Physiological role of aquaporin 5 in salivary glands

Regarding the 13 known mammalian aquaporins (AQPs), their functions in their expressing tissues, effects of their mutation/polymorphisms in humans, and effects of knockout of their genes are summarized in this review article. The roles of AQP5, an exocrine gland-type water channel, in the salivary gland under normal and pathophysiological conditions are reviewed in detail. First, the involvement of AQP5 in water secretion from acinar cells was demonstrated by measuring volume changes of acini/acinar cells, as well as activation energy (E a) in transepithelial water movement by NMR spectrometry, and a functional linkage between AQP5 and TRPV4 was suggested. Next, involvement of the parasympathetic nervous system on the AQP5 levels in the acinar cells of the submandibular and that of a β-adrenergic agonist on those in the parotid gland are described. That is, chorda tympani denervation induces autophagy of the submandibular gland, causing AQP5 degradation/metabolism, whereas isoproterenol, a β-adrenergic agonist, causes first an increase then decrease in AQP5 levels in the parotid gland, which action is coupled with the secretory-restoration cycle of amylase-containing secretory granules. The PG also responded to endotoxin, a lipopolysaccharide that activates NF-κB and MAPK pathways. Elevated NF-κB and AP-1 (c-Fos/c-Jun) form a complex that can bind to the NF-κB-responsive element on the AQP5 promoter and thus potentially downregulate AQP5 transcription. Salivary gland pathologies and conditions involving AQP5 and possible treatments are described as well.