Impaired mitochondria and intracellular calcium transients in the salivary glands of obese rats

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".

Interrelationship between Vitamin D and Calcium in Obesity and Its Comorbid Conditions

Obesity has been linked to vitamin D (VD) deficiency and low calcium (CAL) status. In the last decade, dietary supplementation of vitamin D and calcium (VD–CAL) have been extensively studied in animal experiments and human studies. However, the physiological mechanisms remain unknown as to whether the VD–CAL axis improves homeostasis and reduces biomarkers in regulating obesity and other metabolic diseases directly or indirectly. This review sought to investigate their connections. This topic was examined in scientific databases such as Web of Science, Scopus, and PubMed from 2011 to 2021, and 87 articles were generated for interpretation. Mechanistically, VD–CAL regulates from the organs to the blood, influencing insulin, lipids, hormone, cell, and inflammatory functions in obesity and its comorbidities, such as non-alcoholic fatty liver disease, cardiovascular disease, and type-2 diabetes mellitus. Nevertheless, previous research has not consistently shown that simultaneous VD–CAL supplementation affects weight loss or reduces fat content. This discrepancy may be influenced by population age and diversity, ethnicity, and geographical location, and also by degree of obesity and applied doses. Therefore, a larger prospective cohort and randomised trials are needed to determine the exact role of VD–CAL and their interrelationship.

High Glucose Reduces the Paracellular Permeability of the Submandibular Gland Epithelium via the MiR-22-3p/Sp1/Claudin Pathway

Tight junctions (TJs) play an important role in water, ion, and solute transport through the paracellular pathway of epithelial cells; however, their role in diabetes-induced salivary gland dysfunction remains unknown. Here, we found that the TJ proteins claudin-1 and claudin-3 were significantly increased in the submandibular glands (SMGs) of db/db mice and high glucose (HG)-treated human SMGs. HG decreased paracellular permeability and increased claudin-1 and claudin-3 expression in SMG-C6 cells. Knockdown of claudin-1 or claudin-3 reversed the HG-induced decrease in paracellular permeability. MiR-22-3p was significantly downregulated in diabetic SMGs and HG-treated SMG-C6 cells. A miR-22-3p mimic suppressed claudin-1 and claudin-3 expression and abolished the HG-induced increases in claudin-1 and claudin-3 levels in SMG-C6 cells, whereas a miR-22-3p inhibitor produced the opposite effects. Specificity protein-1 (Sp1) was enhanced in diabetic SMGs and HG-treated SMG-C6 cells, which promoted claudin-1 and claudin-3 transcription through binding to the corresponding promoters. A luciferase reporter assay confirmed that miR-22-3p repressed Sp1 by directly targeting the Sp1 mRNA 3′-untranslated region (3′-UTR). Consistently, the miR-22-3p mimic suppressed, whereas the miR-22-3p inhibitor enhanced, the effects of HG on Sp1 expression. Taken together, our results demonstrate a new regulatory pathway through which HG decreases the paracellular permeability of SMG cells by inhibiting miR-22-3p/Sp1-mediated claudin-1 and claudin-3 expression.

N-acetylcysteine supplementation did not reverse mitochondrial oxidative stress, apoptosis, and inflammation in the salivary glands of hyperglycemic rats

Background/objectives

Previous studies have shown that N-acetylcysteine (NAC) supplementation with the simultaneous inclusion of HFD prevents salivary glands from oxidative stress and mitochondrial dysfunction. In this experiment, we examined if NAC supplementation could reverse the harmful effect of HFD on mitochondrial function, reduce the severity of apoptosis, and the activity of pro-oxidative enzymes in the salivary glands of rats with confirmed hyperglycemia.

Subjects/methods

Wistar rats were fed the standard or high-fat (HFD) diet for 10 weeks. After 6 weeks of the experiment, HFD rats were diagnosed with hyperglycemia and for the next 4 weeks, the animals were given NAC intragastrically. In the mitochondrial fraction of the parotid (PG) and submandibular salivary glands (SMG), we assessed redox status, inflammation, and apoptosis.

Results

The inclusion of NAC increased the activity of mitochondrial complexes I and II + III as well as decreased the concentration of interleukin-1β, tumor necrosis factor α, and caspase-3, but only in the parotid glands of rats with hyperglycemia compared to the HFD group. However, N-acetylcysteine supplementation did not reduce the activity of caspase-9 or the Bax/Bcl-2 ratio in PG and SMG mitochondria. In both salivary glands we observed reduced activity of cytochrome c oxidase, NADPH oxidase, and xanthine oxidase, as well as hindered production of ROS and lower ADP/ATP radio, but the levels of these parameters were not comparable to the control group.

Conclusions

We demonstrated that NAC supplementation restores the glutathione ratio only in the mitochondria of the submandibular salivary glands. The supply of NAC did not significantly affect the other measured parameters. Our results indicate that NAC supplementation provides little protection against free radicals, apoptosis, and inflammation in the salivary gland mitochondria of HFD rats. Stimulated salivary secretion in hyperglycaemic rats supplemented with NAC seems to be closely related to mitochondrial respiratory capacity and appropriate ATP level.

