Regulation of exocrine cell secretion by Rab proteins

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DESCRIPTION (provided by applicant): The long-term objective of the studies described in the present proposal is to elucidate the role of Rab proteins and Rab accessory proteins in regulated exocytosis in exocrine glands. Like Ras, Rab proteins are GTPases. Rab3 isoforms (A, B, C and D) are small GTP-binding proteins believed to be involved with regulated secretion in secretory cells including neurons, exocrine and endocrine cells. For example, in parotid and pancreatic acinar cells Rab3D is localized to secretory granule membranes. Rab27b is another Rab protein believed to play a role in regulated secretion. Rab accessory proteins like rab-GDP-dissociation inhibitor (GDI) and rab escort protein (REP) regulate Rab protein activity by regulating guanine nucleotide binding to Rabs. REP also plays a role in the lipid modification of Rab proteins. Experiments outlined in this proposal are aimed at examining the interactions between Rab proteins and Rab accessory proteins in AR42J cells, a rat pancreatic cell line that displays an acinar cell-like phenotype upon treatment with dexamethasone. The PI believes that the AR42J cell line is a useful model for studying exocytosis because changes in protein expression and localization, as well as granule formation, can be examined as AR42J cells differentiate into acinar-like cells. In addition, work from this laboratory indicates that Rab3D associates with a less dense subpopulation of secretory granules in parotid. In the present proposal, experiments are outlined to further examine this novel finding. The specific aims of this proposal are to: 1) examine the expression, localization and interactions of Rab3 proteins, Rab27b, rab-GDI and REP isoforms in AR42J cells as they develop a regulated secretory pathway; and 2) examine the association of Rab3D with subpopulations of secretory granules in parotid and AR42J cells. Exocrine glands perform many important functions and exocrine gland function may be compromised by disease or with advancing age. Therefore, an understanding of the basic mechanisms that regulate exocrine gland secretion is essential.