ORIGINAL RESEARCH
published: 28 July 2020
doi: 10.3389/fneur.2020.00692
Longitudinal Stroke Recovery
Associated With Dysregulation of
Complement System—A Proteomics
Pathway Analysis
Vinh A. Nguyen 1,2,3,4*, Nina Riddell 2 , Sheila G. Crewther 2 , Pierre Faou 5 ,
Harinda Rajapaksha 5 , David W. Howells 6 , Graeme J. Hankey 7,8 , Tissa Wijeratne 3,9 ,
Henry Ma 10 , Stephen Davis 11 , Geoffrey A. Donnan 11 and Leeanne M. Carey 1,3
1
Department of Occupational Therapy, La Trobe University, Bundoora, VIC, Australia, 2 Department of Psychology and
Counselling, La Trobe University, Bundoora, VIC, Australia, 3 Neurorehabilitation and Recovery, Stroke, The Florey Institute of
Neuroscience and Mental Health, Heidelberg, VIC, Australia, 4 Western Health, Department of Neurology, Sunshine, VIC,
Australia, 5 Department of Biochemistry and Genetics, La Trobe University, Bundoora, VIC, Australia, 6 Medical Sciences
Precinct, University of Tasmania, Hobart, TAS, Australia, 7 Faculty of Health and Medical Sciences, Internal Medicine,
University of Western Australia, Perth, WA, Australia, 8 Clinical Research, Harry Perkins Institute of Medical Research, Perth,
WA, Australia, 9 Department of Medicine, The University of Melbourne, Sunshine, VIC, Australia, 10 Monash Health, Neurology
and Stroke, Clayton, VIC, Australia, 11 Department of Neurology, Royal Melbourne Hospital, Parkville, VIC, Australia
Edited by:
Aristeidis H. Katsanos,
McMaster University, Canada
Reviewed by:
Marios K. Georgakis,
LMU Munich University
Hospital, Germany
Michael Chong,
Population Health Research Institute
(PHRI), Canada
*Correspondence:
Vinh A. Nguyen
v.nguyen@latrobe.edu.au
Specialty section:
This article was submitted to
Stroke,
a section of the journal
Frontiers in Neurology
Received: 18 April 2020
Accepted: 09 June 2020
Published: 28 July 2020
Citation:
Nguyen VA, Riddell N, Crewther SG,
Faou P, Rajapaksha H, Howells DW,
Hankey GJ, Wijeratne T, Ma H,
Davis S, Donnan GA and Carey LM
(2020) Longitudinal Stroke Recovery
Associated With Dysregulation of
Complement System—A Proteomics
Pathway Analysis.
Front. Neurol. 11:692.
doi: 10.3389/fneur.2020.00692
Frontiers in Neurology | www.frontiersin.org
Currently the longitudinal proteomic profile of post-ischemic stroke recovery is relatively
unknown with few well-accepted biomarkers or understanding of the biological systems
that underpin recovery. We aimed to characterize plasma derived biological pathways
associated with recovery during the first year post event using a discovery proteomics
workflow coupled with a topological pathway systems biology approach. Blood samples
(n = 180, ethylenediaminetetraacetic acid plasma) were collected from a subgroup of
60 first episode stroke survivors from the Australian START study at 3 timepoints: 3–7
days (T1), 3-months (T2) and 12-months (T3) post-stroke. Samples were analyzed by
liquid chromatography mass spectrometry using label-free quantification (data available
at ProteomeXchange with identifier PXD015006). Differential expression analysis revealed
that 29 proteins between T1 and T2, and 33 proteins between T1 and T3 were
significantly different, with 18 proteins commonly differentially expressed across the
two time periods. Pathway analysis was conducted using Gene Graph Enrichment
Analysis on both the Kyoto Encyclopedia of Genes and Genomes and Reactome
databases. Pathway analysis revealed that the significantly differentiated proteins
between T1 and T2 were consistently found to belong to the complement pathway.
Further correlational analyses utilized to examine the changes in regulatory effects of
proteins over time identified significant inhibitory regulation of clusterin on complement
component 9. Longitudinal post-stroke blood proteomics profiles suggest that the
alternative pathway of complement activation remains in a state of higher activation from
3-7 days to 3 months post-stroke, while simultaneously being regulated by clusterin and
vitronectin. These findings also suggest that post-stroke induced sterile inflammation and
immunosuppression could inhibit recovery within the 3-month window post-stroke.
Keywords: longitudinal, stroke, proteomics, immune system, complement system, bioinformatics, systems
biology, pathway analysis
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INTRODUCTION
of networks of established biochemical relationships (gene to
gene) using topological omics databases such as Reactome (17)
and KEGG (18) for the analysis of structural representation
of biological pathways in the analytical workflow. This more
novel approach further addresses the regulatory mechanisms
in gene and protein pathways by examining co-expression
and co-regulation networks using correlation analyses (19).
Examining the changes in correlational strength also allows
for quantification of the changes in the regulatory effect of
proteins between timepoints. Although discovery approaches are
hypothesis-free by nature, based on our previous study (14) and
others (20) that suggest inflammatory and immune homeostasis
will be disrupted in the post-stroke recovery timeline, we
hypothesize that the complement system will be dysregulated
when comparing early 3–7 days post-stroke to later 3-month and
12–month post-stroke timepoints.
Ischemic stroke covers a variety of cerebrovascular events
that affect up to 800,000 people in the United States every
year, with 133,000 deaths reported in 2017 (16.74%) (1).
