Disruption of tight junctions contributes to hyposalivation of salivary glands in a mouse model of type 2 diabetes

F-actin/DRP1 axis-mediated mitochondrial fission promotes mitophagy in diabetic submandibular glands

BACKGROUND

Diabetes is accompanied by a high prevalence of hyposalivation, causing severe damage to oral and systemic health. Mitochondrial dynamics play important roles in the pathogenesis of various diabetic complications; however, little is known about their roles in diabetic hyposalivation.

MATERIALS AND METHODS

A diabetic mouse model and a high glucose (HG)-induced diabetic submandibular gland (SMG) cell model were employed.

RESULTS

More mitochondria surrounded by autophagosomes and higher expression of mitophagy-related proteins were detected in the SMGs of diabetic mice and HG-treated SMG cells. In diabetic SMGs, dynamin-related protein 1 (DRP1) was upregulated, whereas mitofusin-2 was downregulated both in vivo and in vitro. Shortened mitochondria and impaired mitochondrial functions were observed in the HG group. A DRP1-specific inhibitor, mdivi-1, suppressed mitochondrial fission and mitophagy, as well as restored mitochondrial functions in the HG condition. Moreover, the interaction of F-actin and DRP1 was enhanced in the diabetic group. Inhibiting F-actin with cytochalasin D repaired the injured effects of HG on mitochondrial dynamics and functions. Conversely, the F-actin-polymerization-inducer jasplakinolide aggravated mitochondrial fission and dysfunction.

CONCLUSIONS

F-actin contributes to HG-evoked mitochondrial fission by interacting with DRP1, which induces mitophagy and impairs mitochondrial function in SMG cells, ultimately damaging the SMG.

Identification of circRNAs expression profiles and functional networks in parotid gland of type 2 diabetes mouse

BACKGROUND

Circular RNAs (circRNAs) are a novel kind of non-coding RNAs proved to play crucial roles in the development of multiple diabetic complications. However, their expression and function in diabetes mellitus (DM)-impaired salivary glands are unknown.

RESULTS

By using microarray technology, 663 upregulated and 999 downregulated circRNAs companied with 813 upregulated and 525 downregulated mRNAs were identified in the parotid glands (PGs) of type2 DM mice under a 2-fold change and P < 0.05 cutoff criteria. Gene ontology (GO) and kyoto encyclopedia of genes and genomes (KEGG) analysis of upregulated mRNAs showed enrichments in immune system process and peroxisome proliferator-activated receptor (PPAR) signaling pathway. Infiltration of inflammatory cells and increased inflammatory cytokines were observed in diabetic PGs. Seven differently expressed circRNAs validated by qRT-PCR were selected for coding-non-coding gene co-expression (CNC) and competing endogenous RNA (ceRNA) networks analysis. PPAR signaling pathway was primarily enriched through analysis of circRNA-mRNA networks. Moreover, the circRNA-miRNA-mRNA networks highlighted an enrichment in the regulation of actin cytoskeleton.

CONCLUSION

The inflammatory response is elevated in diabetic PGs. The selected seven distinct circRNAs may attribute to the injury of diabetic PG by modulating inflammatory response through PPAR signaling pathway and actin cytoskeleton in diabetic PGs.

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.

Mitochondrial malfunction mediates impaired cholinergic Ca2+ signalling and submandibular salivary gland dysfunction in diabetes

Xerostomia (dry-mouth syndrome) is a painful and debilitating condition that frequently occurs in individuals with diabetes and is associated with impaired saliva production and salivary gland hypofunction. Saliva fluid production relies on Ca2+-coupled secretion driven by neurotransmitter stimulation of submandibular acinar cells. Although impairments in intracellular Ca2+ signalling have been reported in various xerostomia models, the specific Ca2+-dependent mechanisms underlying saliva fluid hypofunction in diabetes remain unclear. In this study, we show that diabetic animals exhibit severe xerostomia, evident by reduced saliva flow rate, diminished total protein content, and decreased amylase activity in the saliva secreted by submandibular glands. These impairments remained resistant to exogenous cholinergic stimulation. In submandibular acinar cells, the intracellular Ca2+ signals evoked by cholinergic stimulation were reduced and delayed in diabetes, caused by malfunctioning mitochondria. Upon initiation of cholinergic-evoked Ca2+ signals, mitochondria accumulate higher Ca2+ and fail to redistribute Ca2+ influx and facilitate the store-operated Ca2+ entry effectively. Structural damage to mitochondria was evident in the acinar cells in diabetes. These findings provide insights into the potential targeting of malfunctioning mitochondria for the treatment of diabetic xerostomia as an alternative strategy to the existing pharmacotherapeutic approaches. This article is part of the Special Issue on "Ukrainian Neuroscience".

