Expression of aquaporin 3 and 5 as a potential marker for distinguishing dry mouth from Sjögren’s syndrome

A comprehensive review of Sjögren's syndrome: Classification criteria, risk factors, and signaling pathways

Sjögren's syndrome (SS) is a chronic autoimmune disease that affects the exocrine glands and may lead to a range of systemic symptoms that impact various organs. Both innate and adaptive immune pathways might trigger the disease. Studying the signaling pathways underlying SS is crucial for enhancing diagnostic and therapeutic effectiveness. SS poses an ongoing challenge for medical professionals owing to the limited therapeutic options available. This review offers a comprehensive understanding of the intricate nature of SS, encompassing disease classification criteria, risk factors, and signaling pathways in immunity and inflammation. The advancements summarized herein have the potential to spark new avenues of research into SS.

Molecular mechanisms of saliva secretion and hyposecretion

The exocrine salivary gland secretes saliva, a fundamental body component to maintain oral homeostasis. Saliva is composed of water, ions, and proteins such as amylase, mucins, and immunoglobulins that play essential roles in the digestion of food, lubrication, and prevention of dental caries and periodontitis. An increasing number of people experience saliva hyposecretion due to aging, medications, Sjögren's syndrome, and radiation therapy for head and neck cancer. However, current treatments are mostly limited to temporary symptomatic relief. This review explores the molecular mechanisms underlying saliva secretion and hyposecretion to provide insight into putative therapeutic targets for treatment. Proteins implicated in saliva secretion pathways, including Ca2+ -signaling proteins, aquaporins, soluble N-ethylmaleimide-sensitive factor attachment protein receptors, and tight junctions, are aberrantly expressed and localized in patients with saliva hyposecretion, such as Sjögren's syndrome. Analysis of studies on the mechanisms of saliva secretion and hyposecretion suggests that crosstalk between fluid and protein secretory pathways via Ca2+ /protein kinase C and cAMP/protein kinase A regulates saliva secretion. Impaired crosstalk between the two secretory pathways may contribute to saliva hyposecretion. Future research into the detailed regulatory mechanisms of saliva secretion and hyposecretion may provide information to define novel targets and generate therapeutic strategies for saliva hyposecretion.

Transcriptome analysis of salivary glands of rabies-virus-infected mice

Rabies is a fatal zoonotic disease that poses a threat to public health. Rabies virus (RABV) is excreted in the saliva of infected animals, and is primarily transmitted by bite. The role of the salivary glands in virus propagation is significant, but has been less studied in the pathogenic mechanisms of RABV. To identify functionally important genes in the salivary glands, we used RNA sequencing (RNA-seq) to establish and analyze mRNA expression profiles in parotid tissue infected with two RABV strains, CVS-11 and PB4. The biological functions of differentially expressed genes (DEGs) were determined by Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis, which revealed 3,764 DEGs (678 up-regulated and 3,086 down-regulated) in the CVS-11 infected group and 4,557 DEGs (874 up-regulated and 3,683 down-regulated) in the PB4 infected group. Various biological processes are involved, including the salivary secretion pathway and the phosphatidylinositol 3-kinase-Akt (PI3K-Akt) signaling pathway. This study provides the first mapping of the transcriptome changes in response to RABV infection in parotid tissue, offering new insights into the study of RABV-affected salivary gland function and RABV pathogenic mechanisms in parotid tissue. The salivary gland-enriched transcripts may be potential targets of interest for rabies disease control.

Ontogenetic development of the water channel protein AQP5 in mouse salivary gland tissue

Aquaporins (AQP) are a family of channel proteins expressed in the cell membranes of many tissue types. As water channels, they enable the selective permeation of water molecules and thus play an important role in water transport through the plasma membrane. There are numerous AQP sub-types, among which AQP5 is expressed in the salivary glands. The expression and localization of AQP5 in different salivary gland cells of animal models during fetal development and after birth have enabled the physiological functions of AQP5 to be elucidated, but subsequent changes in the adult phase are unknown. It is known that saliva production tends to decrease with age, but it is unclear how AQP5 activity and function changes developmentally, from young to old including gender differences. In the present study, we sampled the parotid, submandibular, and sublingual glands from young (8 weeks old) and aged (12 months old) mice of both sexes to study the effects of age- and sex-related differences in AQP5 expression. Positive fluorescence immunostaining was detected in the membranes of cells from all gland types, and this was enhanced in juvenile mice from both sexes. Western blot analyses revealed that AQP5 expression levels tended to decrease with age in both male and female animals. Conversely, AQP5 gene expression levels did not change significantly with aging, but were found to be high in submandibular gland cells of both sexes, in parotid gland cells of older female mice, and in the sublingual gland cells of young male mice.