Of the survivors, 30% are reported to experience prolonged
cognitive impairment (2) and depressive symptoms at any
point 5 years post-stroke (3). Currently there are few wellaccepted biomarkers for recovery and comparatively little
literature exploring the biological systems that drive recovery
or even the most optimal times for monitoring biological and
behavioral recovery. Evidence from stroke rehabilitation studies
suggest the greatest efficacy for motor-based rehabilitation is
within this 3 month time window (4), though recovery may
continue at a slower rate over subsequent months and years.
Although there has been increasing research examining the blood
biomarkers of stroke recovery (5), the additional linking of
biomarkers to biological systems remains speculative. Hence,
we aimed to investigate the changes in the molecular profile
of proteins in plasma samples via a mass spectrometry (MS)
based discovery proteomics approach (6). Mass spectrometry and
nuclear magnetic resonance (NMR) based techniques examining
protein expression are among the most versatile techniques
for protein identification and quantification, with the ability to
address a wide range of biological samples, especially plasma,
and serum (7, 8). Proteomics utilizes the advantage of systems
biology techniques to quantify a large number of analytes in
an exploratory fashion, with a computational bioinformatics
approach to further categorize biomarkers into biosystems (9).
Proteomics have recently been used to pursue multiple clinical
questions within stroke research, relating to differentiation of
ischemic from hemorrhagic stroke (10–12) and investigations
of potential biomarkers involved in post-stroke recovery (13,
14). Although traditional bioinformatics methods were originally
developed to accommodate gene expression data, proteomics
studies can utilize these methods to organize and visualize
findings by adopting standardized change scores and using
annotations that are common across proteomics and genomics
(15). Indeed, our laboratory has previously used proteomic
methods and Gene Set Enrichment Analysis (GSEA) to
investigate the relationship between protein changes in plasma at
3-months post-stroke and affective (depression) outcomes (16).
The results indicated that proteins involved in the complement
but not the coagulation pathway of the immune system are
likely to be associated with post-stroke depressive symptoms
(14). The complement system is recognized as an innate immune
pathway that contributes to primary host defence by encouraging
phagocytosis of unwanted cells. This new study aims to expand
upon our earlier single time point study by using discovery
proteomics to identify longitudinal changes in blood plasma
protein expression over the post-stroke timeline of recovery;
specifically 3 timepoints post ischemic stroke: 3–7 days (T1), 3months (T2) and 12-months (T3). We also aimed to improve
upon our previous set-based functional annotation methods by
utilizing Gene Graph Enrichment Analysis (GGEA). The GGEA
approach differs from the GSEA approach by further addition
to traditional set-based functional annotations by incorporation
Frontiers in Neurology | www.frontiersin.org
MATERIALS AND METHODS
Subjects
Data from a subset of ischemic stroke patients was obtained
from the longitudinal stroke cohort known as START, which
comprised participants from START_PrePARE (STroke imAging
pRevention and Treatment: Prediction and Prevention to
Achieve optimal Recovery Endpoints; (21) (Neuroscience
Trials Australia: NTA 0902) and START_EXTEND [STroke
imAging pRevention and Treatment: EXtending the time
for Thrombolysis in Emergency Neurological Deficits (22)
(Neuroscience Trials Australia: NTA 0901, Clinicaltrials.gov
number: NCT01580839)] studies. These prospective, integrated
studies were longitudinal and provided long term plasma
and serum samples at 3 time-points post-stroke. Selection
of the subset ischemic stroke patients for the current
study (n = 106) was initially based on the availability of
ethylenediaminetetraacetic acid (EDTA) treated plasma samples
at each of the 3 timepoints: 3–7 days (T1), 3-months (T2) and
12 months (T3). Laboratory batch processing limitations and
time/cost considerations allowed for 180 EDTA plasma samples
to be processed, i.e., samples from 60 stroke patients across 3
timepoints. We therefore conducted a stratified selection process
involving patients with complete blood samples at the 3 repeated
times and full clinical scores, to ensure a spread of scores across
clinical outcomes of stroke severity, mood, and cognition. As
expected, the current subset contained only participants who
completed the 3 sessions in the 12 months post first acute stroke
event. See Table 1 below for baseline characteristics for patients
(43 male) and Table 2 for clinical characteristics at T1, T2, and
T3. The total START_PrePARE and START_EXTEND cohort
comprised 219 patients, with 21 deaths, 8 lost to follow-up and
2 withdrawals. See Supplementary Table 1 for a comparison
between the selected subset and the total cohort. Healthy control
data was not available as part of the original protocols for the
START_PrePARE and START EXTEND studies. Ethics was
approved by the Human Research Ethics Committee of Austin
Hospital, Heidelberg (HREC code: H2010/03588), and relevant
university and hospital sites.
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and filtering for at least 80% of data present across the
protein, 163 proteins remained (Supplementary Data 1). This
procedure regarding missing values is an expected step in
addressing proteomics data as the protein identification search
on MaxQuant (utilizing an FDR of 1%) will not output as
an identified protein in the final data file unless there is a
single unique peptide, regardless of peptide length. The Limma
R package (version 3.34.9) with Benjamini–Hochberg (BH)
adjustments for familywise error rate was used to compare which
proteins were differentially expressed across T1, T2, and T3.