The role and mechanism of tight junctions in the regulation of salivary gland secretion

Tight junctions (TJs) are cell-cell interactions that localize at the most apical portion of epithelial/endothelial cells. One of the predominant functions of TJs is to regulate material transport through paracellular pathway, which serves as a selective barrier. In recent years, the expression and function of TJs in salivary glands has attracted great interest. The characteristics of multiple salivary gland TJ proteins have been identified. During salivation, the activation of muscarinic acetylcholine receptor and transient receptor potential vanilloid subtype 1, as well as other stimuli, promote the opening of acinar TJs by inducing internalization of TJs, thereby contributing to increased paracellular permeability. Besides, endothelial TJs are also redistributed with leakage of blood vessels in cholinergic-stimulated submandibular glands. Furthermore, under pathological conditions, such as Sjögren's syndrome, diabetes mellitus, immunoglobulin G4-related sialadenitis, and autotransplantation, the integrity and barrier function of TJ complex are impaired and may contribute to hyposalivation. Moreover, in submandibular glands of Sjögren's syndrome mouse model and patients, the endothelial barrier is disrupted and involved in hyposecretion and lymphocytic infiltration. These findings enrich our understanding of the secretory mechanisms that link the importance of epithelial and endothelial TJ functions to salivation under both physiological and pathophysiological conditions.

Gemigliptin Improves Salivary Gland Dysfunction in D-Galactose-Injected Aging Rats

Oral dryness is among the most common conditions experienced by the elderly. As saliva plays a crucial role in maintaining oral health and overall quality of life, the condition is increasingly taking its toll on a rapidly growing aging population. D-galactose (D-gal) stimulates their formation, which in turn cause oxidative stress and accelerate age-related decline in physical function. In this study, we observed a reduction in salivary secretion and amylase levels in aged rats injected with D-gal, confirming salivary gland dysfunction. Treatment with gemigliptin increased DPP-4 inhibition and GLP-1 levels in the salivary glands of aging rats and reduced the expression of AGEs and receptors for advanced glycation end products (RAGE). This effect was caused by the presence of additional reactive oxygen species (ROS) in the salivary glands of the examined rats. Gemigliptin's cytoprotective effect reduced amylase and mucin accumulation and increased AQP5 expression, which are important indicators of salivary gland function. In sum, gemigliptin was shown to improve D-gal-induced decline in the salivary gland function of aged rats through its anti-glycation and antioxidant activities. Gemigliptin shows promise as a treatment strategy for patients experiencing decreased salivary function associated with their advancing age.

Ginger Root Extract Improves GI Health in Diabetic Rats by Improving Intestinal Integrity and Mitochondrial Function

Background Emerging research suggests hyperglycemia can increase intestinal permeability. Ginger and its bioactive compounds have been reported to benefit diabetic animals due to their anti-inflammatory and antioxidant properties. In this study, we revealed the beneficial effect of gingerol-enriched ginger (GEG) on intestinal health (i.e., barrier function, mitochondrial function, and anti-inflammation) in diabetic rats. Methods Thirty-three male Sprague Dawley rats were assigned to three groups: low-fat diet (control group), high-fat-diet (HFD) + streptozotocin (single low dose 35 mg/kg body weight (BW) after 2 weeks of HFD feeding) (DM group), and HFD + streptozotocin + 0.75% GEG in diet (GEG group) for 42 days. Glucose tolerance tests (GTT) and insulin tolerance tests (ITT) were conducted at baseline and prior to sample collection. Total pancreatic insulin content was determined by ELISA. Total RNA of intestinal tissues was extracted for mRNA expression using qRT-PCR. Results Compared to the DM group, the GEG group had improved glucose tolerance and increased pancreatic insulin content. Compared to those without GEG (DM group), GEG supplementation (GEG group) increased the gene expression of tight junction (Claudin-3) and antioxidant capacity (SOD1), while it decreased the gene expression for mitochondrial fusion (MFN1), fission (FIS1), biogenesis (PGC-1α, TFAM), mitophagy (LC3B, P62, PINK1), and inflammation (NF-κB). Conclusions Ginger root extract improved glucose homeostasis in diabetic rats, in part, via improving intestinal integrity and mitochondrial dysfunction of GI health.

Alteration of tight junctions during botulinum toxin type A-inhibited salivary secretion

OBJECTIVES

Tight junctions (TJs) are involved in the regulation of salivary secretion via paracellular pathway. Botulinum toxin type A (BTXA) is widely used for the treatment of hypersecretion diseases such as sialorrhea. This study aimed to investigate the role of TJs in BTXA-inhibited secretion of the submandibular gland (SMG).

MATERIALS AND METHODS

BTXA was injected into the SMGs of rats, and the same amount of saline was injected as a control. Western blot, real-time PCR and immunofluorescence staining were used to detect the expression and distribution of TJ proteins. Paracellular permeability was evaluated using the transepithelial electrical resistance (TER) measurements and fluorescent tracer detection in BTXA-stimulated SMG-C6 cells.