Molecular Mechanisms Linking Inflammation to Autoimmunity in Sjögren's Syndrome: Identification of New Targets

Sjögren's syndrome (SS) is a systemic autoimmune rheumatic disorder characterized by the lymphocytic infiltration of exocrine glands and the production of autoantibodies to self-antigens. The involvement of the exocrine glands drives the pathognomonic manifestations of dry eyes (keratoconjunctivitis sicca) and dry mouth (xerostomia) that define sicca syndrome. To date, the molecular mechanisms mediating pathological salivary gland dysfunction in SS remain to be elucidated, despite extensive studies investigating the underlying cause of this disease, hampering the development of novel therapeutic strategies. Many researchers have identified a multifactorial pathogenesis of SS, including environmental, genetic, neuroendocrine, and immune factors. In this review, we explore the latest developments in understanding the molecular mechanisms involved in the pathogenesis of SS, which have attracted increasing interest in recent years.

C-Terminal Domain of Aquaporin-5 Is Required to Pass Its Protein Quality Control and Ensure Its Trafficking to Plasma Membrane

Aquaporin-5 (AQP5) is selectively expressed in the apical membrane of exocrine glands, such as salivary, lacrimal, and submucosal glands. It is important for the secretory function of exocrine glands because mice with the knockout of AQP5 exhibit a significant reduction in secretion from these glands. Previous reports indicated that the AQP5 C-terminal domain is crucial for the localization of AQP5 at the plasma membrane, but it remains unclear which motif or amino acid residues in the C-terminal domain are essential for this. In this study, we examined the effects of various AQP5 C-terminal deletions or mutations on the expression of AQP5 on the cell surface. AQP5 C-terminal domain mutants did not localize on the plasma membrane, and Leu²⁶² was shown to be crucial for AQP5′s plasma membrane localization. The mutants localized in the autophagosome or lysosome and showed decreased protein stability via lysosomal degradation. Taking these findings together, our study suggests that the C-terminal domain is required for AQP5 to pass protein quality control and be trafficked to the plasma membrane.

A comprehensive review of the influence of Epigallocatechin gallate on Sjögren's syndrome associated molecular regulators of exocytosis (Review)

Sjögren's syndrome (SS) is an autoimmune disorder that affects the salivary glands, leading to reduced secretory functions and oral and ocular dryness. The salivary glands are composed of acinar cells that are responsible for the secretion and production of secretory granules, which contain salivary components, such as amylase, mucins and immunoglobulins. This secretion process involves secretory vesicle trafficking, docking, priming and membrane fusion. A failure during any of the steps in exocytosis in the salivary glands results in the altered secretion of saliva. Soluble N-ethylmaleimide-sensitive-factor attachment protein receptors, actin, tight junctions and aquaporin 5 all serve an important role in the trafficking regulation of secretory vesicles in the secretion of saliva via exocytosis. Alterations in the expression and distribution of these selected proteins leads to salivary gland dysfunction, including SS. Several studies have demonstrated that green tea polyphenols, most notably Epigallocatechin gallate (EGCG), possess both anti-inflammatory and anti-apoptotic properties in normal human cells. Molecular, cellular and animal studies have indicated that EGCG can provide protective effects against autoimmune and inflammatory reactions in salivary glands in diseases such as SS. The aim of the present article is to provide a comprehensive and up-to-date review on the possible therapeutic interactions between EGCG and the selected molecular mechanisms associated with SS.

Aquaporin5-Targeted Treatment for Dry Eye Through Bioactive Compounds and Gut Microbiota