To further understand the functional organization of significant
protein sets, the EnrichmentBrowser R package (version 2.09.17)
was employed with GGEA as the network-based enrichment
method across the KEGG and Reactome databases using default
settings (minimum set size = 3, maximum set size = 500,
permutations = 1000). Correlations with BH corrections were
used to further explore the changes in proteins that regulate
identified pathways, with Fisher r to z transformations conducted
to explore changes in regulatory effect across timepoints.
Statistical significance levels were set to a =.05 after multiple
comparisons adjustment.
TABLE 1 | Baseline sample characteristics (n = 60).
Baseline
Mean
SD
Median
IQR
Age (years)
68.00
14.60
69.00
18.00
NIHSS
4.70
4.76
2.50
3.00
Height (cm)
160.41
26.80
170.00
20.00
Weight (kg)
74.66
19.80
80.00
26.75
Heart rate (per minute)
74.52
11.06
74.50
14.50
Systolic blood pressure (mm Hg)
141.47
23.27
134.50
29.75
78.00
14.00
Diastolic blood pressure (mm Hg)
78.50
11.77
Frequency
Percentage
Large artery atherosclerosis
14
24.14%
Cardioembolism
6
10.34%
Small vessel occlusion
15
25.86%
Stroke of other determined
etiology
3
5.17%
Stroke of undetermined etiology,
two or more causes identified
3
5.17%
Stroke of undetermined etiology,
negative evaluation
3
5.17%
Stroke of undetermined etiology,
incomplete evaluation
14
24.13%
4
6.7%
TOAST Criteria
Data Availability
Comorbidities
Past atrial fibrillation
Hypertension
26
43.3%
Lipid disorder
24
40.0%
Ischemic heart disease
11
18.3%
Diabetes mellitus
9
15.0%
Due to the requirements of ethics and the nature of ongoing
clinical trials with the START cohorts, unidentified patient
clinical data may only be made available upon request. The
mass spectrometry proteomics data have been deposited to
the ProteomeXchange Consortium via the PRIDE (23) partner
repository with the dataset identifier PXD015006.
n = 60; NIHSS, National Institute of Health Stroke Scale; 0, No Stroke symptoms; 1–
4, Minor Stroke; 5–15, Moderate Stroke; 16–20, Moderate to Severe Stroke; 21–42,
Severe Stroke (73); TOAST, Trial of Org 10172 in Acute Stroke Treatment classification
for ischemic stroke subtype.
RESULTS
The differential expression (DE) analysis revealed that 29
proteins significantly differed between T1 and T2, and that
33 proteins significantly differed between T1 and T3 (Table 3)
with 8 proteins at FDR of 0.05 indicating that the effects are
higher than would occur due to chance alone. The changes
between T1 to T2 and T1 to T3 constitute 17.79 and 20.25%,
respectively, in proportion to the total number of proteins
identified. Eleven proteins were uniquely expressed between T1
and T2 and 15 proteins were uniquely expressed between T1
and T3, with 18 of the same proteins expressed both between
T1 and T2 and T1 and T3. There were no significant differences
in protein expression between T2 and T3, potentially suggesting
that the currently identified proteome in post-stroke survivors
does not change significantly between 3- and 12-month times. See
Supplementary Data 1 for a full list of proteins and fold change
values across all comparisons.
The list of DE proteins and the full expression matrix
were submitted to EnrichmentBrowser using GGEA as the
network enrichment method (1000 permutations and α = 0.05).
This revealed significant functional annotation of our set of
proteins in both KEGG and Reactome databases only between
T1 and T2, after BH adjustment of p-values (Table 4) (see
Supplementary Data 2 a full list of nominally significant sets
from KEGG and Reactome). In the case of the Reactome
database, the nested structures in the ontological organization of
Blood Collection and Storage
All samples were collected in Benton-Dickson (BD) EDTAcoated 4 ml vacutainers and were mixed and left to stand in
ambient room temperature for 30 min. Average time to blood
draw was 3.45 days (range = 2.03–7.58 days, SD = 1.174)
after stroke onset for T1. The tubes were then centrifuged at
1100–1300 g at room temperature and the resulting plasma was
aliquoted into cryotubes and immediately stored in a −80o C
freezer. For transport from the central study freezer to the
analysis site, temperature was kept at −70 to −80o C on dry
ice before transfer into a −80o C freezer. This procedure was
consistent for the 3-month and 12-month follow up periods.
Mass Spectrometry
Label-free quantitation (LFQ) proteomics was conducted on
the Q Exactive HF Orbitrap instrument (Thermo-Fisher
Scientific). Details of the sample preparation, instrument
parameters and protein identification are available in the
Supplementary Methods.
Bioinformatics and Statistical Analysis
The initial MaxQuant output consisted of 358 identified proteins
across 180 samples. After removal of site-identified contaminants
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TABLE 2 | Sample Clinical Characteristics at 3–7 days, 3 months and 12 months (n = 60).
3–7 Days (T1)
Mean
SD
Median
3 Months (T2)
IQR
12 Months (T3)
Mean
SD
Median
IQR
Mean
SD
Median
IQR
Weight (kg)
76.30
15.30
78.00
19.00
78.75
13.57
79.50
20.00
Heart rate (per minute)
63.02
21.03
65.00
20.00
61.50
21.01
70.00
19.00
Systolic blood pressure (mm Hg)
125.76
16.92
125.50
21.00
124.28
15.34
124.00
17.25
74.78
10.17
76.00
11.75
72.47
9.78
70.00
15.00
1.18
2.31
0.00
1.00
2.45
12.87
0.00
1.00
1.25
1.31
1.00
2.00
1.12
1.22
1.00
2.00
Diastolic blood pressure (mm Hg)
NIHSS
2.35
3.05
1.00
3.00
mRS*
MoCA
24.21
5.00
26.00
5.00
25.93
4.57
28.00
5.00
25.07
5.02
26.00
4.00
MADRS
6.93
7.31
4.00
11.00
8.72
8.81
5.50
13.75
7.13
7.41
5.00
10.75
NIHSS, National Institute of Health Stroke scale; mRS, modified Rankin Scale; MADRS, Montgomery-Åsberg Depression Rating Scale; MoCA, Montreal Cognitive Assessment.
