GLP-1 expression in von Ebner's gland of diabetic rats

Expression of glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide in the rat submandibular gland is influenced by pre- and post-natal high-fat diet exposure

Background: Incretins, i.e., glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) promote insulin secretion to reduce postprandial blood sugar. Previous studies found incretins in the salivary glands. However, the role of GLP-1 and GIP in the submandibular gland (SMG) is unclear. This study investigates the effects of a high-fat diet (HFD) on the expression of GLP-1 and GIP throughout the development of rat SMG. Methods: Pregnant 11-week-old Wistar rats were divided into two groups: those fed on a standard diet (n = 5) and those fed on a HFD (n = 5). From day 7 of pregnancy and throughout the lactation period, all the rats were fed on either a chow diet or HFD. The newborns were divided into four subgroups (n = 6): standard diet males (SM), HFD males (HM), standard diet females (SF), and HFD females (HF). The SMGs of 3- and 10-week-old rats from each subgroup were collected under general anesthesia. Moreover, body weight, food intake, and fasting blood sugar were measured. The mRNA expression of GLP-1 and GIP was quantified, and the localization was observed using immunohistochemistry (p < 0.05). Results: GLP-1 mRNA expression was statistically significantly more upregulated in HM than in HF at 3 weeks. Moreover, GLP-1 mRNA expression was significantly higher in HM than in both SM and HF at 10 weeks. Although a decreasing trend was observed in GIP mRNA expression in both 3- and 10-week-old rats fed on a HFD, a significant difference between HM and SM only occurred at 3 weeks. Furthermore, the GIP mRNA expression of HM was lower than that of HF at 10 weeks. Immunohistochemical staining revealed GLP-1 and GIP expression mainly in the SMG duct system. Moreover, vacuolated cytoplasm in the duct was observed in rats fed on a HFD. Conclusion: Exposure to HFD during pre- and post-natal periods increased GLP-1 mRNA expression in the SMGs of male rats. However, GIP expression decreased following the HFD in male newborns. Furthermore, a decreasing trend of GIP mRNA expression was observed in male newborns after HFD feeding. Sex influenced incretin hormones secretion and obesity-related conditions. HFD during pre- and post-natal periods reprograms the epigenome, contributing to subsequent disease development.

Liraglutide alters DPP4 in the circumvallate papillae of type 2 diabetic rats

Liraglutide, a human glucagon-like peptide (GLP1) analog that partially inhibits dipeptidyl-peptidase 4 (DPP4), can decrease glucose levels and suppress appetite in patients with type 2 diabetes (T2DM). GLP1 and its receptor (GLP1R) also exist in the taste buds of rodents and regulate taste sensitivity. DPP4, a protease, functions in homeostasis of blood glucose, lipids, and body weight. Interactions among GLP1, GLP1R, and DPP4 likely affect taste and food-intake behavior. The aim of the present study was to investigate DPP4 expression in the taste buds of the circumvallate papillae (CV) in T2DM rats, and determine the effects of liraglutide treatment. Rats were divided into diabetic control (T2DM-C), normal control (NC), and liraglutide-treated diabetic (T2DM+LIR) groups. DPP4 localization and gene expression levels were evaluated by immunohistochemistry and quantitative reverse transcription-polymerase chain reaction (RT-qPCR), respectively. DPP4 immunoreactive cells were localized in the taste buds of the rat CV. RT-qPCR showed significantly higher expression of Dpp4 mRNA in both the taste buds and hypothalamus of T2DM-C rats compared with NC rats. However, in the T2DM+LIR group, Dpp4 expression differed between the taste buds and hypothalamus, with significantly higher and lower levels compared with the T2DM-C group, respectively. Dpp4 mRNA expression is increased in the taste buds of the CV of T2DM rats. Liraglutide simultaneously upregulated (taste buds) and downregulated (hypothalamus) Dpp4 expression in T2DM rats. Therefore, DPP4 may be closely associated with the anorexigenic signaling and weight loss induced by the treatment of liraglutide in type 2 diabetic patients.

Expression of NUCB2/nesfatin-1 in the taste buds of rats

Nesfatin-1, an anorexigenic peptide derived from nucleobindin 2 (NUCB2), is closely involved in feeding behavior, glycometabolism, and satiety regulation. Some studies show that NUCB2/nesfatin-1 is highly expressed and interacts with many appetite-regulating peptides that are co-expressed in the gastrointestinal tract. However, it remains unclear whether nesfatin-1 is expressed and interacts similarly in taste buds. Glucagon-like peptide-1 (GLP-1), a well-known appetite down-regulating peptide, is associated with changes in the expression of nesfatin-1. Therefore, we measured the expression of the NUCB2 gene and the distribution of nesfatin-1-immunoreactive cells and investigated whether these variables change in taste buds of circumvallate papillae (CV) from rats with type 2 diabetes (T2DM) after treatment with liraglutide, a GLP-1 receptor agonist. The results showed that nesfatin-1 immunoreactive cells were localized in the taste buds of rat CV. Quantitative RT-PCR showed a significantly lower expression of NUCB2 mRNA in the taste buds of diabetic control rats (T2DM-C) than in those of the normal control group (NC) and a higher level of NUCB2 in the liraglutide treated group (T2DM + LIR) than either the T2DM-C or the NC groups. Changes in the expression of NUCB2 in the rat hypothalamus were opposite to those in CV taste buds. In summary, we found that rat CV taste buds express NUCB2/nesfatin-1, and that this expression decreases significantly in T2DM and increases after treatment with liraglutide in rat CV. This indicates that nesfatin-1 could be an important factor in the regulation of gustatory function, feeding and perhaps energy homeostasis.

Tight junctions in Gerbil von Ebner's gland: horseradish peroxidase and freeze-fracture studies

The permeability of tight junctions to horseradish peroxidase (HRP) and the freeze-fracture appearance of junctional structures were investigated in the von Ebner's gland of gerbils. In the tracing study, HRP was either administered topically on the dorsal surface of tongues or injected subepithelially into the connective tissue of vallate papillae for 5-30 min. Lingual tissues containing the von Ebner's gland were sectioned and examined by light and electron microscopy. In von Ebner's glands, the reaction product for HRP was found in the intercellular and interstitial spaces, whereas HRP appeared to penetrate the tight junctions and the reaction product was localized in the lumina of serous acini. In contrast, the staining for HRP that delineated the boundary of epithelial cells was frequently observed in the superficial layers of the lingual epithelium but not the underlying tissues while applying HRP topically. Freeze-fracture replicas of acinar cells revealed that the tight junction had a depth of 0.815 ± 0.023 μm, and 4-6 parallel strands on the protoplasmic fracture face, with a branching network of joining strands with interruptions, interconnections and high linear strand density apically, and corresponding grooves on the extracellular face. Quantitative analyses showed a greater number of strands (7.217 ± 0.326) in gerbils compared to those of acinar cells (3.86 ± 0.22) in mice. These results demonstrate that the tight junctions in the gerbil von Ebner's gland is permeable, and that specific species differences in tight junction structures may be associated with the mechanism for survival in an extremely dry environment.