Calcium signaling and cell volume regulation are altered in Sjögren's Syndrome

Loss of STIM1 and STIM2 in salivary glands disrupts ANO1 function but does not induce Sjogren's disease

Sjogren's disease (SjD) is an autoimmune disease characterized by xerostomia (dry mouth), lymphocytic infiltration into salivary glands and the presence of SSA and SSB autoantibodies. Xerostomia is caused by hypofunction of the salivary glands and has been involved in the development of SjD. Saliva production is regulated by parasympathetic input into the glands initiating intracellular Ca 2+ signals that activate the store operated Ca 2+ entry (SOCE) pathway eliciting sustained Ca 2+ influx. SOCE is mediated by the STIM1 and STIM2 proteins and the ORAI1 Ca 2+ channel. However, there are no studies on the effects of lack of STIM1/2 function in salivary acini in animal models and its impact on SjD. Here we report that male and female mice lacking Stim1 and Stim2 ( Stim1/2 K14Cre ) in salivary glands showed reduced intracellular Ca 2+ levels via SOCE in parotid acini and hyposalivate upon pilocarpine stimulation. Bulk RNASeq of the parotid glands of Stim1/2 K14Cre mice showed a decrease in the expression of Stim1/2 but no other Ca 2+ associated genes mediating saliva fluid secretion. SOCE was however functionally required for the activation of the Ca 2+ activated chloride channel ANO1. Despite hyposalivation, ageing Stim1/2 K14Cre mice showed no evidence of lymphocytic infiltration in the glands or elevated levels of SSA or SSB autoantibodies in the serum, which may be linked to the downregulation of the toll-like receptor 8 ( Tlr8 ). By contrast, salivary gland biopsies of SjD patients showed increased STIM1 and TLR8 expression, and induction of SOCE in a salivary gland cell line increased the expression of TLR8 . Our data demonstrate that SOCE is an important activator of ANO1 function and saliva fluid secretion in salivary glands. They also provide a novel link between SOCE and TLR8 signaling which may explain why loss of SOCE does not result in SjD.

Dental pulp stem cell-derived exosomes revitalize salivary gland epithelial cell function in NOD mice via the GPER-mediated cAMP/PKA/CREB signaling pathway

BACKGROUND

Restoration of salivary gland function in Sjogren's syndrome (SS) is still a challenge. Dental pulp stem cells (DPSCs) derived exosomes had shown anti-inflammatory, anti-oxidative, immunomodulatory, and tissue function restorative abilities. However, the salivary gland function restoration potential of DPSCs-derived exosomes (DPSC-Exos) during SS has not been investigated yet.

METHODS

DPSC-Exos was isolated by ultracentrifugation methods and characterized. Salivary gland epithelial cells (SGEC) were treated with interferon-gamma (IFN-γ) to mimic SS in vitro and cultured with or without DPSC-Exos. SGEC survival and aquaporin 5 (AQP5) expression were analyzed. mRNA sequencing and bioinformatics analysis were performed in IFN-γ vs. DPSC-Exos+ IFN-γ treated SGEC. Non-obese diabetic (NOD)/ltj female mice (SS model), were intravenously administered with DPSC-Exos, and salivary gland functions and SS pathogenicity were analyzed. Furthermore, the mRNA sequencing and bioinformatics predicted mechanism of the therapeutic effect of DPSC-Exos was further investigated both in vitro and in vivo using RT-qPCR, Western blot, immunohistochemistry, immunofluorescence, flowcytometry analysis.

RESULTS

DPSC-Exos partially rescued IFN-γ triggered SGEC death. IFN-γ inhibited AQP5 expression in SGEC and DPSC-Exos reversed this effect. Transcriptome analysis showed GPER was the upregulated DEG in DPSC-Exos-treated SGEC with a positive correlation with salivary secretion-related DEGs. Pathway enrichment analysis revealed that DEGs were mainly attributed to estrogen 16 alpha-hydroxylase activity, extracellular exosome function, cAMP signaling, salivary secretion, and estrogen signaling. Intravenous injection of DPSC-Exos in NOD/ltj mice alleviated the SS syndrome as indicated by the increased salivary flow rate, attenuated glandular inflammation, and increased AQP5 expression. GPER was also upregulated in the salivary gland of DPSC-Exos-treated NOD/ltj mice compared with the PBS-treated NOD/ltj mice. IFN-γ+DPSC-Exos-treated SGEC showed higher expression of AQP5, p-PKA, cAMP, and intracellular Ca²⁺ levels compared with IFN-γ-treated SGEC. These effects were reversed by the inhibition of GPER.

