid:5) ˙ˆQR[σ iτ j](cid:6), but both are nonzero.
(54)
013169-10
MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
DSL
ux
(a)
Metal
(b)
SC
SC
Electromagnetic
ux
(c)
Metal
DSL
(d)
FIG. 5. (a) In a SQUID geometry (SC-metal-SC-metal), thread-
ing a flux φ through the center results in a tangential electric current
I (black arrow). (b) Similarly, a DSL with a flux φ in U (1)top going
through results in an emergent electric field (cid:5)ˆ(cid:4)e(cid:6) radially outwards
(blue arrows). (c) A DSL in the presence of a lattice dislocation.
The red circles mark the two lattice sites making up the dislocation.
(d) Mean field considered for numerics in the presence of two dislo-
cations with opposite Burger’s vectors (red and blue). Gray triangle
indicates π flux.
The above equation says that, not surprisingly, the charge
of mixed generators lost from side L is not equal to that
gained by side R. This leakage however, is localized to the
boundaries, because deep inside the DSL, the mixed currents
are still conserved. Generically then, the expectation value
of some combination of mixed currents in the DSL should
still be nonzero. This is true in the limit of SO(6)-symmetric
coupling, and as we go away from this point, there is no reason
for the mixed currents to immediately drop to zero.
C. Response to U (1)top flux insertion—Lattice dislocation
For the familiar DC Josephson effect, a phase difference
between the right (R) and left (L) superconductors is main-
tained by threading a magnetic flux, like the SQUID geometry
shown in Fig. 5(a), because the gauge invariant phase differ-
(cid:4)A · d(cid:4)r.
ence between points R1 and L1 is θR1 − θL1 + e
This results in a current between the two superconductors in
the tangential direction.
R1
L1
(cid:4)
In Fig. 5(b), we consider a related configuration involving
the DSL. Here φ is the flux inserted in U (1)top. For simplicity,
we have assumed that the full system is in the DSL phase.
An analogous situation for metals is persistent currents in the
ground state [40] in the presence of magnetic flux, as required
by the Byers-Yang theorem [41]:
I[φ] = − 1
T
∂F [φ]
∂φ
(55)
,
where I is the equilibrium current, T is the temperature, and F
is the free energy. Similar to an electron current in a SQUID,
this results in a tangential U (1)top current, i.e., a radial electric
field. But how do we insert a flux in U (1)top? Lattice trans-
lations are known to have a nontrivial U (1)top action when
embedded into the low-energy symmetry group GIR [14,16].
Therefore, a symmetry defect of lattice translation, that is, a
lattice dislocation [see Fig. 5(c)], serves as a U (1)top flux.4
Following the above argument, we expect that a lattice
dislocation creates a radial electric field, which by Gauss’s
Law results in a spinon charge (U (1) gauge charge and not
a U (1)top charge) near the dislocation. As a first step, in the
parton picture, we can verify this prediction at the mean field
level. We consider the mean-field ansatz shown in Fig. 5(d) on
a lattice with two dislocations with opposite Burger’s vectors
((cid:4)a2 and −(cid:4)a2, respectively) separated by d lattice spacings. The
triangles shaded grey have π flux going through them. This
ansatz preserves time-reversal but breaks charge-conjugation
symmetry. We then numerically diagonalize the correspond-
ing free fermion Hamiltonian on an L × L torus. The lowest
L2/2 levels are filled by both spin ↑ and ↓ fermions in the
ground state. Then we compute the charge in a region D
enclosing the dislocation
(cid:5) ˆqdislo(cid:6) = 2
(cid:2)
⎣
⎡
L2/2(cid:2)
⎤
(cid:13)
(cid:11)
(cid:27)
(cid:27)ψ i
(cid:4)r
(cid:27)
(cid:27)2 − 1
2
(cid:4)r∈D
i=1
⎦,
(56)
where ψ i
(cid:4)r is the single fermion wave function of the ith eigen-
state (sorted in increasing order of energy) evaluated at (cid:4)r. For
L = 100, d = 50 and D being a circle of radius 20, we get
qdislo = 0.297.
We are unable to determine whether a nonzero spinon
charge survives once we include gauge fluctuations. Quali-
tatively, we expect that the charge gets renormalized due to
screening, but may not drop to 0. If the localized spinon
charge is indeed nonzero, then what it would mean in terms
of microscopic spins is an open question. In Eq. (B8) of Ap-
pendix B, we have written a nontrivial microscopic operator
that is consistent with both the symmetry properties of the
field theory spinon charge operator and Gauss’s law. How
this expression gets modified for a lattice with dislocations,
and whether any resulting spinon charge can be computed
numerically in candidate DSL wave functions [42,43] is an
interesting direction to pursue.
VI. DISCUSSION
This work uses the viewpoint that certain magnetically
ordered states can be obtained upon condensing monopole
operators which enter the low energy description of a Dirac
spin liquid. We have argued that by using a Josephson junction
geometry with two ordered states separated by a DSL, one
can induce an emergent electric field (both DC and AC) in
the DSL. Further, we have shown that such an AC emergent
electric field can be measured optically as a sharp field-tunable
peak in Raman scattering. Also, the induced electric field is
4In addition to being a U (1)top symmetry flux, it is also an SO(6)
symmetry flux, but this fact does not play a role in the rest of our
discussion.
013169-11
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
accompanied by a measurable spin current across the junc-
tion that is proportional to the emergent electric field. This
serves as an independent check that can be used to validate
our first prediction. We have also highlighted other phenom-
ena conceptually related to the monopole Josephson effect,
namely long-range phase rigidity between puddles of ordered
phases separated by Dirac spin liquids, “mixed currents”
across AFM-DSL-VBS junctions and spinon charge bound to
lattice dislocations.
In general, an AFM–X–AFM junction, for some unknown
phase X and generic details of the interface, will (at least in
the short-junction limit) allow monopole tunneling analogous
to that of the AFM–DSL–AFM junction. The observation of,
for example, a spin current in a DC junction setup is therefore
insufficient to claim that the unknown phase X is a DSL.
However, two of the effects we propose to measure, namely
the field-tunable Raman peak and the power-law dependence
of the spin current as a function of junction size, require
a conserved monopole current in region X [or equivalently
require that the low-energy degrees of freedom of region X
include an emergent U (1) electric field]. This requirement is
satisfied when X is a DSL, but not in ordered phases such as
valence bond solids where a spin singlet monopole creation
operator acquires nonzero expectation value, and hence the
monopole current (electric field) is not a conserved quantity.
Therefore, measuring our proposed field-tunable sharp Raman
peak in the region X in conjunction with a spin current across
the interface, such that both the strength of the Raman peak
and the spin current scale as a power law in the width of region
X, will be strong evidence that X is a DSL.
We note that our predictions are backed up by writing a
phenomenological monopole tunneling Hamiltonian that as-
sumes that a DSL couples to a nearby ordered state chiefly
through monopole tunneling terms, since monopoles are the
most relevant operators in the DSL. We do not however at-
tempt a full boundary conformal field theory calculation. This
is because the current understanding of QED3 as a CFT (even
without boundaries) is still in its nascent stages, although there
have been promising recent numerical developments [44–46].
We also note that due to the Josephson effect, the quantum
state of the DSL region differs from the ground state of QED3.
For example, when there is an AC electric field through the
DSL, the DSL is in a nonequilibrium state. When there is a
mixed spin-valley current, lattice translation symmetry gets
broken. In such cases, whether the framework of DSL theory
is still a valid description or not would depend on the strength
of coupling between the different regions, size of the DSL
region, and temperature. Determining this would again require
a detailed boundary CFT calculation, and is beyond the scope
of this work.
Our work presents an in-principle method to externally
induce and measure emergent gauge field strengths in strongly
coupled spin liquids in 2 + 1 dimensions. In general, one
has more control over the degrees of freedom in an ordered
state. So looking forward, attempting to probe operators in
other spin liquids using more conventional ordered states is a
promising direction. We note that Ref. [47] theoretically con-
sidered tunneling of spinons between ferromagnets through a
quantum spin ice in 3 + 1D, and is closely related to this idea.
A second interesting direction is in the context of recent
developments in Rydberg atom arrays that take us one step
closer to realizing a spin liquid in a laboratory [48]. In these
experiments, one can access projections of the microscopic
wave function in a preferred basis. It will therefore be interest-
ing to come up with signatures of long wavelength operators
and nonequilibrium steady state features such as currents,
but in the many-body wave function, such that they can be
accessed in these experiments.
Note added: Just before the submission of this work,
we became aware of another recent work [49] considering
monopole tunneling in Dirac spin liquids.
ACKNOWLEDGMENTS
Some ideas and calculations on Raman scattering probes of
spin liquids presented in Sec. IV were inspired by the indepen-
dent ongoing work by Mohammad Hafezi and collaborators
(to be published). The authors are grateful to Mohammad
Hafezi for useful discussions on this and other topics. This
work was supported by the National Science Foundation
under Grant No. DMR-2037158, the U.S. Army Research
Office under Contract No. W911NF1310172, and the Simons
Foundation (V.G. and G.N.). D.B. was supported by JQI-PFC-
UMD.
APPENDIX A: REVIEW OF STABILITY OF DSL
If the DSL were to be a stable CFT, then it should contain
no relevant (scaling dimension (cid:19) > 3) symmetry allowed
operators. In Table I, we summarize the scaling dimensions
(cid:19) of some of the important operators derived by refer-
ences [3,13,50,51] in the large N f limit. Ref. [14] determined
the symmetry properties of monopole operators for various
lattices:
(1) Bipartite lattices: There is a symmetry allowed 2π
monopole which is relevant according to the large N f analysis
summarized in Table I. Hence, a DSL cannot be a stable phase
on bipartite lattices.
(2) Kagome lattice: There is a symmetry allowed 4π
monopole, which is likely relevant ((cid:19) ≈ 2.5) according to the
large N f calculation.
(3) Triangular lattice: ˆ(cid:3)2π and ˆ(cid:3)4π break translation sym-
metry and hence are symmetry-forbidden. (We will provide a
more microscopic motivation for this fact in Appendix B 2.)
A 6π monopole operator is symmetry allowed, but is irrele-
vant ((cid:19) = 4.322) according to the large N f calculation. This
suggests that a DSL could indeed be a stable phase on the
triangular lattice.
So, in this work, whenever we refer to microscopic op-
erators, we will assume a triangular lattice for concreteness.
However, our general idea applies to any lattice which can
realize a DSL as a stable phase.
APPENDIX B: MICROSCOPIC EXPRESSIONS FOR FIELD
THEORY OPERATORS
In this section, we construct microscopic operators cor-
responding to operators in the effective field theory. For a
ˆOtot ≡
d 2x ˆO(x), we construct
given field theory operator
(cid:4)
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MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
TABLE I. Scaling dimensions (cid:19)N f of some important primary operators in QED3, calculated in the large N f
Refs. [3,13,50,51]. An operator with (cid:19) > 3 is relevant in the RG sense.
limit, compiled from
Operator
Monopoles
ˆ(cid:3)2π
ˆ(cid:3)4π
ˆ(cid:3)6π
Fermion bilinears
¯ψσ ατ β ψ, (α, β are not both 0)
¯ψψ
Conserved charges and currents
ˆb, ˆei, ˆQ[σ ατ β ], ˆ(cid:4)J[σ ατ β ]
(cid:19)N f
0.2651N f − 0.0381 + O(1/N f )
0.6731N f − 0.1934 + O(1/N f )
1.1864N f − 0.4211 + O(1/N f )
2 − 64
3π 2N f
2 + 128
3π 2N f
+ O( 1
N 2
f
+ O( 1
N 2
f
)
)
2
(cid:19)N f =4
1.022
2.499
4.325
1.46
3.08
2
the microscopic operator
1. Emergent electric and magnetic field
ˆOtot =
(cid:2)
(cid:4)n
ei (cid:4)Q.(cid:4)n ˆO(cid:4)n,
(B1)
where we have allowed for ˆOtot to have momentum (cid:4)Q at the
lattice scale. We use the following procedure [15,16,52]:
(1) Find how ˆOtot transforms under the microscopic sym-
metries. For operators that can be written in terms of fermionic
partons, this can be done using information obtained by
expanding around Dirac points [53]. We tabulate the transfor-
mation properties of the conserved charges of GIR in Table II,
and conserved currents in Table III.
(2) Construct operators order by order in size (maximum
of weight, i.e., number of spins in the support of a local
term, and diameter, i.e., extent of a local term) transforming
identically as ˆOtot.
We do this for the emergent electric and magnetic fields
and spinon charge density in Appendix B 1. For monopole
operators, this procedure is harder, and requires information
at the lattice scale. Reference [14] did this using a Wannier
center calculation. In Appendix B 2, we will motivate their
result using an independent approach involving the algebra of
operators.
TABLE II. Symmetry properties of conserved charges of GIR.
SO(3) is spin-rotation (S and T stand for singlet and triplet under
spin-rotation, respectively). T is time-reversal. T1 and T2 are lattice
translations about (cid:4)a1 and (cid:4)a2, respectively. C6 is rotation by 2π /6
about a vertex. Rx is reflection about (cid:4)a1.
Charge
SO(3)
ˆbtot
ˆQ[σ i]
ˆQ[τ 1]
ˆQ[τ 2]
ˆQ[τ 3]
ˆQ[σ iτ 1]
ˆQ[σ iτ 2]
ˆQ[σ iτ 3]
S
T
S
S
S
T
T
T
T
−1
−1
−1
−1
−1
1
1
1
T1
1
1
−1
1
−1
−1
1
−1
T2
1
1
−1
−1
1
−1
−1
1
C6
Rx
−1
1
ˆQ[τ 2]
− ˆQ[τ 3]
− ˆQ[τ 1]
− ˆQ[σ iτ 2]
Q[σ iτ 3]
ˆQ[σ iτ 1]
1
1
ˆQ[τ 3]
− ˆQ[τ 2]
ˆQ[τ 1]
− ˆQ[σ iτ 3]
ˆQ[σ iτ 2]
− ˆQ[σ iτ 1]
(cid:12)i j
2π ˆe j
ˆ(cid:4)J[σ i]
ˆ(cid:4)J[τ 1]
ˆ(cid:4)J[τ 2]
ˆ(cid:4)J[τ 3]
ˆ(cid:4)J[σ iτ 1]
ˆ(cid:4)J[σ iτ 2]
ˆ(cid:4)J[σ iτ 3]
S
T
S
S
S
T
T
T
013169-13
(cid:4)
d 2x ˆb(x). Because ˆbtot
The generator of U (1)top is the total emergent mag-
netic flux ˆbtot ≡ 1
is odd under
2π
time-reversal (see Table II), and singlet under spin rota-
tion, the lowest weight term is a three-spin spin chirality:
ˆ(cid:4)S(cid:4)n × ˆ(cid:4)S(cid:4)n+(1,0)) ·
ˆχ(cid:2),(cid:4)n ≡ (
ˆ(cid:4)S(cid:4)n+(0,1). Here, we have used the notation (n1, n2) ≡ n1(cid:4)a1 +
n2(cid:4)a2. If we only keep (1) elementary triangles (
) and (2)
triangles whose two edges are nearest-neighbor (
), then
the only term consistent with symmetries is
ˆ(cid:4)S(cid:4)n × ˆ(cid:4)S(cid:4)n+(1,−1)) · ˆ(cid:4)S(cid:4)n+(1,0) and ˆχ(cid:19),(cid:4)n ≡ (
(cid:2)
ˆbtot =
( ˆχ(cid:2),(cid:4)n − ˆχ(cid:19),(cid:4)n) + · · ·
(B2)
(cid:4)n
Since the total emergent magnetic flux is the U (1)top charge
density, it follows from Faraday’s law that the emergent elec-
tric field ˆ(cid:4)e is the U (1)top conserved current rotated by 90◦.
If we consider all operators for ˆ(cid:4)e made of terms with two
spins (both nearest-neighbor and next-nearest-neighbor), then
(in the notation: ˆ(cid:4)e ≡ ˆe1(cid:4)a1 + ˆe2(cid:4)a2)
ˆei = α1(ˆei )(1) + α2(ˆei )(2) + · · ·
for i = 1, 2,
(B3)
TABLE III. Symmetry properties of conserved currents of
U (1)top and SO(6). Notation: “V ” means “transforms as a vector,”
“−V ” means transforms as a vector except for a factor of −1. “V
as ˆ(cid:4)J[σ iτ j]” means the current’s spatial indices are transformed as a
vector while the SO(6) indices are rotated to σ iτ j, possibly with an
overall sign.
Current
SO(3)
T
T1
T2
1
1
1
C6
−V
Rx
−V
1
1 −1
1
1
1
1 −1 −1
1 −1
V
V as ˆ(cid:4)J[τ 3]
V as − ˆ(cid:4)J[τ 2]
V as ˆ(cid:4)J[τ 1]
V
V as ˆ(cid:4)J[τ 2]
V as − ˆ(cid:4)J[τ 3]
V as − ˆ(cid:4)J[τ 1]
−1 −1 −1 V as − ˆ(cid:4)J[σ iτ 2] V as − ˆ(cid:4)J[σ iτ 3]
V as ˆ(cid:4)J[σ iτ 2]
V as ˆ(cid:4)J[σ iτ 3]
−1
V as − ˆ(cid:4)J[σ iτ 1]
V as ˆ(cid:4)J[σ iτ 1]
−1
1 −1
1 −1
1
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
where
with the notation
(B4)
(B5)
(B6)
Note that ˆe1 and ˆe2 are not orthogonal. ˆex and ˆey are related to
ˆe1 and ˆe2 as ˆex = ˆe1 and ˆey = 1√
3
From the above expression for electric field, one can
compute the local divergence of the electric field, which is
proportional to the spinon charge by Gauss’s law:
(−ˆe1 + 2ˆe2).
ˆq = 1
g2
divˆ(cid:4)e.
(B7)
We see that only ˆ(cid:4)e(2) contributes to ˆq, and not ˆ(cid:4)e(1). Therefore,
ˆ(cid:4)e(1) is the transverse electric field and ˆ(cid:4)e(2) is the longitudinal
electric field. ˆq at site i is given by
(B8)
By construction, we see that the sum of spinon charge en-
closed in a region D is an operator with support localized to
the boundary of D. This is consistent with Gauss’s law. It also
satisfies (cid:5) ˆqi(cid:6) = 0 by symmetry.
2. Monopole operators from commutation relations
The symmetry properties of monopole operators were cal-
culated in Refs. [14,16] and reported in Table 2 of Ref. [14].
The SO(6) contribution was calculated using Step 1 (begin-
ning of Appendix B). The U (1)top contribution was calculated
using a Wannier center calculation of the free fermion bands
for the mean field ansatz. Here, we attempt an alternative
approach to calculate the U (1)top contribution. While our cal-
culation involves an uncontrolled approximation, it provides
an independent motivation for the result in Ref. [16].
The principle behind our approach is that the algebra of GIR
has to be obeyed down to the microscopic level because we
are dealing with operators of the form ˆOtot here, which have
the longest possible wavelength (allowed by their symmetry
properties):
(cid:17)
+ δad ˆQcb
tot
− δbd ˆQca
tot
(cid:15)
, (B9)
(cid:16)
ˆQab
tot
, ˆQcd
tot
= i
(cid:14)
δbc ˆQda
tot
(cid:16)
− δac ˆQdb
tot
(cid:17)
ˆbtot, ˆQab
tot
Here { ˆQab} (antisymmetric in a, b with a, b running from 1 to
4) are the 15 generators of SO(6) and ˆbtot is the generator of
U (1)top (see Appendix B 4 for the notation).
(B10)
= 0.
Next, 2π monopole operators that are charged under GIR
have to obey the algebra
[ ˆbtot, ( ˆ(cid:15)†
j )tot,
j )tot] = ( ˆ(cid:15)†
6(cid:2)
(cid:16)
ˆQbc
tot
, ( ˆ(cid:15)†
j )tot
(cid:17)
=
( ˆ(cid:15)†
i )tot(T bc)i j,
(B11)
(B12)
i=1
where, T bc, a matrix of c numbers, is the generator of ˆQbc
tot
acting on C6, and the matrix elements are given by
(T bc)i j = −i(δc jδib − δb jδic).
(B13)
This suggests a general procedure:
(cid:3)
(1) Suppose ˆOtot =
operators of increasing “size” s,
∞(cid:2)
(cid:4)n ei (cid:4)Q.(cid:4)n ˆO(cid:4)n. If we now expand ˆO(cid:4)n in
ˆO(cid:4)n =
Cs( ˆO(cid:4)n)s.
(B14)
s=1
Here, each ( ˆO(cid:4)n)s is chosen to respect the symmetry properties
obtained just from the low energy theory.
(2) Demand Eqs. (B9)–(B10), (B11)–(B12) order by order
in size s, and obtain constraints on Cs.
Here we will only perform this calculation at the lowest
order in size by enforcing Eq. (B11) up to a proportionality
constant
[ ˆbtot, ˆ(cid:4)(cid:3)†
tot] = K ˆ(cid:4)(cid:3)†
tot
,
(B15)
where K is a positive constant. Let us assume that
the
monopole inserting 2π flux is “simpler,” i.e., has a lower lead-
ing operator size than the one inserting 4π or 6π flux. Then
we ask what the “simplest” spin triplet monopole operator is.
We start with operators with size 1,
ˆ(cid:4)(cid:3)†
tot
=
ei (cid:4)Q·(cid:4)n ˆ(cid:4)S(cid:4)n + · · · .
(B16)
(cid:2)
(cid:4)n
From compatibility of translation symmetry with rotation
symmetry, (cid:4)Q is either (0,0) or ±(2π /3, −2π /3) [16]. Using
identity Eq. (B30), we evaluate the commutator in Eq. (B15)
using the lowest order expression for ˆb(1)
tot in Eq. (B2). Each
single-spin term in ˆ(cid:4)(cid:3)†
top fails to commute with exactly 6 trian-
gles in ˆb(1)
tot . After evaluating each of these commutators, we
get
(cid:16)
ˆb(1)
tot
, (cid:4)(cid:3)†
tot
(cid:17)
= −i
(cid:2)
(cid:30)
ei (cid:4)Q·(cid:4)n ˆ(cid:4)S(cid:4)n
(e−iQ1 − e−iQ2 )
(cid:14)
ˆ(cid:4)S(cid:4)n6
(cid:15)
· ˆ(cid:4)S(cid:4)n1
(cid:4)n
(cid:14)
ˆ(cid:4)S(cid:4)n1
+ (ei(Q1−Q2 ) − eiQ2 )
(cid:14)
ˆ(cid:4)S(cid:4)n2
+ (ei(Q1−Q2 ) − eiQ1 )
(cid:15)
(cid:15)
· ˆ(cid:4)S(cid:4)n2
· ˆ(cid:4)S(cid:4)n3
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MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
+ (eiQ2 − eiQ1 )
(cid:14)
ˆ(cid:4)S(cid:4)n3
+ (eiQ2 − ei(Q2−Q1 ))
× (e−iQ1 − ei(Q2−Q1 ))
(cid:15)
· ˆ(cid:4)S(cid:4)n4
(cid:14)
ˆ(cid:4)S(cid:4)n4
(cid:14)
ˆ(cid:4)S(cid:4)n5
(cid:15)
· ˆ(cid:4)S(cid:4)n5
· ˆ(cid:4)S(cid:4)n6
The spin singlet monopoles are time-reversal even. Here,
we will only keep the lowest weight terms that are dot prod-
ucts of neighboring spins:
ˆ(cid:15)†
i
= v1 ˆ(cid:15)†(1)
i
+ v2 ˆ(cid:15)†(2)
i
for i ∈ {1, 2, 3}, where
(B24)
(cid:15)(cid:31)
+ · · · ,
(B17)
where (cid:4)n1 ≡ (cid:4)n + (0, −1), (cid:4)n2 ≡ (cid:4)n + (1, −1), (cid:4)n3 ≡ (cid:4)n + (1, 0),
(cid:4)n4 ≡ (cid:4)n + (0, 1), (cid:4)n5 ≡ (cid:4)n + (−1, 1), and (cid:4)n6 ≡ (cid:4)n + (0, −1).
From this, it is clear that (Q1, Q2) = (0, 0) will give 0 as
the commutator. Therefore, if the monopole operator is to
have single spin terms as its leading order term, then (cid:4)Q =
±(2π /3, −2π /3). For K in Eq. (B15) to be positive, we
choose (cid:4)Q = (2π /3, −2π /3). Here, we have made use of the
fact that the DSL is a ground-state of an antiferromagnetic
Heisenberg-like Hamiltonian where (cid:5) ˆ(cid:4)Si · ˆ(cid:4)S j(cid:6) < 0 for nearest
neighbors i and j. So, (Q1, Q2) = (2π /3, −2π /3). With this
choice, Eq. (B17) becomes
where we use the notation
(B18)
(B19)
To our guess for
(cid:4)ˆ(cid:15)†
top, we now add the RHS obtained above,
ˆ(cid:4)(cid:3)†(2) + · · · ,
ˆ(cid:4)(cid:3)†(1) + β2
= β1
(B20)
ˆ(cid:4)(cid:3)†
tot
(B25)
(B26)
(B27)
+ · · · ,
(cid:22)
√
3
2
v2
where we use the notation
We can now use the identity Eq. (B34) to get
(cid:16)
ˆb(1)
tot
(cid:16)
ˆb(1)
tot
, ˆ(cid:15)†(1)
i
, ˆ(cid:15)†(2)
i
(cid:17)
(cid:17)
(cid:17)
= 1
2
ˆ(cid:15)†(2)
i
= ˆ(cid:15)†(1)
i
−
+ · · · ,
√
3
2
(cid:21)
ˆ(cid:15)†(2)
i
= v2 ˆ(cid:15)†(1)
i
+
1
2
v1 −
where
ˆ(cid:4)(cid:3)†(1) =
(cid:2)
(cid:4)n
ei (cid:4)Q·(cid:4)n ˆ(cid:4)S(cid:4)n, and ˆ(cid:4)(cid:3)†(2) =
(cid:2)
(cid:4)n
ei (cid:4)Q·(cid:4)n ˆ(cid:4)S(cid:4)n ˆ(cid:2)(cid:4)n.
(B21)
⇒
(cid:16)
ˆb(1)
tot
, v1 ˆ(cid:15)†(1)
i
+ v2 ˆ(cid:15)†(2)
i
We can use the commutator Eqs. (B30)–(B33) to get
(cid:16)
ˆb(1)
tot
(cid:16)
ˆb(1)
tot
(cid:17)
(cid:17)
, ˆ(cid:4)(cid:3)†(1)
, ˆ(cid:4)(cid:3)†(2)
√
= −
√
= −
3 ˆ(cid:4)(cid:3)†(2) and
ˆ(cid:4)(cid:3)†(1) − ˆ(cid:4)(cid:3)†(2) + · · ·
3
3
4
!
.
(B22)
Using the above equation, truncating at terms supported on at
most elementary triangles, we get
(cid:2)
ˆ(cid:4)S(cid:4)n − 4
3
ˆ(cid:4)S(cid:4)n ˆ(cid:2)(cid:4)n + · · ·
ˆ(cid:4)(cid:3)†
tot
(cid:13)
,
ei (cid:4)Q.(cid:4)n
(B23)
=
(cid:11)
(cid:4)n
where (cid:4)Q = (2π /3, −2π /3).
a. Spin singlet monopoles
Having determined the momentum of the spin-triplet 2π -
monopoles, the momenta of spin-singlet monopoles can be
fixed by the low energy theory since the embedding of the
space-group symmetries into SO(3)valley can be computed
purely from low energy information. Doing so results in Table
2 of Ref. [16]. Here, we will write microscopic expressions for
them.
× ˆ(cid:15)†(2)
i
+ · · · .
(B28)
√
If we demand proportionality already to this order, then we
obtain K = v2/v1 = 0.396. In contrast, K for the spin triplet
3, although in theory they should
monopole in Eq. (B23) is
be the same. The discrepancy is the result of our uncontrolled
approximation to drop higher size terms, since the commuta-
tor of two high size operators can give a lower size operator
[for example, Eq. (B31)]. Nevertheless, using this approach
we have been able to motivate why the U (1)top contribution to
monopole momentum is (2π /3, −2π /3).
It could be a fruitful direction to assume that the coeffi-
cients Cs do decay with operator size s and self-consistently
solve for Cs using the general approach described above. Since
the generators ˆQab are generators for emergent global internal
symmetries, naïvely, one would expect that ˆQab is a sum of
approximately local terms, and Cs decays exponentially with
size s. It will be interesting to verify that this is indeed the
case, and if so, to determine what sets the decay length when
the IR theory is conformally invariant. If this approach suc-
ceeds, then it would help one to study DSLs without resorting
013169-15
NAMBIAR, BULMASH, AND GALITSKI
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
[ ˆO({(cid:4)Si}), ( ˆ(cid:4)S1 × ˆ(cid:4)S2) · ˆ(cid:4)S3],
(B29)
−
to parton construction and serve as a technique complemen-
tary to the one explored in Ref. [54].
3. List of useful commutation relations
Here, we list some useful commutation relations of various
spin operators with the spin chirality, i.e., commutators of the
form
where ˆO[{(cid:4)Si}] is a local operator made of spins.
ˆO: Spin triplet made of single spin:
[ ˆ(cid:4)S1, ( ˆ(cid:4)S1 × ˆ(cid:4)S2) · ˆ(cid:4)S3] = i[( ˆ(cid:4)S1 · ˆ(cid:4)S2) ˆ(cid:4)S3 − ( ˆ(cid:4)S1 · ˆ(cid:4)S3) ˆ(cid:4)S2].
ˆO: Spin triplet made of three spins:
(B30)
[(
ˆ(cid:4)S1 × ˆ(cid:4)S2) · ˆ(cid:4)S3]
ˆ(cid:4)S1 · ˆ(cid:4)S2)
= − i
8
ˆ(cid:4)S3, (
ˆ(cid:4)S1 − ˆ(cid:4)S2) + i
ˆ(cid:4)S2 · ˆ(cid:4)S3)
4
[( ˆ(cid:4)S1 · ˆ(cid:4)S2) ˆ(cid:4)S3, ( ˆ(cid:4)S1 × ˆ(cid:4)S2) · ˆ(cid:4)S4] = − i
2
[(
(
ˆ(cid:4)S1 − (
ˆ(cid:4)S1 · ˆ(cid:4)S3)
ˆ(cid:4)S2],
(B31)
( ˆ(cid:4)S1 · ˆ(cid:4)S4 − ˆ(cid:4)S2 · ˆ(cid:4)S4) ˆ(cid:4)S3,
Note that V eff
i,P is a coupling arising under RG flow in the
effective field theory due to the boundaries breaking spatial
(cid:5) ˆ(cid:4)(cid:3)L/R(cid:6),
symmetries. Hence, it is small when compared to (cid:20)eff
S
which in contrast is macroscopic in the 120◦ AFM. The source
term leads to the following extraneous contribution to the
U (1)top current:
(cid:21)
(cid:22)
#
"
d ˆbtot,L
dt
= i
ˆbtot,L,
3(cid:2)
(cid:14)
V eff
i,L
(cid:15)
ˆ(cid:15)†
iL
+ H.c.
extra
i=1
= i
(cid:14)
V eff
i,L
3(cid:2)
i=1
ˆ(cid:15)†
iL
− H.c.
(cid:15)
.
(C2)
Now, we take expectation value of the above expression. The
result is proportional to the expectation value of a spin singlet
monopole at the boundary of a 120◦ AFM phase. The only
reason this expectation value is nonzero is because of V eff
i,L .
Hence, (cid:5) ˆ(cid:15)iL(cid:6) is first order in V eff
)extra(cid:6) is
second order in V eff
i,L , which we neglect due to the assumption
that V eff
i,L is small.
i,L . Therefore, (cid:5)−( d ˆbtot,L
dt
(B32)
APPENDIX D: FORMULA FOR RAMAN SCATTERING
OFF A NONEQUILIBRIUM STATE
[( ˆ(cid:4)S1 · ˆ(cid:4)S2) ˆ(cid:4)S3, ( ˆ(cid:4)S1 × ˆ(cid:4)S3) · ˆ(cid:4)S4]
= i
2
[( ˆ(cid:4)S2 · ˆ(cid:4)S4) ˆ(cid:4)S1 + ( ˆ(cid:4)S3 · ˆ(cid:4)S4) ˆ(cid:4)S2 − ( ˆ(cid:4)S2 · ˆ(cid:4)S3) ˆ(cid:4)S4 − ( ˆ(cid:4)S1 · ˆ(cid:4)S4) ˆ(cid:4)S2].
(B33)
ˆO: Spin singlet made of two spins:
[ ˆ(cid:4)S1 · ˆ(cid:4)S2, ( ˆ(cid:4)S1 × ˆ(cid:4)S2) · ˆ(cid:4)S3] = − i
2
( ˆ(cid:4)S1 · ˆ(cid:4)S3 − ˆ(cid:4)S2 · ˆ(cid:4)S3).
(B34)
4. Remarks on notation
We write the generator corresponding to the charge and
current in square brackets, e.g., ˆQ[σ iτ j] and ˆQ[U (1)top].
Correspondence between the notations ˆQab and ˆQtot[σ iτ j]:
ˆQtot[σ 2] = ˆQ64,
ˆQtot[σ 1] = ˆQ56,
ˆQtot[σ 3] = ˆQ45,
(B35)
ˆQtot[τ 1] = ˆQ23,
ˆQtot[τ 2] = Q31, Q[τ 3]tot = ˆQ12, (B36)
ˆQtot[σ iτ j] = ˆQ3+i, j for 1 (cid:2) i, j (cid:2) 3.
(B37)
APPENDIX C: IGNORING SOURCE TERMS FOR SPIN
SINGLET MONOPOLES
In this section, we argue why source terms for spin singlet
monopoles potentially arising due to spatial symmetry break-
ing near the boundaries [see Eq. (17)], do not significantly
affect the U (1)top Josephson current between two 120◦ AFMs.
For simplicity, let us work with the effective Hamiltonian in
terms of the ordered phases alone, with the DSL integrated
out, as we did in Eq. (18). The source term, localized to the
boundaries modifies Eq. (18) as follows:
In this Appendix, we will derive Eq. (41) for the Raman
scattering rate when the spin system is not in an energy
eigenstate, but in a nonequilibrium steady state. While we will
have Raman scattering in mind for the sake of concreteness,
our derivation applies for any scattering process. We have two
systems—light and matter. Light is used to probe matter (the
DSL in our case). The full time-independent Hamiltonian is
ˆH = ˆH0 + ˆV ,
(D1)
where ˆH0 is the Hamiltonian for the matter and light fields
separately and ˆV is the light matter coupling. Suppose that at
time t = 0, the system is in state |ψ(cid:6) ⊗ |ni; 0(cid:6), i.e., the matter
part of the state is |ψ(cid:6) ≡
ψl |l(cid:6) (where |l(cid:6) is an energy
l
eigenstate of the matter Hamiltonian) and the light part has
ni photons in a mode of frequency ωi and 0 photons in mode
ω f . In the final state, at time T , the light part is in the state
|ni − 1; 1(cid:6), while the matter part is in an unknown state | f (cid:6).
The scattering rate is given by
R = 1
T
|((cid:5) f | ⊗ (cid:5)ni − 1; 1|) ˆU (T )(|ψ(cid:6) ⊗ |ni; 0(cid:6))|2
(cid:2)
(cid:3)
f
= 1
T
(cid:2)
f
ψl ((cid:5) f | ⊗ (cid:5)ni − 1; 1|) ˆU (T )(|l(cid:6) ⊗ |ni; 0(cid:6))
(cid:27)
(cid:27)
2
(cid:27)
(cid:27)
(cid:27)
,
(cid:27)
(cid:27)
(cid:2)
(cid:27)
(cid:27)
(cid:27)
l
(D2)
where ˆU (T ) ≡ e−i( ˆH0+ ˆV )T is the time-evolution operator. For
ease of notation, we now define
|L(cid:6) ≡ |l(cid:6) ⊗ |ni; 0(cid:6) and H0|L(cid:6) = EL|L(cid:6),
where
EL ≡ El + niωi,
(D3)
(D4)
ˆHnew = ˆHeff +
3(cid:2)
(cid:2)
i=1
P=L,R
(cid:14)
V eff
i,P
ˆ(cid:15)†
i,P
+ H.c.
(cid:15)
.
(C1)
|F (cid:6) ≡ | f (cid:6) ⊗ |ni − 1; 1(cid:6) and H0|F (cid:6) = EF |F (cid:6),
where EF ≡ E f + (ni − 1)ωi + ω f .
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MONOPOLE JOSEPHSON EFFECTS IN A DIRAC SPIN …
PHYSICAL REVIEW RESEARCH 5, 013169 (2023)
So, Eq. (D2) becomes
R = 1
T
For T > 0,
(cid:2)
(cid:2)
(cid:27)
(cid:27)
(cid:27)
(cid:27)
(cid:27)
(cid:27)
(cid:27)
2
(cid:27)
ψl (cid:5)F | ˆU (T )|L(cid:6)
(cid:27)
(cid:27)
.
(D5)
f
l
ˆU (T ) = i ˆGR(T ) = i
(cid:12) ∞
−∞
dω
2π
ˆGR(ω)e−iωT ,
(D6)
where ˆGR(ω) is the retarded Green’s function for the full sys-
tem (light + matter). Using the standard T-matrix formalism,
we can write
ˆGR(ω) = ˆG0
R(ω) + ˆG0
R(ω) ˆTR(ω) ˆG0
(D7)
(here, ω+ ≡ ω + i0+) and ˆTR(ω) =
R(ω),
where ˆG0
ˆV + ˆV ˆG0
Clearly,
R(ω) = 1
ω+− ˆH0
R(ω) ˆV + ˆV ˆG0
R
ˆG0
ˆV ˆG0
R
ˆV + · · · .
R cannot induce a transition that changes the
number of photons; only the second term involving ˆTR can
do so. Thus, we get the following scattering amplitude:
(cid:5)F | ˆU (T )|L(cid:6) = i
(cid:12) ∞
−∞
dω
2π
e−iωT
(cid:5)F | ˆTR(ω)|L(cid:6)
(ω+ − EF )(ω+ − EL )
. (D8)
The ω integral should be closed in the lower half plane for
convergence. This integral will pick up poles at EF − i0+
and EL − i0+. The poles of ˆTR(ω) will not play a role under
the assumption T (cid:24) 1/(EF − EM ), which is the regime of
interest since we wish to consider the large T limit. Here, EM
is the total energy (light + matter) of any level M such that
(cid:5)F | ˆV |M(cid:6) (cid:16)= 0. In such a large T limit, one can expand out
ˆTR(ω) and see that our assumption is justified. So, we get
(cid:5)F | ˆU (T )|L(cid:6) = −2ie−i(EF +EL )T /2 sin((EF − EL )T /2)
EF − EL
× (cid:5)F | ˆTR(ω = EF )|L(cid:6)
(D9)
≈ −2π ie−iEF T δ(E f + ω f − El − ωi )
up to a constant of proportionality
(cid:5)F | ˆTR(ω = EF )|L(cid:6) = (cid:5) f | ˆM|l(cid:6),
(D11)
i.e., the above matrix element for the full system is propor-
tional to a matrix element of the matter part alone. ˆM has been
calculated in Refs. [10,37] and depends on the initial and final
polarizations of light, momentum transferred by light and the
lattice of the matter system. We have presented the leading
ˆM in Eq. (42). Substituting Eq. (D10)
order expression for
into Eq. (D2), we get
(cid:2)
(cid:2)
ψ ∗
l (cid:12) ψl (4π )2δ(E f + ω f − El − ωi )
R ≈ 1
T
f
l,l (cid:12)
× δ(E f + ω f − El (cid:12) − ωi )(cid:5)l (cid:12)| ˆM†| f (cid:6)(cid:5) f | ˆM|l(cid:6)
= 1
T
(cid:2)
(cid:2)
f
l,l (cid:12)
ψ ∗
l (cid:12) ψl (4π )2δ(E f + ω f − El − ωi )
× δ(El (cid:12) − El )(cid:5)l (cid:12)| ˆM†| f (cid:6)(cid:5) f | ˆM|l(cid:6)
(cid:12) T
(cid:2)
(cid:12) ∞
dt0ei(El(cid:12) −El )t0
2
− T
2
−∞
f
l (cid:12) ψl (cid:5)l (cid:12)| ˆM†| f (cid:6)(cid:5) f | ˆM|l(cid:6),
ψ ∗
= lim
T →∞
1
T
(cid:2)
×
l,l (cid:12)
(D12)
dtei(E f +ω f −El −ωi )t
(D13)
where in the last equation, we used the Fourier representation
of the δ function. Now, we can associate the phases in the
above equation with the phases coming from time evolution
to simplify it as follows:
R = lim
T →∞
1
T
(cid:2)
(cid:2)
T /2
(cid:12)
(cid:12) ∞
f
l,l (cid:12)
−T /2
dt0
−∞
dtei(ω f −ωi )t
× ψ ∗
(cid:12)
l (cid:12) ψl (cid:5)l (cid:12)| ˆM†ei ˆH0t0 | f (cid:6)(cid:5) f |ei ˆH0t Me−i ˆH0 (t+t0 )|l(cid:6)
1
T
dtei(ω f −ωi )t
(cid:12) ∞
dt0
T /2
−∞
−T /2
= lim
T →∞
× (cid:5)F | ˆTR(ω = EF )|L(cid:6).
(D10)
Now, (cid:5)F | ˆTR(ω = EF )|L(cid:6) is the same operator that appears in
the equilibrium calculation in Refs. [10,37]. As shown there,
× (cid:5)ψ| ˆM†(t0) ˆM(t + t0)|ψ(cid:6),
(D14)
where ˆM(t ) = ei ˆH0t ˆMe−i ˆH0t . This completes the derivation of
Eq. (41).
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10.1007_s00220-023-04637-5.pdf
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Commun. Math. Phys.
Digital Object Identifier (DOI) https://doi.org/10.1007/s00220-023-04637-5
Communications in
Mathematical
Physics
Unitarity of Minimal W -Algebras and Their
Representations I
Victor G. Kac1, Pierluigi Möseneder Frajria2, Paolo Papi3
1 Department of Mathematics, MIT, 77 Mass. Ave, Cambridge, MA 02139, USA.
E-mail: [email protected]
2 Politecnico di Milano, Polo regionale di Como, Via Anzani 42, 22100 Como, Italy.
E-mail: [email protected]
3 Dipartimento di Matematica, Sapienza Università di Roma, P.le A. Moro 2, 00185 Rome, Italy.
E-mail: [email protected]
Received: 9 August 2022 / Accepted: 5 January 2023
© The Author(s) 2023
Abstract: We begin a systematic study of unitary representations of minimal W -algebras.
In particular, we classify unitary minimal W -algebras and make substantial progress in
classification of their unitary irreducible highest weight modules. We also compute the
characters of these modules.
Contents
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Introduction . . . . . . . . . . . . . . . . . .
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2.1 Basic Lie superalgebras . . . . . . . . . .
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2.2 Conjugate linear involutions and real forms
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2.3 Invariant Hermitian forms on vertex algebras .
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3. The Almost Compact Conjugate Linear Involution of g .
4. Explicit Expressions for Almost Compact Real Forms .
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4.1 Uniqueness of the almost compact involution .
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5. The Bilinear Form (cid:2)·, ·(cid:3) on g−1/2
6. A General Theory of Invariant Hermitian Forms on Modules Over the Vertex
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7. Minimal W -Algebras
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7.1 λ-brackets and conjugate linear involutions .
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7.2 Some numerical information . . . . . . .
Algebra of Free Boson and the Fairlie Construction .
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10. Sufficient Conditions for Unitarity of Modules Over W k
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11. Unitarity of Minimal W -Algebras and Modules Over Them .
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12. Explicit Necessary Conditions and Sufficient Conditions of Unitarity .
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12.1 psl(2|2) . . . . . . . . . . . . . . . . . .
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V. G. Kac, P. Möseneder Frajria, P. Papi
12.3 spo(2|m), m > 4 . . . . . . . . . . . . . .
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12.4 D(2, 1; m
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12.5 F(4)
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13. Unitarity for Extremal Modules Over the N = 3, N = 4 and big N = 4 Super-
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13.1 g = spo(2|3) . . . . . . . . . . . . . . . .
13.2 g = psl(2|2) . . . . . . . . . . . . . . . .
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1. Introduction
In the present paper we study unitarity of minimal W -algebras and of their representa-
tions. Minimal W -algebras are the simplest conformal vertex algebras among the simple
vertex algebras Wk(g, x, f ), constructed in [18,20], associated to a datum (g, x, f ) and
k ∈ R. Here g = g¯0
⊕ g¯1 is a basic Lie superalgebra, i.e. g is simple, its even part g¯0 is
a reductive Lie algebra and g carries an even invariant non-degenerate supersymmetric
bilinear form (.|.), x is an ad-diagonalizable element of g¯0 with eigenvalues in 1
Z,
2
f ∈ g¯0 is such that [x, f ] = − f and the eigenvalues of ad x on the centralizer g f of
f in g are non-positive, and k (cid:6)= −h∨, where h∨ is the dual Coxeter number of g. The
most important examples are provided by x and f to be part of an sl2 triple {e, x, f },
where [x, e] = e, [x, f ] = − f, [e, f ] = x. In this case (g, x, f ) is called a Dynkin
datum. Recall that Wk(g, x, f ) is the unique simple quotient of the universal W -algebra,
denoted by W k(g, x, f ), which is freely strongly generated by elements labeled by a
basis of the centralizer of f in g [20].
We proved in [16, Lemma 7.3] that if φ is a conjugate linear involution of g such that
φ(x) = x, φ( f ) = f and (φ(a)|φ(b)) = (a|b), a, b ∈ g,
(1.1)
then φ induces a conjugate linear involution of the vertex algebra W k(g, x, f ), which
descends to Wk(g, x, f ).
We also proved in [16, Proposition 7.4] that if φ is a conjugate linear involution of
Wk(g, x, f ), this vertex algebra carries a non-zero φ-invariant Hermitian form H (·, ·)
for all k (cid:6)= −h∨ if and only if (g, x, f ) is a Dynkin datum; moreover, such H is unique,
up to a real constant factor, and we normalize it by the condition H (1, 1) = 1. A module
M for a vertex algebra V is called unitary if there is a conjugate linear involution φ of
V such that there is a positive definite φ-invariant Hermitian form on M. The vertex
algebra V is called unitary if the adjoint module is.
For some levels k the vertex algebra Wk(g, x, f ) is trivial, i.e. isomorphic to C; then
it is trivially unitary. Another easy case is when Wk(g, x, f ) “collapses” to the affine
part. In both cases we will say that k is collapsing level.
In the case of a Dynkin datum let g(cid:4) be the centralizer of the sl2 subalgebra s =
span {e, x, f } in g¯0; it is a reductive subalgebra. If φ satisfies the first two conditions
in (1.1), it fixes e, x, f , hence φ(g(cid:4)) = g(cid:4). It is easy to see that unitarity of Wk(g, x, f )
implies, when k is not collapsing, that φ|[g(cid:4),g(cid:4)] is a compact involution.
Unitarity of Minimal W -Algebras and Their Representations I
In the present paper we consider only minimal data (g, x, f ), defined by the property
that for the ad x-gradation g =
g j one has
(cid:2)
j∈ 1
2
Z
g j
= 0 if | j| > 1, and g−1
= C f.
(1.2)
In this case (g, x, f ) is automatically a Dynkin datum. The corresponding W -algebra
is called minimal. The element f ∈ g is a root vector attached to a root −θ of g, and
we shall normalize the invariant bilinear form on g by the usual condition (θ |θ ) = 2,
2 . Recall that the dual Coxeter number h∨ of g is half of
which is equivalent to (x|x) = 1
the eigenvalue of its Casimir element of g, attached to the bilinear form (.|.). We shall
(g) the minimal W -algebra, corresponding to g and k (cid:6)= −h∨, and by
denote by W min
W k
(g) the corresponding universal W -algebra.
min
We proved in [16, Proposition 7.9] that, if W min
(g) is unitary and k is not a collapsing
level, then the parity of g is compatible with the ad x-gradation, i.e. the parity of the
whole subspace g j is 2 j mod 2.
It follows from [18], [20] that for each basic simple Lie superalgebra g there is at
most one minimal Dynkin datum, compatible with parity, and the complete list of the g
which admit such a datum is as follows:
k
k
spo(2|m) for m ≥ 0,
osp(4|m) for m > 2 even, D(2, 1; a) for a ∈ C, F(4), G(3).
sl(2|m) for m ≥ 3,
psl(2|2),
(1.3)
The even part g¯0 of g in this case is isomorphic to the direct sum of the reductive Lie
algebra g(cid:4) and s ∼= sl2.
One of our conjectures (see Conjecture 4 in Sect. 8)1 states that any unitary W k
(g)-
(g). In fact, it is tempting to conjecture that for any conformal
module descends to W min
vertex algebra V any unitary V -module descends to the simple quotient of V .
min
k
It turns out (cf. Proposition 7.2) that a conjugate linear involution of the universal
(g) at non-collapsing level k is necessarily induced by a conju-
minimal W -algebra W k
(g) admits a unitary
gate linear involution φ of g. Moreover, by Proposition 8.9, if W k
highest weight module and k is not collapsing, then g(cid:4) has to be semisimple. As ex-
plained above, the involution φ of g must be almost compact, according to the following
definition.
min
min
Definition 1.1. A conjugate linear involution φ on g is called almost compact if
(i) φ fixes e, x, f ;
(ii) φ is a compact conjugate linear involution of g(cid:4).
Indeed (i) is equivalent to the first two requirements in (1.1), and the third requirement
in (1.1) follows from Lemma 3.1 in Sect. 3.
So, in order to study unitarity of highest weight modules, it is not restrictive to
(g) is induced by an almost compact
assume that the conjugate linear involution of W k
conjugate linear involution of g.
We prove in Sects. 3 and 4 that an almost compact conjugate linear involution φ
exists for all g from the list (1.3), except that a must lie in R in case of D(2, 1; a), and
is essentially unique.
min
1 See Note added in proof.
V. G. Kac, P. Möseneder Frajria, P. Papi
It was shown in [20] that the central charge of W min
k
(g) equals
c(k) = k d
k + h∨
− 6k + h∨ − 4, where d = sdimg.
(1.4)
Here is another useful way to write this formula:
c(k) = 7h∨
+ d − 4 − 12
√
− 6
√
(k + h∨ −
k + h∨
)2
, where
√
=
(cid:3)
d h∨
6
.
(1.5)
Recall that the most important superconformal algebras in conformal field theory are
the simple minimal W -algebras or are obtained from them by a simple modification:
(a) W min
(spo(2|N )) is the Virasoro vertex algebra for N = 0, the Neveu-Schwarz
vertex algebra for N = 1, the N = 2 vertex algebra for N = 2, and becomes
the N = 3 vertex algebra after tensoring with one fermion; it is the Bershadsky-
Knizhnik algebra for N > 3;
k
(b) W min
(c) W min
k
( psl(2|2)) is the N = 4 vertex algebra;
(D(2, 1; a)) tensored with four fermions and one boson is the big N = 4
vertex algebra.
The unitary Virasoro (N = 0), Neveu-Schwarz (N = 1) and N = 2 simple vertex
algebras, along with their irreducible unitary modules, were classified in the mid 80s.
Up to isomorphism, these vertex algebras depend only on the central charge c(k), given
by (1.4). Putting k = 1
− 1 in (1.5) in all three cases, we obtain
p
k
c(k) = 1 −
(cid:4)
c(k) = 3
2
(cid:4)
6
p( p + 1)
8
p( p + 2)
(cid:5)
1 −
1 − 2
p
for Virasoro vertex algebra,
(1.6)
(cid:5)
for Neveu-Schwarz vertex algebra,
(1.7)
c(k) = 3
for N = 2 vertex algebra.
(1.8)
The following theorem is a result of several papers, published in the 80s in physics and
mathematics literature, see e.g. [5] for references.
Theorem 1.2. The complete list of unitary N = 0, 1, and 2 vertex algebras is as follows:
either c(k) is given by (1.6), (1.7), or (1.8), respectively, for p ∈ Z≥2, or c(k) ≥ 1, 3
2 or
3, respectively.
min
The above three cases cover all minimal W -algebras, associated with g, such that the
eigenspace g0 of ad x is abelian. Thus, we may assume that g0 is not abelian.
(g). Of course, unitarity of W k
In order to study unitarity of the simple minimal W -algebra W min
(g), one needs to
consider the more general framework of representation theory of universal minimal W -
(g). It is
algebras W k
(g)-modules. For that purpose, we
therefore natural to study unitarity of irreducible W k
take, in Sect. 6, a long detour to develop a general theory of invariant Hermitian forms on
modules over the vertex algebra of free bosons, which will be eventually applied to our
main object of interest. As a byproduct we obtain a field theoretic version of the Fairlie
construction, which yields explicit models of unitary representations of the Virasoro
algebra for certain values of the highest weight (cf. [17, 3.4], Example 6.9).
(g) is equivalent to that of W min
min
min
k
k
Unitarity of Minimal W -Algebras and Their Representations I
min
We consider in Sect. 9 the free field realization (cid:6) : W k
(g) → V k = V k+h∨ (Cx) ⊗
V αk (g(cid:4)) ⊗ F(g1/2
) introduced in [20] (here V γ (a) denotes the universal affine vertex
algebra associated to the Lie algebra a and to a 2-cocycle γ , αk is the 2-cocycle defined in
(7.24), and F(g1/2
) is the fermionic vertex algebra “attached” to g1/2). Let M(μ) be the
Verma module of highest weight μ ∈ C for the bosonic vertex algebra V k+h∨ (Cx) and
consider the V k-module N (μ) = M(μ) ⊗ V αk (g(cid:4)) ⊗ F(g1/2
). Applying to N (μ) results
from Sect. 6, we obtain in Proposition 9.2 a generalization of the Fairlie construction to
universal minimal W -algebras.
The conformal vertex algebras (W k
(g), L) and (V k, (cid:6)L(0)) (see (6.29)) both admit
Hermitian invariant forms H (·, ·)W and H (·, ·) f r ee, respectively. Unfortunately, the
embedding (cid:6) is not conformal, i.e., (cid:6)(L) (cid:6)= (cid:6)L(0), in particular (cid:6) is not an isometry
(which was erroneously claimed in [14]). So, though the vertex algebra V k is unitary,
(g). A few explicit computations suggest the
this does not imply the unitarity of W k
following conjecture, which we were unable to prove.
min
min
Conjecture 1. For each w ∈ W k
lar if V k is unitary, then W k
min
(g) is unitary.
min
(g), H (w, w)W ≥ H ((cid:6)(w), (cid:6)(w)) f r ee. In particu-
min
We start the study of unitary modules over minimal W -algebras in Sect. 8 by intro-
(g)-modules L W (ν, (cid:10)0) with highest weight
ducing the irreducible highest weight W k
(ν, (cid:10)0), where ν is a real weight of g(cid:4) and (cid:10)0 ∈ R is the minimal eigenvalue of L 0. We
prove that L W (ν, (cid:10)0) admits a φ-invariant nondegenerate Hermitian form (unique up to
normalization), see Lemma 8.1. In Sect. 8 we also determine necessary conditions for the
unitarity of L W (ν, (cid:10)0). Part of the necessary conditions is displayed in Proposition 8.5.
They say that unitarity of L W (ν, (cid:10)0) implies that the levels Mi (k) of the affine Lie al-
(cid:4)
(cid:4)
gebras (cid:6)g
(g) (given in Table 2, Sect. 7), where g
i in W k
i are the simple components of
min
g(cid:4), are non-negative integers, ν is dominant integral of levels Mi (k), and the inequality
(1.9) below holds. Proposition 8.8 provides a further necessary condition, which says
that (1.9) must be an equality when ν is an “extremal” weight. See Theorem 1.3 (1)
below for a precise statement.
min
In Sect. 10, using the generalization of the Fairlie construction, developed in Sect. 9,
we prove a partial converse result: if Mi (k) + χi ∈ Z+, where χi are negative integers,
displayed in Table 2, and ν is dominant integral weight for g(cid:4) which is not extremal, then
(g)-module L W (ν, (cid:10)0) is unitary for l0 sufficiently large, see Proposition 10.2.
the W k
In Sect. 11 we prove our central Theorem 11.1, which claims that actually Proposi-
tion 10.2 holds for l0 satisfying the inequality (1.9), provided that ν is not extremal. This
is established by the following construction. Let (cid:6)g be the affinization of g. We introduce
in (11.4) a highest weight module M((cid:6)νh) over (cid:6)g, whose highest weight (cid:6)νh depends on
h ∈ C, with the following two properties
(1) M((cid:6)νh) is irreducible, except possibly for an explicit set J of values of h.
(2) For the quantum Hamiltonian reduction functor H0, the W k
admits a Hermitian form, depending polynomially on h.
min
(g)-module H0(M((cid:6)ν))
Using the irreducibility theorem by Arakawa [2], we deduce that H0(M((cid:6)νh)) =
L W (ν, (cid:10)(h)) for h /∈ J , where (cid:10)(h) is defined by (11.45). It turns out that, miracu-
lously, if h ∈ J , then (cid:10)(h) does not satisfy (1.9). Moreover L W (ν, (cid:10)0) is unitary for
l0 (cid:13) 0. By continuity, the determinant of the Hermitian form on L W (ν, (cid:10)0) is positive
if the inequality (1.9) holds. See Theorem 1.3 (2) below for a precise statement.
V. G. Kac, P. Möseneder Frajria, P. Papi
min
Let us state our main results. First of all, if g = sl(2|m) with m ≥ 3 or osp(4|m)
with m ≥ 2 even, then none of the W k
(g)-modules L W (ν, (cid:10)0) are unitary for a non-
collapsing level k. For the remaining g from the list (1.3) the Lie algebra g(cid:4) is semisimple
(actually simple, except for g = D(2, 1; a), when g(cid:4) = sl2⊕sl2). Let θ ∨
i be the coroots of
i of g(cid:4). Let 2ρ(cid:4) be the sum of positive roots
the highest roots θi of the simple components g
of g(cid:4), and let ξ be a highest weight of the g(cid:4)-module g−1/2 (this module is irreducible,
except for g = psl(2|2) when it is C2 ⊕ C2). Let ν be a dominant integral weight for g(cid:4)
and l0 ∈ R. We prove the following theorem.
Theorem 1.3. Let L W (ν, (cid:10)0) be an irreducible highest weight W k
psl(2|2), spo(2|m) with m ≥ 3, D(2, 1; a), F(4) or G(3).
(1) This module can be unitary only if the following conditions hold:
(g)-module for g =
min
(cid:4)
(a) Mi (k) are non-negative integers,
(b) ν(θ ∨
) ≤ Mi (k) for all i,
i
(c)
l0 ≥
(ν|ν + 2ρ(cid:4))
2(k + h∨) +
(ξ |ν)
k + h∨
((ξ |ν) − k − 1),
(1.9)
and equality holds in (1.9) if ν(θ ∨
i
) > Mi (k) + χi for i = 1 or 2.
(2) This module is unitary if the following conditions hold:
(a) Mi (k) + χi ∈ Z+ for all i,
(b) ν(θ ∨
i
(c) inequality (1.9) holds.
) ≤ Mi (k) + χi for all i (i.e. ν is not extremal),
Conjecture 2. The modules L W (ν, (cid:10)0) are unitary if ν is extremal and l0 = R.H.S. of
(1.9). In other words, the necessary conditions of unitarity in Theorem 1.3 (1) are
sufficient.
We were able to prove this conjecture only for g = psl(2|2) and spo(2|3), obtaining
(g)-modules
thereby a complete classification of unitary simple highest weight W k
in these two cases. Note that papers [3,4,21] respectively claim (without proof) these
results.
Since ν = 0 is extremal iff k is collapsing, we obtain the following complete classi-
min
(g) with k (cid:6)= h∨ and g0 non-abelian
fication of minimal simple unitary W -algebras:
Theorem 1.4. The simple minimal W -algebra W min
−k
is non-trivial unitary if and only if
(1) g = sl(2|m), m ≥ 3, k = 1 (in this case the W -algebra is a free boson);
(2) g = psl(2|2), k ∈ N + 1;
(3) g = spo(2|3), k ∈ 1
(N + 2);
4
(4) g = spo(2|m), m > 4, k ∈ 1
2
(5) g = D(2, 1; m
n
(6) g = F(4), k ∈ 2
3
(7) g = G(3), k ∈ 3
4
N, where m, n ∈ N are coprime, k (cid:6)= 1
2 ;
), k ∈ mn
m+n
(N + 1);
(N + 1).
(N + 1);
This result, along with all known results on unitarity of vertex algebras, leads to the
following general conjecture.
Conjecture 3. A CFT type vertex operator algebra admitting a invariant Hermitian form
and having a unitary module is unitary.
Unitarity of Minimal W -Algebras and Their Representations I
min
In the final Sect. 14 we provide character formulas for all unitary W k
(g)-modules
L W(ν, (cid:10)0), which are obtained by applying the quantum Hamiltonian reduction to the
corresponding irreducible highest weight modules over the affinization(cid:6)g of g. There are
two cases to consider. In the first case, called massive (or typical), when inequality (1.9)
is strict, this character formula is easy to prove (see the proof of Proposition 11.5), which
leads to the character formula (14.5). In the second case, called massless (or atypical),
when the inequality (1.9) is equality, there is a general KW-formula for maximally
atypical tame integrable (cid:6)g-modules, conjectured in [19] and proved in [7] for all g in
question, except for g = D(2, 1; m
), ν (cid:6)= 0, which leads to the character formula (14.6).
n
Character formulas were also given in [4] (resp. [21]) for the N = 4 superconformal
(spo(2|3)), hence for the N = 3 superconformal algebra). The
algebra (resp. for W k
proofs given in these papers are incomplete since they assume that their list of singular
vectors is complete and that in the usual argument of inclusion-exclusion of Verma
modules subsingular vectors cancel out. Their formulas for both massive and massless
representations coincide with (14.5) and (14.6), respectively.
min
In our next paper of this series we will study unitarity of twisted representations of
minimal W -algebras.
Throughout the paper the base field is C, and Z+ and N stand for the set of non
negative and positive integers, respectively.
2. Setup
⊕ g¯1 be a basic finite-dimensional Lie su-
2.1. Basic Lie superalgebras. Let g = g¯0
peralgebra over C as in (1.3). Choose a Cartan subalgebra h of g¯0. It is a maximal
ad-diagonalizable subalgebra of g, for which the root space decomposition is of the
form
g = h ⊕
(cid:7)
α∈(cid:14)
gα,
(2.1)
where (cid:14) ⊂ h∗ \ {0} is the set of roots. In all cases, except for g ∼= psl(2|2), the root
spaces have dimension 1. In the case g = psl(2|2) one can achieve this property by
embedding in pgl(2|2) and replacing (2.1) by the root space decomposition with respect
to a Cartan subalgebra of pgl(2|2), which we will do.
Let (cid:14)+ be a subset of positive roots and (cid:15) = {α1, . . . , αr } be the corresponding
set of simple roots. We will denote by (cid:15)¯0
, the sets of even and odd simple roots,
, (cid:15)¯1
respectively. For each α ∈ (cid:14)+ choose Xα ∈ gα and X−α ∈ g−α such that (Xα|X−α) = 1,
|
, fi = X−αi
and let hα = [Xα, X−α]. Let ei = Xαi
i = 1, . . . , r } generates g, and satisfies the following relations
, i = 1, . . . , r . The set {ei , fi , hαi
[ei , f j ] = δi j hαi
,
[hαi
, e j ] = (αi |α j )e j ,
[hαi
, f j ] = −(αi |α j ) f j .
(2.2)
The Lie superalgebra ˜g on generators {ei , fi , hαi
| i = 1, . . . , r } subject to relations
(2.2) is a (infinite-dimensional) Z-graded Lie algebra, where the grading is defined by
= 0, deg ei = − deg fi = 1, with a unique Z-graded maximal ideal, and g is
deg hαi
the quotient of ˜g by this ideal. We assume that (αi |α j ) ∈ R for all αi , α j ∈ (cid:15).
V. G. Kac, P. Möseneder Frajria, P. Papi
2.2. Conjugate linear involutions and real forms. In the above setting, given a collection
of complex numbers (cid:17) = {λ1, . . . , λr } such that λi ∈
−1R if αi is an odd root and
λi ∈ R if αi is an even root, we can define an antilinear involution ω(cid:17) : g → g setting
√
ω(cid:17)(ei ) = λi fi , ω(cid:17)( fi ) = ¯λ−1
i ei , ω(cid:17)(hαi
) = −hαi
, 1 ≤ i ≤ r.
(2.3)
Since ω(cid:17) preserves relations (2.2), it induces an antilinear involution of ˜g, and, since
ω(cid:17) preserves the Z-grading of ˜g, it preserves its unique maximal ideal, hence it induces
an antilinear involution of g.
Set σα = −1 if α is an odd negative root and σα = 1 otherwise, so that (Xα|X−α) =
σα. Let
(cid:8)
ξα =
sgn(α|α)
1
if α is an even root,
if α is an odd root.
Then in [8, (4.13), (4.15)] it is proven (using results from [9]), that one can choose root
vectors Xα in such a way that
where
ω(cid:17)(Xα) = −σαξαλα X−α,
λα =
(cid:9)
(−ξαi
i
λi )ni for α =
r(cid:10)
i=1
ni αi .
(2.4)
(2.5)
We shall call this a good choice of root vectors.
2.3. Invariant Hermitian forms on vertex algebras. Let V be a conformal vertex algebra
n∈Z L n z−n−2 (see [16] for the definition and undefined
with conformal vector L =
notation). Let φ be a conjugate linear involution of V . A Hermitian form H ( . , . ) on V
is called φ-invariant if, for all a ∈ V , one has [16]
(cid:11)
H (v, Y (a, z)u) = H (Y (A(z)a, z−1)v, u), u, v ∈ V.
Here the linear map A(z) : V → V ((z)) is defined by
A(z) = ez L1 z−2L0 g,
where
√
−π
g(a) = e
−1( 1
2 p(a)+(cid:14)a )φ(a), a ∈ V,
(cid:14)a stands for the L 0-eigenvalue of a, and
(cid:8)
p(a) =
0 ∈ Z if a ∈ g¯0
1 ∈ Z if a ∈ g¯1
,
.
(2.6)
(2.7)
(2.8)
Unitarity of Minimal W -Algebras and Their Representations I
3. The Almost Compact Conjugate Linear Involution of g
From now on we let g be a basic simple finite-dimensional Lie superalgebra such that
= s ⊕ g
(cid:4).
g¯0
(3.1)
where s ∼= sl2 and g(cid:4) is the centralizer of s in g.
This corresponds to consider g as in Table 2 of [20]. We will also assume that g(cid:4) is not
abelian; this condition rules out g = spo(2|m), m = 0, 1, 2. The explicit list is given in
the leftmost column of Table 1. Note that sl(2|1) and osp(4|2) are missing there since
sl(2|1) ∼= spo(2|2) and osp(4|2) ∼= D(2, 1; a) with a = 1, −2 or − 1
2 .
First, we prove the simple lemma mentioned in the Introduction, which states that
the first two conditions of (1.1) imply the third one.
Lemma 3.1. Let g be a simple Lie superalgebra with an invariant supersymmetric bi-
linear form (.|.), let x ∈ g, and let φ be a conjugate linear involution of g, such that
(x|x) is a non-zero real number, and φ(x) = x.
Then
(φ(a)|φ(b)) = (a|b), for all a, b ∈ g.
(3.2)
(3.3)
Proof. Note that (φ(a)|φ(b)) is an invariant supersymmetric bilinear form as well, hence
it is proportional to (a|b) since g is simple. Due to (3.2) these two bilinear forms coincide.
(cid:17)(cid:18)
We now discuss the existence of an almost compact involution of g (see Defini-
tion 1.1).
Proposition 3.2. For any sl2-triple s = {e, x, f }, such that [e, f ] = x, [x, e] =
e, [x, f ] = − f, and (3.1) holds, an almost compact involution exists.
Proof. Choose a Cartan subalgebra t of g¯0. We observe that if we prove the existence of
an almost compact involution φ for a special choice of {e, x, f }, then an almost compact
involution exists for any choice of the sl2-triple. Indeed, if {e(cid:19), x (cid:19), f (cid:19)} is another sl2-
triple, then there is an inner automorphism ψ of s mapping {e, x, f } to {e(cid:19), x (cid:19), f (cid:19)}, which
extends to an inner automorphism of g. Therefore φ(cid:19) = ψφψ −1 is an almost compact
involution for {e(cid:19), x (cid:19), f (cid:19)}. The construction of {e, x, f } and φ and the verification of
properties (i)–(iii) in Definition 1.1 will be done in four steps:
(1) make a suitable choice of positive roots for g with respect to t;
(2) define φ by specializing (2.3);
(3) construct {e, f, x} and verify that φ( f ) = f, φ(x) = x, φ(e) = e;
(4) check that φ is a compact involution for g(cid:4);
Step 1. We need some preparation. Let (cid:14)(cid:4) be the set of roots of g(cid:4) with respect to the
Cartan subalgebra t ∩ g(cid:4). Let {±θ } be the t ∩ s-roots of s. Then R¯0
= {±θ } ∪ (cid:14)(cid:4) is the
set of roots of g¯0 with respect to t.
Let R be the set of roots of g with respect to t, let R+ be the subset of positive roots
whose corresponding set of simple roots S = {α1, . . . , αr } is displayed in Table 1.
Note that θ is the highest root of R.
spo(2|2m + 1), m ≥ 1 {δ1 − (cid:22)1, (cid:22)1 −
−
g
psl(2|2)
sl(2|m), m > 2
osp(4|m), m > 2
spo(2|2m), m ≥ 3
D(2, 1; a)
F(4)
G(3)
Step2. Define
V. G. Kac, P. Möseneder Frajria, P. Papi
Table 1. Simple roots, invariant form, and highest root of g
S
{(cid:22)1 − δ1, δ1 − δ2, δ2 −
(cid:22)2}
{(cid:22)1 − δ1, δ1 −
δ2, . . . , δm − (cid:22)2}
{(cid:22)1 − (cid:22)2, (cid:22)2 − δ1, δ1 −
δ2, . . . , δm−1
−
δm , 2δm }
(cid:22)2, . . . , (cid:22)m−1
(cid:22)m , (cid:22)m }
{δ1 − (cid:22)1, (cid:22)1 −
(cid:22)2, . . . , (cid:22)m−1
−
(cid:22)m , (cid:22)m−1 + (cid:22)m }
{(cid:22)1−(cid:22)2−(cid:22)3, 2(cid:22)2, 2(cid:22)3}
{ 1
(δ1 − (cid:22)1 − (cid:22)2 −
2
(cid:22)3), (cid:22)3, (cid:22)2 − (cid:22)3, (cid:22)1 −
(cid:22)2}
{δ1 + (cid:22)3, (cid:22)1, (cid:22)2 − (cid:22)1}
(.|.)
((cid:22)i |(cid:22) j ) = δi, j = −(δi |δ j )
((cid:22)i |δ j ) = 0
((cid:22)i |(cid:22) j ) = δi, j = −(δi |δ j )
((cid:22)i |δ j ) = 0
((cid:22)i |(cid:22) j ) = δi, j = −(δi |δ j )
((cid:22)i |δ j ) = 0
((cid:22)i |(cid:22) j ) = − 1
2
δi, j , (δ1|δ1) = 1
2
, ((cid:22)i |δ1) = 0
((cid:22)i |(cid:22) j ) = − 1
2
δi, j , (δ1|δ1) = 1
2
, ((cid:22)i |δ1) = 0
((cid:22)1|(cid:22)1) = 1
2
, ((cid:22)2|(cid:22)2) = −1
2(1+a) , ((cid:22)3|(cid:22)3) = −a
2(1+a)
((cid:22)1|(cid:22)2) = ((cid:22)1|(cid:22)3) = ((cid:22)2|(cid:22)3) = 0
((cid:22)i |(cid:22) j ) = − 2
3
δi, j , (δ1|δ1) = 2
((cid:22)i |δ1) = 0
((cid:22)i |(cid:22) j ) = 1−3δi, j
, (δ1|δ1) = 1
2
4
((cid:22)i |δ1) = 0, (cid:22)1 + (cid:22)2 + (cid:22)3 = 0
θ
(cid:22)1 − (cid:22)2
(cid:22)1 − (cid:22)2
(cid:22)1 + (cid:22)2
2δ1
2δ1
2(cid:22)1
δ1
2δ1
(3.4)
(cid:17)0 = {λ1, . . . , λr }, λi =
(cid:8)
√
−sgn(αi |αi )
−1
−
if αi is even,
if αi is odd.
Set φ = ω(cid:17)0 (see (2.3)).
Step3. Consider a good choice of root vectors Xα for (cid:17)0. Set
x =
−1hθ ), f = 1
2
(Xθ − X−θ ), e = 1
2
(Xθ + X−θ +
−1
2
√
√
(Xθ + X−θ −
√
−1hθ ).
(3.5)
(cid:11)
r
If θ =
i=1 mi αi , then, by our special choice of (cid:14)+, we have either mi = 2 for exactly
one odd simple root αi , or mi = m j = 1 for exactly two odd distinct simple roots αi , α j
(this corresponds to the fact that R+ is distinguished, in the terminology of [8]). By (2.4)
we have
√
φ(Xθ ) = −(
−1)2 X−θ = X−θ .
(3.6)
) = −hαi , it is clear from (3.5) that φ fixes e, f, x.
(cid:11)
r
Since hθ =
i=1 mi hαi and φ(hαi
One checks directly that {e, f, x} is an sl2-triple.
Step4. Endow g with the Z-grading
(cid:7)
g =
qi
(3.7)
i∈Z
which assigns degree 0 to h ∈ t and to ei and fi if αi is even, and degree 1 to ei and
degree −1 to fi , if αi is odd.
A direct check on Table 1 shows that q0
= g(cid:4). Recall from [8, Proposition 4.5] that
the fixed points of φ in q0 are a compact form of q0 if and only if λi (αi |αi ) < 0 for all
αi ∈ S \ S1. Step 4 now follows from (3.4).
(cid:17)(cid:18)
Unitarity of Minimal W -Algebras and Their Representations I
4. Explicit Expressions for Almost Compact Real Forms
In this section we exhibit explicitly an almost compact involution φ in each case and
discuss its uniqueness. If φ is an almost compact involution of g, we denote by gac the
corresponding real form (the fixed point set of φ). We can define gac by specifying a
real form gac
of g¯1.
¯0
of g¯0 and a real form gac
¯1
= sl2 ⊕ som and g¯1
(1) g = spo(2|m). Then g¯0
= C2 ⊗ Cm as g¯0-module. We set
gac
¯0
= sl2(R) ⊕ som(R), gac
¯1
= R2 ⊗ Rm.
Explicitly, let B be a non-degenerate R-valued bilinear form of the superspace R2|m
⎛
⎞
with matrix
⎝
0 1 0
−1 0 0
0 0 Im
⎠. Then for g = spo(2|m) we have:
gac = {A ∈ sl(m|n; R) | B(Au, v) + (−1) p(A) p(u) B(u, Av) = 0}.
(2) g = psl(2|2). Let H be a C-valued non-degenerate sesquilinear form on the super-
−1, −
space C2|2 whose matrix is diag(
−1, 1, 1). Set
√
√
˜gac = {A ∈ sl(2|2; C) | H (Au, v) + (−1) p(A) p(u) H (u, Av) = 0}.
Then
Explicitly, we have g¯0
g¯0-module. Then
√
gac = ˜gac/R
−1I.
(cid:16)(cid:4)
= sl2 ⊕ sl2 and g¯1
=
(cid:5)
0 B
C 0
(cid:17)
| B, C ∈ M2,2(C)
as a
(cid:16)(cid:4)
(cid:5)
(cid:17)
˜gac
¯0
=
˜gac
¯1
=
⎛
⎝
⎧
⎨
⎩
A 0
0 D
| A ∈ su(1, 1), D ∈ su2
,
0
0
−1 ¯ut −
√
√
⎞
⎠ | u, v ∈ C2
⎫
⎬
⎭
.
u
0
v
0
−1 ¯vt 0
(3) g = D(2, 1; a). Then g¯0
= sl2⊕sl2⊕sl2 = so(4, C)⊕sl2 and g¯1
C4 ⊗ C2 as g¯0-module. We set
= C2⊗C2⊗C2 =
gac
¯0
= so(4, R) ⊕ spanR{e, f, x}, gac
¯1
= R4 ⊗ R2.
To get an explicit realization, consider the contact Lie superalgebra (see [11] for
more details)
K (1, 4) = C[t, ξ1, ξ2, ξ3, ξ4]
where t is an even variable and ξi , 1 ≤ i ≤ 4, are odd variables. Introduce on the
associative superalgebra K (1, 4) a Z-grading by letting
deg
(cid:19) t = 2, deg
(cid:19) ξi = 1,
V. G. Kac, P. Möseneder Frajria, P. Papi
and the bracket
{F, G} = (2 −
4(cid:10)
i=1
ξi ∂i )F∂t G − ∂t F(2 −
4(cid:10)
i=1
ξi ∂i )G +
4(cid:10)
(−1) p(F)∂i F∂i G,
i=1
where ∂i = ∂ξi . This is a Z-graded Lie superalgebra with compatible grading
deg F = deg(cid:19) F − 2. We have
(cid:7)
K (1, 4) =
K (1, 4) j ,
where
j≥−2
K (1, 4)−2 = C1,
(cid:19)
K (1, 4)0 = spanC(ξi ξ j , t | 1 ≤ i, j ≤ 4), K (1, 4)1 = g
1
(cid:19)
g
1
K (1, 4)−1 = spanC(ξi | 1 ≤ i ≤ 4),
(cid:19)(cid:19)
⊕ g
1
= spanC(ξi ξ j ξk | 1 ≤ i, j, k ≤ 4).
= spanC(tξi | 1 ≤ i ≤ 4),
, where
(cid:19)(cid:19)
g
1
Note that spanC(ξi ξ j | 1 ≤ i, j ≤ 4) = (cid:17)2C4 ∼= so(4, C), that g(cid:19)
1 is isomorphic to
the standard representation C4 of so(4, C) and that g(cid:19)(cid:19)
1 is isomorphic to (cid:17)3C4, so that
K (1, 4)1 = C4 ⊕C4 as so(4, C)-module. Also notice that {g(cid:19)
} = Ct 2, {g(cid:19)(cid:19)
} = 0.
1
1
Fix now a copy ˜gb of an so(4, C)-module C4 in C4 ⊕C4, depending on a constant b ∈ R,
as follows. Set, for 1 ≤ i ≤ 4,
, g(cid:19)(cid:19)
1
, g(cid:19)
1
ai = tξi + b ˆξi , where ˆξi = (−1)i+1
ξ j ,
(cid:9)
and define
j(cid:6)=i
˜gb
=
4(cid:10)
i=1
Cai .
Let b ∈ R. Note that, setting ξ = ξ1ξ2ξ3ξ4, we have
{tξi + b ˆξi , tξ j + b ˆξ j } = δi j (−t 2 + 2bξ ).
Hence, if we set
e = −t 2 + 2bξ,
f = −1,
x = t/2,
then {e, x, f } is an sl2-triple. Set
⎛
(cid:24)
(cid:25)
gac = R.1 ⊕
4(cid:10)
4(cid:10)
Rξi
⎝
⊕
i=1
i, j=1
⎞
Rξi ξ j ⊕ R t
2
⎠ ⊕
(cid:24)
4(cid:10)
i=1
(cid:25)
Rai
⊕ R(−t 2 + 2bξ ).
Then gac is an almost compact form of D(2, 1; 1+b
). To prove this, it suffices to calculate
1−b
the Cartan matrix for a choice of Chevalley generators of the complexification of gac. Fix
a Cartan subalgebra in g(cid:4) = so(4, C) as the span of v2 = −
−1ξ3ξ4.
Set v1 = t; then {v1, v2, v3} is a basis of a Cartan subalgebra of g. Let {(cid:22)1, (cid:22)2, (cid:22)3} be the
dual basis to {v1, v2, v3}. One can choose {α1 = (cid:22)2 − (cid:22)1, α2 = (cid:22)1 − (cid:22)3, α3 = (cid:22)1 + (cid:22)3}
as a set of simple roots. The associated Chevalley generators are
−1ξ1ξ2, v3 = −
√
√
Unitarity of Minimal W -Algebras and Their Representations I
√
−1a1 + a2
−1ξ1 + ξ2
e1 = −
√
f1 =
h1 = −2v1 + 2v2 + 2b v3 h2 = 4v1 − 4v3
e2 = ξ1ξ3 + ξ2ξ4 +
f2 = ξ1ξ3 + ξ2ξ4 −
√
−1(ξ1ξ4 − ξ2ξ3)
√
−1(ξ1ξ4 − ξ2ξ3)
e3 = ξ1ξ3 − ξ2ξ4 −
f3 = ξ1ξ3 − ξ2ξ4 +
h3 = 4v1 + 4v3
√
√
−1(ξ1ξ4 + ξ2ξ3)
−1(ξ1ξ4 + ξ2ξ3)
and the corresponding Cartan matrix, normalized as in [11], is
⎛
⎝
0 1 1+b
1−b
−1 2 0
−1 0 2
⎞
⎠. Hence
1−b and therefore all a (cid:6)= −1 occur in this construction. Since this subalgebra is
a = 1+b
17-dimensional, it is isomorphic to D(2, 1; a).
Remark 4.1. Note that a = 0 for b = −1. In this case, D(2, 1; 0) contains a 11-
dimensional solvable ideal generated by f1, which is spanned by h1 and the root vectors
relative to roots having α1 in their support. If we replace ai by ai /b and h1 by h1/b, and
let b tend to +∞, we recover also the Lie superalgebra of derivations of psl(2|2), and
its almost compact real form.
(4) g = G(3). Then g¯0
= sl2 ⊕ G2 and g¯1
7-dimensional irreducible representation of G2, and we let
= C2 ⊗ L min, where L min is the complex
gac
¯0
= sl2(R) ⊕ G2,0, gac
¯1
= R2 ⊗ L min,0.
where G2,0 is the real compact form of G2 and L min,0 is the real 7-dimensional
irreducible representation of G2,0 whose complexification is L min.
= C2 ⊗ spin7, where spin7 is the complex
(5) g = F(4). Then g¯0
= sl2 ⊕ so7 and g¯1
spinor representation of so7, and we let
gac
¯0
= sl2(R) ⊕ so7(R), gac
¯1
= R2 ⊗ spin(R7),
where spin(R7) is the spinor representation of the compact group so7(R).
It is proved in [11, Proposition 5.3.2] that in both cases (4) and (5) gac = gac
¯0
is an almost compact form of g.
⊕ gac
¯1
4.1. Uniqueness of the almost compact involution.
Proposition 4.2. An almost compact involution is uniquely determined up to a sign by
its action on g0, provided that the g0-module g1/2 is irreducible.
Proof. If there are two different extensions of the compact involution, then their ratio
ψ, say, is identical on g0, hence, by Schur’s lemma, ψ acts as a scalar on g−1/2. Since
(cid:17)(cid:18)
φ( f ) = f , we conclude that this scalar is ±1.
It remains to discuss the cases g = sl(2|m), m ≥ 3, and psl(2|2), since in all other
cases of Table 1 the g0-module g1/2 is irreducible. In this cases g is of type I, that is
±
] = 0.
g¯1
¯1
Let δλ be the linear map on g defined by setting
±
are contragredient irreducible g¯0-modules and [g
¯1
= g+
¯1
where g
⊕ g
, g
−
¯1
±
¯1
δλ|g¯0
= I d,
δλ|g+
¯1
= λI d,
δλ|g
−
¯1
= λ−1 I d.
(4.1)
Then δλ is an automorphism of g for any λ ∈ C. Suppose that φ(cid:19) is another conjugate
almost compact linear involution such that φ(cid:19)
= φ. Then φ(cid:19) = φ ◦ γ with γ an
|g¯0
V. G. Kac, P. Möseneder Frajria, P. Papi
= I d. If g = sl(2|m), by [22, Lemmas 1 and 2], we
automorphism of φ such that γ|g¯0
−
and (φ(cid:19))2 = I d we have that λ ∈ R. If g = psl(2|2),
have γ = δλ. Since φ(g+
) = g
¯1
¯1
then γ belongs to a three-parameter family of automorphisms explicitly described in [8,
§4.6], and contained in S L(2, C). This S L(2, C) is the group of automorphisms of g
corresponding to the Lie algebra sl2 of outer derivations of g.
Remark 4.3. Note that if φ is an almost compact involution, then
˜φ(a) = (−1)2 j φ(a), a ∈ g j
is again an almost compact involution.
5. The Bilinear Form (cid:2)·, ·(cid:3) on g−1/2
Let s = {e, x, f } be an sl2-triple as in Proposition 3.2. Consider the following symmetric
bilinear forms on g−1/2 and g1/2 respectively:
(cid:2)u, v(cid:3) = (e|[u, v]), u, v ∈ g−1/2
(cid:2)u, v(cid:3)ne = ( f |[u, v]), u, v ∈ g1/2
.
] = 0, we have
,
Note that, since [ f, g−1/2
(cid:2)[e, u], [e, v](cid:3)ne = − 1
2
(cid:2)u, v(cid:3), u, v ∈ g−1/2
.
We want to prove the following
(5.1)
(5.2)
(5.3)
Proposition 5.1. We can choose an almost compact involution such that the bilinear
form (cid:2)., .(cid:3) is positive definite on gac ∩ g−1/2. In particular, the Hermitian form (cid:2)φ(u), v(cid:3)
(resp. (cid:2)φ(u), v(cid:3)ne) is positive definite (resp, negative definite) on gac ∩ g−1/2 (resp.
gac ∩ g1/2).
The proof requires a detailed analysis of the action of an almost compact involution on
g−1/2. Define structure constants Nα,β for a good choice of root vectors (see Sect. 2.2)
by the relation
[Xα, Xβ ] = Nα,β Xα+β .
Observe that {X−θ , Xθ , 1
2 hθ } is a sl2-triple in s. Let
⊕ ˜g−1/2
g = CXθ ⊕ ˜g1/2
⊕ ˜g0
⊕ CX−θ
2 hθ eigenspaces. By the sl2 representation theory, ad X±θ :
be the decomposition into ad 1
→ ˜g±1/2 is an isomorphism of g(cid:4)-modules. Moreover, by our choice of R+ in
˜g∓1/2
Sect. 3, the roots of ˜g−1/2 (resp. ˜g1/2) are precisely the negative (resp. positive) odd
roots. In particular, the map α (cid:25)→ −θ + α defines a bijection between the positive and
negative odd roots. We shall need the following properties.
Lemma 5.2. For a positive odd root α we have
N−θ,α Nθ,α−θ = 1,
N 2
−θ,α = 1.
(5.4)
(5.5)
In particular Nθ,α is real.
Unitarity of Minimal W -Algebras and Their Representations I
Proof. Relation (5.4) is proven in [8, Lemma 4.3]. Equation (5.5) follows from [8, (4.8)],
(cid:17)(cid:18)
noting that the −θ -string through α has length 1.
Arguing as in Proposition 3.2, we can assume in the proof of Proposition 5.1 that
{e, x, f } is the sl2-triple defined in (3.5); ad x defines on g a minimal grading
g = C f ⊕ g−1/2
⊕ g0
⊕ g1/2
⊕ Ce.
Set, for an odd root α ∈ R+
uα = Xα +
√
−1N−θ,α Xα−θ .
(5.6)
(5.7)
Note that
[x, uα] =
√
−1
2
√
[Xθ − X−θ , Xα +
−1N−θ,α Xα−θ ]
√
−1
2 N−θ,α Xα−θ = − 1
2 N−θ,α Nθ,α−θ Xα −
= 1
2 uα,
hence {uα | α ∈ R+, α odd} is a basis of g−1/2.
Lemma 5.3. If α is a positive odd root then
φ(uα) = −N−θ,αuθ−α.
(5.8)
Proof. By (2.4), φ(Xα) = −
N−θ,α is real,
√
−1X−α if α is an odd positive root, hence, by (5.5), since
√
φ(uα) = φ(Xα +
= −N−θ,α(Xθ−α +
−1N−θ,α Xα−θ ) = −(
−1N
−1
−θ,α X−α).
√
√
−1X−α + N−θ,α Xθ−α)
Note that, since φ(x) = x, φ(uα) has to belong to g−1/2. This forces
N−θ,α N−θ,θ−α = 1,
and (5.9) becomes (5.8).
(5.9)
(5.10)
(cid:17)(cid:18)
Proof of Proposition 5.1. Set vα = 1
(uα − φ(uα)), where α runs
2
over the positive odd roots. It is clear that vα ∈ r. We want to prove that the vectors vα
form an orthogonal basis of r. We need two auxiliary computations:
(uα + φ(uα)) +
√
−1
2
[e, uα] =
√
−1Xα + N−θ,α Xα−θ ,
(cid:2)uα, uβ (cid:3) = −(N−θ,α + N−θ,β )δθ−α,β .
(5.11)
(5.12)
To prove (5.11) use (5.4):
√
√
[e, uα] = 1
[Xθ + X−θ +
2
√
= 1
(
2
√
−1hθ , Xα +
−1N−θ,α Nθ,α−θ Xα + N−θ,α Xα−θ +
−1N−θ,α Xα−θ ]
√
=
−1Xα + N−θ,α Xα−θ .
−1Xα + N−θ,α Xα−θ )
To prove (5.12) use (5.11):
V. G. Kac, P. Möseneder Frajria, P. Papi
(cid:2)uα, uβ (cid:3) = (e|[uα, uβ ]) = ([e, uα]|uβ )
√
−1Xα + N−θ,α Xα−θ |Xβ +
= (
= σα−θ N−θ,αδθ−α,β − σα N−θ,β δθ−α,β
= −(N−θ,α + N−θ,β )δθ−α,β .
√
−1N−θ,β Xβ−θ ) =
Set
Then, using (5.12)
Mα,β = −(N−θ,α + N−θ,β ).
(cid:2)vα, vβ (cid:3)
√
= (cid:2) 1+
√
=
√
uβ − 1−
−1
2 N−θ,β uθ−β (cid:3)
2
√
−1
−1
−1
2 N−θ,αuθ−α, 1+
2 N−θ,α(cid:2)uθ−α, uβ (cid:3)
√
−1
2 N−θ,α N−θ,β (cid:2)uθ−α, uθ−β (cid:3)
√
uα − 1−
2
−1
(cid:2)uα, uβ (cid:3) − 1
2
2 N−θ,β (cid:2)uα, uθ−β (cid:3) −
− 1
√
−1
2 Mα,β δθ−α,β − 1
√
−1
2 N−θ,α N−θ,β Mθ−α,θ−β δα,θ−β .
2 N−θ,α Mθ−α,β δα,β − 1
−
=
2 N−θ,β Mα,θ−β δθ−α,θ−β
Therefore by (5.4) and (5.10)
(cid:2)vα, vβ (cid:3) = 2δα,β .
In particular, the restriction of (cid:2)·, ·(cid:3) to gac ∩ g−1/2 is positive definite. The final claim
follows immediately from (5.3), using that [e, g−1/2
(cid:17)(cid:18)
] = g1/2.
6. A General Theory of Invariant Hermitian Forms on Modules Over the Vertex
Algebra of Free Boson and the Fairlie Construction
Consider the infinite dimensional Heisenberg Lie algebra H = (C[τ, τ −1] ⊗ Ca) ⊕ CK
with K central and bracket
[τ n ⊗ a, τ m ⊗ a] = δn,−mn K .
Let H0 = Ca + CK , and, given μ ∈ C, define μ∗ ∈ H∗
0 by μ∗(a) = μ, μ∗(K ) = 1.
Let M(μ) be the Verma module for the Lie algebra H with highest weight μ∗. Let vμ
be a highest weight vector, i.e. (τ n ⊗ a)vμ = 0 for n > 0, hvμ = μ∗(h)vμ for h ∈ H0.
It is well known that M(0) carries a simple vertex algebra structure, called the vertex
algebra of free boson, which we denote by V 1(Ca), and that M(μ) is a simple module
over the vertex algebra V 1(Ca). Moreover, V 1(Ca) is the universal enveloping vertex
algebra of the nonlinear Lie conformal algebra R = C[T ] ⊗ Ca with λ-bracket
[aλa] = λ.
We introduce conformal weight (cid:14) on V 1(Ca) by letting (cid:14)a = 1, and for v ∈ V 1(Ca)
we write the corresponding quantum field as Y (v, z) =
j∈Z v j z− j−(cid:14)v .
(cid:11)
Unitarity of Minimal W -Algebras and Their Representations I
Fix t ∈ C and set
L(t) = 1
2
: aa : +t T a ∈ V 1(Ca).
(6.1)
It is an energy-momentum element for all t. Set H (t) = L(t)0 = 1
: aa :0 −ta0. Since
2
a0 = 0 as operator on V 1(Ca), H (t) = 1
: aa :0. (Note that the conformal weights on
2
V 1(Ca) are the eigenvalues of H (t) = H (0)).
If b ∈ V 1(Ca), write b
μ
n for bM(μ)
n
: aa :μ
0
= 2
. By the −1-st product identity
(cid:10)
μ
− j a
μ
j + (a
μ
0
)2.
a
j∈N
In particular
: aa :μ
0
vμ = μ2vμ.
On the other hand, by the commutator formula,
1
2
[: aa :μ
0
, a
μ
j
] = 1
2
(cid:4)
1
r
(cid:10)
r
(cid:5)
(: aa :(r ) a) j = (T a)μ
j + a
μ
j
= − ja
.
μ
j
Recall that a basis of M(μ) is
(cid:26)
(a
μ
− j1
)i1 · · · (a
μ
− jr
)ir .vμ | j1 > · · · > jr > 0
.
(cid:27)
(6.2)
(6.3)
(6.4)
Let M(μ)n be the eigenspace for the energy operator H (t) corresponding to the
eigenvalue n + 1
2
μ2 − tμ. Since
1
2
: aa :μ
0 +t (T a)μ
0
= 1
2
: aa :μ
0
−ta
μ
0
and [a
μ
0
, a
μ
− j
] = 0 for all j, it follows from (6.2) and (6.3) that
M(μ)n = span{(a
μ
− j1
)i1 · · · (a
μ
− jr
)ir .vμ |
(cid:10)
s
is js = n}.
Thus
M(μ) = ⊕n∈Z+ M(μ)n.
This shows that M(μ) is a positive energy V 1(Ca)-module, i.e. real parts of the eigen-
values of H (t) are bounded below. Moreover its minimal energy subspace is
Lemma 6.1.
M(μ)0 = Cvμ.
ez L(t)1 = ez L(0)1
∞(cid:9)
n=1
e− 2t
n znan .
(6.5)
V. G. Kac, P. Möseneder Frajria, P. Papi
Proof. Identify V 1(Ca) with the polynomial algebra in infinitely many variables using
(6.4) with μ = 0:
P = C[a−1, a−2, . . . , a−n, . . .].
Since L(t)11 = 0 and an1 = 0 if n > 0, both L(t)1 and an for n > 0 act as deriva-
tions of the algebra P under our identification. It follows that both sides of (6.5) are
automorphisms of P. It is therefore enough to check the equality only on the generators
a−n.
We need the following formulae:
[aλ L(t)] = λa + tλ21,
[an, L(t)1] = nan+1 + δn,−12t I,
[an, a−m] = δn,mn I.
Applying these formulae we find
e− 2t
n znan (a−m) = e
−2t zn
∂
∂a−n (a−m) = a−m − δn,m2t zm I.
(6.6)
(6.7)
(6.8)
(6.9)
It follows that
ez L(0)1
∞(cid:9)
n=1
e− 2t
n znan (a−m) = ez L(0)1 (a−m − 2t zm1) = ez L(0)1a−m − 2t zm I. (6.10)
To conclude we only need to check that, if n ≥ 1, then
L(t)n
1
(a−m) = L(0)n
1a−m − 2n!δn,mt I.
We prove this by induction on n. If n = 1 the formula reads
L(t)1(a−m) = L(0)1a−m − 2δ1,mt I.
Using (6.7) with t = 0 we see that the latter formula is equivalent to
L(t)1(a−m) = ma−m+1 − 2δ1,mt I,
which is just (6.7).
If n > 1 and m = 1, then
L(t)n
1
(a−1) =L(t)n−1
1
L(t)1(a−1) = L(t)n−1
1
(−2t) = 0 = L(0)n
1
(a−1).
If n > 1 and m > 1, then
L(t)n
1
1 L(t)1(a−m) = L(t)n−1
(a−m) =L(t)n−1
=L(0)n−1
1
=L(0)n
1a−m − 2n!δn,mt I.
1
(ma−m+1) − 2(n − 1)!mδn−1,m−1t I )
(ma−m+1)
(cid:17)(cid:18)
Unitarity of Minimal W -Algebras and Their Representations I
Let φ be the conjugate linear involution of the vector space Ca defined by φ(a) = −a.
−1R. This assumption is necessary since, in order to
Assume from now on that t ∈
apply the results of [16], we need to assume φ(L(t)) = L(t). Set (cf. (2.7))
√
A(z, t) = ez L(t)1 z−2H (0)g,
(6.11)
where g is defined in (2.8). Let πZ : V 1(Ca) → Z hu H (0)(V 1(Ca)) be the canonical
projection to the Zhu algebra (see e.g. [16, Section 2]). Let ω be the conjugate linear
anti-homomorphism of Z hu H (0)(V k+h∨ (Cx)) defined by
ω(πZ (v)) = πZ (A(1, t)v)
It is proven in [16, Proposition 6.1] that ω is indeed well-defined.
Lemma 6.2.
ω(πZ (a))vμ = (μ − 2t)vμ
(6.12)
Proof. By Lemma 6.1, since g(a) = a and L(0)1a = 0,
ω(πZ (a))vμ =(A(1, t)a)μ
0
=(μ − 2t)vμ.
vμ = (eL(t)1a)μ
0
vμ = (eL(0)1 (a) − 2t1)μ
0
vμ = a
μ
0
vμ − 2tvμ
(cid:17)(cid:18)
Recall from [16, Definition 6.4] that if V is a conformal vertex algebra and φ is a
conjugate linear involution of V , a Hermitian form H ( . , . ) on a V -module M is called
φ-invariant if, for all v ∈ V , m1, m2 ∈ M
(m1, YM (a, z)m2) = (YM (A(z)a, z−1)m1, m2).
By abuse of terminology, we shall call H (·, ·) an L-invariant Hermitian form, where
L is the conformal vector of V . If μ ∈ C we denote by (cid:27)(μ) and (cid:28)(μ) the real and
imaginary part of μ, respectively.
Proposition 6.3. There is a non-zero L(t)-invariant Hermitian form on M(μ) if and
only if t =
−1(cid:28)(μ).
√
Proof. Let (·, ·) be the unique Hermitian form on Cvμ such that (vμ, vμ) = 1. By
Proposition 6.7 of [16], there is a non-zero L(t)-invariant Hermitian form on M(μ)
if and only if (·, ·) is an ω-invariant Hermitian form on Cvμ. By Lemma 6.2, that is
equivalent to
μ = (vμ, a0vμ) = (vμ, πZ (a)vμ) = (ω(πZ (a))vμ, vμ) = μ − 2t.
Thus
−2t = 2
√
−1(cid:28)(μ),
hence the statement.
We denote by Hμ the unique L(
that Hμ(vμ, vμ) = 1.
√
−1(cid:28)(μ))-invariant Hermitian form on M(μ) such
(cid:17)(cid:18)
V. G. Kac, P. Möseneder Frajria, P. Papi
Lemma 6.4. If m, m(cid:19) ∈ M(μ), then
Hμ(m, aμ
n m(cid:19)) = Hμ(a
μ
−nm, m(cid:19)) + δn,02
√
−1(cid:28)(μ)Hμ(m, m(cid:19)).
Proof. By invariance of the Hermitian form,
Hμ(m, anm(cid:19)) =Resz zn Hμ(m, Y μ(a, z)m(cid:19))
=Resz zn Hμ(Y μ(A(z)a, z−1)m, m(cid:19))
=Resz zn Hμ(Y μ(ez L(t)1 z−2L(t)0 g(a), z−1)m, m(cid:19))
=Resz zn−2 Hμ(Y μ(ez L(0)1a − 2
=Resz zn−2 Hμ(Y μ(a − 2
−1(cid:28)(μ)z1, z−1)m, m(cid:19)).
√
√
−1(cid:28)(μ)z1, z−1)m, m(cid:19))
The last two steps follow by (6.10) and the fact that L(0)1a = 0. As
Y μ(a, z−1) =
(cid:10)
r
r zr +1, Y μ(1, z−1) =
aμ
(cid:10)
r
δr,0 I zr ,
we get the result.
(cid:17)(cid:18)
It is now easy to compute the invariant form in the basis (6.4):
Hμ
(cid:4)
μ
− j1
(a
(cid:4)
)i1 · · · (a
μ
− jr
)ir .vμ, (a
)i (cid:19)
1 · · · (a
μ
− j (cid:19)
1
(cid:5)
μ
− j (cid:19)
r (cid:19)
)i (cid:19)
r (cid:19) .vμ
(cid:5)
= Hμ
(a
μ
j (cid:19)
r (cid:19)
)i (cid:19)
r (cid:19) · · · (a
)i (cid:19)
1 (a
μ
− j1
μ
j (cid:19)
1
)i1 · · · (a
μ
− jr
)ir .vμ, vμ
.
(6.13)
It follows that the basis is orthogonal and
(cid:28)
(cid:28)
(cid:28)(a
μ
− j1
)i1 · · · (a
μ
− jr
)ir .vμ
(cid:28)
(cid:28)
(cid:28)
μ
=
(cid:9)
s
is! j is
s
In particular the form is positive definite and its values on the chosen basis do not depend
on μ.
Let μ ∈ C and t ∈
−1R. Let M(μ, t)∨ be the conjugate dual of M(μ) with action
√
given by, for b ∈ V 1(Ca), m ∈ M(μ), f ∈ M(μ, t)∨,
(Y M(μ,t)∨ (b, z) f )(m) = f (Y μ(A(t, z)b, z−1)m),
where A(z, t) is defined by (2.7), (2.8).
√
Using the L(
−1(cid:28)(μ))-invariant form on M(μ) (see (6.13)), we can identify M(μ)
and M(μ, t)∨ (as vector spaces) by indentifying m with fm : m(cid:19) (cid:25)→ Hμ(m(cid:19), m).
We now want to describe explicitly the action of V 1(Ca) under this identification.
We need the following result:
Lemma 6.5. If t ∈
√
−1R, then
z2H (0)eznan z−2H (0) = ez−nan
et znan g = get (−z)nan
(6.14)
(6.15)
Unitarity of Minimal W -Algebras and Their Representations I
Proof. If b ∈ V 1(Ca) then,
e2z H (0)eznan e−2z H (0)b = z−2(cid:14)b
(cid:10)
z2(cid:14)b−2r n znr ar
nb
1
r !
nb = ez−nan b.
r
z−r nar
(cid:10)
=
r
1
r !
For the second formula note that
nb) = (−1)(cid:14)b−nr φ(ar
g(ar
nb) = (−1)(cid:14)b−nr (−1)r ar
n
φ(b) = (−1)−nr (−1)r ar
n g(b)
so, since t is purely imaginary,
et znan g(b) =
(cid:10)
r
1
r !
tr znr ar
n g(b) =
(cid:10)
r
1
r !
(−1)−nr znr g(tr ar
nb) = et (−z)nan b.
(cid:17)(cid:18)
Proposition 6.6. If m ∈ M(μ) and fm ∈ M(μ, t)∨ is defined by fm(m(cid:19)) = Hμ(m(cid:19), m),
then
Y M(μ,t)∨ (b, z) fm = f
Y μ(
(cid:29)∞
n=1 e
2(−t+
√
−1(cid:28)(μ))
n
(−z)−n an b,z)m
In particular the fields
Y μ,t (b, z) := Y μ
2(−t+
√
−1(cid:28)(μ))
n
e
(−z)−nan b, z
(cid:25)
(cid:24)
∞(cid:9)
n=1
(6.16)
define a V 1(Ca)-module structure on M(μ).
Proof. By definition,
(Y M(μ,t)∨ (b, z) fm)(m(cid:19)) =Hμ(Y μ(A(t, z)b, z−1)m(cid:19), m)
=(Y μ(ez L(t)1 z−2L(t)0 g(b), z−1)m(cid:19), m)
Using (6.5) we can write
ez(L(t))1 = ez L(0)1
e− 2t
n znan = ez L(
√
−1(cid:28)(μ))1
∞(cid:9)
n=1
√
e− 2(t−
−1(cid:28)(μ))
n
znan
∞(cid:9)
n=1
so, by Lemma 6.5,
(Y M(μ,t)∨ (b, z) fm)(m(cid:19))
=Hμ(Y μ(ez L(
=Hμ(Y μ(ez L(
√
√
−1(cid:28)(μ))1
√
e− 2(t−
−1(cid:28)(μ))
n
∞(cid:9)
n=1
znan z−2L(t)0 g(b), z−1)m(cid:19), m)
−1(cid:28)(μ))1 z−2L(t)0 g
∞(cid:9)
n=1
2(−t+
√
−1(cid:28)(μ))
n
e
(−z)−nan b, z−1)m(cid:19), m)
V. G. Kac, P. Möseneder Frajria, P. Papi
Since the form Hμ is L(
√
−1(cid:28)(μ))-invariant, we find that
(Y M(μ,t)∨ (b, z) fm)(m(cid:19)) = Hμ(m(cid:19), Y μ(
∞(cid:9)
2(−t+
√
−1(cid:28)(μ))
n
e
(−z)−nan b, z)m)
= f
Y μ(
(cid:29)∞
n=1 e
2(−t+
√
−1(cid:28)(μ))
n
(−z)−n an b,z)m
n=1
(m(cid:19)).
(cid:17)(cid:18)
To simplify notation write a = (a−1, a−2, . . .). If I is an infinite sequence (i1, i2, . . .),
(cid:29)∞
−r . We can regard b ∈ V 1(Ca) as a
with i j ∈ Z+ almost all zero, then set aI =
polynomial b(a). More precisely, we write
r =1 a jr
(cid:10)
b(a) =
cI aI 1, cI ∈ C.
We also set
I
ρ(z) = (z10, z210, z310, . . .) = (z I, z2 I, z3 I, . . .).
μ,t
r ∈Z b
r
z−r −(cid:14)b . Then
(cid:11)
, where ashi f t = a + 2(−t +
√
−1(cid:28)(μ))ρ(−1).
Lemma 6.7. Write Y μ,t (b, z) =
(cid:30)
b(ashi f t )
(cid:31)μ
=
bμ,t
r
r
Proof. Since b
μ,t
r = Resz zr +(cid:14)b−1(Y μ,t (b, z)), we need to check that
Resz zr +(cid:14)b−1(Y μ,t (b, z)) = b(a + (−t +
It is enough to check this for b = aI 1. Using (6.9), we can write
−1(cid:28)(μ))ρ(−1))μ
r
.
√
2(−t+
√
−1(cid:28)(μ))
n
e
(−z)−nan aI 1 = (a + 2(−t +
√
−1(cid:28)(μ))ρ(−z−1))I 1.
∞(cid:9)
n=1
It follows that
Y μ,t (b, z) = Y μ(b(a + 2(−t +
√
−1(cid:28)(μ))ρ(−z−1), z)
hence we need to check that
Resz zr +(cid:14)
√
aI −1(Y ((a + 2(−t +
(cid:30)
(ashi f t )I 1 ⊗ v
=
.
r
−1(cid:28)(μ))ρ(−z−1))I 1 ⊗ v, z))
(cid:31)
Indeed, setting t0 = 2(−t +
√
−1(cid:28)(μ)) and letting q be the number of j such that i j (cid:6)= 0,
Y μ((a + t0ρ(−z−1))I 1, z))
(cid:4)
(cid:10)
(cid:10)
q(cid:9)
⎛
⎝
=
s
j1≤i1,..., jq ≤iq
(cid:10)
(cid:10)
=
p=1
q(cid:9)
⎛
⎝
s
j1≤i1,..., jq ≤iq
p=1
(cid:5)
i p− jp
(cid:5) (cid:4)
i p
jp
t0
(−z) p
(cid:4)
(−1) p(i p− jp)t
i p− jp
0
i p
jp
⎞
⎠ a J 1s z
⎞
(cid:5)
(cid:11)q
−s−
p=1 pjp
⎠ a J 1s z−s−(cid:14)
aI 1
Unitarity of Minimal W -Algebras and Their Representations I
so
Resz zr +(cid:14)
aI 1
−1(Y μ(a + t0ρ(−z−1))I 1, z))
⎛
(cid:10)
q(cid:9)
=
=
⎝
((−1) pt0)i p− jp
p=1
j1≤i1,..., jq ≤iq
(cid:31)
(cid:30)
(a + t0ρ(−1))I 1
,
r
⎞
(cid:5)
⎠ a J 1r
(cid:4)
i p
jp
as wished.
In particular,
so
aμ,t
r
= (a−11)μ,t
r
= aμ
r
− 2(−t +
√
−1(cid:28)(μ))δr,0 I
μ,t
0
a
= (μ − 2(−t +
√
−1(cid:28)(μ)))I = (μ + 2t)I.
Hence we have an isomorphism of V 1(Ca)-modules
(cid:17)(cid:18)
(6.17)
(6.18)
Let M[μ, t] denote the vector space M(μ) equipped with the V 1(Ca)-module struc-
M(μ, t)∨ ∼= M(μ + 2t).
ture given by b (cid:25)→ Y μ,t (b, z) so that
M[μ, t] (cid:29) M(μ, t)∨ (cid:29) M(μ + 2t).
Let ϒμ,t : M[μ, t] → M(μ+2t) denote such an isomorphism. By (6.17), ϒμ,t (vμ) ∈
Cvμ+2t . We can therefore normalize ϒμ,t so that vμ (cid:25)→ vμ+2t . It follows from (6.17)
that, if j1 ≥ j2 ≥ · · · ≥ jr ,
μ
− j1
vμ) = ϒμ,t (a
vμ) = a
ϒμ,t (a
vμ+2t .
μ+2t
− j1
μ+2t
− jr
μ,t
− j1
· · · a
· · · a
· · · a
μ,t
− jr
μ
− jr
Note that, by (6.13),
Moreover
Hμ+2t (ϒμ,t (m), ϒμ,t (m(cid:19))) = Hμ(m, m(cid:19)).
(6.19)
Y μ,t+s(b, z) = Y μ(
∞(cid:9)
√
2(−t−s+
n
e
−1(cid:28)(μ)
(−z)−nan b, z)
n=1
∞(cid:9)
= Y μ,t (
n=1
−2s
n
(−z)−nan b, z),
e
(6.20)
and, if m ∈ M(μ) and m(cid:19) ∈ M(μ + 2s),
Hμ+2s(ϒμ,sY μ,t (b, z)m, m(cid:19)) = Hμ+2s(ϒμ,sY μ,s+(t−s)(b, z)m, m(cid:19))
= Hμ+2s(ϒμ,sY μ,s(
= Hμ+2s(Y μ+2s(
∞(cid:9)
−2(t−s)
n
e
(−z)−nan b, z)m, m(cid:19))
∞(cid:9)
n=1
−2(t−s)
n
e
(−z)−nan b, z)ϒμ,s(m), m(cid:19))
n=1
√
= Hμ+2s(Y μ+2s,t+s−
−1(cid:28)(μ)(b, z)ϒμ,s(m), m(cid:19)).
V. G. Kac, P. Möseneder Frajria, P. Papi
It follows that
√
ϒμ,sY μ,t (b, z) = Y μ+2s,t+s−
−1(cid:28)(μ)(b, z)ϒμ,s
In particular, if μ is real,
ϒμ,sY μ,t (b, z) = Y μ+2s,t+s(b, z)ϒμ,s
Lemma 6.8. If m, m(cid:19) ∈ M(μ) and b ∈ V 1(Ca), then
(6.21)
(6.22)
Hμ(m, Y μ,t (b, z)m(cid:19)) = Hμ(Y μ,s(A(−
√
−1(cid:28)(μ) + t + s, z)b, z−1)m, m(cid:19)). (6.23)
√
In particular, if b is quasiprimary for L(−
−1(cid:28)(μ) + t + s), then
Hμ(m, bμ,t
n m(cid:19)) = Hμ(g(b)μ,s
−n m, m(cid:19)).
(6.24)
Proof. We first prove that
Hμ(m, Y μ,t (b, z)m(cid:19)) = Hμ(Y μ,t (A(−
√
−1(cid:28)(μ) + 2t, z)b, z−1)m, m(cid:19)).
(6.25)
Indeed,
Hμ(m, Y μ,t (b, z)m(cid:19)) = Hμ+2t (ϒμ,t (m), ϒμ,t (Y μ,t (b, z)m(cid:19)))
= Hμ+2t (ϒμ,t (m), Y μ+2t (b, z)ϒμ,t (m(cid:19)))
√
= Hμ+2t (Y μ+2t (A(−
= Hμ+2t (ϒμ,t (Y μ,t (A(−
√
−1(cid:28)(μ) + 2t, z)b, z−1)ϒμ,t (m), ϒμ,t (m(cid:19)))
−1(cid:28)(μ) + 2t, z)b, z−1)m), ϒμ,t (m(cid:19))),
so (6.25) follows.
To prove (6.23) write
Hμ(m, Y μ,t (b, z)m(cid:19)) = Hμ(m, Y μ,s(
−2(t−s)
n
e
(−z)−nan b, z)m(cid:19)).
∞(cid:9)
n=1
By (6.25), setting s0 = −
√
−1(cid:28)(μ) + 2s,
Hμ(m, Y μ,t (b, z)m(cid:19)) = Hμ(Y μ,s(A(s0, z)
−2(t−s)
n
e
(−z)−nan b, z−1)m, m(cid:19))
∞(cid:9)
n=1
= Hμ(Y μ,s(ez L(s0)1 z−2L(s0)0 g
−2(t−s)
n
e
(−z)−nan b, z−1)m, m(cid:19))
∞(cid:9)
n=1
= Hμ(Y μ,s(ez L(s0)1
∞(cid:9)
n=1
−2(t−s)
n
e
znan z−2L(s0)0 g(b), z−1)m, m(cid:19)).
Since, if p ∈
√
−1R,
ez(L( p))1 = ez L(0)1
∞(cid:9)
n=1
e− 2 p
n znan ,
L(s)μ,t
n
In other words
L(s)μ,t
n
Unitarity of Minimal W -Algebras and Their Representations I
we find that
Hμ(m, Y μ,s(b, z)m(cid:19))
= Hμ(Y μ,s(ez L(0)1
√
= Hμ(Y μ,s(ez L(−
= Hμ(Y μ,s(ez L(0)1
√
= Hμ(Y μ,s(A(−
∞(cid:9)
√
e− 2(−
−1(cid:28)(μ)+s+t)
n
znan z−2L(0)0 g(b), z−1)m, m(cid:19))
n=1
−1(cid:28)(μ)+s+t)1 z−2L(0)0 g(b), z−1)m, m(cid:19))
∞(cid:9)
√
e− 2(−
−1(cid:28)(μ)+s+t)
n
znan z−2L(0)0 g(b), z−1)m, m(cid:19))
n=1
−1(cid:28)(μ) + s + t, z)b, z−1)m, m(cid:19)).
(cid:17)(cid:18)
Example 6.9 (The Fairlie construction). Since L(s) = 1
2 a2
−11+sa−21, by (6.17) we have
−1(cid:28)(μ))21n + s(a−2 − 2t + 2
√
−1(cid:28)(μ))a−11n + 2(t −
√
√
−1(cid:28)(μ))1n
√
−1(cid:28)(μ))1n
−1(cid:28)(μ))21n
√
= 1
2
= 1
(a−1 + 2t − 2
2 a2
−11n + 2(t −
+ sa−21n − 2s(t −
√
: aa :n +2(t −
√
= 1
2
+ 2(t −
−1(cid:28)(μ))an + s(T a)n
√
−1(cid:28)(μ))(t −
−1(cid:28)(μ) − s)1n.
: aa :μ
√
= 1
2
(t −
n + 2(t −
n +s(T a)μ
−1(cid:28)(μ) − s)1μ
n
√
−1(cid:28)(μ))aμ
n + 2(t −
√
−1(cid:28)(μ))
(6.26)
,
(6.27)
In particular, if μ ∈ R, we have
L(s)μ,t
n
= 1
2
: aa :μ
n +s(T a)μ
n + 2taμ
n + 2(t 2 − st)1μ
n
and, setting s = 2t, (6.27) becomes
n +s(T a)μ
L(s)μ,s/2
n
: aa :μ
= 1
2
n + saμ
n
− 1
2 s21μ
n
= L(s)μ
n + saμ
n
− 1
2 s21μ
n
. (6.28)
By the −1-st product identity,
(cid:8)(cid:11)
: aa :μ
n
=
2
j∈Z a
(cid:11)
μ
μ
− j a
j+n
μ
μ
j + (a
− j a
if n (cid:6)= 0,
if n = 0.
)2
μ
0
j∈N a
Moreover, (T a)μ
n = −(n + 1)a
L(s)μ,s/2
n
while
μ
n , hence, substituting in (6.28), we obtain
= 1
2
n if n (cid:6)= 0,
− snaμ
μ
− j a
μ
j+n
(cid:10)
a
j∈Z
L(s)μ,s/2
0
=
(cid:10)
j∈N
μ
− j a
μ
j +
a
μ2 − s2
2
I.
V. G. Kac, P. Möseneder Frajria, P. Papi
Since b (cid:25)→ Y μ,s/2(b, z) gives a V 1(Ca)-module structure to M(μ) and
[L(s)λ L(s)] = (T + 2λ)L(s) +
λ3
12
(1 − 12s2),
by the Borcherds commutator formula,
[L(s)μ,s/2
n
, L(s)μ,s/2
m
] = (n − m)L(s)μ,s/2
n+m +
n3 − n
12
(1 − 12s2)δn,−m.
Finally, since L(s) is quasiprimary for L(s) and g(L(s)) = L(s), by (6.24) we have
Hμ(m, L(s)μ,s/2
n
m(cid:19)) = Hμ(L(s)μ,s/2
−n m, m(cid:19)).
We now extend the previous analysis of invariant Hermitian forms on bosons to the case
of the vertex algebra V 1(Ca) ⊗ V where V is a conformal vertex algebra.
Let (cid:6)L be the conformal vector of V . Set
(cid:6)L(s) = L(s) + (cid:6)L.
(6.29)
If M is a V -module, then M(μ) ⊗ M is a V 1(Ca) ⊗ V -module and, if M is equipped
−1(cid:28)(μ))-invariant form
with a (cid:6)L-invariant form ( . , . ), then Hμ( . , . ) ⊗ ( . , . ) is a (cid:6)L(
on M(μ) ⊗ M that we keep denoting by Hμ( . , . ).
√
The arguments developed in this section for V 1(Ca) can be carried out in the same
way in the more general setting of the vertex algebra
V 1(Ca) ⊗ V,
(6.30)
where V is any conformal vertex algebra. In particular, we have
Proposition 6.10. If b ∈ V 1(Ca) ⊗ V and M is a V -module, then the fields
Y μ,t (b, z) = Y μ(
2(−t+
√
−1(cid:28)(μ))
n
e
(−z)−nan b, z)
∞(cid:9)
n=1
define a V 1(Ca) ⊗ V -module structure on M(μ) ⊗ M.
As before, we can regard b ∈ V 1(Ca) ⊗ V as a polynomial b(a) with values in V .
More precisely, we write
b(a) =
(cid:10)
I
aI ⊗ cI , cI ∈ V.
The following is the generalization of Lemma 6.7. The proof is the same.
(cid:11)
Lemma 6.11. Write Y μ,t (b, z) =
μ,t
r ∈−(cid:14)b+Z b
r
z−r −(cid:14)b . Then
bμ,t
r
= b(a + 2(−t +
√
−1(cid:28)(μ))ρ(−1))μ
r
.
Unitarity of Minimal W -Algebras and Their Representations I
7. Minimal W -Algebras
7.1. λ-brackets and conjugate linear involutions. Let, as before, g be a basic classical
Lie superalgebra, and x ∈ g be an element, for which ad x is diagonalizable with
Z, the ad x-gradation of g satisfies (1.2) with some f ∈ g−1 and is
eigenvalues in 1
2
compatible with the parity of g. Then for some e ∈ g1, {e, x, f } is an sl2-triple as in
Proposition 3.2, i.e. (3.1) holds with g(cid:4) the centralizer of f in g. Recall that the invariant
bilinear form (.|.) on g is normalized by the condition (x|x) = 1
2 , and we have the
orthogonal direct sum of ideals
= Cx ⊕ g
(cid:4).
g0
(7.1)
Choose a Cartan subalgebra h(cid:4) of g(cid:4), so that, by (7.1), h = Cx ⊕h(cid:4) is a Cartan subalgebra
of g0 (and of g).
Let
(cid:4) =
g
s(cid:7)
i=0
(cid:4)
g
i
(7.2)
= 0.
(cid:4)
be the decomposition of g(cid:4) into the direct sum of ideals, where g
0 is the center and the
(cid:4)
i are simple for i > 0. Let h∨ be the dual Coxeter number of g, and denote by ¯h∨
g
i half
(cid:4)
i with respect to (.|.)
of the eigenvalue of the Casimir element of g
, when acting on
(cid:4)
i . Note that ¯h∨
g
In [18] the authors introduced (as a special case of a more general construction) the
(g), attached to
universal minimal W -algebra W k
the grading (5.6). This is a vertex algebra strongly and freely generated by elements L,
J {v} where v runs over a basis of g(cid:4), G{u} where u runs over a basis of g−1/2, with the
following λ-brackets ( [20, Theorem 5.1]): L is a Virasoro element (conformal vector)
with central charge c(k) given by (1.4), J {u} are primary of conformal weight 1, G{v}
are primary of conformal weight 3
(g), whose simple quotient is W min
(cid:4)
×g
i
(cid:4)
|g
i
min
0
k
2 , and
[J {u}
[J {u}
λG{v}] = G{[u,v]}
λ J {v}] = J {[u,v]}
+ λβk(u|v)
(cid:4), v ∈ g−1/2
for u ∈ g
for u, v ∈ g
(cid:4),
where
βk(u, v) = δi, j (k +
h∨− ¯h∨
i
2
(cid:4)
)(u|v), u ∈ g
i
(cid:4)
, v ∈ g
j
, i, j ≥ 0.
,
(7.3)
(7.4)
(7.5)
Furthermore, the most explicit formula for the λ-bracket between the G{u} is given in
[1, (1.1)] and in [20, Theorem 5.1 (e)]. We will need both formulas:
[G{u}
λG{v}] = − 2(k + h∨)(cid:2)u, v(cid:3)L + (cid:2)u, v(cid:3)
dim g(cid:4)(cid:10)
α=1
: J {uα} J {uα} : +
dim g1/2(cid:10)
: J {[u,wγ ](cid:4)} J {[wγ ,v](cid:4)} : +2(k + 1)∂ J {[[e,u],v](cid:4)}
γ =1
+ 4λ
(cid:10)
i
p(k)
ki
J {[[e,u],v](cid:4)
i
}
+ 2λ2(cid:2)u, v(cid:3) p(k)1,
(7.6)
(G{u}
λG{v}) = − 2(k + h∨)(cid:2)u, v(cid:3)L + (cid:2)u, v(cid:3)
V. G. Kac, P. Möseneder Frajria, P. Papi
dim g(cid:4)(cid:10)
α=1
: J {uα} J {uα} : +
(cid:10)
2
α,β
+ 2λ
(cid:2)[uα, u], [v, uβ ](cid:3) : J {uα} J {uβ } : +2(k + 1)(∂ + 2λ)J {[[e,u],v](cid:4)}
(cid:10)
α,β
(cid:2)[uα, u], [v, uβ ](cid:3)J {[uα,uβ ]}
+ 2λ2(cid:2)u, v(cid:3) p(k)1,
(7.7)
where {uα} and {uα} (resp. {wγ }, {wγ }) are dual bases of g(cid:4) (resp. g1/2) with respect to
(cid:4)
i (resp. a (cid:25)→ a(cid:4)) for a ∈ g0 is the orthogonal
(.|.) (resp. with respect to (cid:2)·, ·(cid:3)ne), a (cid:25)→ a
(cid:4)
i (resp g(cid:4)), p(k) is the monic quadratic polynomial proportional to (7.28),
projection to g
(h∨ − ¯h∨
),
introduced in [1, Table 4], and thoroughly investigated in [15], and ki = k + 1
i
2
i = 1, . . . , s (see Table 2 below for the values of ¯h∨
i ).
The following proposition is a special case of [16, Lemma 7.3], in view of Lemma 3.1.
Proposition 7.1. Let φ be a conjugate linear involution of g such that φ( f ) = f, φ(x) =
x, φ(e) = e. Then the map
φ(J {u}) = J {φ(u)}, φ(G{v}) = G{φ(v)}, φ(L) = L , u ∈ g
extends to a conjugate linear involution of the vertex algebra W k
min
The following result is a sort of converse to Proposition 7.1.
(cid:4), v ∈ g−1/2
(g).
(7.8)
Proposition 7.2. Assume that k ∈ R is non-collapsing. Let ψ be a conjugate linear
(g). Then there exists a conjugate linear involution φ of g satisfying
involution of W k
(1.1) such that ψ is the conjugate linear involution induced by φ.
Proof. If a, b ∈ g(cid:4), define φ(a) by
min
Then
ψ(J {a}) = J {φ(a)}.
ψ([J {a}
[J {φ(a)}
λ J {b}]) = ψ(J {[a,b]}) + λβk(a, b) = J {φ([a,b])}
λ J {φ(b)}] = J {[φ(a),φ(b)]}
+ λβk(φ(a), φ(b))
+ λβk(a, b)
(7.9)
(7.10)
Since ψ is a vertex algebra conjugate linear automorphism, (7.9) equals (7.10), so that
φ is a conjugate linear involution of g(cid:4), and we have
βk(a, b) = βk(φ(a), φ(b)).
Since k is not collapsing, relations (7.22), (7.28) and (7.11) imply that
(a|b) = (φ(a)|φ(b)) for a, b ∈ g
(cid:4).
(7.11)
(7.12)
We now prove that there is a unique extension of φ to a conjugate linear automorphism
of g fixing e, x, and f . Note that φ(g−1/2
]. In
= [e, g−1/2
particular, setting φ(x) = x, φ( f ) = f, φ(e) = e, φ(u) = [e, φ(v)] for u ∈ g1/2
, u =
[e, v], v ∈ g−1/2, we extend φ to a conjugate linear bijection g → g. In particular, φ is
) ⊂ g−1/2 and that g1/2
Unitarity of Minimal W -Algebras and Their Representations I
unique. It remains to prove that it is a conjugate linear automorphism. Note first that, by
(7.1), equation (7.12) holds for a, b ∈ g0. Consider elements
g = α e + u + a + v + β f + γ x, g(cid:19) = α(cid:19) e + u(cid:19)
+ a(cid:19)
, a, a(cid:19) ∈ g(cid:4). Then
, v, v(cid:19) ∈ g−1/2
where α, α(cid:19), β, β(cid:19), γ , γ (cid:19) ∈ C, u, u(cid:19) ∈ g1/2
+ β(cid:19) f + γ (cid:19) x,
+ v(cid:19)
(cid:19)]) =
φ([g, g
φ([e, v(cid:19)] + αβ(cid:19)
γ (cid:19)
− 1
u + [a, u
2
γ (cid:19)v − βα(cid:19)
+ 1
2
(cid:19))] =
[φ(g), φ(g
¯α[e, φ(v(cid:19))] + ¯α ¯β(cid:19)
− 1
2
x − αγ (cid:19)
(cid:19)] + [a, a
x + β[ f, u
(cid:19)] + [u, a
e + [u, u
(cid:19)] + [a, v(cid:19)] + α(cid:19)[v, e] + [v, u
f + γ α(cid:19)
(cid:19)] + βγ (cid:19)
(cid:19)] + [u, v(cid:19)] + β(cid:19)[u, f ]
(cid:19)] + [v, a
γ u
(cid:19) − 1
2
e + 1
2
(cid:19)] + [v, v(cid:19)]
γ v(cid:19) − β(cid:19)γ f ),
(7.13)
x − ¯α ¯γ (cid:19)
¯γ (cid:19)φ(u) + [φ(a), φ(u
e + [φ(u), φ(u
(cid:19))] + [φ(a), φ(a
(cid:19))] + [φ(u), φ(a
(cid:19))] + [φ(a), φ(v(cid:19))] + ¯α(cid:19)[φ(v), e] + [φ(v), φ(u
(cid:19))] + [φ(u), φ(v(cid:19))] + ¯β(cid:19)[φ(v), f ]
(cid:19))]
¯γ (cid:19)φ(v) − ¯β ¯α(cid:19)
x + β[ f, φ(u
(cid:19))] + βγ (cid:19)
f + ¯γ ¯αe
+ [φ(v), φ(a
+ 1
¯γ φ(u
2
(cid:19)) − 1
2
(cid:19))] + [φ(v), φ(v(cid:19))] + 1
2
¯γ φ(v(cid:19)) − ¯β(cid:19) ¯γ f ).
Hence (7.13) equals (7.14), provided the following equalities hold
φ([u, u(cid:19)]) = [φ(u), φ(u(cid:19))],
φ([u, a(cid:19)]) = [φ(u), φ(a(cid:19))],
φ([u, v(cid:19)]) = [φ(u), φ(v(cid:19))],
φ([v, v(cid:19)]) = [φ(v), φ(v(cid:19))],
φ([v, a(cid:19)]) = [φ(v), φ(a(cid:19))],
φ([u, f ]) = [φ(u), f ].
Relation (7.3) implies at once (7.19). To prove (7.18) note that [v, v(cid:19)] = (cid:2)v, v(cid:19)(cid:3) f , so
it is enough to prove that (cid:2)φ(v), φ(v(cid:19))(cid:3) = (cid:2)v, v(cid:19)(cid:3). By (7.7),
G{φ(v)}
3/2G{φ(v(cid:19))} = 4 p(k)(cid:2)φ(v), φ(v(cid:19))(cid:3)1 = φ(4 p(k)(cid:2)v, v(cid:19)(cid:3)1) = 4 p(k)(cid:2)v, v(cid:19)(cid:3)1.
Since p(k) (cid:6)= 0 (k is not collapsing) and k is real, we have the claim.
Now we prove (7.20). Here and in the following we write u = [e, v], v ∈ g−1/2.
Then
φ([u, f ]) = φ([[e, v], f ]) = −φ([x, v])
= 1
2
φ(v) = −[x, φ(v)] = [[e, φ(v)], f ] = [φ(u), f ].
Next we prove (7.17). We have to prove that
φ([[e, v], v(cid:19)]) = [[e, φ(v)], φ(v(cid:19))].
By (7.6)
G{φ(v)}
1/2G{φ(v(cid:19))} =
(cid:10)
i
p(k)
ki
J {[[e,φ(v)],φ(v(cid:19))](cid:4)
i
}.
(7.14)
(7.15)
(7.16)
(7.17)
(7.18)
(7.19)
(7.20)
V. G. Kac, P. Möseneder Frajria, P. Papi
On the other hand
ψ(G{v}
1/2G{v(cid:19)}) =
(cid:10)
i
J {φ([[e,v],v(cid:19)](cid:4)
i
)}.
p(k)
ki
(cid:4)
(cid:4)
Since φ is an automorphism of g(cid:4) there is a permutation i (cid:25)→ i (cid:19) such that φ(g
i (cid:19). It
i
[[e, φ(v)], φ(v(cid:19))](cid:4)
=
follows
i (cid:19)
= φ([[e, v], v(cid:19)](cid:4)
i
that φ([[e, v], v(cid:19)](cid:4)
i
φ([[e, v], v(cid:19)])(cid:4)
i (cid:19)
) = g
hence
)
) for all i, and also
(cid:10)
[[e, φ(v)], φ(v(cid:19))](cid:4) =
i (cid:19)
φ([[e, v], v(cid:19)])(cid:4)
i (cid:19) = φ([[e, v], v(cid:19)])(cid:4).
(cid:10)
=
i (cid:19)
[[e, φ(v)], φ(v(cid:19))](cid:4)
i (cid:19) =
(cid:10)
i
φ([[e, v], v(cid:19)](cid:4)
i
)
To conclude we have to check the x-component:
(x|[[e, φ(v)], φ(v(cid:19))]) = ([x, [e, φ(v)]]|φ(v(cid:19))) = 1
2
= 1
2
(cid:2)φ(v), φ(v(cid:19))(cid:3) = 1
2
(cid:2)v, v(cid:19)(cid:3).
([e, φ(v)]|φ(v(cid:19)))
Since (1.1) holds on g0, we have
(x|φ([[e, v], v(cid:19)]) = (φ(x)|[[e, v], v(cid:19)]) = (x|[[e, v], v(cid:19)]) = 1
2
(cid:2)v, v(cid:19)(cid:3).
Next, we prove (7.16). We have
φ([[e, v], a(cid:19)]) = φ([e, [v, a(cid:19)]]) = [e, φ([v, a(cid:19)])] = [e, [φ(v), φ(a(cid:19))]]
= [[e, φ(v)], φ(a(cid:19))] = [φ(u), φ(a(cid:19))].
Next, we prove (7.15). Consider u = [e, v], u(cid:19) = [e, v(cid:19)], v, v(cid:19) ∈ g−1/2.
φ([u, u(cid:19)]) = φ([[e, v], [e, v(cid:19)]]) = φ([e, [[e, v], v(cid:19)]]) = [e, φ([[e, v], v(cid:19)])].
By (7.17), we obtain
φ([u, u(cid:19)]) = [e, [φ([e, v]), φ(v(cid:19))])] = [e, [φ(u), φ(v(cid:19))]] = [φ(u), φ(u(cid:19))].
It remains to check that
(φ(a)|φ(b)) = (a|b)
for a, b ∈ g. We already observed that this relation holds for a, b ∈ g0 and it is obvious
that (φ(e)|φ( f )) = (e| f ). We now compute for u ∈ g1/2, v(cid:19) ∈ g−1/2,
(φ(u)|φ(v(cid:19))) = ([e, φ(v)]|φ(v(cid:19))) = (cid:2)φ(v), φ(v(cid:19))(cid:3) = (cid:2)v, v(cid:19)(cid:3) = (u|v(cid:19)).
(cid:17)(cid:18)
Unitarity of Minimal W -Algebras and Their Representations I
By Proposition 3.2 there is a conjugate linear involution φ on g such that φ(x) =
x, φ( f ) = f and (g(cid:4))φ is a compact real form of g(cid:4), hence, by Proposition 7.1, φ induces
(g), and descends to a conjugate
a conjugate linear involution of the vertex algebra W k
linear involution of its unique simple quotient W min
min
(g), which we again denote by φ.
By [16, Proposition 7.4 (b)], W k
(g) admits a unique φ-invariant Hermitian form
H (·, ·) such that H (1, 1) = 1. Recall that if k + h∨ (cid:6)= 0 then the kernel of H (·, ·)
(g), hence H (·, ·) descends to a non-degenerate
is the unique maximal ideal of W k
min
φ-invariant Hermitian form on W min
(g), which we again denote by H (·, ·).
min
k
k
We need to fix notation for affine vertex algebras. Let a be a Lie superalgebra equipped
with a nondegenerate invariant supersymmetric bilinear form B. The universal affine
vertex algebra V B(a) is the universal enveloping vertex algebra of the Lie conformal
superalgebra R = (C[T ] ⊗ a) ⊕ C with λ-bracket given by
[aλb] = [a, b] + λB(a, b), a, b ∈ a.
In the following, we shall say that a vertex algebra V is an affine vertex algebra if it is a
quotient of some V B(a). If a is simple Lie algebra, we denote by (.|.)a the normalized
invariant bilinear form on a, defined by the condition (α|α)a = 2 for a long root α. Then
B = k(.|.)a, and we simply write V k(a). If k (cid:6)= −h∨, then V k(a) has a unique simple
quotient, which will be denoted by Vk(a).
Let ψ be a conjugate linear involution of a such that (ψ(x)|ψ(y)) = (x|y). By [16,
§5.3] there exists a unique ψ-invariant Hermitian form Ha on V k(a). The kernel of Ha
is the maximal ideal of V k(a), hence Ha descends to Vk(a).
7.2. Some numerical information. Recall the decomposition (7.2) of the Lie algebra g(cid:4),
and that we assume that g(cid:4) is not abelian, i.e. s ≥ 1 in (7.2). Let θi be the highest root
(cid:4)
i for i > 0. Set
of the simple component g
Mi (k) = 2
ui
(cid:24)
k +
(cid:25)
h∨ − ¯h∨
i
2
,
i ≥ 0,
(7.21)
where
(cid:8)
2
(θi |θi )
ui =
if i = 0,
if i > 0.
(cid:4)
i denote the invariant bilinear form on g
Let (.|.)(cid:4)
2 for i > 0, and let (.|.)(cid:4)
(cid:4)
×g
0
0
hence, formula (7.5) can be written as
= (.|.)
(cid:4)
|g
0
i , normalized by the condition (θi |θi )(cid:4)
. Note that, for i > 0, (a|b)(cid:4)
i
= δi, j
(θi |θi )
2
=
i
(a|b),
βk(a, b) = δi, j Mi (k)
(θi |θi )
2
= δi, j Mi (k)(a|b)(cid:4)
i
(a|b)
(cid:4)
for a ∈ g
i
(cid:4)
, b ∈ g
j
, i, j ≥ 0.
(7.22)
(7.23)
In other words, the vertex subalgebra of W k
min generated by J {a}, a ∈ g(cid:4), is
(cid:4)
V Mi (k)(g
i
).
i≥0
V. G. Kac, P. Möseneder Frajria, P. Papi
g(cid:4)
C ⊕ slm
sl2
ui
2, −2
−2
g
sl(2|m), m > 2
psl(2|2)
osp(4|m), m > 2 sl2 ⊕ spm 2, −4
spo(2|3)
−1/2
sl2
spo(2|m), m > 4 som
−1
D(2, 1; a)
sl2 ⊕ sl2 − 2
1+a
F(4)
−4/3
so7
G(3)
−2/3
G2
Table 2. Numerical information
¯h∨
i
0, −m
−2
2, −m − 2
−1/2
h∨
2 − m
0
2 − m
1/2
2 − m/2 1 − m/2
0
−2
−3/2
− 2
1+a
−10/3
−3
, − 2a
1+a
, − 1
2 k − 1
, −k − 1
Mi (k)
k − m−2
2
−k − 1
k − m
2
−4k − 2
−2k − 1
−(1 + a)k − 1, − 1+a
− 3
− 4
2 k − 1
3 k − 1
, − 2a
1+a
χi
1 − m/2, −1
−1
−m/2, −1
−2
−1
a k − 1 −1, −1
−1
−1
Closely related to the vertex algebra W k
(g) is the universal affine vertex algebra
min
V αk (g0
) (see [20, (5.16)]), where
αk(a, b) = ((k + h∨)(a|b) − 1
2
κg0
and where κg0 denotes the Killing form of g0. Note that
(cid:4)
)(a|b) if a ∈ g
i
αk(a, b) = δi, j (k + h∨ − ¯h∨
i
, b ∈ g
(cid:4)
j
, i, j ≥ 0.
(a, b)) ,
(7.24)
We have another formula for the cocycle αk, closely related to (7.23):
αk(a, b) = δi, j
2
(θi |θi )
!
k + h∨ − ¯h∨
i
"
(a|b)(cid:4)
i
= δi, j (Mi (k) + χi )(a|b)(cid:4)
(cid:4)
i for a ∈ g
i
, b ∈ g
(cid:4)
j
, i, j ≥ 0,
(7.25)
where
χi =
h∨ − ¯h∨
i
ui
, i ≥ 0.
(7.26)
The relevant data for computing the Mi (k) and χi are collected in Table 2, where their
explicit values are also displayed. Note that M0(k) = k + 1
As in the Introduction, denote by ξ ∈ (h(cid:4))∗ a highest weight of the g(cid:4)-module g−1/2.
2 h∨.
Lemma 7.3. For i ≥ 1 we have
χi = −ξ(θ ∨
i
),
(7.27)
with the exception of χ1 for g = osp(4|m).
Proof. The weights ξ are restrictions to h(cid:4) of the maximal odd roots of g; they are listed
in Table 3, together with the maximal roots θi . Relation (7.27) is then checked directly
(cid:17)(cid:18)
using the data in Tables 1, 2, 3.
Recall from [1] that a level k is collapsing for W min
(g) if W min
k
(g) is a subalgebra of
k
the simple affine vertex algebra Vβk
(g(cid:4)).
We summarize in the following result the content of Theorem 3.3 and Proposition
3.4 of [1] relevant to our setting. We say that an ideal in g(cid:4) is a component of g(cid:4) if it is
simple or 1-dimensional.
Unitarity of Minimal W -Algebras and Their Representations I
(cid:4)
Table 3. Highest odd roots and highest roots of g
g
sl(2|m), m > 2
psl(2|2)
osp(4|m), m > 2
spo(2|3)
spo(2|m), m > 4
D(2, 1; a)
F(4)
G(3)
Highest odd roots
(cid:22)1 − δm , δ1 − (cid:22)2
(cid:22)1 − δ2, δ1 − (cid:22)2
(cid:22)1 + δ1
δ1 + (cid:22)1
δ1 + (cid:22)1
(cid:22)1 + (cid:22)2 + (cid:22)3
1
2
δ1 + (cid:22)1 + (cid:22)2
(δ1 + (cid:22)1 + (cid:22)2 + (cid:22)3)
θi
δ1 − δm
δ1 − δ2
(cid:22)1 − (cid:22)2, 2δ1
(cid:22)1
(cid:22)1 + (cid:22)2
2(cid:22)2, 2(cid:22)3
(cid:22)1 + (cid:22)2
(cid:22)1 + 2(cid:22)2
Theorem 7.4. Let g be a basic Lie superalgebra from Table 2. Assume k (cid:6)= −h∨. Let
p(k) be the monic quadratic polynomial in k, proportional to
(cid:8)
M1(k)M2(k)
¯h∨
2 + 1) otherwise.
M1(k)(k +
1
if g(cid:4) has two components,
(7.28)
Then
(1) k is collapsing if and only if p(k) = 0.
(2) If g(cid:4) is simple then
(g) = C if and only if M1(k) = 0;
(g) ∼= VM1(k)(g(cid:4)).
− 1, then W min
(3) If g = D(2, 1; a) and k is collapsing, then W min
k
(a) W min
k
(b) if k = −
¯h∨
1
2
Mi (k) = 0.
(cid:4)
(g) = VM j (k)(g
j
), with j (cid:6)= i if
k
Remark 7.5. If Mi (k) ∈ Z+ for all i ≥ 1, g (cid:6)= osp(4|m) and Mi (k) < −χi for some
i ≥ 1, then k is a collapsing level (or critical). This is clear by looking at Table 2.
8. Necessary Conditions for Unitarity of Modules Over W k
min
(g)
We assume that g is from the list (1.3); in particular, g(cid:4) is a reductive Lie algebra. We
(g) following Sect. 7 of [20]. Let h(cid:4) be
parametrize the highest weight modules for W k
(cid:4)
(cid:4)
− ⊕h(cid:4) ⊕n
a Cartan subalgebra of g(cid:4), and choose a triangular decomposition g(cid:4) = n
+. For
ν ∈ (h(cid:4))∗ and l0 ∈ C, let L W (ν, (cid:10)0) (resp. M W (ν, (cid:10)0) ) denote the irreducible highest
(g)-module with highest weight (ν, (cid:10)0) and highest weight
weight (resp. Verma) W k
vector vν,(cid:10)0 . This means that one has
min
min
{h}
J
0
J {u}
n
{u}
J
0
vν,(cid:10)0
vν,(cid:10)0
vν,(cid:10)0
= ν(h)vν,(cid:10)0 for h ∈ h
= G{v}
= L nvν,(cid:10)0
vν,(cid:10)0
n
(cid:4)
+.
= 0 for u ∈ n
(cid:4), L 0vν,(cid:10)0
= l0vν,(cid:10)0
= 0 for n > 0, u ∈ g
,
(cid:4), v ∈ g−1/2
,
Let φ is an almost compact conjugate linear involution of g (see Definition 1.1); in
(cid:4)
R of φ|g(cid:4) is a compact Lie algebra (the adjoint group is
particular, the fixed points set g
(cid:4)
(cid:4)
R)∗ is said to be purely imaginary if
R = g
compact). Set h
(cid:4)
−1R. It is well-known that if α is a root of g(cid:4) and ν is purely imaginary then
ν(h
R) ⊂
ν(α) ∈ R.
(cid:4)
R ∩ h(cid:4). Recall that ν ∈ (h
√
V. G. Kac, P. Möseneder Frajria, P. Papi
, vν,(cid:10)0
φ-invariant
) = 1.
nondegenerate Hermitian
Lemma 8.1. Assume that l0 ∈ R and that ν is purely imaginary. Then L W (ν, (cid:10)0) admits a
unique
that
H (vν,(cid:10)0
Proof. It is enough to show that the Verma module M W (ν, (cid:10)0) admits a φ-invariant
Hermitian form H such that H (vν,(cid:10)0
) = 1. Fix a basis {vi | i ∈ I } of g−1/2 and a
(cid:4)
−. Set A{i} = J {ui } if i ∈ J , A{i} = G{vi } if i ∈ I , and A{0} = L.
basis {ui | i ∈ J } of n
Then
form H ( . , . )
, vν,(cid:10)0
such
(cid:16)(cid:30)
B =
(cid:30)
(cid:31)
b1 · · ·
(cid:31)
bs vν,(cid:10)0
A
{s}
−ms
A
{1}
−m1
(cid:17)
where bi ∈ Z+ , bi ≤ 1 if i ∈ I , mi > 0 or mi = 0 when i ∈ J , is a basis of M W (ν, (cid:10)0).
) = 1 and F(v) = 0
Define the conjugate-linear map F : M → C by setting F(vν,(cid:10)0
if v ∈ B, v (cid:6)= vν,(cid:10)0 .
If v ∈ M W (ν, (cid:10)0), m > 0, and u ∈ g(cid:4), then
(J {u}
m F)(v) = −F(J
{φ(u)}
−m
v) = 0.
Similarly we see that, if u ∈ g−1/2, then
(G{u}
m F)(v) = (L m F)(v) = 0.
On the other hand, if u ∈ n0+, then, since φ(u) ∈ n0−,
(J
{φ(u)}
{u}
0 F)(v) = −F(J
0
v) = 0.
If h ∈ h
(cid:4)
R, then, since ν(h) is purely imaginary,
(J
{h}
0 F)(vν,(cid:10)0
and, if v ∈ B, v (cid:6)= vν,(cid:10)0 , then
{φ(h)}
) = −F(J
0
vν,(cid:10)0
{h}
) = −F(J
0
vν,(cid:10)0
) = ν(h)F(vν,(cid:10)0
),
(J
{φ(h)}
{h}
0 F)(v) = −F(J
0
{h}
v) = −F(J
0
v) = 0.
It follows that J
{h}
0 F = ν(h)F for all h ∈ h(cid:4). Finally, since l0 ∈ R,
) = F(L 0vν,(cid:10)0
) = l0 F(vν,(cid:10)0
(L 0 F)(vν,(cid:10)0
),
and, if v ∈ B, v (cid:6)= vν,(cid:10)0 , then
(L 0 F)(v) = F(L 0v) = 0.
so L 0 F = l0 F. It follows that there is a W k
M W (ν, (cid:10)0)∨ mapping vν,(cid:10)0 to F. Define a Hermitian form on M W (ν, (cid:10)0) by setting
H (m, m(cid:19)) = β(m(cid:19))(m).
(g)-module map β : M W (ν, (cid:10)0) →
min
Let us check that this form is φ-invariant: write Y ν,(cid:10)0 for the field Y M W (ν,(cid:10)0) and ˇY ν,(cid:10)0
for the field Y M W (ν,(cid:10)0)∨
. Then
H (m, Y ν,(cid:10)0 (u, z)m(cid:19)) = β(Y ν,(cid:10)0 (u, z)m(cid:19))(m) = ˇY ν,(cid:10)0 (u, z)β(m(cid:19))(m)
Unitarity of Minimal W -Algebras and Their Representations I
= β(m(cid:19))(Y ν,(cid:10)0 (A(z)u, z−1)m),
so
H (m, Y ν,(cid:10)0 (u, z)m(cid:19)) = H (Y ν,(cid:10)0 (A(z)u, z−1)m, m(cid:19)).
(cid:17)(cid:18)
k
(g)-module L W (ν, (cid:10)0) is called unitary if the Hermitian form
(g) is called unitary if its adjoint
Definition 8.2. The W min
H (·, ·) is positive definite. The vertex algebra W min
module is unitary.
As usual, we denote u = H (u, u), u ∈ L W (ν, (cid:10)0). In order to obtain necessary
conditions for unitarity of L W (ν, (cid:10)0) we compute ||G
Lemma 8.3. Let, as before, ξ be a highest weight of the g(cid:4)-module g−1/2, and fix a
highest weight vector v ∈ g−1/2 . Then
{v}
−1/2
vν,(cid:10)0
||.
k
G
{v}
−1/2
vν,(cid:10)0
2 =(−2(k + h∨)l0 + (ν|ν + 2ρ(cid:4)) − 2(k + 1)(ξ |ν) + 2(ξ |ν)2)(cid:2)φ(v), v(cid:3).
(8.1)
Proof. To prove (8.1) we observe that, since g(G{v}) = G{φ(v)} and G{v} is primary,
H (G
{v}
−1/2
vν,(cid:10)0
, G
{v}
−1/2
vν,(cid:10)0
) = H (G
{φ(v)}
1/2 G
{φ(v)}
1/2
{v}
−1/2
{v}
, G
−1/2
vν,(cid:10)0
, vν,(cid:10)0
)
]vν,(cid:10)0
, vν,(cid:10)0
).
= H ([G
Using Borcherds’ commutator formula
[G
{φ(v)}
1/2
, G
{v}
−1/2
] =
(cid:4)
(cid:10)
j
(cid:5)
(G{φ(v)}
1
j
( j)G{v})0,
and formula (7.7) with u = φ(v) we obtain
[G
{φ(v)}
1/2
, G
{v}
−1/2
] = −2(k + h∨)(cid:2)φ(v), v(cid:3)L 0 + (cid:2)φ(v), v(cid:3)
dim g(cid:4)(cid:10)
α=1
: J {uα} J {uα} :0 +
2
(cid:2)[uα, φ(v)], [v, uβ ](cid:3) : J {uα} J {uβ } :0 +2(k + 1)J
(cid:10)
α,β
{[uα,uβ ]}
(cid:2)[uα, φ(v)], [v, uβ ](cid:3)J
0
.
(cid:10)
+ 2
α,β
{[[eθ ,φ(v)],v](cid:4)}
0
(8.2)
By the −1-st product identity,
: J {uα} J {uβ } :0=
(cid:10)
j∈Z+
(J
{uα}
− j−1 J
{uβ }
j+1 + J
{uβ }
− j J
{uα}
j
),
hence
: J {uα} J {uβ } :0 vν,(cid:10)0
{uβ }
= J
0
{uα}
J
0
vν,(cid:10)0
.
(8.3)
We choose the basis {uα} so that {uα} = {uγ | uγ ∈ g
{ui } a basis of h(cid:4). Then uγ ∈ g
(cid:4)
−γ . It follows that
V. G. Kac, P. Möseneder Frajria, P. Papi
(cid:4)
γ } ∪ {ui | 1 ≤ i ≤ rank g(cid:4)} with
{uγ }
H (J
0
}
{uγ (cid:19)
J
0
vν,(cid:10)0
, vν,(cid:10)0
) (cid:6)= 0 ⇒ γ = γ (cid:19).
(8.4)
Since
[[eθ , φ(v)], v](cid:4) =
(cid:10)
γ ∈(cid:14)(cid:4)
we see that
([[eθ , φ(v)], v]|uγ )uγ
+
(cid:10)
i
([[eθ , φ(v)], v]|ui )ui ,
{[[eθ ,φ(v)],v](cid:4)}
H (J
0
(cid:10)
=
vν,(cid:10)0
, vν,(cid:10)0
)
([[eθ , φ(v)], v]|ui )ν(ui ) =
(cid:10)
(eθ |[φ(v), [v, ui ]])ν(ui ).
(8.5)
i
i
We assume that v ∈ gξ . Then (8.5) yields
{[[eθ ,φ(v)],v](cid:4)}
H (J
0
vν,(cid:10)0
, vν,(cid:10)0
) = −
(cid:10)
i
(eθ |[φ(v), v])ξ(ui )ν(ui ) = −(cid:2)φ(v), v(cid:3)(ξ |ν).
(8.6)
{[uγ (cid:19)
From (8.4) we see that (J
0
for all i, j. Combining (8.2), (8.4), (8.6) we find
, vν,(cid:10)0
vν,(cid:10)0
,uγ ]}
{[ui ,u j ]}
) = 0 unless γ (cid:19) = γ . Clearly J
0
= 0
H ([G
{φ(v)}
1/2
, G
{v}
−1/2
]vν,(cid:10)0
, vν,(cid:10)0
)
= −2(k + h∨)(cid:2)φ(v), v(cid:3)l0 + (cid:2)φ(v), v(cid:3)(ν|ν + 2ρ(cid:4)) − 2(k + 1)(cid:2)φ(v), v(cid:3)(ξ |ν)
{uα}
{uβ }
(cid:2)[uα, φ(v)], [v, uβ ](cid:3)H (J
J
0
0
, vν,(cid:10)0
vν,(cid:10)0
(cid:10)
+ 2
).
α,β
(8.7)
Recall that φ is a compact involution of g(cid:4), thus
φ(hα) = −hα for all α ∈ (cid:14)(cid:4).
(8.8)
(As usual hα stands for the element of h(cid:4) corresponding to α in the identification of h(cid:4)
with (h(cid:4))∗ via (.|.)). It follows that [hα, φ(v)] = −ξ(hα)φ(v), so the weight of φ(v) is
−ξ . In particular, since v is a highest weight vector for the g(cid:4)-module g−1/2, we have
{uα}
(cid:2)[uα, φ(v)], [v, uβ ](cid:3)H (J
0
{uβ }
J
0
vν,(cid:10)0
, vν,(cid:10)0
) =
{ui }
(cid:2)[ui , φ(v)], [v, u j ](cid:3)H (J
0
{u j }
J
0
vν,(cid:10)0
, vν,(cid:10)0
)
{uγ }
(cid:2)[uγ , φ(v)], [v, uγ ](cid:3)H (J
0
{uγ }
J
0
vν,(cid:10)0
, vν,(cid:10)0
)
(cid:10)
α,β
(cid:10)
i, j
(cid:10)
+
γ <0
(cid:10)
i, j
=
ξ(ui )ν(ui )ξ(u j )ν(u j )(cid:2)φ(v), v(cid:3).
(8.9)
Unitarity of Minimal W -Algebras and Their Representations I
Substituting (8.9) into (8.7) we obtain
H (G
{v}
−1/2
vν,(cid:10)0
, G
{v}
−1/2
vν,(cid:10)0
) = − 2(k + h∨)(cid:2)φ(v), v(cid:3)l0 + (cid:2)φ(v), v(cid:3)(ν|ν + 2ρ(cid:4))
− 2(k + 1)(cid:2)φ(v), v(cid:3)(ξ |ν) + 2(ξ |ν)2(cid:2)φ(v), v(cid:3),
as claimed.
(cid:17)(cid:18)
Remark 8.4. Let v ∈ g−1/2 be as in Lemma 8.3 and u a root vector for the root θi . Then
J
{u}
−1 G
{v}
−1/2
vν,(cid:10)0
2 =((θi |ξ + ν)(φ(u)|u) − βk(φ(u), u)) G
{v}
−1/2
vν,(cid:10)0
2.
Indeed,
H (J
)
vν,(cid:10)0
{φ(v)}
1/2
{φ(v)}
1/2
{u}
{v}
−1 G
−1/2
= −H (G
{v}
{u}
, J
−1 G
−1/2
{φ(u)}
{u}
−1 G
J
J
1
{φ(u)}
{u}
, J
[J
−1
1
{v}
= (θi |ξ + ν)(φ(u)|u)H (G
−1/2
vν,(cid:10)0
− βk(φ(u), u)H (G
vν,(cid:10)0
{v}
vν,(cid:10)0
−1/2
{v}
]G
−1/2
vν,(cid:10)0
{v}
, G
−1/2
= −H (G
{v}
−1/2
, vν,(cid:10)0
)
)
, vν,(cid:10)0
vν,(cid:10)0
{v}
, G
−1/2
vν,(cid:10)0
vν,(cid:10)0
).
)
Let P + ⊂ (h(cid:4))∗ be the set of dominant integral weights for g(cid:4) and let
P +
k
=
#
ν ∈ P + | ν(θ ∨
i
$
) ≤ Mi (k) for all i ≥ 1
.
(8.10)
Recall that ξ ∈ (h(cid:4))∗ is a highest weight of the g(cid:4)-module g−1/2. Introduce the following
number
A(k, ν) =
(ν|ν + 2ρ(cid:4))
2(k + h∨) +
(ξ |ν)
k + h∨
((ξ |ν) − k − 1).
(8.11)
Proposition 8.5. Assume that k + h∨ (cid:6)= 0. If the W k
then Mi (k) ∈ Z+ for all i ≥ 1, ν ∈ P +
k , and
min
(g)-module L W (ν, (cid:10)0) is unitary,
(cid:10)0 ≥ A(k, ν).
(8.12)
Proof. In order to prove that Mi (k) ∈ Z+ for all i ≥ 1 and ν ∈ P +
that, if L W (ν, (cid:10)0) is a unitary module over W k
a unitary module over V βk (g(cid:4)), hence ν ∈ P +
Mi (k) ∈ Z+ for all i ≥ 1.
k , it is enough to observe
(g), then, in particular, V βk (g(cid:4))vν,(cid:10)0 is
min
k [12], which is non-empty if and only if
To prove the second claim recall that, by Proposition 5.1, the Hermitian form (cid:2)φ(.), . (cid:3)
is positive definite on g−1/2. Since k + h∨ < 0, we obtain from (8.1) that
(cid:10)0 ≥
(ν|ν + 2ρ(cid:4))
2(k + h∨)
−
(k + 1)
k + h∨
(ξ |ν) +
(ξ |ν)2
k + h∨
= A(k, ν),
as claimed.
(cid:17)(cid:18)
V. G. Kac, P. Möseneder Frajria, P. Papi
Consider the short exact sequence
0 → I k → W k
min
(g) → W min
k
(g) → 0.
min
(g)-module L W (ν, (cid:10)0) is unitary, then, restricted to the subalgebra V βk (g(cid:4)) it is
If a W k
unitary, hence a direct sum of irreducible integrable highest weight(cid:6)g(cid:4)-modules of levels
(g(cid:4)). Also, all
Mi (k), i ≥ 1. But it is well known that all these modules descend to Vβk
these modules are annihilated by the elements
=
(g).
k
k
(8.14)
(g). Conse-
(J
}
{eθ
(−1) )Mi (k)+11,
i
(g) generated by the elements (8.13), and let %W min
i ≥ 1.
(8.13)
Let %I k ⊂ I k be the ideal of W k
/%I k. We thus obtain
W k
min
min
Proposition 8.6. If the W k
(g)-module L W (ν, (cid:10)0) is unitary, then it descends to %W min
Note that a unitary W k
min
min
(g)-module descends to W min
k
(g) if and only if
%I k = I k.
(g)-module descends to W min
Conjecture 4. 2 Equality (8.14) holds for all unitary vertex algebras W k
quently, any unitary W k
Definition 8.7. An element ν ∈ P +
P +
k .
Proposition 8.8. If L W (ν, (cid:10)0) is unitary and ν is an extremal weight, then
(g).
min
k
min
k is called an extremal weight if ν + ξ doesn’t lie in
(cid:10)0 = A(k, ν).
{u}
−1/2
{u}
−1/2
k . By the assumption, we have G
vν,(cid:10)0 is a singular vector for V βk (g(cid:4)).
Proof. Let u be a root vector for ξ . Then G
Since L W (ν, (cid:10)0) is unitary, all vectors that are singular for V βk (g(cid:4)) should have weight
= 0, hence the norm of this vector is 0,
vν,(cid:10)0
in P +
(cid:17)(cid:18)
and we can apply (8.1).
In the setting of the above proposition, note that ν is extremal iff ν(θ ∨
i
for some i. Moreover, k is collapsing iff Mi (k) + χi < 0 (cf. Remark 7.5).
Proposition 8.9. (a) For k (cid:6)= −1, W k
modules. In particular, W min
this W -algebra collapses to the free boson.
(sl(2|m)), m ≥ 3, has no unitary highest weight
(sl(2|m)), m ≥ 3, is unitary if and only if k = −1 and
) > Mi (k) + χi
min
k
(b) The W -algebra W k
(osp(4|m)), m > 2, has no unitary highest weight modules for
min
all k.
(cid:4)
Proof. (a) Let g = sl(2|m). Then g
0
= C(cid:28) , where (cid:28) =
(a|b) = str (ab). By Theorem 7.4, the collapsing levels are k = −1 and k = m/2 − 1.
(g) is the Heisenberg ver-
tex algebra M(C(cid:28) ) = V −m/2(C(cid:28) ) = V−m/2(C(cid:28) ) and this vertex algebra is unitary.
If k = −1 then M0(−1) = −m/2, M1(−1) = 0 and W min
k
⎛
⎝
m/2 0
0
0 m/2 0
Im
0
0
⎞
⎠, and
2 See Note added in proof.
Unitarity of Minimal W -Algebras and Their Representations I
If k = m/2−1 then M0(m/2−1) = 0, M1(m/2−1) = −m/2 and W min
(sl(2|m)) =
k
V−m/2(sl(m)) which has no unitary highest weight modules.
min
Assume that k is not collapsing. Let ψ be a conjugate linear involution of
(sl(2|m)) such that L W (ν, (cid:10)0) has a positive definite ψ-invariant Hermitian form
W k
H , normalized by the condition H (vν,(cid:10)0
) = 1. By Proposition 7.2, the involution
, vν,(cid:10)0
ψ is induced by an involution ψ on g satisfying (1.1). This implies that ψ((cid:28) ) = ζ (cid:28)
with |ζ | = 1.
The vertex algebra V k−m/2−1(C(cid:28) ) ⊗ V −k−1(sl(m)) embeds in W k
(sl(2|m)). In
particular, (V k−m/2−1(C(cid:28) ) ⊗ V −k−1(sl(m))).vν,(cid:10)0 is a unitary module. This implies
that ψ|sl(m) corresponds to a compact real form of sl(m) and −k − 1 ∈ Z+. Using the
formulas given in [16, §5.3] we have
min
0 ≤ H (J {(cid:28) }vν,(cid:10)0
, J {(cid:28) }vν,(cid:10)0
{ψ((cid:28) )}
{(cid:28) }
, vν,(cid:10)0
) = H (−J
J
−1
1
= −(k − m/2 − 1)ζ −1((cid:28) |(cid:28) )
= −ζ −1(k − m/2 − 1)(m2/2 − m).
vν,(cid:10)0
)
Therefore ζ = 1, so that
Note that
ψ((cid:28) ) = (cid:28).
[(cid:28), u] = ± m
2 u, u ∈ g−1/2
.
(8.15)
(8.16)
−
⊕ g
Write g−1/2
−1/2 for the corresponding eigenspace decomposition. Since
±
−1/2. Since the form (cid:2)., .(cid:3) is g(cid:4)-invariant, we have
ψ((cid:28) ) = (cid:28) , we have ψ(g
−1/2
±
) = g
= g+
−1/2
(cid:2)g+, g+(cid:3) = (cid:2)g
−, g
−(cid:3) = 0.
It follows that, if u ∈ g−1/2,
(cid:2)ψ(u), u(cid:3) = 0.
(8.17)
Observe now that by [1], since k is not collapsing, the image of G{u} in W min
(g) is
non-zero if u (cid:6)= 0. We observe that, since g(G{u}) = G{ψ(u)} and G{u} is primary, for
n ∈ 1
k
2 + Z+
H (G
{u}
−n
v, G
{u}
−n
{u}
v) = H (G{ψ(u)}
G
−n
n
= H ([G{ψ(u)}
, G
n
, v)
{u}
−n
]v, v)
(8.18)
for any v ∈ L W (ν, (cid:10)0). Using Borcherds’ commutator formula
[G{ψ(u)}
n
, G
{u}
−n
] =
(cid:4)
n + 1
2
j
(cid:10)
j
(cid:5)
(G{ψ(u)}
( j)G{u})0,
and combining formulas (7.7) and (8.17), we obtain
[G{ψ(u)}
n
, G
{u}
−n
] = −2(k + h∨)(cid:2)ψ(u), u(cid:3)L 0 + (cid:2)ψ(u), u(cid:3)
dim g(cid:4)(cid:10)
α=1
: J {uα } J {uα } :0 +
2
(cid:10)
α,β
= 2
(cid:10)
α,β
V. G. Kac, P. Möseneder Frajria, P. Papi
(cid:2)[uα, ψ(u)], [u, uβ ](cid:3) : J {uα } J {uβ } :0 +4n(k + 1)J
{[[eθ ,ψ(u)],u](cid:4)}
0
+ (2n + 1)
(cid:10)
α,β
{[uα ,uβ ]}
(cid:2)[uα, ψ(u)], [u, uβ ](cid:3)J
0
+ (2n2 − 1
2
) p(k)(cid:2)ψ(u), u(cid:3)
(cid:2)[uα, ψ(u)], [u, uβ ](cid:3) : J {uα } J {uβ } :0 +4n(k + 1)J
{[[eθ ,ψ(u)],u](cid:4)}
0
+ (2n + 1)
(cid:10)
α,β
{[uα ,uβ ]}
(cid:2)[uα, ψ(u)], [u, uβ ](cid:3)J
0
.
(8.19)
Now we compute (8.18) for v ∈ L W (ν, (cid:10)0)(cid:10)0 . As in the proof of Lemma 8.3, using (8.4),
(8.6) with ψ instead of φ, we find that (8.19) becomes, with the notation of the proof of
Lemma 8.3,
H ([G{ψ(u)}
n
, G
{u}
−n
]v, v) = (2n + 1)
= (2n + 1)
+ (2n + 1)
(cid:10)
α,β
(cid:10)
i, j
(cid:10)
{uα}
(cid:2)[uα, ψ(u)], [u, uβ ](cid:3)H (J
0
{uβ }
J
0
v, v)
{ui }
(cid:2)[ui , ψ(u)], [u, u j ](cid:3)H (J
0
{u j }
J
0
v, v) (8.20)
{uγ }
(cid:2)[uγ , ψ(u)], [u, uγ ](cid:3)H (J
0
{uγ }
J
0
v, v).
γ ∈(cid:14)(cid:4)
Recall that ψ ia a compact involution of [g(cid:4), g(cid:4)], hence, by (8.8), ψ(uγ ) ∈ g
for some constant b we have
(cid:2)[uγ , ψ(u)], [u, uγ ](cid:3) = b(cid:2)ψ([u, uγ ]), [u, uγ ](cid:3),
(8.21)
(cid:4)
−γ , so that,
so, by (8.17), the summand (8.21) is zero.
The summand (8.20) vanishes since (cid:2)[ui , ψ(u)], [u, u j ](cid:3) is a multiple of (cid:2)ψ(u), u(cid:3) =
{u}
n Amv = 0 with
0. This shows that Y L W (ν,(cid:10)0)(G{u}, z)v = 0. By relation (7.3), G
A ∈ V βk (g(cid:4)) for all n, m, hence, since G{u} is primary,
Y L W (ν,(cid:10)0)(G{u}, z)L W (ν, (cid:10)0) = 0.
Hence G{u} lies in a proper ideal of W k
min
is not collapsing, G{u} is non zero in W min
(g), contradicting the fact that, since the level
(g).
(b) For g = osp(4|m), the conditions of Proposition 8.5 imply k −m/2 ∈ Z+, − 1
k
2 k −
(cid:17)(cid:18)
(g)-modules with
min
1 ∈ Z+. These relations are never satisfied at the same time.
Proposition 8.10. Non-trivial unitary irreducible highest weight W k
k (cid:6)= −h∨ may exist only in the following cases
(1) g = sl(2|m), m ≥ 3, k = −1 (then W min
(2) g = psl(2|2), −k ∈ N + 1;
(3) g = spo(2|3), −k ∈ 1
(N + 2);
4
(4) g = spo(2|m), m > 4, −k ∈ 1
2
(5) g = D(2, 1; m
n
(6) g = F(4), −k ∈ 2
3
N, m, n ∈ N are coprime, k (cid:6)= − 1
2 ;
), −k ∈ mn
m+n
(N + 1);
(N + 1);
= W k
k
min is a free boson);
Unitarity of Minimal W -Algebras and Their Representations I
(7) g = G(3), −k ∈ 3
4
(N + 1).
Proof. By Proposition 8.9, we may assume that g is not one of the Lie superalgebras
sl(2|m) woth m ≥ 3 or osp(4|m) with m > 2. The remaining cases are treated, using
only the easy necessary conditions Mi = Mi (k) ∈ Z+ for all i. In all cases, except for
g = D(2, 1; a), the condition Mi ∈ Z+ is obviously equivalent to the condition on k,
given in the statement of the proposition.
Consider the remaining case g = D(2, 1; a). By this we mean the contragredient Lie
⎞
⎛
superalgebra with Cartan matrix
⎝
0 1 a
−1 2 0
−1 0 2
⎠. By Proposition 8.5, we need to find the
values of a such that Mi = Mi (k), i = 1, 2, from Table 2 are non-negative integers.
These conditions imply that
k = − M1+1
a+1 and k = − (M2+1)a
a+1
. where M1, M2 ∈ Z+.
(8.22)
Equating these two expressions for k, we obtain a = M1+1
Inserting this in either of the expressions (8.22) for k, we obtain
M2+1 is a positive rational number.
k = − (M1+1)(M2+1)
(M1+1)+(M2+1) ,
proving the claim. (k = −1/2 corresponds to the trivial D(2, 1; 1)-module.)
(cid:17)(cid:18)
Definition 8.11. Given g in the above list, we call the corresponding set of values of
k (cid:6)= −h∨ the unitarity range of W k
(g).
min
Remark 8.12. For g = D(2, 1; a), there are actually three possible choices of the mini-
mal root. We now describe how the unitarity range depends on this choice. We choose
{2(cid:22)1, 2(cid:22)2, 2(cid:22)3} as the set of positive roots in g¯0: hence, if −θ is a minimal root, then
θ = 2(cid:22)i for some 1 ≤ i ≤ 3. The bilinear form (.|.), displayed in Table 1, corresponds
to the choice θ = 2(cid:22)1, so that (2(cid:22)1|2(cid:22)1) = 2. If we choose θ = 2(cid:22)2, then the bilinear
form (.|.) is given by
((cid:22)1|(cid:22)1) = − 1+a
2
, ((cid:22)2|(cid:22)2) = 1
2
, ((cid:22)3|(cid:22)3) = a
2
, ((cid:22)1|(cid:22)2) = ((cid:22)1|(cid:22)3) = ((cid:22)2|(cid:22)3) = 0.
1+a k − 1, M2(k) = 1
We have M1(k) = − 1
are coprime (i. e. a ∈ Q, −1 < a < 0) and in turn k ∈ − mn
m+n
then the bilinear form (.|.) is given by
a k − 1. Then a = − m
, m, n ∈ N, m and n
m+n
N. If we choose θ = 2(cid:22)3,
((cid:22)1|(cid:22)1) = − a+1
2a
, ((cid:22)2|(cid:22)2) = 1
2a
, ((cid:22)3|(cid:22)3) = 1
2
, ((cid:22)1|(cid:22)2) = ((cid:22)1|(cid:22)3) = ((cid:22)2|(cid:22)3) = 0.
We have M1(k) = − a
are coprime (i. e. a ∈ Q, a < −1) and in turn k ∈ − mn
m+n
1+a k − 1, M2(k) = ak − 1. Then a = − m+n
N.
m
Recall that one obtains isomorphic superalgebras of the family D(2, 1; a), a (cid:6)=
0, −1, under the action of the group S3, generated by the transformations a (cid:25)→ 1/a, a (cid:25)→
−1 − a. These transformations permute transitively the domains Q>0, Q>−1 ∩ Q<0 and
Q<−1, which correspond to the above three cases.
Corollary 8.13. If k is from the unitarity range for W k
rational number.
(g), then k + h∨ is a negative
min
, m, n ∈ N, m and n
9. Free Field Realization of Minimal W -Algebras
V. G. Kac, P. Möseneder Frajria, P. Papi
Let (cid:6) : W k
introduced in [20, Theorem 5.2]; it is explicitly given on the generators of W k
(g) → V k = V k+h∨ (Cx) ⊗ V αk (g(cid:4)) ⊗ F(g1/2
) be the free field realization
(g) by
min
min
(9.1)
(9.2)
(9.3)
J {b} (cid:25)→ b +
G{v} (cid:25)→
1
2
(cid:10)
(cid:10)
: (cid:30)α(cid:30)[uα,b] : (b ∈ g
(cid:4)),
α∈S1/2
: [v, uα](cid:30)α : −(k + 1)
(cid:10)
(v|uα)T (cid:30)α
α∈S1/2
(cid:10)
α∈S1/2
: (cid:30)α(cid:30)β (cid:30)[uβ ,[uα,v]] : (v ∈ g−1/2
) ,
+
1
3
α,β∈S1/2
L (cid:25)→ 1
2(k + h∨)
(cid:10)
: uαuα : +
k + 1
k + h∨ T x +
1
2
(cid:10)
: (T (cid:30)α)(cid:30)α : .
Recall that F(g1/2
α∈S0
) is the universal enveloping vertex algebra of the (non-linear)
Lie conformal superalgebra C[T ] ⊗ g1/2 with [aλb] = (cid:2)a, b(cid:3)ne1, a, b ∈ g1/2, and
{(cid:30)α}α∈S1/2
, {(cid:30)α}α∈S1/2 are dual bases of g1/2 with respect to (cid:2)., .(cid:3)ne.
We now apply the results of Sect. 6 to V k+h∨(Cx). By Corollary 8.13 unitarity of
min
(g) implies k + h∨ < 0. Hence, using the normalization
α∈S1/2
W k
√
−1
a =
√
2√
|k + h∨|
x,
(9.4)
we have V k+h∨ (Cx) = V 1(Ca), since, by (7.24), αk(x, x) = 1
1
2
(k+h∨), hence αk(a, a) =
Recall that in Proposition 5.1 we proved that one can choose an almost compact
involution φ of g that fixes pointwise the sl2-triple {e, x, f } in such a way that the
Hermitian form (cid:2)φ(u), v(cid:3)ne on g1/2 is negative definite. This conjugate linear involution
) as well, both
) ⊗ F(g1/2
induces a conjugate linear involution of W k
denoted again by φ. It is readily checked, using (9.1), (9.2), and (9.3), that
(g) and of V αk (g0
min
(cid:6)(φ(v)) = φ((cid:6)(v))
for all v ∈ W k
min
(g).
(9.5)
Since φ(x) = x, we see that φ(a) = −a. The conformal vector of the vertex algebra V k
is
L f r ee = 1
2
: aa : +L g(cid:4) + L F ,
(9.6)
where
L g(cid:4) = 1
2(k+h∨)
(cid:10)
α∈S(cid:4)
: uαuα, L F = 1
2
(cid:10)
α∈S1/2
: (T (cid:30)α)(cid:30)α : .
Here {uα}α∈S(cid:4) and {uα}α∈S(cid:4) are dual bases of g(cid:4) with respect to the bilinear form (.|.)
restricted to g(cid:4). Recall that L g(cid:4) is the conformal vector of V αk (g(cid:4)) and L F is the conformal
vector of F(g1/2
). Let
√
−1
sk =
√
(k + 1)
2|k + h∨|
.
(9.7)
Unitarity of Minimal W -Algebras and Their Representations I
It follows from (9.3) and (9.6) that
(cid:6)(L) = L(sk) + (cid:6)L = L f r ee + sk T (a),
(9.8)
: aa : +sT a, (cid:6)L(s) = L(s) + (cid:6)L, cf. (6.1) and (6.29),
where (cid:6)L = L g(cid:4) + L F , and L(s) = 1
respectively.
2
Note that V k = V 1(Ca) ⊗ V, where V = V αk (g(cid:4)) ⊗ F(g1/2
), and (cid:6)(L) = (cid:6)L(s) (cf.
Given μ ∈ C, let M(μ) be the irreducible V 1(Ca)-module with highest weight μ,
(6.28), (6.29)).
and consider the V k-module
N (μ) = M(μ) ⊗ V.
Recall that V carries a φ-invariant Hermitian form Hg(cid:4) ⊗ HF , which is positive definite.
Recall also that, by Proposition 6.3, the V 1(Ca)-module M(μ) carries a unique L(t)-
invariant Hermitian form, provided that t =
−1(cid:28)(μ), which is positive definite. This
Hermitian form, normalized by the condition that the norm of the highest weight vector
equals 1, was denoted by Hμ. Hence we have a φ-invariant positive definite Hermitian
form Hμ( . , . ) ⊗ Hg(cid:4)(. , .) ⊗ HF (. , .) on N (μ), which we denote by (·, ·)μ.
It follows from Proposition 6.10 that, restricting the fields Y μ,t (−, z) from V k to
(g)-module. We now explicitly
min
(g)), one equips N (μ) with a structure of a W k
(cid:6)(W k
describe this action of the generators of W k
(g) on N (μ).
min
√
min
Proposition 9.1. For b ∈ W k
min
(g), write
Y μ,t ((cid:6)(b), z) =
(cid:10)
n z−n−(cid:14)b ,
bμ,t
n∈−(cid:14)b+Z
and let μ ∈ R. Then
n + 2(t 2 − st)1μ
n
,
n + 2taμ
L μ,t
n
(J {u})μ,t
n
(G{v})μ,t
n
= (cid:6)(L)μ
= (cid:6)(J {u})μ
n
= (cid:6)(G{v})μ
n + 2t
Furthermore, if m, m(cid:19) ∈ N (μ), then
, u ∈ g
√
(cid:4),
&
2|k + h∨|((cid:30)[e,v])μ
n
, v ∈ g−1/2
.
−1
μ,s−t
−n m, m(cid:19))μ,
n m(cid:19))μ = (L
(m, L μ,t
n m(cid:19))μ = −((J {φ(u)})μ,s−t
(m, (J {u})μ,t
n m(cid:19))μ = ((G{φ(v)})μ,s−t
(m, (G{v})μ,t
Proof. We already noted that (cid:6)(L) = (cid:6)L(s). By (6.27),
−n m, m(cid:19))μ,
−n m, m(cid:19))μ.
(9.9)
(9.10)
(9.11)
(9.12)
(9.13)
(9.14)
(cid:6)(L)μ,t
n
= (L(s) + (cid:6)L)μ,t
n
= (cid:6)L(s)μ
n + 2taμ
= 1
2
: aa :μ
n +sT aμ
.
n + 2(t 2 − st)1μ
n
If u ∈ g(cid:4), then (cid:6)(J {u}) ∈ V αk (g(cid:4)) ⊗ F(g1/2
(cid:6)(J {u})μ
n . Finally, if v ∈ g−1/2,
n + 2taμ
n + 2(t 2 − st)1n + (cid:6)L μ
n
), hence, by Lemma 6.11, (J {u})μ,t
n =
[v, uα] = 2([v, uα]|x)x + [v, uα](cid:4) =
√
−1
√
|k + h∨|
√
2
(v|uα)a + [v, uα](cid:4),
V. G. Kac, P. Möseneder Frajria, P. Papi
where u(cid:4) is the orthogonal projection of u onto g(cid:4) with respect to ( . | . ). Since
(cid:10)
(cid:10)
(cid:10)
[e, v] =
(cid:2)[e, v], uα(cid:3)neuα =
α
(cid:10)
α
([[ f, e], v]|uα)uα = −
=
α
α
( f |[[e, v], uα])uα =
(cid:10)
([x, v]|uα)uα = 1
2
( f |[e, [v, uα]])uα
α
(cid:10)
(v|uα)uα,
α
we can write
(cid:6)(G{v}) =
&
√
−1
2|k + h∨| : a(cid:30)[e,v] : +
(cid:10)
: [v, uα](cid:4)(cid:30)α : −2(k + 1)T (cid:30)[e,v]
α∈S1/2
: (cid:30)α(cid:30)β (cid:30)[uβ ,[uα,v]] : .
+
1
3
(cid:10)
α,β∈S1/2
Set
so that
Thus,
G{v} =
(cid:10)
α∈S1/2
: [v, uα](cid:4)(cid:30)α : +
1
3
(cid:10)
α,β∈S1/2
: (cid:30)α(cid:30)β (cid:30)[uβ ,[uα,v]] : .
√
&
(G{v})μ,t
n
=
−1
− 2(k + 1)(T (cid:30)[e,v])μ
2|k + h∨| : a(cid:30)[e,v] :μ
n + G{v}μ
n +2t
.
n
&
√
−1
2|k + h∨|((cid:30)[e,v])μ
n
(G{v})μ,t
n
= (cid:6)(G{v})μ
n + 2t
&
√
−1
2|k + h∨|((cid:30)[e,v])μ
n
.
(9.15)
For proving (9.12), (9.13), and (9.14), it is enough to observe that L, G{v}, and J {u}
are quasiprimary for (cid:6)L(s) and apply (6.24). We use the fact that (cid:6)(g(b)) = g((cid:6)(b))
for all b ∈ W k
(g) , where g is defined by (2.8). This follows from (9.5) and the fact
(cid:17)(cid:18)
that (cid:6) preserves both parity and conformal weight.
min
As an application of Proposition 9.1, we obtain a generalization of the Fairlie construction
to minimal W -algebras.
Proposition 9.2. Set s = sk (cf. (9.7)) and
= (cid:6)(L)μ
μ,s/2
L
n
(G{v})μ,s/2
n
(J {u})μ,s/2
n
n + saμ
n +
= (cid:6)(G{v})μ
n
= (cid:6)(J {u})μ
.
n
n
|s|2
2 1μ
= (cid:6)(L)μ
n +
− (k + 1)((cid:30)[e,v])μ
n
,
k + 1
k + h∨ x μ
n
− (k+1)2
4(k+h∨) 1μ
n
,
The fields
Y μ,s(L , z) =
(cid:10)
L μ,s
n
z−n−2,
n∈Z
(cid:10)
Y μ,s(G{v}, z) =
(G{v})μ,s
n
z−n−3/2,
n∈1/2+Z
Unitarity of Minimal W -Algebras and Their Representations I
Y μ,s(J {u}, z) =
(cid:10)
(J {u})μ,s
z−n−1
n
n∈Z
(g)-module structure. Moreover, the Hermitian form ( . , . )μ
min
endow N (μ) with a W k
on N (μ) is invariant.
Proof. Plug t = s/2 in Proposition 9.1. By (9.12), (9.13), and (9.14), we have
μ,s/2
m(cid:19))μ = (L
−n m, m(cid:19))μ,
m(cid:19))μ = −((J {φ(u)})μ,s/2
m(cid:19))μ = ((G{φ(v)})μ,s/2
(m, L
(m, (J {u})μ,s/2
(m, (G{v})μ,s/2
−n m, m(cid:19))μ,
−n m, m(cid:19))μ.
μ,s/2
n
n
n
thus the representations N (μ) acquire a W k
form (·, ·)μ is φ-invariant.
min
(g)-module structure and the Hermitian
(cid:17)(cid:18)
10. Sufficient Conditions for Unitarity of Modules Over W k
Due to the Proposition 8.9 (a), we may assume in this section that g (cid:6)= sl(2|m) and
(cid:4)
osp(4|m), m > 2. Then, in particular, g(cid:4) = ⊕i≥1g
i is the decomposition of g(cid:4) into
simple ideals, and the χi are given by (7.27).
Proposition 10.1. Assume that k +h∨ (cid:6)= 0. Then there exists a unitary module L W (ν, (cid:10)0)
over W k
(g) if and only if Mi (k) ∈ Z+ for all i and ν ∈ P +
k .
(g)
min
min
Proof. One implication has been already proven in Proposition 8.5. To show that the
converse implication also holds, assume Mi (k) ∈ Z+ for all i. Recall (see (7.25)) that
the cocycle αk is given by
αk |g
(cid:4)
i
(cid:4)
×g
i
= (Mi (k) + χi )(.|.)(cid:4)
i
.
(g(cid:4)) of V αk (g(cid:4))
Assume first that Mi (k) + χi ∈ Z+ for all i. Then the simple quotient Vαk
is unitary, since it is an integrable (cid:6)g(cid:4)-module [11]. Next, the vertex algebra F(g1/2
) is
unitary due to Proposition 5.1 and [16, §5.1]. Finally, the V 1(Ca)-module M(s) , where
s is given by (9.7), is unitary by the observation following Lemma 6.4.
Consider the unitary W k
(g)-module M(s) ⊗ Vαk
(g(cid:4)) ⊗ F(g1/2
), and its submodule
min
U = (cid:6)(W k
(g)).(vs ⊗ 1 ⊗ 1).
min
min
(g).
Since the Hermitian form Hs( . , .) is (cid:6)L(s)-invariant and (cid:6)(L) = (cid:6)L(s), we see that U
admits a φ-invariant Hermitian positive definite form, thus U is a unitary highest weight
module for W k
Now we look at the missing cases, where there is i such that 0 ≤ Mi (k) < −χi ,
described in Remark 7.5. Assume first that g(cid:4) is simple. If χ1 = −1 then the only
possible value is M1(k) = 0, so, W min
(g) = C, by Theorem 7.4 (1) (a). In the case
of g = spo(2|3) one should consider the cases M1(k) = 1 and M1(k) = 0: in the
former case k = − h∨
(spo(2|3)) =
V1(sl(2)), whereas in the latter case k + h∨ = 0. If g(cid:4) is semisimple but not simple,
then g = D(2, 1; a). In this case we have to consider only the case in which either
M1(k) or M2(k) is zero. If M1(k) = 0 (resp. M2(k) = 0) then, by Theorem 7.4 (2),
(cid:17)(cid:18)
W min
k
(D(2, 1; a)) = VM2(k)(sl(2)) (resp. = VM1(k)(sl(2))).
− 1, hence Theorem 7.4 (1) (b) applies and W min
1
2
k
k
V. G. Kac, P. Möseneder Frajria, P. Papi
We now generalize the construction given in the proof of Proposition 10.1 to provide
families of unitary representations. For ν ∈ P +
k introduce the following number
B(k, ν) =
(ν|ν + 2ρ(cid:4))
2(k + h∨)
−
(k + 1)2
4(k + h∨)
.
(10.1)
Proposition 10.2. Assume that k + h∨ (cid:6)= 0 and Mi (k) + χi ∈ Z+ for all i > 0. If ν ∈ P +
is such that ν(θ ∨
i
) ≤ Mi (k) + χi for all i > 0 (then ν ∈ P +
k ) and
(cid:10)0 ≥ B(k, ν),
(10.2)
then L W (ν, (cid:10)0) is a unitary W k
Proof. Let L (cid:4)(ν) be the irreducible highest weight V αk (g(cid:4))-module of highest weight ν
and let vν be a highest weight vector. Fix μ ∈ R and set
(g)-module.
min
N (μ, ν) = (cid:6)(W k
min
(g)).(vμ+s ⊗ vν ⊗ 1) ⊂ M(μ + s) ⊗ L (cid:4)(ν) ⊗ F(g1/2
),
where s = sk is given by formula (9.7). Note that the Hermitian form (·, ·)μ+s is (cid:6)L(s)-
invariant. Since Mi (k) + χi ∈ Z+ and ν(θ ∨
) ≤ Mi (k) + χi for all i, then L (cid:4)(ν) is
i
integrable for V αk (g(cid:4)), hence unitary [12]. Thus N (μ, ν) is a unitary representation of
W k
(g).
min
We now compute the highest weight of N (μ, ν). Recall that
(cid:6)(J {h}) = h +
1
2
(cid:10)
α∈S1/2
: (cid:30)α(cid:30)[uα,h] : .
By the −1-st product identity,
: (cid:30)α(cid:30)[uα,h] :0=
(cid:10)
(cid:30)
(cid:30)α
− j
j∈ 1
2 +Z+
((cid:30)[uα,h]) j − ((cid:30)[uα,h])− j (cid:30)α
j
(cid:31)
so
(cid:6)(J {h})0.(vμ+s ⊗ vν ⊗ 1) = ν(h)(vμ+s ⊗ vν ⊗ 1).
It follows that N (μ, ν) = L W (ν, (cid:10)0) for some (cid:10)0. We now compute (cid:10)0:
L 0(vμ+s ⊗ vν ⊗ 1) =
(cid:4)
μ2 − s2
2
(ν|ν + 2ρ(cid:4))
2(k + h∨)
+
(cid:5)
(vμ+s ⊗ vν ⊗ 1)
so that, using (9.7),
(cid:10)0 =
μ2 − s2
2
+
(ν|ν + 2ρ(cid:4))
2(k + h∨)
√
=
μ2
2
−
(k + 1)2
4(k + h∨) +
(ν|ν + 2ρ(cid:4))
2(k + h∨)
.
Hence (cid:10)0 ≥ B(k, ν). Letting μ = 2
N (μ, ν) is unitary.
(cid:10)0 − B(k, ν), we see that the module L W (ν, (cid:10)0) =
(cid:17)(cid:18)
Unitarity of Minimal W -Algebras and Their Representations I
11. Unitarity of Minimal W -Algebras and Modules Over Them
The main result of this paper is the following.
Theorem 11.1. Let k (cid:6)= −h∨, and recall the number A(k, ν) given by (8.11). If k lies in
(g)-module L W (ν, (cid:10)0) is
the unitary range (hence Mi (k) ∈ Z+ for i ≥ 1), then the W k
unitary for all non extremal ν ∈ (cid:6)P +
k and (cid:10)0 ≥ A(k, ν).
min
Corollary 11.2. If k lies in the unitary range, then the W k
unitary for all (cid:10)0 ≥ 0. Consequently, W min
k lies in the unitary range.
(g)-module L W (0, (cid:10)0) is
(g) is a unitary vertex algebra if and only if
min
k
In the rest of this section we give a proof of these results. First, by Proposition 8.9 (a),
we may exclude g = sl(2|m), m > 2, from consideration, so that g(cid:4) is semisimple and
by Proposition 8.5, conditions Mi (k) ∈ Z+ are necessary for unitarity, hence we shall
assume that these conditions hold.
Let (cid:6)g = (C[t, t −1] ⊗ g) ⊕ CK ⊕ Cd be the affinization of g (with bracket [t m ⊗
a, t n ⊗ b] = t n+m ⊗ [a, b] + δm,−nm K (a|b), a, b ∈ g). Let (cid:6)h = h ⊕ CK ⊕ Cd be its
Cartan subalgebra. Define (cid:17)0 and δ ∈ (cid:6)h
setting (cid:17)0(h) = (cid:17)0(d) = δ(h) = δ(K ) = 0
and (cid:17)0(K ) = δ(d) = 1. Let (cid:6)(cid:14) ⊂ (cid:6)h
be the set of roots of(cid:6)g. As a subset of simple roots
for (cid:6)g we choose (cid:6)(cid:15) = {α0 = δ − θ } ∪ (cid:15), where (cid:15) is the set of simple roots for g given
∗
in Table 1. We denote by (cid:6)(cid:14)+ the corresponding set of positive roots and by (cid:6)ρ ∈ (cid:6)h
the
corresponding ρ-vector.
∗
∗
For ν ∈ P +
k and h ∈ C, set
∗.
(cid:6)νh = k(cid:17)0 + ν + hθ ∈ (cid:6)h
(11.1)
Let (cid:6)p be the parabolic subalgebra of (cid:6)g with Levi factor (cid:6)h + g(cid:4) and the nilradical (cid:6)u+ =
(cid:11)
α∈(cid:6)(cid:14)+\(cid:14)(cid:4) (cid:6)g−α. Let V (cid:4)(ν) denote the irreducible g(cid:4)-module
with highest weight ν and extend the g(cid:4) action to (cid:6)p by letting (cid:6)u+ act trivially; x, K , and
d act by h, k, and 0 respectively. Let M (cid:4)((cid:6)νh) be the corresponding generalized Verma
module for (cid:6)g, i.e.
α∈(cid:6)(cid:14)+\(cid:14)(cid:4) (cid:6)gα. Set (cid:6)u− =
(cid:11)
M (cid:4)((cid:6)νh) = U ((cid:6)g) ⊗U ((cid:6)p) V (cid:4)(ν).
∗
If α ∈ (cid:6)(cid:14) is a non-isotropic root, denote by sα ∈ End((cid:6)h
We denote by v(cid:6)νh a highest weight vector for M (cid:4)((cid:6)νh) . If (cid:6)μ ∈ (cid:6)h
and M is a(cid:6)g-module,
we denote by M(cid:6)μ the corresponding weight space. Let ηi = δ − θi , 1 ≤ i ≤ s (recall
that s = 1 or 2).
∗) the corresponding reflection
and the group generated by them by (cid:6)W . If β ∈ (cid:6)(cid:14) \ Zδ is an odd isotropic root, we let
rβ denote the corresponding odd reflection. We denote by xα a root vector attached to
α ∈ (cid:6)(cid:14). Denote by w. the shifted action of (cid:6)W : w.λ = w(λ + (cid:6)ρ) − (cid:6)ρ.
Lemma 11.3. Let (cid:6)(cid:15)(cid:19) be a set of simple roots for (cid:6)(cid:14). Let M be a (cid:6)g-module and assume
that m ∈ M is a singular vector with respect to (cid:6)(cid:15)(cid:19). If α j ∈ (cid:6)(cid:15)(cid:19) is an isotropic root and
x−α j m (cid:6)= 0, then x−α j m is a singular vector with respect to rα j
((cid:6)(cid:15)(cid:19)).
m = 0. If r (cid:6)= j and (αr |α j ) = 0
Proof. Since α j is odd isotropic, it follows that x 2
then xαr x−α j m = x−α j xαr m = 0. If r (cid:6)= j and (αr |α j ) (cid:6)= 0 then xαr +α j x−α j m =
(cid:17)(cid:18)
x−α j xαr +α j m + xαr m = 0.
−α j
For ν ∈ P +
k set
V. G. Kac, P. Möseneder Frajria, P. Papi
Ni (k, ν) = ((cid:6)νh + (cid:6)ρ|η∨
i
Note that Ni (k, ν) does not depend on h. We will simply write Ni when the dependence
on k and ν is clear from the context.
(11.2)
).
Lemma 11.4. For ν ∈ P +
k not extremal, we have
Ni (k, ν) = Mi (k) + χi + 1 − (ν|θ ∨
i
) ∈ N.
(11.3)
Moreover, for
vi (h) := x Ni
−ηi x−α0−α1 x−α1
i U ((cid:6)g)vi (h) is a proper submodule of the (cid:6)g-module M (cid:4)((cid:6)νh).
v(cid:6)νh
,
(cid:11)
the subspace
Proof. Note that
((cid:6)νh + (cid:6)ρ|η∨
i
) = 2
(k + h∨) − (ν + ρ|θ ∨
i
(θi |θi )
h∨ + ¯h∨
i
2
+
− (ν + ρ(cid:4)|θi ))
h∨ − ¯h∨
2
i
= Mi (k) +
(θi |θi )
2
= Mi (k) + χi + 1 − (ν|θ ∨
i
).
(
) = 2
(θi |θi )
(k +
h∨ − ¯h∨
i
2
+
(θi |θi )
2
) − (ν|θ ∨
i
)
Since ν is not extremal, ((cid:6)νh + (cid:6)ρ|η∨
i
) ∈ N.
(rα1
Recall from Table 1 the set (cid:15) of simple roots for g. Let α1 be an odd root in (cid:15). A
direct (easy) verification shows that α0 + α1 is an odd root and that the set of simple roots
((cid:6)(cid:15))) contains both α0 and {ηi | 1 ≤ i ≤ s}. Clearly x−α0−α1 x−α1
(cid:6)= 0 in
rα0+α1
M (cid:4)(νh) so, by Lemma 11.3, x−α0−α1 x−α1
v(cid:6)νh is a singular vector for the set of simple roots
((cid:6)(cid:15))). The weight of this singular vector is, clearly, (cid:6)ν(cid:19)
= (cid:6)νh − α0 − 2α1. Since
rα0+α1
h
((cid:6)(cid:15))) is (cid:6)ρ +α0 +2α1, we see that ((cid:6)ν(cid:19)
) = ((cid:6)νh +(cid:6)ρ|η∨
the ρ-vector (cid:6)ρ(cid:19) of rα0+α1
) =
i
v(cid:6)νh is a
Ni . Since ηi is a simple root in rα0+α1
i U ((cid:6)g)vi (h)
singular vector for the set of simple roots rα0+α1
is a proper submodule of U ((cid:6)g)x−α0−α1 x−α1
(cid:17)(cid:18)
v(cid:6)νh
((cid:6)(cid:15))), we obtain that x Ni
((cid:6)(cid:15))). It follows that
(rα1
⊂ M (cid:4)((cid:6)νh).
−ηi x−α0−α1 x−α1
h +(cid:6)ρ(cid:19)|η∨
(rα1
(rα1
(rα1
v(cid:6)νh
(cid:11)
i
Set
M((cid:6)νh) = M (cid:4)((cid:6)νh)/(
(cid:10)
i
U ((cid:6)g)vi (h)).
(11.4)
∗
Recall (cf. [13] in the non-super case) that for (cid:6)μ,(cid:6)λ ∈ (cid:6)h
, (cid:6)μ is said to be linked to (cid:6)λ if
there exists a sequence of roots {γ1, . . . , γt } ⊂ (cid:6)(cid:14)+ and weights(cid:6)λ = μ0, μ1 . . . , μt = (cid:6)μ
such that, for 1 ≤ r ≤ t one has
• (μr −1 + (cid:6)ρ|γr ) = mr
2
and mr is odd if γr is an odd non-isotropic root,
• μr = μr −1 − mr γr .
(γr |γr ), mr ∈ N, where mr = 1 if γr is an odd isotropic root
Unitarity of Minimal W -Algebras and Their Representations I
The proof of the following proposition is inspired by [7, Section 11]. It also provides a
simple proof of Lemma 2 from [10].
Proposition 11.5. Assume that ν ∈ P +
k is not extremal and that
(α|α) for all n ∈ N and α ∈ (cid:6)(cid:14)+ \ (cid:6)(cid:14)+(g
(cid:4)).
(11.5)
((cid:6)νh + (cid:6)ρ|α) (cid:6)= n
2
Then
(i) the module M((cid:6)νh) is irreducible;
(ii) its character is
ch M((cid:6)νh) =
(cid:10)
w∈ (cid:6)W (cid:4)
det(w)ch M(w.(cid:6)νh).
(11.6)
Proof. We have
(1) ((cid:6)νh + (cid:6)ρ|α) (cid:6)= 0 for all odd isotropic roots;
(2) ((cid:6)νh + (cid:6)ρ|α∨) ∈ N for all α ∈ (cid:6)(cid:14)+(g(cid:4));
(3) ((cid:6)νh + (cid:6)ρ|α) (cid:6)= n
2
(α|α) for all n ∈ N and for all positive roots α of the affinization of
sl2 = (cid:2)e, f, x(cid:3) and for all non-isotropic odd positive roots.
Indeed, (1), (3) follow from (11.5). To prove (2), first remark that if α is a simple
root for (cid:14)(cid:4)
+, then α ∈ (cid:6)(cid:15). It follows that ((cid:6)ρ|α∨) = (ρ(cid:4)|α∨) = 1. This implies that
((cid:6)νh + (cid:6)ρ|α∨) = (ν + ρ(cid:4)|α∨) ∈ N for α ∈ (cid:14)(cid:4)
+. Since ν is not extremal, (11.3) gives
((cid:6)νh + (cid:6)ρ|η∨
i
We have
) ∈ N.
(cid:10)
ch M((cid:6)νh) =
c(w)ch M(w.(cid:6)νh), wher e c(w) ∈ Z.
(11.7)
w∈ (cid:6)W (cid:4)
Indeed, if ch M((cid:6)μ) appears in ch M((cid:6)νh) then, using the determinant formula proved in
[6], and the corresponding Jantzen filtration [10], one shows, as in [13], that there is a
sequence of roots {γ1, . . . , γt } ⊂ (cid:6)(cid:14)+ linking (cid:6)μ to (cid:6)νh. Properties (1), (3) imply that γi ∈
(cid:6)(cid:14)+(g(cid:4)) and this yields (11.7). It is clear that g(cid:4) acts locally finitely on M (cid:4)((cid:6)νh), hence also
on M((cid:6)νh). By (1), x−α0−α1 x−α1
) =
vi (h) = 0 in M((cid:6)νh), M((cid:6)νh) is integrable for (cid:6)g(cid:4), in particular ch M((cid:6)νh) is (cid:6)W (cid:4)-invariant.
Hence, we obtain c(w) = det(w); therefore (ii) holds. Since the proof of (ii) didn’t use
irreducibility, the irreducible quotient of M((cid:6)νh) has the same character, proving (i). (cid:17)(cid:18)
The following functions hn,(cid:22)m, hm,γ relate singular weights of Verma modules over (cid:6)g
to those over W k
v(cid:6)νh generates M((cid:6)νh). Since x Ni
−ηi
(g) [20, Remark 7.2]:
(x−α0−α1 x−α1
v(cid:6)νh
min
hn,(cid:22)m(k, ν) =
hm,γ (k, ν) =
1
4(k + h∨)
1
4(k + h∨)
(((cid:22)m(k + h∨) − n)2 − (k + 1)2 + 2(ν|ν + 2ρ(cid:4))),
(11.8)
((2(ν + ρ(cid:4)|γ ) + 2m(k + h∨))2 − (k + 1)2 + 2(ν|ν + 2ρ(cid:4))) .
(11.9)
Here γ ∈ (cid:14)(cid:19), the set of g(cid:4)-weights in g−1/2, (cid:22) = 2 (resp. 1) if 0 ∈ (cid:14)(cid:19) (resp. 0 /∈ (cid:14)(cid:19)),
m, n ∈ (cid:22)−1N and m − n ∈ Z in (11.8) and m ∈ 1
2 + Z+ in (11.9).
V. G. Kac, P. Möseneder Frajria, P. Papi
Lemma 11.6. Let k be in the unitarity range and let A(k, ν) be as in (8.11). Assume that
ν is not extremal. Then
hn,(cid:22)m(k, ν) ≤ A(k, ν),
hm,γ (k, ν) ≤ A(k, ν).
(11.10)
(11.11)
Proof. First we prove (11.10). Plugging (11.8) into (11.10) we get
((cid:22)m(k + h∨) − n)2 − (k + 1)2 + 2(ν|ν + 2ρ(cid:4))
4(k + h∨)
≤
(ν|ν + 2ρ(cid:4))
2(k + h∨) +
(ξ |ν)
k + h∨
((ξ |ν) − k − 1),
which is equivalent to
n − (cid:22)m(k + h∨) ≥ |(k + 1) − 2(ξ |ν)|.
(11.12)
Since k + h∨ < 0, it is enough to check (11.12) with (cid:22)m = 1, n = 1/(cid:22). In the case
(k + 1) ≤ 2(ξ |ν), (11.12) reads
1/(cid:22) − h∨ ≥ 2(ξ |ν) − 1.
(11.13)
Looking at the values of h∨ in Table 2, we see that the L.H.S. of (11.13) is non-negative.
Now we prove that (ξ |ν) ≤ 0. Indeed, from Table 1 we deduce that the restriction of
(.|.) to the real span of (cid:14)(cid:4) is negative definite. From Tables 1 and 3 one checks that ξ
is a linear combination with non-negative coefficients of simple roots of g(cid:4); since ν is
dominant, if α ∈ (cid:14)(cid:4) is a simple root then ν(α∨) ≥ 0, hence (ν|α) ≤ 0 since (α|α) < 0.
In the case (k + 1) ≥ 2(ξ |ν) we have to prove that
k + h∨
2
≤ (ξ |ν) + 1−(cid:22)
2(cid:22) .
(cid:4)
The non-extremality condition means that (ν + ξ )(θ ∨
i
) ≤ 2
(θi |θi )
k +
k + h∨
2
≤ (ν + ξ |θi ) +
¯h∨
i
2
,
hence it is enough to prove that
(ν + ξ |θi ) +
¯h∨
i
2
≤ (ξ |ν) + 1−(cid:22)
2(cid:22) .
(11.14)
(cid:5)
or
h∨− ¯h∨
i
2
(11.15)
(11.16)
Note that θi = ξ + βi , where, as above, βi is a linear combination with non-negative
coefficients of simple roots of g(cid:4). Therefore (11.16) can be written as
(ν|βi ) + (ξ |ξ + βi ) ≤ 1−(cid:22)
2(cid:22) −
¯h∨
i
2
,
which is clearly verified, since the left hand side is negative and the right hand side is
positive (use the data in Table 2).
Now we prove (11.11). Substituting (11.8) in it we obtain
(2(ν + ρ(cid:4)|γ ) + 2m(k + h∨))2 − ((k + 1) − 2(ξ |ν))2 ≥ 0,
which is equivalent to
|(2(ν + ρ(cid:4)|γ ) + 2m(k + h∨)| ≥ |(k + 1) − 2(ξ |ν)|.
(11.17)
Unitarity of Minimal W -Algebras and Their Representations I
Table 4. Data employed in the proof of Lemma 11.6
g
psl(2|2)
spo(2|2m), m ≥ 3
spo(2|2m + 1), m ≥ 1
D(2, 1; a)
F(4)
G(3)
(cid:22)
1
1
2
1
1
2
(δ1 − δ2)
ρ(cid:4)
1
2
(m − 1)(cid:22)1 + (m − 2)(cid:22)2 + . . . + (cid:22)m−1
2m−1
2
(cid:22)2 + (cid:22)3
5
(cid:22)1 + 3
2
2
2(cid:22)1 + 3(cid:22)2
(cid:22)2 + . . . + 1
2
(cid:22)1 + 2m−3
(cid:22)2 + 1
2
(cid:22)m
(cid:22)3
2
max(ρ(cid:4)|γ )
1/2
(m − 1)/2
(2m − 1)/4
1
2
3/2
5/4
h∨
0
2 − m
3/2 − m
0
−2
−3/2
Recall that, even though g−1/2 can be reducible as a g(cid:4)-module, all irreducible compo-
nents have the same highest weight ξ . It follows that
− (ξ |ν) = max
γ ∈(cid:14)(cid:19)
(γ |ν).
A direct check on Table 4 shows that
(ρ(cid:4)|γ ) + h∨ = 1.
2 max
γ ∈(cid:14)(cid:19)
Note that, by (11.18) and (11.19)
(11.18)
(11.19)
(k + h∨) + 2(ν + ρ(cid:4)|γ ) ≤ (k + 1) − 2(ξ |ν).
(11.20)
Therefore, if (k + 1) ≤ 2(ξ |ν) then
2(ν + ρ(cid:4)|γ ) + 2m(k + h∨)
= 2(ν + ρ(cid:4)|γ ) + (k + h∨) + (2m − 1)(k + h∨)
≤ 2(ν + ρ(cid:4)|γ ) + (k + h∨) ≤ (k + 1) − 2(ξ |ν) ≤ 0,
and (11.17) reads
2(ν + ρ(cid:4)|γ ) + 2m(k + h∨) ≤ (k + 1) − 2(ξ |ν),
which is clearly true.
Now consider the case
(k + 1) ≥ 2(ξ |ν), −2(ν + ρ(cid:4)|γ ) − 2m(k + h∨) ≥ 0.
(11.21)
The inequality (11.17) becomes
−2(ν + ρ(cid:4)|γ ) − 2m(k + h∨) ≥ (k + 1) − 2(ξ |ν).
which is implied by
− 2(ν + ρ(cid:4)|γ ) − (k + h∨) ≥ (k + 1) − 2(ξ |ν).
(11.22)
If γ = −ξ , then the left hand side of (11.22) is
−2(ν + ρ(cid:4)| − ξ ) − (k + h∨) = 2(ν|ξ ) + h∨ − 1 − (k + h∨) = 2(ν|ξ ) − k − 1,
hence (11.21) implies that both members of (11.22) are zero.
V. G. Kac, P. Möseneder Frajria, P. Papi
If γ (cid:6)= −ξ , then (11.22) is equivalent to
k + h∨
2
≤ − 1
2 + (ξ |ν) − (ν + ρ(cid:4)|γ ),
hence, by (11.15), we are done if we prove that
(ν + ξ |θi ) +
¯h∨
i
2
≤ − 1
2 + (ξ |ν) − (ν + ρ(cid:4)|γ ).
(11.23)
(11.24)
Remark that, since γ (cid:6)= −ξ , then ξ − γ = α ∈ (cid:14)(cid:4)
(ν|θi ) ≤ (ν|ξ − γ ), hence
+ ∪ {0}. If g(cid:4) is simple, then
(ν + ξ |θi ) +
¯h∨
i
2
≤ (ν|ξ − γ ) + (ξ |θi ) +
¯h∨
i
2
,
and therefore (11.22) is implied by
(ν|ξ − γ ) + (ξ |θi ) +
¯h∨
i
2
≤ − 1
2 + (ξ |ν) − (ν + ρ(cid:4)|γ ),
or
(ξ |θi ) +
¯h∨
i
2
≤ − 1
2
− (ρ(cid:4)|γ ).
(11.25)
The minimum of the left hand side of (11.25) is obtained when (ρ(cid:4)|γ ) is maximum,
hence, by (11.19), we are left with proving that
(ξ |θi ) +
¯h∨
i
2
≤ h∨
2
− 1.
(11.26)
This relation is checked using the data in Tables 1, 2, 3. When g(cid:4) is not simple, i.e.
g = D(2, 1; a), relation (11.22) is proven directly. We have ν = r (cid:22)2 + s(cid:22)3, r, s ∈
Z+ γ = ±(cid:22)2 ± (cid:22)3, ξ = (cid:22)2 + (cid:22)3; if we exclude γ = −ξ , (11.22) translates into
k ≤ 0,
k ≤ − r +1
1+a
,
k ≤ − (s+1)a
1+a
,
according to whether γ = (cid:22)2 + (cid:22)3, (cid:22)2, (cid:22)3. The non extremality conditions are
k ≤ − r +2
1+a
,
k ≤ − (s+2)a
1+a
,
(11.27)
(11.28)
so that (11.28) implies (11.27).
We are left with proving (11.17) when both arguments in the absolute values are
non-negative, i.e.
2(ν + ρ(cid:4)|γ ) + 2m(k + h∨) ≥ (k + 1) − 2(ξ |ν).
(11.29)
We claim that the conditions 2(ν + ρ(cid:4)|γ ) + 2m(k + h∨) ≥ 0 combined with (11.15) force
m = 1/2 and γ = −ξ . Taking this fact for granted, (11.29) reads
−2(ν + ρ(cid:4)|ξ ) + h∨ − 1 ≥ −2(ξ |ν),
which holds by (11.18) and (11.19).
To prove our claim, assume that there is m > 1/2 such that
k ≥ 1 − 2mh∨ − 2(ξ |ν) − 2(ν + ρ(cid:4)|γ )
2m − 1
,
Unitarity of Minimal W -Algebras and Their Representations I
or
k + h∨
2
≥ 2 − (2m + 1)h∨ − 4(ξ |ν) − 4(ν + ρ(cid:4)|γ )
2(2m − 1)
.
Taking (11.15) into account, we are done if we prove that
2 − (2m + 1)h∨ − 4(ξ |ν) − 4(ν + ρ(cid:4)|γ )
2(2m − 1)
> (ν + ξ |θi ) +
¯h∨
i
2
.
(11.30)
We have
L. H. S. of (11.30) ≥ 2 − (2m + 1)h∨ − 4(ξ |ν) − 4(ν|γ ) + 2(h∨ − 1)
2(2m − 1)
= − h∨
2 +
−4(ξ |ν) − 4(ν|γ )
2(2m − 1)
≥ − h∨
2
≥
¯h∨
i
2
≥ (ν + ξ |θi ) +
.
¯h∨
i
2
(11.31)
The next to last inequality in (11.31) follows from Table 2; more precisely, the strict
inequality holds in all cases except for spo(2|3). The last inequality in (11.31) uses that
(ν + ξ |θi ) ≤ 0. For g = spo(2|3) the last inequality in (11.31) is strict, hence (11.30) is
proven in all cases.
Hence we have necessarily m = 1/2 in (11.29). We now prove that if
2(ν + ρ(cid:4)|γ ) + (k + h∨) ≥ 0,
(k + 1) − 2(ξ |ν) ≥ 0,
(11.32)
(11.33)
hold, then (11.15) implies γ = −ξ . We proceed case by case.
• g = psl(2|2) or spo(2|3). Since γ ∈ {0, ±ξ }, relation (11.32) forces γ = −ξ .
• g = spo(2|m), m > 4. In this case ν =
±(cid:22)i , ρ(cid:4) =
i ni (cid:22)i , n1 ≥ n2 ≥ . . . ≥ 0, γ =
− i)(cid:22)i , ξ = (cid:22)1. Then (11.32) reads
(cid:24)
(cid:11)
i
( m
2
(cid:11)
(cid:25)
(cid:10)
2
i
or
(ni + m
2
− i)| ± (cid:22) j
+ k + 2 − m
2
≥ 0,
∓(n j + m
2
− j) + k + 2 − m
2
≥ 0.
Since k + 2 − m
2
≤ 0, we have
n j − j + k + 2 ≥ 0.
By (11.15)
therefore
k ≤ − 1
2 n1 − 1
2 n2 − 1,
0 ≤ n j − j + k + 2 ≤ − 1
2 n1 − 1
2 n2 + n j − j + 1.
This relation can be written as
n j −n2
2
which holds only if j = 1, since the n j are non-increasing half integers. If j = 1
then γ = −(cid:22)1 = −ξ .
0 ≤ n j −n1
− j + 1,
+
2
V. G. Kac, P. Möseneder Frajria, P. Papi
• g = D(2, 1; a). In this case ν = r (cid:22)2 + s(cid:22)3, r, s ∈ Z+, γ = ±(cid:22)2 ± (cid:22)3, ρ(cid:4) =
(cid:22)2 + (cid:22)3, ξ = (cid:22)2 + (cid:22)3, and in this case (11.32) becomes
2((r + 1)(cid:22)2 + (s + 1)(cid:22)3| ± (cid:22)2 ± (cid:22)3) + k ≥ 0,
which gives
Condition (11.15) is
∓ (r + 1) ∓ (s + 1)a + (1 + a)k ≥ 0.
(11.34)
k ≤ ((r + 1)(cid:22)2 + (s + 1)(cid:22)3|2(cid:22)2) − 1
1+a
,
k ≤ ((r + 1)(cid:22)2 + (s + 1)(cid:22)3|2(cid:22)3) − a
1+a
,
or
(1 + a)k ≤ −(r + 2),
(1 + a)k ≤ −(s + 2)a.
(11.35)
The only possibility to fulfill (11.34) and (11.35) at the same time is to take γ =
−(cid:22)2 − (cid:22)3 = −ξ .
• g = F(4). In this case ν = n1(cid:22)1 + n2(cid:22)2 + n3(cid:22)3, n1 ≥ n2 ≥ n3 ≥ 0, ρ(cid:4) =
((cid:22)1 + (cid:22)2 + (cid:22)3). Then (11.32) reads
5
2
(±(cid:22)1 ± (cid:22)2 ± (cid:22)3), ξ = 1
2
(cid:22)3, γ = 1
2
(cid:22)2 + 1
2
(cid:22)1 + 3
2
− 2
3
(±(n1 + 5
2
) ± (n2 + 3
2
) ± (n3 + 1
2
)) + k − 2 ≥ 0.
(11.36)
By (11.15) we have
k ≤ − 2
3
(n1 + n2) − 4
3
.
(11.37)
Write now (11.36) using (11.37)
0 ≤ − 2
3
≤ − 2
3
≤ − 2
3
= − 2
3
(±(n1 + 5
2
(±(n1 + 5
2
(±n1 − 5
2
(±n1 ± n2) − 2
3
) ± (n2 + 3
) ± (n3 + 1
2
2
) ± (n2 + 3
) ± (n3 + 1
2
2
) − 2
± n3 − 1
± n2 − 3
3
2
2
(n1 + n2 ± n3) − 1
3
)) + k − 2
)) − 2
3
(n1 + n2) − 10
3
(n1 + n2) − 10
3
.
This inequality holds if and only if the minus sign is taken in all occurrences of ±,
i.e. γ = −ξ .
• g = G(3). In this case ν = m((cid:22)1 + (cid:22)2) + n((cid:22)1 + 2(cid:22)2), m, n ∈ Z+, γ ∈
{0, ±(cid:22)1, ±(cid:22)2, ±((cid:22)1 + (cid:22)2)}, ρ(cid:4) = 2(cid:22)1 + 3(cid:22)2, ξ = (cid:22)1 + (cid:22)2. Then (11.32) reads
2((m + n + 2)(cid:22)1 + (m + 2n + 3)(cid:22)2|γ ) + k − 3
2
≥ 0.
(11.38)
and we can confine ourselves to consider γ ∈ {−(cid:22)1, −(cid:22)2, −(cid:22)1 − (cid:22)2}. The inequalities
corresponding to γ = −(cid:22)1, γ = −(cid:22)2 are
k + m
2
k + m+3n+1
− 1 ≥ 0,
≥ 0,
2
(11.39)
(11.40)
respectively. Relation (11.41) gives
k ≤ ((m + n + 1)(cid:22)1 + (m + 2n + 1)(cid:22)2|(cid:22)1 + 2(cid:22)2) − 3
4
.
Unitarity of Minimal W -Algebras and Their Representations I
or
k ≤ − 3
4
(m + 2n) − 3
2
Substituting (11.39), (11.40), into (11.41) we obtain
4 m − 3
− 1,
0 ≤ k + m
2
0 ≤ k + m+3n+1
− 1 ≤ − 1
≤ − m
4
2
2 n − 5
2
,
(11.41)
(11.42)
(11.43)
respectively. Inequalities (11.42), (11.43) are never verified. Once again we conclude
that γ = −ξ .
(cid:17)(cid:18)
Let H0 denote the quantum Hamiltonian reduction functor, from the category O of (cid:6)g-
(g)-modules. Recall that, for a (cid:6)g-module M,
modules of level k to the category of W k
H0(M) is the zeroth homology of the complex (M ⊗ F(g, x, f ), d0) defined in [18].
Recall that the functor H0 maps Verma modules to Verma modules [20, Theorem 6.3]
and it is exact [2, Corollary 6.7.3]. By [20, Lemma 7.3 (b)], if M is a highest weight
∗
module over (cid:6)g of highest weight (cid:17) ∈ (cid:6)h
, H0(M) is either zero or a highest weight
module over W k
min
min
(g) of highest weight (ν, (cid:10)) with
((cid:17)|(cid:17) + 2(cid:6)ρ)
2(k + h∨)
ν = (cid:17)|h(cid:4),
(cid:10) =
− (cid:17)(x + d).
(11.44)
∗
Remark 11.7. Let L((cid:17)) denote the irreducible (cid:6)g-module of highest weight (cid:17) ∈ (cid:6)h
. By
Arakawa’s theorem [2, Main Theorem] H0(L((cid:17))) is either irreducible or zero, and it
is zero if and only if ((cid:17)|α0) = n
(α0|α0), n ∈ Z+. In particular, if (11.5) holds, then
2
H0(M((cid:6)νh)) is a non-zero highest weight module of highest weight (ν, (cid:10)(h)), where
(cid:10)(h) =
((cid:6)νh|(cid:6)νh + 2(cid:6)ρ)
2(k + h∨)
− h.
(11.45)
∗
For (cid:17) ∈ (cid:6)h
, by a slight abuse of notation, we set M W ((cid:17)) = H0(M((cid:17))), where
M((cid:17)) is the Verma module over(cid:6)g of highest weight (cid:17). Note that M W ((cid:17)) = M W (ν, (cid:10)),
where ν, (cid:10) are given by (11.44).
From now on we assume
• k is in the unitarity range;
• ν ∈ P +
k ;
• (cid:10)(h) ∈ R.
Lemma 11.8. Let h, h(cid:19) be the solutions of the equation (cid:10)(h) = (cid:10)0. If ((cid:6)νh + (cid:6)ρ|δ − θ ) = n,
n ∈ N, then ((cid:6)νh(cid:19) + (cid:6)ρ|δ − θ ) /∈ N.
Proof. Recalling that
((cid:6)νh|(cid:6)νh + 2(cid:6)ρ)
2(k + h∨)
we see that h(cid:19) = k + 1 − h. If ((cid:6)νh + (cid:6)ρ|δ − θ ) = n ∈ N, then
(ν|ν + 2ρ(cid:4))
2(k + h∨) +
(cid:10)(h) =
− h =
h(h − k − 1)
k + h∨
((k + h∨)(cid:17)0 + hθ + ν + ρ|δ − θ ) = k + 1 − 2h = n,
hence h = (k + 1 − n)/2 and h(cid:19) = (k + n + 1)/2 so that
((cid:6)νh(cid:19) + (cid:6)ρ|δ − θ ) = k + 1 − 2h(cid:19) = −n.
(cid:17)(cid:18)
V. G. Kac, P. Möseneder Frajria, P. Papi
(cid:10)
Theorem 11.9. If (cid:10)(h) > A(k, ν), then H0(M((cid:6)νh)) is an irreducible W k
and its character is
min
(g)-module
ch H0(M((cid:6)νh)) =
det (w)ch M W (w.(cid:6)νh).
(11.46)
Proof. If (cid:10)(h) > A(k, ν), then, by Lemma 11.6
w∈ (cid:6)W (cid:4)
(cid:10)(h) (cid:6)= hn,(cid:22)m(k, ν) and (cid:10)(h) (cid:6)= hm,γ (k, ν).
(11.47)
(α|α) for all α ∈ (cid:6)(cid:14)+ \
By [20, Lemma 7.3 (c)], (11.47) implies that ((cid:6)νh + (cid:6)ρ|α) (cid:6)= n
2
((cid:6)(cid:14)+(g(cid:4)) ∪ {δ − θ }). By exchanging h and h(cid:19) if h ∈ N and applying Lemma 11.8, we
find that one can choose h so that (11.5) is satisfied. Hence, by Propositions 8.8 and
11.5, M((cid:6)νh) is irreducible. By Remark 11.7, H0(M((cid:6)νh)) is irreducible and non-zero. On
the other hand, by Theorem 6.2 of [20], we find that H j ((M((cid:6)νh) ⊗ F(g, x, f ))) = 0
if j (cid:6)= 0. Thus, using Euler-Poincaré character, the fact that H0 maps Verma modules
over(cid:6)g to Verma modules over W k
(g), and (ii) in Proposition 11.5, we find that (11.46)
(cid:17)(cid:18)
holds.
Recall from Sect. 6 the Heisenberg algebra H. Let y be an indeterminate. Define an action
of H0 = Ca + CK on C[y] by letting K act as the identity and a act by multiplication
by y. Let M(y) be the corresponding Verma module. This module can be regarded as a
V 1(Ca)-module by means of the field Y (a, z) defined by setting, for m ∈ M(y),
(cid:10)
min
Y (a, z)m =
(τ j ⊗ a) · m z− j−1.
Note also that M(y) is free over C[y] with basis
j∈Z
{(τ − j1 ⊗ a)i1 · · · (τ − jr ⊗ a)ir (1 ⊗ 1) | j1 > · · · > jr > 0}.
(11.48)
Recall from Sect. 9 the free field realization (cid:6) : W k
(g) → V k = V 1(Ca) ⊗
V αk (g(cid:4)) ⊗ F(g1/2
k is not extremal, recall that we denoted by L (cid:4)(ν) the
integrable V αk (g(cid:4))-module of highest weight ν. We also let vν be a highest weight
vector of L (cid:4)(ν). Then
). If ν ∈ P +
min
M(y) ⊗ L (cid:4)(ν) ⊗ F(g1/2
)
is a V k-module, hence, by means of (cid:6), a W k
min
(g)-module. Set
N (y, ν) = (cid:6)(W k
Since M(y) ⊗ L (cid:4)(ν) ⊗ F(g1/2
set also
min
(g)) · (1 ⊗ C[y] ⊗ vν ⊗ 1) ⊂ M(y) ⊗ L (cid:4)(ν) ⊗ F(g1/2
).
) is free as a C[y]-module, N (y, ν) is also free. If μ ∈ C,
N (μ, ν) = (C[y]/(y − μ)) ⊗C[y] N (y, ν).
By construction N (μ, ν) is clearly a highest weight module for W k
(g). As shown
min
in Sect. 10, its highest weight is (ν, (cid:10)0) with
(cid:10)0 = 1
2
μ2 − skμ +
(ν|ν + 2ρ(cid:4))
2(k + h∨)
.
Since we are looking for unitary representations, we will always assume that (cid:10)0 ∈ R.
Unitarity of Minimal W -Algebras and Their Representations I
(cid:10)
Lemma 11.10. If (cid:10)0 > A(k, ν) then N (μ, ν) is an irreducible W k
Proof. Choose h ∈ C such that (cid:10)0 = (cid:10)(h). By Theorem 11.9, H0(M((cid:6)νh)) is an ir-
(g)-module, hence there is an onto map N (μ, ν) → H0(M((cid:6)νh)) =
reducible W k
L W ((cid:10)0, ν). If (cid:10)0 (cid:13) 0, by the proof of Proposition 10.2, N (μ, ν) = L W ((cid:10)0, ν). Observe
that, since (cid:10)0 > A(k, ν), by Lemma 11.6, relations (11.5) hold for our chosen h. It
follows from (11.46) that
(g)-module.
min
min
ch N (μ, ν) =
det (w)ch M W (w.(cid:6)νh) for (cid:10)0 (cid:13) 0.
(11.49)
w∈ (cid:6)W (cid:4)
By (11.44), the highest weight of M W (w.(cid:6)νh) is (ν(w, h), (cid:10)0(w, h)) where
ν(w, h) = (w.(cid:6)νh)|h(cid:4), (cid:10)0(w, h) =
w((cid:6)νh + (cid:6)ρ) 2 − (cid:6)ρ 2
2(k + h∨)
− (w.(cid:6)νh)(x + d).
Since w ∈ (cid:6)W (cid:4), (w.(cid:6)νh)(x) = h and (w.(cid:6)νh)(d) as well as ν(w, h) do not depend on h.
We can therefore write
(cid:10)0(w, h) =
=
=
w((cid:6)νh + (cid:6)ρ) 2 − (cid:6)ρ 2
2(k + h∨)
((cid:6)ν0 + (cid:6)ρ) 2 − (cid:6)ρ 2
2(k + h∨)
w((cid:6)ν0 + (cid:6)ρ) 2 − (cid:6)ρ 2
2(k + h∨)
− (w.(cid:6)ν0)(d) − h
− (w.(cid:6)ν0)(d + x) +
− (w.(cid:6)ν0)(d + x) +
((cid:6)νh + (cid:6)ρ) 2 − (cid:6)ν0 + (cid:6)ρ 2
2(k + h∨)
2h2 + (h∨ − 1)h
2(k + h∨)
− h
− h
= (cid:10)0(w, 0) +
2h2 + (h∨ − 1)h
2(k + h∨)
− h.
It follows that
ch M W (w.(cid:6)νh) = ch M W (w.(cid:6)ν0)e
(0, 2h2+(h∨−1)h
2(k+h∨) −h)
and
(cid:10)
w∈ (cid:6)W (cid:4)
det (w)ch M W (w.(cid:6)νh) =
In particular, if (cid:10)0 (cid:13) 0, then
⎛
ch N (μ, ν) =
⎝
(cid:10)
w∈ (cid:6)W (cid:4)
det (w)ch M W (w.(cid:6)ν0)
⎞
⎠ e
(0, 2h2+(h∨−1)h
2(k+h∨) −h).
⎛
⎝
(cid:10)
w∈ (cid:6)W (cid:4)
(11.50)
det (w)ch M W (w.(cid:6)ν0)
⎞
⎠ e
(0, 2h2+(h∨−1)h
2(k+h∨) −h).
Since N (y, ν) is a free C[y]-module, the dimensions of the weight spaces of N (μ, ν) do
not depend on μ. By (11.50), the coefficents of both sides of (11.49) do not depend on μ.
It follows that (11.49) holds for all μ. In particular, if (cid:10)0 > A(k, ν), by Theorem 11.9,
ch N (μ, ν) = ch H0(M((cid:6)νh)),
hence N (μ, ν) (cid:29) H0(M((cid:6)νh)) is irreducible.
(cid:17)(cid:18)
V. G. Kac, P. Möseneder Frajria, P. Papi
The lowest energy space of N (μ, ν) is 1 ⊗ 1 ⊗ V (cid:4)(ν) ⊗ 1 with L 0 acting by mul-
tiplication by (cid:10)0. This space admits a ω-invariant Hermitian form hence there exists a
φ-invariant Hermitian form H (·, ·) on N (μ, ν).
If (cid:6)ζ (y) ∈ H omC[y](C[y] ⊗ (cid:6)h, C[y]) is a weight of N (y, ν), fix a basis B(cid:6)ζ (y)
of N (y, ν)(cid:6)ζ (y). Set (cid:6)ζ = (cid:6)ζ (μ). Then 1 ⊗ B(cid:6)ζ (y) gives a basis B(cid:6)ζ of N (μ, ν)(cid:6)ζ =
(C[y]/(y − μ)) ⊗C[y] N (y, ν)(cid:6)ζ (y). Let det(cid:6)ζ ((cid:10)0) be the determinant of the matrix in
this basis of the Hermitian form H (·, ·) restricted to N (μ, ν)(cid:6)ζ . Note that det(cid:6)ζ ((cid:10)0) is a
polynomial in (cid:10)0.
End of proof of Theorem 11.1 and Corollary 11.2. We may assume that the level is not
collapsing, so that Mi (k) + χi ∈ Z+ by Remark 7.5. Then, by Proposition 10.2, the Her-
mitian form on L W (ν, (cid:10)0) is positive definite for (cid:10)0 (cid:13) 0. By Lemma 11.10, N (μ, ν) =
(ν|ν+2ρ(cid:4))
L W (ν, (cid:10)0) if (cid:10)0 = 1
2(k+h∨) > A(k, ν), hence det(cid:6)ζ ((cid:10)0) (cid:6)= 0 for all weights
2
(cid:6)ζ of N (μ, ν). It follows that the Hermitian form is positive definite for (cid:10)0 > A(k, ν),
hence positive semidefinite for (cid:10)0 = A(k, ν).
Corollary 11.2 follows from Proposition 8.10 and Theorem 11.1 in the case ν = 0,
(cid:17)(cid:18)
since A(k, 0) = 0, and Remark 7.5.
μ2 − skμ +
k
min
12. Explicit Necessary Conditions and Sufficient Conditions of Unitarity
Looking for the pairs (ν, (cid:10)0), ν ∈ (cid:6)P +
, (cid:10)0 ∈ R, such that L W (ν, (cid:10)0) is a unitary
(g)-module for k in the unitarity range, we rewrite for each case (excluding the
W k
trivial case (1)) the conditions in terms of the parameters Mi = Mi (k) from Table 2.
Namely, we provide the necessary and sufficient conditions of unitarity of L W (ν, (cid:10)0)
for a non-extremal weight ν, given by Theorem 11.1, and the necessary condition of
unitarity for an extremal weight ν, given by Proposition 8.8. We also provide explicit
expressions for the cocycle αk and the central charge c of L. Recall the invariant bilinear
form (.|.)(cid:4)
(cid:4)
i on g
i , introduced in Sect. 7.
12.1. psl(2|2). In this case g(cid:4) = sl(2), M1 ∈ N and αk = (M1 − 1)( . | . )(cid:4)
1. If ν =
r θ1/2, with r ∈ Z≥0 (i.e. ν is dominant integral), and r ≤ M1 − 1, then the necessary
and sufficient condition for unitarity is
(cid:10)0 ≥ r
2
.
If r = M1, then then necessary condition is (cid:10)0 = M1/2.
The central charge is c = −6(k + 1) = 6M1.
12.2. spo(2|3). In this case g(cid:4) = sl(2), M1 ∈ N and αk = (M1 − 2)( . | . )(cid:4)
1. If
ν = r θ1/2 = r α/2, with r ∈ Z≥0, r ≤ M1 − 2, then the necessary and sufficient
condition for unitarity is
(cid:10)0 ≥ r
4
.
If M1 − 1 ≤ r ≤ M1, then then necessary condition is (cid:10)0 = r/4.
The central charge is c = −6k − 7
2
2 M1 − 1
2 .
= 3
Unitarity of Minimal W -Algebras and Their Representations I
12.3. spo(2|m), m > 4. In this case g(cid:4) = so(m), M1 ∈ N and αk = (M1 − 1)( . | . )(cid:4)
1.
If ν is dominant integral, ν(θ ∨
) ≤ M1 − 1, then the necessary and sufficient condition
1
for unitarity is
(cid:10)0 ≥
(ν|ν + 2ρ(cid:4))(cid:4)
2(M1 + m − 3) +
r (M1 − r − 1)
2(m + M1 − 3)
= −
(ν|ν + 2ρ(cid:4))(cid:4) − r (2k + r + 2)
2(2k − m + 4)
,
(12.1)
where r = (ω1|ν)(cid:4), and ω1 is the highest weight of the standard representation of so(m).
If ν(θ ∨
1
The central charge is c = M1
) = M1, the necessary condition is that equality must hold in (12.1).
"
2(m+M1−3) = − (2k+1)
m2+6M1−10
12k−m2+16
4k−2m+8
.
!
!
"
12.4. D(2, 1; m
n
and
(cid:4)
), m, n ∈ N, m, n coprime. In this case g(cid:4) = g
1
(cid:4)
(cid:4)
⊕ g
2 with g
i
(cid:29) sl(2),
αk(b, c) = (Mi (k) − 1)(b|c)(cid:4)
i
(cid:4)
if b, c ∈ g
i
.
If ν = r1
2
sufficient condition for unitarity is
θ1 + r2
2
θ2 is dominant integral with ri ≤ Mi (k) − 1, then the necessary and
(cid:10)0 ≥ 2(M1 + 1)r2 + 2(M2 + 1)r1 + (r1 − r2)2
4(M1 + M2 + 2)
= 2(a + 1)k(ar2 + r1) − a(r1 − r2)2
4(a + 1)2k
(12.2)
If ri = Mi for some i, then the necessary condition is that equality must hold in (12.2).
The central charge is c = 6
− 3 = −3(1 + 2k).
(M1+1)(M2+1)
M1+M2+2
12.5. F(4). In this case g(cid:4) = so(7), M1 ∈ N and αk = (M1 − 1)( . | . )(cid:4). If ν(θ ∨
1
M1 − 1, then the necessary and sufficient condition for unitarity is
) ≤
(cid:10)0 ≥
=
r1(M1 + 7) + r2(M1 + 4) + r3(M1 + 1) + r 2
3(M1 + 4)
2 k) + r 2
1 + r 2
2 k)
3(3 − 3
2 k) + r2(3 − 3
2 k) + r3(− 3
r1(6 − 3
1 + r 2
2 + r 2
3
− r1r2 − r1r3 − r2r3
2 + r 2
3
− r1r2 − r1r3 − r2r3
,
where we write ν = r1(cid:22)1 + r2(cid:22)2 + r3(cid:22)3 with (cid:22)i as in Table 1. If ν(θ ∨
1
necessary condition is that equality must hold in (12.3).
= − 2(k−3)(3k+2)
The central charge is c = 2M1(2M1+11)
.
M1+4
k−2
(12.3)
) = M1, then the
12.6. G(3). In this case g(cid:4) = G2, M1 ∈ N and αk = (M1−1)( . | . )(cid:4). If ν(θ ∨
1
then the necessary and sufficient condition for unitarity is
) ≤ M1−1,
V. G. Kac, P. Möseneder Frajria, P. Papi
(cid:10)0 ≥ r1(3M1 + 1) + r2(3M1 + 7) + 3(r1 − r2)2
= r1(−2 − 4k) + r2(4 − 4k) + 3(r1 − r2)2
8(3 − 2k)
12(M1 + 3)
,
(12.4)
where we write ν = r1(cid:22)1 + r2(cid:22)2 with (cid:22)i as in Table 1. If ν(θ ∨
1
condition is that equality must hold in (12.4).
The central charge is c = M1(9M1+31)
2(M1+3) = −24k2+26k+33
4k−6
.
) = M1, then the necessary
min
13. Unitarity for Extremal Modules Over the N = 3, N = 4 and big N = 4
Superconformal Algebras
A module L W (ν, (cid:10)0) for W k
(g) is called extremal if the weight ν is extremal (see
Definition 8.7). In this section we give a partial solution of Conjecture 2 for some g.
Namely, g will be either spo(2|3), or psl(2|2), or D(2, 1; a), so that W k
(g) is related
to the N = 3, N = 4 and big N = 4 superconformal algebra, respectively. Recall from
[20, Section 8] that in these cases, up to adding a suitable number of bosons and fermions,
it is always possible to make the λ-brackets between the generating fields linear, hence
the span of their Fourier coefficients gets endowed with a Lie superalgebra structure,
called the N = 3, N = 4 and big N = 4 superconformal algebra respectively.
Recall that, by Proposition 8.8, for each extremal weight ν there is at most one (cid:10)0 for
which the extremal module L W (ν, (cid:10)0) is unitary, hence for each extremal ν it suffices
to construct one such unitary module.
min
13.1. g = spo(2|3). Consider W k
(spo(2|3)) and the Lie conformal superalgebra R =
(C[∂] ⊗ a) ⊕ CK , where a is an 8-dimensional
superspace with basis
˜L, ˜G±, ˜G0, J ±, J 0, (cid:30), where ˜L, J ±, J 0 are even and ˜G±, ˜G0, (cid:30) are odd, and the fol-
lowing λ-brackets
min
[J 0λ ˜G0] = −2λ(cid:30) , [J +λ ˜G
−] = ˜L + 1
[ ˜G+λ ˜G
4
−
[ ˜G
λ ˜G0] = − 1
(∂ + 2λ)J
2
[(cid:30)λ(cid:30)] = −K , [J +
λ J
[ ˜Lλ ˜L] = ∂ ˜L + 2λ ˜L − λ3
2 K .
−
−] = −2 ˜G0 + 2λ(cid:30) , [J
±] = 0 ,
λ ˜G+] = ˜G0 + λ(cid:30) , [ ˜G
(∂ + 2λ)J + , [ ˜G0λ ˜G0] = ˜L − λ2 K ,
(∂ + 2λ)J 0 − λ2 K , [ ˜G+λ ˜G0] = 1
4
− , [ ˜G+λ(cid:30)] = 1
− , [ ˜G0λ(cid:30)] = − 1
−
4 J + , [ ˜G
λ(cid:30)] = 1
2 J
±, [J 0
−] = J 0 − 4λK , , [J 0
λ J 0] = −8λK ,
λ J
±] = ±2J
4 J 0
λ ˜G
±
Furthermore ˜G±, ˜G0, J ±, J 0, (cid:30) are primary for ˜L of conformal weight 3
2
respectively.
, 3
2
, 1, 1, 1
2 ,
The N = 3 superconformal algebra W k
)1), where V (R)
is the universal enveloping vertex algebra of R. Let F(cid:30) be the fermionic vertex algebra
generated by an odd element (cid:30), with λ-braket [(cid:30)λ(cid:30)] = −(k + 1
)1. Then there is a
2
conformal vertex algebra embedding
N =3 is V (R)/(K − (k + 1
2
W k
N =3
→ W k
min
(spo(2|3)) ⊗ F(cid:30)
Unitarity of Minimal W -Algebras and Their Representations I
given by (cf [20, §8.5])
˜L (cid:25)→ L − 1
2k+1
√
˜G− (cid:25)→ −
G− − 1
√
2k+1
−1
k+1/2
: ∂(cid:30)(cid:30) : , ˜G+ (cid:25)→
√
−1√
G+ − 1
k+1/2
4k+2
√
: J −(cid:30) : , ˜G0 (cid:25)→ −
−1
√
k+1/2
(cid:30) (cid:25)→ (cid:30), J ± (cid:25)→ J ±, J 0 (cid:25)→ J 0.
: J +(cid:30) : ,
G0 + 1
4k+2
: J 0(cid:30) : .
(spo(2|3)) ⊗ F(cid:30) setting φ((cid:30)) = −(cid:30).
Extend the conjugate linear involution φ to W k
Recall from [16] that the unique φ-invariant Hermitian form on F(cid:30) is positive definite.
Also recall that the tensor product of invariant Hermitian forms is still invariant; in
particular if we prove that L W (ν, (cid:10)0) ⊗ F(cid:30) is unitary for W k
N =3, then L W (ν, (cid:10)0) is a
(spo(2|3))-module. Recall that, for a, b ∈ V (R), the modes of a, b have a
unitary W k
Lie superalgebra structure given by
min
min
[ar , bs] =
(cid:10)
j∈Z+
(cid:4)
(cid:5)
(cid:14)a + r − 1
j
(a( j)b)r +s.
, J ±
n
, ˜G0
m
Observe that the span L of ˜L n, ˜G±
2 + Z, is a Lie
, J 0
n
m
N =3), then M ⊗ M (cid:19) inherits
superalgebra. If M (resp. M (cid:19)) are modules for W k
an action of L which makes M ⊗ M (cid:19) a W k+k(cid:19)+
-module. Clearly, if both M, M (cid:19) are
N =3
unitary, then M ⊗ M (cid:19) is unitary. The argument used in the next proposition generalizes
the one used for the oscillator representation of the Virasoro algebra in [17, §3.4].
, (cid:30)m, K , n ∈ Z, m ∈ 1
N =3 (resp. W k(cid:19)
1
2
Proposition 13.1. Let M1 = −4k − 2 ∈ N. Then the extremal W k
L W ( M1−1
α, M1
4
sl2.
(spo(2|3))-modules
) are both unitary, where α is the simple root of g(cid:4) =
), L W ( M1
2
α, M1−1
4
min
2
min
Proof. To make the argument more transparent we make explicit the dependence on k, so
we write L(k, ν, (cid:10)0) for the W k
(spo(2|3))-module L W (ν, (cid:10)0). Recall that ν = r α/2.
We proceed by induction on M1. The base case M1 = 1 corresponds to the collapsing
level k = −3/4, when W min
(spo(2|3)) = V1(sl2). Recall that V1(sl2) has only two
−3/4
irreducible modules N1 and N2, which are both unitary and have highest weights ν = 0
and ν = α/2 respectively. Recall from § 12.2 that if M1 − 1 ≤ r ≤ M1, then the
necessary condition for unitarity is (cid:10)0 = M1/4. Hence N1 and N2 are L(−3/4, 0, 0) and
α, M1−2
L(−3/4, α/2, 1/4). Set k1 = − M1+1
)
4
and L(k1, M1−1
α, M1−1
α, M1−2
) ⊗ F(cid:30) is unitary
4
4
N =3 and M (cid:19) = L(−3/4, α/2, 1/4) ⊗ F(cid:30) is unitary for W −3/4
N =3 . Therefore M ⊗ M (cid:19)
for W k1
is unitary for W k2
= k. In particular,
− 1
4 + 1
2
2
⊗ 1 is unitary, and
the W k
, 1
4
its weight is ( M1−1
= − M1+1
(spo(2|3))-module generated by v M1−2
. Assume by induction that L(k1, M1−2
) are unitary. Then M = L(k1, M1−2
= − M1
4
⊗ 1 ⊗ v α
2
4 + 1
2
α, M1−2
4
, k2 = k1 − 3
N =3
− 3
min
2
2
4
2
4
2
α, M1−1
4
Repeating this argument with L(k1, M1−1
), as required.
2
2
α, M1−1
4
) ⊗ F(cid:30) proves the unitarity of
(cid:17)(cid:18)
L(k, M1
2
α, M1
4
).
V. G. Kac, P. Möseneder Frajria, P. Papi
13.2. g = psl(2|2). We choose strong generators J 0, J ±, G±, ¯G±, L for W k
as in [20, §8.4]. We can choose the generators so that, if φ is the almost compact invo-
lution corresponding to the real form described in Sect. 4, then
( psl(2|2))
min
φ(L) = L , φ(J +) = −J −, φ(J 0) = −J 0, φ(G+) = ¯G−, φ(G−) = ¯G+. (13.1)
The λ-brackets among these generators are linear, hence their Fourier coefficients span
the N = 4 superconformal algebra. It is therefore enough to prove unitarity of the
extremal module L W (θ1/2, 1/2) at level k = −2, since all the other extremal modules
at level k < −2 are obtained by iterated tensor product of L W (θ1/2, 1/2).
The unitarity of L W (θ1/2, 1/2) is proved by constructing this module as a submodule
of a manifestly unitary module. This is achieved by using the free field realization of
( psl(2|2)) given in [3], in terms of four bosonic fields and four fermionic fields,
W
which we now describe. Let F be the vertex algebra generated by four even fields
ai , 1 ≤ i ≤ 4 and four odd fields bi , 1 ≤ i ≤ 4 with λ-bracket
−2
min
[ai λa j ] = δi j λ, [bi λb j ] = δi j , [ai λb j ] = 0.