Dry eye and dry mouth are the principal sources of morbidity for patients with Sjögren's syndrome (SS). There are few effective treatments, particularly systemic ones. Targeting aquaprin-5 (AQP5)-mediated tear secretion has been tested as a novel ancillary strategy and has proved promising. Patients have a great interest in using complementary medicine, including nutraceuticals and bioactive compounds to alleviate their symptoms. Potential mechanisms by which phytocompounds and bioactive compounds may benefit SS ocular and mouth symptoms through modulation of AQP5 activity are presented within this review. Supplementation with prebiotics (such as polyphenols with high bioavailability) in SS patients with lower Firmicutes/Bacteroides (F/B) community ratio phenotype, through administration of butyrate-producing diets, is proposed as ancillary strategy for dry eye and mouth. The potential use of natural bioactive compounds to treat dry eye could also apply to dry mouth occurring in the context of aging and SS. This novel hypothesis could have implications with respect to planning a successful dietary regimen for achieving and maintaining a normal gut microbiota in SS patients. This regimen would include augmenting butyrate-producing foodstuffs and/or polyphenol-rich syrups, and high amounts of some specific probiotic-rich foodstuffs such as yogurt, soy yogurt, or as probiotic supplements. There are applications for pharmaceutical and nutraceutical products aiming to relieve dry eye and mouth.

Role of Aquaporins in Inflammation-a Scientific Curation

Inflammation is a universal response mechanism existing as inter-communicator of biological systems. Uncontrolled or dysregulated inflammation addresses chronic low-grade effects eventually resulting in multimorbidity. Active solute transport across the membrane establishes varying osmotic gradients. Aquaporins (AQPs) are a class of critical ubiquitously expressed transmembrane proteins that aid in fluid and small solute transport via facilitated diffusion over established osmotic gradients. Numerous significant data features the biological functions of AQPs rendering them as an appropriate biomarker of health and diseases. Besides their physiological role in well-balanced inflammatory responses, it is worth noting the dysregulation of AQPs during any undesirable inflammatory event. Most literature to date clearly sets out AQPs as potential drug targets instigating AQP-based therapies. In light of this conception, the current review provides a compendious overview on the propitious and portentous out-turns of AQPs under inflammation.

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.

Characteristics of aquaporin 1, 3, and 5 expression during early murine salivary gland development

Aquaporins (AQPs) are essential to coordinate the transit of water and ions through the cell membrane. In salivary glands (SGs), AQPs have been associated with saliva formation, facilitating water absorption through the epithelium during the formation of hypotonic saliva, which is then secreted into the oral cavity. Different members of the AQP family have been suggested to play distinct roles during embryonic development, highlighted by their specific expression patterns. Here, we have investigated the expression patterns of AQP-1, AQP-3 and AQP-5 by immunofluorescence at key stages of salivary gland development, utilising cultured mouse embryonic submandibular (SMG) and sublingual (SLG) glands. The expression of AQPs was compared to a mitotic marker, phospho-histone 3 (PH3), a myoepithelial marker, smooth muscle actin (SMA), and a vascular marker, CD31. Qualitative analysis revealed that AQP-1 and AQP-3 were primarily expressed during the earlier phases of SG morphogenesis and were associated with cells undergoing mitotic processes (PH3-positive). AQP-5, in contrast, was not associated to mitotic figures, but was predominantly expressed during late stages of SG morphogenesis. Our results highlight that AQPs are expressed from early stages of SG morphogenesis and exhibit complimentary expression patterns that may contribute to the morphogenesis of salivary glands.

Traditional Chinese medicine is a useful and promising alternative strategy for treatment of Sjogren's syndrome: A review

Primary Sjogren's syndrome (pSS) is a chronic autoimmune disease involving exocrine glands. Current studies have found that the occurrence of the disease is closely related to genetic, environmental and neuroendocrine factors, as well as abnormal activation of T and B lymphocytes. The etiology and pathogenesis of pSS is complex, and there is a lack of specific targeted drugs. Traditional Chinese medicines (TCMs) have been comprehensively investigated for their treatment effects on pSS. Through a systematic review of the literature, we summarized the TCMs used to treat pSS, and find that there are four major ways that TCMs are used, including upregulation of aquaporin proteins, suppression of cell apoptosis, suppression of the abnormal activation of B lymphocytes and suppression of the abnormal activation of T lymphocytes (balancing T helper type [Th]1/Th2 & Th17/Treg and suppressing follicular helper T [Tfh] cells). However, there are not enough data about the active constituents, quality control, pharmacokinetics, toxicity and modern preparations of these TCMs; therefore, more investigations are needed. This paper highlights the importance of TCMs for treating pSS and provides guidance for future investigations.