CONCLUSIONS

Our results showed that DPSC-Exos revitalize salivary gland epithelial cell function during SS via the GPER-mediated cAMP/PKA/CREB pathway suggesting the possible therapeutic potential of DPSC-Exos in SS-treatment.

Aquaporins in Glandular Secretion

Exocrine and endocrine glands deliver their secretory product, respectively, at the surface of the target organs or within the bloodstream. The release of their products has been shown to rely on secretory mechanisms often involving aquaporins (AQPs). This chapter will provide insight into the role of AQPs in secretory glands located within the gastrointestinal tract, including salivary glands, gastric glands, duodenal Brunner's glands, liver, gallbladder, intestinal goblets cells, and pancreas, as well and in other parts of the body, including airway submucosal glands, lacrimal glands, mammary glands, and eccrine sweat glands. The involvement of AQPs in both physiological and pathophysiological conditions will also be highlighted.

The integrated stress response is activated in the salivary glands of Sjögren's syndrome patients

Introduction

Primary Sjögren's syndrome (SS) is an autoimmune exocrinopathy that affects the structure and function of salivary and lachrymal glands. Labial salivary gland (LSG) acinar cells from SS patients lose cellular homeostasis and experience endoplasmic reticulum and oxidative stress. The integrated cellular stress response (ISR) is an adaptive pathway essential for restoring homeostasis against various stress-inducing factors, including pro-inflammatory cytokines, and endoplasmic reticulum and oxidative stress. ISR activation leads eIF2α phosphorylation, which transiently blocks protein synthesis while allowing the ATF4 expression, which induces a gene expression program that seeks to optimize cellular recovery. PKR, HRI, GCN2, and PERK are the four sentinel stress kinases that control eIF2α phosphorylation. Dysregulation and chronic activation of ISR signaling have pathologic consequences associated with inflammation.

Methods

Here, we analyzed the activation of the ISR in LSGs of SS-patients and non-SS sicca controls, determining the mRNA, protein, and phosphorylated-protein levels of key ISR components, as well as the expression of some of ATF4 targets. Moreover, we performed a qualitative characterization of the distribution of ISR components in LSGs from both groups and evaluated if their levels correlate with clinical parameters.

Results

We observed that the four ISR sensors are expressed in LSGs of both groups. However, only PKR and PERK showed increased expression and/or activation in LSGs from SS-patients. eIF2α and p-eIF2α protein levels significantly increased in SS-patients; meanwhile components of the PP1c complex responsible for eIF2α dephosphorylation decreased. ATF4 mRNA levels were decreased in LSGs from SS-patients along with hypermethylation of the ATF4 promoter. Despite low mRNA levels, SS-patients showed increased levels of ATF4 protein and ATF4-target genes involved in the antioxidant response. The acinar cells of SS-patients showed increased staining intensity for PKR, p-PKR, p-PERK, p-eIF2α, ATF4, xCT, CHOP, and NRF2. Autoantibodies, focus score, and ESSDAI were correlated with p-PERK/PERK ratio and ATF4 protein levels.

Discussion

In summary, the results showed an increased ISR activation in LSGs of SS-patients. The increased protein levels of ATF4 and ATF4-target genes involved in the redox homeostasis could be part of a rescue response against the various stressful conditions to which the LSGs of SS-patients are subjected and promote cell survival.

Cysteine-Rich Secretory Proteins (CRISP) are Key Players in Mammalian Fertilization and Fertility