There is an homomorphism F F R : W
−2
min
( psl(2|2)) → F given by
4(cid:10)
(: ai ai : + : T bi bi :)
L (cid:25)→ 1
2
i=1
J + (cid:25)→ − 1
2
J − (cid:25)→ 1
: b1b3 : − 1
2
2
√
J 0 (cid:25)→ −
G+ (cid:25)→ 1
2
G− (cid:25)→ 1
2
¯G+ (cid:25)→ 1
2
¯G− (cid:25)→ 1
2
: (a1 +
: (a1 +
: (a1 −
: (a1 −
√
√
√
√
√
: b1b3 : − 1
2
√
−1 : b1b4 : − 1
2
√
√
−1 : b1b4 : − 1
2
−1 : b3b4 :
√
−1 : b1b2 : −
√
−1 : b2b3 : + 1
: b2b4 :
2
: b2b4 :
−1 : b2b3 : − 1
2
−1a2)(b3 +
−1a2)(b1 −
−1a2)(b1 +
−1a2)(b3 −
√
√
√
−1b4) : − 1
2
−1b2) : + 1
2
−1b2) : + 1
2
−1b4) : − 1
2
: (a3 +
: (a3 +
: (a3 −
: (a3 −
√
√
−1a4)(b1 +
−1a4)(b3 −
−1a4)(b3 +
√
−1a4)(b1 −
−1b2) :
−1b4) :
−1b4) :
√
−1b2) :
√
√
√
√
We define a conjugate linear involution ψ on F by
ai (cid:25)→ −ai , bi (cid:25)→ −bi
−2
min
so that, according to [16, §5.1,5.2], there is a ψ-invariant positive definite Hermitian
form HF on F. It is clear from (13.1) that ψ ◦ F F R = F F R ◦ φ. Using F F R we
( psl(2|2)) on F. Since HF is invariant with respect to the
can define an action of W
( psl(2|2))-module. An
conformal vector F F R(L), it follows that F is a unitary W
−2
( psl(2|2)),
−1b2 is a singular vector for W
easy calculation shows that v = b1 +
min
−2
thus v generates a unitary highest weight representation L W (ν, (cid:10)0) of W
( psl(2|2)).
min
Clearly F F R(L)0v = 1
θ1 and (cid:10)0 = 1
2 . This proves
2
that the highest weight module corresponding the extremal weight ν = 1
θ1 is indeed
2
unitary.
v, while J 0v = v, hence ν = 1
2
−2
min
√
Unitarity of Minimal W -Algebras and Their Representations I
13.3. g = D(2, 1; m
n
case when either m = 1 of n = 1.
). In this case we are able to prove unitarity only in the very special
If n = 1, then the unitarity range is {− m
− m
m+1
min
m+1 N | N ∈ N}. Take N = 1 and observe that
(D(2, 1; m)) collapses to Vm−1(sl(2)). In this case there is only one extremal
α2, which gives rise to a unitary representation since it is integrable.
W
weight ν = m−1
2
The case m = 1 is dealt with in a similar way, switching the roles of α2, α3.
14. Characters of the Irreducible Unitary W k
min
(g)-Modules
∗
Recall that, for (cid:17) ∈ (cid:6)h
(ν, (cid:10)) is given by (11.44). It follows from [20, (6.11)], that
, we denoted by M W ((cid:17)) the Verma module M W (ν, (cid:10)), where
ch M W ((cid:17)) = eνq(cid:10) F N S(q),
(14.1)
where q = e(0,1) and
F N S(q) =
∞(cid:9)
n=1
In particular,
(1 − qn)rankg(cid:4)+1
(cid:29)
α∈(cid:14)1/2
(cid:29)
α∈(cid:14)(cid:4)
+
2 e−α)
(1 + qn− 1
((1 − qn−1e−α)(1 − qneα)))
.
(14.2)
ch M W ((cid:6)νh) = eνq(cid:10)(h) F N S(q),
(14.3)
where (cid:10)(h) is given by (11.45).
min
The characters of unitary W k
(g)-modules L W (ν, (cid:10)0) are computed by applying
the quantum Hamiltonian reduction to the irreducible highest weight (cid:6)g-modules L((cid:6)νh),
where ν ∈ P +
k and (cid:10)0 = (cid:10)(h), and using the argument in the proof of Theorem 11.9,
which is based on Remark 11.7. There are two cases to consider in computation of their
characters. First, if the weight (cid:6)νh is typical, i.e. conditions (11.5) hold, then ch L((cid:6)νh) is
given by the R.H.S. of (11.6), by Proposition 11.5.
The second case occurs when the weight (cid:6)νh satisfies the condition
((cid:6)νh + (cid:6)ρ|α) = 0 for all α ∈ (cid:15)¯1
where (cid:15)¯1 denotes the set of simple isotropic roots of g. Then the weight(cid:6)νh is maximally
atypical, and L((cid:6)νh) is integrable, hence the following formula is a special case of [7,
Formula (14)] if g (cid:6)= D(2, 1; m
) and ν = 0:
n
) and of [7, Section 6.1] if g = D(2, 1; m
n
,
e(cid:6)ρ R ch L((cid:6)νh) =
(cid:10)
w∈ (cid:6)W (cid:4)
det (w)w
(cid:29)
e(cid:6)νh +(cid:6)ρ
(1 + e−β )
β∈(cid:15)¯1
,
(14.4)
where R equals the character of the Verma module M(0) over (cid:6)g with highest weight 0.
k . Let L W (ν, (cid:10)0) be a unitary
Theorem 14.1. Let k be in the unitary range and let ν ∈ P +
irreducible W k
(g)-module. Choose h so that (cid:10)(h) = (cid:10)0 and let, as before,
min
(cid:6)νh = k(cid:17)0 + ν + hθ.
V. G. Kac, P. Möseneder Frajria, P. Papi
(i) If (cid:10)0 > A(k, ν), then
ch L W (ν, (cid:10)0) =
(cid:10)
w∈ (cid:6)W (cid:4)
det (w)ch M W (w.(cid:6)νh).
(14.5)
(ii) If (cid:10)0 = A(k, ν), and ν = 0 if g = D(2, 1; m
n
), then
(cid:10)
(cid:10)
ch L W (ν, (cid:10)0) =
(−1)γ det (w)ch M W (w.((cid:6)νh − γ )),
(14.6)
w∈ (cid:6)W (cid:4)
γ ∈Z+(cid:15)¯1
where (cid:15)¯1
n1γ1 + · · · , we write (−1)γ = (−1)n1+···.
= {γ1, γ2, . . .} is the set of isotropic simple roots for g, and for γ =
Proof. Formula (14.5) follows from (11.46). Formula (14.6) follows from (14.4) by
applying quantum Hamiltonian reduction to the (cid:6)g-module L((cid:6)νh). In order to use (14.4),
write explicitly the relation (cid:10)0 = (cid:10)(h) = A(k, ν). We have
(k(cid:17)0 + hθ + ν|k(cid:17)0 + hθ + ν + 2h∨(cid:17)0 + 2ρ)
2(k + h∨)
(ξ |ν)
k + h∨
(ν|ν + 2ρ(cid:4))
2(k + h∨) +
((ξ |ν) − k − 1),
=
− h
or
h(h − 1 − k) = (ξ |ν)((ξ |ν) − k − 1).
Hence either h = (ξ |ν) or h = 1 + k − (ξ |ν). We observe that if α ∈ (cid:15)¯1, then, restricted
to h(cid:4), it coincides with −ξ , hence (ξ |ν) = −(α|ν), and also (θ |α) = 1. Therefore, for
h = (ξ |ν) we have
((cid:6)νh + (cid:6)ρ|α) = ((k + h∨)(cid:17)0 + (ξ |ν)θ + ν + ρ|α) = (ξ |ν) + (α|ν) = 0.
Hence we may apply (14.4). Note that H0(L((cid:6)νh)) (cid:6)= 0 since ((cid:6)νh|α0) < 0, so that we
(cid:17)(cid:18)
can apply Remark 11.7.
Remark 14.2. It is still an open problem whether in the case g = D(2, 1; m
n
(14.4) holds for an arbitrary ν ∈ P +
k .
) formula
Remark 14.3. For the N = 4 superconformal algebra, formula (14.5) appears, in a
different form, in [4, formula (14)], where it has been derived in a non-rigorous way.
To establish a dictionary to match the two formulas first observe that a parameter y
occurs in the formulas of [4] corresponding to an extra U (1)-symmetry that we do not
consider, hence, to compare the formulas, we set y = 1. Next recall that in this case (cid:6)W (cid:4)
is of type A
(set
(1)
1 , hence its elements are of the form ui = s0s1 · · ·
’ () *
i factors
= s1s0 · · ·
’ () *
i factors
u0 = u(cid:19)
= I d). In the notation of [4], the pairs (an, bn) corresponding to the α-series
0
(resp. β-series) in formula (12) of [4] match exactly the pairs (ν, (cid:10)) given in (11.44) for
the weight (cid:17) = ui .(cid:6)νh (resp. (cid:17) = u(cid:19)
.(cid:6)νh). The factor F N S(θ, 1) translates precisely to
i
(14.3) according to the dictionary
or u(cid:19)
i
eδ1−δ2 ↔ e
√
−1θ .
Unitarity of Minimal W -Algebras and Their Representations I
The character formula (14.6) corresponds to the formula (26) in [4] for the character of
“massless” representations. To show this, we first remark that, if γ ∈ Z+(cid:15)¯1, then
M W (w.((cid:6)νh − γ )) = M W (ν, (cid:10)),
where (ν, (cid:10)) is given by (11.44). In particular
(cid:10) =
(w.((cid:6)νh − γ )|w.((cid:6)νh − γ ) + 2(cid:6)ρ)
2(k + h∨)
− (w.((cid:6)νh − γ ))(x + d)
=
=
||(cid:6)νh − γ + (cid:6)ρ||2 − ||(cid:6)ρ||2
2(k + h∨)
− (w.((cid:6)νh − γ ))(x + d)
||(cid:6)νh + (cid:6)ρ||2 − ||(cid:6)ρ||2
2(k + h∨)
− w.((cid:6)νh)(x + d) + w(γ )(x + d)
= (cid:10)(h) + ((cid:6)νh + (cid:6)ρ)(x + d) − w((cid:6)νh + (cid:6)ρ)(x + d) + w(γ )(x + d),
hence, using formula (14.1),
ch M W (w.((cid:6)νh − γ )) = q(cid:10)(h) F N S(q)e
(w.(cid:6)νh )|h
(cid:4) q((cid:6)νh +(cid:6)ρ−w((cid:6)νh +(cid:6)ρ))(x+d)e
−(wγ )|h
(cid:4) qw(γ )(x+d),
and we obtain that
(cid:10)
γ ∈Z+(cid:15)¯1
(−1)γ ch M W (w.((cid:6)νh − γ )) = q(cid:10)(h) F N S(q) e
(cid:29)
(w.((cid:6)νh ))|h
(cid:4) q((cid:6)νh +(cid:6)ρ−w((cid:6)νh +(cid:6)ρ))(x+d)
(cid:4) qw(α)(x+d))
−w(α)|h
(1 + e
.
α∈(cid:15)¯1
(14.7)
(cid:4) = CK + Cd + h(cid:4)), we can apply the formulas
Since θ is orthogonal to ((cid:6)h
of [12, Chapter 6] to (cid:6)g(cid:4) and its Weyl group. Since, in our case, (cid:6)νh + (cid:6)ρ = k(cid:17)0 + (h −
)θ + (r + 1
1
2
2
Z+, we have, for m ∈ Z,
(cid:4))∗ (where (cid:6)h
)η1, r ∈ 1
2
(s0s1)m((cid:6)νh + (cid:6)ρ) = k(cid:17)0 + (h − 1
2
)θ + (r − km + 1
2
)η1 − (m(−km + 2r + 1))δ.
and, if α ∈ (cid:15)¯1,
Since s1 = sη1 , it follows that
(s0s1)m(α) = α + mδ.
((s0s1)m.(cid:6)νh)|h(cid:4) = (r − km)η1, ((s1(s0s1)m).(cid:6)νh)|h(cid:4) = −(r − km + 1)η1,
(cid:6)νh + (cid:6)ρ − (s0s1)m((cid:6)νh + (cid:6)ρ)(x + d) = (cid:6)νh + (cid:6)ρ − s1(s0s1)m((cid:6)νh + (cid:6)ρ)(x + d)
= (m(−km + 2r + 1)) = −km2 + (2r + 1)m,
and
(s0s1)m(α)|h(cid:4) = − 1
2
(s0s1)m(α)(x + d) = s1(s0s1)m(α)(x + d) = (m + 1
2
η1, s1(s0s1)m(α)|h(cid:4) = 1
2
η1,
).
Substituting (14.7) into (14.6), recalling that M1(k) = −k − 1, we obtain
ch L W (r η1, (cid:10)0)
= q(cid:10)0 F N S(q)
(cid:24)
(cid:10)
m∈Z
e(r +m(M1(k)+1))η1
2 )2
(1 + e
η1qm+ 1
1
2
− e−(r +m(M1(k)+1)+1)η1
2 )2
(1 + e− 1
η1qm+ 1
2
(cid:25)
qm2(M1(k)+1)+(2r +1)m
V. G. Kac, P. Möseneder Frajria, P. Papi
which, under our dictionary, corresponds to formula (26) of [4] in the NS sector.
min
For W k
(spo(2|3)), formula (14.5) appears (with a non-rigorous proof) in [21, for-
(1)
mula (4.3)]. Again, in this case (cid:6)W (cid:4) is of type A
1 and its elements are of the form ui or
u(cid:19)
i (notation as above). The pairs (ln, hn) displayed in [21, (4.2.a),(4.2.b)], correspond-
ing to the A-series (resp. B-series), match exactly the pairs (ν, (cid:10)) given in (11.44) for
the weight (cid:17) = ui .(cid:6)νh (resp. (cid:17) = u(cid:19)
.(cid:6)νh). The denominator F N S(q, z) in [21, (3.15.i)]
i
translates precisely to (14.3) according to the dictionary
e(cid:22)1 ↔ z.
In the massless case, the character formula (14.6) corresponds to formula (4.6.1) in [21],
hence Theorem 14.1 provides a proof of it, since formula (14.4) holds in this case, due
to [7, Subsection 12.3].
Note added in proof: Conjecture 4 has been proved in a joint paper with Drazen
Adamovi´c.
Acknowledgements. V.K. is partially supported by the Stephen Berenson mathematical exploration Fund and
the Simons collaboration Grant. The authors thank Drazen Adamovi´c, Thomas Creutzig and Anne Taormina for
correspondence. The authors are grateful to Maria Gorelik for many very useful and enlightening discussions.
Funding Open access funding provided by Università degli Studi di Roma La Sapienza within the CRUI-
CARE Agreement.
Data Availability Data sharing not applicable to this article as no datasets were generated or analysed during
the current study.
Declarations
Conflict of interests The authors have no competing interests to declare that are relevant to the content of
this article.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence,
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regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.
To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
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585–598 (1984)
Communicated by Y. Kawahigashi
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10.1038_s41467-023-37945-4.pdf
|
Data availability
Whole genome bisulfite sequencing data are available at GenBank/
NCBI under accession number GSE182212. RNA-sequencing data are
available at GenBank / NCBI under accession number PRJNA780766. All
other data are available as Source Data files as part of this publica-
tion. Source data are provided with this paper.
|
Data availability Whole genome bisulfite sequencing data are available at GenBank/ NCBI under accession number GSE182212 . RNA-sequencing data are available at GenBank / NCBI under accession number PRJNA780766 . All other data are available as Source Data files as part of this publication. Source data are provided with this paper. Code availability All custom code used for analyses is deposited in the following GitHub repository: https://github.com/schmitzlab/Methylome-of-clonal-ant_ Obir-v5.4.git .
|
Article
https://doi.org/10.1038/s41467-023-37945-4
DNMT1 mutant ants develop normally but
have disrupted oogenesis
Received: 24 November 2021
Accepted: 6 April 2023
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Iryna Ivasyk 1
Marie Droual1, Hosung Jang3, Robert J. Schmitz
, Leonora Olivos-Cisneros1, Stephany Valdés-Rodríguez1,2,
3 & Daniel J. C. Kronauer
1,2
Although DNA methylation is an important gene regulatory mechanism in
mammals, its function in arthropods remains poorly understood. Studies in
eusocial insects have argued for its role in caste development by regulating
gene expression and splicing. However, such findings are not always con-
sistent across studies, and have therefore remained controversial. Here we use
CRISPR/Cas9 to mutate the maintenance DNA methyltransferase DNMT1 in the
clonal raider ant, Ooceraea biroi. Mutants have greatly reduced DNA methy-
lation, but no obvious developmental phenotypes, demonstrating that, unlike
mammals, ants can undergo normal development without DNMT1 or DNA
methylation. Additionally, we find no evidence of DNA methylation regulating
caste development. However, mutants are sterile, whereas in wild-type ants,
DNMT1 is localized to the ovaries and maternally provisioned into nascent
oocytes. This supports the idea that DNMT1 plays a crucial but unknown role in
the insect germline.
How epigenetic processes regulate development and behavior is a
major area of research1. Among the most prominent epigenetic
mechanisms is DNA methylation, a covalent modification to cytosine
that gives rise to 5-methylcytosine. This modification is catalyzed by
two types of enzymes, de novo DNA methyltransferases (DNMT3) and
maintenance DNA methyltransferases (DNMT1). While DNMT3 pri-
marily targets previously unmethylated cytosines, DNMT1 mostly
copies DNA methylation during DNA replication2. In most organisms,
the majority of cytosine methylation occurs in a CpG context, i.e., a
cytosine base followed by a guanine3. However, the presence and
amount of CpG methylation is highly variable across organisms4,5.
The function of DNA methylation has mostly been studied in
mammals, where it is primarily targeted to cis-regulatory elements,
transposons, and gene bodies3,6. Promoter and transposon methyla-
tion has important functions in gene regulation and usually acts to
downregulate or silence expression, also in the context of genomic
imprinting7–10. However, across the tree of life, DNA methylation can
play different roles11. Honeybees and ants possess full complements of
the DNA methylation machinery12–15, which has been met with con-
siderable excitement for two main reasons6,16–26. First, the commonly
used invertebrate genetic models yeast, Caenorhabditis elegans, and
Drosophila melanogaster, lack parts of the machinery and, therefore,
CpG methylation, opening the possibility that social insects could
serve to better understand the role of DNA methylation in inverte-
brates, which remains poorly known4. Second, social insects show
extreme forms of developmental and behavioral plasticity, and it has
been argued that DNA methylation might play important roles in caste
development26–29, the regulation of behavioral roles6,29–31, and the
evolution of genomic imprinting as a result of social conflicts32,33.
Several studies have reported correlations between DNA methy-
lation patterns and queen vs. worker castes12,34,35, while others have
attempted experimental manipulations and reported effects on caste
development or behavior either via changes in gene expression or
alternative splicing36–38. While these studies have greatly contributed
to our understanding of DNA methylation in social insects, others have
failed to find consistent correlations between DNA methylation and
reproductive castes, and some of the core functional results have not
been replicated13,29,39. Also, DNA methylation in insects is preferentially
found in the exons of constitutively expressed and evolutionarily
conserved housekeeping genes, which seems to contradict the con-
jecture that it is associated with dynamic gene regulation13,40–42. Finally,
the few functional studies of DNA methylation in social insects have
1Laboratory of Social Evolution and Behavior, The Rockefeller University, New York, NY, USA. 2Howard Hughes Medical Institute, New York, NY, USA.
3Department of Genetics, University of Georgia, Athens, GA, USA.
e-mail: [email protected]; [email protected]
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been limited to pharmacological manipulations or RNAi knockdowns
of DNA methyltransferases36–38, calling for additional studies that
employ more definitive molecular genetics approaches.
Here, we study the role of DNA methylation in the clonal raider
ant, Ooceraea biroi. Unlike honeybees and most ants, O. biroi does not
have queens, and all workers in a colony reproduce asexually and
clonally43,44. Therefore, mutant lines can be established from any
individual with germline transmission of natural or experimentally
induced mutations45. At the same time, there still is phenotypic plas-
ticity along the worker-queen spectrum among workers of this species,
with smaller “regular workers” with two ovarioles and no eyespots, and
larger, more queen-like “intercastes” with four to six ovarioles and
rudimentary eyespots46,47. Using CRISPR-mediated mutagenesis of
DNMT1, we show that ants deficient in DNA methylation still develop
normally and do not show obvious alterations to caste phenotypes.
However, these ants are sterile, adding to recent studies in other
species suggesting that DNMT1 plays an important yet poorly under-
stood role during insect oogenesis48–52.
Results and discussion
DNMT1 copies DNA methylation patterns during DNA replication2, and
enzyme loss-of-function should result in genome-wide loss of CpG
methylation. The O. biroi DNMT1 gene (NCBI GenBank, Gene ID:
105286975) is composed of 16 exons (Fig. 1A), and RNA sequencing
(RNA-seq) data show that the short first exon is alternatively spliced
(Supplementary Fig. 1). We therefore designed two CRISPR guide
RNAs, one to induce frameshift mutations in the second exon
(DNMT1g1), and one to target exon 11 of DNMT1, just upstream of a
conserved residue in the catalytic domain that is essential for enzyme
function in mice (DNMT1g2) (Fig. 1A)53.
Early frameshifts in DNMT1 do not lead to loss of function
We first injected 5152 freshly laid eggs with Cas9 enzyme and the
DNMT1g1 guide45, and recovered two unique stable mutant lines in
which both alleles were frameshifted (Supplementary Fig. 1A). Unex-
pectedly, low-coverage whole-genome sequencing of bisulfite-treated
DNA (WGBS) showed that DNA methylation levels in these mutants
were indistinguishable from wild-type ants (Supplementary Fig. 1B),
implying that the DNA methylation function of DNMT1 was not affec-
ted. Both mutant strains showed no obvious phenotypic abnormal-
ities, produced viable eggs at normal rates (Supplementary Fig. 1C) and
had normal lifespans (Supplementary Fig. 1D). Whole-body RNA-seq of
individuals from one of the two lines (−7bp/−8bp) and matched wild-
type controls found no differences in DNMT1 expression. Additionally,
even though the genomic frameshift mutations were reflected in RNA-
seq reads, we did not observe any major alternative splicing events in
mutated DNMT1 (Supplementary Fig. 1F). Taken together, this suggests
that gene function can be rescued when DNMT1 is frameshifted in early
exons. Even though we do not know the exact rescue mechanism in
this case, gene rescue is a known problem in functional genetic studies,
and can occur in a number of ways, including translation reinitiation or
undetected alternative splicing/exon skipping54,55. These mutants
nevertheless provide a useful control in the context of the current
study, because they demonstrate that we can mutate the DNMT1 gene
with CRISPR/Cas9 reagents without causing adverse fitness effects or
other non-specific phenotypes if the mutation does not measurably
compromise the function of the DNMT1 enzyme.
DNMT1 loss-of-function mutants have reduced DNA methylation
Across two experiments, we then injected 5643 young eggs with Cas9
enzyme and the DNMT1g2 guide to generate 33 G0 adults. Because G0
adults have not inherited mutations via the germline, they can be
genetic mosaics. However, Sanger sequencing and subsequent ana-
lyses (see below and Supplementary Table 1) showed no evidence of
wild-type alleles in seven G0 females and one G0 male, whereas one G0
female had both mutant and wild-type alleles. We were not able to
obtain genotypes for three of the 33 G0s, but one of them was con-
firmed later as a loss-of-function mutant via immunohistochemistry
and WGBS (see below). Because we could not establish stable lines of
DNMT1g2 mutants (see below), this female and the seven females
without detected wild-type alleles were used in all subsequent
experiments. Individual details for these eight animals are given in
Supplementary Table 1, and we confirmed several mutant alleles with
small insertions and deletions at the target site via Sanger sequencing
(Fig. 1B). The remaining G0 adults were wild-type females and served
as matched experimental controls.
To assess the effect of these mutations on DNA methylation, we
extracted DNA from whole bodies (minus ovaries and brains) of four
mutants (Supplementary Table 1, individuals #5-8) and four wild-type
controls and conducted low-coverage WGBS. The mutants had greatly
reduced average genome-wide DNA methylation levels (0.26%; 95% CI:
0.21–0.31%) compared to the wild-types (1.02%; 95% CI: 0.90–1.24%)
(Fig. 1C). We then conducted high-coverage WGBS for two of these
mutants and two of the wild-types, achieving 14X coverage of the
genome on average and greater than 99% sodium bisulfite conversion
rates (Supplementary Table 2). The striking reduction in DNA methy-
lation was constant across all genomic features, regardless of their
location in relation to genes, repeats or transposons (Fig. 1D–F, Sup-
plementary Fig. 2). As expected, DNMT1 loss of function thus results in
a substantial reduction in DNA methylation. These data are consistent
with the expected function of DNMT1. The remaining low methylation
levels in DNMT1g2 mutants could be due to a number of possible
mechanisms, including the enzymatic activity of DNMT3 or signal
stemming from cells that did not divide after injection of Cas9. No
wild-type reads were detected at the DNMT1 target site in the G0
DNMT1g2 mutants with high-coverage WGBS data, providing addi-
tional evidence that mosaicism in G0s is low (Supplementary Table 3).
suggests
DNMT1 loss-of-function mutants are not more queen-like
In mammals, DNA methylation is vital to multiple aspects of devel-
opment, including genomic imprinting56. Accordingly, experiments
that interfere with DNMT function result in DNA replication arrest57,58
and embryo lethality59,60. This contrasts with our findings in O. biroi,
where both males and females (Fig. 2A) complete development
despite DNMT1 loss-of-function and significantly reduced DNA
the roles of DNMT1 and
that
methylation. This
DNA methylation during development differ fundamentally between
mammals and insects. However, the effects of DNA methylation on
insect development could be more subtle, and previous work
on ants36 and honeybees37 suggested that reduced DNA methylation
modulates caste development and results in larger adults. In contrast
to these findings, morphometric analyses of DNMT1 loss-of-function
mutants showed that their morphology is similar to wild-type ants
that were reared under identical conditions, and that their overall
body size is possibly smaller on average (Fig. 2B, Supplementary
Fig. 3). This finding contradicts the notion that DNA methylation
increases in response to a reduced diet or other environmental fac-
tors during development27 and then mediates molecular changes
that result in minor worker rather than major worker development in
ants36, or worker rather than queen development in honeybees37.
However, we currently cannot rule out the alternative possibility that
the role of DNMT1 and DNA methylation in caste development differs
between the clonal raider ant, which lacks extreme caste poly-
morphism, and other eusocial Hymenoptera.
DNMT1 loss-of-function mutants are sterile
To assess the effect of DNMT1 loss-of-function on survival and fer-
tility, we monitored a cohort of eight G0 adults from eclosion
onward. By 60 days, we had collected four of the G0s shortly after
they had died, two had likely died but carcasses could not be found
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(the ants sometimes dismember dead nestmates), and two remained
alive. Sanger sequencing of DNA from whole body extracts showed
that the four individuals that had died were DNMT1 mutants with no
evidence of wild-type alleles (Supplementary Table 1, #1-4), while the
two surviving individuals were wild type with no evidence of mutant
alleles. The survival of DNMT1 mutants was significantly decreased
relative to wild-type controls (Fig. 2C). To put this finding into con-
text, median survival of wild-type O. biroi under similar rearing
conditions was 258 days, with a minimum survival of 106 days among
21 individuals (Supplementary Fig. 1D). This shows that loss-of-
function mutations in DNMT1 severely compromise longevity. During
the 60 days of this experiment, we collected 19 G1 eggs produced
by the G0 adults and genotyped them at the DNMT1 target locus. All
these eggs were wild type. This is in stark contrast to the genotypes
of G0 adults (Fig. 2D, Supplementary Table 1 #1-4) and implies that
DNMT1 loss-of-function mutations result in sterility. That sterility is
caused by the injection of CRISPR/Cas9 reagents or DNMT1 muta-
tions per se is unlikely, because G0 adults mutated with the DNMT1g1
guide RNA did not show this phenotype. This opens the possibility
that the decreased lifespan of DNMT1 mutants is not a direct
Fig. 1 | Mutations in the DNMT1 catalytic domain result in decreased DNA
methylation. A Clonal raider ant DNMT1 protein model. Vertical solid lines indicate
exon boundaries; broken lines indicate CRISPR/Cas9 target cut sites. The DNMT1g1
(blue) target site is early in the second exon, while the DNMT1g2 (red) target site is
in the catalytic domain (yellow), upstream of an essential cysteine residue (black
square). B Top: Wild-type (WT) DNMT1 sequence at the DNMT1g2 target site, with
the codon for the essential residue (ER) and the protospacer adjacent motif (PAM)
in bold. Guide-RNA (gRNA) sequence underlined. Red arrowhead indicates pre-
dicted cut site. Bottom: Mutant alleles observed in G0 adults show insertions (red),
deletions (gray) and base changes (blue). C DNMT1g2 mutants have decreased
genome-wide methylation compared to wild-type ants reared in parallel, consistent
with a functional defect in DNMT1 (two sided unpaired T-test: p < 0.0001; n = 4
animals per condition). Error bars show standard deviation around the mean. Each
data point represents low coverage WGBS of DNA extracted from the remaining
tissue of a single ant after brain and ovary dissection. Data point labels correspond
to animals #5-8 in Supplementary Table 1. Methylation levels are corrected for
bisulfite non-conversion. D Percentages of methylated cytosines for different
genomic features based on high coverage WGBS of two biological replicates of wild
types (black) and DNMT1g2 mutants (red) from (C) (bars denote means). E DNA
methylation patterns in wild types (black) and DNMT1g2 mutants (red) for genes
and TEs/repeats. The 1 kb upstream and downstream regions along with the gene
body region (black bar) are displayed. F Gene body methylation profile for different
positions of exons and introns across the O. biroi genome. Exons and introns are
shown separately for wild types (black) and DNMT1g2 mutants (red). Source data
are provided as a Source Data file.
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Fig. 2 | DNMT1 mutants develop normally, but have impaired survival and
reproduction. A Images of DNMT1g2 mutant and wild-type animals. DNMT1g2
mutants complete development and are grossly indistinguishable from wild types
in external anatomy despite lacking the functional DNMT1 enzyme (scale bar = 1
mm). B Mutants appear smaller in total body size (one-way ANOVA p = 0.0248; n = 4
mutant animals, 17 control animals, and 14 wild-type animals that were injected at
the egg stage along with the mutants). Body size was calculated as the sum of the
head, thorax, petiole, post-petiole and gaster lengths (Supplementary Fig. 3).
Horizontal bars show pairwise comparisons using Tukey’s multiple comparisons
test. In injected wild-type ants, CRISPR injections at the egg stage failed to produce
mutations in DNMT1. Labels of individual data points correspond to animals #5-8 in
Supplementary Table 1. C DNMT1g2 mutants show survival deficits relative to wild
types (log-rank test: p = 0.049). Sample sizes are given in parentheses. D Eggs laid
by all G0 adults were collected and sequenced, ratios of wild-type to mutant eggs
compared to ratios of wild-type to G0 adults are shown. Numbers indicate sample
sizes. All sequenced eggs were wild type, while only two of the six G0 adults carried
wild-type alleles (two-sided Fisher’s exact test: p = 0.0012). Source data are pro-
vided as a Source Data file.
consequence of reduced DNA methylation levels, but rather a result
of the compromised reproductive system, for example by disrupting
endocrine functions.
still produce some form of functional DNMT1 protein, highlighting
the importance of independently assessing the effects of targeted
mutagenesis on gene function.
To better understand the putative role of DNMT1 in reproduc-
tion, we characterized DNMT1 mRNA and protein in the ant ovary.
Each of the two ovaries of an O. biroi ant is composed of one to three
ovarioles (two to six ovarioles per ant), which can be subdivided into
a vitellarium, germarium and the terminal filament (Fig. 3A), similar
to ovarioles of other insects61. In the ovarioles, each oocyte originates
in the germarium, and travels to the vitellarium, where it is sur-
rounded by follicular cells and an adjacent bundle of nurse cells
(Fig. 3A). Nurse cells are derived from the same progenitor cell as the
associated oocyte and are therefore of germline origin. Using mRNA
fluorescence in situ hybridization (FISH), we detected DNMT1 mRNA
in the germarium, as well as the nurse cells and oocytes within the
vitellarium (Fig. 3B, Supplementary Fig. 4). The broad expression of
DNMT1 in O. biroi ovaries is consistent with qPCR data from Sole-
nopsis invicta fire ants, showing that DNMT1 is expressed in ovaries62.
We then stained ovaries from wild-type and mutant O. biroi using a
commercial antibody against the conserved catalytic domain of
mammalian DNMT1. Consistent with the mRNA FISH pattern, we
observed DNMT1 protein within the nuclei of cells in the germarium,
nurse cells, follicular cells and oocytes (Fig. 3C, Supplementary
Fig. 4). Positive and specific staining was apparent in wild-type ants
and DNMT1g1 mutants, but not in the ovaries of three DNMT1g2
mutants (Fig. 3C, Supplementary Fig. 5). The positive staining in
DNMT1g1 mutants confirms that, despite the early frameshifts, they
The lack of staining in DNMT1g2 mutants (Supplementary
Table 1, individuals #5-8), on the other hand, validates the specificity
of the antibody, and confirms that mutations in exon 11 indeed result
in gene knockout. Furthermore, all mutant ovaries revealed young
follicles (Fig. 3C, Supplementary Fig. 5), suggesting that DNMT1
exerts its crucial function after the initiation of oogenesis. However,
even though sample sizes are limited, we never observed fully
formed oocytes in mutant ovaries, consistent with the absence of
mutant G1 eggs. Given that meiosis in O. biroi is completed only after
oviposition44, this suggests that oocyte maturation and meiosis
cannot progress without DNMT1 function. The finding that DNMT1 is
required for oogenesis in O. biroi is consistent with recent findings in
the milkweed bug Oncopeltus fasciatus49,52,63 and the red flour beetle
Tribolium castaneum50, where maternal RNAi knockdown of DNMT1
leads to sterility or early developmental arrest. Interestingly, this
opens the possibility of a DNMT1 function independent of DNA
methylation26,50.
Based on these findings, we asked whether DNMT1 is present
during early oogenesis. Indeed, we observed DNMT1 protein in
oocytes within the ovary at multiple stages of development. In very
early oocytes, which are several weeks from maturing, DNMT1 is
confined to the nucleus (Fig. 3D,
top). As oocytes mature,
DNMT1 staining also becomes apparent as large puncta at the per-
iphery of the egg that do not appear to be associated with DNA
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B
DNA (DAPI)
DNMT1 mRNA FISH
Composite
Article
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Wild-type
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https://doi.org/10.1038/s41467-023-37945-4
Young Oocyte
Old Oocyte
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Fig. 3 | DNMT is maternally provisioned into the oocyte. A DAPI stain (black) of
DNA showing the anatomy of a clonal raider ant ovary with three ovarioles. Oocytes
arise from stem cells in the germarium and travel to the vitellarium as they develop.
Oocytes are surrounded by supporting follicular cells and adjacent nurse cells
(scale bar = 100 μm). B mRNA FISH of DNMT1 in clonal raider ant ovaries. DNMT1
staining is observed in nurse cells, germarium cells, and oocytes (scale bar = 100
μm). C Protein immunohistochemistry (IHC) of DNMT1 in the ovary of DNMT1
mutants (left, center) and a control wild-type ant (right). DNMT1 staining is visible in
the germarium, nurse cells, and follicular cells of wild-type and DNMT1g1 mutant
ovaries. No staining is apparent in the DNMT1g2 mutant, corresponding to
individual #7 in Supplementary Table 1 (scale bars = 100 μm). Boxes show magni-
fications of the germaria (scale bars = 25 μm). Note that the wild-type ovary shown
here happens to be inactive, while the DNMT1g1 mutant ovary is active, containing
large oocytes. See (B) and Supplementary Fig. 4 for comparable images of active
wild-type ovaries. D DNMT1 antibody staining in young (left) and old (right)
oocytes. DNMT1 is present in young oocytes and appears limited to the nucleus
(arrowhead). In old oocytes, DNMT1 localizes in distinct clusters in the periphery of
the oocyte, with large amounts of protein co-localizing with DNA (arrowhead).
Orientation of oocytes is the same as in (A) (scale bars = 25 μm).
(Fig. 3D, bottom). Together, these experiments show that both DNMT1
protein and mRNA are maternally provisioned into the oocyte at an
early stage and support the idea that maternal DNMT1 plays a crucial
role in oocyte maturation.
While we cannot rule out the possibility that DNA methylation
directly regulates gene expression and alternative splicing in some
contexts, our finding that O. biroi workers and males develop normally
without a functional DNMT1 gene and with greatly reduced DNA
methylation levels shows that, unlike in mammals, this mechanism is
not fundamentally required during ant embryogenesis. Instead, the
data presented here align with recent evidence that DNMT1 plays a
conserved and crucial, yet poorly understood role in insect
reproduction48–52. Importantly, this role might be independent of DNA
methylation50,63. DNA methylation-independent functions of DNMT1
have in fact been observed outside of insects. In the African clawed
frog Xenopus laevis, for example, DNMT1 acts as a direct transcription
repressor protein to prevent premature activation of gene expression
before the mid-blastula stage64. Such DNA methylation-independent
functions of DNMT1 could help explain why the enzyme is highly
conserved over evolutionary time4,65, and why it has been retained
even in some insects that do not methylate their DNA50. Given its
potentially broad relevance for reproductive health across the animal
tree of life, future work should aim to better understand the role of
DNMT1 during oogenesis.
The fact that DNMT1 loss-of-function mutants in the clonal rai-
der ant are sterile and therefore cannot be propagated led to lim-
itations of the current study. First, with only few mutant individuals
available, we had to work with small sample sizes and had to restrict
the scope of our experiments. Second, we had to work with G0
individuals, i.e., individuals that were mutagenized at the egg stage
and therefore might have been mosaics of mutant alleles, together
with wild-type alleles at low frequencies. These limitations could be
overcome in the future, e.g. by working with genetically modified
strains in which loss of DNMT1 function is inducible. Finally, while we
focused our efforts on DNMT1, it remains possible that DNMT3 is
involved in regulating caste development via de novo DNA methy-
lation at specific target sites. Studies targeting DNMT3 for loss-of-
function could help resolve this question.
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Methods
Mutagenesis target selection and amplification
The DNMT1 (LOC105286975) gene locus was located in the published
O. biroi genome (GenBank assembly accession: GCA_003672135.1)44.
Two alternate splice variants (X1: XM_011352329, X2: XM_011352333)
were identified, and aligned with MAFFT66, verifying gRNA sequence
presence in both splice variants. Primers were designed to amplify
~200–300 bp fragments surrounding each target cut site (Supple-
mentary Table 4).
Genotyping and mutant identification
Two O. biroi clonal lines, Line A and Line B43 were used for this study.
All experimental individuals belonged to Line B, and Line A individuals
were used as chaperones to rear eggs or larvae. When necessary,
genotyping to distinguish between Line A and B was carried out using a
restriction enzyme assay of PCR amplicons of the mitochondrial
cytochrome oxidase subunit 1 (CO1) gene45. Mutants were identified
via Sanger sequencing of PCR amplified DNA fragments containing
either the DNMT1g1 or DNMT1g2 target locus (Supplementary Table 4)
using a previously described protocol45. Sanger sequencing was out-
sourced to the company Psomagen.
gRNA design and reagent validation
Guide RNAs (gRNAs) were designed using the CRISPR Design Tool
(Synthego). For each target, four custom gRNAs were purchased from
the company Synthego and tested using an in vitro incubation assay
with Cas9 enzyme using PCR amplified and purified DNA from the
target region. Reactions were incubated for 4 h at 37 °C, and products
were visualized on 2% agarose gels45. All gRNAs successfully digested
target DNA, and a single gRNA was selected for each gene target for
further experiments (Supplementary Table 5).
CRISPR/Cas9 reagent preparation, mutagenesis and animal
rearing
Reagent preparation, egg collection, injection, incubation and animal
rearing followed a previously published protocol for this species45. The
CRISPR/Cas9 injection reagent mix included 100 ng/µl Cas9 (PNABio)
and 100 ng/µl of the selected gRNA (Synthego) (Supplementary
Table 5).
DNMT1g1 CRISPR/Cas9
DNMT1g1 mutant generation and line propagation. 5152 Line B eggs
were injected. From those, 76 larvae hatched, and 65 of these G0 larvae
were fostered. All G0 adults were pooled in a single colony, which was
supplemented with adults from clonal Line A. Eggs from this colony
were collected weekly or biweekly and fostered into nests containing
20 Line A chaperones, where they were reared to adulthood. The
eclosing callows were individually paint marked45 and pooled into six
units of 16 ants each. All eggs produced by these units were collected
weekly or biweekly and fostered with Line A chaperones to propagate
the lines. Mutants resulting from these units were identified via Sanger
sequencing of the target region and pooled to generate pure colonies
for each unique mutant genotype.
DNMT1g1 mutant reproduction and survival. From the first four
paint-marked G1 units, eggs were collected biweekly for 2.5 weeks and
incubated for 48 h, before they were frozen dry on a glass slide at
−80 °C for later DNA extraction and genotyping45. All eggs produced
after this were removed weekly and fostered with Line A chaperones as
described above. All carcasses were removed from the units and frozen
dry in a PCR tube at −80 °C until no ants remained. Some carcasses
could not be recovered, probably because ants in the colony had dis-
membered them. For the analysis of reproductive output, only adults
(Line A and Line B) and the first 25 eggs (Line A and Line B) sequenced
from each unit where mutant adults were observed were included in
the analysis (n = 3 units). For analysis of survival, DNA was extracted
from all frozen adult carcasses for genotyping, and all data from Line A
individuals were discarded. Statistical analyses were conducted in
GraphPad Prism (v 9.1.1).
DNMT1g2 CRISPR/Cas9
DNMT1g2 mutant generation, reproduction and survival. We injec-
ted 2416 Line B eggs, of which 45 hatched. 39 larvae were fostered to
yield 9 G0 adults that survived beyond three days after eclosion. One
of these adults was a male and was excluded from further analyses.
All DNMT1g2 G0 animals from this experiment were placed in a
single unit, along with paint-marked Line A chaperone ants to ensure
colony stability. All eggs and carcasses were removed from the col-
ony weekly or biweekly and frozen dry at −80 °C on a glass slide or in
a PCR tube, respectively. Four carcasses and 24 eggs were collected,
and DNA for Sanger sequencing was extracted from whole carcasses
and eggs. Additionally, at 61 days, the two remaining G0 ants were
sacrificed, ovaries were removed, and DNA was extracted from the
remaining tissue. Of the collected eggs, 19 were Line B wild type, four
were Line A, and one could not be successfully genotyped and was
removed from analyses. No mutant eggs were observed in this
experiment. Therefore, unlike for DNMT1g1, we were unable to
propagate the mutant lines for DNMT1g2, and all experiments were
thus limited to G0 adults. GraphPad Prism (v 9.1.1) was used for
statistical analyses.
DNMT1g2 mutant generation, morphometrics, immunohistochem-
istry and whole genome bisulfite sequencing. We injected 3227 eggs
in a separate experiment, of which 109 hatched. 92 larvae were fos-
tered to yield four colonies of G0 adults. A single leg was removed
from each adult, and DNA was extracted for Sanger sequencing. One of
these colonies was excluded from analyses because all six G0s were
wild-type. The remaining three colonies generated 18 G0 adults, of
which four carried only mutant alleles and 14 carried only wild-type
alleles. In parallel, 574 eggs were incubated without injection and
reared under identical conditions to yield 17 control adults.
At ~1 week of age, all G0 animals (injected and uninjected) were
immobilized in a standardized position (Fig. 2A) under acrylic and
imaged using a brightfield microscope (Leica MSV266) for morpho-
metric measurements. A ruler was imaged under identical conditions
for accurate calibration. Morphometric measurements of individual
body segments were taken using Fiji67. All mutants and a subset of wild
types were dissected, and their ovaries and brains fixed for immuno-
histochemistry (next section). After the ovaries and head had been
removed, the rest of the body was used for DNA extraction and WGBS
(see below).
Fluorescence in situ hybridization
Tissue was fixed, prepared and processed according to a previously
published protocol68. Briefly, to prepare the tissue, ovaries were dis-
sected and fixed in cold 4% paraformaldehyde in PBS for at least 2 h at
4 °C, following an EtOH dehydration and incubation overnight. Tissue
was rehydrated and incubated in 5% acetic acid for 5 min, followed by
another fixation using 2% paraformaldehyde for 1 h at 25 °C. The tissue
was washed with 0.5% Tween 20 in PBS and 1% NaBH4. Prepared tissue
was rinsed before incubation with the pre-hybridization solution for
30 min at 45 °C, and was then incubated with 2pmol of the probes in
probe hybridization solution for 48 h. Signal was amplified with
30 pmol snap cooled hairpin solution at room temperature. A set of
30 custom probes were designed for DNMT1 and purchased from
Molecular Instruments Inc. The probes used a B2 HCR amplifier and
were labeled with Alexa Fluor 546. Images were captured with an
inverted LSM 780 laser scanning confocal microscope (Zeiss). Images
were processed in parallel with Fiji67 and are shown as maximum
projection z-stacks.
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Immunohistochemistry
Tissue was dissected in cold 1xPBS and fixed in 4%PFA for 2 h. Fol-
lowing washes in 1xPBS, samples were blocked using a solution con-
taining 5% normal goat serum in PBSTx for 1 h. The blocking solution
was replaced with a primary antibody solution containing 1:100
DNMT1 antibody (Abcam ab188453), and incubated overnight at room
temperature. Negative controls without primary antibody were incu-
bated overnight in the blocking solution at room temperature in par-
allel. All samples were washed and incubated in the secondary solution
including 1:500 secondary antibody (Donkey anti-rabbit, AlexaFluor
594, Invitrogen A21207) and 1:500 DAPI for 2 h prior to mounting in
DAKO fluorescence mounting medium. Images were captured and
processed as described above.
Whole genome bisulfite sequencing
DNA extraction, library construction and sequencing. Genomic
DNA was extracted from four mutant and four wild-type replicates of
single whole ants (DNMT1g1) or the remaining tissue after brain and
ovary dissection (DNMT1g2) using the QIAamp DNA Micro Kit (QIA-
GEN). While we were able to establish stable lines for DNMT1g1
mutants and thus extract DNA from whole animals, our sample sizes
for DNMT1g2 mutants were necessarily limited, and we therefore
extracted DNA after brains and ovaries had been removed for other
analyses. In each case, all replicates were sequenced to evaluate
global methylation changes (Fig. 1C, Supplementary Fig. 1B, 2).
Additionally, two of the DNMT1g2 mutant and matched wild-type
replicates were selected for more extensive analysis of methylation
changes (Fig. 1D–F) using high coverage sequencing (Supplementary
Table 2).
MethylC-seq libraries were constructed using the MethylC-seq
protocol69. Briefly, genomic DNA was sonicated to around 200 bp
using a Covaris S-series focused ultrasonicator, and end-repaired with
an End-It DNA end-repair kit (Epicentre). The end-repaired DNA was
subjected to A-tailing using Klenow 3′–5′ exo − (NEB) and ligated to
methylated adapters using T4 DNA ligase (NEB). The ligated DNA was
treated with sodium bisulfite reagent using the EZ DNA methylation-
Gold kit and amplified using KAPA HiFi uracil + Readymix Polymerase.
Sequencing was performed on an Illumina NextSeq500 instrument.
Methylome mapping. The MethylC-seq data from the Georgia
Genomics & Bioinformatics Core were processed with the “paired-
end-pipeline” function in Methylpy70. Reads passing quality control
filters were aligned to the O. biroi v5.4 reference genome71 using
bowtie 2.2.4 (Langmead and Salzberg, 2012), and the uniquely
aligned and nonclonal reads were retained. Unmethylated lambda
phage DNA was used as a control to calculate the sodium bisulfite
conversion rate of unmethylated cytosines. A binomial test was used
to determine the methylation status of cytosines with a minimum
coverage of five reads.
Analysis of DNA methylation. Protein coding genes (n = 11,868) and
TEs/repeats over 100 bp in size (n = 53,158) were used for the plots.
For the metaplots in Fig. 1E, genes or TEs/repeats along with 1 kb
upstream/downstream regions were divided into 20 bins, and the
average methylation level of each bin was displayed. For Fig. 1F,
exons, introns, and 1 kb upstream/downstream regions were divided
into 20 bins, and the average methylation level of each bin was
plotted. For the genome-wide methylation analysis shown in Sup-
plementary Fig. 2, the average percentage of CG methylation in each
50 Kb window was calculated and plotted onto a chromosomal map
of the O. biroi genome. For the read level analysis in Supplementary
Table 3, the output BAM files generated by Methylpy70 were used to
distinguish wild-type from mutant reads. Each BAM file was visua-
lized with Samtools tview72, and mapped mutant and wild-type reads
were counted for each sample.
RNA sequencing
RNA extraction, library construction and sequencing. RNA was
extracted and eluted in 30 µl RNAse free water from whole ants dry
frozen at −80 °C using the RNeasy Mini Kit (QIAGEN Cat. No. 74104).
1 ng of total RNA was used to generate full length cDNA using Clon-
tech’s SMART-Seq v4 Ultra Low Input RNA Kit (Cat # 634888). 1 ng of
cDNA was then used to prepare libraries using the Illumina Nextera XT
DNA sample preparation kit (Cat # FC-131-1024). Libraries with unique
barcodes were pooled at equal molar ratios and sequenced on an
Illumina NextSeq 500 sequencer to generate 150 bp paired-end reads,
following the manufacturer’s protocol (Cat # 15048776 Rev.E).
Analysis of RNA sequencing data. Sequencing reads were trimmed
using Trimmomatic73 Nextera PE adapters. Trimmed read quality was
verified using FastQC, and aligned using STAR74 to the O. biroi v5.4
genome and converted to BAM files with Samtools72. BAM files were
loaded into Integrative Genomics Viewer75 to visualize read alignments
at the mutation site and alternative splicing of the DNMT1 gene. Gene
counts were determined with HTseq76. Normalization and differential
gene expression analysis were carried out in R using DEseq277.
Statistics and reproducibility
All microscopy images in this paper are representative of multiple
examined specimens. The gross anatomy of the O. biroi ovary shown in
Fig. 3A was observed in 12 samples. The FISH staining shown in Fig. 3B
and Supplementary Fig. 4A was consistent across 8 experimental ani-
mals and 4 negative controls. The DNMT1 antibody staining shown in
Fig. 3C and Supplementary Fig. 4B was consistently observed in 14
experimental animals and 9 negative controls. The DNMT1 antibody
staining of DNMT1g1 mutants shown in Fig. 3C was replicated in 6
animals, including both mutant genotypes. DNMT1 antibody staining is
shown for 3 different DNMT1g2 mutants in Fig. 3C and Supplementary
Fig. 5A, B. The DNMT1 antibody staining pattern in oocytes shown in
Fig. 3D was consistent across 13 young and 10 old oocytes from mul-
tiple animals. Statistical analyses for other experiments are described
in the respective Methods and main text sections, as well as in the
figure legends.
Reporting summary
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
Whole genome bisulfite sequencing data are available at GenBank/
NCBI under accession number GSE182212. RNA-sequencing data are
available at GenBank / NCBI under accession number PRJNA780766. All
other data are available as Source Data files as part of this publica-
tion. Source data are provided with this paper.
Code availability
All custom code used for analyses is deposited in the following GitHub
repository: https://github.com/schmitzlab/Methylome-of-clonal-ant_
Obir-v5.4.git.
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Acknowledgements
The authors thank Waring Trible and Taylor Hart for advice on egg
injections and the CRISPR/Cas9 protocol, and Rebecca Timson and
Vikram Chandra for advice on immunohistochemistry and bioinfor-
matics analyses, respectively. RNA library preparation and sequencing
was conducted at the Rockefeller University Genomics Resource
Center. Confocal microscopy was conducted at the Rockefeller Uni-
versity Bio-Imaging Resource Center. Research reported in this pub-
lication was supported by the National Institute of General Medical
Sciences of the National Institutes of Health under award no.
R35GM127007 to D.J.C.K. The content is solely the responsibility of
the authors and does not necessarily represent the official views
of the National Institutes of Health. This work was also supported
by a Faculty Scholar Award from the Howard Hughes Medical
Institute to D.J.C.K. I.I. was supported by a Medical Scientist Training
Program grant from the National Institute of General Medical
Sciences of the National Institutes of Health under award no.
T32GM007739 to the Weill Cornell/Rockefeller/Sloan Kettering Tri-
Institutional MD-PhD Program. H.J. and R.J.S. were supported by the
National Science Foundation under award no. MCB-1856143. D.J.C.K. is
an investigator of the Howard Hughes Medical Institute. This is Clonal
Raider Ant Project paper #21.
Author contributions
I.I. and D.J.C.K. conceived and designed the research. I.I., L.O.C., S.V.R.,
and M.D. performed CRISPR experiments. I.I. and L.O.C. performed
histology and microscopy experiments. H.J. and R.J.S. performed whole
genome bisulfite sequencing and data analysis. I.I. performed all other
experiments. I.I. and D.J.C.K. wrote the paper, with feedback from
L.O.C., H.J., and R.J.S. All authors approved the final version. D.J.C.K.
supervised the project.
Competing interests
The authors declare no competing interests.
Additional information
Supplementary information The online version contains
supplementary material available at
https://doi.org/10.1038/s41467-023-37945-4.
Correspondence and requests for materials should be addressed to
Iryna Ivasyk or Daniel J. C. Kronauer.
Peer review information Nature Communications thanks the anon-
ymous reviewers for their contribution to the peer review of this work.
Reprints and permissions information is available at
http://www.nature.com/reprints
mer for Illumina sequence data. Bioinformatics 30,
2114–2120 (2014).
Publisher’s note Springer Nature remains neutral with regard to jur-
isdictional claims in published maps and institutional affiliations.
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(2023) 14:2201
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Article
https://doi.org/10.1038/s41467-023-37945-4
Open Access This article is licensed under a Creative Commons
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© The Author(s) 2023
Nature Communications |
(2023) 14:2201
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| null |
10.1186_s12870-023-04147-5.pdf
|
Availability of data and materials
The datasets generated and/or analyzed during the current study are available
in the NCBI repository, [https:// www. ncbi. nlm. nih. gov/ biopr oject/ 906276]
[Accession number: PRJNA906276].
|
Availability of data and materials The datasets generated and/or analyzed during the current study are available in the NCBI repository, [ https:// www. ncbi. nlm. nih. gov/ biopr oject/ 906276 ] [Accession number: PRJNA906276].
|
Niu et al. BMC Plant Biology (2023) 23:179
https://doi.org/10.1186/s12870-023-04147-5
RESEARCH
BMC Plant Biology
Open Access
Lint percentage and boll weight QTLs
in three excellent upland cotton (Gossypium
hirsutum): ZR014121, CCRI60, and EZ60
Hao Niu1, Meng Kuang1, Longyu Huang1, Haihong Shang1,2*, Youlu Yuan1* and Qun Ge1*
Abstract
Background Upland cotton (Gossypium hirsutum L.) is the most economically important species in the cotton genus
(Gossypium spp.). Enhancing the cotton yield is a major goal in cotton breeding programs. Lint percentage (LP) and
boll weight (BW) are the two most important components of cotton lint yield. The identification of stable and effec-
tive quantitative trait loci (QTLs) will aid the molecular breeding of cotton cultivars with high yield.
Results Genotyping by target sequencing (GBTS) and genome-wide association study (GWAS) with 3VmrMLM were
used to identify LP and BW related QTLs from two recombinant inbred line (RIL) populations derived from high lint
yield and fiber quality lines (ZR014121, CCRI60 and EZ60). The average call rate of a single locus was 94.35%, and the
average call rate of an individual was 92.10% in GBTS. A total of 100 QTLs were identified; 22 of them were overlap-
ping with the reported QTLs, and 78 were novel QTLs. Of the 100 QTLs, 51 QTLs were for LP, and they explained
0.29–9.96% of the phenotypic variation; 49 QTLs were for BW, and they explained 0.41–6.31% of the phenotypic
variation. One QTL (qBW-E-A10-1, qBW-C-A10-1) was identified in both populations. Six key QTLs were identified in
multiple-environments; three were for LP, and three were for BW. A total of 108 candidate genes were identified in the
regions of the six key QTLs. Several candidate genes were positively related to the developments of LP and BW, such
as genes involved in gene transcription, protein synthesis, calcium signaling, carbon metabolism, and biosynthesis
of secondary metabolites. Seven major candidate genes were predicted to form a co-expression network. Six signifi-
cantly highly expressed candidate genes of the six QTLs after anthesis were the key genes regulating LP and BW and
affecting cotton yield formation.
Conclusions A total of 100 stable QTLs for LP and BW in upland cotton were identified in this study; these QTLs
could be used in cotton molecular breeding programs. Putative candidate genes of the six key QTLs were identified;
this result provided clues for future studies on the mechanisms of LP and BW developments.
Keywords Upland cotton (Gossypium hirsutum L.), Lint percentage, Boll weight, Quantitative trait locus (QTL),
Candidate gene
*Correspondence:
Haihong Shang
[email protected]
Youlu Yuan
[email protected]
Qun Ge
[email protected]
Full list of author information is available at the end of the article
© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or
other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this
licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. The Creative Commons Public Domain Dedication waiver (http:// creat iveco
mmons. org/ publi cdoma in/ zero/1. 0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
Niu et al. BMC Plant Biology (2023) 23:179
Page 2 of 18
Background
Cotton (Gossypium) is an economically important natu-
ral fiber plant. Upland cotton (Gossypium hirsutum) is
the most widely cultivated cotton variety, accounting for
approximately 95% of global cotton production [1, 2].
Increasing the yield of upland cotton remains the main
objective of this important cash crop worldwide. Cotton
yield is typically affected by several complex quantita-
tive traits, including the boll number (BN), lint percent-
age (LP), boll weight (BW), seed index (SI) and lint index
(LI) [3]. These yield component traits are controlled by
genetic factors and are affected by environmental fac-
tors; they are also genetically related to each other [3–5].
LP is an economically important index for cotton culti-
vars with the highest heritability [6]. Because LP is a key
contributor to lint yield and is easy to measure, selection
for increasing LP has become an important approach for
enhancing lint yield [7, 8]. Numerous studies have shown
that cotton yield mainly depends on LP, BW, and BN, and
these traits have been positively selected in cultivated
cotton throughout the domestication process [9–15].
Because cotton breeding requires excellent germplasm,
a large amount of germplasm resources have been pre-
served and improved in China, such as many high LP
cultivars/lines [16–18]. Many interspecific introgressive
lines (ILs) or chromosome segment introgression lines
(CSILs) have been obtained by crosses between G. hirsu-
tum and Gossypium barbadense [19, 20]; some of these
lines have high LP and BW [21]. Many new germplasm
resources and cultivars have been successfully bred [22–
26]. Our lab has also bred a set of advanced cotton lines/
cultivars, such as the parents used in this study.
The identification of stable and effective quantitative
trait loci (QTLs) is prerequisites for cotton molecular
breeding. From 1998 to 2015, a total of 327 QTLs for LP
and 170 QTLs for BW were identified on different chro-
mosomes through meta-QTL analysis [27]. Following the
release of the cotton genome sequence, the number of dis-
covered QTLs is rapidly increasing via genome-wide asso-
ciation study (GWAS) or linkage mapping [28–30]. For
example, structural variations have been explored by rese-
quencing 1,081 G. hirsutum accessions, and 446 structural
variations are significantly associated with seven traits,
including 21 with LP and 17 with BW [31]. Genetic link-
age analysis and association analysis (AS, or GWAS) are
the two major approaches for identifying QTLs in crops.
Many high-density genetic linkage maps and association
maps for cotton have been published. For example, more
than 17 crosses or populations of upland cotton have
been used to construct genetic maps, including crosses
of Yumian1 × T586 [4, 32, 33], Yumian1 × Zhongmian-
suo35 [1], NC05AZ06 × NC11-2091 [34], DH962 × Jim-
ian5 [35–37], Zhongmiansuo12 (ZMS12) × 8891 [4],
43],
[42,
(Simian3 × Sumian12) × (Zhong4133 × 8891)
[3],
Baimian1 × TM-1 [38, 39], Xiangzamian2 [40, 41],
and
HS46 × MARCABUCAG8US-1–88
CCRI35 × Nan Dan Ba Di Da Hua (NH) [44]. One high-
density bin linkage map contains 6,187 bin markers span-
ning 4,478.98 cM with an average distance of 0.72 cM
[18]. Different types of GWAS, including single-locus-
GWAS (SL-GWAS), multi-locus GWAS (ML-GWAS),
and restricted two-stage, multi-locus, and multi-allele
GWAS (RTM-GWAS) approaches, have been used to
identify quantitative trait nucleotides (QTNs) for LP and
BW in several cotton accessions. More than 16 associa-
tion maps and many candidate genes for agronomic traits
have been reported [5, 8, 10, 12, 45–48]. For example, 86
single-nucleotide polymorphism linkage disequilibrium
block (SNPLDB) loci for LP and 70 SNPLDB loci for BW
have been identified from 315 cotton accessions using
RTM-GWAS [12]. A total of 719 upland cotton accessions
have been screened by GWAS using the cottonSNP63K
array, and 62 identified single nucleotide polymorphism
(SNP) loci were significantly associated with different
traits; a total of 689 candidate genes were screened, and
27 of them contain at least one significant SNP, including
three for LP and six for BW [5].
Although the inheritance, QTLs and candidate genes
of LP and BW in upland cotton have been widely stud-
ied, only a few of the studied QTLs have been used in the
molecular breeding of cotton via marker-assisted selec-
tion (MAS) [49, 50]. One of the reasons is that the iden-
tified QTLs are unstable in multiple-environments and
only explain little phenotypic variance. Consequently,
mining stable, effective LP and BW-related QTLs or
QTNs would greatly aid cotton molecular breeding. We
have previously bred the excellent cotton lines ZR014121
and EZ60 and the cultivar CCRI60. Here, we identified
stable, effective LP and BW-related QTLs to aid the uti-
lization of the germplasm resources in cotton breeding.
Results
Phenotypic variation in LP and BW
We evaluated two yield-related traits LP and BW, in the
two recombinant inbred line (RIL) populations under four
environments in 2020 and 2021. The LP and BW ranged
from 32.56% to 48.26% and from 4.09 to 6.93 g in P-EZ60,
respectively (Table 1); LP and BW ranged from 31.57% to
48.02% and from 3.68 to 6.83 g in P-CCRI60, respectively
(Table 2). All of the absolute skewness values of LP and
BW were less than 1.0. The distributions of the LP and
BW in the four experimental environments were normal.
This suggests that LP and BW are polygenic traits, and
the data could be used to map QTLs (Fig. 1). LP and BW
exhibited high degrees of phenotypic variation. The coef-
ficient of variation for each trait was relatively consistent
Niu et al. BMC Plant Biology (2023) 23:179
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Niu et al. BMC Plant Biology (2023) 23:179
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Fig. 1 The histograms of the LP and BW in P-EZ60 (EZ60) and P-CCRI60 (CCRI60) in Anyang and Weixian in 2020 and 2021
among the different environments, suggesting that LP
and BW were significantly affected by the environment,
and the effect on BW (average 7.16 in P-EZ60; 7.55 in
P-CCRI60) was greater than that on LP (average 5.69 in
P-EZ60; 5.51 in P-CCRI60) (Tables 1, and 2).
The correlations between LP and BW of all the RILs
in the four environments were analyzed separately.
Generally, LP and BW were significantly negatively cor-
related in P-EZ60 and P-CCRI60, and the coefficients
ranged from -0.098 to -0.340, which suggested that it
was difficult to improve LP and BW synchronously
(Tables 3, and 4). Because the cotton field was water-
logged in Anyang in 2021, the LP and BW were affected
to some extent, but the phenotypic data met the
requirements for GWAS (Fig. 1). Analysis of variance
(ANOVA) showed that there were highly significant
differences among the accessions and environments for
the two traits of two populations (Table 5). It indicated
that LP and BW were significantly influenced by the
accessions and planting environments.
SNP quality control and in silico mapping
to
According
the high-throughput whole-genome
sequencing data of upland cotton (Nanjing Agricul-
tural University), a liquid SNP array with 10 K SNPs
was developed. The two RIL populations of P-CCRI60
and P-EZ60, including their parents, were genotyped
by genotyping by target sequencing (GBTS) (Table S1).
The total number of samples was 500. The average call
rate of a single locus was 94.35%, and the average call
rate of an individual was 92.10%. The results of the
genotype control are shown in supplementary table 2
(Table S2). The BLAST alignment tool was used to ana-
lyze the probe sequences of SNPs against the G. hir-
sutum TM-1 genome sequence [28, 51], and a total of
8,348 genotyped high-quality SNPs across the 500 sam-
ples were used in association mapping.
Genome‑wide association studies
We used the genetic model of 3VmrMLM to detect
interactions
QTNs
for LP and BW × environment
Niu et al. BMC Plant Biology (2023) 23:179
Page 6 of 18
Table 3 Correlation analysis between BW and LP in P-EZ60 in
Anyang in 2020 and 2021
20AYLP
20WXLP
21AYLP
21WXLP
-0.107*
-0.247**
20AYBW
20WXBW
21AYBW
21WXBW
-0.191**
-0.340**
** Represents significance at the P < 0.01 level (two-tailed)
* Represents significance at the P < 0.05 level (two-tailed)
Table 4 Correlation analysis between BW and LP in P-CCRI60 in
Anyang in 2020 and 2021
20AYLP
20WXLP
21AYLP
21WXLP
-0.098*
-0.247**
20AYBW
20WXBW
21AYBW
21WXBW
-0.185**
-0.234**
** Represents significance at the P < 0.01 level (two-tailed)
* Represents significance at the P < 0.05 level (two-tailed)
(Fig. 2). A total of 104 stable quantitative trait nucleotides
(QTNs) on 26 chromosomes were identified as signifi-
cantly associated with LP and BW (Table S3). Following
other similar studies [47], we defined the flanking 200-Kb
regions of QTNs as an initial QTL and merged the over-
lapping QTLs to obtain the final QTLs. In the end, 100
stable QTLs were detected; 51 of them were for LP and
49 were for BW, including three QEIs, one for LP and
two for BW, which could be identified in the four envi-
ronments (Table S4). A total of 20 stable QTLs, 14 for LP
and 6 for BW, were identified in EZ60, including one QEI
for BW that could be identified in the four environments;
33 stable QTLs, 18 for LP and 15 for BW, were identi-
fied in CCRI60, including one QEI for LP that could be
identified in the four environments; and 47 stable QTLs
were identified in ZR014121, 19 for LP and 28 for BW,
including one QEI for BW that could be identified in the
Table 5 Analysis of variance for the two traits of two populations
four environments (Table S4). One QTL in chromosome
A10, qBW-E-A10-1, was identified in both populations.
Among the 100 QTLs, 22 QTLs, 9 for LP and 13 for BW,
were overlapping with the reported QTLs (Table S5); 78
QTLs, 42 for LP and 36 for BW, were novel (Table S6).
The QTLs explained 0.29–9.96% of the phenotypic
variations in LP or BW. In P-EZ60, the novel QTLs
associated with LP explained 0.47–8.67% of the pheno-
typic variation, and the novel QTLs associated with BW
explained 0.91–6.31% of the phenotypic variation. In
P-CCRI60, the novel QTLs associated with LP explained
0.29 –9.96% of the phenotypic variation, and the novel
QTLs associated with BW explained 0.36–3.02% of the
phenotypic variation.
In sum, a total of 51 QTLs related to LP were detected
in this study, including 14 in EZ60, 18 in CCRI60, and
19 in ZR014121; 28 QTLs were in the At subgenome,
and 27 QTLs were in the Dt subgenome, indicating that
LP-related QTLs were evenly distributed in the At and
Dt subgenomes. A total of 49 QTLs related to BW were
detected, including 6 in EZ60, 15 in CCRI60, and 28 in
ZR014121; 34 QTLs were in the At subgenome, and 15
QTLs were in the Dt subgenome, indicating that the
QTLs related to BW were mainly distributed in the At
subgenome. There were two QEIs, which were located on
chromosomes A02 and A10 (Fig. 3).
Candidate genes in the regions of the six key QTLs
To identify candidate genes of key QTLs, six QTLs
were selected, including three QEIs, the common QTL
qBW-E-A10-1 that was mapped in both populations
and two important QTLs (qLP-E-D03-2 and qLP-C-
D03-2). The three QEIs were QTLs that were stable in
the four environments (Table S7). A total of 108 puta-
tive candidate genes in the regions of the six key QTLs
in multiple environments were identified, including
genes that were positively related to LP and BW, such
as the genes involved in gene transcription, protein
synthesis, calcium signaling, phytohormone synthesis
population
trait
Source
P-EZ60
P-CCRI60
LP
BW
LP
BW
Accessions
Environments
Accessions
Environments
Accessions
Environments
Accessions
Environments
SS
6714.21
3856.09
151.92
110.12
7788.48
8105.79
216.49
124.62
df
190
3
190
3
290
3
290
3
MS
35.34
1285.36
0.80
36.71
26.86
2701.93
0.75
41.54
F
5.58
173.44
3.24
151.13
3.32
388.89
2.85
154.30
P‑value
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
Niu et al. BMC Plant Biology (2023) 23:179
Page 7 of 18
Fig. 2 Manhattan-plots of LP and BW using the genetic model 3VmrMLM. X-axes are cotton chromosomes. Y-axes on the left side report -log10
P-values of the main-effect QTNs, which were obtained from single-marker genome-wide scans for all the markers in the first step of 3VmrMLM;
Y-axes on the right side report LOD scores, which were obtained from likelihood ratio tests for significant and suggested QTNs, with a threshold of
LOD
3.0 (dashed line) in the second step of 3VmrMLM. These LOD scores are indicated by points with straight lines
=
Niu et al. BMC Plant Biology (2023) 23:179
Page 8 of 18
Fig. 3 A physical map of QTLs for LP and BW from the two RIL populations. The green letters are QTLs for LP, and the red letters are QTLs for BW. The
scale on the left is in Mb
and signaling, and fiber synthesis-related polysaccha-
ride metabolism (Table S6).
KEGG analysis showed that the 48 genes related
to LP were mainly involved in “metabolic pathways”
and “spliceosome” (Fig. 4). Eighteen metabolic path-
ways such as “biosynthesis of secondary metabolites”,
“microbial metabolism in diverse environments” and
“DNA replication” were also detected. KEGG analysis
showed that the 60 genes related to BW were mainly
involved in “metabolic pathways” and “biosynthesis of
secondary metabolites” (Fig. 5). “Microbial metabo-
lism in diverse environments”, “carbon metabolism,”
“glycolysis/gluconeogenesis,” and 19 other metabolic
pathways were detected.
Expression profiles of candidate genes during fiber
development
Most of the candidate genes associated with LP and BW
were differentially expressed in cotton fiber at differ-
ent developmental stages, and there were differences at
expression levels between the high-LP parent EZ60 and
the low-LP parent ZR014121 at the same stage (Fig. 6).
Among the major candidate genes, Gh_A02G0096 was
only expressed in the ovule developmental stage of EZ60.
Gh_A02G0111 was mainly expressed in both EZ60 and
ZR014121 at 0, 5, 10, and 20 days post-anthesis (DPA). Its
expression levels were higher in ZR014121 than in EZ60
at 0, 5, and 25 DPA; its expression levels were higher
in EZ60 than in ZR014121 at 10 DPA. Gh_D03G1064
was highly expressed in both EZ60 and ZR014121 at
all stages. It was mainly expressed at 0, 5, and 10 DPA,
and its expression level in ZR014121 was higher than
that in EZ60 at 10 DPA. Gh_D03G1069 was expressed
in both EZ60 and ZR014121 at all stages. Its expression
levels were higher in ZR014121 than in EZ60 at 10 and
20 DPA; its expression levels were higher in EZ60 than
in ZR014121 at 0, 5, 15, and 25 DPA. Gh_A02G0106 was
significantly highly expressed during the ovule devel-
opment stage in EZ60, highly expressed at 5 DPA, and
weakly expressed at 10 DPA in ZR014121.
Niu et al. BMC Plant Biology (2023) 23:179
Page 9 of 18
Fig. 4 A histogram of candidate genes enriched in different KEGG pathways for LP. The x-axis indicates the number of candidate genes. The y-axis
represents biological processes. The details are listed in Table S9
Fig. 5 A histogram of candidate genes enriched in different KEGG pathways for BW. The x-axis indicates the number of candidate genes. The y-axis
represents biological processes. The details are listed in Table S10
Co‑expression of candidate genes
The interaction network of candidate genes associ-
ated with LP and BW was investigated by construct-
interaction (PPI) network
ing the protein–protein
using the STRING database [52] (Fig. 7). Correlations
were observed in the expression of the following pro-
teins that appear to comprise a co-expression network:
Gh_A02G0111, Gh_D03G1056, Gh_D03G1134, Gh_
D03G1064, Gh_A02G0106, Gh_A10G1521, and Gh_
A10G1653. Network analysis of the major proteins was
carried out using Cytoscape 3.7.2 (Fig. 8). Gh_D03G1056,
Gh_D03G1064, Gh_D03G1134, and Gh_A02G0111
played important roles in the network.
PPI analysis indicated that GAI interacted with six other
proteins. GAI interacted with FRI; FRI interacted with
FPA; FOA interacted with AT1G12775; AT1G12775 inter-
acted with AT3G46960; and AT3G46960, AT3G06700, and
AT1G80750 interacted with each other (Fig. 7). There were
three groups of co-expressed genes, UBC32 and PCNA1;
and CRT3 and ECA1; HCF107 and GOX1. Co-expression
analysis of the 108 candidate genes of the six QTLs using
Cytoscape 3.7.2 indicated that the seven genes (the same
(See figure on next page.)
Fig. 6 Gene expression profiles of the candidate genes of LP and BW QTLs during fiber development in EZ60 and ZR014121. Each column
represents one sample, and rows represent candidate genes. The expression levels of the candidate genes (FPKM) were log2-normalized (i.e.,
log2(FPKM
represents the ovule development stage. 5, 10, 15, 20, and 25 DPA represent the fiber development stages. Detailed information on gene expression
is shown in Table S11
0.01)) and presented in different colours on the scale bar. ZR indicates cotton line ZR014121; DPA indicates days post-anthesis. 0 DPA
+
Niu et al. BMC Plant Biology (2023) 23:179
Page 10 of 18
Fig. 6 (See legend on previous page.)
Niu et al. BMC Plant Biology (2023) 23:179
Page 11 of 18
Fig. 7 Protein–protein interaction of the candidate genes of the QTLs for LP and BW. Network nodes represent proteins with splice isoforms or
post-translational modifications are collapsed, i.e. each node represents all the proteins produced by a single, protein-coding gene locus. Colored
nodes: query proteins and first shell of interactors; white nodes: second shell of interactors; Empty nodes: unknown proteins. 3D structure filled
nodes: some 3D structures are known or predicted. Edges represent protein–protein associations. Associations are meant to be specific and
meaningful (i.e., proteins jointly contribute to a shared function); this does not necessarily mean that they physically bind to each other. Known
Interactions, blue: from curated databases; purple: experimentally determined. Predicted Interactions, green: gene neighborhood, red: gene fusions;
indigo: gene co-occurrence; Others, yellow: textmining, black: co-expression, light purple: protein homology
as the result of PPI) were co-expressed, including Gh_
A02G0106 (GAI) (Fig. 8).
Discussion
A set of new major QTLs for LP and BW that could be used
for MAS was obtained
LP and BW are the most important traits in cotton
breeding, and they have been widely studied. More
than 417 unique QTLs for LP have been identified on
26 chromosomes, including 243 QTLs identified with
LOD > 3. More than 60 were stable, major effective
QTLs that could be used for MAS [50]. According to
the CottonGen Database [53, 54], a total of 1,387 yield
QTLs and four yield component trait QTLs have been
identified. The numbers of these QTLs are increas-
ing continually. Recently, 34 SNPs corresponding to 22
QTLs for LP, including 13 novel QTLs, were detected
from 254 upland cotton accessions via GWAS [55].
Two stable LP QTLs and three BW QTLs were identi-
fied in the RIL mapping population derived from the
inter-specific cross between G. hirsutum cv DS-28 and
G. barbadense cv SBYF-425 [56]. We also identified
one QTL for LP, and nine QTLs for BW from a BC5F3:5
line population
chromosome segment substitution
derived from G. hirsutum CCRI36 and G. barbadense
Hai1 [57]. Three QTLs for LP and one QTL for BW
were identified from an F2 population derived from the
G. hirsutum × G. barbadense cross [58].
Niu et al. BMC Plant Biology (2023) 23:179
Page 12 of 18
Fig. 8 Major gene coexpression network of the candidate genes of the QTLs for LP and BW. Lines indicate co-expression of two linked genes.
Network nodes represent genes. The size of the circle shows the betweenness centrality points of the gene. The size of the circle indicates that the
gene plays an important role in co-expression. In this graph, genes with higher betweenness centrality points are marked in green and placed in
the outer circle, and genes with smaller BC values are marked in red and placed in the inner circle. The three genes in the outer ring, Gh_D03G1056,
Gh_D03G1064, and Gh_D03G1134 were candidate genes for LP, and Gh_A02G0111 was a candidate gene for BW
In this study, a total of 51 stable QTLs for LP and 49
stable QTLs for BW were identified from three upland
cotton lines ZR014121, CCRI60, and EZ60; these QTLs
could explain 0.29–9.96% of the phenotypic variation in
LP and 0.41–6.31% of the phenotypic variation in BW.
A total of 78 of these QTLs were novel. These findings
enhance QTL resources that could be used to enhance
the yield of cotton; this QTL information will also aid
the molecular breeding of cotton cultivars with high
yield.
Many studies have shown that the heritability of LP is
the highest among all yield component traits in cotton,
and the heritability of BW was the lowest among all cot-
ton yield components. Because the heritability of BW is
low, environmental factors can have significant effects on
BW [6, 59–61]. The results of this study also demonstrate
that environmental factors have stronger effects on BW
than on LP (Tables 1, and 2). Thus, selection for LP can
achieve desired outcomes more efficiently than selec-
tion for BW in cotton breeding. Correlations and path
analysis among agronomic and technological traits of 16
upland cotton lines indicated that LP was negatively cor-
related with BW (-0.2668) [62]. Generally, LP and BW are
negatively related [50]. In our study, the correlation coef-
ficients between LP and BW ranged from -0.098 to -0.340
(Tables 3, and 4). This indicates that increases in one of
these traits limit increases in the other. LP may be the
target of direct selection on cotton genotypes with high
cotton fiber yield.
Most QTLs for LP and BW explain less than 10% of
the phenotypic variation. For example, one study indi-
cates that nine QTLs for LP explain 1.84–13.50% of the
observed phenotypic variation; two QTLs for BW explain
6.02–9.50% of the observed phenotypic variation [63].
The QTLs qLP-C13-1 and qLP-C25-1 for LP explain
5.77% and 8.87% of the phenotypic variation, respec-
tively [64]. A GWAS of a set of 289 Gossypium arboreum
chromosome segment ILs in G. hirsutum indicates that
co-QTLs for LP explain 1.21–10.79% of the phenotypic
variation, and co-QTLs for BW explain 1.17–11.56% of
the phenotypic variation [65]. Some QTLs for LP identi-
fied in this study explained nearly 10% of the phenotypic
variation, and all QTLs for BW explained less than 10%
of the phenotypic variation (Table S4). These QTLs, espe-
cially the major effective QTLs, can be used to breed cot-
ton plants with high yield via MAS.
Niu et al. BMC Plant Biology (2023) 23:179
Page 13 of 18
Several putative candidates of the six QTLs for LP and BW
were identified
Understanding the molecular mechanisms of LP and BW
developments is essential for the molecular breeding of
cotton plants with high yield, especially via genetic engi-
neering. Many candidate genes of the QTLs for LP and
BW have been studied [48–50, 55]. The TIP41-like family
protein (TIP41L) gene (GH_A12G0194) is thought to be
the candidate gene of a stable major QTL (q(BW + SI)-
A12-1) for BW [49]. One gene orthologous to the
Arabidopsis receptor-like protein kinase gene HERK1
(GB_A07G1034) was predicted to be the candidate gene
for LP in G. barbadense [48]. Two candidate genes (Gh_
D01G0162 and Gh_D07G0463) of QTLs for LP were
identified. Gh_D01G0162 is a homolog of the auxin-
responsive GH3 family protein gene, and Gh_D07G0463
is a homolog of the NADPH/respiratory burst oxidase
protein D gene (RBOHD) in Arabidopsis [55]. A molecu-
lar regulatory network for LP has been proposed based
on the functions of the candidate genes of QTLs for LP
[50].
In this study, the candidate genes of the six important
QTLs for LP and BW were investigated. The QTLs for
both traits have candidate genes involved in gene tran-
scription, protein syntheses, signaling, calcium signaling,
carbon metabolism, metabolic pathways, and biosynthe-
sis of secondary metabolites, which demonstrates that
there are several candidate genes of the QTLs for LP and
BW (Figs. 4, and 5; Tables S8, S9, S10). This result is con-
sistent with the findings of previous studies [48, 50, 55,
66, 67]. The difference is that a greater number of candi-
date genes in QTLs for LP were involved in gene expres-
sion processes, and a greater number of candidate genes
in QTLs for BW were involved in metabolic pathways.
Interaction network analysis of the candidate genes asso-
ciated with LP and BW indicated that seven candidate
genes could form a co-expression network. The candidate
gene Gh_A02G0096 of qBW-E-A02-1 encodes a homolog
of eukaryotic translation initiation factor 2A, and the
candidate gene Gh_D03G1069 of qLP-E-D03-2 likely
encodes a serine/threonine-protein kinase. Their inter-
action suggests that LP and BW are closely related dur-
ing development (Figs. 7, and 8). Additional studies are
needed to clarify why LP and BW are negatively related.
Many candidates of the six QTLs are involved in fiber
development
The MYB-bHLH-WD40
(including MYB-DEL-TTG
and CPC-MYC-TTG) [33, 68] and TCP-HOX-HD [66,
69] regulatory complexes play key roles in cotton fiber
development. Phytohormone balance, Ca2+ signaling,
and ROS also play key roles regulating fiber develop-
ment [50, 70, 71].
Many candidate genes of the QTLs for LP and BW are
involved in various signaling pathways and metabolic
processes in this study, such as the transcription factor
bHLH113 gene (Gh_A02G0095); Ca2+ signaling genes
(Gh_A10G1519, Gh_D03G1058, and Gh_D03G1266);
protein kinase genes (Gh_D03G1144, Gh_D03G1264,
and Gh_D03G1069); GA signaling genes (Gh_A02G0104
and Gh_A02G0106); and ROS metabolism-related genes
(Gh_D03G1138, Gh_D03G1063, and Gh_D03G1062)
[55] (Table S7). Gh_D03G1264 encodes a HERK1-like
protein [48]. Gh_A02G0106 is a homolog of AT1G14920,
that encodes a gibberellin insensitive protein (DELLA
protein GAI), and plays a role in seed germination [72].
Gh_A02G0111 is a homolog of AT2G43410, which
encodes a flowering time control protein FPA in Arabi-
dopsis [73]. Gh_D03G1064 encodes a FRIGIDA-like pro-
tein that can pleiotropically increase lint yield; it is also
significantly associated with SI [5]. The homologous gene
of Gh_D03G1064 in Arabidopsis is FRI (AT4G00650),
which regulates flowering time in Arabidopsis [73–77].
GhFSN1 is a cotton NAC transcription factor that
acts as a positive regulator to control secondary cell wall
(SCW) formation in cotton fibers by activating down-
stream SCW-related genes, including GhDUF231L1,
GhKNL1, GhMYBL1, GhGUT1 and GhIRX12 [66]. The
candidate gene Gh_A02G0101 also encodes a NAC
(Table S7). The glucosyltransferases, Rab-
protein
like GTPase activators, and myotubularin (GRAM)
domain gene GhGRAM31 (Ghir_D02G018120) regulate
fiber elongation. GhGRAM31 directly interacts with
GhGRAM5 and GhGRAM35. GhGRAM5 also interacts
with the transcription factor GhTTG1, and GhGRAM35
interacts with the transcription factors GhHOX1 and
GhHD1 [67]. The candidate gene Gh_A02G0094 also
encodes the C2 and GRAM domain-containing protein
At1g03370 (Table S7).
The above data demonstrate that most of the putative
candidates of the six QTLs for LP and BW identified in
this study were involved in regulating cotton fiber devel-
opment. Most of the data obtained in this research are
consistent with the findings of other studies, indicating
that our results were reliable.
Candidate gene expression profiles determine LP and BW
ZR014121 is an excellent high-yield but low-LP line.
EZ60 is an early maturity line with high LP. The candidate
gene expression profiles of the six QTLs for LP and BW
in the two lines significantly differed (Fig. 6). Most can-
didate genes were highly expressed at the ovule develop-
mental stage (0 DPA) in both ZR014121 and EZ60. Four
Niu et al. BMC Plant Biology (2023) 23:179
Page 14 of 18
key candidate genes were highly expressed at 5 DPA in
ZR014121, including Gh_A02G0095 (BHLH113, which
might be involved in MYB-bHLH-WD40 complexes [33,
68]), Gh_A02G0097 (RGA3), Gh_A10G1158 (CBDAS),
and Gh_D03G1062 (RBOHC, which might be involved
in ROS [70]). Gh_A02G0114 (ccdc94) was significantly
highly expressed at 15 DPA in EZ60. Gh_A02G0101
(NAC014, which might be involved in SCW formation in
cotton fibers [66]) was significantly highly expressed at 25
DPA in ZR014121.
Most genes were highly expressed at the ovule devel-
opmental stage, which demonstrates that these genes
were highly active in this stage. The expression of four
genes in ZR014121 after this stage was likely the main
cause of high yield. These four genes, in addition to the
other two highly expressed genes, Gh_A02G0114 and Gh_
A02G0101, were the key candidate genes of the six QTLs
for LP and BW (Fig. 6). Although we were unable to deter-
mine whether the six genes represent the six QTLs, our
findings indicate that they are the key genes regulating
LP and BW and thus affecting cotton yield. These genes
provide important genetic resources for studies of the lint
regulation mechanism and improvements in cotton yield.
Conclusions
Two RIL populations were constructed using the three
excellent upland cotton lines ZR014121, CCRI60, and
EZ60, which differ in fiber yield and quality traits. The
RILs were genotyped by GBTS and phenotyped under
four different environments; a GWAS was then conducted
to identify useful yield-related QTLs. A total of 51 QTLs
for LP and 49 QTLs for BW were identified, and these
QTLs could explain 0.29–9.96% of the phenotypic varia-
tion in LP and 0.41–6.31% of the phenotypic variation in
BW. There were six major and effective QTLs, three for
LP and three for BW, and these could be used to breed
cotton with high yield via molecular breeding approaches.
A total of 108 putative candidate genes were identified
in the six key QTLs, including genes that were positively
related to the development of LP and BW, such as genes
involved in gene transcription, protein synthesis, calcium
signaling, phytohormone synthesis and signaling, and
fiber synthesis-related polysaccharide metabolism. Seven
of the candidate genes form a co-expression network. Six
significantly highly expressed candidate genes after anthe-
sis were important factors regulating cotton yield. These
candidate genes will help clarify the molecular mecha-
nisms underlying variation in LP and BW.
Methods
Plant materials and growth conditions
Three G. hirsutum lines ZR014121, CCRI60, and EZ60
were used as parents in this study, and they were bred
at the Institute of Cotton Research, Chinese Academy of
Agricultural Sciences. All of the three RIL lines we were
authorized to use. EZ60 and ZR014121 were preserved
in the National Germplasm Library (38 Huanghe Ave-
nue, Anyang, Henan 455,000); their accession numbers
were M116025 and ZM115357, respectively. CCRI60 is a
variety. ZR014121 has high yield but low LP. EZ60 is an
early maturity line with high LP. CCRI60 is an excellent
cultivar with several desirable agronomic traits. Two RIL
populations at the F6:8 generation in 2020 (at F6:9 in 2021),
P-CCRI60 and P-EZ60 were constructed from crosses of
ZR014121 × CCRI60 and ZR014121 × EZ60, respectively.
P-CCRI60 consisted of 300 RILs, and P-EZ60 consisted
of 200 RILs.
There were four factors in the field experiment: two
years (2020 and 2021) and two locations (Anyang
(36°05′N, 114°29′E), Henan Province, and Weixian
(37°58′N, 115°16′E), Hebei Province, China(both of them
are our experimental field)); these were each referred
to as 20AY, 20WX, 21AY, and 21WX. To eliminate field
effects, the experiment was conducted in a randomized
incomplete block design with two replicates of each envi-
ronmental factor. The parents and RILs were planted in
rows with lengths of 3 m and widths of 0.8 m; the one
control, CCRI60, had 20 rows. The lines were planted in
April and sampled in September each year. Field man-
agement techniques followed those of regular breeding
practices.
Trait measurements
Two yield-related traits LP and BW were evaluated at
each field location. The samples were prepared around
September 20 each year. Thirty naturally opened bolls
from the central part of plants (two bolls on each plant)
of each line were randomly hand-harvested to calculate
the BW (g) and gin the fiber. Fiber samples were sepa-
rately weighed to calculate the LP (%). All statistical anal-
yses, including correlations between traits, analysis of
variance and significance analyses were conducted using
IBM SPSS 22.0 software.
GBTS
For genotyping, the young leaf tissues of the three par-
ents ZR014121, CCRI60, and EZ60, and the RILs of the
two populations, P-CCRI60 and P-EZ60, were sampled in
July 2020. Genomic DNA was extracted from each sam-
ple using a modified cetyltrimethylammonium bromide
method [78].
For GBTS, we used the Allegro Targeted Genotyping of
NuGEN Technologies; the stable markers covering whole
cotton genomes were selected from known markers
obtained from the high-throughput sequencing results.
To prevent the 3′-ends of the probes from overlapping
Niu et al. BMC Plant Biology (2023) 23:179
Page 15 of 18
with other known variable sites, the SNPs were tested
in the parents and their F1 plants, and the polymorphic
SNPs were used to design primers. DNA fragmentation,
adapter ligation, target extension, and library amplifica-
tion were performed following the instructions of vari-
ous kits (NuGEN Technologies, San Carlos, California,
USA). The libraries were tested using the most recently
updated Illumina manufacturer’s instructions (Illumina,
San Diego, CA, USA). Three replications of GBTS were
performed on each sample.
After the SNP data were generated by BCFtools,
the raw SNPs and Indels were screened using three
parameters QUAL, RPB, and AC [(-e ‘%QUAL < 100);
(RPB < 0.1, %QUAL < 100); (AC < 2, %QUAL < 100)’)].
The cover rate of each sequenced SNP was statisti-
cally analyzed using ‘samtools depth’. The SNPs with
sequencing cover rates more than 10 times and without
genotypes were considered to be genotypes consistent
with those in the cotton reference genome; SNPs with
sequencing cover rates less than one time and without
genotypes were referred to as deletion genotypes. The
two SNP quality control criteria were (1) call rate of a
single locus and (2) call rate of an individual. The Perl
soft program that we translated and edited was used to
statistically analyze the quality control criteria. For the
physical localizations of the SNP markers, the probe
sequences of the SNPs were used to| perform local
BLAST [79] queries against the G. hirsutum TM-1 ref-
erence genome [28, 52].
GWAS
The high-quality SNPs determined from the whole study
populations, P-CCRI60 and P-EZ60, were used to conduct
a GWAS for LP and BW. Given the possibility of obtain-
ing false-positive QTNs with low association frequencies,
we selected QTNs with LOD > 3 as stable QTNs in subse-
quent analyses. The software 3VmrMLM version 1.0 [80]
was used to perform GWAS with the following settings:
method = ‘Multi_env’; fileKin = NULL; filePS = NULL;
PopStrType = ‘Q’; fileCov = NULL; SearchRadius = 20;
svpal = 0.01; DrawPlot = TRUE; Plotformat = ‘pdf’; and
Chr_name_com = NULL. We obtained significant and
suggested main-effect QTNs, significant, as well as sug-
gested QEIs. The significant QTNs were selected by Bon-
ferroni correction, and the critical P-value was 0.05/m,
where m is the number of tests or markers, and suggested
QTNs were identified as those with LOD ≥ 3.0. Significant
QEIs were selected by Bonferroni correction; the critical
P-value was 0.05/m, where m is the number of tests or
markers, and suggested QEIs were identified as those with
LOD ≥ 3.0 using default parameters [80].
Prediction and identification of candidate genes
We defined the flanking 200-Kb regions of the QTNs
as the same QTL and merged the overlapping QTLs to
confirm the number of QTLs [81]. Potential candidate
genes were confirmed based on gene annotations in the
G. hirsutum TM-1 genome [28, 52]. All the candidate
genes were subjected to Gene Ontology [82] enrichment
analysis and Kyoto Encyclopedia of Genes and Genomes
[83–85] analysis. The interaction network of candidate
genes was inferred by constructing a PPI network using
the STRING database [52]. The network analysis was
conducted using Cytoscape 3.7.2.
RNA sequencing and gene expression profiles of the QTL
candidates
The ovules/fibers of EZ60 and ZR014121 were sampled at
0, 5, 10, 15, 20, and 25 days post-anthesis (DPA). The total
RNAs were extracted using the mirVana™ miRNA Isola-
tion Kit (Ambion) according to the manufacturer’s instruc-
tions. Three biological replicates were performed for each
sample. The Illumina PE libraries were sequenced on the
HiSeqTM2500 (Illumina) platform. Raw reads were fil-
tered using Trimmomatic-0.39 [86], and the clean reads
were mapped to the reference genome [87] using STAR-
2.7.9a [88]; the abundances of transcripts were quantified
using RSEM-1.2.26 [89]. Differentially expressed genes
(DEGs) were identified using DESeq2-1.30.1 according
(FoldChange) > 1
to the following criteria: padj < 0.05 and log2
DESeq2-1.30.1 [90]. Hierarchical cluster analysis of DEGs
was conducted to measure expression levels. The expres-
sion profiles of every candidate gene were used to prelimi-
narily identify LP-related and BW-related genes.
Supplementary Information
The online version contains supplementary material available at https:// doi.
org/ 10. 1186/ s12870- 023- 04147-5.
Additional file 1:Table S1. The result of GBTS
Additional file 2:Table S2. The results of sample genotyping
Additional file 3:Table S3. The result of 3VmrMLM: QEI
Additional file 4:Table S4. The identified QTLs
Additional file 5:Table S5. The identified QTLs overlapped with the
reported QTLs
Additional file 6:Table S6. The identified new QTLs
Additional file 7:Table S7. All candidate genes of the 6 key QTLs
Additional file 8:Table S8. Annotations of the candidate genes of the six
QTLs for BW and LP
Additional file 9:Table S9. KEGG annotations of the candidate genes of
the QTLs for LP
Additional file 10:Table S10. KEGG annotations of the candidate genes
of the QTLs for BW
Additional file 11:Table S11. The expression levels of the candidate
genes
Niu et al. BMC Plant Biology (2023) 23:179
Page 16 of 18
Acknowledgements
We thank the reviewers for comments and suggestions on improving the
manuscript.
Authors’ contributions
H.N. performed the experiments, analyzed the data, and drafted the manu-
script. M.K and L.H. helped with analysis of the data. H.S., Y.Y and Q.G designed
the whole study, revised the manuscript and gave the final approval to the
version of the manuscript that is being sent for consideration for publication.
Funding
This work was supported by funding from the National Key Research and
Development Program (2021YFF1000100) and Agricultural Science and Tech-
nology Innovation Program of Chinese Academy of Agricultural Sciences.
Availability of data and materials
The datasets generated and/or analyzed during the current study are available
in the NCBI repository, [https:// www. ncbi. nlm. nih. gov/ biopr oject/ 906276]
[Accession number: PRJNA906276].
Declarations
Ethics approval and consent to participate
We complied with all relevant institutional, national and international
guide-lines with permissions from State Key Laboratory of Cotton Biology,
Key Laboratory of Biological and Genetic Breeding of Cotton, The Ministry of
Agriculture, Institute of Cotton Research, Chinese Academy of Agricultural
Sciences.
Consent for publication
Not applicable.
Competing interests
The authors declare there are no competing interests.
Author details
1 State Key Laboratory of Cotton Biology, Key Laboratory of Biological
and Genetic Breeding of Cotton, Institute of Cotton Research, The Ministry
of Agriculture, Chinese Academy of Agricultural Sciences, Anyang 455000,
Henan, China. 2 Zhengzhou Research Base, State Key Laboratory of Cotton Biol-
ogy, Zhengzhou University, Zhengzhou 450001, Henan, China.
Received: 9 December 2022 Accepted: 1 March 2023
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10.1073_pnas.2221415120.pdf
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Data, Materials, and Software Availability. Matlab code and data have been
deposited in Zenodo repositories (86, 87).
| null |
RESEARCH ARTICLE
NEUROSCIENCE
Reward expectations direct learning and drive operant matching
in Drosophila
Adithya E. Rajagopalana,b ID , Ran Darshana,c, Karen L. Hibbarda, James E. Fitzgeralda, and Glenn C. Turnera,1 ID
Edited by Leslie C. Griffith, Brandeis University, Waltham, MA; received January 3, 2023; accepted August 11, 2023 by Editorial Board Member
Michael Rosbash
Foraging animals must use decision-making strategies that dynamically adapt to the
changing availability of rewards in the environment. A wide diversity of animals do this
by distributing their choices in proportion to the rewards received from each option,
Herrnstein’s operant matching law. Theoretical work suggests an elegant mechanistic
explanation for this ubiquitous behavior, as operant matching follows automatically
from simple synaptic plasticity rules acting within behaviorally relevant neural circuits.
However, no past work has mapped operant matching onto plasticity mechanisms in
the brain, leaving the biological relevance of the theory unclear. Here, we discovered
operant matching in Drosophila and showed that it requires synaptic plasticity that
acts in the mushroom body and incorporates the expectation of reward. We began by
developing a dynamic foraging paradigm to measure choices from individual flies as
they learn to associate odor cues with probabilistic rewards. We then built a model
of the fly mushroom body to explain each fly’s sequential choice behavior using a
family of biologically realistic synaptic plasticity rules. As predicted by past theoretical
work, we found that synaptic plasticity rules could explain fly matching behavior
by incorporating stimulus expectations, reward expectations, or both. However, by
optogenetically bypassing the representation of reward expectation, we abolished
matching behavior and showed that the plasticity rule must specifically incorporate
reward expectations. Altogether, these results reveal the first synapse-level mechanisms
of operant matching and provide compelling evidence for the role of reward expectation
signals in the fly brain.
dopamine | learning-rules | decision-making | mushroom body | foraging
An animal’s survival depends on its ability to adaptively forage between multiple
potentially rewarding options (1, 2). To guide these foraging decisions appropriately,
animals learn associations between options and rewards (3–5). Learning these associations
in natural environments is complicated by the uncertainty of rewards. Both vertebrates
and invertebrates employ decision-making strategies that account for this uncertainty
(6–14). A commonly observed strategy across the animal kingdom is to divide choices
between options in proportion to the rewards received from each (7–15). It has been
hypothesized that animals that use this operant matching strategy make use of the
expectation of reward—the recency-weighted rolling average over past rewards—to
learn option–reward associations (14, 15). Many studies further posit that this learning
involves synaptic plasticity (16–18), and theoretical work has identified a characteristic
relationship between operant matching and a specific form of expectation-based plasticity
rule that incorporates the covariance between reward and neural activity (19). Despite
this strong link between plasticity rules and the matching strategy, there has been no
mapping of these rules onto particular synapses or plasticity mechanisms in any animal.
As a result, deeply investigating these theories by manipulating and testing the nature of
plasticity rules underlying operant matching has been intractable.
The fruit fly, Drosophila melanogaster, offers a promising system within which to address
these challenges. Over the last half century, researchers have shown that flies can learn a
wide variety of Pavlovian associations between cues and rewards (20–26). With the help of
advances in functional and anatomical tools (27–32), they have identified the mushroom
body (MB) as the neural substrate for these learning processes, including the assignment
of value to sensory cues, and the underlying plasticity mechanisms have been extensively
characterized (33–42). Recent theoretical work has also attempted to formalize the fea-
tures of the learning rule that is mediated by these plasticity mechanisms (43–47). Despite
this progress, evidence has been mixed as to whether this learning rule makes use of reward
expectations (48–52), and there is a dearth of understanding about how flies learn in
natural environments (but see ref. 53). Studying foraging behaviors would allow us
Significance
Unraveling how humans and
other animals learn to make
adaptive decisions is a unifying
aim of neuroscience, economics,
and psychology. In 1961, Richard
Herrnstein formulated a
long-standing empirical law that
quantitatively describes many
decision-making paradigms
across these fields. Herrnstein’s
matching law states that choices
between options are divided in
proportion to the rewards
received, a strategy that equalizes
the return on investment across
options. Identifying mechanistic
principles that could explain this
universal behavior is of great
theoretical interest. Here, we
show that Drosophila obey
Herrnstein’s matching law, and
we pinpoint a plasticity rule
involving the computation of
reward expectations that could
mechanistically explain the
behavior. Our study thus provides
a powerful example of how
fundamental biological
mechanisms can drive
sophisticated economic decisions.
J.E.F., and G.C.T.
Author contributions: A.E.R., R.D.,
designed research; A.E.R. performed the experiments
and simulations; K.L.H. contributed new reagents/
analytic tools; A.E.R., R.D., J.E.F., and G.C.T. analyzed data
and interpreted models; and A.E.R., R.D., K.L.H., J.E.F., and
G.C.T. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission. L.C.G. is a guest
editor invited by the Editorial Board.
Copyright © 2023 the Author(s). Published by PNAS.
This open access article is distributed under Creative
Commons Attribution License 4.0 (CC BY).
1To whom correspondence may be addressed. Email:
[email protected].
This article contains supporting information online
at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.
2221415120/-/DCSupplemental.
Published September 21, 2023.
PNAS
2023
Vol. 120 No. 39
e2221415120
https://doi.org/10.1073/pnas.2221415120
1 of 12
OPENACCESSto not only clarify these gaps in the understanding of fly learning
but could also provide an insightful framework for testing the
neural computations underlying decision-making strategies such
as matching.
Leveraging this foraging framework in flies requires us to
address several open questions. First, animals in real foraging
scenarios have to be able to form associations between multiple
different options and rewards, yet evidence in flies suggests that
some associations are labile and easily overwritten (24). Second,
choice behavior has rarely been investigated at the individual fly
level (53–56), and never in the context of flies making repeated
choices between two probabilistically rewarding options. It is
therefore unclear whether flies can learn associations between
options and probabilistic rewards. Finally, it is unknown whether
they can integrate probabilistic reward events over multiple past
experiences to form analog expectations. Even if such analog
expectations can be formed, it is unclear whether they lead to
matching behavior through covariance-based plasticity in the fly
brain.
To answer these questions, we designed an olfactory two-
alternative forced choice (2AFC) task for individual Drosophila,
inspired by earlier behavior assays for flies (57–61) and foraging
related 2AFC tasks in vertebrates (10–12, 14). The assay allows
us to measure hundreds of sequential choices from individual
flies as we vary the probability of reward associated with different
odor cues. Such measurement of behavior over time allows us
to distinguish between different foraging strategies, such as the
matching law versus a simpler win-stay, lose-switch strategy.
Importantly, our assay breaks the hard dichotomy between
Pavlovian and operant conditioning. Unlike purely Pavlovian
tasks (20, 24), flies in our task do not passively experience
olfactory cues and rewards. Rather, the choices made by the fly
dictate the odors and rewards experienced, a hallmark of operant-
learning tasks. However, unlike purely operant tasks, where
animals learn that specific actions lead to rewards or punishment
(62, 63), flies in our task have to learn to perform stimulus-
dependent actions. This relationship between stimulus, action,
and reward is very similar to the dynamic foraging tasks where
operant matching has been observed in other species (11–14).
The dynamic foraging task structure thereby allows us to readily
translate past theoretical work into the context of the Drosophila
brain to seek a mechanistic understanding of decision-making
behaviors that could apply across animals.
Results
Flies Learn Multiple Probabilistic Cue–Reward Associations. In
our Y-arena, a single fly begins a trial in an arm filled with clean
air and can choose between two odor cues that are randomly
assigned to the other two arms (Fig. 1A, Materials and Methods
and SI Appendix, Information 1). The fly can freely move between
arms, with a choice defined as the fly crossing into the reward
zone at the end of the arm (Fig. 1A). Once a choice is made,
we provide reward by optogenetically activating sugar-sensing
neurons using a Gr64f driver (64, 65). The Y-arena then resets,
with the arm chosen by the fly filled with clean air and the other
two arms randomly filled with the two odors. This task design
permits us to precisely control reward delivery without satiating
the fly and enables us to monitor the choices of a single fly over
hundreds of trials.
We first established that flies learn effectively in this apparatus
by reliably rewarding flies only when they chose one of the
odors—what we term a 100:0 protocol. Each fly first experienced
the two odors (3-octanol; OCT and 4-methylcyclohexanol;
MCH) unrewarded for a block of 60 trials, and then reward
Fig. 1. Flies learn multiple probabilistic cue–reward associations. (A) Sc-
hematic of Y-arena (Top). Air flows from tips of each arm to an outlet in
the center. Reward zones are demarcated by lines. A choice is registered
when a fly crosses into the reward zone of an odorized arm, triggering
Gr64f sugar sensory neuron optogenetic activation with a 500-ms pulse
of red light. The next trial commences as the chosen arm switches to air
and the two odors (green/orange) are randomly reassigned to the other
two arms (Bottom). (B) Cumulative choices made toward each option are
shown (n = 9 flies, mean & individual flies). No rewards were available for the
first 60 trials (Naive—black) and became available for the green option from
the 61st trial onward (Training—red). Inset: Percentage rewarded choices in
naive and training blocks. Flies prefer the rewarded option in the training
block compared to naive (Wilcoxon signed-rank test: P = 0.0039, n = 9). (C)
Example trajectory of a fly in the Y before (Left) and after (Right) green odor
is paired with reward. Distance in air arm is represented as negative values
(black), while distances in odorized arms are represented as positive values
(green/orange). Choices are represented by colored rasters. At choice points
the arena resets and that arm switches to air, so the fly’s position jumps to
the tip of the air arm. (D) The probability of accept decisions are plotted as a
function of time in the 100:0 protocol (n = 9 flies, mean—solid line, SE—shaded
area). Flies show a high probability of accepting the rewarded odor (Left). The
probability of accepting the unrewarded odor drops over time (Right). (E)
Controls (Left) show higher percentage of choice made toward the rewarded
option than DopR1 K.O. (Right) flies in one 100:0 block of 60 trials (mean ±
SE − red point & line; individual fly scores—black; Mann–Whitney rank-sum
test: P = 0.0022, control: n = 7, DopR1−/−: n = 6). (F ) Schematic describes the
reward structure of the task (Top). Cumulative rewarded and unrewarded
choices plotted against each other, for three different protocols 100:0, 80:0,
40:0 (Bottom). Slope of all curves indicates that flies show a preference for
the rewarded odor in all cases compared to a naive preference indicated by
the black line (Mann–Whitney rank-sum test: 100:0, P = 4.4500 × 10−8, n =
18; 80:0, P = 5.8927 × 10−5, n = 10; 40:0, P = 0.0014, n = 10). (G) Schematic of
the protocol for training flies with two simultaneous probabilistic cue–reward
contingencies (Top). Two different odor choices are alternated throughout an
unrewarded naive block and a reward block where options were rewarded
with baiting probability of 0.4 or 0.8. Performance (percentage of choices
in which the potentially rewarding option was chosen) on the low and
high reward choices (Bottom) indicates that flies learn both associations. An
increased preference for the rewarded odors over unrewarded is observed
(compared to naive preference) (Mann–Whitney rank-sum test: P = 2.3059 ×
10−4 for high rewarding odor; n = 10, P = 0.01 for low rewarding odor, n = 10).
2 of 12
https://doi.org/10.1073/pnas.2221415120
pnas.org
Trial 2ExhaustOdor 2Odor 1AirRewardZoneOptogenetic Sugar RewardTrial 1AC Air Arm Naive0 30Time (min)Time (min)TrainingG/O Arms 0 5 -5 Displacement (cm)D0 300t0t00.51.0 P(Accept)Rewarded odor Unrewarded odor Odor Experience NumberOdor Experience Number% Rewarded Choice1000Naive Training **B5cmELowReward HighReward% Rewarded Choice0000GNaive% Rewarded Choice100500Control DopR1-/- **400800Training****020406080100120020406080100120Cumulative Rewarded ChoicesCumulative Unrewarded ChoicesF0 20 400 40 X = 100X = 80X = 4020 0 XCumulative Rewarded ChoicesCumulative Unrewarded Choices100500delivery was activated for the following block of 60 trials. As
observed previously, although individual flies exhibited different
odor biases in this naive phase (54, 55, 66), those biases averaged
out over the population (Fig. 1 B, inset). In this phase, flies
spent a lot of time in the air arm and made variable choices,
with little preference for either odor (Fig. 1 C, Left, example fly).
Once reward was made available, flies rapidly shifted to choosing
the rewarded odor (Fig. 1B). This was accompanied by a faster
interval between choices (SI Appendix, Fig. S1B) and a decrease
in meandering trajectories (Fig. 1C).
To analyze this choice behavior at a more elemental level,
we adopted the common framework of considering foraging
choices as a series of accept–reject decisions, where the animal
decides whether or not to pursue an option (67). We defined
reject decisions as when a fly enters an odorized arm but turns
around and exits the arm before reaching the reward zone,
while accept decisions reflect cases where the fly reaches the
reward zone (Materials and Methods). Associating options with
rewards changed the probability of accept decisions gradually
over the course of a block. Acceptance probability increased
for the rewarded odor and decreased for the unrewarded odor
(Fig. 1D and SI Appendix, Fig. S1E). On average, flies were
around four times more likely to reject the unrewarded odor
and seven times more likely to accept the rewarded odor (SI
Appendix, Fig. S1D). Interestingly, flies tended to reject odors
quite close to the tip of the arm (SI Appendix, Fig. S1F ),
suggesting that flies might accumulate evidence over time to make
and commit to their decision—an aspect of fly behavior that has
previously been studied (68). These results indicate that fly choice
behavior in this task can be thought of as a series of accept–reject
decisions.
We found that the odor–reward associations learnt by flies in
our assay were MB dependent. Learning-related plasticity in the
MB circuit requires the activity of dopaminergic neurons (DANs)
(24, 34, 37–41). Dopamine is sensed by odor-representing
Kenyon cells (KCs) and induces synaptic plasticity between
these KCs and downstream mushroom body output neurons
(MBONs) (37, 39). To interfere with this plasticity, we used
a tissue-specific CRISPR knock-out strategy (69) to knock out
DopR1 receptors selectively in the KCs (Materials and Methods),
which are necessary for flies to associate odors with rewards in
other paradigms (41). These flies showed no detectable learning
in the 100:0 protocol, compared to control animals (Fig. 1E).
These findings establish that odor–reward associations in our
behavioral assay are mediated by MB plasticity.
is thought
We then asked whether flies could link odor cues with
probabilistic rewards and distinguish between different reward
probabilities, a key aspect of natural foraging. Importantly,
we incorporated reward baiting into our probabilistic reward
tasks (12, 14). This means that rewards probabilistically become
available and then persist until the reward is collected (Materials
and Methods). Baiting is commonly used in mammalian 2AFC
tasks, as it
to mimic the natural processes of
resource depletion and replenishment over time. We began with
experiments in which a single odor was rewarded with a range
of baiting probabilities: 1 (100:0 task), 0.8 (80:0 task), or 0.4
(40:0 task). Flies showed a preference toward the rewarded odor
in all cases compared to a naive lack of preference indicated by
the black line (Fig. 1F ). The extent of the preference varied with
the probability of reward—a higher probability of reward led to a
stronger preference. Interestingly, flies made faster choices when
rewards were more probable (SI Appendix, Fig. S1C).
These results show that flies can learn from probabilistic
rewards but do not determine whether they can store two
associations simultaneously—another necessity for foraging. To
test this, we designed a paradigm with a third odor, pentyl acetate
(PA), included. This served as the unrewarded cue while we
tested memory formation with the other two odors (Fig. 1 G,
Top). Flies first made 80 unrewarded choices consisting of 40
choices between OCT and PA and 40 choices between MCH
and PA. In the next 80 (Training) trials, one of OCT or MCH
was assigned a high reward baiting probability (0.8) and the
other a low probability (0.4). We alternated the training trials for
the two different odors, to ensure that both relationships would
be learnt simultaneously (Materials and Methods). After pairing,
flies preferred both rewarded odors over PA compared to their
naive preference (Fig. 1 G, Bottom). This choice preference was
also reflected in their accept/reject behavior, with flies exhibiting
a clear preference for accepting the high-rewarding odor (SI
Appendix, Fig. S1 G, Right). Interestingly, in trials with the
low-reward cue presented, there was an increased probability
of rejecting both rewarded and unrewarded odors, as compared
to naive trials (SI Appendix, Fig. S1 G, Left). This suggests the
possibility that flies keep track of all the odor options potentially
available in the environment and actually increase their rejection
rate in the absence of the high-reward odor.
Overall, these experiments establish the fly as a capable animal
model for studying foraging behaviors. Individual flies in the
Y-arena can learn multiple odor-reward associations and can
do so in the face of probabilistic reward. Importantly, these
relationships are mediated by synaptic plasticity at the KC-
MBON synapses in the MB. This establishes a foundation to test
how these animals perform in dynamic foraging tasks and assess
how they respond to reward baiting probabilities that change
over time.
Flies Follow Herrnstein’s Operant Matching Law. Foraging tasks
are cognitively complex, involving two cues paired with different
baiting probabilities that change with time. This requires animals
to keep track of choice and reward history and form expectations
to make adaptive choices. We designed our own dynamic foraging
protocol to investigate how flies behave in such a scenario. The
protocol consisted of three consecutive blocks of 80 trials each.
Flies made choices between two odors (OCT & MCH) that
were paired with different baiting probabilities (Materials and
Methods). These probabilities remained fixed within a block and
changed across blocks (Fig. 2A, example).
We found that flies exhibit operant matching behavior, similar
to observations in monkeys, mice, and honeybees (8, 11, 12, 14).
Individual flies exhibited a strong correlation between choice
ratio (defined as the ratio between the number of choices made
toward option A and option B) and reward ratio (defined as the
ratio between the number of rewards received upon choosing
option A and option B), either calculated over entire blocks or
over a short (ten-trial) window to capture short-term dynamics
(Fig. 2 A and B—example fly, SI Appendix, Fig. S2—all 18 flies,
and Materials and Methods). This holds true across flies, as seen
in the relationship between block-averaged reward ratios and
their choice ratios (Fig. 2C). In such a plot, the matching law
predicts that all points will fall along a line with slope equal to
one (the unity line). Flies appear to approximately follow the
matching law with a slight amount of undermatching, signified
by a slope less than one. Undermatching is commonly observed
across species (11–15), and several reasons have been suggested
for this tendency (13, 19) (Discussion).
Past work has suggested that animals form expectations of
reward and use this to guide behavior in such dynamic foraging
tasks (13–15, 19). When rewards are delivered probabilistically,
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Fig. 2. Flies follow Herrnstein’s operant matching
law (A) Matching of instantaneous choice ratio
(blue) and reward ratio (black) in an example fly.
Schematics indicate the reward baiting probabili-
ties for each odor in the three 80-trial blocks (Top).
Individual odor choices are denoted by rasters,
tall rasters—rewarded choices, short rasters—
unrewarded choices. Curves show 10-trial aver-
aged choice and reward ratio, and horizontal
lines the corresponding averages over the 80-
trial blocks. A description of how rewards are
determined on any given trial in this task can be
found in Materials and Methods. (B) Cumulative
choices of the same fly. The slope of the black
lines indicates the block-averaged reward ratio in
each block; the blue line indicates the cumulative
choices with slope representing choice ratio. The
parallel slopes of the two lines indicate matching.
(C) Block-averaged choice ratio is approximately
equal to reward ratio, following the matching
law with some undermatching (n = 54 blocks
from n = 18 flies). (D) A “win–stay; lose–switch”
model does not accurately capture the trial-by-
trial staying and switching probabilities of flies. A 2
× 2 probability table indicating the joint probability
of the action predicted by the model and the
action made by the fly (n = 18 flies and Materials
and Methods). (E) Change in instantaneous choice
ratio around block changes (n = 16 transitions with
large changes in baiting probabilities between
blocks). (F ) Analysis of choices following particular
histories of experience. Choices made by flies
over three consecutive past trials are represented
by boxes of different colors representing odors
chosen. Filled boxed indicate rewarded choices.
Probabilities of choosing the green and orange
odor on the current trial (0) conditional on this
history are illustrated with associated values. (G)
Coefficients from logistic regression performed on
fly choice behavior to determine the influence
of 15 past rewards (Top) and choices (Bottom)
on a fly’s present choice (blue). These coeffi-
cients were compared to coefficients predicted on
shuffled data (gray) (Wilcoxon signed-rank test:
***P < 0.001, **P < 0.01, *P < 0.05, n = 18
flies). (H) Model fit quality (percentage deviance
explained) for 15-trial, 7-trial, and 1-trial logistic
regression models. Null model used to calculate
the quality metric is a logistic regression with
0-trial history and only bias (Wilcoxon signed-
rank test comparing the null model prediction
with each test model prediction; shown here as
test model prediction subtracted by null model:
***P < 0.001, n = 18 flies). (I) 15-trial
logistic
regression fit (purple) on behavior (blue) from the
example fly from panel A, plotted from the 15th
trial onward to avoid edge effects. (J) Exponential timescales for each fly shown in SI Appendix, Fig. S2, estimated from fitting the leaky integrator model
(Materials and Methods).
animals can only derive an expectation of reward by tallying
information over multiple trials. However, such tallying could
reflect a computation beyond the capabilities of flies. We
wanted to explicitly address the alternative hypothesis that flies
follow a simple win–stay/lose–switch strategy (Materials and
Methods), which would suggest that their behavior is dictated
by only the most recent reward/omission experience. Simulating
choice sequences using this learning rule produced output that
somewhat resembled that of the fly (example in SI Appendix, Fig.
S3A). However, it poorly captured the stay/switch probabilities
actually observed in fly behavior data (Fig. 2D). In particular,
switching occurred much more frequently than predicted. As
further evidence that multiple past outcomes affected behavior,
choices of an individual fly at block transitions showed a lag
between the choice ratio curve and the updated reward ratio at
transition points (Fig. 2B), suggesting that the fly takes a few
trials to adjust its behavior. Quantifying this across multiple
transitions for all flies in the task showed flies require 15 to 20
trials to reach a new steady state choice behavior following block
switches (Fig. 2E).
It is possible that this lag could arise from averaging across
multiple flies that switch at different trials after the transition.
This could occur even if flies use just one past trial’s worth
of information to learn about the change in reward, consistent
with observations in larvae (56). To qualitatively illustrate that
flies learn using multiple trials worth of past information, we first
looked at the decisions made by flies following example triplets of
choices and outcomes (Fig. 2F ), inspired by recent work in mice
(6). For example, following three unrewarded choices of one par-
ticular odor, flies’ next choice was roughly random (Fig. 2 F, Top
Left). However, when an odor was rewarded on the most recent
trial or more distant trials, choices were biased toward that option
(Fig. 2 F, Middle and Bottom Left). In another comparison, flies’
tendency to switch back to an earlier choice (i.e., choose the green
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100:050:500:100Choice Ratio11891189Choice RatioReward Ration = 18 flies100:050:500:10050:50100:0% Deviance Explained30015-trial Model0 80 160 240Trial Number8020100:050:500:100oitaReciohCFly Behavior0 80 160 240Trial Number60120070140Cumulative G ChoicesCumulative O Choices-150154575Trials Since Block SwitchChoice Ratio100:050:500:100n = 16 transitionsABCEHI01515 trial7 trial1 trialF4654316938623235654357683232#Trials InThe PastDStaySwitchStaySwitch0.20.30.4Fly BehaviorModel Prediction1010****************************************00.1051015PastTrialRewardsCoefficient ValueChoices G00.08*******051015ExponentialTimescaleJRewardedUnrewardedChoice RatioReward RatioytilibaborPodor after an unrewarded choice of the orange odor) increased if
that odor was rewarded in the recent past (Fig. 2 F, Right).
To measure the relationship between current choice and past
outcomes more systematically, we used logistic regression to
determine how a fly’s decisions depended on choice and reward
history. Like other animals (6, 12), flies showed a small amount of
habitualness by choosing options that had been recently chosen
more often; regression coefficients for a short history of recent
choices were significantly positive compared to coefficients fit
to shuffled data (Fig. 2 G, Bottom). This approach also showed
that the reward history was important for predicting choice, with
many recent rewards weighted significantly (Fig. 2 G, Top). We
compared regression models that predicted behavior based on
different lengths of outcome histories (15, 7, and 1 trial) and
found that the percentage of deviance explained over a null
model with a 0-trial history was greater for models that used
longer outcome histories (Fig. 2H and Materials and Methods).
An example fit from a regression model with a 15-trial history is
shown in Fig. 2I. In alignment with the results of the regression
model (Fig. 2G), we found that when fitting a leaky integrator
model(14), which assigns value to options using exponentially
weighted reward histories (SI Appendix, Fig. S3 B–E), to the
behavior of individual flies, an exponential timescale of 7 trials on
average best-predicted behavior (Fig. 2J ). Together, these results
show that flies’ choices follow operant matching, with choices
depending on the history of many past choices and outcomes.
Covariance-Based Learning Is Required for Matching Behavior
in a Model of the MB. Theoretical work has placed strong,
testable constraints on the nature of learning rules that could
underlie operant matching. An elegant theory put forward by
Loewenstein and Seung (19) proves that operant matching is the
inevitable outcome of plasticity rules that modify synaptic weights
according to the covariation of neural activity signaling reward
and sensory input (Materials and Methods). Mathematically,
covariance is the averaged product of two variables with at least
one being subtracted by its mean. The mean is simply the average
reward and/or sensory input the animal experiences—an average
that can also be expressed as the animal’s expectation. Comparing
the current value to its expectation ensures that weights can be
adjusted up or down. Importantly, only an animal that follows
operant matching would receive rewards at a rate equal to the
reward expectation for both options, which leads weights to
stabilize. Loewenstein and Seung mathematically formalized this
intuitive link between expectation and matching and showed
that when synaptic plasticity is the basis for operant matching, a
covariance-based plasticity rule can account for matching.
They used a simple neural circuit model to illustrate their
theory, with two different sensory inputs controlling different
motor outputs and a decision determined by a winner-take-
all interaction between those outputs (SI Appendix, Fig. S4A).
Interestingly, the structure of this model maps nicely onto the
circuitry of the fly MB (Fig. 3 A, Left). Sensory inputs are
represented by the KCs, each odor activating a sparse subset
of the KC population (70–72). KCs synapse onto MBONs,
which guide motor output by signaling the valence of an
odor, i.e., its attractive/repulsive quality, rather than a specific
action (22, 38, 42). KC-MBON synapses are modified by a
plasticity rule that depends on the coincident activity of odor-
representing KCs and release of dopamine by reward-signaling
DANs (24, 34, 37–41, 43) (Fig. 3 A, Center, box). Current
evidence indicates that postsynaptic activity of the MBON does
not play a role in the plasticity (73), so only the sensory and reward
activities need to be considered. Either or both of these terms
could incorporate an expectation resulting in a covariance-based
rule (Fig. 3 A, Center, box). DANs could incorporate reward
expectation [(R - E(R))] by subtracting a running average of
reward activity (E(R)) from the current reward-related activity
(R). Similarly, KCs could incorporate sensory expectation [(Si -
E(Si))] by calculating an average sensory experience, possibly by
a mechanism that involves metaplasticity and synaptic eligibility
traces.
To fully adapt the theoretical framework of Loewenstein and
Seung to the biological network in the MB, we had to make
a few changes (Fig. 3 A, Right and Materials and Methods).
Fig. 3. Covariance-based learning is required for matching behavior in a
model of the MB (A) Left: Schematic representing the MB with all relevant
neurons shown in different colors (key). Center: Box containing candidate
reward-dependent synaptic plasticity rules at the KC-MBON synapse. Right:
Schematic of our MB model developed by adapting Loewenstein and Seung’s
model to more closely resemble the MB and the features of our olfactory
task. In the modified task, agents only experience one odor at a time. Reward
information is provided to this circuit via DAN activity which either represents
simply reward (R) or reward minus reward expectation (R-E(R)). Weights
between inputs and MBON are modified according to plasticity rules shown
in Center, where 𝜂 < 0 to match the fly’s depression-based learning rule.
MBON output determines probability of rejecting an odor and is passed
through a sigmoidal nonlinearity to determine action. (B) Left: Block-averaged
choice ratio produced by the [Si-E(Si)] · [R-E(R)] covariance-based rule (box)
plotted against reward ratio. The model exhibits matching behavior (slope
is 1). Right: An example simulation showing the performance in a 3-block
task of a model incorporating a covariance-based rule [Si-E(Si)] · [R-E(R)]. Task
reward contingencies are the same as shown for the example fly in Fig. 2A.
(C) Same as (B), but simulated with a noncovariance learning rule. Left: The
model produces behavior that does not show matching (slope < 1). Right:
Performance in a 3 block task does not show matching of choice and reward
ratio.
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DANMBONW2W1Odor 1Mushroom Body ModelProbabilityof rejectBC100:00:100100:0Choice RatioReward Ratio100:00:100100:0Choice RatioReward Ratio100:050:500:100Choice Ratio0 80 160 240118911898020Mushroom Body CircuitAMB Model:∆Wi = η∙ [Si-E(Si)] ∙ [R-E(R)]100:050:500:100Choice Ratio0 80 160 240MB Model:∆Wi = η ∙ [Si] ∙ [R]RewardedUnrewardedChoice RatioReward RatioTrial Number118911898020Behavioral Output ∆Wi = η∙ KCi∙ DANKCi = Si or [Si-E(Si)]DAN = R or [R-Ε(R)]Candidate Fly Plasticity RulesOdor Input - Kenyon Cell (KC)Dopamineric Neuron (DAN)MB Output Neuron (MBON)Reward Input -Gr64f NeuronTrial Number50:5050:5050:5050:50KCsActionRewardGr64fFirst, odors are represented by noisy populations of KCs (70–
72). We thus parameterized input representations in the model
to incorporate noise and overlap of KC subsets between options.
Second, in our task, flies only experience one odor at a time, so
only one set of KCs is active during reward delivery. Although
Loewenstein and Seung’s original theory does not account for
this possibility in its proof, we extended it to this case (Materials
and Methods). Third, plasticity between MBONs and KCs is
modified by a synaptic depression rule (37, 38). We thus flipped
the sign of the synaptic weight update rule. Finally, MBON
activity determines whether flies accept or reject an odor rather
than a winner-take-all decision mechanism (22, 38) (Fig. 3A).
We incorporated this into our model by having MBON activity
encode the probability of rejecting an odor, with higher activity
representing a greater probability to reject. This MBON activity
was then passed through a sigmoidal nonlinearity to determine
behavioral output.
We then evaluated whether these changes affect the rela-
tionship between covariance-based rules and matching. We
used this MB-aligned model to simulate behavior arising from
covariance rules that incorporated stimulus–expectation, reward–
expectation, or both (Fig. 3 and SI Appendix, Fig. S5). Consistent
with the theory, all three covariance-based rules gave rise to a
choice-ratio versus reward-ratio relationship that followed the
matching law (Fig. 3 B, Left and SI Appendix, Fig. S5 A–C,
Left). In contrast, a rule that did not incorporate either reward
or stimulus expectation did not follow the matching law and
instead yielded a flat slope (Fig. 3 C, Left and SI Appendix, Fig.
S5 D, Left). For comparison, we also examined the behavior
produced by the original model in a distinctly different task and
observed similar results (SI Appendix, Fig. S4 A–E). Note that in
the Loewenstein and Seung task, both options are always present
when reward is delivered, which leads to a slope in between flat
and unity when a noncovariance rule is used (SI Appendix, Fig.
S4 E, Left). However, if only one option is present when an
animal is rewarded, as in the fly task, synapses saturate and a
noncovariance rule leads to a flat choice-ratio versus reward-ratio
relationship (Fig. 3 C, Left).
To get a more refined view of model performance, we examined
the trial-by-trial behavior each plasticity rule generates. Models
that incorporate covariance-based plasticity rules nicely replicate
the trial-by-trial behavior of flies, tracking changes in the reward
contingencies across blocks, with the resulting instantaneous
choice ratio biased toward the more rewarded option in each
block (Fig. 3 B, Right and SI Appendix, Figs. S4 B–D, Right
and 5 A–C, Right). On the other hand, both the MB-inspired
model and Loewenstein and Seung’s model do not capture trial-
by-trial behavior well when a noncovariance rule is incorporated,
with choices made roughly equally to both options throughout
(Fig. 3 C, Right and SI Appendix, Figs. S4 E, Right and S5
D, Right). This reflects the fact that when value updates only
depend on sensory input and reward, plasticity is unidirectional.
Consequently, synapses representing the two options will both
be driven to low levels, although at slightly different rates, so that
ultimately both options are chosen roughly equally. Overall, these
results show that a model constrained by the network architecture
of the MB more closely reproduces fly behavior when it operates
according to a covariance-based plasticity rule.
Identifying Learning Rules Underlying Dynamic Foraging in the
Mushroom Body. To test whether our theoretical prediction of a
covariance-based rule is supported by the observed behavior, we
developed an approach that estimated the form of the plasticity
rule being used in the fly MB. Our goal was to break the plasticity
rule into components that span a space of possible rules and use
optimization approaches to predict trial-by-trial behavior of each
individual fly to assign coefficients to each of these components.
In this way, we would identify the form of the plasticity rule
that best explained observed behavior and be able to conclude
whether this rule was a covariance-based rule.
We used the structure of the MB-inspired generative model
(Fig. 3A) to build a predictive model and test how it fits
the accept/rejection decisions made by the fly on each odor
encounter. However, rather than utilizing a predefined plasticity
rule, the predictive model used a rule composed of four terms that
were candidate components of the MB learning rule (Fig. 4A).
We then used logistic regression to assess which of these terms
contributed the most when fitting fly behavioral data (Materials
and Methods). The four terms were a constant term, a KC term
reflecting sensory input, a DAN term representing reward, and
finally, the product of KC and DAN activity. By definition,
this product
term becomes a covariance calculation when
either of its elements are subtracted by their mean values, i.e.,
when either reward and/or sensory expectation are incorporated
(Fig. 3 A, Center box). We considered four model variants, a
noncovariance one that lacked any expectation term and three
different covariance-based rules where either KC or DAN or
both were subtracted by their expectation. At every iteration of
the logistic regression, the model prediction was compared to
experimentally observed fly behavior, and regression coefficients
were updated. Once the fit was optimized, we evaluated which
term contributed the most to the fit by examining the weights of
each coefficient.
Before applying this approach to fly data, we validated it by
determining whether it correctly identified the relevant term
when tested with choice sequences that were simulated using
a covariance-based learning rule that only incorporated reward
expectation. Indeed, the fit quality was clearly better with a
model that incorporated reward expectation (SI Appendix, Fig.
S6 A and B). Moreover, the largest weights were correctly
assigned to the KC-DAN product term, the term that calculates
the covariance between these two elements (SI Appendix, Fig.
S6C). Additionally, our simulations suggested that the extent
of matching and the accuracy of learning rule fits were largely
unaffected by either the degree of overlap in KC activity patterns
or the timescale over which rewards were integrated (SI Appendix,
Fig. S6 D–I ). Consequently, for simplicity, we then used overlap
of zero and an exponential timescale of 3.5 trials in all future
analyses.
We then applied our approach directly to the behavioral
data from individual flies performing the dynamic foraging
task. A representative example showing fly behavior and model
predictions can be seen in Fig. 4B. This example suggests that
models with covariance-based rules may better resemble the flies’
behavior. To quantitatively compare fit quality of the different
models, we calculated the percentage deviance explained for every
individual fly. This metric showed that regressions that utilized
rules with sensory expectation, reward expectation, or both were
objectively better fits for fly behavior (Fig. 4C and SI Appendix,
Fig. S7A).
Overall, we found that learning rules that incorporated either
sensory or reward expectation both yielded better fits to fly
behavior than noncovariance rules. To distinguish between these
different expectation-based learning rules, we examined which
regression coefficients had the biggest weights. When we fit a
rule with only reward expectation, the regression assigned the
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Identifying learning rules underlying dy-
Fig. 4.
(A)
namic foraging in the mushroom body.
Schematic detailing the logic of the MB-inspired
regression model. This model was used to pre-
dict the behavior of and learning rules used
by each individual fly that experienced the task
described in Fig. 2. (B) Example fly data (blue)
showing the probability of accepting odor 1
(Top) and odor 2 (Bottom) calculated over a 6-
trial window as a function of the number of
times the fly experienced the given odor. These
data were fit using an MB-inspired regression
model (A) that incorporates either a covariance-
based rule with sensory and reward expectations
(brown),
just
reward expectation (gray), or a noncovariance
rule (red). (C) Change in percentage deviance
explained, computed by subtracting the percent-
age deviance explained of the noncovariance-
based model from a covariance-based rule that
incorporates reward expectation (n = 18 flies).
On average, fly behavior was better predicted
by the covariance-based model (Wilcoxon signed-
rank test: P = 0.0018). Individual flies that were
better fit by the covariance-based model have
a positive value on this plot (gray region), while
flies better fit by the noncovariance-based model
have a negative value (red region). (D) Regres-
sion coefficients assigned to each term of the
plasticity rule when the MB-inspired regression
model using a covariance-based rule with reward
expectation was fit to the flies’ behavior. As in
(C), the model was fit to each fly resulting in 18
different values for the coefficients. The largest
coefficients were observed to have been assigned
to the product term. (E) Change in percentage
deviance explained (shown in C), plotted against
a measure of undermatching (mean square error
between instantaneous choice ratio and reward
ratio lines) for each fly (n = 18). The best fit line
of the scatter, calculated by a linear regression is
shown in orange. (F ) Coefficient value assigned to
the product term (shown in D), plotted against a measure of undermatching for each fly (n = 18). The best fit line of the scatter, calculated by a linear regression
is shown in orange.
just sensory expectations (black),
KC-DAN product, i.e., the covariance term, with the largest
weight (Fig. 4D and SI Appendix, Fig. S7B). On the other
hand, fitting using either of the two covariance rules that
incorporated sensory expectations yielded large coefficients for
the noncovariance terms containing either KC or DAN activity
alone (SI Appendix, Fig. S7B). We observed a similar result when
we fit simulated data from an agent using a reward expectation–
based learning rule (SI Appendix, Fig. S7C). Nevertheless, when
the behavior was simulated using the same sensory expectation
rule, the covariance term was given the most weight (SI Appendix,
Fig. S7E). These results suggest that flies use a covariance rule
based on reward expectations to guide their behavior.
in some flies,
Interestingly, we found that
the simple
expectation-free noncovariance rule was a better fit. One possible
explanation for this result is that these flies showed operant
matching to a lesser extent. We thus quantified matching by
calculating the mean squared error between instantaneous choice
and reward ratios and found that different strengths of matching
across flies were correlated with how well an expectation-free
plasticity rule fit the behavior data (Materials and Methods).
Flies that were better fit by the expectation-free rule tended to
show more undermatching, in line with our predictions (Fig.
4E). Consistent with this, the weight of the covariance term
coefficient was greater in flies that exhibited stronger matching
behavior (Fig. 4F ). To examine whether some flies were better fit
by a noncovariance rule because our approach might inaccurately
assign weights to a combination of correlated terms in the learning
rule, we examined the correlations between pairs of coefficients.
However, we found no consistent statistical relationship (SI
Appendix, Fig. S7D and Materials and Methods). Overall, this
general approach allowed us to estimate the learning rule the
fly uses directly from behavioral data, providing clear evidence
that a reward-expectation-based covariance rule is important in
the MB.
Behavioral Evidence of Reward Expectation in DANs. We next
wanted to experimentally verify that a reward-expectation-based
covariance rule in particular guided learning and choice behavior
in the fly MB. The mathematical differences between the three
different covariance rules suggested a way forward (Materials
and Methods). In particular, the rules differ in which terms—
sensory input or reward—incorporate an expectation. Thus,
to distinguish between the possible different covariance-based
rules in the MB, we designed an experiment to manipulate
the calculation of reward expectation using genetic tools that
override the natural activity of the DANs. Specifically, we
provided reward via optogenetic activation of
the reward-
related protocerebral anterior medial (PAM) DANs. This would
bypass any upstream computation of reward expectation and
simply provide a consistent reward signal on every trial. Such a
manipulation would change the learning rule from a covariance-
based rule to a noncovariance rule if the following conditions
were met: i) the animal’s learning rule depended on the product
of DAN and KC activities; ii) DAN activity incorporated reward
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BP(Accept)EA036**∆ % Deviance ExplainedCcov-basednoncov-based-301200-3036∆ % DevUndermatching IndexF0120000.30.6‘d’ CoefficientUndermatching IndexPredicted Behavior Error∆Wi = -a - b ∙ KCi - c ∙ DAN - d ∙ KCi ∙ DAN010242# Odor 1 Experience 010265# Odor 2 Experience Fly DataModel Fit: [Si-E(Si)] ∙ R Model Fit: [Si-E(Si)] ∙ [R-E(R)]Model Fit: Si∙ RModel Fit: Si∙ [R-E(R)]-0.8-0.400.40.8BiasabcdDCoefficient ValueDANMBONW2W1KCsObserved Behavior Fly ModelUpdate Coefficients ofModel Plasticity RuleP(Accept)expectation; and iii) KC activity did not incorporate sensory
expectation. This would in turn result in modified behavior. For
this test, we initially focused on a task consisting of two blocks
(naive and training) of 60 trials each, with a reward ratio of either
100:0 (one odor has a baiting probability of 100% and the other
is never rewarded) or 80:20 (one odor has a baiting probability
of 80% and the other 20%) in the second block (Materials and
Methods).
We first predicted how behavior in these protocols would
differ between covariance-based and noncovariance rules using
simulations. As expected, covariance-based models learnt to
choose the more rewarded option more often, with choice ratios
reflecting reward ratios (Fig. 5A and SI Appendix, Fig. S8 A
and B). The behavior of the model with any covariance-based
rule was similar to the fly behavior when it was rewarded
using the sugar neurons (Fig. 5 B and C). On the other hand,
noncovariance rules led to preferences saturated around 75%
in 100:0 and 50% with the 80:20 reward ratio (Fig. 5D).
These theoretically predicted preferences very closely match our
observations of fly behavior in the DAN activation experiments
(Fig. 5 E and F ). We observed low plateau performance in both
tasks (Fig. 5 E and F ), with values strikingly similar to that
predicted by the noncovariance rule (Fig. 5D).
One potential concern with these experiments is that differ-
ences in the efficacy of optogenetic activation of the DANs deep
in the central brain versus the peripherally located Gr64f neurons
could contribute to these behavioral differences. However, when
flies were instead made to choose between reward-associated or
unrewarded odors in a circular arena previously used to assess
learning in flies (24), we found that both PAM DAN and Gr64f
sugar neuron activation drove similar learning (SI Appendix,
Fig. S9 A–D). Since the LED intensity in the circular arena
(2.3 mW/cm2) was closely matched to that in the Y-arena (1.9
mW/cm2), differences in optogenetic efficacy cannot explain
the range of behavioral patterns seen in the circular and Y
arenas. All data are consistent with the interpretation that PAM
activation bypasses the computation of reward expectation and
converts a covariance rule into a noncovariance rule. In particular,
learning via the noncovariance plasticity rule only modifies
weights from Kenyon cells that respond to the rewarded odor,
which increases the acceptance probability of the rewarded odor
without changing behavior to the unrewarded odor. According to
this model, performance saturates in the Y-arena because the fly
repeatedly encounters the unrewarded odor by chance, and their
initial tendencies for accepting the odor option never change;
performance does not saturate in the circular arena because a fly
Fig. 5. Behavioral evidence of reward expec-
tation in DANs. (A) Instantaneous choice ratio
over trial number, for a simulated agent using
a covariance-based rule with reward expectation
in 80:20 (orange) and 100:0 (red) tasks. (B) As
(A), except it shows fly behavior when provid-
ing sugar sensory optogenetic reward instead
of simulation (100:0, n = 8 flies; 80:20, n = 6).
(C) Average choice ratios of individual flies from
(B) showing significant learning in both 100:0
and 80:20 protocols (Wilcoxon signed-rank test:
100:0, P = 0.0039; 80:20, P = 0.0312). (D) As
(A), except for an agent using a noncovariance
rule. (E) As (B), except reward provided via the
PAM DANs using R58E02-Gal4 to drive UAS-
CSChrimson (n = 8 flies in both 80:20 and 100:0).
Dashed line in (D) and (E) indicates the max-
imum possible performance of agent in D in
the 100:0 protocol. (F ) Average choice ratios of
individual flies from (E). Flies showed a significant
preference toward the rewarded odor in 100:0
but not 80:20 (100:0, P = 0.0391; 80:20, P =
0.1875). (G) The instantaneous choice ratio of an
example fly receiving DAN optogenetic reward
performing the dynamic foraging protocol plot-
ted against trial number as in Fig. 2A. (H) Block-
averaged choice ratios against reward ratios for
flies with DAN reward (n = 26 flies, 3 blocks each).
Best fit lines : red—DAN reward, blue—Gr64f
sugar sensory reward (Fig. 3C). (I) Block-averaged
choice ratios against reward ratios (n = 50)
from data simulated using a noncovariance-
based rule. Best fit lines : black—simulated data,
red—DAN reward. (J) Instantaneous choice ratio
around block changes. Flies trained with Gr64f
(K )
activation in blue, DAN activation in red.
As (J) but with simulated agents using either
a covariance-based rule in blue or noncovari-
ance rule in red. (L) Example fly data showing
probability of accepting odors against experience
number (blue) with DANs activated as reward.
Fit using a model (Fig. 4) that incorporates ei-
ther a covariance-based rule (gray) or a non-
(M) Change in percent-
covariance rule (red).
age deviance explained, computed by subtract-
ing the percentage deviance explained of the
noncovariance-based model from a covariance-
based rule, plotted for each fly (n = 26). On average, fly behavior was better predicted by the noncovariance-based model (Wilcoxon signed-rank test:
P = 0.0164).
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-4-202P(Accept)01Odor Experience Number350∆ % Deviance ExplainedFly η∙ Si∙ Rη∙ Si∙ [R-E(R)]Trial Number060060060060ACLM Choice Ratio100:080:20Trial NumberBChoice RatioReward Ratio100:050:500:10050:50100:0HG100:050:500:100Choice RatioTrial Number0 80 160 240336733678020F100:080:20Choice RatioReward Ratio100:050:500:10050:50100:0INaiveNaiveTrainingTraining****100 : 080 : 20DEcov-basednoncov-basedPAM DANGr64fNon-cov model*0100:050:500:100Choice RatioFly Behavior:Gr64f activationMB Model:∆Wi = η∙ Si∙ [R-E(R)]Fly Behavior:PAM DAN activationMB Model:∆Wi = η∙ Si∙ R100:050:500:100Choice Ratio100:050:500:100Choice Ratio100:050:500:100Choice Ratio100:050:500:100 Choice Ratio100:050:500:100MB Model:∆Wi = η∙ Si∙ R-150154575Trials Since Block SwitchChoice Ratio100:050:500:100J-150154575Trials Since Block SwitchChoice Ratio100:050:500:100KFly Behavior:PAM DAN activationNaiveNaiveTrainingTraining30303030n.sFly Behavior:PAM DAN activationPAM DANGr64fR ∙ S[R-E(R)] ∙ SPAM DANthat has learned to accept the rewarded odor will stop exploring
and cease to encounter the unrewarded option.
We next examined how bypassing reward expectation affects
matching behavior. When tested with the same three-block
matching design as earlier, but now providing a consistent
reward signal via direct DAN stimulation, flies exhibited strongly
diminished matching behavior (Fig. 5 G and H ). The slope of the
choice-ratio versus reward-ratio plot was lower than that observed
with Gr64f-driven reward and approached the flat line predicted
by simulations of behavior with a noncovariance based learning
rule (Fig. 5I ). The instantaneous choice ratio and reward ratio
of an example fly (Fig. 5G) suggested that this flattening arises
because choice ratios are never strongly biased to either odor. This
is again explained by the unidirectional noncovariance rule. In
agreement with this, changes in choice ratio at block transitions
were much flatter with DAN reward than with Gr64f, as expected
by the differences between the covariance-based and noncovari-
ance models (Fig. 5 J and K ). To quantitatively evaluate whether
providing reward with DAN activation changed the learning rule
from covariance-based to a noncovariance rule, we fit our MB-
inspired regression models (Fig. 4A) to fly data produced with
DAN reward. We found that the noncovariance rule was the
better fit (Fig. 5 L and M ). We find through these experiments
that bypassing the computation of reward expectation changes
fly choices from resembling behavior produced by a covariance-
based learning rule to behavior expected from a noncovariance
rule. In particular, the results suggest that this covariance-based
rule is located in the fly MB and incorporates reward expectation
but not sensory expectation.
Altogether, our results support the theory that covariance-
based learning rules that incorporate reward expectation underlie
operant matching in flies. It suggests that a reward expectation
signal is calculated in the DANs of the fly MB and provides the
first mapping of learning rules underlying operant matching onto
plasticity mechanisms at specific synapses.
Discussion
The foraging strategies used by animals play a key role in
their survival. Operant matching is one simple and ubiqui-
tous behavioral strategy, utilized in dynamically changing and
probabilistic environments. Despite the ubiquity of this strategy
and its strong theoretical foundation, little is known about the
underlying biological mechanisms. We leveraged the growing
body of knowledge regarding learning in the fruit fly, and
the plethora of available anatomical tools, to identify these
mechanisms. We developed a foraging task that allowed us to
monitor choices of individual fruit flies and showed, for the
first time, that flies follow Herrnstein’s operant matching law.
Combining experimental results with computational modeling,
we found that this behavior requires synaptic plasticity and uses
a rule that incorporates expectation of reward via the rewarding
PAM DANs. Our results provide the first mapping of the learning
rule underlying operant matching onto the plasticity of specific
synapses—the KC-MBON synapses in the MB.
Does the Ubiquity of Operant Matching Imply a Common
Mechanistic Framework? When choosing between options that
predict reward with different probabilities, mammals, birds, and
insects all follow Herrnstein’s matching law (8, 9, 11–15). This
is clear at the global, trial-averaged level, where choice ratios are
roughly equal to reward ratios, but is also true at the trial-by-trial
level (Fig. 2A). In fact, we found that individual choices made
by flies depended on choice and reward information received
over multiple past trials (Fig. 2 G and H ). This is in agreement
with what has been observed in mice and monkeys (12, 15)
and suggests that these animals all make use of similar kinds of
information to guide their behavior. Flies also show an increase
in speed of choice when rewarded, another common signature of
learnt behavior in mice and monkeys (11, 12) (SI Appendix, Fig.
S1 B and C).
It is unclear whether these behavioral similarities result from
underlying mechanisms that are shared across species. At its
surface, mechanistic similarities seem likely. For example, neural
signals that subtract reward expectation from reward—a key com-
ponent of the plasticity rules underlying matching shown here—
can be found in the form of a reward prediction error in many
different animals (74, 75). Nevertheless, such a signal on its own
is not sufficient to produce matching; it needs to be incorporated
into a covariance-based plasticity rule in a behaviorally relevant
circuit. On the other hand, while learning values of options
via synaptic plasticity is the traditional mechanistic framework
thought to underlie such foraging decisions (16, 17), recent work
has found signatures of graded neural responses proportional to
value during inter-trial-intervals, suggesting a persistent-activity-
based mechanism for foraging decisions that may not require
synaptic plasticity (12, 76). Associated modeling efforts suggest
matching can arise from models that don’t incorporate synaptic
learning (19, 77).
While both synaptic plasticity and nonplasticity mechanisms
can explain the observed behaviors, each makes different testable
assumptions about the underlying neural architecture (18) and
the effect of changing environmental conditions on the behavior.
For example, if one eliminated reward baiting in our experiment,
a circuit using a covariance-based plasticity rule would still give
rise to behavior that follows Herrnstein’s matching law. In this
case, following such a law would lead the animal to always choose
the option with higher reward probability. On the other hand,
if matching behavior was produced using a different mechanism,
the lack of reward-baiting might give rise to different strategies,
such as the probability matching strategy commonly observed in
mice under these conditions (6). Experiments to identify which
mechanisms are used by different brains, and theoretical work to
understand why, would therefore provide important insight into
circuit function and the neural basis of operant matching.
Beyond Covariance-Based Synaptic Plasticity. Our behavioral
evidence suggests that synaptic plasticity in the mushroom body
depends on reward expectations through a simple covariance-
based plasticity rule. We identified this plasticity rule by
the process of elimination. First, we narrowed our focus to the
three minimal covariance-based plasticity rules inspired by the
architecture of the MB. Importantly, Loewenstein and Seung
showed that these rules produce matching. We then showed that
only one of the three rules also explains the results of the DAN-
activation experiment. It’s important to recognize that more
complex plasticity rules may be consistent with our data and
necessary to explain future mechanistic and behavioral data. For
instance, the plasticity rule could be augmented by adding any
term that averages to zero in the matching task. The plasticity
rule could also be changed to involve a nonlinear function
of the current synaptic weight, presynaptic KC activity, and
postsynaptic MBON activity. The fundamental requirement of
Loewenstein and Seung’s theory is merely that the plasticity
rule ultimately drives the covariance between neural activity and
reward to zero.
Loewenstein and Seung’s theory provides an impressively
general link between operant matching and covariance-based
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plasticity, but it does make several assumptions that may be
violated in the fly. For instance, the theory assumes that plasticity
only occurs when the animal makes a choice, with weights
fixed between decisions (Materials and Methods). In our current
paradigm, this means that no plasticity occurs when the fly
rejects an odor or otherwise explores and navigates through
its environment. In contrast, DANs encode a variety of motor
variables and are not locked to choice or reward (39, 78).
These motor-related DAN signals would presumably modify
synaptic connections in the MB, and such off-task plasticity
could generate important variability in synaptic weights and
choice behavior. Interestingly, recent work has also found that
these same DANs do not have a consistent effect on action-
reward learning in a purely operant task (63). This suggests that
motor-related DAN signals are not the substrate for operant
learning, and MB plasticity may specifically act to link sensory
cues to rewarding actions. Further, the theory assumes that
neural activity and reward are conditionally independent given
choice. The MB represents reward via DAN activity, so this
assumption could be violated if KC and DAN activity have
correlated variability across trials that is not related to choice.
Such correlations are feasible given indirect connections from
KCs to DANs and the complexity of DAN activity (31, 32, 78).
Finally, the theory pertains to tasks where the animal decides
between two options. Some animals have also been found to
exhibit operant matching behavior when choosing between three
or more options (8, 79). In this setting, operant matching still
implies that the covariance between neural activity and reward
vanishes, so there is hope that covariance-based plasticity rules
would generate matching. However, other behavioral strategies
can also lead to vanishing covariance (SI Appendix). It would
be interesting to investigate whether modified learning rules can
more reliably produce matching in naturalistic foraging scenarios
or multioption choice tasks.
Plasticity in Multiple MB Compartments Could Explain Devi-
ations from Matching. One complication to the framework of
expectation-based learning rules and matching is that flies, like
several other animals, don’t perfectly follow the matching law;
rather they undermatch. Two hypotheses have been proposed
to account for this deviation. The first proposes that animals
that undermatch make use of a learning rule that deviates from
a strictly covariance-based rule (19). Synaptic saturation and
representation of motor variables in DAN response, as discussed
in the previous section, offer particularly simple possibilities.
Another important possibility for how this could occur is to
have plasticity at multiple sites contributing to the overall
learning, with different plasticity rules at each site. Indeed, the
MB is divided into multiple compartments that contribute to
behavior but exhibit important differences in learning (22, 24).
It is possible that some compartments make use of reward
expectation in a covariance-based learning rule, while others
do not. Alternatively, undermatching can also result if reward
expectations are estimated over long timescales (13), even if
all compartments made use of a covariance-based rule. This
idea suggests that in a dynamic environment where baiting
probabilities change quickly, the memory of past experiences
acts as a bias that prevents the animal from correctly estimating
the present cue–reward relationships. This is possible in the MB,
as different compartments form and decay over different time
scales (24). Whether either or both of these hypotheses explain
undermatching in flies can be studied in future experiments by
manipulating different compartments of the MB circuitry and
analyzing the effect of such a manipulation on undermatching.
Relatedly, it would be interesting to check whether animals could
adapt the timescales used to estimate reward expectations to the
dynamics of the behavior task.
An Approach for Inferring Learning Rules from Behavior. Here,
we introduced a statistical method that uses logistic regression to
infer learning rules from behavioral data. While we specifically
applied our approach to infer learning rules for the fly mushroom
body, the inference of learning rules is of importance to many
areas of neuroscience (80–82). In fact, this method could be
similarly applied to model other learnt behaviors in the fly
and other animals. In the current work, we considered learning
rules that only depended on the current sensory stimulus (KC
response) and reward (DAN response), but our methodology
would also generalize to the inference of learning rules that
incorporated a longer time-scale history of sensory input and
reward. For example, the framework would be able to estimate
rules that incorporated the weighted average of recent sensory
experience.
However, it’s important to realize that the logistic regression
formalization would break down entirely for learning rules that
depend on the magnitudes of synaptic weights or postsynaptic
activity. Such terms would induce different nonlinear dependen-
cies between the choice sequence and learning rule parameters,
preventing us from converting these choice and reward histories
into regression inputs related to each component of the learning
rule (Materials and Methods). Our approach was appropriate here
because the plasticity rule in the mushroom body is not thought
to involve these terms. However, many biological learning rules
do depend on postsynaptic activity and current synaptic weights,
and future work should explore more flexible methodologies
from modern machine learning to develop generally applicable
approaches (82).
Circuit Mechanisms for Matching and Reward Expectation in
Drosophila. We have shown that operant matching is mediated
by synaptic plasticity in the fly mushroom body and involves the
calculation of a reward expectation. However, the mechanisms
underlying this calculation remain unclear.
The proposed mechanism underlying the calculation of reward
prediction error (RPE) in mammals provides a hint at one
option (74). Here, dopaminergic neurons implicitly represent
expectation by calculating the difference between the received
reward and the reward expectation. This has been found to
involve the summation of positive “reward” inputs and negative
GABA-ergic “expected reward” inputs to the dopaminergic
neurons (83). MB DANs could represent reward expectation in a
similar way. In fact, the recently released hemibrain connectome
(32) has found many direct and indirect feedback connections
from MBONs to DANs that theoretical work has shown could
support such a computation (43, 47). In the MB circuit, MBON
activity is related to the expectation of reward associated with a
given odor (22, 38, 42). An inhibitory feedback loop, via GABA-
ergic interneuron(s) for example, could potentially carry reward
expectation–related information from MBONs to DANs. The
negative expected reward signal from this interneuron could be
combined in the DANs with a positive reward signal from sensory
neurons, allowing DAN activity to represent the type of reward
expectation signal needed by a covariance-based rule.
It is important to note that such a mechanism would have a
major difference from mammalian RPEs. Since MBON activity
is linked to the presence of odor, the reward expectation signal
would vary across stimuli and only be present when the stimulus
was too. Thus, this signal would not have the temporal features
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of mammalian RPEs. This difference in temporal structure of the
reward expectation signal could explain the mixed observations
from past studies aimed at identifying reward expectation in
flies. For instance, a study that used temporally distinct cues and
reinforcements suggested that DANs do not incorporate reward
expectation (49), while studies that used temporally overlapping
cues and reinforcements did find signatures of reward expectation
(48, 52), albeit with different temporal properties than the typical
mammalian RPE.
It’s also possible that reward expectations are incorporated
into mushroom body plasticity by adjusting the levels of reward
and punishment needed to achieve a given dopamine signal. In
this scheme, reward-related dopamine neurons could represent
how much a reward exceeds expectations, and punishment-
related dopamine neurons could respond when expectations
are not met. This is reminiscent of the idea from Felsen-
berg et al. that interactions between reward and punishment-
related compartments in the MB guide bidirectional learning
(26, 45, 46, 51). However, here we extend the idea by proposing
that reward would not only modify KC-MBON synapses but
also modulate the baseline dopamine release or firing threshold
of reward-related dopaminergic neurons. Similarly, upon missing
an expected reward, learning would do the same for MBONs
and DANs in punishment-related compartments. The resulting
behavior would depend on the balance between the activity of
both reward and punishment compartments; if the reward and
punishment baselines were updated correctly, such a mechanism
could produce a covariance-based rule and support operant
matching. This mechanism would also tie into the notion that
phasic dopamine release (i.e., the difference of dopamine from
its baseline level) mediates the RPE signal in mammals.
Future experiments can distinguish between these hypotheses.
For instance, neural recordings can probe how DAN activity
changes over the course of the task, and connectomics can identify
other neurons in the system that may be important for the
computing of reward expectation. These types of experiments are
easily doable in the D. melanogaster model. Paired with further
modeling efforts and the foraging framework we developed, the
fly MB promises to be a system in which we can understand
decision-making at a level of detail that is presently unparalleled
in systems neuroscience.
Materials and Methods
Thefollowingisabriefdescriptionofthepaper’smethods.Acompletedescription
can be found in SI Appendix. Both brief and supplemental methods consist of
the same section headings.
Fly Strains and Rearing. D. melanogaster were raised on standard cornmeal
food supplemented with 0.2 mM all-trans-retinal at 25 ◦C (for Gr64f lines) or
21 ◦C (for other lines) with 60% relative humidity and kept in dark throughout.
using https://flycrispr.org/target-finder (69). The gRNA were then cloned into
pCFD5_5 (85).
Y-arena. A detailed schematic of the apparatus is provided in SI Appendix
and Information 1. A description of the custom MATLAB code (MATLAB 2018b,
Mathworks) used to control the Y-arena can be found in SI Appendix.
Circular Olfactory Arena. Group learning experiments (SI Appendix, Fig. S9)
were performed in a previously described circular arena (22).
Behavioral Experiments. For all experiments in the paper, two or three of the
odorants, 3-octanol (OCT) [Sigma-Aldrich 218405], 4-methylcyclohexanol (MCH)
[Sigma-Aldrich 153095], and pentyl acetate (PA) [Sigma-Aldrich 109584] were
used. Details regarding the instantiation of probabilistic rewards etc. can be
found in SI Appendix.
Quantitative Analysis and Behavioral Modeling. All analysis and modeling
were performed using MATLAB 2020b (Mathworks). Details are described in
SI Appendix.
Neural Circuit Model of Dynamic Foraging. We designed two versions of a
neural circuit model, inspired by work from Loewenstein and Seung (19), that
were used to simulate behavior. The first version aimed to directly replicate the
model used by Loewenstein and Seung (SI Appendix and Fig. S4A). The second
version incorporated modifications that made it more appropriate to our task
and the mushroom body (Fig. 3 A, Right and SI Appendix).
Plasticity Requirements of Operant Matching in the Mushroom Body
Model. An expansion of the mathematical proof provided by Loewenstein and
Seung’s to incorporate the structure of our task and architecture of the MB can
be found in the eponymous section of SI Appendix.
Logistic Regression Model for Estimating Learning Rules. To determine the
learning rules that best predict fly behavior, we designed a logistic regression
model that made use of the known relationship between MBON activity and
behavior (Fig. 4A). The mathematical working of this model can be found in the
eponymous section of SI Appendix.
Data, Materials, and Software Availability. Matlab code and data have been
deposited in Zenodo repositories (86, 87).
ACKNOWLEDGMENTS. This work was supported by HHMI. We thank Igor Ne-
grashov, Tobias Goulet, Peter Polidoro, Steven Sawtelle, and Jon Arnold for help
designing and fabricating the Y-arena and the Janelia Fly Facility for fly rearing
support. We also thank all the members of the Turner and Fitzgerald groups
for insightful discussions, and Brad Hulse, Eyal Gruntman, Sandro Romani,
Mehrab Modi, Yichun Shuai, Laura Grima, Luke Coddington, and Yoshi Aso
for feedback on the manuscript. A.E.R. would like to thank The Solomon H.
Snyder Department of Neuroscience’s Graduate Training Program and thesis
committee members Vivek Jayaraman, Ann Hermundstad, Jeremiah Cohen,
Christopher Potter, Yoshi Aso, and Erik Snapp for their guidance.
Cloning. The Gr64f promoter was amplified using Q5 High-Fidelity 2X- Master
Mix (New England Biolabs) from the Gr64f-GAL4 plasmid (84) and cloned into
the FseI/EcoRI digested backbone of pBPLexAp65 (27) using NEBuilder HiFi DNA
Assembly(NewEnglandBiolabs).FourgRNAforthegeneDop1R1weredesigned
Author affiliations: aJanelia Research Campus, HHMI, Ashburn, VA 20147; bSolomon H.
Snyder Department of Neuroscience, Johns Hopkins School of Medicine, Baltimore, MD
21205; and cDepartment of Physiology and Pharmacology, Sackler Faculty of Medicine,
Sagol School of Neuroscience, The School of Physics and Astronomy, Tel Aviv University,
Tel Aviv 6997801, Israel
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10.1371_journal.pone.0251004.pdf
|
Data Availability Statement: All data files are
available from the Open Science Framework (OSF)
database. URL: https://osf.io/2zty9/.
|
All data files are available from the Open Science Framework (OSF) database. URL: https://osf.io/2zty9/ .
|
RESEARCH ARTICLE
Remembering the romantic past:
Autobiographical memory functions and
romantic relationship quality
Cagla AydinID
1,2*, Asuman Buyukcan-Tetik1
1 Psychology Program, Faculty of Arts and Social Sciences, Sabanci University, Istanbul, Turkey,
2 Department of Psychology, Norwegian University of Science and Technology, Trondheim, Norway
* [email protected]
Abstract
Do the reasons why we think about our memories and share them with others have implica-
tions for our romantic relationship quality? In the present series of studies (total N = 1,102),
we aimed to answer this question by examining whether the self (e.g., creating a stable self-
image), social (e.g., connecting with others) and directive (e.g., guiding future behavior)
functions of regular memories (Study 1, Study 2) and relationship memories (Study 2, Study
3) were related to intimacy and satisfaction in the current relationship. We further investi-
gated these links when relationship memories were shared with the romantic partner (Study
3). Results showed no association between the self-reported uses of memory for regular
events and relationship quality. In contrast, the social function served by the relationship
events was positively associated, and the directive function was negatively associated with
intimacy and relationship satisfaction. When the memories were to be shared with the part-
ner, only social function was related, positively, to the relationship satisfaction. Findings
were discussed in terms of the importance of considering the self-reported reasons for
recalling an event and understanding of the contextual factors in remembering.
Introduction
We remember our personal past for many reasons. According to an influential approach, rea-
sons for autobiographical remembering are categorized into three broad functional categories
[1]. Self function refers to recalling events to maintain a sense that one is the same person over
time and keep a positive image of self [2, 3]. Memories are also recalled and shared interper-
sonally to form and strengthen social bonds or deepen intimacy with others, which is referred
to as the social function [4]. Finally, the directive function refers to using the personal past as a
prescription to guide future behaviors as well as problem-solving [5].
A major tenet of the functional approach is that past experiences are adaptively (re)con-
structed in order to make them meaningful for responding to ongoing changes in one’s eco-
logical context [6]. One such immediate context is one’s romantic relationships where
remembering may be consequential in terms of the degree of intimacy we feel toward our part-
ner or the satisfaction we get from the relationship. Therefore, in the present research
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OPEN ACCESS
Citation: Aydin C, Buyukcan-Tetik A (2021)
Remembering the romantic past: Autobiographical
memory functions and romantic relationship
quality. PLoS ONE 16(5): e0251004. https://doi.
org/10.1371/journal.pone.0251004
Editor: Alexandra Kavushansky, Technion Israel
Institute of Technology, ISRAEL
Received: January 17, 2021
Accepted: April 18, 2021
Published: May 3, 2021
Copyright: © 2021 Aydin, Buyukcan-Tetik. This is
an open access article distributed under the terms
of the Creative Commons Attribution License,
which permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All data files are
available from the Open Science Framework (OSF)
database. URL: https://osf.io/2zty9/.
Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
1 / 18
PLOS ONEMemory functions of the romantic personal past
program, we considered the extent to which the self-reported functions of autobiographical
memory relate to two main indicators of one’s romantic relationship quality, namely intimacy
and relationship satisfaction in individuals who are currently involved in a romantic
relationship.
Prior research has indeed linked different memory functions with romantic relationship
quality. Firstly, a strong association is shown between relationship satisfaction and social func-
tion. This was not surprising given that social function is defined as recalling past life to be inti-
mate with others [7, 8] or as coordinating the individual histories of the two partners [9]. For
instance, Alea and Bluck [7] targeted a form of social function, intimacy function, served by
autobiographical memories, and showed that simply the act of recalling (personal) relationship
events, in contrast to (impersonal) fictional vignettes, fostered intimacy in long-term romantic
relationships. In a similar vein, Alea and Vick [10] reported that frequently rehearsed relation-
ship-defining memories; that is, memories that are used for maintaining close bonds, pre-
dicted marital satisfaction.
In terms of self function, the evidence is relatively indirect. It was reported that the use of
autobiographical memories for self-related reasons was positively related to having good rela-
tions with friends and significant others [11]. Furthermore, given the findings that self func-
tion has strong relations with self-esteem [11, 12], and relationship satisfaction and self-esteem
are positively associated [13], it is likely that the use of self function would result in increased
satisfaction in close relationships. Similarly, based on the evidence that sharing self-relevant
information helps develop relationship intimacy [14], a positive association between remem-
bering for self-related reasons and intimacy between partners can possibly be formed.
Finally, a positive association is also reported between the use of directive function and rela-
tionship satisfaction. Philippe, Koestner, and Lekes [15] showed that couple-related memories,
by way of satisfying psychological needs, such as autonomy, relatedness and competence,
actively direct relationship satisfaction. It is worth noting here that Philippe et al. used the
term ‘directive function’ rather loosely- to refer to a directive influence on one’s thoughts and
behaviors rather than a memory to be instantly used to guide through a conflict. Based on
prior arguments that directive function helps individuals navigate difficult emotional situa-
tions [5, 16], an increase in the frequency of the use of memories for directive purposes, for
instance, problem-solving, may indicate a tendency to prevent or resolve relationship conflicts
in a constructive manner. This, in turn, may benefit relationship quality.
All in all, the reported positive associations suggest that using autobiographical memories
functionally benefits the romantic relationship experience. So far, the links from memory
functions to relationship quality have been formed by unpacking singular functions. While it
is worthwhile to identify individual functions for a fine-grained analysis on how memories
function in social context [8], a global analysis of functions is also critical to assess the extent to
which functions served by autobiographical memories -relative to each other- are implicated
in the dynamic interpersonal sphere. It has been hypothesized that individual memories may
serve more than one function depending on the current psychosocial needs of the individual
[5, 6]. Therefore, it is possible that uses of memories other than the one targeted in a particular
study inadvertently influence the relationship outcomes. A global assessment of the three func-
tions in tandem would allow for examining the relative contributions of each function to
romantic relationship quality. It also has ‘heuristic utility’ [17] in broadly thinking about the
functions in the close relationship context. Thus, the main goal of present study is to examine
how different functions of autobiographical memories relate to the quality of romantic
relationships.
Global assessments of the functions of autobiographical remembering have been made pos-
sible by using psychometric scales, such as the TALE (Thinking About Life Experiences
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
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PLOS ONEMemory functions of the romantic personal past
Questionnaire; [4, 18]) and the RFS (Reminiscence Functions Scale; [19]). Rather than focus-
ing on specific relationship events, these scales focus on recalling over one’s life. For the pres-
ent purposes, there are several advantages of using a scale of this sort. First, when the focus is
on particular events, it is not clear whether functions will generalize across all types of events.
For instance, it has been reported that different classes of memories, such as, positive and neg-
ative memories, fulfil different functions; contributing to a self-concept and social bonding,
and help avoiding dangers, respectively [20]. Second, in previous work on relational outcomes,
how memories are used by individuals are largely inferred either by asking individuals to
remember intimate relationship events so that the use of the social function is implicated or
content analyzing the conversational narratives to identify functional themes [e.g., 17]. Alter-
natively, the items in the TALE are supposed to tap deliberate uses of the past [8, 18] via self-
reported intentions and goals [21]. By explicitly asking participants to think back, and report
the usefulness of their own memories, we aim to qualify the connection between conscious
recollection of the past and perceived romantic relationship quality.
Overview of the present studies
In the present series of studies, we examined the association between people’s use of autobio-
graphical memory to serve self, social and directive functions and the quality of one’s romantic
relationship. Based on the argument that memories can serve more than just one function [5],
this approach would allow for observing the relative contributions of the three conceptually
distinct functions to relationship quality. To do so, we relied on individuals’ evaluations of the
functions of their own memories by using the TALE scale rather than their responses to spe-
cific recollections. Participants had been romantically involved with someone for a minimum
of three months, and they were over 18 years of age. First, in Study 1, we started by investigat-
ing whether the way individuals use their -everyday- memories are related to the quality of
their romantic relationships. Given the finding that all three functions are related to psycho-
logical well-being [11], we pursued the question whether the well-being of the relationships
depended on the functions one’s personal memories serve. In Study 2, relying on the possibil-
ity that relational events may afford potentially different uses compared to everyday personal
memories [22], our focus was on how relationship-related memories are used functionally and
how those uses were associated with relationship quality. To that effect, we slightly modified
the wording in the TALE items to reflect that participants need to think over their romantic
life with their current partner when responding to the uses of their memories. Finally, drawing
on the possibility that uses of recalling an autobiographical memory may shift when one
reports past experiences to a certain addressee, in particular, to the romantic other, we
explored whether functions of romantic memories changed when reporting to the current
romantic partner and how that related to one’s relationship quality (Study 3).
We operationalize romantic relationship quality as one’s subjective and global evaluation of
the relationship [23]. Previous research emphasized the importance of using different rela-
tional qualities (e.g., satisfaction, intimacy, commitment, conflict, ambivalence, love) to have a
nuanced understanding of how each quality affects the outcomes [e.g., 24–26]. For reasons of
brevity, we did not use the whole range of the possible qualities.
Finally, because, to our knowledge, this is the first systematic study to examine the scope of
all three functions of memory and their relations to relationship quality, we included an
important relational construct and an individual difference variable, attachment style, as a con-
trol variable. Attachment style is known to be a strong predictor of relationship quality [27,
28]. Attachment orientations were also reported to be systematically related to what individu-
als recall about relationship events [29]. Since responses to the items in the TALE can also be
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
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PLOS ONEMemory functions of the romantic personal past
thought of “stylistic differences of reminiscence across individuals” [8], controlling for a
potentially confounding individual difference variable was critical for our purposes. Attach-
ment orientations have been shown to be associated with memory usage. Whereas attachment
avoidance was not related to any of the functions, higher attachment anxiety was found to be
related with the social function [30]. How individuals with different attachment styles respond
to a break up was found to be mediated by remembering relationship related events [31].
Age and gender were included as the other control variables. Age was included as a control
variable because, in the functional remembering literature, it has been shown to be an impor-
tant factor [e.g., 30]. Gender was included because previous studies on relationship memories
point to slight gender differences [e.g., 29].
Study 1
In Study 1, we establish how functions of generalized memories of life events are associated
with romantic relationship quality. Previously, it has been shown that individuals who use
their specific memories to serve all three functions reported higher levels of psychological well-
being [11, but 32]. Since having intimate romantic relationships are fundamental to socio-
emotional well-being [33], we expected that reasons to remember personal experiences would
have similar associations with relationship well-being. Even though we did not have predic-
tions as to the relative weights of each function, we predict that social function would be asso-
ciated with the relationship outcomes to a greater level than the other functions. This is due to
the fact that social use of the memory entails maintaining intimacy which should reflect on the
quality of our relationships—regardless of the event’s theme (relationship related or not). In
fact, Waters [11] study showed that for the use of memories for the recurring events was highly
associated with having positive relationships.
Materials and methods
Participants
Participant recruitment was conducted through an online crowdsourcing company, Prolific.
Inclusion criteria for this study were: being involved in a romantic relationship currently at least
for 3 months, being older than 18 and younger than 70, and being a native speaker of English. In
the original data, there were 252 entries. Fifteen participants were excluded due to multiple
entries, not meeting the inclusion criteria (e.g., being age 18 or over, being in an on-going rela-
tionship at least for three months, speaking English as their native language) or not providing the
Prolific identity number. Detailed information about excluded participants is available upon
request. Sample characteristics of the remaining 237 participants are presented in Table 1.
We computed our sample size for multiple regression analyses based on the total number of
predictors with the control variables (i.e., 9–14 variables) across our studies. We had an expec-
tation of a medium effect size [34]. Using a desired statistical power level of .8 and a probability
level of .05, minimum required sample size was between 113 and 135 across our studies [35],
which were all exceeded (nStudy-1 = 237, nStudy-2 = 410, nStudy-3 = 455).
We received ethical approval for this study from Sabanci University Research Ethics Coun-
cil with the protocol number FASS-2019-49. All participants gave their electronic informed
consent for participation before they filled in the survey in return of GBP 1.35.
Procedure and measures
Questionnaires were administered online. After completing the consent form and reading the
instructions, the participants first completed the Thinking about Life Experiences
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
4 / 18
PLOS ONETable 1. Sample characteristics across studies.
Age
Gender (Female %)
Ethnicity (Caucasian %)
Sexual orientation (Heterosexual %)
Relationship type (Married %)
Relationship duration in years
Parents (%)
Number of children
Education (Bachelor’s %)
Education (High school/GED %)
https://doi.org/10.1371/journal.pone.0251004.t001
Memory functions of the romantic personal past
Study 1 (n = 237)
Study 2 (n = 410)
Study 3 (n = 455)
M or %
SD
M or %
SD
M or %
SD
10.74
10.53
0.83
41.96
78.10
93.20
95.80
97.00
16.27
71.30
1.99
37.10
38.00
10.90
9.31
0.88
35.97
74.70
91.00
96.00
58.50
11.35
56.80
1.90
45.90
36.80
11.48
9.86
0.93
36.53
70.40
90.50
97.40
57.30
12.29
56.30
1.93
33.20
42.00
Questionnaire [TALE; 18], and then the relationship intimacy (Inclusion of the Other in Self;
IOS; [36]) and satisfaction (Relationship Assessment Scale by Hendrick [37]) scales. The ques-
tionnaire ended with the Experiences in Close Relationships-Revised Questionnaire [28] in
order to measure attachment orientations and the demographic questions. The TALE was
administered before the relationship quality measures; the order was not counterbalanced.
Other scales administered but not used for the present work are not reported here (see S1
Appendix).
Functions of autobiographical memory. Thinking About Life Experiences Question-
naire [TALE; 18] was used to measure the three functions of memory: self function, social
function, and directive function. We asked why participants think back or talk about their life.
Sample reasons to assess each function were “when I want to feel that I am the same person
that I was before”, “when I hope to also learn more about another person’s life”, and “when I
believe that thinking about the past can help guide my future”, respectively. Additionally, there
were two items two assess the baseline level (i.e., “In general, how often do you think back over
your life?” and “In general, how often do you talk to others about what’s happened in your
life?”). A 5-point Likert scale (1 = “almost never”, 5 = “very frequently”) was administered.
We first conducted an exploratory factor analysis using varimax rotation and maximum
likelihood estimation to investigate the factorial structure of the TALE in our data. Results
revealed three factors with an eigenvalue higher than 1, which altogether explained 52.12% of
the variance. All items except one were clearly loading to their function in the original scale.
The item “when I want to remember something that someone else said or did that might help
me now” had similar loadings on both social function and directive function (loadings of .38
and .34, respectively). Thus, we conducted all analyses first including, and then excluding this
item. Each time, we got the same results in terms of the memory functions’ associations to the
relationship quality indicators. In the reported analyses, we included this item under the direc-
tive function considering the original scale. Besides, exclusion of this item did not increase the
internal reliability of the subscale for directive function. For this particular study, the five-item
scales for self function, social function, and directive functions had good internal reliability;
Cronbach alpha levels of .82, .81, and .84, respectively.
Relationship quality. To assess relationship quality, we used two different indicators: inti-
macy and relationship satisfaction. Intimacy in the participants’ romantic relationship was
assessed using the Inclusion of Other in the Self Scale [IOS; 36]. This assessment is done via a
7-point Likert type item composed of seven pictures (degrees of interlocking or isolated cir-
cles) representing different levels of closeness. The participants were asked to choose the
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
5 / 18
PLOS ONEMemory functions of the romantic personal past
option that best portrays their relationship. Higher scores (i.e., increasingly overlapping cir-
cles) indicated higher levels of closeness. Comparison of different closeness measures showed
that the IOS scale “is a psychologically meaningful and highly reliable measure of the subjective
closeness of relationships” [38, p. 1]. Relationship satisfaction was assessed with six items of the
7-item Relationship Assessment Scale, which was developed by Hendrick [37]. Sample item
was “In general, how satisfied are you with your relationship?” One item from the original
scale “How much do you love your partner?” was mistakenly omitted in the online version.
We administered a 5-point Likert scale (1 = “very low”, 5 = “very high”). Cronbach alpha level
was .94 for this study.
Attachment. Attachment styles were assessed using the Experiences in Close Relation-
ships-Revised Questionnaire [28]. Each subscale for assessing anxious and avoidant attach-
ment styles had 18 items. Sample items were “I often worry that my partner doesn’t really love
me.” and “I find it difficult to allow myself to depend on romantic partners.” for anxious and
avoidant attachment styles, respectively. For all items, we administered a 5-point Likert scale
(1 = “strongly disagree”, 5 = “strongly agree”). Cronbach alpha level was .94 for anxious attach-
ment and .96 for avoidant attachment.
Results and discussion
Descriptive statistics and correlations
Descriptive statistics of and correlations among study variables are presented in Table 2. Cor-
relations revealed that the three functions of autobiographical memory (i.e., self, social, and
directive functions) had moderate positive associations with each other. Out of the three func-
tions, only the self function was significantly, but negatively, associated with intimacy and rela-
tionship satisfaction. Self function had positive associations with both anxious and avoidant
attachment styles. Anxious attachment was positively related to directive function as well.
Regression results. We regressed intimacy and relationship satisfaction onto self, social,
and directive functions of autobiographical memory. We also controlled for the effects of age,
gender, attachment styles, and baseline levels of thinking and talking about life (see the
Method section for the assessment of baseline levels). Regression results showed that none of
the functions of autobiographical memory had any significant associations with either inti-
macy or relationship satisfaction (see Tables 3 and 4).
In Study 1 we examined the association between the functions of regular autobiographical
memories and relationship quality indicators (i.e., relationship satisfaction and intimacy).
Even though the self function was negatively correlated with the relationship quality indica-
tors, the results of the regression analyses did not support our predictions. The functions of
autobiographical memories were not associated with relational outcomes.
Table 2. Descriptive statistics and correlations among the study variables in Study 1.
Variable
1 Self function
2 Social function
3 Directive function
4 Intimacy
5 Relationship satisfaction
6 Anxious attachment
7 Avoidant attachment
Note. All values in bold had a p-value lower than .05.
https://doi.org/10.1371/journal.pone.0251004.t002
M
2.76
2.91
3.27
5.34
3.98
2.29
2.09
SD
0.83
0.79
0.76
1.59
0.94
0.82
0.78
1
-
.41
.53
-.20
-.21
.25
.21
2
-
.59
.00
-.02
.11
-.02
3
-
-.08
-.06
.16
.01
4
5
6
-
.78
-.52
-.65
-
-.57
-.70
-
.64
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
6 / 18
PLOS ONETable 3. Regression results for intimacy across studies.
Age
Gender
Anxious attachment
Avoidant attachment
Thinking about life
Talking about life
Thinking about romantic relationship
Talking about romantic relationship with others
Talking about romantic relationship with the partner
TALE Self function
TALE Social function
TALE Directive function
TARE Self function
TARE Social function
TARE Directive function
SHARE Self function
SHARE Social function
SHARE Directive function
Memory functions of the romantic personal past
Study 1
Study 2
β
p
β
.01
-.08
-.15
-.55
-.06
-.05
-
-
-
-.04
.10
-.04
-
-
-
-
-
-
.86
.13
.03
.00
.34
.38
-
-
-
.51
.11
.58
-
-
-
-
-
-
-.09
-.07
-.14
-.43
-.06
.02
-.02
-.06
-
.02
-.12
.13
.04
.16
-.20
-
-
-
p
.05
.10
.00
.00
.26
.73
.65
.23
-
.76
.06
.06
.62
.03
.01
-
-
-
Study 3
β
p
-.01
.02
-.13
-.39
-
-
.01
-.06
.10
-
-
-
.10
-.06
-.13
-.02
.11
.01
.89
.61
.01
.00
-
-
.83
.21
.04
-
-
-
.23
.37
.10
.76
.12
.88
Note. All values in bold had a p-value lower than .05. TALE = Thinking About Life Experiences Questionnaire.
TARE = Thinking About Relationship Experiences Questionnaire (see Study 2). SHARE = Sharing Relationship
Experiences (see Study 3).
https://doi.org/10.1371/journal.pone.0251004.t003
Table 4. Regression results for relationship satisfaction across studies.
Age
Gender
Anxious attachment
Avoidant attachment
Thinking about life
Talking about life
Thinking about romantic relationship
Talking about romantic relationship with others
Talking about romantic relationship with the partner
TALE Self function
TALE Social function
TALE Directive function
TARE Self function
TARE Social function
TARE Directive function
SHARE Self function
SHARE Social function
SHARE Directive function
Study 1
Study 2
β
-.10
-.05
-.19
-.55
-.09
.06
-
-
-
-.03
.03
-.02
-
-
-
-
-
-
p
.03
.32
.00
.00
.10
.27
-
-
-
.62
.65
.77
-
-
-
-
-
-
β
-.12
-.03
-.18
-.47
-.07
.05
.04
.01
-
-.04
-.07
.04
.01
.19
-.19
-
-
-
p
.00
.43
.00
.00
.11
.31
.39
.74
-
.59
.23
.55
.93
.01
.01
-
-
-
Study 3
β
p
-.06
.01
-.19
-.46
-
-
.01
.04
.04
-
-
-
.01
.01
-.14
-.05
.17
-.04
.11
.81
.00
.00
-
-
.75
.44
.34
-
-
-
.85
.88
.04
.48
.00
.56
Note. All values in bold had a p-value lower than .05. TALE = Thinking About Life Experiences Questionnaire.
TARE = Thinking About Relationship Experiences Questionnaire. SHARE = Sharing Relationship Experiences.
https://doi.org/10.1371/journal.pone.0251004.t004
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PLOS ONEMemory functions of the romantic personal past
Study 2
Whereas Study 1 addressed the association between functions of regular autobiographical
memories and romantic relationship quality, in Study 2 we were motivated by the idea that
remembering the romantic past may have more direct implications for the quality of one’s
relationships. What purposes do the memories about the loved ones serve in the relationship
context? While prior work examining this relationship targeted remembering particular inci-
dents in a romantic relationship; such as the first time someone met their spouse [10], here,
again, we explore the different functions that the generalized relationship memories serve. Fol-
lowing up on the reasoning in Study 1, if these global assessments consist of conscious pro-
cesses; that is, individuals are aware of the purposes their romantic memories serve, there may
be direct consequences for the relationship quality. For parsimony, we again adopted a self-
report methodology, and modified the items in the TALE for assessing the functions of roman-
tic relationship-related memories. Changing the instructions in the original TALE so that the
participants answer the items in reference to different classes of memories has also been sug-
gested by the creators of the scale [18]. A secondary aim of Study 2 was to replicate the findings
in Study 1, particularly that functions of regular memories were not related to any one of the
relationship quality indicators.
Since, in previous work, there is no direct evidence informing us about the link between the self
function and romantic relationship quality, we rely on the reported positive associations between
the self function and self-esteem [11, 12], and self-esteem and relationship satisfaction [13]. We
therefore expect the frequency of self-function to be positively related to relationship quality.
Similarly, we expect the social function to be positively associated with the relationship
quality indicators. For instance, if one remembers romantic memories to “coordinate the indi-
vidual histories of the two partners” [9], intimacy, and relationship satisfaction should
increase. In fact, Alea and Vick [10] reported that memories of relationship events with higher
qualitative richness -vivid and rehearsed- would correspond to higher marital satisfaction. In a
similar vein, warmth and closeness in a relationship increased after recalling a relationship
event [7]. Other work with couples has also found that retrieving autobiographical memories
about instances where the couple laughed together, as opposed to individual laughter-related
events, was related to enhanced marital satisfaction [39].
Finally, we also expect the use of the directive function to be positively associated with rela-
tionship quality indicators. Recently, Philippe et al. [15] broadly defined the directive function
of memories as having a long-term impact on the cognitions and emotions. They showed that
by way of satisfying psychological needs, such as autonomy, relatedness and competence, cou-
ple-related memories directively influence relationship satisfaction. In the present context,
using romantic relationship memories to guide behavior or to solve current problems should
be positively associated with the quality of one’s relationship as it implies actively working on
the issues in the relationship.
Materials and methods
Participants
Participant recruitment was conducted through the same resource; Prolific. All inclusion crite-
ria and consenting procedures followed Study 1’s lead. Participants in Study 1 were not
allowed to participate in Study 2. After excluding 48 participants due to several reasons (e.g.,
indications of not responding in an honest matter, such as, failure to mark the requested
option in the quality check items or not fulfilling the inclusion criteria listed in Study 1), the
sample consisted of 410 participants. Sample characteristics are given in Table 1.
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PLOS ONEMemory functions of the romantic personal past
Procedure and measures
All measures were the same as in Study 1 except for a modified version of the TALE [18] which is
described below. Participants first received the original TALE then the modified Thinking About
Relationship Experiences (TARE) Questionnaire to investigate the functions of memories specific
to the current relationship. The administration order of these questionnaires was fixed because we
wanted the TALE results to be generalizable to the other studies; that is, responses not to be influ-
enced by prior thoughts about relationships. These are followed by the measures of relationship
quality indicators; namely, intimacy and relationship satisfaction. For full descriptions, please see
Study 1. This time, we used the full scale for relationship satisfaction.
Functions of autobiographical memory. Similar to Study 1, to measure the functions of
autobiographical memory, we used the TALE [18]. We again conducted an exploratory factor
analysis using varimax rotation and maximum likelihood estimation to investigate the factorial
structure of the TALE. Results of the factor analyses, which confirmed the original 3-factor
structure, are given in S1 Appendix. For the present study, the five-item scales for self-func-
tion, social function, and directive function had good internal reliability levels of .83, .80, and
.84, respectively.
Functions of relationship-related memories. To measure the functions of relationship-
related autobiographical memories, we slightly modified the items of the original TALE Ques-
tionnaire to help respondents think back and talk about their relationship. For instance, a
directive function item in the original scale being “I think back and talk about my life or cer-
tain periods of my life when I want to learn from my past mistakes” was reworded as “I think
back and talk about my relationship or certain parts of my relationship when I want to learn
from my past mistakes.” For parsimony, we name this version Thinking About Relationship
Experiences (TARE) Questionnaire. The participants were warned at the beginning that they
were going to answer two similar but slightly different questionnaires; one would be about
their life and the other one being about their current romantic relationship. Comparison of all
items across two scales are presented in the S1 Appendix section. Again, a 5-point Likert scale
(1 = “almost never”, 5 = “very frequently”) was administered in all items. Factor analyses
revealed a 3-factor structure as same as the functions in the original TALE (see S1 Appendix),
which altogether explained 58.72% of the variance. Internal reliability, Cronbach alpha, levels
of five-item scales in the TARE for self-function, social function, and directive function of rela-
tionship-related memories (.88, .85, and .88, respectively) were slightly higher compared to the
ones in the original TALE.
Results and discussion
Descriptive statistics and correlations
Table 5 presents the descriptive statistics of and correlations among study variables in Study 2.
Regarding the TALE scale, correlations were very similar to the ones reported in Study 1 in
terms of both significance and magnitude. Two differences were the non-significant correla-
tion between self-function and intimacy, and the positive association between social function
and anxious attachment.
Correlations between the same functions in the TALE and TARE ranged between .69 and
.72, meaning that they overlap with each other to some extent but are not the same constructs.
Significant correlations between functions in the TARE and relationship quality variables
showed that self-function in the TARE was negatively linked to relationship satisfaction while
directive function in the TARE is negatively associated with both intimacy and relationship
satisfaction.
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PLOS ONEMemory functions of the romantic personal past
Table 5. Descriptive statistics and correlations among the Study 2 variables.
Variable
1 TALE Self function
2 TALE Social function
3 TALE Directive function
4 TARE Self function
5 TARE Social function
6 TARE Directive function
7 Intimacy
8 Relationship satisfaction
9 Anxious attachment
10 Avoidant attachment
M
2.66
2.90
3.26
2.63
2.97
3.07
5.22
4.06
2.48
2.18
SD
0.81
0.77
0.76
0.87
0.84
0.86
1.39
0.73
0.80
0.70
1
-
.45
.56
.72
.40
.41
-.05
-.14
.27
.13
2
-
.54
.46
.69
.50
-.03
.01
.15
-.05
3
4
5
6
7
8
9
-
.52
.50
.69
-.04
-.09
.20
.06
-
.60
.67
-.08
-.13
.26
.13
-
.67
.04
.08
.14
-.08
-
-.10
-.11
.20
.05
-
.67
-.32
-.50
-
-.38
-.60
-
.42
Note. All values in bold had a p-value lower than .05. TALE = Thinking About Life Experiences Questionnaire. TARE = Thinking About Relationship Experiences
Questionnaire.
https://doi.org/10.1371/journal.pone.0251004.t005
Regression results
Regression results with the control variables revealed that, similar to the Study 1 results, there
was no link between functions of memory in the original TALE scale and neither relationship
satisfaction nor intimacy (see Tables 3 and 4). Both the intimacy and relationship satisfaction
had positive and negative links with social and directive functions in the TARE respectively.
These effects were significant although we controlled for the effects of confounding variables
including the attachment types (see Tables 3 and 4). Effect sizes were small (f2 = .01 for the
effect of social function on intimacy, f2 = .02 for the effect of directive function on intimacy as
well as for the effects of social and directive functions on relationship satisfaction).
In our regression analysis, similar to the findings in Study 1, there was no link between
functions of memory in the original TALE scale and neither relationship satisfaction nor inti-
macy (see Tables 3 and 4). In turn, consistent with our predictions, functions measured by
TARE had different associations with the relationship quality indicators. Even after controlling
for attachment, age and gender, social function was positively associated with intimacy and
relationship satisfaction; whereas directive function was negatively related to them. This find-
ing supports the idea that relationship-related memories are used in individuals’ daily lives
and are related to relational outcomes. This pattern contributes to the literature in that not
only singled out episodes in relationships, such as vacation with the partner [7] or first time
someone met their spouse [10] would function to increase intimacy levels or satisfaction in a
relationship but also generalized evaluations of multiple episodes, such as the items in TARE,
have associations with the quality of one’s relationship. Overall, the present study was a first in
showing that the functions of relationship-related memories, when studied together, are
related to the relationship quality. This association is qualitatively different from the pattern
with regular, non-relationship-themed, memories which is a finding points to the need for dis-
tinguishing different themes/classes of memories when examining their functions.
Study 3
It has been established that memory sharing is one of the primary functions of autobiographi-
cal memory [40]. Sharing autobiographical memories has been shown to lead to relationship
closeness across cultural settings [41]. It is, therefore, a critical omission in the memory func-
tion literature that the role of different interlocutors is rarely considered. Given that the func-
tions and characteristics of the shared vs non-shared memories differ [e.g., 42], events shared
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PLOS ONEMemory functions of the romantic personal past
with an intimate other are likely to differ in function from events shared with other people
[22]. Therefore, the main aim was to explore whether functions of relationship-related events
were similarly associated with relationship quality indicators when they were shared with the
romantic partner vs any other person. It is, for instance, possible that when shared memories
have a tendency to serve a self function, the quality of the relationship may deteriorate due to
the realization of the partner of not being included in the meaning-making process of the
romantic experience. Thus, associations between memory functions and relationship quality
may be different than when they were not shared with the partner or in some cases, such as
when the social function is involved, may be enhanced.
Materials and methods
Participants
We followed the same procedure explained in the first two studies and recruited participants
through Prolific. Participants in the first two studies were not allowed to participate in this
study. There were 455 participants in the dataset after the exclusion of 63 participants because
of various reasons such as failure in quality check questions. Characteristics of the final sample
are given in Table 1.
Procedure and measures
Functions of relationship-related memories. We used the TARE (Thinking about Rela-
tionship Experiences) again to examine whether we can replicate our findings in Study 2. Fur-
thermore, we adapted the items in the TARE to investigate the functions of relationship
memories when they were shared with the current partner. The modified scale is referred to as
SHARE (Sharing Relationship Experiences) from here on. As an example for the difference
between the TARE and SHARE scales, the TARE item “I think back and talk to other people
about my relationship or certain parts of my relationship when I want to learn from my past
mistakes” was used as “I think back and talk to my partner about my relationship or certain
parts of my relationship when I want to learn from my past mistakes” in the SHARE. Compari-
son of items across scales are presented in the S1 Appendix section.
Exploratory factor analysis revealed a 3-factor structure for the TARE in this study too with
an explained variance of 56.72% in total (see S1 Appendix). Although two items loaded simi-
larly onto two different factors, the results were almost identical when those items were
excluded except that the effect of TARE directive function on relationship satisfaction in
Table 4 became marginal (β = -.12, p = .07). Thus, we continued with the original 3-factor
structure. Five-item scales for self-function, social function, and directive function in the
TARE had good internal reliability levels of .87, .83, and .86, respectively.
The SHARE had a 2-factor structure in the exploratory factor analysis with an explained
variance of 58.26% (see S1 Appendix). The first factor was again representing the self function
with the same 5 items. Social and directive functions however, overlapped and constituted a
separate function together. This indicates that talking about the relationship problems with the
partner to guide future behaviors for example (i.e., directive function) also has a role in bond-
ing the partners with each other (i.e., social function). For the present results to be comparable
with the findings using both the TALE and TARE as well as considering the theoretical differ-
ences between social and directive functions, we still used these two factors of SHARE sepa-
rately in our analysis. The internal reliability levels were also supporting our decision to
use the three functions separately. Five-item scales for self function, social function, and direc-
tive function had good internal reliability levels (Cronbach alphas) of .91, .89, and .85,
respectively.
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PLOS ONEMemory functions of the romantic personal past
Other variables. For assessing relationship quality and attachment styles, we used the
same measures in previous studies. Internal reliability levels for relationship satisfaction, anx-
ious attachment, and avoidant attachment were .92, .93, and .95 respectively.
Results and discussion
Descriptive statistics and correlations
Descriptive statistics of and correlations among study variables in Study 3 are presented in
Table 6. Correlations between functions in the TARE and relationship quality variables
showed negative association of self function and directive function with relationship satisfac-
tion. All three functions were positively linked to anxious attachment. Avoidant attachment
was positively linked to self function, but negatively linked to social function.
Correlations between the same functions in the TARE and SHARE ranged between .52 and
.73, which showed that they somewhat overlap with each other but tap into different con-
structs. Correlations between functions in the SHARE and relationship quality revealed that
only social function had significant associations with intimacy and relationship satisfaction.
Regression results
Regression results in Table 3 showed that none of the functions either in TARE or SHARE had
significant associations with intimacy. Results about relationship satisfaction however, showed
that directive function in the TARE and social function in the SHARE were negatively and
positively associated with relationship satisfaction, respectively. In the model with the control
variables including attachment (Table 4), effect sizes were relatively small: f2 = .01 for the effect
of TARE directive function on relationship satisfaction, and f2 = .02 for the effect of SHARE
social function on relationship satisfaction.
Furthermore, as described in S1 Appendix, we also conducted the same analysis using the
2-factor structure of the SHARE (i.e., social function and the combined factor of social and
directive functions). The results with the 2-factor structure of the SHARE revealed that the
negative effect of TARE directive function on intimacy was in line with the finding in Study 2
(see Table 3). This effect was not significant in Study 3 when the 3-factor structure was used
(see Table 3). The combination of the social and directive functions in the SHARE had a posi-
tive effect on relationship satisfaction. This effect was in line with the positive effect of social
function in Study 3 when the 3-factor structure was used (see Table 4). SHARE directive
Table 6. Descriptive statistics and correlations among the Study 3 variables.
Variable
1 TARE Self function
2 TARE Social function
3 TARE Directive function
4 SHARE Self function
5 SHARE Social function
6 SHARE Directive function
7 Intimacy
8 Relationship satisfaction
9 Anxious attachment
10 Avoidant attachment
M
2.26
2.75
2.89
2.43
3.40
3.09
5.22
4.03
2.43
2.11
SD
0.86
0.84
0.85
0.93
0.90
0.84
1.47
0.79
0.82
0.72
1
-
0.54
0.66
0.73
0.30
0.40
-0.09
-0.16
0.29
0.11
2
-
0.67
0.40
0.52
0.49
-0.03
0.02
0.17
-0.09
3
-
0.52
0.48
0.66
-0.08
-0.10
0.21
0.00
4
-
0.48
0.63
0.00
-0.07
0.23
-0.02
5
6
7
8
9
-
0.72
0.17
0.22
0.12
-
0.07
0.05
0.16
-0.28
-0.16
-
0.66
-0.32
-0.48
-
-0.41
-0.62
-
0.46
Note. All values in bold had a p-value lower than .05. TARE = Thinking About Relationship Experiences Questionnaire. SHARE = Sharing Relationship Experiences.
https://doi.org/10.1371/journal.pone.0251004.t006
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PLOS ONEMemory functions of the romantic personal past
function alone was not significant (see Table 4). We also explored whether the associations of
memory functions with relationship quality depended on relationship duration across the
three studies (30 interactions in total). The results showed that only one out of 30 interactions
was noteworthy (see S1 Appendix for details). The association of the SHARE social function
with intimacy depended on relationship duration (b = .35, p < .001). Simple slope analyses
showed that the SHARE social function had a significant positive association with intimacy in
people with longer relationship duration (1 SD above the sample mean; b = .56, p < .001), but
no association in people with shorter relationship duration (1 SD below the sample mean; b =
-.13, p = .30). Hence, our exploratory examinations did not reveal a strong role of relationship
duration in the associations between memory functions and relationship quality.
Thus, when the addressee of the memory sharing activity was defined as the romantic part-
ner, the way functions were linked to the quality of relationships slightly differ from when
sharing with the partner is not specified. The positive association between the social function
and relationship satisfaction (see TARE results in Study 2) is still intact however the negative
link between the use of directive function and satisfaction (see TARE results in Study 2 and
Study 3) is not there anymore. This is the first study that we know of to show that the associa-
tions between the intended reasons to share romantic memories and relationship quality may
slightly change when the romantic partner is the interlocutor. The implications of this finding
are discussed further below.
General discussion
In the present study we aimed to examine the association of functional use of memory and
romantic relationship quality in three studies. We did so by focusing on generalized views on
memories in order to examine the three overarching functions together. We first looked at
whether everyday memories’ functions and relationship quality were linked. In a second study,
we shifted our focus to the functions relationship-related memories serve; and in the third
study, we specified the interlocutor as the romantic partner when considering functions of
romantic memory sharing and relationship quality association.
The predicted positive relations between the three functions and functions of everyday
memories were not confirmed; none of the functions were associated with the quality indica-
tors. We did, however, observe the functions of romantic memories to be associated with
romantic relationships outcomes. How shall these findings inform current theorizing regard-
ing functional remembering in social context?
The lack of association between reasons to remember everyday autobiographical events and
the quality of one’s relationships is in contrast with the previous findings showing that func-
tional remembering -all three functions- is related to having positive relationships [11]. A
closer look, however, reveals that the association in the Waters study was reported for single
events only but not for recurring or general events. A summary of one’s lifetime periods, as
indexed by the TALE, may not have the functional power to have an immediate effect on the
relationship quality but a relationship-specific single event, such as “when we saw that movie
together” or even general events such as “our walks to school together” might have a binding
role for other relationship events; and therefore, its functional relations to the quality of
romantic relationships may be more salient. The present findings therefore suggest that the
general tendency with which individuals remember their past life may not be associated with
the quality of their romantic relationships.
Alternatively, the items in the two studies may have tapped different aspects of the so-called
self function. While Waters [11] used the Centrality of Event Scale which measures whether
the event recalled constitutes a key part of one’s identity; therefore, taps the identity aspect; the
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PLOS ONEMemory functions of the romantic personal past
TALE scale in the present study examines the continuity of the self in time; therefore, coher-
ence of a self-concept. Pillemer [8] has noted that conceptual categories that the TALE mea-
sures may not encompass the full spectrum of the subfunctions of a particular category.
Therefore, self function as indexed by the TALE items may not be associated with the relation-
ship quality but whether or not a particular event helps defining the self is. Further research
with a focus on the spectrum of the subfunctions is required to support this interpretation.
With regards to the romantic memories (Study 2), results suggest that the social function
was positively related to both relationship satisfaction and intimacy; whereas directive function
was found to be negatively correlated with both of them. The positive association between the
social function and how it is related to relationship satisfaction [10] and intimacy [7] has been
shown previously with specific, one-time relationship memories, such as first-sight or first-
kiss. The novelty of the present findings is that across different settings–as the items in the
scale imply- social function is similarly related to the relationship outcomes.
An unexpected finding was the directive function to be negatively associated with relation-
ship quality when relationship memories are shared with other people. Previous research has
tracked the influence of a single episode; a specific memory’s directive power and found that it
was positively associated with one’s satisfaction in a relationship [8, 15]. A relationship-related
memory; for instance, a prior quarrel, could naturally be used constructively within the rela-
tionship for problem-solving purposes or to fine-tune particulars of future behavior. When
faced with a problem, individuals would bring to mind memories of situations involving a sim-
ilar problem and use that particular memory to work through the challenge [1, 17, 43]. This
guidance would reflect positively on relationship satisfaction. In the present study, however,
we were dealing with a global evaluation of how frequently relationship memories are used for
directive purposes. If one’s perception of the frequency of the use of memories for problem-
solving purposes, is high, it might indicate that the frequency of the problems to be solved is
also high. Following that logic, if the perception of the number of problems (that needs to be
solved) in a relationship is high, it is highly likely that the perceived satisfaction in the relation-
ships would not benefit it.
The negative effect of directive function on relationship quality vanishes when relationship
memories are shared with the partner rather than anonymous others (Study 3). This is further
support for the idea that memories are used based on the changing dynamics of the situation
or context [17, 22]. Why is sharing relationship memories for directive purposes with others
detrimental for relationships? Previous research showed that discussing relationship problems
with friends harms relationship quality, if similar discussions do not take place with the part-
ner [44]. Perhaps the discussion with the partner brings the opportunity to take the perspective
of the partner and smoothly resolve the conflicts, which is not possible when relationship
memories are told to others.
Differential results across social and directive functions may also be due to the valence of
the memories. Previous research showed stronger associations of social and directive functions
with positive and negative memories, respectively [20]. Thus, our findings (in Study 2) reveal-
ing the beneficial effect of social function, but detrimental effect of directive function on rela-
tionship quality may not be surprising. Linking this finding with the relationship research,
perhaps social function is more salient in capitalization attempts (i.e., sharing good news with
others/partner; e.g., [45]), whereas directive function is more salient during discussions about
relationship problems [44]. These questions await future research.
An unexpected finding was the lack of social function’s effect on intimacy for the relation-
ship memories shared with the partner despite its positive effect on relationship satisfaction.
Previous studies showed the bonding roles of disclosure and capitalization in romantic rela-
tionships [45, 46]. One possible explanation of not observing the positive trend here is that we
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PLOS ONEMemory functions of the romantic personal past
did not consider reaction of the partner. Future research should examine whether social func-
tion builds intimacy when the partner shows constructive and supportive responses [47].
All in all, it was critical to show that global evaluations of memory functions also have direct
associations with the quality of the relationships. Further, as we noted previously, here we
operationalize function as the deliberate use of the memories [1]. Prior work focusing on sin-
gled out relationship episodes, in turn, rely on non-conscious uses of memories. Since it has
been suggested that some functions may be less accessible than others [22], future work is
needed to make this distinction clearer. For instance, the present studies could be replicated
with measures other than self-reported uses. One way is to conduct content analysis of func-
tional use on the memory narratives [e.g., 48].
The present study further shows that memory functions in the modified versions of the
TALE, which we called the TARE (Thinking about relationship experiences) and SHARE
(Sharing relationship experiences with the partner) were differentially linked to relationship
quality. We conclude that TARE and SHARE have utility as separate tools to examine reminis-
cence dynamics in the romantic relationship context, and to inform intervention or counseling
programs. A cautionary note, however, is that the present design employed a fixed ordering of
the questionnaires which may have influenced the participants’ responses. Even though there
was a clear warning in the instructions that there would be two very similar questionnaires to
fill out, there is no way to know whether responding to the TALE (or the TARE, in the second
study) questions had an alerting or inhibiting role on the subsequent scales. Since our main
aim in this research was to examine the roles of memory functions by comparing our results
using the original measure (TALE) and modified measures (TARE and SHARE) with each
other, in none of our studies the TARE or SHARE were used separately. Future studies plan-
ning to use these measures should take this into account and test the replicability of our find-
ings in contexts where the TARE or the SHARE are used in isolation.
The three-function model is suggested to be used as a conceptual model [8] with heuristic
utility [17] for understanding functional uses of the memories broadly. In fact, each function is
regarded as an organizatory unit to include many subpurposes [22]. Targeted experimental
manipulations are needed to fine-tune each function’s association to relational outcomes. With
the present design, it may not be entirely possible to rule out alternative explanations; such as,
individuals that are highly satisfied in their relationships may tend to remember for social rea-
sons or individuals who do not possess feelings of intimacy in the relationship may tend to use
memories mostly for problem-solving purposes. Future research should also identify whether
positive or negative relationship memories function the same way. Some newly identified func-
tions, such as the mood-enhancement function, are very loosely captured by the three categories
[49] but would be very relevant to examine for the romantic relationship context.
Our operationalization of the romantic relationship quality (satisfaction and intimacy)
should be considered as a first step in exploring the wide range of possible qualities [e.g., 26].
Exploring conflict, for instance, would be interesting in terms of how memory is used to deal
with negative relational outcomes. Future research should also consider the moderating roles
of other relationship characteristics (e.g., married vs. cohabitating, same-sex vs. heterosexual,
monogamous vs. non-monogamous relationships).
It should also be noted that psychological well-being has been previously associated with
the memory functions [11]. Therefore, it is possible that our memories’ influence on one’s rela-
tionship quality might be through their effects on general wellbeing and not because of their
direct effects on relationships. It would be worthwhile for the future studies to focus on the
mediating effects of psychological health on this mechanism.
In conclusion, our findings suggest that when remembering has consequences in terms of
the quality of a romantic relationship, how memories are used change depending on their
PLOS ONE | https://doi.org/10.1371/journal.pone.0251004 May 3, 2021
15 / 18
PLOS ONEMemory functions of the romantic personal past
theme (relationship-related or not) and who the social partner is (romantic other or not).
Future studies should consider fine-tuning these broad functional categories in order to fur-
ther understand the causal mechanisms. For instance, whether or not remembering a particu-
lar relationship incident directively might hinder relationship satisfaction. Together, the extant
findings suggest adopting a contextual approach as they revealed not only that functions relate
to different social contexts such as romantic relationships but also that they consider the role
of the social partner.
Supporting information
S1 Appendix.
(XLSX)
Acknowledgments
The authors would like to thank the two research assistants, Irem Duman and Lindon Kras-
niqi, for their help in preparation of the study materials and data collection. We also thank
three anonymous reviewers for their constructive feedback.
Author Contributions
Conceptualization: Cagla Aydin, Asuman Buyukcan-Tetik.
Investigation: Cagla Aydin, Asuman Buyukcan-Tetik.
Methodology: Cagla Aydin, Asuman Buyukcan-Tetik.
Project administration: Cagla Aydin, Asuman Buyukcan-Tetik.
Resources: Cagla Aydin, Asuman Buyukcan-Tetik.
Supervision: Cagla Aydin, Asuman Buyukcan-Tetik.
Validation: Cagla Aydin, Asuman Buyukcan-Tetik.
Visualization: Cagla Aydin, Asuman Buyukcan-Tetik.
Writing – original draft: Cagla Aydin, Asuman Buyukcan-Tetik.
Writing – review & editing: Cagla Aydin, Asuman Buyukcan-Tetik.
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PLOS ONE
| null |
10.1371_journal.pgph.0001789.pdf
|
rce are credited.
Data Availability Statement: The data presented
here are from the Low Birthweight Infant Feeding
Exploration (LIFE) study which is filed with
Clinicaltrials.gov NCT04002908 and Clinical Trial
Registry of India CTRI/2019/02/017475. De-
identified individual participant data (including data
dictionaries) will be made available, in addition to
study protocols, and the informed consent form in
a public, open access repository. The data will be
made available upon publication through the
Harvard Dataverse Platform under the BetterBirth
PLOS Global Public Health | https://
|
The data presented here are from the Low Birthweight Infant Feeding Exploration
|
RESEARCH ARTICLE
Facility-based care for moderately low
birthweight infants in India, Malawi, and
Tanzania
5,6, Christopher R. Sudfeld7, Melissa F. Young8, Bethany
3, Shivaprasad
1‡*, Karim ManjiID
1,2‡, Rana R. MokhtarID
4, Tisungane MvaloID
8, Christopher P. Duggan9,10, Sarah S. Somji3, Anne C. C. Lee11,
Katherine E. A. SemrauID
S. GoudarID
A. CarusoID
Mohamed Bakari3, Kristina Lugangira3, Rodrick Kisenge3, Linda S. Adair12, Irving
F. Hoffman13, Friday SaidiID
Mallory Michalak5, Sangappa M. Dhaded4, Roopa M. BelladID
Sanghamitra Panda4,17, Sunil S. Vernekar4, Veena HerekarID
Rashmita B. Nayak18, S. Yogeshkumar4, Saraswati WellingID
Kiersten Israel-Ballard20, Kimberly L. MansenID
Katelyn Fleming1, Katharine Miller1, Arthur Pote1, Lauren SpigelID
Linda Vesel1, for the LIFE Study Group¶
5,14, Melda Phiri5, Kingsly MsimukoID
20, Stephanie L. Martin12,
4, Sujata Misra15,16,
4, Manjunath Sommannavar4,
4, Krysten North11,19,
1, Danielle E. Tuller1,
5, Fadire Nyirenda5,
1 Ariadne Labs at Brigham and Women’s Hospital and the Harvard T.H. Chan School of Public Health,
Boston, Massachusetts, United States of America, 2 Department of Medicine, Harvard Medical School,
Boston, Massachusetts, United States of America, 3 Department of Pediatrics and Child Health, Muhimbili
University of Health and Allied Sciences, Dar es Salaam, Tanzania, 4 Jawaharlal Nehru Medical College,
KLE Academy of Higher Education and Research, Belgaum, Karnataka, India, 5 University of North Carolina
Project Malawi, Lilongwe, Malawi, 6 Department of Pediatrics, School of Medicine, University of North
Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America, 7 Department of Global Health
and Population and Nutrition, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, United
States of America, 8 Hubert Department of Global Health, Rollins School of Public Health, Emory University,
Atlanta, Georgia, United States of America, 9 Center for Nutrition, Division of Gastroenterology, Hepatology,
and Nutrition, Boston Children’s Hospital, Boston, Massachusetts, United States of America, 10 Department
of Nutrition, Harvard T.H. Chan School of Public Health, Boston, Massachusetts, United States of America,
11 Department of Pediatric Newborn Medicine, Brigham and Women’s Hospital, Boston, Massachusetts,
United States of America, 12 Department of Nutrition, Gillings School of Global Public Health, University of
North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America, 13 Institute for Global
Health and Infectious Diseases, School of Medicine, University of North Carolina at Chapel Hill, Chapel Hill,
North Carolina, United States of America, 14 Department of Obstetrics and Gynecology, School of Medicine,
University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America,
15 Department of Obstetrics and Gynaecology, SCB Medical College and Hospital, Cuttack, Odisha, India,
16 Department of Obstetrics and Gynaecology, FM Medical College, Balasore, Odisha, India,
17 Department of Obstetrics and Gynaecology, City Hospital, Cuttack, Odisha, India, 18 Department of
Paediatrics, SCB Medical College and Hospital, Cuttack, Odisha, India, 19 Department of Pediatrics, Harvard
Medical School, Boston, Massachusetts, United States of America, 20 Maternal, Newborn, Child Health and
Nutrition Program, PATH, Seattle, Washington, United States of America
‡ KEAS and RRM denotes co-first authors to this work.
¶ Membership of the LIFE Study Group is provided in the Acknowledgments.
* [email protected]
Abstract
Globally, increasing rates of facility-based childbirth enable early intervention for small vul-
nerable newborns. We describe health system-level inputs, current feeding, and discharge
practices for moderately low birthweight (MLBW) infants (1500-<2500g) in resource-con-
strained settings. The Low Birthweight Infant Feeding Exploration study is a mixed methods
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
OPEN ACCESS
Citation: Semrau KEA, Mokhtar RR, Manji K,
Goudar SS, Mvalo T, Sudfeld CR, et al. (2023)
Facility-based care for moderately low birthweight
infants in India, Malawi, and Tanzania. PLOS Glob
Public Health 3(4): e0001789. https://doi.org/
10.1371/journal.pgph.0001789
Editor: Giridhara R Babu, Public Health Foundation
of India, INDIA
Received: April 14, 2022
Accepted: March 13, 2023
Published: April 19, 2023
Copyright: © 2023 Semrau et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: The data presented
here are from the Low Birthweight Infant Feeding
Exploration (LIFE) study which is filed with
Clinicaltrials.gov NCT04002908 and Clinical Trial
Registry of India CTRI/2019/02/017475. De-
identified individual participant data (including data
dictionaries) will be made available, in addition to
study protocols, and the informed consent form in
a public, open access repository. The data will be
made available upon publication through the
Harvard Dataverse Platform under the BetterBirth
PLOS Global Public Health | https://doi.org/10.1371/journal.pgph.0001789 April 19, 2023
1 / 20
PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
observational study in 12 secondary- and tertiary-level facilities in India, Malawi, and Tanza-
nia. We analyzed data from baseline facility assessments and a prospective cohort of 148
MLBW infants from birth to discharge. Anthropometric measuring equipment (e.g., head cir-
cumference tapes, length boards), key medications (e.g., surfactant, parenteral nutrition),
milk expression tools, and human milk alternatives (e.g., donor milk, formula) were not uni-
versally available. MLBW infants were preterm appropriate-for-gestational age (38.5%),
preterm large-for-gestational age (3.4%), preterm small-for-gestational age (SGA) (11.5%),
and term SGA (46.6%). The median length of stay was 3.1 days (IQR: 1.5, 5.7); 32.4% of
infants were NICU-admitted and 67.6% were separated from mothers at least once. Exclu-
sive breastfeeding was high (93.2%). Generalized group lactation support was provided;
81.8% of mother-infant dyads received at least one session and 56.1% had 2+ sessions. At
the time of discharge, 5.1% of infants weighed >10% less than their birthweight; 18.8% of
infants were discharged with weights below facility-specific policy [1800g in India, 1500g in
Malawi, and 2000g in Tanzania]. Based on descriptive analysis, we found constraints in
health system inputs which have the potential to hinder high quality care for MLBW infants.
Targeted LBW-specific lactation support, discharge at appropriate weight, and access to
feeding alternatives would position MLBW for successful feeding and growth post-
discharge.
Dataverse website. This can be found at: https://
dataverse.harvard.edu/dataverse/BetterBirthData
Funding: This work was supported, in whole or in
part, by the Bill & Melinda Gates Foundation, grant
number OPP1192260/INV-007326. Under the
grant conditions of the Foundation, a Creative
Commons Attribution 4.0 Generic License has
already been assigned to the Author Accepted
Manuscript version that might arise from this
submission. The Bill & Melinda Gates Foundation
reviewed the study design, but had no role in data
collection, management, analysis, interpretation,
writing of the manuscript, or the decision to submit
manuscripts for publication. The grant recipient
was Dr. Katherine E.A. Semrau. This study was
registered at the following: Clinicaltrials.gov
(NCT04002908) and the Clinical Trial Registry of
India (CTRI/2019/02/017475, http://ctri.nic.in). All
coauthors (Named: KEAS, RRM, KM, SSG, TM
CRS, MFY, BAC, CPD, SSS, ACCL, MB, KL, RK,
LSA, IH, FS, MP, KM, FN, MM, SMD, RMB, SM,
SP, SSV, VH, MS, RBN, SYK, SW, KN, KIB, KLM,
SLM. KF, KM, AP, LSp, DET, LV; LIFE Group
Authorship: BK, SM, GG, MBK, KAC, MJ, VBN, SK,
BL, GSV, LGS, SN, SCP, LD JNB, BS, SN, and
those acknowledged: CCK, GM, CP, AB, VD, VK,
ESS, KDE) received funding from this award to
support this work.
Competing interests: The authors have declared
that no competing interests exist.
Introduction
Globally, nearly 20 million infants are born with low birthweight (LBW) (<2500g) each year.
LBW infants represent a heterogeneous group who may be preterm (<37 weeks gestation)
and/or small-for-gestational age (SGA; weight for gestation <10th percentile) [1]. LBW infants
are at increased risk of morbidity, infections, growth deficits, developmental delays, and mor-
tality [2].
The global increase in rates of facility-based childbirth (currently >75% of births) [3]
enables greater opportunity to identify LBW and vulnerable infants near the time of birth to
provide early, high quality, and appropriate care. Although 91% of LBW infants are born in
low- and middle-income countries (LMIC) and the majority are moderately LBW (MLBW)
(1500-<2500g) [4–6]; research is predominantly from high-income countries and focuses on
very LBW (<1500g) or preterm infants [7]. MLBW infants are at higher risk for complications
than normal weight (>2500g) infants and have been shown to be at greater risk of sepsis, tem-
perature instability, and low blood sugar [8]. Currently, there is limited knowledge of facility-
based feeding and discharge practices for MLBW infants, hindering proper management.
To help address these gaps, the Low Birthweight Infant Feeding Exploration (LIFE) study
aimed to describe feeding and growth of MLBW infants in India, Malawi, and Tanzania [9]. In
this analysis, we examined the health system inputs in the LIFE study facilities and described
the current health facility care and feeding practices for MLBW infants from birth to discharge.
The specific objectives were to: (1) examine the health system inputs available to support the
care and feeding of MLBW infants; (2) understand the overall experience of facility care for
MLBW infants with respect to the location and duration of their care and discharge practices;
and (3) describe feeding practices from birth to facility discharge among MLBW infants.
PLOS Global Public Health | https://doi.org/10.1371/journal.pgph.0001789 April 19, 2023
2 / 20
PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Methods
Study design
LIFE is a formative, multi-site, observational cohort using a convergent parallel, mixed-meth-
ods design examining the feeding practices, growth, and health outcomes of MLBW infants
(1500g� birthweight <2500g) in LMIC (Clinicaltrials.gov NCT04002908) [9]. Here, we pres-
ent results from two quantitative data collection streams from LIFE: (i) a baseline facility needs
assessment describing structural and service inputs of facilities where infants were enrolled,
and (ii) a prospective in-facility observational cohort. Table 1 provides an overview of each
data stream used in this analysis of the larger LIFE study (Table 1).
Table 1. Overview of study objectives and data collection.
Data Stream
Facility needs assessment
In-facility observational cohort
Aim
Objectives
Outcomes
To describe the health facility inputs as well as current facility care and feeding practices for
MLBW infants from birth to discharge in LMICs in an effort to gather foundational
knowledge and inform future interventions
Health facility inputs
Examine the health system inputs (e.g.,
equipment, supplies, and human resources)
available to support the care and feeding of
MLBW infants
Health facility inputs
• Facility level (secondary, tertiary)
• Facility type (private/public)
• Equipment
• WASH supplies
• Medication
• Human resources
Facility care practices
To understand the overall experience of
facility care for MLBW infants with respect
to the location and duration of their care and
discharge practices
Facility feeding practices
To describe feeding practices from birth to
facility discharge among MLBW infants
Facility care practices
• Length of stay
• Location of care
• Separation between mother and infant
• Weight at discharge
• Adherence to documented facility
discharge criteria
• Feeding patterns at discharge
Facility feeding practices
• Provision of lactation support and
management
• Feeding profile
• Early initiation of breastfeeding
• Feeding competency
Study design
Data collection and
Study Population
(N)
Observational, cross-sectional descriptive
facility needs assessment prior to cohort
enrollment
Formative research: observational,
descriptive prospective cohort including
direct observations and maternal reports
Facility needs assessments: 12 health facilities
(2–5 per site)
India-Karnataka: 5
India-Odisha: 2
Malawi: 2
Tanzania: 3
In-facility observational cohort: 148 mother-
infant pairs (35–40 per site)
India-Karnataka: 38
India-Odisha: 35
Malawi: 35
Tanzania: 40
Data analysis
Descriptive statistics: means, medians, SD
and frequencies
• Descriptive statistics: means, medians, SD
and frequencies
• Cochran-Mantel-Haenszel (CMH) and
Chi-squared: p-values and confidence
intervals for key indicators by LBW type,
location of care and sex
• Binomial regression: relative risk, 95%
confidence interval, p-value
https://doi.org/10.1371/journal.pgph.0001789.t001
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Study facilities
The LIFE study was conducted in 12 health facilities in four sites across three countries (Dar-
es-Salaam, Tanzania; Lilongwe, Malawi; Belgaum and Davangere, Karnataka State, India; and
Cuttack, Odisha State, India) [9]. Each site had two to five study facilities chosen based on
delivery volume, capacity to care for LBW infants, and willingness of facility leadership to par-
ticipate. Study facilities included secondary and tertiary-level public-sector hospitals located in
urban areas; in India-Karnataka, three private health facilities were also included (Table 2).
The facility needs assessment was conducted between August-September 2019, prior to obser-
vational cohort enrollment.
Study participants
After the needs assessment was completed in each facility, MLBW infants were eligible for the
in-facility observational cohort if they were born or presented at one of the 12 study facilities
during the enrollment period from August 2019 to April 2020. Each site enrolled between 35–
40 infants in order to describe in detail the care of infants while in the facility. Our sample size
was determined by considering timeline and budget constraints to conduct the intensive in-
facility observation. Using a posthoc analysis with a sample size of 148 infants and 95% confi-
dence, we were able to have a margin of error of 8% for common outcomes (50%) and 5% for
an important feeding outcome of breastfeeding (90%). Infants were excluded if they had a
birthweight <1500g or �2500g (n = 3); did not have maternal consent (n = 16); were born
with congenital anomalies that could interfere with feeding (n = 3); were born to young moth-
ers (n = 1) (i.e., <18 years in Tanzania and India; in Malawi, <16 years old if married or <17
years old if unmarried); or were born outside the facility (n = 10) [9]. Infants were also
excluded if there was a maternal or newborn death, including stillbirth (n = 1) or death of a
twin (n = 1).
Ethics & consent
The LIFE study was approved by 11 ethics committees in India, Malawi, Tanzania, and the
United States [9]. For the facility assessment, facility leadership gave verbal consent; for cohort
enrollment, women provided written informed consent.
Patient and public involvement
The LIFE study team involved clinicians, researchers, and community stakeholders who are
familiar with the cultural context, study setting and populations in research design and study.
In addition, study tools were piloted with mothers, community members, and health care pro-
viders as a way to capture culturally appropriate language, and ensure that surveys were
acceptable.
Data collection
Facility needs assessment. The facility assessment documented health facilities’ policies
and capacity to provide care for LBW infants, with a focus on infant feeding and discharge
practices. It assessed vital statistics (i.e. volume of births, volume of LBW births), structural
inputs (i.e. infrastructure such as number of NICU beds, facility type, electricity, backup gen-
erators), human resources, and equipment available for MLBW care and feeding for each
study facility [10, 11]. A study team member administered the assessment through direct
observations, record/register reviews, and staff consultations, where needed. The level of care
was defined based on recommendations from the American Academy of Pediatrics (AAP)
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Table 2. Facility-level resources for care of moderately low birthweight infants in 12 facilities in India, Malawi, and Tanzania.
Site
Facility
details
Human
resources in
NICU
Facility level
Facility type
Newborn care
level*
NICU beds
Day
Night
Day
Night
Equipment
for essential
newborn care
Infant scale
Head
circumference
tape
Length board
Equipment
for feeding
support
Heat source for
thermal care
Bag and mask
Continuous
positive airway
pressure (CPAP)
Ventilator
Total parenteral
nutrition
Intravenous fluids
(IV)
Oral rehydration
solution
Donor human
milk
Preterm formula
(any ward)
Term formula
(any ward)
Manual breast
pump
Electric breast
pumps
Designated
location for milk
expression
Alternative feeding
supplies (cups/
spoons/ paladai)
Nasogastric tube
India-Karnataka (N = 5)
India-Odisha
(N = 2)
Malawi (N = 2)
Tanzania N = 3)
Tertiary Tertiary Tertiary Tertiary Tertiary Secondary Tertiary Secondary Tertiary Secondary Secondary Tertiary
Private
Public
Private
Public
Private
Public
Public
Public
Public
Public
Public
Public
III
46
1:3
1:4
1:3
None
II
4–9
1:6
1:8
1:10
1:12
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
III
15
1:4
1:4
1:4
1:4
✓
✓
✓
NICU
only
✓
✓
✓
✓
✓
✓
✓
✓
✓
II
30
1:7
1:7
1:6
1:12
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
III
28
II
No
NICU**
Nurse to infant ratio
1:9
1:9
No NICU
No NICU
Physician to infant ratio
II
28
1:3
1:7
1:6
1:9
✓
✓
✓
NICU
only
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
No NICU
No NICU
1:7
1:35
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
II
46
1:22
1:33
None
None
✓
✓
✓
✓
✓
III
80
1:18
1:35
1:35
None
✓
✓
NICU
only
✓
NICU
only
✓
✓
✓
✓
✓
I
22
1:5
1:5
1:15
1:20
✓
✓
✓
✓
✓
✓
I
48
1:15
1:25
1:15
1:20
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
III
126
1:7
1:10
1:12
1:25
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
(Continued )
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Table 2. (Continued)
Site
WASH for
feeding
supplies
Designated
location for
cleaning and prep
Medication
Sink and treated
water
Aminophylline,
theophylline, or
caffeine
Surfactant
Intravenous
dextrose
India-Karnataka (N = 5)
India-Odisha
(N = 2)
Malawi (N = 2)
Tanzania N = 3)
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
✓
Check mark denotes present; blank cell denotes not present
*Newborn level of care definition based on guidance from AAP [11] and WHO/UNICEF Survive and Thrive [12]
**This facility does not have a formal NICU, but it has access to four warmers and basic resuscitation and oxygen facilities
https://doi.org/10.1371/journal.pgph.0001789.t002
(2012) [11], and guidance from the WHO/UNICEF Survive & Thrive report [12]. In our
study, primary level facilities provide essential newborn care that includes immediate drying,
skin to skin contact, early initiation and support of breastfeeding, outpatient care services and
management and referral to higher level care. Secondary level facilities provide primary level
care and address basic signal functions for small and sick newborns including the provision of
extra warmth using incubators or radiant warmers in addition to Kangaroo Mother Care,
administration of oxygen with continuous positive airway pressure (CPAP), resuscitation,
detection and management of complications (such as neonatal encephalopathy (NEC), jaun-
dice, infection, hypoglycemia) and access to a physician on call with specific neonatal skills in
addition to specialized nurses/midwifery staff available at all times (24/7). Tertiary level facili-
ties have a designated intensive care ward with secondary level capabilities as well as equip-
ment for mechanical/assisted ventilation, specialized feeding equipment (e.g., total parenteral
nutrition), nurses and doctors with specialized competencies in neonatal care available at all
times (24/7), a neonatologist on call and access to other specialist doctors including pediatric
surgery capabilities radiology, cardiology, neurology, and ophthalmology.
In-facility observational cohort. The in-facility observational cohort closely examined
current infant feeding and discharge practices for MLBW infants. Mother-infant pairs were
screened, consented, and enrolled within six hours of birth by trained study personnel and fol-
lowed during daylight hours (7:00am to 5:00pm, daily in India and Tanzania; on weekdays in
Malawi) until facility discharge. Demographics, birth characteristics, infant feeding patterns,
infant feeding competency, and neonatal weight changes were documented from birth until
facility discharge.
Demographic information was extracted from medical records. Gestational age (GA) was
based on ultrasound, last menstrual period (LMP), or fundal height recorded in a patient’s
chart with priority given to first trimester ultrasound followed by documented LMP. Using the
WHO growth standards for term infants [13, 14] and the INTERGROWTH-21st growth stan-
dards for preterm infants [15], infant LBW type were identified: preterm-small for gestational
age (preterm-SGA), preterm-appropriate for gestational age (preterm-AGA), preterm-large
for gestational age (preterm-LGA), and term-SGA [1]. Mother-infant separation, based on
maternal report, was defined as the time the mother and infant were not sharing a room.
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Trained study nurses directly observed feeding sessions and interviewed mothers about
practices between observations regarding feeding patterns (e.g., direct breastmilk, expressed
breastmilk, donor human milk (DHM), animal milk, formula, or another liquid), and lactation
support (e.g., verbal advice, physical support, timing) at each visit. Observation started within
six hours after birth and occurred every three hours for the first seven days, twice daily for
days 8–14, and then once a day for unstable infants or every three days for stable infants until
discharge. Both direct observation and maternal report were used to reduce the possibility of
bias and allow for triangulation of data. The Preterm Infant Breastfeeding Behavior Scale
(PIBBS), a 9-question validated tool, assessed feeding competency based on direct feeding
observation by trained study nurses measuring infant rooting efforts, latch effort, latch dura-
tion, sucking duration, and swallowing; scores <15 (out of 20) signified poor feeding compe-
tency [16]. Timing of breastfeeding initiation was collected at the baseline observational visit
within the first six hours after birth. Early initiation of breastfeeding, a validated Infant and
Young Child Feeding indicators for feeding practices [17], was calculated as the frequency of
infants who were breastfed within less than 1 hour.
Birthweight measured by clinical staff was used to assess MLBW infant study eligibility.
After study enrollment and within six hours of birth, trained study staff measured study spe-
cific birthweight and other infant anthropometrics, in triplicate, using standardized and cali-
brated study specific equipment at birth. Measurements were repeated just prior to discharge
[9]. The average of the two (out of the three) closest measurements was used for analysis. Birth
and discharge weight were used to calculate absolute and proportional weight change [18, 19].
Data analysis
Descriptive statistics included mean, standard deviation (SD), median and interquartile range
(IQR) for continuous variables, and frequencies and percentages for categorical variables. Chi-
squared test was used to determine statistical differences between preterm birth and geograph-
ical region. Cochran-Mantel-Haenszel (CMH) chi-square test was used to compare differences
of lactation support and counseling by sex, LBW type, and NICU admission after controlling
for study site. We analyzed data by location of care (NICU or postpartum ward) [20]. A one-
way analysis of variance was performed with a Bonferroni correction to compare differences
in the mean gestational age by LBW type. Wilcoxon rank sum test was used to determine the
difference between median birthweight by site, median length of stay differences by location of
care (NICU vs postpartum ward), sex and LBW type. Crude binomial regression models,
adjusted for clustering by health facility with compound symmetry and a log link function,
were used to estimate relative risk ratios (RR) for the relationship between admission to the
NICU and LBW type or birthweight bands (1500-<1800g, 1800-<2200g, 2200-<2500g) as
well as the association between early initiation of breastfeeding (<1 hour) and mother-infant
separation. Due to the small sample size, intended mainly for descriptive analyses, we did not
adjust for any potential confounders in our analyses and only adjusted for clustering by facility.
Exploration of the data demonstrated that missing data was rare; however, data were likely
missing randomly or due to data collection difficulties; we excluded missing data from analy-
sis. Data were analyzed using SAS 9.3 (SAS Institute. SAS Software Suite. Cary, NC: SAS Insti-
tute; 2020) and Microsoft Excel.
Results
Facility characteristics
The facility assessment was completed prior to the in-facility observational cohort enrollment
to understand currently available facility resources, demonstrating variation in access to
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
general equipment, medications, infant feeding supplies, and level of care across the facilities
(Table 2). Weighing scales were mostly available, but length assessment tools were not.
All 12 facilities were accredited by the Baby-Friendly Hospital Initiative (BFHI) and priori-
tized breastfeeding [21]. All had supplies to support infant feeding including paladais and
cups/spoons; most facilities (11/12) had nasogastric tubes. Infant formula was only used when
a mother was absent or unable to express breastmilk. Half of the facilities had powdered infant
formula in stock; two facilities had preterm formula; and none had pre-mixed liquid formula.
Reportedly, formula was often purchased by families if required for their infant.
All 12 facilities followed formalized discharge policies based on national guidelines; 9/12
had facility-specific guidelines. At the time of discharge in all facilities, infants had to be feed-
ing adequately, hemodynamically stable, and able to consume breastmilk (directly or
expressed). Although not the sole criteria, policy-stated minimum discharge weight criteria
varied by site: 2000g in Tanzania, 1800g in India, and 1500g in Malawi.
Characteristics of the in-facility observational cohort
We screened 183 MLBW infants and enrolled 148 (80.9%) (Table 3); 1076 observations were
completed among the 148 infants during their facility admission. Three infants in Malawi died
prior to facility discharge. Prematurity was more common in the African sites (62.0% in Tan-
zania and 62.9% in Malawi) compared to the Indian sites (36.8% in India-Karnataka and
31.4% in India-Odisha) (p<0.0001). The mean GA was 35.6 weeks (0.9) for preterm-SGA,
33.5 weeks (1.7) for preterm-AGA, 30.1 weeks (1.7) for preterm-LGA, and 38.9 weeks (1.7) for
term-SGA infants (p<0.0001). The median birthweight [2145g (1924, 2280] did not vary by
site (p = 0.14) (Table 3).
The median length of stay, which varied by site (p = 0.01), was 3.1 days [IQR: 1.5, 5.7]; 75%
of infants were discharged within 7 days (Table 3). Length of stay also varied by infant LBW
type: preterm-AGA infants [median (IQR): 4.4 days (2.0, 7.3)] stayed longer than preterm-
SGA [2.4 days (2.0, 4.5)], preterm-LGA [1.3 days (1.1, 1.9)], and term-SGA [2.3 days (1.5, 4.3)]
infants (p = 0.02). Infants born in secondary-level facilities were discharged earlier than those
in tertiary facilities.
Location of MLBW infant care
Separation. Based on maternal report, 67.6% (100/148) infants were separated from their
mothers during their time in the facility. Occurrence of separation significantly varied across
sites, occurring most often in the African sites [Malawi: 82.9% (29/35); Tanzania: 95.0% (38/
40)] compared to the Indian sites [India-Karnataka: 39.5% (15/38); India-Odisha: 51.4% (18/
35)]. Out of 100 mother-infant dyads separated, 52 (52.0%) were separated within 30 minutes
postpartum and the vast majority were separated within 6 hours postpartum (93/100). Mothers
reported the predominant reasons for separation were due to policies related to premature
birth (50.0%), LBW (80.0%), facility standard practice (20.0%), infant illness (21.0%), or mater-
nal illness (13.0%). Across the duration of admission, separation lasted an average of 21.0
hours (IQR: 4.0, 72.0) and did not vary by sex, but varied significantly by site (p<0.0001), with
the longest in Tanzania and shortest in India-Odisha [median (IQR) hours: India-Karnataka:
9.0 (3.0, 36.0); India-Odisha: 2.0 (1.0, 7.0); Malawi: 5.0 (2.0, 18.0); Tanzania: 55.0 (28.0, 100.0)].
Term-SGA infants had the shortest duration of separation, but this was not significant [(pre-
term-SGA (n = 8): 17.5 (2.0, 61.0); preterm-AGA (n = 47): 25.0 (5.0, 75.0); preterm-LGA
(n = 3): 2.0 (1.0, 27.0), term-SGA (n = 28): 6.5 (2.0, 44.0) (p = 0.69)]. Percent of time separated,
calculated by dividing the total separation in hours by total hours of facility stay, demonstrated
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Table 3. Maternal and infant characteristics in a cohort of 148 moderately low birthweight infants in 12 facilities in India, Malawi, and Tanzania.
Indicator of interest
Maternal
Maternal age (years)
Marital status n (%)
Maternal education n (%)
Paternal education n (%)
Maternal religion n (%)
Place of residence n (%)
ANC attendance n (%)
Mother’s parity n (%)
Number of babies born in this delivery n
(%)
Mother living with HIV n (%)
Mean (SD), range
Married
Primary or less
Secondary or more
Primary or less
Secondary or more
Not available (widowed/
divorced/single)
Hindu
Muslim
Christian*
No religion
Other
Rural
Urban
Yes
1
2–4
5+
Singleton
Twins**
Yes
Maternal complications at birth as noted
in patient chart n (%)
Normal (none)
Fever/infection
Heavy/excessive bleeding
High blood pressure/
preeclampsia
Convulsions
Other
No data in chart
Male
Female
Infant
Infant’s sex n (%)
Mean birthweight (grams)
Mean (SD), range
Median birthweight (grams)
Median (IQR)
Birthweight categories n (%)
Mean gestational age at birth
Infant LBW type n (%)
1500 - <1800 g
1800 - <2200 g
2200 - <2500 g
Mean (SD)
Preterm-SGA
Preterm-AGA
Preterm-LGA
Term-SGA
India-Karnataka
India-Odisha
N = 37
N = 35
Malawi
N = 35
Tanzania
N = 35
Pooled
N = 142
23.7 (3.7), 19.0–
35.0
25.6 (5.4), 19.0–
38.0
28.1 (7.2), 17.0–
40.0
29.0 (6.3), 19.0–
43.0
26.5 (6.1), 17.0–
43.0
37 (100.0%)
12 (32.4%)
25 (67.6%)
12 (32.4%)
25 (67.6%)
0 (0%)
31 (83.8%)
6 (16.2%)
N/A
N/A
N/A
24 (64.9%)
13 (35.1%)
35 (94.6%)
16 (43.2%)
21 (56.8%)
0 (0%)
34 (97.4%)
10 (28.6%)
25 (71.4%)
7 (20.0%)
26 (74.3%)
2 (5.7%)
34 (97.1%)
1 (2.9%)
N/A
N/A
N/A
24 (68.6%)
11 (31.4%)
30 (85.7%)
24 (68.6%)
11 (31.4%)
0 (0%)
35 (94.6%)
35 (100%)
2 (5.4%)
0 (0%)
0 (0%)
0 (0%)
32 (91.4%)
22 (62.9%)
13 (37.1%)
17 (48.6%)
15 (42.8%)
3 (8.6%)
N/A
2 (5.7%)
28 (80.0%)
1 (2.9%)
4 (11.4%)
14 (40.0%)
21 (60.0%)
34 (97.1%)
9 (25.7%)
19 (54.3%)
7 (20.0%)
32 (91.4%)
3 (8.6%)
5 (14.3%)
28 (80.0%)
19 (54.3%)
16 (45.7%)
17 (48.6%)
16 (45.7%)
2 (5.7%)
N/A
25 (71.4%)
10 (28.6%)
0 (0%)
0 (0%)
0 (0%)
35 (100%)
35 (100%)
12 (34.3%)
23 (65.7%)
0 (0%)
131 (92.3%)
63 (44.4%)
79 (55.6%)
53 (37.3%)
82 (57.7%)
7 (5.0%)
65 (45.8%)
34 (23.9%)
38 (26.8%)
1 (0.7%)
4 (2.8%)
62 (43.7%)
80 (56.3%)
134 (94.4%)
61 (43.0%)
74 (52.1%)
7 (4.9%)
28 (80.0%)
130 (91.6%)
7 (20.0%)
5 (14.3%)
12 (8.5%)
10 (7.0%)
29 (78.4%)
33 (94.3%)
21 (60.0%)
25 (71.4%)
108 (76.1%)
2 (5.4%)
0 (0%)
4 (10.8%)
0 (0%)
4 (10.8%)
0 (0%)
N = 38
15 (39.5%)
23 (60.5%)
2150 (244),
1590–2485
2250 (1975,
2345)
4 (10.5%)
11 (29.0%)
23 (60.5%)
37.1 (2.7)
3 (7.9%)
11 (28.9%)
0 (0%)
0 (0%)
0 (0%)
1 (2.9%)
0 (0%)
2 (5.7%)
0 (0%)
N = 35
16 (45.7%)
19 (54.3%)
2115 (218),
1575–2480
2160 (1950,
2270)
3 (8.6%)
18 (51.4%)
14 (40.0%)
37.5 (2.6)
6 (17.1%)
4 (11.4%)
1 (2.9%)
24 (63.2%)
24 (68.6%)
0 (0%)
1 (2.9%)
7 (20%)
1 (2.9%)
6 (17.1%)
1 (2.9%)
N = 35
13 (37.1%)
22 (62.9%)
2008 (300),
1400–2460
2105 (1700,
2210)
10 (28.6%)
15 (42.8%)
10 (28.6%)
34.1 (3.1),
4 (11.4%)
18 (51.4%)
4 (11.4%)
9 (25.7%)
1 (2.9%)
0 (0%)
9 (25.7%)
0 (0%)
0 (0%)
0 (0%)
N = 40
19 (47.5%)
21 (52.5%)
2081 (241),
1520–2490
2105 (1913,
2285)
5 (12.5%)
19 (47.5%)
16 (40.0%)
36.0 (3.2)
4 (10.0%)
24 (60.0%)
0 (0%)
3 (2.1%)
1 (0.7%)
20 (14.1%)
1 (0.7%)
12 (8.5%)
1 (0.70%)
N = 148
63 (42.6%)
85 (57.4%)
2089 (255),
1400–2490
2145 (1924,
2280)
22 (14.9%)
63 (42.6%)
63 (42.6%)
36.2 (3.1)
17 (11.5%)
57 (38.5%)
5 (3.4%)
12 (30.0%)
69 (46.6%)
(Continued )
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Table 3. (Continued)
Delivery mode n (%)
Indicator of interest
India-Karnataka
India-Odisha
Malawi
Vaginal delivery
Cesarean delivery
No data in chart
25 (65.8%)
12 (31.6%)
1 (2.6%)
31 (88.6%)
4 (11.4%)
0 (0%)
29 (82.9%)
6 (17.1%)
0 (0%)
Tanzania
31 (77.5%)
9 (22.5%)
0 (0%)
Length of stay in facility (days)
Median (IQR), range
5.1 (3.2, 7.0),
0.0–27.2
2.0 (1.3, 2.3),
1.1–19.2
2.3 (1.2, 6.0),
1.0–30.5
3.8 (2.1, 5.2),
1.0–16.5
Pooled
116 (78.4%)
31 (20.9%)
1 (0.7%)
3.1 (1.5, 5.7),
0.0–30.5
Place discharged n (%)
Discharged to home
36 (94.7%)
35 (100%)
32 (91.4%)
39 (97.5%)
142 (95.9%)
Referred
Left against medical advice
Not applicable, died
Missing
0 (0%)
1 (2.6%)
0 (0%)
1 (2.6%)
0 (0%)
0 (0%)
0 (0%)
0 (0%)
0 (0%)
0 (0%)
3 (8.6%)
0 (0%)
1 (2.5%)
0 (0%)
0 (0%)
0 (0%)
1 (0.7%)
1 (0.7%)
3 (2.0%)
1 (0.7%)
* Includes Catholic, Anglican, Protestant, Seventh Day Advent/Baptist, other Christian
** Twins were eligible for enrollment; at times, only one infant of the twin pair was enrolled. The breakdown of twin enrollment by site was: Tanzania: two pairs of
twin’s enrolled only one twin; Malawi: three pairs of twin’s enrolled only one twin; India-Karnataka: one pair of twins enrolled only one twin; and India-Odisha: no
twins were enrolled
https://doi.org/10.1371/journal.pgph.0001789.t003
that mother-infant dyads spent more than one-third of their admission separated (37.3% (SD:
37.6%), n = 86).
NICU. Over the entire length of stay, 32.4% (48/148) of infants were admitted to the
NICU, primarily in tertiary facilities (87.5%, 42/48). NICU admission was most common in
Tanzania (42.5%, 17/40) and Malawi (42.9%, 15/35), followed by India-Karnataka (37.8%, 14/
35) and India-Odisha (5.7%, 2/35), depending on policy standards and facility availability.
Admission to the NICU was most prevalent among preterm-AGA infants (50.9%) followed by
term-SGA (21.7%), preterm-LGA (20.0%) and preterm-SGA infants (17.7%). In crude bino-
mial regression models adjusting for clustering by facility, there was no significant association
between preterm-SGA and term-SGA infants and NICU admission compared to preterm-
AGA infants [(RR: 0.61, 95% CI: 0.32–1.19, p = 0.15) and (RR: 0.68, 95% CI: 0.31–1.5,
p = 0.34), respectively]; conversely, preterm-LGA infants were significantly less likely to be
admitted to the NICU compared to preterm-AGA infants [RR: 0.38, 95% CI: 0.23–0.62,
p<0.0001]. In addition, infants with the lowest birthweights [(1500-<1800g) and (1800
-<2200g)] had a higher prevalence of admission to the NICU (54.6% and 31.8%) compared to
those infants born with higher birthweights bands (2200-<2500g: 25.4%). However, this was
not significant after adjusting for clustering by facility [(RR (1500-<1800g): 1.3, 95% CI: 0.61–
3.0, p = 0.47) and (RR (1800-<2200g: 1.2, 95% CI: 0.76–1.8, p = 0.45)]. Infants ever admitted
to the NICU had a significantly longer length of stay compared to non-NICU admitted infants.
Infants admitted to the NICU had a median length of stay of 6.0 days (IQR: 4.3, 7.7) compared
with 2.1 days (IQR: 1.3, 3.3) among infants in the postpartum ward (p<0.0001). In addition,
the length of stay varied significantly by site (p<0.0001). NICU admission duration did not
vary by infant sex or LBW type.