*The mRS is not conducted at 3–7 days as it is a measure of post-stroke disability within previous 30 days.
TABLE 3 | Differentially expressed proteins (BH p < 0.05) detected between T1, T2 and T1, T3.
3–7 Days (T1) to 3 Months (T2)
Gene symbol
3–7 Days (T1) to 12 Months (T3)
log2 fold change
adj p
Gene symbol
log2 fold change
adj p
C8A
0.6657
0.0000
C8A
0.7394
0.0000
ACTB
0.3766
0.0000
APOA4
0.3585
0.0000
0.0000
APOA4
0.3169
0.0000
ACTB
0.3712
A1BG
−0.3210
0.0004
CFI
0.3346
0.0000
C9
−0.2814
0.0027
APOA2
0.2349
0.0003
−0.3231
0.0003
CLEC3B
0.1814
0.0027
C9
FBLN1
0.2953
0.0044
PGLYRP2
0.1863
0.0006
CFI
0.2417
0.0086
CLEC3B
0.1992
0.0006
APOD
0.1584
0.0104
APOB
−0.1754
0.0049
PGLYRP2
0.1634
0.0104
CFD
−0.2793
0.0109
CPN1
APOF
IGFALS
F9
SERPINA3
SAA2-SAA4
0.2459
0.0065
−0.1435
0.0065
0.2243
0.0065
0.1975
0.0131
TF
−0.2180
0.0133
LRG1
−0.2285
0.0066
0.1332
0.0133
RBP4
0.2153
0.0067
0.0068
−0.2462
0.0141
FGA
0.1504
RBP4
0.1836
0.0146
APOD
0.1399
0.0068
TF
0.2026
0.0146
A1BG
−0.2353
0.0073
0.1906
0.0073
APOC1
−0.1699
0.0286
IGFALS
FETUB
0.1995
0.0286
SAA2-SAA4
−0.2728
0.0081
−0.1673
0.0295
A2M
−0.1860
0.0095
PLTP
0.3322
0.0295
AFM
0.1487
0.0160
SAA1
−0.9248
0.0295
C4BPB
−0.2011
0.0160
FGB
−0.2347
0.0308
CSPG4
0.2386
0.0160
0.0187
HPR
APOB
−0.1347
0.0389
C4BPA
0.1206
C8G
−0.1511
0.0389
APCS
−0.2772
0.0198
VTN
0.0723
0.0413
VTN
0.0737
0.0244
−0.2143
0.0420
GPLD1
0.2335
0.0274
HPX
SERPINA10
0.0918
0.0433
ORM1
−0.3497
0.0274
C6
0.0980
0.0493
LBP
−0.2497
0.0282
FBLN1
0.2117
0.0296
SAA1
−0.9380
0.0334
SERPINA3
0.1000
0.0374
HPX
0.0898
0.0440
p < 0.05 adjusted by the Benjamini–Hochberg procedure was considered statistically significant. Lists are ordered by p-value.
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better understand the biochemical pathways impacting/affecting
the post-stroke timeline of recovery, this study employed a
discovery approach utilizing mass spectrometry to examine
protein expression in patients at 3–7 days (T1), 3-months
(T2), and 12-months (T3) post-stroke. Two sets of proteins
were identified to be significantly different based on differential
expression analysis from T1 to T2 and T1 to T3, but
not between T2 and T3. Of these proteins, complement
(C8A, C9, CFI), apolipoproteins (APOA4, APOA2, APOD)
and membrane bound proteins (TF, ACTB) were highly
overexpressed; consistent with high abundance typically found
in human blood samples (24). The lists of significantly
differently expressed proteins were analyzed using the GGEA
bioinformatics algorithm based on the topological consistency
of the observed data sets compared to database defined
reference pathways. This revealed that the proteins identified
in this longitudinal experiment significantly conformed to
pathways central to the complement system in both KEGG and
Reactome databases. To our knowledge, the current study is first
published use of the GGEA approach in clinical proteomics of
human blood.
There is currently limited knowledge of the changes in
complement system in post-stroke recovery, especially relative
to blood plasma concentrations in similar aged individuals
(mean age 68 ± 14 years). Furthermore, most studies examining
the complement system post-stroke have described its damageexacerbating role in acute stroke human and animal models but
have not examined changes in levels longitudinally over time
post-stroke (25–29). Many of the studies exploring complement
in stroke have examined the system in the context of danger
associated molecular pattern (DAMP) signaling and subsequent
neuronal repair in central nervous system (CNS) specific injury
(30), with plasma and serum based studies examining levels in
relation to clinical outcomes (31, 32). The endothelial junctions
forming the blood brain barrier (BBB) have been traditionally
viewed as preventative to the entry of large peripherally derived
molecules such as complement into the cerebral parenchyma (33,
34). The CNS has also been recognized as able to endogenously
biosynthesize complement proteins in glial cells (35). Although
disruption to BBB permeability allows for the passage of
complement proteins from periphery to CNS (34), to our
knowledge, no study has simultaneously examined the levels and
functional activity of the complement system in both domains.