Early diagnosis and treatment for Sjögren's syndrome: current challenges, redefined disease stages and future prospects

There are some challenges and unmet needs in the early diagnosis and management of Sjögren's syndrome (SjS) such as prominent glandular dysfunction at diagnosis and long diagnostic delay. Those challenges are partly attributed to the lack of a good knowledge of the early stages of SjS, which is a major obstacle to delivering appropriate care to SjS patients. Findings from both clinical and experimental studies suggest the plausibility of a redefined SjS course consisting of 4 stages, which includes initiation stage, preclinical stage, asymptomatic SjS stage and overt SjS stage. More studies focusing on the pathological processes and changes during the early stages of SjS are needed. To enable early diagnosis and treatment for SjS, more useful biomarkers of the early stages of SjS need to be identified, and individuals at high risk of SjS development need to be identified. Appropriate screening can be performed to facilitate the early diagnosis of SjS among those high-risk individuals.

Current State of Knowledge on Primary Sjögren's Syndrome, an Autoimmune Exocrinopathy

Primary Sjögren's syndrome (pSS) is a chronic systemic autoimmune rheumatic disease characterized by lymphoplasmacytic infiltration of the salivary and lacrimal glands, whereby sicca syndrome and/or systemic manifestations are the clinical hallmarks, associated with a particular autoantibody profile. pSS is the most frequent connective tissue disease after rheumatoid arthritis, affecting 0.3-3% of the population. Women are more prone to develop pSS than men, with a sex ratio of 9:1. Considered in the past as innocent collateral passive victims of autoimmunity, the epithelial cells of the salivary glands are now known to play an active role in the pathogenesis of the disease. The aetiology of the "autoimmune epithelitis" still remains unknown, but certainly involves genetic, environmental and hormonal factors. Later during the disease evolution, the subsequent chronic activation of B cells can lead to the development of systemic manifestations or non-Hodgkin's lymphoma. The aim of the present comprehensive review is to provide the current state of knowledge on pSS. The review addresses the clinical manifestations and complications of the disease, the diagnostic workup, the pathogenic mechanisms and the therapeutic approaches.

Insight into Salivary Gland Aquaporins

The main role of salivary glands (SG) is the production and secretion of saliva, in which aquaporins (AQPs) play a key role by ensuring water flow. The AQPs are transmembrane channel proteins permeable to water to allow water transport across cell membranes according to osmotic gradient. This review gives an insight into SG AQPs. Indeed, it gives a summary of the expression and localization of AQPs in adult human, rat and mouse SG, as well as of their physiological role in SG function. Furthermore, the review provides a comprehensive view of the involvement of AQPs in pathological conditions affecting SG, including Sjögren's syndrome, diabetes, agedness, head and neck cancer radiotherapy and SG cancer. These conditions are characterized by salivary hypofunction resulting in xerostomia. A specific focus is given on current and future therapeutic strategies aiming at AQPs to treat xerostomia. A deeper understanding of the AQPs involvement in molecular mechanisms of saliva secretion and diseases offered new avenues for therapeutic approaches, including drugs, gene therapy and tissue engineering. As such, AQP5 represents a potential therapeutic target in different strategies for the treatment of xerostomia.

Hypoxia-Induced Aquaporin-3 Changes Hepatocellular Carcinoma Cell Sensitivity to Sorafenib by Activating the PI3K/Akt Signaling Pathway

Purpose

Hypoxia-induced changes are primarily activated in patients with hepatocellular carcinoma (HCC) and long-term sorafenib exposure, thereby reducing the sensitivity to the drug. Aquaporin-3 (AQP3), a member of the aquaporin family, is a hypoxia-induced substance that affects the chemosensitivity of non-hepatocellular tumors. However, its expression and role in the sensitivity of hypoxic HCC cells to sorafenib-induced apoptosis remain unclear. The purpose of this study was to detect changes in AQP3 expression in hypoxic HCC cells and to determine whether these changes alter the sensitivity of these cells to sorafenib.

Materials and Methods

Huh7 and HepG2 hypoxic cell models were established and AQP3 expression was detected using quantitative real-time polymerase chain reaction (qPCR) and Western blotting. Furthermore, the role of AQP3 in cell sensitivity to sorafenib was evaluated via flow cytometry, Western blotting, and a CCK-8 assay.