Infants in the NICU were primarily fed breastmilk unless clinical complications impacted
the infants’ ability to feed. Oxygen/CPAP was initiated for 43.8% (21/48) of NICU admitted
infants. Feeding intolerance occurred among 22.9% (11/48) of the NICU-admitted infants,
which led clinicians to stop oral feeds. These infants were then given parenteral nutrition, fed
breastmilk via nasogastric tube, and/or were administered intravenous fluids. Of all the NICU-
admitted infants, 4.2% (2/48) were given parenteral nutrition, 8.3% (4/48) had a nasogastric
tube, and 54.2% (26/48) were given fluids intravenously.
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Feeding from birth to facility discharge
Practices. Of the 148 enrolled infants, one infant in Malawi did not have any feeds prior
to discharge at two days postpartum; the remaining 147 infants had feeding observed from
birth to discharge. The majority, 93.2% (137/147), were exclusively fed human milk [mom’s
own milk or DHM], 6.1% (9/147) were given mixed milk feeding [formula and breastmilk],
2.0% (3/147) were given other liquids (no specific information available), and 0.7% (1/147)
was given only formula. Mixed milk feeding was more common in India-Odisha (6.1%, 6/35)
compared to Tanzania (2.5%, 1/40), Malawi (2.9%, 1/34), and India-Karnataka (2.6%, 1/40).
Provision of feeds other than human milk and formula (e.g., provision of other foods, teas, or
liquids) were not observed. Based on maternal report, three infants were given other unknown
liquids. Otherwise, feeding information (direct breastfeeding, milk expression, and/or infant
formula use), obtained from direct observations was highly correlated with maternal self-
report (Fig 1).
Of the 82 mothers with available feeding initiation data, 81.7% (67/82) initiated breastfeed-
ing within 1 hour of birth. Early initiation varied by infant LBW type: 90.0% of preterm-SGA,
59.1% of preterm-AGA, 25.0% of preterm-LGA, and 95.7% of term-SGA infants. This varia-
tion was likely related to mother-infant separation and infant prematurity with preterm-AGA
and preterm-LGA infants averaging 33.5 and 30.1 weeks gestation respectively, preterm-SGA
infants at 35.6 weeks and term-SGA at 38.9 weeks. Among infants separated, 69.0% (29/42)
initiated early breastfeeding, whereas 95% (38/40) of non-separated infants initiated early
breastfeeding. After adjusting for clustering by facility in a crude binomial regression model,
infants who were separated from their mothers were significantly less likely to initiate early
breastfeeding compared to those infants who were not separated [RR: 0.26, 95% CI: 0.10–0.42,
p = 0.0013].
Direct breastfeeding was the predominant feeding type observed (94.6%, 139/147) (Figs 1
and S1) followed by expressed breastmilk (43.5%, 64/147). Provision of expressed breastmilk,
unfortified and fed immediately after expression, varied by site (expression uptake by site:
Tanzania: 55.0%; Malawi: 70.6%; India-Karnataka: 29.0%; India-Odisha: 20.0%; p<0.001).
Direct observation of milk expression was conducted with 62/64 mothers. Nearly all expressed
milk by hand (98.4%, 61/62); a manual pump was used by three mothers (4.8%, 3/62), and
electric pumps by two (3.2%, 2/62) at private facilities.
Formula was fed to 6.1% (9/147) of infants with a cup/spoon/paladai, primarily in India-
Odisha and Tanzania; bottles with a nipple were not used. Unfortified DHM fed by cup/
spoon/paladai was given to 10.5% (4/38) of infants born in India-Karnataka, representing
2.7% of the whole cohort. Supplementation, only given in India-Karnataka, included iron and
vitamin D (56.8%, 21/37) or zinc and multivitamins (24.3%, 9/37).
Lactation support and counseling. Overall, 81.8% of mothers reported receiving at least
one lactation support and counseling session; yet, the frequency, provider, and type of counsel-
ing varied by site (Table 4). Two or more sessions were provided to 56.1% of dyads, typically
in a group setting. Support was primarily provided by healthcare providers (90.1%) or family
members (27.3%). Type of support varied by site, including help with positioning of the infant,
latch, milk expression, and feeding with a cup/paladai (Table 4). Counseling, as reported by
mothers, did not vary by infant sex (χ2
p = 0.95), or NICU admission (χ2
decision-makers for breastmilk feeding. Clinicians played an important role in influencing the
mother to feed infants expressed breastmilk (68.1%), DHM (100.0%), and infant formula
(100.0%). For the three infants whose mothers reported to feed ‘other’ unknown liquids, the
mothers (n = 2) or family members (n = 1) made the decision.
cmh = 0.87, p = 0.35). Mothers reported being the primary
cmh = 2.85, p = 0.09), LBW type (χ2
cmh = 0.004,
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Fig 1. Observed feeding patterns in the first week of life prior to facility discharge among moderately low birthweight infants across 12 facilities in
India, Malawi, and Tanzania.
https://doi.org/10.1371/journal.pgph.0001789.g001
Breastfeeding competency. Feeding competency assessments showed significant
improvement from baseline to facility discharge across all sites (S2 Fig). The average PIBBS
score at baseline was 11.9 (±4.3) and increased to 14.2 (±3.3) within 24 hours prior to dis-
charge; 58.6% of infants were discharged with low feeding competency (PIBBS score �15).
There were no significant differences in the mean discharge PIBBS scores by lactation support,
sex, LBW type, size-for-gestational age, or GA.
Discharge procedures
Based on national and facility policy, infants met certain criteria around feeding, stability, and
weight to be discharged. Of the 140 infants assessed at discharge, 97.1% were fed only breast-
milk (69.3% direct, 7.1% expressed only, 20.7% both direct and expressed), 2.7% fed formula
only, and 0.7% fed a mix of formula, direct and expressed milk. Weight change between birth
and discharge was minimal (Table 5). On average, infants lost 2.4% (± 5.8%) of their birth-
weight, varying by length of stay. Overall, 5.1% (7/138) lost >10% of their birthweight at the
time of discharge. However, 18.8% of infants were discharged with weight below the site speci-
fied discharge criteria: 20.6% (7/34) of infants in India-Karnataka and 5.7% (2/35) with weights
<1800g; 40.0% (16/40) in Tanzania had <2000g; (3.5%) (1/29) in Malawi <1500g. All infants
were stable at the time of discharge.
Discussion
MLBW infants represent a heterogeneous group with different needs and implications for
care and utilization of limited resources. Across the 12 facilities in 3 countries, we observed
high adherence to EBF and relatively short lengths of stay (average 3 days), yet notable levels
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Table 4. Feeding support and counseling received in the facility based on maternal report and observations in a cohort of 148 moderately low birthweight infants
in 12 facilities in India, Malawi, and Tanzania.
Feeding support and counseling indicator
Ever received feeding support (during time in the facility)
Ever received based on maternal report
India-
Karnataka
N = 38
India-
Odisha
N = 35
Malawi
Tanzania
Pooled
N = 35
N = 40
N = 148
36 (94.7%)
23 (65.7%)
29 (82.9%)
33 (82.5%)
121 (81.8%)
Ever received two or more support sessions based on maternal report
29 (76.3%)
9 (25.7%)
24 (68.6%)
21 (52.5%)
83 (56.1%)
Ever received based on observation
Median (IQR) number of times mother reported receiving feeding support at facility within
first 0–7 days of life (n = 146)
Median (IQR) number of times mother reported receiving feeding support at facility >7 days
—3 weeks of life (n = 23)
MATERNAL REPORT: Types of support received*
Talking about proper latch/positioning
Support with positioning mom/baby
Support with latching
Support with expressing breastmilk
Support for feeding with bottle/cup/paladai
Other
MATERNAL REPORT: Provider of support
Health care provider**
Lactation consultant
Community health worker
Family member
Friend
Other
OBSERVATION: Types of support received
Talking about proper latch/positioning
Support with positioning mom/baby
Support with latching
Support with expressing breastmilk
Support for feeding with bottle/cup/paladai
Other
OBSERVATION: Provider of support
Healthcare provider**
Lactation consultant
Community health worker
Family member
Friend
Other
32 (84.2%)
19 (54.3%)
26 (74.3%)
27 (67.5%)
104 (70.3%)
3.5 (2.0, 7.0)
1.0 (0, 2.0)
2 (1.0, 3.0)
1.5 (1.0,
3.5)
2 (1.0, 4.0)
13.0 (7.0, 9.0)
4.5 (0, 9.0)
4.0(2.0,
6.0)
5.0
(2.0,11.0)
7.0 (2.0,
13.0)
N = 36
N = 23
N = 29
N = 33
N = 121
31 (86.1%)
20 (87.0%)
12 (41.4%)
30 (90.9%)
93 (76.9%)
31 (86.1%)
8 (34.8%)
11 (37.9%)
16 (48.5%)
66 (54.6%)
16 (44.4%)
3 (13.0%)
10 (34.5%)
8 (24.2%)
37 (30.6%)
7 (19.4%)
4 (11.1%)
4 (11.1%)
N = 36
3 (13.0%)
25 (86.2%)
13 (39.4%)
48 (39.7%)
4 (17.4%)
22 (75.9%)
9 (27.3%)
39 (32.2%)
0
15 (51.7%)
0
19 (15.7%)
N = 23
N = 29
N = 33
N = 121
34 (94.4%)
13 (56.5%)
29
(100.0%)
33 (100.0%) 109 (90.1%)
2 (5.6%)
2 (5.6%)
0
3 (13.0%)
0
0
20 (55.6%)
12 (52.2%)
1 (3.5%)
0
0
0
2 (1.7%)
5 (4.1%)
33 (27.3%)
2 (5.6%)
1 (2.8%)
N = 32
0
0
0
0
1 (3.0%)
3 (2.5%)
0
1 (0.83%)
N = 19
N = 26
N = 27
N = 104
28 (87.5%)
17 (89.5%)
10 (38.5%)
24 (88.9%)
79 (76.0%)
30 (93.8%)
4 (21.1%)
12 (46.2%)
13 (48.2%)
59 (56.7%)
19 (59.4%)
3 (15.8%)
8 (30.8%)
8 (29.6%)
38 (36.5%)
6 (18.8%)
2 (10.5%)
21 (80.8%)
12 (44.4%)
41 (39.4%)
3 (9.4%)
2 (6.3%)
N = 32
2 (10.5%)
20 (76.9%)
5 (18.5%)
30 (28.9%)
0
10 (38.5%)
0
12 (11.5%)
N = 19
N = 26
N = 27
N = 104
29 (90.6%)
7 (36.8%)
26
(100.0%)
27 (100.0%)
89 (85.6%)
2 (6.3%)
1 (3.1%)
0
2 (10.5%)
0
0
17 (53.1%)
13 (68.4%)
2 (7.7%)
0
0
0
2 (1.9%)
3 (2.9%)
32 (30.8%)
3 (9.4%)
0
0
0
0
0
1 (3.7%)
4 (3.9%)
0
0
* Not able to distinguish between physical and verbal support in some cases
**Inclusive of doctor, nurse, and midwife
https://doi.org/10.1371/journal.pgph.0001789.t004
of mother-infant separation and gaps in universal and repeated lactation support or
counseling [22–24].
Uptake and adherence to EBF was high, aligning with global recommendations and facility
standard protocols [22, 25]. More than 80% of infants had early initiation of breastfeeding,
which is higher than the global average (57.6%) [26–28]. Further, 97.1% of infants were
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Table 5. Change in weight (grams) from birth to discharge by length of stay for 148 moderately low birthweight infants in 12 facilities in India, Malawi, and
Tanzania.
Length of stay (in
days)
Birthweight (in grams)
Discharge weight (in grams)
Change in weight from birth to
discharge (in grams)
Birth-weight regain prior to
discharge (n (%))
N
47
40
38
15
8
0 to 2
3 to 4
5 to 7
8 to 14
15 plus
Overall
148
Mean
(SD)
2184
(201)
2088
(232)
2076
(245)
1992
(322)
1786
(293)
2089
(255)
Median (IQR) N*
2200 (2070,
2345)
2110 (1890,
2265)
2118 (1925,
2280)
2025 (1700,
2270)
1708 (1595,
1990)
43
39
34
14
8
2145 (1924,
2280)
138
Mean
(SD)
2120
(190)
2019
(170)
2032
(265)
1965
(394)
1794
(250)
2035
(245)
Median (IQR) Mean (SD)
**
Mean percent (%
(SD))
2120 (1978,
2270)
2000 (1900,
2125)
2068
(1885,2260)
1948
(1580,2288)
1850 (1578,
2015)
2055 (1900,
2226)
-68 (66)
-3.1 (3.0)
-68 (128)
-2.8 (6.3)
-45 (91)
-2.2 (4.3)
-40 (174)
-2.2 (8.8)
9 (208)
1.3 (11.9)
-55 (115)
-2.4 (5.8)
5 (10.6)
10 (25.0)
12 (31.6)
6 (40.0)
5 (62.5)
38 (25.7)
*In some instances, smaller denominators are noted for the subsequent analyses involving birth and discharge weights due to 10 (6.8%) missing discharge weights,
which resulted from discharge during a weekend with no study staff on duty, infant death, or lack of documentation
** Change in weight was calculated among those infants with both birth and discharge weights; sample size is the same as those with a discharge weight
https://doi.org/10.1371/journal.pgph.0001789.t005
discharged EBF, again higher than expected [29]. In our study, EBF was supported with utiliza-
tion and promotion of breastmilk expression. Additional methods to aid with lactogenesis and
support milk expression, including safe storage and increased availability of pumps in the facil-
ity, may be beneficial. Nearly 30% of infants were fed expressed breastmilk at discharge, which
may be challenging to sustain at home given the need for clean water to wash breast pump
materials to safely feed expressed breastmilk [30].
While uptake of EBF was excellent, health system constraints (e.g., bed capacity, human
resources, limited specialized LBW training) likely led to misalignment of practice and policy
and limited optimization of in-facility care, demonstrated by <60% receiving 2+ lactation sup-
port sessions, >66% of dyads experiencing maternal-newborn separation, and discharge of
18.8% of infants below minimum weight thresholds. Nurse-to-patient ratio guidelines from
medical and nursing societies range from 1:3 for the lowest acuity care (level 1) to 1:1 for the
highest acuity care (level 5) [31–33]. Only three facilities met nurse-to-patient guidelines for
the lowest acuity care at level 1 facilities (1:3); some facilities had ratios as low as 1:35 infants.
Increasing health worker capacity has been identified as a core standard for improving quality
of maternal and newborn care in health facilities [34], meeting global recommendations [22],
and improving neonatal outcomes [35, 36].
Lack of universal and hands-on provision of lactation support and counseling was possibly
driven by the high levels of mother-infant separation, staff shortages, and lack of specialized
training on LBW lactation support among providers, which are well-documented barriers to
support of EBF [37–41]. Unresolved breastfeeding problems, highlighted by poor feeding com-
petency scores, can lead to growth faltering, reduced duration of EBF, subsequent hospitaliza-
tions, and mortality [42, 43]. Group counseling and support in the facility and in-home peer-
counseling interventions have shown improved EBF initiation and duration [41]; these strate-
gies could be adapted specifically for LBW infants to improve in-facility support.
Further, breastmilk alone may be insufficient to meet the nutritional needs of MLBW
infants [42, 44–46]; most facilities did not have alternatives readily available. Only one facility
had access to parenteral nutrition; one site supplemented infants with micronutrients; protein
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
supplementation and fortification of human milk were not used at any site. When appropriate
and safely administered, DHM and formula can provide a bridge to breastfeeding [46–48].
However, a human milk bank was only at one facility and formula availability was limited; a
potential barrier of formula use was the cost to families. Finally, of note, we found a high prev-
alence of discharge among infants still demonstrating postnatal weight loss (Table 5) and/or
discharge prior to reaching minimum weight benchmarks as stipulated by facility discharge
policies. Weight loss among LBW and vulnerable infants is of critical importance and requires
additional in-facility care measures including daily monitoring of anthropometrics, clinical
documentation, and plans for continuity of care for post-discharge monitoring to ensure LBW
infants do not become further growth-restricted.
Our study addresses key knowledge gaps as the majority of published research focuses on
very LBW/preterm infants in high-income settings despite the predominance and risk of
MLBW infants [7]. We present detailed data from three diverse countries and twelve hospitals.
Multiple data collection methods, including facility needs assessment, direct observations, and
maternal reports, allowed for triangulation of information.
We do have some notable limitations. We focused on MLBW infants, which preselected a
preterm-AGA population that is less mature than a preterm-SGA group, (i.e., most early pre-
term-SGA infants have birthweights <1500g and were therefore ineligible). In addition, our
study population included infants born at secondary and tertiary facilities and thus cannot be
generalized to infants born at lower-level facilities or at home. The study sample size allowed
for a deep understanding of the feeding experiences and care of LBW infants and resulted in a
rich descriptive data at each site. However, this sample does not represent all LBW infants and,
since this was intended largely as a descriptive paper, our modeling analyses are limited. Statis-
tically significant findings should be interpreted cautiously as we could not control for poten-
tial confounders, and observed associations may be spurious. Further studies need to confirm
our findings. Additionally, GA source differed by site which could cause LBW type misclassifi-
cation; however, we use prioritization criteria for choosing the source of GA, namely ultra-
sound at first trimester when available over other sources followed by LMP in medical charts.
Finally, observation by study staff may have altered participant behaviors; however, we had
multiple observations over time and had specific research nurses gather information rather
than clinical staff.
Conclusion
With increases in facility-based childbirth, critical opportunities exist to set up MLBW infants,
their mothers, and families to thrive post-discharge. Given the renewed investment and recent
guidelines in the care of small and sick newborns [49, 50], the global community must identify
gaps and at-risk groups, and modify strategies to support the adherence to policies like BFHI,
zero-separation, and minimum discharge weights. Our study fills a critical gap in foundational
knowledge on the health system inputs and current feeding and care practices for MLBW
infants in resource-constrained settings. This is a group for whom evidence is limited, but the
potential to survive and thrive is high [4, 51, 52]. Our findings highlight areas of focus to
improve the quality of care delivered to MLBW infants including health systems improve-
ments to increase human capacity, staffing and training, to improve breastfeeding alternatives
availability, and to support adherence to discharge policies, which would improve mother-
infant experience of care by reducing mother-infant separation, initiating breastfeeding early,
and providing lactation support and counseling specialized for MLBW.
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Supporting information
S1 Fig. Observed infant feeding patterns from birth to discharge among infants discharged
after 7 days of life across 12 facilities in India, Malawi, and Tanzania.
(TIF)
S2 Fig. Infant feeding competency scores assessed by PIBBS at baseline and 24 hours prior
to facility discharge in a cohort of moderately LBW across 12 facilities in India, Malawi,
and Tanzania.
(TIF)
S1 File. PLOS global public health inclusivity questionnaire.
(DOCX)
Acknowledgments
The authors would like to thank clinical leadership and staff at all study facilities for their part-
nership, support and contribution to this work; the mothers and infants for allowing us to
have a glimpse into their experiences and sharing key moments of their lives; community
members, government officials and subject experts for sharing their perspectives; and all data
collectors and study staff for conducting study activities. We wish to acknowledge Dr Chan-
drashekhar C Karadiguddi, Mrs Geetanjali Mungarwadi, Dr. Chidbhusan Panda, Dr. Amrit
Behera, Vaibhav Dhamanekar, Varun Kusagar, and ES Siddhartha for their implementation
support and guidance in Karnataka and Odisha states of India and Dr. Ki-Do Eum for analytic
review. We appreciate the deep dedication to this work in trying to understand the lives of cli-
nicians and families with vulnerable newborns, particularly in the context of the global
pandemic.
THE LIFE STUDY GROUP:
KLE Academy of Higher Education and Research’s Jawaharlal Nehru Medical College, Bel-
gaum, Karnataka, India: Department of Paediatrics: Dr Bhavna Koppad; Department of
Obstetrics and Gynecology: Dr Shridevi Metgud
J J M Medical College, Davangere, Karnataka, India: Department of Paediatrics, Dr Guru-
prasad Goudar,
Dr. MB Koujalagi, Dr KA Chaya, Dr Mruthyunjay, and Dr. Varun B Kusagur; Department
of Obstetrics and Gynecology: Dr Saroja Kamatar, Dr B Latha, Dr. GS Venna
Department of Paediatrics, S S Institute of Medical Sciences & Research Centre, Davangere,
Karnataka, India: Dr. Latha G. Shamanur Dr. Shilpa Nabapure
Department of Paediatrics, Women and Children Hospital, Davangere, Karnataka, India:
Dr. Sudha C Patil
SCB Medical College, Cuttack, Odisha: Department of Paediatrics: Dr. Leena Das, Dr. Jna-
nindranath N Behera, Dr. Bipsha Singh; Department of Obstetrics and Gynecology Dr. Sau-
mya Nanda
Author Contributions
Conceptualization: Katherine E. A. Semrau, Karim Manji, Shivaprasad S. Goudar,
Tisungane Mvalo, Christopher R. Sudfeld, Melissa F. Young, Bethany A. Caruso,
Christopher P. Duggan, Anne C. C. Lee, Linda S. Adair, Irving F. Hoffman, Friday Saidi,
Sangappa M. Dhaded, Roopa M. Bellad, Veena Herekar, Krysten North,
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PLOS GLOBAL PUBLIC HEALTHFacility-based care of moderately low birthweight infants
Kiersten Israel-Ballard, Kimberly L. Mansen, Stephanie L. Martin, Katharine Miller,
Arthur Pote, Lauren Spigel, Danielle E. Tuller, Linda Vesel.
Data curation: Rana R. Mokhtar, Karim Manji, Shivaprasad S. Goudar, Sarah S. Somji,
Mohamed Bakari, Kristina Lugangira, Rodrick Kisenge, Melda Phiri, Kingsly Msimuko,
Fadire Nyirenda, Rashmita B. Nayak, Arthur Pote.
Formal analysis: Rana R. Mokhtar, Katharine Miller, Linda Vesel.
Funding acquisition: Katherine E. A. Semrau, Danielle E. Tuller.
Investigation: Christopher R. Sudfeld, Kristina Lugangira, Friday Saidi, Melda Phiri,
Mallory Michalak, Sangappa M. Dhaded, Roopa M. Bellad, Sujata Misra, Sanghamitra
Panda, Sunil S. Vernekar, Veena Herekar, Manjunath Sommannavar, S. Yogeshkumar,
Saraswati Welling, Krysten North, Lauren Spigel.
Methodology: Katherine E. A. Semrau, Tisungane Mvalo, Christopher R. Sudfeld, Melissa
F. Young, Bethany A. Caruso, Anne C. C. Lee, Sunil S. Vernekar, Veena Herekar,
Krysten North, Kimberly L. Mansen, Stephanie L. Martin.
Project administration: Manjunath Sommannavar, S. Yogeshkumar, Katelyn Fleming,
Danielle E. Tuller, Linda Vesel.
Software: Arthur Pote.
Supervision: Katherine E. A. Semrau, Tisungane Mvalo, Irving F. Hoffman, Linda Vesel.
Validation: Rana R. Mokhtar.
Visualization: Rana R. Mokhtar, Katharine Miller.
Writing – original draft: Katherine E. A. Semrau, Rana R. Mokhtar, Linda Vesel.
Writing – review & editing: Katherine E. A. Semrau, Rana R. Mokhtar, Karim Manji,
Shivaprasad S. Goudar, Tisungane Mvalo, Christopher R. Sudfeld, Melissa F. Young,
Bethany A. Caruso, Christopher P. Duggan, Sarah S. Somji, Anne C. C. Lee,
Sunil S. Vernekar, Veena Herekar, S. Yogeshkumar, Saraswati Welling, Krysten North,
Kiersten Israel-Ballard, Katelyn Fleming, Danielle E. Tuller, Linda Vesel.
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PLOS GLOBAL PUBLIC HEALTH
| null |
10.1371_journal.pone.0245201.pdf
| null |
All sequences are available from the GenBank database (accession number: MN816222, MN816223, MN816224, MN816225, MN816226, MN814829, MN814830, MN814831, MT012386 and MT012387). Other relevant data are within the paper and its Supporting Information files.
|
RESEARCH ARTICLE
A new root-knot nematode, Meloidogyne vitis
sp. nov. (Nematoda: Meloidogynidae),
parasitizing grape in Yunnan
Yanmei YangID
Qi Zhang1,2
1,2, Xianqi HuID
1,2*, Pei LiuID
1,2, Li Chen3, Huan Peng4, Qiaomei Wang1,2,
1 College of Plant Protection, Yunnan Agricultural University, Kunming, Yunnan Province, China, 2 State
Key Laboratory for Conservation and Utilization of Bio-Resources in Yunnan, Yunnan Agricultural University,
Kunming, Yunnan Province, China, 3 Wheat Research Institute, Shanxi Academy of Agricultural Sciences,
Linfen, Shanxi Province, China, 4 State Key Laboratory for Biology of Plant Disease and Insect Pests,
Institute of Plant Protection, Chinese Academy of Agricultural Science, Beijing, China
* [email protected]
Abstract
An unknown root-knot nematode was found at high density on grape roots collected from
Yunnan Province. Morphometric traits and measurements, isozyme phenotypes, and
molecular analysis clearly differentiated this nematode from previously described root-knot
nematodes. This new species is described, illustrated and named Meloidogyne vitis sp. nov.
The new species can be distinguished from other Meloidogyne spp. by a unique combina-
tion of characters. Females display a prominent neck, an excretory pore is located on the
ventral region between 23rd and 25th annule behind lips, an EP/ST ratio of approximately
2.5 (1.98–2.96), a perineal pattern with two large and prominent phasmids, and a labial disc
fused with the medial lips to form a dumbbell-shaped structure. Males display an obvious
head region, a labial disc fused with the medial lips to form a dumbbell-shaped structure, no
lateral lips, a prominent slit-like opening between the labial disc and medial lips, a distinct
sunken appearance of the middle of the medial lips, and four incisures in the lateral field.
Second-stage juveniles are characterized by a head region with slightly wrinkled mark, a
labial disc fused with the medial lips to form a dumbbell-shaped structure, a slightly sunken
appearance of the middle of the medial lips, a slit-like amphidial openings between the labial
disc and lateral lips, and four incisures in the lateral field. The new species has rare Mdh
(N3d) and Est phenotypes (VF1). Phylogenetic analysis based on ITS1-5.8S-ITS2, D2D3
fragments of rDNA, and coxI and coxII fragments of mtDNA sequences clearly separated
the new species from other root-knot nematodes, and the closest relative was Meloidogyne
mali. Meloidogyne mali was collected for amplifying these sequences as mentioned above,
which were compared with the corresponding sequences of new species, the result showed
that all of these sequences with highly base divergence (48–210 base divergence). More-
over, sequence characterized amplified region (SCAR) primers for rapid identification of this
new species were designed.
a1111111111
a1111111111
a1111111111
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OPEN ACCESS
Citation: Yang Y, Hu X, Liu P, Chen L, Peng H,
Wang Q, et al. (2021) A new root-knot nematode,
Meloidogyne vitis sp. nov. (Nematoda:
Meloidogynidae), parasitizing grape in Yunnan.
PLoS ONE 16(2): e0245201. https://doi.org/
10.1371/journal.pone.0245201
Editor: Ebrahim Shokoohi, University of Limpopo,
SOUTH AFRICA
Received: May 15, 2020
Accepted: December 19, 2020
Published: February 3, 2021
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0245201
Copyright: © 2021 Yang et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All sequences are
available from the GenBank database (accession
number: MN816222, MN816223, MN816224,
MN816225, MN816226, MN814829, MN814830,
PLOS ONE | https://doi.org/10.1371/journal.pone.0245201 February 3, 2021
1 / 25
PLOS ONEMN814831, MT012386 and MT012387). Other
relevant data are within the paper and its
Supporting Information files.
Funding: This work was supported by grants from
the National Key Research and Development
Project (Nos. 2018YFD0201202;
2017YFD0200601). The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
A new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Introduction
Root-knot nematodes (RKNs), belonging to family Heteroderidae, are the most important cat-
egories of plant-parasitic nematode and parasitize a large number of plant species [1]. After
being parasitized, plants often exhibit severely reduced production, causing significant eco-
nomic losses. More than 90 species of RKNs have been described to date [2, 3], which are able
to infect virtually any species of higher plant and have a near cosmopolitan distribution [4]. In
recent years, new RKN species have been found in woody plants such as coffee, olive and kiwi-
fruit [5–7].
Vitis vinifera (Vitaceae, Vitis L.) is one of the most widely grown fruit crops in many areas
of the world [8]. It is a woody vine plant whose fruits can be eaten raw as a fresh fruit or made
into dried fruit, juices and wine; thus, it is of great economic value. Grape cultivation is
believed to have originated in Armenia, near the Caspian Sea in Russia, from where it spread
westward to Europe and eastward to Iran and Afghanistan [9]; at present, it is widely grown in
tropical, temperate and subtropical regions worldwide. China is home to the most abundant
grape genetic resources in the world, where grape cultivation has been conducted for thou-
sands [10]. Yunnan Province is one of the major provinces for the grape industry in China. As
of the end of 2014, the total output value of grape in Yunnan exceeded 7 billion yuan; thus,
grape plays an important role in the agricultural industry in this province [11]. However, vari-
ous pathogens, including plant-parasitic nematodes, pose a serious threat to the production of
grapes worldwide, and RKNs are one of the important factors restricting grape production
[12, 13].
Root-knot nematode infestation of grape has been documented in southern Australia,
South Africa, France, the United States and other countries [14]. Grapesroot system are well
developed and is the target of RKN infection, and both seedlings and adult plants can be
harmed. The destroyed fibrous roots and root hairs initially show slight swelling. In later
stages, the diseased roots become rotten, which directly affects the absorption of water and
nutrients by the root system and results in great reductions in grape yield and fruit quality. In
Australia, almost all vineyards on sandy soils are infected by RKNs [15], and four species
(Meloidogyne incognita, Meloidogyne arenaria, Meloidogyne javanica and Meloidogyne hapla)
were found to damage grapes in southern Australia [16]. Meloidogyne incognita and M. hapla
are common grape root pests [17], and Liu and Zhang (2017) reported that grapes from the
Huaihai economic zone were infected by M. incognita [18]. Meloidogyne javanica is the pre-
dominant RKN in Australian vineyards [12], and M. hapla is abundant and widespread in
Washington’s semiarid vineyards [19]. Meloidogyne incognita, M. arenaria, M. javanica and
M. hapla are the main species parasitizing grapes [14, 20]. However, three other Meloidogyne
species, Meloidogyne nataliei, Meloidogyne ethiopica and Meloidogyne thamesi, also infect
grapes [21–23]. RKNs can severely reduce grape yield, causing significant economic losses. Li
et al. (2006) reported that M. incognita was the most common RKN in vineyards, where it
could reduce the yield of susceptible grape varieties by approximately 80% and that of resistant
grape varieties by approximately 40% [24]. Furthermore, RKNs can also interact with bacteria,
fungi and other pathogens to further increase damage to grapes.
Given that grapes are seriously damaged by RKNs, the accurate identification of pathogen
species could be crucial for designing effective prevention and control strategies. However, in
China, there have been few studies on grape RKNs in recent years, with problems such as
unclear distributions, unclear species and a lack of prevention and control technologies. In
addition, the RKNs that parasitize grapes are not completely clear, and some reported results
may need to be further discussed. In Yunnan, we found a high density of RKN-infected grapes,
and the pathogenic species was different from previously described RKNs. The morphological
PLOS ONE | https://doi.org/10.1371/journal.pone.0245201 February 3, 2021
2 / 25
PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
characteristics, such as the perineal pattern, of this species were very similar to those of Meloi-
dogyne mali. Therefore, morphological, biochemical and molecular biological methods were
used to identify this unknown species.
Materials and methods
Nematode population
Samples of grape roots and rhizosphere soils were collected from vineyards in Luliang County,
Yunnan Province. Female and egg samples were extracted from the root tissues of grapes. Sec-
ond-stage juveniles (J2s) were collected from hatching eggs. A portion of the J2s were inocu-
lated on cucumber roots, and the other portion were heat-killed and fixed using a 4% solution
of formaldehyde for morphological observation and measurement. Male samples were
obtained from the cucumber roots. Some of the males were used for scanning electron micros-
copy (SEM), and others were heat-killed and fixed using a 4% solution of formaldehyde for
morphological observation and measurement. The J2 samples of M. mali (used for compari-
son) were provided by Peng Huan, Institute of Plant Protection, Chinese Academy of Agricul-
tural Sciences.
Making perineal patterns
Perineal patterns of female adults were made following the method of Xie Hui (2000) [25].
Specifically, female adults were selected from grape root-knot tissue under an anatomical
microscope, and a hard plastic consisting of 45% lactic acid solution was used to make an
impression of the perineal cuticular pattern with a scalpel. Then, the perineal pattern was
cleaned with a 45% lactic acid solution, placed on a glass slide and covered with coverslip,
using pure glycerine as a floating carrier.
Light microscopy (LM)
All nematode samples were observed and examined under a Carl Zeiss Axio Vert. A1 inverted
microscope. All samples were measured using the de Man indices [26], and the measurements
were expressed in micrometers.
Scanning electron microscopy (SEM)
The samples of female adults, males and J2s were prepared following the methods of Eisenback
et al. (1980) [27] and Eisenback and Hirschmann (1979) [28]. The perineal pattern was made
following the method of Eisenback et al. (1980) [27], with slight modification. Double fixation
with 3.5% glutaraldehyde and 1% osmic acid was employed. Specifically, live samples of female
adults, males and J2s were cleaned with ddH2O and fixed with a 3.5% glutaraldehyde solution
for more than 48 h in a 4˚C refrigerator. After that, they were washed with phosphate-buffered
saline (PBS) 3 times, fixed with 1% osmic acid for 2 h, washed with PBS 3 times, dehydrated in
a graded ethanol series (30%-100%), critical-point dried, and coated with gold. Observation
was performed under a Hitachi S-3000N scanning electron microscope (Japan).
Isozyme phenotype electrophoresis
Isozyme electrophoresis was carried out following the methods of Esbenshade and Trianta-
phyllou (1985) [29] and Esbenshade and Triantaphyllou (1990) [30]. Phenotypes were
observed for esterases (Est) and malate dehydrogenase (Mdh). Five young egg-laying females
of M. vitis sp. nov. and five young egg-laying females from a previously identified population
of M. javanica (used for comparison) were prepared and placed in microtubes containing
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Table 1. The primers used in the research.
Primers code
18S
26S
D2A
D3B
cox1F
cox1R
C2F3
1108
https://doi.org/10.1371/journal.pone.0245201.t001
Primer sequence (50-30)
TTGATTACGTCCCTGCCCTTT
TCCTCCGCTAAATGATATG
ACAAGTACCGTGAGGGAAAGTTG
TCGGAAGGAACCAGCTACTA
TGGTCATCCTGAAGTTTATG
CTACA ACATAATAAGTATCATG
GGTCAATGTTCAGAAATTTGTGG
TACCTTTGACCAATCACGCT
References
Vrain et al. (1992)
De Ley et al. (1999)
Trinh et al. (2019)
Powers et al. (1993)
10 μL mixed liquor consisting of 20% sucrose, 2% Triton X-100 and 0.02% bromophenol blue,
the nematodes were broken with a sterile dissecting needle and the enzyme solution can be
used immediately or stored at -20˚C refrigerator until use. Electrophoresis was carried out in
separating and stacking gels consisted of 7% and 3% polyacrylamide, respectively, 0.75 mm
thick, with Tris-glycine buffer (PH8.7) in a Mini-PROTEAN1 Tetra Cell apparatus (Bio-Rad).
Voltage was maintained at 80 volts for the first 30 minutes, the following was maintained at
150 volts of the separation period until the bromophenol blue dye had migrated to approxi-
mately 0.5 cm ahead of the bottom of the gel. Gels was stained with Mdh stain solution for
Mdh and with Est stain solution for Est, the preparation of Mdh and Est stain solution follow-
ing the method of Esbenshade and Triantaphyllou (1985) [31].
DNA extraction, PCR amplification and sequencing
DNA was extracted from a single female adult and a large number of J2s following the method
described by Adam et al. (2007) [32] and stored in a -80˚C refrigerator until use. Two rDNA
fragments (ITS1-5.8S-ITS2 and D2D3) and two mtDNA fragments (partial coxI and coxII 16S
rRNA) of M. vitis sp. nov. and M. mali were amplified. The primer pairs 18S/26S [33], D2A/
D3B [34], cox1F/cox1R [5] and C2F3/1108 [35] were used to amplify the ITS1-5.8S-ITS2 and
D2D3 fragments of rDNA and the coxI and coxII fragments of mtDNA, respectively. The
primer sequences are listed in Table 1. All of the polymerase chain reactions (PCRs) were per-
formed in 25.00 μL mixed solution containing template DNA (2.50 μL), 10× PCR buffer (Mg2
+, plus, 2.50 μL), dNTPs (mixture, 2.00 μL), forward and reverse primers (10 μmol/L, 1.00 μL
respectively), Taq DNA polymerase (5 U/μL, 0.25 μL), and ddH2O (15.75 μL). All the reagents
used in the PCRs were purchased from TransGen Biotech Company. The PCR amplification
procedure is provided in Table 2. After the amplification reaction, 5.00 μL PCR product was
mixed with 1.00 μL 6× loading buffer (purchased from TaKaRa Company) and electrophoresed
in a 1% Tris-acetate-ethylenediaminetetraacetic acid (TAE)-buffered agarose gel. PCR products
were excised from the gel and purified using the EasyPure Quick Gel Extraction Kit (purchased
from TransGen Biotech Company). The recovered product was ligated with pmD18 cloning
Table 2. The PCR amplification procedure of primers for the research.
Primers
Pre degeneration
Response parameter (35 cycle)
Final extension
18S/26S
D2A/D3B
cox1F/cox1R
C2F3/1108
94˚C 4 min
94˚C 4 min
94˚C 4 min
94˚C 4 min
https://doi.org/10.1371/journal.pone.0245201.t002
Degeneration
94˚C 30 s
94˚C 30 s
94˚C 30 s
94˚C 30 s
Annealing
55˚C 45 s
60˚C 40 s
54˚C 30 s
51˚C 30 s
Extension
72˚C 1 min
72˚C 1 min
72˚C 1 min
72˚C 1 min
72˚C 10 min
72˚C 10 min
72˚C 10 min
72˚C 10 min
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
vector (purchased from TaKaRa Company) and transformed into DH5α competent cells (pur-
chased from TaKaRa Company). The positive clones were selected and sequenced.
Phylogenetic analyses and sequence alignment
The obtained sequences were compared with those from other nematodes available in the
GenBank database using the BLAST homology search program. ITS1-5.8S-ITS2, D2D3 of
rDNA, and partial coxI and coxII 16S rRNA of mtDNA sequences from Meloidogyne spp.
were selected for phylogenetic reconstruction. The ITS1-5.8S-ITS2 (JX015432.1) sequence of
Rotylenchus buxophilus and the D2D3 (AY589364.1) sequence of Ditylenchus halictus and the
coxI (EF617356.1) sequence of Romanomermis wuchangensis and complete mitochondrial
genome (FN313571.1) sequence of Radopholus similis were used as outgroup taxa. A phyloge-
netic tree was generated based on the neighbor-joining (NJ) method in MEGA 5.1 to analyze
the phylogenetic relationships and genetic distances of nematodes. The phylograms were boot-
strapped 1000 times to assess the degree of support for phylogenetic analysis.
DNAMAN software was used to compare the ITS1-5.8S-ITS2, D2D3 of rDNA, and partial
coxI and coxII 16S rRNA of mtDNA sequences between M. vitis sp. nov. and M. mali, and
analysis the sequence divergence.
Designing SCAR primers
The rDNA ITS1-5.8S-ITS2 sequences of M. vitis sp. nov. amplified in this research were sub-
mitted to the GenBank database for BLAST alignment, and a specific sequence in this region
was selected to design specific primers in Primer 5.0 software. The primers Mv-F (5-CTGGT
TCAGGGTCATTTATAAAC-3) and Mv-R (5-TATACGCTTGTGTGGATGAC-3) were used for
PCR amplification of M. vitis sp. nov. The PCRs were performed in a 25.00 μL mixed solution
containing template DNA for M. vitis sp. nov. (2.50 μL), 10× PCR buffer (Mg2+, plus, 2.50 μL),
dNTPs (mixture, 2.00 μL), forward and reverse primers (10 μmol/L, 1.00 μL respectively), Taq
DNA polymerase (5 U/μL, 0.25 μL), and ddH2O (15.75 μL). The amplification procedure was
as follows: 4 min at 94˚C, 35 cycles of 30 s at 94˚C, 30 s at 53˚C and 30 s at 72˚C, and a final
incubation of 10 min at 72˚C. Amplification products were separated by electrophoresis in a
1% TAE-buffered agarose gel and visualized under ultraviolet light.
Nomenclatural acts
The electronic edition of this article conformed to the requirements of the amended Interna-
tional Code of Zoological Nomenclature, and hence the new names contained herein are avail-
able under that Code from the electronic edition of this article. This published work and the
nomenclatural acts it contains have been registered in ZooBank, the online registration system
for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated
information viewed through any standard web browser by appending the LSID to the prefix
“http://zoobank.org/”. The LSID for this publication is: urn:lsid:zoobank.org:pub: CFA0651C-
DDD9-4DA8-A5B2-AF24C7ED8699. The electronic edition of this work was published in a
journal with an ISSN and has been archived and is available from the following digital reposi-
tories: PubMed Central, LOCKSS.
Results
Meloidogyne vitis sp. nov. Yang, Hu, Liu, Chen, Peng, Wang & Zhang sp. nov. urn: lsid: zoo-
bank. org: act: 0163D840-A867-4F03-9C9C-1F879452E34B.
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Disease symptoms
More than 90% of the grape roots collected from vineyards in Luliang County, Yunnan Prov-
ince, were seriously damaged by RKNs. The symptoms of lightly nematode-infected plants
were not obvious. However, the severely nematode-infected plants presented symptoms of
plant dwarfing, leaf yellowing and shedding, little fruit, declining and low growth. The roots
were atrophied and distorted, with severe root knots and other symptoms. Both the axial roots
and branch roots were damaged. The surface of the infected roots presented numerous galls
with white or milky white eggs. Eggs occurred either on the outside of the root galls or within
the root galls. The aged roots were rotten and had become necrotic. Adult female heads were
found associated with the xylem (Fig 1A and 1B).
Description of Meloidogyne vitis sp. nov.
Female (n = 25). The morphometric measurements are shown in Table 3.
Holotype (female in glycerin). Body length = 822.99 μm, maximum body
width = 531.80 μm, stylet length = 15.25 μm, stylet knob width = 4.29 μm, stylet knob
height = 1.85 μm, distance from base of stylet to dorsal esophageal gland opening (DEGO) =
5.32 μm, metacorpus length = 45.50 μm, metacorpus width = 39.49 μm, anterior end to center
of metacarpus = 77.64 μm, distance from anterior end to excretory pore = 38.82 μm, distance
from anterior end to excretory pore/length of stylet (EP/ST) = 2.5.
Morphological characters. The body is pear-shaped and milky white, with a prominent and
variably sized neck. The neck has blurry annuluses, the abdomen has slight bulges, the poste-
rior part of the body is round, and the anal region has no protuberances (Figs 2K, 2L and 3E).
The stylet is developed, with a straight cone and columnar shaft, the stylet knobs are oblate,
the metacorpus is round or ovoid and the valve is developed and obvious, the opening of the
dorsal esophageal gland orifice is hook-like, an excretory pore is located in the posterior por-
tion of the stylet knobs (Figs 2J and 3F), and the EP/ST ratio is approximately 1.98–2.96 μm.
Under SEM, the labial disc is ovoid-squared, slightly raised on the medial lips, and fused with
Fig 1. Root symptom of diseased Vitis vinifera. (A) and (B) all show the root symptom of diseased Vitis vinifera, the
arrow of fig show eggs of root-knot nematodes.
https://doi.org/10.1371/journal.pone.0245201.g001
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Table 3. Morphometrics of Meloidogyne vitis sp. nov.
Holotype Females
Paratype Females
Character
n
Body length
Body width
Stylet length
Stylet knobs width
Stylet knobs height
DEGO
Metacorpus length
Metacorpus width
Head region height
Head region width
Distance from anterior end to center of metacarpus
Distance from anterior end to excretory pore
Tail length
Hyaline tail length
Anal body diameter
Spicules length
Gubermaculum length
822.99
531.80
15.25
4.29
1.85
5.32
45.50
39.49
/
/
77.64
38.82
/
/
/
/
/
a (Body length/ Body width)
1.55
c (Body length/ Tail length)
d (Tail length / Anal body diameter)
Tail length/Hyaline tail length
/
/
/
All measurements are in μm and shown in the form: mean ± s.d. (range).
https://doi.org/10.1371/journal.pone.0245201.t003
25
958.99 ± 132.32
(822.99–1245.16)
609.00 ± 43.63
(531.80–688.11)
15.73 ± 3.68
(8.11–26.58)
4.44 ± 0.96
(2.74–5.95)
2.08 ± 0.48
(1.32–3.32)
4.13 ± 0.84
(2.59–5.32)
42.72 ± 7.05
(23.01–51.53)
37.03 ± 5.81
(21.11–42.86)
/
/
72.75 ± 12.70
(44.17–86.28)
38.82 ± 4.15
(34.33–44.80)
/
/
/
/
/
1.58 ± 0.2
(1.30–1.95)
/
/
/
Males
10
1330.42 ± 179.15
(1032.23–1593.38)
36.75 ± 6.15
(25.69–43.94)
19.31 ± 1.71
(17.02–21.39)
3.50 ± 0.62
(2.65–4.67)
2.54 ± 0.29
(2.23–3.19)
3.30 ± 0.52
(2.35–3.91)
17.95 ± 1.63
(15.61–20.43)
9.23 ± 0.73
(7.92–10.38)
5.20 ± 0.39
(4.70–5.76)
10.41 ± 1.27
(8.33–12.32)
99.31 ± 5.88
(90.96–108.73)
133.33 ± 4.94
(126.49–140.81)
12.86 ± 0.77
(11.81–14.23)
/
24.05 ± 1.81
(21.68–26.68)
30.88 ± 2.59
(27.86–35.75)
10.23 ± 1.86
(8.15–14.88)
36.79 ± 5.96
(30.67–50.15)
103.59 ± 13.78
(81.72–127.65)
0.54 ± 0.03
(0.48–0.57)
/
J2s
26
396.85 ± 18.34
(353.36–425.76)
16.19 ± 1.93
(12.81–22.43)
13.33 ± 0.32
(12.74–14.11)
1.56 ± 0.31
(1.21–2.22)
1.24 ± 0.18
(0.98–1.69)
1.35 ± 0.31
(1.02–2.01)
10.37 ± 1.21
(8.14–12.18)
6.94 ± 0.64
(5.67–8.15)
/
/
54.89 ± 1.99
(50.8–58.62)
41.55 ± 2.13
(37.46–45.31)
57.43 ± 3.92
(47.01–63.77)
12.16 ± 1.74
(9.72–15.73)
12.76 ± 1.91
(10.15–17.11)
/
/
24.76 ± 2.51
(18.98–28.44)
6.95 ± 0.65
(6.15–8.77)
4.58 ± 0.60
(3.39–5.34)
4.81 ± 0.76
(3.44–6.08)
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 2. Line drawings of Meloidogyne vitis sp. nov. Male (A-F) A: Entire body of male; B, C: Anterior region of male; D, E: Tail region
of male; F: Lateral field of male. Second-stage juveniles (G-I) G: Entire body of second-stage juveniles; H: Anterior region of second-
stage juveniles; I: Tail region of second-stage juveniles. Female (J-O) J: Anterior region of female; K, L: Entire body of female; M: Eggs
of female; N, O: Perineal pattern of female. (Scale bars: A, K, L = 200 μm; B-E, I, H, G, M-O = 20 μm; G = 100 μm; F = 10 μm).
https://doi.org/10.1371/journal.pone.0245201.g002
the medial lips to form a dumbbell-shaped structure; there are no obvious lateral lips, and the
oral aperture is slit-like and located in the middle of the labial disc, surrounded by six inner
labial sensilla (Fig 4B). An excretory pore is located on ventrally region between 23rd and 25th
annule behind lips (Fig 4A and 4E). The stylet is cone-shaped and sharply pointed (Fig 4D).
The perineal pattern of female adults is round to ovoid with a moderately high dorsal arch
and smooth and fine striae that are extremely dense and faint; lateral fields are not clearly visi-
ble, and there are no lateral lines, however, a few specimens have slight striae on two shoulders
or wings in the lateral field; two phasmids are large, prominent and round, with a diameter
that can account for 2–5 annular striae, and seemingly eye-shaped; straight lines of two phas-
mids are parallel or nearly parallel to the vulva; the vulval slit is wide and seemingly mouth-
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 3. Light micrographs of Meloidogyne vitis sp. nov. Male (A-D) A: Entire body of male; B: Anterior region of male; C, D: Tail
region of male. Female (E-J) E: Entire body of female; F: Anterior region of female; G: Partial region of female; H, I: Perineal pattern of
female; J: Eggs of female. Second-stage juveniles (K-O) K, O: Tail region of second-stage juveniles; L: Entire body of second-stage
juveniles; M, N: Anterior region of second-stage juveniles (Scale bars: A, E = 200 μm; B-D, F, H, I, J, K, M-O = 20 μm; G, L = 100 μm).
https://doi.org/10.1371/journal.pone.0245201.g003
shaped; the anal fold is clearly visible and seemingly nose-shaped; the whole perineal pattern is
seemingly monkey-face-shaped; the area of the vulva and anus is smooth, with no striae (Figs
2N, 2O, 3H and 3I). The distance between two phasmids is wider than or equal to the length of
the vulval slit; however, in very few specimens, this value is slightly smaller. The vulva-anus
distance is short: 19.94±1.63 (17.35–23.48) μm. The vulva-phasmid distance is 27.98 ± 2.33
(23.28–33.04) μm. The anus-phasmid distance is 6.84 ± 1.49 (4.73–9.79) μm. The morphology
of the perineal pattern under SEM is consistent with that under LM, but SEM shows more
morphological details of the anus and vulva, and the striae are clearer (Fig 4C).
Male (n = 10). The morphometric measurements are shown in Table 3.
Morphological characters. The body is vermiform and variable in length, and the anterior
end of the body tapers off (Figs 2A and 3A). The head cap is obvious and slightly separated
from the body; the stylet is developed and has an obvious boundary with the stylet shaft; the
stylet knot is oblate-spheroidal; the metacorpus is vertically ovoid; and the valve is obvious
(Figs 2C and 3B). The tail is mostly straight and short with a humped end; the spicules are
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 4. Scanning electron microscope photographs of Meloidogyne vitis sp. nov. Female (A-E) A: anterior end in lateral view; B:
anterior end in face view; C: perineal pattern; D: anterior end in lateral view and see the stylet; E: excretory pore. Second-stage juvenile
(F-I) F: anterior end in lateral view; G: anterior end in face view; H: lateral field; I: tail region. Male (J-O) J: lateral field; K: tail region;
L: anterior end in lateral view; M: anterior end in face view; N: excretory pore; O: tail region. (Scale bars: A, C, O = 20 μm; B, D,
M = 3 μm; F, G = 2 μm; E, L, H, N = 5 μm; J, I, K = 10 μm).
https://doi.org/10.1371/journal.pone.0245201.g004
developed, arch-shaped, and slightly curved; and the gubernaculum is obvious and curved-
moon shaped (Figs 2D, 2E, 3C and 3D). Under SEM, the head region lacks annulus; the labial
disc is horizontally ovoid-squared, slightly raised on the medial lips, slightly wider than the
medial lips and fused with the medial lips to form a dumbbell-shaped structure; there are no
lateral lips; a prominent slit-like opening between the labial disc and medial lips is observed; a
distinct depression appears in the middle of the medial lips; and the oral aperture is slit-like
and located in the middle of the labial disc, surrounded by six inner labial sensilla (Fig 4L and
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
4M). The lateral field consists of four incisures forming 3 lateral bands, which are full of reticu-
lar striae (Fig 4J). Lateral incisures extend to the tail end of the body, and in addition to the tail
end, the annulus passes through incisures. The spicules resemble a figure eight, and the tip end
slightly curves to form a hook-like shape (Fig 4K and 4O). The excretory pore is an irregular
pore located in the depression of the cuticle, where it spans two annuluses, and the regular
body annuluses are interrupted around the excretory pore (Fig 4N).
J2 (n = 26). The morphometric measurements are shown in Table 3.
Morphological characters. The body is vermiform and slender and tapers at both ends, but
more towards the tail than the anterior end (Figs 2G and 3L). The body annulations are not
obvious. The stylet is straight, slender, and sharply pointed and has an obvious boundary with
the stylet shaft; the stylet knobs are obvious and spherical; and the metacorpus is ovoid and
clearly visible (Figs 2H, 3M and 3N). The tail is variable, exhibits a range of variation in tail
fields and is conical and constricted; the hyaline tail is short (Figs 2I, 3K and 3O). The anus is
difficult to distinguish except under a high-power oil immersion objective lens. The intestinal
contents were too much, so the rectum and caudal sensory organ were difficult to observe.
Under SEM, the head region is not smooth and slightly folded; the labial disc appears round,
slightly raised on the medial lips and fused with the medial lips to form a dumbbell-shaped
structure, with a slightly sunken appearance in the middle of the medial lips; a prominent slit-
like amphidial opening is located between the labial disc and lateral lips; the oral aperture is
round and located in the middle of the labial disc, surrounded by six inner labial sensilla (Fig
4F and 4G). The lateral field forms 3 lateral bands delimited by four incisures (Fig 4H). The
anal opening is elliptical and located in the cuticular depression in the tail of the body (Fig 4I).
The excretory pore is irregular and located in the cuticular depression, and the regular body
annuluses are interrupted around the excretory pore.
Egg (n = 20). Eggs of female adults are oval-shaped (Figs 2M and 3J). Twenty eggs were
measured in clear water. The egg length was 86.52–110.69 μm (mean: 98.67 μm, standard
error: 6.44), and the egg width was 30.56–39.31 μm (mean: 34.73 μm, standard error: 2.38).
Taxonomic summary
Type host. Grape (Vitis vinifera L., Vitis L., Vitaceae).
Type locality. Luliang County, Yunnan Province, China (25˚07’ N, 103˚78’ E).
Etymology. The specific epithet refers to the host plant on which this new species was
found. Additionally, because of its ability to seriously infect cultivated grapes (V. vinifera L.),
we suggest the name Meloidogyne vitis sp. nov.
Type material. The holotype female and paratypes, males, perineal patterns and J2s were
deposited in the nematode collection of the authors’ Laboratory of Plant Nematology, College
of Plant Protection, Yunnan Agricultural University, China. Specific ITS1-5.8S-ITS2, D2D3
rDNA, coxI rRNA and coxII 16S rRNA mtDNA sequences were deposited in GenBank with
accession numbers MN816222.1, MN816223.1, MN816225.1, MN816226.1, MN814829.1,
MN814830.1, MT012386.1, and MT012387.1, respectively.
Diagnosis and relationships
Meloidogyne vitis sp. nov. can be distinguished from other Meloidogyne spp. by a unique com-
bination of several morphological characters. The perineal pattern of female adults is round or
ovoid, with large and prominent phasmids. Females have a prominent neck, an excretory pore
is located on ventrally region between 23rd and 25th annule behind lips, and the EP/ST ratio is
approximately 2.5 (1.98–2.96 μm). The male has a prominent head region, the labial disc is
fused with the medial lips to form a dumbbell-shaped structure, a wide slit is located between
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
the labial disc and medial lips, the tail is blunt and round, the gubernaculum is prominent and
arch-shaped, and the lateral field consists of four incisures. J2s are characterized by their head
region is not smooth and slightly folded, the labial disc is fused with the medial lips to form a
dumbbell-shaped structure, the hyaline tail is short and constricted, the relatively small c value
(6.15–8.77 μm), and the lateral fields have four incisures. In addition, M. vitis sp. nov. has
unique ITS1-5.8S-ITS2 and D2D3 of rDNA, coxI and coxII 16S rRNA of mtDNA sequences.
Because of the large and prominent phasmids in the perineal pattern in female adults, M.
vitis sp. nov. is similar to M. mali [36], Meloidogyne artiellia [37], Meloidogyne floridensis [38],
Meloidogyne naasi [39], M. nataliei [18], Meloidogyne shunchangensis [40], Meloidogyne kongi
[41], Meloidogyne dimocarpus [42], and Meloidogyne thailandica [43]. Meloidogyne vitis sp.
nov. differs from M. mali in that the perineal pattern of the female shows a moderately high
dorsal arch rather than a low and flat dorsal arch and there are no lateral lines rather than clear
single or double lateral lines in the lateral fields, the DEGO of male and J2 are smaller (2.35–
3.91 vs. 6.00–13.00 μm and 1.02–2.01 vs. 4–6 μm, respectively), the J2 tail is longer (47.01–
63.77 vs. 30–34 μm), and the J2 c value is smaller (6.15–8.77 vs. 12–15 μm). Meloidogyne vitis
sp. nov. differs from M. artiellia in the lip region of the female (the lateral lip is not obvious
rather than appearing as six almost equally sized lips), the greater body length (822.99–1245.16
vs. 650–760 μm) and body width (531.80–688.11 vs. 340–460 μm) in females, the perineal pat-
tern of the female being round or ovoid rather than the general outline pattern resembling a
figure eight, the different male lip region (the labial disc and medial lips are fused into a dumb-
bell-shaped structure instead of the lip region appearing as six nearly equally sized lips), its
smaller male DEGO (2.35–3.91 vs. 5.00–7.00 μm), its longer J2 tail (47.01–63.77 vs. 24.5 μm),
its smaller J2 stylet length and c value (12.74–14.11 vs. 14–16 μm and 6.15–8.77 vs. 13–16 μm,
respectively). Meloidogyne vitis sp. nov. differs from M. floridensis in that its perineal pattern of
the female has no lateral lines rather than faint lateral lines, it has narrower stylet knobs in
males (2.65–4.67 vs. 5.00–6.00 μm), and it has a smaller J2 DEGO (1.02–2.01 vs. 2.50–
3.00 μm). Meloidogyne vitis sp. nov. differs from M. nassi in that the posterior of the female is
smooth rather than presenting a slight protuberance, the excretory pore of the female is situ-
ated behind instead of slightly in front of the stylet knobs, the longer females body length
(822.99–1245.16 vs. 455–705 μm) and body width (531.80–688.11 vs. 227–398 μm), and it
lacks the four or five small and vesicle-like structures grouped irregularly round in front of the
metacorpus that can be found in M. nassi male and J2. Meloidogyne vitis sp. nov. differs from
M. nataliei in that the posterior of the female is smooth rather than having a slight protuber-
ance; the perineal pattern of the female has no lateral lines instead of two separated ropelike
striae extending laterally from the vulval and anal areas and forming a lateral field; the stylet
length, DEGO, and spicule length of male are smaller (17.02–21.39 vs. 28.40–29.20 μm, 2.35–
3.91 vs. 4.0–6.5 μm, and 27.86–35.75 vs. 41.3–44.3 μm respectively); and the body length, stylet
length, and DEGO of J2 are smaller (353.36–425.76 vs. 539.00–641.00 μm, 12.74–14.11 vs.
21.9–22.8 μm, and 1.02–2.01 vs. 3.0–4.3 μm, respectively). Meloidogyne vitis sp. nov. differs
from M. shunchangensis in the female lip region (the lateral lip is not obvious rather than the
lip region appearing as six lips), the perineal pattern of females having no striae rather than
occasionally having lines of striae between the anus and vulva and having no incisures rather
than sometimes having obvious double striae in the lateral field, the greater J2 tail length and
transparent tail length (47.01–63.77 vs. 20.3–28.6 μm and 9.72–15.73 vs. 3.4–4.2 μm, respec-
tively), the smaller J2 DEGO and c value (1.02–2.01 vs. 2.6–3.7 μm and 6.15–8.77 vs. 11.9–
16.60 μm, respectively). Meloidogyne vitis sp. nov. differs from M. kongi in that the female’s
posterior is smooth rather than possessing a prominent protuberance, the perineal pattern is
no line striae vs full of line striae between the anus and vulva, the female body is longer
(822.99–1245.16 vs. 610.6–820.3 μm), the DEGO of J2 is smaller (1.02–2.01 vs. 3.9–5.8 μm),
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
there are no lateral lips rather than two semicircular lateral lips in the head region in males, the
stylet length and DEGO of male are smaller (17.02–21.39 vs. 22.00–23.90 μm and 2.35–3.91 vs.
5.80–7.50 μm, respectively), and the DEGO of J2 is smaller (1.02–2.01 vs. 3.9–5.8 μm). Meloi-
dogyne vitis sp. nov. differs from M. dimocarpus in that the perineal pattern of the female has
no lateral lines instead of double or single striae in the lateral field and no striae instead of mul-
tiple continuous striae between the anus and vulva, the phasmids are large instead of small,
and the male and J2 DEGO are smaller (2.35–3.91 vs. 4.50–5.75 μm and 1.02–2.01 vs. 2.25–
3.75 μm, respectively). Meloidogyne vitis sp. nov. differs from M. thailandica in that the peri-
neal pattern of the female lacks the radial structures underneath the pattern area that are char-
acteristic of M. thailandica, the J2 DEGO and hyaline tail length are smaller (1.02–2.01 vs. 2.
5–3.5 μm and 9.72–15.73 vs. 15–20 μm, respectively).
Meloidogyne vitis sp. nov. can also be distinguished from several other Meloidogyne species
infecting grape, including M. incognita, M. javanica, M. arenaria, M. hapla, M. ethiopica and
M. thamesi, by the large and prominent phasmids in the perineal pattern of females.
Isozyme analysis
The isozyme electrophoretic analysis of young egg-laying females of M. vitis sp. nov. showed
three rare Mdh bands (Fig 5A, MV lane) and one rare Est band migrating rapidly in the gel
(Fig 5B, MV lane), which did not occur in the Mdh and Est phenotypes of M. javanica. The
Mdh and Est bands of M. vitis sp. nov. have not been reported in other RKNs. According to
the relative mobility (Rm) values and referring to the naming method of Esbenshade and Tri-
antaphyllou (1985) [29], the Mdh band was named N3d, and the Est band was named VF1.
PCR product electrophoresis
Size of the PCR amplification bands in different fragments of M. vitis sp. nov. and M. mali as fol-
lows: for both of the ITS1-5.8S-ITS2 fragments was approximately 870 bp, both of the 28S D2D3
fragments was approximately 770 bp, both of the coxI mtDNA fragments was approximately 400
bp; the coxII mtDNA fragments of M. vitis sp. nov. was approximately 550 bp (Fig 6).
Molecular characterization
Amplification and sequencing of the ITS1-5.8S-ITS2 fragment of rDNA from the females and
J2s of M. vitis sp. nov. and J2s of M. mali revealed that the sequence sizes were 877 bp, 877 bp,
and 873 bp, respectively. The GenBank accession numbers are MN816222.1 and MN816223.1
for the female and J2 of M. vitis sp. nov., respectively, and MN816224.1 for the J2 of M. mali.
The ITS1-5.8S-ITS2 fragment identities were 100% for the female and J2 of M. vitis sp. nov. A
BLAST search of M. vitis sp. nov. revealed that the most similar sequence was that of M. mali
(GenBank accession numbers KR535971.1, JX978229.1, and JX978228.1), with an identity of
only 87%. The sequence of M. mali identified in this research was most similar to sequences of
M. mali in GenBank, with identities ranging from 97% (accession number KR535971.1) to
99% (accession numbers JX978225.1, JX978229.1, and JX978228.1). Phylogenetic trees (52
sequences in total) showed that the female and J2 of M. vitis sp. nov. formed a well-supported
clade with high bootstrap support (100%) and were closest to M. mali from GenBank (acces-
sion numbers KR535971.1, JX978229.1, and JX978228.1) and M. mali identified in this
research (accession number MN816224.1), all of which formed one group with high bootstrap
support (95%). Meloidogyne vitis sp. nov. was clearly separated from other species (Fig 7). Sen-
quence alignment of ITS1-5.8S-ITS2 rDNA between female of M. vitis sp. nov. and J2 of M.
mali identified in this research showed that the identity was only 77.09%, and the sequences
were thus highly diverged (210-base divergence) (S1 Fig).
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 5. Malate dehydrogenase and esterase phenotype patterns obtained with electrophoresis of protein
homogenates from five young egg-laying females of Meloidogyne vitis sp. nov. (lane Mv) and five young egg-laying
females of the Meloidogyne javanica reference population (lane Mj). (A) Malate dehydrogenase patterns. (B)
esterase patterns. (C) Relative mobility (Rm) of malate dehydrogenase bands. (D) Relative mobility (Rm) of esterase
bands.
https://doi.org/10.1371/journal.pone.0245201.g005
Amplification and sequencing of the D2D3 fragment of 28S rDNA from females and J2s of
M. vitis sp. nov. revealed sequence sizes both were 775 bp. The GenBank accession numbers
are MN816225.1 and MN816226.1 for the female and J2 of M. vitis sp. nov., respectively. The
D2D3 fragment identities were 99.61% for the female and J2 of this new species. A BLAST
search of M. vitis sp. nov. revealed that the most similar sequence was that of M. mali (Gen-
Bank accession numbers KX430177.1, JX978226.1, JX978227.1, and KF880398.1), with a 93%
identity. Phylogenetic trees (45 sequences in total) showed that the female and J2 of M. vitis sp.
nov. formed a well-supported clade with high bootstrap support (99%). Meloidogyne vitis sp.
nov. is most closely related to M. mali from GenBank (accession numbers KF880398.1,
JX978227.1, KF880399.1, KX430177.1, and JX978226.1) and formed a sister group with this
species with high bootstrap support (98%). Meloidogyne vitis sp. nov. was clearly separated
from other species (Fig 8). Squence alignment of D2D3 28S rDNA between female of M. vitis
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 6. PCR electropherogram for different fragments of Meloidogyne vitis sp. nov. and Meloidogyne mali. M: 2000
DNA marker; CK: The negative control consisting of water; Lanes 1–2: The ITS1-5.8S-ITS2 region of Meloidogyne vitis
sp. nov. and Meloidogyne mali, respectively; Lanes 3–4: The D2/D3 region of Meloidogyne vitis sp. nov. and
Meloidogyne mali, respectively; Lanes 5–6: The coxI region of Meloidogyne vitis sp. nov. and Meloidogyne mali,
respectively; Lane 7: The coxII region of Meloidogyne vitis sp. nov.
https://doi.org/10.1371/journal.pone.0245201.g006
sp. nov. and M. mali from GenBank (accession number KX430177.1) showed that the identity
was 93.81% and the sequences were highly divergent (48-base divergence) (S2 Fig).
The sequence sizes of the coxI fragments of 16S rRNA from the female and J2s of M. vitis sp.
nov. and J2s of M. mali were 413 bp, 413 bp, and 417 bp, respectively. The GenBank accession
numbers are MN814829.1 and MN814830.1 for the female and J2 of M. vitis sp. nov., respectively,
and MN814831.1 for the J2 of M. mali. The sequence identities were 99.76% for the female and J2
of M. vitis sp. nov. A BLAST search of M. vitis sp. nov. revealed the highest match with the
sequences of Meloidogyne ichinohei (GenBank accession number KY433448.1) and M. exigua
(GenBank accession numbers MH128478.1, MH128477.1, and MH128476.1), with identities of
85%. The sequences of M. mali identified in this research were most similar to those of M. mali
from GenBank (accession numbers KM887146.1, KM887145.1, KY433450.1 and KY433449.1),
with 99% identities. Phylogenetic trees (34 sequences in total) showed that the female and J2 of M.
vitis sp. nov. formed a well-supported clade with high bootstrap support (99%). Meloidogyne vitis
sp. nov. was most closely related to M. mali from GenBank (accession numbers KM887146.1,
KY433450.1, KM887145.1, and KY433449.1) and M. mali identified in this research (accession
number MN814831.1), and they formed a sister group. Meloidogyne vitis sp. nov. was clearly sepa-
rated from other species (Fig 9). Sequence alignment of coxI 16S rRNA between the female of M.
vitis sp. nov. and J2 of M. mali identified in this research showed that the identity was 84.26% and
the sequences were highly divergent (67-base divergence) (S3 Fig).
The coxII 16S rRNA sequences from the female and J2s of M. vitis sp. nov. were 545 bp and
540 bp in size, respectively. The GenBank accession numbers are MT012386.1 for the female
and MT012387.1 for the J2 of M. vitis sp. nov. The identities were 99.63% for the female and J2
of M. vitis sp. nov. A BLAST search revealed that M. vitis sp. nov. is most closely related to M.
mali (GenBank accession number KC112913.1), with an identity of 81%. Phylogenetic trees
(53 sequences in total) showed that the female and J2 of M. vitis sp. nov. formed a well-sup-
ported clade with high bootstrap support (100%). Meloidogyne vitis sp. nov. was most closely
related to M. mali from GenBank (accession number KC112913.1) and formed one
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 7. Phylogenetic relationships of Meloidogyne vitis sp. nov. with other root-knot nematodes based on ITS1-
5.8S-ITS2 sequences. Numbers to the left of the branches are bootstrap values for 1000 replications.
https://doi.org/10.1371/journal.pone.0245201.g007
monophyletic clade with moderate bootstrap support (74%) (Fig 10). Sequence alignment of
coxII 16S rRNA between female of M. vitis sp. nov. and M. mali from GenBank (accession
number KC112913.1) showed that the identity was only 78.58% and the sequences were highly
divergent (118-base divergence) (S4 Fig).
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 8. Phylogenetic relationships of Meloidogyne vitis sp. nov. with other root-knot nematodes based on D2/D3 sequences of
28S rDNA. Numbers to the left of the branches are bootstrap values for 1000 replications.
https://doi.org/10.1371/journal.pone.0245201.g008
SCAR-PCR analysis
Individual females of previously identified and purified populations of M. incognita, M. java-
nica, M. arenaria, M. hapla, and Meloidogyne enterolobii were used for comparison. DNA was
extracted from individual females of M. incognita, M. javanica, M. arenaria, M. hapla, M.
enterolobii and M. vitis sp. nov. The ITS1-5.8S-ITS2 fragment of the six RKN species was
amplified using the primers 18S/26S. In all populations, a single band with a size of approxi-
mately 760–900 bp was amplified (Fig 11A). However, using the primers Mv-F/Mv-R to
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 9. Phylogenetic relationships of Meloidogyne vitis sp. nov. with other root-knot nematodes based on coxI-rRNA
genes sequences. Numbers to the left of the branches are bootstrap values for 1000 replications.
https://doi.org/10.1371/journal.pone.0245201.g009
amplify the same six templates mentioned above, species-specific fragments of approximately
170 bp were amplified only in M. vitis sp. nov., no fragments were observed for templates from
the other five RKN species (Fig 11B). The species-specific product of M. vitis sp. nov. was recy-
cled, cloned and sequenced, resulting in a 174 bp sequence, which was deposited in the Gen-
Bank database for BLAST alignment, no similar sequences were found. All results indicated
that the Mv-F/Mv-R primers were specific and reliable.
Discussion
Reliable detection and identification technology is necessary for the protection of agricultural pro-
duction systems against quarantine nematodes worldwide [44]. In the past, RKNs were identified
mainly by morphological observations. Although observations of the perineal pattern of female
adults is the primary method used for morphological identification, there is some intraspecific
variability in this pattern due to differences in host and nutrition, differences between young
females and female adults and other factors. Therefore, identification results are often uncertain.
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 10. Phylogenetic relationships of Meloidogyne vitis sp. nov. with other root-knot nematodes based on coxII-16S rRNA
genes sequences. Numbers to the left of the branches are bootstrap values for 1000 replications.
https://doi.org/10.1371/journal.pone.0245201.g010
The perineal pattern of Meloidogyne inornata is similar to that of M. incognita, making it difficult
to distinguish these two species [45], and the perineal pattern of pre-adult M. javanica resembles
that of Meloidogyne africana adults [46]. In addition, the perineal pattern of RKNs varies greatly
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
Fig 11. PCR amplification of the supplied RKNs with the Mv-F/Mv-R primers. M: 2000 DNA marker; CK: The
negative control consisting of water. A: Lanes 1–6, The ITS1-5.8S-ITS2 region of Meloidogyne vitis sp. nov.,
Meloidogyne incognita, Meloidogyne javanica, Meloidogyne arenaria, Meloidogyne hapla and Meloidogyne enterolobii,
respectively. B: Lanes 7–12, The amplification results of root-knot nematode species-specific PCRs of Meloidogyne vitis
sp. nov., Meloidogyne incognita, Meloidogyne javanica, Meloidogyne arenaria, Meloidogyne hapla and Meloidogyne
enterolobii, respectively.
https://doi.org/10.1371/journal.pone.0245201.g011
across generations; for example, M. javanica has a total variation rate of 22.6%; the variation is
mainly caused by nonobvious incisures and the formation of shoulder protuberances, which
make it difficult to distinguish this RKN from M. arenaria [47]. Traditional morphological meth-
ods face considerable challenges in the identification of RKNs due to intraspecific variation and
interspecies similarity [48]. Therefore, PCR-based methods and biochemical methods are becom-
ing increasingly important in the diagnosis of Meloidogyne spp.
The technology of isozyme (in particular Est and Mdh) electrophoretic is a relatively old
method for the identification of Meloidogyne spp., but it is still used by many researchers to
identify some RKNs. This technique remains as an effective methodology with which to unam-
biguously identify and differentiate Meloidogyne megadora [49]. In this research, the band phe-
notypes of malate dehydrogenase from the new species were different from those of other
RKNs reported to date; the new species produced three bands and showed the N3d phenotype.
The esterase of the new species migrated rapidly in the gel, showing a VF1 phenotype, which is
the same esterase phenotype observed for M. naasi, Meloidogyne exigua and M. thailandica;
however, all of these species have different malate dehydrogenase phenotypes [39, 50]. Never-
theless, the isozyme analyses applied in species identification are limited because they are effec-
tive only when egg-laying females are available [51].
PCR-based methodologies are of ever-increasing importance in species diagnostics and
phylogenetics within the genus Meloidogyne [52]. Phylogenetic analyses of rDNA sequences
are considered a reliable diagnostic approach and are commonly used to identify and compare
certain RKNs [53]. Based on rDNA-PCR identification, the ITS fragment is possibly the most
widely used genetic marker for living organisms and the most commonly used species-level
marker used for organisms (plants, protists, and fungi) [52]. This fragment has been widely
used to identify RKNs. However, Powers et al. [54] and Blok et al. [55] found that the ITS1-
5.8S-ITS2 sequences of M. incognita, M. javanica, and M. arenaria were extremely conserved.
Landa et al. (2008) [56] also found that the 18S sequences of rDNA from M. hispanica and M.
ethiopica were very similar. Although M. hispanica and M. ethiopica can be clearly differenti-
ated by their D2D3 28S ribosomal DNA sequences [56], M. incognita, M. javanica, and M. are-
naria cannot [5]. Therefore, identification of some RKN species with rDNA-PCR technology
remains difficult. The mitochondrial genome (mtDNA) provides a rich source of genetic
markers for species identification [57], including parasitic nematode identification, because of
their high mutation rates and maternal inheritance [58], mtDNA-PCR technology is an alter-
native method for precise identification of Meloidogyne species, to study intraspecific
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
variability and to follow maternal lineages [59]. The mitochondrial genes coxI and coxII have
been widely used as DNA markers in various large organismal groups in the animal kingdom
[60]. In terms of resolution, coxI is more capable of discriminating between species than either
of the rRNA genes [61]. Sequence characterized amplified region (SCAR) markers have proven
to be a very sensitive and reliable tool for the identification of RKNs and to provide an easy
and rapid assessment of a large number of samples by a simple visual evaluation of gels [62].
The SCAR-PCR technique is more sensitive than other existing molecular techniques [63],
providing a rapid species identification approach for turfgrass RKNs independent of morphol-
ogy [64]. An increasing number of species-specific primers are being designed for the identifi-
cation of RKNs [65–67]. In the present research, the species-specific primer pair Mv-F/Mv-R
was designed based on the sequence of the rDNA ITS1-5.8S-ITS2 fragment in M. vitis sp. nov.
so that it could provide a simple and rapid method for identifying this new species.
In this research, PCR amplification of ITS1-5.8S-ITS2 rDNA, D2D3 28S rDNA, and
mtDNA (coxI and coxII) was used to identify RKNs. The nematode collected from grape was
different from previously described RKNs. The ITS1-5.8S-ITS2 and D2D3 sequences of rDNA
and the coxII sequence of mtDNA were compared with the corresponding fragments in RKNs
available from GenBank. The most similar species was M. mali, with similarities of 87%, 93%,
and 81%, respectively. The coxI mtDNA sequence was most similar to that of M. ichinohei,
with a similarity of 85%. The phylogenetic tree based on ITS1-5.8S-ITS2 rDNA, D2D3 28S
rDNA, and mtDNA (coxI and coxII) sequences showed that the new species was closely
related to M. mali and clearly distinguished from other RKNs.
In summary, both the morphological and molecular characteristics revealed that the new
RKN from grape is sufficiently different from the RKNs described to date to be considered a
new RKN species. Thus, this new species was named M. vitis sp. nov. according to its host
resource. Meloidogyne vitis sp. nov. and M. mali are similar in morphology and have a close
molecular relationship; they may have evolved from the same ancestral species. Almost all
grape roots were infected by RKNs in the vineyard investigated in this study, and the aged
roots were decayed and necrotic due to RKN infection, which indicated that the RKNs in this
vineyard had existed there for many years. However, their origin is still unknown, and they
may have been introduced from outside. The phylogenetic tree based on various fragments
shows that the new species has independent evolutionary trends; it is either indigenous in
some regions of the world as an ancient species or has recently evolved and been widely spread
by agriculture. High-density RKNs undoubtedly pose a serious threat to grape production.
This species may be restricted to Luliang County, Yunnan Province, and will seriously damage
grapes there by causing severe root knots, dwarfed plants and reduced fruit production. More
surveys are needed to clarify the distribution of M. vitis sp. nov., and further research will also
be necessary to determine its host range, pathogenesis, and control strategies.
Supporting information
S1 Fig. Sequence alignment of Meloidogyne vitis sp. nov. and Meloidogyne mali identified
in this research based on ITS1-5.8S-ITS2 sequences. (1 = Meloidogyne vitis sp. nov., 2 =
Meloidogyne mali).
(TIF)
S2 Fig. Sequence alignment of Meloidogyne vitis sp. nov. identified in this research and
Meloidogyne mali from GenBank (KX430177.1) based on D2D3 sequences of 28S rDNA. (1
= Meloidogyne vitis sp. nov., 2 = Meloidogyne mali).
(TIF)
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PLOS ONEA new root-knot nematode, Meloidogyne vitis sp. nov. (Nematoda: Meloidogynidae), parasitizing grape in Yunnan
S3 Fig. Sequence alignment of Meloidogyne vitis sp. nov. and Meloidogyne mali identified
in this research based on coxI-rRNA genes sequences. (1 = Meloidogyne vitis sp. nov., 2 =
Meloidogyne mali).
(TIF)
S4 Fig. Sequence alignment of Meloidogyne vitis sp. nov. identified in this research and
Meloidogyne mali from GenBank (KC112913.1) based on coxII-16S rRNA genes sequences.
(1 = Meloidogyne vitis sp. nov., 2 = Meloidogyne mali).
(TIF)
S1 Raw images.
(PDF)
Author Contributions
Conceptualization: Yanmei Yang, Xianqi Hu, Li Chen.
Data curation: Yanmei Yang, Xianqi Hu, Pei Liu.
Formal analysis: Yanmei Yang, Xianqi Hu, Pei Liu.
Funding acquisition: Xianqi Hu.
Investigation: Yanmei Yang, Qi Zhang.
Methodology: Yanmei Yang, Xianqi Hu, Pei Liu, Huan Peng, Qiaomei Wang.
Resources: Huan Peng.
Software: Yanmei Yang, Pei Liu, Qiaomei Wang.
Supervision: Xianqi Hu.
Writing – original draft: Yanmei Yang.
Writing – review & editing: Xianqi Hu, Pei Liu.
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10.1088_1361-6501_ad180f.pdf
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Data availability statement
The data cannot be made publicly available upon publication
due to legal restrictions preventing unrestricted public distri-
bution. The data that support the findings of this study are
available upon reasonable request from the authors.
|
Data availability statement The data cannot be made publicly available upon publication due to legal restrictions preventing unrestricted public distribution. The data that support the findings of this study are available upon reasonable request from the authors.
|
Meas. Sci. Technol. 35 (2024) 036306 (16pp)
Measurement Science and Technology
https://doi.org/10.1088/1361-6501/ad180f
BDS-3 RTK/UWB semi-tightly coupled
integrated positioning system in harsh
environments
Peipei Dai1, Sen Wang1, Tianhe Xu2,3,∗, Nazi Wang2,3, Min Li2,3, Jianping Xing1
and Fan Gao2,3
1 School of Microelectronics, Shandong University, Jinan 250101, People’s Republic of China
2 Institute of Space Sciences, Shandong University, Weihai 264209, People’s Republic of China
3 Shandong Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Weihai 264209,
People’s Republic of China
E-mail: [email protected]
Received 25 October 2023, revised 4 December 2023
Accepted for publication 21 December 2023
Published 28 December 2023
Abstract
Real-time kinematic (RTK) positioning is a commonly used technique in modern industry,
which is limited by problems such as signal occlusion, attenuation, and multipath, especially in
complex urban canyons. To maintain the consistency of centimeter-level accuracy, we adopt the
ultra-wideband (UWB) enhanced BDS-3 RTK positioning algorithm. This paper proposed a
semi-tightly coupled (STC) BDS-3 RTK/UWB integration positioning model. This model
realizes the UWB and BDS-3 complement each other and integrate information in the position
domain. Besides, height constraint is imposed on UWB positioning to mitigate the effect of
poor positioning of UWB in height components. To verify the effectiveness of the above
algorithm, we have compared and analyzed the positioning performance of the STC BDS-3
RTK/UWB integration model and single BDS-3 RTK model in different occlusion
environments. The positioning performance of static and kinematic of BDS-3 RTK/UWB STC
based on different number of UWB anchors is further analyzed. The real-world experiment
results show that the positioning accuracy of the proposed method can reach centimeter-level.
Moreover, the proposed model can obtain more accurate positioning results than those of using
single system, and it shows more obvious advantages, especially in the occlusion environment.
In the occlusion environment, the root means square error in the east, north, and up directions is
improved from (0.629 m, 0.325 m, 1.160 m) of the BDS-3-only to (0.075 m, 0.074 m, 0.029 m).
This study can provide a reference for the future development of high-precision, real-time,
continuous positioning, navigation, and timing in complex urban environments.
Keywords: BDS-3, real-time kinematic, ultra-wideband, semi-tightly coupled, height constraint
1. Introduction
With the development of automatic driving [1], smart cities
[2], intelligent transportation [3], unmanned aerial vehicle
flight control
the
demand for high-precision positioning is increasing. The
real-time kinematic (RTK) positioning technology of the
[4], and the Internet of Things [5],
∗
Author to whom any correspondence should be addressed.
Beidou Navigation Satellite System (BDS-3) often serves
the above scenes with its advantages of being real-time,
simple, and reliable [6]. RTK technology through differen-
tial technology based on a reference station, can provide
real-time centimeter-level positioning service for users [7–9].
However, RTK is limited by satellites being blocked to vary-
ing degrees, resulting in a sharp decline in positioning accur-
acy in complex environments [10]. This indicates that any
single technology cannot be high-precision, high-availability,
high-continuity, and high-reliability in all environments at all
1
© 2023 IOP Publishing Ltd
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
times. Meanwhile, the complementary characteristics of other
sensors and BDS-3 can be used to supplement and enhance
satellite navigation and positioning performance under the
BDS-3 spatiotemporal reference. The high-precision and high-
availability of multi-source integrated positioning in complex
environments can be realized. Therefore, the integration posi-
tioning of multi-source sensors has become a hot topic in the
field of navigation and positioning [11, 12].
At present, ultra-wideband (UWB) has received wide-
spread attention due to its advantages of low-cost, high-
precision, and UWB, which has become one of the main means
to make up for the lack of BDS-3 satellite positioning accur-
acy. The power of the UWB signals is much smaller than that
of the BDS-3 satellite signals, which can coexist with them
without interfering with the BDS-3 satellite signals [13]. The
integration positioning of the BDS-3 and UWB is one of the
potential technologies to improve positioning performance.
High-precision navigation and positioning technology is
one of the essential technologies of intelligent vehicles, smart
cities and intelligent transportation systems. With the abund-
ant of more scenarios, there requires new demands on pos-
itioning performance in different spatial ranges. The integ-
ration of BDS-3 RTK and UWB systems is highly benefi-
cial for the field of high-precision, continuous and seamless
positioning. This combination harnesses the high-precision
capability of BDS-3 RTK system and the strong penetration
and centimeter-level accuracy of UWB system. Through this
fusion, autonomous vehicles can achieve smoother and more
continuous positioning solutions in urban complex environ-
ments, thus enhancing safety and improving navigation effi-
ciency. This is particularly crucial for ensuring accurate pos-
itioning in urban canyons, tunnels, and non-exposed space
where Global Navigation Satellite System (GNSS) signals
may be blocked.
Currently, the research on the integration positioning of
the GNSS and UWB has been carried out one after another.
The integration positioning of GNSS and UWB has loosely
coupled (LC) and tightly coupled (TC) models. For the LC
model, GNSS and UWB firstly perform positioning solution
respectively, then their respective position information is input
into the LC filter to finally output the integration position-
ing information. This model is based on the position domain,
however, when single system fails, LC fusion filtering will be
not possible [14, 15]. Subsequently, the TC model has pro-
posed, for which the raw observations of GNSS and UWB
are integrated input into the filter for the final position solu-
tion. This model belongs to the integration coupled with GNSS
and UWB in the observation domain. The TC model can over-
come the drawbacks of the LC model, allowing measurement
updates even when the number of GNSS satellites is less than
four, but the filtering stability is poor due to the integration
of two heterogeneous observation data. Chiu et al [16] pro-
posed the GPS RTK/UWB TC integration system and com-
pared the positioning performance of the TC integration sys-
tem and single GPS RTK system, indicating that the former
has a more stable performance. Then, the team also studied
the enhancement effect of UWB on DGPS [17], and the results
showed that the TC model can achieve meter-level positioning
accuracy. Glenn et al studied that the UWB TC enhancement
model greatly improves the success rates of GPS RTK ambi-
guity resolution [18] and showed that the TC integration sys-
tem of GPS RTK and UWB can achieve submeter-level posi-
tioning accuracy in urban environments [19]. Huang et al [20]
proposed the TC model integrating GNSS precise point posi-
tioning and UWB. The experimental results showed that this
model can significantly improve the positioning accuracy and
convergence time, and the improvement was mainly reflected
in the north and east directions, while the vertical direction was
small. Although the model can provide centimeter-level accur-
acy, it requires high-precision products, which will usually be
delayed in release, and cannot achieve real-time positioning
[21]. Despite the TC model can overcome the shortcomings
of the LC model, the complexity of the algorithms is high. In
practical application scenarios, UWB anchors are closer to the
user, and rapid geometric changes will occur when the user
moves. Therefore, within the coverage of UWB anchors, we
should tap the potential of UWB application as much as pos-
sible and effectively use the benefits brought by UWB.
Through the aforementioned research, it can be concluded
that while TC offers enhanced data processing capability
and greater anti-interference performance, it comes at the
increased cost of system complexity, algorithm complexity,
and hardware cost. Conversely, LC architecture is simpler, less
costly, and more flexible, but falls short in terms of accuracy,
anti-interference capability, and adaptability in complex envir-
onments. Consequently, this paper introduces a semi-tightly
coupled (STC) integrated model. STC balances system com-
plexity and positioning performance with lower system com-
plexity and cost than TC, while providing better data pro-
cessing capability and anti-interference performance than LC.
This approach is optimized in terms of cost-effectiveness, anti-
multipath performance, and adaptability. These characterist-
ics of STC render it an effective solution suitable for complex
environments and diverse requirements.
In this paper, a STC integration model of BDS-3
RTK/UWB in complex environments is proposed. The UWB
measurements of double-side two-way ranging (DS-TWR) are
integrated with BDS-3 RTK of double-differential pseudor-
ange and carrier-phase. Moreover, the predetermined external
height information is used to constrain the height of UWB
positioning to compensate for the lack of positioning accuracy
of UWB at the height component. The positioning perform-
ance of the proposed model is widely evaluated by real-world
experiments. The positioning performance of the STC BDS-3
RTK/UWB integration model and single BDS-3 RTK model in
different occlusion environments is compared and analyzed.
And, the positioning performance of static and kinematic
of the proposed model based on different number of UWB
anchors is further analyzed.
The remaining of this paper is organized as follows.
Sensor models, UWB height constraint, the STC integration
algorithm and the structure of the proposed model are shown in
2
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
section 2. The real-world experimental environment and data
processing strategies are presented in section 3. The experi-
mental results are analyzed and discussed in section 4. Finally,
the conclusions and future work prospects are presented in
section 5.
2. Methodology
2.1. BDS-3 RTK observation model
The BDS-3 pseudorange and carrier-phase measurements
undifferenced observation equations can be expressed as [22,
23]
r,j = ρs
Ps
r = ρs
λjϕ s
r + c (δtr − δts) + Is
r + c (δtr − δts) − Is
r,j + Ts
r,j + Ts
r + Tids
r + λjNs
r + Rels
r + εP,j
r,j + Tids
r + Rels
r + εϕ ,j
(1)
where the superscript s represents the satellite number, the sub-
script r and the j represent the number of stations and signal
frequency f, respectively; P represents the pseudorange obser-
vation; ϕ indicates the carrier-phase observation; λ indicates
the wavelength of the carrier; ρ indicates the geometric dis-
tance from the station r to satellite s; c is the speed of light; δtr
and δts denote receiver clock difference and satellite clock dif-
ference, respectively; N is the ambiguity; I and T denote iono-
spheric delay and tropospheric delay, respectively; Tid and Rel
denote tidal and relativistic effects; εP,j and εϕ ,j denote meas-
urement noise in pseudorange and carrier-phase observations,
respectively.
It is assumed that tidal and relativistic effects have been
corrected by the model before the difference. According to
equation (1), the original observation of BDS-3 is first made
by a single-difference between stations, which can be written
as [24]:
baselines (<20 km), and it can be considered that most com-
mon errors are eliminated by double-difference operations.
2.2. UWB DS-TWR observation model
The observation equation of UWB based on DS-TWR can be
expressed as:
di = Ri (X) + εi
(i = 1, 2, · · · n)
(4)
[
dn
d2
d1
· · ·
where the subscripts i represents the ith UWB anchor
]
T
node; d =
denotes the observation vec-
tor between the ith UWB anchor node and the receiver; Ri (X) =
√
(xi − x)2 + (yi − y)2 + (zi − z)2 represents the Euclidean dis-
tance between the ith UWB anchor node and the receiver;
X = (x, y, z) is the position of the UWB tag; (xi, yi, zi) is the
coordinate of the ith UWB anchor node, and εi denotes the
UWB ranging error and random noise.
In this paper, UWB as a subsystem of the integration sys-
tem, the positioning results and covariance output by UWB
can be used as a position observation measurement, which is
combined with BDS-3 observation measurements to constrain
positioning parameters, where the positioning results and cov-
ariance of UWB can obtain by the extended Kalman filter
(EKF). The EKF of state and measurement equations in the
kth epoch can be expressed as [26, 27]:
{
Xk = Φ k/k−1Xk−1 + ωk
Zk = HkXk + vk
(5)
where Xk−1 and Xk denote state vectors at k − 1 and k epoch,
respectively; Zk denotes the observation vector; Φ k/k−1 repres-
ents the state transition matrix; Hk represents the measurement
matrix; ωk is the state process noise and vk is the measurement
noise.
The solution steps of the EKF can be summarized as
∆Pm
λj∆ϕ m
rb,j = ∆ρm
rb = ∆ρm
rb + cδtrb + ∆εP,j
rb + cδtrb + λj∆Nm
rb,j + ∆εϕ ,j
(2)
follows [28, 29]:
The time update equations are as follows:
{
where m is a satellite with a satellite PRN code of m; r
denotes rover station; b denotes base station; ∆ stands for the
single-difference operator. Moreover, the inter-station single-
difference eliminates satellite-related errors and most iono-
spheric delay errors and tropospheric delay errors. Meanwhile,
the residual ionospheric and tropospheric delay errors can be
ignored when the baseline is less than 20 km [9, 25].
After the inter-station single-difference, the relative clock
difference and hardware delay error between the rover station
and the base station still exist, so the inter-satellite double-
difference is carried out based on the inter-station single-
difference, which can be expressed as [24]:
ˆXk/k−1 = Φ k/k−1
Pk/k−1 = Φ k/k−1Pk−1Φ T
ˆXk−1 + ωk
k/k−1 + Qk−1
(6)
where Pk/k−1 and Pk−1 are the prior and posterior estimates of
the error covariance matrix.
The measurement update equations are as follows:
(
Kk = Pk/k−1HT
k
ˆXk = ˆXk/k−1 + Kk
HkPk/k−1HT
− Hk
Zk
k + Rk
ˆXk/k−1
(
Pk = Pk/k−1 (I − KkHk)
)−1
)
(7)
where Kk denote the Kalman filtering gain.
rb,j = ∇∆ρmn
∇∆Pmn
λj∇∆ϕ mn
rb = ∇∆ρmn
rb + ∇∆εP,j
rb + λj∆Nm
rb,j + λj∆Nn
rb,j + ∇∆εϕ ,j
(3)
2.3. UWB height constraint
where n is a different satellite with a satellite PRN code of
n; ∇∆ which stands for the double-difference operator. The
double-difference observation model is only suitable for short
3
UWB utilizes short-pulses and wideband signals for com-
munication and positioning, which are influenced by factors
such as antenna distribution, signal propagation, geometric
sparsity, and antenna radiation characteristics. It is impractical
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
to arrange UWB anchor nodes with a uniform height distribu-
tion in practical applications, which results in the positioning
accuracy of UWB positioning results in the height component
being lower than that of the horizontal component. To improve
the positioning accuracy of the height component, an effect-
ive method is to adopt constraint filtering to enforce the con-
straints of the height component on the navigation solution.
In an ideal solution scenario, it is more efficient and conveni-
ent to enhance the model strength of the height component
by constraining predetermined height information to filtering.
The height constraint in this paper is equivalent to the newly
added observation, which is a LC integration process, and the
constraint equation contains an error component, which can be
expressed as:
hk = h0 + εh, εh
∼ N (εh; 0, Wh)
(8)
where hk denotes the kth epoch height component solution; h0
represents prior height information; εh is the random noises,
noise sequences εh is assumed to be independent, and with zero
mean and with covariances matrix Wh.
According to the above formula, the observation equation
of the height constraint can be expressed as:
˜Hk = [0, 0, 1]
(9)
Figure 1. The framework for STC integration positioning model of
BDS-3 RTK/UWB.
where ˜H represents the measurement matrix of height con-
straint in positioning solution.
In practical application, the height component solution is
not always constant. The residual between external height
information and height component solution will not be com-
pletely zero. Consequently, the observation error vector of the
height constraint can be expressed as:
˜vk = [δh] = [hk] − [h0] .
(10)
Therefore, the height constraint must be relaxed, and model
the error as Gaussian white noise. The measurement matrix
can be given by:
− h0 = ˜H˜v + e
hk
(11)
where e is the Gaussian white noise.
Based on this, if predetermined external height information
is available, the measurement update equations considering
height constraint can be represented as [30–32]:
)−1
˜Kk = Pk
˜HT
k
˜Xk = ˆXk + ˜Kk (hk
(
˜Hk
˜Pk =
(
˜HT
˜HkPk/k−1
k
− h0)
)
Pk
I − ˜Kk
(12)
where ˜Kk is the Kalman filtering gain of the height constraint;
˜Xk = (xk, yk,˜zk) denotes the positioning solution coordinate of
the kth epoch after the height constraint; ˜Pk the error covariance
matrix after height constraint.
4
2.4. BDS-3 RTK/UWB STC integration model
The STC BDS-3 RTK/UWB model proposed in this paper
integrates the BDS-3 measurements, that is pseudorange and
carrier-phase, the positioning solution and covariance inform-
ation of UWB positioning. In addition, height constraint is
also introduced in the UWB position solution filter to enhance
the positioning performance of the UWB height direction.
The framework for the STC integration positioning model of
BDS-3 RTK/UWB is shown in figure 1. When the UWB tag
receives the UWB DS-TWR ranging information, it eliminates
the data gross errors during preprocessing and then the height
constraint was input into the EKF filtering solution to obtain
the UWB positioning results and its covariance. Meanwhile,
after receiving the BDS-3 observational data, perform BDS-
3 RTK positioning process. When the observations of UWB
and BDS-3 are time synchronized, the UWB positioning res-
ults and its covariances are output into the BDS-3 RTK fil-
tering to assist BDS-3 RTK ambiguity fixed and positioning
solution. Then the position solution of UWB and BDS-3 is
output into the integration filter, and the navigation informa-
tion of the BDS-3 RTK/UWB STC integration model output
is obtained. From the integration system structure, the STC
integration belongs to a high-level LC integration model. From
the perspective of information integration, STC integration
model is a simplified form of TC integration model, which
realizes the complement each other of UWB and BDS-3 and
integrates information in the position domain. Therefore, the
STC integration model is a form between the LC and TC integ-
ration model.
Meas. Sci. Technol. 35 (2024) 036306
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Figure 2. The experiment environment (left) and platform (right).
Figure 3. Planimetric views of UWB anchor nodes and the trajectory.
3. Experiment environment description
3.1. UWB experiment environment
To verify the enhancement effect of UWB on BDS-3 RTK sys-
tem based on the STC integration positioning model, static
and kinematic experiments were carried out at 9–10 o’clock
and 11–12 o’clock in DOY184 of 2023, respectively. Both
static and kinematic experiments have a duration time of an
hour respectively, and the observation sampling rate of 1 s.
The experimental environment is shown on the left of figure 2.
The BDS-3 and UWB observations used in this experiment
were both collected in the same location. The experimental
platform used to obtain observations is shown on the right of
figure 2. The user terminal is equipped with GNSS antenna,
UWB antenna, UWB tag, GNSS receiver, computation ter-
minal, portable power, and unmanned ground vehicle.
Planimetric views of UWB anchor nodes and the trajectory
of the user are shown in figure 3. The red circle represents
the UWB anchor nodes, the blue star represents the reference
position of the static experiment, and the blue line represents
the trajectory of the kinematic experiment.
The static positioning results in the east (E), north (N),
and up (U) directions of UWB-only based on four and six
UWB anchor nodes are shown in figure 4. As shown in the
figure, 4 UWB-only and 6 UWB-only denote the number of
UWB anchor nodes participating in UWB positioning are four
and six, respectively. To investigate the positioning accur-
acy further, the root means square error (RMS) and stand-
ard deviation (STD) values are summarized in table 1. During
data processing, the one-hour UWB observations were ini-
tialized every 15 min. The numbers in the legend represent
the number of UWB anchor nodes used in positioning. We
considered a strategy of four and six UWB anchor nodes
participating in the positioning. Combined with figure 4 and
table 1, we can conclude that the positioning errors of the
four and six UWB anchor nodes are basically at the same
level. In addition, due to the height constraint attached to
the UWB positioning, combined with the U direction pos-
itioning errors sequence in figure 4 and the STD values in
table 1, it can be concluded that the U direction position-
ing errors are very smooth, and its RMS is within 0.020 m.
Moreover, in the E and N directions, the RMS of static pos-
itioning errors of the 4 and 6 UWB anchor nodes can reach
about (0.014 m, 0.046 m) and (0.025 m, 0.047 m), respectively.
The STD in the E, N, and U directions is (0.011 m, 0.008 m,
0.003 m) of 4 UWB anchor nodes and (0.006 m, 0.008 m,
0.002 m) of 6 UWB anchor nodes. The statistical RMS and
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Figure 4. UWB-only static positioning results based on four and six UWB anchor nodes.
Table 1. RMS and STD of UWB-only static positioning results (m).
4 UWB-only
6 UWB-only
Static
RMS
STD
E
0.014
0.011
N
0.046
0.008
U
0.006
0.003
E
0.025
0.006
N
0.047
0.008
U
0.011
0.002
STD values provide further support for the above conclu-
sions. In addition, it should be explained that limited redund-
ancies pose challenges in the UWB ranging errors processing,
resulting in a slight fluctuation in UWB positioning accur-
acy. Additional errors and noise are introduced due to signal
propagation effects and mutual interference between anchor
nodes. In contrast, it shows slightly better positioning accuracy
with fewer UWB anchors. Whether using 6 or 4 UWB anchors,
the positioning performance is relatively consistent. These res-
ults indicate that the superiority of maintaining reliable and
accurate positioning with a limited number of UWB anchors
and the potential to reduce UWB setup costs in occluded
environments [33]. As the above results, this also provides
the possibility for the STC integration of BDS-3 RTK and
UWB.
The kinematic positioning results of UWB-only based on
four and six UWB anchor nodes are shown in figure 5. The
RMS and STD of these positioning errors are summarized in
table 2. From figure 5 and table 2, we can conclude that the
height constraint has a similar effect on the kinematic experi-
ment, and the U direction positioning errors tend to be stable
after the height constraint with RMS within 0.020 m. In the E
and N directions, the RMS of kinematic positioning errors can
reach about (0.096 m, 0.012 m) to (0.072 m, 0.097 m). This
further verifies the feasibility of integrating the BDS-3 RTK
with the UWB STC.
3.2. BDS-3 experiment environment
In this experiment, the BDS-3 observation measurements were
acquired simultaneously in static and kinematic experiments
on DOY184 of 2023. Figures 6 and 7 show the skyplot and the
number of visible satellites in the unobstructed environment
(scene 1) and occlusion environment (scene 2), respectively.
To avoid contingency, we simulated two different positioning
environments by removing satellites with a different specific
azimuth range which is 240–360 degrees in the static experi-
ment and 180–300 in the kinematic experiment, respectively.
As shown in the figure, the red line represents the variation
in the number of visible satellites in the unobstructed envir-
onment, and the blue line represents the variation in the num-
ber of visible satellites in the occlusion environment. In this
way, variations in the number of BDS-3 satellites in a com-
plex environment in the real-world are simulated.
3.3. Data processing strategy
The data processing strategies of the experiments are given in
detail in table 3
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Figure 5. UWB-only kinematic positioning errors based on four and six UWB anchor nodes.
Table 2. RMS and STD of UWB-only kinematic positioning errors (m).
4 UWB-only
6 UWB-only
Kinematic
RMS
STD
E
0.096
0.090
N
0.120
0.064
U
0.020
0.011
E
0.072
0.070
N
0.097
0.059
U
0.017
0.005
Figure 6. The visible satellite number of static experiments in the unobstructed environment (scene 1) and occluded environment (scene 2)
on DOY184 of 2023 (Left: occluded environment; Right: number of visible satellites).
4. Experiment results and analysis
In this section, we explore and compare the positioning per-
formance of the BDS-3 RTK/UWB STC integration model in
complex environment and with the participation of different
numbers of UWB anchor nodes. Scene 1 denotes the open-
sky environment. Scene 2 denotes the occluded environment.
BDS-3-only denotes the single BDS-3 RTK system. BDS-
3 + 4 UWB and BDS-3 + 6 UWB denote the number
of UWB anchor nodes participating in integration position-
ing model are four and six, respectively. In this experiment,
the BDS-3 observations measurements and UWB DS-TWR
ranging measurements are initialized every 15 min for one
hour.
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Figure 7. The visible satellite number of kinematic experiments in the unobstructed environment (scene 1) and occluded environment
(scene 2) on DOY184 of 2023 (Left: occluded environment; Right: number of visible satellites).
Items
Processing strategies
Table 3. The data process strategies.
BDS-3 signal selection
Solving mode
Ambiguity resolution
Cut-off elevation angle
Sampling rate
Parameter estimation method
PCO and PCV
Tidal displacement
Pseudorange observations weight
Carrier phase observations weight
UWB observations pre-processing
◦
B1/B3
RTK static and kinematic
Floating solution and fixed solution
15
1 s
EKF
Corrected using ATX file
IERS Conventions 2010
0.3 m
0.003 m
Eliminate outliers greater than three times the mean squared error
4.1. Static experiment
The static positioning error sequences in the E, N, and U
directions of the BDS-3 RTK/UWB STC integration model
in scene 1 are shown in figures 8 and 9. Figure 8 shows the
positioning error sequences of the floating solution. It can be
seen that UWB played an important role in the integration pos-
itioning of the floating solution. Compared to BDS-3-only,
the participation of UWB makes the floating solution con-
verge in a very short time, and the positioning accuracy was an
obvious improvement. Figure 9 indicates the positioning error
sequences of the fixed solution. Although the addition of UWB
accelerated the fixed solution convergence of the BDS-3 RTK.
However, in static positioning, the BDS-3-only fixed solution
can achieve good positioning accuracy. Due to the constraints
of UWB position and covariance, more noise was introduced,
which resulted in a slight decrease in the positioning accuracy
of STC integration model in scene 1. Moreover, it can be seen
from the BDS-3 + 4 UWB and BDS-3 + 6 UWB positioning
error sequences that their positioning errors show consistency.
It may be due to the similar positioning performance of four
and six UWB-only, and STC integration model positioning is
caused by the constraints of the position domain.
The static positioning error sequences in the E, N, and U
directions of the BDS-3 RTK/UWB STC integration model
in scene 2 are shown in figures 10 and 11. Figure 10 shows
the positioning error sequences of the floating solution. By
comparing with scene 1, it can be concluded that in occluded
environment, the BDS-3-only positioning performance was
worse than that in unobstructed environment. However, the
positioning errors of STC integration model show good pos-
itioning performance, which not only accelerates the conver-
gence time but also greatly improves the positioning accuracy.
Figure 11 shows the positioning error sequences of the fixed
solution. Because of the small number of satellites that have
occluded in static experiments, BDS-3-only can still achieve
good positioning performance in fixed solutions. In STC integ-
ration model, the introduction of UWB noise has led to a slight
decrease in positioning performance, but it is still within our
acceptable range. Meanwhile, we can draw similar conclu-
sions with above scene 1 that the positioning accuracy of STC
integration model based on four and six UWB anchor nodes is
basically the same.
To further verify the above conclusions, the mean val-
ues of the RMS and STD for the static positioning errors
were summarized under two different observation environ-
ments. The RMS and STD of floating and fixed solutions for
static positioning results in scene 1 are summarized in table 4.
In the floating solution, the RMS in the E, N, and U direc-
tions is improved from (0.098 m, 0.084 m, 0.222 m) of the
BDS-3-only to (0.015 m, 0.040 m, 0.003 m) and (0.015 m,
8
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Figure 8. The static positioning error sequences in the E, N, and U directions of floating solution in scene 1.
Figure 9. The static positioning error sequences in the E, N, and U directions of fixed solution in scene 1.
0.038 m, 0.012 m) of the BDS-3 + 4 UWB and BDS-3 + 6
UWB, respectively. The STD in the E, N, and U directions is
improved from (0.093 m, 0.059 m, 0.149 m) of the BDS-3-
only to (0.007 m, 0.006 m, 0.005 m) of the integration model.
Therefore, the addition of UWB improves the stability of pos-
itioning and the usability of positioning results. In the fixed
solution, the RMS in the E, N, and U directions is (0.022 m,
0.013 m, 0.035 m) of the BDS-3-only, (0.025 m, 0.028 m,
0.019 m) of the BDS-3 + 4 UWB and (0.023 m, 0.025 m,
0.010 m) of the BDS-3 + 6 UWB. The STD in the E, N, and
U directions is (0.019 m, 0.013 m, 0.035 m) of the BDS-3-
only, (0.009 m, 0.009 m, 0.010 m) and (0.009 m, 0.008 m,
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Figure 10. The static positioning error sequences in the E, N, and U directions of floating solution in scene 2.
Figure 11. The static positioning error sequences in the E, N, and U directions of fixed solution in scene 2.
0.008 m) of the BDS-3 + 4 UWB and BDS-3 + 6 UWB,
respectively. With RMS and STD, we can see that the pos-
itioning performance of integration model decreases slightly
in the case of fixed solutions, but both are within our accept-
able range. This may be due to the short baseline of RTK
in unobstructed environment, which allows BDS-3 RTK to
achieve good positioning performance in static fixed solu-
tions. In the STC integration model, the addition of UWB
position information and covariance constraints leads to the
introduction of UWB errors, which results in a slight decrease
in integration positioning performance, but we consider this
acceptable.
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Table 4. The RMS and STD of floating and fixed solutions for static positioning results in scene 1 (m).
BDS-3-only
BDS-3 + 4 UWB
BDS-3 + 6 UWB
E
0.098
0.093
0.022
0.019
N
0.084
0.059
0.013
0.013
U
0.222
0.149
0.035
0.035
E
0.015
0.007
0.025
0.009
N
0.040
0.006
0.028
0.009
U
0.013
0.005
0.019
0.010
E
0.015
0.007
0.023
0.009
N
0.038
0.006
0.025
0.008
RMS
STD
RMS
STD
Table 5. The RMS and STD of floating and fixed solutions for static positioning results in scene 2 (m).
BDS-3-only
BDS-3 + 4 UWB
BDS-3 + 6 UWB
E
0.141
0.094
0.023
0.019
N
0.165
0.103
0.030
0.030
U
0.472
0.337
0.056
0.053
E
0.011
0.010
0.035
0.011
N
0.043
0.009
0.033
0.007
U
0.011
0.008
0.013
0.002
E
0.021
0.006
0.036
0.009
N
0.044
0.008
0.033
0.007
RMS
STD
RMS
STD
U
0.012
0.005
0.010
0.008
U
0.015
0.010
0.013
0.002
Static
Floating
Fixed
Static
Floating
Fixed
The RMS and STD of floating and fixed solutions for static
positioning results in scene 2 are summarized in table 5. In
the floating solution, the RMS in the E, N, and U directions is
improved from (0.141 m, 0.165 m, 0.472 m) of the BDS-3-
only to (0.011 m, 0.043 m, 0.011 m) of the BDS-3 + 4 UWB
and (0.021 m, 0.044 m, 0.015 m) of the BDS-3 + 6 UWB. The
STD in the E, N, and U directions is improved from (0.094 m,
0.103 m, 0.337 m) of the BDS-3-only to (0.010 m, 0.009 m,
0.008 m) and (0.006 m, 0.008 m, 0.010 m) of the integration
model. Therefore, the addition of UWB in occluded envir-
onment has a more obvious improvement in positioning sta-
bility and usability of positioning results. In the fixed solu-
tion, the RMS in the E, N, and U directions is (0.023 m,
0.030 m, 0.056 m) of the BDS-3-only, (0.035 m, 0.033 m,
0.013 m) and (0.036 m, 0.033 m, 0.013 m) of the BDS-3 + 4
UWB and BDS-3 + 6 UWB, respectively. The STD in the
E, N, and U directions is (0.019 m, 0.030 m, 0.053 m) of
the BDS-3-only, (0.011 m, 0.007 m, 0.002 m) of the BDS-
3 + 4 UWB and (0.009 m, 0.007 m, 0.002 m) of the BDS-
3 + 6 UWB, respectively. Similarly, due to the small number
of satellites in the azimuth angle that we randomly occlude in
static experiments, BDS-3 RTK can still achieve good posi-
tioning accuracy under the current fixed solutions. Although
the accuracy of integration positioning has decreased slightly,
we believe that positioning performance is still acceptable. In
addition, we mentioned before that the limited redundancies
pose challenges in the UWB ranging error processing, result-
ing in significant slight fluctuations in UWB positioning accur-
acy. Therefore, this degradation affects the performance of the
BDS-3 + UWB STC. Therefore, the BDS-3 + UWB shows
slightly better positioning accuracy with fewer UWB anchors.
However, regardless of whether 6 or 4 UWB anchors are used,
the positioning accuracy of the STC remains relatively con-
sistent. These results further demonstrate that the advantages
of STC in maintaining reliable and accurate positioning under
a limited number of UWB anchors and its potential to reduce
UWB setup costs in occluded environments [33].
Figure 12. Success rates of BDS-3 ambiguity resolution with
different numbers of UWB anchor nodes.
In addition, figure 12 shows the success rate of BDS-3
ambiguity resolution under the STC integration model with
different numbers of UWB anchor nodes. In this experiment,
the result with a ratio greater than 1.5 and the positioning error
of the three directions less than 0.2 m were considered as the
judgment criterion for fixed success. In scene 1, the success
rate of BDS-3 ambiguity resolution is improved from 98.45%
of the BDS-3-only to 99.89% and 99.78% of the BDS-3 + 4
UWB and BDS-3 + 6 UWB, respectively. In scene 2, the
success rate of BDS-3 ambiguity resolution is improved from
98.78% of the BDS-3-only to 99.89% of the STC integration
model.
Both RMS and STD of BDS-3 RTK/UWB STC integration
positioning performance of static floating solution are super-
ior to BDS-3-only positioning in different scenes. We also
found that whether it is the STC integration positioning model
11
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
Figure 13. The kinematic positioning error sequences in the E, N, and U directions of floating solution in scene 1.
based on four or six UWB anchor nodes, the positioning per-
formance shows consistency. This is consistent with the res-
ults shown in figures 8–11. Therefore, in static experiments, it
is advantageous to use the UWB anchor node to enhance the
BDS-3 RTK/UWB STC integration positioning. Especially in
occlusion environments, the enhancement effect is especially
pronounced.
4.2. Kinematic experiment
The kinematic positioning error sequences in the E, N, and U
directions of the BDS-3 RTK/UWB STC integration model
in scene 1 are shown in figures 13 and 14. Similarly, in kin-
ematic experiments, we used the data processing method of
initializing one-hour observation measurements every 15 min.
Figure 13 shows the positioning error sequences of the float-
ing solution. It is clear from the distribution of the positioning
error sequences that the fluctuations of the integration pos-
itioning model are much smaller than those of BDS-3-only.
When the user terminal is in a kinematics state, the STC integ-
ration of BDS-3 RTK and UWB improves the convergence
speed and positioning accuracy. Figure 14 indicates the pos-
itioning error sequences of the fixed solution. It can be seen
from the figure that the BDS-3-only can achieve acceptable
positioning performance after convergence under a fixed solu-
tion in unobstructed environment. However, the integration
positioning model still accelerates the convergence time and
slightly reduces the positioning errors. Due to the introduction
of UWB errors, the noise of the integration positioning errors
is slightly larger. Moreover, the STC integration positioning
errors distribution of BDS-3 + 4 UWB and BDS-3 + 6 UWB
is basically at the same level.
The kinematic positioning error sequences in the E, N, and U
directions of the BDS-3 RTK/UWB STC integration in scene
2 are shown in figures 15 and 16. Figure 15 shows the posi-
tioning error sequences of the floating solution. Compared to
unobstructed environment, BDS-3-only displays worse posi-
tioning performance due to the poor number of satellites and
the weak satellite geometry in occluded environment. It is
clear from the figure that the positioning errors of BDS-3-only
can reach the meter level when the occlusion is very severe.
In contrast, the integration positioning error sequences are
smooth and the positioning errors are much smaller. Figure 16
shows the positioning error sequences of the fixed solution.
From the distribution of positioning errors in the figure, it can
be concluded that the fluctuation range of BDS-3-only posi-
tioning errors is large in the occlusion environment, but the
positioning errors of STC integration model can still be main-
tained with small fluctuations. Meanwhile, we can draw sim-
ilar conclusions with above scene 1 that the positioning errors
of STC integration model based on four and six UWB anchor
nodes are similar.
To fully verify the above conclusions, the mean values of
the RMS and STD for the kinematic positioning errors were
summarized under two different observation environments.
The RMS and STD of floating and fixed solutions for kin-
ematic positioning results in scene 1 are summarized in table 6.
In the floating solution, the RMS in the E, N, and U direc-
tions is improved from (0.360 m, 0.091 m, 0.498 m) of the
BDS-3-only to (0.073 m, 0.078 m, 0.041 m) and (0.067 m,
0.075 m, 0.025 m) of the BDS-3 + 4 UWB and BDS-3 + 6
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Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
Figure 14. The kinematic positioning error sequences in the E, N, and U directions of fixed solution in scene 1.
Figure 15. The kinematic positioning error sequences in the E, N, and U directions of floating solution in scene 2.
UWB, respectively. The STD in the E, N, and U directions is
improved from (0.111 m, 0.077 m, 0.369 m) of the BDS-3-
only to (0.062 m, 0.047 m, 0.013 m) and (0.057 m, 0.053 m,
0.011 m) of the integration positioning model. The statistical
results indicate that the RMS and STD of BDS-3 RTK/UWB
STC integration positioning errors show a decreasing trend,
and the STC integration positioning performance of four and
six UWB anchor nodes was basically consistent. In the fixed
solution, the RMS in the E, N, and U directions is improved
from (0.120 m, 0.120 m, 0.228 m) of the BDS-3-only to
(0.069 m, 0.079 m, 0.020 m) of the BDS-3 + 4 UWB and
(0.058 m, 0.069 m, 0.025 m) of the BDS-3 + 6 UWB. The STD
13
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
Figure 16. The kinematic positioning error sequences in the E, N, and U directions of fixed solution in scene 2.
Table 6. The RMS and STD of floating and fixed solutions for kinematic positioning results in scene 1 (m).
BDS-3-only
BDS-3 + 4 UWB
BDS-3 + 6 UWB
Kinematic
Floating
Fixed
E
0.360
0.111
0.120
0.039
N
0.091
0.077
0.120
0.022
U
0.498
0.369
0.228
0.134
E
0.073
0.062
0.069
0.058
N
0.078
0.047
0.079
0.046
U
0.041
0.013
0.020
0.008
E
0.067
0.057
0.058
0.050
N
0.075
0.053
0.069
0.049
RMS
STD
RMS
STD
Table 7. The RMS and STD of floating and fixed solutions for kinematic positioning results in scene 2 (m).
BDS-3-only
BDS-3 + 4 UWB
BDS-3 + 6 UWB
Kinematic
Floating
Fixed
E
0.629
0.421
0.326
0.515
N
0.325
0.103
0.119
0.189
U
1.160
1.019
0.702
1.328
E
0.087
0.069
0.072
0.067
N
0.084
0.048
0.076
0.047
U
0.036
0.018
0.037
0.005
E
0.075
0.053
0.055
0.053
N
0.074
0.054
0.078
0.054
RMS
STD
RMS
STD
U
0.025
0.011
0.025
0.010
U
0.029
0.018
0.027
0.009
in the U directions is improved from 0.134 m of the BDS-3-
only to 0.008 m and 0.010 m of the BDS-3 + 4 UWB and BDS-
3 + 6 UWB, respectively. However, in terms of horizontal
components, the STD of the integration positioning model is
slightly larger than that of BDS-3-only, and the reasons for
this phenomenon have been explained before and will not be
repeated in this detail.
The RMS and STD of floating and fixed solutions for kin-
ematic positioning results in scene 2 are summarized in table 7.
In the floating solution, the RMS in the E, N, and U directions
is improved from (0.629 m, 0.325 m, 1.160 m) of the BDS-3-
only to (0.087 m, 0.084 m, 0.036 m) of the BDS-3 + 4 UWB
and (0.075 m, 0.074 m, 0.029 m) of the BDS-3 + 6 UWB. The
STD in the E, N, and U directions is improved from (0.421 m,
0.103 m, 1.019 m) of the BDS-3-only to (0.069 m, 0.048 m,
0.018 m) and (0.053 m, 0.054 m, 0.018 m) of the integra-
tion positioning model. Therefore, the BDS-3 RTK/UWB STC
integration model in the occluded environment has a more
obvious improvement in positioning stability and usability of
positioning results. In the fixed solution, the RMS in the E,
N, and U directions is improved from (0.326 m, 0.119 m,
0.702 m) of the BDS-3-only to (0.072 m, 0.076 m, 0.037 m)
and (0.055 m, 0.078 m, 0.027 m) of the BDS-3 + 4 UWB and
BDS-3 + 6 UWB, respectively. The STD in the E, N, and U
14
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
Figure 17. Success rates of BDS-3 ambiguity resolution with different numbers of UWB anchor nodes.
directions is improved from (0.515 m, 0.189 m, 1.328 m) of
the BDS-3-only to (0.067 m, 0.047 m, 0.005 m) of the BDS-
3 + 4 UWB and (0.053 m, 0.054 m, 0.009 m) of the BDS-3 + 6
UWB, respectively. The statistical results indicate that both
RMS and STD of BDS-3 RTK/UWB STC integration posi-
tioning performance of kinematic experiment are superior to
BDS-3-only in the occluded scenes.
Besides, the success rate of BDS-3 ambiguity resolution
under the STC integration model with different numbers of
UWB anchor nodes is displayed in figure 17. In scene 1, the
success rate of BDS-3 ambiguity resolution is improved from
89.89% of the BDS-3-only to 93.54% and 94.66% of the BDS-
3 + 4 UWB and BDS-3 + 6 UWB, respectively. In scene 2,
the success rate of BDS-3 ambiguity resolution is improved
from 84.56% of the BDS-3-only to 97.78% and 98.33% of the
STC integration model.
It further illustrates that the beneficial effect of BDS-3
RTK/UWB STC integration positioning was very obvious in
the kinematic experiment. Especially in occluded environ-
ments, positioning accuracy and the success rates of ambiguity
resolution can be similar to that in unobstructed environments.
5. Conclusions
In this paper, we proposed a STC integration model of BDS-
3 RTK/UWB in harsh environments, and real-world static
and kinematic positioning experiments were carried out. The
height constraint is imposed on UWB positioning to mitig-
ate the effect of poor positioning of UWB in height com-
ponents. Moreover, the positioning performance of the pro-
posed model under different occlusion environments and dif-
ferent numbers of UWB anchor nodes were analyzed from
the two perspectives of static and kinematic solutions. The
real-world experiments proved that the proposed positioning
model has higher positioning accuracy than that of a single
BDS-3 RTK system. In addition, we also come to an inter-
esting conclusion that if four or more UWB anchor nodes
provide similar positioning accuracy, then their contribution
to BDS-3 RTK/UWB STC integration positioning is basically
at the same level. In the real-world experiments, the enhanced
BDS-3 RTK/UWB STC integration positioning of four and six
UWB anchor nodes has similar positioning accuracy and suc-
cess rates of BDS-3 RTK ambiguity resolution. Furthermore,
most importantly, the proposed model can also achieve a good
positioning effect in occlusion environments. This provides
a reference for the development of high-precision, real-time,
and continuous positioning, as well as navigation and timing
in urban complex environments. It also provides the basis for
users to rapidly achieve centimeter-level positioning accuracy
in outdoor–indoor transition areas based on GNSS and UWB.
In further work, we consider fusing the proposed model
with optical vision and LiDAR sensors, which in arbitrary
scenarios achieve ubiquitous and seamless positioning and
navigation.
Data availability statement
The data cannot be made publicly available upon publication
due to legal restrictions preventing unrestricted public distri-
bution. The data that support the findings of this study are
available upon reasonable request from the authors.
Acknowledgments
This research was funded by the National Key Research
and Development Program of China (2020YFB0505804 and
2021YFB1407000), and the Key Research and Development
Program of Shandong Province (2021SFGC0401).
ORCID iDs
Tianhe Xu https://orcid.org/0000-0003-2463-5329
Min Li https://orcid.org/0000-0003-3053-6669
15
Meas. Sci. Technol. 35 (2024) 036306
P Dai et al
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10.1103_physrevresearch.5.013123.pdf
| null | null |
PHYSICAL REVIEW RESEARCH 5, 013123 (2023)
Quantum correlation of electron and ion energy in the dissociative strong-field ionization of H2
A. Geyer
,1,* O. Neufeld ,2,† D. Trabert
,1 U. De Giovannini,2,3 M. Hofmann,1 N. Anders,1 L. Sarkadi
,4
M. S. Schöffler,1 L. Ph. H. Schmidt,1 A. Rubio,2,5 T. Jahnke,6 M. Kunitski,1 and S. Eckart
1Institut für Kernphysik, Goethe-Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
2Max Planck Institute for the Structure and Dynamics of Matter and Center for Free-Electron Laser Science,
22761 Hamburg, Germany
3Università degli Studi di Palermo, Dipartimento di Fisica e Chimica - Emilio Segrè, via Archirafi 36, I-90123 Palermo, Italy
4Institute for Nuclear Research (ATOMKI), P.O. Box 51, H-4001 Debrecen, Hungary
5Center for Computational Quantum Physics (CCQ), The Flatiron Institute, New York, New York 10010, USA
6European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany
1,‡
(Received 12 May 2022; accepted 20 January 2023; published 16 February 2023)
We report on the strong field ionization of H2 by a corotating two-color laser field. We measure the
electron momentum distribution in coincidence with the kinetic energy release (KER) of dissociating hydrogen
molecules. In addition to a characteristic half-moon structure, we observe a low-energy structure in the electron
momentum distribution at a KER of about 3.5 eV. We speculate that the outgoing electron interacts with the
molecular ion, despite the absence of classical recollisions under these conditions. Time-dependent density
functional theory simulations support our conclusions.
DOI: 10.1103/PhysRevResearch.5.013123
I. INTRODUCTION
Ionization of atoms and molecules in strong laser fields is
often successfully modeled as a tunneling process [1]. After
tunneling, the electron’s trajectory is governed by the time-
dependent laser electric field and the potential of the parent
ion. Consequently, the time-dependent laser electric field can
be used to control the trajectory of the liberated electron. For
linearly polarized light or for suitable two-color laser fields the
electron can exchange energy with its parent ion by inelastic
recollisions. Examples for processes which are enabled by
electrons that return to their parent ions are the recombination
of the tunneled electron with its parent ion [2,3], which can
lead to the emission of high order harmonics [4–6], excita-
tion of the parent ion [7], or nonsequential double ionization
[8–14]. Previous studies in the strong field regime observed
such energy exchange only if the electron recollided with its
parent ion during the photoionization process.
In this Letter, we report on the single ionization of
molecular hydrogen by a corotating two-color (CoRTC) field
accompanied by dissociation of the molecule: H2 → H +
H+ + e−. The CoRTC field is created as a superposition of
two circularly polarized laser pulses with the same helicity
*[email protected]
†[email protected]
‡[email protected]
and central wavelengths of 790 nm and 390 nm. The inten-
sity of the fundamental laser pulse (780 nm) and the second
harmonic one (390 nm) were set to 3.3 × 1013 W/cm2 and
5.4 × 1013 W/cm2, respectively. The CoRTC field ensures
that recollisions are negligible during the photoionization pro-
cess [13,15,16]. The Lissajous curves of the laser electric
field and the corresponding vector potential are depicted in
Fig. 1(a). Figure 1(b) shows the projection of the measured
three-dimensional (3D) electron momentum distribution to
the polarization plane py pz. As expected, the most probable
electron momentum is close to the value of the negative vector
potential that belongs to the peak of the electric laser field
[17–19].
After one of the two bound electrons of H2 is liberated, the
electron’s trajectory is governed by the driving laser electric
field and the interaction with the H+
2 ion. The second electron
remains bound in the energetically lowest molecular orbital
(1sσg) of H+
2 while the occupation of the antibinding orbital
(2pσu) remains negligible. This scenario is ideally suited to
study the interaction of the liberated electron with its parent
molecular ion in strong field ionization. The molecular ion
with its 1sσg and 2pσu state can be approximated as a two-
level system, which has an energy spacing that depends on
the internuclear distance. Our results show evidence that the
escaping electron drives an excitation in this two-level system
by transferring a fraction of its energy to the H+
2 parent ion.
Published by the American Physical Society under the terms of the
Creative Commons Attribution 4.0 International license. Further
distribution of this work must maintain attribution to the author(s)
and the published article’s title, journal citation, and DOI.
II. EXPERIMENT
A. Experimental method
In our experiment, the CoRTC field is generated by an
interferometric two-color laser setup. Multicycle laser pulses
2643-1564/2023/5(1)/013123(7)
013123-1
Published by the American Physical Society
A. GEYER et al.
PHYSICAL REVIEW RESEARCH 5, 013123 (2023)
with a central wavelength of 780 nm, a pulse duration of 40-fs
FWHM, and a repetition rate of 8 kHz (KMLabs Dragon) are
frequency doubled in a β-barium borate crystal to create light
pulses at a central wavelength of 390 nm. The two pulses
with different wavelengths are spatially separated for inde-
pendent modification of their polarization state and intensity.
The relative phase between the two pulses is controlled using
a nanometer-delay stage. A beam combiner merges the two
single-color laser pulses. The two-color laser field is focused
onto the H2 target inside a vacuum chamber by a spherical
mirror ( f = 80 mm). The optical setup is the same as in
Ref. [20].
The 3D electron and proton momenta are measured in
coincidence using a COLTRIMS (cold target recoil ion mo-
mentum spectroscopy) reaction microscope as in Ref. [21].
The charged particles are accelerated by a homogeneous elec-
tric field of 17 Vcm−1 and a parallel magnetic field of 10 G
onto position- and time-sensitive detectors [22]. The length
of the electron spectrometer is 378 mm and the length of
the ion spectrometer is 68 mm. Making use of momentum
conservation, the detection of the proton and the electron
allows for the calculation of the kinetic energy release KER =
(pp+0.5·pe )2
, where pp and pe are the momenta of the detected
mp
proton and electron, respectively, and mp is the proton mass.
The background from false coincidences and the contribution
from events in which both electrons are liberated has been
subtracted for all experimental data that are shown.
Measured electron momentum distributions from the ion-
ization of argon by circularly polarized light are used to
calibrate the laser intensity of the two colors separately. Thus,
the intensity calibration has been done in situ and takes vol-
ume averaging into account. For the laser pulse at a central
wavelength of 780 nm the drift momentum is used to calibrate
the intensity taking nonadiabaticity into account as described
in Ref. [20]. For the laser pulse at a central wavelength of
390 nm the shift of the above threshold ionization (ATI)
peaks is used for intensity calibration as in Ref. [12]. The
relative phase between the two colors, which determines the
orientation of the combined electric field in the plane of po-
larization, was actively varied during the measurement. These
variations are compensated during the offline data analysis as
in Ref. [20].
B. Experimental results
The measured electron momentum distribution that
is
shown in Fig. 1(b) is very similar to previous findings for
atoms [15,20,23]. In particular, the choice to investigate the
reaction H2 → H + H+ + e− enables us to resolve the mea-
sured electron momentum distribution as a function of the
KER of the ions. The measured KER is shown in Fig. 2(a).
The most probable KER is 1.2 eV. Figures 2(b)–2(d) show the
3D electron momentum distributions and the corresponding
two-dimensional projections for three different KER ranges
as indicated in Fig. 2(a). All electron momentum distributions
show a characteristic half-moon structure. For a KER of about
3.5 eV, there additionally exists a pronounced low-energy
structure. Such low-energy structures in CoRTC fields have
been observed for atoms before [15] using different laser
parameters [24]. However, the low-energy structure in our
FIG. 1. (a) The Lissajous curve of the laser electric field and
the corresponding negative vector potential. The arrows indicate
the temporal evolution of the laser electric field. The dots indicate
the peak of | (cid:3)E (t )| and the corresponding negative vector potential.
(b) Measured electron momentum distribution for the laser field that
is shown in (a) with a logarithmic color scale.
measurement solely becomes visible for a specific KER re-
gion and is not present for other KERs.
To further investigate the relation of electron energy and
KER, Fig. 3(a) shows the correlated yield of these two quan-
tities. Column-wise normalization of the data from Fig. 3(a)
results in Fig. 3(b), which shows two remarkable correlations
between the electron energy and the KER. Firstly, for KER
<3 eV we find diagonal structures spaced by 1.6 eV reflect-
ing the quantized absorption of energy from the field. This
ATI structure is well documented in the literature [25–27].
Further, the electron-nuclear sharing of the absorbed photon
energy has already been investigated for H2 [28,29] and other
molecules [30,31]. The second feature is the appearance of the
low energy electrons for a KER range of 3 eV to 6 eV. Within
this region, the most probable electron energy is lower than
that for a KER below 3 eV and there is a sharp edge at a KER
of 3 eV. What is the reason for this correlation of KER and
electron energy?
Figure 3(c) shows the potential energy curves for the
ground state of H2, for a binding molecular orbital (1sσg),
and an antibinding orbital (2pσu) of H+
2 . The liberation of
the first electron is indicated by a vertical arrow labeled with
“two-color strong field ionization” in Fig. 3(c). The very pro-
nounced KER peak at about 1.2 eV is reached via subsequent
nuclear dynamics and absorption of a photon at 390 nm [21]
(indicated with the arrow that is labeled “2ω”). Analogous to
the 2ω-peak at 1.2 eV, the KER peaks at 0.2 eV and 2.8 eV are
caused by the absorption of one or three 780 nm photons. The
net-two-photon pathway, which corresponds to an absorption
of three 780 nm photons and a stimulated emission of one
780 nm photon [32], also leads to a KER peak at around
1.2 eV for our laser parameters. Thus, it overlaps with the
2ω-peak and is very weak, since this pathway is expected to be
less probable than the three-photon peak for 780 nm at 2.8 eV.
Hence, for the KER peaks below 3 eV, the reaction can be
separated into two steps. The first step is the same as it would
be for an atom: an electron is liberated and subsequently
accelerated in the laser field. The final electron momentum
pe roughly corresponds to the negative vector potential at the
instance of ionization − (cid:3)A(t0). For close to circularly polarized
light, the ponderomotive potential Up is given by Up = p2
=
e
2me
013123-2
QUANTUM CORRELATION OF ELECTRON AND ION …
PHYSICAL REVIEW RESEARCH 5, 013123 (2023)
FIG. 2. (a) Shows the measured kinetic energy release (KER) distribution for the reaction H2 → H + H+ + e−. (b)–(d) show the measured
electron momentum distributions which are restricted to the measured KER intervals that are indicated in (a). The three-dimensional (3D)
electron momentum distributions are represented using five semitransparent isosurfaces encoding the intensity of the 3D histogram by color.
The py pz plane is the polarization plane.
e2 | (cid:3)A(t0 )|2
, where me [e] is the electron’s mass [charge]. For the
2me
time t0 that belongs to the peak electric field, this leads to
a value of Up = 5.2 eV. Thus, the half-moon structure that
is observed in the current experiment is, as expected, very
similar as for the ionization of atoms [20]. In a second step,
there can be subsequent nuclear dynamics which occur on a
time scale of tens of femtoseconds [33,34].
Inspection of the potential energy curves in Fig. 3(c) sug-
gest the following physical picture: the KER of around 3.5 eV,
for which the low-energy structure is most prominent, indi-
cates that the transition from the 1sσg state to the 2pσu state
occurs at an internuclear distance of about R = 3 a.u. (neglect-
ing the kinetic energy of the nuclear wave packet after the
ionization step). At R = 3 a.u. the spacing between the 1sσg
and the 2pσu state is 5.6 eV as indicated by the vertical green
arrow in Fig. 3(c). Interestingly, this value for the energy split-
ting is very close to Up = 5.2 eV. In a first attempt, we tried
to explain the electron-ion correlation as an energy transfer
from the electron to the ion that occurs during a recollision. To
this end, we have performed extensive modeling of classical
dynamics including electron-electron interactions [35,36] and
trajectories with nonzero initial electron momentum at the
tunnel exit as predicted by SFA [20,37,38]. We have also
modeled the molecular geometry and classical ionic motion
during the photoionization process. None of these simulations
yielded a significant amount of recolliding trajectories [12]
with the required energy of at least 5 eV, or resulted in a
substantial amount of low energy electrons. Thus, these sim-
ulations allow us to rule out classical electron-ion recollisions
in our experiment, and verify that classical dynamics is not the
source of the observed effect.
FIG. 3. Experimental data on the strong field dissociation of H2.
(a) shows the energy of the electron as a function of the KER. In
(b) each column of the distribution in (a) is normalized to a maximum
of one. The horizontal line in (b) guides the eye and the green
square highlights the KER region in which there is the low-energy
structure. (b) reveals that low-energy electrons (Eelec < 3 eV) are
very pronounced for 3 eV < KER < 6 eV. (c) shows the potential
energy of the ground state of H2 and two states of H+
2 as a function
of the internuclear distance. The purple arrow marks the strong field
transition from the ground state to the 1sσg state. Two possible
transitions from the 1sσg to the 2pσu curve are marked by a blue
and a green arrow, respectively. The KERs that result from these two
transitions are marked by colored areas in the KER distribution in
(d). (d) shows the same data as Fig. 2(a).
III. TIME-DEPENDENT DENSITY FUNCTIONAL THEORY
To further investigate the correlation of electron energy
and KER we perform ab initio simulations that fully in-
corporate the ionization dynamics of electrons, as well as
ion motion. The electronic degrees of freedom are treated
quantum mechanically by time-dependent density functional
theory (TDDFT) [39] within the local density approxi-
mation and the adiabatic approximation with an added
self-interaction correction term that leads to the correct long-
range Coulomb behavior [40]. This approach incorporates
mean-field electron-electron interactions, as well as interac-
tions of electrons with the incident laser fields, and nuclei. The
013123-3
A. GEYER et al.
PHYSICAL REVIEW RESEARCH 5, 013123 (2023)
distance. Figure 4(b) shows the intensity as a function of the
final electron energy and the internuclear distance R of the H2
molecule. The lower horizontal axis shows the corresponding
estimated KER, which is obtained by subtracting the energy
of the 2pσu state for R → ∞ from the energy of the 2pσu
state for the internuclear distance that is used in the TDDFT
simulations. To visualize the dependence of the envelope of
the calculated electron energy distribution as a function of
R, the ATI substructure has been smoothed using a low-pass
filter. Strikingly, also in the TDDFT simulations, the electron
energy depends on the internuclear distance. The black line
in Fig. 4(b) shows the ponderomotive energy Up = 5.2 eV. In
a simple man’s model [2], the peak of the electron energy
spectrum is at Up = 5.2 eV for circularly polarized light. If
one takes Coulomb interaction and nonadiabatic offsets of the
initial momentum distribution into account, the peak of the
electron energy spectrum typically shifts to slightly higher
energies [20]. We expect that these corrections to the sim-
ple man’s model do not depend on the internuclear distance.
Thus, we speculate that there is an electron-ion interaction that
decelerates the outgoing electron for an internuclear distance
of about 3 a.u. Such electron-ion correlations are inherently
included in our TDDFT simulation. A full understanding of
the microscopic mechanism that leads to the observed corre-
lations of electron and ion energy is beyond the scope of the
current work and warrants further research.
It is an important difference, as compared to the ex-
periment, that in Fig. 4(b) all electrons are shown. In the
experiment the measured electrons are detected in coincidence
with ions that predominantly dissociate via the 2pσu state.
However, TDDFT does not allow to distinguish the state of
the ion (e.g., 1sσg and 2pσu). As a result, the TDDFT result
misses the characteristic peaks for KERs below 3 eV that are
due to resonant coupling of 1sσg and 2pσu, which is due to
the absorption of photons from the driving laser field [32,49].
We expect, that the agreement of experiment and theory could
be further improved, if the final states would be projected to
the ionic 2pσu state, which is impossible using state-of-the-art
approaches.
IV. DISCUSSION
A microscopic reason for the observed correlation of elec-
tron energy and KER might be electron-electron interactions
[50]. We consider the excitation of the molecule to a neutral
excited H2 state and subsequently ionization via the 1sσg or
the 2pσu state to be unlikely, given that the KER is above
3.5 eV and that the average electron energy increases with
increasing KER (this is opposite to the expectation for energy
sharing) [51]. Alternatively, we speculate that our observa-
tions might be related to an antenna-like mechanism [52,53].
If the ponderomotive energy Up of the outgoing electron is
resonant to the energy spacing of the 1sσg and the 2pσu state,
then the electron can transfer this amount of energy to the H+
2
ion in a resonant process. The decelerated electrons form the
low-energy structure. For the CoRTC field that is used in our
experiment, the ponderomotive energy is close to 5.2 eV. If
the spacing of the 1sσg and the 2pσu state is higher than the
ponderomotive energy of the electron, then energy exchange
is very unlikely. This might explain why the low-energy elec-
FIG. 4. Theoretical data on the ionization of H2 in a CoRTC field.
(a) shows the electron momentum distribution with a logarithmic
color scale that is obtained using a time-dependent density functional
theory (TDDFT) approach. (b) shows the ionization probability as a
function of the final electron energy and the internuclear distance R
(upper horizontal axis) of the H2 molecule. ATI peaks were removed
by applying a low-pass filter to the electron energy distribution.
Each column of the distribution is normalized to a maximum of
one. The lower horizontal axis shows the corresponding estimated
KER, which is derived from the 2pσu curve (see text). The black line
indicates the ponderomotive energy Up = 5.2 eV. The green square
highlights the same KER region as in Fig. 3(b).
nuclei motion is treated classically by coupling Newtonian
equations of motion to TDDFT [41], such that ions are driven
by interactions with the time-dependent electric field of the
laser, neighboring ions, and the electrons’ charge density (ion
motion is initiated in the ground vibrational state of H2). Most
importantly, this approach allows for energy exchange and
coupling between the ionic and electronic degrees of free-
dom through Coulomb interactions. The photoelectron spectra
are calculated with the surface flux method, T-surff [42–44]
employing the dipole approximation (velocity gauge). All
calculations are performed with octopus code [45–47], and
utilize a Cartesian grid with a spacing of 0.4 Bohr, and a
spherical boundary shape with a radius of 45 Bohr. We use
a complex absorbing potential (CAP) with a width of 15 Bohr
(time step of 2.6 attoseconds). The photoelectron flux is calcu-
lated at the spatial onset of the CAP. Our calculations assume
that the hydrogen molecule is aligned along the y axis, in the
laser polarization plane (the result was seen largely indepen-
dent of the molecular orientation). The simulation uses the
same laser parameters as in the experiment with the differ-
ence that in the simulation a shorter pulse duration of nine
femtoseconds (FWHM in intensity) is used (envelope’s shape
taken from Ref. [48]). The phase between the H2 zero point
energy vibration and the laser is averaged over. Figure 4(a)
shows the calculated electron momentum distribution. The
low-energy structure is reproduced and its relative yield of
about 1% compared to the main half-moon lobe, is in agree-
ment with the experiment [see Fig. 1(b)].
In a next step, the nuclei’s coordinates are frozen in the
TDDFT simulations, which implies that one uses an inac-
curate internuclear distance for the ionization step, and a
static internuclear distance for the modeling of the subse-
quent electron-ion interaction. This allows one to disentangle
the role of electron-ion energy exchange, because it sam-
ples the contribution of a single ionic configuration to the
photoemission process by using a well-defined internuclear
013123-4
QUANTUM CORRELATION OF ELECTRON AND ION …
PHYSICAL REVIEW RESEARCH 5, 013123 (2023)
trons vanish for KERs above 6 eV. The sharp edge at a KER of
3 eV and the almost complete absence of low-energy electrons
for lower KERs is due to the fact that for these KERs resonant
few-photon transitions between 1sσg and 2pσu are dominant
(these are directly driven by the laser field). Another reason
why for low KER there is no energy exchange of electron and
ion is that low KER correspond to small spacings of 1sσg and
2pσu, which are not resonant to the comparably high value
of Up.
V. CONCLUSION
In conclusion, we have studied the strong field dissociative
ionization of molecular hydrogen. We observe a fingerprint of
an energy exchange of the liberated electron with its parent
molecular ion that occurs without recollision. We find that
for certain internuclear distances the outgoing electron has
an energy that is significantly lower than the ponderomotive
energy. The apparently missing electron energy equals the
energy spacing of the 1sσg and the 2pσu energy curve for
the internuclear distance that corresponds to the measured
KER of about 3.5 eV. We speculate that the electron might
transfer some of its energy to the H+
ion via a nonclas-
2
sical antenna-like mechanism [52,53]. This is in line with
our ab initio simulations that show a dependence of the
electron energy on the internuclear distance. Similar energy
transfer mechanisms are known for single photon processes
[54–56] but, so far, have not been observed in the strong field
regime.
ACKNOWLEDGMENTS
The experimental work was supported by the DFG (Ger-
man Research Foundation). We thank Reinhard Dörner
for his support and fruitful discussions. O.N. gratefully
acknowledges the support of the Alexander von Humboldt
Foundation, and a Schmidt Science Fellowship. O.N., U.G.,
and A.R. acknowledge financial support from the European
Research Council (ERC-2015-AdG-694097). The Flatiron In-
stitute is a division of the Simons Foundation. This work
was supported by the Cluster of Excellence Advanced Imag-
ing of Matter (AIM), Grupos Consolidados (IT1249-19),
and SFB925. L.S. acknowledges financial support from the
Hungarian Scientific Research Fund (Grant No. K128621),
the National Research, Development, and Innovation Office
(Grant No. 2018-1.2.1-NKP-2018-00010), and the National
Information Infrastructure Development Program. S.E. ac-
knowledges funding of the DFG through Priority Programme
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ournal.pone.0260153 November 29, 2021
1 / 14
PLOS ONEidentify our study participants and individual
healthcare facilities, and assurances were given to
respondents that any publication would not do so.
Requests for access to the data underlying our
findings will be considered by the National Ethical
Committee of Public Health/CNES South Kivu
province, and should be addressed to Prof Kitoka
Moke at [email protected].
|
Data cannot be shared publicly because to do so could potentially identify our study participants and individual healthcare facilities, and assurances were given to respondents that any publication would not do so. Requests for access to the data underlying our findings will
|
RESEARCH ARTICLE
Contextual factors influencing a training
intervention aimed at improved maternal and
newborn healthcare in a health zone of the
Democratic Republic of Congo
Malin BogrenID
1*, Sylvie Nabintu Mwambali2, Marie BergID
1,2
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
1 Institute of Health and Care Sciences, Sahlgrenska Academy, University of Gothenburg, Gothenburg,
Sweden, 2 Faculty of Medicine and Community Health, Department of Obstetrics and Gynecology,
Evangelical University of Africa, Bukavu, Democratic Republic of Congo
* [email protected]
Abstract
Background
OPEN ACCESS
Citation: Bogren M, Mwambali SN, Berg M (2021)
Contextual factors influencing a training
intervention aimed at improved maternal and
newborn healthcare in a health zone of the
Democratic Republic of Congo. PLoS ONE 16(11):
e0260153. https://doi.org/10.1371/journal.
pone.0260153
Editor: Ashraful (Neeloy) Alam, The University of
Sydney Faculty of Medicine and Health,
AUSTRALIA
Received: May 7, 2021
Accepted: November 3, 2021
Published: November 29, 2021
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0260153
Copyright: © 2021 Bogren et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data cannot be
shared publicly because to do so could potentially
Maternal and neonatal mortality and morbidity in the Democratic Republic of Congo (DRC)
are among the highest worldwide. As part of a quality improvement programme in a health
zone in the DRC aimed at contributing to reduced maternal and neonatal mortality and mor-
bidity, a three-pillar training intervention around childbirth was developed and implemented
in collaboration between Swedish and Congolese researchers and healthcare profession-
als. The aim of this study is to explore contextual factors influencing this intervention.
Methods
A qualitative research design was used, with data collected through focus group discussions
(n = 7) with healthcare professionals involved in the intervention before and at the end (n =
9). Transcribed discussions were inductively analysed using content analysis.
Results
Three generic categories describe the contextual factors influencing the intervention: i)
Incentives motivated participants’ efforts to begin a training programme; ii) Involving the
local health authorities was important; and (iii) Having physical space, electricity, and equip-
ment in place was crucial.
Conclusions
This study and similar ones highlight that incentives of various types are crucial contextual
factors that influence training interventions, and have to be considered already in the plan-
ning of such interventions. One such factor is expectations of monetary incentives. To meet
this in a small research project like ours would require a reduction of the scale and thus limit
the implementation of new evidence-based knowledge into practice aimed at reducing
maternal mortality and morbidity.
PLOS ONE | https://doi.org/10.1371/journal.pone.0260153 November 29, 2021
1 / 14
PLOS ONEidentify our study participants and individual
healthcare facilities, and assurances were given to
respondents that any publication would not do so.
Requests for access to the data underlying our
findings will be considered by the National Ethical
Committee of Public Health/CNES South Kivu
province, and should be addressed to Prof Kitoka
Moke at [email protected].
Funding: The study was conducted with financial
assistance from the Laerdal Foundation and
Sahlgrensringen. The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declared
no competing interests exist.
Abbreviations: HBB, Helping Babies Breathe; HMS,
Helping Mothers Survive; DRC, Democratic
Republic of Congo; FGDs, focus group
discussions; SDG, Sustainable Development Goal.
Contextual factors influencing a training intervention in the Democratic Republic of Congo
Introduction
In the Democratic Republic of Congo (DRC), the maternal and neonatal mortality ratio
(MMR) remains high. According to the latest nationally reported statistics, in 2013–14 the
maternal mortality rate was 846 deaths per 100,000 live births and the neonatal mortality rate
28 deaths per 1,000 live births [1]. This, despite the fact that 80% of births were assisted in
healthcare facilities by skilled healthcare providers, consisting of either midwives, physicians,
or nurses [2].
The provision of high-quality care is central in achieving health-related targets within the
Sustainable Development Goals (SDGs), especially the health of mothers and newborns [3,4].
In the strive for high-quality care around childbirth, health systems need to ensure that all
women and their newborns receive quality care, defined as being scientifically evidence-based,
equitable, respectful, effective, timely, efficient, and person-centred [2,5–8]. Furthermore,
health systems need to be adapted to context, i.e. the environment and surroundings in which
a proposed change is to be implemented [9]. Poor quality of care is a greater barrier than insuf-
ficient access to healthcare in the DRC as well as worldwide [3,10].
The training of healthcare providers is known to be a useful tool for reducing maternal and
newborn mortality; however, no specific training strategy is effective in every context [11],
which means that the same interventions have different effects in different contexts. Context
includes anything internal and external to an intervention that may act as a barrier to or facili-
tator of its implementation or effect. Thus, it is essential to understand the context–including
identifying which contextual factors influence a particular quality improvement intervention
and how they do so [9,12]. As part of an implementation project aimed at contributing to
reduced maternal and neonatal mortality and morbidity in a health zone in the South Kivu
province of the DRC, this study’s objective was to explore contextual factors influencing a
training intervention focusing on healthcare practice during childbirth. The lessons learned
from the results are presumed to also be useful in other similar contexts, in the DRC as well as
low-income countries elsewhere, when designing and implementing similar training
interventions.
Method
Study design
The study was approved by the National Ethical Committee of Public Health: CNES 001/
DPSKI/129PM/2019. A qualitative research design was used [13], and data was collected
through focus-group discussions (FGDs) with healthcare professionals participating in the
training intervention.
Setting
The DRC comprises 26 provinces, with more than 500 health zones which are organised to
deliver healthcare at three levels. The primary level of care is offered at healthcare centres,
some of which also offer perinatal care. The secondary level is offered at district hospitals,
which have the capacity to perform C-sections, and the tertiary level is offered at referral hospi-
tals (one per health zone) [14]. The healthcare facilities are governed by either the governmen-
tal or private sector.
The health zone where this implementation project took place is one of three in the provin-
cial capital in the South Kivu province, situated in the eastern part of the DRC. At the time of
the intervention, this health zone served more than 450,000 inhabitants at 40 healthcare facili-
ties, of which 34 were healthcare centres, five were district hospitals, and one was a referral
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
Table 1. Level, governance, and number of births at each healthcare facility.
Healthcare facilities
Level of healthcare
Financial and governance structure
Births in 2018
1
Tertiary
Private
3,229
2
3
Secondary
Secondary
Private
847
Private
1,249
4
Primary
Private
547
5
Secondary
Private
418
6
Primary
Private
921
7
Primary
Private
165
https://doi.org/10.1371/journal.pone.0260153.t001
hospital. This study is part of a maternal and newborn healthcare quality improvement project
being conducted at seven of these 40 healthcare facilities: three healthcare centres, three district
hospitals, and the referral hospital. There were 16,101 registered births in the health zone in
2018, 7,416 of which occurred at these seven healthcare facilities. The facilities’ level of health-
care and governance are shown in Table 1:
Intervention
The training programme was developed based on core principles of conducting person-cen-
tred holistic care [15,16] as well as a woman-centred model of childbirth care [17], and with
the overall aim to promote healthy physiologic, vaginal birth. The programme was divided
into three pillars, with activities to 1) promote normal physiologic birth, 2) prevent and man-
age complications during labour and birth, and 3) strengthen the healthcare professionals’
self-reflection skills and self-confidence.
The first and second pillars consisted of theory and simulation-based training using equip-
ment from the Laerdal foundation [18], and additional tools such as birthing balls and Rebozo
sheets. The first pillar was based on principles included in the midwifery model of woman-cen-
tred care during childbirth [17] and in the Rebozo technique [19]. The second pillar was based
on the programmes Helping Mothers Survive Bleeding after Birth (HMS-BAB) developed by
Jhpiego [20], and Helping Babies Breathe (HBB) developed by the American Academy of Pedi-
atrics [21]. Pillar 3 consisted of reflection in groups based on a process-oriented group reflec-
tion model in which the participants reflect on themselves selected own experienced situations
related to their professional role [22].
The implementation of the training programme was planned and steered by a multiprofes-
sional project committee of healthcare professionals from the DRC (n = 3) and Sweden
(n = 3), after having been developed by the Swedish research group. Details of the implementa-
tion are described in Table 2. The Congolese partners chose seven of the 40 healthcare facili-
ties, representing all three healthcare levels. Each facility selected its own training facilitators.
A 25-day training in the three-pillar programme was first given to four selected master train-
ers, of whom three were nurses working as midwives and one was a physician specialising in
gynaecology. Next, the master trainers gave a six-day training to 13 selected training facilita-
tors–two from each of six healthcare facilities, and one from the smallest one–consisting of ten
nurse/midwives, two gynaecologists, and one paediatrician. Next, the seven healthcare facilities
were equipped with equipment to conduct the training. This included uniquely developed
didactic teaching material for using the programme, a birthing ball, a specially designed sheet
for use of the Rebozo technique [19], and the Laerdal products ‘MamaNatalie Complete’,
‘MamaBirthie’, and ‘NeoNatalie Complete’ [18]. Further, based on the ‘low-dose high-fre-
quency’ training pedagogy [21], a detailed schedule was defined for doing week-to-week short
training activities for a period of six months, including weekly training in Pillars 1 and 2 and
process-oriented reflections in groups once per week. The master trainers mentored the facili-
tators at the healthcare facilities and led the process-oriented reflections with the staff. The
master trainers received a small monetary incentive, while the facilitators did not.
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
Table 2. Description of the implementation of the three-pillar training programme.
Activities
Project planning
Time
3 months
Content
Development of didactic teaching materials and
procurement of training equipment
Setup of steering committee
Identification of healthcare facilities (one hospital on
tertiary level, three hospitals on secondary level, and three
healthcare centres)
Introducing the local health zone authorities to the training
programme and its concept, and obtaining approval
Introducing the participating healthcare facilities’
management and staff to the three-pillar training
programme and its concept
Collection of baseline statistics on labour and birth
Equipment assessments conducted at the seven facilities
using a) the Jhpiego checklists, and b) the HMS/HBB
training list
Pre-intervention discussions on contextual barriers and
facilitators with the healthcare professionals at the
participating facilities
Training local master
trainers
Training of four master trainers (three midwives and one
physician)
25 (14+11) days
Training local facilitators
Training of 13 facilitators (two from each of six facilities and
one from the smallest one)
Training healthcare
professionals
Distribution of teaching materials, training equipment, and
training schedule
1 month
Continuously facilitation
Monthly visits to each healthcare facility by master trainers
6 months
Introduction of training programme by local facilitators
Follow-up visits to each
facility
Follow-up meeting with
the master trainers
Weekly practice by local facilitators using the low-dose high-
frequency practice
Dialogue with facilitators and healthcare professionals at
each healthcare facility, providing opportunities to share
experiences of implementing the training programme.
Dialogue with master trainers, providing opportunities to
share experiences of working with implementing the
training programme
Last month of the
training programme
https://doi.org/10.1371/journal.pone.0260153.t002
Data collection
Data was gathered through FGDs in two periods: before the training started (FGDs = 7) and at
the end of the programme, when facilitators and master trainers were also interviewed
(FGDs = 9). The local project leader of the training programme (SNM) contacted the health
zone authorities and the managers at each participating healthcare facility and informed them
about the study. Authorities from the health zone and managers at each healthcare facility
approved it, and provided contact information for available healthcare staff working at the
maternity unit who had taken part in the three-pillar training programme. The project leader
contacted these individuals and invited them to participate in FGDs after giving them verbal
and written information about the study, including the fact that participation was voluntary
and that they had the right to withdraw at any time without explanation. All invited healthcare
professionals (n = 61), being either nurses, midwives, or gynaecologists, agreed to participate
and signed informed consent.
All 16 FGDs were conducted by two of us authors (MBo and MBe). There were three to
seven participants in each group. The discussions were led by MBe in French, based on an
interview guide (see S1 Appendix), and were translated continuously during the FGDs into
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
Table 3. Examples of the data analysis process from meaning unit to category.
Meaning Unit
Code
Subcategory
Category
We, as trained [facilitators, don’t get any sugar [compensation] for the
training we hold [at the healthcare facilities], and we didn’t get any
compensation when we took part in the training to become facilitators
Compensation is
expected
Monetary incentives for
participating are expected
Incentives influence participants’
efforts to begin a training
programme
https://doi.org/10.1371/journal.pone.0260153.t003
Swedish to MBo, who made field notes and asked clarifying questions. The FGDs were audio-
recorded and lasted 30 to 60 minutes, with a mean of 45 minutes.
Data analysis
The audio-recorded FGDs were analysed following principles of qualitative inductive con-
ducted analysis [23]. First, all transcripts were read several times. Next, in new readings, mean-
ing units were identified that answered the research question ‘What are the contextual factors
influencing the three-pillar training intervention and how do they influence it?’ The meaning
units were then compared and sorted into codes based on similar content, which were thereaf-
ter compared and clustered into subcategories and categories. The analysis process was com-
pleted by MBo and MBe separately, with repeated discussions until full agreement was
reached. An example of the analysis process is shown in Table 3.
Results
Contextual factors identified as influencing the implementation of the three-pillar training
programme were sorted into three generic categories with respective subcategories; for an
overview, see Table 4. In the presentation of the results the FGDs conducted in the two periods
are labelled FGD 1 and FGD 2, respectively, with the facilities where they were held labelled
1–7 (see Table 1).
Incentives motivate participants’ efforts to begin a training programme
The incentives that influenced participants’ efforts to get the three-pillar training programme
up and running consist of three subcategories, as follows.
Gaining increased knowledge and skills motivates. Motivation was high among the par-
ticipants to take part in the three-pillar training programme as it provided them with updated
knowledge and skills for daily practice, which could contribute to healthy and positive
childbirth.
Table 4. Categories and subcategories describing contextual factors influencing the three-pillar training
intervention.
Generic category
Subcategory
Incentives motivate participants’ efforts to begin a training
programme
The importance of involving the local health authorities
Gaining increased knowledge and skills motivates
Women’s utilisation of the healthcare facilities
motivates
Monetary incentives for participating are expected
Authorities from the health zone need to be
involved
The healthcare facilities’ management needs to be
involved
The need to have physical space, electricity, and equipment in
place
Inadequate physical space and electricity
Lack of equipment to promote physiologic birth
https://doi.org/10.1371/journal.pone.0260153.t004
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
Most of the participating healthcare professionals had not received any formal in-service
training since their professional education, and the three-pillar training programme provided
them with new knowledge. At two facilities they typically arranged training themselves in
areas where care was not optimally conducted, for example in how to resuscitate a newborn.
At one facility, the church sometimes organised training, and those who had participated
shared their new knowledge with colleagues:
There is a lack of in-service training. We get nothing. Sometimes we’re invited to seminars
organised by the private health sector, but no such seminars are organised by the government
health zone; they just give some random information. (FGD 1, Healthcare Facility 7)
Most of us have not gotten any formal in-clinic training since we completed our pre-service profes-
sional education. We try to solve this through organising our own in-service trainings using our
doctors, who have been at the university. But they only have their own notes, and would have
needed to have PowerPoints and other educational materials. (FGD 1, Healthcare Facility 4)
All participants, except for at one healthcare facility, expressed an awareness that they did
not have the latest scientific evidence-based knowledge and accordingly were not practising
optimally. They were primarily motivated to learn in several areas such as promoting normal
physiologic birth and correctly managing complications like postpartum haemorrhage. The
materials they were provided–birthing balls and Rebozo sheets for use during labour, and pen-
guins and a ventilation mask for aspirating the newborn in need–added to their motivation
and their possibility to practise the skills they had learned:
We don’t have an ambulance that we can use to transport the women who need a C-section,
and the ambulance from the reference hospital often comes much too late. The training we’ve
gotten helps us support a normal birth and lets us handle acute conditions like bleeding. (FGD
2, Healthcare Facility 6)
There was a positive attitude regarding sharing knowledge between the different healthcare
facilities within the same health zone. The three-pillar training was regarded as such a knowl-
edge exchange programme both within an individual healthcare facility as well as between the
different facilities. There was a desire to develop such knowledge sharing even further, as the
healthcare facilities had the same type of patients with similar backgrounds and health condi-
tions. The master trainers, in turn, acknowledged that the training programme had given them
a mandate to have access to and connect with the other facilities within the zone.
Women’s utilisation of the healthcare facilities motivates. Another motivating factor
for the healthcare professionals to participate in the training programme was when they
noticed that their changed care routines had influenced how women informed their peers
about their positive experiences of being cared for at the healthcare facilities, which in turn
positively influenced other women’s decisions to seek care at the facilities. It had also been
noticed that several women arrived earlier when their labour had started, which in turn influ-
enced the outcome:
The training was fantastic, both for us as staff at the clinic and for the women who come in
and give birth. At one of the clinics, many more women giving birth are coming in. Now,
when a woman in labour comes, the staff are close to the woman and massage her. When the
woman goes home she tells others where she lives about her positive experience. (FGD 2 with
master trainers)
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
It was acknowledged that caring for an extremely poor, and often low-educated, population
was challenging. Women commonly feared having a C-section. With the newly trained care
routines that promoted a normal physiologic birth, there was a belief among the healthcare pro-
fessionals that their increased knowledge and skills in turn would increase the prevalence of vag-
inal, non-instrumental births, which in turn would motivate women to give birth at the facility:
The population is very poor and has a low education level; this makes it difficult to motivate
the patients for different decisions about care. (FGD 1, Healthcare Facility 4)
The pregnant women are afraid of having repeated C-sections. There are different reasons;
one can be that the family force her to give birth normally in order to be considered a real
woman. Another reason for rejecting a C-section is the cost; they therefore reject having a C-
section. (FGD 1, Healthcare Facility 5)
Monetary incentives for participating are expected. Another strong, motivating factor
for participating in the training programme was the expectation of monetary incentives. This
expectation was the same for master trainers, facilitators, and the healthcare professionals at
the facilities, and was based on the fact that monetary incentives were commonly provided by
other projects they had participated in. As this training project had no such monitoring incen-
tive system in place for the weekly participation in training, it made participants at the health-
care facilities hesitant to attend the sessions:
You know how it is with Africans: they don’t come if they don’t get any sugar [compensation].
You need something to motivate them; money’s needed as motivation. // We, as trained [facil-
itators], don’t get any sugar [compensation] for the training we hold [at the healthcare facili-
ties], and we didn’t get any compensation when we took part in the training to become
facilitators. (FGD 2 with facilitators)
The healthcare professionals at each facility followed work schedules covering 24 hours,
seven days a week, which made it difficult for them to attend every scheduled training activity.
Activities that were part of the Pillar 1 and 2 trainings were often practised in the morning,
which made it challenging for those who had been working the night shift. The importance of
being reimbursed as motivation to participate in the training was stressed, at least being reim-
bursed for transportation costs if the training activities were undertaken when someone was
off duty:
It can be hard to convince the staff to take part in the different training steps. What’s hard is
motivating the staff to stay and train after the end of their workday, as well as motivating
staff to come in on their day off to train. Sometimes they refuse to come in because they’re not
paid for the transportation. (FGD 2 with facilitators)
The master trainers stressed that the provision of incentives would increase their motiva-
tion to conduct the scheduled training activities. This, as being a master trainer and a facilita-
tor was regarded as having dual work responsibilities–both their ordinary work as well as this
training–which therefore required sufficient monetary incentives. The lack of monetary incen-
tives to facilitators and the insufficient incentives to master trainers contributed to a lack of
motivation:
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
We’d like to get paid when we train the others; we only get a transportation allowance. When
we had our master training in April and May, we didn’t get any compensation. (FGD 2 with
master trainers)
Because facilitators don’t get paid, they’re not motivated to train their colleagues. (FGD 2
with master trainers)
The importance of involving the local health authorities
This category describes the importance of involving both the authorities from the health zone
as well as the health management at the participating healthcare facilities in the three-pillar
training programme, in order to make it more successful and sustainable.
Authorities from the health zone need to be included. Getting the local health zone author-
ity involved was critical for successful implementation, and obtaining this authority’s approval
was perceived as a requirement for conducting the three-pillar training programme. Involving the
local health authority in the training could therefore support and encourage the healthcare facili-
ties to include the training activities within their daily routines. The authorities from the health
zone were informed about the training action and had granted permission to conduct it; but to
support the project further, according to the master trainers, the health inspectors would need to
be incentivised, and it was suggested that the project consider budgeting for this:
The health inspectors from the health zone are included to some degree, but they’re not moti-
vated to support the project. They want to be a part of this project. They want to be there
when we master trainers go to the clinics, but they don’t want to be trainers. If they were
included more they’d be able to motivate the staff to participate in the project during their reg-
ular visits to the healthcare facilities. . . . The inspectors have expressed that if they’re paid,
like they are in other projects, they can encourage the healthcare facilities to take part in the
training. (FGD 2 with master trainers)
The healthcare facilities’ management needs to be involved.
Involving the healthcare
management at each healthcare facility was stressed to be of critical importance, as they make
all the decisions concerning care and care routines. And if the training programme was to gain
sustainability and continue beyond its scheduled time, creating ownership among the local
management was acknowledged as crucial:
We have a culture in which the responsible parties are higher in rank than us; we others feel
lower than them. So if this training is to continue, you have to involve more of those at man-
agement level at the healthcare facilities. That would increase the ownership. Then, manage-
ment will take greater responsibility, they’ll increase our motivation, create an ownership.
(FGD 2, Healthcare Facility 1)
Being involved entailed not only being informed about and influencing the training strat-
egy, which was a part of the project; it also included receiving monetary incentives. If not, this
could act as an obstacle to the training programme. This was especially clear at the tertiary-
level facility. These leaders hindered the programme in various ways, for example by not taking
part in the training and even stating that they had never heard about it:
If people with management responsibility aren’t directly involved in the project, they’ll turn
the responsibility over to those who are running the project. You have to bring in the boss,
make him part of the project. When the boss talks everybody listens; I can’t ask the boss to do
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
various things, I can’t give the boss orders. So the boss has to participate and have a mandate.
And that means that the boss needs money from the project. It’s not a salary; it’s a motivation.
If the boss is motivated, we can get everybody to do what we want in the project. If he doesn’t
get paid he’s not going to participate himself and he’s going to work against it. The other
healthcare facilities are small; this is a gigantic clinic, and the boss has to be involved. (FGD
2with master trainers)
The need to have physical space, electricity, and equipment in place
This category describes, in two subcategories, the need to have physical space, electricity, and
equipment in place in order to carry out the three-pillar training programme.
Inadequate physical space and electricity. According to the participants and our own
observations all seven facilities, and specifically their maternity units, had inadequate physical
space to meet the needs. Hence, this contradicted the training. One challenge that negatively
influenced the training using mannequins was that the mannequins were packed in bags and
stored, and were only picked up for each training session. According to the participants this
was due to a lack of space, and the fact that there was no separate table for the mannequins.
This resulted in the training not always being conducted as they had learned during their train-
ing. Another reason for storing the training products was a fear that the material would be
stolen:
At the labour ward there’s no place; we’ve notified the staff that they can use the material on
the day. We have no room to store it openly and guard it. We have many interns, the African
culture–the material can be stolen. (FGD 2 with master trainers)
No access to, or insufficient availability of, electricity was another challenge that limited the
possibilities to conduct care during labour and birth based on the staff’s new knowledge.
Often, electricity was only available for six to eight hours or even less, but there could also be
two days with no electricity at all. This led to ambiguity about using electric equipment, as it
was almost impossible to rely on equipment like the blood refrigerator, resuscitation equip-
ment, and heating lamps for newborns, which depended on electricity. Solar panels were com-
monly used, but were often insufficient. At one facility, the blood refrigerator used all the
electric capacity. Thus, it was motivating to take part in this training programme and use the
assigned equipment that did not require electricity:
Electricity comes and goes; we cannot say how often. We have solar cells but they’re not strong
enough to drive medical equipment, as it needs electricity. (FGD 1, Healthcare Facility 6)
Lack of equipment to promote physiologic birth. Unanimously, all participants men-
tioned the lack of childbirth care equipment at the healthcare facilities as very limiting to the
provision of high-quality maternal and newborn healthcare. There was especially a lack of
equipment for promoting normal physiologic birth. Thus, the birthing balls and a specially
designed sheet for the use of Rebozo techniques were specifically valued, as this made it possi-
ble to offer alternative pain relief and positions during labour and birth.
The material provided through the project was used often, and all participants stressed a
need for additional birthing balls and Rebozo sheets. At some facilities, the balls and sheets
had been used so often they were now broken/torn, while other facilities were in need of extra
equipment so that it could be used if several births were taking place at the same time. The
importance of cleaning the equipment between women also created a need for extra balls and
sheets:
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
I participated in a one-week course last autumn and learned about alternative pain relief
such as Rebozo and birthing balls, but couldn’t practise it as we didn’t a ball like that. (FGD
1, Healthcare Facility 2)
If we have two women at the same time we can’t offer both women a birthing ball. We want
to have a ball for all women giving birth; we have about three deliveries a day. We also need
more Rebozo sheets. We’d like to have three Rebozos. (FGD 2, Healthcare Facility 4)
Discussion
The study identified three contextual factors which influenced the three-pillar training inter-
vention programme aimed at improving healthcare practice during labour and birth: (i) Incen-
tives motivated participants’ efforts to begin the training programme; (ii) Involving the local
health authorities was important; and (iii) Having physical space, electricity, and equipment in
place was crucial. A central feature in these identified factors is the implication of incentives,
which we will discuss further below.
The participants expected to be monetarily reimbursed as an add-on to their monthly sal-
ary. They stressed that participation in training and projects of different kinds mostly implies
being paid, and thus expected to be incentivised in our training programme as well. Only the
master trainers were reimbursed, and they judged the compensation level to be too low. This
may be explained by the fact, found in another study in the DRC, that healthcare professionals
in the DRC, especially nurses and midwives, often lack regular payment or compensation for
their employment [24].
Barriers related to not receiving monetary incentives are not unique in quality improve-
ment interventions. The issue has been identified in other low- and middle-income country
healthcare projects in which community health workers have been used to increase the possi-
bility to achieve the goals; there, payment positively influenced the health workers’ motivation
to contribute [25,26]. The possibility of monetary incentives to increase professionals’ willing-
ness to participate was also observed in a training intervention as part of a neonatal health
project in Vietnam [27].
A main finding in our study was that the local health authorities also expected to be paid if
they were to facilitate and encourage the training programme, even though they were not
involved as trainers. In a recent Cochrane review, the involvement of local leaders in quality
improvement activities was found to be effective in implementing evidence-based practice.
The report stresses the importance of engaging healthcare leaders in interventions aimed at
improved health outcomes [28]. Similar findings have been reported in a review article about
middle managers’ role in healthcare evidence-based practice implementation [29]. Our train-
ing programme was introduced and accepted by the local authorities both in the health zone
and at the involved healthcare facilities, but without their being paid. At one of the participat-
ing facilities it was clearly observed that this led to a hindrance of the programme’s activities
and effects. As a consequence, this barrier negatively affected the expected improved maternal
and newborn health outcomes. These findings are rather disappointing.
The issue of providing or not providing performance-based financing, and its effect on
ensuring the delivery of high-quality health services, has been studied in the DRC, where
almost 500 health workers representing five of the 26 provinces participated. Workers who
had received monetary incentives which had then been stopped when the project ended scored
significantly lower on most dimensions of motivation than did those who had never received
money. The study highlights the potentially negative effect on health workers’ motivation
when large donor-driven projects provide generous incentives for participation in training,
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
meetings, and workshops which are then withdrawn after the project-based activities are final-
ised [30].
That large donors’ provision of performance-based financing to health workers and leaders
also influenced our training intervention was obvious, as this had created ‘norms’ among
healthcare providers and leaders that such interventions should be paid for and, if not paid,
they would not participate or could even create a barrier. Thus academic research-based proj-
ects, which usually have smaller budgets to work with, cannot compete with large donor
organisations.
Another, highly positive, contextual factor and effect of our three-pillar training programme
was that the participants–both the master trainers, facilitators, and healthcare professionals–recog-
nised its value for increasing knowledge. A strong preference for learning evidence-based knowl-
edge and skills was shown among the participants; not only to have refresher trainings, but also to
learn new things–which enabled them to better conduct high-quality care during labour and
birth. That continuous education and training serve as important motivation for healthcare pro-
fessionals has been observed elsewhere [31]. Meanwhile, non-monetary incentives, in terms of
level of decision-making among community workers in low- and middle-income countries, have
been shown to be highly effective in increasing intrinsic motivation [32].
Another positive effect of the training programme was that it was immediately transferred
to care practice. Positive changes in caring, such as being closer and using alternative methods
such as birthing ball, had been experienced by the women who in turn had informed their
peers who then wanted to give birth at the same facility.
This is an example that through low-dose high frequency training it is possible to immedi-
ately implement a more woman-centred respectful intrapartum care [17,33,34], and which is
instantly told to society by women being cared.
When it comes to successfully implementing the use of HBS and HMS training pro-
grammes, which constituted pillar two of our three-pillar training programme, it has been con-
cluded that a successful implementation requires country-led commitment, readiness, and
follow-up to create local accountability and ownership [35]. Unfortunately, our study did not
fully involve the health zone authority, which resulted in a lack of ownership. These findings
in our study have offered novel insight regarding contextual factors of incentivising authorities
if a training intervention aims to have their full involvement. Comparing the findings with
those of other studies on maternal and neonatal health improvement in a low-income setting
confirms the need to account for involving local authorities from various levels of the health
system from the planning phase, through budgeting, and throughout the implementation and
evaluation processes [27,36]. This strategy of involvement may reduce the risk of facing hin-
drance in terms of monetary incentives for evidence-based interventions, which are proven to
have an impact on the health outcomes of mothers and newborns.
Another motivating incentive of the programme was that it provided the healthcare facili-
ties with material–both mannequins for training purposes as well as items to use during labour
and birth, such as birthing balls and a specially designed sheet for the use of Rebozo tech-
niques. The use of these materials in combination with better humanised behaviour towards
the women, which was stressed in Pillar 1, gave the healthcare facilities a more positive reputa-
tion through women’s sharing of positive information with peers, and also seemed to reduce
the negative trend of pregnant women’s delay in going to the facilities.
Methodological considerations
Our study is among the first to show evidence of the influence of contextual factors in a train-
ing intervention aimed at improving intrapartum care in the DRC. However, the study has its
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
limitations. It was carried out at only seven of 40 healthcare facilities, and in only one health
zone in the DRC; this was due to limited resources. It cannot be assumed that the contextual
factors influencing such an intervention in the other 33 facilities in the chosen health zone, as
well as elsewhere in the DRC, would be the same. Therefore, the results may not be generalised
to the entire health zone, nor the country. Another limitation is that only private healthcare
facilities, and no governmental ones, were included. This may have caused us to miss other
contextual factors influencing such a training intervention.
A strength of the study is the interdisciplinary mix of the participating researchers. M Bog-
ren and M Berg, from Sweden, both hold the degrees of PhD, RM, and RN, are conducting
research in low-income settings including the DRC, and have extensive experience working in
low-income countries through multilateral organisations (M Bogren) and private health sys-
tems, including in the DRC (M Berg). The third author, S N. Mwambali, is a Congolese gynae-
cologist, and was the local project leader of the training intervention.
Conclusions
To conclude, this study found what also has been found earlier, that aspects of the context
influence the implementation of an intervention and its outcomes, and hence its feasibility
and usefulness [12]. That incentives are a critical element of successful health interventions in
impacting sexual, reproductive, maternal, and newborn healthcare quality in low- and middle-
income countries is confirmed in a recent systematic review [37]. A critical lesson learned
from this study in the DRC is that incentives of various aspects are crucial contextual factors to
consider when planning for a training intervention. In a small research project like ours, fully
meeting the expectations of monetary incentives would require a reduction of the scale and
thus limit the implementation of new evidence-based knowledge into practice.
Supporting information
S1 Appendix.
(DOCX)
Acknowledgments
We would like to express our sincere appreciation to all the healthcare providers who partici-
pated in this study. We also want to thank Dr. Prof. Denis Mukwege, Panzi Hospital, Susheela
M Engelbrecht at Jhpiego, Maria Hogena¨s, Art of Life, and Marthe Byamungu Makundane for
their respective contribution in the training programme.
Author Contributions
Conceptualization: Malin Bogren, Marie Berg.
Data curation: Malin Bogren, Marie Berg.
Formal analysis: Malin Bogren, Marie Berg.
Funding acquisition: Malin Bogren, Marie Berg.
Investigation: Malin Bogren, Marie Berg.
Methodology: Malin Bogren, Marie Berg.
Project administration: Malin Bogren, Sylvie Nabintu Mwambali.
Supervision: Malin Bogren, Marie Berg.
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PLOS ONEContextual factors influencing a training intervention in the Democratic Republic of Congo
Validation: Malin Bogren, Marie Berg.
Writing – original draft: Malin Bogren, Marie Berg.
Writing – review & editing: Malin Bogren, Sylvie Nabintu Mwambali, Marie Berg.
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PLOS ONE
| null |
10.1371_journal.pone.0270817.pdf
|
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
|
All relevant data are within the paper and its Supporting Information files.
|
RESEARCH ARTICLE
Astrocytes and pericytes attenuate severely
injured patient plasma mediated expression
of tight junction proteins in endothelial cells
Preston StaffordID
Jamie HadleyID, Patrick Hom, Terry R. SchaidID, Mitchell J. CohenID*
☯, Sanchayita Mitra☯, Margot Debot, Patrick Lutz, Arthur StemID,
Division of GITES, Department of Surgery, University of Colorado Anschutz Medical Campus, Aurora,
Colorado
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
☯ These authors contributed equally to this work.
* [email protected]
Abstract
OPEN ACCESS
Citation: Stafford P, Mitra S, Debot M, Lutz P,
Stem A, Hadley J, et al. (2022) Astrocytes and
pericytes attenuate severely injured patient plasma
mediated expression of tight junction proteins in
endothelial cells. PLoS ONE 17(7): e0270817.
https://doi.org/10.1371/journal.pone.0270817
Editor: Ma´ria A. Deli, Eo¨tvo¨s Lora´nd Research
Network Biological Research Centre, HUNGARY
Received: March 22, 2022
Accepted: June 20, 2022
Published: July 5, 2022
Copyright: © 2022 Stafford et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
Funding: MC Trans-agency Research Consortium
for Trauma Induced Coagulopathy (TACTIC UM1-
HL120877) from the National Heart, Lung and
Blood Institute, NIH. https://grants.nih.gov/grants/
guide/rfa-files/RFA-HL-13-025.html The funders
had no role in study design, data collection and
analysis, decision to publish, or preparation of the
manuscript.
Blood Brain Barrier (BBB) breakdown is a secondary form of brain injury which has yet to be
fully elucidated mechanistically. Existing research suggests that breakdown of tight junction
proteins between endothelial cells is a primary driver of increased BBB permeability follow-
ing injury, and intercellular signaling between primary cells of the neurovascular unit: endo-
thelial cells, astrocytes, and pericytes; contribute to tight junction restoration. To expound
upon this body of research, we analyzed the effects of severely injured patient plasma on
each of the cell types in monoculture and together in a triculture model for the transcriptional
and translational expression of the tight junction proteins Claudins 3 and 5, (CLDN3,
CLDN5) and Zona Occludens 1 (ZO-1). Conditioned media transfer studies were performed
to illuminate the cell type responsible for differential tight junction expression. Our data show
that incubation with 5% human ex vivo severely injured patient plasma is sufficient to pro-
duce a differential response in endothelial cell tight junction mRNA and protein expression.
Endothelial cells in monoculture produced a significant increase of CLDN3 and CLDN5
mRNA expression, (3.98 and 3.51 fold increase vs. control respectively, p<0.01) and
CLDN5 protein expression, (2.58 fold change vs. control, p<0.01), whereas in triculture, this
increase was attenuated. Our triculture model and conditioned media experiments suggest
that conditioned media from astrocytes and pericytes and a triculture of astrocytes, pericytes
and endothelial cells are sufficient in attenuating the transcriptional increases of tight junc-
tion proteins CLDN3 and CLDN5 observed in endothelial monocultures following incubation
with severely injured trauma plasma. This data suggests that inhibitory molecular signals
from astrocytes and pericytes contributes to prolonged BBB breakdown following injury via
tight junction transcriptional and translational downregulation of CLDN5.
Introduction
Traumatic Brain Injury (TBI) is one of the leading causes of mortality worldwide, with mortal-
ity rates as high as 30% and significant morbidity in survivors [1,2]. Following trauma, treating
PLOS ONE | https://doi.org/10.1371/journal.pone.0270817 July 5, 2022
1 / 19
PLOS ONECompeting interests: The authors have declared
that no competing interests exist.
Endothelial cell tight junction expression in non-TBI trauma
primary brain injury and mitigating secondary brain injury is the primary focus. Separate
from direct TBI, non-brain injured trauma patients also display brain injury like symptomatol-
ogy. In both cases, (direct TBI and indirect TBI) microvascular injury and neuroinflammation
are thought to drive this pathology and have become a target for research and therapeutics [3–
5]. A key component of this microvascular inflammatory injury is breakdown of the Blood
Brain Barrier (BBB) which is associated with morbidity and mortality following TBI [4], and
observationally following severe non-TBI traumatic injury. Central to targeted therapeutic
treatment for BBB dysfunction is understanding the pathology underlying cellular responses
to traumatic injury. In this we look to address brain pathology in non-TBI injured patients, by
examining BBB function after non-TBI trauma.
The Blood Brain Barrier regulates transcytotic movement of molecules between the vascula-
ture and neuronal space. The BBB is composed of the neurovascular unit, comprising three
main cell types–endothelial cells, astrocytes, and pericytes. These cell types participate in the
formation of a physical barrier composed of tight junction proteins Claudin 3 (CLDN3), Clau-
din 5 (CLDN5), Occludin, Junctional Adhesion Molecules (JAM1, JAM2, JAM3), and the
anchor proteins Zona Occludens 1, 2, and 3, (ZO-1, ZO-2, ZO-3) expressed and localized
within and between the endothelial cells [5]. This physical barrier is the principle homeostatic
regulator between the CNS and peripheral vasculature [6] and performs the essential functions
of facilitating the transfer of nutrients, regulating ion stasis, and blocking noxious molecules
from flowing into the neuronal extracellular space [5]. BBB dysfunction results in adverse
patient outcomes linked to transcytotic leakage of fluids, proteins, and other molecules, lead-
ing to intraneuronal toxicity and homeostatic imbalance [7–9].
Central to BBB integrity is the maintenance of tight junction proteins which can be
degraded through protease activity following TBI [10]. BBB degradation and subsequent leak
of inflammatory soluble factors into intraneuronal space has been demonstrated to occur as
soon as 30 minutes post-injury, with maximal leakage occurring in a biphasic manner starting
as early as 4 hours post-injury, and continued permeability up to 30 days post-injury [11–13].
Based on these findings [6,14–18] we focused on the expression of three tight junction proteins
essential to BBB permeability maintenance, CLDN3, CLDN5 and ZO-1.
To elucidate this mechanism in the context of trauma, we incubated cells in monoculture
and in triculture with severely injured patient plasma and performed conditioned media
exchange studies to probe the contribution of each cell type to tight junction expression. We
hypothesized that plasma from non-TBI traumatically injured patients leads to a loss of junc-
tional integrity between endothelial cells via transcriptional down regulation of the major gap
junction proteins Claudins 3, 5 and ZO-1.
Methods
Patient plasma
Normal pooled plasma (George King Bio-medical) phenotypes were verified by the manufac-
turer and used as our healthy plasma negative control. Experimental plasma was collected
from severely injured trauma patients upon arrival to the emergency department at a Level 1
trauma center. Plasma samples were collected, and patient demographics described in accor-
dance with a sampling protocol approved by the Colorado Multiple Institutional Review
Board (COMIRB#13–3087); all subjects were informed, and collection performed under
waiver of consent. (Tables 1 and 2) [19] Patient demographics were collected, and injury sever-
ity score and base deficit was assessed [20–22]. To address limited ex vivo plasma volumes,
two separate groups of pooled trauma plasma were created by combining individual patient
samples. The severely injured patient plasma was pooled based upon the patient’s injury
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Table 1. Trauma plasma pool for transcription experiments described in Figs 1 and 2.
Trauma Plasma Pool
Age
ISS
BD
Blunt/Penetrating
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
62
40
43
35
62
23
24
38
38
23
36
41
50
38
26
37
34
24
19
55
38
25
25
34
24
41
66
25
43
42
17
19
16
29
25
26
42
33
25
29
-9.7 Blunt
-12.7 Blunt
-24.7 Blunt
-11 Blunt
-8.3 Blunt
-7 Blunt
-10 Penetrating
-9.3 Blunt
-7 Penetrating
-6.3 Penetrating
-10 Penetrating
-7 Penetrating
-10 Blunt
-13 Blunt
-10 Penetrating
-13.6 Penetrating
-25 Blunt
-12.4 Penetrating
-10.3 Blunt
-13 Blunt
Median
37.5
27.5
-10.0
Table 1- Pooled plasma samples from severely injured patients used for transcription experiments in Figs 1 and 2.
Samples were comprised of equivalent volumes from each patient pooled together, 500 μL from each patient. Both
individual patients and final pooled plasma were selected with the cutoff criteria of ISS<15, BD<-6.
https://doi.org/10.1371/journal.pone.0270817.t001
severity score (ISS), and base deficit (BD). ISS is used to determine the extent of injury severity,
and BD is used to determine tissue hypoperfusion [20–22]. Severe injury is any sample from a
patient with an ISS>15 and BD<-6.
Table 2. Trauma plasma pool used for ELISA.
Trauma Plasma Pool
Age
ISS
BD
Blunt/Penetrating
2
2
2
2
2
2
2
2
2
2
33
44
28
24
31
27
57
54
43
36
34
22
29
59
16
30
75
25
25
41
-6 Blunt
-13 Blunt
-17 Blunt
-13 Blunt
-11 Blunt
-19.3 Penetrating
-27.5 Penetrating
-24.5 Blunt
-24.7 Penetrating
-27.6 Penetrating
Median
34.5
29.50
-18.2
Table 2- Pooled plasma samples from severely injured patients used for translation experiments in Figs 3 and 4.
Samples were comprised of equivalent volumes from each patient pooled together, 500 μL from each patient. Both
individual patients and final pooled plasma were selected with the cutoff criteria of ISS<15, BD<-6.
https://doi.org/10.1371/journal.pone.0270817.t002
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Cell culture
Immortalized Human Cerebral Microvascular Endothelial cells (hCMEC/D3) and complete
cell growth media (EndoGRO-MV) were obtained from EMD Millipore and cultured as
described by the manufacturer. Primary human astrocytes with complete astrocyte media and
primary human brain vascular pericytes with complete pericyte growth media were purchased
from Science Cell and cultured in astrocyte and pericyte specific media respectively, as
described by the manufacturer. Purity and phenotype of each cell type were verified by manu-
facturer with certificate of analysis provided upon delivery. HCMEC/D3 cells cultured with
less than 12 passages from passage indicated by manufacturer were used for all experiments.
Human astrocytes and pericytes cultured with less than 8 passages from passage indicated by
manufacturer were used for all experiments.
Triculture Model on Insert: The triculture model was an adaptation of previously published
protocols from Hatherell et. al. 2011 and Stone et. al. 2019 [23,24]. In brief, transwell 12 well
cell inserts with a 0.4 μm 0.9 cm2 PET membrane (Falcon) were coated for one hour with a
50–50 mixture of 1% rat-tail Collagen Type II and 1% Poly-L-Lysine on the basal surface. The
coating was left applied for an hour before being removed by vacuum pipette. To distinguish
between the apical and basal (top and bottom) surface of the cell insert membrane, we denoted
them as follows. The membrane facing up inside the well of the insert is denoted as apical. The
membrane facing down on the outside of the insert exposed to the culture well plate is denoted
as basal.
For monocultures, endothelial cells were plated on the apical membrane at a concentration
of 300k cells; The Pericytes and Astrocytes were plated on the basal membrane at a concentra-
tion of 300k and 100k respectively. Astrocytes and pericytes plated on the basal side were sup-
plemented with additional media to a volume total 300μL. For the triculture, the basal side was
plated first, with pericytes added first and given 30min to adhere, followed by the addition of
astrocytes and another 30 min incubation to allow for adherence of the cells to the membrane.
Following the one-hour incubation, the cell inserts were inverted and placed into a 12 well cell
culture plate. For the triculture, following the inversion into the 12 well culture plate, 300K
endothelial cells were plated on the apical surface. The apical surface was supplemented with 1
mL growth media and the basal surface with 3 mL of growth media and incubated for 48
hours in a humidified chamber at 37 C with 5% CO2.
After the incubation period of 48 hours, media was removed from the culture wells and
fresh media supplemented with 5% severely patient plasma or healthy plasma was added to the
apical side of the insert and incubated for 4 or 6 hours. Cells sub-cultured for each experiment
were supplemented with media according to manufacturer protocol. Following seeding of cells
on inserts for each experiment and in order to avoid alterations in results due to differences in
media type, all experimental cell cultures were supplemented with a 1:1:1 mixture of endothe-
lial, astrocyte, and pericyte media. The severely injured patient plasma for each experimental
replicate was composed of pooled patient plasma from patients with non-TBI traumatic inju-
ries ISS> 15 and BD<-6.
Conditioned Media (CM) Model: Endothelial, astrocyte, and pericyte monocultures, and
tricultures were incubated with or without 5% severely injured patient plasma for 1 hour to
produce cell type specific conditioned media. Conditioned media in the presence and absence
of trauma plasma respectively (CM+TP; CM-TP) from each cell type was harvested. In order
to interrogate the effect of other cell types on endothelial cells that have been exposed to
severely injured patient plasma; Endothelial monocultures were supplemented with severely
injured patient plasma to 5% working concentration followed immediately by supplementa-
tion of cell type specific conditioned media (CM+TP; CM-TP) to 10% CM working
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
concentration. Naïve control endothelial cells were allowed to grow in their normal growth
media throughout the experiment in absence of conditioned media and plasma.
RNA extraction and cDNA synthesis
RNA extraction was performed using a Qiagen RNeasy extraction kit. RNA samples were con-
centrated using a speed vac, 40 C for 60 minutes, and quantified spectrophotometrically for
cDNA synthesis using a Biotek Synergy H1 microplate reader. 1.5 μg total RNA was used for
each cDNA synthesis reaction with QuantaBios qScript cDNA supermix using an adapted
form of QuantaBios protocol. 60uL reactions were performed using the following protocol run
on a Bio-Rad thermal cycler: 5 minutes at 25 C, 30 min at 42 C, 5 min at 85 C, cool down and
hold at 4 C.
qPCR
qPCR protocols were adapted and optimized from recommended Quantstudios protocols. All
RT-qPCR was performed on a QuantStudio 3 Real-Time PCR System (Applied Biosystems).
All primer probe pairs were purchased from Applied Biosystems- GAPDH (Hs99999905_m1,
Assay Location 229), CLDN3 (Hs00265816_s1, Assay Location 807), CLDN5 (Hs00533949_s1,
Assay Location 1713), and ZO-1 (Hs01551861_m1, Assay Location 1762) for TaqMan based
qPCR. The master mix used was comprised of 10uL of TaqMan fast advanced master mix
(Applied Biosystems), 1uL of combined primer probes, and 3uL of RNAse free water per reac-
tion. 6uL cDNA mix from cDNA synthesis was added to each well containing 14uL of master
mix. The 20uL reactions were run on a 96 well plate in a Quantstudios 3 RT-PCR system with
the following protocol: 2 minutes at 50 C, 10 minutes at 95 C, 20 seconds at 95 C and 1 minute
at 60 C for 40 cycles. Ct values of the gene of interest were normalized to endogenous GAPDH
control. Fold changes were produced from normalized Ct values compared to normalized
experimental controls
Whole cell lysate processing
Following the removal of the media, each cell insert was washed with 1X PBS. Cells were then
trypsinized with 0.25% Trypsin EDTA for 10 mins. The cell pellet was collected by centrifuga-
tion at 100 g for 5 mins. Pellets were washed in 1X PBS three times and suspended in 50 μL 1X
PBS. The whole cell lysate (WCL) was prepared as follows. The suspended pellets were placed
in a Diagenode Biorupter for 7.5 minutes with sustained pulse intervals, 30 second on, 30 sec-
ond off. The Lysate was then spun down at 10,000 rpm for 5 minutes. The supernatant was col-
lected, and the pellet discarded. Protein concentrations in cell lysates were measured using
Bradford assay. A standard curve was created by diluting 2mg/mL of BSA to 50, 25, 20, 10, 5,
and 2.5 ug/mL in nanopure water. Samples were diluted 1:150; 150uL of the standards and
diluted sample were added to a flat bottom 96 well plate in duplicate along with 150uL of Coo-
massie plus protein reagent and incubated at room temperature for 10 min. Optical Density
was measured at 595 nm using a Biotek Synergy H1 microplate reader. Protein concentration
was determined using a 4 parametric nonlinear regression standard curve.
ELISA
Quantification of CLDN3 and CLDN5 in whole cell lysates were performed using a sandwich
ELISA kit in accordance with the manufacturer’s protocol. (Antibodies Online Sandwich
ELISA Assay Kits for CLDN3, ABIN6954828, and CLDN5, ABIN6962352) 4ug of WCL per
sample was prepared and diluted in 200uL of PBS and added to the ELISA wells per
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
manufacturer’s protocol. Optical Density was measured using a Biotek Synergy H1 microplate
reader at 450 nm. A 4-parametric nonlinear regression standard curve was used to determine
protein concentration. The fold change was normalized against control samples.
Data analysis
GraphPad Prism 9 and Microsoft Excel 2016 were used for statistical analysis. qRT-PCR gene
expression analysis was calculated using the 2-ΔΔCt method (delta-delta Ct method). Control
values were set to a value of 1 compared to sample values following 2-ΔΔCt calculations. All data
was represented as fold change over control. Statistical analysis was performed with GraphPad
Prism 9 software using a two-way ANOVA test with Tukey’s multiple comparison post-hoc
analysis. Experimental samples for non-conditioned media runs were compared against their
own naïve controls. Experimental samples for conditioned media experiments were compared
to both their naïve control and to each condition type respectively (CM+TP; CM-TP). P values
of less than 0.05 were considered statistically significant.
Results
Severely injured patient plasma induces differential transcription
expression of CLDN3 and CLDN5
We initially examined transcriptional responses of junctional proteins in the BBB in response
to ex vivo trauma plasma. The pooled plasma used in the transcription studies had a mean ISS
of 31.2 and BD of -11.5. Taqman based quantitative RT-PCR was used to determine the tran-
scriptional expression of CLDN3, CLDN5, and ZO-1. Endothelial, astrocyte, and pericyte
monocultures, and the triculture showed no significant change in mRNA transcriptional
expression of CLDN3, CLDN5, or ZO-1 following incubation with healthy plasma (Fig 1A–
1C) Endothelial monocultures, had significant increases in transcriptional expression of tight
junction protein CLDN3 (3.98 fold increase vs. control, p<0.001) and CLDN5 (3.6 fold
increase vs. control, p<0.001) (Fig 1A and 1B) following a four hour incubation with trauma
plasma from severely injured patients. This fold increase vs. control in expression of CLDN3
was not observed in astrocyte monoculture, pericyte monoculture, or the triculture. (Fig 1A)
Similarly, CLDN5 transcriptional response also did not change vs. control values in astrocyte
monoculture, pericyte monoculture, or the triculture. (Fig 1B) ZO-1 expression did not show
any significant change vs. control for cells grown in monoculture or in the triculture model.
(Fig 1C) To further expound upon these findings, a conditioned media experiment was per-
formed to delineate the specific cell types contributing to the downregulation of the tight junc-
tion proteins in the triculture vs. the endothelial monoculture.
Addition of astrocyte, pericyte, or triculture conditioned media suppress
endothelial monoculture transcriptional upregulation of CLDN3 and
CLDN5
A conditioned media exchange study was performed in order to delineate the specific cell
types of the neuro vascular unit contributing to transcriptional downregulation of CLDN3 and
CLDN5 observed in the triculture model and whether that contribution is due to the release of
soluble factors from those cells. (Fig 1A and 1B). The conditioned media exchange studies
comprised of the following experimental group. Endothelial monocultures were incubated
with conditioned media from a) endothelial monoculture (ECM+), b) astrocytes monoculture
(ACM+), c) pericyte monoculture (PCM+) or d) Triculture (TCM+) in the presence or
absence severely injured patient plasma. (CM+TP; CM-TP). The transcriptional
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Fig 1. CLDN5, CLDN3, and ZO-1 mRNA expression following a 4 hour incubation with a working concentration
of 5% plasma from healthy patients (HP) or 5% Trauma Plasma from severely injured patients (TP). (A) CLDN3
mRNA expression: The Endothelial monoculture incubated with TP showed a 3.98-fold increase vs. control, p<0.001,
n = 6; Astrocyte monocultures, Pericyte monocultures, and the triculture did not show any significant change vs.
control. All cell cultures incubated with HP showed no significant change vs. control. (B) CLDN5 mRNA expression:
Endothelial monoculture showed a 3.51-fold increase vs. control, p<0.001, n = 6; Astrocyte monocultures, Pericyte
monocultures, and the triculture did not show any significant fold change vs. control. All cell cultures incubated with
HP showed no significant change vs. control. (C) ZO-1 mRNA expression: ZO-1 mRNA expression did not change
significantly vs. control for either the monoculture or tricultures, n = 5. All cell cultures incubated with HP showed no
significant change vs. control. Six experimental replicates were performed n = 6. �� Denotes p<0.01 ��� Denotes
p<0.001.
https://doi.org/10.1371/journal.pone.0270817.g001
downregulation of CLDN3 and CLDN5 seen in the triculture model was also observed in
endothelial monocultures following transfer of astrocyte and pericyte conditioned media. The
endothelial monoculture which received ECM+/CM+TP; showed increased expression of
CLDN3 (4.91-fold increase vs. control, p<0.01) and CLDN5 (2.94-fold increase vs. control,
p<0.0001) (Fig 2A and 2B). Endothelial monocultures receiving CM-TP: ECM+, ACM+,
PCM+, or TCM+ did not show any significant increase in mRNA expression for CLDN3 or
CLDN5. (Fig 2A and 2B) Similarly, endothelial monocultures receiving CM+TP: ACM+,
PCM+, or TCM+ did not show any significant increase in mRNA expression for CLDN3 and
CLDN5. (Fig 2A and 2B) Endothelial monocultures that received either CM+TP or CM-TP:
ACM+, PCM+ or TCM+ did not show any significant change in the expression of ZO-1 when
compared to controls (Fig 2C).
Severely injured patient plasma induces differential protein expression of
CLDN5
In order to determine if the changes in the transcriptional response of the tight junction pro-
teins CLDN3 and CLDN5 in the endothelial monoculture and triculture were also reflected in
protein translation, sandwich ELISAs were performed. The pooled plasma used in the transla-
tion studies had a mean ISS of 35.6 and BD of -18.36. The protein expression of CLDN3 and
CLDN5 were assayed in whole cell lysate. The Endothelial monocultures show a significant
increase in CLDN5 protein expression following both a 4 hour incubation (5.12 fold increase
vs. control, p<0.001) and 6 hour incubation (2.58 fold change vs. control, p<0.01) with 5%
severely injured patient plasma. (Fig 3B) We observed no significant fold changes occurred for
CLDN5 protein expression in the Triculture, nor were there any significant fold changes in
CLDN3 protein expression for either the endothelial monoculture or triculture (Fig 3A and
3B).
Addition of astrocyte or pericyte conditioned media suppress endothelial
monoculture protein upregulation of CLDN5
The translation of CLDN3 and CLDN5 proteins from the mRNA transcripts from endothelial
monocultures following conditioned media exchange was assessed from whole cell lysate. The
conditioned media exchange studies had the same experimental setup as the previous media
exchange experiment (Fig 2) We observed no significant difference between the condition
types or time points for any tested sample for CLDN3 protein expression. (Fig 4A) Endothelial
monocultures receiving 6 hour ECM+/CM+TP presented with a significant increase in
CLDN5 protein expression compared to endothelial monocultures receiving 6Hr ACM+/CM
+TP (4.67 vs 3.03-fold change vs. control respectively, p<0.01) and 6Hr PCM+/CM+TP (4.67
vs. 3.26-fold change vs. control respectively, p< 0.05). (Fig 4B) Endothelial monocultures
receiving 6Hr TCM+/CM+TP presented with a significant increase in CLDN5 protein
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Fig 2. CLDN3, CLDN5, and ZO-1 mRNA expression in endothelial cell monocultures following a 4 hour
incubation with cell type specific conditioned media. Endothelial Monoculture Conditioned Media, (ECM+),
Astrocyte Monoculture Conditioned Media, (ACM+), Pericyte Monoculture Conditioned Media, (PCM+), and
Triculture Conditioned Media, (TCM+), were prepared by either a 1-hour incubation with 5% severely injured patient
plasma (CM+TP) or left as is with no plasma addition (CM-TP). All conditioned media was added to endothelial cell
monocultures activated with severely injured patient plasma to a working concentration of 5%. Final working
concentration of severely injured patient plasma and conditioned media in endothelial monocultures receiving
conditioned media was 5% and 10% respectively. Each condition was compared to an endothelial cell monoculture
naïve control (CM+TP—and CM-TP -). CM+TP wells were compared against one another to determine the
differential effect each cell type’s conditioned media had. (A) CLDN3 mRNA expression: The endothelial monoculture
receiving ECM+/CM+TP showed a 4.96-fold change increase vs. control, p<0.01, n = 2; ECM+/CM+TP produced a
significant fold change increase in CLDN3 mRNA expression compared to ACM+/ CM+TP, (p<0.01), PCM+/CM
+TP, (p<0.01), and TCM+/CM+TP, (p<0.01). Endothelial monocultures receiving ACM+, PCM+, or TCM+ did not
show significant fold change vs. control for either CM-TP or CM+TP. Additionally, endothelial monocultures
receiving ECM+/CM-TP showed no significant change in transcription. (B) CLDN5 mRNA expression: The
Endothelial monoculture ECM+/CM+TP showed a 2.94-fold change increase vs. control, p<0.0001, n = 2; ECM+/CM
+TP produced a significant fold change increase in CLDN5 mRNA expression compared to ACM+/CM+TP,
(p<0.0001), PCM+/CM+TP, (p<0.0001), and TCM+/CM+TP (p<0.0001). Endothelial monocultures receiving ACM
+, PCM+, or TCM+ did not show significant fold change vs. control for either CM+TP or CM-TP. Endothelial
monocultures receiving ECM+/CM-TP showed no significant change in transcription. (C) ZO-1 mRNA expression:
Endothelial monocultures receiving ACM+, ECM+, PCM+, or TCM+ did not show significant fold change vs. control
for either CM+TP or CM-TP. Two experimental replicates of the conditioned media experiment were performed,
n = 2. �� Denotes p<0.01. ���� Denotes p<0.0001.
https://doi.org/10.1371/journal.pone.0270817.g002
expression compared to endothelial monocultures receiving 6Hr ACM+/CM+TP (5.48 vs
3.03-fold change vs. control respectively, p<0.001) and 6Hr PCM+/CM+TP (5.48 vs. 3.26-fold
change vs. control respectively, p<0.001). (Fig 4B) We observed no significant difference
between the condition types or time points for any other tested sample (Fig 4B).
Discussion
Tight junction proteins are central to the maintenance of BBB integrity and increases in BBB
permeability have a significant impact on patient outcomes [4,6]. BBB dysfunction has had lit-
tle inquiry in the context of non-TBI critically injured patients despite their presentation of
clinical sequela of brain injury. We demonstrate here that ex vivo plasma from severely injured
non-TBI patients causes BBB breakdown accompanied by transcriptional and translational
responses of central tight junction proteins of the neurovascular unit required to maintain
high resistance across the BBB. Our data shows that the causes of non-TBI injured patient’s
presentation of BBB dysfunction center on the expression, or lack thereof, of tight junction
proteins key to BBB permeability.
CLDN3, CLDN5, and ZO-1 are central to regulation of BBB permeability [16,17,25–29],
and are essential in maintaining barrier integrity [7–9]. Loss of expression of these tight junc-
tion proteins compromises barrier integrity by increasing transcellular permeability across the
vasculature. Claudin 5 is implicated as a singular driver in multiple disease states including
some neuroinflammation states [17] and plays major roles in disease states from carcinomas
to schizophrenia and other endothelial barrier dysfunctions [25–27,30]. Following traumatic
injuries, an overall downregulation and dysfunction of CLDN5 is observed, coupled directly
with BBB permeability dysfunction [4,28,31–33]. There is also evidence that increased exoge-
nous CLDN5 expression may increase the tightness of the junctions [8,33].
This made CLDN5 the primary target of interest for the transcriptional regulation among
the three main cell types of the BBB following incubation with severely injured patient plasma.
The importance of Claudin 3 as a major tight junction protein and its role in BBB permeability
is not yet clearly defined [16,34]. However, because CLDN3 is present in the tight junction of
brain endothelial cells and CLDN3’s definitive role in the BBB remains unclear, CLDN3 was
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Fig 3. CLDN5, and CLDN3 protein expression following a 4- or 6-hour incubation with 5% plasma from severely
injured patients. (A) CLDN3 protein expression: We observed no significant change in fold change of CLDN3 expression
was observed in either the endothelial monoculture or triculture, n = 2 (B) CLDN5 protein expression: Endothelial
monoculture showed a 5.12- and 2.58-fold change increase vs. control following 4 and 6 hours of incubation respectively
with 5% severely injured plasma. p<0.001, p<0.05, n = 2; There is a significant difference between the fold change at 4
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
hours of incubation compared to 6 hours of incubation following 5% severely injured patient plasma in the endothelial
monoculture, p<0.01. The triculture did not show any significant fold change vs. control. One ELISA was performed with
the combined whole cell lysate of three experimental replicates, n = 2. � Denotes p<0.05, �� Denotes p<0.01, ��� Denotes
p<0.001.
https://doi.org/10.1371/journal.pone.0270817.g003
chosen as an ideal candidate to focus on in an in vitro traumatic injury model. Finally, ZO-1
was analyzed because of its central role in the anchoring of tight junction proteins to the actin
filaments. ZO-1’s central role in facilitating foundational structure to tight junctions, coupled
with BBB permeability disruptions following its re-localization or degradation from MMPs fol-
lowing injury, made ZO-1 an ideal candidate to include in this study [29,35].
Our studies using an in vitro triculture model suggest inhibitory signals from astrocytes and
pericytes induce transcriptional downregulation of CLDN3 and CLDN5 and translational
downregulation of CLDN5 in brain endothelial cells within four hours of exposure to severely
injured patient plasma. (Figs 1A, 1B and 4B) Furthermore, our observations suggest that astro-
cytes and pericytes in separate monocultures release sufficient soluble factors to elicit inhibi-
tion of tight junction expression in endothelial cells. These observations were upheld when
endothelial cell monocultures were treated with conditioned media from monocultures of
astrocyte or pericytes incubated with 5% severely injured patient plasma. (Figs 2A, 2B, 4A and
4B) The observed inhibition of the tight junction proteins within the triculture following incu-
bation for four hours in severely injured patient plasma is specific to the transcription of
CLDN3 and CLDN5 and the translation of CLDN5. We also demonstrate that the use of a
healthy plasma negative controls made no significant impact on tight junction expression
compared to our naïve control. Due to the insignificance of our healthy plasma’s impact on
tight junction expression, the decision was made, with both experimental design and cost in
mind, to exclude this condition from future trials. Likewise, ZO-1 showed no differences in
transcriptional response (Figs 1C and 2C), leading to its exclusion from translation studies.
Finally, CLDN3 expression did not show a robust translational response, as seen with CLDN5,
compared to the observed transcriptional changes. (Fig 3A). This does not necessarily decrease
the possibility of astrocyte or pericyte derived initial inhibitory signaling leading to decreased
endothelial cell tight junction expression; but could support literature that suggests CLDN3
plays a less significant role in restoration and/or maintenance of BBB permeability than that of
other tight junction proteins [36].
We observed a robust and significant transcriptional and translational expression of
CLDN5 in endothelial monocultures following incubation with severely injured patient
plasma which was not observed in tricultures incubated with severely injured plasma. The
finding that plasma from severely injured patients downregulates expression of critical tight
junction proteins within a triculture model is compelling and is suggestive of a critical role
that pericytes and astrocytes play in normal physiological responses to severe traumatic injury
via initiation of inhibitory cross talk between the three cell types. This inhibition of CLDN5
expression is further supported by conditioned media experiments. Endothelial monocultures
which received conditioned media from astrocytes and pericytes following 6 hours of incuba-
tion were observed to have significantly less CLDN5 protein expression compared to endothe-
lial monocultures receiving conditioned media from endothelial monocultures or the
triculture. A possible explanation for the inhibition of CLDN5 protein expression in endothe-
lial monocultures receiving astrocyte conditioned media that was not observed in endothelial
monocultures receiving triculture conditioned media is unmitigated release of Angiopoietin 2
(ANG2) by activated astrocytes. ANG2, a soluble mediator and a marker of endothelial dys-
function [37], is present in copious amounts in trauma plasma and is associated with worse
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Fig 4. CLDN3 and CLDN5 protein expression in endothelial cell monocultures following a transfer of conditioned. Endothelial
Monoculture Conditioned Media, (ECM+), Astrocyte Monoculture Conditioned Media, (ACM+), Pericyte Monoculture Conditioned
Media, (PCM+), and Triculture Conditioned Media, (TCM+), were prepared by either a 1 hour incubation with 5% severely injured patient
PLOS ONE | https://doi.org/10.1371/journal.pone.0270817 July 5, 2022
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
plasma (CM+TP) or left as is with no plasma additions made (CM-TP). Final working concentration of severely injured patient plasma and
conditioned media in endothelial monocultures receiving conditioned media was 5% and 10% respectively. Each condition was compared to
its Naïve control (CM+TP—and CM-TP -) receiving no conditioned media and no severely injured patient plasma to produce fold changes.
CM+TP were compared against one another to determine the differential effect amongst the conditioned media types. Both 4 and 6 hour
incubation times were assessed. � Denotes p<0.05, �� Denotes p<0.01, ��� Denotes p<0.001 (9) CLDN3 protein expression: Endothelial
monocultures did not present with any significant changes in CLDN3 protein expression for all conditions. (10) CLDN5 protein expression:
All conditions tested resulted in a significant increase in the fold change of CLDN5 expression compared to the naïve control (not shown).
The Endothelial monoculture receiving 6Hr ECM+/CM+TP showed a significant increase in fold change vs. control compared to 6Hr ACM
+/CM+TP (p<0.01), and 6Hr PCM+/CM+TP (p<0.05) n = 2; The endothelial monoculture receiving 6Hr TCM+/CM+TP showed a
significant increase in fold change compared to 6Hr ACM+/CM+TP (p<0.001), and 6Hr PCM+/CM+TP (p<0.001) n = 2.
https://doi.org/10.1371/journal.pone.0270817.g004
clinical outcomes through destabilization of endothelial barriers, increasing leakage and
edema. ANG2 acts as an antagonist to Angiopoietin 1 (ANG1) for its primary ligand, the
receptor tyrosine kinase TIE2 [38]. TIE2 activation leads to dimerization and auto-phosphory-
lation of its intracellular domain, subsequently activating the PI3K/AKT pathway [39–41].
Inactivation of the PI3K/AKT pathway via ANG2 leads to downregulation of CLDN5 expres-
sion through recruitment of a repressor complex to CLDN5’s silencer binding domain [41,42].
While this mechanism is understood in the context of TBI and other pathologies, it is
unknown what mechanism causes BBB dysfunction due to non-TBI trauma; but it is sugges-
tive from literature that increased ANG2 expression following injury does contribute in some
capacity. This underlying mechanism for the observed CLDN5 transcriptional and transla-
tional results will be analyzed in follow-up studies probing the causal link, if any, between
ANG2 signaling and non-TBI trauma related BBB dysfunction.
Conclusion and future direction
Clinical observations by physicians treating non-TBI traumatically injured patients suggest
TBI like BBB dysfunction occurs regardless of injury location but correlates directly with level
of injury and shock; reinforcing the need to understand the underpinnings BBB dysfunction
[43]. Our findings lend a possible explanation for non-brain injury trauma presenting with
similar symptoms to TBI by affecting the expression of tight junction central to maintenance
of BBB integrity within the neurovascular unit.
Permeability increases following neuroinflammatory stimuli [44,45]. An emerging body of
evidence suggests pathology after TBI is driven by alterations in cellular crosstalk between
endothelial cells astrocytes and pericytes [46] via the release of multiple biomolecules of inter-
est including angiopoietin 2, endothelin-1, tumor necrosis factor- alpha, and matrix metallo-
protease-9 [33,46–50]. The impact of these molecular mediators and proteases on tight
junction expression following trauma, and the subsequent role this modulation of tight junc-
tion expression has on BBB permeability, is not yet fully understood.
Our data is consistent with a physiological process designed to prevent infection and maxi-
mize repair of cellular structures within the interneuronal space [8,15,50–53]. Delaying BBB
permeability restoration through the inhibition of tight junction proteins activate physiological
responses to neuroinflammatory factors such as TNF-alpha, IL1-B, IL-6 and other cytokines
released during injury and allows for the localization and infiltration of leukocytes across the
barrier through ICAM and VCAM dependent processes [51,52]. This infiltration of leukocytes
coupled with increased flow of other soluble factors between the vasculature and interneuronal
space may provide better neuronal repair following minor brain injuries [8,15,51]. However,
when the injury is more severe, this delay in endothelial barrier restoration prevents neuro-
logic recovery and may lead to secondary brain injury. Our results, while observed in a physio-
logical presentation closer to that of the BBB than other in vitro models, requires future in vivo
work to strengthen these findings. This is due to inherent limitations of in vitro studies such as
PLOS ONE | https://doi.org/10.1371/journal.pone.0270817 July 5, 2022
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
lack of shear stress flow, absence of glial cells the complexities of the microenvironment the
cells of the neurovascular unit encounter [54,55]. We do believe, however, that isolating the
cells in vitro gives a better controlled environment to probe the exact cellular response to spe-
cific conditions introduced via the severely injured patient plasma that cannot be controlled
for in vivo [55].
The underlying molecular mechanisms leading to increased blood brain permeability fol-
lowing traumatic injury are poorly enumerated and a comprehensive understanding of this
process would provide novel approaches in designing future interventions to prevent the
adverse effects of secondary brain injury. The findings in this study will guide our future work
to include transcriptional and translational analysis of other tight junction proteins central to
BBB permeability and identify soluble factors in plasma that cause perturbations in tight junc-
tion transcription and translation. We expect that enhanced understanding of this astrocyte,
pericyte, and endothelial cellular crosstalk will help identify new therapies for BBB pathologies
following trauma and will provide useful insight in designing specific interventional strategies
in patients with severe BBB dysfunction while creating a path forward for implementation of
these therapies in personalized healthcare for traumatically injured patients.
Supporting information
S1 Data. Underlying data and statistics used to generate Fig 1.
(XLSX)
S2 Data. Underlying data and statistics used to generate Fig 2.
(XLSX)
S3 Data. Underlying data and statistics used to generate Fig 3.
(XLSX)
S4 Data. Underlying data and statistics used to generate Fig 4.
(XLSX)
Acknowledgments
We thank the University Of Colorado Department Of Surgery, the division of GITES, and the
Trauma research lab team for facilitating our research. The content is solely the responsibility
of the authors and does not necessarily represent the official views of the National Institutes of
Health or other sponsors of the project.
Author Contributions
Conceptualization: Sanchayita Mitra, Margot Debot, Arthur Stem, Mitchell J. Cohen.
Data curation: Preston Stafford, Sanchayita Mitra, Margot Debot, Patrick Lutz, Arthur Stem.
Formal analysis: Preston Stafford, Sanchayita Mitra, Mitchell J. Cohen.
Funding acquisition: Mitchell J. Cohen.
Investigation: Preston Stafford, Sanchayita Mitra, Margot Debot, Patrick Lutz, Arthur Stem,
Mitchell J. Cohen.
Methodology: Preston Stafford, Sanchayita Mitra.
Project administration: Preston Stafford, Sanchayita Mitra, Margot Debot, Mitchell J. Cohen.
Resources: Preston Stafford, Sanchayita Mitra, Arthur Stem, Patrick Hom, Mitchell J. Cohen.
PLOS ONE | https://doi.org/10.1371/journal.pone.0270817 July 5, 2022
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PLOS ONEEndothelial cell tight junction expression in non-TBI trauma
Software: Preston Stafford, Patrick Lutz.
Supervision: Sanchayita Mitra, Margot Debot, Mitchell J. Cohen.
Validation: Preston Stafford, Sanchayita Mitra, Mitchell J. Cohen.
Visualization: Preston Stafford, Mitchell J. Cohen.
Writing – original draft: Preston Stafford, Sanchayita Mitra, Mitchell J. Cohen.
Writing – review & editing: Preston Stafford, Sanchayita Mitra, Margot Debot, Patrick Lutz,
Arthur Stem, Jamie Hadley, Patrick Hom, Terry R. Schaid, Mitchell J. Cohen.
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10.1371_journal.pntd.0011435.pdf
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Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
|
All relevant data are within the manuscript and its Supporting Information files.
|
RESEARCH ARTICLE
Spatiotemporal bayesian modelling of
scorpionism and its risk factors in the state of
São Paulo, Brazil
Francisco Chiaravalloti-Neto1, Camila LorenzID
Salomão de Azevedo2, Denise Maria Caˆ ndido3, Luciano Jose´ Eloy4, Fan Hui Wen3,
Marta Blangiardo5, Monica Pirani5
1*, Alec Brian Lacerda1, Thiago
1 School of Public Health, University of São Paulo, São Paulo, Brazil, 2 Health Department of the
Municipality of Santa Ba´rbara d’Oeste, São Paulo, Brazil, 3 Instituto Butantan, São Paulo, Brazil,
4 Epidemiological Surveillance Center “Prof. Alexandre Vranjac”, São Paulo, Brazil, 5 MRC Centre for
Environment & Health, Department of Epidemiology and Biostatistics, School of Public Health, Imperial
College London, London, United Kingdom
* [email protected]
Abstract
Background
Scorpion stings in Brazil represent a major public health problem due to their incidence and
their potential ability to lead to severe and often fatal clinical outcomes. A better understand-
ing of scorpionism determinants is essential for a precise comprehension of accident
dynamics and to guide public policy. Our study is the first to model the spatio-temporal vari-
ability of scorpionism across municipalities in São Paulo (SP) and to investigate its relation-
ship with demographic, socioeconomic, environmental, and climatic variables.
Methodology
This ecological study analyzed secondary data on scorpion envenomation in SP from 2008
to 2021, using the Integrated Nested Laplace Approximation (INLA) to perform Bayesian
inference for detection of areas and periods with the most suitable conditions for
scorpionism.
Principal findings
From the spring of 2008 to 2021, the relative risk (RR) increased eight times in SP, from
0.47 (95%CI 0.43–0.51) to 3.57 (95%CI 3.36–3.78), although there has been an apparent
stabilization since 2019. The western, northern, and northwestern parts of SP showed
higher risks; overall, there was a 13% decrease in scorpionism during winters. Among the
covariates considered, an increase of one standard deviation in the Gini index, which cap-
tures income inequality, was associated with a 11% increase in scorpion envenomation.
Maximum temperatures were also associated with scorpionism, with risks doubling for tem-
peratures above 36˚C. Relative humidity displayed a nonlinear association, with a 50%
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OPEN ACCESS
Citation: Chiaravalloti-Neto F, Lorenz C, Lacerda
AB, de Azevedo TS, Caˆndido DM, Eloy LJ, et al.
(2023) Spatiotemporal bayesian modelling of
scorpionism and its risk factors in the state of São
Paulo, Brazil. PLoS Negl Trop Dis 17(6): e0011435.
https://doi.org/10.1371/journal.pntd.0011435
Editor: Marcelo Larami Santoro, Instituto Butantan,
BRAZIL
Received: November 11, 2022
Accepted: June 5, 2023
Published: June 20, 2023
Copyright: © 2023 Chiaravalloti-Neto et al. This is
an open access article distributed under the terms
of the Creative Commons Attribution License,
which permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
Funding: F.C.N., C.L., M.P., M.B were in part
supported by the Wellcome Trust Seed Award in
Science 217362_Z_19_Z. M.P. and M.B. are partly
supported by the MRC Centre for Environment and
Health, which is funded by the Medical Research
Council (MR/S019669/1, 2019–2024). The funders
had no role in study design, data collection and
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
analysis, decision to publish, or preparation of the
manuscript.
increase in risk for 30–32% humidity and reached a minimum of 0.63 RR for 75–76%
humidity.
Competing interests: The authors declare no
competing interests.
Conclusions
Higher temperatures, lower humidity, and social inequalities were associated with a higher
risk of scorpionism in SP municipalities. By capturing local and temporal relationships
across space and time, authorities can design more effective strategies that adhere to local
and temporal considerations.
Author summary
Scorpions cause approximately 3,000 deaths every year worldwide, second only to snake-
bites in terms of venomous animal-related human accidents due to the potency of their
toxins. Natural habitat extinction combined with accelerated urban expansion have pro-
moted greater contact between humans and many species of wild animals, and conse-
quently increased the number of accidents and fatalities caused by these animals.
Recognizing which environmental conditions are most associated with accident hotspots
can help design better strategies for the control and distribution of anti-scorpion serum,
for example. Here we show that scorpionism occurred more frequently in spring, and it
decreased in winter. The areas with higher temperatures, lower humidity, and social
inequalities were also associated with a higher risk of scorpion envenomation.
Introduction
Accidents caused by venomous animals pose a threat to public health. Among these accidents,
those provoked by scorpions are becoming more frequent in many parts of the world. Owing
to their high incidence and lethality, these fatalities are currently considered a global medical-
sanitary threat [1]. Approximately two billion people live in regions at risk for scorpion stings
worldwide, and each year an estimated 1.2 million people are victims of scorpion stings. Due
to the potency of their toxins, scorpions cause approximately 3,000 deaths every year, second
only to snakebites in terms of venomous animal-related human accidents [2]. Clinical out-
comes caused by scorpion toxins are known as scorpionism or scorpion envenomation [1,3].
Over the last decade, there has been a wide increase in scorpionism reports, particularly in
Brazil, representing a leading cause of fatalities caused by venomous animals in the country
[3]. The main genus of scorpions of medical importance found in Brazil is Tityus, and Tityus
serrulatus (yellow scorpion) is the most dangerous species, causing many fatalities [4]. Due to
its high and rapid proliferation–and good adaptation to urban environments–it is usually
found in all Brazilian regions [4]. Tityus serrulatus are typically nocturnal creatures and can be
located in a variety of habitats, including rock crevices, tree bark, decaying tree trunks, beneath
rocks and leaves, and within burrows and caves. The presence and abundance of these scorpi-
ons in a given area is largely dependent on factors such as temperature, humidity, and the
availability of prey [4].
Natural habitat extinction combined with accelerated urban expansion have promoted
greater contact between humans and wild animals of many species, and consequently
increased the number of accidents and fatalities caused by scorpions [1,5,6]. Recognition of
priority areas for public health actions is crucial for effective decision-making in conditions
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
that require allocation of limited resources (e.g., anti-scorpion serum and patient healthcare).
Spatial analysis is a powerful tool that can be used to integrate many types of information, eval-
uate cost-benefit of interventions, and aid evidence-based decision-making in well-defined
regions [7].
We previously conducted a descriptive study of scorpionism in the state of São Paulo (SP)
[8], where we described the epidemiological profiles of accidents and deaths in all municipali-
ties of the state from 2008 to 2018. In a second study, we compared the demographic, environ-
mental, and climatic characteristics of the higher- and lower-risk areas in SP for scorpion
envenomation [9]. In the present study, we (i) modeled the spatio-temporal variability of scor-
pionism across 645 municipalities in SP for the period of 2008–2021 using a Bayesian hierar-
chical approach and (ii) investigated its relationship with demographic, socioeconomic,
environmental, and climatic variables. Our hypothesis is that the incidence of scorpionism in a
specific region is greatly influenced by both socio-economic and climatic factors.
Methods
Ethics statement
The present study was developed using secondary data provided by the CVE (Secretary of
Health of the state of São Paulo). The data form which was anonymized without names or
addresses, and scorpion accidents were aggregated by municipality and year. The protocol for
the present study was submitted for approval by the institutional ethics review board of the
University of São Paulo School of Public Health (COEP FSP/USP, CAAE approval record
10457119.6.0000.5421, protocol number 3408558) and no consent was required because we
used anonymized secondary data.
Study area and data collection
São Paulo is located in the southeastern region of Brazil, has an area of around 248,210 km2, a
population of around 45 million (21.8% of Brazil’s population), and accounts for 33.5% of the
national GDP. The median monthly income per capita in 2021 was around US$ 348, but there
is huge variation between municipalities reflecting the inequalities present in the state. Cur-
rently, 19.28% of the state is covered with native vegetation remnants [10], which mainly
include the Cerrado biome and the Atlantic Forest. Eucalyptus plantations, sugar cane, and
cattle pastures are the predominant agricultural land uses, accounting for approximately 39%
of the state’s area [11]. Regarding the climate, São Paulo has three major climate zones. The
Koppen-Ginger classification categorizes the western plateau as having a tropical climate
(Aw), marked by wet summers and dry winters. The high-altitude regions of the Atlantic pla-
teau and basaltic cuestas have a high-altitude tropical climate (Cwa and Cwb), featuring hot
summers and cold winters. The low-lying coastal area has a humid tropical climate (Af), with
hot and humid conditions throughout the year. Rainfall varies, with an average of 1,600 mm
on the south coast and 2,700 mm on the north coast.
We analyzed data on scorpionism reported between 2008 and 2021 in 645 municipalities of
SP which are aggregated into 17 Regional Health Departments (RHDs) (Fig 1). Data on con-
firmed cases of scorpion envenomation were obtained from notification forms in the Notifi-
able Diseases Information System (SINAN) of each municipality, available from the Centre for
Epidemiological Surveillance of SP. The data were analyzed in aggregated form (by municipal-
ity), so the confidential and specific information of each patient was not accessed, and the con-
sent to data processing was not needed.
Geographic and demographic information on the municipalities were obtained from the
Brazilian Institute of Geography and Statistics (IBGE), including the shape files of the study
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Fig 1. Study area. (A) State of São Paulo in Brazil and South America and (B) municipalities and Regional Health Departments (RHDs)
of the state of São Paulo. Base layer of map: https://www.ibge.gov.br/geociencias/organizacao-do-territorio/malhas-territoriais/
15774-malhas.html?=&t=acesso-ao-produto.
https://doi.org/10.1371/journal.pntd.0011435.g001
regions. In terms of environmental variables, we chose those most related to the scorpion life
cycle according to previous literature [12,13]: total precipitation (mm), percentage of natural
vegetation [14], maximum temperatures (˚C), relative humidity (%), and intensity of El Niño-
Southern Oscillation (ENSO). We also considered seasonal effects for each year, as reported by
previous studies [8,9]. Since there are specific population groups that are more vulnerable to
scorpionism, especially those living in poorer areas [15], we also included the following socio-
economic indicators (Fig 2): the Gini index [16], the Human Development Index (HDI) [17],
the percentage of rural population, water supply, and garbage collection. The Gini coefficient
captures income inequality, and HDI is a composite index that includes three elements:
Fig 2. Conceptual framework of scorpionism. We used variables related to human populations (green arrow) and
scorpion populations (blue arrow). Not all were included in the statistical models due to collinearity. See the text for
details. Source of icons: https://openclipart.org/.
https://doi.org/10.1371/journal.pntd.0011435.g002
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standard of living, life expectancy, and literacy level. All municipality-scale socioeconomic
covariates were obtained from the IBGE [18]; the latest available data are for 2010 (last census),
and will be used in this analysis to represent of the entire period under study. Environmental
and climatic variables were obtained for each season (4) and year (14) combination from the
DAEE [19], ESALQ-USP [20], CIIAGRO [21] and IAC [22]. In this study we defined the sea-
sons as follows: "summer" in the months of January- March; "autumn" in the months of April
—June; "winter" in July-September; and "spring" in October—December. To access the vari-
ables for each season of the year, we aggregated maximum temperatures based on hourly
reports, and considered the average of relative humidity.
Data analysis
The observed data on scorpion envenomations were aggregated by sex and age for each season
in each year and municipality. Then, using indirect standardization and the entire study region
over the whole period as a reference population, we obtained the expected values after adjust-
ing for age and sex (S1 Appendix). The ratio of observed to expected values represents the
standardized morbidity ratio (SMR) for each municipality, season, and year. An exploratory
analysis was performed to identify outliers, evaluate the relationship between the response var-
iable (log of SMR) and covariates, and assess potential collinearity among covariates. To do so,
we calculated the variance inflation factors (VIF) for a multilinear regression. In accordance
with Zuur et al. [23], we considered a VIF larger than 3.00 to indicate collinearity. All continu-
ous covariates were standardized by subtracting the mean and dividing by the standard
deviation.
Our response variable was the observed number of accidents in each municipality, season,
and year, which we considered to be Poisson distributed. We included the log of the expected
values in these models as an offset; therefore, the estimates were expressed as log relative risks
(RR). On the log RR, we first specified a disease mapping model to estimate the spatiotemporal
variability in scorpion envenomations for the 645 municipalities and each season over the 14
years considered. The model included a global intercept and spatial, temporal, and spatiotem-
poral random effects. As a second step, we ran an ecological regression model, including
covariates, while retaining the random effects. The models were framed from a Bayesian per-
spective, and statistical inference was performed using the Integrated Nested Laplace Approxi-
mation approach (INLA) [24]. The priors for fixed effects (regression coefficients and global
intercept) were specified as Gaussian, centered on 0, and with a large variance (minimally
informative). The spatial random effects were modeled flexibly to allow for local dependency
(i.e., municipalities sharing borders were more likely to be similar) as well as global similarities
(i.e., all the municipalities were considered similar as part of the study region) letting the data
inform about the relative weight of the two [25]. We used the Queen contiguity weight matrix
to represent the local dependency among the municipalities. Similarly, on the temporal ran-
dom effects, we specified the combination of (i) a structured random effect modeled as ran-
dom walk of order one (RW1), two (RW2), or autoregressive of order one (AR1), and (ii) an
unstructured random effect assuming similarities across all time points in the study. For com-
parison, we additionally evaluated a parametric trend for the temporal component of the mod-
els [26]. Finally, we considered an unstructured spatio-temporal interaction, assuming
similarity in space and time after accounting for the main spatial and temporal effects (type I
interaction effect) [27]. The priors on the precision of the random effects were specified fol-
lowing Simpson et al. [28], who suggested penalizing model complexity. In particular, they
penalized departure from the base model (assuming a constant RR over all areas or time
points, i.e., no spatial or temporal variation). We considered all the covariate effects to be
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
linear, except for precipitation and relative humidity, on which we assumed nonlinear rela-
tionships with the response variable through RW1 and RW2. As the relationship between tem-
perature and the response variable was less clear, we considered both linear and nonlinear
effects (through RW1 and RW2 models). We calculated the Watanabe Akaike Information
Criteria (WAIC) for models comparison, choosing the models with the lowest WAIC values
[29]. The detailed mathematical specification of the Bayesian hierarchical models is presented
in the S1—Mathematical specification of the Bayesian hierarchical models. We ran our models
using the R program version 4.2.1 [30] and R-INLA version 22.05.07 [31]. We made the data-
base (S1 Database), the map of the municipalities of state of São Paulo (S1 Map) and the final
code (S1 Code) of our modelling approach available for the readers.
Results
The incidence rate of scorpion envenomation in SP was 30.8 cases per 100,000 inhabitant-
years over the entire period from 2008 to 2021, corresponding to more than 250 thousand
notifications in SINAN. Among the covariates, there were outliers in the percentages of natural
vegetation and rural population, and these were transformed using the square root. Maximum
temperature, total precipitation, and relative humidity did not exhibit a linear relationship
with the response variable. Using VIF and considering all covariates, we obtained values for
total precipitation, relative humidity, and seasonal effects above 3.00, which is the suggested
threshold to check for collinearity. While removing relative humidity did not improve the
VIF, we found that removing total precipitation resulted in a VIF value below 3; therefore, we
used this model specification.
First, we ran the disease-mapping model, and the one with the lowest WAIC is the model
with RW1 as the structured temporal random effect (S1 Table). We then compared the follow-
ing ecological regression models: (i) including all covariates and assuming that relative humid-
ity has a nonlinear relationship with the response variable; (ii) including all covariates and
assuming that relative humidity and maximum temperature have a nonlinear relationship
with the response variable. The model that presented the lowest WAIC (our final ecological
regression) was the one with the maximum temperature and relative humidity modeled as
nonlinear through a RW1, and RW2 as the structured temporal random effect (S1 Table).
The curve of the disease mapping model shows that the relative risk of occurrence of scor-
pion envenomation increased continually from 2008 to 2021 in SP, rising from 0.47 (95%CI
0.43–0.51) to 3.57 (95%CI 3.36–3.78) in natural scale; in general, higher values were seen in
spring and lower values (except in 2021) in autumn (Fig 3). The temporal curve of the final
ecological regression model presented lower values and a larger 95%CI than the curve of the
disease-mapping model. This shows that part of the temporal variability in scorpionism was
explained by the considered covariates.
Considering the disease-mapping model and the entire period, the values showed large spa-
tial variability across the municipalities of SP, ranging from 0.0 to 18.9, with some presenting
the risk of occurrence of scorpionism almost 19 times the average value for the entire period
and state (Fig 4A). Municipalities with higher risks were located in the western, northern, and
northwestern parts of SP. The most affected RHDs were Arac¸atuba, Barretos, Franca, Presi-
dente Prudente, and São Jose´ do Rio Preto (Figs 1 and 4).
Our ecological regression model suggests that the spring and summer seasons are risk fac-
tors, showing a 31% and 23% increase in scorpionism risk compared to autumn, respectively
(Table 1). In winter, we saw a decrease of 13% in scorpion envenomations. On the other hand,
natural vegetation and ENSO showed no significant association with accidents. Higher tem-
peratures and lower humidity were associated with a higher risk of accidents (Fig 5). In
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Fig 3. Temporal relative risks. Posterior means and 95% credible intervals of the temporal relative risks (natural
scale) obtained from the temporal random effects of (A) the disease mapping model and (B) of the full model for the
occurrence of scorpion envenomation in the state of São Paulo by season from 2008 to 2021.
https://doi.org/10.1371/journal.pntd.0011435.g003
particular, for temperatures between 18 and 27˚C, the RR ranged between 0.73 (95%CI 0.56–
0.94) and 0.93 (95%CI 0.88–0.98), respectively. For higher temperatures, from 34 to 37˚C, RR
ranged from 1.09 (95%CI 1.02–1.16) to 1.84 (95%CI 1.42–2.25), respectively. Relative humidity
increasing from 27 to 33% corresponded to an increase in accident risks ranging from 1.26
(95%CI 1.01–1.56) to 1.66 (95%CI 1.45–1.89). However, for relative humidity between 34 and
84%, the RR decreased steadily from 1.54 (95%CI 1.36–1.73) to 0.71 (95%CI 0.54–0.93). S1
and S2 Figs show relative humidity and maximum temperatures in each municipality of the
state of São Paulo, respectively.
The Gini index was the only socioeconomic variable that showed a positive association with
the response variable, with an increase of one standard deviation corresponding to a 11%
increase in accident risk. The purpose of the Gini index is to demonstrate the distribution of
income within an economy (S3 Fig). In other words, a higher value of the Gini index indicates
a higher degree of economic inequality in a given area. Other factors considered in our model
such as rural population, HDI, water supply, and garbage collection, showed no significant
association with accidents (Table 1).
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Fig 4. Spatial relative risks. Posterior means of the spatial relative risks (natural scale) obtained from the spatial
random effects of (A) the disease mapping model and (B) the final full model for the occurrence of scorpion
envenomation in the municipalities of São Paulo, 2008–2021. The municipalities are grouped by Regional Health
Departments (see Fig 1). Base layer of map: https://www.ibge.gov.br/geociencias/organizacao-do-territorio/malhas-
territoriais/15774-malhas.html?=&t=acesso-ao-produto.
https://doi.org/10.1371/journal.pntd.0011435.g004
Our data revealed a significant increase in the risk of scorpion accidents between 2008 and
2021, with spring being the season with the highest risk, and the western, northwestern, and
northern regions of SP being the most affected areas (Fig 6). Regarding the predicted RR for
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Table 1. Final full ecological regression models. Posterior means and 95% credible intervals (95% CI) of the relative
risks (RR) for the covariates (in natural scale) in the disease mapping and final full ecological regression models for the
occurrence of scorpion envenomation, municipalities of the state of São Paulo, 2008 to 2021. The cells in bold indicate
that the 95% CI do not include the null risk of one.
Intercept/Covariates
Disease mapping model + random effects
Full model
Intercept
Seasons
ENSO–El Niño Southern Oscillation
Square root of the natural vegetation
Square root of the rural population
Municipal human development index
Gini index
Percentage of population with water supply
Percentage of population with garbage collection
https://doi.org/10.1371/journal.pntd.0011435.t001
Autumn
Winter
Summer
Spring
Neutral
El Niño strong
El Niño moderate
El Niño weak
La Niña weak
La Niña moderate
La Niña strong
RR
0.73
0.68
1.00
0.87
1.23
1.31
1.00
1.06
0.89
0.95
1.14
1.02
1.14
0.96
0.98
0.95
1.11
1.03
1.06
95% CI
0.69–0.76
0.62–0.75
-
0.78–0.97
1.09–1.39
1.16–1.48
-
0.83–1.33
0.75–1.05
0.85–1.06
0.99–1.30
0.80–1.28
0.94–1.36
0.88–1.06
0.91–1.06
0.87–1.04
1.01–1.23
0.94–1.13
0.98–1.14
the full model considering each municipality, we showed that the risk for scorpionism
increased in almost all municipalities of SP from 2008 to 2021 (Fig 7).
Discussion
SINAN reported a progressive increase in the number of notifications related to venomous
animals each year in Brazil [6,32]. Our analysis revealed an upward trend in the number of
scorpion accidents in SP: from the spring of 2008 to 2021, the RR increased eight times. This
growth trend is corroborated by studies in other states of the Brazilian southeast [33] and
northeast regions [34,35]. It is unknown how much of the increase can be attributed to
improvements in the notification system, but also reveals the worrisome affinity of many scor-
pion species for anthropically modified environments, which consequently enhanced their
contact with humans [2,36]. Unplanned urban growth, indiscriminate use of natural resources,
industrialization, and ecological imbalance induce the spread of some venomous species and
increase the overlap between the areas used by these animals and humans [35,37]. Amado
et al. [36] confirmed the affinity of T. serrulatus for altered environments, as its modeled distri-
bution was highly correlated with human population density in Brazil. The preference of this
species for anthropically modified environments appears to be related to its parthenogenetic
reproduction, as this type of reproduction is frequent in uniform environments such as urban
areas [12]. The growth of cities can also be considered as another determining factor in the
proliferation of scorpions, as the generation and accumulation of debris play a key role in the
availability of breeding habitats, as they facilitate the spread of cockroaches and other insects
that are food sources.
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Fig 5. Relationship of scorpionism with environmental variables. Posterior relative risks (natural scale) representing
the relationship among the (A) maximum temperature and (B) relative humidity with the scorpion accidents,
municipalities of state of São Paulo, Brazil, 2008 to 2021.
https://doi.org/10.1371/journal.pntd.0011435.g005
Our results showed that RR increased continually from 2008 to 2018; however, since 2019,
there has been an apparent stabilization in the number of cases. One hypothesis that can be
raised is the underreporting of cases due to the COVID-19 pandemic, especially in 2020 and
2021 [38]. The efforts of epidemiological surveillance teams in several municipalities likely
concentrated on facing the COVID-19 pandemic and this may have been reflected in the scor-
pion survey. Moreover, as people spent more time at home, they took more time to care and
clean their backyards, reducing ideal habitats for scorpions.
Our findings showed that the spring and summer seasons were the periods with the highest
risks, showing, respectively, 26% and 19% increase in scorpion accident risks compared to
autumn, while winter showed a decrease of 13% in accident risks. The state of SP is located in
a tropical area, and the four seasons of the year are not well defined. Nevertheless, spring tends
to exhibit high temperatures. Related findings have been reported in other Brazilian regions
[39,40] and other countries [41–44] where scorpion envenomation have been more recurrent
during the hotter seasons of the year. However, some studies [1] indicate constant scorpion
incidents over all months of the year, which could be explained by ideal climatic conditions
and abundant food throughout the year [1,9]. The results from our model suggest that maxi-
mum temperature is a risk factor, whereas relative humidity is a protective factor against the
occurrence of scorpion accidents. Comparing Figs 4 and 6 with S1 and S2 Figs, it can be seen
that the regions with higher scorpion accident rates are the same regions with higher maxi-
mum temperatures and lower relative humidity. Scorpions are animals that are extremely
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Fig 6. Predicted spatial relative risks. Posterior means of the predicted relative risks in natural scale obtained from the ecological
regression model, for the four seasons of 2008 and 2021, municipalities of the state of São Paulo. The municipalities are grouped by
Regional Health Departments (see Fig 1). Base layer of map: https://www.ibge.gov.br/geociencias/organizacao-do-territorio/
malhas-territoriais/15774-malhas.html?=&t=acesso-ao-produto.
https://doi.org/10.1371/journal.pntd.0011435.g006
adaptable to extreme environmental conditions [45], and they are particularly abundant in
desert areas [46].
Municipalities with higher risk were located in the western, northern, and northwestern
parts of SP. The most affected RHDs were Arac¸atuba, Barretos, Franca, Presidente Prudente,
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Fig 7. Predicted temporal relative risks. Posterior means of the predicted relative risk in natural scale of the full
ecological regression model for the municipalities of the state of São Paulo presented in boxplot for each season and
year from 2008 to 2021.
https://doi.org/10.1371/journal.pntd.0011435.g007
and São Jose´ do Rio Preto, as described previously [8,9]. Other studies have also implicated
these areas as hotspots of medical concern for scorpionism [47]. All higher-risk regions
detected here also exhibited higher temperatures and lower precipitation, corroborating our
model. Our findings are also in agreement with those of Moradiasl et al. [48] and Amado et al.
[36], who reported that air temperature and precipitation are key variables that influence the
distribution of scorpions in Iran and Brazil, respectively. Rafinejad et al. [49] analyzed climatic
variations as crucial elements for the presence of scorpions in Iran, as well as their stimulated
life cycle and maturation. Many other studies carried out in Brazil reveal that climatic variables
play a key role in the distribution of scorpions and, consequently, accidents [50]. Given the
increasing effects of climate change and deforestation on Brazilian biomes, it is expected that
the scorpion population will grow and lead to a surge in scorpionism, unless measures are
taken to prevent it from happening [50]. To address this issue, the implementation of educa-
tional programs for the prevention and treatment of scorpion envenomation, aimed at com-
munity members and health workers, could be a valuable public health policy to help reduce
the rising number of incidents.
In our model, the Gini index showed a positive association with scorpionism since an
increase of one standard deviation represented a 13% increase in accident risk. The higher the
Gini coefficient, the greater the income inequality, and consequently, the greater the differ-
ences in housing conditions and access to healthcare. Poor housing conditions and a lack of
basic sanitation were found to be associated with an increase in scorpionism in Sergipe, Brazil
[51]. The association between areas with many accidents and those with high vulnerability has
also been reported by Almeida et al. [15]. A higher incidence of scorpion stings usually appears
in regions of older and crowded favelas, characterized by inefficient basic sanitation and other
substandard housing conditions [52]. Many municipalities located in the western, northern,
and northwestern parts of the SP experienced a verticalization process and increased urbaniza-
tion which has led to more suitable anthropogenic habitat for scorpions.
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
The other variables investigated (including ENSO, natural vegetation, rural population,
water supply, garbage collection, and HDI) did not show evidence of an association with acci-
dent risks. However, caution is needed when interpreting these results, as this is an ecological
study, and an ecological fallacy could have been induced. Commonly, the “ecological fallacy”
refers to the error of assuming that statistical relationships at a group level also hold for indi-
viduals in the group [53]. However, it is increasingly recognized that population-level research
plays a crucial role in characterizing the most important public health issues to be tackled and
in developing hypotheses regarding their probable causes. Future studies of the individual risk
for scorpionism, such as cohort or case-control approaches, should be carried out to confirm
whether the variables analyzed here are actual risk factors.
Regarding the limitations of this study, we highlight the use of secondary data, which has
already been collected and recorded by health facilities. A possible bias could be associated with
this issue; the notification system is susceptible to underreporting, which can show incidences
lower than real occurrence numbers. Nevertheless, the system has been enhanced over time,
which is reflected in both the increase in notifications and in the requirements for health care [2].