Therefore, the results of this study have been interpreted as
referring to the circulating fluid phase proteins that enhance
the decay of complement proteins by cleaving active proteins
through allosteric binding (25).
The complement pathway has three traditional modes
of activation, the classical, lectin and alternative pathways.
The classical and lectin pathways are activated by antigen
and pathogen binding complexes, respectively, whereas the
alternative pathway is an internal biomechanical activation loop
that regulates downstream pathway activity (36). In the classical
pathway of complement activation, upregulation of SERPING1
or the C1 inhibitor protein is responsible for regulation of
the C1 complex. Specifically, SERPING1 strongly binds to the
C1r and C1s proteases and also to the activation units of the
TABLE 4 | Significantly Enriched Gene Pathways between T1 (3–7 days) and T2
(3-month) timepoints in Stroke Survivors.
Gene Set
Identifier
Normed Score
adj p
hsa04610
1.91
0.003
Immune system
R-HSA-168256
0.497
0.0009
Innate immune system
R-HSA-168249
0.497
0.0009
Complement cascade
R-HSA-166658
0.497
0.0009
KEGG
Complement and coagulation
cascades
Reactome
p < 0.05 adjusted by the Benjamini–Hochberg procedure was considered
statistically significant.
the pathways can lead to redundancy resulting in identification
of multiple pathways, especially as the “Complement Cascade”
is located as a subset of the “Innate Immune System” and the
“Immune System.” Further interpretation is needed to account
for the number and type of identified proteins between the
reference pathways and experimental data and should occur at
the level closest to the relevant biological processes. Both KEGG
and Reactome databases concurred on the identification of the
complement system as the significantly enriched network in these
plasma samples.
A visual illustration of this enriched pathway on the
Reactome database reveals the changes and interactions
of the detected molecules on the complement pathway
(Supplementary Figure 1), displaying information on nodes
(proteins) and edges (the correlation between nodes). Similarly,
the results from the KEGG database includes elements of
the complement pathway but also the coagulation cascades
(Supplementary Figure 2). As the complexity of information
is difficult to interpret without a significant degree of system
specific understanding in these pathway diagrams, a composite
network pathway was created to amalgamate the statistical
relationships found in both diagrams pertaining to the
complement system as shown in Figure 1.
Some of the regulatory proteins in the complement pathway
(CLU & VTN) were not included in the pathway diagrams
for KEGG and Reactome databases used in this analysis.
Therefore, a correlation matrix with BH corrected p-values was
created to explore the direction and strength of the regulatory
effects between regulator and related proteins (Table 5). Fisher’s
transformation of Pearson’s r to normally distributed z scores
was computed to compare changes in regulatory effects between
T2 and T1. While not overall presenting significant results,
examination of the rdiff (T2-T1) complement regulators on their
target proteins show that the middle stage complement regulators
shifted their effects forward, resulting in a reduced inhibitory
effect. Only the inhibitory effect of CLU on C9 was significantly
increased from T1 to T2, with VTN also showing a large effect
size for changes in C9 regulation.
DISCUSSION
Currently the interaction of the many biological mechanisms
limiting post-stroke recovery remain poorly understood. To
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Stroke Recovery Longitudinal Proteomics
FIGURE 1 | A pathway diagram contrasting information from differential proteomic analysis of KEGG and Reactome GGEA complement system pathways at 3–7
days (T1) and 3-months (T2) post-stroke. This pathway is activated by the C1q complex antigen linked immunoglobulin through the classical pathway and pathogen
surface linked mannose binding linked (MBL) proteins through the MASP complex. The alternative pathway of complement activation follows a constant and low state
of activation by a feedback loop that is heavily regulated by complement factor I (CFI) and complement factor H (CFH). Simultaneous upregulation of complement
factor D (CFD) and CFI suggest that the alternative pathway is undergoing activation but also being regulated to prevent autoimmune insult. Recently, the link between
coagulation and complement has been established through thrombin, plasmin and factor XIIa, these factors were not identified or significantly related to complement
in this model. All activation pathways lead to the promotion of cleavage of fluid phase complement component 3 (C3) in the bloodstream, with heavier β chains
forming convertases downstream and lighter α chains such as C3a and complement component 5α (C5a) able to signal for potent local and systemic inflammation
responses, with the most extreme being anaphylactic shock. The final stage of complement activation is the membrane attack complex (MAC), a large protein
complex that is constructed on cells to disrupt the outer membrane and promotes active cytolysis. The profile of significant differences in MAC proteins such as C8G
and C9 suggest plasma complement regulation by vitronectin (VTN) or clusterin (CLU). Image adapted from KEGG [map04610: Complement and coagulation
cascades (homo sapiens)] with permission.
can further inhibit leukocyte-endothelial adhesion and vascular
rolling via interference of cell adhesion molecules (CAMs)
or selectins, the first physiological requirements for transendothelial leukocyte infiltration (38). Although a fold change of
0.008 or 0.8% for SERPING1 was detected between T1 and T2
lectin pathway [mannose binding lectin, MBL, and associated
proteases MASP1 and MASP2 (37)], effectively inhibiting the
initial activation effects of the complement system in both antigen
and pathogen related pathways. In addition to the effects on
the complement system, high concentrations of this protease
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TABLE 5 | Pearson’s Correlation matrix with Fisher r to z transformations for complement proteins and their regulators between T1 and T2.