Mammalian fertilization is a complex process involving a series of successive sperm-egg interaction steps mediated by different molecules and mechanisms. Studies carried out during the past 30 years, using a group of proteins named CRISP (Cysteine-RIch Secretory Proteins), have significantly contributed to elucidating the molecular mechanisms underlying mammalian gamete interaction. The CRISP family is composed of four members (i.e., CRISP1-4) in mammals, mainly expressed in the male tract, present in spermatozoa and exhibiting Ca2+ channel regulatory abilities. Biochemical, molecular and genetic approaches show that each CRISP protein participates in more than one stage of gamete interaction (i.e., cumulus penetration, sperm-ZP binding, ZP penetration, gamete fusion) by either ligand-receptor interactions or the regulation of several capacitation-associated events (i.e., protein tyrosine phosphorylation, acrosome reaction, hyperactivation, etc.) likely through their ability to regulate different sperm ion channels. Moreover, deletion of different numbers and combination of Crisp genes leading to the generation of single, double, triple and quadruple knockout mice showed that CRISP proteins are essential for male fertility and are involved not only in gamete interaction but also in previous and subsequent steps such as sperm transport within the female tract and early embryo development. Collectively, these observations reveal that CRISP have evolved to perform redundant as well as specialized functions and are organized in functional modules within the family that work through independent pathways and contribute distinctly to fertility success. Redundancy and compensation mechanisms within protein families are particularly important for spermatozoa which are transcriptionally and translationally inactive cells carrying numerous protein families, emphasizing the importance of generating multiple knockout models to unmask the true functional relevance of family proteins. Considering the high sequence and functional homology between rodent and human CRISP proteins, these observations will contribute to a better understanding and diagnosis of human infertility as well as the development of new contraceptive options.

McH-lpr/lpr-RA1 mice: A novel spontaneous mouse model of autoimmune sialadenitis

Many studies of the autoimmune disease Sjögren's syndrome have been performed using spontaneous mouse models. In the present study, we describe the characteristics of McH/lpr-RA1 mice and propose their use as a novel murine model of autoimmune sialadenitis. The McH/lpr-RA1 mouse is a recombinant congenic strain derived from generation F54 or more of MRL-Faslpr x (MRL- Faslpr x C3H- Faslpr) F1. We show for the first time that this mouse spontaneously develops autoimmune sialadenitis and vasculitis in submandibular gland tissues. Sialadenitis was accompanied by extensive inflammatory cell infiltration and tissue destruction. Immunohistochemical studies revealed that the salivary gland lesions strongly expressed four sialadenitis-related molecules: SSA and SSB (autoantigens of Sjögren's syndrome), gp91phox (an accelerator of reactive oxygen species production) and single strand DNA (a marker of apoptotic cells). In contrast, expression of aquaporin-5 (AQP5), which stimulates salivary secretion was weak or negligible. Statistical correlation analyses indicated that the apoptosis of salivary gland cells provoked by oxidative stress contributed to the severe sialadenitis and reduced expression of AQP5. Our study has demonstrated that McH/lpr-RA1 mice spontaneously develop the pathognomonic features of autoimmune sialadenitis and thus could be used as a new animal model of Sjögren's syndrome.

Characterization of Lipids in Saliva, Tears and Minor Salivary Glands of Sjögren's Syndrome Patients Using an HPLC/MS-Based Approach

The diagnostic work-up of primary Sjögren’s syndrome (pSS) includes quantifying saliva and tear production, evaluation of autoantibodies in serum and histopathological analysis of minor salivary glands. Thus, the potential for further utilizing these fluids and tissues in the quest to find better diagnostic and therapeutic tools should be fully explored. Ten samples of saliva and tears from female patients diagnosed with pSS and ten samples of saliva and tears from healthy females were included for lipidomic analysis of tears and whole saliva using high-performance liquid chromatography coupled to time-of-flight mass spectrometry. In addition, lipidomic analysis was performed on minor salivary gland biopsies from three pSS and three non-SS females. We found significant differences in the lipidomic profiles of saliva and tears in pSS patients compared to healthy controls. Moreover, there were differences in individual lipid species in stimulated saliva that were comparable to those of glandular biopsies, representing an intriguing avenue for further research. We believe a comprehensive elucidation of the changes in lipid composition in saliva, tears and minor salivary glands in pSS patients may be the key to detecting pSS-related dry mouth and dry eyes at an early stage. The identified differences may illuminate the path towards future innovative diagnostic methodologies and treatment modalities for alleviating pSS-related sicca symptoms.

