Background
Sjögren’s syndrome (SjS) is a chronic autoimmune disorder that affects the exocrine glands and multiple organ systems. The pathobiology of SjS involves the activation of the innate immune system and development of autoimmunity to exocrine tissues, resulting in severe dryness. In general, SjS can be divided into primary or secondary SjS (pSjS or sSjS) depending on co-morbid rheumatologic or autoimmune diseases [
1]. In addition, although SjS is the second most common autoimmune condition to be diagnosed next to rheumatoid arthritis (RA), SjS diagnosis usually is difficult due to multiple symptoms resembling other conditions and a lack of disease-specific diagnostic markers [
2,
3]. Devastating consequences of SjS have prompted research endeavors to expedite SjS diagnosis and guide SjS intervention strategies.
The precise mechanisms and sequence of events leading to salivary and ocular dysfunction are still unclear. Inflammatory responses in autoimmune conditions are assumed to be limited based on the tissue microenvironment, where tissue homeostatic processes and regulation of inflammation are critical in curtailing chronic inflammation. The innate immune cells involved in these processes, such as macrophages, dendritic cells (DCs), and epithelial cells, have been shown to shape lymphocyte proliferation and functions during adaptive immune regulation [
4]. Monocytes and their derivatives of macrophages and DCs have abnormalities in autoimmune diseases such as systemic lupus erythematosus (SLE), RA, and SjS [
5]. Monocytes represent a relatively homogenous population of important precursor cells to peripheral macrophages and DCs. Monocytes have innate immune functions including immunomodulatory functions, cytokine production, and phagocytosis [
6]. Among SjS patient cell subsets, monocytes secrete increased levels of pro-inflammatory cytokines such as interleukin(IL)-6 and B cell-activating factor (BAFF) upon stimulation [
7], express type I interferon (IFN)-regulated genes [
5,
8‐
10], reduced NFκB inhibitor (IκBα) [
11], and show decreased function in phagocytosis of apoptotic cells [
12]. Thus, monocytes reflect the inflammatory state in SjS patients [
10] and mature monocytes are proposed to contribute to salivary gland inflammation in SjS [
13].
MicroRNAs (miRNAs) are naturally occurring post-transcriptional regulators of their targeted genes and can directly inhibit protein translation by a variety of mechanisms including direct cleavage of messenger RNAs (mRNAs) and translational repression due to disruption of translation initiation or premature translation termination [
14,
15]. MiRNAs can regulate a wide range of processes including pro-apoptotic pathways, sensitization of innate immune receptor signaling, and cytokine expression in a variety of autoimmune processes and disease [
16,
17]. One of the most intriguing aspects is how miRNAs function to cooperatively modulate cell processes by limiting specific pathway components, and thus in recent years extensive research has been performed to characterize miRNAs and their regulation of immune responses and immune cell development. Ours and other studies reported miRNAs are associated with SjS salivary gland tissue inflammation [
18‐
21] and are shown to be upregulated in SjS peripheral blood mononuclear cells [
20‐
23] and in long-term cultured salivary gland-derived epithelial cells [
21]. Due to the complexity of cell subsets and miRNA-mRNA interactions involved in autoimmunity, it is still not well understood how miRNA dysregulation contributes to autoimmune disease pathogenesis in human patients.
Considering possible roles of miRNAs as mediators of inflammation, we hypothesized profiling of miRNAs in SjS monocytes provides insight into functional implications of miRNAs in shaping the observed dysregularities. No miRNA studies have been conducted on SjS monocytes to date, let alone clarified implications of miRNAs in SjS pathogenesis. Therefore, the objective of this study was to profile miRNAs in SjS monocytes, focusing on identifying SjS-specific miRNAs and predicting their potential roles in SjS pathogenesis. Our study is the first to characterize the differentially expressed miRNAs in SjS in comparison to HC, SLE, and RA autoimmune conditions and identify a potential SjS-specific miRNA profile in SjS monocytes. Elucidation of the pathobiology of SjS is critical not only to guide intervention strategies, but also to expedite SjS diagnosis.
Discussion
Research on gene expression in SjS CD14
+ monocytes has yielded valuable findings for the role of inflammatory genes in SjS [
5]. However, identifying master genes to be targeted to reverse disease processes has been a challenge in the field of SjS. MiRNAs can modulate protein expression levels by inhibiting gene translation into protein. MiRNA expression dysregulation has been reported in a variety of autoimmune diseases, including SjS [
18‐
20,
22,
23]. However, the vast majority of previous studies evaluated miRNA expression levels in mixed cell populations or tissues, which may confound the further evaluation of specific miRNA-mRNA target relationships. Therefore, our study utilized miRNA microarray analyses to screen for differentially expressed miRNAs in SjS CD14
+ monocytes in comparison with HC, RA, and SLE. In addition, we selected six SjS-associated miRNAs to be validated by RT-PCR and performed intensive pathway analyses to identify potential target pathways of SjS-associated miRNAs in CD14
+ monocytes.