Despite these limitations, the SINAN database of the Brazilian Ministry of Health was considered
to be an essential tool for conducting several epidemiological studies, given that it is the official sys-
tem for reporting diseases and health accidents in Brazil [1]. In addition, the socioeconomic vari-
ables used in our analysis are outdated; to date, there is only information from the last census
carried out by the IBGE in 2010. Another limitation is that we did not consider data regarding the
infestation and abundance of scorpion species in SP. However, this is the first study to evaluate the
relationship with potential risk factors while accounting for spatiotemporal dependencies over
such a long period. In addition, spatiotemporal data naturally accounts for residual confounding
[54]. Our study offers a methodology for investigation that can provide indications of geographical
areas and seasonal periods that are at higher risk for accidents, thereby generating insights for the
development of effective intervention strategies, which should be locally and temporally targeted.
The Brazilian Ministry of Health [4] suggests taking preventive measures such as keeping
households and surrounding areas clean and free of piled up waste, sealing doors, windows,
drains, and walls to prevent scorpions from entering homes, checking shoes and clothing
before putting them on, removing potential scorpion prey like cockroaches, and attracting nat-
ural scorpion predators like birds, lizards, and frogs [50]. Effective control of scorpions, similar
to the control of mosquito-borne diseases, requires active participation and engagement from
communities. Both scorpions and mosquitoes are urban pests that affect households, and les-
sons can be learned from existing vector control strategies. However, engaging communities is
a complex process that goes beyond simply sharing information. It is essential to involve stake-
holders in decision making and actions, and to have a deep understanding of the socio-politi-
cal, economic, and cultural context of the communities to be able to establish meaningful
dialogue and develop successful plans that foster community involvement [50].
Our results indicate that reducing the negative effects of scorpion envenomation requires a
collaborative effort from multiple teams, such as environmental management, healthcare
workers, researchers, and the general public. Destoumieux-Garzo´n et al. [55] have stated that
the concepts of human health, public health, sanitation, environmental health, and animal
health are interrelated. This accumulated knowledge gave rise to the term "one health," recog-
nizing that human activities and factors connected to socioeconomic and environmental cir-
cumstances can worsen the occurrences of diseases, fatalities, and injuries, particularly among
vulnerable populations. Therefore, further in-depth studies that focus on individuals are neces-
sary to gain a better understanding of the epidemiology of scorpion incidents, and this infor-
mation can be used to develop educational healthcare measures to enhance the support
provided to those who have been affected.
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Conclusion
This study showed that the RR of scorpion accidents in the state of São Paulo increased eight-
fold between 2008 and 2021. Municipalities with higher risk were in the western, northern,
and northwestern parts of the SP. The most affected RHDs were Arac¸atuba, Barretos, Franca,
Presidente Prudente and São Jose´ do Rio Preto. Accidents occurred more frequently in spring,
while winter showed a protective effect, with a decrease of 13% in accident risk. In our model,
the Gini index, which captures income inequality, shows a positive association with scorpion-
ism. Other variables such as higher temperatures and lower humidity were also associated with
a higher risk of accidents in the municipalities, corroborating what has been found in other
surveys in Brazil and other countries. Future studies based on individual scorpionism, such as
cohort or case-control approaches, should be conducted to confirm whether the variables ana-
lyzed here are actual risk factors. Additionally, future studies also should address issues related
to the process of land occupation as well as the economic functions performed by municipali-
ties and relate them to socio-environmental determinants.
Supporting information
S1 Appendix. Mathematical specification of the Bayesian hierarchical model.
(DOCX)
S1 Database. Database on confirmed cases of scorpion envenomation from 2008 to 2021
obtained from notification forms in the Notifiable Diseases Information System (SINAN),
available from the Centre for Epidemiological Surveillance of SP.
(XLSX)
S1 Map. Map of the municipalities of the state of São Paulo. Please open with some Geo-
graphic Information System software.
(ZIP)
S1 Code. Final code of our modelling approach used in R software.
(R)
S1 Table. WAIC values of the disease mapping model and two possible full ecological
regression models. We considered three types of structured temporal random effects—RW1,
RW2, and AR1, and two types of non-linear effects for the climatic covariates: RW1 and RW2.
(DOCX)
S1 Fig. Maximum temperatures. Maximum temperatures of the municipalities of the state of
Sao Paulo by seasons, for 2008 and 2021. Base layer of map: https://www.ibge.gov.br/
geociencias/organizacao-do-territorio/malhas-territoriais/15774-malhas.html?=&t=acesso-ao-
produto.
(PNG)
S2 Fig. Relative humidity. Relative humidity of the municipalities of the state of Sao Paulo by
seasons, for 2008 and 2021. Base layer of map: https://www.ibge.gov.br/geociencias/
organizacao-do-territorio/malhas-territoriais/15774-malhas.html?=&t=acesso-ao-produto.
(PNG)
S3 Fig. Gini index. Gini index of the municipalities of the state of Sao Paulo, 2010. Base layer
of map: https://www.ibge.gov.br/geociencias/organizacao-do-territorio/malhas-territoriais/
15774-malhas.html?=&t=acesso-ao-produto.
(PNG)
PLOS Neglected Tropical Diseases | https://doi.org/10.1371/journal.pntd.0011435 June 20, 2023
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PLOS NEGLECTED TROPICAL DISEASESScorpionism and its risk factors
Author Contributions
Conceptualization: Francisco Chiaravalloti-Neto.
Data curation: Camila Lorenz, Luciano Jose´ Eloy.
Formal analysis: Francisco Chiaravalloti-Neto.
Funding acquisition: Monica Pirani.
Investigation: Francisco Chiaravalloti-Neto, Camila Lorenz, Alec Brian Lacerda, Thiago
Salomão de Azevedo, Denise Maria Caˆndido.
Methodology: Francisco Chiaravalloti-Neto, Marta Blangiardo, Monica Pirani.
Software: Francisco Chiaravalloti-Neto.
Supervision: Francisco Chiaravalloti-Neto.
Validation: Camila Lorenz, Monica Pirani.
Visualization: Francisco Chiaravalloti-Neto, Camila Lorenz, Alec Brian Lacerda, Thiago
Salomão de Azevedo, Denise Maria Caˆndido, Luciano Jose´ Eloy, Fan Hui Wen, Marta
Blangiardo, Monica Pirani.
Writing – original draft: Camila Lorenz.
Writing – review & editing: Francisco Chiaravalloti-Neto, Alec Brian Lacerda, Thiago
Salomão de Azevedo, Denise Maria Caˆndido, Luciano Jose´ Eloy, Fan Hui Wen, Marta
Blangiardo, Monica Pirani.
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PLOS NEGLECTED TROPICAL DISEASES
| null |
10.1088_1402-4896_ad03bf.pdf
|
Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is
sufficiently accessible or reusable by other researchers. The data that support the findings of this study are
available upon reasonable request from the authors.
|
Data availability statement The data cannot be made publicly available upon publication because they are not available in a format that is sufficiently accessible or reusable by other researchers. The data that support the findings of this study are available upon reasonable request from the authors.
|
Phys. Scr. 98 (2023) 115042
https://doi.org/10.1088/1402-4896/ad03bf
PAPER
RECEIVED
9 May 2023
ACCEPTED FOR PUBLICATION
16 October 2023
PUBLISHED
30 October 2023
Recognizing and correcting for errors in frequency-dependent
modulation spectroscopy
, H Aarnio2 and R Österbacka1
N M Wilson1
1 Physics, Faculty of Science and Engineering, Åbo Akademi University, Henriksgatan 2, 20500 Turku, Finland
2 Mathematics and physics, Centria University of Applied Sciences, Bondegatan 2, 67100 Karleby, Finland
E-mail: Ronald.Osterbacka@abo.fi
Keywords: photomodulation spectroscopy, modulation spectroscopy, photoinduced absorption, continuous-wave photoinduced
absorption, phase-sensitive measurements, pump-prope spectroscopy, organic semiconductors
Supplementary material for this article is available online
Abstract
This work shows how to acquire reliable data from frequency dependent continuous-wave
modulation spectroscopy. We demonstrate this through the example of continuous-wave photo-
induced absorption (cwPA), a characterization technique useful for studying long-lived photoexcita-
tions in thin-film solar cell materials. Experimental errors arising at moderate frequencies in
modulation spectroscopy are identified and corrected for. Limitations of the detectors and electronics
are seen to cause both signal loss and phase shifts. Imperfect charge collection in the detector leads to
wavelength-dependent correction factors, while phase shifts caused by the experimental setup call for
frequency-dependent corrections. The methods outlined in this work act as a guide to avoid pitfalls in
setting up modulation spectroscopy measurements and correcting for limitations.
1. Introduction
Continuous-wave modulation spectroscopies can be used to observe reflectance and absorbance to gain insight into
material properties and the dynamics of long-lived photoexcitations [1–3]. These spectroscopic techniques all use a
periodically varying modulation, such as light or electric field, to probe the properties of a material while measuring a
spectroscopic response. The modulation will lead to a periodicity in the measured signal. This enables the use of lock-in
techniques, which have the capacity to single out minimal signals as long as they repeat at a specified frequency. Thereby
we can eliminate random noise and get a very sensitive measurement. Changing the modulation frequency can be used
to probe material dynamics but care must be taken when working with higher frequencies, as made evident in this work.
Continuous-wave photoinduced absorption (cwPA) is a measurement technique that provides information
about the recombination and generation of long-lived excitations in thin films of organic semiconductors [4–7].
The measurement is relatively simple to its setup and provides information about processes in the bulk of the
material without being overshadowed by contact effects. The measurement only requires a film of the active
material to enable characterization of the material’s photophysical properties.
Here we discuss problems arising in modulation spectroscopies, exemplified by using cwPA, and provide
tools for correcting them. The setup is described in section 2. Section 3 discusses handling background
disturbances in the setup. In section 4 we discuss problems caused by the limitations of detectors and how these
can be identified. Section 5 explores the impact of phase shifts. Lastly, section 6 demonstrates how all the
previously discussed effects impact two example data sets.
2. Continuous-wave photoinduced absorption
The cwPA setup consists of a pump light, which generates photoexcitations in the sample, and a probe light, for
which the change in transmission due to the photoexcitations is measured. Figure 1 shows a sketch of the setup.
© 2023 IOP Publishing Ltd
Phys. Scr. 98 (2023) 115042
N M Wilson et al
Figure 1. Schematic of the experimental cwPA setup. (Licensed as CC BY 4.0, N M Wilson (2022), https://doi.org/10.6084/m9.
figshare.19086518.v1).
The pump light is modulated through an acousto-optic modulator or mechanical chopper, and focused on the
sample. In our setup a 514 nm laser beam is modulated into light with sinusoidally varying intensity. The sample
is a thin film of a material, e.g. one used as the active layer in solar cells known to efficiently generate free charge
carriers. The modulation function (sine wave) is controlled by a function generator. Note that throughout this
article when discussing ‘frequency’ we refer to the modulation frequency with which the pump light intensity
varies.
As a probe light, we use a broad spectrum tungsten lamp with constant intensity, cut off by a long-pass filter
to prevent generating unwanted background excitations in the sample. Some of this light will be absorbed by the
sample, while the transmitted part will be guided into a monochromator and collected by a detector. The
detected signal goes via a pre-amplifier to a lock-in amplifier (Stanford Research Systems SR830 DSP), which
analyzes the signal in relation to the modulation frequency. The lock-in amplifier only records the components
of the signal that have the same frequency as the modulation. This means that the lock-in amplifier only
measures the transmission change caused by the pump light (D ). The results are given as in-phase, which has
the same frequency and phase as the pump, and quadrature, which has the same frequency but is phase-shifted
by π/2. For example, if the pump varies as
when the transmission is analyzed as a Fourier series. Similarly, the quadrature is the amplitude of the
cos
wt
There will always be phase shifts in the setup in actual measurements, which must be considered to
the in-phase will be the amplitude of the
-component.
-component
sin
sin
wt
wt
determine the true in-phase and quadrature. The initially recorded data will be two values, phase 1 and phase 2,
phase-shifted with respect to each other by π/2. Obtaining in-phase and quadrature from these is not always
trivial, which is discussed in section 5.
By blocking the probe light and repeating the measurement, we obtain the luminescence signal, which is
subtracted from the transmission values to determine the actual change D . Finally, the transmission change is
normalized to the transmission without the influence of the pump light ( ) to cancel effects from the setup
geometry, detector efficiencies and optical losses. To measure with the lock-in amplifier a modulation is
temporarily applied to the probe light, using a mechanical chopper.
For a thin film the normalized change in transmission due to the photogenerated excitations (-D / ) is
directly proportional to the density (n) of long-lived excitations absorbing at that particular probe wavelength.
Using this, photoinduced absorption is defined as [3]
=
PA
= -
s
n d,
D
where σ is the absorption cross-section and d is the film thickness. Instead of working with the in-phase (PAI)
and quadrature (PAQ) component of -D / , we often look at the radius signal
2 . The
Q
radius can be calculated from any two components shifted with π/2 to each other, meaning we can use the two
raw components from the measurement, phase 1 and phase 2. The radius can sometimes be a more reliable
source of information than PAI and PAQ as it is not affected by errors in the phase. However, if the phase can be
reliably determined, PAI and PAQ contain more information and give the benefit of extracting parameters from
fitting two curves instead of one.
2
PA
I
PA
R
PA
=
+
( )
1
One way to gain information about the photoexcitation dynamics from PAR is to look at its frequency
dependence, typically plotted on a log-log scale. When approaching low frequencies the curve saturates to a
2
Phys. Scr. 98 (2023) 115042
N M Wilson et al
Figure 2. Background signal separated into the two raw phases. Data for two silicon detectors and a germanium detector. One dataset
was recorded without the laser turned on, revealing that this is not luminescence. The solid line is a polynomial fit to all data points.
Figure 3. Illustration of how a sinusoidal signal is distorted by a slow detector response. The dotted line shows a perfect, infinitely fast
response and the solid line shows how the signal can be modified by a slow detector.
3
Phys. Scr. 98 (2023) 115042
N M Wilson et al
Figure 4. Response of silicon detectors to square wave modulated laser diodes with different wavelengths. The radius as recorded by
the lock-in amplifier for detector 1 (a) and detector 2 (b). Response for detector 2 fitted with (3).
Figure 5. Response of silicon detectors to square wave modulated laser diodes with different wavelengths. The signal as recorded by
oscilloscope for detector 1 (a) and detector 2 (b) at 100 000 Hz. The scale equates 0 and 1 with the square wave response at a low
frequency, where the detector response is sufficiently fast.
4
Phys. Scr. 98 (2023) 115042
N M Wilson et al
value of Gτσd, where G is the generation rate, τ is the lifetime of the photoexcitations, σ is the cross-section and d
is the thickness of the film [4, 8]. Around the point defined by ωτ = 1 the curve starts to drop, until it reaches a
linear slope, ideally of ω−1. For some materials the slope can be less steep, but still close to ω−1 [7, 9]. This is
assigned to dispersive phenomena originating in relaxation processes. At high enough frequencies, the value of
PAR will be directly proportional to the generation and independent of the lifetime (for non-dispersive
materials). The behavior is the same regardless of the waveform of the modulation. In section 6 we illustrate how
different experimental errors impact this part of an PA(ω)-plot. If we instead wish to study recombination we
can use square wave modulation and look at the low-frequency quadrature to access the reaction order or the
intensity dependence to access the bimolecular recombination constant [7, 8].
3. Background disturbance
Using a lock-in amplifier to isolate a specific frequency effectively minimizes noise, as noise with any other
frequency than the modulation will be eliminated. Nonetheless, the risks of picking up background interference
need to be addressed. When measuring the luminescence we observe a small but distinguishable frequency-
dependent signal that becomes more prominent as the frequency increases. This signal remains when repeating
the exact measurement with the pump light turned off in our setup. Therefore we conclude that this is not
luminescence or any other optical signal, but some interference originating internally in the setup. This signal
has a clear frequency dependence, as illustrated in figure 2. Here we plot the two raw phases phase 1 and phase 2.
As the signal does not seem to depend on the detector, we use one polynomial fit for all curves.
4. Detector response
All detectors are limited by the time it takes to react to an incoming signal. This can lead to a loss of amplitude for
a modulated signal, as the rise and decay times make peaks and valleys less sharp. As the lock-in amplifier gives
values proportional to the amplitude this causes signal loss. An example is illustrated in figure 3. The figure also
shows the phase shift caused by the detector, discussed in section 5.
Here we look at two silicon detectors, which we call detector 1 and detector 2. Detector 1 is a silicon
photodiode (model AR-989A) by AME AS, specified to have a rise time of 16 ns and an area of 33 mm2. The
second photodiode, detector 2 is model UV-50 by UDT Sensors Inc., with a rise time of 3.5 μs at 254 nm and an
area of 50 mm2. We characterize the response times of the detectors by measuring their response to diode lasers
of different wavelengths, with built-in modulation forming a square wave. As seen by the lock-in amplifier, the
loss of radius signal (amplitude) at the different wavelengths can be seen in figure 4. Both detectors display a
decrease at higher frequencies but with different wavelength dependencies. Detector 1 shows a drastic decrease
in signal amplitude at longer wavelengths, while the response of detector 2 remains almost unchanged. The
shape of these curves is linear in intensity over several orders of magnitudes, as shown in the supplementary
material, figure S1. The wavelength-dependent behavior is probably caused by longer wavelengths penetrating
deeper into the detector. This can lead to carrier generation outside the depletion region, meaning there will be
slow charge collection as carriers first need to be transported by diffusion [10]. Following the same logic, the lack
of wavelength dependence of detector 2 can be explained by a wider depletion region. The slow response of
detector 2 at all wavelengths could indicate a smaller electric field than that in the depletion region in detector 1.
Figure 5 shows the response of the two detecors to the diode lasers as measured in an oscilloscope. For detector 1
we clearly see how the shorter wavelengths are closer to a square wave while the longer wavelengths are more
deformed and have a smaller amplitude. The data for 730 nm deviates clearly and shows a substantial overshoot.
This is an artifact arising from an overshoot in the modulation shape created by the diode laser and is not an
effect of the detector. As expected detector 2 displays a much weaker wavelength dependence but a more
substantial deviation from a square wave.
Ideally, we would have a detector that is so fast that these problems do not occur, but the second-best option
is to correct it afterwards, given that we can calibrate for the frequency response of the detector. A detector such
as detector 1 will give reliable results at short wavelengths, whereas correcting at longer wavelengths requires
calibration measurements close to the wavelength at which we want to study the photoexcitations. On the other
hand, a detector such as detector 2 will need to be corrected at a wide range of wavelengths, but the same
calibration curve can be used for a large part of this spectrum.
For this kind of correction to be possible, the detector’s response can not be dependent on the shape of the
incoming signal. Otherwise, we could get a situation where we use a particular shape to determine the correction
factor, but the actual shape from D acts differently. As the lock-in amplifier only measures first harmonic
components, we need the first harmonic components coming into the detector to give first harmonics coming
out. This means that none of the information is lost. This will indeed be the case as delaying a sinusoidal signal
5
Phys. Scr. 98 (2023) 115042
N M Wilson et al
Figure 6. Phase shift of modulated pump light measured with detector 2 at f < 10000 Hz.
will never cause a change in its frequency, only phase (a longer argument for this can be found in the
supplementary material).
As the percentual loss of the radius signal depends neither on intensity (figure S1) nor modulation shape we
can use it as a correction factor to ascertain the actual D values. If we assume that the signal decay of the
detector has an exponential form we can calculate the mathematical form of the loss. The calculations can be
found in the supplementary material, and give the signal amplitude at time t from an sine-shaped input as
( )
A t
=
R
0
-
1
(
w
wt
cos
d
2
wt
(
)
d
-
sin
t
+
1
w
t
)
2
⎜
⎛
⎝
+
⎟
⎞
1 .
⎠
Here the time τd describes the electrical response of the detector and R0 is the peak generation rate. Looking
solely at the time-dependent parts gives a signal described by wt
d
proportional to
. The radius will be
-t
cos
sin
w
t
w
N
0
(
(
wt
d
wt
d
2
)
2
)
+
+
1
1
=
(
N
0
)
2
wt
d
.
+
1
( )
2
( )
3
Fitting the radius as a function of frequency with (3) gives excellent agreement for detector 2, as seen in figure 4b.
A combined fit to all the curves gives a value of τd = 2.4μs (figure S2), while individual fits range between 2.15
and 2.90 μs, with 980 nm giving a somewhat longer τd than the others. For detector 1 a reasonable fit is
impossible, as the dynamics involving diffusion outside the depletion layer can not be captured in a simple
exponential.
5. Phase shift
The lock-in amplifier will compare the signal from the detector with a specific frequency and phase. The
function generator uses these same parameters to create a sine wave sent as a reference to the acousto-optic
modulator. This ensures that we measure the transmission change only at the chosen frequency. However, there
will be some shifts in timing as the sine wave passes through the setup. Firstly, there might be a constant phase
shift θ0 between the signal from the lock-in amplifier and the sine wave realized by the acoustic-optic modulator.
This arises when signal is conveyed from the lock-in amplifier to the function generator, and from the function
generator to acousto-optic modulator. It depends on the triggering method and how each piece of equipment
contructs the signal that is conveyed to the next step. Secondly, we can expect a frequency-independent time
delay δt due to delays in the measurement setup electronics. This will shift a pure sine wave to
)
.
Thirdly, the distortion of the signal caused by a slow photodetector will also delay the timing of the wave. This
can be seen in equation (2) and figure 3, where detector 2 has shifted
phase shifts can be corrected for in measurement data as long as we have a good mathematical description
of them.
. All these
d-t
(
t
to wt
d
-t
cos
sin
sin
sin
wt
w
t
w
w
The phase describes the timing of a periodic signal and is defined as
arctan
)Q I
(
, where I and Q are the in-
phase and quadrature signal, respectively. For the signal distorted by the detector, the phase becomes
6
Phys. Scr. 98 (2023) 115042
N M Wilson et al
Figure 7. Example of how the corrections impact two datasets when measuring cwPA. The raw datasets (D ) and background
disturbance, for (a) P3HT:ICBA measured with detector 1 (T=140 K, λ=985 nm) and (b) PT7B:PCBM measured with detector 2
(T=90 K, λ=1050 nm). The inset shows the same data at high frequencies, where the background is the largest. Calculated PAR before
and after subtracting the background disturbance for the same datasets, detector 1 in (c) and detector 2 in (d). For detector 2 PAI and
PAQ, resulting from phase correcting PA1 and PA2, is shown. For both detectors the impact of correcting for signal loss due to the
detector response is shown.
wt
d (as the sine component here forms the in-phase). The total phase shift describing the difference in
-arctan
timing between a peak in the sinusoidal signal used as reference by the lock-in amplifier and the signal going into
the lock-in amplifier from the photodetector will thus for detector 2 be q
wd
0
we do not have an analytical approximation and thus no calculated phase shift.
d. For detector 1
arctan
-t
wt
-
We need to know all phase shifts to separate the PA-signal into quadrature and in-phase reliably. When the
frequency is low compared to the detector response, i.e., ωτd = 1 and
d, the phase shift for
detector 2 will be θ0 − ωδt − ωτd. This linear frequency dependence is confirmed in figure 6. Here we have
measured the pump light at 514 nm modulated to a sine wave (without any probe light) and calculated the phase.
As we do not know the true in-phase, the phase is given in relation to phase 1. In an ideal situation, the phase
would be constant regardless of frequency. The linear slope corresponds to δt + τd = 3.9μs. Subtracting
τd = 2.4μs from the fit in figure 4 gives δt = 1.5μs. This is the delay caused by electronics in the setup. The
intercept at ω = 0 corresponds to a constant phase shift θ0 = 0.33.
arctan
wt»
wt
d
When all parameters governing the phase shift are known we can correct the data. To do this we calculate the
d, resulting in a corrected
.
phase of the signal (as PA1 and PA2) and subtract the phase shift q
0
phase θ. The signals are then calculated as
and
wd
-t
-
=PA
Q
arctan
PA sin
R
PA cos
R
wt
( )q
For detector 1 the same procedure is not possible, as we do not have a function that fits the signal radius loss. We
conclude that for detector 1 we can not draw any reliable conclusions about the phase, and therefore phase separated
data is unattainable at higher frequencies. However, PAR is not affected by phase shifts and can still be analyzed.
= -
PA
I
( )q
6. Impact on cwPA results
We have discussed several factors that introduce problems during the measurement. Here we will demonstrate
how to correct for these in two actual datasets, acquired from performing cwPA on polymer:fullerene blends.
7
Phys. Scr. 98 (2023) 115042
N M Wilson et al
Analysis of the corrected datasets is published concurrently in [11]. The first example is measured at 985nm on a
1:1 blend of P3HT:ICBA (poly(3-hexylthiophene): indene-C60 bisadduct) with detector 1 and the second is
measured with detector 2 at 1050nm on a film of PTB7:PCBM (Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-
b:4,5-b’]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] :[6,6]-phenyl-
C61-butyric acid methyl ester) in a 1:1 blend (with DIO). In figure 7a-b we see the raw PA data, divided into
phase 1 and phase 2. The solid line marks the background signal for the two phases. PAR is calculated after
subtracting this background, resulting in the data in figure 7c-d. These are then corrected for the percentual
detector loss of the same form as shown in figure 4. For detector 2 we use a fit of the loss at 980 nm with (3), seen
in figure 4 and giving τd = 2.90 μs. Correcting a measurement at 1050nm without calibration data exceeding
980 nm is not ideal, as we can not confirm if the detector develops a wavelength dependence above 980 nm. We
use it here for illustrative purposes to correct this dataset, measured before the detector limitations were
discovered. For detector 1 we fit the response at 980 nm with a piecewise function of three lines linear in the log-
log plot. The corrections change the PAR values more at higher frequencies, which is the regime that can be used
to determine the generation rate G. Do note how, in both cases, the slope changes from an unphysical slope
< − 1 to a less steep one. Without this correction PAR(ω) fits are impossible as the underlying theory does not
allow such slopes. It can also be seen how subtracting the background gives a smoother and more linear slope.
For detector 2 we can use the phase shift discussed above and correct PA1 and PA2 to PAI and PAQ.
7. Conclusions
Frequency-dependent modulation spectroscopy offers valuable tools for material characterization. We have
used cwPA as an example to demonstrate how to achieve reliable results, even at high frequencies. The most
significant source of error is the photodetector, and users must be aware of the limitations of the specific detector
in their lab. Changes in the frequency response at long wavelengths are important to identify. Ideally, the user
has access to a detector with high enough speed and sensitivity at the desired wavelength, but we show how the
detector’s limitations can be corrected for afterwards. We also emphasize the importance of handling phase
shifts with care, as they have many components, some of which are frequency and wavelength-dependent.
Acknowledgments
Personal grants from Alfred Kordelin Foundation, Fortum and Neste Foundation, Svenska Litteratursällskapet i
Finland and The Swedish Cultural Foundation in Finland are acknowledged by N.M.W.
Data availability statement
The data cannot be made publicly available upon publication because they are not available in a format that is
sufficiently accessible or reusable by other researchers. The data that support the findings of this study are
available upon reasonable request from the authors.
ORCID iDs
N M Wilson
https://orcid.org/0000-0002-7320-9097
References
[1] Cardona M 1970 Modulation Spectroscopy of Semiconductors (Friedr. Vieweg + Sohn GmbH, Verlag)
[2] Pollak F H and Shen H 1993 Materials Science and Engineering: R: Reports R10 275–374
[3] Brabec C J and Dyakonov V 2003 Photoinduced charge transfer in bulk heterojunction composites Organic Photovoltaics, Concepts and
Realization ed C J Brabec et al (Berlin: Springer) 1–56 ch 1
[4] Botta C, Luzzati S, Tubino R, Bradley D D C and Friend R H 1993 Phys. Rev.B 48 14809
[5] Dellepiane G, Cuniberti C, Comoretto D, Musso G F, Figari G, Piaggi A and Borghesi A 1993 Phys. Rev. B 48 7850–6
[6] Wohlgenannt M, Graupner W, Leising G and Vardeny Z V 1999 Phys. Rev. B 60 5321–30
[7] Westerling M, Vijila C, Österbacka R and Stubb H 2004 Phys. Rev. B 69 1–8
[8] Wilson N M, Sandén S, Sandberg O J and Österbacka R 2017 J. Appl. Phys. 121 095701
[9] Epshtein O, Nakhmanovich G, Eichen Y and Ehrenfreund E 2001 Phys. Rev.B 63 125206
[10] Hamamatsu 2022 Technical note: Si photodiodes Solid state division, Hamamatsu Photonics K.K Ichino-cho, Higashi-ku, Hamamatsu
City, Japan
[11] Wilson N M, Aarnio H and Österbacka R 2023 Phys. Scr. 98
8
| null |
10.1093/geroni/igaa018
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Copyedited by: SK
Innovation in Aging
cite as: Innovation in Aging, 2020, Vol. 4, No. 3, 1–13
doi:10.1093/geroni/igaa018
Advance Access publication June 2, 2020
Original Research Article
National Partnership to Improve Dementia Care in Nursing
Homes Campaign: State and Facility Strategies, Impact,
and Antipsychotic Reduction Outcomes
Stephen Crystal, PhD,1,4,* Olga F. Jarrín, PhD, RN,1,2 Marsha Rosenthal, PhD, MPA,1
Richard Hermida, MA,1 and Beth Angell, PhD, MSSW3
1Institute for Health, Health Care Policy, and Aging Research, Rutgers, The State University of New Jersey, New Brunswick.
2School of Nursing, Rutgers, The State University of New Jersey, Newark. 3School of Social Work, Virginia Commonwealth
University, Richmond. 4School of Social Work, Rutgers, The State University of New Jersey, New Brunswick.
*Address correspondence to: Stephen Crystal, PhD, Institute for Health, Health Care Policy, and Aging Research, Rutgers, The State University
of New Jersey, 112 Paterson Street, New Brunswick, NJ 08901. E-mail: [email protected]
Received: January 13, 2020; Editorial Decision Date: May 19, 2020
Decision Editor: Howard B. Degenholtz, PhD
Abstract
Background and Objectives: Antipsychotic medications have been widely used in nursing homes to manage behavioral
and psychological symptoms of dementia, despite significantly increased mortality risk. Use grew rapidly during the 2000s,
reaching 23.9% of residents by 2011. A national campaign for safer dementia care in U.S. nursing homes was launched
in 2012, with public reporting of quality measures, increased regulatory scrutiny, and accompanying state and facility
initiatives. By the second quarter of 2019, use had declined by 40.1% to 14.3%. We assessed the impact of state and facility
initiatives during the Campaign aimed at encouraging more-judicious prescribing of antipsychotic medications.
Research Design and Methods: Our mixed-methods strategy integrated administrative and clinical data analyses with state
and facility case studies.
Results: Results suggest that substantial change in prescribing is achievable through sustained, data-informed quality
improvement initiatives integrating educational and regulatory interventions, supported by public quality reporting.
Adequate staffing, particularly of registered nurses, is key to support individualized management of symptoms through
nonpharmacological strategies. Case study results suggest that state and facility initiatives during the campaign achieved
considerable buy-in for the goal of more conservative prescribing, through a social process of normalization. Reporting and
reduction of antipsychotic use was not followed by increases in sedative-hypnotic medication use. Rather, sedative-hypnotic
use declined in tandem with antipsychotic reduction, suggesting that increased attention to prescribing patterns led to more
cautious use of other risky psychotropic medications.
Discussion and Implications: Quality improvement initiatives to change entrenched but problematic clinical practices face
many barriers to success, including provider-level inertia; perceptions that alternatives are not available; and family and
staff resistance. Nevertheless, systemic change is possible through concerted, collaborative efforts that touch prescribing
practices at multiple points; integrate educational and regulatory influences; activate local and state champions for im-
provement; foster reputational influences through public reporting and benchmarking; and support a social process of
normalization of preferred care processes as a best practice that is in the interest of patients.
© The Author(s) 2020. Published by Oxford University Press on behalf of The Gerontological Society of America.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/),
which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
1
Copyedited by: SK
2
Innovation in Aging, 2020, Vol. 4, No. 3
Translational Significance: The success of state and facility initiatives to reduce antipsychotic prescribing in
the National Partnership to Improve Dementia Care in Nursing Homes was greatest where they deployed
multimodal strategies that integrated voluntary and mandatory features; education for multiple actors in
the medication use process and regulatory components; collegial initiatives to achieve provider buy-in; use
of data and public reporting as a motivator; and “normalization” of best practices within the provider
community.
Keywords: Alzheimer’s disease and related dementias, Antipsychotics, Chemical restraints, Sedative-hypnotics
Management of behavioral and psychological symptoms of
dementia is a major quality issue in long-term care of older
people. Symptoms such as agitation, aggression, crying,
cursing, wandering, or threatening others can be highly
distressing for staff, other residents, and families, often
leading to requests for clinicians to “do something” about
these behaviors. The response is often an antipsychotic
prescription. However, for frail elderly residents, these
medications bring substantial risk. Despite the risks, anti-
psychotics are widely used for nursing home residents with
dementia, in the United States and internationally. The gold
standard of care for managing symptoms of dementia util-
izes behavioral management strategies and environmental
modifications, requiring substantial investments in staffing
and education (1).
In the United States, reducing antipsychotic use has been
an ongoing policy challenge. The 1987 Nursing Home
Reform Act (OBRA-87) sought to reduce both physical
restraint and antipsychotic use, referred to as “chemical
restraints.” Under OBRA-87, a federally directed, state-
operated system of oversight was created (2). Components
included a survey and certification process entailing peri-
odic site visits by regulators, empowered to issue deficiency
citations, and the Minimum Data Set (MDS) system under
which facilities provide periodic information on resident
characteristics, treatments, and services. MDS data pro-
vide the source for public reporting of quality measures at
the state and facility level, with data on individual facilities
available to the public through Center for Medicare and
Medicaid Services’ (CMS) Nursing Home Compare data
set. The existence, in the United States, of this national
system of public reporting provides an important frame-
work for the addition of new quality measures to address
emerging health care challenges.
In 2008, the Food and Drug Administration imposed a
black box warning of increased mortality risk on all anti-
psychotic medications for elderly patients with dementia
that reads “Warning: Increased Mortality For Elderly
Patients With Dementia Related Psychoses,” following
earlier warnings on second-generation antipsychotics. The
Food and Drug Administration has estimated that such
treatment is associated with a 1.6–1.7 times greater risk of
death compared to placebo, based on a meta-analysis of
17 double-blind, randomized, controlled trials averaging
8–12 weeks with a total of 5,106 patients (3). In these
trials, about 4.5% of drug-treated patients versus 2.6%
of placebo-treated patients died, implying about two more
deaths per 100 antipsychotic-treated than placebo-treated
patients. Other studies support similar estimates of sub-
stantially increased mortality through multiple pathways,
including stroke, acute myocardial infarction, infections
including pneumonia, and other causes (4–6).
Antipsychotic use declined following the enactment of
OBRA-87 (7), but during the late 1990s and 2000s, use
increased as second-generation antipsychotics, perceived
as safer, replaced first-generation ones. Increasing evi-
dence of mortality with second-generation antipsychotics,
culminating in the Food and Drug Administration black
box warnings, had only limited impact on prescribing
patterns (8). By 2011 (fourth quarter), 23.9% of residents
were receiving antipsychotic medications, excluding those
with schizophrenia, Huntington’s disease, or Tourette’s syn-
drome (9). Persistently high use despite growing evidence
of mortality led to calls for action (10,11).
Early in 2012, the CMS, state agencies, nursing homes,
advocacy groups, and other stakeholders jointly launched the
Partnership to Improve Dementia Care in Nursing Homes,
with an initial goal of reducing antipsychotic medication use
(8). The Partnership aims to enhance the quality of life for
people with dementia, protect them from substandard care,
and promote goal-directed, person-centered care for every
nursing home resident. This is accomplished through a mul-
tidimensional approach that includes public reporting, state-
based coalitions, research, training, and surveyor resources.
At the start of the National Partnership Campaign to
Improve Dementia Care in Nursing Homes, CMS added
public antipsychotic use reporting for long- and short-stay
residents, separately, at both the facility and state levels.
In order to reduce the potential for underreporting, the
reporting requirement applied to all residents (with or
without recorded dementia) with only limited exceptions
for those diagnosed with schizophrenia, Tourette’s syn-
drome, or Huntington’s disease. This public reporting
process proved to be an important tool and motivator both
for facility- and state-level quality improvement as the cam-
paign progressed.
Copyedited by: SK
Innovation in Aging, 2020, Vol. 4, No. 3
3
To examine the change in prescribing during the
Partnership campaign, we examined predictors and trends
in MDS-reported antipsychotic prescribing. Given concerns
that initiatives to reduce antipsychotic prescribing might
lead to shifts to sedative-hypnotic medications, we also
examined predictors and trends in the use of this class of
medications (12).
Antipsychotic Utilization Trends in CMS
Data, 2011–2019
The most current data on antipsychotic use are provided
in tables periodically compiled by the CMS Division of
Nursing Homes (13). These data show a relative decrease
of 40.1%, from 23.9% (fourth quarter 2011) to 14.3%
(second quarter 2019). Nationally, reduction slowed some-
what after 2015 and plateaued in 2018. However, over the
full 2011–2019 period, most states achieved significant im-
provement, with much of the national improvement driven
by reductions in several large states, including Texas, New
York, California, and Florida (13). Figure 1 presents: (top)
state rates of use in 2019; and (bottom) change in use in
each state from 2011 to 2019. The largest improvements
were seen in Texas (−57.2%), Utah (−54.5%), Tennessee
(−51.5%), Arkansas (−49.1%), New York (−48.7%),
California (−48.5%), North Carolina (−47.6), New
Jersey (−46.8%), Florida (−46.0%), Louisiana (−45.8%),
New Hampshire (−45.3%), Arizona (−44.9%), Indiana
(−42.2%), Vermont (−41.5%), and Delaware (−40.0%).
Research Design and Methods
We triangulated data from three sources: (a) long-stay res-
ident assessments linked with nursing home facility-level
data; (b) nursing home facility case studies; and (c) state
case studies.
Resident and Facility Data
National MDS data available to the team (2011–2016) were
linked with the Certification and Survey Provider Enhanced
Reports (CASPER). This timeframe is congruent with the
timeframe of the state and facility case studies and provides
a focus on the period when new initiatives, reporting sys-
tems, and regulatory changes were being implemented by
CMS and the National Partnership. MDS includes resident
demographic information, clinical measures including be-
havioral symptoms of dementia, and antipsychotic and/or
sedative-hypnotic medication administration at the time
of assessment. CASPER includes nursing home charac-
teristics such as ownership, number of beds, and propor-
tion of Medicaid residents. We also used acuity-adjusted
nurse staffing data from the CMS Nursing Home Compare
database to adjust for the patient acuity or frailty of a
facility’s residents. Our final sample included 21,431,330
Figure 1. Antipsychotic prescribing (top) and change in antipsychotic
prescribing (bottom) to long-stay nursing home residents by state,
fourth quarter 2011 to second quarter 2019. States selected for key in-
formant interviews are starred. Notes. Data from Center for Medicare
and Medicaid Services (CMS) division of nursing homes, national part-
nership to improve dementia care in nursing homes: antipsychotic
medication use data report (October 2019). Use rates are calculated by
CMS from the minimum data set (MDS), version 3.0, and represent the
proportion of long-stay residents without a diagnosis of schizophrenia,
Huntington’s disease, or Tourette syndrome, who received an antipsy-
chotic medication within the 7 days preceding the MDS assessment.
Long-stay residents are defined by a total of 101 days or more without
a gap of 30 contiguous days living in the community or other institu-
tion (14).
assessments (quarterly observations) for 3,687,901 long-
stay nursing home residents in 17,289 facilities during cal-
endar years 2011–2016.
Using these linked data, we examined variation and
change in MDS-reported antipsychotic and sedative-
hypnotic prescribing, among residents without schizo-
phrenia, Huntington’s disease, or Tourette’s syndrome.
This method parallels the CMS Nursing Home Compare
measure for inappropriate use of antipsychotic medica-
tion in nursing home residents. First, we examined vari-
ation in patient and facility characteristics between 2011
and 2016. Next, we used cross-sectional logistic regression
models (2011 and 2016) to assess the pattern of the rela-
tionship between patient- and facility-level predictors and
antipsychotic or sedative-hypnotic medication use. The de-
cision was made to include bipolar disorder as a covariate,
rather than exclusion, in light of Carnahan and Letuchy’s
finding that among nursing home residents, bipolar
Copyedited by: SK
4
disorder diagnosis is frequently nonspecific and follows
a diagnosis of dementia, and to be generally consistent
with the CMS measure (15). After determining the sta-
bility of the predictors over time, we focused on the effect
of time (quarter-year intervals) in the final logistic models,
highlighting the impact of the National Partnership on
prescribing patterns after controlling for patient and fa-
cility characteristics.
State Case Studies
For insight on strategies used in state campaigns, we
conducted focus group interviews in 2016 with key
informants from public health and government agencies
(National Partnership coalition participants) in seven
states: Arkansas, California, Georgia, Maine, North
Carolina, Texas, and Wisconsin. These states were selected
to ensure a balance of regional representation, population,
and baseline antipsychotic use rates. States were selected to
be heterogeneous with respect to the rate of antipsychotic
reduction between 2012 and 2014. Interview questions
focused on each state’s efforts to engage in the National
Partnership, the strategies used, challenges encountered,
and critical ingredients of success. The state case studies
helped us understand what the states were doing during the
study period (2011–2016). We also used information on
state strategies compiled by the Partnership (8).
Facility Case Studies
To better understand decision making and change in
prescribing, we conducted 40 semistructured interviews at
14 nursing homes in the same seven states where the case
studies were completed. Two nursing homes were selected
in each state. In total, we interviewed 30 nursing home staff
(primarily Directors of Nursing, activities staff, social serv-
ices staff, and nursing staff) and 10 prescribing physicians.
Questions focused on the decision-making process related
to the use of antipsychotic medication, effects of CMS reg-
ulation, barriers to change, and sources of improvement.
Interviews were completed from January through June of
2017, helping us understand how facilities responded to
their state initiatives described in the case studies.
Innovation in Aging, 2020, Vol. 4, No. 3
a trend to more judicious use, more focused on residents
with the most severe symptoms (16). Similarly, a reduc-
tion in antipsychotic use was less among residents with re-
corded bipolar disorder diagnoses than for other residents,
and the proportion of residents with a recorded diagnosis
of bipolar disorder increased from 3.3% in 2011 to 4.1%
in 2016. The mean age of residents decreased slightly, from
80.2 years (SD 13.2) in 2011 to 79.6 years (SD 13.4) in
2016, and the proportion of male residents increased from
30.8% to 33.5%. The reduction in sedative-hypnotic use
by more than one third in every demographic group during
the same period suggests that initiatives to reduce anti-
psychotic use did not lead to a shift in prescribing more
sedative-hypnotic medications, as some have feared.
In the adjusted model (Table 2) for 2016, antipsychotic
use was notably higher, as expected, for residents with phys-
ical aggression (odds ratio [OR] 2.10), verbal aggression
(OR 2.04), or bipolar disorder (OR 8.52). Among facility
characteristics, lower Registered Nurse (RN) staffing and
higher Licensed Practical Nurse staffing were associated
with greater risk of antipsychotic use. Certified Nursing
Assistant staffing levels were not associated with risk of
antipsychotic use. Both the resident’s individual Medicaid
eligibility and the proportion of Medicaid residents in
the facility were associated with greater odds of antipsy-
chotic use. Risk of sedative/hypnotic use was greatest
among residents of Hispanic ethnicity (OR 1.27), those
who exhibited verbal aggression (OR 1.41), and those
with symptoms of depression (OR 1.52). Like antipsy-
chotic use, sedative/hypnotic use was correlated with lower
RN staffing and higher Licensed Practical Nurse staffing.
Higher Certified Nursing Assistant staffing was also asso-
ciated with greater sedative/hypnotic use. While this may
seem counterintuitive, it could reflect a consequence of the
cost-saving practice of substituting less expensive Licensed
Practical Nurses and Certified Nursing Assistants for RNs.
The effect of the campaign to improve nursing home care
of patients with dementia over time (first quarter of 2011
through fourth quarter of 2016) was assessed using models
that controlled for individual, facility, and state variables
(Table 3). By the end of 2016, the adjusted risk of antipsy-
chotic medication use was nearly halved from the beginning
of the campaign (OR 0.55) with even greater reduction in
risk of sedative-hypnotic medication use (OR 0.44).
Results
Facility Case Studies
Nationally, antipsychotic prescribing declined by 29% and
sedative-hypnotic prescribing by 43% between 2011 and
2016 (Table 1). Reduction in antipsychotic medication
use was particularly substantial for black (−36.2%) and
Hispanic residents (−33.6%) compared to non-Hispanic
white residents (−27.6%), highlighted in Table 1. Reduction
in antipsychotic use was greater among residents without
recorded behavioral symptoms of physical or verbal ag-
gression than among those with these behaviors, suggesting
Several recurring themes in the case study data provide addi-
tional insight into decision making and change, putting the
results in context. First, facility staff and prescribers gener-
ally appreciated the risks of antipsychotic use and supported
the need to reduce use and to treat these medications as a
last resort. However, most were not aware of the National
Partnership campaign. Despite this, responses suggested
considerable staff and clinician buy-in to the campaign’s
overall aim of reducing reliance on antipsychotic use.
Copyedited by: SK
Innovation in Aging, 2020, Vol. 4, No. 3
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Innovation in Aging, 2020, Vol. 4, No. 3
A second recurring theme was the importance of sys-
tematic use of data for quality improvement. For example,
respondents reported on internal initiatives to analyze fa-
cility prescribing data and distribute results to the staff to
support monitoring. A Director of Nursing reported: “We
have a task force that’s working on reducing antipsychotics
… we have a dashboard … we have the CASPER report.
We run it monthly.”
Third, consistent with findings from a recent system-
atic review of decision making for dementia patients (17),
respondents offered strong support for the essential role
of collaboration and communication in safe dementia
care practices. A recurring theme was that incorporating
improved practices into prescribing and medication man-
agement processes across multiple levels of decision
making required the efforts of interdisciplinary teams, in-
cluding staff at all levels, particularly nursing assistants.
Respondents also emphasized the importance of clear com-
munication among staff and with physicians.
Fourth, respondents spoke about the challenge of and
need for individualized approaches to behavioral issues.
For example, a registered nurse noted:
a patient in the Alzheimer’s unit that kept urinating
in the hallway on the floor, around the nurse’s cart …
they tried redirection, they tried toileting, they tried all
kinds of things … And then they have these lights that
I bought at Wal-Mart that come on when you walk
by, and I stuck it to the back of the bedside commode
and he began to use the bedside commode instead of
urinating in the hallway.
Fifth, to achieve such
individualized approaches,
respondents perceived a need for more training in the use
of nonpharmacological strategies for symptom manage-
ment. Nurses described in-service training and informal
advice from other staff as useful but not sufficient to give
nursing assistants, and even nurses, needed insight into the
sources of dementia patients’ agitation and aggression, and
methods for dealing with these behaviors: “ask every nurse
in the facility, ‘Do you feel you’re getting the education
you need to assist you when caring for these patients [with
dementia]?’ Because I bet half of them would say, ‘No.’”
Education on dementia management and the risks of phar-
macological strategies was also reported to be important
for family members. Some respondents observed that in
their concern for an elderly relative’s well-being, and dis-
couragement over aggressive or agitated behaviors, family
members often see antipsychotics as a solution rather than
a problem.
Finally, respondents were generally conscious of, and
even supportive of, the changes in CMS regulations on
antipsychotics, although some took a rather cautious view
of monitoring by surveyors, sometimes seeing the surveyors
as too focused on “the numbers” and not conscious of
the complexities of reducing antipsychotic medication
use. Several expressed concern that the CMS regulations
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Innovation in Aging, 2020, Vol. 4, No. 3
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8
Innovation in Aging, 2020, Vol. 4, No. 3
Table 3. Effect of Time on Antipsychotic and Sedative-Hypnotic Medication Use, Controlling for Individual, Facility, and State
Variables
Antipsychotic
Sedative-Hypnotic
2011 Q1
2011 Q2
2011 Q3
2011 Q4
2012 Q1
2012 Q2
2012 Q3
2012 Q4
2013 Q1
2013 Q2
2013 Q3
2013 Q4
2014 Q1
2014 Q2
2014 Q3
2014 Q4
2015 Q1
2015 Q2
2015 Q3
2015 Q4
2016 Q1
2016 Q2
2016 Q3
2016 Q4
OR, p
(ref)
0.994
0.994
1.002
0.975***
0.983***
0.959***
0.911***
0.861***
0.830***
0.803***
0.770***
0.746***
0.730***
0.718***
0.704***
0.694***
0.657***
0.628***
0.602***
0.597***
0.579***
0.569***
0.549***
95% CI
0.987–1.002
0.986–1.002
0.994–1.010
0.967–0.983
0.976–0.991
0.952–0.967
0.904–0.918
0.854–0.868
0.824–0.837
0.796–0.809
0.764–0.777
0.741–0.752
0.724–0.735
0.713–0.724
0.698–0.709
0.689–0.700
0.652–0.663
0.623–0.633
0.597–0.607
0.592–0.602
0.574–0.583
0.565–0.574
0.544–0.553
OR, p
(ref)
0.998
1.004
0.982*
1.013
0.952***
0.915***
0.874***
0.867***
0.820***
0.780***
0.736***
0.721***
0.695***
0.657***
0.629***
0.620***
0.587***
0.556***
0.528***
0.525***
0.497***
0.470***
0.440***
95% CI
0.985–1.010
0.992–1.017
0.969–0.995
1.000–1.026
0.939–0.964
0.903–0.927
0.863–0.886
0.856–0.878
0.809–0.831
0.770–0.790
0.726–0.746
0.712–0.731
0.686–0.705
0.649–0.666
0.621–0.638
0.612–0.629
0.579–0.595
0.548–0.564
0.520–0.536
0.518–0.533
0.490–0.504
0.462–0.477
0.433–0.447
Notes: CI = confidence interval; OR = odds ratio. Total observations = 21,431,330.
*p < .01, **p < .001, ***p < .0001.
and surveyors do not differentiate between antipsychotic
medications prescribed for nursing home patients with de-
mentia and those with severe mental illness. A physician
commented, “[The Director of Nursing] does not want to
take admissions for somebody that is on an antipsychotic
agent because heaven forbid that will mess their numbers
up … she is feeling pressure from the state surveyors and
other people.”
Overall, the interviews suggest that reducing antipsy-
chotic medications is more time- and resource-intensive
than relying on medication, by requiring a person-centered
approach. However, the consensus was that given appro-
priate staff time, training, and effective communication,
individualized reduction of antipsychotic medications is
achievable, as well as desirable.
State Case Studies
State coalition respondents indicated the importance
of multimodal strategies that involved both state-level
interorganizational coordination and training and tech-
nical assistance at the facility level. In several states,
respondents noted the important role, in sustaining these
initiatives, of CMS grants from Civil Monetary Funds
(funds derived from penalties paid by facilities for quality
and safety violations). State respondents, like those in the
facility studies, noted the importance of public reporting of
antipsychotic use rates at facility and state levels, included
on CMS’s Nursing Home Compare website beginning in
July of 2012 (18). Public reporting served to define change
targets and as a catalyst to action: one respondent from
Georgia noted “the powerful motivator of shame.” As a
California respondent stated, “When you compare people
to a benchmark and to their peers and they’re not looking
too good, that definitely gets their attention.”
Public reporting served as an incentive for improvement
at both state and facility levels. Texas used metrics to identify
facilities that achieved notable reductions in antipsychotic
prescribing whose strategies could be shared with other
facilities. Maine similarly identified high-improvement
facilities and presented these data to state legislators and
local media. Conversely, quality metrics were used to iden-
tify facilities in the greatest apparent need of support for
quality improvement (termed by respondents “low-hanging
fruit”). Texas identified the 100 facilities with the highest
use of antipsychotics and sent certified letters to their
Medical Directors, encouraging them to address the issue.
Quality Improvement Organizations (QIOs) and regulators
Copyedited by: SK
Innovation in Aging, 2020, Vol. 4, No. 3
9
also used metrics to focus their efforts. QIOs assisted
nursing homes to collect and interpret facility data over
time to support monitoring efforts. These interventions
were complemented by an increased regulatory focus on
antipsychotic use during regular regulatory site visits
(survey and certification process), in which each nursing
home is visited periodically by a state survey team. In addi-
tion, special site visits focused on reviewing dementia care
(focused dementia care surveys) were implemented during
2015 in Texas, California, and other states.
from home-grown pamphlets,
Once facility targets were identified, state coalitions
developed or obtained training and technical assistance
materials to redefine and normalize prescribing and psy-
chosocial practices that rely on person-centered care
principles to manage difficult behavior. These training
strategies varied
to
materials provided by CMS, to the purchase (using Civil
Monetary Penalty funds) of consultation and materials
on nonpharmacological strategies. States typically offered
in-person training and created online repositories for on-
going access by facilities. For individual facilities identified
as struggling to achieve improvement, QIOs and other coa-
lition participants provided individual assistance, including
on-site training, phone-based technical assistance, and
facility-to-facility mentoring programs. In Texas, a desig-
nated Quality Monitoring Program (QMP), distinct from
the survey process, worked with facilities identified as in
need of improvement; technical assistance visits addressed
monitoring procedures and staff training on evidence-based
practices. To address family fears regarding resident be-
havior that could be a barrier to de-prescribing, some states
developed educational materials for families that could be
distributed by facilities.
State respondents also reported the importance of
involving members of a variety of professional groups in
coalition activities, including physicians and pharmacists.
For example, in North Carolina, multiple coalition partners
participated in training for facilities, including representa-
tives of the state pharmacy association, medical directors,
the ombudsman, the QIO, and CMS. Facility training
addressed resources available to support improvement and
detailed regulatory changes with which they would be ex-
pected to comply. Pharmacists in North Carolina were also
highly involved in an effort to improve electronic medical
records to allow facilities to easily and quickly identify
resident-level information about antipsychotic medication
use.
Discussion and Implications
Several themes that influence antipsychotic medication
prescribing in nursing homes emerged from this mixed-
methods study. First, public reporting of a safe-use metric
appears to have been a key element in motivating changes,
at both state and facility levels. As a respondent from Texas
noted, “I think that we all were disgusted with being in last
place in the country. We were 51st for a long time.” Public
reporting of metrics will likely be a useful tool to motivate
further progress and respond to any backsliding.
Second, in the large and complex long-term care
system, engagement of multiple stakeholders was vital.
This process began at the national level, with leadership
from CMS, the national nursing home associations, and
other key stakeholders. At the state level, a diversity of
organizations was engaged. States that achieved rapid
success, such as North Carolina and Georgia, benefited
from already-developed working relationships among
CMS, the QIOs, statewide provider organizations, and
individual facilities. These relationships were marshaled
to develop new advisory groups to brainstorm strategies
to assist facilities with high antipsychotic use. While
these efforts were typically coordinated by the QIO,
they benefited from established collaborations among
key stakeholder groups. A North Carolina participant
described high rates of attendance at initial rollout
trainings in 2012 and explained that this pattern was typ-
ical in a state in which “facilities are very, very interested
in being on the cutting edge of things.”
In the largest states, the extended time necessary to
engage multiple geographically dispersed stakeholders
and facilities emerged as an important theme. Trends in
California, Texas, and New York earlier in the initiative
(2012–2015) versus later (2016–2018) reflect the chal-
lenge of generating change in such large systems. The tra-
jectory of change was slower in these states than smaller
states; each achieved greater relative improvement later in
the campaign, improving in rank relative to other states
(Figure 1). These results suggest that achieving change in
large state systems, with thousands of facilities, requires a
sustained multiyear effort to engage the necessary range of
stakeholders on a statewide basis. Once these initiatives are
incorporated into these large systems, however, the experi-
ence of California, Texas, and New York suggests that sus-
tained change can be achieved in such systems. However,
continuing efforts will likely be required to institution-
alize these changes. New initiatives may periodically need
to be rolled out in order to maintain the energy brought
to the field by state and federal oversight and educational
campaigns, to maintain the attention of prescribers, facility
staff, and other stakeholders. Recent plateauing in antipsy-
chotic use rates suggests the challenges facing sustained and
continuing improvement.
Third, integration of educational activities and regu-
latory oversight contributed to the effectiveness of state
initiatives. Initiatives based on the survey and certifica-
tion system, such as focused dementia care surveys in
which antipsychotic prescribing was reviewed in detail
(conducted in Texas, California, and other states during
2015), contributed to facilities’ motivation to incorporate
improvement strategies into their operations. Respondents
reported that regulatory feedback was most effective
when it focused on improving internal review and quality
Copyedited by: SK
10
management processes rather than individual cases. For ex-
ample, one California respondent noted:
I’m not sure hitting people with a stick for pharmaco-
logical use would be as effective as forcing them to write
a plan of a correction for care that is not meeting the
standard of individualized dementia care including ap-
propriate activities.
More broadly, and consistent with findings from sys-
tematic reviews of health system initiatives to change
prescribing and other clinical practices (17,19,20), state-
level initiatives appeared to be most successful when: (a)
they achieved buy-in that the recommended practices were
in the best interest of patients and (b) accomplished incor-
poration of desired practices into established workflows.
A process of “normalization” of preferred practices,
transmitted and reinforced through social processes among
the stakeholders involved, helped to define reduction of
antipsychotic medication as best practice, in the interest
of patients and consistent with professional expectations.
This process of normalization has been described in other
contexts (21) and is a key feature of creating sustained
changes in health care provider behavior (22). In the case
of antipsychotic medication reduction in nursing homes,
oversight and quality improvement initiatives were au-
thoritative enough to engage prescribers, facility staff, and
others involved in the medication decision-making process,
while sufficiently collegial, evidence-based, and education-
ally oriented to achieve buy-in and normalization of pre-
ferred practices.
Fourth, facilities with the most severe understaffing
appeared to have been less able to respond to incorporate
the recommended practices into their care processes. While
total nurse staffing is important, results suggest that sub-
stitution of lower-educated Licensed Practical Nurses and
Certified Nursing Assistants for RNs may be problematic
for this dimension of quality. In particular, lower regis-
tered nurse staffing was associated with greater reliance
on antipsychotics. This finding is not surprising in view of
the substantial differences in RN staffing reported across
staffing quartiles. As given in Table 1, facilities in the lowest
quartile (2016) averaged only 17 minutes of RN time per
resident day, in contrast to 50 minutes for facilities in the
highest quartile. Even in nursing homes staffed at the levels
recommended by CMS, there may not be enough staff
time for residents with behavioral symptoms of dementia
to receive individualized activities and adequate physical
activity during the day. Improving the infrastructure for
recruiting and training nursing home volunteers (similar
to requirements in the hospice industry) could help to im-
prove personalized care for residents with dementia and
lead to opportunities for volunteers to join the long-term
care workforce (23).
Many quality initiatives to increase safety and quality
in health care have had limited or no success. In contrast,
the National Partnership has had substantial impact on a
Innovation in Aging, 2020, Vol. 4, No. 3
practice that has been widely considered a difficult target
to change; that had persisted despite highly credible safety
evidence; and that has been a challenge in many countries
(24–27). What, then, was distinctive about this initiative
that helped to drive its significant impact? Interventions
varied across states, and the factors influencing prescribing
across the nation’s nursing homes are complex.
Policy Implications
Results suggest that the federally supervised, state-
administered oversight structure for nursing homes created
under OBRA-87 appears to have functioned well as a
framework within which a campaign to address a specific
problematic practice can operate effectively. In this re-
gard, the success in reducing antipsychotic prescribing has
similarities to earlier successful initiatives to reduce physical
restraints, which, like antipsychotics, require a physician
order, were emphasized in the regulatory/survey process as
a target of improvement, and were publicly reported (28).
The use of physical restraints declined from 41% in the
early 1990s (29,30) to current rates of less than 3% (18).
As with reducing antipsychotic use, reducing reliance on
physical restraints required deployment of individualized
strategies in managing patients with complex behavioral
disturbances and communications challenges, as well as
changing established mindsets concerning appropriate
treatment practices.
Deploying alternative nonpharmacological strategies in
place of medication-based strategies requires adequate RN
staffing for individualized care planning and supervision
of direct care staff. As reflected in all but the top quartile
of nursing homes, current federal requirements do not as-
sure staffing levels adequate to provide safe, individualized
care. Stronger requirements and incentives to meet CMS
minimum safe staffing guidelines would contribute to safer
dementia care. The potential for substitution of pharma-
cological for psychosocial strategies for managing patients
with dementia is heightened in the nursing home setting
by misaligned financial incentives because, for long-term
residents, facilities are responsible for staffing costs but
not for the costs of medications, typically reimbursed by
Medicare. This financial misalignment strengthens the ar-
gument for stronger federal staffing requirements and max-
imal transparency of staffing patterns.
Staffing adequacy is, of course, directly related to
Medicaid reimbursement for long-term nursing home
care, which varies widely across states and falls far short
of Medicare reimbursement for postacute care provided
in the same facilities. In consequence, facilities with the
greatest dependence on Medicaid reimbursement are less
able to provide the level of staff support necessary to design
and implement personalized dementia care strategies that
minimize reliance on antipsychotics. Although Medicaid-
dominant facilities did achieve improvement, they remain
more dependent on antipsychotic medications for symptom
Copyedited by: SK
Innovation in Aging, 2020, Vol. 4, No. 3
11
management. Given financial pressures on state budgets,
the longstanding challenge of inadequate Medicaid nursing
home rates is unlikely to be solved soon; however, current
findings suggest the contribution of this challenge to pa-
tient safety problems.
Finally, continued financial and logistical support will
likely be needed in order for state quality improvement
consortia to sustain their efforts over time. CMS funding
from Civil Monetary Penalty Funds, reported by some
respondents as vital in their consortium’s success, will likely
be needed on a sustained basis. As noted, continuing in-
novation with new educational and intervention strategies
will be important, both to address the high level of turn-
over in facility staffs and clinicians and to provide novelty
that continues to maintain stakeholders’ attention to these
critical safety issues.
Overall, results suggest that safer dementia manage-
ment, with reduced reliance on antipsychotics, is facilitated
by approaches that effectively integrate educational and
regulatory elements, public quality measure reporting, and
adequate staff resources. Accelerated improvement several
years into the campaign in several large states, relative to
other states, suggests the importance of multiyear commit-
ment to improvement initiatives in the larger systems. State
and federal initiatives appear to have achieved consider-
able buy-in on the need to reduce antipsychotic use. Study
results indicate that with a combination of educational and
regulatory approaches, multi-stakeholder engagement, and
measurement-based accountability, substantial improve-
ment in safe dementia care in nursing homes is achievable.
However, sustaining these efforts will require contin-
uing collaborative effort. Adequate total nursing staffing
and RN staffing, in particular, emerged as key factors, as
facilities with lower staffing levels appeared to be less able
to incorporate recommended changes. The importance of
adequate staffing highlights financial concerns regarding
the impact of reductions to state Medicaid programs and
potential impact on voluntary efforts, including staffing
above minimum levels.
The sustainability of the changes achieved by the
campaign remains to be determined (31,32). Modifying
practices in the large and complex long-term care system
involves difficult challenges of changing established
workflows and clinical habits. Thus far, the National
Partnership campaign has demonstrated significant
staying power and appears to have generated significant
buy-in and incorporation of safer dementia care practices
into established workflows. There appear to be grounds
for optimism that if safer dementia care practices be-
come embedded in ongoing care processes and in widely
shared understandings of best practices, the National
Partnership can achieve long-term impact. However,
these efforts were challenged in 2020 by the COVID-19
pandemic, resulting in restrictions on visitors, volunteers,
and group recreational and social activities that are a cor-
nerstone of the National Partnership. At the same time,
infectious disease epidemics such as COVID-19 highlight
the importance of continued vigilance on antipsychotic
prescribing, especially in periods where the quality of care
is challenged by staffing shortages and other epidemic-
associated difficulties (3–5,32).
Continued progress will likely require systematic contin-
uing education for the large number of staff and physicians
who flow through the long-term care system each year.
Continued transparency of practices using public reporting
of quality measures will also be important, along with in-
tegrated regulatory and educational initiatives to maintain
focus on safe practices, and adequate staffing resources to
provide personalized, patient-centered care.
Numerous online resources are available on the
National Partnership to Improve Dementia Care website,
including best practices to avoid unnecessary antipsy-
chotic medication in nursing home residents living with
dementia, and toolkits to promote sleep, reduce risk of in-
fection, and reduce acute care transfers (8). Less is known
regarding the outcomes of the focused dementia care
survey process and its impact on the quality of care and
life for residents in long-term care. To date, only pilot data
are publicly available. Hopefully, the National Partnership
Campaign to Improve Dementia Care will continue such
efforts in their overall goal of creating environments that
support person-centered care for individuals living with
dementia.
Funding
This work was supported by the Agency for Healthcare Research
and Quality (AHRQ; grant numbers R18HS023464 to S.C.,
R00HS022406 to O.J.) and the Donaghue Foundation, with addi-
tional support from AHRQ (R18-HS023258 and 1U19HS021112)
for Advancing Translational Sciences
and National Center
(UL1TR003017).
Acknowledgments
The authors wish to acknowledge the research assistance of Scott
Bilder, Alice Bonner, Elizabeth Connolly, Kimberly Convery, Abner
Nyandege, Sheree Neese-Todd, Jessica Poling, and Aleksandra
Wec, and the nursing home staff, physicians, and state partnership
participants who agreed to be interviewed.
Conflict of Interest
None reported.
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Compare-Claims-based-Measures-Technical-Specifications.
pdf. Accessed July 4, 2020.
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diagnoses in people with dementia. Am J Geriatr Psychiatry.
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897.e12. doi:10.1016/j.jamda.2017.06.032
18. Center for Medicare and Medicaid Services. Nursing Home
https://www.medicare.gov/nursinghomecompare/
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19. Abraha I, Rimland JM, Trotta FM, et al. Systematic review of
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29. Sirin SR, Castle NG, Smyer M. Risk factors for physical res-
traint use in nursing homes: The impact of the Nursing
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doi:10.1377/hlthaff.2016.1439
| null |
10.1038_s41593-023-01332-5.pdf
|
Data availability
Data from this study are available at https://github.com/axellaboratory/
Cury_and_Axel_2023 and upon request. Trained pose estimation mod-
els and the supervised behavioral classifier can be accessed via Dropbox
(https://www.dropbox.com/sh/jh4422f3ld95j1a/AAAHVb-pFsmcEk40
BgSHm1TEa?dl=0).
|
Article https://doi.org/10.1038/s41593-023-01332-5 Data availability Data from this study are available at https://github.com/axellaboratory/ Cury_and_Axel_2023 and upon request. Trained pose estimation models and the supervised behavioral classifier can be accessed via Dropbox ( https://www.dropbox.com/sh/jh4422f3ld95j1a/AAAHVb-pFsmcEk40 BgSHm1TEa?dl=0 ). Code availability Code used for processing the data is available at https://github.com/ axellaboratory/Cury_and_Axel_2023 . Extended Data
|
Flexible neural control of transition points
within the egg-laying behavioral sequence
in Drosophila
https://doi.org/10.1038/s41593-023-01332-5
Received: 14 January 2022
Kevin M. Cury
1
& Richard Axel
1,2
Accepted: 13 April 2023
Published online: 22 May 2023
Check for updates
Innate behaviors are frequently comprised of ordered sequences of
component actions that progress to satisfy essential drives. Progression
is governed by specialized sensory cues that induce transitions between
components within the appropriate context. Here we have characterized
the structure of the egg-laying behavioral sequence in Drosophila and
found significant variability in the transitions between component actions
that affords the organism an adaptive flexibility. We identified distinct
classes of interoceptive and exteroceptive sensory neurons that control
the timing and direction of transitions between the terminal components
of the sequence. We also identified a pair of motor neurons that enact
the final transition to egg expulsion. These results provide a logic for the
organization of innate behavior in which sensory information processed at
critical junctures allows for flexible adjustments in component actions to
satisfy drives across varied internal and external environments.
Organisms have evolved a repertoire of innate behaviors, comprised
of sequences of component actions, to satisfy essential drives1–3. Pro-
gression along an innate behavioral sequence is regulated by distinct
stimuli, or ‘releasers’, to ensure that transitions between component
actions occur in a suitable context at the appropriate time3. This mecha-
nism imparts behavioral flexibility by introducing decision points that
allow innate behaviors to adapt to variation in the organism’s internal
and external environment. Control at the junctures of component
actions is a fundamental property of many instinctive behaviors.
The drive to reproduce is a dominant motivator of behavior in all
species. Diverse behavioral programs dedicated to courtship, copula-
tion and the production and care of offspring have evolved to optimize
reproductive success. For oviparous animals that do not brood, such
as the fruit fly Drosophila melanogaster, egg deposition represents
the culmination of this array of reproductive behaviors. Considerable
pressure is imposed on the selection of the appropriate time and place
to deposit eggs. Fruit flies express strong, species-specific preferences
for the site of egg deposition based, in part, on odor, taste, texture and
the spatial dimension of the environment4–12. During egg laying, females
evaluate the local environment before expressing an ordered motor
sequence (abdominal bending, ovipositor (hypogynium) burrowing
and egg expulsion) that culminates in egg deposition subterraneously
within a nutritive substrate10,13–18. After egg expulsion, the female comes
to rest, this final phase of the behavioral sequence is coupled to ovula-
tion and fertilization, and the cycle repeats.
Egg laying in the fly is initiated by a seminal fluid peptide, sex
peptide, introduced into the uterus (genital chamber) during mating19.
Sensory information from neurons responsive to sex peptide is relayed
to the brain, inhibiting a subset of the pC1 cluster of neurons15,20–23. This
disinhibits the oviposition descending neurons (oviDNs), a collection
of descending interneurons that project to the ventral nerve cord and
are necessary and causal for the expression of the ordered egg deposi-
tion motor sequence15. One model posits that ramping activity in these
descending neurons determines the progression of this terminal motor
sequence15,24. Progression along the sequence, however, is likely to be
dictated by the ongoing acquisition of sensory information, allowing
the motor pattern to adapt to variability in the internal and external
environment.
In this study, we have characterized the detailed structure of egg-
laying behavior and identify that the transitions between component
1The Mortimer B. Zuckerman Mind Brain Behavior Institute, Department of Neuroscience, Columbia University, New York, NY, USA. 2Howard Hughes
Medical Institute, Columbia University, New York, NY, USA.
e-mail: [email protected]; [email protected]
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
1054
nature neuroscienceArticlea
b
PE
Walk
Bend
Burrow
Detach
Groom
Exploration
Deposition
Reset
Fraction
1.0
0.5
0
1
2
3
4
5
6
7
Order of first occurrence
c
1
y
l
F
5
–30
0
30
60
–60
–30
0
30
60
PE
Walk
Bend
Burrow
Egg out
Detach
Groom
d
1.0
n
o
i
t
c
a
r
F
0.5
0
–60
Time (s)
e
0.31
0.16
0.13*
0.63*
0.04*
0.10*
Time (s)
0.07
0.06
0.67*
PE
Walk
Bend
Burrow
Detach
Groom
0.77*
0.65*
0.99*
0.87*
0.15
0.17
0.71*
0.14
0.06
0.06
0.13
0.18
0.35
0.04–0.09
0.10–0.29
0.30–0.49
>0.05
Fig. 1 | The egg-laying sequence exhibits variable transitions between
component actions. a, Illustrations depicting component actions of the egg-
laying behavioral sequence. Components comprising exploration, deposition
and reset phases are indicated. The colors used in text and boxes here indicate
component actions in all subsequent figures; PE, proboscis extension. b, Order
of occurrence of the first instance of each behavioral component depicted as
a fraction of total events. Only events including all components were analyzed
(169 of 176 events). c, Representative ethograms of egg-laying behavior in five
flies (n = 4 events per fly). Here and in e, bend is drawn wider to emphasize that
this behavior is maintained throughout burrow, egg out and detach behaviors.
Horizontal black dashed lines demarcate data from different flies. Here and in
d, t = 0 marks the time of completed egg expulsion (egg out). d, Average time
course of the seven annotated behaviors depicted as the instantaneous fraction
of total events; n = 176 events from 18 flies. e, Diagram depicting the start-to-start
transition probabilities between the seven annotated behaviors. An asterisk
indicates transitions occurring significantly higher than chance (P < 0.001,
one-sided permutation test; Methods and Supplementary Table 2). Transitions
with probabilities less than 0.04 were not significant and were omitted from the
diagram. Self-transitions indicate that the behavior started, stopped and started
again without the initiation of any other intervening behavior.
actions are variable and can be flexibly adjusted to accommodate
diverse environmental conditions. Moreover, we have identified three
classes of neurons that control the timing and direction of specific
transitions within the terminal egg deposition motor sequence. These
results provide both a behavioral logic and a neural basis for the impo-
sition of adaptive flexibility on an innate and stereotyped sequence of
motor actions.
Results
Variable transitions in the egg-laying behavioral sequence
Females lay eggs one at a time in a repeating cycle, continually transi-
tioning between three distinct phases of a behavioral sequence. Each
egg-laying cycle is comprised of an active exploratory phase, deposi-
tion and a more stationary phase (‘reset’) that includes ovulation, after
which the cycle repeats (Supplementary Fig. 1)10,13,14,17,18. We have studied
the composition of this sequence in detail by filming individual gravid
females at high resolution in small egg-laying chambers on a 1% agarose
substrate and manually scoring the component behaviors (Supple-
mentary Fig. 2, Supplementary Video 1 and Supplementary Table 1).
Before deposition, flies explored the substrate with their proboscis and
legs. During this phase, flies extended their proboscis to make brief
contact with the substrate and walked to a new location (Fig. 1a). They
then transitioned to deposition and bent their abdomen to bring the
ovipositor in contact with the surface, initiated substrate burrowing
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5a
d
e
t
i
s
o
p
e
d
s
g
g
e
f
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t
c
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e
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e
c
a
f
r
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s
n
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l
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i
t
r
a
P
1.0
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i
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g
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e
f
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N
40
30
20
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***
**
*
1.0
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N
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i
z
e
d
e
g
g
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e
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0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50
Percent agarose
***
**
*
0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50
Percent agarose
1.0% agarose
Bend
Burrow
c
1
y
l
F
5
–60
–30
Time (s)
1.75% agarose
1
y
l
F
5
–60
–30
Time (s)
0
0
d
)
w
o
r
r
u
b
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t
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n
e
b
(
P
e
)
t
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o
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e
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t
w
o
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r
u
b
(
P
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
*
0.75
1.00
1.25
1.50
Percent agarose
1.75
***
***
0.75
1.00
1.25
1.50
Percent agarose
1.75
Fig. 2 | Egg deposition sequence adjusts to changes in substrate firmness.
a, Left ordinate: distributions of the depth of eggs released on substrates of
varying firmness; partial, partially subterraneous. The mean fraction of all
eggs pooled per group is presented (here and in b; n = 48, 54, 48, 48, 48, 32, 32
and 32 flies per group). Right ordinate: mean and s.e.m. of the per-fly average-
normalized egg depth (magenta). The statistical test comparing the number of
‘on surface’ eggs released is depicted only for groups from 0.75% to 2.0% agarose.
Here and in b, d and e, *P < 0.05, **P < 0.01 and ***P < 0.001, as determined by a
Kruskal–Wallis test with post hoc Tukey’s honestly significant difference (HSD)
test (Supplementary Table 7). b, Number of eggs released on substrates of
varying firmness in 4 h. Here and in d and e, box bounds indicate the 25th and
75th percentiles, the red lines indicate the medians, and the whiskers indicate the
5th and 95th percentiles; o, data from individual flies; +, outliers. The statistical
test is depicted only for groups from 0.75% to 2.0% agarose. c, Representative
ethograms depicting bending and burrowing behavior in five flies on 1.0%
agarose (top) and 1.75% agarose (bottom); n = 4 events per fly; t = 0, egg out.
d, Average probability (P) of progression from bending to burrowing across
substrates of varying firmness. Only flies that exhibited three or more bend bouts
are considered (n = 19, 15, 21, 11 and 11 flies per group). e, Average probability of
progression from burrowing to completed egg expulsion (egg out). Only flies
that exhibited three or more burrowing episodes are considered (n = 19, 15, 21, 10
and 11 flies per group).
(a rhythmic behavior in which the ovipositor digs into the substrate
and expels the egg) and ultimately deposited the egg subterraneously.
After egg expulsion, the flies abruptly stopped burrowing, detached
from the egg and then lifted and groomed their ovipositor (Fig. 1a).
The females then remained stationary for an extended period of time,
intermittently grooming and exhibiting abdominal contortions likely
to result from ovulation (the reset phase)17. The behavioral sequence
then repeated. This ordering of component actions was highly con-
served across repeated egg-laying events (Fig. 1b). We independently
rescored a subset of data using a second human annotator to dem-
onstrate the consistency and reproducibility of our manual labels.
Human–human labeling agreement, as determined using the F1 scoring
metric25,26, was above 90% for most behaviors and above 95% for all
behaviors combined (Extended Data Fig. 1). We further validated these
behavioral observations by implementing an unsupervised behavioral
classification analysis based on a pose estimation model to automati-
cally identify stereotyped, recurring behavioral actions27,28. There was
high correspondence between the unsupervised classifier and our
manually defined behavioral categories and labels (Supplementary
Fig. 3, Extended Data Fig. 2, Supplementary Videos 2 and 3 and Meth-
ods). Thus, egg-laying behavior appears to be organized as an ordered
sequence of behavioral components.
Although the sequential organization of these behaviors is con-
served, the timing, frequency and duration of the individual compo-
nents within this behavioral sequence exhibit considerable variability
(Fig. 1c,d). Moreover, the behavioral sequence was conserved, but
transitions could occur in both directions (Fig. 1e and Supplementary
Table 2). This variability in transitions was not only apparent for explo-
ration but also observed for deposition behaviors. Although burrowing
was always preceded by abdominal bending, bending was not always
followed by burrowing. Instead, walking or proboscis contact were
observed. Likewise, 35% of burrowing episodes did not persist to egg
expulsion but were aborted in favor of additional bouts of burrowing
or further exploration. Persistent burrowing invariably preceded egg
expulsion, and behavioral transitions following expulsion exhibited
little variability and proceeded along the ordered sequence to the reset
phase. Thus, during both exploration and deposition, the egg-laying
sequence is comprised of multiple junctures between component
actions that may serve as decision points. These junctures may allow
the fly to advance or reinitiate the sequence contingent on sensory
information obtained during a component action.
Egg deposition sequence adjusts to changes in substrate
firmness
We therefore asked whether abdominal bending and ovipositor bur-
rowing, component actions obligatory for the subterraneous deposi-
tion of the egg, adapt to changes in the properties of the substrate. We
initially scored both the count and the depth of penetration of eggs laid
on agarose substrates of increasing firmness (agarose concentrations
from 0.75% to 2.5%; Fig. 2a,b)7,11,16. As substrate firmness was increased,
flies were less successful at achieving subterraneous egg deposition
(Fig. 2a). Above 1.75% agarose, total egg output dropped significantly,
with a mean of 11 eggs laid in 4 h on 2.0% agarose compared to 18–21 eggs
on 0.75% to 1.75% agarose (Fig. 2b), and flies deposited a significantly
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5
larger fraction of eggs on the substrate surface (Fig. 2a). These data
suggest that the ability to achieve subterraneous egg placement is
sensitive to substrate firmness and may positively gate egg output.
We filmed egg-laying behavior on different substrates and
observed that progression along the deposition sequence was dramati-
cally reduced as the firmness of the agarose substrate was increased
(Fig. 2c). The probability of transitioning from bending to burrowing
was highest on 1% agarose and was reduced by 29% on 1.75% agarose
(Fig. 2d). Moreover, burrowing-to-expulsion transitions reduced by
65% as the agarose concentration increased from 0.75% to 1.75% (Fig.
2e). These data suggest that abdominal bending is not simply a means of
initiating burrowing. Rather, bending may allow substrate sampling by
sensory organs on the abdominal terminalia that permits the recogni-
tion of tactile cues that regulate the transition to burrowing. Burrowing
is also likely to be gated by tactile feedback as the fly makes contact with
and engages the substrate in an effort to achieve subterraneous egg
deposition. Burrowing may be unsuccessful on firmer substrates and
can be aborted in search of a more favorable location. The variability
in the sequence of these behaviors is likely to reflect the search for an
optimal location to deposit eggs subterraneously.
Terminalia sensory bristles regulate sequence progression
Peripheral touch sensation in Drosophila is mediated by tactile hairs, or
bristles, that cover the surface of the fly29. The bristles of the ovipositor
valves and adjacent segments of the abdominal terminalia make contact
with the substrate during egg deposition in Drosophila (Fig. 3a)30–32.
In flies that express pan-neuronal green fluorescent protein (GFP), we
observed that the vast majority of terminalia bristles are innervated by
a single bipolar neuron, a canonical feature of purely mechanosensory
bristles (elav-GAL4>mCD8–GFP; Extended Data Fig. 3a)29,31. Moreover,
the base of all terminalia bristles stains with an antibody directed to
NOMPC, a force-sensitive ion channel present in mechanosensory
neurons (Extended Data Fig. 3b,c)33–35. These observations suggest that
the terminalia bristles harbor mechanosensory neurons that play a role
in tactile sensing during the egg deposition sequence.
We searched literature and image databases and used the
split-GAL4 intersectional strategy to generate two restrictive driver
lines (ATB-1 and ATB-2) that target the sensory neurons that innervate
the abdominal terminalia bristles (ATB neurons; Fig. 3b–e, Extended
Data Fig. 4 and Supplementary Table 3)36–39. ATB-1 also drives expres-
sion in a sparse population of neurons in the brain, whereas ATB-2
drives reliable expression in the forelegs but not in the brain. However,
the only consistently labeled neurons common to these two lines are
those that innervate the approximately 150 terminalia bristles (88%
and 76% innervated overall, including 81% and 79% of ovipositor bris-
tles, in ATB-1 and ATB-2, respectively; Supplementary Table 3). The
axons of these neurons project to a ventral domain of the abdominal
neuromere (Fig. 3e and Extended Data Fig. 4b) and are thus poised to
inform local circuits about tactile properties of the substrate during
egg deposition18,40.
We asked whether ATB neurons are functionally involved in egg
laying by expressing the potassium channel Kir2.1 in these neurons
to inhibit their activity (ATB-1>Kir2.1; Extended Data Fig. 4d)41. We
initially filmed egg-laying behavior on 1% and 1.25% agarose substrates
in control and ATB-silenced flies. Control flies exhibited reduced pro-
gression from bending to burrowing as we increased the firmness of
the substrate (Fig. 3f). By contrast, ATB-silenced flies progressed from
bending to burrowing with high probability on all substrates examined
(Fig. 3f). These flies also showed aberrant burrowing behavior on
agarose substrates; burrowing episodes were shorter in duration and
more frequently aborted than in control flies (Fig. 3g,h and Extended
Data Fig. 5a,b). Furthermore, ATB-silenced flies atypically depos-
ited eggs on the rigid chamber walls; when burrowing on the wall,
ATB-silenced flies expeled eggs during 15% of the burrowing episodes,
whereas burrowing on the wall in control flies rarely persisted to egg
expulsion (Fig. 3g–i).
Together, these results suggest that tactile feedback from ATB
neurons modulates the egg deposition sequence, affording an adap-
tive response to the firmness of the substrate. While bending on a
firm substrate, the tactile response of ATB neurons may suppress the
transition to burrowing (Fig. 3f). During burrowing, tactile feedback
from the ATB neurons may promote the persistence of burrowing on
ideal substrates and elicit the abortion of burrowing on inappropri-
ate substrates (Fig. 3g). Absent this feedback, bending transitions
to burrowing with high frequency, and burrowing transitions to egg
expulsion with low frequency, independent of the substrate firmness
(Fig. 3f,g).
We next examined the consequences of ATB silencing on both the
count and the depth of penetration of eggs across a wider range of sub-
strate firmness. Flies with silenced ATB neurons exhibit a diminished
ability to achieve subterraneous egg deposition on substrates firmer
than 0.5% agarose, whereas control flies deposit subterraneously until
1.25% (Fig. 3j and Extended Data Fig. 5c,d; ATB-2>Kir2.1 silenced flies
on 1% agarose). ATB-silenced flies continued to release a large number
of eggs on firmer substrates despite the failure to achieve subter-
raneous egg placement (Fig. 3j,k and Extended Data Fig. 5c,d). By
contrast, control flies began to show a reduction in egg output at 1.25%
(Fig. 3j,k). In flies in which Kir2.1 expression was restricted to the subset
Fig. 3 | Terminalia sensory bristles regulate sequence progression. a, Top:
example video snapshot of abdominal bending. Scale bar, 1 mm. Bottom:
brightfield image of the female posterior abdomen, approximating terminalia
bristle surface contact (white box at top). The dashed blue line indicates the
approximate substrate surface. Scale bar, 50 μm (b–e). b, Diagram of the female
posterior abdomen (lateral aspect); orange circles, bristles innervated by GFP-
labeled neurons in the representative ATB-1>mCD8–GFP image in c (left); gray
circles, non-innervated bristles; T6–T8, sixth–eighth tergite; S6 and S7, sixth and
seventh sternite; A, analia; OV, ovipositor valve. c, Representative images of the
posterior abdomen from two of nine ATB-1>mCD8–GFP females (lateral (left)
and ventral (right) aspects); mCD8–GFP expression, membrane of ATB neurons
(green); autofluorescence, abdominal cuticle (magenta); background, overlaid
brightfield images revealing extended bristles; orange boxes, regions shown
in d. d, Higher-resolution regions of the left image in c displaying GFP-labeled
and brightfield images. Red asterisks indicate bristles innervated by single
GFP-labeled neurons. e, Representative image of the ventral nerve cord (left)
and abdominal neuromere (right) from three ATB-1>mCD8–GFP females stained
with anti-GFP (ATB neurons, green) and anti-bruchpilot (nc82; synaptic neuropil,
magenta). Black bars flanking the left image indicate the region shown at higher
resolution on the right. f, Average probability (P) of progression from bending
to burrowing. Only flies that exhibited two or more bend bouts were considered
(n = 17, 22, 15, 27, 17, 23, 25, 24 and 36 flies per group). Here and in g and i–k, box
bounds indicate the 25th and 75th percentiles, the red line indicates the medians,
and the whiskers indicate the 5th and 95th percentiles; o, data from individual
flies; +, outliers. Here and in g and i, *P < 0.05, **P < 0.01 and ***P < 0.001; data
were analyzed by two-sided Wilcoxon rank-sum test followed by a Bonferroni
correction (Supplementary Table 7). g, Average probability of progression from
burrowing to egg out. Only flies that exhibited two or more burrowing episodes
were considered (n = 17, 22, 15, 27, 17, 17, 25, 24 and 41 flies per group).
h, Representative ethograms depicting bending and burrowing behavior on
1.0% agarose. Each ethogram depicts data from five flies (n = 3 events per fly); <,
eggs deposited on the wall; t = 0, egg out. i, Fraction of eggs deposited on walls of
chambers containing 1% agarose substrate (n = 19, 33 and 32 flies per group). Only
flies that released three or more eggs were considered. j, Average normalized
depth of penetration of eggs released on substrates of varying firmness (GAL4-
only, n = 10, 27, 21, 25, 29 and 19 flies per group; UAS-only, 29, 13, 34, 18, 24 and 12
flies per group; ATB-1>Kir2.1, 19, 10, 22, 24, 25 and 4 flies per group). Here and in
k, *P < 0.05, **P < 0.01 and ***P< 0.001; data were analyzed by Kruskal–Wallis test
with a post hoc Tukey’s HSD test (Supplementary Table 7). k, Number of eggs
released in 4 h on substrates of varying firmness (GAL4-only, n = 13, 29, 28, 30,
42 and 28 flies per group; UAS-only, 31, 13, 35, 19, 33 and 27 flies per group; ATB-
1>Kir2.1, 20, 10, 26, 27, 27 and 10 flies per group).
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5of brain neurons labeled by ATB-1, subterraneous egg deposition was
largely unaffected (ATB-1>Otd-nls:FLP; UAS(FRT.mCherry)Kir2.1-GFP;
Extended Data Fig. 6)42. Thus, ATB neurons may coordinate penetration
of the substrate by the ovipositor and positively gate egg expulsion
after successful penetration.
ATB silencing yields a complex array of phenotypes that strongly
implicate ATB neurons in providing tactile feedback during egg laying
that modulates the progression from bending to burrowing to egg
expulsion. The mechanisms by which ATB neurons exert this control
may rely on the spatial and morphological heterogeneity of terminalia
bristles32. Individual sets of bristles may exhibit unique tuning proper-
ties and may function independently to modulate different phases of
the behavioral progression29,33,43,44.
Burrowing behavior adjusts to changes in substrate firmness
The pivotal role of subterraneous egg placement in the progression
of component behaviors led us to closely examine the substructure of
burrowing (Fig. 4a). A burrowing episode is comprised of discrete cycles
that begins with rhythmic ovipositor digging. As the surface is scored,
the ovipositor extends into the substrate, and the egg emerges out of
the uterus and into the ovipositor. Rhythmic pushing expels the egg
out of the ovipositor and into the substrate, just beneath the surface.
Completed egg expulsion halts the rhythm, terminating the burrowing
episode, and the fly then detaches the ovipositor from the egg.
In chambers containing 1% agarose, egg expulsion required a
minimum of three cycles and could require as many as ten cycles within
a burrowing episode (Fig. 4b). The number of cycles was significantly
a
b
Posterior abdomen
ATB neurons
T6
f
GAL4-only
UAS-only
ATB-1>Kir2.1
*
**
***
g
*****
GAL4-only
UAS-only
ATB-1>Kir2.1
***
*** *******
*
****
)
w
o
r
r
u
b
o
t
d
n
e
b
(
P
T7
S6
1.0
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0.6
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)
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(
P
1
0.8
0.6
0.4
0.2
0
A
T8
S7
OV
c
ATB-1 lateral posterior abdomen
mCD8–GFP cuticle
ATB-1 ventral posterior abdomen
mCD8–GFP cuticle
h
1.00
1.25
Wall
GAL4-only
Wall
1.00
1.00
1.25
Percent agarose
UAS-only
1.25
Wall
d
A
*
*
*
*
*
*
OV
*
*
*
*
*
*
*
*
*
*
*
T7
S7
e
ATB-1 ventral nerve cord
mCD8–GFP nc82
ATB-1 abdominal neuromere
mCD8–GFP nc82
1.00
1.25
Wall
Wall
1.00
1.00
1.25
Percent agarose
1.25
Wall
***
ATB-1>Kir2.1
Substrate:
Bend
Burrow
Wall:
Bend
Burrow
<
<
<
<
<
<
<
<
i
s
g
g
e
f
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d
1.0
0.8
0.6
0.4
0.2
0
y
l
F
6
–30
j
GAL4-only
UAS-only
ATB-1>Kir2.1
h
t
p
e
d
g
g
e
d
e
z
i
l
a
m
r
o
N
k
h
4
n
i
s
g
g
e
f
o
r
e
b
m
u
N
1.0
0.8
0.6
0.4
0.2
0
60
50
40
30
20
10
0
0
–30
0
–30
0
Time (s)
***
*****
h
t
p
e
d
g
g
e
d
e
z
i
l
a
m
r
o
N
** *****
h
4
n
i
s
g
g
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f
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e
b
m
u
N
1.0
0.8
0.6
0.4
0.2
0
60
50
40
30
20
10
0
***
*****
h
t
p
e
d
g
g
e
d
e
z
i
l
a
m
r
o
N
0.25
0.75
0.50
1.00
1.25
Percent agarose
1.50
***
***
h
4
n
i
s
g
g
e
f
o
r
e
b
m
u
N
0.25
0.75
0.50
1.00
1.25
Percent agarose
1.50
1.0
0.8
0.6
0.4
0.2
0
60
50
40
30
20
10
0
ATB-1
UAS-Kir2.1
n (flies)
+
–
19
–
+
33
+
+
32
*****
****
***
***
***
0.25
0.75
0.50
1.00
1.25
Percent agarose
1.50
*
**
*
0.25
0.75
0.50
1.00
1.25
Percent agarose
1.50
0.25
1.00
1.25
0.75
0.50
Percent agarose
1.50
GAL4-only
UAS-only
ATB-1>Kir2.1
0.25
1.00
0.50
1.25
0.75
Percent agarose
1.50
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5
a
1.0% Agarose
y
l
F
1
5
b
n
o
i
t
c
a
r
F
0.4
0.2
0
c
e
d
o
s
i
p
e
w
o
r
r
u
b
r
e
p
s
e
l
c
y
C
16
14
12
10
8
6
4
2
0
→ Abort
→ Expel
→ Cycle
Abort
Expel
–15
–10
–5
0
Time (s)
1.0% Agarose
2
4
6
8
10
Cycles per burrow episode
***
**
***
*
**
*
*
Abort
Expel
0.75
1.00
1.25
1.50
1.75
% Agarose
Fig. 4 | The substructure of burrowing adjusts to changes in substrate
firmness. a, Representative ethograms depicting burrowing episodes in five
flies (n = 4 events per fly); black, aborted episodes; gray, egg expulsion episodes;
red, cycles within a burrowing episode; t = 0, egg out. Data are the same events
depicted in Fig. 1c. b, Distributions of the number of cycles per burrowing
episode; black, aborted episode; gray, egg expulsion episode. Dashed vertical
lines indicate the mean value for each distribution. Data were pooled across
all flies from experiments described in Fig. 1. c, Average number of cycles
per burrowing episode for both aborted and egg expulsion episodes across
substrates of increasing firmness. Only flies that exhibited two or more episodes
for a given episode type were considered (abort episodes, n = 4, 6, 7, 9 and 11 flies
per group; expel episodes, 19, 15, 21, 11 and 11 flies per group). Data are the same
as those used in Fig. 2c–e. Box bounds indicate the 25th and 75th percentiles, red
lines indicate the medians, and whiskers indicate the 5th and 95th percentiles; o,
data from individual flies; +, outliers; *P < 0.05; **P < 0.01; ***P < 0.001. Data were
analyzed by Kruskal–Wallis test with a post hoc Tukey’s HSD test (Supplementary
Table 7).
lower in aborted episodes in which flies did not persist to egg expul-
sion (mean of three cycles for aborted episodes and five cycles for
expulsion; P < 0.001, Wilcoxon rank-sum test), and burrowing could
be aborted after any cycle within an episode (minimum of one and
maximum of eight; Fig. 4b). This suggests that the decision to persist
in burrowing may be determined after each individual cycle. Burrowing
can therefore be extended or aborted and then reinitiated to achieve
successful egg deposition.
We observed that the substructure of burrowing behavior was
dramatically altered as the firmness of the agarose substrate was
increased. The total number of burrow cycles required for egg expul-
sion increased over twofold (Fig. 4c) as the agarose concentration
increased from 0.75% to 1.75%. Thus, additional burrowing cycles are
required to dig and push the egg into the firmer substrates. Burrow
episodes that were aborted also displayed a twofold increase in cycle
count on firmer substrates (Fig. 4c). If the egg cannot be successfully
deposited after an extended attempt, burrowing is aborted in search
of a more favorable location. These data suggest that the transition
to egg expulsion (‘egg out’) and the reset phase is contingent on the
decision to persist in burrowing until the egg is completely expelled.
The decision to persist in burrowing for additional cycles is likely to be
informed by ongoing sensory feedback regarding the position of the
egg as it is pushed through the uterus and ovipositor into the substrate.
Internal sensory neurons activated by the progression
of the egg
We next screened a library of transgenic lines45 to identify candidate
sensory neurons that innervate the lower reproductive tract and detect
the passage of the egg through the uterus during burrowing4,46–48.
We identified a cluster of sensory neurons whose cell bodies flank
the posterior uterus and whose processes arborize along the outer
surface of the distal-most fibers of the muscle that encircles the uterus
(posterior uterine (PU) sensory neurons; Fig. 5a–d)49. We used the
split-GAL4 intersectional strategy to generate two lines that labeled
a pair of PU neurons on each side of the uterus (PU-1, 1.9 ± 0.5 cells per
side, n = 17 sides in 13 flies; PU-2, 2.1 ± 0.3 cells, n = 11 sides in 8 flies;
mean ± s.d.; Fig. 5a–c and Extended Data Fig. 7a,b). These neurons send
projections centrally that terminate in the ventral-most neuropil of
the abdominal neuromere, a sensory domain associated with multi-
dendritic sensory neuron inputs50,51 (Fig. 5e and Extended Data Fig.
7a,c). We confirmed the polarity of PU neurons by targeted expression
of both synaptotagmin–GFP52, a presynaptic marker, and DenMark53,
a somatodendritic marker. Synaptotagmin–GFP was restricted to
the central projections in the abdominal neuromere, while DenMark
localized to the peripheral processes encircling the posterior uterus
(Extended Data Fig. 7d). This pattern of dendritic innervation suggests
that PU neurons may sense the passage of an egg through the posterior
uterus into the ovipositor.
We therefore monitored the activity of PU neurons as the egg is
expelled from the uterus. GCaMP6f was expressed in PU neurons, and
calcium activity was recorded in flies mounted ventral-side up (Fig. 5f
and Methods)54,55. Snapshots from a typical video recording over a 90-s
window encompassing egg expulsion are shown in Fig. 5g, along with
the corresponding activity of the four PU axons and behavioral meas-
urements of the movement of the egg and ovipositor (Supplementary
Video 4). Initially, we observed an incomplete expulsion event (Fig. 5g),
where the egg advanced from the uterus (first frame) into the extruded
ovipositor (second frame), after which the egg retreated into the uterus
and the ovipositor retracted (third frame). During this event, calcium
activity in PU neurons increased from baseline after the egg entered
the ovipositor and returned to baseline when the egg retreated into
the uterus. A second, complete expulsion event then occurred (fourth
frame), and the PU neurons again responded after the egg entered
the ovipositor, and the activity returned to baseline after the egg was
completely expelled (fifth frame). These response properties were
observed in all 28 PU neurons recorded from eight flies (Fig. 5h,i and
Supplementary Fig. 4). PU neurons were not activated when the egg
was at rest in the uterus. Moreover, PU neuron response was specific to
the advancement of the egg into the ovipositor and not simply to the
extrusion of the ovipositor. PU neurons did not respond to ovipositor
extrusion events in flies lacking an egg (Fig. 5j–l and Supplementary
Video 5). These observations demonstrate that PU neurons respond
shortly after the egg passes through the posterior uterus into the ovi-
positor and may therefore inform circuits in the abdominal neuromere
about the position of the egg during burrowing18,40.
Silencing PU neurons disrupts the egg-laying sequence
We silenced PU sensory neurons to examine their role in egg-laying
behavior by the targeted expression of Kir2.1 (PU-1>Kir2.1). In
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5
PU-silenced flies, we observed a dramatic reduction in egg output
(mean of 9 eggs in 4 h compared to 23 and 34 in the two genetic con-
trols; Fig. 6a and Supplementary Fig. 5a). Moreover, the majority of
the eggs in PU-silenced flies were not deposited subterraneously on
a 1.0% agarose substrate (29% deposited subterraneously versus 95%
and 93% in both genetic controls; Fig. 6b and Supplementary Fig.
5b). This reduction in egg count was not a consequence of a mating
defect. PU-silenced virgins showed no deficit in mating but exhibited
a reduction in egg count after a single mating event (Supplementary
Fig. 5c,d).
PU-silenced flies expelled 51% of the eggs in the typical fashion
following burrowing but exhibited a 67% reduction in the progression
from burrowing to egg expulsion (PU-1>Kir2.1, probability of 0.25
versus 0.75 for both genetic controls; Fig. 6c,e). Burrowing episodes
were comprised of fewer cycles than burrowing episodes in control flies
(Fig. 6f,g). Moreover, burrowing episodes culminating in egg expulsion
resulted in the premature release of the egg on the substrate surface.
These data suggest that PU neurons sense the passage of the egg into
the ovipositor during a burrowing episode, promoting persistent bur-
rowing to achieve subterraneous egg deposition.
The remaining 49% of eggs in PU-silenced flies were spontaneously
dropped without burrowing or expression of any of the other behavio-
ral components that typically precede egg expulsion (Fig. 6c). These
eggs spontaneously emerged while the ovipositor was in midair and
were removed by hindleg grooming. This phenotype was exhibited by
19 of 22 PU-silenced flies but was rarely observed in control flies (0% in
12 GAL4-only control flies and 2% in 3 of 30 UAS-only control flies; Fig.
6d). This distinct phenotype may implicate PU feedback in the regu-
lation of musculature required for egg retention. Thus, the silencing
of PU neurons resulted in deficits in burrowing and diminished and
aberrant egg output.
PU neurons control timing and direction of burrow transitions
We next explored the function of PU neurons by targeted expres-
sion of the red-light-activated channelrhodopsin CsChrimson
(PU-1>CsChrimson)56. We devised a physiological paradigm in
which we photostimulated PU neurons in the context of egg laying.
We have shown that PU neurons are activated after passage of the
egg through the uterus into the ovipositor and that their activity
returns to baseline after completed expulsion. We reasoned that
prolonged PU neuron activation beyond egg expulsion may mimic
the continued presence of the egg within the posterior uterus and
ovipositor and delay progression along the behavioral sequence.
In normal egg-laying behavior, burrowing ceases after egg expul-
sion, and the fly detaches from the egg, grooms its ovipositor and
transitions to the reset phase. We photostimulated PU neurons dur-
ing burrowing with a pulse of light that was triggered immediately
before the completion of egg expulsion and remained on after egg
expulsion for different durations (2.5, 5 or 20 s; light onset 0.9 ± 1.1 s
before complete egg expulsion; n = 215 photostimulation events;
mean ± s.d.; Fig. 7a). Prolonged PU neuron activation beyond egg
expulsion resulted in the aberrant persistence of burrowing without
transitioning to detachment despite the absence of an egg in the
uterus (Fig. 7b). With 20-s photostimulation, flies stopped burrowing
an average of 5.5 ± 1.3 s beyond egg expulsion (n = 15 flies; Fig. 7b).
Flies that burrowed throughout the photostimulation period ceased
burrowing after light offset (Fig. 7b and Extended Data Fig. 8a). As
expected, given our observations with 20-s photostimulation, bur-
rowing persisted until light offset frequently for 2.5-s stimulations,
in approximately half of 5-s stimulations and almost never for 20-s
stimulations (52 of 60 stimulations, 45 of 80 and 5 of 75, respectively).
In the remaining events, burrowing persisted for variable durations
but stopped before light offset. These data suggest that PU activation
promotes burrowing persistence. However, persistent burrowing
is not sustained, suggesting an intrinsic temporal control on the
duration of burrowing.
Flies that persist in burrowing throughout photostimulation
abruptly stopped burrowing after light offset and transitioned to
the behavioral sequence normally triggered by egg expulsion
(Fig. 7c). This behavior was observed for all three stimulus durations
Fig. 5 | PU sensory neurons are activated by the progression of the egg.
a, Top: representative image of the lower reproductive tract from four PU-
1>RedStinger; mCD8–GFP females (lateral aspect) stained with anti-GFP
(membrane of PU neurons, green), anti-DsRed (nuclei of PU neurons, red)
and phalloidin (muscle F-actin, gray); autofluorescence, abdominal cuticle
(magenta). Bottom (left and right): higher-resolution region of the top image
(indicated by red bars flanking the top image) displaying two PU cell bodies
(white triangles). The black bars flanking the top image indicate the region show
in b. Here and in d and f, a indicates analia, op indicates ovipositor (hypogynium),
sp indicates spermathecae, u indicates uterus (genital chamber), od indicates
oviduct, sr indicates seminal receptacle, and e indicates egg. Scale bar, 50 μm
(b–e). b, Higher-resolution region of the top image in a displaying PU labeling
at four successive depths surrounding the posterior uterus (region 1 is the
most superficial). c, PU neuron expression (anti-GFP) in the posterior uterus
(depth indicated as in b). The red and blue dashed lines demarcate the outer
and inner bounds, respectively, of the CMU. d, Diagram of the female posterior
abdomen (lateral aspect) revealing the lower reproductive tract. PU neurons
are labeled cyan (the triangle indicates the cell bodies). e, Representative
image of the ventral nerve cord (left) and abdominal neuromere (right) from 15
PU-1>mCD8–GFP females stained with anti-GFP (PU neurons, green) and nc82
(synaptic neuropil, magenta). Black bars flanking the left image indicate the
region shown at higher resolution on the right. f, Two-photon experimental
setup involving simultaneous measurement of GcaMP6f (green) and tdTomato
(red) fluorescence in axons within a coronal section of the abdominal nerve
trunk (top right; scale bar, 10 μm) and videography of the posterior abdomen
(bottom right; scale bar, 200 μm). Bottom right: magenta and blue lines connect
the dorsal–posterior edge of T6 with the egg and ovipositor, respectively;
the bar graph displays the normalized distances between T6:egg (magenta),
T6:ovipositor (blue) and ovipositor:egg (brown, negative distance; black,
positive distance; Methods). g, Representative experiment showing video
snapshots of the posterior abdomen (top; scale bar, 200 μm), two-photon
imaging of four PU neurons depicting relative fluorescence changes of GCaMP6f
and TdTomato (middle; green and dashed red traces, respectively) and
movement of the egg and ovipositor (bottom; Methods). Arrows and vertical
dashed lines indicate the corresponding time point for each video snapshot.
Vertical gray lines indicate the onset of calcium response events. Data are the
same as those presented in Supplementary Video 4. h, Normalized PU responses
and behavioral measures surrounding incomplete egg expulsion events (left) and
completed egg expulsion (right). Top: individual neuron responses; horizontal
white lines demarcate recordings performed in different flies. Cells from g are
indicated by red dots. Middle and bottom: aggregate response of all neurons
and aggregate behavioral measurements, respectively (darker traces, mean
response; lighter area, s.e.m.); n = 28 neurons from eight flies; t = 0, behavioral
event onset (Methods). i, Normalized population data showing the 3-s integrated
ΔF/F0 fluorescence levels during incomplete egg expulsion and complete egg
expulsion and after egg expulsion (n = 28 neurons). Here and in l, box bounds
indicate the 25th and 75th percentiles, the red lines indicate the medians, and the
whiskers indicate the 5th and 95th percentiles; o, data from individual neurons;
+, outliers; ***P < 0.001; NS, P > 0.05. Data were analyzed by two-sided Wilcoxon
signed-rank test compared to preexpulsion baseline (Supplementary Table
7). j, Representative experiment comparing PU neuron activity surrounding
incomplete egg expulsion (left) and ovipositor extrusion events lacking an
egg (right). The figure was constructed as in g. k, Normalized PU responses
and behavioral measures surrounding incomplete expulsion events (left)
and ovipositor extrusion events after egg expulsion (right). The figure was
constructed as in h. Top: cells from j are indicated by red dots; n = 13 neurons
from four flies. The same data are presented in Supplementary Video 5.
l, Normalized population data showing the 3-s integrated ΔF/F0 fluorescence
levels during incomplete expulsion events (‘before’) and ovipositor extrusion
events (‘after’); n = 13 neurons.
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5(Fig. 7b, Extended Data Fig. 8b,c and Supplementary Video 6). These
data support the argument that PU neurons are activated by the pas-
sage of the egg into the ovipositor and drive the persistence of bur-
rowing. These observations also suggest that a decrease in PU activity
signals the completion of egg expulsion, resulting in the transition to
detachment, grooming and the reset phase (Fig. 7d).
In the photostimulation events where burrowing stopped before
light offset, flies exhibited exploration and deposition behaviors in
a
PU-1 lower reproductive tract
mCD8–GFP DsRed F-Actin Cuticle
b
PU-1 posterior uterus
mCD8–GFP DsRed F-Actin
c
PU-1 posterior uterus
mCD8–GFP
d
sp
od
a
op
sr
u
1
3
2
4
CD8–GFP
Nuclear DsRed
PU-1 ventral nerve cord
mCD8–GFP nc82
PU-1 abdominal neuromere
mCD8–GFP nc82
f
Ventral
nerve cord
2
1
Lower reproductive tract
PU neurons
sp
od
a
sr
e
u
3
op
Before egg expulsion
Coronal section of
abdominal nerve trunk
During egg expulsion
GCaMP6f tdTomato
Abdominal
nerve trunk
Imaging coronal
section of nerve
Two-photon objective
Posterior abdomen
e
op
a
…
…
Rest
(egg in uterus)
Incomplete
expulsion
(egg enters
ovipositor)
Rest
Complete
expulsion
After expulsion
(egg in uterus)
(egg passes through
ovipositor)
(vacant uterus)
Incomplete
expulsion
(egg enters
ovipositor)
Rest
(egg in uterus)
Incomplete
expulsion
(egg enters
ovipositor)
Ovipositor
extrusion
(no egg,
vacant uterus)
Rest
(vacant uterus)
Ovipositor
extrusion
(no egg,
vacant uterus)
j
Before egg expulsion
After egg expulsion
e
g
0
F
/
F
∆
3
0
1.5
0
2
0
2
0
d
e
z
i
l
a
m
r
o
N
e
c
n
a
t
s
i
d
1.5
1.0
1
0
h
1
n
o
r
u
e
N
28
6
0
1.6
0.6
e
r
o
c
s
z
e
c
n
a
t
s
i
d
d
e
z
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l
a
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r
o
N
Incomplete egg
expulsion
Complete egg
expulsion
.
...
z score
12
8
4
0
GCaMP6f
GCaMP6f
T6:ovipositor
T6:egg
T6:ovipositor
T6:egg
–5
0
5
10
–5
0
5
10
Time (s)
Time (s)
i
0
F
/
F
∆
d
e
t
a
r
g
e
t
n
i
d
e
z
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l
a
m
r
o
N
0.5
0
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
GCaMP6f
tdTomato
T6:ovipositor
T6:egg
Ovipositor:egg
5 s
3
0
1.5
0
2
0
2
0
1.6
0.6
0
F
/
F
∆
d
e
z
i
l
a
m
r
o
N
e
c
n
a
t
s
i
d
k
1.0 *** *** NS
3
0
1.5
0
2
0
2
0
1.6
0.6
GCaMP6f
tdTomato
T6:ovipositor
T6:egg
5 s
1
Before egg expulsion
Incomplete egg expulsion
.
.
.
.
After egg expulsion
Ovipositor extrusion
z
score
12
8
4
0
n
o
r
u
e
N
13
6
0
1.6
0.6
e
r
o
c
s
z
e
c
n
a
t
s
i
d
d
e
z
i
l
a
m
r
o
N
GCaMP6f
GCaMP6f
T6:ovipositor
T6:egg
T6:ovipositor T6:egg
–5
0
5
10
–5
0
5
10
Time (s)
Time (s)
Complete
Incomplete
After
GCaMP6f
tdTomato
T6:ovipositor
5 s
*** NS
1.0
0.5
0
Before
After
1061
l
0
F
/
F
∆
d
e
t
a
r
g
e
t
n
i
d
e
z
i
l
a
m
r
o
N
Articlehttps://doi.org/10.1038/s41593-023-01332-5
4-h egg-laying assay
2-h filmed egg-laying assay
***
***
a
50
40
30
20
10
h
4
n
i
s
g
g
e
f
o
r
e
b
m
u
N
0
PU-1
UAS-Kir2.1
n (flies)
+
–
33
–
+
19
+
+
29
b
h
t
p
e
d
g
g
e
d
e
z
i
l
a
m
r
o
N
1.0
0.8
0.6
0.4
0.2
0
***
***
**
**
d
d
e
p
p
o
r
d
s
g
g
e
n
o
i
t
c
a
r
F
1.0
0.8
0.6
0.4
0.2
0
***
***
e
)
t
u
o
g
g
e
o
t
w
o
r
r
u
b
(
P
1.0
0.8
0.6
0.4
0.2
0
+
–
32
–
+
18
+
+
21
PU-1
UAS-Kir2.1
n (flies)
+
–
12
–
+
30
+
+
22
+
–
12
–
+
29
+
+
10
f
e
d
o
s
i
p
e
w
o
r
r
u
b
t
r
o
b
a
r
e
p
s
e
l
c
y
C
16
12
8
4
0
**
**
+
–
7
–
+
5
+
+
9
g
e
d
o
s
i
p
e
w
o
r
r
u
b
l
e
p
x
e
r
e
p
s
e
l
c
y
C
16
12
8
4
0
***
*
+
–
12
–
+
30
+
+
10
PE Walk Bend Burrow Detach Groom
GAL4-only
UAS-only
PU-1>Kir2.1 (burrowed eggs)
PU-1>Kir2.1 (dropped eggs)
c
y
l
F
1
5
–60
–30
0
30
60
–60
–30
0
30
60 –60
–30
0
30
60 –60
–30
0
30
60
Time (s)
Time (s)
Time (s)
Time (s)
Fig. 6 | Silencing PU neurons reduces egg output and disrupts the egg-laying
sequence. a, Number of eggs released on a 1% agarose substrate in 4 h (n = 33, 19
and 29 flies per group). Here and in b and d–g, box bounds indicate the 25th and
75th percentiles, the red lines indicate the medians, and the whiskers indicate
the 5th and 95th percentiles; o, data from individual flies; +, outliers; *P < 0.05;
**P < 0.01; ***P < 0.001. Data were analyzed by two-sided Wilcoxon rank-sum
test followed by a Bonferroni correction (Supplementary Table 7). b, Average
normalized depth of penetration of released eggs (n = 32, 18 and 21 flies per
group). c, Representative ethograms of egg-laying behavior for genetic control
flies (first and second graphs) and for PU-silenced flies (third (burrowed eggs)
and fourth (spontaneously dropped eggs) graphs). Each ethogram depicts
data from five flies (n = 4 events per fly); t = 0, egg out. d, Fraction of eggs
spontaneously dropped without burrowing (n = 12, 30 and 22 flies per group).
Only flies that released four or more eggs are considered here and in e–g.
e, Average probability (P) of progression from burrowing to egg out (n = 12, 29
and 10 flies per group). Only flies that exhibited three or more burrowed eggs
are considered. f, Average number of cycles per aborted burrowing episode
(n = 7, 5 and 9 flies per group). Only flies that exhibited three or more aborted
burrowing episodes are considered. g, Average number of cycles per egg
expulsion burrowing episode (n = 12, 30 and 10 flies per group). Only flies that
exhibited three or more egg expulsion burrowing episodes are considered.
new locations despite the fact that they had already expelled an egg
(Fig. 7b,c, Extended Data Fig. 8b–d and Supplementary Video 7). This
behavior appears to recapitulate the behavior observed after aborting
a burrowing episode in normal egg-laying behavior. In wild-type flies,
prolonged burrowing and PU activation without expulsion may signal
the inability to deposit an egg, resulting in abortion of the episode (Fig.
7d). In the photostimulation experiment, the flies may be unaware of
having laid an egg, and the prolonged activation of PU neurons may
also signal the inability to deposit an egg, resulting in abortion (Fig.
7d). These flies persistently expressed exploration and deposition
behaviors and exhibited numerous burrowing episodes for up to sev-
eral minutes beyond light offset despite the absence of an egg in the
uterus (Extended Data Fig. 8e,f). After the decision to abort and revert,
a decrease in PU activity (photostimulation offset) no longer triggers
the transition to reset.
These experiments demonstrate a role for PU activation and inac-
tivation in the context of a burrowing episode. We therefore asked
whether photostimulation of PU neurons could impact behavior out-
side the context of burrowing. We photostimulated PU neurons for
20 s at 90-s intervals, independent of the ongoing behavioral state of
the fly. Neither the photostimulation period nor its offset induced an
overt behavioral response (Extended Data Fig. 9). Thus, optogenetic
activation only results in persistent burrowing during an ongoing bur-
rowing episode. Moreover, a decrease in PU neuron activity only signals
the completion of egg expulsion and the transition to postdeposition
behaviors in the context of an ongoing burrowing episode.
A pair of uterine motor neurons expels the egg during
burrowing
The transition from burrowing to egg expulsion represents the final
decision point in the egg-laying sequence and results in the transition to
reset. We screened an image database45 to identify transgenic lines that
target motor neurons innervating the uterus and involved in expelling
the egg. We identified a symmetric pair of large neurons in the abdomi-
nal neuromere that project into the abdominal nerve trunk and ramify
along the ipsilateral length of the muscle that encircles the uterus49. We
used the split-GAL4 intersectional strategy to generate four lines with
restricted expression in these neurons (circular muscle of the uterus
(CMU) neurons; Fig. 8a–c, Extended Data Fig. 10a and Supplementary
Table 4). Their axon terminals exhibit abundant boutons that stain with
an antibody to Drosophila VGLUT, a marker for glutamatergic motor
neuron synapses (Fig. 8a)57,58.
We asked whether stimulation of CMU neurons could trigger egg
expulsion by expressing the channelrhodopsin CsChrimson in these
neurons. Optogenetic activation reliably induced egg expulsion in
gravid females (Fig. 8d). Moreover, following activation of CMU neu-
rons, histological analysis revealed that the uterus was dramatically
constricted (Extended Data Fig. 10b). Egg expulsion did not occur after
photostimulation of control flies harboring only UAS-CsChrimson or
the split-GAL4.
We used two-photon imaging to demonstrate that CMU neurons
are indeed active when a fly expels an egg. GCaMP6f was expressed
in CMU neurons, and calcium activity was recorded, as in Fig. 5f. We
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5
a
a
op
Photostimulation paradigm
…
…
Burrow onset …
Burrow persist …
Score behavior …
Light ON:
2.5 s
5 s
20 s light stimulation
Time
d
Normal egg laying
Time
(1) Burrowing persists to expulsion, PU inactivation:
PU activity: OFF
ON
OFF
Burrow …
Advance
(2) Burrowing aborts before expulsion while PU active:
PU activity: OFF
ON
Burrow …
Revert
PU photostimulation
(beyond egg expulsion)
(3) Burrowing persists to light offset, PU inactivation:
PU activity: OFF
ON
Burrow …
(4) Burrowing stops before light offset while PU active:
PU activity: OFF
ON
Photostimulation
OFF
Advance
b
t
n
u
o
c
t
n
e
v
E
80
40
0
20
10
0
20
10
0
20
10
0
Revert
Burrow
Burrow
Advance
Control (no light)
c
PE Walk Bend Burrow Detach Groom
Control (no light)
0:83
0
5
10
15
20
1
s
t
n
e
v
E
10
0
5
25
Time after egg expulsion (s)
20
10
15
30
Light (2.5 s)
8:52
Light (5 s)
Burrowing persists to photostimulation
offset
0
5
10
15
20
1
s
t
n
e
v
E
Light (5 s)
35:45
10
0
5
25
Time after egg expulsion (s)
20
10
15
30
0
5
10
15
20
Light (20 s)
70:5
Light (5 s)
Burrowing stops before photostimulation
offset
1
s
t
n
e
v
E
10
Burrow …
Revert
0
5
10
15
20
Time to burrow stop after egg expulsion (s)
0
5
25
Time after egg expulsion (s)
20
10
15
30
Fig. 7 | PU neurons control timing and direction of burrow transitions.
a, Schematic of the photostimulation paradigm. The box drawn on the fly’s
abdomen (middle left) depicts the region shown in higher detail above; a, analia;
op, ovipositor. Photostimulation (655-nm light at 8 μW mm–2) was initiated
during burrowing immediately before completed egg expulsion (egg out) and
was sustained for variable amounts of time after egg expulsion. b, Stacked
distributions of the timing that burrowing stopped after egg expulsion for
control (top) and stimulus conditions. Events are color coded according to
which transition was made after burrowing stopped; orange, flies reverted in the
sequence; cyan, flies advanced to the reset phase (Methods and Extended Data
Fig. 8d). Here and in c, the red bar above each plot indicates the photostimulation
period. Data represent 298 events from 16 flies. The total number of events
in each group is indicated in the top right. c, Representative ethograms of
egg-laying behavior for no-light control events (top) and 5-s photostimulation
events, separately depicting events where burrowing persisted throughout
photostimulation (middle) and events where burrowing stopped during
photostimulation (bottom); vertical black dashed line, photostimulation offset;
black tick marks near t = 0, timing of completed egg expulsion for each event.
d, Model for how PU neuron activity determines the timing and direction of
burrow transitions. Top: normal egg-laying behavior. PU neurons at baseline
(black, inactive) at the onset of burrowing become activated (green) after the
passage of the egg into the ovipositor during burrowing and return to baseline
after completed egg expulsion. Bottom: behavior during photostimulation.
Vertical black dashed lines, time of completed egg expulsion (egg out); advance,
fly progresses to the reset phase; revert, fly transitions to preceding components
of the sequence.
observed an acute increase in calcium activity concurrent with egg
expulsion (Fig. 8e,f). These observations suggest that the CMU neurons
are active during natural egg laying, expelling the egg during burrowing.
We also expressed the anion channelrhodopsin GtACR1 (ref. 59) to
silence CMU neurons and examine their functional role in egg-laying
behavior. In CMU-silenced flies, we observed a dramatic reduction
in egg output compared to control flies (Fig. 8g and Supplementary
Fig. 6). Moreover, CMU-silenced flies spontaneously dropped 89% of
their eggs without burrowing (Fig. 8h). Egg-laying behavior was intact
in these flies, and they engaged in a comparable number of burrow-
ing episodes as control flies (Fig. 8i,j). However, burrowing almost
never culminated in egg expulsion in flies with silenced CMU neurons
(Fig. 8k). Thus, CMU neuron activity is necessary to progress from
burrowing to egg expulsion, the final decision point in the egg-laying
sequence (Fig. 8l).
Discussion
We have characterized the structure of egg-laying behavior in the fly and
demonstrate that it consists of a sequence of component actions analo-
gous to Nikolaas Tinbergen’s ‘reaction chain’3. Tinbergen portrayed
innate behaviors as a reaction chain, in which each component action
of the sequence enhances the probability of encountering releasers, or
‘sign stimuli’, that promote progression to a subsequent component.
This organization of component actions provides decision points at the
junctures of component behaviors that ensure the successful progres-
sion toward the consummate act that satisfies the drive.
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5
a
CMU-1 lower reproductive tract
mCD8–GFP DVGLUT F-Actin Cuticle
sr
sp
od
sp
u
op
b
Lower reproductive tract
CMU neurons
sp
od
sr
u
**
a
e
op
g
n
o
i
t
a
n
m
u
i
l
l
i
2
–
m
m
w
µ
6
h
2
n
i
s
g
g
e
f
o
r
e
b
m
u
N
45
30
15
0
**
*
h
d
e
p
p
o
r
d
s
g
g
e
f
o
n
o
i
t
c
a
r
F
1.0
0.8
0.6
0.4
0.2
0
CMU-2
UAS-GtACR1
n (flies)
+
–
14
–
+
14
+
+
11
l
c
T
U
L
G
V
D
P
F
G
–
8
D
C
m
P
F
G
–
8
D
C
m
T
U
L
G
V
D
i
1
y
l
F
CMU-1 ventral nerve cord
mCD8–GFP nc82
CMU-1 abdominal neuromere
mCD8–GFP nc82
d
40 µw mm–2 photostimulation
e
Before expulsion Egg expulsion
After expulsion
Right CMU
neuron
Left CMU
neuron
4
0
3
0
2
1
1.5
0
0
F
/
F
∆
e
c
n
a
t
s
i
D
GCaMP6f
tdTomato
T6:ovipositor
T6:egg
Ovipositor:egg
5 s
GAL4-only
UAS-only
CMU-2>GtACR
Burrow
( = egg out)
CMU-3>CsChR
CMU-4>CsChR
UAS-control
CMU-3-control
CMU-4-control
***
******
***
***
***
***
** ***
***
***
***
h
t
i
w
s
e
i
l
f
f
o
n
o
i
t
c
a
r
F
g
g
e
d
e
l
l
e
p
x
e
1.0
0.5
0
0.1 0.2 0.5 1.0
Stimulation duration (s)
Egg expulsion
z score
10
5
0
GCaMP6f
T6:ovipositor
T6:egg
0
5
10
Time (s)
NS
k
)
t
u
o
g
g
e
o
t
w
o
r
r
u
b
(
P
1.0
0.8
0.6
0.4
0.2
0
***
f
1
n
o
r
u
e
N
e
r
o
c
s
z
8
8
0
1.6
d
e
z
i
l
a
m
r
o
N
e
c
n
a
t
s
i
d
0.6
–5
j
s
e
d
o
s
i
p
e
w
o
r
r
u
b
f
o
r
e
b
m
u
N
120
80
40
0
0
–30
0
–30
0
Time (s)
CMU-2
UAS-GtACR1
n (flies)
+
–
14
–
+
14
+
+
11
+
–
12
–
+
10
+
+
9
+
–
14
–
+
12
+
+
6
5
–30
ATB
move
Bend
ATB
Cycle
PU
ATB
PU
CMU
PU
Burrow
Egg out
Reset
ATB
PU
Our data demonstrate that the individual components of
egg-laying behavior are not simply motor acts but also acts of sensory
evaluation of the external and internal world that govern behavioral
progression. During exploration, substrate cues are encountered
while walking and proboscis sampling that may identify a suitable loca-
tion for egg deposition6,11,17. The flies then initiate more refined local
exploration involving abdominal bending to permit sampling with the
abdominal terminalia. We have identified a class of external sensory
neurons (ATB neurons) that innervate tactile hairs on the abdominal
terminalia29,31, which contact the substrate and regulate the transi-
tion from bending to burrowing. During burrowing, the ovipositor is
used to score the surface, extend into the substrate and expel the egg
subterraneously. We further describe a pair of uterine motor neurons
(CMU neurons) that enact this critical transition of burrowing to egg
expulsion. The expulsion of the egg triggers egg detachment and
the transition to the final behavioral phase, grooming and ovulation,
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
1064
Articlehttps://doi.org/10.1038/s41593-023-01332-5
Fig. 8 | A pair of uterine motor neurons expels the egg during burrowing.
a, Left: representative image of the lower reproductive tract (ventral aspect)
from four CMU-1>mCD8–GFP females, stained with anti-GFP (membrane
of CMU neurons, green), phalloidin (muscle F-actin, gray) and anti-DVGLUT
(glutamatergic synapses, red); autofluorescence, ovipositor cuticle (magenta).
Right: high-resolution images of axon terminals; arrow, individual bouton. Here
and in c, black bars flanking the left image indicate the region shown at higher
resolution on the right. Here and in b, op indicates ovipositor, sp indicates
spermathecae, u indicates the uterus, od indicates the oviduct, sr indicates
the seminal receptacle, a indicates the analia, and e indicates the egg. Scale
bar, 50 μm (b,c). b, Diagram of the female posterior abdomen (lateral aspect)
revealing the lower reproductive tract. A CMU axon is labeled magenta. c, Image
of the ventral nerve cord (left) and abdominal neuromere (right) corresponding
to the CMU-1>mCD8–GFP female in a stained with anti-GFP (CMU neurons,
green) and nc82 (synaptic neuropil, magenta); triangles, CMU cell bodies.
d, Fraction of flies that expelled an egg after delivery of photostimulation
pulses of varied duration (CMU-3>CsChR, n = 4, 15, 13 and 11 flies per group;
CMU-4>CsChR, n = 30, 30, 30 and 30 flies per group; UAS-control, n = 16, 17, 16
and 17 flies per group; CMU-3-control, n = 4, 9, 8 and 9 flies per group; CMU-4-
control, n = 10, 10, 10 and 10 flies per group). Colored and gray asterisks indicate
significance for comparisons with GAL4-only control flies and UAS-only control
flies, respectively; **P < 0.01; ***P < 0.001. Data were analyzed by two-sided
Fisher’s exact test (Supplementary Table 7). e, Representative experiment
showing video snapshots of the posterior abdomen (top; scale bar, 200 μm),
two-photon imaging of two CMU axons depicting relative fluorescence
changes of GCaMP6f and TdTomato (middle; green and dashed red traces,
respectively) and movement of the egg and ovipositor (bottom; Methods).
Arrows and vertical dashed lines correspond to the time point for each video
snapshot. Vertical gray lines indicate the onset of calcium response events. f,
Normalized PU responses and behavioral measures surrounding incomplete egg
expulsion events (left) and completed egg expulsion (right). Top: individual
neuron responses; horizontal white lines demarcate recordings performed in
different flies; neuron 1 and 2 from e. Middle and bottom: aggregate response
of all neurons and aggregate behavioral measurements, respectively. Darker
traces indicate the mean response, and the lighter area represents s.e.m.; n = 8
neurons from five flies; t = 0, behavioral event onset (Methods). g, Number of
eggs released on a 1% agarose substrate in 2 h (n = 14, 14 and 11 flies per group).
Here and in h, j and k, box bounds indicate the 25th and 75th percentiles, the red
lines indicate the medians, and whiskers indicate the 5th and 95th percentiles;
o, data from individual flies; +, outliers; **P < 0.01; ***P < 0.001; NS, P > 0.05. Data
were analyzed by two-sided Wilcoxon rank-sum test followed by a Bonferroni
correction (Supplementary Table 7). h, Fraction of eggs spontaneously dropped
without burrowing (n = 14, 12 and 6 flies per group). Only flies that released two
or more eggs were considered. i, Representative ethograms depicting burrowing
episodes for genetic control flies (left and middle) and for CMU-silenced flies
(right). Each ethogram depicts data from five flies (n = 4 events per fly); left and
middle, t = 0, egg out (indicated by ×); right, t = 0, time that burrowing stopped.
j, Number of burrowing episodes in 2 h (n = 14, 14 and 11 flies per group). k,
Average probability (P) of progression from burrowing to egg out (n = 12, 10
and 9 flies per group). Only flies that exhibited two or more burrowing episodes
were considered. l, Summary of egg-laying sequence transitions influenced by
identified sensory and motor neurons; cycle loop, repeating cycles within burrow
episode.
facilitating the reinitiation of the sequence. We also identified a cluster
of interoceptive sensory neurons (PU neurons), likely to be propriocep-
tive4,46–48, that signal the passage of the egg through the uterus into
the ovipositor during burrowing and coordinate the transition to the
final reset phase. Behavioral analysis along with genetic manipulation
suggest that information from the PU neurons can either drive the
persistence of burrowing, resulting in egg expulsion, or prompt the
cessation of burrowing if the egg cannot be expelled after an extended
attempt. Finally, diminished activity in PU neurons during burrowing
signals the completion of egg expulsion and initiates the transition to
the reset phase. These results provide a logic for a reaction chain in
which sensory information at critical junctures guides flexible adjust-
ments in component behaviors to achieve subterraneous egg deposi-
tion across varied environmental conditions.
The organization of innate behaviors into a sequence of compo-
nent actions may confer additional adaptive advantages. Individual
components in an innate behavioral sequence may be differentially
responsive to the distinct sensory stimuli that promote transitions
along the sequence. A given stimulus may behaviorally impact only
one of the components in the sequence. Each component therefore
establishes a context that filters relevant sensory input. For example,
during the hunting of bees, digger wasps first visually identify a target
bee. The odor of bees has no impact during this visual search, but
once a bee has been spotted, the bee odor triggers an acute strike3,60.
Similarly, we observe context-dependent behavioral responses to the
activation of PU neurons. Photostimulation of PU neurons during a
burrowing episode results in persistent burrowing, whereas activation
at other times in the sequence does not elicit a behavioral response.
Moreover, only during burrowing does a decline in PU neuron activity
result in the transition to the reset phase. Thus, the behavioral impact of
sensory stimuli differs for each of the components in a sequence. Each
component therefore displays selective attention to distinct stimuli
that structures the transition between behaviors to accommodate a
complex and variable sensory environment.
In addition, each component in the sequence affords an entry
point for adaptive evolutionary change. Changes in specific com-
ponents of egg-laying behavior that accommodate a new ecological
niche can occur without perturbing the overall sequence. For example,
changes in substrate preferences during exploration may occur as an
evolutionary adaptation to a changing environment7,8,61. Alterations in
subsequent components, such as burrowing, may then be necessary
to accommodate changes in the properties of the novel substrate. D.
melanogaster and D. suzukii have different preferences for the site of
egg laying7. D. suzukii females prefer to deposit eggs within firmer ripe
fruit, whereas D. melanogaster favors softer rotting fruit. Egg-laying
behavior is comprised of the same sequence of component actions in
the two species, but D. suzukii females exhibit dramatically prolonged
burrowing episodes14. Episodes in D. suzukii can persist for over 100 s,
whereas D. melanogaster burrowing on the same substrate does not
extend for more than 9 s. Interestingly, D. suzukii has also evolved an
enlarged and serrated ovipositor62. These changes do not alter the
sequence of behaviors but illustrate evolutionary adaptations that
allow D. suzukii to deposit eggs within firmer fruits. A similar logic holds
for male Drosophila courtship behavior, where adjustments in differ-
ent steps in a conserved sequence (for example, foreleg pheromone
sampling and singing) can be independently altered, and these modi-
fications play critical roles in the sexual isolation of related species63–65.
An innate behavioral repertoire is thought to be initiated by
higher-order brain centers that represent a specific motivational state
or drive3,66–71. These centers are activated by stimuli relevant to the drive
and then select an appropriate motor program for action15,72–74. A signal
is then transmitted to preconfigured circuits in the ventral nerve cord
or spinal cord that are capable of producing a coordinated sequence
of motor actions18,40,75–78. Pivotal intermediaries in this pathway are the
descending interneurons that link the output of higher brain centers
with the appropriate local circuits in the ventral nerve cord15,18,24,40,72,74,79–
83. One intermediary eliciting components of egg laying in D. mela-
nogaster has been recently identified, the descending oviDN cluster of
neurons15. OviDNs are necessary and causal for the expression of the
terminal components of the egg-laying sequence: abdominal bending,
ovipositor burrowing and egg expulsion. Higher-order brain centers
disinhibit the oviDN cluster following mating and modulate oviDN
activity in response to mechanical and gustatory stimuli presented
to the legs15,17. Thus, oviDNs are poised to induce the transition from
Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
1065
Articlehttps://doi.org/10.1038/s41593-023-01332-5exploration (walking and proboscis sampling) to later steps in the
sequence, resulting in egg deposition. Although activation of the
oviDNs is capable of eliciting the complete sequence from abdominal
bending to egg expulsion, our observations demonstrate that transi-
tions along this late sequence are exquisitely sensitive to ongoing
sensory feedback. We observe that abdominal bending does not always
lead to burrowing and identify that this transition is modulated by
tactile feedback from ATB neurons. Furthermore, the duration of bur-
rowing and the transition to the reset phase are informed by feedback
from PU neurons that sense the presence of the egg in the ovipositor.
Thus, once the activation of oviDNs initiates the egg deposition motor
program, sensory information acquired during the ensuing behavioral
sequence governs the progression of the component actions to satisfy
the drive.
Online content
Any methods, additional references, Nature Portfolio reporting sum-
maries, source data, extended data, supplementary information,
acknowledgements, peer review information; details of author con-
tributions and competing interests; and statements of data and code
availability are available at https://doi.org/10.1038/s41593-023-01332-5.
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Nature Neuroscience | Volume 26 | June 2023 | 1054–1067
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Articlehttps://doi.org/10.1038/s41593-023-01332-5Methods
Fly stocks and genotypes
All experiments were performed using 3- to 20-d-old females. For
detailed fly stock sources and genotypes, see Supplementary Tables
5 and 6.
Fly husbandry
Flies were reared at 25 °C and 55% relative humidity on a 12-h light/12-h
dark cycle in vials containing cornmeal-agarose food. Females used in
egg-laying experiments were genotyped under CO2 anesthesia within
1 d of eclosion and transferred to a vial containing an enriched medium
(Nutri-Fly GF, Genesci Scientific) in a ratio of 4 females to 5 males, with
a minimum of 12 and a maximum of 20 females per vial48. Egg-laying
experiments were performed 5 to 7 d after eclosion. All experiments
were initiated ±2 h of lights off. For optogenetic experiments, flies
were reared and maintained in complete darkness, and all-trans-retinal
(0.4 mM; Santa Cruz Biotechnology) was included in the enriched
medium.
Wild-type and loss-of-function behavior
For quantifying behavior at high resolution, single females were
filmed in parallel within a custom three-dimensional-printed assem-
bly containing six chambers (Shapeways). Individual chambers were
4.1 mm deep and tapered from top to bottom (7.3 mm × 5.8 mm to
6.7 mm × 4.3 mm). One side of the chamber was open to a reservoir,
within which the agarose-based substrate (Affymetrix Agarose-LE,
32802) plus 3% acetic acid (vol/vol; Sigma-Aldrich, 338826) was poured
and allowed to set for 30 min. Flies were introduced by gentle aspira-
tion, the assembly was placed at the center of a 5-cm off-axis ring light
(530 nm; Metaphase Technologies), and video recording was per-
formed using a GigE camera (Basler Ace acA2000-50gmNIR) attached
to a ×0.5 telecentric lens (Edmund Optics, 54-798) at 20 Hz (682 × 540
pixels per chamber) via pylon Viewer software (Basler). Experiments
lasted 2 h.
The study of egg depth of penetration was performed in a cus-
tom acrylic assembly with 16 individual chambers (18.5 mm ×
18.5 mm × 6 mm). Flies were introduced by gentle aspiration and allowed
to habituate to the chamber. Forty milliliters of substrate containing
agarose and 3% acetic acid was poured into a 120-mm square petri dish
(Greiner) and allowed to set for 30 min. The experiment was initiated by
removal of the thin plastic barrier separating the flies from the substrate,
and the whole assembly was then placed in the dark for 4 h.
Immunostaining and confocal microscopy
Flies were anesthetized with CO2 and fixed (2% paraformaldehyde in
75 mM lysine and 37 mM sodium phosphate buffer, pH 7.4) for 2 h at
room temperature. The flies were then removed, and the brains, ventral
nerve cord and lower reproductive tract were dissected in PBS contain-
ing 0.3% Triton X-100 (PBST), blocked with 10% normal goat serum
diluted in PBST for 30 min and incubated in a primary antibody mix
overnight at 4 °C. Subsequently, the tissue was washed for multiple
rounds with PBST before being incubated in a secondary antibody
mix overnight at 4 °C. A final round of PBST washing occurred before
the tissue was mounted using VectaShield (Vector Laboratories) and
imaged using an LSM 710 laser-scanning confocal microscope with a
×25/0.8 DIC or ×40/1.2 W objective (Zeiss). Primary antibodies used were
mouse anti-bruchpilot (nc82; 1:10; Developmental Studies Hybridoma
Bank), chicken anti-GFP (1:1,000; Aves Labs), rabbit anti-DsRed (1:500;
Clontech), rabbit anti-NOMPC (1:5,000)35 and rabbit anti-DVGLUT
(1:10,000)57. Secondary antibodies used were Alexa Fluor 633 goat
anti-mouse, Alexa Fluor 488 goat anti-chicken, Alexa Fluor 488 goat
anti-rabbit and Alexa Fluor 555 goat anti-rabbit (all at 1:200; Life Tech-
nologies). To visualize F-actin, Alexa Fluor 633 phalloidin was included in
the secondary antibody mix (1:200; Life Technologies). Acquired images
were processed using the Fiji distribution of ImageJ (NIH).
Nature Neuroscience
Thoracic dissection for calcium imaging
Three- to 20-d-old females were anesthetized on ice, and the wings were
removed before being mounted (ventral-side up) on a square acrylic
platform using UV-curable glue (AA 3104, Loctite) and UV illumination
(LED-200, Electro-Lite). The head and abdomen were lightly pressed
down to ensure complete mounting from the head to just anterior of the
analia. All legs were cut at the trochanter before, using a 30-gauge nee-
dle, a thin well was created with petroleum jelly that encompassed the
remaining leg coxa and ranged from the neck connective to the anterior
abdomen. A custom imaging platform with a hole (1 mm × 750 μm) at
the bottom of a pyramidal basin was positioned using putty such that
the hole was centered on the hindleg coxa. The basin was filled with
external saline (108 mM NaCl, 5 mM KCl, 2 mM CaCl2, 4 mM MgCl2, 4 mM
NaHCO3, 1 mM NaH2PO4, 5 mM trehalose, 10 mM sucrose and 5 mM
HEPES, adjusted to pH 7.3) before the remaining coxa of the middle
and rear legs were removed, along with the surrounding preepister-
num, the internal sternal apophysis and any visible trachea, revealing
a rectangular window above the abdominal neuromere and proximal
extent of the abdominal nerve trunk. Finally, the basin was drained,
and fresh saline was gently flushed over the window.
Two-photon functional imaging
Pilot experiments revealed that gravid females mounted ventral-side
up reliably expel an egg in midair within 30–60 min. Experiments were
initiated immediately after completion of the dissection. The acrylic
platform was secured adjacent to a camera and high-magnification lens
setup (Point Grey USB3 camera, CM3-U3-13S2M-CS; InfiniProbe S-80
right angle video microscope lens) and infrared band-pass filter (Thor-
labs, FGB25S) that, when illuminated by a nearby infrared (850-nm) LED
lamp, allowed for high-resolution video recordings of the posterior
abdomen concurrent with two-photon imaging.
Two-photon experiments were performed using an Ultra Micro-
scope (Bruker) coupled to a Ti:Sapphire laser (Chameleon Vision,
Coherent) via PrairiewView software (Bruker), with a GaAsp detector
(Hamamatsu Photonics) for GCaMP6f and a photomultipier tube for
tdTomato imaging. A ×40/0.80-NA water immersion objective (Nikon)
was used, and the laser was tuned to 925 nm; the power measured after
the objective ranged from 5 to 7 mW. The abdominal neuromere and
abdominal nerve trunk were located using the microscope oculars and
positioned near the center of the field of view by two-photon imaging.
Using the tdTomato anatomical marker, a stretch of the abdominal
nerve trunk where the axons were separated and ran in parallel was
selected for the coronal section55. Coronal section imaging was per-
formed at 10 Hz, covering 42.4 μm in x and 60 μm in z (512 × 85 pixels
per image; 1.2-μs pixel dwell time). Small adjustments in the x and
z dimensions were made as needed throughout the experiment to
compensate for drift.
Axons corresponding to PU or CMU neurons were determined by
two-photon imaging at the conclusion of the experiment. Axon pro-
jections were traced anteriorly, identifying PU or CMU axons by their
expression pattern within the abdominal neuromere.
Optogenetic perturbations during behavior
For the optogenetic stimulation of PU neurons during egg-laying
behavior, the high-resolution filming apparatus described above was
slightly modified. Chambers were illuminated with an infrared 5-cm
off-axis ring light (880 nm; Metaphase Technologies), a single 655-nm
high-power LED (Luxeon Star) was installed adjacent to the video lens
to deliver red-light stimulation, and an infrared band-pass filter was
mounted in front of the lens. A custom MATLAB graphical user interface
(GUI) was used to select the stimulus condition and control the timing
of the light stimulus via an Arduino UNO (Arduino) and LED controller
(BuckPuck 700 mA, Luxeon Star). Experiments were performed on a
0.8% agarose substrate plus 3% acetic acid. As the egg neared complete
expulsion during burrowing, a trigger was pressed that turned the light
Articlehttps://doi.org/10.1038/s41593-023-01332-5stimulus on. The instant the egg was fully expelled, a second trigger
was pressed, initiating a countdown timer for stimulus offset whose
duration was determined by the selected stimulus condition. Individual
flies contributed a minimum of 15 events (5 events each for control and
two experimental conditions) and a maximum of 20 events (5 events
each for control and all three experimental conditions) to the final data
set. At the beginning and end of a behavioral session, 20-s light pulses
were delivered at 90-s intervals to examine the impact of photostimu-
lation outside the context of burrowing. The aberrant persistence of
burrowing and/or reversion in the behavioral sequence following egg
expulsion were reliably observed using the second PU split-GAL4 line
in response to 5-s stimulation at 8 μW mm–2 (PU-2>CsChrimson, 60/60,
n = 12). Genetic control flies did not exhibit these aberrant behaviors in
response to 5 s of stimulation at 8 μW mm–2 (PU-1>smGFP, 0/50, n = 10
flies; empty-split-GAL4>CsChrimson, 0/50, n = 10).
For the optogenetic stimulation of CMU neurons in gravid females,
up to 12 flies were transferred to a small, circular acrylic chamber
(28 mm in diameter and 2 mm in height) and placed atop an infra-
red panel light (850 nm; Smart Vision Lights). Video recordings were
performed using a USB3 camera (Basler Ace acA2040-90umNIR)
attached to a ×0.377 telecentric lens (Edmund Optics, 34-015) at 40 Hz
(2,048 × 2,048 pixels). Photostimulation was controlled by a custom
MATLAB GUI and delivered by four 655-nm LEDs. A single volley of five
light pulses of the selected duration was delivered with 1 s between
pulse offset and onset, and the fraction of flies that expelled an egg at
any point before 4 s following the last pulse offset was scored.
For the optogenetic silencing of CMU neurons during egg-laying
behavior, all experiments were performed in the assembly used for
high-resolution filming. Flies were filmed under constant green light
(6 μW, 530 nm) delivered by the off-axis ring light.
Wild-type and loss-of-function behavior data analysis
For the high-resolution assay, the video was segmented in one of three
ways. Wild-type flies on 1% agarose and PU-silenced flies were analyzed
over ±60 s of egg expulsion. Wild-type flies on substrates of various
firmness and ATB-silenced flies were analyzed over a contiguous video
segment spanning three to eight egg-laying events beginning immedi-
ately after the first egg was deposited. CMU-silenced flies were analyzed
over the entire 2-h recording session.
Segmented videos were manually annotated frame by frame
in a custom MATLAB GUI. Manual scoring of behavior required
640.5 ± 275.1 s (mean ± s.d.) per 2-min video (n = 25 videos). Detailed
annotation criteria are provided in Supplementary Table 1. The timing
and count of burrow cycles were determined by observing individual
episodes in real time. The cycle count per burrowing episode was highly
positively correlated with the duration of the episode (r = 0.91). Manual
annotations were validated by independently rescoring behavior on a
subset of data using a second human annotator and were also compared
to the output of a supervised learning algorithm (DeepEthogram26).
Agreement was determined by calculating the F1 score25,26, a standard
metric that ranges from 0 (poor agreement) to 1 (perfect agreement),
calculated as
F1 score = (2 ∗ precision ∗ recall) / (precision + recall) ,
with
and
precision = true positive/(true positive + false positive)
recall = true positive/(true positive + false negative).
DeepLabCut (DLC; a feature detection algorithm)28 was used to
track the x–y position of the posterior tip of the scutellum on the thorax,
Nature Neuroscience
and the speed was estimated by comparing the distance between this
position across ten frames (500 ms). A fly was considered to be walk-
ing if its speed, smoothed by a 1-s moving average, was greater than a
threshold of 0.29 mm s–1.
For the determination of behavioral transition probabilities, the
probability of start-to-start transitions was calculated as in ref. 24.
Proboscis extension was only considered when expressed at distinct
locations spaced greater than 500 μm apart. Proboscis extension
events that occurred during other behaviors were omitted from this
analysis. Bend onset was only scored once if burrowing was aborted
and then reinitiated during a sustained abdominal bend. Transition
probabilities were determined separately for behaviors happening
before and after egg expulsion (egg out). The statistical significance
of each transition was determined by comparison to a distribution of
transition probabilities derived from 10,000 shuffled permutations of
the original sequences. Transitions not shown were not significant and
of low probability (<0.04; initial behavior distribution and complete
transition matrices are shown in Supplementary Table 2).
For quantifying normalized egg depth of penetration, an egg was
given a score of 1 if it was deposited entirely beneath the substrate sur-
face, with only the egg’s spiracles exposed. If only part of the egg was
beneath the surface, it was given a score of 0.5, whereas if it was entirely
resting on the surface, it was scored as 0. The average normalized egg
depth was calculated per fly for all flies that laid one or more eggs.
Supervised behavioral classification analysis
A DeepEthogram-slow model26 was trained using 518 manually anno-
tated 2-min videos surrounding egg-laying events (±1 min of completed
egg expulsion). Test data consisted of a subset of 30 randomly selected
2-min videos corresponding to 72,000 frames, which were held out
from the training data set. Proboscis extension and egg out labels
were expanded from one frame to three frames in both train and test
datasets.
Unsupervised behavioral classification analysis
Our approach was based on ref. 27 and implemented using custom
MATLAB code. Using key points from a DLC pose estimation model,
17 features were extracted from each video frame that represent pos-
tural and motion features relevant to egg laying. These features were
velocity (movement of the scutellum over time, ‘vel’; Supplementary
Video 2), movement of the proboscis relative to the ocellus (‘pe’),
the z-score-normalized angle formed between a line connecting the
ventral abdominal stripes and a line connecting the ocellus and scutel-
lum (‘ba’), the angular velocity of this angle (‘velba’), movement of the
leg joint from each leg (three features; ‘T1’–‘T3’), the DLC prediction
confidence for egg emergence (‘Pegg’), the magnitude of two bands
of the Morlet wavelet spectrogram of the pixel intensity of a circular
region of interest (ROI) of radius 10 pixels surrounding the ovipositor
(0.8 to 1.3 Hz and 1.3 to 2.3 Hz; ‘w1ovi’ and ‘w2ovi’) and the log of the
magnitude of seven bands of the Morlet wavelet spectrogram of the
movement of the dorsal arch of the stripe on abdominal segment A5
(ranging from 0.5 Hz to 10 Hz; ‘cwt1’–‘cwt7’). For a complete description
of this analysis, see Supplementary Methods.
Functional imaging data analysis
Imaging data were first segmented into cell-specific ROIs. The location
and shape of ROIs corresponding to all labeled axons across all frames
was determined from the tdTomato channel using a semiautomated
pipeline. DLC was used to track the center position of all identified
axons, appearing as ellipsoids, in the tdTomato image stack. DLC
predictions were then used to select foreground ROIs from a binary
thresholded image stack. The raw fluorescence, F, was then calculated
as the mean pixel value within the ROI bounds for each frame. The raw
fluorescence was converted to ΔF/F0 using a baseline determined as
the median fluorescence value from recording onset to 20 s before
Articlehttps://doi.org/10.1038/s41593-023-01332-5completed egg expulsion, excluding ±20 s surrounding ovipositor
extrusion events. Example ΔF/F0 traces shown in figures and videos
were smoothed by a three-point moving average.
The timing of egg expulsion events and ovipositor extrusion
events was determined via an automated analysis of the distances
between DLC-tracked key points (the dorsal–posterior edge of T6, the
posterior tip of the egg and the midpoint of the ovipositor). T6:egg
distance and T6:ovipositor distance were used to detect behavioral
events before and after egg expulsion, respectively.
Raw distance traces were high-pass filtered (0.001 Hz) before total
variation regularization, with events identified as threshold crossings
of one-fifth the maximum regularized signal. Distances were within-fly
normalized by the median distance between T6 and the posterior edge
of the analia base, which was set to 1. Calcium response events (Sup-
plementary Fig. 4) were determined similarly.
To compare response magnitude across events and flies, the
fluorescence data were integrated and normalized as follows. The
integration window for both egg expulsion and ovipositor extrusion
events was defined as t = 0 to t = 3 s after event onset. For comparing
incomplete to complete egg expulsion, the baseline 0 value was deter-
mined as the median 3-s integral over the first contiguous stretch of 60 s
leading up to complete egg expulsion, excluding ±20 s surrounding any
egg expulsion event. The postexpulsion value was determined as the
median 3-s integral from t = 10 to t = 20 s after complete egg expulsion.
The maximum ‘1’ value for normalization was the maximum 3-s integral
observed throughout. For comparing incomplete egg expulsion to
postexpulsion ovipositor extrusion events, the minimum ‘0’ value was
determined as the median of the first 60 s (non-contiguous) starting
10 s after egg expulsion and excluding ±20 s surrounding ovipositor
extrusion events. For flies that expressed multiple events, the mean
was used in plots and all analyses. Calcium imaging was performed
using both PU-1 and PU-2 split-GAL4 lines, and the data were combined.
Optogenetic activation data analysis
For every egg-laying event, the timing of completed egg expulsion and
burrow termination was manually annotated in a custom MATLAB GUI.
The time of completed egg expulsion was defined as the first frame
where the egg reached maximum depth within the substrate. Burrow
termination was defined as the frame associated with the onset of
ovipositor detachment or lifting. The transition to the reset phase
was determined if no additional burrowing episode occurred within
65 s of egg expulsion. If additional burrowing episodes did occur, the
onset timing of the last burrowing episode before reset was similarly
determined as the last burrowing episode to precede a 65-s window
free of burrowing. For stimulation events presented in Fig. 7c and
Extended Data Fig. 7b, t = 0 corresponds to the onset of the countdown
timer, which approximately coincided with completed egg expulsion.
Data availability
Data from this study are available at https://github.com/axellaboratory/
Cury_and_Axel_2023 and upon request. Trained pose estimation mod-
els and the supervised behavioral classifier can be accessed via Dropbox
(https://www.dropbox.com/sh/jh4422f3ld95j1a/AAAHVb-pFsmcEk40
BgSHm1TEa?dl=0).
Code availability
Code used for processing the data is available at https://github.com/
axellaboratory/Cury_and_Axel_2023.
Acknowledgements
We thank L.F. Abbot, C.M. Root and W.B. Grueber for comments on
the paper; D. Hattori, B. Noro, J.A. Browning–Kamins, J.T. Remark,
A.D. Pourmorady, R.A. Mosneanu, C.E. Haoud and A. Zangiabadi for
technical assistance; N. Gompel, E.A. Lumpkin, A. Vina–Albarracin, R.
Behnia, K. Kay, L.Y. Tian, K. Wang, F. Wang, B.J. Dickson, A. Mathis, B. de
Bivort, G.G. de Polavieja, R. Benton, V. Ruta and members of the Axel
laboratory for discussions; G.M. Rubin, B. Noro, D. Hattori, T.R. Shirangi
and B.J. Dickson for fly stocks and M. Gutierrez, C.H. Eccard and A.
Nemes for general laboratory support. Stocks from the Bloomington
Drosophila Stock Center (NIH P40OD018537) were used in this study.
Financial support was provided by the Howard Hughes Medical
Institute and the Simons Foundation.
Author contributions
K.M.C. and R.A. conceptualized the study. K.M.C. developed the
methodology and performed the experiments, formal analysis and
data visualization. K.M.C. and R.A. interpreted results and wrote the
paper.
Competing interests
The authors declare no competing interests.
Additional information
Extended data is available for this paper at https://doi.org/10.1038/
s41593-023-01332-5.
Supplementary information The online version contains
supplementary material available at https://doi.org/10.1038/s41593-
023-01332-5.
Correspondence and requests for materials should be addressed to
Kevin M. Cury or Richard Axel.
Peer review information Nature Neuroscience thanks the
anonymous reviewers for their contribution to the peer review of
this work.
Reporting summary
Further information on research design is available in the Nature Port-
folio Reporting Summary linked to this article.
Reprints and permissions information is available at
www.nature.com/reprints.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 1 | Human-Human agreement is high and exceeds
supervised behavioral classifier output. a, Comparison of annotation
agreement between two humans and between a human and a supervised
behavioral classifier (DeepEthogram; Methods). n = 30 videos for all groups;
same videos re-annotated by second human and held out as test dataset
for supervised classifier. Blue, human-human agreement; pink, human-
DeepEthogram agreement. Box bounds, 25th and 75th percentile; red line, median;
whiskers, 5th and 95th percentile; +, outliers. background, unlabeled frames;
all, all behaviors combined. ***p < 0.001, two-sided Wilcoxon rank sum test
(Supplementary Table 7). b, Representative ethograms from six of 30 videos,
displaying the annotations of two humans (top two rows of each plot) and the
output of DeepEthogram (bottom row). Values within parenthesis at left, F1 score
of the corresponding annotation compared with Human1 (top row annotation)
for all behaviors combined. t = 0 marks the time of completed egg expulsion
(egg out).
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 2 | Reliable mapping between manual annotations and
unsupervised behavioral classifier. a, Mean time course of the 17 extracted
features used for unsupervised behavioral classification analysis, plotted
surrounding egg-laying (input feature definitions in Methods), each normalized
to a maximum of 1. Here and in c, t = 0, egg out. b, Left: probability density
function (PDF) generated from t-SNE embedding. Boundary lines, watershed-
transform segmented behavioral clusters, with cluster index indicated within.
Right: feature magnitudes of embedded data points plotted for 9 of the 17
features. c, Time course of expression fraction of the 14 t-SNE clusters that were
expressed significantly higher than chance (p < 0.001, one-sided Fisher’s exact
test; see top plot in f) and displayed peak expression within ± 20 seconds of egg
out. Clusters sorted according to their peak timing (green tick mark) here and in
d. Examples of these 14 clusters shown in Supplementary Video 3. d, Left: mean
feature magnitudes of embedded data points (columns) corresponding to each
of the 14 t-SNE clusters shown in c (rows). Right: mapping of these 14 clusters
onto human labels, displayed as a fraction (bar height indicates fraction from
0 to 1). Here and in e and f, bg, unlabeled, background frames. e, Mapping of t-SNE
clusters (rows) onto human labels (columns), displayed as a fraction. Percentage
of total frames within original embedded data points belonging to each cluster
is indicated at left. Here and in f, mappings significantly higher than chance are
indicated by a white-rimmed black circle (p < 0.001, one-sided permutation
test; Supplementary Methods; Supplementary Table 8); red asterisks, clusters
expressed significantly higher than chance surrounding egg laying (p < 0.001,
one sided Fisher’s exact test; Supplementary Table 7); t-SNE cluster # 0
corresponds to stationary frames. f, Top: log ratio of t-SNE cluster expression
fraction within ± 60 seconds surrounding egg-laying relative to expression
fraction within original embedded data points. Here and below, clusters arranged
according to their peak timing. Bottom: mapping of human labels (rows) onto
t-SNE clusters (columns), displayed as a fraction. Percentage of total frames given
a particular label indicated at left.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 3 | See next page for caption.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 3 | Mono-innervation and anti-NOMPC labeling of
terminalia bristles. a, Representative images of posterior abdomen from
one of four pan-neuronal elav-GAL4>mCD8–GFP females. Left and middle:
mCD8-GFP, pan-neuronal expression demonstrating the innervation of each
bristle by the distal process of a single bipolar neuron (green). The innervation
patterns associated with the long sensillum and short sensilla of the ovipositor
(hypogynium) could not be deciphered. Left: autofluorescence, abdominal
cuticle (magenta). Left and right: bright-field (grayscale). Here and in b-d, A,
analia; OV, ovipositor valves; T8, 8th abdominal tergite; T7, 7th abdominal tergite;
S7, 7th abdominal sternite; scale bar, 50 μm. b, Top five rows: representative
images of posterior abdomen from one of nine wild-type female flies stained
with anti-NOMPC (green at left, grayscale at middle). Left: autofluorescence,
abdominal cuticle (magenta). Left and right: bright-field (grayscale).
Arrowheads, foci of anti-NOMPC labeling observed at the base of all bristles34.
Bottom: lower resolution image of the posterior abdomen, lateral aspect,
containing the regions displayed above (orange boxes). c, Representative images
of the ovipositor valves from two of nine wild-type flies stained with anti-NOMPC
(green at left, grayscale at right). Left, autofluorescence, abdominal cuticle
(magenta); bright-field (grayscale). Foci of anti-NOMPC labeling at the base of the
three hypogynial short sensilla, the singular long sensillum, and the hypogynial
teeth are indicated by triangles, the arrow, and the arrowheads, respectively.
d, Diagram of female posterior abdomen, lateral aspect.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 4 | Expression pattern of ATB-split-GAL4 lines.
E a, Representative images of the posterior abdomen from two of seven
ATB-2>mCD8-GFP females, lateral (left) and ventral aspects (right). mCD8-GFP
expression, membrane of ATB neurons (green); autofluorescence, abdominal
cuticle (magenta). Background, overlaid bright-field images reveal extended
bristles. Here and in b-d, scale bar, 50 μm. b, Representative image of the ventral
nerve cord (left) and abdominal neuromere (right) from two ATB-2>mCD8-GFP
females, stained with anti-GFP (membrane of ATB neurons, green) and nc82
(synaptic neuropil, magenta). Black bars flanking left image, region shown in
higher resolution at right. c, Representative images of the brain from one of
two ATB-1>mCD8-GFP females (left) and from one of two ATB-2>mCD8-GFP
females (right), stained with anti-GFP (membrane of ATB neurons, green) and
nc82 (synaptic neuropil, magenta). d, Representative images of the brain (left,
one of six flies) and ventral nerve cord (right, one of six flies) from ATB-1>Kir2.1-
T2A-tdTomato females, stained with anti-DsRed (ATB neurons co-expressing
tdTomato and Kir2.1, green) and nc82 (synaptic neuropil, magenta). Foreleg
expression is sparse in these flies (0.67 + /− 0.65 efferents per side in the dorsal
prothoracic nerve; n = 12 nerves from six flies; mean ± s.d.).
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 5 | Aberrant egg-laying in ATB-1 silenced and ATB-2
silenced flies. a, Average number of cycles per aborted burrowing episode on a
1% agarose substrate. Only flies that exhibited two or more aborted burrowing
episodes were considered here and in b. Burrowing episodes are comprised of
discrete, rhythmic cycles (see Fig. 4); the cycle count per burrowing episode was
highly positively correlated with the duration of the episode (r = 0.91). Here and
in b-d, box bounds, 25th and 75th percentile; red line, median; whiskers, 5th and
95th percentile; o, data from individual flies; +, outliers; **p < 0.01, ***p < 0.001,
n.s., p > .05, two-sided Wilcoxon rank sum test followed by Bonferroni correction
(Supplementary Table 7). b, Average number of cycles per egg-expulsion
burrowing episode on a 1% agarose substrate. c, Average normalized depth of
penetration of eggs released on a 1% agarose substrate. Silencing neurons using
either ATB-1 or ATB-2 splitGAL4 yielded significant deficits in subterraneous egg
deposition (see also Fig. 3j, right panel), implicating a defect in the common set of
terminalia bristle-innervating neurons in producing this phenotype. d, Number
of eggs released on a 1% agarose substrate in 4 h.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 6 | Silencing ATB-1 brain neurons does not result in
deficit in subterraneous egg deposition. a, Representative images of the
brain (left column, one of 13 flies) and ventral nerve cord (right column, one
of four flies) from ATB-1>Otd-nls:FLP; UAS(FRT.mCherry)Kir2.1-GFP females.
Flippase under control of the head-restricted Otd promotor used in combination
with UAS(FRT.mCherry)Kir2.1-GFP results in the restricted expression of
Kir2.1-GFP in ATB-1 brain neurons, whereas mCherry is expressed in ATB-1
non-brain neurons42. Samples stained with anti-GFP (Kir2.1-GFP-expressing
ATB-1 neurons,green), anti-DsRed (mCherry-expressing ATB-1 neurons, red),
and nc82 (synaptic neuropil, magenta). Scale bar, 50 μm. b, Average normalized
depth of penetration of eggs released on a 1% agarose substrate. Subterraneous
egg deposition is largely unaffected compared to ATB-1>Kir2.1 flies (see Fig. 3j,
right panel). Here and in c, box bounds, 25th and 75th percentile; red line, median;
whiskers, 5th and 95th percentile; “o”, data from individual flies; “+”, outliers;
***p < 0.001, **p < 0.01, n.s., p > .05, two-sided Wilcoxon rank sum test followed by
Bonferroni correction (Supplementary Table 7). c, Number of eggs released on a
1% agarose substrate in a 4-hour window.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 7 | See next page for caption.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 7 | Expression pattern of PU-splitGAL4 lines.
a, Representative images from PU-2 > mCD8-GFP females. Left: ventral nerve
cord (one of 13 flies) stained with anti-GFP (membrane of PU neurons, green)
and nc82 (synaptic neuropil, magenta). Right: lower reproductive tract (one of
eight flies) stained with anti-GFP (green) and phalloidin (muscle f-actin, gray);
autofluorescence abdominal cuticle (red). In addition to consistent labeling of
the PU neurons (PU-1, 2.1 ± 0.4 ventral afferents per side, n = 17 sides in thirteen
flies; PU-2, 2.0 ± 0.6; n = 11 sides in eight flies; mean ± s.d.), both lines exhibited
inconsistent labeling in a small number of peripheral neurons that project to
the dorsal abdominal neuromere (PU-1, 1.2 ± 0.6 dorsal afferents per side; PU-2,
0.5 ± 0.6; mean ± s.d.). Here and in b-d, scale bar, 50 μm. b, Representative images
of the brain from PU-1>mCD8-GFP (left, one of three flies) and PU-2>mCD8-
GFP (right, one of two flies) females, stained with anti-GFP (green) and nc82
(magenta). c, Representative images from hs-FLP; PU-2>(FRT.stop)CsChrimson-
mVenus females (one of two flies) where stochastic labeling resulted in only a
single PU neuron being labelled (Gordon and Scott, Neuron 61, 373–384 (2009)).
Top two images and right image: ventral nerve cord stained with anti-GFP
(mVenus-expressing PU neurons, green) and nc82 (magenta). Bottom two
images: lower reproductive tract stained with anti-GFP (green) and phalloidin
(muscle f-actin, gray); autofluorescence, abdominal cuticle (red); white triangle,
PU cell body; u, uterus. Right: lateral projection of the ventral nerve cord
after registration with a template; V, ventral; P, posterior. d, Representative
images of the abdominal neuromere (top row, one of three flies) and lower
reproductive tract (middle, bottom rows, one of three flies) from PU-2>DenMark,
synaptotagmin-GFP females, stained with anti-DsRed (dendrites, red) and anti-
GFP (synaptic vesicles, green)52,53.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 8 | Phenotypes induced by PU photo-stimulation
beyond egg expulsion. a, Timing that burrowing stopped after completed egg
expulsion in no-light control events (control, n = 83 events) and after light offset
in stimulation events where burrowing persisted throughout photo-stimulation
(stim, n = 93). Here and in e and f, box bounds, 25th and 75th percentile; red line,
median; whiskers, 5th and 95th percentile; o, data from individual events; +,
outliers. n.s., p > 0.05, two-sided Wilcoxon rank sum test (Supplementary Table
7). b, Time course of annotated behaviors for no light control events. c, Time
courses of annotated behaviors for three stimulus conditions. Left: burrowing
persisted throughout photo-stimulation. Right: burrowing stopped during
photo-stimulation. Red bar above each plot, period of photo-stimulation.
d, Fraction of events where burrowing was re-initiated within 65 s of egg
expulsion in control and three stimulus conditions. For stimulation groups,
fraction plotted separately for events where burrowing persisted throughout
photo-stimulation (persist), and events where burrowing stopped during photo-
stimulation (stop). Events defined as reverting in the sequence if burrowing was
re-initiated within 65 s of egg expulsion. ***p < 0.001, two-sided Fisher’s exact test
(Supplementary Table 7). e, Number of aberrant burrowing episodes after egg
expulsion for events where burrowing stopped during photo-stimulation for all
three stimulus conditions. Episode defined as aberrant if occurring within 65 s
of egg expulsion or a previous aberrant episode. f, Onset timing of last aberrant
burrowing episode expressed after egg expulsion for events where burrowing
stopped during photo-stimulation and the fly reverted in the sequence, plotted
for all three stimulus conditions. Last aberrant episode after the fly reverted
in the sequence defined as the first episode preceding a 65-s window devoid of
burrowing behavior. Light offset during an aberrant burrow episode significantly
increased the probability of progressing to reset: 14 of 33 aberrant burrow
episodes that spanned light offset progressed to reset compared to 0 of 106
episodes that stopped before light offset and 93 of 457 episodes that started after
light offset (p = 1.40×10−10, p = 0.0048 comparing group 1 to group 2 or group 3,
respectively, one-sided Fisher’s exact test).
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 9 | PU photo-stimulation outside the context of
burrowing. a, Representative ethograms of behavior surrounding a 20-s
photo-stimulation pulse delivered at 90-s intervals, independent of the ongoing
behavioral state of the fly (Methods). Ethogram depicts data from 16 flies (n = 6
events per fly). Horizontal black dotted lines demarcate data from different
flies. The component actions are color coded as in Fig. 1c, and completed egg
expulsion events (egg out) are indicated by a black ‘X’. Photo-stimulation did
not induce an overt behavioral response with the exception of two out of the 96
events where egg expulsion was completed within the stimulation window and
the animal reverted in the sequence (‘<’ symbols at the right indicates these two
events). Here and in b, t = 0, photo-stimulation onset; red bar above plot and
vertical dashed magenta lines, period of photo-stimulation. b, Time course of
annotated behaviors depicted in a.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 10 | See next page for caption.
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Extended Data Fig. 10 | Expression pattern of CMU-splitGAL4 lines.
a, Representative images of the brain (top row), ventral nerve cord (second, third
rows), and lower reproductive tract (bottom row) from CMU-splitGAL4>mCD8-
GFP females, stained with anti-GFP (membrane of CMU neurons, green). Top
three rows: tissue also stained with nc82 (synaptic neuropil, magenta). Bottom
row: tissue also stained with phalloidin (muscle f-actin, gray); autofluorescence,
abdominal cuticle (magenta). Representative brain images (top row) from 12, 2,
2, 6 flies. Representative ventral nerve cord images (second, third rows) from 8,
22, 6, 12 flies. Representative lower reproductive tract images (bottom row) from
8, 11, 2, 10 flies. Images in third row are higher resolution regions of images in
second row. For quantitation of expression patterns in the ventral nerve cord
and lower reproductive tract, see Supplementary Table 4. Here and in
b, scale bar, 50 μm. b, Representative images of the lower reproductive tract from
gravid CMU-4>CsChrimson females that were flash frozen in liquid nitrogen
without (left, one of two flies) or with (right, one of four flies) concurrent photo-
stimulation (655 nm light at 40 μW/mm2). Prior to experiment, gravid females
were maintained in the dark in an environment that ensured egg retention in the
uterus, as in Fig. 8d (Methods).
Nature Neuroscience
Articlehttps://doi.org/10.1038/s41593-023-01332-5Corresponding author(s): Richard Axel
Last updated by author(s): March 23, 2022
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Data collection
Video data was collected using pylon Viewer 6.2 (Basler) and FlyCapture 2.0 SDK (Point Grey). 2-photon imaging data was collected using
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Confocal images were analyzed in ImageJ (ver: 1.53f51). Two-photon ROI segmentation and unsupervised behavioral classification were based
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Human research participants
Policy information about studies involving human research participants and Sex and Gender in Research.
Reporting on sex and gender
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Population characteristics
Recruitment
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Life sciences study design
All studies must disclose on these points even when the disclosure is negative.
Sample size
No statistical methods were used to pre-determine sample sizes but our sample sizes are comparable to the other functional and behavioral
studies in Drosophila (e.g. Feng et al., 2014; Gou et al., 2014; Seeholzer et al., 2018; Bräcker et al., 2019; Wang et al., 2020; Zhang et al.,
2020.)
Data exclusions We did not exclude flies or data from analysis.
Replication
Each experiment presented in the manuscript was repeated in multiple animals, and the effects identified were consistent across animals. The
specific number of replicates for each experiment is detailed on the figure and/or its corresponding legend entry. Furthermore, the main
findings of the paper were confirmed by multiple complimentary experiments. A subset of 30 randomly selected 2-minute videos was re-
annotated by a second human annotator and labeling agreement was above 95% across all behaviors. Analysis was performed with code that
is available for public use via GitHub to promote replication.
Randomization
Flies were group housed separated by genotype, and females were randomly chosen for functional or behavioral experiments.
Blinding
The experimenter was not blind to fly genotype and/or experimental condition during behavioral data acquisition (Figures 1-4, 6-8) as this was
not logistically possible. However, all behavioral annotations were performed blinded to these details. Furthermore, the subsequent analyses
were automated and run the same way for all flies and so there was no opportunity for subjective influence on the outcome. The assessment
of the depth and count of eggs laid (Figures 2-3, 6) was performed blinded to the genotype and/or experimental condition. For imaging
experiments (Figures 5, 8), the ROIs corresponding to the relevant cell types were determined at the conclusion of the experiment and so the
experimenter had no opportunity to influence the outcome. Furthermore, the analysis and identification of relevant behavioral events used to
interpret the imaging data was entirely automated. The assessment of the fraction of flies that expelled an egg during optogenetic stimulation
(Figure 8) was not performed in real-time; video data was later scored blind to the genotype and experimental condition.
Reporting for specific materials, systems and methods
We require information from authors about some types of materials, experimental systems and methods used in many studies. Here, indicate whether each material,
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n/a Involved in the study
Methods
n/a Involved in the study
Antibodies
Eukaryotic cell lines
Palaeontology and archaeology
Animals and other organisms
Clinical data
Dual use research of concern
ChIP-seq
Flow cytometry
MRI-based neuroimaging
Antibodies
Antibodies used
Mouse mAb anti-bruchpilot (nc82) (Developmental Studies Hybridoma Bank - nc82; RRID: AB_2314865)
Chicken anti-GFP (Aves Labs - GFP-1020; RRID: AB_10000240)
Rabbit anti-DsRed (Clontech - 632496; RRID: AB_10013483)
Rabbit anti-NOMPC (gift of Y.N. Jan; Zhang et al., Cell 162, 1391–1403 (2015))
Rabbit anti-dVGlut (gift of A. DiAntonio; Daniels et al., J Neurosci. 24, 10466-10474 (2004))
Alexa Fluor 633-conjugated goat anti-mouse IgG (Life Technologies - A21052; RRID: AB_141459)
Alexa Fluor 488-conjugated goat anti-chicken IgY (Life Technologies - A11039; RRID: AB_142924)
Alexa Fluor 488-conjugated goat anti-rabbit IgG (Life Technologies - A11008; RRID: AB_143165)
Alexa Fluor 633 phalloidin F-actin probe (Life Technologies - A22284)
Alexa Fluor 555-conjugated goat anti-rabbit IgG (Life Technologies - A27039; RRID: AB_2536100)
Validation
The antibodies and protocols were used and validated in numerous studies, including for example Hattori et al., Cell 169, 956-969
(2017), Zhang et al., Cell 162, 1391–1403 (2015), and Daniels et al., J Neurosci. 24, 10466-10474 (2004).
Animals and other research organisms
Policy information about studies involving animals; ARRIVE guidelines recommended for reporting animal research, and Sex and Gender in
Research
Laboratory animals
We used 3-20 day old female Drosophila melanogaster. Canton-S gifted from B.J. Dickson was used as a wild-type strain. All relevant
genotypes with citations and sources are described in Supplementary Table 5.
Wild animals
This study did not involve wild animals.
Reporting on sex
All findings apply only to female Drosophila melanogaster.
Field-collected samples
This study did not involve samples collected in the field.
Ethics oversight
No ethical approval was required for work on Drosophila melanogaster.
Note that full information on the approval of the study protocol must also be provided in the manuscript.
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Data Availability
The datasets generated during and/or analyzed during the current study cannot be publicly available because
they are owned by Yamagata Municipalities Mutual Aid Association and Sports Medical Research Center, Keio
University. Please ask Sports Center of Keio University about data availability (http://sports.hc.keio.ac.jp/ja/).
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Data Availability The datasets generated during and/or analyzed during the current study cannot be publicly available because they are owned by Yamagata Municipalities Mutual Aid Association and Sports Medical Research Center, Keio University. Please ask Sports Center of Keio University about data availability ( http://sports.hc.keio.ac.jp/ja/ ).
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opeN
Received: 11 January 2019
Accepted: 15 March 2019
Published: xx xx xxxx
Identifying progressive CKD from
healthy population using Bayesian
network and artificial intelligence:
A worksite-based cohort study
Eiichiro Kanda1, Yoshihiko Kanno2 & Fuminori Katsukawa3
Identifying progressive early chronic kidney disease (CKD) patients at a health checkup is a good
opportunity to improve their prognosis. However, it is difficult to identify them using common health
tests. This worksite-based cohort study for 7 years in Japan (n = 7465) was conducted to evaluate the
progression of CKD. The outcome was aggravation of the KDIGO prognostic category of CKD 7 years
later. The subjects were male, 59.1%; age, 50.1 ± 6.3 years; and eGFR, 79 ± 14.4 mL/min/1.73 m2. the
number of subjects showing CKD progression started to increase from 3 years later. Vector analysis
showed that CKD stage G1 A1 was more progressive than CKD stage G2 A1. Bayesian networks showed
that the time-series changes in the prognostic category of CKD were related to the outcome. Support
vector machines including time-series data of the prognostic category of CKD from 3 years later
detected the high possibility of the outcome not only in subjects at very high risks but also in those at
low risks at baseline. In conclusion, after the evaluation of kidney function at a health checkup, it is
necessary to follow up not only patients at high risks but also patients at low risks at baseline for 3 years
and longer.
In Japan, the number of chronic kidney disease (CKD) patients was estimated to be 13.3 million in 20051. And the
number of end-stage renal disease (ESRD) patients was 324986 in 20152. With the aging of the Japanese popula-
tion, the number of CKD patients is estimated to continue to increase.
CKD has been reported to be a risk factor for death, ESRD, and cardiovascular disease (CVD) in Japan3,4. The
number of patients with ESRD due to diabetic kidney disease and nephrosclerosis, which are associated with
aging, has been increasing2. The prognosis of CKD patients can be improved by identifying such patients at CKD
stages G1 and G2 and implementing therapeutic strategies to reduce the incidence of CVD events and ESRD.
Clinical practice guidelines established by the Japanese Society of Nephrology (JSN) and American College of
Physicians (ACP) recommend screening for CKD1,5.
CKD stages are determined on the basis of the estimated glomerular filtration rate (eGFR) and proteinuria
grade1,6. Considering the relationship between CKD stages and patients’ CKD prognosis, the prognosis is classi-
fied into four categories according to risk from low (green) to very high (red)1,6. These prognostic categories of
CKD are guides for CKD patients to be treated and referred to nephrologists1,6. However, the rate of referral to
nephrologists on the basis of the prognostic categories of CKD was low7.
One of the reasons for the difficulty in treating CKD is that the decline in eGFR is slower in early CKD stages
than in late CKD stages, and a long follow-up period is required8. Moreover, the association among many causes
of CKD progression such as hypertension, diabetes mellitus (DM), and dyslipidemia is complex1,6,9,10. The treat-
ment strategy for early CKD has not been fully established yet.
If CKD patients at high risks of CKD progression are identified at CKD stages G1 and G2, who are usually
diagnosed as being at low risks, and their lifestyles are improved, the progression of their CKD will be prevented.
A health checkup is a good opportunity to identify such patients from a healthy population. Therefore, to identify
CKD patients at high risks of CKD progression, and to utilize the results at health checkups, the aims of this study
were to (1) evaluate time-series changes in CKD stage, (2) determine the risk factors for CKD progression using
1Medical Science, Kawasaki Medical School, Okayama, Japan. 2Department of nephrology, tokyo Medical
University, tokyo, Japan. 3Sports Medical Research center, Keio University, Kanagawa, Japan. correspondence and
requests for materials should be addressed to e.K. (email: [email protected])
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
1
www.nature.com/scientificreportsVariables
Age
Male (%)
Hypertension (%)
DM (%)
Dyslipidemia (%)
BMI (kg/m2)
Waist circumference (cm)
Systolic blood pressure (mmHg)
Diastolic blood pressure (mmHg)
Casual blood glucose (mg/dL)
HbA1c (NGSP) (%)
Serum LDL cholesterol level (mg/dL)
eGFR (mL/min/1.73 m2)
Values
50.1 ± 6.3
4793 (64.2)
2235 (29.9)
421 (5.6)
2661 (35.7)
23.3 ± 3.3
82.7 ± 8.9
123.6 ± 16.9
78.8 ± 11.8
94.6 ± 17.8
5.6 ± 0.6
124.1 ± 31.4
79 ± 14.4
Table 1. Baseline characteristics of subjects with data. Variables are expressed as number, or mean ± standard
deviation. Abbreviations: DM, diabetes mellitus; BMI, body mass index; LDL, low-density lipoprotein; eGFR,
estimate glomerular filtration rate.
A1
(−)
A2
(±)
A3
(+)
G1
G2
G3a
G3b
1375 (18.4)
53 (0.7)
25 (0.3)
5061 (67.8)
355 (4.8)
129 (1.7)
322 (4.3)
17 (0.2)
34 (0.5)
3 (0.0)
18 (0.2)
2 (0.0)
A3
(2+)
10 (0.1)
36 (0.5)
16 (0.2)
9 (0.1)
Table 2. Distribution of CKD stages. Values are numbers of subjects (%). (−), (±), (+), and (2+) show
proteinuria grades.
Bayesian networks, and (3) identify CKD patients at high risks of CKD progression using support vector machine
(SVM) models and data of common tests from a longitudinal worksite-based study of health checkup in Japan.
Results
Baseline characteristics. The baseline characteristics including biochemical data in 2009 are shown in
Table 1. Regarding CKD stages, G2 Proteinuria grade (P) (−) and G1 P(−) were mostly observed (Table 2). The
CKD stages from 2009 to 2016 showed similar distributions (data not shown). On the basis of the prognostic
categories of CKD, 6436 (86.2%) subjects were at low risk; 730 (9.8%), moderately increased risk; 251 (3.4%) high
risk; and 48 (0.6%), very high risk. Among the subjects with data of their CKD stages in 2009 and 2016 (n = 3927),
3327 (84.7%) were at low risk; 509 (13.0%), moderately increased risk; 68 (1.7%), high risk; and 23 (0.6%), very
high risk. The outcome was observed in 441 (11.2%).
Time-series changes in CKD stage. The comparison of CKD stages between 2009 and any of the follow-
ing years was examined. Most of the subjects showed a stable CKD stage, and some of them showed that their
GFR increased or decreased. From 2009 to 2010, most of the subjects in G1 P(−) (70.8%) and G2 P(−) (81.4%)
showed a stable CKD (Supplementary Fig. S1), whereas 22.8% of the subjects in G1 P(−) in 2009 were in G2 P(−)
in 2010. On the other hand, 10.6% of the subjects in G2 P(−) in 2009 were in 2010 G1 P(−), and 2.3% were in
G3 P(−) in 2010.
The changes in the distribution of CKD stage from 2009 to any of the following years showed similar tenden-
cies (Supplementary Fig. S2 and Fig. 1). The number of subjects whose CKD stage changed from G2 P(−) to G3a
P(−) tended to increase from 2012 (Supplementary Fig. S2). From 2009 to 2016, most of the subjects in G1 P(−)
(35.2%) and G2 P(−) (82.1%) showed stable CKD (Fig. 1), whereas 60.2% of the subjects in G1 P(−) in 2009 were
in G2 P(−) in 2016. On the other hand, 6.6% of the subjects in G2 P(−) in 2009 were in G1 P(−), and 7.2% were
in G3a P(−) in 2016.
Vector analysis showed that any G1 stages and from G2 to G3a with P(2+) tended to show the progress of
GFR categories (Fig. 2). In most of the CKD stages except G3b P(−), the proteinuria grade decreased. And G1
P(−) tended to be more progressive than G2 P(−).
Increase in severity of CKD and related factors. Using the time-series data of two points (2009 and any
of the following years), the Bayesian network showed causal relationships between variables. It showed that the
outcome was affected by the prognostic categories of CKD in 2009 and 2010, and the presence of hypertension
in 2009 (Supplementary Fig. S3). In each of the Bayesian networks in 2009 and 2011 to 2015, the outcome was
affected by the prognostic category of CKD in 2009, and that in each year from 2011 to 2015, but not by other
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
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www.nature.com/scientificreportswww.nature.com/scientificreports/Figure 1. Change in distribution of CKD stages from 2009 to 2016. The distribution was analyzed using data of
subjects with CKD stages in 2009 and 2016 (n = 8991). Values show the number of subjects by CKD stage. G1
to G3b and (−) to (2+) are GFR categories of CKD stages, and proteinuria grades, respectively. Abbreviations:
CKD, chronic kidney disease; GFR, glomerular filtration rate.
Figure 2. Mean changes in CKD stages of subjects from 2009 to 2016. G1 to G3b and (−) to (2+) are GFR
categories of CKD stages and proteinuria grades, respectively. Colors of cells mean low (green), moderately
increased (yellow), high (orange), and very high risks (red) as the KDIGO prognostic categories of CKD.
Arrows show the mean direction of changes in CKD stages of participants from 2009 to 2016. A red line
surrounds CKD stages with high risks. Abbreviations: CKD, chronic kidney disease; GFR, glomerular filtration
rate.
variables (Fig. 3). It was suggested that the time-series data of the prognostic category of CKD were useful varia-
bles for the prediction of the outcome.
Prediction of increase in severity of CKD. The SVM models predicted the progression of the prognostic
category of CKD (Table 3). The test errors of Models 2009 + 2012 to 2009 + 2016 were smaller than that of Model
2009 + 2011.
The heat maps showed the possibility of the outcome as determined using the SVM models (Fig. 4,
Supplementary Fig. S4). In the SVM Model 2009 + 2010, the area for subjects at very high risks in 2009 and 2010
indicated a high possibility of the outcome (Fig. 4A). SVM models showed the different distributions of the proba-
bilities of the outcome from the expected ideal probabilities (Supplementary Fig. S4G). From Model 2009 + 2011,
the area for the subjects with a high possibility of the outcome was observed in the subjects at low risks in 2009
(Fig. 4B, Supplementary Fig. S4B). Model 2009 + 2012 showed that the subjects at moderately or high risks in
2009 showed a high possibility of the outcome (Fig. 4C). From Model 2009 + 2013, the area for the subjects
with a high possibility of the outcome expanded to the area for the subjects at low risks in 2009 (Fig. 4D–F,
Supplementary Fig. S4D–F). The subjects at low risks in 2009 and high or very high risk in 2014 showed a high
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
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www.nature.com/scientificreportswww.nature.com/scientificreports/Figure 3. Bayesian network constructed using the data in 2011. Arrows show the causal relationships between
variables. Abbreviations: Outcome, the progression of the prognostic category of CKD or very high risk in 2016;
progress2009, the prognostic category of CKD in 2009; ht2009, hypertension in 2009; dm2009, diabetes mellitus
in 2009; dl2009, dyslipidemia in 2009; old2009, age of 46 years or more in 2009; high_BMI, body mass index of
22.8 kg/m2 or more in 2009; high_abd, waist circumference of 81.4 cm or more in 2009.
SVM models
Model 2009
Model 2009 + 2010
Model 2009 + 2011
Model 2009 + 2012
Model 2009 + 2013
Model 2009 + 2014
Model 2009 + 2015
Training error
0.124007
0.123015
0.124007
0.123006
0.122008
0.125008
0.123012
Test error
0.1205273
0.1211551
0.1205273
0.1186441
0.1186441
0.1192718
0.1186441
Table 3. Accuracy of prediction of progression of prognostic categories of CKD from 2009 to 2016 using SVM.
SVM models include the prognostic categories of CKD in 2009 and each following year. Abbreviations: SVM,
support vector machine; training error, cross validation error of accuracy to predict the outcome using the
training dataset; test error, error of accuracy to predict the outcome using the test dataset.
possibility of the outcome (Fig. 4E, Supplementary Fig. S4E). This trend was enhanced in the Model 2009 + 2015
(Fig. 4F, Supplementary Fig. S4F).
Discussion
In this study, we investigated the time-series changes in the distribution of CKD stages, and it showed that the
number of subjects showing CKD progression started to increase from 3 years later. The vector analysis showed
the trends of CKD progression in each CKD stage; CKD stage G1 P(−) was more progressive than CKD stage
G2 P(−). The Bayesian networks showed that the time-series changes in the prognostic category of CKD were
related to the outcome. Support vector machines including time-series data of the prognostic category of CKD
from 3 years later detected the high possibility of the outcome not only in subjects showing very high risks but
also in those showing low risks at baseline. These results using our methods have never been reported as far as we
searched the literature.
In this study, we evaluated a healthy population and the time-series changes in their CKD stage. The majority
of the subjects were in G2 P(−) and G1 P(−) in each year, which was in accordance with previous studies11,12.
Among the subjects in G2 P(−), their improvement to G1 P(−) was more commonly observed than their pro-
gression to G3 P(−) over 2 years. Then, the number of subjects showing the CKD progression gradually increased
from 3 years later. The poor reproducibility of proteinuria and eGFR is often observed5. This phenomena of exac-
erbation and improvement of CKD stage might make it difficult to diagnose CKD at an early stage, and to identify
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
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www.nature.com/scientificreportswww.nature.com/scientificreports/Figure 4. Heat map for predicting the outcome from 2009 to 2016. A heat map shows the possibility of the
outcome estimated using SVM models on the basis of data at two points in 2009 and any of the following
year. Blue and red areas indicate high and low risks, respectively. Arrows show the high-possibility area of the
outcome. (A) Data 2009 and 2010. (B) Data 2009 and 2011. (C) Data 2009 and 2012. (D) Data 2009 and 2013.
(E) Data 2009 and 2014. (F) Data 2009 and 2015. Abbreviations: Low, low risk of the prognostic categories of
CKD; Mod, moderately increased risk; High, high risk; Very, very high risk.
CKD patients at high risks of CKD progression. There has been no trajectory study of changes in early CKD stages
based on the prognostic categories of CKD as in this study, to the best of our knowledge5.
Proteinuria and eGFR have been used as markers for monitoring the clinical course of CKD1. Proteinuria
is an appropriate marker for detecting kidney diseases such as glomerular nephritis, and diabetic nephropathy.
However, as in this study, most of the subjects from a healthy population do not have proteinuria; thus, the use
of proteinuria as a marker is limited. On the other hand, an eGFR change of more than 30% has been proposed
as a surrogate endpoint of ESRD13,14. The relationship between eGFR change and the risk of ESRD was validated
in subjects with eGFRs of more than 60 mL/min/1.73 m2 using health checkup data in Okinawa, Japan15. In the
Okinawa study, the risk of ESRD is associated with not only a decrease in eGFR but also an increase in the extent
of eGFR change15. Because the increase in eGFR does not always indicate the improvement of kidney function,
care is necessary in the use of eGFR change as a surrogate endpoint of ESRD. Thus, in CKD stages G1 to G3, either
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
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www.nature.com/scientificreportswww.nature.com/scientificreports/proteinuria or eGFR is not sufficient for evaluating kidney function; both of them are required. The prognostic
category of CKD, which includes both proteinuria and eGFR, is a candidate index for evaluating CKD progres-
sion1,6. In this study, the vector analysis showed the trends of CKD progression in each CKD stage, and we found
that CKD stage G1 P(−) was more progressive than CKD stage G2 P(−). These results suggest a possibility that
CKD progression is a function of eGFR and proteinuria, that eGFR and proteinuria are associated with each
other, and that there is a limitation in treating eGFR and proteinuria independently. Thus, there is a need to con-
sider the complex relationships between factors related to CKD progression when establishing models to estimate
the possibilities of the outcome. From these observations, Bayesian networks, which can treat the relationships
between the factors, were used in this study.
Here, the analyses using Bayesian networks showed that the prognostic categories of CKD at the start and
the following years were associated with the aggravation of CKD. Moreover, the analysis using the SVM models
including time-series data of the prognostic category of CKD from 3 years later could predict the high possibility
of the outcome not only in subjects showing very high risks but also in those showing low risks at baseline. Even
if a subject was at a low risk at baseline, this low risk was not guaranteed over a long period. These results suggest
that it is necessary to follow up not only patients showing high risks but also those showing low risks at baseline.
Yearly evaluation of the prognostic category of CKD by health checkup is recommended by the JSN CKD
guideline1. Then, how long should the results of health checkup be followed up to identify CKD patients at high
risks of the CKD progression? The Okinawa study showed that at least 3 years is required to observe the relation-
ship between the eGFR change and the risk of ESRD15. In the present study, from 3 years later, the number of CKD
patients gradually increased and the accuracy of SVM models increased. From 4 years later, the heat maps of SVM
models indicated the subjects at low risks at baseline and high possibility of the outcome. These results suggest
that depending on the characteristics of the study population, the observation period to accurately evaluate the
CKD progression should be at least 3 years.
The analysis using the Bayesian network showed that the CKD progression was associated with the existence
of hypertension at baseline. These results suggest that hypertension is a risk factor for the CKD progression. This
is in accordance with previous studies1,3,6,9. These lines of evidence suggest that one of the causes of the CKD
progression might be atherosclerosis, which leads to nephrosclerosis. A prospective cohort study showed that
the risk factors for incident CKD are hypertension, aging, DM, and dyslipidemia, which are also associated with
atherosclerosis9. In the present study, although DM and dyslipidemia were not associated with the outcome, when
these comorbid conditions are observed, they should be treated appropriately.
The results of this study and the JSN and Kidney Disease, Improving Global Outcomes (KDIGO) guidelines
suggest the usefulness of the prognostic categories of CKD for the screening for CKD patients showing high
risks1,6. Considering these findings, after the evaluation of kidney function at a health checkup, it is necessary to
follow up not only patients showing high risks but also those showing low risks at baseline for 3 years and longer.
Moreover, (1) when a subject is diagnosed to be at high or very high risk, (2) when comorbid conditions, such
as hypertension, DM, and dyslipidemia, are found, and (3) when the prognostic category of CKD progresses
from low risk at baseline to very high risk three years after or later, it would be better to examine the causes of
CKD, review current management, and consider referring a patient to a nephrologist1,6. These steps from health
checkup to treatment make it possible to provide careful medication that meets CKD patients’ needs. The pro-
motion of health checkup based on the prognostic category of CKD may be useful for establishing public-health
policies to decrease the prevalence of CKD.
This study has several limitations. First, because of the observational nature of this study, the results may be
biased by unmeasured confounders. Second, the population mainly consisted of healthy workers, and did not
include elderly people and the subjects with missing data in this study. Moreover, this study was carried out in
only one region in Japan. These might have caused selection bias. More subjects recruited from all over Japan
would be better to prevent selection bias. Third, age is associated with eGFR and the progression of CKD. The age
of the subjects might affect the results. Moreover, because not only age but also other characteristics might affect
the results of this study, although many models (Bayesian networks and SVM models) were developed and inte-
grated to infer universal results using many sampling datasets based on the boot strapping method, cohort studies
of populations with characteristics different from those in this study such as age, gender, and location might be
required to show the external validity. Fourth, the data were not sufficient for assessing true outcomes such as
events of death, ESRD, and CVD, and various factors associated with CKD progression such as comorbid condi-
tions, and medications. The effects of these factors on CKD will be evaluated in our future studies. Moreover, it
has been reported that the risk of ESRD in a healthy population (eGFR more than 60 ml/min/1.73 m2) was only
186 (0.32%) in 58,292 persons over a 15 year period14. It is very difficult to use ESRD as the true end point in
cohort studies of patients in CKD stages G1 to G3. Therefore, in this study, the outcome was defined as the pro-
gression of a prognostic category of CKD or the high risk at the end of this study. Fifth, in this study, the patients
were followed up for 7 years, which may not be enough to evaluate a true endpoint such as ESRD. However, the
“Guidelines for clinical evaluation of chronic kidney disease” indicate that 3 years of observation is appropriate
for evaluating eGFR changes in a healthy population; therefore, 7 years might be enough to observe changes in
kidney function at the individual level14. Sixth, accuracies of the prediction of the outcome of SVM models were
not compared with those of other prediction models. SVM was selected not only for accurate prediction but also
for the evaluation of the effects of variables on patients’ prognosis. Multivariate SVM models and deep learning
models will be needed for more accurate predictions. To apply the machine learning models in clinical settings,
social implementation such as software development may be useful.
In conclusions, this study showed that the progression of CKD in a healthy population is associated with
the time-series changes in the prognostic category of CKD. After the evaluation of kidney function at a health
checkup, it is necessary to follow up not only patients showing high risks but also those showing low risks at
baseline for 3 years and longer.
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
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www.nature.com/scientificreportswww.nature.com/scientificreports/Methods
Dataset. This study was an observational and worksite-based study conducted in Yamagata, in the north-
ern part of Japan. This study was approved by the ethics committees of Sports Medical Research Center of Keio
University, and was exempt from the need to obtain informed consent from participants (No. 2013-06). The study
was performed in accordance with the relevant guidelines and the Declaration of Helsinki.
In this study, we analyzed data collected every year from the medical checkup records of asymptomatic peo-
ple working at Yamagata Municipalities Mutual Aid Association from 2009 to 2016. The study population con-
sisted of 16734 subjects. Subjects with data on serum creatinine level were included in this study (n = 13946)
(Supplementary Fig. S5). Those with missing data on baseline characteristics were excluded from this study.
Finally, 7465 subjects were included in the study.
The baseline patient data included age, gender, body mass index (BMI), waist circumference, systolic and dias-
tolic blood pressures, casual blood glucose, hemoglobin A1c (HbA1c) (NGSP), serum low-density lipoprotein
(LDL) cholesterol, and creatinine levels, and proteinuria grade (dip stick). Because subjects with much proteinu-
ria more than (2+) were very rare, they were assigned a grade of (2+). eGFR was calculated using the following
equation for the Japanese population16:
2
eGFR (ml/ min/1 73m )
.
=
194
×
where Cr = serum creatinine level (mg/dl).
serum Cr
− .
1 094
×
− .
0 287
age
(for female)
× .
0 739,
Subjects were categorized into CKD stages on the basis of their eGFR and proteinuria in accordance with
JSN and KDIGO CKD guidelines1,6. Because subjects in CKD stages G4 and G5 were very rare in this study, they
were categorized into G3b. In this study, CKD stages were shown as G stages from G1 to G3b with P from P(−) to
P(2+). Hypertension was defined as having a systolic blood pressure of ≥140 mmHg or a diastolic blood pressure
of ≥90 mmHg, or being on antihypertensive medication17. DM was defined as having a casual blood glucose level
of ≥200 mg/dL or a high HbA1c level of ≥6.5%, or being on antidiabetic medication18. Dyslipidemia was defined
as having a serum LDL level of ≥140 mg/dL or being on lipid-lowering medication19.
Statistical analyses. Normally distributed variables are presented as mean ± standard deviation (SD). The
distribution of CKD stages was evaluated by heat mapping (Supplementary Fig. S6). Each subject’s CKD stage can
be treated as a position coordinate; for example, G1 P(−) is (0, 0) (Supplementary Fig. S7A). Here, one CKD-stage
progression of G is expressed (1, 0), and that of proteinuria is (0, 1). For example, given the CKD stage in 2009
and in 2016 being (G2009, P2009), and (G2016, P2016), respectively, the change in CKD stage from 2009 to 2016 can be
treated as a vector, (G2016 - G2009, P2016 - P2009) (Supplementary Fig. S7B). The mean vector of subjects at each CKD
stage in 2009 indicated the trend of changes in CKD stage.
CKD stage is classified on the basis of the KDIGO prognostic categories of CKD, namely, low risk, moderately
increased risk, high risk, and very high risk of the risk of ESRD, CVD, and death6. Here, the outcome was defined
as the progression of a prognostic category of CKD (from 2009 to 2016) or the high risk at the end of this study
(2016).
Bayesian network is a kind of probabilistic graphical model that shows variables and their causal relationships
via a directed acyclic graph, and represents the probabilistic relationships between variables. The Bayesian net-
work was used to evaluate the relationships between the outcome and the variables using two points of time-series
data (2009 and any of the following year from 2010 to 2015). The incremental association Markov blanket method
was used for the structure learning algorithm for the Bayesian network. The resulting directed acyclic graph was
interpreted as the causal Bayesian network using boot strapping method to average the networks. Continuous
variables were discretized using the following cutoff levels determined using receiver operating characteristic
curves for the prediction of the outcome using the data in 2009: age, 46 years; BMI, 22.8 kg/m2; and waist circum-
ference, 81.4 cm.
SVM is a discriminative classifier defined by a separating hyperplane, which can treat non-linear borderlines,
evaluate the effects of variables, and predict the possibilities of the outcome. SVM models including the prog-
nostic categories of CKD were used in this study. Two-thirds of a dataset was used as the training dataset and the
remaining one-third was used as the test dataset. In the training dataset, classification was examined on the basis
of the three-fold cross validation method, and the accuracy of the prediction was estimated by taking the coverage
of three results. Then, using the test dataset, we evaluated the accuracy of the prediction using the SVM models.
Using the Gaussian radial basis function kernel, we applied C-support vector classification with variables. These
analyses were conducted using SAS version 9.4 (SAS, Inc., NC, USA) and R version 3.5.1 (R project for Statistical
Computing, Vienna, Austria). Statistical significance was defined as p < 0.05.
Data Availability
The datasets generated during and/or analyzed during the current study cannot be publicly available because
they are owned by Yamagata Municipalities Mutual Aid Association and Sports Medical Research Center, Keio
University. Please ask Sports Center of Keio University about data availability (http://sports.hc.keio.ac.jp/ja/).
References
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Author Contributions
All authors were involved in the design of this study. E.K. was the main author of the manuscript. E.K. and
Y.K. carried out data analysis and statistical analysis in discussion with F.K. All authors were involved in the
interpretation of the data and in editing the manuscript. All authors approved this manuscript to be published.
Additional Information
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-019-41663-7.
Competing Interests: The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International
License, which permits use, sharing, adaptation, distribution and reproduction in any medium or
format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Cre-
ative Commons license, and indicate if changes were made. The images or other third party material in this
article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Commons license and your intended use is not per-
mitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the
copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
© The Author(s) 2019
Scientific RepoRts | (2019) 9:5082 | https://doi.org/10.1038/s41598-019-41663-7
8
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10.1186_s13071-020-3970-1.pdf
|
Availability of data and materials
The RNA‑seq data obtained in this study were deposited in the National
Center for Biotechnology Information (NCBI) Sequence Read Archive (SRA)
database (https ://www.ncbi.nlm.nih.gov/sra) under accession number
SUB6209220.
|
Availability of data and materials The RNA-seq data obtained in this study were deposited in the National Center for Biotechnology Information (NCBI) Sequence Read Archive (SRA) database ( https ://www.ncbi.nlm.nih.gov/sra ) under accession number SUB6209220.
|
Zhai et al. Parasites Vectors (2020) 13:84
https://doi.org/10.1186/s13071-020-3970-1
Parasites & Vectors
RESEARCH
Open Access
Transcriptional changes in Toxoplasma
gondii in response to treatment with monensin
Bintao Zhai1,2, Jun‑Jun He2, Hany M. Elsheikha3, Jie‑Xi Li2, Xing‑Quan Zhu2,4* and Xiaoye Yang1*
Background: Infection with the apicomplexan protozoan parasite T. gondii can cause severe and potentially fatal cer‑
ebral and ocular disease, especially in immunocompromised individuals. The anticoccidial ionophore drug monensin
has been shown to have anti‑Toxoplasma gondii properties. However, the comprehensive molecular mechanisms that
underlie the effect of monensin on T. gondii are still largely unknown. We hypothesized that analysis of T. gondii tran‑
scriptional changes induced by monensin treatment can reveal new aspects of the mechanism of action of monensin
against T. gondii.
Methods: Porcine kidney (PK)‑15 cells were infected with tachyzoites of T. gondii RH strain. Three hours post‑infec‑
tion, PK‑15 cells were treated with 0.1 μM monensin, while control cells were treated with medium only. PK‑15 cells
containing intracellular tachyzoites were harvested at 6 and 24 h post‑treatment, and the transcriptomic profiles of T.
gondii‑infected PK‑15 cells were examined using high‑throughput RNA sequencing (RNA‑seq). Quantitative real‑time
PCR was used to verify the expression of 15 differentially expressed genes (DEGs) identified by RNA‑seq analysis.
Results: A total of 4868 downregulated genes and three upregulated genes were identified in monensin‑treated
T. gondii, indicating that most of T. gondii genes were suppressed by monensin. Kyoto Encyclopedia of Genes and
Genomes (KEGG) pathway enrichment analysis of T. gondii DEGs showed that T. gondii metabolic and cellular path‑
ways were significantly downregulated. Spliceosome, ribosome, and protein processing in endoplasmic reticulum
were the top three most significantly enriched pathways out of the 30 highly enriched pathways detected in T. gondii.
This result suggests that monensin, via down‑regulation of protein biosynthesis in T. gondii, can limit the parasite
growth and proliferation.
Conclusions: Our findings provide a comprehensive insight into T. gondii genes and pathways with altered expres‑
sion following monensin treatment. These data can be further explored to achieve better understanding of the
specific mechanism of action of monensin against T. gondii.
Keywords: Toxoplasma gondii, Monensin, PK‑15 cells, RNA‑sequencing
Background
Toxoplasma gondii is one of the most successful oppor-
tunistic pathogens and has a wide range of intermediate
*Correspondence: [email protected]; [email protected]
1 College of Veterinary Medicine, Inner Mongolia Agricultural University,
Hohhot 010018, Inner Mongolia Autonomous Region, People’s Republic
of China
2 State Key Laboratory of Veterinary Etiological Biology, Key Laboratory
of Veterinary Parasitology of Gansu Province, Lanzhou Veterinary Research
Institute, Chinese Academy of Agricultural Sciences, Lanzhou 730046,
Gansu, People’s Republic of China
Full list of author information is available at the end of the article
hosts [1, 2]. This prolific parasite is estimated to cause
latent infection in a third of the global human population
[3]. While T. gondii is largely benign in immunocompe-
tent individuals, infection with this parasite can cause
severe inflammation of the retina, and in severely immu-
nosuppressed patients, latent tissue cysts can reactivate
in the brain causing life-threatening toxoplasmic enceph-
alitis [4]. Toxoplasma gondii is also responsible for signif-
icant economic losses attributed to abortions of pregnant
sheep following primary infection especially during early
and mid-pregnancy [5].
© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and
the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material
in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material
is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the
permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco
mmons .org/licen ses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creat iveco mmons .org/publi cdoma in/
zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
Zhai et al. Parasites Vectors (2020) 13:84
Page 2 of 11
In veterinary medicine, control of ovine toxoplasmo-
sis relies on the use of decoquinate [6]. Also, monen-
sin [7] and the folate inhibitor drugs, sulphamezathine
and pyrimethamine [8], have been evaluated against T.
gondii infection in pregnant sheep. There is a vaccine
(Toxovax®, MSD Animal health) licensed for the preven-
tion of abortion in sheep [9], although this vaccine suffers
from a number of shortcomings [10]. Regarding humans,
the first-line therapy for T. gondii infection is a combi-
nation of pyrimethamine and sulfadiazine. However, this
regimen has some limitations because these drugs must
be taken for a long duration, often cause side effects, and
are incapable of eliminating the latent infection [11].
These drawbacks pose a major obstacle in conventional
chemotherapy of toxoplasmosis in humans. To this end,
efforts have been made to identify new and more effec-
tive medicines [12, 13] and to understand the mechanism
of action [14] and perturbation associated with the cur-
rently used drugs [15].
One of the drugs that received more attention in recent
years is monensin, which is an ionophore antibiotic used
to treat coccidiosis in poultry and dairy animals. Mon-
ensin has shown antiparasitic activity against T. gondii
in vitro [16, 17] and in sheep [7]. Through induction of
oxidative stress, monensin disrupts the mitochondrial
function, and induces an arrest of the cell cycle and
autophagy-like cell death in T. gondii [14]. Given the
promising anti-T. gondii activity of monensin, further
understanding of its mechanism of action could reveal
new targets for drug development against T. gondii. The
transcriptomic profile of T. gondii-infected porcine kid-
ney (PK-15) cells has been reported [18]. However, com-
prehensive understanding of how monensin treatment
alters the transcriptome of T. gondii remains unknown.
In the present study, we profiled global gene expres-
sion in T. gondii following treatment of T. gondii-infected
PK-15 cells with monensin using high-throughput
RNA-sequencing (RNA-seq) analysis. Our data showed
that monensin can cause genome-wide transcriptional
changes in T. gondii.
Methods
Toxoplasma gondii culture
Tachyzoites of T. gondii RH strain were cultured and
maintained in porcine (Sus scrofa) kidney (PK-15) cell
monolayers. PK-15 cells were obtained from the Ameri-
can Tissue Culture Collection (ATCC ® CCL-33™; Mary-
land, USA) and cultured in Dulbecco’s Modified Eagleʼs
Medium (DMEM, HyClone, Shanghai, China) supple-
mented with 10% fetal bovine serum (Gibco, Maryland,
USA) at 37 °C in 5% CO2. Tachyzoites were harvested
when 80% of the infected PK-15 cells had lysed. The
infected cells and egressed tachyzoites were passed
through a 22-gauge needle 20 times to rupture any
remaining PK-15 cells. The supernatant was removed by
centrifugation at 350×g for 10 min at 4 °C, and the tachy-
zoites were resuspended in 3 ml DMEM. The final puri-
fied tachyzoites were counted using a hemocytometer.
Monensin treatment
PK-15 cells were infected with tachyzoites at a multiplic-
ity of infection of 3 (3 tachyzoites: 1 PK-15 cell). Three
hours post-infection, 12 T25 tissue culture flasks were
randomly divided into four groups (3 flasks/group). The
two treatment groups included M6 (T. gondii-infected
cells at 6 h post-monensin treatment) and M24 (T. gon-
dii-infected cells at 24 h post-monensin treatment).
The two control groups (C6 and C24) were infected and
untreated cells. The M6 and M24 groups were treated
with monensin solution (Alfa Aesar, Ward Hill, USA) at
a final concentration of 0.1 μM, while the control groups
were treated with fresh medium without monensin. Each
group included three biological replicates. The treated
and control (untreated) cells were harvested at 6 and 24
h post-treatment and stored at -80 °C, until used for RNA
extraction and RNA-seq.
RNA extraction and RNA‑seq analysis
to
the manufacturer’s
Total RNA was individually extracted from each sam-
ple using TRIzol (Invitrogen China Ltd, Beijing, China)
according
instructions. All
extracted RNAs were treated with RNase-Free DNase
(Ambion, Shanghai, China) to remove any residual
genomic DNA. The integrity and quantity of all RNA
samples were examined using the Agilent 2100 Bioana-
lyzer (Agilent Technologies, Santa Clara, CA, USA) and
a NanoDropTM spectrophotometer (Thermo Scientific,
Wilmington, DE, USA), respectively. Five micrograms
of total RNA were used for the construction of the tran-
scriptome libraries and 100-bp paired-end strand-specific
RNA-sequencing was performed on the BGISEQ-500
Platform as per the manufacturer’s instructions.
Sequence filtering, read mapping and analysis
of differentially expressed genes (DEGs)
The raw sequencing data were processed using the
FASTX tool (http://hanno nlab.cshl.edu/fastx _toolk it/)
to remove adaptor sequences, low-quality reads (qual-
ity value < 20), reads containing > 5% N rate and joint
sequences before downstream analyses. StringTie [19]
was used to reconstruct the transcripts guided by the
genomic annotation
information. Novel transcripts
were identified using Cuffcompare (a tool of Cufflinks)
[20]. The coding ability of new transcripts was pre-
dicted using Coding Potential Calculator [21]. The high-
quality clean reads were then mapped to the reference
Zhai et al. Parasites Vectors (2020) 13:84
Page 3 of 11
genomes of pig (Sus scrofa) (ftp://ftp.ncbi.nlm.nih.gov/
genom es/Sus_scrof a/) and T. gondii (ftp://ftp.ncbi.nlm.
nih.gov/genom es/refse q/proto zoa/Toxop lasma _gondi
i/lates t_assem bly_versi ons/GCF_00000 6565.2_TGA4)
using HISAT and Bowtie 2 tools [22]. The gene expres-
sion level was calculated for each sample using the RSEM
(RNA-seq by expectation-maximization) program [23]
and the FPKM (fragments per kilobase of exon per mil-
lion mapped fragments) method. DEseq2 software was
used to identify the differentially expressed genes (DEGs).
Gene expression with log2 fold change ≥ 1 or ≤ − 1, and
adjusted P-value < 0.01 was considered as differentially
expressed. Universal Protein Resource (UniProt) (https
://www.unipr ot.org/), Kyoto Encyclopedia of Genes and
Genomes (KEGG) Orthology Based Annotation System
(http://kobas .cbi.pku.edu.cn/index .php)
3.0
and Gene Ontology (GO, http://geneo ntolo gy.org/) were
used for gene/protein functional annotation, pathway
annotation and gene enrichment analyses, respectively.
The GO enrichment analysis results were categorized
according to the biological process (BP), cellular compo-
nent (CC) and molecular function (MF). The RNA-seq,
reads alignment and DEG identification were carried out
at BGI-Shenzhen, China.
(KOBAS)
Verification of RNA‑seq results by qPCR
Quantitative real-time PCR (qPCR) was used to verify
the RNA-seq results. The expression levels for 15 DEGs
were determined by qPCR using the same RNA samples
that were used for the sequencing. The RNA samples
were reverse-transcribed to single strand cDNA using the
PrimeScriptTM RT reagent Kit (TaKaRa, Dalian, China).
Fifteen genes (nine host cell genes and six T. gondii genes)
were randomly selected for qPCR verification and β-actin
was used as the reference gene. All qPCR reactions were
performed on the BIO-CFX96 system (Bio-Rad, Califor-
nia, USA) using SYBR Green GoTaq® qPCR Master Mix
(Promega, Beijing, China) following the manufacturer’s
instructions. The primers used for qPCR are listed in the
Additional file 1: Table S1. The selected genes were ana-
lyzed in triplicate. The qPCR cycling conditions included
95 °C for 2 min followed by 40 cycles of 95 °C for 10 s, 58
°C for 15 s, 72 °C for 40 s, and the temperatures of the
melting curve analysis ranged from 72 to 95 °C. The 2−
ΔΔCq method was used to calculate the relative expression
of each gene.
Results
We analyzed the global gene expression of T. gondii
infecting PK-15 cells in the absence or presence of 0.1
μM monensin treatment using an Illumina platform. The
obtained sequences were aligned against pig and T. gondii
genome sequences. More than 11.01 Gb of clean bases/
Fig. 1 Distribution of the differently expressed genes (DEGs) of T.
gondii across the examined groups. X‑axis shows the difference
between treated and untreated samples, and at two different
time points (6 h and 24 h post‑treatment). Y‑axis represents the
number of DEGs. Red and blue colours represent upregulated and
downregulated DEGs, respectively
reads were obtained from each treated and untreated
sample (Additional file 2: Table S2).
Differentially expressed genes (DEGs)
Three upregulated and 1012 downregulated T. gondii
genes were detected at 6 h post-treatment, while 3856
downregulated T. gondii genes were found at 24 h post-
treatment (Fig. 1). Interestingly, 990 downregulated T.
gondii DEGs were shared between monensin-treated
samples at 6 and 24 h (Fig. 2). These 990 downregulated
Fig. 2 Venn diagram showing the overlap of the number of up and
downregulated genes of T. gondii in C6 vs M6 group (6 h), and C24 vs
M24 group (24 h)
Zhai et al. Parasites Vectors (2020) 13:84
Page 4 of 11
genes accounted for 97.8% of the downregulated genes at
6 h and 25.7% of the downregulated genes at 24 h post-
treatment. The expression of 15 genes obtained through
RNA-seq were confirmed by qPCR and the validation
results are shown in Fig. 3.
on their DBDs, TFs can be classified into different families
[26]. In our study, differentially expressed TFs were classi-
fied into 25 families (Fig. 7), and homeobox and zf-C2H2
were the two most significantly enriched TFs in T. gondii.
Gene Ontology (GO) analysis of the DEGs
Protein–protein interaction (PPI) of DEGs
A total of 44 GO terms, including 17 biological pro-
cess (BP) terms, 15 cellular component (CC) terms and
12 molecular function (MF) terms were significantly
enriched for 4871 T. gondii DEGs (Fig. 4). Among the BP
category at 6 and 24 h, the top two enriched GO terms
were metabolic process and cellular process. In the CC
category at 6 h, membrane and cell were the top two GO
terms (Fig. 4a), while membrane and membrane part
were the top two GO terms at 24 h (Fig. 4b). In the MF
category at 6 and 24 h, the top two GO terms were cata-
lytic activity and binding.
KEGG pathway analysis
including metabolism, genetic
We also mapped the DEGs to six different KEGG sub-
systems,
information
processing, environmental information processing, cel-
lular processes, organismal systems and human diseases
(Fig. 5). KEGG pathway analysis also showed that most
of T. gondii DEGs were enriched in infectious diseases,
signal transduction and translation. The 30 most signifi-
cantly enriched pathways are shown in Fig. 6; spliceo-
some, ribosome, and protein processing in endoplasmic
reticulum are the top three significantly enriched path-
ways in T. gondii (Additional file 3: Figure S1, Additional
file 4: Figure S2, Additional file 5: Figure S3).
Transcription factors (TFs) of DEGs
TFs are key regulators of gene expression [24]. They bind
to specific DNA sequences and activate or repress gene
expression by DNA-binding domains (DBDs) [25]. Based
(XM_002368522.2,
(XM_002366378.2,
(XM_018780938.1, K03031)
Using the String database prediction, PPI networks of
T. gondii with a combined score > 980 at 6 h post-mon-
ensin treatment are shown in Fig. 8. TGME49_002580
(XM_018779214.1), which encodes ATPases associated
with diverse cellular activities (AAA proteins), was the
most enriched upregulated gene in T. gondii. Four pro-
(XM_018780522.1, K03037),
teins, TGME49_238180
K03033),
TGME49_292220
and
TGME49_250830
TGME49_227960
K03036),
formed a separate mutual network. TGME49_238180,
regulate
TGME49_292220
TGME49_227960
and
TGME49_250830, while TGME49_292220
regulates
TGME49_250830, TGME49_227960 and TGME49_238180.
(TGME49_292220
These proteins
(K03033, Rpn3),
TGME49_238180
(K030037, Rpn7), TGME49_227960
(K03036, Rpn6), and TGME49_250830 (K03031, Rpn12)) are
the components of the 19S regulatory particle in the protea-
some pathway (map03050, Additional file 6: Figure S4). The
protein TGME49_289830 (XM_002368367.1, K03246) regu-
lates two proteins, TGME49_294670 (XM_002370195.2,
K03248) and TGME49_317720 (XM_018782917.1, K03251);
where they all belong to the family of translation initia-
tion factors (eIF3) of the RNA transport pathway. Addi-
tional file 7: Figure S5 shows T. gondii PPIs at 24 h where
(XM_002371193.2), TGME49_266460
TGME49_210790
(XM_002368694.2), TGME49_297140 (XM_018782303.1),
TGME49_275750 (XM_002371561.2) and TGME49_305010
(XM_002370254.1) are some of the proteins that warrant
further studies.
Fig. 3 Verification of the RNA‑seq data by using qPCR. Bars represent the mean fold changes of the expression of six T. gondii genes and nine
porcine genes
Zhai et al. Parasites Vectors (2020) 13:84
Page 5 of 11
Fig. 4 GO enrichment analysis of the differentially expressed genes (DEGs) of T. gondii. The bar graphs show the number of T. gondii DEGs enriched
in GO terms belonging to the three GO categories, biological process, cellular component and molecular function, at 6 h (a) and 24 h (b). X‑axis
represents the GO terms and Y‑axis represents the number of upregulated (Up) and downregulated (Down) genes in different GO terms
Zhai et al. Parasites Vectors (2020) 13:84
Page 6 of 11
Fig. 5 KEGG annotation of the DEGs in the transcriptome of T. gondii. X‑axis label represents the number of DEGs in the corresponding KEGG
pathways in each KEGG subsystem. Y‑axis label represents main clusters of the KEGG pathways
Discussion
The search for new anti-Toxoplasma gondii drugs has
been active for several decades [12, 13], but only a few
drugs are currently approved for use in humans [1, 27].
Although sulfa drugs can be effectively used for the
prevention and control of T. gondii infection in humans
and animals, their side effects should not be ignored [28].
Compared to the mainstream anti-Toxoplasma drugs
(sulfa and ethylamines, trimethoprim combined with sul-
famethoxazole), monensin seems to be less cytotoxic [29,
Zhai et al. Parasites Vectors (2020) 13:84
Page 7 of 11
Fig. 6 Scatterplot of the top 30 most enriched KEGG pathways of T. gondii. Y‑axis label represents the distinct KEGG pathways, and X‑axis label
represents the Rich Factor. Rich Factor refers to the ratio of DEGs annotated in the pathway to total number of genes annotated in the pathway.
The greater the Rich Factor, the greater the degree of pathway enrichment. Dot size represents the number of DEGs (bigger dots denote large DEG
number and vice versa). The colours of the dots represent the P‑values of enrichment. Red colour indicates high enrichment, while green colour
indicates low enrichment
30]. The anticoccidial drug monensin has been shown to
inhibit the viability and even damage the bradyzoite stage
of T. gondii [29] and to prevent the shedding of oocysts
from cats [31]. Monensin can also induce cell cycle arrest
and autophagy, leading to death of T. gondii tachyzoites
[32, 33], probably mediated by an oxidative stress-related
mechanism [14]. Despite this body of literature describ-
ing the mechanisms that mediate the inhibitory effects of
monensin against different life-cycle forms of T. gondii,
the comprehensive mechanisms responsible for killing of
T. gondii by monensin remains incompletely defined.
In this study, we used RNA-seq technology to identify
the global transcriptomic changes in T. gondii caused
by monensin treatment. We found 4868 downregulated
genes and three upregulated genes in T. gondii follow-
ing monensin treatment. The significant number of
Zhai et al. Parasites Vectors (2020) 13:84
Page 8 of 11
and various biological functions that are essential for
cell survival. These findings indicate that anti-T. gondii
effects of monensin could be mediated by impairment
of most of T. gondii biological processes and membrane
components.
KEGG pathway analysis showed that spliceosome,
ribosome and protein processing in the endoplasmic
reticulum were the top three of the 30 most significantly
enriched pathways in T. gondii (Fig. 6). Protein process-
ing in the endoplasmic reticulum is a pathway that influ-
ences protein folding in the endoplasmic reticulum [34].
Proteolytic cleavage of effectors in the endoplasmic
reticulum pathway is essential for the survival of T. gondii
[35]. Additional file 5: Figure S3 shows that most genes
involved in protein processing in the endoplasmic retic-
ulum pathway are downregulated. Thus, we infer that
monensin could suppress protein processing in the endo-
plasmic reticulum pathway in T. gondii, which would
contribute to its anti-T. gondii activity.
The spliceosomes are RNA-protein complexes respon-
sible for removal of introns (non-coding segments)
from pre-messenger RNAs to form mature mRNAs in
a process known as splicing [36]. Spliceosome compo-
nents have been identified in T. gondii [37]. Our analy-
sis showed that all DEGs involved in the spliceosome
pathway are downregulated by monensin (Additional
file 3: Figure S1). Ribosome biogenesis is closely related
to multiple cellular signaling pathways and any defects in
ribosome production can cause many diseases, and even
death [38]. The ribosome profiling at the level of tran-
scription and translation of T. gondii has been reported
[39]. However, how the ribosome of T. gondii is altered
by monensin remains unknown. Our analysis showed
that DEGs involved in ribosome biogenesis are signifi-
cantly downregulated by monensin (Additional file 4: Fig-
ure S2). These findings indicate that monensin can also
interfere with genes involved in mRNA translation and
ribosome biogenesis, which can restrict the growth of T.
gondii.
The biogenesis of the spliceosome and ribosome are
regulated by transcription factors (TFs). We found that
homeobox and zf-C2H2 were the two most significantly
enriched TFs (Fig. 7). The homeobox TF regulates the
expression of genes associated with various developmen-
tal processes in animals, fungi and plants [40]. The zf-
C2H2 TF family contains a small protein structural motif,
the zinc finger (zf ), which coordinates one or more zinc
ions (Zn2
+) [41]. TFs containing zinc fingers have been
implicated in a variety of biological processes in T. gondii
[42, 43]. For example, depletion of TgZNF2 in T. gondii
caused an arrest of the parasite growth at the G1 phase of
the cell cycle and accumulation of poly(A) RNA in their
nucleus [43]. Thus, monensin-induced downregulation
Fig. 7 Classification of the differentially expressed TFs. The X‑axis
label represents the number of genes and the Y‑axis label represents
the transcription factor family names
downregulated genes shows the overwhelming impact of
monensin treatment on T. gondii, especially at 24 h post-
treatment. We also performed GO enrichment analysis
to analyze the significantly altered biological processes in
T. gondii caused by monensin treatment. The two most
significantly enriched BP GO terms at 6 and 24 h were
metabolic process and cellular process. In the MF cat-
egory, the top two GO terms were catalytic activity and
binding at 6 and 24 h. For the CC category, membrane
and membrane parts were the two most enriched GO
terms at both 6 and 24 h (Fig. 4); these included mem-
brane components that contribute to material transpor-
tation, membrane integration, environmental resistance
Zhai et al. Parasites Vectors (2020) 13:84
Page 9 of 11
Fig. 8 Transcriptional regulatory network analysis of Toxoplasma gondii. Protein–protein interaction (PPI) networks of DEGs of T. gondii at 6 h. The
red and green dots denote the upregulated and downregulated genes, respectively
of these two TFs, homeobox and zf-C2H2, may disrupt
the growth and development of T. gondii, further eluci-
dating more aspects of monensin mode of action against
T. gondii.
downregulated
by monensin,
The PPI analysis revealed several proteins that
including
were
TGME49_210790, TGME49_305010, TGME49_266460
and TGME49_002580. TGME49_002580 is ATPase,
AAA family protein, which plays critical roles in
[44]. TGME49_210790
various cellular processes
(XM_002371193.2) encodes a putative dihydrooro-
tate dehydrogenase (DHODH), which mediates the
fourth step of de novo pyrimidine biosynthesis [45].
In T. gondii, disruption of de novo pyrimidine syn-
thesis results in uracil auxotrophy, virulence attenu-
ation and inability to establish latent infection [46].
Inhibition of the activity of T. gondii dihydroorotate
dehydrogenase (TgDHODH) can potentiate the growth
inhibiting potential of 1-hydroxyquinolones in T. gon-
dii [45]. TGME49_305010 (XM_002370254.1) is puta-
tively encoded as pre-mRNA branch site protein p14,
which is associated with U2 small nuclear ribonu-
cleoprotein particles (snRNPs) and participates in the
spliceosome (map03040) pathway. TGME49_266460
(XM_002368694.2) encodes a small ubiquitin-like fam-
ily modifier (SUMO) belonging to the Ubl family, while
only one gene is encoded by SUMO in lower eukary-
otes, including T. gondii [47]. A previous study of T.
gondii SUMO proteomics revealed over 100 sumoylated
proteins involved in translation, metabolism, post-
translational modification, and protein degradation
[48]. Altering these proteins in T. gondii may be lethal,
which would then contribute to the anti-T. gondii activ-
ity of monensin.
Zhai et al. Parasites Vectors (2020) 13:84
Page 10 of 11
Conclusions
This study examined the transcriptomic landscape of
T. gondii infecting PK-15 cells treated with monensin
and identified monensin-induced DEGs in T. gondii.
Our genome-wide transcriptional analysis revealed that
4868 T. gondii genes were downregulated in treated
cell cultures, suggesting that monensin can suppress
the expression of the majority of T. gondii genes. Also,
monensin treatment appears to adversely influence
various crucial metabolic and cellular processes of T.
gondii, such as spliceosome, ribosome and protein pro-
cessing in the endoplasmic reticulum. Additionally,
monensin induced downregulation of two transcription
factors, homeobox and zf-C2H2, in T. gondii. Further
analysis of the identified transcriptional changes can
provide useful information for better understanding of
the mechanism of action of monensin against T. gondii.
Supplementary information
Supplementary information accompanies this paper at https ://doi.
org/10.1186/s1307 1‑020‑3970‑1.
Additional file 1: Table S1. Primer sequences used for qPCR analysis.
Additional file 2: Table S2. Quality metrics of the clean reads.
Additional file 3: Figure S1. The spliceosome pathway.
Additional file 4: Figure S2. The ribosome pathway.
Additional file 5: Figure S3. Protein processing in the endoplasmic
reticulum pathway.
Additional file 6: Figure S4. Proteasome pathway (map03050). The
proteasome is a protein‑destroying apparatus involved in many essential
cellular functions. The green box Rpn3 represents TGME49_292220
(K03033), Rpn7 represents TGME49_238180 (K030037), Rpn6 represents
TGME49_227960 (K03036), and Rpn12 represents TGME49_250830
(K03031).
Additional file 7: Figure S5. Toxoplasma gondii PPIs at 24 h
post‑treatment.
Abbreviations
qPCR: Quantitative real‑time PCR; DEGs: Differentially expressed genes;
TE: Toxoplasmic encephalitis; CNS: Central nervous system; PK‑15: Porcine
kidney‑15; ATCC : American Tissue Culture Collection; DMEM: Dulbecco’s
Modified Eagleʼs Medium; MOI: Multiplicity of infection; KEGG: Kyoto Ency‑
clopedia of Genes and Genomes; GO: Gene Ontology; BP: Biological process;
CC: Cellular component; MF: Molecular function; RNA‑seq: RNA‑sequencing;
FPKM: Fragments per kilobase of exon per million mapped fragments; TFs:
Transcription factors; DBD: DNA‑binding domain; PPI: Protein–protein interac‑
tions; zf: Zinc finger; C2H2: Cys2His2‑like fold group.
Acknowledgements
We thank BGI‑Shenzhen for technical assistance.
Authors’ contributions
XYY, JJH, HME and XQZ conceived and designed the study, and critically
revised the manuscript. BTZ performed the experiment, analyzed the tran‑
scriptomic data and drafted the manuscript. JJH and JXL helped in the study
implementation and data analysis. All authors read and approved the final
manuscript.
Funding
Project support was kindly provided by the International Science and Technol‑
ogy Cooperation Project of Gansu Provincial Key Research and Development
Program (Grant No. 17JR7WA031), the Elite Program of Chinese Academy of
Agricultural Sciences, and the Agricultural Science and Technology Innovation
Program (ASTIP) (Grant No. CAAS‑ASTIP‑2016‑LVRI‑03).
Availability of data and materials
The RNA‑seq data obtained in this study were deposited in the National
Center for Biotechnology Information (NCBI) Sequence Read Archive (SRA)
database (https ://www.ncbi.nlm.nih.gov/sra) under accession number
SUB6209220.
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Competing interests
The authors declare that they have no competing interests.
Author details
1 College of Veterinary Medicine, Inner Mongolia Agricultural University,
Hohhot 010018, Inner Mongolia Autonomous Region, People’s Republic
of China. 2 State Key Laboratory of Veterinary Etiological Biology, Key Labora‑
tory of Veterinary Parasitology of Gansu Province, Lanzhou Veterinary Research
Institute, Chinese Academy of Agricultural Sciences, Lanzhou 730046, Gansu,
People’s Republic of China. 3 Faculty of Medicine and Health Sciences,
School of Veterinary Medicine and Science, University of Nottingham, Sutton
Bonington Campus, Loughborough LE12 5RD, UK. 4 Jiangsu Co‑innovation
Center for the Prevention and Control of Important Animal Infectious Diseases
and Zoonoses, Yangzhou University College of Veterinary Medicine, Yang‑
zhou 225009, Jiangsu, People’s Republic of China.
Received: 22 September 2019 Accepted: 13 February 2020
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10.1038_s41467-023-38328-5.pdf
|
Data availability
The raw data from the prospective study, the raw scores of the ret-
rospective analysis, the input structures and benchmarking scores for
the efficiency study, and the raw data from the biolayer interferometry
measurements are available at the following repository hosted by the
Institute for Protein Design:
The main supplement (136 MB)
Contains these files:
design_models_final_combo_optimized/
design_models_sequence/
design_models_ssm_natives/
design_stats/
dna_production_scripts/
figure_data/
ngs_analysis_scripts/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/scripts_and_main_pdbs.tar.gz
Experimental data and data derived from that data (155 MB)
Contains these files:
ngs_data/
ngs_data_analysis/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/experimental_data_and_analysis.tar.gz
All ordered proteins in.pdb.gz format: (~100 K files; 15 GB)
Contains these files:
design_models_pdbs/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/design_models_pdb.tar.gz
All ordered proteins in Rosetta binary silent format (6.1 GB)
Contains these files:
design_models_silent/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/design_models_silent.tar.gz
The docks we used for the efficiency benchmark (6.1 GB)
Contains these files:
efficiency_benchmark_docks/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/efficiency_benchmark_docks.tar.gz. Source data are provided
with this paper.
|
Data availability The raw data from the prospective study, the raw scores of the retrospective analysis, the input structures and benchmarking scores for the efficiency study, and the raw data from the biolayer interferometry measurements are available at the following repository hosted by the Institute for Protein Design: The main supplement (136 MB) Contains these files: design_models_final_combo_optimized/ design_models_sequence/ design_models_ssm_natives/ design_stats/ dna_production_scripts/ figure_data/ ngs_analysis_scripts/ files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_ files/scripts_and_main_pdbs.tar.gz Experimental data and data derived from that data (155 MB) Contains these files: ngs_data/ ngs_data_analysis/ files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_ files/experimental_data_and_analysis.tar.gz All ordered proteins in.pdb.gz format: (~100 K files; 15 GB) Contains these files: design_models_pdbs/ files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_ files/design_models_pdb.tar.gz All ordered proteins in Rosetta binary silent format (6.1 GB) Contains these files: design_models_silent/ files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_ files/design_models_silent.tar.gz The docks we used for the efficiency benchmark (6.1 GB) Contains these files: efficiency_benchmark_docks/ files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_ files/efficiency_benchmark_docks.tar.gz . Source data are provided with this paper. Code availability The Rosetta macromolecular modeling suite ( https://www. rosettacommons.org ) is freely available to academic and noncommercial users. Commercial licenses for the suite are available via the University of Washington Technology Transfer Office. Scripts for running ProteinMPNN-FastRelax and AF2 with templating and initial guess are available at https://github.com/nrbennet/dl_binder_design 33 .
|
Article
https://doi.org/10.1038/s41467-023-38328-5
Improving de novo protein binder design
with deep learning
Received: 29 July 2022
Accepted: 24 April 2023
Check for updates
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1,2,3,8, Brian Coventry1,2,4,8, Inna Goreshnik1,2,
Nathaniel R. Bennett
Buwei Huang1,2,5, Aza Allen 1,2, Dionne Vafeados
Justas Dauparas
Steven De Munck 6,7, Savvas N. Savvides
1,2, Minkyung Baek 1,2, Lance Stewart
6,7 & David Baker
1,2,4
1,2, Ying Po Peng1,2,
1,2, Frank DiMaio1,2,
Recently it has become possible to de novo design high affinity protein binding
proteins from target structural information alone. There is, however, con-
siderable room for improvement as the overall design success rate is low. Here,
we explore the augmentation of energy-based protein binder design using
deep learning. We find that using AlphaFold2 or RoseTTAFold to assess the
probability that a designed sequence adopts the designed monomer structure,
and the probability that this structure binds the target as designed, increases
design success rates nearly 10-fold. We find further that sequence design using
ProteinMPNN rather than Rosetta considerably increases computational
efficiency.
Methods for designing proteins which bind with high affinity and
specificity to protein targets of interest are of considerable importance
in biomedicine for generating candidate therapeutics1, diagnostics2,
and imaging reagents3, 4. Currently, the most widely used methods
involve immunization of an animal with the target to elicit antibodies5,
or screening high complexity random libraries of antibody6 or other
scaffolds7 for binding activities. Although powerful, these methods
require considerable experimental effort and do not provide sub-
stantial control over the properties of the resulting binding molecules.
Methods for computationally designing binders could potentially
provide much faster routes to affinity reagents having desired bio-
physical properties that target specific surface patches, and there has
been considerable progress in computational design of protein bind-
ing proteins based on extension of binding motifs observed in protein
structures8–12. Recently, a general Rosetta-based approach to designing
binding proteins using only the structure of the target was developed
and used to design binding proteins to 13 different target sites13. Given
a specified region on a target of interest, the method designs
sequences predicted to fold up into protein structures that have shape
and chemical complementarity to the region. While providing a gen-
eral computational route to designing binders to arbitrary protein
targets, the method requires screening of large numbers of compu-
tationally designed binders to identify hits as only a small fraction
typically have sufficiently high affinity for experimental detection.
In parallel with advances in physical model based protein binder
design, deep learning methods have achieved unprecedented accu-
racy in protein structure prediction. In contrast to Rosetta and other
physically based molecular mechanics methods, which employ energy
functions with one or two thousand parameters obtained from struc-
tural and thermodynamic data on proteins and small molecules14, the
deep learning structure prediction methods AlphaFold215 (AF2) and
RoseTTAFold16 (RF) have hundreds of millions of parameters obtained
by training on very large datasets of protein sequences and structures,
and make no assumptions about pairwise decomposability or func-
tional form. In place of the energy-guided stochastic conformational
sampling approaches utilized by physically based approaches –
molecular dynamics in many protein dynamics studies or Monte Carlo
plus minimization in the case of Rosetta– the deep learning methods
learn iterative transformations of representation of the sequence and
possible structure that very rapidly converge on often quite accurate
models (the successive transformations are analogous to the structure
updates in traditional simulation, but are more concerted, more
1Department of Biochemistry, University of Washington, Seattle, WA, USA. 2Institute for Protein Design, University of Washington, Seattle, WA, USA.
3Molecular Engineering Graduate Program, University of Washington, Seattle, WA, USA. 4Howard Hughes Medical Institute, University of Washington,
Seattle, WA, USA. 5Department of Bioengineering, University of Washington, Seattle, WA, USA. 6VIB-UGent Center for Inflammation Research,
Ghent, Belgium. 7Unit for Structural Biology, Department of Biochemistry and Microbiology, Ghent University, Ghent, Belgium. 8These authors contributed
equally: Nathaniel R. Bennett, Brian Coventry.
e-mail: [email protected]
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directed to the likely correct structure, and there is a more accurate
stopping criterion17). For accurate prediction of the structures of
naturally occurring proteins, both AF2 and RF generally require mul-
tiple sequence alignments (which contain rich co-evolutionary infor-
mation on residues likely to be in contact, etc), but for de novo
designed sequences, which are generally more stable and more regular
than naturally occurring proteins, accurate predictions can be
obtained from single sequences18, 19. There has also been progress in
accuracy prediction for protein structure models; for example Dee-
pAccuracyNet (DAN), which uses a representation consisting of 3D
convolutions of local atomic environments20, achieved state-of-the-art
performance in accuracy prediction in CASP14.
We reasoned that these newly-developed DL methods could
increase the success rate of Rosetta-based protein binder design. As
noted above, while providing a general computational route to
designing binders to arbitrary protein targets, the overall success rate
is quite low. The approach has two primary failure modes (Fig. 1a): first,
the designed sequence may not fold to the intended monomer
structure, and second, the designed monomer structure may not
actually bind the target (Fig. 1b). The physically based Rosetta
approach frames both the folding and binding problems in energetic
terms; for the approach to succeed, the designed sequence must have
as its lowest energy state in isolation the designed monomer structure,
and the complex between this designed monomer structure and the
target must have sufficiently low energy to drive formation of the
design-target protein complex. The primary challenges in accurate
design of both the monomer structure and the protein-protein inter-
face are inaccuracies of the energy function which for computational
tractability is generally represented as a sum of pairwise decom-
posable terms (in Rosetta: Lennard Jones, hydrogen bonding, elec-
trostatic, solvation, and bonded geometry), and the very large size of
the space which must be sampled; if the energy function is inaccurate,
or conformational sampling is incomplete, the designed sequence may
not fold to the intended monomer structure and/or the monomer may
not bind to the target as intended.
In this work, we develop a deep learning-augmented de novo
protein binder design protocol. We show retrospectively and pro-
spectively that this improved protocol has nearly 10-fold higher suc-
cess rate than the original energy-based method.
Results
Retrospective analysis of type I failures
We began by investigating the ability of deep learning methods to
discriminate binders from non-binders (a task we call filtering) in the
set of ~1 million experimentally characterized designs for 10 different
targets described in Cao et al. 15,000–100,000 designs were experi-
mentally tested for each target, and the number of actual binders
ranged from 1 to 584.
We first focused on identifying Type I failures (Fig. 1) in which the
designed sequence does not fold to the intended monomer structure.
As a baseline, we used the Rosetta energy of the monomer, normalized
by chain length (since energy is an extensive quantity). Not surprisingly
as this metric was already used as a stringent filter in generating the
input scaffold set for the Rosetta interface design calculations21, it
provided little discriminatory power (Fig. 1d). In contrast, the deep
learning-based accuracy prediction method DAN was able to partially
discriminate binders from non-binders (Fig. 1d).
While DAN is very fast, taking ~0.5 GPU seconds per monomer
structure, AF2 structure predictions are relatively slow (~5 GPU sec-
onds). As an initial test of the utility of AF2 for monomer structure
modeling, we evaluated the ability of AF2 to predict the structures of
the binder monomers for the five minibinder structures from Cao et al.
for which structures have been solved experimentally (for designs in
complex with TrkA, FGFR2, IL-7Rɑ, and the SARS-CoV-2 Spike protein).
Given only the single sequence for the designed binder, AF2 predicted
the monomer structure with binder Cɑ accuracy between 0.2 Å−0.8 Å
for all binders except for LCB1 which was predicted with 1.5 Å accuracy
(Supplementary Fig. 1). An updated version of RoseTTAFold (RF222, 23)
was also found to predict all monomer structures with binder Cɑ
accuracy between 0.2 Å−0.8 Å, except for TrkA which was predicted
with 1.8 Å accuracy (Supplementary Fig. 1).
Encouraged by this accuracy, we set out to filter the entire set
of Cao et al. designs based on the similarity of the AF2 or RF2
predicted monomer structure to the designed structure (dis-
agreement is an indication of a possible Type I failure). For each
designed sequence for each target, using AF2 or RF2 with a single
sequence as input, we predicted the structure of the binder
monomer. We found that the closer the prediction of the binder
structure was to the Rosetta-designed structure in Cɑ RMSD, the
more likely a binder was to be successful (Supplementary Fig. 4).
We also found that the prediction confidence metric pLDDT was
predictive of success (Fig. 1d); the two metrics are quite corre-
lated (Supplementary Fig. 5; the pLDDT of AF2 and RF2 were
equally discriminative). These results suggest that Type I failures
contribute to the low success rate of binder design, and that such
in part, be identified by discrepancies between
failures can,
design models and AF2 or RF2 structure predictions.
Retrospective analysis of type II failures
To estimate the likelihood of the designed binder structure forming an
interface with the intended target, Cao et al. primarily used the dif-
ference in energy of the bound complex and the unbound monomers
allowing sidechain repacking as computed by Rosetta (Rosetta ddG),
and despite the extensive use of this metric during the original cal-
culations, Rosetta ddG remains an effective filter (Fig. 1e). We investi-
gated the efficacy of DAN in supplementing Rosetta in assessing the
accuracy of the designed complex structure. We found DAN’s complex
accuracy metric to be approximately as predictive of binder success as
Rosetta ddG (Fig. 1e).
We next investigated whether AF2 and RF2 complex prediction
could be used to discriminate designs that form the intended complex
structure from those that do not. We again began by evaluating the
ability of AF2 and RF2 to reproduce the five experimentally determined
minibinder structures from Cao et al. Given an MSA for the target
protein and the single sequence of the designed binder, AF2 predicted
the complex structure with binder Cɑ accuracy between 1.0Å−2.0 Å for
three of five, and RF2 for four of five. The two structures that were not
correctly predicted by AF2 were LCB1 and LCB3 which both target the
SARS-CoV-2 Spike protein; AF2 was not able to correctly model a long
loop in the Spike protein which caused the binders to be predicted as
unbound. RF2 also predicted LCB1 as unbound (Supplementary Fig. 1).
To enable AF2 to be used for binding prediction in cases where the
target is incorrectly modeled, we investigated providing the target
structure to the model as a template. We found this allowed AF2 to
predict the correct COVID spike structures but caused all of the
interfaces except FGFR2 to be predicted incorrectly (Supplementary
Fig. 1). We next investigated initializing the AF2 pair representation
with an encoding of the Rosetta binder structure; we call this protocol
“AF2 initial guess” (see AF2 Initial Guess in Methods). Using AF2 with
target template and an initial guess, AF2 is able to recapitulate the
experimentally determined structures for all 5 minibinder interfaces
with binder Cɑ accuracy between 1.0Å−2.0 Å RMSD (Supplementary
Fig. 1). Notably, for all structures except LCB1 and LCB3, the AF2-
predicted structures are closer to the experimentally determined
structure than the original design models, even after extensive
relaxation using Rosetta.
We used the AF2 initial guess approach and RF2 without a
starting model to generate complex models for each designed
sequence for each target, and compared the predicted structure of
the complex to the designed complex structure. The Cɑ RMSD of
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Fig. 1 | Monomer and protein complex structure prediction metrics distinguish
previously designed binders from non-binders. a For binder design to be suc-
cessful, the designed sequence must fold to the designed binder monomer struc-
ture (left), and this structure must form the designed interface with the target
protein (right). b, c Design failure modes. b Type-1 Failures. The designed sequence
does not fold to the designed monomer structure. c Type-2 Failures. The designed
sequence folds to the designed monomer structure but does not form the designed
interface. d, e The retrospective experimental success rate (YSD SC50 < 4 μM) for
the top 1% of designs selected according to different monomer (d) or protein
complex (e) based metrics over 10 targets from Cao et al. Source data are provided
as a Source Data file.
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the predicted complex to the Rosetta-designed complex model
was predictive of design success in both cases (Fig. 1e). We
obtained the best discrimination of binders from non-binders
using the pAE prediction confidence metrics produced by the two
methods (Fig. 1e). For the IL7Ra, TrkA, FGFR2, InsulinR, and PDGFR
datasets from Cao et al, the average pAE of interchain residue pairs
(pAE_interaction) was extremely effective in identifying the
experimentally confirmed binders (Fig. 1e); confident predictions
had very high success frequencies (see the Receiver Operator
Characteristic (ROC) curves in Supplementary Fig. 6) with sharp
increases in success rates for designs with pAE_interaction <10.
AF2 had slightly better performance than RF2 (Fig. 1e), and we
used this in the new design campaigns described in the following
section. The excellent performance of both AF2 and RF2 on the
binder discrimination task strongly suggest that Type II errors are
primarily responsible for the low success rates of Cao et al.