Long non-coding RNA and mRNA profile analysis in the parotid gland of mouse with type 2 diabetes

AIMS

Salivary gland dysfunction is a common complication of diabetes mellitus (DM). Long non-coding RNA (lncRNA) is evidenced to involve in the functional regulation of salivary gland, however, its role in DM-impaired gland is unknown. Therefore, this study aimed to investigate the expression profiles and functional networks of lncRNA in the parotid glands (PGs) of DM mice.

MAIN METHODS

Microarray was used to detect lncRNA and messenger RNA (mRNA) expression profiles in the PGs from db/db and db/m mice. Eleven differently expressed (DE) lncRNAs validated by qRT-PCR were selected for coding-non-coding gene co-expression (CNC) and competing endogenous RNA (ceRNA) network analysis, as well as the following Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Pearson's coefficient correlation analysis was used to analyze the correlations between DE lncRNAs expression and DM pathology.

KEY FINDINGS

By using a 2-fold change and P < 0.05 as the cutoff criteria, 1650 DE lncRNAs (758 upregulated and 892 downregulated) and 1073 mRNAs (563 upregulated and 510 downregulated) were identified in the PGs of db/db mice compared to db/m mice. GO and KEGG analysis of DE mRNA suggested that activated inflammation response and downregulated ion transport might count for the dysfunction of diabetic PG. CNC and ceRNA networks analysis of 11 DE lncRNAs showed that the inflammation process and its related signaling pathways including advanced glycation end product (AGE)-receptor for AGE (RAGE) signaling pathway in diabetic complications, cytokine-cytokine receptor interaction, chemokine signaling pathway, apoptosis, and cell adhesion molecules were significantly enriched. The alterations of lncRNAs were closely correlated with higher blood glucose and serum insulin levels in mice.

SIGNIFICANCE

We identified multiple lncRNAs/mRNAs and several signaling pathways that may involve in the pathogenesis of diabetic salivary injury, providing new insight into potential target of diabetic hyposalivation.

Re-establishing normal diet following high fat-diet-induced obesity reverses the altered salivary composition in Wistar rats

Background Obesity has been implicated in impaired salivary secretion. This study aimed at evaluating the influence of diet-induced obesity on salivary secretion and how re-feeding with normal diet would affect changes in salivary secretion associated with diet-induced obesity. Methods Weaning rats weighing 55-65 g were randomly divided into three groups (control, diet-induced obese, re-fed obese) of seven rats each. The diet-induced obese group was fed a high-fat diet for 15 weeks, whereas the re-fed obese group received normal diet for another 15 weeks following the 15 weeks of high-fat diet. After treatment, blood and stimulated saliva samples were collected for the analyses of total protein, electrolytes, amylase, Immunoglobulin A (IgA), leptin and ghrelin. Tissue total protein, nitric oxide level, expressions of Na+/K+-ATPase, muscarinic (M3) receptor and aquaporin 5 in the submandibular glands were determined. Data were presented as mean±SEM and compared using independent student t-test and ANOVA with Tukey's post-hoc test. Results Results indicated increases in the levels of salivary calcium, phosphate, bicarbonate and leptin, whereas the levels of salivary amylase and ghrelin showed reduction in the obese group compared with the control. Most of these changes were reversed in the re-fed obese group. There were no significant differences in salivary lag time, flow rate, levels of tissue total protein, nitric oxide and the relative expressions of M3 receptor, Na++/K+-ATPase and aquaporin 5 in the submandibular glands between the obese and control groups. Conclusions Diet-induced obesity lead to some changes in salivary factors which were reversed by returning to normal diet.

Type 2 diabetes-induced hyposalivation of the submandibular gland through PINK1/Parkin-mediated mitophagy

Diabetes is often accompanied by dysfunction of salivary glands. However, the molecular mechanism remains unclear. The mechanisms that underlie diabetic hyposalivation were studied by db/db mice and SMG‐C6 cells. We found morphological changes and decreased stimulated salivary flow rates of the submandibular gland (SMG) in diabetic mice. We observed structural changes and dysfunction of mitochondria. More mitophagosomes and higher expression of autophagy‐related proteins were detected. Increased levels of proteins PINK1 and Parkin indicate that PINK1/Parkin‐mediated mitophagy was activated in diabetic SMG. Consistently, high glucose (HG) induced mitochondrial dysfunction and PINK1/Parkin‐mediated mitophagy in cultivated SMG‐C6 cells. HG also increased reactive oxygen species (ROS) and lessened activation of antioxidants in SMG‐C6 cells. In addition, HG lowered ERK1/2 phosphorylation and HG‐induced mitophagy was decreased after ERK1/2 was activated by LM22B‐10. Altogether, these data suggest that ROS played a crucial role in diabetes‐induced mitochondrial dysfunction and PINK1/Parkin‐mediated mitophagy and ERK1/2 was required in HG‐induced mitophagy in SMG.