Stage
Early
Mid
Late
Regulator
Complement protein
r (T1)
r (T2)
r diff (T2-T1)
zdiff (T2-T1)
adj p
0.71
SERPING1
C1QA
−0.01
0.06
0.07
0.37
SERPING1
C1QB
0.13
0.16
0.03
0.15
0.88
SERPING1
C1QC
0.25
−0.01
−0.26
−1.44
0.15
0.53
SERPING1
C1R
−0.11
0.00
0.12
0.62
SERPING1
C1S
−0.18
−0.15
0.02
0.12
0.90
C4BPA
C4A
−0.03
0.17
0.20
1.08
0.28
−0.08
0.08
0.16
0.86
0.39
0.04
0.06
0.01
0.08
0.94
−0.13
0.17
0.31
1.64
0.10
C4BPA
C4B
C4BPA
C2
C4BPB
C4A
C4BPB
C4B
−0.15
−0.08
0.07
0.40
0.69
C4BPB
C2
−0.26*
−0.05
0.21
1.13
0.26
CFH
C3
0.37*
0.54*
0.16
1.10
0.27
CFI
C3
0.15
0.27*
0.12
0.67
0.50
CLU
C6
0.13
0.05
−0.07
−0.40
0.69
CLU
C7
0.01
0.04
0.03
0.14
0.89
CLU
C8A
0.10
−0.03
−0.13
−0.68
0.50
CLU
C8B
0.35*
0.14
−0.20
−1.15
0.25
CLU
C8G
−0.24
−0.18
0.06
0.31
0.76
CLU
C9
−0.02
−0.49*
−0.47**
−2.74
0.01
VTN
C6
0.25
0.19
−0.06
−0.32
0.75
VTN
C7
0.23
0.10
−0.12
−0.68
0.49
VTN
C8A
0.42*
0.28*
−0.14
−0.85
0.40
VTN
C8B
0.24
0.29*
0.05
0.28
0.78
VTN
C8G
0.10
0.15
0.04
0.24
0.81
VTN
C9
0.04
−0.30*
−0.34
−1.87
0.06
The early stage of the complement pathway consists of activation units, the middle stage consists of protein activity related to cleaving C3 and C5, with the late stage consisting of
proteins involved in membrane attack complex (MAC) formation. These correlations only show the effect of a single regulator on the related protein and does not take into consideration
the downstream pathways. p was BH adjusted *p < 0.05, **p < 0.01.
in this study, previous reports have indicated that a fold change
can be observed during acute inflammation and that higher
levels may be therapeutic in preventing autoimmune injury by
providing a stop mechanism to complement system activation
(39, 40). These acute properties have been demonstrated in
animal models of post-stroke middle cerebral artery occlusion
(MCAO) and shown to attenuate ischemic reperfusion injury
(41, 42). Limited evidence for longitudinal changes in levels
of SERPING1 in human cardiovascular disease have previously
been suggested as a link to low-grade levels of chronic
inflammation (43). Furthermore, the incidence rates of poststroke immunological trajectory of post-stroke recovery may also
be explained by the immune inhibitory effects of SERPING1,
following the initial phase of post-stroke immune flux and
subsequent immunosuppression (44).
The cleavage of C3 is the central amplification step of the
whole complement pathway, with active C3b forming the C5
convertase and C3a inducing further phagocytotic chemotaxis
(29). Given that C3 is central in all complement activation
pathways, especially with the alternative pathway requiring C3
hydrolysis, much of the theorized regulatory activity in this
pathway is focused on inhibition of C3 convertase formation
and decay rate (36). Of the complement regulatory proteins,
Frontiers in Neurology | www.frontiersin.org
CFH and CFI were overexpressed, with only plasma CFI levels
shown to be statistically different. Additionally, the changes in
regulatory effect for middle stage complement proteins show
that even though CFH and CFI are upregulated between T1
and T2, their inhibitory effects were reduced. Although CFI
and CFH are cofactors of C3 convertase (not identified in
this study) inhibition and not C3 itself, the overall profile
suggests a relative lack of effectiveness in the functional ability
of these proteins to regulate alternative complement pathway
activity in this older group. Furthermore, this profile may
also be indicative of overexpression in levels of CD55 or
decay accelerating factor (DAF), a protein that inhibits C3
and C5 convertase formation, albeit on cell surfaces (45).
Complement C3a has previously been shown to be acutely
elevated from 1 to 28 days post-stroke (46, 47). Indeed, ongoing
anaphylatoxin signaling from components such as C3a and C5a
are damaging to host systems, especially in cerebral ischemia
(25). The quantification of properdin, a protein that acts as a
positive regulator of C3 and C5 convertases (48), would further
ameliorate understanding of alternative pathway activation in
this sample.
Complement component C5 is the important functional
unit of the pathway. Cleavage of soluble C5 protein into
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Nguyen et al.