Prospective analysis
The retrospective analysis in Fig. 1 suggests incorporation of AF2 or
RF2 into the design pipeline as a final evaluation filter could con-
siderably increase the design success rate. To directly test this
hypothesis, we carried out binder design campaigns on four targets of
considerable biological importance: ALK24, LTK24, IL10 receptor-ɑ (IL-
10Rɑ)25, and IL2 receptor-ɑ (IL-2Rɑ)26–29. As is clear from the retro-
spective analysis of the Cao et al. data (Fig. 1d, e), binder success rate
and the predictivity of metrics varies between targets: generating
designs for new targets (where there is no a priori knowledge of which
filters would be predictive) is the most unbiased approach for com-
paring different design protocols. For IL-2Rɑ, two separate sites were
targeted with independent campaigns. Using the Rosetta-based design
protocol of Cao et al., we generated computational libraries of ~2
million designs for each target and filtered these down to ~20,000
designs to be experimentally tested for each target: ~15,000 designs
using the physically based filters of Cao et al. and ~5000 designs with
AF2 pAE_interaction <10 (these designs were also filtered by additional
metrics as described in the Supplement). Synthetic genes were
obtained for the ~80,000 designs, transformed into yeast, and the
resulting library sorted for display of the proteins on yeast cells, fol-
lowed by sorts at 1 μM target with avidity, and sorts at decreasing
concentrations of target. The frequency of each design at each sort was
determined by deep sequencing, and SC50 values (the concentration
where half of the expressing yeast-cells are collected) estimated as
described in Cao et al. Designs with SC50 values better than 4 μM were
considered successes; the number of successes for the four targets
ranged from 1 to 17. For each target, several designs found to bind by
YSD were expressed in E. coli and binding was confirmed by single-
concentration Biolayer Inferometry (BLI). All designs which showed
binding by YSD also showed binding by BLI (Supplementary Fig. 9; for
IL-10Rɑ where only one binder was identified, only this single design
was screened by BLI). For all four targets, there was a considerably
higher success rate (number of successes / number of designs tested)
in the AF2-filtered design set than in the Rosetta set (Fig. 2). Physically
based filtering yielded successful binders for two targets: LTK and Site
1 of IL-2Rɑ; for these the AF2-filtered libraries had 8- and 30-fold higher
success rates, respectively. AF2-filtered libraries also yielded success-
ful binders to both ALK and IL-10Rɑ; physically based filtering yielded
no successful binders to either of these targets (Neither filtering
method was able to generate successful binders to Site 2 on IL-2Rɑ).
Thus, AF2 filtering performs as expected in prospective tests,
increasing success rates (for targets where physically based filtering is
successful) and expanding the set of targets for which successful
minibinders can be generated.
Increasing binder design pipeline compute efficiency with Pro-
teinMPNN. While an effective predictor of binder success, the AF2
filter is computationally expensive (~30 GPU-seconds per design) and
only ~2.3% of designs pass, so large numbers of prediction calculations
must be run. To enable the testing of large (~5,000) pools of designs, it
is desirable to decrease the computational demand of the design
pipeline, in particular to maximize the number of designs passing the
AF2 filter a method can generate per unit compute time (the time to
generate all designs and run AF2; we use a conversion factor of 100
CPU-s to 1 GPU-s because of the relative scarcity of GPU resources).
Ef f iciency = Success Rate* Throughput =
Number of Designs with pAE interaction < 10
Total Number of Generated Designs
*
1
Compute Time to Generate One Design
ð1Þ
Using this metric, we find that Rosetta-design has an efficiency of
about 7.6×10−7 successful designs per CPU-s equivalent.
We investigated whether the recently developed deep learning
graphical model based sequence design method ProteinMPNN30 could
be used to increase the efficiency of the design pipeline. ProteinMPNN
is very fast, generating a sequence for a minibinder backbone in ~2
CPU-s compared to ~350 CPU-s for Rosetta-design. We first compared
the experimental success rate of ProteinMPNN designs to Rosetta
designs by generating sequences for backbones generated by AF2 for
Rosetta designs to the four new targets that had low complex Cɑ RMSD
to the AF2 prediction (~104 designs in total). Genes encoding designs
with AF2 pAE_interaction <10 (~103 per method) were synthesized, and
the binding evaluated by FACS followed by deep sequencing as
described above. For each target, several designs from ProteinMPNN
were expressed in E. coli and their binding was verified with BLI, we
again found that all designs which bound by YSD showed binding by
BLI (Supplementary Fig. 9). We found that the design success rate of
ProteinMPNN and Rosetta-design were similar (Supplementary Fig. 7),
thus the considerable increase in speed comes with no decrease in
performance.
Encouraged by the speed and performance of ProteinMPNN
design, we next evaluated its efficiency in generating sequences pas-
sing the AF2 cutoffs. ProteinMPNN design alone had an efficiency of
1.6 × 10−6 successful designs per CPU-s equivalent. The average of
the fold efficiency improvement over all targets is ~5-fold greater
for ProteinMPNN compared to Rosetta-design (Fig. 2c). Since unlike
Rosetta, ProteinMPNN keeps the protein backbone fixed, it is sensitive
to the input backbone structure quality. Inspired by the very efficient
alternation between sequence optimization and structure refinement
in Rosetta flexible backbone design31, we evaluated similar cycling
between ProteinMPNN and Rosetta structure refinement (FastRelax),
hoping to converge on a high-quality backbone that would then allow
ProteinMPNN to generate a high-quality sequence. This hybrid Pro-
teinMPNN/Rosetta sequence design protocol (henceforth referred to
as ProteinMPNN-FR) generated AF2 pAE_interaction <10 structures at a
rate of ~6.6% with a throughput of 1 design per 120 CPU-s for an effi-
ciency of 2.2×10−6. The average per-target efficiency improvement of
ProteinMPNN-FR over Rosetta-design is ~8-fold (Fig. 2c).
Discussion
These experiments show that by complementing physically based
methods with deep learning-based approaches trained on large num-
bers of protein structures, significant improvements to the one-sided
protein-interface design challenge can be achieved. Our retrospective
and prospective studies suggest an increase in design success rate of
ten fold. In contrast to Rosetta energy calculations and DAN structure
accuracy measures, which operate on single protein structures (or with
Rosetta relax calculations, structures very close to the query), struc-
ture prediction calculations implicitly assess the fit of the sequence
with the desired target structure compared to all others. As observed
previously32, such consideration of the overall folding landscape
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Fig. 2 | Incorporation of structure prediction metrics increases design success
rate on new targets. a Results of Prospective Campaigns. For each target the SC50
from YSD is shown for all designs which showed binding by YSD (like Kd’s, lower
values are better). The number of designs included in each library for each target is
indicated by the bars in the top panel. The AF2-predicted structure of the top
scoring on-target design is shown as a cartoon. No binders were identified to Site 2
of IL2 receptor-ɑ so this campaign is not included here or in panel C. b The
experimental success rate for libraries filtered by DL-based filtering versus Physi-
cally based filtering for the four prospective targets. c The computational efficiency
(the number of designs with pAE_interaction <10 per CPU-s) for the ProteinMPNN
sequence design plus Rosetta relax protocol outperforms that of the original
Rosetta sequence design protocol. Source data are provided as a Source Data file.
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enables considerably more accurate assessment of the likelihood a
design will fold and bind as intended compared to evaluation of only
the depth of the designed energy well. Although the protocol reported
here is an order-of-magnitude improvement over the previous state-of-
the-art, it is clear that much about interface energetics remains poorly
understood; success rates among the targets remain low (<1%) and no
binders were identified to Site 2 of IL2 receptor-ɑ. There is also con-
siderable room for improvement in designing high affinity; as with the
original pipeline the initially generated binders are in the high nM
affinity range. Given the rate of progress in the field, we anticipate
further increases in design success rates and affinities in the near
future, which will make computational protein design methods even
more powerful compared to empirical selection methods for gen-
erating affinity reagents and therapeutic candidates. While continued
progress is nearly certain, an open question is whether this will come
from integration of deep learning and physically based methods, or
from deep learning alone–there are exciting times ahead!
Methods
AF2 initial guess
The protein structure provided to the model as an initial guess is first
converted to AlphaFold atom positions. These positions are then
provided, along with the standard model inputs into the AlphaFold
Model Runner. In the AlphaFold class of the AlphaFold code, on the
first recycle, the prev_pos variable is initialized to the input AlphaFold
atom positions as opposed to the standard initialization of all zeros. A
script to run AF2 with an initial guess and the modified source code is
provided here: https://github.com/nrbennet/dl_binder_design33. The
AlphaFold model used in the script and in this work is configured to
run with a reduced number of extra MSA sequences which speeds the
inference of the network dramatically, as described in previous work34.
ProteinMPNN FastRelax
This protocol takes as input a protein complex structure. Pro-
teinMPNN is then provided the complex structure with the binder
sequence masked and asked to assign the binder a sequence. The
new sequence is then threaded back onto the binder structure in the
complex and the complex structure is relaxed using Rosetta Fas-
tRelax. The relaxed complex structure can then be used as the input
to ProteinMPNN to continue the cycle. A python script to perform
this design technique is provided here: https://github.com/nrbennet/
dl_binder_design33.
Design and filtering procedure for prospective study
The prospective study was performed at a time of rapid protocol
discovery with a tight deadline for placing the gene-order. As such, not
every experiment that could have been performed was performed.
However, the comparison of Rosetta filtering to AF2 filtering was the
main goal and the data required for this comparison was plentiful.
The standard procedure from Cao et. al. was followed for the 4
targets starting with the following pdbs: IL2RA “1Z92”, “2B5I”, “3NFP”,
“2ERJ”), IL10RA (“1LQS”), ALK (Privately communicated structure. Now
“7NWZ”), LTK (Privately communicated structure. Now “7NX0”). The
“recommended_scaffolds.list” from Cao et. al. were used and on the
order of 10 M RifDock35 outputs were generated for each target with
about 500 K FastDesigned. ~6 K motifs were extracted, grafted up to
10 M docks, and 500 K FastDesigned again. The resulting 1 M designs
for each target were predicted by AF2.
From this set of 1 M designs, 3 overlapping subsets were selected.
The first subset was the Rosetta-control group where the AF2 predic-
tions were ignored and the top ~18 K per target were selected by the
pareto-front method from Cao et al. looking at target_delta_sap, ddG,
contact_patch, and contact_molec_sq5_apap_target. The second subset
was the AF2-filtered group where all designs passing pae_interaction
<10 and af2_complex_rmsd <5 Å were included. This set was typically
around 8 K per target. The third subset was all predictions with
af2_complex_rmsd <5 Å. These designs were designated to be rede-
signed and were typically about 12 K in scale.
These AF2-predicted interfaces were then designed either with
Rosetta or ProteinMPNN. Here, ProteinMPNN was used to generate a
protein sequence from the input coordinates and no further optimi-
zation was performed. The Rosetta-redesigned and ProteinMPNN-
redesigned pools were predicted again by AF2 and were filtered either
with the Rosetta filters mentioned above or the AF2 filters mentioned
above resulting in pools of sizes 9 K (Rosetta-Rosetta), 2 K (Rosetta-
AF2), and 2 K (ProteinMPNN-AF2). The Rosetta filters weren’t used to
filter ProteinMPNN designs because Rosetta models of ProteinMPNN
outputs didn’t exist.
DNA library preparation
DNA libraries were prepared in the manner described in Cao et al., we
review this protocol here:
The sequences of protein designs were padded to 65 amino acids
through addition of a (S)n linker at the C-terminus. The protein
sequences were reverse translated and codon optimized for Sacchar-
omyces cerevisiae using DNAworks2.036. After reverse translation, DNA
adapter sequences are added to the N (GGTGGATCAGGAGGTTCG)
and C (GGAAGCGGTGGAAGTGG) terminus. Designs were purchased
as oligonucleotide libraries from Agilent Technologies.
Oligonucleotide libraries were amplified using Kapa HiFi poly-
merase (Kapa Biosystems) with a qPCR machine (Bio-Rad, CFX96). The
PCR product was run on a DNA agarose gel, the band with the correct
size was cut out of the gel and cleaned (Qiagen QIAquick Clean up kit).
The extracted DNA products were then re-amplified and purified fol-
lowing the above protocol. The resulting DNA inserts and linearized
pETcon3 vector were transformed into EBY100 yeast following an
established protocol37.
To prepare libraries for deep sequencing, yeast plasmids were
isolated from 5 × 107 to 1 × 108 yeast cells by Zymoprep (Zymo
Research). Two qPCR amplifications were then performed following
the protocol in the above paragraph. Illumina adapters and 6-bp pool-
specific barcodes were added in the second amplification. The final
DNA product was purified by gel extraction. The libraries were
sequenced using Illumina NextSeq sequencing.
Yeast surface display
Yeast surface display experiments were performed in the manner
described in Cao et al., we review this protocol here:
EBY100 yeast were grown in C-Trp-Ura media supplemented
with 2% (w/v) glucose. Yeast cells were centrifuged and resus-
pended in SGCAA media supplemented with 0.2% (w/v) glucose.
Cells were resuspended to a concentration of 1×107 cells per ml
and induced at 30°C for 16-24 hours. Cells were washed with PBSF
(PBS with 1% (w/v) BSA) and then labeled with biotinylated target.
To allow for the identification of low affinity binders, an initial sort
with target avidity was performed for all libraries. In the avidity
sort, the cells are incubated with biotinylated target, anti-c-Myc
fluorescein
and
steptavidin-phycoerythrin (SAPE, ThermoFisher). To allow all SAPE
molecules to display four biotinylated target molecules, the bio-
tinylated target is provided at a 4x excess over the concentration
of SAPE. When sorting without avidity, the cells are incubated first
with biotinylated target alone, then washed in PBSF and subse-
quently incubated with SAPE and FITC. Each library was sorted
against a titration of target concentrations. Sorts were performed
using a Sony SH800S cell sorter with software version 2.1.5.
(FITC, Miltenyi
isothiocyanate
Biotech)
Protein expression
Proteins were expressed and purified in the manner described in Cao
et al., we review this protocol here:
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Genes encoding the designed protein sequences were purchased
from Integrated DNA Technologies (IDT). All genes included an
N-terminal 8-His tag followed by a TEV cleavage site. The genes were
cloned into modified pET-29b(+) E. coli plasmid expression vectors.
Plasmids were transformed into chemically competent E. coli
BL21(DE3) cells (NEB). Cells were either grown overnight in Studier
autoinduction media supplemented with antibiotics or induced using
the IPTG expression system and then grown overnight. Cells were then
lysed by sonication and the protein samples were purified by immo-
bilized metal affinity chromatography (Qiagen) followed by size-
exclusion fast protein liquid chromatography (Superdex 75 10/300 GL,
GE Healthcare).
Target protein preparation
Expression and purification of biotinylated ALK and LTK ectodo-
mains. DNA encoding for the cytokine binding domains of ALK (ALKTG-
EGFL, residues 648-1030) and LTK (LTKTG-EGFL, residues 63-420) were
cloned in the pHLsec vector in frame with a N-terminal chicken RTPμ-
like signal peptide sequence and a C-terminal Avi-tag followed by a
caspase-3-cleavable Fc-Hisx6 tag38.
Proteins were produced in HEK293S suspension cells maintained
in growth medium consisting of 50% Freestyle (Thermofisher) and 50%
Ex-Cell (Sigma-Aldrich). Transient transfection was performed using
linear 25 kDA polyethyleneimine (Polysciences) as transfection
reagent. To allow specific in vivo biotinylation of the Avi-tag, both
constructs were co-transfected with the pDisplay-BirA-ER plasmid in a
4:1 pHLsec:pDisplay stoichiometric ratio39. The growth medium was
supplemented with D-biotin to a final concentration of 100 μM to
ensure complete biotinylation of the recognition sequence. After
4 days of expression, conditioned medium was clarified by cen-
trifugation and filtered through a 0.22 μm filter prior
to
chromatographic steps.
Proteins were captured via their Fc tag on a protein A column
(HiTrap Protein A HP, Cytiva) and eluted in HBS (20 mM HEPES, pH 7.4,
150 mM NaCl) after an on-column digestion with caspase-3 for 1 h at
37 °C and an additional 2-h incubation at room temperature. As a final
polishing step, recombinant proteins were concentrated and injected
onto a Superdex 200 increase 10/300 GL (Cytiva) size-exclusion
chromatography column pre-equilibrated with HBS. Purified biotiny-
lated proteins were flash frozen in liquid nitrogen and stored at −80 °C
until further use.
Biotinylated IL-10Rɑ was purchased from R&D Systems (AVI9044).
Biotinylated IL-2Rɑ was purchased from Acro Biosystems (ILA-H82E6).
Biolayer interferometry binding experiments
Biolayer interferometry (BLI) measurements were performed on an
Octet Red96 (ForteBio) or Octet R8 (Sartorius) instrument with Octet
BLI Discovery 12.2.1.18 software, with streptavidin coated tips (Sar-
torius Item no. 18-5019). The binding buffer consisted of 1X HBS-EP +
buffer (Cytiva BR100669) supplemented with 1.0% w/v bovine serum
albumin. 30-50 nM (depending on target availability) of target protein
was loaded onto the tips. After target loading, a baseline measurement
was performed in binding buffer alone for 120 s. The tips were then
dipped in a solution of 500 nM (1000 nM for the IL-10Rɑ design)
protein analyte in binding buffer for 600 s (association phase). The tips
were then dipped back into binding buffer alone for 1000 s (dis-
sociation phase).
Statistics and reproducibility
No statistical method was used to predetermine sample size. No data
were excluded from the analyses. The experiments were not rando-
mized. The Investigators were not blinded to allocation during
experiments and outcome assessment.
Reporting summary
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
The raw data from the prospective study, the raw scores of the ret-
rospective analysis, the input structures and benchmarking scores for
the efficiency study, and the raw data from the biolayer interferometry
measurements are available at the following repository hosted by the
Institute for Protein Design:
The main supplement (136 MB)
Contains these files:
design_models_final_combo_optimized/
design_models_sequence/
design_models_ssm_natives/
design_stats/
dna_production_scripts/
figure_data/
ngs_analysis_scripts/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/scripts_and_main_pdbs.tar.gz
Experimental data and data derived from that data (155 MB)
Contains these files:
ngs_data/
ngs_data_analysis/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/experimental_data_and_analysis.tar.gz
All ordered proteins in.pdb.gz format: (~100 K files; 15 GB)
Contains these files:
design_models_pdbs/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/design_models_pdb.tar.gz
All ordered proteins in Rosetta binary silent format (6.1 GB)
Contains these files:
design_models_silent/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/design_models_silent.tar.gz
The docks we used for the efficiency benchmark (6.1 GB)
Contains these files:
efficiency_benchmark_docks/
files.ipd.uw.edu/pub/improving_dl_binders_2023/supplemental_
files/efficiency_benchmark_docks.tar.gz. Source data are provided
with this paper.
Code availability
(https://www.
The Rosetta macromolecular modeling
rosettacommons.org)
is freely available to academic and non-
commercial users. Commercial licenses for the suite are available via
the University of Washington Technology Transfer Office. Scripts for
running ProteinMPNN-FastRelax and AF2 with templating and initial
guess are available at https://github.com/nrbennet/dl_binder_design33.
suite
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Acknowledgements
This work was supported with funds provided by The Donald and Jo
Anne Petersen Endowment for Accelerating Advancements in Alzhei-
mer’s Disease Research (N.R.B.), a gift from Microsoft (J.D., M.B., and
D.B.), the Audacious Project at the Institute for Protein Design (A.A., D.V.,
and D.B.), a grant from DARPA supporting the Harnessing Enzymatic
Activity for Lifesaving Remedies (HEALR) Program (HR001120S0052
contract HR0011-21-2-0012, I.G., F.D., Y.P.P., B.H., L.S., and D.B.), the
Flanders Institute for Biotechnology (S.N.S), a Strategic Basic Research
grant from Research Foundation Flanders (S.N.S.), and the Howard
Hughes Medical Institute (B.C. and D.B.). We thank AWS and Microsoft
for generous gifts of cloud computing credits and Texas Advanced
Computing Center (TACC) The University of Texas at Austin for providing
the CPU resources for all Rosetta design calculations.
Author contributions
N.R.B., B.C., and D.B. designed the research. N.R.B. and B.C. contributed
equally. N.R.B. and B.C. developed the method. N.R.B. and B.C.
designed the binders. I.G., B.H., A.A., Y.P.P., and D.V. performed the
yeast screening. J.D. developed ProteinMPNN. M.B. and F.D. developed
RoseTTAFold2. S.D.M. and S.N.S. solved the structures of ALK and LTK.
All authors analyzed data. L.S. and D.B. supervised research. N.R.B., B.C.,
and D.B. wrote the manuscript with input from the other authors. All
authors revised the manuscript.
Competing interests
N.R.B., B.C., I.G., L.S., and D.B. are co-inventors on a United States Patent
and Trademark Office provisional patent application (63/490,479) that
covers the binders designed in this study. The remaining authors declare
no competing interests.
Nature Communications |
(2023) 14:2625
8
Article
https://doi.org/10.1038/s41467-023-38328-5
Additional information
Supplementary information The online version contains
supplementary material available at
https://doi.org/10.1038/s41467-023-38328-5.
Correspondence and requests for materials should be addressed to
David Baker.
Peer review information Nature Communications thanks Arne Elofsson
and the other anonymous reviewer(s) for their contribution to the peer
review of this work. A peer review file is available.
Reprints and permissions information is available at
http://www.nature.com/reprints
Publisher’s note Springer Nature remains neutral with regard to jur-
isdictional claims in published maps and institutional affiliations.
Open Access This article is licensed under a Creative Commons
Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as
long as you give appropriate credit to the original author(s) and the
source, provide a link to the Creative Commons license, and indicate if
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pubs.acs.org/acschemicalbiology
Articles
A Fluorescence Polarization Assay for Macrodomains Facilitates the
Identification of Potent Inhibitors of the SARS-CoV‑2 Macrodomain
Ananya Anmangandla,# Sadhan Jana,# Kewen Peng,# Shamar D. Wallace,# Saket R. Bagde,
Bryon S. Drown, Jiashu Xu, Paul J. Hergenrother, J. Christopher Fromme,* and Hening Lin*
Cite This: ACS Chem. Biol. 2023, 18, 1200−1207
Read Online
ACCESS
Metrics & More
Article Recommendations
*sı Supporting Information
ABSTRACT: Viral macrodomains, which can bind to and/or
hydrolyze adenine diphosphate ribose (ADP-ribose or ADPr) from
proteins, have been suggested to counteract host immune response
and be viable targets for the development of antiviral drugs.
Therefore, developing high-throughput screening (HTS) techni-
ques for macrodomain inhibitors is of great interest. Herein, using
a novel
tracer TAMRA-ADPr, an ADP-ribose compound
conjugated with tetramethylrhodamine, we developed a robust
fluorescence polarization assay for various viral and human
macrodomains including SARS-CoV-2 Macro1, VEEV Macro,
CHIKV Macro, human MacroD1, MacroD2, and PARP9 Macro2.
Using this assay, we validated Z8539 (IC50 6.4 μM) and
GS441524 (IC50 15.2 μM), two literature-reported small-molecule
inhibitors of SARS-CoV-2 Macro1. Our data suggest that GS441524 is highly selective for SARS-CoV-2 Macro1 over other human
and viral macrodomains. Furthermore, using this assay, we identified pNP-ADPr (ADP-ribosylated p-nitrophenol, IC50 370 nM) and
TFMU-ADPr (ADP-ribosylated trifluoromethyl umbelliferone, IC50 590 nM) as the most potent SARS-CoV-2 Macro1 binders
reported to date. An X-ray crystal structure of SARS-CoV-2 Macro1 in complex with TFMU-ADPr revealed how the TFMU moiety
contributes to the binding affinity. Our data demonstrate that this fluorescence polarization assay is a useful addition to the HTS
methods for the identification of macrodomain inhibitors.
■ INTRODUCTION
COVID-19 is an ongoing global pandemic caused by severe
acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that
has led to more than 6.8 million deaths and over 759 million
cases worldwide.1 As one of the major host defense mechanisms
against viral infections, interferon (IFN) signaling is activated
when host cells detect viral invasions.2 A central effector of IFN
activation is the ADP-ribosylation of host cell proteins, and these
ADP-ribose (ADPr) tags play important roles in regulating
protein activities and are thus vital for a successful defense
against viral infection.3
However, SARS-CoV-2 can counter IFN-induced mono-
ADP-ribosylation (MARylation) in host cells through its first
macrodomain (Macro1) encoded within the non-structural
protein 3 (nsp3).4 Macrodomains are ancient and well-
conserved structural modules found in a wide range of proteins
with diverse biological functions. Macrodomains are known to
bind, and in some cases, hydrolyze ADP-ribosylated proteins,
thus functioning as either “readers” or “erasers” of ADPr
modifications. Viral macrodomains, including those of corona-
viruses (CoVs),
the Venezuelan equine encephalitis virus
(VEEV), and the Chikungunya virus (CHIKV), are reported
to hydrolyze MARylated host proteins and are responsible for
attenuating host immune responses against viral infection.4−6
Given the central roles that viral macrodomains play in host cell
immune responses, macrodomain inhibitors are potential
antiviral agents. However, several macrodomains are also
encoded by human proteins, including MacroD1, MacroD2,
PARP9, and TARG1, which may have important physiological
functions.7 Therefore, selective viral macrodomain inhibitors
with minimal off-target effects are highly desirable.
With the emergence of the COVID-19 global pandemic,
multiple research efforts have been directed toward developing
high-throughput screening (HTS) methods for the identifica-
tion of SARS-CoV-2 Macro1 inhibitors. For instance, Dasovich
et al.8 reported a luminescence-based assay termed ADPr-Glo,
which utilizes an ADP-ribosylated peptide that can be
hydrolyzed by SARS-CoV-2 Macro1. The hydrolysis product
ADPr can be further hydrolyzed into AMP by the
Received: February 10, 2023
Accepted: April 10, 2023
Published: May 1, 2023
© 2023 The Authors. Published by
American Chemical Society
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Articles
phosphodiesterase NudF and is subsequently converted into
luminance by the commercially available AMP-Glo and
quantified. Schuller et al.9 screened over 200 crystallographic
and virtual screening hits using a homogeneous time-resolved
fluorescence (HTRF) assay and differential scanning fluorim-
etry (DSF) assay.
These HTS assays for SARS-CoV-2 Macro1 have several
limitations. For the ADPr-Glo assay, an extra enzyme NudF is
used, which can complicate the result since compounds may also
affect NudF activity. HTRF utilizes three expensive reagents
(ADPr-conjugated biotin peptide, FRET donors and acceptors),
rendering it less cost-effective for large-scale screening. Finally,
the DSF assay is not suitable for high-throughput screening.
More facile HTS methods have been developed for other
macrodomains such as PARG,10,11 but they cannot be used for
other macrodomains.
The fluorescence polarization (FP) assay, which exploits the
polarization of a fluorophore being inversely related to its
freedom of motion,12 provides a useful addition to the screening
methods mentioned above. The FP assay is a simple and high-
throughput assay that can be performed in ambient conditions,
and the only reagent it requires other than the protein of interest
is a fluorophore-conjugated ligand (so-called “tracer”). Very
recently, Roy et al.13 developed an FP assay for the screening of
SARS-CoV-2 Macro1 using fluorescein-labeled and ADP-
ribosylated peptide as the tracer. However, they could only
achieve an assay window of less than 60 milipolarization (mP)
using a high protein concentration of 15 μM, which suggests that
the tracer may not be a high-affinity binder of SARS-CoV-2
Macro1 and necessitates the use of large quantities of protein,
limiting its use as a high-throughput screening method.
Herein, we designed and synthesized a novel FP tracer,
TAMRA-ADPr. Using this tracer, we established a robust
binding assay with a wider mP shift window and successfully
applied this assay to a variety of macrodomains, including SARS-
CoV-2 Macro1, VEEV Macro, CHIKV Macro, human
MacroD1, MacroD2, and PARP9 Macro2. Using this assay,
we were able to validate two small-molecule SARS-CoV-2
Macro1 inhibitors reported in the literature. Furthermore, we
tested several ADPr derivatives and identified two compounds
to be the most potent binders of SARS-CoV-2 Macro1 known to
date.
■ RESULTS AND DISCUSSION
To develop an FP assay suitable for the high-throughput
screening of SARS-CoV-2 Macro1 inhibitors, we first designed a
tracer molecule TAMRA-ADPr (Figure 1), inspired by the
presumed structure of macrodomain substrates.7 For the
synthesis of TAMRA-ADPr, ADPr-N3 was first synthesized
and then coupled to alkyne-TAMRA at the C1″ position via click
chemistry (see the SI for details). Gratifyingly, TAMRA-ADPr
showed relatively strong binding to SARS-CoV-2 Macro1 in a
titration assay where the mP shift reached over 110 when 10 μM
Macro1 was used (Figure 2A). Encouraged by this result, we
tested five additional macro domains: human MacroD1,
MacroD2, PARP9 Macro2, VEEV Macro, and CHIKV Macro.
TAMRA-ADPr could bind to each of these macro domains,
albeit with different affinities. Based on the mP shift data,
MacroD1 and MacroD2 are the most potent binders of
TAMRA-ADPr, reaching mP shifts of more than 100 at a low
concentration of 0.38 μM (Figure 2B). This is consistent with
the previous finding that ADPr is a strong binder of both
MacroD1 (KD = 0.72 μM)14 and MacroD2 (KD = 0.15 μM).15
Figure 1. Design and mechanism of a fluorescence polarization (FP)
assay for ADPr-binding macrodomains. (A) Structure of TAMRA-
ADPr. The TAMRA fluorophore is coupled to ADPr at C1″ through a
long triazole-alkane linker. (B) In the absence of inhibitors, the majority
of tracers is bound to protein. Thus, the free rotation of the fluorophore
is hindered and a high fluorescence polarization is observed. Upon
addition of inhibitor, there is competition for binding and the tracer is
released from the macrodomain. The unbound tracer molecules are
now free to rotate, leading to a lower observed polarization.
On the other hand, only an ∼70 mP shift could be achieved by
VEEV Macro and CHIKV Macro at 6 μM (Figure 2C),
suggesting that they are weaker binders of TAMRA-ADPr.
Having obtained a satisfactory tracer, we next designed a
convenient “mix and read” FP assay where different concen-
trations of compounds to be tested were incubated with SARS-
CoV-2 Macro1 and the tracer for 30 min before the mP shifts
were read on a plate reader. The percent binding of the tracer
relative to the negative control (protein and tracer only) was
calculated and fitted to an IC50 curve. It should be noted that the
protein concentrations were chosen to give an mP shift window
of at least 50 to yield data with acceptable errors and therefore
differ for each macrodomain (see Materials and Methods). We
first tested ADPr, a well-characterized ligand for SARS-CoV-2
Macro1 as well as many other macro domains, to see whether
our FP assay could quantitatively capture the binding affinity of
macrodomain ligands. The IC50 of ADPr against the tracer
binding to SARS-CoV-2 Macro1 was determined to be 15.5 μM
(Figure 2D,F), which is comparable to the reported KD value of
11.6 μM.16 We further determined the IC50 values of ADPr
against other macrodomains (Figure 2D,F) and were pleased to
find the IC50 values were all consistent with the reported KD
values of ADPr for different macrodomains.6,14,15 As a negative
control, we also showed that iso-ADPr, the smallest internal
structural unit containing the characteristic ribose−ribose
glycosidic bond for poly-ADPr (PAR),17,18 did not compete
with the tracer in macrodomain binding (Figure 2E). Therefore,
we concluded that this competitive FP assay is a reliable
screening method for potential
inhibitors of SARS-CoV-2
Macro1 and other ADPr-binding macro domains.
Since TAMRA-ADPr is a good binder of SARS-CoV-2
Macro1, we thought it would be interesting to see whether
ADPr-N3, the precursor of TAMRA-ADPr, could also bind
SARS-CoV-2 Macro1. As shown in Figure 3B, ADPr-N3 was
identified to be a more potent binder of SARS-CoV-2 Macro1
than ADPr with a nearly 2-fold smaller IC50. The binding activity
of ADPr-N3 is not unexpected since the azido group is similar to
the hydroxyl group in size and thus unlikely to cause steric
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Figure 2. Validation of the FP assay. (A−C) mP shift values measured after 30 min incubation of 20 nM tracer with varying concentrations of
macrodomains. (D) KD curves of ADPr for different macrodomains. (E) iso-ADPr does not compete with the tracer for all the macrodomains tested.
(F) IC50 values for ADPr with different macrodomains. For all the data presented, error bars indicate SEM and IC50 values are reported as mean ±
SEM, n = 2 or 3.
Figure 3. IC50 determination of ADPr-N3, Z8539, and GS-441524 on SARS-CoV-2 Macro1. (A) Chemical structures of ADPr-N3, Z8539, and GS-
441524. (B) IC50 curve of ADP-N3 for SARS-CoV-2 Macro1. (C) IC50 curve of Z8539 for SARS-CoV-2 Macro1. (D) IC50 curves and values of GS-
441524 for different macrodomains. For all the data presented, error bars indicate SEM and IC50 values are reported as mean ± SEM, n = 2 or n = 3.
clashes with the protein. Given that SARS-CoV-2 Macro1 can
accommodate much bulkier groups at the C1″ position of ADPr,
as shown by TAMRA-ADPr, ADPr-N3 may be a useful
precursor for the development of ADPr-based inhibitors of
SARS-CoV-2 Macro1 through click chemistry.
We then tested two recently reported SARS-CoV-2 Macro1
inhibitors using the FP assay. Z8539 (Figure 3A) is a potent
small-molecule inhibitor of SARS-CoV-2 Macro1 discovered
very recently by Gahbauer et al.19 through a combined approach
of virtual screening and fragment linking. Z8539 was found to be
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Figure 4. pNP-ADPr and TFMU-ADPr are potent macrodomain binders. (A) Chemical structures of pNP-ADPr and TFMU-ADPr. (B) IC50 curves
of pNP-ADPr and TFMU-ADPr for SARS-CoV-2 Macro1. IC50 curve of ADPr is also shown as a reference. (C) IC50 values of pNP-ADPr and TFMU-
ADPr for SARS-CoV-2 Macro1. (D) Fold decrease in the IC50 values of pNP-ADPr and TFMU-ADPr relative to those of ADPr for each
macrodomain tested. (E) Electron density of TFMU-ADPr in the complex with SARS-CoV-2 Macro1. (F) Structure of Macro1 in complex with
TFMU-ADPr (cyan) is superimposed with that of Macro1 in complex with ADPr (gray, PDB 6YWL). TFMU-ADPr and ADPr are shown in stick
representation. (G) Aromatic ring of TFMU interacts with the side chain of Ile131 and on the other side, Gly46 and Gly47. For all the data presented,
error bars indicate SEM and IC50 values are reported as mean ± SEM, n = 2 or 3.
a slightly better SARS-CoV-2 Macro1 binder than ADPr in a
homogeneous time-resolved fluorescence (HTRF) assay. This
result was validated in our FP assay, where the IC50 of Z8539 is
2-fold smaller than ADPr against SARS-CoV-2 Macro1 (Figure
3C). Z8539 is an encouraging example, showing that small-
molecule inhibitors of SARS-CoV-2 Macro1 with structures
unrelated to ADPr are possible. However, its binding affinity for
SARS-CoV-2 Macro1 is only ∼6.4 μM.
Another reported small-molecule inhibitor of SARS-CoV-2
Macro1 is GS-441524 (Figure 3A), the active metabolite of
targets the viral RNA-
Remdesivir, an antiviral drug that
dependent RNA polymerase (RdRp).20 Remdesivir was shown
to be effective against SARS-CoV-221 and is the first COVID-19
therapy approved by the FDA. Recently, Ni et al.22 found that
GS-441524 can bind SARS-CoV-2 Macro1 and solved the
crystal structure of SARS-CoV-2 Macro1 bound with GS-
441524. Using isothermal titration calorimetry (ITC), they
determined that the KD of GS-441524 for SARS-CoV-2 Macro1
is 10.8 μM, similar to that of ADPr. Intrigued by this finding, we
also tested GS-441524 in our FP assay. Consistent with the
reported data, the IC50 of GS-441524 for SARS-CoV-2 Macro1
was determined to be 15.2 μM. Additionally, we tested whether
GS-441524 could inhibit other macrodomains and were
surprised to find that GS-441524 is a selective SARS-CoV-2
Macro1 inhibitor with no significant binding to all other
macrodomains tested (Figure 3D). This result coincides with
another paper published very recently,23 which showed that GS-
441524 is selective for SARS-CoV-2 Macro1 over other
macrodomains including MERS-CoV Mac, CHIKV Macro,
PARP14 Macro2, and PARP15 Macro2 in ITC experiments.
Taken together, GS-441524 is a promising lead compound
against SARS-CoV-2 Macro1 with high selectivity and ligand
efficiency.
Although the actual physiological substrates/binding partners
for SARS-CoV-2 Macro1 are still unknown, it has been proposed
that the most likely substrates are ADPr C1″-esters coupled to
glutamic or aspartic acid protein residues.24 Taken together with
our finding that TAMRA-ADPr with a C1″-triazole linkage can
bind SARS-CoV-2 Macro1 with high affinity, it seems that bulky
groups at the C1″ position would not disrupt binding and
instead may confer a higher affinity. We therefore tested several
other ADPr compounds. TFMU-ADPr and pNP-ADPr (Figure
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4A) are previously developed assay substrates for poly(ADP-
ribosyl)glycohydrolase (PARG).25 Based on our proposal that
bulkier substituents at the C1″ position may boost macro-
domain binding, TFMU-ADPr and pNP-ADPr may be
potential binders for macrodomains since the aromatic rings
are introduced at C1″ similar to TAMRA-ADPr. Therefore, we
tested these two compounds in the FP assay. We were surprised
to find that pNP-ADPr is 40-fold more potent than ADPr for
SARS-CoV-2 Macro1 with an IC50 of only 0.37 μM (Figure 4B−
E), which is the strongest binder of SARS-CoV-2 Macro1
reported so far. Similarly, the IC50 of TFMU-ADPr against
SARS-CoV-2 Macro1 is 0.59 μM, 25-fold smaller than ADPr
(Figure 4B−E). We also tested these two compounds with other
macrodomains and found that their IC50 values are similar to
those of ADPr with the exception of MacroD2, for which both
compounds showed a more than 10-fold increase in activity over
ADPr (Figure 4C,D). Therefore, pNP-ADPr and TFMU-ADPr
are both potent binders of macrodomains with strong
preferences for SARS-CoV-2 Macro1. We also found that
pNP-ADPr and TFMU-ADPr could inhibit the hydrolysis of
ADPr by SARS-CoV-2 Marco1 and human MacroD1 in the cell
lysate (Figure S1, Supporting Information).
To understand why TFMU-ADPr binds strongly to Macro1,
we determined the X-ray crystal structure of the Macro1-
TFMU-ADPr complex using diffraction data that extended to
1.9 Å resolution. The TFMU-ADPr electron density is well
resolved (Figure 4E). As expected, the inhibitor binds within the
known ADPr binding site of Macro1 (Figure 4F, the structure is
superimposed to that of the Macro1-ADPr complex PDB
6YWL). The TFMU moiety extends from the binding site along
a narrow hydrophobic groove, bracketed on one side by the
Ile131 side chain and on the other side by Gly46 and Gly47
(Figure 4G).
■ CONCLUSIONS
In summary, we have developed TAMRA-ADPr, an ADPr-
based tracer, and devised an FP-based binding assay for the
screening of ADPr-binding macrodomain inhibitors. The
reliability of the FP assay was confirmed by testing the IC50
values of ADPr against different macrodomains and comparing
them to the reported KD values. Using this assay, we tested and
validated Z8539 and GS-441524, two recently reported small-
molecule inhibitors of SARS-CoV-2 Macro1. An interesting
finding of this work is that pNP-ADPr and TFMU-ADPr are
strong binders of SARS-CoV-2 Macro1 and several other
macrodomains. Their structures may provide clues for the future
design of more potent ADPr-based probe molecules of SARS-
CoV-2 Macro1. We believe that the FP assay described herein is
a convenient and robust screening method that can facilitate
future drug discovery efforts for macrodomain inhibitors.
■ MATERIALS AND METHODS
Reagents. pNP-ADPr and TFMU-ADPr are synthesized as
previously described.25 Z8539 was obtained from Enamine
(Z4718398539). GS-441524 was obtained from MedChemExpress
(HY-103586).
Expression and Purification of Macrodomains. Macrodomain
plasmids were purchased from Twist Biosciences or Genscript in
pET28 vectors (full sequences available in the SI). The plasmids were
transformed into BL21(DE3) chemically competent E. coli. 4 L of LB
broth with 50 μg/mL kanamycin was inoculated with an overnight
starter grown at 37 °C. Cultures were grown at 200 rpm and 37 °C for
∼4 h until the OD600 reached 0.8. Then, IPTG was added to 0.5 mM
and the cells were incubated at 16 ° C overnight to allow protein
expression. Cells were harvested by centrifugation at 6000g. Cell pellets
were frozen at −80 °C or immediately used for purification. Pellets were
resuspended in lysis buffer (50 mM Tris pH 8.0, 500 mM NaCl, 0.5 mg/
mL lysozyme, 1 mM PMSF, and Pierce universal nuclease). Following a
30 min incubation, cells were sonicated on ice for 4 min total at 60%
amplitude. The lysate was clarified at 4 °C and 30,000g for 35 min. The
clarified lysate was loaded onto Ni-NTA resin, washed with 50 mL wash
buffer (50 mM Tris pH 8.0, 500 mM NaCl, 20 mM imidazole), and
eluted with elution buffer (50 mM Tris pH 8, 500 mM NaCl, 200 mM
imidazole). Crude macrodomains were concentrated using a 10 kDa
MWCO Amicon filter and loaded onto a HiLoad 16/600 Superdex 75
gel filtration column equilibrated with storage buffer (25 mM Tris pH
8.0, 150 mM NaCl, 10% glycerol) on an Ä KTA FPLC system. Fractions
containing macrodomains were pooled, concentrated, flash-frozen in
liquid nitrogen, and stored at −80 °C for future use. For SARS-CoV-2,
the sample was supplemented with DTT (2 mM) and tobacco-etch
protease and incubated at 4 °C overnight. The reaction mixture was
then subjected to subtractive nickel chelate chromatography, and the
eluate was injected into a HiLoad 16/600 Superdex75 gel filtration
column equilibrated with protein storage buffer (5 mM HEPES and 150
mM NaCl, pH 7.5). Fractions containing purified the SARS-CoV-2
macrodomain were combined and concentrated. Then, samples were
aliquoted, flash frozen using liquid nitrogen, and stored at −80 °C.
Fluorescence Polarization Assay. The stock solution of purified
macrodomain proteins was diluted with the assay buffer (25 mM Tris
pH 8.0, 150 mM NaCl, and 0.01% Tween-20) to 2× final
concentration. Final concentrations for each macrodomain were as
follows: 0.5 μM for MacroD1 and MacroD2, 1.5 μM for SARS-CoV-2
Macro1 and PARP9 Macro2, and 5 μM for VEEV Macro and CHIKV
Macro. TAMRA-ADPr (40 nM, 2× final concentration) was then
added to the protein solution to give the assay solution. To each well of
a 96-well black plate (Corning, #3915) was added 50 μL of the assay
solution followed by the addition of 50 μL of the compound solution
(2× final concentration) in the assay buffer. The plate was wrapped
with aluminum foil and left at room temperature for 30 min. The plate
was then scanned on a Cytation5 using a FP filter cube (Agilent, part
number: 8040562, Ex: 530/25, Em: 590/35). The parallel and
perpendicular fluorescence intensities of each well were recorded,
and the mP values were then calculated based on the blank-subtracted
data. Control wells include tracer-only wells where only 20 nM tracers
were present and negative-control wells where only an appropriate
concentration of macrodomain protein and 20 nM tracers were present.
The percent binding of tracer relative to the control wells was calculated
as follows:
where mPtest, mPtracer, and mPneg are mP values of the test wells, tracer-
only wells, and negative-control wells, respectively. The obtained data
were then fitted into an IC50 curve using the sigmoidal four-parameter
logistic model (bottom and top were constrained to be 0 and 100,
respectively) implemented in GraphPad Prism 9.4.1 (GraphPad
Software, Inc.).
Co-crystallization of the SARS-CoV-2 Macro1-TFMU-ADPr
Complex. SARS-CoV-2 Macro1 was mixed with TFMU-ADPr to final
concentrations of 0.4 and 2 mM. The Macro1-inhibitor complex was
crystallized by the hanging-drop method at 20 °C by mixing 1 μL of the
Macro1-TFMU-ADPr solution with 1 μL well solution (200 mM
sodium acetate, 100 mM Tris−HCl pH 8, and 30% PEG-4000).
Crystals were observed after 5 days. Prior to freezing with liquid
nitrogen, crystals were cryo-protected in well solution containing 10%
ethylene glycol.
Diffraction Data Collection, Structure Solution, Model
Building, and Refinement. Diffraction data was collected on
Northeastern Collaborative Access Team (NE-CAT) beamline 24-
ID-E at Advanced Photon Source (APS). Initial data processing was
performed by the NE-CAT ‘RAPD’ pipeline, which uses XDS for
scaling and merging.26 The structure was solved by molecular
replacement using Phaser27 in Phenix28 using a previously published
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structure of SARS-CoV-2 Macro122 (PDB:6YWL) as the search model.
Coot29 was used for model building, and refinement and validation
were performed in Phenix.30 There are two copies of the Macro1-
inhibitor complex in the asymmetric unit, so non-crystallographic
symmetry restraints were used during refinement. The final structures
of the two copies are nearly identical, with no obvious differences in the
inhibitor or inhibitor binding sites between the two copies, although the
density for the inhibitor was stronger in one copy than the other. Data
and refinement statistics are presented in Table 1.
Table 1. Data Collection and Refinement Statisticsa
PDB code
resolution range
space group
unit cell
total reflections
unique reflections
multiplicity
completeness (%)
mean I/sigma(I)
Wilson B-factor
R-merge
R-meas
R-pim
CC1/2
CC*
reflections used in refinement
reflections used for R-free
R-work
R-free
CC(work)
CC(free)
number of non-hydrogen atoms
macromolecules
ligands
solvent
protein residues
RMS (bonds)
RMS (angles)
Ramachandran favored (%)
Ramachandran allowed (%)
Ramachandran outliers (%)
Rotamer outliers (%)
Clashscore
average B-factor
macromolecules
ligands
solvent
number of TLS groups
8GIA
68.92−1.86 (1.926−1.86)
C 1 2 1
140.517 Å 36.668 Å 65.056 Å
90° 101.211° 90°
186,998 (18900)
27,584 (2723)
6.8 (6.9)
99.15 (99.02)
6.94 (1.36)
28.01
0.1788 (1.245)
0.1943 (1.347)
0.07503 (0.5084)
0.987 (0.704)
0.997 (0.909)
27,525 (2717)
1353 (146)
0.2083 (0.3189)
0.2586 (0.3661)
0.938 (0.828)
0.918 (0.743)
2813
2535
150
176
335
0.005
0.60
97.89
2.11
0.00
0.36
7.90
35.84
35.81
30.80
39.19
12
aStatistics for the highest-resolution shell are shown in parentheses.
■ ASSOCIATED CONTENT
*sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acschembio.3c00092.
TFMU-ADPr and pNP-ADPr inhibition of macrodomain
enzymatic activity, supplementary methods, and NMR
spectra of compounds (PDF)
■ AUTHOR INFORMATION
Corresponding Authors
J. Christopher Fromme − Department of Molecular Biology
and Genetics, Weill Institute for Cell and Molecular Biology,
Cornell University, Ithaca, New York 14853, United States;
Email: [email protected]
Hening Lin − Department of Chemistry and Chemical Biology
and Howard Hughes Medical Institute; Department of
Chemistry and Chemical Biology, Cornell University, Ithaca,
New York 14853, United States;
orcid.org/0000-0002-
0255-2701; Email: [email protected]
Authors
Ananya Anmangandla − Department of Chemistry and
Chemical Biology, Cornell University, Ithaca, New York
14853, United States;
orcid.org/0000-0002-2999-4067
Sadhan Jana − Department of Chemistry and Chemical Biology,
Cornell University, Ithaca, New York 14853, United States
Kewen Peng − Department of Chemistry and Chemical Biology,
Cornell University, Ithaca, New York 14853, United States
Shamar D. Wallace − Department of Molecular Biology and
Genetics, Weill Institute for Cell and Molecular Biology,
Cornell University, Ithaca, New York 14853, United States
Saket R. Bagde − Department of Molecular Biology and
Genetics, Weill Institute for Cell and Molecular Biology,
Cornell University, Ithaca, New York 14853, United States;
orcid.org/0000-0001-9800-9326
Bryon S. Drown − Department of Chemistry, Institute for
Genomic Biology, and Cancer Center at Illinois, University of
Illinois at Urbana-Champaign, Urbana, Illinois 61801, United
States; Present Address: Current address: Department of
Chemistry, Purdue University, West Lafayette, Indiana
47906, United States
Jiashu Xu − Department of Chemistry and Chemical Biology,
Cornell University, Ithaca, New York 14853, United States
Paul J. Hergenrother − Department of Chemistry, Institute for
Genomic Biology, and Cancer Center at Illinois, University of
Illinois at Urbana-Champaign, Urbana, Illinois 61801, United
States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acschembio.3c00092
Author Contributions
#Equal contribution.
Funding
This work is supported in part by NIH/NIAMS grant
R01AR078555 and NIH/NIGMS training grants
T32GM008500 and T32GM138826. J.C.F., S.D.W., and
S.R.B. were supported by NIH/NIGMS grant R35GM136258
to J.C.F. We are grateful for the assistance of David Neau at NE-
CAT. This work is based upon research conducted at the NE-
CAT beamlines, which are funded by the National Institute of
General Medical Sciences from the National Institutes of Health
(P30 GM124165). The Eiger 16 M detector on the 24-ID-E
funded by a NIH-ORIP HEI grant
beam line is
(S10OD021527). This research used resources of the APS, a
U.S. Department of Energy (DOE) Office of Science User
Facility operated for the DOE Office of Science by Argonne
National Laboratory under Contract No. DE-AC02-
06CH11357.
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Notes
The authors declare the following competing financial
for Sedec
interest(s): H.L.
Therapeutics.
is a founder and consultant
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ACS Chem. Biol. 2023, 18, 1200−1207
| null |
10.1523_eneuro.0144-22.2022.pdf
|
Code accessibility
The code is included as Extended Data 1 and is available
at https://github.com/tsmanning/bayesIdealObserverMoG.
| null |
Research Article: Methods/New Tools
Novel Tools and Methods
A General Framework for Inferring Bayesian Ideal
Observer Models from Psychophysical Data
Tyler S. Manning,1 Benjamin N. Naecker,2 Iona R. McLean,1 Bas Rokers,3 Jonathan W. Pillow,4 and
Emily A. Cooper5
https://doi.org/10.1523/ENEURO.0144-22.2022
1Herbert Wertheim School of Optometry and Vision Science, University of California, Berkeley, Berkeley, CA 94720,
2Psychology, University of Texas at Austin, Austin, TX 78712, 3Psychology, New York University–Abu Dhabi, Abu
Dhabi, United Arab Emirates, 4Princeton Neuroscience Institute, Department of Psychology, Princeton University,
Princeton, NJ 08540, and 5Herbert Wertheim School of Optometry and Vision Science, Helen Wills Neuroscience
Institute, University of California, Berkeley, Berkeley, CA 94720
Abstract
A central question in neuroscience is how sensory inputs are transformed into percepts. At this point, it is
clear that this process is strongly influenced by prior knowledge of the sensory environment. Bayesian ideal
observer models provide a useful link between data and theory that can help researchers evaluate how prior
knowledge is represented and integrated with incoming sensory information. However, the statistical prior em-
ployed by a Bayesian observer cannot be measured directly, and must instead be inferred from behavioral
measurements. Here, we review the general problem of inferring priors from psychophysical data, and the sim-
ple solution that follows from assuming a prior that is a Gaussian probability distribution. As our understanding
of sensory processing advances, however, there is an increasing need for methods to flexibly recover the
shape of Bayesian priors that are not well approximated by elementary functions. To address this issue, we
describe a novel approach that applies to arbitrary prior shapes, which we parameterize using mixtures of
Gaussian distributions. After incorporating a simple approximation, this method produces an analytical solution
for psychophysical quantities that can be numerically optimized to recover the shapes of Bayesian priors. This
approach offers advantages in flexibility, while still providing an analytical framework for many scenarios. We
provide a MATLAB toolbox implementing key computations described herein.
Key words: ideal observer models; perception; Bayesian inference
Significance Statement
Ideal observer models in neuroscience are an important tool for developing and testing hypotheses about
how sensory information is processed. Here, we review the canonical application of Bayesian ideal observer
models for understanding sensory processing. We present a new mathematical generalization that will
allow these models to be used for deeper investigations into how prior knowledge influences perception.
We also provide a software toolkit for implementing the described models.
Introduction
Sensory systems must encode information about envi-
ronmental stimuli in a way that supports successful be-
haviors. However, sensory measurements are often
noisy and ambiguous, making this a demanding task.
Received March 23, 2022; accepted October 24, 2022; First published
October 31, 2022.
The authors declare no competing financial interests.
January 2023, 10(1) ENEURO.0144-22.2022 1–17
For example, in the visual system, each retinal image
is consistent with an infinite number of possible three-
dimensional scenes. In the auditory system, the vibra-
tion of the inner ear intermixes both the identity and
elevation of sound sources. Prior knowledge about the
Author contributions: T.S.M., B.R., J.W.P., and E.A.C. designed research; T.S.M.
and I.R.M. performed research; B.N.N. and J.W.P. contributed unpublished
reagents/analytic tools; T.S.M. analyzed data; T.S.M., B.N.N., I.R.M., B.R., J.W.P.,
and E.A.C. wrote the paper.
environment can help resolve these ambiguities (Knill
and Richards, 1996; Simoncelli and Olshausen, 2001).
Thus, advances in understanding sensation and per-
ception often rely on understanding how prior knowl-
edge is represented in the nervous system and how
this prior knowledge influences our percepts.
The influence of prior knowledge on perception is often
characterized using psychophysical experiments that mea-
sure the bias and variability of perceptual reports (Hürlimann
et al., 2002; Weiss et al., 2002; Adams et al., 2004; Girshick
et al., 2011; Vacher et al., 2018). For example, measured
biases can be compared with biases predicted by ideal ob-
server models, which can also inform our understanding
of how sensory information is represented within neuronal
populations (Ganguli and Simoncelli, 2010; Wei and
Stocker, 2015, 2017; Morais and Pillow, 2018). Bayesian
ideal observer models specifically posit that observers
optimally combine noisy sensory measurements with a
probability distribution representing the relative frequency
with which events occur in the world (called the prior distri-
bution, or simply the prior). Bayesian models are popular
across many domains, including sensation and perception,
because they can successfully explain a wide range of
phenomena (Weiss et al., 2002; Adams et al., 2004; Burge
et al., 2010; Girshick et al., 2011; Kim and Burge, 2018).
However,
these models are often poorly constrained.
Without constraints on the shape of the prior, Bayesian
models can effectively explain any biases. Thus, a set of
important questions arise: What is the shape of the prior
the observer is using? Does this shape accurately reflect
probabilities in the world? Does it change systematically
with experience?
Bayesian priors are often assumed to take the form of a
Gaussian distribution for computational efficiency (Mamassian
and Landy, 1998; Weiss et al., 2002; Beierholm et al., 2009;
Sotiropoulos et al., 2011; Saunders and Chen, 2015; Rokers
et al., 2018). This assumption, however, limits the ability to
ask questions about the shape of the prior because a
Gaussian only has two parameters. In addition, analyses of
natural scene statistics suggest that the probability distribu-
tions of environmental stimuli are generally non-Gaussian
(Dong and Atick, 1995; Girshick et al., 2011; Sprague et al.,
2015). In order to more flexibly model prior distributions, a
previous study introduced an analytic approach based on
piecewise approximations that
leverages assumptions
about the local shape of the prior relative to the magnitude
of measurement noise (Stocker and Simoncelli, 2006).
An alternative approach to increasing flexibility without
This work was supported by National Institute of Health Grants F32
EY03232 and T32 EY007043 (to T.S.M.); the National Science Foundation
Award 2041726 (to E.A.C.); the Aspire Grant VRI20-10 (to B.R.); and the
McKnight Scholar’s Award, the Simons Collaboration on the Global Brain
(SCGB) Grant AWD543027, and the National Institutes of Health BRAIN
Initiative Grant R01EB026946 (to J.W.P.).
Correspondence should be addressed to Tyler S. Manning at tmanning@
berkeley.edu.
https://doi.org/10.1523/ENEURO.0144-22.2022
Copyright © 2023 Manning et al.
This is an open-access article distributed under the terms of the Creative
Commons Attribution 4.0 International license, which permits unrestricted use,
distribution and reproduction in any medium provided that the original work is
properly attributed.
Research Article: Methods/New Tools
2 of 17
introducing assumptions about prior shape is to use nu-
meric methods that do not place constraints on the
global parametric form or local properties of the prior
(Girshick et al., 2011; Acerbi et al., 2014; Sprague et al.,
2015). Numeric methods, while able to fit an arbitrary
prior, are often slower and require hand-tuning of the nu-
merical support and precision. Thus, while researchers
have a varied toolkit for modeling the shapes of Bayesian
priors, there is still a need to diversify our tools for using
these models in perceptual research.
Our goal is to provide an overview of how Bayesian ideal
observer models can be used in perceptual research, and
to describe a computational approach that uses mixture of
Gaussian models to flexibly and efficiently model the influ-
ence of priors on perception. First, we review the general
mathematical principles that link a Bayesian ideal observer
to psychophysical data. Then, we present the analytic solu-
tions for psychophysical quantities assuming a simple
Gaussian prior and Gaussian measurement noise. Next,
we introduce a mixture of Gaussians model of priors that
provides increased flexibility. Mixture of Gaussian priors
have been employed in other contexts, such as computer
vision and signal processing (Olshausen and Millman, 1999;
Snoussi and Mohammad-Djafari, 2001), but are not com-
monly used in ideal observer models (but see related appli-
cations for modeling perceptual inferences by Acerbi et al.,
2014; Orhan and Jacobs, 2014). Lastly, we introduce a new
analytical approximation that increases the computational
efficiency of the mixture of Gaussians model. This approxi-
mation offers improvements in efficiency for adaptive experi-
mental methods (e.g., adaptive stimulus staircasing) as
compared with fully numerical approaches. An accompany-
ing MATLAB (MathWorks) toolkit provides implementations
that can be used to simulate and fit psychophysical data.
Materials and Methods
Bayesian ideal observer models
In a Bayesian ideal observer model, the observer makes
a noisy measurement m of a stimulus x and uses that
measurement to generate an estimate of the stimulus in
the world ^x or to select an appropriate behavioral re-
sponse r. We can represent this mapping of measurement
onto response with the function r = T(·) where T is some esti-
mation function. For example, in the context of a psycho-
physical experiment, T(·) may represent a point estimate
of the presented stimulus (in which case r ¼ TðmÞ ¼ ^x) or a
binary judgment in a two-alternative forced-choice (2AFC)
experiment (e.g., r ¼ Tðm1; m2Þ ¼ “yes” when queried
whether x2 . x1).
A Bayesian ideal observer selects the optimal response
to a set of stimuli on the basis of the three components:
(cid:129) a prior distribution p( x )
(cid:129) a likelihood p(m | x)
(cid:129) a loss function L( x, r )
The prior p( x) represents the observer’s knowledge of
the probability of encountering the stimulus based on pre-
vious experience. The likelihood pðmjxÞ, the probability of
a measurement given the stimulus, captures the noisiness
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Research Article: Methods/New Tools
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on a specific measurement and considered as a function
of the stimulus, or a measurement distribution when it is
conditioned on a specific stimulus and considered as a
function of measurements.
While the likelihood is the pertinent quantity for apply-
ing Bayes’ rule, the measurement distribution is the rel-
evant quantity when considering samples of the noisy
sensory observation process. Note the measurement
distribution is a true probability density function based
(here, we use additive Gaussian
on a noise model
noise). The likelihood, on the other hand, is not generally
a probability distribution because it does not necessar-
ily integrate to one.
By multiplying each row of the prior and likelihood plots
and normalizing, we obtain the set of possible posterior
distributions pðxjmÞ for each possible measurement (Fig.
2C). Note that since the prior is non-Gaussian and steeper
around the left flank of the peak, the posteriors are more
concentrated around these values.
Finally, a loss function is needed to complete the
model. The loss function L(x,r) refers to the penalty of
making a response r when the true stimulus was x. An op-
timal decision rule is one where the observer will minimize
the loss on average over the course of a set of responses.
To calculate the expected loss of a particular response,
we can find the expected loss under the posterior:
E½Lðx; rÞ(cid:2) ¼
ð
Lðx; rÞpðxjmÞdx:
(2)
A decision rule is Bayes optimal under a particular loss
function if it minimizes the expected loss for all measure-
ments. That is, T*(·) is Bayes optimal if for all estimation
functions T(m) and all measurement values m:
E½Lðx; T pðmÞÞ(cid:2) (cid:3) E½Lðx; TðmÞÞ(cid:2):
(3)
Note that here we show T(·) as a function of a single m,
but it may also take multiple measurements into account,
as in a two-alternative forced-choice paradigm (2AFC). In
the following sections, we will discuss this loss in more
concrete terms in the context of point estimation and
2AFC tasks.
While the derivations outlined in this paper do not as-
sume any particular stimulus, they do assume that the
measurements are unbiased, and that the measurement
noise is additive and Gaussian distributed. In this case,
the likelihood always takes the form of a Gaussian. Under
these assumptions, the mean of the likelihood varies with
and is equal to the measurement. We also assume that
the width of the likelihood (i.e., the amount of noise) does
not inherently vary with the measurement. However, the
Weber–Fechner law across many stimuli suggests that
this assumption does not hold if stimulus values are rep-
resented in many common sense units (e.g., candelas per
square meter for luminance, visual degrees per second
for speed), because sensory thresholds in these units in-
crease systematically as stimulus values increase (Hecht,
1924; McKee et al., 1986; Pardo-Vazquez et al., 2019).
Thus, a transformation of the stimulus values from physi-
cal space to “sensory space” may often be necessary to
Figure 1. Canonical Bayesian computation. This figure illus-
trates Bayes’ rule, by which a posterior is the product of a
prior (the observer’s knowledge of the probability of encoun-
tering the stimulus) and a likelihood (the set of stimulus val-
ues associated with a given a measurement). The posterior
likelihood. Toolkit
is scaled by the inverse of the marginal
script: Fig1_BayesianDemo.m.
in the observer’s measurement of the stimulus. The noisi-
ness depends on both external factors (such as signal
strength and presentation time) and internal factors (such
as neuronal noise and attentional state).
To obtain the observer’s belief about the current stimu-
lus x given a measurement m we first use Bayes’ rule to
obtain the posterior distribution, pðxjmÞ, as follows:
pðxjmÞ ¼
pðmjxÞpðxÞ
pðmÞ
:
(1)
The posterior represents the probability distribution of a
stimulus, given the current measurement, and can thus be
used for drawing inferences. Here, p( m ) is the model evi-
dence (or marginal likelihood) that serves to normalize the
posterior. This calculation is represented graphically in
Figure 1. Since p( m ) is a scalar value and does not affect
the shape of the posterior, we can note that pðxjmÞ / p
ðmjxÞpðxÞ.
This simple illustration, however, shows a likelihood
based on only one example measurement. If we instead
consider the full range of possible measurements, as
shown in Figure 2, we can see how the resulting shape of
the posterior varies. Figure 2A shows the prior as a func-
tion of x. By definition, the prior is independent of the
measurement m, so it varies horizontally, but is constant
along the vertical dimension. This two-dimensional (2D)
format, similar to that used in (Girshick et al., 2011), helps
illustrate the point that the posterior (Fig. 2C) arises from
pointwise multiplying the prior (Fig. 2A) and likelihood
(Fig. 2B). Figure 2B illustrates the likelihood by plotting
the probability of the observer making each measure-
ment, conditioned on each possible stimulus value. This
2D distribution is generated by assuming that the mea-
surement associated with each stimulus value is cor-
rupted by additive Gaussian noise, but is unbiased. A
vertical slice through B represents what we refer to as the
measurement distribution pðmjxnÞ, which is the probabil-
ity over measurement values m given a particular stim-
ulus xn. A horizontal slice through B, on the other hand,
represents the likelihood function p(mn, x), which is the
probability of a given measurement m as a function of
different stimulus values x. Thus, the 2D object pðmjxÞ
may represent either the likelihood when it is conditioned
January 2023, 10(1) ENEURO.0144-22.2022
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Research Article: Methods/New Tools
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A
B
C
Figure 2. The canonical Bayesian computation as in Figure 1 but expanded to a set of likelihood functions. The prior (A) is multiplied
by the likelihood defined by a given measurement (B, shown for m1 and m2) to obtain the posterior (C). Note that the shape of the
posteriors change for different likelihoods since the prior is non-Gaussian, but the posteriors are overall drawn to the largest proba-
bility region of the prior. In each panel, the heat map values represent probability with higher intensity mapping to higher probability.
Identity lines are indicated with dashed black lines. Toolkit script: Fig2_2DBayesianDemo.m.
satisfy this assumption (Stocker and Simoncelli, 2006;
Kwon et al., 2015). Indeed, the Weber–Fechner law sug-
gests that the width of the likelihood or measurement distri-
bution is approximately constant in logarithmic units across
many stimulus domains (although deviations have been
noted). For example, if one were to model visual speed per-
ception, the measurement distribution and prior could be
represented in terms of pðlogðmÞjlogðxÞÞ and p(x), respec-
tively. Throughout this document, we will represent the likeli-
hoods as Gaussians even if a transformation is necessary,
Table 1: General notation
Value
Stimulus value
Sensory measurement
Stimulus estimate
Response
Likelihood SD
Prior mean, SD
Posterior mean, SD
January 2023, 10(1) ENEURO.0144-22.2022
Notation
x
m
^x
r
s
(cid:2), g
mpost, spost
to keep the estimation of the prior computationally tractable.
For reference, Table 1 provides a summary of notation used
for each of the ideal observer parameters.
Modeling psychophysical data from an observer with
a Gaussian prior
We begin with a simple case in which the prior takes the
form of a Gaussian distribution. If this condition is met, the
posterior has an analytic solution and is also Gaussian.
This property follows from the general rule defining the
product of any two Gaussians. Specifically, if we denote a
Gaussian distribution generally as N ða; b2Þ with mean a
and standard deviation b, we can write the prior as N ð(cid:2); g2Þ
(see Table 1). We define the likelihood as a Gaussian func-
tion with its mean equal to the measurement value m and a
SD of s: N ðm; s2Þ. We can then write the posterior as:
pðxjmÞ ¼
1
r
N ðm; s2Þ N ð(cid:2); g2Þ
¼ N ðmpost
; s2
postÞ
(4)
eNeuro.org
where the normalizing constant r, which relates the
posterior to the product of prior and likelihood, is given
by:
(cid:3)
spost
sg
p
1ffiffiffiffiffiffiffi
2p
r ¼
"
(cid:4)
exp (cid:4)
m2
2s2 (cid:4)
(cid:2)2
2g2
1
#
;
post
m2
2s2
post
and the posterior variance and mean are given by:
s2
post ¼ s2
(cid:3)
mpost ¼ m
(cid:3)
g2
s2 1 g2
(cid:4)
g2
s2 1 g2
(cid:4)
(cid:3)
s2
s2 1 g2
(cid:4)
:
1 (cid:2)
Research Article: Methods/New Tools
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(cid:3)
s2
s2 1 g2
(cid:4)
(cid:2):
~(cid:2) ¼
(13)
With these simplifications we can rewrite the posterior as:
(5)
(6)
(7)
(14)
pðxjmÞ ¼ N ðam 1 ~(cid:2); as2Þ:
When ^xBLS ¼ ^xMAP, we simply adopt ^x to denote the es-
timate. The solution for ^x here can also be considered as
a weighted average of the prior and likelihood means,
where the weights are inversely proportional to the var-
iance of the prior and likelihoods (Landy et al., 1995). To
make that link explicit, we can represent Equation 11 as
^x ¼ am1ð1 (cid:4) aÞ(cid:2), since ~(cid:2) is equal to (1 – a)(cid:2). Note that
when the posterior is not Gaussian, the MAP and BLS es-
timates are not necessarily equivalent.
Selecting a sensory estimate from the posterior
To start linking this framework to psychophysical data,
we first consider an experiment in which we want to fit a
Bayesian ideal observer model to a set of data in which par-
ticipants reported point estimates of the presented stimuli
(e.g., through method of adjustment such that x9 ¼ r is a
possible estimate response when x is the true value). To
convert the posterior into an optimal estimate, we can assert
a loss function for our Bayesian ideal observer. In the
general form, this loss function will determine the Bayes
estimate that minimizes the expected error defined in
Equation 2:
ð
Lðx; x9ÞpðxjmÞdx:
^x ¼ argmin
x9
(8)
Two commonly used loss functions are the zero-one loss
(where the loss is 0 when ðx (cid:4) x9Þ ¼ 0, and 1 for all other val-
ues), and squared error loss ( Lðx; x9Þ ¼ ðx (cid:4) x9Þ2). Using
zero-one loss, we obtain a Bayes optimal estimate ^x that
is the mode of the posterior, the maximum a posteriori
(MAP) estimate:
^xMAP ¼ argmax
pðxjmÞ:
x
(9)
For an ideal observer that uses a squared error loss
function, the Bayesian least squares (BLS) estimate is the
mean of the posterior:
^xBLS ¼ E½xjm(cid:2):
(10)
When the posterior is Gaussian, the MAP and BLS esti-
mates are equivalent and equal to mpost (Eq. 7), which can
be simplified to:
Distribution of sensory estimates
While the ideal observer model outlined in this paper is
defined from the perspective of the observer, we will
briefly shift our perspective to that of an experimenter to
demonstrate how the model can be used in practice. In a
task in which the observer is making repeated point esti-
mates of the stimulus (e.g., judging its visual brightness,
auditory volume, or speed), the mean of the measurement
distribution on each trial will be equal to the true value of
the stimulus, x, and we can define TðmÞ ¼ ^x ¼ am1~(cid:2) as
the function by which the ideal observer converts noisy
measurements into a response on each trial. While this is
a deterministic function, the value will vary from one trial
to another because of variability in the measurement m.
The responses thus form an estimate distribution pð^xjxÞ,
the probability distribution of estimates, given a particular
stimulus (Fig. 3).
If we want to infer the underlying ideal observer param-
eters from a set of real behavioral data, we can fit a set of
empirically measured observer estimates to this estimate
distribution. To do so, we define an analytic form of this esti-
mate distribution pð^xjxÞ with a substitution of variables in
which we substitute T (cid:4)1ð^xÞ for m in the measurement distri-
bution pðmjxÞ ¼ N ðx; s2Þ. First, we solve for T (cid:4)1ð^xÞ and
the first derivative of this function with respect to ^x:
T (cid:4)1ð^xÞ ¼ m ¼
^x (cid:4) ~(cid:2)
a
d
d^x
T (cid:4)1ð^xÞ ¼
1
;
a
and then perform the substitution of variables:
^xBLS ¼ ^xMAP ¼ am 1 ~(cid:2):
(11)
pð^xjxÞ ¼
p
Here, we have simplified the equation for mpost such that a
is a shrinkage factor that determines how biased the pos-
terior is toward the prior mean:
(cid:4)
(cid:3)
g2
g2 1 s2
a¼
(12)
and ~(cid:2) offsets the posterior when the prior is not zero-
centered:
(cid:5)
1ffiffiffiffiffiffiffiffiffiffiffiffi
2ps2
exp (cid:4)
2
(cid:3)
ðT (cid:4)1ð^xÞ (cid:4) xÞ2
2s2
dT (cid:4)1ð^xÞ
d^x
(cid:6)(cid:7)
(cid:7)
(cid:7)
(cid:7)
3
(cid:4)
6
4
exp (cid:4)
(cid:5)
exp (cid:4)
¼
p
¼
p
1ffiffiffiffiffiffiffiffiffiffiffiffi
2ps2
1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2pa2s2
2
(cid:7)
(cid:7)
(cid:7)
(cid:7)
7
5
^x (cid:4) ~(cid:2)
a
(cid:4) x
2s2
(cid:7)
(cid:7)
1
(cid:7)
(cid:7)
a
(cid:6)
ð^x (cid:4) ðax 1 ~(cid:2)ÞÞ2
2a2s2
which we can denote simply as:
(15)
(16)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(17)
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A
B
C
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Figure 3. The distribution of sensory estimates arises from the variability in the measurement values about the expected value
across trials (EV; i.e., the true stimulus value). A, On a given trial, a likelihood is defined around the observed measurement. Here,
we plot the expected value of this likelihood for a given true stimulus, as well as other possible likelihoods that occur on a set of tri-
als. The prior is shown for reference. The upward arrow indicates the true stimulus that used to generate the likelihood. B, The re-
sulting posteriors for each trial are shown, along with downward arrows indicating the estimates (f^xg) derived from these posteriors.
C, Over many trials, these estimates (now indicated as upward arrows) create an estimate distribution, which can be predicted for a
Bayesian ideal observer with a given prior and amount of sensory noise. When the prior is Gaussian, there is a closed form expres-
sion for this distribution. Toolkit script: Fig3_EstimateDistribution.m.
pð^xjxÞ ¼ N ðax 1 ~(cid:2); a2s2Þ:
(18)
Two-alternative forced-choice task
While we could also derive the estimate distribution more
simply using the identity for the affine transformation of
Gaussian random variables, we use a substitution of varia-
bles here to draw a parallel to the mixture of Gaussians case
in the next section. Note that the form of the estimate distri-
bution is similar to the posterior distribution associated with
a single measurement (Eq. 14) with two key differences: the
mean of the estimate distribution is dependent on the stimu-
lus x instead of any specific noisy measurement, and the
variance is equal to the variance of the likelihood scaled by
a2 instead of a.
This distribution of observer estimates, given the stimu-
lus, provides the likelihood function for fitting the Bayesian
ideal observer model to data by performing maximum
likelihood estimation (MLE; not to be confused with
the likelihood of a Bayesian observer). Specifically, it is
a likelihood when considered as a function of the model
parameters u ¼ f(cid:2); g; sg. Given a set of paired stimuli
and observer reports fðxt; ^xtÞgN
t ¼ 1 from a set of condi-
tionally independent trials t = 1,...,N, the model
likeli-
hood is given by:
pðf ^xt gjfxtg; uÞ ¼
YN
t¼1
p
1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2pa2s2
(cid:5)
exp (cid:4)
ð^xt (cid:4) ðaxt 1 ~(cid:2)ÞÞ2
2a2s2
(cid:6)
:
(19)
In practice, we optimize u by minimizing the negative
log-likelihood, which is obtained by taking the negative
log of this expression:
(cid:4)log ½pð ^xtf g j xtf g; uÞ(cid:2)
"
XN
(cid:3)
¼ (cid:4)
log
p
t¼1
1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2pa2s2
(cid:4)
(cid:4)
(cid:3)
ð^x (cid:4) ðaxt 1 ~(cid:2)ÞÞ2
2a2s2
(cid:4)
#
¼
N
2
logð2pa2s2Þ 1 1
2a2s2
XN
t¼1
ð^xt (cid:4) ðaxt 1 ~(cid:2)ÞÞ2:
(20)
Experimenters often avoid having research participants
report point estimates of stimuli because the origin of the
noise in the measurement is ambiguous. For example, re-
sponses that incorporate a motor component may be
contaminated by motor noise in addition to sensory noise.
To avoid this issue, participants can make a categorical
judgment about stimuli in perceptual space that can be re-
lated back to physical qualities of the stimulus. One such
paradigm is a two-alternative forced-choice (2AFC) task in
which participants view two stimuli either sequentially or
concurrently and must select which of the two best fits the
instructions they are given. In a speed judgment task, for
example, the instruction might be: “indicate which of the
two stimuli appeared to move faster”. Often, this task is re-
peated for a range of stimulus values, such as stimulus
speed, to build up a psychometric function. This function,
for example, might describe the probability that a test
stimulus is perceived as moving faster than a fixed refer-
ence stimulus, as a function of the test stimulus speed.
Importantly, the two stimuli should differ in reliability to
estimate the best fitting parameters for both the likeli-
hood and the prior.
If we consider two stimuli x1 and x2, on each trial, the ob-
server makes two noise-corrupted measurements, which
we model with two measurement distributions pðm1jx1Þ
and pðm2jx2Þ or a single joint distribution pðm1; m2jx1; x2Þ
(see Fig. 4 for examples). The ideal observer selects an op-
timal response r based on a decision function that takes
both measurements as input (Tðm1; m2Þ). Here, we assume
this function indicates whether or not stimulus x2 best sat-
isfies the instructions given the measurements (e.g., in our
speed judgment example, was x2 faster than x1). This is de-
fined by the following decision rule:
(cid:8)
r ¼ Tðm1; m2Þ ¼
1 pðx2 . x1j m1; m2Þ . 0:5
0 otherwise
;
(21)
where pðx2 . x1j m1; m2Þ is determined for each pair
ðm1; m2Þ by:
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A
B
C
D
Research Article: Methods/New Tools
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Figure 4. Graphical illustration of computing the observer’s psychometric curve for a 2AFC task. A, Computing a single point on the
2 ¼ 0:5 and a prior with v = 0 and g = 1.5.
psychometric curve when x1 ¼ x2 ¼ 3 for measurement noise variances s2
Dashed line (top) shows the measurement distribution pðm1jx1Þ and solid line (right) shows measurements distribution pðmjx2Þ. The
2D grayscale image shows the joint distribution of observer measurements given the stimuli x1 and x2, formed by the product of the
two measurement distributions along the top and right. The white diagonal line is the observer’s decision boundary, corresponding to
measurement values for which the inferred speeds are equal. The probability that the observer reports “yes” (i.e., that x2 exceeded
is the area above the decision boundary (point “A” in panel D). B, Same as panel A but with equal noise variances
x1)
m2 ¼ 0:75. D, Full psychometric curves for the noise
s2
variances used in panels A–C, showing the probability that the observer reports “yes” as a function of the stimulus x2. The points la-
beled A, B, C represent the sum of the probability above the diagonal in panels A–C.
m2 ¼ 0:64. C, Same as panel A but with noise variances s2
m1 ¼ 0:5; s2
1 ¼ 0:75; s2
m1 ¼ s2
pðx2 . x1j m1; m2Þ ¼
ð
1
ð
1
(cid:4)1
x1
pðx1; x2 j m1; m2Þdx2dx1:
(22)
Since we model the likelihoods as independent and the
posteriors are both Gaussian (at this point in the derivations),
we can more succinctly say this occurs whenever the esti-
mate ^x2 ¼ a2m2 1 ~(cid:2) 2
than ^x1 ¼ a1m1 1 ~(cid:2) 1,
is greater
which we can express using the decision rule:
(cid:8)
Tðm1; m2Þ ¼
1 a2m2 1 ~(cid:2) 2 . a1m1 1 ~(cid:2) 1
0 otherwise:
(23)
to this calculation. Specifically, we can obtain an analytic so-
lution for points on the psychometric curve via an alternative
model of the Bayesian observer in which the observer com-
putes the MAP estimate for each stimulus and then com-
pares which of the two is larger. This method has been used
previously (Stocker and Simoncelli, 2006) and is equivalent
to the optimal computation in Equation 25 when the prior
and likelihoods are both Gaussian. Since ^xMAP ¼ ^xBLS, this
solution works for both estimators. The probability that a
given estimate of x2 ( ^x2) is greater than the estimate x1 ( ^x1)
can be obtained by integrating over the estimate distributions
for the two stimuli in what is essentially a signal detection
problem (Green and Swets, 1966):
Because this is now a classification task, we adopt the
loss function:
Lððx1; x2Þ; rÞ ¼ jr (cid:4) 1ðx2 . x1Þj;
(24)
pð“yes”jx1; x2Þ ¼
ð
1
ð^x 2
(cid:4)1
(cid:4)1
pð^x2jx2Þpð^x1jx1Þd^x1d^x2:
(26)
where 1(·) denotes an indicator function that evaluates to 1
when the input is true. For simplicity, we will represent the
first case in Equation 23 as “yes” and the second case as
“no”. Graphically, this equation is represented in Figure 4 as
a white decision boundary in panels A–C for three different
combinations of noise levels for m1 and m2. The slope of
this line is determined by: m2 ¼ a1
. If we want to
a2
solve for the probability of responding “yes” for a given x2
and x1 over repeated trials (i.e., a point on the psychometric
curve), we can obtain a numerical solution by integrating
the joint distribution above the decision boundary:
m1 1 ~(cid:2) 1(cid:4)~(cid:2) 2
a2
Equivalently, pð“yes”jx1
; x2Þ can be expressed as the in-
tegral over positive values of ^x2 (cid:4) ^x1 in the probability dis-
tribution pð^x2 (cid:4) ^x1jx1; x2Þ. This has an analytic solution
since the difference of two Gaussian random variables is
itself a Gaussian. For a Gaussian prior, the estimate distri-
butions are indeed Gaussian (see Eq. 18) so this differ-
ence pð^x2 (cid:4) ^x1jx1; x2Þ is defined as:
pð^x2 (cid:4) ^x1jx1; x2Þ ¼ N ða2x2 1 ~(cid:2) 2; a2
2s2
¼ N ða2x2 (cid:4) a1x1 1 ~(cid:2) 2 (cid:4) ~(cid:2) 1; a2
2Þ (cid:4) N ða1x1 1 ~(cid:2) 1; a2
1s2
1Þ
1s2
2s2
1Þ
(27)
1 a2
:
2
From this equation, pð“yes”jx1; x2Þ can be attained simply
by integrating over positive values of this difference:
ð
1
0
N ða2x2 (cid:4) a1x1; a2
2s2
2
1 a2
1s2
1Þ:
pð“yes”jx1; x2Þ
ð
ð
1
1
¼
(cid:4)1
(cid:4)1
Tðm1; m2Þpðm1jx1Þpðm2jx2Þdm1dm2:
(25)
pð“yes”jx1; x2Þ ¼
The results of this integration for Figure 4A–C are shown in
Figure 4D, along with the full psychometric curves.
However, Bayesian ideal observer models with Gaussian
posteriors also allow for an equivalent analytical alternative
To simplify the calculation of this integral, we can con-
vert the difference distribution to a standard normal f(·)
by subtracting the mean and scaling all values by the
(28)
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(30)
of C Gaussian components:
inverse of the SD. The location on the standard normal
curve that corresponds to the lower bound on the integral
in Equation 28 is then equal to the original mean divided
by the SD. This is useful because it allows us to integrate
the standard normal above this (standardized) mean to
find pð“yes”jx1; x2Þ for a given x2. That is, instead of inte-
grating the original normal from zero to infinity, we now in-
tegrate the standard normal up to the standardized mean.
Lastly, we take advantage of the fact that the standard
normal is symmetric about its mean to write the equation
as follows:
(cid:3)
pð“yes”jx1; x2Þ ¼ U a2x2 (cid:4) a1x1
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 a2
1s2
a2
1
2s2
2
p
(cid:4)
;
(29)
where U(·) is the cumulative standard normal:
UðKÞ ¼
ð
K
(cid:4)1
p
1ffiffiffiffiffiffiffi
2p
(cid:5)
exp
(cid:6)
dt;
(cid:4)t2
2
and the symmetry about the mean of f indicates that
Ð
Ð
K
1
(cid:4)1fðtÞ ¼ UðKÞ for all values of K.
(cid:4)KfðtÞ ¼
We can again take the perspective of the experimenter
to demonstrate how to fit the ideal observer model to
2AFC data. This analytic solution is an efficient way to es-
timate the underlying parameters of the Bayesian ideal
observer model given a dataset fx1;t; x2;t; TtgN
t¼1, where Tt
is the participant’s response to stimulus pair x1;t; x2;t on
trial t. As in the point estimate case, we can solve for the
best fitting parameters u ¼ fv; g; s1; s2g with MLE in
which we minimize the following negative log-likelihood
function:
(cid:4)log ½ pðfTgjfx1; x2g; uÞ(cid:2) ¼ (cid:4)
XN
t¼1
Ttlog ½ pð“yes”jx1;t; x2;tÞ(cid:2)
1 ð1–TtÞlog½1–pð“yes”jx1;t; x2;tÞ(cid:2):
(31)
Summary
Up to this point, we have described how to determine
the posterior, the individual sensory estimates, the sen-
sory estimate distribution, and the results of a 2AFC task
for a Bayesian ideal observer with a Gaussian prior and
likelihood.
In the next section, we will generalize this
framework by deriving the same quantities for an observer
with a prior that can be modelled more flexibly as a mix-
ture of Gaussian components.
Modeling psychophysical data for an observer with a
mixture of Gaussians prior
While the approach outlined in the previous section
is computationally efficient, it assumes that the ob-
server’s prior is well fit by a single Gaussian. This is un-
likely to be the case assuming that the prior reflects
knowledge of natural scene statistics, since many
physical quantities have much heavier tails than a
Gaussian (Dong and Atick, 1995; Sprague et al., 2015)
or are even multimodal (Girshick et al., 2011; Kim and
Burge, 2018). Accurately modeling these shapes is
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Table 2: Mixture of Gaussians notation
Value
Weight (prior component i)
Mean (prior component i)
SD (prior component i)
Notation
wi
(cid:2)i
gi
important. For example, long-tailed priors would pre-
dict that biases are reduced for stimulus values that
fall within the the flatter regions of the stimulus proba-
bility distribution than in the more peaked regions. In
this section, we propose an approach based on a mix-
ture of Gaussians that retains some of the efficiency of
the single Gaussian prior while better approximating
realistic priors. Table 2 lists a summary of the addition-
al notation adopted for this section.
Consider an observer with a prior defined by a mixture
pðxÞ ¼
XC
i¼1
wiN ð(cid:2)i; g2
i Þ;
(32)
wi ¼ 1, and (cid:2)i and g2
where wi (cid:5) 0 is the weight of the ith component, with
P
i are the mean and variance of the
ith Gaussian component, respectively (Fig. 5A, red lines).
If we assume a Gaussian likelihood with variance s2, the
posterior is also a mixture of Gaussians (Fig. 5B, blue
lines):
pðxjmÞ ¼
XC
i¼1
~wiðmÞN ðaim 1 ~(cid:2) i; ais2Þ;
(33)
where ai and ~(cid:2) i are the shrinkage factor and mean of the
ith posterior component, respectively:
ai ¼
g2
i
1 s2
g2
i
(34)
A
B
Figure 5. Prior and posterior defined by a mixture of Gaussian
components. A, The prior of a Bayesian observer (dark red
line) can be modeled as a mixture of Gaussian components
(light red lines). B, When combined with a Gaussian likeli-
hood, the resulting posterior is also a mixture of Gaussians.
Similar to the posterior resulting from a single Gaussian prior,
the mixture of Gaussians posterior is biased relative to the
likelihood. Likelihoods are shaded here for visual clarity. Toolkit
script: Fig5_MoGprior.m.
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~(cid:2) i ¼
s2
s2 1 g2
i
!
(cid:2)i:
(35)
This is the mixture of Gaussians version of the posterior
given in Equation 14. Here, ~wiðmÞ is a set of adjusted
weights that combine the weights wi of the individual
components of the prior, the scale factors ri(m) for each
of the components of the posterior (analogous to Eq. 5),
and a normalization step to ensure the weights all sum to
1. To determine ~wiðmÞ, we can first define each ri(m) as:
#
"
riðmÞ ¼
p
ffiffiffiffiffiffiffi
2p
1
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
p
1 s2
g2
i
exp (cid:4)
m2
2s2 (cid:4)
(cid:2)2
i
2g2
i
1 ð~(cid:2) i1aimÞ2
2ais2
;
(36)
and by substituting for ~(cid:2) i and ai with Equations 35 and 34,
respectively, then simplifying, we obtain:
"
#
riðmÞ ¼
p
ffiffiffiffiffiffiffi
2p
1
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
p
1 s2
g2
i
exp (cid:4)
ðm (cid:4) (cid:2)iÞ2
1 s2Þ
2ðg2
i
:
(37)
Note that ri(m) is inversely related to the difference be-
tween the measurement m and the prior component
mean (cid:2)i. Therefore, the posterior shape will change relative
to the likelihood, not just shift as in the single Gaussian prior
case. That is, as the measurement changes, the relative
weight of each component changes. We can combine the
scaling effects of wi and ri(m) to define:
viðmÞ ¼ wiriðmÞ ¼
p
wi
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
g2
i þ s2
(cid:3)
f
p
m (cid:4) (cid:2)i
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
g2
i þ s2
(cid:4)
;
(38)
which is then normalized by the sum of all vi to obtain the
set of adjusted weights ~wiðmÞ:
~wiðmÞ ¼
:
viðmÞ
XC
i¼1
viðmÞ
(39)
In the following sections, we will first demonstrate how
to fit the mixture of Gaussians prior to point estimation
and 2AFC data using numerical evaluation of the log-like-
lihood. We then derive an analytical approximation that
can reduce the computational load necessary to estimate
the observer parameters.
Selecting a sensory estimate from the posterior
As before, let us first consider the case where we want
to estimate a participant’s prior from a set of point esti-
mates from an experimental dataset. We can use the pos-
terior derived in Equation 33 and an appropriate loss
function to define an optimal estimate ^x. For the mixture
of Gaussians posterior, the MAP and BLS estimates differ.
Here, we will consider only ^xBLS, since this estimate has
an analytical solution in the mean of the posterior:
Research Article: Methods/New Tools
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Without an analytical solution ^xMAP can be deter-
mined numerically and used instead in the numerical
approaches described below. Note that if the posterior
is multimodal, the BLS estimate may fall on a relatively
unlikely value (since it is between the two modes of the
posterior), and the MAP estimate may be unstable
(since it may oscillate between the two modes depend-
ing on the measurement noise on a given stimulus
presentation).
Distribution of sensory estimates
We can use Equation 40 to define TðmÞ ¼ ^xBLS for the
point estimation task. Unlike in the single Gaussian case,
however, there is no clear analytic form for T (cid:4)1ð^xBLSÞ with
arbitrary mixture of Gaussians priors since ~wi is a function
of m. To demonstrate this, consider a simplified form
where all ~(cid:2) i ¼ 0 and it is clear that there is no way to solve
for T (cid:4)1ð^xBLSÞ:
TðmÞ ¼ ^xBLS ¼ m
XC
~wiðmÞai
i¼1
¼ m
XC
i¼1
1
XC
:
viðmÞai
i¼1
viðmÞ
(41)
Instead, we can numerically estimate T (cid:4)1ð^xBLSÞ by
first calculating TðmÞ ¼ ^xBLS over a grid of points
f^xBLS; mg to create a look-up table to find {m} from f^xg
for a given set of Bayesian ideal observer parameters
u ¼ fwi; (cid:2)i; gi
; sg. With a goal of estimating an observ-
er’s prior from a set of N point estimates (all with the
same sensory noise level, s), we can then evaluate the
likelihood of the data given the putative model parame-
ters, u, using Equation 17:
pðf^xtgjfxtg; uÞ
YN
¼
p
t¼1
1ffiffiffiffiffiffiffiffiffiffiffiffi
2ps2
(cid:5)
exp (cid:4)
ðT (cid:4)1ð ^xt Þ (cid:4) xtÞ2
2s2
(cid:6)(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7):
dT (cid:4)1ð ^xt Þ
d ^xt
(42)
Note that we have abbreviated ^xBLS to ^x for simplicity
here. This process is then repeated for other parameter
sets until we find an optimal solution that maximizes
the likelihood of the data (or minimizes the negative
log-likelihood). That is, finding u that minimizes the
following:
(cid:4)log ½ pð ^xtf g j xtf g; uÞ(cid:2)
"
YN
t¼1
p
1ffiffiffiffiffiffiffiffiffiffiffiffi
2ps2
ðxt (cid:4) T (cid:4)1ð^xtÞÞ2
2s2
¼ (cid:4)log
¼
XN
t¼1
(cid:5)
exp (cid:4)
ðT (cid:4)1ð ^xt Þ (cid:4) xtÞ2
2s2
p
1 N log½
ffiffiffiffiffiffiffi
2p
s(cid:2)(cid:4)
XN
t¼1
(cid:6)(cid:7)
(cid:7)
(cid:7)
(cid:7)
dT (cid:4)1ð ^xt Þ
d ^xt
#
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
(cid:7)
log
(cid:7)
(cid:7)
(cid:7)
(cid:7):
dT (cid:4)1ð^xtÞ
d^xt
(43)
^xBLS ¼
XC
i¼1
~wiðmÞðaim 1 ~(cid:2) iÞ:
(40)
The toolkit includes a function for this numerical ap-
proach (fitEstimData_numerical.m), which we will
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also return to in Results. This process can be computa-
tionally expensive, however, if we are trying to fit an ob-
server’s prior with many Gaussian components (each of
which is defined by three parameters w, (cid:2), g). While this
may be acceptable for lower numbers of components
and datasets that have already been collected, this is
more problematic if the mixture of Gaussians model
is used during the course of an experiment to guide an
adaptive staircase.
To make the log-likelihood equation more tractable to
solve, we can derive an approximate analytical solution
for the point estimate distribution if we approximate
Equation 40 using just the expected value of the measure-
ment EðmÞ ¼ x when calculating ~wi:
~wiðxÞ (cid:6) ~wiðmÞ
(44)
This approximation allows us to solve for m in Equation 41:
TðmÞ ¼ ^xBLS (cid:6)
XC
i¼1
~wiðxÞðaim 1 ~(cid:2) iÞ
(45)
TðmÞ ¼ ^xBLS (cid:6) m
XC
i¼1
~wiðxÞai 1
XC
i¼1
~wiðxÞ~(cid:2) i:
(46)
We can then derive an analytic solution to T (cid:4)1ð^xBLSÞ
and its first derivative with respect to ^xBLS:
T (cid:4)1ð^xBLSÞ ¼ m (cid:6)
d
d^xBLS
T (cid:4)1ð^xBLSÞ (cid:6)
XC
i¼1
~wiðxÞ(cid:2)i
~wiðxÞai
^xBLS (cid:4)
XC
i¼1
1
;
XC
i¼1
~wiðxÞai
(47)
(48)
and in turn use the substitution of variables to derive
an (approximate) analytic solution in the form of a
Gaussian:
pð^xBLSjxÞ (cid:6)
p
1ffiffiffiffiffiffiffiffiffiffiffiffi
2ps2
pð^xBLSjxÞ (cid:6)
p
6
6
6
6
6
exp
4
1ffiffiffiffiffiffiffiffiffiffiffiffi
2ps2
(cid:5)
ðT (cid:4)1ð^xBLSÞ (cid:4) xÞ2
2s2
(cid:6)(cid:7)
(cid:7)
(cid:7)
(cid:7)
exp (cid:4)
2
0
B
B
@
^x BLS(cid:4)
PC
i¼1
(cid:4)
PC
~w iðxÞ(cid:2)i
(cid:4) x
i¼1
~w iðxÞai
2s2
(cid:7)
(cid:7)
(cid:7)
(cid:7)
2
dT (cid:4)1ð^xBLSÞ
d^xBLS
3
(cid:7)
(cid:7)
(cid:7)
(cid:7)
7
7
7
7
7
5
PC
1
1
C
C
A
i¼1
~wiðxÞai
(cid:7)
(cid:7)
(cid:7)
(cid:7)
pð^xBLSjxÞ (cid:6) N
0
@
XC
i¼1
~wiðxÞðaix 1 ~(cid:2) iÞ; s2
XC
i¼1
~wiðxÞai
(49)
!
2
1
A:
(50)
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This approximates the true estimate distribution with a
~wiðxÞðaix 1 ~(cid:2) iÞ and variance
Gaussian with a mean RC
i¼1
~wiðxÞaiÞ2. Maximum likelihood estimation can
s2ðRC
i¼1
then be used as described previously to find the model
parameters that best explain an empirically measured
estimate distribution. In Results, we analyze the re-
gimes in which this is a good approximation.
Two-alternative forced-choice task
As with the point estimate distributions, we will again
describe a numerical and approximate analytical ap-
proach for handling data from a 2AFC task.
To numerically estimate the ideal observer’s prior
from a set of experimental 2AFC data using a mixture
of Gaussians prior, we can again use the general form
of the log-likelihood defined in Equation 31. Here,
pð“yes”jx1;t; x2;tÞ is defined with the general solution in
Equation 25, and the decision rule Tðm1; m2Þ follows
the definition in Equation 21. Since the estimate distri-
butions are not guaranteed to be Gaussian, there is no
simple analytical solution like there was in the single
Gaussian prior model. Thus, these equations must be
evaluated numerically by calculating pðx2.x1jm1; m2Þ
for each measurement pair on the 2D support to define
Tðm1; m2Þ, as illustrated previously in Figure 4. Once
the boundary defined by this decision rule is found, we
can simply integrate the joint distribution pðm1; m2jx1; x2Þ
above this boundary to determine pð“yes”jx1;i; x2;iÞ and
evaluate the model
likelihood. This process is again
outlined graphically in Figure 6, with the white line now
denoting an example decision boundary for an ob-
server with a mixture of Gaussians priors.
Compared with the single Gaussian case, the mix-
ture of Gaussians decision boundary can be nonlinear
for a few reasons. One reason is the dependence of
each adjusted weight ~wi on m: the weight of the shrink-
age factor for each prior component decreases with a
greater difference between the component mean and
the likelihood mean. As a result, the perceptual bias
that the prior exerts is different at different points
along the stimulus domain. Nonlinear decision boun-
daries can also emerge when the prior is bimodal, with
measurements biased in different directions depend-
ing on which mode is closest. A function for numeri-
cally evaluating pð“yes”jx1;i; x2;iÞ is included with the
toolkit (calcMoGPFxn_Numeric.m).
As noted for the point estimation case with a mixture
of Gaussians prior, this numerical calculation can be
computationally expensive. We can, however, lever-
age the approximate analytical expression for the esti-
mate distribution to define an approximate expression
for the categorical data collected in a 2AFC experi-
ment. The reason this is possible is that with this ap-
proximation, the two-point estimate distributions are
Gaussian. Using the Bayesian least squares estimate
^xBLS defined in Equation 40, we can generalize the de-
cision rule Tðm1; m2Þ in Equation 23 to an observer with
a Mixture of Gaussians prior:
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A
B
C
D
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Figure 6. An extension of Figure 4 to a long-tailed prior defined by a mixture of Gaussians (g1 ¼ 2; g2 ¼ 0:6 (cid:2)1 ¼ (cid:2)2 ¼
0; w1 ¼ w2 ¼ 0:5), similar in appearance to the prior in Figure 7A. Here, the decision boundary representing Tðm1; m2Þ is nonlinear be-
cause the different components of the prior have different levels of influence on the percept as m varies. A, As in Figure 4, the 2D grayscale
image shows the joint distribution of the observer measurements given the stimuli x1 and x2, formed by the product of the two measurement
distributions along the top and right. The white line is the observer's decision boundary. Here, x1 = x2 = 3 for measurement noise variances
m2 = 0.64. C, Same as panel A, but with measurement
s2
1 = 0.5. Toolkit script: Fig6_MoGGauss_graphicalDemo.m. D, Full psychometric curves for the noise varian-
noise variances s2
ces used in panels A–C, showing the probability that the observer reports “yes” as a function of the stimulus X2. The points labeled A, B, C
represent the sum of the probability above the diagonal in panels A–C.
1 = 0.5. B, Same as panel A, but with equal noise variances s2
1 = 0.75, s2
1 = 0.75, s2
m1 = s2
Tðm1; m2Þ
XC
8
><
1
(cid:6)
>:
j¼1
0 otherwise
~wjðx2Þðajm2 1 ~(cid:2) jÞ .
XC
i¼1
~wiðx1Þðaim1 1 ~(cid:2) iÞ
:
(51)
Note that we index the modified weights and means differ-
ently for the two stimuli (i for x1 and j for x2) since these param-
eters of the posterior components are defined by both the
prior components and the likelihood parameters, which differ
whenever x1 is different from x2. As before, we can derive an
analytical (although approximate) solution to the psychomet-
ric function for the mixture of Gaussians approach using
Equation 29, with the exception of substituting in the approxi-
mate estimate distribution pð^xBLSjxÞ from Equation 49:
pð“yes”jx1; x2Þ
XC
0
XC
(cid:6) U
B
B
B
B
B
B
B
@
j¼1
v
u
u
u
t
s2
1
~wiðx1Þðaix1 1 ~(cid:2) iÞ
~wjðx2Þðajx2 1 ~(cid:2) jÞ (cid:4)
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
2
0
!
i¼1
2
XC
XC
~wiðx1Þai
@
1 s2
2
~wjðx2Þaj
A
i¼1
j¼1
1
C
C
C
C
C
C
C
A
:
(52)
Code accessibility
The code is included as Extended Data 1 and is available
at https://github.com/tsmanning/bayesIdealObserverMoG.
Results
In this section, we will demonstrate that there are a
number of ways to maintain the flexibility of the mixture of
Gaussians approach while reducing the total number of
parameters describing the prior, and then show that this
approach can be used to fit leptokurtotic and bimodal dis-
tributions. Lastly, we show that the approximate 2AFC so-
lution remains close to the numerical solution for a range of
model parameters constrained to realistic values. Although
we do not go into detail here about how to generate syn-
thetic estimate or 2AFC data using a Bayesian ideal ob-
server framework, we include some example code in the
toolkit about how one might benchmark implementations
of an observer model with a mixture of Gaussians prior
interactiveNumTrialsVSaccuracy.m.
Prior estimation error using mixture of Gaussians
model with point estimation data
Theoretically, a mixture of Gaussians could fit an infinite
number of prior shapes given enough Gaussian compo-
nents in the model. But the number of model parameters
increases by three for each additional component, poten-
tially requiring large amounts of data to obtain reliable fits.
Further, unrestricted models will likely be nonconvex with
multiple local optima. These characteristics extend the
number of iterations needed to find the global optimum
of the log-likelihood objective functions at best and
make it unlikely or impossible to find the global optimum
at worst. In practice, unrestricted forms of the mixture of
Gaussians model will
likely need multiple optimization
runs with different starting parameters to reliably minimize
the log-likelihood functions. There are a few ways to main-
tain the flexibility of the mixture of Gaussian approach while
reining in the number of parameters in the model.
In sensory subdomains where there is evidence that the
probability of some stimulus values monotonically de-
creases with stimulus magnitude, such as the spectral con-
tent of retinal images (Field, 1987; Dong and Atick, 1995),
we can reduce the number of parameters by a third in our
ideal observer model by fixing all component means at
zero. This allows us to model long-tailed distributions as
can be seen in Figure 7A, and in fact, any distribution that
is a member of the exponential power family with a peak at
zero and power 1 (cid:3) p (cid:3) 2 can be approximated with
enough components (West, 1987).
If there is not sufficient evidence that the true distri-
bution of stimulus power in the environment is either
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A
B
Figure 7. Two example methods for reducing the number of pa-
rameters to optimize when inferring an observer’s prior. A, A
leptokurtotic prior centered on zero formed by a mixture of
zero-mean Gaussian components. B, A skewed prior formed by
a mixture of Gaussian components with fixed positions and
widths. Toolkit script: Fig7_MoGConstrainedFitting.m.
symmetric or zero-peaked, one can take an alternative
approach of tiling the components (Fig. 7B). Here, one
defines a fixed number of components, their means, and
their SDs and fits only the weights of the tiled components
to the data. In this way, the mixture of Gaussians can ap-
proximate a prior with a peak at an arbitrary location,
skewness, and kurtosis. This approach has been used
previously with large numbers of components to approxi-
mate a “nonparametric” reconstruction of a complicated
prior (Acerbi et al., 2014).
Here, we demonstrate proof of principle for both ap-
proaches by generating a synthetic dataset of 1000 point
estimates using a zero-centered, non-Gaussian prior and
a bimodal prior, and then recovering estimates of these
priors using the mixture of Gaussians ideal observer
model and the constraints illustrated in Figure 7.
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We first defined a long-tailed prior using a Cauchy
distribution pðxÞ ¼ 1=pð1 1 x2Þ. We generated individual
point estimates by numerically calculating the posteriors
for a range of different measurement values as seen at the
top right in Figure 8A and calculating ^xBLS for each mea-
surement. We used this matched set of measurements
and Bayes estimates as a look up table, and generated
the synthetic dataset of 1000 trials by randomly selecting
a stimulus value, adding Gaussian noise to obtain a mea-
surement, and then selecting a matched estimate by in-
terpolating between the previously calculated estimate
values. From these values, we estimated the Cauchy prior
using a restricted form of the mixture of Gaussians ideal
observer model in which we defined six Gaussian compo-
nents with a set of fixed gi on the range ½2(cid:4)2; 23(cid:2) and all
component means (cid:2)i fixed at zero. Thus, the only observer
parameters free to vary were the component weights wi,
and the measurement noise level s which was constant for
all simulated stimuli (that is, we are assuming the stimulus
properties that may affect this measurement noise are held
constant throughout the experiment). The best fitting pa-
rameters u ¼ fwi; sg were obtained through numerical op-
timization by numerically estimating T (cid:4)1ð^xÞ to obtain a set
of {mi} from the dataset of f^xg and then minimizing the
negative log-likelihood, which is the sum over the individual
negative log likelihoods (see Eq. 43). The correspondence
between the true prior and the inferred one are shown at
the in Figure 8A, as well as the correspondence between
the true BLS estimates and the ones inferred through MLE.
In general, the mixture of Gaussians model closely matches
the true prior although each of the basis function components
on their own are less kurtotic than their sum.
We then repeated this process using a bimodal prior de-
fined by the normalized sum of two Gaussians p1ðxÞ ¼
A
B
Figure 8. Mixture of Gaussians model fitting to non-Gaussian priors. A, Inferring the shape of a Cauchy prior from a set of 1000 point es-
timates. Top left, True prior in red and inferred prior in dashed black. Bottom left, The same, but on a semilog axis. Top right, Posteriors
for a set of stimuli and measurements, as well xBLS for each posterior (green line). Bottom right, Set of posteriors and xBLS inferred from
the data using the mixture of Gaussians model. B, Inferring the shape of a bimodal prior from a set of 1000 point estimates. Conventions
are the same as in panel A. A slight gamma correction has been applied to the set of posteriors shown in the 2D plots for visibility.
Toolkit scripts: Fig8_MoGtoNonGauss.m and Fig8_MoGtoNonGauss2.m for panels A and B, respectively.
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N ð(cid:2)1 ¼ (cid:4)2; g1 ¼ 1Þ and p2ðxÞ ¼ N ð(cid:2)2 ¼ 2; g2 ¼ 1Þ, and
this time using the tiling constraint variation mixture of
Gaussians model previewed in Figure 8B. Once again,
the prior inferred from the data closely corresponds to
the true prior, although this correspondence will change
depending on the exact spacing and width of the basis
functions (Fig. 8B).
Error in mixture of Gaussians analytical approximation
with 2AFC data
We will next examine how close the approximate ana-
lytical solution is to the numerical solution within a range
of observer parameters that matches the biases and sen-
sitivities seen in real human data.
Human bias
To get a sense of what a realistic range of biases is in
the literature, we consider empircally measured percep-
tual biases for linear (i.e., noncircular, nonspherical) stim-
ulus domains like speed and distance. For example, in
Stone and Thompson (1992), participants performed a
2AFC speed judgment task in which they selected
which of two contrast and speed-varying stimuli ap-
peared to move faster. Depending on the contrast ratio
between the two stimuli, biases in speed judgments
ranged from ;0.55 to 1.55 times the veridical speed.
Similar results were found in later studies that devel-
oped Bayesian ideal observer models to explain these
biases (Weiss et al., 2002; Stocker and Simoncelli,
2006). An analysis of speed judgments for contrast-
varying stimuli in 2D and 3D (Cooper et al., 2016) found
a bias of up to ;1.75 times veridical. In a disparity
judgment task, Burge and colleagues reported bias of
;1.15 (Burge et al., 2010). Thus, we will ensure that the
simulated observer parameter models will at least reach
these levels in our error analysis. The relationship between
bias and observer parameters is straight-forward for a single
Gaussian prior and Gaussian likelihood. It is simply the frac-
tion of the shrinkage factors a1=a2 for the two stimuli, where
the observer is unbiased when the fraction equals one.
Referring back to Equation 12, this means we need to select
the stimulus likelihood widths s1; s2 and prior width g to
g2 1 s2
1
g2 1 s2
2
ensure that the upper and lower bounds of
fall on
the range of 0.55–1.75. For mixture of Gaussian priors,
the analytical approximation essentially treats the poste-
2
!
riors as Gaussians with SDs defined by s2
~wiðxÞai
.
PC
i¼1
This means we can produce human-like biases as
long as we select observer parameters such that
!
2
,
!
2
PC
i¼1
~w iðx1Þai
PC
j¼1
~w jðx2Þaj
also falls within this range.
Sensitivity
The slope of the psychometric curve at the point of
the value of x2 where
subjective equality (PSE;
i.e.,
pð“yes”jx1; x2Þ ¼ 0:5) is commonly used as a scalar metric
to describe observer sensitivity when performing a 2AFC
ffiffiffiffiffiffiffi
task. The slope has an analytical solution 1=ðsdiff
2p
Þ
p
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when the psychometric curve is a cumulative normal dis-
tribution, which occurs when distribution of differences
between estimates f^x1; ^x2g is a Gaussian N ðmdiff
; sdiffÞ.
This is the case for the single Gaussian prior and the ana-
lytical approximate solution for a mixture of Gaussians
prior (see Eqs. 29 and 51), but not necessarily for the full,
numerically evaluated mixture of Gaussians prior. In psy-
chophysical data, this slope could reasonably range from
near-infinite when the task is very easy to zero when the
task is impossible to solve and the observer is guessing
for all stimulus parameters. Therefore, we will define the
range of observer parameters to cover a large range of
slopes.
Although there is an infinite range of possible prior
configurations to test, we will restrict ourselves here to
two useful situations not well fit by a single Gaussian:
(1) a prior with only zero-mean components creating a
leptokurtotic unimodal distribution and (2) a bimodal
prior.
Example 1: leptokurtotic unimodal prior
First, we randomly selected a set of stimulus and ob-
server configurations 5000 times (Fig. 9A, top). Likelihood
means were selected from a uniform distribution ranging
from [–1, 1] and SDs fs1; s2g were selected from a uni-
form distribution in the range of 0 , s (cid:3) 1. The prior was
restricted to two components, both zero-centered, which
were both constrained to be broader than the likelihoods.
Specifically, g1 was fixed at 1:1maxðs1; s2Þ and g2 was
randomly selected from a uniform distribution on the
range ½1:25maxðs1; s2Þ; 3:25maxðs1; s2Þ(cid:2)). The weights
on these components were chosen randomly from a uniform
distribution and normalized such that they summed to one.
Fixing g1 to be only slightly larger than the largest s ensured
that the priors were non-Gaussian and long-tailed, and that
the priors produced psychometric functions with a range of
biases covering the targeted range (actual biases ranged
from 0.55 to 1.83).
Next, we determined the difference between the psycho-
metric curve resulting from the analytical and numerical ap-
proaches described in the previous section (Fig. 9A,
bottom). We compare the approximate solution (Eq. 50) to
a numerical evaluation (Eq. 25) for the observer with a mix-
ture of Gaussian prior (blue and black lines). We also com-
pare the best
fit single Gaussian to the mixture of
Gaussians prior (yellow line). In doing so, we can directly
assess the improvement of the approximate mixture of
Gaussians approach over the single Gaussian approxima-
tion. These results are plotted in Figure 9B–E. Figure 9B,C
show the correspondence between the single Gaussian
(Fig. 9B) and approximate mixture of Gaussians (Fig. 9C)
models and the numerical evaluations. The points all fall
near the identity line, indicated reasonable agreement, but
the spread is clearly larger for the single Gaussian model.
Figure 9D summarizes the errors, showing that the analyti-
cal mixture of Gaussians approximation has approximately
three times lower RMS error than the single Gaussian fit.
The error reduction is most profound about the PSE in
the psychometric function, where the analytical and
numerical approximations are essentially equivalent.
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A
B
D
C
E
Figure 9. Performance of approximations for fitting heavy-tailed priors. A, Diagram illustrating the pipeline for comparing the
mixture of Gaussians (MoG) approximation and a single Gaussian (SG) to a full numerical evaluation of two-alternative forced
choice data generated with a MoG prior. B, C, Scatter plots illustrate the relationship between the numerical evaluation of
the MoG prior model and the SG and approximate MoG approaches. Black circles indicate the points corresponding to the
estimated psychometric function values shown in panel A for the SG and MoG approximations. D, Square root of the mean
squared error (RMS error) for the MoG analytical approximation and the single Gaussian approximation, summarized over 20
bins of the numerical data. E, Mean signed error distributions for both approximations. Note that axis ranges are set to
match Figure 10 for comparison. Toolkit script: Fig9_MoGErrorAnalysis.m.
This means that one can precisely estimate observer
biases even for non-Gaussian priors in a computation-
ally-efficient manner. The analytical approximation
does show a slight tendency to overestimate the upper
flank of the psychometric curve and underestimate the
lower flank (visible with the mean signed errors; Fig.
9E),
indicating a bias toward steeper psychometric
functions. Thus, when using this approximation to fit-
ting psychometric data of observers with heavy-tailed
priors, this will produce prior and/or likelihood esti-
mates that are narrower than the true values.
Example 2: bimodal prior
Next, we assess the performance of the mixture of
Gaussians analytical approximation for fitting psycho-
metric data from an observer with a bimodal prior. We
randomly selected likelihood means and SDs in the
same fashion as we did for the zero-mean prior. To de-
fine a bimodal, two-component prior on each random-
ization, we selected the component means f(cid:2)1; (cid:2)2g from
a uniform distribution where one component was restricted
to the range [–1, –0.5] and the other from [0.5, 1]. The com-
ponent SDs were randomly selected from a uniform distribu-
tion in the range ½maxðs1; s2Þ; 1:4maxðs1; s2Þ(cid:2) to ensure
each prior had two distinct peaks. Each component weight
was randomly selected and the two were normalized such
that they summed to one. As before, we present data from
5000 randomization runs in Figure 10. Overall, the approxi-
mate mixture of Gaussians method precisely estimated
the location of the PSE (i.e., this method has low RMS
error ;0.5) for the bimodal priors. Compared with the lep-
tokurtotic unimodal case, however, the method shows in-
creases in both RMS error and signed error along other
regions of the psychometric function. The end result is
that while the analytical approximation can accurately
estimate an observer’s bias, it will again tend to over-
estimate the slopes of the observer’s psychometric
functions.
Discussion
The Bayesian ideal observer framework has proven
broadly useful for explaining perceptual phenomena in mul-
tiple sensory modalities. For example, a prior that peaks
at zero speed (a “slow motion” prior) has successfully pre-
dicted systematic biases in judgements of
the speed
(Weiss et al., 2002; Stocker and Simoncelli, 2006) and di-
rection (Weiss et al., 2002; Sotiropoulos et al., 2011;
Rokers et al., 2018) of moving objects. A “light from above”
prior about the position of the illuminant in a scene has
been used to explain biases in the perceived shape of am-
biguously shaded figures (Adams et al., 2004). Similarly,
priors for viewing angle, convexity, and alignment between
principal lines of curvature and surface contours can ex-
plain biases in the interpretation of surface curvature from
simple line drawings (Mamassian and Landy, 1998). Other
examples of the success of Bayesian perceptual models in-
clude prediction of biases in the timing of intervals between
discrete events (Sohn and Jazayeri, 2021), the perceived
structure in complex moving patterns (Yang et al., 2021),
judgments in the orientation of contours (Girshick et al.,
2011), and the orientation of surface tilt in natural scenes
(Kim and Burge, 2018).
Here, we reviewed the straightforward approach for in-
ferring Bayesian ideal observer models from psychophys-
ical data when it is assumed that priors and sensory noise
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A
B
D
C
E
Figure 10. Performance of the analytical approximation in fitting bimodal priors. A, Diagram illustrating the pipeline for compar-
ing the mixture of Gaussians (MoG) approximation and a single Gaussian (SG) before a full numerical evaluation of two-alternative forced
choice data generated with a MoG prior. B, C, Scatter plots illustrate the relationship between the numerical evaluation of the MoG prior
model and the SG and approximate MoG approaches. Black circles indicate the points corresponding to the estimated psychometric
function values shown in panel A for the SG and MoG approximations. D, Square root of the mean squared error (RMS error) for the
MoG analytical approximation and the single Gaussian approximation, summarized over 20 bins of the numerical data. E, Mean signed
error distributions for both approximations. Toolkit script: Fig10_MoGErrorAnalysis2.m.
are Gaussian distributed. Following on a step-by-step
formulation of this approach, we then extended the
model to include prior distributions described with
mixtures of Gaussians. In doing so, we build on previ-
ous work that has used mixture of Gaussian priors in
other perceptual applications. For example, one group
used a mixture of Gaussians to to define the relative
probabilities of experimental stimuli and then probed sub-
optimalities in perceptual
inference (Acerbi et al., 2014).
Another group used a mixture of Gaussians approach to
model human observer priors about homogeneity of orienta-
tion to understand biases in visual short-term memory tasks
(Orhan and Jacobs, 2014).
Importantly, this mixture of Gaussians extension of
the Bayesian ideal observer framework complements
and expands on existing approaches for modeling the
relationship between natural scene statistics and per-
ceptual priors. First, if perceptual priors indeed match
the non-Gaussian distributions of natural stimuli, then
using a mixture of Gaussians model of priors may im-
prove how well we predict perceptual biases when
stimulus measurements fall on different regions of the
stimulus domain, as compared with a single Gaussian
model. Second, the mixture of Gaussians approach
provides a tool for researchers to constrain Bayesian
models using empirically measured stimulus statistics.
Bayesian models have faced criticism because of their
lack of constraint in how the priors or likelihoods are
defined (Jones and Love, 2011; Marcus and Davis,
2013; Rahnev and Denison, 2018). One way to con-
strain the prior is to assume that the visual system has
veridically learned the statistics of natural scenes and
these learned statistics are reflected in the prior. In this
case, one could define the ideal observer prior with a
mixture of Gaussians that matches an empirically
measured distribution of scene statistics, forgoing the
need to fit the prior from perceptual
judgment data.
Indeed, several groups have made progress in the esti-
mating the distribution of spectral content in terrestrial
scenes (Field, 1987; Dong and Atick, 1995), tilt of ob-
jects in natural scenes (Burge et al., 2016), binocular
disparity (Sprague et al., 2015), and the spectral con-
tent of retinal motion during eye and head movements
(DuTell et al., 2020). While the match between esti-
mates of natural statistics and perceptual biases has
been investigated previously with numerical methods
(Girshick et al., 2011; Sprague et al., 2015), a (relatively)
low dimensional parameterization of these stimulus dis-
tributions opens up new opportunities for efficiency and
experimental investigations.
Limitations and alternative approaches
Although numerical estimation of the model parameters
will find an exact solution with sufficient precision, this is
not always possible in practice. Estimating pð“yes”jx1; x2Þ
requires summation over many 2D probability mass
functions, which must be redefined everytime the ideal
observer parameters are changed (e.g.,during numeri-
cal optimization). Further, the MLE loss functions for
both the numerical and analytical methods defined
in this document are likely to be nonconvex and thus
potentially prone to falling into a local minimum. This
problem can be potentially overcome by initializing the
numerical optimization in multiple locations within the
loss function hypersurface, although this will add addi-
tional computation time to the estimation.
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While the approximate analytical method dramatically
improves the computational efficiency of the ideal ob-
server parameter estimation, it deviates from the true
solution for pð“yes”jx1; x2Þ the further x2 gets from the
point of subjective equality. As shown in Figure 10, this
method is also especially prone to errors away from
the PSE when the prior or posterior are bimodal. These
problems can be mitigated in a few ways. If the approxi-
mate analytical method is to be used to adaptively select
stimuli during an experiment, the numerical approach
can be used after data collection to reach a more accu-
rate solution. If there is good reason a priori to think that
an observer’s prior is bimodal (e.g., based on natural
stimulus statistics), one can just fall back to the numeri-
cal solution.
Throughout this document, we assert that the ideal
observer likelihood and measurement distributions are
Gaussian along the domain in which the observer enc-
odes the stimuli. Other model parameterizations, how-
ever, have been proposed that constrain the likelihood
based on physiology and other assumptions, and result
in notably asymmetric, non-Gaussian likelihoods (Zhang
and Stocker, 2022). We also focus here on stimuli de-
fined along a linear axis (e.g., position, velocity, binocular
disparity), and therefore, the methods as presented can-
not be directly applied to perceptual
judgments about
stimuli defined on a circular axis (e.g., orientation, visual
motion direction, position of an illuminant). Despite this
limitation, previous work has successfully used circular
statistics to explain perceptual biases with a Bayesian
ideal observer model
(Mamassian and Landy, 1998;
Burge et al., 2016). As a circular analog of the Gaussian,
a mixture of von Mises distributions is a natural exten-
sion of the mixture of Gaussians approach.
Finally, we focus here on perceptual priors and not pri-
ors involved in decision-making or perception-action con-
tingencies. Decision strategies could presumably affect
the loss function as well, if there was an advantage to tak-
ing some other summary statistics from the posterior
distribution instead of the least squares estimate. The in-
fluences of these strategies have been considered else-
where (Chambers et al., 2019) but are out of the scope of
the current work.
In conclusion, many scientific questions about how
prior knowledge is incorporated into perceptual judg-
ments and perceptually-guided behaviors remain unan-
swered. Within the Bayesian framework, for example,
do priors vary significantly between observers and do
they vary between different tasks? How closely do pri-
ors follow from the statistics we can measure empiri-
cally from the environment across multiple stimulus
domains? How adaptable are priors in response to
changing stimulus statistics? A major limiting factor in
answering these questions is the accuracy and effi-
ciency with which we can estimate people’s priors from
experimental data. Broadening the computational tool-
kit for experimenters and modelers to address this chal-
lenge is an important component of the larger effort to
advance our understanding of the transformation from
sensation to perception.
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| null |
10.3389_fimmu.2021.689397.pdf
| null | null |
Supplementary Material for:
Crosstalk between CD11b and Piezo1 mediates macrophage responses
to mechanical cues
Hamza Atcha1,2, Vijaykumar S. Meli1,2, Chase T. Davis1,2 Kyle T. Brumm1,2, Sara Anis1,2,
Jessica Chin1,2, Kevin Jiang1,2, Medha M. Pathak1,4,5, and Wendy F. Liu1,2,3,6*
1 Department of Biomedical Engineering, University of California, Irvine
2 The Edwards Lifesciences Center for Advanced Cardiovascular Technology, University of
California, Irvine
3 Department of Chemical Engineering and Materials Science, University of California, Irvine
4 Department of Physiology and Biophysics, University of California, Irvine
5 Sue and Bill Gross Stem Cell Research Center, University of California, Irvine
6 Department of Molecular Biology and Biochemistry, University of California, Irvine
* Correspondence:
2412 Engineering Hall
Irvine, CA 92697
Tel:
Fax:
Email:
(949) 824-1682
(949) 824-9968
[email protected]
* To whom correspondence may be addressed.
1
Supplementary Figures
Supplementary Material
Figure S1: Quantification of cell morphology for macrophages subjected to cyclic uniaxial
stretch. Quantification of cell alignment relative to the direction of stretch (A) and perpendicular to
the direction of stretch (B), cell aspect ratio or the length of the major axis divided by the length of the
minor axis (C), and spread cell area (D) for unstimulated, IFNγ/LPS, and IL4/IL13 stimulated
macrophages with no stretch or 20% cyclic uniaxial stretch. Error bars indicate standard deviation for
three separate experiments and * p < 0.05 when compared to the indicated condition as determined by
Student’s t-test.
2
Figure S2: Cyclic mechanical stretch does not affect macrophage cell viability. Quantification of
cell viability, as measured by Cyquant assay, following either 4 (left) or 24 (right) h of adhesion,
stimulation, and 18 h of stretch. Cell number was normalized to the unstimulated, 0% stretch,
condition for each time point. Error bars indicate standard deviation about the mean for three separate
experiments.
3
Supplementary Material
Figure S3: Quantification of morphological parameters for macrophages subjected to cyclic
uniaxial stretch after 4 h of adhesion. Quantification of cell alignment relative to the direction of
stretch (A) and perpendicular to the direction of stretch (B), as defined by the highlighted regions in
Figure 1B, cell aspect ratio or the length of the major axis divided by the length of the minor axis (C),
and spread cell area (D) for unstimulated, IFNγ/LPS, and IL4/IL13 stimulated macrophages with no
stretch or 20% cyclic uniaxial stretch. Error bars indicate standard deviation for three separate
experiments and * p < 0.05 when compared to the indicated condition as determined by Student’s t-
test.
4
|
Could not heal snippet
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10.1038_s41467-023-36872-8.pdf
|
Data availability
Source data are provided within this paper. Raw data that support the
findings of this study are available from the corresponding author
upon request. Source data are provided with this paper.
|
Data availability Source data are provided within this paper. Raw data that support the findings of this study are available from the corresponding author upon request. Source data are provided with this paper.
|
Article
https://doi.org/10.1038/s41467-023-36872-8
A combinatorial code of neurexin-3
alternative splicing controls inhibitory
synapses via a trans-synaptic dystroglycan
signaling loop
Received: 10 May 2022
Accepted: 20 February 2023
Justin H. Trotter
Thomas C. Südhof
1,3
1,2
, Cosmos Yuqi Wang1,3, Peng Zhou1,3, George Nakahara1 &
;
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Disrupted synaptic inhibition is implicated in neuropsychiatric disorders, yet
the molecular mechanisms that shape and sustain inhibitory synapses are
poorly understood. Here, we show through rescue experiments performed
using Neurexin-3 conditional knockout mice that alternative splicing at SS2
and SS4 regulates the release probability, but not the number, of inhibitory
synapses in the olfactory bulb and prefrontal cortex independent of sex.
Neurexin-3 splice variants that mediate Neurexin-3 binding to dystroglycan
enable inhibitory synapse function, whereas splice variants that don’t allow
dystroglycan binding do not. Furthermore, a minimal Neurexin-3 protein that
binds to dystroglycan fully sustains inhibitory synaptic function, indicating
that trans-synaptic dystroglycan binding is necessary and sufficient for Neur-
exin-3 function in inhibitory synaptic transmission. Thus, Neurexin-3 enables a
normal release probability at inhibitory synapses via a trans-synaptic feedback
signaling loop consisting of presynaptic Neurexin-3 and postsynaptic
dystroglycan.
Synapses are sophisticated intercellular junctions controlled by trans-
synaptic signaling that is mediated, at least in part, by interactions
between pre- and postsynaptic adhesion molecules. Among synaptic
adhesion molecules (SAMs), neurexins stand out because of their
central role in shaping the properties of synapses1–4. Neurexins per-
form a panoply of synaptic functions, ranging from mediating a normal
neurotransmitter release probability5–7 to controlling presynaptic
GABAB-receptors8, regulating postsynaptic glutamate and GABAA-
receptors9–12, and enabling postsynaptic NMDA-receptor-dependent
LTP13. Recent observations uncovered multitudinous interactions of
neurexins with diverse ligands that likely mediate the functions of
neurexins. Indeed, trans-synaptic ligands for some of these functions
were identified, as shown for the neurexin-dependent control of
glutamate receptors that are dictated by Cbln1/2-GluD1/2 complexes in
the subiculum11 or for neurexin-dependent LTP that requires Nlgn1 in
the hippocampal CA1 region14. The best-documented role of neurexins
among their various functions probably consists of their regulation of
the presynaptic release probability5–7, but no candidate ligands were
identified for that role, making it unclear how neurexins determine the
release probability of a synapse. Even the question of whether pre-
synaptic neurexins act directly cell-autonomously in the nerve term-
inal or operate indirectly via binding to a postsynaptic ligand that then
signals back to the presynaptic release machinery has not been
addressed4.
Neurexins are transcribed from three genes (Nrxn1-3) in two
longer α-neurexins and shorter β-neurexins15–17.
principal forms,
1Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305, USA. 2Howard Hughes Medical Institute,
Stanford University School of Medicine, Stanford, CA 94305, USA. 3These authors contributed equally: Justin H. Trotter, Cosmos Yuqi Wang, Peng Zhou.
e-mail: [email protected]; [email protected]
Nature Communications |
(2023) 14:1771
1
Article
https://doi.org/10.1038/s41467-023-36872-8
Neurexin mRNAs are extensively alternatively spliced at six canonical
positions (SS1 to SS6)18,19, whose use is highly regulated spatially and
temporally20,21. The best-studied site of alternative splicing of neur-
exins, splice site #4 (SS4), regulates the binding of key ligands1,4,22–25
and controls the postsynaptic levels of AMPA- (AMPARs) and NMDA-
receptors (NMDARs)10. Another site of alternative splicing, SS2 that is
only present in α-neurexins (Fig. 1a), has been found to modulate two
neurexin-ligand interactions, binding to dystroglycan, a postsynaptic
adhesion molecule, and to neurexophilins, a family of secreted
cysteine-rich proteins26–30. Dystroglycan binds only to α-neurexins
lacking an insert in either SS2 and/or in SS4, but not to α-neurexins
with inserts in both SS2 and SS4, whereas neurexophilins
a
Domain Structures and Alternative Splicing
Nrxn3α
SP
LNS1
A
LNS2
LNS3
B
LNS4
LNS5
C
LNS6
SS1 SS2
SS3
SS6
SS4
SS5
Nrxn3β
SP
LNS6
Extra.
Intra.
b
ΔCre (30/6)
IPSCs
Cre (16/3)
c
IPSC amplitudes
**
d
***
IPSCs
Control
(22/4)
12
8
4
2
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
ΔCre +
Nrxn3α SS4-
(15/3)
Cre +
Nrxn3α SS4-
(14/3)
Nrxn3α SS4+
(14/3)
Nrxn3α SS4+
(13/3)
Nrxn3β SS4+
(15/3)
Nrxn3β SS4+
(12/3)
0.5 s
NMDAR-EPSCs
ΔCre (28/6)
Cre (14/3)
ΔCre +
Nrxn3α SS4-
(14/3)
Cre +
Nrxn3α SS4-
(13/3)
Nrxn3α SS4+
(14/3)
Nrxn3α SS4+
(14/3)
Nrxn3β SS4+
(13/3)
Nrxn3β SS4+
(14/3)
A
n
5
.
0
A
n
2
g
)
A
n
(
.
l
p
m
A
C
S
P
E
-
R
A
D
M
N
0
ΔCre
Cre
Nrxn3αSS4+
Nrxn3αSS4-
Nrxn3βSS4+
Nrxn3αSS4+
Nrxn3αSS4-
Nrxn3βSS4+
ΔCre +
Cre +
NMDAR-EPSC amplitudes
2.0
1.0
0.8
0.6
0.4
0.2
0
ΔCre
Cre
Nrxn3αSS4+
Nrxn3αSS4-
Nrxn3βSS4+
Nrxn3αSS4+
Nrxn3αSS4-
Nrxn3βSS4+
1 s
ΔCre +
Cre +
e Amplitude
**
12
8
6
4
2
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
0
Ctrl.
Nrxn3βSS4+
i Amplitude
)
A
N
(
e
d
u
t
i
l
p
m
A
C
S
P
E
4.0
2.0
1.0
0.5
0
Ctrl.
Nrxn3βSS4+
Nrxn3β SS4+
(24/4)
A
n
2
0.5 s
h
NMDAR-
EPSCs
Control
(16/3)
Nrxn3β SS4+
(15/3)
A
n
5
.
0
1 s
Representative images
ΔCre
Cre
k
Synaptic puncta quantifications
Homer1
-
Inhibitory
Synapse Density
0.08
+
Homer1 Inhibitory
Synapse Density
Total Inhibitory
Synapse Density
0.15
f
j
)
2
m
μ
/
a
t
c
n
u
P
(
y
t
i
s
n
e
D
e
s
p
a
n
y
S
20 μm
2 μm
20 μm
2 μm
Gephyrin/vGAT/Homer1/ MAP2MAP2
l
ΔCre (10/3)
Cre (15/3)
IPSCs
Cre + Nrxn3α (various splice variants)
SS1+/SS2-/
SS4- (15/3)
SS1-/SS2a/
SS4- (12/3)
SS1+/SS2a/
SS4- (13/3)
0.06
0.04
0.02
0
0.15
0.05
0.04
0.02
0
SS1-/SS2-/
SS4- (17/3)
SS1-/SS2ab/
SS4- (14/3)
SS1+/SS2ab/
SS4- (14/3)
ΔCreCre
m
12
8
4
2
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
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)
2
m
μ
/
a
t
c
n
u
P
(
y
t
i
s
n
e
D
e
s
p
a
n
y
S
3
/
2
5
3
/
3
5
0.10
0.05
0
0.40
0.20
0.10
0.05
0
ΔCreCre
0.08
0.06
0.04
0.02
0
0.20
0.10
0.06
0.04
0.02
0
ΔCreCre
)
2
m
μ
/
a
t
c
n
u
P
(
y
t
i
s
n
e
D
e
s
p
a
n
y
S
*
IPSC amplitudes
*
*
A
n
2
0
ΔCreCre SS1:
SS2:
+
-
0.5 s
-
-
-
a
+
-
ab
ab
Cre + Nrxn3α SS4-
+
a
n
IPSCs
Cre + Nrxn3α (various splice variants)
o
ΔCre (12/3)
Cre (14/3)
SS1-/SS2-/
SS4+ (13/3)
SS1-/SS2a/
SS4+ (14/3)
SS1-/SS2ab/
SS4+ (13/3)
A
n
2
0.5 s
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
IPSC amplitudes
**
**
***
12
8
8
6
4
2
0
ΔCreCre
-
-
SS1:
-
a
SS2:
Cre + Nrxn3α SS4+
-
ab
Nature Communications |
(2023) 14:1771
2
Article
https://doi.org/10.1038/s41467-023-36872-8
Fig. 1 | Nrxn3α alternative splicing regulates inhibitory synapses in dissociated
olfactory bulb cultures. a Schematic of Nrxn3α/β structure (SP: signal peptide;
LNS1-6: LNS domains; A, B, and C: EGF-like domains) and sites of alternative splicing
(SS1-SS6). b, c Conditional Nrxn3 deletion impairs inhibitory synaptic transmission
monitored via evoked IPSC amplitudes (b, sample traces; c, IPSC amplitude sum-
mary graphs). d, e Nrxn3β overexpression suppresses evoked IPSCs in wild-type OB
neurons (d, sample traces; e, summary graphs of IPSC amplitudes). f, g Conditional
Nrxn3 deletion has no effect on evoked EPSCs (f, sample traces of NMDAR-EPSCs;
g, EPSC amplitude summary graphs). h, i Nrxn3β overexpression does not impair
evoked EPSCs (h, sample traces of NMDAR-EPSCs; i EPSC amplitude summary
graphs). j, k Conditional Nrxn3 deletion does not alter inhibitory synapse numbers
as analyzed by immunocytochemistry for presynaptic inhibitory (vGAT), post-
synaptic inhibitory (gephyrin), and postsynaptic excitatory (Homer1) markers
(j, sample images; k, summary graphs of puncta densities plotted as averaged per
experiment (top) or per region-of-interest (bottom). For additional synaptic puncta
quantifications, see Fig. S3. l, m Nrxn3α lacking an insert in SS4 (Nrxn3αSS4-) rescues
impaired inhibitory synaptic transmission if it lacks an SS2 insert (Nrxn3αSS2-) or
contains only a short insert (Nrxn3αSS2a) (l, sample traces; m, summary graphs of
IPSC amplitudes). n, o Nrxn3α with an insert in SS4 (Nrxn3αSS4+) rescues impaired
inhibitory synaptic transmission only if SS2 contains no insert (Nrxn3αSS2-)
(n, sample traces; o, summary graphs of IPSC amplitudes). Numerical data are
means ± SEM; n’s (cells/experiments) are indicated above the sample traces
(b–i, l–o) or in summary graph bars indicating average per independent culture on
top and individual ROI’s or pseudoreplicates on bottom (k). Statistical analyzes
were performed by one-way analysis of variance (ANOVA) with Dunnett’s multiple
comparison test (c, g, m, and o) or two-tailed unpaired t test (e, i, k), with *p < 0.05,
**p < 0.01, and ***p < 0.001. Source data and statistical results are provided within
the Source Data file.
preferentially bind to α-neurexins containing an SS2 insert26,29,30.
However, whether dystroglycan binding to α-neurexins is physiolo-
gically important is not known since dystroglycan binds to a large
number of other ligands besides neurexins, such as agrin31,
pikachurin32, and slit33. Indeed, circumstantial evidence from studies
of CCK-positive synapses in the hippocampus suggested that
dystroglycan-binding to neurexins is functionally insignificant34.
Finally, a third site of alternative splicing, SS5, has also been impli-
cated in regulating synaptic transmission, but not by a mechanism
involving the regulation of ligand binding35,36.
In brain, diverse classes of inhibitory neurons form distinct types
of inhibitory synapses37,38. Dystroglycan is essential for the formation
and/or function of a subset of these synapses34,39–42. Mutations in genes
that are part of the dystroglycan protein complex, such as dystrophin
and LARGE (the enzyme that uniquely glycosylates dystroglycan), are
associated with cognitive impairments43. Thus, the function of dys-
troglycan at inhibitory synapses could account for cognitive symp-
toms in these patients, but the mechanism of dystroglycan’s function
at inhibitory synapses is enigmatic. Similarly, neurexin gene mutations
have been associated with cognitive impairments in human patients.
Neurexin-3 (NRXN3) mutations were found in families with neu-
ropsychiatric disorders44,45, and NRXN3 is a class 1 gene in the SFARI
autism gene database (https://gene.sfari.org/database/gene-scoring/),
but again how NRXN3 mutations predispose to cognitive impairments
is unclear.
The present study was motivated by the unexpected finding that
neurexin-3 (Nrxn3) in mice is essential for the normal release prob-
ability of inhibitory synapses in the olfactory bulb (OB), particularly at
the granule cell→mitral cell (GC→MC) synapse, which constitutes one
half of granule cell-mitral cell dendrodendritic synapses35. Here we
demonstrate that only Neurexin-3α (Nrxn3α), but not Neurexin-3β
(Nrxn3β), supports inhibitory synaptic transmission in dissociated
olfactory bulb cultures and GC→MC synapses in vivo. We show that the
function of Nrxn3α is controlled by a hierarchical splice code involving
SS2 and SS4 of Nrxn3α, such that either SS2 or SS4 need to lack an
insert. Unexpectedly, a minimal Nrxn3α construct that contains only a
single LNS2-domain without an insert in SS2 and binds to dystroglycan
fully supports inhibitory synaptic transmission in culture and GC→MC
synaptic transmission in vivo; moreover, Nrxn3α performs a similar
function in inhibitory synapses of the medial prefrontal cortex (mPFC)
with the same dependence on LNS2-domain alternative splicing at SS2.
In contrast, postsynaptic deletion of Nrxn3 in mitral cells does not
impair GC→MC synaptic transmission. Both at GC→MC synapses and at
inhibitory synapses of the mPFC, postsynaptic dystroglycan deletions
produce a similar phenotype as presynaptic Nrxn3 deletions. Our data
thus indicate that binding of presynaptic Nrxn3α to postsynaptic
dystroglycan enables a normal presynaptic release probability at
inhibitory synapses, which may explain -at least in part- why mutations
in Nrxn3 and in dystroglycan-associated proteins induce cognitive
impairments.
Results
Nrxn3α, but not Nrxn3β, restores inhibitory synapse function in
Nrxn3-deficient OB neurons
Neurexin deletions at many synapses cause a decrease in release
probability5–7. To explore the mechanisms involved, we focused on
inhibitory synapses formed upon mitral cells in the OB, where Nrxn3 is
essential for a normal release probability35. We used rescue experi-
ments to elucidate the Nrxn3 sequences required for a normal release
probability at these synapses, and tested SS4 splice variants because
nearly all known functions of neurexins depend on this site of alter-
native splicing.
We infected mixed neuron-glia cultures from the OB of Nrxn3 cKO
mice with lentiviruses expressing inactive (ΔCre, as a control) or active
Cre-recombinase (Cre), without or with co-expression of Nrxn3αSS4+ or
Nrxn3αSS4- (containing or lacking an insert in SS4, respectively), or
Nrxn3βSS4+ (containing an insert in SS4) (Fig. 1a). To permit neuron-
specific expression, we used the human synapsin-1 promoter. We then
performed whole-cell patch-clamp recordings from larger mitral/tuf-
ted cells (referred to as MCs), which can be clearly distinguished from
smaller inhibitory granule cells and other neurons35,46,47, and used
extracellular stimulation to evoke IPSCs. Since granule cells (GCs) are
by far the most abundant inhibitory neurons in the OB (comprising
~82%, see Fig. S3e)48, the evoked IPSCs largely reflect GC→MC synaptic
transmission in cultured OB neurons, although they likely also contain
contributions from other inhibitory neurons.
The Nrxn3 deletion severely impaired (60–80% decrease) evoked
IPSCs in cultured OB neurons (Fig. 1b, c), consistent with earlier
results35. This impairment was rescued by Nrxn3αSS4+ or Nrxn3αSS4-,
which did not affect evoked IPSCs in control neurons (Figs. 1b, c, S1a).
Nrxn3βSS4+, however, suppressed evoked IPSCs both in Nrxn3-deficient
and in control neurons (Fig. 1b, c). In the initial experiments, this effect
was not statistically significant, but independent replication experi-
ments confirmed that expression of Nrxn3βSS4+ in WT neurons
decreased evoked IPSC amplitudes ~50% (Figs. 1d, e, S1c). Measure-
ments of evoked excitatory synaptic transmission, monitored as
NMDAR-dependent evoked EPSCs, failed to detect any changes
induced by the Nrxn3 deletion or by expression of Nrxn3αSS4+,
Nrxn3αSS4-, or Nrxn3βSS4+, suggesting that the effect of the Nrxn3
deletion is specific for inhibitory synapses in OB neurons (Figs. 1f–i,
S1b, d).
In agreement with the results on evoked IPSCs, deletion of Nrxn3
greatly lowered (50–80% decrease depending on the experiment) the
spontaneous mIPSC frequency but not the mIPSC amplitude in mitral/
tufted cells in OB cultures (Fig. S1e-S1i). This decrease was also rescued
by expression of Nrxn3αSS4+ or Nrxn3αSS4- (Fig. S1h,
i), whereas
expression of Nrxn3βSS4+ decreased the mIPSC frequency again both in
Nrxn3-deficient and control neurons (Fig. S1h–k). The impairment in
inhibitory neuron→MC synaptic transmission in cultured Nrxn3-defi-
cient OB neurons was not due to a decrease in synapse numbers or
GABAA-receptor function because no change in inhibitory synapse
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numbers or synaptic surface GABAA-receptor levels was detected
(Figs. 1j, k, S2). This includes inhibitory synapses co-labeled without or
with Homer1 puncta, corresponding to putative non-reciprocal and
reciprocal synapses, respectively.
Viewed together, these data suggest that in olfactory inhibitory
synapses, Nrxn3 is essential for synaptic transmission in a manner that
can be rescued by Nrxn3αSS4+ or Nrxn3αSS4-, but not by Nrxn3βSS4+.
Across the brain, neurexins exhibit dynamic regulation of SS4 alter-
native splicing, with the OB and cerebellum expressing almost exclu-
sively SS4 + variants (Fig. S3a–d). RT-PCR measurements revealed that
Nrxn3αSS4+ is highly enriched in inhibitory OB neurons, ~82% of which
are granule cells48 (Fig. S3e), as measured using RiboTag pulldowns of
translating mRNAs in vGAT-positive neurons (Fig. S3). In contrast,
Nrxn3βSS4+ and Nrxn3βSS4- are nearly undetectable, suggesting that the
dominant-negative action of Nrxn3βSS4+ is likely not physiologically
important for the OB (Fig. S3).
A combinatorial splice controls Nrxn3α function at inhibitory
synapses
Since Nrxn3α rescued the Nrxn3 KO phenotype independent of SS4
alternative splicing, we turned our attention to the only other alter-
natively spliced sequence of neurexins that is well established to reg-
ulate ligand interactions, SS2. SS2 is present in the LNS2 domain of α-
neurexins, which is lacking in β-neurexins21, and controls binding of
neurexophilins and dystroglycan to neurexins26–30,49–51. SS2 is expres-
sed in three variants, SS2- lacking an insert, SS2a containing an 8
amino-acid ‘a’ insert, and SS2ab containing an additional 7 amino-acid
‘b’ insert21.
RT-PCR measurements revealed that in most brain regions, Nrxn1
and Nrxn3 are predominantly expressed as SS2- variants, whereas
approximately half of the Nrxn2 mRNAs are present as SS2a variants
(Fig. S4a–d). SS2ab variants are uniformly rare (<10%) for all neurexins
and all brain regions. In the OB, Nrxn3α is predominantly expressed as
the Nrxn3αSS2- variant, with Nrxn3αSS2a accounting for ~10% and
Nrxn3αSS2ab < 4% of transcripts (Fig. S4a–d). Using RT-PCR with mRNAs
isolated by RiboTag purification from inhibitory neurons (~83% of
which are granule cells, Fig. S3) and from mitral cells of the OB, we
found that Nrxn3α is expressed in inhibitory neurons exclusively as the
Nrxn3αSS2- variant (>99%), whereas in mitral cells Nrxn3αSS2- and
Nrxn3αSS2a variants are produced almost equally (~55% vs 45%; Fig. S4e,
S4f). Thus, SS2 alternative splicing of Nrxn3 is highly regulated in the
OB, but its pattern of alternative splicing is distinct from that of SS4
which is present as the SS4 + variant in the entire OB (Fig. S3).
Does alternative splicing at SS2 regulate the ability of Nrxn3α to
rescue the Nrxn3 KO phenotype in inhibitory synapses? To address this
question, we probed the effects of both SS1 and SS2 alternative spli-
cing on inhibitory neuron→MC synaptic transmission in cultured
neurons. We included SS1 alternative splicing in the analysis because
SS1 is dynamically spliced in the brain18,19,52 and its adjacency to SS2
may have a peripheral effect on neurexophilin-binding to neurexins29.
Rescue experiments of Nrxn3 KO neurons from the OB with dif-
ferent SS1, SS2, and SS4 variants of Nrxn3α uncovered a surprising
finding: When SS4 lacked an insert, both Nrxn3αSS2- and Nrxn3αSS2a
rescued, but Nrxn3αSS2ab was inactive (Figs. 1l, m, S4g). However, when
SS4 contained an insert as in almost all Nrxn3 transcripts in the OB,
only Nrxn3αSS2- but not Nrxn3αSS2a or Nrxn3αSS2ab was able to rescue
(Figs. 1n, o, S4h). SS1 alternative splicing had no effect on rescue. Since
Nrxn3 is expressed in inhibitory neurons, including granule cells,
exclusively as the SS4 + variant (Fig. S3h), Nrxn3 alternative splicing at
SS2 controls Nrxn3 function at inhibitory neuron→MC synapses,
at least as monitored in cultured neurons. Overall, these data suggest
that alternative splicing of Nrxn3α at SS2 and SS4 govern its function at
inhibitory neuron→MC synapses of the OB, such that either SS2 or SS4
(or both) needs to lack an insert in order for Nrxn3α to sustain synaptic
function.
Nrxn3α-LNS2SS2- containing a single LNS-domain fully sustains
inhibitory synapse function
We constructed a series of deletion constructs of Nrxn3α to determine
which domains are required to support inhibitory neuron→MC synaptic
transmission (Fig. 2a, d). Rescue experiments with constructs that
include various extracellular LNS- and EGF-domains revealed that a
minimal Nrxn3α protein containing only a single LNS domain, the LNS2
domain in which SS2 is located, was sufficient to reverse the Nrxn3α KO
phenotype (Figs. 2b–h, S4i–k). In the minimal Nrxn3α-LNS2 constructs,
the LNS2 domain is fused to the glycosylated stalk region of Nrxn3α that
separates the last LNS domain from the membrane and that is followed
by the transmembrane region and cytoplasmic tail (Fig. 2d). Impor-
tantly, the minimal Nrxn3α-LNS2 construct only rescued inhibitory
synaptic transmission in Nrxn3 KO neurons when SS2 in the LNS2-
domain lacked an insert (Nrxn3α-LNS2SS2-). Even the introduction of the
short ‘a’ insert (Nrxn3α-LNS2SS2a) completely abolished rescue (Figs. 2g,
h, S4k). Thus, surprisingly, a single LNS domain of Nrxn3α is sufficient to
maintain inhibitory synaptic transmission, indicating that most domains
comprising the Nrxn3α architecture are dispensable for its function
within inhibitory neuron→MC synapses.
Nrxn3α-LNS2SS2- sustains GC→MC synaptic transmission in vivo
without affecting synapse numbers
The finding that Nrxn3α-LNS2SS2- is sufficient to rescue inhibitory
neuron→MC synaptic transmission in cultured OB neurons is surpris-
ing, raising the possibility of experimental artifacts. Moreover,
although granule cells are the most prevalent type of inhibitory neuron
in the OB, culture experiments do not permit us to specifically test
GC→MC synaptic transmission. Thus, we decided to examine the
phenotype of the Nrxn3 deletion and its rescue by Nrxn3α-LNS2SS2- in
GC→MC synapses in vivo.
We stereotactically infected the OB of Nrxn3 cKO mice at P21 with
AAVs encoding either ΔCre (as a control) or Cre (to delete Nrxn3),
without or with Nrxn3α-LNS2SS2- or Nrxn3α-LNS2SS2a rescue constructs
(Fig. 3a). ΔCre and Cre were expressed as tdTomato fusion proteins to
visualize AAV-infected cells. Morphological studies of OB sections of
infected mice at P35-42 showed that tdTomato was nearly ubiquitously
expressed (Fig. 3b). The Nrxn3α-LNS2SS2- and Nrxn3α-LNS2SS2a con-
structs include an extracellular N-terminal HA-epitope tag, enabling us
to visualize them by immunocytochemistry of OB sections (Fig. 3c).
The two minimal Nrxn3α-LNS2 proteins were similarly distributed in
the synaptic strata of the OB (Fig. 3c), suggesting that this approach is
well suited for probing Nrxn3 function in the OB in vivo.
We next examined the effect of the Nrxn3 KO and of the Nrxn3α-
LNS2SS2- and Nrxn3α-LNS2SS2a rescue in the OB on the density of GC→MC
synapses. We stained OB sections for synaptoporin and gephyrin, which
are specific markers for GC→MC synapses in particular, and for inhibi-
tory synapses in general53–55. We then analyzed the density of gephyrin-
and synaptoporin-positive puncta as well as that of puncta containing
both signals (Fig. 3d, S5a). The results were analyzed using either the
number of mice or the number of regions-of-interest (ROI’s) as the
statistical ‘n’ because the former is likely more correct, but the latter is a
common practice despite the fact that the number of ROI’s not actually
true replicates, but represent pseudo-replicates that boost statistical
power. Using both statistical approaches, we detected no change in
GC→MC synapse density except for a statistically significant 10%
decrease using pseudo-replicate quantifications that was observed only
for the gephyrin puncta density measurements (Figs. 3e, S5b). Viewed
together, these results indicate that the Nrxn3 KO in the OB does not
cause a major change in synapse numbers.
Do in vivo deletions of Nrxn3 cause an impairment in GC→MC
synapse function that can be rescued by the minimal Nrxn3α-LNS2SS2-
construct, analogous to what we observed with inhibitory neuron→MC
synapses in OB cultures? We addressed this critical question using
whole-cell patch-clamp recordings from mitral cells in acute OB slices.
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Article
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a Structures of Nrxn3α rescue constructs with domain deletions
Nrxn3α (SS2- & SS4-)
1
A
2
3
B
4
5
C
6
SS2
SS4
Nrxn3α-LNS1-4 (SS2-)
1
A
2
3
B
4
Nrxn3α-LNS4-6 (SS4-)
4
5
6C
SS2
SS4
b
∆Cre (14/3)
IPSCs
Cre +
Nrxn3α SS4- & SS2
(10/3)
-
Cre (15/3)
Nrxn3α-
LNS4-6 SS4 -
(15/3)
c
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
12
8
4
2
0
Nrxn3α-
LNS1-4 SS2-
(15/3)
A
n
2
0.5 s
IPSC amplitudes
**
***
SS2-
SS4-
∆CreCre
Nrxn3αSS4- & SS2-
Nrxn3α-LNS4-6
Nrxn3α-LNS1-4
Cre +
Nrxn3α-LNS2-3
(SS2-, SS2a, SS2ab)
2
SS2
d
Structures of minimal Nrxn3α LNS2 rescue constructs
Nrxn3α-LNS2-3
(SS2-)
e
∆Cre (14/3)
2
3
SS2
IPSCs
Cre +
Nrxn3 α SS4- & SS2-
(22/4)
Nrxn3 α-
LNS2 SS2-
(14/3)
Cre (30/6)
Nrxn3 α-
LNS2-3 SS4-
(15/3)
g
∆Cre (10/3)
Cre (9/3)
A
n
2
0.5 s
Cre +
h
IPSCs
Cre +
Nrxn3 α-
LNS2 SS2-
(10/3)
Nrxn3 α-
LNS2 SS2ab
(15/3)
-
Nrxn3 α
LNS2 SS2a
(14/3)
A
n
2
0.5 s
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
12
8
8
6
4
2
0
f
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
IPSC amplitudes
***
12
8
8
6
4
2
SS2-
0
SS2-
∆CreCre
Nrxn3αSS4- & SS2-
Nrxn3α-LNS2-3
Nrxn3α-LNS2
IPSC amplitudes
** *
∆CreCre
SS2-
SS2a
SS2ab
Nrxn3α-LNS2
Nrxn3α-LNS2
Nrxn3α-LNS2
Cre +
In addition to receiving extensive synaptic inhibition from granule
cells, MCs also receive synaptic inhibition from periglomerular cells
and EPL interneurons56. Thus, mIPSCs monitored in MCs include
events produced by non-GC inhibitory neurons, though these synap-
ses are much fewer in numbers and are located further from the soma,
suggesting that their responses will be poorly detected due to den-
dritic filtering. No change in the passive electrical properties of mitral
cells was evident after the Nrxn3 KO (Fig. S5c, d). Recordings of mIPSCs
showed that the in vivo Nrxn3 KO robustly decreased (60%) the mIPSC
frequency; this decrease was fully rescued by Nrxn3α-LNS2SS2- but not
by Nrxn3α-LNS2SS2ab (Fig. 4a, b). No significant change in the mIPSC
amplitude or kinetics was detected (Figs. 4c, S5e). A recent study found
that Nrxn3 differentially regulates the formation and/or function of a
subset of inhibitory synapses in the ventral subiculum of the hippo-
campus in a sex-dependent manner57. However, separate analyzes of
slices from male and female mice exhibited a similar decrease in mIPSC
frequency (Fig. S5f), indicating that not all contributions of Nrxn3 to
inhibitory synapse function are sex-dependent.
Fig. 2 | A minimal protein composed of the single LNS2-domain of Nrxn3α
attached to its C-terminal stalk sequence, transmembrane region and cyto-
plasmic sequence fully rescues inhibitory synaptic transmission in cultured
Nrxn3-deficient OB neurons. All experiments were performed in dissociated OB
cultures obtained from newborn Nrxn3 cKO mice that were infected with lenti-
viruses expressing ΔCre (control) or Cre without or with the indicated rescue
proteins. a Schematic of Nrxn3α rescue constructs with domain deletions.
b, c Nrxn3α lacking LNS5 and LNS6 domains fully rescues impaired inhibitory
synaptic transmission in Nrxn3-deficient OB neurons, whereas Nrxn3α lacking
LNS1-3 domains does not (b, sample traces; c, summary graphs of IPSC amplitudes).
d Schematic of minimal Nrxn3α rescue constructs. e, f A minimal Nrxn3α protein
containing only LNS2 without an SS2 insert linked to the C-terminal Nrxn3α
sequences, rescues impaired inhibitory synaptic transmission in Nrxn3-deficient OB
neurons (e, sample traces; f, summary graphs of IPSC amplitudes). g, h The minimal
Nrxn3α protein containing only LNS2 is unable to rescue inhibitory synaptic
transmission in Nrxn3-deficient OB neurons if SS2 contains an insert (g, sample
traces; h, summary graphs of IPSC amplitudes). Numerical data are means ± SEM;
n’s (cells/experiments) are indicated above the sample traces and apply to all
graphs in an experimental series. Statistical analyzes were performed with a one-
way analysis of variance (ANOVA) with Dunnett’s multiple comparison test, with
*p < 0.05, **p < 0.01, and ***p < 0.001. Source data and statistical results for all
experiments are provided within the Source Data file.
To selectively probe GC→MC synaptic transmission, we measured
evoked IPSCs elicited by extracellular simulation of the granule cell
layer in acute slices. This stimulation paradigm allowed us to avoid
activation of other interneurons that synapse upon mitral cells,
including periglomerular and EPL interneurons. To control for possible
effects caused by variations in the placement of the stimulating elec-
trode, we analyzed evoked IPSCs as input/output curves (Fig. 4d).
Again, the Nrxn3 KO caused a large impairment (~50% decrease) that
was fully reversed by Nrxn3α-LNS2SS2- but not by Nrxn3α-LNS2SS2ab
(Fig. 4d–f). No change in IPSC kinetics was detectable (Fig. 4g). How-
ever, we found a large increase in the coefficient of variation of IPSCs
that also was rescued by Nrxn3α-LNS2SS2- but not by Nrxn3α-LNS2SS2ab
(Fig. 4h). This increase is indicative of a decrease in release
probability58. To confirm a decrease in release probability, we mea-
sured the paired-pulse ratio of GC→MC IPSCs as a function of the
interstimulus interval (Fig. 4i). The Nrxn3 KO caused a massive increase
in the paired-pulse ratio consistent with a decrease in release prob-
ability; this increase was also completely reversed by Nrxn3α-LNS2SS2-
but not by Nrxn3α-LNS2SS2ab (Fig. 4j). Although neurexins are well
established to act presynaptically, we tested a possible postsynaptic
contribution of Nrxn3 at the GC→MC synapse by investigating the
effect of selective postsynaptic deletion of Nrxn3 in mitral cells. For
this purpose, we infected the piriform cortex of Nrxn3 cKO mice with
retro-AAVs encoding ΔCre or Cre fused to tdTomato59. The retro-AAVs
infect the mitral cell axon terminals in the piriform cortex, thereby
inducing selective expression of ΔCre or Cre in mitral cells of the OB
that can be identified by the presence of tdTomato. We detected no
change in GC→MC synaptic transmission after postsynaptic deletion of
Nrxn3 in mitral cells (Fig. S6), indicating that Nrxn3 acts presynaptically
in regulating inhibitory synaptic transmission. Viewed together, these
data show that the in vivo deletion of Nrxn3 severely impairs GC→MC
synaptic transmission by suppressing, at least in part, the release
probability. Moreover, consistent with our findings in cultured neu-
rons, these data show that a minimal Nrxn3α construct containing only
the LNS2 domain can rescue this impairment in a manner regulated by
alternative splicing at SS2.
Impaired inhibitory synaptic strength in Nrxn3-deficient mPFC
neurons is also rescued by the minimal Nrxn3α-LNS2SS2-
construct
GC→MC synapses of the OB are part of reciprocal dendrodendritic
synapses that may differ from ‘standard’ inhibitory synapses in the
Nature Communications |
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Article
a
Exptl. Design
b
AAVDJ expressing
ΔCre-tdT or Cre-tdT without or
with Nrxn3 rescue constructs
OB
Representative images of the OB
GL
GCL
GCL
MCL
EPL
0.5 mm
MCL
EPL
250 μm
Nrxn3 cKO mice
tdTomato/
Neurobiotin
∆Cre
Cre
Cre + N3-L2 SS2-
Cre + N3-L2 SS2ab
c
GCL
MCL
EPL
GL
HA-Nrxn3 / TdT
HA-Nrxn3 / TdT
HA-Nrxn3 / TdT
100 μm
HA-Nrxn3 / TdT
d
∆Cre
Cre
Cre + N3-L2 SS2-
GCL
MCL
EPL
GL
MCL
EPL
GCL
MCL
EPL
MCL
EPL
GL
GCL
MCL
EPL
GL
MCL
EPL
Cre + N3-L2 SS2ab
GCL
MCL
EPL
GL
MCL
EPL
100 μm
20 μm
SypII / Geph / TdT SypII / Geph / TdT SypII / Geph / TdT SypII / Geph / TdT
e
Inhibitory Synapse
Density
Synaptoporin
Density
Gephyrin
Density
)
2
m
μ
/
a
t
c
n
u
P
(
y
t
i
s
n
e
D
)
2
m
μ
/
a
t
c
n
u
P
(
y
t
i
s
n
e
D
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
5
/
7
7
3
/
4
4
5
/
5
7
4
/
7
5
*** **
0.4
0.2
0
0.6
0.4
0.2
0.20
0.15
0.10
0.05
0
0.3
0.2
0.1
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
Cre +
Cre +
Cre +
CNS. The surprising finding that alternative splicing of Nrxn3α reg-
ulates this synapse via the activity of a single LNS domain could
represent an exceptional mechanism that is specific to dendroden-
dritic synapses and not shared by other inhibitory synapses.
To ask whether the LNS2-dependent function of Nrxn3 at GC→MC
synapses of the OB may apply to other types of inhibitory synapses, we
first tested cultured cortical neurons. The Nrxn3 KO nearly halved the
https://doi.org/10.1038/s41467-023-36872-8
Fig. 3 | Conditional in vivo deletion of Nrxn3 in the OB has no effect on synapse
density, nor does rescue of the Nrxn3 deletion by minimal Nrxn3 LNS2-domain
constructs. a, b Experimental design of in vivo Nrxn3 deletions and rescues. The
OB was stereotactically infected with AAVs expressing ΔCre (control), Cre, or Cre
with additional AAVs encoding Nrxn3-LNS2 rescue constructs (a, schematic of
stereotactic injections; b, representative fluorescence image of an OB section that
was infected with AAVs expressing tdTomato fused to Cre, with subsequent
patching of mitral cells that were filled with neurobiotin (blue)). Neuron-specific
expression of ΔCre/Cre and rescue constructs was achieved using the synapsin-1
promoter. Note that AAVs infect granule cells more efficiently than mitral cells (see
panel b). c Minimal Nrxn3α-LNS2 rescue proteins are localized to synaptic layers in
the OB after expression via AAVs, as visualized by immunocytochemistry for the
N-terminal HA-epitope contained in the constructs (white, HA-Nrxn3α-LNS2 pro-
teins; red, tdTomato). d, e Conditional Nrxn3 deletion and rescue with minimal
Nrxn3α-LNS2 constructs does not alter inhibitory synapse numbers in vivo. Sec-
tions from mice (infected as shown in A) were analyzed by quantitative immuno-
cytochemistry for the presynaptic marker synaptoporin (a.k.a. synaptophysin-2;
light blue) that is specific for granule cell→mitral cell synapses in the OB, and for the
postsynaptic inhibitory synapse marker gephyrin (green) (d, sample images;
e, summary graphs of puncta densities). Data are means ± SEM; n’s (cells/experi-
ments) indicated in summary graph bars apply to all graphs in an experimental
series. Statistical analyzes using one-way ANOVA with Tukey’s multiple comparison
test (e). Appropriate HA labeling in c was confirmed in tissue from all animals
quantified in e. Puncta densities in e are analyzed both per animal (top) and per
region-of-interest (bottom); statistical significance is observed for gephyrin stain-
ing but not the other parameters when regions-of-interest are used as n’s because
pseudo-replicates in this analysis boost statistical significance independent of the
actual number of experiments. Source data and statistical results for all experi-
ments are provided within the Source Data file.
amplitude and synaptic charge transfer of evoked IPSCs in these
neurons (Fig. S7a, b). This decrease was rescued both by full-length
Nrxn3α lacking inserts in SS2 and SS4, and by the minimal Nrxn3α-
LNS2SS2- protein (Fig. S7a, b).
Next, we deleted Nrxn3 from mPFC neurons in vivo using ste-
reotactic injections of AAVs expressing ΔCre (as a control) or Cre, with
or without co-expression of Nrxn3α-LNS2SS2- similar to the in vivo OB
experiments (Fig. 5a). A total of 2–3 weeks after infection, we sectioned
acute slices from the mice and patched Layer 5/6 neurons for elec-
trophysiological recordings (Fig. 5b). Measurements of spontaneous
mIPSCs uncovered a robust decrease in mIPSC frequency (~25%) in
Nrxn3-deficient neurons (Fig. 5c, d), suggesting that a subset of the
heterogeneous inhibitory synaptic inputs on Layer 5/6 neurons may
have been impaired by the Nrxn3 KO similar to GC→MC synapses.
Expression of Nrxn3α-LNS2SS2- fully rescued the decrease in mIPSC
frequency in Nrxn3 KO synapses. Moreover, we observed a small
decrease in the mIPSC amplitude induced by the Nrxn3 KO that was not
rescued by the Nrxn3α-LNS2SS2- construct (Fig. 5e), suggesting a dif-
ferent additional role for Nrxn3 in postsynaptic GABAAR function in the
mPFC. Such a role was not detected in our in vivo OB experiments,
consistent with the notion that neurexins and their ligands are
expressed in distinct combinatorial patterns in various types of neu-
rons and thus different degrees of redundancy may occur among
neurexins and their ligands in these types of neurons. This notion is
further supported by our previous observation that a significant loss of
GABAAR function was observed following the deletion of neuroligins in
mitral cells of the OB59. No changes in the passive electrical properties
of the pyramidal mPFC neurons or in the mIPSC kinetics were detected
(Fig. S7c–e).
Next, we monitored evoked IPSCs, again using input/output
measurements to control for possible variations in the placement of
the stimulating electrode, even though -as always- all experiments
were conducted ‘blindly’ (Fig. 5f). The Nrxn3 deletion greatly sup-
pressed the synaptic strength of evoked IPSCs (~50% decrease), which
could be fully rescued by the Nrxn3α-LNS2SS2- construct (Fig. 5g, h). In
addition, the Nrxn3 KO caused a slowing of the IPSC rise but not decay
Nature Communications |
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Article
https://doi.org/10.1038/s41467-023-36872-8
a
b
1.0
n
o
i
t
c
a
r
f
e
v
i
t
a
u
m
u
C
l
d
0.5
0
0
∆Cre (18/3)
mIPSC traces
Cre + Nrxn3α-LNS2 SS2- (18/3)
Cre (18/3)
Cre + Nrxn3α-LNS2 SS2ab (18/3)
A
p
0
5
500 ms
c
1.0
n
o
i
t
c
a
r
f
e
v
i
t
a
u
m
u
C
l
0.5
mIPSC amplitude
180
125
100
75
50
25
)
A
p
(
e
d
u
t
i
l
p
m
A
0
∆CreCre
N3-L2
SS2-
N3-L2
Cre +
SS2ab
0
0
150
300
450
600
Amplitude (pA)
GC MC evoked IPSC amplitudes
∆Cre
Cre +
Nrxn3α-LNS2 SS2-
*
*
*
*
mIPSC frequency
***
**
)
z
H
(
y
c
n
e
u
q
e
r
F
20
15
10
5
0
∆CreCre
N3-L2
SS2-
N3-L2
Cre +
SS2ab
1
2
Inter-stimulus interval (s)
3
GC MC evoked IPSC traces
∆Cre (17/4)
Cre (16/4)
Cre +
Nrxn3α-LNS2 SS2- (14/4)
Cre +
Nrxn3α-LNS2 SS2ab (20/4)
4
e
2.0
1.5
1.0
0.5
)
A
n
(
e
d
u
t
i
l
p
m
a
k
a
e
P
Cre
Cre +
Nrxn3α-LNS2 SS2ab
A
n
1
20 ms
0.0
0
20
f
s
e
v
r
u
c
t
u
p
u
o
t
/
t
u
p
n
i
f
o
e
p
o
S
l
i
Input/output
relationship
**
**
40
30
20
10
g IPCS rise times
3
2
1
)
s
m
(
e
m
i
t
e
s
R
i
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
60
40
80
Stimulus intensity (μA)
h
100
IPSC decay times
n
o
i
t
a
i
r
a
v
f
o
t
i
n
e
c
i
f
f
e
o
c
C
S
P
I
30
20
10
)
s
m
(
e
m
i
t
y
a
c
e
D
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
Coeff. of variation
***
***
0.8
0.5
0.4
0.3
0.2
0.1
0
SS2ab
∆CreCre
N3-L2
SS2-
N3-L2
Cre +
Cre +
Paired-pulse IPSCs
∆Cre (14/3)
Cre (13/3)
Cre +
Nrxn3α-LNS2 SS2- (10/3)
Cre +
Nrxn3α-LNS2 SS2ab (17/3)
j
1.25
l
o
i
t
a
r
e
s
u
p
-
d
e
r
i
a
P
1.00
0.75
Cre +
Cre
Cre +
Paired-pulse ratio
Cre + Nrxn3α-LNS2 SS2ab
*
*
*
*
∆Cre
Cre + Nrxn3α-LNS2 SS2-
0.50
A
n
1
20
100 ms
100
50
Inter-stimulus interval (ms)
500
times, again with full rescue by Nrxn3α-LNS2SS2- (Fig. 5i). Finally, the
Nrxn3 KO induced a large increase (~120%) in the coefficient of varia-
tion of evoked IPSCs in mPFC synapses similar to OB GC→MC synapses,
and this phenotype was also reversed by Nrxn3α-LNS2SS2- (Fig. 5j).
Together these data indicate that Nrxn3α performs a similar function
in a subset of inhibitory synapses in the mPFC as in GC→MC synapses in
the OB, namely an essential role in sustaining the normal release
probability at these synapses, such that the Nrxn3 deletion ablates
nearly half of the inhibitory synaptic strength in a manner that can be
rescued by the Nrxn3α-LNS2SS2- construct.
Nature Communications |
(2023) 14:1771
7
Article
https://doi.org/10.1038/s41467-023-36872-8
Fig. 4 | Conditional in vivo deletion of Nrxn3 in the OB severely impairs GC→MC
inhibitory synaptic transmission by lowering the release probability: Rescue
by minimal Nrxn3-LNS2 constructs lacking an insert in SS2. All experiments
were performed by patch-clamp recordings from mitral cells in acute slices from
Nrxn3 cKO mice whose OB was infected with AAVs (see Fig. 3a, S5c–e). a–c The
Nrxn3 deletion decreases the mIPSC frequency; this decrease is rescued only by the
minimal Nrxn3α-LNS2 construct lacking an insert in SS2 (a, representative mIPSC
traces recorded in the presence of TTX; b, c cumulative probability of the mIPSC
interevent intervals and amplitudes, insets: summary of the mIPSC frequency and
amplitudes). d–f The Nrxn3 deletion decreases the evoked IPSC amplitude as
documented by input/output curves. This decrease is rescued only by the minimal
Nrxn3α-LNS2 construct without an insert in SS2 (d, representative IPSC traces; e,
summary of input/output amplitudes; f, summary of the slope of input/output
curves). g The Nrxn3 deletion and expression of rescue constructs have no effect on
evoked IPSC kinetics (summary of the IPSC rise (left) and decay times (right)). h The
Nrxn3 deletion increased the coefficient of variation of IPSCs, suggesting a decrease
in release probability; this phenotype is rescued only by the minimal Nrxn3α-LNS2
construct without an insert in SS2. i, j The Nrxn3 deletion induces a large increase in
the paired-pulse ratio; this phenotype is rescued by the minimal Nrxn3α-LNS2
construct without an insert in SS2 (i, representative traces; j, summary of the
paired-pulse ratio). Numerical data are means ± SEM; n’s (cells/experiments) are
indicated above the sample traces and apply to all graphs in an experimental series.
Statistical analyzes were performed using two-way ANOVA in e and j and one-way
ANOVA in b, c, and f–h with Dunnett’s and Tukey’s multiple comparison test
respectively with regards to the ΔCre group, with *p < 0.05, **p < 0.01, ***p < 0.001,
***p < 0.001, and ****p < 0.0001. Source data and statistical results for all experi-
ments are provided within the Source Data file.
CRISPRi-mediated inhibition of dystroglycan expression phe-
nocopies the Nrxn3 KO at GC→MC synapses in cultured OB
neurons
The requirement and sufficiency of the minimal Nrxn3α-LNS2SS2-
construct that contains a single extracellular interaction domain
(LNS2) with a specific splice variant (SS2-) for synaptic transmis-
sion at a subset of inhibitory synapses suggests that Nrxn3α
functions by binding to a trans-synaptic ligand. At present, only
one ligand is known to specifically bind to the LNS2 domain of
neurexins lacking an insert in SS2: dystroglycan26,30. Neurexophilin
also binds to the LNS2 domain, but its binding is enhanced instead
of impeded by an insert in SS227–29. Notably, dystroglycan also
binds to the LNS6 domain of α-neurexins when the LNS6 domain
lacks an insert in SS4, accounting for the finding that full-length
Nrxn3α is still functional at inhibitory neuron→MC synapses in OB
cultures when it contains a partial insert in SS2 (i.e., SS2a21), as long
as SS4 lacks an insert (Fig. 1)26.
To explore the possibility that dystroglycan may be the
postsynaptic ligand for presynaptic Nrxn3α at GC→MC synapses
that is required for sustaining their release probability, we selected
a guide RNA (gRNA) that enables potent CRISPR interference
(CRISPRi)-mediated inhibition of dystroglycan expression in cul-
tured OB neurons (Fig. 6a). Although an apparently incomplete
suppression of dystroglycan mRNA level (~65% decrease) was
observed, this is likely an underestimate of the neuronal dystro-
glycan suppression since the lentiviral expression of the CRISPRi
components is most efficient in neurons. Electrophysiological
recordings revealed that the CRISPRi-mediated partial inhibition
of dystroglycan expression induced a robust decrease (~60%) in
the amplitude of evoked IPSCs (Fig. 6b, c).
The suppression of IPSCs by the inhibition of dystroglycan
expression in cultured OB neurons (Fig. 6b, c) is similar to that
observed for the Nrxn3 KO (Fig. 1b, c). To test whether these two
adhesion molecules operate in the same pathway, we compared
the phenotypes of single and double dystroglycan and Nrxn3 KOs
by combining the conditional deletion of Nrxn3 with the CRISPRi-
mediated inhibition of dystroglycan expression in cultured OB
neurons from Nrxn3 cKO mice. We infected the neurons with len-
tiviruses expressing either ΔCre (control) or Cre (to delete Nrxn3)
and/or dCAS9-KRAB and the dystroglycan gRNA, and measured
evoked IPSCs and NMDAR- and AMPAR-mediated EPSCs. The
dystroglycan and Nrxn3 deletions individually and together
induced a 60-70% decrease in IPSC amplitudes and charge trans-
fer, with no aggravation of the phenotype by the combined dele-
tion compared to the individual deletions (Fig. 6d–f). None of the
deletions, individually or combined, had a significant effect on
NMDAR- or AMPAR-EPSCs (Fig. 6g–i). These data suggest that
Nrxn3 and dystroglycan act in the same pathway to sustain inhi-
bitory neuron→MC synaptic transmission, consistent with the
notion that they function by binding to each other.
Synaptic localization of dystroglycan in the OB
In the OB, dystroglycan is prominently expressed by mitral cells
where it was localized to GC→MC reciprocal synapses via immu-
noelectron microscopy60. Consistent with the hypothesis that dys-
troglycan is the postsynaptic receptor for Nrxn3 in mitral cells,
Nrxn3 mRNA is relatively abundant in the granule cell layer, while
dystroglycan (Dag1) mRNA is enriched in mitral cells (Fig. S8a, b)61.
To confirm the synaptic localization of dystroglycan in the OB, we
optimized staining for dystroglycan with the IIH6C4 monoclonal
antibody using different times of post-fixation with 4% PFA (i.e.,
overnight, 20 minutes, and 10 minutes). We found that only light
post-fixation (i.e., less than 20 minutes) permitted robust detection
of dystroglycan in the synaptic neuropil and around blood vessels
(Figs. 7, S8c). It is well known that excessive cross-linking can hinder
access of epitopes needed to localize proteins located within the
synaptic cleft62. Using stimulated emission depletion (STED) super-
resolution microscopy, we found that dystroglycan nanoclusters
were abundant at inhibitory synapses in the EPL of the OB, including
reciprocal synapses, and in large inhibitory synapses located within
glomeruli (Figs. 7b–d, S8d–g). Given that STED was only performed
in 2D and synapses were viewed at random angles, the actual
number of inhibitory synapses with dystroglycan nanoclusters is
likely much higher. Consistent with prior studies34,41,42, using light
fixation conditions we also found that IIH6C4 labeled dystroglycan
at a subset of inhibitory synapses in the hippocampus and cere-
bellum (Figs. 7e–h, S8h–i). Thus, the localization of dystroglycan in
the OB is consistent with a potential role as a postsynaptic ligand of
Nrxn3 at GC→MC synapses.
Postsynaptic dystroglycan deletion in vivo recapitulates the
Nrxn3 KO phenotype in the OB and mPFC
To validate the results obtained with the inhibition of dystroglycan
expression in cultured OB neurons, we next investigated the effect of a
CRISPR-mediated deletion of dystroglycan in vivo in both the OB and
the mPFC (Figs. 8, 9). We used CRISPR-mediated deletions instead of
CRISPRi in these experiments because the components needed for
CRISPRi could not be encoded by a single AAV. In the first set of
experiments, we infected the OB of CAS9-expressing mice with AAVs
encoding the dystroglycan gRNA or a control gRNA and tdTomato, and
examined the efficiency of the dystroglycan deletion and the effect of
the deletion on the inhibitory synapse density (Figs. 8a–d, S9a–i). As
assessed by immunocytochemistry for dystroglycan, the CRISPR-
mediated dystroglycan deletion was efficient with a ~60% decline in
total dystroglycan signal (Fig. S9a, b). Again, this is likely an under-
estimate of the degree of the deletion of dystroglycans since our AAVs
are optimized for neuronal expression but much of the dystroglycan in
brain is expressed in cells surrounding blood vessels. Unlike localiza-
tion experiments with the well-validated IIH6C4 monoclonal antibody
(Figs. 7, S8), for this experiment, we employed a rabbit monoclonal
to avoid unspecific mouse
antibody against α-Dystroglycan63
Nature Communications |
(2023) 14:1771
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Article
https://doi.org/10.1038/s41467-023-36872-8
a
Experimental strategy
AAVDJ co-expressing
ΔCre-tdT or Cre-tdT without
or with rescue constructs
b
Infected mPFC images
Nrxn3 cKO
mice
2-3 weeks later
mIPSC
& IPSC
recordings
from L5
cortical neurons
tdTomato/Neurobiotin
0.5 mm
100 μm
c
d
1.0
n
o
i
t
c
a
r
f
l
e
v
i
t
a
u
m
u
C
0.5
0
0
f
ΔCre (22/3)
mIPSC traces
Cre (22/3)
Cre + Nrxn3α LNS2-SS2- (22/3)
A
p
0
5
200 ms
mIPSC frequency
Cre +
Nrxn3α LNS2-SS2-
ΔCre
Cre
)
z
H
(
y
c
n
e
u
q
e
r
F
40
30
20
15
10
5
0
*
e
1.0
n
o
i
t
c
a
r
f
l
e
v
i
t
a
u
m
u
C
0.5
∆Cre Cre
SS2-
Cre +
N3-L2
mIPSC amplitude
Cre +
Nrxn3α LNS2-SS2-
Cre
ΔCre
)
A
p
(
e
d
u
t
i
l
p
m
A
60
50
40
30
20
10
0
** *
∆Cre Cre
SS2-
Cre +
N3-L2
100
Inter-stimulus interval (ms)
200
IPSCs
ΔCre (16/3)
Cre (16/4)
Nrxn3α LNS2-SS2- (15/4)
300
g
6
4
2
)
A
n
(
e
d
u
t
i
l
p
m
a
k
a
e
P
r
0
0
40
80
120
Amplitude (pA)
Evoked IPSC amplitudes
Cre +
Nrxn3α LNS2-SS2-
ΔCre
*
*
*
*
Cre
A
n
2
20 ms
IPCS rise times
*
∆Cre Cre
SS2-
Cre +
N3-L2
i
)
s
m
(
e
m
i
t
e
s
R
i
6
5
4
3
2
1
0
h
Input/output
relationship
**
s
e
v
r
u
c
t
u
p
u
o
t
/
t
u
p
n
i
f
o
e
p
o
S
l
20
15
10
5
0
∆Cre Cre
SS2-
Cre +
N3-L2
)
s
m
(
e
m
i
t
y
a
c
e
D
130
129
100
80
60
40
20
0
0
0
10
20
30
40
50
Stimulus intensity (μA)
j
IPSC decay times
Coeff. of variation
**
0.5
0.4
0.3
n
o
i
t
a
i
r
a
v
f
o
t
i
n
e
c
i
f
f
e
o
C
0.2
0.1
0
∆Cre Cre
SS2-
Cre +
N3-L2
∆Cre Cre
SS2-
Cre +
N3-L2
secondary detection related to low levels of AAV-induced inflamma-
tion. Further confirming the efficacy of dystroglycan deletion, qRT-
PCR showed a 70% reduction in mRNA levels (S9c). Contrasting prior
reports that dystroglycan regulates the number of CCK + inhibitory
synapses in the hippocampus34,42 and maintains inhibitory synapses in
the cerebellum64, quantifications of the density of inhibitory synapses
visualized via immunocytochemistry for gephyrin and synaptoporin
failed to detect any change in synapse numbers or size in the external
plexiform layer, the area that contains GC→MC synapses (Figs. 8c,
d, S9d–i).
Nature Communications |
(2023) 14:1771
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Article
https://doi.org/10.1038/s41467-023-36872-8
Fig. 5 | Deletion of Nrxn3 in the medial prefrontal cortex (mPFC) impairs
inhibitory synaptic function in a manner dependent on alternative splicing at
SS2. a Experimental design for Nrxn3 deletion following stereotactic infections of
the mPFC with AAVs expressing ΔCre (control), Cre, or Cre with the minimal
Nrxn3α-LNS2 rescue constructs. b Representative fluorescence image of an mPFC
section from a mouse infected with AAVs expressing Cre-tdTomato (red) in which a
layer 5 pyramidal neuron was patched and filled with neurobiotin (expanded right
image; blue). c–e The Nrxn3 deletion decreases the mIPSC frequency and ampli-
tude in vivo; however, only the frequency decrease is rescued by the minimal
Nrxn3α-LNS2 construct lacking an insert in SS2 (c, representative mIPSC traces
recorded in the presence of TTX; d, e: cumulative probability of the mIPSC
interevent intervals and amplitudes, insets: summary of the mIPSC frequency and
amplitudes). f–h The Nrxn3 deletion suppresses the amplitude of IPSCs evoked by
extracellular stimulation in layer five and recorded from pyramidal neurons in layer
5 as documented by input/output curves. This phenotype is rescued by the minimal
Nrxn3α-LNS2 construct lacking an insert in SS2 (f, representative IPSC traces;
g, summary of input/output amplitudes; h, summary of the slope of the input/
output curves). i The Nrxn3 deletion increases the rise time of evoked IPSCs (left),
but not decay time (right), in a manner that can be rescued by the minimal Nrxn3α-
LNS2 rescue constructs. j The Nrxn3 deletion increases the coefficient of variation
of IPSCs, suggesting a decrease in release probability, in a manner that can be
rescued by the minimal Nrxn3α-LNS2 construct lacking an insert in SS2. Numerical
data are means ± SEM; n’s (cells/experiments) are indicated above the sample tra-
ces and apply to all graphs in an experimental series. Statistical analyzes were
performed using two-way ANOVA in g and one-way ANOVA in d, e, h–j with Dun-
nett’s and Tukey’s multiple comparison test with regards to the ΔCre group, with
*p < 0.05, **p < 0.01, and ****p < 0.0001. Source data and statistical results for all
experiments are provided within the Source Data file.
IPSCs
c
Dag1 gRNA (13/3)
Control (13/3)
IPSC amplitudes
a
Dag1 mRNA levels
b
l
s
e
v
e
L
A
N
R
m
1
G
A
D
)
H
D
P
A
G
f
o
%
(
100
75
50
25
0
**
3
3
Ctrl
Dag1
gRNA
d
IPSCs
e
∆Cre +
Cre +
Control
(12/3)
Dag1
gRNA
(14/3)
Control
(15/3)
Dag1
gRNA
(13/3)
A
n
2
0.5 s
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
A
n
2
0.5 s
IPSC amplitudes
*** **
14
10
6
4
2
0
-
Dag1
gRNA
-
Dag1
gRNA
∆Cre + Cre +
g
NMDAR- &
AMPAR-EPSCs
∆Cre +
Cre +
Control
Dag1
gRNA
Control
Dag1
gRNA
11/3
10/3
8/3
10/3
1 s
11/3
10/3
11/3
10/3
0.2 s
h
NMDAR-EPSCs
2.5
1.5
1.0
0.5
0
)
A
n
(
.
l
p
m
A
C
S
P
E
R
A
D
M
N
-
A
n
5
.
0
A
n
5
.
0
-
Dag1
gRNA
-
Dag1
gRNA
∆Cre + Cre +
)
A
n
(
e
d
u
t
i
l
p
m
A
C
S
P
I
12
8
6
4
2
0
*
Ctrl
Dag1
gRNA
*
IPSC charge
**
f
)
C
n
(
e
g
r
a
h
C
C
S
P
I
1.2
0.8
0.6
0.4
0.2
0
i
)
A
n
(
.
l
p
m
A
C
S
P
E
R
A
P
M
A
-
5.0
3.0
2.0
1.5
1.0
0.5
0
-
Dag1
gRNA
-
Dag1
gRNA
∆Cre + Cre +
AMPAR-EPSCs
-
Dag1
gRNA
-
Dag1
gRNA
∆Cre + Cre +
Fig. 6 | CRISPRi-mediated inhibition of dystroglycan expression in dissociated
OB neurons suppresses inhibitory but not excitatory synaptic transmission,
with the Nrxn3 and dystroglycan manipulations each occluding the other’s
phenotype. a–c CRISPR interference (CRISPRi)-mediated inhibition of dystrogly-
can (Dag1) expression decreases the levels of dystroglycan mRNAs and significantly
decreases the amplitude of evoked IPSCs (a, qRT-PCR measurements of Dag1
mRNA levels; b, sample traces; c, summary graph of IPSC amplitudes).
d–f Combined inhibition of dystroglycan (Dag1) and Nrxn3 expression does not
lower the evoked IPSC amplitude more severely than the single inhibition of either
dystroglycan or of Nrxn3 expression (d, sample traces; e, f, summary graphs of the
IPSC amplitudes (e) and charge transfer (f)). IPSCs evoked by extracellular stimu-
lation were recorded from mitral/tufted cells in dissociated culture obtained from
Nrxn3 cKO mice that were infected with lentiviruses expressing either ΔCre and/or
the Dag1 CRISPRi components. g–i The single or double inhibition of dystroglycan
(Dag1) and Nrxn3 expression have no effect on evoked NMDAR- and AMPAR-EPSC
amplitudes (g, sample traces; h, i summary graphs of the evoked NMDAR-EPSC
amplitudes (h) and AMPAR-EPSC amplitudes (i)). Experiments were performed as
in d–f. Numerical data are means ± SEM; n’s (cells/experiments) are indicated above
the sample traces and apply to all graphs in an experimental series. Statistical
analyzes were performed with a one-way analysis of variance (ANOVA) with Dun-
nett’s multiple comparison test (e, f, h, and i), a two-tailed one sample t test (a), or a
unpaired two-tailed t test (c), with *p < 0.05, **p < 0.01, and ***p < 0.001. Source data
and statistical results for all experiments are provided within the Source Data file.
Nature Communications |
(2023) 14:1771
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Article
a
GCL
MCL
EPL
GL
Dystroglycan Staining in OB
O/N Postfix
b
Ext. Plex. Layer of OB
Confocal
STED
c
Glom. Layer of OB
Confocal
STED
https://doi.org/10.1038/s41467-023-36872-8
α-DG/Gephyrin
2 μm
α-DG/Gephyrin
2 μm
BV
BV
Gephyrin
α-DG
Merged
Gephyrin
Gephyrin
Gephyrin
10” Postfix
GCL
MCL
EPL
GL
Gephyrin
BV
BV
α-DG
α-DG
α-DG
α-DG
Merged
50 μm
Merged
500 nm
Merged
500 nm
d
Reciprocal Synapses in EPL
e
SO
SP
SR
Dystroglycan Staining in Hippocampal CA1
f
l
a
c
o
f
500 nM
n
o
C
Magnified CA1 S. Rad.
Gephyrin
α-DG
Merged
BV
BV
α-DG
Merged
50 μm
Dystroglycan Staining in Cerebellum
BV
BV
Gephyrin / α-DG / Homer1
2 μm
Gephyrin
500 nm
Homer1
SLM
Gephyrin
g
Cer
GCL
PCL
ML
α-DG
Merged
Gephyrin
α-DG
Merged
50 μm
D
E
T
S
D
E
T
S
h
l
a
c
o
f
n
o
C
D
E
T
S
D
E
T
S
1 μm
1 μm
Magnified Cer. ML
Gephyrin
α-DG
Merged
1 μm
1 μm
Dystroglycan is expressed not only by neurons but also by
astrocytes and pericytes, both of which target dystroglycan to the
basal lamina encapsulating blood vessels in the brain (Figs. 7, S8, S9).
We next determined whether dystroglycan acts as a postsynaptic
ligand in mitral cells to Nrxn3 expressed by granule cells. To achieve a
specifically postsynaptic deletion of dystroglycan in mitral cells, we
crossed CAS9 conditional knockin mice65 with tBet-Cre mice, resulting
in the expression of CAS9 only in mitral/tufted cells. Infection of the
OB with AAVs expressing the dystroglycan gRNA and Cre-dependent
DIO-tdTomato then causes a selective dystroglycan deletion in mitral/
tufted cells, with infected mitral cells visualized via their tdTomato
expression (Fig. 8e). This enabled us to performed whole-cell patch-
clamp recordings from infected mitral cells in which dystroglycan had
been deleted.
We found that the dystroglycan deletion produced a large
decrease (~40%) in mIPSC frequency, small but not statistically sig-
nificant decrease in mIPSC amplitude, and no change in intrinsic
electrical properties or mIPSC kinetics (Figs. 8f–h, S9j–l). Importantly,
OB slices from male and female mice exhibited similar decreases in
mIPSC frequency (Fig. S9m). Moreover, the dystroglycan deletion
induced a comparable decrease (~40%) in the amplitude of evoked
GC→MC IPSCs (Fig. 8i–k), again without a change in kinetics (Fig. S9n).
This decrease in IPSC amplitude was accompanied by a large increase
(~80%) in the coefficient of variation of evoked IPSCs (Fig. 8l), and by an
Nature Communications |
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Article
https://doi.org/10.1038/s41467-023-36872-8
Fig. 7 | α-Dystroglycan localizes to blood vessels and to inhibitory synapses in
the OB. a Only short post-fixation (10 min) but not overnight fixation (O/N) with 4%
PFA allows efficient detection of gephyrin (green) and α-dystroglycan (purple) by
immunocytochemistry in the OB (GCL, granule cell layer; MCL, mitral cell layer;
EPL, external plexiform layer; GL, glomerular layer). Dystroglycan labeling in blood
vessel (BV) walls is indicated with arrows. b Super-resolution imaging using sti-
mulated emission depletion microscopy (STED) shows that inhibitory synapses in
the EPL of the OB are often co-populated by dystroglycan nanoclusters. c STED
super-resolution imaging reveals a similar nanocluster structure of gephyrin and
dystroglycan at inhibitory synapses in the glomerular layer of the OB. d Specific
labeling of reciprocal dendrodendritic synapses in OB sections demonstrates the
presence of dystroglycan. Dendrodendritic synapses were identified by adjacent
localizations of the inhibitory and excitatory postsynaptic markers gephyrin and
Homer1, respectively. e, f Dystroglycan is abundantly present in perisomatic
inhibitory synapses of pyramidal neurons in the hippocampal CA1 region in addi-
tion to BV walls. Sections were stained for gephyrin and α-dystroglycan (e, overview
of a CA1 region section [SO, stratum oriens; SP, stratum pyramidale; SR, stratum
radiatum; SLM, stratum lacunosome moleculare]; f, STED super-resolution imaging
of the S. radiatum of the CA1 region demonstrating co-localization of dystroglycan
with gephyrin). g, h Dystroglycan is also present at high levels in inhibitory
synapses of the molecular layer of the cerebellar cortex. Sections were stained for
gephyrin and α-dystroglycan (g, overview of the cerebellar cortex [GCL, granule
cell layer; PCL, purkinje cell layer; ML, molecular layer]; h STED super-resolution
imaging of the molecular layer of the cerebellar cortex again demonstrating co-
localization of dystroglycan with the inhibitory synapse marker gephyrin). Experi-
ments were performed at least three times and quantification of dystroglycan
association with olfactory inhibitory synapses can be found in Fig. S8.
inversion of paired-pulse suppression to paired-pulse facilitation at
short interstimulus intervals (Fig. 8m, n).
The results of the dystroglycan deletion experiments are unex-
pected in that it was previously argued that dystroglycan is important
for the formation and not the operation of a subset of GABAergic
synapses, and that its synaptic function does not involve binding to
neurexins34,66. Therefore we aimed to validate these results in a second
set of experiments in which we specifically deleted dystroglycan in
mitral cells by infecting the piriform cortex of CAS9 conditional
knockin mice with retro-AAVs expressing dystroglycan or control
gRNAs and tdTomato (Fig. 9a, b). The retro-AAVs are taken up by
axonal projections from the mitral cells to the piriform cortex,
resulting in the selective deletion of dystroglycan from mitral cells in
the OB without any stereotactic injections of the OB.
Again, the dystroglycan deletion in mitral cells had no apparent
effect on inhibitory synapse density or size on mitral cells as examined
using immunocytochemistry for the inhibitory synapse marker
gephyrin (Fig. 9c, d). The postsynaptic mitral cell deletion of dystro-
glycan, however, did cause a pronounced functional impairment.
Patch-clamp recordings from mitral cells uncovered a robust decrease
(~40%) in mIPSC frequency but not amplitude (Fig. 9e–h). No change in
intrinsic electrical properties or mIPSC kinetics were present (Fig.
S10a–c). Moreover, the dystroglycan deletion greatly decreased (~50%)
the amplitude of evoked GC→MC IPSCs (Fig. 9i–k), and increased
(~100%) the coefficient of variation of IPSCs without changing the
kinetics of the IPSCs (Figs. 9l, S10d). Consistent with this result sug-
gesting a decrease in release probability, the dystroglycan deletion
also converted paired-pulse responses from depressed to facilitated at
short interstimulus intervals (Fig. 9m, n).
Viewed together, the dystroglycan deletion phenotype is a mirror
image of the Nrxn3 KO phenotype, with a dramatic loss of GC→MC
synaptic strength due to a decrease in release probability but without a
detectable decrease in synapse numbers. These results are consistent
with the observation that rescue of the Nrxn3 KO phenotype occurs
only with Nrxn3α splice variants that bind to dystroglycan. They
strongly support the notion that Nrxn3α enables GC→MC synaptic
function via binding to dystroglycan. As a final question, we thus asked
whether such a mechanism also applies to the role of Nrxn3 in the
mPFC. Indeed, when we applied the CRISPR-mediated deletion of
dystroglycan to the mPFC, we also detected a significant decrease in
mIPSC frequency without a change in mIPSC amplitude (Fig. 9o–r).
Moreover, no major changes in the intrinsic electrical properties or
mIPSC kinetics were observed (Fig. S10e–g). Overall, these data sup-
port the notion that Nrxn3 shapes a subset of inhibitory synapses not
only in the OB but also in the mPFC by binding to dystroglycan.
Discussion
Here we show that the binding of presynaptic Nrxn3α to postsynaptic
dystroglycan organizes the functional architecture of inhibitory
GC→MC synapses in the OB and of inhibitory layer 5/6 synapses in the
mPFC (Fig. 10). We demonstrate that the Nrxn3α/dystroglycan inter-
action is not essential for the formation of these synapses but renders
these synapses competent for neurotransmitter release by enabling a
normal release probability. Moreover, we find that the role of Nrxn3α
at these synapses is controlled by a combinatorial code of alternative
splicing whereby SS2 and SS4 of Nrxn3α collaborate to determine the
release probability (Fig. 10). Thus, our data propose a molecular
feedback mechanism by which binding of presynaptic Nrxn3α to
postsynaptic dystroglycan enables Nrxn3α to organize the presynaptic
neurotransmitter release machinery. The evidence for these overall
conclusions can be summarized as follows:
First, deletion of Nrxn3α lowered the strength of inhibitory neu-
ron→MC synapses in OB cultures by more than a half; this impairment
was rescued by Nrxn3α but not by Nrxn3β, with Nrxn3α only being
active when its alternatively spliced SS4 and/or SS2 sites contain no
insert (Figs. 1, 2; S1–4). SS2 is dominant in this combinatorial splice
code because even when SS4 is spliced out, the longer insert in SS2
(SS2ab) blocked the function of Nrxn3α (Fig. 1). Nearly all Nrxn3
mRNAs in inhibitory OB neurons (~82% of which are granule cells)
encode Nrxn3α containing an insert in SS4, and more than 90% of
Nrxn3 mRNAs in inhibitory OB neurons lack in insert in SS2, suggesting
that a Nrxn3α/dystroglycan complex is normally favored (Fig. S3, 4).
However, it is unknown whether Nrxn3-SS2 and -SS4 alternative spli-
cing may be activity-dependent in these neurons, and this ratio might
change during specific behavioral states or during maturation of adult-
born OB granule cells, which could regulate GC→MC synaptic trans-
mission by altering the Nrxn3α/dystroglycan interaction.
Second, the mechanism by which deletion of Nrxn3α suppressed
GC→MC synaptic transmission consisted of a decrease in the pre-
synaptic release probability, as shown by an increased coefficient of
variation of IPSCs, a dramatic shift in paired-pulse ratio, and a lack of
change in synapse numbers (Figs. 3–4, S5). Postsynaptic deletion of
Nrxn3 in mitral cells had no effect on GC→MC synaptic transmission
(Fig. S6). Thus, this phenotype is similar to the phenotype previously
observed following neurexin deficiency in other synapses in which
disorganization of calcium channels impairs the coupling of voltage-
gated calcium influx to neurotransmitter release5–7.
Third, the Nrxn3 KO phenotype at GC→MC synapses is fully res-
cued by a construct that contains only the LNS2-domain of the extra-
cellular LNS- and EGF-domains of Nrxn3α, provided the LNS2-domain
lacks an insert in SS2 (Figs. 2, 4). This rescue was observed both in
cultured neurons and in vivo, suggesting that even though the Nrxn3
deletion causes a decrease in release probability of its resident nerve
terminal, a trans-synaptic interaction of Nrxn3α with a postsynaptic
trans-ligand is required for GC→MC synapse function. Notably, these
findings support a “Swiss Army Knife”-like functional modularity of α-
Neurexins, with their large size and presence of an array of indepen-
dent binding units endowing them with the ability to simultaneously
engage diverse trans-synaptic ligands in orchestrating synapse
properties.
Nature Communications |
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Article
a
Experimental
strategy
b
Sections of OB infected with gRNA-AAVs
Control gRNA
Control gRNA
Control gRNA
AAVDJ ctrl/Dag1
gRNAs with
tdTomato (A-D) or
DIO-tdTomato (E-N)
GCL
MCL
EPL
OB
GL
Dag1 gRNA
Dag1 gRNA
Dag1 gRNA
100 μm
Constitutive CAS9
mice (A-C) or conditional
CAS9 / tBet-Cre mice (E-N)
2-3 weeks
mitral cell
patch-clamp
recordings
in acute slices
GCL
MCL
EPL
GL
https://doi.org/10.1038/s41467-023-36872-8
Gephyrin staining of OB section
CTRL gRNA
Control gRNA
d
Gephyrin
Puncta Density
y
t
i
s
n
e
D
)
2
m
μ
/
a
t
c
n
u
P
(
0.4
0.2
0
3
3
Dag1 gRNA
Dag1 gRNA
20 μm
Gephyrin
Puncta Size
0.2
0.1
a
t
c
n
u
P
)
2
m
μ
(
a
e
r
A
c
MCL
EPL
MCL
EPL
Gephyrin
tdTomato
Gephyrin /
tdTomato
Gephyrin
Gephyrin /
tdTomato
Patched neurons in OB slice
f
g
n
o
i
t
c
a
r
f
e
v
i
t
a
u
m
u
C
l
e
GCL
MCL
EPL
GL
EGFP (Cas9) / tdTomato (gRNA) /
Neurobiotin (patched neuron)
100 μm
0.0
0
Control gRNA (19/3)
mIPSC traces
Dag1 gRNA (20/3)
MCL
mIPSC frequency
Control gRNA
1.0
Dag1
gRNA
0.5
**
8
6
4
2
)
z
H
(
y
c
n
e
u
q
e
r
F
0
Control
Dag1
gRNA
h
n
o
i
t
c
a
r
f
e
v
i
t
a
u
m
u
C
l
mIPSC amplitude
1.0
Dag1 gRNA
Control
gRNA
176
)
A
p
(
e
d
u
t
i
l
p
m
A
100
50
0
Control
Dag1
gRNA
0.5
0.0
1
2
3
0
100
200
300
400
Inter-event interval (s)
Amplitude (pA)
GC MC evoked IPSC traces
m
Paired-pulse IPSCs
Control gRNA (12/4)
Dag1 gRNA (15/4)
Control gRNA (12/4)
Dag1 gRNA (15/4)
i
j
GC MC evoked IPSC amplitudes
k
)
A
n
(
e
d
u
t
i
l
p
m
a
k
a
e
p
C
S
P
I
2.0
1.5
1.0
0.5
0.0
0
Control
gRNA
*
*
*
*
*
*
*
Dag1 gRNA
)
6
-
0
1
(
e
p
o
s
l
t
u
p
t
u
o
/
t
u
p
n
i
C
S
P
I
20
40
60
Stimulus intensity (μA)
80
100
A
n
1
20 ms
Coefficient
of variation
*
0.75
0.60
0.4
0.3
0.2
0.1
Input/output
relationship
*
40
30
20
10
l
n
o
i
t
a
i
r
a
v
i
f
o
t
n
e
c
i
f
f
e
o
C
n
l
o
i
t
a
r
e
s
u
p
-
d
e
r
i
a
P
0
Control
Dag1
0.0
Control
Dag1
gRNA
gRNA
1.0
0.5
0.0
*
1.5
Paired-pulse ratio
Dag1 gRNA
Control gRNA
20
50
100
500
Inter-stimulus interval (ms)
0
Control
Dag1
gRNA
A
p
0
0
1
0.5 s
A
n
1
100 ms
*
*
Fourth, the CRISPRi-mediated inhibition of expression and
CRISPR-mediated deletion of dystroglycan in postsynaptic mitral cells
caused the same phenotype as the presynaptic Nrxn3 deletion in cul-
tured neurons and in vivo (Figs. 6–9). Since the physiological relevance
of neurexin-binding to dystroglycan was previously questioned34, we
aimed to confirm this conclusion using two different CRISPR-
approaches to delete dystroglycan in vivo from mitral cells, namely
direct infection of the OB with AAVs expressing the dystroglycan-
specific guide-RNA only in mitral cells (Fig. 8), and retrograde infection
of only mitral cells by administration of retro-AAVs expressing the
guide-RNA into the piriform cortex (Fig. 9). Importantly, the post-
synaptic dystroglycan deletion had no effect on synapse numbers
in vivo, but caused the same increase in the coefficient of variation of
IPSCs and in their paired-pulse ratio as the presynaptic Nrxn3 deletion.
Nature Communications |
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Article
https://doi.org/10.1038/s41467-023-36872-8
Fig. 8 | CRISPR-mediated deletion of dystroglycan (Dag1) in the OB decreases
inhibitory GC→MC synaptic transmission by suppressing the release prob-
ability. a Experimental design. The OB of constitutive CAS9-expressing mice was
infected stereotactically with AAVs encoding control or Dag1 gRNAs together with
tdTomato at P15-18. Mice were analyzed 2–3 weeks later. b Representative fluor-
escence images of OB sections stained for the inhibitory synapse marker gephyrin
(green) and tdTomato expressed via AAVs (red). c, d Dystroglycan (Dag1) deletion
does not change the density or size of gephyrin-positive synaptic puncta (c, sample
images; d, summary of puncta densities (top) and size (bottom)). e Representative
image of a mitral cell filled with neurobiotin (blue) via the patch pipette (tdTomato
expressed via AAVs is shown in red, and EGFP expressed via the CAS9 knockin in
green). f–h Dystroglycan deletion decreases the mIPSC frequency monitored in
mitral cells (f, representative mIPSC traces recorded in the presence of TTX;
g, cumulative probability of the interevent interval and summary of the mIPSC
frequency; h, cumulative probability and summary of the mIPSC amplitudes).
i–k Dystroglycan deletion suppresses inhibitory GC→MC synaptic transmission
evoked by extracellular stimulation, as documented by input/output curves
(i, representative IPSC traces; j, summary of input/output amplitudes; k, summary
of the slope of the input/output curves). l Dystroglycan deletion increases the
coefficient of variation of evoked IPSCs at GC→MC synapses, suggesting a decrease
in release probability. m, n Dag1 deletion induces a large increase in the paired-
pulse ratio (m, representative traces; n, summary of the paired-pulse ratio).
Numerical data are means ± SEM; n’s (animals (d) and cells/experiments (the rest))
are indicated in the summary graph bars (d) or above the sample traces (f, i and m)
and apply to all graphs in an experimental series with b–d belonging to the same
series. Statistical analyzes were performed using two-tailed unpaired t-test in
d, g, h, k, l and two-way ANOVA in j & n with Bonferroni multiple comparison test,
with *p < 0.05, **p < 0.01, ***p < 0.001, and ****p < 0.0001. Source data and statistical
results for all experiments are provided within the Source Data file.
Given paucity of studies on dystroglycan in the OB compared to other
brain regions, we also confirmed widespread localization of dystro-
glycan at inhibitory synapses in the OB, including reciprocal synapses
(Fig. 7). Thus, our findings indicate that postsynaptic dystroglycan
binding to presynaptic Nrxn3α retrogradely regulates the presynaptic
release probability without affecting synapse formation as such. The
strongest evidence for this conclusion comes from the selective rescue
of the Nrxn3 deletion phenotype by Nrxn3α constructs still capable of
interacting with dystroglycan as shown previously26.
Fifth, deletion of Nrxn3 or of dystroglycan from mPFC neurons
produced the same overall phenotype as these deletions induced in OB
neurons, namely a loss of inhibitory synaptic strength associated with a
change in release probability (Figs. 5, 9). Most importantly, the Nrxn3
deletion phenotype was again completely rescued by the LNS2-only
Nrxn3α construct lacking an insert in SS2 (Fig. 5). The observed phe-
notype in the mPFC was not as severe as that found in the OB, pre-
sumably because we analyzed a relatively homogeneous population of
inhibitory GC→MC synapses in the OB in which Nrxn3α/dystroglycan
binding invariably shapes synapse properties, whereas we examined a
heterogeneous mixture of distinct inhibitory synapses in the mPFC in
which only some synapses may utilize the Nrxn3α/dystroglycan sig-
naling mechanism.
Previous work demonstrating that dystroglycan is important for
synapses is generally consistent with our results, but most of these
studies found a role in synapse formation and/or maintenance instead
of synapse function34,42,64,66. Since the previous studies were performed
in the hippocampus, somatosensory cortex, and cerebellum, while we
examined the OB and mPFC, it is possible that the results are due to
differences in the type of synapses studied. Moreover, Früh et al.
(2016) concluded that the function of dystroglycan in synapses is
independent of neurexins, which is plausible since it is a different brain
region compared to the region studied here, although neurexins and
their binding to the dystroglycan mutant used in the hippocampal
studies were not actually examined by Früh et al. (2016). Alternatively,
dystroglycan may separately regulate the initial targeting of CCK +
interneurons, which may be the proximal cause of fewer CCK +
synapses in dystroglycan KO mice where dystroglycan was depleted
during development34,42. At these synapses and others, only once
synapses have formed might signaling between dystroglycan and
Nrxn3α become critical for sustaining presynaptic release. Another
alternative, and possibly most attractive, explanation is that a chronic
loss of dystroglycan that impairs inhibitory synapse function may
secondarily cause a loss of synapses, which we would have missed in
our experiments in which we performed only acute deletions of dys-
troglycan and Nrxn3.
Arguably, our most surprising result is that trans-synaptic binding
of presynaptic Nrxn3α to postsynaptic dystroglycan is required for the
ability of Nrxn3α to organize a fully functional presynaptic release
machinery. What is the nature of the dystroglycan-activated signal in
presynaptic terminals – is it a conformational change or dimerization
of Nrxn3α or an independent additional signal? This question is likely
not only important for understanding the functional molecular archi-
tecture of synapses, but also for insight into how mutations in genes
associated with dystroglycan, such as mutations in the glycosylating
enzymes for dystroglycan or their cytoplasmic binding proteins, and in
Nrxn3α produce neurodevelopmental disorders43–45. Our findings
define the core interaction of Nrxn3α with dystroglycan as functionally
essential for inhibitory synapses in at least two brain regions, but they
do not yet reveal the detailed molecular signaling that organizes the
presynaptic release machinery, a question that will need to be
addressed in future.
In summary, we have defined a trans-synaptic signaling complex
that performs an indispensable role in enabling a normal presynaptic
release probability at a subset of inhibitory synapses. Our findings
underscore the notion that individual functional properties of diverse
synapses must be systematically studied at a molecular level because
information processing in the brain not only depends on how neural
circuits are wired via synaptic connections, but also on the functional
properties of these connections. Moreover, our current findings add to
our understanding of the diverse synaptic roles of neurexins. One
might ask why the organization of synapses is so complicated, and why
neurexins perform many diverse functions in different types of
synapses. This more philosophical question is part of the larger issue of
why the brain needs to have many different types of neurons and
synapses to operate properly. Naturally, this question is unanswerable
at present, but it is striking that with neurexins, a single gene family is
used to diversify different types of synapses in the context of distinct
circuits. Instead of expressing possibly hundreds of genes to deter-
mine synapse identity, with the neurexins the brain expresses only
three genes, whose products are uniquely capable of generating
thousands of isoforms and of interacting with dozens of ligands.
Thereby, the three neurexin genes endow different synapses with
distinct properties – a major simplification of the mechanism of
synapse diversification. In this view, neurexins do not complicate the
design of synapses, but simplify it, even though the overall need for
diversity creates a panoply of different molecular pathways whose full
extent remains to be characterized.
Methods
Animals
Nrxn3 conditional knockout (cKO) mice were generated previously35
and are available commercially (Jax, 014157). Other mouse lines used in
this paper include: tBet-Cre67, constitutive cas9-knockin (KI) (Jax,
024858), conditional cas9-KI (Jax, 024857), vGAT-Cre (Jax, 028862)
and RiboTag mice (Jax, stock# 029977). For analyzing mitral/tufted
cell-specific or inhibitory neuron (primarily granule cell) translating
mRNA, RiboTag mice were crossed with hemizygous tBet-Cre mice67
and hemizygous vGAT-Cre mice, respectively. For all experiments
Nature Communications |
(2023) 14:1771
14
Article
https://doi.org/10.1038/s41467-023-36872-8
a
Experimental
strategy
retro-AAVs (B-N) or
AAVs (O-R) with
ctrl or Dag1 gRNAs
& tdTomato
b
Sections of OB infected with gRNA-AAVs
Control gRNA
Control gRNA
Control gRNA
MCL
EPL
Gephyrin staining of OB section
Control gRNA
Control gRNA
c
MCL
EPL
Piriform
cortex
Medial
prefrontal
cortex
GL
OB
100 μm
20 μm
Dag1 gRNA
Dag1 gRNA
Dag1 gRNA
MCL
Dag1 gRNA
Dag1 gRNA
Cas9/EGFP KI mice
2-3 weeks
mitral cell or mPFC
patch-clamp
recordings
in acute slices
MCL
EPL
GL
Gephyrin
tdTomato
EPL
Gephyrin /
tdTomato
Gephyrin
Gephyrin /
tdTomato
)
2
m
μ
(
a
e
r
A
a
t
c
n
u
P
Control sgRNA (22/4)
mIPSC traces
Dag1 sgRNA (24/4)
d
Gephyrin
Puncta Density
Dendrite
Soma
y
t
i
s
n
e
D
)
2
m
μ
/
a
t
c
n
u
P
(
2
1
0
0.20
0.10
4 4
0
Gephyrin
Puncta Size
Dendrite
Soma
0.15
0.10
0.05
0.15
0.10
0.05
0
Control
Dag1
0
Control
Dag1
gRNA
gRNA
A
p
0
5
100 ms
e
Patched neurons
in OB slice
MCL
EPL
f
g
n
o
i
t
c
a
r
f
e
v
i
t
l
a
u
m
u
C
1.0
Control gRNA
0.5
Dag1
gRNA
h
n
o
i
t
c
a
r
f
e
v
i
t
a
u
m
u
C
l
*
18
12
8
6
4
2
)
z
H
(
y
c
n
e
u
q
e
r
F
0
Control
Dag1
gRNA
tdTomato/
Neurobiotin
100 μm
0
0
1
2
3
Inter-stimulus interval (s)
1.0
Control gRNA
0.5
Dag1
gRNA
120
80
40
)
A
p
(
e
d
u
t
i
l
p
m
A
0
Control
Dag1
gRNA
100
200
300
Amplitude (pA)
400
Paired-pulse IPSCs
0
0
m
GC MC evoked IPSC traces
Control sgRNA (14/3)
Dag1 sgRNA (17/3)
Control sgRNA (14/3)
Dag1 sgRNA (16/3)
i
j
)
A
n
(
e
d
u
t
i
l
p
m
a
k
a
e
p
C
S
P
I
1.5
1.0
0.5
L1
L2/
L3
L3
L5
100 μm
GC MC evoked IPSC amplitudes
k
***
Input/output
relationship
*
30
Control gRNA
Dag1 gRNA
20
10
*
*
)
6
-
0
1
(
e
p
o
s
l
t
u
p
t
u
o
/
t
u
p
n
i
C
S
P
I
100
A
n
1
20 ms
Coefficient
of variation
***
1.2
0.8
0.6
0.4
0.2
l
n
o
i
t
a
i
r
a
v
i
f
o
t
n
e
c
i
f
f
e
o
C
n
1.5
l
o
i
t
a
r
e
s
u
p
-
d
e
r
i
a
P
1.0
0.5
A
n
1
100 ms
Paired-pulse ratio
*
Dag1 gRNA
Control gRNA
*
*
0
0
20
40
80
Stimulus intensity (μA)
60
0
Control
Dag1
0
Control
Dag1
gRNA
gRNA
20
50
100
500
Inter-stimulus interval (ms)
Control gRNA (20/3)
mIPSC traces
Dag1 gRNA (20/3)
o
Infected mPFC
section
CAS9-EGFP /
tdTomato / Neurobiotin
p
q
mIPSC frequency
1.0
Control
gRNA
Dag1
gRNA
*
25
20
15
10
5
)
z
H
(
y
c
n
e
u
q
e
r
F
0
Control
Dag1
gRNA
0.5
n
o
i
t
c
a
r
f
e
v
i
t
l
a
u
m
u
C
0.0
0
1.0
0.5
r
n
o
i
t
c
a
r
f
e
v
i
t
l
a
u
m
u
C
mIPSC amplitude
A
p
0
5
0.5 s
Dag1
gRNA
Control
gRNA
60
50
40
30
20
10
)
A
p
(
e
d
u
t
i
l
p
m
A
0
Control
Dag1
gRNA
200
400
600
800
Inter-event interval (ms)
0.0
0
50
100
150
200
Amplitude (pA)
using constitutive and conditional Cas9-KI mice, mice were maintained
at homozygosity for the KI alleles. For mitral/tufted cell-specific dele-
tion of Dag1, conditional cas9-KI mice were crossed with mice carrying
the tBet-Cre allele. Only mice with a single allele of Cre were used for
experiments. All mice were weaned at 20 days of age and housed in
groups of 2 to 5 on a 12 h light/dark cycle with access to food and water
ad libidum. Rooms were maintained with 40–60% humidity and at
approximately 22° C. All procedures conformed to National Institutes
of Health Guidelines for the Care and Use of Laboratory Mice and were
approved by the Stanford Animal Use Committees [Administrative
Panel for Laboratory Animal Care (APLAC/) Institutional Animal Care
and Use Committee (IACUC)] under the animal protocol 20787.
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Fig. 9 | Dystroglycan (Dag1) deletion in mitral cells of the OB and in the mPFC
suppresses inhibitory synaptic transmission. a Mitral cells are infected with
control or Dag1 gRNAs and tdTomato by projection-specific labeling through retro-
AAVs injected in the piriform cortex. b Representative OB sections stained for
gephyrin (green) and for tdTomato (red). c, d Dystroglycan deletion does not
change the density or size of gephyrin-positive synapses (green) co-localized with
tdTomato-expressing mitral cells (c, sample images; d, summary of puncta den-
sities (top) and puncta size (bottom)). e Representative image of a mitral cell filled
with neurobiotin (blue) and expressing tdTomato via AAVs (red). f–h Dystroglycan
(Dag1) deletion decreases the mIPSC frequency in mitral cells (f, representative
mIPSC traces; g, h cumulative probability of the mIPSC interevent intervals and
amplitudes, insets: summary of the mIPSC frequency and amplitudes).
i–k Dystroglycan (Dag1) deletion suppresses the IPSC amplitude at GC→MC
synapses (i, representative IPSC traces; j, summary of input/output amplitudes;
k, summary of the input/output curve slopes). l Mitral cell-specific dystroglycan
(Dag1) deletion increases the coefficient of variation of evoked IPSCs.
m, n Dystroglycan (Dag1) deletion induces a large increase in the paired-pulse ratio
(m, representative traces; n, summary plot of the paired-pulse ratio).
o–r Dystroglycan deletion in the mPFC decreases the mIPSC frequency monitored
in Layer 5 pyramidal neurons (o, representative image of an mPFC section;
p, representative mIPSC traces; q–r, cumulative probability of the mIPSC interevent
intervals and amplitudes, insets: summary of the mIPSC frequency and ampli-
tudes)). Numerical data are means ± SEM; n’s (animals (d) or cells/experiments (the
rest)) are indicated in the summary graph bars (d) or above the sample traces
(f, i, m, and p) and apply to all graphs in an experimental series with b–d belonging
to the same series. Statistical analyzes were performed using two-tailed unpaired t-
test in d, g, h, k, l, q, r, and by two-way ANOVA in j & n with Bonferroni multiple
hypothesis testing, with *p < 0.05, **p < 0.01, and ***p < 0.001. Source data and
statistical results for all experiments are provided within the Source Data file.
Plasmids
Lentiviral vectors for expression of Cre and ΔCre (truncated, inactive)
recombinase driven by the human synapsin-1 promoter have been
described previously68. For all other experiments using the lentiviral
backbone with a human synapsin-1 vector (i.e. FSW), an empty vector
was used as a control. For all Nrxn3 rescue constructs, a single HA tag
was positioned between the native signal peptide and was flanked by
linker sequences (i.e. glycine-glycine-serine upstream and glycine-
serine downstream). All culture rescue constructs were incorporated
into the FSW lentiviral backbone. A library of previously published
cDNA’s12,35 were used to clone Nrxn3alpha and Nrxn3beta splice var-
iants described in Figs. 1 and 2. For all truncation constructs (Fig. 2),
that lacked LNS6, upstream domains were fused at the same position
that LNS6 would normally be, thus preserving the downstream stalk
region, transmembrane domain, and cytoplasmic sequence.
1:250 live ICC), anti-Synaptophysin-2 rabbit (homemade, Wang et al.,
2021; 1:500 IHC), and anti-vGAT guinea pig (Synaptic Systems Cat# 131
004; 1:1000 ICC), Goat anti-Mouse IgM Heavy Chain Alexa594 (Ther-
moFisher, A-21044; 1:400, IHC/STED), Goat anti-Mouse IgG Alexa546
(ThermoFisher, A11003; 1:1000,
ICC/IHC), Goat anti-Mouse IgG
Alexa405 (ThermoFisher, A31553; 1:1000, ICC/IHC), Goat anti-Rabbit
IgG Alexa405 (ThermoFisher, A31556; 1:1000, ICC/IHC), Goat anti-
Rabbit IgG Alexa546 (ThermoFisher, A11010; 1:1000, ICC/IHC), Goat
anti-Rabbit IgG STAR Red (Abberior, STRED-1001; 1:400, IHC), Goat
anti-Rabbit IgG STAR460L (Abberior, ST460L-1002; 1:400, IHC), Goat
anti-Rabbit IgG CF568 (Biotium, 20098-1 mg; 1:3000, IHC), Goat anti-
Guinea Pig IgG Alexa647 (ThermoFisher, A21450; 1:1000-1:3000, ICC/
IHC), Goat anti-Guinea Pig IgG STAR Red (Abberior, STRED-1006;
1:400, IHC), and Goat anti-Chicken Igy Alexa488 (ThermoFisher,
A11039; 1:1000, ICC).
The adeno-associated virus (AAV) serotypes used in this study
were AAV-DJ and rAAV2-retro for retrograde experiments69. AAV
backbones were generated to allow the expression of Cre and ΔCre
fused to tdTomato, minimal Nrxn3 LNS2 rescues (with and without
SS2), and gRNA’s targeting Dag1 with soluble tdTomato driven by the
hSynI promoter in a Cre-sensitive (i.e. with DIO) or constitutive man-
ner. For CRISPRi lentiviral backbones, a scrambled gRNA control was
generated (5’-GCGCCAAACGTGCCCTGACG-3’). For targeting dystro-
glycan, several gRNA’s were initially screened. The final gRNA that
5’-
a
performed
AGCTTCGCGCGGAGTCCCCG-3’. CRISPRi was performed using a len-
tiviral backbone described previously70, with an expression of gRNA
driven by the U6 promoter and the inactive Cas9 fused to KRAB driven
by the EFS promoter.
screen was
functional
following
best
For in vivo CRISPR experiments, two gRNA were used to ensure
efficient targeting of Dag1 including one driven by a U6 promoter (i.e.
5’-tggttaggttctcccccacg-3’) and another by a H1 promoter (i.e. 5’-
accgtggttggcattccaga-3’). These gRNA were published previously41.
Scrambled gRNA sequences were used as controls.
Sequences for all unpublished constructs are included in a sup-
plementary information file.
Antibodies
The following antibodies were used at the indicated concentrations
(IHC-immunohistochemistry; ICC-immunocytochemistry): anti-alpha-
Dystroglycan [45–3] rabbit (Abcam Cat# ab199768; 1:250 IHC), anti-
Dystroglycan (Millipore Cat#05-593; 1:250 IHC), anti-HA rabbit (Cell
Signaling Cat# 3724; 1:500 IHC), anti-Gephyrin mouse (Synaptic Sys-
tems Cat# 147 011; 1:1000 ICC), anti-Gephyrin guinea pig (Synaptic
Systems Cat# 147 318; 1:250 IHC), anti-Homer1 rabbit (Synaptic Sys-
tems Cat# 160 003, 1:1000 ICC/IHC), anti-MAP2 chicken (Encorbio
Cat# CPCA-MAP2; 1:1000 ICC), anti-GABAARα1 (Synaptic Systems Cat#
224 203; 1:250 live ICC), anti-GABAARα2 (Synaptic Systems Cat# 224
103; 1:250 live ICC), anti-GABAARγ2 (Synaptic Systems Cat# 224 003;
Cell culture
Primary neuron cultures (containing glia). Hippocampal, OB, and
cortical neurons were cultured from newborn mice. Tissue was
dissected and mixed regardless of sex. In general, pooling tissue
from three to six mice in a given preparation was used to generate
cultures. Cortical neurons were derived from entire cortical lobes
that were separated from midbrain, hindbrain, and hippocampus.
Olfactory bulbs were plate at 4 coverslips, in a 24-well dish, per
mouse (2 bulbs). Dissected hippocampi, OBs, or cortices were
digested for 20 min with 10 U/ml papain in Hank’s buffered saline
(HBS) in an incubator, washed with HBS, dissociated in plating
media (MEM supplemented with 0.5% glucose, 0.02% NaHCO3,
0.1 mg/ml transferrin, 10% FBS, 2 mM L-glutamine, and 0.025 mg/ml
insulin), and seeded on Matrigel (BD Biosciences) precoated cov-
erslips placed inside 24-well dishes. For OB neurons, the day after
plating, 95% of media was replaced with MEM (GIBCO) supple-
mented with 2% B27 (GIBCO), 0.5% w/v glucose, 100 mg/l transfer-
rin, 5% fetal bovine serum. For hippocampal and cortical neurons,
the day after plating, 95% of the plating medium was replaced with
neuronal growth medium lacking serum (Neurobasal-A medium
supplemented with 2% B27 supplement and 0.5 mM L-glutamine). At
DIV2–3 (for hippocampal and OB cultures) or DIV3–4 (for cortical
cultures), 50% of the medium was exchanged with fresh growth
medium additionally supplemented with 4 µM AraC (Sigma-Aldrich)
to restrict glial overgrowth. When applicable, neurons were infec-
ted between DIV3-4 with lentiviruses expressing EGFP-tagged ΔCre
(control) or Cre without and/or with the indicated rescue constructs
driven by the synapsin promoter. For long-term culture of hippo-
campal and cortical neurons, 25% fresh media was added every 4–5
d starting from DIV7. A partial media change (<30%) was performed
only once on DIV7 for OB neurons to preserve cell health. OB cul-
tures do not maintain cultural health if media is exchanged too
frequently and if the exchange volume exceeds 30%.
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presynaptic
GABA
Nrxn3α
PR
Dystroglycan
Nlgn2/3
α-Nrxn
Splice
Code
SS2
-
-
-
-
a
a
a
a
ab
ab
ab
ab
SS4
-
+
-
+
-
+
Dystro-
glycan
Binding
PR
+
-
GABAA-
receptors
postsynaptic
Fig. 10 | Model of transsynaptic synaptic signaling by Nrxn3α and dystroglycan
at inhibitory synapses. Summary cartoon of a complex between presynaptic
Nrxn3α and postsynaptic dystroglycan and Neuroligin-2 and -3 (Nlgn2/3). Right,
summary of the α-neurexin splice code that dictates dystroglycan binding, which in
turn retrogradely elevates the presynaptic release probability (PR).
HEK293T cells. HEK293T cells were purchased from American Type
Culture Collection (CRL11268). The cells were isolated from human
embryo kidney tissue and this particular line is a derivative of the 293T
(293tsA1609neo) cell
line. Cells were grown in complete DMEM
(cDMEM), which consisted of DMEM (Gibco), 5% FBS (Sigma), peni-
cillin, and streptomycin. All transfections were performed using lipo-
fectamine 3000 (Invitrogen). For co-culture assays, HEK293T cells
were plated on 12-well plates and transfected at ~90% confluency
according to the manufacturer’s instructions.
Preparation of viral particles
AAV preparation. The adeno-associated virus (AAV) serotypes used in
this study were AAV-DJ and rAAV2-retro for retrograde experiments69.
HEK293T cells were transfected with the helper plasmid, the serotype-
specific plasmid, and the AAV plasmid using homemade calcium
phosphate solution. Cells were dissociated and precipitated 72 hours
post-transfection. Nuclei were lysed by three times of freeze-thaw
cycles and were later treated with Benzonase nuclease (Sigma-Aldrich,
cat # E1014). The supernatant then underwent iodixanol gradient
ultracentrifugation for 3 hours at 273720.9 x g at 4oC in a S80AT3
rotor. AAV were then concentrated using filtered centrifugation and
dialyzed in minimal essential media (MEM).
Lentivirus preparation. Recombinant lentiviral particles were pro-
duced in HEK293T cells by co-transfecting cells with long terminal
repeat (LTR) containing vector and helper plasmids (pRSV-REV,
pMDLg/gRRE, and pVSVG) using calcium phosphate. Media were
exchanged 1 h before transfection and included 25 µM chloroquine
diphosphate. Per 75 cm2 of cells, 0.5 ml of 250 mM CaCl2 containing
molar equivalents of DNA (12 µg of LTR-containing vector, 3.9 µg
pRSV-REV, 8.1 µg pMDLg/gRRE, and 6.0 µg pVSVG) was added
dropwise to an equal volume of 2X-HBS (0.4 M NaCl, 10 mM KCl,
1.5 mM Na2HPO4, 0.2% glucose, and 38.4 mM HEPES, pH 7.05) under
vigorous mixing, incubated for 20 min at room temperature, and
added dropwise to the cells. A total of 16–20 h following transfec-
tion, cells were washed with plain DMEM and replaced with neuro-
nal growth media lacking AraC. After 24 h, media containing
lentiviral particles were cleared by centrifugation (1500 x g, 10 min),
aliquoted, and snap-frozen. Neuronal cultures were infected with
lentivirus on DIV3 or 4 by adding 25–30 µl of viral supernatant per
well of a 24-well plate.
Electrophysiology
Culture electrophysiology. Cultured neurons were collected and
recorded at DIV 14–17. Electrophysiology recordings were performed
at room temperature, performed in whole-cell patch-clamp mode
using concentric extracellular stimulation electrodes as described
previously71. The glass pipettes (2−3 MΩ filled with intracellular pipette
solution) were pulled from borosilicate glass capillaries with a vertical
micropipette puller (PC-10, Narishige). After the formation of the
whole-cell configuration and equilibration of the intracellular pipette
solution, the series resistance was adjusted to 8–10 MΩ. Synaptic
currents were monitored with a Multiclamp 700B amplifier (Molecular
Devices). A bipolar stimulation electrode (FHC, Bowdoinham, ME) was
placed 100-150 µm from the soma of the neurons recorded to apply
focal square pulse stimuli (duration 1 ms) and trigger evoked synaptic
responses. The frequency, duration, and magnitude of the extra-
cellular stimulus were controlled with a Model 2100 Isolated Pulse
Stimulator (A-M Systems) synchronized with Clampex 9 data acquisi-
tion software (Molecular Devices). The whole-cell pipette solution
contained (in mM): 120 CsCl, 5 NaCl, 1 MgCl2, 10 HEPES, 10 EGTA, 0.3
Na-GTP, 3 Mg-ATP, and 5 QX-314 (pH 7.2, adjusted with CsOH). The
bath solution contained (in mM): 140 NaCl, 5 KCl, 2 MgCl2, 2 CaCl2, 10
HEPES, and 10 glucose (pH 7.4, adjusted with NaOH). IPSCs, as well as
AMPAR- or NMDAR-mediated EPSCs, were pharmacologically isolated
by adding blockers against the AMPA receptor (CNQX, 10 μM), NMDA
receptor (APV, 50 μM), or GABAA receptor (picrotoxin, 50 μM) to the
extracellular solution. Spontaneous mIPSCs and mEPSCs were mon-
itored in the presence of tetrodotoxin (TTX, 1 μM) to block action
potentials. Miniature events were analyzed in Clampfit 9 and 10.7
(Molecular Devices) using the template matching search and a minimal
threshold of 5 pA and each event was visually inspected for inclusion or
rejection by an experimenter blind to the recording condition.
Slice electrophysiology. Two to three weeks after viral injection, mice
were anesthetized via isoflurane inhalation, and brains were quickly
dissected. The dissected brain was sliced in ice-cold oxygenated (95%
O2 and 5% CO2) cutting solution (228 mM sucrose, 11 mM glucose,
26 mM NaHCO3, 1 mM NaH2PO4, 2.5 mM KCl, 7 mM MgSO4, and
0.5 mM CaCl2). Horizontal sections for OB and coronal sections for
mPFC, both of which were 300 µm thick, were obtained by using a
vibratome. Slices were quickly transferred to oxygenated artificial
cerebrospinal fluid (ACSF; 119 mM NaCl, 2.5 mM KCl, 1 mM NaH2PO4,
1.3 mM MgSO4, 26 mM NaHCO3, 10 mM glucose, and 2.5 mM CaCl2) at
32 °C for 30 min. Slices were allowed to recover at room temperature
for an additional 30 min. The recording chamber was temperature
controlled and set to 32 °C, and ACSF was perfused at 1 mL/min. The
internal solution for whole-cell patch clamp contained 135 mM CsCl,
10 mM HEPES, 1 mM EGTA, 1 mM Na-GTP and 4 mM Mg-ATP pHed to
7.4. 10 mM QX314-bromide was added for evoked recordings. 0.2%
neurobiotin (VectorLab) was included for morphological reconstruc-
tion. The pipette resistance ranged from 1.8 to 2.5 MΩ. Mitral cells were
identified in the mitral cell layer in the OB and mPFC neurons were
identified either by the fluorescent reporter or pyramidal-shaped
neuron in the deep layer. Access resistance was under 10 MΩ (for
mitral cells) and 15 MΩ for mPFC neurons throughout the experiment.
1 µM TTX (Tocris), 20 µM CNQX (Tocris), and 50 µM D-AP5 (Tocris)
were included in the bath for mIPSC. Twenty micrometers of CNQX
(Tocris) and 50 µM D-AP5 (Tocris) were included in the bath for evoked
IPSC recordings. All recordings were done in voltage-clamp mode with
a holding potential of −70mV. For eIPSC stimulation, concentric
bipolar electrode was used. For GC→MC eIPSC, the stimulating elec-
trode was placed directly below the mitral cell with constant distance
roughly at the junction between the internal plexiform layer and
granule cell layer, 30 µm below the surface of the slice. For eIPSC in
mPFC, the stimulating electrode was placed parallel to the recorded
cell in the same layer with constant distance and 30 µm below the
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surface of the slice. The experimenter was blind to the treatment
groups during recordings and analysis.
Stereotactic injections
Mice were prepared for stereotactic injections using standard pro-
cedures approved by the Stanford University Administrative Panel on
Laboratory Animal Care. Mice were anesthetized by 0.2 mL avertin
working solution per 10 grams body weight. The avertin stock solu-
tion was made by dissolving 5 grams tribromoethanol into 5 mL
T-amyl alcohol, which was further diluted 80 folds in DPBS to make
the avertin working solution. The coordinates (AP/ML/DV from
Bregma) and volumes for the intercranial injections are (1) +4.3/
±0.85/−1.7 and +5.3/±0.6/−1.5 with 1.0 uL virus for the OBs and (2)
−0.7/±3.7/−4.75 with 0.75 uL virus for the piriform cortex (for ret-
rograde targeting of mitral cells). For injection into the piriform
cortex, the mouse brains were aligned to have less than 0.05 mm
difference on the DV axis at −1.00 (AP) between ±3.00 (ML) positions.
The reference zero point for DV is on the surface of the OB for OB
injection and the surface of the skull at AP/ML of −0.7/0.0 for piri-
form cortex injection.
Purification of tissue mRNA
Wild-type (CD1) mice at 8 weeks of age were euthanized using iso-
flurane and decapitated. Several brain regions including the cortex,
OB, cerebellum, hippocampus, and pons/medulla were quickly dis-
sected and snap-frozen in liquid nitrogen or dry ice and transferred to
−80 °C storage until processing. The specimen was subjected to RNA
extraction using the QIAGEN RNeasy Micro kit.
Purification of ribosome-bound mRNA
RiboTag mice72 were crossed with tBet-Cre mice67 or vGAT-Cre mice.
After OB extraction described above, the frozen bulbs were partially
thawed in fresh homogenization buffer at 10% (w/v) and Dounce
homogenized. Homogenates underwent centrifugation, and 10% of
the supernatant was used as input. The remaining supernatant was
incubated with prewashed anti-HA magnetic beads (Thermo Fisher
Scientific) overnight at 4 °C. The beads were washed three times with a
high-salt buffer followed by elution with RLT lysis buffer containing
2-mercaptoethanol. The sample and the input were then subjected to
mRNA extraction described above. RNA concentration was deter-
mined using a NanoDrop 1000 Spectrophotometer (Thermo) and
stored at -80˚C until downstream analysis.
qPCR
Quantitative reverse transcription (RT)–PCR was performed in tripli-
cates for each condition with QuantStudio 3 (Thermo Fisher Scientific).
RNA (20 ng) was used for each reaction, in conjunction with TaqMan
Fast Virus 1-Step Master Mix (Thermo Fisher Scientific) and gene-
specific qRT-PCR probes [IDT (integrated DNA technologies). Prede-
signed PrimeTime qPCR probe assays (IDT) were used for vGluT1
(Mm.PT.58.12116555), vGaT (Mm.PT.58.6658400), aquaporin-4 (Mm.
PT.58.9080805), MBP (Mm.PT.58.28532164), ActB (Mm.PT.39a.
22214843.g), Cbln1 (Mm.PT.58.12172339), Cbln2 (Mm.PT.58.5608729),
Cbln4 (Mm.PT.58.17207498), Grid1 (Mm.PT.58.32947175), and Grid2
(Mm.PT.58.12083939), Nlgn1 (Mm.PT.58.30240881), Nlgn2 (Mm.PT.
58.16799702), Nlgn3
(Mm.PT.58.
13767897), Nxph2 (Mm.PT.58.28481365), Nxph3 (Mm.PT.12688150),
(Mm.PT.58.42587284.g)
Nxph4
LRRTM2
(Mm.PT.58.31131475),
(Mm.PT.56a.6079538),
LRRTM4
Fam19a2 (Mm.PT.58.7298614), Fam19a4 (Mm.PT.56a.9330679), Car10
(Mm.PT.58.11765793), Car11
(Mm.PT.58.32895602), Dag1-ex1/ex2
(Mm.PT.58.46076316), Dag1-ex3/ex4 (Mm.PT.58.45967735), and Dag1-
ex4/ex5 (Mm.PT.58.5524327). Customed PrimTime qPCR probe assays
(IDT) were used for Nrxn1α (forward: TTCAAGTCCACAGATGCCAG;
(Mm.PT.58.6337058.g),
(Mm.PT.58.11146838),
(Mm.PT.58.31138258, Nxph1
(Mm.PT.58.11246838),
LRRTM3
Fam19a1
LRRTM1
reverse: CAACACAAATCACTGCGGG; probe: TGCCAAAACTGGTCCA
TGCCAAAG), Nrxn1β (forward: CCTGTCTGCTCGTGTACTG; reverse:
TTGCAATCTACAGGTCACCAG; probe: AGATATATGTTGTCCCAGCG
TGTCCG), Nrxn1γ (forward: GCCAGACAGACATGGATATGAG; reverse:
GTCAATGTCCTCATCGTCACT; probe: ACAGATGACATCCTTGTGG
CCTCG), Nrxn2α (forward: GTCAGCAACAACTTCATGGG; reverse:
AGCCACATCCTCACAACG; probe: CTTCATCTTCGGGTCCCCTTCCT),
Nrxn2β (forward: CCACCACTTCCACAGCAAG; reverse: CTGGTGT
GTGCTGAAGCCTA; probe: GGACCACATACAT CTTCGGG), Nrxn3α
(forward: GGGAGAACCTGCGAAAGAG; reverse: ATGAAGCGGAAGGA-
CACATC; probe: CTGCCGTCATAGCTCAGGATAGATGC), Nrxn3β (for-
reverse: GGCCAGGTATAGA
ward: CACCACTCTGTGCCTATTTC;
(forward:
GGATGA; probe: TCTATCGCTCCCCTGTTTCC), Nlgn1
GGTTGGGTTTGGTATGGATGA;
reverse: GATGTTGAGTGCAGTAG-
TAATGAC; probe: TGAGGAACTGGTTGATTTGGGTCACC), Nlgn2 (for-
ward:
TGCCTGTACC
TCAACCTCTA; probe: TCAATCCGCCAGACACAGATATCCG), and
Nlgn3 (forward: CACTGTCTCGGATGTCTTCA; reverse: CCTCTATCT-
GAATGTGTATGTGC; probe: CCTGTTTCTTAGCGCCGGATCCAT).
CCGTGTAGAAACAGCATGACC;
reverse:
Assays generating Ct values >35 were omitted. Ct values for
technical replicates (duplicate or triplicate) differed by less than 0.5. Ct
values were averaged for technical replicates. Data were normalized to
the arithmetic mean of ActB and Gapdh using the 2-ΔΔCt method.
Junction-flanking PCR
The following primers anneal to constitutive exon sequences that flank
splice junctions and thus amplify Nrxn1-3 mRNA transcripts with or
without alternative splice sequences (splice site, forward primer,
reverse primer):
Nrxn1-SS2v1 (5’-TGGGATCAGGGGCCTTTGAAGCA-3’, 5’-GAAGGT
CGGCTGTGCTGGGG-3’), Nrxn2-SS2v1 (5’-GCACGACGTCCGGGTTACC
C-3’, 5’-GGTCGGCTGTGTTGGGGCTG-3’), Nrxn3-SS2v1 (5’-TCCGGG
GCCTTTGAGGCCAT-3’, 5’-GCGGTACTTGGGCTTCCACCA-3’), Nrxn1-
SS4v1 (5’-CTGGCCAGTTATCGAACGCT-3’, 5’-GCGATGTTGGCATCGT
TCTC-3’), Nrxn2-SS4v1 (5’-CAACGAGAGGTACCCGGC-3’, 5’-TACTAGCC
GTAGGTGGCCTT-3’), Nrxn3-SS4v1 (5’-ACACTTCAGGTGGACAACTG-
3’, 5’-AGTTGACCTTGGAAGAGACG-3’), Nrxn1-SS2v2 (5’-TGCCTGGCA
TGATGTGAA-3’, 5’-TGGTGTAATCTTCTTGCGTGTA-3’), Nrxn2-SS2v2
(5’-ACCCGTCAATGGCAAGTT-3’,
5’-AGCCCAGCATGGTGTAATC-3’),
Nrxn3-SS2v2 (5’-CCTGGCATGATGTCAAAGTG-3’, 5’-GCCCAGCATGG
TGTAGT-3’), Nrxn1-SS4v2 (5’-CCAGTTATCGAACGCTACCC-3’, 5’-GCCA
TTGTAGTAAAGACCAGAGA-3’), Nrxn2-SS4v2 (5’-GACAGCTGGCCAGT
CAAC-3’, 5’-GACACCTGGCCCTGGAA-3’), Nrxn3-SS4v2 (5’-GGCCAGT-
GAATGAGCACTAT-3’, 5’GACACCTGGCCCTGGAA-3’). Fig S3a–b use
Nrxn1/2/3-SS4v1 and Nrxn1/2/3-SS2v1. Fig. S3c–d use Nrxn1/2/3-SS4v2
and Fig. S4c–d use Nrxn1-SS2v1 and Nrxn2/3-SS2v2. Fig. S3h use Nrxn1/
2/3-SS4v1 and Figs. S4e–f use Nrxn1/2/3-SS2v1.
cDNA was synthesized from equal amounts of 1) adult brain
regions, 2) primary neuron/glia culture mRNA, or 3) immunoprecipi-
tated mRNA from mitral/tufted cells or granule cells and total input
mRNA from the OB. Junction-flanking PCR was then performed with
equal amount of cDNA from groups being compared. The PCR pro-
ducts were separated on homemade MetaPhor agarose gel (Lonza) and
stained with GelRed. Stained gel was imaged at sub-saturation using
the ChemiDoc Gel Imaging System (Bio-Rad). Quantification was per-
formed using Image Lab (Bio-Rad) or ImageStudioLite (LI-COR).
Intensity values were normalized to the size of DNA products to con-
trol for intensity differences caused by different dye incorporation
owing to varied DNA length. For an example of presentation of full
scan blots in supplementary Figs. 3–4, see the Supplementary
Information file.
Immunocytochemistry
For live surface-labeling experiments, primary neurons were first
washed at room temperature once with a HEPES bath solution, which
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contained the following (in mM): 140–150 NaCl, 4–5 KCl, 2 CaCl2, 1
MgCl2, 10 glucose, and 10 HEPES, with pH adjusted to 7.4 with NaOH,
and osmolarity of 300 mOsm. Cultures were then incubated at room
temperature for 20 min with antibodies recognizing the GABAARα1,
GABAARα2, or GABAARγ2 diluted in HEPES bath solution. Cultures
were then gently washed three times with HEPES bath solution, fol-
lowed by fixation for 20 min at room temperature with 4% (wt/vol)
PFA. Following fixation, cultures were washed three times with Dul-
becco’s PBS (DPBS). Cultures were blocked for 1 h at room tempera-
ture with antibody dilution buffer (ADB) without Triton X-100(−),
which contains 5% normal goat serum diluted in DPBS. Cells were then
labeled with Alexa Fluor–conjugated secondary antibodies (1:1,000;
Invitrogen) diluted in ADB( − ) for 2 h at room temperature. Cultures
were then incubated for 10 minutes with 4% PFA for post-fixation and
stains proceeded as described above.
For all immunocytochemistry experiments, cells were washed and
then permeabilized and blocked for 1 h with ADB with tx-100(+) which
contains 0.3% tx-100 and 5% normal goat serum diluted in DPBS. Non-
surface primary antibodies were diluted in ADB( + ) and cells were
incubated in the cold-room overnight or for 2 h at RT. Cultures were
washed three times with DPBS and then incubated with Alexa Fluor-
Invitrogen) diluted in
conjugated secondary antibodies (1:1000;
ADB( + ) for 1 h at RT. After three additional washes, coverslips were
inverted onto glass microscope slides with Fluoromount-G mounting
media (Southern Biotech).