Evaluating All Potential Oral Complications of Diabetes Mellitus

Diabetes mellitus (DM) is associated with several microvascular and macrovascular complications, such as retinopathy, nephropathy, neuropathy, and cardiovascular diseases. The pathogenesis of these complications is complex, and involves metabolic and hemodynamic disturbances, including hyperglycemia, insulin resistance, dyslipidemia, hypertension, and immune dysfunction. These disturbances initiate several damaging processes, such as increased reactive oxygen species (ROS) production, inflammation, and ischemia. These processes mainly exert their damaging effect on endothelial and nerve cells, hence the susceptibility of densely vascularized and innervated sites, such as the eyes, kidneys, and nerves. Since the oral cavity is also highly vascularized and innervated, oral complications can be expected as well. The relationship between DM and oral diseases has received considerable attention in the past few decades. However, most studies only focus on periodontitis, and still approach DM from the limited perspective of elevated blood glucose levels only. In this review, we will assess other potential oral complications as well, including: dental caries, dry mouth, oral mucosal lesions, oral cancer, taste disturbances, temporomandibular disorders, burning mouth syndrome, apical periodontitis, and peri-implant diseases. Each oral complication will be briefly introduced, followed by an assessment of the literature studying epidemiological associations with DM. We will also elaborate on pathogenic mechanisms that might explain associations between DM and oral complications. To do so, we aim to expand our perspective of DM by not only considering elevated blood glucose levels, but also including literature about the other important pathogenic mechanisms, such as insulin resistance, dyslipidemia, hypertension, and immune dysfunction.

Protein expression in submandibular glands of young rats is modified by a high-fat/high-sugar maternal diet

OBJECTIVE

Maternal diet has consequences on many organs of the offspring, but salivary glands have received little attention despite the importance of the saliva secretory function in oral health and control of food intake. The objective of this work was therefore to document in rats the impact of maternal high-fat/high-sugar diet (Western Diet) on submandibular glands of the progeny.

DESIGN

Sprague-Dawley rat dams were fed either a Western diet or control diet during gestation and lactation and their pups were sacrificed 25 days after birth. The pups' submandibular gland protein content was characterized by means of 2D-electrophoresis followed by LC-MS/MS. Data were further analyzed by Gene Ontology enrichment analysis and protein-protein interactions mapping. The expression of two specific proteins was also evaluated using immunohistochemistry.

RESULTS

Combining both male and female pups (n = 18), proteome analysis revealed that proteins involved in protein quality control (e.g. heat shock proteins, proteasome sub-units) and microtubule proteins were over-expressed in Western diet conditions, which may translate intense metabolic activity. A cluster of proteins controlling oxidative stress (e.g. Glutathione peroxidases, peroxiredoxin) and enhancement of the antioxidant activity molecular function were also characteristic of maternal Western diet as well as under-expression of annexin A5. The down-regulating effect of maternal Western diet on Annexin A5 expression was significant only for males (p < 0.05).

CONCLUSIONS

A maternal Western diet modifies the protein composition of the offspring's salivary glands, which may have consequences on the salivary function.

Reduced salivary amylase activity in metabolic syndrome patients with obesity could be improved by treatment with a dipeptidyl peptidase IV inhibitor

OBJECTIVES

The present study is to investigate the salivary gland function of metabolic syndrome (MetS) patients, as indicated by salivary flow rate, amylase activity, and salivary oxidative stress, by measuring MDA level.

MATERIALS AND METHODS

One hundred and eighty-one MetS patients from Maharaj Nakorn Chiang Mai Hospital were enrolled onto this study. The metabolic parameters of each patient were collected and evaluated. Unstimulated saliva was also collected for 5 min. Salivary gland functions, including salivary flow rate, amylase activity, and salivary MDA levels, were investigated.

RESULTS

High levels of triglycerides, high-density lipoprotein, blood pressure, and waist circumference in MetS patients did not show a correlation with altered salivary gland function. However, a decrease in salivary flow rate was observed in MetS patients with hyperglycemia. In addition, decreased amylase activity was found in MetS patients with obesity (BMI ≥ 23 kg/m²). Salivary amylase activity of MetS patients treated with dipeptidyl peptidase IV (DPP-IV) inhibitor was significantly greater than that observed in MetS patients without a DPP-IV inhibitor. Moreover, the salivary amylase activity in MetS patients was found to be independently positively correlated with DPP-IV inhibitor therapy (r = 0.708, p < 0.01).

CONCLUSION

These findings suggest that obesity and hyperglycemia in MetS patients were associated with the impairment of salivary glands. Treatment with a DPP-IV inhibitor was found to exert beneficial effects on the salivary gland.

CLINICAL RELEVANCE

This study demonstrated the impairment of salivary glands of MetS patients and the beneficial effect of DPP-IV inhibitor treatment in the salivary glands.