RESULTS

BTXA injection into the SMGs of rats led to increased expression of claudin (Cldn) -1 and Cldn3. Immunofluorescence staining showed no significant changes in the distribution of TJ proteins. In vitro, BTXA increased the TER values and significantly reduced the permeability of fluorescent tracer, suggesting that BTXA decreased the paracellular permeability. The expression levels of Cldn1, Cldn3 and Cldn4 were upregulated after BTXA treatment.

CONCLUSION

The expression of TJ proteins changed in both animal models and SMG-C6 cells after BTXA treatment, which may contribute to the inhibition of salivary secretion.

Micronutrient Improvement of Epithelial Barrier Function in Various Disease States: A Case for Adjuvant Therapy

The published literature makes a very strong case that a wide range of disease morbidity associates with and may in part be due to epithelial barrier leak. An equally large body of published literature substantiates that a diverse group of micronutrients can reduce barrier leak across a wide array of epithelial tissue types, stemming from both cell culture as well as animal and human tissue models. Conversely, micronutrient deficiencies can exacerbate both barrier leak and morbidity. Focusing on zinc, Vitamin A and Vitamin D, this review shows that at concentrations above RDA levels but well below toxicity limits, these micronutrients can induce cell- and tissue-specific molecular-level changes in tight junctional complexes (and by other mechanisms) that reduce barrier leak. An opportunity now exists in critical care—but also medical prophylactic and therapeutic care in general—to consider implementation of select micronutrients at elevated dosages as adjuvant therapeutics in a variety of disease management. This consideration is particularly pointed amidst the COVID-19 pandemic.

Polydatin Alleviates Diabetes-Induced Hyposalivation through Anti-Glycation Activity in db/db Mouse

Polydatin (resveratrol-3-O-β-mono-D-glucoside) is a polyphenol that can be easily accessed from peanuts, grapes, and red wine, and is known to have antiglycation, antioxidant, and anti-inflammatory effects. Diabetes mellitus is a very common disease, and diabetic complications are very common complications. The dry mouth symptom is one of the most common oral complaints in patients with diabetes mellitus. Diabetes mellitus is thought to promote hyposalivation. In this study, we aimed to investigate the improvement effect of polydatin on diabetes-induced hyposalivation in db/db mouse model of type 2 diabetes. We examined salivary flow rate, TUNEL assay, PAS staining, and immunohistochemical staining for AGEs, RAGE, HMGB1, 8-OHdG, and AQP5 to evaluate the efficacy of polydatin in the submandibular salivary gland. Diabetic db/db mice had a decreased salivary flow rate and salivary gland weight. The salivary gland of the vehicle-treated db/db mice showed an increased apoptotic cell injury. The AGEs were highly accumulated, and its receptor, RAGE expression was also enhanced. Expressions of HMGB1, an oxidative cell damage marker, and 8-OHdG, an oxidative DNA damage marker, increased greatly. However, polydatin ameliorated this hypofunction of the salivary gland and inhibited diabetes-related salivary cell injury. Furthermore, polydatin improved mucin accumulation, which is used as a damage marker for salivary gland acinar cells, and decreased expression of water channel AQP5 was improved by polydatin. In conclusion, polydatin has a potent protective effect on diabetes-related salivary gland hypofunction through its antioxidant and anti-glycation activities, and its AQP5 upregulation. This result suggests the possibility of the use of polydatin as a therapeutic drug to improve hyposalivation caused by diabetes.

Pilocarpine improves submandibular gland dysfunction in irradiated rats by downregulating the tight junction protein claudin-4

OBJECTIVES

To investigate the effects of radiation on paracellular pathway of rat submandibular glands (SMGs) and the mechanism of increasing secretion following treatment with pilocarpine.

MATERIALS AND METHODS

In situ irradiation models of SMGs in Wistar rats were conducted, and the glands were exposed to X-radiation at a single dose of 20 Gy. Pilocarpine was intraperitoneally injected 60 min prior to radiation and continuous 6 days postirradiation for a total of 7 days. Salivary secretion, histological changes, pro-inflammatory cytokines, alterations in tight junctions (TJs), and functional membrane proteins aquaporin-5 (AQP5) and claudin-4 mediated by the muscarinic acetylcholine M3 subtype receptor were determined at 1 and 12 weeks after irradiation.

RESULTS

Salivary secretion of the irradiated glands was reduced at 1 and 12 weeks. As well, acinar cell numbers, TJ width, and the levels of M3 receptor and AQP5 were decreased. In contrast, tumor necrosis factor-α, interleukin 6, interleukin 1α, and the expression of the TJ protein claudin-4 were significantly increased in irradiated SMGs. Notably, all the alterations were attenuated by pilocarpine treatment.

CONCLUSIONS

Pilocarpine could improve the secretory function of irradiated rat SMGs via reducing inflammation, ameliorating the structural injury of TJs, and attenuating the up-regulation of claudin-4 expression.