Stroke Recovery Longitudinal Proteomics
C5b is able to create the porous membrane attack complex
(MAC) on targeted cells to induce cell death by cytolysis while
C5a is able to produce local and system wide inflammatory
cascades that are 20 times more potent than C3a (49). The
profile here shows that C5 is upregulated at 3-months, despite
reduced C3 levels. This suggests that the reduction in C3 levels
may be indicative of active cleaving to produce downstream
increased C3b and C3a levels to produce or maintain constant
immune system homeostasis. The detection of increased C5
may also imply involvement of the novel complement pathway
or the extrinsic complement pathway, linking coagulation, and
complement systems. This pathway was originally thought to
be activated based on thrombin functionally substituting for the
C3 dependent C5 convertase (50). Recently, a study has also
demonstrated that in thrombosis, interactions with plasmin in
both surface bound and fluid phase complement is capable of
upregulating C5a and C5b whether in venous or arterial thrombi
(51). In our analysis however, plasminogen was not found to be
significantly differentially expressed or related to the complement
pathway (Supplementary Figure 2).
Our methodology identified and quantified all the molecular
components in MAC formation pathway. Our findings
demonstrate that C6 and C8A were significantly upregulated
and C8G and C9 were significantly downregulated between
T1 and T2. Regulators of MAC formation exhibit function by
preventing polymerisation of the C8 or C9 complexes, thereby
inhibiting cytolytic function on cell surfaces (52). Pore formation
is heavily regulated by CD59 or MAC-inhibitory protein on host
cell surfaces to prevent further immune insult resulting from
early tissue induced hyperinflammation and gradual immune
dysregulation (53). As this expression profile was obtained
in plasma, the expression profile here is suggestive of fluid
phase regulation of MAC formation, dependent on vitronectin
(VTN or S-protein) and clusterin (CLU or apolipoprotein J).
To create a successful MAC, from 12 to 16 molecules of C9
are needed as the final step to form a cyclical pore in addition
to 1 of each previous MAC protein in the chain (54). Both
VTN and CLU inhibit the formation of the MAC insertion
C5b-7 complex and also inhibit the ability of C9 to bind to
inserted MAC complexes to create functioning lysis pores (54).
VTN and CLU were both found to be overexpressed, with
the increases in VTN shown to be statistically significant (36).
Overall, the profile of the complement system here appears
to be dysregulated, with an increase in alternative pathway
activation by CFB, and reduced inhibitory effectiveness of all
middle stage complement proteins but especially CFI and CFH.
The apparent contradiction in expected abundances of MAC
proteins suggests that the pathway is also being inhibited by CLU
and VTN.
Although few studies have presented this specific profile
of complement expression, and currently none in a complex
proteomics pathway model, there are some interpretations
relevant to stroke survival and recovery that can be made here.
From the upregulation of C5 and initial MAC proteins, it is
possible that C5 is being continually cleaved to produce the proinflammatory chemokine C5a. This can be understood in theory
of post-stroke immune profiles where both central and peripheral
Frontiers in Neurology | www.frontiersin.org
systems never fully recover to pre-stroke levels and establish new
albeit dysfunctionally higher inflammation profiles (44).
The profile of complement activation described in this study
is likely to indicate that
1) the internal activation loop that readies the immune system
for pathological threats and tissue damage (the alternative
pathway) is activated,
2) middle stage complement regulators that are normally
expected to be activated to prevent indiscriminatory
autoimmune damage are overexpressed but their intended
inhibitory effects (CFI –> C3) are reduced and
3) although early chain MAC proteins (C6, C7, C8A, C8B)
trend toward overexpression by time T2 are likely due
to alternative pathway activation, CLU and VTN are
also overexpressed, possibly as a compensatory mechanism
for alternative pathway activation and thereby preventing
autoimmune MAC formation by inhibiting C9 expression on
host/friendly cells.
Our results did not find/demonstrate an association between C8G
expression and CLU or VTN, although downregulation of C8G
is theorized to be coincidental with immunosuppression (55).
It is speculated that based on the positional role of C8G as the
protein that provides a binding site for the first C9 molecule in the
structural formation of the MAC pore [see (56) for an overview
of MAC protein structure], the significant downregulation of
C8G between T1 and T2 can likely be attributed to inhibitory
CD55 or CD59 (that was not present in our proteomics
analysis). Furthermore, there are additional molecules and
mechanisms that may regulate MAC formation through C8G
expression such as anti-apoptotic processes that promote cell
survival (57). This may also explain the immunosuppressed
state of post-stroke patients and infections (58), whereby the
system is not able to react as efficiently to acquired pathogens
such as viral pneumonia due to increased fluid phase MAC
regulation (59).
Limitations and Future Research
These results are limited to interpretation based on the sample
characteristics of a cohort of mild stroke patients over a 12month period post-stroke. During this period, it is recognized
that other potential concurrent processes such as recurrent
stroke, infections, venous thromboembolism could impact on
the results: however, we were not able to control or adjust for
such factors in the current analyses. The lack of an age matched
control sample represents a challenge to external validity, and
it is uncertain if these changes could also occur in people
without stroke, as a process of senescence and aging or other
comorbidities. Additionally, our results were not controlled
for by age and baseline stroke as the analyses using the
EnrichmentBrowser package did not have a streamlined function
to address confounds. The profile of complement expression in
this sample may also have been influenced by the myriad of
post-stroke medications that currently include aspirin, warfarin,
and clopidergel. Although these pharmacological agents target
aspects of the coagulation system, it is possible that they
also exhibit regulatory effects on the complement system (60)
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Nguyen et al.
Stroke Recovery Longitudinal Proteomics
CONCLUSION
via the intersection between hemostasis and innate immunity
(61). These confounds remain difficult to statistically control
in the context of bioinformatics models, and should form
part of the conceptualization in the design phase in future
studies (62). While the focus of the current study was to
explore evidence of change in biological pathways over the
first-year post-stroke, it is recommended that future studies
investigate the relationship between changes in these pathways
and changes with age as well as in neurological and functional
recovery profiles.