Results

The results of qPCR and Western blotting showed that AQP3 was overexpressed in the Huh7 and HepG2 hypoxic cell models. Furthermore, AQP3 protein levels were positively correlated with hypoxia-inducible factor-1α (HIF-1α) levels. Compared with cells transfected with lentivirus-GFP (Lv-GFP), hypoxic cells transfected with lentivirus-AQP3 (Lv-AQP3) were less sensitive to sorafenib-induced apoptosis. However, the sensitivity to the drug increased in cells transfected with lentivirus-AQP3RNAi (Lv-AQP3RNAi). Akt and Erk phosphorylation was enhanced in Lv-AQP3-transfected cells. Compared with UO126 (a Mek1/2 inhibitor), LY294002 (a PI3K inhibitor) attenuated the AQP3-induced insensitivity to sorafenib observed in hypoxic cells transfected with Lv-AQP3. Combined with LY294002-treated cells, hypoxic cells transfected with Lv-AQP3RNAi were more sensitive to sorafenib.

Conclusion

The study results show that AQP3 is a potential therapeutic target for improving the sensitivity of hypoxic HCC cells to sorafenib.

Aquaporin3 (AQP3) expression in oral epithelium in oral lichen planus

Lichen Planus is an autoimmune disease of skin and mucous membranes, including oral mucosa and genital organs, with the potential to undergo a malignant transformation. Xerostomia and the salivary gland hypofunction are common manifestations and complications in oral lichen planus (OLP), whose exact cause has not been well defined yet. Aquaporins (AQP) are a family of membrane channel cell proteins which play an important role in the intercellular water permeability. This study for the first time compares the expression of the aquaporin-3 (AQP3) gene in oral tissues of a group of OLP patients and a control group. In this study, 30 OLP patients and 30 healthy individuals were selected. The expression of AQP3 gene was measured using Real-Time PCR method. The expression of the gene in the OLP patients was more than the control group. It could be argued that decreased salivation increases the level of expression of the AQP3 gene in an effort to compensate for this deficiency and prevent the onset of xerostomia (compensatory up-regulation).

Aquaporin 1, 3, and 5 Patterns in Salivary Gland Mucoepidermoid Carcinoma: Expression in Surgical Specimens and an In Vitro Pilot Study

Salivary gland aquaporins (AQPs) are essential for the control of saliva production and maintenance of glandular structure. However, little is known of their role in salivary gland neoplasia. Salivary gland tumors comprise a heterogeneous group of lesions, featuring variable histological characteristics and diverse clinical behaviors. Mucoepidermoid carcinoma (MEC) is the most common salivary gland malignancy. The aim of this study was to evaluate the expression of AQP1, AQP3, and AQP5 in 24 MEC samples by immunohistochemistry. AQP1 expression was observed in vascular endothelium throughout the tumor stroma. AQP3 was expressed in epidermoid and mucosal cells and AQP5 was expressed in mucosal cells of MEC. These proteins were expressed in the human MEC cell line UH-HMC-3A. Cellular ultrastructural aspects were analyzed by electron microscopy to certificate the tumor cell phenotype. In summary, our results show that, despite the fact that these molecules are important for salivary gland physiology, they may not play a distinct role in tumorigenesis in MEC. Additionally, the in vitro model may offer new possibilities to further investigate mechanisms of these molecules in tumor biology and their real significance in prognosis and possible target therapies.

Dynamics of Salivary Gland AQP5 under Normal and Pathologic Conditions

AQP5 plays an important role in the salivary gland function. The mRNA and protein for aquaporin 5 (AQP5) are expressed in the acini from embryonic days E13-16 and E17-18, respectively and for entire postnatal days. Ligation-reopening of main excretory duct induces changes in the AQP5 level which would give an insight for mechanism of regeneration/self-duplication of acinar cells. The AQP5 level in the submandibular gland (SMG) decreases by chorda tympani denervation (CTD) via activation autophagosome, suggesting that its level in the SMG under normal condition is maintained by parasympathetic nerve. Isoproterenol (IPR), a β-adrenergic agonist, raised the levels of membrane AQP5 protein and its mRNA in the parotid gland (PG), suggesting coupling of the AQP5 dynamic and amylase secretion-restoration cycle. In the PG, lipopolysaccharide (LPS) is shown to activate mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) signalings and potentially downregulate AQP5 expression via cross coupling of activator protein-1 (AP-1) and NF-κB. In most species, Ser-156 and Thr-259 of AQP5 are experimentally phosphorylated, which is enhanced by cAMP analogues and forskolin. cAMP-dependent phosphorylation of AQP5 does not seem to be markedly involved in regulation of its intracellular trafficking but seems to play a role in its constitutive expression and lateral diffusion in the cell membrane. Additionally, Ser-156 phosphorylation may be important for cancer development.

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.