Epithelial-immune cell interplay in primary Sjögren syndrome salivary gland pathogenesis

In primary Sjögren syndrome (pSS), the function of the salivary glands is often considerably reduced. Multiple innate immune pathways are likely dysregulated in the salivary gland epithelium in pSS, including the nuclear factor-κB pathway, the inflammasome and interferon signalling. The ductal cells of the salivary gland in pSS are characteristically surrounded by a CD4⁺ T cell-rich and B cell-rich infiltrate, implying a degree of communication between epithelial cells and immune cells. B cell infiltrates within the ducts can initiate the development of lymphoepithelial lesions, including basal ductal cell hyperplasia. Vice versa, the epithelium provides chronic activation signals to the glandular B cell fraction. This continuous stimulation might ultimately drive the development of mucosa-associated lymphoid tissue lymphoma. This Review discusses changes in the cells of the salivary gland epithelium in pSS (including acinar, ductal and progenitor cells), and the proposed interplay of these cells with environmental stimuli and the immune system. Current therapeutic options are insufficient to address both lymphocytic infiltration and salivary gland dysfunction. Successful rescue of salivary gland function in pSS will probably demand a multimodal therapeutic approach and an appreciation of the complicity of the salivary gland epithelium in the development of pSS.

Expression of NGAL-specific cells and mRNA levels correlate with inflammation in the salivary gland, and its overexpression in the saliva, of patients with primary Sjögren's syndrome

Salivary gland involvement is a characteristic feature of primary Sjögren's syndrome (pSS), where tissue destruction is mediated by infiltrating immune cells, and may be accompanied by the presence of adipose tissue. Optimally diagnosing this multifactorial disease requires the incorporation of additional routines. Screening for disease-specific biomarkers in biological fluid could be a promising approach to increase diagnostic accuracy. We have previously investigated disease biomarkers in saliva and tear fluid of pSS patients, identifying Neutrophil gelatinase-associated lipocalin (NGAL) as the most upregulated protein in pSS. In the current study, we aimed to explore for the first time NGAL expression at the site of inflammation in the pSS disease target organ. Immunohistochemical staining was conducted on minor salivary gland biopsies from 11 pSS patients and 11 non-SS sicca subjects, targeting NGAL-specific cells. Additional NGAL/PNAd double staining was performed to study NGAL expression in high endothelial venules, known as specialised vascular structures. Moreover, NGAL mRNA expression was measured utilising quantitative real-time polymerase chain reaction (qRT-PCR) on minor salivary gland biopsies from 15 pSS patients and 7 non-SS sicca individuals that served as tissue controls. Our results demonstrated NGAL expression in acinar and ductal epithelium within the salivary gland of pSS patients, where significantly greater levels of acinar NGAL were observed in pSS patients (p < .0018) when compared to non-SS subjects. Also, acinar expression positively correlated with focus score values (r ² = 0.54, p < .02), while ductal epithelial expression showed a negative such correlation (r ² = 0.74, p < .003). Some PNAD⁺ endothelial venules also expressed NGAL. An increase in NGAL staining with increased fatty replacement was also observed in pSS patients. Concurringly, a 27% increase in NGAL mRNA levels were also detected in the minor salivary glands of pSS patients when compared to non-SS tissue control subjects. In conclusion, there is a positive association between increase in NGAL expression and inflammation in the pSS disease target organ, which also coincides with its previously demonstrated upregulation in the saliva of pSS patients. Additional functional analyses are needed to better understand the immunological implications of this potential biomarker.

The role of stroma and epithelial cells in primary Sjögren's syndrome

Primary SS (pSS) is a chronic autoimmune condition characterized by infiltration of the exocrine glands and systemic B cell hyperactivation. This glandular infiltration is associated with loss of glandular function, with pSS patients primarily presenting with severe dryness of the eyes and mouth. Within the affected glands, the infiltrating lymphocytes are organized in tertiary lymphoid structures. Tertiary lymphoid structures subvert normal tissue architecture and impact on organ function, by promoting the activation and maintenance of autoreactive lymphocytes. This review summarizes the current knowledge about the role of stromal cells (including endothelium, epithelium, nerves and fibroblasts) in the pathogenesis of pSS, in particular the interactions taking place between stromal cells and infiltrating lymphocytes. We will provide evidences pointing towards the driving role of stromal cells in the orchestration of the local inflammatory milieu, thus highlighting the need for therapies aimed at targeting this compartment alongside classical immunosuppression in pSS.