In general, primary and secondary SjS patients were undifferentiated from each other and showed the highest level of miRNA expression, whereas SLE patients tended to have an intermediate level of expression in comparison to SjS patients. Unsupervised hierarchical clustering of these miRNAs showed SjS and SLE patients clustered together independent from HCs and RA patient miRNA profiles (Fig.
1). These findings indicate miRNA alterations in monocytes are more common in SjS regardless of co-morbid autoimmune conditions and their dysregulation may play a role in SjS and SLE. In addition, although the healthy control group tended to be much younger than autoimmune groups (Additional file
1), they did not differ significantly based on miRNA associations with age (Additional file
8), which suggests age is not likely to confound our results.
It is important to note the monocytes utilized in this study were freshly isolated from peripheral blood. Monocytes once activated will rapidly adhere and migrate into affected tissues. Therefore, it was unsurprising a variety of previously reported miRNAs associated with monocyte activation and differentiation into DCs and macrophages, including miR-155-5p, miR-146a-5p, miR-146b-5p, miR-21-5p, miR-22-3p, miR-221-3p, miR-222-3p, and miR-424-5p [
35,
36] were not differentially expressed miRNAs between SjS and control groups in our study. Indicating upregulated SjS-associated miRNAs in monocytes are most likely independent of immune activation stimuli controlling monocyte maturation and differentiation.
Validation of six selected SjS-associated miRNAs showed miR-34b-3p was upregulated in SjS patient monocytes in comparison to HCs and RA patient monocytes (Fig.
2a). MiRNAs significantly upregulated in SjS patient monocytes in comparison to RA patients were miR-3162-3p and miR-4701-5p (Fig.
2b,e). Notably, miR-300 and miR-609 tended to be elevated in SjS samples, although two RA patients from the validation cohort expressed higher levels of miR-300 and miR-609 (Fig.
2c,d). Conceivably, this observed overlap in upregulated SjS-associated miRNAs for SLE and RA patients could be related to the observed co-aggregation and similarities in serologic profiles of SjS patients with SLE and RA [
37,
38].
Because SjS-associated miRNAs were similarly upregulated in SjS patients, we hypothesized SjS-associated miRNAs may also be co-expressed in individuals. Interestingly, 43.3 % of SjS patients, in comparison to 5.8 % SLE and 5.6 % RA patients, had upregulated expression of at least two of the six SjS-associated miRNAs analyzed (Fig
3a). Furthermore, regression analyses indicated definite pairs of miRNAs showed positive associations in SjS patients (Fig.
3b). Therefore, our data suggests co-expression of SjS-associated miRNAs may be especially useful in distinguishing cases of SjS. In addition, since these SjS-associated miRNAs are not genetically linked (Additional file
5), co-expression and upregulation of these miRNAs in SjS may indicate shared functions of regulating molecular pathways in monocytes.
In general, co-expressed miRNAs can share functions either by co-targeting individual genes or by targeting different components of the same pathway. Our in-depth analyses of target prediction algorithms and verified miRNA-mRNA interactions were utilized to reconstruct potential pathways targeted by SjS-associated miRNAs. Interestingly, canonical TGFβ signaling pathway showed the greatest coverage of predicted targets, averaging 40.5 % ± 23.1 (SD) pathway coverage for the six SjS-associated miRNAs (Fig.
4a). Moreover, TGFβ receptors are well known to activate a variety of associated signal transduction cascades, including the canonical MAP kinase and p 38/JNK MAP kinase pathways [
34]. Targeting of the MAPK pathways averaged 27.4 % ± 16.4 coverage for the six SjS-associated miRNAs tested (Fig.
4a). Previously, miR-34 family members [
39,
40], miR-609 [
40], and miR-300 [
41] were independently predicted to target the TGFβ signaling pathway. Specifically, experimental data support miR-34b-3p directly targets cAMP response element-binding (CREB)-1 transcription factor [
42] and bone morphogenetic protein (BMP)-2 [
43]. In addition, miR-609 directly targeted the full-length gene transcript of TGFβ1 [
44] and
TGFBR3 gene transcript [
29,
45]. MiR-300 overexpression inhibited the TGFβ-mediated epithelial-to-mesenchymal transition in tumor cells [
46]. And miR-877-3p has been shown to directly target
SMAD2 gene transcript [
29,
47]. Furthermore, we observed considerable overlap in evidence for SjS-associated miRNAs to target
ACVR2B,
SMAD2,
SMAD3, and
SMAD4 (Fig.
4b).
Based on our target prediction analyses,
SMAD2,
SMAD3 and
SMAD4 were selected for initial screening for changes in gene expression in SjS monocytes compared with HCs. Intriguingly,
SMAD2 and
SMAD3 gene expression indicated significant increases in SjS monocytes compared with HCs (Fig.
5a), which has not been previously reported. Both
SMAD2 and
SMAD3 3’UTRs contain multiple alternative polyadenylation sites upstream of predicted miRNA binding sites according to AceView gene database [
33], which we speculate could result in failure in miRNA-directed inhibition of gene transcript levels. Alternatively, miRNAs can also regulate protein translation by mechanisms other than transcript degradation [
15]. However, at this time our study is limited to RNA-based analyses and monocytes are collected analyses. Relationships between miRNAs and their post-transcriptional inhibition of target genes will be addressed as soon as sufficient human monocytes are collected for protein analyses.