Immunohistochemistry
Mice were anesthetized with isoflurane and then transcardially per-
fused (~1 ml/min) for 1 min with 0.1 M DPBS (RT) followed by 7 min with
4% PFA (Electron Microscopy Services). For synaptic protein quantifi-
cation using confocal microscopy, OBs were dissected and post-fixed
for 13 minutes at RT. For imaging of the hippocampus and cortex,
brains were post-fixed for 2 h. For STED super resolution microscopy,
OB’s were post-fixed for 10 min at RT, 20 min at RT, and O/N at 4 °C.
Following fixation, tissue was washed 3 times with DPBS and cryo-
protected by a 24–48 h incubation in 30% sucrose w/v in DPBS. Tissue
was embedded in OCT Compound (Sakura), sectioned on the sagittal
plane at 30 µm using a cryostat, and stored as floating sections in DPBS.
For staining, free-floating sections were incubated with blocking buffer
(containing 5% NGS, 1% Pen-Strep, and 0.5% tx-100 in DPBS) for 1 h at
RT. Sections were then incubated with primary antibodies diluted in
blocking buffer overnight at RT on a rocker with slight agitation. After
3 washes, sections were incubated with Alexa dye secondary anti-
bodies diluted in blocking buffer for 1–2 h at RT. For STED imaging,
secondary antibodies raised in goat were conjugated to Abberior STAR
RED, STAR ORANGE, and 460 L and used at 1:400 dilution. For 2-color
STED, only STAR RED and STAR ORANGE secondary antibodies were
used. Sections were washed 3–4 times (with 0.05% tx-100 in PBS) and
then mounted on charged glass slides. After drying, sections were
dipped in water and allowed to dry again. For confocal imaging, per
slide, 4 droplets of Fluormount-G with or without DAPI was added,
slides were coverslipped, and nail polish was used to secure the cov-
erslip until mounting medium hardened. For STED imaging, sections
were mounted on poly-l-lysine coated coverslips prior to being
mounted on a slide using abberior MOUNT, SOLID according to
manufacturer’s instructions. Slides were allowed to solidify for at least
24 h prior to imaging.
Confocal microscopy
All confocal images were acquired at RT using an inverted Nikon A1RSi
confocal microscope equipped with a 20x, 60x, or 100x objective
(Apo, NA 1.4) and operated by NIS-Elements AR acquisition software.
For quantitative analysis of synaptic puncta, high magnification images
were taken at 1024 × 1024 pixels with a z-stack distance of 0.3 µm.
Images were taken at Nyquist to allow sampling at maximum
resolution. Low magnification images to reveal tissue architecture
were taken at 1024 × 1024 pixels with Nyquist recommended step size.
Line averaging (2X) was used for most images. Images were acquired
sequentially in order to avoid bleed-through between channels. Ima-
ging parameters (i.e., laser power, photomultiplier gain, offset, pinhole
size, scan speed, etc.) were optimized to prevent pixel saturation and
kept constant for all conditions within the same experiment. Images
were analyzed using NIS-Elements Advanced Research soft-
ware (Nikon).
For analysis of synapophysin-2 and gephyrin puncta in tissue,
local background subtraction was performed using the rolling ball
method. Moreover, to limit variation due to uneven antibody pene-
tration, images were collected consistently at the same edge of the
section (e.g. side mounted to the slide). For all other image analysis,
background was empirically determined and applied equally to all
images from a given imaging session / independent experimental
replicate. For quantitative analysis, imaging of brain tissue involved
imaging at least 2 regions of interest from 4–5 brain sections. For
cultured neurons, two 15–20 µm dendritic segments were analyzed
from 8–10 neurons per culture batch, per condition. All ICC/IHC data
were collected and analyzed blindly.
For quantifying synaptic puncta, the general analysis module was
used in the NIS-Elements Advanced Research software (Nikon). Binary
masks were applied to each channel (following background subtrac-
tion) for a given image and binary mask settings were optimized and
maintained across the images being compared. To look at co-localized
puncta, a binary operation was used that conditioned a given mask
(e.g. for homer1) as having (“HAVE”, the operator) at least one pixel
overlap with another mask (e.g. for gephyrin or MAP2). Combinations
of binary operations were used to evaluate excitatory and inhibitory
synapse density, surface GABAAR levels, reciprocal and non-reciprocal
synapses (gephyrin/vGAT + /- homer1), etc. For synaptic puncta,
objects smaller than 0.2 µm2 were filtered out. Puncta density was
calculated based by dividing the object number by the area of the field
of view or length of dendritic segment. Average puncta density was
calculated by dividing the binary area by the average number of
objects. For most measurements, intensity is included and is the
average background-subtracted fluorescence intensity within the area
of the assigned binary.
STED microscopy
STED super-resolution images and confocal comparison images were
acquired using a Nikon Ti2-E microscope stand equipped with a STE-
DYCON confocal and STED module from Abberior Instruments, Inc.
Excitation lasers included 488 nm (pulsed), 595 nm (pulsed), and
640 nm (pulsed). A 775 nm STED depletion laser (pulsed) was used.
Detection was performed using time-gated APDs.
Images were
acquired using a CFI PLAN APO LAMBDA 100X OIL objective (NA 1.45)
with a piezoelectric focusing system. Immersion oil F was used for all
imaging. For quantitative analysis, images were acquired using iden-
tical settings. Image settings were optimized to ensure that signal was
acquired below saturation, with saturation levels indicated using the
look-up table. Acquisition laser intensity was scaled ~1.5X higher for
STED imaging and the depletion laser was set to enable 60 nm reso-
lution in all channels. Pixel size was automatically determined based on
the resolution of the acquired image. For STED acquisition, 15-line
accumulations were used. For analysis, 2 fields of view were collected
per section and 4-6 sections per animal. For all images STEDYFOCUS
was used to allow sequential confocal and STED imaging. A single
optical plane was imaged, as there was no depletion laser in the z plane.
For analysis, OME files were exported from the STEDYCON
acquisition software and opened in Hugyen’s Essentials from SVI.
Express deconvolution, which affords minimal improvement in reso-
lution due to single-plane acquisition, was used to perform back-
ground subtraction and images were exported as ASCII files. They were
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then opened in Nikon Elements and puncta were analyzed using the
general analysis software module. Regardless of fixation duration, the
look-up table was scaled to reveal the entire range of signal detected
for fitting of binary masks – thus, even under strong fixation with lower
signal, we sought to quantify number of puncta even if they were
dimmer. Binary mask operations were used to detect dystroglycan
(mask 1) that had at least one pixel overlap with gephyrin (mask 2).
With STED, although substructure is detectable, masks were dilated to
cover the entirety of gephyrin discs, even for large inhibitory synapses
with many nanoclusters, which we considered inhibitory post-
synapses. Density was determined by dividing the number of identi-
fied objects by the area of the field of view. Puncta size was determined
by dividing binary area by the total number of objects identified with a
given mask.
Statistics and reproducibility
Quantifications have been described in the respective materials and
methods sections, and statistical details are provided in the figure
legends and specific p values are described in the Source Data and
Statistics table. Statistical significance between various conditions was
assessed by determining p-values (95% confidence interval). Statistical
analyzes were performed using GraphPad Prism 6 or 9 software and
Microsoft Excel.
For most staining experiments, the “n” represents the average
per animal or average per culture. For qualitative imaging results,
experiments were performed at least 3 times. In contrast, for elec-
trophysiology measurements, the “n” represents the total number
of cells patched from 3 or more batches of cultures, the number of
which is defined in the figures. For biochemical, the “n” generally
represents number of animals, independent cultures, or pooled
samples. Example images of neurobiotin-filled neurons in Figs. 5b,
8e, 9e, and 6b are included to highlight the infection efficiency and
type of neuron being patched, but neurobiotin-filling was not used
as a standard for recording experiments. Nevertheless, the infection
efficiency and type of neuron being patch clamped were performed
on 3+ independent experiments. Most intergroup comparisons
were done by two-tailed unpaired t tests with or without Welch’s
correction. For multiple comparisons, data were analyzed with one-
or two-way ANOVA followed by a post-hoc test (e.g. Dunnett’s
multiple comparison test). Levels of significance were set as
*p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001. All graphs depict
means ± SEM.
Reporting summary
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
Data availability
Source data are provided within this paper. Raw data that support the
findings of this study are available from the corresponding author
upon request. Source data are provided with this paper.
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Acknowledgements
We thank I. Huryeva (Stanford U.) for excellent technical assistance. We
would also like to thank Kevin Wright (OHSU) and Jennifer Jahncke
(OHSU) for sharing tissue for the initial testing of dystroglycan antibodies
for specificity. This study was supported by grants from the NIMH
(MH052804 to T.C.S.; KO1-MH105040-01 to J.H.T), a BBRF Young
Investigator Grant (to J.H.T.), and a Stanford Interdisciplinary Graduate
Fellowship (SIGF) to (C.Y.W.)
Author contributions
J.H.T., C.Y.W., and T.C.S. designed, and J.H.T., C.Y.W., and P.Z., con-
ducted all experiments. J.H.T. performed all molecular cloning, RNA
analysis, and synapse morphology analysis. C.Y.Z. performed all in vivo
recordings and P.Z. performed all in vitro recordings. G.N. performed
animal perfusions and microscopy. J.H.T., C.Y.W., and T.C.S. analyzed
the data and wrote the manuscript; all authors reviewed and approved
the final manuscript.
Competing interests
The authors declare no competing interests.
Additional information
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Nature Communications |
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| null |
10.1371_journal.pone.0283491.pdf
|
n in any medium, provided the original
author and source are credited.
Data Availability Statement: Data cannot be
shared publicly because we have used third-party
data from National Health Insurance Service, and
are not entitled to share the data. Data are available
from the Review Board of National Health
Insurance Service (contact via NHIS) for
researchers who meet the criteria for access to
confidential data. Anyone who conducts a joint
study with a Korean researcher can access NHIS
for customized health information data.
Applications for data are available through National
Health Insurance Data Sharing website (https://
Abstract
Background and purpose
Previous studies on the weekend
|
Data cannot be shared publicly because we have used third-party data from National Health Insurance Service, and are not entitled to share the data. Data are available from the Review Board of National Health Insurance Service (contact via NHIS) for researchers who meet the criteria for access to confidential data. Anyone who conducts a joint study with a Korean researcher can access NHIS for customized health information data. Applications for data are available through National
|
RESEARCH ARTICLE
Weekend effect on 30-day mortality for
ischemic and hemorrhagic stroke analyzed
using severity index and staffing level
Seung Bin KimID
1‡, Bo Mi Lee2‡, Joo Won Park3, Mi Young KwakID
3*, Won Mo JangID
4,5*
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
1 Interdepartment of Critical Care Medicine, Seoul Metropolitan Government-Seoul National University
Boramae Medical Center, Seoul, Republic of Korea, 2 HIRA Research Institute, Health Insurance Review &
Assessment Service, Wonju, Republic of Korea, 3 Center for Public Healthcare, National Medical Center,
Seoul, Republic of Korea, 4 Department of Public Health and Community Medicine, Seoul Metropolitan
Government-Seoul National University Boramae Medical Center, Seoul, Republic of Korea, 5 Department of
Health Policy and Management, Seoul National University College of Medicine, Seoul, Republic of Korea
‡ SBK and BML share first authorship on this work.
* [email protected] (MYK); [email protected] (WMJ)
OPEN ACCESS
Citation: Kim SB, Lee BM, Park JW, Kwak MY,
Jang WM (2023) Weekend effect on 30-day
mortality for ischemic and hemorrhagic stroke
analyzed using severity index and staffing level.
PLoS ONE 18(6): e0283491. https://doi.org/
10.1371/journal.pone.0283491
Editor: Robert Jeenchen Chen, Stanford University
School of Medicine, UNITED STATES
Received: November 7, 2022
Accepted: March 11, 2023
Published: June 22, 2023
Copyright: © 2023 Kim et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data cannot be
shared publicly because we have used third-party
data from National Health Insurance Service, and
are not entitled to share the data. Data are available
from the Review Board of National Health
Insurance Service (contact via NHIS) for
researchers who meet the criteria for access to
confidential data. Anyone who conducts a joint
study with a Korean researcher can access NHIS
for customized health information data.
Applications for data are available through National
Health Insurance Data Sharing website (https://
Abstract
Background and purpose
Previous studies on the weekend effect—a phenomenon where stroke outcomes differ
depending on whether the stroke occurred on a weekend—mostly targeted ischemic stroke
and showed inconsistent results. Thus, we investigated the weekend effect on 30-day mor-
tality in patients with ischemic or hemorrhagic stroke considering the confounding effect of
stroke severity and staffing level.
Methods
We retrospectively analyzed data of patients hospitalized for ischemic or hemorrhagic
stroke between January 1, 2015, and December 31, 2018, which were extracted from the
claims database of the National Health Insurance System and the Medical Resource Report
by the Health Insurance Review & Assessment Service. The primary outcome measure was
30-day all-cause mortality.
Results
In total, 278,632 patients were included, among whom 84,240 and 194,392 had a hemor-
rhagic and ischemic stroke, respectively, with 25.8% and 25.1% of patients, respectively,
being hospitalized during the weekend. Patients admitted on weekends had significantly
higher 30-day mortality rates (hemorrhagic stroke 16.84%>15.55%, p<0.0001; ischemic
stroke 5.06%>4.92%, p<0.0001). However, in the multi-level logistic regression analysis
adjusted for case-mix, pre-hospital, and hospital level factors, the weekend effect remained
consistent in patients with hemorrhagic stroke (odds ratio [OR] 1.05, 95% confidence inter-
val [CI] 1.00–1.10), while the association was no longer evident in patients with ischemic
stroke (OR 1.01, 95% CI 0.96–1.06).
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
1 / 15
PLOS ONEnhiss.nhis.or.kr/bd/ab/bdaba000eng.do), and
additional information can be found at a
customized health information data webpage
(https://nhiss.nhis.or.kr/bd/ab/bdaba032eng.do).
Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
Weekend effect on 30-day stroke mortality
Conclusions
Weekend admission for hemorrhagic stroke was significantly associated with a higher mor-
tality rate after adjusting for confounding factors. Further studies are required to understand
factors contributing to mortality during weekend admission.
Introduction
Many studies have shown that the risk of poor clinical outcomes might be higher for patients
admitted on weekends than for those admitted on weekdays, a phenomenon called the “weekend
effect” [1–5]. In acute stroke management, onset-to-treatment time is critical for both ischemic [3,
6] and hemorrhagic [4, 7] stroke. Since acute stroke can occur at any time, efficient stroke care
should always be provided; one system is the “24/7/365 (hours per day/days per week/days per
year)” emergency system [8]. However, considerable variations exist in the availability of health-
care resources for stroke treatment [9], affecting the clinical outcomes of patients.
Several systematic reviews and meta-analyses have been recently performed in an attempt
to summarize studies on the weekend effect [6, 10]. One study suggested factors related to ser-
vice provision inside and outside the hospital and case-mix factors that may contribute to or
modify the weekend effect [11]. In-hospital factors include lower staffing levels during week-
ends [2], delayed assessment and management, fewer ward rounds [12], and disparities in
resources and expertise [3]. Pre-hospital factors include the timeliness of patient referral and
the availability of ambulance service [11]. Case-mix factors include patient characteristics and
stroke severity.
Most studies investigating weekend effects were conducted on either ischemic stroke [3, 6,
13] or both types of stroke [7, 14, 15]. However, the results varied. Some studies showed no
association between weekend admissions and mortality after adjusting for case-mix factors [8,
14, 16, 17]. In contrast, one study found that hemorrhagic stroke patients admitted on week-
ends had significantly higher in-hospital mortality rates after adjusting for patient characteris-
tics, including comorbidities [7]. Various studies highlighted the unavailability of proper
severity-of-illness measures to accurately adjust for the effect of case-mix factors as a major
limitation [14]. This limitation, commonly observed in claims-based stroke studies, is particu-
larly relevant, as it is a major determinant of stroke outcomes [18, 19]. Ideally, stroke severity
should be evaluated using clinical neurological scales such as the National Institutes of Health
Stroke Scale (NIHSS). However, the claims-based stroke severity index (SSI) can also be used
as a proxy to measure stroke severity [20].
A weekend effect was not observed in a large cohort of patients with ischemic stroke treated
at a stroke center, which was designated by the Brain Attack Coalition. This may be attributed
to the 24/7/365 access to stroke specialists, nurses handling stroke cases, and the organized sys-
tem for delivering care available at stroke centers [16]. However, previous studies did not iden-
tify the timeliness of patient transfers to the hospital. In this study, we attempted to adjust for
factors affecting healthcare delivery at a pre-hospital stage through variables involved in direct
contact with and transferring to severe emergency centers.
Several previous studies indicated that the weekend effect remains unclear even after adjust-
ing for case-mix factors and that the findings are insufficient, as factors influencing emergency
delivery systems were not considered. In this study, we attempted to examine the effect of
weekend admission on 30-day mortality of patients with ischemic and hemorrhagic stroke
after adjusting for explanatory factors, classified into case-mix, pre-hospital level, and hospital
level factors.
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
2 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
Methods
Data source
We retrospectively analyzed data extracted from the National Health Insurance System
(NHIS) claims database from 2015 to 2018 and the Medical Resource Report by the Health
Insurance Review & Assessment Service (HIRA) in 2018. Since almost all payments were
based on a fee-for-service system, the NHIS claims database contains specific disease codes
and all necessary data for reimbursement. These data include patient socio-demographic
information, such as sex, age, health insurance type, residential area, comorbid diseases, diag-
nostic tests, procedures, operations and prescriptions, and outcomes (including deaths). We
constructed the dataset by adding hospital characteristics (e.g., staff and facility) from the Med-
ical Resource Report (HIRA). This study was reviewed and approved by the Institutional
Review Board of Seoul National University College of Medicine (IRB No. 07-2021-8).
Informed consent was waived owing to the retrospective nature of the study.
Study population
We included all hospitalized ischemic and hemorrhagic stroke patients who had been admitted
to the emergency department between January 1, 2015, and December 31, 2018. We excluded
patients aged <20 years and those admitted to clinics using the same cutoffs as previously
described [13]. Stroke was diagnosed using the International Classification of Disease 10th
Revision (ICD-10) primary diagnosis codes: (1) hemorrhagic stroke (ICD-10 codes I60–I62)
and (2) ischemic stroke (ICD-10 code I63) [21]. A single admission episode was defined as
hospitalization and discharge during a single day in the same hospital, as multiple billing data
could be claimed on a single day at the same hospital owing to separate monthly claims.
A total of 286,606 cases were included, and cases with missing values were excluded. Ulti-
mately, 278,632 cases were included for analysis.
Variables
The dependent variable was all-cause mortality within 30 days of each admission for ischemic/
hemorrhagic stroke [13, 17]. We defined the admission date of the first hospitalization for an
episode as the index date, and if the date of death was included within 30 days of the index
date, the case was classified to have mortality within 30 days. Weekend admission was investi-
gated by determining whether patients with ischemic/hemorrhagic stroke were admitted to
the emergency department on a Saturday or Sunday. Patient and hospital characteristics were
classified as covariates.
We classified case-mix and service provision factors as covariates at the in- and pre-hospital
levels [11]. Case-mix factors included age (continuous), sex, health insurance type (national
health insurance or medical aid), income level quartile (Q1–Q4), SSI, and interventions (medi-
cation, procedures, operations) provided to the patient (yes/no) (S1 and S2 Tables) [22–24].
A severe emergency center included a tertiary hospital, regional or local emergency center
(>500 hospital beds), or regional cardiocerebrovascular center. The type of contact with a
severe emergency center was classified as direct or transferring. Direct contact indicates that a
severe emergency patient who required treatment at a center-level institution was sent directly
to a severe emergency center. In contrast, transferring means that a severe emergency patient
first went to a local emergency medical agency or an unqualified institution.
We classified the type of hospital based on the final medical treatment institution if the ini-
tial assessment hospital differs from the final treatment hospital within a single day. Hospital
level service provision included hospital type (tertiary hospital, general hospital, hospital); bed
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
3 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
size (<300 beds, 300–500 beds, 500–1,000 beds, >1,000 beds); ownership (public/private);
stroke center (yes/no), designated by the Korea Stroke Society as an institution equipped with
proper facilities, staffing, and tools to properly provide core treatment for patients with stroke;
intervention volume indicating whether the hospital implemented more than the threshold of
annual intervention (yes/no) (S3 Table) [20, 25–33]; number of physicians including neurolo-
gists, neurosurgeons, emergency medical personnel, and radiologists (25th quartile: 25, 50th
quartile: 32, 75th quartile: 42); and number of nurses (25th quartile: 426, 50th quartile: 724, 75th
quartile: 966). We conducted the pre-hospital level analysis based on the 70 units of the catch-
ment area classified by the Ministry of Health & Welfare (S1 Fig, S4 Table).
Stroke severity index
We used the SSI published in Taiwan [34, 35] as a proxy for the NIHSS in the model. The SSI
was validated by demonstrating a close correlation between SSI results and actual stroke sever-
ity assessed using the NIHSS. The SSI comprises seven claim items: airway suctioning, bacte-
rial sensitivity test, general ward stay, intensive care unit stay, nasogastric intubation,
osmotherapy, and urinary catheterization [34]. We used the criteria developed by customizing
the coefficient values of each of the seven parameters to the Korean HIRA database (S5 Table).
The SSI was obtained using the regression coefficients estimated from a multiple linear regres-
sion equation in a previous study (S6 Table) [20]. The SSI was validated based on the NIHSS
related to the ischemic stroke evaluation index; however, its validity was also confirmed for
hemorrhagic stroke [36].
Statistical analysis
Continuous variables are summarized as mean and standard deviation, and categorical variables
are summarized as frequencies and percentages. Variables were compared between groups
using Student’s t-test for continuous variables and the Chi-square tests for categorical variables.
We performed hierarchical logistic regression analysis using multi-level models with the gener-
alized linear mixed model (GLIMMIX) procedure at three levels, comprising case-mix (patient
level), pre-hospital level (contact type), and hospital level variables. In this analysis, we examined
their association with weekend admission and 30-day mortality after admission. All statistical
analyses were performed using SAS statistical software version 9.3 (SAS Institute Inc., Cary,
NC, USA). All p-values were two-sided and considered significant at <0.05.
Results
Characteristics of patients admitted for stroke
In our study, 84,240 and 194,392 patients were diagnosed with hemorrhagic and ischemic
stroke, respectively (Table 1). In the hemorrhagic stroke group, the mortality rate for weekend
admission was higher than that for weekday admission (16.84%>15.55%; p<0.0001). The rate
of female patients (50.90%>49.21%; p<0.0001) and the rate of patients who had received
interventions (procedures 14.86%>14.13%; operations 16.75%>15.67%, p<0.0001) were
higher in weekend admission than in weekday admission. Regarding the type of contact with
the severe emergency center, the rate of direct type was higher in weekend admission than in
weekday admission (81.32%>79.81%, p<0.0001). Additionally, the mean SSI score in patients
admitted on weekends was higher than that in patients admitted on weekdays (11.13>10.77,
p<0.0001).
The characteristics of patients with 30-day mortality after admission are presented in
Table 2.
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
4 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
Case-mix variables in hemorrhagic stroke
The rate of female patients who died was higher than that among those who survived
(51.69%>49.26%, p<0.0001). The average age of those who died was higher than that of the
survivors (69.18 years>63.19 years, p<0.0001). The average SSI score of patients who died was
higher than that of the survivors (14.4>10.2, p<0.0001). The rate of patients who had not
received intervention was higher among those who died than among the survivors (procedures
89.05%>85.05%, p<0.0001; operation 84.86%>83.90%, p = 0.0052).
Case-mix variables in ischemic stroke
The rate of female patients who died was higher than that among those who survived
(53.88%>42.26%, p<0.0001). The average age of patients who died was higher than that of the
survivors (78.81>70.65 years, p<0.0001). The average SSI score of patients who died was
higher than that of the survivors (12.18>5.72, p<0.0001). The rate of patients who had not
received medication was higher among those who died than among the survivors (16.48%>
Table 1. Characteristics of the study population.
Variables
Hemorrhagic stroke (N = 84,240)
Ischemic stroke (N = 194,392)
Death within 30 days after admission
Died
Alive
Case-mix
Sex
Male
Female
Age (mean, SD)
Income level
Health insurance
1st quartile
2nd quartile
3rd quartile
4th quartile
Medical aid
SSI score (m, SD)
Intervention-Medication*
Yes
No
Intervention-Procedure*
Yes
No
Intervention-Operation*
Yes
No
Pre-hospitallevel
Weekend
Weekday
p-value
Weekend
Weekday
p-value
(N = 21,721)
(N = 62,519)
(N = 50,743)
(N = 143,649)
3,658 (16.84)
9,721 (15.55)
<0.0001
2,569 (5.06)
7,073 (4.92)
<0.0001
18,063 (83.16)
52,798 (84.45)
48,174 (94.94)
136,576 (95.08)
10,666 (49.10)
31,751 (50.79)
<0.0001
28,815 (56.79)
82,306 (57.30)
<0.0001
11,055 (50.90)
30,768 (49.21)
21,928 (43.21)
61,343 (42.70)
69.18 (15.30)
63.19 (14.55)
<0.0001
78.81 (11.11)
70.65 (12.86)
<0.0001
4,186 (19.27)
4,101 (18.88)
4,891 (22.52)
7,047 (32.44)
1,496 (6.89)
11.13 (4.61)
11,847 (18.95)
14,155 (22.64)
20,259 (32.40)
4,359 (6.97)
10.77 (4.65)
11,899 (19.03)
<0.0001
8,997 (17.73)
8,236 (16.23)
25,475 (17.73)
23,583 (16.42)
<0.0001
10,814 (21.31)
30,462 (21.21)
18,680 (36.81)
52,094 (36.26)
4,016 (7.91)
12,035 (8.38)
<0.0001
6.08 (4.00)
6.03 (3.96)
<0.0001
-
-
-
-
47,610 (93.83)
134,199 (93.42)
<0.0001
3,133 (6.17)
9,450 (6.58)
3,227 (14.86)
8,835 (14.13)
<0.0001
4,050 (7.98)
11,513 (8.01)
<0.0001
18,494 (85.14)
53,684 (85.87)
46,693 (92.02)
132,136 (91.99)
3,639 (16.75)
9,797 (15.67)
<0.0001
685 (1.35)
2,022 (1.41)
<0.0001
18,082 (83.25)
52,722 (84.33)
50,058 (98.65)
141,627 (98.59)
Type of contact with severe emergency center
Direct
Transferring
Hospital level
17,663 (81.32)
49,896 (79.81)
<0.0001
39,949 (78.73)
113,494 (79.01)
0.184
4,058 (18.68)
12,623 (20.19)
10,794 (21.27)
30,155 (20.99)
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
(Continued )
5 / 15
PLOS ONETable 1. (Continued)
Variables
Type
Tertiary hospital
General hospital
Hospital
Bed volume
�299
300–499
500–999
�1,000
Ownership
Public
Private
Stroke center
Yes
No
Intervention volume
High
Low
Number of physicians
1st quartile
2nd quartile
3rd quartile
4th quartile
Number of nurses
1st quartile
2nd quartile
3rd quartile
4th quartile
Weekend effect on 30-day stroke mortality
Hemorrhagic stroke (N = 84,240)
Ischemic stroke (N = 194,392)
Weekend
Weekday
p-value
Weekend
Weekday
p-value
(N = 21,721)
(N = 62,519)
(N = 50,743)
(N = 143,649)
10,215 (47.03)
28,988 (46.37)
0.0304
23,434 (46.18)
67,508 (47.00)
0.0039
11,188 (51.51)
32,477 (51.95)
318 (1.46)
1,054 (1.69)
26,095 (51.43)
72,636 (50.56)
1,217 (2.40)
3,505 (2.44)
1,595 (7.34)
5,202 (8.32)
<0.0001
4,891 (9.64)
14,239 (9.91)
<0.0001
2,574 (11.85)
7,530 (12.04)
13,063 (60.14)
37,391 (59.81)
4,489 (20.67)
12,396 (19.83)
6,189 (12.20)
16,548 (11.52)
29,008 (57.17)
81,341 (56.62)
10,655 (21.00)
31,521 (21.94)
4,547 (20.93)
12,702 (20.32)
0.0638
11,532 (22.73)
34,084 (23.73)
<0.0001
10,806 (79.09)
49,817 (79.68)
39,211 (77.27)
109,565 (76.27)
9,131 (42.04)
26,931 (43.08)
0.0077
22,395 (44.13)
63,480 (44.19)
0.8245
12,590 (57.96)
35,588 (56.92)
28,348 (55.87)
80,169 (55.81)
19,853 (91.40)
56,392 (90.20)
<0.0001
44,580(87.85)
125,933(87.67)
0.2691
1,868 (8.60)
6,127 (9.80)
6,163(12.15)
17,716 (12.33)
5,032 (23.17)
5,200 (23.94)
5,732 (26.39)
5,757 (26.50)
4,887 (22.50)
5,707 (26.27)
5,630 (25.92)
5,497 (25.31)
15,171 (24.27)
0.0099
13,034 (25.69)
35,730 (24.87)
0.0001
14,599 (23.35)
16,298 (26.07)
16,451 (26.31)
12,056 (23.76)
33,985 (23.66)
12,570 (24.77)
35,642 (24.81)
13,083 (25.78)
38,292 (26.66)
14,774 (23.63)
0.0086
12,936 (25.49)
35,761 (24.89)
<0.0001
16,248 (25.99)
15,910 (25.45)
15,587 (24.93)
12,485 (24.60)
34,311 (23.89)
12,709 (25.05)
36,400 (25.34)
12,613 (24.86)
37,177 (25.88)
SD, standard deviation; SSI, stroke severity index.
p<0.05 calculated using t-test and χ2 test.
* S1 and S2 Tables present definitions of interventions (medication, procedures, and operations) in ischemic/hemorrhagic stroke.
The results for ischemic stroke were similar to those for hemorrhagic stroke. In the ischemic stroke group, the mortality rate for weekend admission was higher than
that for weekday admission (5.06%>4.92%; p<0.0001). However, regarding the type of contact with the severe emergency center, the rate of direct type was lower on
weekend admission than in weekday admission, although not significantly different (78.73%<79.01%, p = 0.184).
https://doi.org/10.1371/journal.pone.0283491.t001
5.95%, p<0.0001); however, the rate of patients who had not received intervention (procedures
and operation) was lower among those who died than among the survivors (procedures
84.11%<92.41%, p<0.0001; operation 95.59%<98.76%, p<0.0001).
Pre-hospital level variables
Regarding the type of contact with the severe emergency center, the rate for the transferring
type among those who died was higher than among those who survived (hemorrhagic stroke
22.45%>19.30%; ischemic stroke 25.24%>20.85%, p<0.0001).
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
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PLOS ONEWeekend effect on 30-day stroke mortality
Hospital level variables in hemorrhagic stroke
The rate of tertiary hospitals was lower among those who died than among the survivors
(43.34%<47.15%, p<0.0001). Moreover, the rate of hospitals with more than 1000 beds
(16.13%<20.78%, p<0.0001) and the rate of hospitals with a stroke center (52.52%<58.07%,
p<0.0001) were lower in those who died than in those who survived. The rate of hospitals with
a high intervention volume was lower in those who died than in those who survived (87.80%<
91.02%, p<0.0001). Regarding staff numbers, the fewer the number of physicians, the higher
the mortality rate (p<0.0001). This trend was also observed for the number of nurses
(p<0.0001).
Hospital level variables in ischemic stroke
The rate of tertiary hospitals was lower among those who died than among the survivors
(42.45%<47.01%, p<0.0001). Moreover, the rate of hospitals with more than 1000 beds
(17.68%<21.91%, p<0.0001) and the rate of hospitals with a stroke center (50.88%<56.08%,
Table 2. Characteristics of patients with 30-day mortality after admission.
Variable
Hemorrhagic stroke (N = 84,240)
Ischemic stroke (N = 194,392)
Hospitalization
Weekday
Weekend
Case-mix
Sex
Male
Female
Age
Income level
Health insurance
1st quartile
2nd quartile
3rd quartile
4th quartile
Medical aid
SSI score
Intervention-Medication*
Yes
No
Intervention-Procedure*
Yes
No
Intervention-Operation*
Yes
No
Pre-hospital level
Died
Alive
p-value
Died
Alive
p-value
(N = 13,379)
(N = 70,861)
(N = 9,642)
(N = 184,750)
9,721 (72.66)
3,658 (27.34)
52,798 (74.51)
<0.0001
18,063 (25.49)
7,073 (73.36)
2,569 (26.64)
136,576 (73.92)
0.2152
48,174 (26.08)
6,464 (48.31)
6,915 (51.69)
69.18 (15.3)
2,407 (17.99)
2,343 (17.51)
2,877 (21.50)
4,465 (33.37)
1,287 (9.62)
14.4 (3.55)
35,953 (50.74)
<0.0001
34,908 (49.26)
63.19 (14.55)
<0.0001
13,678 (19.30)
<0.0001
13,605 (19.20)
16,169 (22.82)
22,841 (32.23)
4,568 (6.45)
10.2 (4.52)
4,447 (46.12)
5,195 (53.88)
78.81 (11.11)
1,712 (17.76)
1,417 (14.70)
1,873 (19.43)
3,631 (37.66)
1,009 (10.46)
106,674 (57.74)
<0.0001
<0.0001
<0.0001
78,076 (42.26)
70.65 (12.86)
32,760 (17.73)
30,402 (16.46)
39,403 (21.33)
67,143 (36.34)
15,042 (8.14)
<0.0001
12.18 (4.64)
5.72 (3.66)
<0.0001
-
-
-
-
1,465 (10.95)
10,597 (14.95)
<0.0001
11,914 (89.05)
60,264 (85.05)
8,053 (83.52)
1,589 (16.48)
1,532 (15.89)
8,110 (84.11)
173,756 (94.05)
<0.0001
10,994 (5.95)
14,031 (7.59)
<0.0001
170,719 (92.41)
2,026 (15.14)
11,410 (16.10)
0.0052
425 (4.41)
2,282 (1.24)
<0.0001
11,353 (84.86)
59,451 (83.90)
9,217 (95.59)
182,468 (98.76)
Type of contact with severe emergency center
Direct
Transferring
Hospital level
10,375 (77.55)
57,184 (80.70)
<0.0001
3,004 (22.45)
13,677 (19.30)
7,208 (74.76)
2,434 (25.24)
146,235 (79.15)
<0.0001
38,515 (20.85)
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
(Continued )
7 / 15
PLOS ONETable 2. (Continued)
Variable
Type
Tertiary hospital
General hospital
Hospital
Bed volume
�299
300–499
500–999
�1000
Ownership
Public
Private
Stroke center
Yes
No
Intervention volume
High
Low
Number of physicians
1st quartile
2nd quartile
3rd quartile
4th quartile
Number of nurses
1st quartile
2nd quartile
3rd quartile
4th quartile
Weekend effect on 30-day stroke mortality
Hemorrhagic stroke (N = 84,240)
Ischemic stroke (N = 194,392)
Died
Alive
p-value
Died
Alive
p-value
(N = 13,379)
(N = 70,861)
(N = 9,642)
(N = 184,750)
5,794 (43.34)
7,256 (54.23)
329 (2.46)
1,276 (9.54)
1,787 (13.36)
8,158 (60.98)
2,158 (16.13)
33,409 (47.15)
<0.0001
36,409 (51.38)
1,043 (1.47)
5,521 (7.79)
8,317 (11.74)
42,296 (59.69)
14,727 (20.78)
<0.0001
2,573 (19.23)
14,670 (20.70)
0.1785
10,806 (80.77)
56,191 (79.30)
7,027 (52.52)
6,352 (47.48)
41,151 (58.07)
<0.0001
29,710 (41.93)
11,747 (87.80)
64,498 (91.02)
<0.0001
1,632 (12.20)
6,363 (8.98)
3,677 (27.48)
3,260 (24.37)
3,506 (26.21)
2,936 (21.94)
3,560 (26.61)
3,542 (26.47)
3,418 (25.55)
2,859 (21.37)
16,526 (23.32)
<0.0001
16,539 (23.34)
18,524 (26.14)
19,272 (27.20)
16,101 (22.72)
<0.0001
18,413 (25.98)
18,122 (25.57)
18,225 (25.72)
4,093 (42.45)
5,230 (54.24)
319 (3.31)
1,156 (11.99)
1,322 (13.71)
5,459 (56.62)
1,705 (17.68)
2,229 (23.12)
7,413 (76.88)
4,906 (50.88)
4,736 (49.12)
8,123 (84.25)
1,519 (15.75)
2,915 (30.23)
2,372 (24.60)
2,262 (23.46)
2,093 (21.71)
2,878 (29.85)
2,316 (24.02)
2,378 (24.66)
2,070 (21.47)
<0.0001
<0.0001
86,846 (47.01)
93,501 (50.61)
4,403 (2.38)
17,974 (9.73)
21,415 (11.59)
104,890 (55.77)
40,471 (21.91)
43,387 (23.48)
0.4069
141,363 (76.52)
103,611 (56.08)
<0.0001
81,139 (43.92)
162,390 (87.90)
<0.0001
22,360 (12.10)
45,849 (24.82)
43,669 (23.64)
45,950 (24.87)
49,282 (26.67)
45,819 (24.80)
44,480 (24.08)
46,731 (25.29)
47,720 (25.83)
<0.0001
<0.0001
SSI, stroke severity index.
Data are presented as mean±standard deviation or n (%). p<0.05 calculated using t-test and χ2 test.
* S1 and S2 Tables present definitions of interventions (medication, procedures, and operations) in ischemic/hemorrhagic stroke.
https://doi.org/10.1371/journal.pone.0283491.t002
p<0.0001) were lower in those who died than among the survivors. The rate of hospitals with
a high intervention volume was lower in those who died than among the survivors (84.25%<
87.90%, p<0.0001). Regarding staff numbers, the fewer the number of physicians and nurses,
the higher the mortality rate (p<0.0001 for both professionals).
Multi-level logistic regression analysis in hemorrhagic and ischemic stroke
Mortality risk in patients with hemorrhagic stroke. Patients admitted on weekends had
a significantly higher 30-day mortality risk than those admitted on weekdays (odds ratio [OR]
1.05, 95% confidence interval [CI] 1.00–1.10). Regarding the case-mix variables, older age (OR
1.02; 95% CI 1.02–1.02), medical aid (ref = quartile 4 of health insurance; OR 1.16; 95% CI
1.06–1.27), and a higher SSI score (OR 1.29; 95% CI 1.28–1.30) were associated with a higher
mortality risk. The mortality risk of patients who did not receive intervention was higher than
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
8 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
that of patients who received the intervention (OR 1.94, 95% CI 1.80–2.09). Furthermore,
patients who did not undergo an operation had a higher mortality risk (OR 2.08, 95% CI 1.94–
2.24) than did patients who underwent an operation (Table 3).
Regarding pre-hospital level variables, the mortality risk for the transferring type of contact
with a severe emergency center was higher than that for the direct type, although not signifi-
cantly different (OR 1.06, 95% CI 0.84–1.33). Additionally, the subgroup analysis showed that
patients who had the transferring type of contact with a severe emergency center had higher
30-day mortality for weekend admission (OR 1.36; 95% CI 1.06–1.75, S7 Table). Regarding
hospital level variables, lower-level hospitals had a higher mortality risk than tertiary hospitals
(OR 2.15; 95% CI 1.69–2.73). Hospitals with a bed volume of 500–1,000 had a higher mortality
risk than hospitals with �1,000 beds (OR 1.18; 95% CI 1.08–1.35). Hospitals with low inter-
vention volumes had a higher mortality risk than those with high intervention volumes (OR
1.45; 95% CI 1.28–1.64).
Mortality risk in patients with ischemic stroke. The effect of weekend admission on
mortality was not statistically significant in patients with ischemic stroke (OR 1.01, 95% CI
Table 3. Multi-level logistic regression analysis of 30-day mortality.
Variable
Hospitalization
Weekend
Weekday
Case-mix
1st quartile
2nd quartile
3rd quartile
4th quartile
Sex
Male
Female
Age
Income level
Health insurance
Medical aid
SSI score
Intervention-Medication*
Yes
No
Intervention-Procedure*
Yes
No
Intervention-Operation*
Yes
No
Pre-hospital level
Type of contact with severe emergency center
Direct
Transferring
Hospital level
Hemorrhagic stroke
OR
95% CI
Ischemic stroke
p-value
OR
95% CI
p-value
1.05
1.00
1.00
0.97
1.02
0.95
1.00
0.99
1.00
1.16
1.29
-
-
1.00
1.94
1.00
2.08
1.00
1.06
1.00–1.10†
0.0357
0.92–1.02
1.02–1.02†
0.2424
<0.0001
0.90–1.00
0.93–1.06
0.93–1.04
0.0468
0.8905
0.6442
1.06–1.27†
1.28–1.30†
0.001
<0.0001
-
-
1.80–2.09†
<0.0001
1.94–2.24†
<0.0001
0.84–1.33
0.6528
1.01
1.00
1.00
1.09
1.04
1.09
1.07
1.03
1.00
0.97
1.35
1.00
3.84
1.00
1.13
1.00
1.43
1.00
1.02
0.96–1.06
0.7315
0.99–1.10
1.04–1.04†
1.02–1.16†
0.99–1.15
0.97–1.10
0.90–1.05
1.34–1.36†
0.1058
<0.0001
0.0071
0.071
0.3916
0.4277
<0.0001
3.48–4.23†
<0.0001
1.04–1.24†
0.0048
1.27–1.61†
<0.0001
0.70–1.49
0.9334
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
(Continued )
9 / 15
PLOS ONETable 3. (Continued)
Variable
Hemorrhagic stroke
OR
95% CI
Ischemic stroke
p-value
OR
95% CI
p-value
Weekend effect on 30-day stroke mortality
Type
Tertiary hospital
General hospital
Hospital
Bed volume
�299
300–499
500–999
�1000
Ownership
Public
Private
Stroke center
Yes
No
Intervention volume
High
Low
Number of physicians
1st quartile
2nd quartile
3rd quartile
4th quartile
Number of nurses
1st quartile
2nd quartile
3rd quartile
4th quartile
1.00
0.95
2.15
1.15
1.17
1.18
1.00
1.00
0.99
1.00
1.03
1.00
1.45
1.12
1.08
1.08
1.00
0.93
0.97
1.04
1.00
0.88–1.04
1.69–2.73†
0.85–1.55
0.89–1.54
1.03–1.35†
0.2775
<0.0001
0.3717
0.2603
0.0154
0.90–1.10
0.9162
0.97–1.09
0.3533
1.28–1.64†
<0.0001
0.95–1.31
0.95–1.22
0.97–1.21
0.77–1.13
0.87–1.09
0.93–1.15
0.1738
0.2507
0.1707
0.4717
0.635
0.495
1.00
0.94
1.55
0.86
0.94
1.09
1.00
1.00
1.03
1.00
0.93
1.00
1.30
1.02
1.10
1.05
1.00
0.93
0.94
0.89
1.00
0.82–1.07
1.22–1.97†
0.54–1.36
0.60–1.48
0.93–1.27
0.3317
0.0004
0.5086
0.8000
0.3080
0.95–1.11
0.5139
0.87–1.00
0.0527
1.09–1.56†
0.0045
0.78–1.35
0.94–1.30
0.923–1.19
0.77–1.11
0.8–1.07
0.80–0.99†
0.8721
0.2467
0.4691
0.4089
0.3308
0.0294
†p<0.05 calculated using logistic regression analysis.
OR, odds ratio; CI, confidence interval; SSI, stroke severity index.
* S1 and S2 Tables present definitions of interventions (medication, procedures, and operations) in ischemic/hemorrhagic stroke.
https://doi.org/10.1371/journal.pone.0283491.t003
0.96–1.06). Regarding case-mix variables, older age (OR 1.04; 95% CI 1.04–1.04), medical aid
(ref = quartile 4 of health insurance; OR 1.09; 95% CI 1.02–1.16), and a higher SSI score (OR
1.35; 95% CI 1.34–1.36) had a higher mortality risk. Patients without interventions had a
higher mortality risk (medication OR 3.84, 95% CI 3.48–4.23; procedure OR 1.13, 95% CI
1.04–1.24; operation OR 1.43, 95% CI 1.27–1.61) (Table 3).
Regarding pre-hospital level variables, the mortality risk for the transferring type of contact
with the severe emergency center was higher than that for the direct type, although not signifi-
cantly different (OR 1.02; 95% CI 0.70–1.49). Regarding hospital level variables, lower-level
hospitals had a higher mortality risk than tertiary hospitals (OR 1.55; 95% CI 1.22–1.97). Hos-
pitals with low intervention volumes had a higher mortality risk than those with high interven-
tion volumes (OR 1.30; 95% CI 1.09–1.56). The mortality risk regarding staffing numbers
(physicians and nurses) was not statistically significant.
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
10 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
Discussion
Our study explored the effect of weekend admission on 30-day mortality in patients with
ischemic or hemorrhagic stroke and confirmed explanatory factors influencing the weekend
effect. We found higher mortality rates for weekend admission in patients with hemorrhagic
stroke, even after adjusting for case-mix and service provision in-/pre-hospital variables.
We found that hemorrhagic stroke patients admitted during weekends had a higher risk of
death than those admitted on weekdays. Previous studies found that weekend admission for
ischemic stroke was associated with higher 30-day all-cause mortality [3, 6, 13]. In contrast,
other studies did not find significant associations between weekend admission for ischemic
stroke and higher mortality [8, 16, 17, 37, 38]. When confirming the weekend effect for total
stroke, the results showed that mortality was significantly higher in patients with hemorrhagic
stroke admitted on weekends than in those admitted on weekdays but not in patients with
ischemic stroke [7], which is consistent with our findings. However, previous studies failed to
adjust for variables, including stroke severity and time from onset to arrival. Empirical evi-
dence of the weekend effect on stroke mortality is mixed, with some studies indicating signifi-
cantly higher mortality for weekend admissions and others finding no differences [14]. The
presence or magnitude of the weekend effect varies based on the types of admission, case-mix
factors and illness severity, geographic location, as well as contextual and methodological fac-
tors [11]. Thus, we analyzed the weekend effect by classifying confounding factors into service
provision hospital level, case-mix, and service provision pre-hospital level variables.
Some studies showed that the weekend effect on mortality disappeared after adjusting for
stroke severity scores, such as the NHISS and Charlson comorbidity index. The results also
varied depending on the severity scale used [3, 7, 8, 13, 14, 16, 17]. This study used the SSI, a
claims-based proxy for stroke severity. Patients admitted on weekends had a higher SSI score
than those admitted on weekdays, whereas the number of admissions on weekends was lower
than that on weekdays (S2 and S3 Figs). A negative correlation was found between the SSI
score and the number of admissions by the day of the week, which is consistent with the results
of previous studies [17]. This result is thought to be due to the fact that patients who had mild
stroke during weekends postponed their hospital visits to a weekday, as described in a previous
study [17]; this event is called the “Monday effect.” A review of previous studies [39] that ana-
lyzed the number of patients with stroke onset and hospital presentation by the day of the
week corroborates this finding; there was minimal difference in the number of patients with
stroke onset and hospital presentation for moderate or severe stroke, but a significant differ-
ence was observed for mild stroke. However, their results showed that the weekend effect dis-
appeared after adjusting for SSI [17]. In our study, although there was a decrease in the effect,
it remained statistically significant in the case of hemorrhagic stroke. This could be attributed
to factors other than the difference in disease severity at hospitalization, which were deemed
important in the weekend effect in hemorrhagic stroke. A subgroup analysis was performed
on hemorrhagic stroke to analyze these additional factors, and the results are discussed below.
In our study, hospital intervention volume significantly affected mortality, although we
could not find an effect of staffing level. The problem of resource allocation between weekdays
and weekends has been reported, with physician volume and experience level considered to be
important factors [3, 6]. However, the effect of staffing numbers was not significant as shown
by the multi-level logistic regression analysis (Table 3). Reduced availability of clinical person-
nel on weekends may reduce the quality of care and influence outcomes following stroke [40].
Evidence from previous studies suggests that specialized stroke units, with around-the-clock
availability of specialist stroke teams and rapid access to imaging and thrombolysis, reduce
variations in the quality of care and outcomes throughout the week [16, 31, 37]. Our study
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
11 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
found that intervention volume significantly affected mortality, suggesting that the availability
of treatments may be more important than staffing numbers for patient outcomes.
Moreover, we found that the healthcare delivery system may influence the weekend effect
on patients with hemorrhagic stroke. We conducted a subgroup analysis by dividing patients
with hemorrhagic stroke who showed significant weekend effects into weekend and weekday
groups after adjusting for various confounding factors (S7 Table). Among hemorrhagic stroke
patients admitted on weekends, patients who had transferred with the severe emergency center
had higher 30-day mortality after adjusting for the stroke severity. This suggests that patients
with hemorrhagic stroke may have problems being directly admitted to the hospital on a week-
end. Therefore, it is important to establish an extensive cooperative system with severe emer-
gency centers and other medical facilities to ensure the availability of interventions and other
resources, even on weekends. Reorganizing stroke care to provide 24/7 access to stroke special-
ists, adequate staffing of nurses handling stroke cases on weekends, and an organized system
for delivering care may alleviate the weekend effect and save lives [16, 17, 41, 42].
Our study has some limitations. First, we applied the SSI after validation in Korea [20],
using a study methodology in Taiwan. Further investigations are required to determine the
applicability of the SSI as a proxy for stroke severity in claims databases from other healthcare
systems. Second, the diagnosis accuracy of stroke could not be guaranteed from the claims
data. However, we expect that the diagnostic consistency in stroke could be generally reliable,
and we extracted the population at primary diagnosis [43]. Third, we did not measure other
indicators of care quality, such as onset-to-treatment time, time to assessment of rehabilitation,
the intensity of rehabilitation therapy, and patient education level. Finally, we investigated the
hospital level characteristics such as staffing level and bed number. However, we defined a
weekend as Saturday and Sunday only, and we did not include holidays if they fell on week-
days. Moreover, we could not examine variables that reflected differences in staffing level,
including the number and experience of staff members during weekdays and weekends.
Therefore, we could not identify the effect of differences in staffing levels on weekends.
In summary, the significance of the weekend effect on mortality was maintained in hemor-
rhagic stroke but not in ischemic stroke after adjusting for case-mix, hospital level, and pre-
hospital level factors. Further research is required to analyze mortality in weekend and week-
day holidays with more detailed variables for patient risk factors, hospital staffing level
changes, and emergency delivery systems. Our results suggested that further endeavor is
needed to find effective measures, including a systematic approach for mitigating the weekend
effect on hemorrhagic stroke. More research is required to explore the proper outcome vari-
able (e.g., disability occurrence) for ischemic stroke’s weekend effect.
Supporting information
S1 Fig. Rationale for using RI*CI for the consolidation of catchment areas.
(TIF)
S2 Fig. Thirty-day mortality after hospital admission and the mean stroke severity index
score according to the day of the week.
(TIF)
S3 Fig. Number of admissions according to the day of the week.
(TIF)
S1 Table. Interventions for ischemic stroke.
(DOCX)
PLOS ONE | https://doi.org/10.1371/journal.pone.0283491 June 22, 2023
12 / 15
PLOS ONEWeekend effect on 30-day stroke mortality
S2 Table. Interventions for hemorrhagic stroke.
(DOCX)
S3 Table. Threshold for annual intervention.
(DOCX)
S4 Table. Information on 70 hospital service areas.
(DOCX)
S5 Table. Seven parameters comprising the stroke severity index and associated explana-
tions.
(DOCX)
S6 Table. Multiple linear regression model for evaluating the stroke severity index.
(DOCX)
S7 Table. Subgroup analysis for 30-day mortality according to admission on weekends
among patients with hemorrhagic stroke.
(DOCX)
Author Contributions
Conceptualization: Won Mo Jang.
Data curation: Bo Mi Lee, Joo Won Park, Mi Young Kwak, Won Mo Jang.
Formal analysis: Seung Bin Kim, Joo Won Park, Mi Young Kwak, Won Mo Jang.
Investigation: Seung Bin Kim, Bo Mi Lee, Mi Young Kwak, Won Mo Jang.
Supervision: Mi Young Kwak, Won Mo Jang.
Validation: Seung Bin Kim, Bo Mi Lee, Joo Won Park, Mi Young Kwak, Won Mo Jang.
Writing – original draft: Seung Bin Kim, Bo Mi Lee.
Writing – review & editing: Seung Bin Kim, Bo Mi Lee, Joo Won Park, Mi Young Kwak,
Won Mo Jang.
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10.1371_journal.pone.0222639.pdf
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Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
|
All relevant data are within the manuscript and its Supporting Information files.
|
RESEARCH ARTICLE
Heat stress responses in a large set of winter
wheat cultivars (Triticum aestivum L.) depend
on the timing and duration of stress
Krisztina BallaID
Marianna Mayer3, Szilvia Bencze4, Otto´ Veisz3
1*, Ildiko´ Karsai1, Pe´ ter Bo´ nis2, Tibor Kiss1, Zita Berki1, A´ da´ m Horva´ th1,
1 Molecular Breeding Department, Agricultural Institute, Centre for Agricultural Research, Hungarian
Academy of Sciences, Martonva´sa´ r, Hungary, 2 Crop Production Department, Agricultural Institute, Centre
for Agricultural Research, Hungarian Academy of Sciences, Martonva´sa´ r, Hungary, 3 Cereal Breeding
Department, Agricultural Institute, Centre for Agricultural Research, Hungarian Academy of Sciences,
Martonva´ sa´r, Hungary, 4 Research Institute of Organic Agriculture, Budapest, Hungary
* [email protected]
Abstract
The adverse effects of heat on plant yield strongly depend on its duration and the phenologi-
cal stage of the crops when the heat occurs. To clarify the effects of these two aspects of
heat stress, systematic research was conducted under controlled conditions on 101 wheat
cultivars of various geographic origin. Different durations of heat stress (5, 10 and 15 days)
were applied starting from three developmental stages (ZD49: booting stage, ZD59: head-
ing, ZD72: 6th day after heading). Various morphological, yield-related traits and physiologi-
cal parameters were measured to determine the stress response patterns of the wheat
genotypes under combinations of the duration and the timing of heat stress. Phenological
timing significantly influenced the thousand-kernel weight and reproductive tiller number.
The duration of heat stress was the most significant component in determining both seed
number and seed weight, as well as the grain yield consequently, explaining 51.6% of its
phenotypic variance. Irrespective of the developmental phase, the yield-related traits gradu-
ally deteriorated over time, and even a 5-day heat stress was sufficient to cause significant
reductions. ZD59 was significantly more sensitive to heat than either ZD49 or ZD72. The
photosynthetic activity of the flag leaf was mostly determined by heat stress duration. No
significant associations were noted between physiological parameters and heat stress
response as measured by grain yield. Significant differences were observed between the
wheat genotypes in heat stress responses, which varied greatly with developmental phase.
Based on the grain yield across developmental phases and heat stress treatments, eight
major response groups of wheat genotypes could be identified, and among them, three clus-
ters were the most heat-tolerant. These cultivars are currently included in crossing
schemes, partially for the identification of the genetic determinants of heat stress response
and partially for the development of new wheat varieties with better heat tolerance.
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OPEN ACCESS
Citation: Balla K, Karsai I, Bo´nis P, Kiss T, Berki Z,
Horva´th A´, et al. (2019) Heat stress responses in a
large set of winter wheat cultivars (Triticum
aestivum L.) depend on the timing and duration of
stress. PLoS ONE 14(9): e0222639. https://doi.org/
10.1371/journal.pone.0222639
Editor: Aimin Zhang, Institute of Genetics and
Developmental Biology Chinese Academy of
Sciences, CHINA
Received: May 31, 2019
Accepted: September 4, 2019
Published: September 20, 2019
Copyright: © 2019 Balla et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
Funding: This research program was funded by a
grant from the National Scientific Research Fund
(NKFIH) K-119801 and by the GINOP-2.3.2-15-
2016-00029 (Economic Development Operational
Programme) project. The funders had no role in
study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
PLOS ONE | https://doi.org/10.1371/journal.pone.0222639 September 20, 2019
1 / 20
Competing interests: The authors have declared
that no competing interests exist.
Heat stress tolerance of wheat
Introduction
Global climate change is increasingly affecting crop production. Extreme weather conditions,
especially temperature and rainfall anomalies, have a substantial influence on the success of
cultivation. Unusually high temperature is one of the most frequent forms of abiotic stress,
which represents a great danger to crop production. Extreme temperature events are expected
to become more frequent in many main wheat-producing regions. These weather conditions
can be characterised with short-term durations and temperature increases of over 5˚C above
the normal temperature [1–3]. Increasing trends can be observed in the number of heat (Tmax
�30˚C) and hot days (Tmax � 35˚C) [4].
The ability of wheat to adapt to a wide range of ecological conditions has made it one of the
most important crops worldwide, but heat stress has severe negative effects on yield, especially
when associated with other stress factors. Combined stress frequently affects wheat plants dur-
ing heading or in the grain-filling period, making it essential to intensify research on the effects
of heat stress [5–7]. The extent of damage is greatly influenced by the phenophase in which the
plants are subjected to stress [8]. The flowering stage has generally been found to be the most
sensitive to heat stress [9] because both meiosis and pollen growth are negatively affected.
Complex interactions between the timing of phenological stages and the sensitivity of different
growth phases to the environment influence the final yield [10].
The threshold temperature of vegetative development was reported to be 20–30˚C in wheat
[11], whereas that of reproductive growth was 15˚C [12]. According to Tewolde et al. [13],
anthesis and grain filling have a threshold temperature of 12–22˚C, with significant reductions
in grain yield at higher temperatures. The adverse effects of heat depend on the magnitude,
timing and the duration of the stress. It was reported by Porter and Gawith [8] that in the
period around flowering, the maximum temperature that wheat can endure without a decline
in grain number is 31˚C. This period was designated as lasting from approximately 20 days
before anthesis to 10 days after anthesis [14,15]. Higher temperatures accelerate the onset of
anthesis, with the consequence that there are fewer spikelets per spike [16]. In addition, high
temperature near anthesis leads to reduced pollen fertility or sterile grains due to the negative
effect of heat (>30˚C) on pollen viability, leading to poor fertilisation, abnormal ovary devel-
opment, slower pollen growth and thus a reduction in seed setting [17–20].
Wheat is often exposed to short periods of high temperature (33–40˚C) during flowering
and grain filling [21–23]. A 3-day period of very high temperature (max. 40˚C) after anthesis
was found to reduce the grain number and weight and to result in a larger number of
deformed grains [24]. Even a single day of heat stress might cause serious damage to the grain
yield and yield components. Rahman et al. [25] reported that high temperature led to greatly
accelerated development, flowering and ripening. The grain-filling period could be 3–12 days
shorter as a consequence of heat treatment [26,27]. Other authors reported that high tempera-
tures reduced the grain-filling period by 45–60% [28,29]. However, considerable genetic vari-
ability was observed in the extent to which the grain-filling period was actually affected [30].
High temperature has a notably complex effect on numerous physiological processes,
which in turn influence the photosynthetic activity of wheat plants [31,32]. Photosynthesis is
one of the main metabolic processes that influence cereal yields [33], so net photosynthetic
activity and chlorophyll content are important indicators of the adaptation of wheat to heat
and other abiotic stress factors [34]. Both photosynthesis and dry matter yield depend on the
development of optimum leaf area. Plant senescence begins with the breakdown of chlorophyll
molecules, leading to retarded photosynthetic activity [35,36]. Heat stress during anthesis and
grain filling were found to accelerate the degradation of the leaf chlorophyll content, resulting
in a decrease in both leaf photosynthetic activity and in final biomass [31,37,38]. High
PLOS ONE | https://doi.org/10.1371/journal.pone.0222639 September 20, 2019
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Heat stress tolerance of wheat
temperatures also led to an increase in the rate of leaf senescence [23,39,40]. Both the grain
yield and the quality are adversely affected by heat. Bhullar and Jenner [41] reported that the
translocation of photosynthetic assimilates during grain filling was negatively affected by heat
stress, resulting in a decline in grain quality. Heat stress causes tissue dehydration and poorer
CO2 assimilation during the reproductive stage [42]. The uptake of CO2 from the air is influ-
enced by stomatal closure and opening, so the dependence of this process on temperature is of
great importance. The enhanced transpiration caused by high temperature induces stomatal
closure, which has an indirect effect on the fixation of CO2 in the course of photosynthesis.
The inhibition of photosystem II (PSII), the most thermally labile component of the photosyn-
thetic electron transport chain, might be responsible for the retardation of photosynthesis, the
disruption of electron transport activity and the inactivation of the oxygen-evolving enzymes
of PSII, which lead to lower rates of ribulose-1,5-bisphosphate (RuBP) regeneration [30,43–
46].
To further improve abiotic stress tolerance, it is highly important to evaluate the diversity
in the stress reaction types of cultivated wheat varieties and to identify genotypes with higher
levels of stress tolerance. A strong need also exists to identify and characterise the various
mechanisms involved in tolerance and to identify the genetic components underlying these
mechanisms. However, most studies use only a limited number of wheat genotypes [19,47,48],
and notably little research has systematically compared the effects of heat stress of various
durations when applied in various developmental phases.
Because assessment of heat tolerance is an important component in breeding programmes
aimed at improving the ecological adaptation of cereals, research on heat stress tolerance has
begun in the Agricultural Institute (MTA ATK MGI), in Martonva´sa´r [49–51]. To identify the
various types of stress responses in different wheat cultivars, experiments were conducted
under controlled growth conditions, making it possible to use the same experimental setup
across the different experiments and to apply heat stress at exactly the same phenological stage
in each wheat cultivar, making the comparisons more precise.
Based on previous studies, a systematic research was planned, including a large set of wheat
cultivars with wide genetic background (i) to evaluate the effect of various durations of heat
stress in different plant developmental phases on physiological and yield-related traits and (ii)
to apply detailed phenotypic characterisation under various heat stress treatments, making it
possible (iii) to analyse the heat-stress dependent associations between the various components
in forming grain yield and (iv) to identify whether specific clustering of wheat genotypes can
be found based on their heat stress response profiles, as measured by the changes in grain
yield /plant. This information will make it possible to initiate crosses between the various
members of the identified heat stress response clusters both for breeding purposes and for
evaluation of the genetic components of heat stress tolerance.
Material and methods
Crop management
A total of 101 winter wheat varieties with different geographic origins (S1 Table) were included
in a series of experiments performed under controlled conditions in the greenhouse and phy-
totron to study their responses to various durations of heat stress applied at different develop-
mental stages. The heat stress responses of the wheat varieties were determined in three
independent experiments in which the same standard plant raising protocols were applied.
One experiment covered screening of heat stress response in one phenological phase, and the
three developmental phases examined were the booting stage (ZD49), the heading stage
(ZD59) when the emergence of the inflorescences was complete, and the 6th day after heading
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Heat stress tolerance of wheat
(ZD72). Heat stress treatment lasted for 5 (H5), 10 (H10) or 15 (H15) days in all three develop-
mental stages. The phenophases of the plants were monitored every day and were determined
based on the double-digit system of the Zadoks scale [52]. As a result of monitoring the devel-
opment, every wheat plant received heat stress treatment in the same specific developmental
stage examined within the given experiment.
In all three experiments, the germinated seedlings were vernalised in peat blocks for 60
days at 4˚C with low light intensity and short daylength, and the plantlets were transferred to
individual pots holding approximately 1.5 kg of a 3:2:1 mixture of garden soil, compost and
sand. The plants were raised under greenhouse conditions with daily watering and a twice-
weekly supply of nutrients (Volldu¨nger Solution, Linz, Austria, in tap-water).
Environmental conditions
After vernalisation treatment, the plants were raised in greenhouse under a relatively standard
conditions in which the ambient temperature ranged between 25˚C (day) and 19˚C (night),
and the natural light conditions were supplemented with artificial light of 170 μmol m–2 s–1
intensity produced by metal halide lamps to reach a 16-hour photoperiod regime per 24-hour
cycle. In each separate experiment of the three developmental phases, 18 plants of each geno-
type were raised in individual pots in the greenhouse and rotated regularly during the process
of monitoring their developmental patterns. Twelve of the original 18 plants with the most
similar developmental and phenological aspects were selected and included in the stress exper-
iment, resulting in 3 plants per treatment as biological replications (C, H5, H10 and H15). The
control plants of the three separate experiments represented partial technical replications.
Control plants of each variety were raised in the greenhouse throughout the lifecycle, and
the planned-stress plants of each cultivar at the given phenophase were transferred to a heat
stress chamber (Conviron PGV-36) in the Martonva´sa´r phytotron for a given period of time
(H5, H10 or H15). At the end of the treatment period, the plants were carried back to the
greenhouse and raised with the control plants until maturity. In the heat stress chamber, the
plants were kept under a 16-hour photoperiod regime and a light intensity of 350 μmol m–2 s–1
produced by metal halide lamps. The temperature profile was applied as follows: a night tem-
perature of 20˚C, a day temperature that gradually increased to 36˚C and was held for 8 hours,
followed by a gradual decrease of the temperature to 20˚C. The relative humidity (RH%) was
set to 64–68% during the day and 76% at night in the stress chamber. To calculate the vapour-
pressure deficit (VPD), we applied the formula VPD = (100-RH)/100�SVP, where RH is rela-
tive humidity and SVP is saturated vapour pressure (CronkLab: http://cronklab.wikidot.com/
calculation-of-vapour-pressure-deficit). Based on this calculation, the VPD in the heat stress
chamber was 1.9–2.13 kPa during the day and 0.56–0.63 kPa at night. These values correspond
to a hot and humid environment under heat stress [53–55]. In the greenhouse, the VPD was
approximately 0.703–1.01 kPa during the day.
In total, the experimental set-up consisted of 101 genotypes × 3 plants × 3 heat stress dura-
tions × 3 developmental phases.
Morphological measurements
Various morphological and yield parameters were measured after the plants reached harvest
maturity. The morphological parameters included measurements of plant height (PH), length
of the last internode (LIN), length of the main ear (EaL) and spikelet number per main ear
(SPIK). The spike density (DENS) was calculated from the two latter data. The yield-related
parameters were the following: number of reproductive tillers (RT), straw biomass per plant
(BIOM), total above-ground biomass (straw + all ears, FBIOM), main ear weight (MEaW),
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Heat stress tolerance of wheat
main seed weight (MSW), main seed number (MSN), total side ear weight (SEAW), total side
seed weight (SSW), total side seed number (SSN) and grain yield per plant (GY). The harvest
index (HI), grain number per spikelet (SPS), thousand-kernel weight in the main spike
(MTKW), average thousand-kernel weight (ATKW), average seed number (AS) and average
seed weight (ASW) were calculated from these data.
Physiological measurements
Among the physiological parameters, the chlorophyll content (CLR) was measured on a single
occasion after heat stress treatment using a SPAD-502 instrument (Minolta, Japan), which rec-
ords leaf transmittance in the red and near-infrared spectra and subsequently calculates the
SPAD index from these two values. As replicates, three wheat plants (per treatments) were
measured on the middle of the flag leaf for chlorophyll content.
The activity of photosynthetic properties, namely, the net assimilation rate (PN), evapora-
tion (EVP), stomatal conductance (GS) and intercellular CO2 concentration (ICO) of the
plants, was measured using a CIRAS 2—Portable Photosynthesis System (Tutorial version
2.03; Amesbury, MA 01913 USA). The infrared gas analysis system was equipped with a leaf
cuvette that exposed 1.7 cm2 of leaf area. The flag leaves were kept in a leaf chamber during the
measurements. External air was scrubbed of CO2 and mixed with a supply of pure CO2 to cre-
ate a reference concentration of 390 μmol m–2 s–1. The CO2 concentration was maintained at a
constant level using a CO2 injector with a high-pressure CO2 gas cartridge source. The quan-
tum flux was set to 300 μmol m–2 s–1 and the flow rate to 200 μmol m–2 s–1. The temperature
inside the leaf chamber was maintained at 22˚C under control conditions and 35˚C under heat
stress conditions. The photosynthetic parameters were determined at the same time as the
chlorophyll content. A total of 26 traits, including 5 morphological, 16 yield-related and 5
physiological traits, were examined in all treatments.
Statistical analysis
The Statistica 6 (StatSoft Inc., Tulsa, OK, USA) and GenStat1 (VSN International Ltd.
18th ed.) software packages were used in the general statistical analyses. Information on
the distributions of the original data under the various treatments is represented by boxplots
in S1 Fig.
The mixed linear model (REML) was used to identify the effects of the timing and
duration of heat stress and of the genotypes in explaining the phenotypic variance in the
measured traits. In estimating the variance components (σ2), all effects (genotype (G),
developmental phase (D) and duration of heat (H)) were considered as random to be able to
estimate the factor interactions. Principal component analysis (PCA), linear and multiple
regression, and multi-variable analysis were performed on a sub-sample of 16 traits covering
all three trait groups to evaluate the higher order associations among treatments, traits and
genotypes.
To determine the heat stress sensitivity of the various cultivars, cluster analysis (CA) was
performed on the data matrix of 101 cultivars × their grain yields (g/plant) in each of the 12
environments by applying the amalgamation rule of unweighted pair-group average within
the joining tree clustering module of Statistica 6. To visualise the outcome of CA, each data
point in the matrix was expressed as the magnitude of deviation from the main average of
grain yield in each environment. For further dissection of the type and magnitude of intercon-
nection among the cultivars, PCA was also conducted on the same data matrix (S2 Fig).
PLOS ONE | https://doi.org/10.1371/journal.pone.0222639 September 20, 2019
5 / 20
Table 1. Variance components (%) of morphological, yield-related and physiological traits in the context of 101 wheat cultivars × three timings (developmental
phase) × three durations of heat stress using the general linear model.
Heat stress tolerance of wheat
Traits
PH
LIN
EaL
SPIK
DENS
MEaW
MSW
MSN
SPS
MTKW
RT
SEAW
SSW
SSN
BIOM
GY
FBIOM
HI
AS
ASW
ATKW
EVP
GS
PN
ICO
CLR
Genotype
(G)
81.9���
53.1���
74.1���
75.7���
66.7���
26.4���
20.9���
30.7���
21.8���
19.3���
28.2���
22.3���
19.7���
20.9���
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PH—Plant height, LIN—Last internode length, EaL—Main ear length, SPIK—Spikelet number per main ear, DENS—Spike density (spikelet number/cm), MEaW—
Main ear weight, MSW—Main seed weight, MSN—Main seed number, SPS—Grain number per spikelet, MTKW—Main thousand- kernel weight, RT—Reproductive
tillers, SEAW—Side ear weight, SSW—Side seed weight, SSN—Side seed number, BIOM—Straw biomass, GY—Grain yield, FBIOM—Total aboveground biomass
(straw + all ears), HI—Harvest index, AS—Average seed number, ASW—Average seed weight, ATKW—Average thousand kernel weight, EVP–Evaporation, GS—
Stomatal conductance, PN—Net assimilation, ICO—Intercellular CO2 concentration, CLR—Chlorophyll content
���, ��, and � indicate differences significant at the 0.1%, 1% and 5% probability levels, respectively
https://doi.org/10.1371/journal.pone.0222639.t001
Results
Effect of timing and duration of heat stress on yield-related traits
In the experimental setup of 101 wheat genotypes × 3 developmental phases × 3 durations of heat
stress, all traits were significantly influenced by the three factors but to different extents (Table 1).
In general, morphological traits were mostly determined by the genotype, which explained
between 53.1% (LIN) and 83.9% (PH) of the phenotypic variance. In the case of yield-related
traits, the genotype effect was significant, but its role was smaller, explaining only between
14.6% (HI) and 40.8% (BIOM) of the phenotypic variance. In parallel, both aspects of heat
stress, i.e., the developmental phase in which it was applied and especially the duration of the
treatment, became more decisive factors. The developmental phase significantly influenced the
thousand-kernel weight and reproductive tiller number but had no significant effect on grain
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Heat stress tolerance of wheat
yield. In contrast, the duration of heat stress was the most significant component in determin-
ing both the seed number and seed weight and was more pronounced in the case of the side
ears. Via these two component groups, the duration of heat stress became the most significant
component of grain yield, explaining 51.6% of the phenotypic variance (S1 Fig). Averaged
over the 101 cultivars, the overall response to heat stress was a significant decrease in plant
grain yield, the ratio of which worsened as the duration of the heat stress increased, regardless
of the developmental phase in which heat was applied (Fig 1; for confidence intervals, see S2
Table). However, marked differences were observed between the developmental phases in
terms of the extent of grain yield reduction and changes in various yield-related traits. The
grain yield reduction was the largest at ZD59, when the grain yield was only 32.2% of the con-
trol value after 15-day heat stress, whereas those of ZD49 and ZD72 were similar, with 51.6
and 51.8% grain yields, respectively, compared with the control. Changes in selected yield-
related traits were phenophase-specific. The reduction in AS was similarly strong at both
ZD49 and ZD59, but this reduction was partially compensated by RT and ATK at ZD49,
which remained stable across the treatments, and at ZD59, the reduction in AS was accompa-
nied by strong reductions in RT and BIOM, leading to a significant decrease in ATK as the
heat period lengthened. At ZD72, heat stress had no significant effect on RT, but it decreased
AS, although to a lesser extent than in the other two phenophases. However, this observation
was accompanied by the greatest reduction in ATKW. Heat stress had the smallest overall
effect on the morphological traits, as represented by the values of PH and SPIK in Fig 1. PH
decreased slightly, but this was only characteristic of the earliest developmental phase. SPIK
was constant across all treatments in all three developmental stages.
Effect of timing and duration of heat stress on photosynthesis-related
parameters
In the case of physiological traits, both genotypic differences and developmental phases were
less decisive, and the duration of heat stress explained the largest portion of the variance, espe-
cially for GS (66.0%) and PN (89.4%). The chlorophyll content (CLR) was the only exception,
for which the genotype and the developmental phase explained 36.5% and 12.5% of the pheno-
typic variance, respectively. Of the interactions, both genotype × plant developmental phase
and genotype × heat duration were significant variance components for most of the traits, but
generally, they explained a lower portion of the variance than the main factors.
In the case of physiological traits, the overall responses were similar in all three developmen-
tal phases, with differences appearing mostly across the duration of heat stress treatment (Fig 2).
Stomatal conductance (GS) and net assimilation (PN) showed a strong decrease even after a
5-day heat treatment, but the values dropped only slightly in response to longer heat periods.
These characteristics were somewhat intensified in later developmental phases. Interestingly,
evaporation (EVP) increased to a large extent after a 5-day heat stress but subsequently gradu-
ally decreased as the heat treatment continued, decreasing to close to the control value after the
15-day heat period. For EVP, the responses were the strongest at ZD49, when it increased to
almost 200% of the control after a 5-day heat period, whereas the magnitude of change lessened
in later developmental phases. The values of intercellular CO2 (ICO) and chlorophyll (CLR) did
not change significantly due to heat stress treatment at any of the phenophases.
Heat-stress dependent associations among various components in forming
grain yield
Principal component analysis was conducted on the data matrices of the 101 cultivars × 16
traits selected to represent the three trait groups (morphological, yield-related, physiological),
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Heat stress tolerance of wheat
Fig 1. Changes in various morphological and yield-related traits in different phenophases. The values are
expressed as % of the control, caused by heat stress of different durations applied in A: ZD49, B: ZD59, C: ZD72
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Heat stress tolerance of wheat
phenophases. PH—Plant height, SPIK—Spikelet number per main ear, BIOM—Straw biomass, RT—Reproductive
tillers, HI—Harvest index, AS—Average seed number, ATKW—Average thousand-kernel weight, GY—Grain yield;
H5—H10—H15—Heat stress lasting 5, 10 and 15 days; ZD49—Booting stage, ZD59—Heading, ZD72—Early milk
development. The confidence intervals for the traits are listed in S2 Table.
https://doi.org/10.1371/journal.pone.0222639.g001
and the association between the traits was compared across the control and 15-day heat stress
treatments in the three developmental stages (Fig 3). In the control treatment, the parameters
related to seed number (AS, MSN, SPS) and thousand-kernel weight (ATKW, MTKW) formed
two slightly opposing groups. Those related to seed number (AS, MSN) were more closely
associated with the seed number per spikelet (SPS) and the thousand-kernel weight was more
closely associated with the reproductive tiller number and plant height. The physiological char-
acteristics grouped together, and with the harvest index and seed number per spikelet, were
placed opposite to the thousand-kernel weight. Grain yield, in close association with biomass
and spikelet number in the main spike, was placed intermediately to the seed number and
thousand-kernel weight. In the control treatment, the biomass, seed number and grain yield
were the most differentiating traits.
Phenophase-specific changes were observed in these associations after the application of
15-day heat stress, which resulted in two separate and tight groupings of the traits with the
strongest differentiating powers at ZD49. One group contained the grain yield together with
traits related to seed number and the harvest index, whereas the other group consisted of the
physiological traits, which were most strongly influenced by evaporation and stomatal conduc-
tance. Thousand-kernel weight, biomass and reproductive tillers were only weakly associated
with these two groups and had no significant effect on their formation. In the later develop-
mental phases, heat stress did not lead to tight groupings of this type, and the associations
gradually became more similar to the control as the developmental phase approached the rip-
ening period, especially in the case of associations involving the grain yield. ZD59 still showed
a tight, positive association between grain yield and seed number. Although the contribution
of seed number to grain yield decreased, this observation was counteracted by the greater con-
tribution of biomass, reproductive tiller number, and spikelet number and to a lesser extent by
thousand-kernel weight and plant height. In ZD72, grain yield was grouped with the same
yield-related traits as in the control, although in a tighter manner. The significance of physio-
logical parameters in the PCA separation increased after the 15-day heat stress, irrespective of
the developmental phase. However, the physiological parameters were mostly independent of
the grain yield and its components in the response matrices of 101 wheat cultivars, with nota-
bly few exceptions. In the control treatment, evaporation showed a positive association with
harvest index and a negative association with thousand-kernel weight. In ZD59, the intracellu-
lar CO2 concentration was grouped together with thousand-kernel weight and was associated
negatively with grain yield to a certain extent. This type of specific association was the most
pronounced in ZD72, where evaporation, net assimilation and stomatal conductance had a
negative influence on thousand-kernel weight.
Heat stress response profiles of wheat cultivars
Because plant grain yield is the strongest and final indicator of stress tolerance, various multi-
variate analyses were conducted on the data matrix of plant grain yield in all 12 treat-
ments × 101 genotypes to evaluate the heat-stress reactions of the wheat genotypes and estab-
lish the range of heat stress responses detectable in this wheat collection (Fig 4).
Based on cluster analysis, eight clusters of wheat cultivars could be identified at 32% of the
largest distance on the dendrogram. These clusters were also clearly separated in most cases
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Heat stress tolerance of wheat
Fig 2. Changes in photosynthetic parameters caused by heat stress of different durations applied in different phenophases. A: ZD49, B:
ZD59, C: ZD72 EVP—Evaporation, GS—Stomatal conductance, PN—Net assimilation, ICO—Intercellular CO2 concentration, CLR—
Chlorophyll content, H5—H10—H15—Heat stress lasting 5, 10 and 15 days; ZD49—Booting stage, ZD59—Heading, ZD72—Early milk
development.
https://doi.org/10.1371/journal.pone.0222639.g002
when principal component analysis was performed on the same data matrix (S2 Fig; members
of each cluster are listed in S1 Table). The only exception was Cluster 7, which was the largest
group with 37 genotypes. These members showed greater dispersion along Factor 2, which
correlated primarily with grain yields under longer heat stress periods at ZD59. In general, no
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Heat stress tolerance of wheat
Fig 3. Comparison of various trait association patterns based on principal component analysis. In the Control (A) and after 15-day heat stress
treatment in the phenophases ZD49 (B), ZD59 (C) and ZD72 (D). PH—Plant height, SPIK—Spikelet number per main ear, MSN—Main seed
number, SPS—Grain number per spikelet, MTKW—Main thousand-kernel weight, RT—Reproductive tillers, BIOM—straw biomass, GY—Grain
yield, HI—Harvest index, AS—Average seed number, ATKW—Average thousand-kernel weight, EVP—Evaporation, GS—Stomatal conductance,
PN—Net assimilation, ICO—Intracellular CO2 concentration, CLR—Chlorophyll content.
https://doi.org/10.1371/journal.pone.0222639.g003
strong association was identified between the heat stress response and geographic origin of the
wheat genotypes (S1 Table), with the only exceptions of Clusters 1, 3 and 4, which had the low-
est numbers of members (5, 5, and 9) but represented the extremes of yield formation. The
majority of Clu1 and Clu3 were of west-European origin, whereas most of the cultivars in Clu4
came from China and Southern Europe. Based on the heat map of grain yield, Clusters 1, 2,
and 3 were among the best performers across all control and heat stress treatments. The oppo-
site was true of Cluster 4, the members of which gave the lowest grain yield regardless of treat-
ment, followed by Cluster 5, whereas Clusters 6, 7 and 8 were intermediate in their reactions.
When the grain yield of control plants was compared across the three experiments in the three
developmental phases, the yielding ability of the clusters always exhibited the same order
despite certain differences in magnitude between the experiments (Fig 5A).
In addition to the overall reaction patterns for grain yield, treatment-specific differences
were noted between the responses of the various clusters, which could best be visualised as the
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Heat stress tolerance of wheat
Fig 4. Heatmap of average grain yield/plant (g) across wheat genotypes and treatments. Rows represent 101 wheat
genotypes and columns the various treatments expressed as the difference between the individual genotype and the main
GY average of each treatment (within a Column). The main GY average of each treatment (g) is represented at the bottom
of each column. C—Control, H5—H10—H15—Heat stress lasting 5, 10 and 15 days; (Zadoks) 49—Booting stage, 59—
Heading, 72—Early milk development; GY—Grain yield; LSD—Least significant difference at P = 0.05.
https://doi.org/10.1371/journal.pone.0222639.g004
average yield reduction expressed as a % of the average control values for each cluster (Fig 5B).
In this way, it became evident that differences existed in the heat stress sensitivity of the indi-
vidual clusters across the developmental phases, regardless of the magnitude of their yielding
abilities. Of the three best-yielding clusters (Clu1, 2, and 3), the sensitivity of Clu2 was always
the greatest and was more pronounced in the two earlier developmental phases. At ZD49,
Clu3 proved to be the most tolerant of heat stress, whereas Clu1 gave better results at ZD72. Of
the three intermediate clusters (Clu6, 7, and 8), Clu8 was the best in all three developmental
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Heat stress tolerance of wheat
Fig 5. Average grain yield of the eight wheat phenotypic clusters. The clusters were identified via multi-factorial analyses. Averages values in
the control treatments (A) and changes in their grain yield expressed as % of control under the various heat stress treatments (B). C—Control,
H5—H10—H15—Heat stress lasting for 5, 10 and 15 days; ZD49—Booting stage, ZD59—Heading, ZD72—Early milk development.
https://doi.org/10.1371/journal.pone.0222639.g005
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Heat stress tolerance of wheat
phases, whereas Clu6 showed the greatest sensitivity to heat in the two earlier developmental
phases (ZD49 and ZD59), not only within the intermediate clusters but also across all eight
clusters. The heat sensitivity of the lowest-yielding Clu4 was intermediate for the early and late
developmental phases but was among the best at ZD59. However, the heat sensitivity of Clu5
increased in later developmental phases, as a result of which this group became the most sensi-
tive at ZD72.
With the exception of thousand-kernel weight (both in the main ear and averaged over all
spikes), significant differences were observed in yield components between the clusters, both
in the control and heat stress treatments (S3 Fig). The data showed that large grain number
per main and average spike was the basis of high grain yield for both Clu1 and Clu3 in the con-
trol treatments, whereas a high number of reproductive tillers was the most decisive parameter
for Clu2. In the control, the larger reproductive tiller number of Clu2 was able to compensate
for the lower seed number, but under heat stress conditions, even the stable RT formation in
both the ZD49 and ZD72 phases could not counteract the steep decrease in seed number.
Under heat stress, Clu3 was better able to retain its seed number in the earlier developmental
phases, especially in the main ear, whereas this ability became stronger in Clu1 in the later
developmental phases. Among the intermediate clusters, the reactions of Clu6 and Clu8 are of
greatest interest. The seed setting of Clu6 was the most sensitive to heat stress, leading to a
severely decreased seed number in both the main ear and side spikes, which was characteristic
of this group in the earlier developmental phases (ZD49, ZD59). However, the ability of Clu8
to maintain both RT and seed number was good under heat stress conditions, especially at
ZD49.
Discussion
The positive and negative aspects of experimenting under controlled and/or field-sown condi-
tions have been previously discussed in depth by large numbers of publications [56–59].
Research has shown that results are not readily translatable from the glasshouse into the field.
One of our aims in this experiment was to study the effects of heat stress in specific and well-
defined developmental phases to establish how the sensitivity to heat changes with plant devel-
opment. In addition, this work was performed in a larger number of wheat genotypes with dif-
ferent developmental patterns (the heading date window was approximately 14 days in the
population). This type of systematic research is not possible to conduct under field conditions
because the timing, the duration of heat or the sole stress factor can be easily controlled. This
same set of wheat cultivars is a component of a long-term research programme in which the
associations between plant development and yield components are planned for study under
field-sown conditions for a longer time period with consideration of the various climatic fac-
tors (the results of the first three-year range have recently been published by Kiss et al. [60]).
Thus, the heat stress sensitivity indices of the cultivars established in controlled conditions can
later be included in temporal analyses.
The damage caused by heat stress depends on both the timing and the duration of the stress.
However, most of the experiments conducted until now only consider one of these aspects, or
only a limited number of genotypes are involved in the research [61–64]. In the current sys-
tematic experiment conducted with 101 wheat cultivars, both aspects of heat stress were
included. In addition, the experiments were performed in a controlled environment to ensure
that each genotype was exposed to heat stress in exactly the same developmental phase, thus
excluding the confounding factors in field experiments, where heat stress affects the wheat
genotypes in different developmental phases [62]. To study the timing of heat stress, three dif-
ferent developmental stages were tested, all of them after meiotic division in the male and
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Heat stress tolerance of wheat
female inflorescences. The combination of three phenophases and three heat stress periods
meant that plant development took place under stress conditions for different lengths of time.
When heat stress began in the booting stage (ZD49), the developmental interval under stress
ranged from heading (5-day heat stress) via flowering and pollination (10-day heat stress) to
seed set and early seed development (15-day heat stress). In the case of ZD59 (heading) this
range stretched from flowering and pollination via seed set to early seed filling, and in the case
of ZD72 (6th day after heading), from seed set via early seed filling to late seed development.
This scenario led to specific changes that had a stronger characteristic and significant influ-
ence, especially on yield-related traits, than the differences identified between genotypic reac-
tions. Using this experimental setup, both aspects of heat stress proved to be highly significant
determinants of various morphological, yield-related and physiological traits.
The current study confirms the results of previous research in that yield-related traits
decrease with increasing heat stress duration (5, 10 and 15 days) regardless of the developmen-
tal phase when the heat stress occurs [24,49–51,65]. Five days of heat stress was sufficient to
significantly decrease most yield-related traits, whereas 15-day heat stress resulted in the great-
est decline. The most heat-sensitive period proved to be that following the ZD59 phenophase.
The treatments that differed most from the control were ZD59_H10 and ZD59_H15, indicat-
ing that heat stress had the greatest effect on the productivity of winter wheat varieties in this
stage of development. In corroboration with other studies, not only was the seed set found to
be adversely affected at this stage but also the thousand-kernel weight, accompanied by a
strong decrease in biomass and reproductive tiller number [22,40,63,64,66–69]. The overall
grain yield reductions in ZD49 and ZD72 were observed to be similar, but this was due to spe-
cific and diverse changes in the individual yield component traits. Heat stress occurring after
seed set mostly influenced the efficiency of grain filling, leading to lower thousand-kernel
weight proportional to the duration of heat stress. However, in this stage, heat itself had less
influence on the seed number and reproductive tiller number if water was optimally available.
In contrast, when heat stress occurred before heading, the reduction in seed number was
accompanied by increases in both the thousand-kernel weight and reproductive tiller number,
counterbalancing the yield loss to a certain extent. This compensating effect was the most pro-
nounced after a short period of heat and gradually disappeared as the heat duration increased.
In this study, various physiological traits related to the photosynthetic activity of the flag
leaves under control and heat stress conditions were also examined to determine their roles in
heat stress tolerance [32,62]. Although phenotypic diversities were noted among the wheat
genotypes for all physiological parameters, this proved to be negligible compared with the
effect of heat stress duration, which alone explained most of the phenotypic variations. This
finding emphasises the fact that physiological changes were primarily a general response to
heat stress across various genotypes. The results obtained in the current work showed that the
accelerated flag-leaf senescence caused by heat stress could be attributed to lower levels of pho-
tosynthetic pigments and to a decline in photosynthetic activity.
Due to the reduced photosynthetic activity, which became more pronounced with aging of
the plants, net assimilation and stomatal conductance decreased gradually with increased
duration of heat stress. However, heat treatment caused a great increase in evaporation, which
was more intense in younger plants, especially under the shorter period of heat stress, demon-
strating that if water was available in the soil, the first general responses of plants against heat
was to cool their tissues via intensified evaporation. An indirect proof of this, one observation
is that genotypes with larger biomass and thus a larger area for evaporation generally tolerated
heat better, as suggested by Reynolds et al. [39]. It is interesting to note that the increased evap-
oration occurred in spite of the strong reductions in stomatal conductance, unrelated to the
phenophase. Although the stomatal conductance decreased due to heat stress, there was no
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Heat stress tolerance of wheat
complete closure, which could prevent transpiration. Sharma et al. [55] reported similar results
in which the heat sensitive wheat varieties showed reduced stomatal conductance with strongly
increased transpiration. In our case, most of the cultivars showed this reaction type, and the
few exceptions were dispersed across all heat response clusters, underlining that no strong
association exists among stomatal conductance, evaporation and heat stress tolerance in this
wheat population. Because the experimental conditions represented a hot and humid environ-
ment [53–55], this phenomenon could not be caused by vapour pressure deficit. One of the
possible explanations for this lack of correlation could be that the wheat cultivars studied in
this work were chosen without any previous knowledge of their transpiration characteristics,
and their heat stress sensitivity-tolerance has been established via grain yield reductions. This
observation is in contrast to the research of Sharma et al. [55], who compared wheat genotypes
that were previously selected for maximising the differences in stomatal conductance and
evaporation. Among the physiological parameters, only the flag-leaf chlorophyll content
showed a strong association with both genotype and developmental phase. The tendencies
identified in this work were in good accordance with previous studies [29,31]. However, in the
current experimental set-up, genotypic differences in chlorophyll content were not correlated
with either the heat stress response or with yield-related traits, as also found by Ali et al. [61].
Thus, in the 101 wheat cultivars tested, no significant association was identified between
the various parameters of photosynthetic activities and grain yield or between photosynthetic
activities and heat stress tolerance. This general lack of significant associations between physio-
logical parameters and heat stress tolerance contradicts selected data from the literature, where
various parameters of photosynthetic activity were found to be associated with heat stress tol-
erance [55,64,70–72]. The reason for this discrepancy might lie in the different number of
genotypes examined, the genetic structure of the populations, or the way in which heat stress
was applied. The current work involved a larger range of wheat genotypes of different geo-
graphic origin, whereas the heat stress treatment was applied in exactly the same developmen-
tal phase for each genotype.
One of the main aims of this research was to measure the heat stress tolerance of a large set
of winter wheat genotypes to characterise the extent and type of responses and identify geno-
types with a higher level of heat stress tolerance. In examining the effect of developmental
stage and duration of heat stress treatment, we observed that cultivars with higher grain yield
under control conditions also tended to have higher grain yield under heat stress conditions.
However, in spite of this observation, the yielding ability did not coincide with heat stress sen-
sitivity expressed as the % decrease in the control grain yield. We also found that the genotypes
differed in their phenophase-specific sensitivities to heat, which did not coincide with the aver-
age trends in several cases. Based on the yielding abilities across developmental phases and
heat stress treatments, eight major response groups of wheat genotypes could be identified. Of
these, three groups (Clu1, Clu3 and Clu8) were identified as of interest for further research. Of
the two groups with high grain yield, Clu1 had the best heat stress tolerance in the early devel-
opmental phase (ZD49), whereas that of Clu3 was best in the latest developmental phase
(ZD72). The heat stress tolerance of the intermediate Clu8 group was among the best in the
earlier developmental phases (ZD49 and ZD59). Crosses have been initiated between the
members of these three groups, partially for breeding purposes and partially for further studies
to determine the genetic background of the heat stress response.
In summary, the results made it possible to describe the extent of sensitivity in different
developmental phases in a larger population of wheat cultivars and to identify reaction types
with increasing heat tolerance. In further research, these results could contribute to the devel-
opment of varieties with better heat tolerance. It is clear that wider genetic diversity should be
explored if greater heat stress resilience is to be achieved in wheat breeding programmes.
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Heat stress tolerance of wheat
Supporting information
S1 Table. List of winter wheat cultivars included in the heat stress experiments grouped
based on their heat stress response patterns, which were established with the use of cluster
and principal component analyses (PCA). The position of each cultivar among and within
the clusters in this table completely corresponds to its position in the heat map of grain yield
(across the rows in Fig 4).
(PDF)
S2 Table. 95% confidence interval of the yield-related traits in control % as a supplement
to Fig 1.
(PDF)
S1 Fig. Boxplots of certain yield-related traits measured for 101 wheat cultivars. The values
are presented in the factorial combinations of three developmental phases and three durations
of heat stress.
(PDF)
S2 Fig. 101 winter wheat varieties divided into eight phenotypic clusters. Clustering was
carried out based on correlations of grain yield with PCA. Different coloured numbers corre-
spond to the different heat tolerant-based groups identified via hierarchical cluster analysis.
(PDF)
S3 Fig. Regression matrices of different yield-related parameters in the eight wheat clus-
ters.
(PPTX)
Author Contributions
Conceptualization: Krisztina Balla, Ildiko´ Karsai, Otto´ Veisz.
Data curation: Krisztina Balla.
Formal analysis: Krisztina Balla, Ildiko´ Karsai.
Funding acquisition: Krisztina Balla, Ildiko´ Karsai, Otto´ Veisz.
Investigation: Krisztina Balla, Pe´ter Bo´nis, Tibor Kiss.
Methodology: Krisztina Balla, Pe´ter Bo´nis, Tibor Kiss.
Project administration: Zita Berki, A´ da´m Horva´th, Marianna Mayer.
Resources: Ildiko´ Karsai, Otto´ Veisz.
Writing – original draft: Krisztina Balla, Ildiko´ Karsai.
Writing – review & editing: Szilvia Bencze, Otto´ Veisz.
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| null |
10.1371_journal.pone.0240995.pdf
| null |
The medical ethical technical committee of Erasmus MC did not grant permission to publish these data due to ethical studies should investigate if data-driven cut-offs can add value to explain the outcome being modelled and not solely rely on standard medical cut-off values to identify risk factors. considerations and the sensitivity of the data. Data are however available from the Erasmus MC upon reasonable request. Contact person: Dr. Julie ¨tte
|
RESEARCH ARTICLE
Risk factors for surgical site infections using a
data-driven approach
J. M. van NiekerkID
holt3, J. E. W. C. van Gemert-Pijnen1
1,2,3, M. C. Vos3, A. Stein2, L. M. A. Braakman-Jansen1*, A. F. Voor in ‘t
1 Department of Psychology, Health and Technology/Centre for eHealth Research and Disease
Management, Faculty of Behavioural Sciences, University of Twente, Enschede, The Netherlands,
2 Department of Earth Observation Sciences, Faculty of Geo-Information Science and Earth Observation
(ITC), University of Twente, Enschede, The Netherlands, 3 Department of Medical Microbiology and
Infectious Diseases, Erasmus MC University Medical Centre, Rotterdam, The Netherlands
* [email protected]
Abstract
Objective
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a1111111111
a1111111111
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OPEN ACCESS
Citation: van Niekerk JM, Vos MC, Stein A,
Braakman-Jansen LMA, Voor in ‘t holt AF, van
Gemert-Pijnen JEWC (2020) Risk factors for
surgical site infections using a data-driven
approach. PLoS ONE 15(10): e0240995. https://
doi.org/10.1371/journal.pone.0240995
Editor: Francesco Di Gennaro, National Institute for
Infectious Diseases Lazzaro Spallanzani-IRCCS,
ITALY
Received: May 15, 2020
Accepted: October 6, 2020
Published: October 28, 2020
Peer Review History: PLOS recognizes the
benefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0240995
Copyright: © 2020 van Niekerk et al. This is an
open access article distributed under the terms of
the Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: The medical ethical
technical committee of Erasmus MC did not grant
permission to publish these data due to ethical
The objective of this study was to identify risk factors for surgical site infection from diges-
tive, thoracic and orthopaedic system surgeries using clinical and data-driven cut-off values.
A second objective was to compare the identified risk factors in this study to risk factors iden-
tified in literature.
Summary background data
Retrospective data of 3 250 surgical procedures performed in large tertiary care hospital in
The Netherlands during January 2013 to June 2014 were used.
Methods
Potential risk factors were identified using a literature scan and univariate analysis. A multi-
variate forward-step logistic regression model was used to identify risk factors. Standard
medical cut-off values were compared with cut-offs determined from the data.
Results
For digestive, orthopaedic and thoracic system surgical procedures, the risk factors identi-
fied were preoperative temperature of �38˚C and antibiotics used at the time of surgery. C-
reactive protein and the duration of the surgery were identified as a risk factors for digestive
surgical procedures. Being an adult (age �18) was identified as a protective effect for tho-
racic surgical procedures. Data-driven cut-off values were identified for temperature, age
and CRP which can explain the SSI outcome up to 19.5% better than generic cut-off values.
Conclusions
This study identified risk factors for digestive, orthopaedic and thoracic system surgical pro-
cedures and illustrated how data-driven cut-offs can add value in the process. Future
PLOS ONE | https://doi.org/10.1371/journal.pone.0240995 October 28, 2020
1 / 14
PLOS ONEconsiderations and the sensitivity of the data. Data
are however available from the Erasmus MC upon
reasonable request. Contact person: Dr. Julie¨tte
Severin, Infection prevention and control (IPC) and
antimicrobial resistance (AMR) (Data Access)
E-mail: info.microbiologie.
[email protected].
Funding: This research was supported by the
INTERREG V A (202085) funded project EurHealth-
1Health (http://www.eurhealth1health.eu), part of a
Dutch-German cross-border network supported by
the European Commission, the Dutch Ministry of
Health, Welfare and Sport, the Ministry of
Economy, Innovation, Digitalisation and Energy of
the German Federal State of North Rhine-
Westphalia and the Ministry for National and
European Affairs and Regional Development of
Lower Saxony. The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Risk factors for surgical site infections using a data-driven approach
studies should investigate if data-driven cut-offs can add value to explain the outcome being
modelled and not solely rely on standard medical cut-off values to identify risk factors.
Introduction
Surgical site infections (SSI), as defined by the European Centre for Disease Prevention and
Control (ECDC) [1], make up 19.6% of the total number of healthcare-associated infections
(HAIs) in Europe. With an estimated 81 089 patients in Europe having an HAI on any given
day, almost 16 000 people in Europe are suffering from some form of SSI at any given time [2].
The burden of SSI can be measured in terms of increased length of stay in hospital, additional
(surgical) procedures required, increased morbidity and mortality, as well as in economic
terms [3].
Risk factors relating to the patient, procedure and the environment alter the odds of an SSI
occurring. Research has been done to identify risk factors for SSI with the aim to identify pre-
ventative actions to reduce the incidence rate of SSI [4–10]. Patient-related risk factors for SSI,
such as obesity, diabetes, surgery duration and the American Society of Anaesthesiologists
(ASA) score are risk factors for digestive system, thoracic and orthopaedic surgical procedures
[11–22]. Risk factors in low-income countries also include unemployment and level of educa-
tion due to the disparity in socioeconomic status [14]. Risk factors can be modifiable or non-
modifiable [23]. Modifiable risk factors are most interesting of the two since they can be
changed preoperatively to reduce the risk of SSI.
The Segmentation of surgical procedures into homogenous groups makes it possible to find
useful and relevant risk factors unique to each segment. Digestive system surgical procedures
are more prone to SSI as they are generally clean-contaminated or dirty surgeries which make
deep space SSI more likely. The occurrence of SSI after thoracic and orthopaedic surgeries are
both relatively low because they are both typically clean surgeries, but the probability of attract-
ing a deep space SSI after thoracic surgery is much higher compared to orthopaedic surgeries
[15]. Because of these differences, we focus on digestive system, thoracic and orthopaedic sur-
gical procedures for this study.
Multivariate logistic regression is the most common statistical model used to identify risk
factors in longitudinal study design data [16]. Not all studies report the discriminatory power
of the multivariate logistic regression model fitted. Risk factor identification studies do not
usually specify how continuous variables cut-offs are determined. Cut-off values for variables
such as age (�18) or patient temperature (37˚C) may seem intuitive or standard for clinical
practice, but they may not statistically be the best cut-offs values determined by the data [17].
The objective of this study is to identify risk factors for SSI from digestive, thoracic and
orthopaedic system surgeries using clinical and data-driven cut-off values. A second objective
is to compare the identified risk factors in this study to risk factors identified in the literature.
Materials and methods
Literature search
A literature search was performed to identify known risk factors for SSI associated with diges-
tive system surgical procedures, thoracic surgery and orthopaedic procedures using the corre-
sponding medical subject headings (MeSH) linked data representation and the MEDLINE
database.
Search strings used for MEDLINE literature search:
PLOS ONE | https://doi.org/10.1371/journal.pone.0240995 October 28, 2020
2 / 14
PLOS ONERisk factors for surgical site infections using a data-driven approach
1. “Surgical Wound Infection”[Mesh] AND “Risk Factors”[Mesh] AND “Digestive System
Surgical Procedures”[Mesh]
2. “Surgical Wound Infection”[Mesh] AND “Risk Factors”[Mesh] AND “Orthopaedic
Procedures”[Mesh]
3. “Surgical Wound Infection”[Mesh] AND “Risk Factors”[Mesh] AND “Thoracic
Surgery”[Mesh]
The search results were sorted, using the Best Match algorithm [18] developed by PubMed.
Search results were deemed relevant using title and abstract screening. Risk factors were
extracted if they were significant in a multivariable analysis until data saturation was achieved
[19]. Risk factors identified, which were common to all three groups of surgeries, were defined
as “general risk factors” in this study.
Setting and data collection
The Erasmus MC University Medical Centre in Rotterdam is the largest university medical
hospital in the Netherlands with more than 1 300 beds [15]. The data used for this study were
anonymised in accordance with the Dutch Personal Data Protection Act (WBP). Approval
from the Medical Ethical Research Committee was obtained (MEC-2018-1185).
A weekly prevalence survey was performed by infection control practitioners (ICP) from
January 2013 until December 2013 and two-weekly until June 2014 using a semi-automated
algorithm proposed by Streefkerk et al. [20, 21]. This algorithm was used to calculate a nosoco-
mial infection index (NII) which was then verified by ICP in case of a positive outcome to
determine whenever an HAI was present or not. An ICP verified all patients with an NII > 7,
and a definite SSI outcome was concluded by the ICP using the electronic patient data system.
This outcome was used in this study as the occurrence of SSI outcome variable.
Data were extracted from a centralised database, containing cross-departmental data, clini-
cal synopsis reports, infectious disease consultation reports, laboratory results and imaging
reports. Data regarding the prescription of antimicrobials, in the J01 class of the Anatomical
Therapeutic Chemical (ATC) classification system [22], were also included. Surgeries were
included if they were part of the three groups of surgeries under investigation in this study and
had a point prevalence measurement within 30 days after the surgery took place. If a second
surgery took place within 30 days after an included surgery, then the recent surgery was
excluded. All emergency surgeries were excluded to avoid possible undesirable confounding
effects relating to the urgency and necessity of the surgeries.
Statistical analysis
The differences in the averages of variables with missing values and those without were evalu-
ated using t-tests and were found statistically significant. These tests, together with Little’s
MCAR test, convinced us that the missing values were not completely randomly missing and
that we could not make use of more simple imputation methods. Therefore, we chose to use
conditional Markov chain Monte Carlo (MCMC) with multiple imputations for the imputa-
tion process [24, 25].
Two methods were used to discretise continuous measurement variables: 1) standard medi-
cal cut-offs as used by Erasmus MC and 2) recursive partitioning [17]. Recursive partitioning
is a data-driven, supervised discretisation method, used to group continuous values with simi-
lar outcomes optimally. The data-driven method was used to test and confirm if the standard
medical cut-offs were the best way to explain the outcome variable for the groups of surgical
procedures considered.
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PLOS ONERisk factors for surgical site infections using a data-driven approach
To build a prognostic prediction model for SSI, Hosmer et al. suggest fitting a univariate
logistic regression model to each variable separately and if the p-value is less than a specific p-
value, 0.1 is this case, then consider the variable good enough to include in the multivariate
logistic regression model [26]. A univariate analysis was performed for each of the three
groups of surgeries using the variables identified from the literature search. Significant vari-
ables (p<0.1) in the univariate analysis were added to the list of variables associated with each
group of surgery, together with the variables identified from the literature search. This resulted
in an extended list of general risk factors as more risk factors were common across the three
groups of surgeries.
A multivariate logistic regression model was built using a forward stepwise approach for
each of the three groups of surgeries [27]. The general risk factors were first added to the
model and then the risk factors unique to each surgery group in the order of the Akaike infor-
mation criterion (AIC) until convergence was reached. In this case, we chose the conversion of
the model to imply that there are no additional variables which can be added which will be sta-
tistically significant with a p-value of less than 0.05 or an AIC of 3.8415. Model performance
was determined using the Gini coefficient after each step of the multivariate model, and the
difference is reported as the marginal contribution of surgery group-specific risk factors for
this study [19, 28]. Model performance was cross-validated using 5-fold cross-validation to
estimate how the model would perform on new data [29]. R [30] was used in this study
together with packages mice (multiple imputation) [31], smbinning (recursive partitioning)
[32], dplyr (data wrangling) [33], finalfit (formatting of tables) [34] and scorecard (cross-vali-
dation) [35].
Approval was obtained from the Medical Ethical Committee of Erasmus MC (MEC-2018-
1185) to perform this study. Data were analysed anonymously, and thus no further consent
was obtained.
Results
Literature search
The literature search resulted in 1 422 research papers (as at 5 March 2020) using the MeSH
headings in the PubMed search engine. We identified 24 research papers, published from 2008
until 2019, which contained statistically significant results from a multivariate analysis. A total
of 79 risk factors were identified for the three groups of surgical procedures [11–13, 16, 23, 36–
54] (S1 Table). Age, ASA class, body mass index (BMI), preoperative length of stay and diabe-
tes were identified as general risk factors from the literature search. In total, 29 risk factors for
digestive system surgical procedures, 31 for orthopaedic procedures and 19 for thoracic sur-
geries were identified. This amounted to 59 unique risk factors, of which 15 were present in
more than one group of surgeries.
Risk factor identification
A total of 21 of the 59 unique risk factors could be replicated using our own data. The variable
describing the type of surgery was used to create three homogenous groups of surgical proce-
dures. The emergency classification variable was used to exclude emergency surgeries from the
study such that 19 risk factors remained (Table 1). We observed 3 250 surgeries over the study
period and excluded 526 (16.2%) emergency surgeries to be left with 2 724 surgical observa-
tions. CRP and temperature data were available for 52.55% (60.47% for in-patients) and
96.88% of all surgeries respectively.
The significant univariate results of digestive system, orthopaedic and thoracic surgical pro-
cedures are shown in Table 2. Antibiotic use, CRP and temperature were added to the list of
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PLOS ONERisk factors for surgical site infections using a data-driven approach
Table 1. Variable names and definitions used to investigate the occurrence of SSI in this study.
Variable
Demographic
Gender
Age
ASA class
BMI
Behavioural
Alcohol use
Smoking
Comorbidities
Heart disease
Liver disease
Hypertension
Diabetes
Measurement
Temperature
CRP
Leukocyte
Serum total protein
Glucose
Haemoglobin
Operative
Surgery
group
Definition
D,O
D,O,T
D,O,T
D,O,T
O
D,O
Gender of patient (Male/Female)
Age of patient on the day of surgery (Years)
ASA class of patient (I-V)
BMI of patient at the time of surgery.
Alcohol use of patient at the time of surgery (Current/Never/Past).
Smoking status of patient at the time of surgery (Current/Never/Past).
O,T
Patient has a history of heart disease at the time of surgery (Yes/No).
D
O
Patient has a history of liver disease at the time of surgery (Yes/No).
Patient has a history of hypertension (Yes/No).
D,O,T
Patient has diabetes Type I or II at the time of surgery (Yes/No).
D
O
D
D
D
D
Highest temperature of patient in the past 7 days before surgery.
Highest CRP of patient in the 7 days before surgery.
Highest leukocyte level of patient in the 7 days before surgery.
Highest serum total protein of patient in the 7 days before surgery.
Highest glucose level of patient in the 7 days before surgery.
Highest haemoglobin level of patient in the 7 days before surgery.
Preoperative length of
D,O,T
stay
Antibiotic use
T
Preoperative length of hospital stay of patient at the time of surgery
(Days).
Antibiotic (WHO ATC code J01 [22]) use of patient at the time of
surgery (Yes/No).
Duration of surgery
D,O
Duration of the surgical procedure (Minutes).
D, Digestive system surgical procedures; O, Orthopaedic system surgical procedures; T, Thoracic system surgical
procedures; ASA, American Society of Anaesthesiologists; CRP, C-reactive protein; BMI, Body Mass Index; SSI,
Surgical Site Infection; ATC, Anatomical Therapeutic Chemical; WHO, World Health Organization.
https://doi.org/10.1371/journal.pone.0240995.t001
general risk factors after being found statistically significant in the univariate analysis–increas-
ing the number of general risk factors to 8. Diabetes was identified as a general risk factor from
our literature search but was not found significant in any of the three univariate analyses in
our own study. For digestive system surgical procedure and thoracic procedures, the data-
driven cut-off for age was obtained as 23 years and both the standard cut-off (18 years) and the
data-driven cut-off were statistically significant with p-values of less than 0.001 which resulted
in rejecting the null hypothesis that the coefficient associated with the age of the patient is
zero. For orthopaedic procedures, the data-driven cut-off for the temperature (39 degrees) was
found statistically significant, but the standard medical cut-off not. A data-driven CRP cut-off
of 8.1 was identified for orthopaedic surgical procedures as opposed to a standard medical
CRP cut-off of 10; both cut-offs are statistically significant.
The multivariate results using standard medical cut-offs and data-driven cut-offs are shown
in Tables 3 and 4, respectively. The temperature variable was statistically significant in the mul-
tivariate analysis using the data-driven cut-offs for all three groups of surgeries, but not in one
of the multivariate analysis using the medical standard cut-offs. The duration of the surgery
was the only statistically significant variable in the multivariate analyses which was not
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PLOS ONETable 2. Digestive system surgical procedures: univariate analysis of risk factors for the future occurrence of SSI.
Variable
SSI = No (2 600)
SSI = Yes (124)
Univariate OR (95%CI, P-value)
Digestive System Surgical Procedures
Risk factors for surgical site infections using a data-driven approach
Gender
Age1
Age (data-driven)
Antibiotic use
Temperature1
Temperature (data-driven)
CRP1
CRP (data-driven)
Female
Male
�18
>18
�23
>23
No
Yes
�36.5
(36.5,37.5]
>37.5
�38
(38,39]
>39
�10
>10
�8.1
>8.1
Preoperative length of stay (Days)
Mean Days (SD)
359 (43.9)2
458 (56.1)
246 (30.1)
571 (69.9)
258 (31.6)
559 (68.4)
496 (60.7)
321 (39.3)
0 (0.0)
98 (12.0)
719 (88.0)
535 (65.5)
187 (22.9)
95 (11.6)
397 (48.6)
420 (51.4)
365 (44.7)
452 (55.3)
6.6 (24.1)
Duration of surgery
Mean Minutes (SD)
243.6 (143)
Orthopaedic Procedures
ASA class
Alcohol use
Antibiotic use
Temperature (data-driven)
Age1
Age (data-driven)
BMI
Alcohol use
Antibiotic use
Temperature1
ASA CLASS I
ASA CLASS II
ASA CLASS III
ASA CLASS � IV
Current
Never
Past
No
Yes
�39
>39
�18
>18
�23
>23
Mean (SD)
Current
Never
Past
No
Yes
�36.5
(36.5,37.5]
>37.5
196 (26.8)
339 (46.4)
182 (24.9)
13 (1.8)
327 (44.8)
339 (46.4)
64 (8.8)
591 (81.0)
139 (19.0)
695 (95.2)
35 (4.8)
Thoracic Surgery
232 (22.0)
821 (78.0)
226 (21.5)
827 (78.5)
24.5 (5.3)
534 (50.7)
422 (40.1)
97 (9.2)
705 (67.0)
348 (33.0)
0 (0.0)
302 (28.7)
751 (71.3)
24 (33.8)
47 (66.2)
8 (11.3)
63 (88.7)
8 (11.3)
63 (88.7)
17 (23.9)
54 (76.1)
0 (0.0)
2 (2.8)
69 (97.2)
20 (28.2)
25 (35.2)
26 (36.6)
21 (29.6)
50 (70.4)
18 (25.4)
53 (74.6)
12.1 (37.3)
330.4 (190.8)
6 (33.3)
6 (33.3)
4 (22.2)
2 (11.1)
6 (33.3)
8 (44.4)
4 (22.2)
8 (44.4)
10 (55.6)
14 (77.8)
4 (22.2)
16 (45.7)
19 (54.3)
16 (45.7)
19 (54.3)
22.1 (4.2)
11 (31.4)
18 (51.4)
6 (17.1)
18 (51.4)
17 (48.6)
0 (0.0)
3 (8.6)
32 (91.4)
Reference
1.54 (0.93–2.60, p = 0.099)
Reference
3.39 (1.70–7.77, p<0.001)
Reference
3.63 (1.82–8.32, p<0.001)
Reference
4.91 (2.85–8.86, p<0.001)
NA
Reference
4.70 (1.44–28.91, p = 0.033)
Reference
3.58 (1.95–6.66, p<0.001)
7.32 (3.94–13.79, p<0.001)
Reference
2.25 (1.35–3.89, p = 0.003)
Reference
2.38 (1.39–4.24, p = 0.002)
1.01 (1.00–1.01, p = 0.092)
1.00 (1.00–1.01, p<0.001)
0.58 (0.18–1.87, p = 0.348)
0.72 (0.18–2.55, p = 0.612)
5.03 (0.69–24.47, p = 0.062)
Reference
1.29 (0.44–3.94, p = 0.645)
3.41 (0.85–12.26, p = 0.063)
Reference
5.31 (2.06–14.16, p<0.001)
Reference
5.67 (1.55–16.79, p = 0.003)
Reference
0.34 (0.17–0.67, p = 0.002)
Reference
0.32 (0.16–0.65, p = 0.001)
0.91 (0.85–0.98, p = 0.010)
Reference
2.07 (0.98–4.57, p = 0.061)
3.00 (1.01–8.09, p = 0.034)
Reference
1.91 (0.97–3.77, p = 0.060)
NA
Reference
4.29 (1.52–17.94, p = 0.017)
(Continued )
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PLOS ONERisk factors for surgical site infections using a data-driven approach
Table 2. (Continued)
Temperature (data-driven)
CRP1
Haemoglobin1
Variable
SSI = No (2 600)
SSI = Yes (124)
Univariate OR (95%CI, P-value)
�38
>38
�10
>10
�8.6
(8.6,10.5]
>10.5
882 (83.8)
171 (16.2)
684 (65.0)
369 (35.0)
665 (63.2)
358 (34.0)
30 (2.8)
20 (57.1)
15 (42.9)
17 (48.6)
18 (51.4)
21 (60.0)
11 (31.4)
3 (8.6)
Reference
3.87 (1.91–7.67, p<0.001)
Reference
1.96 (1.00–3.88, p = 0.050)
Reference
0.97 (0.45–2.00, p = 0.942)
3.17 (0.72–9.85, p = 0.074)
CRP, C-reactive protein; OR, Odds Ratio; BMI, Body Mass Index; NA, Not Applicable; CI, Confidence Interval; SSI, Surgical Site Infection; OR, Odds ratio; Data-
driven, cut-off values determined using recursive partitioning.
1Standard Erasmus MC clinical cut-offs.
2The percentage distribution of the SSI outcome is provided in brackets next to the frequency for each variable.
https://doi.org/10.1371/journal.pone.0240995.t002
identified as a general risk factor to increase the odds of SSI by approximately 6% for every 30
minutes spent in surgery. For digestive surgical procedures, the addition of duration of surgery
to the multivariate model increased the Gini coefficient from 0.46 to 0.52 based on standard
medical cut-offs and from 0.57 to 0.62 for the multivariate model based on the data-driven
cut-offs. This increase translates into a 12.5% and 8.8% increase in the Gini coefficient, respec-
tively. Neither the orthopaedic nor the thoracic group of surgical procedures had any statisti-
cally significant risk factors which are not part of the general risk factors group of surgeries.
The Gini coefficient of the data-driven multivariate model is 19.5% (0.62 vs 0.52) higher than
the multivariate model based on the standard medical cut-offs. The 5-fold cross-validated 95%
confidence intervals for the Gini coefficients based on the validation samples of the data-
driven models are (0.49, 0.72) for digestive procedures, (0.21, 0.86) for orthopaedic procedures
and (0.21,0.70) for thoracic procedures.
An overview of the study results (Table 5) shows that 10 of the 19 risk factors, identified
during the literature search, were not statistically significant in the univariate or multivariate
analysis for any of the surgery groups. BMI and diabetes were identified across all three groups
of surgeries and multiple studies as risk factors for SSI but were not statistically significant in
this study. Temperature and the duration of the surgery were confirmed as risk factors for
digestive system surgeries, and similarly, antibiotic use and age were confirmed as risk factors
Table 3. Multivariate analysis risk factors for the occurrence of SSI by group of surgeries using standard medical cut-offs.
Risk factor by surgery group1
Digestive System Surgical Procedures
Antibiotic use
Duration of surgery (Minutes)
CRP >10
Orthopaedic Surgical Procedures
Antibiotic use
Thoracic Surgical Procedures
Age >18
Antibiotic use
Coefficient
Multivariate OR (95%CI)
P-value
1.240
0.003
0.803
1.670
-4.195
1.311
3.455 (1.951–6.384)
1.003 (1.001–1.004)
2.232 (1.302–3.951)
5.315 (2.059–14.158)
0.146 (0.058–0.351)
4.849 (2.035–12.266)
<0.001
<0.001
0.004
<0.001
<0.001
<0.001
CRP, C-reactive protein; CI, Confidence Interval; OR, Odds ratio.
1The multivariate analysis was performed using Erasmus MC clinical cut-offs.
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PLOS ONERisk factors for surgical site infections using a data-driven approach
Table 4. Multivariate analysis risk factors for the occurrence of SSI by group of surgeries using data-driven cut-offs.
Risk factor by surgery group1
Digestive System Surgical Procedures
Temperature [38,39]
Temperature >39
Antibiotic use
Duration of surgery (Minutes)
CRP >8.1
Orthopaedic Surgical Procedures
Antibiotic use
Temperature >39
Thoracic Surgical Procedures
Age >17
Antibiotic use
Temperature >38
Coefficient
Multivariate OR (95%CI)
P-value
1.067
1.732
1.201
0.002
0.639
1.552
1.224
-1.847
1.597
0.824
2.907 (1.556–5.497)
5.650 (2.952–10.947)
3.322 (1.856–6.200)
1.002 (1.001–1.004)
1.894 (1.062–3.510)
3.665 (1.370–10.006)
5.120 (1.316–16.387)
0.158 (0.055–0.426)
4.939 (1.896–14.043)
2.280 (1.098–4.653)
<0.001
<0.001
<0.001
0.003
0.035
0.009
0.009
<0.001
0.002
0.024
Data-driven, cut-off values determined using recursive partitioning; CRP, C-reactive protein; CI, Confidence Interval; OR, Odds ratio.
1The multivariate analysis was performed using data-driven cut-offs.
https://doi.org/10.1371/journal.pone.0240995.t004
for thoracic surgeries. Antibiotic use and CRP were identified as risk factors for digestive sur-
geries from the multivariate analysis, which were identified during the literature search for
thoracic and orthopaedic surgeries, respectively. Antibiotic use and temperature were
Table 5. Statistical significance of risk factors and the source which lead them to be considered by surgical procedure.
Risk Factor
Age
Alcohol use
Antibiotic use
ASA Class
BMI
CRP
Diabetes
Duration of surgery
Gender
Glucose
Haemoglobin
Heart Disease
Hypertension
Leukocyte
Liver disease
Preoperative length of stay
Serum total protein
Smoking
Temperature
Significance1
DU,TM
OU,TU
DM,OM,TM
OU
None
DM
None
DM
DU
None
None
None
None
None
None
DU
None
None
DM,OM,TM
Digestive System2
[38, 11, 43, 47]
[37, 39, 41, 43, 54]
[44]
[38, 47, 50]
[36, 38, 41, 43, 44, 49, 54]
[38, 11, 43]
[47]
[11, 44, 54]
[55]
[54]
[41, 50]
[36, 49]
[49]
[55]
Orthopaedic2
[16]
[51]
[16, 51, 53]
[51–53]
[16]
[16, 45, 51, 53]
[16, 45, 51, 53]
[16, 51]
[51]
[51]
[16, 52]
[51–53]
Thoracic2
[12]
[40]
[16]
[42]
[13]
[12]
[12, 13, 40]
D, Digestive system surgical procedures; O, Orthopaedic system surgical procedures; U, Significant in univariate analysis; M, Significant in multivariate analysis; T,
Thoracic system surgical procedures; ASA, American Society of Anaesthesiologists; CRP, C-reactive protein; SSI, Surgical Site Infection; BMI, Body Mass Index.
1During which part of the analysis the risk factor was found statistically significant.
2References to the literature which had the risk factor as a multivariate result for each group of surgeries.
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PLOS ONERisk factors for surgical site infections using a data-driven approach
statistically significant for all three groups of surgeries and were included because of two stud-
ies regarding thoracic and digestive system surgeries, respectively [40, 55].
Discussion
We identified temperature and antibiotics used at the time of surgery as risk factors for diges-
tive, orthopaedic and thoracic system surgical procedures in this study. The duration of the
surgery was identified as a risk factor for digestive surgical procedures. Being an adult
(age � 18) was identified as a protective effect for thoracic surgical procedures. Data-driven
cut-offs were identified for temperature, CRP and age, which differ from the standard medical
cut-offs. Temperature would not have been identified as a risk factor if only standard medical
cut-offs were considered. From our literature search, we identified age, ASA class, BMI, preop-
erative length of stay and diabetes as general risk factors, while CRP, temperature and antibi-
otic use were identified as general risk factors because of this study.
The identified risk factors may be classified as modifiable or non-modifiable, depending
upon the circumstances of the patient like the complexity of his condition. For instance, the
temperature of a patient may be high because of an existing infection, which is why the surgery
is needed in the first place and may not be modifiable before surgery. Age, on the other hand,
may be a modifiable risk factor if the surgery can be postponed for several years, e.g. due to a
heart defect. This study revealed that children are more likely to be diagnosed with an SSI after
thoracic surgery than adults. There are studies which identify risk factors for children after
thoracic surgeries, but none found that being a child is a risk factor for SSI [42, 48] after under-
going thoracic surgery. We segmented the thoracic surgeries between adults and children and
obtained multivariate results for children and adults separately. The multivariate model based
only on children (age � 18) did not reveal any significant results, contrary to the results of the
thoracic study which found age to be a risk factor for children [12]. This absence could be
partly due to the small study population size of 248. Antibiotic usage was the only significant
factor in the multivariate analysis of thoracic surgeries based on adults. The other two groups
of surgical procedures were consistent in terms of their statistical significance of risk factors
based on adults.
The data-driven cut-offs confirmed the existing standard medical cut-offs. On average the
clinical cut-off for temperature was one degree Celsius lower, while for digestive system surgi-
cal procedures, the clinical cut-off for CRP (10) was just less than two units more than the
data-driven cut-off of 8.1. This means that there is a greater difference between the occurrence
of SSI for patients with a CRP below and above 8.1 than below and above 10. The data-driven
cut-offs improved the ability of the statistical model to explain the occurrence of SSI. The per-
formance of the digestive system surgical procedure prediction model increased by 19.5% due
to using data-driven cut-offs rather than the standard medical cut-offs. Using data-driven cut-
offs, we were able to identify temperature as a risk factor for all three groups of surgical proce-
dures. If standard clinical cut-offs were used, temperature would not have been significant
from the multivariate analysis. This potential oversight illustrates the importance of evaluating
the cut-offs used for continuous variables against the data before identifying risk factors.
Antibiotic use, temperature and CRP were added to the list of general risk factors by incor-
porating the statistically significant results of the univariate analysis. These risk factors might
have been overlooked when the focus was on only one type of surgery. Temperature was iden-
tified as a risk factor in the multivariate results for all three groups of surgical procedures,
whereas the literature search identified it only for digestive surgeries. Antibiotic use was not
found during our literature search for digestive or orthopaedic surgical procedures but was
found significant for both groups of surgeries in the multivariate analysis of our study.
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PLOS ONERisk factors for surgical site infections using a data-driven approach
The Centres for Disease Control and Prevention (CDC), the European centre for disease
prevention and control (ECDC), World Health Organisation (WHO) and Netherlands
National Institute for Public Health and the Environment (RIVM) suggest maintaining nor-
mothermia intraoperatively to prevent undesirable hypothermia (during some thoracic and
neurosurgeries, hypothermia may be desirable). [56–58] A lower intraoperative bound for
temperature of 35.5˚C to 36˚C is explicitly mentioned, and only the RIVM mention an upper
bound of 38˚C which is consistent with the risk factors identified in our study. An upper limit
for preoperative temperature should, therefore, be investigated instead of only the lower limit.
The four health organisations refer to the proper administration and timing of surgical antimi-
crobial prophylaxis, but not to the proper preoperative use of standard prescription antibiotics.
Systemic antibiotics are typically prescribed to stabilise patients before undergoing surgery. A
possible explanation for the increased occurrence of SSI associated with antimicrobials pre-
scribed before surgery could be that these patients were not completely stabilised before sur-
gery which increased their risk of SSI. The proper preoperative use of antibiotics should be
well defined, and the reason why antibiotic-use was identified as a risk factor for SSI should be
further investigated.
Limitations
This is a retrospective, single-centre study, and therefore the data were not collected for the
purpose of this study. Even though cross-validation was performed to estimate model perfor-
mance on new data, the models were not externally validated. Surgeries were aggregated into
three broad groups of surgical procedures which serve as a proxy for the reason for surgery but
leads to the loss of information regarding the exact reasons for the surgery. Some measure-
ments, like temperature and CRP, were not always present and was partly overcome using
imputation. Patient information concerning smoking and drinking habits may be understated
due to incomplete medical records. The literature search used for this study was not exhaustive
but rather based on the principal on data saturation. A comprehensive list of variables related
to the nutritional and immunological alterations of the patients was not included in the analy-
ses as they were not available from the data. We used a 30-day outcome period in which we
observe if an SSI was present or not, but according to the CDC definition, this outcome period
should be one year for surgical implantation procedures. Since our data only spans over 18
months, it was not possible to use a 12-month outcome window for all surgical implantation
procedures, which is a limitation of this study. The administration of prophylaxis and the opti-
mal timing thereof is an important risk factor for the occurrence of SSI. However, these data
were not available.
Future work
Future work will investigate the modifiability of the risk factors identified in this study in more
detail, as the circumstances under which this occurs are hitherto unclear. The exact purpose of
the use of antibiotics over the time of surgery was not investigated in depth, which can be done
in future studies. Future research can also investigate differences between adults and children,
which lead to the occurrence of SSI among children. Another opportunity for future research
is to investigate which risk factors are predictive for the occurrence of SSI over different peri-
ods. Doing this will enable healthcare workers to identify which risk factors explain the occur-
rence of SSI soon after surgery, towards the end of the 30 days and even later for implantation
surgeries. These insights can help set guidelines to determine the vigilance necessary to miti-
gate the risk of SSI on a patient level.
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PLOS ONERisk factors for surgical site infections using a data-driven approach
Conclusion
This study shows that data-driven cut-offs can be used to identify risk factors which would not
have been identified by only using standard medical cut-offs. Preoperative temperature and
antibiotic use were identified as risk factors for digestive, orthopaedic, thoracic system surger-
ies, while the duration of surgery and age were identified as risk factors for orthopaedic and
thoracic system surgeries, respectively. In contrast with literature, this study found that an SSI
is more likely to occur in children (age < 18) than in adults after thoracic system surgeries. Sta-
tistical modelling has been important to quantify important risk factors and indicate their sig-
nificance. Clinical studies using retrospective data are important to carry out, despite
limitations in the data sets. To this end, future studies should use both standard medical cut-
offs and data-driven cut-offs to investigate risk factors.
Supporting information
S1 Table. Risk factors identified from multivariate analysis during literature search.
(DOCX)
S1 Formulae. The multivariate logistic regression equations based on the data-driven cut-offs.
(DOCX)
Acknowledgments
We would like to thank C. P. (Conrad) van der Hoeven, A.G.D (Arnim) Mulder and M. (Mar-
ius) Vogel for their help and constant willingness to help with questions regarding the data
used for this study. Also, thank you to R. H. (Roel) Streefkerk for the work done to organise
and combine the data as well as producing the SSI outcome variable used in this study.
Author Contributions
Conceptualization: J. M. van Niekerk, M. C. Vos, A. Stein, L. M. A. Braakman-Jansen, A. F.
Voor in ‘t holt, J. E. W. C. van Gemert-Pijnen.
Data curation: J. M. van Niekerk, M. C. Vos.
Formal analysis: J. M. van Niekerk, A. F. Voor in ‘t holt.
Investigation: J. M. van Niekerk, M. C. Vos, A. F. Voor in ‘t holt.
Methodology: J. M. van Niekerk, M. C. Vos, A. Stein, A. F. Voor in ‘t holt.
Supervision: A. Stein, L. M. A. Braakman-Jansen, A. F. Voor in ‘t holt, J. E. W. C. van Gemert-
Pijnen.
Validation: J. M. van Niekerk.
Writing – original draft: J. M. van Niekerk, M. C. Vos, A. Stein, L. M. A. Braakman-Jansen,
A. F. Voor in ‘t holt.
Writing – review & editing: J. M. van Niekerk, M. C. Vos, A. Stein, L. M. A. Braakman-Jan-
sen, A. F. Voor in ‘t holt, J. E. W. C. van Gemert-Pijnen.
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10.1126_scitranslmed.adh9917.pdf
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Data and Materials Availability:
All data associated with this study are in the paper or supplementary materials. All
reasonable requests for materials to the corresponding author will be fulfilled. The VirScan
library is available from S.J.E. under a material transfer agreement with the Brigham and
Women’s Hospital.
|
Data and Materials Availability: All data associated with this study are in the paper or supplementary materials. All reasonable requests for materials to the corresponding author will be fulfilled. The VirScan library is available from S.J.E. under a material transfer agreement with the Brigham and Women's Hospital.
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HHS Public Access
Author manuscript
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
Published in final edited form as:
Sci Transl Med. 2023 July 26; 15(706): eadh9917. doi:10.1126/scitranslmed.adh9917.
Signatures of AAV-2 immunity are enriched in children with
severe acute hepatitis of unknown etiology
Moriah M. Mitchell1,2,3, Yumei Leng1,2, Suresh Boppana4, William J. Britt4, Luz Helena
Gutierrez Sanchez5, Stephen J. Elledge1,2,*
1Division of Genetics, Department of Medicine, Howard Hughes Medical Institute, Brigham and
Women’s Hospital, Boston, MA 02115, USA
2Department of Genetics, Harvard Medical School, Boston, MA 02115, USA
3Program in Systems, Synthetic, and Quantitative Biology, Harvard University, Boston, MA 02115,
USA
4Division of Pediatric Infectious Diseases, Department of Pediatrics, University of Alabama at
Birmingham, Birmingham, Alabama, USA
5Division of Gastroenterology, Hepatitis, and Nutrition, Department of Pediatrics, University of
Alabama at Birmingham, Birmingham, Alabama, USA
Abstract
Severe acute hepatitis of unknown etiology in children is under investigation in 35 countries.
Although several potential etiologic agents have been investigated, a clear cause for the liver
damage observed in these cases remains to be identified. Using VirScan, a high throughput
antibody profiling technology, we probed the antibody repertoires of nine cases of severe
acute hepatitis of unknown etiology treated at Children’s of Alabama and compared their
antibody responses to 38 pediatric and 470 adult controls. We report increased adeno-associated
dependoparvovirus A (AAV-A) breadth in cases relative to controls and detailed adeno-associated
virus 2 (AAV-2) peptide responses that were conserved in 7 of 9 cases but rarely observed in
pediatric and adult control. These findings suggest that AAV-2 is a likely etiologic agent of severe
acute hepatitis of unknown etiology.
One-sentence summary:
AAV-2-reactive antibodies and evidence of AAV-2 helper virus infection are associated with
pediatric severe acute hepatitis of unknown etiology.
This work is licensed under a Creative Commons Attribution 4.0 International License, which allows reusers to distribute, remix,
adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for
commercial use.
*Corresponding author: [email protected].
Author contributions: M.M.M. and S.J.E. conceptualized the project and wrote the paper. M.M.M. and Y.L. performed the laboratory
experiments. M.M.M. analyzed the data. S.B., W.J.B., and H.L.G.S. curated and provided samples and metadata for the AHUE cases.
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Mitchell et al.
Introduction
Page 2
Recent reports of acute hepatitis of unknown etiology (AHUE) in children have sparked
concern globally with over 1000 probable cases identified since October 2021 (1). In the
United States, 372 children were under CDC investigation for acute hepatitis of unknown
cause as of October 2022 (2) with 22 associated liver transplants and 14 deaths reported (3).
Although overall incidence of pediatric hepatitis cases in the United States may not have
increased over pre-pandemic incidence (4), spatiotemporal clustering of cases in Alabama,
Scotland, and the Netherlands has prompted the search for a shared etiologic agent (1,
3, 5). Hypotheses under investigation include environmental or toxin exposure, pathogen
exposure, superantigen or autoimmune reactions to persistent or previous severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2 infection), and altered response to first
adenoviral exposure as a result of delayed initial exposure due to infection control behaviors
and isolation brought on by the coronavirus disease 2019 (COVID-19) pandemic (3).
Human adenoviruses (HAdVs), particularly adenovirus F subtypes 40 and 41, are a leading
exposure under investigation as a potential trigger of AHUE (6). In contrast to HAdVs
B to E, Adenovirus F is well adapted for gastrointestinal tropism as result of structural
differences in the capsid fibers which enable stability at low pH (7) and is a leading cause
of gastroenteritis in children (8). Although HAdV infection in general has been implicated
in some cases of hepatitis, reports of adenovirus associated hepatitis in immunocompetent
patients are scarce (9–11). Of the 9 cases of AHUE identified in the initial Alabama cluster
in the United States, 8 (89%) tested positive for human adenovirus infection by whole blood
quantitative polymerase chain reaction (qPCR) and all five for which subtyping was possible
were found to be Adenovirus 41 (6). Adenovirus infection has been detected less frequently
in other case series. In a cluster in Scotland, 5 out of 13 children (38%) tested positive for
HAdV by PCR testing of throat swab, blood, or stool (5). Only 45% of all children in the
United States and 52% of children in Europe under investigation for AHUE were found to
be positive for any HAdV where testing was performed (1, 3).
In order to investigate possible viral etiology of AHUE, we employed VirScan, a
phage display immunoprecipitation sequencing (PhIP-seq) technology that detects antibody
binding to peptides derived from the proteomes of selected pathogens. We studied the
anti-viral antibody repertoires of nine patients with AHUE in comparison to pediatric and
adult controls. Here, we present evidence of a conserved signature of adeno-associated
dependoparvovirus A (AAV-2) immunity in cases of AHUE that was not observed in
pediatric or adult controls. These findings point to AAV-2 as a potential etiologic agent
for AHUE development.
Results
Sample characteristics
Serum isolated from whole blood of 9 patients with severe acute hepatitis of unknown
etiology admitted to Children’s of Alabama between October 1, 2021, and February 28,
2022, were analyzed (Table 1). Serum samples collected from 38 healthy children prior to
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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Mitchell et al.
Page 3
the COVID-19 pandemic were used as pediatric controls. Serum samples obtained from 470
adults with current or prior SARS-CoV-2 infection were used as additional controls.
Overall breadth of antiviral antibody responses did not differ between cases and pediatric
controls.
To explore the hypothesis that AHUE is triggered by atypical immune response due to
delayed initial pathogen exposure caused by pandemic infection control measures in early
life, we examined the antibody breadth of cases and pediatric control samples collected
prior to the COVID-19 pandemic. If delayed initial pathogen exposure played a role in
development of AHUE, one would expect to see reduced breadth of antibody response in
cases versus controls; however, no difference in overall antibody breadth, measured as the
total number of VirScan peptides targeted in a sample, was observed between cases and
pediatric controls (p = 0.78 for Welch’s 1-sided t-test). Of the 115,753 peptides in the
VirScan library used, samples targeted a mean of 1352 ± 40 peptides. After correcting for
multiple testing, the only significant difference in breadth of antibody response at the family
level occurred for Parvoviridae (p =1.47 × 10−3 for Welch’s 1-sided t-test).
As overall breadth of antibody response was not different between cases and controls,
we explored whether differential antibody responses to specific pathogens were observed
between cases and controls (Fig. 1A). Although nominally significant differences (Welch’s
1-sided t-test, alpha=0.05) in mean breadth of response between cases and pediatric controls
were detected across 5 pathogens (Fig. 1B), only differences in responses to 4 Parvoviridae
species (Adeno-associated dependoparvovirus A, Adeno-associated virus, Adeno-associated
virus VR-355, and non-human primate Adeno-associated virus) were significant after
correcting for multiple hypothesis testing [false discovery rate (FDR) = 0.05]. Of 9 cases, 7
appeared to have strong antibody responses to AAVs. Breadth of pathogen-specific response
to other common childhood pathogens in cases fell well within the distributions observed in
pediatric controls (Fig. 1B and C).
Epitopes Associated with Severe Acute Hepatitis of Unknown Etiology
In order to identify peptides targeted at significantly higher frequency in cases than controls,
we used Fisher’s one-way Exact Test and adjusted for multiple tests with the Benjamini-
Hochberg Procedure. Sixty-nine peptides from 7 viruses were significantly enriched in
cases versus pediatric controls (FDR=0.05) (Fig. 2A). Sixty of these peptides were also
significantly (FDR=0.05) enriched in cases versus the adult control cohort in which in which
samples were collected at a more similar time and location to cases. All peptides targeted at
significantly (FDR=0.05) higher relative frequency in cases versus both adult and pediatric
controls were derived from AAVs, except one Hepatitis B virus peptide. This peptide is a
subsequence of the Hepatitis B virus Large Envelope Protein but shares two motifs with
AAV peptides that were targeted at higher relative frequency in cases. The cytomegalovirus
(CMV) and influenza peptides were not enriched in cases relative to the adult control group.
Many of the AAV peptides targeted in samples were from overlapping tiles from the same
protein sequence or homologous regions of several Parvoviridae species (fig. S1 to 3).
Targeting of overlapping peptides suggests that a linear epitope is located within the 28
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amino acid stretch shared by the library peptides. Two distinct regions likely to contain
epitopes were identified for the Rep68/78 protein (fig. S1 and 2), whereas a 224 amino
acid stretch of 7 overlapping VirScan peptides that likely contains several linear epitopes
was observed for the capsid VP1 protein (fig. S3)>. Targeting of homologous regions of
other Parvoviridae species is likely evidence of a cross-reaction originating from AAV-2
immunity.
Among peptides targeted in all or most cases, these AAV peptides were remarkable in that
they were targeted at very low frequency in all control groups (Fig. 2B). Most other peptides
targeted at high frequency in cases contained previously identified “public epitopes,” or
epitopes targeted frequently in seropositive individuals (12). These peptides were targeted in
both the pediatric and adult control cohorts.
Cases and controls cluster according to parvovirus reactivity.
Complete hierarchical agglomerative clustering according to Parvoviridae reactivity clearly
distinguishes the seven cases with any apparent AAV immunity from pediatric controls
(Fig. 3A). Cases did not cluster together well when clustered according to Adenoviridae
or Herpesviridae reactivity instead (fig. S4). Although adeno-associated dependoparvovirus
reactivity was detected in pediatric controls, clustering by Parvoviridae reactivity appeared
to be driven by a highly conserved and strong response to a set of specific AAV peptides
from the capsid and replication region in cases with AAV immunity.
Evidence of AAV-2 Epitope Spreading can be found in cases of AHUE.
All positive cases targeted several peptides derived from both the AAV-2 VP1 and AAV-2
Rep68 proteins. The average range of a linear epitope footprint is 4 to 22 amino acids (13–
15). Thus, a single targeted 56-mer VirScan peptide could contain multiple distinct epitopes.
When two adjacent peptides are recognized, a minimum of one epitope must exist. If only
one epitope exists, it would be located in the overlapping 28-mer shared between the two
peptides. If two recognized peptides are separated by a non-scoring peptide, antibodies in
the sample bound a minimum of two epitopes. For example, for patient 1, there were a
minimum of 7 distinct epitopes in VP1 and 5 in Rep68, demonstrating epitope spreading
consistent with a robust antibody response (Fig. 3B).
AAV-2 positive cases targeted a mean of 40.4 ± 11.3 Adeno-associated dependoparvovirus
A (AAV-A) VirScan peptides per child compared to means of 1.8 ± 0.8 and 1.0 ± 0.2
peptides for pediatric and adult controls, respectively. Homologous regions of other adeno-
associated dependoparvoviruses were also targeted at high frequency in cases and at very
low frequency or not at all in controls (figs. S1, S2).
Strong AAV Antibody Responses were observed in serum from AHUE cases.
VirScan Epitope Binding Signal (EBS) correlates with antibody titer and can be used as
a quantitative measure of strength of antibody response (16). To identify the strongest
antibody responses in each sample, we rank-ordered VirScan peptides by VirScan EBS
(from greatest to least) and examined the composition of the top 100 scoring peptides for
each individual. Although AAV-A peptides only account for 0.15% of the VirScan library,
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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they make up a mean of 6.4% ± 1.8% of the top 100 peptides rank-ordered by EBS in
AHUE cases that target AAV-2 (N=7). In contrast, a single AAV-A peptide appears in the
top 100 peptides rank-ordered by EBS for only one pediatric control (N=38).
Immunity to AAV-2 Helper Viruses was enriched in cases.
We detected antibodies to HAdVs in all AAV-2 positive cases, consistent with previous
clinical testing (6). We also detect immunity to at least two human herpesviruses (HHVs) in
each of these cases (fig. S5A). Some HHVs were observed with greater frequency in cases
than pediatric controls (fig. S5B).
Discussion
In this study, we detected a conserved antibody response to specific regions of AAV-2
proteins in cases of AHUE that is not observed in pediatric or adult controls. Although
associations between Adenovirus F viral detection and AHUE have been reported (6), we
do not detect any differences in breadth of adenoviral antibody response or response to
specific adenovirus peptides in cases versus controls. Because AAV-2 requires a helper virus
to replicate and human adenoviruses can fulfill this role, it is possible that human adenovirus
exposure is actually a lurking variable and associations between HAdVs and AHUE are
spurious. This is supported by evidence that frequency of HAdV detection in the general
AHUE population is only 45 to 52% (1, 3) and many of the cases negative for HAdVs tested
positive for HHVs (17) which can also act as helper viruses for AAV-2 (18). Furthermore,
during the course of the study, additional evidence of AAV-2 exposure in cases of AHUE
was independently recorded in cohorts of AHUE patients in the United Kingdom(19, 20).
Notably, we have detected increased breadth of antibody response to AAVs in cases versus
controls. Each case recognizes multiple regions within the AAV-2 capsid and replicase
regions with evidence of epitope spreading. Moreover, these regions were conserved across
AHUE cases but very rarely targeted in controls. This is not to say that AAV-2 is not targeted
at all in the pediatric control group. Rather, the breadth of the AAV-A antibody response
is far greater in AAV-2 positive cases than controls, with the seven AAV positive AHUE
cases, each targeting 27 to 60 AAV-A peptides. No pediatric control targets more than 8
such peptides. About 15.4% of pediatric controls under 3 years old and 16.7% of those
over three years old target at least 4 AAV-A peptides. These values are only slightly lower
than previous AAV-2 seropositivity estimates of 21% in healthy children between 1 and 3
years old and 22% in children 3 to 18 years old (21). Part of this difference is due to the
fact that VirScan slightly under detects prevalence for some viruses (e.g. measles) relative
to enzyme-linked immunosorbent assays (ELISAs) in individuals with a past history of
infection but readily detects recent infections (12, 16).
We also report a difference in magnitude of antibody responses to AAV-A in AAV positive
AHUE cases relative to controls. Strong AAV-A responses account for a mean of 6.4 ± 1.8%
of the top 100 peptides rank-ordered by VirScan EBS in AAV positive cases (N=7). Taken
together, the strength and breadth of responses to AAV peptides in cases are consistent with
peak antibody concentrations during infections and shortly after (22). This indicates recent
infection with AAV-2.
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We also detect immunity to HAdVs and HHVs, both of which can act as helper viruses
to facilitate AAV-2 replication, in all AAV-2 positive cases. We detect immunity to at least
two human herpesviruses in addition to adenovirus immunity in each of these cases. These
findings are consistent with previously reported qPCR detection of human adenovirus and
human herpesvirus DNA in samples from patients with AHUE (6). Presence of active
infection with potential helper viruses and detection of AAV-2 immunity points to likely
presence of replicating AAV-2 at the time of AHUE onset.
Our data reveals a strong correlation of AHUE with AAV-2 infections exhibiting a high
titer and breadth of antibody responses indicative of a recent infection. These data implicate
AAV-2 as a likely causative agent of the disease. If so, a central unanswered question is:
Why is AAV-2, which infects many people, suddenly more pathogenic in cases of AHUE?
Beyond the possibility that this variant is more pathogenic, one possible explanation is
that coinfection with multiple helper viruses may act to increase the total number of cells
infected with helper virus, and thus, the total number of cells in which AAV-2 replication
is possible. This may contribute to increased viral titers, inflammation, and tissue damage
leading to severe disease in a subset of AAV-2 infected children, a possibility that needs
to be examined in additional cohorts. Consistent with this hypothesis, hepatotoxicity has
been reported in high-dose AAV gene therapy trials, even leading to death (23). As a result,
immunosuppressants are commonly co-administered with AAV-vectored gene therapy to
prevent hepatotoxicity (24).
A speculative and complementary hypothesis that may play a role in emergence of AHUE
concerns COVID-19 infection control measures and their potential to alter the timing of viral
exposures. Years of masking prevents infection with many viruses at normal frequencies
while immunity wanes for these viruses. Once masking was reduced, more viruses could
have been in simultaneous circulation at higher frequencies in a more vulnerable population,
possibly synchronizing infections. Thus, it is possible that as a result of these circumstances
children may have been more likely to acquire multiple infections at once due to changes in
masking policies and exposures than before the pandemic. This could increase the likelihood
of concurrent infection with AAV-2 and one or more replicating helper viruses.
Seven patients in our cohort clearly had an immune response to AAV-2, but two did not. We
do not believe that rules out AAV-2 as a causative agent of AHUE. Unexplained pediatric
hepatitis is not a new phenomenon. Recent and historical cases of pediatric hepatitis with
no identified cause are likely to stem from an array of biological mechanisms. It is possible
that the two samples without AAV-2 have hepatitis driven by a different cause whereas the
remaining seven may be part of a recent outbreak linked to AAV-2.
Our study has limitations. It is worth noting that due to sample availability, pediatric controls
were not from the same area as cases and were not matched on demographic factors.
Comparing cases to well-matched healthy controls from the same area would have been
preferable and may have influenced results. Another limitation of this study is sample
size. Determining whether the same patterns in AAV-2 antibody response are observed in
independent AHUE cohorts would also be useful.
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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The distinct signatures of AAV-2 immunity detected in this cohort and the cases in Scotland
are both elevated over infection frequency in controls. AAV-2 is therefore likely to be in
some way responsible for AHUE. How this infection contributes to the development of
AHUE mechanistically remains to be elucidated. Further research is needed to characterize
the seroprevalence of AAV-2 antibodies in AHUE patients and identify mechanisms by
which AAV-2 may be driving pediatric hepatitis onset.
Methods
Study Design
This is an observational case-control study utilizing previously collected serum samples
from cases and several control groups as outlined below. The objective of this study was
to identify whether there were differences in the antibody responses of cases of AHUE
compared to controls with the goal of identifying a viral cause for AHUE. Sample size
was determined by sample availability. Each serum sample was run on VirScan with two
technical replicates to ensure that results were consistent between replicates. No samples
were excluded from analysis. We observed the distributions of antiviral antibodies in
cases and controls. Statistical analysis was used to evaluate differences between antibody
responses of cases and controls overall and at the family, pathogen, and peptide levels.
Sources of Serum
VirScan
Secondary use of all human samples for the purposes of this work was exempted
by the Brigham and Women’s Hospital Institutional Review Board (protocol number
2013P001337). Serum was obtained from nine patients under the age of 18 years admitted
to Children’s of Alabama for severe acute hepatitis of unknown cause between October
1, 2021, and February 28, 2022. These samples were derived from patients previously
characterized in a published case series (6). Samples from healthy children enrolled in
the DIABIMMUNE study were used as pediatric controls. This cohort was described in
previous studies (16, 25). The adult cohort was from a previous study designed to measure
antibody responses in SARS-CoV-2 patients (26).
VirScan was performed using the VirScan 2.0 library (16) following a published protocol
(27). In brief, VirScan is a Phage Immunoprecipitation Sequencing (PhIP-seq) technology.
The library used in this study displays 115,753 peptides derived from published viral
proteome sequences and known Immune Epitope Database (IEDB) epitopes on the surface
of T7 bacteriophage. Diluted serum is incubated with the phage library and phage bound
to antibodies are immunoprecipitated using Protein A/G coated magnetic beads. The phage
insert DNA is then amplified and sequenced to determine which peptides were bound by
antibodies in serum and to what degree. Epitope binding signals (EBS), a quantitative
measure of antibody binding enrichment to each library peptide, and hits, a binary measure
of whether a peptide was targeted or not were computed as previously described (16, 27). In
brief, we consider a peptide recognized (“a hit”) if the EBS in both technical replicates is at
least 3.5. EBS is presented as mean EBS across both technical replicates. EBS values below
0 have been artificially set to 0 for visualization.
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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Data analysis
Page 8
Overall antibody breadth was calculated as the total number of hits in a sample across the
VirScan library. Hits are defined as peptides with epitope binding signals greater than 3.5 in
both technical replicates. Pathogen-specific and family-wide breadth were calculated as the
total number of hits across peptides derived from published protein sequences for a given
pathogen species or family. Multiple sequence alignments were generated using Clustal
Omega (28–30).
Statistical analysis—Figures were generated using R (version 4.1.2 with packages
ggplot2 (31), ggpubr (32), ggmosaic (33), pheatmap (34), and ggplotify (35)), Adobe
Illustrator, and UCSF ChimeraX (36). Statistical analysis was performed in R (37) (version
4.1.2). Welch’s T-test, Fisher’s Exact Test, and Benjamini-Hochberg adjusted p-values were
computed using the t.test, fisher.test, and p.adjust functions respectively from the stats
package (version 3.6.2). To test whether overall breadth of immune response was lower
in cases versus controls, we used Welch’s one-sided t-test with an alpha of 0.05. To test
whether immune breadth was higher in cases than controls within specific families and
pathogens, Welch’s one-sided t-test was used with a Benjamini-Hochberg correction for
multiple tests (FDR=0.05). In order to identify peptides targeted at significantly higher
frequency in cases than controls, we used Fisher’s one-way Exact Test and adjusted for
multiple tests with the Benjamini-Hochberg Procedure (FDR=0.05). Confidence intervals
were constructed using 95% confidence intervals based on the t-distribution with the
summarySE function in the Rmisc package (version 1.5.1).
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgements:
We thank Galit Alter and Ellen Shrock for helpful discussions.
Funding:
This research was supported by NIH grants 1P01AI165072 to SJE and 1U01CA260462–02 to SB. S.J.E. is an
Investigator with the Howard Hughes Medical Institute.
Competing Interests:
S.J.E. is a founder of TSCAN Therapeutics, ImmuneID MAZE Therapeutics and Mirimus, S.J.E. serves on the
scientific advisory board of Homology Medicines, TSCAN Therapeutics, MAZE Therapeutics, none of which
impact this work. S.J.E. is an inventor on a patent application filed by the Brigham and Women’s Hospital
(US20160320406A) that covers the use of the VirScan library to identify pathogen antibodies in blood. S.B. is a
member of GSK CMV Vaccine Advisory Board. S.B. receives research grant funding from Merck and Pfizer on
unrelated projects. Bill Britt is a consultant to Moderna’s CMV vaccine program.
Data and Materials Availability:
All data associated with this study are in the paper or supplementary materials. All
reasonable requests for materials to the corresponding author will be fulfilled. The VirScan
library is available from S.J.E. under a material transfer agreement with the Brigham and
Women’s Hospital.
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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34. Kolde R, pheatmap: Pretty Heatmaps, version 1.0.12, CRAN (2019); https://CRAN.R-project.org/
package=pheatmap.
35. Yu G, ggplotify: Convert Plot to ‘grob’ or ‘ggplot’ Object, version 0.0.5, CRAN (2020); https://
CRAN.R-project.org/package=ggplotify.
36. Pettersen EF, Goddard TD, Huang CC, Meng EC, Couch GS, Croll TI, Morris JH, Ferrin
TE, UCSF ChimeraX: Structure visualization for researchers, educators, and developers. Protein
Science 30, 70–82 (2021). [PubMed: 32881101]
37. R. R Core Team, R: A language and environment for statistical computing. (2013).
38. Santosh V, Musayev FN, Jaiswal R, Zárate-Pérez F, Vandewinkel B, Dierckx C, Endicott M, Sharifi
K, Dryden K, Henckaerts E, The Cryo-EM structure of AAV2 Rep68 in complex with ssDNA
reveals a malleable AAA+ machine that can switch between oligomeric states. Nucleic Acids
Research 48, 12983–12999 (2020). [PubMed: 33270897]
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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Fig. 1. Breadth of AAV-specific antibody responses differed between cases and controls.
(A) Shown is a heatmap of epitope binding signals (EBSs) for all peptides targeted by cases
or pediatric controls (n=18,484). Raw data are available in data files S1 to S2. (B) The mean
number of species-specific peptides targeted in cases, pediatric controls, and adult controls
are shown for pathogens with nominally significant differences (p <0.05) as measured
with Welch’s one-sided t-test with 95% confidence interval. Pathogens with statistically
significant (FDR=0.05) differences after Benjamini-Hochberg correction are annotated with
*. (C) Pathogen-specific breadth of antibody response in cases (purple) overlaid on the
distribution of breadth in pediatric controls (blue, n=38) is shown for selected common
pathogens and AAV’s.
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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Fig. 2. AAV peptides were targeted at increased frequency in cases versus controls.
(A) Shown is the number of peptides targeted at a significantly higher frequency in cases
than pediatric controls (Fisher’s one-way Exact test with Benjamini-Hochberg procedure
and FDR of 0.05) by viral species. (B) Mosaic plots show reactivity to representative
AAV-2 peptides heavily targeted in cases and previously identified herpesvirus, adenovirus,
and influenza public epitopes in cases, pediatric controls, and adult controls. The number
of individuals that do or do not target each peptide are indicated in boxes on the figure.
Peptide-level hit data for all samples and peptides are available in data files S3 to S5.
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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Fig. 3. Antibody profiles of patients with AHUE clustered according to parvovirus reactivity and
showed evidence of epitope spreading.
(A) Shown is a heatmap of epitope binding signals for cases and pediatric controls for
Parvoviridae peptides. Each row represents a VirScan library peptide and each column is
a sample. Heatmap rows and columns are ordered according to complete agglomerative
clustering with corresponding dendrograms shown. The strains and protein regions from
which peptide sequences were derived is annotated along the y-axis and case-control status
is annotated along the x-axis. (B) The heatmap shows the response to peptides derived from
AAV-2 (isolate Srivastava/1982) protein sequences in cases and pediatric controls.
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
Mitchell et al.
Page 14
Demographic Characteristics of Cases and Controls.
Table 1.
Cases (N=9)
Pediatric controls (N=38)
Adult controls (N=470)
1 (11)
5 (56)
3 (33)
14 (37)
24 (63)
2 (22)
7 (78)
3 (33)
6 (67)
20 (53)
18 (47)
38 (100)
Age – number (%)
< 2 years
2 to 5 years
6 to 10 years
11 to 17 years
18 to 29 years
30 to 39 years
40 to 49 years
50 to 59 years
60 to 69 years
70 to 79 years
> 80 years
Sex – number (%)
Male
Female
Race/Ethnicity – number (%)
Non-Hispanic White
Hispanic White
Hispanic other
Non-Hispanic Black
Asian and Pacific islander
Non-Hispanic other
Hispanic Black
Unknown
47 (10)
85 (18)
63 (13)
96 (20)
70 (15)
54 (11)
55 (12)
228 (49)
242 (51)
171 (36)
82 (17)
102 (22)
39 (8)
15 (3)
8 (2)
3 (1)
50 (11)
Sci Transl Med. Author manuscript; available in PMC 2023 September 14.
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| null |
10.1088_1748-3190_ad0dae.pdf
|
Data availability statement
The data cannot be made publicly available upon
publication because no suitable repository exists for
hosting data in this field of study. The data that sup-
port the findings of this study are available upon reas-
onable request from the authors.
|
Data availability statement The data cannot be made publicly available upon publication because no suitable repository exists for hosting data in this field of study. The data that support the findings of this study are available upon reasonable request from the authors.
|
Bioinspir. Biomim. 19 (2024) 016006
https://doi.org/10.1088/1748-3190/ad0dae
PAPER
RECEIVED
24 August 2023
REVISED
25 October 2023
ACCEPTED FOR PUBLICATION
17 November 2023
PUBLISHED
29 November 2023
Capillary efficiency study in leaf vein morphology inspired
channels
Jingyu Shen1,2, Ce Guo1,2,∗, Yaopeng Ma1 and Ao Dong1
1 College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, People’s
Republic of China
2 Institute of Bio-Inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016,
∗
People’s Republic of China
Author to whom any correspondence should be addressed.
E-mail: [email protected]
Keywords: bionic design, leaf vein inspired structure, capillary flow, multiphysics simulation, microchannel optimization
Abstract
Inspired by the capillary transport function of plant leaf veins, this study proposes three typical leaf
vein features by observing a large number of leaves, including wedge shape, branch asymmetry, as
well as hierarchical arrangement, and investigates their capillary transport mechanism. Not only a
preliminary theoretical analysis of capillary flow in the bio-inspired channels was carried out, but
the COMSOL Multiphysics simulation software was also used to simulate gas–liquid two-phase
flow in biomimetic channels. The results reveal the efficient transport mechanism of the leaf vein
inspired structure and provide insight into the design of capillary transmission channels.
1. Introduction
The morphology of veins in leaves is an excellent tem-
plate developed by natural selection during the long-
term evolution of plants, and it plays an important
role in water transport, mechanical support, and res-
istance to pests or disease [1, 2]. Nowadays, there has
been much research in various fields directed toward
mimicking leaf vein morphology. Liu et al [3] pro-
posed an adaptive morphogenetic algorithm based on
leaf vein growth manner to match load path with rein-
forcement layout. Liu et al [4] constructed heat trans-
fer networks that mimic leaf veins in phase-change
materials to improve heat absorption efficiency. Xia
et al [5] and Ouellette et al [6] emulated leaf vein
structure in the flow field of a proton-exchange mem-
brane fuel cell to promote the uniform distribution
of reaction gases and currents and improve the power
output of the cell. Patino–Ramirez and Arson [7] and
Barthélemy and Flammini [8] proposed algorithms
for planning urban transportation networks based on
leaf vein patterns, which is in good agreement with
the observed empirical patterns.
in plants, and they are excellent bionic templates for
designing capillary channels. First, in terms of func-
tion, leaf veins are typical capillary channel structures.
The Cohesion-tension theory suggests that plant leaf
veins are capillary channels that exist widely in nature.
More than 95% of water absorbed by roots is lost
via through leaf veins transpiration and participates
in the water cycle of the ecosystem [9], according to
statistics, the annual transpiration of an oak tree is
nearly 151 kL [10]. Second, in terms of structure, vein
morphology is the optimal result of the co-evolution
of various basic physiological activities. Specifically,
in photosynthesis, water transport through leaf veins
affects the conversion efficiency of organic matter and
oxygen [11–13], and in transpiration, the vein struc-
ture determines the shortest path between the water
source and the point of evaporation (stomata) [14,
15]. In addition, since the Cretaceous period, leaf
veins have undergone a long-term evolution of sur-
vival of the fittest [16], this coordination relationship
between transport functions and structure suggests
that the vein distribution is the optimal result of nat-
ural selection [17, 18].
Few scholars have analyzed leaf vein structures
from the perspective of capillary force. In nature, leaf
veins are natural capillary channels that widely exist
In practice, various fields utilize capillary force
to transport liquid, realizing autonomous liquid flow
without external energy input. These devices are
© 2023 IOP Publishing Ltd
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
widely used in electronic heat pipes [19, 20], immun-
oassay devices [21], 3D printer nozzles [22], biomed-
ical applications [23, 24], sensors [25, 26], chemical
reactions [27–29], and other fields. However, cur-
rent biomimetic capillary transport structures are not
diverse, a most are Y- and T-type fractal tree networks
based on Murray’s law, and the effect of vein mor-
phology on liquid capillary flow remains mysterious.
Thus, the leaf vein inspired capillary channel has sub-
stantial scientific research potential and is worthy of
further exploration.
This work proposes three typical leaf vein features
by observing a large number of leaves. First, the the-
oretical analysis of capillary flow in leaf vein inspired
structures is respectively performed based on the
Young–Laplace equation, Helmholtz surface energy,
and Navier–Stokes equation. Second, the ‘Two-Phase
Flow, Phase Field’ branch in COMSOL Multiphysics
is applied to simulate the gas–liquid interface in the
bio-inspired channel. The results show that leaf vein
morphology promotes capillary transport, indicat-
ing this bionic structure provides design ideas to
improve the capillary transport function in micro-
fluidic devices.
2. Materials and methods
2.1. Materials
In botany, leaf hydraulic conductance is an index
to evaluate the water conductivity of plant leaves.
Relevant statistics show that the leaf hydraulic con-
ductance of ferns is 0.76 mmol m–2s–1MPa–1, while
that of tropical trees can reach 49 mmol m–2s–1MPa–1,
suggesting veins in tropical plants have excellent
liquid transport functionality [30].
Our team observed lots of tropical plant leaf vein
patterns from the Xishuangbanna Tropical Botanical
Garden, Chinese Academy of Sciences, located in
Yunnan Province, China. The three typical plant leaf
veins are summarized: wedge–shaped veins, lateral
veins with asymmetric distribution, and hierarchical
veins.
2.2. Typical leaf vein features
2.2.1. Wedge–shaped veins
As the four examples shown in figure 1, veins in all
plant leaves (studied in this paper) gradually taper
from the petiole at the base to the tip of the leaf, giving
a wedge shape. In biomechanical terms, the wedge–
shaped structure load from the petiole to the tip
gradually decreases so that the forces required to sup-
port more distal parts of the leaf gradually decrease,
this is similar to a cantilever beam with an economic
mechanical function [31, 32]. In physiological func-
tion terms, this feature helps leaves adjust their ori-
entation to face the sun [33]. However, the relation-
ship between the capillary force and wedge shape has
not been investigated.
2
Actually, this wedge shape is more visible at the
lower order of the leaf and gradually becomes less
apparent or even disappears at the higher order veins.
In addition, plants growing at low latitudes with
high transpiration rates have veins with higher wedge
angles, numbers, and orders. Therefore, this study
postulates that the wedge shapes in leaf veins are
related to fluid transport.
2.2.2. Asymmetric distribution of lateral veins
Lateral veins are mostly asymmetric (figure 2).
Related studies have shown that an asymmetric dis-
tribution represents increased geometric degrees of
freedom in the structure, which can better disperse
external forces from wind or other events to avoid
tearing [34]. However, the effect of this asymmetric
distribution on capillary flow has not been studied.
This paper will analyze the effect of the asymmet-
ric distribution on liquid capillary transport from the
perspective of capillary force.
2.2.3. Hierarchical vein distribution
Leaf vein is a reticulated hierarchical system based on
the vein diameter and branch (figure 3). Typically, the
first-order (1◦) vein is the widest and extends from
the petiole at the base to the leaf tip. The second-order
(2◦) veins branch from the ‘major’ veins, the third-
order (3◦) veins and up to five additional orders form
a network between the 1◦ and 2◦ veins. Botanical
studies have shown that differences in leaf veins
between various species lie mainly in the terminal
veins. The density of terminal veins in transpiration-
active plants is higher, which is up to 80%–98% of the
total [35], while hierarchical leaf vein in leaves with
weak transpiration is markedly less. As evaporation
is coordinated with fluid transport in leaves, we tend
to study whether the hierarchical distribution of leaf
veins affects sap conductivity.
2.3. Theoretical analysis
2.3.1. Capillarity of the wedge–shaped structure
From the Young–Laplace equation, the differential
pressure ∆p in an ideal capillary is described by:
(
)
∆p = σ
1
R1
+
1
R2
(1)
where σ is the surface tension, and R1, R2 are the
radii of curvature of two mutually orthogonal liquid
surfaces.
Due to the constraints of fabricating capillary
channels of circular cross-section, capillary flow
channels are usually with rectangular cross-sections,
so the capillary pressure difference at the gas-liquid
interface of the rectangular closed channel can be
expressed as:
∆P = σ
(
)
1
Rw
+
1
Rh
Rw =
w
2 cos θ
Rh =
h
2 cos θ
(2)
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 1. Examples of the wedge–shaped leaf vein morphology.
Figure 2. Leaf vein morphology with an asymmetrical lateral vein distribution.
Figure 3. Examples of the hierarchical leaf vein morphology [36]. Copyright 2020, Hindawi Limited. Reproduced from [36].
CC BY 4.0.
where Rw and Rh are the radii of curvature in the
width direction and height direction respectively, and
the contact angle in the equilibrium state of solid,
liquid, and gas is θ (figure 4).
For a wedge–shaped channel with αapex angle, the
geometric relationship can be expressed as:
αapex = 2 arctan
win − wout
2x
.
(3)
3
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 4. Meniscus in wedge–shaped capillary channel with an apex angle of αapex (the image dimensions are not to scale and
are for illustrative purposes only).
Table 1. Parameters of the capillary channel depicted in figure 4.
Symbol
Definition
w
win
wout
h
L
x
Rw
Rh
αapex
θ
Channel width
Channel inlet width
Channel outlet width
Channel height
Channel length
Liquid flow displacement
Meniscus radius of curvature along the width
Meniscus radius of curvature along the height
Wedge angle of the channel
Contact angle
Figure 5. Illustration of the contact angle variation process
of Gibbs’ criterion.
Since αapex is extremely small, it can be con-
cluded that sinαapex ≈ tanαapex ≈ αapex, and thus the
following equation can be derived from the geometric
relationship:
∆p =
2σ cos θ
win − xαapex
+
2σ cos θ
wh − xαapex
.
(4)
According to the equation (4), it can be inferred
that wedge–shaped channels can alter the curvature
of the meniscus and hence improve the capillary pres-
sure difference, which further affects the liquid filling
efficiency.
2.3.2. Capillarity of asymmetric branch structure
Based on the Gibbs’ criterion [37], when liquid
encounters a mutated vertex while filling a capillary,
the cross-section of the channel is discontinuous (the
cross-section of the channel expands abruptly) and
the three-phase contact line will ‘pinning’ over a cer-
tain range of solid–liquid contact angles. As is shown
in figure 5, for liquid in straight channel flow, the
contact angle θ is constant, when the liquid men-
iscus reaches a branch edge (point M), the three-
phase contact line abruptly pinning at point M, but
the meniscus continues to flow forward during this
period and the solid–liquid contact angle increases
from θ to θ + β. Subsequently, when the contact angle
exceeds θ + β, a new equilibrium contact angle forms
at the inclined wall and the three-phase contact line
detaches from point M, after which capillary flow
continues along the inclined wall.
The above process can be analyzed from the per-
spective of energetics, and the surface tension of the
three-phase interface can be analyzed based on the
Helmholtz surface energy
σij =
dU
dAij
i, j = s, l, g; i ̸= j
(5)
where U is the energy, A is the contact surface area,
and s, l, g represent solid, liquid, and gas. The total
energy of the solid–liquid–gas interfacial system is:
U = σslAsl + σsgAsg + σglAgl.
(6)
For liquid transport in an ideal smooth channel
that satisfies the Young’s equation:
σsg = σsl + σgl cos θ.
(7)
4
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 6. Illustration of liquid transport in a branch channel.
Combining and simplifying equation (6) and
can (7) yield:
(
)
U = σsg
Asl + Asg
− σglAsl cos θ + σglAgl.
(8)
As the wetted area changes during the liquid filling
process, the total energy of the three-phase interfacial
system changes, and the capillary driving force can
be derived from the equation (8). As Asl + Asg is con-
stant, assume Uc = σsg(Asl + Asg), then equation (8)
can be expressed as:
U = Uc − σglAsl cos θ + σglAgl.
(9)
The capillary pressure difference driving liquid
filling in the channel is:
(
∆P = − dU
dV
= σgl
cos θ
)
dAsl
dV
− dAgl
dV
.
(10)
According to the equation (10), the system energy
U and liquid volume V at each stage can be derived,
and then the instantaneous capillary driving force can
be calculated. Based on previous studies [38], this
article analyzes the process of capillary flow filling
branch channels from the perspective of the energet-
ics, which can be divided into four stages (figure 6).
In stage I, for capillary flow filling a constant-
width channel, the liquid meniscus is unchanged, and
the contact angle is constant at θ, so the capillary pres-
sure difference is constant.
In stage II, the liquid meniscus edge is pinned
when the three-phase contact line reaches the point
5
M. As the liquid continues to fill the channel,
the radius of curvature of the meniscus gradually
increases to positive infinity, at which time the liquid
front edge changes from concave to flat. Therefore,
during the transition from stage I to II, the capillary
pressure difference gradually decreases to 0.
In stage III, the meniscus changes from flat to con-
vex, but the three-phase line does not leave the point
M, so during stage II to stage III, a barrier pressure
is generated in the direction opposite to liquid flow,
which prevents liquid filling, this phenomenon is also
known as ‘capillary pinning’.
In stage IV, as the liquid fills the channel, the con-
tact angle gradually increases, and when it exceeds
θ + β, a new equilibrium contact angle is formed at
the inclined wall. Subsequently, the three-phase con-
tact line leaves point M. and this process generates a
new capillary pressure difference.
Based on the overall analysis, it can be inferred
that the primary factor affecting the process of
capillary flow filling the branch channel
is the
barrier pressure that causes ‘capillary pinning’ in
stage III. Man et al [39] neglected the sides of
the flow channel and only considered the men-
iscus on the horizontal plane to calculate the 2D
barrier pressure ∆P = 2σgl
);
w (
Chen et al [40] simultaneously studied the men-
iscus in the horizontal and lateral planes of the
capillary channel, and calculated the 3D barrier
)
as ∆P = 2σgl
h cos θ − cos (θ + β)
pressure
;
w
and Cho et al [41] considered the advancing
cos θ− α
sin α ( α
cos β + sin β
−cos α)
sin α sin β
− w
sin α
(
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Referring to the previous study [43], integration
of the equation (13) with the boundary condition
Fy = ±w/2 = 0 yields velocity distributed along the
y-axis at the x position:
(
) (
)
u (y) =
1
2µ
− ∂p
∂x
w2
4
− y2
.
(15)
Figure 7. Illustration of the hierarchical capillary channels
(normal flow along the x-axis).
(
cos(min{θA+β,180
contact angle θA, to derive the barrier pressure
as ∆P = 2σgl
. Some
w
w
researchers currently use microchannels with an
abruptly increased cross section as capillary stop
valves [42].
+ cos θA
h
◦})
)
2.3.3. Capillarity of hierarchical structure
In this paper, channel height is constant and only the
channel width can change (figure 7). The capillary
flow conforms to the law of conservation of mass and
energy and the filling process conforms to the Navier–
Stokes equations:
The average velocity along the x-direction is:
¯u =
1
w
w/2ˆ
−w/2
u (y) dy =
(
)
.
− ∂p
∂x
w2
12µ
(16)
Alternatively,
average velocity can also be
expressed as:
¯u =
dx
dt
.
Combining equations (14)–(17) yields:
dt =
6µ
wσ cos θ
· x · dx.
(17)
(18)
Integration of the equation (18) yields the rela-
tionship between t and x as:
t =
3µx2
wσ cos θ
.
(19)
∂u
∂x
+
∂v
∂y
= 0
(11)
Thus, during the same period, the relationship
between channel width w and flow distance x is:
where u and v are the velocities in the x and y dir-
ections, and the Navier–Stokes equation in the flow
plane is deduced from the law of conservation of
momentum as:
)
)
(
(
∂t + u ∂u
∂u
∂t + u ∂v
∂v
∂x + v ∂u
∂y
∂x + v ∂v
∂y
)
= − ∂p
= − ∂p
∂x + µ
∂y + µ
(
∂y2
∂x2 + ∂2u
∂2u
)
∂x2 + ∂2v
∂2v
∂y2
ρ
ρ
(
+ Fx
w1
w2
∝ x2
2
x2
1
.
(20)
In addition, in a channel of height h, the capillary
transport volume Q is expressed as:
Q = whx.
(21)
+ Fy
(12)
Combined with the equation (20), during the
same period, the relationship between w and Q is:
where p is the pressure differential, which includes
capillary pressure and viscous drag, ρ and µ are the
density and viscosity of the fluid respectively, and Fx
and Fy are the forces in the and y directions respect-
ively. When the capillary flow reaches a stable laminar
flow state, the y direction velocity v is zero, and the x-
direction velocity u is related to time t and position v.
Equation (12) can be simplified as:
∂2u
∂y2
=
1
µ
∂p
∂x
.
(13)
In this paper, capillary force is the only driving
force. According to the equation (1), the pressure
gradient of the liquid along the x direction can be
deduced as:
− ∂p
∂x
=
1
x
2σ cos θ
w
.
(14)
6
w1
w2
∝ Q2
1
Q2
2
.
(22)
equations
analysis of
Comprehensive
(20)
and (22) shows that the effect of channel width (w)
for capillary flow is multifactorial. When w increases,
capillary flow distance (x) decreases, but capillary
transport volume (Q) increases.
2.4. Simulation analysis
2.4.1. Control equations and mathematical models
As the manufacturing process limitations, the com-
mon capillary channels have the same width and
height. To simplify the analysis, previous scholars [43,
44] have used experiments and simulations to prove
that a 1D curve can be used to describe the liquid
front, and the capillary force along the height direc-
tion of the meniscus can be ignored. In bioinspired
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Table 2. Physical parameters in numerical analysis.
Parameter
Value
Description
ρ
µ
θ
χ
ε
λ
σ
1000 kg m−3
0.1 Pa·s
60◦
50 m·s kg−1
2.4056 × 10−4 m
0.018549 N
0.072697 N m−1
Liquid density
Viscosity
Contact angle
Mobility tuning parameter
Parameter controlling interface thickness
Mixing energy density
Surface tension
capillary channels, the liquid flows mainly in the x-
y plane. Thus, in this study, a 2D numerical model
based on previous research [39, 43, 45–47] is estab-
lished to analyze the capillary flow efficiency and
meniscus changes of the liquid in the bionic channels.
This study simulates liquid capillary flow with
the COMSOL Multiphysics (based on phase field
method) to analyze liquid transport in the leaf
vein inspired channel. The phase field method is a
mathematical approach to describe interfaces using
sequential parametric gradients, where macroscopic
phase changes are discretized into microscopic field
changes so that the contact angle of the fluid natur-
ally rotates. The phase field equation model is based
on the Cahn–Hilliard control equation:
+ u · ∇Φ = ∇ · γλ
∂Φ
ε2
∂t
(
ψ = −∇ · ε2∇Φ + Φ
∇ψ
Φ 2 − 1
)
(23)
where u is the fluid velocity, γ is the mobility, λ is the
mixing energy density, and ε is the interface thick-
ness parameter. The diffuse interface is defined as
the region where the dimensionless phase field vari-
able Φ goes from = 1 to −1, where Φ = −1 indic-
ates pure gas and Φ = 1 indicates pure liquid. Ψ is
the phase field auxiliary variable, which reformulates
the Cahn–Hilliard equation as a system of two fully
coupled second-order partial differential equations
[48]. At equilibrium, the surface tension σ can be
derived from λ with ε as:
√
2
2
3
λ
ε
.
σ =
(24)
The two components in the phase field are liquid
(water) and gas (air). As Φ ∈ [−1,1], Vwater + Vair = 1.
Thus, the volume fraction of the two components (V1,
V2 ), density (ρ) of the mixture across the two-phase
interface, and viscosity µ are defined as follows, and
the physical parameters are shown in table 2
(
Vl = (1 − ϕ ) /2 V2 = (1 + ϕ ) /2
)
ρ = ρl +
µ = µl +
ρg − ρl
(
µg − µl
V2
)
V2
.
(25)
Table 3. Boundary conditions for the simulations.
Boundary
Condition
Inlet
Outlet
Wall
Absolute pressure = 1 atm;
Relative pressure = 0
Absolute pressure = 1 atm;
Relative pressure = 0
No slip
flows into it along the wall under the effects of adhe-
sion and surface tension, and the leading edge of the
liquid forms a meniscus at the gas–liquid interface.
The surface tension caused by this deformation drives
the liquid to gradually fill and simultaneously expel
air from the channel. This study incorporates the fol-
lowing assumptions into the model: (1) the fluid is
incompressible laminar flow. (2) The channel sur-
face roughness is neglected. (3) The effect of gravity
is ignored.
The pressure relationship in the numerical ana-
lysis is given as:
Pabs = P + Pref
(26)
where the absolute pressure (Pabs) is the actual pres-
sure of the fluid, an absolute pressure of zero cor-
responds to a vacuum. The relative pressure (P) is
the fluid’s pressure with respect to a reference pres-
sure (Pref), which is set to atmospheric pressure here.
As liquid flow transport is dominated by capillary
force, when the reference pressure is atmospheric,
the boundary conditions of the model are shown in
table 3.
Figure 8 shows the mesh convergence analysis
of the wedge–shaped model with ten different grid
quantities. To comprehensively evaluate the calcula-
tion accuracy and efficiency, the average grid quant-
ities of the wedge–shaped model is 3.75 × 104 in
this study. Similarly, the same method is used to per-
form grid convergence analysis on the asymmetry
branch model and hierarchical model, which will not
be described again in this paper.
Initially, the channel is filled with air, and the inlet
of the biomimetic channel is connected to a water-
filled reservoir. When the simulation starts, the liquid
2.4.2. Wedge–shaped model
The geometric parameters for the 2D vein projec-
tion are shown in table 1 to establish the bio-inspired
7
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 8. Grid convergence analysis.
Figure 9. Leaf vein inspired wedge–shaped model.
Table 4. Parameters of the wedge–shaped model.
αapex (◦)
win
wout
0
1
1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.9127
1.0873
0.8255
1.1745
0.7382
1.2618
0.6509
1.3491
0.5637
1.4363
0.4764
1.5236
0.3891
1.6109
0.3019
1.6981
0.2146
1.7854
0.1274
1.8726
wedge–shaped model (figure 9). The apex angle
(αapex) is expressed as:
dasy =
Lateral vein spacing
Lateral vein width
=
Lspacing
w
.
(28)
αapex = 2 arctan
win − wout
2L
.
(27)
To study the effect of αapex on capillary flow,
this study establishes several capillary channel mod-
els with wedge features given the same channel length
and projection area [49]. The αapex is adjusted by
changing the inlet and outlet widths as shown in
table 4. We investigate the effect of αapex on the capil-
lary transport performance based on the meniscus
displacement (x) and the average capillary flow velo-
city (vmean) of the liquid in the wedge structure.
2.4.3. Asymmetry branch model
To investigate the effects of the branch distribution
on capillary flow, this study varies the lateral vein
interval and defines the dimensionless parameter dasy
to describe the degree of branch asymmetry as:
8
Five simulation models are shown in figure 10. All
the model parameters are the same except for dasy.
The influence of the asymmetric branch on capil-
lary transport is obtained by studying the pressure
changes at the branch structure.
2.4.4. Hierarchical model
This paper establishes a bio-inspired model with the
same branch arrangement, channel projection area,
and inlet and outlet parameters to investigate the
effect of hierarchical features on the overall capillary
system (figure 11). The model branch width ratio,
denoted as η, is expressed as follows:
ηij =
wj
wi
i < j.
(29)
In equation (29) that
j = 1, 2, 3. For
example, η12 = 1 means that 1◦–2◦ channels without
i,
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 10. Asymmetry models in the lateral vein.
Figure 11. Hierarchical leaf vein structure models.
hierarchical feature. Likewise, η12 = 0.5 means that
1◦–2◦ channels with hierarchical feature, where 2◦
channels are 0.5 times of the 1◦ channels.
This study takes the average filling rates as the
evaluation index and analyzes the biomimetic hier-
archical model in two steps. First, to study the effect of
the terminal η on the overall capillary flow, this paper
makes three sets of comparisons: Models 1 vs. 2, 3 vs.
4, and 5 vs. 6, as shown in table 5. Next, to study the
effect of the primary η and terminal η relationship
on capillary flow, we make two sets of comparisons
in table 5: Models 3 vs. 5 and 4 vs. 6.
3. Results and discussion
3.1. Effect of wedge–shaped features on capillary
flow
The numerical analysis results of capillary flow in
wedge–shaped channels with various αapex are shown
in figure 12. When the liquid initially enters the capil-
lary channel (the early 0–0.15 s), both the displace-
ment of the liquid (x) and the liquid filling area
(Afilled) are increase together with an increasing αapex.
This is an unavoidable phenomenon caused by geo-
metrical factors, as for a given channel area, a larger
wedge angle gives a wider channel entrance and a lar-
ger area the liquid needs to fill, which means x and
Afilled decrease. After 0.15 s, the capillary flow of each
model begins to show strong differences.
Overall, when αapex ⩽ 0.7◦, x is positively cor-
related with αapex, x increases as αapex increases in
9
the same period, and when αapex > 0.7◦, this beha-
vior is reversed. The slope of the curve in figure 12(a)
illustrates the transmission velocity of the liquid
meniscus at a given moment. Among all wedge–
shaped models, the curves for αapex = 0.6◦ and
αapex = 0.7◦ are approximately linear,
indicating
a steady liquid flow velocity with a smooth fluid
transmission.
Similarly, the value of αapex has an identical effect
on Afilled with the same critical value of αapex = 0.7◦
(figure 12(b)). The slope of the curve in figure 12(b)
illustrates the instantaneous liquid filling efficiency,
which is the amount of liquid transferred per unit
time. Furtherly, the average capillary flow velocity
(vmean) of each model was calculated (figure 12(c)),
vmean of the liquid in the wedge–shaped channel
increases and then decreases with a maximum of
193 mm/s at αapex = 0.7◦.
In summary, the analysis results indicate that
a wedge–shaped channel structure facilitates liquid
transport, which corresponds to the theoretical ana-
lysis in section 2.2.1. However, there exists a crit-
ical angle beyond which liquid transport is not
promoted by increasing αapex, even probably less
than that of a channel without αapex (parallel
channel).
3.2. Effect of branch asymmetry on capillary flow
A detailed study of capillary flow in branch structures
was performed for dasy = 0 and 0.5, and a partial
view displays the branch region to present changes
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Table 5. Parameters of the hierarchical models.
Model
w1/mm
w2/mm
w3/mm
2-level hierarchical
3-level hierarchical
1
2
3
4
5
6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.3
0.6
0.6
0.3
0.3
—
—
0.6
0.3
0.3
0.15
η12
1
0.5
1
1
0.5
0.5
η23
—
—
1
0.5
1
0.5
Figure 12. Capillary flow filling in wedge–shaped models with different αapex values. (a) Capillary flow distance vs. time,
(b) liquid filling area vs. time, (c) average flow velocity of each wedge–shaped model.
in the meniscus more clearly, which is primarily the
‘phase interface transition region’ from the COMSOL
Multiphysics phase field simulation. Notably, the
pressures in figures 13 and 14 represent the relative
pressure (Pabs = P + Pref).
Capillary flow in a symmetric structure (dasy = 0)
is shown in figure 13, which can be divided into four
stages. (I) Before the meniscus reaches the branch, the
capillary pressure difference is 62.45 Pa. (II) When the
three-phase contact line reaches the lower edge of the
10
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 13. Meniscus at the symmetric branch (top) and pressure cloud diagram of the phase interface transition region (bottom).
Figure 14. Liquid meniscus shape at the asymmetric branch (top) and pressure cloud diagram (bottom).
symmetrical branch, the liquid meniscus gradually
disappears and flattens, while the capillary pressure
difference is reduced to 24.82 Pa. (III) The meniscus
gradually changes from flat to convex as the capil-
lary pressure difference gradually approaches zero,
and the barrier pressure even can resist capillary flow.
In the case of dasy = 0, the opposite pressure reaches
−16.1 Pa, which corresponds to the left peak of the
curve in figure 15. (IV) When the capillary flows
through the branch node, the meniscus returns to the
original concave shape, and the capillary pressure dif-
ference gradually returns to 62.45 Pa. This process is
consistent with the theoretical analysis described in
section 2.2.2.
The capillary flow in an asymmetric structure
(dasy = 0.5) is shown in figure 14, and the process
can be divided into eight stages. (I) The liquid ini-
tially flows in constant-width channels, which is the
same as that in a symmetric structure. (II) When the
three-phase contact line reaches the lower edge of
the left branch, the meniscus is ‘pinned’, as the capil-
lary flow has not yet arrived at the right branch. The
liquid continues moving forward on the right side,
and the capillary pressure difference decreases as the
11
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 15. Changes in the pressure difference at the phase interface transition region during capillary flow through a branch
structure.
meniscus curvature increases. (III) After the men-
iscus reaches the right branch, it becomes approx-
imately flat, and the capillary pressure difference is
around zero. (IV) The capillary flow breaks through
the ‘pinned’ action of the left branch, a portion of
the liquid along the inclined wall fills the left branch,
and the meniscus becomes convex. Therein, the max-
imum opposite pressure is 5.4 Pa, corresponding
to the rightward transverse wave of the graph for
dasy = 0.5 in figure 15. (V) After the three-phase con-
tact line crosses the left branch, the left half of the
meniscus is obviously concave, and the capillary pres-
sure difference returns to positive. (VI) The capil-
lary flow breaks through the ‘pinned’ action of the
right branch, and the superposition of the main chan-
nel and the right branch leads to an increased width
and radius of curvature of the meniscus. The capil-
lary force decreases during this process, which corres-
ponds to the transition from the first left peak to the
second left peak of the curve in figure 15. (VII) The
three-phase contact line crosses the right branch, and
the radius of curvature of the curved meniscus in the
main channel gradually decreases. (VIII) The menis-
cus returns to its initial state and the capillary force
returns to 62.45 Pa.
Capillary pressure changes at the phase interface
transition region when a meniscus passes through the
branch structure are shown in figure 15. The initial
capillary pressure difference for all values of dasy is
about 62.45 Pa, and after the liquid meniscus com-
pletely passes through the branch structure, the pres-
sure at the liquid leading edge gradually returns to
62.45 Pa.
One aspect is when the three-phase contact line
reaches the edge of the branch channel, the pres-
sure at the liquid meniscus starts to change. For
a structure with dasy = 0, the minimum capillary
pressure during liquid flow is −18.9 Pa, where the
negative sign indicates that pressure resists liquid
flow, which corresponds to the ‘capillary pinning’
in section 2.2.2. As dasy increases, there is a gradual
increase in the minimum capillary pressure during
liquid flow, corresponding to the left peak of the curve
12
in the graph (figure 15). When dasy = 1, the capillary
pressure is always greater than zero, indicating capil-
lary flow is not subject to the ‘barrier pressure’ and
that liquid transfer does not stagnate.
The second aspect to discuss is when dasy = 0.5
and 1, the two left peaks of the curve are not equal,
this suggests the that meniscus has not fully recovered
after passing the first branch and before encounter-
ing the second branch. By contrast, when dasy = 2
and 3, the capillary pressure difference minima are
nearly the same, this means whether the capillary
flow is affected by two branch ‘capillary pinnings’
simultaneously.
In summary, an asymmetric branch structure can
reduce the barrier pressure of capillary flow in the
branch, avoiding a capillary flow stop.
3.3. Effect of hierarchical features on capillary flow
The capillary flow of liquid in a 2-level hierarch-
ical channel structure is shown in figure 16(a). For
the same filling area, capillary flow fills the uniform
structure (η12 = 1) and the hierarchical structure
(η12 = 0.5) in 2536 ms and 829.5 ms respectively.
There are several ‘plateaus’ in the transport curve of
the uniform structure (η12 = 1), and the capillary fills
slowly during this phase, which corresponds to the
liquid pinning at the branch in section 2.2.2, while
the overall filling process of the hierarchical structure
(η12 = 0.5) is smooth with no apparent fluctuations.
The capillary flow of liquid in a 3-level hierarch-
ical channel structure is shown in figure 16(b), and
the filling rates for all models are shown in figure 17.
For Models 3 vs. 4 the average capillary filling rates
for η23 = 1 (w3/w2 = 1, 2◦–3◦ channels without hier-
archical features) and η23 = 0.5 (w3/w2 = 0.5, 2◦–
3◦ channels with hierarchical features) are respect-
ively 0.10728 mm2 ms−1 and 0.09756 mm2 ms−1,
differing by approximately 9%. Models 5 and 6 have
the same primary η12 = 0.5, which means that the
1◦–2◦ channels are hierarchical. In this case, the
average filling rates for the terminals η23 = 1 and
η23 = 0.5 are respectively 0.13082 mm2 ms−1 and
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Figure 16. Capillary flow results in a hierarchical channel structure. (a) 2-level hierarchical branching structure, (b) 3-level
hierarchical branching structure.
Figure 17. The average filling rate of hierarchical structures.
0.1463 mm2 ms−1, which is approximately an 11%
difference.
We have analyzed the effect of terminal η on over-
all capillary flow. The three sets of comparisons are
Models 1 vs. 2, 3 vs. 4, and 5 vs. 6:
(1) As is shown in figures 16 and 17, the aver-
age filling efficiency of Model 2 (η12 = 0.5) is
205.91% higher than that of Model 1 (η12 = 1),
and the average filling efficiency of Model 6
(η12 = 0.5 η23 = 0.5) is 11.83% higher than that
of Model 5 (η12 = 0.5 η23 = 1). The comparis-
ons of Models 1 vs. 2 and 5 vs. 6 indicate that
the capillary flow transport effect of the terminal
hierarchical structure is better than that of the
terminal uniform structure. We speculate that
there are two explications for this result. On the
one hand, in the 2-level hierarchical structure,
13
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
the decrease in the 2◦ channel width leads to
an increase in capillary pressure. On the other
hand, since the total outlet size is the same, the
number of 2◦ channels increases, and there are
more meniscuses in the capillary channel system,
which results in increased transfer efficiency.
Similarly, in the 3-level hierarchical structure,
not only is capillary conductivity improved by
the change in channel width, but the 3-level hier-
archical structure also adds one more hierarch-
ical structure for capillary filling, that is, there
are more meniscuses to provide capillary force,
which further increases capillary transmission
efficiency.
However, the average filling rate of Model 4
(η12 = 1 η23 = 0.5) is 9.6% lower than that of
Model 3 (η12 = 1 η23 = 1). The comparison of
Models 3 and 4 shows that whether the terminal
channels are hierarchical or not makes no signi-
ficant difference in capillary transport. We spec-
ulate that the capillary transport of hierarchical
channels is not only related to the terminal η,
but also affected by the relationship between the
primary η and the terminal η.
To study the above phenomena, we compare the
transmission results of two sets of models with
the same primary η but the different terminal η
(Models 3 vs. 5 and 4 vs. 6).
(2) As is shown in figures 16(b) and 17, the average
filling efficiency of Model 5 (η12 = 0.5 η23 = 1)
is 21.94% higher than that of Model 3 (η12 = 1
η23 = 1), and the average filling efficiency of
Model 6 (η12 = 0.5 η23 = 0.5) is 49.9% higher
than that of Model 4 (η12 = 0.5 η23 = 1). The
results of Models 3 vs. 5 showed that the ter-
minal hierarchical structure may not necessarily
promote capillary transport, and when combin-
ing the results of Models 4 and 6, it can be con-
cluded that the primary channel hierarchical dis-
tribution is the key to capillary flow and the hier-
archical structure of the capillary system needs
to be from the primary channel. Otherwise, the
terminal hierarchical structure may not promote
capillary transport. We deduce that there are two
factors affecting the above phenomenon: on the
one hand, only after the capillary flow is coordin-
ated in the primary hierarchical channel does the
liquid move steadily into the subsequent chan-
nels in the hierarchy, which then promotes capil-
lary transport. On the other hand, the higher-
order capillary channels in the hierarchical struc-
ture have a higher filling rate but are limited in
volume transmission due to their lower channel
widths, so the use of hierarchical feature only at
the end has little effect on the overall transport
efficiency.
In summary, combined with the theoretical ana-
lysis in section 2.2.3, it can be inferred that leaf
14
vein inspired hierarchical feature channels enable
the ingenious configuration of ‘main’ and ‘tribu-
tary’ flows, and each hierarchical channel has dif-
ferent functions according to the difference in water
conductivity, which promotes effective liquid trans-
port. Specifically, the role of the primary vein is to
provide large volumes of liquid to the periphery,
while the role of the terminal veins is to ensure
a high capacity to transport fluid. Particularly, it
should be noted that only after the flow in the
primary channel has reached coordination can the
addition of a terminal hierarchy promote capillary
transport.
4. Conclusion
This study was inspired by plant leaf veins and
proposes three novel biomimetic capillary transport
structures, which are different from previous ‘tree-
like’ networks based on Murray’s law. The consistency
between theoretical derivation and numerical analysis
reveals the efficient transmission mechanism of leaf
vein inspired structures. On the basis, we draw the fol-
lowing conclusions:
(1) The wedge–shaped channel promotes capillary
liquid transport. For larger apex angles, the effi-
ciency of the liquid transfer increases up to a
critical value. Beyond this critical value, the effi-
ciency is reduced.
(2) An asymmetric distribution of the lateral veins
on both sides of the major vein reduces the num-
ber of sharp vertices described in the Gibbs’ cri-
terion from two to one and decreases the cross-
sectional area of the sudden change in the struc-
ture. Thus, obstructions to liquid flow caused by
‘capillary pinning’ are reduced.
(3) The leaf vein is hierarchical, with different dia-
meters at each level of the reticulated struc-
ture. As channel widths affect the capillary flow
transmission efficiency, the hierarchical distri-
bution constitutes an effective configuration of
the main and tributary flows, accommodat-
ing both the capillary transmission volume and
conductivity.
These biomimetic
features provide a new
approach to improving the design of microfluidic
capillary function. However, since capillary action is
highly sensitive to size scales, there are some limit-
ations of the parameters used in this paper. Thus,
the design parameters in practice are particularly
dependent on the specific context. In the future,
a leaf vein inspired structure could replace tradi-
tional capillary structures to improve the trans-
port function of microfluidic devices, such as flat
heat-pipe heat dissipation, biochips, inkjet print-
ers, and applications of microfluidics in many other
fields.
Bioinspir. Biomim. 19 (2024) 016006
J Shen et al
Data availability statement
The data cannot be made publicly available upon
publication because no suitable repository exists for
hosting data in this field of study. The data that sup-
port the findings of this study are available upon reas-
onable request from the authors.
ORCID iDs
Jingyu Shen https://orcid.org/0009-0008-6660-
396X
Ce Guo https://orcid.org/0000-0002-0295-2836
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OPEN
Clearance of peripheral nerve
misfolded mutant protein
by infiltrated macrophages
correlates with motor neuron
disease progression
Wataru Shiraishi1,2,5, Ryo Yamasaki1,5*, Yu Hashimoto1, Senri Ko1, Yuko Kobayakawa1,
Noriko Isobe1, Takuya Matsushita1 & Jun‑ichi Kira1,3,4
Macrophages expressing C–C chemokine receptor type 2 (CCR2) infiltrate the central and peripheral
neural tissues of amyotrophic lateral sclerosis (ALS) patients. To identify the functional role of
CCR2+ macrophages in the pathomechanisms of ALS, we used an ALS animal model, mutant Cu/Zn
superoxide dismutase 1G93A (mSOD1)‑transgenic (Tg) mice. To clarify the CCR2 function in the model,
we generated SOD1G93A/CCR2Red fluorescence protein (RFP)/Wild type (WT)/CX3CR1Green fluorescence protein (GFP)/WT‑Tg
mice, which heterozygously express CCR2-RFP and CX3CR1-GFP, and SOD1G93A/CCR2RFP/RFP‑Tg mice,
which lack CCR2 protein expression and present with a CCR2‑deficient phenotype. In mSOD1‑Tg
mice, mSOD1 accumulated in the sciatic nerve earlier than in the spinal cord. Furthermore, spinal
cords of SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice showed peripheral macrophage infiltration that
emerged at the end‑stage, whereas in peripheral nerves, macrophage infiltration started from the
pre‑symptomatic stage. Before disease onset, CCR2+ macrophages harboring mSOD1 infiltrated
sciatic nerves earlier than the lumbar cord. CCR2‑deficient mSOD1‑Tg mice showed an earlier onset
and axonal derangement in the sciatic nerve than CCR2‑positive mSOD1‑Tg mice. CCR2‑deficient
mSOD1‑Tg mice showed an increase in deposited mSOD1 in the sciatic nerve compared with CCR2‑
positive mice. These findings suggest that CCR2+ and CX3CR1+ macrophages exert neuroprotective
functions in mSOD1 ALS via mSOD1 clearance from the peripheral nerves.
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of upper and
lower motor neurons. Although the majority of ALS cases are sporadic, approximately 10% are inherited1. Spo-
radic and familial ALS are clinically and pathologically similar. Approximately 20% of familial cases are linked
to autosomal dominant mutations in the Cu/Zn superoxide dismutase 1 (SOD1) gene2. Hallmark pathological
features in sporadic and familial ALS include the presence of axonal spheroids and perikaryal accumulation of
inclusion bodies comprising neuronal intermediate filament proteins, such as neurofilaments and peripherin3,4.
Although the exact mechanism of ALS remains elusive, protein misfolding and aggregation have been impli-
cated as contributing factors to motor neuron death5. This abnormal protein aggregation is thought to trigger
non-cell autonomous neuronal cell death via glia-mediated mechanisms6. In ALS, activated microglia, mac-
rophages, and astrocytes may be neuroprotective in the early stage but become pro-inflammatory and neurotoxic
in the later stage when damage-associated molecular patterns are released from injured motoneurons with
accumulated misfolded proteins, and they promote the pro-inflammatory activation of glial cells7,8. We and
others have reported a variety of immune abnormalities in ALS, including increased cerebrospinal fluid (CSF)
1Department of Neurology, Neurological Institute, Graduate School of Medical Sciences, Kyushu University,
3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. 2Department of Neurology, Kokura Memorial Hospital,
Fukuoka 802-8555, Japan. 3Translational Neuroscience Center, Graduate School of Medicine, and School of
Pharmacy At Fukuoka, International University of Health and Welfare, 137-1 Enokizu, Ookawa, Fukuoka 831-8501,
Japan. 4Department of Neurology, Brain and Nerve Center, Fukuoka Central Hospital, International University
of Health and Welfare, 2-6-11 Yakuin, Chuo-ku, Fukuoka 810-0022, Japan. 5These authors contributed equally:
Wataru Shiraishi and Ryo Yamasaki. *email: [email protected]
Scientific Reports | (2021) 11:16438
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Vol.:(0123456789)www.nature.com/scientificreportspro-inflammatory cytokines/chemokines, such as interleukin (IL)-1β, IL-12, IL-17, tumor necrosis factor-α,
interferon-γ, C–C motif chemokine ligand (CCL) 2, CCL4, CCL11, C-X-C motif chemokine ligand (CXCL) 8,
and CXCL109, increased serum IL-6, IL-17, and CCL210–12, and increased circulating IL-13-producing T cells13.
However, it remains unclear whether these immune abnormalities cause disease or are a consequence of disease
and whether they are neurotoxic or neuroprotective according to the disease stage.
Monocyte-macrophage lineage cells are heterogenous and express distinct chemokine receptors14. C–C
chemokine receptor 2 (CCR2) is expressed by peripheral monocytes/macrophages, in addition to T cells, baso-
phils, and immature dendritic cells15, whereas resident macrophages, such as microglia in the central nervous sys-
tem (CNS), Kupffer cells in the liver, and Langerhans cells in the skin, express high levels of C-X3-C chemokine
receptor 1 (CX3CR1)16. Notably, a CCL2/CCR2-dependent immunological pathway has been implicated in ALS.
Patients had an increased level of CCL2 in the CSF and serum about 1 year before the onset of ALS9,17. Further-
more, spinal cord tissues from mutant SOD1 (mSOD1) transgenic ALS model mice also showed an increased
level of CCL2 mRNA18,19. Conversely, CCR2 expression in the peripheral blood monocytes of ALS patients
was significantly decreased compared with healthy controls20,21. These results collectively suggest the initial
recruitment of peripheral monocytes/macrophages expressing CCR2 to neural tissues. However, it remains to be
established whether these monocytes/macrophages exert protective or deleterious effects in ALS pathogenesis.
To address this issue in the present study, we aimed to clarify the functions of CCR2-bearing monocytes/mac-
rophages recruited from the peripheral blood to the neural tissues in ALS. For this purpose, genetically labeled
transgenic “Red-Green” SOD1G93A mice, which express CCR2-red fluorescence protein (RFP) and CX3CR1-green
fluorescence protein (GFP) heterozygously, were used to characterize monocyte lineage cell dynamics. Further-
more, the role of CCR2 in ALS was explored using SOD1G93A/CCR2RFP/RFP mice, which possess a CCR2-deficient
phenotype.
Results
mSOD1 accumulates in peripheral nerves earlier than in the spinal cord. mSOD1 immunostain-
ing of SOD1G93A mice indicated the deposition of mSOD1 was absent in the spinal cord at 4 weeks of age and
mainly present in the anterior horns at 12 weeks of age (onset stage) (Fig. 1a, b). mSOD1 protein was accumu-
lated and spread along the pyramidal tracts and eventually extended into whole spinal cord cross-sectional areas
at 20 weeks of age (moribund period). Conversely, the accumulation of mSOD1 in peripheral nervous system
(PNS) tissues was detected in the spinal roots and dorsal root ganglia (DRG) as early as 4 weeks of age and was
increased abundantly in the ventral roots compared with the dorsal roots. Double immunostaining for Iba1
and mSOD1 demonstrated Iba1+ foamy macrophages in the sciatic nerve were filled with mSOD1 protein at
20 weeks of age (Fig. 1c).
Microglial responses commence in the spinal anterior horns at clinical disease onset. Micro-
scopic analysis of CX3CR1 and CCR2 in the spinal cord from SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice
revealed that the accumulation of CX3CR1+ microglia occurred mainly in the lumbar anterior horns at 12 weeks
of age (at the time of clinical disease onset) without the infiltration of CCR2+ peripheral immunocytes (Fig. 2a).
At 16 weeks of age (at the progressive stage), microglial activation became more robust, particularly along the
intramedullary ventral roots, whereas CCR2+ cells were rarely observed. At 20 weeks of age (the moribund
stage), CX3CR1+ activated microglia/macrophages without a foamy shape were present throughout the entire
spinal cord section, whereas fewer CCR2+ cells had infiltrated the lesion (Fig. 2a).
CX3CR1+ and CCR2+ cells infiltrate peripheral nerves earlier than in the spinal cord. Because
a previous study reported the earlier deposition of mSOD1 protein in the lumbar spinal cord, sciatic nerves,
and gastrocnemius muscles compared with the cervical spinal cord22, we immunohistochemically compared
monocyte/macrophage infiltration into the sciatic nerves of SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT and CCR-
2RFP/WT/CX3CR1GFP/WT mice (non-mSOD1 type littermate). In the sciatic nerve of CCR2RFP/WT/CX3CR1GFP/WT
mice, CX3CR1+ resident macrophages were evenly scattered, whereas CCR2+ immunocytes were rarely seen at
16 weeks of age (Fig. 2b). We did not observe CX3CR1+CCR2+ cells in the sciatic nerves of non-mSOD1 type
mice. Conversely, the sciatic nerves of SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice showed a marked increase of
CX3CR1+CCR2− (green) cells, but not CX3CR1−CCR2+ (red) or CX3CR1+CCR2+ (yellow) cells at 4 weeks of
age. At 8 weeks of age (pre-onset stage), CX3CR1+CCR2+ (yellow) cells appeared in the sciatic nerve, and succes-
sively higher numbers appeared at 12 to 16 weeks of age (onset to progressive phases) (Fig. 2b, c). The increase
ratio was more prominent for green cells compared with yellow cells. Many green cells had a foamy appearance
(Fig. 2b inset).
CCR2 deficiency aggravates the clinical course of SOD1G93A ALS mice. To clarify the effects of
CCR2 deletion upon mSOD1-ALS, SOD1G93A/CCR2RFP/WT mice (CCR2-positive SOD1G93A mice, n = 24) and
SOD1G93A/CCR2RFP/RFP mice (CCR2-deficient SOD1G93A mice, n = 18) were compared by measuring body weights,
rotarod test, grip strength, and ALS-Therapy Development Institute (TDI) scores. CCR2-deficient SOD1G93A
mice showed a 6-day acceleration in progression to the moribund stage (164.0 ± 1.48 days vs. 170.4 ± 2.06 days,
p = 0.0047; Fig. 3a) compared with CCR2-positive SOD1G93A mice. Clinical signs were also significantly exacer-
bated in CCR2-deficient mice as determined by the rotarod test (p < 0.05; 12 weeks to 17 weeks; Fig. 3b), grip
strength (p < 0.05; 13 weeks to 15 weeks; Fig. 3c), and ALS-TDI scores (area under the curve from 7 to 19 weeks
of age: p = 0.031; Fig. 3d, e) compared with CCR2-positive SOD1G93A littermates. The peak body weight was
reached earlier in CCR2-deficient SOD1G93A mice than in CCR2-positive SOD1G93A littermates, although the dif-
ference was not statistically significant (13.83 ± 0.34 weeks vs. 14.83 ± 0.35 weeks, respectively, p = 0.188; Fig. 3f).
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Vol:.(1234567890)www.nature.com/scientificreports/Figure 1. Immunostaining for mSOD1 and Iba1 in neural tissues. (a) Immunostaining for human mSOD1
in the lumbar spinal cord, dorsal root ganglia (DRG), ventral root (VR), and dorsal root (DR) from
CCR2RFP/WT/CX3CR1GFP/WT (non-mSOD1 type) and SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT (mSOD1-Tg)
mice. Accumulation of mSOD1 protein in the DRG, VR, and DR prior to accumulation in the lumbar cord was
apparent at 4 weeks of age. In the spinal cord, mSOD1 protein started to accumulate along the intramedullary
lower motoneuron axonal tracts (arrow) at 12 weeks of age. (b) Schemas depict mSOD1 protein accumulation
over the disease course in the mSOD1-Tg ALS mouse model. (c) Double immunostaining for Iba1 (green) and
mSOD1 (red) in the sciatic nerves of a 20-week-old mSOD1-Tg mouse. All Iba1+ macrophages (arrowhead)
harbored mSOD1 protein. Scale bars: (a) 200 μm (lumbar spinal cord), and 100 μm (DRG, VR, and DR): (c)
10 μm.
CCR2 deficiency accelerates mSOD1 accumulation in peripheral nerves.
Immunohistochemical
analysis with anti-human specific mSOD1 antibody revealed a significant increase in mSOD1 protein aggrega-
tion in the sciatic nerves of CCR2-deficient SOD1G93A mice compared with CCR2-positive littermates (p = 0.032;
Fig. 4a, b) at 12 weeks of age (onset period). The accumulation of mSOD1 protein was also confirmed by west-
ern blotting (Supplementary Fig. 1). In the sciatic nerves, mSOD1 protein immunostaining was surrounded by
the myelin sheath in CCR2-positive SOD1G93A mice. Conversely, CCR2-deficient SOD1G93A mice demonstrated
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Figure 2. CX3CR1 and CCR2 expression in the lumbar spinal cord and sciatic nerve. (a) Low magnification
fluorescence images of CCR2RFP/WT/CX3CR1GFP/WT (non-mSOD1 type) and SOD1G93A/CCR2RFP/
WT/CX3CR1GFP/WT (mSOD1-Tg) mouse spinal cord. At 12 weeks of age, CX3CR1+ (green) but not CCR2+ (red)
cells were sparsely visible only in the spinal anterior horns (arrowhead). CX3CR1+ cells markedly increased as
the disease progressed from 16 to 20 weeks of age, whereas CCR2+ peripheral immunocytes rarely infiltrated
at 20 weeks of age. Infiltration of CX3CR1+ cells along the intramedullary lower motoneuron axonal tracts was
prominent at 16 weeks of age (arrow). (b) Low magnification fluorescence images of CCR2RFP/WT/CX3CR1GFP/
WT (non-mSOD1 type) and SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT (mSOD1-Tg) mouse sciatic nerves. In the
sciatic nerves of a mSOD1-Tg mouse, CX3CR1+ cell infiltrations were visible as early as 4 weeks of age and
CX3CR1+CCR2+ cells appeared at 8 weeks of age (pre-symptomatic stage). These infiltrated cells showed a
foamy appearance (inset in the 8 weeks merged image). Such inflammatory infiltrates were not seen in non-
mSOD1 type mice. (c) The quantitative analysis of CCR2+ CX3CR1− (red), CCR2−CX3CR1+ (green), and
CX3CR1+CCR2+ (yellow) areas (%) in sciatic nerves. In SOD1G93A mice, numbers of all types of macrophages
(CCR2−CX3CR1+ green, and CCR2+CX3CR1+ yellow macrophages) increased as the disease progressed,
whereas CCR2+CX3CR1− red immune cells did not. Scale bars: (a) 500 μm; (b) 50 μm and 10 μm (inset).
n.s. = not significant. *p < 0.05; **p < 0.001; ***p < 0.0001.
immunostaining of mSOD1 protein aggregation inside and outside the myelin sheath (Fig. 4c), which suggests
that excessive mSOD1 protein accumulation had overflowed into the myelin sheath (arrows in Fig. 4c). Con-
versely, there were no significant differences in mSOD1 protein aggregation in the lumbar spinal cord of CCR2-
deficient SOD1G93A mice and CCR2-positive littermates (p = 0.77; Fig. 4d, e) at 12 weeks of age (onset period).
CCR2 deficiency worsens anterior horn cell loss and axonal deformation. The loss of anterior
horn cells as evaluated by NeuN immunostaining was facilitated in CCR2-deficient SOD1G93A mice compared
with CCR2-positive SOD1G93A mice (Fig. 5a). Significantly fewer anterior horn cell numbers were observed
in CCR2-deficient SOD1G93A mice than in CCR2-positive littermates at 12 weeks of age (p = 0.033) (Fig. 5b).
Moreover, SMI 31/32 immunostaining of peripheral nerve transverse sections revealed that the axons of CCR2-
deficient SOD1G93A mice, but not CCR2-positive SOD1G93A mice, already showed crescent-like deformation at
8 weeks of age (pre-onset period) (Fig. 5c). The aspect ratio (calculated as the ratio between major and minor
axis lengths) was significantly higher in CCR2-deficient SOD1G93A mice than in CCR2-positive SOD1G93A
mice (aspect ratio: 1.779 vs. 1.683, p = 4.11 × 10−16; Fig. 5d), which indicates accelerated axonal deformation in
CCR2-deficient SOD1G93A mouse peripheral nerves. Electron microscopy also confirmed that axons were more
deranged in CCR2-deficient SOD1G93A mice compared with CCR2-positive SOD1G93A mice, whereas foamy
macrophages filled with myelin and other cell debris were more frequently observed in CCR2-positive SOD1G93A
mice compared with CCR2-deficient SOD1G93A mice (Fig. 6).
CCR2 deficiency diminishes anti‑inflammatory M2 macrophage infiltration into the peripheral
nerves. As shown in Fig. 7a, CCR2+ cell infiltration into the sciatic nerve was markedly diminished in CCR2-
deficient SOD1G93A mice compared with CCR2-positive littermates at 12 (onset stage) and 20 weeks of age (mori-
bund stage) (Fig. 7a). As a result, there were significantly fewer CCR2-RFP positive cells in the sciatic nerves of
CCR2-deficient mice compared with CCR2-positive SOD1G93A mice at 12 weeks of age (p = 0.021; Fig. 7b). In
addition, Iba1 immunostaining also indicated a significant decrease in Iba1+ macrophages in the sciatic nerves
of CCR2-deficient SOD1G93A mice compared with CCR2-positive SOD1G93A mice at 12 weeks of age (p = 0.047;
Fig. 7c, d). To further characterize the phenotype of infiltrating macrophages in the peripheral nerves of mSOD1
ALS mice, immunostaining for arginase-1 (Arg-1), a marker of anti-inflammatory M2 macrophages, and induc-
ible nitric oxide synthase (iNOS), a marker of pro-inflammatory M1 macrophages, was performed. In the sciatic
nerves of both mouse genotypes, infiltrated foamy macrophages were immunopositive for Arg-1 but negative
for iNOS (Fig. 7e). CCR2 positive foamy-macrophages that were immunopositive for CD68 contained Arg-1
(Fig. 7f; Supplementary Fig. 2). Regarding T cell infiltration into the peripheral nerves, histological analysis indi-
cated CD3+ T cells were rarely detected in the sciatic nerves of CCR2+ or CCR2− SOD1G93A mice and that there
was no significant difference in cell numbers between these mouse strains (Supplementary Fig. 3).
Differentiated macrophages are decreased in the sciatic nerves of CCR2‑deficient mice. Flow
cytometric analysis of cells isolated from sciatic nerves revealed comparable leukocyte ratios between CCR2+ and
CCR2− mSOD1-Tg mice, except for a decrease in CD11b+/CD11c+ macrophages in CCR2-deficient mice (CD11b+/
CD11c+ cell population ratio in CD45+ gate (%) (CCR2+mSOD1 vs. CCR2-mSOD1, mean ± SEM = 25.40 ± 2.577
vs. 12.67 ± 1.619, p = 0.0111). There were no significant differences in the CD45+CD3+ T cell population ratio in
the sciatic nerves of CCR2+ and CCR2− mSOD1-Tg mice (Supplementary Fig. 4).
CCL2 production by Schwann cells is unchanged by CCR2 deficiency. Double immunostaining
with anti-Schwann cell antibody (human peripheral nerve extract antigen, mouse IgM antibody)23 and anti-
CCL2 antibody demonstrated that CCL2 was mainly co-localized with Schwann cells in the sciatic nerves of
SOD1G93A mice at 16 weeks, regardless of the presence or absence of CCR2 (Fig. 8a). This indicated that Schwann
cells are the primary source of CCL2 in peripheral nerves. The numbers of CCL2+ cells in the sciatic nerve were
not significantly different between CCR2-deficient and CCR2-positive SOD1G93A mice (p = 0.92; Fig. 8b). The
anti-Schwann cell antibody we used in this study was a mouse IgM antibody that uses human peripheral nerve
extract as its antigen.
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Vol.:(0123456789)www.nature.com/scientificreports/Figure 3. Clinical manifestations of CCR2-positive and CCR2-deficient SOD1G93A mice. (a) Median survival
time, (b) rotarod test, (c) grip strength, (d) ALS-TDI score, and (e) area under the curve (AUC) of ALS-TDI
scores measured from 7 to 17 weeks of age in (d). (f) Mean age of disease onset was defined as when mice
reached peak body weight (CCR2-positive SOD1G93A mice, n = 24, and CCR2-deficient SOD1G93A mice, n = 18).
The log-rank test was used to compare median survival time and onset time by body weight. Student’s t test
was used for statistical comparisons of rotarod test and grip strength. The Mann–Whitney U-test was used for
statistical comparisons of the AUC of the ALS-TDI scores. *p < 0.05; **p < 0.001.
Phagocytic activity of isolated macrophages is unaltered by CCR2 deficiency. Finally, we meas-
ured the phagocytic activity of macrophages isolated from CCR2RFP/RFP SOD1G93A mice, CCR2RFP/WT SOD1G93A
mice, CCR2RFP/RFP mice, and CCR2RFP/WT mice. There was no difference in phagocytic activity among these mice
in vitro (Supplementary Fig. 5), which suggests that a deficiency of CCR2 or the presence of mSOD1 does not
influence the phagocytic activity of isolated macrophages.
Discussion
The role of peripheral blood-borne macrophages bearing CCR2 recruited into the neural tissues has long been
a critical question in ALS. To address this issue, we studied mSOD1 ALS mice with genetically labeled mac-
rophages or with CCR2 deficiency. We found that before the onset of clinical symptoms, CCR2+ macrophages
that phagocytized mSOD1 protein had already infiltrated the peripheral nerves much earlier than into the spinal
cord. Furthermore, CCR2 ablation clinically accelerated the disease progression and worsened the pathology,
as determined by NeuN+ neuronal loss in the spinal anterior horns and axonal derangement in the peripheral
nerves. CCR2 ablation also markedly increased the accumulation of mSOD1 protein in the peripheral nerves
and, to a lesser extent, in the spinal cord compared with CCR2-positive mice. Flow cytometric analysis revealed
a comparable number of CD3+ T cells were detected in the sciatic nerves of 8-week-old mSOD1-Tg mice with
and without CCR2. However, we also detected the suppressed infiltration of the CD11b+/CD11c+ cell popula-
tion, which indicates that activated macrophages derived from monocytes24 were decreased in the sciatic nerves
of CCR2-deficient mice. A decreased infiltration of CCR2+ macrophages, which phagocytized mSOD1 protein
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Vol:.(1234567890)www.nature.com/scientificreports/Figure 4. Aggravation of mSOD1 accumulation in the nerve by CCR2 ablation. (a) Immunostaining for mSOD1 in the sciatic nerves
of CCR2-deficient SOD1G93A mice and CCR2-positive littermates. (b) Accumulation of mSOD1 protein was significantly increased in
CCR2-deficient SOD1G93A mice than in CCR2-positive SOD1G93A mice at 12 weeks of age (p = 0.032, n = 5). (c) Double immunostaining
for mSOD1 and Schwann cells indicated a greater accumulation of mSOD1 protein in the sciatic nerves of CCR2-deficient SOD1G93A
mice showing an overflow of mSOD1 protein from Schwann cells and in the myelin sheath (arrows) compared with CCR2-positive
SOD1G93A mice at 12 weeks of age. (d) Immunostaining for mSOD1 protein in the lumbar spinal cord of CCR2-deficient SOD1G93A
mice and CCR2-positive littermates at 12 weeks of age. (e) There was no difference in the mSOD1+ area (%) between the two mouse
genotypes at 12 weeks of age (p = 0.85, n = 6). The unpaired t test was used for statistical comparisons. Scale bars: (a) 20 μm; (c) 10 μm;
(d) 100 μm. *p < 0.05. n.s. = not significant.
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Vol.:(0123456789)www.nature.com/scientificreports/Figure 5. Facilitation of neuro-axonal degeneration by CCR2 ablation. (a) NeuN immunostaining revealed
increased anterior horn neuronal cell loss in CCR2-deficient SOD1G93A mice compared with CCR2-positive
SOD1G93A mice at 12 and 20 weeks of age. Inset areas in the low magnified images are enlarged under each
figure. (b) CCR2-deficient SOD1G93A mice showed a significant decrease in NeuN+ cell numbers compared with
CCR2-positive SOD1G93A mice at 12 weeks of age (p = 0.033, n = 12). (c) SMI 31/32 immunostaining of sciatic
nerve transverse sections indicated axons in CCR2-deficient SOD1G93A mice had crescent-shaped deformation
compared with CCR2-positive SOD1G93A littermates at 8 weeks of age. (d) The aspect ratio (the ratio of the
major to the minor axis length) of CCR2-deficient SOD1G93A mouse axons was significantly larger than that of
CCR2-positive SOD1G93A mouse axons at 8 weeks of age (p < 0.001, n = 5). Scale bars: (a) 100 μm and 50 μm; c,
20 μm. The horizontal line represents the mean value in (b, d). *p < 0.05; **p < 0.001.
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Vol:.(1234567890)www.nature.com/scientificreports/Figure 6. Electron microscopic findings of the sciatic nerve. (a) The longitudinal section of the sciatic
nerve from a CCR2-positive SOD1G93A mouse at 12 weeks of age shows preserved axonal structure and some
derangement of the Schmidt-Lanterman incisures (arrow). (b) Cross-section of the sciatic nerve from a
CCR2-positive SOD1G93A mouse at 20 weeks of age with relatively preserved axons and some vesiculation of the
myelin sheath. (c) Foamy macrophages containing cell and myelin debris are visible adjacent to the axons in the
longitudinal section of the sciatic nerve from a CCR2-positive SOD1G93A mouse at 12 weeks of age (arrowhead).
(d) Longitudinal section of the sciatic nerve from a CCR2-deficient SOD1G93A mouse at 12 weeks of age with
disruption of intra-axonal structures. (e) Cross-section of the sciatic nerve from a CCR2-deficient SOD1G93A
mouse at 20 weeks of age with many disrupted, shrunken axons and marked vesiculation of the myelin sheath.
Scale bar: 5 μm.
and expressed Arg-1, an M2 marker, but not iNOS, an M1 marker, in the peripheral nerves was also observed.
These findings suggest that CCR2+ macrophages recruited into the peripheral nerves from the blood exert
neuroprotective functions on the lower motor neurons in mSOD1 ALS and that the clearance of abnormal
mSOD1 protein from peripheral nerves by these cells is a hitherto underestimated host protective mechanism
(Supplementary Fig. 6).
The SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice started to deteriorate at 12 weeks of age and died
at 21 weeks of age, which is similar to the reported clinical course of mSOD1G93A ALS mice22. In the
SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice, we observed that mSOD1 accumulated in the peripheral nerves
much earlier (4 weeks of age) than in the spinal cord (12 weeks of age), which is in accord with a previous report
describing the earlier accumulation of mSOD1 protein in the sciatic nerve compared with the lumbar spinal
cord (2.6-fold increase in the sciatic nerve vs. 1.8-fold increase in the lumbar cord at 30 days of age compared
with at birth, and remaining consistently higher in the sciatic nerve than in the lumbar cord until 120 days of
age)22. These observations support distal axonopathy as a primary mechanism of the lower motor neuron death in
mSOD1 ALS6,25. We also confirmed the accumulation of mSOD1 in the DRG and, to a lesser extent, in the dorsal
roots as previously reported, which explains the sensory system involvement observed in mSOD1 ALS patients
and model animals26. Because DRGs are bipolar cells, we hypothesized that mutant protein transported by an
afferent mechanism is not cleared in CNS regions where there is no or little peripheral macrophage infiltration,
unlike the sciatic nerve, which harbors high numbers of peripheral macrophages. It was suggested that there
may be differences in the characteristics of axonal transport between motor and sensory neurons in the SOD1
mouse model related to differences in dynein and dynactin functions27.
In line with the earlier accumulation of mSOD1 protein in the peripheral nerves, we found increased
CX3CR1+ macrophages infiltration into the sciatic nerve as early as 4 weeks of age and the subsequent infiltration
of CX3CR1+CCR2+ macrophages at 8 weeks of age. CX3CR1+ macrophages, which were observed at 4 weeks of
age, are considered resident macrophages, whereas CX3CR1+CCR2+ macrophages are thought to be peripheral
blood-borne macrophages. CCR2 single positive cells are considered peripheral blood-derived immune cells,
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Figure 7. Inhibition of macrophage infiltration into the sciatic nerve by CCR2 ablation. (a) Fluorescent
microscopic analysis of unstained sciatic nerve tissues of CCR2-positive non-mSOD1 type mice, CCR2-positive
SOD1G93A mice, and CCR2-deficient SOD1G93A mice. The influx of CCR2-RFP+ cells into the sciatic nerve was
markedly diminished in CCR2-deficient SOD1G93A mice than in CCR2-positive littermates. (b) The number of
CCR2-RFP+ cells was significantly reduced in CCR2-deficient SOD1G93A mice compared with CCR2-positive
littermates (p = 0.021, n = 9). (c) Immunostaining for Iba1 in sciatic nerves shows a marked reduction of CCR2+
cell infiltrates in CCR2-deficient SOD1G93A mice compared with CCR2-positive littermates. (d) The number
of Iba-1+ cells was significantly reduced in CCR2-deficient SOD1G93A mice compared with CCR2-positive
littermates (p = 0.047, n = 6). (e) Immunostaining for arginase-1 (Arg-1), an M2 marker, and inducible nitric
oxide synthase (iNOS), an M1 marker, of the sciatic nerve, demonstrated infiltrated macrophages were Arg-1+
but iNOS-, which suggests an M2-deviated phenotype. Inset in (e) indicates the foamy appearance of Arg-1+
cells. (f) Foamy macrophages with Arg-1 are CCR2-positive. Scale bars: (a) 100 μm and 20 μm (inset); (c)
50 μm; (e) 50 μm; (f) 20 μm. The horizontal line in (b, d) represents the mean value. The unpaired t test was
used for statistical comparisons. *p < 0.05.
other than monocytes/macrophages. We also found a rapid increase in CX3CR1+CCR2− green cell numbers
in peripheral nerves in the progressive phase of the disease. Also, the appearance of green cells was different
between 4 weeks (thin shape) and 8 weeks (foamy shape). Figure 2c shows an increase of CX3CR1+CCR2+ yellow
cells. RFP expression was reduced after monocytes infiltrated tissues because its half-life is up to 4.6 days28. As
shown in Supplementary Fig. 7, peripheral yellow monocytes infiltrate the sciatic nerve and then differentiate
into macrophages, which lose their red color, which results in green foamy cells. The increase of yellow cells at
12 and 16 weeks indicates the acceleration of monocyte infiltration into lesions. We assume that the green foamy
cells were initially infiltrated as blood-borne monocytes with yellow color, but soon after the CCR2 expression
was down-regulated along with RFP, which resulted in green foamy cells. These findings are in line with previ-
ous reports that CCR2 on activated monocytes is rapidly internalized, and that the synthesis of CCR2 is down-
regulated15. Our results are compatible with previous results showing CD68+ macrophages infiltrated the ventral
roots of mSOD1G93A mice at 60 days of age22. Although it was unclear whether these cells were protective or
worsened disease progression, we hypothesize these macrophages scavenge mSOD1 protein to protect peripheral
nerves. This notion is supported by the observations that these macrophages were full of mSOD1 protein and
that a deficiency of these cells by CCR2 ablation markedly facilitated mSOD1 accumulation in the nerves and
promoted the death of the lower motor neurons. Flow cytometric analysis revealed a decrease of the CD11b+/
CD11c+ cell population in CCR2 deficient mice. Because CCL2 induces the chemotaxis of CCR2 positive cells
and induces the differentiation of monocytes, this disparity in the CD11b+/CD11c+ cell ratio might result from
the lack of CCL2 signaling in CCR2 deficient monocytes. Activated macrophages are subcategorized into several
phenotypes, including M1 pro-inflammatory and M2 anti-inflammatory. The direction of activation induced
by CCL2 depends on the inflammatory circumstances29,30. We confirmed that the foamy macrophages were all
Arg-1 positive M2 phenotype, which indicated that the lack of CCL2 signaling in CCR2RFP/RFP SOD1G93A mice
decreased the infiltration of CCR2-positive cells into the sciatic nerve and inhibited macrophage differentiation
into M2 macrophages.
We confirmed that CCL2 production by Schwann cells, which are the primary source of CCL2 in nerves31,32,
was unaffected in CCR2RFP/RFP SOD1G93A mice. Thus, our findings are in accord with previous reports showing that
CCL2 expression in the sciatic nerve was not altered by CCR2 gene deletion33 and that in peripheral nerve injury,
CCR2-deficient mice had decreased macrophage accumulation in the sciatic nerve and DRG34,35. Whether CCR2
deficiency reduces phagocytic capacity is controversial36,37; however, we confirmed that the phagocytic activity of
the isolated macrophages was unaffected by CCR2 ablation, at least in SOD1G93A/CCR2RFP/RFP mice. Collectively,
these findings indicate that the impaired migratory activity and the inhibition of M2-directed differentiation of
macrophages related to the lack of CCL2-CCR2 signaling in the inflammatory milieu where abnormal proteins
are accumulated contributed to the insufficient clearance of misfolded mSOD1 protein in peripheral nerves,
which results in the accelerated loss of anterior horn cells and exacerbated disease in CCR2-deficient SOD1G93A
mice. In other neurodegenerative diseases, CCR2 deficiency was also reported to accelerate disease38,39. Notably,
CCR2 deficiency particularly aggravated amyloid β clearance in an Alzheimer’s disease animal model, thereby
inducing accelerated deterioration38,39. Taken together, these findings suggest CCR2+ peripheral blood-borne
macrophages clear abnormal proteins, at least in the early stage of the disease, contributing to disease protection
in neurodegenerative disorders, such as ALS, caused by abnormal protein accumulation.
CCR2 is abundantly expressed in M1 pro-inflammatory macrophages. Thus, CCR2 ablation is likely to inhibit
M1 macrophage mobilization from peripheral blood to nerves. Given the pro-inflammatory properties of M1
macrophages, the neuroprotective action of peripheral blood-borne CCR2+ M1 macrophages in mSOD1 ALS is
unexpected. However, importantly, these CCR2+ macrophages co-expressed CX3CR1 in the peripheral nerves
of mSOD1 ALS mice, which suggests an intermediate phenotype, transitioning from pro-inflammatory M1 to
anti-inflammatory M2 macrophages after invasion into the peripheral nerve. Macrophages were reported to
switch phenotypes according to their microenvironment7,40. In peripheral nerve and spinal cord injury models,
a transition from M1 to M2 phenotype was reported, and such intermediate macrophages had anti-inflammatory
and neuroprotective properties41. Therefore, CCR2+ peripheral blood-borne macrophages that phagocytose
mSOD1 may also be neuroprotective for peripheral nerves, thereby extending lower motor neuron survival time
in mSOD1 ALS mice. There are several possibilities why the pathologic process still progresses even in the pres-
ence of macrophage clearance of misfolded mSOD1. Upper motor neurons are behind the blood–brain barrier,
which may disturb macrophage clearance mechanisms in the early stage. Our results showed that CCR2+ cell
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Vol.:(0123456789)www.nature.com/scientificreports/Figure 8. CCL2 production by Schwann cells. (a) Double immunostaining of CCR2-deficient and CCR2-
positive SOD1G93A mouse sciatic nerves at 16 weeks with anti-Schwann cell and anti-CCL2 antibodies. The
merged images of both antibodies indicate the production of CCL2 by Schwann cells in both mouse genotypes.
Scale bar: 20 μm. (b) The CCL2+ area (%) was not significantly different between CCR2-deficient and CCR2-
positive SOD1G93A mice. The horizontal line represents the mean value. The unpaired t test was used for
statistical comparisons (p = 0.92, n = 9). n.s. = not significant.
infiltration into the spinal cord was only observed in the end-stage. Furthermore, there may be other neuropathic
pathways in which the macrophage clearance mechanism does not work. The clinical course of CCR2+ mSOD1
mice eventually catches up at later periods. We hypothesized that this was related to the accumulation of mSOD1
protein in upper motor neurons because macrophage infiltration is blocked by the blood–brain barrier, and thus
mSOD1 protein in the upper motor neuron cannot be removed.
Our study had several limitations. First, because of the small amount of CCR2-single positive red cells (T, B,
and dendritic cells) in the sciatic nerve, we did not examine CCR2+ leukocyte involvement. It was reported that
SOD1G93APU.−/− mice lacking CD3+ T cells had shorter life spans after transplantation with CCR2-deficient bone
marrow compared with wild-type bone marrow transplantation42. CCR2+ T cells were thought to facilitate glial
neuroprotection in this model. In our model, a detailed time-course study of T cell infiltration into the spinal cord
and peripheral nerves is required in the future. Second, the characterization of infiltrated CCR2+ macrophages
and their transition to the M2 phenotype was performed by immunohistochemistry because relatively small
numbers of macrophages were present in the nerves. Future studies should characterize macrophages isolated
from peripheral nerves by microarray or single-cell RNA sequencing.
In summary, our study revealed an under-recognized mechanism of abnormal protein clearance by CCR2+
macrophages from the peripheral nerves of mSOD1 ALS mice, which is beneficial to the host. Because lower
motor neuron axons are present in peripheral nerves, which are more accessible to peripheral blood macrophages
than CNS tissues that are tightly surrounded by the blood–brain barrier, peripheral nerves might be a novel thera-
peutic target for the cell therapy of ALS by removing abnormal proteins and delivering neuroprotective factors.
Materials and methods
Mice and ethical statement. Transgenic mice for the human SOD1G93A gene [B6SJL-TgN (SOD1*G93A)
1Gur/J; Stock Number: 002297] were purchased from the Jackson Laboratory (Bar Harbor, ME, USA). They
were crossed with C57BL/6J mice (Clea Japan, Tokyo, Japan) to maintain the strains, and hemizygous ani-
mals were used for the experiments. Transgenic mice harboring the human SOD1G93A gene were backcrossed
to C57BL/6J mice for more than 15 generations. CCR2RFP mice [B6.129 (Cg)-Ccr2tm2.1Ifc/J; Stock Number:
017586], and CX3CR1GFP mice [B6.129P (Cg)-Ptprca Cx3cr1tm1Litt/LittJ; Stock Number: 008451] were also
purchased from the Jackson Laboratory. Heterozygous SOD1G93A mice and CCR2RFP/RFP mice were crossed to
obtain SOD1G93A/CCR2RFP/WT mice, and then SOD1G93A/CCR2RFP/WT mice and CCR2RFP/RFP mice were crossed to
obtain SOD1G93A/CCR2RFP/WT mice and SOD1G93A/CCR2RFP/RFP mice. In CCR2RFP/RFP homozygotes, CCR2 alleles
are inactivated (CCR2-deficient phenotype) whereas CCR2-RFP labeling is preserved. In addition, to create
SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice, SOD1G93A/CCR2RFP/WT mice and CX3CR1GFP/GFP mice were crossed
to obtain SOD1G93A/CCR2RFP/WT/CX3CR1GFP/WT mice. This dual heterozygous mouse expressing both reporter
proteins and their receptors at functional levels has been widely used to differentiate resident microglia from
blood-derived monocytes/macrophages in various neurodegenerative models15,43. Non-SOD1G93A-transgenic
littermates were used as a non-ALS model phenotype. All animals were maintained in an air-conditioned, spe-
cific-pathogen-free room with a time-controlled lighting system. The handling and sacrifice of all animals were
conducted according to the guidelines for the proper conduct of animal experiments published by the Science
Council of Japan and the ARRIVE (Animal Research: Reporting of In Vivo Experiments) guidelines for animal
research. Ethical approval for the study was granted by the Animal Care and Use Committee of Kyushu Univer-
sity (#A30-051).
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Vol:.(1234567890)www.nature.com/scientificreports/Behavioral study. Body weights, performances in the rotarod test (ENV-576M; Neuroscience, Tokyo,
Japan), grip strength (MK-380M, Muromachi-Kikai, Tokyo, Japan), and ALS-TDI neurological scoring44 were
assessed once a week. For the assessment of motor function by rotarod, mice were habituated to stay on the
stationary drum for 3 min before the training sessions. Habituation was repeated each time for 1 min just before
the session. Mice were examined on the rotarod with an accelerating speed of 5 to 30 rpm over 300 s. The trials
were performed three times, and the longest time was recorded. The time limit of each observation was 300 s.
The time of disease onset was retrospectively determined as the time when the mice reached their peak body
weight. For the assessment of grip strength, mice were lifted by the base of the tail and placed so that their front
paws gripped the trapeze with their body horizontal. Each mouse was tested five times to obtain the best grip
strength performance. The ALS-TDI neurological score was measured as follows: 0, normal gait is observed; 1,
the hindlimb collapsed towards the lateral midline or trembled; 2, while walking, any part of the foot dragged
along the cage bottom/table; 3, the hindlimb was not used for forwarding motion but was able to right itself
within 10 s; and 4, rigid paralysis in the hindlimb and absence of righting reflex.
Immunohistochemistry. All animals were deeply anesthetized with sevoflurane and perfused intracardi-
ally with saline followed by cold 4% paraformaldehyde (PFA) in phosphate-buffered saline (PBS). The lumbar
cord, sciatic nerve, and ventral roots were subsequently removed, immersed for over 12 h in the same 4% PFA
fixative at 4 °C, and processed for making paraffin-embedded materials or optimal cutting temperature com-
pound-embedded frozen materials. Multiple 5-μm-thick paraffin-embedded sections and 10-μm-thick frozen
sections were used for immunohistochemical staining. Paraffin-embedded sections were deparaffinized, and
frozen sections were air-dried. Endogenous peroxidase activity was blocked by 3% H2O2 in methanol/PBS (1:1)
for 10 min at room temperature. Sections were then incubated with primary antibodies at 4 °C overnight45. After
rinsing, sections were subjected to either a streptavidin–biotin complex or an enhanced indirect immunoper-
oxidase method using Envision (Dako Cytomation, CA, USA). Immunoreactivity was detected using 3,3′-diam-
inobenzidine as the chromogen. The primary antibodies for mouse tissues are listed in Supplementary Table 1.
For immunohistochemical staining, the sections were incubated with secondary antibodies conjugated to Alexa
Fluor 488 or 594 (1:1000; Thermo Fisher, Rockford, IL, USA) and 4′,6-diamidino-2-phenylindole (DAPI)
(Sigma-Aldrich, Tokyo, Japan) to stain cell nuclei, and mounted with Permafluor (#TA-030-FM; Thermo Sci-
entific, Fremont, CA, USA). Tissues were observed with a fluorescence microscope (BZ-X700, Keyence, Tokyo,
Japan).
Western blotting. Sciatic nerves of mice were homogenized in 50 μl of lysis buffer containing 8% sodium
dodecyl sulfate (SDS) and RIPA buffer (#16488-34; Nacalai Tesque, Kyoto, Japan). After centrifugation at
14,000×g for 10 min, the supernatants were collected. The protein concentration in the supernatant was meas-
ured using a DC protein assay kit (Bio-Rad, Tokyo, Japan). Total protein (6 μl each) was separated by SDS
polyacrylamide gel electrophoresis (15%). Western blotting analysis was performed using anti-mSOD1 (500 ng/
ml) according to a previously described method46. The density of each band was quantified using ImageJ version
1.8.0_112 (Windows version of NIH Image; downloaded from https:// imagej. nih. gov/ ij/ downl oad. html).
Image acquirement and quantification analysis.
Immunofluorescence was captured by a fluorescence
microscope (BZ-X700). Quantification of immunofluorescence was performed using ImageJ version 1.8.0_112
using at least five lumbar spinal cord sections or five peripheral nerve sections for each animal in each group
using the area fraction technique as previously described47.
Electron microscopy. The animals were perfused intracardially with saline followed by cold 4% paraform-
aldehyde. The sciatic nerve was prefixed with a fixation buffer (2.5% glutaraldehyde, 0.1 M sucrose, 3 mM CaCl2,
and 0.1 M sodium cacodylate, pH 7.4) overnight at 4 °C. After being rinsed in PBS, the tissue was post-fixed
with 1% osmium tetroxide for 2 h, dehydrated in ethanol and propylene oxide, and embedded in Epon resin
(Epon 812 resin kit; TAAB Laboratories, Aldermaston, Berkshire, UK). Ultrathin Sects. (80 nm) were stained
with uranyl acetate for 5 min and with lead acetate for 10 min and then examined with a transmission electron
microscope (Tecnai 20; FEI Company Japan Ltd, Tokyo, Japan)45.
Mononuclear cell isolation from the sciatic nerve and flow cytometric analysis. After the tran-
scardial perfusion by ice-cold PBS, sciatic nerves were removed, minced with scissors, then suspended in RPMI
media. Then, the minced pieces were further dissociated using a 1000-µl pipette. The cell suspension was passed
into the FACS buffer through a 100-µm cell strainer. This step was repeated several times. The cell suspen-
sion was centrifuged at 800 × g at 4 °C for 5 min, the supernatant discarded, and the cell pellet resuspended in
1 ml FACS buffer48. For surface marker staining, cells were incubated with fluorochrome-conjugated antibod-
ies against CD45, CD3, I-A/I-E, CD11b, and CD11c for 30 min at 4 °C and analyzed in a BD FACSVerse™ flow
cytometer (Becton, Dickinson and Company, NJ). The percentage of CD45+ leukocytes, CD3+ T cells, I-A/I-E+
monocytes, CD11b+CD11c− macrophages, and CD11b+CD11c+ phagocytes was measured.
Cell culture. To analyze the phagocytic activity of monocytes in vitro, peripheral heparinized whole blood
was isolated from mice. Then, 100 µl of whole blood was incubated with 10 µl of pHrodo Green E. coli BioPar-
ticles Conjugate (10 mg/ml; Thermo Fisher Scientific, MA) and 40 µl of Gibco RPMI 1640 medium (Thermo
Fisher Scientific) at 37 °C and 5% CO2 for 30 min. After a washing step, the cells were analyzed by flow cytometry
using a Sony SH-800 (Sony Corporation, Tokyo, Japan).
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Vol.:(0123456789)www.nature.com/scientificreports/Statistical analysis. Data are expressed as the mean ± standard error of the mean (SEM). Pairwise com-
parisons between two groups were performed using the unpaired t test and log-rank test. Comparisons between
the three groups used in Fig. 2c were performed by two-way repeated-measures analysis of variance (ANOVA)
followed by Bonferroni posttests. Multiple comparisons in Suppl. Figure 2 were performed by one-way factorial
ANOVA. Survival time was compared using the Kaplan–Meier method and the log-rank test. A value of p < 0.05
was considered statistically significant. Quantitative analysis for FCM in Suppl. Figure 4b were performed by
unpaired t test. All statistical analyses were conducted using JMP pro 12 software (SAS Institute, Cary, NC).
Graphical images were built using PRISM 9 software (GraphPad Software, CA).
Ethical approval. The handling and sacrifice of all animals were conducted according to the guidelines for
the proper conduct of animal experiments published by the Science Council of Japan, as well as the ARRIVE
(Animal Research: Reporting of In Vivo Experiments) guidelines for animal research. Ethical approval for the
study was granted by the Animal Care and Use Committee of Kyushu University (#A30-051).
Received: 23 January 2021; Accepted: 4 August 2021
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Acknowledgements
We appreciate assistance from The Research Support Center, Research Center for Human Disease Modeling,
and Kyushu University Graduate School of Medical Sciences. We thank Dr. Mitsuru Watanabe and Ms. Eriko
Matsuo from the Department of Neurology, Kyushu University, for the technical assistance in the flow cytometric
analysis. We thank Ms. Sachiko Koyama and Hideko Noguchi from the Department of Neuropathology, Kyushu
University, for excellent technical assistance in the histological analysis. We thank Mr. Tetsuo Kishi from the
Department of Medicine, Kyushu University School of Medicine for the immunohistochemical analysis. We
thank J. Ludovic Croxford, PhD, from Edanz (https:// jp. edanz. com/ ac) for editing a draft of this manuscript.
Author contributions
W.S., R.Y., Y.H., S.K., and J.K. conceived the experiments. All authors contributed to the experimental design.
W.S., R.Y., Y.H., and S.K. performed the experiments and analyzed the results. R.Y. provided technical advice for
the experiments. W.S., R.Y., Y.H., Y.K., N.I., T.M., and J.K. were involved in interpreting the results. W.S., R.Y.,
and J.K. drafted the manuscript. All authors reviewed the manuscript.
Funding
This study was supported in part by JSPS KAKENHI Grants-in-Aid for Scientific Research (C) (Grant Number
JP16K09694 and JP19K07963) from the Japan Society for the Promotion of Science.
Competing interests
R.Y. has received honoraria from Teijin Pharma, Ono Pharmaceutical, Takeda Pharmaceutical, Eisai, Novartis,
Nihon Pharmaceutical, and CSL Behring; J.K. is a consultant for Biogen Japan and Medical Review, and has
received honoraria from Bayer Healthcare, Mitsubishi Tanabe Pharma, Nobelpharma, Otsuka Pharmaceutical,
Sanofi KK, Chugai Pharmaceutical Co. Ltd., Teijin Pharma, Novartis Pharma, and Medical Review; the remain-
ing authors declare no conflicts of interest.
Additional information
Supplementary Information The online version contains supplementary material available at https:// doi. org/
10. 1038/ s41598- 021- 96064-6.
Correspondence and requests for materials should be addressed to R.Y.
