Abstract
Every day, salivary glands produce about 0.5 to 1.5 L of saliva, which contains salivary proteins that are essential for oral health. The contents of saliva, 0.3% proteins (1.5 to 4.5 g) in fluid, help prevent oral infections, provide lubrication, aid digestion, and maintain oral health. Acinar cells in the lobular salivary glands secrete prepackaged secretory granules that contain salivary components such as amylase, mucins, and immunoglobulins. Despite the important physiological functions of salivary proteins, we know very little about the regulatory mechanisms of their secretion via exocytosis, which is a process essential for the secretion of functional proteins, not only in salivary glands, but also in other secretory organs, including lacrimal and mammary glands, the pancreas, and prostate. In this review, we discuss recent findings that elucidate exocytosis by exocrine glands, especially focusing on the salivary glands, in physiological and pathological conditions.
Highlights
The major and minor salivary glands produce 0.5–1.5 L of saliva daily, which is essential for oral health
Vesicle-associated membrane protein 8 (VAMP8) expression in the lacrimal glands is altered, from the apical to the basal side, in individuals with dry eyes [48]; in Sjögren’s syndrome, the expression of the gene and protein is decreased in the acinar cells of labial salivary glands
Mutations in SEC23B have been reported in congenital dyserythropoietic anemia type II (CDAII) [129,130], and mice deficient for Sec23b (Sec23bgt/gt mice) display developmental defects in pancreatic acinar cells such as degeneration of acinar cells, absence of zymogene granules, accumulation of exocrine proteins in the ER, and dilated ER since the contents of the zymogen granules cannot exit from the ER via COPII vesicles [27]
Summary
The major (submandibular, sublingual, and parotid) and minor salivary glands produce 0.5–1.5 L of saliva daily, which is essential for oral health. Acinar cells are a major cell type within the salivary glands, responsible for the production and secretion of prepackaged secretory granules that contain key functional salivary components, such as amylase, mucins, and immunoglobulins [5] These salivary components are functionally important for the digestion and taste of foods, lubrication, buffering, and prevention of dental caries, periodontitis, candidiasis and halitosis (bad breath). Several studies show that secretion of salivary proteins is reduced, and the contents of saliva are changed, in patients with dry mouth syndromes (e.g., Sjögren’s syndrome) and in a mouse model for Sjögren’s syndrome (e.g., non-obese diabetic (NOD) mice) [1,7,8,9,10] This evidence suggests that exocytosis may be altered in various conditions, and the altered salivary contents may constitute a risk factor for major oral health issues [11,12]. While the pathogenesis of Sjögren’s syndrome remains elusive, various factors, including environmental, genetic and hormonal factors, seem to be involved, and either immune cells or exocrine gland cells are primarily damaged or disorganized to induce inflammation in the salivary and lacrimal glands [17,18,19]
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