Submandibular gland-specific inflammaging-induced hyposalivation in the male senescence-accelerated mouse prone -1 line (SAM-P1)

Aging has pronounced effects on mammalian tissues and cells, but the impacts of aging on salivary gland function are relatively unknown. This study aims to evaluate the effects of aging on submandibular gland (SMG) and parotid gland (PG) functions in the male senescence-accelerated mouse. In vivo analysis at the systemic level revealed that salivary secretion induced by pilocarpine, a muscarinic agonist, from the SMG was significantly decreased in aged mice, whereas salivary secretion from the PG was not affected. To evaluate organ-level function, the SMG was perfused with the muscarinic agonists carbachol and calcium ionophore A23187 ex vivo to induce salivary secretion, and decreased saliva production was also observed in the aged SMG. Histological analysis revealed the presence of CD4-positive lymphocytes infiltrating the aged SMG. Furthermore, real-time PCR revealed that the aged SMG exhibited accelerated cell aging, increased levels of the inflammatory cytokine interleukin-6, and decreased mRNA levels of the water channel protein aquaporin-5 (AQP5). In summary, these results demonstrate that SMG function in aged mice was diminished, and that cell senescence, chronic inflammation, and the decreased gene expression of AQP5 are the likely causes of hyposalivation in the SMG of aged mice.

Long-term dexamethasone treatment diminishes store-operated Ca2+ entry in salivary acinar cells

Corticosteroids are used in the treatment of many diseases; however, they also induce various side effects. Dexamethasone is one of the most potent corticosteroids, and it has been reported to induce the side effect of impaired salivary gland function. This study aimed to evaluate the effects of dexamethasone on mouse submandibular gland function to gain insight into the mechanism of dexamethasone-induced salivary hypofunction. The muscarinic agonist carbachol (CCh) induced salivary secretion and was not affected by short-term dexamethasone treatment but was decreased following long-term dexamethasone administration. The expression levels of the membrane proteins Na⁺-K⁺-2Cl^- cotransporter, transmembrane member 16A, and aquaporin 5 were comparable between the control and long-term dexamethasone treatment groups. The CCh-induced increase in calcium concentration was significantly lower in the presence of extracellular Ca²⁺ in the long-term dexamethasone treatment group compared to that in the control group. Furthermore, CCh-induced salivation in the absence of extracellular Ca²⁺ and Ca²⁺ ionophore A23187-induced salivation was comparable between the control and long-term dexamethasone treatment groups. Moreover, salivation induced by the Ca²⁺-ATPase inhibitor thapsigargin was diminished in the long-term dexamethasone treatment group. In summary, these results demonstrate that short-term dexamethasone treatment did not impair salivary gland function, whereas long-term dexamethasone treatment diminished store-operated Ca²⁺ entry, resulting in hyposalivation in mouse submandibular glands.

Involvement of Aquaporins in the Pathogenesis, Diagnosis and Treatment of Sjögren's Syndrome

Sjögren’s syndrome (SS) is a chronic autoimmune disease characterized by lymphocytic infiltration of salivary and lacrimal glands resulting in diminished production of saliva and tears. The pathophysiology of SS has not yet been fully deciphered. Classically it has been postulated that sicca symptoms in SS patients are a double step process whereby lymphocytic infiltration of lacrimal and salivary glands (SG) is followed by epithelial cell destruction resulting in keratoconjunctivitis sicca and xerostomia. Recent advances in the field of the pathophysiology of SS have brought in new players, such as aquaporins (AQPs) and anti AQPs autoantibodies that could explain underlying mechanistic processes and unveil new pathophysiological pathways offering a deeper understanding of the disease. In this review, we delineate the link between the AQP and SS, focusing on salivary glands, and discuss the role of AQPs in the treatment of SS-induced xerostomia.