The sample preparation methods used in this study did
not include fractionation or depletion of the plasma samples,
leading to a low number of proteins identified relative to
the blood proteome. Blood samples represent a very complex
sample with high dynamic range for proteomics analysis,
with a proportionally high abundance of proteins such as
albumin, haptoglobin, fibrinogen, and immunoglobulin G (63).
Future studies could increase the range of identified proteins
by depleting abundant plasma proteins with various methods
before MS analysis (64, 65); although some popular depletion
columns also remove proteins identified here as differentially
expressed such as those within the complement system (66).
In addition to this, limitations to the discovery approach
imply that there is a degree of uncertainty in the ability
to identify a complete group of proteins (67, 68), especially
where set and network-based bioinformatics are the intended
analyses. Therefore, after pathways are identified by robust
techniques such as GGEA, the integrity of these pathways
can be enhanced by examining proteins that are missing in
the pathway and directly related to pathway activation and
function by targeted proteomics techniques such as single or mass
reaction monitoring (SRM or MRM) in MS with immunoaffinity
enrichment (69).
In this study, the LC-MS methodology did not detect the
specific formation of C3 or C5 chains or convertase protein
complexes. Therefore, the pathway representations of relative
abundance for C3 and C5 may not necessarily reflect the
abundance of downstream protein chains of the active products.
Specifically, C3a and C5a would be interesting proteins to
target as they have potent effects on inflammation physiology
and would be of concern especially if shown to be elevated
at 3 months post-stroke. Fluid phase identification of inactive
pre-MAC complexes with protein S such as sC5b-7, 8 and
9 in addition to VTN and CLU may also provide further
information on the regulatory activity of the MAC in stroke
survivors. Furthermore, the longitudinal profile of complement
in this study and others (25, 47) suggests that complement
dysregulation begins in the first week post-stroke and continues
for at least 3 months, and may set a post-stroke level of
immune homeostasis. This finding needs to be further validated
at other timepoints within this window, especially in relation
to infection based mortality (70). To this extent, it is possible
that the post-stroke immune profile is settled at 3 weeks
to 1 month post-stroke when adaptive immunity is engaged
in phagocytizing dead cells (44) and resolution of cerebral
microedema (71).
Frontiers in Neurology | www.frontiersin.org
The biology of post-stroke recovery is not well-understood,
with patients exhibiting varying profiles based on factors such
as their age, location of lesion, and degree of stroke damage
(72). This study aimed to characterize common peripheral
biological systems involved in post-stroke recovery by examining
the longitudinal proteomics profile of EDTA plasma in stroke
survivors. Specifically, elements of the alternative pathway of
complement system and MAC proteins, i.e., CFI, C6, C8A, C8G,
C9 were found to be activated but also undergoing regulation
at 3 months post-stroke. This increased turnover may lead to
the upregulation of anaphylatoxins C3a and C5a that could
explain the prolonged sterile inflammation profile of post-stroke
survivors. These results suggest a biomechanism for post-stroke
immunosuppression by complement system regulatory proteins.
This knowledge may be useful to guide assessment in the clinical
setting for post-stroke infections and immune recovery in the
3-month recovery window. For example, it may be used to
inform future recommendations for in-hospital and post-stroke
infections by suggesting clinical utility of complement panel
blood tests. Future investigations should examine the clinical
implications of this biological profile and determine the temporal
trajectory of immune homeostasis post-stroke.
DATA AVAILABILITY STATEMENT
The datasets presented in this study can be found in
online repositories. The names of the repository/repositories
and accession number(s) can be found below: http://www.
proteomexchange.org/, PXD015006.
ETHICS STATEMENT
The studies involving human participants were reviewed and
approved by Human Research Ethics Committee of Austin
Health, all participating Hospitals and La Trobe University. The
patients/participants provided their written informed consent to
participate in this study.
AUTHOR CONTRIBUTIONS
VN, NR, SC, LC, and GH contributed to project conception,
data analysis, and draft editing. PF and HR contributed to data
analysis and project conception. TW, HM, DH, SD, and GD
contributed to project conception. All authors contributed to the
article and approved the submitted version.
FUNDING
We acknowledge financial support for conduct of the
START_EXTEND and START_PrePARE studies from the
Commonwealth Scientific and Industrial Research Organization
(CSIRO) of Australia, Flagship Collaboration Fund through the
Preventative Health Flagship and support for analysis, write
up and researchers from the James S. McDonnell Foundation
9
July 2020 | Volume 11 | Article 692
Nguyen et al.
Stroke Recovery Longitudinal Proteomics
ACKNOWLEDGMENTS
21st Century Science Initiative in Cognitive Rehabilitation Collaborative Award (# 220020413); NHMRC Centre of Research
Excellence in Stroke Rehabilitation and Brain Injury (#1077898);
NHMRC program grant Saving brain and changing practice
in stroke (#1113352); Victorian Government’s Operational
Infrastructure Support Program; an Australian Research Council
Future Fellowship awarded to LC [#FT0992299]; and a La
Trobe University Post Graduate Scholarship supported by the
Understanding Diseases research focus group awarded to VN.
The funding sources had no role in the conduct of this study or
the writing of this report.
The authors acknowledge the stroke patients and all
research personnel associated with the START_EXTEND
and START_PrePARE teams.
SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/fneur.
2020.00692/full#supplementary-material
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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.
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