Further evidence for AQP8 expression in the myoepithelium of rat submandibular and parotid glands

Abstract

Previously (Wellner et al., Pflugers Arch 441:49-56, 2000) we suggested that the localization of the aquaporins (AQPs) AQP5 and AQP8 in the apical and basolateral membranes of rat submandibular gland (SMG) acinar cells, respectively, provides for transcellular water flow during saliva formation. While the localization of AQP5 in this gland has been verified in several laboratories, there have been differing reports regarding AQP8 localization. Other investigators subsequently reported that AQP8 is not expressed in the acinar or ductal cells of the major salivary glands of the rat, but in the myoepithelium of each gland. Thus, we have carried out additional studies: (1) to reassess the localization of AQP8 in the rat SMG and (2) to assess the localization of AQP8 in the rat parotid gland (PG). Initially, we compared the localizations of AQP8 with recognized basolateral markers in acinar cells [the Na+,K+-ATPase and the Na+-K+-2Cl- cotransporter (NKCC1)]. Our results indicated that Na+,K+-ATPase localized in both the basal and lateral membranes of rat SMG acinar cells, whereas AQP8 was detected only in the basal regions of the acini. In the rat PG, AQP8 was invested near intercalated ducts and adjacent acini, whereas NKCC1 localized in the basolateral membranes of acinar cells. As these results were suggestive of myoepithelial localization in both glands, we compared AQP8 localization with the localization of smooth muscle actin, a myoepithelial marker. We found that AQP8 and smooth muscle actin colocalized in both the rat SMG and PG, providing additional strong support for a myoepithelial localization of AQP8. Thus, in agreement with an earlier report by other investigators (Elkjaer et al., Am J Physiol Renal Physiol 281:F1047-F1057, 2001), we report that AQP8 is expressed in the myoepithelial cells, but not in the acinar cells, of both the rat SMG and PG.

Water Channels and Zymogen Granules in Salivary Glands

“…AQP4 is not detected, however, in immunofluorescence microscopy images of salivary gland cells (17). AQP8 is also a 28-kDa protein, and it is abundant in the testis (28), pancreas, and liver (19) and is expressed in the myoepithelial cells of rat submandibular and parotid glands (18). These findings indicate that AQP5 has a significant role in water secretion from serous acinar cells and interlobular duct cells of parotid and submandibular glands.…”

“…The AQP family consists of three subsets: aquaporins (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8); aquaglyceroporins (AQP3, AQP7, AQP9, and AQP10); and superaquaporins (AQP11 and AQP12) (15,16). Among these members of the AQP family, AQP1, AQP3, AQP4, AQP5 (17), and AQP8 (18,19) are present in mammalian salivary glands. Hybridization studies indicate that AQP1, AQP3, and AQP5 mRNA are present at significant levels, but AQP4 is not detected (17).…”

Changes of Chloride Channels in the Lacrimal Glands of a Rabbit Model of Sjögren Syndrome

“…However, studies in salivary glands, another exocrine glands that were very similar to LG anatomically and physiologically, showed that NKCC1 26,27 , CFTR 28 , and ClC2γ 22 play an important role in salivary secretion. Evans and colleagues 26 demonstrated that NKCC1 was localized to the basolateral membranes of acinar cells, but not duct cells, from parotid gland of wild-type mice.…”

Roles of lysosomal proteolytic systems in AQP5 degradation in the submandibular gland of rats following chorda tympani parasympathetic denervation

“…Among them, AQP1 is known to be localized in endothelial and myoepithelial cells (1,24) and AQP8 in myoepithelial cells (11,57), although their functions in the salivary gland are not fully understood. On the other hand, AQP5 is known to be localized at the apical membrane of secretory acinar cells of the submandibular, parotid, and sublingual glands (1,37) and is believed to be the molecule that plays a major role in the salivary secretion process (23,35,41,43).…”