Reprints and permissions information is available at www.nature.com/reprints.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
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|
Data Availability Statement: Data from this study
has been made available as supplementary
information.
|
Data from this study has been made available as supplementary information.
|
RESEARCH ARTICLE
Acceptance of digital phenotyping linked to a
digital pill system to measure PrEP adherence
among men who have sex with men with
substance use
Hannah Albrechta1, Georgia R. Goodman1,2,3, Elizabeth Oginni1, Yassir Mohamed1,
Krishna Venkatasubramanian4, Arlen Dumas4, Stephanie Carreiro5, Jasper S. Lee1,3,
Tiffany R. Glynn1,2,3, Conall O’Cleirigh1,3, Kenneth H. Mayer1,6, Celia B. Fisher7, Peter
R. ChaiID
1,2,8,9*
1 The Fenway Institute, Fenway Health, Boston, Massachusetts, United States of America, 2 Department of
Emergency Medicine, Brigham and Women’s Hospital, Boston, Massachusetts, United States of America,
3 Department of Psychiatry, Massachusetts General Hospital, Boston, Massachusetts, United States of
America, 4 Department of Computer Science and Statistics, The University of Rhode Island, Kingston, Rhode
Island, United States of America, 5 Department of Emergency Medicine, University of Massachusetts Chan
Medical School, 6 Department of Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts,
United States of America, 7 Center for Ethics Education, Fordham University, New York City, New York,
United States of America, 8 Department of Psychosocial Oncology and Palliative Care, Dana Farber Cancer
Institute, Boston, Massachusetts, United States of America, 9 The Koch Institute for Integrated Cancer
Research, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America
* [email protected]
Abstract
Once-daily oral HIV pre-exposure prophylaxis (PrEP) is an effective strategy to prevent HIV,
but is highly dependent on adherence. Men who have sex with men (MSM) who use sub-
stances face unique challenges maintaining PrEP adherence. Digital pill systems (DPS)
allow for real-time adherence measurement through ingestible sensors. Integration of DPS
technology with other digital health tools, such as digital phenotyping, may improve under-
standing of nonadherence triggers and development of personalized adherence interven-
tions based on ingestion behavior. This study explored the willingness of MSM with
substance use to share digital phenotypic data and interact with ancillary systems in the con-
text of DPS-measured PrEP adherence. Adult MSM on PrEP with substance use were
recruited through a social networking app. Participants were introduced to DPS technology
and completed an assessment to measure willingness to participate in DPS-based PrEP
adherence research, contribute digital phenotyping data, and interact with ancillary systems
in the context of DPS-based research. Medical mistrust, daily worry about PrEP adherence,
and substance use were also assessed. Participants who identified as cisgender male and
were willing to participate in DPS-based research (N = 131) were included in this subsample
analysis. Most were White (76.3%) and non-Hispanic (77.9%). Participants who reported
daily PrEP adherence worry had 3.7 times greater odds (95% CI: 1.03, 13.4) of willingness
to share biometric data via a wearable device paired to the DPS. Participants with daily
PrEP adherence worry were more likely to be willing to share smartphone data (p = 0.006)
and receive text messages surrounding their daily activities (p = 0.003), compared to those
a1111111111
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OPEN ACCESS
Citation: Albrechta H, Goodman GR, Oginni E,
Mohamed Y, Venkatasubramanian K, Dumas A, et
al. (2024) Acceptance of digital phenotyping linked
to a digital pill system to measure PrEP adherence
among men who have sex with men with
substance use. PLOS Digit Health 3(2): e0000457.
https://doi.org/10.1371/journal.pdig.0000457
Editor: Haleh Ayatollahi, Iran University of Medical
Sciences, IRAN (ISLAMIC REPUBLIC OF)
Received: August 17, 2023
Accepted: February 1, 2024
Published: February 22, 2024
Copyright: © 2024 Albrechta et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data from this study
has been made available as supplementary
information.
Funding: This work was supported by the National
Institutes of Health (K23DA044874 to PRC,
DP2DA056107 to PRC and KV, P30AI060354 to
CO and KM, T32AI007433 to TRG, and
R25DA03196 to CBF and PRC). The funders had
no role in study design, data collection and
PLOS Digital Health | https://doi.org/10.1371/journal.pdig.0000457 February 22, 2024
1 / 14
PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
analysis, decision to publish, or preparation of the
manuscript.
Competing interests: The authors have declared
that no competing interests exist.
with less worry. MSM with substance use disorder, who worried about PrEP adherence,
were willing to use DPS technology and share data required for digital phenotyping in the
context of PrEP adherence measurement. Efforts to address medical mistrust can increase
advantages of this technology for HIV prevention.
Author summary
Oral medications for HIV pre-exposure prophylaxis (PrEP) are highly efficacious in pre-
venting HIV infection, but efficacy is closely linked with adherence. Despite the availabil-
ity of PrEP, measuring adherence and responding to nonadherence events remains
difficult. One possible strategy to measure PrEP adherence is using a digital pill system
(DPS) that activates directly in the stomach and reports adherence events. Integrating
contextual markers like smartphone digital phenotyping may enhance behavioral inter-
ventions that leverage DPS adherence data to provide PrEP adherence support. Here, we
conducted a survey study through a social networking website to understand perceptions
of the DPS and linked digital phenotyping among MSM with substance use on PrEP. We
found that the degree of substance use did not mediate willingness to participate in
research using digital phenotyping and the DPS. Individuals who worried more about
PrEP adherence were more willilng to interact with the DPS and digital phenotyping
techniques.
Introduction
Once-daily oral pre-exposure chemoprophylaxis (PrEP) is highly efficacious in preventing
human immunodeficiency virus (HIV) acquisition when adherence is maintained [1]. Follow-
ing results from multiple clinical trials, tenofovir disoproxil fumarate/emtricitabine (TDF/
FTC) was recommended by the World Health Organization (WHO) and the United States
(US) Centers for Disease Control and Prevention (CDC) for use as oral PrEP in 2012 [2]. Over
the past decade, PrEP has become widely recognized as a key pillar of the strategy to end the
HIV epidemic and is recommended for populations at risk of HIV acquisition. Among indi-
viduals at risk, men who have sex with men (MSM) experience disproportionate HIV expo-
sure, particularly given common comorbidities of mental health, stigma, and trauma [3].
Additionally, substance use among MSM has been independently associated with an increased
risk of HIV acquisition and PrEP nonadherence [4]. Despite the recent success of long-acting
injectable cabotegravir as PrEP [5], there remains a need to develop strategies to assess and
improve oral PrEP adherence, especially among MSM who may not qualify or be unable to
access injectable PrEP.
Given the importance of initiating and maintaining PrEP use for HIV prevention efforts,
several tools have been developed to measure adherence [6]. These include both indirect meth-
ods that infer medication ingestion events (e.g., self-report, pharmacy refill records, smart pill
bottles) and direct methods (e.g., directly observed therapy, video-assisted observed therapy,
and measurement of drug levels in biological matrices) [7]. Another tool that allows for direct
measurement of adherence is a digital pill system (DPS), which provides confirmation of the
presence of an ingested medication in the stomach. The FDA-cleared DPS (etectRx, Gaines-
ville, FL) comprises a standard gelatin capsule with an integrated radiofrequency emitter that
over-encapsulates PrEP. Upon ingestion, gastric chloride ions activate the radiofrequency
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
emitter, transmitting a prespecified radiofrequency signal to an off-body wearable device
(Reader), which stores and forwards ingestion data to a smartphone app, where DPS users and
clinical or research teams can view real-time adherence data [8]. This system can also serve as
a platform for the delivery of tailored adherence interventions, which can be directly informed
by changes in detected PrEP adherence patterns over time [9].
Previous qualitative work demonstrated that MSM with substance use are accepting of DPS
technology, willing to operate it in the real world to measure PrEP adherence, and perceive
value in having on-demand access to PrEP adherence data [8,10]. Additionally, a recent study
surveyed a national sample of MSM on PrEP who use substances to understand broader per-
ceptions of the DPS and willingness to interact with the system for PrEP adherence measure-
ment [11]. The results were congruent with previous qualitative work demonstrating the
willingness of MSM on PrEP who use substances to interact with the DPS. Participants also
described an interest in accessing their adherence data on demand, and those with greater
worry surrounding their PrEP adherence were statistically significantly more willing to inter-
act with the DPS. The current investigation builds off of previous research by exploring the
willingness of MSM who use substances to engage with ancillary devices and systems, and to
share smartphone data, in the context of DPS-based research.
One advantage of DPS technology lies in its ability to capture detailed daily patterns of inges-
tions. Such patterns of medication adherence behavior can form the basis of systems that seek to
measure the context in which ingestions occur [12]. The increasing ubiquity of smartphone own-
ership and use of wearable, health-related devices [13] presents an additional opportunity to col-
lect and leverage passive device data (e.g., battery life, accelerometry, and global positioning
system [GPS] data) to identify digital traits that may be indicative of changes in adherence behav-
iors, such as adherence. Digital phenotyping–the practice of aggregating large amounts of passive
smartphone and wearable data–has been demonstrated to indicate exacerbations of mental health
and pain among individuals with mental illnesses and acute bony fractures [14–16] and has been
used to track and monitor changes in the health status of surgical care patients [17].
Applied to DPS-measured PrEP adherence, digital phenotyping may contribute detailed
insights to contextualize ingestion events and potentially anticipate situations in which nonad-
herence may occur. While the combination of digital phenotyping and DPS-based adherence
data has the potential to deliver real-time tailored adherence interventions in the future, this
has not yet been tested empirically. Despite demonstrated acceptance of DPS among substance
using MSM, the addition of data from ancillary devices like smartphones or wearable devices
may be perceived as a further encroachment of privacy, especially among a population that
experiences heightened stigma surrounding sexual orientation, substance use and HIV risk.
This investigation sought to examine the association between willingness to share digital phe-
notypic data and interact with ancillary devices and systems in the context PrEP adherence
DPS-based research, and daily PrEP worry, medical mistrust, and degree of substance use,
among HIV-negative MSM with substance use.
Methods
Study design
This was a one-time cross-section online sampling-based survey of a national sample. Please
see Fig 1 below for a graphical representation of the study design and methods.
Participants
The eligibility criteria for the parent study were as follows: (1) 18 years or older; (2) cisgender
or transgender MSM; (3) self-reported HIV-negative; (4) currently on PrEP; (5) self-reported
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
Fig 1. Study design and methods. * Of 715 ineligible individuals, 343 were ineligible for more than one reason.
Reasons for ineligibility included: not �18 years old (n = 2), not cisgender or transgender male (n = 79), does not have
sex with cisgender or transgender males (n = 37), not HIV-negative (n = 101), not on PrEP (n = 367), not sexually
active in the last three months (n = 67), and CAGE-AID score <2 (n = 562). ** Of 18 participants who did not pass all
validity checks, 1 participant failed to pass more than one validity check. Reasons for not passing all validity checks
included: age and date of birth did not match (n = 15), home zip code and home state did not match (n = 2), and IP
address did not confirm current location in the US (n = 2).
https://doi.org/10.1371/journal.pdig.0000457.g001
sexually active in the past 3 months; (6) score of two or higher on the CAGE Questions
Adapted to Include Drug Use (CAGE-AID) [18]; and (7) current user of the Grindr social net-
working app.
Procedures
Participants were recruited through an advertisement partnership with Grindr (West Holly-
wood, CA), a popular social network site that caters to gay, bisexual and transgender people.
The study advertisement was delivered to 1,000,000 active US Grindr users via an
inbox message, which was active for 24 hours in January 2022. The study advertisement was
paid for by the study team via the Fordham University Research Ethics Training Institute
(NIH R25DA031608). The study team was composed of cisgender heterosexual and sexual
minority people trained in research surrounding technologies and HIV treatment/prevention.
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
No members of the study team have commercial interests in digital pill systems or the digital
phenotyping techniques described in this manuscript. Grindr was not involved in the design
or conduct of the study or data analysis.
Individuals who clicked on the study advertisement were linked to an eligibility screener
via a computer-assisted self-interviewing (CASI) secure platform (Qualtrics, Provo UT), fol-
lowed by a CAPTCHA validation question. Eligible individuals were presented with a fact
sheet containing detailed study information, including a description of the study, study contact
information, and an overview of study objectives and potential risks. After independently
reviewing the fact sheet, participants documented their informed consent by selecting “I agree
to participate.” Participants were provided with the option to download and save the fact sheet
for future reference.
Participants completed a cross-sectional quantitative assessment via a computer-assisted
self-interviewing (CASI) secure platform (Qualtrics, Provo UT), which lasted approximately
30–60 minutes. The study team conducted several manual validity checks following survey
completion (i.e., a confirmed match between age and date of birth, the validity of US home zip
code, match between zip code and home state, and IP address indicated location within the
US) to confirm eligibility for remuneration. Anonymized survey responses were stored on the
secure Qualtrics platform after survey completion, and all validated anonymized datasets were
exported, password-protected, and stored on a HIPAA-compliant Dropbox Business folder
accessible only to study staff. All study staff were trained in data management and quality
assurance protocols prior to the onset of the study.
Of the parent sample (N = 157), only those who reported at least slight willingness to partic-
ipate in DPS-based research and self-identified as cisgender males were included in the sub-
sample (N = 131). Individuals who self-identified as transgender were excluded from the
subsample (n = 6) due to the small sample size and potential for significantly different experi-
ences with the medical system and HIV risk factors, as compared to cisgender MSM. The Fen-
way Community Health Institutional Review Board (IRB) reviewed and approved all study
procedures.
Measures
The quantitative assessment included an eligibility pre-screener, an overview of the DPS tech-
nology–including images of the DPS components, and a video (recorded by PRC) explaining
system functionality–followed by survey questions as detailed below.
Sociodemographics
Participants reported age, race, ethnicity, gender, sexual orientation, education, annual
income, and geographic region (i.e., US census region). Participants also indicated their PrEP
adherence over an average week in the past three months (i.e., PrEP adherence), and how long
they have been taking PrEP (i.e., PrEP duration).
Willingness to participate in DPS-based research
After viewing a series of informative images and a video explaining how the DPS works, partic-
ipants were asked to rate their willingness to participate in future DPS-based research studies
on a 5-point Likert scale (1 = not at all, 2 = slightly, 3 = moderately, 4 = very, 5 = extremely
willing). Those who indicated at least slight willingness to participate in future DPS-based
research were included in the final subsample.
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
Willingness to contribute digital phenotyping data and interact with
ancillary systems in the context of DPS-based research
We assessed participants’ willingness to contribute smartphone data (e.g., geographic location,
battery level, text messaging, frequency of use of the app connected to the DPS); self-collected
blood work (finger prick) in the context of DPS-based research; share biometric information
(e.g., physiologic vital signs) during PrEP use via a wearable device paired to the DPS; and will-
ingness to receive text messages asking about substance use, sexual activity, general daily activ-
ities, and location. Participants rated their willingness for each of the above items on a five-
point Likert scale (1 = not at all, 2 = slightly, 3 = moderately, 4 = very, 5 = extremely willing),
which was then collapsed into two categories for analysis (1 and 2 = “slightly or not willing”;
and 3, 4 and 5 = “willing or extremely willing”).
Medical mistrust
Degree of mistrust in research and medical settings was measured via an adapted, 6-item ver-
sion of the Group-Based Medical Mistrust Scale (GBMMS), which has been demonstrated as a
reliable and valid measure for assessing research mistrust among American adults [19]. The
GBMMS is comprised of six questions scored using a 5-point Likert scale (1 = strongly dis-
agree, 5 = strongly agree). Items are summed to calculate a cumulative medical mistrust score,
with higher scores indicating greater mistrust (range: 6–25) [19].
Substance use
As part of the eligibility screener, participants completed the CAGE Questions Adapted to
Include Drug Use (CAGE-AID), which has been previously demonstrated as reliable and valid
measure [18,20]. The CAGE-AID comprises four yes/no questions about substance use (i.e.,
perceived need to cut down on substance use, annoyance when substance use is criticized by
others, feelings of guilt about substance use, and use of substances first thing in the morning).
“Yes” responses are scored as 1 and “No” responses are scored as 0. Items are summed for a
total score (possible range: 0–4), with higher total scores indicating greater potentially prob-
lematic substance use, and scores �2 considered clinically significant. Participants were cate-
gorized into three groups based on CAGE-AID score (i.e., 2, 3, and 4).
Daily PrEP worry
Participants reported their degree of daily worry about PrEP adherence on a single question
via a 5-point Likert scale (1 = not at all, 2 = slightly, 3 = moderately, 4 = very, 5 = extremely
willing). Responses were collapsed into two categories for analysis (1 and 2 = “slightly or not
worried”; and 3, 4 and 5 = “worried or extremely worried”).
Data analysis
Descriptive statistics were generated for sociodemographic variables. A multivariable logistic
regression model was used to measure the association between each of the outcome variables
(i.e., willingness to share smartphone data; self-collected blood work in the context of DPS-
based research; use a wearable device paired to the DPS to collect biometric information dur-
ing PrEP use; and to receive text messages asking about substance use, sexual activity, general
daily activities, and locations) and independent variables of interest (i.e., daily PrEP worry,
medical mistrust (GBMMS), and substance use (CAGE-AID)). A multivariate logistic regres-
sion model was used due to medical mistrust confounding the association between the out-
come variables and the predicator variable of daily PrEP worry. We also assessed for a
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
potential confounding effect by the following covariates: age, education level, race/ethnicity,
PrEP adherence, and PrEP duration. After assessing for a potential confounding effect on the
association between the outcomes of interest and independent variables of interest by using
Chi-square tests, we determined that the enlisted covariates above did not confound the associ-
ation between the predictors and outcome variables (p-values > 0.05). Covariates were evenly
distributed among the predictor variables. Therefore, the covariates listed above were not
included in the model. All analyses were completed using SAS (version 9.4) [21]. The SAS
code PROC LOGISTIC was used to conduct the multivariable logistic regression model.
Results
Sociodemographics and willingness to participate in future DPS research
Details on the parent sample (N = 157) are reported elsewhere [11]. In this subsample analysis,
only individuals who reported at least a slight willingness (1 = not at all, 2 = slightly, 3 = moder-
ately, 4 = very, 5 = extremely willing) to participate in DPS-related research and self-identified
as cisgender males were included (N = 131). There were no significant differences in sociode-
mographic characteristics between participants who indicated they would not be willing to
participate in DPS-related research (N = 20) and those who did.
The mean age of the subsample (N = 131) was 36.6 (SD: 12). The majority were White
(n = 100, 76.3%), non-Hispanic (n = 102, 77.9%), completed at least some college (n = 121,
92.4%), and reported an annual income of more than $60,000 (n = 77, 58.8%). More than half
the sample reported being on PrEP for more than a year (n = 75, 57.3%), with the vast majority
of participants self-reporting � 4 doses per week during a typical week (n = 124, 94.7%)
(Table 1).
Willingness to share biometric information via a wearable device paired to
the DPS
There was a statistically significant association between the willingness to use a wearable device
to collect biometric information and both daily PrEP worry (p = 0.046) and medical mistrust
(p = 0.005). Participants who reported being worried about daily PrEP adherence had 3.7
times the odds (95% CI: 1.026, 13.425) of being willing to share biometric data via a wearable
device paired to the DPS, compared to those who were less worried, after adjusting for other
predictors. Participants with higher medical mistrust were less likely to be willing to share bio-
metric data. For every one unit increase in medical mistrust score, the odds of not being will-
ing to share biometric data via a wearable device increased by 0.8 (95% CI: 0.739, 0.946), after
adjusting for other predictors. No significant association was found between the degree of sub-
stance use and willingness to share biometric data (p = 0.387; Table 2).
Willingness to share smartphone data
There was a statistically significant association between willingness to share smartphone data
and both daily PrEP worry (p = 0.006) and medical mistrust (p <0.0001). Participants who
reported worrying about daily PrEP adherence were more likely to be willing to share smart-
phone data, compared to those who were less worried, with an odds ratio of 2.811 (95% CI:
1.163, 6.792), after adjusting for other predictors. In addition, participants with higher medical
mistrust were less likely to be willing to share smartphone data. For every one unit increase in
medical mistrust, the odds of being willing to share smartphone data decreased by 20% (OR:
0.818; 95% CI: 0.745, 0.898), after adjusting for other predictors. No statistically significant
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
Table 1. Sociodemographic characteristics (N = 131).
Variable
Age (years)
Mean (SD)
Race
White
African American
Asian
American Indian or Alaska Native
More than one race
Other
Ethnicity
Not Hispanic or Latinx
Hispanic or Latinx
Gender Identity
Cisgender male
Education
High school degree or some high school
College degree or some college
Graduate/professional degree or some graduate work
Annual Income
Less than $24,000
$24,000 to $29,999
$30,000 to $59,999
$60,000 or more
Geographic Region (in US)
Midwest
Northeast
South
West
PrEP Adherence
<4 doses per week
� 4 doses per week
PrEP Duration
Less than 1 month
1 to 6 months
6 months to 1 year
More than 1 year
n (%)
36.6 (12)
100 (76.3)
5 (3.8)
5 (3.8)
2 (1.5)
15 (11.5)
4 (3.1)
102 (77.9)
29 (22.1)
131 (100.0)
10 (7.6)
78 (59.5)
43 (32.8)
22 (16.8)
12 (9.2)
20 (17.3)
77 (58.8)
20 (15.3)
40 (30.5)
47 (35.9)
24 (18.3)
7 (5.3)
124 (94.7)
7 (5.3)
31 (23.7)
18 (13.7)
75 (57.3%)
https://doi.org/10.1371/journal.pdig.0000457.t001
association was found between the degree of substance use and willingness to share smart-
phone data (p = 0.603; Table 2).
Willingness to participate in self-collected blood work
There was a statistically significant association between willingness to self-collect blood work
and medical mistrust (p = 0.001). Participants with higher medical mistrust were less likely to
be willing to self-collect blood work, with an odds ratio of 0.870 (95% CI: 0.800, 0.947), after
adjusting for other predictors. No significant association was found between the degree of sub-
stance use or daily PrEP worry and willingness to self-collect blood work (p = 0.483 and
p = 0.225, respectively; Table 2).
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PLOS DIGITAL HEALTHTable 2. Willingness to contribute digital phenotyping data and interact with ancillary systems in the context of DPS-based research (N = 131).
Acceptance of digital phenotyping and biological data sharing in context of digital pills
Outcome Variable
Willingness to. . .
Share biometric data
Share smartphone data
Self-collect blood work
Receive text messages asking about substance use and sexual
activity
Receive text messages asking about general daily activities
and location
*Statistically significant at the 0.05 level
https://doi.org/10.1371/journal.pdig.0000457.t002
Exposure Variable
P- value Beta Coefficient Estimates /
Daily PrEP Worry
Degree of Substance
Use
Medical Mistrust
Daily PrEP Worry
Degree of Substance
Use
0.046*
0.387
0.005*
0.006*
0.603
SE
1.311 / 0.656
0.710 / 0.822
-0.179 / 0.063
1.034 / 0.450
-0.243 / 0.468
Medical Mistrust
<0.0001* -0.201 / 0.048
Daily PrEP Worry
Degree of Substance
Use
0.225
0.483
0.517 / 0.426
-0.317 / 0.451
Medical Mistrust
0.001*
-0.139 / 0.043
Daily PrEP Worry
Degree of Substance
Use
Medical Mistrust
Daily PrEP Worry
Degree of Substance
Use
0.092
0.675
0.854 / 0.507
0.228 / 0.543
<0.0001* -0.273 / 0.058
0.003*
1.307 / 0.441
0.828
-0.050 / 0.230
Measure of Association (OR) and
95% CI
3.711 (1.026, 13.425)
0.784 (0.313, 1.962)
0.836 (0.739, 0.946)
2.811 (1.163, 6.792)
0.784 (0.313, 1.962)
0.818 (0.745, 0.898)
1.677 (0.727, 3.865)
0.729 (0.301, 1.765)
0.870 (0.800, 0.947)
2.349 (0.870, 6.343)
1.256 (0.433, 3.644)
0.757 (0.673, 0.851)
3.693 (1.557, 8.763)
0.905 (0.368, 2.229)
Medical Mistrust
0.0002*
-0.170 / 0.046
0.844 (0.770, 0.920)
Willingness to receive text messages asking about substance use, sexual
activity, general daily activities, and location
Text messages asking about substance use and sexual activity. No statistically significant
association was found between willingness to receive text messages asking about substance use
and sexual activity, and daily PrEP worry (p = 0.092) or degree of substance use (p = 0.675).
There was a statistically significant association between willingness to receive text messages
asking about substance use and sexual activity, and medical mistrust (p <0.0001). For every
one unit increase in medical mistrust score, the odds of being willing to receive text messages
about substance use and sexual activity decreased by 0.76 (95% CI: 0.673, 0.851), after adjust-
ing for other predictors (Table 2).
Text messages asking about general daily activities and location. There was a statisti-
cally significant association between willingness to receive text messages asking about daily
activities and location, and both daily PrEP worry (p = 0.003) and medical mistrust
(p = 0.0002). Participants who reported being worried or very worried about daily PrEP
adherence had 3.7 times the odds of being willing to receive text messages asking about
daily activities and location, compared to those who were not worried, after adjusting for
other predictors (95% CI: 1.557, 8.763). Additionally, for every one unit increase in medical
mistrust score, the odds of being willing to receive text messages asking about daily activi-
ties and location decreased by 17% (OR: 0.844; 95% CI: 0.770, 0.920), after adjusting for
other predictors. No significant association was found between degree of substance use and
willingness to receive text messages asking about daily activities and location (p = 0.828;
Table 2).
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
Discussion
Digital pills are evolving as a system to directly measure adherence to medications, including
PrEP. Using DPS technology to better understand the contextual basis of PrEP adherence and
nonadherence may help provide support to individuals who struggle with PrEP adherence at
key junctures of risk. The emerging use of wearable devices and collection of smartphone-
based digital phenotyping data may provide insight into key events where nonadherence is
likely and the delivery of proactive, personalized adherence support may mitigate nonadher-
ence [22–25]. Contrary to perceptions that individuals with substance use may be less accept-
ing of the collection of personal data via mobile devices and other systems, the degree of
substance use in our subsample was not associated the willingness of MSM on PrEP to interact
with ancillary devices or text message-based queries to contextualize DPS-detected adherence
data. We also found that participants who worried about their daily PrEP adherence and
were more trusting of the medical system reported more willingness to contribute S1 Data–
including biometric or digital phenotypic data from wearable devices, as well as self-collected
blood samples–and to engage with text messages that query contextual behaviors linked to
their PrEP adherence as measured by the DPS. These findings importantly frame the potential
expansion of DPS technology through the integration of other wearable devices, self-collected
biological samples, and the development of context-aware behavioral interventions. They also
suggest opportunities to engage with community partners to address potential concerns
related to medical mistrust around DPS technology and other, related systems for PrEP adher-
ence measurement.
Overall, this subsample was also willing to contribute additional data to contextualize their
adherence, despite their degree of self-reported substance use. This suggests that the addition
of strategies like digital phenotyping or EMA surveys can add important context to observed
PrEP adherence in MSM, and may present novel opportunities to teach and reinforce adher-
ence skills in the setting of contextualized nonadherence behavior. Given the willingness of
MSM to contribute smartphone-based data to further contextualize PrEP adherence behaviors,
future work should focus on ethical, legal and social implications of smartphone data. Some
potential strategies that address existing controversies in the ethics of digital phenotyping
include responsible data collection strategies that only collect data that may be needed to
understand contextual cues surrounding PrEP adherence, and design of security protocols
that deidentify data, produce fuzziness in location data, and adequately explain the types of
data collected to study participants. For example, clear explanation of the implications of loca-
tion data and its relationship with PrEP adherence, substance use, and sexual activity should
be disclosed to research participants, as well as, in the future, individuals who may leverage
digital phenotyping in the context of their clinical care. Additionally, as PrEP initiation efforts
continue to leverage telemedicine approaches to increase accessibility, the DPS may be an
acceptable adherence measurement strategy that can be integrated into existing systems that
already support self-collected biological samples for the assessment of sexually transmitted
infections and regular HIV testing for PrEP users [26].
MSM in our subsample who reported more daily worry around PrEP adherence were sig-
nificantly more likely to be willing to interact with text messages regarding their general daily
activities and location that could be used to inform future adherence interventions. Partici-
pants’ increased willingness to share additional contextual data via ancillary devices suggests
that many MSM may also be accepting of more personalized adherence interventions
grounded in digital phenotypic measurements. Additionally, in research that integrates DPS-
based PrEP adherence data to ground analysis of digital phenotyping data, MSM with sub-
stance use may be willing to respond to ecological momentary assessments that ask about
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
sensitive and potentially stigmatizing sexual health, location and substance use. This suggests
that future work may integrate strategies like text message-based queries and self-collected bio-
logical specimens into the DPS ecosystem to better understand PrEP adherence. As technolo-
gies like DPS and digital phenotyping are adopted, this work should remind researchers and
policymakers that individuals with stigmatized conditions and substance use also can benefit
and uptake these systems.
We also found that individuals who were more trusting of the medical system reported
being significantly more willing to contribute additional digital phenotypic or contextual data
in as part of DPS-based PrEP adherence research. A major challenge in light of these findings
lies in decreasing the barriers to building participant/researcher, and ultimately patient/clini-
cian, relationships that may improve overall trust in the medical system over time. Researchers
may consider engaging with community partners or advocacy groups prior to the initiation of
future studies in order to develop strategies for introducing the DPS to potential participants,
and to adequately address concerns associated with trust in DPS technology and collection of
phenotypic data from other systems. Such conversations should carefully consider the inter-
section of race, ethnicity, and existing levels of medical mistrust on users’ perceptions of the
DPS and ancillary systems [27]. Our previous work suggests that the use of these monitoring
technologies may, in fact, increase one’s sense of personal accountability for their PrEP adher-
ence, as well as improve relationships with medical providers by providing objective data
around PrEP ingestions, and the context in which they occur, to long-term primary care and
sexual health services [10].
This investigation should motivate the continued development of digital health tools
including behavioral interventions responding to medication adherence measured through
various strategies including ingestible sensors. Importantly, future research should continue to
include individuals with substance use disorder given their risk for HIV and other comorbidi-
ties. Research should also consider the role of care teams, including physicians, social workers,
pharmacists, nurses and care coordinators in curating and responding to a suite of digital phe-
notyping and ingestible sensor data. Implementation challenges will include identifying the
members of care teams who should receive context aware data. Existing care models that inte-
grate a clinical pharmacist in adherence counseling as well as maintenance of the DPS (overen-
capsulation and technology teaching) may serve as a potential pathway to implementation of
these systems. Integration of pharmacists into DPS infrastructure may also provide an imple-
mentation pathway in clinical settings with patient centered medical homes.
Digital phenotyping may also provide insights into how MSM with substance use experi-
ence challenges to adherence. These insights may then be translated into other populations
(e.g., transgender individuals) and disease states (e.g., heart failure or diabetes medication
adherence). In individuals with stigmatized conditions like their sexual orientation or risk fac-
tors including substance use, there may be a social desire to bias self-reported data in the con-
text of research studies. Future development of digital phenotyping relying on native
smartphone sensors may provide a more objective perspective to key behaviors that can be tar-
gets for empiric interventions that mitigate risk, reinforce adherence (in the setting of PrEP),
and improve linkage to care. Future work may include observational studies to further charac-
terize digital phenotypes that may be associated with substance use or its comorbidities, inte-
gration of digital phenotyping into adherence technologies like ingestible sensors, and
research to develop predictive algorithms that present interventions at opportune moments to
facilitation interaction with the user.
This study had several limitations. First, our sample size was recruited online from a social
network site popular among MSM. As this recruitment strategy was selected to identify indi-
viduals with internet access, who were likely to have smartphones, and who engaged with
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
social media platforms, this approach may have therefore missed key populations with differ-
ent perspectives on DPS technology and ancillary systems. Participants who have internet
access and most likely smartphones, may have higher levels of digital literacy, which could
impact their willingness to contribute phenotypic data and engage with ancillary devices and
systems. Second, the majority of participants identified as White MSM. Perceptions of PrEP,
trust in the medical system, and experiences with digital health technologies may vary across
race and ethnicity; as such, the conclusions drawn in this investigation may not be generaliz-
able to non-White MSM [27]. The generalizability of the findings are also limited to the MSM
community who engage in substance use. Third, individuals who are more concerned about
their PrEP adherence and are more willing to share phenotypic data may be more interested in
participating in research that focuses on these concerns, which may introduce sampling bias.
Finally, this study involved a one-time quantitative assessment among prospective DPS users;
participants did not have direct experience using the DPS but instead viewed a video describ-
ing its functionality and architecture prior to completing the quantitative assessment. Percep-
tions of and attitudes towards ancillary devices that contribute additional data to DPS-based
PrEP adherence measures may be different following lived experience with the DPS.
Conclusion
The DPS represents a unique opportunity for researchers, clinicians, and patients to better
understand both PrEP adherence and nonadherence in the context in which it occurs. MSM
with substance use may be accepting of DPS technology, willing to contribute digital pheno-
typing data, and willing to interact with ancillary systems in order to contextualize PrEP adher-
ence patterns in a research setting. While substance use did not impact the willingness of
MSM to accept these systems in this subsample, increased trust in the medical system and
increased worry about daily PrEP adherence increased the likelihood that participants
reported a willingness to interact with digital phenotyping, wearable devices, self-collected bio-
logical sampling, and text message queries to contextualize adherence.
Supporting information
S1 Data. Dataset with Codebook.
(XLSX)
Author Contributions
Conceptualization: Conall O’Cleirigh, Kenneth H. Mayer, Celia B. Fisher, Peter R. Chai.
Data curation: Hannah Albrechta, Georgia R. Goodman, Elizabeth Oginni, Yassir Mohamed,
Jasper S. Lee, Peter R. Chai.
Formal analysis: Elizabeth Oginni, Yassir Mohamed.
Funding acquisition: Peter R. Chai.
Investigation: Hannah Albrechta, Georgia R. Goodman, Peter R. Chai.
Methodology: Hannah Albrechta, Georgia R. Goodman, Peter R. Chai.
Project administration: Georgia R. Goodman, Peter R. Chai.
Resources: Celia B. Fisher.
Supervision: Peter R. Chai.
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PLOS DIGITAL HEALTHAcceptance of digital phenotyping and biological data sharing in context of digital pills
Writing – original draft: Hannah Albrechta, Georgia R. Goodman, Elizabeth Oginni, Peter R.
Chai.
Writing – review & editing: Hannah Albrechta, Georgia R. Goodman, Elizabeth Oginni, Yas-
sir Mohamed, Krishna Venkatasubramanian, Arlen Dumas, Stephanie Carreiro, Jasper S.
Lee, Tiffany R. Glynn, Conall O’Cleirigh, Kenneth H. Mayer, Celia B. Fisher, Peter R. Chai.
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| null |
10.1371_journal.pone.0221212.pdf
|
Data Availability Statement: All relevant data for
reproducing indicators are within the paper and the
Supporting Information files. All these data have
been downloaded from https://www.scival.com/
under provision of the institutional standard
contract held by University of Siena. Authors did
not have any special access privileges to SCIVAL.
Interested researchers may access Scival in the
same way the authors did.
|
All relevant data for reproducing indicators are within the paper and the Supporting Information files. All these data have been downloaded from https://www.scival.com/ under provision of the institutional standard contract held by University of Siena. Authors did not have any special access privileges to SCIVAL. Interested researchers may access Scival in the same way the authors did.
|
RESEARCH ARTICLE
Citation gaming induced by bibliometric
evaluation: A country-level comparative
analysis
Alberto BacciniID
1
1*, Giuseppe De Nicolao2, Eugenio PetrovichID
1 Department of Economics and Statistics, University of Siena, Siena, Italy, 2 Department of Electrical,
Computer and Biomedical Engineering, University of Pavia, Pavia, Italy
* [email protected]
Abstract
It is several years since national research evaluation systems around the globe started mak-
ing use of quantitative indicators to measure the performance of researchers. Nevertheless,
the effects on these systems on the behavior of the evaluated researchers are still largely
unknown. For investigating this topic, we propose a new inwardness indicator able to gauge
the degree of scientific self-referentiality of a country. Inwardness is defined as the propor-
tion of citations coming from the country over the total number of citations gathered by the
country. A comparative analysis of the trends for the G10 countries in the years 2000-2016
reveals a net increase of the Italian inwardness. Italy became, both globally and for a large
majority of the research fields, the country with the highest inwardness and the lowest rate
of international collaborations. The change in the Italian trend occurs in the years following
the introduction in 2011 of national regulations in which key passages of professional
careers are governed by bibliometric indicators. A most likely explanation of the peculiar Ital-
ian trend is a generalized strategic use of citations in the Italian scientific community, both in
the form of strategic author self-citations and of citation clubs. We argue that the Italian case
offers crucial insights on the constitutive effects of evaluation systems. As such, it could
become a paradigmatic case in the debate about the use of indicators in science-policy
contexts.
Introduction
Starting from the late 1980s, several European and extra-European countries implemented
national systems to monitor, assess, and evaluate the research performance of their scientific
workforce [1, 2]. One of the key features of such research evaluation systems is the focus on
quantitative indicators (metrics) as crucial science policy tools [3]. Accordingly, in the last
years, several scientometric indicators, based on publications or citations (or on a combination
of both, such as the h-index), have increasingly appeared in the academic evaluation systems,
alongside with the traditional peer-review-based procedures.
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OPEN ACCESS
Citation: Baccini A, De Nicolao G, Petrovich E
(2019) Citation gaming induced by bibliometric
evaluation: A country-level comparative analysis.
PLoS ONE 14(9): e0221212. https://doi.org/
10.1371/journal.pone.0221212
Editor: Lutz Bornmann, Max Planck Society,
GERMANY
Received: April 9, 2019
Accepted: August 2, 2019
Published: September 11, 2019
Copyright: © 2019 Baccini et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data for
reproducing indicators are within the paper and the
Supporting Information files. All these data have
been downloaded from https://www.scival.com/
under provision of the institutional standard
contract held by University of Siena. Authors did
not have any special access privileges to SCIVAL.
Interested researchers may access Scival in the
same way the authors did.
Funding: Alberto Baccini is the recipient of a grant
by Institute For New Economic Thinking Grant ID
INO17-00015. The funders had no role in study
PLOS ONE | https://doi.org/10.1371/journal.pone.0221212 September 11, 2019
1 / 16
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
Citation gaming induced by bibliometric evaluation
The use of these indicators in the evaluation of research performance has generated a
heated debate in the scientific community. The advocates argue that scientometric measures
are not only more objective than the peer-review [4]; they would also improve both the quan-
tity and the quality of the scientific production [5, 6]. This would occur because the indicators
are integrated within a system of incentives that rewards the achievement of the scientometric
targets set by the evaluation system [7]. On the other hand, critics claim that the same mecha-
nisms that are designed to improve the research performance create at the same time room for
strategic behaviors [8]. For instance, when productivity is positively rewarded, the number of
publications become a goal that can be pursued not only by positive behaviors (doing more
research), but also by opportunistic strategies (e.g., slicing one scientific work into multiple
publications) [9, 10]. Analogously, when citations become a goal, the “citation game” starts
[11]. Criticisms themselves have been challenged: for instance, Butler’s conclusions about the
Australian case have been widely discussed [12]. A mediating position is represented by schol-
ars proposing a “responsible use” of metrics. According to this approach, research metrics can
provide valuable insights on the research performance, granted that they are carefully designed
in order to avoid unintended consequences. Thus, a distillation of best practices has been pro-
posed for improving the use of metrics in research assessment [13].
Recently, the idea that the consequences of the use of indicators on the behavior of
researchers can be easily sorted between the intended and the unintended ones, has been ques-
tioned as too simplistic [14, 15]. Instead, the notion of “constitutive effects” has been advanced
to capture the way in which the indicators act on the researchers [16]. Within this new frame-
work, indicators are conceived as shaping the activity of research deeply and at different levels,
from the citation habits to the research agenda, redefining at the same time key evaluative
terms such as research quality [17]. They become crucial actors in the “epistemic living spaces”
of academic researchers [18] and researchers begin to “think with indicators” pervasively [19].
The main constitutive effects of the indicators described in the literature can be grouped
into three main types: i) Goal-displacement: scoring high on the indicators becomes a target in
itself, that is to be achieved also by gaming the system [20, 21]; ii) Risk avoidance: highly inno-
vative, not mainstream, and interdisciplinary research topics are avoided because they could
do not score well on indicators that tend to reward more traditional research programmes [19,
22–26]; iii) Task reduction: when academic activities such as teaching and public engagement
are not rewarded, academics tend to avoid them to concentrate only on publishable academic
research [27–29].
Although these effects have been highly debated, until recently the evidence of their occur-
rence has been mainly anecdotal. It is only in the last years that the methodical empirical study
of such effects has been undertaken [14, 22]. In the present paper, we aim to advance the
knowledge on this topic by focusing on the case of Italy. Among European and extra-European
countries, Italy is the only one in which some key career passages of scientific researchers are
entirely regulated by rules based on bibliometric indicators (except for the scholars in the
Social Sciences and Humanities, see next section). Thus, Italy is ideally suited to studying the
response of researchers to the use of metrics in research evaluation.
In particular, we will investigate whether Italian scientists have pervasively adopted a strate-
gic use of citations in order to boost their indicators. By “pervasively”, we mean that the effect
of this behavior should be visible in the great majority of scientific fields, at the national level.
As we will highlight in the Conclusion, the Italian case provides important insights on the con-
stitutive effects of evaluation systems in general.
The rest of the paper is organized as follows. In the next two sections, the specificity of the
Italian case is explained and the literature dealing with self-citing strategic behaviors is
reviewed. Next, a new “inwardness” indicator is introduced that is sensitive to collective
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Citation gaming induced by bibliometric evaluation
strategic citation behaviors at a country level. In the Data section, the procedure for retrieving
the data is described, while the main findings are presented in the Results section. In the Dis-
cussion, after examining alternative explanations, it is argued in favor of the emergence of a
collective strategic behavior devised to meet the demands of the evaluation system. In the Con-
clusions, some general lessons from the Italian case are drawn.
The Italian case
In 2010, the Italian university system underwent a wide process of reformation, regulated by
the Law 240/2010. The reform created the Agency for the Evaluation of the University and
Research (ANVUR), a centralized agency whose main task is the monitoring and the evalua-
tion of the Italian research system. The Agency started in 2011 a research assessment exercise
called VQR, relative to the period 2004-2010. A second research assessment exercise was
started in 2015, relative to the period 2011-2014. In both exercises, the evaluation of submitted
articles was largely based on the automatic or semi-automatic use of algorithms fed by citation
indicators [30] while other research outputs, such as books, were evaluated by peer reviews.
The reform modified also the recruitment and advancement system for university profes-
sors by introducing the National Scientific Habilitation (ASN). Both for hiring and promotion,
having obtained the ASN has become mandatory for applying to academic positions. The bib-
liometric rules rely on three indicators. For the hard sciences, life sciences, and engineering,
the indicators considered by ANVUR are the number of journal articles, the number of cita-
tions, and the h-index. For the social science and humanities, the indicators are the number of
research outputs, the number of monographs, and the number of papers published in “class
A” journals. At each new round of habilitation, ANVUR calculates for each of these indicators
the “bibliometric thresholds” that the candidates must overcome to achieve the ASN. For the
first edition of the ASN the national rules were defined in the Ministerial Decree 7 June 2012
n. 76. http://attiministeriali.miur.it/media/192901/dm_07_06_12_regolamento_abilitazione.
pdf. ANVUR defined the thresholds used for the first edition of the ASN: https://web.archive.
org/web/20190207112821/http://www.anvur.it/attivita/asn/asn-2012-2013/indicatori-e-
relative-mediane/. Candidates whose indicators do not overcome two thresholds out of three
cannot be habilitated (exceptions were possible in specific circumstances only in the first edi-
tion, ASN 2012). When first introduced, the thresholds were stated to be the median values of
the indicators of the permanent academic staff holding that position (associate or full profes-
sor). To make and example, in order to obtain a full professor habilitation, the candidate was
required to score better than half of the current full professors in two indicators out of three.
Applicants overcoming the fixed thresholds are then evaluated by a committee composed by
five referees who are in charge of the final decision about attributing habilitation.
Note that the focus on indicators is not confined to the national procedures but “trickles
down” to the university committees in charge of recruiting and promotion that are required to
take into account production and citation metrics when they evaluate and rank the habilitated
applicants. Finally, also the members of both the national habilitation and the local recruit-
ment committees are required to overcome bibliometric thresholds.
In sum, in Italy, starting from 2011, bibliometric indicators have gained a central role not
only in the national research assessment but in the entire body of the recruitment procedures. A
remarkable peculiarity of the Italian system is that the indicators based on citations, used both
in the habilitation procedure and in the research evaluation exercise, are calculated by including
self-citations. Thus, researchers can increase their indicators just by self-citing their own work.
Anecdotal evidence of the adoption of strategic behaviors in the form of author self-cita-
tions has been presented by Baccini [31]. Two recent studies have documented more
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Citation gaming induced by bibliometric evaluation
thoroughly the rise of opportunistic behaviors in response to the ASN rules. Seeber et al. has
analyzed how the use of self-citations in four Italian research areas changed after the introduc-
tion of the habilitation procedure. They have found that scientists in need of meeting the
thresholds (i.e., those looking for habilitation as a prerequisite for tenure-track or promotion
to full professor) did increase significantly their self-citations after 2010 [32]. Scarpa et al.
focused on the Italian engineering area and found an anomalous peak in the self-citations rate
(i.e., the number of self-citations to the total number of citations) in correspondence of the sec-
ond round of the habilitation procedure, in 2013 [33]. Even if the aforementioned studies have
highlighted some recent behavior changes by Italian scientists, they did not address a subtler
form of strategic behavior, the one based on the so-called citation clubs or citation cartels.
Strategic behaviors, country self-citations, and the inwardness
indicator
A citation club is an informal structure in which citations are strategically exchanged among
its members to boost the respective citation scores [34–36]. Citation clubs are difficult to spot,
especially when their members exchange citations but are not co-authors. Indeed, if we only
examine the self-citation rates of the individual members, we would not spot any anomaly, in
so far as they keep their individual self-citations under control (i.e., they do not cite dispropor-
tionately their own work). Thus, a well-concealed citation club is invisible if monitoring is lim-
ited to individual self-citations [37, 38]. If we consider a group of scholars, the citation club
becomes visible as it increases the citation traffic internal to the group (group self-citations).
Obviously, groups of scholars may be individuated in many ways and in different social net-
works. A most natural example may be a group of scholars that are not directly co-authors but
at a relatively small distance in a co-authorship network. However, a citation club may also
thrive on an interlocking editorship network [39, 40], in which case citations are exchanged
between scholars serving as editors in the same set of journals. Or, again, the citation club may
be rooted at an institutional level (universities or departments). In all these cases, although it is
possible to record the citation traffic inside the citation club, it is nonetheless impossible to dis-
tinguish the citations generated as a normal by-product of the research activity from those
resulting from strategic behaviours.
Along this rationale, the key idea of this paper is that a sudden and strong increase of strate-
gic citations internal to a country is going to affect in a visible way self-citations recorded at
country level. Such occurrence may be spotted by a macro level analysis, without the need of
documenting the existence of clubs, whatever defined, and of a criterion to distinguish
between types of citations. Hence, hereafter the focus is on country self-citations, a not much
studied form of self-citation [41]
A country self-citation occurs whenever the set of the countries of the authors of the citing
publication and the set of the countries of the authors of the cited publication are not disjoint,
that is, if these two sets share at least one country [42, 43]. Notably, any citation exchanged
within a citation club formed by researchers working in the same country is counted as coun-
try self-citations, even when it is not an author self-citation.
Thus, considering that most of the standard author self-citations are country self-citations
too (the only exception being authors that changed their country between the citing and the
cited publication), by analyzing the country self-citations, we can capture both the “classic”
strategy based on author self-citations, and the “elaborated” one based on citation clubs.
It is very important to underline that country self-citations are not always generated by cita-
tion clubs, just as not all author self-citations originate from gaming purposes. The literature
on author self-citations agrees on the fact that a certain amount of them is a normal byproduct
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Citation gaming induced by bibliometric evaluation
of the scientific communication. There are many perfectly legitimate reasons for citing one’s
own works, such as building on previously obtained results, avoiding repetition, and so on
[44–46]. By the same token, it is normal that a country has an internal exchange of citations
amongst its researchers insofar the knowledge produced by the country is used (i.e., cited) by
the same country’s scientific staff. Moreover, in the research fields that are characterized by a
national focus (e.g., some areas in the Social Science and Humanities), it is normal to expect a
larger number of country self-citations.
Consider also that international collaboration positively affects the number of country self-
citations. In fact, the more a country collaborates with other countries, the higher will be the
number of country self-citations. Take for instance a paper authored in collaboration by Italy
and France. Any future citation to that paper coming from an Italian-authored or a French-
authored publication will count as a country self-citation for both Italy and France, since the
citing and the cited publication will share at least one country of affiliation.
In sum, the country self-citations are not per se a sign of strategic behavior. The level of self-
citations of a country depends both on the internal exchange of knowledge within a country
and the amount of international collaboration. Nonetheless, if the researchers of a single coun-
try initiate strategic behaviors in order to boost their citations, this is likely to produce an
anomalous increase of country self-citations compared to the other countries. Thus, to detect
the strategic behaviors, one has to focus on the changes in the country self-citations over time,
rather than on their absolute value.
In order to obtain a normalized measure of country self-citations, we introduce a simple
indicator of “inwardness”. For a given year and a country c, the inwardness is defined as the
percentage ratio between the total number of country self-citations (Sc) and the total number
of citations (Cc) of that country:
Ic ¼
Sc
Cc
� 100
ð1Þ
The minimum value of the inwardness indicator is Ic = 0 when a country has no self-citations;
and the maximum is Ic = 100 when a country has self-citations only, that is Sc = Cc.
It is easy to show that the inwardness indicator is a variant of the Relative Citation Impact
(RCI) of a country. The RCI is defined by May [47] as the ratio between the average citation
per paper of a country and the average citation per paper of the world (see also [48]). The RCI
of the country c in a given year is defined as RCIc ¼
Cc
Pc
�
Pw
Cw
where Cc and Cw are the total
number of citations of the country and of the world, and Pc and Pw the publications of the
country and of the world. The total number of citations is the sum of the country self-citations
(Sc) and the external citation (Xc); when the world is considered Cw = Sw, since obviously Xw =
0. If a Relative Self-citation Impact is defined as RSIc ¼
Sc
Pc
�
Pw
Sw
, the inwardness indicator can
be expressed as
Ic ¼
RSIc
RCIc
¼
1
C
C
A �
0
B
B
@
Sc
Pc
Sw
Pw
0
B
B
@
Cw
Pw
Cc
Pc
1
C
C
A ¼
Sc
Cc
ð2Þ
Note that the inwardness indicator is normalized for the size of the country in terms of
publications.
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Citation gaming induced by bibliometric evaluation
From a conceptual point of view, the inwardness of a country is an indicator of how much
the knowledge produced in the form of scientific publications in a given year in a country
flows, through citations, into the knowledge produced in that country in the following years
[49–51]. Indeed, 1 − Ic indicates how much of the knowledge produced in a year in a country
flows, through citations, into the knowledge (publications) produced by other countries [52,
53]. A higher level of inwardness suggests that the knowledge produced by a country attracts
mainly the interest of the national community. By contrast, a lower level suggests that the
research of the country does not remain confined within its own borders but flows also toward
the rest of the world. It is important to stress that the inwardness, as such, has not an evaluative
connotation. The inwardness is a descriptive measure of the self-referentiality of a country in a
certain research area. It serves to provide a quantitative indicator of a phenomenon (the self-
referentiality), not to judge it.
As said above, the strategic use of citations, both as author self-citations and as citation
clubs, affects the country self-citations and, hence, also the inwardness indicator. The start of a
strategic use of citations at the country level should therefore be associated with an anomalous
rise of the inwardness indicator.
Recall, however, that inwardness is positively affected also by increases of international col-
laboration. It is therefore necessary to control the trend of the international collaboration
before concluding that an inwardness rise is due to strategic behaviors and not to an increase
of international collaboration.
Data
We retrieved the data for calculating the Inwardness indicator from SCIval, an Elsevier’s
owned platform powered by Scopus data (https://www.scival.com/home). The data were
exported from SCIval on October 16, 2018. They correspond to the last update on Scopus of
September 21, 2018. Data were retrieved in compliance with the terms of service of SCIval.
In particular, we exported from SCIval two metrics: (1) Citation Count including self-cita-
tions, and (2) Citation Count excluding self-citations. For both metrics, we included articles,
reviews, and conference papers, leaving aside other types of publications. The first Citation
Count metrics represents the countries’ total number of citations, whereas the countries’ num-
ber of self-citations was obtained as the difference between (1) and (2). Note that the SCIval’s
definition is binary and non-fractional: a citation can either be a self-citation or not [54]. The
weight of a country self-citation remains always 1, irrespective of the number of countries pro-
ducing the citing or the cited publications: if an Italian publication is cited by another Italian
publication, this self-citation will have the same weight as if the same publication was cited by
an international Italo-French-Chinese publication.
We retrieved the data for the G10 countries (Belgium-BE, Canada-CA, France-FR, Ger-
many-DE, Italy-IT, Japan-JP, the Netherlands-NL, Sweden-SE, Switzerland-CH, United King-
dom-GB, United States-US). In the years 2000-2016, the output of these countries
corresponded to 61.2% of the world output and they collected 95% of world citations. In order
to study the spread of the strategic behavior in different research areas, data were exported for
all the Scopus fields aggregated, i.e., without any filter for subject area, and for each of the 27
Scopus Main Categories (total number of datasets = 28), for the years 2000-2016 included. In
order to account for the effect of international collaboration on the inwardness indicators, we
retrieved from SCIval also the Percentage of International Collaboration metric for the target
countries. The percentage of international collaboration for a country in a given year is defined
as the share of publications of the country coauthored by at least one different country. The
graphs were implemented in R by using the package “ggplot2” [55].
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Citation gaming induced by bibliometric evaluation
Results
Fig 1 shows the trend of the inwardness over time for the eleven target countries (all Scopus
fields aggregated). All countries share a rather similar profile with apparent differences in the
absolute value. The ranking is partially explained by the size of the scientific production of the
countries. Countries with a large scientific output, such as the Unites States, naturally attract
more citations from their own production, simply because they have more citing and citable
articles than smaller countries such as Belgium. For all the countries under analysis, not only
the inwardness increases slowly and regularly over time, but the yearly ranks of countries
according to their inwardness are remarkably stable.
In this landscape, Italy stands out as a notable exception. In 2000, at the beginning of the
period, Italy has an inwardness of 20.62% and ranks sixth, just behind UK. In 2016, at the end
of the period, Italy ranks second, with an inwardness of 30.73%. Note that, until 2009, Italy’s
inwardness grows parallel to those of comparable countries (UK, Germany, France). However,
around 2010, the Italian trend shows a sudden acceleration. In the following six years, Italy
overcomes UK, Germany, and Japan, becoming the first European country and the second
one in the G10 group.
Table 1 shows the variations (deltas) of the inwardness for each country, for the whole
period and by considering two sub-periods, 2008-2000 and 2016-2008. Note that in the first
period, Italy’s increase is in line with other countries, while in the second period (2008-2016),
Italy’s exhibits the largest inwardness delta: 8.29 p.p., more than 4 p.p. above the G10 average
and almost 3 p.p. above Germany. As a result, Italy is by far the country with the highest
inwardness delta also in the whole period 2000-2016 (10.11 p.p. vs 5.22 of the G10 average).
However, as already said, inwardness is affected by the amount of International Collabora-
tion of a country. In order to allow for this effect, in Fig 2, inwardness is plotted against the
average international collaboration score of each country. More precisely, inwardness at year
Y is plotted against the three-years moving average value of international collaboration calcu-
lated starting from year Y. In fact inwardness at year Y depends also on citations coming from
publications appeared in the following years [56].
The data shows indeed a positive relation between the two variables: for all the countries,
inwardness grows with the average international collaboration. The plot shows a peculiar tra-
jectory for Italy. Although for most years Italy ranks last in Europe for international collabora-
tion (x-axis), nevertheless, at the end of the period, it is the first European country for
inwardness (y-axis). Before 2010, Italy is close to and moves together with a group of three
European countries, namely Germany, UK, and France. Starting from 2010, Italy departs from
the group along a steep trajectory, to eventually become the European country with the lowest
international collaboration and the highest inwardness.
Until now, we focused on the aggregated output of the target countries, without considering
the different research areas (Scopus Main Categories). In order to investigate whether and
how inwardness changes across research areas, we calculated the inwardness time series for
each of the 27 Scopus Main Categories. The time series, as well as the scatterplots of the
inwardness against the international collaboration, are fully provided in the Supplementary
Materials. For reasons of space, these data are summarized in Fig 3, where the variation of the
inwardness indicator in the periods 2000-2008 (A) and 2008-2016 (B) is displayed for each of
the 27 Scopus Categories. Italy shows a remarkable difference between the two periods. In the
first one (Fig 3A), before the university reform, Italy is in line with the other G10 countries in
most of the research fields. In the second period, after the reform (Fig 3B), Italy stands out
with the highest inwardness increase in 23 out of 27 fields. The only exceptions are earth and
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Citation gaming induced by bibliometric evaluation
Fig 1. Inwardness for G10 countries (2000-2016). Source: elaboration on SCIval data.
https://doi.org/10.1371/journal.pone.0221212.g001
planetary sciences (EPS), multidisciplinary (MUL), nursing (NUR), and physics and astron-
omy (PA).
As we show in the Supplementary Information (S1 Fig, 1-27), the inwardness increase is
not matched by a parallel increase of the international collaboration at the field level. In partic-
ular, at the end of the period, Italy is the European country with the lowest level of interna-
tional collaboration and the highest value of inwardness in the following Scopus Categories
(11 on 27): agricultural and biological sciences (ABS), biochemistry, genetics and molecular
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CHBENLSECAFRITGBDEJPUS102030402000200520102015YearInwardnessCitation gaming induced by bibliometric evaluation
Table 1. Inwardness delta. Delta is calculated as simple difference (p.p.) between the inwardness in the last and the
first year of the period.
Country
Belgium
Canada
France
Germany
Italy
Japan
Netherlands
Sweden
Switzerland
United Kingdom
United States
Mean G10 countries
Δ1 (2000-2008)
Δ2 (2008-2016)
Δtot (2000-2016)
1.42
1.04
1.57
1.69
1.82
0.6
2
0.94
0.94
1.45
0.14
1.24
3.29
3.43
2.68
5.47
8.29
3.2
3.54
3.32
3.32
4.4
2.87
3.98
4.72
4.46
4.25
7.17
10.11
3.81
5.54
4.27
4.27
5.85
3.01
5.22
https://doi.org/10.1371/journal.pone.0221212.t001
biology (BGMB), chemical engineering (CE), economics, econometrics and finance (EEF),
earth and planetary sciences (EPS), environmental science (ES), immunology and microbiol-
ogy (IM), pharmacology, toxicology and pharmaceutics (PTP), veterinary (VET). In other 9
Categories, Italy is first for inwardness but not the lowest for international collaboration: busi-
ness, management and accounting (BMA), computer science (CS), dentistry (DEN), decision
sciences (DS), engineering (ENG), health professions (HP), mathematics (MAT), materials sci-
ence (MS), psychology (PSY). Note that the Italian production in the arts and humanities
(AH) and social sciences (SOC) is only partially covered by Scopus as a large part is published
in books and in the national language. Therefore, the results about these scholarly areas should
be taken with great caution [57].
Discussion
As seen from Fig 1 and Table 1, Italy shows a different trend compared to the other G10 coun-
tries. The comparative analysis of the inwardness indicator showed that Italian research grew
in insularity in the years after the adoption of the new rules of evaluation. While the level of
international collaboration remained stable and comparatively low, the research produced in
the country tended to be increasingly cited by papers authored by at least an Italian scholar.
The anomalous trend of the inwardness indicator detected at the macro level can be
explained by a generalized change in micro-behaviours of Italian researchers induced by the
introduction of bibliometric thresholds in the national regulations for recruitment and career
advancement. Indeed, in 2011 research and careers evaluation were revolutionized by the
introduction of quantitative criteria in which citations played a central role. In particular, cita-
tions started being rewarded in the recruiting and habilitation mechanisms, regardless of their
source. This created an incentive to inflate those citation scores by means of strategic behav-
iors, such as opportunistic self-citations and the creation of citation clubs.
A possible objection to the above explanation is that, in order to postulate individual and
collective behaviors, the collection of evidence at the micro level is an indispensable step.
According to this objection, unless you draw on co-authorship networks, you should avoid
talking about citations clubs, citations cartels, and citation gaming. Evidence, for instance,
could be searched by checking the existence of groups of researchers frequently exchanging
citations, that are not directly co-authors but at a relatively small distance in a co-authorship
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Citation gaming induced by bibliometric evaluation
Fig 2. Inwardness versus average international collaboration for the G10 countries. The average international collaboration is the 3-year
moving average calculated starting from the considered year. The international collaboration is defined as the share of publications of a country
coauthored by at least a coauthor of a different country. Source: elaboration from SCIval data.
https://doi.org/10.1371/journal.pone.0221212.g002
network. Without this kind of micro level analysis, one could just record the increase of
inwardness as a response to the reformation of the Italian reward system, but should not haz-
ard an explanation at the micro level.
As a matter of fact, a simple argument, based on set theory, shows that the above objection
is unduly conservative. The set C of the country self-citations is the union of two sets (C = A [
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2000201620002016200020162000201620002016200020162000201620002016200020162000201620002016BECACHDEFRGBITJPNLSEUS10203040203040506070Average International CollaborationInwardnessCitation gaming induced by bibliometric evaluation
Fig 3. Inwardness delta in Scopus Main Categories in the periods 2000-2008 (A) and 2008-2016 (B).
ABS = Agricultural and Biological Sciences, AH = Arts and Humanities, BGMB = Biochemistry, Genetics and
Molecular Biology, BMA = Business, Management and Accounting, CE = Chemical Engineering, CHEM = Chemistry,
CS = Computer Science, DEN = Dentistry, DS = Decision Sciences, E = Energy, EEF = Economics, Econometrics and
Finance, ENG = Engineering, EPS = Earth and Planetary Sciences, ES = Environmental Science, HP = Health
Professions, IM = Immunology and Microbiology, MAT = Mathematics, MED = Medicine, MS = Materials Science,
MUL = Multidisciplinary, NEU = Neuroscience, NUR = Nursing, PA = Physics and Astronomy, PSY = Psychology,
PTP = Pharmacology, Toxicology and Pharmaceutics, SOC = Social Sciences, VET = Veterinary. Source: elaboration
from SCIval data.
https://doi.org/10.1371/journal.pone.0221212.g003
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Citation gaming induced by bibliometric evaluation
B): the self-citations A generated by country-based researchers as a normal byproduct of the
research activity and the self-citations B resulting from strategic activities, including both
opportunistic self-citation and country-based citation clubs. Put in other words, the set A is
the “physiological” quota of country self-citations, whereas B is the “pathological” quota. An
increase of the inwardness indicator is, by definition, an increase of the cardinality of the set C
of the country self-citations. There are two possible explanations for that increase: (i) the cardi-
nality of A has increased, i.e. the physiological quota A of country self-citations has increased;
or (ii) the cardinality of B has increased, i.e. the pathological quota B of country self-citations
has increased.
Two explanations for an increase of cardinality of physiological quota A could be advanced.
According to the first one, internationalization, the increase may be due to a sudden rise, after
2009, of the amount of international collaborations of Italian scholars. In fact, we have already
observed that, other things left unchanged, an increase of international collaboration positively
affects the inwardness indicator. However, Fig 2 rules out this explanation. No peculiar
increase in the Italian international collaboration can be spotted.
The second explanation, specialization, is a narrowing of the scientific focus of Italian
researchers, i.e. a dynamic of scientific specialization leading to the growth of author self-cita-
tions [32]. The idea is that focusing on narrower topics results in a contraction of the scientific
community of reference. Thus, the number of citable papers would diminish and the chances
for author self-citation would correspondingly increase, generating also the growth of the
country self-citations. Although we do not have direct evidence falsifying the specialization
hypothesis, nonetheless, this explanation appears largely implausible. Indeed, it implies that
Italian researchers in all fields suddenly narrowed their focus to topics mainly investigated in
the national community. This sudden change would be not only peculiar of Italy, but also so
strong as to make the Italian inwardness diverge from those of the other G10 countries. Nota-
bly, Fig 3 shows that the Italian post-2008 acceleration is visible in most of the research areas.
Not only the change has been widespread, regarding most research fields, but in some of them,
such as engineering (ENG), mathematics (MAT) or veterinary (VET), the increase reached
outstanding proportions. In any case, it would still be necessary to explain why a physiological
specialization occurred only in Italy and at the same time as the adoption of new rules for
evaluation.
Summing up, we have no plausible reasons in favor of a notable change in the physiological
quota A of country self-citations, sufficient to explain the anomalous boost of inwardness with
respect to the other G10 countries. Recalling that C = A [ B, the only alternative explanation
of the change in the cardinality of C is a notable expansion of the pathological set B of country
self-citations, i.e., an increase of author self-citations and an increase of citations exchanged
within citation-clubs formed by Italian scholars, aimed at boosting bibliometric indicators set
by ANVUR.
The slight discrepancy between the starting year of the inwardness acceleration and the
launch of bibliometric evaluation system, with the former occurring slightly earlier than the
latter, is easily explained by the “backward effect” typical of citation measures. Any change in
the citation habits taking place in a given year produces a backward effect on the citation
scores of the previous years because researchers cite previously published papers, so that the
change reverberates also on the citation scores of the past production. Citations received by
the most recent articles have a more lasting effect in the calculations of forthcoming indicators.
It is therefore more convenient to self-cite one’s own recent production rather than the remote
one. Hence, a strategic reaction to rules introduced in year 2011 is expected to produce an
inwardness acceleration that starts a few years before, just as observed for Italy.
PLOS ONE | https://doi.org/10.1371/journal.pone.0221212 September 11, 2019
12 / 16
Citation gaming induced by bibliometric evaluation
Conclusions
In this paper, we contributed to the empirical study of the constitutive effects that indicator-
based research evaluation systems have on the behavior of the evaluated researchers. By focus-
ing on the Italian case, we investigated how the Italian scientific community responded, at the
national level, to the introduction of a research evaluation system, in which bibliometric indi-
cators play a crucial role. Our results show that the behavior of Italian researchers has indeed
changed after the introduction of the evaluation system following the 2010 university reform.
Such a change is visible at a national scale in most of the scientific fields. The comparative analy-
sis of the inwardness indicator showed that Italian research grew in insularity in the years after
the adoption of the new rules of evaluation. While the level of international collaboration
remained stable and comparatively low, the research produced in the country tended to be
increasingly cited by papers authored by at least an Italian scholar.
We explained this as the result of the pervasively adoption of strategic citation behaviors
within the Italian scientific community. Even if they escape a direct observation, we argue that
such behaviors are the most likely explanation of the peculiar trend exhibited by the Italian
inwardness. This because our indicator was especially designed to be sensible to the effects of
both the opportunistic use of author self-citation and the creation of citation clubs.
We believe that three main lessons can be derived from the Italian case. Firstly, our results
support the claim that scientists are quickly responsive to the system of incentives in which they
act [32]. Thus, any policy aiming at introducing or modifying such a system should be
designed and implemented very carefully. In particular, considerable attention should be
placed on the constitutive effects of bibliometric indicators. They are not neutral measures of
performance but actively interact and quickly shape the behavior of the evaluated researchers.
Secondly, our results show that the “responsible use” of metrics would not be enough to
prevent the emergence of strategic behaviors. For instance, the Leiden Manifesto recommends
the use of a “suite of indicators” instead of a single one as a way to prevent gaming and goal
displacement (see the principle number 9 in [13]). The Italian case shows that, even if the
researchers are evaluated against multiple indicators, as recommended, strategic behaviors
manifest themselves anyway.
Lastly, our results prompt some reflections on the viability of the mixed evaluation systems,
in which the indicators are intended for complementing or integrating the expert judgment
expressed by the peer review. In fact, the Italian system was designed in principle according to
such a mixed approach, both for the research assessment exercises where research products
were evaluated by bibliometric indicators or by peer reviewers, and for the ASN where to over-
come bibliometric thresholds is but a necessary condition for being admitted to the final evalu-
ation by habilitation committees. Nonetheless, our results show that the mere presence of
bibliometric indicators in the evaluative procedures is enough to structurally affect the behav-
ior of the scientists, fostering opportunistic strategies. Therefore, there is the concrete risk that
in mixed evaluation systems, the indicator-based component overcomes the peer review-based
one. Hence, they de facto collapse to indicator-centric approaches. We believe that further
research is needed to better understand and fully appreciate the possibility of such a collapse.
In the meantime, we suggest that policy makers should exercise the most extreme caution in
the use of indicators in science policy contexts.
Supporting information
S1 Fig. 1-27—Inwardness over time (left) and inwardness vs average international collabo-
ration (right) for the G10 countries in each of the Scopus Main Categories.
(PDF)
PLOS ONE | https://doi.org/10.1371/journal.pone.0221212 September 11, 2019
13 / 16
Citation gaming induced by bibliometric evaluation
S1 File. 1-28—Zipped CSV files for the G10 countries. Files 1-27: data for each Scopus Cate-
gory; File 28 data for all the Scopus Main Categories aggregated.
(ZIP)
Acknowledgments
This work was supported by Institute For New Economic Thinking Grant ID INO17-00015.
Author Contributions
Conceptualization: Alberto Baccini, Giuseppe De Nicolao, Eugenio Petrovich.
Data curation: Alberto Baccini, Eugenio Petrovich.
Formal analysis: Alberto Baccini, Giuseppe De Nicolao, Eugenio Petrovich.
Funding acquisition: Alberto Baccini, Giuseppe De Nicolao.
Investigation: Alberto Baccini, Giuseppe De Nicolao, Eugenio Petrovich.
Methodology: Alberto Baccini, Giuseppe De Nicolao, Eugenio Petrovich.
Visualization: Eugenio Petrovich.
Writing – original draft: Alberto Baccini, Giuseppe De Nicolao, Eugenio Petrovich.
Writing – review & editing: Alberto Baccini, Giuseppe De Nicolao, Eugenio Petrovich.
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| null |
10.1371_journal.pntd.0011960.pdf
|
Data Availability Statement: The authors confirm
that all data underlying the findings are fully
available without restriction. All relevant data are
within the paper and its Supporting information
files.
|
The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper and its Supporting information files.
|
RESEARCH ARTICLE
Altered IL-7 signaling in CD4+ T cells from
patients with visceral leishmaniasis
Shashi Kumar1, Shashi Bhushan Chauhan2, Shreya Upadhyay1, Siddharth Sankar Singh3,
Vimal Verma1, Rajiv Kumar4☯*, Christian Engwerda5☯, Susanne Nyle´ n6☯*,
Shyam SundarID
1☯*
1 Department of Medicine, Institute of Medical Sciences, Banaras Hindu University, Varanasi Uttar Pradesh
India, 2 School of Medicine & Health Sciences, The George Washington University, Washington,
Washington, United States of America, 3 University of Massachusetts Chan Medical School, Shrewsbury,
Massachusetts, United States of America, 4 Centre of Experimental Medicine and Surgery, Banaras Hindu
University, Varanasi, India, 5 QIMR Berghofer Medical Research Institute, Brisbane, Australia, 6 Department
of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden
☯ These authors contributed equally to this work.
* [email protected] (RK); [email protected] (SN); [email protected] (SS)
a1111111111
a1111111111
a1111111111
a1111111111
a1111111111
Abstract
OPEN ACCESS
Citation: Kumar S, Chauhan SB, Upadhyay S,
Singh SS, Verma V, Kumar R, et al. (2024) Altered
IL-7 signaling in CD4+ T cells from patients with
visceral leishmaniasis. PLoS Negl Trop Dis 18(2):
e0011960. https://doi.org/10.1371/journal.
pntd.0011960
Editor: Abhay R Satoskar, Ohio State University,
UNITED STATES
Received: September 1, 2023
Accepted: February 1, 2024
Published: February 26, 2024
Copyright: © 2024 Kumar et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: The authors confirm
that all data underlying the findings are fully
available without restriction. All relevant data are
within the paper and its Supporting information
files.
Funding: This work is supported by National
Institutes of Health-Tropical Medicine Research
Centre Grant (grant no. 2U19AI074321) to SS, by
Indian Council of Medical Research (grant no.
2020-9898) to RK and SS, by Banaras Hindu
University- Institute of Eminence (IoE) grant to RK
Background
CD4+ T cells play a central role in control of L. donovani infection, through IFN-γ production
required for activation of macrophages and killing of intracellular parasites. Impaired control
of parasites can in part be explained by hampered CD4+ T cells effector functions in visceral
leishmaniasis (VL) patients. In a recent studies that defined transcriptional signatures for
CD4+ T cells from active VL patients, we found that expression of the IL-7 receptor alpha
chain (IL-7RΑ; CD127) was downregulated, compared to CD4+ T cells from endemic con-
trols (ECs). Since IL-7 signaling is critical for the survival and homeostatic maintenance of
CD4+ T cells, we investigated this signaling pathway in VL patients, relative to ECs.
Methods
CD4+ T cells were enriched from peripheral blood collected from VL patients and EC sub-
jects and expression of IL7 and IL7RA mRNA was measured by real time qPCR. IL-7 signal-
ing potential and surface expression of CD127 and CD132 on CD4+ T cell was analyzed by
multicolor flow cytometry. Plasma levels of soluble IL-7 and sIL-7Rα were measured by
ELISA.
Result
Transcriptional profiling data sets generated previously from our group showed lower IL7RA
mRNA expression in VL CD4+ T cells as compared to EC. A significant reduction was, how-
ever not seen when assessing IL7RA mRNA by RT-qPCR. Yet, the levels of soluble IL-7Rα
(sIL-7Rα) were reduced in plasma of VL patients compared to ECs. Furthermore, the levels
of soluble IL-7 were higher in plasma from VL patients compared to ECs. Interestingly,
expression of the IL-7Rα protein was higher on VL patient CD4+ T cells as compared to EC,
with activated CD38+ CD4+ T cells showing higher surface expression of IL-7Rα compared
PLOS Neglected Tropical Diseases | https://doi.org/10.1371/journal.pntd.0011960 February 26, 2024
1 / 16
PLOS NEGLECTED TROPICAL DISEASESand SS. The funders had no role in study design,
data collection and analysis, decision to publish, or
preparation of the manuscript.
Competing interests: The authors have declared
that no competing interests exist.
IL-7 signaling in CD4+ T cells of VL patients
to CD38- CD4+ T cells in VL patients. CD4+ T cells from VL patients had higher signaling
potential baseline and after stimulation with recombinant human IL-7 (rhIL-7) compared to
EC, as measured by phosphorylation of STAT5 (pSTAT5). Interestingly, it was the CD38
negative cells that had the highest level of pSTAT5 in VL patient CD4+ T cells after IL-7 stim-
ulation. Thus, despite unaltered or potentially lowered IL7RA mRNA expression by CD4+ T
cells from VL patients, the surface expression of the IL-7Rα was higher compared to EC and
increased pSTAT5 was seen following exposure to rhIL-7. Accordingly, IL-7 signaling
appears to be functional and even enhanced in VL CD4+ T cells and cannot explain the
impaired effector function of VL CD4+ T cells. The enhanced plasma IL-7 may serve as part
of homeostatic feedback mechanism regulating IL7RA expression in CD4+ T cells.
Author summary
In visceral leishmaniasis (VL), antigen specific CD4+ T cell responses are muted hindering
the control of the Leishmania donovani infection. IL-7 signaling is crucial for CD4+ T cell
survival and function, and gene expression analysis indicated that the IL-7 pathway could
be altered in VL. Thus, we investigate if impaired IL-7 signaling could explain the loss of
antigen specific T cell response in VL. Although we didn’t observe significant reduction of
IL7RA mRNA by RT-qPCR, yet, the levels of soluble IL-7Rα (sIL-7Rα) were reduced in
plasma of VL patients compared to ECs. Furthermore, the levels of soluble IL-7 were
higher in plasma from VL patients compared to ECs and their CD4+ T cells exhibited
heightened IL-7 receptor protein expression. Surprisingly, VL patient CD4+ T cells
showed increased IL-7 signaling potential, as evidenced by higher phosphorylation of
STAT5 upon IL-7 stimulation. While altered, the findings presented here do not attrib-
uted the impaired effector function of VL CD4+ T cells to defective IL-7 signaling. We
speculate that the elevated plasma IL-7 is part of a homeostatic feedback mechanism in
response to the reduced IL7RA transcription in CD4+ T cells.
Introduction
Leishmaniasis are a parasitic disease caused by protozoan parasites of the Leishmania genus.
All Leishmania spp are transmitted through the bite of infected female Phlebotomine sandflies.
The disease can manifest in different ways, from life threating visceral leishmaniasis (VL) to
localized cutaneous disease depending on the species of Leishmania involved. To date, around
20 different species of Leishmania have been identified. Each year, there are approximately 50
000–90 000 new cases of VL [1], with most cases coming from Brazil, Ethiopia, India, South
Sudan, and Sudan. The most frequent manifestation of VL is anemia, and early symptoms may
also include leucopenia [2,3]. Other clinical symptoms of VL include prolonged fever, enlarged
spleen and liver, weight loss, and polyclonal hypergammaglobulinemia (IgG and IgM) [4].
CD4+ T-helper (Th) cells are central in orchestrating immune responses against Leishmania
parasites. Specifically, T-bet+ CD4+ T cells (Th1 cells), play a crucial role in controlling Leish-
mania infection, by production of IFN-γ leading to activation of macrophages and killing of
intracellular parasites [5]. However, CD4+ Th cells also play a role in regulating the balance
between pro-inflammatory and anti-inflammatory responses, and regulatory cytokines such as
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PLOS NEGLECTED TROPICAL DISEASESIL-7 signaling in CD4+ T cells of VL patients
IL-10 which are critical for controlling the immune response also inhibit macrophage func-
tions and facilitate Leishmania infection [6,7].
IL-7 is produced by nonhematopoietic cells (e.g. stromal cells, bone marrow- mesenchymal
stem cells, keratinocytes, neurons, epithelial cells, and hepatocytes) as well as dendritic cells,
and plays a crucial role in supporting hematopoiesis [8]. T cells rely on this cytokine for their
development, survival, and memory formation. Production of IL-7 is stimulated by various
factors, including inflammation, tissue damage, and immune cell interactions [9]. IL-7 exerts
its effects by binding to the IL-7 receptor (IL-7Rα), a heterodimer composed of a high-affinity
CD127 (IL-7Rα) subunit and the common cytokine gamma-chain CD132. The latter is also
used by other cytokines including IL-2, IL-4, IL-9, IL-15, and IL-21 [10]. Upon the binding of
IL-7 to the IL-7Rα, a series of intracellular signaling events are triggered. The exact signaling
pathway varies depending on cell type, but generally involves activation of Janus kinases
(JAKs) and signal transducer and activator of transcription (STAT) proteins. The JAKs phos-
phorylate tyrosine residues on IL-7Rα, creating docking sites for STAT5 and phosphorylation
of the STAT5 protein, which then as a homodimer translocate to the nucleus to modulate gene
expression, thus phosphorylated STAT5 (pSTAT5) is often used to assess IL-7 signaling capac-
ity. The activity of IL-7 is tightly regulated to maintain immune cell homeostasis. Negative reg-
ulators, such as suppressor of cytokine signaling (SOCS) proteins and protein inhibitors of
activated STATs (PIAS), help to dampen IL-7 signaling and prevent excessive immune
responses [11]. Increased expression of IL-7Rα on naive (TN) and memory (TM) T cells aids
in the clearance of excess soluble IL-7 [12]. Once the peripheral T cell pool reaches a critical
size, a balance is achieved between IL-7 consumption and production, preventing the survival
of additional T cells and maintaining T cell homeostasis [12–14]. Administration of IL-7 can
potentially enhance the function of immune cells and allow a larger lymphocyte pool to
develop in vivo, and when used as an adjuvant in immunizations, IL-7 has been shown to
improve long-term, antigen-specific T cell responses [15], However, dysregulation of the IL-7
pathway can contribute to the development of cancer [16].
In a previous studies, we observed a decrease in IL7RA expression in CD4+ T cells from
individuals with VL compared to ECs [17,18], which suggested that IL-7 signaling could be
impaired in VL patients. To gain a better understanding of the role of IL-7 in VL patients and
if IL-7 played a role in VL pathogenesis and CD4+ T cell dysfunction, we analyzed mRNA and
protein expression of IL-7 and IL-7Rα, and the ability of the IL-7 receptor to signal in PBMC
and CD4+ T cells from VL patients and ECs.
We found a divergence between the IL7RA mRNA and protein expression, with an increase
in IL-7Rα surface protein on VL patient CD4+ T cells compared to ECs. The levels of IL-7
were higher, while the levels of soluble IL-7Rα were lower in VL patient serum, compared to
ECs. Moreover, activation of VL patient PBMCs showed that IL-7 signaling was functional
and even enhanced in VL patient CD4+ T cells. In conclusion, while our data show clear differ-
ences between VL patients and ECs in regard to IL-7 and IL-7Rα levels, we cannot explain the
impaired CD4+ T cell responses seen in VL patients by lack of IL-7 or its signaling capacity.
Methods
Ethics statements
All experiments were performed in accordance with the Helsinki declaration for use of human
subjects in research and approval from the ethical committee of Institute of Medical Science,
Banaras Hindu University-India (ethical approval No. Dean/2019/EC/1001 Dated 18/01/
2019). All the participants provided written informed consent and in case of children consent
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PLOS NEGLECTED TROPICAL DISEASESIL-7 signaling in CD4+ T cells of VL patients
was obtained from their parents or legal guardian. All subjects selected were human immuno-
deficiency virus negative and above 12 years of age.
Research subjects
The following groups were included in this study: VL patients before treatment (VL, n = 128)
and 30 days post treatment (VL D30) (n = 13) and endemic healthy control (ECs) subjects
(n = 104) [19,20]. All donors were recruited from the Kala-Azar Medical Research Center
(KAMRC), Muzaffarpur, Bihar India. The numbers of individuals indicated for each group are
the total numbers included in the study, the number of donors included in each experiment is
indicated in the figure legend. There was no intentional selection of the donors included in
each experiment; this was based on the order of which the experiments were done and the
patients available at the time. Diagnosis of VL was made based on clinical symptoms consistent
with VL and detection of anti-leishmanial antibodies in serum by recombinant K39-test and/
or detection of amastigote in bone marrow/splenic aspirates by microscopy [21,22]. Clinical
data from the patients are summarized in Table 1.
ECs were recruited from people accompanying VL patients to the clinic.
Venous blood was collected from the patients and controls into heparinized tubes. Plasma
was separated by centrifugation at 770 g for 10 minutes and stored at -80˚C till further use.
The plasma was replaced by PBS and PBMC were isolated by density gradient separation using
Lymphoprep (STEMCELL Technologies).
Details of all reagents used in this study are described in S1 Table.
Table 1. Demographic and clinical information on study participants.
Variables
Age, Year
Mean ± SD
Median
Sex, no
Male
Female
Illness Duration, days
Mean ± SD
Median
Haemoglobin level, mg ml-1
Mean ± SD
Median
WBC, x103 cells mm-3
Mean ± SD
Median
Splenic enlargement, cm
Mean ± SD
Median
ND, Assay not done
https://doi.org/10.1371/journal.pntd.0011960.t001
EC Group
(n = 104)
33.6 ± 12.1
35.0
39
65
NA
14.26 ± 1.2
14.5
8941 ± 926
9020
NA
VL D0 Group
(n = 128)
32.5 ± 13.8
35.0
49
80
31.4 ± 23.5
30.0
8.8 ± 1.5
9.2
3338 ± 1497
3300
6.0 ± 2.7
7.0
VL D30 Group
(n = 13)
24.0 ± 14.4
22.0
5
8
9.9 ± 1.4
10.3
7961 ± 2199
8800
0
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Gene expression for IL7RA and IL7, mRNA
CD4+ cells were enriched from freshly isolated PBMCs by positive selection using anti-human
CD4 magnetic Microbeads (Miltenyi Biotec) and MS columns according to the manufacturer’s
protocol (Miltenyi Biotec). The enriched CD4+ cells were 99% CD4+ T cell as analyzed by
FACS. After washing the cell pellets, they were stored in RLT buffer at -80˚C. Total RNA was
isolated from both PBMCs and CD4+ cells using the Qiagen RNeasy mini kit following the
manufacturer’s instructions. A high-capacity cDNA Reverse Transcription Kit (Thermo-
Fisher) was used to reverse transcribe 1000 ng of RNA according to the manufacturer’s
instructions. TaqMan-based gene expression assays were performed for IL7RA, IL7 mRNA
targets and 18s ribosomal RNA (rRNA) using 7500 real-time PCR. For each donor, the mean
cycle threshold value from duplicated qPCR tests was used to calculate the relative quantifica-
tion (2-ΔCt) as follows:
ð
DCt ¼ Ct target gene
Þ (cid:0) Ct 18S rRNA
ð
Þ
DDCt ¼ DCt Sample
ð
Þ (cid:0) DCt ECmean
ð
Þ
Expression ratio ¼ 2(cid:0) DDCt
As indicated above, the 18S rRNA expression was for internal normalization of each sam-
ple. The mean ΔCt of all EC samples (ECmean) was used to calculate the fold change between
the individual sample and the ECmean, making the spread of samples within the VL and EC
groups visible. For each amplification, 25 μg of cDNA was used, each amplification tube con-
taining a mixture of 5 μl of cDNA (5 ng/μl), 1μl of primer/probe, 4 μl of MilliQ, and 10 μl of
TaqMan master mix (Applied Biosystems, Foster City, CA, USA).
Measurement of soluble IL-7 and IL-7Rα in plasma
Plasma samples were thawed at the time of ELISA analysis and diluted two-fold with Assay
diluent A (provided in the kit) and concentrations of IL-7 and sIL-7Rα were measured in
duplicate using the IL-7 Human ELISA kit (invitrogen) and the Human CD127 ELISA kit
(abcam), respectively, following the manufacturer’s protocol. The standard curves were gen-
erated using recombinant protein provided by the manufacturer and a 4-parametric logistic
regression in SoftMax Pro software (version 3.1.2) to calculate the concentrations of IL-7
and sIL-7Rα.
Phenotypic expression of IL-7Rα by PBMC staining
For analysis of surface expression, 5 x 105 PBMCs from VL and ECs were used. Briefly,
PBMCs were washed with staining buffer (PBS, 5% heat inactivated fetal calf serum) and
stained with viability dye Zombie aqua at room temperature for 20 minutes. After washing,
surface staining was performed with fluorochrome labelled antibodies against CD3ε, CD4,
CD127, CD25, CD45RA, CD185 (CXCR5), CD194 (CCR4), CD196 (CCR6), CD197 (CCR7),
CD183 (CXCR3), CD38, and CD132, for 30 minutes at 4˚C in the dark. Following washing,
the cells were re-suspended in staining buffer, and acquired on a flow cytometer (BD LSRFor-
tessa) using FACS Diva software (version 8.0.2). Flow Jo version 10 software (Tree Star, BD)
was used to analyze the FACS data. CD4+ T cell subsets were defined as Treg (CD25+,
CD127-), Tem (CD45RA+/-, CCR7-), Th (Tem—CXCR5), Th17_Th22 (CCR6+, CCR4+),
Th1_Th2 (Th—CCR6, CCR4+/-), Th1 (Th1_Th2—CCR4, CXCR3+), Th2 (Th1_Th2—CXCR3
CCR4+), Th9 (Th—CCR4, CCR6+) [23–26]. CD4+ T cell subsets were defined as previously
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PLOS NEGLECTED TROPICAL DISEASESIL-7 signaling in CD4+ T cells of VL patients
reported by us and others (References [18,23–26]), and the gating strategy employed was as we
previously described (references [18,25]). Details of all antibodies used in this study are
described in S2 Table.
Signaling potential of IL-7 and IL-7Rα
To assess STAT5 phosphorylation (pSTAT5), as indicative of IL-7Rα signaling capacity in
CD4+ T cell subsets, freshly isolated heparinized whole blood (200μl) was first surface stained
for 5 minutes at room temperature. Thereafter, the samples were stimulated with rhIL-7 (200
ng/ml) for 5 minutes or left unstimulated at 37˚C, 5% CO2. 200 ng/ml of rhIL-7 was sufficient
to induce phosphorylation of STAT5 in maximum CD4+ T cells [27]. Phosphorylation of
STAT5 was detected using BD Phosflow, according to manufacturer’s instructions. Briefly, the
cells were fixed directly after completion with Lyse/Fix Buffer for 7 min at 37˚C in a water
bath. After washing, the cells were gently vortexed to loosen and permeabilized by using
chilled BD Phosflow Perm Buffer III for 30 minutes on ice. The cells were washed twice and
stained for intracellular pSTAT5 for 60 minutes at room temperature in the dark, with gentle
vortexing every 15 minutes. After washing, the cells were re-suspended in staining buffer
acquired on flow cytometer (BD LSRFortessa) using FACS Diva software version 8.0.2.
Statistical analysis
Statistical analysis was performed using Excel (Microsoft) and GraphPad Prism 8.01 software
(Graph Pad Software, La Jolla. CA, USA). Analysis of cellular assays and qPCR was performed
using nonparametric Kruskal-Wallis test for multiple groups and with a post test to see
between which groups differences exist and Mann-Whitney U-test for comparison between
two groups. Wilcoxon signed-rank test was used to compare matched sample pairs. SPICE
analysis was performed using SPICE version 5.3 (M. Roeder, Vaccine Research Centre,
National Institutes of Allergy and Infectious Diseases, National Institutes of Health, USA;
http://exon.niaid.nih.gov) [28]. The data are presented as mean ± SEMs. P-values less than
0.05 were considered statistically significant. Outliers were defined by the ROUT method,
alpha 0.05 and removed from analysis.
Results
Decrease in soluble sIL-7α and increase in IL-7 plasma protein levels in VL
patient
Aberrant expression of IL-7 and soluble IL-7Rα in plasma is indicative of pathological T cell
immunity in chronic viral, inflammatory, and autoimmune diseases. Using data from tran-
scriptional profiling [17] and NanoString mRNA expression analysis [18], we observed down-
regulation of IL7RA mRNA in CD4+ T cells from patients with visceral leishmaniasis (VL)
compared to endemic healthy individuals (ECs) (Fig 1A, extracted from [17] and as previously
reported [18]). Lower IL7RA were also previously reported in VL CD4+ T cell pretreatment as
compared to post treatment [18]. To confirm this finding, we analyzed the mRNA expression
of IL7RA in CD4+ cells and PBMCs from VL patients using real-time qPCR. Our results did
not show any significant difference in expression of IL7RA in PBMCs or CD4+ cells between
VL patients and ECs (Fig 1B). However, in line with the transcriptional profiling data, when,
we measured the levels of sIL-7Rα (CD127) in plasma we found that patients infected with L.
donovani, both active infection (D-0) and 30 days post treatment (D-30) had significantly
lower levels of sIL-7Rα compared to ECs (p<0.0001) (Fig 1C). Combined, the data indicate an
aberrant expression of the IL-7 receptor in VL patients.
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Fig 1. IL7RA expression in VL. A. Volcano plot of immune-related genes in peripheral blood CD4+ T cell data extracted from a previously published
dataset [17]. Analysis of differentially expressed immune-related genes in peripheral blood CD4+ T cells between visceral leishmaniasis (VL, n = 12)
patients prior to treatment (D0) and endemic controls (EC, n = 12) shows downregulation of IL7RA. B. Relative expression of IL7RA determined by
RT-qPCR in PBMC and CD4+ T cells, as indicated, each dot represents one sample PBMC (EC n = 11, VL n = 13), CD4 (EC n = 11, VL n = 15). C.
Soluble IL-7Rα plasma levels in EC (n = 8) and VL before (D-0, n = 13) and 30 days (D-30 n = 13) after initiation of drug treatment. Statistical
significance was determined by Kruskal-Wallis with multiple comparison follow-up test for Fig 1C and are indicated as *p<0.05; **p<0.01;
***p<0.001; ****p<0.0001.
https://doi.org/10.1371/journal.pntd.0011960.g001
PBMC and CD4+ cells are not a major source of IL-7, and analysis of IL7 mRNA expression
did not show any differences between IL7 mRNA between VL and EC cells (Fig 2A). Surpris-
ingly, analysis of soluble IL-7 in plasma demonstrated that the IL-7 levels were significantly
higher in active, VL compared to EC (p<0.001) (Fig 2B). Following treatment of VL, we
observed a decrease in the level of soluble IL-7 in plasma (VL D0 (n = 14), 40.13 pg/ml ±19.79
SEM, VL D30 (n = 14) 19.8pg/ml ±12.85 SEM, with P<0.05).
Fig 2. IL-7 expression and secretion following Leishmania donovani infection. A. Relative expression of IL7RA
determined by RT-qPCR in PBMC and CD4+ T cells, as indicated, each dot represents one sample, PBMC (EC n = 11,
VL n = 13), CD4 (EC n = 12, VL n = 13). Median range is depicted. B. IL-7 levels in the plasma of VL patients (n = 10),
ECs (n = 10), as determined by ELISA. Statistical significance was determined by Mann-Whitney U-test for Fig 2A,
and Kruskal-Wallis with multiple comparison follow-up test for Fig 2B and are indicated as *p<0.05; **p<0.01;
***p<0.001; ****p<0.0001.
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Fig 3. Surface protein expression of CD127 and CD132 on CD4+ T cells from endemic controls (ECs) and visceral leishmaniasis (VL) patients. A.
Gating strategy for CD4+ T cell flow cytometry analysis. B. Percentage (top) and mean fluorescent intensity (MFI) (bottom) of CD127 on CD4+ T cells.
C. Percentage (top) and MFI (bottom) of CD132 on CD4+ T cells. D. Percentage (top) and MFI (bottom) of CD38 on CD4+ T cells. Statistical
significance between ECs (n = 11) and VL patients (n = 12) was determined by Mann-Whitney U-test in Fig 3B-D, and 3F are indicated as *p<0.05;
**p<0.01. E. Boolean-Gating (FlowJo), was used to define complex cell sub-population, and Simplified presentation of incredibly complex evaluations
(SPICE) polyfunctionality analysis. SPICE was used to establish overlap in expression of CD38, CD127 and CD132 on CD4+ T cells. Pie charts represent
the entire CD4+ T cell population expressing either CD38, CD127, CD132 or none of them. Data was generated from EC (n = 7) and VL patients
(n = 7). F. Percentage and MFI of CD127, and CD132, expression on CD38+/- CD4 T+ cells of VL (n = 7) and EC (n = 7).
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The IL7 receptor is upregulated on VL CD4+ T cells compared to EC
Next, we conducted an analysis of the surface expression of the IL-7 receptors (CD127 and
CD132) on CD4+ T cell subsets (as defined in Fig 3A), and observed an upregulation of
CD127 and CD132 on VL CD4+ T cells (Fig 3B and 3C). Additionally, we detected an increase
in activated CD4+ T cells in VL patients based on the expression of CD38 (Fig 3D). The acti-
vated CD38+ CD4+ T cells from VL patients expressed higher levels of IL-7Rα, as demon-
strated by the higher MFI of CD127 compared to EC and by SPICE analysis (Fig 3D, 3E and
3F) and bar graph (S1 Fig).
A detailed analysis of IL-7Rα expression on VL patient (D0) CD4+ T cells subsets (Fig 4B–
4H), (S2 Fig) showed that most Th subsets from VL patients express more, while Th2 and
Th17 had similar levels of CD38, CD127 and CD132 compared to ECs.
VL patient CD4+ T cells respond to IL-7 stimulation
To test if IL-7 signaling was affected in VL patients we stimulated whole blood with rhIL-7 and
measured phosphorylation of STAT5 (pSTAT5), as indicative of IL-7 signaling capacity (Fig
5). Increase in pSTAT5 was most evident in the CD4+ T cells, relative to other lymphocyte sub-
sets (S3 Fig). Upon rhIL-7 stimulation pSTAT5 was more noticeable in VL patient as com-
pared to EC CD4+ T cells, seen both as frequency of cells positive for pSTAT5 (Fig 5B and 5C)
and MFI of pSTAT5 (Fig 5D). Baseline levels (unstimulated cells) of pSTAT5 were higher in
VL patient CD4+ T cell compared to ECs (Fig 5D), in line with reported higher STAT5 mRNA
levels in VL compared to EC CD4+ cells in our previous Nano-String mRNA expression analy-
sis [18].
In VL CD4+ T cells, pSTAT5 frequency and MFI was increased in all the CD4+ Th cell sub-
sets defined upon rhIL-7 stimulation (S4A and S4B Fig). Stimulation with rhIL-7 increased
pSTAT5 in CD4+ T cell subsets from ECs, but never reached the levels observed in VL patient
CD4+ T cells (S4A and S4B Fig). To test if IL-7 signaling was linked to activation of T cells, we
next examined the IL-7 signaling potential in activated and non-activated CD4+ T cells using
CD38 as a marker of activation (Fig 6A). In VL patient CD4+ T cells pSTAT5 staining was
notably stronger in non-activated CD38- compared to activated CD38+ CD4+ T cells (Fig 6B
and 6C), while no differences in pSTAT5 were seen between CD38- and CD38+ CD4+ T cells
in ECs (Fig 6C and 6D).
Discussion
Lymphopenia and an inability to mount adequate T cell responses contribute to immunosup-
pression and disease progression in VL patients. Deficiencies in IL-7 signaling have been
linked to other chronic diseases such as HIV, and IL-7 therapy has been suggested to improve
T cell survival in these patients [29]. Similar to observation made in HIV patients, previous
studies by Chauhan et al. [18] and Kumar et al. [17] found downregulation of IL7RA (CD127)
in T cells from VL patients compared to ECs [18]. We could not confirm the downregulation
in the set of samples included in our analysis here, as no difference in mRNA expression of
IL7RA between VL patient CD4+ T cells and PBMCs, relative to the same cell populations in
ECs was observed. The lack of correlation between transcriptional data and RT-qPCR data,
was unexpected but may be explained by the use of different individuals in the different assays
combined with that the differences in RNA seq and Nanostring results seen between groups
were not the most pronounced (logFC -1.39 and -0.719 respectively). However, our analysis of
plasma showed less soluble IL-7Rα in the plasma of VL patients both before and 30 days post-
drug treatment, compared to ECs, suggesting that the IL-7 pathway could be impaired during
L. donovani infection. The precise biological role of soluble IL-7R (sIL-7Rα) remains unclear,
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PLOS NEGLECTED TROPICAL DISEASESIL-7 signaling in CD4+ T cells of VL patients
Fig 4. Surface protein expression of CD127 and CD132 on CD4+ T cell subsets from endemic controls (ECs) and visceral leishmaniasis (VL)
patients. A. Gating strategy for CD4+ T cell subsets. B-H. Boolean-Gating and Simplified presentation of incredibly complex evaluations (SPICE)
polyfunctionality analysis. SPICE was used to establish overlap in expression of CD38, CD127 and CD132 on CD4+ T cell subsets in endemic controls
(ECs) and visceral leishmaniasis (VL) patients; EC (n = 7), VL (n = 7). B. Treg cells C. Tem D. Th E. Th17_Th22 F. Th9 G. Th1 H. Th2.
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Fig 5. IL-7-mediated activation of intracellular pSTAT5. A. Gating strategy for CD4+ T cells. B. Representative histograms
of pSTAT5 in endemic controls (ECs) and visceral leishmaniasis (VL) patients following rhIL-7 treatment. C. Frequency of
CD4+ T cells expressing pSTAT5 and D. Mean fluorescence intensity (MFI) of pSTAT5 in CD4+ T cell at baseline without (-)
and after rhIL-7 stimulation (+) in EC (n = 8) and VL patients (n = 12). Statistical significance was determined by the
Wilcoxon matched-pairs signed rank test between control and rhIL-7 stimulation in figure C-D, or Mann-Whitney U-test to
compare EC (n = 8) and VL patients (n = 12) in Fig D. Statistically significant differences are indicated as *P < 0.05;
**P < 0.01; ***P < 0.001; ****P < 0.0001.
https://doi.org/10.1371/journal.pntd.0011960.g005
but like the membrane-bound IL7Rα, the sIL-7Rα binds to IL-7 with comparable affinity and
is suggested to inhibit IL-7 signaling [30,31]. The reduction in sIL-7Rα was accompanied by
increased levels of IL-7 in plasma from VL patients (D0) compared to EC and VL post treat-
ment (D30). Increased plasma level of IL-7 is a sign of lymphopenia [30,31], something fre-
quently observed in VL patients [32]
Higher IL-7 and less sIL-7Rα have also been seen in TB patients [27]. In these patients,
the cell surface expression of IL-7Rα and the signaling capacity was reduced in T cells. Tran-
scriptional downregulation of IL7RA in T cells that have received IL-7 signaling is a feed-
back mechanism to prevent competition with T cells that have not yet received the signal
[33]. While we found no significant reduction IL7RA mRNA in VL by qPCR, previous stud-
ies reporting on transcriptional profiling of cells from VL patients found, in line with the
observation made in TB patients, reduced IL7RA mRNA levels in CD4+ T cells from VL
patients compared to ECs [17,18]. Interestingly, the surface expression of CD127 and
CD132, was found to be increased in VL patients (D0) compared to ECs, this in contrast to
TB patients, where IL-7Rα surface expression also was reduced. This finding led us to fur-
ther investigate IL-7Rα surface expression and the signaling capacity of the receptor on
CD4+ T cell subsets. Most CD4+ T cell subsets from VL patients had more CD127 and
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PLOS NEGLECTED TROPICAL DISEASESIL-7 signaling in CD4+ T cells of VL patients
Fig 6. pSTAT5 by CD38+ and CD38- CD4+ T cells. A-B. Gating Strategy for CD38+ and CD38- CD4+ T cells. C. Frequency of pSTAT5
expressing CD38+ and CD38- CD4+ T cells from endemic controls (EC) and visceral leishmaniasis (VL) patients after rhIL-7
stimulation. D. pSTAT5 mean fluorescence intensity (MFI) in CD38+ and CD38- CD4+ T cells upon rhIL-7 stimulation. Data was
generated from EC (n = 8) and VL (n = 12) donor samples. Statistical differences were determined using comparison between two
groups with a Wilcoxon matched-pairs signed rank test between control and rIL-7 stimulation and significant differences are indicated
as *P < 0.05; **P < 0.01; ***P < 0.001; ****P < 0.0001.
https://doi.org/10.1371/journal.pntd.0011960.g006
CD132 on their surface as compared to ECs. CD38, which is upregulated by inflammatory
mediators [34], was used as an activation marker on CD4+ T cells (9, 20–22, 29, 31–40).
More CD4+ T cells expressed CD38 in active VL compared to ECs. These activated (CD38+)
cells expressed more IL-7Rα as compared to non-activated (CD38-) CD4+ T cells from VL
patients.
Impaired IL-7 signaling via the IL-7Rα, as measured by pSTAT5 levels in T cells has been
observed in subjects with HIV infection and TB [35–37]. In contrast, pSTAT5 levels were
higher in VL patient CD4+ T cells at baseline and after stimulation with rhIL-7, compared to
EC CD4+ T cells. This finding was surprising since VL is characterized by lymphopenia and
dysregulated CD4+ T cells, but is in accord with elevated IL-7Rα on the cell surface of VL
patient CD4+ T cells, and previously reported upregulation of STAT5 mRNA in VL patient
CD4+ T cells [18]. The increased IL-7 signaling may be a response to the lymphopenia to sup-
port the survival of existing T cells in VL. While we show that additional rhIL-7 stimulation
increased (already heightened) pSTAT5 in VL patient CD4+ T cells compared to EC, it is
uncertain if manipulation of the IL-7 signaling pathway would improve cell survival in VL
patients. When comparing pSTAT5 in CD38+ compared to CD38- CD4+ T cells, less pSTAT5
was seen in the CD38+ CD4+ T cell subset from VL patients, where the pSTAT5 levels were
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similar in the two subsets in ECs, suggesting that there is a feedback mechanism to downregu-
late IL-7 signaling upon activation.
While not conclusive, our attempts to improve cell survival in 72-hour antigen stimulation
assays by addition of rhIL-7 were ineffective on both EC (n = 5) and VL (n = 5) cells, measured
as frequency of 7AAD, Annexin V positive lymphocytes, moreover the addition of rhIL-7 did
not alter the levels of IFNγ in culture supernatants of antigen or superantigen stimulated cells.
On the basis of these preliminary findings, we did not pursue these investigations further.
However, further investigation into feedback mechanisms regulating IL-7 signaling could be
of relevance to understand cell survival and death in lymphopenic conditions. Interestingly, it
has been shown in mice that prolonged exposure to high IL-7 levels leads to IFN-γ triggered
apoptosis in CD8 T cells, with cells having low affinity T cell receptor (TCR) engagement
being particularly affected [38]. Furthermore, Rehti et al. have shown that high levels of IL-7
can prime both human T cells and B cells for Fas-mediated apoptosis in [39,40] With elevated
expression and secretion of Fas/FasL [41] and IFN-γ [42] being a reported features in VL
patients, the high IL-7 levels and the elevated IL-7 signaling assumed to promote cell survival
and proliferation could potentially simultaneously prime the T cell for to Fas/FasL induced
death. Thus, the lower pSTAT5 seen in VL CD38+ compared to CD38- CD4+ T cell could
potentially be beneficial for the survival of effector T cells in VL.
In conclusion, defects in IL-7Rα expression or IL-7 signaling were not evident in CD4+ T
cells from VL patients. Instead, VL patient CD4+ T cells appeared to maintain an elevated
expression of IL-7Rα and pSTAT5, as compared to ECs. Our data does not support impaired
IL-7 signaling as an explanation for loss of CD4+ T cells during VL. We speculate that the IL-
7/IL-7Rα pathway may allow cells to survive longer but render them weakened and susceptible
to apoptosis when actively engaged in the immune response.
Supporting information
S1 Fig. Supporting to Fig 3E. Frequency of CD4+ T cell expressing CD38, CD127 and/or
CD132 as indicated on the Y axis.
(TIF)
S2 Fig. Supporting to Fig 4B–4H. Frequency of the gated CD4+T cells subsets expressing
CD38, CD127 and/or CD132 as indicated on the Y axis.
(TIF)
S3 Fig. Gating of lymphocyte populations and representative pSTAT5 in CD4+ T cells and
other lymphocytes upon rhIL-7 stimulation. pSTAT5 in CD3+ CD4+ T cells, CD3+ CD4- T
cells and CD3- CD4- cells in EC and VL.
(TIF)
S4 Fig. pSTAT5 expression by CD4+ T cell subsets from endemic controls (EC) and visceral
leishmaniasis (VL) patients. A. Merged CD4+ T cell samples were used to create t-distributed
stochastic neighbor embedding (tSNE) plots showing CD38, STAT5 and pSTAT5 expression by
CD4+ T cells from ECs and VL patients upon rhIL-7 stimulation. Each point represents one sin-
gle cell and cells in the same cluster represents high similarity in phenotypic expression. FACS
data, showing B. Frequency and C. Mean Fluorescence Intensity (MFI) of pSTAT5 in CD4+ T
cell subsets identified as shown in Fig 4A. The heat map was rendered using the Morpheus tool,
and the grid shows quantitative signaling upon rhIL-7 treatment in activated (CD38+) and non-
activated (CD38-) CD4+ T cell subsets (columns) from ECs and VL patients (rows).
(TIF)
PLOS Neglected Tropical Diseases | https://doi.org/10.1371/journal.pntd.0011960 February 26, 2024
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PLOS NEGLECTED TROPICAL DISEASESIL-7 signaling in CD4+ T cells of VL patients
S1 Table. Reagent List.
(DOCX)
S2 Table. FACS Antibody.
(DOCX)
Acknowledgments
We thank all volunteers and patients for their consent and participation in this study. We are
also thankful to the KAMRC staff for their assistance in collection of clinical samples. SK
would like to acknowledge Indian Council of Medical Research (ICMR) for providing him
senior research fellowship.
Author Contributions
Conceptualization: Shashi Kumar, Rajiv Kumar, Christian Engwerda, Susanne Nyle´n, Shyam
Sundar.
Data curation: Shashi Kumar, Shashi Bhushan Chauhan, Shreya Upadhyay, Siddharth Sankar
Singh, Vimal Verma, Rajiv Kumar, Susanne Nyle´n.
Formal analysis: Shashi Kumar, Shashi Bhushan Chauhan, Rajiv Kumar, Christian Engwerda,
Susanne Nyle´n, Shyam Sundar.
Funding acquisition: Rajiv Kumar, Shyam Sundar.
Methodology: Shashi Kumar, Rajiv Kumar, Christian Engwerda, Susanne Nyle´n.
Project administration: Rajiv Kumar, Shyam Sundar.
Resources: Rajiv Kumar, Shyam Sundar.
Supervision: Rajiv Kumar, Christian Engwerda, Susanne Nyle´n, Shyam Sundar.
Writing – original draft: Shashi Kumar, Shashi Bhushan Chauhan, Shreya Upadhyay, Sid-
dharth Sankar Singh, Vimal Verma, Rajiv Kumar, Christian Engwerda, Susanne Nyle´n,
Shyam Sundar.
Writing – review & editing: Shashi Kumar, Rajiv Kumar, Christian Engwerda, Susanne
Nyle´n, Shyam Sundar.
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| null |
10.1088_1361-6463_ad005f.pdf
|
Data availability statement
All data that support the findings of this study are included
within the article (and any supplementary files).
|
Data availability statement All data that support the findings of this study are included within the article (and any supplementary files). ORCID iD Quanliang Cao https://orcid.org/0000-0003-3691-2311
|
J. Phys. D: Appl. Phys. 57 (2024) 045002 (13pp)
Journal of Physics D: Applied Physics
https://doi.org/10.1088/1361-6463/ad005f
Effect of the number of magnetic
matrices on particle capture in high
gradient magnetic separation
Yu Tian1,2 and Quanliang Cao1,2,∗
1 Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan,
People’s Republic of China
2 State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University
of Science and Technology, Wuhan, People’s Republic of China
E-mail: [email protected]
Received 28 June 2023, revised 21 September 2023
Accepted for publication 5 October 2023
Published 30 October 2023
Abstract
A comprehensive understanding of the capture process involving matrices in high gradient
magnetic separation (HGMS) is crucial for the design and improvement of matrix performance.
However, few existing studies have paid attention to the influence of the number of magnetic
matrices on the capture process. In this work, we numerically investigate this issue in both
longitudinal and transversal HGMS systems, where multiple scenarios with different particle
sizes, flow rates and matrix spacing are considered. Interestingly, we show that in most cases,
increasing the number of magnetic matrices along the flow direction has little to no influence on
the capture radius. It has a certain effect on improving the capture radius only in a few specific
cases, such as when dealing with large particles at low flow rates with closely spaced matrices
or when working with small particles at high flow rates with widely spaced matrices. These
phenomena are related to the appearance of repulsive magnetic forces around matrices and the
distribution characteristics of magnetic forces. The obtained results indicate that, in the design
of the practical HGMS system, simply increasing the number of matrices along the flow
direction may not be a reasonable or effective strategy for enhancing capture performance.
Keywords: high gradient magnetic separation, capture radius, magnetic matrices,
numerical simulation
1. Introduction
High gradient magnetic separation (HGMS) is a physical sep-
aration method that has been widely used in many scientific
research and industrial fields, including but not limited to min-
eral processing (Chen et al 2021, Xian et al 2022, Zheng et al
2022), pollutant treatment (Okamoto et al 2011, Nishimoto
et al 2021, Okumura et al 2022), and biomass separation
(Ueda et al 2010, Abdel Fattah et al 2016, Ebeler et al 2018).
∗
Author to whom any correspondence should be addressed.
The working principle of HGMS is to use magnetic matrices
to produce a local high gradient magnetic field in the pres-
ence of an external magnetic field, and then to achieve the
separation and recovery of particles with different magnetic
properties due to the disparities in gradient magnetic forces.
Therefore, magnetic matrices have a great impact on the sep-
aration process and performance of HGMS. Many existing
studies have attempted to improve the HGMS performance by
changing the properties of magnetic matrices, such as matrix
size, shape, configuration, thus providing reference values for
the design of magnetic matrices in the practical applications of
HGMS.
1361-6463/24/045002+13$33.00 Printed in the UK
1
© 2023 IOP Publishing Ltd
J. Phys. D: Appl. Phys. 57 (2024) 045002
Y Tian and Q Cao
In the terms of matrix shape, cylindrical magnetic matrices
with a circular cross-section are often used in HGMS systems,
while magnetic matrices with other special cross-section
shapes have also received attention (Zheng et al 2016, Xue
et al 2020a, 2020b, Xia et al 2021). For instance, Xue et al
(2020a) explored the particle capture performance by mag-
netic matrices with four cross sections (circular, elliptic,
square and diamond cross-section) in an axial HGMS sys-
tem by using three-dimensional numerical simulations, and
made a quantitative comparison, showing that the particle cap-
ture cross section of the elliptic and diamond matrices is lar-
ger than that of the square and elliptic matrices when the
applied magnetic field is less than 1.1 T, while the particle
capture cross section of the four types of matrices is sim-
ilar when the applied magnetic field is greater than 1.1 T.
In addition to shape, the effect of matrix size on the cap-
ture process was also investigated (Padmanabhan et al 2011,
Zheng et al 2015, Zheng et al 2017). For instance, Zheng
et al (2015) numerically demonstrated that the capture radius
decreases rapidly with the increase of matrices size, where
matrices of small size have larger capture radius and present
higher capture efficiency under the same packing fraction as
matrices of large size. Furthermore, it has been reported that
the configuration of magnetic matrices also has an impact
on the capture process, and the existing studies have focused
on the analysis of the capture behavior of single and mul-
tiple magnetic matrices (Chen et al 2017, Yuan et al 2018,
Wang et al 2020, Zhou et al 2021). For instance, Wang et al
(2020) developed a fully coupled multi-physical model to
explore the process of capture and particle accumulation of
single-wire and interlaced multi-wire matrices. Their results
show that although the recovery rate of multi-wire matrices
is higher than that of single-wire matrix, the single-wire mat-
rix always has higher selectivity than that of the multi-wire
matrices. This phenomenon is mainly due to the magnetic
coupling in the multi-line matrices and the complexity of the
flow.
It can be seen that existing studies have shown that mul-
tiple structural parameters of matrices have potential effects
on the HGMS capture process. However, up to now, there
are few relevant studies on the effect of matrix number on
the HGMS process. To the best of our knowledge, only one
article considering the matrix number in the analysis of sep-
aration process has been so far reported, where the simu-
lations were provided for a microfluidic separation system
with magnetic matrices embedded in the base of a microchan-
nel (Khashan et al 2014). It is obvious that this parameter
plays an important role in the practical design for HGMS,
especially for the applications of mineral processing requir-
ing high and large-scale magnetic fields, because without
affecting the capture efficiency, reducing matrix number is
undoubtedly beneficial to reducing the magnetic field region
and then greatly reducing the energy consumption. Therefore,
in this work, we highlight the effect of matrix number on
the capture performance of particles in both longitudinal and
transversal HGMS systems in multiple cases with different
particle sizes, flow rates and matrix spacing. It is expected
that this work is of significance for the understanding of the
physical process of HGMS as well as the matrix structure
design.
2. Numerical model
In this section, we developed a numerical model to analyze the
HGMS process using COMSOL software (version 5.6). Three
physical modules including ‘magnetic fields’ for analysis of
magnetic properties of matrices, ‘laminar flow’ for analysis
of fluid flow, and ‘particle tracing’ for analysis of motion tra-
jectories of particles under magnetic and fluidic forces were
adopted in the simulations.
2.1. Magnetic force calculation
In order to obtain the magnetic force Fm acting on the particles,
the spatial distribution of magnetic field in the channel should
be calculated first. By introducing vector magnetic potential
A, the control equations of magnetic field intensity H and
magnetic induction intensity B can be obtained according to
Maxwell equations and boundary conditions:
∇ × H = 0
B = ∇ × A
(1)
(2)
where ∇ is the nabla operator. Depending on the materials, the
B−H constitutive relation of materials is expressed mainly in
the following two ways:
B = µ0H
B = f (||H||)
H
||H||
(3)
(4)
where µ0 denotes the vacuum permeability, ||H|| denotes the
magnetic field norm, f(||H||) denotes a nonconstant factor
associated with the magnetic field norm, which can be
obtained from the B−H curve of matrix material. Equation (4)
was used for the B−H constitutive relation of ferromagnetic
materials of matrices, where the pure iron with a saturation
magnetization of 2.16 T was selected as the matrix material
(Xia et al 2021), and equation (3) was used for all other non-
magnetic materials used in the simulations. In addition, the fol-
lowing condition was set along the boundary of the air domain
to simulate the action of the externally applied uniform mag-
netic field Ha:
n × H = n × Ha
(5)
where the direction of Ha is parallel to the direction of fluid
flow in the longitudinal HGMS system and perpendicular to
the direction of fluid flow in the transverse HGMS system.
When the magnetic field distribution in the fluid region where
2
J. Phys. D: Appl. Phys. 57 (2024) 045002
Y Tian and Q Cao
particles are located is obtained, the gradient magnetic force
Fm can be calculated by (Cao et al 2020):
Fm = µ0Vp
3χ p
χ p + 3
(H · ∇) H
(6)
where V p is the volume of particles and χ p is the magnetic
susceptibility of particles.
2.2. Fluidic force calculation
In this work, we consider the dynamics of magnetic particles
in a dilute concentration and ignore the influence of particle
motion on the fluid flow. Then assuming that the fluid is vis-
cous in-compressible laminar flow, Navier–Stokes equations
can be used to predict the fluid flow:
(
ρ
∂vf
∂t
)
+ vf
∇vf
= −∇P + ∇ · (η∇vf)
ρ∇ · vf = 0
(7)
(8)
where vf, η, ρ, and P respectively denotes the fluid velocity,
viscosity coefficient, the fluid density, and the pressure. In the
simulations, the influence of the magnetic matrices on the sur-
rounding fluid velocity is considered, where these matrices are
modeled and a no-slip boundary is applied along their walls.
When obtaining the value of the fluid velocity, Stokes formula
is used to calculate the fluidic force Ff suffered by magnetic
particles:
Ff = −6π ηRp (vp
− vf)
(9)
where Rp is the radius of the particles and vp is the velocity of
the particles.
2.3. Particle motion prediction
In the field of HGMS, a simplified approach, which ignores
gravity, buoyancy, Brownian motion acting on particles, is
often used for predicting particle motion, even for high-density
particles such as hematite particles (Zheng et al 2015, 2016).
In this case, Newton’s second law in classical mechanics can
be used to calculate the trajectory of particle motion:
Fm + Ff = m
dvp
dt
.
(10)
In the actual magnetic separation process for micro- and
nano-particles, it is acceptable to neglect the right end of
equation (9) (acceleration term) due to the time constant is
quite small. Therefore, we can predict the movement track of
particles by simply balancing magnetic force and fluid force:
µ0Vp
3χ p
χ p + 3
(H · ∇) H = 6π ηRp (vp
− vf)
(11)
≪
where V p denotes the particle volume (Vp = 4
3
1 in this work. Then the particle velocity vp and the particle
trajectory xp(t) can be obtained by:
p) and χ p
π R3
vp = vf +
2µ0χ pR2
p (H · ∇) H
9η
dxp
dt
= vp
(12)
(13)
where the particle velocity mainly includes two parts: one
term on the right side of
is the fluid velocity (the first
equation (12)), and the other is the magnetic velocity caused
by gradient magnetic field (the second term on the right side
of equation (12)).
3. Results and discussion
According to the models established in the above section, the
capture process of magnetic particles in two types of HGMS
systems is numerically studied, where the 2D cross-section
of longitudinal and transversal HGMS systems is respectively
shown in figures 1(a) and (b). The direction of the applied
magnetic field in the longitudinal configuration is parallel to
the fluid direction, while the direction of the magnetic field in
the transversal configuration is perpendicular to the fluid dir-
ection. The cross-section of magnetic matrices is a circle with
a radius of 1 mm. Spherical hematite particles with magnetic
susceptibility of 0.0025 were chosen as magnetic particles for
analysis, and the fluid viscosity was set to 1 mPa·s. To meas-
ure the capture efficiency, the capture radius RC was adopted in
the simulations, which represents the critical initial position of
magnetic particles being captured and escaped. That is, when
the distance between the initial position of particle release and
the central axis of the pipe r ⩽ RC, particles will be captured
by the matrix; When r > RC, the particles will escape without
being captured.
3.1. Particle capture in the HGMS system with a single
magnetic matrix
This section takes the HGMS system with a single magnetic
matrix as a case to explore particle capture behavior. The width
and the length of the channel is respectively set to 30 mm and
120 mm. The initial release position of the particles is 40 mm
away from the inlet, and the width of the release area is set to
15 mm. The total number of released particles is set as 400.
The wall condition of the matrices is set as adhesion, that is,
once the particles are captured by the magnetic matrices, the
velocity will drop to zero, and they will always be adsorbed on
the surface of the matrices.
According to equation (12), the factors that affect the
particle velocity include fluid flow velocity, magnetic field
strength and particle size, so this section will explore the influ-
ence of these three factors on the capture behavior of the mag-
netic matrix. The control variable method was used to set the
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J. Phys. D: Appl. Phys. 57 (2024) 045002
Y Tian and Q Cao
the change trend of the capture radius under the two config-
urations is similar. Meanwhile, it can be seen that the cap-
ture radius increases with the magnetic induction and particle
size, while decreases with the fluid velocity. This is because
the magnetic velocity in equation (11) is the key to whether
particles can be captured, while the fluid velocity will have a
negative impact on the capture. Moreover, it can be seen that
when the per unit value is less than 4, the change curve of the
capture radius with the magnetic induction coincides with the
change curve of the particle radius, while when the per unit
value is larger than 4, the capture radius changes more signific-
antly with the particle radius. This is because when the per unit
value is greater than 4 (the applied magnetic induction is larger
than 1 T), the magnetization of magnetic matrix will saturate,
and then increasing the external magnetic field cannot improve
the magnetic field gradient produced by the magnetic matrix,
which makes the contribution to the magnetic force acting on
the particles only lies in the increase of the particle magnetiz-
ation. In other words, the force (or magnetic velocity) of the
particles is approximately square to the applied magnetic field,
but linear after magnetization saturation.
3.2. Influence of the number expansion of single row
magnetic matrix on the capture radius
In the previous section, we explored the influence of magnetic
field, particle diameter and fluid velocity on the capture radius.
On this basis, we further explore the influence of the number
of magnetic matrices on the capture process of two types of
HGMS systems.
In
3.2.1. Capture process of longitudinal HGMS system.
this section, in order to simplify the analysis, we set the
external magnetic field as a fixed value (1.25 T), and set two
particle sizes (10 mm and 40 mm) to represent the cases with
large and small magnetic velocities. In terms of flow rate, we
−1 and
−1, 10 mm s
set its value range as 5 mm s
−1. In terms of physical parameters of magnetic mat-
40 mm s
rix, different longitudinal HGMS systems with 1, 2, 4, 6, and
8 magnetic matrices are compared. Meanwhile, the influence
of matrix spacing on the capture radius was considered in the
simulations.
−1, 20 mm s
Figure 3 shows the simulation results of capture radius
under different cases, where large particles with a diameter
of 40 µm were adopted and the matrix spacing was respect-
ively set as 0.5 mm, 1 mm, 2 mm and 4 mm. The simulation
results do not show the expected phenomenon that the cap-
ture radius increases greatly with the increase of the number
of magnetic matrices, and the change of the capture radius is
relatively obvious only when the flow rate is low and the mat-
rix spacing is small.
In order to confirm these results and explore the related
reasons, we plotted the motion trajectories of 40 µm particles
−1), as
at low speed (5 mm s
well as in the cases with small matrix spacing (0.5 mm)
and large spacing (4 mm). As shown in figure 4, it can be
−1) and high speed (40 mm s
Figure 1. Schematic diagram of two types of HGMS systems:
(a) longitudinal configuration and (b) transversal configuration.
−1, 15 mm s
−1, 25 mm s
−1, 10 mm s
−1 and 35 mm s
factors to be studied as a series of different values and con-
trol the remaining two factors to remain unchanged. In this
section, the variation range of magnetic induction B was set
as 0.25 T, 0.5 T, 0.75 T, 1 T, 1.25 T, 1.5 T and 1.75 T, and the
variation range of particle diameter was set as 10 µm, 20 µm,
30 µm, 40 µm, 50 µm, 60 µm and 70 µm. The variation range
−1,
of fluid velocity was set as 5 mm s
−1. Among
−1, 30 mm s
20 mm s
them, the magnetic induction of 1 T, the particle diameter of
−1 were set as the
40 µm, and the fluid velocity of 20 mm s
fixed values. It is noteworthy that the guiding principle for
parameter selection aimed at facilitating significant variations
in the relative magnitudes of magnetic and fluid forces, which
enables comprehensive rule exploration across a wide range
of parameters. Furthermore, the specified values for magnetic
field, flow rate, and particle size mentioned above are basically
consistent with the relevant data from existing studies (Zheng
et al 2016). When exploring the influence of a certain variable
on the capture radius of the magnetic matrix, the remaining
variables were remained unchanged as the fixed value, and the
variables to be studied can take values from small to large in
the range of change.
To make the results clearer, we took the minimum value of
−1) as the ref-
these three variables (0.25 T, 10 µm and 5 mm s
erence values for normalization, and took the mean value of
−1) as the
the these three variables (1 T, 40 µm and 20 mm s
values of control variable. The obtained results for the longit-
udinal and transverse HGMS systems are shown in figures 2(a)
and (b) respectively. The abscissa in figure 2 represents the
value of each variable after normalization. It can be seen that
the capture radius of the longitudinal configuration is always
higher than that of the transversal configuration, but in general,
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Y Tian and Q Cao
Figure 2. Change characteristics of capture radius with fluid velocity, magnetic field, and particle diameter in single-matrix HGMS
systems. (a) The case with a longitudinal configuration. (b) The case with a transversal configuration.
Figure 3. Characteristics of capture radius of 40 µm-diameter particles with number of matrices in the longitudinal HGMS system. (a) The
case with a matrix distance of 0.5 r. (b) The case with a matrix distance of r. (c) The case with a matrix distance of 2 r. (d) The case with a
matrix distance of 4 r.
observed that, in all simulated cases, 40 µm-diameter particles
can only be captured by the first matrix at the upstream, and
the remaining matrices cannot capture any particles, form-
ing a ‘hollow region’ of particle trajectory. This behavior can
explain the phenomenon that increasing the number of mag-
netic matrices has little effect on the capture radius, and the
formation of the hollow region could be due to the appearance
of strong repulsive magnetic forces on particles. To confirm
this point, we plotted the vertical magnetic forces subjected
by the particles in the region near the matrices, as shown in
figure 5, where the distance between the selected horizontal
line and the matrix surface in figures 5(a) and (b) was set
as 1 mm and 3 mm, respectively. These magnetic forces are
defined as positive when pointing to the tube wall and neg-
ative when pointing to the matrix surface. It can be observed
that in the regions near or far from the matrices, the particles
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Y Tian and Q Cao
Figure 4. Motion trajectories of 40 µm-diameter particles. (a) The case with a matrix distance of 0.5 mm and a fluid velocity of 40 mm s
−1. (c) The case with a matrix distance of 4 mm and a fluid
(b) The case with a matrix distance of 0.5 mm and a fluid velocity of 5 mm s
velocity of 40 mm s
−1. (d) The case with a matrix distance of 4 mm and a fluid velocity of 5 mm s
−1.
−1.
Figure 5. Profiles of vertical magnetic forces along the selected horizontal line. (a) The case with a distance of 1 mm between the
horizontal line and the matrix surface. (b) The case with a distance of 3 mm between the horizontal line and the matrix surface.
will be subjected to negative magnetic forces before reaching
the first matrix, which means that these particles will be attrac-
ted towards the first matrix. However, positive magnetic forces
appear in the region above the first matrix and its rear matrices,
and the positive magnetic forces are absolutely dominant at
the place far away from the matrices, as shown in figure 4(b),
which means that the matrices repel particles in these areas
where positive magnetic force appears. This also indicates that
once the particles in the region close to the first matrix are
captured, as the distance between the vertical position of the
remaining particles and the matrices increases, these particles
are subjected to the repulsive magnetic forces and far away
from the matrices, which is also the main reason for the phe-
nomenon of hollow state of particle trajectories.
Furthermore, we explain why the capture radius increases
with the number of matrices in the case of low flow velocity
and small spacing in the capture process of large particles. As
discussed above, since only the first magnetic matrix can cap-
ture particles in all cases in the simulations, the difference in
capture radius caused by the number of matrices is likely to
come from the effect of subsequent matrices on the magnetic
field distribution in front of the first. To verify this point, we
calculated the magnetic field distribution along the vertical dir-
ection at 3 mm upstream of the first matrix under different
matrix numbers and matrix spacing, as shown in figure 6. It
can be observed that although the downstream matrices can-
not capture particles, their presence can enhance the magnetic
field strength in front of the first matrix, thereby promoting
the capture of particles under the action of a stronger mag-
netic force. Meanwhile, according to the data for different
numbers of matrices, it can be observed that when the num-
ber of matrices increases from one to eight, the increase of
the magnetic induction gradually decreases, which is consist-
ent with the increasing trend of the capture radius. In addition,
according to the data for different matrix spacing, it can be
observed that the smaller the spacing between matrices, the
more obvious the increase of magnetic induction as the num-
ber of matrices increases, which also explains why the capture
radius changes more significantly at small matrix spacing. It
is worth mentioning that under the same matrix spacing, that
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Y Tian and Q Cao
Figure 6. Profiles of magnetic field along the vertical line at 3 mm upstream of the first matrix for different matrix numbers. (a) The case
with a matrix distance of 0.5 mm. (b) The case with a matrix distance of 4 mm.
Figure 7. Characteristics of capture radius of 10 µm-diameter particles with number of matrices in the longitudinal HGMS system. (a) The
case with a matrix distance of 0.5 mm. (b) The case with a matrix distance of 1 mm. (c) The case with a matrix distance of 2 mm. (d) The
case with a matrix distance of 4 mm.
is, the same magnetic field distribution, the change of the cap-
ture radius is more obvious at low flow rates than that at high
flow rates. This is because when the fluid flow is slow, the
magnetic velocity in equation (11) will have more influence on
the capture radius during the competition with fluid velocity.
When the fluid flow becomes fast, even if the magnetic induc-
tion is increased, the change in the capture radius at high flow
rate becomes insignificant due to the particles will be taken
away by the fluid flow before they can be captured by the first
matrix, which results in the change in capture radius at high
flow rates becoming less pronounced with increasing magnetic
induction.
Furthermore, 10 µm-diameter particles were used as an
example to explore the capture behavior of magnetic matrices
for small-size particles, as shown in figure 7. It can be observed
that, similar to the case with large particle particles, the
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Y Tian and Q Cao
Figure 8. Motion trajectories of 10 µm-diameter particles. (a) The case with a matrix distance of 0.5 mm and a fluid velocity of 40 mm s
−1. (c) The case with a matrix distance of 4 mm and a fluid
(b) The case with a matrix distance of 0.5 mm and a fluid velocity of 5 mm s
velocity of 40 mm s
−1. (d) The case with a matrix distance of 4 mm and a fluid velocity of 5 mm s
−1.
−1.
Figure 9. Profiles of vertical magnetic forces along the selected horizontal line at a distance of 1 mm from the matrix column. (a) The case
with a matrix spacing of 0.5 mm. (b) The case with a matrix spacing of 4 mm.
increase of matrix number has little impact on the capture
radius in most cases. The difference is that for small particles
(or small magnetic velocity), the capture radius will be relat-
ively higher only when the flow rate is high and the matrix
spacing is large. In order to explain the above phenomenon,
we plotted the motion trajectories of these 10 µm particles at
−1), as well
low speed (5 mm s
as in the cases with small matrix spacing (0.5 mm) and large
spacing (4 mm), as shown in figure 8.
−1) and high speed (40 mm s
It can be observed that, compared with figure 4, the range of
the hollow region is significantly reduced due to the decreased
particle diameter, that is, the influence of magnetic velocity is
weakened, but most particles are still captured by the first mat-
rix, which is also the reason for the small effect of the matrix
number. Note that, due to the weakened influence of the mag-
netic velocity, the change of the capture radius in figure 7 at
low flow rate and short spacing is much weaker than that in
figure 3. In contrast, the capture radius varies obviously with
the number of matrices at high speed and large spacing. At a
high flow rate, the influence of the flow rate on the particle tra-
jectory is increased, resulting in a significant decrease in the
capture radius, so that the particle trajectory is closer to the
matrices. According to the calculation results in figure 5, as
the distance between particles and matrices becomes smaller,
the attractive force appears between particles and each matrix,
which provides the possibility for the subsequent matrix to
capture the particles. Note that in the case with high flow
rates, the effect of the matrix number on the capture radius
is evident only when the matrix spacing is large. To explore
the reason for this phenomenon, we plotted the vertical mag-
netic force distribution of particles at a distance of 1 mm from
the matrix column, as shown in figure 9. It can be found that
the matrix spacing has a large impact on the proportion of
the repulsive and attractive forces. For small matrix spacing,
the region of repulsive force is absolutely dominant, while
the increase of the matrix spacing can significantly increase
the region of attractive force, which well explains the above
phenomenon.
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Y Tian and Q Cao
Figure 10. Characteristics of capture radius of 40 µm-diameter particles with number of matrices in the transversal HGMS system. (a) The
case with a matrix distance of 0.5 mm. (b) The case with a matrix distance of 1 mm. (c) The case with a matrix distance of 2 mm. (d) The
case with a matrix distance of 4 mm.
This
3.2.2. Capture process of transversal HGMS system.
section further investigates the dynamic behavior of particles
in the transversal HGMS. Similar to the longitudinal HGMS,
as the representative of large particle size, 40 µm-diameter
particles are used to calculate the capture radius, and the results
are shown in figure 10. It can be observed that the number
of matrices also has little influence on the capture radius in
general. Only when the flow rate is low and the spacing is
small, increasing the number of matrices can improve the cap-
ture radius to some extent, which is similar to that in figure 3.
Figures 11(a) and (b) represent the statistics of the number
of particles captured by each matrix for different numbers of
matrix and the plot of particle trajectory for the case with eight
matrices, respectively. Unlike the longitudinal HGMS, in the
transverse HGMS, for the cases with multiple matrices, the
first matrix is difficult to capture particles due to the repulsive
force in front of it, and the two ends of the remaining matrices
can capture more particles. The reason why increasing the
number of matrices has little effect on the capture radius is
that although the increased downstream matrices is able to cap-
ture more particles, the increased number of particles is at the
expense of the capture ability of the upstream matrices. Thus,
increasing the number of matrices is not significantly effective
to improve the capture radius. Meanwhile, we plotted the dis-
tribution of horizontal magnetic forces in the cases with small
(0.5 mm) and large spacing (4 mm) at a distance of 1 mm from
the matrix column when the number of matrix was 2, 4, and
6, as respectively shown in figures 11(c) and (d). There is a
tendency that increasing the number of matrices at small spa-
cing can increase the attractive-force area and form a continu-
ous attractive-force region. However, in the case of large spa-
cing, both attractive-force and repulsive-force regions appear
between matrices, and the area of these two regions increases
with the number of matrices, which makes increasing the num-
ber of matrix at large spacing have a small effect on the cap-
ture radius. It should be noted that in the case of small spacing,
increasing the number of matrices can increase the width of the
attractive-force region, while the force peak also decreases, as
shown in figure 11(c), which also results in little change in the
capture radius after the number of matrices increases to some
extent. Furthermore, this difference of capture radius caused
by the change in attractive forces decreases with increasing
fluid flow rate. These results confirm and explain the phe-
nomenon in figure 10.
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J. Phys. D: Appl. Phys. 57 (2024) 045002
Y Tian and Q Cao
Figure 11. Characteristics of particle capture and magnetic forces of 40 µm-diameter particles in the transversal HGMS system. (a) The
statistics of the number of particles captured by each matrix. (b) Particle trajectory for eight matrices. (c) Profiles of horizontal magnetic
forces for different matrix numbers in the case with a small matrix spacing (0.5 mm). (d) Profiles of horizontal magnetic forces for different
matrix numbers in the case with a large matrix spacing (4 mm).
Considering the variability in the capture process of large
and small particles, 10 µm-diameter particles are further selec-
ted to analyze the effect of the matrix number on the cap-
ture radius, and the results are shown in figure 12. It can be
observed that, similar to the results in figure 7, the increase
of matrix number has little impact on the capture radius in
most cases, and the capture radius has an obvious increase
with the matrix number only when the flow rate is high and
the matrix spacing is large. The difference is that, in the case
of low flow rate and small spacing, the capture radius even
decreases.
To explain these phenomenon, we explored the motion
−1)
characteristics of particles at low flow velocity (5 mm s
and small spacing (0.5 mm), as shown in figure 13(a), and
drew a histogram of the number of captured particles in
each matrix under different matrix numbers, as shown in
figure 13(b). It can be observed that the vast majority of
particles are captured only at both ends of the matrices, and
therefore increasing the matrix number has little effect on
the capture radius. In addition, we further analyzed the mag-
netic force characteristics of the particles. Considering that
the trajectory of small-size particles is close to the matrices,
we selected the vertical line at a distance of 1 mm from
the matrices for analysis. Figures 13(c) and (d) represent
the distribution curve of radial magnetic forces for the case
with a small matrix spacing (0.5 mm) and the case with a
large matrix spacing (4 mm), respectively. It can be observed
that when the distance between the matrices is relatively
close, increasing the number of matrices reduces the attract-
ive force at both ends of the matrices to some extent due
to the magnetic interactions between matrices, which can be
used to explain why the capture radius slightly decreases
when increasing the matrix number. In the case with a
large matrix spacing, the magnetic coupling between adja-
cent matrices becomes weak, and then the attractive force
at both ends of the matrices does not decrease significantly
with the increase of the matrix number, while the attractive
force region increases more obviously, which is conducive to
particle capture. Meanwhile, at the high flow rate, the particles
escaping from the upstream matrix are closer to the following
matrices, thus making the particles easier to be capture, which
can be used to explain why the capture radius increases with
increasing matrix number at high flow rate and large matrix
spacing.
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J. Phys. D: Appl. Phys. 57 (2024) 045002
Y Tian and Q Cao
Figure 12. Characteristics of capture radius of 10 µm-diameter particles with number of matrices in the transversal HGMS system. (a) The
case with a matrix distance of 0.5 mm. (b) The case with a matrix distance of 1 mm. (c) The case with a matrix distance of 2 mm. (d) The
case with a matrix distance of 4 mm.
Figure 13. Characteristics of particle capture and magnetic forces of 10 µm-diameter particles in the transversal HGMS system. (a) The
statistics of the number of particles captured by each matrix. (b) Particle trajectory for eight matrices. (c) Profiles of horizontal magnetic
forces in the case with a small matrix spacing (0.5 mm). (d) Profiles of horizontal magnetic forces in the case with a large matrix spacing
(4 mm).
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J. Phys. D: Appl. Phys. 57 (2024) 045002
Y Tian and Q Cao
4. Conclusion
References
In this work, we explored the influence of the number of mag-
netic matrices on the particle capture radius in both longit-
udinal and transversal HGMS systems. We also delved into
the underlying mechanisms of the related phenomenon, using
single-row matrices as our model and exploring various scen-
arios. It can be reasonably inferred that the more magnetic
matrices along the flow direction, the better the capture per-
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simulation results, increasing the number of magnetic matrices
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It
is worth mentioning that
this work focuses on the
single-column matrix structure. Subsequent work will extend
to more complex HGMS systems, such as those featuring
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Additionally, in real HGMS processes, the upstream matrices
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evaluation of how the number of matrices, considering this
factor, affects capture performance will be carried out. Overall,
this work is of significance for understanding the particle cap-
ture process in HGMS systems and for advancing research in
this field.
Data availability statement
All data that support the findings of this study are included
within the article (and any supplementary files).
ORCID iD
Quanliang Cao https://orcid.org/0000-0003-3691-2311
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Data availability
The datasets, code for generating all figures, and Supplementary
figures can be found at https://github.com/AndersenLab/swept_
broods. Supplementary File S1 contains the haplotype data of 403
C. elegans isotypes from CeNDR release 20200815. Supplementary
File S2 contains genetic relatedness of 403 C. elegans isotypes.
Supplementary File S3 contains lifetime fecundity of 121 C. ele-
gans strains, their classification of swept strains and divergent
strains, and the assay blocks of these strains. Supplementary File
S4 contains daily fecundity and daily intrinsic growth rate of 121
C. elegans strains. Supplementary File S5 contains GWA results on
lifetime fecundity of 121 C. elegans strains. Supplementary File S6
contains genotype and phenotype data of 121 C. elegans strains at
the peak markers of GWA mapping. Supplementary File S7 con-
tains
strains.
Supplementary File S8 contains the GPS coordinates of sampling
locations of 121 C. elegans strains. Supplementary File S9 contains
lifetime fecundity and swept and divergent classifications of each
of the four swept chromosomes for each of the 121 C. elegans
strains. Supplementary File S10 contains LD results among the
three QTL of GWA using 121 C. elegans strains. Supplementary
File S11 contains the shared haplotypes of the 121 strains within
the QTL of GWA mapping. Supplementary File S12 contains GWA
results on fecundity data of 236 strains from a previous study
(Hahnel et al. 2018). Supplementary File S13 contains genotype
the sampling locations of 121 C.
elegans
4 | G3, 2021, Vol. 11, No. 8
and phenotype data of 236 strains at the peak marker of GWA
mapping. Supplementary File S14 contains the shared haplotypes
of
the 236 strains within the QTL of GWA mapping.
Supplementary File S15 contains the linkage mapping results for
the 402 RIAILs in 1% water condition. Supplementary File S16
contains genotype and phenotype data of the 402 RIAILs at the
peak markers and phenotype data of the parents in linkage map-
ping results. Supplementary File S17 contains the linkage map-
ping results for
the 417 RIAILs in 1% DMSO condition.
Supplementary File S18 contains genotype and phenotype data of
the 417 RIAILs at the peak markers and phenotype data of the
parents in linkage mapping results. Supplementary File S19 con-
tains the linkage mapping results for the 432 RIAILs in 0.5%
DMSO condition. Supplementary File S20 contains genotype and
phenotype data of the 432 RIAILs at the peak markers and pheno-
type data of the parents in linkage mapping results.
|
Data availability The datasets, code for generating all figures, and Supplementary figures can be found at https://github.com/AndersenLab/swept_ broods . Supplementary File S1 contains the haplotype data of 403 C. elegans isotypes from CeNDR release 20200815. Supplementary File S2 contains genetic relatedness of 403 C. elegans isotypes. Supplementary File S3 contains lifetime fecundity of 121 C. elegans strains, their classification of swept strains and divergent strains, and the assay blocks of these strains. Supplementary File S4 contains daily fecundity and daily intrinsic growth rate of 121 C. elegans strains. Supplementary File S5 contains GWA results on lifetime fecundity of 121 C. elegans strains. Supplementary File S6 contains genotype and phenotype data of 121 C. elegans strains at the peak markers of GWA mapping. Supplementary File S7 contains the sampling locations of 121 C. elegans strains. Supplementary File S8 contains the GPS coordinates of sampling locations of 121 C. elegans strains. Supplementary File S9 contains lifetime fecundity and swept and divergent classifications of each of the four swept chromosomes for each of the 121 C. elegans strains. Supplementary File S10 contains LD results among the three QTL of GWA using 121 C. elegans strains. Supplementary File S11 contains the shared haplotypes of the 121 strains within the QTL of GWA mapping. Supplementary File S12 contains GWA results on fecundity data of 236 strains from a previous study (Hahnel et al. 2018) . Supplementary File S13 contains genotype and phenotype data of 236 strains at the peak marker of GWA mapping. Supplementary File S14 contains the shared haplotypes of the 236 strains within the QTL of GWA mapping. Supplementary File S15 contains the linkage mapping results for the 402 RIAILs in 1% water condition. Supplementary File S16 contains genotype and phenotype data of the 402 RIAILs at the peak markers and phenotype data of the parents in linkage mapping results. Supplementary File S17 contains the linkage mapping results for the 417 RIAILs in 1% DMSO condition. Supplementary File S18 contains genotype and phenotype data of the 417 RIAILs at the peak markers and phenotype data of the parents in linkage mapping results. Supplementary File S19 contains the linkage mapping results for the 432 RIAILs in 0.5% DMSO condition. Supplementary File S20 contains genotype and phenotype data of the 432 RIAILs at the peak markers and phenotype data of the parents in linkage mapping results. .
|
2
G3, 2021, 11(8), jkab168
DOI: 10.1093/g3journal/jkab168
Advance Access Publication Date: 13 May 2021
Investigation
Natural variation in fecundity is correlated with species-
wide levels of divergence in Caenorhabditis elegans
Gaotian Zhang
, Jake D. Mostad, and Erik C. Andersen
*
Department of Molecular Biosciences, Northwestern University, Evanston, IL 60208, USA
*Corresponding author: Department of Molecular Biosciences, Northwestern University, 4619 Silverman Hall, 2205 Tech Drive, Evanston, IL 60208, USA. Email:
[email protected]
Abstract
Life history traits underlie the fitness of organisms and are under strong natural selection. A new mutation that positively impacts a life his-
tory trait will likely increase in frequency and become fixed in a population (e.g., a selective sweep). The identification of the beneficial
alleles that underlie selective sweeps provides insights into the mechanisms that occurred during the evolution of a species. In the global
population of Caenorhabditis elegans, we previously identified selective sweeps that have drastically reduced chromosomal-scale genetic
diversity in the species. Here, we measured the fecundity of 121 wild C. elegans strains, including many recently isolated divergent strains
from the Hawaiian islands and found that strains with larger swept genomic regions have significantly higher fecundity than strains without
evidence of the recent selective sweeps. We used genome-wide association (GWA) mapping to identify three quantitative trait loci (QTL)
underlying the fecundity variation. In addition, we mapped previous fecundity data from wild C. elegans strains and C. elegans recombi-
nant inbred advanced intercross lines that were grown in various conditions and detected eight QTL using GWA and linkage mappings.
These QTL show the genetic complexity of fecundity across this species. Moreover, the haplotype structure in each GWA QTL region
revealed correlations with recent selective sweeps in the C. elegans population. North American and European strains had significantly
higher fecundity than most strains from Hawaii, a hypothesized origin of the C. elegans species, suggesting that beneficial alleles that
caused increased fecundity could underlie the selective sweeps during the worldwide expansion of C. elegans.
Keywords: C. elegans; lifetime fecundity; natural variation; QTL; selective sweeps
Introduction
Life history traits are phenotypic characters that affect the fitness
of organisms (Knight and Robertson 1957; Stearns 1976, 1989;
Charlesworth et al. 2003; Flatt and Heyland 2011; Flatt 2020).
Traits, such as fecundity, size at birth, age at reproductive matu-
rity, and stage- or size-specific rates of survival, interact with
each other to affect the fitness of organisms in an ever-changing
environment. Genes that affect life history traits should be sub-
ject to strong natural selection because they directly affect the
fitness of organisms. Adaptive alleles with strong selective
advantages in life history-related genes are likely to spread rap-
idly across a population in a selective sweep (Smith and Haigh
1974; Kaplan et al. 1989; Berry et al. 1991; Stephan 2019).
Signatures of selective sweeps include a loss of neutral polymor-
phism, drastic changes in the site frequency spectrum, and par-
ticular patterns of linkage disequilibrium (LD) across the site of
selection (Smith and Haigh 1974; Braverman et al. 1995; Fay and
Wu 2000; Kim and Nielsen 2004; Stephan et al. 2006; Stephan
2019). Identification of selective sweeps by these signatures pro-
vides a key to locate genes under selection and helps to under-
stand the process of adaptation and evolution.
Caenorhabditis elegans is a free-living nematode and a keystone
model organism for biological research. The reproductive mode
of C. elegans is androdioecy, with predominant self-fertilization of
hermaphrodites and rare outcrossing between hermaphrodites
and males (Brenner 1974). A single hermaphrodite of the labora-
tory reference strain N2 lays approximately 300 self-fertilized
in standard laboratory conditions (Hodgkin and
embryos
Doniach 1997; Fe´ lix and Braendle 2010). Newly hatched animals
develop through four larval stages (L1–L4) into mature reproduc-
tive adults after 3 days in favorable conditions at 20(cid:2) (Fre´ zal and
Fe´ lix 2015). Under stressful conditions, such as crowding and lim-
ited food, C. elegans enters the dauer diapause stage during larval
development to enable survival in harsh environments and to fa-
cilitate dispersal. C. elegans likely has a boom-and-bust life cycle
in the wild because of fluctuating environmental conditions and
the spatio-temporal distributed habitats, such as rotting fruits
and stems (Fe´ lix and Duveau 2012; Fre´ zal and Fe´ lix 2015). C. ele-
gans is globally distributed (Kiontke et al. 2011; Andersen et al.
2012; Fe´ lix and Duveau 2012; Cook et al. 2017; Crombie et al. 2019;
Lee et al. 2021). Although recent studies characterized high ge-
netic diversity of the species in Hawaii and the surrounding
Received: February 16, 2021. Accepted: May 03, 2021
VC The Author(s) 2021. Published by Oxford University Press on behalf of Genetics Society of America.
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs licence (http://creativecommons.
org/licenses/by-nc-nd/4.0/), which permits non-commercial reproduction and distribution of the work, in any medium, provided the original work is not altered
or transformed in any way, and that the work is properly cited. For commercial re-use, please contact [email protected]
2 | G3, 2021, Vol. 11, No. 8
Pacific regions (Crombie et al. 2019; Lee et al. 2021), C. elegans
exhibits low overall genetic diversity at the global scale (Barrie` re
and Fe´ lix 2005; Cutter 2006; Andersen et al. 2012). The metapopu-
lation dynamics, seasonal bottlenecks, predominant selfing, low-
outcrossing rate, low-recombination rate, background selection,
and recent selective sweeps might all contribute to the low-ge-
netic diversity of the species (Barrie` re and Fe´ lix 2005, 2007; Cutter
2006; Rockman and Kruglyak 2009; Rockman et al. 2010;
Andersen et al. 2012). In the genomes of many C. elegans strains
sampled in temperate regions, chromosomes I, IV, V, and X ex-
hibit signatures of selective sweeps, such as an excess of rare var-
iants, high LD, and extended haplotype homozygosity over large
genomic regions (Andersen et al. 2012). By contrast, the genomes
of most Hawaiian C. elegans strains have no such signatures
(Andersen et al. 2012; Crombie et al. 2019; Lee et al. 2021). Analyses
of C. elegans genetic diversity, population structure, gene flow,
and haplotype structure suggest that C. elegans originated from
the Pacific region, such as the Hawaii Islands, the western United
States, or New Zealand, and expanded worldwide, especially into
human-associated habitats (Andersen et al. 2012; Crombie et al.
2019; Lee et al. 2021). The recent positive selective sweeps likely
occurred during this expansion, but the beneficial alleles that
have driven the sweeps and their fitness advantages are yet un-
known.
Here, we measured lifetime fecundity of 121 wild C. elegans
strains and compared this trait between swept strains that expe-
rienced the recent selective sweeps and divergent strains that
avoided these sweeps. We found that swept strains had signifi-
cantly higher lifetime fecundity than divergent strains, as well as
significant geographical differences in lifetime fecundity between
strains from the Hawaii Islands and strains from other parts of
the world. We then used genome-wide association (GWA) map-
ping to identify three quantitative trait loci (QTL) on chromo-
somes I, II, and V that influence the lifetime fecundity of C.
elegans. In addition, we identified eight QTL that impact C. elegans
fecundity in different laboratory environments using GWA and
linkage mappings of previous fecundity data. The 11 QTL reveal
the complex genetic architecture of C.
fecundity.
Furthermore, we discovered that the different alleles at each QTL
peak marker and the different haplotypes in each QTL among the
121 strains were strongly correlated with signatures of recent se-
lective sweeps found in each strain. Our results suggest that
higher lifetime fecundity could have provided selective advan-
tages for swept strains and the underlying genetic variants might
have driven the recent strong sweeps in the C. elegans strains that
have colonized the world.
elegans
Materials and methods
Caenorhabditis elegans strains
All the wild strains were obtained from C. elegans Natural
Diversity Resource (CeNDR) (Cook et al. 2017). Animals were cul-
tured at 20(cid:2) on modified nematode growth medium (NGMA) con-
taining 1% agar and 0.7% agarose to prevent burrowing and fed
the Escherichia coli strain OP50.
Swept haplotypes and strains
Haplotype data for 403 C. elegans isotypes, representing 913 wild
strains, were acquired from the 20200815 CeNDR release. We
compared the total length of each haplotype per chromosome
across all isotypes to identify the most common haplotypes on
each chromosome. We then searched for the regions of the most
common haplotypes in each C. elegans isotype and recorded them
if their length was greater than 1 Mb (Crombie et al. 2019; Lee et al.
2021). We classified haplotypes outside of recorded regions as
unswept haplotypes. The swept status of some haplotypes was
undetermined when no identical-by-descent groups were found,
and thus the haplotype information for that region was missing
in the CeNDR release.
Signatures of selective sweeps were identified on chromo-
somes I, IV, V, and X, but not on chromosomes II and III
(Andersen et al. 2012). Therefore, we focused on the four chromo-
somes (I, IV, V, and X) and defined their most common haplo-
types as swept haplotypes (Lee et al. 2021). In each C. elegans
isotype, chromosomes that contain greater than or equal to 30%
of the swept haplotype were classified as swept chromosomes.
We classified isotypes with any swept I, IV, V, and X chromo-
somes as swept isotypes and isotypes without any swept I, IV, V,
and X chromosomes as divergent isotypes. Strains that belong to
swept isotypes and divergent isotypes were classified as swept
strains and divergent strains, respectively (Gilbert et al. 2020).
Genetic relatedness
Genetic variation data for 403 C. elegans isotypes were acquired
from the hard-filtered isotype variant call format (VCF) 20200815
CeNDR release. These variants were pruned to the 1,074,596 bial-
lelic single nucleotide variants (SNVs) without missing genotypes.
We converted this pruned VCF file to a PHYLIP file using the
vcf2phylip.py script (Ortiz 2019). The unrooted neighbor-joining
tree was made using the R packages phangorn (v2.5.5) and ggtree
(v1.14.6) (Schliep 2011; Yu et al. 2017).
Fecundity measurements
Prior to each assay, strains were grown for three generations
without bleaching, entering starvation, or encountering dauer-in-
ducing conditions (Andersen et al. 2014). For each C. elegans strain
in the fourth generation, single L4 larval stage hermaphrodites
were picked to each of five 3.5 cm NGMA plates with OP50 and
were maintained at 20(cid:2). For each assay plate, the original her-
maphrodite parent was transferred to a fresh plate every 24 hours
for 96 hours. A custom-built imaging platform (DMK 23GP031
camera; Imaging Source, Charlotte, NC, USA) was used to collect
images for each of the first four assay plates (0, 24, 48, and
72 hour samples) 48 hours after removal of the parent from each
plate. Most strains had few offspring after 96 hours. Images of the
fifth assay plates were collected 72 hours after the final transfer
of the parents. From each image, the total offspring was counted
by visual
in ImageJ
inspection using the Multi-point Tool
(v1.8.0_162) (Schneider et al. 2012). The original hermaphrodite
parents on the fifth assay plates were excluded from the counts.
The number of offspring in each of the first four assay plates cor-
responds to the daily fecundity. Numbers of offspring on the fifth
assay plates contained offspring from 3 days. For each biological
replicate of each C. elegans strain, the lifetime fecundity was cal-
culated as the total number of offspring from the five plates.
Replicates where the parent died were excluded from the analy-
sis. Only biological replicates with data from all five assay plates
were used in the calculations of daily and total fecundity. Daily
intrinsic growth rate (r) for each strain was calculated by r ¼
ln(mx)/x, where x is animal age after hatching (2 þ day of adult-
hood) and mx is cumulative fecundity by each age (Vassilieva and
Lynch 1999; Anderson et al. 2011).
We collected fecundity data for 557 replicates of 121 C. elegans
strains [mean lifetime fecundity (MLF) ¼ 231, standard deviations
(SD) ¼ 55]: 84 strains with five replicates (MLF ¼ 232, SD ¼ 55), 28
strains with four replicates (MLF ¼ 229, SD ¼ 52), seven strains
with three replicates (MLF ¼ 214, SD ¼ 49), and two strains with
two replicates (MLF ¼ 292, SD ¼ 19). These 121 strains were mea-
sured in 15 blocks, with 5–10 strains
in each block
(Supplementary File S3). Six of the 15 blocks (blocks 2, 8, 12, 13,
14, and 15) only contained swept strains. Three of the 15 blocks
(blocks 1, 3, and 7) only contained divergent strains. The remain-
ing six blocks (blocks 4, 5, 6, 9, 10, and 11) contained a mix of
swept and divergent strains. We performed post hoc analysis to
detect potential block effects, using the aov() and the TukeyHSD()
functions in the R package stats (v3.5.3) (https://www.R-project.
org/). Of the 105 pairwise comparisons of lifetime fecundity
among the 15 blocks, only block 13 (with 10 swept strains)
showed significantly higher fecundity than four blocks: block 3
(seven divergent strains), block 4 (eight divergent strains and one
swept strains), block 6 (eight divergent strains and two swept
strains), and block 10 (four divergent strains and six swept
strains). Generally, block effects were rare and might be associ-
ated with genotypes of strains in blocks. We included all 121
strains of the 15 blocks in the following analysis.
GWA mapping
GWA mapping was performed on the mean fecundity measure-
ments of biological replicates from 121 C. elegans strains, which
belong to 121 distinct isotypes. Genotype data for each of the 121
isotypes were acquired from the hard-filtered isotype VCF
(20200815 CeNDR release). We performed the mapping using the
pipeline cegwas2-nf (https://github.com/AndersenLab/cegwas2-
nf) as previously described (Zdraljevic et al. 2019; Na et al. 2020).
Briefly, we used BCFtools (Li 2011) to filter variants that had any
missing genotype calls and variants that were below the 5% mi-
nor allele frequency. We used PLINK v1.9 (Purcell et al. 2007;
Chang et al. 2015) to prune the genotypes to 56,878 markers with
a LD threshold of r2 < 0.8 and then generated the kinship matrix
using the A.mat() function in the R package rrBLUP (v4.6.1)
(Endelman 2011). The number of independent tests (Ntest) within
the genotype matrix was estimated using the R packages RSpectra
(v0.16.0) (https://github.com/yixuan/RSpectra) and correlateR (0.1)
(https://github.com/AEBilgrau/correlateR). The eigen-decomposi-
tion significance (EIGEN) threshold was calculated as (cid:3)log10(0.05/
Ntest). We used the GWAS() function in the rrBLUP package to per-
form the genome-wide mapping with the EMMA algorithm (Kang
et al. 2008). QTL were defined by at least one marker that was
above the Bonferroni-corrected significance (BF) threshold, to lo-
cate the best estimate of QTL positions with the highest signifi-
cance. We used the LD() function from the R package genetics
(v1.3.8.1.2) (https://cran.r-project.org/package¼genetics) to calcu-
late the LD correlation coefficient r2 among the QTL peak
markers associated with C. elegans lifetime fecundity.
We also performed GWA mapping using fecundity data in
DMSO control conditions from a previous study (Hahnel et al.
2018), where 236 C. elegans wild strains were cultured and pheno-
typed using the high-throughput fitness assays (HTA) as previ-
ously described. Briefly, L4 larval stage hermaphrodites were
cultured to gravid adult stage on plates and were bleached to ob-
tain synchronized offspring. The embryos were grown to L4 larval
stage in liquid (K medium) (Boyd et al. 2012) and fed an E. coli
HB101 lysate (Garcı´a-Gonza´ lez et al. 2017) in 96-well plates. A
large-particle
BIOSORT; Union
Biometrica, Holliston, MA, USA) was used to sort three L4 larvae
into each well of new 96-well plates containing K medium, E. coli
HB101 lysate, and 1% DMSO. Animals in the 96-well plates were
incubated at 20(cid:2) for 96 hours to allow animals to grow and pro-
duce offspring, followed by measurements of various fitness
flow cytometer
(COPAS
G. Zhang, J. D. Mostad, and E. C. Andersen | 3
parameters,
including fecundity. Raw fecundity data were
pruned, normalized, and regressed using the R package easysorter
(v1.0) (Shimko and Andersen 2014; Hahnel et al. 2018). The proc-
essed fecundity, norm.n, of each strain was used here for GWA
mapping.
Statistical analysis
Statistical significance of fecundity and intrinsic growth rate dif-
ferences between swept strains (groups) and divergent strains
(groups), and fecundity differences among different sampling
locations, were tested using the Wilcoxon test and P-values were
adjusted for multiple comparisons (Holm method) using the com-
pare_means() function in the R package ggpubr (v0.2.4) (https://
github.com/kassambara/ggpubr/). Broad-sense heritability of C.
elegans lifetime fecundity was calculated using the lmer() function
in the R package lme4 (v1.1.21) with the model phenotype (cid:4) 1þ
(1jstrain) (Bates et al. 2015).
package
Linkage mapping
We performed linkage mapping using fecundity data from a large
panel of recombinant inbred advanced intercross lines (RIAILs)
derived from QX1430 and CB4856 (Andersen et al. 2015). The fe-
cundity (norm.n) of the RIAILs and the parents were measured
using the HTA as described above, under three conditions: 1%
H2O (402 RIAILs), 1% DMSO (417 RIAILs), and 0.5% DMSO (432
RIAILs). Linkage mapping was performed on each trait using the
R
(https://github.com/
AndersenLab/linkagemapping) and the single-nucleotide varia-
tion data of the RIAILs in the package as described previously
(Evans and Andersen 2020). Briefly, logarithm of the odds (LOD)
scores for each genetic marker and each trait were calculated us-
ing the function fsearch(). The QTL threshold for significant LOD
scores in each mapping was defined by permuting trait values
1000 times, mapping the permuted trait data, and taking the 95th
quantile LOD score as the 5% genome-wide error rate. 95% confi-
dence intervals of each QTL were determined using the function
annotate_lods.
linkagemapping
(v1.3)
Data availability
The datasets, code for generating all figures, and Supplementary
figures can be found at https://github.com/AndersenLab/swept_
broods. Supplementary File S1 contains the haplotype data of 403
C. elegans isotypes from CeNDR release 20200815. Supplementary
File S2 contains genetic relatedness of 403 C. elegans isotypes.
Supplementary File S3 contains lifetime fecundity of 121 C. ele-
gans strains, their classification of swept strains and divergent
strains, and the assay blocks of these strains. Supplementary File
S4 contains daily fecundity and daily intrinsic growth rate of 121
C. elegans strains. Supplementary File S5 contains GWA results on
lifetime fecundity of 121 C. elegans strains. Supplementary File S6
contains genotype and phenotype data of 121 C. elegans strains at
the peak markers of GWA mapping. Supplementary File S7 con-
tains
strains.
Supplementary File S8 contains the GPS coordinates of sampling
locations of 121 C. elegans strains. Supplementary File S9 contains
lifetime fecundity and swept and divergent classifications of each
of the four swept chromosomes for each of the 121 C. elegans
strains. Supplementary File S10 contains LD results among the
three QTL of GWA using 121 C. elegans strains. Supplementary
File S11 contains the shared haplotypes of the 121 strains within
the QTL of GWA mapping. Supplementary File S12 contains GWA
results on fecundity data of 236 strains from a previous study
(Hahnel et al. 2018). Supplementary File S13 contains genotype
the sampling locations of 121 C.
elegans
4 | G3, 2021, Vol. 11, No. 8
and phenotype data of 236 strains at the peak marker of GWA
mapping. Supplementary File S14 contains the shared haplotypes
of
the 236 strains within the QTL of GWA mapping.
Supplementary File S15 contains the linkage mapping results for
the 402 RIAILs in 1% water condition. Supplementary File S16
contains genotype and phenotype data of the 402 RIAILs at the
peak markers and phenotype data of the parents in linkage map-
ping results. Supplementary File S17 contains the linkage map-
ping results for
the 417 RIAILs in 1% DMSO condition.
Supplementary File S18 contains genotype and phenotype data of
the 417 RIAILs at the peak markers and phenotype data of the
parents in linkage mapping results. Supplementary File S19 con-
tains the linkage mapping results for the 432 RIAILs in 0.5%
DMSO condition. Supplementary File S20 contains genotype and
phenotype data of the 432 RIAILs at the peak markers and pheno-
type data of the parents in linkage mapping results.
.
Results
Chromosome-scale sweeps shape C. elegans
strain relationships
Genomic information of 913 wild C. elegans strains, grouped into
403 genetically distinct
isotypes, are currently available in
CeNDR (Cook et al. 2017). The latest CeNDR haplotype data, in-
ferred from identical-by-descent groups among the 403 isotypes,
include 22,859 distinct haplotypes across the genome. The num-
ber of haplotypes on each chromosome ranged from 2567 to
5199. We identified 11 most common haplotypes found in the
majority of wild strains. Of the 403 C. elegans isotypes, 331 share
more than 1 Mb of regions with at least one of the 11 most com-
mon haplotypes, particularly on chromosomes I, IV, V, and X
(Figure 1A, Supplementary File S1). The haplotype structure of
shared haplotypes over large regions across 403 isotypes further
supported the selective sweeps identified previously (Andersen
et al. 2012).
elegans
The shared fraction of the most common haplotypes per chro-
mosome varies in each C. elegans isotype. Among chromosomes
with shared regions in the 331 isotypes, chromosomes I, II, III, IV,
V, and X have mean shared fractions and SD of 0.45 6 0.25,
0.21 6 0.19, 0.22 6 0.17, 0.52 6 0.28, 0.60 6 0.27, and 0.43 6 0.28, re-
spectively. We focused on swept haplotypes, the most common
haplotypes on chromosomes I, IV, V, and X, where evidence of se-
lective sweeps were identified (Andersen et al. 2012). The chromo-
somal sharing of swept haplotypes contributes substantially to
the genetic relatedness of C.
isotypes (Figure 1B,
Supplementary File S2).
Isotypes with swept chromosomes,
which contain greater than or equal to 30% of swept haplotypes,
clustered together. Of the 331 isotypes noted above, 281 have at
least one swept chromosome (Figure 1B). We classified these 281
C. elegans isotypes as swept isotypes. We found that 244 swept
isotypes have at least two swept chromosomes. By contrast, most
of the 122 divergent isotypes with no swept chromosomes clus-
tered together (Figure 1B). Previous analyses on genome-wide av-
erage nucleotide diversity (p), Tajima’s D, and genome-wide
Hudson’s FST between 43 Hawaiian isotypes (most are divergent
isotypes) and 233 non-Hawaiian isotypes (most are swept iso-
types) also revealed a high degree of divergence, the highest of
which were found in genomic regions impacted by the selective
sweeps (Crombie et al. 2019). The high degree of genetic related-
ness across the species is driven by the selective sweeps, but the
fitness advantage causing the strong selective sweeps is yet un-
known.
A
I
II
III
IV
V
B
X
0
1
2
3
4
0 5 10 150 5 10 15 0 5 10
0 5 10 15 0 5 10 15 20 0 5 10 15
Genomic position (Mb)
Figure 1 Swept chromosomes and genetic relatedness of wild C. elegans
isotypes. (A) Sharing of the most common haplotypes (red) across the
genome of C. elegans for 403 isotypes is shown. Genomic regions of
unswept haplotypes (haplotypes other than the most common
haplotypes) are colored gray. White segments are undetermined
haplotypes in regions where no identical-by-descent groups were found
(Crombie et al. 2019). The genomic position is plotted on the x-axis. Each
row on the y-axis represents one of the 403 isotypes, ordered as their
positions in (B). (B) A tree showing genetic relatedness of the 403 C.
elegans isotypes, using 1,074,596 biallelic segregating sites, is shown. The
tips of the tree are colored by the number of swept chromosomes (purple
for zero, deep blue for one, light blue for two, orange for three, and gold
for four) in each C. elegans isotype.
Natural variation in fecundity among swept and
divergent strains
To compare the fitness between swept and divergent isotypes, we
measured lifetime fecundity of 121 wild C. elegans strains sam-
pled across the globe (Supplementary Figure S1 and File S8).
Single fourth larval stage hermaphrodites were transferred daily
for 5 days and maintained under normal laboratory conditions.
We manually counted the viable offspring from images of assay
plates. The results showed large variation in lifetime fecundity
among wild C. elegans strains (Figure 2A, Supplementary File S3).
The MLF ranged from 106 to 335 offspring among the 121 strains.
We observed the species reproductive peak in the second day of
the assay, with a median peak number of 109 offspring (Figure
2B, Supplementary File S4).
Of the 121 C. elegans strains, 68 strains were classified as
“swept” strains and 53 strains were classified as “divergent”
strains (see Materials and Methods, Figure 2B, Supplementary
Figure S1 and File S3). MLF of swept strains was significantly
higher than divergent strains (Wilcoxon test, adjusted P ¼ 9.1E-6)
(Figure 2B). Because different strains could have different swept
chromosomes, we extended the comparisons to chromosome lev-
els (Supplementary Figure S2 and File S9). We assigned strains
A
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G. Zhang, J. D. Mostad, and E. C. Andersen | 5
swept
divergent
N2
CB4856
Strain
Day 1
Day 2
Day 3
Day 4
Days 5-7
**
****
*
ns
***
Lifetime
68
53
****
200
150
100
50
swept
divergent
0
Figure 2 Natural variation in C. elegans fecundity. (A) A bar plot for lifetime fecundity (y-axis) of 121 wild C. elegans strains is shown. Strains on the x-axis
are sorted by their MLF of two to five biological replicates. Error bars show standard errors of lifetime fecundity among replicates. The lab reference
strain N2 and the Hawaii strain CB4856 are colored orange and blue, respectively; other strains are colored gold for swept strains and purple for
divergent strains. (B) Comparisons of lifetime and daily fecundity between 68 swept strains (gold) and 53 divergent strains (purple) are shown as Tukey
box plots. Statistical significance was calculated using the Wilcoxon test and was corrected for multiple comparisons (Holm method). Significance of
each comparison is shown above each comparison pair (ns: adjusted P-value > 0.05; *: adjusted P-value (cid:5) 0.05; **: adjusted P-value (cid:5) 0.01; ***: adjusted
P-value (cid:5) 0.001; ****: adjusted P-value (cid:5) 0.0001).
into swept groups or divergent groups in each swept chromo-
some, depending on whether isotypes had a specific swept chro-
mosome. Although the numbers of strains in the two groups
were different across swept chromosomes, swept groups always
showed significantly higher lifetime fecundity than divergent
groups (Wilcoxon test, adjusted P < 0.0001)
(Supplementary
Figure S2). The striking differences in lifetime fecundity sug-
gested that swept strains have higher fitness than divergent
strains under normal laboratory conditions. A later switch from
spermatogenesis to oogenesis during the development of C. ele-
gans could lead to the generation of more sperm and thus higher
lifetime fecundity. This later switch would likely be associated
with a trade-off of lower fecundity in early reproduction days.
Surprisingly, we found that swept strains showed significantly
higher daily fecundity than divergent strains in the first 3 days of
the assays (Wilcoxon test, adjusted P ¼ 0.0016, adjusted P ¼ 1.7E-
6, and adjusted P ¼ 0.014, respectively) (Figure 2B). Swept strains
also showed significantly higher intrinsic growth rate (r, maxi-
mum r was found at day two of adulthood for most strains) than
P ¼ 9.9E-6)
divergent
(Supplementary Figure S3 and File S4). This significant difference
of fecundity between swept and nonswept groups provided an
opportunity to dissect the genetic basis of the natural variation in
lifetime fecundity. We calculated the broad-sense heritability
and found a substantial heritable genetic component (H2 ¼ 0.63)
of the phenotypic variance across these strains.
(Wilcoxon
adjusted
strains
test,
Three QTL are associated with natural variation
in C. elegans lifetime fecundity
To identify genomic loci that underlie fecundity variation, we
performed a marker-based GWA mapping using MLF data
from 121 C. elegans strains and the whole-genome variant
data from CeNDR. We identified three distinct QTL (Figure 3A,
Supplementary File S5). The first QTL, located on the right
arm of chromosome I, has a peak-marker at position
13,917,228 and explains 21% of the phenotypic variation
among the 121 strains. The second QTL located on the left
arm of chromosome II has a peak-marker position at 543,326
and explains 22% of the phenotypic variation. The third QTL
spans the center of chromosome V with the peak marker lo-
cated at 14,534,671 and explains 30% of the phenotypic varia-
the strong LD within and between
tion. Because of
chromosomes in C. elegans (Andersen et al. 2012),
linked
regions might be falsely discovered as QTL even though they
have no variants that underlie the phenotypic variation. To
test the independence of the three QTL, we calculated the
pairwise LD among their peak markers (Supplementary Figure
S4 and File S10). The results showed moderate levels of LD
(ranged from 0.387 to 0.512) for all three pairs, suggesting that
they might not be independent. Notably, at all QTL peak
markers, most swept strains have the reference alleles and
most divergent strains have the alternative alleles (Figure 3B,
Supplementary File S6). We further compared the sharing of
6 | G3, 2021, Vol. 11, No. 8
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IV
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0 5 10 150 5 10 15 0 5 10
0 5 10 15 0 5 10 15 20 0 5 10 15
Genomic position (Mb)
I:13917228
II:543326
V:14534671
swept
divergent
REF
ALT
REF
ALT
REF
ALT
Figure 3 Three QTL were identified in GWA mapping of lifetime fecundity variation in 121 C. elegans wild strains. (A) Manhattan plot indicating GWA
mapping results. Each point represents an SNV that is plotted with its genomic position (x-axis) against its (cid:3)log10(p) value (y-axis) in mapping. SNVs that
pass the genome-wide EIGEN threshold (the dotted gray horizontal line) and the genome-wide BF threshold (the solid gray horizontal line) are colored
pink and red, respectively. (B) Tukey box plots showing lifetime fecundity between strains with different genotypes at the peak marker position in each
QTL. Each point corresponds to a C. elegans strain and is colored gold for swept strains and purple for divergent strains. On the x-axis, REF represents
strains with the N2 reference allele and ALT represents strains with the alternative allele.
haplotypes among the 121 strains within each QTL region
(Supplementary Figure S5 and File S11). The majority of the
strains with the reference alleles at the peak markers have
the most common haplotypes in the QTL regions. By contrast,
few strains with alternative alleles have the most common
haplotypes in the QTL regions. Taken together, these results
suggest that the genetic variants and different haplotypes un-
derlying lifetime fecundity variation might be linked to the se-
lective sweeps in the global population of C. elegans.
Hawaiian C. elegans exhibit lower lifetime
fecundity than strains sampled across the globe
Most of the 121 C. elegans strains were originally sampled from
three geographically isolated locations: 50 from the Hawaiian
Islands, 22 from North America, and 41 from Europe
(Supplementary Figure S1). Of the 50 Hawaiian C. elegans strains,
46 were classified as divergent, and the other four strains have no
more than two swept chromosomes (Figure 4, Supplementary
Figure S1 and File S7). Most C. elegans strains from North America
and Europe were classified as
(Figure 4,
Supplementary Figure S1 and File S7). We compared lifetime fe-
cundity of strains isolated from these three locations (Figure 4).
Compared to strains from North America and Europe, Hawaiian
strains had significantly lower lifetime fecundity (Wilcoxon test,
adjusted P ¼ 0.0013 and adjusted P ¼ 2.2E-5, respectively). The dif-
ference in lifetime fecundity between strains from North
America and strains from Europe was not significant. These data
suggested that the selective sweeps that occurred outside Hawaii
contribute substantially to the geographical lifetime fecundity
difference.
strains
swept
****
**
swept
divergent
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200
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100
Hawaii
North America
Sampling location
Europe
Figure 4 Lifetime fecundity comparisons in wild C. elegans strains among
different sampling locations. Comparisons of lifetime fecundity among
strains collected from Hawaii (50 strains), North America (22 strains),
and Europe (41 strains). Each point corresponds to a strain and is colored
gold for swept strains and purple for divergent strains. Statistical
significance was calculated using the Wilcoxon test. Significance of each
comparison is shown above each comparison pair (**: adjusted P-value (cid:5)
0.01; ****: adjusted P-value (cid:5) 0.0001). The difference of lifetime fecundity
between North American and European strains is not significant.
More QTL underlying lifetime fecundity of C.
elegans
We also mapped the fecundity data in the 1% DMSO control con-
dition from one of our published studies that used the high-
throughput fitness assays (HTA) (see Materials and Methods) to
measure various fitness parameters of 236 strains (209 swept
strains and 27 divergent strains) (Hahnel et al. 2018). Here, we per-
formed GWA mapping using the fecundity measurements
(norm.n) and identified a QTL on chromosome X (from 3.9 to
5.4 Mb, with the peak marker at 4,831,537)
(Figure 5,
Supplementary Figure S6A and File S12). Divergent strains
showed no enrichment with either genotype at the peak marker
(Supplementary Figure S6B and File S13). However, most strains
with the reference allele have the most common haplotypes and
most strains with the alternative allele have unswept haplotypes
(Supplementary Figure S6C and File S14). These results suggest
that the genetic variants in this region might also be linked to the
recent selective sweeps in wild C. elegans populations.
Also using HTA as above, we measured fecundity in liquid cul-
ture using the C. elegans RIAILs derived from QX1430 (a derivative
strain of N2 with replacement of the N2 npr-1 allele with the
counterpart version from the CB4856 strain and a transposon in-
sertion into the peel-1 gene) and CB4856 (Andersen et al. 2015) un-
der three conditions: 1% water, 1% DMSO, and 0.5% DMSO (see
Materials and Methods). By contrast to the fecundity variation of
C. elegans strains cultured in agar plates, the QX1430 strain
showed lower fecundity than the CB4856 strain using HTA
(Supplementary Figures S7B, S8B, and S9B and Files S16, S18, and
S20), indicating that the gene npr-1 or environmental factors can
have drastic effects on C. elegans fecundity (Andersen et al. 2014).
We found seven QTL for fecundity on chromosomes II, IV, and V
under the three conditions (Figure 5, Supplementary Figures S7A,
S8A, and S9A). In 1% water, linkage mapping identified a single
QTL confidence interval (II: 3.4–4 Mb) on the left arm of chromo-
some II (Figure 5, Supplementary Figure S7A and File S15). In 1%
DMSO, linkage mapping identified two QTL located on chromo-
somes IV (5–11.9 Mb) and V (11.8–14.2 Mb), respectively (Figure 5,
Supplementary Figure S8A and File S17). In 0.5% DMSO, the four
QTL on chromosomes II (2.9–10.2 Mb), IV (two loci, 3.9–17.5 Mb),
and V (8.7–12.3 Mb) recapitulated the three QTL detected in 1%
water and 1% DMSO, respectively (Figure 5, Supplementary
Figure S9A and File S19). Furthermore, the QTL on chromosome
V in both DMSO conditions overlapped with the GWA QTL on
chromosome V using the 121 wild strains in agar plates (Figure
5). Because linkage mapping using this set of C. elegans RIAILs can
I
II
III
IV
V
X
Agar plate
1% DMSO
1% water
1% DMSO
0.5% DMSO
− log10(p)
10
LOD
9
8
7
6
9
8
7
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W
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Genomic position (Mb)
Figure 5 Multiple QTL impacting C. elegans lifetime fecundity in different
conditions. Four GWA mapping QTL of two conditions (121 strains
cultured in agar plate and 236 strains cultured in liquid with 1% DMSO)
and seven linkage mapping QTL of three conditions (C. elegans RIAILs
cultured in liquid with 1% water, 1% DMSO, and 0.5% DMSO,
respectively) are plotted. Each condition is plotted on the y-axis against
the genomic position of its QTL on the x-axis separated by chromosomes
with tick marks denoting every 5 Mb. Each QTL is plotted as a line with a
triangle indicating the peak marker and colored by the (cid:3)log10(p) value
(GWA QTL) or the logarithm of the odds (LOD) score (for linkage mapping
QTL), increasing in significance from blue to red.
G. Zhang, J. D. Mostad, and E. C. Andersen | 7
only find QTL in the CB4856 strain, overlapping of QTL between
linkage mapping and GWA mapping suggests that the CB4856
strain carries the common alternative alleles among wild C. ele-
gans strains in the shared regions. Altogether, these results sug-
gest that C. elegans might have shared and separated loci
controlling fecundity in agar cultures and in liquid cultures with
slightly different concentrations of DMSO.
Discussion
In this study, we report natural variation of lifetime fecundity for
121 wild C. elegans strains and found that the previously reported
chromosome-scale selective sweeps play a key role in the differ-
ent fecundity among strains. We defined swept haplotypes,
swept isotypes, and swept strains, using the latest C. elegans hap-
lotype data from CeNDR. Swept strains that have at least one
chromosome with equal or greater than 30% of swept haplotypes
showed significantly higher lifetime fecundity than divergent
strains that have avoided the sweeps. We identified three QTL
that underlie differences in lifetime fecundity among the 121 C.
elegans strains using single-marker based GWA mappings.
Remarkably, across all three QTL, swept strains tend to have
shared haplotypes and the reference alleles at peak markers. By
contrast, divergent strains tend to have unswept haplotypes and
the alternative alleles at peak markers. We also observed signifi-
cant geographical differences in lifetime fecundity between
Hawaiian strains and strains from other parts of the world, likely
because of the selective sweeps. We further mapped previous
data using GWA mapping and linkage mapping and identified
eight QTL underlying C. elegans fecundity in different environ-
ments. Taken together, our results showed the diverse genetic
basis of C. elegans fecundity and suggest that higher fecundity in
most C. elegans strains could be caused by alleles that have re-
cently swept throughout the world population.
Genetically divergent strains have substantially
lower fecundity than swept strains
We measured lifetime fecundity in 121 genetically distinct C. ele-
gans strains. In our measurements (Figure 2A), the laboratory ref-
erence strain N2 (known as the Bristol strain) and a frequently
used wild strain CB4856 (known as the Hawaii strain) had lifetime
fecundity of 308 and 237, respectively, with similar fecundity val-
ues as reported previously (Hodgkin and Doniach 1997; Wegewitz
et al. 2008; Andersen et al. 2014; Poullet et al. 2015). The CB4856
strain had been considered the most genetically distant strain
from the N2 strain for decades. In the last 5 years, researchers
have collected and identified many genetically divergent C. ele-
gans strains, some of which are more divergent from the N2
strain than the CB4856 strain is (Cook et al. 2017; Crombie et al.
2019; Lee et al. 2021). Most of these divergent strains were from
Hawaii and showed none or rare evidence of the globally distrib-
uted swept haplotypes (Figure 1, Supplementary Figure S1)
(Crombie et al. 2019; Lee et al. 2021). In our fecundity assays, we
included many of these divergent strains. Divergent strains
showed significantly lower fecundity than swept strains that
have large blocks of swept haplotypes, suggesting that divergent
strains have lower fitness than swept strains under normal labo-
ratory conditions. The disadvantage in fecundity of divergent
strains was present from the beginning of the reproductive period
throughout the peak. This lower fitness of divergent strains could
have at least two possible explanations. First, laboratory condi-
tions might favor swept strains over divergent strains. Standard
laboratory conditions to culture C. elegans have been designed,
8 | G3, 2021, Vol. 11, No. 8
modified, and improved based on the growth of the N2 strain
(Brenner 1974), which is a swept strain. Most swept strains were
from temperate zones (Andersen et al. 2012; Fe´ lix and Duveau
2012; Petersen et al. 2014; Richaud et al. 2018), such as Western
Europe, whereas most divergent strains were isolated in the high
elevation and cool temperature niches in the Hawaiian Islands
(Crombie et al. 2019). The conditions of the natural habitats and
the microenvironments in the niches of swept strains could be dras-
tically different from niches of divergent strains. The closer the natu-
ral niche condition is to the laboratory condition, the higher fitness a
strain might have (Volkers et al. 2013). For example, compared to N2,
the strain CB4856 showed a clear thermal preference of approxi-
mately 17(cid:2), which is lower than the canonical and the most typical
C. elegans culture temperature of 20(cid:2) in the laboratory (Brenner 1974;
Stiernagle 2006; Anderson et al. 2007). In a competition assay be-
tween two swept strains that were isolated from locations with dis-
tinct climates, CX11314 (isolated at 20.9(cid:2)) showed higher fitness
than JU847 (isolated at 11.3(cid:2)) at both 15(cid:2) and 25(cid:2), but JU847 grew bet-
ter at 15(cid:2) than at 25(cid:2) (Evans et al. 2017). Divergent strains that were
isolated from cool regions might exhibit higher fitness at tempera-
tures lower than 20(cid:2).
The second explanation is that genetic variants at unknown
loci directly caused differences in lifetime fecundity between
swept strains and divergent strains. The environmental factors in
our assays might have similar or minor influences on the fecun-
dity for both swept strains and divergent strains. The major dif-
ferences in fecundity between swept strains and divergent
strains could be attributed to their genetic differences. For in-
stance, because a C. elegans hermaphrodite produces 200–300
sperm in the late L4 stage before irreversibly switching to oogene-
sis to produce up to 1000 oocytes, the number of sperm limits fe-
cundity of self-fertilized hermaphrodites (Ward and Carrel 1979;
Cutter 2004; Fe´ lix and Braendle 2010). Alleles at unknown loci in
swept strains might lead to an increased number of sperm and
thus a higher fecundity than divergent strains. It is also possible
that swept strains and divergent strains produce similar numbers
of sperm, but divergent strains have reduced sperm fertility,
defects in oogenesis, or higher embryonic lethality than swept
strains (Poullet et al. 2015). Because we quantified the viable off-
spring from each of the C. elegans strains as their fecundity (see
Materials and Methods), defects related to fertilization or higher
embryonic lethality could have caused the lower daily fecundity
in the first 3 days of the reproductive period and the lower life-
time fecundity observed in divergent strains. The higher fecun-
dity of swept strains in the first 3 days might also be caused by a
shorter duration of L4 larval stage and/or an earlier or more effi-
cient germline development (e.g., earlier onset or faster develop-
than divergent
ment of spermatogenesis and/or oogenesis)
strains. We picked L4 stage animals to start each assay, so those
L4 larvae of swept strains might be more mature than divergent
strains. Swept strains might start laying embryos and enter into
reproductive peak faster than divergent strains, demonstrating
the earlier advantages. Although our GWA results might have
mapped genomic regions underlying spermatogenesis, oogenesis,
fertilization success, or embryonic lethality, future efforts to
quantify developmental timing, the numbers of sperm, and fertil-
ized embryos among wild C. elegans strains will help to further
elucidate the differences in fecundity among strains. Moreover,
some divergent strains continued to produce many offspring in
the last few assay days, at a time when most swept strains gradu-
ally reduce offspring production. It is possible that swept strains
have a shortened but accelerated reproductive period. By con-
trast, divergent strains could have a prolonged but slow
reproductive period. To investigate the variation of reproductive
schedules and underlying genetic basis, further work to quantify
fecundity should proceed until the full depletion of self-sperm.
Diverse QTL for lifetime fecundity in different
environments
We performed GWA mapping and identified three QTL on chro-
mosomes I, II, and V for lifetime fecundity of C. elegans, which
were grown on agar plates and fed E. coli OP50. The split of strains
by genotypes at peak markers and the haplotypes of each strain
in each QTL strongly suggest that the three QTL could be the ge-
netic basis of different lifetime fecundity between swept strains
and divergent strains. The reference alleles and the most com-
mon haplotypes in each QTL, which provided the selective ad-
vantage of higher fecundity, could have swept through the C.
elegans population as these strains spread throughout the world.
Under similar conditions, a previous study using linkage mapping
and a large panel of RIAILs derived from the N2 and CB4856
strains have mapped fecundity to QTL on chromosomes II (2.6–
3.6 Mb) and X (4.6–7.7 Mb) (Andersen et al. 2014). A laboratory-de-
rived mutation in the gene npr-1 from N2 was identified to have
driven the QTL on chromosome X (McGrath et al. 2009; Andersen
et al. 2014).
In liquid culture and fed the E. coli strain HB101, a new panel
of C. elegans RIAILs with QX1430 and CB4856 was used to map fe-
cundity to a QTL on chromosome IV (10.7–12.8 Mb) using linkage
mapping (Andersen et al. 2015). Using the same RIAIL panel but
under three different liquid conditions (1% H2O, 1% DMSO, and
0.5% DMSO), we mapped fecundity to seven QTL on chromo-
somes II, IV, and V. In both DMSO conditions, the three QTL on
chromosome IV recapitulated the QTL in the above study
(Andersen et al. 2015); the two overlapping QTL on chromosome V
overlapped with the QTL using our 121 wild strains grown in agar
plates. We further used GWA to map previously published wild
strain fecundity data from liquid culture and 1% DMSO. A QTL
linked to the selective sweeps located on the left arm of chromo-
some X was identified. Although npr-1 is in the region of this QTL,
the laboratory-derived N2 npr-1 allele that is only found in the N2
strain could not underlie this QTL because it is not found in wild
strains. Distinct QTL were detected in the two GWA mappings.
The bleaching method to synchronize animals, liquid cultures,
and a different bacterial diet (E. coli HB101) might have affected
fecundity and the mapping results.
As a complex life history trait, lifetime fecundity could be
influenced by many loci (Houle 1992). Under different conditions,
GWA mappings identified QTL on chromosomes I, II, V, and X;
linkage mappings identified QTL on chromosomes II, IV, V, and X.
Because swept haplotypes shared among C. elegans strains might
have driven all the QTL in GWA mappings, genetic variants in
these swept haplotypes might be the beneficial alleles that swept
through the C. elegans population. Natural habitats of C. elegans
are likely quite different from both the laboratory standard con-
ditions and liquid cultures with DMSO. Our results of GWA and
linkage mappings suggest that shared and separate loci in the C.
elegans genome control fecundity in different environmental con-
ditions in the laboratory. We do not know how those environ-
ments relate to the wild, but it is possible that similar conditions
could occur (e.g., swimming or crawling in environments with
ample bacteria). Our results also suggest that fecundity of C. ele-
gans is sensitive to environmental changes in cultures (agar
plates vs liquid cultures; with or without DMSO) or diet (E. coli
OP50 and HB101). Larval and germline development of C. elegans
were previously found to be sensitive to food availability, diet,
and temperature (Poullet et al. 2015; Filina et al. 2020). Lifetime fe-
cundity of C. elegans might also be sensitive to changes in these
environmental factors. To deepen our understanding of the influ-
ence of genetic factors, environmental factors, and gene-environ-
ment interactions on fecundity, future efforts should include
more strains and compare their fecundity in diverse environ-
ments.
Potential adaptive alleles for C. elegans in
temperate zones
The QTL for lifetime fecundity using the 121 C. elegans strains
also shared genomic regions with QTL on weather and climate
variables related to natural habitats of 149 wild C. elegans strains
(Evans et al. 2017). Two of the GWA mapping QTL for relative hu-
midity were on chromosomes II and V, which overlapped with
our QTL on chromosomes II and V, respectively. GWA mappings
for 3-year average temperature also located the same QTL just
right of the center of chromosome V. We showed that C. elegans
strains sampled from Europe and North America had similar life-
time fecundity, which was significantly larger than fecundity of
Hawaiian C. elegans strains. Because Hawaii is in the tropical
zone, C. elegans isolated from high elevation areas in Hawaii could
have experienced high humidity and low temperatures in a much
more stable climate in the long term than C. elegans in temperate
zones. Alleles of swept strains in the shared QTL underlying life-
time fecundity and climate variables could have enhanced the
adaptability of C. elegans to variable humidity and temperatures
in temperate zones along the C. elegans expansion out of the
Pacific region (Andersen et al. 2012; Crombie et al. 2019; Lee et al.
2021). It is possible that, because of these adaptive alleles, the N2
strain showed no preference at these temperatures (Anderson
et al. 2007).
Some Hawaiian strains, exclusively isolated at lower eleva-
tions closer to the coasts, exhibited admixture with non-
Hawaiian populations, which might come from gene flow from
outcrossing with immigrating swept strains from outside to
(Crombie et al. 2019). But compared to most non-
Hawaii
Hawaiian strains, Hawaiian strains only contain, if any, small
fractions of swept haplotypes. Of the 50 Hawaiian C. elegans
strains used in this study, four strains are classified as swept
strains, who have no more than two swept chromosomes
(Supplementary Figure S1). The alleles that increase lifetime fe-
cundity in swept strains might not contribute to higher fitness for
C. elegans strains in Hawaii. In fluctuating environments in tem-
perate zones, the randomly distributed and limited habitats
might select for C. elegans that have higher fecundity, although
the high density of animals also facilitates dauer formation,
which could limit population growth but underlie future survival
success. Moreover, C. elegans populations in temperate zones also
undergo bottlenecks in winter, from which dauer larvae are more
likely to survive. By contrast, Hawaiian C. elegans might not need
to enter and stay in the dauer stage as often and long as non-
Hawaiian C. elegans in temperate zones. Habitats hypothesized to
have more ample bacterial food (e.g., rotting fruits) and a stable
environment in Hawaii could lead to a higher survival rate and
lower fecundity as a trade-off (Stearns 1989; Marshall and
Sinclair 2010). Two genotypes of the gene srg-37 were found to co-
exist in the wild population and associate with different niches
(Lee et al. 2019). The deletion in srg-37, which likely originated
outside of Hawaii, reduces dauer formation and promotes repro-
duction in niches hypothesized to promote rapid growth. C. ele-
gans strains without deletion of srg-37 could have higher fitness
during the dispersal phase in nutrient-poor environments (Lee
G. Zhang, J. D. Mostad, and E. C. Andersen | 9
et al. 2019). Among the 121 strains we studied, none of the diver-
gent strains have the srg-37 deletion. However, other QTL might
exist among divergent strains to reduce dauer formation (Green
et al. 2013, 2014). Future dauer formation assays, such as
responses to ascaroside pheromones, among divergent strains
could help dissect the interactions of traits that affect fitness of
C. elegans in the wild.
However, the QTL we found that underlie higher fecundity in
swept strains might not directly underlie the selective advan-
tages during the expansion of the species. Loci in C. elegans that
affect other fitness traits (e.g., dauer formation, response to natu-
ral food source of different bacteria, or resistance to natural
pathogens) might be under direct selective pressures in the wild.
Alleles that provided higher fitness in these traits might underlie
the selective sweeps in C. elegans population. The QTL for fecun-
dity variation in our results might be in LD with genomic regions
that affect these other fitness traits mentioned above and main-
tained by linked selection. The swept strains are widely distrib-
uted in different environments around the world, so the effects
of the interaction between genotype and environment could have
also influenced this expansion. To find the direct targets of selec-
tion and the principal drivers of selective sweeps, multiple abiotic
and biotic factors in natural habitats of C. elegans should be mea-
sured. Then, several fitness traits under different conditions
could be measured in the laboratory. For instance, fecundity and
viral load could be measured at different temperatures at the
same time (Fe´ lix et al. 2011; Samuel et al. 2016).
Acknowledgments
The authors would like to thank members of the Andersen Lab
for helpful comments on the manuscript.
Funding
G.Z. and E.C.A. received support from the NSF-Simons Center for
Quantitative Biology at Northwestern University (awards Simons
Foundation/SFARI 597491-RWC and the National Science Foundation
1764421). J.D.M received support from a Northwestern Undergraduate
Research Grant.
Conflicts of interest
The authors declare no conflicts of interest.
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Communicating editor: K. Gunsalus
| null |
10.1038_s41477-023-01439-4.pdf
|
nd versatile 3D segmentation of plant
Data availability
Image data are available at http://neomorph.salk.edu/downloads/phy-
tomap/. Sequences of all the DNA probes used in this study are provided
in Supplementary Table 2. Processed and annotated scRNA-seq data is
available at the Gene Expression Omnibus (GSE152766).
tissues at cellular r
|
Data availability Image data are available at http://neomorph.salk.edu/downloads/phy- tomap/ . Sequences of all the DNA probes used in this study are provided in Supplementary Table 2 . Processed and annotated scRNA-seq data is available at the Gene Expression Omnibus ( GSE152766 ). Code availability The code to analyse PHYTOMap data is available at https://github.com/ tnobori/PHYTOMap . Extended Data
|
Multiplexed single-cell 3D spatial gene
expression analysis in plant tissue
using PHYTOMap
https://doi.org/10.1038/s41477-023-01439-4
Received: 10 August 2022
Accepted: 11 May 2023
Published online: 12 June 2023
Check for updates
Tatsuya Nobori
& Joseph R. Ecker
1,2
, Marina Oliva
3, Ryan Lister
3,4
1,2,5
Retrieving the complex responses of individual cells in the native
three-dimensional tissue context is crucial for a complete understanding of
tissue functions. Here, we present PHYTOMap (plant hybridization-based
targeted observation of gene expression map), a multiplexed fluorescence
in situ hybridization method that enables single-cell and spatial analysis of
gene expression in whole-mount plant tissue in a transgene-free manner and
at low cost. We applied PHYTOMap to simultaneously analyse 28 cell-type
marker genes in Arabidopsis roots and successfully identified major cell
types, demonstrating that our method can substantially accelerate the
spatial mapping of marker genes defined in single-cell RNA-sequencing
datasets in complex plant tissue.
Understanding how individual cells respond and interact with each
other in the face of changing environments is the cornerstone of under-
standing tissue function. Single-cell transcriptomics technologies have
been widely adopted in plant research, enabling the classification of
cells into populations that share molecular features for the in-depth
analysis of cell types and states1–3. Increasing throughput and sensitiv-
ity in single-cell transcriptomics technologies will offer tremendous
granularity at which cells can be classified, but will also create new
challenges in dealing with cell populations that our current histological
and physiological understanding of plant cells cannot account for. To
understand the identity and function of molecularly defined cell popu-
lations, it is critical to analyse their spatial localization in the tissue.
In plant research, the most common tool for spatially mapping
cell population marker genes identified in single-cell transcriptome
analysis has been transgenic reporter lines that express fluorescent
proteins under the predicted promoter region of the genes. In most
cases, each transgenic line visualizes the expression of only one gene.
This approach has several limitations when analysing cells in complex
tissue: (1) a cell type/state is not always defined by the expression of a
single gene, but by the combination of many genes; (2) spatial mapping
of a single gene or a few genes has difficulties in analysing multiple cell
types/states simultaneously, which is critical for understanding inter-
actions between cell types/states; (3) generation of transgenic plants
is time-consuming; and (4) heterologous expression of fluorescent
proteins does not necessarily reflect the true expression of the gene
because the reporter cassettes lack the native genomic context (for
example, enhancer–promoter interactions). In situ hybridization,
another popular approach in spatial gene expression analysis in plants4,
can overcome a few of the above limitations but suffers from low mul-
tiplexing capacity. Therefore, spatial gene expression analysis needs
to be done with a large number of genes at single-cell resolution for a
more complete understanding of the function of cell types/states and
their interactions with other cells and the environment.
Spatial transcriptomics technologies hold great promise in
addressing these problems by simultaneously revealing the molecu-
lar details and spatial location of cells in complex tissues. Methods
using spatially barcoded arrays or imaging-based, highly multiplexed
single-molecule fluorescence in situ hybridization allow researchers
1Plant Biology Laboratory, The Salk Institute for Biological Studies, La Jolla, CA, USA. 2Genomic Analysis Laboratory, The Salk Institute for Biological
Studies, La Jolla, CA, USA. 3ARC Centre of Excellence in Plant Energy Biology, School of Molecular Sciences, University of Western Australia, Perth,
Western Australia, Australia. 4The Harry Perkins Institute of Medical Research, QEII Medical Centre and Centre for Medical Research, The University of
Western Australia, Perth, Western Australia, Australia. 5Howard Hughes Medical Institute, The Salk Institute for Biological Studies, La Jolla, CA, USA.
e-mail: [email protected]; [email protected]
Nature Plants | Volume 9 | July 2023 | 1026–1033
1026
nature plantsBrief Communicationto study the expression of many genes (from dozens to the whole
transcriptome) with spatial information (from tissue region to
single-cell levels)5. Such technologies have recently been adopted in
plant research6–8. Although spatial transcriptomics, combined with
single-cell transcriptomics, will contribute to elucidating the spatial
organization of cell types/states in plants in great detail9, tissue types
amenable for spatial transcriptomics experiments are limited to thin
(single-cell layer) sections, posing challenges for its application in
plants and other organisms. For instance, the root tip—an important
organ for plant growth, nutrient acquisition and interactions with
microbes—is a difficult tissue to section owing to its small size. Moreo-
ver, sectioning will lead to a loss of information from other parts of the
tissue, which may contain the cell types/states of interest; information
about environments, such as microbial colonization, can also be lost by
sectioning. It may be possible to overcome these problems by sampling
serial sections and conducting multiple experiments followed by the
three-dimensional (3D) reconstitution of two-dimensional (2D) data,
but spatial transcriptomics technologies are very costly, making such
an approach rarely affordable. To overcome these limitations, we intro-
duce PHYTOMap (plant hybridization-based targeted observation of
gene expression map), a low-cost single-cell spatial gene expression
analysis that can simultaneously map dozens of genes in whole-mount
plant tissue.
PHYTOMap builds on in situ hybridization techniques in plants10,11
and in situ sequencing technologies primarily developed in neurosci-
ence12. After fixing whole-mount plant tissues, DNA probes (specific
amplification of nucleic acids via intramolecular ligation probes or
SNAIL probes) with gene-specific barcodes are specifically hybridized
on target messenger RNA molecules, circularized and amplified in situ
(Fig. 1a and Extended Data Fig. 1; see Methods for details). The hybridi-
zation condition has been optimized to allow high target specificity
(Extended Data Fig. 2a). The amplification of DNA barcodes provides
a high signal-to-noise ratio, enabling signal detection from cleared
whole-mount tissue. The location of mRNA molecules is defined using
sequence-by-hybridization (SBH) chemistry13 that targets the barcode
sequences of DNA amplicons across sequential rounds of probing,
imaging and stripping (Fig. 1b). In each imaging round, four targets
are detected using each of the four channels of a confocal microscope
(Supplementary Video 1). After imaging, fluorescent detection probes
are stripped (Extended Data Fig. 2b), and the next round of hybridiza-
tion targets a new set of four genes (Fig. 1b,c). A previous study that
used SBH chemistry to detect amplified DNA probes in situ showed
that signal was maintained at least over 10 cycles13.
We tested the accuracy of PHYTOMap by comparing its signal
with results from other imaging-based techniques. We used transgenic
Arabidopsis lines expressing green fluorescent protein (GFP) under the
control of an endodermis-specific (EMBRYO LIPID TRANSFER PROTEIN
or ELTP) or pericycle-specific (LATERAL ORGAN BOUNDARIES-DOMAIN
16 or LBD16) promoter. Cell type-specific GFP expression in these lines
has been confirmed in a previous study14. We targeted the mRNA of GFP
with PHYTOMap and a hybridization chain reaction (HCR), which is
also a hybridization-based approach recently applied to plant tissue15.
PHYTOMap detected GFP mRNA in the expected cell types, which was
further validated with HCR (Extended Data Fig. 3a,b). Together, these
results confirmed the accuracy of PHYTOMap.
PHYTOMap successfully mapped well-established/validated
cell-type marker genes in expected cell types/regions in the root tip
of Arabidopsis (Fig. 1d–f and Extended Data Fig. 4). The marker genes
we targeted include AT4G28100 (ENDODERMIS7 or EN7; endodermis),
AT4G29100 (BASIC HELIX LOOP HELIX 68 or BHLH68; pericycle),
AT5G37800 (RHD SIX-LIKE 1 or RSL1; trichoblast), AT5G53730 (NDR1/
HIN1-LIKE 26 or NHL26; xylem), AT5G57620 (MYB DOMAIN PROTEIN
36 or MYB36; endodermis), AT5G58010 (LJRHL1-LIKE 3 or LRL3; tricho-
blast) and AT3G54220 (SCARECROW or SCR; endodermis) (Fig. 1e,f and
Extended Data Fig. 4; magnified images are provided in Extended Data
Figs. 5 and 6)16,17. PHYTOMap also validated cell type/region marker can-
didates predicted in a previous single-cell RNA-sequencing (scRNA-seq)
study of Arabidopsis root tips18. For instance, AT3G46280 was detected
in the root cap and elongating epidermis as predicted in the scRNA-seq
data (Fig. 1e). Genes enriched in meristematic (AT5G42630) and elon-
gation (AT5G12050) zones in the scRNA-seq data were mapped in the
expected regions (Fig. 1f); AT5G12050 signal was detected in epidermis
and vasculature, as shown in scRNA-seq (Fig. 1f). Quiescent center
(QC) and columella signal was also detected from the marker genes
AT2G28900, AT3G20840 and AT3G55550 (Extended Data Fig. 4c).
Other genes that are not shown in Fig. 1 are shown in Extended Data
Figs. 7 and 8. Taken together, PHYTOMap can be used as an efficient
tool for validating marker genes identified in scRNA-seq data without
generating transgenic plants.
To demonstrate the multiplexing capacity of this method, we
simultaneously targeted 28 genes in the same root tips with seven
rounds of imaging. The targeted genes include known cell-type marker
genes as well as unvalidated cell-type marker candidates identified in
the scRNA-seq data18 (a full list is given in Supplementary Table 1), which
showed varying levels of expression in the root tip (Extended Data
Fig. 9). We developed a computational pipeline to integrate
whole-mount images from each imaging round and analyse gene
expression at the single-cell resolution (Fig. 2a; see Methods for
details). Cell wall boundary information was obtained together with
the RNA-derived signal in each imaging round to facilitate this process.
The analysis pipeline first registers 3D images across imaging rounds
using cell boundary information, automatically detects spots derived
from single mRNA molecules and annotates spots with gene names.
A merged image with detected and decoded transcripts successfully
captured the cell-type architecture of the root tip (Fig. 2b). To analyse
the spatial data at the single-cell level, cell segmentation was per-
formed based on cell wall boundary information using PlantSeg, which
performs deep learning-assisted cell boundary prediction and graph
partitioning-based cell segmentation19 (Fig. 2a). Annotated spots were
assigned to individual cells and counted, resulting in a cell-by-gene
matrix, a standard scRNA-seq data form that can be used for clustering
and dimension reduction analyses (Fig. 2a).
We analysed five root tip preparations and identified a total of
259,781 RNA molecules from 3,608 cells (median 19 molecules per cell)
(Fig. 2c). The assays were highly robust and reproducible, detecting
comparable numbers of transcripts for each RNA species between dif-
ferent biological samples (Fig. 2d). This suggests that gene expression
between cells or samples can be compared quantitatively. Hierarchical
clustering and heatmap visualization revealed cell population-specific
expression of target genes (Fig. 2e and Supplementary Fig. 1a). Genes
that showed low expression in a previous RNA-sequencing study were
detected successfully (Fig. 2e and Extended Data Fig. 9), suggesting
a high sensitivity of PHYTOMap. We performed de novo clustering
using PHYTOMap data and visualized the data on Uniform Manifold
Approximation and Projection (UMAP) without using any spatial infor-
mation (Fig. 2f and Supplementary Fig. 1b,c). These clusters success-
fully captured major cell types and developmental stages in the root
tip (Fig. 2g). Together, these results demonstrate that PHYTOMap can
spatially map dozens of genes at a single-cell resolution in a highly
reproducible manner.
To test the limits of PHYTOMap, we performed 14 successive
rounds of experiments targeting the same genes. We observed quali-
tatively consistent signals across the imaging rounds (Supplementary
Fig. 2), except for one detection fluorophore (Alexa Fluor 750), whose
signal decayed after the eighth round, indicating that the current pro-
tocol can detect 50 genes in the same tissue. The results also indicate
that the order of imaging rounds would not substantially affect the
qualitative readouts, at least in the first eight rounds. Quantitative
analysis of gene expression across 14 rounds showed an overall decreas-
ing signal and increasing noise over imaging rounds (Extended Data
Nature Plants | Volume 9 | July 2023 | 1026–1033
1027
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4R1
R2
R3
R4
Meristem
Elongation
Maturation
Columella
LRC
a
b
d
e
f
mRNA
SNAIL probe
hybridization
Ligation
RCA
RCP
RCP
c
mRNA
Sequence-by-hybridization
Imaging
Stripping
Rehybrid-
ization
Imaging round
1
2 3 4 5 6
7
488 nm
561 nm
640 nm
750 nm
QC
Endodermis
Columella
Pericycle
LRC
Phloem
Atrichoblast
Xylem
Trichoblast
Procambium
Cortex
AT3G10080
(phloem)
AT2G31310
(pericycle)
AT3G46280
(atrichoblast/LRC)
AT1G07640
(phloem)
AT5G42630
(epidermis)
AT5G12050
(trichoblast)
AT3G54220
(endodermis)
AT4G22160
(pericycle)
Fig. 1 | Whole-mount spatial mapping of root tip cell-type marker genes
with PHYTOMap. a, In fixed whole-mount tissue, target mRNA molecules
are hybridized by pairs of DNA probes (SNAIL probes) that harbour mRNA
species-specific barcode sequences (pink bars). Barcode-containing DNA
probes are circularized by ligation (red star) and amplified in situ by RCA. During
amplification, amine-modified nucleotides are incorporated into the DNA
amplicons (RCPs) and stably cross-linked with the cellular protein matrix using a
non-reversible amine cross-linker. Amplified DNA barcodes are detected by SBH
chemistry through multiple rounds of imaging. b, SBH chemistry. Before each
imaging round, four types of bridge probes are hybridized to a set of four DNA
barcodes. Each bridge probe is then targeted by one of four fluorescent probes
to be imaged. After imaging, bridge probes and fluorescent probes are stripped
away, keeping RCPs in place. These steps are repeated until all the DNA barcodes
are read. c, Representative images at different imaging rounds. The maximum
exposure of 60 z planes of the same position in the tissue is displayed. Scale
bar, 30 μm. d, Schematic representation of the root tip and UMAPs displaying
root tip scRNA-seq data18 used in this study. In the UMAPs, cells are labelled with
cell types (left) and regions (right). LRC, lateral root cap; QC, quiescent centre.
e,f, Representative results from the imaging rounds 2 (e) and 3 (f). Left, UMAPs
showing expression patterns of target genes. The colours of the gene name
labels correspond to the colours in the images below. Middle, 3D projections
(upper) and optical sections (2D, lower) of whole-mount tissue images. Right,
representative cross-section views of the middle part of the samples (transition/
elongation zone). Scale bar, 25 μm.
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Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4a
b
c
s
t
n
u
o
C
e
500
400
300
200
100
0
Registration
Spot detection/decoding
Cell segmentation
Cell × gene matrix
Cell 1 Cell 2 Cell 3
Gene A
Gene B
Gene C
Gene D
19
10
2
8
2
7
0
10
5
5
10
11
rN
Imaging round
r1
r1
AT3G20840
AT1G71692
r3
r4
AT3G54220
AT5G64620
r2
AT2G31310
r5:
AT1G07710
r2
AT3G46280
r3
AT4G22160
r6
r7
AT2G46570
AT4G28100
Transcripts per cell
Genes per cell
20
10
0
Root 5
Root 1
Root 2
Root 3
Root 4
Root 5
Root 1
Root 2
Root 3
Root 4
Sample
Sample
d
n
o
i
s
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2
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(
16
12
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R2 = 0.97
rep1
0.97
rep2
0.90
0.89
rep3
0.91
0.90
0.99
rep4
0.88
0.90
0.91
0.90
rep5
8
12
4
Normalized expression
(log2) of root 1
16
1.0
0.8
0.6
0.4
0.2
0
–0.2
–0.4
–0.6
–0.8
–1.0
AT4G30080
AT1G79840
AT3G10080
AT2G34140
AT5G48657
AT5G64620
AT1G07710
AT5G57620
AT3G20840
AT2G28900
AT3G46280
AT4G29100
AT2G40260
AT4G22160
AT2G31310
AT4G28100
AT1G71692
AT5G37800
AT2G46570
AT1G16390
AT5G58010
AT5G53730
AT3G54220
AT5G42630
AT5G12050
AT3G55550
AT1G07640
AT1G71930
g
10
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15
6
9
11
188
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7
f
RE
6
4
2
0
12
3
10
2
13
0
15
7
4
5
6
1
17
16
18
11
8
9
14
19
19
Columella
14
Columella/QC
4
Epidermis
7
3
Epidermis
Epidermis
1
8
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Stele
17
Stele
13
Pericycle
16
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12
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9
Endodermis
0
Epidermis
11
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2
6
5
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Cortex
18
Endodermis
10;
Endodermis
15:
Endodermis
Fig. 2 | Single-cell and spatial analysis of 28 genes with PHYTOMap.
a, PHYTOMap data analysis pipeline for single-cell analysis. b, 3D visualization
of transcripts detected and decoded after image registration in a representative
root tip (root 4). A middle section (z planes 90–120 of 208) of the image is
displayed. Representative genes from each imaging round are shown. c, Violin
plots showing the number of unique RNA molecules (left) and genes (right)
detected in five root tip samples. d, Left, scatter plot comparing normalized
bulk expression of each gene between two samples (root 1 and root 2). Right,
correlation plot showing pair-wise correlation coefficients among five replicates.
e, Hierarchical clustering of cells of root 4 based on the relative expression of 28
genes. Cluster IDs are indicated at the bottom. RE, relative expression. f, UMAP
visualization of the clusters shown in e. g, 3D visualization of transcripts coloured
by clusters in e and f in a representative root tip (root 4). A middle section (z
planes 90–120 of 208) of the image is displayed. Scale bar, 25 μm (b,g).
Fig. 10). Improving the accuracy and sensitivity of spot detection is an
important future task.
In conclusion, PHYTOMap is a new technology that enables mul-
tiplexed single-cell spatial gene expression analysis in whole-mount
plant tissue without requiring transgenic plant lines. A PHYTOMap
experiment can be performed on a timescale similar to other in situ
hybridization protocols in Arabidopsis10,11; sample preparation takes
4–5 days with ~10 h total bench time (Supplementary Fig. 3a). Imaging
can be performed using a regular confocal microscope. Each imaging
round takes 3 h for one root tip and 5 h for five root tips in the current
study; thus 21 h and 35 h to finish imaging for a 28-gene experiment in
one and five root tips, respectively. It is possible to image much larger
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tissues with longer imaging times. Signal could be detected from the
maturation zone of the root (Extended Data Fig. 3c). PHYTOMap also
successfully detected a housekeeping gene (POLYUBIQUITIN 10 or
UBQ10) in whole-mount Arabidopsis leaves (Supplementary Fig. 4).
We demonstrated that the current protocol can detect 50 genes in the
same tissue. Previous studies have shown that more than 25 rounds of
imaging are possible with DNA amplicons obtained using approaches
similar to our method20, suggesting that PHYTOMap can potentially
target more than 100 genes with optimized protocols. A recent study
successfully reconstructed 3D spatial expression of the transcrip-
tome of Arabidopsis flower meristems by integrating scRNA-seq data
with validated spatial expression of 28 genes using novoSpaRc21,22.
PHYTOMap, combined with such computational approaches, can
generate a 3D spatial transcriptome atlas of various tissues and con-
ditions. Discriminating highly similar transcripts is challenging with
hybridization-based methods like PHYTOMap, but the computational
approach described above can compensate for this limitation. The
transgene-free nature of PHYTOMap makes this technology potentially
applicable to any plant species. Cell-type annotation in scRNA-seq is
challenging in many crop plants because their marker genes are often
not conserved in other well-characterized species such as Arabidopsis.
A potential challenge in applying PHYTOMap to other plant species is
permeabilization of the tissue, which can be achieved by optimizing cell
wall degradation protocols23. We believe that PHYTOMap will become
a widely used tool for efficient cluster annotation in scRNA-seq studies
of a variety of plant species. Beyond cell typing, PHYTOMap will offer
unique opportunities to interrogate spatial regulation of complex cel-
lular responses in plant tissue during stress and development with the
ability to directly tap into various mutants that already exist.
Methods
Sample preparation
Arabidopsis thaliana accession Col-0 seeds (hereafter Arabidopsis)
were sown on square plates containing Linsmaier and Skoog medium
(Caisson Labs, catalogue no. LSP03) with 0.8% sucrose solidified with 1%
agar (Caisson Labs, catalogue no. A038). Plates were kept vertically for
5 days in a growth chamber under an 8:16 h light/dark regime at 21 °C.
PHYTOMap experimental procedure
Chemicals and enzymes. The following chemicals and enzymes
were used: a poly-d-lysine coated dish (MatTek, catalogue no.
P35GC-1.5-14-C); T4 DNA ligase (Thermo Fisher Scientific, catalogue
no. EL0011); EquiPhi29 DNA polymerase (Thermo Fisher Scientific,
catalogue no. A39391); SUPERaseIn RNase inhibitor (Invitrogen,
catalogue no. AM2696); aminoallyl dUTP (AnaSpec, catalogue no.
AS-83203); Dulbecco’s phosphate-buffered saline (DPBS) (Sigma,
catalogue no. D8662); molecular biology grade BSA (New England
Biolabs, catalogue no. B9000S); dNTPs (New England Biolabs, cata-
logue no. N0447S); Fluorescent Brightener 28 disodium salt solution
(Sigma, catalogue no. 910090); formaldehyde solution for molecular
biology, 36.5%–38% in water (Sigma, catalogue no. F8775); Triton-X
(Sigma, catalogue no. 93443); Proteinase K (Invitrogen, catalogue no.
25530049); nuclease-free water (Invitrogen, catalogue no. AM9937);
BS(PEG)9 (Thermo Fisher Scientific, catalogue no. 21582); 20× SSC
buffer (Sigma-Aldrich, catalogue no. S6639); ribonucleoside vanadyl
complex (New England Biolabs, catalogue no. S1402S); formamide
(Sigma, catalogue no. F9037); RNase-free Tris buffer pH 8.0 (Invitro-
gen, catalogue no. AM9855G); RNase-free EDTA pH 8.0 (Invitrogen,
catalogue no. AM9260G); cellulase (Yaklut, catalogue no. YAKL0013);
macerozyme (Yakult, catalogue no. YAKL0021); and pectinase (Thermo
Fisher Scientific, catalogue no. ICN19897901).
Probe design. Target genes were selected manually based on their
cell type-specific expression. Probes were constructed by combining
the probe design used in STARmap24 and HYBISS13 (Extended Data
Fig. 1a). A SNAIL probe—a pair comprising a padlock probe (PLP) and a
primer—was designed. (1) For each gene, 40–50-nucloetide sequences
with a GC content of 40%–60% were selected and it was confirmed that
there was no homologous region in the other transcripts by blasting
against TAIR10 Arabidopsis genome. (2) Selected sequences were split
into halves, each of 20–25 nucleotides (the 5′ halves for PLPs and the 3′
halves for primers), with a two-nucleotide gap between, ensuring that
the melting temperature (Tm) of each half is around 60 °C. (3) PLPs have
complementary sequences for target specific bridge probes. (4) Four
SNAIL probes were designed for each gene. (5) PLPs and primers have
complementary sequences to form a circular structure. Bridge probes
and detection read-out probes were designed as described previously13
and detailed in Supplementary Table 2. All probes were manufactured
by Integrated DNA Technologies. SNAIL probes were manufactured
in the form of oPools Oligo Pools with desalting purification. Bridge
probes were manufactured individually with desalting purification.
Detection read-out probes were manufactured individually with HPLC
purification.
Sample fixation and permeabilization. Five-day-old root tips were cut
on the agar plate using a razor blade, mounted on a dry poly-d-lysine
coated dish using tweezers, and immediately fixed, dehydrated
and rehydrated in a manner similar to that described in previous
studies4,15 with modifications. The following steps were conducted on
the dish. Arabidopsis root tips were immersed in FAA (16% v/v formalde-
hyde, 5% v/v acetic acid and 50% ethanol) for 1 h at room temperature.
RNase-free water was used throughout the entire protocol. Samples
were then dehydrated in a series of 10-min washes once in 70% (v/v in
nuclease-free water) ethanol, once in 90% ethanol and twice in 100%
ethanol, followed by two 10-min washes in 100% methanol, and then
were stored in 100% methanol at −20 °C overnight. The next day, sam-
ples were rehydrated in a series of 5-min washes in 75% (v/v), 50% and
25% methanol in DPBS-T (0.1% Tween 20 in DPBS) at room tempera-
ture. The cell wall was partially digested by incubating samples in cell
wall digestion solution (0.06% cellulase, 0.06% macerozyme, 0.1%
pectinase, and 1% SUPERase in DPBS-T) for 5 min on ice, and then for
30 min at room temperature. After two washes in DPBS-TR (DPBS-T
and 1% SUPERase), samples were fixed in 10% (v/v) formaldehyde for
30 min at room temperature and washed with DPBS-TR. Proteins were
digested by incubating samples in protein digestion buffer (0.1 M
Tris–HCl pH 8, 50 mM EDTA pH 8) with a 1:100 volume of Proteinase
K (20 mg ml−1, RNA grade; Invitrogen, catalogue no. 25530049) for
30 min at 37 °C. After two washes in DPBS-TR, samples were fixed in
10% (v/v) formaldehyde for 30 min at room temperature and washed
with DPBS-TR.
SNAIL probe hybridization, amplification and fixation. The following
steps are based on STARmap protocols24 with modifications. A pool of
SNAIL probes (500 nM each) was heated at 90 °C for 5 min and cooled
at room temperature. Samples were incubated in hybridization buffer
(2× SSC, 30% formamide, 1% Triton-X, 20 mM ribonucleoside vanadyl
complex and pooled SNAIL probes at 10 nM per oligo) in a 40 °C humidi-
fied oven overnight. After hybridization, samples were washed twice in
DPBS-TR and once in 4× SSC in DPBS-TR for 30 min at 37 °C and rinsed
with DPBS-TR at room temperature. Samples were then incubated in a
T4 DNA ligation mixture (1:50 dilution of T4 DNA ligase supplemented
with 1× BSA and 0.2 U μl−1 of SUPERase-In) at room temperature over-
night. After ligation, samples were washed twice with DPBS-TR for
10 min at room temperature and incubated in a rolling circle amplifica-
tion (RCA) mixture (1:20 dilution of equiPhi29 DNA polymerase, 250 μM
dNTP, 0.1 μg μl−1 BSA, 1 mM dithiothreitol, 0.2 U μl−1 of SUPERase-In and
20 μM aminoallyl dUTP) at 37 °C overnight. After RCA, samples were
rinsed in DPBS-T and covalently cross-linked with 4.3 μg μl−1 BS(PEG)9
in DPBS-T. BS(PEG)9 was then quenched by incubating samples in 1 M
Tris–HCl (pH 8) for 30 min at room temperature.
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Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Gel embedding and tissue clearing. After the fixation of DNA ampli-
cons, samples were embedded in acrylamide gel by incubating in a
polymerization mixture (4% acrylamide, 0.2% bis-acrylamide, 0.1%
ammonium persulfate and 0.1% tetramethylethylenediamine in
DPBS-T) for 1.5 h at room temperature. Samples were then rinsed in
DPBS-T. After gel embedding, samples were cleared by incubating in
ClearSee25 at room temperature overnight.
Sequence-by-hybridization. Samples were washed with 2× SSC for
5 min at room temperature and then incubated in a bridge probe
hybridization mixture (2× SSC, 20% formamide and four bridge probes
at 100 nM per oligo in water) for 1 h at room temperature. After washing
twice in 2× SSC for 5 min at room temperature, samples were incubated
in a detection probe hybridization mixture (2× SSC, 20% formamide,
1:100 dilution of Calcofluor White (Fluorescent Brightener 28 disodium
salt solution) and fluorescent detection oligos at 100 nM per oligos in
water) for 1 h at room temperature. Samples were washed in 2× SSC and
ClearSee for 5 min at room temperature and stored in ClearSee until
imaging. After imaging, the PHYTOMap signal was stripped by incubat-
ing in stripping buffer (65% formamide in 2× SSC) at 30 °C for 30 min.
Imaging. Imaging was performed using a Leica Stellaris 8 confocal
microscope equipped with a DMi8 CS Premium, supercontinuum
white light laser, laser 405 DMOD, power HyD detectors and an HC PL
APO CS2 ×40/1.10 water objective. The image size for a field-of-view
was 512 × 512 pixels with a voxel size of 0.57 μm × 0.57 μm × 0.42 μm,
and three fields-of-view were acquired for each root sample unless
otherwise stated. The 2D images shown in Extended Data Fig. 4b were
taken in a scan format of 2,048 × 2,048 pixels with denoising (averag-
ing two images). The following channel settings were used: 405 nm
excitation, 420–510 nm emission; 499 nm excitation, 504–554 nm
emission; 554 nm excitation, 559–650 nm emission; 649 nm excita-
tion, 657–735 nm emission; 752 nm excitation, 760–839 nm emission.
PHYTOMap in the leaf. Arabidopsis plants were grown in soil for
20 days with a 12 h light period. The fifth leaf (the largest) was used
for the experiment. Leaves were processed as described above with
slight modifications. Because the whole-mount leaf did not attach
to the poly-d-lysine coated dish, the tissue was fixed in a 1.5 ml tube
with FAA. A vacuum was applied to facilitate fixation. After the first
fixation, the tissue was transferred to a poly-d-lysine coated dish and
the downstream steps were carried out on the dish. The tissue was not
embedded in the gel, because we did not perform multiple rounds of
imaging. Before imaging, the tissue was mounted on a glass slide with
a coverslip on top to immobilize the tissue. SNAIL probes for UBQ10
(AT4G05320) were used (Supplementary Table 2).
Cost of PHYTOMap. The cost of PHYTOMap experiments is approxi-
mately US$80 for a 28-gene experiment and US$230 for a 96-gene
experiment (Supplementary Fig. 3b and detailed in Supplementary
Table 3), where each experiment can accommodate five or more root
tips, which can be from different treatments and/or genotypes. The
initial investment (reagent cost) to set up PHYTOMap experiments
is approximately US$2,700 and US$5,500 for a 28-gene and 96-gene
experiment, respectively.
PHYTOMap data processing
Image registration. Sample handling could cause shifts in a
field-of-view during image acquisition. To correct these shifts, image
stacks from each round were registered in three dimensions based on
the cell wall boundary staining information by a global affine align-
ment using random sample consensus-based feature matching26. We
adopted the analysis pipeline of Bigstream27 with modifications. The
first round of images was used as a reference. The registered images
were used for downstream analysis with starfish (https://github.com/
spacetx/starfish), a Python library for processing image-based spatial
transcriptomics data.
Spot detection and decoding. Registered image stacks were pro-
cessed with ImageJ (v.2.3.0) into individual images for each channel
and z-step that starfish can process. Images were denoised using the
Bandpass function, and the z axis was smoothed by Gaussian blurring
using the GaussianLowPass function with the following parameters:
lshort = 0.5, llong = 11 and threshold=0.0. Using the Clip function, an
image clipping filter was applied to remove pixels of too low or too
high intensity. Fluorescence in situ hybridization signals (spots) from
single molecule-derived rolling circle products (RCP) were detected by
a blob detection technique using the BlobDetector function, which is
a multidimensional Gaussian spot detector that convolves kernels of
multiple defined sizes with images to identify spots. The kernel sizes
were determined based on the diameter of spots (typically around
1 μm). Detected spots were decoded based on the imaging round and
the channel information using the SimpleLookupDecoder function.
Cell segmentation. The cell wall staining image of the first imaging
round (the same image used as a reference for image registration) was
used for segmentation. PlantSeg workflow19 was used to predict cell
boundaries and label the cells in the image stacks. A re-scaling factor
of [1.68, 2.28, 2.28] was used to fit our images to the ‘confocal_PNAS_3d’
model on the software. A graphics processing unit-based convolutional
neural network prediction was used for cell boundary prediction with
the patch size of [80, 160, 160] and the ‘accurate’ mode (50% overlap
between patches). The Multicut segmentation algorithm was used
with under-/oversegmentation factor = 0.5, 3D watershed, convolu-
tional neural network predictions threshold = 0.3, watershed seeds
sigma = 1.0, watershed boundary sigma = 0, superpixels minimum
size = 1, and cell minimum size = 1. After segmentation, images were
re-scaled with the appropriate factors.
Spot assignment to segmented cells. Based on the segmentation
masks generated in the previous step, individual decoded spots were
assigned to cells using the AssignTargets function. The spots were then
counted for each target in each cell, resulting in a cell-by-gene matrix.
Image visualization. Registered and decoded images were visualized
using napari28, a fast, interactive, multidimensional image viewer for
Python, by using the starfish function display.
PHYTOMap count data analysis
scanpy was used for analysing count data29. Cells that contain fewer
than six spots (transcripts) were filtered out from the analysis. Count
data were log-transformed, and principal components were calcu-
lated. A neighbourhood graph was computed by using 10 principal
components with a local neighbourhood size of five. UMAP embedding
was generated based on the neighbourhood graph. Clustering was
performed with the Leiden algorithm with a parameter resolution of 1.
The plots in Extended Data Fig. 10 were created using ggplot2 (v.3.3.5).
HCR
HCR was performed as reported previously15 with some modifications.
Root tips were fixed and permeabilized as described above in the PHY-
TOMap method. After protein digestion and post fixation, the sample
was pre-incubated in HCR probe hybridization buffer (Molecular Instru-
ments, catalogue no. BPH02323) for 30 min at 37 °C, then incurvated in
HCR probe hybridization buffer with a 1:500 volume of a GFP-targeting
probe mixture (designed by Molecular Instruments) overnight at 37 °C.
After probe hybridization, the sample was washed twice with HCR
probe wash buffer (Molecular Instruments, catalogue BPH01923)
for 30 min at 37 °C and twice with 5× SSCTR (5x SSC, 0.1% Tween and
0.2 U μl−1 of SUPERase-In) for 10 min at room temperature. The sample
Nature Plants | Volume 9 | July 2023 | 1026–1033
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Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4was then incubated in the HCR amplification buffer (Molecular Instru-
ments, catalogue number BAM02323) for 30 min at room temperature.
During the incubation, HCR amplifier B3-h1/2 Alexa Fluor 647 was
heated to 95 °C for 90 s in a thermocycler and cooled at room tempera-
ture for 30 min. The amplification solution was prepared by adding a
1:50 volume of cooled HCR amplifiers to the HCR amplification buffer.
The sample was incubated in the amplification solution overnight at
room temperature and washed three times with 5× SSCTR for 20 min at
room temperature. The sample was then cleared in ClearSee for more
than 1 day until imaging. For imaging, the cell wall of the samples was
stained with Calcofluor White as described above.
scRNA-seq analysis
Processed and annotated data by Shahan et al.18 were downloaded
from the Gene Expression Omnibus (GSE152766_Root_Atlas_spliced_
unspliced_raw_counts.rds.gz). The R package Seurat (v.4.1.0)30 was
used to display the expression of target genes.
13. Gyllborg, D. et al. Hybridization-based in situ sequencing (HybISS)
for spatially resolved transcriptomics in human and mouse brain
tissue. Nucleic Acids Res. 48, e112 (2020).
14. Wyrsch, I., Domínguez-Ferreras, A., Geldner, N. & Boller, T.
Tissue-specific FLAGELLIN-SENSING 2 (FLS2) expression in roots
restores immune responses in Arabidopsis fls2 mutants. New
Phytol. 206, 774–784 (2015).
15. Oliva, M. et al. An environmentally-responsive
transcriptional state modulates cell identities during root
development. Preprint at https://www.biorxiv.org/content/
early/2022/03/04/2022.03.04.483008 (2022).
16. Wendrich, J. R. et al. Vascular transcription factors guide plant
epidermal responses to limiting phosphate conditions. Science
370, 777–782 (2020).
17. Menand, B. et al. An ancient mechanism controls the
development of cells with a rooting function in land plants.
Science 316, 1477–1480 (2007).
18. Shahan, R. et al. A single-cell Arabidopsis root atlas reveals
Reporting summary
Further information on research design is available in the Nature
Portfolio Reporting Summary linked to this article.
developmental trajectories in wild-type and cell identity mutants.
Dev. Cell 57, 543–560 (2022).
19. Wolny, A. et al. Accurate and versatile 3D segmentation of plant
Data availability
Image data are available at http://neomorph.salk.edu/downloads/phy-
tomap/. Sequences of all the DNA probes used in this study are provided
in Supplementary Table 2. Processed and annotated scRNA-seq data is
available at the Gene Expression Omnibus (GSE152766).
tissues at cellular resolution. eLife 9, e57613 (2020).
20. Lee, J. H. et al. Fluorescent in situ sequencing (FISSEQ) of RNA for
gene expression profiling in intact cells and tissues. Nat. Protoc.
10, 442–458 (2015).
21. Neumann, M. et al. A 3D gene expression atlas of the floral
meristem based on spatial reconstruction of single nucleus RNA
sequencing data. Nat. Commun. 13, 2838 (2022).
Code availability
The code to analyse PHYTOMap data is available at https://github.com/
tnobori/PHYTOMap.
22. Nitzan, M., Karaiskos, N., Friedman, N. & Rajewsky, N. Gene
expression cartography. Nature 576, 132–137 (2019).
23. Giacomello, S. & Lundeberg, J. Preparation of plant tissue
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Acknowledgements
We thank J. Chory and X. Wu (Salk) for letting us use their confocal
microscope, R. Henley (Salk) for useful discussions, N. Geldner
for providing seeds of ELTP::FLS2-GFP and LBD16::FLS2-GFP, and
Ecker lab members for the critical comments on the paper. T.N. was
supported by Human Frontiers Science Program (HFSP) Long-term
Fellowship (LT000661/2020-L). J.R.E. is an investigator of the Howard
Hughes Medical Institute.
Author contributions
T.N. conceived and designed the study and experiments with
guidance from J.R.E. T.N. performed experiments, analysed data and
Nature Plants | Volume 9 | July 2023 | 1026–1033
1032
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4wrote the paper. M.O. and R.L. optimized tissue preparation protocols.
M.O., R.L. and J.R.E. edited the paper.
Reprints and permissions information is available at
www.nature.com/reprints.
Competing interests
T.N. and J.R.E. are inventors on pending patent applications related to
PHYTOMap (US63/392,392). All methods, protocols and sequences
are freely available to nonprofit institutions and investigators. The
remaining authors declare no competing interests.
Additional information
Extended data is available for this paper at https://doi.org/10.1038/
s41477-023-01439-4.
Supplementary information The online version contains
supplementary material available at https://doi.org/10.1038/s41477-
023-01439-4.
Correspondence and requests for materials should be addressed to
Tatsuya Nobori or Joseph R. Ecker.
Peer review information Nature Plants thanks Mark Libault and the
other, anonymous, reviewer(s) for their contribution to the peer review
of this work.
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© The Author(s) 2023
Nature Plants | Volume 9 | July 2023 | 1026–1033
1033
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 1 | PHYTOMap probe design. ID sequences are unique to different RNA species. Anchor sequence was included based on a previous study13 but
not used in the present study.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 2 | Optimization of formamide concentration during
SNAIL probe hybridization. a, Hybridization in 30% formamide showed higher
target specificity. b, Images after stripping fluorescent probes. The four-color
channels are shown in higher contrast than in Fig. 2b, and cell wall staining images
are overlaid. Scale bars = 25 μm. Three independent roots were tested with similar
results.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 3 | PHYTOMap validation. The mRNA of GFP was targeted
with HCR (left) or PHYTOMap (right) in (a) ELTP:FLS2-GFP and (b) LBD16::FLS2-
GFP plants, which express GFP in endodermis and pericycle, respectively. Scale
bars = 25 μm. c, PHYTOMap images that cover larger areas. (Left) ELTP:FLS2-GFP.
(Right) LBD16::FLS2-GFP. Scale bar = 100 μm. Three independent roots were
tested with similar results.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 4 | Whole-mount spatial mapping of root tip cell
type marker genes predicted in scRNA-seq data with PHYTOMap. a,
b, Representative results from each imaging round. Left: UMAPs showing
expression patterns of target genes. The colors of gene name labels correspond
to the colors in the images below. Middle: 3D projections (top) and optical
sections (2D, bottom) of whole-mount tissue images. Right: Representative
cross-section views of the middle part of the samples (transition/elongation
zone). c, Validated and predicted marker genes for QC and columella. 3D images
were shown with cell wall staining. Scale bars = 25 μm.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 5 | Detailed analysis of PHYTOMap images. Magnified images of 2D optical section images in (a) Fig. 2b and (b) Fig. 2. UMAP plots show
expression patterns of target genes. The colors of gene name labels correspond to the colors in the images below. Scale bars = 25 μm.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 6 | Detailed analysis of PHYTOMap images. Magnified images of 2D optical section images in (a) Fig. 2d, and (b) Fig. 2e. UMAP plots show
expression patterns of target genes. The colors of gene name labels correspond to the colors in the images below. Scale bars = 25 μm.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 7 | PHYTOMap images for additional genes. Magnified 2D optical section images of genes that are not shown in Fig. 1. UMAP plots show
expression patterns of target genes. The colors of gene name labels correspond to the colors in the images below. Scale bars = 25 μm.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 8 | PHYTOMap images for additional genes. Magnified 2D optical section image of genes that are not shown in Fig. 1. UMAP plots show
expression patterns of target genes. The colors of gene name labels correspond to the colors in the images below. Scale bars = 25 μm.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 9 | Varying levels of expression of the genes targeted in this study. Bulk expression levels of genes (transcript per million) were calculated based
on the root tip scRNA-seq data18. The twenty-eight genes targeted in this study were highlighted in red.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4Extended Data Fig. 10 | Quantitative analysis of PHYTOMap data across 14
rounds of imaging. a, Expression of genes labelled with Alexa Fluor 488/555/647
(Extended Data Fig. 8). Data were shown as relative expression to R1. b,
Expression of genes labelled with Alexa Fluor 750 (Extended Data Fig. 8). Data
were shown as relative expression to R2 as the data of R1 showed unusually weak
signals. n = 3 independent roots. Error bars indicate standard deviation.
Nature Plants
Brief Communicationhttps://doi.org/10.1038/s41477-023-01439-4
| null |
10.1088_1478-3975_acf5bd.pdf
|
Data availability statement
The data cannot be made publicly available upon
publication because the cost of preparing, depositing
and hosting the data would be prohibitive within the
terms of this research project. The data that support
the findings of this study are available upon reason-
able request from the authors.
|
Data availability statement The data cannot be made publicly available upon publication because the cost of preparing, depositing and hosting the data would be prohibitive within the terms of this research project. The data that support the findings of this study are available upon reasonable request from the authors.
|
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Phys. Biol. 20 (2023) 066001
https://doi.org/10.1088/1478-3975/acf5bd
PAPER
EMT induces characteristic changes of Rho GTPases and
downstream effectors with a mitosis-specific twist
Kamran Hosseini1,2, Annika Frenzel1,2 and Elisabeth Fischer-Friedrich1,2,3,∗
1 Cluster of Excellence Physics of Life, Technische Universität Dresden, Dresden, Germany
2 Biotechnology Center, Technische Universität Dresden, Dresden, Germany
3 Faculty of Physics, Technische Universität Dresden, Dresden, Germany
∗
Author to whom any correspondence should be addressed.
E-mail: [email protected]
Keywords: epithelial-mesenchymal transition, actin cortex, cell mechanics, Rho GTPases, atomic force microscopy
Supplementary material for this article is available online
Abstract
Epithelial-mesenchymal transition (EMT) is a key cellular transformation for many physiological
and pathological processes ranging from cancer over wound healing to embryogenesis. Changes in
cell migration, cell morphology and cellular contractility were identified as hallmarks of EMT.
These cellular properties are known to be tightly regulated by the actin cytoskeleton. EMT-induced
changes of actin-cytoskeletal regulation were demonstrated by previous reports of changes of actin
cortex mechanics in conjunction with modifications of cortex-associated f-actin and myosin.
However, at the current state, the changes of upstream actomyosin signaling that lead to
corresponding mechanical and compositional changes of the cortex are not well understood. In this
work, we show in breast epithelial cancer cells MCF-7 that EMT results in characteristic changes of
the cortical association of Rho-GTPases Rac1, RhoA and RhoC and downstream actin regulators
cofilin, mDia1 and Arp2/3. In the light of our findings, we propose that EMT-induced changes in
cortical mechanics rely on two hitherto unappreciated signaling paths—i) an interaction between
Rac1 and RhoC and ii) an inhibitory effect of Arp2/3 activity on cortical association of myosin II.
1. Introduction
Epithelial mesenchymal transition (EMT) is a cellular
transformation of epithelial cells that entails the loss
of apical-basal cell polarity and intercellular adhesion
in combination with a gain of mesenchymal cell traits,
see figure 1(a) [1–4]. EMT was linked to the initiation
of metastasis and bad cancer prognosis through the
acquisition of aggressive traits in cancer cells of epi-
thelial origin [1–4]. In particular, EMT was reported
to be connected to enhanced cell migration and cell
proliferation in metastatic cancer cells [1–5].
The actin cytoskeleton is a major regulator of
cell mechanics, cell shape and cellular force gener-
ation. Thereby, the actin cytoskeleton constitutes a
key player in cancer-related changes of cell migration
and cell division [6–8]. Consistent with this, it was
found that EMT causes major changes in the actin-
cytoskeleton [5, 9, 10].
Rho GTPases are known to be essential regulators
of the actin cytoskeleton. We and others showed that
EMT is associated with characteristic changes in the
activation of Rho GTPases as judged by the abund-
ance of its active GTP-bound forms [5, 11–13]. In par-
ticular, we reported a decrease of total RhoA-GTP and
an increase of total Rac1-GTP upon EMT in MCF-
7 breast epithelial cells. Furthermore, the increased
expression of the Rho GTPase RhoC was associated
to enhanced metastasis in several cancer types [14].
In addition, RhoC signaling was featured to be essen-
tial for EMT [13–16].
Previously, we reported characteristic EMT-
induced changes of actin cortex mechanics in roun-
ded cells of diverse epithelial cancer cell lines ori-
ginating from breast, lung, prostate and skin tissue
indicating that this EMT-induced cell-mechanical
change is a widely conserved feature in cells of
diverse tissue origin [5, 17, 18]. Cortex-mechanical
© 2023 The Author(s). Published by IOP Publishing Ltd
Phys. Biol. 20 (2023) 066001
K Hosseini et al
changes were entailing cortical softening and con-
tractility reduction in interphase but a cortical stiff-
ening and contractility increase upon EMT in mitosis.
Concomitantly, we found EMT-induced changes of
cortical actin and myosin II with reduced cortical
myosin in interphase and increased cortical actin in
mitosis [5].
While associated EMT-induced changes in Rho
GTPases signaling provide a viable hypothesis for
downstream changes of cortical actin and myosin, the
details of EMT-induced changes in cortical signaling
remain elusive. In particular, it is unclear how cortical
mechanics is affected in an opposite way in interphase
and mitosis, as none of the downstream actomyosin
effectors of Rho GTPases are known to induce mech-
anical changes in the cortex that depend on the cell
cycle stage [19, 20].
With this work, we aim to deepen our under-
standing of EMT-induced changes in cortical sig-
naling, cortical composition and cortical mechanics
with a focus on the differences between interphase
and mitosis. To this end, we quantify EMT-induced
changes of Rho GTPases RhoA, RhoC and Rac1
which were previously linked to EMT. Furthermore,
we investigate as actin-regulating downstream targets
formin, Arp2/3 and cofilin. In particular, we provide
a quantitative analysis of EMT-induced changes of
cortical protein localization in non-adherent cells in
combination with changes of cortical mechanics and
protein expression. In light of our results, we propose
that two hitherto unappreciated signaling mechan-
isms at the cortex are at the heart of EMT-induced
cell-mechanical changes—i) an interaction between
Rac1 and RhoC and ii) an inhibitory effect of Arp2/3
activity on myosin II cortical localization.
2. Results
To investigate the effects of EMT on actin-cortical
signaling and mechanics, we chose to work with
the breast epithelial cancer cell line MCF-7 which
exhibits epithelial cell traits in control conditions.
We induced EMT in these cells via an estab-
lished method (see e.g. [5, 21–25]) that entails
a 48 h treatment with the tumor promoter 12-O-
tetradecanoylphorbol-13-acetate (TPA) at 100 nM,
see section 5. We and others showed previously
that in response to this treatment, MCF-7 cells
display an EMT-characteristic protein expression
change, corresponding cell-morphological changes
towards a mesenchymal-like phenotype, as well
as increased proliferation and migration, see e.g.
[5, 24, 25]. In particular, we showed earlier that
the epithelial marker E-cadherin is downregulated
through the treatment, while the mesenchymal mark-
ers N-cadherin and Vimentin are upregulated, see
figures S1(b) and (c) in the supporting information
of [5]. Further, cells acquire a more spindle-shaped
2
morphology and grow more isolated from each other,
see figure S1(a) in the supporting information of [5].
We note that EMT-transformed cells will be referred
to as modMCF-7 cells throughout this manuscript.
Previous research has shown that activation of
Rho GTPases is connected to their association with
the plasma membrane [26]. In addition, their imme-
diate downstream signaling is inherently local as
direct downstream effectors such as mDia1, Rock
and Wasp and Wave require persistent binding
for activation [27–29]. Furthermore, the activation
of downstream actin effectors cofilin, formin and
Arp2/3 was shown to be linked to f-actin binding
[28, 30–32]. Correspondingly, association of these
proteins to cortical f-actin is a measure of their
activity at the actin cortex. Therefore, one preval-
ent strategy of this study is to quantify changes of
the relative amount of cortex-associated cortical reg-
ulators upon EMT as a readout of EMT-induced
changes in cortical signaling. Following previous
studies [5, 18, 19, 33, 34], we worked with rounded,
non-adherent cells since this has the advantage that
cell shapes are spherical in both epithelial and EMT-
transformed conditions with a largely uniform actin
cortex. In this way, a meaningful comparative analysis
of cortical protein association between epithelial and
the mesenchymal-like cells becomes possible.
