Saliva formation depends on the coordinated action of multiple processes involving the regulation of paracellular junctions as well as precise regulation of ion and water transporters in the acinar and ductal cells of the salivary gland. Ion fluxes across the apical membrane in acinar cells are thought to generate an osmotic gradient that drives paracellular water transport while creating an isotonic primary secretion. The lack of proper salivary secretion results in dry mouth, which may be idiopathic or associated with disorders such as Sjogren's syndrome. A significant number of individuals in the older population also suffer from medication-induced dry mouth in the US. Recent evidence suggests that water is transported in salivary acinar cells via a transcellular route through water channels, or aquaporins (AQPs). At least four AQPs are expressed in salivary glands. These include AQP1 (capillary endothelium), AQP4 (basolateral membrane of ductal cells), AQP5 (apical membrane of acinar cells) and AQP8 (cellular distribution unknown). Direct evidence in support of a role for AQP5 in fluid secretion in the salivary gland has been demonstrated in Aqp5 null mice, which secrete low volume hypertonic viscous saliva in response to supra-maximal muscarinic stimulation (Ma et al, 1999). This evidence suggests that transepithelial transport through water channels provides an important route for water flow through salivary secretion, and that the AQP5 water channel plays a major role in the process. The identification of aberrant salivary secretion in the AQP5 null mice is therefore the first direct evidence for requiring AQP5 in normal salivary secretion. The tasks ahead are to understand the regulatory, functional and physiological roles played byAQP5 in saliva secretion, which remains unknown at this time. In this proposal, plans are put forth to use a combination of cell biological, physiological and genomic approaches to analyze a mouse in which the AQP5 gene has been """"""""knocked out"""""""" by gene targeting in order to elucidate the role, and to identify the mechanisms, by which AQP5 regulates the formation of saliva. This multidisciplinary approach will refine current understanding of the water transport mechanisms involved in the production of saliva, and provide valuable information on targets for the development of treatments for salivary gland dysfunction.