Calcium signaling defects underlying salivary gland dysfunction

Salivary glands secrete saliva, a mixture of proteins and fluids, which plays an extremely important role in the maintenance of oral health. Loss of salivary secretion causes a dry mouth condition, xerostomia, which has numerous deleterious consequences including opportunistic infections within the oral cavity, difficulties in eating and swallowing food, and problems with speech. Saliva secretion is regulated by stimulation of specific signaling mechanisms within the acinar cells of the gland. Neurotransmitter-stimulated increase in cytosolic [Ca²⁺] ([Ca²⁺]_i) in acinar cells is the primary trigger for salivary fluid secretion from salivary glands, the loss of which is a critical factor underlying dry mouth conditions in patients. The increase in [Ca²⁺]_i regulates multiple ion channel and transport activities that together generate the osmotic gradient which drives fluid secretion across the apical membrane. Ca²⁺ entry mediated by the Store-Operated Ca²⁺ Entry (SOCE) mechanism provides the essential [Ca²⁺]_i signals to trigger salivary gland fluid secretion. Under physiological conditions depletion of ER-Ca²⁺ stores is caused by activation of IP3R by IP3 and this provides the stimulus for SOCE. Core components of SOCE in salivary gland acinar cells are the plasma membrane Ca²⁺ channels, Orai1 and TRPC1, and STIM1, a Ca²⁺-sensor protein in the ER, which regulates both channels. In addition, STIM2 likely enhances the sensitivity of cells to ER-Ca²⁺ depletion thereby tuning the cellular response to agonist stimulation. Two major, clinically relevant, conditions which cause irreversible salivary gland dysfunction are radiation treatment for head-and-neck cancers and the autoimmune exocrinopathy, Sjögren's syndrome (pSS). However, the exact mechanism(s) that causes the loss of fluid secretion, in either condition, is not clearly understood. A number of recent studies have identified that defects in critical Ca²⁺ signaling mechanisms underlie salivary gland dysfunction caused by radiation treatment or Sjögren's syndrome (pSS). This chapter will discuss these very interesting and important studies.

Signalling pathways identified in salivary glands from primary Sjögren's syndrome patients reveal enhanced adipose tissue development

A characteristic feature of primary Sjögren's syndrome (pSS) is the destruction of salivary and lacrimal glands mediated by mononuclear cell infiltration. Adipocytes can also occupy a large portion of the salivary gland (SG) tissue area, although little is known about their significance in pSS. We have previously investigated adipose tissue infiltration in SG biopsies from pSS patients and non-SS sicca controls. Our findings indicated the distinct incidence of adipose tissue replacement in pSS patients, where adipocytes were detected in interleukin (IL) 6 rich regions. We now aimed to examine the development of adipocytes in the SG microenvironment, and delineate their possible involvement in immune reactions. A microarray analysis was performed on SG from 6 pSS patients and 6 non-SS controls, where the expression levels of genes involved in adipose tissue development, inflammatory responses, and lymphoma development were assessed. Real-time PCR was carried out on SG from 14 pSS patients and 15 non-SS controls to account for IL6, IL10, and IL17 mRNA levels. Immunohistochemical staining of frozen SG tissue using IL17 was also conducted. Our results indicate signalling pathways identified in SG of pSS patients displayed genes leading to prominent adipose tissue development and reduced mitochondrial fatty acid beta-oxidation (ARID5B, OXCT1, BDH1, SOX8, HMGCS2, FTO, ECHS1, PCCA, ACADL and ACADVL), inflammatory responses (IL1R1, IL7R, IL10RA, IL15, IL18RAP, CCL2, CCL5, CCL22, CXCR6, CD14, and CD48), and lymphoma development via JAK-STAT signalling (STAT2, TYK2, EBI3, FAS, TNFRSF1B, MAP3K8, HMOX1, LTB, TNF, STAT1, and BAK1). Genes involved in interferon production and signalling were also detected (IRF1, IRF9, and IRF7), in addition to IL6, IL10, and IL17. Higher mRNA levels of IL6, IL17 and IL10 were observed in the SG of pSS patients compared to controls. Moreover, IL17 positive cells were detected mostly interstitially in the SG and around adipocytes, also within the focal infiltrates. In conclusion, adipocyte development seems to be more prominent in the SG of pSS patients, where adipose tissue replacement is also evident. Whether this is due to disease progression, or the repair process, remains to be investigated. Detection of IL17 positive adipocytes in the target organ suggests their involvement in immune reactions.

How stress contributes to autoimmunity-lessons from Sjögren's syndrome

A large body of clinical evidence on the association between stressful life events and autoimmune diseases suggests that stress may play an important role in the pathogenesis of these disorders. In this article, we discuss the effects of stress, not on the immune system but on specific cell populations against which the autoimmune reactivity is directed. Using Sjögren's syndrome as a model autoimmune disease, we review the role of stress in the initiation and perpetuation of autoimmune reactivity. We present data that reveal the effects of stress on salivary gland epithelial cells, suggesting that stress can become immunogenic through its various effects on salivary gland epithelium.