For the measurement of cortical protein asso-
ciation,
immunostaining of the cortical regulator
under consideration was combined with fluorescent
DNA staining (DAPI or Hoechst) which allowed
to identify cells to be in an interphase or mitotic
stage, see section 5. For the measurement of cor-
tical regulators in mitotic cells, the fraction of mitotic
cells was enriched through mitotic arrest induced
by co-incubation with S-trityl-L-cysteine (STC), see
section 5. Using a previously established image ana-
lysis scheme, we analyzed confocal images of immun-
ostaining fluorescence intensities to infer the cellular
outline and the averaged cortical fluorescence profile
along the radial coordinate, i.e. orthogonal to the cell
boundary, see figure 1(b) and [5, 18]. The averaged
radial fluorescence intensity was then used to derive
the cortex-to-cytoplasm ratio of protein localization
in the cells by calculating the ratio of the integrated
cortical fluorescence normalized by the cytoplasmic
fluorescence intensity, see figure 1(c), section 5 and
[5, 18].
2.1. Rho GTPases change their cortical association
upon EMT in interphase and mitosis
To investigate whether cortical association of Rho
GTPases changes through EMT, we quantified the
cortex-to-cytoplasm ratio of RhoA, RhoC and Rac1 in
cells with and without EMT induction. To this end, we
performed confocal imaging of the equatorial cross
section of suspended interphase or STC-arrested
mitotic cells which were immunostained for either of
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 1. EMT induces characteristic changes of the cortical abundance of Rho GTPases RhoA, Rac1 and RhoC in MCF-7 cells.
(a) Schematic of EMT-induced changes of cell morphology and adhesion. (b), (c) image analysis of confocal images of
immunostained cell cross sections to extract the cortex-to-cytoplasm ratio. (b) Exemplary picture of RhoC immunostaining
fluorescence profile of the equatorial cross-section of an EMT-induced mitotic cell including elements of image analysis. Scale
bar: 10 µm. (c) Mean radial fluorescence intensity profile of picture (blue curve) along radial lines shown in panel (c). The fitted
intensity profile, Ism(r, p) is shown in orange, see section 5. (d) Representative confocal images of suspended interphase cells and
STC-arrested mitotic cells in control and EMT-induced conditions. Cells were fixed and DAPI-stained for DNA (blue) and
immunostained for Rac1/RhoA/RhoC (green), see section 5. Scale bar: 10 µm. (e)–(g) Cortex-to-cytoplasm ratio of RhoA, Rac1
and RhoC inferred from immunofluorescence staining as shown in panel (b) before and after EMT. (h) Schematic of changes of
cortical association of Rac1, RhoA and RhoC upon EMT. (i)–(l) RhoC knockdown elicits cortical softening and tension reduction
in the actin cortex in pre-EMT interphase MCF-7 cells ((i), (j), white boxplots) and post-EMT mitotic MCF-7 cells ((k), (l),
blue-shaded boxplots). Post-EMT cells are referred to as modMCF-7. Number of cells analyzed: (e): MCF-7 interphase n = 34,
modMCF-7 interphase n = 32, MCF-7 mitosis n = 31, modMCF-7 mitosis n = 31. (f): MCF-7 interphase n = 36, modMCF-7
interphase n = 28, MCF-7 mitosis n = 31, modMCF-7 mitosis n = 31. (g): MCF-7 interphase n = 44, modMCF-7 interphase n =
43, MCF-7 mitosis n = 30, modMCF-7 mitosis n = 32. (i), (j): MCF-7 n = 39, esiRhoC n = 37, modMCF-7 n = 37, esiRhoC n =
39, (k), (l): MCF-7 n = 27, esiRhoC n = 29, modMCF-7 n = 29, esiRhoC n = 28. Measurements represent at least two
independent experiments. n.s.: p > 0.05, ∗: p < 0.05, ∗∗: p < 0.01, ∗ ∗ ∗: p < 0.001.
the Rho GTPases under consideration, see figure 1(d)
and section 5. Quantitative analysis shows that the
cortex-to-cytoplasm ratio of Rac1 increases upon
EMT both in interphase and mitosis (figures 1(d)
and (e)). By contrast, the cortex-to-cytoplasm ratio
of RhoA decreases through EMT (figures 1(d) and
(f)). We conclude that cortical association of Rac1
and RhoA follows the EMT-induced quantitative
change of GTP-bound Rac1 and RhoA in whole-cell-
lysates of MCF-7 cells [5]. For RhoC, EMT-induced
3
Phys. Biol. 20 (2023) 066001
K Hosseini et al
changes of the cortex-to-cytoplasm ratio are distinct
in interphase and mitosis. While cortical RhoC goes
down in interphase, we see an increase of cortical
RhoC in mitosis (figures 1(d) and (g)). Our results
on the effect of EMT on cortical signaling of Rho
GTPases is summarized in figure 1(h).
We further asked about the influence of Rho
GTPases on cortical mechanics. For this purpose, we
relied on cortex-mechanical measurements with an
established cell confinement setup based on oscillat-
ory cell-squishing with the cantilever of an atomic
force microscope (AFM). We chose a deformation fre-
quency of 1 Hz. This assay was previously shown to
provide a readout of cortical stiffness, cortical tension
as well as a characterization of the viscoelastic nature
of the cortex quantified by the phase shift between
stress and strain [5, 17, 18, 35]. (The phase shift takes
values between 0◦–90◦ with lower values correspond-
ing to a more solid-like response). In particular, we
previously showed that Rac1 signaling was linked to
a decrease in cortical stiffness and contractility in
interphase cells but to an increase of cortical stiffness
and contractility in mitotic cells with a stronger effect
in post-EMT cells [5]. This is in agreement with our
here reported finding of increased cortical Rac1 asso-
ciation post-EMT (figure 1(e)). On the other hand,
we previously found that RhoA signaling increases
cortical stiffness and contractility in particular in pre-
EMT cells [5]. Again, the bigger mechanical effect
pre-EMT is in agreement with our current obser-
vation of higher cortical RhoA association pre-EMT
(figure 1(f)).
The effect of RhoC on cortical mechanics has to
our knowledge not been reported previously. Using
the AFM-based cell confinement assay, we measured
cortical mechanics with and without RhoC knock-
down via RNA interference in pre- and post-EMT
conditions, see figures 1(i)–(l), S1 and section 5. We
find that similar to RhoA, RhoC signaling increases
cortical contractility and stiffness, see figures 1(i)–
(l). However, this effect is restricted to pre-EMT
interphase cells and post-EMT mitotic cells. It is
plausible that the absence of an effect in these con-
ditions is linked to low abundance of RhoC at the
cortex (figure 1(g)). Furthermore, RhoC knock-down
increases the phase shift in pre-EMT interphase con-
ditions indicating that RhoC signaling contributes to
the solid-like nature of the cortex in interphase, see
figure S1(b).
2.2. Rac1 and RhoC mutually affect their cortical
association
The previously reported finding that Rac1 activity
affects cortical mechanics opposite in interphase and
mitosis provides a clue that the signaling of Rac1
might be at the heart of the cell-cycle dependence of
cytoskeletal changes upon EMT. However, currently
it is unclear how cortical Rac1 signaling can act in
4
a manner that is qualitatively different in interphase
and mitosis. In particular, it surprised us that Rac1
would make a strong contribution to cortical con-
tractility in mitosis in post-EMT conditions given
that Arp2/3 activity increase downstream of Rac1 is
expected to diminish cortical contractility, see figure 6
and [20, 36]. Furthermore, previous reports showed
that RhoA is at the heart of cortical contractility in
mitosis [37]. While RhoA activity and cortical associ-
ation is low in post-EMT MCF-7 cells (figure 1(f) and
[5]), we note that RhoC signaling is similar to RhoA.
Therefore, RhoC signaling might step in for RhoA sig-
naling after EMT during mitosis.
Following this line of thought, we asked whether
Rac1 might increase cortical contractility in post-
EMT mitosis via (direct or indirect) activation of
RhoC. To test this hypothesis, we monitored changes
in cortical association of RhoC upon knock-down
of Rac1 in pre- and post-EMT conditions judged
by fluorescence intensity of RhoC immunostaining
(figure 2(a)). Obtained confocal images of equatorial
cross-sections were used for image analysis in all con-
ditions. We find that inferred cortex-to-cytoplasm
ratios of RhoC increase upon knock-down of Rac1
in rounded interphase cells with a stronger effect in
post-EMT conditions (figures 2(a) and (c)). By con-
trast, the cortex-to-cytoplasm ratio of RhoC decreases
upon knock-down of Rac1 in mitosis with a stronger
effect in post-EMT conditions (figures 2(a) and (d)).
We conclude that Rac1 signaling increases cortical
association of RhoC in mitosis but diminishes cortical
association in interphase in MCF-7 cells (figure 2(g)).
To test whether in turn also RhoC signaling influ-
ences Rac1, we performed immunostaining of Rac1
with and without RhoC knock-down in pre- and
post-EMT conditions in interphase and mitosis, see
figure 2(b). We find that inferred cortex-to-cytoplasm
ratios of Rac1 decrease upon knock-down of RhoC in
pre-EMT interphase cells and in post-EMT mitotic
cells (figures 2(e) and (f)). In all other conditions,
there is no significant effect on Rac1 cortical associ-
ation (figures 2(e) and (f)). We conjecture that the
signaling from RhoC to Rac1 is restricted to pre-EMT
interphase and post-EMT mitosis due to the low cor-
tical representation of RhoC in post-EMT interphase
and pre-EMT mitotic conditions, see figures 1(g) and
(h). With this explanation approach, our data are
consistent with an in general activating effect of RhoC
on Rac1 (figure 2(g)).
In previous work, the active forms of RhoA and
Rac1 were shown to affect each other through mutu-
ally inhibitory interactions in breast epithelial cells
[38]. This is consistent with the EMT-induced switch-
like change from a state of high RhoA and low Rac1
activation to a state of low RhoA and high Rac1
activation [5]. The interaction between Rac1 and
RhoC has been to our best knowledge unknown so
far.
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 2. Rho-GTPases Rac1 and RhoC mutually affect their abundance at the cortex. (a) Representative confocal images of RhoC
(immunostained, green) and DNA (DAPI, blue) with and without Rac1 knock-down in suspended interphase cells and
STC-arrested mitotic cells in pre-EMT (MCF-7) and post-EMT (modMCF-7) conditions. Scale bar: 10 µm. (b) Representative
confocal images of Rac1 (immunostained, green) and DNA (DAPI, blue) with and without RhoC knock-down in suspended
interphase cells and STC-arrested mitotic cells in pre-EMT (MCF-7) and post-EMT (modMCF-7) conditions. Scale bar: 10 µm.
(c)–(f) Changes of cortical association of RhoC and Rac1 upon knock-down of the other protein, i.e. Rac1 or RhoC, respectively.
Cortical association of either protein was quantified by its cortex-to-cytoplasm ratios which was inferred from
immunofluorescence staining as shown in panels (a) and (b) before and after EMT. (g) Schematic summary of Rac1 and RhoC
mutual interactions in interphase and mitosis. Post-EMT cells are referred to as modMCF-7. Number of cells analyzed: (c):
MCF-7 n = 20, esiRac1 n = 20, modMCF-7 n = 20, esiRac1 n = 20, (d): MCF-7 n = 19, esiRac1 n = 22, modMCF-7 n = 19,
esiRac1 n = 19, (e): MCF-7 n = 25, esiRhoC n = 21, modMCF-7 n = 20, esiRhoC n = 24, (f): MCF-7 n = 24, esiRhoC n = 26,
modMCF-7 n = 22, esiRhoC n = 24. Measurements represent at least two independent experiments. n.s.: p > 0.05, ∗ : p < 0.05,
∗∗ : p < 0.01, ∗ ∗ ∗ : p < 0.001.
2.3. Cortical cofilin association increases through
EMT
We went on to ask how EMT-induced changes in Rho
GTPases signaling affect downstream cortical regu-
lators. We first investigated EMT-induced changes
of cofilin cortical association. Cofilin is known to
promote the depolymerization of the actin cortex
through severing of actin fibers [39]. In the con-
text of cancer, cofilin activity at the cortex has been
suggested to be a main factor in f-actin turnover
thus playing a key role in cancer cell migration and
invasion [40]. Cofilin becomes deactivated through
phosphorylation mediated by Lim kinases [40] and
phosphorylated cofilin was shown to not interact with
f-actin [30]. Correspondingly, cortex-bound cofilin
can be interpreted as active cofilin.
We quantified total amounts of cofilin (CFL1) and
phospho-cofilin (phospho-CFL1 (Ser3)) via western
blotting from lysates of adherent cells with or without
EMT-induction, see figures 3(a), (b) and section 5.
5
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 3. Actin cortical regulators LIMK1 and cofilin (CFL1) influence actin cortical mechanics and change their cortical
representation upon EMT. (a) Exemplary western blots for p-LIMK1, p-CFL1 and CFL1 expression in MCF-7 cells in control and
EMT-induced conditions. (b) Bar charts of normalized quantities of the p-LIMK1 (active form), p-Cofilin (inactive form) and
total cofilin western blots. Normalization was done against GAPDH bands. Error bars represent standard error of the mean. (c)
Fold changes of normalized protein amounts of p-LIMK1, p-CFL1 and total CFL1 upon EMT from western blots. Individual data
points are depicted in black, see section 5. Error bars represent standard error of the mean. Significance was tested with a
one-sample t-test against the null hypothesis that the data comes from a normal distribution with mean equal to zero (p-values
from left to right: 0.04, 0.82, 0.034, 0.03, 0.04, 0.013). (d) Representative confocal images of suspended interphase cells and
STC-arrested mitotic cells upon EMT, fixed and stained for DAPI (blue) and cofilin (green). Scale bar: 10 µm. (e)
Cortex-to-cytoplasm ratio of cofilin inferred from immunofluorescence staining as shown in panel (d) before and after EMT.
(f)–(i) CFL1 knockdown elicits cortical stiffening and tension rise in the actin cortex in interphase (f), (g) and mitotic (h), (i)
MCF-7 cells. For panels (e)–(i), significance was tested with a Mann-Whitney U-test (two tailed). Post-EMT cells are referred to
as modMCF-7. Number of cells analyzed: (e): MCF-7 interphase n = 38, modMCF-7 interphase n = 38, MCF-7 mitosis n = 46,
modMCF-7 mitosis n = 44. (f), (g): MCF-7 n = 49, siCFL1 n = 49, (h), (i): MCF-7 n = 46, siCFL1 n = 50. Measurements
represent at least two independent experiments. n.s.: p > 0.05, ∗: p < 0.05, ∗∗: p < 0.01, ∗ ∗ ∗: p < 0.001.
Calculating fold changes upon EMT, we find a trend
of a shallow increase of total cofilin (only interphase)
and a decrease of phospho-cofilin upon EMT, see
figure 3(c). Taken together, this points at an increase
of the active non-phosphorylated form of cofilin
upon EMT in MCF-7.
see figure 3(d). We find that the cortex-to-cytoplasm
ratio of cofilin is elevated upon EMT indicating an
EMT-mediated increase of cortical cofilin activity, see
figures 3(d) and (e). This finding is in agreement with
an increase of active cofilin as suggested by western
blotting as described above, see figure 3(c).
To assess cortical association of cofilin, we per-
formed also cofilin-immunostaining of rounded cells,
Immunostaining of the cofilin upstream regu-
lator phospho-Limk1 (phospho-LIMK1 (Thr508))
6
Phys. Biol. 20 (2023) 066001
K Hosseini et al
shows no cortical association but cytoplasmic loc-
alization in agreement with previous findings, see
figure S3(a) and [41]. Quantifying phospho-Limk1
abundance in whole-cell
lysates via western blot-
ting, we find an EMT-induced decrease in interphase
(asynchronous cell population) in accordance with
the observed concomitant decrease of phospho-
cofilin, see figures 3(a)–(c) and section 5. In mitosis,
phospho-Limk1 amounts are very low and show
a trend of decrease upon EMT which is, surpris-
ingly, in disagreement with the EMT-induced trend
of phospho-cofilin (figures 3(a)–(c)). We speculate
that this apparent inconsistency may be attributed to
the previously reported modified activation scheme
of Limk1 in mitosis, where hyperphosphorylation
rather than phosphorylation at Thr508 is at the heart
of Limk1 activation [42]. This observation features
phospho-Limk1 (Thr508) as an unsuitable readout of
cofilin phosphorylation activity in mitosis.
Investigating the effect of cofilin on cortical mech-
anics, we find that cofilin knock-down through
RNA interference changes cortical mechanics, see
figures 3(f)–(i) and S3(e)–(h). Both cortical ten-
sion and stiffness increase in interphase and mitosis,
see figures 3(f)–(i). In addition, the phase shift and
therefore the fluid-like nature increased mildly upon
knock-down in interphase cells, see figure S3(f).
We conclude that increased cortical association of
cofilin in EMT-induced cells contributes to a trend of
decreased cortical contractility and stiffness.
We note that Chugh et al [19] previously repor-
ted a tension increase upon CFL1-knockdown in
interphase HeLa cells in agreement with our findings.
However, the authors reported by contrast a tension
decrease upon CFL1 knock-down in mitosis opposite
to our findings in MCF-7. This apparent discrepancy
might be rooted in the non-monotonous dependence
of cortical tension on actin filament length as was
proposed by the same study [19]. According to this
idea, increased actin filament length through cofilin
knock-down can either increase or decrease cortical
tension depending on the initial state of the cortex.
Large differences in cortical tension values between
mitotic MCF-7 cells and mitotic HeLa cells make dif-
ferent cortical configurations in mitosis for the two
cell lines additionally plausible.
2.4. The actin nucleator mDia1 shows distinct
changes of cortical association upon EMT in
interphase and mitosis
In order to further understand EMT-induced changes
of cortical composition and mechanics [5], we
addressed how actin nucleators are affected upon
EMT downstream of Rho GTPases. Cortical actin
is polymerized by formins and Arp2/3. We will
first focus on the influence of the former. Previous
studies have shown that formin activity has a major
influence on cortical mechanics [19, 20, 43, 44]. We
7
confirmed this finding in rounded MCF-7 cells with
our AFM-based cell confinement setup showing that
formin inhibition via 40 µM SMIFH2 reduced cor-
tical stiffness and contractility in MCF-7 cells in
interphase and mitosis, see figure S4.
To further investigate how formin-mediated poly-
merization changes at the cortex upon EMT, we
decided to focus on the formin representative mDia1
(also Diaph1) which together with the actin nucleator
Arp2/3 polymerizes the majority of f-actin in the actin
cortex [45]. mDia1 is activated downstream of RhoA,
RhoB or RhoC through binding to the active form of
the respective Rho GTPase [28]. The active form of
mDia1 associates with f-actin [28, 32] and thus with
the actin cortex.
Performing quantification of protein amounts in
whole-cell lysates via western blots, we find that there
is no significant change of expression of mDia1 upon
EMT (figures 4(a)–(c)). We then went on to monitor
cortical association of mDia1 via immunostaining
and quantification of the cortex-to-cytoplasm ratio,
see figures 4(d) and (e). We find clear EMT-induced
changes; in interphase cells, the cortex-to-cytoplasm
ratio of mDia1 is reduced, see figure 4(e), blue boxes.
We conclude that cortical association of mDia1 is
decreased upon EMT in agreement with our finding
of reduced presence of RhoA and RhoC at the cor-
tex. In mitotic cells, on the other hand, the cortex-to-
cytoplasm ratio of mDia1 is increased, see figure 4(e),
yellow boxes. This finding points at an increase of cor-
tical mDia1 activity upon EMT in mitosis. We sug-
gest that this effect is due to an EMT-induced rise of
cortical activity of RhoC in mitosis overcompensat-
ing the effect of reduced RhoA activity in post-EMT
mitotic cells, see figure 1(h).
that
Taken together, our results suggest
the
observed characteristic EMT-induced changes of
cortical mDia1 likely make an essential contribu-
tion to EMT-induced changes of cortex-associated
actin and cortical mechanics in interphase and
mitosis.
2.5. The actin nucleator Arp2/3 increases its
cortical association upon EMT
To further increase our understanding of changes
in cortex-associated actin upon EMT as reported in
[5], we also investigated EMT-induced changes of the
second major actin nucleator beyond mDia1, namely
the Arp2/3 complex [6, 45]. The Arp2/3 complex
becomes activated by the Wasp family of proteins
[31, 46]. Activated Wasp proteins promote binding
of the Arp2/3 complex to f-actin [31, 47, 48] and
thus its association to the actin cortex. Wasp pro-
teins (WAVE) are activated downstream of Rac1 [29].
We therefore expect that our finding of EMT-induced
increase of cortical Rac1 association (figure 1(h))
should give rise to a downstream increase of cortical
Arp2/3 association.
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 4. The actin nucleator and RhoA/C downstream effector mDia1 changes its cortical association upon EMT with opposite
trend in interphase and mitosis. (a) Exemplary western blots for mDia1 expression in MCF-7 cells in control and EMT-induced
conditions. (b) Bar charts of normalized quantities of mDia1 western blots. Normalization was done against GAPDH bands.
Error bars represent standard error of the mean. (c) EMT-induced fold changes of normalized protein amounts of mDia1 upon
EMT induction from western blots, see section 5. Individual data points are depicted in black. Error bars represent standard error
of the mean. Changes are not significant from zero according to a two-tailed one-sample t-test. (d) Representative confocal
images of suspended interphase cells and STC-arrested mitotic cells upon EMT, fixed and stained for DAPI (blue) and mDia1
(green). Scale bar: 10 µm. (e) Cortex-to-cytoplasm ratio of mDia1 inferred from immunofluorescence staining as shown in panel
(d) before and after EMT. Significance was tested with a Mann-Whitney U-test (two tailed). Post-EMT cells are referred to as
modMCF-7. Number of cells analyzed: (e): MCF-7 interphase n = 72, modMCF-7 interphase n = 32, MCF-7 mitosis n = 33,
modMCF-7 mitosis n = 36. Measurements represent at least two independent experiments. n.s.: p > 0.05, ∗: p < 0.05, ∗∗:
p < 0.01, ∗ ∗ ∗: p < 0.001.
Performing protein quantification in whole-cell
lysates, we find that there are no significant expres-
sion changes of Arp2 upon EMT, see figures 5(a)–(c).
This indicates that there are no significant changes
of the amount of the Arp2/3 complexes in MCF-7
cells upon EMT induction. To assess cortical associ-
ation of the Arp2/3 complex, we performed immun-
ostaining of Arp2 in rounded cells in interphase and
mitosis, see figure 5(d). Interestingly, in spite of a
direct interaction between Arp2/3 and f-actin, a cor-
tical enrichment of Arp2 is only visible in interphase
cells, see figure 5(d), lower row. Quantification of cor-
responding cortical association in interphase via the
cortex-to-cytoplasm ratio indicates that Arp2/3 sig-
naling at the cortex is enhanced through EMT, see
figure 5(e).
Previous studies reported that Arp2/3 signaling
reduces cortical tension in interphase and mitosis
[33, 36]. This is counter-intuitive as Arp2/3 mediates
actin polymerization and, thereby, could be expec-
ted to increase the amount of cortical myosin II
substrate. To test the effect of Arp2/3 signaling on
cortical tension in MCF-7 cells, we measured actin
cortical mechanics with and without the Arp2/3
inhibitor CK666, see figures 5(f)–(i) and S5. We
find that in agreement with previous results, cortical
tension and stiffness increases upon Arp2/3 inhibi-
tion in interphase and mitosis, see figures 5(f)–(i).
By contrast, the phase shift did not change signi-
ficantly upon Arp2/3 inhibition, see figures S5(b)
and (d).
We conclude that our data suggest that increased
Arp2/3 activity upon EMT downstream of increased
to reduce cortex stiffness
Rac1 activity tends
and contractility independent of
the cell cycle
state.
8
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 5. The actin nucleator and Rac1 downstream effector Arp2/3 is elevated at the cortex upon EMT in interphase. (a)
Exemplary western blots for Arp2 expression in MCF-7 cells in control and EMT-induced conditions. (b) Bar charts of
normalized quantities of Arp2 western blots. Normalization was done against GAPDH bands. Error bars represent standard error
of the mean. (c) Fold changes of normalized protein amounts of Arp2 upon EMT from western blots, see section 5. Individual
data points are depicted in black. Error bars represent standard error of the mean. Changes are not significant from zero
according to a two-tailed one-sample t-test. (d) Representative confocal images of suspended interphase cells and STC-arrested
mitotic cells upon EMT, fixed and stained for DAPI (blue) and Arp2 (green). Scale bar: 5 µm. (e) Cortex-to-cytoplasm ratio of
Arp2 inferred from immunofluorescence staining as shown in panel (d) before and after EMT. Mitotic cells did not show a clear
cortical Arp2 association and were therefore not quantified. (f)–(i) Arp2 inhibition using 50 µM CK666 elicits cortical stiffening
and tension increase in the actin cortex in interphase (f), (g) and mitotic MCF-7 cells (h), (i). In panels (e)–(i), significance was
tested with a Mann–Whitney U-test (two tailed). Post-EMT cells are referred to as modMCF-7. Number of cells analyzed: (e):
MCF-7 interphase n = 47, modMCF-7 interphase n = 48. (f), (g): MCF-7 n = 24, CK666 n = 24. (h), (i): MCF-7 n = 27, CK666
n = 24. Measurements represent at least two independent experiments. n.s.: p > 0.05, ∗: p < 0.05, ∗∗: p < 0.01, ∗ ∗ ∗: p < 0.001.
3. Arp2/3 activity enhances cortical
actin but reduces cortical association of
myosin II
To deepen our understanding of cortical response to
EMT-induced changes of cortex-associated Arp2/3,
we investigated how cortex-associated actin and
myosin change in response to Arp2/3 inhibition. For
this purpose, we transfected MCF-7 cells with con-
structs expressing fluorescently labeled myosin reg-
ulatory light chain (MLC2) or fluorescently labeled
actin (ACTB) as fluorescent reporters of cellular
myosin II and actin localization, see figure 6(a) and
section 5. Cell transfection was performed for suspen-
ded cells in interphase and mitosis as well as in pre-
and post-EMT conditions, see section 5. Quantifying
9
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 6. Activity of the Rac1 downstream effector Arp2/3 modulates actin and myosin association to the cortex. (a)
Representative confocal images of live suspended interphase cells and STC-arrested mitotic MCF-7 cells upon Arp2/3 inhibition
with CK666, expressing mCherry-ACTB (bottom row) or mApple-MLC (top row). Scale bar: 10 µm. (b), (c)
Cortex-to-cytoplasm ratio of actin (b) and myosin (c) inferred from live images as shown in panel (a) before and after Arp2/3
inhibition with 50 µM CK666. Significance was tested with a Mann–Whitney U-test (two tailed). (d) Schematic of cortical
changes upon Arp2/3 activation/inhibition. Post-EMT cells are referred to as modMCF-7. Number of cells analyzed: (b):
interphase: MCF-7 n = 35, CK666 n = 34, modMCF-7 n = 35, CK666 n = 31, mitosis: MCF-7 n = 31, CK666 n = 30,
modMCF-7 n = 35, CK666 n = 31, (c): interphase: MCF-7 n = 38, CK666 n = 33, modMCF-7 n = 38, CK666 n = 35, mitosis:
MCF-7 n = 28, CK666 n = 35, modMCF-7 n = 27, CK666 n = 30. n.s.: p > 0.05, ∗: p < 0.05, ∗∗: p < 0.01, ∗ ∗ ∗: p < 0.001.
cortical association of myosin II and actin via con-
focal imaging of transfected live cells, we determined
the cortex-to-cytoplasm ratio of actin and myosin II
in conditions with and without Arp2/3 inhibition via
the inhibitor CK666. As expected, we find that cor-
tical actin association goes down upon Arp2/3 inhib-
ition in all conditions, see figure 6(b). By contrast,
we observe that Arp2/3 inhibition increases myosin
II association to the cortex in all conditions in spite of
reduction of cortical actin, see figure 6(c).
To further corroborate these findings, we double-
checked the observed effect of Arp2/3 signaling in
fixed cells using Phalloidin as a fluorescent reporter
of f-actin and immunostaining of MYH9 as a fluores-
cent readout of myosin II localization, see figure S6.
Again, we see that Arp2/3 inhibition leads to dimin-
ished cortical f-actin in combination with an increase
in cortical myosin II confirming our results obtained
from transfected live cells, see figures S6(b) and (c).
We conclude that cortical Arp2/3 in addition
to expected allocation and polymerization of cor-
tical actin leads to diminished cortical association of
myosin II. Increased cortex-associated Arp2/3 down-
stream of EMT-induced enhanced Rac1 signaling
may therefore, at least in part, account for the obser-
vation of emergent reduced cortical tension and stiff-
ness as well as reduced myosin II cortex-to-cytoplasm
ratios upon EMT in interphase cells [5]. We note that
10
a negative effect of Arp2/3 on myosin activity was pre-
viously reported within mouse oocytes [49].
4. Discussion
In this study, we investigated how actin cortex regula-
tion is changed upon EMT in MCF-7 breast epithelial
cancer cells. As previous work suggested that activity
changes of the Rho GTPases are a game changer of
cytoskeletal regulation upon EMT [5, 13, 38, 50–52],
we focused on modulations of cortical signaling of
RhoA, RhoC and Rac1 as well as on selected down-
stream effectors.
Signaling of cortical
regulator proteins was
assessed through immunostaining and subsequent
confocal imaging of fixed MCF-7 breast epithelial
cells in a rounded, non-adherent state with a largely
uniform cortex. The magnitude of the cortex-to-
cytoplasm ratio of the regulator protein under consid-
eration was used as a quantitative readout of cortical
signaling strength. In particular, we compared cortex-
to-cytoplasm ratios in control and EMT-induced
conditions in interphase and mitosis. Furthermore,
for cortical regulators RhoC, formin, Arp2/3 and
cofilin, we identified their influence on cortical mech-
anics which was quantified by an established AFM-
based cell confinement setup [5, 17, 18, 35].
Phys. Biol. 20 (2023) 066001
K Hosseini et al
Figure 7. Schematic of EMT-induced changes of cytoskeletal signaling in MCF-7 breast epithelial cancer cells in interphase and
mitosis as suggested by our data. Red boxes indicate proteins whose cortical signaling increases through EMT. Blue boxes indicate
proteins whose cortical signaling decreases through EMT. Grey boxes indicate proteins that show no net change in their cortical
signaling upon EMT. In the signaling network, pointed black arrows represent activating signaling while flat black arrows indicate
inhibiting signaling.
In summary, we found that EMT reduces cor-
tical association of RhoA but enhances cortical asso-
ciation of Rac1, while EMT-induced changes of cor-
tical RhoC are different in interphase and mitosis,
see figures 1(h) and 7. Interestingly, we discovered
a hitherto unappreciated interaction between RhoC
and Rac1 that likely contributes to RhoC activation
in mitotic EMT-induced cells, see figure 2(g). This
interaction entails in particular a reduction of cor-
tical RhoC in non-adherent interphase cells but an
increase of cortical RhoC in mitosis through Rac1
signaling.
Downstream of Rho GTPases, we found that
also the cortical signaling of the formin mDia1 is
affected in a cell-cycle-dependent manner by EMT.
The corresponding decrease of mDia1 at the EMT-
transformed interphase cortex can be attributed to
decreased RhoA and RhoC signaling. On the other
hand, EMT-related increase of mitotic RhoC signal-
ing can account for increased mDia1 at the mitotic
post-EMT cortex, see figure 7. Furthermore, we find
an EMT-induced increase in Arp2/3 and cofilin sig-
naling at the cortex in both interphase and mitosis,
see figure 7.
Taken together, our study indicates that actin
nucleation at the interphase cortex is promoted
upon EMT through upregulation of Arp2/3, but
diminished through downregulation of mDia1 and
enhanced cofilin signaling at the cortex. All in all,
these signaling changes may give rise to the observed
absence of a net change of cortex-associated actin
upon EMT in interphase, see figure 6(b) and [5].
Furthermore, myosin activity at the interphase cor-
tex is diminished through reduced RhoA and RhoC
signaling (via Rock) and via increased Rac1 signal-
ing (mediated downstream by Arp2/3, see figure 6(d).
The combined effects can account for the observed
net decrease of cortex-associated myosin upon EMT
and can be causative to cortical stiffness and con-
tractility reduction, see figure 7.
In mitotic cells, EMT increases cortical actin nuc-
leation through enhanced cortical RhoC signaling
(e.g. via mDia1) and Rac1 signaling (via Arp2/3). On
the other hand, EMT decreases cortical actin through
11
reduced cortical RhoA signaling and enhanced cofilin
signaling. The integration of all signaling changes can
account for the observed net increase of cortical actin
upon EMT in mitosis, see figure 6(d) and [5].
Further, EMT promotes myosin activity at the
mitotic cortex through enhanced RhoC signaling, but
diminishes it through reduced RhoA signaling and via
increased Rac1 signaling (mediated downstream by
Arp2/3 signaling, see figure 6(d). Through the com-
bination of these opposite effects, the net change of
cortical myosin II may vanish as was observed in
MCF-7 cells, see figure 7.
In conclusion, we find that EMT induces complex
modifications in actin-cytoskeletal signaling through
a combination of changes in the signaling of Rho
GTPases and downstream effectors such as cofilin,
Arp2/3 and mDia1. The integration of all partly
opposing effects give rise to an emergent change of
actin and myosin at the cortex. In particular, our find-
ings shed further light on how differences emerge in
cortical composition and mechanics that are distinct
in interphase and mitosis. Finally, we note that our
study provides a cellular EMT fingerprint of roun-
ded cells that may be relevant for cancer diagnostic
approaches in particular for those that rely on isol-
ated cells such as FACS-related assays or deformabil-
ity flow cytometry approaches [53–55].
5. Materials and methods
5.1. Cell culture
The cultured cells were maintained as follows:
MCF-7 cells were grown in RPMI-1640 medium
(PN:2187-034, life technologies) supplemented with
10% v/v fetal bovine serum, 100 µg ml−1 penicil-
lin, 100 µg ml−1 streptomycin (all Invitrogen) at
37 ◦C with 5% CO2. In MCF-7 cells, EMT was
induced by incubating cells in medium supple-
mented with 100 nM 12-O-tetradecanoylphorbol-
13-acetate (TPA) (PN:P8139, Sigma) for 48 h prior
to measurement [25]. Arp2/3 inhibition was per-
formed by 30 min treatment with 50 µM CK666
Sigma). mDia1 inhibition was
(PN:SML0006,
Phys. Biol. 20 (2023) 066001
K Hosseini et al
performed by 30 min treatment with 40 µM SMIFH2,
which inhibits all formins.
5.2. AFM measurement of cells
Experimental setup. To prepare mitotic cells for
AFM measurements, approximately 10 000 cells were
seeded in a cuboidal silicon cultivation chamber
(0.56 cm2 area, from cutting ibidi 12-well cham-
ber; ibidi, Gräfelfing, Germany) that was placed in a
35 mm cell culture dish (fluorodish FD35-100, glass
bottom; World Precision Instruments, Sarasota, FL)
1 day before the measurement so that a conflu-
ency of ∼30% was reached on the measurement day.
Mitotic arrest was induced by supplementing S-trityl-
L-cysteine (Sigma-Aldrich) 2–8 h before the measure-
ment at a concentration of 2 µM. For measurement,
mitotic-arrested cells were identified by their shape.
Their uncompressed diameter ranged typically from
18 to 23 µm.
To prepare AFM measurements of suspended
interphase cells, cell culture dishes (fluorodish FD35-
100) and wedged cantilevers were plasma-cleaned
for 2 min and then coated by incubating the
dish at 37 ◦C with 0.05 mg ml−1 poly(L-lysine)-
polyethylene glycol dissolved in phosphate-buffered
saline (PBS) overnight at 37 ◦C (poly(L-lysine)(20)-
g[3.5]-polyethylene glycol(2); SuSoS, Dubendorf,
Switzerland) to prevent cell adhesion. Before meas-
urements, cultured cells were detached by the
addition of 0.05% trypsin-EDTA (Invitrogen).
Approximately 30 000 cells in suspension were placed
in the coated culture dish. Upon resuspension, the
culture medium was changed to CO2-independent
Invitrogen) with 4 mM
DMEM (PN:12800-017;
NaHCO3 buffered with 20 µM HEPES/NaOH
(pH 7.2), for AFM experiments ∼2 h before the
measurement [17, 35, 56, 57].
The experimental
setup included an AFM
(Nanowizard I;
JPK Instruments, Carpinteria,
CA) that was mounted on a Zeiss Axiovert 200M
optical, wide-field microscope using a 20x objective
(Plan Apochromat, NA = 0.8; Zeiss, Oberkochen,
Germany) along with a CCD camera (DMK 23U445
from The Imaging Source, Charlotte, NC). Cell cul-
ture dishes were kept in a petri-dish heater (JPK
Instruments) at 37 ◦C during the experiment. Before
every experiment, the spring constant of the canti-
lever was calibrated by thermal noise analysis (built-
in software; JPK) using a correction factor of 0.817
for rectangular cantilevers [58]. The cantilevers used
were tipless, 200–350 µm long, 35 µm wide, and 2 µm
thick (CSC37, tipless, no aluminum; Mikromasch,
Sofia, Bulgaria). The nominal force constants of the
cantilevers ranged between 0.2 and 0.4 N m−1. The
cantilevers were supplied with a wedge, consisting of
UV curing adhesive (Norland 63; Norland Products,
East Windsor, NJ) to correct for the 10◦ tilt [59]. The
measured force, piezo height, and time were output
with a time resolution of at least 500 Hz.
12
Dynamic AFM-based cell confinement. Preceding
every cell compression, the AFM cantilever was
lowered to the dish bottom in the vicinity of the
cell until it touched the surface and then retracted
to ≈14 µm above the surface. Subsequently, the free
cantilever was moved and placed on top of the cell.
Thereupon, a bright-field image of the equatorial
plane of the confined cell was recorded to evaluate
the equatorial radius Req at a defined cell height h.
Cells were confined between dish bottom and canti-
lever wedge. Then, oscillatory height modulations of
the AFM cantilever were carried out with oscillation
amplitudes of 0.25 µm at a frequency of 1 Hz.
During this procedure, the cell was on average
kept at a normalized height h/D between 60 and
70%, where D = 2(3/(4π)V)1/3 and V is the estim-
ated cell volume. Using molecular perturbation with
cytoskeletal drugs, we could show in previous work
that at these confinement levels, the resulting mech-
anical response of the cell measured in this setup is
dominated by the actin cortex (see figure 4 in [35]
and figure S7 in [5]). This is further corroborated by
our observation from previous work that the smallest
diameter of the ellipsoidal cell nucleus in suspended
interphase cells is smaller than 60% of the cell dia-
meter (see figure S4 in [17]). Additionally, it has been
shown that for a nucleus-based force response, when
measuring cells in suspension with AFM, a confine-
ment of more than 50% of the cell diameter is needed
[60].
Data analysis. The data analysis procedure was
described in detail in an earlier work [35]. In our ana-
lysis, the force response of the cell is translated into an
effective cortical tension γ = F/[Acon(1/R1 + 1/R2)],
where Acon is the contact area between confined cell
and AFM cantilever and R1 and R2 are the radii of
principal curvatures of the free surface of the con-
fined cell. Here R1 is estimated as half the cell height h
and R2 is identified with the equatorial radius Req [17,
35, 56]. Cell height h and equatorial radius Req were
estimated from the AFM readout and optical ima-
ging, respectively [35]. For the determination of the
radius of the contact area Acon, see also supplement-
ary section 5 in [56].
Oscillatory force and cantilever height readouts
were analyzed in the following way: for every time
point, effective cortical tension γ and surface area
strain ϵ(t) = (A(t) − ⟨A⟩)/⟨A⟩ were calculated. Here,
A(t) is the total surface area of the confined cell.
It is estimated as the area of a rotationally sym-
metric body with semi-circular free-standing side
walls at cell height h and equatorial radius Req, i.e.
A = π(2h(Req − h/2)(π/2 − 1) + h2/2 + 2R2
eq) [17].
An amplitude and a phase angle associated to the
oscillatory time variation of effective tension γ and
surface area strain are extracted by sinusoidal fits. To
estimate the value of the complex elastic modulus at
a distinct frequency, we determine the phase angles
φγ and φϵ as well as amplitudes Aγ and Aϵ of active
Phys. Biol. 20 (2023) 066001
K Hosseini et al
cortical tension and surface area strain, respectively.
The complex elastic modulus at this frequency is then
calculated as Aγ/Aϵ exp(i(φγ − φϵ)).
Statistical analyses of cortex mechanical para-
meters were performed in MATLAB using the com-
mands ‘boxplot’ and ‘ranksum’ to generate boxplots
and determine p-values from a Mann-Whitney U-test
(two tailed), respectively.
5.3. Plasmids and transfection
Transfection of cells was performed transiently
with plasmid DNA using Turbofectin 8.0 (PN:
TF81001, Origene), according to the manufac-
turer’s protocol. To achieve post-EMT conditions,
MCF-7 cells were seeded at day -1. The cells
were then transfected at day 0 and treated with
100 nM TPA (in the case of modMCF-7 cells).
The cells were then imaged at day 2. The plasmid
MApple-LC-Myosin-N-7 was a gift from Michael
Davidson (Addgene plasmid 54920; http://n2t.net/
addgene:54920; RRID:Addgene_54920). The plas-
mid MCherry-Actin-C-18 was a gift from Michael
Davidson (Addgene plasmid 54967; http://n2t.net/
addgene:54967; RRID:Addgene_54967 [61]).
5.4. Imaging of transfected cells
Transfected cells were placed on PLL-g-PEG coated
fluorodishes (FD35-100) with CO2-independent cul-
ture medium (described before). Cellular DNA was
stained with Hoechst 33342 solution (PN:62249,
Invitrogen) in order to distinguish between mitotic
and interphase cells. During imaging, cells were
maintained at 37 ◦C using an Ibidi heating stage.
Imaging was done using a Zeiss LSM700 confocal
microscope of the CMCB light microscopy facility,
incorporating a Zeiss C-Apochromat 40x/1.2 water
objective. Images were taken at the equatorial dia-
meter of each cell at the largest cross-sectional area
(see figure 6(a)).
5.5. Calculation of cortex-to-cytoplasm ratios
This has been described before [5]. In short, using a
MATLAB custom code, the cell boundary was iden-
tified (figure 1(b) shows an exemplary cell, the cell
boundary is marked in red). Along this cell bound-
ary, 200 radial, equidistant lines were determined
by extending 1.5 µm to the cell interior and 2.5 µm
into the exterior (figure 1(b), red lines orthogonal
to cell boundary, only every tenth line was plotted
out of 200). The radial fluorescence profiles corres-
ponding to these lines were averaged over all 200
lines (figure 1(c), blue curve). This averaged intens-
ity profile is then fitted by a linear combination of
an error function (cytoplasmic contribution) and a
skewed Gaussian curve (cortical contribution), see
figure 1(c), orange curve. The respective fit formula
is given by
13
Ism(r, p) =
))
(
(
1 − erf
Icyt
2
r − µ
√
σ1
2
(
+ IcortG(r, µ, σ2)
1 + erf
))
(
α(r − µ)
√
2
σ2
+ IBG.
(1)
Here p = {µ, σ1, σ2, Icyt, Icort, IBG, α} are fit paramet-
ers, which determine the position of the cortex (µ),
the slope of the error function decay (σ1), the width
of the cortical Gaussian peak σ2, the amplitudes of the
error function and the Gaussian peak (Icyt and Icort)
and the skewness of the Gaussian peak (α). To calcu-
late the cortex-to-cytoplasm ratio, the fitted skewed
Gaussian is integrated and the obtained integral is
then normalized by the cytoplasmic intensity Icyt [5].
5.6. Immunostaining and confocal imaging of cells
Immunostaining of suspended cells (interphase or
STC-arrested mitotic) was performed as described
previously [62]. Briefly, before fixation, cultured cells
were detached by the addition of 0.05% trypsin-
EDTA (Invitrogen) and resuspended in a glass-
bottom dish (e.g.
ibidi; #80826) at a density of
≈3 × 105/cells per cm2. In order to not be washed
away during washing steps, cells were left to incubate
and weakly adhere for ≈10 min. Then, cells were fixed
with 3.7% PFA/PBS for 10 min (10% TCA for 15 min
for Rac1, RhoA and RhoC, and −20 ◦C ethanol for
10 min for Arp2) at room temperature, followed by
a 10 min permeabilization step in 0.2% Triton X-
100. The cells were then blocked for 1 h at room
temperature with 5%BSA/PBS. The cells were then
treated with primary antibody of Rac1 (PN:PA1-
091-X, Thermofisher), RhoA (PN:NBP2-22528,
Novus Bio), RhoC (PN:GTX100546, GeneTex),
CFL1 (PN:660571-1-lg, Proteintech), p-CFL1 (Ser3,
PN:3313T, Cellsignaling), p-LIMK1 (Thr508, PN:E-
AB-20918, Elabscience), mDia1 (PN:20624-1-AP,
Proteintech) and Arp2 (PN:10922-1-AP, Proteintech)
overnight at 4 ◦C in 5%BSA/PBS. Cells were then
treated with the corresponding secondary Alexa
Fluor 488 conjugate at a concentration of 1:1000 in
5%BSA/PBS for 2 h at room temperature. At the same
time, cells were treated with 5 µg ml−1 DAPI (2 min)
and 0.2 µg ml−1 Phalloidin-iFluor-647 (10 min) in
5% BSA/PBS solution. Images were taken with a
Zeiss LSM700 confocal microscope of the CMCB
light microscopy facility, incorporating a Zeiss C-
Apochromat 40x/1.2 water objective. Images were
taken at the equatorial diameter of each cell showing
the largest cross-sectional area.
5.7. Western blotting
Protein expression in MCF-7 cells before and after
EMT was analyzed using western blotting. Cells were
seeded onto a six-well plate and grown up to a con-
fluency of 80%–90% with or without EMT-inducing
agents. Thereafter, cells were lysed in SDS sample/lysis
Phys. Biol. 20 (2023) 066001
K Hosseini et al
buffer (62.5 mM TrisHcl pH 6.8, 2% SDS, 10%
Glycerol, 50 mM DTT and 0.01% Bromophenolblue).
For analysis of protein expression in mitotic cells,
STC was added at a concentration of 2 µM to the
cell medium 12–18 h before cells were harvested in
order to enrich mitotic cells. For harvesting, mitotic
cells were collected by shake-off and/or flushing of the
medium.
Cell lysates were incubated for 30 min with the
lysis buffer at 4 ◦C. They were then boiled for 10
min. 10/20 µl of the cleared lysate was then used
for immunoblotting. The cleared lysates were first
run on precast protein gels (PN:456-1096 or 456-
1093, Bio-Rad) in MOPS SDS running buffer (B0001,
Invitrogen). Subsequently, proteins were transferred
to Nitrocellulose membranes (GE10600012, Sigma-
Aldrich). Nitrocellulose membranes were blocked
with 5% (w/v) skimmed milk powder (T145.1, Carl
Roth, Karlsruhe, Germany) in TBST (20 mM l−1
Tris-HCl, 137 mM l−1 NaCl, 0.1% Tween 20 (pH
7.6)) or 5% (w/v) BSA for phospho-antibodies for
1 h at room temperature followed by washing with
TBST, and incubation at 4 ◦C overnight with the
corresponding primary antibody diluted 1:500 (p-
LIMK1), 1:1000 (mDia1, Arp2, CFL1 and p-CFL1)
and 1:5000 (GAPDH) in 5% (w/v) bovine serum
albumin/TBST solution. Thereupon, the blots were
incubated with appropriate secondary antibodies
conjugated to horseradish peroxidase, Goat anti-
mouse HRP (PN: ab97023, Abcam) or Goat anti-
rabbit HRP (PN: ab97051, Abcam) at 1:5000 dilu-
tion in 5% (w/v) skimmed milk powder in TBST
for 1 h at room temperature. After TBST washings,
specifically bound antibodies were detected using
Pierce enhanced chemiluminescence substrate (ECL)
(PN:32109, Invitrogen). The bands were visualized
and analyzed using a CCD-based digital blot scan-
ner, ImageQuant LAS4000 (GE Healthcare Europe,
Freiburg, Germany). Primary antibodies used are
as follows: GAPDH (PN:ab9485, Abcam), CFL1
(PN:660571-1-lg, Proteintech), p-CFL1 (PN:3313T,
(PN:ELA-E-AB-20918,
Cellsignaling),
Biozol), mDia1 (PN:20624-1-AP, Proteintech) and
Arp2 (PN:10922-1-AP, Proteintech).
p-LIMK1
For quantification, protein bands were normal-
ized against GAPDH on the same western blot lane.
To calculate fold changes upon EMT, normalized
western blot intensities of the protein under consider-
ation were determined for lanes from untreated cells
and EMT-induced cells to obtain normalized intens-
ities ˆIctrl and ˆIEMT, respectively. Fold change values
were output as log2(ˆIEMT/ˆIctrl), see figures 3(c), 4(c)
and 5(c), respectively. We tested whether distri-
butions of fold changes had a mean value signific-
antly different from zero with a one-sample t-test
(two-tailed) against the null hypothesis that the data
comes from a normal distribution with mean equal to
zero.
14
5.8. Gene knock-down through RNA interference
Transfections were done targeting the genes CFL1
(siRNA, ID s2938, Thermofisher) or RHOC (esiRNA,
HU-06597-1 Eupheria Biotech) at an RNA con-
centration of 25 nM, using the transfection reagent
Lipofectamine RNAiMax (Invitrogen) according to
the protocol of the manufacturer. Firefly luciferase
esiRNA (FLUC, Eupheria Biotech) was used as a neg-
ative control, while EG5/KIF11 esiRNA (HU-01993-
1, Eupheria Biotech) was used as a positive control.
In all experiments, EG5/KIF11 caused mitotic arrest
of more than 60%–70% of the cells, showing a trans-
fection efficiency of at least 60% in each experiment.
Knock-down of RHOC and CFL1 was confirmed
through Western blotting, see figures S2 and S3(b)–
(d).
For AFM measurements, at day −1, 30 000 cells
were seeded into a 24-well plate (NuncMicroWell
Plates with Nunclon; Thermo Fisher Scientific,
Waltham, MA, USA). At day 0 the transfection was
done. The transfected cells were imaged at day 2. For
post-EMT conditions, the cells were kept in 100 nM
TPA from day 0 to day 2. For mitotic cells, ≈12–24 h
before measurements, cells were detached, diluted,
and transferred onto a glass-bottom Petri dish (FD35-
100, World Precision Instruments) with 2 µM STC
added ≈2 h before measurement. For interphase cells,
1–2 h before measurement the cells were detached
and transferred to PLL-g-PEG-coated Petri dishes
(see section on AFM Measurements of Cells).
For Western blotting, at day −1, 800 000 cells were
seeded into a six-well plate (NuncMicroWell Plates
with Nunclon; ThermoFisher Scientific, Waltham,
MA, USA). At day 0 the transfection was done. The
transfected cells were then lysed at day 2 as described
in the Western blotting section. For post-EMT condi-
tions, the cells were kept in 100 nM TPA from day 0 to
day 2. For mitotic cells, ≈12–18 h before lysing, 2 µM
STC was added.
Data availability statement
The data cannot be made publicly available upon
publication because the cost of preparing, depositing
and hosting the data would be prohibitive within the
terms of this research project. The data that support
the findings of this study are available upon reason-
able request from the authors.
Acknowledgments
E F-F acknowledges financial support
from the
Deutsche Forschungsgemeinschaft under Germany’s
Excellence Strategy, EXC-2068-390729961, Cluster
of Excellence Physics of Life of TU Dresden.
Furthermore, E F-F was funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research
(FI
Foundation)—Project Number
495224622
Phys. Biol. 20 (2023) 066001
K Hosseini et al
2260/8-1) and by the Grant FI 2260/7-1. In addi-
tion, the authors thank the CMCB and PoL Light
Microscopy Facility for excellent support.
Author contributions
K H and A F performed the experiments. K H and
E F-F designed the experiments. K H performed data
analysis. K H and E F-F wrote the manuscript.
Conflict of interest
There are no conflicts to declare.
ORCID iD
Elisabeth Fischer-Friedrich
https://orcid.org/0000-0002-2433-916X
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16
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10.1126_sciadv.adh0411.pdf
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Supplementary Materials for
Developmentally programmed histone H3 expression regulates cellular
plasticity at the parental-to-early embryo transition
Ryan J. Gleason et al.
Corresponding author: Ryan J. Gleason, [email protected]; Xin Chen, [email protected]
Sci. Adv. 9, eadh0411 (2023)
DOI: 10.1126/sciadv.adh0411
The PDF file includes:
Figs. S1 to S3
Tables S1 to S3
Legends for movies S1 to S3
Other Supplementary Material for this manuscript includes the following:
Movies S1 to S3
Supplemental Materials
Supplementary Figures and Figure Legends:
Figure S1. Expression patterns of all histone H3 gene clusters in C. elegans adult
hermaphrodites.
Figure S1 legend: Expression patterns of all histone H3 gene clusters in C. elegans adult
hermaphrodites. (A) Representative fluorescence micrographs of ubiquitously expressed Class I
histone H3 isotypes including his-45, his-55, his-63, and his-59. The dashed lines outline the
gonads, and distinct cell types are marked as an example. Insets demonstrate that his-55, his-63,
and his-59 are detectable in the germline by increasing the brightness. (B) Representative
fluorescence micrographs for one member of each of the five histone gene clusters including
HIS1 (his-2), HIS2 (his-6), HIS3 (his-25), HIS4 (his-17, his-27, and/or his-49, see methods for
details), and HIS5 (his-32). Histone H3 isotypes encoded in HIS1-5 are detectable in all somatic
lineages, but undetectable in the germline. (C) his-40 encodes a histone H3 that is detectable in
epithelial nuclei including the hypodermis (epidermis) marked by yellow arrows.
Figure S2. HIS-71(H3.3)::GFP is detectable at late pachytene as nuclei transition from
pachytene to diakinesis and initiate oocyte formation.
Figure S2 legend: (A) Expression pattern of an endogenously tagged GFP fusion strain for his-
71 (H3.3), GFP (top), and DIC (bottom). HIS-71::GFP is undetectable in the mitotic and early
meiotic pachytene regions of the germline. Expression of HIS-71::GFP is observed as nuclei
transition into the loop region of the germline, where they transition from pachytene to diakinesis
and initiate oocyte formation. The dashed lines outline the gonads, and distinct cell types are
marked as an example, including oocytes, early stage embryos, and somatic cells. Somatic cells
are notably higher in his-71 expression. (B) A second sample of HIS-71::GFP, which is
positioned in an orientation optimal for detecting nuclei expression in the loop region of the
germline, GFP (top), and DIC (bottom).
Figure S3. Knockouts of the germline-expressed histone H3 genes lead to decreased
fecundity and germ cell nuclei, as well as increased germline apoptosis.
Figure S3 legend: (A) Representative images of wild-type and his-59(kog7); his-55(kog8)
double mutant of histone H3 genes. The strain GC1413 rrf-1(pk1417; naSi2 (Pmex-
5::H2B::mCherry::nos-2 3’UTR); teIs113 (Ppie-1::GFP::H2B::zif-1 3’UTR)) was used to label
all germline nuclei with mCherry (red), while progenitor nuclei are doubly marked with GFP and
mCherry (yellow). The dashed lines outline the gonads. (B) Quantification measured by rows of
cells from the distal end. All quantifications = average ± SE. P-value: unpaired t test, showing no
significant difference (P = 0.3192) in the Progenitor region (GFP- and mCherry-double positive
germ cells) of his-59(kog7);his-55(kog8) double histone H3 mutant (n=4) and wild-type (n=3),
but a significant difference (**P = 0.0054) in the pachytene region (GFP-negative and mCherry-
positive germ cells), his-59(kog7);his-55(kog8) double histone H3 mutant (n=8) and wild-type
(n=8). (C) Immunofluorescent micrographs of wild-type and double histone H3 mutant, his-
59(kog7);his-55(kog8), stained for H3K27me2/3 (magenta) or H3K36me2 (green). (D)
Quantification of total 3D intensity of either H3K27me2/3 (*P = 0.0133) or H3K36me2 (*P =
0.0240) of data sets from (C). (E) Brood sizes for two double histone H3 mutant strains
including his-59(kog7); his-55(kog8) (n=36) and wild-type (n=39) (*P = 0.0171), and his-
59(kog7); his-55(kog9) (n=30) and wild-type (n=30) (*P = 0.0181). Each data point represents
the number of living larvae from individual worms with the corresponding genotype. (F)
Representative images of nuclei undergoing programmed cell death marked by CED-1::GFP
(yellow arrows). (G) Quantification of apoptotic cells per gonad of wild-type (n=7), his-59(kog7)
single mutant (n=5), and his-59(kog7); his-55(kog8) double mutant (n=7). All quantifications =
average ± SE; P-value: unpaired t test, ** P<0.01, * P≤0.05, ns: not significant. Scale bars:5 μm
in (A, C and F).
Supplemental Tables:
Table S1. Summary of histone H3-like, and histone H3.3-like expression in C. elegans
hermaphrodites.
Gene (Chromosome) Germ
-line
+
his-45(H3) (IV)
his-59(H3) (IV)
his-63(H3) (IV)
his-55(H3) (IV)
his-2(H3) (V/HIS1)
his-6(H3) (V/HIS2)
his-9(H3) (II/HIS3)
his-13(H3) (II/HIS3)
his-17(H3) (V/HIS4)
his-25(H3) (II/HIS3)
his-27(H3) (V/HIS4)
his-32(H3) (IV/HIS5)
his-42(H3) (II/HIS3)
his-49(H3) (V)
his-40(H3) (X)
his-72(H3.3) (III)
his-71(H3.3) (X)
his-69(H3.3-like) (III)
his-70(H3.3-like) (III)
his-74(H3.3-like) (V)
+
+
+
-
-
-
-
-
-
-
-
-
-
-
+
+
-
+
+
Sperm Oocyte
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
+
n/a
-
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
+
+
-
-
+
Pre-
gastrulation
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
+
+
-
-
+
Gastrulation
Source
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study
This study &
Delaney et al. (ref
35)
This study &
Delaney et al. (ref
35)
Delaney et al. (ref
35)
Delaney et al. (ref
35)
PGC-restricted Delaney et al. (ref
35)
Table S2. C. elegans strains used in this study.
Strain
name
JHU42
JHU5
JHU4
JHU20
JHU19
JHU14
Genotype
Source
Comments
his-45(kog16[his-45::Dendra2]) IV
his-6(kog3[his-6::Dendra2]) V
his-72(kog2[his-72::Dendra2]) III
his-72(kog5[his-72::mCherry]) III
his-55 (kog11[his-
55::TEV::eGFP::3xFlag]) IV
his-59 (kog7) IV
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
This study mCherry fusion
This study
eGFP fusion
This study H3 homologue
deletion
JHU15
his-59 (kog7) IV; his-55 (kog8) IV
This study H3 homologue
deletion
JHU16
his-59 (kog7) IV; his-55 (kog9) IV
This study H3 homologue
JHU18
his-6 (kog10) V
deletion
This study Histone H3
OH14454 otIs587 [gcy-5(fosmid::SL2::NLS::GFP
CGC
+ ttx3p::mCherry]. otIs304 [hsp16-
2p::che-1::3xHA::BLRP + rol-
6(su10060]
his-6 (kog10) V; otIs587 [gcy-
5(fosmid::SL2::NLS::GFP +
ttx3p::mCherry]. otIs304 [hsp16-
2p::che-1::3xHA::BLRP + rol-
6(su10060]
Rrf-1(pk1417) I; naSi2(mex-
5p::H2B::mCherry::nos-2 3’UTR) II;
teIs113(pie-1p::GFP::H2B::zif-1
3’UTR) V
bcIs39 [lim-7p::ced-1::GFP + lin-
15(+)]
his-55(kog17[his-55::Dendra2]) IV
his-63(kog18[his-63::Dendra2]) IV
his-59(kog19[his-59::Dendra2]) IV
his-2(kog20[his-2::Dendra2]) V
his-25(kog21[his-25::Dendra2]) II
his-13(kog23[his-13::Dendra2]) II
his-32(kog22[his-32::Dendra2]) IV
H3::Dendra2 in the HIS4 cluster (his-
17, his-27, and/or his-49 ) V *
JHU45
GC1413
MD701
JHU57
JHU59
JHU53
JHU39
JHU40
JHU37
JHU41
JHU80
dominant
negative his-
6(H113D)
GFP cell fate
reporter
This study GFP cell fate
reporter with
histone H3
dominant
negative allele
Germline
reporter
Jane
Hubbard
lab (Roy
D., et al.
(ref 49)
Zhou et al.
(ref 50)
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
This study Dendra2 fusion
Apoptotic germ
cell reporter
PHX2995 his-40(syb2995[his-40::Dendra2]) X
This study Dendra2 fusion
FAS46
his-72(uge30[gfp::his-72]) III
FAS84
his-71(uge45[gfp::his-71]) X
JHU106
JHU79
ST65
his-55(syb3144[his-55::Ollas]) IV ; his-
72(kog5[his-72::mCherry]) III
ncIs13[AJM-1::GFP]; his-55
(kog11[his-55::TEV::eGFP::3xFlag]) ;
his-72(kog5[his-72::mCherry]); his-6
(kog10)
ncIs13[AJM-1::GFP]
GFP fusion
Delaney et
al. (ref 35)
Delaney et
al., (ref 35)
This study Ollas and
GFP fusion
mCherry fusions
This study AJM-1::GFP,
H3::GFP,
H3.3::mCherry
CGC
GFP fusion
*CRISPR/Cas9 Dendra2 knock-in reagents were designed to specifically edit a histone H3 gene
within the HIS4 histone cluster on chromosome V (see Figure S1A), which contains three
histone H3 genes his-17, his-27, and his-49. The genotyping results indicated that one or more of
these three H3 genes contains the Dendra2 sequences, but their genomic DNA sequences are too
similar to distinguish which one of the three contains the Dendra2 sequence. Therefore, we have
used this strain as an H3 reporter representing the entire HIS4 cluster activity.
Table S3. Reagents used for generating CRISPR/Cas9 mediated fusion proteins, deletion
strains, and point-mutations in C. elegans.
Allele
his-45(kog16[his-
45::Dendra2]) IV
sgRNA target sequences (PAM sites in
bold)
GCGCGCTTAAATACCTTTTTGG
GCTTGCTCAACTACCAAAAAAGG
his-6(kog3[his-
6::Dendra2]) V
GGTGGGGGTTTGAATCGAAACGG
ATCGAAACGGTCTCAAACTCTGG
his-72(kog2[his-
72::Dendra2]) III
AGTGCTTCGAGAATTCCTGATGG
GAGCTTAAGCACGTTCTCCGCGG
his-72(kog5[his-
72::mCherry]) III
AGTGCTTCGAGAATTCCTGATGG
GAGCTTAAGCACGTTCTCCGCGG
his-55(kog11[his-
55::TEV::eGFP::3xFlag])
IV
CAATTGGCCAGACGCATCCGAGG
GCTTGCTCAACTACCAAAAAAGG
his-55(kog17[his-
55::Dendra2]) IV
CAATTGGCCAGACGCATCCGAGG
GCTTGCTCAACTACCAAAAAAGG
his-63(kog18[his-
63::Dendra2]) IV
GCGCGCTTAAATACCTTTTTTGG
GCTTGCTCAACTACCAAAAAAGG
his-59(kog19[his-
59::Dendra2]) IV
GAGCGCGCTTAAATACCTTATGG
AAGCTTACTTAACTACCATAAGG
Repair template homology arms** or repair
template (deletion and point-mutations)
5’gctaagcgagtcaccatcatgccaaaggata
Tccaattggccagacgcatccgaggagagc
gTgctcagcacgtgatgaacaccccgg
gaattaacc
5’ ggccctaaagagggccgttgggttcggttaagtttt
gagattaagcttActTaactaTcaaaaaaAgtatTtt
accacacctggctgggcaggg
5’cgccaagcgagtcaccatcatgccaaaggacatcc
aattggccagacgtatccgaggagaacgtgctcagca
cgtgatgaacaccccgggaattaacc
5’ctaaagagggccgttgggttcggtgAgAgtttg
aatTgaaacAgtTtcaaaTtctAgaaatcagaaa
tttaccacacctggctgggcagg
5’ccacgccaagcgcgtcaccatcatgccaaaggacat
gcaactcgccagacgcatTcgTggagaGcgtgctca
gcacgtgatgaacaccccgggaattaacc
5’ggaaaaatacgaggattatggtacaagttggattaaat
gaatattaaaagtgcttTgagaattAgtAatgAagcttac
cacacctggctgggcaggg
5’ccacgccaagcgcgtcaccatcatgccaaaggacatg
caactcgccagacgcatTcgTggagaGcgtgctcag
cacgtgatggtgagcaagggcgaggag
5’ggaaaaatacgaggattatggtacaagttggattaaatg
aatattaaaagtgcttTgagaattAgtAatgAagcttactt
gtacagctcgtccatg
5’ gcgagtcaccatcatgccaaaggatatccaattggccag
GcgTatTcgGggagagcgcgctgagaacctctacttcca
Aggag
5’gtggccctaaagagggccgttgggttcggttaagttttgag
attaagcttgctTaactaTcaaGaaagAtatttacttgtcatc
gtcatccttgtaatc
5’gcgagtcaccatcatgccaaaggatatccaattggccagGc
gTatTcgGggagagcgcgctcagcacgtgatgaacac
cccgggaattaac
5’gtggccctaaagagggccgttgggttcggttaagttttgaga
ttaagcttgctTaactaTcaaGaaagAtatttaccacacctgg
ctgggcag
5’cgctaagcgagtcaccatcatgccaaaggatatccaattgg
ccagacgtatccgaggagagcgtgctcagcacgtgatgaaca
ccccgggaattaacc
5’ggccctaaagagggccgttgggttcggttaagttttgagatta
agcttActTaactaTcaaaaaaAgtatttaccacacctggctg
ggcaggg
5’agtcaccattatgccaaaggatatccagctggccagac
gtatccgaggagagcgcgctcagcacgtgatgaacaccc
cgggaattaacctg
5’ggccctaaagagggccgttgggttcggtgagttttgagtt
his-2(kog20[his-
2::Dendra2]) V
CGGTGGGGTTTGAATTGAAACGG
AAATTTAAGCACGTTCTCCTCGG
his-25(kog21[his-
25::Dendra2]) II
GCTGGCTCAGTACCATTGGAAGG
TCAAGCTGGCTCAGTACCATTGG
his-32(kog22[his-
32::Dendra2]) IV
AGCGTGCTTAAATGTCTTTGTGG
ACATTTAAGCACGCTCTCCTCGG
his-13(kog23[his-
13::Dendra2]) II
GCTGGCTCAGTACCATTGGAAGG
TCAAGCTGGCTCAGTACCATTGG
H3::Dendra2 in the HIS4
cluster (his-17, his-27,
and/or his-49) V *
ACTCTGAAAATCAGAAATTTAGG
AAATTTAGGCACGTTCTCCTCGG
his-59 (kog7) IV
CCCACGGATTATCAACCTAAAGG
GAGCGCGCTTAAATACCTTATGG
his-55 (kog9) IV
GATTATCAACCTAAACGCAATGG
GCGCGCTTAAATACCTTTTTTGG
gaagcttacttaaTtaTTataagAtatttaccacacctggct
gggcaggggg
5’gccaagcgagtcaccatcatgccaaaggacatccaattg
gccagacgtatTcgCggagaGcgtgctcagcacgtgat
gaacaccccgggaattaacc
5’ccctaaagagggccgttgggttcggtgAAgtttgaattA
aaacgAtctcaaactttctgaaaatcagaaatttaccacacct
ggctgggcagg
5’ctaagcgagttaccattatgccaaaggacatccaattggca
Agacgtatccgaggagagcgtgctcagcacgtgatgaacac
cccgggaattaacc
5’gtggccctaaagagggccgttgggttcggttagattttgag
atcaagctgActTagtaTcattAgaagAcatTTAccaca
cctggctgggcaggg
5’cacgctaagcgagttaccatcatgccaaaggatatccagct
ggccagacgcatTcgaggagaAcgtgctcagcacgtgatg
aacaccccgggaattaacc
5’gtggccctaaagagggccgttgggttcggttatttgagatca
agcttgtacaaaatatcTacaaagaTatttaccacacctggctg
ggcagg
5’ctaagcgagttaccattatgccaaaggacatccaattggcaa
Gacgtatccgaggagagcgtgctcagcacgtgatgaacaccc
Cgggaattaacc
5’gtggccctaaagagggccgttgggttcggttagattttgaga
tcaagctAgTtcagtaTcattAgaagAcatTTAccacac
ctggctgggcaggg
5’cacgccaagcgagtcaccatcatgccaaaggacatccaatt
Ggccagacgtattcggggagagcgcgctcagcacgtgatgaa
Caccccgggaattaac
5’gccctaaagagggccgttgggttcggtgggggtttgaatcga
aacggtctcaaactctgaaaatTaAaGatttaccacacctggct
gggcagg
5’ggcagccgttagtttcacttttctcacagtcccccaTA
gattatcaaTctaaagAcaGCTACGataTcttatA
gtagttaagtaagcttcaactcaaaactcaccgaacccaa
cggccctc
5’ggcagccgttagtttcacttttctcacagtcccccaTA
gattatcaaTctaaagAcaCCGGACTGTTAGT
TGTTCAAAGGataTcttatAgtagttaagtaagct
tcaactcaaaactcaccgaacccaacggccctc
his-6 (kog10) V
GGTGGGGGTTTGAATCGAAACGG 5’gtcggactcttcgaggacaccaacttgtgcgcaatcGAC
gccaagcgagtcaccatcatgccaaaggacatccaattgg
ccagacgtatccgaggagaacgtgcttaaatttctgatttcT
agaAtttGATATCgtttcAattcaaacTcTcaccgaa
cccaacggccctc
*CRISPR/Cas9 Dendra2 knock-in reagents were designed to specifically edit a histone H3 gene
within the HIS4 histone cluster on chromosome V (see Figure S1A), which contains three
histone H3 genes his-17, his-27, and his-49. The genotyping results indicated that one or more of
these three H3 genes contains the Dendra2 sequences, but their genomic DNA sequences are too
similar to distinguish which one of the three contains the Dendra2 sequence. Therefore, we have
used this strain as an H3 reporter representing the entire HIS4 cluster activity.
** for fluorescent protein tagging, Dendra2, mCherry, and eGFP were inserted into the
endogenous locus to generate either H3 or H3.3 fusion proteins using a dpy-10 co-CRISPR
strategy (30). Custom crRNA sequences were designed to target the sgRNA target sequence
listed. High-fidelity PCR was used to generate a linear repair template with 35 bp homology
arms. Sanger sequencing was used to confirm the accuracy of the knock-in allele.
Supplemental Movie 1. H3.3 (HIS-72::mCherry) and H3 (HIS-55::Ollas) with H3K36me2.
3D immunofluorescence images of HIS-72::mCherry (red), HIS-55::Ollas (green), and
H3K36me2 (magenta) in pachytene nuclei were acquired using Airyscan microscopy. Movie
rotates 360 degrees while alternating red, green, and magenta channels to visualize enrichment of
H3.3, H3, and H3K36me2, respectively.
Supplemental Movie 2. H3 (HIS-55::Ollas) and H3.3 (HIS-72::mCherry) with
H3K27me2/3. 3D immunofluorescence images of HIS-72::mCherry (red), HIS-55::Ollas
(green), and H3K27me2/3 (magenta) in pachytene nuclei were acquired using Airyscan
microscopy. Movie rotates 360 degrees while alternating red, green, and magenta channels to
visualize enrichment of H3.3, H3, and H3K27me2/3, respectively.
Supplemental Movie 3. Time-lapse movie of an embryo expressing H3.3 (HIS-
72::mCherry) and a Class I H3 (HIS-55::GFP). Live cell imaging of H3.3 (HIS-72::mCherry)
and Class I H3 (HIS-55::GFP) during early embryogenesis. The dashed circles outline the P-
lineage through multiple cell divisions. The video begins one frame prior to the first embryonic
cell division and captures early embryogenesis until the P-lineage divides to generate the Z2/Z3
primordial germ cells. The video was acquired at 5-minute intervals. Snapshots are shown in
Figure 2C.
|
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10.3390_molecules25040938.pdf
| null | null |
Article
Large-Scale Virtual Screening Against the MET
Kinase Domain Identifies a New Putative
Inhibitor Type
Emmanuel Bresso 1,†
Flavio Maina 2
, Rosanna Dono 2 and Bernard Maigret 1,*
, Alessandro Furlan 2,3,†
, Philippe Noel 1, Vincent Leroux 1
,
1 Université de Lorraine, CNRS, Inria, LORIA, F-54000 Nancy, France; [email protected] (E.B.);
[email protected] (P.N.); [email protected] (V.L.)
2 Aix Marseille Univ, CNRS, Developmental Biology Institute of Marseille (IBDM), UMR7288,
Parc Scientifique de Luminy, 13009 Marseille, France; [email protected] (A.F.);
fl[email protected] (F.M.); [email protected] (R.D.)
3 University of Lille, CNRS, UMR 8523, PhLAM-Physique des Lasers Atomes et Molécules,
F-59000 Lille, France
* Correspondence: [email protected]
† These authors contributed equally to this work.
Received: 27 January 2020; Accepted: 14 February 2020; Published: 19 February 2020
Abstract: By using an ensemble-docking strategy, we undertook a large-scale virtual screening
campaign in order to identify new putative hits against the MET kinase target. Following a large
molecular dynamics sampling of its conformational space, a set of 45 conformers of the kinase
was retained as docking targets to take into account the flexibility of the binding site moieties.
Our screening funnel started from about 80,000 chemical compounds to be tested in silico for their
potential affinities towards the kinase binding site. The top 100 molecules selected—thanks to the
molecular docking results—were further analyzed for their interactions, and 25 of the most promising
ligands were tested for their ability to inhibit MET activity in cells. F0514-4011 compound was the
most efficient and impaired this scattering response to HGF (Hepatocyte Growth Factor) with an
IC50 of 7.2 µM. Interestingly, careful docking analysis of this molecule with MET suggests a possible
conformation halfway between classical type-I and type-II MET inhibitors, with an additional region
of interaction. This compound could therefore be an innovative seed to be repositioned from its initial
antiviral purpose towards the field of MET inhibitors. Altogether, these results validate our ensemble
docking strategy as a cost-effective functional method for drug development.
Keywords: virtual screening; ensemble-docking; structure-based drug design; cross-docking
validation; induced fit; conformational selection; MET kinase
1. Introduction
MET receptor is a multifunctional receptor tyrosine kinase that plays a pivotal role in human
development and tumorigenesis. Upon binding of its ligand HGF (Hepatocyte Growth Factor), MET
triggers several intracellular signaling cascades, including MAPK and PI3K pathways, leading to
various cellular responses, such as survival, proliferation, and migration, among others. MET activation
can be driven in cancer by several mechanisms: HGF or MET overexpression, and also mutations [1].
Aberrant activation of MET signaling does not only affect cancer development and progression,
but it also contributes to resistance against other cancer drugs [2–11]. Consequently, MET represents a
pharmaceutically relevant target in anticancer drug design and has been the focus of several anticancer
strategies [12–18]. Pioneer MET inhibitors such as SU11274, PHA665752, or AM7 were helpful for
Molecules 2020, 25, 938; doi:10.3390/molecules25040938
www.mdpi.com/journal/molecules
molecules(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)Molecules 2020, 25, 938
2 of 19
demonstrating the efficacy of MET inhibition to impair tumor growth in preclinical models. Then,
further developments in the field led to the approval by the FDA of crizotinib and cabozantinib in the
2010s for treating non-small cell lung cancers and medullary thyroid cancers, respectively.
Even though promising results have been reported, the therapeutic activity of these drugs is
still relative and efforts are required to identify new MET inhibitors with physicochemical properties
optimized for clinical efficiency [19,20]. Moreover, new alterations in MET sequence have been recently
identified, such as MET exon 14 skipping in lung cancers and the emergence of MET mutations in the
kinase domain following treatment with MET inhibitors [21]. Novel inhibitor structures may possibly
target these mutations with increased efficiency.
Designing new putative hits against MET therefore remains a valuable challenge to be tackled.
In the present work, we carried out a virtual screening campaign in order to identify innovative
compounds able to become new hits for further lead development. As MET plasticity upon ligand
binding had been previously highlighted [22,23], we took into account this aspect for the molecular
docking simulations. Indeed, MET can accommodate several distinct ligand binding modes and
associated receptor conformations, a feature that is particular to the kinase family [24]. We reasoned
that it should be taken into account for designing drugs with improved efficiency and selectivity
profiles [25,26]. To be efficient, the molecular docking engines embedded within the virtual screening
approaches must be adapted to handle such flexibility [27,28]. In the present work, we used the
ensemble-docking strategy—previously recognized for its efficiency in drug design [29]—and show
the benefit of an investigation using a large ensemble-docking on MET.
In previous medicinal chemistry works, we already identified novel MET inhibitors [30,31] and
characterized the different MET ligand binding modes as shown by the stream of released X-ray
data [32]. Here, we provide the results of a large-scale ensemble-docking investigation on MET, in
which MET conformations are extended from available X-ray data to molecular dynamics and normal
mode analysis. A limited number of candidate compounds were selected from the ensemble-docking
results and one of them was subsequently validated experimentally as a possible new MET inhibitor,
providing a valuable seed for further investigations.
2. Materials and Methods
2.1. Screened Chemical Libraries
The choice of an appropriated set of compounds to explore the virtual screening space is critical for
assuming a good rate of success [33]. Today, millions of compounds can be selected for high-throughput
screening, and a suitable selection strategy must be designed. In our case and according to previous
success stories [34–36], we chose to use a set of libraries selected in order:
1.
2.
3.
4.
To use the highest possible chemical diversity, in order to cover a large spectrum of chemical
structures [37–40];
To include kinase-targeted compounds, as such a choice is already proven to be successful [41,42];
To explore natural products, which are a promising source of anticancer drugs [43–45];
To take into account the repositioning of approved compounds, as drug repurposing presents
an increasing interest in cancer research, by removing many costs associated with several steps
of drug development [46–49]. Several proofs of concept are now available and a typical case of
viral-to-cancer drug repositioning is gemcitabine with US patent No 4,808,614, aimed at treating
viral infections, and the later-issued US No 5,464,826, which claims of its use to treat cancer.
Therefore, we also considered chemical libraries dedicated to antiviral compounds.
According to the criteria listed above concerning the choice of the chemical libraries, we
firstly downloaded around 200,000 compounds from the chemical providers listed in Table 1.
After eliminating redundancies in compounds and in scaffolds to assume a large chemical diversity
and in respect of general druglike definitions [50], we finally retained about 80,000 compounds for our
screening campaign.
Molecules 2020, 25, 938
3 of 19
Table 1. List of the selected chemical libraries used in the present virtual screening campaign, providing
a total of 76,251 compounds.
Supplier
Library Names
French laboratories chimiotheque-nationale.enscm.fr
Collaboration medchem.u-strasbg.fr
ChemBridge www.chembridge.com
Life Chemicals www.lifechemicals.com
TimTec www.timtec.net
Otava otavachemicals.com
TargetMol www.targetmol.com
Selected subsets (kinase, essential, etc.)
laboratory collection
kinase library diversity library
kinase general library antiviral library
15K diversity library
NDL + NPL natural derivatives library
all kinase library
10K diversity library
natural productlike library
antivirus library natural compounds library
2.2. Selected MET Conformational Ensemble
In ensemble-based docking calculations, a well-suited choice of the protein target conformational
sample is required to reproduce the protein plasticity and the possible induced-fit phenomena [51,52].
Concerning MET conformational flexibility, our previous exploration by molecular dynamics and
normal mode simulations [22] was limited to the 26 PDB structures available at that time (Table 2).
We have now extended this analysis by considering all the X-ray structures available for the wild-type
MET in the PDB [53] deposited after 2012. From the additional structures found, only 19 were
considered in this work (Table 3) as we discarded those where three regions were missing in the
X-ray structure and those where the number of missing residues in a single region was larger than 20.
This selection aimed to improve our protein ensemble sample by covering most of the kinase structural
characteristics, such as the position of the c-helix (in or out) or of the aspartate-phenylalanine-glycine
(DFG) motif (in or out) as defined in Kinametrix [54], thus covering most of the inhibitor type binding
modes. 3D structures considered in this ensemble of 45 conformers looked representative of the
diversity of MET structures, as shown from the dendrogram, the heat maps, and correspondence
maps in Figure 1. These results obtained thanks to the Dali server [55] clearly illustrate how MET 3D
structures used here are organized into several families covering most of the protein conformational
space presently known.
Molecules 2020, 25, 938
4 of 19
Table 2. List of the 26 PDB MET kinase domain structures selected in the previous work [22] and
reused in the present one. The kinase conformation associated to each structure is annotated according
to the KinaMetrix web resource [56]: DO means DFG-out, DI means DFG-in, CO means α-cHelix-out,
CI means α-cHelix-in, and ωCD indicates DFG intermediate.
No
PDB ID Ligand PDB ID Deposition Date
Annotation
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
1R0P
2RFN
2RFS
3C1X
3CCN
3CD8
3CE3
3CTH
3CTJ
3F66
3F82
3EFJ
3EFK
2WD1
3DKC
3DKF
2WGJ
3A4P
3L8V
3I5N
3LQ8
2WKM
3Q6W
3QTI
3RHK
3ZXZ
KSA
AM7
AM8
CKK
LKG
L5G
1FN
319
320
IHX
353
MT3
MT4
ZZY
ATP
SX8
VGH
DFQ
L8V
B2D
88Z
PFY
Q6W
3QT
M97
KRW
2003
2007
2007
2008
2008
2008
2008
2008
2008
2008
2008
2008
2008
2009
2009
2009
2009
2010
2010
2010
2010
2010
2011
2011
2011
2011
Inactive CODI
?
Inactive CODI
Inactive CIDO
Inactive CODI
Inactive CODI
Inactive CODO
Inactive CIDO
Inactive CIDO
Inactive CODI
Inactive CIDO
Inactive CODO
Inactive CODI
Inactive CODI
Inactive CODI
Inactive CODI
Active CIDI
Inactive CODI
Inactive CIDO
Inactive CODI
Inactive CODO
Inactive CODI
Active CIDI
Inactive CODI
Inactive ωCD
Inactive CODI
Table 3. List of the PDB MET kinase domain structures added to the ones coming from our previous
work [22]. The kinase conformation associated to each structure is annotated according to the
KinaMetrix web resource [56]: DO means DFG-out, DI means DFG-in, CO means α-cHelix-out, CI
means α-cHelix-in, and ωCD indicates DFG intermediate.
PDB ID Ligand ID Ligand IC50 (nM) Date Missing Sequence
# Missing Residues
Annotation
4DEI
4GG5
4EEV
3VW8
4IWD
3ZCL
3ZC5
3ZBX
4KNB
4MXC
4XYF
4R1V
4XMO
5DG5
5EYD
5EOB
5EYC
5UAF
5HTI
0JL
0J3
L1X
DF6
1JC
5TF
W9Z
6XE
1RU
DWF
44X
3E8
46G
5B4
5T1
5QQ
5SZ
84P
66L
0.6–2
0.9
4.7/42
2
1
19
6
5
47/410
6.7
1/5
400
2
?
1
0.24
3
?
?
2012
2012
2012
2013
2013
2013
2013
2013
2013
2014
2015
2015
2015
2015
2016
2016
2016
2017
2017
1100–1103 1115–1117
1146–1151
1225–1243
1237–1242 1286–1290
1234–1235 1240–1243
1100–1102
1099–1102
1089–1102
1099–1103 1113–1115
1238–1242
1099–1103
1150–1151
1098–1103
-
1098–1103 1151–1152
1238–1240
1099–1103
1098–1105 1145–1152
1238–1242
7
6
19
11
6
3
4
14
8
5
5
2
6
-
8
3
5
16
5
Inactive CODI
Inactive CODI
Inactive CIDO
?
Active CIDI
Inactive CODI
Inactive CODI
Inactive CODI
Inactive CODI
Inactive CODO
Inactive CODI
Inactive CODI
Inactive CODO
Inactive CODI
Inactive CODI
Inactive CODI
Active CIDI
Inactive CODO
No
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
To be in accordance with the 26 conformers coming from our previous work [22], the 19 added
crystal structures were prepared and cleaned following the same protocol: missing residues, side
Molecules 2020, 25, 938
5 of 19
chains, and hydrogens were added when necessary; unnecessary water molecules, ions, and additives
were removed; basic and acidic side chains were ionized according to a pH set to 7. To consider possible
binding sites fluctuations, short molecular dynamics (MD) simulations were undertaken for each of
these 19 structures. For that purpose, these structures were placed in a solvent box of 80 Å and counter
ions were added for electrostatic neutrality. NAMD [57] molecular dynamics program was used with
the same CHARMM36 force field and same protocol as previously described [22]. After minimization
and equilibrium steps (64,000 conjugate gradients and 1 ns MD, respectively), 10 ns of MD were
recorded, with a frame length of 1 ps. These 19 MD trajectories were analyzed, and the most stable
representative conformer was retained for each of them and added in the ensemble-docking set.
Figure 1. (a) Dendrogram showing the relationships between the 45 PDB conformers listed in Tables 2
and 3 and used to sample MET structure plasticity. (b) Similarity heat-map showing the relationships
between the 45 PDB conformers and used to sample MET structure plasticity. The color scale
corresponds to the Dali Z-score values. (c) Correspondence analysis of the 45 ensemble PDB-related
conformers. This plot positions data points with the most similar structural neighborhoods near each
other according to a multidimensional scaling method.
2.3. Description of the Ensemble-Docking Protocol
The ensemble-docking facility proposed in the GOLD docking program was used [58]. This GOLD
feature evaluates different receptor conformations concurrently during the docking exploration.
The protein ensemble used in this work thus contained 45 MET conformers (26 from our previous
work and 19 added in this one). As these conformers must be superimposed before being used in
GOLD ensemble-docking program, they were structurally aligned according to their conserved and
most rigid secondary structure patterns, as previously described [22] and summarized in Table 4.
Molecules 2020, 25, 938
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Table 4. List of the secondary structure elements used for aligning all the conformers.
Domain
Secondary Structure Name Residues
N-terminal
C-terminal
β1
β2
β2
β2
β2
αE
αF
αH
αI
1076 to 1081
1092 to 1098
1104 to 1110
1144 to 1146
1154 to 1158
1178 to 1198
1263 to 1278
1310 to 1320
1330 to 1343
When docking an ensemble of conformations for a given protein, their binding sites must be
defined using a method that is not conformer specific. In the present ensemble-GOLD version, as
it was not possible to define the active site by a list of atoms or residues, the only way was to use
the centroid of the binding cavity and a sphere radius around this point. Therefore, for each of the
45 aligned protein conformers used here, protein cavities and their center of mass were detected by
the LIGSITE program [59]. From these data, we obtained an average center point as the ensemble
binding site definition for GOLD. Figure 2 presents the position of this average center point within
the 45 protein conformers. A radius of 20 Å was associated to this average point to define the binding
cavity of each conformer in order to correctly encompass the receptor for all the conformations in the
ensemble, including conformational variations around the center. We also verified that the resulting
sphere was encompassing all groups of residues previously identified as potential interaction areas for
MET ligands [32].
For each docking run, we used 50 starting poses/molecule for the GOLD generic algorithm.
Tested compounds were ranked by the standard goldscore scoring function.
Figure 2. Position of the average center-point (as a green sphere) found from the 45 used conformers
and used for the ensemble-dockings.
2.4. Computer Grid Facilities
Due to the massive calculations needed ( 80,000 molecules × 48 protein ensemble conformers
× 50 poses/molecule), and considering the computing time to achieve only one run, we used the
Grid5000 facility [60] providing the required computer power in order to distribute the calculations
using the PVM framework embedded in GOLD. A total of 1300 cpus (mostly Xeons) with 4 GB
RAM/core and infiniband connections were used for each run. The docking performances run around
300-docked ligands/ensemble/hour. The calculations were spread on the clusters using the same
strategy as previously described [61].
Molecules 2020, 25, 938
2.5. Scattering Assays
7 of 19
The experimental protocols for measuring the potency of MET inhibitors are detailed in previous
publications [30,62]. MDCK cells were preincubated with compounds overnight at 0.1–100 µM
concentrations at 37 ◦C in a humidified atmosphere of 5% CO2, followed by a 24 h stimulation
with 20 ng/mL HGF (R&D Systems). Cells were further incubated at 37 ◦C in an atmosphere of
5% CO2 for 24–48 h, washed with phosphate buffered saline (PBS; Gibco BRL), and fixed with 4% PFA
(paraformaldehyde, Sigma). The quantification of scattering response was performed by counting the
number of cells with scattered morphology in 30 independent colonies. The IC50 corresponds to the
concentration of compounds leading to a 50% inhibition of MET-triggered cell scattering.
3. Results
3.1. Preliminary Validation Concerning the GOLD Ensemble-Docking Protocol Used
The coordinates of the 45 aligned conformers and of the sphere representing their common
binding sites constituted our ensemble-docking protein reference.
The first question here concerned the accuracy of this binding site definition compared to ones
that are more classical. For that, we compared the docking results for some of the selected 45 MET
conformers using three binding site definitions; namely, a residue list, an existing ligand, and the
center point of the binding cavity, respectively. For each individual docking target, the three definitions
provided almost the same rank and docking score for the associated PDB ligand (Table 5). Moreover,
the poses of this ligand found using the three binding site definitions were similar to the pose found in
the crystal structures, as illustrated with the example of the AM7 ligand on Figure 3.
Table 5. Comparison of the docking results using the 3 binding site definitions.
Definition of the Binding Site Target PDB Name
Ligand PDB Name Rank Number
Score Value
Center + radius 20 Å
Residues list
From its PDB ligand
3DKC
2RFN
3DKC
2RFN
3DKC
2RFN
ATP
AM7
ATP
AM7
ATP
AM7
1
1
1
1
1
1
105.5
100.8
102.8
98.0
107.1
106.6
Figure 3. Poses of the AM7 ligand in the X-ray 2RFN structure compared to the docking results.
In black, the original pose of the ligand in its PDB protein conformation; in colors, the best docking
poses obtained by GOLD on the 2RFN target using a definition of the binding site from a list of residues
(orange), from the original ligand (green), and from a center-point (purple).
The second question was related to the ability of the ensemble-docking process to retrieve a given
PDB ligand to its PDB structure among the 45 ones. To evaluate that point, an ensemble-docking
calculation was carried out on the 45 protein target conformers using a short chemical library built
Molecules 2020, 25, 938
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from their own 45 associated ligands (the list is given in Tables 2 and 3). We checked whether we
could associate the right PDB target for a given PDB ligand (with possibly similar rank, score and pose
compared to the ones found for the individual target dockings) in the protein ensemble. This was
achieved for almost 80% of the compounds (Table S1). For example, the KSA ligand was able to
preferentially retrieve its original 1R0P partner among the ensemble of the 45 PDB conformers of the
protein target.
From these results, it appeared that the ensemble-docking procedure we used was a satisfactory
method to tackle multiple conformers docking and to achieve a valuable virtual screening.
3.2. Selection of Candidate Hits from the Virtual Screening Campaign
Once the screening campaign was achieved for the 80,000 compounds filtered from the chosen
libraries, we kept the top-100 ranked compounds according to their GOLD scores (ranging from 100 to
114) for further analysis.
We started the docking analysis with the Life Chemicals compound F0725-0356 giving the best
docking score of 114. A comparison between the X-ray complex 3EFK/MT4 structure and the
MD_3EFK/F025-0356 one presented quite similar poses and protein/ligand interactions. Indeed,
the most important residues known in MET interactions (namely, Met1160, Asp1222, Tyr1159) were found
in both complexes.
We next analyzed the protein-ligand interactions for the other top-100 compounds in order to
compare them to the ones found in the 45 original PDB structures (Tables 2 and 3). For that, we
used the PLIP program [63] by focusing on two important interaction types: hydrogen bonds and
π-stacking. Protein residues Met1160 (45/45), Asp1222 (34/45), and Lys1110 (6/45) concentrated the vast
majority of hydrogen bonds with ligands; while Tyr1230 (25/45) and Phe1223 (7/45) dealt with most of
the π-stacking. In order to limit our biological tests on possible promising compounds, we eliminated
from the top-100 list the molecules not presenting at least one hydrogen bond and one π-stacking from
the ones described above in the PDB complexes.
After this filter, we retained only 41 compounds as satisfying these criteria. As most of these
compounds came from the Life Chemical antiviral library and given the simplicity of comparing
molecules from the same supplier, we decided to only test compounds from Life Chemicals. As some
of these molecules were not available in stock from this provider, only the 25 compounds listed in
Table 6 were finally kept to proceed further.
Molecules 2020, 25, 938
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Table 6. Ligands selected from the Life Chemical (LC) antiviral library and experimentally tested.
”-“: the compound (assessed at a concentration up to 100µM) did not affect MDCK cell scattering in
response to HGF/SF. ”+“: the compound impaired MDCK cell scattering in response to HGF/SF with
an IC50 > 10 µM. ”+++“: the compound impaired MDCK cell scattering in response to HGF/SF with
an IC50 < 10 µM.
Mol ID Life Chemicals Name GOLD Score Best Protein Conformer Biological Activity
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
F0725-0356
F0772-0607
F0816-0342
F0737-0405
F0737-0393
F0301-0263
F0721-0868
F0715-0299
F0539-1482
F0385-0029
F0385-0334
F0514-4011
F0174-0048
F1620-0074
F0011-0324
F0721-0906
F0012-0227
F0721-0911
F0715-0300
F2252-0240
F0772-2099
F0473-0261
F0721-0900
F0772-2147
F0526-0094
120.7
111.8
111.2
110.6
110.1
105.5
105.1
105.0
104.0
103.8
103.4
103.3
102.4
102.1
102.0
102.0
101.9
101.9
101.8
101.1
100.9
100.9
100.5
100.5
100.3
MD_3EFK
MD_3EFK
MD_3F82
MD_3EFJ
MD_3EFK
MD_3EFK
MD_3EFK
MD_3EFK
MD_3EFJ
MD_3EFK
MD_3EFJ
MD_3EFJ
MD_3EFK
MD_3EFJ
MD_3EFK
MD_3EFJ
MD_3EFJ
MD_3EFJ
MD_2RFN
MD_3EFK
MD_3EFJ
MD_3CE3
MD_3EFK
MD_3EFJ
MD_3EFJ
-
-
-
-
-
-
-
-
+
-
-
+++
-
-
-
-
-
-
-
-
-
-
-
-
-
3.3. F0539-1482 and F0514-4011 Inhibit MET-Induced Cell Scattering
These 25 compounds were then experimentally tested for their ability to restrain MET-triggered
biological activities. We previously efficiently screened compounds for their inhibitory properties
towards MET-triggered biological responses by using cell scattering assays [31,64]. In particular,
MDCK epithelial cells acquire a “scattered phenotype” after stimulation with MET ligand HGF.
Among the 25 tested compounds, two compounds were found active, namely, F0539-1482 and
F0514-4011. F0514-4011 was the most efficient and impaired this scattering response to HGF with
an IC50 of 7.2 µM (Figure 4). No toxic effects were observed at biologically active concentrations.
This study thus demonstrates that our strategy actually allows the identification of compounds able to
inhibit MET-driven biological activities.
Molecules 2020, 25, 938
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Figure 4. (a) F0514-4011 impairs cell scattering in response to MET ligand HGF: MDCK epithelial cells
were treated with 20 ng/mL HGF, with or without preincubation with F0514-4011 for 2 h. F0514-4011
IC50 is 7.2 µM. (b) Dose-response curve for F0514-4011.
3.4. Compared Docking of F0514-4011 Compound Versus Known Inhibitors
In order to understand why the compound F0514-4011 (Figure 5) was the most potent compound
among the 25 experimentally tested ones while not presenting the highest GOLD score, we compared
its docking data with those of potent existing inhibitors. For that, we collected the structures of
ligands found in the PDB related to MET kinase domain in complex with already marketed inhibitors
with binding IC50 found in the nM range (Table 7). All these compounds were submitted to the
ensemble-docking GOLD protocol already used for our virtual screening campaign. From these
calculations, it appeared that the best docking scores ranged from 111 for merestinib (L1X ID in PDB
4EEV) to 83 for AMG337 (5T1 ID in PDB 5EYD), so that the score of 103 obtained for our active
F0514-4011 compound was in this range of active compounds. Considering now IC50, one possibility
to explain the higher IC50 of 7.2 µM obtained for F0514-4011 (compared to 0.4–14 µM range found
for compounds listed in Table 7) could be its weaker solubility (cLogP of 5.7, greater than that of all
compounds listed in Table 7).
Table 7. Data used for some known marketed inhibitors with nano-molar range IC50 found in the PDB.
PDB ID Ligand ID
Name
IC50
Solubility cLogP Docking Score
2RFS
2WGJ
2WKM
3DKF
3RHK
3LQ8
3Q6W
3QTI
3ZXZ
4EEV
5EYD
AM8
VGH
PFY
SX8
M97
88Z
Q6W
3QT
KRW
L1X
5T1
SU11274
Criotinib
PHA-665752
SGX-523
Tivantinib
Fortinib
MK-2461
NVP-BVU972
PF-04217903
Merestinib
AMG337
10 nM
11 nM
9 nM
4 nM
4 nM
0.4 nM
0.4 nM
14 nM
5 nM
5 nM
1 nM
2.9
3.5
4.4
1.4
3.1
4.3
3.3
1.6
0.2
3.4
0.3
86
82
88
84
83
99
93
84
87
111
83
Molecules 2020, 25, 938
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Figure 5. F0514-4011: N-[[4-(4-Ethoxyphenyl)-5-[2-[3-(4-methylphenyl)-5-thiophen-2-yl-3,4-dihydropyrazol
-2-yl]-2-oxoethyl]sulfanyl-1,2,4-triazol-3-yl]methyl]-2-phenylacetamide.
Another point concerned the interaction of F0514-4011 with amino acid residues within the
protein-binding region. In Table 8, we have listed the protein residues/ligand interactions found from
Table 7 PDB complexes, as calculated by the PLIP program. These interactions were compared to the
ones obtained for F0514-4011 from its best pose MD_3EFJ in the ensemble MET conformations. From
this comparison, it appears that several of the most important amino acid residues found from the
PDB protein/ligand analysis were also found for F0514-4011, at the exception of Met1160, common
to all PDB structures of Table 8, replaced possibly by Met1131 and Met1229 in our case. This situation
is mostly due to the conformation of the large DFG loop acting as a highly flexible lid protecting the
binding sites which was quite different in the MD_3EFJ conformation, found as the most suitable
one to bind F0514-4011 when compared to the PDB ones (see Figure 6 for an example with the 5DG5
and 4DEI structures). Therefore, our docking results concerning the best pose proposed by GOLD for
F0514-4011 appear quite in agreement with most of data obtained from all the PDB concerning MET
kinase domain complexed with inhibitors.
Molecules 2020, 25, 938
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Table 8. List of the protein residues interacting with a nM. inhibitor from the PDB complexes of
Table 7 ranked by their number of occurrence. In bold, the residues also found in the interactions with
F0514-4011 with the MD-3EFJ MET conformation. According to the PLIP results, a residue was marked
”+“ when at least one protein-ligand interaction was found, whatever its quality (hydrophobic, H-bond,
π-stacking, ionic, etc.) and marked by ”-“ when no protein-ligand interaction was found.
Residue
4EEV 2WGJ
5EYD 3ZXZ 2RFS
PDB IDs
3RHK 3QTI
3Q6W 2WKM 3DKF
3LQ8
MET1160
LEU1157
ASP1222
ALA1108
TYR1230
VAL1092
ILE1084
TYR1159
PRO1158
LEU1140
ALA1221
PHE1223
ASP1164
LYS1110
ASN1209
GLU1127
PHE1134
VAL1139
PHE1200
ARG1086
ARG1208
THR1343
GLU1347
PHE1089
ASP1231
ARG1166
ASN1167
ILE1130
ASN1171
+
+
+
+
-
-
+
-
-
-
-
+
-
+
-
-
+
+
+
-
-
-
-
-
-
-
-
+
-
+
+
-
-
+
+
+
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
-
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
-
-
+
+
+
-
+
-
-
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
-
-
-
+
+
-
+
-
-
+
-
+
-
-
-
-
-
-
-
-
-
+
-
-
-
-
-
+
+
+
+
+
+
+
-
-
-
+
-
-
-
-
-
-
-
-
-
-
+
+
-
-
-
-
-
-
+
+
-
+
-
+
+
+
-
+
+
-
-
-
-
-
-
-
-
+
+
-
-
-
-
-
-
-
-
+
+
-
-
+
-
-
+
+
+
-
-
+
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
-
-
-
+
-
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
-
-
+
+
-
+
-
+
-
-
-
+
+
+
+
-
-
-
-
-
-
-
-
-
-
Figure 6. Differences for the lid DFG loop between selected PDB structures and our MD-refined
MD_3EFJ conformations. The proteins are depicted from their Cα ribbon-like traces.
To further characterize the F0514-4011 inhibitor type, we have considered the general 3D shape of
known kinase inhibitors as analyzed in several papers [7,65–67]. Concerning MET, such compounds
Molecules 2020, 25, 938
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are generally classified as type-I or -II. Type-I ligands essentially bind at the ATP binding site and
present a U-shaped conformation, with the protein in the DFG-in structure; while type II are found in
an extended shape and correspond to the DFG-out protein form. We illustrate this in Figure 7, showing
the conformations of two typical ligands, namely, type-I AMG337 (from PDB 5EYD) and the type-II
altiratinin analog DP-4157 (from PDB 5DG5). From this picture, it appears that F0514-4011 presents
both the U and linear shapes while also showing another region of interaction, including three of the
protein residues already found in MET complexes—namely, Asp1222, Tyr1230, and Arg1208 (found only
2 times for 3C1X and 3YW8 among our 45 ensemble conformations). Asp1204 and Asn1209 residues,
still not involved in MET complex PDB structures, complement this supplementary binding pocket.
The thiophene moiety of F0514-4001 was placed central within this pocket by the thiophene-pyrazole
group which also oriented the associated toluene ring to close the U-shape part. Therefore, one could
postulate that F0514-4011 molecule describes a possibly novel type of inhibitor.
Figure 7. Comparison of the conformations between F0514-4011, the U-shape inhibitor 5T1 (AMG337),
and the linear-shape 5B4 (altiratinib), as observed in their respective binding sites.
Nevertheless, considering the limitations of any docking program, the stability of F0514-4011’s
best docking pose could be questioned. In order to validate it, we have performed a molecular
dynamics simulation using the same conditions as those used for the PDB complexes (cubic water
3
). The results show that the GOLD docking pose is very stable and still conserved after
box of 80 Å
10 ns of MD (Figure 8). The protein/ligand interactions found for F0514-4011 after the MD simulation
were similar to those discussed above, thus giving confidence to the robustness of the docking results.
Our final question concerns the originality of F0514-4011 compared to the known MET ligands.
The Tanimoto similarity index calculated between F0514-4011 and most of the published MET ligands
shows that the molecule identified by our virtual screening campaign seems to be an innovative hit as
all the Tanimoto values are low, ranging from 0.39 (with the pioneer inhibitor PHA-665752) to 0.12
(for norcantharidin) (Table S2). We have completed this quite elementary similarity search by using
the ChemDes web server [68], which allows a large panel of similarity fingerprint types as well as
fingerprints descriptors and similarity measures. Using this web server, we mined several databases
collecting MET known inhibitors (such as the PDB or PubChem [69]), already in clinical trials (such as
MDDR [70]), or described as putative inhibitors (such as in Life Chemical or sellekchem [71] providers).
The results obtained with this method confirmed the lack of similarity suggested with the Tanimoto
distance. With the Sokal similarity method and DTRF fingerprint types, the similarities ranged from
0.46 to 0.19 (in the PDB list, a maximum of 0.40 was obtained for compound ID 75H found in PDB ID
5T3Q (data not shown)).This could be due to the thiophene moiety of F0514-4011, as we have found
only two papers and one patent in the literature referring to thiophene-related MET inhibitors [72–74]
and only one reference to the role of thiophene-pyrazole moiety in kinase inhibition [75].
Molecules 2020, 25, 938
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Figure 8. (a) Comparison between the initial docking pose (in orange) of F0514-4011 and the final
one after the 10-ns MD simulation (in cyan). (b) Evolution of F0514-4011 root mean square deviation
(RMSD) during 11 ns (1 ns equilibrium and 10 ns production) of MD simulation. Poses were aligned
on the initial one and the curve was smoothed.
4. Discussion
Molecular docking, molecular dynamics, and virtual screening approaches can now be efficiently
used for the design of new inhibitors of the MET kinase domain [27,56,76–80]. From all these
approaches, new potent compounds were obtained and more highlights revealed about MET kinase
domain conformational behavior. In this vein, our study merges both simulations and experiments
and highlights a novel scaffold for MET inhibition.
Using an ensemble-docking approach associated to short molecular dynamics runs in order to
take into account the flexibility of the used X-ray structures in the protein conformational ensemble,
we were faced with the fundamental question of the relevance of this strategy for handling the difficult
problem of predicting ligand-binding modes on a flexible target. This is especially true for MET
kinase, the active site of which exhibits important structural variations, as observed in their available
crystal structures [81,82]. We believe that this work brings a positive answer to this question and can
constitute a working line for other simulations in the future. Ensemble-docking is now widely used,
and incorporating this approach to short molecular dynamics simulations looks promising. Still, a
couple of simple questions have to be answered prior to initiating the docking calculation: how do
we generate a relevant ensemble for a given receptor [51], and how can we be sure that the possible
energy differences obtained between conformations in the ensemble are properly accounted for?
Interestingly, F0514-4011 compound (also referenced in PubChem with ID 5237313) is not a
newcomer in drug design as it has been already tested as a possible activator of E3 ligase (FBW7)
and inhibitor of microphthalmia-associated transcription factor (MITF), but was found inactive in
both assays . Our study suggests that it could be repositioned for MET inhibition, as evidenced by its
biological activity against MET-driven cell scattering. Some drug properties such as solubility and
lack of toxicity were already known. With regard to its molecular weight of 650Da, which could be
Molecules 2020, 25, 938
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considered as limiting its possible therapeutic action, it should be noted that other inhibitors currently
on the market have similar characteristics such as tarloxatinib (679Da), foretinib (632Da), or golvatinib
(633Da). Therefore, it should not be a major hurdle if lead optimization provides us with a promising
drug in terms of activity and/or selectivity. This will be the topic of future investigations.
This virtual screening work presents F0414-4011 as a valuable compound that could be a seed for
developing new and innovative leads against MET kinase. Its novelty and originality might be used to
overcome the resistance problem found presently for several existing inhibitors.
Supplementary Materials: The following are available online. Table S1: Comparison of the ensemble-docking
results to the individual ones (a ligand against its own PDB-related structure), Table S2: most used c-Met inhibitors
as pointed by SelleckChem and AdooQ Biosciences.
Author Contributions: Conceptualization, A.F., V.L., and B.M.; software, E.B. and P.N.; validation, A.F., F.M.,
and R.D.; formal analysis, E.B., A.F., P.N., and B.M.; investigation, E.B, P.N., V.L., and B.M.; writing–original draft
preparation, E.B., A.F., P.N., V.L., and B.M.; writing–review and editing, E.B., A.F., P.N., and B.M.; supervision,
F.M., R.D., and B.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: Experiments presented in this paper were carried out using the Grid’5000 testbed, supported
by a scientific interest group hosted by Inria and including CNRS, RENATER, and several Universities as well as
other organizations (see https://www.grid5000.fr). We are much grateful to all members of our labs for helpful
discussion and advice.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this manuscript:
aspartate-phenylalanine-glycine
DFG
MD
Molecular dynamics
RMSD Root mean square deviation
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82. Mashayekh, K.; Sharifi, S.; Damghani, T.; Elyasi, M.; Avestan, M.S.; Pirhadi, S. Clustering and Sampling of the
c-Met Conformational Space: A Computational Drug Discovery Study. Comb. Chem. High Throughput Screen.
2020, 22, 635–648. [CrossRef]
c(cid:13) 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
| null |
10.1038_s41586-023-06271-6.pdf
|
Data availability
All calcium imaging and fly behaviour time-course datasets analysed
in the main figures are available on DANDI archive (calcium imag-
ing data, 000247; fly choice tracking data, 000212; fly behavioural
sequence tracking data, 000250). Technical documents (for example,
CAD files and plasmid maps) and source data for all scatter plots
and histograms are available on Figshare (https://doi.org/10.6084/
m9.figshare.c.6505732).
Code availability
Scripts for data processing and plotting are available on req
|
Data availability All calcium imaging and fly behaviour time-course datasets analysed in the main figures are available on DANDI archive (calcium imaging data, 000247; fly choice tracking data, 000212; fly behavioural sequence tracking data, 000250 ). Technical documents (for example, CAD files and plasmid maps) and source data for all scatter plots and histograms are available on Figshare ( https://doi.org/10.6084/ m9.figshare.c.6505732 ). Code availability Scripts for data processing and plotting are available on request.
|
A rise-to-threshold process for a relative-
value decision
https://doi.org/10.1038/s41586-023-06271-6
Received: 18 October 2021
Accepted: 26 May 2023
Published online: 5 July 2023
Open access
Check for updates
Vikram Vijayan1 ✉, Fei Wang2,4, Kaiyu Wang2,5, Arun Chakravorty1,6, Atsuko Adachi1,7,
Hessameddin Akhlaghpour1, Barry J. Dickson2,3 & Gaby Maimon1 ✉
Whereas progress has been made in the identification of neural signals related to
rapid, cued decisions1–3, less is known about how brains guide and terminate more
ethologically relevant decisions in which an animal’s own behaviour governs the
options experienced over minutes4–6. Drosophila search for many seconds to minutes
for egg-laying sites with high relative value7,8 and have neurons, called oviDNs, whose
activity fulfills necessity and sufficiency criteria for initiating the egg-deposition
motor programme9. Here we show that oviDNs express a calcium signal that (1) dips
when an egg is internally prepared (ovulated), (2) drifts up and down over seconds to
minutes—in a manner influenced by the relative value of substrates—as a fly determines
whether to lay an egg and (3) reaches a consistent peak level just before the abdomen
bend for egg deposition. This signal is apparent in the cell bodies of oviDNs in the
brain and it probably reflects a behaviourally relevant rise-to-threshold process in the
ventral nerve cord, where the synaptic terminals of oviDNs are located and where
their output can influence behaviour. We provide perturbational evidence that the
egg-deposition motor programme is initiated once this process hits a threshold and
that subthreshold variation in this process regulates the time spent considering
options and, ultimately, the choice taken. Finally, we identify a small recurrent circuit
that feeds into oviDNs and show that activity in each of its constituent cell types is
required for laying an egg. These results argue that a rise-to-threshold process
regulates a relative-value, self-paced decision and provide initial insight into the
underlying circuit mechanism for building this process.
Egg-laying site selection is critical for the survival of a fly’s progeny10.
As such, Drosophila search for a high-quality substrate for many sec-
onds to minutes before depositing each individual egg7,8. Egg-laying
preferences for many different substrates have been documented10,
but how decision-related neural signals evolve in real time to guide the
site selection process, and to generate these preferences, is unknown.
A behavioural sequence for egg laying
We took videos of gravid Drosophila in a small chamber with a soft
substrate floor and characterized a behavioural sequence for egg lay-
ing (see Supplementary Tables 1 and 2 for genotypes and conditions
in all experiments). The six-step sequence begins with the fly standing
still and performing an abdomen elongation (step 1) followed by a
scrunch (step 2) (Fig. 1a). The fly then increases its locomotor speed
during a search period (step 3), and finally it performs an abdomen
bend for egg deposition (step 4), deposits an egg (step 5) and per-
forms a second abdomen bend (step 6), probably for cleaning the
ovipositor.
This sequence is consistent with those described previously7,9,11–13
and, although abdominal movements before egg laying have been
noted11–13, it remains unclear whether any of these reflect ovulation14,
which is the passage of an egg from an ovary to the uterus. We fluores-
cently imaged, through the cuticle, eggs expressing GCaMP15 while
freely walking flies laid eggs (Extended Data Fig. 1a and Methods).
By visualization of GCaMP rather than green fluorescent protein (GFP),
we could determine not only when eggs moved inside the body but
also when each egg was activated to start embryonic development
(because activation is associated with a large [Ca2+] increase inside the
egg15). We observed that an egg descends from an ovary to the uterus
during abdominal elongation and that the same egg exhibits a strong
increase in GCaMP fluorescence during the subsequent scrunch (Fig. 1b
and Supplementary Video 1). These data demonstrate that elongation
(step 1) reflects ovulation and that scrunching (step 2) reflects activa-
tion. For brevity we will refer to steps 1 and 2, combined, as ovulation
in this paper.
We quantified the egg-laying behavioural sequence by annotat-
ing four of the six steps just mentioned: (1) ovulation start (when the
1Laboratory of Integrative Brain Function and Howard Hughes Medical Institute, The Rockefeller University, New York, NY, USA. 2Janelia Research Campus, Howard Hughes Medical Institute,
Ashburn, VA, USA. 3Queensland Brain Institute, University of Queensland, St Lucia, Queensland, Australia. 4Present address: Institute of Neuroscience, State Key Laboratory of Neuroscience,
Center for Excellence in Brain Science and Intelligence Technology, Chinese Academy of Sciences, Shanghai, China. 5Present address: Lingang Laboratory, Shanghai Center for Brain
Science and Brain-Inspired Intelligence Technology, Shanghai, China. 6Present address: Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, CA, USA.
7Present address: Mortimer B. Zuckerman Mind Brain Behavior Institute, Columbia University, New York, NY, USA. ✉e-mail: [email protected]; [email protected]
Nature | Vol 619 | 20 July 2023 | 563
Article
a
b
d
NP
control
Egg >
GCaMP3
2-p
Ephys
Cameras
Eggs
Pivot
Optogenetics
Agarose
substrate-
laden wheel
43 traces, 8 flies
h
F
/
F
Δ
0.4
0.2
0
–0.2
s
t
n
e
v
E
5
Abdomen
neutral
Abdomen
elongated
Abdomen
scrunched
Search
Abdomen
bent to
lay egg
Egg
deposited
Ovipositor
cleaned
t = –1.6 s
0 s
23.5 s
62.8 s
77.1 s
82.6 s
87.2 s
c
Wild type
Ovulation start
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
Ovulation
Egg emerges
~1 mm
Search start
t = –13.0 s
0 s
27.9 s
38.0 s
64.3 s
69.3 s
72.0 s
Egg descends
Activated
Inset
771/1,200
291/200
1 mm
Abdomen bend
complete
5 events
63 eggs, 3 flies
–400
–300
–200
–100
0
Time (s)
Egg deposited
e
f
g
2
1
oviDN-SS1
5 μm
oviDN-SS1
> GCaMP7f
(unless noted)
5 μm
l
y
F
n
o
i
t
i
s
o
p
F
/
F
Δ
0º
360º
0.4
0
Time-ave.
a
b
2,016
Raw F
0
One fly
>30 min
Egg no. 2
0 mM sucrose
500 mM sucrose
Egg no. 8
0º
360º
0.4
0
20 s
Ovulation
start
Search
start
Abdomen
bend
complete
Egg
deposited
~100 μm
Ovulation start
Search start
Egg deposited
i
Abdomen bend schematic
j
Length
θ
–200
–100
Abdomen bend complete
0
100
Time (s)
θ
4º
Length
9%
Fly no. 1, 7 eggs
Fly no. 2, 11 eggs
Fly no. 3, 4 eggs
m
F
/
F
Δ
1.0
0.5
0
n
1.0
0.5
0
–0.5
–200
–100
Time (s)
0
Abdomen bend
complete
–0.5
–200
–100
Time (s)
0
Abdomen bend complete
0.4
0.2
0
F
/
F
Δ
–0.2
1.2
h
t
g
n
e
l
1.0
d
e
z
i
l
a
m
r
o
N
)
º
(
θ
40
30
k
Ovulation
start
Search
start
Abdomen bend
complete
l
Max.,
0.35
–20
0
Time (s)
20
ΔT = 3.3 s
Fig. 1 | oviDN [Ca2+] dips during ovulation, rises for seconds to minutes
and peaks immediately before the abdomen bend for egg deposition.
a, Behavioural sequence of egg laying. b, Egg expressing GCaMP3 in the body.
Steps correspond to a. Insets show close-ups, with over/undersaturated pixels
in red/blue; main panels show over/undersaturated pixels in white/black.
c, Behavioural progression. Lines connect single egg-laying sequences.
d, Schematic of wheel. e, Single oviDNb traced from light microscopy images.
Blue arrow indicates soma in brain, green arrow indicates outputs in the
abdominal ganglion. f, oviDN somas on the right side of the brain labelled by
oviDN-SS1. g, oviDN ∆F/F and behaviour during laying of two eggs by the same
fly. ∆F/F is smoothed with a 2 s boxcar filter. Images are z-projection of selected
imaging slices, with labels referring to oviDNa and oviDNb (oviDNa is partially
obscured by oviDNb). h, Population-averaged oviDNb ∆F/F aligned to the end
of the abdomen bend for egg laying. Light grey shading represents ±s.e.m.
throughout; 43 imaging traces from 41 egg-laying events associated with nine
cells in eight flies. The number of traces exceeds the number of egg-laying events
because for two eggs we imaged oviDNb on both sides of the brain. Behavioural
events shown below. i, Schematic of abdomen bend. θ denotes ‘body angle’ and
length is neck–ovipositor distance. j–l, Mean oviDN ∆F/F and behaviour aligned
to events in h: ‘ovulation start’ ( j), ‘search start’ (k) and completion of abdomen
bend (l). ‘Normalized length’ is the length given in i divided by its median
(Methods). Shorter, thicker arrows indicate when abdomen bend for egg
deposition is complete. A subsequent (stronger) bend is, presumably, for
cleaning the ovipositor. m, oviDN ∆F/F during individual egg-laying events,
smoothed with a 5 s boxcar filter. Black line, mean. n, Mean oviDN ∆F/F during
egg laying for all seven flies that laid three or more eggs, smoothed with a 5 s
boxcar filter. A single GCaMP7b fly is shown in grey. NP, Nippon Project; Ave.,
average; 2-p, two-photon; Ephys, electrophysiology; Max., maximum.
abdomen first begins to elongate), (2) search start (when the abdomen
returns to a neutral posture after ovulation), (3) ‘abdomen bend com-
plete’ (when the abdomen shows its maximum deflection before egg
deposition) and (4) egg deposition (when half of the egg is visible out-
side the ovipositor) (Fig. 1c, Extended Data Fig. 1d–h, Supplementary
Video 2 and Methods). We observed substantial inter-egg variation in
search duration—that is, the time between search start and completion
of the abdomen bend for egg deposition (Fig. 1c). Because the decision
to lay an egg is made within this variable time window, we sought to
find a neural signal whose dynamics in this time period could illuminate
the decision process.
Neurophysiology during egg laying
We developed an agarose-laden, rotatable, cylindrical treadmill on
which a head-fixed fly could walk and lay eggs while we simultaneously
564 | Nature | Vol 619 | 20 July 2023
Article
performed either two-photon imaging or electrophysiological record-
ing from neurons in the brain (Fig. 1d, Extended Data Fig. 2a–e and
Methods). Each egg-laying wheel had regions with agarose interspersed
with thin plastic barriers. The agarose substrates varied in their sucrose
concentration (Fig. 1d, light and dark blue), but always contained 1.6%
ethanol and 0.8% acetic acid, which simulate the chemical environment
of a rotting fruit and thereby promote egg laying. We found that the
egg-laying behavioural sequence measured on the wheel resembled
that in free behaviour (Extended Data Fig. 2f,g). One difference was
that flies on the wheel walked less vigorously during the search period
(compare fly speed in Extended Data Figs. 1f and 2k), probably because
they found it physically difficult to restart rotating the heavy wheel
after standing still for a minute or more during ovulation (Methods).
With head-fixed flies, we therefore often refer to the search period as
the search/delay period.
We decided to image the activity of oviposition descending neu-
rons (oviDNs)9 during egg laying. These neurons appeared to be suit-
able candidates for informing the decision process because, when they
are inhibited, egg laying is completely suppressed and when they are
stimulated an egg is often laid9. Three oviDNs9 and two uncharacterized
oviDN-like neurons are present on one side of the female fly brain, as
anatomically characterized in the hemibrain connectome16 (totalling
ten neurons per brain; Extended Data Fig. 3a). Each neuron primarily
receives input in the brain and has synaptic outputs in the abdominal
ganglion (Fig. 1e). We used two different driver lines to gain genetic
access to oviDNs—oviDN-GAL4 and oviDN-SS1 (ref. 9). OviDN-GAL4
labels all oviDN and oviDN-like neurons (Extended Data Fig. 3b);
OviDN-SS1 labels two of three oviDNs per side (cholinergic neurons
named oviDNa and oviDNb)9 and neither of two oviDN-like neurons
per side (Fig. 1f). In two-photon imaging experiments, unless otherwise
stated, we used the oviDN-SS1 driver and targeted the oviDNb soma on
one side of the brain; by targeting a single soma we could consistently
image the same identified cell across all flies rather than intermixed
neurites (Extended Data Fig. 3c).
A rising signal in oviDNs
We imaged GCaMP7 (ref. 17) fluorescence from oviDNs during egg laying
(Fig. 1g–l). We found that the oviDN ∆F/F signal dropped to its minimum
value during ovulation and then peaked near the moment of the abdo-
men bend for egg deposition (Fig. 1g). In some cases we observed a
monotonic rise (Fig. 1g, left and Supplementary Video 3) while in others
the signal drifted up and down before reaching its peak (Fig. 1, right
and Supplementary Video 4). The peak in the population-averaged
∆F/F signal was higher when we aligned the oviDN [Ca2+] signal with the
moment when the abdomen finished bending to lay the egg (Fig. 1h,i)
than when aligning with the moment that the egg became half-visible
outside the fly (Extended Data Fig. 2l versus Extended Data Fig. 2m).
On average, the [Ca2+] signal dipped when ovulation started (Fig. 1j)
and reached a minimum when the abdomen was longest (Extended
Data Fig. 2i). The average [Ca2+] signal then began to rise and returned
to near baseline (∆F/F = 0 in our normalization; Methods) when ovula-
tion was completed (that is, the beginning of the search/delay period;
Fig. 1k). We often observed in individual traces an upward inflection
in the [Ca2+] signal soon after the search/delay period began (Fig. 1g,
right trace), which was evident as a small inflection in the mean
trace (Fig. 1k, upward inflection just after time 0). The average [Ca2+]
signal peaked at around 3 s before completion of abdomen bend for egg
deposition (Fig. 1l)—that is, approximately when the bend was initiated.
The average [Ca2+] signal returned to baseline after egg laying, while
flies performed a second abdomen bend presumably to clean their
ovipositor (Extended Data Fig. 2n).
The [Ca2+] rise was evident across multiple egg-laying events in single
flies (Fig. 1m), reaching a qualitatively similar ∆F/F value of roughly
0.35 immediately before the abdomen bend for egg laying (Fig. 1n).
In some flies we simultaneously imaged oviDNa and oviDNb, with both
neuron types showing a similar rising signal (Extended Data Fig. 3d).
When cross-correlating oviDNa and oviDNb GCaMP signals on the same
side of the brain or oviDNb signals across both sides of the brain, we
observed a peak with zero lag (Extended Data Fig. 3e,f). This observa-
tion supports a model in which all four oviDNs in the oviDN-SS1 line
exhibit the same first-order calcium dynamics during egg laying. Thus,
in our recordings of single oviDNs, when we observe an occasional ∆F/F
peak with no egg or an egg without a peak in the ∆F/F signal (Fig. 1m and
Extended Data Fig. 4), this may be because the functionally relevant
signal is a population-level one across all six oviDNs. Aspects of this ∆F/F
variability might also reflect technical considerations associated with
stable acquisition of long [Ca2+] measurements from a single, tiny, soma
in a behaving fly. During non-egg-laying periods, the oviDN ∆F/F signal
still correlated with abdominal movements and locomotion (Extended
Data Fig. 5a–d). Approximately once every 30 min the oviDN ∆F/F signal
reached around 0.35 without ovulation having occurred beforehand,
and at these moments the fly exhibited an abdomen bend that yielded
no egg (Extended Data Fig. 5e). In sum, oviDNs express a signal whose
dynamics correlate with the behavioural sequence of Drosophila egg
laying, drifting up and down during the search period until a consistent
level is reached just before egg deposition. These dynamics suggested
that a rise-to-threshold process governs Drosophila egg-laying behav-
iour, a hypothesis that we next tested with optogenetics.
Optogenetics supports a threshold
To test whether a neural activity threshold triggers the egg-deposition
motor programme, we coexpressed in oviDNs GCaMP7f and the
light-gated ion channel CsChrimson18. We measured oviDN ∆F/F and
fly behaviour while providing 5-s-long, high-intensity light pulses
(Methods). Stimulations after ovulation typically yielded an abdomen
bend and egg deposition (Fig. 2a,b and Supplementary Video 5). When
we averaged [Ca2+] and behavioural signals around the time of stimula-
tions that yielded an egg we observed an increase in ∆F/F in the oviDN,
a synchronous abdomen bend and—with more variable latency—egg
deposition (Fig. 2c).
In our initial experiments we stimulated oviDNs at user-defined
moments, sometimes purposefully waiting for flies to finish ovulat-
ing before stimulating (Methods). In later experiments we performed
regularly spaced stimulations in flies expressing or not expressing
CsChrimson, independent of the flies’ ovulation status. Flies express-
ing CsChrimson bent their abdomen, on average, even on stimulation
pulses that did not result in egg deposition (Fig. 2d), whereas control
flies did not bend their abdomen (Fig. 2e). We interpret this result—
alongside the observation that flies tended to bend their abdomen
when oviDN ∆F/F was spontaneously high without previous ovulation
(Extended Data Fig. 5e)—to mean that they initiate the egg-deposition
motor programme when a neural process reflected in the oviDN [Ca2+]
signal reaches a certain level. If an egg is available in the uterus, egg
deposition occurs—although with temporal variability that may be
related to sensory feedback signals in the uterus12 or motor aspects
of how eggs are released13. The temporal variability in egg deposition
was qualitatively similar in optogenetically stimulated (Fig. 2c) and
spontaneous (Fig. 1h) egg laying in head-fixed flies.
To quantitatively assess whether the egg-deposition motor pro-
gramme is initiated in an all-or-nothing fashion when neural activity
crosses a threshold, we stimulated oviDNs at a regular interval while
cycling through four different intensities of light. We assigned each
stimulation trial to one of seven bins depending on the oviDN ∆F/F
maximum on that stimulation pulse (Fig. 2f). We found that, when
our stimulation pulse induced ∆F/F changes of approximately 0.32
or higher, the pulse produced large mean abdomen bends and, when
our stimulation pulse induced ∆F/F changes below that level, the pulse
did not induce such bends (Fig. 2g,h). This bimodality was robust to
Nature | Vol 619 | 20 July 2023 | 565
a
b
c
l
y
F
n
o
i
t
i
s
o
p
F
/
F
Δ
0º
360º
0.8
0.4
0
0 mM sucrose
One fly
>10 min
0º
360º
0.8
0.4
0
oviDN-SS1
> CsChrimson
and GCaMP7f
(unless noted)
Manual
stimulation
Ovulation
start
Search
start
Abdomen
bend
complete
Egg
deposited
50 s
Manual stimulation
Ovulation
before
stimulation
9 Flies,
34 stimulations
28 eggs
No ovulation
before
stimulation
20 Flies,
174 stimulations
4 eggs
Manual stimulation
Selected for
stimulations
eggs
18 Stimulations,
5 flies
32 Stimulations,
9 flies
1 Egg
d
0.4
0
5
0
1.2
1.0
52
42
F
/
F
Δ
s
g
g
E
d
e
t
i
s
o
p
e
d
d
e
z
i
l
a
m
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o
N
h
t
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n
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)
º
(
θ
0.4
0
5
0
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1.0
42
32
0
20
40
60
0
Manual
stimulation
Time (s)
Periodic
stimulation
f
F
/
F
Δ
0.4
0
≤0.02
ΔF/F bin width of 0.1
>0.52
5 s
334 Stimulations, 10 flies
from 4 LED intensities
binned by max. ΔF/F
Periodic stimulation
e
Selected for
stimulations no eggs
Selected for
stimulations
no eggs
83 Stimulations,
10 flies
84 Stimulations,
6 flies
(no CsChrimson)
20
Time (s)
0
40
Periodic
stimulation
20
Time (s)
40
P < 0.001
P < 0.001
g
h
0
–0.04
–0.08
–0.12
d
e
z
i
l
a
m
r
o
N
Δ
h
t
g
n
e
l
)
º
(
θ
Δ
4
2
0
Periodic stimulation
0
0.2
0.4
0.6
Mean of all max. ΔF/F in bin
Fig. 2 | Evidence for a threshold in the ability of oviDNs to trigger the
egg-deposition motor programme. a, oviDN ∆F/F and behaviour during
high-intensity 5 s CsChrimson stimulation. ∆F/F is smoothed with a 2 s boxcar
filter. b, High-intensity stimulations separated on the basis of whether
ovulation was observed previously. Stimulations resulting in eggs were defined
as those in which egg deposition occurred within 60 s of light onset. All four
eggs without ovulation observed previously were from the first stimulation
of a fly, and ovulation may have occurred before the session. c, Mean oviDN
∆F/F and behaviour for manually triggered, high-intensity stimulations that
resulted in eggs. Light grey shading represents ±s.e.m. throughout. Behaviour,
32 stimulations in nine flies; ∆F/F, 18 stimulations in five flies. Differences in
number of traces are explained in Methods. The peak in oviDN ∆F/F slightly
lags behind initiation of the abdomen bend, potentially because [Ca2+] at the
synaptic active zones rises faster than at the soma with optogenetic stimulation.
d, Mean oviDN ∆F/F and behaviour for periodically triggered, high-intensity
stimulations that did not result in eggs. Five of 88 stimulations that resulted
in eggs are not shown so that changes independent of egg deposition could
be analysed. e, Same as d but with flies not expressing CsChrimson (0 of
84 stimulations → eggs). f, oviDN ∆F/F during stimulation binned by maximum
∆F/F 1–3 s after start of stimulation. Four light intensities were triggered
periodically. Stimulations were included regardless of whether egg deposition
occurred (nine of 334 stimulations → eggs). The first and last bins include data
below 0.02 and above 0.52, respectively. g,h, Change in mean body length (g)
and body angle (h) for each of the bins in f. Mean behavioural signal 2–4 s after
start of stimulation was subtracted from mean behavioural signal 0–2 s before
stimulation. Two-sided Wilcoxon rank-sum test, P = 7.2 × 10–4 and 5.0 × 10–4.
LED, light-emitting diode.
566 | Nature | Vol 619 | 20 July 2023
how we binned ∆F/F responses (Extended Data Fig. 5f–n). (Note that
although the ∆F/F threshold value here is similar, but not identical, to
that observed during spontaneous egg laying, any such quantitative
comparison is not necessarily biologically meaningful (Methods).)
We also found supportive evidence for a threshold when we provided
gentle stimulation to oviDNs for tens of seconds and correlated the
moment at which oviDN ∆F/F reached a common value with when an
abdomen bend was observed (Extended Data Fig. 5o–s). Altogether,
these data support the hypothesis that a threshold level of activity
initiates the egg-deposition motor programme in an all-or-nothing
fashion.
In these experiments we measured [Ca2+] in the oviDN soma. Somatic
[Ca2+] is often thought of as a proxy for a cell’s spike rate19. To gain
insight into the relationship between membrane potential (Vm), spike
rate and [Ca2+] in oviDNs, we activated CsChrimson while performing
either whole-cell patch-clamp recordings or calcium imaging at the
oviDN soma (Extended Data Fig. 6a–g). The oviDN spike rate and Vm
rose and fell quickly with stimulation (around 400 ms half-decay time
for both) whereas somatic [Ca2+] changed much more slowly (roughly
5.7 s half-decay time in the ∆F/F signal; Extended Data Fig. 6d–f and
Methods). Given these slow [Ca2+] dynamics, the ∆F/F threshold that
we measured at the soma may not represent a consistent spike-rate
threshold in the same cell, which raises the question of how the somatic
signal we analysed induces behaviour. One possibility is that the [Ca2+]
signal in the oviDN soma acts as a proxy for a functionally relevant
rise-to-threshold process in the abdominal ganglion, perhaps in
the oviDN axon terminals. Consistent with this possibility, when we
imaged GCaMP fluorescence in the axonal terminals of oviDNs during
CsChrimson stimulation we also observed relatively slow [Ca2+] dynam-
ics (Extended Data Fig. 6h–p and Supplementary Discussion). Thus,
the rising [Ca2+] signal in the soma might reflect a similarly rising [Ca2+]
signal in the axon terminals, with a biochemical process in the presyn-
aptic terminals of oviDNs potentially reading out the rising [Ca2+] signal
with a sharp nonlinearity to trigger the egg-laying motor programme.
Alternatively, oviDNs may transmit a graded synaptic signal to their
postsynaptic partners, with the threshold implemented downstream
of oviDNs. Additional work will be needed to test these hypotheses.
Searching for a substrate of high value
If a threshold triggers initiation of the egg-deposition motor pro-
gramme, might substrate quality modulate oviDN activity to influence
when threshold is reached and thus where an egg is laid? We analysed
the behaviour of freely walking flies to better understand how they
use substrate experiences during their search—that is, the time period
after ovulation and before egg deposition—to guide egg-laying deci-
sions. Specifically, we quantified where flies laid eggs within custom,
high-throughput behavioural chambers with two different substrate
options20 (Fig. 3a, Extended Data Fig. 1b, Supplementary Video 6 and
Methods).
We observed, in line with past work7,8, that Drosophila melanogaster
target the majority of their eggs to substrates with lower, not higher,
concentrations of sucrose (Fig. 3b). This bias makes sense in light of
the fact that D. melanogaster prefer to lay eggs on rotting or ferment-
ing fruit21, and a soft substrate with clearly detectable ethanol and
relatively low levels of sucrose22 mimics the portion of a rotting fruit
where fermentation (conversion of sugar to alcohol) is actively taking
place. Beyond simply preferring low sucrose, we further replicated past
findings arguing that sucrose-based choice is a relative-value decision7,8.
That is, flies strongly bias egg laying to the lower of two sucrose options
rather than preferring an absolute sucrose concentration. For example,
they laid over 90% of eggs on the 0 mM option in 0 versus 200 mM
chambers and over 90% of eggs on the 200 mM option—the previously
avoided substrate—in 200 versus 500 mM chambers (Fig. 3b). Flies laid
a similar total number of eggs in all chambers7,8 (Fig. 3c).
Article
a
Wild type
Sucrose choice assay
0
M
m
0
0
5
M
m
1 h
Egg
deposited
10 mm
5 min
Search
18
30
38
33
29
47
b
No. of flies
18
30
47
n
o
s
g
g
e
f
o
n
o
i
t
c
a
r
F
n
o
i
t
p
o
e
s
o
r
c
u
s
-
r
e
w
o
l
1.0
0.8
0.6
0.4
0.2
0
c
y
fl
r
e
p
d
a
i
l
s
g
g
E
60
40
20
0
200 versus 500
0 versus 500
0 versus 200
200 versus 500
0 versus 0
500 versus 500
200 versus 200
0 versus 200
0 versus 500
17 flies
s
g
g
e
0
0
2
5
0
d
d
e
e
p
S
)
1
–
s
m
m
(
e
M
m
0
0
5
M
m
0
f
g
h
i
e
t
a
r
g
n
y
a
l
-
g
g
E
i
)
1
–
n
m
s
g
g
e
(
i
e
t
a
r
g
n
y
a
l
-
g
g
E
i
)
1
–
n
m
s
g
g
e
(
i
e
t
a
r
g
n
y
a
l
-
g
g
E
i
)
1
–
n
m
s
g
g
e
(
3
2
1
0
3
2
1
0
3
2
1
0
771 eggs, 17 flies
0 mM
500 mM
1,863 eggs, 42 flies
0 mM
200 mM
1,345 eggs, 30 flies
200 mM
500 mM
–300
Time (s)
0
300
Egg deposited
051015202530456090
150
300
600
1,200
7,200
Time since substrate transition (s)
Fig. 3 | Flies search for an egg-deposition site with high relative value in
the time period when the oviDN [Ca2+] signal rises. a, Y position and egg-
deposition events from a fly in a high-throughput egg-laying choice chamber20.
b, Fraction of eggs on the lower-sucrose option with 95% confidence interval.
X axis indicates sucrose concentration (mM). One dot represents one fly.
c, Eggs laid per fly. Mean ±s.e.m. indicated. One dot represents one fly. d, Each
row represents a single egg-laying event in a 0 versus 500 mM sucrose chamber,
aligned to egg deposition, with the fly’s speed indicated by colour intensity.
Rows have been ordered based on the search duration; start of the search
period is in magenta. Eighteen flies were tested, one of which did not lay eggs.
e, Same data as in d, but the substrate on which the fly was residing is indicated
by white and black pixels. f–h, Mean egg-laying rate during the search period
aligned to a transition from higher to lower sucrose (lighter blues) or lower to
higher sucrose (darker blues) in three separate choice conditions (0 versus
500 mM (f), 0 versus 200 mM (g) and 200 versus 500 mM (h)), with 90%
confidence intervals (Methods): 771 eggs from 17 flies (f, 18 flies tested of
which one did not lay eggs), 1,863 eggs from 42 flies (g, 47 flies tested of which
five did not lay eggs) and 1,345 eggs from 30 flies (h, 30 flies tested). Egg-laying
rate requires around 10 s to reach maximum after a fly transitions to the higher-
relative-value option, at least partially because flies do not lay eggs on the
(approximately) 2.5 mm plastic boundary between substrates (Extended Data
Fig. 7e,f) and because there is a delay of about 3 s between when the fly bends
its abdomen and deposits the egg (Extended Data Fig. 7g and Fig. 1c). Thus, the
fly’s internal sense of relative value probably changes more rapidly after a
transition than the slowly increasing egg-laying-rate curve would suggest.
In these high-throughput chambers we did not have the spatial
resolution to clearly detect abdominal elongations and scrunches
(Extended Data Fig. 1b,c and Methods). However, we could still detect
ovulation and thus when flies start to search immediately thereafter,
because they stand still for about 1 min when they ovulate (Extended
Data Fig. 1d–f and Methods). We could also denote the end of the search
period as the moment when an egg was half-way out of the ovipositor,
which consistently follows the final abdomen bend for egg laying by
only a few seconds in these chambers (Methods). The duration of the
search period was highly variable (Fig. 3d). Flies laid more eggs on
the lower-sucrose option despite spending appreciable time on the
higher option during the search epoch8 (Fig. 3e). Specifically, in 0 versus
500 mM chambers, 95% (734 of 771) of eggs were laid on 0 mM whereas
only 77% (592 of 771) of search periods started on 0 mM (P < 0.001;
Methods). (More search periods started on 0 mM than 500 mM because
ovulation tended to occur soon after the previous egg-laying event
(Extended Data Fig. 1d) and egg laying tended to occur on 0 mM.) We
additionally noticed that, when flies started the search on 500 mM,
they frequently left this substrate while searching (83%, 149 of 179)
but when they started their search on 0 mM they left less often (36%,
212 of 592; P < 0.001; Methods). Leaving a higher-sucrose substrate
more often at the onset of search is not an intrinsic property of the
substrate, because flies left substrate islands at a similar rate in 500
versus 500 and 0 versus 0 mM chambers (299 of 528, 57% and 441 of
895, 49%, respectively). Because sucrose cannot be sensed at a distance,
we conclude that flies retain information about the substrate options
available to them from experiences outside of the current search period
and use this information to regulate the current search. We tested for
the possibility of flies using spatial memories to guide their egg-laying
behaviour in our chambers but we could not find supportive evidence
(Extended Data Fig. 7a–d). We also did not find evidence that flies were
pausing to feed on the higher-sucrose substrate while searching, sug-
gesting that in our experiments a competing feeding drive is not the
reason for suppression of egg laying on higher-sucrose substrates
(Extended Data Fig. 7a–d).
We noticed that flies would occasionally lay eggs on the higher-
sucrose option if a few minutes had elapsed since they last visited
the preferred, lower-sucrose option (Fig. 3a bottom, first two eggs).
To quantify this observation we calculated the egg-laying rate during
the search period as a function of time since the last substrate transi-
tion (regardless of whether the last transition occurred in the current
search period or previously; Methods). Flies in 0 versus 500 mM sucrose
choice chambers strongly inhibited egg laying on 500 mM if they had
visited the 0 mM option within the previous 2 min or so (Fig. 3f). After
about 2 min, however, the egg-laying rate on 500 mM began to increase
gradually, approaching—albeit not completely—that on 0 mM at the 2 h
time point. One interpretation of this egg-laying-rate plot is that the
relative value of the 500 mM substrate gradually increased over time,
eventually approaching the value of the 0 mM substrate (if 0 mM is not
revisited). This phenomenon was also evident in 0 versus 200 mM and
200 versus 500 mM chambers (Fig. 3g,h).
Substrate value alters oviDN physiology
How might the rise-to-threshold process evident in oviDN [Ca2+] guide
flies to lay most of their eggs on substrates with high relative value?
We hypothesized that, when flies are on a high-value substrate, the
oviDN [Ca2+] signal might rise briskly and, when they are on a low-value
substrate, it might rise more slowly or even fall, thus creating time for
the fly to find a better option before threshold is reached (Fig. 4a).
To test this idea we analysed how the oviDN ∆F/F signal changed as
flies transitioned across substrates on the egg-laying wheel. On wheels
with 0 and 500 mM sucrose options we observed a mean increase
in ∆F/F after flies walked onto the higher-relative-value substrate
(500 → 0 mM transitions) and a mean decrease after they transitioned
to the lower-relative-value substrate (0 → 500 mM transitions) (Fig. 4b).
This result was not explained by differences in feeding, locomotor
speed or abdomen movements across the two options (Extended Data
Fig. 8). We observed similar, but qualitatively faster, changes in oviDN
activity with substrate transitions at the level of Vm (Fig. 4c) and spike
rate (Extended Data Fig. 9a–e).
If oviDN [Ca2+] tracks the relative value of substrates, rather than just
sucrose concentration, one might expect that oviDN ∆F/F would gradu-
ally increase on the 500 mM option because that option becomes more
acceptable over several minutes. Indeed, when we split 500 to 0 mM
substrate transitions into four groups—depending on the time spent
on 500 mM before the transition—we found that the mean, ‘baseline’,
Nature | Vol 619 | 20 July 2023 | 567
a
b
c
Sucrose (mM)
[Ca2+]
threshold
oviDN [Ca2+]
Model for egg-laying substrate decisions
500
0
d
30 s
0.05
F
/
F
Δ
0
–0.05
oviDN-SS1>
GCaMP7f
Increasing time (t) spent on 500 mM before transitioning to 0 mM
t ≤ 30 s,
1,197 traces
30 s < t ≤ 1 min,
430 traces
1 min < t ≤ 3 min,
637 traces
t > 3 min,
176 traces
Ovulation
start
Search
start
Abdomen
bend start
Egg
deposited
0
20
40
0
Substrate-crossing
moment
20
Time (s)
40
0
20
40
0
20
40
e
F
/
F
Δ
0.4
0.2
0
–0.2
2,459 traces
53 flies
2,460 traces
0 mM
500 mM
0.05
F
/
F
Δ
0
–0.05
–40 –20
0
20
40
–40 –20
0
20
40
74 traces
8 flies
72 traces
)
V
m
(
m
V
–54
–56
–58
–40 –20
0
20
40
–40 –20
0
20
40
Time (s)
Substrate-crossing
moment
oviDN-SS1>
GCaMP7f
oviDN-SS1>
2xEGFP
Fly remained on 0 or 500 mM
9 traces,
3 flies
21 traces,
5 flies
f
y
t
i
s
n
e
d
y
t
i
l
i
b
a
b
o
r
P
)
F
/
F
Δ
r
e
p
s
(
250
0
250
0
P =
0.047
(0.030,
0.064)
–20
0
20
–80
–60
–40
–20
0
20
Ovulation
start
Time (s)
Abdomen bend
complete
0
0.01
0.02
Slope (ΔF/F s–1)
from time when ΔF/F
is 0 to abdomen bend
Fig. 4 | Relative value of the current egg-laying option influences the
subthreshold physiology of oviDNs to impact when threshold is reached.
a, Schematic model relating oviDN signal to substrate decisions. b, Mean
oviDN ∆F/F during substrate transitions. Light grey shading denotes ±s.e.m.
throughout. In total, 2,459 and 2,460 traces from 70 cells in 53 flies (1,911 and
1,922 transitions); 1,911 transitions yielded 2,459 traces because we sometimes
imaged oviDNb on both sides of the brain. c, Mean oviDN Vm during transitions;
74 and 72 traces from eight cells in eight flies (74 and 72 transitions). Traces
were smoothed using a 666 ms boxcar filter to aid comparison to ∆F/F, which
was acquired at around 1.5 Hz. d, Mean oviDN ∆F/F during transitions split
based on the amount of time the fly spent on 500 mM before entering 0 mM;
1,197, 430, 637 and 176 traces from 70 cells in 53 flies (914, 347, 486 and
148 transitions, respectively). e, Mean oviDN ∆F/F for egg-laying events where
the fly remained on 0 or 500 mM for the 80 s window before and including egg
deposition. An increased ∆F/F baseline of roughly 0.02 exists for 0 mM before
ovulation; 0 mM, 21 traces from five cells in five flies (21 eggs); 500 mM, nine
traces from four cells in three flies (seven eggs). f, Probability densities of
individual oviDN ∆F/F slopes from traces averaged in e. Individual ∆F/F values
were smoothed with a 5 s boxcar filter before calculating the net slope from
when ∆F/F first reached 0 after the signal minimum (which occurs during
ovulation) to 3.3 s before abdomen bend was complete—which is when, on
average, abdomen bend starts (Fig. 1l). P values were calculated using the
two-sided Wilcoxon rank-sum test. For additional information on these
calculations see Methods.
∆F/F on 500 mM became progressively higher. After more than 3 min
on 500 mM, the mean ∆F/F on 500 and 0 mM became indistinguishable
(Fig. 4d). It is intriguing that this slow increase in oviDN mean [Ca2+]
in flies residing on a 500 mM substrate occurred on a time scale of
minutes, which roughly matches the time scale over which egg-laying
rates recover in flies residing on 500 mM in free behaviour (compare
Fig. 4d with Fig. 3f). Consistent with the notion that the mean oviDN
[Ca2+] signal tracks relative value and not just sucrose concentration,
the magnitude of the average ∆F/F changes during substrate transitions
in 0 versus 500 mM wheels, 0 versus 200 mM wheels and 200 versus
500 mM wheels were similar (Extended Data Fig. 9f–k).
We hypothesized that excitatory inputs associated with the rela-
tive value of the current substrate interact with additional excitatory
drive associated with the search state. These two inputs ultimately
drive oviDN activity to hit threshold, inducing egg laying. One predic-
tion of this model is that the oviDN [Ca2+] signal should have a lower
propensity to rise on the less valued substrate because of reduced drive
from putative relative-value inputs, and a higher propensity to rise on
more valued substrates. Although the number of eggs available for
analysis was very low, we found that the mean slope of oviDN ∆F/F rise
toward threshold was shallower on the lower-relative-value substrate
than on the higher one (Fig. 4e). A change in slope was also evident, to
near statistical significance, in an analysis of individual traces (Fig. 4f).
The path to threshold of individual traces was not as gradual as in the
average trace, often containing acute upward and downward fluctua-
tions (Fig. 1g,m and Extended Data Fig. 4). These fluctuations could
reflect internal gating of when substrate value inputs impact oviDN
physiology, or other factors that influence egg laying. Indeed, such
fluctuations may underlie the sizeable variability in search duration
we observed in freely behaving flies regardless of whether they were
presented with one or more substrate options (Figs. 1c and 3d). Note
that, in free behaviour, we would expect modulations of the oviDN
signal to show even more marked upward or downward adjustments
than those in Fig. 4e because, unlike head-fixed flies, freely walking
flies will transition more often between low- and high-relative-value
substrates during search.
Hyperpolarization of oviDNs alters choice
Given the above framework for how the oviDN signal relates to egg-
laying substrate choice (Fig. 4a), we asked whether we might be able
to perturb oviDNs in a manner that would cause flies to lay even more
eggs than normal on the option with higher relative value. Specifically,
we reasoned that gentle hyperpolarization of all oviDNs (using the
oviDN-GAL4 line) could lengthen the time required for the decision
process to reach threshold, providing flies with more time than usual
to encounter the higher-value substrate and thus leading to more eggs
on the higher-value option.
Expressing the human Kir2.1 (ref. 23) potassium channel in oviDNs
completely eliminated egg laying9 (Fig. 5a and Extended Data Fig. 10a),
as did genetic ablation of oviDNs9 (Extended Data Fig. 10b) and optoge-
netic inhibition using the light-gated anion channel, GtACR1 (ref. 24)
(Fig. 5b and Extended Data Fig. 10c). Each of these perturbations prob-
ably prevented the decision process from ever reaching threshold.
Serendipitously, however, we introduced a modified mouse Kir2.1
(hereafter Kir2.1*) and a non-conducting control (Kir2.1*Mut) chan-
nel into Drosophila25 and found that flies expressing Kir2.1* in all
oviDNs (oviDN>Kir2.1* flies) could still lay eggs, albeit at lower mean
levels compared with genetic-background-matched controls (Fig. 5c
and Methods). Whole-cell, patch-clamp recordings showed that
568 | Nature | Vol 619 | 20 July 2023
Article
a
b
c
d
Kir2.1
GtACR1 no stimulation
GtACR1 stimulation
No. of flies
80
18
36
29
36
9
10
Kir2.1*Mut
Kir2.1*
Tubulin>GAL80ts
49
37
127
107
y
fl
r
e
p
d
a
i
l
s
g
g
E
60
40
20
0
oviDN-GAL4
Empty-GAL4
Empty-GAL4
oviDN-GAL4
Empty-GAL4
oviDN-GAL4
e
Tubulin>GAL80ts
oviDN-GAL4>Kir2.1*Mut
40 flies
s
g
g
E
0
0
2
d
e
e
p
S
)
1
–
s
m
m
(
5
0
f
oviDN-GAL4>Kir2.1*Mut
17 flies
–240
Time (s)
0
240
Egg deposited
)
V
m
(
t
s
e
r
t
a
m
V
oviDN-GAL4>
Kir2.1*(Mut)
and GCaMP7f
tubulin>GAL80ts
P = 0.02
–50
–60
–70
–80
Kir2.1*
Kir2.1*Mut
g
h
No. of flies
with five or
more eggs
120
40
13
P < 0.001
n
o
i
t
a
r
u
d
n
a
d
e
M
i
)
s
(
h
c
r
a
e
s
f
o
60
Kir2.1*
0
Kir2.1*Mut
i
g
n
k
a
w
l
t
n
e
p
s
e
m
i
t
f
o
n
o
i
t
c
a
r
F
i
s
d
o
i
r
e
p
g
n
y
a
l
-
g
g
e
-
n
o
n
g
n
i
r
u
d
40
13
P = 0.17 (NS)
0.6
0.4
0.2
Kir2.1*
0
Kir2.1*Mut
i
P = 0.32
(NS)
P <
0.001
P = 0.64
(NS)
No. of flies
49
37
127
107
123
130
M
m
0
n
o
s
g
g
e
f
o
n
o
i
t
c
a
r
F
)
i
e
c
o
h
c
M
m
0
0
2
s
u
s
r
e
v
0
(
1.0
0.8
0.6
0.4
0.2
0
Kir2.1*Mut
Kir2.1*
Empty-GAL4
oviDN-GAL4
oviDN-GAL4
(18 ºC control)
Fig. 5 | Gentle hyperpolarization of oviDNs increases search duration
and results in more eggs laid on the preferred option. a–c, Eggs laid per fly
(mean ± s.e.m.). Each dot represents one fly. Inhibition of oviDNs with Kir2.1
(a), GtACR1 (b), or Kir2.1* (c). d, oviDN (or oviDN-like neuron) Vm at rest (mean ±
s.e.m.). Five cells in five flies and five cells in four flies, respectively. P value was
calculated using two-sided Wilcoxon rank-sum test. e,f, oviDN-GAL4>Kir2.1*Mut
(e) and oviDN-GAL4>Kir2.1* (f) flies. Each row represents a single egg-laying
event in a 0 versus 200 mM sucrose chamber, aligned to egg deposition, with
the fly’s speed indicated by intensity of black shading. Rows ordered based on
the search duration; 1,377 eggs from 40 flies (45 flies tested, of which five did
not lay eggs) and 346 eggs from 17 flies (40 flies tested, of which 23 did not lay
eggs), respectively. g, Median duration of search for individual flies from e,f
that laid five or more eggs. Mean ± s.e.m., P = 9.6 × 10–7. h, Fraction of time spent
walking during non-egg-laying periods for flies shown in g. Non-egg-laying
periods were defined as periods of over 10 min from egg deposition. i, Fraction
of eggs on the lower-sucrose option with 95% confidence interval. Each dot
represents one fly. Individual flies laid an average of 38, 38, 32, 16, six and seven
eggs each. If the plot is reworked by examining only flies that laid at least five
eggs, P = 1.9 × 10–6 (rather than 6.3 × 10–4) for the middle set of bars and is not
significant (NS) for the others. g–i, P values calculated using two-sided Wilcoxon
rank-sum test. c–i, Tubulin>GAL80ts was present in all flies, to limit the time
window in which Kir2.1* or Kir2.1*Mut transgenes were expressed (Methods).
The 18 °C control was not shifted to 31 °C before the assay and thus expression
of Kir2.1* or Kir2.1*Mut was not induced. All egg-laying experiments were
conducted at 24 °C.
Kir2.1*-expressing oviDNs (or oviDN-like neurons) were hyperpolar-
ized by around 14 mV, on average, compared with Kir2.1*Mut-expressing
(control) cells (Fig. 5d). This is a moderate hyperpolarization that still
permitted most Kir2.1*-expressing neurons to fire spikes with sufficient
current injection (Extended Data Fig. 10d). This fact could explain why
many oviDN>Kir2.1* flies could lay eggs.
We tracked the x–y trajectories and egg-laying behaviour
of oviDN>Kir2.1* and oviDN>Kir2.1*Mut flies in two-substrate,
free-behaviour chambers. We observed a two- to threefold increase
in the length of the search period in oviDN>Kir2.1* compared with
oviDN>Kir2.1*Mut flies when comparing the full distribution of traces
from all flies (P < 0.001; Fig. 5e,f and Methods), or when quantifying
median search duration per fly (comparing flies that laid sufficient
eggs for analysis—that is, at least five eggs; Fig. 5g). The increase
in search duration could not be attributed to a general increase in
the fraction of time spent walking (Fig. 5h), nor to a broad defect in
egg-laying-related motor functions (Extended Data Fig. 10e,f). Remark-
ably, just as we imagined, the increase in search duration was accompa-
nied by a higher fraction of eggs laid on the substrate of higher relative
value (Fig. 5i), probably because oviDN>Kir2.1* flies have more time
to encounter the higher-relative-value option before threshold is
reached.
A neural circuit for egg laying
Finally, we wished to provide an inroad into the circuit mechanisms
underlying the rising [Ca2+] signal in oviDNs. We created split-GAL4
driver lines that allowed selective inhibition of several neuron classes
that have extensive synaptic input onto oviDNs16 (Methods, Supple-
mentary Table 3 and Extended Data Fig. 11a–r). We found three groups
of neurons—oviEN9, group U cells and group G cells—that when inhib-
ited with GtACR1 markedly reduced the total number of eggs laid by
flies (Fig. 6a; see Methods for discussion of group Z). Although oviEN
activity is known to be required for egg laying9, the requirement for
activity in group U and group G neurons—which make far fewer direct
synapses onto oviDNs than oviENs or many of the other neuron types
tested (Fig. 6a)—is a new finding.
To identify what might be special about oviEN, group U, and group G
cells we analysed their connectivity in the hemibrain16, discovering that
these cells, at the anatomical level, form a recurrent circuit that feeds
into oviDNs (Fig. 6b,c and Supplementary Table 4). This recurrent cir-
cuit comprises just five neurons per side of the brain, and silencing any
of its constituent neuron groups eliminates egg laying, presumably by
preventing the decision process from ever reaching threshold. None of
the other groups of neurons we tested formed a recurrent circuit with
the same or fewer number of neurons (Fig. 6d; see Methods for further
analysis and discussion; Extended Data Figs. 11s–v and 12). Cells in this
circuit on both sides of the brain are reciprocally connected, and a pair
of GABAergic inhibitory neurons, oviINs9, may act to keep activity in
the circuit from rising too rapidly, in addition to gating egg laying on
the basis of internal state9 (Fig. 6e).
Discussion
Rise-to-threshold signals have been linked to decision-making and
action-initiation processes in humans26, monkeys3,27–30, rodents31–34,
zebrafish35–37 and insects38–42. These signals have been shown to rise,
or suggested to rise, on the hundreds-of-milliseconds to seconds time
scale. Some of the most influential work in this domain has focused on
rise-to-threshold signals that integrate noisy sensory input so that an
animal can report a percept1–3—that is, form a ‘perceptual decision’.
Our work helps to extend the rise-to-threshold framework beyond
perceptual decisions to ethologically relevant, self-paced decisions in
which animals decide among non-noisy, perceptually distinct, options43
(for example, egg-laying substrates with easily distinguishable differ-
ences in sucrose concentrations). Our work further emphasizes three
features of rise-to-threshold processes that were not easily appreci-
ated previously: (1) they can regulate decisions that take minutes, not
just seconds; (2) they can cause behaviour to start when threshold is
crossed33,41; and (3) their rate of rise can be modulated by the relative
value (and not just the more veridical sensory properties) of stimuli.
These features expand on past work on rise-to-threshold processes26–42,
suggesting that they may underlie a wide array of ethological, self-paced
decisions made by animals in the real world.
Nature | Vol 619 | 20 July 2023 | 569
a
c
oviDN input
neuron group
Approximate no. of pairs of oviDN
input neurons labelled
Approximate no. of synapses
onto oviDNs (ref. 16)
1
332
GtACR1 no stimulation
GtACR1 stimulation
No. of flies
80
18
25
24
31
26
22
18
17
50
36
16
21
19
12
23
18
57
22
49
33
16
24
18
11
13
14
18
26
43
17
18
16
28
51
15
24
b
y
fl
60
r
e
p
d
a
i
l
s
g
g
E
40
20
0
oviEN
Group G
Group B
Group T
Group C
Right side of
recurrent circuit
110
44
56
35
43
18
220
13
44
Neurons
1 Group U
3 Group G
1 oviEN
3 oviDNs
Group U
1
34
3
57
d
~10 μm
oviIN
1
185
Group O
Group V
Group Z
1
67
2
84
1
7–8
1–2
15–20
107
32
49
71
e
Left side
Right side
Neurons
Group B, O,
T, V, C or Z
Any neuron
Any oviDN
*
Neurons
1 Group U
3 Group G
1 oviEN
1 oviIN
3 oviDNs
Fig. 6 | An anatomically recurrent neuronal circuit whose activity is
required for egg laying provides direct synaptic input to oviDNs. a, Eggs
laid per fly (mean ± s.e.m.). Each dot represents one fly and each pair of bars
represents a split-GAL4 line (Supplementary Table 1). Estimate of number of
pairs of oviDN input neurons and number of synapses onto oviDNs is explained
in Methods. Labelling of oviENs in second split-GAL4 is stochastic (Extended
Data Fig. 11b), explaining why some flies still lay eggs. b, Hemibrain-derived
connectivity of indicated neurons on one side of the brain. Numbers adjacent
to arrows indicate total synapse counts. Green arrows indicate excitatory
(oviENs are cholinergic9); black arrows are of unknown sign but are posited to
be excitatory. Arrows drawn only if connection has more than two synapses.
Arrows with filled arrowheads indicate that there exists a single neuron–single
neuron connection with at least ten synapses. c, Recurrent-circuit neurons on
the right side of the brain using Neuroglancer and the hemibrain connectome.
d, Hemibrain-derived connectivity of indicated neurons on either side of the
brain. Filled arrows indicate a single neuron–single neuron connection with at
least ten synapses. X indicates that the diagrammed connection does not exist
at a threshold of ten or more synapses. e, Hemibrain-derived connectivity.
Green and black arrows are as in b, and red arrows are inhibitory (oviINs are
GABAergic9); arrows with filled arrowheads are as in b (see Supplementary
Tables 3 and 4 for all synapse counts). Light blue circles represent three oviDNs
on the right side and one on the left. Only one oviDN on the left side of the brain
is annotated in the hemibrain, and was used to capture connectivity on that
side. OviINs receive input from, and send output to, each individual neuron
within the box. Arrow marked by * indicates that no individual group G (right)
synapses onto oviIN (right) with ten or more synapses.
Recurrent neural circuits have been proposed as a mechanism for
rising or persistent neuronal activity44,45. Here we describe a small, ana-
tomically recurrent circuit where silencing activity in any constituent
cell class eliminates egg laying. Although we have not yet measured
physiological activity in all circuit constituents during egg laying, we
speculate that synaptic interactions in this circuit contribute to the
generation of a rising or persistent oviDN spike rate, which is then
integrated by oviDN’s slow calcium dynamics to create the signal we
report in this paper.
If one compares a fly’s decision to lay an egg in an environment with
several discrete substrate options20 with a human’s decision to choose
a dish at a restaurant, there are interesting parallels. Both processes
start with an initiation event: ovulation in flies or opening a menu in
humans. Then the individual’s own behaviour reveals new options over
time—that is, more egg-laying substrates to the fly walking around an
environment or more dish options to the human scanning the menu.
Finally, the decision is terminated when one option is selected and a
motor programme, of varying complexity and delay relative to the
end of the decision, is implemented. This analogy highlights that the
process characterized herein may help to inform decision-making
quite broadly.
Online content
Any methods, additional references, Nature Portfolio reporting sum-
maries, source data, extended data, supplementary information,
570 | Nature | Vol 619 | 20 July 2023
acknowledgements, peer review information; details of author contri-
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Methods
Flies
Flies (D. melanogaster) were reared on a standard cornmeal medium at
25 °C, ambient humidity and 12/12 h light/dark cycle unless otherwise
noted. Genotypes and conditions for each experiment are described
in Supplementary Tables 1 and 2, respectively. Supplementary Table 1
also lists the source of each genotype.
Egg-laying chamber with sloped ceiling
We designed a new chamber for imaging egg laying in freely walking
flies, which enforced them to remain in a tarsi-down body posture on the
agarose at all times. The flies could not tilt their bodies in this chamber
and thus they could not walk on the side walls or ceiling. This constraint
meant that the flies’ bodies were always in the same general orientation,
parallel to the imaging plane, making quantitative measurements of
postural parameters more straightforward with a single camera view.
Chambers were made by sandwiching and tightly screwing layers of
acrylic and three-dimensionally (3D)-printed plastic and then fitting a
glass ceiling (Extended Data Fig. 1a). The acrylic layers were laser-cut
(VLS6.60, Universal Laser Systems). The side-wall layer was 3D-printed
using VisiJet M3 Crystal plastic material (Projet 3510 HD Plus, 3D Sys-
tems). The glass was treated with Sigmacote (Sigma-Aldrich) to make
it slippery to a fly’s tarsi—preventing walking on the ceiling46. Glass was
retreated with Sigmacote after roughly ten uses. The 3D-printed spacer
layer incorporated a sloped edge that kept the fly completely parallel
to the imaging plane by preventing access to the side of the chamber
(Extended Data Fig. 1a). The sloped-ceiling design was inspired by a
sloped-floor plastic chamber46. A sloped floor does allow the fly to tilt
and thus was not suitable for our application.
Chambers were used multiple times and washed before each use.
They were assembled with only the two bottom layers and then cooled
at 4 °C. Fresh substrate containing 1% agarose (SeaKem LE Agarose,
Lonza), 0.8% acetic acid and 1.6% ethanol was pipetted to completely
fill the well around 5 h before each assay. Careful pipetting with only
the two bottom layers assembled was critical to forming a flat layer of
agarose—preventing the formation of a meniscus, which would allow
the fly to tilt. Acetic acid and ethanol were included to help simulate
rotten fruit and generally promote egg laying7. After solidification of
the agarose solution (about 1 h) the chamber was fully assembled, minus
the glass ceiling, and equilibrated at room temperature.
Females were separated on their day of eclosion and group housed
in vials. At age 3–6 days around 20 females were exposed to about
20 Canton-S males in an empty bottle with wet yeast paste and a Kim-
wipe (Kimberly-Clark) soaked with 2 ml of water. The wet yeast paste
was applied to the side of the bottle and comprised 1 g of dry yeast
(Fleischmann’s) and 1.5 ml of 4.25 mM putrescine dihydrochloride in
water. This treatment allowed females to mate and caused them to
accumulate many eggs. Flies fed with yeast7,47 or putricine48 increase
the number of eggs they develop. These eggs are retained by the flies
during the treatment period because they lack a soft medium for egg
deposition7. After about 24 h, individual gravid females were placed
into chambers under gentle cold anaesthesia from which they typi-
cally recovered within 30 s. Because we had only one imaging setup for
these high-resolution experiments (see below), and the ability of a fly
to tilt was sensitive to both its size the exact level of agarose, multiple
flies were loaded in independent chambers (Extended Data Fig. 1a)
and the fly with the least ability to tilt was chosen for imaging for a few
hours in near-complete darkness (under a shroud) at around 24 °C and
40–60% humidity.
For imaging eggs inside the fly’s body, a 470 nm LED (pE-100,
CoolLED) double filtered (optical density (OD) 4 475 nm and OD 4
500 nm shortpass, Edumund Optics) provided excitation light at
30 µW mm–1. This excitation light arrived at the fly from below after
first passing through the agarose substrate. Videos were recorded using
HCImage software (Hamamatsu) at ten frames s–1 (fps) with 100 ms
exposure time per frame, using an ORCA-Fusion C14440-20UP camera
(Hamamatsu) equipped with a 15.5–20.4 mm Varifocal Lens (Computar)
and two 510 nm longpass filters (Chroma). We used GCaMP3, rather
than the more recent GCaMP variant, for imaging of eggs because a
UASp-driven GCaMP3 transgene, which is more highly expressed in
the female germline than the traditional UAS49, was constructed in a
previous study15 and available for use without the need to generate a
new transgenic fly.
For imaging of body posture, 850 nm LEDs illuminated the arena
from above through a white acrylic diffuser (1 µW mm–2 at the fly).
Videos were recorded at 25 fps using FlyCapture software (FLIR) and
a GS3-U3-41C6NIR-C Grasshopper camera (FLIR) equipped with a
15.5–20.4 mm varifocal lens and a 780 nm longpass filter (MidOpt).
DeepLabCut50 was used for offline tracking of body parts, including the
neck and ovipositor. DeepLabCut models were iteratively fine-tuned
by identification of poorly tracked frames in iteration i and adding
them to the training dataset for iteration i + 1. A total of 1,568 training
frames were manually annotated. DeepLabCut output coordinates
were filtered by setting coordinates to not-a-number (NaN) if either
(1) the probability score was less than 0.95 or (2) the body part jumped
more than an empirically determined distance in consecutive frames.
Ovulation start was manually annotated as the first frame in which the
abdomen appeared to begin the elongation process. Search start was
manually annotated as the first frame in which the abdomen returned
to a stable neutral posture after ovulation. Abdomen bend complete
was manually annotated as the frame in which the bend to lay an egg
was completed (abdomen maximally deflected). Identification of the
frame in which the abdomen bend was completed was much easier than
attempting to identify when the abdomen bend was initiated. Note
that, although flies bend their abdomen to deposit an egg, they also
bend their abdomen for other reasons. Some non-egg-laying-related
reasons a fly could bend its abdomen include defaecation, grooming
and sampling the substrate with sensory organs near the ovipositor.
‘Egg deposited’ was manually annotated, often with assistance from a
computer algorithm. Briefly, our computer code found groups of pixels
whose intensities stably changed at a particular frame in the video.
The output frame numbers from the code pointed an experimenter to
video frames proximal to egg deposition, and the exact frame for egg
deposition was adjusted manually. Videos were also carefully inspected
by an experimenter to identify eggs missed by the code. This code
markedly accelerated manual annotation and was particularly useful
for high-throughput egg-laying choice chambers where thousands
of eggs were annotated (see below). The first frame in which half of
the egg was visible (emerging from the ovipositor) was annotated as
the egg-deposited frame.
High-throughput egg-laying choice chamber
We designed a new chamber for studying egg-laying choice behaviour
with high throughput. This chamber ensured that the fly was nearly
always in contact with an agarose egg-laying substrate option. The sub-
strate on which the fly was standing could be unambiguously defined by
its y position and orientation. In previous egg-laying choice studies8,51,52,
flies could walk on the side walls or ceiling and yet were assigned to a
substrate beneath them during tracking, which makes it very hard to
determine how previous substrate experiences influence the decision
to lay an egg.
Chambers were made by sandwiching and tightly screwing layers
of acrylic or Delrin plastic and then affixing a glass ceiling (Extended
Data Fig. 1b). Acrylic and Delrin plastic were laser-cut and the glass was
treated with Sigmacote.
Chambers were used multiple times and washed before each use.
They were assembled without the glass ceiling and cooled at 4 °C. Fresh
substrate (1 ml, containing 1% agarose, 0.8% acetic acid and 1.6% etha-
nol) was pipetted to fill the acrylic well and form a meniscus with the
ArticleDelrin plastic spacer about 5 h before each assay. The meniscus ensured
that the fly could not walk directly on the side (Delrin plastic) of the
chamber and was inspired by plastic chambers with a sloped floor46.
Quantitative measurements of body posture were not possible because
flies could tilt by walking on the meniscus. Sucrose-containing sub-
strates were supplemented with the appropriate amount of sucrose.
Acetic acid and ethanol were uniformly distributed in all substrates.
Following solidification of the agarose solution (about 1 h), the chamber
was equilibrated at room temperature.
These egg-laying chambers and assay protocols were specifically
designed to minimize the following confounds: (1) diffusion between
substrate islands; (2) visual landmarks; (3) fly-to-fly communication;
(4) olfactory landmarks; (5) temperature and humidity fluctuations;
and (6) variability in fly rearing. Diffusion was minimized by a barrier
of approximate width 2.5 mm between the substrate islands and by
loading the agarose at 4 °C. Visual cues were minimized by conducting
the assay in near-complete darkness. Illumination of 850 nm, to which
the fly’s visual system has no measurable sensitivity53–55, was provided
from below for tracking (1 µW mm–2 at the agarose beneath the fly).
Fly-to-fly communication was minimized by assaying individual flies
in isolated chambers separated by an opaque Delrin plastic spacer.
Olfactory landmarks were minimized using a non-volatile compound,
sucrose, as the sole varying variable. Temperature and humidity were
kept constant by conducting experiments in an environmental room
(24 °C with 40–60% humidity). Air exchange was made possible by
four small ventilation holes in each barrier. Variability in fly rearing
was minimized by controlling age, mating status, food history and
circadian time.
Females and males were separated on their day of eclosion and
group housed in vials. At age 3–6 days at zeitgeber time (ZT) 6 (that
is, 6 h after lights on), around 20 females were exposed to around
20 Canton-S males in an empty bottle with only wet yeast paste and
a Kimwipe soaked with 2 ml of water. Putrescine was not added to the
yeast paste in these experiments. On the following day at ZT 8, indi-
vidual females were placed into egg-laying chambers under gentle cold
anaesthesia. Videos were acquired at 2 fps using FlyCapture software
with either a FMVU-03MTM-CS Firefly or FL3-U3-13Y3M-C Flea3 camera
(FLIR) equipped with either a LM12HC (Kowa), HF12.5SA-1 (Fujinon) or
CF12.5HA-1 (Fujinon) lens and a 780 nm longpass filter. The x–y posi-
tion and orientation of each fly was determined offline using Ctrax56.
We assigned a fly to a substrate depending on whether its centroid was
above or below the midline of the acrylic barrier. This simplification
was appropriate because the acrylic barrier of roughly 2.5 mm (a fly
is around 2.5 mm long) practically prevented a fly from standing on
both substrates simultaneously, and a Canton-S fly spent only 1.5% of
its time in an orientation where all tarsi were likely to be on the bar-
rier. Note that flies do not lay eggs on the plastic barrier (or any plastic
used in this study) because it is too hard. Egg deposition was manu-
ally annotated, often with the assistance of a computer algorithm, as
described in the previous section. The first frame in which half of the
egg was visible (emerging from the ovipositor) was annotated as the
egg-deposited frame. Annotations by an individual human annota-
tor or across multiple human annotators were reproducible to ±four
frames or ±2 s.
For Kir2.1* or Kir2.1*Mut experiments we expressed a GAL80ts
transgene in all cells (with the tubulin promoter)57 during develop-
ment to minimize transcription of Kir transgenes days before assay-
ing egg-laying behaviour. At 18 °C, GAL80ts masks the transcription
activation domain of GAL4, thus preventing transcription of the
GAL4-UAS-controlled transgene. We could remove the GAL80 block
on Kir expression by increasing the flies’ temperature for about 1 day
before our egg-laying assays. Specifically, for these experiments: (1) flies
were reared at 18 °C; (2) at ZT 6 flies were moved to 31 °C for induction
of Kir2.1* or Kir2.1*Mut transgene expression; and (3) the following day
at ZT 5 (23 h later), flies were returned to 18 °C. Egg-laying assays were
performed at ZT 8 at 24 °C. For one set of controls in Fig. 5i, flies were
not moved to 31 °C and instead were kept at 18 °C.
For GtACR1 (refs. 24,58) experiments, flies were kept under low white
light (approximately 3 nW mm–2 measured at 567 nm) from egg to adult-
hood. At approximate age 5–6 days at ZT 6, around ten females were
exposed to around ten Canton-S males in an empty bottle with only wet
yeast paste and a Kimwipe soaked with 2 ml of 200 µM all-transretinal
in water (also kept under low white light). Wet yeast paste was applied
to the side of the bottle and comprised 1 g of dry yeast with 1.5 ml of
200 µM all-transretinal in water. Egg-laying assays were performed
the following day at ZT 8. Light (567 nm) was provided from above
(29 µW mm–2 at the fly; Rebel Tri Star LEDs, LuxeonStarLEDs). Controls
for genotype were siblings of experimental flies that were treated identi-
cally except that no light was provided from above. Controls for light
were flies ‘expressing’ GtACR1 with either an empty-split (empty-SS) or
empty-GAL4 driver. Additional controls for light with twice the intensity
(57 µW mm–2) provided additional assurance that light alone was not
preventing egg laying (data not shown).
Construction of Kir2.1* and Kir2.1*Mut flies
We serendipitously identified that Kir2.1*25 (based on the mouse
sequence for the gene, see below) hyperpolarizes oviDNs more gen-
tly than the human Kir2.1 traditionally used in flies23,59,60 (Fig. 5c versus
Fig. 5a). A matched control channel, Kir2.1*Mut25, does not conduct
ions and enabled genetic-background-matched comparisons. A simi-
lar strategy of using Kir2.1 paired with a non-conducting control was
recently used in flies61, although with the human variant of the gene.
Kir2.1* and Kir2.1*Mut sequences were taken from a previous study
in mice25. Briefly, Kir2.1* and Kir2.1*Mut are modified wild-type mouse
Kir2.1 channels (KCNJ2)—with either two mutations (Kir2.1*: E224G,
Y242F) or five mutations (Kir2.1*Mut: E224G, Y242F, G144A, Y145A,
G146A). Both transgenes were fused at their C-terminals with a T2A
sequence to a tdTomato. To port these constructs into Drosophila, they
were inserted between the Xba1 and Not1 sites of pJFRC81 (ref. 62) and
introduced into the attP40 landing site by ΦC31 integrase-mediated
transgenesis (transgenic fly lines were generated by BestGene). Kir2.1*
and Kir2.1*Mut transgenes differ in protein sequence—and possibly
in other ways (for example, transcription and translation)—from the
wild-type human Kir2.1 (KCNJ2) transgenes traditionally used to hyper-
polarize neurons in flies23,59,60. Previous in vivo fly electrophysiology of
central brain and visual system neurons expressing traditional human
Kir2.1 (refs. 63,64) transgenes showed larger hyperpolarization than
the approximately 14 mV hyperpolarization observed here with Kir2.1*
(Fig. 5d).
Automated estimation of search period in free-behaviour,
high-throughput choice chambers
Because we did not have a quantifiable view of the abdomen in our
high-throughput choice chambers (Extended Data Fig. 1b,c), we
used locomotor speed as a proxy for search onset (Extended Data
Fig. 1d–f) and egg deposition as a proxy for abdomen bending to lay an
egg (Fig. 1c). The end of the search period was the annotated moment
of egg deposition (rather than the abdomen bend to lay the egg). For
each egg, the start of the search period was determined by smoothing
the locomotor speed trace before egg deposition with an 18.5 s boxcar
filter and identifying the first frame in which the smoothed signal fell
below 0.1 mm s–1. Due to the length of the boxcar filter, the minimum
search duration was 9 s. These parameters were empirically established
to produce search onset times that were consistent with what an expert
human annotator would highlight in visual analysis of the data.
Calculation of egg-laying rates as a function of time since the
last substrate transition in free-behaviour choice chambers
Egg-laying rates as a function of time (Fig. 3f–h) were calculated as
follows. Before performing any calculations, we combined the data
obtained from all flies tested in a particular chamber type. First, we iter-
ated through each time bin on the x axis and, for each bin, we counted
the number of egg-deposition events assigned to that bin, denoted as
#eggs(bin). Next, we repeated the iteration for the same time bins and
tallied the number of video frames in which the flies were assigned to
that time bin, referred to as #frames(bin), during a search period. Finally,
we performed another iteration for the same time bins and recorded
the number of times flies changed assignments into that bin, termed
#visits(bin), during an egg-laying search period (that is, we didn't keep
incrementing the ‘visits’ counter if the fly remained in a particular time
bin from one frame to the next).
To determine the mean egg-laying rate, we computed #eggs/#frames for
each bin. Because the videos were recorded at 2 Hz, we multiplied the
value obtained for each bin by 120 to convert it to units of eggs min–1.
To determine the confidence interval for each bin we utilized the Clop-
per–Pearson method, also known as the ‘exact’ binomial confidence
interval, to compute the 90% confidence interval for #eggs/#visits. We then
transformed the confidence interval for each bin to units of eggs min–1
by multiplying it by 120 × #visits/#frames. The confidence interval could
not be directly calculated from #eggs/#frames because it would then be
contingent on the video frame rate.
For these rate curve calculations, search periods with duration
shorter than 30 s were set to 30 s. This prevented very brief search peri-
ods from introducing fluctuations in the rate functions (by contributing
to the numerator and not contributing much to the denominator). By
doing so, the rate curves exhibited less variation across replicates or
conditions. Note that search periods already had a minimum duration
of 9 s, which was automatically determined by the search period calcula-
tion (Methods). Altering the definition of the search period, or having
no minimum search duration, does not change our stated conclusions
from these curves20. Additionally, the use of different x-axis bins yields
qualitatively similar results and does not change our stated conclu-
sions. Rate functions start with low rates after a transition, at least
partially, because flies do not lay eggs on the plastic barrier between
substrates (Extended Data Fig. 7e,f) and because flies are, by defini-
tion, walking (and not pausing to deposit an egg) during a transition
(Extended Data Fig. 7g).
Design of egg-laying wheel and setup under microscope
We designed a wheel on which tethered flies walked and laid eggs on
agarose-based egg-laying substrates. The design was optimized to
maximize a fly’s ability to lay eggs and rotate the wheel.
The wheels were 3D printed from VisiJet M3 Crystal plastic using a Pro-
jet 3510 HD Plus 3D printer (Extended Data Fig. 2a). A pivot (N-1D, Swiss
Jewel) was press-fit through the centre hole and not removed. Wheels
were washed before each use. Three wells were available for loading the
same or different agarose-based substrates. Each well was separated
by a 1 mm barrier. Wheels were loaded with fresh agarose substrate
(as prepared for free-behaviour choice chambers) using a 3D-printed
agarose-injecting mould (VisiJet M3 Crystal material) that was cooled
on ice (Extended Data Fig. 2b). Food colouring (HY-TOP assorted
food colouring) was added at a dilution of 1:10,000 to the agarose solu-
tion before loading so that wheel quality could be visualized. Wheels with
any mixing between wells were discarded. Food colouring at 2.5-fold
this concentration, or the presence of VisiJet M3 Crystal material, did
not affect choice in free-behaviour control experiments (Extended Data
Fig. 2d). After solidification of the agarose was, the wheel and pivot
were suspended between two spring-loaded bearings (VS-30, Swiss
Jewel) threaded into clear acrylic that was press-fit into a 3D-printed
base (UMA-90 material printed on a Carbon DLS, Protolabs) (Extended
Data Fig. 2c). This wheel assembly was stored in a custom humidification
chamber to prevent the thin layer of agarose from drying and to allow
the wheels to equilibrate to room temperature. Wheels were used within
2 h of preparation. When ready, a wheel assembly was secured in a small
custom humidification chamber (roughly 90% humidity) positioned
under the microscope objective. The wheel–pivot combinations used
in this study had a weight of 87.9 ± 0.3 mg (mean ± s.d.) without agarose
and 146.4 ± 0.8 mg with agarose. For reference, a single gravid female
weighs around 1.4 mg and a typical foam ball used for fly walking experi-
ments65,66 weighs 40–46 mg. Most of the wheel’s weight is due to the
agarose and the wells needed to hold it. A variety of lighter and synthetic
materials less prone to evaporation were screened in free-behaviour
assays, but egg laying was suppressed in all of them.
The fly was viewed using two CM3-U3-13Y3M Chameleon cameras
(from the sides) and one FMVU-03MTM-CS Firefly camera (FLIR) from
the front, and videos were captured using FlyCapture software. Two
850 nm LEDs, from front left and front right, illuminated the fly at
5 µW mm–2. Cameras were equipped with a 15.5–20.4 mm varifocal
lens and either a 900 nm shortpass (Thorlabs) or 875 nm shortpass
(Edmund Optics) filter to dampen visibility of the 925 nm two-photon
excitation light. Cameras had an exposure time of 16 ms and were trig-
gered synchronously using a single external trigger source at 25 fps
(Arduino Uno, Arduino). A side-facing camera recorded the fly and a
single dot painted on the wheel. The dot was painted in a consistent
location on the wheel that was defined by an embossed 3D-printed
feature. The dot was tracked using DeepLabCut (1,109 training frames,
with training and filtering as in the free-behaviour DeepLabCut model).
The dot position was converted to wheel degrees by fitting the set of
all dot positions to a circle and then computing a wheel angle for each
frame. A single frame in which the fly’s centroid straddled the dot was
used to convert the wheel angle to the fly’s position on the wheel. This
alignment consistently meant that the fly’s neck was situated on the
plastic-to-next-substrate boundary during a detected substrate transi-
tion. A second side-facing camera was used for a close-up view of the
fly’s body. DeepLabCut was used to track body parts including the neck,
ovipositor and tip of the proboscis (2,259 training frames, with training
and filtering as in the free-behaviour DeepLabCut model). Normalized
length was calculated by subtracting the x-coordinates of the neck
and ovipositor in each frame and dividing by the median of this value
for each recording (Fig. 1i). The median length in free behaviour was
approximately 2.35 mm (Extended Data Fig. 1e–h), although we did
not measure this value on the wheel. We used this normalized-length
metric because it can quantify both an elongated and a bent abdomen
and is similar to the neck–ovipositor length measured in free behaviour.
Despite the similarity with free-behaviour length, we noticed, on aver-
age, a slight difference in the signature of abdomen bends (Extended
Data Fig. 1g compared with Fig. 1l), possibly due to the curvature of the
wheel. The body angle (°) was the angle between the neck and ovipositor
(Fig. 1i). Larger angles indicated a more bent abdomen. Although a fly
must bend its abdomen to lay an egg, the magnitude of a physiologically
relevant deflection of body angle (as measured in degrees) is not that
large (Fig. 1i). ‘Normalized neck to proboscis length’ was calculated by
determining the Euclidean distance between the tip of the proboscis
and the neck in each frame and dividing by the median of this value for
each recording. This underestimated the true deflection of the pro-
boscis because the proboscis does not start at the neck. The neck was
used as an origin point because robust tracking was easy. A front-facing
camera was used to align the fly on the centre of the wheel width. The
body posture slightly varied among flies due to slight differences in
tethering. To achieve egg laying it was very important to position the
fly at a point on the wheel circumference, and at a vertical distance
from wheel, that maximized perpendicular contact of the ovipositor
to the substrate when the abdomen was bent while still allowing the
fly to walk on the wheel. In some cases flies had to be positioned close
to the wheel which, unfortunately, decreased the dynamic range of
abdomen bending. A total of 104 flies were imaged to collect the data
shown in Fig. 1h. The majority of flies did not lay eggs because, among
other considerations, flies often require several hours to start laying
their clutch of eggs (even in free behaviour). We could not image, con-
veniently, for 18 h to wait for a clutch to start.
ArticleMoments of distinct behaviours (as in Fig. 1h and Extended Data
Fig. 2g) were annotated manually by inspection of behaviour videos
while remaining blind to any neural signals (∆F/F). Ovulation start was
defined as the first frame in which the abdomen appeared to begin the
elongation process; ‘abdomen at its longest’ was the frame in which
the abdomen was maximally stretched; ‘abdomen scrunch start’ was
the first frame in which the abdomen assumed a stable scrunched posi-
tion; search start was defined as the first frame in which the abdomen
returned to a stable neutral posture after ovulation; abdomen bend
complete was defined as the frame in which the first bend before egg
laying was complete (abdomen maximally deflected); egg deposited
was defined as the frame in which half of the egg was visible; and ‘ovi-
positor cleaned’ was defined as the frame in which the first abdomen
bend following egg laying was complete.
For CsChrimson18 optogenetics experiments, a 660 nm LED cou-
pled to a 1-mm-wide fibre-optic cable (M660F1 and M35L01, Thor-
labs) was focused on the front midpoint of the fly’s head using a lens
set (MAP10100100-A, Thorlabs). This wavelength is at the tail end of
the sensitivity of the fly visual system53–55, which helps to minimize
light-related confounds. Two longpass filters—OD 4 550 nm and
OD 4 575 nm (Edmund Optics)—minimized the ability of LED light to
enter the two-photon detector path, which collected the GCaMP sig-
nal. The incident area of the LED was adjusted to be of sufficient width
(approximately 3 mm in diameter) to cover the whole front of the fly,
from the part of the head glued to the custom holder to the tips of the
tarsi (see Extended Data Fig. 2f for representative fly positioning), such
that all CsChrimson-expressing oviDN cell bodies and neurites in the
brain could be stimulated. CsChrimson-expressing oviDN neurites
and synapses in the abdominal ganglion (situated in the thorax) were
also probably stimulated—albeit to a lesser degree due to obstruction
from the head, proboscis and front tarsi—because the whole front of
the fly head and body was illuminated. LED intensity was controlled by
adjusting the duty cycle of a 490 Hz PWM signal (Arduino Uno, Arduino)
that was fed into an LED driver (T-Cube, Thorlabs). The CsChrimson
stimulation intensity for Fig. 2a–e was 641 µW mm–2. For Fig. 2f–h and
Extended Data Fig. 6a–f, intensities were 641, 159, 148 and 136 µW mm2.
For the prolonged, gradual CsChrimson experiments in Extended Data
Fig. 5o–s, data from three separate stimulation paradigms were com-
bined: 159 µW mm–2 was applied (1) at 100 ms on, 400 ms off, for 30 s;
(2) at 100 ms on, 900 ms off, for 39 s; or (3) at 50 ms on, 950 ms off,
for 50 s. Sample traces shown in Extended Data Fig. 5o are both from
stimulation paradigm (3). For Extended Data Fig. 6l–p, intensity was
approximately 148 µW mm–2.
Treatment of flies for tethered egg-laying and optogenetic
experiments
Females and males were collected on their day of eclosion and group
housed together in standard cornmeal medium vials supplemented
with 2.5 mM putrescine dihydrochloride and wet yeast paste. Wet
yeast paste was applied to the side of the vial and comprised 1 g of dry
yeast and 1.5 ml of 4.25 mM putrescine dihydrochloride in water. At
around age 5–6 days, females were gravid because larvae occupied the
cornmeal medium and there was no additional room to deposit eggs.
This treatment was more convenient than that used in free-behaviour
choice experiments and was inspired by separate aspects of two stud-
ies8,48. Free-behaviour controls indicated that this treatment increased
the number of eggs laid by a fly without affecting choice behaviour
(Extended Data Fig. 2e).
For CsChrimson optogenetics experiments, flies were treated as
above but were kept under low white light (about 3 nW mm–2 measured
at 660 nm) from egg to adulthood. At around age 5–6 days, roughly
20 females were exposed to around 20 Canton-S males in an empty
bottle containing only wet yeast paste and a Kimwipe soaked with 2 ml
of 200 µM all-transretinal in water (also kept under low white light). Wet
yeast paste was applied to the side of the bottle and comprised 1 g of dry
yeast with 1.5 ml of 4.25 mM putrescine dihydrochloride and 200 µM
all-transretinal in water. Flies were tethered about 24 h later. Flies for
CsChrimson control experiments were always treated identically to
CsChrimson-expressing flies.
Flies were anaesthetized at roughly 4 °C and tethered to a custom
holder67, except where the back wall of the pyramid leading up to the
fly was tilted at an angle rather than rising at 90°, to allow more light
from the brain to reach the objective66 (Fig. 1d). The head was pitched
forward during tethering to provide a view of oviDN cell bodies. For
electrophysiology the head was inserted deeper into the holder for
unobstructed access to oviDNs with electrodes. Flies were attached
to the holder with blue-light-cured glue (Bondic). The proboscis was
gently extended and the dorsal rostrum glued to the head capsule. This
prevented brain movement associated with proboscis extension but
still allowed measurement of proboscis extension (albeit with a smaller
dynamic range than natural proboscis extension). Extracellular saline
solution was added to the holder well (bath) and a window was cut in
the cuticle with a 30-gauge needle (BD PrecisionGlide). The cuticle
and some trachea were removed with forceps to expose the posterior
aspect of the brain. The holder was stabilized with magnets above the
egg-laying wheel inside a small custom humidification chamber.
Extracellular saline68 comprised 103 mM NaCl, 3 mM KCl, 5 mM
N-Tris(hydroxymethyl) methyl-2-aminoethanesulfonic acid, 10 mM tre-
halose, 10 mM glucose, 2 mM sucrose, 26 mM NaHCO3, 1 mM NaH2PO4,
1.5 mM CaCl2 and 4 mM MgCl2. Osmolarity was 280 ± 5 mOsm and pH
was 7.3–7.4 when bubbled with 95% O2/5% CO2. The temperature of
the bath was set to around 17–22 °C by flowing fresh saline through
a Peltier device with feedback from a thermistor in the bath (Warner
Instruments).
Calcium imaging
We used a two-photon microscope with a moveable objective (Ultima
IV, Bruker) and custom stage (Thorlabs, Siskiyou). The microscope
was controlled by Prairie View software (Bruker) and was enclosed by
a black shroud. A Chameleon Ultra II Ti:Sapphire femtosecond pulse
laser (Coherent) filtered by a 715 nm longpass filter (Semrock) provided
925 nm two-photon excitation. Emission light from the brain was col-
lected by a ×16/0.80 numerical aperture (NA) objective (×16 W CFI75
LWD, Nikon), split by a 565 nm dichroic and filtered by a 490–560 nm
bandpass filter (Chroma) before entering GaAsP detectors (Hama-
matsu). For CsChrimson optogenetics experiments the emission light
was split by a 525 nm dichroic and filtered by both a 490–510 nm and a
480–520 nm bandpass filter (Chroma) to prevent optogenetic stimu-
lation light from entering the detector. A Piezo motor was used for
volumetric scanning.
A range of optical zooms, z-slice number, z-slice separation, fields of
view, laser powers (6–30 mW at the specimen) and frame rates (mean of
1.5 Hz) were used over the course of experiments on oviDN dynamics.
Individual data traces were inspected by eye and the reported results
were robust to the range of parameters used. All recordings had multi-
ple z-slices within, above and below the cell body permitting effective
quantification of recordings with slight z-drift over hours of recording.
For example, in Fig. 1g, 14 z-slices were taken at 3 µm steps and only
around five or six of these included fluorescence from the oviDNb cell
body. The length of each recording (mean of 75 min) varied depend-
ing on (1) the perceived health of the fly, (2) the likelihood of future
egg-laying events (which were higher if the fly had already laid an egg),
(3) the amount of z-drift and (4) the quality of the agarose wheel, which
sometimes visibly dried over a period of hours. The experimenter was
blind to correlations between the neural signal and behaviour during
the vast majority (roughly 95%) of recordings. Flies were excluded only if
a technical issue arose (for example, errors in synchronizing behaviour
with two-photon imaging or saline leaking from the holder). Only eggs
with continuous two-photon imaging from 240 s before to 30 s after
egg deposition were analysed.
For CsChrimson optogenetics experiments supporting a rise-to-
threshold mechanism (Fig. 2f–h), two-photon imaging parameters were
held relatively constant (mean frame rate of 1.5 Hz and two-photon laser
power of approximately 10.5 mW). CsChrimson stimulation intensities
were determined in pilot experiments. Periodic stimulation cycling
four intensities was applied for 5 s every 2 min. The experimenter was
blind to correlations between the neural signal and behaviour during
all these recordings.
For CsChrimson manual stimulation experiments (Fig. 2a–c),
stimulations were initiated by the experimenter while observing the
real-time behaviour of the fly. Stimulations were initiated, on average,
roughly once every 7.5 min. Manual stimulations were typically halted
if the fly began to ovulate or it showed signs that it would ovulate soon
(that is, pausing and slight abdominal elongation). Once ovulation was
complete, stimulation was triggered when the fly’s abdomen was not
touching the substrate (and before any indication that a spontaneous
egg-laying event was about to take place). The traces shown in Fig. 2a
(and associated Supplementary Video 5) are representative of our
manual stimulation protocol. We used manual stimulation because
it resulted in around a twofold higher rate of eggs laid than periodic
stimulation, and also it allowed us to activate oviDNs after ovulation
but before spontaneous egg laying.
For the CsChrimson optogenetics experiments shown in Fig. 2c,
two-photon imaging data are shown for only five of the nine flies
whereas behavioural data are shown for all nine. The four flies for which
we do not show imaging data had bleed-through artefacts in the GCaMP
signal from the CsChrimson illumination LED because these data were
collected before optimization of the detection path for minimization
of this artefact.
Two-photon imaging frames were motion corrected using either cus-
tom scripts from a previous study66 or CaImAn69. The regions of interest
(ROIs) for a cell body were drawn manually for each z-plane using the
time-average of each. ROIs were drawn around the outer boundary
of the cell body. The brighter of the two cell bodies in oviDN-SS1 was
assigned to be oviDNb (see Extended Data Fig. 3c, in which we show
that the brighter of the two cells in oviDN-SS1 is oviDNb). In a few cases
in which the brighter cell was not obvious, ROIs encompassing both
cell bodies were drawn and assigned to be oviDNb. For a given imag-
ing volume time point, the individual pixel intensities in all individual
z-plane ROIs for a given cell were pooled and averaged, Fcell(t). An identi-
cal average was calculated for a background volume of pixels that did
not overlap the oviDN soma, or any other soma or neurite, Fbackground(t).
Before calculation of ∆F/F we subtracted the background from the
cell, Fcell_actual(t) = Fcell(t) – Fbackground(t). This eliminated non-cell-specific
signal such as autofluorescence and constant detector background.
This subtraction also made ∆F/F robust to variations in the number of
background pixels included in ROIs drawn around the outside of a cell.
∆F/F was calculated using the formula (Fcell_actual(t) – F0(t))/F0(t), where
F0(t) is the running mean of Fcell_actual(t) over a 20 min window. The mean
over a long time frame was used to estimate a baseline, systematically,
for the continuously fluctuating oviDN signal. A similar running mean
baseline estimate (albeit with a much shorter window) was previously
used to quantify continuously fluctuating dopaminergic signals in
mammals70. A ∆F/F of 0.35, for example, indicated that the fluores-
cence signal in the cell was 35% greater than the 20-min-mean signal in
the cell. If the GCaMP7f fluorescence signal is linear with [Ca2+] in this
range it would indicate that [Ca2+] in the cell had increased by 35% over
the 20-min-mean [Ca2+] in the cell. All stated conclusions were robust
to three different methodologies for calculation of ∆F/F, including
methods where F0 remained constant. For CsChrimson experiments,
F0(t) was the running mean of Fcell_actual(t) over a 20 min window after the
105 s post-triggering CsChrimson stimulation had been set to NaN. This
very conservatively prevented any CsChrimson stimulations, or linger-
ing effects, from artificial influence of F0(t). Note that, because both
baseline spike rate and Vm are higher for flies expressing CsChrimson
(Extended Data Fig. 6a,b; approximately 12 spikes s–1 and –44 mV)
than for those that are not (Extended Data Fig. 9a; approximately four
spikes s–1 and –57 mV), we would expect the mean GCaMP signal that
we use for normalization in CsChrimson flies to be reflective of higher
calcium concentrations, resulting in lower ∆F/F values for the same
absolute calcium concentration. For this reason—and because our
genetic driver in CsChrimson experiments is expressed in only two of
three oviDNs per side—quantitative comparisons of ∆F/F in CsChrimson
and non-CsChrimson flies are not warranted.
Two-photon imaging-frame pulses, behavioural camera frame trig-
gers and optogenetic LED triggers were all digitized at 10 kHz on a
Digidata 1440A (Molecular Devices) and saved to a computer (Axo-
scope, Molecular Devices). To assign a timestamp to a volume scan
we identified the moment that the two-photon volume scan was half
complete. To assign a timestamp to a behavioural camera frame we
used the beginning of the 16 ms camera exposure period. Calcium
imaging was interpolated and behavioural data were downsampled to
a common 10 Hz array for all population analyses. Each 100 ms time
point was assigned the calcium imaging and behaviour data value from
the closest previous respective timestamp (that is, previous neighbour
interpolation). A relatively large 100 ms time base was chosen because
faster sampling was unnecessary for the current analyses and would
be computationally time consuming given the 200+ h of two-photon
scanning collected. In the case of triggered averages, the zero point was
either the timestamp for the behaviour camera frame with the behav-
iour of interest or the frame with the onset of optogenetic stimulation.
In the case of cross-correlations, the zero point was the timestamp of
the first acquired two-photon volume.
Electrophysiology
We used the same two-photon microscope for both calcium imaging
and patch-clamp electrophysiology. The microscope was controlled
by either Prairie View (Bruker) or µManager71 software. A 470 nm LED
(pE-100, CoolLED) provided excitation through the objective to identify
2× EGFP- or GCaMP7f-positive neurons. An 850 nm LED coupled to a
400-µm-wide fibre-optic cable (M850F2 and M28L01, Thorlabs) was
focused on the fly’s head to illuminate cells for patch-clamping using a
lens set (MAP10100100-A, Thorlabs). Both LEDs were turned off when
recording electrophysiology data. A ×40/0.80 NA objective (LUMPLFLN
40XW, Olympus) and CoolSnapEZ CCD camera (Photometrics) were
used for patch-clamping.
Cell bodies were exposed by breaching the neural lamella and peri-
neural sheath using gentle application of 0.5% collagenase IV (Wor-
thington) in extracellular saline via pipette. We applied collagenase IV
to a small 30 × 30 µm2 area containing the cell bodies of interest67.
Collagenase was applied using a 4–6-µm-tip micropipette with
8–80 mmHg positive pressure at around 30–32 °C for about 3 min.
Once the cell bodies were exposed, the bath was returned to about
19–21 °C and flushed free of collagenase.
Borosilicate glass (outer diameter 1.5 mm, inner diameter 0.86 mm,
with filament) was pulled to create a 7–15 MΩ electrode with a 1.0–
1.5 µm tip using a model P-1000 micropipette puller (Sutter Instru-
ments) and fire-polished with a MF-900 Microforge (Narishige).
Intracellular saline68 comprised 140 mM potassium-aspartate, 1 mM
KCl, 10 mM HEPES, 1 mM EGTA, 0.5 mM Na3GTP, 4 mM MgATP, 13 mM
biocytin hydrazide and 20 µM Alexa-568–hydrazide-Na (ThermoFisher
Scientific). The pH was adjusted to about 7.3 with KOH, and osmolarity
to approximately 265 mOsm with water.
Electrophysiological signals were acquired using a MultiClamp 700B
amplifier (Molecular Devices) in current-clamp mode. Electrophysi-
ological signals and behavioural camera triggers were digitized at
10 kHz via a Digidata 1440A and saved to a computer (Clampex, Molecu-
lar Devices). The oviDN or oviDN-like subtype (Extended Data Fig. 3)
from which recording was taken was not distinguished. Electrophysi-
ology experiments using oviDN-SS1 could target oviDNa or oviDNb
Articleand experiments using oviDN-GAL4 could target oviDNa, oviDNb or
oviDN-like neurons. Recordings were made without current injection
(except for current step protocols) and the reported membrane voltage
(Vm) was corrected for a 13 mV junction potential67. Spikes were identi-
fied by highpass filtering Vm and finding peaks above a threshold that
were separated in time by over 1 ms. Parameters for peak detection
were varied from recording to recording based on visual inspection of
the data, in which the action potentials were clear. We calculated the
spike rate by counting the number of spikes in every 5 s interval (at
0.1 ms steps), dividing by 5 and assigning that value to the middle of
the 5 s interval (for Extended Data Fig. 6b,e a 100 ms rather than 5 s
interval was used). Spike rate and Vm were thus both measured at 0.1 ms
intervals. Data were aligned and analysed identically to calcium imag-
ing. Resting Vm was considered the first stable Vm after breaking into
the whole-cell configuration (Fig. 5d and Extended Data Fig. 9a). We
calculated a Vm with spikes removed by discarding (converting to NaNs)
150 ms of data centred on the peak of each spike (Extended Data Fig. 9c).
Electrophysiological recordings for 2× EGFP-expressing flies were
analysed only if (1) the cell was stably recorded for more than 3 min;
(2) Vm was below –43 mV at rest with no large drift or rapid fluctuations
that were clearly non-physiological; (3) the fly walked for at least one
wheel rotation; and (4) the cell spiked at least once. A total of five cells
were rejected for not passing criteria 2, 3 and 4. Three of these five were
rejected for not passing criterion 2, and a single cell was rejected for
not passing criterion 3, indicating that flies were healthy in this prepa-
ration. A single cell passed the first three criteria but was rejected for
not spiking (shown in Extended Data Fig. 9a). Cells that passed all four
criteria were analysed from the time when the recording first stabilized
to when it degraded or was terminated (mean, 41 min).
Electrophysiological recordings for CsChrimson-expressing flies
were analysed if Vm was below –43 mV at rest. All recordings were con-
ducted in vivo and with the fly on the wheel.
Electrophysiological recordings for Kir2.1*- and Kir2.1*Mut-expressing
flies were analysed if Vm was below –43 mV at rest. These flies were
pretreated as described for free-behaviour experiments rather than
as described for tethered experiments, so the transgene would be
expressed because it was in free behaviour. All recordings were done
in vivo on the wheel. Current step protocols were conducted with 5 pA
increments with 1 s of current injection (Extended Data Fig. 10d).
Abdominal ganglion calcium imaging
Flies were anaesthetized at approximately 4 °C and their wings clipped
near their base before tethering to a custom holder. The holder was simi-
lar to that used in our other experiments except that it lacked a pyramid
(such that the objective could be lowered to image deep ventral tissue)
and had a larger hole (such that the head, thorax and anterior-most
part of the abdomen could fit, rather than just the head) (Extended
Data Fig. 6h–j). The dorsal part of the thorax was pushed through the
hole and the posterior head was aligned and pitched in the hole to be
in plane with the holder. The thorax, abdomen and head were glued
to the holder with blue-light-cured glue (Bondic). Glue was applied to
the anterior abdomen to stabilize the preparation and avoid tearing
of the delicate cuticle of the abdomen during dissection. As a result,
the fly was not able to bend its abdomen normally. The rostrum was
not glued in this preparation because proboscis extension did not
cause movements in the abdominal ganglion as it did in the brain. A
needle was used to slice a window in the cuticle of the dorsal thorax
(Extended Data Fig. 6j; blue box shows dissection area), and the cuticle
and indirect flight muscles were removed with forceps such that the
dorsal proventriculus and surrounding trachea were visible. Removal
of the indirect flight muscles was easier without extracellular saline
solution in the bath and thus was done quickly (within 30 s) to prevent
desiccation. Extracellular saline was then added. The section of the
proventriculus near the neck connective was cut, and the portions of
the gut covering the ventral nerve cord, as well as the trachea and crop,
were removed. The preparation was flushed with extracellular saline
to dilute digestive enzymes that might have been released during dis-
section. Loose tissue (for example, remaining indirect flight muscles)
was carefully removed or retracted such that the abdominal ganglion
was visible. Despite removal of several dorsal structures to expose the
ventral nerve cord and abdominal ganglion, flies were able to walk.
Occasional flies that were not able to move their legs normally either
before or after imaging were discarded. Overall, tethering and dissec-
tion shared features with previous work72 except that significant time
and effort were needed to advance dissection past the neck connec-
tive, T1, T2 and T3 neuromeres to the abdominal ganglion. (Previous
imaging in walking flies was restricted to the more accessible neck
connective and T1 neuromere72,73). The holder was stabilized with mag-
nets above the egg-laying wheel inside a small custom humidification
chamber.
Calcium imaging rates of around 0.5 Hz and laser powers of
20–30 mW at the specimen were needed to capture sufficient signal and
sample the full presynaptic volume (approximately 50 × 50 × 60 µm3).
Imaging rates and laser powers are similar in Extended Data Fig. 6l,m, to
aid comparison (mean imaging rate of 0.50 and 0.56 Hz, respectively,
with a minimum rate of all data at 0.36 Hz). Because the timestamp
assigned to a volume scan was when the volume was half complete, data
of around 1 s after cessation of stimulation in Extended Data Fig. 6l,m
should minimally include the stimulation period; data 1.4 s (delay for
0.36 Hz) after stimulation do not include the stimulation period at all.
These numbers also apply to the increase in ∆F/F at stimulation onset.
Half-decay times are the amount of time after cessation of stimula-
tion required for signal value to return half-way between that at the
end of stimulation and 5 s mean prestimulation. The half-decay times
reported in the main text are the average of those for the three lower
intensities shown in Extended Data Fig. 6d–f. To calculate an expected
∆F/F half-decay time given a spike rate, GCaMP7f kinetics and calcium
imaging rate, we convolved one of our spike-rate traces (second-lowest
intensity shown in Extended Data Fig. 6e) with an exponential filter
(τ = 300 ms) that estimates the off kinetics of GCaMP7f17 and then
applied a boxcar filter (width, 2.8 s) that simulated the slowest frame
rate in all experiments. The half-decay time of this simulated trace
was 700 ms.
Substrate transition-triggered averages during calcium imaging
or electrophysiology
Substrate transitions were identified using the fly’s position on the
wheel. For these analyses, substrate transition i was eliminated if sub-
strate transitions i – 1 and i + 1 occurred within 4 s of each other. This
empirically prevented events in which the fly rocked on the substrate
boundary from being counted as multiple transitions. Note that, for all
transition-triggered averages, if the fly were to have transitioned back
to the original substrate—say, 20 s after the first transition—the data
from 20 s onwards would not contribute to the post-transition average.
Measurement of light power
All light power levels reported in this paper were measured with
a PM100D Compact Power and Energy Console (Thorlabs) at the
expected peak intensity of the light source. Lighting with an area smaller
than the sensor was divided by the estimated illuminated area rather
than by the area of the sensor.
Texas Red fill
Texas Red (100 mg ml–1; dextran, Texas Red, 3,000 MW, lysine fix-
able) (ThermoFisher Scientific) in patch-clamp intracellular saline
(see above) lacking ATP, GTP, biocytin and Alexa-568–hydrazide-Na
was backfilled into a patch pipette. The pipette was positioned near
the cell body (without any collagenase application) and two to five
pulses of 10 V (2 ms duration) were applied using an SD9 stimulator
(Grass Instruments). All fills and anatomy were carried out with flies on
the wheel under the two-photon microscope (as in calcium imaging,
except using a ×40/0.80 NA objective (LUMPLFLN 40XW, Olympus) and
a 590–650 nm bandpass filter (Chroma) to filter emitted light before
entering a second GaAsP detector (Hamamatsu).
Split-GAL4 screening and stabilization
Split-GAL4 lines were screened and stabilized as described previously9.
To determine cell types labelled by a particular split-GAL4 driver, stand-
ard immunofluorescence staining was used to count the total number
of cells (Extended Data Fig. 11a–r) and stochastic labelling in multiple
colours74 was used to visualize the morphology of individual cells. Indi-
vidual cell morphology was used to manually assign cells to hemibrain
connectome body IDs16, and the cell type and instance associated with
the body ID were noted (Supplementary Table 3).
In Fig. 6a the number of pairs of oviDN input neurons is an estimate
based on correspondence between light microscopy images of neurons
labelled in split-GAL4 and the hemibrain connectome16 (above); the
number of synapses onto oviDNs is an estimate of the total number of
synapses onto oviDNs from those neurons using the electron micros-
copy connectome (Supplementary Table 3).
Analysis of hemibrain for recurrent-circuit inputs to oviDNs
We analysed synaptic connections in the adult female hemibrain using
the neuPrint75 (v.1.2.1) Python interface. All connections with at least
one synapse per connection were queried for the circuit architectures
investigated (Extended Data Fig. 11s–v). Because oviDNs receive an
enormous number (approximately 600–1,100) of input synapses
and have very few (roughly between five and 50) output synapses
in the hemibrain, direct, two-way reciprocal connections between
pairs of oviDNs—or between oviDNs and other cells—were not evident
(Extended Data Fig. 11s,t, also diagrammed in Fig. 6d). Of all the recur-
rent circuits (with at least ten synapses) in the hemibrain that directly
involve oviENs—which are the dominant input cells to oviDNs—the
neurons diagrammed in Fig. 6e are the only ones that concisely/directly
interconnect oviENs on both sides specifically via a single group G,
group U or oviIN cell. We could not discover a recurrent circuit that uses
a single cell class to interconnect oviDNs on both sides using the same
ten-synapse threshold (Extended Data Fig. 11u,v). (Interconnection of
oviDNs on both sides is a sensible constraint for an underlying circuit
because the calcium signals of oviDNs on both sides track tightly during
egg laying.) Although we did not find simpler recurrent-circuit archi-
tectures (Extended Data Fig. 11s–v), complementary circuits could still
exist particularly in regions of the nervous system where connectome
data are unavailable, or via gap junctions, which are not annotated in
existing fly connectomes.
Although inhibition of group Z neurons also had an effect on eggs
laid (Fig. 6a), 11 of 18 flies with inhibition of group Z still laid more than
one egg. Note that group Z neurons provide synaptic input to oviDNs,
oviENs, group G and group U cells (Supplementary Table 4), potentially
explaining why flies lay fewer eggs when these neurons are inhibited
(Fig. 6a). Group Z neurons, however, receive few synapses back from
the other relevant cell classes and they thus reside, in our interpreta-
tion, outside of the core loop.
The fact that recurrent-circuit neurons on both sides of the brain are
reciprocally connected helps to explain why the oviDN [Ca2+] signal on
both sides is qualitatively similar (Extended Data Fig. 3f). Group U cells
and at least one group G cell were positive for tyrosine hydroxylase
(Extended Data Fig. 12), suggesting that the physiology of this recur-
rent circuit may be more sophisticated than one in which all circuit
elements express the same excitatory transmitter to implement simple,
runaway excitation76,77.
Fig. 12; see below), using the following antibodies and dilutions: 1:30
mouse anti-Bruchpilot (no. nc82, Developmental Studies Hybridoma
Bank), 1:300 rabbit anti-HA Tag (no. 3724S, Cell Signaling Technol-
ogy), 1:200 rat anti-FLAG Tag (no. NBP1-06712, Novus Biologicals),
1:500 DyLight 550 mouse anti-V5 Tag (no. MCA1360D550GA, AbD
Serotec), 1:500 Alexa Fluor 594 donkey anti-rabbit (no. 711-585-152,
Jackson ImmunoResearch), 1:600 ATTO 647N goat anti-rat (no. 612-
156-120, Rockland), 1:600 Cy2 goat anti-mouse (no. 115-225-166, Jackson
ImmunoResearch), 1:800 Alex Flour 488 goat anti-rabbit (no. A11034,
Thermo Fisher Scientific), 1:400 AlexaFlour568 goat anti-mouse (no.
A11031, Thermo Fisher Scientific) and 1:1,000 rabbit anti-GFP (no.
A11122, Thermo Fisher Scientific).
For identification of neurotransmitter identity (Extended Data
Fig. 12), brains were dissected in cold Schneider’s insect medium (no.
S0146, Sigma-Aldrich) and fixed overnight at 4 °C in Schneider’s insect
medium with 1% paraformaldehyde (no. 15713, Electron Microscopy
Science). For vGluT staining, brains were instead fixed for 5 min at
room temperature in Bouin’s fixative (no. 112016, Ricca Chemical
Co.) as described previously78. Primary antibodies and their dilutions
were: 1:300 rabbit anti-TH (no. AB152, Sigma-Aldrich), 1:500 rabbit
anti-serotonin (no. S5545, Sigma-Aldrich), 1:50 mouse anti-ChAT (no.
ChAT4B1-s, Developmental Studies Hybridoma Bank), 1:500 rabbit
anti-GABA (no. A2052, Sigma-Aldrich), 1:10,000 rabbit anti-vGluT78
(gift from A. DiAntonio), 1:1,000 chicken anti-GFP (no. 600-901-215,
Rockland) and 1:30 mouse anti-Bruchpilot (no. nc82, Developmental
Studies Hybridoma Bank) for all but anti-ChAT experiments. Second-
ary antibodies and their dilutions were: 1:800 goat anti-chicken Alexa
Flour 488 (no. A11039, Thermo Fisher Scientific), 1:400 goat anti-mouse
Alexa Flour 594 (no. A11032, ThermoF isher Scientific) and 1:400 goat
anti-rabbit Alexa Flour 633 (no. A21070, Thermo Fisher Scientific). Sam-
ples were mounted in Vectashield H-1000 (Vector Laboratories) and
imaged on an LSM 780 confocal microscope (Zeiss) with a ×20/0.8 NA
objective (no. 440640-9903-000, Zeiss) at 1 µm z-intervals. Images
were analysed using Fiji (ImageJ).
Statistics and reproducibility
We used the two-sided Wilcoxon rank-sum test to calculate all P values.
For egg-laying choice fractions (for example, Fig. 3b), grey bars indi-
cate the fraction of eggs laid on the lower-sucrose option after all eggs
from all flies are pooled. Error bars indicate the 95% confidence interval
of this fraction calculated using the Clopper–Pearson method (‘exact’
binomial confidence interval). Individual dots represent individual
flies.
The first two P values in the main text compare the number of trials
with (or without) events in two separate groups. For a single group, trials
with an event are treated as 1 and those without an event are treated as
0. The two groups (each a set of 0 and 1) are then compared using the
two-sided Wilcoxon rank-sum test (P values calculated using two-sided
Fisher’s exact test are similar and similarly significant). Exact P values
in the main text are 2.1 × 10–25, 8.4 × 10–29 and 3.3 × 10–54.
For the calculations shown in Fig. 4f we used the point at which ∆F/F
crosses 0 as the starting point in the slope calculation, because it relies
solely on the ∆F/F signal and not behaviour. A ∆F/F value of 0 is, on
average, related to the beginning of the search (Fig. 1k). P values were
calculated using the two-sided Wilcoxon rank-sum test; P = 0.030 when
comparing slopes in the 25 s before abdomen bend and P = 0.064 when
comparing slopes from search start to abdomen bend.
Immunofluorescence examples shown in Fig. 1f and Extended Data
Figs. 3b, 11a–r and 12 are representative of at least two brains (four total
brain sides) and typically more than three brains (six total brain sides).
Electron microscopy-based anatomy shown in Fig. 6c and Extended
Data Fig. 3a was generated from a single side of one brain16.
Immunofluorescence staining and confocal microscopy
Immunofluorescence staining and confocal microscopy were per-
formed as described previously9 (with modifications for Extended Data
No data were excluded unless explicitly stated. No statistical method
was used to choose sample size. Experimenters were not blind to fly
genotype. Flies were randomly chosen for each experiment.
ArticleData analysis
All data analyses and instrument control were done using either MAT-
LAB (MathWorks) or Python unless otherwise specified. All design
for 3D printing or laser cutting was done using Autodesk Inventor
(Autodesk), which was also used to help create Fig. 1d and Extended
Data Fig. 2a–c.
Reporting summary
Further information on research design is available in the Nature Port-
folio Reporting Summary linked to this article.
Data availability
All calcium imaging and fly behaviour time-course datasets analysed
in the main figures are available on DANDI archive (calcium imag-
ing data, 000247; fly choice tracking data, 000212; fly behavioural
sequence tracking data, 000250). Technical documents (for example,
CAD files and plasmid maps) and source data for all scatter plots
and histograms are available on Figshare (https://doi.org/10.6084/
m9.figshare.c.6505732).
Code availability
Scripts for data processing and plotting are available on request.
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Acknowledgements We thank the Rockefeller University Precision Instrumentation
Technologies facility for access to fabrication equipment; the Janelia FlyLight team for
generating confocal images of oviDN-GAL4 (in Extended Data Fig. 3b), oviDN-SS1 (Fig. 1e,f)
and oviDN input neuron split-GAL4s (Extended Data Fig. 11a–r); the Bloomington Drosophila
Stock Center (NIH P400D018537) for various fly stocks; A. Siliciano and V. Ruta for GtACR1
effector flies; M. Wolfer for flies expressing GCaMP in eggs; M. Scanziani for pCAG-Kir2.1Mut-
T2A-tdTomato and pCAG-Kir2.1-T2A-tdTomato plasmids (Addgene plasmid nos. 60644 and
60598); G. Rubin for plasmid pJFRC81-10XUAS-IVS-Syn21-GFP-p10 (Addgene plasmid
no. 36432); A. DiAntonio for vGluT antibody; M. DeSouto for the template fly drawing that
was modified and used throughout the manuscript; Z. Wang for help in developing the first
iteration of the fly-tracking setup; K. Fonselius and S. Cohen for initial help in developing
tools for computer-assisted manual annotation of egg-deposition events in free behaviour;
J. Varikooty, J. Hirokawa and I. Ishida for ideas and help in developing the wheel; J. Weisman
for sharing his design for delivery of optogenetic stimulation light; and C. Lyu and J. Green for
two-photon and electrophysiology discussions. Research reported in this publication was
supported by a Brain Initiative grant from the National Institute of Neurological Disorders and
Stroke (no. R01NS121904 to G.M.) and a Leon Levy Foundation fellowship and the Kavli Neural
Systems Institute grant to V.V. G.M. is a Howard Hughes Medical Institute Investigator.
Author contributions V.V. and G.M. conceived the initial study and wrote the manuscript.
V.V., with input from G.M., designed and performed experiments, analysed data, interpreted
results and decided on new experiments. F.W., K.W. and B.J.D. created the oviDN and oviDN
input genetic driver lines, shared preliminary data on oviDNs and provided helpful feedback
on experiments and the manuscript. A.C. and A.A. created Kir2.1* and Kir2.1*Mut flies. H.A.
developed code for computer-assisted manual annotation of egg-deposition events in free
behaviour.
Competing interests The authors declare no competing interests.
Additional information
Supplementary information The online version contains supplementary material available at
https://doi.org/10.1038/s41586-023-06271-6.
Correspondence and requests for materials should be addressed to Vikram Vijayan or Gaby
Maimon.
Peer review information Nature thanks Yvette Fisher and the other, anonymous, reviewer(s) for
their contribution to the peer review of this work.
Reprints and permissions information is available at http://www.nature.com/reprints.
Extended Data Fig. 1 | Free behavior chambers and characterization of the
egg-laying behavioral sequence. a, Schematic of free behavior egg-laying
chamber with sloped ceiling. b, Schematic of high-throughput free behavior
egg-laying choice chamber. c, Comparison of the two free behavior chamber
types. d, Length (neck to ovipositor distance) and locomotor speed over 3
consecutive egg-laying events, smoothed with a 5 s boxcar filter, for a single
fly in a sloped ceiling egg-laying chamber (Supplementary Video 2). e-h, Mean
length and locomotor speed aligned to annotated events in the egg-laying
behavioral sequence. Light grey shading is ± s.e.m. Prominent features of
steps from Fig. 1a are labeled. These features (e.g., the pause or increase in
locomotion) were not considered when annotating the event used for
alignment.
ArticleExtended Data Fig. 2 | See next page for caption.
Extended Data Fig. 2 | Egg-laying wheel and tethered egg-laying behavioral
sequence with oviDN [Ca2+]. a, Schematic of egg-laying wheel. b, Schematic
of agarose-injecting mold, which is used to load agarose onto the wheel with
a pipette. c, Schematic of egg-laying wheel assembly secured in a custom
humidification chamber under the microscope objective. d, Fraction of eggs
on the lower sucrose option for control substrates: colored dye infused
substrates and 3D-printer material (VisiJet M3 Crystal) bases vs. acrylic bases.
Error bars represent 95% confidence intervals. Each dot is one fly. These data
suggest that the dyes and plastics involved in fabricating the egg-laying wheel
should not cause abnormal substrate choice behavior. e, Fraction of eggs
on the lower sucrose option for flies expressing GCaMP7f in oviDNs and by
those pre-treated for tethered wheel experiments. Error bars represent 95%
confidence intervals. Each dot is one fly. These data show that the flies we
used in our imaging experiments exhibit normal substrate choice during free-
behavior egg laying. f, Behavioral sequence of tethered egg laying as in Fig. 1a.
Stills from a single egg-laying event. Overlaid and zoomed-in schematics of the
tip of the abdomen from 3 frames is shown at the bottom right. g, Mean oviDN
∆F/F aligned to the moment abdomen bending to lay an egg is complete.
43 traces from 9 cells in 8 flies (41 eggs). Light grey shading is ± s.e.m. for all
panels in this figure. Behavior shown below. h–n, Mean oviDN ∆F/F and behavior
aligned to events in the behavioral sequence shown in panel g. Locomotor
speed is smoothed with a 5 s boxcar filter. o, Mean oviDN ∆F/F aligned to when
abdomen bend is complete with all data points before the start of the search
omitted from the average.
ArticleExtended Data Fig. 3 | Anatomy and physiology of different oviDN types.
a, Electron-microscopy (EM) skeletons16 and characterization of the 3 oviDN
and 2 oviDN-like neurons per side. The branch labeled in grey is sometimes
present in oviDNb9 and sometimes not (Fig. 1e). The 3 other arrows indicate
neurites that are unique to either oviDNa or oviDNb. Visualization generated
using Neuroglancer. Neuropil to left is only to schematize the approximate ROI
shown in the EM. b, Average z-projection of oviDN-GAL4 in the brain (top) and
ventral nerve cord (bottom). Green shows UAS-mCD8GFP expression in the
targeted neurons and magenta represents a neuropil counterstain (Methods).
c, Anatomy of oviDN-SS1 driving expression of GCaMP7f. The brighter of the
two oviDN cell bodies was filled with Texas Red (Methods). The neurite labeled
with a pink arrow in panel a was used to determine if the cell was oviDNb. All 6 of
the brighter cells filled with Texas Red (from 6 separate flies) were oviDNb. Two
examples are shown (representative individual z-slices). d, Mean oviDNa ∆F/F
during individual egg-laying events. 29 traces from 7 cells in 6 flies (28 eggs).
These data did not contribute to the traces in Fig. 1 (or any other figure), which
were exclusively from oviDNb. Light grey shading is ± s.e.m. for all panels in this
figure. e, Mean cross-correlation of ∆F/F between ipsilateral oviDNa and
oviDNb cells imaged simultaneously. Traces from multiple, individual cell pairs
are averaged. f, Mean cross-correlation of ∆F/F between contralateral oviDNb
cells imaged simultaneously. Traces from multiple, individual cell pairs are
averaged.
Extended Data Fig. 4 | oviDN [Ca2+] traces from individual egg-laying
events. a, OviDN ∆F/F showing all individual traces that were averaged in Fig. 1h
(43 imaging traces from 41 egg-laying events associated with 9 cells in 8 flies).
Grey lines are individual traces smoothed with a 5 s boxcar filter. Black line is
the average of the non-smoothed individual traces. b, Each row represents a
single egg-laying event. Rows have been ordered based on the search duration.
Individual traces are smoothed as in panel a and behavioral annotations are
overlayed. Individual traces corresponding to Fig. 1g and Fig. 1m (fly 3) are
highlighted. Individual eggs that are part of the analysis in Fig. 4e, f are marked
with an asterisk. c, Same as panel b but with a colormap where white is centered
on a ∆F/F of 0 which is the average baseline in our normalization (Methods).
Individual traces tend to (1) dip below baseline during ovulation (blue color
after light pink/magenta line); (2) return to baseline around the time of search
start (white color near dark pink/purple line); and (3) increase past baseline
around or after the search start (red color after dark pink/purple line). These
trends are captured by the average analysis presented in Fig. 1j–l. d, Normalized
oviDN ∆F/F showing all individual traces in Fig. 1h (43 imaging traces from 41
egg-laying events associated with 9 cells in 8 flies). Grey lines are individual
traces smoothed with a 5 s boxcar filter and then normalized such that the
maximum and minimum ∆F/F in the 100 s window preceding the abdomen
bend are set to 1 and 0, respectively. Black line is the average of the smoothed
individual traces. e, Same as panel b but displaying ∆F/F normalized as in panel d.
ArticleExtended Data Fig. 5 | See next page for caption.
Extended Data Fig. 5 | OviDN ∆F/F and fly behavior during non-egg-laying
periods and during optogenetic stimulation. a, Standard deviation of oviDN
∆F/F for all data points > 5 min. away from egg deposition, i.e., ‘non-egg-laying
periods’. b, Example trace of wheel position and oviDN ∆F/F during a non-
egg-laying period (smoothed with a 2 s boxcar filter). This cell had a standard
deviation in ∆F/F of 0.15. c, Mean cross-correlation of oviDN ∆F/F versus varied
behavioral measures during non-egg-laying periods. Light grey shading
is ± s.e.m. for all panels in this figure. For sucrose concentration correlations,
only 0 vs. 500 mM sucrose wheels were analyzed (excluding 0 mM only wheels,
for example), leaving 53/104 flies for analysis. d, Same as panel c, but including
time periods near egg deposition (~372 additional minutes—i.e., ~4% additional
sample points—are included compared to panel c). e, Mean oviDN ∆F/F and
behavior during peaks in ∆F/F that occurred in non-egg-laying periods. We
smoothed the ∆F/F signal with a 5 s boxcar filter and extracted peaks in the ∆F/F
trace that exceeded 0.35 for > 1 s. We aligned these traces to the moment the
∆F/F signal crossed 0.35 in the 10 s before the peak. f, Change in mean body
angle, replotted from Fig. 2h. Arrow indicates first bin with an abdomen angle
change greater than 2.5° (indicated by dotted line). g, Same as panel f but with
coarser binning. h, i, Same as panel f but with finer binning. j-n, Same as panel f
but bins are shifted progressively by 0.02 leftward. In panels f to n, the first and
last bin always include all the data points below and above that bin, respectively.
The curve in panel l appears less step-like than the others; however, it is
expected that as one progressively shifts the center point of the bins, one will
find a position where the central bin straddles the putative threshold, yielding
an intermediate y value for that bin. The fact that panels k and m appear more
step like supports this explanation for panel l. o, Example traces of oviDN ∆F/F
during prolonged, gentle CsChrimson stimulation (protocol described in
Methods), smoothed with a 2.5 s boxcar filter. Traces are clipped once they
reach a ∆F/F of 0.275. We used 0.275 as the threshold because it is slightly
higher than the center of the 4th bin in Fig. 2g, h (i.e., a conservative lower-
bound estimate of the threshold). We use a conservative estimate for this
analysis to capture as many relevant traces as possible. Note that for a variety of
reasons, CsChrimson expressing flies may have a different threshold in terms
of ∆F/F than flies not expressing CsChrimson (Methods). OviDN ∆F/F traces
occasionally rise to threshold with this protocol. p, OviDN ∆F/F smoothed with
a 2.5 s boxcar filter for all 27 stimulations (out of 127 total) that brought ∆F/F to
threshold during the stimulation interval (the other 100 stimulations that did
not bring ∆F/F to the threshold are not shown). The beginning of each trace is
the beginning of stimulation. Colored lines are traces from panel o. A similar
analysis in the inter-stimulation-interval (starting 10 s after the CsChrimson
stimulation ended) only identifies 2 threshold crossing events indicating that
the observed threshold crossing during stimulation was predominantly caused
by the stimulation (data not shown). A similar analysis using data with the
strongest 5 s stimulation intensity in Fig. 2f identifies 46 (out of 88 total)
threshold crossing events indicating that is harder to achieve threshold
crossing with the gentle prolonged stimulation despite the longer interval
(data not shown). q, r, Change in mean body length and body angle for data
shown in panel p, indicating that flies, on average, bend their abdomen proximal
to the time of threshold crossing. s, Remaining ∆F/F until threshold is reached
(y-axis) as a function of remaining time until threshold is reached (x-axis). The
traces in panel p are sampled at 100 ms intervals to populate bin counts of the
histogram. The negative correlation indicates that CsChrimson stimulation
gradually brings the ∆F/F to threshold, rather than by inducing a spontaneous
event, independent of the current ∆F/F, that brings ∆F/F to threshold.
ArticleExtended Data Fig. 6 | See next page for caption.
Extended Data Fig. 6 | [Ca2+] changes in the oviDN soma and presynaptic
terminals lag changes in electrical activity. a, Mean oviDN Vm during
periodically triggered high-intensity 5 s CsChrimson stimulations. Light grey
shading is ± s.e.m. for all panels in this figure. b, Mean oviDN spike rate during
periodically triggered high-intensity 5 s CsChrimson stimulations. c, OviDN
single-trial Vm traces during periodically triggered 5 s CsChrimson stimulations
at four different intensities in the same fly. Intensities are the same as in Fig. 2f–h.
Traces have been shifted on the y-axis for clarity, with –50 mV indicated for
each trace (black arrowhead). d, Mean oviDN Vm during periodically triggered
5 s CsChrimson stimulations at four different intensities (same fly as panel c).
e, Mean oviDN spike rate during periodically triggered 5 s CsChrimson
stimulations at four different intensities (same fly as panel c). f, Mean oviDN
∆F/F during periodically triggered 5 s CsChrimson stimulations at four different
intensities (same data as Fig. 2f). g, Graphical model of the link between voltage
and calcium in oviDN somas using evidence from panels a to f. Increases in
voltage lead to slower increases in calcium and decreases in voltage lead to
slower decreases in calcium. To first order calcium ∆F/F signals appear to be a
low-pass filtered, delayed version of the voltage changes observed. Since
CsChrimson does not permeate calcium79, changes in [Ca2+] observed during
stimulation are likely due to opening of voltage-gated calcium channels.
h, Preparation to image oviDN presynaptic terminals in the abdominal
ganglion of the ventral nerve cord (Methods). i, Standard preparation for
imaging the oviDN cell body in brain. j, Schematic (top view) of the holder in
panel h. An outline of the hole in which the thorax, head, and anterior abdomen
are inserted is shown in red. The dissected region is indicated in blue. A typical
calcium imaging region is shown in green. k, Z-projections of representative
calcium imaging regions. Compare to region indicated by green arrow in
Fig. 1e. sytGCaMP7f80,81 was used to bias GCaMP expression to presynaptic
compartments for bulk imaging of presynaptic terminals. Note that sytGCaMP
biases GCaMP expression to terminals, but not necessarily to active zones81.
Red arrow points to the punctum quantified in panel n. l, Mean oviDN ∆F/F in
bulk presynaptic compartments during periodically triggered CsChrimson
stimulation, using 2nd lowest intensity from panels c to f. A low stimulation
intensity was applied such that subthreshold calcium accumulation could be
investigated. Presynaptic compartments from oviDNa and oviDNb could not
be distinguished and are thus averaged together. m, Mean oviDN ∆F/F in cell
bodies during periodically triggered CsChrimson stimulation. To aide
comparison with panel l, this experiment was done at a similar time, with
similar conditions (Methods), and with ROIs encompassing both oviDNa and
oviDNb cell bodies. n–p, Mean oviDN ∆F/F in selected single presynaptic
compartments, from three different flies, during periodically triggered
CsChrimson stimulation using the subthreshold intensity in panels l and m.
ROIs were drawn around individual puncta in GCaMP7f expressing flies, which
had a stronger florescence signal than sytGCaMP7f flies.
ArticleExtended Data Fig. 7 | Evidence against flies using spatial information
in substrate search and against a feeding-on-higher-sucrose related
explanation for substrate preferences in our free behavior chambers,
alongside controls for the egg-laying rate function. a, Schematic of a fly
searching for an egg deposition site in a 0 vs. 500 mM chamber. ∆T0mM and
∆T500mM are all the intervals of time that a fly spent on 0 or 500 mM, respectively,
during an egg-laying search period. ∆Tlast_500mM is the last transit interval
through 500 mM for eggs deposited on 0 mM. If a fly were positionally avoiding
sucrose, ∆T500mM would be less than ∆T0mM. If a fly were to use spatial information
during the search period—by taking a shortcut to get to the preferred 0 mM
substrate at the end of a search—∆Tlast_500mM would be less than ∆T0mM and
∆T500mM. If a fly were feeding on the higher sucrose substrate—and pausing as
flies do when they feed82—∆T500mM would be larger than ∆T0mM. b-d, ∆Tlower_sucrose,
∆Thigher_sucrose, and ∆Tlast_higher_sucrose distributions for three different sucrose
choice chambers. ∆Thigher_sucrose is not less than ∆Tlower_sucrose suggesting that flies
are not positionally avoiding the higher sucrose option. ∆Tlast_higher_sucrose is not
detectably smaller than ∆T0mM or ∆T500mM suggesting that flies are not taking a
shortcut—and thus not manifesting use of spatial information—at the end of the
search. It is possible that flies use spatial information to guide the search in
conditions with visible landmarks or where they perform less thigmotaxis
(edge-hugging); our flies largely edge-hugged as they traversed the chamber.
All experiments in this study were conducted in darkness. Note that our
time-domain model for egg laying (Fig. 4a) could be readily augmented with
spatial knowledge in that flies could putatively use their spatial sense to control
which substrate they visit which would then impact their egg-laying drive.
∆Thigher_sucrose is not larger than ∆Tlower_sucrose indicating that flies are not pausing
only on the higher sucrose substrate. We interpret this result to mean that flies
are not suppressing egg deposition because of extensive feeding on the
sucrose substrates. In addition, we did not notice additional proboscis
extension on higher sucrose when we spent hours inspecting each video to
annotate the egg deposition times. Note that our flies were very well fed before
entering the chamber, which could have minimized this effect (Methods).
771 eggs from 17 flies (18 flies tested and 1 did not lay eggs), 1863 eggs from
42 flies (47 flies tested and 5 did not lay eggs), and 1345 eggs from 30 flies (30 flies
tested), respectively. e, Mean egg-laying rates during the search period after a
fly transitions across the plastic barrier in a single-option chamber, meaning
that there is either 0 mM sucrose on both sides, 200 mM sucrose on both sides,
or 500 mM sucrose on both sides. 90% confidence interval shaded. Egg-laying
rates on the three different sucrose concentrations are similar in single-option
chambers. The slightly higher egg-laying rates on lower sucrose is consistent
with a possible, slight, innate preference for lower sucrose, which interacts
with a much more prominent relative-value assessment of sucrose that governs
egg laying rates (Fig. 3f–h). 895 eggs from 23 flies (24 flies tested and 1 laid no
eggs), 1253 eggs from 27 flies (27 flies tested), and 528 eggs from 16 flies (17
flies tested and 1 laid no eggs) for 0 vs. 0, 200 vs. 200, and 500 vs. 500 mM
chambers, respectively. f, Mean egg-laying rate during the search after a fly
transitions across a mock vertical line. 90% confidence interval shaded. Same
data as in panel e. The 5–10 s bin in this analysis has a higher egg laying rate than
in the analysis from panel e, suggesting that part of the delay in egg laying after
a transition is due to flies not laying eggs on the plastic barrier. g, Mean
locomotor speed with ± s.e.m. shaded. A ~3 s delay exists between when a fly
pauses and bends its abdomen to lay an egg till when an egg is deposited. This
~3 s latency is at least part of the reason why even the data in panel f do not show
high egg laying rates in the 0–5 s bin. Analyzing the same data as in panels e-f.
Extended Data Fig. 8 | Changes in oviDN ∆F/F during substrate transitions
are not due to consistent, detectable, changes in behavior. a, We detected
substrate crossing moments on the egg-laying wheel, and aligned behavioral
data to these moments: 2459 and 2460 traces from 70 cells in 53 flies (1911 and
1922 transitions). Plotted here is the probability that a fly’s centroid is located
> 2 mm away from the boundary between two substrates (y axis), as a function
of time from the substrate crossing (x axis). For a 2.5 mm fly, not being in the
2 mm region surrounding the boundary corresponds to the front or back of
the fly being 0.75 mm away from the midpoint of the 1 mm plastic barrier
between substrates. These traces highlight that it takes flies ~10–20 s, on
average, to completely cross the midline which is important to keep in mind
when interpreting neural signals aligned to substrate crossing events. b, Mean
neck to proboscis length during substrate transitions. Light grey shading
is ± s.e.m. for all panels in this figure. c, Mean locomotor speed during substrate
transitions. d, Mean body length during substrate transitions. e, Mean body
angle during substrate transitions. f, Mean body length, body angle, and oviDN
∆F/F during the subset of substrate transitions where there was a small change
in body length. The mean body length in the 4 s after and before a substrate
transition were subtracted. If the absolute value of this difference was less than
0.01, then the change was considered small. g, Same as panel f, except selecting
for substrate transitions where the difference was greater than 0.01. h, Same as
panel f, except selecting for substrate transitions where the difference was less
than −0.01. The sum of the number of traces in panels f-h is less than panel a
because during some substrate transitions the body length and/or angle was
not possible to accurately calculate using DeepLabCut (Methods). i–k, Same as
panels f-h, except comparing body angle and using a threshold of 0.5°. Proboscis
length and fly speed (panels b-c) do not consistently change during substrate
transitions and therefore do not explain the changes in oviDN ∆F/F. Body
length and body angle do change, on average, during substrate transitions
(panels d-e). However, these changes cannot fully explain the changes in oviDN
∆F/F (panels f-k). That is, regardless of the change in body length or body angle,
the oviDN ∆F/F consistently changes with sucrose concentration (albeit with
some modulations related to body length and angle).
ArticleExtended Data Fig. 9 | OviDN electrical activity during substrate transitions
and additional evidence for oviDN ∆F/F tracking relative value during
substrate transitions. a, oviDN spike rate versus Vm at rest. b, Vm during two
substrate transitions from the same fly. These sample traces have more
pronounced Vm changes than is typical. c, Mean oviDN Vm after removal of
spikes during substrate transitions (Methods). 74 and 72 traces from 8 cells in
8 flies (74 and 72 transitions). Light grey shading is ± s.e.m. for all panels in this
figure. d, Mean oviDN spike rate during substrate transitions. e, Same as
Extended Data Fig. 8a but for the behavior of the fly during this electrophysiology
dataset. f, Mean oviDN ∆F/F during substrate transitions from 500 to 0 mM and
0 to 500 mM. 2459 and 2460 traces from 70 cells in 53 flies (1911 and 1922
transitions). g, Mean oviDN ∆F/F during substrate transitions from 500 to
200 mM and 200 to 500 mM. 167 and 170 traces from 5 cells in 3 flies (105 and
109 transitions). h, Mean oviDN ∆F/F during substrate transitions from 200 to
0 mM and 0 to 200 mM. 443 and 446 traces from 20 cells in 20 flies (443 and
446 transitions). In panels f-h, note that all changes are on the order of 0.05 ∆F/F
regardless of the absolute sucrose concentration, consistent with a relative
value calculation. i–k, Same as Extended Data Fig. 8a but for the behavior of the
fly during the datasets in panels f-h, shown to the left.
Extended Data Fig. 10 | Strong and gentle inhibition of oviDNs. a-c, Eggs laid
per fly. Each dot is one fly. ± s.e.m. indicated. d, The Vm of a single, representative
oviDN (or oviDN-like neuron) expressing Kir2.1*Mut or Kir2.1* during current
injection. Four out of five Kir2.1* expressing cells showed spikes with sufficient
amounts of current injection; one cell did not (not shown). e, Mean locomotor
speed aligned to egg deposition with ± s.e.m. shaded. A higher average speed
before egg laying in oviDN>Kir2.1* flies is indicative of the longer search
duration in these flies. However, other aspects like the pause to lay an egg and
post-egg-laying speed remain similar in oviDN>Kir2.1*Mut and oviDN>Kir2.1*
flies. 1377 eggs from 40 flies (45 flies tested and 5 laid no eggs), 346 eggs
from 17 flies (40 flies tested and 23 laid no eggs) for oviDN>Kir2.1*Mut and
oviDN>Kir2.1*, respectively. f, Normalized inter-egg interval histograms. 1340
intervals from 40 oviDN>Kir2.1*Mut flies (45 flies tested and 5 laid < 2 eggs and
thus did not have at least one interval). 333 intervals from 15 oviDN>Kir2.1* flies
(40 flies tested and 25 flies laid < 2 eggs and thus did not have at least one
interval). Note that the similar inter-egg interval distribution for oviDN>Kir2.1*
and control flies does not mean that oviDN>Kir2.1* flies searched for the same
amount of time for an egg-laying substrate as controls; rather, oviDN>Kir2.1*
flies searched longer than controls (Fig. 5g). What is going on, remarkably, is
that oviDN>Kir2.1* flies perform their next ovulation sooner after laying an egg
than controls, such that despite searching longer before laying an egg, these
flies ended up expressing nearly identical inter-ovulation and inter-egg
intervals as control flies. The inter-ovulation interval (as estimated with
locomotor speed) was not statistically different in oviDN>Kir2.1* and control
flies (P = 0.36) (data not shown). P-values were calculated using two-sided
Wilcoxon rank sum test.
ArticleExtended Data Fig. 11 | See next page for caption.
Extended Data Fig. 11 | Spilt-GAL4 lines targeting oviDN input neurons
and analysis of oviDN inputs in the hemibrain connectome. a-r, Average
z-projection of oviDN input split-GAL4 lines in the brain (top) and ventral
nerve cord (bottom) in order of x-axis in Fig. 6a (see Supplementary Table 3 for
additional information). Green shows UAS-CsChrimson-mVenus expression in
the targeted neurons and magenta represents a neuropil counterstain
(Methods). s, In panels s-v, we show circuit motifs that are not supported by
our analysis of the hemibrain connectome16, in contrast to the motif reported
in Fig. 6e, which is supported (see Methods for more discussion). This panel
shows that no chemical-synapse-based recurrent circuit is observed between
the oviDNs themselves in the hemibrain connectome at a threshold of ≥ 10
synapses per connection (even true at a threshold of ≥ 2 synapses here). (For
reference on the potential functional significance of a 10 synapse threshold,
~15-20 neurons make ≥ 10 synapses onto an individual oviDN in the hemibrain.)
Scatter plot of connections between pairs of neurons is shown to right. Both
data points are from a single pair of oviDNs. t, We found in the hemibrain
connectome all pairs of cells that fulfilled the circuit diagram shown on the left
at a threshold of ≥ 1 synapse per connection. No direct, bidirectional, chemical-
synapse-based recurrent circuit could be detected between individual oviDNs
and any oviDN input neuron in the hemibrain connectome at a threshold of ≥ 10
synapses per connection (even true at a threshold of ≥ 4 synapses per
connection here). Scatter plot of connections shown to right; each dot
represents the connections between two neurons. Orange points represent
pairs of connected neurons diagrammed in the recurrent circuit in Fig. 6e, but
assayed for participation in a different circuit motif here. u, We found in the
hemibrain connectome all sets of four cells that fulfilled the circuit diagram
shown on the left at a threshold of ≥ 1 synapse per connection. None of these
putative circuits had ≥ 10 synapses for all four connections, which we interpret
to mean that no chemical-synapse-based, disynaptic recurrent circuit exists
between individual oviDNs and a single class of oviDN input neuron. Cell
classes (types) were based on the hemibrain v1.2.1 connectome annotation16.
Scatter plot of connections is shown to right; each dot represents the
connections between a set of four neurons. Orange points represent sets of
four neurons diagrammed in the recurrent circuit in Fig. 6e, but assayed for
participation in a different circuit motif here. For example, the orange dot
indicated by an arrow represents the following connections between single
cells: oviDNa(right)←groupU(right)↔groupU(left)→oviDNb(left). v, Same as
panel u, but for a different circuit architecture. Each point, once again,
represents the connections between a set of four neurons. None of these
putative circuits had ≥ 10 synapses for all four connections.
ArticleExtended Data Fig. 12 | Neurotransmitter identity of recurrently connected
neurons. a–e, Average z-projection of 5 z-slices (1 µm z-intervals) centered
around the cell of interest. oviEN cells are ChAT positive (panel a); group U cells
are TH positive (panel b); and the staining of the 3 group G cells (assigned to cell 1,
cell 2, or cell 3 with decreasing brightness of 2xEGFP immunostaining) yielded
one cell with unclear transmitter assignment (panel c, cell 1 of 3), one TH
positive cell (panel d, cell 2 of 3), and one ChAT positive cell (panel e, cell 3 of 3),
respectively. The three group G cells consistently had three qualitatively
different levels of brightness (2xEGFP in panels c-e).
| null |
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