Abstract
Aquaporin 5 (AQP5) plays an important role in the salivary gland function. The mRNA and protein for 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.
Highlights
Aquaporins (AQPs) are serpentine-type membrane proteins which facilitate water movement across the biological membrane
Since the discovery of the first AQP, AQP1 [1], 13 AQPs, AQP0–12, are presently known to exist in mammals. These AQPs have been divided into 4 major subfamilies; the water-selective AQPs (AQP0, 1, 2, 4, 5, and 6), aquaglyceroporins (AQP3, 7, 9, and 10), superaquaporins, unorthodox AQPs (AQP11 and 12), and AQP8 which have an unusual structure with a long N-terminus, short C-terminus [2]
In Aquaporin 5 (AQP5) of most species, 2 consensus motifs are conserved as protein kinase A (PKA)-target phosphorylation sites: One is RRTS-156 located at an intracellular loop D; the other is RKKT-259 located at C-terminal region, which is corresponding to the motif including Ser-256 of AQP2
Summary
Aquaporins (AQPs) are serpentine-type membrane proteins which facilitate water movement across the biological membrane. Sci. 2020, 21, 1182 mature acinar cells (Figure 1a) These observations provide basic data regarding the relation between development of tissue morphology and functional expression of AQP5, in the salivary gland. AQP5 mRNA has been shown to be increased in expression till the birth, which results are similar to our previous data shown in the rat SMG [15] They detected AQP5 protein at first on the embryonic day E17; its level peaked around birth and at least decreased in the early postnatal day. Expression level of AQP5 was decreased by means of Western blotting, whereas, AQP5 was still localized at the apical membranes of the remaining such shrank acinar cells (Figure 1b) in mouse experiments [27]. Membrane AQP2 abundance regulated by exocytosis and endocytosis are controlled by a cAMP-dependent phosphorylation and ubiquitination, respectively
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