Aquaporins and Gland Secretion

Aquaporins (AQPs ) are expressed in most exocrine and endocrine secretory glands. Consequently, summarizing the expression and functions of AQPs in secretory glands represents a daunting task considering the important number of glands present in the body, as well as the number of mammalian AQPs - thirteen. The roles played by AQPs in secretory processes have been investigated in many secretory glands. However, despite considerable research, additional studies are clearly needed to pursue our understanding of the role played by AQPs in secretory processes. This book chapter will focus on summarizing the current knowledge on AQPs expression and function in the gastrointestinal tract , including salivary glands, gastric glands, Duodenal Brunner's gland, liver and gallbladder, intestinal goblets cells, exocrine and endocrine pancreas, as well as few other secretory glands including airway submucosal glands, lacrimal glands, mammary glands and eccrine sweat glands.

Investigation of age-related changes in the expression of aquaporin-1 and aquaporin-5 in the salivary glands of mice

CONCLUSION

The increased AQP5 expression associated with ageing in glands, which mainly secreted a serous solution, suggests a compensation for the decreased amount of saliva secretion associated with age progression.

OBJECTIVE

To investigate the change in aquaporin-1 (AQP1) and aquaporin-5 (AQP5) expression in the salivary glands in young and elder mice.

MATERIALS AND METHOD

Twelve female mice from the Balb/C genus (30-50 g) were used. The mice were separated into two groups: Group I had 2-month-old mice and Group II had 18-month-old mice. Salivary glands (glandula parotidea, glandula sublungualis, glandula submaxillaris) were excised and examined immunohistochemically and histopathologically. AQP1 and AQP5 expression of young and elder mice was evaluated using the H-score. A p-value less than 0.05 was considered statistically significant.

RESULTS

Upon histopathological examination, the acini of glands were found to be atrophic in elder mice. The number and diameter of intercalated ducts were increased. Indeed, the amount of adipose tissue in the gland was increased. Upon immunohistochemical examination, both AQP1 and AQP5 levels in sublingual glands of elder mice were increased (p < 0.01 and p < 0.001, respectively). However, only AQP5 levels were increased in the parotid gland of elder mice (p < 0.01).

Pro-inflammatory cytokines enhance ERAD and ATF6α pathway activity in salivary glands of Sjögren's syndrome patients

Salivary gland (SG) acinar-cells are susceptible to endoplasmic reticulum (ER) stress related to their secretory activity and the complexity of synthesized secretory products. SGs of Sjögren's syndrome patients (SS)-patients show signs of inflammation and altered proteostasis, associated with low IRE1α/XBP-1 pathway activity without avert increases in apoptosis. Acinar-cells may avoid apoptosis by activation of the ATF6α pathway and ER-associated protein degradation (ERAD). The aim of this study was to evaluate the role of pro-inflammatory cytokines in ATF6α pathway/ERAD activation and cell viability in labial salivary glands (LSG) of SS-patients. In biopsies from SS-patients increased ATF6α signaling pathway activity, as evidenced by generation of the ATF6f cleavage fragment, and increased expression of ERAD machinery components, such as EDEM1, p97, SEL1L, gp78, UBE2J1, UBE2G2, HERP and DERLIN1, were observed compared to controls. Alternatively, for pro- (active-caspase-3) and anti-apoptotic (cIAP2) markers no significant difference between the two experimental groups was detected. Increased presence of ATF6f and ERAD molecules correlated significantly with increased expression of pro-inflammatory cytokines. These observations were corroborated in vitro in 3D-acini treated with TNF-α and/or IFN-γ, where an increase in the expression and activation of the ATF6α sensor and ERAD machinery components was detected under ER stress conditions, while changes in cell viability and caspase-3 activation were not observed. Cytokine stimulation protected cells from death when co-incubated with an ERAD machinery inhibitor. Alternatively, when cytokines were eliminated from the medium prior to ERAD inhibition, cell death increased, suggesting that the presence of pro-inflammatory cytokines in the medium is essential to maintain cell viability. In conclusion, the ATF6α pathway and the ERAD machinery are active in LSG of SS-patients. Both were also activated by TNF-α and IFN-γ in vitro in 3D-acini and aided in preventing apoptosis. IFN-γ levels were elevated in SS-patients and UPR responses triggered in vitro by this cytokine closely matched those observed in LSG from SS-patients, suggesting that cytokines may induce ER stress.