Nevertheless,
SMAD4 gene expression exhibited repression in a subpopulation of pSjS patients (Fig.
5a). The SMAD4 protein of the family of TGFβ signaling molecules acts as an essential common mediator for receptor-regulated SMADs to enter the nucleus. MiR-300 and miR-609, which showed the strongest co-association, are likely to target
SMAD4 directly based on target prediction analyses (Fig.
4b). Four miR-300 putative interaction sites are upstream of the first alternative polyadenylation site and the best predicted interaction of miR-300 with the lowest minimum free energy (MFE) -15.1 kcal/mole begins at nucleotide position 833 of the
SMAD4 3’UTR as is shown in Fig.
4b. The best predicted interaction for miR-609, MFE -28.6 kcal/mole, is relatively nearby downstream to miR-300 predicted target sites and begins at position 2810 of the
SMAD4 3’UTR (Fig.
4b). Regression analyses showed
SMAD4 gene expression is significantly associated with both miR-300 and miR-609 (Fig.
4c), indicating possible cooperative targeting of
SMAD4. Although further studies to identify target protein levels are required to define specific targets and functions of SjS-associated miRNAs in monocytes, our results indicate SjS-associated miRNAs could cooperate to modulate gene expression.
Our extensive data analyses pinpointing the TGFβ signaling network prompted us to revisit potential roles of TGFβ signaling in SjS pathogenesis. Until now the involvement of TGFβ in SjS patients has been largely unappreciated due to previous inconsistencies in reported TGFβ protein expression levels in SjS patient lip biopsy tissues [
48‐
52] and in circulation [
53,
54]. Conditional knockout of TGFβ receptors in mice have been utilized to better identify roles of TGFβ signaling in autoimmunity. Interestingly, TGFβ receptor 1 (TGFβR1) conditional knockout in mice salivary glands developed glandular inflammation resembling SjS only in female mice [
55]. Of interest to our current study, bone marrow-derived DC-specific deletion of TGFβ receptor 2 (TGFβR2) resulted in multi-organ inflammation and activation of autoreactive T and B cells [
56]. TGFβR2-deficient DCs are more pro-inflammatory (increased stimulated TNF, IL-6, IL-12, IL-1β, and IFNγ gene expression [
56,
57]) and also have impaired IL-4 polarization of M2 alternatively activated regulatory macrophages [
58]. Altogether these studies signify intact TGFβ signal transduction is critical for maintaining a regulatory immune phenotype and reinforcing DC/macrophage control of autoimmune progression.
Our target prediction analyses also identified SjS-associated miRNAs were predicted to target members of the JAK-STAT signaling family, averaging 20.0 % ± 16.4 coverage of signaling molecules (Fig.
4a). The MAPK and JAK-STAT signaling pathways are well known to affect a variety of pro-inflammatory and immune-modulatory cascades beyond TGFβ family signaling. As such, we propose that SjS-associated miRNAs also function by limiting the availability of shared signaling molecules, which creates conditions for competition that would ultimately favor pathways with greater abundance of signaling molecules. Substrate competition is found in many types of biological processes and can influence dynamics and steady state concentrations of a pathway [
59] as well as serve as a general signal integration strategy for networks where enzymes interact with multiple regulators and substrates [
60].
Interestingly, pro-inflammatory IL-12 and IL-23 signaling pathway molecules
IL12RB1,
IL12RB2,
IL23R,
TYK2, and
STAT4, were not predicted to be targeted by SjS-associated miRNAs (Fig.
4d). Previously, IL-12p40 secretion in SjS patient monocyte-derived DCs was increased and correlated with increased NFκB/RELB protein levels [
61]. Likewise, our analyses of the NFκB signaling pathway showed relatively low level of pathway coverage (7.3 % ± 5.8) by SjS-associated miRNAs (Fig.
4a,e). Intriguingly, SjS patient monocytes are previously shown to have increased gene expression of NFκB transcription factors (
NFKB1 and
NFKB2) and display increased inflammatory responses to BAFF and IFNγ [
7,
9,
61]. Notably, SjS patients’ monocytes also exhibit reduced levels of inhibitor IκBα, which is a central regulator of NFkB activation [
11] and TGFβ signaling is critical for IκBα maintenance [
62‐
64]. Therefore, pro-inflammatory IL-12 and NFκB signaling pathways are presumed to be well maintained, whereas SjS-associated miRNAs skew regulatory TGFβ signaling responses in SjS monocytes.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
AEGW contributed to the experimental design, acquisition of the data set, analysis and interpretation of the work presented, and drafting and revision of the manuscript. KC acquired the data from qRT-PCR and assisted drafting the manuscript. YL contributed to data interpretation and manuscript revision. ALC, WHR, and MRB made contributions to study design, and acquisition of clinical data. CMS contributed to the design of the study. SC made substantial contributions to the study’s conception and design, coordination, drafting and revision of the manuscript. All authors read and approved the final manuscript.