Epigallocatechin gallate stimulates the neuroreactive salivary secretomotor system in autoimmune sialadenitis of MRL-Fas(lpr) mice via activation of cAMP-dependent protein kinase A and inactivation of nuclear factor κB

The water channel aquaporin 5 (AQP5) plays a crucial role in regulating salivary flow rates. Xerostomia is often observed in patients with Sjögren's syndrome, and this is attributed to reduced AQP5 expression in the salivary glands. Recently, anti-type 3 muscarinic cholinergic receptors (M3R) autoantibodies and nuclear factor κB (NF-κB) have been found to be negative regulators of AQP5 expression in the salivary gland. Anti-M3R autoantibodies desensitize M3R to salivary secretagogues in Sjögren's syndrome, while activated NF-κB translocates to nuclei and binds to the AQP5 gene promoter, resulting in the suppression of AQP5 expression. We previously documented that epigallocatechin gallate (EGCG), which is a robust antioxidant contained in green tea, ameliorates oxidative stress-induced tissue damage to the salivary glands of MRL/MpJ-lpr/lpr (MRL-Fas(lpr)) mice, which are widely used as a model of Sjögren's syndrome. Reactive oxygen species (ROS) can activate NF-κB and inactivate protein kinase A (PKA), which is a key driver of AQP5 expression. In this study, we examined the effects of administering EGCG to MRL-Fas(lpr) mice with autoimmune sialadenitis on the levels of AQP5, activated NF-κB p65 subunit, activated PKA, activated c-Jun N-terminal kinase (JNK) (an activator of NF-κB), inhibitor κB (IκB) and histone deacetylase 1 (HDAC1) (an inhibitor of NF-κB). In EGCG-treated mice, intense aster-like immunostaining for AQP5 was observed on the apical plasma membranes (APMs) of submandibular gland acinar cells. Likewise, PKA, IκB and HDAC1 were highly expressed in salivary gland tissues, whereas the expression of JNK and NF-κB p65 was negligible. Rank correlation and partial correlation analyses revealed that treatment with EGCG upregulated AQP5 expression on the APM of acinar cells through activation of PKA and inactivation of NF-κB, while IκB and HDAC1 played a pivotal role in the induction of AQP5 expression by PKA. Our study indicates that EGCG may have therapeutic potential for Sjögren's syndrome patients.

Aquaporins in Salivary Glands: From Basic Research to Clinical Applications

Salivary glands are involved in saliva secretion that ensures proper oral health. Aquaporins are expressed in salivary glands and play a major role in saliva secretion. This review will provide an overview of the salivary gland morphology and physiology of saliva secretion, and focus on the expression, subcellular localization and role of aquaporins under physiological and pathophysiological conditions, as well as clinical applications involving aquaporins. This review is highlighting expression and localization of aquaporins in human, rat and mouse, the most studied species and is pointing out possible difference between major salivary glands, i.e., parotid, submandibular and sublingual glands.

IP3R deficit underlies loss of salivary fluid secretion in Sjögren's Syndrome

The autoimmune exocrinopathy, Sjögren’s syndrome (SS), is associated with secretory defects in patients, including individuals with mild lymphocytic infiltration and minimal glandular damage. The mechanism(s) underlying the secretory dysfunction is not known. We have used minor salivary gland biopsies from SS patients and healthy individuals to assess acinar cell function in morphologically intact glandular areas. We report that agonist-regulated intracellular Ca²⁺ release, critically required for Ca²⁺ entry and fluid secretion, is defective in acini from SS patients. Importantly, these acini displayed reduction in IP3R2 and IP3R3, but not AQP5 or STIM1. Similar decreases in IP3R and carbachol (CCh)-stimulated [Ca²⁺]_i elevation were detected in acinar cells from lymphotoxin-alpha (LTα) transgenic (TG) mice, a model for (SS). Treatment of salivary glands from healthy individuals with LT α, a cytokine linked to disease progression in SS and IL14α mice, reduced Ca²⁺ signaling. Together, our findings reveal novel IP3R deficits in acinar cells that underlie secretory dysfunction in SS patients.